Emeritus Faculty, Acad Council, Chemical and Systems Biology
PhD, Brown University
Insulin is one of the primary regulators of rapid anabolic responses in the body. Defects in the synthesis and/or ability of cells to respond to insulin results in the condition known as diabetes mellitus. To better design methods of treatment for this disorder, we have been focusing our research on how insulin elicits its various biological responses. We are utilizing the techniques of immunology, molecular biology, and biochemistry to study:
(i) How does the insulin receptor initiate the response to insulin? Like various oncogenes, the insulin receptor has an intrinsic enzymatic activity; it phosphorylates various proteins on tyrosine residues. This enzymatic activity has been found to be critical for insulin to elicit its various responses. The receptor kinase tyrosine phosphorylates various endogenous proteins. These proteins bind and activate a lipid kinase called a phosphatidylinositol 3-kinase. This kinase activates a serine/threonine kinase called Akt or PKB. A major focus is to understand the role of this serine kinase in eliciting various biological responses. Novel substrates for this kinase are being isolated and genes regulated by this kinase are being identified.
(ii) How is the response to insulin modulated? Cells from non-insulin dependent diabetics (the most common form of diabetes, ~5 million in the US) exhibit a profound resistance to insulin. This resistance can be mimicked in cell cultures by stimulating the serine phosphorylation of the insulin receptor and/or various substrates of the insulin recpetor tyrosine kinase. We are therefore exploring the hypothesis that excessive serine phosphorylation of the insulin receptor and/or insulin receptor substrates in these individuals causes this insulin resistance. We are determining the serine residues phosphorylated in the receptor and insulin receptor substrates, the enzymes response for this phosphorylation, and the consequences of these phosphorylations;
(iii) How is the response to insulin terminated? We have purified to homogeneity a protease with a high specificity for insulin and capable of cleaving insulin at the same sites as those identified in insulin cleaved in intact cells. We have also isolated the cDNA which encodes for this protease and are overexpressing this protease in mammalian cells to determine whether it will affect the termination of the insulin response; and
(iv) What is the relationship of the insulin receptor to the receptor for other insulin-like growth factors? We are comparing the abilities of these different receptors to stimulate various biological responses.
The branched-chain amino acid leucine stimulates muscle protein synthesis in part by directly activating the mTOR signaling pathway. Furthermore, leucine, if given in conjunction with resistance exercise, enhances the exercise-induced mTOR signaling and protein synthesis. Here we tested whether leucine can activate the mTOR anabolic signaling pathway in uremia and whether it can enhance work overload (WO)-induced signaling through this pathway. Chronic kidney disease (CKD) and control rats were studied after 7 days of surgically induced unilateral plantaris muscle WO and a single leucine or saline load. In the basal state, 4E-BP1 phosphorylation was modestly depressed in non-WO muscle of CKD rats, whereas rpS6 phosphorylation was nearly completely suppressed. After oral leucine mTOR, S6K1 and rpS6 phosphorylation increased similarly in both groups, whereas the phospho-4E-BP1 response was modestly attenuated in CKD. WO alone activated the mTOR signaling pathway in control and CKD rats. In WO CKD, muscle leucine augmented mTOR and 4E-BP1 phosphorylation, but its effect on S6K1 phosphorylation was attenuated. Taken together, this study has established that the chronic uremic state impairs basal signaling through the mTOR anabolic pathway, an abnormality that may contribute to muscle wasting. However, despite this abnormality, leucine can stimulate this signaling pathway in CKD, although its effectiveness is partially attenuated, including in skeletal muscle undergoing sustained WO. Thus, although there is some resistance to leucine in CKD, the data suggest a potential role for leucine-rich supplements in the management of uremic muscle wasting.
View details for DOI 10.1152/ajpendo.00068.2011
View details for Web of Science ID 000296585400015
View details for PubMedID 21791619
View details for Web of Science ID 000287479400446
Muscle wasting in chronic renal failure is associated with increased morbidity and mortality; however, resistance exercise is effective at increasing muscle mass while improving muscle strength and function. To study the mechanism by which this occurs, we compared uremic and control rats where work overload was surgically induced unilaterally in the plantaris muscle. We found that work overload increases muscle insulin-like growth factor-1 and mechano-growth factor expression. This in addition to direct mechanical activation of signaling was likely the cause of the observed increased in the protein levels and phosphorylation of the mediators of these growth factors, the insulin receptor substrate-1/phosphoinositide 3-kinase/Akt pathway. The mechanical enhancement of signal transduction appeared to be mediated in part by increased signal protein levels and decreased SOCS2 mRNA expression (suppressor of cytokine signaling-2). Despite impaired basal signaling, the work-induced signaling response was similar to that observed in nonuremic rats and produced changes consistent with decreased muscle protein degradation, increased protein synthetic capacity, and an increased number of multinucleated muscle cells. Our studies suggest that these work-induced changes account for the improved uremic muscle mass reaching levels comparable to those seen in normal rats.
View details for DOI 10.1038/sj.ki.5002801
View details for Web of Science ID 000254085100011
View details for PubMedID 18216779
Mammalian target of rapamycin (mTOR) functions in two distinct signaling complexes, mTORC1 and mTORC2. In response to insulin and nutrients, mTORC1, consisting of mTOR, raptor (regulatory-associated protein of mTOR), and mLST8, is activated and phosphorylates eukaryotic initiation factor 4E-binding protein (4EBP) and p70 S6 kinase to promote protein synthesis and cell size. Previously we found that activation of mTOR kinase in response to insulin was associated with increased 4EBP1 binding to raptor. Here we identify prolinerich Akt substrate 40 (PRAS40) as a binding partner for mTORC1. A putative TOR signaling motif, FVMDE, is identified in PRAS40 and shown to be required for interaction with raptor. Insulin stimulation markedly decreases the level of PRAS40 bound by mTORC1. Recombinant PRAS40 inhibits mTORC1 kinase activity in vivo and in vitro, and this inhibition depends on PRAS40 association with raptor. Furthermore, decreasing PRAS40 expression by short hairpin RNA enhances 4E-BP1 binding to raptor, and recombinant PRAS40 competes with 4E-BP1 binding to raptor. We, therefore, propose that PRAS40 regulates mTORC1 kinase activity by functioning as a direct inhibitor of substrate binding.
View details for DOI 10.1074/jbc.M702376200
View details for Web of Science ID 000247650600077
View details for PubMedID 17510057
cGMP-inhibited cAMP phosphodiesterase 3A (PDE3A) is expressed in mouse oocytes, and its function is indispensable for meiotic maturation as demonstrated by genetic ablation. Moreover, PDE3 activity is required for insulin/insulin-like growth factor-1 stimulation of Xenopus oocyte meiotic resumption. Here, we investigated the cAMP-dependent protein kinase B (PKB)/Akt regulation of PDE3A and its impact on oocyte maturation. Cell-free incubation of recombinant mouse PDE3A with PKB/Akt or cAMP-dependent protein kinase A catalytic subunits leads to phosphorylation of the PDE3A protein. Coexpression of PDE3A with constitutively activated PKB/Akt (Myr-Akt) increases PDE activity as well as its phosphorylation state. Injection of pde3a mRNA potentiates insulin-dependent maturation of Xenopus oocytes and rescues the phenotype of pde3(-/-) mouse oocytes. This effect is greatly decreased by mutation of any of the PDE3A serines 290-292 to alanine in both Xenopus and mouse. Microinjection of myr-Akt in mouse oocytes causes in vitro meiotic maturation and this effect requires PDE3A. Collectively, these data indicate that activation of PDE3A by PKB/Akt-mediated phosphorylation plays a role in the control of PDE3A activity in mammalian oocytes.
View details for DOI 10.1038/sj.emboj.7601431
View details for Web of Science ID 000242891100008
View details for PubMedID 17124499
View details for PubMedCentralID PMC1698880
The IRS-1 PH and PTB domains are essential for insulin-stimulated IRS-1 Tyr phosphorylation and insulin signaling, while Ser/Thr phosphorylation of IRS-1 disrupts these signaling events. To investigate consensus PKC phosphorylation sites in the PH-PTB domains of human IRS-1, we changed Ser24, Ser58, and Thr191 to Ala (3A) or Glu (3E), to block or mimic phosphorylation, respectively. The 3A mutant abrogated the inhibitory effect of PKCdelta on insulin-stimulated IRS-1 Tyr phosphorylation, while reductions in insulin-stimulated IRS-1 Tyr phosphorylation, cellular proliferation, and Akt activation were observed with the 3E mutant. When single Glu mutants were tested, the Ser24 to Glu mutant had the greatest inhibitory effect on insulin-stimulated IRS-1 Tyr phosphorylation. PKCdelta-mediated IRS-1 Ser24 phosphorylation was confirmed in cells with PKCdelta catalytic domain mutants and by an RNAi method. Mechanistic studies revealed that IRS-1 with Ala and Glu point mutations at Ser24 impaired phosphatidylinositol-4,5-bisphosphate binding. In summary, our data are consistent with the hypothesis that Ser24 is a negative regulatory phosphorylation site in IRS-1.
View details for DOI 10.1016/j.bbrc.2006.08.158
View details for Web of Science ID 000240907800015
View details for PubMedID 16970908
Akt1 is frequently up-regulated in human tumors and has been shown to accelerate cell proliferation and to suppress programmed cell death; consequently, inhibition of the activity of Akt1 has been seen as an attractive target for therapeutic intervention. Paradoxically, hyperactivation of the Akt1 oncogene can also prevent the invasive behavior that underlies progression to metastasis. Here we show that overexpression of activated myr-Akt1 in human breast cancer cells phosphorylates and thereby targets the tumor suppressor tuberous sclerosis complex 2 (TSC2) for degradation, leading to reduced Rho-GTPase activity, decreased actin stress fibers and focal adhesions, and reduced motility and invasion. Overexpression of TSC2 rescues the migration phenotype of myr-Akt1-expressing tumor cells, and high levels of TSC2 in breast cancer patients correlate with increased metastasis and reduced survival. These data indicate that the functional properties of genes designated as oncogenes or tumor suppressor genes depend on the context of the cell type and the tissues studied, and suggest the need for caution in designing therapies targeting the function of individual genes in epithelial tissues.
View details for DOI 10.1073/pnas.0511342103
View details for Web of Science ID 000236429300038
View details for PubMedID 16537497
View details for PubMedCentralID PMC1390746
Although a number of studies and approaches have indicated that activation of the Ser/Thr kinase called Akt/protein kinase B is critical for the insulin-stimulated increase of glucose uptake in adipocytes, other studies have indicated that this enzyme may play an ancillary role. For example, a recent study indicated that neomycin would allow insulin-stimulated Glut4 translocation and glucose transport in the presence of the phosphatidylinositol (PI) 3-kinase inhibitor, wortmannin, a known inhibitor of Akt activation (James, D. J., Salaun, C., Brandie, F. M., Connell, J. M. C., and Chamberlain, L. H. (2004) J. Biol. Chem. 279, 20567-20570). To better understand this observation, we examined a number of downstream targets of Akt. As previously reported, treatment of 3T3-L1 adipocytes with neomycin prevented the wortmannin inhibition of insulin-stimulated glucose transport. However, in the presence of neomycin, wortmannin did not inhibit the insulin-stimulated phosphorylation of several downstream targets of Akt including a proline-rich Akt substrate of 40 kDa, ribosomal protein S6, and glycogen synthase kinase-3. In addition, neomycin did not prevent the ability of a structurally unrelated PI 3-kinase inhibitor, LY294002, to inhibit the insulin-stimulated activation of glucose uptake. Moreover, neomycin reversed the inhibitory effect of wortmannin but not LY294002 on insulin stimulation of Akt kinase activity. Finally, neomycin was found to inactivate in vitro the PI 3-kinase inhibitory actions of wortmannin but not LY294002. These results indicate that the effects of neomycin in adipocytes are not mediated via its ability to sequester phosphatidylinositol 4,5-bisphosphate but are instead caused by the ability of neomycin to inactivate wortmannin.
View details for DOI 10.1074/jbc.M411540200
View details for Web of Science ID 000225960800038
View details for PubMedID 15504741
The Akt kinase is a serine/threonine protein kinase that has been implicated in mediating a variety of biological responses. Studies show that high Akt activity in breast carcinoma is associated with a poor pathophenotype, as well as hormone and chemotherapy resistance. Additionally, high Akt activity is associated with other features of poor prognosis. Thus, a chemotherapeutic agent directed specifically toward tumors with high Akt activity could prove extremely potent in treating those breast tumors with the most aggressive phenotypes. Several studies have demonstrated that rapamycin, which inhibits mammalian target of rapamycin (mTOR), a downstream target of Akt, sensitizes certain resistant cancer cells to chemotherapeutic agents. This study evaluated the efficacy of mTOR inhibition in the treatment of tamoxifen-resistant breast carcinoma characterized by high Akt activity. We found that MCF-7 breast cancer cell lines expressing a constitutively active Akt are able to proliferate under reduced estrogen conditions and are resistant to the growth inhibitory effects of tamoxifen, both in vitro as well as in vivo in xenograft models. Cotreatment with the mTOR inhibitor rapamycin in vitro, or the ester of rapamycin, CCI-779 (Wyeth) in vivo, inhibited mTOR activity and restored sensitivity to tamoxifen, suggesting that Akt-induced tamoxifen resistance is mediated in part by signaling through the mTOR pathway. Although the mechanism underlying the synergism remains to be understood, the results were associated with rapamycin's ability to block transcriptional activity mediated by estrogen receptor alpha, as assessed by reporter gene assays with estrogen-responsive element luciferase. These data corroborate prior findings indicating that Akt activation induces resistance to tamoxifen in breast cancer cells. Importantly, these data indicate a novel mechanism for tamoxifen resistance and suggest that blockage of the phosphatidylinositol 3'-kinase/Akt signaling pathway by mTOR inhibition effectively restores the susceptibility of these cells to tamoxifen. These data may have implication for future clinical studies of mTOR inhibition in breast carcinoma.
View details for DOI 10.1158/1078-0432.CCR-04-0035
View details for Web of Science ID 000225673200033
View details for PubMedID 15585641
Non-esterified fatty acid (free fatty acid)-induced activation of the novel PKC (protein kinase C) isoenzymes PKCdelta and PKCtheta correlates with insulin resistance, including decreased insulin-stimulated IRS-1 (insulin receptor substrate-1) tyrosine phosphorylation and phosphoinositide 3-kinase activation, although the mechanism(s) for this resistance is not known. In the present study, we have explored the possibility of a novel PKC, PKCdelta, to modulate directly the ability of the insulin receptor kinase to tyrosine-phosphorylate IRS-1. We have found that expression of either constitutively active PKCdelta or wild-type PKCdelta followed by phorbol ester activation both inhibit insulin-stimulated IRS-1 tyrosine phosphorylation in vivo. Activated PKCdelta was also found to inhibit the IRS-1 tyrosine phosphorylation in vitro by purified insulin receptor using recombinant full-length human IRS-1 and a partial IRS-1-glutathione S-transferase-fusion protein as substrates. This inhibition in vitro was not observed with a non-IRS-1 substrate, indicating that it was not the result of a general decrease in the intrinsic kinase activity of the receptor. Consistent with the hypothesis that PKCdelta acts directly on IRS-1, we show that IRS-1 can be phosphorylated by PKCdelta on at least 18 sites. The importance of three of the PKCdelta phosphorylation sites in IRS-1 was shown in vitro by a 75-80% decrease in the incorporation of phosphate into an IRS-1 triple mutant in which Ser-307, Ser-323 and Ser-574 were replaced by Ala. More importantly, the mutation of these three sites completely abrogated the inhibitory effect of PKCdelta on IRS-1 tyrosine phosphorylation in vitro. These results indicate that PKCdelta modulates the ability of the insulin receptor to tyrosine-phosphorylate IRS-1 by direct phosphorylation of the IRS-1 molecule.
View details for DOI 10.1042/BJ20031493
View details for Web of Science ID 000189290600012
View details for PubMedID 14583092
View details for PubMedCentralID PMC1223928
Expression of constitutively active Akt3 was found to increase the size of MCF-7 cells approximately twofold both in vitro and in vivo. A regulatable version of Akt1 (MER-Akt) was also found capable of inducing a twofold increase in the size of H4IIE rat hepatoma cells. Rapamycin, a specific inhibitor of mTOR function, was found to inhibit the Akt-induced increase in cell size by 70%, presumably via inhibition of the Akt-induced increase in protein synthesis. To determine whether Akt could be inhibiting protein degradation, thereby contributing to its ability to induce an increase in cell size, we conducted protein degradation experiments in the H4IIE cell line. Activation of MER-Akt was found to inhibit protein degradation to a degree comparable to insulin treatment. The effects of these two agents on protein degradation were not additive, thereby suggesting that they were acting on a similar pathway. An inhibitor of the phosphatidylinositol 3-kinase pathway, LY-294002, blocked both insulin- and Akt-induced inhibition of protein degradation, again consistent with the hypothesis that both agents were acting on the same pathway. In contrast, rapamycin did not block the ability of either agent to inhibit protein degradation. These results indicate that Akt increases cell size through both mTOR-dependent and -independent pathways and that the latter involves inhibition of protein degradation. These studies are also consistent with the hypothesis that insulin's ability to regulate protein degradation is to a large extent mediated via Akt.
View details for DOI 10.1152/ajpendo.00239.2003
View details for Web of Science ID 000185822500005
View details for PubMedID 12876075
Inorganic polyphosphate (poly P), chains of hundreds of phosphate residues linked by "high-energy" bonds as in ATP, has been conserved from prebiotic times in all cells. Poly P is essential for a wide variety of functions in bacteria, including virulence in pathogens. In this study, we observe the unique and many-fold stimulation by poly P in vitro of the protein kinase mTOR (mammalian target of rapamycin). To explore the role of poly P in mammalian cells, a yeast polyphosphatase, PPX1, was inserted into the chromosomes of MCF-7 mammary cancer cells. The transfected cells are markedly deficient in their response to mitogens, such as insulin and amino acids, as seen in their failure to activate mTOR to phosphorylate one of its substrates, PHAS-I (the initiation factor 4E-binding protein). In addition, the transfected cells are severely reduced in their growth in a serum-free medium. On the basis of these findings, we suggest that poly P (and/or PPX1) serves as a regulatory factor in the activation of mTOR in the proliferative signaling pathways of animal cells.
View details for DOI 10.1073/pnas.1534805100
View details for Web of Science ID 000185685700016
View details for PubMedID 12970465
View details for PubMedCentralID PMC208743
Prior studies had suggested that Akt activity is elevated in a subset of breast cancers. In this study, to test the effect of active Akt-3 on estrogen receptor function, we have produced MCF-7 cells, which express active Akt-3 and examined the estrogen responsiveness of these cells in vivo and in vitro. Experimental Design: MCF-7 cells expressing active Akt-3 were studied for estradiol (E2) responsiveness in vitro by both using an estrogen receptor element reporter construct as well as looking at induction of endogenous genes. These cells were also studied in vivo after injection into nude, ovariectomized mice by following tumor growth rates in the presence or absence of E2, tamoxifen, or the pure antiestrogen, ICI 182,780 (fulvestrant).Akt-3-expressing cells were found to produce tumors in mice in the absence of E2 that were approximately equivalent in size to control cells in mice given E2. Moreover, the formation of tumors by the Akt-3 cells was greatly suppressed by E2, stimulated by tamoxifen, and unaffected by ICI 182,780. In the in vitro assays for gene induction by E2, the Akt-3-expressing cells exhibited similar E2 and tamoxifen responsiveness as the control cells.These results indicate that expression of active Akt-3 in MCF-7 cells results in E2-independent tumor growth. Moreover, the growth of these tumors is inhibited by E2 and enhanced by tamoxifen. Finally, these tumors are resistant to ICI 182,780. These findings suggest that the amount of active Akt present in breast cancers may be important in the relative efficacy of different treatments.
View details for Web of Science ID 000184680200010
View details for PubMedID 12912939
To determine which genes may be regulated by Akt and participate in the transformation of cells, we have examined by microarray analyses genes turned on in the prostate cancer cell line, PC3, when Akt activity was induced. PC3 cells, which lack the lipid phosphatase PTEN, were treated overnight with a reversible inhibitor of the phosphatidylinositol 3-kinase, LY294002 (a treatment which was found to reversibly decrease Akt enzymatic activity). The inhibitor was then washed out and mRNA collected 2, 6, and 10 h later and compared by microarray analyses with mRNAs present immediately after removal of the inhibitor. One of the identified induced mRNAs, Fra-1, was further studied by transient transfections of a reporter construct containing its 5' regulatory region. This construct was found to be directly induced 4- to 5-fold by co-transfection with constitutively active Akt3 but not kinase dead Akt. The regulation by Akt3 was found to be due to two specific regions in the Fra-1 regulatory sequence which match Sp1 consensus sites. Finally, gel shift studies showed that the binding of Sp1 to one of these sites was dependent on the PI 3-kinase pathway. These results indicate that LY294002 treatment and washout is a useful method to study the activation of Akt in the context of a tumor cell. Moreover, the identification of Fra-1 as an Akt-regulated gene may have implications for the ability of Akt to transform cells since Fra-1 has been implicated in cell growth and the aggressiveness of tumors.
View details for Web of Science ID 000182325600008
View details for PubMedID 12692267
Akt (also called protein kinase B) is one of the major downstream targets of the phosphatidylinositol 3-kinase pathway. This protein kinase has been implicated in insulin signaling, stimulation of cellular growth, and inhibition of apoptosis as well as transformation of cells. Although a number of cellular proteins have been identified as putative targets of the enzyme, additional substrates may play a role in the varied responses elicited by this enzyme. We have used a combination of 14-3-3 binding and recognition by an antibody to the phosphorylation consensus of the enzyme to identify and isolate one of the major substrates of Akt, which is also a 14-3-3 binding protein. This 40-kDa protein, designated PRAS40, is a proline-rich Akt substrate. Demonstration that it is a substrate of Akt was accomplished by showing that 1) PRAS40 was phosphorylated in vitro by purified Akt on the same site that was phosphorylated in insulin-treated cells; 2) activation of an inducible Akt was alone sufficient to stimulate the phosphorylation of PRAS40; and 3) cells lacking Akt1 and Akt2 exhibit a diminished ability to phosphorylate this protein. Thus, PRAS40 is a novel substrate of Akt, the phosphorylation of which leads to the binding of this protein to 14-3-3.
View details for DOI 10.1074/jbc.M210837200
View details for Web of Science ID 000181777500028
View details for PubMedID 12524439
Ser/Thr phosphorylation of insulin receptor substrate-1 (IRS-1) is a negative regulator of insulin signaling. One potential mechanism for this is that Ser/Thr phosphorylation decreases the ability of IRS-1 to be tyrosine-phosphorylated by the insulin receptor. An additional mechanism for modulating insulin signaling is via the down-regulation of IRS-1 protein levels. Insulin-induced degradation of IRS-1 has been well documented, both in cells as well as in patients with diabetes. Ser/Thr phosphorylation of IRS-1 correlates with IRS-1 degradation, yet the details of how this occurs are still unknown. In the present study we have examined the potential role of different signaling cascades in the insulin-induced degradation of IRS-1. First, we found that inhibitors of the phosphatidylinositol 3-kinase and mammalian target of rapamycin block the degradation. Second, knockout cells lacking one of the key effectors of this cascade, the phosphoinositide-dependent kinase-1, were found to be deficient in the insulin-stimulated degradation of IRS-1. Conversely, overexpression of this enzyme potentiated insulin-stimulated IRS-1 degradation. Third, concurrent with the decrease in IRS-1 degradation, the inhibitors of the phosphatidylinositol 3-kinase and mammalian target of rapamycin also blocked the insulin-stimulated increase in Ser(312) phosphorylation. Most important, an IRS-1 mutant in which Ser(312) was changed to alanine was found to be resistant to insulin-stimulated IRS-1 degradation. Finally, an inhibitor of c-Jun N-terminal kinase, SP600125, at 10 microm did not block IRS-1 degradation and IRS-1 Ser(312) phosphorylation yet completely blocked insulin-stimulated c-Jun phosphorylation. Further, insulin-stimulated c-Jun phosphorylation was not blocked by inhibitors of the phosphatidylinositol 3-kinase and mammalian target of rapamycin, indicating that c-Jun N-terminal kinase is unlikely to be the kinase phosphorylating IRS-1 Ser(312) in response to insulin. In summary, our results indicate that the insulin-stimulated degradation of IRS-1 via the phosphatidylinositol 3-kinase pathway is in part dependent upon the Ser(312) phosphorylation of IRS-1.
View details for DOI 10.1074/jbc.M209153200
View details for Web of Science ID 000181466800059
View details for PubMedID 12510059
The phosphatidylinositol 3-kinase/Akt pathway plays an important role in the signaling of insulin and other growth factors, which reportedly attenuate the interleukin-6 (IL-6)-mediated stimulation of acute phase plasma protein genes. We investigated the effect of the protein kinase Akt on IL-6-mediated transcriptional activation. The transient expression of constitutively active Akt inhibited the IL-6-dependent activity of the alpha(2)-macroglobulin promoter in HepG2 cells, whereas expression of an inactive mutant of phosphatidylinositol-dependent kinase 1 had the opposite effect. Since Akt is known to regulate gene expression through inactivation of the transcription factor FKHR (forkhead in rhabdomyosarcoma), we examined the effect of FKHR on STAT3-mediated transcriptional regulation. Indeed, the overexpression of FKHR specifically enhanced the activity of STAT3-dependent promoters but not that of a STAT5-responsive promoter. The effect of FKHR required the presence of functional STAT3 and was abrogated by the expression of dominant negative STAT3 mutants. Furthermore, FKHR and STAT3 were shown to coimmunoprecipitate and to colocalize in the nuclear regions of IL-6-treated HepG2 cells. Our results indicate that FKHR can modulate the IL-6-induced transcriptional activity by acting as a coactivator of STAT3.
View details for DOI 10.1074/jbc.M205403200
View details for Web of Science ID 000180968900113
View details for PubMedID 12456685
In the present study, we have characterized the Xenopus Akt expressed in oocytes from the African clawed frog Xenopus laevis and tested whether its activity is required for the insulin- and progesterone-stimulated resumption of meiosis. A cDNA encoding the Xenopus Akt was isolated and sequenced, and its expression in the Xenopus oocyte was confirmed by reverse transcription PCR and Northern blotting. Using phosphospecific antibodies and enzyme assays, a large and rapid activation of the Xenopus Akt was observed upon insulin stimulation of the oocytes. In contrast, progesterone caused a modest activation of this kinase with a slower time course. To test whether the activation of Akt was required in the stimulation of the resumption of meiosis, we have utilized two independent approaches: a functional dominant negative Akt mutant and an inhibitory monoclonal antibody. Both the mutant Akt, as well as the inhibitory monoclonal antibody, completely blocked the insulin-stimulated resumption of meiosis. In contrast, both treatments only partially inhibited (by approx. 30%) the progesterone-stimulated resumption of meiosis when submaximal doses of this hormone were utilized. These data demonstrate a crucial role for Akt in the insulin-stimulated cell cycle progression of Xenopus oocytes, whereas Akt may have an ancillary function in progesterone signalling.
View details for Web of Science ID 000180715900003
View details for PubMedID 12374568
Abeta is the major component of amyloid plaques characterizing Alzheimer's disease (AD). Abeta accumulation can be affected by numerous factors including increased rates of production and/or impaired clearance. Insulin-degrading enzyme (IDE) has been implicated as a candidate enzyme responsible for the degradation and clearance of Abeta in the brain. We have previously shown that AD patients exhibit abnormalities in insulin metabolism that are associated with apoliprotein E (APOE) status. The possible association of IDE with AD, as well as the link between APOE status and insulin metabolism, led us to examine the expression of IDE in AD. We report that hippocampal IDE protein is reduced by approximately 50% in epsilon4+ AD patients compared to epsilon4- patients and controls. The allele-specific decrease of IDE in epsilon4+ AD patients is not associated with neuronal loss since neuron-specific enolase levels were comparable between the AD groups, regardless of APOE status. Hippocampal IDE mRNA levels were also reduced in AD patients with the epsilon4 allele compared to AD and normal subjects without the epsilon4 allele. These findings show that reduced IDE expression is associated with a significant risk factor for AD and suggest that IDE may interact with APOE status to affect Abeta metabolism.
View details for Web of Science ID 000180009800032
View details for PubMedID 12507914
View details for PubMedCentralID PMC1851126
Expression of the catalytic subunit of glucose-6-phosphatase (G6Pase) has recently been shown to be transactivated by the transcription factor FKHR. Insulin and conditions of energy depletion are known repressors of the G6Pase gene. Whereas insulin is known to inhibit G6Pase expression by phosphorylation and nuclear exclusion of FKHR, the mechanism of repression of G6Pase by energy depletion is unknown. Here, we have studied the effect of glucose starvation and AICAR, an activator of AMP-activated protein kinase (AMPK) on G6Pase expression and the expressional level of FKHR-protein in hepatic cells. Using a H4-hepatoma cell line stably overexpressing FKHR, we found that both glucose starvation and treatment of cells with AICAR strongly repressed G6Pase expression and led to an almost complete disappearance of the FKHR protein, whereas the levels of control proteins and FKHR mRNA were not affected. Our data suggest that AICAR and glucose starvation inhibit G6Pase expression by a reduction of the cellular level of FKHR, presumably mediated by specific degradation of the protein.
View details for Web of Science ID 000176999600042
View details for PubMedID 12130586
View details for Web of Science ID 000177465301443
In order to study the effect of the peroxisome proliferator-activated receptor gamma (PPARgamma) agonist troglitazone on the insulin-induced expression of fatty acid synthase (FAS) in adipocytes, we generated a 3T3-L1 cell line stably expressing a FAS reporter gene construct. In this cell line, a low concentration of troglitazone (250 nM) increased the effect of insulin on the FAS promoter activity and the expression of FAS protein about 1.5- to 2-fold. Since the effect of insulin on the expression of FAS is believed to be mediated by activation of protein kinase B (PKB), we investigated the effect of troglitazone on the regulation of PKB. Troglitazone (250 nM) increased the maximal effect of insulin on PKB activity about twofold without significantly affecting its EC(50) (1.4+/-0.5 nM vs. 2.2+/-0.6 nM in controls). Higher concentrations of troglitazone (> or =1 microM) inhibited both insulin-stimulated PKB activity and expression of FAS. In summary, our data indicate a dual effect of troglitazone on the insulin-induced FAS gene expression in 3T3-L1 cells. The therapeutic, stimulatory effect is produced by low concentrations of troglitazone (250 nM), and is presumably mediated by enhanced activation of PKB.
View details for DOI 10.1007/s00210-002-0529-y
View details for Web of Science ID 000174970100006
View details for PubMedID 11919653
View details for PubMedID 28201650
Hormonally stimulated lipolysis occurs by activation of cyclic AMP-dependent protein kinase (PKA) which phosphorylates hormone-sensitive lipase (HSL) and increases adipocyte lipolysis. Evidence suggests that catecholamines not only can activate PKA, but also the mitogen-activated protein kinase pathway and extracellular signal-regulated kinase (ERK). We now demonstrate that two different inhibitors of MEK, the upstream activator of ERK, block catecholamine- and beta(3)-stimulated lipolysis by approximately 30%. Furthermore, treatment of adipocytes with dioctanoylglycerol, which activates ERK, increases lipolysis, although MEK inhibitors decrease dioctanoylglycerol-stimulated activation of lipolysis. Using a tamoxifen regulatable Raf system expressed in 3T3-L1 preadipocytes, exposure to tamoxifen causes a 14-fold activation of ERK within 15-30 min and results in approximately 2-fold increase in HSL activity. In addition, when differentiated 3T3-L1 cells expressing the regulatable Raf were exposed to tamoxifen, a 2-fold increase in lipolysis is observed. HSL is a substrate of activated ERK and site-directed mutagenesis of putative ERK consensus phosphorylation sites in HSL identified Ser(600) as the site phosphorylated by active ERK. When S600A HSL was expressed in 3T3-L1 cells expressing the regulatable Raf, tamoxifen treatment fails to increase its activity. Thus, activation of the ERK pathway appears to be able to regulate adipocyte lipolysis by phosphorylating HSL on Ser(600) and increasing the activity of HSL.
View details for Web of Science ID 000172406700143
View details for PubMedID 11581251
View details for Web of Science ID 000172372500113
Estrogen acting through the estrogen receptor (ER) is able to regulate cell growth and differentiation of a variety of normal tissues and hormone-responsive tumors. Ligand-activated ER binds DNA and transactivates the promoters of estrogen target genes. In addition, ligand-activated ER can interact with other factors to alter the physiology and growth of cells. Using a yeast two-hybrid screen, we have identified an interaction between ER alpha and the proapoptotic forkhead transcription factor FKHR. The ER alpha-FKHR interaction depends on beta-estradiol and is reduced significantly in the absence of hormone or the presence of Tamoxifen. A glutathione S-transferase pull-down assay was used to confirm the interaction and localized two interaction sites, one in the forkhead domain and a second in the carboxyl terminus. The FKHR interaction was specific to ER alpha and was not detected with other ligand-activated steroid receptors. The related family members, FKHRL1 and AFX, also bound to ER alpha in the presence of beta-estradiol. FKHR augmented ER alpha transactivation through an estrogen response element. Conversely, ER alpha repressed FKHR-mediated transactivation through an insulin response sequence, and cell cycle arrest induced by FKHRL1 in MCF7 cells was abrogated by estradiol. These results suggest a novel mechanism of estrogen action that involves regulation of the proapoptotic forkhead transcription factors.
View details for Web of Science ID 000170910200037
View details for PubMedID 11435445
The insulin responsive H4IIEC3 rat hepatoma cell line (H4 cells) was used in order to determine the role of the transcription factor FKHR in the regulation of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase). Both PEPCK and G6Pase contain putative FKHR binding sites in their promoter sequence. Using a retroviral expression system, we stably overexpressed FKHR in H4-cells. FKHR was phosphorylated in a PI 3-kinase- and Akt-dependent manner, and was translocated from the nucleus to the cytoplasm in response to insulin. Furthermore, overexpression of FKHR markedly increased the expression of the catalytic subunit of G6Pase (basal about 2.5-fold, dexamethasone/cAMP stimulated about fivefold, respectively). In contrast, both basal and dexamethasone/cAMP-induced levels of PEPCK mRNA were unaffected by FKHR-overexpression. These data suggest a specific function for FKHR in the regulation of hepatic gluconeogenesis at the level of G6Pase, but not PEPCK gene expression.
View details for Web of Science ID 000170110800008
View details for PubMedID 11467835
Thyrotropin (TSH)-initiated cell cycle progression from G1 to S phase in FRTL-5 thyroid cells requires serum, insulin, or insulin-like growth factor 1 (IGF-1) and involves activation of 3-hydroxy-3-methylglutaryl-CoA reductase, geranylgeranylation of RhoA, p27Kip1 degradation, and activation of cyclin-dependent kinase (cdk) 2. In the present report, we show that the serine-threonine kinase Akt is an important mediator of insulin/IGF-1/serum effects on cell cycle progression in FRTL-5 thyroid cells. The phosphoinositol (OH) 3 kinase inhibitors, Wortmannin (WM) and Ly294002 (LY), block the ability of insulin/IGF-1 to reduce p27 expression, to induce expression of cyclins E, D1, and A as well as cdk 2 and 4, and to phosphorylate retinoblastoma protein. They also inhibit insulin/IGF-1-increased DNA synthesis and cell cycle entrance (S+G2/M). Insulin/IGF-1 rapidly induced activation of Aktl in a PI3 kinase-dependent manner, and increased Aktl RNA levels. Most importantly, FRTL-5 cells transfected with a constitutively active form of Aktl have higher basal rates of DNA synthesis and no longer require exogenous insulin/IGF-1 or serum for TSH-induced growth. In sum, Aktl appears to have an important role in insulin/IGF-1 regulation of FRTL-5 thyroid cell growth and cell cycle progression.
View details for Web of Science ID 000168507400006
View details for PubMedID 11349832
The pleckstrin homology (PH) domain of the insulin receptor substrate-1 (IRS-1) plays a role in directing this molecule to the insulin receptor, thereby regulating its tyrosine phosphorylation. In this work, the role of the PH domain in subsequent signaling was studied by constructing constitutively active forms of IRS-1 in which the inter-SH2 domain of the p85 subunit of phosphatidylinositol 3-kinase was fused to portions of the IRS-1 molecule. Chimeric molecules containing the PH domain were found to activate the downstream response of stimulating the Ser/Thr kinase Akt. A chimera containing point mutations in the PH domain that abolished the ability of this domain to bind phosphatidylinositol 4,5-bisphosphate prevented these molecules from activating Akt. These mutations also decreased by about 70% the amount of the constructs present in a particulate fraction of the cells. These results indicate that the PH domain of IRS-1, in addition to directing this protein to the receptor for tyrosine phosphorylation, functions in the ability of this molecule to stimulate subsequent responses. Thus, compromising the function of the PH domain, e.g. in insulin-resistant states, could decrease both the ability of IRS-1 to be tyrosine phosphorylated by the insulin receptor and to link to subsequent downstream targets.
View details for Web of Science ID 000167474900057
View details for PubMedID 11145958
Inhibitors of signalling pathways were used to dissect the mechanism of insulin action on expression of the gene encoding glucokinase in cultured rat hepatocytes. Wortmannin and LY 294002 completely prevented the insulin-induced increase in glucokinase mRNA seen in unhibited cells, indicating that the phosphoinositide 3-kinase module has a key role. A ligand inducible protein kinase B (PKB, also termed cAkt) fusion protein was expressed by using adenoviral transduction of hepatocytes in primary culture. The PKB activity of this protein was shown to be activated in transduced hepatocytes within 30 min of the addition of 4-hydroxytamoxifen and to stay high for 8 h, as a result of serine phosphorylation at position 473 of PKB. The increase in PKB activity was reflected in the hyperphosphorylation of phosphorylated, heat and acid stable regulated by insulin protein (PHAS-I; also termed 4E-BP1, for eukaryotic initiation factor 4E-binding protein 1), a protein involved in the regulation of translation initiation. These effects were comparable to the insulin-induced activation of endogenous PKB and phosphorylation of PHAS-I in non-transduced hepatocytes. The addition of tamoxifen to transduced hepatocytes resulted in an induction of glucokinase mRNA with kinetics and magnitude similar to those of insulin-induced mRNA accumulation. The effect of tamoxifen depended on stimulated PKB activity because it did not occur in hepatocytes that were transduced with a mutant PKB fusion protein that was refractory to activation with tamoxifen. These results establish that acute activation of PKB is sufficient to produce an insulin-like induction of glucokinase in isolated hepatocytes. Together with the inhibition by phosphoinositide 3-kinase inhibitors, they suggest that the activation of PKB might be critical in mediating the induction of glucokinase by insulin. In addition, experiments showed that PD98059 decreased by half the increase in glucokinase mRNA brought about by insulin, suggesting a contributory role of the mitogen-activated protein kinase cascade.
View details for DOI 10.1042/0264-6021:3510621
View details for Web of Science ID 000165227000010
View details for PubMedID 11042116
View details for PubMedCentralID PMC1221401
The Akt/PKB protein kinase is implicated in the control of cell cycle progression and the suppression of apoptosis in cancer cells. Here we describe the use of a conditionally active form of Akt/PKB (M+ Akt:ER*) to study the ability of this protein to influence biological processes that are central to the process of oncogenic transformation of mammalian cells. Activation of M+ Akt:ER* in Rat1 cells elicited alterations in cell morphology and promoted anchorage-independent growth in agarose with high efficiency. Consistent with these observations, activation of M+ Akt:ER* suppressed the apoptosis of Rat1 cells that occurs after the detachment of these cells from extracellular matrix. Furthermore, activation of M+ Akt:ER* was sufficient to promote the progression of quiescent Rat1 cells into the S and G2-M phases of the cell cycle. In accord with this is the observation that activation of M+ Akt:ER* led to decreased expression of the cyclin-dependent kinase inhibitor p27Kip1 with a concomitant increase in cyclin-dependent kinase-2 activity. Perhaps surprisingly, activation of M+ Akt:ER* or expression of a constitutively active form of Akt led to rapid activation of MAP/ERK Kinase (MEK) and the extracellular signal-regulated kinase (ERK)/mitogen-activated protein (MAP) kinases in Rat1 cells. However, pharmacological inhibition of MEK by PD098059 did not inhibit the morphological alterations of Rat1 cells that occur after M+ Akt:ER* activation. These data suggest that M+ Akt:ER* can activate a number of pathways in Rat1 cells, leading to significant alterations in a number of biological processes. The conditional transformation system described here will allow further elucidation of the ability of Akt to contribute to both the normal response of cells to mitogenic stimulation and the aberrant proliferation observed in cancer cells.
View details for Web of Science ID 000088116900001
View details for PubMedID 10910095
Loss of the tumor suppressor MMAC1 has been shown to be involved in breast, prostate and brain cancer. Consistent with its identification as a tumor suppressor, expression of MMAC1 has been demonstrated to reduce cell proliferation, tumorigenicity, and motility as well as affect cell-cell and cell-matrix interactions of malignant human glioma cells. Subsequently, MMAC1 was shown to have lipid phosphatase activity towards PIP3 and protein phosphatase activity against focal adhesion kinase (FAK). The lipid phosphatase activity of MMAC1 results in decreased activation of the PIP3-dependent, anti-apoptotic kinase, AKT. It is thought that this inhibition of AKT culminates with reduced glioma cell proliferation. In contrast, MMAC1's effects on cell motility, cell - cell and cell - matrix interactions are thought to be due to its protein phosphatase activity towards FAK. However, recent studies suggest that the lipid phosphatase activity of MMAC1 correlates with its ability to be a tumor suppressor. The high rate of mutation of MMAC1 in late stage metastatic tumors suggests that effects of MMAC1 on motility, cell - cell and cell - matrix interactions are due to its tumor suppressor activity. Therefore the lipid phosphatase activity of MMAC1 may affect PIP3 dependent signaling pathways and result in reduced motility and altered cell - cell and cell - matrix interactions. We demonstrate here that expression of MMAC1 in human glioma cells reduced intracellular levels of inositol trisphosphate and inhibited extracellular Ca2+ influx, suggesting that MMAC1 affects the phospholipase C signaling pathway. In addition, we show that MMAC1 expression inhibits integrin-linked kinase activity. Furthermore, we show that these effects require the catalytic activity of MMAC1. Our data thus provide a link of MMAC1 to PIP3 dependent signaling pathways that regulate cell - matrix and cell - cell interactions as well as motility. Lastly, we demonstrate that AKT3, an isoform of AKT highly expressed in the brain, is also a target for MMAC1 repression. These data suggest an important role for AKT3 in glioblastoma multiforme. We therefore propose that repression of multiple PIP3 dependent signaling pathways may be required for MMAC1 to act as a tumor suppressor.
View details for Web of Science ID 000084844400004
View details for PubMedID 10644997
In the present studies, we demonstrate that heregulin is a potent and rapid activator of the serine/threonine kinase called Akt in the MCF-7 breast cancer cell line but not in 3 other breast cancer cell lines (T47D, HBL-100, and MDA-231). The extent of activation of Akt in the 4 cell lines correlated with the ability of heregulin to activate phosphatidylinositol 3-kinase and inhibition of the kinase blocked Akt activation. A monoclonal antibody to HER2 inhibited the ability of heregulin to activate Akt in the MCF-7 cells. BT474, a breast cancer cell line which overexpresses HER2, had high basal Akt enzymatic activity. This high basal activity was lowered when cells were pre-incubated with an anti-HER2 monoclonal antibody which is used to treat breast cancer patients. Our results indicate that heregulin is a potent activator of Akt and that overexpression of HER2 in breast cancers could also lead to activation of Akt.
View details for Web of Science ID 000082014900060
View details for PubMedID 10441522
We measured the insulin-stimulated amount of Akt1, Akt2, and Akt3 enzymatic activities in four breast cancer cell lines and three prostate cancer cell lines. In the estrogen receptor-deficient breast cancer cells and the androgen-insensitive prostate cells, the amount of Akt3 enzymatic activity was approximately 20-60-fold higher than in the cells that were estrogen- or androgen-responsive. In contrast, the levels of Akt1 and -2 were not increased in these cells. The increase in Akt3 enzyme activity correlated with an increase in both Akt3 mRNA and protein. In a prostate cancer cell line lacking the tumor suppressor PTEN (a lipid and protein phosphatase), the basal enzymatic activity of Akt3 was constitutively elevated and represented the major active Akt in these cells. Finally, reverse transcription-PCR was used to examine the Akt3 expression in 27 primary breast carcinomas. The expression levels of Akt3 were significantly higher in the estrogen receptor-negative tumors in comparison to the estrogen receptor-positive tumors. To see if the increase in Akt3 could be due to chromosomal abnormalities, the Akt3 gene was assigned to human chromosome 1q44 by fluorescence in situ hybridization and radiation hybrid cell panel analyses. These results indicate that Akt3 may contribute to the more aggressive clinical phenotype of the estrogen receptor-negative breast cancers and androgen-insensitive prostate carcinomas.
View details for Web of Science ID 000081721100009
View details for PubMedID 10419456
We used mouse hepatoma (Hepa1c1c7) cells to study the role of the serine/threonine kinase Akt in the induction of GLUT1 gene expression. In order to selectively turn on the Akt kinase cascade, we expressed a hydroxytamoxifen-regulatable form of Akt (myristoylated Akt1 estrogen receptor chimera (MER-Akt1)) in the Hepa1c1c7 cells; we verified that hydroxytamoxifen stimulates MER-Akt1 activity to a similar extent as the activation of endogenous Akt by insulin. Our studies reveal that stimulation of MER-Akt1 by hydroxytamoxifen induces GLUT1 mRNA and protein accumulation to levels comparable to that induced by insulin; therefore, activation of the Akt cascade suffices to induce GLUT1 gene expression in this cell system. Furthermore, expression of a kinase-inactive Akt mutant partially inhibits the response of the GLUT1 gene to insulin. Additional studies reveal that the induction of GLUT1 mRNA by Akt and by insulin reflects increased mRNA synthesis and not decreased mRNA degradation. Our findings imply that the GLUT1 gene responds to insulin at the transcriptional level and that Akt mediates a step in the activation of GLUT1 gene expression in this system.
View details for Web of Science ID 000081438300040
View details for PubMedID 10400647
To characterize the contribution of glycogen synthase kinase 3beta (GSK3beta) inactivation to insulin-stimulated glucose metabolism, wild-type (WT-GSK), catalytically inactive (KM-GSK), and uninhibitable (S9A-GSK) forms of GSK3beta were expressed in insulin-responsive 3T3-L1 adipocytes using adenovirus technology. WT-GSK, but not KM-GSK, reduced basal and insulin-stimulated glycogen synthase activity without affecting the -fold stimulation of the enzyme by insulin. S9A-GSK similarly decreased cellular glycogen synthase activity, but also partially blocked insulin stimulation of the enzyme. S9A-GSK expression also markedly inhibited insulin stimulation of IRS-1-associated phosphatidylinositol 3-kinase activity, but only weakly inhibited insulin-stimulated Akt/PKB phosphorylation and glucose uptake, with no effect on GLUT4 translocation. To further evaluate the role of GSK3beta in insulin signaling, the GSK3beta inhibitor lithium was used to mimic the consequences of insulin-stimulated GSK3beta inactivation. Although lithium stimulated the incorporation of glucose into glycogen and glycogen synthase enzyme activity, the inhibitor was without effect on GLUT4 translocation and pp70 S6 kinase. Lithium stimulation of glycogen synthesis was insensitive to wortmannin, which is consistent with its acting directly on GSK3beta downstream of phosphatidylinositol 3-kinase. These data support the hypothesis that GSK3beta contributes to insulin regulation of glycogen synthesis, but is not responsible for the increase in glucose transport.
View details for Web of Science ID 000080974300076
View details for PubMedID 10364240
The family of protein kinases called Akt, protein kinase B (PKB), or related to A and C kinase (RAC) have been implicated in numerous biological processes including adipocyte and muscle differentiation, glycogen synthesis, glucose uptake, apoptosis and cellular proliferation. There are 3 known isoforms of this enzyme in mammalian cells (1/alpha, 2/beta and 3/gamma). Akt1 and 2 contain a key regulatory serine phosphorylation site in the carboxy-terminal region of the protein. However, the reported sequence of the rat Akt3 protein differed significantly from this in that it lacked 25 amino acids in the C-terminal region, including this key regulatory serine phosphorylation site (Biochem. Biophys. Res. Commun. 216, 526-534). In the present studies we show that the deduced sequence of human Akt3 contains this serine and that it is phosphorylated in response to insulin. These results indicate that human Akt3 is regulated similarly to Akt1 and Akt2.
View details for Web of Science ID 000079925300047
View details for PubMedID 10208883
Serine/threonine phosphorylation of insulin receptor substrate 1 (IRS-1) has been implicated as a negative regulator of insulin signaling. Prior studies have indicated that this negative regulation by protein kinase C involves the mitogen-activated protein kinase and phosphorylation of serine 612 in IRS-1. In the present studies, the negative regulation by platelet-derived growth factor (PDGF) was compared with that induced by endothelin-1, an activator of protein kinase C. In contrast to endothelin-1, the inhibitory effects of PDGF did not require mitogen-activated protein kinase or the phosphorylation of serine 612. Instead, three other serines in the phosphorylation domain of IRS-1 (serines 632, 662, and 731) were required for the negative regulation by PDGF. In addition, the PDGF-activated serine/threonine kinase called Akt was found to inhibit insulin signaling. Moreover, this inhibition required the same IRS-1 serine residues as the inhibition by PDGF. Finally, the negative regulatory effects of PDGF and Akt were inhibited by rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), one of the downstream targets of Akt. These studies implicate the phosphatidylinositol 3-kinase/Akt kinase cascade as an additional negative regulatory pathway for the insulin signaling cascade.
View details for Web of Science ID 000079451800032
View details for PubMedID 10092613
To determine whether defects in the insulin signal transduction pathway to glucose transport occur in a muscle fiber type-specific manner, post-receptor insulin-signaling events were assessed in oxidative (soleus) and glycolytic (extensor digitorum longus [EDL]) skeletal muscle from Wistar or diabetic GK rats. In soleus muscle from GK rats, maximal insulin-stimulated (120 nmol/l) glucose transport was significantly decreased, compared with that of Wistar rats. In EDL muscle from GK rats, maximal insulin-stimulated glucose transport was normal, while the submaximal response was reduced compared with that of Wistar rats. We next treated diabetic GK rats with phlorizin for 4 weeks to determine whether restoration of glycemia would lead to improved insulin signal transduction. Phlorizin treatment of GK rats resulted in full restoration of insulin-stimulated glucose transport in soleus and EDL muscle. In soleus muscle from GK rats, submaximal and maximal insulin-stimulated insulin receptor substrate (IRS)-1 tyrosine phosphorylation and IRS-1-associated phosphatidylinositol (PI) 3-kinase activity were markedly reduced, compared with that of Wistar rats, but only submaximal insulin-stimulated PI 3-kinase was restored after phlorizin treatment. In EDL muscle, insulin-stimulated IRS-1 tyrosine phosphorylation and IRS-1-associated PI-3 kinase were not altered between GK and Wistar rats. Maximal insulin-stimulated Akt (protein kinase B) kinase activity is decreased in soleus muscle from GK rats and restored upon normalization of glycemia (Krook et al., Diabetes 46:2100-2114, 1997). Here, we show that in EDL muscle from GK rats, maximal insulin-stimulated Akt kinase activity is also impaired and restored to Wistar rat levels after phlorizin treatment. In conclusion, functional defects in IRS-1 and PI 3-kinase in skeletal muscle from diabetic GK rats are fiber-type-specific, with alterations observed in oxidative, but not glycolytic, muscle. Furthermore, regardless of muscle fiber type, downstream steps to PI 3-kinase (i.e., Akt and glucose transport) are sensitive to changes in the level of glycemia.
View details for Web of Science ID 000078827100034
View details for PubMedID 10078575
A rat hepatoma cell line, H4IIE, was stably transfected with a tamoxifen regulatable Akt-1 construct. Treatment of these cells with tamoxifen caused a rapid stimulation of Akt enzymatic activity that was comparable with the activity observed with the endogenous Akt after insulin stimulation. Prior studies have extensively documented that insulin can repress the glucocorticoid and cAMP-stimulated increase in phosphoenolpyruvate carboxykinase (PEPCK) gene transcription. Activation of this regulatable Akt with tamoxifen was found to mimic the dominant inhibitory effect of insulin on PEPCK gene transcription. Dose response curves to insulin and tamoxifen demonstrated that this response was very sensitive to Akt activation although the maximal response observed with tamoxifen activation was slightly less than that observed with insulin, indicating that the response to insulin may also involve other signaling cascades. The regulation of PEPCK transcription via Akt was, like that previously described for insulin, not dependent upon 70 kDa S6 kinase activity in that it was not inhibited by rapamycin. Finally, the expression of a kinase dead Akt was able to partially inhibit the ability of insulin to stimulate this response. In summary, the present results indicate that activation of Akt alone is sufficient to repress the glucocorticoid and cAMP-stimulated increase in PEPCK gene transcription.
View details for Web of Science ID 000076448000045
View details for PubMedID 9765258
The molecular signaling mechanisms by which muscle contractions lead to changes in glucose metabolism and gene expression remain largely undefined. We assessed whether exercise activates MAP kinase proteins (ERK1/2, SEK1, and p38 MAP kinase) as well as Akt and PYK2 in skeletal muscle from healthy volunteers obtained during and after one-leg cycle ergometry at approximately 70% VO2max. Exercise led to a marked increase in ERK1/2 phosphorylation, which rapidly decreased to resting levels upon recovery. Exercise increased phosphorylation of SEK1 and p38 MAP kinase to a lesser extent than ERK1/2. In contrast to ERK1/2, p38 MAP kinase phosphorylation was increased in nonexercised muscle upon cessation of exercise. Phosphorylation of the transcription factor CREB was increased in nonexercised muscle upon cessation of exercise. Exercise did not activate Akt or increase tyrosine phosphorylation of PYK2. Thus, exercise has divergent effects on parallel MAP kinase pathways, of which only p38 demonstrated a systemic response. However, our data do not support a role of Akt or PYK2 in exercise/contraction-induced signaling in human skeletal. Activation of the different MAP kinase pathways by physical exercise appears to be important in the regulation of transcriptional events in skeletal muscle.
View details for Web of Science ID 000076402000013
View details for PubMedID 9761781
In the present work a chimeric receptor containing the intracellular domain of the insulin receptor-related receptor (IRR) and the extracellular domain of the colony stimulating factor-1 (CSF-1) receptor was expressed in 3T3-L1 adipocytes and compared with the parallel chimeric receptor containing the cytoplasmic domain of the insulin receptor (IR). Both chimeric receptors exhibited CSF-stimulated tyrosine kinase activity when assayed in vitro after in vivo activation comparable to that of the endogenous IR present in these cells. No cross-activation of the expressed chimeric and endogenous receptors was observed. The cytoplasmic domain of the IRR was found to 1) mediate activation of the Ser/Thr kinase Akt/PKB, 2) stimulate glucose uptake, 3) inhibit lipolysis, and 4) stimulate glycogen synthase, all with a potency comparable to those of the expressed CSF-1R/IR chimera and the endogenous insulin receptors. These results indicate that despite the extensive differences in sequence between the cytoplasmic domains of the IRR and IR, the elements required for insulin-specific responses have been conserved in this distinct member of the insulin receptor family.
View details for Web of Science ID 000074922600027
View details for PubMedID 9681510
The serine/threonine kinase Akt (PKB/Rac) has been implicated as playing a role in the insulin-signaling pathway to glucose transport. Little is known regarding the regulation of Akt kinase activity in insulin-sensitive tissues, such as skeletal muscle, or whether this regulation is altered in insulin-resistant states such as NIDDM. We examined the effect of insulin on Akt kinase activity in skeletal muscle from six NIDDM patients and six healthy subjects. Whole-body insulin sensitivity, assessed by the euglycemic-hyperinsulinemic clamp, was significantly lower in NIDDM subjects (P < 0.001), and this was accompanied by impaired in vitro insulin-stimulated glucose transport in skeletal muscle. In both groups, insulin induced a significant increase in Akt kinase activity, but the response to maximal insulin (60 nmol/l) was markedly reduced in skeletal muscle from NIDDM subjects (66% of control levels, P < 0.01). Impaired Akt kinase activity was not accompanied by decreased protein expression of Akt. Instead, a trend toward increased Akt expression was noted in skeletal muscle from NIDDM subjects (P < 0.1). These parallel defects in insulin-stimulated Akt kinase activity and glucose transport in diabetic skeletal muscle suggest that reduced Akt kinase activity may play a role in the development of insulin resistance in NIDDM.
View details for DOI 10.2337/diabetes.47.8.1281
View details for Web of Science ID 000075177400016
View details for PubMedID 9703329
Transient expression of oncogenic Ha-Ras (Ras:V12) stimulates endocytosis. Using NIH3T3 cells expressing constitutively active protein kinase B/akt (PKB/akt) or kinase-dead PKB/akt, we show that PKB/akt mediates the stimulatory effect of Ras on endocytosis. Fluid phase endocytosis of horseradish peroxidase in cells expressing the constitutively active form of PKB/akt was elevated and insensitive to phosphatidylinositol 3-kinase inhibitors. However, expression of dominant negative Rab5:N34 blocked endocytosis in cells expressing the constitutively active form of PKB/akt. Transient expression of either Rab5:wt or Rab5:L79, a GTPase deficient mutant of Rab5, in cells expressing constitutively activated PKB/akt further increased endocytic rate. However, in cells expressing kinase-dead PKB/akt, endocytic rate was not affected by transient expression of Rab5:wt. Rab5:L79, on the other hand, increased endocytosis in cells expressing kinase-dead PKB/akt. Similar results were obtained using an in vitro endosome fusion reconstitution assay with cytosol prepared from cells expressing the activated PKB/akt or kinase-dead PKB/akt. Both Rab5:wt and Rab5:L79 stimulated endosome fusion when assayed in cytosol containing the activated PKB/akt, whereas only Rab5:L79 activated fusion when the assay utilized cytosol from kinase-dead expressing cells. We conclude that Ras activation of endocytosis requires both PKB/akt and Rab5 and that active kinase is required for activation Rab5.
View details for Web of Science ID 000075125200003
View details for PubMedID 9677351
The activation of protein kinase B/Akt is thought to be a critical step in the phosphoinositide 3-kinase pathway that regulates cell growth and differentiation. Because insulin-like growth factor 1 stimulates the resumption of meiosis in Xenopus laevis oocytes via phosphoinositide 3-kinase activation, we investigated the Akt involvement in this process. Injection of mRNA coding for a constitutively active Akt in Xenopus oocytes induced germinal vesicle breakdown (GVBD) to the same extent as progesterone or insulin treatment. Injection of mRNA coding for the wild type Akt kinase was less effective in stimulating GVBD, whereas Akt bearing a lysine mutation in the catalytic domain that abolishes the kinase activity had no effect. A mutant Akt lacking a membrane-targeting sequence did not induce GVBD, despite high levels of expression and activity. As previously reported for insulin, induction of GVBD by Akt was prevented by incubating the oocytes with cilostamide, an inhibitor specific for the type 3 phosphodiesterase (PDE3), suggesting that the activity of a PDE is required for Akt action. That an increase in PDE activity in the oocyte is sufficient to induce meiotic resumption was demonstrated by expression of an active PDE protein. In addition, the constitutively active Akt caused a 2-fold increase in the activity of the endogenous PDE. These data demonstrate that Akt is in the pathway controlling resumption of meiosis in the Xenopus oocyte and that regulation of the activity of a PDE3 is a step distal to the kinase activation.
View details for Web of Science ID 000074974700008
View details for PubMedID 9668041
The effects of insulin on the mammalian target of rapamycin, mTOR, were investigated in 3T3-L1 adipocytes. mTOR protein kinase activity was measured in immune complex assays with recombinant PHAS-I as substrate. Insulin-stimulated kinase activity was clearly observed when immunoprecipitations were conducted with the mTOR antibody, mTAb2. Insulin also increased by severalfold the 32P content of mTOR that was determined after purifying the protein from 32P-labeled adipocytes with rapamycin.FKBP12 agarose beads. Insulin affected neither the amount of mTOR immunoprecipitated nor the amount of mTOR detected by immunoblotting with mTAb2. However, the hormone markedly decreased the reactivity of mTOR with mTAb1, an antibody that activates the mTOR protein kinase. The effects of insulin on increasing mTOR protein kinase activity and on decreasing mTAb1 reactivity were abolished by incubating mTOR with protein phosphatase 1. Interestingly, the epitope for mTAb1 is located near the COOH terminus of mTOR in a 20-amino acid region that includes consensus sites for phosphorylation by protein kinase B (PKB). Experiments were performed in MER-Akt cells to investigate the role of PKB in controlling mTOR. These cells express a PKB-mutant estrogen receptor fusion protein that is activated when the cells are exposed to 4-hydroxytamoxifen. Activating PKB with 4-hydroxytamoxifen mimicked insulin by decreasing mTOR reactivity with mTAb1 and by increasing the PHAS-I kinase activity of mTOR. Our findings support the conclusion that insulin activates mTOR by promoting phosphorylation of the protein via a signaling pathway that contains PKB.
View details for Web of Science ID 000074436400096
View details for PubMedID 9636226
View details for PubMedCentralID PMC22753
Akt is a serine/threonine kinase that requires a functional phosphatidylinositol 3-kinase to be stimulated by insulin and other growth factors. When directed to membranes by the addition of a src myristoylation sequence, Akt becomes constitutively active. In the present study, a conditionally active version of Akt was constructed by fusing the Akt containing the myristoylation sequence to the hormone binding domain of a mutant murine estrogen receptor that selectively binds 4-hydroxytamoxifen. The chimeric protein was expressed in NIH3T3 cells and was shown to be stimulated by hormone treatment 17-fold after only a 20-min treatment. This hormone treatment also stimulated an approximate 3-fold increase in the phosphorylation of the chimeric protein and a shift in its migration on SDS gels. Activation of this conditionally active Akt resulted in the rapid stimulation of the 70-kDa S6 kinase. This conditionally active Akt was also found to rapidly stimulate in these cells the phosphorylation of properties of PHAS-I, a key protein in the regulation of protein synthesis. The conditionally active Akt, when expressed in 3T3-L1 adipocytes, was also stimulated, although its rate and extent of activation was less then in the NIH3T3 cells. Its stimulation was shown to be capable of inducing glucose uptake into adipocytes by stimulating translocation of the insulin-responsive glucose transporter GLUT4 to the plasma membrane.
View details for Web of Science ID 000073536700076
View details for PubMedID 9565622
Prior studies have established a role in insulin action for the tyrosine phosphorylation of substrates and their subsequent complexing with SH2 containing proteins. More recently, SH2 proteins have been identified which can tightly bind to the tyrosine phosphorylated insulin receptor. The major protein identified so far (called Grb-IR or Grb10) of this type appears to be present in at least 3 isoforms, varying in the presence of a pleckstrin homology domain and in the sequence of its amino terminus. The binding of this protein to the insulin receptor appears to inhibit signalling by the receptor. The present review will discuss the current knowledge of the structure and function of this protein.
View details for Web of Science ID 000073343400009
View details for PubMedID 9609116
A method to detect the biological activity of serum insulin has been developed. This method, called a bioactive insulin assay, determines the ability of serum insulin to stimulate the autophosphorylation of insulin receptors in an intact cell system. For this, intact Chinese hamster ovary cells which overexpress the human insulin receptor are treated with serum and then lysed. Autophosphorylation of the insulin receptors is then measured by a two-site immunofluorometric assay using monoclonal anti-insulin receptor antibodies and europium-labeled anti-phosphotyrosine antibodies. The detection limit of this assay is 1 microU/ml of insulin. Dilution and recovery test inter- and intraassay coefficient variations are permissible. The amount of insulin determined by this assay correlated well with the amount of insulin detected by a traditional immunological assay for insulin (r = 0.94, P < 0.001). In the case of a mutant insulin, the insulin from a Wakayama subject, the biologically active insulin was found to constitute 9% of the immunologically reactive insulin. Since this assay specifically measures the amount of biologically active insulin present in serum, it should be particularly useful in monitoring active insulin in patients with various mutant insulins.
View details for Web of Science ID 000072634000007
View details for PubMedID 9514773
In the present studies, we have compared the properties of two members of the Akt family of ser/thr kinases, Akt1 and Akt3. First, we demonstrate that both 3T3-L1 fibroblasts and adipocytes express Akt3 mRNA by RT-PCR and sequencing of the resultant PCR product. Second, we show that insulin stimulates the enzymatic activity of Akt1 and Akt3 15- and 7-fold, respectively. We then investigated the ability of protein kinase C to regulate Akt1 and 3. Neither enzyme was activated by stimulation of protein kinase C, however, the insulin-stimulated increases in activity of both isozymes were found to be comparably inhibited by prior protein kinase C activation. Since this inhibition could have resulted from an interaction of the pleckstrin homology domain of the Akt with protein kinase C, we also examined the ability of a mutant Akt1 lacking this domain to be regulated by this enzyme. The insulin-stimulated increase in enzymatic activity of this mutant Akt was regulated by PKC activation like the wild type enzyme. These results indicate that Akt1 and 3 are similarly stimulated by insulin and this stimulation is inhibited by prior activation of protein kinase C through a mechanism that is independent of the presence of the pleckstrin homology domain.
View details for Web of Science ID 000072130300031
View details for PubMedID 9480839
The recently identified 53-kDa substrate of the insulin receptor family was further characterized in several retroviral-generated stable cell lines overexpressing the wild type and various mutant forms of the protein. To facilitate the study of its subcellular localization in NIH3T3 cells overexpressing insulin receptor, a myc epitope-tag was added to the carboxy terminus of the 53-kDa protein. Like the endogenous protein in Chinese hamster ovary cells, the expressed myc-tagged 53-kDa protein was found partially in the particulate fraction and was tyrosine phosphorylated in insulin-stimulated cells. Immunofluorescence studies showed for the first time that a fraction of the 53-kDa protein was localized to the plasma membrane. Confocal microscopy of cells double-labeled with antibodies to the insulin receptor and the myc epitope showed the two proteins co-localize at the plasma membrane at the level of light microscopy. Further analyses of the protein sequence of the 53-kDa substrate revealed the presence of a putative SH3 domain and two proline-rich regions, putative binding sites for SH3 and WW domains. Disruption of these three motifs by the introduction of previously characterized point mutations did not affect the membrane localization of the 53-kDa protein, its ability to serve as substrate of the insulin receptor, or its colocalization with the insulin receptor, suggesting these domains are not important in the subcellular targeting of the protein and instead may function in the interaction with subsequent signaling proteins.
View details for Web of Science ID 000071325200001
View details for PubMedID 9443070
Increased serine phosphorylation of insulin receptor substrate-1 (IRS-1) has been observed in several systems to correlate with a decreased ability of the insulin receptor to tyrosine-phosphorylate this endogenous substrate and to inhibit its subsequent association with phosphatidylinositol 3-kinase. In the present studies we have examined the potential role of the mitogen-activated protein (MAP) kinase in the increased serine phosphorylation of IRS-1 observed in human embryonic kidney cells treated with an activator of protein kinase C, phorbol 12-myristate 13-acetate. First, recombinantly produced kinase was shown to phosphorylate intact IRS-1 in a way that decreased the ability of isolated insulin receptor to phosphorylate the tyrosines recognized by the SH2 domains of the phosphatidylinositol 3-kinase. Second, an inhibitor of MAP kinase activation, PD98059, blocked the phorbol 12-myristate 13-acetate-induced inhibition of the insulin-stimulated increase in IRS-1 associated phosphatidylinositol 3-kinase. Third, activation of MAP kinase in intact cells via a regulatable upstream kinase, a RAF:estrogen receptor construct, could also inhibit the insulin-stimulated increase in IRS-1-associated phosphatidylinositol 3-kinase. Fourth, an in gel kinase assay showed that MAP kinase was the primary renaturable kinase in cell extracts capable of phosphorylating an IRS-1 fusion protein. Finally, IRS-1 was found to associate in coprecipitation studies with endogenous MAP kinase. These studies implicate MAP kinase as one of the kinases capable of phosphorylating and regulating IRS-1 tyrosine phosphorylation.
View details for Web of Science ID A1997YL41900030
View details for PubMedID 9395471
The serine/threonine kinase Akt (protein kinase B [PKB] or related to A and C protein kinase [RAC]) has recently been implicated to play a role in the signaling pathway to glucose transport. However, little is known concerning the regulation of Akt activity in insulin-sensitive tissues such as skeletal muscle. To explore the role of hyperglycemia on Akt kinase activity in skeletal muscle, normal Wistar rats or Goto-Kakizaki (GK) diabetic rats were treated with phlorizin. Phlorizin treatment normalized fasting blood glucose and significantly improved glucose tolerance (P < 0.001) in GK rats, whereas in Wistar rats, the compound had no effect on glucose homeostasis. In soleus muscle from GK rats, maximal insulin-stimulated (120 nmol/l) Akt kinase activity was reduced by 68% (P < 0.01) and glucose transport was decreased by 39% (P < 0.05), compared with Wistar rats. Importantly, the defects at the level of Akt kinase and glucose transport were completely restored by phlorizin treatment. There was no significant difference in Akt kinase protein expression among the three groups. At a submaximal insulin concentration (2.4 nmol/l), activity of Akt kinase and glucose transport were unaltered. In conclusion, improved glucose tolerance in diabetic GK rats by phlorizin treatment fully restored insulin-stimulated activity of Akt kinase and glucose transport. Thus, hyperglycemia may directly contribute to the development of muscle insulin resistance through alterations in insulin action on Akt kinase and glucose transport.
View details for Web of Science ID A1997YH80500032
View details for PubMedID 9392506
Activation of the endogenous protein kinase Cs in human kidney fibroblast (293) cells was found in the present study to inhibit the subsequent ability of insulin to stimulate the tyrosine phosphorylation of an expressed insulin receptor substrate-1. This inhibition was also observed in an in vitro phosphorylation reaction if the insulin receptor and its substrate were both isolated from cells in which the protein kinase C had been activated. To test whether serine phosphorylation of the insulin receptor substrate-1 was contributing to this process, serine 612 of this molecule was changed to an alanine. The insulin-stimulated tyrosine phosphorylation and the associated phosphatidylinositol 3-kinase activity of the expressed mutant were found to be comparable to those of the expressed wild-type substrate. However, unlike the wild-type protein, activation of protein kinase C did not inhibit the insulin-stimulated tyrosine phosphorylation of the S612A mutant nor its subsequent association with phosphatidylinositol 3-kinase. Tryptic peptide mapping of in vivo labeled IRS-1 and the S612A mutant revealed that PMA stimulates the phosphorylation of a peptide from wild-type IRS-1 that is absent from the tryptic peptide maps of the S612A mutant. Moreover, a synthetic peptide containing this phosphoserine and its nearby tyrosine was found to be phosphorylated by the insulin receptor to a much lower extent than the same peptide without the phosphoserine. Activation of protein kinase C was found to stimulate by 10-fold the ability of a cytosolic kinase to phosphorylate this synthetic peptide as well as the intact insulin receptor substrate-1. Finally, cytosolic extracts from the livers of ob/ob mice showed an 8-fold increase in a kinase activity capable of phosphorylating this synthetic peptide, compared to extracts of livers from lean litter mates. These results indicate that activation of protein kinase C stimulates a kinase which can phosphorylate insulin receptor substrate-1 at serine 612, resulting in an inhibition of insulin signaling in the cell, posing a potential mechanism for insulin resistance in some models of obesity.
View details for Web of Science ID A1997YD31500033
View details for PubMedID 9335553
The exact mechanism by which insulin reverses impaired wound healing is unknown. Previous investigators have shown that insulin is degraded in experimental wounds, suggesting that the action of insulin may be locally modified. The following study corroborates these findings and identifies the major proteinase responsible for insulin degradation in wound fluid (WF). Adult male Fisher rats were wounded by subcutaneous implantation of polyvinyl alcohol sponges while under pentobarbital sodium anesthesia. WF and serum were collected on 1, 5, 10, and 14 days postinjury. Decreased insulin concentration in late WF correlated with an increased insulin-degrading activity. Multiple proteinases appear to participate in the overall degradation of insulin in WF. However, the primary enzyme responsible for insulin degradation in WF was characterized by immunoprecipitation and immunoblotting and identified as the neutral thiol-dependent metalloproteinase, insulin-degrading enzyme (EC 220.127.116.11). Exogenous steroid administration caused a decrease in WF insulin-degrading activity. Glucagon and adrenocorticotrophin degradation was also observed, whereas minimal degradation of insulin-like growth factors I and II and epidermal growth factor was detected in WF. The ability to extracellularly degrade insulin may represent a unique mechanism for the regulation of this hormone's role in healing wounds.
View details for Web of Science ID A1997YA43500001
View details for PubMedID 9357792
The exact mechanism by which insulin reverses impaired wound healing is unknown. Previous investigators have shown that insulin is degraded in experimental wounds, suggesting that the action of insulin may be locally modified. The following study corroborates these findings and identifies the major proteinase responsible for insulin degradation in wound fluid (WF). Adult male Fisher rats were wounded by subcutaneous implantation of polyvinyl alcohol sponges while under pentobarbital sodium anesthesia. WF and serum were collected on 1, 5, 10, and 14 days postinjury. Decreased insulin concentration in late WF correlated with an increased insulin-degrading activity. Multiple proteinases appear to participate in the overall degradation of insulin in WF. However, the primary enzyme responsible for insulin degradation in WF was characterized by immunoprecipitation and immunoblotting and identified as the neutral thiol-dependent metalloproteinase, insulin-degrading enzyme (EC 18.104.22.168 ). Exogenous steroid administration caused a decrease in WF insulin-degrading activity. Glucagon and adrenocorticotrophin degradation was also observed, whereas minimal degradation of insulin-like growth factors I and II and epidermal growth factor was detected in WF. The ability to extracellularly degrade insulin may represent a unique mechanism for the regulation of this hormone's role in healing wounds.
View details for DOI 10.1152/ajpendo.1997.273.4.E657
View details for PubMedID 29585251
The expression of the ob gene product leptin in adipose tissues has been previously described to be regulated by insulin in vivo and vitro. Akt, a ser/thr kinase with a pleckstrin homology domain, has recently been identified to function in the insulin receptor signaling cascade. The aim of this study was to investigate the role of Akt in the production of leptin by adipocytes. Therefore, we examined leptin production by 3T3-L1 adipocytes stably expressing a myristoylated version of Akt which is constitutively active. Leptin levels in the supernatants of serum starved, nonstimulated 3T3-L1 adipocytes were determined by radioimmunoassay (RIA). Expression of the constitutively active Akt was found to induce a more than 20-fold increase in leptin levels whereas a control non-myristoylated Akt had no effect. Leptin mRNA levels as determined by either RNase protection assay or reverse transcriptase (RT)-polymerase chain reaction (PCR) were not elevated by the constitutively active Akt. These results indicate that Akt can induce leptin production in 3T3-L1 adipocytes via a non-transcriptional mechanism.
View details for Web of Science ID A1997XL84200060
View details for PubMedID 9231812
View details for PubMedID 28200227
Prior studies have shown that Madin-Darby canine kidney cells (MDCK) overexpressing the human insulin receptor bind and respond normally to insulin (T.C. Yeh, R.A. Roth, Diabetes 43 (1994) 1297-1303). Moreover, the insulin receptor preferentially localizes to the basolateral membrane of these cells. In the present studies, insulin was added to either the apical or the basolateral side of these cells and the extent of degradation of the insulin was assessed. Radioactive insulin added to either side was bound to its receptor and the radioactivity which reached the other side of the cell was to a large extent degraded fragments. Insulin added to the apical side was degraded to a larger extent (83%) than when added to the basolateral side (49%) although the basolateral side has much more insulin receptors than the apical side. This degradation process was not inhibitors of either lysosomal enzymes, the proteasome complex or cathepsins. The degradation process could however, be potently inhibited by the sulfhydryl alkylating agent N-ethylmaleimide. Further, cell surface biotinylation study showed that the insulin degrading enzyme was preferentially localized on the apical membranes. These results suggest that insulin added on the apical side of MDCK cells are more closely linked to the degradation process than that added on the basolateral side.
View details for Web of Science ID A1997XQ99000001
View details for PubMedID 9279478
Eph-related receptor tyrosine kinases have been implicated in the control of axonal navigation and fasciculation. To investigate the biochemical mechanisms underlying such functions, we have expressed the EphB2 receptor (formerly Nuk/Cek5/Sek3) in neuronal NG108-15 cells, and have observed the tyrosine phosphorylation of multiple cellular proteins upon activation of EphB2 by its ligand, ephrin-B1 (formerly Elk-L/Lerk2). The activated EphB2 receptor induced the tyrosine phosphorylation of a 62-64 kDa protein (p62[dok]), which in turn formed a complex with the Ras GTPase-activating protein (RasGAP) and SH2/SH3 domain adaptor protein Nck. RasGAP also bound through its SH2 domains to tyrosine-phosphorylated EphB2 in vitro, and complexed with activated EphB2 in vivo. We have localized an in vitro RasGAP-binding site to conserved tyrosine residues Y604 and Y610 in the juxtamembrane region of EphB2, and demonstrated that substitution of these amino acids abolishes ephrin-B1-induced signalling events in EphB2-expressing NG108-15 cells. These tyrosine residues are followed by proline at the + 3 position, consistent with the binding specificity of RasGAP SH2 domains determined using a degenerate phosphopeptide library. These results identify an EphB2-activated signalling cascade involving proteins that potentially play a role in axonal guidance and control of cytoskeletal architecture.
View details for Web of Science ID A1997XJ99000014
View details for PubMedID 9233798
View details for PubMedCentralID PMC1170012
To investigate the role of insulin degrading enzyme (insulysin, EC 22.214.171.124) in insulin signaling, Chinese hamster ovary cells overexpressing the human insulin receptor were genetically engineered to also stably overexpress the rat insulin degrading enzyme. In comparison to the parental cells, these cells expressed 2.7-fold elevated levels of enzyme and insulin degradation was also increased 2-fold. These cells also exhibited a more rapid decrease in receptor tyrosine phosphorylation after removal of insulin. Moreover, low concentrations of insulin were less effective at stimulating proliferation of the cells overexpressing the enzyme. Finally, a fraction of the overexpressed enzyme as well a fraction of the endogenous enzyme could be detected on the plasma membrane surface of these cells. These results support the hypothesis that this enzyme may function in insulin signaling by degrading the insulin molecule.
View details for Web of Science ID A1997WH35500036
View details for PubMedID 9070242
Akt is a serine/threonine kinase that requires a functional phosphatidylinositol 3-kinase to be stimulated by insulin and other growth factors. When directed to membranes by the addition of a src myristoylation sequence, Akt becomes constitutively active. In the present studies, the constitutively active Akt and a nonmyristoylated control mutant were expressed in 3T3-L1 cells that can be induced to differentiate into adipocytes. The constitutively active Akt induced glucose uptake into adipocytes in the absence of insulin by stimulating translocation of the insulin-responsive glucose transporter 4 to the plasma membrane. The constitutively active Akt also increased the synthesis of the ubiquitously expressed glucose transporter 1. The increased glucose influx in the 3T3-L1 adipocytes directed lipid but not glycogen synthesis. These results indicate that Akt can regulate glucose uptake and metabolism.
View details for Web of Science ID A1996VW68600056
View details for PubMedID 8940145
Prior studies have demonstrated that a juxtamembrane tyrosine (tyrosine 972) in the insulin receptor is required for the receptor to elicit various biological responses and to stimulate the tyrosine phosphorylation of two endogenous substrates, the insulin receptor substrate-1 and the adaptor protein called Shc. In the present studies the role of this tyrosine was examined in the insulin-stimulated tyrosine phosphorylation of a group of 60-kDa endogenous proteins. These include a 60-kDa protein which, when phosphorylated, becomes associated with the GTPase activating protein of Ras, a distinct 60-kDa protein that associates with either the phosphatidylinositol 3-kinase or the tyrosine phosphatase Syp, as well as a 58/53-kDa protein that is tyrosine phosphorylated in response to insulin but has no known associated protein. In each case, a mutant insulin receptor in which tyrosine 972 has been changed to phenylalanine was found to be defective in its ability to phosphorylate these three endogenous substrates, although the mutant receptor exhibited the same level of insulin-stimulated autophosphorylation as the wild type receptor. These results further demonstrate the critical role that the juxtamembrane tyrosine 972 plays in downstream signaling by the insulin receptor.
View details for Web of Science ID A1996VU35100021
View details for PubMedID 8940353
Akt is a serine/threonine kinase that is stimulated by receptor tyrosine kinases and contains a pleckstrin homology domain. One model proposed to explain this activation suggests that receptor tyrosine kinases stimulate a phosphatidylinositol 3-kinase whose lipid products directly activate Akt kinase by interacting with its pleckstrin homology domain. In the present study, we show, in three cell types, that Akt does not require its pleckstrin homology domain to respond to either insulin or platelet-derived growth factor. Moreover, attachment of the src myristoylation signal to target Akt, without its pleckstrin homology domain, to the membrane constitutively activates Akt by causing an increase in its basal level of phosphorylation. This constitutively active form of Akt can also activate p70(S6K), indicating that the pleckstrin homology domain is not necessary for downstream interactions. Fusion of the inter src homology 2 domain from the p85 regulatory subunit of the phosphatidylinositol 3-kinase to Akt also constitutively activated Akt and induced an association with the lipid kinase. Phosphorylation of this fusion protein still critically contributes toward its increased activity. The sum of these results indicates that the primary mechanism of Akt activation is via protein phosphorylation.
View details for Web of Science ID A1996VF61200037
View details for PubMedID 8702995
A monoclonal antibody has been produced which immunoprecipitates 58- and 53-kDa proteins which are rapidly tyrosine phosphorylated in insulin-treated cells. These proteins can also be tyrosine phosphorylated in vitro by the isolated human insulin receptor. Increased tyrosine phosphorylation of these proteins is also observed in cells expressing a transforming chicken c-Src (mutant Phe-527) and in cells with the activated tyrosine kinase domains of the Drosophila insulin receptor, human insulin-like growth factor I receptor, and human insulin receptor-related receptor. P58/53 did not appear to associate with either the GTPase activating protein of Ras (called GAP) or the phosphatidylinositol 3-kinase by either co-immunoprecipitation experiments or in Far Westerns with the SH2 domains of these two proteins. Since p58/53 did not appear, by immunoblotting, to be related to any previously described tyrosine kinase substrate such as the SH2 containing proteins SHC and the tyrosine phosphatase Syp, the protein was purified in sufficient amounts to obtain peptide sequence. This sequence was utilized to isolate a cDNA clone that encodes a previously uncharacterized 53-kDa protein which, when expressed in mammalian cells, is tyrosine phosphorylated by the insulin receptor.
View details for Web of Science ID A1996TU69100010
View details for PubMedID 8621681
In the present study we describe a nonradioactive assay for measuring the intrinsic tyrosine kinase activity of the insulin receptor. This assay utilizes as an exogenous substrate a biotinylated peptide based on the sequence of the endogenous substrate insulin receptor substrate-1 (IRS-1). To separate the tyrosine phosphorylated peptide from the nonphosphorylated peptide, immobilized recombinantly produced SH2 domain of the p85 subunit of the phosphatidylinositol 3-kinase is utilized to bind the tyrosine-phosphorylated peptide. The amount of bound peptide is then detected by the use of peroxidase-conjugated streptavidin and a colorimetric assay. This assay has been used to measure the tyrosine kinase activity of receptor which was immunocaptured from lysates of various cells overexpressing the human insulin receptor as well as the endogenous insulin receptors in the parental cells. In this in vitro assay, no decrease in tyrosine kinase activity was observed in receptors from cells with activated overexpressed protein kinase C alpha or after high glucose treatment although a decrease in in situ phosphorylation of IRS-1 was observed with the activation of protein kinase C alpha. These results indicate that this assay may be a useful new method for monitoring the enzymatic activity of the insulin receptor kinase as well as other tyrosine kinases.
View details for Web of Science ID A1995TJ97900008
View details for PubMedID 8594980
To identify potential signaling molecules involved in mediating insulin-induced biological responses, a yeast two-hybrid screen was performed with the cytoplasmic domain of the human insulin receptor (IR) as bait to trap high-affinity interacting proteins encoded by human liver or HeLa cDNA libraries. A SH2-domain-containing protein was identified that binds with high affinity in vitro to the autophosphorylated IR. The mRNA for this protein was found by Northern blot analyses to be highest in skeletal muscle and was also detected in fat by PCR. To study the role of this protein in insulin signaling, a full-length cDNA encoding this protein (called Grb-IR) was isolated and stably expressed in Chinese hamster ovary cells overexpressing the human IR. Insulin treatment of these cells resulted in the in situ formation of a complex of the IR and the 60-kDa Grb-IR. Although almost 75% of the Grb-IR protein was bound to the IR, it was only weakly tyrosine-phosphorylated. The formation of this complex appeared to inhibit the insulin-induced increase in tyrosine phosphorylation of two endogenous substrates, a 60-kDa GTPase-activating-protein-associated protein and, to a lesser extent, IR substrate 1. The subsequent association of this latter protein with phosphatidylinositol 3-kinase also appeared to be inhibited. These findings raise the possibility that Grb-IR is a SH2-domain-containing protein that directly complexes with the IR and serves to inhibit signaling or redirect the IR signaling pathway.
View details for Web of Science ID A1995TB46700072
View details for PubMedID 7479769
View details for PubMedCentralID PMC40781
Stimulation of the activity of protein kinase C by pretreatment of cells with phorbol esters was tested for its ability to inhibit signaling by four members of the insulin receptor family, including the human insulin and insulin-like growth factor-I receptors, the human insulin receptor-related receptor, and the Drosophila insulin receptor. Activation of overexpressed protein kinase C alpha resulted in a subsequent inhibition of the ligand-stimulated increase in antiphosphotyrosine-precipitable phosphatidylinositol 3-kinase mediated by the kinase domains of all four receptors. This inhibition varied from 97% for the insulin receptor-related receptor to 65% for the Drosophila insulin receptor. In addition, the activation of protein kinase C alpha inhibited the in situ ligand-stimulated increase in tyrosine phosphorylation of the GTPase-activating protein-associated p60 protein as well as Shc mediated by these receptors. The mechanism for this inhibition was further studied in the case of the insulin-like growth factor-I receptor. Although the in situ phosphorylation of insulin-receptor substrate-1 and p60 by this receptor was inhibited by prior stimulation of protein kinase C alpha, the in vitro tyrosine phosphorylation of these two substrates by this receptor was not decreased by prior stimulation of the protein kinase C alpha in the cells that served as a source of the substrates. Finally, the insulin-like growth factor-I-stimulated increase in cell proliferation was found to be inhibited by prior activation of protein kinase C alpha.(ABSTRACT TRUNCATED AT 250 WORDS)
View details for Web of Science ID A1995RU75700029
View details for PubMedID 7545165
In the present study, insulin is shown to rapidly stimulate by 8- to 12-fold the enzymatic activity of RAC-PK alpha, a pleckstrin homology domain containing ser/thr kinase. In contrast, activation of protein kinase C by phorbol esters had almost no effect on the enzymatic activity of RAC-PK alpha. Insulin activation was accompanied by a shift in molecular weight of the RAC-PK alpha protein, and the activated kinase was deactivated by treatment with a phosphatase, indicating that insulin activated the enzyme by stimulating its phosphorylation. This insulin-induced shift in RAC-PK was also observed in primary rat epididymal adipocytes, as well as in a muscle cell line called C2C12 cells. The insulin-stimulated increase in RAC-PK alpha activity was inhibited by wortmannin (an inhibitor of phosphatidylinositol 3-kinase) in a dose-dependent manner with a half-maximal inhibition of 10 nM, but not by 20 ng/ml of rapamycin. Activation of RAC-PK alpha activity was also observed in a variant RAC lacking the pleckstrin homology domain. These results indicate that RAC-PK alpha activity can be regulated by the insulin receptor. RAC-PK alpha may therefore play a general role in intracellular signaling mediated by receptor tyrosine kinases.
View details for Web of Science ID A1995RU75100018
View details for PubMedID 7556070
View details for PubMedCentralID PMC394513
Chimeric receptors encoding either the whole or a portion of the cytoplasmic domain of the drosophila insulin receptor (IR) with the extracellular domain of the human IR were expressed either transiently in COS cells or stably in Chinese hamster ovary cells and compared with the wild-type human IR. All three receptors bound insulin equally and exhibited an insulin-activated tyrosine kinase activity. The ability of the drosophila cytoplasmic domain to mediate the tyrosine phosphorylation of insulin receptor substrate 1, stimulate cell proliferation, and activate MAP kinase was found to be indistinguishable from that of the human IR. The chimeric drosophila receptors did not bind more phosphatidylinositol 3-kinase than the human IR, despite containing a C-terminal extension with potential tyrosine phosphorylation sites in the motif recognized by the SH2 domain of this enzyme. Thus, the essential signal-transducing abilities of the IR appear to have been conserved from invertebrates to mammals, despite the considerable differences in the sequences of these receptors.
View details for Web of Science ID A1995QU26000007
View details for PubMedID 7711018
In the present studies, insulin was found to stimulate in a rat hepatoma cell line (called FAO cells) the tyrosine phosphorylation of the 60-kilodalton p21ras GTPase-activating protein (GAP)-associated protein called p60. Surprisingly, the tyrosine phosphorylation of this protein was also almost equally stimulated by an activator of protein kinase C (PKC), the phorbol ester phorbol 12-myristate 13-acetate (PMA). The tyrosine phosphorylation of p60 induced by either agent correlated with the formation of the GAP-p60 complex in situ and an increase in the ability of p60 to directly bind to the SH2 domain of GAP in vitro. Several lines of evidence indicated that the PMA-induced tyrosine phosphorylation of p60 occurred through a different mechanism than that induced by insulin. First, the stimulation of tyrosine phosphorylation of p60 by maximal concentrations of the two agents was almost additive. Second, down-regulation of PKC or pretreatment with a specific inhibitor of PKC abolished the ability of PMA to stimulate tyrosine phosphorylation of p60 but had no effect on the insulin stimulation. And third, long-term pretreatment with insulin abolished the insulin response but did not affect the response to PMA. The PMA effect did seem to be mediated via a tyrosine kinase, since it was blocked by quercetin, an inhibitor of tyrosine kinases. These results indicate that both PMA and insulin can equally stimulate in FAO cells the tyrosine phosphorylation of p60 and its association with GAP, although these two agents seem to act via different signaling systems.
View details for Web of Science ID A1995QD58600014
View details for PubMedID 7835279
A gene encoding a putative third member of the insulin receptor family (called the insulin receptor-related receptor or IRR) was isolated in 1989. However, the naturally occurring protein product encoded by this gene has yet to be described. In the present studies, we have generated four monoclonal antibodies to a recombinantly expressed chimera, which contains the extracellular domain of human IRR. These antibodies were found to specifically recognize the chimeric IRR (and not the insulin or insulin-like growth factor I receptors), and two of the antibodies were capable of acting as partial agonists in the cells expressing the chimeric IRR. These antibodies have therefore been utilized to study the expression and properties of the native receptor. In contrast to the two other members of this receptor family, the endogenous IRR protein had only a very limited expression, being detected only in neuroblastomas. In primary neuroblastomas, the levels of the receptor were highest in samples from stage A tumors (those which are generally more highly differentiated and have higher levels of the nerve growth factor receptor). The endogenous IRR could also be detected in a neuroblastoma cell line (called IMR-5 cells). In these cells, IRR could be shown to be partly present as a hybrid with the insulin and insulin-like growth factor-I receptors but not with the receptor for nerve growth factor. The intrinsic tyrosine kinase activity of this endogenous IRR was activated by the agonist monoclonal antibody to IRR but not by nerve growth factor, insulin-like growth factor I, or insulin. Finally, this monoclonal antibody was found to stimulate mitogen-activated protein kinase activity in these cells. In summary, these studies demonstrate for the first time that the IRR protein is normally expressed, that its levels are highest in neuronal tissues, and that it can form hybrid receptors with the two other members of this receptor family but not with the more distantly related nerve growth factor receptor.
View details for Web of Science ID A1995QD20400060
View details for PubMedID 7829525
Protein(s) which bind polyphosphatidylinositol phosphates (PI 3,4,5-P3 and PI 4,5-P2) were identified in the wheat-germ agglutinin bound fraction of cells and tissues. The binding of this protein(s) to the phospholipid could be demonstrated in two ways, either by a shift in the migration of the lipid by size exclusion column chromatography or directly by binding to the protein after capture on wheat-germ agglutinin-coupled beads. Of the rat tissues tested (muscle, spleen, brain, heart, kidney and liver), the activity was highest in liver. The protein(s) was purified more than 5000-fold by sequential chromatography on columns of wheat-germ agglutinin, phosphocellulose, Blue-Sepharose, Mono Q and Superose 6. The peak of activity appeared to have a molecular weight on this latter column of approx. 240,000. The protein(s) bound PI 3,4,5-P3, PI 3,4-P2, and PI 3-P in the ratio of 4:2:1. The binding of 3-phosphorylated PI phosphates to the protein(s) was not significantly inhibited by 36 micrograms/ml of either phosphatidylinositol or phosphatidylcholine, but was inhibited 10% and 65% by 36 micrograms/ml of PI 4-P and PI 4,5-P2, respectively. Since these results suggested that the binding protein(s) could also bind PI 4,5-P2, binding of this lipid was directly tested and found to be comparable to that of PI 3,4,5-P3. These results suggest that this protein(s) could be involved in the signaling mechanism elicited by these polyphosphoinositides.
View details for Web of Science ID A1994QA37700029
View details for PubMedID 7803513
A monoclonal antibody to a 60-kDa substrate of the insulin receptor tyrosine kinase is utilized in the present studies to examine this molecule in 3T3 cells expressing either the transforming chicken c-Src (mutant Phe-527), the wild type molecule, or the parental cells. The tyrosine phosphorylation of this 60-kDa protein was greatly increased in cells expressing transforming Src and partially increased in cells expressing wild type enzyme. This tyrosine phosphorylation correlated with an increased association with the GTPase-activating protein of p21ras (GAP). However, this 60-kDa protein did not react with antibodies to another 62-kDa tyrosine-phosphorylated protein previously isolated from Src-transformed cells (Wong, G., Muller, O., Clark, R., Conroy, L., Moran, M. F., Polakis, P., and McCormick, F. (1992) Cell 69, 551-558), although this latter antibody did react with a 62-kDa protein in anti-phosphotyrosine precipitates from cells expressing transforming c-Src but not the parental cells. These two proteins could also be distinguished by their subcellular location, the ability of the latter but not the former protein to bind RNA, and their migration in SDS gels. Moreover, the 62-kDa RNA-binding phosphoprotein could be almost completely depleted from cell lysates with poly(U)-Sepharose without affecting the amount of either the GAP-associated 60-kDa tyrosine-phosphorylated protein or the protein precipitated with the monoclonal antibody. When the two proteins were phosphorylated in vitro with purified c-Src, they were both found to bind directly to the amino-terminal SH2 domain of GAP, although the RNA-binding protein was found to have a weaker affinity. These results indicate that two distinct 60-kDa proteins are substrates for the Src tyrosine kinase, one which binds RNA and the other which constitutes the major GAP-associated 60-kDa phosphoprotein.
View details for Web of Science ID A1994PU28400047
View details for PubMedID 7525585
We have developed and characterized a line of Madin-Darby canine kidney (MDCK) cells overexpressing the human insulin receptor. The expressed receptor was found to be processed normally, and its intrinsic tyrosine kinase was determined to be functional from both in vitro and in vivo phosphorylation studies. The expressed receptor was able to mediate an insulin-stimulated increase in both anti-phosphotyrosine-precipitable and anti-insulin receptor substrate 1-precipitable phosphatidylinositol 3-kinase activity. Moreover, insulin-induced glycogen synthase activity was greater and more sensitive to insulin in the transfected cells than in the parental cells. Interestingly, insulin promoted tubule-like growth in cells overexpressing the insulin receptor but not in the parental cells. Another advantage of this cell system lies in its ability to polarize into distinct basolateral and apical membrane compartments. With the use of biotinylation and Western analysis, the expressed insulin receptor was found to be preferentially expressed in the basolateral membrane (fivefold greater) in comparison with the apical membrane. Therefore, MDCK cells overexpressing the insulin receptor represent a novel system to study not only the pathway of insulin signaling, but also this pathway in the context of cell polarity.
View details for Web of Science ID A1994PN28100005
View details for PubMedID 7926303
Maturation of the insulin proreceptor in a late Golgi compartment requires cleavage at an Arg-Lys-Arg-Arg processing site, suggesting involvement of furin, a transmembrane serine protease of the Kex2 family of processing enzymes. A genetically engineered secreted, soluble form of human furin (ss-furin), expressed by infection of insect cells with a recombinant baculovirus, was purified to near homogeneity. ss-Furin exhibited rapid and efficient cleavage of both isoforms of the human insulin proreceptor in solubilized extracts of cultured mammalian cells expressing preproreceptor cDNA. Proreceptor cleavage occurred at the physiological processing site as judged by the effects of mutations in this site on cleavage by purified ss-furin. Moreover, purified ss-furin exhibited specificity for proreceptor cleavage identical to that of the endogenous insulin proreceptor-processing enzyme. Furin thus displays the properties expected of an insulin proreceptor-processing enzyme in that it (i) cleaves the proreceptor efficiently and at the correct site; (ii) exhibits the same specificity in processing variant proreceptors as the endogenous enzyme; (iii) appears to be localized in the correct secretory compartment; and (iv) has the same broad pattern of tissue distribution as the insulin proreceptor.
View details for Web of Science ID A1994PQ49100088
View details for PubMedID 7929288
A new site of serine phosphorylation (Ser-1035/1037) has been identified in the kinase domain of the insulin receptor. Mutant receptors missing these two serines were expressed in Chinese hamster ovary cells overexpressing protein kinase C alpha. These mutant receptors lacked a phorbol ester-stimulated phosphoserine containing tryptic peptide as demonstrated by both high percentage polyacrylamide/urea gel electrophoresis and two-dimensional tlc. Moreover, a synthetic peptide with the sequence of this tryptic peptide was phosphorylated by isolated protein kinase C alpha and co-migrated with the phosphopeptide from in vivo labeled receptor. These results indicate that serine-1035 and/or 1037 in the kinase domain of the insulin receptor are phosphorylated in response to activation of protein kinase C alpha.
View details for Web of Science ID A1994PK72700029
View details for PubMedID 7926007
The insulin receptor-related receptor (IRR) has recently been identified as a member of the insulin receptor tyrosine kinase family; however, its endogenous ligand and biological function are still unknown. In contrast to the very widespread pattern of expression of the homologous insulin and IGF-I receptors, IRR demonstrates a very restricted cellular distribution. Using in situ hybridization and immunohistochemistry, we now show that the expression of this receptor is selectively concentrated in a subset of neurons where its appearance is closely associated with that of the NGF receptor TRK. IRR and TRK demonstrate synchronized patterns of coexpression in neural crest-derived sensory and sympathetic neurons and in non-neural crest basal forebrain and striatal neurons. Both appear early in the embryonic development of dorsal root and trigeminal neurons and somewhat later, near the time of birth, in sympathetic neurons. Expression of both IRR and TRK appears perinatally in basal forebrain neurons, reaching maximal levels about postnatal day 20. This association is highly selective, since TRK mRNA is not detected anywhere in the developing nervous system in the absence of coordinate IRR expression, and the same is true for IRR expression with respect to TRK. In the adult rat, the majority of TRK-positive sensory neurons still express IRR mRNA, and coexpression in sympathetic and forebrain neurons continues without evidence of diminution. These findings are consistent with a functional linkage of the IRR and TRK receptors in NGF-sensitive neurons.
View details for Web of Science ID A1994PA95300007
View details for PubMedID 8046442
Insulin, in the presence of phorbol esters, was observed to stimulate the tyrosine phosphorylation of a major 80 kDa protein by immunoblotting with anti-phosphotyrosine antibodies in Chinese hamster ovary cells overexpressing the insulin receptor and protein kinase C alpha. The protein was specifically immunoprecipitated by antibodies to protein kinase C and anti-phosphotyrosine antibodies were capable of immunoprecipitating protein kinase C enzymatic activity from these cells. When this tyrosine phosphorylated protein kinase C was treated with a tyrosine-specific phosphatase, a 35% decrease in its enzymatic activity was observed and this inhibition was blocked by inclusion of a tyrosine phosphatase inhibitor, vanadate, in the reaction mixture. These results indicate that under certain conditions insulin can stimulate the tyrosine phosphorylation of protein kinase C and this phosphorylation can affect its enzymatic activity.
View details for Web of Science ID A1994NL38800057
View details for PubMedID 7514404
A 60-kDa tyrosine-phosphorylated protein has been observed after insulin treatment of cells in immunoprecipitations of the GTPase-activating protein of Ras (called GAP) as well as the phosphatidylinositol 3-kinase. In the present studies, these two 60-kDa proteins have been shown to differ by limited proteolytic digestions as well as by immunoprecipitation with a monoclonal antibody. This monoclonal antibody was also utilized to show that the 60-kDa GAP-associated protein was rapidly phosphorylated in intact cells after insulin stimulation and to associate with GAP only after insulin treatment of the cells. In addition, the 60-kDa protein was found to be phosphorylated in vitro by the insulin receptor. Finally, the 60-kDa protein immunoprecipitated by this antibody was found not to react with a polyclonal antibody directed against a 62-kDa tyrosine-phosphorylated GAP-associated protein previously observed in src-transformed cells. These studies indicate that insulin stimulates the tyrosine phosphorylation of at least two distinct 60-kDa proteins, one that becomes associated with GAP and appears to be a direct substrate of the insulin receptor kinase and another that associates with the phosphatidylinositol 3-kinase.
View details for Web of Science ID A1994NF96600077
View details for PubMedID 7512567
A line of Chinese hamster ovary cells overexpressing protein kinase C alpha was transfected with cDNAs encoding either the wild-type human insulin receptor or one of two mutant insulin receptors with either Ser-967 and -968 or -974 and -976 in the juxtamembrane region changed to alanine. Both mutant receptors exhibited normal insulin-activated tyrosine kinase activity as assessed by either autophosphorylation or insulin-stimulated increases in anti-phosphotyrosine-precipitable phosphatidylinositol 3-kinase. The wild-type and mutant insulin receptors were also examined for serine and threonine phosphorylation in response to insulin and activation of protein kinase C. To visualize Ser/Thr-phosphorylation sites of the receptor better in response to insulin, the receptor from in vivo-labelled insulin-treated cells was first treated with a tyrosine-specific phosphatase to remove all tyrosine phosphorylation. Phosphopeptides from the three receptors were analysed by high-percentage polyacrylamide/urea gel electrophoresis and two-dimensional t.l.c. The mutant receptor lacking Ser-967 and -968 but not the mutant lacking Ser-974 and -976 was found to be missing phosphorylated peptides in response to insulin and, to a lesser extent, after activation of protein kinase C. However, the insulin-stimulated increase in anti-phosphotyrosine-precipitable phosphatidylinositol 3-kinase was inhibited to the same extent by activation of protein kinase C in cells expressing the two mutant receptors as in cells expressing the wild-type receptor. These results indicate that these four serine residues in the juxtamembrane region are not major regulatory sites of the intrinsic tyrosine kinase activity of the insulin receptor by protein kinase C, although Ser-967 and/or -968 appear to be phosphorylated in response to insulin.
View details for Web of Science ID A1994NA07300033
View details for PubMedID 8135757
View details for PubMedCentralID PMC1137964
Insulin-degrading enzyme (IDE) hydrolyzes both insulin and IGFs and has been proposed to play a role in signal termination after binding of these peptides to their receptors. In situ hybridization was used to investigate the cellular distribution of IDE mRNA and to compare it with insulin receptor (IR) and IGF-I receptor (IGFR) gene expression in serial thin sections from a variety of tissues in embryonic and adult rats. IDE mRNA is highly abundant in kidney and liver, tissues known to play a role in insulin degradation. IDE and IR mRNAs are highly coexpressed in brown fat and liver. The highest level IDE gene expression, on a per cell basis, is found in germinal epithelium. IDE and IGFR mRNAs are colocalized in oocytes, while IDE is colocalized with the IGF-II receptor in spermatocytes, suggesting that IDE may be involved with degradation of IGF-II in the testis. In summary, IDE expression demonstrates significant anatomical correlation with insulin/IGF receptors. These data are compatible with a role for IDE in degrading insulin and IGFs after they bind to and are internalized with their respective receptors and may also suggest a novel role for IDE in germ cells.
View details for Web of Science ID A1994NB19500010
View details for PubMedID 8132782
View details for PubMedCentralID PMC294007
View details for PubMedID 8205066
Chinese hamster ovary (CHO) cells were transfected with a cDNA encoding protein kinase C alpha (PKC) and a cell line (CHO-PKC alpha) expressing approximately 7-fold greater amounts of PKC as the parental cells were isolated. Activation of PKC by 12-O-tetradecanoylphorbol-13-acetate in the CHO-PKC alpha cells inhibited by approximately 75% the: 1) insulin-stimulated increase in antiphosphotyrosine precipitable phosphatidylinositol 3-kinase activity in these cells; 2) insulin-stimulated increase in PI 3-kinase activity associated with insulin receptor substrate-1; and 3) tyrosine phosphorylation of the endogenous substrate, insulin receptor substrate-1. In contrast, 12-O-tetradecanoylphorbol-13-acetate treatment did not inhibit any of these responses in the parental CHO cells. These results indicate that excessive PKC activity can interfere in a very early step in insulin receptor signaling and are consistent with the hypothesis that excessive PKC activity may contribute to some states of insulin resistance.
View details for Web of Science ID A1994MT06400006
View details for PubMedID 7512195
The insulin receptor tyrosine kinase is required for insulin to elicit subsequent biological signalling. Recent studies have identified several endogenous substrates of the insulin receptor kinase, including one called insulin receptor substrate 1 (IRS-1). Tyrosine phosphorylation of this substrate results in its being bound by various proteins containing src homology 2 (SH2) sites, including a phosphatidylinositol 3-kinase and a ras activator complex containing GRB2 and son of sevenless (SOS) 1. Decreases in the insulin receptor tyrosine kinase activity have been observed in various insulin-resistant states, such as non-insulin-dependent diabetes mellitus. A model of insulin resistance has recently been described in which the insulin receptor is expressed in Chinese hamster ovary cells along with the phospholipid- and calcium-activated serine/threonine kinase called protein kinase C. In this model system, activation of protein kinase C is shown to interfere with insulin receptor signalling by inhibiting tyrosine phosphorylation of IRS-1 and its subsequent binding by phosphatidylinositol 3-kinase. Such a model system may be further utilized to determine the detailed biochemical basis for insulin resistance.
View details for Web of Science ID A1994NV61500014
View details for PubMedID 8088704
In rat adipocytes, insulin dose-response curves were determined for the following effects in the same cells under the same conditions: glucose uptake, binding to insulin receptors (IR), IR autophosphorylation in vivo and in vitro, IR tyrosine kinase activity and insulin-stimulated phosphatidylinositol (PI) kinase. All the EC50 values were essentially the same (mean +/- S.E.M. was 7 +/- 1 nM), except for glucose uptake, which was 170 pM. Using an improved method, we were able to measure PI kinase activity at picomolar concentrations of insulin (> 30 pM) corresponding to the EC50 for glucose uptake. These experiments showed that insulin-stimulated increase in glucose uptake was associated with an increase in antiphosphotyrosine antibody precipitable PI kinase activity, consistent with the view that IR tyrosine kinase activity may be involved in insulin-mediated signaling of glucose uptake. Small peptides (17-25 residues long) derived from major histocompatibility complex class I have previously been shown to inhibit IR internalization without any effect on the affinity of insulin to the receptor. It is now shown that the peptide-mediated inhibition of internalization, which doubles the number of insulin-occupied receptors at an insulin concentration of 70 pM, also results in a corresponding enhancement of PI kinase activity and glucose uptake. Thus, the receptors arrested on the cell surface by the peptide are biologically active.
View details for Web of Science ID A1993MH94100058
View details for PubMedID 8246175
The insulin receptor-related receptor (IRR) demonstrates striking structural homology to the insulin receptor (IR) and insulin-like growth factor-I receptor (IGFR), suggesting that IRR is a member of the IR family. However, the endogenous ligand and biological role for this "orphan" receptor are unknown. To identify potential sites of action for the IRR, in situ hybridization was employed to reveal cellular patterns of IRR gene expression in the developing and adult rat and in the adult human kidney. From embryonic days 15-20, IRR mRNA is most abundant in sensory neurons of the trigeminal and dorsal root ganglia and, to a lesser extent, neurons of the autonomic system. IRR gene expression diminishes in the majority of sensory neurons postnatally, but remains abundant in a subpopulation of adult rat trigeminal and dorsal root ganglion neurons. IRR mRNA is localized in peripheral autonomic ganglia localized in the adrenal medulla and renal hilum in the adult. From birth to maturity, IRR mRNA is abundant in renal epithelial cells focally localized in the distal tubule in both rat and human kidney. The specificity of this pattern of IRR gene expression was demonstrated by hybridization of serial tissue sections with two different nonoverlapping cRNA probes. Nonspecific signal, as measured by IRR sense probe hybridization, was negligible. This highly restricted pattern of IRR gene expression is in marked contrast to the very widespread pattern of gene expression demonstrated by the IR and IGFR. This study showed that IRR, IR, and IGFR mRNAs were colocalized in some sensory neurons, suggesting the possibility for hybrid receptor formation in these cells. In summary, IRR gene expression is focally localized in sensory and autonomic neurons and renal distal tubule cells. These observations suggest that the IRR, in contrast to the related IR and IGFR, serves a narrow cell-specific role.
View details for Web of Science ID A1993LK83600002
View details for PubMedID 8319578
A novel active site has been identified in a family of zinc-dependent metalloendopeptidases that includes bacterial proteinase III, the human and Drosophila insulin-degrading enzymes, and the processing-enhancing protein subunit of the mitochondrial processing proteinase. None of these enzymes contains the conserved active site described in most other metalloendopeptidases, HEXXH; instead, all four contain an inversion of this motif, HXXEH. Prior mutagenesis studies of proteinase III indicate that the two histidines are essential for co-ordinating the zinc atom, while all three residues are required for enzyme activity. To identify the third zinc-binding residue in this protein family, three glutamates downstream from the active site were mutated to glutamine in proteinase III. The mutant proteins were expressed and their ability to degrade insulin was compared with the wild-type enzyme. The glutamate-204 mutant was as active as the wild-type protein, the glutamate-162 mutant retained 20% of the activity of the wild-type enzyme and the glutamate-169 mutant was completely devoid of insulin-degrading activity. The purified wild-type and glutamate-204 mutant enzymes were found to contain nearly stoichiometric levels of zinc by atomic absorption spectrophotometry, whereas the glutamate-162 mutant had a slight reduction in the level of zinc, and the glutamate-169 mutant retained less than 0.3 mol of zinc/mol of enzyme. These findings are consistent with glutamate-169 being the third zinc-binding residue in proteinase III.
View details for Web of Science ID A1993LD11300021
View details for PubMedID 8099278
View details for PubMedCentralID PMC1134279
Insulin stimulated tyrosine phosphorylation of SHC, a SH2 containing protein, was demonstrated in Chinese hamster ovary cells overexpressing the insulin receptor by immunoblotting with antiphosphotyrosine antibodies and in vivo labeling. Insulin induced tyrosine phosphorylation of SHC occurred very rapidly (within 1 min) with a dose curve which paralleled the autophosphorylation of the insulin receptor. Phosphorylation of SHC appeared to occur to a high stoichiometry since insulin induced the majority of SHC to shift to a higher molecular weight. The tyrosine phosphorylated SHC was not bound by the GTPase activating protein of Ras although a distinct 62 kDa tyrosine phosphorylated protein was found to be associated in the same experiments. It also was not bound to the insulin receptor, phosphatidylinositol 3-kinase or insulin receptor substrate-1.
View details for Web of Science ID A1993LC53100046
View details for PubMedID 7685165
Chinese hamster ovary cells overexpressing the human insulin receptor were transfected with cDNAs encoding protein kinase C isoenzymes alpha, beta I, gamma, and epsilon as well as an inactive alpha. Overexpression of these protein kinase Cs did not affect expression of the insulin receptor or insulin-stimulated tyrosine phosphorylation of the receptor. However, in response to phorbol esters, cells overexpressing isoenzymes alpha, beta I, and gamma, but not epsilon or inactive alpha, exhibited 3-4-fold higher levels of insulin receptor phosphorylation. This increased phosphorylation occurred exclusively on serines and threonine. Tryptic peptide maps indicated that this phosphorylation was primarily on serines 1305/1306 and threonine 1348 as well as several other unidentified sites. This phorbol ester-stimulated phosphorylation did not inhibit activation of the insulin receptor kinase when the receptor was activated in situ but assayed in vitro. However, in cells overexpressing protein kinase C alpha, it did inhibit an in vivo monitor of the activation of the insulin receptor kinase, the insulin-stimulated increase in anti-phosphotyrosine-precipitable phosphatidylinositol 3-kinase activity. These results indicate that increased protein kinase C alpha activity can inhibit insulin-stimulated responses and support the hypothesis that excessive protein kinase C is involved in the insulin resistance observed in non-insulin-dependent diabetics.
View details for Web of Science ID A1993KT36800041
View details for PubMedID 8454604
The activation of insulin-stimulated protein-serine/threonine kinases has been investigated in CHO cell lines transfected with cDNAs encoding either wild-type or mutant human insulin receptors. (1) Insulin treatment of CHO cells over-expressing wild-type insulin receptors resulted in the rapid and substantial (5-10-fold) activation of cytosolic protein kinases which phosphorylated myelin basic protein, Kemptide and two peptide substrates based on sites phosphorylated on ribosomal protein S6 in vivo. (2) Further fractionation of cytosolic extracts by MonoQ chromatography revealed two peaks of insulin-stimulated myelin basic protein kinase activity which were highly related to the previously described mitogen-activated protein (MAP) kinases ERK1 and ERK2. In addition, at least two major peaks of S6 kinase activity were resolved, which exhibited properties similar to the 70 kDa and 90 kDa S6 kinases described by others; the predominant effect of insulin was on the activity of the 90 kDa enzyme and was in excess of 10-fold. (3) MonoQ fractionation of extracts from parental CHO cells, or cells expressing kinase-deficient receptors, showed all insulin-stimulated peaks of activity to be almost completely absent. (4) Further studies demonstrated that substitution of tyrosine residues 1162 and 1163 (or 1162 alone) with phenylalanine led to a substantial reduction in the ability of insulin to stimulate these protein kinase activities when assayed in cytosolic extracts. In contrast, deletion of 69 amino acids from the C-terminus of the insulin receptor beta-subunit caused a leftward shift in the insulin dose-response curve of the MAP kinase activity, but apparently not in that of the 90 kDa S6 kinase activity.
View details for Web of Science ID A1992JR75500029
View details for PubMedID 1329727
View details for PubMedCentralID PMC1133144
In 1989, Shier and Watt identified a gene which was predicted to encode a new member of the insulin receptor (IR) family, and they called it the insulin receptor-related receptor (IRR) (Shier, P., and Watt, V. M. (1989) J. Biol. Chem. 264, 14605-14608). However, the tissues expressing this receptor, its ligand binding specificity and its signaling capability have remained unknown. In the present studies we report Northern blot analyses and polymerase chain reaction data, which indicate that the IRR mRNA is expressed in a variety of tissues, including the human kidney, heart, skeletal muscle, liver, and pancreas. In order to examine the ligand(s) recognized by IRR, we constructed a chimeric receptor with the extracellular domain of the IR replaced with that of IRR. This chimera was found not to bind radioactively labeled insulin, insulin-like growth factor I (IGF-I), or IGF-II. These ligands and relaxin, the only other known member of the mammalian insulin family, also failed to stimulate the tyrosine kinase activity of this chimeric receptor. A second chimeric receptor with the extracellular domain of IR and the kinase domain of IRR was also constructed and utilized to study the signaling capabilities of the kinase domain of IRR. This chimera exhibited high affinity insulin binding and insulin-stimulated tyrosine kinase activity. The kinase domains of the IR and IRR were found capable of phosphorylating the same spectrum of exogenous and endogenous substrates. However, Chinese hamster ovary (CHO) cells stably overexpressing the kinase domain of IRR exhibited elevated basal thymidine incorporation and 2-deoxyglucose uptake compared with CHO cells and CHO cells overexpressing wild-type IR. We conclude that: 1) IRR is expressed in the human kidney, heart, skeletal muscle, liver, and pancreas, 2) IRR does not appear to be the receptor of any known member of the insulin family, and 3) the tyrosine kinase of IRR appears to be similar to that of IR in both the spectrum of substrates phosphorylated and the biological responses stimulated.
View details for Web of Science ID A1992JN50200015
View details for PubMedID 1326521
We obtained 20 mouse monoclonal antibodies specific for human type I insulin-like growth factor (IGF) receptors, using transfected cells expressing high levels of receptors (IGF-1R/3T3 cells) as immunogen. The antibodies immunoprecipitated receptor.125I-IGF-I complexes and biosynthetically labeled receptors from IGF-1R/3T3 cells but did not react with human insulin receptors or rat type I IGF receptors. Several antibodies stimulated DNA synthesis in IGF-1R/3T3 cells, but the maximum stimulation was only 25% of that produced by IGF-I. The antibodies fell into seven groups recognizing distinct epitopes and with different effects on receptor function. All the antibodies reacted with the extracellular portion of the receptor, and epitopes were localized to specific domains by investigating their reaction with a series of chimeric IGF/insulin receptor constructs. Binding of IGF-I was inhibited up to 90% by antibody 24-60 reacting in the region 184-283, and by antibody 24-57 reacting in the region 440-586. IGF-I binding was stimulated up to 2.5-fold by antibodies 4-52 and 16-13 reacting in the region 62-184, and by antibody 26-3 reacting downstream of 283. The latter two groups of antibodies also dramatically stimulated insulin binding to intact IGF-1R/3T3 cells (by up to 50-fold), and potentiated insulin stimulation of DNA synthesis. Scatchard analysis indicated that in the presence of these antibodies, the affinity of the type I IGF receptor for insulin was comparable with that of the insulin receptor. These data indicate that regions both within and outside the cysteine-rich domain of the receptor alpha-subunit are important in determining the affinity and specificity of ligand binding. These antibodies promise to be valuable tools in resolving issues of IGF-I receptor heterogeneity and in studying the structure and function of classical type I receptors and insulin/IGF receptor hybrids.
View details for Web of Science ID A1992HZ48300084
View details for PubMedID 1377676
An unusual active site has been identified in a family of zinc metalloendopeptidases that includes bacterial protease III and the human and Drosophila insulin-degrading enzymes. All of these enzymes have been characterized as metalloendopeptidases and purified protease III has been shown to contain stoichiometric levels of zinc. However, all three proteases lack the consensus sequence (HEXXH) described in the active site of other zinc metalloendopeptidases. Instead, these proteases contain an inversion of this motif, HXXEH. To determine whether this region could represent the active site in these proteins, the two histidines in protease III were individually mutated to arginine and the glutamate was mutated to glutamine. All three mutants were devoid of proteolytic activity toward an exogenous substrate, insulin, as compared to the wild-type protease. Three lines of evidence indicate that this loss of activity in the mutants is not due to distortion of the three-dimensional structure of the protein: (i) the mutants are secreted into the periplasmic space and chromatograph normally; (ii) all three mutants are expressed at levels nearly identical to wild-type protein and do not appear to have an increased susceptibility to proteolysis in the bacteria; and (iii) the mutants compete equally with wild-type protein in a radioimmunoassay. The purified wild-type and glutamate mutants were found to contain stoichiometric amounts of zinc by atomic absorption spectrophotometry, whereas both histidine mutants had negligible zinc signals. These findings are consistent with this region being the active site in this protein, with the histidine residues coordinating the essential zinc atom and the glutamate involved in catalysis.
View details for Web of Science ID A1992HR85300034
View details for PubMedID 1570301
The enzymatic and biochemical properties of human insulin-degrading enzyme and Escherichia coli protease III have been compared. Both enzymes were found to degrade insulin in such a way that its receptor binding activity was rapidly lost but its precipitability in trichloracetic acid was only slightly decreased. Both enzymes were also found to be inhibited by chelating agents. The bacterial enzyme, which could be purified in large amounts, was found to contain 0.6 mol of zinc per mol of enzyme but no detectable manganese. The mammalian enzyme but not the bacterial one was inhibited by a sulfhydryl alkylating agent. The two enzymes also differed in substrate specificity. The mammalian enzyme degraded insulin much better than insulin-like growth factor II, whereas the bacterial enzyme degraded them equally. The mammalian enzyme could be labeled by cross-linking to insulin = bombyxin II much greater than insulin-like growth factor I and II much greater than relaxin, while the bacterial enzyme was labeled by insulin-like growth factor II greater than insulin = insulin-like growth factor I much greater than relaxin much greater than bombyxin. Finally, sucrose gradient centrifugation and cross-linking studies both in vitro and in vivo indicated that active human enzyme partially existed as a homo- or heterodimer, whereas the bacterial enzyme was active as a monomer.
View details for Web of Science ID A1992HB53200048
View details for PubMedID 1733942
In the last few years several potential substrates of the insulin receptor tyrosine kinase have been identified, purified, and their cDNAs isolated. These putative substrates include: 1) pp15, a fatty acid-binding protein; 2) pp120, a plasma membrane ecto-ATPase; 3) pp42, a MAP serine/threonine kinase; 4) pp85, a subunit of the Type 1 phosphatidylinositol kinase; and 5) pp185, a phosphatidylinositol kinase binding protein. Although the tyrosine phosphorylation of several of these substrates correlates with the signalling capabilities of various mutant receptors, the role of these substrates in mediating any one of insulin's many biological responses is still unknown. In addition, recent data indicate that the tyrosine phosphorylation of pp42 may in fact be due to autophosphorylation, thereby removing it from the list of putative substrates of the insulin receptor kinase. Finally, the present review discusses the question of whether signalling occurs as a result of the tyrosine phosphorylation of substrates or via the formation of signalling complexes.
View details for Web of Science ID A1992HA50400003
View details for PubMedID 1316356
View details for Web of Science ID A1992GY44002352
View details for PubMedID 15336083
Insulin promotes insulin receptor beta-subunit phosphorylation on tyrosine, serine, and threonine residues in a variety of cells, including simian COS cells which transiently express human insulin receptors following transfection with a cDNA encoding the wild-type receptor protein. To examine the potential roles of serines 1305 and 1306 and threonine 1348 as sites of insulin-stimulated phosphorylation in these cells, these residues (i.e. either serines 1305 and 1306, or threonine 1348) were replaced with neutral (alanine) or negatively charged (aspartate) amino acids. Following transient expression of each of these mutant receptors in COS cells, two-dimensional phosphopeptide mapping reveals that threonine 1348 is the major, if not the only, insulin-stimulated threonine phosphorylation site. In contrast, while serines 1305 and/or 1306 are phosphorylated in an insulin-dependent manner, these sites comprise only a minor proportion of insulin receptor serine phosphorylation in these cells. Substitution of either serines 1305 and 1306 or threonine 1348 with neutral or negatively charged amino acids has no effect on insulin-stimulated tyrosine autophosphorylation of these mutant receptors in intact cells. Furthermore, insulin-stimulated exogenous protein-tyrosine kinase activity of the mutant receptors is unaffected, as assessed following either phosphorylation of receptors in intact cells or following immunopurification of receptors and their autophosphorylation in vitro.
View details for Web of Science ID A1991GP80400073
View details for PubMedID 1939203
Chimeric receptors containing different portions of the homologous human insulin receptor, insulin-like growth factor I receptor, and insulin receptor-related receptor were utilized to identify the epitopes recognized by various anti-insulin receptor antibodies. The antibodies studied included 12 monoclonal antibodies to the extracellular domain of the human insulin receptor as well as 15 patients' sera with autoimmune anti-insulin receptor antibodies. All of the patients' sera and all 8 monoclonal antibodies that inhibit insulin binding were found to recognize an epitope contained within residues 450-601 of the alpha subunit of the receptor. In contrast, 2 monoclonal antibodies that do not inhibit insulin binding were found to recognize the cysteine-rich region of the alpha subunit. Chimeric insulin receptors that had residues 450-601 replaced with the homologous residues of the insulin-like growth factor I receptor exhibited a decreased ability to bind insulin. In contrast, insulin-like growth factor I receptors that have had the comparable region replaced with that of the insulin receptor showed no decrease in their ability to bind ligand. These results indicate that residues 450-601 of the insulin receptor are important for insulin binding and include the major site for recognition by inhibitory monoclonal antibodies and patients' autoimmune anti-receptor antibodies.
View details for Web of Science ID A1991GM78100101
View details for PubMedID 1719540
Chinese hamster ovary cells and NIH 3T3 cells overexpressing mutant human insulin receptors were examined for the presence of hybrid receptors composed of human and rodent insulin receptors. In the present studies, most of the endogenous rodent receptors were found to be immunoprecipitated from the transfected cells but not the parental cells with a monoclonal antibody specific for human receptor. These data indicate that in these transfected cells, most of the endogenous rodent receptors are in a hybrid complex with the overexpressed human receptor. These results together with the in vitro studies of Treadway et al. (Treadway, J.L., Morrison, B.D., Soos, M.A., Siddle, K., Olefsky, J., Ullrich, A., McClain, D.A., and Pessin, J.E. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 214-218) showing that hybrid receptors exhibit transdominant inhibition explain the prior finding indicating that overexpression of defective insulin receptors interferes with the normal signaling of endogenous receptors.
View details for Web of Science ID A1991GB97700009
View details for PubMedID 1651915
We constructed and expressed chimeric receptor cDNAs with insulin receptor exon 3 (residues 191-297 of the cysteine-rich region) replaced with either the comparable region of the insulin-like growth factor I receptor (IGF-IR) or the insulin receptor related receptor (IRR). Both chimeric receptors still could bind insulin with as high affinity as the wild-type receptor. In addition, chimeric receptors containing exon 3 of the IGF-IR could also bind with high affinity both IGF-I and IGF-II. In contrast, chimeric receptors containing exon 3 of IRR did not bind either IGF-I, IGF-II, or relaxin. These results indicate that (1) the high affinity of binding of insulin to its receptor can occur in the absence of insulin receptor specific residues encoded by exon 3, the cysteine-rich region; (2) the cysteine-rich region of the IGF-I receptor can confer high-affinity binding to both IGF-I and IGF-II; and 3) the IRR is unlikely to be a receptor for either IGF-I, IGF-II, or relaxin.
View details for Web of Science ID A1991FN55200001
View details for PubMedID 1645190
Serum-free medium conditioned by BSC-40 cells was analyzed for the presence of transforming growth factor-beta 2 (TGF beta 2)-related proteins. Western blot analysis was performed using site-specific antipeptide antibodies directed against the pro- and mature regions of the TGF beta 2 precursor. When conditioned medium was analyzed by polyacrylamide gel electrophoresis under reducing conditions, proteins with mol wt of 53 kDa (containing both mature and proregion sequences), 34-38 kDa (containing proregion sequences only), and 12 kDa (containing mature sequences) were detected. Under nonreducing conditions, complexes of 60- to 80-kDa, 160- to 200-kDa, as well as 24-kDa mature dimers were seen. Cleavage of mature TGF beta 2 from its precursor was inhibited by monensin and chloroquin, but not by ammonium chloride or methylamine. Two peaks of bioactivity were detected after fractionation on a TSK column corresponding to mol wt of 130 and 400 kDa. These peaks contained TGF beta 2 and pro-TGF beta 2 proteins. Partial purification of the 130-kDa complex followed by N-glyconase digestion indicated that the pro-TGF beta 2 proteins were glycosylated. These data demonstrate that BSC-40 cells secrete mature TGF beta 2 complexed with proregion-containing proteins and suggest that this association may contribute to the latency phenomena observed with respect to this growth regulator.
View details for Web of Science ID A1991FJ39600014
View details for PubMedID 1902166
In the present studies mutant insulin receptors with regulatory tyrosine residues 1162 and 1163 changed to phenylalanines were tested for tyrosine kinase activity. In agreement with prior studies, this mutant receptor was found to exhibit almost no insulin-stimulated exogenous kinase activity when assayed in vitro. In contrast, this mutant receptor was found in situ to have a significant, albeit reduced, ability to mediate the tyrosine phosphorylation of various endogenous proteins, as assessed by Western blotting with antiphosphotyrosine antibodies. In addition, extracts of insulin-treated cells overexpressing this mutant receptor exhibited increased amounts of tyrosine phosphorylated phosphatidylinositol 3-kinase compared to control cells. Finally, this mutant receptor, like the wild-type receptor, was found to mediate an increase in the activity of a membrane-associated phosphatidylinositol 4,5-biphosphate kinase. These results indicate that 1) in vitro assessments of the tyrosine kinase activity of mutant insulin receptors may not accurately reflect their in vivo activities; and 2) the ability of the mutant receptor lacking tyrosine autophosphorylation sites 1162 and 1163 to mediate insulin-stimulated tyrosine phosphorylation of various endogenous substrates may account for the reported ability of this receptor to mediate various biological responses.
View details for Web of Science ID A1991FA90700005
View details for PubMedID 1710030
A previous study showed that the human insulin receptor (IR) could be activated by insertion of a 3' portion of the cDNA encoding the beta subunit into a retrovirus genome to form a Gag-IR fusion protein. While capable of transforming cells in culture, this IR cDNA-containing virus, called UIR, was not able to induce tumors in animals. Subsequently, we isolated a spontaneous sarcomagenic variant called UIR19t from the parental UIR. UIR19t was molecularly cloned, sequenced, and found to harbor two mutations. A 44-amino acid deletion immediately upstream from the transmembrane domain of the Gag-IR fusion protein removes all the extracellular sequence of the IR remaining in the original UIR construct. In addition, a single nucleotide deletion at the 3' end results in truncation and replacement of the carboxyl-terminal 12 amino acids by 4 new amino acids. The specific kinase activity of UIR19t is 4- to 5-fold higher than that of the parental UIR. However, no new cellular substrates were detected in UIR19t-transformed cells as compared to UIR cells. Viruses containing either the 5' or the 3' deletion mutation were constructed and assessed for their biological function. Our data indicate that the 5' deletion alone is sufficient to confer tumorigenic ability. We conclude that sequence immediately upstream from the transmembrane domain imposes a negative effect on the transforming and tumorigenic potential of the Gag-IR fusion protein.
View details for Web of Science ID A1991EW40000041
View details for PubMedID 1846965
Autophosphorylation of the insulin receptor has been previously documented to activate the phosphotransferase activity of the receptor from 20- to 200-fold. Biochemical studies have correlated activation of the receptor kinase with the autophosphorylation of tyrosines residues 1158, 1162, and 1163. To further assess the role of these 3 tyrosines in the activation process, we have studied the effect of their substitution with either the neutral amino acids phenylalanine or alanine or with the negatively charged amino acids aspartate and glutamate. In several other proteins, it has been shown that substitution of phosphorylated residues with negatively charged amino acids can mimic the phosphorylation state of the protein. In agreement with previous studies, tyrosines at positions 1162 and 1163 were found to be crucial in the kinase activation process. In contrast, mutant receptors with tyrosine 1158 changed to either phenylalanine or aspartate were still activated to the same extent as the wild-type receptor. An increased basal exogenous kinase activity was observed upon replacement of tyrosines 1162 and 1163 with, in increasing order of potency, aspartate = glutamate less than alanine = phenylalanine. These results indicate that phosphorylation of tyrosines 1162/1163 but not 1158 play a critical role in the activation of the receptor kinase and that the mechanism of activation of the receptor kinase by autophosphorylation is more complex than just an introduction of a cluster of negative charges in this region of the receptor. In addition, the finding of an increased basal kinase activity in receptors lacking tyrosines 1162 and 1163 could explain the reported ability of this receptor to mediate certain biological responses.
View details for Web of Science ID A1991ET17700051
View details for PubMedID 1846000
Latent recombinant transforming growth factor-beta 2 (LrTGF-beta 2) complex has been purified from serum-free media conditioned by Chinese hamster ovary cells transfected with a plasmid encoding the TGF-beta 2 (414) precursor. Under neutral conditions, LrTGF-beta 2 had an apparent molecular weight of 130 kDa. The complex contained both mature and pro-region sequences. Acidification of LrTGF-beta 2 resulted in the release of mature 24 kDa TGF-beta 2 from the high molecular weight pro-region-containing complex, suggesting that TGF-beta 2 was non-covalently associated with this complex. These results were confirmed by crosslinking experiments performed on partially purified LrTGF-beta 2. Protein sequence analysis of the purified TGF-beta 2 pro-region indicated that signal peptide cleavage occurred between ser(20) and leu(21). The pro-region, which previously was found to contain mannose-6-phosphate, bound to the mannose-6-phosphate receptor. Proteolytic cleavage of mature TGF-beta 2 from pro-TGF-beta 2 was inhibited by monensin and chloroquine suggesting that binding to this receptor and subsequent transport to acidic vesicles may be involved in the processing of rTGF-beta 2 precursor.
View details for Web of Science ID A1991ET74000016
View details for PubMedID 1848562
We have previously shown that the protein encoded by a human insulin-like growth factor I (IGF-I) receptor cDNA binds both IGF-I and II with high affinity. In the present studies, we show that a monoclonal antibody to the IGF-I receptor, alpha IR-3, inhibits the binding of IGF-I but not IGF-II to the expressed receptor in intact cells and after solubilization. Surprisingly, this monoclonal antibody inhibits the ability of both IGF-I and II to stimulate thymidine synthesis in cells with the expressed receptor. Moreover, this antibody inhibits the ability of both IGF-I and II to stimulate the kinase activity of the IGF-I receptor in intact cells. These results indicate that alpha IR-3 binds to the IGF-I receptor in such a way that it does not inhibit the binding of IGF-II but does inhibit the subsequent ability of the receptor to be activated to transmit a signal.
View details for Web of Science ID A1990EA55400050
View details for PubMedID 2171510
Insulin-degrading enzyme (IDE) hydrolyzes insulin at a limited number of sites. Although the positions of these cleavages are known, the residues of insulin important in its binding to IDE have not been defined. To this end, we have studied the binding of a variety of insulin analogues to the protease in a solid-phase binding assay using immunoimmobilized IDE. Since IDE binds insulin with 600-fold greater affinity than it does insulin-like growth factor I (25 nM and approximately 16,000 nM, respectively), the first set of analogues studied were hybrid molecules of insulin and IGF I. IGF I mutants [insB1-17,17-70]IGF I, [Tyr55,Gln56]IGF I, and [Phe23,Phe24,Tyr25]IGF I have been synthesized and share the property of having insulin-like amino acids at positions corresponding to primary sites of cleavage of insulin by IDE. Whereas the first two exhibit affinities for IDE similar to that of wild type IGF I, the [Phe23,Phe24,Tyr25]IGF I analogue has a 32-fold greater affinity for the immobilized enzyme. Replacement of Phe-23 by Ser eliminates this increase. Removal of the eight amino acid D-chain region of IGF I (which has been predicted to interfere with binding to the 23-25 region) results in a 25-fold increase in affinity for IDE, confirming the importance of residues 23-25 in the high-affinity recognition of IDE. A similar role for the corresponding (B24-26) residues of insulin is supported by the use of site-directed mutant and semisynthetic insulin analogues. Insulin mutants [B25-Asp]insulin and [B25-His]insulin display 16- and 20-fold decreases in IDE affinity versus wild-type insulin.(ABSTRACT TRUNCATED AT 250 WORDS)
View details for Web of Science ID A1990DV41700022
View details for PubMedID 2271531
We have recently described the isolation of a cDNA encoding an enzyme thought to be involved in the degradation of insulin by insulin-responsive tissues. This enzyme, referred to as insulin-degrading enzyme (IDE), is a cytosolic proteinase of 110,000 mol wt which shares structural and functional homology with bacterial protease III. The enzyme may function in the termination of the insulin response. We report here the mapping of the human and mouse IDE genes to human chromosome 10 and mouse chromosome 19, respectively, and evidence for the existence of a single complex IDE gene. We also describe the stable transfection of Chinese hamster ovary cells with a plasmid containing the IDE cDNA under the transcriptional control of the SR alpha promoter. The recombinant protein synthesized by these cells is indistinguishable from the isolated human enzyme in both its size and immunoreactivity and degrades insulin with a specific activity similar to that of the purified proteinase. Overexpression of IDE using this system should allow for a functional test of the role of IDE in insulin action. In addition, expression of various site-directed mutants of IDE will aid in identifying the residues of IDE and protease III that are essential to the activity of this unique family of proteinases.
View details for Web of Science ID A1990DU70300005
View details for PubMedID 2293021
Insulin was found to stimulate the serine/threonine kinase activity of the proto-oncogene product Raf-1. This stimulation was observed in HeLa, NIH 3T3, and Chinese hamster ovary cells, all overexpressing the human insulin receptor. In the HeLa cells, 100 pM insulin gave a significant increase in Raf-1 kinase activity, and 100 nM insulin caused a maximal 2-5-fold increase in activity. The increase in activity was detected after 2 min of insulin treatment and peaked after 5 min. In addition to stimulating Raf-1 kinase activity, insulin caused a shift in the electrophoretic mobility of the Raf-1 protein and an increase in the amount of serine phosphorylation of Raf-1. Moreover, a serine/threonine-specific phosphatase, phosphatase 1, but not two tyrosine-specific phosphatases, was found to deactivate the insulin-activated Raf-1 kinase activity. These findings indicate that insulin activates the serine/threonine kinase activity of the Raf-1 proto-oncogene by increasing its content of phosphoserine.
View details for Web of Science ID A1990DP45800002
View details for PubMedID 2197270
In the present studies, nine different monoclonal antibodies to the extracellular domain of the insulin receptor were tested in three different cell types for their ability to stimulate the intrinsic tyrosine kinase activity of the receptor. Previous studies had suggested that several of these monoclonal antibodies stimulate biological responses without stimulating the intrinsic tyrosine kinase activity of the receptor (Hawley, D. M., Maddux, B. A., Patel, R. G., Wong, K. Y., Manula, P. W., Firestone, G. L., Brunetti, A., Verspohl, E., and Goldfine, I. D. (1989) J. Biol. Chem. 264, 2438-2444 and Soos, M. A., O'Brien, R. M., Brindle, N. P. J., Stigter, J. M., Okamoto, A. K., Whittaker, J., and Siddle, K. (1989) Proc. Natl. Acad. Sci. U. S. A. 86, 5217-5221). In the present study, a more sensitive assay was utilized, and these same monoclonal antibodies, when added to intact cells, were found to stimulate the phosphotransferase activity of the receptor. This increase in activity was reversed by phosphatase treatment of the receptor. In contrast, monoclonal antibodies which had no insulin-mimetic activities did not stimulate the receptor's kinase activity. In addition, Western blot analyses of lysates with anti-phosphotyrosine antibodies showed that insulin-mimetic, but not non-insulin-mimetic antibodies, stimulated tyrosine phosphorylation of the receptor as well as an endogenous substrate (phosphoprotein Mr = 160,000). Finally, these antibodies were found to stimulate the tyrosine phosphorylation of another endogenous substrate of the insulin receptor kinase, the type I phosphatidylinositol kinase. These studies support the hypothesis that monoclonal antibodies, like insulin, stimulate biological responses via their ability to stimulate the tyrosine kinase activity of the receptor.
View details for Web of Science ID A1990DF14200077
View details for PubMedID 1693150
Previous studies of the substrate specificity of the purified insulin receptor tyrosine kinase using synthetic random polymers have demonstrated that the receptor kinase phosphorylates poly (Glu, Tyr) 4:1 but not poly (Glu, Tyr) 1:1. In the present study, insulin treatment of Chinese hamster ovary cells overexpressing the human insulin receptor was found to stimulate the ability of their membrane extracts to phosphorylate poly (Glu, Tyr) 1:1. It was concluded that this activity was due to the receptor itself because: 1) it was precipitated with a monoclonal antibody to the receptor; 2) the addition of various membrane extracts to purified insulin receptor preparations stimulated the ability of these preparations to phosphorylate poly (Glu, Tyr) 1:1; and 3) certain purified proteins, including bovine serum albumin and casein, were also capable of stimulating the purified receptor to phosphorylate poly (Glu, Tyr) 1:1. The effect of albumin was dose-dependent; 0.5 and 10 mg/ml bovine serum albumin stimulated the phosphorylation of poly (Glu, Tyr) 1:1 by 2- and 230-fold, respectively. In contrast, albumin had no effect on the phosphorylation of poly (Glu, Tyr) 4:1. These results indicate that the activity of the insulin receptor kinase on certain substrates can be modulated by the presence of other proteins.
View details for Web of Science ID A1990CM41800008
View details for PubMedID 2153593
The tyrosine kinase activity intrinsic to the insulin receptor is thought to be important in eliciting the intracellular responses to insulin; however, it has been difficult to determine the biochemical functions of the proteins which are substrates for this receptor. Treatment of Chinese hamster ovary (CHO) cells overexpressing the human insulin receptor (CHO.T) with insulin results in a 38 +/- 11 (mean +/- S.E., n = 9)-fold increase in a phosphatidylinositol (PtdIns) kinase activity in anti-phosphotyrosine immunoprecipitates of whole cell lysates. One minute of treatment of cells with insulin causes a dramatic increase in the PtdIns kinase activity in the anti-phosphotyrosine immunoprecipitates; the activity peaks within 5 min and remains elevated for at least 60 min after addition of insulin to the cells. This response is only slightly delayed compared with the time course we observe for activation of the insulin receptor tyrosine kinase. The insulin dose-response curves are also very similar for the activation of the insulin receptor tyrosine kinase activity and for the appearance of PtdIns kinase in the anti-phosphotyrosine immunoprecipitates. Stimulation of the endogenous insulin receptor of CHO cells also results in the association of PtdIns kinase activity with phosphotyrosine-containing proteins. However, CHO cells are less sensitive to insulin than CHO.T cells, and the maximal PtdIns kinase activity in antiphosphotyrosine immunoprecipitates from CHO cells is one-sixth that of CHO.T cells. In contrast, immunoprecipitates from CHO.T cells made with anti-insulin receptor antibodies do not contain significant levels of PtdIns kinase activity. This demonstrates that the PtdIns kinase is either a substrate for the insulin receptor tyrosine kinase or is tightly associated with another tyrosine phosphoprotein, which is not the insulin receptor.
View details for Web of Science ID A1990CF66200060
View details for PubMedID 1688432
The insulin receptor is a large cell surface glycoprotein that concentrates insulin at the site of action and also initiates responses to insulin. The receptor is a disulfide-linked oligomer comprised of two alpha and two beta subunits. Signal transduction through the insulin receptor appears to require the activation of an intrinsic tyrosine-specific protein kinase activity. A variety of disorders, both acquired and genetic, are associated with the development of insulin resistance and are frequently the result of cellular defects in insulin receptor structure, function, and action. The recent cloning of several mutant receptors from patients with genetic forms of extreme insulin resistance has increased our understanding of insulin resistance on the molecular level.
View details for Web of Science ID A1990CX21800011
View details for PubMedID 2184752
View details for Web of Science ID A1990BR15Q00043
View details for Web of Science ID A1990BQ80E00007
Insulin and insulin-like growth factor (IGF)-I inhibit intracellular protein degradation in a variety of different cell types. In the present studies, the IGF-I-induced inhibition of protein metabolism in Chinese hamster ovary (CHO) cells was found to be blocked by polyclonal antibodies to the IGF-II/mannose-6-phosphate phosphate (Man-6-P) receptor, but not by control immunoglobulin. In contrast, these antibodies had no effect on the ability of IGF-I to stimulate glucose uptake in the same cells. The antibodies to the IGF-II/Man-6-P receptor also inhibited the effect of IGF-I and insulin on protein catabolism in human foreskin fibroblasts and human hepatoma cells, respectively. Moreover, CHO cells overexpressing a cDNA coding for the IGF-II/Man-6-P receptor were found to exhibit an increased effect of insulin on protein catabolism. In contrast, the insulin stimulation of glucose uptake is the same in these transfected cells as in the parental CHO cells. These results implicate the IGF-II/Man-6-P receptor in the insulin- and IGF-I-induced inhibition of protein catabolism.
View details for Web of Science ID A1989AB31400003
View details for PubMedID 2544802
Recombinant transforming growth factor (TGF)-beta 1 precursor was recently found to contain mannose 6-phosphate (Purchio et al., 1988, J. Biol. Chem. 263, 14211-14215). In the present study, recombinant TGF-beta 1 precursor was shown to bind to the insulin-like growth factor (IGF)-II/mannose 6-phosphate (man6P) receptor on the plasma membrane of cells since: 1) Insulin, which induces an increase in cell surface IGF-II/man6P receptors on adipocytes, caused a 2.7-fold increase in TGF-beta 1 precursor binding to adipocytes; 2) Chinese hamster ovary cells selected for overexpression of the IGF-II/man6P receptor exhibited an increased binding of TGF-beta 1 precursor in comparison to the parental cells; and 3) the binding of 125I-TGF-beta 1 precursor to these transfected cells and adipocytes was largely inhibited by man6P. After 15 minutes at 37 degrees C, 75% of the recombinant TGF-beta 1 precursor was found to be internalized in the transfected cells. Additional studies with latent TGF-beta 1 isolated from platelets indicated that this material could also bind to the isolated IGF-II/man6P receptor.
View details for Web of Science ID A1989U182100059
View details for PubMedID 2540751
Chinese hamster ovary cell lines expressing either the wild-type human insulin receptor or a hybrid molecule in which the tyrosine kinase domain of the insulin receptor is replaced with that of the oncogene, v-ros were examined for their ability to internalize and degrade insulin. Cells expressing the hybrid receptor were found to internalize and degrade insulin at approximately half the rate of cells expressing the native insulin receptor. Moreover, insulin was incapable of inducing the internalization of the cell-surface hybrid molecule. In contrast, the constitutive rate of receptor internalization was found to be the same for the hybrid and wild-type receptors. These results obtained were similar to those with cells expressing either wild-type or mutant receptors lacking kinase activity. In conclusion, the substitution of the specificity of tyrosine kinase of the insulin receptor with that of the v-ros oncogene product results in defective internalization and degradation of insulin, and loss of ligand-induced receptor internalization.
View details for Web of Science ID A1989T316600012
View details for PubMedID 2645866
A proteinase with high affinity for insulin has been proposed to play a role in the cellular processing of this hormone. A complementary DNA (cDNA) coding for this enzyme has been isolated and sequenced. The deduced amino acid sequence of the enzyme contained the sequences of 13 peptides derived from the isolated protein. The cDNA could be transcribed in vitro to yield a synthetic RNA that in cell-free translations produced a protein that coelectrophoresed with the native proteinase and could be immunoprecipitated with monoclonal antibodies to this enzyme. The deduced sequence of this proteinase did not contain the consensus sequences for any of the known classes of proteinases (that is, metallo, cysteine, aspartic, or serine), but it did show homology to an Escherichia coli proteinase (called protease III), which also cleaves insulin and is present in the periplasmic space. Thus, these two proteins may be members of a family of proteases that are involved in intercellular peptide signaling.
View details for Web of Science ID A1988R247900033
View details for PubMedID 3059494
Recombinant transforming growth factor-beta 1 (TGF-beta 1) precursor produced and secreted by a clone of Chinese hamster ovary cells was found to be glycosylated and phosphorylated. Treatment of 32P-labeled precursor protein with N-glycanase indicated that phosphate was incorporated into asparagine-linked complex carbohydrate moieties. Fractionation of 32P-labeled glycopeptides followed by amino acid sequence analysis indicated that greater than 95% of the label was incorporated into two out of three glycosylation sites at Asn-82 and Asn-136 of the TGF-beta 1 precursor. Two-dimensional electrophoretic analysis of acid hydrolyzed precursor protein and precursor protein-derived glycopeptides indicated that 32P was incorporated as mannose 6-phosphate. Binding studies with the purified receptor for mannose 6-phosphate indicated that the TGF-beta 1 precursor could bind to this receptor and the binding was specifically inhibited with mannose 6-phosphate.
View details for Web of Science ID A1988Q306000039
View details for PubMedID 2971654
Stable transfectants of Chinese hamster ovary (CHO) cells were developed that expressed the protein encoded by a human insulin-like growth factor I (IGF-I) receptor cDNA. The transfected cells expressed approximately 25,000 high affinity receptors for IGF-I (apparent Kd of 1.5 X 10(-9) M), whereas the parental CHO cells expressed only 5,000 receptors per cell (apparent Kd of 1.3 X 10(-9) M). A monoclonal antibody specific for the human IGF-I receptor inhibited IGF-I binding to the expressed receptor and immunoprecipitated polypeptides of apparent Mr values approximately 135,000 and 95,000 from metabolically labeled lysates of the transfected cells but not control cells. The expressed receptor was also capable of binding IGF-II with high affinity (Kd approximately 3 nM) and weakly recognized insulin (with about 1% the potency of IGF-I). The human IGF-I receptor expressed in these cells was capable of IGF-I-stimulated autophosphorylation and phosphorylation of endogenous substrates in the intact cell. This receptor also mediated IGF-I-stimulated glucose uptake, glycogen synthesis, and DNA synthesis. The extent of these responses was comparable to the stimulation by insulin of the same biological responses in CHO cells expressing the human insulin receptor. These results indicate that the isolated cDNA encodes a functional IGF-I receptor and that there are no inherent differences in the abilities of the insulin and IGF-I receptors to mediate rapid and long term biological responses when expressed in the same cell type. The high affinity of this receptor for IGF-II also suggests that it may be important in mediating biological responses to IGF-II as well as IGF-I.
View details for Web of Science ID A1988P671300076
View details for PubMedID 2969892
The insulin-like growth factor II (IGF-II) is a polypeptide hormone with structural homologies to insulin and insulin-like growth factor I (IGF-I). In contrast to these other hormones, the in vivo function of IGF-II is not known. Although IGF-II can stimulate a broad range of biological responses in isolated cells, these responses have usually been found to be mediated by the insulin and IGF-I receptors. Recently, the receptor for IGF-II was found to also be the receptor for mannose-6-phosphate. Since this latter receptor has been implicated in targeting of lysosomal enzymes, the question is now raised of whether the same protein can also mediate metabolic responses to IGF-II.
View details for Web of Science ID A1988M425400020
View details for PubMedID 2964085
View details for Web of Science ID A1988P776800004
Recently, the sequence of the human receptor for insulin-like growth factor II (IGF-II) was found to be 80% identical [Morgan et al., (1987) Nature 329, 301-307] to the sequence of a partial clone of the bovine cation-independent mannose-6-phosphate receptor [Lobel et al., (1987) Proc. Natl. Acad. Sci. USA 84, 2233-2237]. In the present study, the purified receptor for insulin-like growth factor II (IGF-II) was found to react with two different polyclonal antibodies to the purified mannose-6-phosphate receptor. Moreover, mannose-6-phosphate was found to stimulate the binding of labeled IGF-II to the IGF-II receptor by two-fold. This effect had the same specificity and affinity as the reported binding of mannose-6-phosphate to its receptor; mannose-1-phosphate and mannose had no effect on the binding of labeled IGF-II to its receptor, and the half-maximally effective concentration of mannose-6-phosphate was 0.3 mM. Also, mannose-6-phosphate did not affect labeled IGF-II binding to the insulin receptor. These results support the hypothesis that a single protein of Mr-250,000 binds both IGF-II and mannose-6-phosphate. Furthermore, they indicate that mannose-6-phosphate can modulate the interaction of IGF-II to its receptor.
View details for PubMedID 2962576
The internalization and degradation of insulin was assessed in Chinese hamster ovary cell lines expressing either the wild-type receptor or mutated receptors lacking kinase activity. The mutated receptors included receptors which differed from the wild-type receptor by a single amino acid (substitution of an arginine for lysine at position 1030, a site critical for ATP binding) as well as receptors which had a deletion of 112 amino acids at the carboxyl terminus. Cells expressing mutated receptors lacking kinase activity were found to internalize and degrade insulin at about half the rate of cells expressing wild-type receptors with kinase activity. Moreover, insulin was found incapable of inducing the internalization of the mutated receptors, whereas it could stimulate the internalization of the wild-type receptor. Finally, the constitutive rate of receptor internalization was found to be the same for the mutant and wild-type receptors. These results implicate the intrinsic tyrosine-specific kinase activity of the insulin receptor in the ligand-induced, but not the constitutive, internalization of this receptor.
View details for Web of Science ID A1987K886800006
View details for PubMedID 3316198
Insulin-like growth factor II (IGF-II) shares sequence homology and predicted three-dimensional structure with insulin and IGF-I. IGF-II can bind, therefore, to a limited extent with the receptors for these two other hormones, as well as to a distinct receptor for IGF-II. Previous studies have been unable to attribute a particular response of IGF-II through its own receptor. In the present studies, the IGF-II receptor is shown to mediate the stimulation of glycogen synthesis in human hepatoma cells since: (i) IGF-II is found to be capable of stimulating a response at concentrations in which it would primarily interact with its own receptor; (ii) the response to IGF-II was not blocked by monoclonal antibodies which inhibit the responses of cells through the insulin and IGF-I receptors; and (iii) polyclonal antibodies to the IGF-II receptor were found to mimic the ability of IGF-II to stimulate glycogen synthesis. These results indicate that the IGF-II receptor mediates a particular biological response--stimulation of glycogen synthesis in hepatoma cells. Furthermore, a monovalent Fab fragment of the polyclonal antibody to the IGF-II receptor was also shown to stimulate glycogen synthesis in these cells. These data indicate that clustering of the IGF-II receptor is not required to stimulate a biological response.
View details for PubMedID 2828025
The primary structure of human insulin-like growth factor II receptor, predicted from the complementary DNA sequence, reveals a transmembrane receptor molecule with a large extracellular domain made up of fifteen repeat sequences and a small region homologous to the collagen-binding domain of fibronectin. The structural and biochemical features of the IGF-II receptor appear identical to those of the cation-independent mannose-6-phosphate receptor.
View details for Web of Science ID A1987K110200043
View details for PubMedID 2957598
A retrovirus containing part of the human insulin receptor (hIR) gene was constructed by replacing ros sequences in the avian sarcoma virus UR2 with hIR cDNA sequences coding for 46 amino acids of the extracellular domain and the entire transmembrane and cytoplasmic domains of the beta subunit of hIR. The resulting virus, named UIR, contains the hIR sequence fused to the 5' portion of the UR2 gag gene coding for p19. UIR is capable of transforming chicken embryo fibroblasts and promoting formation of colonies in soft agar; however, it does not form tumors in vivo. A variant that arose from the parental UIR is capable of efficiently inducing sarcomas in vivo. UIR-transformed cells exhibit higher rates of glucose uptake and growth than normal cells. The 4-kilobase UIR genome codes for a membrane-associated, glycosylated gag-hIR fusion protein of 75 kDa designated P75gag-hir. P75gag-hir contains a protein tyrosine kinase activity that is capable of undergoing autophosphorylation and of phosphorylating foreign substrates in vitro; it is phosphorylated at both serine and tyrosine residues in vivo.
View details for Web of Science ID A1987J650800044
View details for PubMedID 3039503
A hybrid receptor molecule composed of the extracellular ligand-binding domain of the human insulin receptor and the transmembrane and cytoplasmic (protein-tyrosine kinase) domains of the chicken sarcoma virus UR2 transforming protein p68gag-ros has been constructed and expressed in Chinese hamster ovary (CHO) cells. The hybrid is processed normally into alpha and hybrid beta subunits, is expressed on the cell surface at high levels, and binds insulin with near-wild-type affinity. Furthermore, insulin stimulates the phosphorylation on tyrosine residues of the hybrid beta subunit in vivo and the phosphorylation of an exogenous substrate [poly(Glu,Tyr)] in vitro. Thus the hybrid is capable of heterologous transmembrane signaling. However, the hybrid mediates neither the insulin-activated uptake of 2-deoxyglucose nor the incorporation of [3H]thymidine into DNA, suggesting that the physiological response(s) mediated by ligand-activated protein-tyrosine kinases may utilize distinct intracellular mechanisms for postreceptor signaling.
View details for Web of Science ID A1987J472700004
View details for PubMedID 3299376
To test the proposed role of protein disulfide isomerase in the synthesis of immunoglobulins (Ig), intact lymphocytes were treated with a thiol-cleavable, bifunctional cross-linking agent and lysed, and the lysates were immunoprecipitated with antibodies to either Ig or enzyme. When the immunoprecipitates were analyzed on polyacrylamide-sodium dodecyl sulfate gels, protein disulfide isomerase was found to be cross-linked to immunoglobulins. The extent of cross-linking was dependent upon the concentration of cross-linker added and the class of Ig. For IgMs and high concentrations of cross-linker, approximately one molecule of Ig was coupled per two molecules of enzyme. For IgGs, the extent of cross-linking was less. Finally, depletion of the intracellularly reduced glutathione by diamide was found to also result in the linkage of protein disulfide isomerase to IgM. These results therefore support the hypothesis that protein disulfide isomerase functions in the in vivo synthesis of immunoglobulins.
View details for Web of Science ID A1987J320400001
View details for PubMedID 3663584
Insulin receptor down-regulation was studied in various Chinese hamster ovary (CHO) cell lines expressing transfected human insulin receptor cDNAs. In addition to a cell line expressing the normal receptor (CHO.T line), three lines expressing mutated receptors were studied: the CHO.T-t line, which expresses a receptor with a degraded cytoplasmic domain due to the removal of the C-terminal 112 amino acids, and the CHO.YF1 and CHO.YF3 lines, in which important autophosphorylation sites of the receptor kinase (tyrosines-1162 and -1163) have been replaced by phenylalanine. A monoclonal anti-receptor antibody, but not insulin itself, was found to down-regulate cell surface receptor levels in all four cell lines by 60-80% after 18-h treatment at 37 degrees C. Down-regulation of the CHO.T and CHO.T-t receptors occurred at similar antibody concentrations and with a similar time course, although the maximum level of CHO.T-t down-regulation (60%) was generally lower than the level of CHO.T down-regulation (80%). Pulse-chase labeling of these two cell types with [35S]methionine revealed that antibody treatment of both CHO.T and CHO.T-t cells resulted in a similar increase in the rate of degradation of mature receptor subunits. These results indicate that antibody-induced down-regulation of the insulin receptor in these cells can occur in the absence of various autophosphorylation sites of the receptor and that the mechanism of antibody-induced down-regulation is different from that for insulin.
View details for Web of Science ID A1987H575400001
View details for PubMedID 3607002
The oncogene protein product (p21) of the ras gene has been implicated in mediating the effects of a variety of growth factors and hormones. Microinjection of monoclonal antibody 6B7, which is directed against a synthetic peptide corresponding to a highly conserved region of p21 (amino acids 29 to 44) required for p21 function, specifically inhibited Xenopus oocyte maturation induced by incubation with insulin. The inhibition was dose-dependent and specific since (i) the same antibody had no effect on progesterone-induced maturation, (ii) immunoprecipitation and Western blotting indicated that the antibody recognized a single protein of molecular weight 21,000 in oocyte extracts, and (iii) inhibition was not observed with identical concentrations of normal immunoglobulin. Thus, p21 appears to be involved in mediating insulin-induced maturation of Xenopus oocytes. Furthermore, the mechanism may involve phosphorylation of p21, as p21 was found to be a substrate of the insulin receptor kinase.
View details for Web of Science ID A1987H282100038
View details for PubMedID 3554510
To test whether the tyrosine kinase activity of the insulin receptor is crucial for insulin action, we have constructed mutations of the human insulin receptor at Lys-1030, which is in the presumed ATP-binding region. By using oligonucleotide-directed mutagenesis, this lysine residue was replaced with either methionine, arginine, or alanine. Chinese hamster ovary cells were transfected by mutant cDNAs and the expressed insulin receptors were characterized. We show here that none of these mutants exhibited insulin-activated autophosphorylation and kinase activity in vitro. They also do not mediate insulin- and antibody-stimulated uptake of 2-deoxyglucose. The tyrosine kinase activity is thus required for a key physiological response of insulin.
View details for Web of Science ID A1987F976300021
View details for PubMedID 3101064
View details for PubMedCentralID PMC304284
Insulin stimulates the autophosphorylation of the beta-subunit of the insulin receptor (IR) on tyrosine residues. Mutations which compromise IR autophosphorylation in vivo result in a decrease of the insulin-activated uptake of 2-deoxyglucose. These results are consistent with previous results which implicate IR autophosphorylation in the generation of the insulin response by cells. To further explore the specificity of the IR tyrosine phosphokinase (TPK) domain in IR function, we have altered the human IR (hIR) cDNA to encode truncated insulin-independent TPKs, which are expressed in chinese hamster ovary (CHO) cells as either membrane-anchored or cytosolic proteins. Both mutant hIRs exhibit TPK activity in vitro, although the cytosolic form is approximately 20 times more active. The carbohydrate moiety of the membrane-anchored form is of the high mannose type, consistent with an intracellular localization for this mutant hIR. The two mutant hIRs mediate very different physiological responses in transfected cells: the membrane-anchored, but not the cytosolic, hIR TPK mediates a constitutively elevated (135% the maximum insulin-stimulated response in CHO cells) insulin-independent uptake of 2-deoxyglucose. These results thus suggest that the hIR TPK is in fact specific for this aspect of IR function and, when membrane-associated, can mediate the insulin-independent uptake of 2-deoxyglucose. Neither of these mutant hIRs appears to transform CHO cells.
View details for Web of Science ID A1987K944200002
View details for PubMedID 2842659
Highly purified insulin receptor was shown to be a substrate for cAMP kinase. Approximately 1 phosphate was incorporated per molecule of receptor, and the cAMP kinase's affinity for the receptor was at least as high as its affinity for histone. The sites phosphorylated by cAMP kinase seemed distinct from those phosphorylated by the protein kinase C. Phosphorylation by cAMP kinase had no effect on the ability of several monoclonal antibodies to recognize the receptor or on the insulin-binding activity of the receptor. However, cAMP phosphorylation partially inhibited the tyrosine kinase activity of the receptor (approximately 25%). These results suggest that catecholamine-induced resistance to insulin may be partly due to a direct phosphorylation of the receptor by cAMP kinase and a subsequent inhibition of the ability of the receptor kinase to be activated by insulin.
View details for Web of Science ID A1987F436600020
View details for PubMedID 3539674
View details for Web of Science ID A1987H773000014
The human insulin receptor (hIR) is an integral transmembrane glycoprotein comprised of two alpha and two beta subunits. An immediate consequence of insulin binding to the extracellular alpha subunit is the autophosphorylation of tyrosine residues on the intracellular domain of the beta subunit. The placental hIR cDNA has been cloned and sequenced, providing the primary structural features of the protein. In order to investigate the functions of the beta subunit and particularly the role of autophosphorylation and tyrosine phosphokinase (TPK) activity (a feature shared by other receptors and oncogene proteins) in transmembrane signalling, we designed an expression system of the hIR cDNA in eucaryotic cells. Superexpressing CHO cell lines that contain about 10(6) functional hIR/cell have been developed. In these cells half maximum stimulation of glucose uptake occurs at 5 X 10(-10)M insulin, whereas normal CHO cells require 5 X 10(-12)M insulin. In this expression system we have carried out site-directed mutagenesis experiments in which domains of the molecule have been deleted or particular amino acids have been replaced by others. The replacement of either or both the tyrosine residues 1162 and 1163 compromise an autophosphorylated site that is important for kinase function and the insulin response. Expression of an isolated membrane-bound form of the beta-subunit produces a 6 fold increase in glucose uptake. This insulin-independent effect disappears if the twin tyrosines are mutated or if the beta subunit is expressed in the cytoplasm. These studies also show that the C terminal 112 amino acid portion of the beta subunit is important for the stability of this protein.
View details for Web of Science ID A1987J166500020
View details for PubMedID 3305910
The role of the insulin receptor tyrosine kinase (protein-tyrosine kinase, EC 126.96.36.199) in various rapid insulin effects was studied by injecting four different cell types (by osmotic lysis of pinocytotic vesicles) with a monoclonal antibody that specifically inhibits the kinase activity of the insulin receptor and the closely related receptor for insulin-like growth factor (IGF)-I. Injection of this inhibitory antibody resulted in a decreased ability of insulin to stimulate the uptake of 2-deoxyglucose in Chinese hamster ovary cells and freshly isolated rat adipocytes, ribosomal protein S6 phosphorylation in CHO cells, and glycogen synthesis in the human hepatoma cell line HepG2. The ability of insulin, IGF-I, and IGF-II to stimulate glucose uptake in TA1 mouse adipocytes was also inhibited. Studies with CHO cells demonstrated that these effects of the inhibitory antibody were specific, since there was no change in phorbol ester-stimulated glucose uptake and injection of a noninhibiting antibody to the kinase had no effect on insulin action. These studies indicate that the tyrosine kinase activity of the insulin receptor is important in mediating several rapid insulin effects in a variety of different cell types.
View details for Web of Science ID A1987F667900009
View details for PubMedID 3540958
A radioimmunoassay for the insulin receptor has been developed. In this assay, unlabeled receptor competes with 125I-labeled receptor for binding to monoclonal anti-receptor antibodies immobilized on microtiter wells coated with affinity-purified anti-mouse immunoglobulin G. This assay was highly reproducible and could detect 7 ng (14 fmol) of insulin receptor. By utilizing monoclonal antibodies to various antigenic regions of the receptor, different parts of the receptor molecule could be examined. By utilizing antibodies to the cytoplasmic domain of the receptor, an assay was developed which was not influenced by the presence of insulin and could equally detect the insulin receptor from different species (rat and human) and different tissues (placenta and brain). By utilizing antibodies to an autophosphorylation site of the receptor, the assay was shown capable of detecting the extent of phosphorylation of the receptor. Finally, this assay was utilized to monitor the decrease in insulin receptors in lysates of insulin-treated human lymphocytes. This radioimmunoassay should be useful for monitoring both the number and status of the insulin receptor under a variety of physiological conditions.
View details for Web of Science ID A1986F165800025
View details for PubMedID 3812995
A hybrid receptor has been constructed that is composed of the extracellular domain of the human insulin receptor fused to the transmembrane and cytoplasmic domains of the bacterial aspartate chemoreceptor. This hybrid protein can be expressed in rodent (CHO) cells and displays several functional features comparable to wild-type insulin receptor. It is localized to the cell surface, binds insulin with high affinity, forms oligomers, and is recognized by conformation-specific monoclonal antibodies. Although most of the expressed protein accumulates as a 180-kDa proreceptor, some processed 135-kDa receptor can be detected on the cell surface by covalent cross-linking. Expression of the hybrid receptor inhibits the insulin-activated uptake of 2-deoxyglucose by CHO cells. Thus, this hybrid is partially functional and can be processed; however, it is incapable of native transmembrane signaling. The results indicate that the intact domains of different types of receptors can retain some of the native features in a hybrid molecule but specific requirements will need to be satisfied for transmembrane signaling.
View details for Web of Science ID A1986E643100025
View details for PubMedID 3022282
Specific receptors for insulinlike growth factors I and II (IGF-I and IGF-II) were found on cultured human myoblasts and myotubes. In contrast, myotubes but not myoblasts specifically bound insulin and were stimulated by nanomolar concentrations of insulin to take up deoxyglucose. In addition, in myoblasts, physiological concentrations of IGF-I and -II and, to a lesser extent, insulin stimulated two- to threefold the uptake of the nonmetabolizable amino acid analogue methylaminoisobutyric acid (MAIB). In myotubes, uptake of MAIB was stimulated preferentially by IGF-I. Monoclonal antibodies that preferentially recognize either the insulin receptor or the IGF-I receptor were utilized to examine which receptors mediated the biological effects of these hormones. The effects of insulin on both myoblasts and myotubes appeared to be mediated in part by the insulin receptor and in part by the IGF-I receptor. In myotubes, the effects of IGF-I and -II both appeared to be mediated through the IGF-I receptor. In myoblasts, the effects of the two IGFs appeared to be in part mediated by the IGF-I receptor and in part mediated by either the IGF-II receptor or another type of IGF-I receptor. The present results suggest that cultured human muscle cells provide a useful model system in which to study the biological actions of insulin and the IGFs.
View details for Web of Science ID A1986E863700039
View details for PubMedID 2946238
The receptor for insulin-like growth factor I (IGF-I) was purified from the rat liver cell line BRL-3A by a combination monoclonal anti-receptor antibody column and a wheat germ agglutinin column. Analyses of these receptor preparations on reduced sodium dodecyl sulfate-polyacrylamide gels yielded protein bands of Mr 136K (alpha subunit) and Mr 85K and 94K (beta subunit). These receptor preparations bound 5 times more IGF-I than insulin, and the binding of both labeled ligands was more potently inhibited by unlabeled IGF-I than by insulin. These results indicate that these receptor preparations contained predominantly the IGF-I receptor. This highly purified receptor preparation was found to possess an intrinsic kinase activity; autophosphorylation of the receptor beta subunit was stimulated by low concentrations of IGF-I (half-maximal stimulation at 0.4 nM IGF-I). Twentyfold higher concentrations of insulin were required to give comparable levels of stimulation. A monoclonal antibody that inhibits the insulin receptor kinase was found to inhibit the IGF-I receptor kinase with the same potency with which it inhibits the insulin receptor. In contrast, monoclonal antibodies to other parts of the insulin receptor only poorly recognized the IGF-I receptor. A comparison of V8 protease digests of the insulin and IGF-I receptors again revealed some similarities and also some differences in the structures of these two receptors. Thus, the IGF-I receptor is structurally, antigenically, and functionally similar to but not identical with the insulin receptor.
View details for Web of Science ID A1986E242300032
View details for PubMedID 2946318
Removal of insulin from the peritubular vessels involves binding of insulin to specific receptors in the basolateral membranes (BLM); this is followed by phosphorylation of the receptor which may mediate the actions of the hormone. In most tissues receptor number is regulated by plasma insulin levels and is increased in insulinopenic diabetics. To determine whether cortical BLM insulin receptors are similarly regulated, we studied insulin binding to receptors in BLM from normal control rats and rats with streptozotocin diabetes of varying severity. Specific binding of insulin did not differ between control and modestly insulinopenic diabetics but was increased significantly in the severely insulinopenic diabetics. Insulin treatment returned binding to normal. Scatchard analysis suggested an increase in the binding capacity of the severe diabetic BLM rather than an increase in affinity for insulin. This latter was confirmed by competitive experiments in which similar displacement curves were obtained with control and diabetic membranes. Insulin removed by glomerular filtration binds to specific receptors in the luminal membranes but unlike BLM receptors, phosphorylation of these luminal receptors has not been observed. To determine whether luminal and BLM receptors differ structurally, binding sites in both membranes were affinity labelled with 125I-insulin and the cross linking agent, disuccinimidyl suberate, and subjected to SDS-polyacrylamide gel electrophoresis in the presence of a reducing agent. Autoradiograms revealed that the major specifically labelled subunit in both membranes is a 135,000 Mr species which is more abundant in the BLM. We conclude that insulin receptors in cortical BLM respond to severe insulinopenic diabetes as do receptors in most other tissues.(ABSTRACT TRUNCATED AT 250 WORDS)
View details for Web of Science ID A1986D725200009
View details for PubMedID 3537445
A monoclonal antibody was identified which equally inhibits 125I-labeled insulin and insulin-like growth factor I (IGF-I) binding to their respective receptors in human IM-9 lymphoid cells and solubilized placenta receptor preparations. In contrast, this monoclonal antibody inhibits insulin but not IGF-I binding to human hepatoma (HepG2) cells, fibroblasts and muscle cells. These results indicate that there are two distinct species of the type I insulin-like growth factor receptor (which we have named type IA and type IB) and suggest that this monoclonal antibody may be useful in determining whether different biological effects are mediated through these two receptors.
View details for Web of Science ID A1986D760500048
View details for PubMedID 3019341
The beta subunit of purified insulin receptor is phosphorylated on a serine residue by purified preparations of protein kinase C (ATP: protein phosphotransferase, EC 188.8.131.52). This phosphorylation is inhibited by antibodies to protein kinase C and stimulated by phospholipids, diacylglycerol, and Ca2+. The phosphorylation of the receptor by protein kinase C does not affect its insulin-binding activity but does inhibit by 65% the receptor's intrinsic tyrosine-specific protein kinase activity (ATP: protein-tyrosine O-phosphotransferase, EC 184.108.40.206). These results indicate that activators of protein kinase C, such as phorbol esters, desensitize cells to insulin by direct protein kinase C action on the insulin receptor.
View details for Web of Science ID A1986D754900016
View details for PubMedID 3526339
View details for PubMedCentralID PMC386387
Insulin stimulates the autophosphorylation of tyrosine residues of the beta subunit of the insulin receptor (IR); this modified insulin-independent kinase has increased activity toward exogenous substrates in vitro. We show here that replacement of one or both of the twin tyrosines (residues 1162 and 1163) with phenylalanine results in a dramatic reduction in or loss of insulin-activated autophosphorylation and kinase activity in vitro. In vivo, these mutations not only result in a substantial decrease in insulin-stimulated IR autophosphorylation but also in a parallel decrease in the insulin-activated uptake of 2-deoxyglucose. Furthermore, a truncated IR protein (lacking the last 112 amino acids) has an unstable beta subunit; this mutant has no kinase activity in vitro or in vivo and does not mediate insulin-stimulated uptake of 2-deoxyglucose. IR autophosphorylation is thus implicated in the regulation of IR activities, with tyrosines 1162 and 1163 as major sites of this regulation.
View details for Web of Science ID A1986C698200010
View details for PubMedID 3518947
Four monoclonal antibodies were identified by their ability to bind to 125I-labeled insulin covalently linked to a cytosolic insulin-degrading enzyme from human erythrocytes. All four antibodies were also found to remove more than 90% of the insulin-degrading activity from erythrocyte extracts. These antibodies were shown to be directed to different sites on the enzyme by mapping studies and by their various properties. Two antibodies recognized the insulin-degrading enzyme from rat liver; one inhibited the erythrocyte enzyme directly; and two recognized the enzyme after gel electrophoresis and transfer to nitrocellulose filters. By this latter procedure and immunoprecipitation from metabolically labeled cells, the enzyme from a variety of tissues was shown to be composed of a single polypeptide chain of apparent Mr 110,000. Finally, these monoclonal antibodies were microinjected into the cytoplasm of a human hepatoma cell line to assess the contribution of this enzyme to insulin degradation in the intact cell. In five separate experiments, preloading of cells with these monoclonal antibodies resulted in an inhibition of insulin degradation of 18-54% (average 39%) and increased the amount of 125I-labeled insulin associated with the cells. In contrast, microinjection of control antibody or an extraneous monoclonal antibody had no effect on insulin degradation or on the amount of insulin associated with the cells. Moreover, the monoclonal antibodies to the insulin-degrading enzyme caused no significant inhibition of degradation of another molecule, low density lipoprotein. Thus, these results support a role for this enzyme in insulin degradation in the intact cell.
View details for Web of Science ID A1986C835000009
View details for PubMedID 2424018
An insulin-degrading enzyme (IDE) was purified from the cytosol of human erythrocytes via the use of ammonium sulfate precipitation and chromatography on columns composed of DEAE-Sephadex, pentylagarose, hydroxylapatite, chromatofocusing resins, and Ultrogel AcA-34. The final preparation was purified greater than 50,000-fold and exhibited a single protein band of Mr = 110,000 on reduced sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Cross-linking of 125I-labeled insulin to the enzyme preparation labeled a protein of the same molecular weight, indicating that this band was in fact the enzyme. Intact insulin, insulin B chain, and glucagon inhibited this cross-linking half-maximally at concentrations of 0.1, 1, and 1.5 microM, respectively. Under nondenaturing conditions, the enzyme had an Mr = 300,000, suggesting that the enzyme may exist under physiological conditions as a dimer or timer. The purified enzyme was inhibited by both sulfhydrylmodifying reagents and chelating agents, indicating that a free thiol and metal were both required for the activity of the enzyme. The purified enzyme was found to degrade physiological concentrations of intact insulin more rapidly than insulin B chain, although at high substrate concentrations (greater than 1 microM) the enzyme degraded B chain to a greater extent. Additional characteristics of the enzyme were a pl of 5.2 and a pH optimum of 7.0. These properties of the red blood cell (RBC) enzyme were very similar to those reported for IDEs from other tissues. Moreover, a polyclonal antiserum to the IDE from skeletal muscle was found to recognize the RBC enzyme.
View details for Web of Science ID A1986C645500008
View details for PubMedID 3519322
TA1 cells, like 3T3-L1 cells, undergo a differentiation process in vitro from a fibroblast to an adipocyte phenotype. The TA1 pre-adipocytes were found to have low numbers of insulin receptors but high numbers of receptors for insulin-like growth factors (IGF) I and II. Also, the pre-adipocytes were more responsive to IGF than insulin as measured by either stimulation of glucose or amino acid uptake. After differentiation, the adipocytes had higher numbers of insulin receptors and a better responsitivity to insulin than to IGF-I. These results indicate that insulin-like growth factors are the primary regulators of the pre-adipocytes whereas insulin regulates the adipocytes.
View details for Web of Science ID A1986C606800080
View details for PubMedID 3013194
A panel of 37 monoclonal antibodies to the human insulin receptor has been used to characterize the receptor's major antigenic regions and their relationship to receptor functions. Three antibodies recognized extracellular surface structures, including the insulin binding site and a region not associated with insulin binding. The remaining 34 monoclonal antibodies were directed against the cytoplasmic domain of the receptor beta subunit. Competitive binding studies demonstrated that four antigenic regions (beta 1, beta 2, beta 3, and beta 4) are found on this domain. Sixteen of the antibodies were found to be directed against beta 1, nine against beta 2, seven against beta 3, and two against beta 4. Antibodies to all four regions inhibited the receptor-associated protein kinase activity to some extent, although antibodies directed against the beta 2 region completely inhibited the kinase activity of the receptor both in the autophosphorylation reaction and in the phosphorylation of an exogenous substrate, histone. Antibodies to the beta 2 region also did not recognize autophosphorylated receptor. In addition, antibodies to this same region recognized the receptor for insulin-like growth factor I (IGF-I) as well as the insulin receptor. In contrast, antibodies to other cytoplasmic regions did not recognize the IGF-I receptor as well as the insulin receptor. These results indicate that the major immunogenic regions of the insulin receptor are located on the cytoplasmic domain of the receptor beta subunit and are associated with the tyrosine-specific kinase activity of the receptor. In addition, these results suggest that a portion of the insulin receptor is highly homologous to that of the IGF-I receptor.
View details for Web of Science ID A1986A562900026
View details for PubMedID 2421765
The insulin receptor from human brain cortex was purified by a combination monoclonal antibody affinity column and a wheat germ agglutinin column. This purified receptor preparation exhibited major protein bands of apparent Mr = 135,000 and 95,000, molecular weights comparable to those for the alpha and beta subunits of the purified human placental and rat liver receptors. A minor protein band of apparent Mr = 120,000 was also observed in the brain receptor preparation. Crosslinking of 125I-insulin to all three receptor preparations was found to preferentially label a protein of apparent Mr = 135,000. In contrast, cross-linking of 125I-labeled insulin-like growth factor I to the brain preparation preferentially labeled the protein of apparent Mr = 120,000. The purified brain insulin receptor was found to be identical with the placental insulin receptor in the amount of neuraminidase-sensitive sialic acid and reaction with three monoclonal antibodies to the beta subunit of the placental receptor. In contrast, a monoclonal antibody to the insulin binding site recognized the placental receptor approximately 300 times better than the brain receptor. These results indicate that the brain insulin receptor differs from the receptor in other tissues and suggests that this difference is not simply due to the amount of sialic acid on the receptor.
View details for Web of Science ID A1986A425300042
View details for PubMedID 3005298
Our experiments with the hIR protein have been designed to address a very general question of transmembrane receptor structure and function: What are the roles and interactions of the various deduced structural domains of such molecules in the initiation of the response of cells to extracellular signals? All of the evidence to date supports the previous hypothesis based on biochemical data that the IR requires ligand-activated TPK functions to initiate the insulin response by cells (for review, see Kahn 1985). Thus, mutations that compromise hIR TPK activity (site-directed point mutations or deletions) result in a concomitant decrease in at least one aspect of insulin action (glucose uptake; Ellis et al. 1986a). Other studies utilizing microinjection of antibodies to inhibit the receptor kinase have extended this conclusion to include a critical role for the receptor kinase in insulin's ability to stimulate ribosomal protein S6 phosphorylation in CHO cells, glycogen synthetase in hepatoma cells, glucose uptake in adipocytes (Morgan and Roth 1987), and frog oocyte maturation (Morgan et al. 1986). Second, analyses of cell lines that express experimentally truncated hIR TPKs demonstrate that, when membrane-anchored, this TPK domain is in fact capable of autonomous hormone-independent IR function: Such cells exhibit a constitutively elevated, insulin-independent uptake of 2-deoxyglucose (Ellis et al. 1987). Finally, by substitution of a homologous TPK for that of hIR, we find that although such a hybrid is capable of insulin-dependent transmembrane signaling (phosphorylation of the hybrid beta-subunit on tyrosine residues), the hybrid IR.ros molecule does not function as an IR in such cells: It mediates neither short-term (uptake of 2-deoxyglucose) nor long-term (incorporation of [3H]thymidine) effects of insulin (L. Ellis et al., in prep.). Together, these results suggest that (1) the hIR TPK domain conveys a substrate specificity for the insulin response and (2) that a functional hIR extracellular domain alone is not sufficient for generation of the insulin response (e.g., ligand-induced aggregation, or simple delivery of insulin into the cell). With the linking of the extracellular and cytoplasmic domains of the hIR molecule has evolved a cellular mechanism for the control of hIR TPK activity; the result is that cells which express the IR are now insulin responsive, and the physiological responses associated with the hormone are ligand-activated. Thus, the uncontrolled state of autonomous TPK activity, with the associated constitutive physiological response (e.g., as exhibited by the spBam hIR mutant), is circumvented.(ABSTRACT TRUNCATED AT 400 WORDS)
View details for Web of Science ID A1986H050100008
View details for PubMedID 3472760
View details for PubMedID 2944813
View details for PubMedID 3613365
Thirty-six monoclonal antibodies to the human insulin receptor were produced. Thirty-four bound the intracellular domain of the receptor beta subunit, the domain containing the tyrosine-specific kinase activity. Of these 34 antibodies, 33 recognized the rat receptor and 1 was shown to precipitate the receptors from mice, chickens, and frogs with high affinity. Another of the antibodies inhibited the kinase activities of the human and frog receptors with equal potencies. This antibody inhibited the kinase activities of these receptors by more than 90%, whereas others had no effect on either kinase activity. Microinjection of the inhibiting antibody into Xenopus oocytes blocked the ability of insulin to stimulate oocyte maturation. In contrast, this inhibiting antibody did not block the ability of progesterone to stimulate the same response. Furthermore, control immunoglobulin and a noninhibiting antibody to the receptor beta subunit did not block this response to insulin. These results strongly support a role for the tyrosine-specific kinase activity of the insulin receptor in mediating this biological effect of insulin.
View details for Web of Science ID A1986AYP2200029
View details for PubMedID 3455769
An insulin degrading enzyme from cultured human lymphocytes, IM-9 cells, has been purified and characterized. The biochemical, enzymatic and immunological characteristics of this enzyme were all found to be similar to the characteristics of insulin degrading enzymes previously isolated from rat and pig skeletal muscle. Furthermore, this insulin degrading enzyme was found to have no effect on the structure of the insulin receptor nor to be linked to the insulin receptor either on the plasma membrane of cells or when they are shed into the media. The present studies suggest that the IM-9 lymphocytes, which have been extensively used to study the human insulin receptor, may also be a good system for studying human insulin degrading enzymes.
View details for PubMedID 3915257
We have placed human insulin receptor cDNA into a vector under the control of the simian virus 40 (SV40) early promoter and tested its function by transient expression in microinjected Xenopus oocytes and by expression in stably transformed CHO cells. The precursor and the alpha and beta subunits of the receptor were detected by immunoprecipitation from extracts of these cells. The human insulin receptor expressed in CHO cells specifically binds 125I-labeled insulin but not insulin-like growth factor I, displays insulin-stimulated autophosphorylation of the beta subunit, and mediates insulin-stimulated 2-deoxyglucose uptake. We conclude that the human insulin receptor is synthesized, processed normally, and functional in this heterologous cell system.
View details for Web of Science ID A1985AVT4000044
View details for PubMedID 3906655
The formation of disulphide bonds is essential to the structure and function of proteins. These bonds rapidly form either cotranslationally or immediately post-translationally in the lumen of the endoplasmic reticulum. Native disulphide pairing for such proteins has been achieved in vitro; however, the rates of reassembly are slow and the conditions non-physiological. To account for these observations, Anfinsen et al. proposed that a 'disulphide interchange protein' was the in vivo catalyst of disulphide bond rearrangement. Other groups discovered an activity with similar characteristics that catalysed the reductive cleavage of insulin and may be associated with insulin degradation, although this result has been disputed. The enzyme involved, protein disulphide isomerase (PDI; EC 220.127.116.11), may be the in vivo catalyst of disulphide bond formation. Here we describe the sequence of cloned rat liver PDI complementary DNA which predicts a protein with two distinct regions homologous with Escherichia coli thioredoxin, a known cofactor in oxidation-reduction reactions. Each of these regions contains the presumed active site sequence Trp-Cys-Gly-His-Cys-Lys, suggesting that PDI, similar in action to thioredoxin, catalyses disulphide bond interchange via an internal disulphide-sulphydryl interchange. The cDNA predicts a signal peptide consistent with the view that PDI is a luminal endoplasmic reticulum protein. PDI messenger RNA, although ubiquitous, is more highly concentrated in secretory cells.
View details for Web of Science ID A1985AQV6800060
View details for PubMedID 3840230
A cloned approximately 5 kb cDNA (human placenta) contains the coding sequences for the insulin receptor. The nucleotide sequence predicts a 1382 amino acid precursor. The alpha subunit comprises the N-terminal portion of the precursor and contains a striking cysteine-rich "cross-linking" domain. The beta-subunit (the C-terminal portion of the precursor) contains a transmembrane domain and, in the intracellular region, the elements of a tyrosine phosphokinase: an ATP-binding site and a possible tyrosine autophosphorylation site or sites. The overall structure is reminiscent of the EGF receptor; the cross-linking domain of the alpha subunit and several regions of the beta subunit exhibit sequence homology with the EGF receptor. The phosphokinase domain also exhibits homology with some oncogenic proteins that have tyrosine phosphokinase activity, in particular, a striking homology with v-ros. Southern blotting experiments suggest that the coding region spans more than 45 kb. The insulin receptor gene is located on chromosome 19.
View details for Web of Science ID A1985AGH5400005
View details for PubMedID 2859121
A human insulin degrading enzyme purified from IM-9 lymphocytes was tested for its ability to degrade insulin-like growth factor I (IGF-I) and insulin-like growth factor II (IGF-II). Degradation of these molecules was assessed by trichloroacetic acid precipitation, binding to specific receptors and chromatography on Sephadex G-50. All three techniques indicated that the enzyme readily degraded IGF-II and slightly degraded IGF-I. The IGF-II degrading activity chromatofocused with the insulin degrading activity and was absorbed by specific antibodies to the insulin degrading enzyme. These studies indicate, therefore, that a human insulin degrading enzyme can degrade IGF-II and, to a lesser extent, IGF-I.
View details for Web of Science ID A1984TK73300002
View details for PubMedID 6389104
Insulin and the insulinlike growth factors (IGF-I and IGF-II) are members of a family of hormones that regulate the metabolism and growth of many tissues. Cultured HEP-G2 cells (a minimal deviation human hepatoma) have insulin receptors and respond to insulin by increasing their glycogen metabolism. In the present study with HEP-G2 cells, we used 125I-labeled insulin, IGF-I, and IGF-II to identify distinct receptors for each hormone by competition-inhibition studies. Unlabeled insulin was able to inhibit 125I-IGF-I binding but not 125I-IGF-II binding. A mouse monoclonal antibody to the human insulin receptor that inhibits insulin binding and blocks insulin action inhibited 75% of 125I-insulin binding, but inhibited neither 125I-IGF-I nor 125I-IGF-II binding. When glycogen metabolism was studied, insulin stimulated [3H]glucose incorporation into glycogen in a biphasic manner; one phase that was 20-30% of the maximal response occurred over 1-100 pM, and the other phase occurred over 100 pM-100 nM. The anti-receptor monoclonal antibody inhibited the first phase of insulin stimulation but not the second. Both IGF-I and IGF-II stimulated [3H]glucose incorporation over the range of 10 pM-10 nM; IGF-I was three to fivefold more potent. The monoclonal antibody, however, was without effect on IGF regulation of glycogen metabolism. Therefore, these studies indicate that insulin as well as the IGFs at physiological concentrations regulate glycogen metabolism in HEP-G2 cells. Moreover, this regulation of glycogen metabolism is mediated by both the insulin receptor and the IGF receptors.
View details for Web of Science ID A1984TM66300035
View details for PubMedID 6090502
View details for PubMedCentralID PMC425312
Rat liver thiol:protein-disulfide oxidoreductase/glutathione-insulin transhydrogenase (glutathione:protein disulfide oxidoreductase, EC 18.104.22.168) was purified and found to give two bands on sodium dodecyl sulfate polyacrylamide gel electrophoresis. A monoclonal antibody was produced against this enzyme preparation and found to remove all the insulin degrading activity of purified preparations of the enzyme. This monoclonal antibody was also found to react with the two different forms of the enzyme observed on gel electrophoresis. These results suggest that glutathione-insulin transhydrogenase can exist in more than one state.
View details for Web of Science ID A1984TF86500005
View details for PubMedID 6743666
The serum from a patient with lupus nephritis, insulin resistance, and hypoglycemia was studied. This serum both inhibits the binding of 125I-insulin to its receptor and has insulin-like activity on fat cells (see refs. 1 and 2). The IgG fraction from this patient's serum one-half maximally inhibited 125I-insulin binding to IM-9 cells at 1 microM, but did not markedly inhibit 125I-monoclonal antibody binding even at concentrations as high as 4 microM. The IgG was then subjected to affinity chromatography on a protein A-Sepharose column. Four protein peaks were eluted from this column by a step pH gradient from 5.5 to 2.3. Three of the four peaks inhibited 125I-insulin binding to its receptors, but none was more potent than the unfractionated IgG itself. One IgG peak, however, was able to inhibit 125I-monoclonal antibody binding at tenfold lower concentrations than the unfractionated IgG. When the ability of the four IgG fractions to stimulate 2-deoxy[3H]-D-glucose transport in rat adipocytes was studied, two fractions showed stimulatory activity. Compared with unfractionated IgG, one had a weak ability to inhibit 125I-insulin binding, but tenfold more potency to mimic insulin action. The other had a strong ability to inhibit 125I-insulin binding but less potency to mimic insulin action. These studies indicate, therefore, that the serum contains multiple populations of antibodies to the insulin receptor, or portions of the plasma membrane adjacent to the receptor, which have different biologic effects.
View details for Web of Science ID A1984SE63000018
View details for PubMedID 6365664
ATP, in a dose-dependent manner, inhibited the binding of 125I-insulin to its receptor in rat liver and human placental membranes. With rat liver plasma membranes an effect of ATP was detected at concentrations between 1.0 and 2.5 mmol/L, and maximal effects were seen at 10.0 mmol/L where binding was decreased by approximately 40%. The effect of ATP was one half-maximal within 10 minutes and maximal within 60 minutes. Scatchard analysis indicated that ATP was acting primarily to change the binding affinity of the insulin receptor. The effect of ATP was mimicked by CTP, GTP, and UTP, but not by ADP, 5'-AMP, 3'-AMP, 3'5'-cyclic AMP and adenosine. The ATP analog AMP-PNP had a potency approximately 10% that of ATP. The effect of ATP was not significantly influenced by inhibitors of phosphoprotein kinases and phosphoprotein phosphatases. In human placental membranes, ATP had a similar effect in inhibiting 125I-insulin binding to its receptor. Moreover, ATP was active in inhibiting insulin binding to purified human placental insulin receptors at 0.01 mmol/L, a concentration 1/100 of that needed for inhibiting binding to intact membranes. These studies indicate, therefore, that ATP and other nucleoside triphosphates influence the ability of the insulin receptor to bind insulin.
View details for Web of Science ID A1984SW94400015
View details for PubMedID 6374370
Certain covalently linked insulin dimers have previously been found to have a greater ability to bind to the insulin receptor than to stimulate lipogenesis in adipocytes. The present report presents data indicating that the same insulin dimers also have a greater ability to bind to the receptor than to stimulate the kinase activity of the insulin receptor. In particular, one such covalently linked insulin dimer had less than 1% the potency of native insulin in stimulating the receptor kinase although it could bind to the solubilized receptor with 30% the potency of native insulin. In contrast, this dimer could down regulate the insulin receptor with approximately 30% the potency of native insulin. These results suggest that stimulation of the receptor kinase may require more than simple occupancy of the receptor binding site whereas down regulation of the receptor may require only the binding of ligand to the receptor.
View details for Web of Science ID A1984SU14600028
View details for PubMedID 6327387
In the present study, we have used isolated mouse pancreatic acini to investigate the relationship between 125I-insulin binding and its degradation in order to probe the nature and cellular localization of the degradative process. In these cells, the proteolysis of 125I-insulin was dependent on time and cell concentration, and was saturated by unlabeled insulin with a Km of 290 nM. Since this value was much higher than the Kd for insulin binding to its receptor (1.1 nM), the data indicated that 125I-insulin degradation by acini occurred primarily via nonreceptor mechanisms. Several lines of evidence suggested that insulin was being degraded by the neutral thiol protease, insulin degrading enzyme (IDE). First, insulin degradation was inhibited by thiolreacting agents such as N-ethylmaleimide and p-chloromercuribenzoate. Second, the Km for degradation in acini was similar to the reported Km for IDE in other tissues. Third, the enzyme activity had a relative mol wt of approximately 130,000 by gel filtration, a value similar to that reported for purified IDE. Fourth, the degrading activity was removed with a specific antibody to IDE. Other lines of evidence suggested that enzymes located on the cell surface played a role in insulin degradation by acini. First, the nonpenetrating sulfhydryl reacting agent 5,5' dithiobis-2-nitrobenzoic acid blocked 125I-insulin degradation. Second, a specific antibody to IDE identified the presence of the enzyme on the cell surface. Third, chloroquine, leupeptin and antipain, agents that inhibit lysosomal function, did not influence 125I-insulin degradation. Fourth, highly purified pancreatic plasma membranes degraded 125I-insulin.
View details for Web of Science ID A1984SB51700012
View details for PubMedID 6360769
Collagenase preparations (a mixture of enzymes including collagenase, clostripain, and a casein-degrading protease) degraded the beta subunit (Mr = 95,000) of the purified insulin receptor into fragments of Mr less than 15,000, without degrading the alpha subunit. The resulting beta-digested insulin receptor preparations were found to bind insulin as well as control insulin receptor, as assessed by either cross-linking of 125I-insulin to the digested receptor or by separating insulin bound to receptor from free insulin by high performance liquid chromatography. Moreover, the beta-digested insulin receptor preparations were still precipitated by a monoclonal antibody directed against the insulin-binding site. In contrast, the beta-digested insulin receptor lacked protein kinase activity since it no longer phosphorylated either itself, or an exogenous substrate, calf thymus histone. These results support the identification of the beta subunit of the insulin receptor as a protein kinase.
View details for PubMedID 6315728
In the present study, we investigated the ability of a monoclonal antibody to the insulin receptor to regulate the expression of the insulin receptor of IM-9 lymphocytes. Previously, this antibody was shown to be a competitive antagonist of insulin action on severe metabolic functions. In the present study, we report that preincubation of IM-9 cells with the monoclonal antibody caused a dose- and time-dependent decrease in the subsequent ability of these cells to bind 125I-insulin, a phenomenon termed down regulation. The antibody was approximately 100 times more potent than insulin at down regulating the receptor. In contrast, the antibody was 5 times less potent than insulin in competing for binding to insulin receptors and dissociated 4 times more rapidly than insulin from IM-9 cells. Three lines of evidence suggested that the mechanism of down regulation by the antibody was the same as the one used by insulin. First, both agents caused a rapid initial decrease in insulin binding to cells followed by a slower, gradual decrease in binding. Second, the down regulation caused by both was reversible, and this reversibility required new protein synthesis. Third, the antibody, like insulin, accelerated receptor degradation. Since the antibody does not mimic the other effects of insulin on metabolic processes, these results suggest that the mechanism of insulin receptor down regulation is different from the mechanism of insulin action on other cellular functions.
View details for PubMedID 6355081
Insulin-ricin B chain conjugate, a hybrid molecule consisting of insulin covalently linked to the binding chain of ricin, was tested for insulin-like biological activity in HTC and H35 rat hepatoma cells, rat adipocytes, rat thymocytes, and human fibroblasts. In H35 cells and adipocytes, cells that have abundant insulin receptors and are very sensitive to insulin (ED50 of 30 pM and 50 pM, respectively), the conjugate had 5% the biological activity of native insulin (ED50 of 500 pM and 1000 pM, respectively). Since the insulin portion of the conjugate has 5% the potency of native insulin in binding to the insulin receptor, these observations suggested that (in these cells) the conjugate was acting via the insulin receptor. Moreover lactose and galactose, potent inhibitors of ricin binding to its receptor, had no effect on the action of the conjugate on H35 cells. In contrast, in thymocytes, HTC cells, and fibroblasts cells that have relatively few insulin receptors and require high concentrations of insulin to elicit biological actions (ED50 of 10 nM, 20 nM, and 1 nM, respectively), the conjugate had more biological activity than was predicted on the basis of its ability to bind to the insulin receptor. In addition, in these three insulin-insensitive cell types, the activity of the conjugate was inhibited by either lactose or galactose. Thus, these observations indicate that in cells which are relatively insensitive to insulin, the biological effects of the conjugate require the interaction of the ricin B chain moiety with the ricin receptor. In addition, in fibroblasts, the activity of the conjugate was inhibited by the addition of a monoclonal antibody to the insulin receptor which inhibits the response of fibroblasts to insulin. These data suggest, therefore, that in insulin-insensitive cells the binding of the ricin B chain moiety to its receptor enhances the interaction of the insulin portion of the conjugate with the insulin receptor.
View details for Web of Science ID A1983QR79600048
View details for PubMedID 6343061
The polypeptide hormone insulin and the binding unit of cholera toxin (CTB) were coupled via a disulfide bond. This hybrid molecule had 1/30 the ability of native insulin to bind to the insulin receptor and 1/30 the biological activity of native insulin in H35 rat hepatoma cells and rat adipocytes. Thus, in these two cell types that are very sensitive to insulin, the biological activity of the hybrid molecule was as predicted on the basis of the ability of the molecule to interact with the insulin receptor. In contrast, in HTC rat hepatoma cells and rat thymocytes, two poorly responsive cell types, the insulin-CTB conjugate had 1/3 the biological activity of native insulin, a value 10 times greater than its insulin receptor binding potency. This increased activity of the conjugate did not appear to be due to cholera toxin in the preparation, since a control of uncoupled CTB had no biological activity. Furthermore, native cholera toxin increased intracellular levels of cAMP by 20-fold, whereas the conjugate had no effect on cAMP levels. The CTB moiety did, however, contribute to the biological activity of the conjugate, since the activity of the hybrid molecule, like cholera toxin, was inhibited by gangliosides, whereas the activity of native insulin was not. Finally, the binding to thymocytes of insulin-CTB conjugate, but not insulin, was inhibited by gangliosides. Thus, a hybrid hormone molecule has been constructed which has insulin-like biological activity with the receptor specificity of cholera toxin in poorly responsive cells.
View details for Web of Science ID A1983QP10600007
View details for PubMedID 6132923
Three agents which mimic insulin action in intact cells (concanavalin A, wheat germ agglutinin, and polyclonal insulin receptor antibody), mimicked insulin's ability to stimulate the kinase activity of purified insulin receptors. In contrast, monoclonal insulin receptor antibody, an antagonist of insulin action, did not stimulate the phosphorylation of the insulin receptor either in intact IM-9 cells or in purified receptor preparations. This antibody, however, antagonized the ability of insulin to stimulate the phosphorylation of the receptor both in intact cells and in the purified receptor. These studies with insulin mimickers and an insulin antagonist are consistent with a role for the kinase activity of the receptor mediating the actions of insulin.
View details for Web of Science ID A1983RF54000037
View details for PubMedID 6351861
Highly purified preparations of insulin receptor catalyzed the phosphorylation of the 95,000-dalton subunit of the insulin receptor. This subunit of the insulin receptor was also labeled with [alpha-32P]8-azidoadenosine 5'-triphosphate, a photoaffinity label for adenosine triphosphate binding sites. The identity of the 95,000-dalton band was confirmed in both cases by precipitation with a monoclonal antibody to the insulin receptor. These results suggest that the insulin receptor is itself a protein kinase.
View details for Web of Science ID A1983PX64500030
View details for PubMedID 6849137
View details for Web of Science ID A1983RT93700067
Antibodies to the insulin receptor were prepared in BALB/c mice by immunization with IM-9 human lymphocytes, a cell type that has a large number of plasma membrane insulin receptors. The spleens of these mice were then removed, and their lymphocytes were fused to a mouse myeloma cell line, FO cells. After screening over 1,200 resulting hybrids, one stable hybrid was obtained that produced IgG1 antibodies directed towards the insulin receptor. This antibody blocked 125I-labeled insulin binding to its receptor by more than 90% in three human tissues: IM-9 cultured lymphocytes, freshly isolated adipocytes, and placenta membranes. In contrast, the antibody did not inhibit insulin binding to rat adipocytes and rat liver plasma membranes, suggesting that the antibody was species specific. In IM-9 cells, which had their proteins prelabeled with [35S]methionine, the antibody precipitated two polypeptides with molecular weights of 135,000 and 95,000; these molecular weights are identical to those previously identified as the alpha and beta subunits of the insulin receptor. The monoclonal antibody inhibited the actions of insulin on both human adipocytes and fibroblasts, suggesting that the antibody was an antagonist of insulin action. The present studies suggest, therefore, that monoclonal antibodies to the insulin receptor may provide new insights into the structure of the insulin receptor and its interaction with insulin.
View details for Web of Science ID A1982PT40700049
View details for PubMedID 6185950
View details for PubMedCentralID PMC347329
The polypeptide hormone insulin and the binding portion of ricin toxin, the B chain, were linked via a disulfide bond. This insulin-ricin B chain conjugate bound to insulin receptors with a potency one-twentieth that of native insulin. Rat HTC hepatoma cells, a cultured cell line that has relatively few insulin receptors, bound the conjugate to a much greater degree than insulin. Binding occurred predominantly via the ricin B chain portion of the conjugate since binding was not inhibited by insulin but was inhibited by galactose, a known inhibitor of the interaction of ricin B chain to its receptor. In HTC cells, the insulin-ricin B chain conjugate at 330 nM stimulated amino acid uptake to 225% of controls, a value higher than that for insulin which stimulated uptake to only 167% of controls. The conjugate also stimulated tyrosine aminotransferase activity in HTC cells with a potency value approximately one-half that of insulin. Both of these activities of the insulin-ricin B chain conjugate in HTC cells were inhibited by 100 mM galactose (90% and 80%, respectively), whereas the ability of insulin to stimulate these activities was not inhibited significantly by this sugar. The results suggest, therefore, that one can construct hybrid molecules consisting of binding proteins and polypeptide hormones and that these hybrid molecules can have binding and biological activities which are different from the parent hormone molecule.
View details for Web of Science ID A1981LS97800011
View details for PubMedID 7016852
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