Research & Scholarship
Current Research and Scholarly Interests
The p53 tumor suppressor gene plays a crucial role in protecting organisms from developing cancer. Our research is aimed at dissecting the mechanism of p53 action and the role of target genes it activates in apoptosis and tumor suppression, using the mouse as a model system. Our strategy is to use combination of biochemical, cell biological, and mouse genetic approaches, starting by generating hypotheses about p53 using primary mouse embryo fibroblasts (MEFs), and then testing them using knockout technology in the mouse. An understanding of p53 function in the mouse will ultimately be useful for understanding how inactivation of p53 in humans leads to cancer and for designing therapies for the many tumors disrupted in the p53 pathway.
Specific projects include:
1) Analysis of PERP, a p53 apoptosis-specific target gene
In a screen to identify genes activated during p53-mediated apoptosis but not G1 arrest, we previously isolated a novel gene known as PERP (P53 apoptosis Effector Related to PMP-22). PERP induction correlates with p53-dependent apoptosis and not p53-independent cell death, and PERP overexpression can induce cell death in p53-deficient MEFs, suggesting that it could be a downstream mediator of p53 function in apoptosis. PERP displays sequence similarity to PMP-22/gas3 (Peripheral Myelin Protein 22/growth arrest specific 3), a tetraspan membrane protein that is commonly mutated in human hereditary peripheral neuropathies (i.e. Charcot Marie Tooth), and that appears to function in part by regulating cell proliferation and apoptosis. We are currently characterizing PERP and its encoded protein in two ways. We are constructing a PERP conditional knockout mouse to examine the function of PERP in development and tumorigenesis. We are also dissecting the mechanism by which overexpression of PERP induces cell death. PERP is a multipass transmembrane protein that could act potentially either as a signalling molecule or as an ion channel. The generation and analysis of PERP mutants will be helpful for determining how PERP exerts its effects on known steps of the apoptotic process, thus defining a link between p53 and the execution of apoptosis.
2) Characterization of additional targets of p53 in apoptosis
Another research direction will to be to identify and characterize other components of the p53-activated cell death pathway. A comprehensive search for additional p53 apoptosis-specific target genes will be carried out by the hybridization of G1-arrested and apoptotic MEF RNA populations to DNA microarrays. Genes upregulated in apoptotic cells will be characterized similarly to PERP, first in vitro, by cell biological and biochemical approaches to identify those that are most interesting, and then ultimately in vivo by gene targeting approaches.
3) Generation of p53 knock-in mice and cells
- The Basic Science of Radiation Therapy
RADO 202 (Spr)
Independent Studies (9)
- Directed Reading in Cancer Biology
CBIO 299 (Aut, Win, Spr, Sum)
- Directed Reading in Radiation Oncology
RADO 299 (Aut, Win, Spr, Sum)
- Early Clinical Experience in Radiation Oncology
RADO 280 (Aut, Win, Spr, Sum)
- Graduate Research
CBIO 399 (Aut, Win, Spr, Sum)
- Graduate Research
RADO 399 (Aut, Win, Spr, Sum)
- Medical Scholars Research
RADO 370 (Aut, Win, Spr, Sum)
- Out-of-Department Advanced Research Laboratory in Experimental Biology
BIO 199X (Aut, Win, Spr)
- Readings in Radiation Biology
RADO 101 (Aut, Win, Spr, Sum)
- Undergraduate Research
RADO 199 (Aut, Win, Spr, Sum)
- Directed Reading in Cancer Biology
TRP53 activates a global autophagy program to promote tumor suppression.
2013; 9 (9): 1440-1442
The mechanisms by which the TP53/TRP53 transcription factor acts as a tumor suppressor remain incompletely understood. To gain new insights into TP53/TRP53 biology, we used ChIP-seq and RNA-seq technologies to define global TRP53 transcriptional networks in primary cells subjected to DNA damage. Intriguingly, we identified a TRP53-regulated autophagy program, which can be coordinately regulated by the TRP53 family members TRP63 and TRP73 in certain settings. While autophagy is not involved in TRP53-dependent cell cycle arrest, it contributes to both TRP53-driven apoptosis in response to DNA damage and TRP53-mediated transformation suppression. Collectively, our genome-wide analyses reveal a profound role for TRP53 in regulating autophagy, through an extensive transcriptional network, and have demonstrated an important role for this program in promoting TRP53-mediated apoptosis and tumor suppression.
View details for DOI 10.4161/auto.25833
View details for PubMedID 23899499
- Not all p53 gain-of-function mutants are created equal. Cell death and differentiation 2013; 20 (7): 855-857
Tumor Suppression: p53 Alters Immune Surveillance to Restrain Liver Cancer.
2013; 23 (12): R527-30
The p53 tumor suppressor governs multiple cell-intrinsic programs, including cell-cycle arrest and apoptosis, to curb neoplastic growth. A new study reveals that p53 also acts through a novel non-cell-autonomous mechanism, by stimulating the innate immune system to maintain tissue homeostasis and suppress tumorigenesis.
View details for DOI 10.1016/j.cub.2013.04.076
View details for PubMedID 23787049
- Global genomic profiling reveals an extensive p53-regulated autophagy program contributing to key p53 responses GENES & DEVELOPMENT 2013; 27 (9): 1016-1031
RB goes mitochondrial.
Genes & development
2013; 27 (9): 975-979
The retinoblastoma tumor suppressor RB is well known for its capacity to restrict cell cycle progression at the G1/S transition of the cell cycle by controlling the transcription of cell cycle genes. In this issue of Genes & Development, Hilgendorf and colleagues (pp. 1003-1015) have identified a novel tumor suppressor function for RB independent of its role as a transcriptional regulator, in which RB directly activates the apoptosis regulator Bax at the mitochondria to promote cell death.
View details for DOI 10.1101/gad.219451.113
View details for PubMedID 23651852
Global genomic profiling reveals an extensive p53-regulated autophagy program contributing to key p53 responses.
Genes & development
2013; 27 (9): 1016-1031
The mechanisms by which the p53 tumor suppressor acts remain incompletely understood. To gain new insights into p53 biology, we used high-throughput sequencing to analyze global p53 transcriptional networks in primary mouse embryo fibroblasts in response to DNA damage. Chromatin immunoprecipitation sequencing reveals 4785 p53-bound sites in the genome located near 3193 genes involved in diverse biological processes. RNA sequencing analysis shows that only a subset of p53-bound genes is transcriptionally regulated, yielding a list of 432 p53-bound and regulated genes. Interestingly, we identify a host of autophagy genes as direct p53 target genes. While the autophagy program is regulated predominantly by p53, the p53 family members p63 and p73 contribute to activation of this autophagy gene network. Induction of autophagy genes in response to p53 activation is associated with enhanced autophagy in diverse settings and depends on p53 transcriptional activity. While p53-induced autophagy does not affect cell cycle arrest in response to DNA damage, it is important for both robust p53-dependent apoptosis triggered by DNA damage and transformation suppression by p53. Together, our data highlight an intimate connection between p53 and autophagy through a vast transcriptional network and indicate that autophagy contributes to p53-dependent apoptosis and cancer suppression.
View details for DOI 10.1101/gad.212282.112
View details for PubMedID 23651856
- Loss of the p53/p63 target PERP is an early event in oral carcinogenesis and correlates with higher rate of local relapse ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY ORAL RADIOLOGY 2013; 115 (1): 95-103
Engaging the p53 metabolic brake drives senescence.
2013; 23 (6): 739-40
Emerging evidence suggests that the ability of p53 to regulate metabolism is important for its tumor suppressor activity. A recent study published in Nature reveals a novel connection between p53 and metabolism: p53 transcriptionally represses the expression of malic enzymes and associated NADPH production, which in turn triggers a positive feedback loop resulting in sustained p53 activation, cellular senescence, and tumor suppression.
View details for PubMedID 23478296
Loss of the p53/p63 target PERP is an early event in oral carcinogenesis and correlates with higher rate of local relapse.
Oral surgery, oral medicine, oral pathology and oral radiology
2013; 115 (1): 95-103
PERP is a p53/p63-regulated gene encoding a desmosomal protein that plays a critical role in cell-cell adhesion and tumor suppression.We evaluated PERP expression in different grades of oral dysplasia (34 cases) and at different stages of invasive squamous cell carcinoma (SCC), and correlated the latter with clinical outcome. A tissue microarray consisting of nondysplastic mucosa, carcinoma in situ, SCC, and nodal metastases from 33 patients with human papilloma virus-negative SCC was stained for PERP and E-cadherin.Complete loss of PERP expression was associated with worse local control in patients with SCC. The 5-year local control rate was 91% for patients with partial PERP loss versus 31% for those with complete loss (P = .01).This is the first study to show that loss of PERP expression correlates with the transition to SCC and with increased local relapse in patients with oral cavity SCC.
View details for DOI 10.1016/j.oooo.2012.10.017
View details for PubMedID 23217540
Unimpaired Skin Carcinogenesis in Desmoglein 3 Knockout Mice
2012; 7 (11)
The contribution of adherens junction inactivation, typically by downregulation or mutation of the transmembrane core component E-cadherin, to cancer progression is well recognized. In contrast, the role of the desmosomal cadherin components of the related cell-cell adhesion junction, the desmosome, in cancer development has not been well explored. Here, we use mouse models to probe the functional role of desmosomal cadherins in carcinogenesis. Because mice lacking the desmosomal cadherin Desmoglein 3 (Dsg3) have revealed a crucial role for Dsg3 in cell-cell adhesion in stratified epithelia, we investigate the consequence of Dsg3 loss in two models of skin carcinogenesis. First, using Dsg3-/- keratinocytes, we show that these cells display adhesion defects in vitro and compromised tumor growth in allograft assays, suggesting that Dsg3 enables tumor formation in certain settings. In contrast, using an autochthonous model for SCC development in response to chronic UVB treatment, we discover a surprising lack of enhanced tumorigenesis in Dsg3-/- mice relative to controls, unlike mice lacking the desmosomal component Perp. Accordingly, there is no defect in the apoptotic response to UVB or enhanced immune cell infiltration upon Dsg3 loss that could promote tumorigenesis. Thus, Dsg3 does not display a clear function as a tumor suppressor in these mouse skin cancer models. Continued unraveling of the roles of Dsg3 and other desmosomal constituents in carcinogenesis in different contexts will be important for ultimately improving cancer diagnosis, prognostication, and treatment.
View details for DOI 10.1371/journal.pone.0050024
View details for Web of Science ID 000311821000167
View details for PubMedID 23185521
Deconstructing p53 transcriptional networks in tumor suppression
TRENDS IN CELL BIOLOGY
2012; 22 (2): 97-106
p53 is a pivotal tumor suppressor that induces apoptosis, cell-cycle arrest and senescence in response to stress signals. Although p53 transcriptional activation is important for these responses, the mechanisms underlying tumor suppression have been elusive. To date, no single or compound mouse knockout of specific p53 target genes has recapitulated the dramatic tumor predisposition that characterizes p53-null mice. Recently, however, analysis of knock-in mice expressing p53 transactivation domain mutants has revealed a group of primarily novel direct p53 target genes that may mediate tumor suppression in vivo. We present here an overview of well-known p53 target genes and the tumor phenotypes of the cognate knockout mice, and address the recent identification of new p53 transcriptional targets and how they enhance our understanding of p53 transcriptional networks central for tumor suppression.
View details for DOI 10.1016/j.tcb.2011.10.006
View details for Web of Science ID 000300870600004
View details for PubMedID 22154076
Deficiency of the p53/p63 target Perp alters mammary gland homeostasis and promotes cancer
BREAST CANCER RESEARCH
2012; 14 (2)
Perp is a transcriptional target of both p53 during DNA damage-induced apoptosis and p63 during stratified epithelial development. Perp-/- mice exhibit postnatal lethality associated with dramatic blistering of the epidermis and oral mucosa, reflecting a critical role in desmosome-mediated intercellular adhesion in keratinocytes. However, the role of Perp in tissue homeostasis in other p63-dependent stratified epithelial tissues is poorly understood. Given that p63 is essential for proper mammary gland development and that cell adhesion is fundamental for ensuring the proper architecture and function of the mammary epithelium, here we investigate Perp function in the mammary gland.Immunofluorescence and Western blot analysis were performed to characterize Perp expression and localization in the mouse mammary epithelium throughout development. The consequences of Perp deficiency for mammary epithelial development and homeostasis were examined by using in vivo mammary transplant assays. Perp protein levels in a variety of human breast cancer cell lines were compared with those in untransformed cells with Western blot analysis. The role of Perp in mouse mammary tumorigenesis was investigated by aging cohorts of K14-Cre/+;p53fl/fl mice that were wild-type or deficient for Perp. Mammary tumor latency was analyzed, and tumor-free survival was assessed using Kaplan-Meier analysis.We show that Perp protein is expressed in the mammary epithelium, where it colocalizes with desmosomes. Interestingly, although altering desmosomes through genetic inactivation of Perp does not dramatically impair mammary gland ductal development, Perp loss affects mammary epithelial homeostasis by causing the accumulation of inflammatory cells around mature mammary epithelium. Moreover, we show reduced Perp expression in many human breast cancer cell lines compared with untransformed cells. Importantly, Perp deficiency also promotes the development of mouse mammary cancer.Together, these observations demonstrate an important role for Perp in normal mammary tissue function and in mammary cancer suppression. In addition, our findings highlight the importance of desmosomes in cancer suppression and suggest the merit of evaluating Perp as a potential prognostic indicator or molecular target in breast cancer therapy.
View details for DOI 10.1186/bcr3171
View details for Web of Science ID 000304771800040
View details for PubMedID 22515648
Full p53 transcriptional activation potential is dispensable for tumor suppression in diverse lineages
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2011; 108 (41): 17123-17128
Over half of all human cancers, of a wide variety of types, sustain mutations in the p53 tumor suppressor gene. Although p53 limits tumorigenesis through the induction of apoptosis or cell cycle arrest, its molecular mechanism of action in tumor suppression has been elusive. The best-characterized p53 activity in vitro is as a transcriptional activator, but the identification of numerous additional p53 biochemical activities in vitro has made it unclear which mechanism accounts for tumor suppression. Here, we assess the importance of transcriptional activation for p53 tumor suppression function in vivo in several tissues, using a knock-in mouse strain expressing a p53 mutant compromised for transcriptional activation, p53(25,26). p53(25,26) is severely impaired for the transactivation of numerous classical p53 target genes, including p21, Noxa, and Puma, but it retains the ability to activate a small subset of p53 target genes, including Bax. Surprisingly, p53(25,26) can nonetheless suppress tumor growth in cancers derived from the epithelial, mesenchymal, central nervous system, and lymphoid lineages. Therefore, full transactivation of most p53 target genes is dispensable for p53 tumor suppressor function in a range of tissue types. In contrast, a transcriptional activation mutant that is completely defective for transactivation, p53(25,26,53,54), fails to suppress tumor development. These findings demonstrate that transcriptional activation is indeed broadly critical for p53 tumor suppressor function, although this requirement reflects the limited transcriptional activity observed with p53(25,26) rather than robust transactivation of a full complement of p53 target genes.
View details for DOI 10.1073/pnas.1111245108
View details for Web of Science ID 000295973800051
View details for PubMedID 21969549
The pro-longevity gene FoxO3 is a direct target of the p53 tumor suppressor
2011; 30 (29): 3207-3221
FoxO transcription factors have a conserved role in longevity, and act as tissue-specific tumor suppressors in mammals. Several nodes of interaction have been identified between FoxO transcription factors and p53, a major tumor suppressor in humans and mice. However, the extent and importance of the functional interaction between FoxO and p53 have not been fully explored. Here, we show that p53 regulates the expression of FoxO3, one of the four mammalian FoxO genes, in response to DNA damaging agents in both mouse embryonic fibroblasts and thymocytes. We find that p53 transactivates FoxO3 in cells by binding to a site in the second intron of the FoxO3 gene, a genomic region recently found to be associated with extreme longevity in humans. While FoxO3 is not necessary for p53-dependent cell cycle arrest, FoxO3 appears to modulate p53-dependent apoptosis. We also find that FoxO3 loss does not interact with p53 loss for tumor development in vivo, although the tumor spectrum of p53-deficient mice appears to be affected by FoxO3 loss. Our findings indicate that FoxO3 is a p53 target gene, and suggest that FoxO3 and p53 are part of a regulatory transcriptional network that may have an important role during aging and cancer.
View details for DOI 10.1038/onc.2011.35
View details for Web of Science ID 000293006800001
View details for PubMedID 21423206
Distinct p53 Transcriptional Programs Dictate Acute DNA-Damage Responses and Tumor Suppression
2011; 145 (4): 571-583
The molecular basis for p53-mediated tumor suppression remains unclear. Here, to elucidate mechanisms of p53 tumor suppression, we use knockin mice expressing an allelic series of p53 transcriptional activation mutants. Microarray analysis reveals that one mutant, p53(25,26), is severely compromised for transactivation of most p53 target genes, and, moreover, p53(25,26) cannot induce G(1)-arrest or apoptosis in response to acute DNA damage. Surprisingly, p53(25,26) retains robust activity in senescence and tumor suppression, indicating that efficient transactivation of the majority of known p53 targets is dispensable for these pathways. In contrast, the transactivation-dead p53(25,26,53,54) mutant cannot induce senescence or inhibit tumorigenesis, like p53 nullizygosity. Thus, p53 transactivation is essential for tumor suppression but, intriguingly, in association with a small set of novel p53 target genes. Together, our studies distinguish the p53 transcriptional programs involved in acute DNA-damage responses and tumor suppression-a critical goal for designing therapeutics that block p53-dependent side effects of chemotherapy without compromising p53 tumor suppression.
View details for DOI 10.1016/j.cell.2011.03.035
View details for Web of Science ID 000290560800010
View details for PubMedID 21565614
Desmosomes: new perpetrators in tumour suppression
NATURE REVIEWS CANCER
2011; 11 (5): 317-323
Adherens junctions, which are intercellular adhesive complexes that are crucial for maintaining epithelial homeostasis, are downregulated in many cancers to promote tumour progression. However, the role of desmosomes - adhesion complexes that are related to adherens junctions - in carcinogenesis has remained elusive. Recent studies using mouse genetic approaches have uncovered a role for desmosomes in tumour suppression, demonstrating that desmosome downregulation occurs before that of adherens junctions to drive tumour development and early invasion, suggesting a two-step model of adhesion dysfunction in cancer progression.
View details for DOI 10.1038/nrc3051
View details for Web of Science ID 000289779800009
View details for PubMedID 21508970
PERP regulates enamel formation via effects on cell-cell adhesion and gene expression
JOURNAL OF CELL SCIENCE
2011; 124 (5): 745-754
Little is known about the role of cell-cell adhesion in the development of mineralized tissues. Here we report that PERP, a tetraspan membrane protein essential for epithelial integrity, regulates enamel formation. PERP is necessary for proper cell attachment and gene expression during tooth development, and its expression is controlled by P63, a master regulator of stratified epithelial development. During enamel formation, PERP is localized to the interface between the enamel-producing ameloblasts and the stratum intermedium (SI), a layer of cells subjacent to the ameloblasts. Perp-null mice display dramatic enamel defects, which are caused, in part, by the detachment of ameloblasts from the SI. Microarray analysis comparing gene expression in teeth of wild-type and Perp-null mice identified several differentially expressed genes during enamel formation. Analysis of these genes in ameloblast-derived LS8 cells upon knockdown of PERP confirmed the role for PERP in the regulation of gene expression. Together, our data show that PERP is necessary for the integrity of the ameloblast-SI interface and that a lack of Perp causes downregulation of genes that are required for proper enamel formation.
View details for DOI 10.1242/jcs.078071
View details for Web of Science ID 000287307000010
View details for PubMedID 21285247
Loss of the p53/p63 Regulated Desmosomal Protein Perp Promotes Tumorigenesis
2010; 6 (10)
Dysregulated cell-cell adhesion plays a critical role in epithelial cancer development. Studies of human and mouse cancers have indicated that loss of adhesion complexes known as adherens junctions contributes to tumor progression and metastasis. In contrast, little is known regarding the role of the related cell-cell adhesion junction, the desmosome, during cancer development. Studies analyzing expression of desmosome components during human cancer progression have yielded conflicting results, and therefore genetic studies using knockout mice to examine the functional consequence of desmosome inactivation for tumorigenesis are essential for elucidating the role of desmosomes in cancer development. Here, we investigate the consequences of desmosome loss for carcinogenesis by analyzing conditional knockout mice lacking Perp, a p53/p63 regulated gene that encodes an important component of desmosomes. Analysis of Perp-deficient mice in a UVB-induced squamous cell skin carcinoma model reveals that Perp ablation promotes both tumor initiation and progression. Tumor development is associated with inactivation of both of Perp's known functions, in apoptosis and cell-cell adhesion. Interestingly, Perp-deficient tumors exhibit widespread downregulation of desmosomal constituents while adherens junctions remain intact, suggesting that desmosome loss is a specific event important for tumorigenesis rather than a reflection of a general change in differentiation status. Similarly, human squamous cell carcinomas display loss of PERP expression with retention of adherens junctions components, indicating that this is a relevant stage of human cancer development. Using gene expression profiling, we show further that Perp loss induces a set of inflammation-related genes that could stimulate tumorigenesis. Together, these studies suggest that Perp-deficiency promotes cancer by enhancing cell survival, desmosome loss, and inflammation, and they highlight a fundamental role for Perp and desmosomes in tumor suppression. An understanding of the factors affecting cancer progression is important for ultimately improving the diagnosis, prognostication, and treatment of cancer.
View details for DOI 10.1371/journal.pgen.1001168
View details for Web of Science ID 000283647800022
View details for PubMedID 20975948
A Large Intergenic Noncoding RNA Induced by p53 Mediates Global Gene Repression in the p53 Response
2010; 142 (3): 409-419
Recently, more than 1000 large intergenic noncoding RNAs (lincRNAs) have been reported. These RNAs are evolutionarily conserved in mammalian genomes and thus presumably function in diverse biological processes. Here, we report the identification of lincRNAs that are regulated by p53. One of these lincRNAs (lincRNA-p21) serves as a repressor in p53-dependent transcriptional responses. Inhibition of lincRNA-p21 affects the expression of hundreds of gene targets enriched for genes normally repressed by p53. The observed transcriptional repression by lincRNA-p21 is mediated through the physical association with hnRNP-K. This interaction is required for proper genomic localization of hnRNP-K at repressed genes and regulation of p53 mediates apoptosis. We propose a model whereby transcription factors activate lincRNAs that serve as key repressors by physically associating with repressive complexes and modulate their localization to sets of previously active genes.
View details for DOI 10.1016/j.cell.2010.06.040
View details for Web of Science ID 000280609100018
View details for PubMedID 20673990
- p53 at a glance JOURNAL OF CELL SCIENCE 2010; 123 (15): 2527-2532
In vivo analysis of p53 tumor suppressor function using genetically engineered mouse models
2010; 31 (8): 1311-1318
p53 is a crucial tumor suppressor, as evidenced by the high propensity for p53 mutation during human cancer development. Already more than a decade ago, p53 knockout mice confirmed that p53 is critical for preventing tumorigenesis. More recently, a host of p53 knock-in mouse strains has been generated, with the aim of either more precisely modeling p53 mutations in human cancer or better understanding p53's regulation and downstream activities. In the first category, several mouse strains expressing mutant p53 proteins corresponding to human-tumor-derived mutants have demonstrated that mutant p53 is not equivalent to loss of p53 but additionally exhibits gain-of-function properties, promoting invasive and metastatic phenotypes. The second class of p53 knock-in mouse models expressing engineered p53 mutants has also provided new insight into p53 function. For example, mice expressing p53 mutants lacking specific posttranslational modification sites have revealed that these modifications serve to modulate p53 responses in vivo in a cell-type- and stress-specific manner rather than being absolutely required for p53 stabilization and activation as suggested by in vitro experiments. Additionally, studies of p53 mouse models have established that both p53-driven cell-cycle arrest and apoptosis responses contribute to tumor suppression and that activation of p53 by oncogenic stress imposes an important barrier to tumorigenesis. Finally, the use of mouse strains expressing temporally regulatable p53 has demonstrated that p53 loss is not only required for tumor development but also required for tumor maintenance, suggesting that p53 restoration in human cancer patients may be a promising therapeutic strategy. These sophisticated p53 mouse models have taught us important lessons, and new mouse models will certainly continue to reveal interesting and perhaps surprising aspects of p53's complex biology.
View details for DOI 10.1093/carcin/bgp331
View details for Web of Science ID 000280703800001
View details for PubMedID 20097732
Loss of the desmosomal component perp impairs wound healing in vivo.
Dermatology research and practice
2010; 2010: 759731-?
Epithelial wound closure is a complex biological process that relies on the concerted action of activated keratinocytes and dermal fibroblasts to resurface and close the exposed wound. Modulation of cell-cell adhesion junctions is thought to facilitate cellular proliferation and migration of keratinocytes across the wound. In particular, desmosomes, adhesion complexes critical for maintaining epithelial integrity, are downregulated at the wound edge. It is unclear, however, how compromised desmosomal adhesion would affect wound reepithelialization, given the need for a delicate balance between downmodulating adhesive strength to permit changes in cellular morphology and maintaining adhesion to allow coordinated migration of keratinocyte sheets. Here, we explore the contribution of desmosomal adhesion to wound healing using mice deficient for the desmosomal component Perp. We find that Perp conditional knockout mice display delayed wound healing relative to controls. Furthermore, we determine that while loss of Perp compromises cell-cell adhesion, it does not impair keratinocyte proliferation and actually enhances keratinocyte migration in in vitro assays. Thus, Perp's role in promoting cell adhesion is essential for wound closure. Together, these studies suggest a role for desmosomal adhesion in efficient wound healing.
View details for DOI 10.1155/2010/759731
View details for PubMedID 20628490
Differential PERP Regulation by TP63 Mutants Provides Insight Into AEC Pathogenesis
AMERICAN JOURNAL OF MEDICAL GENETICS PART A
2009; 149A (9): 1952-1957
Ankyloblepharon Ectodermal Dysplasia and Cleft Lip/Palate (AEC) or Hay-Wells Syndrome is an autosomal dominant disorder characterized by a variety of phenotypes in ectodermal derivatives, including severe skin erosions, ankyloblepharon, coarse and wiry hair, scalp dermatitis, and dystrophic nails. AEC is caused by mutations in the gene encoding the TP63 transcription factor, specifically in the Sterile Alpha Motif (SAM) domain. The exact mechanism, however, by which these specific TP63 mutations lead to the observed spectrum of phenotypes is unclear. Analysis of individual TP63 target genes provides a means to understand specific aspects of the phenotypes associated with AEC. PERP is a TP63 target critical for cell-cell adhesion due to its participation in desmosomal adhesion complexes. As PERP null mice display symptoms characteristic of ectodermal dysplasia syndromes, we hypothesized that PERP dysfunction might contribute to AEC. Using luciferase reporter assays, we demonstrate here that PERP induction is in fact compromised with some, but not all, AEC-patient derived TP63 mutants. Through analysis of skin biopsies from AEC patients, we show further that a subset of these display aberrant PERP expression, suggesting the possibility that PERP dysregulation is involved in the pathogenesis of this disease. These findings demonstrate that distinct AEC TP63 mutants can differentially compromise expression of downstream targets, providing a rationale for the variable spectra of symptoms seen in AEC patients. Elucidating how specific TP63 target genes contribute to the pathogenesis of AEC will ultimately help design novel approaches to diagnose and treat AEC.
View details for DOI 10.1002/ajmg.a.32760
View details for Web of Science ID 000269678800013
View details for PubMedID 19353588
SKP-ing TAp63: Stem Cell Depletion, Senescence, and Premature Aging
CELL STEM CELL
2009; 5 (1): 1-2
The p53 family member p63 comprises multiple isoforms and is critical for stratified epithelial development. In this issue of Cell Stem Cell, by generating isoform-specific knockout mice, Su et al. (2009) reveal pivotal roles for TAp63 in the maintenance of dermal and epidermal precursors, genomic stability, and organismal longevity.
View details for DOI 10.1016/j.stem.2009.06.015
View details for Web of Science ID 000267879200001
View details for PubMedID 19570504
Loss of the Desmosomal Protein Perp Enhances the Phenotypic Effects of Pemphigus Vulgaris Autoantibodies
JOURNAL OF INVESTIGATIVE DERMATOLOGY
2009; 129 (7): 1710-1718
Pemphigus vulgaris (PV) is an autoimmune bullous disease in which autoantibodies against proteins of the desmosomal adhesion complex perturb desmosomal function, leading to intercellular adhesion defects in the oral mucosa and skin. Previous studies have demonstrated a central role for downregulation of the desmosomal cadherin desmoglein 3 (DSG3) in the pathogenesis of PV. However, the effects of non-cadherin desmosomal proteins in modulating the cellular manifestations of PV remain poorly understood. Here, we characterize the expression and functional importance of Perp, a newly discovered tetraspan desmosomal protein, in PV. Our data demonstrate that PV autoantibodies disrupt Perp expression at the membrane and trigger its internalization along with DSG3 into the endosomal pathway, where it is ultimately targeted to the lysosome for degradation. We further show that Perp deficiency exacerbates the pathogenic effects of PV autoantibodies on keratinocytes by enhancing both the depletion of desmosomal DSG3 and intercellular adhesion defects. Together, our findings highlight the importance of non-cadherin desmosomal proteins in modulating PV phenotypes and provide new insight into Perp's role in the desmosome.
View details for DOI 10.1038/jid.2008.419
View details for Web of Science ID 000267270300019
View details for PubMedID 19158843
Ribosomal mutations cause p53-mediated dark skin and pleiotropic effects
2008; 40 (8): 963-970
Mutations in genes encoding ribosomal proteins cause the Minute phenotype in Drosophila and mice, and Diamond-Blackfan syndrome in humans. Here we report two mouse dark skin (Dsk) loci caused by mutations in Rps19 (ribosomal protein S19) and Rps20 (ribosomal protein S20). We identify a common pathophysiologic program in which p53 stabilization stimulates Kit ligand expression, and, consequently, epidermal melanocytosis via a paracrine mechanism. Accumulation of p53 also causes reduced body size and erythrocyte count. These results provide a mechanistic explanation for the diverse collection of phenotypes that accompany reduced dosage of genes encoding ribosomal proteins, and have implications for understanding normal human variation and human disease.
View details for DOI 10.1038/ng.188
View details for Web of Science ID 000258026900012
View details for PubMedID 18641651
The metastasis-associated gene Prl-3 is a p53 target involved in cell-cycle regulation
2008; 30 (3): 303-314
The p53 tumor suppressor restricts tumorigenesis through the transcriptional activation of target genes involved in cell-cycle arrest and apoptosis. Here, we identify Prl-3 (phosphatase of regenerating liver-3) as a p53-inducible gene. Whereas previous studies implicated Prl-3 in metastasis because of its overexpression in metastatic human colorectal cancer and its ability to promote invasiveness and motility, we demonstrate here that Prl-3 is an important cell-cycle regulator. Consistent with a role in DNA damage-induced cell-cycle arrest, Prl-3 overexpression induces G(1) arrest downstream of p53 by triggering a PI3K-Akt-activated negative feedback loop. Surprisingly, attenuation of Prl-3 expression also elicits an arrest response, suggesting that basal level Prl-3 expression is pivotal for normal cell-cycle progression. Our findings highlight key dose-dependent functions of Prl-3 in both positive and negative regulation of cell-cycle progression and provide insight into Prl-3's role in cancer progression.
View details for DOI 10.1016/j.molcel.2008.04.002
View details for Web of Science ID 000255761200008
View details for PubMedID 18471976
Knockin mice expressing a Chimeric p53 protein reveal mechanistic differences in how p53 triggers apoptosis and senescence
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2008; 105 (4): 1215-1220
The contribution of transcriptional activation to the p53 effector functions critical for tumor suppression, apoptosis and cellular senescence, remains unclear because of p53's ability to regulate diverse cellular processes in a transactivation-independent manner. Dissociating the importance of transactivation from other p53 functions, including regulating transcriptional repression, DNA replication, homologous recombination, centrosome duplication, and mitochondrial function, has been difficult because of overlapping motifs for these functions in the amino terminus. To determine the relative contribution of these activities and transactivation to p53 function, we generated knockin mice expressing a p53 mutant lacking domains involved in these transactivation-independent functions, while remaining competent for transactivation through fusion to the Herpes Simplex Virus VP16 transactivation domain. This chimeric mutant, termed p53(VP16), robustly activates the transcription of a range of p53 targets involved in both apoptosis and senescence. Intriguingly, despite being transactivation-competent, this chimeric protein shows selectivity in p53 effector function in mouse fibroblasts, with a capacity to trigger senescence but not apoptosis under a variety of conditions. Our study highlights the central role of p53 transactivation for senescence while suggesting that transactivation is insufficient for apoptosis, and provides insight into the mechanisms by which p53 serves as a tumor suppressor.
View details for DOI 10.1073/pnas.0706764105
View details for Web of Science ID 000252873900025
View details for PubMedID 18216268
Siva is an apoptosis-selective p53 target gene important for neuronal cell death
CELL DEATH AND DIFFERENTIATION
2007; 14 (7): 1374-1385
p53 plays a central role in neuronal cell death resulting from acute injury or disease. To define the pathway by which p53 triggers apoptosis, we used microarray analysis to identify p53 target genes specifically upregulated during apoptosis but not cell cycle arrest. This analysis identified a small subset of targets highly selective for the p53 apoptotic response, including Siva, a proapoptotic protein whose function is not well understood. Siva's expression pattern suggests that it plays an instructive role in apoptosis, and accordingly, we demonstrate that Siva is essential for p53-dependent apoptosis in cerebellar granule neurons. In addition, we determine that endogenous Siva is associated with the plasma membrane and that Caspase-8 and Bid are important for neuronal apoptosis. Our studies highlight the participation of membrane signaling events in p53's apoptotic program in primary neurons and have significant implications for understanding the mechanisms underlying pathogenesis after neuronal injury and in neurodegenerative diseases.
View details for DOI 10.1038/sj.cdd.4402128
View details for Web of Science ID 000247373400020
View details for PubMedID 17464332
A healthy tan?
New England journal of medicine
2007; 356 (21): 2208-2210
View details for PubMedID 17522405
Dominant-negative but not gain-of-function effects of a p53.R270H mutation in mouse epithelium tissue after DNA damage
2007; 67 (10): 4648-4656
p53 alterations in human tumors often involve missense mutations that may confer dominant-negative or gain-of-function properties. Dominant-negative effects result in inactivation of wild-type p53 protein in heterozygous mutant cells and as such in a p53 null phenotype. Gain-of-function effects can directly promote tumor development or metastasis through antiapoptotic mechanisms or transcriptional activation of (onco)genes. Here, we show, using conditional mouse technology, that epithelium-specific heterozygous expression of mutant p53 (i.e., the p53.R270H mutation that is equivalent to the human hotspot R273H) results in an increased incidence of spontaneous and UVB-induced skin tumors. Expression of p53.R270H exerted dominant-negative effects on latency, multiplicity, and progression status of UVB-induced but not spontaneous tumors. Surprisingly, gain-of-function properties of p53.R270H were not detected in skin epithelium. Apparently, dominant-negative and gain-of-function effects of mutant p53 are highly tissue specific and become most manifest upon stabilization of p53 after DNA damage.
View details for DOI 10.1158/0008-5472.CAN-06-4681
View details for Web of Science ID 000246778500012
View details for PubMedID 17510390
P63, cell adhesion and survival
2007; 6 (3): 255-261
The development of stratified epithelia and their derivatives is a complex process requiring a multifaceted transcriptional program. p63, the p53-related transcription factor, is fundamental to this process. However, the underlying mechanisms by which p63 exerts its influence on stratified epithelial development and integrity remain elusive. Recent work from our laboratories has demonstrated that p63 mediates its effects on stratified epithelial function at least in part via its ability to regulate multiple aspects of epithelial cellular adhesion and survival. The identification of cell-cell and cell-matrix adhesion subprograms downstream of p63 provides an initial understanding of p63's role in epithelial development, integrity and homeostasis.
View details for Web of Science ID 000245495300005
View details for PubMedID 17297292
- Adult mice lacking the p53/p63 target gene Perp are not predisposed to spontaneous tumorigenesis but display features of ectodermal dysplasia syndromes CELL DEATH AND DIFFERENTIATION 2006; 13 (9): 1614-1618
- Dissecting p53 tumor suppressor function in vivo through the analysis of genetically modified mice CELL DEATH AND DIFFERENTIATION 2006; 13 (6): 902-908
Genome-wide analysis of p53 under hypoxic conditions
MOLECULAR AND CELLULAR BIOLOGY
2006; 26 (9): 3492-3504
Hypoxia is an important nongenotoxic stress that modulates the tumor suppressor activity of p53 during malignant progression. In this study, we investigated how genotoxic and nongenotoxic stresses regulate p53 association with chromatin, p53 transcriptional activity, and p53-dependent apoptosis. We found that genotoxic and nongenotoxic stresses result in the accumulation and binding of the p53 tumor suppressor protein to the same cognate binding sites in chromatin. However, it is the stress that determines whether downstream signaling is mediated by association with transcriptional coactivators. In contrast to p53 induced by DNA-damaging agents, hypoxia-induced p53 has primarily transrepression activity. Using extensive microarray analysis, we identified families of repressed targets of p53 that are involved in cell signaling, DNA repair, cell cycle control, and differentiation. Following our previous study on the contribution of residues 25 and 26 to p53-dependent hypoxia-induced apoptosis, we found that residues 25-26 and 53-54 and the polyproline- and DNA-binding regions are also required for both gene repression and the induction of apoptosis by p53 during hypoxia. This study defines a new role for residues 53 and 54 of p53 in regulating transrepression and demonstrates that 25-26 and 53-54 work in the same pathway to induce apoptosis through gene repression.
View details for DOI 10.1128/MCB.26.9.3492-3504.2006
View details for Web of Science ID 000236993300013
View details for PubMedID 16611991
The requirement for Perp in postnatal viability and epithelial integrity reflects an intrinsic role in stratified epithelia
JOURNAL OF INVESTIGATIVE DERMATOLOGY
2006; 126 (1): 69-73
Mice lacking the desmosome protein Perp exhibit blistering in their stratified epithelia and display postnatal lethality. However, it is unclear if these phenotypes are strictly related to Perp function in stratified epithelia, as Perp expression is not restricted to these tissues during embryogenesis, and certain desmosomal blistering diseases such as pemphigus vulgaris and pemphigus foliaceus have non-cell-intrinsic bases. Furthermore, we show here that Perp is expressed in the heart, raising the possibility that defects in heart function could account for lethality in the Perp-deficient mice. To determine conclusively if Perp function in stratified epithelia is crucial for postnatal survival and epithelial adhesion, we specifically ablated Perp in stratified epithelia by breeding conditional Perp knockout mice to keratin 5 (K5)-Cre transgenic mice. We found that the majority of mice lacking Perp in stratified epithelia die within 10 days after birth, accompanied by blistering and hyperproliferation in the epithelia, similar to the constitutive Perp null mice. Together, these findings indicate that Perp's requirement for both viability and epithelial integrity reflects a role in the stratified epithelial compartment.
View details for DOI 10.1038/sj.jid.5700032
View details for Web of Science ID 000238943900014
View details for PubMedID 16417219
Probing p53 biological functions through the use of genetically engineered mouse models
MUTATION RESEARCH-FUNDAMENTAL AND MOLECULAR MECHANISMS OF MUTAGENESIS
2005; 576 (1-2): 4-21
The p53 tumor suppressor gene is rendered dysfunctional in the majority of human cancers. To model the effects of p53 dysfunction in an experimentally manipulable organismal context, genetically engineered inbred mice have been the models of choice. Transgenic and knock-out technologies have been utilized to generate an array of different p53 germ line alterations. As expected, many (though not all) of the mutant p53 mouse models are susceptible to enhanced spontaneous and carcinogen-induced tumors of a variety of types. A number of different variables affect the incidence and spectrum of tumors in p53 mutant mice. These include strain background, the nature of the p53 mutation, the presence of wild-type p53 (in addition to mutant p53), exposure to physical and chemical mutagens, or introduction of other cancer-associated genes into the mutant p53 background. In addition to their role in furthering our understanding of the mechanisms of cancer initiation and progression, these models have led to unexpected insights into p53 function in embryogenesis and aging. With the development of ever more sophisticated methods for manipulating the mouse genome, new p53 models are on the horizon, which should deliver advances that will provide not only important mechanistic insights but also discoveries of great clinical relevance.
View details for DOI 10.1016/j.mrfmmm.2004.08.022
View details for Web of Science ID 000231100900002
View details for PubMedID 16038709
Mice lacking the p53/p63 target gene perp are resistant to papilloma development
2005; 65 (15): 6551-6556
Perp is a target of the p53 tumor suppressor involved in the DNA damage-induced apoptosis pathway. In addition, Perp is a target of the p53-related transcription factor p63 during skin development, where it participates in cell-cell adhesion mediated through desmosomes. Here we test the role of Perp in tumorigenesis in a two-step skin carcinogenesis model system. We find that mice lacking Perp in the skin are resistant to papilloma development, displaying fewer and smaller papillomas than wild-type mice. Proliferation levels, apoptotic indices and differentiation patterns are similar in the skin of treated Perp-deficient and wild-type mice. Instead, impaired adhesion through aberrant desmosome assembly may explain the diminished tumor development in the absence of Perp. These studies indicate that in certain contexts, Perp is required for efficient carcinogenesis and suggest a role for intact cell-cell adhesion in supporting tumor development in these settings.
View details for Web of Science ID 000230837900013
View details for PubMedID 16061634
A new Perp in the lineup - Linking p63 and desmosomal adhesion
2005; 4 (7): 873-876
The process of stratified epithelial development depends upon a transcriptional program directed by the p53-related transcription factor p63. p63 is required for the commitment of the ectoderm to stratification and for the completion of terminal differentiation in stratified epithelia, and mutations in p63 have been identified in multiple developmental disorders affecting ectoderm-derived tissues. Recent work from our laboratory has determined that the p53 target gene Perp is required for the integrity of the stratified epithelia specified by p63, and that expression of Perp in these structures depends on the presence of p63. In these tissues, Perp is a critical component of the desmosome, a cell-cell adhesion complex whose constituents are frequently mutated in human diseases affecting the skin and hair. Perp's position downstream of p63 and p53, as well as its essential role in normal desmosome function, suggest that it, like other adhesion proteins, may be a target for mutation in human blistering diseases or cancer.
View details for Web of Science ID 000230986600007
View details for PubMedID 15970683
Pathways connecting telomeres and p53 in senescence, apoptosis, and cancer
BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS
2005; 331 (3): 881-890
The ends of eukaryotic chromosomes are protected by specialized structures termed telomeres that serve in part to prevent the chromosome end from activating a DNA damage response. However, this important function for telomeres in chromosome end protection can be lost as telomeres shorten with cell division in culture or in self-renewing tissues with advancing age. Impaired telomere function leads to induction of a DNA damage response and activation of the tumor suppressor protein p53. p53 serves a critical role in enforcing both senescence and apoptotic responses to dysfunctional telomeres. Loss of p53 creates a permissive environment in which critically short telomeres are inappropriately joined to generate chromosomal end-to-end fusions. These fused chromosomes result in cycles of chromosome fusion-bridge-breakage, which can fuel cancer initiation, especially in epithelial tissues, by facilitating changes in gene copy number.
View details for Web of Science ID 000229135900023
View details for PubMedID 15865944
p53(QS) - An old mutant teaches us new tricks
2005; 4 (6): 731-734
The p53 protein functions as a tumor suppressor, preventing aberrant cellular proliferation in response to various genotoxic and non-genotoxic stress signals. Although p53's ability to induce apoptosis is critical to its capacity to suppress tumorigenesis, the role of transcriptional activation in p53's apoptotic function has been highly controversial. To address this issue, our laboratory generated a p53 mutant knock-in mouse strain in which residues 25 and 26, previously shown to be critical for p53's transactivation function, were mutated from leucine and tryptophan to glutamine and serine, respectively. Our analysis of cells derived from these mice provided significant insight into p53 activity at both the molecular and cellular level. In particular, our data suggest that p53 utilizes discrete mechanisms to transactivate different target genes, and that p53 employs distinct mechanisms to induce apoptosis in response to different stresses. This p53QS mutant mouse strain represents a powerful means to dissect p53 function in vivo.
View details for Web of Science ID 000229658100001
View details for PubMedID 15908788
Perp is a p63-regulated gene essential for epithelial integrity
2005; 120 (6): 843-856
p63 is a master regulator of stratified epithelial development that is both necessary and sufficient for specifying this multifaceted program. We show here that Perp, a tetraspan membrane protein originally identified as an apoptosis-associated target of the p53 tumor suppressor, is the first direct target of p63 clearly involved in mediating this developmental program in vivo. During embryogenesis, Perp is expressed in an epithelial pattern, and its expression depends on p63. Perp-/- mice die postnatally, with dramatic blistering in stratified epithelia symptomatic of compromised adhesion. Perp localizes specifically to desmosomes, adhesion junctions important for tissue integrity, and numerous structural defects in desmosomes are observed in Perp-deficient skin, suggesting a role for Perp in promoting the stable assembly of desmosomal adhesive complexes. These findings demonstrate that Perp is a key effector in the p63 developmental program, playing an essential role in an adhesion subprogram central to epithelial integrity and homeostasis.
View details for Web of Science ID 000228067500013
View details for PubMedID 15797384
Opinion - The role of apoptosis in cancer development and treatment response
NATURE REVIEWS CANCER
2005; 5 (3): 231-237
The inactivation of programmed cell death, or apoptosis, is central to the development of cancer. This disabling of apoptotic responses might be a major contributor both to treatment resistance and to the observation that, in many tumours, apoptosis is not the main mechanism for the death of cancer cells in response to common treatment regimens. Importantly, this suggests that other modes of cell death are involved in the response to therapy.
View details for DOI 10.1038/nrc1570
View details for Web of Science ID 000227302700017
View details for PubMedID 15738985
The p53(QS) transactivation-deficient mutant shows stress-specific apoptotic activity and induces embryonic lethality
2005; 37 (2): 145-152
The role of transcriptional activation in p53 function is highly controversial. To define this role in vivo, we generated a Trp53 knock-in construct encoding a protein carrying mutations of two residues that are crucial for transactivation (L25Q,W26S). Here we show that these mutations have selective effects on the biological functions of p53. Although its ability to activate various p53 target genes is largely compromised, the p53(QS) protein retains the ability to transactivate the gene Bax. The ability of the p53(QS) mutant protein to elicit a DNA damage-induced G1 cell cycle-arrest response is also partially impaired. p53(QS) has selective defects in its ability to induce apoptosis: it is completely unable to activate apoptosis in response to DNA damage, is partially unable to do so when subjected to serum deprivation and retains substantial apoptotic activity upon exposure to hypoxia. These findings suggest that p53 acts through distinct, stimulus-specific pathways to induce apoptosis. The importance of the biological activity of p53(QS) in vivo is underscored by our finding that expression of p53(QS), which cannot bind mdm2, induces embryonic lethality. Taken together, these results suggest that p53 has different mechanisms of action depending on specific contextual cues, which may help to clarify the function of p53 in preventing cancer.
View details for DOI 10.1038/ng1498
View details for Web of Science ID 000226690100020
View details for PubMedID 15654339
The role of p53-mediated apoptosis as a crucial anti-tumor response to genomic instability: lessons from mouse models
MUTATION RESEARCH-FUNDAMENTAL AND MOLECULAR MECHANISMS OF MUTAGENESIS
2005; 569 (1-2): 145-157
Genomic instability is a major force driving human cancer development. A cellular safeguard against such genetic destabilization, which can ensue from defects in telomere maintenance, DNA repair, and checkpoint function, is activation of the p53 tumor suppressor protein, which commonly responds to these DNA damage signals by inducing apoptosis. If, however, p53 becomes inactivated, as is typical of many tumors and pre-cancerous lesions, then cells with compromised genome integrity pathways survive inappropriately, and the accrual of oncogenic lesions can fuel the carcinogenic process. Studies of mouse models have been instrumental in providing support for this idea. Mouse knockouts in genes important for telomere function, DNA damage checkpoint activation and DNA repair - both non-homologous end joining and homologous recombination - are prone to the development of genomic instability. As a consequence of these DNA damage signals, p53 becomes activated in cells of these mutant mice, leading to the induction of apoptosis, sometimes at the expense of organismal viability. This apoptotic response can be rescued through crosses to p53-deficient mice, but has dire consequences: mice predisposed to genomic instability and lacking p53 are susceptible to tumorigenesis. Thus p53-mediated apoptosis provides a crucial tumor suppressive mechanism to eliminate cells succumbing to genomic instability.
View details for DOI 10.1016/j.mfrmmm.2004.04.019
View details for Web of Science ID 000226211100012
View details for PubMedID 15603759
Developmental context determines latency of MYC-induced tumorigenesis
2004; 2 (11): 1785-1798
One of the enigmas in tumor biology is that different types of cancers are prevalent in different age groups. One possible explanation is that the ability of a specific oncogene to cause tumorigenesis in a particular cell type depends on epigenetic parameters such as the developmental context. To address this hypothesis, we have used the tetracycline regulatory system to generate transgenic mice in which the expression of a c-MYC human transgene can be conditionally regulated in murine hepatocytes. MYC's ability to induce tumorigenesis was dependent upon developmental context. In embryonic and neonatal mice, MYC overexpression in the liver induced marked cell proliferation and immediate onset of neoplasia. In contrast, in adult mice MYC overexpression induced cell growth and DNA replication without mitotic cell division, and mice succumbed to neoplasia only after a prolonged latency. In adult hepatocytes, MYC activation failed to induce cell division, which was at least in part mediated through the activation of p53. Surprisingly, apoptosis is not a barrier to MYC inducing tumorigenesis. The ability of oncogenes to induce tumorigenesis may be generally restrained by developmentally specific mechanisms. Adult somatic cells have evolved mechanisms to prevent individual oncogenes from initiating cellular growth, DNA replication, and mitotic cellular division alone, thereby preventing any single genetic event from inducing tumorigenesis.
View details for DOI 10.1371/journal.pbio.0020332
View details for Web of Science ID 000225160300011
View details for PubMedID 15455033
Increased sensitivity to UV radiation in mice with a p53 point mutation at Ser389
MOLECULAR AND CELLULAR BIOLOGY
2004; 24 (20): 8884-8894
Phosphorylation is important for p53 protein stabilization and activation after DNA damage. Serine 389 of p53 is specifically phosphorylated after UV irradiation, whereas gamma radiation activates p53 through a different pathway. To study the in vivo significance of p53 phosphorylation at serine 389, we generated a physiological mouse model in which p53 phosphorylation at serine 389 is abolished by alanine substitution. Homozygous mutant p53.S389A mice are viable and have an apparently normal phenotype. However, cells isolated from these mice are partly compromised in transcriptional activation of p53 target genes and apoptosis after UV irradiation, whereas gamma radiation-induced responses are not affected. Moreover, p53.S389A mice show increased sensitivity to UV-induced skin tumor development, signifying the importance of serine 389 phosphorylation for the tumor-suppressive function of p53.
View details for DOI 10.1128/mcb.24.20.8884-8894.2004
View details for Web of Science ID 000224239100008
View details for PubMedID 15456863
Perp-etrating p53-dependent apoptosis
2004; 3 (3): 267-269
The induction of apoptosis is a fundamental mechanism by which the p53 transcriptional activator protein suppresses tumor development. Recently, the roles of several p53 target genes in mediating the p53 apoptotic response have been queried through loss-of-function analysis with knockout mouse models. These studies have demonstrated that the p53 targets Noxa, Puma, and Perp play cell type-specific roles in p53-mediated apoptosis. Perp, a tetraspan protein localizing to the plasma membrane, rather than to mitochondria, is a novel type of p53 effector that may stimulate apoptosis through a different mechanism from the BH3-containing proteins Noxa, Puma, and Bax.
View details for Web of Science ID 000222361400012
View details for PubMedID 14726658
Activation of the p53-dependent G1 checkpoint response in mouse embryo fibroblasts depends on the specific DNA damage inducer
2004; 23 (4): 973-980
The p53 tumor suppressor protein inhibits proliferation by inducing either cell cycle arrest or apoptosis in response to cellular stresses. Mouse embryo fibroblasts (MEFs) provide a primary cell model system in which to examine both functions of p53. MEFs treated with gamma-rays undergo p53-dependent G1 arrest, while oncogene-expressing MEFs treated with a variety of DNA-damaging agents undergo p53-dependent apoptosis. Although the p53-dependent G1 arrest checkpoint response to gamma-rays in MEFs has been well characterized, the response to other DNA-damaging agents has not. Here, we examine the effects of commonly utilized chemotherapeutics, including doxorubicin, etoposide, and cisplatin, on cell cycle arrest in MEFs, and we define the p53 dependence of these effects. In addition, we examine the response of MEFs to ultraviolet light (UVC), as a representative agent acting by inducing pyrimidine dimers. Although p53 is clearly activated by all the agents examined, as measured by p21 induction, there are surprising differences in the activities of these agents. For example, doxorubicin but not cisplatin can effectively induce a p53-dependent G1 arrest. UVC, in contrast, induces a p53-independent G1 arrest response. Thus, the exact response of cells to DNA damage depends on the specific agent used.
View details for DOI 10.1038/sj.onc.1207026
View details for Web of Science ID 000188486600013
View details for PubMedID 14749764
- Conquering the complexity of p53 NATURE GENETICS 2004; 36 (1): 7-8
Multiple response elements and differential p53 binding control perp expression during apoptosis
MOLECULAR CANCER RESEARCH
2003; 1 (14): 1048-1057
The p53 tumor suppressor gene responds to cellular stress by activating either cell cycle arrest or apoptosis. A growing number of target genes involved in each of these pathways have been identified. However, the mechanism by which the apoptosis versus arrest decision is made remains to be elucidated. Perp is a proapoptotic target gene of p53 expressed to high levels in apoptotic cells compared with those undergoing cell cycle arrest. This pattern of expression is unusual among p53 target genes, many of which are induced to similar levels during arrest and apoptosis. Here, we describe the regulation of the Perp gene by p53 through at least three response elements in the Perp promoter and first intron. These sites are occupied in vivo in E1A-expressing mouse embryo fibroblasts undergoing apoptosis but not cell cycle arrest, in contrast to the p21 5' response element, which is occupied during both. The apoptosis-deficient p53 point mutant, p53V143A, displays a selective deficit in binding to the Perp elements, demonstrating that p53 can distinguish between Perp and p21 at the level of DNA binding. These results provide mechanistic insight into the selective expression of Perp during apoptosis and may provide a useful model for studying the p53-dependent cell cycle arrest versus apoptosis decision.
View details for Web of Science ID 000187889000007
View details for PubMedID 14707288
Perp is a mediator of p53-dependent apoptosis in diverse cell types
2003; 13 (22): 1985-1990
The induction of apoptosis by the p53 protein is critical for its activity as a tumor suppressor. Although it is clear that p53 induces apoptosis at least in part by direct transcriptional activation of target genes, the set of p53 target genes that mediate p53 function in apoptosis in vivo remains to be well defined. The Perp (p53 apoptosis effector related to PMP-22) gene is highly expressed in cells undergoing p53-dependent apoptosis as compared to cells undergoing p53-dependent G1 arrest. Perp is a direct p53 target, and its overexpression is sufficient to induce cell death in fibroblasts, implicating it as an important component of p53 apoptotic function. Here, through the generation of Perp-deficient mice, we analyze the role of Perp in the p53 apoptosis pathway in multiple primary cell types by comparing the cell death responses of Perp null cells to those of wild-type and p53 null cells. These experiments demonstrate the involvement of Perp in p53-mediated cell death in thymocytes and neurons but not in E1A-expressing MEFs, indicating a cell type-specific role for Perp in the p53 cell death pathway. In addition, we show that Perp is not required for proliferation-associated functions of p53. Thus, Perp selectively mediates the p53 apoptotic response, and the requirement for Perp is dictated by cellular context.
View details for DOI 10.1016/j.cub.2003.10.055
View details for Web of Science ID 000186558100026
View details for PubMedID 14614825
Targeted disruption of the three Rb-related genes leads to loss of G(1) control and immortalization
GENES & DEVELOPMENT
2000; 14 (23): 3037-3050
The retinoblastoma protein, pRB, and the closely related proteins p107 and p130 are important regulators of the mammalian cell cycle. Biochemical and genetic studies have demonstrated overlapping as well as distinct functions for the three proteins in cell cycle control and mouse development. However, the role of the pRB family as a whole in the regulation of cell proliferation, cell death, or cell differentiation is not known. We generated embryonic stem (ES) cells and other cell types mutant for all three genes. Triple knock-out mouse embryonic fibroblasts (TKO MEFs) had a shorter cell cycle than wild-type, single, or double knock-out control cells. TKO cells were resistant to G(1) arrest following DNA damage, despite retaining functional p53 activity. They were also insensitive to G(1) arrest signals following contact inhibition or serum starvation. Finally, TKO MEFs did not undergo senescence in culture and do possess some characteristics of transformed cells. Our results confirm the essential role of the Rb family in the control of the G(1)/S transition, place the three Rb family members downstream of multiple cell cycle control pathways, and further the link between loss of cell cycle control and tumorigenesis.
View details for Web of Science ID 000165788600011
View details for PubMedID 11114892
PERP, an apoptosis-associated target of p53, is a novel member of the PMP-22/gas3 family
GENES & DEVELOPMENT
2000; 14 (6): 704-718
The p53 tumor suppressor activates either cell cycle arrest or apoptosis in response to cellular stress. Mouse embryo fibroblasts (MEFs) provide a powerful primary cell system to study both p53-dependent pathways. Specifically, in response to DNA damage, MEFs undergo p53-dependent G(1) arrest, whereas MEFs expressing the adenovirus E1A oncoprotein undergo p53-dependent apoptosis. As the p53-dependent apoptosis pathway is not well understood, we sought to identify apoptosis-specific p53 target genes using a subtractive cloning strategy. Here, we describe the characterization of a gene identified in this screen, PERP, which is expressed in a p53-dependent manner and at high levels in apoptotic cells compared with G(1)-arrested cells. PERP induction is linked to p53-dependent apoptosis, including in response to E2F-1-driven hyperproliferation. Furthermore, analysis of the PERP promoter suggests that PERP is directly activated by p53. PERP shows sequence similarity to the PMP-22/gas3 tetraspan membrane protein implicated in hereditary human neuropathies such as Charcot-Marie-Tooth. Like PMP-22/gas3, PERP is a plasma membrane protein, and importantly, its expression causes cell death in fibroblasts. Taken together, these data suggest that PERP is a novel effector of p53-dependent apoptosis.
View details for Web of Science ID 000086224700008
View details for PubMedID 10733530
The role of p53 in tumour suppression: lessons from mouse models
CELLULAR AND MOLECULAR LIFE SCIENCES
1999; 55 (1): 48-63
The use of mouse models has greatly contributed to our understanding of the role of p53 in tumour suppression. Mice homozygous for a deletion in the p53 gene develop tumours at high frequency, providing essential evidence for the importance of p53 as a tumour suppressor. Additionally, crossing these knockout mice or transgenic expression p53 dominant negative alleles with other tumour-prone mouse strains has allowed the effect of p53 loss on tumour development to be examined further. In a variety of mouse models, absence of p53 facilitates tumorigenesis, thus providing a means to study how the lack of p53 enhances tumour development and to define genetic pathways of p53 action. Depending on the particular model system, loss of p53 either results in deregulated cell-cycle entry or aberrant apoptosis (programmed cell death), confirming results found in cell culture systems and providing insight into in vitro function of p53. Finally, as p53 null mice rapidly develop tumours, they are useful for evaluating agents for either chemopreventative or therapeutic activities.
View details for Web of Science ID 000078544100006
View details for PubMedID 10065151
Absence of p53 in a mouse mammary tumor model promotes tumor cell proliferation without affecting apoptosis
CELL GROWTH & DIFFERENTIATION
1997; 8 (8): 829-838
Loss or mutation of p53 may have multiple biological and genetic effects that result in accelerated tumor progression. Loss of p53 in some tumors has been correlated with a marked decrease in tumor cell apoptosis. p53 loss may also accelerate tumor growth through an increase in cell proliferation rates. To examine the effects of p53 loss on tumor progression in a controlled experimental context, we previously crossed p53-deficient mice to mammary tumor-susceptible Wnt-1 transgenic (TG) mice. The resulting female Wnt-1 TG offspring of this cross all developed mammary tumors, regardless of p53 status (p53+/+, p53+/-, or p53-/-). However, female p53-/- Wnt-1 TG mice developed tumors much sooner than their p53+/+ counterparts. In this report, we demonstrate that the average growth rates of tumors missing (p53-/-) or losing p53 (p53+/- with loss of heterozygosity) are accelerated compared to tumors with both wild-type p53 alleles (p53+/+). This accelerated growth rate appears to be due primarily to increases in rates of tumor cell proliferation. Tumor cell apoptotic levels were modest and were not measurably different in the presence or absence of wild-type p53. These results differ substantially from other mouse tumor models in which p53 loss was closely correlated with accelerated growth rates through attenuated apoptosis. Thus, the mechanisms by which p53 loss influences tumor progression may differ, depending on the tissue type and/or the oncogenic pathways involved.
View details for Web of Science ID A1997XQ82300001
View details for PubMedID 9269892
Transcriptional activation by p53, but not induction of the p21 gene, is essential for oncogene-mediated apoptosis
1996; 15 (14): 3693-3701
The p53 tumor suppressor limits cellular proliferation by inducing either G1 arrest or apoptosis, depending on the cellular context. To determine if these pathways are mechanistically distinct, we have examined the effects of different p53 mutants in p53 null primary mouse embryo fibroblasts. We chose this system as it is highly physiological and ensures that the interpretation of the results will not be confounded by the presence of endogenous p53 or oncoproteins which target p53. Using single cell microinjection assays for both G1 arrest and apoptosis, with loss-of-function and chimeric gain-of-function mutants, we have demonstrated that transcriptional activation is critical for both processes. Replacement of the p53 activation domain with that of VP16, or replacement of the p53 oligomerization domain with that of GCN4, reconstituted both G1 arrest and apoptosis activities. However, despite the importance of transcriptional activation in both processes, the target gene requirements are different. The p21 cyclin-dependent kinase inhibitor, which has been shown to be a direct target of p53 and a component of the radiation-induced G1 arrest response, is dispensable for oncogene-induced apoptosis, suggesting that these two p53-dependent transcriptional pathways are distinct.
View details for Web of Science ID A1996UY92200023
View details for PubMedID 8758936
- Transcriptional Activation by p53, but not Induction of the p21 Gene, is Essential for Oncogene-Mediated Apoptosis Embo J 1996; 15
A SUBSET OF P53-DEFICIENT EMBRYOS EXHIBIT EXENCEPHALY
1995; 10 (2): 175-180
Defects in neural tube formation are among the most common malformations leading to infant mortality. Although numerous genetic loci appear to contribute to the defects observed in humans and in animal model systems, few of the genes involved have been characterized at the molecular level. Mice lacking the p53 tumour suppressor gene are predisposed to tumours, but the viability of these animals indicates that p53 function is not essential for embryonic development. Here, we demonstrate that a fraction of p53-deficient embryos in fact do not develop normally. These animals display defects in neural tube closure resulting in an overgrowth of neural tissue in the region of the mid-brain, a condition known as exencephaly.
View details for Web of Science ID A1995RA85700014
View details for PubMedID 7663512