Amato Giaccia

Research in my lab focuses on the cellular response to stresses found in the tumor microenvironment, especially low oxygen conditions (tumor hypoxia). Our long-term goal is to identify and characterize the molecular and physiological changes induced by the tumor microenvironment that influence the malignant progression of transformed cells. Thus, by understanding the molecular changes which are important for tumor cells to survive in nutrient and oxygen depleted conditions, potentially new targets for cancer therapy will be identified. For the last few years, we have been studying three phenotypes of cells invoked under hypoxic conditions: 1) cessation of cell proliferation under low oxygen conditions, 2) cell death induced by low oxygen conditions, and 3) the angiogenic switch of dormant tumors to rapidly expanding tumors. Let me briefly discuss each of these projects and their potential clinical significance.1) Hypoxia induces a G1/early S-phase cell-cycle arrest. This arrest is different from the cell-cycle arrest induced by gentoxic stress, but may share some of the same genetic determinants such as the retinoblastoma (Rb) tumor suppressor gene. Phosphorylation of Rb is essential for G1 to S-phase progression in the cell-cycle and is controlled by cyclin-cdk activity. Cyclin-cdk activity is controlled by a group of small molecular weight inhibitors which bind to these complexes and inhibit their kinase activity. At present, our data suggest that two of these inhibitors, p21 and p27, are essential in controlling hypoxia/reoxygenation induced cell-cycle arrest. We are continuing to use a combination of genetic and biochemical approaches to examine the state of this pathway in transformed tumor cells to determine whether loss of this checkpoint plays any role in genomic instability. In addition, we are also investigating the complexity of this arrest to determine whether other pathways also contribute to this arrest at different oxygen tensions.2) Hypoxia is a potent activator of the p53 tumor suppressor gene. We have demonstrated that hypoxia induces apoptosis in oncogene transfected rodent and human cells, and that the magnitude of cell killing both in cell culture and in experimental tumors is influenced by the presence of functional p53. We have also shown that hypoxia can select for the expansion of cells which possess diminished apoptotic programs through the inactivation of p53 or through over expression of anti-apoptotic genes such as bcl-2. These studies offer a possible explanation why many solid tumors at the time of clinical presentation possess either mutant p53 or a diminished apoptotic program as hypoxia would select for tumor cells possessing such genotypes. Hypoxia induced apoptosis does not require the transcriptional activity of p53, suggesting that p53 is signaling apoptosis by acting as a transcriptional repressor of anti-apoptotic genes and/or through its interaction with another protein. Both of these possibilities are being investigated.3) Previous studies have indicated that the PTEN tumor suppressor gene plays important roles in modulating apoptotic cell death and cell-cycle progression. We became interested in PTEN as it is mutated or inactivated in a large percentage of glioblastomas, tumors that possess median pO2 levels way below those found in other solid tumors. Since the PI(3) kinase pathway is important for angiogenesis as well as apoptosis, we hypothesized that PTEN would serve to check the hypoxia induced stimulation of the PI(3) kinase-HIF-VEGF.We find that PTEN inhibits the hypoxia induced activation of the PI(3) kinase downstream effector Akt. PTEN also inhibits endogenous vascular endothelial growth factor induction by hypoxia to the same extent as wortmannin, a potent PI(3) kinase inhibitor. Using reporter gene assays, PTEN inhibits VEGF induction induced by hypoxia or insulin-like growth factor in a PI(3) kinase dependent manner. In co-transfection experiments, PTEN inhib