HYPOXIA INDUCES ACCUMULATION OF P53 PROTEIN, BUT ACTIVATION OF A G(1)-PHASE CHECKPOINT BY LOW-OXYGEN CONDITIONS IS INDEPENDENT OF P53 STATUS

It has been convincingly demonstrated that genotoxic stresses cause th e accumulation of the tumor suppressor gene p53. One important consequ ence of increased p53 protein levels in response to DNA damage is the activation of a G(1)-phase cell cycle checkpoint. It has also been sho wn that G(1)-phase cell cycle checkpoints are activated in response to other stresses, such as lack of oxygen. Were we show that hypoxia and heat, agents that induce cellular stress primarily by inhibiting oxyg en-dependent metabolism and denaturing proteins, respectively, also ca use an increase in p53 protein levels. The p53 protein induced by heat is localized in the cytoplasm and forms a complex with the heat shock protein hsc70. The increase in nuclear p53 protein levels and DNA-bin ding activity and the induction of reporter gene constructs containing p53 binding sites following hypoxia occur in cells that are wild type for p53 but not in cells that possess mutant p53. However, unlike ion izing radiation, the accumulation of cells in G, phase by hypoxia is n ot strictly dependent on wild-type p53 function. In addition, cells ex pressing the human papillomavirus E6 gene, which show increased degrad ation of p53 by ubiquitination and fail to accumulate p53 in response to DNA-damaging agents, do increase their p53 levels following heat an d hypoxia. These results suggest that hypoxia is an example of a ''non genotoxic'' stress which induces p53 activity by a different pathway t han DNA-damaging agents.