Hypoxia induces p53 accumulation through MDM2 down-regulation and inhibition of E6-mediated degradation

Hypoxia, a result of DNA-damaging agents such as ionizing radiation, induce s the nuclear accumulation of the p53 tumor suppressor protein. However, un like the effect in ionizing radiation, hypoxia readily induces the nuclear accumulation of p53 in HPV E6-infected cells. In HPV-infected cells, a key regulator of p53 protein levels is the E6 oncoprotein. In association with the endogenous cellular protein E6-associated protein (E6AP), E6 can accele rate the degradation of p53 under aerobic conditions. To better define the mechanism of p53 induction in E6-infected cells by hypoxia, we studied the expression and association of E6 and E6AP with p53 in vivo. We found that h ypoxia did not alter the protein levels of E6 or E6AP as compared with thos e found under aerobic growth conditions, indicating that protein inhibition of E6 or E6AP alone is not sufficient to explain the increased accumulatio n of p53 under hypoxic conditions. However, p53 did fail to coprecipitate w ith E6AP under hypoxia, indicating that hypoxia uncouples the interaction o f p53 with E6 and E6AP, We also present evidence to indicate that hypoxia d ecreases the expression of the endogenous cellular regulator of p53 protein , the human MDM2 protein, resulting in an inhibition of p53 export from the nucleus to the cytoplasm for degradation. Taken together, these results su ggest that the hypoxic induction of p53 is attributable to the down-regulat ion of MDM2 protein levels and uncoupling of p53 from its interaction with the E6/E6AP complex.