Dephosphorylated hypoxia-inducible factor 1α as a mediator of p53-dependent apoptosis during hypoxia

Under hypoxia, HIF-1α binds to aryl hydrocarbon receptor nuclear translocator (ARNT, also called HIF-1β) to activate expression of genes important for cell survival. Alternatively, HIF-1α can bind to the tumor suppressor p53 and promote p53-dependent apoptosis. Here we show that the opposite functions of HIF-1α are distinguished by its phosphorylation status. Two distinguishable forms of HIF-1α, phosphorylated and dephosphorylated, were induced during hypoxia-induced apoptosis. The phosphorylated HIF-1α was the major form that bound to ARNT. Ectopically expressed ARNT was consistently able to enhance HIF-1α phosphorylation in a binding-dependent manner. In contrast, the dephosphorylated HIF-1α was the major form that bound to p53. Depletion of the dephosphorylated HIF-1α, by using the Hsp90 inhibitor geldanamycin A that had little effect on the phosphorylated HIF-1α expression, suppressed p53 induction and subsequent apoptosis. Depletion of dephosphorylated HIF-1α also prevented hypoxia-induced nuclear accumulation of HDM2, a negative regulator of p53. Our results indicate that the functions of HIF-1α varied with its phosphorylation status and that dephosphorylated HIF-1α mediated apoptosis by binding to and stabilizing p53.