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

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