Hsp90 regulates a von Hippel Lindau-independent hypoxia-inducible factor-1α-degradative pathway

HIF-1alpha is a normally labile proangiogenic transcription factor that is stabilized and activated in hypoxia. Although the von Hippel Lindau (VHL) gene product, the ubiquitin ligase responsible for regulating HIF-1alpha protein levels, efficiently targets HIF-1alpha for rapid proteasome-dependent degradation under normoxia, HIF-1alpha is resistant to the destabilizing effects of VHL under hypoxia. HIF-1alpha also associates with the molecular chaperone Hsp90. To examine the role of Hsp90 in HIF-1alpha function, we used renal carcinoma cell (RCC) lines that lack functional VHL and express stable HIF-1alpha protein under normoxia. Geldanamycin (GA), an Hsp90 antagonist, promoted efficient ubiquitination and proteasome-mediated degradation of HIF-1alpha in RCC in both normoxia and hypoxia. Furthermore, HIF-1alpha point mutations that block VHL association did not protect HIF-1alpha from GA-induced destabilization. Hsp90 antagonists also inhibited HIF-1alpha transcriptional activity and dramatically reduced both hypoxia-induced accumulation of VEGF mRNA and hypoxia-dependent angiogenic activity. These findings demonstrate that disruption of Hsp90 function 1) promotes HIF-1alpha degradation via a novel, oxygen-independent E3 ubiquitin ligase and 2) diminishes HIF-1alpha transcriptional activity. Existence of an Hsp90-dependent pathway for elimination of HIF-1alpha predicts that Hsp90 antagonists may be hypoxic cell sensitizers and possess antiangiogenic activity in vivo, thus extending the utility of these drugs as therapeutic anticancer agents.