Hsp90 Regulates a von Hippel Lindau-independent Hypoxia-inducible Factor-1α-degradative Pathway

HIF-1α 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-1α protein levels, efficiently targets HIF-1α for rapid proteasome-dependent degradation under normoxia, HIF-1α is resistant to the destabilizing effects of VHL under hypoxia. HIF-1α also associates with the molecular chaperone Hsp90. To examine the role of Hsp90 in HIF-1α function, we used renal carcinoma cell (RCC) lines that lack functional VHL and express stable HIF-1α protein under normoxia. Geldanamycin (GA), an Hsp90 antagonist, promoted efficient ubiquitination and proteasome-mediated degradation of HIF-1α in RCC in both normoxia and hypoxia. Furthermore, HIF-1α point mutations that block VHL association did not protect HIF-1α from GA-induced destabilization. Hsp90 antagonists also inhibited HIF-1α 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-1α degradation via a novel, oxygen-independent E3 ubiquitin ligase and 2) diminishes HIF-1α transcriptional activity. Existence of an Hsp90-dependent pathway for elimination of HIF-1α predicts that Hsp90 antagonists may be hypoxic cell sensitizers and possess antiangiogenic activityin vivo, thus extending the utility of these drugs as therapeutic anticancer agents.