Reduction of hypoxia-induced transcription through the repression of hypoxia-inducible factor-1α/aryl hydrocarbon receptor nuclear translocator DNA binding by the 90-kDa heat-shock protein inhibitor radicicol

Under low oxygen tension, cells increase the transcription of specific genes involved in angiogenesis, erythropoiesis, and glycolysis. Hypoxia-induced gene expression depends primarily on stabilization of the α subunit of hypoxia-inducible factor-1 (HIF-1α), which acts as a heterodimeric trans-activator with the nuclear protein known as the aryl hydrocarbon receptor nuclear translocator (Arnt). The resulting heterodimer (HIF-1αArnt) interacts specifically with the hypoxia-responsive element (HRE), thereby increasing transcription of the genes under HRE control. Our results indicate that the 90-kDa heat-shock protein (Hsp90) inhibitor radicicol reduces the hypoxia-induced expression of both endogenous vascular endothelial growth factor (VEGF) and HRE-driven reporter plasmids. Radicicol treatment (0.5 μg/ml) does not significantly change the stability of the HIF-1α protein and does not inhibit the nuclear localization of HIF-1α. However, this dose of radicicol significantly reduces HRE binding by the HIF-1α/Arnt heterodimer. Our results, the first to show that radicicol specifically inhibits the interaction between the HIF-1α/Arnt heterodimer and HRE, suggest that Hsp90 modulates the conformation of the HIF-1α/Arnt heterodimer, making it suitable for interaction with HRE. Furthermore, we demonstrate that radicicol reduces hypoxia-induced VEGF expression to decrease hypoxia-induced angiogenesis.