Heat Induction of the Unphosphorylated Form of Hypoxia-inducible Factor-1α Is Dependent on Heat Shock Protein-90 Activity

Hypoxia-inducible factor (HIF)-1α is the oxygen-sensitive subunit of HIF-1, a transcriptional master regulator of oxygen homeostasis. Oxygen-dependent prolyl hydroxylation targets HIF-1α for ubiquitinylation and proteasomal degradation. Unexpectedly, we found that exposing mice to elevated temperatures resulted in a strong HIF-1α induction in kidney, liver, and spleen. To elucidate the molecular mechanisms responsible for this effect, HepG2 hepatoma cells were exposed to different temperatures (34–42 °C) under normoxic (20% O2) or hypoxic (3% O2) conditions. Heat was sufficient to stabilize mainly a phosphatase-resistant, low molecular weight form of HIF-1α (termed HIF-1αa). Heat-induced HIF-1αaaccumulated in the nucleus but neither bound to DNA nortrans-activated reporter or target gene expression, demonstrating the need for post-translational modifications for these functions. The protein banding pattern of heat-induced HIF-1α in immunoblot analyses was clearly distinct from the HIF-1α pattern after prolyl hydroxylase inhibition (by hypoxia or iron chelation/replacement) or following proteasome inhibition, suggesting that heat stabilizes HIF-1α by a novel mechanism. Inhibition of the ATP-dependent chaperone activity of HSP90 by novobiocin or geldanamycin prevented heat-induced as well as hypoxia-induced HIF-1α accumulation, indicating a common role of the HSP90 chaperone activity in HIF-1α stabilization by these two environmental parameters.