Hsp70 and CHIP Selectively Mediate Ubiquitination and Degradation of Hypoxia-inducible Factor (HIF)-1α but Not HlF2α.

Hypoxia-inducible factors (HIFs) are transcription factors that mediate adaptive responses to reduced oxygen availability. HIF-α subunits are stabilized under conditions of acute hypoxia. However, prolonged hypoxia leads to decay of HIF-1α but not HIF-2α protein levels by unknown mechanisms. Here, we identify Hsp70 and CHIP (carboxyl terminus of Hsc70-interacting protein) as HIF-1α-interacting proteins. Hsp70, through recruiting the ubiquitin ligase CHIP, promotes the ubiquitination and proteasomal degradation of HIF-1α but not HIF-2α, thereby inhibiting HIF-1-dependent gene expression. Disruption of Hsp70-CHIP interaction blocks HIF-1α degradation mediated by Hsp70 and CHIP. Inhibition of Hsp70 or CHIP synthesis by RNA interference increases protein levels of HIF-1α but not HIF-2α and attenuates the decay of HIF-1α levels during prolonged hypoxia. Thus, Hsp70- and CHIP-dependent ubiquitination represents a molecular mechanism by which prolonged hypoxia selectively reduces the levels of HIF-1α but not HIF-2α protein. [ABSTRACT FROM AUTHOR]