RACK1 competes with HSP90 for binding to HIF-1α and is required for O2-independent and HSP90 inhibitor-induced degradation of HIF-1α

Hypoxia-inducible factor 1 (HIF-1) regulates transcription in response to changes in 02 concentration. O-2-dependent degradation of the HIF-1 a subunit is mediated by prolyl hydroxylase (PHD), the von Hippel-Lindau (VHL)/Elongin-C/Elongin-B E3 ubiquitin ligase complex, and the proteasome. Inhibition of heat-shock protein 90 (HSP90) leads to O-2/PHDNHL-independent degradation of HIF-1 . We have identified the receptor of activated protein kinase C (RACK1) as a HIF-1 alpha-interacting protein that promotes PHD/VHL-independent proteasomal degradation of HIF-1 alpha. RACK1 competes with HSP90 for binding to the PAS-A domain of HIF-1 alpha in vitro and in human cells. HIF-1 alpha degradation induced by the HSP90 inhibitor 17-allyl-aminogeldanamycin is abolished by RACK1 loss of function. RACK1 binds to Elongin-C and promotes ubiquitination of HIF-1 alpha. Elongin-C-binding sites in RACK1 and VHL show significant sequence similarity. Thus, RACK1 is an essential component of an O-2/PHD/VHL-independent mechanism for regulating HIF-loc stability through competition with HSP90 and recruitment of the Elongin-C/B ubiquitin ligase complex.