Calcineurin promotes hypoxia-inducible factor 1α expression by dephosphorylating RACK1 and blocking RACK1 dimerization

Oxygen homeostasis represents an essential organizing principle of metazoan evolution and biology. Hypoxia-inducible factor 1 (HIF-1) is a master regulator of transcriptional responses to changes in O₂ concentration. HIF-1 is a heterodimer of HIF-1α and HIF-1β subunits. O₂- dependent degradation of the HIF-1α subunit is mediated by prolyl hydroxylase, von Hippel-Lindau protein (VHL)/Elongin-C E3 ubiquitin ligase, and the proteasome. O₂-independent degradation of HIF-1α is regulated by the competition of RACK1 and HSP90 for binding to HIF-1α. RACK1 binding results in the recruitment of the Elongin-C E3 ubiquitin ligase, leading to VHL-independent ubiquitination and degradation of HIF-1α. In this report, we show that calcineurin inhibits the ubiquitination and proteasomal degradation of HIF-1α. Calcineurin is a serine/threonine phosphatase that is activated by calcium and calmodulin. The phosphatase activity of calcineurin is required for its regulation of HIF-1α. RACK1 binds to the catalytic domain of calcineurin and is required for HIF-1α degradation induced by the calcineurin inhibitor cyclosporine A. Elongin-C and HIF-1α each bind to RACK1 and dimerization of RACK1 is required to recruit Elongin-C to HIF-1α. Phosphorylation of RACK1 promotes its dimerization and dephosphorylation by calcineurin inhibits dimerization. Serine 146 within the dimerization domain is phosphorylated and mutation of serine 146 impairs RACK1 dimerization and HIF-1α degradation. These results indicate that intracellular calcium levels can regulate HIF-1α expression by modulating calcineurin activity and RACK1 dimerization.