Immunophilin-ligands FK506 and CsA inhibit HIF1α expression by a VHL- and ubiquitin-independent mechanism

Hypoxia inducible factor-1alpha (HIF1α) plays a key role in the regulation of genes controlling oxygen supply, glucose metabolism and angiogenesis. Its expression in tumors appears to confer an adaptive advantage to their hypoxic microenvironment. We have evaluated the effect of the immunophilin ligands FK506 and cyclosporin A on HIF1α levels in different tumor cell lines. Our results indicate that both drugs are potent suppressors of HIF1α expression by accelerating the proteasomal degradation of the protein. Unexpectedly, the suppressive effect of these compounds was found to be independent of the presence of von Hippel Lindau factor and the degree of hydroxylation of the HIF1α protein. Moreover, HIF1α degradation induced by these compounds did not required ubiquitination, as it was also induced in E1 ligase-incompetent cells.