Increased chemoresistance induced by inhibition of HIF-prolyl-hydroxylase domain enzymes.

The hypoxia-inducible factor (HIF) is the master regulator for oxygen-dependent gene expression. The HIF signal transduction pathway can be manipulated by inhibiting the activity of the HIFalpha-regulating prolyl-4-hydroxylase domain (PHD) enzymes. The consequence of inhibiting the PHD activity for chemoresistance was studied. Inhibiting the PHD activity with the 2-oxoglutarate analog dimethyloxaloylglycine (DMOG) results in increased chemoresistance towards etoposide but not carboplatin in HeLa cells. Evidence for an etoposide-specific resistance, which develops as a consequence of inhibiting the PHD activity, was further supported in a tetracycline-inducible PHD2 knockdown HeLa cell model. The etoposide-resistance was mediated by HIF-1alpha as shown in mouse embryonic fibroblast HIF-1alpha(+/+) and HIF-1alpha(-/-) cells. Decreased cellular cytotoxicity after etoposide treatment inversely correlated with a dimethyloxaloylglycine (DMOG)-inducible, HIF-1alpha-dependent enhanced MDR-1 expression and efflux activity as determined by RT-PCR, immunoblots, and with the fluorescent dye DiOC2. Taken together, our data indicate that PHD inhibitors might increase chemoresistance of tumor cells in a HIF-1-dependent manner.