Non-hypoxic activation of the negative regulatory feedback loop of prolyl-hydroxylase oxygen sensors.

Hypoxia inducible factors (HIF) coordinate cellular responses towards hypoxia. HIFs are mainly regulated by a group of prolyl-hydroxylases (PHDs) that in the presence of oxygen, target the HIFalpha subunit for degradation. Herein, we studied the role of nitric oxide (NO) in regulating PHD activities under normoxic conditions. In the present study we show that different NO-donors initially inhibited endogenous PHD2 activity which led to accumulation of HIF-1alpha subsequently to enhance HIF-1 dependent increased PHD2 promoter activity. Consequently PHD2 abundance and activity were strongly induced which caused downregulation of HIF-1alpha. Interestingly, upregulation of endogenous PHD2 activity by NO was not found in cells that lack an intact pVHL dependent degradation pathway. Recovery of PHD activity required intact cells and was not observed in cell extracts or recombinant PHD2. In conclusion induction of endogenous PHD2 activity by NO is dependent on a feedback loop initiated despite normoxic conditions.