Nitric Oxide Modulates Oxygen Sensing by Hypoxia-inducible Factor 1-dependent Induction of Prolyl Hydroxylase 2

The transcription factor complex hypoxia-inducible factor 1 (HIF-1) plays a crucial role in cellular adaptation to low oxygen availability. O2-dependent HIF prolyl hydroxylases (PHDs) modify HIF-1α, which is sent to proteasomal degradation under normoxia. Reduced activity of PHDs under hypoxia allows stabilization of HIF-1α and induction of HIF-1 target gene expression. Like hypoxia, nitric oxide (NO) was found to inhibit normoxic PHD activity leading to HIF-1α accumulation. In contrast under hypoxia, NO reduced HIF-1α levels due to enhanced PHD activity. Herein, we studied the role of NO in regulating PHD expression and the consequences thereof for HIF-1α degradation. We report a biphasic response of HIF-1α and PHDs to NO treatment both under normoxia and hypoxia. In the early phase, NO inhibits PHD activity that leads to HIF-1α accumulation, whereas in the late phase, increased PHD levels reduce HIF-1α. NO induces expression of PHD2 and -3 mRNA and protein under normoxia and hypoxia in a strictly HIF-1-dependent manner. NO-treated cells with elevated PHD levels displayed delayed HIF-1α accumulation and accelerated degradation of HIF-1α upon reoxygenation. Subsequent suppression of PHD2 and -3 expression using small interfering RNA revealed that PHD2 was exclusively responsible for regulating HIF-1α degradation under NO treatment. In conclusion, we identified the induction of PHD2 as an underlying mechanism of NO-induced degradation of HIF-1α.