Nitric oxide impairs normoxic degradation of HIF-1alpha by inhibition of prolyl hydroxylases.

Hypoxia inducible factor-1 (HIF-1) is the master regulator of metabolic adaptation to hypoxia. It is appreciated that HIF-1alpha accumulation is achieved under normoxic conditions by e.g., nitric oxide. We determined molecular mechanisms of HIF-1alpha accumulation under the impact of S-nitrosoglutathione (GSNO). In human embryonic kidney cells GSNO provoked nuclear accumulation of HIF-1alpha. This appeared unrelated to gene transcription and protein translation, thus pointing to inhibition of HIF-1alpha degradation. Indeed, GSNO as well as the hypoxia mimic CoCl2 decreased ubiquitination of HIF-1alpha and GSNO-induced HIF-1alpha failed to coimmunoprecipitate with pVHL (von Hippel Lindau protein). Considering that HIF-1alpha-pVHL interactions require prolyl hydroxylation of HIF-1alpha, we went on to demonstrate inhibition of HIF-1alpha prolyl hydroxylases (PHDs) by GSNO. In vitro HIF-1alpha-pVHL interactions revealed that GSNO dose-dependently inhibits PHD activity but not the interaction of a synthetic peptide resembling the hydroxylated oxygen-dependent degradation domain of HIF-1alpha with pVHL. We conclude that GSNO-attenuated prolyl hydroxylase activity accounts for HIF-1alpha accumulation under conditions of NO formation during normoxia and that PHD activity is subject to regulation by NO.