Akt-Mediated Activation of HIF-1 in Pulmonary Vascular Endothelial Cells by S-Nitrosoglutathione

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Abstract
S-nitrosoglutathione (GSNO) stabilizes the alpha-subunit of hypoxia inducible factor-1 (HIF-1) in normoxic cells, but not in the presence of PI3K inhibitors. In this report, the biochemical pathway by which GSNO alters PI3K/Akt activity to modify HIF-1 expression was characterized in Cos cells and primary pulmonary vascular endothelial cells. GSNO increased Akt kinase activity--and downstream HIF-1alpha protein accumulation and DNA-binding activity--in a dose- and time-dependent manner. The PI3K inhibitors, wortmannin and LY294002, blocked these responses. Neither glutathione nor 8-bromo-cyclic GMP mimicked the GSNO-induced increases in Akt kinase activity. GSNO-induced Akt kinase activity and downstream HIF-1alpha stabilization were blocked by acivicin, an inhibitor of gamma-glutamyl transpeptidase (gammaGT), a transmembrane protein that can translate extracellular GSNO to intracellular S-nitrosocysteinylglycine. Dithiothreitol blocked GSNO-induced Akt kinase activity and HIF-1alpha stabilization. Moreover, the 3'-phosphatase of phosphoinositides, PTEN (phosphatase and tensin homolog deleted on chromosome ten) was S-nitrosylated by GSNO in pulmonary arterial endothelial cells, which was reversed by dithiothreitol and ultraviolet light. Interestingly, the abundance of S-nitrosylated PTEN also correlated inversely with PTEN activity. Taken together, these results suggest that GSNO induction of Akt appears to be mediated by S-nitrosylation chemistry rather than classic NO signaling through guanylate cyclase/cGMP. We speculate that gammaGT-dependent activation of Akt and subsequent activation of HIF-1 in vascular beds may be relevant to the regulation of HIF-1-dependent gene expression in conditions associated with oxyhemoglobin deoxygenation, as opposed to profoundly low Po(2), in the pulmonary vasculature.