Nitric Oxide Induces Hypoxia-inducible Factor 1 Activation That Is Dependent on MAPK and Phosphatidylinositol 3-Kinase Signaling

Hypoxia-inducible factor-1 (HIF-1) is a master regulator of cellular adaptive responses to hypoxia. Levels of the HIF-1alpha subunit increase under hypoxic conditions. Exposure of cells to certain nitric oxide (NO) donors also induces HIF-1alpha expression under nonhypoxic conditions. We demonstrate that exposure of cells to the NO donor NOC18 or S-nitrosoglutathione induces HIF-1alpha expression and transcriptional activity. In contrast to hypoxia, NOC18 did not inhibit HIF-1alpha hydroxylation, ubiquitination, and degradation, indicating an effect on HIF-1alpha protein synthesis that was confirmed by pulse labeling studies. NOC18 stimulation of HIF-1alpha protein and HIF-1-dependent gene expression was blocked by treating cells with an inhibitor of the phosphatidylinositol 3-kinase or MAPK-signaling pathway. These inhibitors also blocked NOC18-induced phosphorylation of the translational regulatory proteins 4E-BP1, p70 S6 kinase, and eIF-4E, thus providing a mechanism for the modulation of HIF-1alpha protein synthesis. In addition, expression of a dominant-negative form of Ras significantly suppressed HIF-1 activation by NOC18. We conclude that the NO donor NOC18 induces HIF-1alpha synthesis under conditions of NO formation during normoxia and that hydroxylation of HIF-1alpha is not regulated by NOC18.