Hydrogen peroxide activates endothelial nitric-oxide synthase through coordinated phosphorylation and dephosphorylation via a phosphoinositide 3-kinase-dependent signaling pathway

Endothelial nitric-oxide synthase (eNOS) is an important component of vascular homeostasis. During vascular disease, endothelial cells are exposed to excess reactive oxygen species that can alter cellular phenotype by inducing various signaling pathways. In the current study, we examined the implications of H2O2-induced signaling for eNOS phosphorylation status and activity in porcine aortic endothelial cells. We found that H2O2 treatment enhanced eNOS activity and NO bioactivity as determined by the conversion of L-[H-3]arginine to L-[H-3]citrulline and cellular eGMP content. Concomitant with eNOS activation, H2O2 also activated Akt, increased eNOS phosphorylation at Ser-1177, and decreased eNOS phosphorylation at Thr-495. H2O2-induced promotion of eNOS activity and modulation of the eNOS phosphorylation status at Ser-1177 and Thr-495 were significantly attenuated by selective inhibitors of Src kinase, the ErbB receptor family, and phosphoinositide 3-kinase (PI 3-K). We found that Akt activation, eNOS Ser-1177 phosphorylation, and eNOS activation by H2O2 were calcium-dependent, whereas eNOS dephosphorylation at Thr-495 was not, suggesting a branch point in the signaling cascade downstream from PI 3-K. Consistent with this, overexpression of a dominant negative isoform of Akt inhibited H2O2-induced phosphorylation of eNOS at Ser-1177 but not dephosphorylation of eNOS at Thr-495. Together, these data indicate that H2O2 promotes calcium-dependent eNOS activity through a coordinated change in the phosphorylation status of the enzyme mediated by Src- and ErbB receptor-dependent PI 3-K activation. In turn, PI 3-K mediates eNOS Ser-1177 phosphorylation via a calcium- and Akt-dependent pathway, whereas eNOS Thr-495 dephosphorylation does not involve calcium or Akt. This response may represent an attempt by endothelial cells to maintain NO bioactivity under conditions of enhanced oxidative stress.