Phosphorylation of Thr(495) regulates Ca2+/calmodulin-dependent endothelial nitric oxide synthase activity

The activity of the endothelial nitric oxide synthase (eNOS) can be regulat ed independently of an increase in Ca2+ by the phosphorylation of Ser(1177) but results only in a low nitric oxide (NO) output. In the present study, we assessed whether the agonist-induced (Ca2+-dependent, high-output) activ ation of eNOS is associated with changes in the phosphorylation of Thr(495) in the calmodulin (CaM)-binding domain. eNOS Thr(495) was constitutively p hosphorylated in porcine aortic endothelial cells and was rapidly dephospho rylated after bradykinin stimulation. In the same cells, bradykinin enhance d the phosphorylation of Ser(1177), which was maximal after 5 minutes, and abolished by the CaM-dependent kinase II (CaMKII) inhibitor KN-93. Bradykin in also enhanced the association of CaMKII with eNOS. Phosphorylation of Th r(495) was attenuated by the protein kinase C (PKC) inhibitor Ro 31-8220 an d after PKC downregulation using phorbol 12-myristate 13-acetate. The agoni st-induced dephosphorylation of Thr(495) was completely Ca2+-dependent and inhibited by the PP1 inhibitor calyculin A. Little CaM was bound to eNOS im munoprecipitated from unstimulated cells, but the agonist-induced dephospho rylation of Thr(495) enhanced the association of CaM. Mutation of Thr(495) to alanine increased CaM binding to eNOS in the absence of cell stimulation , whereas the corresponding Asp(495) mutant bound almost no CaM. Accordingl y, NO production by the Ala(495) mutant was more sensitive to Ca2+/CaM than the aspartate mutant. These results suggest that the dual phosphorylation of Ser(1177) and Thr(495) determines the activity of eNOS in agonist-stimul ated endothelial cells. Moreover, the dephosphorylation of Thr(495) by pp1 precedes the phosphorylation of Ser(1177) by CaMKII. The full text of this article is available at http://www.circresaha.org.