eNOS Regulation by Phosphorylation and Protein-Protein Interactions

Abstract

Endothelial nitric oxide synthase (eNOS) catalyzes the conversion of L-arginine to L-citrulline and nitric oxide (NO). Protein phosphorylation and protein-protein interactions are two major mechanisms for eNOS regulation at the post-translational level, three aspects of which have been investigated in this study. The first aspect of eNOS regulation that we have examined is whether endostatin (ES) is a novel eNOS-activating agonist responsible for stimulating multi-site eNOS phosphorylation in endothelial cells. We show that ES induces acute endothelial NO release accompanied by eNOS phosphorylation events in cultured bovine aortic endothelial cells (BAECs). ES also induces relaxation of rat aortic rings. The second aspect of eNOS regulation that we have examined is the role of individual eNOS serine and threonine phosphorylation sites in the regulation of eNOS activity in BAECs. We mutated all five Thr- and Ser- sites of eNOS phosphorylation to aspartate or alanine and overexpressed the proteins in BAECs using adenoviral-mediated gene transfer. We show that mimicking phosphorylation of Ser-116 and Thr-497 is inhibitory, and mimicking phosphorylation of Ser-617, Ser-635 and Ser-1179 is stimulatory. Mimicking phosphorylation of Ser-635 and Ser-1179 together does not show synergistic effects on endothelial NO release. In addition, removal of any of the five Ser/Thr phosphorylation sites does not affect thapsigargin- or VEGF-stimulated NO release. A final aspect of eNOS regulation that we have investigated is the role of protein-protein interactions of eNOS with the CAT (cationic amino acid transporter)-1 arginine transporter. We show that eNOS interacts directly with CAT-1 and that overexpression of CAT-1 proteins in BAECs results in significant increases in NO release which is not altered by the CAT-1 inhibitor, L-lysine, suggesting that NO production in this in vitro model is independent of CAT-1 mediated arginine transport. Furthermore, eNOS enzymatic activity is increased in lysates of CAT-1-overexpressing cells accompanied by increased eNOS association with CAT-1, alterations of eNOS phosphorylation and eNOS association with caveolin-1. The present study adds to the knowledge of the regulation of eNOS by multi-site phosphorylation and protein-protein interactions.