Interaction of the endothelial nitric oxide synthase with the CAT-1 arginine transporter enhances NO release by a mechanism not involving arginine transport.

eNOS (endothelial nitric oxide synthase) catalyses the conversion of L-arginine into L-citrulline and NO. Evidence has been presented previously that eNOS is associated with the CAT (cationic amino acid transporter)-1 arginine transporter in endothelial caveolae, and it has been proposed that eNOS-CAT-1 association facilitates the delivery of extracellular L-arginine to eNOS. Definitive proof of a protein-protein interaction between eNOS and CAT-1 is lacking, however, and it is also unknown whether the two proteins interact directly or via an adaptor protein. In the present study, we raised a polyclonal antibody against CAT-1, and show using reciprocal co-immunoprecipitation protocols that eNOS and CAT-1 do indeed form a complex in BAECs (bovine aortic endothelial cells). In vitro binding assays with GST (glutathione S-transferase)-CAT-1 fusion proteins and eNOS show that the two proteins interact directly and that no single CAT-1 intracellular domain is sufficient to mediate the interaction. Overexpression of CAT-1 in BAECs by adenoviral-mediated gene transfer results in significant increases in both L-arginine uptake and NO production by the cells. However, whereas increased L-arginine transport is reversed completely by the CAT-1 inhibitor, L-lysine, increased NO release is unaltered, suggesting that NO production in this in vitro model is independent of CAT-1-mediated transport. Furthermore, eNOS enzymic activity is increased in lysates of CAT-1-overexpressing cells accompanied by increased phosphorylation of eNOS at Ser-1179 and Ser-635, and decreased association of eNOS with caveolin-1. Taken together, these data suggest that direct interaction of eNOS with CAT-1 enhances NO release by a mechanism not involving arginine transport.

Acute activation and phosphorylation of endothelial nitric oxide synthase by HMG-CoA reductase inhibitors.

3-Hydroxy-3-methylglutaryl (HMG)-CoA reductase inhibitors, statins, provide beneficial effects independent of their lipid-lowering effects. One beneficial effect appears to involve acute activation of endothelial nitric oxide (NO) synthase (eNOS) and increased NO release. However, the mechanism of acute statin-stimulated eNOS activation is unknown. Therefore, we hypothesized that eNOS activation may be coupled to altered eNOS phosphorylation. Bovine aortic endothelial cells (BAECs), passages 2-6, were treated with either lovastatin or pravastatin from 0 to 30 min. eNOS phosphorylation was examined by Western blot by use of phosphospecific antibodies for Ser-1179, Ser-635, Ser-617, Thr-497, and Ser-116. Statin stimulation of BAECs increased eNOS phosphorylation at Ser-1179 and Ser-617, which was blocked by the phosphatidylinositol 3-kinase (PI3-kinase)/Akt inhibitor wortmannin, and at Ser-635, which was blocked by the protein kinase A (PKA) inhibitor KT-5720. Statin treatment of BAECs transiently increased NO release by fourfold, measured by cGMP accumulation, and was attenuated by N-nitro-l-arginine methyl ester, wortmannin, and KT-5720 but not by mevalonate. In conclusion, these data demonstrate that eNOS is acutely activated by statins independent of HMG-CoA reductase inhibition and that in addition to Ser-1179, eNOS phosphorylation at Ser-635 and Ser-617 through PKA and Akt, respectively, may explain, in part, a mechanism by which eNOS is activated in response to acute statin treatment.