Objective-We demonstrated recently that neuronal NO synthase (NOS) is expressed in arteriosclerotic lesions and exerts important vasculoprotective effects in vivo. In this study, we examined the molecular mechanism(s) for vascular neuronal NOS (nNOS) expression. Methods and Results-In cultured rat aortic smooth muscle cells, treatment with platelet-derived growth factor (PDGF) selectively upregulated nNOS expression but not inducible NOS (iNOS) or endothelial NOS (eNOS) expression. Treatment with PDGF also significantly caused activation of mitogen-activated protein kinase (MAPK) family, including extracellular signal-regulated kinase (ERK), p38MAPK, and c-Jun N-terminal kinase (JNK). ERK kinase (MAPK kinase [MEK]) inhibitors inhibited PDGF-induced nNOS expression, whereas a p38MAPK inhibitor or JNK inhibitor was without effects. Importantly, gene transfer of MEK per se elicited nNOS induction, and gene transfer of dominant-negative MEK abolished PDGF-induced nNOS expression. In isolated aortas of wild-type, eNOS Ϫ/Ϫ , and iNOS Ϫ/Ϫ mice, but not in those of nNOS Ϫ/Ϫ mice, treatment with PDGF significantly enhanced nNOS expression and nitrite plus nitrate production, both of which were again attenuated by a MEK inhibitor.
Conclusions-These results provide the first evidence that vascular nNOS expression is upregulated selectively in responseto PDGF through the MEK/ERK pathway. Upregulated nNOS may play an important compensatory role under arteriosclerotic/inflammatory conditions associated with eNOS dysfunction to maintain vascular homeostasis. Key Words: ERK Ⅲ mitogen-activated protein kinase Ⅲ neuronal nitric oxide synthase Ⅲ nitric oxide Ⅲ platelet-derived growth factor N O has multiple important actions that contribute to the maintenance of vascular homeostasis. [1][2][3][4] NO is synthesized by 3 different isoforms of NO synthase (NOS): neuronal NOS (nNOS), inducible NOS (iNOS), and endothelial NOS (eNOS). eNOS and nNOS are constitutively expressed mainly in endothelial cells and nitrergic nerves, respectively, synthesizing a small amount of NO in a calcium-dependent manner either under basal conditions or on stimulation. [1][2][3][4] In contrast, iNOS is expressed by inflammatory stimuli such as microbial endotoxins and certain proinflammatory cytokines, producing a large amount of NO in a calcium-independent manner. [1][2][3][4] Although the roles of eNOS 5-8 and iNOS 8 -11 in the development of arteriosclerosis have been investigated extensively, little is known about the role of nNOS. We demonstrated recently that nNOS also exerts important vasculoprotective effects in vivo. 12,13 In a carotid artery ligation model, nNOS Ϫ/Ϫ mice exhibited accelerated neointimal formation and constrictive vascular remodeling caused by blood flow disruption. 12,13 In a rat balloon injury model, selective inhibition of nNOS activity potently enhanced vasoconstrictor responses to calcium-mobilizing stimuli, and exacerbated neointimal formation. 12,13 In these models, nNOS was upregulated in vascular lesions and was expre...