Inaugural Article: Role of the arginine-nitric oxide pathway in the regulation of vascular smooth muscle cell proliferation

Nitric oxide (NO) is a physiological mediator of numerous cellular and organ functions (1), including inhibition of cell proliferation (2, 3). The mechanism by which NO inhibits cell proliferation appears to be multifaceted in that both cGMP-dependent and cGMP-independent mechanisms may be involved (2–5). After critical evaluation of the literature, we concluded that the evidence for involvement of cGMP in expression of the cytostatic effect of NO was indirect and inconsistent. Unpublished experiments in this laboratory consistently revealed cGMP-independent cytostatic effects of NO in various mammalian cell types including Caco-2 tumor cells, murine macrophages, rat aortic endothelial cells (RAEC), and rat aortic smooth muscle cells (RASMC). Therefore, our focus of attention has been on cGMP-independent mechanisms by which NO and the arginine-NO pathway inhibit cell proliferation. One such mechanism appears to be inhibition of two critical enzymes in the arginine-polyamine pathway, resulting in decreased polyamine production and consequent interference with cell proliferation (2, 3, 6, 7). NO is not the only product of NO synthase (NOS) that inhibits cell proliferation. N-hydroxyarginine (NOHA), the principal intermediate in the NOS-catalyzed conversion of arginine to NO plus citrulline, is also a potent competitive inhibitor of arginase (7, 8) and inhibitor of tumor cell proliferation (2). NOHA appears to interfere with tumor cell proliferation by cGMP-independent mechanisms involving the inhibition of arginase (2). NO has been found to be an inhibitor of ornithine decarboxylase (ODC) (2, 3, 6) and, like α-difluoromethylornithine (DFMO), can inhibit tumor cell proliferation by this cGMP-independent mechanism. Therefore, two products of NOS are capable of interfering with tumor cell proliferation by inhibiting two sequential steps in the arginine-polyamine pathway by cGMP-independent mechanisms. Arginase, which catalyzes the conversion or arginine to ornithine plus urea, is important not only in the urea cycle in the liver (arginase I isoform) but also in biochemical pathways essential to cell growth and wound healing in all cells (arginase I and II isoforms). Ornithine is, in turn, converted to putrescine by ODC, following which putrescine is converted to spermidine and spermine. The three polyamines (putrescine, spermidine, spermine) are required for mammalian cell growth (9). Atherosclerosis is a complex inflammatory condition involving the arterial vascular bed, and early vascular lesions are characterized by increased monocyte/macrophage invasion, proliferation, and activation as well as increased vascular smooth muscle proliferation (10). Another feature of early lesions in atherogenesis is distinct vascular endothelial cell dysfunction characterized by impaired endothelium-derived NO production and impaired endothelium-dependent vasodilation (10–12). Considering the possibility that one function of the arginine-NO pathway may be to inhibit or modulate cell proliferation, a deficiency in endothelial NO production could account, at least in part, for the increased proliferation of monocytes/macrophages and vascular smooth muscle that occurs in atherosclerosis. NO as well as activation of NOS have been reported to inhibit vascular smooth muscle cell proliferation (13, 14). Therefore, the objective of the present study was to elucidate the mechanisms by which NO and the arginine-NO pathway bring about inhibition of vascular smooth muscle cell proliferation in an in vitro cell culture model using RASMC.