Mechanisms of inhibition of vascular smooth muscle cell function by nitric oxide.

Abstract

Nitric oxide (NO), an important biologic regulator of vascular function, is known to inhibit the growth of cultured vascular smooth muscle cells. The anti-proliferative mechanisms and cell cycle effects of nitric oxide remain undefined. This thesis tested the hypothesis that nitric oxide inhibits smooth muscle proliferation by inhibiting progression at a specific point in the cell cycle. The inhibitory effect of exogenous nitric oxide occurred during S phase in synchronized rat aortic smooth muscle cells. This S phase inhibition occurred after the restriction point of commitment to cell proliferation late in G1 phase, which is distinct from the point of inhibition of other endogenous inhibitors of proliferation. This S phase effect is not mimicked by cGMP, a second messenger for NO. The mechanism of S phase inhibition was partially reversed by bypass of ribonucleotide reductase, suggesting that inhibition by NO occurs at this point. In cycling cells NO caused immediate inhibition of DNA synthesis, confirming the effects seen in synchronized cells. Prolonged treatment with NO did not synchronize cells at the G1/S border, as hydroxyurea did, but instead induced a G0 state characterized by a prolonged delay before DNA synthesis resumed after removal of NO. This G0 state was not due to inhibition of either protein synthesis or mitochondrial respiration. NO donors also inhibited migration of smooth muscle cells in culture independent of inhibition of proliferation. This inhibition of migration was mimicked by an exogenous cGMP analog, suggesting the involvement of guanylate cyclase in mediating this effect. Inhibition of smooth muscle cell migration may be another mechanism by which NO inhibits neointimal hyperplasia in vivo. DNA synthesis and proliferation of cultured endothelial cells was also inhibited by NO donors at concentrations which inhibit smooth muscle proliferation. This may be one mechanism by which pathologic production of NO in diseased vessels inhibits re-endothelialization. NO is the first endogenous substance which inhibits cell growth in S phase, a process that may be important in regulation of proliferation of cells that have lost normal G1 mechanisms of growth control.