INSULIN-LIKE GROWTH-FACTOR-I INHIBITS INDUCTION OF NITRIC-OXIDE SYNTHASE IN VASCULAR SMOOTH-MUSCLE CELLS

Experiments were designed to examine whether or not insulinlike growth factor I (IGF-I), which is produced by vascular cells in response to injury, affects the production of nitric oxide evoked by the inducible nitric oxide synthase in cultures of smooth muscle cells from the rat aorta. Nitric oxide production was assessed indirectly by the measure ment of nitrite accumulation and nitric oxide synthase activity by det ermining the formation of L-citrulline from L-arginine. Nitric oxide s ynthase was induced in vascular smooth muscle cells that had been expo sed to interleukin-1 beta (IL-1 beta) or tumor necrosis factor-a (TNF- alpha). IGF-I inhibited, in a concentration-dependent manner, the prod uction of nitrite and L-citrulline evoked by IL-1 beta or TNF-alpha. T he inhibition caused by IGF-I required the presence of the growth fact or during the induction of nitric oxide synthase. Two IGF-I-related pr oteins, ICE-Il and insulin, also inhibited, but to a smaller extent, t he release of nitrite and the formation of L-citrulline stimulated by IL-1 beta. Under bioassay conditions, the perfusates from columns cont aining IL-1 beta-treated smooth muscle cells relaxed rings of rat aort a without endothelium that had been contracted with phenylephrine; the se relaxations were reversed by nitro-L-arginine. Addition of IL-1 bet a-treated vascular smooth muscle cells to indomethacin-treated platele ts inhibited their aggregation to thrombin; methylene blue prevented t his inhibition. Control smooth muscle cells or cells exposed to IGF-I alone did not have such effects. Smooth muscle cells that had been exp osed simultaneously to IL-1 beta and IGF-I also relaxed detector rat a ortic rings, but to a smaller extent, and minimally affected the aggre gation, IL-1 beta caused the expression of inducible nitric oxide synt hase mRNA levels in vascular smooth muscle cells; this response was re duced in cells treated with IL-1 beta in combination with IGF-I. These observations indicate that IGF-I inhibits the cytokine-induced produc tion of nitric oxide by preventing the induction of nitric oxide synth ase. They further suggest that IGF-I may be an important modulator of the production of nitric oxide at sites of vascular injury.