Nitric oxide inhibits inducible nitric oxide synthase mRNA expression in RAW 264.7 macrophages

Using cultured murine RAW 264.7 macrophages, the present study investigates the influence of nitric oxide (NO) on the expression of the inducible NO s ynthase (iNOS) enzyme at the transcriptional level. Incubation of cells wit h lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) led to a marked increase in iNOS mRNA levels. Inhibition of LPS/IFN-gamma-induced NO synth esis with the L-arginine analogue N-G-monomethyl-L-arginine (L-NMMA) was ac companied by a significant up-regulation of iNOS mRNA that was reversed in the presence of the NO donor sodium nitroprusside (SNP). Treatment of cells with SNP alone decreased LPS/IFN-gamma-induced iNOS mRNA levels in a conce ntration-dependent manner. The inhibitory effect of SNP on iNOS mRNA expres sion was not prevented by 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (ODQ) , a selective inhibitor of the soluble guanylyl cyclase. In agreement with this finding, incubation of cells with the membrane-permeable cyclic GRIP a nalogue 8-bromo cyclic GMP left LPS/IFN-gamma-induced iNOS mRNA expression virtually unaltered. Together, our results demonstrate that both iNOS-deriv ed and exogenous NO exert an inhibitory effect on the expression of iNOS by a mechanism independent of the soluble guanylyl cyclase/cyclic GRIP pathwa y. In conclusion, NO may control the extent of iNOS mRNA expression by a ne gative autoregulatory feedback. (C) 2000 Academic Press.