NICOTINAMIDE AND DEXAMETHASONE INHIBIT INTERLEUKIN-1-INDUCED NITRIC-OXIDE PRODUCTION BY RINM5F CELLS WITHOUT DECREASING MESSENGER-RIBONUCLEIC-ACID EXPRESSION FOR NITRIC-OXIDE SYNTHASE

Nitric oxide (NO) generation may be a final common pathway for beta-cell damage in early insulin-dependent diabetes mellitus. Insulin-producing cells express an inducible form of NO synthase (iNOS), which is similar to that observed in activated macrophages. Induction of iNOS mRNA in these cells depends on protein synthesis. To further characterize the regulation of iNOS induction in insulin-producing cells, RINm5F cells (RIN cells) were exposed for 6 h to human recombinant interleukin-1beta (rlL-1beta; 1 ng/ml) alone or in combination with either nicotinamide (10, 20, or 50 mm) or dexamethasone (1 or 5 mum). These agents have been previously shown to prevent activation of iNOS in macrophages, fibroblasts, and hepatocytes. rIL-1beta induced the expression of iNOS mRNA in RIN cells and a 12- to 13-fold increase in medium nitrite accumulation, the latter indicating NO production. Nicotinamide decreased nitrite production in a dose-dependent way. Thus, 10 mm nicotinamide decreased rIL-1beta-induced nitrite formation by 30%, 20 mm by 60%, and 50 mm by 90%. The highest concentration of nicotinamide also prevented rIL-1beta-induced iNOS mRNA, an effect associated with inhibition of total protein biosynthesis. However, 10 or 20 mm nicotinamide did not modify rIL-1beta-induced iNOS mRNA expression or inhibit protein biosynthesis. Dexamethasone also decreased rIL-1beta-induced nitrite production without affecting iNOS mRNA expression. As a whole, these data suggest that both nicotinamide and dexamethasone may prevent NO accumulation in insulin-producing cells by posttranscriptional mechanisms. It is also possible that these drugs induce direct inhibition of iNOS enzymatic activity and/or scavenge NO. Higher concentrations of nicotinamide might also inhibit iNOS mRNA expression, possibly by blocking protein biosynthesis.