Interferon-gamma increases the sensitivity of islets of Langerhans for inducible nitric-oxide synthase expression induced by interleukin 1

The purpose of this study was to evaluate the effects of interferon-gamma (IFN-gamma) alone and in combination with interleukin 1 beta (IL-1 beta) on inducible nitric-oxide synthase (iNOS) mRNA and protein expression, nitrite production, and insulin secretion by islets of Langerhans. Treatment of rat islets with IL-1 beta results in a concentration-dependent increase in the production of nitrite that is maximal at 5 units/ml. Individually, 0.1 unit/ml IL-1 beta or 150 units/ml rat IFN-gamma do not stimulate iNOS expression or nitrite production by rat islets; however, in combination, these cytokines induce the expression of iNOS and the production of nitrite to levels similar in magnitude to the individual effects of 5 units/ml IL-1 beta. The islet beta-cell, selectively destroyed during insulin-dependent diabetes mellitus, appears to be one islet cellular source of iNOS as 150 units/ml rat IFN-gamma and 0.1 unit/ml IL-1 beta induced similar effects in primary beta-cells purified by fluorescence-activated cell sorting and in the rat insulinoma cell line, RINm5F. iNOS expression and nitrite production by rat islets in response to 150 units/ml rat IFN-gamma and 0.1 unit/ml IL-1 beta are correlated with an inhibition of insulin secretion and islet degeneration that are prevented by the iNOS inhibitor aminoguanidine. The mechanism by which IFN-gamma increases the sensitivity of beta-cells for IL-1-induced iNOS expression appears to be associated with an increase in the stability of iNOS mRNA. Last, cellular damage during physical dispersion of islets results in the release of sufficient amounts of IL-1 beta to induce iNOS expression and nitrite production in the presence of exogenously added rat IFN-gamma. The cellular source of IL-1 beta under these conditions is believed to be resident islet macrophages as depletion of macrophages prior to dispersion prevents IFN-gamma-induced iNOS expression and nitrite formation by dispersed islet cells, These studies show that the T lymphocyte cytokine, IFN-1 beta increases the sensitivity of rat islets to the effects of IL-<1beta> on iNOS expression and nitrite production by 10-fold, in part, through the stabilization of iNOS mRNA. Our studies also support an effector role for IFN-gamma, in concert with resident islet macrophage release of IL-1 beta, in mediating beta-cell destruction during the development of autoimmune diabetes.