Tyrosine kinase inhibitors prevent cytokine-induced expression of iNOS and COX-2 by human islets

Insulin-dependent diabetes mellitus (IDDM) is an autoimmune disease that is characterized by selective destruction of insulin-secreting β-cells. Cytokines have been implicated as effector molecules that participate in both islet inflammation and β-cell destruction during the development of IDDM. In this study, the effects of cytokines on the expression of inducible nitric oxide synthase (iNOS) and inducible cyclooxygenase (COX-2) by human islets were examined. In combination, the cytokines, human recombinant interleukin- 1β (IL-1β), human recombinant tumor necrosis factor-α (TNF-α), and human recombinant interferon-γ (IFN-γ), induce the time-dependent formation of nitrite and prostaglandin E₂ (PGE₂) by human islets. The nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA) completely inhibits cytokine-induced nitrite formation and attenuates PGE₂ production by human islets. L-NMMA does not inhibit cytokine-induced expression of COX-2 by human islets, suggesting that nitric oxide may directly activate cyclooxygenase, an effect that has been previously demonstrated for isolated rat islets. This combination of cytokines (IL-1β, TNF-α, and IFN-γ) also induces the expression of iNOS mRNA by human islets as demonstrated by both reverse transcriptase-polymerase chain reaction and Northern blot analysis. We further show that the tyrosine kinase inhibitors genistein and herbimycin A prevent IL-1β plus IFN-γ-induced expression of COX-2 and iNOS and the production of PGE₂ and nitric oxide by human islets. These results demonstrate that cytokines induce the expression of iNOS and COX-2 by human islets and that cytokine-induced expression of both COX-2 and iNOS by human islets appears to require the activation of a tyrosine kinase(s).