INTERFERON-GAMMA-INDUCED INTERFERON REGULATORY FACTOR-I (IRF-1) EXPRESSION IN RODENT AND HUMAN ISLET CELLS PRECEDES NITRIC-OXIDE PRODUCTION

The radical nitric oxide (NO) may be a mediator of beta-cell damage in IDDM. The cytokines IFN-alpha and IL-1 beta are required for expressi on of the enzyme nitric oxide synthase (iNOS), and NO production by hu man pancreatic islets. In this study, possible mechanisms by which IFN -gamma participates in iNOS messenger RNA (mRNA) expression were evalu ated in both rodent and human islets cells. Addition of IFN-gamma, bef ore or after arrest of IL-1 beta-induced iNOS gene transcription by ac tinomycin D, did not prolong iNOS mRNA halflife in the rat insulin-pro ducing cell line RINm5F (RIN cells). IFN-gamma also failed to modify I L-1 beta-induced activation of the transcription factor kappa B (NF-ka ppa B) in RIN cells, as determined by electrophoretic mobility shift a ssay. However, IFN-gamma induced an early (30 min-1 h) increase in int erferon regulatory factor-1 (IRF-1) mRNA expression and a later (2 h) 19-fold increase in RIN cell nuclear IRF-1 protein content, an effect further potentiated by IL-1 beta. The total cellular content of IRF-1 protein increased by 30- to 50-fold in human islets exposed for 2-8 h to IFN-gamma or IFN-beta + IL-1 beta. IL-1 beta alone induced a margin al and transient increase in IRF-1. It has been previously reported th at nicotinamide prevents IL-1 beta-induced IRF-1 expression in rat pan creatic islets. However, nicotinamide (20 mM) presently failed to prev ent IL-1 beta + IFN-beta-induced IRF-1 protein expression in human pan creatic islets. In conclusion, the effects of IFN-gamma on iNOS expres sion can neither be explained by iNOS mRNA stabilization nor increased NF-kappa B activation. However, IFN-gamma induces an early increase i n cellular IRF-1 content, and this may contribute to increased iNOS mR NA expression.