Interferon-γ-induced interferon regulatory factor-1 (IRF-1) expression in rodent and human islet cells precedes nitric oxide production

Abstract

The radical nitric oxide (NO) may be a mediator of β-cell damage in IDDM. The cytokines IFN-γ, and IL-1β are required for expression of the enzyme nitric oxide synthase (iNOS), and NO production by human pancreatic islets. In this study, possible mechanisms by which IFN-γ participates in iNOS messenger RNA (mRNA) expression were evaluated in both rodent and human islets cells. Addition of IFN-γ, before or after arrest of IL-1β-induced iNOS gene transcription by actinomycin D, did not prolong iNOS mRNA half life in the rat insulin-producing cell line RINm5F (RIN cells). IFN-γ, also failed to modify IL-1β-induced activation of the transcription factor κB (NF-κB) in RIN cells, as determined by electrophoretic mobility shift assay. However, IFN-γ induced an early (30 min-1 h) increase in interferon 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-1β. The total cellular content of IRF-1 protein increased by 30- to 50-fold in human islets exposed for 2-8h to IFN-γ or IFN-γ + IL- 1β. IL-1β alone induced a marginal and transient increase in IRF-1. It has been previously reported that nicotinamide prevents IL-1β-induced IRF-1 expression in rat pancreatic islets. However, nicotinamide (20 mM) presently failed to prevent IL-1β + IFN-γ-induced IRF-1 protein expression in human pancreatic islets. In conclusion, the effects of IFN-γ on iNOS expression can neither he explained by iNOS mRNA stabilization nor increased NF-κB activation. However, IFN-γ induces an early increase in cellular IRF-1 content, and this may contribute to increased iNOS mRNA expression.