Prolonged STAT1 Activation Is Associated with Interferon-γ Priming for Interleukin-1-induced Inducible Nitric-oxide Synthase Expression by Islets of Langerhans

In this study, the ability of interferon-γ (IFN-γ) to prime rat and nonobese diabetic (NOD) mouse islets for interleukin-1 (IL-1)-stimulated expression of inducible nitric-oxide synthase (iNOS) has been examined. IL-1-induced iNOS expression by rat islets is concentration-dependent with maximal expression occurring in response to 1.0 unit/ml. Individually, neither 0.1 unit/ml IL-1 nor 150 units/ml IFN-γ stimulates iNOS expression or nitrite production by rat islets. However, a 30–60-min pulse of rat islets with IFN-γ, followed by washing to remove the cytokine and continued culture with 0.1 unit/ml IL-1 for 40 h, results in iNOS expression and nitrite production to levels similar in magnitude to the individual effects of 1.0 unit/ml IL-1. A 1-h pulse with IFN-γ primes for IL-1-induced islet degeneration that is mediated by the expression of iNOS and increased production of nitric oxide. IFN-γ also primes for IL-1-induced iNOS expression and nitrite formation by NOD mouse islets. The priming actions of IFN-γ appear to be selective for β-cells, as IFN-γ primes for IL-1-induced nitrite formation by primary β-cells and RINm5F insulinoma cells, but not primary α-cells. The priming actions of IFN-γ for IL-1-induced iNOS expression do not requirede novo protein synthesis as preincubation of RINm5F cells with cycloheximide does not inhibit iNOS mRNA accumulation under priming conditions. The priming actions of IFN-γ on IL-1-induced iNOS expression persists for extended periods of up to 7 days and are associated with persistent signal transducers and activators of transcription (STAT)-1 activation. A 30-min pulse of rat islets with IFN-γ stimulates STAT1 phosphorylation, and STAT1 remains phosphorylated for up to 7 days following IFN-γ removal. In addition, STAT1 remains nuclear for up to 7 days after IFN-γ removal. These results indicate that IFN-γ primes for IL-1-induced islet degeneration via a nitric oxide-dependent mechanism. These findings also provide evidence that the priming actions of IFN-γ for IL-1-induced iNOS expression by islets are associated with the prolonged phosphorylation and activation of STAT1.