Regulation of cytokine-induced nitric oxide production in insulin-producing cells

Abstract: Cytokine-induced expression of the inducible form of nitric oxide synthase (iNOS) and production of nitric oxide (NO) may contribute to pancreatic β-cell damage during the development of type 1 diabetes. iNOS catalyses the conversion of arginine into citrulline and NO, a reaction regulated by the amount of expressed iNOS enzyme and by the cellular availability of arginine. Activation of the transcriptional regulator nuclear factor κB (NF-κB) is required for the expression of iNOS in diverse rodent cells. Activation of NF-κB was presently observed to be necessary but not sufficient for the induction of iNOS by cytokines (IL-1β + IFN-γ + TNF-α) in human islets. This suggests that other transcription factors may play a role in this process. IFN-γ alone or in combination with IL-1β induced the expression of the transcription factor interferon regulatory factor-l (IRF-1) both in rodent and human islets. IRF-1 is necessary for iNOS expression by murine macrophages and may therefore also contribute to the expression of iNOS in insulin-producing cells. Synthesis of NO by cytokine-exposed purified rat β-cells, rodent and human islets was found to depend on the presence of extracellular arginine or citrulline, with arginine levels similar to that observed in plasma (80-200 µM) limiting NO-production. Cytokine-stimulated purified rat β-cells and human and rat islets used citrulline as efficiently as arginine for NO-synthesis, suggesting an intracellular generation of arginine by a citrulline-NO cycle. In line with this hypothesis, IL-1β or IFN-γ + TNF-α enhanced mRNA expression and enzyme activity of argininosuccinate synthetase (AS), the rate-limiting enzyme in the citrulline-NO cycle, in insulin-producing cells. This was paralleled by the induction of iNOS mRNA. Moreover, IL-1β increased arginine accumulation by purified rat β-cells.It is concluded that NF-κB, and possibly IRF-1, contribute to the expression of iNOS in cytokine-exposed insulin-producing rat β-cells and intact human and rat islets. Cytokines also increase the β-cell capacity to accumulate arginine and activate the citrulline-NO cycle. This adaptive response in cytokine-exposed insulin-producing cells may be of relevance to assure an adequate arginine supply for durable NO synthesis.