ACTA UNIVERSITATIS UPSALIENSIS
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine 735
Distributor: Uppsala University Library, Uppsala, Sweden
Malin Flodström
Regulation of Cytokine-Induced Nitric Oxide Production in Insulin-Producing Cells
Dissertation in Medical Cell Biology to be publicly examined in the lecture hall B21, Biomedical Centre, Uppsala University, on February 6, 1998, at 9.15 a.m., for the Degree of Doctor of Medical Science. The examination will be conducted in English.
ABSTRACT
Flodström, M. 1998. Regulation of cytokine-induced nitric oxide production in insulin-producing cells. Acta Universitatis Upsaliensis. Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine 735. 75 pp. Uppsala. ISBN 91-554-4120-3.
Cytokine-induced expression of the inducible form of nitric oxide synthase (iNOS) and production of nitric oxide (NO) may contribute to pancreatic b-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 kB (NF-kB) is required for the expression of iNOS in diverse rodent cells. Activation of NF-kB was presently observed to be necessary but not sufficient for the induction of iNOS by cytokines (IL-1b + IFN-g + TNF-a) in human islets. This suggests that other transcription factors may play a role in this process. IFN-g alone or in combination with IL-1b induced the expression of the transcription factor interferon regulatory factor-1 (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 b-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 b-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-1b or IFN-g + TNF-a 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-1b increased arginine accumulation by purified rat b-cells.
It is concluded that NF-kB, and possibly IRF-1, contribute to the expression of iNOS in cytokine-exposed insulin-producing rat b-cells and intact human and rat islets. Cytokines also increase the b-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.
Key words: Diabetes, pancreatic islets, insulin-producing cells, cytokines, nitric oxide, inducible nitric oxide synthase, nuclear factor-kappaB, interferon regulatory factor-1, arginine, citrulline, argininosuccinate synthetase, insulin release.
Malin Flodström, Department of Medical Cell Biology, Uppsala University, Box 571, S-751 23 Uppsala, Sweden
