GLUCAGON DECREASES CYTOKINE INDUCTION OF NITRIC OXIDE SYNTHASE AND ACTION ON INSULIN SECRETION IN RIN5F CELLS AND RAT AND HUMAN ISLETS OF LANGERHANS

Abstract

Nitric oxide synthase, induced by cytokines in insulin-containing cells, produces nitric oxide which inhibits function and may promote cell killing. Since glucagon was shown to prevent inducible nitric oxide synthase (iNOS) expression in rat hepatocytes it was of interest to examine the action of glucagon (and cyclic AMP) on iNOS induction in insulin-producing cells. Cultured RIN5F cells and primary rat and human islets of Langerhans were treated with interleukin 1β (IL-1β) or a combination of cytokines, and were co-treated or pre-treated with glucagon. In RIN5F cells, the activity of iNOS induced by IL-1β (10pM, 24h), was significantly reduced by glucagon (1000nM), which raises cyclic AMP, and by forskolin (1–10μM), a non specific activator of adenylate cyclase. Glucagon and forskolin also decreased iNOS expression in RIN5F cells, and rat and human islets, as shown by Western blotting. The inhibitory action of IL-1β (100pM, 24h) on rat islet insulin secretion was partially reversed by 1-h pre-treatment with glucagon (10–1000nM), while the contrasting stimulatory effect of 48-h treatment with cytokines on insulin secretion from human islets was similarly prevented by glucagon (1000nM) pre-treatment. These results suggest that glucagon inhibits iNOS expression in insulin-containing cells and imply that glucagon could modulate the inhibitory effects of cytokines.