Dual Functional Effects of Interleukin-1β on Purine Nucleotides and Insulin Secretion in Rat Islets and INS-1 Cells

Abstract

Interleukin-1β (IL-1β) has been shown to inhibit glucose-induced insulin secretion from rat islets and purified β-cells, primarily through the generation of nitric oxide (NO). However, the mechanisms by which NO exerts its effects remain unclear. To examine the role of purine nucleotides, we cultured intact rat islets or INS-1 (glucose-responsive transformed rat) β-cells for 18 h in the presence or absence of IL-1β. In islets, the exposure to IL-1β (100 pmol/l) inhibited subsequent glucose-induced insulin secretion by 91% with no significant effect on insulin content or basal insulin release. IL-1β also diminished insulin secretion induced by pure mitochondrial fuels, 40 mmol/l K+, or a phorbol ester. Concomitantly, IL-1β significantly decreased islet ATP (–45%), GTP (–33%), ATP/ADP (–54%), and GTP/GDP (–46%). These effects were totally reversed by provision of Nω-nitro-L-arginine methyl ester (NAME) in arginine-free media that inhibited NO production. In contrast, in INS-1 cells, IL-1β (10 or 100 pmol/l) reduced both basal and glucose-induced insulin secretion by 50%, but insulin content was also reduced by 35%. Therefore, the INS-1 cells were still able to respond to glucose stimulation with a 1.8–2.0–fold increase in insulin release in either the presence or absence of IL-1β. Concomitantly, in INS-1 cells, IL-1β had no effect on ATP/ADP or GTP/GDP ratios, although it modestly decreased ATP (–25%) and GTP (–22%). As in islets, all effects of IL-1β in INS-1 cells were prevented by NAME. Thus, in rat islets, IL-1β (via the generation of NO) abolishes insulin exocytosis in association with large decreases in the ATP/ADP (and GTP/GDP) ratio, implying the impairment of mitochondrial function. Furthermore, IL-1β inhibits cytosolic synthesis of new purine nucleotides (via the salvage pathway), as assessed by a decrease in their specific activity after labeling with [3H]hypoxanthine. In contrast, in INS-1 cells, IL-1β appears to impair cytosolic synthesis of purine nucleotides and insulin biosynthesis selectively (both possibly reflecting decreased glycolysis) with little direct effect on insulin exocytosis itself.