Interleukin-1 alpha exerts glucose-dependent stimulatory and inhibitory effects on islet cell phosphoinositide hydrolysis and insulin secretion

Abstract

Isolated rat islets were incubated with myo-[2-3H]inositol to label their phosphoinositides (PI). Labeling was carried out in the presence of various glucose levels (2.75-10 mM) with or without human recombinant interleukin-1 alpha (IL-1). After the labeling period, insulin release, [3H]inositol efflux, and the accumulation of labeled inositol phosphates in perfused islets were assessed under various conditions. The following major observations were made. 1) In islets labeled for 2 h with [3H]inositol in the presence of 2.75 mM glucose, subsequent perifusion with 5.0 nM IL-1 increased insulin output, [3H]inositol efflux, and [3H]inositol phosphate accumulation in the simultaneous presence of 7 mM, but not 2.75 mM, glucose. 2) Mannoheptulose, a competitive inhibitor of islet glucokinase, blocked the stimulatory effects of IL-1 noted in the presence of 7 mM glucose. In other experiments, the conditions used during the 2-h labeling period with myo-[2-3H]inositol were varied. The following major observations were made in islets subsequently stimulated during the perifusion with 20 mM glucose. 3) Islets labeled with [3H]inositol in the presence of 2.75 mM glucose with or without 5.0 nM IL-1 responded with similar increases in PI hydrolysis and insulin output. 4) Compared to that with 2.75 mM glucose alone, labeling in the presence of 7 mM glucose alone was without any adverse effect on the subsequent PI and insulin responses of perifused islets to 20 mM glucose. 5) Labeling in the presence of 7 mM glucose plus 5.0 nM IL-1 resulted in a significant reduction in the subsequent PI and insulin responses. 6) These inhibitory effects of IL-1 were abolished if mannoheptulose was included during the 2-h incubation with 7 mM glucose plus 5.0 nM IL-1. 7) The diacylglycerol kinase inhibitor 1-monooleoylglycerol (100 microM) significantly restored insulin output after IL-1 exposure (with 7 mM glucose). 8) Similar to the results obtained with 7 mM glucose plus IL-1, incubation of islets with 8-10 mM glucose alone produced dose-dependent impairments of [3H]inositol efflux patterns and inositol phosphate accumulation. Insulin secretion was also impaired. These results demonstrate that IL-1 has glucose-dependent stimulatory and inhibitory effects on beta-cell function. Both effects appear to involve alterations in islet PI hydrolysis.