GLUCOSE STIMULATES ISLET BETA-CELL MITOGENESIS THROUGH GTP-BINDING PROTEINS AND BY PROTEIN-KINASE C-DEPENDENT MECHANISMS

Glucose is a cardinal secretory and mitogenic stimulus for the insulin -producing pancreatic beta-cell both in vitro and in vivo, but the mec hanisms by which the sugar acts mitogenically remain largely elusive. In this study, the intracellular pathways that convey glucose-induced mitogenic and secretory signaling in beta-cells were investigated. For this purpose, fetal rat pancreatic islets enriched in beta-cells were cultured in 3.3 or 16.7 mmol/l glucose for 3 days. It was found that glucose stimulated beta-cell replication, insulin secretion, and cAMP content. These effects were mimicked by agonists of cAMP-dependent pro tein kinases but not by guanosine-3',5'-cyclic monophosphate (cGMP). A ntagonists of cAMP-dependent protein kinases failed to block the gluco se-induced increments in beta-cell replication and insulin secretion. Glucose is known to activate protein kinase C, and a protein kinase C- activating phorbol ester was found to promote beta-cell mitogenesis an d insulin secretion. Conversely when protein kinase C was inhibited, t he mitogenic (but not secretory) response to glucose was attenuated. T here were no additive or synergistic effects on beta-cell replication when cAMP and phorbol ester were combined, whereas insulin secretion w as potentiated by this combination. Artificially causing Ca2+ inlet by glibenclamide or ionomycin did not result in a stimulated mitogenic r esponse, and preventing Ca2+ influx by blocking plasma membrane Ca2+ c hannels did not abolish the mitogenicity of glucose, although it reduc ed insulin secretion. Pretreatment of islets with pertussis toxin, kno wn to regulate transduction of signals through heterotrimeric GTP-bind ing proteins, completely prevented the stimulatory effect of glucose o n beta-cell mitogenesis but not on insulin secretion. We conclude that specific activation of protein kinase C or cAMP synthesis is sufficie nt to increase beta-cell mitogenesis and insulin secretion, whereas cG MP appears not to affect these processes. However, cAMP does not seem to mediate the mitogenicity or secretory action of glucose. The result s instead suggest that signaling through GTP-binding proteins and prot ein kinase C activation is required for transduction of the mitogenic, but not secretory, message of the sugar in the beta-cell.