Glucose Stimulates Islet β-Cell Mitogenesis Through GTP-Binding Proteins and by Protein Kinase C–Dependent Mechanisms

Abstract

Glucose is a cardinal secretory and mitogenic stimulus for the insulin-producing pancreatic ö-cell both in vitro and in vivo, but the mechanisms by which the sugar acts mitogenically remain largely elusive. In this study, the intracellular pathways that convey glucose-induced mitogenic and secretory signaling in ö-cells were investigated. For this purpose, fetal rat pancreatic islets enriched in ö-cells were cultured in 3.3 or 16.7 mmol/l glucose for 3 days. It was found that glucose stimulated ö-cell replication, insulin secretion, and cAMP content. These effects were mimicked by agonists of cAMP-dependent protein kinases but not by guanosine- 3′,5′-cyclic monophosphate (cGMP). Antagonists of cAMP-dependent protein kinases failed to block the glucose-induced increments in β-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 β-cell mitogenesis and insulin secretion. Conversely, when protein kinase C was inhibited, the mitogenic (but not secretory) response to glucose was attenuated. There were no additive or synergistic effects on β-cell replication when cAMP and phorbol ester were combined, whereas insulin secretion was potentiated by this combination. Artificially causing Ca2+ inlet by glibenclamide or ionomycin did not result in a stimulated mitogenic response, and preventing Ca2+ influx by blocking plasma membrane Ca2+ channels did not abolish the mitogenicity of glucose, although it reduced insulin secretion. Pretreatment of islets with pertussis toxin, known to regulate transduction of signals through heterotrimeric GTP-binding proteins, completely prevented the stimulatory effect of glucose on +-cell mitogenesis but not on insulin secretion. We conclude that specific activation of protein kinase C or cAMP synthesis is sufficient to increase β-cell mitogenesis and insulin secretion, whereas cGMP appears not to affect these processes. However, cAMP does not seem to mediate the mitogenicity or secretory action of glucose. The results instead suggest that signaling through GTP-binding proteins and protein kinase C activation is required for transduction of the mitogenic, but not secretory, message of the sugar in the β-cell.