Glucose-Regulated Proinsulin Gene Expression Is Required for Adequate Insulin Production during Chronic Glucose Exposure

Psammomys obesus, an animal model of type 2 diabetes, shows rapid and marked depletion of pancreatic insulin content as hyperglycemia develops when fed a high-calorie diet. P. obesus islets do not increase proinsulin gene expression when exposed to high glucose, which may be related to absence of the conserved form of the transcription factor insulin promoter factor 1/pancreatic-duodenal homeobox 1. The present study assesses the importance of regulation of proinsulin gene expression by glucose for insulin production. Islets of diabetes-prone P. obesus and diabetes-resistant Wistar rats, cultured at various glucose concentrations for up to 24 h, were analyzed for proinsulin mRNA by quantitative RT-PCR, proinsulin biosynthesis by leucine incorporation into proinsulin, and insulin content and secretion by RIA. No increase in proinsulin mRNA was observed in P. obesus islets during 24-h exposure to increasing concentrations of glucose. In contrast, rat islets exposed to high glucose responded with a 2- to 3-fold stimulation of proinsulin mRNA. The failure of P. obesus islets to increase proinsulin mRNA was accompanied by a reduced proinsulin biosynthetic response: after 24 h, maximal proinsulin biosynthesis was blunted, associated with depletion of islet insulin content. Inhibition of glucose-stimulated proinsulin gene transcription in rat islets by actinomycin D did not affect the early proinsulin biosynthetic response, which, however, was reduced to the level of P. obesus islets after 24 h in culture. We conclude that stimulation of proinsulin gene transcription by glucose is necessary for maintaining proinsulin biosynthesis and hence conserving pancreatic insulin stores, under conditions of sustained secretory drive, but not for short-term regulation of proinsulin biosynthesis Our findings support the hypothesis that inadequate regulation of proinsulin gene expression by glucose contributes to the failure of P. obesus to cope with the increased demand for insulin associated with caloric excess, leading to depletion of insulin stores and diabetes.