GLUCOSE-REGULATED TRANSLATIONAL CONTROL OF PROINSULIN BIOSYNTHESIS WITH THAT OF THE PROINSULIN ENDOPEPTIDASES PC2 AND PC3 IN THE INSULIN-PRODUCING MIN6 CELL-LINE

In the short term (<2 h), proinsulin biosynthesis is predominately glu cose regulated at the translational level; however, the details at the molecular level behind this mechanism are not well defined, One of th e major hindrances for gaining a better understanding of the proinsuli n biosynthetic mechanism has been a lack of au abundant source of beta -cells that express a phenotype of regulated proinsulin biosynthesis i n the appropriate 2.8-16.7 mmol/l glucose range as defined in normal p ancreatic islets, In this study, we demonstrate that in the MIN6 cell line, specific glucose-regulated translational control of proinsulin b iosynthesis is present in the appropriate glucose concentration range, In addition to that of proinsulin, the biosynthesis of the two proins ulin conversion endopeptidases, PC2 and PC3, was coordinately glucose regulated in MING cells, whereas that of the exopeptidase, carboxypept idase H, was unaffected by glucose, Proinsulin, PC2, and PC3 biosynthe sis was specifically stimulated over that of total MIN6 cell protein s ynthesis above a threshold of 4 mmol/l glucose that reached a maximum rate between 8 and 10 mmol/l glucose, Glucose-induced proinsulin, PC2, and PC3 biosynthesis was rapid (occurring after a 20-min lag period b ut reaching a maximum by 60 min), unaffected by the presence of actino mycin D; and in parallel experiments, stimulatory glucose concentratio ns did not alter MING cell total preproinsulin, PC2, or PC3 mRNA level s. Thus, short-term (<2 h) glucose stimulation of proinsulin, PC2, and PC3 biosynthesis in MIN6 cells, Like that in isolated islets, was med iated at the translational level, Intracellular signals for mediating glucose-stimulated proinsulin PC2 and PC3 biosynthesis translation in MING cells also appeared to be similar to those in pancreatic islets, requiring glucose metabolism and a supporting role for protein kinase A, However, protein kinase C or a Ca2+-dependent protein kinase did no t appear to be required for glucose-regulated proinsulin biosynthesis in MING cells, as in islets, MIN6 cells are the first beta-cell line t hat indicate glucose-regulated proinsulin biosynthesis translation ess entially identical to that in differentiated islet beta-cells and will be an important experimental model to better define the mechanism of proinsulin biosynthesis in detail.