THE ROLE OF PROTEIN-KINASE-C IN INSULIN-BIOSYNTHESIS

Activation of protein kinase C (PKC) by the phorbol ester 4beta-phorbo l myristate acetate (4beta-PMA) stimulated (pro)insulin biosynthesis i n collagenase-isolated rat islets of Langerhans, as assessed by measur ing the incorporation of [S-35]cysteine into proinsulin and insulin af ter fractionation by high performance liquid chromatography. The stimu latory effects of 4beta-PMA were observed at a substimulatory concentr ation of glucose (2 mM) but were not additive to the stimulatory effec ts of 20 mM glucose on insulin biosynthesis. Prolonged exposure to 4be ta-PMA caused a marked down-regulation of PKC activity in islets. PKC- depleted islets showed a much reduced biosynthetic response to 20 mM g lucose, but this was caused, at least in part, by an enhanced basal ra te of (pro)insulin synthesis. These elevations in the basal rate of in sulin synthesis were not secondary to an increase in the amount of pre proinsulin mRNA in PKC-depleted islets since Northern blot analysis sh owed that prolonged exposure to 4beta-PMA, and the subsequent loss of PKC activity, did not detectably alter basal levels of preproinsulin m RNA. These results suggest that the activation of PKC stimulates (pro) insulin synthesis in rat islets by enhancing translation of existing p reproinsulin mRNA, and that this may play some part in the biosyntheti c responses of beta-cells to glucose.