Glucose stimulates proinsulin synthesis in pancreatic islets without a concomitant increase in proinsulin mRNA synthesis

In a previous paper (N. Itoh and H. Okamoto (1980) Nature 283, 100-102), we showed that the proinsulin mRNA level in rat pancreatic islets remained unchanged during the period of proinsulin induction by glucose. Here, we examined the transcriptional activity of the proinsulin gene in islets using a recombinant DNA containing rat proinsulin cDNA sequence as a probe. Although proinsulin synthesis was stimulated 8.4-fold by glucose, proinsulin mRNA synthesis was stimulated only 1.8-fold. From these results, we concluded that the stimulation of proinsulin synthesis by glucose was mainly regulated at the translation level. The glucose stimulation at the transcription level may play a part in the maintenance of the proinsulin mRNA level in islets.