The Acute Effects of Glucose on the Insulin Biosynthetic- Secretory Pathway in a Simian Virus 40-Transformed Hamster Pancreatic Islet β-Cell Line*

Abstract

The acute regulatory effects of glucose on proinsulin synthesis at the translational level and the subsequent processing of newly made hormone through the insulin biosynthetic-secretory pathway in a clonal cell line (HIT-T15 cells) of simian virus 40-transformed hamster pancreatic islet beta-cells were examined. Proinsulin synthesis in HIT-T15 cells, assayed by immunoprecipitation, comprised 2.5-5% of the total protein synthesis during a 40-min pulse label period, though preproinsulin mRNA levels were only 0.25-0.29% of the total mRNA. There was no specific enhancement of proinsulin synthesis in response to increasing concentrations of glucose or other tested metabolites or by agents stimulating the cAMP or protein kinase-C second messenger systems during a 40-min pulse label period. Compared to that in normal beta-cells, processing of proinsulin to insulin was markedly retarded. Unlike that in normal beta-cells, newly synthesized hormone in HIT cells was secreted at a very low rate that was not enhanced by a high glucose concentration and was not preferentially secreted compared with preformed hormone. Thus, although a glucose sensor was present in HIT cells, since secretion of preformed insulin stores was stimulated 8-fold between glucose concentrations of 0 and 11 mM, regulation at several stages in the passage of newly made insulin through the biosynthetic-secretory pathway did not display the acute glucose sensitivity seen in normal nontransformed beta-cells.