Age-Related Impairment in the Short Term Regulation of Insulin Biosynthesis by Glucose in Rat Pancreatic Islets*

Abstract

The pancreatic beta-cell is differentiated to express the insulin biosynthetic secretory pathway and to regulate this pathway principally by sensing the extracellular glucose concentration. With aging, glucose stimulation of both insulin synthesis and secretion is significantly impaired. The mechanism of the 40-50% decrease in the short term stimulation of insulin synthesis by glucose in rat pancreatic islets was studied in an effort to elucidate the specific stage at which the impairment occurs. In isolated islets from both young (4- to 5-month-old) and old (18- to 28-month-old) rats at normal (5.5 mM) and elevated (11 mM) glucose concentrations, insulin I was the predominant mature insulin species that accumulated during a 4-h labeling period. After preincubation of islets for 16 h in Minimum Essential Medium (with 5.5 mM glucose), total preproinsulin mRNA levels were similar in old and young islets; after preincubation in an enriched medium, RPMI-1640 (with 11 mM glucose), old and young levels were still similar, although preproinsulin mRNA levels were 6-fold higher. Proinsulin synthesis during a 30-min pulse label period was stimulated 10-fold in young islets and 7-fold in old islets at 16.7 mM glucose compared with synthesis at 2.8 mM glucose and was not affected by 5 micrograms/ml actinomycin-D. Furthermore, stimulation of the two major known intracellular second messenger systems in beta-cells, the cAMP system and the protein kinase-C system, with 8-bromo-cAMP, 3-isobutyl-1-methylxanthine, or a phorbol ester did not further increase proinsulin synthesis over a 30-min pulse label period in islets already stimulated by 16.7 mM glucose. Thus, the impairments in the short term effects of glucose on proinsulin synthesis in old islets are posttranscription of preproinsulin mRNA and likely to lie at the level of enhancement of translation of preexisting preproinsulin mRNA by factors activated or inhibited by the ambient glucose concentration. These age-impaired factors are not modulated through the cAMP or protein kinase-C system.