Insulin secretory granule biogenesis. Co-ordinate regulation of the biosynthesis of the majority of constituent proteins

Abstract

Two-dimensional gel-electrophoretic analysis combined with fluorography and densitometric quantification was used to examine the effects of glucose on the biosynthesis of rat pancreatic islet proteins. An increase in the medium glucose concentration from 2.8 to 16.7 mm produced a 10-20-fold stimulation in the synthesis of 10 out of 260 detected islet proteins, as judged by incorporation of [35S]methionine during a 20 min incubation. The synthetic rates of the majority of the remaining proteins were stimulated by 2-4-fold. Greater resolution achieved by pulse-chase labelling and subcellular fractionation showed that, of 32 major proteins localized to insulin secretory granules, the biosynthesis of 25 were stimulated 15-30-fold by glucose. By contrast, only eight of 160 proteins in the soluble fraction showed a response of similar magnitude. It is concluded that there is a major and co-ordinated activation of the biosyntheses of proteins destined for secretory granules, which most likely occurs at the level of translational initiation and signal-recognitionparticle-mediated translocation into the endoplasmic reticulum lumen. However, it is clear that not all granule proteins, or the majority of proteins translocated across the endoplasmic reticulum membrane, are affected in an equivalent manner. In addition, the synthesis of a small number of cytosolic proteins may be increased markedly by insulinotropic stimuli.

Different Regulated Expression of the Tyrosine Phosphatase-Like Proteins IA-2 and Phogrin by Glucose and Insulin in Pancreatic Islets

“…We previously described a progressive increase with age in the islet IA-2 detected by immunohistochemistry on pancreas tissue sections of rats during the first 2 weeks of life (5). We set out to confirm these findings at the molecular level and to extend our observations to the closely related protein, phogrin (IA-2␤).…”

“…Understanding the influence of insulin on pancreatic ␤-cell function requires identification of proteins relevant to the secretory process whose expression or activity is regulated by insulin. Glucose stimulates biosynthesis of a number of ␤-cell proteins, many of which localize to insulin secretory granules and are therefore likely to participate in insulin storage or secretion (5). The identities of these glucose-response proteins and the contribution of insulin signaling to their increased synthesis are largely unknown.…”

Sorting and processing of secretory proteins

“…As will become apparent, there is considerable overlap between these models, and in reality the sorting process probably involves aspects of each. Secretory granules of the regulated pathway carry many proteins aside from regulatory peptides or zymogens [113,[115][116][117][118][119]. These other proteins (both soluble and transmembrane) are of central importance not only to granule integrity but also to granule function and to the secretory process itself.…”

Glucose-stimulated Insulin Biosynthesis Depends on Insulin-stimulated Insulin Gene Transcription

“…Glucose stimulation of insulin-producing cells has been shown to result in an increase in the biosynthesis rate of more than 50 proteins including insulin (16). To what extent this effect is due to a "glucose"effect, "insulin" effect, or effect on transcription or translation (or both) remains to be clarified for each individual protein.…”