Proteasome inhibition alters glucose-stimulated (pro)insulin secretion and turnover in pancreatic β-cells

Abstract

Metabolic labeling studies were conducted in freshly isolated mouse islets and a β-cell line (MIN6) to examine the effects of proteasome inhibition on glucose-stimulated (pro)insulin synthesis and secretion. Glucose-stimulated (pro)insulin synthesis, as determined by the incorporation of [ 3H]tyrosine, decreased significantly by 90% in islets and 71% in MIN6 cells pretreated with the proteasome inhibitor lactacystin (10 μM) for 2 h. To follow the fate of newly synthesized (pro)insulin, islets were pulse-labeled with [3H]tyrosine (40 μCi) for 20 min and chased ± lactacystin (10 μM) for up to 4 h. The release of newly synthesized (pro)insulin ([3H]tyrosine-labeled) was similar between lactacystin-treated and control islets despite a 51% decrease (p <0.05) in total immunoreactive (pro)insulin secretion by lactacystin-treated islets. The specific radioactivity of [3H]tyrosine-labeled (pro)insulin in the extracellular medium of lactacystin-treated islets (0.52 ± 0.16 cpm/microunits) was 2-fold greater relative to control islets (0.25 ± 0.06 cpm/ microunits). Induction of the unfolded protein response by lactacystin, as evidenced by the up-regulation of endoplasmic reticulum (ER) chaperones (GRP78/BiP, GRP94, protein disulfide isomerase) and induction of the stress-inducible transcription factor C/EBP-homologous protein/GADD153 (CHOP/GADD153), likely contributed to the release of newly synthesized (pro)insulin to relieve ER stress. The present data indicate proteasome inhibition did not prevent, but increased (p <0.05), the intracellular degradation of [3H]tyrosine-labeled (pro)insulin from 8 to 24% in islets. Collectively, these data indicate β-cells may balance glucose-stimulated (pro)insulin synthesis and secretion with the activity of the proteasome to regulate protein concentrations in the ER. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.