Proteomic Analysis of Restored Insulin Production and Trafficking in Obese Diabetic Mouse Pancreatic Islets Following Euglycemia

Abstract

For the treatment of patients with prediabetes or diabetes, clinical evidence has emerged that β-cell function can be restored by glucose-lowering therapeutic strategies. However, little is known about the molecular mechanisms underlying this functional adaptive behavior of the pancreatic β-cell. This study examines the dynamic changes in protein expression and phosphorylation state associated with (pro)insulin production and secretory pathway function mediated by euglycemia to induce β-cell rest in obese/diabetic db/db islet β-cells. Unbiased quantitative profiling of the protein expression and phosphorylation events that occur upon β-cell adaption during the transition from hyperglycemia to euglycemia was assessed in isolated pancreatic islets from obese diabetic db/db and wild-type (WT) mice using quantitative proteomics and phosphoproteomics together with bioinformatics analysis. Dynamic changes in the expression and phosphorylation of proteins associated with pancreatic β-cell (pro)insulin production and complementary regulated-secretory pathway regulation were observed in obese diabetic db/db islets in a hyperglycemic environment, relative to WT mouse islets in a normal euglycemic environment, that resolved when isolated db/db islets were exposed to euglycemia for 12 h in vitro. By similarly treating WT islets in parallel, the effects of tissue culture could be mostly eliminated and only those changes associated with resolution by euglycemia were assessed. Among such regulated protein phosphorylation-dependent signaling events were those associated with COPII-coated vesicle-dependent ER exit, ER-to-Golgi trafficking, clathrin-coat disassembly, and a particular association for the luminal Golgi protein kinase, FAM20C, in control of distal secretory pathway trafficking, sorting, and granule biogenesis. Protein expression and especially phosphorylation play key roles in the regulation of (pro)insulin production, correlative secretory pathway trafficking, and the restoration of β-cell secretory capacity in the adaptive functional β-cell response to metabolic demand, especially that mediated by glucose.

Keywords: ER homeostasis; ER-to-Golgi trafficking; d/d; euglycemia; insulin; insulin biosynthesis; mass spectrometry; obesity; pancreatic islet β-cell; phosphoproteomics; secretory pathway; translation; type 2 diabetes; β-cell rest.