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

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.