Overexpression of mitochondrial FAD-linked glycerol-3-phosphate dehydrogenase does not correct glucose-stimulated insulin secretion from diabetic GK rat pancreatic islets

Abstract

Glucose-stimulated insulin secretion is impaired in GK (Goto-Kakizaki) rats, perhaps because of abnormalities in glucose metabolism in pancreatic islet beta cells. The glycerol phosphate shuttle plays a major role in glucose metabolism by reoxidizing cytosolic NADH generated by glycolysis. In the pancreatic islets of GK rats, the activity of mitochondrial FAD-linked glycerol-3-phosphate dehydrogenase (mGPDH), the key enzyme of the glycerol phosphate shuttle, is decreased and this abnormality may be responsible, at least in part, for impaired glucose-stimulated insulin secretion. To investigate this possibility, we overexpressed mGPDH in islets isolated from GK rats via recombinant adenovirus-mediated gene transduction, and examined glucose-stimulated insulin secretion. In islets isolated from diabetic GK rats at 8 to 10 weeks of age, glucose-stimulated insulin secretion was severely impaired, and mGPDH activity was decreased to 79% of that in non- diabetic Wistar rats. When mGPDH was overexpressed in islets from GK rats, enzyme activity and protein content in- creased 2- and 6-fold, respectively. Basal (3 mmol/l glucose) and glucose-stimulated (20 mmol/l) insulin secretion from the Adex1CAlacZ-infected GK rat islets were, respectively, 4.4 ± 0.7 and 8.1 ± 0.7 ng · islet-1 · 30 min-1, and those from mGPDH- overexpressed GK rat islets 4.7 ± 0.3 and 9.1 ± 0.8 ng · islet-1 · 30 min-1, in contrast to those from the Adex1CAlacZ-infected non-diabetic Wistar rat islets (4.7 ± 1.6 and 47.6 ± 11.9 ng · islet-1 · 30 min-1). Thus, glucose-stimulated insulin secretion is severely impaired in GK rats even in the stage when mGPDH activity is modestly decreased, and at this stage, overexpression of mGPDH cannot restore glucose-stimulated insulin secretion. We conclude that decreased mGPDH activity in GK rat islets is not the defect primarily responsible for impaired glucose-stimulated insulin secretion.