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

Abstract

In the GK (Goto-Kakizaki) rat, a spontaneous diabetic rodent model produced by repeated selective breeding of the Wistar rat, the insulin secretory response to glucose is selectively impaired [1,2] as in human non-insulin-dependent diabetes mellitus (NIDDM) patients [3]. Because the defect is specific for glucose-stimulated insulin secretion, the metabolic glucose signal appears to be impaired [4,5]. Glucose must enter beta cells and be metabolized via glycolytic and mitochondrial oxidative pathways in order to stimulate insulin release. The resulting changes in the intracellular ATP/ADP ratio close the ATP dependent potassium channel (K-ATP), leading to membrane depolarization, Ca 2+ entry and insulin exocytosis. In GK rats, it was suggested that the step responsible for the metabolic dysfunction of beta cells is located within the glycolytic pathway before Diabetologia (1998) Summary 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 increased 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 Adex1-CAlacZ-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. [Diabetologia (1998) 41: 649±653]

β-Cell Adaptation to Insulin Resistance

“…However, null mice for mGPD were found to have no defect in insulin secretion unless combined with pharmacological disruption of the malate-aspartate shuttle (34). Furthermore, overexpressing mGPD in islets of diabetic GK rats failed to restore glucoseinduced insulin secretion (47). Thus, defective activity of the glycerol phosphate shuttle alone is not sufficient for impaired glucose-induced insulin secretion, which is consistent with our findings in the ZF islets.…”

Overexpression of monocarboxylate transporter and lactate dehydrogenase alters insulin secretory responses to pyruvate and lactate in β cells

“…To explore the significance of the documented low activities of LDH and MCT in the β cell relative to islet non-β cells (6,8) and other cell types (8,16), we have investigated the consequences of overexpression of LDH-A and MCT-1 on glucose-, pyruvate-, or lactate-stimulated insulin secretion in INS-1 cells and isolated rat islets. Because the chicken actin promoter used in our recombinant adenovirus constructs (27) is active in both β cells and non-β cells (28), this approach diminishes differences in expression levels of LDH and MCT between these cells.…”

Phenotyping of Individual Pancreatic Islets Locates Genetic Defects in Stimulus Secretion Coupling to Niddm1i Within the Major Diabetes Locus in GK Rats

“…Glycerol phosphate dehydrogenase was mapped to rat chromosome 3 in the vicinity of a region linked to diabetes in GK rats (34). However, overexpression of the enzyme in islets from GK rats was not able to alleviate the impaired glucose-induced insulin secretion (35). Although glucose metabolism may be altered in the GK rat, normal Krebs cycle function has been observed in GK islets (36,37), which is compatible with our observation of a glucose-induced lowering of pO 2 in GK and Niddm1i islets.…”