title: Overexpression of constitutively activated glutamate dehydrogenase induces insulin secretion through enhanced glutamate oxidation.
abstract: Glutamate dehydrogenase (GDH) catalyzes reversible oxidative deamination of l-glutamate to alpha-ketoglutarate. Enzyme activity is regulated by several allosteric effectors. Recognition of a new form of hyperinsulinemic hypoglycemia, hyperinsulinism/hyperammonemia (HI/HA) syndrome, which is caused by gain-of-function mutations in GDH, highlighted the importance of GDH in glucose homeostasis. GDH266C is a constitutively activated mutant enzyme we identified in a patient with HI/HA syndrome. By overexpressing GDH266C in MIN6 mouse insulinoma cells, we previously demonstrated unregulated elevation of GDH activity to render the cells responsive to glutamine in insulin secretion. Interestingly, at low glucose concentrations, basal insulin secretion was exaggerated in such cells. Herein, to clarify the role of GDH in the regulation of insulin secretion, we studied cellular glutamate metabolism using MIN6 cells overexpressing GDH266C (MIN6-GDH266C). Glutamine-stimulated insulin secretion was associated with increased glutamine oxidation and decreased intracellular glutamate content. Similarly, at 5 mmol/l glucose without glutamine, glutamine oxidation also increased, and glutamate content decreased with exaggerated insulin secretion. Glucose oxidation was not altered. Insulin secretion profiles from GDH266C-overexpressing isolated rat pancreatic islets were similar to those from MIN6-GDH266C, suggesting observation in MIN6 cells to be relevant in native beta-cells. These results demonstrate that, upon activation, GDH oxidizes glutamate to alpha-ketoglutarate, thereby stimulating insulin secretion by providing the TCA cycle with a substrate. No evidence was obtained supporting the hypothesis that activated GDH produced glutamate, a recently proposed second messenger of insulin secretion, by the reverse reaction, to stimulate insulin secretion.
authors.affiliation: Division of Molecular Analysis of Human Disorders, Department of Bio-Signal Analysis, Yamaguchi University Graduate School of Medicine, Ube, Yamaguchi 755-8505, Japan.
authors.forename: Takatoshi
authors.initials: T
authors.lastname: Anno
authors.forename: Shunsuke
authors.initials: S
authors.lastname: Uehara
authors.forename: Hideki
authors.initials: H
authors.lastname: Katagiri
authors.forename: Yasuharu
authors.initials: Y
authors.lastname: Ohta
authors.forename: Kohei
authors.initials: K
authors.lastname: Ueda
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Mizuguchi
authors.forename: Yoshinori
authors.initials: Y
authors.lastname: Moriyama
authors.forename: Yoshitomo
authors.initials: Y
authors.lastname: Oka
authors.forename: Yukio
authors.initials: Y
authors.lastname: Tanizawa
mesh_terms: D000818:Animals; D019556:COS Cells; D002460:Cell Line; D002522:Chlorocebus aethiops; D004789:Enzyme Activation / Q000502:physiology; D005969:Glutamate Dehydrogenase / Q000378:metabolism*; D018698:Glutamic Acid / Q000378:metabolism*; D005973:Glutamine / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000378:metabolism; D051379:Mice; D010084:Oxidation-Reduction; D051381:Rats
chemical_list: D007328:Insulin; D005973:Glutamine; D018698:Glutamic Acid; D005969:Glutamate Dehydrogenase