Glucose regulation of glutaminolysis and its role in insulin secretion.

Abstract

Leucine or the nonmetabolized leucine analog +/- 2-amino-2-norbornane-carboxylic acid (BCH) (both at 10 mmol/l) induced biphasic insulin secretion in the presence of 2 mmol/l glutamine (Q2) in cultured mouse islets pretreated for 40 min without glucose but with Q2 present. The beta-cell response consisted of an initial peak of 20- to 25-fold above basal and a less marked secondary phase. However, BCH produced only a delayed response, while leucine was totally ineffective when islets were pretreated with 25 mmol/l glucose plus Q2. With Q2, 10 mmol/l BCH or leucine caused a nearly threefold increase, a twofold increase, or had no effect on cytosolic Ca2+ levels in islets pretreated for 40 min with 0, 5, or 15 mmol/l glucose, respectively. Thus, pretreatment of islets with high glucose inhibited BCH- and leucine-induced cytosolic Ca2+ changes and insulin release. Glucose decreased glutamine oxidation in cultured rat islets when BCH was present at 10 mmol/l, but not in its absence, with a lowest effective level of approximately 0.1 mmol/l, a maximum of 18-30 mmol/l, and an inhibitory concentration, 50%, of approximately 3 mmol/l. The data are consistent with the hypothesis that glucose inhibits glutaminolysis in pancreatic beta-cells in a concentration-dependent manner and hence blocks leucine-stimulated insulin secretion. We postulate that in the basal interprandial state, glutaminolysis of beta-cells is partly turned on because glutamate dehydrogenase (GDH) is activated by a decreased P-potential due to partial fuel depletion and sensitization to endogenous activators such as leucine. Additionally, it may contribute significantly to basal insulin release, which is known to be responsible for about half of the insulin released daily. The data explain "leucine-hypersensitivity" of beta-cells during hypoglycemia and contribute to the elucidation of the GDH-linked syndrome of hyperinsulinism associated with elevated serum ammonia levels. Thus, understanding the precise regulation and role of beta-cell glutaminolysis is probably central to our concept of normal blood glucose control.

Insulin secretion profiles are modified by overexpression of glutamate dehydrogenase in pancreatic islets

“…Therefore, islet glutamate levels can be markedly increased by exposure to extracellular glutamine and, because of scarce conversion of glutamate to α-ketoglutarate [24], without stimulation of insulin secretion [24,55]. Activation of GDH by L-leucine or its non-metabolisable analogue BCH increases glutamine oxidation and insulin secretion, essentially by enhancing the oxidative deamination of glutamate [25,27,28,29,32,33,56]. GDH activity is under the tight control of allosteric effectors, although the flux direction depends chiefly on the relative supply of substrates and cofactors, i.e.…”

“…glutamate, α-ketoglutarate, and NAD(P) + /H respectively. Accordingly, glutamine oxidation stimulated by the presence of an allosteric activator of GDH is inhibited by glucose [28], which increases the carbon pool on the TCA cycle side. The glutamate and α-ke toglutarate pathway is operated either by GDH or alternatively by transamination reactions [57,58].…”

“…Indeed, glutamine oxidation can be prompted by allosteric activation of GDH, an effect correlating with stimulation of insulin secretion [25,27]. However, glucose exposure inhibits glutamine oxidation [28] and glutamate formation from glutamine [29]. Therefore, GDH could play a dual role in glutamate handling [30].…”

Functional Hyperactivity of Hepatic Glutamate Dehydrogenase as a Cause of the Hyperinsulinism/Hyperammonemia Syndrome: Effect of Treatment

“…5 Recently, it has been shown that insulin secretion can be stimulated by leucine, which activates GDH. 20 The differential diagnosis of infantile nonketotic hypoglycemia has been extended. It is now clear that in every case of suspected hyperinsulinism blood ammonia levels should be determined, preferably together with plasma amino acids.…”

Glutamine and glutamate as vital metabolites

“…Glutamine has been reported to enhance glucose-or leucine-stimulated insulin secretion from pancreatic ß-cells (located in the endocrine islets of Langerhans), but does not promote insulin secretion by itself due to tight regulation of glutamate dehydrogenase activity (56,57). Glutamine may act as an anaplerotic substrate in the ß-cell, via formation of glutamate and 2-oxoglutarate, subsequently stimulating a catalytic enhancement of glucose oxidation (58).…”