Increased Uncoupling Protein-2 Levels in β-cells Are Associated With Impaired Glucose-Stimulated Insulin Secretion

Abstract

In pancreatic ␤-cells, glucose metabolism signals insulin secretion by altering the cellular array of messenger molecules. ATP is particularly important, given its role in regulating cation channel activity, exocytosis, and events dependent upon its hydrolysis. Uncoupling protein (UCP)-2 is proposed to catalyze a mitochondrial inner-membrane H ؉ leak that bypasses ATP synthase, thereby reducing cellular ATP content. Previously, we showed that overexpression of UCP-2 suppressed glucose-stimulated insulin secretion (GSIS) in isolated islets (1). The aim of this study was to identify downstream consequences of UCP-2 overexpression and to determine whether insufficient insulin secretion in a diabetic model was correlated with increased endogenous UCP-2 expression. In isolated islets from normal rats, the degree to which GSIS was suppressed was inversely correlated with the amount of UCP-2 expression induced. Depolarizing the islets with KCl or inhibiting ATP-dependent K ؉ (K ATP ) channels with glybenclamide elicited similar insulin secretion in control and UCP-2-overexpressing islets. The glucose-stimulated mitochondrial membrane (⌿ m ) hyperpolarization was reduced in ␤-cells overexpressing UCP-2. ATP content of UCP-2-induced islets was reduced by 50%, and there was no change in the efflux of Rb ؉ at high versus low glucose concentrations, suggesting that low ATP led to reduced glucose-induced depolarization, thereby causing reduced insulin secretion. Sprague-Dawley rats fed a diet with 40% fat for 3 weeks were glucose intolerant, and in vitro insulin secretion at high glucose was only increased 8.5-fold over basal, compared with 28-fold in control rats. Islet UCP-2 mRNA expression was increased twofold. These studies provide further strong evidence that UCP-2 is an important negative regulator of ␤-cell insulin secretion and demonstrate that reduced ⌬⌿ m and increased activity of K ATP channels are mechanisms by which UCP-2-mediated effects are mediated. These studies also raise the possibility that a pathological upregulation of UCP-2 expression in the prediabetic state could contribute to the loss of glucose responsiveness observed in obesity-related type 2 diabetes in humans.

Increasing uncoupling protein-2 in pancreatic beta cells does not alter glucose-induced insulin secretion but decreases production of reactive oxygen species

“…Among the UCPs, UCP2 is the only uncoupling protein present in rodent and human pancreatic beta cells [6,7]. As GSIS depends on ATP production, the uncoupling state that may be induced by increased UCP2 level and/or activity is predicted to impair insulin secretion.…”

“…As GSIS depends on ATP production, the uncoupling state that may be induced by increased UCP2 level and/or activity is predicted to impair insulin secretion. Indeed, several studies have associated an increase in UCP2 with inhibition of GSIS and indications of mitochondrial uncoupling [6][7][8]. An increase in UCP2 protein levels has been measured after long-term exposure of beta cells to high concentrations of fatty acids [9][10][11][12], with indications of mitochondrial uncoupling.…”

Uncoupling protein-2: evidence for its function as a metabolic regulator

“…Overexpression of UCP2 mRNA in rat islets [15,109] or clonal beta cells [109,110] blunts GSIS and lowers cellular ATP, which is suggestive of the uncoupling properties of UCP2 [15]. Moreover, overexpression of UCP2 in beta cells leads to depolarization of the mitochondrial membrane potential [109], a decrease in the ADP:O ratio, increased state 4 respiration [110] and reduced closure of ATP-dependent K + (K ATP ) channels [109].…”

“…Moreover, overexpression of UCP2 in beta cells leads to depolarization of the mitochondrial membrane potential [109], a decrease in the ADP:O ratio, increased state 4 respiration [110] and reduced closure of ATP-dependent K + (K ATP ) channels [109]. Support for the hypothesis that UCP2 is a negative regulator of insulin secretion comes from investigations using UCP2 ±/± mice.…”

“…Rats fed a high fat diet for 3 weeks [109] and hyperlipidaemic, diabetic ob/ob mice [111] have increased islet UCP2 mRNA expression. Such lipid-associated upregulation could contribute to the development of insulin insufficiency and subsequent hyperglycaemia.…”