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

Increased Uncoupling Protein-2 Levels in β-cells Are Associated With Impaired Glucose-Stimulated Insulin Secretion Mechanism of Action Catherine B. Chan 1 , Domenica De Leo 3 , Jamie W. Joseph 3 , Timothy S. McQuaid 1 , Xiao Fang Ha 3 , Fang Xu 3 , Robert G. Tsushima 3 , Peter S. Pennefather 2 , Anne Marie F. Salapatek 3 and Michael B. Wheeler 3 1 Department of Anatomy and Physiology, Atlantic Veterinary College, University of Prince Edward Island, Charlottetown, Prince Edward Island; and the Departments of 2 Physiology and Pharmaceutical Sciences and the Departments of 3 Medicine and Physiology, Faculty of Medicine, University of Toronto, Toronto, Ontario, Canada Abstract In pancreatic β-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 (Ψ m ) hyperpolarization was reduced in β-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 re