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Molecular Endocrinology

Year: 2009  |  Volume: 23  |  Issue: 6  |  Page No.: 893 - 900

Glutaredoxin-1 Mediates NADPH-Dependent Stimulation of Calcium-Dependent Insulin Secretion

T. M Reinbothe, R Ivarsson, D. Q Li, O Niazi, X Jing, E Zhang, L Stenson, U Bryborn and E. Renstrom

Abstract

Nicotinamide adenine dinucleotide phosphate (NADPH) enhances Ca2+-induced exocytosis in pancreatic β-cells, an effect suggested to involve the cytosolic redox protein glutaredoxin-1 (GRX-1). We here detail the role of GRX-1 in NADPH-stimulated β-cell exocytosis and glucose-stimulated insulin secretion. Silencing of GRX-1 by RNA interference reduced glucose-stimulated insulin secretion in both clonal INS-1 832/13 cells and primary rat islets. GRX-1 silencing did not affect cell viability or the intracellular redox environment, suggesting that GRX-1 regulates the exocytotic machinery by a local action. By contrast, knockdown of the related protein thioredoxin-1 (TRX-1) was ineffective. Confocal immunocytochemistry revealed that GRX-1 locates to the cell periphery, whereas TRX-1 expression is uniform. These data suggest that the distinct subcellular localizations of TRX-1 and GRX-1 result in differences in substrate specificities and actions on insulin secretion. Single-cell exocytosis was likewise suppressed by GRX-1 knockdown in both rat β-cells and clonal 832/13 cells, whereas after overexpression exocytosis increased by approximately 40%. Intracellular addition of NADPH (0.1 mm) stimulated Ca2+-evoked exocytosis in both cell types. Interestingly, the stimulatory action of NADPH on the exocytotic machinery coincided with an approximately 30% inhibition in whole-cell Ca2+ currents. After GRX-1 silencing, NADPH failed to amplify insulin release but still inhibited Ca2+ currents in 832/13 cells. In conclusion, NADPH stimulates the exocytotic machinery in pancreatic β-cells. This effect is mediated by the NADPH acceptor protein GRX-1 by a local redox reaction that accelerates β-cell exocytosis and, in turn, insulin secretion.

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