Glucose-stimulated insulin secretion (GSIS) from pancreatic islet ␤-cells is central to control of mammalian fuel homeostasis.Glucose metabolism mediates GSIS in part via ATP-regulated K ؉ (K ATP ) channels, but multiple lines of evidence suggest participation of other signals. Here we investigated the role of cytosolic NADP-dependent isocitrate dehydrogenase (ICDc) in control of GSIS in ␤-cells. Delivery of small interfering RNAs specific for ICDc caused impairment of GSIS in two independent robustly glucose-responsive rat insulinoma (INS-1-derived) cell lines and in primary rat islets. Suppression of ICDc also attenuated the glucose-induced increments in pyruvate cycling activity and in NADPH levels, a predicted by-product of pyruvate cycling pathways, as well as the total cellular NADP(H) content. Metabolic profiling of eight organic acids in cell extracts revealed that suppression of ICDc caused increases in lactate production in both INS-1-derived cell lines and primary islets, consistent with the attenuation of pyruvate cycling, with no significant changes in other intermediates. Based on these studies, we propose that a pyruvate cycling pathway involving ICDc plays an important role in control of GSIS.Glucose metabolism in pancreatic islet ␤-cells generates signals for acute stimulation of insulin secretion, and it is widely accepted that an increase in the ATP:ADP ratio brought about by glucose flux through glycolysis and the tricarboxylic acid cycle is central to this process. The rise in the ATP:ADP ratio results in closure of ATP-regulated K ϩ (K ATP ) 2 channels, plasma membrane depolarization, activation of voltage-dependent Ca 2ϩ channels, and subsequent influx of Ca 2ϩ to stimulate insulin release (1, 2). This K ATP channel-dependent pathway has been suggested to be especially important for the acute first phase of glucose-stimulated insulin secretion (GSIS). In the second and more prolonged phase of GSIS, glucose-derived factors in addition to ATP have been implicated (3, 4), including glutamate (5), malonyl-CoA/cytosolic long chain acyl-CoA esters (6 -9), and transport of NADH reducing equivalents into the mitochondria via hydrogen shuttles (10), but data arguing against a role for some of these events have also been presented (11-13).Pyruvate carboxylase (PC), which catalyzes the conversion of pyruvate to oxaloacetate, is highly active in ␤-cells and accounts for ϳ40 -50% of pyruvate entry into mitochondrial metabolism at stimulatory glucose concentrations (14 -17). The high PC activity in ␤-cells is remarkable in light of the absence of gluconeogenesis (18) and relatively low lipogenic activity (19) in these cells. Furthermore, it has been estimated that only 25% of the glucose-derived carbons entering the tricarboxylic acid cycle via PC are channeled into protein synthesis (17). These findings suggest that PC-catalyzed entry of metabolites into mitochondrial metabolic pathways (anaplerosis) may play other roles in ␤-cell function.Previously, we examined glucose flux in a set of INS-derived ce...

Thiazolidinediones (TZDs) are effective therapies for type 2 diabetes, which has reached epidemic proportions in industrialized societies. TZD treatment reduces circulating free fatty acids (FFAs), which oppose insulin actions in skeletal muscle and other insulin target tissues. Here we report that TZDs, acting as ligands for the nuclear receptor peroxisome proliferator-activated receptor (PPAR)-gamma, markedly induce adipocyte glycerol kinase (GyK) gene expression. This is surprising, as standard textbooks indicate that adipocytes lack GyK and thereby avoid futile cycles of triglyceride breakdown and resynthesis from glycerol and FFAs. By inducing GyK, TZDs markedly stimulate glycerol incorporation into triglyceride and reduce FFA secretion from adipocytes. The 'futile' fuel cycle resulting from expression of GyK in adipocytes is thus a novel mechanism contributing to reduced FFA levels and perhaps insulin sensitization by antidiabetic therapies.