A Role for Cytosolic Isocitrate Dehydrogenase as a Negative Regulator of Glucose Signaling for Insulin Secretion in Pancreatic ß-Cells

Abstract

Cytosolic NADPH may act as one of the signals that couple glucose metabolism to insulin secretion in the pancreatic ß-cell. NADPH levels in the cytoplasm are largely controlled by the cytosolic isoforms of malic enzyme and isocitrate dehydrogenase (IDHc). Some studies have provided evidence for a role of malic enzyme in glucose-induced insulin secretion (GIIS) via pyruvate cycling, but the role of IDHc in ß-cell signaling is unsettled. IDHc is an established component of the isocitrate/α–ketoglutarate shuttle that transfers reducing equivalents (NADPH) from the mitochondrion to the cytosol. This shuttle is energy consuming since it is coupled to nicotinamide nucleotide transhydrogenase that uses the mitochondrial proton gradient to produce mitochondrial NADPH and NAD+ from NADP+ and NADH. To determine whether flux through IDHc is positively or negatively linked to GIIS, we performed RNAi knockdown experiments in ß-cells. Reduced IDHc expression in INS 832/13 cells and isolated rat islet ß-cells resulted in enhanced GIIS. This effect was mediated at least in part via the KATP-independent amplification arm of GIIS. IDHc knockdown in INS 832/13 cells did not alter glucose oxidation but it reduced fatty acid oxidation and increased lipogenesis from glucose. Metabolome profiling in INS 832/13 cells showed that IDHc knockdown increased isocitrate and NADP+ levels. It also increased the cellular contents of several metabolites linked to GIIS, in particular some Krebs cycle intermediates, acetyl-CoA, glutamate, cAMP and ATP. The results identify IDHc as a component of the emerging pathways that negatively regulate GIIS.

Identification of a mammalian glycerol-3-phosphate phosphatase: Role in metabolism and signaling in pancreatic β-cells and hepatocytes

“…Similarly, in INS832/13 cells, Gro3P concentration was calculated to be ∼1.5 mM at 10 mM glucose concentration (31). These cellular concentrations of Gro3P are quite favorable for G3PP/PGP to use this substrate effectively.…”

“…The MS was operated in negative electrospray ionization mode using a turbo ion spray source. Transitions used were described previously (31), with additional selected ion monitoring at mass/charge ratios of 154.75/78.79, for 2-PG and glycolate. Quantification was performed as described before (31).…”

The Pancreatic β-Cell: A Bioenergetic Perspective

“…Since this loop parallels, and reenters, the matrix citric acid cycle, it is free of the logistic problems with the pyruvate/citrate cycle discussed above, and could account for the massive increase in citric acid cycle intermediates when glucose is raised. Cytosolic isocitrate dehydrogenase is required for the cycle, but while Jensen et al (240) reported that its suppression partially inhibited GSIS in INS-1 832/13 cells and rat islets, Guay et al (195) came to an opposite conclusion, namely, that IDH1 is a negative regulator of GSIS.…”

Isocitrate-to-SENP1 signaling amplifies insulin secretion and rescues dysfunctional β cells

“…These include a glycerolipid/ free fatty acid cycle (15), which generates monoacylglycerol as a coupling factor (16); a phosphoenolpyruvate and mitochondrial GTP-dependent pathway (17,18); and so-called "pyruvate cycles" (19)(20)(21), which export reducing equivalents from the mitochondria to the cytosol. The mitochondrial export of citrate and isocitrate and subsequent engagement of isocitrate with the cytosolic NADP + -dependent isocitrate dehydrogenase (IDH1, hereafter referred to as ICDc) has been shown to regulate insulin secretion (22,23), but the metabolites generated by this pathway that engage with the Insulin secretion from β cells of the pancreatic islets of Langerhans controls metabolic homeostasis and is impaired in individuals with type 2 diabetes (T2D).…”