The malonyl-CoA/long-chain acyl-CoA (LC-CoA) model of glucose-induced insulin secretion (GIIS) predicts that malonyl-CoA derived from glucose metabolism inhibits fatty acid oxidation, thereby increasing the availability of LC-CoA for lipid signaling to cellular processes involved in exocytosis. For directly testing the model, INSr3 cell clones overexpressing malonylCoA decarboxylase in the cytosol (MCDc) in a tetracycline regulatable manner were generated, and INS(832/ 13) and rat islets were infected with MCDc-expressing adenoviruses. MCD activity was increased more than fivefold, and the malonyl-CoA content was markedly diminished. This was associated with enhanced fat oxidation at high glucose, a suppression of the glucoseinduced increase in cellular free fatty acid (FFA) content, and reduced partitioning at elevated glucose of exogenous palmitate into lipid esterification products. MCDc overexpression, in the presence of exogenous FFAs but not in their absence, reduced GIIS in all ␤-cell lines and in rat islets. It also markedly curtailed the stimulation of insulin secretion by other fuel and nonfuel secretagogues. In the absence of MCDc overexpression, the secretory responses to all types of secretagogues were amplified by the provision of exogenous fatty acids. In the presence of exogenous FFAs, the fatty acyl-CoA synthetase inhibitor triacsin C reduced secretion in response to glucose and nonfuel stimuli. The data show the existence of important links between the metabolic coupling factor malonyl-CoA, the partitioning of fatty acids, and the stimulation of insulin secretion to both fuel and nonfuel stimuli. Diabetes 53:

Aims/hypothesis The aim of this study was to determine the role of fatty acid signalling in islet beta cell compensation for insulin resistance in the Zucker fatty fa/fa (ZF) rat, a genetic model of severe obesity, hyperlipidaemia and insulin resistance that does not develop diabetes. Materials and methods NEFA augmentation of insulin secretion and fatty acid metabolism were studied in isolated islets from ZF and Zucker lean (ZL) control rats. Results Exogenous palmitate markedly potentiated glucosestimulated insulin secretion (GSIS) in ZF islets, allowing robust secretion at physiological glucose levels (5-8 mmol/l).Exogenous palmitate also synergised with glucagon-like peptide-1 and the cyclic AMP-raising agent forskolin to enhance GSIS in ZF islets only. In assessing islet fatty acid metabolism, we found increased glucose-responsive palmitate esterification and lipolysis processes in ZF islets, suggestive of enhanced triglyceride-fatty acid cycling. Interruption of glucose-stimulated lipolysis by the lipase inhibitor Orlistat (tetrahydrolipstatin) blunted palmitate-augmented GSIS in ZF islets. Fatty acid oxidation was also higher at intermediate glucose levels in ZF islets and steatotic triglyceride accumulation was absent. Conclusions/interpretationThe results highlight the potential importance of NEFA and glucoincretin enhancement of insulin secretion in beta cell compensation for insulin resistance. We propose that coordinated glucose-responsive fatty acid esterification and lipolysis processes, suggestive of triglyceride-fatty acid cycling, play a role in the coupling mechanisms of glucose-induced insulin secretion as well as in beta cell compensation and the hypersecretion of insulin in obesity.

Inhibition of glycogen synthase kinase 3β (GSK3β) is a shared action believed to be involved in the regulation of behavior by psychoactive drugs such as antipsychotics and mood stabilizers. However, little is known about the identity of the substrates through which GSK3β affects behavior. We identified fragile X mental retardation-related protein 1 (FXR1P), a RNA binding protein associated to genetic risk for schizophrenia, as a substrate for GSK3β. Phosphorylation of FXR1P by GSK3β is facilitated by prior phosphorylation by ERK2 and leads to its down-regulation. In contrast, behaviorally effective chronic mood stabilizer treatments in mice inhibit GSK3β and increase FXR1P levels. In line with this, overexpression of FXR1P in the mouse prefrontal cortex also leads to comparable mood-related responses. Furthermore, functional genetic polymorphisms affecting either FXR1P or GSK3β gene expression interact to regulate emotional brain responsiveness and stability in humans. These observations uncovered a GSK3β/FXR1P signaling pathway that contributes to regulating mood and emotion processing. Regulation of FXR1P by GSK3β also provides a mechanistic framework that may explain how inhibition of GSK3β can contribute to the regulation of mood by psychoactive drugs in mental illnesses such as bipolar disorder. Moreover, this pathway could potentially be implicated in other biological functions, such as inflammation and cell proliferation, in which FXR1P and GSK3 are known to play a role.

Elevated glucose and saturated fatty acids synergize in inducing apoptosis in INS832/13 cells and in human islet cells. In order to gain insight into the molecular mechanism(s) of glucolipotoxicity (Gltox), gene profiling and metabolic analyses were performed in INS832/13 cells cultured at 5 or 20 mm glucose in the absence or presence of palmitate. Expression changes were observed for transcripts involved in mitochondrial, lipid, and glucose metabolism. At 24 h after Gltox, increased expression of lipid partitioning genes suggested a promotion of fatty acid esterification and reduced lipid oxidation/detoxification, whereas changes in the expression of energy metabolism genes suggested mitochondrial dysfunction. These changes were associated with decreased glucose-induced insulin secretion, total insulin content, ATP levels, AMP-kinase activity, mitochondrial membrane potential and fat oxidation, unchanged de novo fatty acid synthesis, and increased reactive oxygen species, cholesterol, ceramide, and triglyceride levels. However, the synergy between elevated glucose and palmitate to cause ss-cell toxicity in term of apoptosis and reduced glucose-induced insulin secretion only correlated with triglyceride and ceramide depositions. Overexpression of endoplasmic reticulum glycerol-3-phosphate acyl transferase to enhance lipid esterification amplified Gltox at intermediate glucose (11 mm), whereas reducing acetyl-coenzyme A carboxylase 1 expression by small interfering RNA to shift lipid partitioning to fat oxidation reduced Gltox. The results suggest that Gltox entails alterations in lipid partitioning, sterol and ceramide accumulation, mitochondrial dysfunction, and reactive oxygen species production, all contributing to altering ss-cell function. The data also suggest that the early promotion of lipid esterification processes is instrumental in the Gltox process.

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.