Pancreatic β-cell apoptosis induced by palmitate requires high glucose concentrations. Ceramides have been suggested to be important mediators of glucolipotoxicity-induced β-cell apoptosis. In INS-1 β-cells, 0.4 mM palmitate with 5 mM glucose increased the levels of dihydrosphingosine and dihydroceramides, two lipid intermediates in the de novo biosynthesis of ceramides, without inducing apoptosis. Increasing glucose concentrations to 30 mM amplified palmitate-induced accumulation of dihydrosphingosine and the formation of (dihydro)ceramides. Of note, glucolipotoxicity specifically induced the formation of C(18:0), C(22:0) and C(24:1) (dihydro)ceramide molecular species, which was associated with the up-regulation of CerS4 (ceramide synthase 4) levels. Fumonisin-B1, a ceramide synthase inhibitor, partially blocked apoptosis induced by glucolipotoxicity. In contrast, apoptosis was potentiated in the presence of D,L-threo-1-phenyl-2-palmitoylamino-3-morpholinopropan-1-ol, an inhibitor of glucosylceramide synthase. Moreover, overexpression of CerS4 amplified ceramide production and apoptosis induced by palmitate with 30 mM glucose, whereas down-regulation of CerS4 by siRNA (short interfering RNA) reduced apoptosis. CerS4 also potentiates ceramide accumulation and apoptosis induced by another saturated fatty acid: stearate. Collectively, our results suggest that glucolipotoxicity induces β-cell apoptosis through a dual mechanism involving de novo ceramide biosynthesis and the formation of ceramides with specific N-acyl chain lengths rather than an overall increase in ceramide content.

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Ceramide is a critical mediator to the development of T2D linked to obesity. Targeting proteins involved in ceramide's deleterious action has not been possible due to their involvement in many other intracellular signaling pathways. A possible means of counteracting ceramide action would be to prevent the accumulation of the specific ceramide species involved in both insulin resistance and β-cell apoptosis/dysfunction. Another possibility would be to adjust the dynamic balance between ceramide and sphingoid base phosphate, both known to display opposing properties on the development of T2D-linked obesity.

One main mechanism of insulin resistance (IR), a key feature of type 2 diabetes, is the accumulation of saturated fatty acids (FAs) in the muscles of obese patients with type 2 diabetes. Understanding the mechanism that underlies lipid-induced IR is an important challenge. Saturated FAs are metabolized into lipid derivatives called ceramides, and their accumulation plays a central role in the development of muscle IR. Ceramides are produced in the endoplasmic reticulum (ER) and transported to the Golgi apparatus through a transporter called CERT, where they are converted into various sphingolipid species. We show that CERT protein expression is reduced in all IR models studied because of a caspase-dependent cleavage. Inhibiting CERT activity in vitro potentiates the deleterious action of lipotoxicity on insulin signaling, whereas overexpression of CERT in vitro or in vivo decreases muscle ceramide content and improves insulin signaling. In addition, inhibition of caspase activity prevents ceramide-induced insulin signaling defects in C2C12 muscle cells. Altogether, these results demonstrate the importance of physiological ER-to-Golgi ceramide traffic to preserve muscle cell insulin signaling and identify CERT as a major actor in this process.

Pancreatic β cells secrete insulin in order to maintain glucose homeostasis. However, various environmental stresses such as obesity have been shown to induce loss of secretory responsiveness in pancreatic β cells and pancreatic β cell apoptosis which can favor the development of type 2 diabetes (T2D). Indeed, elevated levels of free fatty acids (FFAs) have been shown to induce β cell apoptosis. Importantly, the chronic adverse effects of FFAs on β cell function and viability are potentiated in the presence of hyperglycaemia, a phenomenon that has been termed gluco-lipotoxicity. The molecular mechanisms underlying the pathogenesis of gluco-lipotoxicity in pancreatic β cells are not completely understood. Recent studies have shown that sphingolipid metabolism plays a key role in gluco-lipotoxicity induced apoptosis and loss of function of pancreatic β cells. The present review focuses on how the two main sphingolipid mediators, ceramides and sphingoid base-1-phosphates, regulate the deleterious effects of gluco-lipotoxicity on pancreatic β cells. The review highlights the role of a sphingolipid biostat on the dysregulation of β cell fate and function induced by gluco-lipotoxicity, offering the possibility of new therapeutic targets to prevent the onset of T2D.

Aims/hypothesis Dietary n-3 polyunsaturated fatty acids, especially docosahexaenoic acid (DHA), are known to influence glucose homeostasis. We recently showed that Elovl2 expression in beta cells, which regulates synthesis of endogenous DHA, was associated with glucose tolerance and played a key role in insulin secretion. The present study aimed to examine the role of the very long chain fatty acid elongase 2 (ELOVL2)/DHA axis on the adverse effects of palmitate with high glucose, a condition defined as glucolipotoxicity, on beta cells. Methods We detected ELOVL2 in INS-1 beta cells and mouse and human islets using quantitative PCR and western blotting. Downregulation and adenoviral overexpression of Elovl2 was carried out in beta cells. Ceramide and diacylglycerol levels were determined by radio-enzymatic assay and lipidomics. Apoptosis was quantified using caspase-3 assays and poly (ADP-ribose) polymerase cleavage. Palmitate oxidation and esterification were determined by [U-14 C]palmitate labelling. Results We found that glucolipotoxicity decreased ELOVL2 content in rodent and human beta cells. Downregulation of ELOVL2 drastically potentiated beta cell apoptosis induced by glucolipotoxicity, whereas adenoviral Elovl2 overexpression and supplementation with DHA partially inhibited glucolipotoxicity-induced cell death in rodent and human beta cells. Inhibition of beta cell apoptosis by the ELOVL2/DHA axis was associated with a decrease in ceramide accumulation. However, the ELOVL2/DHA axis was unable to directly alter ceramide synthesis or metabolism. By contrast, DHA increased palmitate oxidation but did not affect its esterification. Pharmacological inhibition of AMP-activated protein kinase and etomoxir, an inhibitor of carnitine palmitoyltransferase 1 (CPT1), the rate-limiting enzyme in fatty acid β-oxidation, attenuated the protective effect of the ELOVL2/DHA axis during glucolipotoxicity. Downregulation of CPT1 also counteracted the anti-apoptotic action of the ELOVL2/DHA axis. By contrast, a mutated active form of Cpt1 inhibited glucolipotoxicity-induced beta cell apoptosis when ELOVL2 was downregulated. Conclusions/interpretation Our results identify ELOVL2 as a critical pro-survival enzyme for preventing beta cell death and dysfunction induced by glucolipotoxicity, notably by favouring palmitate oxidation in mitochondria through a CPT1-dependent mechanism.

Fatty acid (FA)-sensitive neurons are present in the brain, especially the hypothalamus, and play a key role in the neural control of energy homeostasis. Through neuronal output, FA may modulate feeding behaviour as well as insulin secretion and action. Subpopulations of neurons in the ventromedial and arcuate hypothalamic nuclei are selectively either inhibited or activated by FA. Molecular effectors of these FA effects probably include chloride or potassium ion channels. While intracellular metabolism and activation of the ATP-sensitive K + channel appear to be necessary for some of the signalling effects of FA, at least half of the FA responses in ventromedial hypothalamic neurons are mediated by interaction with FAT/CD36, an FA transporter/receptor that does not require intracellular metabolism to activate downstream signalling. Thus, FA or their metabolites can modulate neuronal activity as a means of directly monitoring ongoing fuel availability by brain nutrient-sensing neurons involved in the regulation of energy and glucose homeostasis. Recently, the role of lipoprotein lipase in FA sensing has also been shown in animal models not only in hypothalamus, but also in hippocampus and striatum. Finally, FA overload might impair neural control of energy homeostasis through enhanced ceramide synthesis and may contribute to obesity and/or type 2 diabetes pathogenesis in predisposed subjects. Keywords: energy balance, FAT/CD36, hypothalamus, potassium channel Date submitted 28 March 2014; date of final acceptance 6 May 2014 IntroductionThe central nervous system (CNS) is a key player in the regulation of energy balance in mammals [1]. This process involves a combination of signals arising from the periphery, including hormones (leptin, insulin, ghrelin, etc.) and nutrients (glucose and fatty acids), which are detected by specialized areas such as the hypothalamus and brain stem [2,3]. Since the work of Oomura et al. [4], there is a growing body of evidence suggesting that hypothalamic FA sensing plays a role in the regulation of energy balance, including insulin secretion and action, hepatic glucose production (HGP), linear growth, adipose deposition and food intake [5][6][7][8] [10] and mesolimbic structures [11]. We will review the mechanisms of lipid actions on CNS areas controlling energy homeostasis (with a focus on the hypothalamus) at the molecular, cellular and integrated levels under physiological situations. Furthermore, dysregulation of brain FA sensing may contribute to the further deterioration of energy balance and finally to obesity, with or without type 2 diabetes, as a complicating factor [12,13]. A better understanding of these mechanisms, as well as further characterization of FA-sensitive neurons and their role in physiological and pathological processes, might lead to identification of novel pharmacological targets for the prevention and treatment of diabetes and obesity. Transport of FA into the Brain and NeuronsCerebral lipids are an essential component of both membranes and intracellular...

Aging is one of the strongest risk factor for Alzheimer's disease (AD). However, several data suggest that dyslipidemia can either contribute or serve as co-factors in AD appearance. AD could be examined as a metabolic disorder mediated by peripheral insulin resistance. Insulin resistance is associated with dyslipidemia, which results in increased hepatic ceramide generation. Hepatic steatosis induces pro-inflammatory cytokine activation which is mediated by the increased ceramides production. Ceramides levels increased in cells due to perturbation in sphingolipid metabolism and upregulated expression of enzymes involved in ceramide synthesis. Cytotoxic ceramides and related molecules generated in liver promote insulin resistance, traffic through the circulation due to injury or cell death caused by local liver inflammation, and because of their hydrophobic nature, they can cross the blood-brain barrier and thereby exert neurotoxic responses as reducing insulin signaling and increasing pro-inflammatory cytokines. These abnormalities propagate a cascade of neurodegeneration associated with oxidative stress and ceramide generation, which potentiate brain insulin resistance, apoptosis, myelin degeneration, and neuro-inflammation. Therefore, excess of toxic lipids generated in liver can cause neurodegeneration. Elevated homocysteine level is also a risk factor for AD pathology and is narrowly associated with metabolic diseases and non-alcoholic fatty liver disease. The existence of a homocysteine/ceramides signaling pathway suggests that homocysteine toxicity could be partly mediated by intracellular ceramide accumulation due to stimulation of ceramide synthase. In this article, we briefly examined the role of homocysteine and ceramide metabolism linking metabolic diseases and non-alcoholic fatty liver disease to AD. We therefore analyzed the expression of mainly enzymes implicated in ceramide and sphingolipid metabolism and demonstrated deregulation of de novo ceramide biosynthesis and S1P metabolism in liver and brain of hyperhomocysteinemic mice.