Role of Sphingosine Kinase 1 and Sphingosine-1-Phosphate Axis in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is primarily diagnosed in the latter stages of disease progression and is the third leading cause of cancer deaths worldwide. Thus, there is a need to find biomarkers of early HCC as well as the development of more effective treatments for the disease. Sphingosine-1-phosphate (S1P) is a pleiotropic lipid signaling molecule produced by two isoforms of sphingosine kinase (SphK1 and SphK2) that is involved in regulation of many aspects of mammalian physiology and pathophysiology, including inflammation, epithelial and endothelial barrier function, cancer, and metastasis, among many others. Abundant evidence indicates that SphK1 and S1P promote cancer progression and metastasis in multiple types of cancers. However, the role of SphK/S1P in HCC is less well studied. Here, we review the current state of knowledge of SphKs and S1P in HCC, including evidence for the correlation of SphK1 expression and S1P levels with progression of HCC and negative outcomes, and discuss how this information could lead to the design of more effective diagnostic and treatment modalities for HCC

Nuclear topology of the mammary epithelial cells

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Gluco-lipotoxicity inhibits ceramide transport between endoplasmic reticulum and Golgi apparatus in pancreatic beta cells

Background and aims: In type 2 diabetes (TD2) the chronic adverse effects of fatty acids (FA) on beta cell function and viability have been correlated to hyperglycaemia. Gluco-lipotoxicity refers to the combined, deleterious effects of elevated glucose and fatty acid levels on pancreatic beta-cell function and survival. Gluco-lipotoxicity caused beta-cell apoptosis and may thus contribute to progressive beta-cell loss in TD2. The molecular pathways and regulators involved in the detrimental effects of chronic exposure to FA, in particular palmitate, include the de novo synthesis of ceramide (Cer) in the endoplasmic reticulum (ER). However, increasing Cer levels in ER can also be due to a reduction of Cer utilization for complex sphingolipids synthesis. Therefore, we studied the effect of gluco-lipotoxicity on Cer metabolism in beta cells and its impact on beta-cell apoptosis. Materials and methods: INS-1 cells were cultured with 0.4 mM palmitate and 5 or 30 mM of glucose. INS-1 cell apoptosis was determined by caspase 3/7 activity assay. Sphingolipid metabolites were analyzed by liquid chromatography mass spectrometry. Metabolism of Cer was measured in INS-1 beta cells with [3H]sphingosine, a precursor of sphingolipid biosynthesis. Transport of C5-Bodipy-ceramides between ER and Golgi apparatus was analysed by fluorescence confocal microscopy. Down-regulation of Cer transporter CERT was made by specific siRNA. Results: Both nutrients taken separately did not induce INS-1 cell death, whereas the combined treatment with palmitate and glucose resulted in an extensive beta cell apoptosis and this was associated to a significant increase of Cer levels. The presence of fumonisin-B1, an inhibitor of Cer biosynthesis, partially reversed the apoptosis induced by the combined treatment with palmitate and glucose. Metabolic studies using [3H]sphingosine as precursor of sphingolipid biosynthesis show that treatment with palmitate results in a small but significant increase of [3H]Cer associated to a decrease of [3H] sphingomyelin (SM) at 5 mM glucose. Inhibition of Cer utilization for SM biosynthesis was significantly potentiated by the presence of 30 mM glucose. Lipidomic analysis showed that gluco-lipotoxicity inhibited biosynthesis of complex sphingolipids. Fluorescence microscopy studies using C5-Bodipy- Cer show that at high gluco-lipotoxicity reduces the fluorescence accumulation in the perinuclear region representative of the Golgi apparatus. Interestingly, this was associated with an inhibition of the Cer transporter CERT function. Inhibition of CERT was mediated by a decrease of its expression and an increase of its phosphorylation status. Finally, selective silencing of CERT expression increased INS-1 cell apoptosis induced by palmitate. Conclusion: Altogether these data suggest that gluco-lipotoxicity increased Cer accumulation in the ER through also through a decreased utilization of newly synthesized Cer for SM biosynthesis. Moreover, these results support a role of Cer transport between ER and Golgi apparatus in the regulation of beta cell death induced by gluco-lipotoxicity

Role of palmitate-induced sphingoid base-1-phosphate biosynthesis in INS-1 β-cell survival

Sphingoid base-1-phosphates represent a very low portion of the sphingolipid pool but are potent bioactive lipids in mammals. This study was undertaken to determine whether these lipids are produced in palmitate-treated pancreatic \u3b2 cells and what role they play in palmitate-induced \u3b2 cell apoptosis. Our lipidomic analysis revealed that palmitate at low and high glucose supplementation increased (dihydro)sphingosine-1-phosphate levels in INS-1 \u3b2 cells. This increase was associated with an increase in sphingosine kinase 1 (SphK1) mRNA and protein levels. Over-expression of SphK1 in INS-1 cells potentiated palmitate-induced accumulation of dihydrosphingosine-1- phosphate. N,N-dimethyl-sphingosine, a potent inhibitor of SphK, potentiated \u3b2-cell apoptosis induced by palmitate whereas over-expression of SphK1 significantly reduced apoptosis induced by palmitate with high glucose. Endoplasmic reticulum (ER)-targeted SphK1 also partially inhibited apoptosis induced by palmitate. Inhibition of INS-1 apoptosis by over-expressed SphK1 was independent of sphingosine-1-phosphate receptors but was associated with a decreased formation of pro-apoptotic ceramides induced by gluco-lipotoxicity. Moreover, over-expression of SphK1 counteracted the defect in the ER-to-Golgi transport of proteins that contribute to the ceramide-dependent ER stress observed during gluco-lipotoxicity. In conclusion, our results suggest that activation of palmitate-induced SphK1-mediated sphingoid base-1-phosphate formation in the ER of \u3b2 cells plays a protective role against palmitate-induced ceramide-dependent apoptotic \u3b2 cell death

Physiological and pathophysiological implications of lipid sensing in the brain

International audienceFatty 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