Sphingosine kinase 1 knockdown reduces insulin synthesis and secretion in a rat insulinoma cell line

Role of Sphingosine Kinase 1 in Glucolipotoxicity-Induced Early Activation of Autophagy in INS-1 Pancreatic β Cells

Insulin-producing pancreatic β cells play a crucial role in the regulation of glucose homeostasis, and their failure is a key event for diabetes development. Prolonged exposure to palmitate in the presence of elevated glucose levels, termed gluco-lipotoxicity, is known to induce β cell apoptosis. Autophagy has been proposed to be regulated by gluco-lipotoxicity in order to favor β cell survival. However, the role of palmitate metabolism in gluco-lipotoxcity-induced autophagy is presently unknown. We therefore treated INS-1 cells for 6 and 24 h with palmitate in the presence of low and high glucose concentrations and then monitored autophagy. Gluco-lipotoxicity induces accumulation of LC3-II levels in INS-1 at 6 h which returns to basal levels at 24 h. Using the RFP-GFP-LC3 probe, gluco-lipotoxicity increased both autophagosomes and autolysosmes structures, reflecting early stimulation of an autophagy flux. Triacsin C, a potent inhibitor of the long fatty acid acetyl-coA synthase, completely prevents LC3-II formation and recruitment to autophagosomes, suggesting that autophagic response requires palmitate metabolism. In contrast, etomoxir and bromo-palmitate, inhibitors of fatty acid mitochondrial β-oxidation, are unable to prevent gluco-lipotoxicity-induced LC3-II accumulation and recruitment to autophagosomes. Moreover, bromo-palmitate and etomoxir potentiate palmitate autophagic response. Even if gluco-lipotoxicity raised ceramide levels in INS-1 cells, ceramide synthase 4 overexpression does not potentiate LC3-II accumulation. Gluco-lipotoxicity also still stimulates an autophagic flux in the presence of an ER stress repressor. Finally, selective inhibition of sphingosine kinase 1 (SphK1) activity precludes gluco-lipotoxicity to induce LC3-II accumulation. Moreover, SphK1 overexpression potentiates autophagic flux induced by gluco-lipotxicity. Altogether, our results indicate that early activation of autophagy by gluco-lipotoxicity is mediated by SphK1, which plays a protective role in β cells.

Sphingosine 1-Phosphate Regulates Obesity and Glucose Homeostasis

One of the major global health and welfare issues is the treatment of obesity and associated metabolic disorders, such as type 2 diabetes mellitus and nonalcoholic fatty liver disease. Obesity, caused by the excessive accumulation of triglycerides in adipose tissues, induces adipocyte dysfunction, followed by inflammation, in adipose tissues and lipotoxicity in nonadipose tissues. Several studies have shown that obesity and glucose homeostasis are influenced by sphingolipid mediators, including ceramide and sphingosine 1-phosphate (S1P). Cellular accumulation of ceramide impairs pancreatic β-cell survival, confers insulin resistance in the liver and the skeletal muscle, and deteriorates adipose tissue inflammation via unknown molecular mechanisms. The roles of S1P are more complicated, because there are five cell-surface S1P receptors (S1PRs: S1P1-5) which have altered functions, different cellular expression patterns, and inapparent intracellular targets. Recent findings, including those by our group, support the notable concept that the pharmacological activation of S1P1 or S1P3 improves obesity and associated metabolic disorders, whereas that of S1P2 has the opposite effect. In addition, the regulation of S1P production by sphingosine kinase (SphK) is an essential factor affecting glucose homeostasis. This review summarizes the current knowledge on SphK/S1P/S1PR signaling in and against obesity, insulin resistance, and associated disorders.

Characterization of a S1PR2 specific 11C-labeled radiotracer in streptozotocin-induced diabetic murine model

BACKGROUND

Diabetes mellitus is a chronic progressive metabolic disorder that affects millions of people worldwide. Emerging evidence suggests the important roles of sphingolipid metabolism in diabetes. In particular, sphingosine-1-phosphate (S1P) and S1P receptor 2 (S1PR2) have important metabolic functions and are involved in several metabolic diseases. In diabetes, S1PR2 can effectively preserve β cells and improve glucose/insulin tolerance in high-fat diet induced and streptozotocin (STZ)-induced diabetic mouse models. We previously developed a group of potent and selective S1PR2 ligands and radioligands.

METHODS

In this study, we continued our efforts and characterized our leading S1PR2 radioligand, [11C]TZ34125, in a STZ-induced diabetic mouse model. [11C]TZ34125 was radiosynthesized in an automated synthesis module and in vitro saturation binding assay was performed using recombinant human S1PR2 membrane. In vitro saturation autoradiography analysis was also performed to determine the binding affinity of [11C]TZ34125 against mouse tissues. Type-1 diabetic mouse model was developed following a single high dose of STZ in C57BL/6 mice. Ex vivo biodistribution was performed to evaluate the distribution and amount of [11C]TZ34125 in tissues. In vitro autoradiography analysis was performed to compare the uptake of [11C]TZ34125 between diabetic and control animals in mouse spleen and pancreas.

RESULTS

Our in vitro saturation binding assay using [11C]TZ34125 confirmed [11C]TZ34125 is a potent radioligand to recombinant human S1PR2 membrane with a Kd value of 0.9 nM. Saturation autoradiographic analysis showed [11C]TZ34125 has a Kd of 67.5, 45.9, and 25.0 nM to mouse kidney, spleen, and liver tissues respectively. Biodistribution study in STZ-induced diabetic mice showed the uptake of [11C]TZ34125 was significantly elevated in the spleen (~2 fold higher) and pancreas (~1.4 fold higher) compared to normal controls. The increased uptake of [11C]TZ34125 was further confirmed using autoradiographic analysis in the spleen and pancreases of STZ-induced diabetic mice, indicating S1PR2 can potentially act as a biomarker of diabetes in pancreases and inflammation in spleen. Future mechanistic analysis and in vivo quantitative assessment using non-invasive PET imaging in large animal model of diabetes is worthwhile.

CONCLUSIONS

Overall, our data showed an increased uptake of our lead S1PR2-specific radioligand, [11C]TZ34125, in the spleen and pancreases of STZ-induced diabetic mice, and demonstrated [11C]TZ34125 has a great potential for preclinical and clinical usage for assessment of S1PR2 in diabetes and inflammation.

Sphingosine-1-phosphate in mitochondrial function and metabolic diseases

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid metabolite. The past decade has witnessed exponential growth in the field of S1P research, partly attributed to drugs targeting its receptors or kinases. Accumulating evidence indicates that changes in the S1P axis (i.e., S1P production, transport, and receptors) may modify metabolism and eventually mediate metabolic diseases. Dysfunction of the mitochondria on a master monitor of cellular metabolism is considered the leading cause of metabolic diseases, with aberrations typically induced by abnormal biogenesis, respiratory chain complex disorders, reactive oxygen species overproduction, calcium deposition, and mitophagy impairment. Accordingly, we discuss decades of investigation into changes in the S1P axis and how it controls mitochondrial function. Furthermore, we summarize recent scientific advances in disorders associated with the S1P axis and their involvement in the pathogenesis of metabolic diseases in humans, including type 2 diabetes mellitus and cardiovascular disease, from the perspective of mitochondrial function. Finally, we review potential challenges and prospects for S1P axis application to the regulation of mitochondrial function and metabolic diseases; these data may provide theoretical guidance for the treatment of metabolic diseases.

To Be or Not to Be: The Divergent Action and Metabolism of Sphingosine-1 Phosphate in Pancreatic Beta-Cells in Response to Cytokines and Fatty Acids

Sphingosine-1 phosphate (S1P) is a bioactive sphingolipid with multiple functions conveyed by the activation of cell surface receptors and/or intracellular mediators. A growing body of evidence indicates its important role in pancreatic insulin-secreting beta-cells that are necessary for maintenance of glucose homeostasis. The dysfunction and/or death of beta-cells lead to diabetes development. Diabetes is a serious public health burden with incidence growing rapidly in recent decades. The two major types of diabetes are the autoimmune-mediated type 1 diabetes (T1DM) and the metabolic stress-related type 2 diabetes (T2DM). Despite many differences in the development, both types of diabetes are characterized by chronic hyperglycemia and inflammation. The inflammatory component of diabetes remains under-characterized. Recent years have brought new insights into the possible mechanism involved in the increased inflammatory response, suggesting that environmental factors such as a westernized diet may participate in this process. Dietary lipids, particularly palmitate, are substrates for the biosynthesis of bioactive sphingolipids. Disturbed serum sphingolipid profiles were observed in both T1DM and T2DM patients. Many polymorphisms were identified in genes encoding enzymes of the sphingolipid pathway, including sphingosine kinase 2 (SK2), the S1P generating enzyme which is highly expressed in beta-cells. Proinflammatory cytokines and free fatty acids have been shown to modulate the expression and activity of S1P-generating and S1P-catabolizing enzymes. In this review, the similarities and differences in the action of extracellular and intracellular S1P in beta-cells exposed to cytokines or free fatty acids will be identified and the outlook for future research will be discussed.

Barrier maintenance by S1P during inflammation and sepsis

Sphingosine 1-phosphate (S1P) is a multifaceted lipid signaling molecule that activates five specific G protein-coupled S1P receptors. Despite the fact that S1P is known as one of the strongest barrier-enhancing molecules for two decades, no medical application is available yet. The reason for this lack of translation into clinical practice may be the complex regulatory network of S1P signaling, metabolism and transportation.In this review, we will provide an overview about the physiology and the network of S1P signaling with the focus on endothelial barrier maintenance in inflammation. We briefly describe the physiological role of S1P and the underlying S1P signaling in barrier maintenance, outline differences of S1P signaling and metabolism in inflammatory diseases, discuss potential targets and compounds for medical intervention, and summarize our current knowledge regarding the role of S1P in the maintenance of specialized barriers like the blood-brain barrier and the placenta.

Sphingosine 1-phosphate metabolism and insulin signaling

Insulin is the main anabolic hormone secreted by β-cells of the pancreas stimulating the assimilation and storage of glucose in muscle and fat cells. It modulates the postprandial balance of carbohydrates, lipids and proteins via enhancing lipogenesis, glycogen and protein synthesis and suppressing glucose generation and its release from the liver. Resistance to insulin is a severe metabolic disorder related to a diminished response of peripheral tissues to the insulin action and signaling. This leads to a disturbed glucose homeostasis that precedes the onset of type 2 diabetes (T2D), a disease reaching epidemic proportions. A large number of studies reported an association between elevated circulating fatty acids and the development of insulin resistance. The increased fatty acid lipid flux results in the accumulation of lipid droplets in a variety of tissues. However, lipid intermediates such as diacylglycerols and ceramides are also formed in response to elevated fatty acid levels. These bioactive lipids have been associated with the pathogenesis of insulin resistance. More recently, sphingosine 1-phosphate (S1P), another bioactive sphingolipid derivative, has also been shown to increase in T2D and obesity. Although many studies propose a protective role of S1P metabolism on insulin signaling in peripheral tissues, other studies suggest a causal role of S1P on insulin resistance. In this review, we critically summarize the current state of knowledge of S1P metabolism and its modulating role on insulin resistance. A particular emphasis is placed on S1P and insulin signaling in hepatocytes, skeletal muscle cells, adipocytes and pancreatic β-cells. In particular, modulation of receptors and enzymes that regulate S1P metabolism can be considered as a new therapeutic option for the treatment of insulin resistance and T2D.

Sphingolipids in Type 1 Diabetes: Focus on Beta-Cells

Type 1 diabetes (T1DM) is a chronic autoimmune disease, with a strong genetic background, leading to a gradual loss of pancreatic beta-cells, which secrete insulin and control glucose homeostasis. Patients with T1DM require life-long substitution with insulin and are at high risk for development of severe secondary complications. The incidence of T1DM has been continuously growing in the last decades, indicating an important contribution of environmental factors. Accumulating data indicates that sphingolipids may be crucially involved in T1DM development. The serum lipidome of T1DM patients is characterized by significantly altered sphingolipid composition compared to nondiabetic, healthy probands. Recently, several polymorphisms in the genes encoding the enzymatic machinery for sphingolipid production have been identified in T1DM individuals. Evidence gained from studies in rodent islets and beta-cells exposed to cytokines indicates dysregulation of the sphingolipid biosynthetic pathway and impaired function of several sphingolipids. Moreover, a number of glycosphingolipids have been suggested to act as beta-cell autoantigens. Studies in animal models of autoimmune diabetes, such as the Non Obese Diabetic (NOD) mouse and the LEW.1AR1-iddm (IDDM) rat, indicate a crucial role of sphingolipids in immune cell trafficking, islet infiltration and diabetes development. In this review, the up-to-date status on the findings about sphingolipids in T1DM will be provided, the under-investigated research areas will be identified and perspectives for future studies will be given.

Sphingosine-1-Phosphate Metabolism in the Regulation of Obesity/Type 2 Diabetes

Obesity is a pathophysiological condition where excess free fatty acids (FFA) target and promote the dysfunctioning of insulin sensitive tissues and of pancreatic β cells. This leads to the dysregulation of glucose homeostasis, which culminates in the onset of type 2 diabetes (T2D). FFA, which accumulate in these tissues, are metabolized as lipid derivatives such as ceramide, and the ectopic accumulation of the latter has been shown to lead to lipotoxicity. Ceramide is an active lipid that inhibits the insulin signaling pathway as well as inducing pancreatic β cell death. In mammals, ceramide is a key lipid intermediate for sphingolipid metabolism as is sphingosine-1-phosphate (S1P). S1P levels have also been associated with the development of obesity and T2D. In this review, the current knowledge on S1P metabolism in regulating insulin signaling in pancreatic β cell fate and in the regulation of feeding by the hypothalamus in the context of obesity and T2D is summarized. It demonstrates that S1P can display opposite effects on insulin sensitive tissues and pancreatic β cells, which depends on its origin or its degradation pathway.

Role of Sphingosine Kinase in Type 2 Diabetes Mellitus

Sphingolipids are a class of essential lipids, functioning as both cell membrane constituents and signaling messengers. In the sphingolipid metabolic network, ceramides serve as the central hub that is hydrolyzed to sphingosine, followed by phosphorylation to sphingosine 1-phosphate (S1P) by sphingosine kinase (SphK). SphK is regarded as a "switch" of the sphingolipid rheostat, as it catalyzes the conversion of ceramide/sphingosine to S1P, which often exhibit opposing biological roles in the cell. Besides, SphK is an important signaling enzyme that has been implicated in the regulation of a wide variety of biological functions. In recent years, an increasing body of evidence has suggested a critical role of SphK in type 2 diabetes mellitus (T2D), although a certain level of controversy remains. Herein, we review recent findings related to SphK in the field of T2D research with a focus on peripheral insulin resistance and pancreatic β-cell failure. It is expected that a comprehensive understanding of the role of SphK and the associated sphingolipids in T2D will help to identify druggable targets for future anti-diabetes therapy.

Sphingosine-1-phosphate induces islet β-cell proliferation and decreases cell apoptosis in high-fat diet/streptozotocin diabetic mice

Sphingosine-1-phosphate (S1P) has been reported to enhance the function of islet β-cells, providing a potential therapeutic target for diabetes mellitus. In the present study, the effects of S1P on the proliferation and apoptosis of β-cells in type 2 diabetic mice were investigated. The mice were administered intraperitoneal S1P solution daily at a dose of 20 µg/kg for three weeks. The intraperitoneal glucose tolerance test (IPGTT) and homeostatic model assessment of insulin resistance (HOMA-IR) index determination were carried out. Immunohistochemical staining was used to detect the protein expression of insulin, antigen Ki-67 and S1P receptor isoforms (S1PR1/S1PR2/S1PR3) in pancreatic islets. Compared with the diabetic control (DC) group, the IPGTT results and HOMA-IR index in the S1P treatment group were decreased. The islets in the S1P group exhibited higher insulin immunostaining intensity than the DC group, as well as higher proliferation (P<0.05) and lower apoptosis rates (P<0.05). Positive staining for the S1P receptors S1PR1, S1PR2 and S1PR3 was observed in the cytoplasm and membrane of the islet cells. S1PR1 and S1PR2 proteins showed increased expression in the S1P and DC groups compared with the normal control group (P<0.01 and P<0.05, respectively), whereas no significant difference was observed in the expression of S1PR3 among these groups. In conclusion, extracellular S1P can induce islet β-cell proliferation and decrease cell apoptosis in diabetic mice. S1P function may be mediated via S1PR1 and S1PR2; therefore, targeting S1P/S1PR signalling pathways may be a novel therapeutic strategy for diabetes mellitus.

Mammalian sphingosine kinase (SphK) isoenzymes and isoform expression: challenges for SphK as an oncotarget

The various sphingosine kinase (SphK) isoenzymes (isozymes) and isoforms, key players in normal cellular physiology, are strongly implicated in cancer and other diseases. Mutations in SphKs, that may justify abnormal physiological function, have not been recorded. Nonetheless, there is a large and growing body of evidence demonstrating the contribution of gain or loss of function and the imbalance in the SphK/S1P rheostat to a plethora of pathological conditions including cancer, diabetes and inflammatory diseases. SphK is expressed as two isozymes SphK1 and SphK2, transcribed from genes located on different chromosomes and both isozymes catalyze the phosphorylation of sphingosine to S1P. Expression of each SphK isozyme produces alternately spliced isoforms. In recent years the importance of the contribution of SpK1 expression to treatment resistance in cancer has been highlighted and, additionally, differences in treatment outcome appear to also be dependent upon SphK isoform expression. This review focuses on an exciting emerging area of research involving SphKs functions, expression and subcellular localization, highlighting the complexity of targeting SphK in cancer and also comorbid diseases. This review also covers the SphK isoenzymes and isoforms from a historical perspective, from their first discovery in murine species and then in humans, their role(s) in normal cellular function and in disease processes, to advancement of SphK as an oncotarget.

Local Sphingosine Kinase 1 Activity Improves Islet Transplantation

Pancreatic islet transplantation is a promising clinical treatment for type 1 diabetes, but success is limited by extensive β-cell death in the immediate posttransplant period and impaired islet function in the longer term. Following transplantation, appropriate vascular remodeling is crucial to ensure the survival and function of engrafted islets. The sphingosine kinase (SK) pathway is an important regulator of vascular beds, but its role in the survival and function of transplanted islets is unknown. We observed that donor islets from mice deficient in SK1 (Sphk1 knockout) contain a reduced number of resident intraislet vascular endothelial cells. Furthermore, we demonstrate that the main product of SK1, sphingosine-1-phosphate, controls the migration of intraislet endothelial cells in vitro. We reveal in vivo that Sphk1 knockout islets have an impaired ability to cure diabetes compared with wild-type controls. Thus, SK1-deficient islets not only contain fewer resident vascular cells that participate in revascularization, but likely also a reduced ability to recruit new vessels into the transplanted islet. Together, our data suggest that SK1 is important for islet revascularization following transplantation and represents a novel clinical target for improving transplant outcomes.

n-3 Polyunsaturated Fatty Acids Improve Inflammation via Inhibiting Sphingosine Kinase 1 in a Rat Model of Parenteral Nutrition and CLP-Induced Sepsis

The sphingosine kinase 1 (SphK1)/sphingosine-1-phosphate (S1P) pathway plays a key role in inflammation. Parenteral nutrition containing n-3 polyunsaturated fatty acids (n-3 PUFA) may regulate inflammatory reactions. The aim of this study is to determine whether n-3 PUFA may improve inflammatory responses by neutralizing SphK1 signaling. Rat models of parenteral nutrition, cecal ligation and puncture (CLP)-induced sepsis were generated. Male Sprague-Dawley rats were operated for CLP on day 2 after venous catheterization. The rats were randomized to receive normal saline (NS; n = 20), parenteral nutrition (PN; n = 20), or PN + fish oil (FO; n = 20) for 5 days. The daily intake of fish oil (1.25-2.82 g EPA and 1.44-3.09 g DHA per 100 ml) in the FO group was approximately 1.8 g/kg body weight/day. Rats in the control group (n = 10) were subjected to sham operation and received a chow diet. Spleen tissues were collected for SphK1 and S1P receptor expression analysis. Our data showed that n-3 PUFA ameliorated the survival rate. SphK1 expression and its enzymatic activity were significantly upregulated in sepsis rats. Furthermore, mRNA and protein levels of S1PR3, but not S1PR1, were also facilitated after CLP. However, PN + FO dramatically decreased SphK1 mRNA level and its enzymatic activity. S1PR3 expression was also attenuated by FO addition. In conclusion, the anti-inflammatory effect of n-3 PUFA may be linked to the inhibition of the SphK1/S1P pathway in a rat model of parenteral nutrition and CLP-induced sepsis.

Estrogen regulates excitatory amino acid carrier 1 (EAAC1) expression through sphingosine kinase 1 (SphK1) transacting FGFR-mediated ERK signaling in rat C6 astroglial cells

Excitatory amino acid carrier 1 (EAAC1) is one important subtype of the excitatory amino acid transporters (EAATs), and its absence can increase the vulnerability to oxidative stress in neural tissue. Enhanced expression of EAAC1 can provide neuroprotection in multiple disorders, including ischemia and multiple sclerosis. However, the mechanism regulating EAAC1 expression is not fully understood. Using rat C6 astroglial cells, which specifically express EAAC1, we found that 17β-estradiol (E2) and (±)-1-[(3aR(∗),4S(∗),9bS(∗))-4-(6-bromo-1,3-benzodioxol-5-yl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinolin-8-yl]-ethanone (G1), an agonist of the G-protein-coupled estrogen receptor (GPR30), strongly increased EAAC1 protein levels and protected cells from hydrogen peroxide (H2O2) toxicity. We further found that E2/G1 activated sphingosine kinase 1 (SphK1) via GPR30, resulting in the transcription of fibroblast growth factor 2 (FGF2), which stimulated its receptor (FGFR) and led to the phosphorylation of FGFR substrate 2α (FRS2α). This triggered downstream ERK1/2 signaling for the expression of EAAC1. Both the knockdown of FGF2 by siRNA and the pharmacological suppression of the FGFR-ERK cascade abolished the E2/G1 effect on EAAC1 expression. Overall, our work characterizes a signaling pathway by which E2 transactivates FGFR-ERK to induce EAAC1 expression in an FGF2-dependent manner. This occurs through SphK1 activation via GPR30 and leads to a resistance to H2O2 toxicity. This signal transduction pathway may provide novel insights into our understanding of the neuroprotective effects of E2 and may reveal new therapeutic targets or drugs for regulating the oxidative toxicity effects of various neurological diseases.

Switching the sphingolipid rheostat in the treatment of diabetes and cancer comorbidity from a problem to an advantage

Cancer and diabetes are among the most common diseases in western societies. Epidemiological studies have shown that diabetic patients have a significantly higher risk of developing a number of different types of cancers and that individuals with comorbidity (cancer and diabetes/prediabetes) have a poorer prognosis relative to nondiabetic cancer patients. The increasing frequency of comorbidity of cancer and diabetes mellitus, mainly type 2 diabetes, has driven the development of therapeutic interventions that target both disease states. There is strong evidence to suggest that balancing the sphingolipid rheostat, ceramide--sphingosine--sphingosine-1-phosphate (S1P) is crucial in the prevention of diabetes and cancer and sphingosine kinase/S1P modulators are currently under development for the treatment of cancer and diabetes. This paper will highlight some of the complexities inherent in the use of the emerging sphingosine kinase/S1P modulators in the treatment of comorbidity of diabetes and cancer.

Loss of sphingosine kinase 1 predisposes to the onset of diabetes via promoting pancreatic β-cell death in diet-induced obese mice

Lipotoxic stress-induced β-cell death (lipotoxicity) is recognized as a key contributor to the development of type 2 diabetes mellitus (T2DM). The current study reports a critical role of sphingosine kinase 1 (SphK1) in β-cell survival under lipotoxic conditions. In an attempt to investigate the role of SphK1 in lipotoxicity in vivo, we fed Sphk1(-/-) and wild-type (WT) mice with a high-fat diet (HFD) or normal chow diet. Remarkably, while HFD-fed WT mice developed glucose intolerance and compensatory hyperinsulinemia, all HFD-fed Sphk1(-/-) mice manifested evident diabetes, accompanied by a nearly 3-fold reduction in insulin levels compared with the WT mice. Pancreatic β-cell mass was increased by 140% in HFD-fed WT mice but decreased to 50% in HFD-fed Sphk1(-/-) mice, in comparison with the chow diet control groups, respectively. Accordingly, by blocking the enzyme activity, expression of a dominant negative form of SphK1 markedly promoted palmitate-induced cell death in MIN6 and INS-1 β-cell lines. Moreover, primary islets isolated from Sphk1(-/-) mice exhibited higher susceptibility to lipotoxicity than WT controls. Of note, sphingosine 1-phosphate (S1P) profoundly abrogated lipotoxicity in β cells or the cells lacking SphK1 activity and Sphk1(-/-) islets, highlighting a pivotal role of S1P in β-cell survival under lipotoxic conditions. These findings could suggest a new therapeutic strategy for preventing β-cell death and thus the onset of T2DM.

Knockdown of pyruvate carboxylase or fatty acid synthase lowers numerous lipids and glucose-stimulated insulin release in insulinoma cells

We previously showed that knockdown of the anaplerotic enzyme pyruvate carboxylase in the INS-1 832/13 insulinoma cell line inhibited glucose-stimulated insulin release and glucose carbon incorporation into lipids. We now show that knockdown of fatty acid synthase (FAS) mRNA and protein also inhibits glucose-stimulated insulin release in this cell line. Levels of numerous phospholipids, cholesterol esters, diacylglycerol, triglycerides and individual fatty acids with C14-C24 side chains were acutely lowered about 20% in glucose-stimulated pyruvate carboxylase knockdown cells over a time course that coincides with insulin secretion. In FAS knockdown cells glucose carbon incorporation into lipids and the levels of the subclasses of phospholipids and cholesterol ester species were lower by 20-30% without inhibition of glucose oxidation. These studies suggest that rapid lipid modification is essential for normal glucose-stimulated insulin secretion.