Plasma ceramides are elevated in obese subjects with type 2 diabetes and correlate with the severity of insulin resistance

The effect of short-term overfeeding on serum lipids in healthy humans

OBJECTIVES

While chronic obesity is associated with alterations in circulating glycerolipids, sphingolipids and plasmalogens, the effects of short-term overfeeding in humans are unclear.

DESIGN AND METHODS

Healthy individuals (n = 40) were overfed by 1,250 kcal day(-1) for 28 days. Insulin sensitivity (hyperinsulinemic-euglycemic clamp), abdominal fat distribution and serum lipidomics (mass spectrometry) were assessed.

RESULTS

Overfeeding increased liver fat, insulin resistance, serum C-reactive protein and urinary F2-isoprostanes. HDL increased (11% ± 2%, P < 0.001) while LDL, triglycerides and nonesterified fatty acids were unchanged. Three hundred and thirty three serum lipids were detected, of which 13% increased and 20% decreased with overfeeding. Total diacylglycerol and lysoalkylphosphatidylcholine (LPC(O)) concentrations decreased (P < 0.01), while total ceramide, Cer22:0 and Cer24:0 increased (P ≤ 0.01). The most notable increases were observed in the HDL-associated phosphatidylethanolamine-based plasmalogens and their precursors alkylhosphatidylethanolamine (18 ± 5% and 38 ± 8% respectively, P ≤ 0.01).

CONCLUSIONS

Overfeeding led to weight gain and changes in the serum lipid profile. Increases in ceramides were noted, which left unchecked may promote systemic insulin resistance. Uniform increases were observed in plasmalogens and their precursors. Because plasmalogens are powerful antioxidants, this may be an appropriate response against increased oxidative stress generated by over-nutrition. The metabolic consequences of changes in concentrations of many circulating lipid species with overfeeding require further study. Copyright © 2013 The Obesity Society.

PKCζ mediates breakdown of outer blood-retinal barriers in diabetic retinopathy

Aims/hypothesis

Diabetic macular edema represents the main cause of visual loss in diabetic retinopathy. Besides inner blood retinal barrier breakdown, the role of the outer blood retinal barrier breakdown has been poorly analyzed. We characterized the structural and molecular alterations of the outer blood retinal barrier during the time course of diabetes, focusing on PKCζ, a critical protein for tight junction assembly, known to be overactivated by hyperglycemia.

Methods

Studies were conducted on a type2 diabetes Goto-Kakizaki rat model. PKCζ level and subcellular localization were assessed by immunoblotting and immunohistochemistry. Cell death was detected by TUNEL assays. PKCζ level on specific layers was assessed by laser microdissection followed by Western blotting. The functional role of PKCζ was then evaluated in vivo, using intraocular administration of its specific inhibitor.

Results

PKCζ was localized in tight junction protein complexes of the retinal pigment epithelium and in photoreceptors inner segments. Strikingly, in outer segment PKCζ staining was restricted to cone photoreceptors. Short-term hyperglycemia induced activation and delocalization of PKCζ from both retinal pigment epithelium junctions and cone outer segment. Outer blood retinal barrier disruption and photoreceptor cone degeneration characterized long-term hyperglycemia. In vivo, reduction of PKCζ overactivation using a specific inhibitor, restored its tight-junction localization and not only improved the outer blood retinal barrier, but also reduced photoreceptor cell-death.

Conclusions

In the retina, hyperglycemia induced overactivation of PKCζ is associated with outer blood retinal barrier breakdown and photoreceptor degeneration. In vivo, short-term inhibition of PKCζ restores the outer barrier structure and reduces photoreceptor cell death, identifying PKCζ as a potential target for early and underestimated diabetes-induced retinal pathology.

A functional, genome-wide evaluation of liposensitive yeast identifies the "ARE2 required for viability" (ARV1) gene product as a major component of eukaryotic fatty acid resistance

The toxic subcellular accumulation of lipids predisposes several human metabolic syndromes, including obesity, type 2 diabetes, and some forms of neurodegeneration. To identify pathways that prevent lipid-induced cell death, we performed a genome-wide fatty acid sensitivity screen in Saccharomyces cerevisiae. We identified 167 yeast mutants as sensitive to 0.5 mm palmitoleate, 45% of which define pathways that were conserved in humans. 63 lesions also impacted the status of the lipid droplet; however, this was not correlated to the degree of fatty acid sensitivity. The most liposensitive yeast strain arose due to deletion of the "ARE2 required for viability" (ARV1) gene, encoding an evolutionarily conserved, potential lipid transporter that localizes to the endoplasmic reticulum membrane. Down-regulation of mammalian ARV1 in MIN6 pancreatic β-cells or HEK293 cells resulted in decreased neutral lipid synthesis, increased fatty acid sensitivity, and lipoapoptosis. Conversely, elevated expression of human ARV1 in HEK293 cells or mouse liver significantly increased triglyceride mass and lipid droplet number. The ARV1-induced hepatic triglyceride accumulation was accompanied by up-regulation of DGAT1, a triglyceride synthesis gene, and the fatty acid transporter, CD36. Furthermore, ARV1 was identified as a transcriptional of the protein peroxisome proliferator-activated receptor α (PPARα), a key regulator of lipid homeostasis whose transcriptional targets include DGAT1 and CD36. These results implicate ARV1 as a protective factor in lipotoxic diseases due to modulation of fatty acid metabolism. In conclusion, a lipotoxicity-based genetic screen in a model microorganism has identified 75 human genes that may play key roles in neutral lipid metabolism and disease.

Ceramides and glucosylceramides are independent antagonists of insulin signaling

Inhibitors of sphingolipid synthesis protect mice from diet induced-insulin resistance, and sphingolipids such as ceramides and glucosylated-ceramides (e.g., GM3) are putative nutritional intermediates linking obesity to diabetes risk. Herein we investigated the role of each of these sphingolipids in muscle and adipose tissue and conclude that they are independent and separable antagonists of insulin signaling. Of particular note, ceramides antagonize insulin signaling in both myotubes and adipocytes, whereas glucosyceramides are only efficacious in adipocytes: 1) In myotubes exposed to saturated fats, inhibitors of enzymes required for ceramide synthesis enhance insulin signaling, but those targeting glucosylceramide synthase have no effect. 2) Exogenous ceramides antagonize insulin signaling in myotubes, whereas ganglioside precursors do not. 3) Overexpression of glucosylceramide synthase in myotubes induces glucosylceramide but enhances insulin signaling. In contrast, glucosylated ceramides have profound effects in adipocytes. For example, either ganglioside addition or human glucosylceramide synthase overexpression suppresses insulin signaling in adipocytes. These data have important mechanistic implications for understanding how these sphingolipids contribute to energy sensing and the disruption of anabolism under conditions of nutrient oversupply.

Inclusion of plasma lipid species improves classification of individuals at risk of type 2 diabetes

Background

A significant proportion of individuals with diabetes or impaired glucose tolerance have fasting plasma glucose less than 6.1 mmol/L and so are not identified with fasting plasma glucose measurements. In this study, we sought to evaluate the utility of plasma lipids to improve on fasting plasma glucose and other standard risk factors for the identification of type 2 diabetes or those at increased risk (impaired glucose tolerance).

Methods and Findings

Our diabetes risk classification model was trained and cross-validated on a cohort 76 individuals with undiagnosed diabetes or impaired glucose tolerance and 170 gender and body mass index matched individuals with normal glucose tolerance, all with fasting plasma glucose less than 6.1 mmol/L. The inclusion of 21 individual plasma lipid species to triglycerides and HbA1c as predictors in the diabetes risk classification model resulted in a statistically significant gain in area under the receiver operator characteristic curve of 0.049 (p<0.001) and a net reclassification improvement of 10.5% (p<0.001). The gain in area under the curve and net reclassification improvement were subsequently validated on a separate cohort of 485 subjects.

Conclusions

Plasma lipid species can improve the performance of classification models based on standard lipid and non-lipid risk factors.

Plasma lipid profiling shows similar associations with prediabetes and type 2 diabetes

The relationship between lipid metabolism with prediabetes (impaired fasting glucose and impaired glucose tolerance) and type 2 diabetes mellitus is poorly defined. We hypothesized that a lipidomic analysis of plasma lipids might improve the understanding of this relationship. We performed lipidomic analysis measuring 259 individual lipid species, including sphingolipids, phospholipids, glycerolipids and cholesterol esters, on fasting plasma from 117 type 2 diabetes, 64 prediabetes and 170 normal glucose tolerant participants in the Australian Diabetes, Obesity and Lifestyle Study (AusDiab) then validated our findings on 1076 individuals from the San Antonio Family Heart Study (SAFHS). Logistic regression analysis of identified associations with type 2 diabetes (135 lipids) and prediabetes (134 lipids), after adjusting for multiple covariates. In addition to the expected associations with diacylglycerol, triacylglycerol and cholesterol esters, type 2 diabetes and prediabetes were positively associated with ceramide, and its precursor dihydroceramide, along with phosphatidylethanolamine, phosphatidylglycerol and phosphatidylinositol. Significant negative associations were observed with the ether-linked phospholipids alkylphosphatidylcholine and alkenylphosphatidylcholine. Most of the significant associations in the AusDiab cohort (90%) were subsequently validated in the SAFHS cohort. The aberration of the plasma lipidome associated with type 2 diabetes is clearly present in prediabetes, prior to the onset of type 2 diabetes. Lipid classes and species associated with type 2 diabetes provide support for a number of existing paradigms of dyslipidemia and suggest new avenues of investigation.

Plasma sphingosine-1-phosphate is elevated in obesity

Background

Dysfunctional lipid metabolism is a hallmark of obesity and insulin resistance and a risk factor for various cardiovascular and metabolic complications. In addition to the well known increase in plasma triglycerides and free fatty acids, recent work in humans and rodents has shown that obesity is associated with elevations in the bioactive class of sphingolipids known as ceramides. However, in obesity little is known about the plasma concentrations of sphinogsine-1-phosphate (S1P), the breakdown product of ceramide, which is an important signaling molecule in mammalian biology. Therefore, the purpose of this study was to examine the impact of obesity on circulating S1P concentration and its relationship with markers of glucose metabolism and insulin sensitivity.

Methodology/Principal Findings

Plasma S1P levels were determined in high-fat diet (HFD)-induced and genetically obese (ob/ob) mice along with obese humans. Circulating S1P was elevated in both obese mouse models and in obese humans compared with lean healthy controls. Furthermore, in humans, plasma S1P positively correlated with total body fat percentage, body mass index (BMI), waist circumference, fasting insulin, HOMA-IR, HbA1c (%), total and LDL cholesterol. In addition, fasting increased plasma S1P levels in lean healthy mice.

Conclusion

We show that elevations in plasma S1P are a feature of both human and rodent obesity and correlate with metabolic abnormalities such as adiposity and insulin resistance.

The adipokine/ceramide axis: key aspects of insulin sensitization

Until recently, sphingolipid physiology was primarily the domain of oncologists and immunologists. However, mounting evidence implicates ceramides and their derivatives in various aspects of metabolism via directly impacting the insulin receptor as well as modulating cell survival and proliferation. More recent observations suggest a strong link between a number of adipokines and ceramide catabolism. Here, we aim to briefly review the available data on the established metabolic effects of sphingolipids in various cell types and will discuss how adipokines exert a critical influence on the steady state levels of these lipid mediators.

Lipidomics in longevity and healthy aging

The role of classical lipids in aging diseases and human longevity has been widely acknowledged. Triglyceride and cholesterol concentrations are clinically assessed to infer the risk of cardiovascular disease while larger lipoprotein particle size and low triglyceride levels have been identified as markers of human longevity. The rise of lipidomics as a branch of metabolomics has provided an additional layer of accuracy to pinpoint specific lipids and its association with aging diseases and longevity. The molecular composition and concentration of lipid species determine their cellular localization, metabolism, and consequently, their impact in disease and health. For example, low density lipoproteins are the main carriers of sphingomyelins and ceramides, while high density lipoproteins are mostly loaded with ether phosphocholines, partly explaining their opposing roles in atherogenesis. Moreover, the identification of specific lipid species in aging diseases and longevity would aid to clarify how these lipids alter health and influence longevity. For instance, ether phosphocholines PC (O-34:1) and PC (O-34:3) have been positively associated with longevity and negatively with diabetes, and hypertension, but other species of phosphocholines show no effect or an opposite association with these traits confirming the relevance of the identification of molecular lipid species to tackle our understanding of healthy aging and disease. Up-to-date, a minor fraction of the human plasma lipidome has been associated to healthy aging and longevity, further research would pinpoint toward specific lipidomic profiles as potential markers of healthy aging and metabolic diseases.

Plasma sphingolipids are biomarkers of metabolic syndrome in non-human primates maintained on a Western-style diet

BACKGROUND

The intake of a Western diet enriched in animal fat has been shown to be a major risk factor for Type 2 diabetes and obesity. Previous rodent studies have indicated that these conditions may be triggered by the accumulation of the sphingolipid ceramide in insulin-sensitive tissues. However, data are lacking in this regard from both humans and non-human primates.

OBJECTIVE

Here we have investigated the relationship between plasma ceramides and metabolic syndrome in Rhesus macaques fed a high-fat and high-fructose (HFFD) 'western' diet.

METHODS

We investigated this relationship in cohorts of monkeys fed a HFFD for a period of 8 months to 5 years. Animals were classified as control, pre-diabetic or diabetic based on fasting plasma parameters and insulin sensitivity.

RESULTS

HFFD treatment produced significant increases in body weight and body fat and also resulted in a decline in insulin sensitivity. In parallel to the reduction in insulin sensitivity, significant increases in both plasma ceramide and dihydroceramide levels were observed, which further increased as animals progressed to the diabetic state. Plasma levels of the rare sphingolipid C18:0 deoxysphinganine, a marker of increased metabolic flux through serine palmitoyl transferase (SPT), were also elevated in both pre- and diabetic animals. Furthermore, plasma serine levels were significantly elevated in diabetic monkeys, which may indicate a shift in SPT substrate selectivity from serine to alanine or glycine. In contrast, branch chain amino acids were unchanged in pre-diabetic non-human primates, and only plasma valine levels were elevated in diabetic animals.

CONCLUSION

Together, these data indicate that HFFD induces de novo synthesis of ceramides in non-human primates, and that increased production of plasma ceramides is significantly correlated with the decline in insulin sensitivity.

Development and validation of LC-MS/MS method for determination of very long acyl chain (C22:0 and C24:0) ceramides in human plasma

Ceramide is a key metabolite in both anabolic and catabolic pathways of sphingolipids. The very long fatty acyl chain ceramides N-(docosanoyl)-sphing-4-enine (Cer(22:0)) and N-(tetracosanoyl)-sphing-4-enine (Cer(24:0)) are associated with multiple biological functions. Elevated levels of these sphingolipids in tissues and in the circulation have been associated with insulin resistance and diabetes. To facilitate quantification of these very long chain ceramides in clinical samples from human subjects, we have developed a sensitive, accurate, and high-throughput assay for determination of Cer(22:0) and Cer(24:0) in human plasma. Cer(22:0) and Cer(24:0) and their deuterated internal standards were extracted by protein precipitation and chromatographically separated by HPLC. The analytes and their internal standards were ionized using positive-ion electrospray mass spectrometry, then detected by multiple-reaction monitoring with a tandem mass spectrometer. Total liquid chromatography-tandem mass spectrometry (LC-MS/MS) runtime was 5 min. The assay exhibited a linear dynamic range of 0.02-4 and 0.08-16 μg/ml for Cer(22:0) and Cer(24:0), respectively, in human plasma with corresponding absolute recoveries from plasma at 109 and 114 %, respectively. The lower limit of quantifications were 0.02 and 0.08 μg/ml for Cer(22:0) and Cer(24:0), respectively. Acceptable precision and accuracy were obtained for concentrations over the calibration curve ranges. With the semi-automated format and short LC runtime for the assay, a throughput of ∼200 samples/day can easily be achieved.

Mechanisms of obesity-induced inflammation and insulin resistance: insights into the emerging role of nutritional strategies

Obesity and associated chronic inflammation initiate a state of insulin resistance (IR). The secretion of chemoattractants such as MCP-1 and MIF and of cytokines IL-6, TNF-α, and IL-1β, draw immune cells including dendritic cells, T cells, and macrophages into adipose tissue (AT). Dysfunctional AT lipid metabolism leads to increased circulating free fatty acids, initiating inflammatory signaling cascades in the population of infiltrating cells. A feedback loop of pro-inflammatory cytokines exacerbates this pathological state, driving further immune cell infiltration and cytokine secretion and disrupts the insulin signaling cascade. Disruption of normal AT function is causative of defects in hepatic and skeletal muscle glucose homeostasis, resulting in systemic IR and ultimately the development of type 2 diabetes. Pharmaceutical strategies that target the inflammatory milieu may have some potential; however there are a number of safety concerns surrounding such pharmaceutical approaches. Nutritional anti-inflammatory interventions could offer a more suitable long-term alternative; whilst they may be less potent than some pharmaceutical anti-inflammatory agents, this may be advantageous for long-term therapy. This review will investigate obese AT biology, initiation of the inflammatory, and insulin resistant environment; and the mechanisms through which dietary anti-inflammatory components/functional nutrients may be beneficial.

An FGF21-adiponectin-ceramide axis controls energy expenditure and insulin action in mice

FGF21, a member of the fibroblast growth factor (FGF) superfamily, has recently emerged as a regulator of metabolism and energy utilization. However, the exact mechanism(s) whereby FGF21 mediates its actions have not been elucidated. There is considerable evidence that insulin resistance may arise from aberrant accumulation of intracellular lipids in insulin-responsive tissues due to lipotoxicity. In particular, the sphingolipid ceramide has been implicated in this process. Here, we show that FGF21 rapidly and robustly stimulates adiponectin secretion in rodents while diminishing accumulation of ceramides in obese animals. Importantly, adiponectin-knockout mice are refractory to changes in energy expenditure and ceramide-lowering effects evoked by FGF21 administration. Moreover, FGF21 lowers blood glucose levels and enhances insulin sensitivity in diabetic Lep(ob/ob) mice and diet-induced obese (DIO) mice only when adiponectin is functionally present. Collectively, these data suggest that FGF21 is a potent regulator of adiponectin secretion and that FGF21 critically depends on adiponectin to exert its glycemic and insulin sensitizing effects.

A lipidomics analysis of the relationship between dietary fatty acid composition and insulin sensitivity in young adults

Relative to diets enriched in palmitic acid (PA), diets rich in oleic acid (OA) are associated with reduced risk of type 2 diabetes. To gain insight into mechanisms underlying these observations, we applied comprehensive lipidomic profiling to specimens collected from healthy adults enrolled in a randomized, crossover trial comparing a high-PA diet to a low-PA/high-OA (HOA) diet. Effects on insulin sensitivity (SI) and disposition index (DI) were assessed by intravenous glucose tolerance testing. In women, but not men, SI and DI were higher during HOA. The effect of HOA on SI correlated positively with physical fitness upon enrollment. Principal components analysis of either fasted or fed-state metabolites identified one factor affected by diet and heavily weighted by the PA/OA ratio of serum and muscle lipids. In women, this factor correlated inversely with SI in the fasted and fed states. Medium-chain acylcarnitines emerged as strong negative correlates of SI, and the HOA diet was accompanied by lower serum and muscle ceramide concentrations and reductions in molecular biomarkers of inflammatory and oxidative stress. This study provides evidence that the dietary PA/OA ratio impacts diabetes risk in women.

Inhibition of sphingolipid synthesis improves dyslipidemia in the diet-induced hamster model of insulin resistance: evidence for the role of sphingosine and sphinganine in hepatic VLDL-apoB100 overproduction

Sphingolipids have emerged as important bioactive lipid species involved in the pathogenesis of type 2 diabetes and cardiovascular disease. However, little is known of the regulatory role of sphingolipids in dyslipidemia of insulin-resistant states. We employed hamster models of dyslipidemia and insulin resistance to investigate the role of sphingolipids in hepatic VLDL overproduction, induction of insulin resistance, and inflammation. Hamsters were fed either a control chow diet, a high fructose diet, or a diet high in fat, fructose and cholesterol (FFC diet). They were then treated for 2 weeks with vehicle or 0.3 mg/kg myriocin, a potent inhibitor of de novo sphingolipid synthesis. Both fructose and FFC feeding induced significant increases in hepatic sphinganine, which was normalized to chow-fed levels with myriocin (P < 0.05); myriocin also lowered hepatic ceramide content (P < 0.05). Plasma TG and cholesterol as well as VLDL-TG and -apoB100 were similarly reduced with myriocin treatment in all hamsters, regardless of diet. Myriocin treatment also led to improved insulin sensitivity and reduced hepatic SREBP-1c mRNA, though it did not appear to ameliorate the activation of hepatic inflammatory pathways. Importantly, direct treatment of primary hamster hepatocytes ex vivo with C2 ceramide or sphingosine led to an increased secretion of newly synthesized apoB100. Taken together, these data suggest that a) hepatic VLDL-apoB100 overproduction may be stimulated by ceramides and sphingosine and b) inhibition of sphingolipid synthesis can reduce circulating VLDL in hamsters and improve circulating lipids--an effect that is possibly due to improved insulin signaling and reduced lipogenesis but is independent of changes in inflammation.

Inflammation links excess fat to insulin resistance: the role of the interleukin-1 family

A growing body of evidence suggests that cytokines of the interleukin-1 (IL-1) family, particularly IL-1β but also IL-1Ra and IL-18, are involved in obesity-associated inflammation. IL-1β is produced via cleavage of pro-IL-1β by caspase-1, which in turn is activated by a multiprotein complex called the inflammasome. The components of the NLRP3 inflammasome are involved in sensing obesity-associated danger signals, both in mice and in human (obese) subjects, with caspase-1 seemingly the most crucial regulator. Autophagy is upregulated in obesity and may function as a mechanism to control IL-1β gene expression in adipose tissue to mitigate chronic inflammation. All these mechanisms are operative in human adipose tissue and appear to be more pronounced in human visceral compared to subcutaneous tissue. In animal studies, blocking caspase-1 activity results in decreased weight gain, decreased inflammation, and improved insulin sensitivity. Human intervention studies with IL-1Ra (anakinra) have reported beneficial effects in patients with diabetes, yet without significant changes in insulin sensitivity. Clearly, the IL-1 family of cytokines, especially IL-1β, plays an important role in obesity-associated inflammation and insulin resistance and may represent a therapeutic target to reverse the detrimental metabolic consequences of obesity.

Ceramides contained in LDL are elevated in type 2 diabetes and promote inflammation and skeletal muscle insulin resistance

Dysregulated lipid metabolism and inflammation are linked to the development of insulin resistance in obesity, and the intracellular accumulation of the sphingolipid ceramide has been implicated in these processes. Here, we explored the role of circulating ceramide on the pathogenesis of insulin resistance. Ceramide transported in LDL is elevated in the plasma of obese patients with type 2 diabetes and correlated with insulin resistance but not with the degree of obesity. Treating cultured myotubes with LDL containing ceramide promoted ceramide accrual in cells and was accompanied by reduced insulin-stimulated glucose uptake, Akt phosphorylation, and GLUT4 translocation compared with LDL deficient in ceramide. LDL-ceramide induced a proinflammatory response in cultured macrophages via toll-like receptor-dependent and -independent mechanisms. Finally, infusing LDL-ceramide into lean mice reduced insulin-stimulated glucose uptake, and this was due to impaired insulin action specifically in skeletal muscle. These newly identified roles of LDL-ceramide suggest that strategies aimed at reducing hepatic ceramide production or reducing ceramide packaging into lipoproteins may improve skeletal muscle insulin action.

The role of sphingolipids in drug metabolism and transport

INTRODUCTION

Sphingolipids represent a diverse class of lipid molecules. In addition to their function as membrane structural components, they serve as signaling molecules involved in various biological processes such as cell metabolism, growth, differentiation, stress and inflammatory responses and apoptosis. Sphingolipids may modulate the activity and/or expression of cytochrome P450s (CYPs) and transporters, which suggests that they may affect drug metabolism and excretion.

AREAS COVERED

In this review, the authors provide an overview of the properties of sphingolipid structures and metabolism. They also describe the effects of sphingolipids on the activity and expression of CYPs and transporters. In addition, the authors discuss the pathologic conditions where the sphingolipid metabolism is dysregulated particularly in association with inflammation and cancer.

EXPERT OPINION

Sphingolipidomic approaches have become accessible with the aid of advances in analytical technology. Sphingolipid profiles are modified by diseases, genetic disorders or certain drug treatment. The consequent changes in sphingolipid contents may alter the activities of detoxifying enzymes and those associated with cell viability. Since CYPs and transporters play roles in xenobiotics metabolism and excretion, sphingolipidomic information may be of use in understanding drug effect and toxicity.

Sphingolipid metabolism and obesity-induced inflammation

Obesity is a metabolic disorder developed by overnutrition and a major cause for insulin resistance and cardiovascular events. Since adipose tissue is one of the major sites for the synthesis and secretion of cytokines, enlarged adipose tissue in obese condition alters inflammatory state leading to pathophysiological conditions such as type 2 diabetes and increased cardiovascular risk. A plausible theory for development of metabolic dysregulation is that obesity increases secretion of inflammatory cytokines from adipose tissue and causes a chronic inflammation in the whole body. Additionally accumulation of lipids in non-adipose tissues elevates the cellular levels of bioactive lipids that inhibit the signaling pathways implicated in metabolic regulation together with activated inflammatory response. Recent findings suggest that obesity-induced inflammatory response leads to modulation of sphingolipid metabolism and these bioactive lipids may function as mediators for increased risk of metabolic dysfunction. Importantly, elucidation of mechanism regarding sphingolipid metabolism and inflammatory disease will provide crucial information to development of new therapeutic strategies for the treatment of obesity-induced pathological inflammation.

Sphingolipids in obesity, type 2 diabetes, and metabolic disease

Metabolic disease, including obesity and type 2 diabetes, constitutes a major emerging health crisis in Western nations. Although the symptoms and clinical pathology and physiology of these conditions are well understood, the molecular mechanisms underlying the disease process have largely remained obscure. Sphingolipids, a lipid class with both signaling and structural properties, have recently emerged as key players in most major tissues affected by diabetes and are required components in the molecular etiology of this disease. Indeed, sphingolipids have been shown to mediate loss of insulin sensitivity, to promote the characteristic diabetic proinflammatory state, and to induce cell death and dysfunction in important organs such as the pancreas and heart. Furthermore, plasma sphingolipid levels are emerging as potential biomarkers for the decompensation of insulin resistance to frank type 2 diabetes. Despite these discoveries, the roles of specific sphingolipid species and sphingolipid metabolic pathways remain obscure, and newly developed experimental approaches must be employed to elucidate the detailed molecular mechanisms necessary for rational drug development and other clinical applications.

Plasma ceramides are elevated in female children and adolescents with type 2 diabetes

Accumulation of ceramides within tissues induces insulin resistance. Moreover, adiponectin exerts its beneficial metabolic effects at least partially through ceramide catabolism. We hypothesized that specific plasma ceramide subspecies are elevated in obese children and adolescents with type 2 diabetes (T2D), and that they inversely correlate with adiponectin and measures of insulin sensitivity. This was a cross-sectional study. Participants included 14 obese female subjects with T2D, ages 10-17, and 14 lean healthy controls of the same age and gender. Fasting plasma ceramide subspecies were measured by quantitative tandem mass spectrometry. Subjects with T2D had higher concentrations of C22:0 and C20:0 ceramides, with a 2-fold increase in C18:0 ceramide and C24:1 dihydroceramide (p<0.05). C22:0, C20:0 and C18:0 ceramide correlated with decreased adiponectin concentrations, increased HOMA-IR, BMI Z-score, triglyceride and fasting blood glucose concentrations (p<0.05). Plasma levels of C18:0, C20:0 and C22:0 ceramide, as well as C24:1 dihydroceramide, were elevated in obese female children and adolescents with T2D. This may be a reflection of tissue insulin resistance and could be a result of low adiponectin levels.

Sphingolipid content of human adipose tissue: relationship to adiponectin and insulin resistance

Ceramides (Cer) are implicated in obesity-associated skeletal muscle and perhaps adipocyte insulin resistance. We examined whether the sphingolipid content of human subcutaneous adipose tissue and plasma varies by obesity and sex as well as the relationship between ceramide content and metabolic indices. Abdominal subcutaneous adipose biopsies were performed on 12 lean adults (males = 6), 12 obese adults (males = 6) for measurement of sphingolipid content and activity of the main ceramide metabolism enzymes. Blood was sampled for glucose, insulin (to calculate homeostasis model assessment-estimated insulin resistance (HOMA(IR))) adiponectin, and interleukin-6 (IL-6) concentrations. Compared to lean controls, total ceramide content (pg/adipocyte) was increased by 31% (P < 0.05) and 34% (P < 0.05) in obese females and males, respectively. In adipocytes from obese adults sphingosine, sphinganine, sphingosine-1-phosphate, C14-Cer, C16-Cer, and C24-Cer were all increased. C18:1-Cer was increased in obese males and C24:1-Cer in obese females. For women only, there was a negative correlation between C16-Cer ceramide and plasma adiponectin (r = -0.77, P = 0.003) and a positive correlation between total ceramide content and HOMA(IR) (r = 0.74, P = 0.006). For men only there were significant (at least P < 0.05), positive correlations between adipocyte Cer-containing saturated fatty acid and plasma IL-6 concentration. We conclude that the sexual dimorphism in adipose tissue behavior in humans extends to adipose tissue sphingolipid content its association with adiponectin, IL-6 and insulin resistance.

Could plasma sphingolipids be diagnostic or prognostic biomarkers for Alzheimer's disease?

Understanding the etiopathological processes of Alzheimer's disease (AD) in the preclinical and early clinical stages will be important in developing new therapeutic targets and biomarkers. There is growing consensus that nonamyloid targets will be necessary to reverse or slow AD progression. Lipidomic, metabolomic and targeted approaches have identified pathways and products of sphingolipid metabolism that are altered early in the course of AD and contribute to the neuropathological alterations associated with AD, including amyloid-β production, tau formation and neurodegeneration. In this article, we briefly review the current literature on the role of sphingolipids in the underlying pathophysiology of AD, and then discuss the current state of translating these findings to clinical populations and the potential utility of plasma sphingolipids as diagnostic and/or prognostic indicators of AD.

Regulation of plasma ceramide levels with fatty acid oversupply: evidence that the liver detects and secretes de novo synthesised ceramide

AIMS/HYPOTHESIS

Plasma ceramide concentrations correlate with insulin sensitivity, inflammation and atherosclerotic risk. We hypothesised that plasma ceramide concentrations are increased in the presence of elevated fatty acid levels and are regulated by increased liver serine C-palmitoyltransferase (SPT) activity.

METHODS

Lean humans and rats underwent an acute lipid infusion and plasma ceramide levels were determined. One group of lipid-infused rats was administered myriocin to inhibit SPT activity. Liver SPT activity was determined in lipid-infused rats, and obese, insulin resistant mice. The time and palmitate dose-dependent synthesis of intracellular and secreted ceramide was determined in HepG2 liver cells.

RESULTS

Plasma ceramide levels were increased during lipid infusion in humans and rats, and in obese, insulin-resistant mice. The increase in plasma ceramide was not associated with changes in liver SPT activity, and inhibiting SPT activity by ~50% did not alter plasma ceramide levels in lipid-infused rats. In HepG2 liver cells, palmitate incorporation into extracellular ceramide was both dose- and time-dependent, suggesting the liver cells rapidly secreted the newly synthesised ceramide.

CONCLUSIONS/INTERPRETATION

Elevated systemic fatty acid availability increased plasma ceramide but this was not associated with changes in hepatic SPT activity, suggesting that liver ceramide synthesis is driven by substrate availability rather than increased SPT activity. This report also provides evidence that the liver is sensitive to the intracellular ceramide concentration, and an increase in liver ceramide secretion may help protect the liver from the deleterious effects of intracellular ceramide accumulation.

Expression of ceramide-metabolising enzymes in subcutaneous and intra-abdominal human adipose tissue

Background

Inflammation and increased ceramide concentrations characterise adipose tissue of obese women with high liver fat content compared to equally obese women with normal liver fat content. The present study characterises enzymes involved in ceramide metabolism in subcutaneous and intra-abdominal adipose tissue.

Methods

Pathways leading to increased ceramide concentrations in inflamed versus non-inflamed adipose tissue were investigated by quantifying expression levels of key enzymes involved in ceramide metabolism. Sphingomyelinases (sphingomyelin phosphodiesterases SMPD1-3) were investigated further using immunohistochemistry to establish their location within adipose tissue, and their mRNA expression levels were determined in subcutaneous and intra-abdominal adipose tissue from both non-obese and obese subject.

Results

Gene expression levels of sphingomyelinases, enzymes that hydrolyse sphingomyelin to ceramide, rather than enzymes involved in de novo ceramide synthesis, were higher in inflamed compared to non-inflamed adipose tissue of obese women (with high and normal liver fat contents respectively). Sphingomyelinases were localised to both macrophages and adipocytes, but also to blood vessels and to extracellular regions surrounding vessels within adipose tissue. Expression levels of SMPD3 mRNA correlated significantly with concentrations of different ceramides and sphingomyelins. In both non-obese and obese subjects SMPD3 mRNA levels were higher in the more inflamed intra-abdominal compared to the subcutaneous adipose tissue depot.

Conclusions

Generation of ceramides within adipose tissue as a result of sphingomyelinase action may contribute to inflammation in human adipose tissue.

Role of ceramides in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is a chronic disease with a histological spectrum ranging from steatosis alone, to nonalcoholic steatohepatitis (NASH). The latter is associated with an increased risk for progression to cirrhosis. Ceramides are a lipid species that exert biological effects through cellular proliferation, differentiation, and cell death, and interact with several pathways involved in insulin resistance, oxidative stress, inflammation, and apoptosis, all of which are linked to NAFLD. We propose a mechanism through which ceramides contribute to the development of NAFLD and progression to NASH, due in part to second messenger effects via tumor necrosis factor (TNF)-α. A better understanding of the role of ceramides in steatohepatitis has both diagnostic and therapeutic implications for the treatment of fatty liver disease.

New insights into ER stress-induced insulin resistance

Insulin resistance is a major characteristic of obesity and type 2 diabetes (T2DM). During the last decade, endoplasmic reticulum (ER) stress has emerged as a new player in this field and a considerable number of recent studies have pointed out its role in the onset of insulin resistance (IR). ER stress appears to act directly as a negative modulator of the insulin signaling pathway but also indirectly by promoting lipid accumulation. This review aims to summarize and decipher the abundant new literature concerning the emerging and multifaceted involvement of ER stress in the development of metabolic dysfunctions in insulin target tissues.

Mechanisms involved in cellular ceramide homeostasis

Sphingolipids are ubiquitous and critical components of biological membranes. Their biosynthesis starts with soluble precursors in the endoplasmic reticulum and culminates in the Golgi complex and plasma membrane. Ceramides are important intermediates in the biosynthesis of sphingolipids, such as sphingomyelin, and their overload in the membranes is injurious to cells. The major product of ceramide metabolism is sphingomyelin. We observed that sphingomyelin synthase (SMS) 1 or SMS2 deficiencies significantly decreased plasma and liver sphingomyelin levels. However, SMS2 but not SMS1 deficiency increased plasma ceramides. Surprisingly, SMS1 deficiency significantly increased glucosylceramide and ganglioside GM3, but SMS2 deficiency did not. To explain these unexpected findings about modest to no significant changes in ceramides and increases in other sphingolipids after the ablation of SMS1, we hypothesize that cells have evolved several organelle specific mechanisms to maintain ceramide homeostasis. First, ceramides in the endoplasmic reticulum membranes are controlled by its export to Golgi by protein mediated transfer. Second, in the Golgi, ceramide levels are modulated by their enzymatic conversion to different sphingolipids such as sphingomyelin, and glucosylceramides. Additionally, these sphingolipids can become part of triglyceride-rich apolipoprotein B-containing lipoproteins and be secreted. Third, in the plasma membrane ceramide levels are maintained by ceramide/sphingomyelin cycle, delivery to lysosomes, and efflux to extracellular plasma acceptors. All these pathways might have evolved to ensure steady cellular ceramide levels.

Plasma lysophosphatidylcholine levels are reduced in obesity and type 2 diabetes

Background

Obesity and type 2 diabetes (T2DM) are associated with increased circulating free fatty acids and triacylglycerols. However, very little is known about specific molecular lipid species associated with these diseases. In order to gain further insight into this, we performed plasma lipidomic analysis in a rodent model of obesity and insulin resistance as well as in lean, obese and obese individuals with T2DM.

Methodology/Principal Findings

Lipidomic analysis using liquid chromatography coupled to mass spectrometry revealed marked changes in the plasma of 12 week high fat fed mice. Although a number of triacylglycerol and diacylglycerol species were elevated along with of a number of sphingolipids, a particularly interesting finding was the high fat diet (HFD)-induced reduction in lysophosphatidylcholine (LPC) levels. As liver, skeletal muscle and adipose tissue play an important role in metabolism, we next determined whether the HFD altered LPCs in these tissues. In contrast to our findings in plasma, only very modest changes in tissue LPCs were noted. To determine when the change in plasma LPCs occurred in response to the HFD, mice were studied after 1, 3 and 6 weeks of HFD. The HFD caused rapid alterations in plasma LPCs with most changes occurring within the first week. Consistent with our rodent model, data from our small human cohort showed a reduction in a number of LPC species in obese and obese individuals with T2DM. Interestingly, no differences were found between the obese otherwise healthy individuals and the obese T2DM patients.

Conclusion

Irrespective of species, our lipidomic profiling revealed a generalized decrease in circulating LPC species in states of obesity. Moreover, our data indicate that diet and adiposity, rather than insulin resistance or diabetes per se, play an important role in altering the plasma LPC profile.

Serum sphingolipids and inflammatory mediators in adolescents at risk for metabolic syndrome

The purpose of this study was to determine low-grade inflammation associated with obesity that is mediated partially by TNF-α, an adipocytokine which stimulates sphingomyelinase activity in adipocytes. Circulating ceramide (Cer) and sphingosine 1-phosphate (S1P) are elevated in genetically obese (ob/ob) mice. We aimed to determine whether serum sphingolipid concentrations correlate with measures of obesity, insulin resistance, and lipid profiles in overweight versus lean adolescents. This cross-sectional study recruited 30 healthy overweight (body mass index, BMI ≥ 85%) and 15 lean (BMI 10-84%) adolescents. Anthropometric measurements and fasting blood samples were collected at one clinic visit. Serum glucose, insulin, and fasting lipid profiles were measured. Serum adipocytokine concentrations were measured by ELISA or colorimetric assay and sphingolipids were measured by HPLC-mass spectrometry. Between group differences in serum sphingolipid concentrations were assessed. Correlations between sphingolipid concentrations and (i) body mass index, (ii) calculated homeostasis model assessment of insulin resistance (HOMA-IR), (iii) adipocytokines, and (iv) lipoproteins were determined. The results showed that significant differences in HOMA-IR (4.5 ± 3.2 vs. 1.2 ± 0.7), free fatty acids (0.8 ± 0.3 mmol/l vs. 0.4 ± 0.3 mmol/l), and adiponectin (6.4 ± 3.8 vs. 12.6 ± 9.9 μg/ml) were seen between groups (overweight vs. lean). There were significant correlations between Cer and TNF-α (r = 0.429), S1P and TNF-α (r = 0.288), Cer and adiponectin (r = 0.321), Cer:S1P and adiponectin (r = 0.324), Cer and HOMA-IR (r = 0.307), and Cer:S1P and LDL cholesterol (r = 0.453); these associations persisted after adjustment for BMI Z-score, sex, and Tanner stage. We concluded that elevated sphingolipid concentrations correlate with TNF-α, adiponectin, lipoprotein profiles, and HOMA-IR. Ceramide is associated with atherogenic lipid profiles and the development of insulin resistance in obese adolescents, similar to adults.

Lipid metabolism in skeletal muscle: generation of adaptive and maladaptive intracellular signals for cellular function

Fatty acids derived from adipose tissue lipolysis, intramyocellular triacylglycerol lipolysis, or de novo lipogenesis serve a variety of functions in skeletal muscle. The two major fates of fatty acids are mitochondrial oxidation to provide energy for the myocyte and storage within a variety of lipids, where they are stored primarily in discrete lipid droplets or serve as important structural components of membranes. In this review, we provide a brief overview of skeletal muscle fatty acid metabolism and highlight recent notable advances in the field. We then 1) discuss how lipids are stored in and mobilized from various subcellular locations to provide adaptive or maladaptive signals in the myocyte and 2) outline how lipid metabolites or metabolic byproducts derived from the actions of triacylglycerol metabolism or β-oxidation act as positive and negative regulators of insulin action. We have placed an emphasis on recent developments in the lipid biology field with respect to understanding skeletal muscle physiology and discuss unanswered questions and technical limitations for assessing lipid signaling in skeletal muscle.

Silencing of enzymes involved in ceramide biosynthesis causes distinct global alterations of lipid homeostasis and gene expression

Dysregulation of ceramide synthesis has been associated with metabolic disorders such as atherosclerosis and diabetes. We examined the changes in lipid homeostasis and gene expression in Huh7 hepatocytes when the synthesis of ceramide is perturbed by knocking down serine pal mitoyltransferase subunits 1, 2, and 3 (SPTLC123) or dihydroceramide desaturase 1 (DEGS1). Although knocking down all SPTLC subunits is necessary to reduce total ceramides significantly, depleting DEGS1 is sufficient to produce a similar outcome. Lipidomic analysis of distribution and speciation of multiple lipid classes indicates an increase in phospholipids in SPTLC123-silenced cells, whereas DEGS1 depletion leads to the accumulation of sphingolipid intermediates, free fatty acids, and diacylglycerol. When cer amide synthesis is disrupted, the transcriptional profiles indicate inhibition in biosynthetic processes, downregulation of genes involved in general endomembrane trafficking, and upregulation of endocytosis and endosomal recycling. SPTLC123 silencing strongly affects the expression of genes involved with lipid metabolism. Changes in amino acid, sugar, and nucleotide metabolism, as well as vesicle trafficking between organelles, are more prominent in DEGS1-silenced cells. These studies are the first to provide a direct and comprehensive understanding at the lipidomic and transcriptomic levels of how Huh7 hepatocytes respond to changes in the inhibition of ceramide synthesis.

Nuclear receptors reverse McGarry's vicious cycle to insulin resistance

Several pathways and pathologies have been suggested as connections between obesity and diabetes, including inflammation of adipose and other tissues, toxic lipids, endoplasmic reticulum stress, and fatty liver. One specific proposal is that insulin resistance induces a vicious cycle in which hyperinsulinemia increases hepatic lipogenesis and exacerbates fatty liver, in turn further increasing insulin resistance. Here I suggest that reversing this cycle via suppression of the lipogenic transcription factor SREBP-1c is a common thread that connects the antidiabetic effects of a surprising number of nuclear hormone receptors, including CAR, LRH-1, TRβ, ERα, and FXR/SHP.

A ceramide-centric view of insulin resistance

The recent implementation of genomic and lipidomic approaches has produced a large body of evidence implicating the sphingolipid ceramide in a diverse range of physiological processes and as a critical modulator of cellular stress. In this review, we discuss from a historical perspective the most important discoveries produced over the last decade supporting a role for ceramide and its metabolites in the pathogenesis of insulin resistance and other obesity-associated metabolic diseases. Moreover, we describe how a ceramide-centric view of insulin resistance might be reconciled in the context of other prominent models of nutrient-induced insulin resistance.

Ameliorative effects of polyunsaturated fatty acids against palmitic acid-induced insulin resistance in L6 skeletal muscle cells

Background

Fatty acid-induced insulin resistance and impaired glucose uptake activity in muscle cells are fundamental events in the development of type 2 diabetes and hyperglycemia. There is an increasing demand for compounds including drugs and functional foods that can prevent myocellular insulin resistance.

Methods

In this study, we established a high-throughput assay to screen for compounds that can improve myocellular insulin resistance, which was based on a previously reported non-radioisotope 2-deoxyglucose (2DG) uptake assay. Insulin-resistant muscle cells were prepared by treating rat L6 skeletal muscle cells with 750 μM palmitic acid for 14 h. Using the established assay, the impacts of several fatty acids on myocellular insulin resistance were determined.

Results

In normal L6 cells, treatment with saturated palmitic or stearic acid alone decreased 2DG uptake, whereas unsaturated fatty acids did not. Moreover, co-treatment with oleic acid canceled the palmitic acid-induced decrease in 2DG uptake activity. Using the developed assay with palmitic acid-induced insulin-resistant L6 cells, we determined the effects of other unsaturated fatty acids. We found that arachidonic, eicosapentaenoic and docosahexaenoic acids improved palmitic acid-decreased 2DG uptake at lower concentrations than the other unsaturated fatty acids, including oleic acid, as 10 μM arachidonic acid showed similar effects to 750 μM oleic acid.

Conclusions

We have found that polyunsaturated fatty acids, in particular arachidonic and eicosapentaenoic acids prevent palmitic acid-induced myocellular insulin resistance.

The cellular and signaling networks linking the immune system and metabolism in disease

It is now recognized that obesity is driving the type 2 diabetes epidemic in Western countries. Obesity-associated chronic tissue inflammation is a key contributing factor to type 2 diabetes and cardiovascular disease, and a number of studies have clearly demonstrated that the immune system and metabolism are highly integrated. Recent advances in deciphering the various cellular and signaling networks that participate in linking the immune and metabolic systems together have contributed to understanding of the pathogenesis of metabolic diseases and may also inform new therapeutic strategies based on immunomodulation. Here we discuss how these various networks underlie the etiology of the inflammatory component of insulin resistance, with a particular focus on the central roles of macrophages in adipose tissue and liver.

Sphingosine 1-phosphate (S1P) regulates glucose-stimulated insulin secretion in pancreatic beta cells

Recent studies suggest that sphingolipid metabolism is altered during type 2 diabetes. Increased levels of the sphingolipid ceramide are associated with insulin resistance. However, a role for sphingolipids in pancreatic beta cell function, or insulin production, and release remains to be established. Our studies in MIN6 cells and mouse pancreatic islets demonstrate that glucose stimulates an intracellular rise in the sphingolipid, sphingosine 1-phosphate (S1P), whereas the levels of ceramide and sphingomyelin remain unchanged. The increase in S1P levels by glucose is due to activation of sphingosine kinase 2 (SphK2). Interestingly, rises in S1P correlate with increased glucose-stimulated insulin secretion (GSIS). Decreasing S1P levels by treatment of MIN6 cells or primary islets with the sphingosine kinase inhibitor reduces GSIS. Moreover, knockdown of SphK2 alone results in decreased GSIS, whereas knockdown of the S1P phosphatase, Sgpp1, leads to a rise in GSIS. Treatment of mice with the sphingosine kinase inhibitor impairs glucose disposal due to decreased plasma insulin levels. Altogether, our data suggest that glucose activates SphK2 in pancreatic beta cells leading to a rise in S1P levels, which is important for GSIS.

Analysis of sphingolipids in extracted human plasma using liquid chromatography electrospray ionization tandem mass spectrometry

We have developed an analytical method used to quantify sphingolipids, including deoxysphingoid bases, in lipid extracts prepared from human plasma. In total, 39 analytes were identified and analyzed in a single chromatographic run in less than 5 min. The new method is 4-8 times faster and more sensitive than previously published methods. We also describe a simple sample preparation method that allows medium-throughput screening of human plasma samples. Mass spectrometric analyses were performed online using liquid chromatography electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) in the positive multiple reaction monitoring mode. Samples were extracted using a one-phase extraction method (methanol-dichloromethane) with appropriate internal standards. Sphingolipid analytes were linear over a wide range of concentrations, from 0.01 to 50 ng/ml, with a high correlation coefficient (r2 = 0.999). We successfully applied this method to analyze the levels of sphingolipid metabolites in healthy human plasma. The ceramide, dihydroceramide, hexosylceramide, and GM3 levels observed in females were slightly higher than those observed in males.

Endoplasmic reticulum stress does not mediate palmitate-induced insulin resistance in mouse and human muscle cells

AIMS/HYPOTHESIS

Recent experiments in liver and adipocyte cell lines indicate that palmitate can induce endoplasmic reticulum (ER) stress. Since it has been shown that ER stress can interfere with insulin signalling, our hypothesis was that the deleterious action of palmitate on the insulin signalling pathway in muscle cells could also involve ER stress.

METHODS

We used C2C12 and human myotubes that were treated either with palmitate or tunicamycin. Total lysates and RNA were prepared for western blotting or quantitative RT-PCR respectively. Glycogen synthesis was assessed by [¹⁴C]glucose incorporation.

RESULTS

Incubation of myotubes with palmitate or tunicamycin inhibited insulin-stimulated protein kinase B (PKB)/ v-akt murine thymoma viral oncogene homologue 1 (Akt). In parallel, an increase in ER stress markers was observed. Pre-incubation with chemical chaperones that reduce ER stress only prevented tunicamycin but not palmitate-induced insulin resistance. We hypothesised that ER stress activation levels induced by palmitate may not be high enough to induce insulin resistance, in contrast with tunicamycin-induced ER stress. Indeed, tunicamycin induced a robust activation of the inositol-requiring enzyme 1 (IRE-1)/c-JUN NH₂-terminal kinase (JNK) pathway, leading to serine phosphorylation of insulin receptor substrate 1 (IRS-1) and a decrease in IRS-1 tyrosine phosphorylation. In contrast, palmitate only induced a very weak activation of the IRE1/JNK pathway, with no IRS1 serine phosphorylation.

CONCLUSIONS/INTERPRETATION

These data show that insulin resistance induced by palmitate is not related to ER stress in muscle cells.

Antihypertensive treatment differentially affects vascular sphingolipid biology in spontaneously hypertensive rats

Background

We have previously shown that essential hypertension in humans and spontaneously hypertensive rats (SHR), is associated with increased levels of ceramide and marked alterations in sphingolipid biology. Pharmacological elevation of ceramide in isolated carotid arteries of SHR leads to vasoconstriction via a calcium-independent phospholipase A₂, cyclooxygenase-1 and thromboxane synthase-dependent release of thromboxane A₂. This phenomenon is almost absent in vessels from normotensive Wistar Kyoto (WKY) rats. Here we investigated whether lowering of blood pressure can reverse elevated ceramide levels and reduce ceramide-mediated contractions in SHR.

Methods and Findings

For this purpose SHR were treated for 4 weeks with the angiotensin II type 1 receptor antagonist losartan or the vasodilator hydralazine. Both drugs decreased blood pressure equally (SBP untreated SHR: 191±7 mmHg, losartan: 125±5 mmHg and hydralazine: 113±14 mmHg). The blood pressure lowering was associated with a 20–25% reduction in vascular ceramide levels and improved endothelial function of isolated carotid arteries in both groups. Interestingly, losartan, but not hydralazine treatment, markedly reduced sphingomyelinase-induced contractions. While both drugs lowered cyclooxygenase-1 expression, only losartan and not hydralazine, reduced the endothelial expression of calcium-independent phospholipase A₂. The latter finding may explain the effect of losartan treatment on sphingomyelinase-induced vascular contraction.

Conclusion

In summary, this study corroborates the importance of sphingolipid biology in blood pressure control and specifically shows that blood pressure lowering reduces vascular ceramide levels in SHR and that losartan treatment, but not blood pressure lowering per se, reduces ceramide-mediated arterial contractions.

Gastric bypass surgery reduces plasma ceramide subspecies and improves insulin sensitivity in severely obese patients

Bariatric surgery is associated with near immediate remission of type 2 diabetes and hyperlipidemia. The mechanisms underlying restoration of normal glucose tolerance postoperatively are poorly understood. Herein, we examined the effect of Roux-en-Y gastric bypass surgery (RYGB) on weight loss, insulin sensitivity, plasma ceramides, proinflammatory markers, and cardiovascular risk factors before and at 3 and 6 months after surgery. Thirteen patients (10 female; age 48.5 ± 2.7 years; BMI, 47.4 ± 1.5 kg/m(2)) were included in the study, all of whom had undergone laparoscopic RYGB surgery. Insulin sensitivity, inflammatory mediators and fasting lipid profiles were measured at baseline, 3 and 6 months postoperatively, using enzymatic analysis. Plasma ceramide subspecies (C14:0, C16:0, C18:0, C18:1, C20:0, C24:0, and C24:1) were quantified using electrospray ionization tandem mass spectrometry after separation with HPLC. At 3 months postsurgery, body weight was reduced by 25%, fasting total cholesterol, triglycerides, low-density lipoproteins, and free fatty acids were decreased, and insulin sensitivity was increased compared to presurgery values. These changes were all sustained at 6 months. In addition, total plasma ceramide levels decreased significantly postoperatively (9.3 ± 0.5 nmol/ml at baseline vs. 7.6 ± 0.4 at 3 months, and 7.3 ± 0.3 at 6 months, P < 0.05). At 6 months, the improvement in insulin sensitivity correlated with the change in total ceramide levels (r = -0.68, P = 0.02), and with plasma tumor necrosis factor-α (TNF-α) (r = -0.62, P = 0.04). We conclude that there is a potential role for ceramide lipids as mediators of the proinflammatory state and improved insulin sensitivity after gastric bypass surgery.

Weight loss amelioration of non-alcoholic steatohepatitis linked to shifts in hepatic ceramide expression and serum ceramide levels

AIM

  Non-alcoholic steatohepatitis (NASH) is associated with increased hepatic insulin resistance. Ceramides and other toxic sphingolipids promote inflammation, lipotoxicity and insulin resistance; however, the role of ceramides in the pathogenesis of NASH has not been determined. This study characterizes expression of ceramide-related genes in human livers with NASH and examines the effects of weight loss on NASH and pro-ceramide gene expression in liver.

METHODS

  Liver biopsies were obtained to assess the histopathological status of non-alcoholic fatty liver disease/NASH prior to and following completion of a 1-year course of implementing either lifestyle changes or a standard enrichment protocol designed to encourage weight loss. Liver biopsy samples were used to measure pro-ceramide gene expression by quantitative reverse transcriptase polymerase chain reaction analysis (qRT-PCR), and serum was used to measure ceramide immunoreactivity.

RESULTS

  At baseline, serine palmitoyltransferase (SPTLC)2 (P = 0.02) and ceramide synthase (CER)1 (P = 0.001) mRNA transcripts were less abundantly expressed in livers with NASH relative to normal controls. After weight loss (average 9.3%), SPTLC1 (P = 0.005) and uridine diphosphate glucose ceramide glucosyltransferase (UGCG) (P = 0.001) expression significantly declined while CER1 increased (P = 0.001) among subjects randomized to the lifestyle change subgroup. Reductions in calorie and fat consumption were significantly correlated with changes in ceramide-related gene expression. Finally, both net and relative reductions in serum ceramide levels were significantly greater in the lifestyles compared with the standard enrichment (control) protocol group (both P < 0.005).

CONCLUSION

  NASH is associated with increased insulin resistance and altered ceramide gene expression in liver. Weight loss-mediated reversal of NASH is associated with reduced pro-ceramide gene expression in liver.