Ceramides and sphingomyelins in skeletal muscles of the rat: content and composition. Effect of prolonged exercise

Abdominal obesity-related lipid metabolites may mediate the association between obesity and glucose dysregulation

BACKGROUND

Children with obesity is associated with a higher risk of cardiovascular disease (CV) risk in adulthood. This study is to explore the obesity-related lipid metabolites and identify the associations of lipid metabolites with selected CV risk in children and adolescents.

METHODS

A case-control study was designed to include a total of 197 children (aged 9-13 years, male 56.34%, 99 children in the obesity group). The lipidomics profiling was measured by ultra-high-performance liquid tandem chromatography quadrupole time-of-flight mass spectrometry.

RESULTS

Four FDR-significant abdominal obesity-related lipid metabolites were identified. Compared to the lean group, decreased phosphatidylcholine O-21:2 level (q = 0.010) and sphingomyelins d21:1 (q = 0.029) were found and two lipid metabolites levels were higher in the obese group, including phosphatidylglycerol 43:6 and one did not match with any candidate compounds in databases. After adjusting for covariates, PC3 (O-21:2) and SM (d21:1) were significantly associated with blood glucose. Mediation analysis showed that all three lipid metabolites may mediate the association between abdominal obesity and glucose regulation.

CONCLUSIONS

This study identified several novel central obesity-related lipid metabolites, and we found that PC3 (O-21:2) and SM (d21:1) were significantly associated with blood glucose, and all these lipid metabolites can mediate the association between abdominal obesity and glucose dysregulation.

IMPACT

Serum lipidomic profiles in children with abdominal obesity and their associations with selected CV risk factors were examined. Our study identified 4 lipid metabolites associated with abdominal obesity, including PC3 (O-21:2), SM (d21:1), PG (43:6), and one did not match with any candidate compounds in the databases. PC3 (O-21:2) and SM (d21:1) were significantly associated with blood glucose. Mediation analysis showed that all three lipid metabolites [PC3 (O-21:2), SM (d21:1), PG (43:6)] may mediate the association between abdominal obesity and abnormal glucose regulation. This study identified several novel obesity-related lipid metabolites.

Fumonisin B Series Mycotoxins' Dose Dependent Effects on the Porcine Hepatic and Pulmonary Phospholipidome

Male weaned piglets n = 6/group were fed Fumonisin B₁₊₂₊₃ (FBs) mycotoxins at 0, 15, or 30 mg/kg diet for 3 weeks to assess the fatty acid (FA) composition of membrane lipid classes, lipid peroxidation, and histomorphological changes in the liver and lung. Growth performance and lipid peroxidation were unaltered, but histomorphological lesion scores increased in the liver. Linear dose-response was detected in liver phosphatidylcholines for C16:1n7, C18:1n9, and total monounsaturation and in lungs for C22:6n3, total n-3 and n-3:n-6, in pulmonary phosphatidylserines C20:0 and C24:0. Alterations associated with the highest FBs dose were detected in sphingomyelins (liver: total saturation ↓, total monounsaturation ↑), phosphatidylcholines (liver: total n-6 ↓, n-6:n-3 ↑; in lungs: total monounsaturation ↑, total polyunsaturation ↑), phosphatidylethanolamines (liver: total n-3 ↓; in lungs: total monounsaturation ↑ and n-6:n-3 ↑), phosphatidylserines (liver: n-6:n-3 ↑; in lungs: total saturation ↓, total polyunsatuartion ↑, and total n-6 and its ratio to n-3 ↑), and phosphatidylinositol (n-6:n-3 ↑; lungs: C22:1n9 ↑, C22:6n3 ↓, total saturation ↓, total monounsaturaion ↑). In conclusion, FBs exposures neither impaired growth nor induced substantial lipid peroxidation, but hepatotoxicity was proven with histopathological alterations at the applied exposure period and doses. FA results imply an enzymatic disturbance in FA metabolism, agreeing with earlier findings in rats.

The ketogenic diet prevents steatosis and insulin resistance by reducing lipogenesis, diacylglycerol accummulation, and PKC activity in male rat liver

KEY POINTS

The accumulation of diacylglycerol (DAG), ceramides and inflammation are key factors that cause insulin resistance and nonalcoholic fatty liver (NAFLD). This study provides evidence that a ketogenic diet (KD) rich in fat and devoid of carbohydrate reduced DAG content and preserved insulin signalling in the liver. The KD shifted metabolism away from lipogenesis by enhancing genes involved in mitochondrial biogenesis and fatty acid oxidations in the liver. The KD also promoted the production of beneficial very long-chain ceramides instead of potentially harmful long-chain ceramides. Through multiple mechanisms, the KD exerted anti-steatogenic and insulin-sensitizing effects in the liver, which supports the use of this dietary intervention to treat NAFLD.

ABSTRACT

Obesity-associated insulin resistance plays a major role in the pathogenesis of nonalcoholic fatty liver disease (NAFLD). The accumulation of diacylglycerol (DAG), ceramides and inflammation are key factors that cause NAFLD. In recent years, the ketogenic diet (KD) has emerged as an effective non-pharmacological intervention for the treatment of NAFLD and other obesity-related metabolic disorders. What remains undetermined is how the KD affects DAG and ceramides content and insulin sensitivity in the liver. Thus, this research was designed to assess these variables, as well as glucose and fat metabolism and markers of inflammation in livers of rats exposed for 8 weeks to one of the following diets: standard chow (SC), obesogenic high-fat, sucrose-enriched diet (HFS), or a KD. Despite having a higher fat content than the HFS diet, the KD did not cause steatosis and preserved hepatic insulin signalling. The KD reduced DAG content and protein kinase C epsilon (PKCε) activity, but markedly increased liver ceramides content. However, whereas the KD increased ceramide synthase 2 (CerS2) expression, it suppressed CerS6 expression, an effect that promoted the production of beneficial very long-chain ceramides instead of harmful long-chain ceramides. The KD also enhanced the liver expression of key genes involved in mitochondrial biogenesis and fatty acid oxidation (Pgc-1α and Fgf21), suppressed inflammatory genes (Tnfα, Nf-kb, Tlr4, and Il6), and shifted substrate away from de-novo lipogenesis. Thus, through multiple mechanisms the KD exerted anti-steatogenic and insulin-sensitizing effects in the liver, which supports the use of this dietary intervention to treat NAFLD. Abstract figure legend This article is protected by copyright. All rights reserved.

Sphingomyelin profiling in patients with diabetes could be potentially useful as differential diagnostics biomarker: A pilot study

PURPOSE

Autoimmune diabetes (AD) in adults includes both the classical form of type 1 diabetes mellitus (T1DM) and latent autoimmune diabetes in adults (LADA). LADA shares clinical and metabolic features with type 1 and type 2 diabetes mellitus (T2DM). Ceramide (Cer) levels negatively correlate with insulin sensitivity in humans and animal models. However, only a few studies have focused on other sphingolipids, including sphingomyelin (SM). Therefore, we determined sphingolipids in patients with newly diagnosed diabetes as possible diagnostic biomarkers.

MATERIALS AND METHODS

We evaluated sphingolipids in a cohort of 59 adults with newly diagnosed diabetes without prior hypoglycemic pharmacotherapy to distinguish diabetes mellitus types and for precise LADA definition. All patients with newly diagnosed diabetes were tested for the concentrations of individual Cer and SM species by gas-liquid chromatography. The study included healthy controls and patients with T1DM, T2DM and LADA.

RESULTS

SM species were significantly altered in patients with newly diagnosed diabetes compared to healthy controls. SM-C16:0, C16:1, -C18:0, -C18:1, -C18:2, -C18:3, -C20:4, and -C22:6 species were found to be significantly elevated in LADA patients. In contrast, significant differences were observed for Cer species with saturated acyl chains, especially Cer-C14:0, -C16:0, -C18:0 (AD and T2DM), -C22:0, and -C24:0 (T1DM). Following ROC analysis, SM-C16:0, and particularly -C18:1, and -C20:4 may be supportive diagnostic markers for LADA.

CONCLUSION

SM profiling in patients with newly diagnosed diabetes could be potentially helpful for differential diagnosis of LADA, T1DM, and T2DM in more challenging cases.

Distinct mechanisms involving diacylglycerol, ceramides, and inflammation underlie insulin resistance in oxidative and glycolytic muscles from high fat-fed rats

This study investigated whether oxidative and glycolytic rat skeletal muscles respond differently to a high-fat (HF) sucrose-enriched diet with respect to diacylglycerol (DAG) and ceramides accumulation, protein kinase C (PKC) activation, glucose metabolism, and the expression of inflammatory genes. HF diet (8 weeks) suppressed insulin-stimulated glycogen synthesis and glucose oxidation in soleus (Sol), extensor digitorum longus (EDL) and epitrochlearis (Epit) muscles. However, DAG and ceramides levels increased in Sol and EDL, but not in Epit muscles of HF-fed rats. Additionally, membrane-bound PKC-delta and PKC-theta increased in Sol and EDL, whereas in Epit muscles both PKC isoforms were reduced by HF diet. In Epit muscles, HF diet also increased the expression of tumor necrosis factor-α (TNF-α) receptors (CD40 and FAS), toll-like receptor 4 (TLR4), and nuclear factor kappa light polypeptide gene enhancer in B cells (NF-kB), whereas in Sol and EDL muscles the expression of these inflammatory genes remained unchanged upon HF feeding. In conclusion, HF diet caused DAG and ceramides accumulation, PKC activation, and the induction of inflammatory pathways in a fiber type-specific manner. These findings help explain why oxidative and glycolytic muscles similarly develop insulin resistance, despite major differences in their metabolic characteristics and responsiveness to dietary lipid abundance.

The Effects of Mixed Fusarium Mycotoxins at EU-Permitted Feed Levels on Weaned Piglets' Tissue Lipids

At exactly the individual permitted EU-tolerance dietary limits, fumonisins (FB: 5 mg/kg diet) and mixed fusariotoxins (DZ: 0.9 mg deoxynivalenol + 0.1 mg zearalenone/kg diet, and FDZ: 5 mg fumonisins + 0.9 mg deoxynivalenol + 0.1 mg zearalenone/kg diet) were administered to piglets (n = 6/group) for three weeks. Bodyweights of intoxicated piglets increased, while feed conversion ratios decreased. In FDZ, both the absolute and relative weight of the liver decreased. In the renal-cellular membrane, the most pronounced alterations were in FDZ treatment, followed by individual FB exposure. In both treatments, high proportions of C20:0 and C22:0 with low fatty acid (FA) unsaturation were found. In hepatocyte phospholipids, FDZ toxins exerted antagonistic interactions, and FB had the strongest increasing effect on FA monounsaturation. Among all investigated organs, the spleen lipids were the least responsive, in which FDZ expressed synergistic reactions on C20:0 (↑ FDZ vs. FB) and C22:0 (↓ FDZ vs. DZ). The antioxidant defense of the kidney was depleted (↓ glutathione concentration by FB-exposure). Blood plasma indicated renal injury (profound increase of urea and creatinine in FB vs. DZ and FDZ). FB strongly increased total-cholesterol and low density lipoprotein concentrations, whereas FDZ synergistically increased gamma-glutamyltransferase, alkaline-phosphatase, calcium and phosphorus levels. Summarized, individual and combined multiple fusariotoxins modified the membrane lipid profile and antioxidant defense of splanchnic organs, and serum biochemicals, without retarding growth in piglets.

Development and validation of a RPLC-MS/MS method for the quantification of ceramides in human serum

Ceramides are key-role lipids involved in numerous central cellular processes. A plethora of studies have demonstrated that the levels of ceramides in blood circulation are related to different disease states, such as type 2 diabetes, cardiovascular diseases, ovarian cancer, multiple sclerosis and others. Herein, a RPLC-MS/MS method for the rapid quantification of ceramides Cer(d18:1/16:0), Cer(d18:1/18:0), Cer(d18:1/24:0) and Cer(d18:1/24:1) in human blood serum was developed and validated. Different sample preparation strategies including SLE, LLE and QuECheRS were tested with the aim to attain effective, accurate and reproducible determination of ceramides in serum samples. Intra and inter-day accuracy were found to be between 80.0-111% and 87.8-106%, respectively, for all ceramides, while intra and inter-day precision were found to vary from 0.05% to 10.2% %RSD and 2.2% to 14.0% %RSD, respectively. The lower limits of quantification were 2.3 ng/mL for Cer(d18:1/16:0) and Cer(d18:1/18:0) and 1.4 ng/mL for Cer(d18:1/24:0) and Cer(d18:1/24:1). The method was evaluated in accordance to bioanalytical method guidelines and was used for the determination of serum ceramides of patients with coronary artery disease to evaluate its utility in clinical analyses.

Extraction and Quantification of Sphingolipids from Hemiptera Insects by Ultra-Performance Liquid Chromatography Coupled to Tandem Mass Spectrometry

Sphingolipids are major structural components of endomembranes and have also been described as an intracellular second messenger involved in various biological functions in all eukaryotes and a few prokaryotes. Ceramides (Cer), the central molecules of sphingolipids, have been depicted in cell growth arrest, cell differentiation, and apoptosis. With the development of lipidomics, the identification of ceramides has been analyzed in many species, mostly in model insects. However, there is still a lack of research in non-model organisms. Here we describe a relatively simple and sensitive method for the extraction, identification, and quantification of ceramides in Hemiptera Insects (brown planthooper), followed by Ultra-Performance Liquid Chromatography coupled to tandem mass spectrometry (UPLC-MS/MS). C18 is used as the separation column for quantitative detection and analysis on the triple quadruple liquid mass spectrometer. In this protocol, the standard curve method is adopted to confirm the more accurate quantification of ceramides based on the optional detection conditions.

Podocytopathy and Nephrotic Syndrome in Mice with Podocyte-Specific Deletion of the Asah1 Gene: Role of Ceramide Accumulation in Glomeruli

Lysosomal acid ceramidase (Ac) has been shown to be critical for ceramide hydrolysis and regulation of lysosome function and cellular homeostasis. In the present study, we generated a knockout mouse strain (Asah1^fl/fl/Podo^Cre) with a podocyte-specific deletion of the α subunit (main catalytic subunit) of Ac. Although no significant morphologic changes in glomeruli were observed in these mice under light microscope, severe proteinuria and albuminuria were found in these podocyte-specific knockout mice compared with control genotype littermates. Transmission electron microscopic analysis showed that podocytes of the knockout mice had distinctive foot process effacement and microvillus formation. These functional and morphologic changes indicate the development of nephrotic syndrome in mice bearing the Asah1 podocyte-specific gene deletion. Ceramide accumulation determined by liquid chromatography-tandem mass spectrometry was demonstrated in isolated glomeruli of Asah1^fl/fl/Podo^Cre mice compared with their littermates. By crossbreeding Asah1^fl/fl/Podo^Cre mice with Smpd1^-/- mice, we also produced a double knockout strain, Smpd1^-/-/Asah1^fl/fl/Podo^Cre, that also lacks Smpd1, the acid sphingomyelinase that hydrolyzes sphingomyelin to ceramide. These mice exhibited significantly lower levels of glomerular ceramide with decreased podocyte injury compared with Asah1^fl/fl/Podo^Cre mice. These results strongly suggest that lysosomal Ac in podocytes is essential for the maintenance of the structural and functional integrity of podocytes.

Porcine Hepatic Response to Fumonisin B1 in a Short Exposure Period: Fatty Acid Profile and Clinical Investigations

Scarce studies have investigated the impact of fumonisin B1 (FB1) on the hepatic tissue fatty acid (FA) profile, and no study is available on piglets. A 10-day in vivo experiment was performed on seven piglets/group: control and FB1-fed animals (diet was contaminated with fungal culture: 20 mg FB1/kg diet). Independent sample t-test was carried out at p < 0.05 as the significance level. Neither growth, nor feed efficiency, was affected. The hepatic phospholipid (PL) fatty acids (FAs) were more susceptible for FB1, while triglyceride (TG) was less responsive. The impact of FB1 on hepatic PL polyunsaturated fatty acids (PUFAs) was more pronounced than on saturated fatty acids. Among all PUFAs, predominant ones in response were docosapentaenoicacid (DPA) (↓), docosahexaenoic DHA (↓) and arachidonic acids (↑). This led to a higher omega-6:omega-3 ratio, whereas a similar finding was noted in TGs. Neither total saturation (SFA) nor total monousaturation (MUFA) were affected by the FB1 administration. The liver showed an increase in malondialdehyde, as well as antioxidant capacity (reduced glutathione and glutathione peroxidase). The plasma enzymatic assessment revealed an increase in alkaline phosphatase (ALP), while alanine transaminase (ALT), aspartate transaminase (AST), lactate dehydrogenase (LDH), and gamma-glutamyltransferase (GGT) were not influenced. The microscopic sections provided evidence of vacuolar degeneration of the hepatocytes' cytoplasm, but it was not severe. Furthermore, the lung edema was developed, while the kidney was not affected. In conclusion, regarding FB1-mediated hepatotoxicity in piglets, the potential effect of slight hepatotoxicity did not compromise growth performance, at least at the dose and exposure period applied.

Changes in Membrane Ceramide Pools in Rat Soleus Muscle in Response to Short-Term Disuse

Lipid raft disruption is an early event during skeletal muscle unloading. Ceramide (Cer) serves as a signaling lipid that can contribute to lipid raft disturbance and muscle atrophy. Using biochemical and fluorescent approaches, the distribution of Cer and related molecules in the rat soleus muscle subjected to 12 h of hindlimb suspension (HS) was studied. HS led to upregulation of TNFα receptor 1 (TNFR1), Cer-producing enzymes, and acid and neutral sphingomyelinase (SMase) in detergent-resistant membranes (lipid rafts), which was accompanied by an increase in Cer and a decrease in sphingomyelin in this membrane fraction. Fluorescent labeling indicated increased Cer in the sarcoplasm as well as the junctional (synaptic) and extrajunctional compartments of the suspended muscles. Also, a loss of membrane asymmetry (a hallmark of membrane disturbance) was induced by HS. Pretreatment with clomipramine, a functional inhibitor of acid SMase, counteracted HS-mediated changes in the Cer/sphingomyelin ratio and acid SMase abundance as well as suppressed Cer accumulation in the intracellular membranes of junctional and extrajunctional regions. However, the elevation of plasma membrane Cer and disturbance of the membrane asymmetry were suppressed only in the junctional compartment. We suggest that acute HS leads to TNFR1 and SMase upregulation in the lipid raft fraction and deposition of Cer throughout the sarcolemma and intracellularly. Clomipramine-mediated downregulation of acid SMase can suppress Cer accumulation in all compartments, excluding the extrajunctional plasma membrane.

Genetic Variation in Acid Ceramidase Predicts Non-completion of an Exercise Intervention

Genetic variation is associated with a number of lifestyle behaviours; it may be associated with adherence and individual responses to exercise training. We tested single nucleotide polymorphisms (SNPs) in the acid ceramidase gene (ASAH1) for association with subject adherence and physiologic benefit with exercise training in two well-characterised randomised, controlled 8-month exercise interventions: STRRIDE I (n = 239) and STRRIDE II (n = 246). Three ASAH1 non-coding SNPs in a linkage disequilibrium block were associated with non-completion: rs2898458(G/T), rs7508(A/G), and rs3810(A/G) were associated with non-completion in both additive (OR = 1.8, 1.8, 2.0; P < 0.05 all) and dominant (OR = 2.5, 2.6, 3.5; P < 0.05 all) models; with less skeletal muscle ASAH expression (p < 0.01) in a subset (N = 60); and poorer training response in cardiorespiratory fitness (peak VO₂ change rs3810 r² = 0.29, P = 0.04; rs2898458 r² = 0.29, P = 0.08; rs7508 r² = 0.28, p = 0.09); and similar in direction and magnitude in both independent exploratory and replication studies. Adherence to exercise may be partly biologically and genetically moderated through metabolic regulatory pathways participating in skeletal muscle adaptation to exercise training.

Individual and Combined Effects of Fumonisin B₁, Deoxynivalenol and Zearalenone on the Hepatic and Renal Membrane Lipid Integrity of Rats

(1) Background and (2) Methods: A 14-day in vivo, multitoxic (pure mycotoxins) rat experiment was conducted with zearalenone (ZEA; 15 μg/animal/day), deoxynivalenol (DON; 30 μg/animal/day) and fumonisin B₁ (FB₁; 150 μg/animal/day), as individual mycotoxins, binary (FD, FZ and DZ) and ternary combinations (FDZ), via gavage in 1 mL water boluses. (3) Results: Body weight was unaffected, while liver (ZEA↑ vs. DON) and kidney weight (ZEA↑ vs. FDZ) increased. Hepatocellular membrane lipid fatty acids (FAs) referred to ceramide synthesis disturbance (C20:0, C22:0), and decreased unsaturation (C22:5 n3 and unsat. index), mainly induced by DON and to a lesser extent by ZEA. The DON-FB₁ interaction was additive on C20:0 in liver lipids. In renal phospholipids, ZEA had the strongest effect on the FA profile, affecting the saturated (C18:0) and many n6 FAs; ZEA was in an antagonistic relationship with FB₁ (C18:0) or DON (C18:2 n6, C20:1 n9). Hepatic oxidative stress was the most expressed in FD (reduced glutathione and glutathione peroxidase), while the nephrotoxic effect was further supported by lipid peroxidation (malondialdehyde) in the DON treatment. (4) Conclusions: In vivo study results refer to multiple mycotoxin interactions on membrane FAs, antioxidants and lipid peroxidation compounds, needing further testing.

Effect of plasma free fatty acid supply on the rate of ceramide synthesis in different muscle types in the rat

Ceramide is a key compound in sphingolipid metabolism. Dynamics of ceramide synthesis is important in the several biological processes, such as induction of apoptosis or insulin resistance. So far, its de novo synthesis rate was evaluated indirectly, based on the content of the compound, its intermediates and the activity of respective enzymes. The aim of the present study was to directly measure ceramide synthesis rate (FSR) in different muscle types under varied plasma FFA supply in rat with the use of [U-¹³C] palmitate tracer and LC/MS/MS. The experiments were carried out on male Wistar rats, divided into three groups: 1-control, 2-with elevated plasma free fatty acid (FFA) concentration by means of intralipid and heparin, 3-with reduced plasma FFA concentration by means of nicotinic acid. The stable plasma FFA concentration and plasma [U-¹³C] palmitate enrichment was maintained for two hours by simultaneous infusion of the tracer and the respective compounds. At the end of the experiment, samples of blood from the abdominal aorta, the heart, diaphragm, soleus and white section of the gastrocnemius were taken. Muscle sphinganine, sphingosine and ceramide content and enrichment and plasma palmitate enrichment was measured with the use of LC/MS/MS. Plasma FFA concentration and composition was measured by means of gas-liquid chromatography. Under basal conditions ceramide FSR in the heart and the diaphragm was higher than in the soleus and the white gastrocnemius. Elevation in the plasma FFA concentration increased the FSR and ceramide content in each muscle, which correlated with increased HOMA-IR. The highest FSR was noted in the heart. Reduction in the plasma FFA concentration decreased ceramide FSR in each muscle type, which was accompanied by marked reduction in HOMA-IR. It is concluded that ceramide FSR depends on both the muscle type and the plasma FFA supply and is correlated with whole body insulin sensitivity under varying plasma FFA supply.

Muscle sphingolipids during rest and exercise: a C18:0 signature for insulin resistance in humans

AIMS/HYPOTHESES

Ceramides and other sphingolipids comprise a family of lipid molecules that accumulate in skeletal muscle and promote insulin resistance. Chronic endurance exercise training decreases muscle ceramides and other sphingolipids, but less is known about the effects of a single bout of exercise.

METHODS

We measured basal relationships and the effect of acute exercise (1.5 h at 50% [Formula: see text]) and recovery on muscle sphingolipid content in obese volunteers, endurance trained athletes and individuals with type 2 diabetes.

RESULTS

Muscle C18:0 ceramide (p = 0.029), dihydroceramide (p = 0.06) and glucosylceramide (p = 0.03) species were inversely related to insulin sensitivity without differences in total ceramide, dihydroceramide, and glucosylceramide concentration. Muscle C18:0 dihydroceramide correlated with markers of muscle inflammation (p = 0.04). Transcription of genes encoding sphingolipid synthesis enzymes was higher in athletes, suggesting an increased capacity for sphingolipid synthesis. The total concentration of muscle ceramides and sphingolipids increased during exercise and then decreased after recovery, during which time ceramide levels reduced to significantly below basal levels.

CONCLUSIONS/INTERPRETATION

These data suggest ceramide and other sphingolipids containing stearate (18:0) are uniquely related to insulin resistance in skeletal muscle. Recovery from an exercise bout decreased muscle ceramide concentration; this may represent a mechanism promoting the insulin-sensitising effects of acute exercise.

Development and validation of a high-throughput LC-MS/MS assay for routine measurement of molecular ceramides

Monitoring the levels of the ceramides (Cer) d18:1/16:0, Cer d18:1/18:0, Cer d18:1/24:0, and Cer d18:1/24:1 and ratios thereof in human plasma empowers the prediction of fatal outcome of coronary artery disease (CAD). We describe a validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) methodology for clinical-scaled measurement of the four distinct ceramides. Rapid plasma precipitation was accomplished in 96-well format. Excellent extraction recoveries in the range of 98-109% were achieved for each ceramide. Addition of corresponding D7-labeled ceramide standards facilitated precise quantification of each plasma ceramide species utilizing a novel short 5-min LC-MS/MS method. Neither matrix interference nor carryover was observed. Robust intra- and inter-assay accuracy and precision <15% at five different concentrations were obtained. Linear calibration lines with regressions, R(2) > 0.99, were achieved for all analytes. Short-term bench top, long-term plasma, and extract stability demonstrated that the distinct ceramides were stable in the conditions evaluated. The validity of the methodology was demonstrated by determining the precise ceramide concentrations in a small CAD case-control study. Thus, our LC-MS/MS methodology features simple sample preparation and short analysis time for accurate quantification of Cer d18:1/16:0, Cer d18:1/18:0, Cer d18:1/24:0, and Cer d18:1/24:1, designed for routine analysis.

Myriocin prevents muscle ceramide accumulation but not muscle fiber atrophy during short-term mechanical unloading

Bedridden patients in intensive care unit or after surgery intervention commonly develop skeletal muscle weakness. The latter is promoted by a variety of prolonged hospitalization-associated conditions. Muscle disuse is the most ubiquitous and contributes to rapid skeletal muscle atrophy and progressive functional strength reduction. Disuse causes a reduction in fatty acid oxidation, leading to its accumulation in skeletal muscle. We hypothesized that muscle fatty acid accumulation could stimulate ceramide synthesis and promote skeletal muscle weakness. Therefore, the present study was designed to determine the effects of sphingolipid metabolism on skeletal muscle atrophy induced by 7 days of disuse. For this purpose, male Wistar rats were treated with myriocin, an inhibitor of de novo synthesis of ceramides, and subjected to hindlimb unloading (HU) for 7 days. Soleus muscles were assayed for fiber diameter, ceramide levels, protein degradation, and apoptosis signaling. Serum and liver were removed to evaluate the potential hepatoxicity of myriocin treatment. We found that HU increases content of saturated C16:0 and C18:0 ceramides and decreases soleus muscle weight and fiber diameter. HU increased the level of polyubiquitinated proteins and induced apoptosis in skeletal muscle. Despite a prevention of C16:0 and C18:0 muscle accumulation, myriocin treatment did not prevent skeletal muscle atrophy and concomitant induction of apoptosis and proteolysis. Moreover, myriocin treatment increased serum transaminases and induced hepatocyte necrosis. These data highlight that inhibition of de novo synthesis of ceramides during immobilization is not an efficient strategy to prevent skeletal muscle atrophy and exerts adverse effects like hepatotoxicity.

Palmitic acid-induced neuron cell cycle G2/M arrest and endoplasmic reticular stress through protein palmitoylation in SH-SY5Y human neuroblastoma cells

Obesity-related neurodegenerative diseases are associated with elevated saturated fatty acids (SFAs) in the brain. An increase in SFAs, especially palmitic acid (PA), triggers neuron cell apoptosis, causing cognitive function to deteriorate. In the present study, we focused on the specific mechanism by which PA triggers SH-SY5Y neuron cell apoptosis. We found that PA induces significant neuron cell cycle arrest in the G₂/M phase in SH-SY5Y cells. Our data further showed that G₂/M arrest is involved in elevation of endoplasmic reticular (ER) stress according to an increase in p-eukaryotic translation inhibition factor 2α, an ER stress marker. Chronic exposure to PA also accelerates beta-amyloid accumulation, a pathological characteristic of Alzheimer’s disease. Interestingly, SFA-induced ER stress, G₂/M arrest and cell apoptosis were reversed by treatment with 2-bromopalmitate, a protein palmitoylation inhibitor. These findings suggest that protein palmitoylation plays a crucial role in SFA-induced neuron cell cycle G₂/M arrest, ER stress and apoptosis; this provides a novel strategy for preventing SFA-induced neuron cell dysfunction.

Neutral sphingomyelinase-3 mediates TNF-stimulated oxidant activity in skeletal muscle

Aims

Sphingolipid and oxidant signaling affect glucose uptake, atrophy, and force production of skeletal muscle similarly and both are stimulated by tumor necrosis factor (TNF), suggesting a connection between systems. Sphingolipid signaling is initiated by neutral sphingomyelinase (nSMase), a family of agonist-activated effector enzymes. Northern blot analyses suggest that nSMase3 may be a striated muscle-specific nSMase. The present study tested the hypothesis that nSMase3 protein is expressed in skeletal muscle and functions to regulate TNF-stimulated oxidant production.

Results

We demonstrate constitutive nSMase activity in skeletal muscles of healthy mice and humans and in differentiated C2C12 myotubes. nSMase3 (Smpd4 gene) mRNA is highly expressed in muscle. An nSMase3 protein doublet (88 and 85 kD) is derived from alternative mRNA splicing of exon 11. The proteins partition differently. The full-length 88 kD isoform (nSMase3a) fractionates with membrane proteins that are resistant to detergent extraction; the 85 kD isoform lacking exon 11 (nSMase3b) is more readily extracted and fractionates with detergent soluble membrane proteins; neither variant is detected in the cytosol. By immunofluorescence microscopy, nSMase3 resides in both internal and sarcolemmal membranes. Finally, myotube nSMase activity and cytosolic oxidant activity are stimulated by TNF. Both if these responses are inhibited by nSMase3 knockdown.

Innovation

These findings identify nSMase3 as an intermediate that links TNF receptor activation, sphingolipid signaling, and skeletal muscle oxidant production.

Conclusion

Our data show that nSMase3 acts as a signaling nSMase in skeletal muscle that is essential for TNF-stimulated oxidant activity.

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First measures of endogenous nSMase3 protein in muscle.

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Detection of nSMase3 splice variant proteins.

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Identification of a functional role for nSMase3 in redox signaling.

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Identification of an intermediate in TNF/redox signaling.

Inhibition of neutral sphingomyelinases in skeletal muscle attenuates fatty-acid induced defects in metabolism and stress

Background

Chronic metabolic overload leads to insulin resistance in a variety of tissues. It has been shown that exposure to saturated fatty acid palmitate can cause insulin resistance in skeletal muscle cells. Fatty acid induced synthesis of ceramide is considered to be one of the major causes for insulin resistance. Both de novo synthesis and sphingomyelin hydrolysis by sphingomyelinase are implicated for ceramide generation. Aim of this study was to evaluate the impact of neutral sphingomyelinase (nSMase) inhibition on saturated fatty acid induced lipotoxicity and insulin resistance in skeletal muscle myotubes.

Results

Treatment of saturated fatty acid (palmitate) but not unsaturated fatty acid (oleate) caused an up-regulation in expression of various nSMase genes which are associated with ceramide synthesis through the salvage pathway. Inhibition of nSMase by a pharmacological inhibitor (GW4869) partially reverted the palmitate induced insulin resistance in C2C12 myotubes. Inhibition of nSMase improved metabolic functions of myotubes as measured by improved oxidative capacity in terms of increased mitochondrial number, PGC1α expression and ATP levels with concomitant decrease in intramyocellular triglyceride levels. Palmitate induced inflammatory response was also reduced by nSMase inhibitor. GW4869 treatment reduced palmitate induced oxidative and endoplasmic reticulum stress and improved cell survival.

Conclusion

In this study, we provide evidences that inhibition of nSMase can protect skeletal muscles from saturated fatty acid induced insulin resistance, metabolic dysfunction, cellular stress and inflammation.

Electronic supplementary material

The online version of this article (doi:10.1186/2193-1801-3-255) contains supplementary material, which is available to authorized users.

Metabolic syndrome and insulin resistance: underlying causes and modification by exercise training

Metabolic syndrome (MS) is a collection of cardiometabolic risk factors that includes obesity, insulin resistance, hypertension, and dyslipidemia. Although there has been significant debate regarding the criteria and concept of the syndrome, this clustering of risk factors is unequivocally linked to an increased risk of developing type 2 diabetes and cardiovascular disease. Regardless of the true definition, based on current population estimates, nearly 100 million have MS. It is often characterized by insulin resistance, which some have suggested is a major underpinning link between physical inactivity and MS. The purpose of this review is to: (i) provide an overview of the history, causes and clinical aspects of MS, (ii) review the molecular mechanisms of insulin action and the causes of insulin resistance, and (iii) discuss the epidemiological and intervention data on the effects of exercise on MS and insulin sensitivity.

Sphingolipid profiles are altered in prefrontal cortex of rats under acute hyperglycemia

Diabetes type 1 is a common autoimmune disease manifesting by insulin deficiency and hyperglycemia, which can lead to dementia-like brain dysfunctions. The factors triggering the pathological processes in hyperglycemic brain remain unknown. We reported in this study that brain areas with different susceptibility to diabetes (prefrontal cortex (PFC), hippocampus, striatum and cerebellum) revealed differential alterations in ceramide (Cer) and sphingomyelin (SM) profiles in rats with streptozotocin-induced hyperglycemia. Employing gas-liquid chromatography, we found that level of total Cer increased significantly only in the PFC of diabetic animals, which also exhibited a broad spectrum of sphingolipid (SLs) changes, such as elevations of Cer-C16:0, -C18:0, -C20:0, -C22:0, -C18:1, -C24:1 and SM-C16:0 and -C18:1. In opposite, only minor changes were noted in other examined structures. In addition, de novo synthesis pathway could play a role in generation of Cer containing monounsaturated fatty acids in PFC during hyperglycemia. In turn, simultaneous accumulation of Cers and their SM counterparts may suggest that overproduced Cers are converted to SMs to avoid excessive Cer-mediated cytotoxicity. We conclude that broad changes in SLs compositions in PFC induced by hyperglycemia may provoke membrane rearrangements in some cell populations, which can disturb cellular signaling and cause tissue damage.

Imaging mass spectrometry visualizes ceramides and the pathogenesis of dorfman-chanarin syndrome due to ceramide metabolic abnormality in the skin

Imaging mass spectrometry (IMS) is a useful cutting edge technology used to investigate the distribution of biomolecules such as drugs and metabolites, as well as to identify molecular species in tissues and cells without labeling. To protect against excess water loss that is essential for survival in a terrestrial environment, mammalian skin possesses a competent permeability barrier in the stratum corneum (SC), the outermost layer of the epidermis. The key lipids constituting this barrier in the SC are the ceramides (Cers) comprising of a heterogeneous molecular species. Alterations in Cer composition have been reported in several skin diseases that display abnormalities in the epidermal permeability barrier function. Not only the amounts of different Cers, but also their localizations are critical for the barrier function. We have employed our new imaging system, capable of high-lateral-resolution IMS with an atmospheric-pressure ionization source, to directly visualize the distribution of Cers. Moreover, we show an ichthyotic disease pathogenesis due to abnormal Cer metabolism in Dorfman–Chanarin syndrome, a neutral lipid storage disorder with ichthyosis in human skin, demonstrating that IMS is a novel diagnostic approach for assessing lipid abnormalities in clinical setting, as well as for investigating physiological roles of lipids in cells/tissues.

Increased ceramide content and NFκB signaling may contribute to the attenuation of anabolic signaling after resistance exercise in aged males

One of the most fundamental adaptive physiological events is the response of skeletal muscle to high-intensity resistance exercise, resulting in increased protein synthesis and ultimately larger muscle mass. However, muscle growth in response to contraction is attenuated in older humans. Impaired contractile-induced muscle growth may contribute to sarcopenia: the age-associated loss of muscle mass and function that is manifested by loss of strength, contractile capacity, and endurance. We hypothesized that the storage of ceramide would be increased in older individuals and this would be associated with increases in NFκB signaling and a decreased anabolic response to exercise. To test this hypothesis we measured ceramides at rest and anabolic and NFκB signaling after an acute bout of high-intensity resistance exercise in young and older males. Using lipidomics analysis we show there was a 156% increase in the accumulation of C16:0-ceramide (P < 0.05) and a 30% increase in C20:0-ceramide (P < 0.05) in skeletal muscle with aging, although there was no observable difference in total ceramide. C16:0-ceramide content was negatively correlated (P = 0.008) with lower leg lean mass. Aging was associated with a ~60% increase in the phosphorylation of the proinflammatory transcription factor NFκB in the total and nuclear cell fractions (P < 0.05). Furthermore, there was an attenuated activation of anabolic signaling molecules such as Akt (P < 0.05), FOXO1 (P < 0.05), and S6K1 (P < 0.05) after an acute bout of high-intensity resistance exercise in older males. We conclude that ceramide may have a significant role in the attenuation of contractile-induced skeletal muscle adaptations and atrophy that is observed with aging.

Early-stage Pathogenic Sequence of Jaw Osteoradionecrosis in vivo

The mechanism underlying jaw osteoradionecrosis (ORN) is not fully understood, particularly in the early stages. To investigate bone and vessel pathogenesis in the early stages of jaw ORN, we generated a mandibular ORN model in miniature pigs (minipigs) by applying a combination of single-dose 25-Gy irradiation (IR) and tooth extraction. We studied 6 ORN model minipigs and 6 control, non-irradiated minipigs. We measured dynamic morphological changes, bone-remodeling-associated gene expression, sphingomyelinase activity, and local blood flow. Bone remodeling, including bone resorption and new bone formation, was observed within 15 days post-IR. Later, an ORN-related imbalance in bone metabolism gradually occurred, with loss of bone regeneration capacity, collagen collapse, and microvascular obliteration. Within 24 hrs post-IR, sphingomyelinase significantly increased in irradiated tissues. At 1 wk post-IR, local blood flow increased, but at 15 days post-IR, it significantly decreased to 50% below normal levels. This study provided details of the sequential occurrences in early-stage ORN in a large animal model. Our results suggested that reduced local blood flow and consequent hypovascularity may have caused an imbalance in bone remodeling. This suggested that microvessel damage may play a key role in the initiation of ORN.

Ceramide profiles in the brain of rats with diabetes induced by streptozotocin

Diabetes is associated with disturbances of brain activity and cognitive impairment. We hypothesize that ceramides may constitute an important contribution to diabetes-linked neuro-dysfunction. In our study we used rats injected with streptozotocin (STZ) as a model of severe hyperglycemia. Using the gas-liquid chromatography technique we found a significant increase of ceramide content in brains and a decrease in plasma of diabetic rats. The inhibitor of serine palmitoyltransferase, myriocin, reduced ceramide generation in hyperglycemic brains, although injected alone it exerted a paradoxical effect of ceramide upregulation. Myriocin had no impact on ceramide concentration in the plasma of either control or diabetic rats. The level of ceramide saturated fatty acids was elevated whereas the level of ceramide poly-unsaturated fatty acids was downregulated in brains of all experimental groups. The concentration of ceramide mono-unsaturated fatty acids remained unchanged. The pattern of individual ceramide species was altered depending on treatment. We noted an STZ-evoked increase of brain ceramide C16:0, C18:0 and C20:0 and a strong decline in ceramide C18:2 fatty acid levels. Some changes of brain ceramide pattern were modified by myriocin. We found a decreased amount of total ceramide-ω-6 fatty acids in STZ-treated rat brains and no changes in ceramide-ω-3 concentration. We conclude that ceramides may be important mediators of diabetes-accompanied brain dysfunction.

Bioactives of Artemisia dracunculus L. mitigate the role of ceramides in attenuating insulin signaling in rat skeletal muscle cells

Ectopic lipids in peripheral tissues have been implicated in attenuating insulin action in vivo. The botanical extract of Artemisia dracunculus L. (PMI 5011) improves insulin action, yet the precise mechanism is not known. We sought to determine whether the mechanism by which PMI 5011 improves insulin signaling is through regulation of lipid metabolism. After differentiation, cells were separately preincubated with free fatty acids (FFAs) and ceramide C2, and the effects on glycogen content, insulin signaling, and ceramide profiles were determined. The effect of PMI 5011 on ceramide accumulation and ceramide-induced inhibition of insulin signaling was evaluated. FFAs resulted in increased levels of total ceramides and ceramide species in L6 myotubes. Saturated FFAs and ceramide C2 inhibited insulin-stimulated phosphorylation of protein kinase B/Akt and reduced glycogen content. PMI 5011 had no effect on ceramide formation or accumulation but increased insulin sensitivity via restoration of Akt phosphorylation. PMI 5011 also attenuated the FFA-induced upregulation of a negative inhibitor of insulin signaling, i.e., protein tyrosine phosphatase 1B (PTP1B), and increased phosphorylation of PTP1B. PMI 5011 attenuates the reduction in insulin signaling induced by ceramide accumulation, but the mechanism of improved insulin signaling is independent of ceramide formation.

Sphingolipid metabolism, oxidant signaling, and contractile function of skeletal muscle

SIGNIFICANCE

Sphingolipids are a class of bioactive lipids that regulate diverse cell functions. Ceramide, sphingosine, and sphingosine-1-phosphate accumulate in tissues such as liver, brain, and lung under conditions of cellular stress, including oxidative stress. The activity of some sphingolipid metabolizing enzymes, chiefly the sphingomyelinases, is stimulated during inflammation and in response to oxidative stress. Ceramide, the sphingomyelinase product, as well as the ceramide metabolite, sphingosine-1-phosphate, can induce the generation of more reactive oxygen species, propagating further inflammation.

RECENT ADVANCES

This review article summarizes information on sphingolipid biochemistry and signaling pertinent to skeletal muscle and describes the potential influence of sphingolipids on contractile function.

CRITICAL ISSUES

It encompasses topics related to (1) the pathways for complex sphingolipid biosynthesis and degradation, emphasizing sphingolipid regulation in various muscle fiber types and subcellular compartments; (2) the emerging evidence that implicates ceramide, sphingosine, and sphingosine-1-phosphate as regulators of muscle oxidant activity, and (3) sphingolipid effects on contractile function and fatigue.

FUTURE DIRECTIONS

We propose that prolonged inflammatory conditions alter ceramide, sphingosine, and sphingosine-1-phosphate levels in skeletal muscle and that these changes promote the weakness, premature fatigue, and cachexia that plague individuals with heart failure, cancer, diabetes, and other chronic inflammatory diseases.

Increased availability of endogenous and dietary oleic acid contributes to the upregulation of cardiac fatty acid oxidation

In the present study, we tested the hypothesis that cardiac substrate utilization is affected by oleic acid originating from the endogenous conversion of stearate by stearoyl-CoA desaturase and from the diet. Here, we show that the cardiac oleate content is increased in tristearate (TS)- and trioleate (TO)-fed rats when compared with chow-fed rats. TS or TO feeding increases mitochondrial fatty acid oxidation via activation of expression of the oxidative genes, activation of the AMP-activated protein kinase pathway, and a decrease in glucose uptake. These results suggest that oleic acid, both dietary and de novo synthesized, affects substrate utilization in the heart. Furthermore, our data show that the endogenous synthesis of oleate in the heart can compensate for a deficiency of this fatty acid in the diet.

A single prior bout of exercise protects against palmitate-induced insulin resistance despite an increase in total ceramide content

Ceramide accumulation has been implicated in the impairment of insulin-stimulated glucose transport in skeletal muscle following saturated fatty acid (FA) exposure. Importantly, a single bout of exercise can protect against acute lipid-induced insulin resistance. The mechanism by which exercise protects against lipid-induced insulin resistance is not completely known but may occur through a redirection of FA toward triacylglycerol (TAG) and away from ceramide and diacylglycerol (DAG). Therefore, in the current study, an in vitro preparation was used to examine whether a prior bout of exercise could confer protection against palmitate-induced insulin resistance and whether the pharmacological [50 μM fumonisin B(1) (FB1)] inhibition of ceramide synthesis in the presence of palmitate could mimic the protective effect of exercise. Soleus muscle of sedentary (SED), exercised (EX), and SED in the presence of FB1 (SED+FB1) were incubated with or without 2 mM palmitate for 4 h. This 2-mM palmitate exposure impaired insulin-stimulated glucose transport (-28%, P < 0.01) and significantly increased ceramide, DAG, and TAG accumulation in the SED group (P < 0.05). A single prior bout of exercise prevented the detrimental effects of palmitate on insulin signaling and caused a partial redistribution of FA toward TAG (P < 0.05). However, the net increase in ceramide content in response to palmitate exposure in the EX group was not different compared with SED, despite the maintenance of insulin sensitivity. The incubation of soleus from SED rats with FB1 (SED+FB1) prevented the detrimental effects of palmitate and caused a redirection of FA toward TAG accumulation (P < 0.05). Therefore, this research suggests that although inhibiting ceramide accumulation can prevent the detrimental effects of palmitate, a single prior bout of exercise appears to protect against palmitate-induced insulin resistance, which may be independent of changes in ceramide content.

Aerobic training in rats increases skeletal muscle sphingomyelinase and serine palmitoyltransferase activity, while decreasing ceramidase activity

Sphingolipids are important components of cell membranes that may also serve as cell signaling molecules; ceramide plays a central role in sphingolipid metabolism. The aim of this study was to examine the effect of 5 weeks of aerobic training on key enzymes and intermediates of ceramide metabolism in skeletal muscles. The experiments were carried out on rats divided into two groups: (1) sedentary and (2) trained for 5 weeks (on a treadmill). The activity of serine palmitoyltransferase (SPT), neutral and acid sphingomyelinase (nSMase and aSMase), neutral and alkaline ceramidases (nCDase and alCDase) and the content of sphingolipids was determined in three types of skeletal muscle. We also measured the fasting plasma insulin and glucose concentration for calculating HOMA-IR (homeostasis model assessment) for estimating insulin resistance. We found that the activities of aSMase and SPT increase in muscle in the trained group. These changes were followed by elevation in the content of sphinganine. The activities of both isoforms of ceramidase were reduced in muscle in the trained group. Although the activities of SPT and SMases increased and the activity of CDases decreased, the ceramide content did not change in any of the studied muscle. Although ceramide level did not change, we noticed increased insulin sensitivity in trained animals. It is concluded that training affects the activity of key enzymes of ceramide metabolism but also activates other metabolic pathways which affect ceramide metabolism in skeletal muscles.

Inhibition of de novo ceramide synthesis reverses diet-induced insulin resistance and enhances whole-body oxygen consumption

OBJECTIVE

It has been proposed that skeletal muscle insulin resistance arises from the accumulation of intramyocellular lipid metabolites that impede insulin signaling, including diacylglycerol and ceramide. We determined the role of de novo ceramide synthesis in mediating muscle insulin resistance.

RESEARCH DESIGN AND METHODS

Mice were subjected to 12 weeks of diet-induced obesity (DIO), and then treated for 4 weeks with myriocin, an inhibitor of serine palmitoyl transferase-1 (SPT1), the rate-limiting enzyme of de novo ceramide synthesis.

RESULTS

After 12 weeks of DIO, C57BL/6 mice demonstrated a doubling in gastrocnemius ceramide content, which was completely reversed (141.5 ± 15.8 vs. 94.6 ± 10.2 nmol/g dry wt) via treatment with myriocin, whereas hepatic ceramide content was unaffected by DIO. Interestingly, myriocin treatment did not alter the DIO-associated increase in gastrocnemius diacyglycerol content, and the only correlation observed between lipid metabolite accumulation and glucose intolerance occurred with ceramide (R = 0.61). DIO mice treated with myriocin showed a complete reversal of glucose intolerance and insulin resistance which was associated with enhanced insulin-stimulated Akt and glycogen synthase kinase 3β phosphorylation. Furthermore, myriocin treatment also decreased intramyocellular ceramide content and prevented insulin resistance development in db/db mice. Finally, myriocin-treated DIO mice displayed enhanced oxygen consumption rates (3,041 ± 124 vs. 2,407 ± 124 ml/kg/h) versus their control counterparts.

CONCLUSIONS

Our results demonstrate that the intramyocellular accumulation of ceramide correlates strongly with the development of insulin resistance, and suggests that inhibition of SPT1 is a potentially promising target for the treatment of insulin resistance.

Effects of endurance exercise and high-fat diet on insulin resistance and ceramide contents of skeletal muscle in sprague-dawley rats

Background

We evaluated the effects of endurance exercise and a high-fat diet on insulin resistance and ceramide contents of skeletal muscle in Sprague-Dawley rats.

Methods

We randomly divided 32 rats into four groups: control (CON, n = 8), high fat diet (HF, n = 8), exercise (Ex, 24 m/min for 2 hours, 5 days/wk, n = 8), HF/Ex (n = 8). After 4-week treatments, plasma lipid profiles, glucose and insulin concentrations were measured. The triglycerides (TG), ceramide, and glucose transporter 4 (GLUT-4) contents were measured in the skeletal muscle. The rate of glucose transport was determined under submaximal insulin concentration during the muscle incubation.

Results

Free fatty acid levels were significantly higher in CON and HF than Ex (P = 0.032). Plasma glucose levels in HF were significantly higher than the two Ex groups (P = 0.002), and insulin levels were significantly higher in HF than in other three groups (P = 0.021). Muscular TG concentrations were significantly higher in HF than CON and Ex and also in HF/Ex than Ex, respectively (P = 0.005). Hepatic TG concentrations were significantly higher in HF than other three groups but Ex was significantly lower than HF/Ex (P = 0.000). Muscular ceramide content in HF was significantly greater than that in either Ex or HF/Ex (P = 0.031). GLUT-4 levels in CON and HF were significantly lower than those in Ex and HF/Ex (P = 0.009, P = 0.003). The glucose transport rate in submaximal insulin concentration was lower in CON than in either Ex or HF/Ex (P = 0.043), but not different from HF.

Conclusion

This study suggests that high fat diet for 4 weeks selectively impairs insulin resistance, but not glucose transport rate, GLUT-4 and ceramide content in skeletal muscle per se. However, endurance exercise markedly affects the content of ceramide and insulin resistance in muscle.

Testosterone affects hormone-sensitive lipase (HSL) activity and lipid metabolism in the left ventricle

Fatty acids, which are the major cardiac fuel, are derived from lipid droplets stored in cardiomyocytes, among other sources. The heart expresses hormone-sensitive lipase (HSL), which regulates triglycerides (TG) breakdown, and the enzyme is under hormonal control. Evidence obtained from adipose tissue suggests that testosterone regulates HSL activity. To test whether this is also true in the heart, we measured HSL activity in the left ventricle of sedentary male rats that had been treated with testosterone supplementation or orchidectomy with or without testosterone substitution. Left ventricle HSL activity against TG was significantly elevated in intact rats supplemented with testosterone. HSL activity against both TG and diacylglyceride was reduced by orchidectomy, whereas testosterone replacement fully reversed this effect. Moreover, testosterone increased left ventricle free fatty acid levels, caused an inhibitory effect on carbohydrate metabolism in the heart, and elevated left ventricular phosphocreatine and ATP levels as compared to control rats. These data indicate that testosterone is involved in cardiac HSL activity regulation which, in turn, may affect cardiac lipid and carbohydrate metabolism.

Role of sphingolipid mediator ceramide in obesity and renal injury in mice fed a high-fat diet

The present study tested a hypothesis that excess accumulation of sphingolipid, ceramide, its metabolites, or a combination contributes to the development of obesity and associated kidney damage. Liquid chromatography/mass spectrometry analysis demonstrated that C57BL/6J mice on the high-fat diet (HFD) had significantly increased plasma total ceramide levels compared with animals fed a low-fat diet (LFD). Treatment of mice with the acid sphingomyelinase (ASMase) inhibitor amitriptyline significantly attenuated the HFD-induced plasma ceramide levels. Corresponding to increase in plasma ceramide, the HFD significantly increased the body weight gain, plasma leptin concentration, urinary total protein and albumin excretion, glomerular damage index, and adipose tissue ASMase activity compared with the LFD-fed mice. These HFD-induced changes were also significantly attenuated by treatment of mice with amitriptyline. In addition, the decline of plasma glucose concentration after an intraperitoneal injection of insulin (0.15 U/kg b.wt.) was more sustained in mice on the HFD with amitriptyline than on the HFD alone. Intraperitoneal injection of glucose (3 g/kg b.wt.) resulted in a slow increase followed by a rapid decrease in the plasma glucose concentration in LFD and HFD plus amitriptyline-treated mice, but such blood glucose response was not observed in HFD-fed mice. Immunofluorescence analysis demonstrated a decrease in the podocin and an increase in the desmin in the glomeruli of HFD-fed mice compared with the LFD and HFD plus amitriptyline-treated mice. In conclusion, our results reveal a pivotal role for ceramide biosynthesis in obesity, metabolic syndrome, and associated kidney damage.

Sphingomyelinase stimulates oxidant signaling to weaken skeletal muscle and promote fatigue

Sphingomyelinase (SMase) hydrolyzes membrane sphingomyelin into ceramide, which increases oxidants in nonmuscle cells. Serum SMase activity is elevated in sepsis and heart failure, conditions where muscle oxidants are increased, maximal muscle force is diminished, and fatigue is accelerated. We tested the hypotheses that exogenous SMase and accumulation of ceramide in muscle increases oxidants in muscle cells, depresses specific force of unfatigued muscle, and accelerates the fatigue process. We also anticipated that the antioxidant N-acetylcysteine (NAC) would prevent SMase effects on muscle function. We studied the responses of C2C12 myotubes and mouse diaphragm to SMase treatment in vitro. We observed that SMase caused a 2.8-fold increase in total ceramide levels in myotubes. Exogenous ceramide and SMase elevated oxidant activity in C2C12 myotubes by 15-35% (P < 0.05) and in diaphragm muscle fiber bundles by 58-120% (P < 0.05). The SMase-induced increase in diaphragm oxidant activity was prevented by NAC. Exogenous ceramide depressed diaphragm force by 55% (P < 0.05), while SMase depressed maximal force by 30% (P < 0.05) and accelerated fatigue--effects opposed by treatment with NAC. In conclusion, our findings suggest that SMase stimulates a ceramide-oxidant signaling pathway that results in muscle weakness and fatigue.

Quantification of ceramide species in biological samples by liquid chromatography electrospray ionization tandem mass spectrometry

We present an optimized and validated liquid chromatography electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) method for the simultaneous measurement of concentrations of different ceramide species in biological samples. The method of analysis of tissue samples is based on Bligh and Dyer extraction, reverse-phase high-performance liquid chromatography separation, and multiple reaction monitoring of ceramides. Preparation of plasma samples also requires isolation of sphingolipids by silica gel column chromatography prior to LC-ESI-MS/MS analysis. The limits of quantification were in a range of 0.01-0.50ng/ml for distinct ceramides. The method was reliable for inter- and intraassay precision, accuracy, and linearity. Recoveries of ceramide subspecies from human plasma, rat liver, and muscle tissue were 78 to 91%, 70 to 99%, and 71 to 95%, respectively. The separation and quantification of several endogenous long-chain and very-long-chain ceramides using two nonphysiological odd chain ceramide (C17 and C25) internal standards was achieved within a single 21-min chromatographic run. The technique was applied to quantify distinct ceramide species in different rat tissues (muscle, liver, and heart) and in human plasma. Using this analytical technique, we demonstrated that a clinical exercise training intervention reduces the levels of ceramides in plasma of obese adults. This technique could be extended for quantification of other ceramides and sphingolipids with no significant modification.

Modulating serine palmitoyl transferase (SPT) expression and activity unveils a crucial role in lipid-induced insulin resistance in rat skeletal muscle cells

Saturated fatty acids, such as palmitate, promote accumulation of ceramide, which impairs activation and signalling of PKB (protein kinase B; also known as Akt) to important end points such as glucose transport. SPT (serine palmitoyl transferase) is a key enzyme regulating ceramide synthesis from palmitate and represents a potential molecular target in curbing lipid-induced insulin resistance. In the present study we explore the effects of palmitate upon insulin action in L6 muscle cells in which SPT expression/activity has been decreased by shRNA (small-hairpin RNA) or sustained incubation with myriocin, an SPT inhibitor. Incubation of L6 myotubes with palmitate (for 16 h) increases intramyocellular ceramide and reduces insulin-stimulated PKB activation and glucose uptake. PKB inhibition was not associated with impaired IRS (insulin receptor substrate) signalling and was ameliorated by short-term treatment with myriocin. Silencing SPT expression (approximately 90%) by shRNA or chronic cell incubation with myriocin (for 7 days) markedly suppressed SPT activity and palmitate-driven ceramide synthesis; however, challenging these muscle cells with palmitate still inhibited the hormonal activation of PKB. This inhibition was associated with reduced IRS1/p85-PI3K (phosphoinositide 3-kinase) coupling that arises from diverting palmitate towards greater DAG (diacylglycerol) synthesis, which elevates IRS1 serine phosphorylation via activation of DAG-sensitive PKCs (protein kinase Cs). Treatment of SPT-shRNA cells or those treated chronically with myriocin with PKC inhibitors antagonized palmitate-induced loss in insulin signalling. The findings of the present study indicate that SPT plays a crucial role in desensitizing muscle cells to insulin in response to incubation with palmitate. While short-term inhibition of SPT ameliorates palmitate/ceramide-induced insulin resistance, sustained loss/reduction in SPT expression/activity promotes greater partitioning of palmitate towards DAG synthesis, which impacts negatively upon IRS1-directed insulin signalling.

Effect of exercise duration on the key pathways of ceramide metabolism in rat skeletal muscles

Ceramide is the key compound on crossroads of sphingolipid metabolism. The content and composition of ceramides in skeletal muscles have been shown to be affected by prolonged exercise. The aim of this study was to examine the effect of exercise on the activity of key enzymes of ceramide metabolism in skeletal muscles. The experiments were carried out on male Wistar rats (200-250 g) divided into four groups: sedentary, exercised for 30 min, 90 min, and until exhaustion. The activity of serine palmitoyltransferase (SPT), neutral and acid sphingomyelinase (nSMase and aSMase), neutral and alkaline ceramidases (nCDase and alCDase) and the content of ceramide, sphingosine, sphinganine and sphingosine-1-phosphate were determined in three types of muscle. We have found that the activity and expression of SPT increase gradually in each muscle with duration of exercise. These changes were followed by elevation in the content of sphinganine. These data indicate that exercise increases de novo synthesis of ceramide. The aSMase activity gradually decreased with duration of exercise in each type of muscle. After exhaustive exercise the activity of both isoforms of ceramidase were reduced in each muscle. The ceramide level depends both on duration of exercise and muscle type. The ceramide level in the soleus and white gastrocnemius decreased after 30 min of running. After exhaustive exercise it was elevated in the soleus and red gastrocnemius. It is concluded that exercise strongly affects the activity of key enzymes involved in ceramide metabolism and in consequence the level of sphingolipid intermediates in skeletal muscles.

Human skeletal muscle ceramide content is not a major factor in muscle insulin sensitivity

AIMS/HYPOTHESIS

In skeletal muscle, ceramides may be involved in the pathogenesis of insulin resistance through an attenuation of insulin signalling. This study investigated total skeletal muscle ceramide fatty acid content in participants exhibiting a wide range of insulin sensitivities.

METHODS

The middle-aged male participants (n=33) were matched for lean body mass and divided into four groups: type 2 diabetes (T2D, n=8), impaired glucose tolerance (IGT, n=9), healthy controls (CON, n=8) and endurance-trained (TR, n=8). A two step (28 and 80 mU m(-2) min(-1)) sequential euglycaemic-hyperinsulinaemic clamp was performed for 120 and 90 min for step 1 and step 2, respectively. Muscle biopsies were obtained from vastus lateralis at baseline, and after steps 1 and 2.

RESULTS

Glucose infusion rates increased in response to insulin infusion, and significant differences were present between groups (T2D<IGT<CON<TR). At baseline, muscle ceramide content was 108+/-7, 95+/-6, 126+/-12 and 156+/-25 nmol total ceramide fatty acids/g wet weight of tissue in the T2D, IGT, CON and TR groups, respectively, and muscle ceramide content was higher (p<0.01) in the TR than the IGT group. Muscle ceramide content was not influenced by insulin infusion. Interestingly, a positive correlation (r=0.42, p<0.05) was present between muscle ceramide content at baseline and insulin sensitivity.

CONCLUSIONS/INTERPRETATION

Total muscle ceramide content was similar between individuals showing marked differences in insulin sensitivity, and therefore does not seem to be a major factor in muscle insulin resistance. Furthermore, aerobic capacity does not appear to influence muscle ceramide content.

Protection from high fat diet-induced increase in ceramide in mice lacking plasminogen activator inhibitor 1

Obesity increases the risk for metabolic and cardiovascular disease, and adipose tissue plays a central role in this process. Ceramide, the key intermediate of sphingolipid metabolism, also contributes to obesity-related disorders. We show that a high fat diet increased ceramide levels in the adipose tissues and plasma in C57BL/6J mice via a mechanism that involves an increase in gene expression of enzymes mediating ceramide generation through the de novo pathway (e.g. serine palmitoyltransferase) and via the hydrolysis of sphingomyelin (acid sphingomyelinase and neutral sphingomyelinase). Although the induction of total ceramide in response to the high fat diet was modest, dramatic increases were observed for C16, C18, and C18:1 ceramides. Next, we investigated the relationship of ceramide to plasminogen activator inhibitor-1 (PAI-1), the primary inhibitor of plasminogen activation and another key player in obesity. PAI-1 is consistently elevated in obesity and thought to contribute to increased artherothrombotic events and more recently to obesity-mediated insulin resistance. Interestingly, the changes in ceramide were attenuated in mice lacking PAI-1. Mechanistically, mice lacking PAI-1 were protected from diet-induced increase in serine palmitoyltransferase, acid sphingomyelinase, and neutral sphingomyelinase mRNA, providing a mechanistic link for decreased ceramide in PAI-1-/- mice. The decreases in plasma free fatty acids and adipose tumor necrosis factor-alpha in PAI-1-/- mice may have additionally contributed indirectly to improvements in ceramide profile in these mice. This study has identified a novel link between sphingolipid metabolism and PAI-1 and also suggests that ceramide may be an intermediary molecule linking elevated PAI-1 to insulin resistance.

The role of skeletal muscle sphingolipids in the development of insulin resistance

Insulin resistance is an important risk factor for type 2 diabetes, obesity, cardiovascular disease, polycystic ovary syndrome and other diseases. The most important stage in the development of insulin resistance is impairment of insulin-stimulated skeletal muscle glucose uptake. There is evidence that intramyocellular lipids might be responsible for this process through inhibition of insulin signaling. One of the important intracellular lipid pools is associated with the sphingomyelin signaling pathway. The second messenger in this pathway is ceramide. In vitro data indicate that ceramide inhibits insulin signaling, mainly through inactivation of protein kinase B. In vivo data suggest that ceramide accumulation within muscle cells might be associated with the development of insulin resistance. In this review, we discuss both in vitro and in vivo evidence for the role of muscle ceramide in the impairment of insulin action with particular focus on the question whether findings from animal studies are applicable to humans. We describe problems that are unresolved so far and topics of potential interest for future research.

Effect of exercise duration on ceramide metabolism in the rat heart

AIM

We aimed at gaining more insight into the mechanisms underlying exercise-induced alterations in myocardial ceramide (CER) content by employing physical activity of various durations and examining all key pathways of CER metabolism.

METHODS

The experiments were carried out on male Wistar rats divided into four groups (n = 6 in each case): control, exercised for 30 and 90 min and until exhaustion on the electrically driven treadmill moving with a speed of 1200 m h(-1) and set at +10 degrees incline. The animals were anaesthetized and samples of the heart's left ventricle were excised.

RESULTS

Thirty-minute exercise decreased the level of CER in the heart by 15%. However, after 90 min of running it returned to the baseline and at the point of exhaustion it exceeded that of the control animals by 26%. The initial reduction in the content of CER was probably a result of its augmented degradation, as a concomitant elevation in the activity of acid ceramidase and the level of sphingosine was observed. The transition from reduction in CER content after 30 min of exercise to its accumulation at the point of exhaustion was a consequence of gradual reduction in the activity of acid ceramidase and simultaneous increase in the rate of de novo CER synthesis, as evidenced by progressive activation of serine palmitoyltransferase and accumulation of sphinganine.

CONCLUSION

We conclude that the effect of physical effort on myocardial CER content and metabolism depends to a large extent on exercise duration.