Elevation of ceramide in serum lipoproteins during acute phase response in humans and mice: role of serine-palmitoyl transferase

From lipids to glucose: Investigating the role of dyslipidemia in the risk of insulin resistance

Dyslipidemia is recognized as one of the most prevalent metabolic disorders and is frequently associated with other prevalent conditions, particularly diabetes mellitus. There appears to be a bidirectional connection between these two metabolic disorders. While considerable research has focused on how insulin resistance can lead to lipid abnormalities, the reverse relationship specifically, how dyslipidemia could assist in developing insulin resistance and diabetes mellitus has received relatively less attention. This review aims to comprehensively evaluate the mechanisms through which dyslipidemia can induce insulin resistance. Dyslipidemia is primarily classified into three main categories: hypercholesterolemia, hypertriglyceridemia, and low levels of HDL. These conditions may promote insulin resistance across multiple pathways, including the accumulation of lipid metabolites, dysfunction of pancreatic β-cells, increased reactive oxygen species, endoplasmic reticulum stress and inflammation, endothelial dysfunction, alterations in adiponectin levels, changes in bile acid composition and concentration, and dysbiosis of gut microbiota. However, further investigation is required to fully elucidate the cellular and molecular mechanisms underlying the relationship between lipid disorders and insulin resistance. Emphasizing such research could facilitate the development of therapeutic strategies targeting both conditions simultaneously.

Sphingolipid-Induced Bone Regulation and Its Emerging Role in Dysfunction Due to Disease and Infection

The human skeleton is a metabolically active system that is constantly regenerating via the tightly regulated and highly coordinated processes of bone resorption and formation. Emerging evidence reveals fascinating new insights into the role of sphingolipids, including sphingomyelin, sphingosine, ceramide, and sphingosine-1-phosphate, in bone homeostasis. Sphingolipids are a major class of highly bioactive lipids able to activate distinct protein targets including, lipases, phosphatases, and kinases, thereby conferring distinct cellular functions beyond energy metabolism. Lipids are known to contribute to the progression of chronic inflammation, and notably, an increase in bone marrow adiposity parallel to elevated bone loss is observed in most pathological bone conditions, including aging, rheumatoid arthritis, osteoarthritis, and osteomyelitis. Of the numerous classes of lipids that form, sphingolipids are considered among the most deleterious. This review highlights the important primary role of sphingolipids in bone homeostasis and how dysregulation of these bioactive metabolites appears central to many chronic bone-related diseases. Further, their contribution to the invasion, virulence, and colonization of both viral and bacterial host cell infections is also discussed. Many unmet clinical needs remain, and data to date suggest the future use of sphingolipid-targeted therapy to regulate bone dysfunction due to a variety of diseases or infection are highly promising. However, deciphering the biochemical and molecular mechanisms of this diverse and extremely complex sphingolipidome, both in terms of bone health and disease, is considered the next frontier in the field.

PCPro: a clinically accessible, circulating lipid biomarker signature for poor-prognosis metastatic prostate cancer

BACKGROUND

Using comprehensive plasma lipidomic profiling from men with metastatic castration-resistant prostate cancer (mCRPC), we have previously identified a poor-prognostic lipid profile associated with shorter overall survival (OS). In order to translate this biomarker into the clinic, these men must be identifiable via a clinically accessible, regulatory-compliant assay.

METHODS

A single regulatory-compliant liquid chromatography-mass spectrometry assay of candidate lipids was developed and tested on a mCRPC Discovery cohort of 105 men. Various risk-score Cox regression prognostic models of OS were built using the Discovery cohort. The model with the highest concordance index (PCPro) was chosen for validation and tested on an independent Validation cohort of 183 men.

RESULTS

PCPro, the lipid biomarker, contains Cer(d18:1/18:0), Cer(d18:1/24:0), Cer(d18:1/24:1), triglycerides and total cholesterol. Within the Discovery and Validation cohorts, men who were PCPro positive had significantly shorter OS compared to those who were PCPro negative (Discovery: median OS 12.0 months vs 24.2 months, hazard ratio (HR) 3.75 [95% confidence interval (CI) 2.29-6.15], p < 0.001, Validation: median OS 13.0 months vs 25.7 months, HR = 2.13 [95% CI 1.46-3.12], p < 0.001).

CONCLUSIONS

We have developed PCPro, a lipid biomarker assay capable of prospectively identifying men with mCRPC with a poor prognosis. Prospective clinical trials are required to determine if men who are PCPro positive will benefit from therapeutic agents targeting lipid metabolism.

Circulating Sphingolipids and Glucose Homeostasis: An Update

Sphingolipids are a family of lipid molecules produced through different pathways in mammals. Sphingolipids are structural components of membranes, but in response to obesity, they are implicated in the regulation of various cellular processes, including inflammation, apoptosis, cell proliferation, autophagy, and insulin resistance which favors dysregulation of glucose metabolism. Of all sphingolipids, two species, ceramides and sphingosine-1-phosphate (S1P), are also found abundantly secreted into the bloodstream and associated with lipoproteins or extracellular vesicles. Plasma concentrations of these sphingolipids can be altered upon metabolic disorders and could serve as predictive biomarkers of these diseases. Recent important advances suggest that circulating sphingolipids not only serve as biomarkers but could also serve as mediators in the dysregulation of glucose homeostasis. In this review, advances of molecular mechanisms involved in the regulation of ceramides and S1P association to lipoproteins or extracellular vesicles and how they could alter glucose metabolism are discussed.

Ceramide concentrations in liver, plasma, and very low-density lipoproteins of humans with severe obesity

We investigated the relationships between ceramide species concentrations in liver, plasma and very low-density lipoproteins (VLDL) particles of humans with obesity as well as the relationships between hepatic fat content and hepatic ceramide concentrations and proportional distribution. Twenty-five obese (body mass index >35 kg/m² ) adults participated in this study. Plasma, VLDL and hepatocellular ceramide concentrations were measured by liquid chromatography/tandem mass spectrometry. The proportionate distribution of measured ceramide species differed between liver, whole plasma and the VLDL fraction. We found significant, positive correlations between the proportion of C14:0, C18:0, C20:0 and C24:1 ceramide in the liver and whole plasma (γ = 0.491, p = 0.013; γ = 0.573, p = 0.003; γ = 0.479, p = 0.015; γ = 0.716, p = 0.00006; respectively). In contrast, only the proportional contribution of C24:1 ceramide correlated positively between VLDL and liver (γ = 0.425, p = 0.013). The percent hepatic fat correlated positively with the proportion of C18:1, C18:0 and C20:0 hepatic ceramides (γ = 0.415, p = 0.039; γ = 0.426, p = 0.034; γ = 0.612, p = 0.001; respectively), but not with total hepatic ceramide concentration. The proportions of whole plasma ceramide subspecies, especially C14:0, C18:0, C20:0 and C24:1chain length, are reflective of those of hepatic ceramide subspecies in obese humans; these appear to be markers of hepatic ceramide species composition.

Ceramides as Mediators of Oxidative Stress and Inflammation in Cardiometabolic Disease

Ceramides, composed of a sphingosine and a fatty acid, are bioactive lipid molecules involved in many key cellular pathways (e.g., apoptosis, oxidative stress and inflammation). There is much evidence on the relationship between ceramide species and cardiometabolic disease, especially in relationship with the onset and development of diabetes and acute and chronic coronary artery disease. This review reports available evidence on ceramide structure and generation, and discusses their role in cardiometabolic disease, as well as current translational chances and difficulties for ceramide application in the cardiometabolic clinical settings.

Overcoming enzalutamide resistance in metastatic prostate cancer by targeting sphingosine kinase

Background

Intrinsic resistance to androgen receptor signalling inhibitors (ARSI) occurs in 20–30% of men with metastatic castration-resistant prostate cancer (mCRPC). Ceramide metabolism may have a role in ARSI resistance. Our study's aim is to investigate the association of the ceramide-sphingosine-1-phosphate (ceramide-S1P) signalling axis with ARSI resistance in mCRPC.

Methods

Lipidomic analysis (∼700 lipids) was performed on plasma collected from 132 men with mCRPC, before commencing enzalutamide or abiraterone. AR gene aberrations in 77 of these men were identified by deep sequencing of circulating tumour DNA. Associations between circulating lipids, radiological progression-free survival (rPFS) and overall survival (OS) were examined by Cox regression. Inhibition of ceramide-S1P signalling with sphingosine kinase (SPHK) inhibitors (PF-543 and ABC294640) on enzalutamide efficacy was investigated with in vitro assays, and transcriptomic and lipidomic analyses of prostate cancer (PC) cell lines (LNCaP, C42B, 22Rv1).

Findings

Men with elevated circulating ceramide levels had shorter rPFS (HR=2·3, 95% CI=1·5–3·6, p = 0·0004) and shorter OS (HR=2·3, 95% CI=1·4–36, p = 0·0005). The combined presence of an AR aberration with elevated ceramide levels conferred a worse prognosis than the presence of only one or none of these characteristics (median rPFS time = 3·9 vs 8·3 vs 17·7 months; median OS time = 8·9 vs 19·8 vs 34·4 months). SPHK inhibitors enhanced enzalutamide efficacy in PC cell lines. Transcriptomic and lipidomic analyses indicated that enzalutamide combined with SPHK inhibition enhanced PC cell death by SREBP-induced lipotoxicity.

Interpretation

Ceramide-S1P signalling promotes ARSI resistance, which can be reversed with SPHK inhibitors.

Aberrations in circulating ceramide levels are associated with poor clinical outcomes across localised and metastatic prostate cancer

BACKGROUND

Dysregulated lipid metabolism is associated with more aggressive pathology and poorer prognosis in prostate cancer (PC). The primary aim of the study is to assess the relationship between the plasma lipidome and clinical outcomes in localised and metastatic PC. The secondary aim is to validate a prognostic circulating 3-lipid signature specific to metastatic castration-resistant PC (mCRPC).

PATIENTS AND METHODS

Comprehensive lipidomic analysis was performed on pre-treatment plasma samples from men with localised PC (N = 389), metastatic hormone-sensitive PC (mHSPC)(N = 44), or mCRPC (validation cohort, N = 137). Clinical outcomes from our previously published mCRPC cohort (N = 159) that was used to derive the prognostic circulating 3-lipid signature, were updated. Associations between circulating lipids and clinical outcomes were examined by Cox regression and latent class analysis.

RESULTS

Circulating lipid profiles featuring elevated levels of ceramide species were associated with metastatic relapse in localised PC (HR 5.80, 95% CI 3.04-11.1, P = 1 × 10^-6), earlier testosterone suppression failure in mHSPC (HR 3.70, 95% CI 1.37-10.0, P = 0.01), and shorter overall survival in mCRPC (HR 2.54, 95% CI 1.73-3.72, P = 1 × 10^-6). The prognostic significance of circulating lipid profiles in localised PC was independent of standard clinicopathological and metabolic factors (P < 0.0002). The 3-lipid signature was verified in the mCRPC validation cohort (HR 2.39, 95% CI 1.63-3.51, P = 1 × 10^-5).

CONCLUSIONS

Elevated circulating ceramide species are associated with poorer clinical outcomes across the natural history of PC. These clinically actionable lipid profiles could be therapeutically targeted in prospective clinical trials to potentially improve PC outcomes.

Keep Your Friends Close, but Your Enemies Closer: Role of Acid Sphingomyelinase During Infection and Host Response

Breakdown of the inert and constitutive membrane building block sphingomyelin to the highly active lipid mediator ceramide by extracellularly active acid sphingomyelinase is tightly regulated during stress response and opens the gate for invading pathogens, triggering the immune response, development of remote organ failure, and tissue repair following severe infection. How do one enzyme and one mediator manage all of these affairs? Under physiological conditions, the enzyme is located in the lysosomes and takes part in the noiseless metabolism of sphingolipids, but following stress the protein is secreted into circulation. When secreted, acid sphingomyelinase (ASM) is able to hydrolyze sphingomyelin present at the outer leaflet of membranes to ceramide. Its generation troubles the biophysical context of cellular membranes resulting in functional assembly and reorganization of proteins and receptors, also embedded in highly conserved response mechanisms. As a consequence of cellular signaling, not only induction of cell death but also proliferation, differentiation, and fibrogenesis are affected. Here, we discuss the current state of the art on both the impact and function of the enzyme during host response and damage control. Also, the potential role of lysosomotropic agents as functional inhibitors of this upstream alarming cascade is highlighted.

Circulating Ceramides- Are Origins Important for Sphingolipid Biomarkers and Treatments?

Biomarkers are important tools for describing the adequacy or inadequacy of biological processes (to allow for the early and accurate diagnosis) and monitoring the biological effects of intervention strategies (to identify and develop optimal dose and treatment strategies). A number of lipid biomarkers are implicated in metabolic disease and the circulating levels of these biomarkers are used in clinical settings to predict and monitor disease severity. There is convincing evidence that specific circulating ceramide species can be used as biological predictors and markers of cardiovascular disease, atherosclerosis and type 2 diabetes mellitus. Here, we review the existing literature that investigated sphingolipids as biomarkers for metabolic disease prediction. What are the advantages and disadvantages? Are circulating ceramides predominantly produced in the liver? Will hepatic sphingolipid inhibitors be able to completely prevent and treat metabolic disease? As sphingolipids are being employed as biomarkers and potential metabolic disease treatments, we explore what is currently known and what still needs to be discovered.

The Role of Ceramides in Diabetes and Cardiovascular Disease Regulation of Ceramides by Adipokines

Metabolic dysfunction is intertwined with the pathophysiology of both diabetes and cardiovascular disease. Recently, one particular lipid class has been shown to influence the development and sustainment of these diseases: ceramides. As a subtype of sphingolipids, these species are particularly central to many sphingolipid pathways. Increased levels of ceramides are known to correlate with impaired cardiovascular and metabolic health. Furthermore, the interaction between ceramides and adipokines, most notably adiponectin and leptin, appears to play a role in the pathophysiology of these conditions. Adiponectin appears to counteract the detrimental effects of elevated ceramides, largely through activation of the ceramidase activity of its receptors. Elevated ceramides appear to worsen leptin resistance, which is an important phenomenon in the pathophysiology of obesity and metabolic syndrome.

Screening for Susceptibility-Related Factors and Biomarkers of Xianling Gubao Capsule-Induced Liver Injury

Although increasing reports from the literature on herbal-related hepatotoxicity, the identification of susceptibility-related factors and biomarkers remains challenging due to idiosyncratic drug-induced liver injury (IDILI). As a well-known Chinese medicine prescription, Xianling Gubao Capsule (XLGB) has attracted great attention due to reports of potential liver toxicity. But the mechanism behind it is difficult to determine. In this paper, we found that XLGB-induced liver injury belongs to IDILI through the analysis of clinical liver injury cases. In toxicological experiment assessment, co-exposure to XLGB and non-toxic dose of lipopolysaccharide (LPS) could cause evident liver injury as manifested by significantly increased plasma alanine aminotransferase activity and obvious liver histological damage. However, it failed to induce observable liver injury in normal rats, suggesting that mild immune stress may be a susceptibility factor for XLGB-induced idiosyncratic liver injury. Furthermore, plasma cytokines were determined and 15 cytokines (such as IL-1β, IFN-γ, and MIP-2α etc) were acquired by receiver operating characteristic (ROC) curves analysis. The expression of these 15 cytokines in LPS group was significantly up-regulated in contrast to the normal group. Meanwhile, the metabolomics profile showed that mild immune stress caused metabolic reprogramming, including sphingolipid metabolism, phenylalanine metabolism, and glycerophospholipid metabolism. 8 potential biomarkers (such as sphinganine, glycerophosphoethanolamine, and phenylalanine etc.) were identified by correlation analysis. Therefore, these results suggested that intracellular metabolism and immune changes induced by mild immune stress may be important susceptibility mechanisms for XLGB IDILI.

Relationships between Very Low-Density Lipoproteins-Ceramides, -Diacylglycerols, and -Triacylglycerols in Insulin-Resistant Men

This short report describes the relationships between concentrations of ceramides (CER), diacylglycerols (DAG), triacylglycerols (TAG) in very low-density lipoproteins (VLDL) particles, and hepatic lipid accumulation. VLDL particles were isolated from male subjects (n = 12, mean ± SD, age 42.1 ± 5.4 years, BMI 37.4 ± 4.1 kg/m² , ALT 45 ± 21 U/L) and apolipoprotein B100 (apoB100), VLDL-TAG, -CER, and -DAG quantified. The contents of all three lipids were highly correlated with VLDL particle number (r ≥ 0.768, p ≤ 0.003). The molar quantity of VLDL-TAG was 3× that of DAG and 137× that of CER (14,053 ± 5714, 5004 ± 2714, and 105 ± 49 mol/mol apoB100, respectively). Reduced VLDL-CER concentrations were associated with both higher insulin levels (r = -0.645, p = 0.024) and intrahepatic-TAG (r = -0.670, p = 0.017). In fatty liver, the secretion of hepatic TAG, CER, and DAG may be suppressed and contribute to intrahepatic lipotoxicity. The mechanisms by which hepatic-CER and -DAG synthesis and assembly into VLDL is coordinately controlled with TAG will be important in understanding the emerging role of elevated CER contributing to cardiometabolic disease.

Sphingolipids in Alzheimer's disease, how can we target them?

Altered levels of sphingolipids and their metabolites in the brain, and the related downstream effects on neuronal homeostasis and the immune system, provide a framework for understanding mechanisms in neurodegenerative disorders and for developing new intervention strategies. In this review we will discuss: the metabolites of sphingolipids that function as second messengers; and functional aberrations of the pathway resulting in Alzheimer's disease (AD) pathophysiology. Focusing on the central product of the sphingolipid pathway ceramide, we describ approaches to pharmacologically decrease ceramide levels in the brain and we argue on how the sphingolipid pathway may represent a new framework for developing novel intervention strategies in AD. We also highlight the possible use of clinical and non-clinical drugs to modulate the sphingolipid pathway and sphingolipid-related biological cascades.

Sphingolipid Metabolism and Signaling in Skeletal Muscle: From Physiology to Physiopathology

Sphingolipids represent one of the major classes of eukaryotic lipids. They play an essential structural role, especially in cell membranes where they also possess signaling properties and are capable of modulating multiple cell functions, such as apoptosis, cell proliferation, differentiation, and inflammation. Many sphingolipid derivatives, such as ceramide, sphingosine-1-phosphate, and ganglioside, have been shown to play many crucial roles in muscle under physiological and pathological conditions. This review will summarize our knowledge of sphingolipids and their effects on muscle fate, highlighting the role of this class of lipids in modulating muscle cell differentiation, regeneration, aging, response to insulin, and contraction. We show that modulating sphingolipid metabolism may be a novel and interesting way for preventing and/or treating several muscle-related diseases.

Hepatic Fatty Acid Balance and Hepatic Fat Content in Humans With Severe Obesity

OBJECTIVE

Nonalcoholic fatty liver disease can lead to hepatic inflammation/damage. Understanding the physiological mechanisms that contribute to excess hepatic lipid accumulation may help identify effective treatments.

DESIGN

We recruited 25 nondiabetic patients with severe obesity scheduled for bariatric surgery. To evaluate liver export of triglyceride fatty acids, we measured very-low-density lipoprotein (VLDL)-triglyceride secretion rates the day prior to surgery using an infusion of autologous [1-14C]triolein-labeled VLDL particles. Ketone body response to fasting and intrahepatic long-chain acylcarnitine concentrations were used as indices of hepatic fatty acid oxidation. We measured intraoperative hepatic uptake rates of plasma free fatty acids using a continuous infusion of [U-13C]palmitate, combined with a bolus dose of [9,10-3H]palmitate and carefully timed liver biopsies. Total intrahepatic lipids were measured in liver biopsy samples to determine fatty liver status. The hepatic concentrations and enrichment from [U-13C]palmitate in diacylglycerols, sphingolipids, and acyl-carnitines were measured using liquid chromatography/tandem mass spectrometry.

RESULTS

Among study participants with fatty liver disease, intrahepatic lipid was negatively correlated with VLDL-triglyceride secretion rates (r = -0.92, P = 0.01) but unrelated to hepatic free fatty acid uptake or indices of hepatic fatty acid oxidation. VLDL-triglyceride secretion rates were positively correlated with hepatic concentrations of saturated diacylglycerol (r = 0.46, P = 0.02) and sphingosine-1-phosphate (r = 0.44, P = 0.03).

CONCLUSION

We conclude that in nondiabetic humans with severe obesity, excess intrahepatic lipid is associated with limited export of triglyceride in VLDL particles rather than increased uptake of systemic free fatty acids.

Elevated ceramides 18:0 and 24:1 with aging are associated with hip fracture risk through increased bone resorption

We assessed whether circulating ceramides, which play a role in a number of degenerative changes with aging, significantly differed according to fragility hip fracture (HF) status. We also performed a human study using bone marrow (BM) aspirates, directly reflecting the bone microenvironment, in addition to in vitro experiments. Peripheral blood and BM samples were simultaneously collected from 74 patients 65 years or older at hip surgery for either HF (n = 28) or for other causes (n = 46). Ceramides were measured by liquid chromatography-tandem mass spectrometry. Age was correlated positively with circulating C16:0, C18:0, and C24:1 ceramide levels. Patients with fragility HF had 21.3%, 49.5%, 34.3%, and 22.5% higher plasma C16:0, C18:0, C18:1, and C24:1 ceramide levels, respectively, than those without HF. Higher C16:0, C18:0, C18:1, and C24:1 ceramide levels were positively related to bone resorption markers in both blood and BM samples. Furthermore, in vitro studies showed that C18:0 and C24:1 ceramides directly increased osteoclastogenesis, bone resorption, and expression levels of osteoclast differentiation markers. These results suggested that the association of increased ceramides, especially C18:0 and C24:1, with adverse bone phenotypes in elderly people could be explained mainly by the increase in osteoclastogenesis and bone resorption.

The Liver as an Endocrine Organ-Linking NAFLD and Insulin Resistance

The liver is a dynamic organ that plays critical roles in many physiological processes, including the regulation of systemic glucose and lipid metabolism. Dysfunctional hepatic lipid metabolism is a cause of nonalcoholic fatty liver disease (NAFLD), the most common chronic liver disorder worldwide, and is closely associated with insulin resistance and type 2 diabetes. Through the use of advanced mass spectrometry "omics" approaches and detailed experimentation in cells, mice, and humans, we now understand that the liver secretes a wide array of proteins, metabolites, and noncoding RNAs (miRNAs) and that many of these secreted factors exert powerful effects on metabolic processes both in the liver and in peripheral tissues. In this review, we summarize the rapidly evolving field of "hepatokine" biology with a particular focus on delineating previously unappreciated communication between the liver and other tissues in the body. We describe the NAFLD-induced changes in secretion of liver proteins, lipids, other metabolites, and miRNAs, and how these molecules alter metabolism in liver, muscle, adipose tissue, and pancreas to induce insulin resistance. We also synthesize the limited information that indicates that extracellular vesicles, and in particular exosomes, may be an important mechanism for intertissue communication in normal physiology and in promoting metabolic dysregulation in NAFLD.

Invited review: Sphingolipid biology in the dairy cow: The emerging role of ceramide

The physiological control of lactation through coordinated adaptations is of fundamental importance for mammalian neonatal life. The putative actions of reduced insulin sensitivity and responsiveness and enhanced adipose tissue lipolysis spare glucose for the mammary synthesis of milk. However, severe insulin antagonism and body fat mobilization may jeopardize hepatic health and lactation in dairy cattle. Interestingly, lipolysis- and dietary-derived fatty acids may impair insulin sensitivity in cows. The mechanisms are undefined yet have major implications for the development of postpartum fatty liver disease. In nonruminants, the sphingolipid ceramide is a potent mediator of saturated fat-induced insulin resistance that defines in part the mechanisms of type 2 diabetes mellitus and nonalcoholic fatty liver disease. In ruminants including the lactating dairy cow, the functions of ceramide had remained virtually undescribed. Through a series of hypothesis-centered studies, ceramide has emerged as a potential antagonist of insulin-stimulated glucose utilization by adipose and skeletal muscle tissues in dairy cattle. Importantly, bovine data suggest that the ability of ceramide to inhibit insulin action likely depends on the lipolysis-dependent hepatic synthesis and secretion of ceramide during early lactation. Although these mechanisms appear to fade as lactation advances beyond peak milk production, early evidence suggests that palmitic acid feeding is a means to augment ceramide supply. Herein, we review a body of work that focuses on sphingolipid biology and the role of ceramide in the dairy cow within the framework of hepatic and fatty acid metabolism, insulin function, and lactation. The potential involvement of ceramide within the endocrine control of lactation is also considered.

Circulating low-density lipoprotein ceramide concentrations increase in Holstein dairy cows transitioning from gestation to lactation

Low-density lipoprotein (LDL) ceramide causes insulin resistance in obese diabetic nonruminants. Because previous work suggests that liver-derived ceramide may impair insulin action in postpartum cows, our objectives were to characterize peripartal changes in lipoprotein ceramides. We further studied the effects of prepartum adiposity on lipoprotein ceramide levels. Twenty-eight pregnant Holstein cows (parity = 3.65 ± 1.62) with lean (body condition score, BCS = 2.97 ± 0.16; body weight, BW = 630 ± 55.2 kg; n = 15) or overweight (BCS = 3.93 ± 0.27; BW = 766 ± 46.1 kg; n = 13) body condition 28 d before expected parturition were evaluated. Sampling occurred on d -20.5 ± 1.74, -13.8 ± 1.71, -7.84 ± 4.07, -6.71 ± 1.00, -3.92 ± 0.64, and -1.28 ± 0.61 (before parturition); daily until d 8 postpartum; and on d 10, 12, 14, 21, and 28. Adipose tissue and liver were biopsied on d -7.84 ± 4.07 and 10. Postpartum insulin sensitivity was assessed using the hyperinsulinemic-euglycemic clamp. Lipoprotein fractions were isolated using liquid chromatography. Sphingolipids were quantified using mass spectrometry. Data were analyzed using a mixed model with repeated measures. Overweight cows had a higher BCS and BW at enrollment relative to lean cows, but BCS and BW were similar postpartum. Overweight cows lost more body condition (0.97 ± 0.36 vs. 0.55 ± 0.16 BCS units) and BW (291 ± 67.3 vs. 202 ± 54.5 kg) during transition relative to lean cows. Adipocyte volume and counts declined from prepartum to postpartum (50.4 and 13.7%, respectively), and adipocyte volume was greater (48.2%) in overweight cows prepartum relative to lean cows. Although DMI was comparable between BCS groups, milk yield tended to be greater in overweight cows. Plasma free fatty acid and β-hydroxybutyrate and liver lipid levels were 40, 16, and 37% greater, respectively, in overweight cows compared with lean cows. Glucose infusion rate during the hyperinsulinemic-euglycemic clamp tended to be lower in overweight cows. Ceramide levels within triacylglycerol-rich lipoprotein fractions declined postpartum, whereas LDL ceramide increased postpartum. Overweight cows had lower triacylglycerol-rich lipoprotein C16:0-ceramide levels relative to lean cows. Prepartum LDL C24:0-ceramide levels were greater in overweight cows relative to lean cows. Independent of prepartum adiposity, we concluded that serum LDL ceramide levels are elevated in early-lactation cows experiencing adipose tissue free fatty acid mobilization and hepatic steatosis.

Very Long-Chain C24:1 Ceramide Is Increased in Serum Extracellular Vesicles with Aging and Can Induce Senescence in Bone-Derived Mesenchymal Stem Cells

Extracellular vesicles (EVs), including exosomes and microvesicles, function in cell-to-cell communication through delivery of proteins, lipids and microRNAs to target cells via endocytosis and membrane fusion. These vesicles are enriched in ceramide, a sphingolipid associated with the promotion of cell senescence and apoptosis. We investigated the ceramide profile of serum exosomes from young (24⁻40 yrs.) and older (75⁻90 yrs.) women and young (6⁻10 yrs.) and older (25⁻30 yrs.) rhesus macaques to define the role of circulating ceramides in the aging process. EVs were isolated using size-exclusion chromatography. Proteomic analysis was used to validate known exosome markers from Exocarta and nanoparticle tracking analysis used to characterize particle size and concentration. Specific ceramide species were identified with lipidomic analysis. Results show a significant increase in the average amount of C24:1 ceramide in EVs from older women (15.4 pmol/sample) compared to those from younger women (3.8 pmol/sample). Results were similar in non-human primate serum samples with increased amounts of C24:1 ceramide (9.3 pmol/sample) in older monkeys compared to the younger monkeys (1.8 pmol/sample). In vitro studies showed that primary bone-derived mesenchymal stem cells (BMSCs) readily endocytose serum EVs, and serum EVs loaded with C24:1 ceramide can induce BMSC senescence. Elevated ceramide levels have been associated with poor cardiovascular health and memory impairment in older adults. Our data suggest that circulating EVs carrying C24:1 ceramide may contribute directly to cell non-autonomous aging.

Quantitative Systems Pharmacology Modeling of Acid Sphingomyelinase Deficiency and the Enzyme Replacement Therapy Olipudase Alfa Is an Innovative Tool for Linking Pathophysiology and Pharmacology

Acid sphingomyelinase deficiency (ASMD) is a rare lysosomal storage disorder with heterogeneous clinical manifestations, including hepatosplenomegaly and infiltrative pulmonary disease, and is associated with significant morbidity and mortality. Olipudase alfa (recombinant human acid sphingomyelinase) is an enzyme replacement therapy under development for the non-neurological manifestations of ASMD. We present a quantitative systems pharmacology (QSP) model supporting the clinical development of olipudase alfa. The model is multiscale and mechanistic, linking the enzymatic deficiency driving the disease to molecular-level, cellular-level, and organ-level effects. Model development was informed by natural history, and preclinical and clinical studies. By considering patient-specific pharmacokinetic (PK) profiles and indicators of disease severity, the model describes pharmacodynamic (PD) and clinical end points for individual patients. The ASMD QSP model provides a platform for quantitatively assessing systemic pharmacological effects in adult and pediatric patients, and explaining variability within and across these patient populations, thereby supporting the extrapolation of treatment response from adults to pediatrics.

Efficacy of nutritional interventions to lower circulating ceramides in young adults: FRUVEDomic pilot study

The 2010 USDA Dietary Guidelines for Americans (DGA) recommends a diet largely composed of fruit and vegetables. Consuming a diet high in fruit and vegetables and low in refined carbohydrates and saturated fat may reduce an individual's risk for type 2 diabetes, nonalcoholic fatty liver disease, low‐grade chronic inflammation, and metabolic syndrome (MetS). Several recent studies have implicated the bioactive sphingolipid ceramide as an associative and causative biomarker for the development of these conditions. Considering that the intake of fruit and vegetables is frequently inadequate in young adults, we performed a pilot investigation to assess the efficacy of a free‐living fruit and vegetable intervention on overall metabolic health, circulating ceramide supply, and inflammatory status in young adults. We discovered that adoption of the recommended DGA for fruit and vegetable intake for 8 weeks decreased waist circumference, systolic blood pressure, and circulating cholesterol. Lipidomics analysis revealed that nutritional intervention can lower circulating ceramides, including C24:0 ceramide, a known inhibitor of insulin signaling. Unexpectedly, we observed an increase in C16:0 ceramide, suggesting that this form of ceramide in circulation is not associated with metabolic disease in humans. We also observed an improved inflammatory status with enhanced fruit and vegetable intake that was correlated with ceramide concentrations. These data suggest that adopting the recommended DGA is associated with a reduction of many, but not all, ceramide species and may help to prevent or mitigate MetS. Future research needs to assess whether the ceramide‐lowering ability of nutritional intervention is associated with reduced risk of developing metabolic disease.

Plasma ceramide levels are altered in low and normal birth weight men in response to short-term high-fat overfeeding

Low birth weight (LBW) individuals have an increased risk of developing insulin resistance and type 2 diabetes compared with normal birth weight (NBW) individuals. We hypothesised that LBW individuals exhibit an increased fatty acid flux into lipogenesis in non-adipose tissue with a resulting accumulation of lipotoxic lipids, including ceramides, in the blood. Therefore, we measured fasting plasma levels of 27 ceramides in 18 young, healthy, LBW men and 25 NBW controls after an isocaloric control diet and a 5-day high-fat, high-calorie diet by HPLC-HRMS. LBW men did not show elevated plasma ceramide levels after the control or high-fat, high-calorie diet. An increased fatty acid oxidation rate in these individuals during both diets may limit ceramide synthesis and thereby compensate for a likely increased fatty acid load to non-adipose tissue. Interestingly, LBW and NBW men decreased d18:0-18:1/d18:1-18:0 and d18:1-24:2/d18:2-24:1 levels and increased the d18:0-24:1a level in response to overfeeding. Plasma d18:0-24:1a and total ceramide levels were positively associated with the fasting blood glucose level and endogenous glucose production after the control diet, and the total ceramide level was in addition positively associated with hepatic insulin resistance. Further studies are needed to determine if lipotoxicity contributes to insulin resistance in LBW individuals.

Alterations of specific lipid groups in serum of obese humans: a review

Obesity is a major contributor to the dysfunction of liver, cardiac, pulmonary, endocrine and reproductive system, as well as a component of metabolic syndrome. Although development of obesity-related disorders is associated with lipid abnormalities, most previous studies dealing with the problem in question were limited to routinely determined parameters, such as serum concentrations of triacylglycerols, total cholesterol, low-density and high-density lipoprotein cholesterol. Many authors postulated to extend the scope of analysed lipid compounds and to study obesity-related alterations in other, previously non-examined groups of lipids. Comprehensive quantitative, structural and functional analysis of specific lipid groups may result in identification of new obesity-related alterations. The review summarizes available evidence of obesity-related alterations in various groups of lipids and their impact on health status of obese subjects. Further, the role of diet and endogenous lipid synthesis in the development of serum lipid alterations is discussed, along with potential application of various lipid compounds as risk markers for obesity-related comorbidities.

Serum sphingolipidomic analyses reveal an upregulation of C16-ceramide and sphingosine-1-phosphate in hepatocellular carcinoma

We have recently shown that major alterations of serum sphingolipid metabolites in chronic liver disease associate significantly with the stage of liver fibrosis in corresponding patients. In the current study we assessed via mass spectrometry serum concentrations of sphingolipid metabolites in a series of 122 patients with hepatocellular carcinoma (HCC) compared to an age- and sex-matched series of 127 patients with cirrhosis. We observed a highly significant upregulation of long and very long chain ceramides (C16-C24) in the serum of patients with HCC as compared to patients with cirrhosis (P < 0.001). Accordingly, dihydro-ceramides, synthetic precursors of ceramides and notably sphingosine, sphingosine-1-phosphate (S1P) and sphinganine-1-phosphate (SA1P) were upregulated in patients with HCC (P < 0.001). Especially the diagnostic accuracy of C16-ceramide and S1P, assessed by receiver operating curve (ROC) analysis, showed a higher area under the curve (AUC) value as compared to alpha fetoprotein (AFP) (0.999 and 0.985 versus 0.823, P < 0.001 respectively). In conclusion, serum levels of sphingolipid metabolites show a significant upregulation in patients with HCC as compared to patients with cirrhosis. Particularly C16-ceramide and S1P may serve as novel diagnostic markers for the identification of HCC in patients with liver diseases. Our data justify further investigations on the role of sphingolipids in HCC.

Association between Plasmatic Ceramides Profile and AST/ALT Ratio: C14:0 Ceramide as Predictor of Hepatic Steatosis in Adolescents Independently of Obesity

OBJECTIVE

To assess the association between plasma ceramides and hepatic steatosis (HS) in adolescents, independently of obesity.

MATERIALS AND METHODS

Ninety-four adolescents from two previous studies conducted and published by our crew were included. Study subjects were stratified in three groups: normal weight (n = 18), obesity (n = 34), and obesity + HS (n = 42). The presence of HS was defined when ALT/AST ratio was <1. Ceramides subspecies (C14:0, C16:0, C18:0, C24:0, and C24:1) were determined by LC/MS.

RESULTS

All ceramides correlated directly with ALT levels and inversely with ALT/AST ratio; the strongest correlation was observed among C14:0 ceramide (r = 0.41 and r = -0.54, resp.; P < 0.001). Furthermore, significant correlations were observed between cholesterol and all ceramides except for C24:1 ceramide. Interestingly ceramides C14:0, C18:0, and C24:1 correlated directly with both fasting insulin and HOMA-IR index. For assessing HS, a cut-off point of 10.3 nmol/L for C14:0 ceramide reported a sensitivity of 92.7% and a specificity of 73.5% when normal weight and obesity groups (n = 52) were compared against obesity + HS group (n = 42). Positive and negative predictive values were 77.5% and 90.2%, respectively.

CONCLUSIONS

Plasma ceramides are closely associated with hepatic steatosis in adolescents. C14:0 ceramide could be a novel biomarker of HS independently of obesity.

Ceramides and mitochondrial fatty acid oxidation in obesity

Obesity is an epidemic, complex disease that is characterized by increased glucose, lipids, and low-grade inflammation in the circulation, among other factors. It creates the perfect scenario for the production of ceramide, the building block of the sphingolipid family of lipids, which is involved in metabolic disorders such as obesity, diabetes, and cardiovascular disease. In addition, obesity causes a decrease in fatty acid oxidation (FAO), which contributes to lipid accumulation within the cells, conferring more susceptibility to cell dysfunction. C16:0 ceramide, a specific ceramide species, has been identified recently as the principal mediator of obesity-derived insulin resistance, impaired fatty acid oxidation, and hepatic steatosis. In this review, we have sought to cover the importance of the ceramide species and their metabolism, the main ceramide signaling pathways in obesity, and the link between C16:0 ceramide, FAO, and obesity.-Fucho, R., Casals, N., Serra, D., Herrero, L. Ceramides and mitochondrial fatty acid oxidation in obesity.

Palmitic acid feeding increases ceramide supply in association with increased milk yield, circulating nonesterified fatty acids, and adipose tissue responsiveness to a glucose challenge

Reduced insulin action is a key adaptation that facilitates glucose partitioning to the mammary gland for milk synthesis and enhances adipose tissue lipolysis during early lactation. The progressive recovery of insulin sensitivity as cows advance toward late lactation is accompanied by reductions in circulating nonesterified fatty acids (NEFA) and milk yield. Because palmitic acid can promote insulin resistance in monogastrics through sphingolipid ceramide-dependent mechanisms, palmitic acid (C16:0) feeding may enhance milk production by restoring homeorhetic responses. We hypothesized that feeding C16:0 to mid-lactation cows would enhance ceramide supply and ceramide would be positively associated with milk yield. Twenty multiparous mid-lactation Holstein cows were enrolled in a study consisting of a 5-d covariate, 49-d treatment, and 14-d posttreatment period. All cows were randomly assigned to a sorghum silage-based diet containing no supplemental fat (control; n=10; 138±45 d in milk) or C16:0 at 4% of ration dry matter (PALM; 98% C16:0; n=10; 136±44 d in milk). Blood and milk were collected at routine intervals. Liver and skeletal muscle tissue were biopsied at d 47 of treatment. Intravenous glucose tolerance tests (300mg/kg of body weight) were performed at d -1, 24, and 49 relative to start of treatment. The plasma and tissue concentrations of ceramide and glycosylated ceramide were determined using liquid chromatography coupled with tandem mass spectrometry. Data were analyzed as repeated measures using a mixed model with fixed effects of treatment and time, and milk yield served as a covariate. The PALM treatment increased milk yield, energy-corrected milk, and milk fat yield. The most abundant plasma and tissue sphingolipids detected were C24:0-ceramide, C24:0-monohexosylceramide (GlcCer), and C16:0-lactosylceramide. Plasma concentrations of total ceramide and GlcCer decreased as lactation advanced, and ceramide and GlcCer were elevated in cows fed PALM. Palmitic acid feeding increased hepatic ceramide levels, a response not observed in skeletal muscle tissue. Plasma ceramides (e.g., C24:0-ceramide) were positively correlated with plasma NEFA and milk yield, and positively correlated with NEFA levels following a glucose challenge. Our data demonstrate a remodeled plasma and hepatic sphingolipidome in mid-lactation dairy cows fed PALM. The potential involvement in ceramide in homeorhetic nutrient partitioning to support lactation requires further consideration.

ORMDL proteins regulate ceramide levels during sterile inflammation

The bioactive sphingolipid metabolite, ceramide, regulates physiological processes important for inflammation and elevated levels of ceramide have been implicated in IL-1-mediated events. Although much has been learned about ceramide generation by activation of sphingomyelinases in response to IL-1, the contribution of the de novo pathway is not completely understood. Because yeast ORM1 and ORM2 proteins negatively regulate ceramide levels through inhibition of serine palmitoyltransferase, the first committed step in ceramide biosynthesis, we examined the functions of individual mammalian ORM orthologs, ORM (yeast)-like (ORMDL)1-3, in regulation of ceramide levels. In HepG2 liver cells, downregulation of ORMDL3 markedly increased the ceramide precursors, dihydrosphingosine and dihydroceramide, primarily from de novo biosynthesis based on [U-(13)C]palmitate incorporation into base-labeled and dual-labeled dihydroceramides, whereas downregulation of each isoform increased dihydroceramides [(13)C]labeled in only the amide-linked fatty acid. IL-1 and the IL-6 family cytokine, oncostatin M, increased dihydroceramide and ceramide levels in HepG2 cells and concomitantly decreased ORMDL proteins. Moreover, during irritant-induced sterile inflammation in mice leading to induction of the acute-phase response, which is dependent on IL-1, expression of ORMDL proteins in the liver was strongly downregulated and accompanied by increased ceramide levels in the liver and accumulation in the blood. Together, our results suggest that ORMDLs may be involved in regulation of ceramides during IL-1-mediated sterile inflammation.

Plasma ceramides are elevated in overweight Holstein dairy cows experiencing greater lipolysis and insulin resistance during the transition from late pregnancy to early lactation

Insulin resistance is a homeorhetic adaptation to parturition in dairy cows transitioning from late pregnancy to early lactation. An increase in prepartum adiposity can predispose periparturient cows to greater lipolysis and insulin resistance, thus increasing the risk for metabolic disease. Mechanisms mediating the development of insulin resistance in overweight peripartal dairy cows may depend on ceramide metabolism. The sphingolipid ceramide accumulates in plasma and tissues of overweight monogastric animals, and facilitates saturated fatty acid-induced insulin resistance. Considering this evidence, we hypothesized that plasma ceramides would be elevated in periparturient dairy cattle and that these sphingolipids would correlate with the magnitude of lipolysis and insulin resistance. To test our central hypothesis, multiparous Holstein cows were allocated into 2 groups according to their body condition score (BCS) at d -30 prepartum: lean (BCS <3.0; n=10) or overweight (BCS >4.0; n=11). Blood samples were collected at d -45, -30, -15, and -7, relative to expected parturition, and at d 4 postpartum. Plasma glucose, insulin, nonesterified fatty acids (NEFA), and β-hydroxybutyrate (BHBA) concentrations were measured, and insulin sensitivity was estimated. The concentrations of individual plasma ceramide and glycosylated ceramide were determined using liquid chromatography-based mass spectrometry. Results demonstrated that greater adiposity was associated with a greater loss in body condition during late pregnancy. Overweight cows had greater circulating concentrations of glucose, insulin, and NEFA, and lower insulin sensitivity relative to lean cows. We detected 30 different sphingolipids across 6 lipid classes with acyl chains ranging from 16 to 26 carbons. The most abundant plasma sphingolipids detected were C24:0-ceramide, C24:0-monohexosylceramide, and C16:0-lactosylceramide. Plasma concentrations of total ceramide and monohexosylceramide increased as lactation approached, and saturated ceramide and monohexosylceramide were elevated in cows with greater adiposity relative to those with a lean phenotype. Plasma ceramides (e.g., C24:0-ceramide) were positively correlated with plasma NEFA and inversely correlated with insulin sensitivity. Our data demonstrate a remodeled plasma sphingolipidome in dairy cows transitioning from late pregnancy to lactation characterized by a concomitant increase in plasma ceramides with the development of peripartal insulin resistance.

Serum Sphingolipid Variations Associate with Hepatic Decompensation and Survival in Patients with Cirrhosis

Background

Sphingolipids constitute bioactive molecules with functional implications in liver homeostasis. Particularly, ablation of very long chain ceramides in a knockout mouse model has been shown to cause a severe hepatopathy.

Methods

We aimed to evaluate the serum sphingolipid profile of 244 patients with cirrhosis prospectively followed for a median period of 228±217 days via mass spectrometry.

Results

We thereby observed a significant decrease of long and very long chain ceramides, particularly of C24ceramide, in patients with increasing severity of cirrhosis (p<0.001). Additionally, hydropic decompensation, defined by clinical presentation of ascites formation, was significantly correlated to low C24ceramide levels (p<0.001) while a significant association to hepatic decompensation and poor overall survival was observed for low serum concentrations of C24ceramide (p<0.001) as well. Multivariate analysis further identified low serum C24ceramide to be independently associated to overall survival (standard beta = -0.001, p = 0.022).

Conclusions

In our current analysis serum levels of very long chain ceramides show a significant reciprocal correlation to disease severity and hepatic decompensation and are independently associated with overall survival in patients with cirrhosis. Serum sphingolipid metabolites and particularly C24ceramide may constitute novel molecular targets of disease severity, hepatic decompensation and overall prognosis in cirrhosis and should be further evaluated in basic research studies.

Improved insulin sensitivity after exercise training is linked to reduced plasma C14:0 ceramide in obesity and type 2 diabetes

OBJECTIVE

To assess the effect of exercise training on insulin sensitivity and plasma ceramides in obesity and type 2 diabetes (T2D).

METHODS

Twenty-four adults with obesity and normal glucose tolerance (NGT, n = 14) or diabetes (n = 10) were studied before and after a 12-week supervised exercise-training program (5 days/week, 1 h/day, 80-85% of maximum heart rate). Changes in body composition were assessed using hydrostatic weighing and computed tomography. Peripheral tissue insulin sensitivity was assessed by a 40 mU/m(2) /min hyperinsulinemic euglycemic clamp. Plasma ceramides (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.

RESULTS

Plasma ceramides were similar for the subjects with obesity and NGT and the subjects with diabetes, despite differences in glucose tolerance. Exercise significantly reduced body weight and adiposity and increased peripheral insulin sensitivity in both groups (P < 0.05). In addition, plasma C14:0, C16:0, C18:1, and C24:0 ceramide levels were reduced in all subjects following the intervention (P < 0.05). Decreases in total (r = -0.51, P = 0.02) and C14:0 (r = -0.56, P = 0.009) ceramide were negatively correlated with the increase in insulin sensitivity.

CONCLUSIONS

Ceramides are linked to exercise training-induced improvements in insulin sensitivity, and plasma C14:0 ceramide may provide a specific target for investigating lipid-related insulin resistance in obesity and T2D.

Variations in serum sphingolipid levels associate with liver fibrosis progression and poor treatment outcome in hepatitis C virus but not hepatitis B virus infection

Ablation of very-long-chain ceramides (Cers) with consecutive elevations in sphinganine levels has been shown to cause a severe hepatopathy in a knockout mouse model. We have recently shown that serum sphingolipids (SLs) are deregulated in patients with chronic liver disease. However, their role as possible biomarkers in liver fibrosis remains to date unexplored. We assessed, using liquid chromatography/tandem mass spectrometry, serum concentrations of various SL metabolites in 406 patients with chronic viral hepatitis, 203 infected with genotype 1 hepatitis C virus (HCV) and 203 with hepatitis B virus (HBV), respectively. We observed significant variations of serum SLs, with sphingosine and sphinganine being, both in univariate (P<0.05) as well as in multivariate analysis, significantly associated to severity of liver fibrosis in HCV-infected patients (odds ratio [OR]: 1.111; confidence interval [CI]: 1.028-1.202; P=0.007 and OR, 0.634; CI, 0.435-0.925; P=0.018, respectively). Serum SLs correlated significantly with serum triglyceride and cholesterol levels as well as with insulin resistance, defined by the homeostatic model assessment index, in HCV patients. Sustained viral response rates in HCV patients were independently predicted by serum C24Cer (OR, 0.998; CI, 0.997-0.999; P=0.001), its unsaturated derivative C24:1Cer (OR, 1.001; CI, 1.000-1.002; P=0.059), and C18:1Cer (OR, 0.973; CI, 0.947-0.999; P=0.048), together with ferritin (OR, 1.006; CI, 1.003-1.010; P<0.001), alkaline phosphatase (OR, 1.020; CI, 1.001-1.039; P=0.032), and interleukin-28B genotype (OR, 9.483; CI, 3.139-28.643; P<0.001).

CONCLUSION

Our study demonstrates a tight interaction between variations in serum SL levels and progression of liver fibrosis as well as responsiveness to antiviral therapy. Particularly, sphingosine, sphinganine, and C24Cer appear as promising novel biomarkers in chronic HCV infection and should be further evaluated within the noninvasive prediction of liver fibrosis.

Defect of insulin signal in peripheral tissues: Important role of ceramide

In healthy people, balance between glucose production and its utilization is precisely controlled. When circulating glucose reaches a critical threshold level, pancreatic β cells secrete insulin that has two major actions: to lower circulating glucose levels by facilitating its uptake mainly into skeletal muscle while inhibiting its production by the liver. Interestingly, dietary triglycerides are the main source of fatty acids to fulfill energy needs of oxidative tissues. Normally, the unconsumed fraction of excess of fatty acids is stored in lipid droplets that are localized in adipocytes to provide energy during fasting periods. Thus, adipose tissue acts as a trap for fatty acid excess liberated from plasma triglycerides. When the buffering action of adipose tissue to store fatty acids is impaired, fatty acids that build up in other tissues are metabolized as sphingolipid derivatives such as ceramides. Several studies suggest that ceramides are among the most active lipid second messengers to inhibit the insulin signaling pathway and this review describes the major role played by ceramide accumulation in the development of insulin resistance of peripherals tissues through the targeting of specific proteins of the insulin signaling pathway.

Serum acid sphingomyelinase is upregulated in chronic hepatitis C infection and non alcoholic fatty liver disease

Sphingolipids constitute bioactive molecules with functional implications in homeostasis and pathogenesis of various diseases. However, the role of sphingolipids as possible disease biomarkers in chronic liver disease remains largely unexplored. In the present study we used mass spectrometry and spectrofluorometry methods in order to quantify various sphingolipid metabolites and also assess the activity of an important corresponding regulating enzyme in the serum of 72 healthy volunteers as compared to 69 patients with non-alcoholic fatty liver disease and 69 patients with chronic hepatitis C virus infection. Our results reveal a significant upregulation of acid sphingomyelinase in the serum of patients with chronic liver disease as compared to healthy individuals (p<0.001). Especially in chronic hepatitis C infection acid sphingomyelinase activity correlated significantly with markers of hepatic injury (r=0.312, p=0.009) and showed a high discriminative power. Accumulation of various (dihydro-) ceramide species was identified in the serum of patients with non-alcoholic fatty liver disease (p<0.001) and correlated significantly to cholesterol (r=0.448, p<0.001) but showed a significant accumulation in patients with normal cholesterol values as well (p<0.001). Sphingosine, a further bioactive metabolite, was also upregulated in chronic liver disease (p<0.001). However, no significant correlation to markers of hepatic injury was identified.

CONCLUSION

Chronic hepatitis C virus infection and non-alcoholic fatty liver disease induce a significant upregulation of serum acid sphingomyelinase which appears as a novel biomarker in chronic hepatopathies. Further studies are required to elucidate the potential of the sphingolipid signaling pathway as putative therapeutic target in chronic liver disease.

The Induction of Cytokine Release in Monocytes by Electronegative Low-Density Lipoprotein (LDL) Is Related to Its Higher Ceramide Content than Native LDL

Electronegative low-density lipoprotein (LDL(−)) is a minor modified LDL subfraction that is present in blood. LDL(−) promotes inflammation and is associated with the development of atherosclerosis. We previously reported that the increase of cytokine release promoted by this lipoprotein subfraction in monocytes is counteracted by high-density lipoprotein (HDL). HDL also inhibits a phospholipase C-like activity (PLC-like) intrinsic to LDL(−). The aim of this work was to assess whether the inhibition of the PLC-like activity by HDL could decrease the content of ceramide (CER) and diacylglycerol (DAG) generated in LDL(−). This knowledge would allow us to establish a relationship between these compounds and the inflammatory activity of LDL(−). LDL(−) incubated at 37 °C for 20 h increased its PLC-like activity and, subsequently, the amount of CER and DAG. We found that incubating LDL(−) with HDL decreased both products in LDL(−). Native LDL was modified by lipolysis with PLC or by incubation with CER-enriched or DAG-enriched liposomes. The increase of CER in native LDL significantly increased cytokine release, whereas the enrichment in DAG did not show these inflammatory properties. These data point to CER, a resultant product of the PLC-like activity, as a major determinant of the inflammatory activity induced by LDL(−) in monocytes.

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.

Cardiomyocyte specific deficiency of serine palmitoyltransferase subunit 2 reduces ceramide but leads to cardiac dysfunction

The role of serine palmitoyltransferase (SPT) and de novo ceramide biosynthesis in cardiac ceramide and sphingomyelin metabolism is unclear. To determine whether the de novo synthetic pathways, rather than ceramide uptake from circulating lipoproteins, is important for heart ceramide levels, we created cardiomyocyte-specific deficiency of Sptlc2, a subunit of SPT. Heart-specific Sptlc2-deficient (hSptlc2 KO) mice had a >35% reduction in ceramide, which was limited to C18:0 and very long chain ceramides. Sphingomyelinase expression, and levels of sphingomyelin and diacylglycerol were unchanged. But surprisingly phospholipids and acyl CoAs contained increased saturated long chain fatty acids. hSptlc2 KO mice had decreased fractional shortening and thinning of the cardiac wall. While the genes regulating glucose and fatty acid metabolism were not changed, expression of cardiac failure markers and the genes involved in the formation of extracellular matrices were up-regulated in hSptlc2 KO hearts. In addition, ER-stress markers were up-regulated leading to increased apoptosis. These results suggest that Sptlc2-mediated de novo ceramide synthesis is an essential source of C18:0 and very long chain, but not of shorter chain, ceramides in the heart. Changes in heart lipids other than ceramide levels lead to cardiac toxicity.

Acidic sphingomyelinase induced by electrophiles promotes proinflammatory cytokine production in human bladder carcinoma ECV-304 cells

Electrophiles in environmental pollutants or cigarette smoke are high risk factors for various diseases caused by cell injuries such as apoptosis and inflammation. Here we show that electrophilic compounds such as diethyl malate (DEM), methyl mercury and cigarette smoke extracts significantly enhanced the expression of acidic sphingomyelinase (ASMase). ASMase activity and the amount of ceramide of DEM-treated cells were approximately 6 times and 4 times higher than these of non-treated cells, respectively. Moreover, we found that DEM pretreatment enhanced the production of IL-6 induced by TNF-α. Knockdown of ASMase attenuated the enhancement of TNF-α-dependent IL-6 production. On the other hand, enhancement of TNF-α-induced IL-6 production was observed in ASMase-overexpressing cells without DEM. Fractionation of the lipid raft revealed that the TNF receptor 1 (TNFR1) was migrated into the lipid raft in DEM-treated cells or ASMase-overexpressing cells. The TNF-α-induced IL-6 expression required the clustering of TNFR1 since IL-6 expression were decreased by the destruction of the lipid raft with filipin. These results demonstrated a new role for ASMase in the acceleration of the production of TNF-induced IL-6 as a pro-inflammatory cytokine and indicated that electrophiles could potentiate inflammation response by up-regulating of ASMase expression following formation of lipid rafts.

Endotoxin activates de novo sphingolipid biosynthesis via nuclear factor kappa B-mediated upregulation of Sptlc2

Sphingolipids are membrane components and are involved in cell proliferation, apoptosis and metabolic regulation. In this study we investigated whether de novo sphingolipid biosynthesis in macrophages is regulated by inflammatory stimuli. Lipopolysaccharide (LPS) treatment upregulated Sptlc2, a subunit of serine palmitoyltransferase (SPT), mRNA and protein in Raw264.7 and mouse peritoneal macrophages, but Sptlc1, another subunit of SPT, was not altered. SPT activation by LPS elevated cellular levels of ceramides and sphingomyelin (SM). Pharmacological inhibition of nuclear factor kappa B (NFκB) prevented LPS-induced upregulation of Sptlc2 while transfection of p65 subunit of NFκB upregulated Sptlc2 and increased cellular ceramide levels. In contrast, MAP kinases were not involved in regulation of sphingolipid biosynthesis. Analysis of Sptlc2 promoter and chromatin immunoprecipitation (ChIP) assay showed that NFκB binding sites are located in Sptlc2 promoter region. Our results demonstrate that inflammatory stimuli activate de novo sphingolipid biosynthesis via NFκB and may play a critical role in lipid metabolism in macrophages.

Quantitative sphingosine measurement as a surrogate for total ceramide concentration-preclinical and potential translational applications

Biomarkers are an increasingly important constituent of the drug development process, offering the potential of increased efficiency through reduced compound attrition and earlier proof of mechanism and/or efficacy. Assays developed for compound screening that can be directly translated for clinical trials are especially valuable, but their successful adoption requires a careful balance between assay performance and implementation costs. One such 'fit-for-purpose' biomarker assay, the indirect measurement of pharmacological modulation of sphingolipid biosynthesis and disposition, is presented here. Among sphingolipids, numerous ceramide species are readily detectable in different lipoprotein fractions of mammalian plasma, but their parallel quantification can be prohibitively expensive and time consuming. Ceramides differ in their fatty acid moiety, which is readily removed by hydrolysis, yielding a common sphingosine derivative, the measurement of which serves as an indicator of total ceramide. When followed by liquid chromatography tandem mass spectrometry (LC/MS/MS) for detection, robust analyte quantification becomes relatively straightforward. The practical utility of a method developed to be fit for the purpose of rapidly and quantitatively measuring treatment-induced variations in total ceramide from hamster plasma and individual lipoprotein fractions is described. With a linear calibration range from 0.003 to 33.4 microm sphingosine, precision and accuracy error in plasma-based quality controls spiked with ceramides was less than 15%. The specificity of the assay for ceramides was also assessed. The simplicity of the method would allow for its potential translation to other preclinical species, as well as for clinical applications in later-stage drug development.

Studies on the role of acid sphingomyelinase and ceramide in the regulation of tumor necrosis factor alpha (TNFalpha)-converting enzyme activity and TNFalpha secretion in macrophages

Acid sphingomyelinase (ASMase) has been proposed to mediate lipopolysaccharide (LPS) signaling in various cell types. This study shows that ASMase is a negative regulator of LPS-induced tumor necrosis factor alpha (TNFalpha) secretion in macrophages. ASMase-deficient (asm(-/-)) mice and isolated peritoneal macrophages produce severalfold more TNFalpha than their wild-type (asm(+/+)) counterparts when stimulated with LPS, whereas the addition of exogenous ceramides or sphingomyelinase reduces the differences. The underlying mechanism for these effects is not transcriptional but post-translational. The TNFalpha-converting enzyme (TACE) catalyzes the maturation of the 26-kDa precursor (pro-TNFalpha) to an active 17-kDa form (soluble (s)TNFalpha). In mouse peritoneal macrophages, the activity of TACE was the rate-limiting factor regulating TNFalpha production. A substantial portion of the translated pro-TNFalpha was not processed to sTNFalpha; instead, it was rapidly internalized and degraded in the lysosomes. TACE activity was 2-3-fold higher in asm(-/-) macrophages as compared with asm(+/+) macrophages and was suppressed when cells were treated with exogenous ceramide and sphingomyelinase. Indirect immunofluorescence analyses revealed distinct TNFalpha-positive structures in the close vicinity of the plasma membrane in asm(-/-) but not in asm(+/+) macrophages. asm(-/-) cells also had a higher number of early endosomal antigen 1-positive early endosomes. Experiments that involved inhibitors of TACE, endocytosis, and lysosomal proteolysis suggest that in the asm(-/-) cells a significant portion of pro-TNFalpha was sequestered within the early endosomes, and instead of undergoing lysosomal proteolysis, it was recycled to the plasma membrane and processed to sTNFalpha.

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.

HDL lipids and insulin resistance

There is renewed interest in high-density lipoproteins (HDLs) due to recent findings linking atherosclerosis to the formation of dysfunctional HDL. This article focuses on the universe of HDL lipids and their potential protective or proinflammatory roles in vascular disease and insulin resistance. HDL carries a wide array of lipids including sterols, triglycerides, fat-soluble vitamins, and a large number of phospholipids, including phosphatidylcholine, sphingomyelin, and ceramide with many biological functions. Ceramide has been implicated in the pathogenesis of insulin resistance and has many proinflammatory properties. In contrast, sphingosine-1-phosphate, which is transported mainly in HDL, has anti-inflammatory properties that may be atheroprotective and may account for some of the beneficial effects of HDL. However, the complexity of the HDL lipidome is only beginning to reveal itself. The emergence of new analytical technologies should rapidly increase our understanding of the function of HDL lipids and their role in disease states.

Lipids on trial: the search for the offending metabolite in Niemann-Pick type C disease

Niemann-Pick disease type C is a complex lysosomal storage disorder caused by mutations in either the NPC1 or NPC2 genes that is characterized at the cellular level by the storage of multiple lipids, defective lysosomal calcium homeostasis and unique trafficking defects. We review the potential role of each of the individual storage lipids in initiating the pathogenic cascade and propose a model of NPC1 and NPC2 function based on the current knowledge.