Myriocin Restores Metabolic Homeostasis in dAGE-Exposed Mice via AMPK-PGC1α-Mediated Mitochondrial Activation and Systemic Lipid/Glucose Regulation

Background: Diet-derived advanced glycation end products (dAGEs) are closely associated with obesity and metabolic disorders. This study investigates the therapeutic potential of myriocin (Myr), a sphingolipid synthesis inhibitor, in counteracting dAGE-induced obesity and its underlying mechanisms. Methods: Male C57BL/6J wild-type mice were randomly assigned to receive either a low-AGE diet or a high-AGE diet with or without the administration of myriocin for a duration of 24 weeks. At the end of the experimental period, blood samples, whole livers, and adipose tissues were harvested for subsequent biochemical, histological, and molecular analyses. Results: Using a 24-week high-AGE diet mouse model, we demonstrate that Myr significantly reduces body weight gain (by 76%) and adipose tissue accumulation, while alleviating hepatic steatosis. Myr improves glucose homeostasis by lowering fasting blood glucose (a 44.5% reduction), enhancing oral glucose tolerance, and restoring hepatic glycolysis/gluconeogenesis balance via upregulating glucokinase and suppressing G6pc. Notably, Myr reduces serum LDL-C, TG, and TC levels by 52.3%, 51.8%, and 48.8%, respectively, and ameliorates liver dysfunction as evidenced by normalized ALT/AST activities. Metabolomics reveal Myr reshapes amino acid, carbohydrate, and lipid metabolism pathways. Mechanistically, Myr suppresses lipogenesis by downregulating Srebp1, Fasn, and Acc, while activating AMPK-PGC1α signaling to enhance mitochondrial biogenesis (a 2.1-fold increase in mtDNA) and thermogenesis via Ucp1 upregulation in brown and white adipose tissues. Conclusions: Our findings unveil Myr as a novel dual regulator of lipid and glucose metabolism through AMPK-PGC1α-mediated mitochondrial activation, providing the first evidence of sphingolipid inhibition as a therapeutic strategy against dAGE-induced metabolic syndrome. This study establishes a multifaceted mechanism involving hepatic lipid regulation, adipose browning, and systemic metabolic reprogramming, advancing potential clinical applications for obesity-related disorders.

Reversed role of CD36 deficiency in high-fat diet or methionine/choline-deficient diet-induced hepatic steatosis and steatohepatitis

Introduction

Cluster of differentiation 36 (CD36) is highly expressed in the liver of patients with metabolic dysfunction-associated fatty liver disease (MAFLD) or metabolic dysfunction-associated steatohepatitis (MASH). However, the precise role of CD36 in MAFLD/MASH is controversial. In the current study, we aimed to uncover the role of CD36 in the early stage of MAFLD/MASH induced by high-fat diet (HFD) and methionine/choline-deficient (MCD) diet.

Methods

CD36-/- mice and littermate control mice were fed a normal food diet (NCD); HFD or MCD diet for 6 weeks.

Results

We determined that CD36 deficiency attenuated HFD-induced hepatic steatosis while exacerbating MCD diet-induced steatohepatitis. Mechanistically, CD36 deficiency reduced HFD-induced expression of fatty acid synthase (FASN), sterol regulatory element binding protein 1c (SREBP1c), and acetyl-CoA carboxylase alpha (ACC1), thereby inhibiting de novo fatty acid synthesis. The expression of superoxide dismutase and genes involving fatty acid oxidation was inhibited by MCD diet. CD36 deficiency reduced expression of genes involving fatty acid oxidation, while MCD diet had no effect on these genes expression in CD36-/- mice. Meanwhile, MCD diet-reduced superoxide dismutase expression was further inhibited by CD36 deficiency. Thus, MCD-induced liver ROS and inflammation were further enhanced by CD36 deficiency. By liver lipidomic analysis, we found that the levels of triglyceride (TG), diacylglycerols (DG), acylcarnitine (AcCA), ceramide (Cer) and LPC were increased, while phosphatidylcholine/phosphatidylethanolamine (PC/PE) were decreased in MCD diet-treated CD36-/- mice compared with MCD diet-treated wild type mice. Indeed, the expression of serine palmitoyltransferase 2 (SPTLC2), the key rate-limiting enzyme of ceramide synthesis, was higher in CD36-/- mice.

Discussion

CD36 deficiency improves HFD-induced MAFLD by inhibiting fatty acid synthesis, while accelerating MCD diet-induced MASH via promoting Cer, LPC, TG and DG accumulation to accelerate liver inflammation. The complex role of CD36 in MAFLD/MASH needs more investigation to discover the precise and effective strategy when targeting CD36.

Altered sphingolipid biosynthetic flux and lipoprotein trafficking contribute to trans-fat-induced atherosclerosis

Dietary fat drives the pathogenesis of atherosclerotic cardiovascular disease (ASCVD), particularly through circulating cholesterol and triglyceride-rich lipoprotein remnants. Industrially produced trans-unsaturated fatty acids (TFAs) incorporated into food supplies significantly promote ASCVD. However, the molecular trafficking of TFAs responsible for this association is not well understood. Here, we demonstrate that TFAs are preferentially incorporated into sphingolipids by serine palmitoyltransferase (SPT) and secreted from cells in vitro. Administering high-fat diets (HFDs) enriched in TFAs to Ldlr-/- mice accelerated hepatic very-low-density lipoprotein (VLDL) and sphingolipid secretion into circulation to promote atherogenesis compared with a cis-unsaturated fatty acid (CFA)-enriched HFD. SPT inhibition mitigated these phenotypes and reduced circulating atherogenic VLDL enriched in TFA-derived polyunsaturated sphingomyelin. Transcriptional analysis of human liver revealed distinct regulation of SPTLC2 versus SPTLC3 subunit expression, consistent with human genetic correlations in ASCVD, further establishing sphingolipid metabolism as a critical node mediating the progression of ASCVD in response to specific dietary fats.

Plasma 16:0 ceramide as a marker of cardiovascular risk estimated by carotid intima-media thickness in people with type 2 diabetes

AIM

New tools are required to better assess cardiovascular risk in individuals with type 2 diabetes mellitus (T2DM). Plasma ceramides emerge as promising candidates, given their substantial influence on the pathogenesis of both T2DM and atherosclerosis. The current study aimed to investigate whether plasma ceramides in patients with T2DM are a predictive factor for carotid intima-media thickness (CIMT), a well-established noninvasive marker for atherosclerosis that predicts adverse cardiovascular outcomes.

METHODS

A lipidomic analysis was carried out on the circulating ceramides of a large cohort consisting of 246 patients with T2DM who underwent a high-resolution real-time B ultrasonography to measure CIMT.

RESULTS

Both plasma 16:0 ceramide and the 16:0/24:0 ceramide ratio were positively associated with CIMT, even after adjustment for traditional cardiovascular risk factors [standardized β ± standard error: 0.168 ± 0.072 (P = 0.020) and 0.180 ± 0.068 (P = 0.009), respectively]. Similar independent associations were found with respect to the prediction of CIMT ≥ 0.80 mm [β = 8.07 ± 3.90 (P = 0.038) and 16.5 ± 7.0 (P = 0.019), respectively]. The goodness-of-fit for multivariate models in predicting CIMT was 5.7 and 7.6 times higher when plasma 16:0 ceramide or the 16:0/24:0 ceramide ratio were included in combination with traditional cardiovascular risk factors (P = 0.020 and 0.015, respectively). This reached a 3.1 and 10.0-fold increase regarding the ability to predict CIMT ≥ 0.80 mm (P = 0.039 and 0.008, respectively).

CONCLUSIONS

Our findings suggest that 16:0 ceramide and the 16:0/24:0 ceramide ratio may serve as plasma biomarkers to improve cardiovascular risk assessment in individuals with T2DM.

Dysregulation of Ceramide Metabolism Is Linked to Iron Deposition and Activation of Related Pathways in the Aorta of Atherosclerotic Miniature Pigs

The miniature pig is a suitable animal model for investigating human cardiovascular diseases. Nevertheless, the alterations in lipid metabolism within atherosclerotic plaques of miniature pigs, along with the underlying mechanisms, remain to be comprehensively elucidated. In this study, we aim to examine the alterations in lipid composition and associated pathways in the abdominal aorta of atherosclerotic pigs induced by a high-fat, high-cholesterol, and high-fructose (HFCF) diet using lipidomics and RNA-Seq methods. The results showed that the content and composition of aortic lipid species, particularly ceramide, hexosyl ceramide, lysophosphatidylcholine, and triglyceride, were significantly altered in HFCF-fed pigs. Meanwhile, the genes governing sphingolipid metabolism, iron ion homeostasis, apoptosis, and the inflammatory response were significantly regulated by the HFCF diet. Furthermore, C16 ceramide could promote iron deposition in RAW264.7 cells, leading to increased intracellular reactive oxygen species (ROS) production, apoptosis, and activation of the toll-like receptor 4 (TLR4)/nuclear Factor-kappa B (NF-қB) inflammatory pathway, which could be mitigated by deferoxamine. Our study demonstrated that dysregulated ceramide metabolism could increase ROS production, apoptosis, and inflammatory pathway activation in macrophages by inducing iron overload, thus playing a vital role in the pathogenesis of atherosclerosis. This discovery could potentially provide a new target for pharmacological therapy of cardiovascular diseases such as atherosclerosis.

Lipidomic Analysis to Assess the Correlation between Ceramides, Stress Hyperglycemia, and HbA1c in Acute Myocardial Infarction

Ceramides have been associated with cardiometabolic disease (e.g., acute myocardial infarction (AMI) and type 2 diabetes (T2D)) and adverse outcomes. Acute admission hyperglycemia (AH) is a transient glucose alteration in response to stress. As glycated hemoglobin (HbA1c) reflects the glycemia over a longer period of time, its use may be helpful in distinguishing between the AH and hyperglycemia associated with T2D in the AMI setting. The aim was to assess the correlation of ceramides with both AH (defined as an admission glucose level ≥140 mg/dL in the absence of T2D) and HbA1c-T2D and other demographic, clinical, and inflammatory-related biomarkers in AMI. High-performance liquid chromatography-tandem mass spectrometry was used to identify nine ceramide species, and their three ratios, in 140 AMI patients (FTGM coronary unit, Massa, Italy). The ceramides did not correlate with stress hyperglycemia, but specific species were elevated in T2D-AMI. Moreover, some ceramides were associated with other cardiometabolic risk factors. Ceramides assessment may be helpful in better understanding the pathogenic molecular mechanisms underlying myocardial acute events and cardiometabolic risk, as a basis for the future evaluation of their role as prognostic predictors and therapeutic targets in T2D-AMI patients.

You aren't IMMUNE to the ceramides that accumulate in cardiometabolic disease

Obesity leads to persistent increases in immune responses that contribute to cardiometabolic pathologies such as diabetes and cardiovascular disease. Pro-inflammatory macrophages infiltrate the expanding fat mass, which leads to increased production of cytokines such as tumor necrosis factor-alpha. Moreover, saturated fatty acids enhance signaling through the toll-like receptors involved in innate immunity. Herein we discuss the evidence that ceramides-which are intermediates in the biosynthetic pathway that produces sphingolipids-are essential intermediates that link these inflammatory signals to impaired tissue function. We discuss the mechanisms linking these immune insults to ceramide production and review the numerous ceramide actions that alter cellular metabolism, induce oxidative stress, and stimulate apoptosis. Lastly, we evaluate the correlation of ceramides in humans with inflammation-linked cardiometabolic disease and discuss preclinical studies which suggest that ceramide-lowering interventions may be an effective strategy to treat or prevent such maladies.

Comprehensive Metabolic Profiling of Inflammation Indicated Key Roles of Glycerophospholipid and Arginine Metabolism in Coronary Artery Disease

Background

Systemic immune inflammation is a key mediator in the progression of coronary artery disease (CAD), concerning various metabolic and lipid changes. In this study, the relationship between the inflammatory index and metabolic profile in patients with CAD was investigated to provide deep insights into metabolic disturbances related to inflammation.

Methods

Widely targeted plasma metabolomic and lipidomic profiling was performed in 1,234 patients with CAD. Laboratory circulating inflammatory markers were mainly used to define general systemic immune and low-grade inflammatory states. Multivariable-adjusted linear regression was adopted to assess the associations between 860 metabolites and 7 inflammatory markers. Least absolute shrinkage and selection operator (LASSO) logistic-based classifiers and multivariable logistic regression were applied to identify biomarkers of inflammatory states and develop models for discriminating an advanced inflammatory state.

Results

Multiple metabolites and lipid species were linearly associated with the seven inflammatory markers [false discovery rate (FDR) <0.05]. LASSO and multivariable-adjusted logistic regression analysis identified significant associations between 45 metabolites and systemic immune-inflammation index, 46 metabolites and neutrophil-lymphocyte ratio states, 32 metabolites and low-grade inflammation score, and 26 metabolites and high-sensitivity C-reactive protein states (P < 0.05). Glycerophospholipid metabolism and arginine and proline metabolism were determined as key altered metabolic pathways for systemic immune and low-grade inflammatory states. Predictive models based solely on metabolite combinations showed feasibility (area under the curve: 0.81 to 0.88) for discriminating the four parameters that represent inflammatory states and were successfully validated using a validation cohort. The inflammation-associated metabolite, namely, β-pseudouridine, was related to carotid and coronary arteriosclerosis indicators (P < 0.05).

Conclusions

This study provides further information on the relationship between plasma metabolite profiles and inflammatory states represented by various inflammatory markers in CAD. These metabolic markers provide potential insights into pathological changes during CAD progression and may aid in the development of therapeutic targets.

AdipoAtlas: A reference lipidome for human white adipose tissue

Obesity, characterized by expansion and metabolic dysregulation of white adipose tissue (WAT), has reached pandemic proportions and acts as a primer for a wide range of metabolic disorders. Remodeling of WAT lipidome in obesity and associated comorbidities can explain disease etiology and provide valuable diagnostic and prognostic markers. To support understanding of WAT lipidome remodeling at the molecular level, we provide in-depth lipidomics profiling of human subcutaneous and visceral WAT of lean and obese individuals. We generate a human WAT reference lipidome by performing tissue-tailored preanalytical and analytical workflows, which allow accurate identification and semi-absolute quantification of 1,636 and 737 lipid molecular species, respectively. Deep lipidomic profiling allows identification of main lipid (sub)classes undergoing depot-/phenotype-specific remodeling. Previously unanticipated diversity of WAT ceramides is now uncovered. AdipoAtlas reference lipidome serves as a data-rich resource for the development of WAT-specific high-throughput methods and as a scaffold for systems medicine data integration.

Ceramides in Metabolism: Key Lipotoxic Players

The global prevalence of metabolic diseases such as type 2 diabetes mellitus, steatohepatitis, myocardial infarction, and stroke has increased dramatically over the past two decades. These obesity-fueled disorders result, in part, from the aberrant accumulation of harmful lipid metabolites in tissues not suited for lipid storage (e.g., the liver, vasculature, heart, and pancreatic beta-cells). Among the numerous lipid subtypes that accumulate, sphingolipids such as ceramides are particularly impactful, as they elicit the selective insulin resistance, dyslipidemia, and ultimately cell death that underlie nearly all metabolic disorders. This review summarizes recent findings on the regulatory pathways controlling ceramide production, the molecular mechanisms linking the lipids to these discrete pathogenic events, and exciting attempts to develop therapeutics to reduce ceramide levels to combat metabolic disease.

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.

Insights into the prognosis of lipidomic dysregulation for death risk in patients with coronary artery disease

BACKGROUND

Dyslipidaemia contributes to the progression of coronary artery disease (CAD) toward adverse outcomes. Plasma lipidomic measure may improve the prognostic performances of clinical endpoints of CAD. Our research is designed to identify the correlations between plasma lipid species and the risks of death, major adverse cardiovascular event (MACE) and left ventricular (LV) remodeling in patients with CAD.

METHODS

A total of 1569 Chinese patients with CAD, 1011 single-centre patients as internal training cohort, and 558 multicentre patients as external validation cohort, were enrolled. The concentration of plasma lipids in both cohorts was determined through widely targeted lipidomic profiling. Least absolute shrinkage and selection operator Cox and multivariate Cox regressions were used to develop prognostic models for death and MACE, respectively.

RESULTS

Ten (Cer(d18:1/20:1), Cer(d18:1/24:1), PE(30:2), PE(32:0), PE(32:2), PC(O-38:2), PC(O-36:4), PC(16:1/22:2), LPC(18:2/0:0) and LPE(0:0/24:6)) and two (Cer(d18:1/20:1) and LPC(20:0/0:0)) lipid species were independently related to death and MACE, respectively. Cer(d18:1/20:1) and Cer(d18:1/24:1) were correlated with LV remodeling (P < .05). The lipidic panel incorporating 10 lipid species and two traditional biomarkers for predicting 5-year death risk represented a remarkable higher discrimination than traditional model with increased area under the curve from 76.56 to 83.65%, continuous NRI of 0.634 and IDI of 0.131. Furthermore, the panel was successfully used in differentiating multicentre patients with low, middle, or high risks (P < .0001). Further analysis indicated that the number of double bonds of phosphatidyl choline and the content of carbon atoms of phosphatidyl ethanolamines were negatively associated with death risk.

CONCLUSIONS

Improvement in the prediction of death confirms the effectiveness of plasma lipids as predictors to risk classification in patients with CAD. The association between the structural characteristics of long-chain polyunsaturated fatty acids and death risk highlights the need for mechanistic research that characterizes the role of individual lipid species in disease pathogenesis.

Ceramides in Adipose Tissue

Adipose tissue is a key nutrient-sensing depot that regulates excess energy storage and consumption. Adipocytes, the key components of the adipose tissue, have unique ability to store excess energy in the form of triglycerides, sense systemic energy demands, and secrete factors (lipids, peptides, cytokines, and adipokines) to regulate other metabolic tissues. The presence of various types of adipocytes (white, brown, and beige) characterized by the number/size of lipid droplets, mitochondrial density, and thermogenic capacity, further highlights how intricate is the communication of these cell-types with other metabolic tissues to sense nutrients. In obesity the inherent capacity of adipose tissue to store and sense nutrients is compromised, causing spillover of the intermediate lipid metabolites into circulation and resulting in their ectopic deposition in tissues not suitable for lipid storage, a phenomenon known as lipotoxicity. This results in a spectrum of cellular dysfunction, that underlies various metabolic diseases. Of the numerous lipid classes implicated in eliciting lipotoxicity, sphingolipid: ceramides are among the most deleterious as they modulate signaling pathways involved in regulating glucose metabolism, triglyceride synthesis, apoptosis, and fibrosis. Notably, recent experimental studies have strongly implicated ceramides in the development of numerous metabolic diseases such as insulin resistance, diabetes, cardiomyopathy, hepatic-steatosis, and atherosclerosis. Herein we discuss and summarizes recent findings that implicate ceramides as a key contributor to adipocyte dysfunction underlying metabolic diseases and how depletion of ceramides can be exploited to improve metabolic health.

Inhibition of sphingolipid synthesis improves outcomes and survival in GARP mutant wobbler mice, a model of motor neuron degeneration

Numerous mutations that impair retrograde membrane trafficking between endosomes and the Golgi apparatus lead to neurodegenerative diseases. For example, mutations in the endosomal retromer complex are implicated in Alzheimer's and Parkinson's diseases, and mutations of the Golgi-associated retrograde protein (GARP) complex cause progressive cerebello-cerebral atrophy type 2 (PCCA2). However, how these mutations cause neurodegeneration is unknown. GARP mutations in yeast, including one causing PCCA2, result in sphingolipid abnormalities and impaired cell growth that are corrected by treatment with myriocin, a sphingolipid synthesis inhibitor, suggesting that alterations in sphingolipid metabolism contribute to cell dysfunction and death. Here we tested this hypothesis in wobbler mice, a murine model with a homozygous partial loss-of-function mutation in Vps54 (GARP protein) that causes motor neuron disease. Cytotoxic sphingoid long-chain bases accumulated in embryonic fibroblasts and spinal cords from wobbler mice. Remarkably, chronic treatment of wobbler mice with myriocin markedly improved their wellness scores, grip strength, neuropathology, and survival. Proteomic analyses of wobbler fibroblasts revealed extensive missorting of lysosomal proteins, including sphingolipid catabolism enzymes, to the Golgi compartment, which may contribute to the sphingolipid abnormalities. Our findings establish that altered sphingolipid metabolism due to GARP mutations contributes to neurodegeneration and suggest that inhibiting sphingolipid synthesis might provide a useful strategy for treating these disorders.

Machine learning reveals serum sphingolipids as cholesterol-independent biomarkers of coronary artery disease

BACKGROUNDCeramides are sphingolipids that play causative roles in diabetes and heart disease, with their serum levels measured clinically as biomarkers of cardiovascular disease (CVD).METHODSWe performed targeted lipidomics on serum samples from individuals with familial coronary artery disease (CAD) (n = 462) and population-based controls (n = 212) to explore the relationship between serum sphingolipids and CAD, using unbiased machine learning to identify sphingolipid species positively associated with CAD.RESULTSNearly every sphingolipid measured (n = 30 of 32) was significantly elevated in subjects with CAD compared with measurements in population controls. We generated a novel sphingolipid-inclusive CAD risk score, termed SIC, that demarcates patients with CAD independently and more effectively than conventional clinical CVD biomarkers including serum LDL cholesterol and triglycerides. This new metric comprises several minor lipids that likely serve as measures of flux through the ceramide biosynthesis pathway rather than the abundant deleterious ceramide species that are included in other ceramide-based scores.CONCLUSIONThis study validates serum ceramides as candidate biomarkers of CVD and suggests that comprehensive sphingolipid panels should be considered as measures of CVD.FUNDINGThe NIH (DK112826, DK108833, DK115824, DK116888, and DK116450); the Juvenile Diabetes Research Foundation (JDRF 3-SRA-2019-768-A-B); the American Diabetes Association; the American Heart Association; the Margolis Foundation; the National Cancer Institute, NIH (5R00CA218694-03); and the Huntsman Cancer Institute Cancer Center Support Grant (P30CA040214).

Ceramides: Nutrient Signals that Drive Hepatosteatosis

Ceramides are minor components of the hepatic lipidome that have major effects on liver function. These products of lipid and protein metabolism accumulate when the energy needs of the hepatocyte have been met and its storage capacity is full, such that free fatty acids start to couple to the sphingoid backbone rather than the glycerol moiety that is the scaffold for glycerolipids (e.g., triglycerides) or the carnitine moiety that shunts them into mitochondria. As ceramides accrue, they initiate actions that protect cells from acute increases in detergent-like fatty acids; for example, they alter cellular substrate preference from glucose to lipids and they enhance triglyceride storage. When prolonged, these ceramide actions cause insulin resistance and hepatic steatosis, 2 of the underlying drivers of cardiometabolic diseases. Herein the author discusses the mechanisms linking ceramides to the development of insulin resistance, hepatosteatosis and resultant cardiometabolic disorders.

Metabolic Messengers: ceramides

Ceramides are products of metabolism that accumulate in individuals with obesity or dyslipidaemia and alter cellular processes in response to fuel surplus. Their actions, when prolonged, elicit the tissue dysfunction that underlies diabetes and heart disease. Here, we review the history of research on these enigmatic molecules, exploring their discovery and mechanisms of action, the evolutionary pressures that have given them their unique attributes and the potential of ceramide-reduction therapies as treatments for cardiometabolic disease.

Sulforaphane Prevents Hepatic Insulin Resistance by Blocking Serine Palmitoyltransferase 3-Mediated Ceramide Biosynthesis

Sulforaphane (SFA), a naturally active isothiocyanate compound from cruciferous vegetables used in clinical trials for cancer treatment, was found to possess potency to alleviate insulin resistance. But its underlying molecular mechanisms are still incompletely understood. In this study, we assessed whether SFA could improve insulin sensitivity and glucose homeostasis both in vitro and in vivo by regulating ceramide production. The effects of SFA on glucose metabolism and expression levels of key proteins in the hepatic insulin signaling pathway were evaluated in insulin-resistant human hepatic carcinoma HepG2 cells. The results showed that SFA dose-dependently increased glucose uptake and intracellular glycogen content by regulating the insulin receptor substrate 1 (IRS-1)/protein kinase B (Akt) signaling pathway in insulin-resistant HepG2 cells. SFA also reduced ceramide contents and downregulated transcription of ceramide-related genes. In addition, knockdown of serine palmitoyltransferase 3 (SPTLC3) in HepG2 cells prevented ceramide accumulation and alleviated insulin resistance. Moreover, SFA treatment improved glucose tolerance and insulin sensitivity, inhibited SPTLC3 expression and hepatic ceramide production and reduced hepatic triglyceride content in vivo. We conclude that SFA recovers glucose homeostasis and improves insulin sensitivity by blocking ceramide biosynthesis through modulating SPTLC3, indicating that SFA may be a potential candidate for prevention and amelioration of hepatic insulin resistance via a ceramide-dependent mechanism.

Regulation of Sphingolipid Metabolism by MicroRNAs: A Potential Approach to Alleviate Atherosclerosis

The rapidly expanding field of bioactive lipids is exemplified by the many sphingolipids, which are structurally and functionally diverse molecules with significant physiologic functions. These sphingolipids are main constituents of cellular membranes and have been found associated with plasma lipoproteins, and their concentrations are altered in several metabolic disorders such as atherosclerosis, obesity, and diabetes. Understanding the mechanisms that regulate their biosynthesis and secretion may provide novel information that might be amenable to therapeutic targeting in the treatment of these diseases. Several sphingolipid synthesis genes have been targeted as potential therapeutics for atherosclerosis. In recent years, significant progress has been made in studying the role of microRNAs (miRNAs) in lipid metabolism. However, little effort has been made to investigate their role in sphingolipid metabolism. Sphingolipid biosynthetic pathways involve various enzymes that lead to the formation of several key molecules implicated in atherosclerosis, and the identification of miRNAs that regulate these enzymes could help us to understand these complex pathways better and may prove beneficial in alleviating atherosclerosis.

Distinct lipidomic profiles in models of physiological and pathological cardiac remodeling, and potential therapeutic strategies

Cardiac myocyte membranes contain lipids which remodel dramatically in response to heart growth and remodeling. Lipid species have both structural and functional roles. Physiological and pathological cardiac remodeling have very distinct phenotypes, and the identification of molecular differences represent avenues for therapeutic interventions. Whether the abundance of specific lipid classes is different in physiological and pathological models was largely unknown. The aim of this study was to determine whether distinct lipids are regulated in settings of physiological and pathological remodeling, and if so, whether modulation of differentially regulated lipids could modulate heart size and function. Lipidomic profiling was performed on cardiac-specific transgenic mice with 1) physiological cardiac hypertrophy due to increased Insulin-like Growth Factor 1 (IGF1) receptor or Phosphoinositide 3-Kinase (PI3K) signaling, 2) small hearts due to depressed PI3K signaling (dnPI3K), and 3) failing hearts due to dilated cardiomyopathy (DCM). In hearts of dnPI3K and DCM mice, several phospholipids (plasmalogens) were decreased and sphingolipids increased compared to mice with physiological hypertrophy. To assess whether restoration of plasmalogens could restore heart size or cardiac function, dnPI3K and DCM mice were administered batyl alcohol (BA; precursor to plasmalogen biosynthesis) in the diet for 16weeks. BA supplementation increased a major plasmalogen species (p18:0) in the heart but had no effect on heart size or function. This may be due to the concurrent reduction in other plasmalogen species (p16:0 and p18:1) with BA. Here we show that lipid species are differentially regulated in settings of physiological and pathological remodeling. Restoration of lipid species in the failing heart warrants further examination.

Repairing Mitochondrial Dysfunction in Disease

Mitochondria are essential organelles for many aspects of cellular homeostasis, including energy harvesting through oxidative phosphorylation. Alterations of mitochondrial function not only impact on cellular metabolism but also critically influence whole-body metabolism, health, and life span. Diseases defined by mitochondrial dysfunction have expanded from rare monogenic disorders in a strict sense to now also include many common polygenic diseases, including metabolic, cardiovascular, neurodegenerative, and neuromuscular diseases. This has led to an intensive search for new therapeutic and preventive strategies aimed at invigorating mitochondrial function by exploiting key components of mitochondrial biogenesis, redox metabolism, dynamics, mitophagy, and the mitochondrial unfolded protein response. As such, new findings linking mitochondrial function to the progression or outcome of this ever-increasing list of diseases has stimulated the discovery and development of the first true mitochondrial drugs, which are now entering the clinic and are discussed in this review.

Sphingolipids and phospholipids in insulin resistance and related metabolic disorders

Obesity, insulin resistance, type 2 diabetes mellitus and cardiovascular disease form a metabolic disease continuum that has seen a dramatic increase in prevalence in developed and developing countries over the past two decades. Dyslipidaemia resulting from hypercaloric diets is a major contributor to the pathogenesis of metabolic disease, and lipid-lowering therapies are the main therapeutic option for this group of disorders. However, the fact that dysfunctional lipid metabolism extends far beyond cholesterol and triglycerides is becoming increasingly clear. Lipidomic studies and mouse models are helping to explain the complex interactions between diet, lipid metabolism and metabolic disease. These studies are not only improving our understanding of this complex biology, but are also identifying potential therapeutic avenues to combat this growing epidemic. This Review examines what is currently known about phospholipid and sphingolipid metabolism in the setting of obesity and how metabolic pathways are being modulated for therapeutic effect.

Insulin treatment increases myocardial ceramide accumulation and disrupts cardiometabolic function

Background

States of hyperinsulinemia, particularly insulin resistance and type 2 diabetes mellitus, are becoming remarkably common, with roughly half a billion people likely to suffer from the disorder within the next 15 years. Along with this rise has been an associated increased burden of cardiovascular disease. Considering type 2 diabetics treated with insulin are more likely to suffer from heart complications, we sought to determine the specific effect of insulin on ceramide-dependent cardiometabolic risk factors, including insulin resistance and altered heart mitochondrial physiology.

Methods

H9c2 cardiomyocytes and adult mice were treated with insulin with or without myriocin to inhibit ceramide biosynthesis. Insulin and glucose changes were tracked throughout the study and mitochondrial bioenergetics was determined in permeabilized cardiomyocytes and myocardium.

Results

Herein, we demonstrate that insulin is sufficient to disrupt heart mitochondrial respiration in both isolated cardiomyocytes and whole myocardium, possibly by increasing mitochondrial fission. Further, insulin increases ceramide accrual in a time-dependent manner, which is necessary for insulin-induced alterations in heart mitochondrial respiration and insulin resistance.

Conclusions

Collectively, these observations have two implications. First, they indicate a pathological role of insulin in heart complications stemming from mitochondrial disruption. Second, they identify ceramide as a possible mediator of insulin-related heart disorders.

ORMDL orosomucoid-like proteins are degraded by free-cholesterol-loading-induced autophagy

Eukaryotic cells have evolved robust mechanisms to counter excess cholesterol including redistribution of lipids into different compartments and compensatory up-regulation of phospholipid biosynthesis. We demonstrate here that excess cellular cholesterol increased the activity of the endoplasmic reticulum (ER) enzyme serine palmitoyl-CoA transferase (SPT), the rate-limiting enzyme in sphingomyelin synthesis. This increased SPT activity was not due to altered levels of SPTLC1 or SPTLC2, the major subunits of SPT. Instead, cholesterol loading decreased the levels of ORMDL1, a negative regulator of SPT activity, due to its increased turnover. Several lines of evidence demonstrated that free-cholesterol-induced autophagy, which led to increased turnover of ORMDL1. Cholesterol loading induced ORMDL1 redistribution from the ER to cytoplasmic p62 positive autophagosomes. Coimmunoprecipitation analysis of cholesterol-loaded cells showed increased association between ORMDL1 and p62. The lysosomal inhibitor chloroquine or siRNA knockdown of Atg7 inhibited ORMDL1 degradation by cholesterol, whereas proteasome inhibitors showed no effect. ORMDL1 degradation was specific to free-cholesterol loading as autophagy induced by serum starvation or general ER stress did not lead to ORMDL1 degradation. ORMDL proteins are thus previously unidentified responders to excess cholesterol, exiting the ER to activate SPT and increase sphingomyelin biosynthesis, which may buffer excess cellular cholesterol.

The consequences of genetic and pharmacologic reduction in sphingolipid synthesis

A new therapy based on substrate synthesis reduction in sphingolipidoses is showing promise. The consequences of decreasing sphingolipid synthesis depend on the level at which synthetic blockage occurs and on the extent of the blockage. Complete synthetic blockage may be lethal if it includes all sphingolipids, such as in a global knockout of serine palmitoyltransferase. Partial inhibition of sphingolipid synthetic pathways is usually benign and may have beneficial effects in a number of lysosomal diseases and in more common pathologies, as seen in animal models for atherosclerosis, polycystic kidney disease, diabetes, and asthma. Studies of various forms of sphingolipid synthesis reduction serve to highlight not only the cellular role of these lipids but also the potential risks and therapeutic benefits of pharmacological agents to be used in therapy for human diseases.

Inhibition of glycosphingolipid synthesis ameliorates atherosclerosis and arterial stiffness in apolipoprotein E-/- mice and rabbits fed a high-fat and -cholesterol diet

BACKGROUND

Glycosphingolipids, integral components of the cell membrane, have been shown to serve as messengers, transducing growth factor-initiated phenotypes. Here, we have examined whether inhibition of glycosphingolipid synthesis could ameliorate atherosclerosis and arterial stiffness in transgenic mice and rabbits.

METHODS AND RESULTS

Apolipoprotein E(-/-) mice (12 weeks of age; n=6) were fed regular chow or a Western diet (1.25% cholesterol, 2% fat). Mice were fed 5 or 10 mg/kg of an inhibitor of glycosphingolipid synthesis, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), solubilized in vehicle (5% Tween-80 in PBS); the placebo group received vehicle only. At 20 and 36 weeks of age, serial echocardiography was performed to measure aortic intima-media thickening. Aortic pulse-wave velocity measured vascular stiffness. Feeding mice a Western diet markedly increased aortic pulse-wave velocity, intima-media thickening, oxidized low-density lipoprotein, Ca(2+) deposits, and glucosylceramide and lactosylceramide synthase activity. These were dose-dependently decreased by feeding D-PDMP. In liver, D-PDMP decreased cholesterol and triglyceride levels by raising the expression of SREBP2, low-density lipoprotein receptor, HMGCo-A reductase, and the cholesterol efflux genes (eg, ABCG5, ABCG8). D-PDMP affected very-low-density lipoprotein catabolism by increasing the gene expression for lipoprotein lipase and very-low-density lipoprotein receptor. Rabbits fed a Western diet for 90 days had extensive atherosclerosis accompanied by a 17.5-fold increase in total cholesterol levels and a 3-fold increase in lactosylceramide levels. This was completely prevented by feeding D-PDMP.

CONCLUSIONS

Inhibition of glycosphingolipid synthesis ameliorates atherosclerosis and arterial stiffness in apolipoprotein E(-/-) mice and rabbits. Thus, inhibition of glycosphingolipid synthesis may be a novel approach to ameliorate atherosclerosis and arterial stiffness.

Myriocin significantly increases the mortality of a non-mammalian model host during Candida pathogenesis

Candida albicans is a major human pathogen whose treatment is challenging due to antifungal drug toxicity, drug resistance and paucity of antifungal agents available. Myrocin (MYR) inhibits sphingosine synthesis, a precursor of sphingolipids, an important cell membrane and signaling molecule component. MYR also has dual immune suppressive and antifungal properties, potentially modulating mammalian immunity and simultaneously reducing fungal infection risk. Wax moth (Galleria mellonella) larvae, alternatives to mice, were used to establish if MYR suppressed insect immunity and increased survival of C. albicans-infected insects. MYR effects were studied in vivo and in vitro, and compared alone and combined with those of approved antifungal drugs, fluconazole (FLC) and amphotericin B (AMPH). Insect immune defenses failed to inhibit C. albicans with high mortalities. In insects pretreated with the drug followed by C. albicans inoculation, MYR+C. albicans significantly increased mortality to 93% from 67% with C. albicans alone 48 h post-infection whilst AMPH+C. albicans and FLC+C. albicans only showed 26% and 0% mortalities, respectively. MYR combinations with other antifungal drugs in vivo also enhanced larval mortalities, contrasting the synergistic antifungal effect of the MYR+AMPH combination in vitro. MYR treatment influenced immunity and stress management gene expression during C. albicans pathogenesis, modulating transcripts putatively associated with signal transduction/regulation of cytokines, I-kappaB kinase/NF-kappaB cascade, G-protein coupled receptor and inflammation. In contrast, all stress management gene expression was down-regulated in FLC and AMPH pretreated C. albicans -infected insects. Results are discussed with their implications for clinical use of MYR to treat sphingolipid-associated disorders.

Sphingomyelin depletion impairs anionic phospholipid inward translocation and induces cholesterol efflux

The phosphatidylserine (PS) floppase activity (outward translocation) of ABCA1 leads to plasma membrane remodeling that plays a role in lipid efflux to apolipoprotein A-I (apoAI) generating nascent high density lipoprotein. The Tangier disease W590S ABCA1 mutation has defective PS floppase activity and diminished cholesterol efflux activity. Here, we report that depletion of sphingomyelin by inhibitors or sphingomyelinase caused plasma membrane remodeling, leading to defective flip (inward translocation) of PS, higher PS exposure, and higher cholesterol efflux from cells by both ABCA1-dependent and ABCA1-independent mechanisms. Mechanistically, sphingomyelin was connected to PS translocation in cell-free liposome studies that showed that sphingomyelin increased the rate of spontaneous PS flipping. Depletion of sphingomyelin in stably transfected HEK293 cells expressing the Tangier disease W590S mutant ABCA1 isoform rescued the defect in PS exposure and restored cholesterol efflux to apoAI. Liposome studies showed that PS directly increased cholesterol accessibility to extraction by cyclodextrin, providing the mechanistic link between cell surface PS and cholesterol efflux. We conclude that altered plasma membrane environment conferred by depleting sphingomyelin impairs PS flip and promotes cholesterol efflux in ABCA1-dependent and -independent manners.

2H2O-based high-density lipoprotein turnover method for the assessment of dynamic high-density lipoprotein function in mice

OBJECTIVE

High-density lipoprotein (HDL) promotes reverse cholesterol transport from peripheral tissues to the liver for clearance. Reduced HDL-cholesterol (HDLc) is associated with atherosclerosis; however, as a predictor of cardiovascular disease, HDLc has limitations because it is not a direct marker of HDL functionality. Our objective was to develop a mass spectrometry-based method for the simultaneous measurement of HDLc and ApoAI kinetics in mice, using a single (2)H2O tracer, and use it to examine genetic and drug perturbations on HDL turnover in vivo.

APPROACH AND RESULTS

Mice were given (2)H2O in the drinking water, and serial blood samples were collected at different time points. HDLc and ApoAI gradually incorporated (2)H, allowing experimental measurement of fractional catabolic rates and production rates for HDLc and ApoAI. ApoE(-/-) mice displayed increased fractional catabolic rates (P<0.01) and reduced production rates of both HDLc and ApoAI (P<0.05) compared with controls. In human ApoAI transgenic mice, levels and production rates of HDLc and human ApoAI were strikingly higher than in wild-type mice. Myriocin, an inhibitor of sphingolipid synthesis, significantly increased both HDL flux and macrophage-to-feces reverse cholesterol transport, indicating compatibility of this HDL turnover method with the macrophage-specific reverse cholesterol transport assay.

CONCLUSIONS

(2)H2O-labeling can be used to measure HDLc and ApoAI flux in vivo, and to assess the role of genetic and pharmacological interventions on HDL turnover in mice. Safety, simplicity, and low cost of the (2)H2O-based HDL turnover approach suggest that this assay can be scaled for human use to study effects of HDL targeted therapies on dynamic HDL function.

Characterization of secretory sphingomyelinase activity, lipoprotein sphingolipid content and LDL aggregation in ldlr-/- mice fed on a high-fat diet

The propensity of LDLs (low-density lipoproteins) for aggregation and/or oxidation has been linked to their sphingolipid content, specifically the levels of SM (sphingomyelin) and ceramide. To investigate this association in vivo, ldlr (LDL receptor)-null mice (ldlr^−/−) were fed on a modified (atherogenic) diet containing saturated fats and cholesterol. The diet led to significantly elevated SM content in all serum lipoproteins. In contrast, ceramide increased only in the LDL particles. MS-based analyses of the lipid acyl chain composition revealed a marked elevation in C_16:0 fatty acid in SM and ceramide, consistent with the prevalence of palmitic acid in the modified diet. The diet also led to increased activity of the S-SMase [secretory SMase (sphingomyelinase)], a protein that is generated by ASMase (acid SMase) and acts on serum LDL. An increased macrophage secretion seemed to be responsible for the elevated S-SMase activity. ASMase-deficient mice (asm^−/−/ldlr^−/−) lacked S-SMase activity and were protected from diet-induced elevation in LDL ceramide. LDL from asm^−/−/ldlr^−/− mice fed on the modified diet were less aggregated and oxidized than LDL from asm^+/+/ldlr^−/− mice. When tested in vitro, the propensity for aggregation was dependent on the SM level: only LDL from animals on modified diet that have high SM content aggregated when treated with recombinant S-SMase. In conclusion, LDL-SM content and S-SMase activity are up-regulated in mice fed on an atherogenic diet. S-SMase mediates diet-induced changes in LDL ceramide content and aggregation. S-SMase effectiveness in inducing aggregation is dependent on diet-induced enrichment of LDL with SM, possibly through increased hepatic synthesis.

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

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

Sphingolipids and atherosclerosis

The atherosclerotic lesion contains a high amount of sphingolipids, a large group of structurally diverse lipids that regulate distinct biological functions beyond their role as structural membrane components. Assessment of their role in atherogenesis has been enabled after genes that regulate their metabolism had been identified and facilitated by the more wide availability of mass spectrometry. Here we discuss recent mechanistic insights obtained in animal and epidemiological studies that have greatly enhanced our understanding of mechanisms how sphingolipids affect the atherosclerotic process.

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

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

Inhibition of serine palmitoyltransferase delays the onset of radiation-induced pulmonary fibrosis through the negative regulation of sphingosine kinase-1 expression

The enforcement of sphingosine-1-phosphate (S1P) signaling network protects from radiation-induced pneumonitis. We now demonstrate that, in contrast to early postirradiation period, late postirradiation sphingosine kinase-1 (SphK1) and sphingoid base-1-phosphates are associated with radiation-induced pulmonary fibrosis (RIF). Using the mouse model, we demonstrate that RIF is characterized by a marked upregulation of S1P and dihydrosphingosine-1-phosphate (DHS1P) levels in the lung tissue and in circulation accompanied by increased lung SphK1 expression and activity. Inhibition of sphingolipid de novo biosynthesis by targeting serine palmitoyltransferase (SPT) with myriocin reduced radiation-induced pulmonary inflammation and delayed the onset of RIF as evidenced by increased animal lifespan and decreased expression of markers of fibrogenesis, such as collagen and α-smooth muscle actin (α-SMA), in the lung. Long-term inhibition of SPT also decreased radiation-induced SphK activity in the lung and the levels of S1P-DHS1P in the lung tissue and in circulation. In vitro, inhibition or silencing of serine palmitoyltransferase attenuated transforming growth factor-β1 (TGF-β)-induced upregulation of α-SMA through the negative regulation of SphK1 expression in normal human lung fibroblasts. These data demonstrate a novel role for SPT in regulating TGF-β signaling and fibrogenesis that is linked to the regulation of SphK1 expression and S1P-DHS1P formation.

A ceramide-centric view of insulin resistance

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

Myriocin-mediated up-regulation of hepatocyte apoA-I synthesis is associated with ERK inhibition

Sphingolipids including sphingomyelin have been implicated as potential atherogenic lipids. Studies in apoE (apolipoprotein E)-null mice have revealed that the serine palmitoyltransferase inhibitor myriocin reduces plasma levels of sphingomyelin, ceramide, sphingosine-1-phosphate and glycosphingolipids and that this is associated with potent inhibition of atherosclerosis. Interestingly, hepatic apoA-I (apolipoprotein A-I) synthesis and plasma HDL (high-density lipoprotein)-cholesterol levels were also increased in apoE-null mice treated with myriocin. Since myriocin is a known inhibitor of ERK (extracellular-signal-related kinase) phosphorylation, we assessed the possibility that myriocin may be acting to increase hepatic apoA-I production via this pathway. To address this, HepG2 cells and primary mouse hepatocytes were treated with 200 muM myriocin for up to 48 h. Myriocin increased apoA-I mRNA and protein levels by approx. 3- and 2-fold respectively. Myriocin also increased apoA-I secretion up to 3.5-fold and decreased ERK phosphorylation by approx. 70%. Similar findings were obtained when primary hepatocytes were isolated from apoE-null mice that were treated with myriocin (intraperitoneal injection at a dose of 0.3 mg/kg body weight). Further experiments revealed that the MEK (mitogen-activated protein kinase/ERK kinase) inhibitor PD98059 potently inhibited ERK phosphorylation, as expected, and increased primary hepatocyte apoA-I production by 3-fold. These results indicate that ERK phosphorylation plays a role in regulating hepatic apoA-I expression and suggest that the anti-atherogenic mechanism of action for myriocin may be linked to this pathway.

Inhibition of glycosphingolipid synthesis induces a profound reduction of plasma cholesterol and inhibits atherosclerosis development in APOE*3 Leiden and low-density lipoprotein receptor-/- mice

OBJECTIVE

The iminosugar N-(5'-adamantane-1'-yl-methoxy)-pentyl-1-deoxynoijirimycin (AMP-DNM), an inhibitor of the enzyme glucosylceramide synthase catalyzing glycosphingolipid (GSL) biosynthesis, ameliorates diabetes and reduces liver steatosis in ob/ob mice. Because an accumulation of sphingolipids, including sphingomyelin and GSLs, has been reported in atherosclerotic lesions in animal models and in humans, the objective of this study was to determine whether AMP-DNM also exerts beneficial effects on the development of atherosclerosis.

METHODS AND RESULTS

APOE*3 Leiden mice, maintained on a high-cholesterol diet, were treated for up to 18 weeks with AMP-DNM. The iminosugar prevented hyperlipidemia, generated a less atherogenic lipid profile, and induced a dramatic reduction in the development of atherosclerotic lesions. At the highest dose, no lesions were detectable. The effect of AMP-DNM was associated with a decrease in liver cholesterol, an increase in bile secretion, and enhanced excretion of cholesterol in the feces. Similar effects of AMP-DNM were observed in mice deficient for the low-density lipoprotein receptor.

CONCLUSION

By lowering plasma cholesterol, the iminosugar AMP-DNM dramatically reduces the development of atherosclerosis in APOE*3 Leiden and low-density lipoprotein receptor -/- mice. Thus, targeting GSL synthesis may be a new treatment modality to prevent cardiovascular disease.

Lipid oversupply, selective insulin resistance, and lipotoxicity: molecular mechanisms

The accumulation of fat in tissues not suited for lipid storage has deleterious consequences on organ function, leading to cellular damage that underlies diabetes, heart disease, and hypertension. To combat these lipotoxic events, several therapeutics improve insulin sensitivity and/or ameliorate features of metabolic disease by limiting the inappropriate deposition of fat in peripheral tissues (i.e. thiazolidinediones, metformin, and statins). Recent advances in genomics and lipidomics have accelerated progress towards understanding the pathogenic events associated with the excessive production, underutilization, or inefficient storage of fat. Herein we review studies applying pharmacological or genetic strategies to manipulate the expression or activity of enzymes controlling lipid deposition, in order to gain a clearer understanding of the molecular mechanisms by which fatty acids contribute to metabolic disease.

Thieno[2,3-c]isoquinolin-5-one, a potent poly(ADP-ribose) polymerase inhibitor, promotes atherosclerotic plaque regression in high-fat diet-fed apolipoprotein E-deficient mice: effects on inflammatory markers and lipid content

We recently showed that poly(ADP-ribose) polymerase (PARP) is activated within atherosclerotic plaques in an animal model of atherosclerosis. Pharmacological inhibition of PARP or reduced expression in heterozygous animals interferes with atherogenesis and may promote factors of plaque stability, possibly reflecting changes in inflammatory and cellular factors consistent with plaque stability. The current study addresses the hypothesis that pharmacological inhibition of PARP promotes atherosclerotic plaque regression. Using a high-fat diet-induced atherosclerosis apolipoprotein E(-/-) mouse model, we demonstrate that administration of the potent PARP inhibitor, thieno[2,3-c]isoquinolin-5-one (TIQ-A), when combined with a regular diet regimen during treatment, induced regression of established plaques. Plaque regression was associated with a reduction in total cholesterol and low-density lipoproteins. Furthermore, plaques of TIQ-A-treated mice were highly enriched with collagen and smooth muscle cells, displayed thick fibrous caps, and exhibited a marked reduction in CD68-positive macrophage recruitment and associated foam cell presence. These changes correlated with a significant decrease in expression of monocyte chemoattractant protein-1 and intercellular cell adhesion molecule-1, potentially as a result of a robust reduction in tumor necrosis factor expression. The PARP inhibitor appeared to affect cholesterol metabolism by affecting acyl-coenzymeA/cholesterol acyltransferase-1 expression but exerted no effect on cholesterol influx or efflux as assessed by an examination of the ATP-binding cassette transporter-1 and the scavenger receptor-A expression levels in the different experimental groups. In accordance, PARP inhibition may prove beneficial not only in preventing atherogenesis but also in promoting regression of preexisting plaques.

SPTLC1 binds ABCA1 to negatively regulate trafficking and cholesterol efflux activity of the transporter

ABCA1 transport of cholesterol and phospholipids to nascent HDL particles plays a central role in lipoprotein metabolism and macrophage cholesterol homeostasis. ABCA1 activity is regulated both at the transcriptional level and at the post-translational level. To explore mechanisms involved in the post-translational regulation of the transporter, we have used affinity purification and mass spectrometry to identify proteins that bind ABCA1 and influence its activity. Previously, we demonstrated that an interaction between beta1-syntrophin stimulated ABCA1 activity, at least in part, be slowing the degradation of the transporter. This work demonstrates that one subunit of the serine palmitoyltransferase enzyme, SPTLC1, but not subunit 2 (SPTLC2), is copurified with ABCA1 and negatively regulates its function. In human THP-I macrophages and in mouse liver, the ABCA1-SPTLC1 complex was detected by co-immunoprecipitation, demonstrating that the interaction occurs in cellular settings where ABCA1 activity is critical for HDL genesis. Pharmacologic inhibition of SPTLC1 with myriocin, which resulted in the disruption of the SPTLC1-ABCA1 complex, and siRNA knockdown of SPTLC1 expression both stimulated ABCA1 efflux by nearly 60% ( p < 0.05). In contrast, dominant-negative mutants of SPTLC1 inhibited ABCA1 efflux, indicating that a reduced level of sphingomyelin synthesis could not explain the effect of myriocin on ABCA1 activity. In 293 cells, the SPTLC1 inhibition of ABCA1 activity led to the blockade of the exit of ABCA1 from the endoplasmic reticulum. In contrast, myriocin treatment of macrophages increased the level of cell surface ABCA1. In composite, these results indicate that the physical interaction of ABCA1 and SPTLC1 results in reduction of ABCA1 activity and that inhibition of this interaction produces enhanced cholesterol efflux.

Biodiversity of sphingoid bases ("sphingosines") and related amino alcohols

"Sphingosin" was first described by J. L. W. Thudichum in 1884 and structurally characterized as 2S,3R,4E-2-aminooctadec-4-ene-1,3-diol in 1947 by Herb Carter, who also proposed the designation of "lipides derived from sphingosine as sphingolipides." This category of amino alcohols is now known to encompass hundreds of compounds that are referred to as sphingoid bases and sphingoid base-like compounds, which vary in chain length, number, position, and stereochemistry of double bonds, hydroxyl groups, and other functionalities. Some have especially intriguing features, such as the tail-to-tail combination of two sphingoid bases in the alpha,omega-sphingoids produced by sponges. Most of these compounds participate in cell structure and regulation, and some (such as the fumonisins) disrupt normal sphingolipid metabolism and cause plant and animal disease. Many of the naturally occurring and synthetic sphingoid bases are cytotoxic for cancer cells and pathogenic microorganisms or have other potentially useful bioactivities; hence, they offer promise as pharmaceutical leads. This thematic review gives an overview of the biodiversity of the backbones of sphingolipids and the broader field of naturally occurring and synthetic sphingoid base-like compounds.

Reduction of plasma glycosphingolipid levels has no impact on atherosclerosis in apolipoprotein E-null mice

Glycosphingolipids (GSLs) have been implicated as potential atherogenic lipids. Studies in apolipoprotein E-null (apoE(-/-)) mice indicate that exacerbated tissue GSL accumulation resulting from alpha-galactosidase deficiency promotes atherosclerosis, whereas the serine palmitoyl transferase inhibitor myriocin (which reduces plasma and tissue levels of several sphingolipids, including sphingomyelin, ceramide, sphingosine-1-phosphate, and GSLs) inhibits atherosclerosis. It is not clear whether GSL synthesis inhibition per se has an impact on atherosclerosis. To address this issue, apoE(-/-) mice maintained on a high-fat diet were treated with a potent glucosylceramide synthesis inhibitor, d-threo-1-ethylendioxyphenyl-2-palmitoylamino-3-pyrrolidino-propanol (EtDO-P4), 10 mg/kg/day for 94 days, and lesion development was compared in mice that were treated with vehicle only. EtDO-P4 reduced plasma GSL concentration by approximately 50% but did not affect cholesterol or triglyceride levels. Assessment of atherosclerotic lesions at four different sites indicated that EtDO-P4 had no significant impact on lesion area. Thus, despite the previously observed positive correlations between plasma and aortic GSL concentrations and the development of atherosclerosis, and the in vitro evidence implying that GSLs may be pro-atherogenic, our current data indicate that inhibition of GSL synthesis does not inhibit atherosclerosis in vivo.