Sphingolipids: players in the pathology of metabolic disease

The role of sphingolipids in drug metabolism and transport

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

Dysfunctional pro-ceramide, ER stress, and insulin/IGF signaling networks with progression of Alzheimer's disease

In Alzheimer's disease (AD), brain insulin and insulin-like growth factor (IGF) resistance and deficiency begin early, and worsen with severity of disease. The factors mediating progression of brain insulin/IGF resistance in AD are not well understood. We hypothesize that AD progression is mediated via negative cross-talk that promotes toxic ceramide generation and endoplasmic reticulum (ER) stress. The rationale is that insulin resistance dysregulates lipid metabolism and promotes ceramide accumulation, and thereby increases inflammation and stress. Consequences include disruption of cytoskeletal function and AβPP-Aβ secretion. The present study correlates AD stage with activation of pro-ceramide genes, ceramide levels, and molecular indices of ER stress in postmortem human brain tissue. The results demonstrated that in AD, brain insulin/IGF resistance was associated with constitutive activation of multiple pro-ceramide genes, increased ceramide levels, and increased expression of pro-ER stress pathway genes and proteins. Expression of several pro-ceramide and pro-apoptotic ER stress pathway molecules increased with AD severity and brain insulin/IGF resistance. In contrast, ER stress molecules that help maintain homeostasis with respect to unfolded protein responses were mainly upregulated in the intermediate rather than late stage of AD. These findings support our hypothesis that in AD, a triangulated mal-signaling network initiated by brain insulin/IGF resistance is propagated by the dysregulation of ceramide and ER stress homeostasis, which themselves promote insulin resistance. Therefore, once established, this reverberating loop must be targeted using multi-pronged approaches to disrupt the AD neurodegeneration cascade.

OSBP-related proteins (ORPs) in human adipose depots and cultured adipocytes: evidence for impacts on the adipocyte phenotype

Oxysterol-binding protein (OSBP) homologues, ORPs, are implicated in lipid homeostatic control, vesicle transport, and cell signaling. We analyzed here the quantity of ORP mRNAs in human subcutaneous (s.c.) and visceral adipose depots, as well as in the Simpson-Golabi-Behmel syndrome (SGBS) adipocyte cell model. All of the ORP mRNAs were present in the s.c and visceral adipose tissues, and the two depots shared an almost identical ORP mRNA expression pattern. SGBS adipocytes displayed a similar pattern, suggesting that the adipose tissue ORP expression pattern mainly derives from adipocytes. During SGBS cell adipogenic differentiation, ORP2, ORP3, ORP4, ORP7, and ORP8 mRNAs were down-regulated, while ORP11 was induced. To assess the impacts of ORPs on adipocyte differentiation, ORP3 and ORP8, proteins down-regulated during adipogenesis, were overexpressed in differentiating SGBS adipocytes, while ORP11, a protein induced during adipogenesis, was silenced. ORP8 overexpression resulted in reduced expression of the aP2 mRNA, while down-regulation of adiponectin and aP2 was observed in ORP11 silenced cells. Furthermore, ORP8 overexpression or silencing of ORP11 markedly decreased cellular triglyceride storage. These data identify the patterns of ORP expression in human adipose depots and SGBS adipocytes, and provide the first evidence for a functional impact of ORPs on the adipocyte phenotype.

Palmitate promotes monocyte atherogenicity via de novo ceramide synthesis

Elevated plasma free fatty acids (FAs) are associated with increased risk of cardiovascular disease. This study investigates the effects of the saturated FA palmitate and unsaturated FA oleate on monocyte phenotype and function. Incubation of human U937 and THP-1 monocytes with palmitate for 24h increased cell surface expression of integrin CD11b and scavenger receptor CD36 in a concentration-dependent manner with some decrease in mitochondrial reducing capacity at high concentration (300 μM). Monocytes incubated with palmitate, but not oleate, showed increased uptake of oxidized LDL and increased adhesion to rat aortic endothelium, particularly at bifurcations. The palmitate-induced increase in CD11b and CD36 expression was associated with increased cellular C16 ceramide and sphingomyelin, loss of reduced glutathione, and increased reactive oxygen species (ROS). Increased monocyte surface CD11b and CD36 was inhibited by fumonisin B1, an inhibitor of de novo ceramide synthesis, but not by the superoxide dismutase mimetic MnTBap. In contrast, MnTBap prevented the mitochondrial ROS increase and metabolic inhibition due to 300μM palmitate. This study demonstrates that in viable monocytes, palmitate but not oleate increases expression of surface CD11b and CD36. Palmitate increases monocyte adhesion to the aortic wall and promotes uptake of oxidized LDL and this involves de novo ceramide synthesis.

Insulin resistance, ceramide accumulation, and endoplasmic reticulum stress in human chronic alcohol-related liver disease

Background. Chronic alcohol-related liver disease (ALD) is mediated by insulin resistance, mitochondrial dysfunction, inflammation, oxidative stress, and DNA damage. Recent studies suggest that dysregulated lipid metabolism with accumulation of ceramides, together with ER stress potentiate hepatic insulin resistance and may cause steatohepatitis to progress. Objective. We examined the degree to which hepatic insulin resistance in advanced human ALD is correlated with ER stress, dysregulated lipid metabolism, and ceramide accumulation. Methods. We assessed the integrity of insulin signaling through the Akt pathway and measured proceramide and ER stress gene expression, ER stress signaling proteins, and ceramide profiles in liver tissue. Results. Chronic ALD was associated with increased expression of insulin, IGF-1, and IGF-2 receptors, impaired signaling through IGF-1R and IRS1, increased expression of multiple proceramide and ER stress genes and proteins, and higher levels of the C14, C16, C18, and C20 ceramide species relative to control. Conclusions. In human chronic ALD, persistent hepatic insulin resistance is associated with dysregulated lipid metabolism, ceramide accumulation, and striking upregulation of multiple ER stress signaling molecules. Given the role of ceramides as mediators of ER stress and insulin resistance, treatment with ceramide enzyme inhibitors may help reverse or halt progression of chronic ALD.

High fat diet induced hepatic steatosis and insulin resistance: Role of dysregulated ceramide metabolism

AIM

  Non-alcoholic fatty liver disease (NAFLD) is an insulin resistance disease that can progress to cirrhosis and liver failure. We hypothesized that in NAFLD, insulin resistance dysregulates lipid metabolism, increasing production of cytotoxic lipids including ceramides, which exacerbate hepatic insulin resistance and injury.

METHODS

  Long Evans rats were pair-fed low (LFD) or high (HFD) fat diets for 8 weeks. Livers were used to measure lipids, gene expression, insulin receptor binding, integrity of insulin signaling, and pro-inflammatory cytokines. In vitro experiments characterized effects of ceramides on Huh7 cell viability, mitochondrial function, and insulin signaling.

RESULTS

  High fat diet feeding caused NAFLD with peripheral and hepatic insulin resistance, increased hepatic expression of pro-ceramide genes, sphingomyelinase activity, and lipid peroxidation, and increased serum ceramide. Ceramide treatment impaired Huh7 cell viability, mitochondrial function, and insulin signaling.

CONCLUSIONS

  Increased hepatic ceramide generation and release may mediate both hepatic and peripheral insulin resistance in NAFLD.

Insulin-sensitive obesity in humans - a 'favorable fat' phenotype?

In most humans, obesity and insulin resistance coexist. However, a unique group of obese individuals, who exhibit better insulin sensitivity than expected for their adiposity, has been the focus of recent research interest. We critically examine cross-sectional and lifestyle intervention studies in obese humans classified as 'insulin-sensitive' versus 'insulin-resistant' and review the few longitudinal studies comparing rates of cardiovascular disease, type 2 diabetes and all-cause mortality in these groups of individuals. We suggest that reduced deposition of fat, particularly of bioactive lipid intermediates, in muscle and liver is potentially protective. We propose that dynamic interventional studies in insulin-sensitive obese humans may increase understanding of the metabolic factors that play a role in obesity-associated insulin resistance in humans.

Quantitation of multiple sphingolipid classes using normal and reversed-phase LC-ESI-MS/MS: comparative profiling of two cell lines

Sphingolipids are an important class of compounds that regulate signal transduction and other vital cellular processes. Herein, we report sensitive normal and reversed phase LC-MS/MS methods for quantitation of multiple sphingolipid classes. In the normal-phase ESI/MS/MS method, a high content of organic solvents was utilized, which, although it included hexane, ethyl acetate, acetonitrile containing 2% methanol, 1-2% acetic acid, and 5 mM ammonium acetate, resulted in a very efficient electrospray ionization of the ceramides (Cers) and hexosylceramides (MHCers). Three normal-phase LC-MS/MS methods using segmented phases were developed to specifically target Cers, MHCers, or sphingomyelins (SMs). This segmentation scheme increases the number of data points acquired for a given analyte and enhances the sensitivity and specificity of the measurements. Nine separate reversed phase chromatography methods were developed for the three classes of compounds. These assays were used for comparing the levels of Cers, SMs, and MHCers from mouse embryonic fibroblast (pMEF) and human embryonic kidney (HEK293) cells. These findings were then compared with the reported data from RAW264.7 mouse macrophage cells, BHK21 hamster cells, and human plasma and serum samples. The analysis of cell lines, using both normal and reversed phase chromatography, revealed discrimination based on the type of chromatography chosen, while sphingolipid assays of samples containing different amounts of protein showed different results, even after normalizing for protein content. Also, LC/MS/MS profiles were provided for the classes and individual compounds so that they could be used as "molecular profiles" for class or individual sample analysis.

Plasma deoxysphingolipids: a novel class of biomarkers for the metabolic syndrome?

AIMS/HYPOTHESIS

Sphingolipid synthesis is typically initiated by the conjugation of L-serine and palmitoyl-CoA, a reaction catalysed by serine palmitoyltransferase (SPT). SPT can also metabolise other acyl-CoAs (C(12) to C(18)) and other amino acids such as L-alanine and glycine, giving rise to a spectrum of atypical sphingolipids. Here, we aimed to identify changes in plasma levels of these atypical sphingolipids to explore their potential as biomarkers in the metabolic syndrome and diabetes.

METHODS

We compared the plasma profiles of ten sphingoid bases in healthy individuals with those of patients with the metabolic syndrome but not diabetes, and diabetic patients (n = 25 per group). The results were verified in a streptozotocin (STZ) rat model. Univariate and multivariate statistical analyses were used.

RESULTS

Deoxysphingolipids (dSLs) were significantly elevated (p = 5 × 10⁻⁶) in patients with the metabolic syndrome (0.11 ± 0.04 μmol/l) compared with controls (0.06 ± 0.02 μmol/l) but did not differ between the metabolic syndrome and diabetes groups. Levels of C(16)-sphingosine-based sphingolipids were significantly lowered in diabetic patients but not in patients with the metabolic syndrome but without diabetes (p = 0.008). Significantly elevated dSL levels were also found in the plasma and liver of STZ rats. A principal component analysis revealed a similar or even closer association of dSLs with diabetes and the metabolic syndrome in comparison with the established biomarkers.

CONCLUSIONS/INTERPRETATION

We showed that dSLs are significantly elevated in patients with type 2 diabetes mellitus and non-diabetic metabolic syndrome compared with healthy controls. They may, therefore, be useful novel biomarkers to improve risk prediction and therapy monitoring in these patients.

Moving beyond "good fat, bad fat": the complex roles of dietary lipids in cellular function and health: session abstracts

The International Life Science Institute North America and the American Society for Nutrition annual Functional Foods for Health Symposium was held 9 April 2011. Evidence that foods and their components offer health benefits beyond basic nutrition continues to captivate the interest of the scientific community, government agencies, and the general public. This paper is comprised of extended abstracts from the session and addresses issues related to emerging lipid nutrition science, including active roles of lipids in modulating physiological pathways. Identified pathways underlie the development of obesity, cognitive development, and inflammation, the latter of which is thought to relate to multiple disease processes. These data point to a new way of thinking about the role of lipids in health and disease.

Blood sphingolipids in homeostasis and pathobiology

Sphingolipids have emerged as key signaling molecules involved in the regulation of a variety of cellular functions including cell growth and differentiation, proliferation and apoptotic cell death. Sphingolipids in blood constitute part of the circulating lipoprotein particles (HDL, LDL and VLDL), carried by serum albumin and also present in blood cells and platelets. Recent lipidomic and proteomic studies of plasma lipoproteins have provided intriguing data concerning the protein and lipid composition of lipoproteins in the context of disease. Sphingolipids have been implicated in several diseases such as cancer, obesity, atherosclerosis and sphingolipidoses; however, efforts addressing blood sphingolipidomics are still limited. The development of methods to determine levels of circulating bioactive sphingolipids in humans and validation of these methods to be a routine clinical laboratory test could be a pioneering approach to diagnose disease in the population. This approach would probably evolve to be analogous in implication to determining "good" and "bad" cholesterol and triglyceride levels in lipoprotein classes.

Classifying human brain tumors by lipid imaging with mass spectrometry

Brain tissue biopsies are required to histologically diagnose brain tumors, but current approaches are limited by tissue characterization at the time of surgery. Emerging technologies such as mass spectrometry imaging can enable a rapid direct analysis of cancerous tissue based on molecular composition. Here, we illustrate how gliomas can be rapidly classified by desorption electrospray ionization-mass spectrometry (DESI-MS) imaging, multivariate statistical analysis, and machine learning. DESI-MS imaging was carried out on 36 human glioma samples, including oligodendroglioma, astrocytoma, and oligoastrocytoma, all of different histologic grades and varied tumor cell concentration. Gray and white matter from glial tumors were readily discriminated and detailed diagnostic information could be provided. Classifiers for subtype, grade, and concentration features generated with lipidomic data showed high recognition capability with more than 97% cross-validation. Specimen classification in an independent validation set agreed with expert histopathology diagnosis for 79% of tested features. Together, our findings offer proof of concept that intraoperative examination and classification of brain tissue by mass spectrometry can provide surgeons, pathologists, and oncologists with critical and previously unavailable information to rapidly guide surgical resections that can improve management of patients with malignant brain tumors.

Time-dependent changes in lipid metabolism in mice with methionine choline deficiency-induced fatty liver disease

Methionine and choline-deficient diet (MCD)-induced fatty liver is one of the best-studied animal models of fatty liver disease. The present study was performed to clarify the relative contributions of individual lipid metabolic pathways to the pathogenesis of MCD-induced fatty liver. Hepatic lipogenesis mediated by the sterol regulatory element-binding protein (SREBP-1c) was increased at 1 week, but not at 6 weeks, of MCD feeding. On the other hand, (14)C-palmitate oxidation did not change at 1 week, but significantly decreased at 6 weeks. This decrease was associated with increased expression of fatty acid translocase, a key enzyme involved in fatty acid uptake. Expression of endoplasmic reticulum stress markers was increased in mice given MCD for both 1 and 6 weeks. These findings suggest the presence of time-dependent differences in lipid metabolism in MCD-induced fatty liver disease: SREBP-1c-mediated lipogenesis is important in the early stages of fatty liver disease, whereas increased fatty acid uptake and decreased fatty acid oxidation become more important in the later stages.

Imaging of phospholipids in formalin fixed rat brain sections by matrix assisted laser desorption/ionization mass spectrometry

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is a valuable tool for the analysis of molecules directly from tissue. Imaging of phospholipids is gaining widespread interest, particularly as these lipids have been implicated in a variety of pathologic processes. Formalin fixation (FF) is the standard protocol used in histology laboratories worldwide to preserve tissue for analysis, in order to aid in the diagnosis and prognosis of diseases. This study assesses MALDI imaging of phospholipids directly in formalin fixed tissue, with a view to future analysis of archival tissue. This investigation proves the viability of MALDI-MSI for studying the distribution of lipids directly in formalin fixed tissue, without any pretreatment protocols. High quality molecular images for several phosphatidylcholine (PC) and sphingomyelin (SM) species are presented. Images correspond well with previously published data for the analysis of lipids directly from freshly prepared tissue. Different ionization pathways are observed when analyzing fixed tissue compared with fresh, and this change was found to be associated with formalin buffers employed in fixation protocols. The ability to analyze lipids directly from formalin fixed tissue opens up new doors in the investigation of disease profiles. Pathologic specimens taken for histologic investigation can be analyzed by MALDI-MS to provide greater information on the involvement of lipids in diseased tissue.

Discovery and evaluation of inhibitors of human ceramidase

The ceramide/sphingosine-1-phosphate (S1P) rheostat has been hypothesized to play a critical role in regulating tumor cell fate, with elevated levels of ceramide inducing death and elevated levels of S1P leading to survival and proliferation. Ceramidases are key enzymes that control this rheostat by hydrolyzing ceramide to produce sphingosine and may also confer resistance to drugs and radiation. Therefore, ceramidase inhibitors have excellent potential for development as new anticancer drugs. In this study, we identify a novel ceramidase inhibitor (Ceranib-1) by screening a small molecule library and describe the synthesis of a more potent analogue (Ceranib-2). In a cell-based assay, both compounds were found to inhibit cellular ceramidase activity toward an exogenous ceramide analogue, induce the accumulation of multiple ceramide species, decrease levels of sphingosine and S1P, inhibit the proliferation of cells alone and in combination with paclitaxel, and induce cell-cycle arrest and cell death. In vivo, Ceranib-2 was found to delay tumor growth in a syngeneic tumor model without hematologic suppression or overt signs of toxicity. These data support the selection of ceramidases as suitable targets for anticancer drug development and provide the first nonlipid inhibitors of human ceramidase activity.

Desorption electrospray ionization mass spectrometry for lipid characterization and biological tissue imaging

Desorption electrospray ionization mass spectrometry (DESI-MS) imaging of biological samples allows untargeted analysis and structural characterization of lipids ionized from the near-surface region of a sample under ambient conditions. DESI is a powerful and sensitive MS ionization method for 2D and 3D imaging of lipids from direct and unmodified complex biological samples. This review describes the strengths and limitations of DESI-MS for lipid characterization and imaging together with the technical workflow and a survey of applications. Included are discussions of lipid mapping and biomarker discovery as well as a perspective on the future of DESI imaging.

A view on sphingolipids and disease

Sphingolipid and glycosphingolipid levels and expression of sphingolipid metabolizing enzymes are altered in a variety of diseases or in response to drug treatment. Inherited defects of enzymes and other proteins required for the lysosomal degradation of these lipids lead to human sphingolipidoses. Also genetic defects that affect sphingolipid biosynthesis are known. Although the molecular details are often far from clear, (glyco)sphingolipids have been implicated to play a role in atherosclerosis, insulin resistance, cancer, and infections by pathogens. More general aspects of selected diseases are discussed.

Identification of a potential biomarker for FABP4 inhibition: the power of lipidomics in preclinical drug testing

The fatty acid binding protein 4 (FABP4) belongs to the family of lipid chaperones that control intracellular fluxes and compartmentalization of their respective ligands (e.g., fatty acids). FABP4, which is almost exclusively expressed in adipocytes and macrophages, contributes to the development of insulin resistance and atherosclerosis in mice. Lack of FABP4 protects against the development of insulin resistance associated with genetic or diet-induced obesity in mice. Furthermore, total or macrophage-specific FABP4 deficiency is protective against atherosclerosis in apolipoprotein E-deficient mice. The FABP4 small-molecule inhibitor BMS309403 has demonstrated efficacy in mouse models for type 2 diabetes mellitus and atherosclerosis, resembling phenotypes of mice with FABP4 deficiency. However, despite the therapeutically attractive long-term effects of FABP4 inhibition, an acute biomarker for drug action is lacking. The authors applied mass spectrometry lipidomics analysis to in vitro and in vivo (plasma and adipose tissue) samples upon inhibitor treatment. They report the identification of a potential biomarker for acute in vivo FABP4 inhibition that is applicable for further investigations and can be implemented in simple and fast-flow injection mass spectrometry assays. In addition, this approach can be considered a proof-of-principle study that can be applied to other lipid-pathway targeting mechanisms.

Inflammatory induction of human resistin causes insulin resistance in endotoxemic mice

OBJECTIVE

Although adipocyte-derived murine resistin links insulin resistance to obesity, the role of human resistin, predominantly expressed in mononuclear cells and induced by inflammatory signals, remains unclear. Given the mounting evidence that obesity and type 2 diabetes are inflammatory diseases, we sought to determine the relationship between inflammatory increases in human resistin and insulin resistance.

RESEARCH DESIGN AND METHODS

To investigate the role of human resistin on glucose homeostasis in inflammatory states, we generated mice lacking murine resistin but transgenic for a bacterial artificial chromosome containing human resistin (BAC-Retn), whose expression was similar to that in humans. The metabolic and molecular phenotypes of BAC-Retn mice were assessed after acute and chronic endotoxemia (i.e., exposure to inflammatory lipopolysaccharide).

RESULTS

We found that BAC-Retn mice have circulating resistin levels within the normal human range, and similar to humans, lipopolysaccharide markedly increased serum resistin levels. Acute endotoxemia caused hypoglycemia in mice lacking murine resistin, and this was attenuated in BAC-Retn mice. In addition, BAC-Retn mice developed severe hepatic insulin resistance under chronic endotoxemia, accompanied by increased inflammatory responses in liver and skeletal muscle.

CONCLUSIONS

These results strongly support the role of human resistin in the development of insulin resistance in inflammation. Thus, human resistin may link insulin resistance to inflammatory diseases such as obesity, type 2 diabetes, and atherosclerosis.

Sphingolipid metabolism and analysis in metabolic disease

Sphingolipids are an important class of structural and signaling molecules within the cell. As sphingolipids have been implicated in the development and pathogenesis of insulin resistance and the metabolic syndrome, it is important to understand their regulation and metabolism. Although these lipids are initially produced through a common pathway, there is no "generic" sphingolipid. Indeed, the biophysical and signaling properties of lipids may be manipulated by the subunit composition or isoform of their synthetic enzymes, via regulation of substrate integration. Functionally distinct pools of chemically-equivalent lipids may also be generated by de novo synthesis and recycling of existing complex sphingolipids. The highly integrated metabolism of the many bioactive sphingolipids means that manipulation of one enzyme or metabolite can result in a ripple effect, causing unforeseen changes in metabolite levels, enzyme activities, and cellular programmes. Fortunately, a suite of techniques, ranging from thin-layer chromatography to liquid chromatography-mass spectrometry approaches, allows investigators to undertake a functional characterization of all or part of the sphingolipidome in their systems of interest.

Hepatic accumulation of intestinal cholesterol is decreased and fecal cholesterol excretion is increased in mice fed a high-fat diet supplemented with milk phospholipids

Background

Milk phospholipids (PLs) reduce liver lipid levels when given as a dietary supplement to mice fed a high-fat diet. We have speculated that this might be due to reduced intestinal cholesterol uptake.

Methods

Mice were given a high-fat diet for 3 or 5 weeks that had no added PL or that were supplemented with 1.2% by wt PL from cow's milk. Two milk PL preparations were investigated: a) a PL-rich dairy milk extract (PLRDME), and b) a commercially-available milk PL concentrate (PC-700). Intestinal cholesterol uptake was assessed by measuring fecal and hepatic radioactivity after intragastric administration of [¹⁴C]cholesterol and [³H]sitostanol. Fecal and hepatic lipids were measured enzymatically and by ESI-MS/MS.

Results

Both PL preparations led to significant decreases in total liver cholesterol and triglyceride (-20% to -60%, P < 0.05). Hepatic accumulation of intragastrically-administered [¹⁴C]cholesterol was significantly less (-30% to -60%, P < 0.05) and fecal excretion of [¹⁴C]cholesterol and unlabeled cholesterol was significantly higher in PL-supplemented mice (+15% to +30%, P < 0.05). Liver cholesterol and triglyceride levels were positively correlated with hepatic accumulation of intragastrically-administered [¹⁴C]cholesterol (P < 0.001) and negatively correlated with fecal excretion of [¹⁴C]cholesterol (P < 0.05). Increased PL and ceramide levels in the diet of mice supplemented with milk PL were associated with significantly higher levels of fecal PL and ceramide excretion, but reduced levels of hepatic PL and ceramide, specifically, phosphatidylcholine (-21%, P < 0.05) and monohexosylceramide (-33%, P < 0.01).

Conclusion

These results indicate that milk PL extracts reduce hepatic accumulation of intestinal cholesterol and increase fecal cholesterol excretion when given to mice fed a high-fat diet.

Anti-apoptosis and cell survival: a review

Type I programmed cell death (PCD) or apoptosis is critical for cellular self-destruction for a variety of processes such as development or the prevention of oncogenic transformation. Alternative forms, including type II (autophagy) and type III (necrotic) represent the other major types of PCD that also serve to trigger cell death. PCD must be tightly controlled since disregulated cell death is involved in the development of a large number of different pathologies. To counter the multitude of processes that are capable of triggering death, cells have devised a large number of cellular processes that serve to prevent inappropriate or premature PCD. These cell survival strategies involve a myriad of coordinated and systematic physiological and genetic changes that serve to ward off death. Here we will discuss the different strategies that are used to prevent cell death and focus on illustrating that although anti-apoptosis and cellular survival serve to counteract PCD, they are nevertheless mechanistically distinct from the processes that regulate cell death.

Cardiac diacylglycerol accumulation in high fat-fed mice is associated with impaired insulin-stimulated glucose oxidation

AIMS

the molecular processes leading to cardiac insulin resistance induced via a high-fat diet (HFD) remain unclear. We examined the changes in cardiac insulin sensitivity and the potential mechanism(s) involved following HFD in mice.

METHODS AND RESULTS

C57BL/6 mice were fed either a low-fat diet (LFD, 4% kcal fat) or a HFD (60% kcal fat) for 3 or 10 weeks. Insulin-stimulated glucose oxidation in isolated working hearts was decreased at 10 weeks of HFD compared with mice on LFD (249 ± 19 to 399 ± 46 vs. 551 ± 97 to 1464 ± 243 nmol/g dry wt/min; P < 0.05). The accumulation of myocardial diacylglycerol (DAG; 479 ± 174 vs. 266 ± 29 micromol/g wet wt; P < 0.05), but not long-chain acyl CoA, ceramide, or triacylglycerol, correlated with the development of insulin resistance. The accumulation of DAG occurred concomitantly with an increase in glycerol phosphate acyltransferase activity, a decrease in DAG acyltransferase activity, as well as an increase in the translocation of protein kinase C-α (PKCα) and phosphorylation of p70s6k. Neither HFD-induced accumulation of cardiac DAG nor up-regulation of phosphorylated p70s6k occurred in mice lacking malonyl CoA decarboxylase which are resistant to the development of HFD-induced insulin resistance.

CONCLUSION

the activation of myocardial p70s6k and PKCα is closely associated with cardiac insulin resistance in which the accumulation of intra-myocardial DAG could be responsible.

Blood sphingolipidomics in healthy humans: impact of sample collection methodology

We used a HPLC-MS/MS methodology for determination of a basic metabolomic profile (18:1,18:0 sphingoid backbone, C(14)-C(26) N-acyl part) of "normal" sphingolipid levels in human serum and plasma. Blood was collected from healthy males and nonpregnant females under fasting and nonfasting conditions with and without anticoagulants. Sphingolipids analyzed included sphingoid bases, sphingosine and dihydrosphingosine, their 1-phosphates (S1P and dhS1P), molecular species (C(n)-) of ceramide (Cer), sphingomyelin (SM), hexosylceramide (HexCer), lactosylceramide (LacCer), and Cer 1-phosphate (Cer1P). SM, LacCer, HexCer, Cer, and Cer1P constituted 87.7, 5.8, 3.4, 2.8, and 0.15% of total sphingolipids, respectively. The abundant circulating SM was C(16)-SM (64.0 µM), and it increased with fasting (100 µM). The abundant LacCer was C(16)-LacCer (10.0 µM) and the abundant HexCer was C(24)-HexCer (2.5 µM). The abundant Cer, C(24)-Cer (4.0 µM), was not influenced by fasting; however, levels of C(16)-C(20) Cers were decreased in response to fasting. S1P levels were higher in serum than plasma (0.68 µM vs. 0.32 µM). We also determined levels of sphingoid bases and SM species in isolated lipoprotein classes. HDL(3) was the major carrier of S1P, dhS1P, and Sph, and LDL was the major carrier of Cer and dhSph. Per particle, VLDL contained the highest levels of SM, Cer, and S1P. HPLC-MS/MS should provide a tool for clinical testing of circulating bioactive sphingolipids in human blood.

A rapid and quantitative LC-MS/MS method to profile sphingolipids

Sphingolipids comprise a highly diverse and complex class of molecules that serve not only as structural components of membranes but also as signaling molecules. To understand the differential role of sphingolipids in a regulatory network, it is important to use specific and quantitative methods. We developed a novel LC-MS/MS method for the rapid, simultaneous quantification of sphingolipid metabolites, including sphingosine, sphinganine, phyto-sphingosine, di- and trimethyl-sphingosine, sphingosylphosphorylcholine, hexosylceramide, lactosylceramide, ceramide-1-phosphate, and dihydroceramide-1-phosphate. Appropriate internal standards (ISs) were added prior to lipid extraction. In contrast to most published methods based on reversed phase chromatography, we used hydrophilic interaction liquid chromatography and achieved good peak shapes, a short analysis time of 4.5 min, and, most importantly, coelution of analytes and their respective ISs. To avoid an overestimation of species concentrations, peak areas were corrected regarding isotopic overlap where necessary. Quantification was achieved by standard addition of naturally occurring sphingolipid species to the sample matrix. The method showed excellent precision, accuracy, detection limits, and robustness. As an example, sphingolipid species were quantified in fibroblasts treated with myriocin or sphingosine-kinase inhibitor. In summary, this method represents a valuable tool to evaluate the role of sphingolipids in the regulation of cell functions.

Sphingosine 1-phosphate lyase deficiency disrupts lipid homeostasis in liver

The cleavage of sphingoid base phosphates by sphingosine-1-phosphate (S1P) lyase to produce phosphoethanolamine and a fatty aldehyde is the final degradative step in the sphingolipid metabolic pathway. We have studied mice with an inactive S1P lyase gene and have found that, in addition to the expected increase of sphingoid base phosphates, other sphingolipids (including sphingosine, ceramide, and sphingomyelin) were substantially elevated in the serum and/or liver of these mice. This latter increase is consistent with a reutilization of the sphingosine backbone for sphingolipid synthesis due to its inability to exit the sphingolipid metabolic pathway. Furthermore, the S1P lyase deficiency resulted in changes in the levels of serum and liver lipids not directly within the sphingolipid pathway, including phospholipids, triacyglycerol, diacylglycerol, and cholesterol. Even though lipids in serum and lipid storage were elevated in liver, adiposity was reduced in the S1P lyase-deficient mice. Microarray analysis of lipid metabolism genes in liver showed that the S1P lyase deficiency caused widespread changes in their expression pattern, with a significant increase in the expression of PPARgamma, a master transcriptional regulator of lipid metabolism. However, the mRNA expression of the genes encoding the sphingosine kinases and S1P phosphatases, which directly control the levels of S1P, were not significantly changed in liver of the S1P lyase-deficient mice. These results demonstrate that S1P lyase is a key regulator of the levels of multiple sphingolipid substrates and reveal functional links between the sphingolipid metabolic pathway and other lipid metabolic pathways that may be mediated by shared lipid substrates and changes in gene expression programs. The disturbance of lipid homeostasis by altered sphingolipid levels may be relevant to metabolic diseases.

Insulin resistance is associated with higher intramyocellular triglycerides in type I but not type II myocytes concomitant with higher ceramide content

OBJECTIVE

We tested the primary hypotheses that sphingolipid and diacylglycerol (DAG) content is higher within insulin-resistant muscle and that the association between intramyocellular triglycerides (IMTG) and insulin resistance is muscle fiber type specific.

RESEARCH DESIGN AND METHODS

A nested case-control analysis was conducted in 22 obese (BMI >30 kg/m(2)) women who were classified as insulin-resistant (IR; n = 12) or insulin-sensitive (IS; n = 10), determined by hyperinsulinemic-euglycemic clamp (>30% greater in IS compared with IR, P < 0.01). Sphingolipid and DAG content was determined by high-performance liquid chromatography-tandem mass spectrometry. Fiber type-specific IMTG content was histologically determined. Gene expression was determined by quantitative PCR.

RESULTS

Total (555 +/- 53 vs. 293 +/- 54 pmol/mg protein, P = 0.004), saturated (361 +/- 29 vs. 179 +/- 34 pmol/mg protein, P = 0.001), and unsaturated (198 +/- 29 vs. 114 +/- 21 pmol/mg protein, P = 0.034) ceramides were higher in IR compared with IS. DAG concentrations, however, were similar. IMTG content within type I myocytes, but not type II myocytes, was higher in IR compared with IS subjects (P = 0.005). Insulin sensitivity was negatively correlated with IMTG within type I myocytes (R = -0.51, P = 0.026), but not with IMTG within type II myocytes. The proportion of type I myocytes was lower (41 vs. 59%, P < 0.01) in IR subjects. Several genes involved in lipid droplet and fatty acid metabolism were differentially expressed in IR compared with IS subjects.

CONCLUSIONS

Human skeletal muscle insulin resistance is related to greater IMTG content in type I but not type II myocytes, to greater ceramide content, and to alterations in gene expression associated with lipid metabolism.

Torpor induction in mammals: recent discoveries fueling new ideas

When faced with a harsh climate or inadequate food, some mammals enter a state of suspended animation known as torpor. A major goal of torpor research is to determine mechanisms that integrate environmental cues, gene expression and metabolism to produce periods of torpor lasting from hours to weeks. Recent discoveries spanning the Metazoa suggest that sirtuins, the mammalian circadian clock, fibroblast growth factor 21 (FGF21) and lipids are involved in torpor induction. For example, sirtuins link cellular energy status to the mammalian circadian clock, oxidative stress and metabolic fuel selection. In this review, we discuss how these recent discoveries form a new hypothesis linking changes in the physical environment with changes in the expression of genes that regulate torpor induction.