Lipid mediators of insulin resistance: ceramide signalling down-regulates GLUT4 gene transcription in 3T3-L1 adipocytes

Adipose Tissue Dysfunction Determines Lipotoxicity and Triggers the Metabolic Syndrome: Current Challenges and Clinical Perspectives

The adipose tissue organ is organised as distinct anatomical depots located all along the body axis, and it is constituted of three different types of adipocytes: white, beige and brown, which are integrated with vascular, immune, neural, and extracellular stroma cells. These distinct adipocytes serve different specialised functions. The main function of white adipocytes is to ensure healthy storage of excess nutrients/energy and its rapid mobilisation to supply the demand of energy imposed by physiological cues in other organs, whereas brown and beige adipocytes are designed for heat production through uncoupling lipid oxidation from energy production. The concerted action of the three types of adipocytes/tissues ensures an optimal metabolic status. However, when one or several of these adipose depots become dysfunctional because of sustained lipid/nutrient overload, then insulin resistance and associated metabolic complications ensue. These metabolic alterations close a vicious cycle that negatively affects the adipose tissue functionality and compromises global metabolic homeostasis. Optimising white adipose tissue expandability and ensuring its functional metabolic flexibility and/or promoting brown/beige mediated thermogenic activity are complementary strategies that counteract obesity and its associated lipotoxic metabolic effects. However, the development of these therapeutic approaches requires a deep understanding of adipose tissue in all broad aspects. In this chapter, we will discuss the characteristics of the different adipose tissue depots with respect to origins and precursors recruitment, plasticity, cellular composition, and expandability capacity potential as well as molecular and metabolic characteristic signatures in both physiological and pathophysiological conditions. Current antilipotoxic strategies for future clinical application are also discussed in this chapter.

Cell starvation regulates ceramide-induced autophagy in mouse preimplantation embryo development

Ceramide induces autophagy upon starvation via downregulation of nutrient transporters. To elucidate the mechanism by which starvation regulates autophagy in mouse embryos, the present study investigated nutrient transporter expression and the effect of C2-ceramide on in vitro embryo development, apoptosis, and autophagy. The transcript levels of the glucose transporters Glut1 and Glut3 were high at the 1- and 2-cell stages, and gradually decreased at the morula and blastocyst (BL) stages. Similarly, expression of the amino acid transporters L-type amino transporter-1 (LAT-1) and 4F2 heavy chain (4F2hc) gradually decreased from the zygote to the BL stage. Upon ceramide treatment, expression of Glut1, Glut3, LAT-1, and 4F2hc was significantly reduced at the BL stage, while expression of the autophagy-related genes Atg5, LC3, and Gabarap and synthesis of LC3 were significantly induced. Ceramide-treated embryos exhibited significantly reduced developmental rates and total cell numbers per blastocyst, and increased levels of apoptosis and expression of Bcl2l1 and Casp3 at the BL stage. Ceramide treatment significantly decreased the average mitochondrial DNA copy number and mitochondrial area at the BL stage. In addition, ceramide treatment significantly decreased mTOR expression. These results suggest that ceramide-induced autophagy promotes apoptosis by following downregulation of nutrient transporters during mouse embryogenesis.

Ceramide synthesis regulates biogenesis and packaging of exosomal MALAT1 from adipose derived stem cells, increases dermal fibroblast migration and mitochondrial function

BACKGROUND

The function of exosomes, small extracellular vesicles (sEV) secreted from human adipose-derived stem cells (ADSC), is becoming increasingly recognized as a means of transferring the regenerative power of stem cells to injured cells in wound healing. Exosomes are rich in ceramides and long noncoding RNA (lncRNA) like metastasis-associated lung adenocarcinoma transcript 1 (MALAT1). We identified putative ceramide responsive cis-elements (CRCE) in MALAT1. We hypothesized that CRCE respond to cellular ceramide levels to regulate sEV MALAT1 packaging. MALAT1 levels by many cells exceed those of protein coding genes and it's expression is equally high in exosomes. Ceramide also regulates exosome synthesis, however, the contents of exosome cargo via sphingomyelinase and ceramide synthase pathways has not been demonstrated.

METHODS

ADSC were treated with an inhibitor of sphingomyelinase, GW4869, and stimulators of ceramide synthesis, C2- and C6-short chain ceramides, prior to collection of conditioned media (CM). sEV were isolated from CM, and then used to treat human dermal fibroblast (HDF) cultures in cell migration scratch assays, and mitochondrial stress tests to evaluate oxygen consumption rates (OCR).

RESULTS

Inhibition of sphingomyelinase by treatment of ADSC with GW4869 lowered levels of MALAT1 in small EVs. Stimulation of ceramide synthesis using C2- and C6- ceramides increased cellular, EVs levels of MALAT1. The functional role of sEV MALAT1 was evaluated in HDF by applying EVs to HDF. Control sEV increased migration of HDF, and significantly increased ATP production, basal and maximal respiration OCR. sEV from GW4869-treated ADSC inhibited cell migration and maximal respiration. However, sEV from C2- and C6-treated cells, respectively, increased both functions but not significantly above control EV except for maximal respiration. sEV were exosomes except when ADSC were treated with GW4869 and C6-ceramide, then they were larger and considered microvesicles.

CONCLUSIONS

Ceramide synthesis regulates MALAT1 EV content. Sphingomyelinase inhibition blocked MALAT1 from being secreted from ADSC EVs. Our report is consistent with those of MALAT1 increasing cell migration and mitochondrial MALAT1 altering maximal respiration in cells. Since MALAT1 is important for exosome function, it stands that increased exosomal MALAT1 should be beneficial for wound healing as shown with these assays. Video Abstract.

Optimization of the Sulfo-Phospho-Vanillin Assay for Total Lipid Normalization in Untargeted Quantitative Lipidomic LC-MS/MS Applications

Liquid chromatography (LC)-mass spectrometry (MS)/MS lipidomic normalization is generally performed by equalizing pre-extraction sample materials or via DNA or protein pre-quantitation methods, which have known measurement inaccuracies. We propose the use of the sulfo-phospho-vanillin assay (SPVA), a total lipid colorimetric analysis, as a pre-quantitation method to normalize lipids in lipidomic LC-MS/MS applications. The assay has been applied to a 300 μL well volume in a 96-well plate and tested using Avanti total lipid standards of porcine brain and E. coli. Assay parameters for lipid sample volume, sulfuric acid, vanillin/phosphoric acid, post-reaction incubation time, and wavelength are optimized for robust application to biologically sourced lipid samples. Standard test samples were prepared using three concentrations covering approximately 100 μg/mL range. The optimized assay yielded test sample errors less than 10%, indicating a precise and accurate assay performance. The test samples were then analyzed by LC-MS/MS and normalized using SPVA pre-quantitation and pseudo-mass normalization. The detected lipids showed smaller standard deviations and greater relative concentration differences compared to the pseudo-mass normalized lipids, showing promise as a normalization method.

Glycosphingolipids in Diabetes, Oxidative Stress, and Cardiovascular Disease: Prevention in Experimental Animal Models

Diabetes contributes to about 30% morbidity and mortality world-wide and has tidal wave increases in several countries in Asia. Diabetes is a multi-factorial disease compounded by inflammation, dyslipidemia, atherosclerosis, and is sometimes accompanied with gains in body weight. Sphingolipid pathways that interplay in the enhancement of the pathology of this disease may be potential therapeutic targets. Thus, the application of advanced sphingolipidomics may help predict the progression of this disease and therapeutic outcomes in man. Pre-clinical studies using various experimental animal models of diabetes provide valuable information on the role of sphingolipid signaling networks in diabetes and the efficacy of drugs to determine the translatability of innovative discoveries to man. In this review, we discuss three major concepts regarding sphingolipids and diabetes. First, we discuss a possible involvement of a monosialodihexosylceramide (GM3) in insulin-insulin receptor interactions. Second, a potential role for ceramide (Cer) and lactosylceramide (LacCer) in apoptosis and mitochondrial dysfunction is proposed. Third, a larger role of LacCer in antioxidant status and inflammation is discussed. We also discuss how inhibitors of glycosphingolipid synthesis can ameliorate diabetes in experimental animal models.

Phosphatase protector alpha4 (α4) is involved in adipocyte maintenance and mitochondrial homeostasis through regulation of insulin signaling

Insulin signaling is mediated via a network of protein phosphorylation. Dysregulation of this network is central to obesity, type 2 diabetes and metabolic syndrome. Here we investigate the role of phosphatase binding protein Alpha4 (α4) that is essential for the serine/threonine protein phosphatase 2A (PP2A) in insulin action/resistance in adipocytes. Unexpectedly, adipocyte-specific inactivation of α4 impairs insulin-induced Akt-mediated serine/threonine phosphorylation despite a decrease in the protein phosphatase 2A (PP2A) levels. Interestingly, loss of α4 also reduces insulin-induced insulin receptor tyrosine phosphorylation. This occurs through decreased association of α4 with Y-box protein 1, resulting in the enhancement of the tyrosine phosphatase protein tyrosine phosphatase 1B (PTP1B) expression. Moreover, adipocyte-specific knockout of α4 in male mice results in impaired adipogenesis and altered mitochondrial oxidation leading to increased inflammation, systemic insulin resistance, hepatosteatosis, islet hyperplasia, and impaired thermogenesis. Thus, the α4 /Y-box protein 1(YBX1)-mediated pathway of insulin receptor signaling is involved in maintaining insulin sensitivity, normal adipose tissue homeostasis and systemic metabolism.

The Effect of Sex and Obesity on the Gene Expression of Lipid Flippases in Adipose Tissue

Molecular mechanisms behind obesity and sex-related effects in adipose tissue remain elusive. During adipocyte expansion, adipocytes undergo drastic remodelling of lipid membrane compositions. Lipid flippases catalyse phospholipid translocation from exoplasmic to the cytoplasmic leaflet of membranes. The present study aimed to analyse the effect of sex, obesity, and their interactions on the gene expression of two lipid flippases—ATP8A1 and ATP8B1—and their possible microRNA (miR) modulators in visceral adipose tissue (VAT). In total, 12 normal-weight subjects (5 premenopausal women and 7 men) and 13 morbidly obese patients (7 premenopausal women and 6 men) were submitted to surgery, and VAT samples were obtained. Gene expression levels of ATP8A1, ATP8B1, miR-548b-5p, and miR-4643 were measured in VAT. Our results showed a marked influence of obesity on VAT ATP8A1 and ATP8B1, although the effects of obesity were stronger in men for ATP8A1. Both genes positively correlated with obesity and metabolic markers. Furthermore, ATP8B1 was positively associated with miR-548b-5p and negatively associated with miR-4643. Both miRs were also affected by sex. Thus, lipid flippases are altered by obesity in VAT in a sex-specific manner. Our study provides a better understanding of the sex-specific molecular mechanisms underlying obesity, which may contribute to the development of sex-based precision medicine.

Insulin resistance induced by de novo pathway–generated C16-ceramide is associated with type 2 diabetes in an obese population

Background

Obesity and diabetes are two chronic metabolic diseases whose prevalence is increasing at an alarming rate globally. A close association between obesity, diabetes, and insulin resistance has been identified, and many studies have pinpointed obesity as a causal risk factor for insulin resistance. However, the mechanism underlying this association is not entirely understood. In the past decade, ceramides have gained attention due to their accumulation in certain tissues and their suggested role in initiating insulin resistance. This study aims to determine the association of specific ceramides and their major metabolizing enzymes with obesity-associated insulin resistance.

Methods

The samples comprised subcutaneous adipose tissues collected from three cohorts: lean non-diabetic (controls; n = 20), obese-non-diabetic (n = 66), and obese-diabetic (n = 32). Ceramide levels were quantified using LC-MS/MS and mRNA expression level for different enzymes were estimated using real-time PCR-based RNA expression analysis.

Results

C16-ceramide (P = 0.023), C16-dihydro-ceramide (P < 0.005), C18-dihydro-ceramide (P = 0.009) and C24-ceramide (P = 0.040) levels were significantly increased in the obese cohort compared to the control group. However, stratification of the obese group revealed a significant increase in the C16-ceramide levels (P = 0.027) and mRNA over expression of the serine palmitoyl transferases enzyme subunit SPT1 (P < 0.005) in the obese-diabetic cohort compared to the obese-non-diabetic cohort.

Conclusions

The present study indicates that C16-ceramide plays a pivotal role in inducing insulin resistance. Overexpression of SPT1 in the obese-diabetic group and its positive correlation with C16-ceramide suggest that C16-ceramide was generated through the de novo pathway.

The Key Role of Akt Protein Kinase in Metabolic-Inflammatory Pathways Cross-Talk: TNF-α Down-Regulation and Improving of Insulin Resistance in HepG2 Cell Line

BACKGROUND

Elevation of plasma free fatty acids as a principal aspect of type 2 diabetes maintains etiologically insulin insensitivity in target cells. TNF-α inhibitory effects on key insulin signaling pathway elements remain to be verified in insulinresistant hepatic cells. Thus, TNF-α knockdown effects on the key elements of insulin signaling were investigated in the palmitate-induced insulin-resistant hepatocytes. The Akt serine kinase, a key protein of the insulin signaling pathway, phosphorylation was monitored to understand the TNF-α effect on probable enhancing of insulin resistance.

METHODS

Insulin-resistant HepG2 cells were produced using 0.5 mM palmitate treatment and shRNA-mediated TNF-α gene knockdown and its down-regulation confirmed using ELISA technique. Western blotting analysis was used to assess the Akt protein phosphorylation status.

RESULTS

Palmitate-induced insulin resistance caused TNF-α protein overexpression 1.2-, 2.78, and 2.25- fold as compared to the control cells at post-treatment times of 8 h, 16 h, and 24 h, respectively. In the presence of palmitate, TNF-α expression showed around 30% reduction in TNF-α knockdown cells as compared to normal cells. In the TNF-α down-regulated cell, Akt phosphorylation was approximately 62% more than control cells after treatment with 100 nM insulin in conjugation with 0.5 mM palmitate.

CONCLUSIONS

The obtained data demonstrated that TNF-α protein expression reduction improved insulin-stimulated Akt phosphorylation in the HepG2 cells and decreased lipidinduced insulin resistance of the diabetic hepatocytes.

Ganoderma lucidum Extract Reduces Insulin Resistance by Enhancing AMPK Activation in High-Fat Diet-Induced Obese Mice

Ganoderma lucidum is used widely in oriental medicine to treat obesity and metabolic diseases. Bioactive substances extracted from G. lucidum have been shown to ameliorate dyslipidemia, insulin resistance, and type 2 diabetes in mice via multiple 5' AMP-activated protein kinase (AMPK)-mediated mechanisms; however, further studies are required to elucidate the anti-obesity effects of G. lucidum in vivo. In this study, we demonstrated that 3% G. lucidum extract powder (GEP) can be used to prevent obesity and insulin resistance in a mouse model. C57BL/6 mice were provided with a normal diet (ND) or a high-fat diet (HFD) supplemented with 1, 3, or 5% GEP for 12 weeks and the effect of GEP on body weight, liver, adipose tissue, adipokines, insulin and glucose tolerance (ITT and GTT), glucose uptake, glucose-metabolism related proteins, and lipogenesis related genes was examined. GEP administration was found to reduce weight gain in the liver and fat tissues of the mice. In addition, serum parameters were significantly lower in the 3% and 5% GEP mice groups than in those fed a HFD alone, whereas adiponectin levels were significantly higher. We also observed that GEP improved glucose metabolism, reduced lipid accumulation in the liver, and reduced adipocyte size. These effects may have been mediated by enhanced AMPK activation, which attenuated the transcription and translation of lipogenic genes such as fatty acid synthase (FAS), stearoyl-CoA desaturase 1 (SCD1), and sterol regulatory element-binding protein-1c (SREBP1c). Moreover, AMP-activated protein kinase (AMPK) activation increased acetyl-CoA carboxylase (ACC), insulin receptor (IR), IR substrate 1 (IRS1), and Akt protein expression and activation, as well as glucose transporter type 1/4 (GLUT1/4) protein production, thereby improving insulin sensitivity and glucose metabolism. Together, these findings demonstrate that G. lucidum may effectively prevent obesity and suppress obesity-induced insulin resistance via AMPK activation.

Lipidomics of brown and white adipose tissue: Implications for energy metabolism

Adipose tissue exerts multiple vital functions that critically maintain energy balance, including storing and expending energy, as well as secreting factors that systemically modulate nutrient metabolism. Since lipids are the major constituents of the adipocytes, it is unsurprising that the lipid composition of these cells plays a critical role in maintaining their functions and communicating with other organs and cells. In both positive and negative energy balance conditions, lipids and free fatty acids secreted from adipocytes exert either beneficial or detrimental effects in other tissues, such as the liver, pancreas and muscle. The way the adipocytes communicate with other organs tightly depends on the nature of their lipidome composition. Notwithstanding, the lipidome composition of the adipocytes is affected by physiological factors such as adipocyte type, gender and age, but also by environmental cues such as diet composition, thermal stress and physical activity. Here we provide an updated overview on how the adipose tissue lipidome profile is shaped by different physiological and environmental factors and how these changes impact the way the adipocytes regulate whole-body energy metabolism.

Obesity, Bioactive Lipids, and Adipose Tissue Inflammation in Insulin Resistance

Obesity is a major risk factor for the development of insulin resistance and type 2 diabetes. The exact mechanism by which adipose tissue induces insulin resistance is still unclear. It has been demonstrated that obesity is associated with the adipocyte dysfunction, macrophage infiltration, and low-grade inflammation, which probably contributes to the induction of insulin resistance. Adipose tissue synthesizes and secretes numerous bioactive molecules, namely adipokines and cytokines, which affect the metabolism of both lipids and glucose. Disorders in the synthesis of adipokines and cytokines that occur in obesity lead to changes in lipid and carbohydrates metabolism and, as a consequence, may lead to insulin resistance and type 2 diabetes. Obesity is also associated with the accumulation of lipids. A special group of lipids that are able to regulate the activity of intracellular enzymes are biologically active lipids: long-chain acyl-CoAs, ceramides, and diacylglycerols. According to the latest data, the accumulation of these lipids in adipocytes is probably related to the development of insulin resistance. Recent studies indicate that the accumulation of biologically active lipids in adipose tissue may regulate the synthesis/secretion of adipokines and proinflammatory cytokines. Although studies have revealed that inflammation caused by excessive fat accumulation and abnormalities in lipid metabolism can contribute to the development of obesity-related insulin resistance, further research is needed to determine the exact mechanism by which obesity-related insulin resistance is induced.

Ovariectomy and obesity have equal impact in causing mitochondrial dysfunction and impaired skeletal muscle contraction in rats

OBJECTIVE

Previous studies have demonstrated that either an obese-insulin resistance condition or a condition involving loss of estrogen impaired skeletal muscle function as indicated by a decrease in muscle contraction. The differing effects of combined estrogen deficiency over obese-insulin resistance on skeletal muscle function have, however, not yet been determined. Our hypothesis was that estrogen deficiency aggravates skeletal muscle dysfunction in obese-insulin resistant rats, via increased muscle oxidative stress and mitochondrial dysfunction.

METHODS

Twenty-four female Wistar rats were divided into 2 groups and animals in each group were fed either a normal diet (ND) or a high-fat diet (HFD) for 24 weeks. At week 13, rats in each group were subdivided into 2 subgroups: sham-operated or ovariectomized (n = 6/subgroup). At the end of the experimental period the contraction of the gastrocnemius muscles was tested before the rats were sacrificed. Skeletal muscle was removed to assess oxidative stress and mitochondrial function.

RESULTS

We found that an obese-insulin resistant condition was observed in sham-operated HFD-fed rats, ovariectomized ND-fed rats, and ovariectomized HFD-fed rats. Skeletal muscle contractile function (peak-force ratio [g/g]; 25.40 ± 2.03 [ovariectomized ND-fed rats], 22.44 ± 0.85 [sham-operated HFD-fed rats] and 25.06 ± 0.61 [ovariectomized HFD-fed rats]), skeletal muscle mitochondrial function, and oxidative stress were equally significantly impaired in all 3 groups, when compared with those of sham-operated ND-fed rats (31.12 ± 1.88 g/g [NDS]; P < 0.05). Surprisingly, loss of estrogen did not aggravate these dysfunctions of skeletal muscles in HFD-fed rats.

CONCLUSIONS

These findings suggest that skeletal muscle dysfunction may occur due to increased muscle oxidative stress and mitochondrial dysfunction as a result of ovariectomy and obese-insulin resistance. Loss of estrogen, however, did not aggravate these impairments in the muscle of rats with obese-insulin resistant condition.

Beyond adiponectin and leptin: adipose tissue-derived mediators of inter-organ communication

The breakthrough discoveries of leptin and adiponectin more than two decades ago led to a widespread recognition of adipose tissue as an endocrine organ. Many more adipose tissue-secreted signaling mediators (adipokines) have been identified since then, and much has been learned about how adipose tissue communicates with other organs of the body to maintain systemic homeostasis. Beyond proteins, additional factors, such as lipids, metabolites, noncoding RNAs, and extracellular vesicles (EVs), released by adipose tissue participate in this process. Here, we review the diverse signaling mediators and mechanisms adipose tissue utilizes to relay information to other organs. We discuss recently identified adipokines (proteins, lipids, and metabolites) and briefly outline the contributions of noncoding RNAs and EVs to the ever-increasing complexities of adipose tissue inter-organ communication. We conclude by reflecting on central aspects of adipokine biology, namely, the contribution of distinct adipose tissue depots and cell types to adipokine secretion, the phenomenon of adipokine resistance, and the capacity of adipose tissue to act both as a source and sink of signaling mediators.

The RNA binding protein HuR influences skeletal muscle metabolic flexibility in rodents and humans

BACKGROUND

Metabolic flexibility can be assessed by changes in respiratory exchange ratio (RER) following feeding. Though metabolic flexibility (difference in RER between fasted and fed state) is often impaired in individuals with obesity or type 2 diabetes, the cellular processes contributing to this impairment are unclear.

MATERIALS AND METHODS

From several clinical studies we identified the 16 most and 14 least metabolically flexible male and female subjects out of >100 participants based on differences between 24-hour and sleep RER measured in a whole-room indirect calorimeter. Global skeletal muscle gene expression profiles revealed that, in metabolically flexible subjects, transcripts regulated by the RNA binding protein, HuR, are enriched. We generated and characterized mice with a skeletal muscle-specific knockout of the HuR encoding gene, Elavl1 (HuR^m-/-).

RESULTS

Male, but not female, HuR^m-/- mice exhibit metabolic inflexibility, with mild obesity, impaired glucose tolerance, impaired fat oxidation and decreased in vitro palmitate oxidation compared to HuR^fl/fl littermates. Expression levels of genes involved in mitochondrial fatty acid oxidation and oxidative phosphorylation are decreased in both mouse and human muscle when HuR is inhibited.

CONCLUSIONS

HuR inhibition results in impaired metabolic flexibility and decreased lipid oxidation, suggesting a role for HuR as an important regulator of skeletal muscle metabolism.

Ceramide and sphingosine 1-phosphate in adipose dysfunction

The increased adipose tissue mass of obese individuals enhances the risk of metabolic syndrome, type 2 diabetes and cardiovascular diseases. During pathological expansion of adipose tissue, multiple molecular controls of lipid storage, adipocyte turn-over and endocrine secretion are perturbed and abnormal lipid metabolism results in a distinct lipid profile. There is a role for ceramides and sphingosine 1-phosphate (S1P) in inducing adipose dysfunction. For instance, the alteration of ceramide biosynthesis, through the de-regulation of key enzymes, results in aberrant formation of ceramides (e.g. C_16:0 and C_18:0) which block insulin signaling and promote adipose inflammation. Furthermore, S1P can induce defective adipose tissue phenotypes by promoting chronic inflammation and inhibiting adipogenesis. These abnormal changes are discussed in the context of possible therapeutic approaches to re-establish normal adipose function and to, thereby, increase insulin sensitivity in type 2 diabetes. Such novel approaches include blockade of ceramide biosynthesis using inhibitors of sphingomyelinase or dihydroceramide desaturase and by antagonism of S1P receptors, such as S1P₂.

The Role of Ceramides in Insulin Resistance

Resistance to insulin is a pathophysiological state related to the decreased response of peripheral tissues to the insulin action, hyperinsulinemia and raised blood glucose levels caused by increased hepatic glucose outflow. All the above precede the onset of full-blown type 2 diabetes. According to the World Health Organization (WHO), in 2016 more than 1.9 billion people over 18 years of age were overweight and about 600 million were obese. Currently, the primary hypothesis explaining the probability of occurrence of insulin resistance assigns a fundamental role of lipids accumulation in adipocytes or nonadipose tissue (muscle, liver) and the locally developing chronic inflammation caused by adipocytes hypertrophy. However, the major molecular pathways are unknown. The sphingolipid ceramide is the main culprit that combines a plethora of nutrients (e.g., saturated fatty acids) and inflammatory cytokines (e.g., TNFα) to the progression of insulin resistance. The accumulation of sphingolipid ceramide in tissues of obese humans, rodents and Western-diet non-human primates is in line with diabetes, hypertension, cardiac failure or atherosclerosis. In hypertrophied adipose tissue, after adipocytes excel their storage capacity, neutral lipids begin to accumulate in nonadipose tissues, inducing organ dysfunction. Furthermore, obesity is closely related to the development of chronic inflammation and the release of cytokines directly from adipocytes or from macrophages that infiltrate adipose tissue. Enzymes taking part in ceramide metabolism are potential therapeutic targets to manipulate sphingolipids content in tissues, either by inhibition of their synthesis or through stimulation of ceramides degradation. In this review, we will evaluate the mechanisms responsible for the development of insulin resistance and possible therapeutic perspectives.

The Role of Oestrogen Receptor Beta (ERβ) in the Aetiology and Treatment of Type 2 Diabetes Mellitus

INTRODUCTION

Challenges facing the treatment of type 2 diabetes necessitate the search for agents which act via alternative pathways to provide better therapeutic outcomes. Recently, an increasing body of evidence implicates the activation of oestrogen receptors (ERα and ERβ) in the development and treatment of underlying conditions in type 2 diabetes. This article summarizes available evidence for the involvement of oestrogen receptors in insulin secretion, insulin resistance as well as glucose uptake and highlights the potential of ERβ as a therapeutic target.

BACKGROUND

Recent studies indicate an association between the activation of each of the isoforms of ER and recent findings indicate that ERβ shows promise as a potential target for antidiabetic drugs. In vitro and in vivo studies in receptor knockout mice indicate beneficial actions of selective agonists of ERβ receptor and underscore its therapeutic potential.

CONCLUSION

Studies are needed to further elucidate the exact mechanism underlying the role of ERβ activation as a therapeutic approach in the management of type 2 diabetes.

Adipose Tissue Function and Expandability as Determinants of Lipotoxicity and the Metabolic Syndrome

The adipose tissue organ is organised as distinct anatomical depots located all along the body axis and it is constituted of three different types of adipocytes : white, beige and brown which are integrated with vascular, immune, neural and extracellular stroma cells. These distinct adipocytes serve different specialised functions. The main function of white adipocytes is to ensure healthy storage of excess nutrients/energy and its rapid mobilisation to supply the demand of energy imposed by physiological cues in other organs, whereas brown and beige adipocytes are designed for heat production through uncoupling lipid oxidation from energy production. The concert action of the three type of adipocytes/tissues has been reported to ensure an optimal metabolic status in rodents. However, when one or multiple of these adipose depots become dysfunctional as a consequence of sustained lipid/nutrient overload, then insulin resistance and associated metabolic complications ensue. These metabolic alterations negatively affects the adipose tissue functionality and compromises global metabolic homeostasis. Optimising white adipose tissue expandability and its functional metabolic flexibility and/or promoting brown/beige mediated thermogenic activity counteracts obesity and its associated lipotoxic metabolic effects. The development of these therapeutic approaches requires a deep understanding of adipose tissue in all broad aspects. In this chapter we will discuss the characteristics of the different adipose tissue depots with respect to origins and precursors recruitment, plasticity, cellular composition and expandability capacity as well as molecular and metabolic signatures in both physiological and pathophysiological conditions.

Markers of sympathetic nervous system activity associate with complex plasma lipids in metabolic syndrome subjects

BACKGROUND AND AIMS

Plasma sphingolipids including ceramides, and gangliosides are associated with insulin resistance (IR) through effects on insulin signalling and glucose metabolism. Our studies of subjects with metabolic syndrome (MetS) showed close relationships between IR and sympathetic nervous system (SNS) activity including arterial norepinephrine (NE). We have therefore investigated possible associations of IR and SNS activity with complex lipids that are involved in both insulin sensitivity and neurotransmission.

METHODS

We performed a cross-sectional assessment of 23 lipid classes/subclasses (total 339 lipid species) by tandem mass spectrometry in 94 overweight untreated subjects with IR (quantified by HOMA-IR, Matsuda index and plasma insulin).

RESULTS

Independently of IR parameters, several circulating complex lipids associated significantly with arterial NE and NEFA (non-esterified fatty acids) and marginally with heart rate (HR). After accounting for BMI, HOMA-IR, systolic BP, age, gender, and correction for multiple comparisons, these associations were significant (p < 0.05): NE with ceramide, phosphatidylcholine, alkyl- and alkenylphosphatidylcholine and free cholesterol; NEFA with mono- di- and trihexosylceramide, G_M3 ganglioside, sphingomyelin, phosphatidylcholine, alkyl- and alkenylphosphatidylcholine, phosphatidylinositol and free cholesterol; HR marginally (p = or <0.1>0.05) with ceramide, G_M3 ganglioside, sphingomyelin, lysophosphatidylcholine, phosphatidylinositol, lysophosphatidylinositol and free cholesterol. Multiple subspecies of these lipids significantly associated with NE and NEFA. None of the IR biomarkers associated significantly with lipid classes/subclasses after correction for multiple comparisons.

CONCLUSIONS

This is the first demonstration that arterial norepinephrine and NEFA, that reflect both SNS activity and IR, associate significantly with circulating complex lipids independently of IR, suggesting a role for such lipids in neural mechanisms operating in MetS.

Targeting ceramide metabolism in obesity

Obesity is a major health concern that increases the risk for insulin resistance, type 2 diabetes (T2D), and cardiovascular disease. Thus, an enormous research effort has been invested into understanding how obesity-associated dyslipidemia and obesity-induced alterations in lipid metabolism increase the risk for these diseases. Accordingly, it has been proposed that the accumulation of lipid metabolites in organs such as the liver, skeletal muscle, and heart is critical to these obesity-induced pathologies. Ceramide is one such lipid metabolite that accumulates in tissues in response to obesity, and both pharmacological and genetic strategies that reduce tissue ceramide levels yield salutary actions on overall metabolic health. We will review herein why ceramide accumulates in tissues during obesity and how an increase in intracellular ceramide impacts cellular signaling and function as well as potential mechanisms by which reducing intracellular ceramide levels improves insulin resistance, T2D, atherosclerosis, and heart failure. Because a reduction in skeletal muscle ceramide levels is frequently associated with improvements in insulin sensitivity in humans, the beneficial findings reported for reducing ceramides in preclinical studies may have clinical application in humans. Therefore, modulating ceramide metabolism may be a novel, exciting target for preventing and/or treating obesity-related diseases.

LA and ALA prevent glucose intolerance in obese male rats without reducing reactive lipid content, but cause tissue-specific changes in fatty acid composition

While the cause of Type 2 diabetes remains poorly defined, the accumulation of reactive lipids within white adipose tissue, skeletal muscle, and liver have been repeatedly implicated as underlying mechanisms. The ability of polyunsaturated fatty acids (PUFAs) to prevent the development of insulin resistance has gained considerable interest in recent years; however, the mechanisms-of-action remain poorly described. Therefore, we determined the efficacy of diets supplemented with either linoleic acid (LA) or α-linolenic acid (ALA) in preventing insulin resistance and reactive lipid accumulation in key metabolic tissues of the obese Zucker rat. Obese Zucker rats displayed impaired glucose homeostasis and reduced n-3 and n-6 PUFA content in the liver and epididymal white adipose tissue (EWAT). After the 12-wk feeding intervention, both LA- and ALA-supplemented diets prevented whole body glucose and insulin intolerance; however, ALA had a more pronounced effect. These changes occurred in association with n-3 and n-6 accumulation in all tissues studied, albeit to different extents (EWAT > liver > muscle). Triacylglycerol (TAG), diacylglycerol (DAG), ceramide, and sphingolipid accumulation were not attenuated in obese animals supplemented with either LA or ALA, suggesting that preservation of glucose homeostasis occurred independent of changes in reactive lipid content. However, PUFA-supplemented diets differentially altered the fatty acid composition of TAGs, DAGs, and PLs in a tissue-specific manner, suggesting essential fatty acid metabolism differs between tissues. Together, our results indicate that remodeling of the fatty acid composition of various lipid fractions may contribute to the improved glucose tolerance observed in obese rats fed PUFA-supplemented diets.

Estrogen Metabolite 16α-Hydroxyestrone Exacerbates Bone Morphogenetic Protein Receptor Type II-Associated Pulmonary Arterial Hypertension Through MicroRNA-29-Mediated Modulation of Cellular Metabolism

BACKGROUND

Pulmonary arterial hypertension (PAH) is a proliferative disease of the pulmonary vasculature that preferentially affects women. Estrogens such as the metabolite 16α-hydroxyestrone (16αOHE) may contribute to PAH pathogenesis, and alterations in cellular energy metabolism associate with PAH. We hypothesized that 16αOHE promotes heritable PAH (HPAH) via microRNA-29 (miR-29) family upregulation and that antagonism of miR-29 would attenuate pulmonary hypertension in transgenic mouse models of Bmpr2 mutation.

METHODS AND RESULTS

MicroRNA array profiling of human lung tissue found elevation of microRNAs associated with energy metabolism, including the miR-29 family, among HPAH patients. miR-29 expression was 2-fold higher in Bmpr2 mutant mice lungs at baseline compared with controls and 4 to 8-fold higher in Bmpr2 mice exposed to 16αOHE 1.25 μg/h for 4 weeks. Blot analyses of Bmpr2 mouse lung protein showed significant reductions in peroxisome proliferator-activated receptor-γ and CD36 in those mice exposed to 16αOHE and protein derived from HPAH lungs compared with controls. Bmpr2 mice treated with anti-miR-29 (20-mg/kg injections for 6 weeks) had improvements in hemodynamic profile, histology, and markers of dysregulated energy metabolism compared with controls. Pulmonary artery smooth muscle cells derived from Bmpr2 murine lungs demonstrated mitochondrial abnormalities, which improved with anti-miR-29 transfection in vitro; endothelial-like cells derived from HPAH patient induced pluripotent stem cell lines were similar and improved with anti-miR-29 treatment.

CONCLUSIONS

16αOHE promotes the development of HPAH via upregulation of miR-29, which alters molecular and functional indexes of energy metabolism. Antagonism of miR-29 improves in vivo and in vitro features of HPAH and reveals a possible novel therapeutic target.

Impaired mitochondrial fat oxidation induces adaptive remodeling of muscle metabolism

The correlations between intramyocellular lipid (IMCL), decreased fatty acid oxidation (FAO), and insulin resistance have led to the hypothesis that impaired FAO causes accumulation of lipotoxic intermediates that inhibit muscle insulin signaling. Using a skeletal muscle-specific carnitine palmitoyltransferase-1 KO model, we show that prolonged and severe mitochondrial FAO inhibition results in increased carbohydrate utilization, along with reduced physical activity; increased circulating nonesterified fatty acids; and increased IMCLs, diacylglycerols, and ceramides. Perhaps more importantly, inhibition of mitochondrial FAO also initiates a local, adaptive response in muscle that invokes mitochondrial biogenesis, compensatory peroxisomal fat oxidation, and amino acid catabolism. Loss of its major fuel source (lipid) induces an energy deprivation response in muscle coordinated by signaling through AMP-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1α) to maintain energy supply for locomotion and survival. At the whole-body level, these adaptations result in resistance to obesity.

Targeting sphingolipid metabolism in the treatment of obesity/type 2 diabetes

INTRODUCTION

Obesity is a major factor that is linked to the development of type 2 diabetes (T2D). Excess circulating fatty acids (FAs), which characterize obesity, induce insulin resistance, steatosis, β cells dysfunction and apoptosis. These deleterious effects have been defined as lipotoxicity.

AREAS COVERED

FAs are metabolized to different lipid species, including ceramides which play a crucial role in lipotoxicity. The action of ceramides on tissues, such as muscle, liver, adipose tissue and pancreatic β cells, during the development of T2D will also be reviewed. In addition, the potential antagonist action of other sphingolipids, namely sphingoid base phosphates, on lipotoxicity in skeletal muscle and β cells will be addressed.

EXPERT OPINION

Ceramide is a critical mediator to the development of T2D linked to obesity. Targeting proteins involved in ceramide's deleterious action has not been possible due to their involvement in many other intracellular signaling pathways. A possible means of counteracting ceramide action would be to prevent the accumulation of the specific ceramide species involved in both insulin resistance and β-cell apoptosis/dysfunction. Another possibility would be to adjust the dynamic balance between ceramide and sphingoid base phosphate, both known to display opposing properties on the development of T2D-linked obesity.

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

Background

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

Results

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

Conclusion

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

Electronic supplementary material

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

On ceramides, other sphingolipids and impaired glucose homeostasis

In most people with type 2 diabetes, progression from obesity to diabetes is accompanied by elevated tissue exposures to a variety of lipids. Among these lipid species, ceramides and more complex sphingolipids have gained recent attention as being pathophysiologically relevant for the development of insulin resistance and impaired glycemic control. Upon excess intake of saturated fat, ceramides accumulate in insulin sensitive tissues either as a consequence of de novo synthesis or through mobilization from complex sphingolipids. Clinical studies have confirmed positive correlation between plasma and tissue levels of several ceramide species and insulin resistance. At the cellular level, it has been demonstrated that ceramides impair insulin signaling and intracellular handling of glucose and lipids with resulting deleterious effects on cellular metabolism. Hence, we are reviewing whether therapeutic interventions aiming at reducing tissue exposure to ceramides or other sphingolipids represent viable therapeutic approaches to improve glucose metabolism in people with diabetes.

Adiposopathy: how do diet, exercise and weight loss drug therapies improve metabolic disease in overweight patients?

An increase in bodyweight is generally associated with an increased risk of excessive fat-related metabolic diseases (EFRMD), including Type 2 diabetes mellitus, hypertension and dyslipidemia. However, not all patients who are overweight have EFRMD, and not all patients with EFRMD are significantly overweight. The adipocentric paradigm provides the basis for a unifying, pathophysiological process whereby fat gain in susceptible patients leads to fat dysfunction ('sick fat'), and wherein pathological abnormalities in fat function (adiposopathy) are more directly related to the onset of EFRMD than increases in fat mass (adiposity) alone. But just as worsening fat function worsens EFRMD, improved fat function improves EFRMD. Peroxisome proliferator-activated receptor-gamma agonists increase the recruitment, proliferation and differentiation of preadipocytes ('healthy fat') and cause apoptosis of hypertrophic and dysfunctional (including visceral) adipocytes resulting in improved fat function and improved metabolic parameters associated with EFRMD. Weight loss interventions, such as a hypocaloric diet and physical exercise, in addition to agents such as orlistat, sibutramine and cannabinoid receptor antagonists, may have favorable effects upon fat storage (lipogenesis and fat distribution), nutrient metabolism (such as free fatty acids), favorable effects upon adipose tissue factors involved in metabolic processes and inflammation, and enhanced 'cross-talk' with other major organ systems. In some cases, weight loss therapeutic agents may even affect metabolic parameters and adipocyte function independently of weight loss alone, suggesting that the benefit of these agents in improving EFRMD may go beyond their efficacy in weight reduction. This review describes how adiposopathy interventions may affect fat function, and thus improve EFRMD.

Carnitine Palmitoyltransferase 1b Deficient Mice Develop Severe Insulin Resistance After Prolonged High Fat Diet Feeding

Background

Carnitine palmitoyltransferase 1 (CPT1) is the rate-limiting enzyme governing the entry of long-chain acyl-CoAs into mitochondria. Treatments with CPT1 inhibitors protect against insulin resistance in short-term preclinical animal studies. We recently reported that mice with muscle isoform CPT1b deficiency demonstrated improved insulin sensitivity when fed a High Fat-Diet (HFD) for up to 5 months. In this follow up study, we further investigated whether the insulin sensitizing effects of partial CPT1b deficiency could be maintained under a prolonged HFD feeding condition.

Methods

We investigated the effects of CPT1b deficiency on HFD-induced insulin resistance using heterozygous CPT1b deficient (Cpt1b^+/−) mice compared with Wild Type (WT) mice fed a HFD for a prolonged period of time (7 months). We assessed insulin sensitivity using hyperinsulinemic-euglycemic clamps. We also examined body composition, skeletal muscle lipid profile, and changes in the insulin signaling pathways of skeletal muscle, liver, and adipose tissue.

Results

We found that Cpt1b^+/− mice became severely insulin resistant after 7 months of HFD feeding. Cpt1b^+/− mice exhibited a substantially reduced glucose infusion rate and skeletal muscle glucose uptake. While Cpt1b^+/− mice maintained a slower weight gain with less fat mass than WT mice, accumulation of lipid intermediates became evident in the muscle of Cpt1b^+/− but not WT mice after 7 months of HFD feeding. Insulin signaling was impaired in the Cpt1b^+/− as compared to the WT muscles.

Conclusion

Partial CPT1b deficiency, mimicking CPT1b inhibition, may lead to impaired insulin signaling and insulin sensitivity under a prolonged HFD feeding condition. Therefore, further studies on the potential detrimental effects of prolonged therapy with CPT1 inhibition are necessary in the development of this potential therapeutic strategy.

The adipokine/ceramide axis: key aspects of insulin sensitization

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

Role of ceramide in diabetes mellitus: evidence and mechanisms

Diabetes mellitus is a metabolic disease with multiple complications that causes serious diseases over the years. The condition leads to severe economic consequences and is reaching pandemic level globally. Much research is being carried out to address this disease and its underlying molecular mechanism. This review focuses on the diverse role and mechanism of ceramide, a prime sphingolipid signaling molecule, in the pathogenesis of type 1 and type 2 diabetes and its complications. Studies using cultured cells, animal models, and human subjects demonstrate that ceramide is a key player in the induction of β-cell apoptosis, insulin resistance, and reduction of insulin gene expression. Ceramide induces β-cell apoptosis by multiple mechanisms namely; activation of extrinsic apoptotic pathway, increasing cytochrome c release, free radical generation, induction of endoplasmic reticulum stress and inhibition of Akt. Ceramide also modulates many of the insulin signaling intermediates such as insulin receptor substrate, Akt, Glut-4, and it causes insulin resistance. Ceramide reduces the synthesis of insulin hormone by attenuation of insulin gene expression. Better understanding of this area will increase our understanding of the contribution of ceramide to the pathogenesis of diabetes, and further help in identifying potential therapeutic targets for the management of diabetes mellitus and its complications.

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

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

Increased bioactive lipids content in human subcutaneous and epicardial fat tissue correlates with insulin resistance

Obesity is a risk factor for metabolic diseases. Intramuscular lipid accumulation of ceramides, diacylglycerols, and long chain acyl-CoA is responsible for the induction of insulin resistance. These lipids are probably implicated in obesity-associated insulin resistance not only in skeletal muscle but also in fat tissue. Only few data are available about ceramide content in human subcutaneous adipose tissue. However, there are no data on DAG and LCACoA content in adipose tissue. The aim of our study was to measure the lipids content in human SAT and epicardial adipose tissue we sought to determine the bioactive lipids content by LC/MS/MS in fat tissue from lean non-diabetic, obese non-diabetic, and obese diabetic subjects and test whether the lipids correlate with HOMA-IR. We found, that total content of measured lipids was markedly higher in OND and OD subjects in both types of fat tissue (for all p < 0.001) as compared to LND group. In SAT we found positive correlation between HOMA-IR and C16:0-Cer (r = 0.79, p < 0.001) and between HOMA-IR and C16:0/18:2 DAG (r = 0.56, p < 0.001). In EAT we found a strong correlation between C16:0-CoA content and HOMA-IR (r = 0.73, p < 0.001). The study showed that in obese and obese diabetic patients, bioactive lipids content is greater in subcutaneous and epicardial fat tissue and the particular lipids content positively correlates with HOMA-IR.

Effects of diet-induced obesity on protein expression in insulin signaling pathways of skeletal muscle in male Wistar rats

BACKGROUND

The prevalence of diet-induced obesity is increasing globally, and posing significant health problems for millions of people worldwide. Diet-induced obesity is a major contributor to the global pandemic of type 2 diabetes mellitus. The reduced ability of muscle tissue to regulate glucose homeostasis plays a major role in the development and prognosis of type 2 diabetes. In this study, an animal model of diet-induced obesity was used to elucidate changes in skeletal muscle insulin signaling in obesity-induced diabetes.

METHODS

Adult male Wistar rats were randomized and assigned to either a control group or to a test group. Controls were fed a standard laboratory pellet diet (chow-fed), while the test group had free access to a highly palatable diet (diet-fed). After 8 weeks, the diet-fed animals were subdivided into three subgroups and their diets were altered as follows: diet-to-chow, diet-fed with addition of fenofibrate given by oral gavage for a further 7 weeks, or diet-fed with vehicle given by oral gavage for a further 7 weeks, respectively.

RESULTS

Untreated diet-fed animals had a significantly higher body weight and metabolic profile than the control chow-fed animals. Intramuscular triacylglyceride levels in the untreated obese animals were significantly higher than those in the control chow-fed group. Expression of protein kinase C beta, phosphatidylinositol 3, Shc, insulin receptor substrate 1, ERK1/2, and endothelial nitric oxide synthase was significantly increased by dietary obesity, while that of insulin receptor beta, insulin receptor substrate 1, and protein kinase B (Akt) were not affected by obesity.

CONCLUSION

These data suggest that diet-induced obesity affects insulin signaling mechanisms, leading to insulin resistance in muscle.

Ceramide metabolism is affected by obesity and diabetes in human adipose tissue

Ceramide is involved in development of insulin resistance. However, there are no data on ceramide metabolism in human adipose tissue. The aim of our study was to examine sphingolipid metabolism in fat tissue from obese nondiabetic (n = 11), obese diabetic (n = 11), and lean nondiabetic (n = 8) subjects. The content of ceramide (Cer), dihydroceramide (dhCer), sphingosine (SPH), sphinganine (SPA), sphingosine-1-phosphate (S1P; pmol/mg of protein), the expression (mRNA) and activity of key enzymes responsible for Cer metabolism: serine palmitoyltransferase (SPT), neutral and acidic sphingomyelinase (nSMase and aSMase, respectively), and neutral and acidic ceramidase (nCDase and aCDase, respectively) were examined in human adipose tissue. The contents of SPA and Cer were significantly lower whereas the content of dhCer was higher in both obese groups than the respective values in the lean subjects. The expression of examined enzymes was elevated in both obese groups. The SPT and CDases activity increased whereas aSMase activity deceased in both obese groups. We have found correlation between adipose tissue Cer content and plasma adiponectin concentration (r = 0.69, P < 0.001) and negative correlation between total Cer content and HOMA-IR index (homeostasis model of insulin resistance) (r = -0.67, P < 0.001). We have found that both obesity and diabetes affected pathways of sphingolipid metabolism in the adipose tissue.

Glycosphingolipids and insulin resistance

Glycosphingolipids are structural membrane components, residing largely in the plasma membrane with their sugar-moieties exposed at the cell's surface. In recent times a crucial role for glycosphingolipids in insulin resistance has been proposed. A chronic state of insulin resistance is a rapidly increasing disease condition in Western and developing countries. It is considered to be the major underlying cause of the metabolic syndrome, a combination of metabolic abnormalities that increases the risk for an individual to develop Type 2 diabetes, obesity, cardiovascular disease, polycystic ovary syndrome and nonalcoholic fatty liver disease. As discussed in this chapter, the evidence for a direct regulatory interaction of glycosphingolipids with insulin signaling is still largely indirect. However, the recent finding in animal models that pharmacological reduction of glycosphingolipid biosynthesis ameliorates insulin resistance and prevents some manifestations of metabolic syndrome, supports the view that somehow glycosphingolipids act as critical regulators, Importantly, since reductions in glycosphingolipid biosynthesis have been found to be well tolerated, such approaches may have a therapeutic potential.

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.

Soybean and sunflower oil-induced insulin resistance correlates with impaired GLUT4 protein expression and translocation specifically in white adipose tissue

Free fatty acids are known for playing a crucial role in the development of insulin resistance. High fat intake is known for impairing insulin sensitivity; however, the effect of vegetable-oil injections have never been investigated. The present study investigated the effects of daily subcutaneous injections (100 microL) of soybean (SB) and sunflower (SF) oils, during 7 days. Both treated groups developed insulin resistance as assessed by insulin tolerance test. The mechanism underlying the SB- and SF-induced insulin resistance was shown to involve GLUT4. In SB- and SF-treated animals, the GLUT4 protein expression was reduced approximately 20% and 10 min after an acute in vivo stimulus with insulin, the plasma membrane GLUT4 content was approximately 60% lower in white adipose tissue (WAT). No effects were observed in skeletal muscle. Additionally, both oil treatments increased mainly the content of palmitic acid ( approximately 150%) in WAT, which can contribute to explain the GLUT4 regulations. Altogether, the present study collects evidence that those oil treatments might generate insulin resistance by targeting GLUT4 expression and translocation specifically in WAT. These alterations are likely to be caused due to the specific local increase in saturated fatty acids that occurred as a consequence of oil daily injections.

Sphingolipids and insulin resistance: the five Ws

PURPOSE OF REVIEW

Inhibition of sphingolipid synthesis increases insulin sensitivity, resolves hepatic steatosis, and prevents the onset of diabetes in obese rodents. I herein review these interventional studies, aiming to summarize the five Ws - the 'Who, What, Where, When, and Why' questions that need to be addressed to understand roles of sphingolipids in the pathogenesis of diabetes.

RECENT FINDINGS

Who: ceramides and glucosylceramides are likely to be independent antagonists of insulin action. Where: recent data suggest that ceramides may inhibit insulin action in skeletal muscle, whereas glucosylceramides may be more efficacious in adipose tissue. In contrast, sphingolipid accumulation in the liver appears to be insufficient to induce insulin resistance. What: ceramides and glucosylceramides inhibit different insulin signaling events, but it is unclear whether these actions account for the broad spectrum of therapeutic benefits resulting from sphingolipid depletion. When: recent data suggest that obesity-induced inflammation is important for the induction of sphingolipid synthesis. Why: sphingolipids have an evolutionarily conserved role to starve cells of nutrients, and the inhibition of insulin action is possibly a component of this broader action.

SUMMARY

Despite considerable attention to the question of how sphingolipids induce metabolic disease, there exist enormous gaps in knowledge. Further elucidation of these molecular details will be essential for the development of new therapeutic strategies for inhibiting sphingolipid action and ameliorating metabolic diseases.

Interactive changes between macrophages and adipocytes

Obesity is associated with a proinflammatory state, with macrophage infiltration into adipose tissue. We tested the hypothesis that communication between macrophages and adipocytes affects insulin resistance by disrupting insulin-stimulated glucose transport, adipocyte differentiation, and macrophage function. To test this hypothesis, we cocultured 3T3-L1 adipocytes with C2D macrophages or primary peritoneal mouse macrophages and examined the impacts of macrophages and adipocytes on each other. Adipocytes and preadipocytes did not affect C2D macrophage TNF-alpha, IL-6, or IL-1beta transcript concentrations relative to those obtained when C2D macrophages were incubated alone. However, preadipocytes and adipocytes increased PEC-C2D macrophage IL-6 transcript levels, while preadipocytes inhibited IL-1beta transcript levels compared to those obtained when PEC-C2D macrophages were incubated in medium alone. We found that adipocyte coculture increased macrophage consumption of tumor necrosis factor alpha (TNF-alpha), interleukin 1beta (IL-1beta), and, in some cases, IL-6. C2D macrophages increasingly downregulated GLUT4 transcript levels in differentiated adipocytes. Recombinant TNF-alpha, IL-1beta, and IL-6 also downregulated GLUT4 transcript levels relative to those for the control. However, only IL-6 was inhibitory at concentrations detected in macrophage-adipocyte cocultures. IL-6 and TNF-alpha, but not IL-1beta, inhibited Akt phosphorylation within 15 min of insulin stimulation, but only IL-6 was inhibitory 30 min after stimulation. Lastly, we found that adipocyte differentiation was inhibited by macrophages or by recombinant TNF-alpha, IL-6, and IL-1beta, with IL-6 having the most impact. These data suggest that the interaction between macrophages and adipocytes is a complex process, and they support the hypothesis that the macrophage-adipocyte interaction affects insulin resistance by disrupting insulin-stimulated glucose transport, adipocyte differentiation, and macrophage function.

Carnitine and type 2 diabetes

Studies in humans and animals demonstrate that "lipid over supply" causes or worsens insulin resistance via multiple mechanisms involving the accumulation of intracellular lipids in multiple tissues. In particular, the accumulation of fatty acyl CoA derivatives/metabolites in muscle inhibits both insulin signaling and glucose oxidation. Therefore agents that ameliorate the accumulation of fatty acyl CoA derivatives and/or their metabolites would be beneficial in the treatment or prevention of insulin resistance and T2D. Hyperinsulemic/euglycemic clamp studies in humans and carnitine supplementation studies in rodents provide "proof-of-concept" that carnitine is effective at improving insulin-stimulated glucose utilization and in reversing abnormalities of fuel metabolism associated with T2D. Carefully controlled clinical trials are warranted to determine the efficacy dietary carnitine supplementation as an adjunctive treatment for type 2 diabetes.

Regulation of ceramide-induced neuronal death: cell metabolism meets neurodegeneration

The present review explores the role of ceramides in neuronal apoptosis, as well as the recent discovery of the signaling pathways involved in this process placing particular emphasis on the correlation between cellular metabolism and neuronal death. Endogenous levels of ceramides are increased following various pro-apoptotic stimuli which have been identified as potential causes of chronic and acute neurodegenerative diseases. Ceramides induce changes in multiple enzymes and cell signaling components. The early inhibition of the neuronal survival pathway regulated by phosphatidil-inositol-3-kinase/protein kinase B or AKT mediated by ceramide may be a relevant early event in the decision of neuronal survival/death. It may perturb several molecular and metabolic functions. In particular it might decrease glycolysis through rapid modulation of hexokinase activity. This would in turn generate limited amounts of mitochondrial substrates leading to mitochondrial dysfunction and neuronal apoptosis. Subtle and early metabolic alterations caused by inhibition of the PI3K/AKT pathway mediated by ceramide may potentially work with genes associated with neurodegenerative diseases such as Parkinson's and Alzheimer's disease. Together they may be determinant steps in downstream events leading to neuronal apoptosis. Therefore, reinforcement of the PI3K/AKT pathway could constitute an important neuroprotective strategy.

Sphingolipids, insulin resistance, and metabolic disease: new insights from in vivo manipulation of sphingolipid metabolism

Obesity and dyslipidemia are risk factors for metabolic disorders including diabetes and cardiovascular disease. Sphingolipids such as ceramide and glucosylceramides, while being a relatively minor component of the lipid milieu in most tissues, may be among the most pathogenic lipids in the onset of the sequelae associated with excess adiposity. Circulating factors associated with obesity (e.g., saturated fatty acids, inflammatory cytokines) selectively induce enzymes that promote sphingolipid synthesis, and lipidomic profiling reveals relationships between tissue sphingolipid levels and certain metabolic diseases. Moreover, studies in cultured cells and isolated tissues implicate sphingolipids in certain cellular events associated with diabetes and cardiovascular disease, including insulin resistance, pancreatic beta-cell failure, cardiomyopathy, and vascular dysfunction. However, definitive evidence that sphingolipids contribute to insulin resistance, diabetes, and atherosclerosis has come only recently, as researchers have found that pharmacological inhibition or genetic ablation of enzymes controlling sphingolipid synthesis in rodents ameliorates each of these conditions. Herein we will review the role of ceramide and other sphingolipid metabolites in insulin resistance, beta-cell failure, cardiomyopathy, and vascular dysfunction, focusing on these in vivo studies that identify enzymes controlling sphingolipid metabolism as therapeutic targets for combating metabolic disease.

TNF-alpha and adipocyte biology

Dyslipidemia and insulin resistance are commonly associated with catabolic or lipodystrophic conditions (such as cancer and sepsis) and with pathological states of nutritional overload (such as obesity-related type 2 diabetes). Two common features of these metabolic disorders are adipose tissue dysfunction and elevated levels of tumour necrosis factor-alpha (TNF-alpha). Herein, we review the multiple actions of this pro-inflammatory adipokine on adipose tissue biology. These include inhibition of carbohydrate metabolism, lipogenesis, adipogenesis and thermogenesis and stimulation of lipolysis. TNF-alpha can also impact the endocrine functions of adipose tissue. Taken together, TNF-alpha contributes to metabolic dysregulation by impairing both adipose tissue function and its ability to store excess fuel. The molecular mechanisms that underlie these actions are discussed.

Thiazolidinediones: effects on insulin resistance and the cardiovascular system

Thiazolidinediones (TZDs) have been used for the treatment of hyperglycaemia in type 2 diabetes for the past 10 years. They may delay the development of type 2 diabetes in individuals at high risk of developing the condition, and have been shown to have potentially beneficial effects on cardiovascular risk factors. TZDs act as agonists of peroxisome proliferator-activated receptor-gamma (PPAR-gamma) primarily in adipose tissue. PPAR-gamma receptor activation by TZDs improves insulin sensitivity by promoting fatty acid uptake into adipose tissue, increasing production of adiponectin and reducing levels of inflammatory mediators such as tumour necrosis factor-alpha (TNF-alpha), plasminogen activator inhibitor-1(PAI-1) and interleukin-6 (IL-6). Clinically, TZDs have been shown to reduce measures of atherosclerosis such as carotid intima-media thickness (CIMT). However, in spite of beneficial effects on markers of cardiovascular risk, TZDs have not been definitively shown to reduce cardiovascular events in patients, and the safety of rosiglitazone in this respect has recently been called into question. Dual PPAR-alpha/gamma agonists may offer superior treatment of insulin resistance and cardioprotection, but their safety has not yet been assured.