CerS1-Derived C18:0 Ceramide in Skeletal Muscle Promotes Obesity-Induced Insulin Resistance

Intramuscular diacylglycerol accumulates with acute hyperinsulinemia in insulin-resistant phenotypes

Elevated skeletal muscle diacylglycerols (DAGs) and ceramides can impair insulin signaling, and acylcarnitines (acylCNs) reflect impaired mitochondrial fatty acid oxidation, thus, the intramuscular lipid profile is indicative of insulin resistance. Acute (i.e., postprandial) hyperinsulinemia has been shown to elevate lipid concentrations in healthy muscle and is an independent risk factor for type 2 diabetes (T2D). However, it is unclear how the relationship between acute hyperinsulinemia and the muscle lipidome interacts across metabolic phenotypes, thus contributing to or exacerbating insulin resistance. We therefore investigated the impact of acute hyperinsulinemia on the skeletal muscle lipid profile to help characterize the physiological basis in which hyperinsulinemia elevates T2D risk. In a cross-sectional comparison, endurance athletes (n = 12), sedentary lean adults (n = 12), and individuals with obesity (n = 13) and T2D (n = 7) underwent a hyperinsulinemic-euglycemic clamp with muscle biopsies. Although there were no significant differences in total 1,2-DAG fluctuations, there was a 2% decrease in athletes versus a 53% increase in T2D during acute hyperinsulinemia (P = 0.087). Moreover, C18 1,2-DAG species increased during the clamp with T2D only, which negatively correlated with insulin sensitivity (P < 0.050). Basal muscle C18:0 total ceramides were elevated with T2D (P = 0.029), but not altered by clamp. Acylcarnitines were universally lowered during hyperinsulinemia, with more robust reductions of 80% in athletes compared with only 46% with T2D (albeit not statistically significant, main effect of group, P = 0.624). Similar fluctuations with acute hyperinsulinemia increasing 1,2 DAGs in insulin-resistant phenotypes and universally lowering acylcarnitines were observed in male mice. In conclusion, acute hyperinsulinemia elevates muscle 1,2-DAG levels with insulin-resistant phenotypes. This suggests a possible dysregulation of intramuscular lipid metabolism in the fed state in individuals with low insulin sensitivity, which may exacerbate insulin resistance.NEW & NOTEWORTHY Postprandial hyperinsulinemia is a risk factor for type 2 diabetes and may increase muscle lipids. However, it is unclear how the relationship between acute hyperinsulinemia and the muscle lipidome interacts across metabolic phenotypes, thus contributing to insulin resistance. We observed that acute hyperinsulinemia elevates muscle 1,2-DAGs in insulin-resistant phenotypes, whereas ceramides were unaltered. Insulin-mediated acylcarnitine reductions are also hindered with high-fat feeding. The postprandial period may exacerbate insulin resistance in metabolically unhealthy phenotypes.

Weight-loss maintenance is accompanied by interconnected alterations in circulating FGF21-adiponectin-leptin and bioactive sphingolipids

Weight loss is often followed by weight regain. Characterizing endocrine alterations accompanying weight reduction and regain may disentangle the complex biology of weight-loss maintenance. Here, we profile energy-balance-regulating metabokines and sphingolipids in adults with obesity undergoing an initial low-calorie diet-induced weight loss and a subsequent weight-loss maintenance phase with exercise, glucagon-like peptide-1 (GLP-1) analog therapy, both combined, or placebo. We show that circulating growth differentiation factor 15 (GDF15) and C16:0-C18:0 ceramides transiently increase upon initial diet-induced weight loss. Conversely, circulating fibroblast growth factor 21 (FGF21) is downregulated following weight-loss maintenance with combined exercise and GLP-1 analog therapy, coinciding with increased adiponectin, decreased leptin, and overall decrements in ceramide and sphingosine-1-phosphate levels. Subgroup analyses reveal differential alterations in FGF21-adiponectin-leptin-sphingolipids between weight maintainers and regainers. Clinically, cardiometabolic health outcomes associate with selective metabokine-sphingolipid remodeling signatures. Collectively, our findings indicate distinct FGF21, GDF15, and ceramide responses to diverse phases of weight change and suggest that weight-loss maintenance involves alterations within the metabokine-sphingolipid axis.

Ceramides as Emerging Players in Cardiovascular Disease: Focus on Their Pathogenetic Effects and Regulation by Diet

Impaired lipid metabolism is a pivotal driver of cardiovascular disease (CVD). In this regard, the accumulation of ceramides within the circulation as well as in metabolically active tissues and atherosclerotic plaques is a direct consequence of derailed lipid metabolism. Ceramides may be at the nexus between impaired lipid metabolism and CVD. Indeed, although on one hand ceramides have been implicated in the pathogenesis of CVD, on the other specific ceramide subspecies have also been proposed as predictors of major adverse cardiovascular events. This review will provide an updated overview of the role of ceramides in the pathogenesis of CVD, as well as their pathogenetic mechanisms of action. Furthermore, the manuscript will cover the importance of ceramides as biomarkers to predict cardiovascular events and the role of diet, both in terms of nutrients and dietary patterns, in modulating ceramide metabolism and homeostasis.

Ceramides are fuel gauges on the drive to cardiometabolic disease

Ceramides are signals of fatty acid excess that accumulate when a cell's energetic needs have been met and its nutrient storage has reached capacity. As these sphingolipids accrue, they alter the metabolism and survival of cells throughout the body including in the heart, liver, blood vessels, skeletal muscle, brain, and kidney. These ceramide actions elicit the tissue dysfunction that underlies cardiometabolic diseases such as diabetes, coronary artery disease, metabolic-associated steatohepatitis, and heart failure. Here, we review the biosynthesis and degradation pathways that maintain ceramide levels in normal physiology and discuss how the loss of ceramide homeostasis drives cardiometabolic pathologies. We highlight signaling nodes that sense small changes in ceramides and in turn reprogram cellular metabolism and stimulate apoptosis. Finally, we evaluate the emerging therapeutic utility of these unique lipids as biomarkers that forecast disease risk and as targets of ceramide-lowering interventions that ameliorate disease.

Myofiber-specific lipidomics unveil differential contributions to insulin sensitivity in individuals of African and European ancestry

Aims

Individuals of African ancestry (AA) present with lower insulin sensitivity compared to their European counterparts (EA). Studies show ethnic differences in skeletal muscle fiber type (lower type I fibers in AA), muscle fat oxidation capacity (lower in AA), whilst no differences in total skeletal muscle lipids. However, skeletal muscle lipid subtypes have not been examined in this context. We hypothesize that lower insulin sensitivity in AA is due to a greater proportion of type II (non-oxidative) muscle fibers, and that this would result in an ancestry-specific association between muscle lipid subtypes and peripheral insulin sensitivity. To test this hypothesis, we examined the association between insulin sensitivity and muscle lipids in AA and EA adults, and in an animal model of insulin resistance with muscle-specific fiber types.

Methods

In this cross-sectional study, muscle biopsies were obtained from individuals with a BMI ranging from normal to overweight with AA (N = 24) and EA (N = 19). Ancestry was assigned via genetic admixture analysis; peripheral insulin sensitivity via hyperinsulinaemic-euglycemic clamp; and myofiber content via myosin heavy chain immunohistochemistry. Further, muscle types with high (soleus) and low (vastus lateralis) type I fiber content were obtained from high-fat diet-induced insulin resistant F1 mice and littermate controls. Insulin sensitivity in mice was assessed via intraperitoneal glucose tolerance test. Mass spectrometry (MS)-based lipidomics was used to measure skeletal muscle lipid.

Results

Compared to EA, AA had lower peripheral insulin sensitivity and lower oxidative type 1 myofiber content, with no differences in total skeletal muscle lipid content. Muscles with lower type I fiber content (AA and vastus from mice) showed lower levels of lipids associated with fat oxidation capacity, i.e., cardiolipins, triacylglycerols with low saturation degree and phospholipids, compared to muscles with a higher type 1 fiber content (EA and soleus from mice). Further, we found that muscle diacylglycerol content was inversely associated with insulin sensitivity in EA, who have more type I fiber, whereas no association was found in AA. Similarly, we found that insulin sensitivity in mice was associated with diacylglycerol content in the soleus (high in type I fiber), not in vastus (low in type I fiber).Conclusions; Our data suggest that the lipid contribution to altered insulin sensitivity differs by ethnicity due to myofiber composition, and that this needs to be considered to increase our understanding of underlying mechanisms of altered insulin sensitivity in different ethnic populations.

Stem cell secretome treatment improves whole-body metabolism, reduces adiposity, and promotes skeletal muscle function in aged mice

Aging coincides with the progressive loss of muscle mass and strength, increased adiposity, and diminished physical function. Accordingly, interventions aimed at improving muscle, metabolic, and/or physical health are of interest to mitigate the adverse effects of aging. In this study, we tested a stem cell secretome product, which contains extracellular vesicles and growth, cytoskeletal remodeling, and immunomodulatory factors. We examined the effects of 4 weeks of 2×/week unilateral intramuscular secretome injections (quadriceps) in ambulatory aged male C57BL/6 mice (22-24 months) compared to saline-injected aged-matched controls. Secretome delivery substantially increased whole-body lean mass and decreased fat mass, corresponding to higher myofiber cross-sectional area and smaller adipocyte size, respectively. Secretome-treated mice also had greater whole-body physical function (grip strength and rotarod performance) and had higher energy expenditure and physical activity levels compared to control mice. Furthermore, secretome-treated mice had greater skeletal muscle Pax7+ cell abundance, capillary density, collagen IV turnover, reduced intramuscular lipids, and greater Akt and hormone sensitive lipase phosphorylation in adipose tissue. Finally, secretome treatment in vitro directly enhanced muscle cell growth and IL-6 production, and in adipocytes, it reduced lipid content and improved insulin sensitivity. Moreover, indirect treatment with secretome-treated myotube culture media also enhanced muscle cell growth and adipocyte size reduction. Together, these data suggest that intramuscular treatment with a stem cell secretome improves whole-body metabolism, physical function, and remodels skeletal muscle and adipose tissue in aged mice.

Ceramides and mitochondrial homeostasis

Lipotoxicity arises from the accumulation of lipid intermediates in non-adipose tissue, precipitating cellular dysfunction and death. Ceramide, a toxic byproduct of excessive free fatty acids, has been widely recognized as a primary contributor to lipotoxicity, mediating various cellular processes such as apoptosis, differentiation, senescence, migration, and adhesion. As the hub of lipid metabolism, the excessive accumulation of ceramides inevitably imposes stress on the mitochondria, leading to the disruption of mitochondrial homeostasis, which is typified by adequate ATP production, regulated oxidative stress, an optimal quantity of mitochondria, and controlled mitochondrial quality. Consequently, this review aims to collate current knowledge and facts regarding the involvement of ceramides in mitochondrial energy metabolism and quality control, thereby providing insights for future research.

Associations of plasma sphingolipids with measures of insulin sensitivity, β-cell function, and incident diabetes in Japanese Americans

BACKGROUND AND AIMS

To prospectively investigate associations of plasma sphingolipids with insulin sensitivity, β-cell function, and incident diabetes in the Japanese American Community Diabetes Study.

METHODS AND RESULTS

Baseline plasma samples from adults without diabetes (n = 349; mean age 56.7 years, 51 % men) were assayed for circulating ceramide and sphingomyelin species. Adjusted regression models examined cross-sectional and longitudinal associations with insulin sensitivity (HOMA2-%S), β-cell function (oral disposition index: DIo) and with incident diabetes over 5 years follow-up. Concentrations of four species (Ceramide C16:0, C18:0, C20:0, and C22:0) were inversely associated with HOMA2-%S at baseline (all P values < 0.05, Q values < 0.05) and change in HOMA2-%S over 5 years (all P values < 0.05, Q values < 0.05). No sphingolipids were associated with baseline or change in DIo. Of the four species associated with HOMA2-%S, only Ceramide C18:0 was significantly and positively associated with incident diabetes (RR/1SD 1.44, 95 % CI 1.10-1.80, P = 0.006, Q = 0.024). The association of plasma Ceramide C18:0 with the risk of diabetes was partially mediated by change in HOMA2-%S between baseline and 5 years (mediation proportion: 61.5 %, 95 % CI 21.1%-212.5 %).

CONCLUSION

Plasma Ceramide C18:0 was associated with higher risk of incident diabetes which was partially mediated through a decrease in insulin sensitivity between baseline and five years. Circulating Ceramide C18:0 could be a potential biomarker for identifying those at risk of developing diabetes.

Ceramides: a potential cardiovascular biomarker in young adult childhood cancer survivors?

Aims

The aim of this study was to investigate circulating ceramides involved in cardiovascular disease (CVD) in young adult childhood cancer survivors (CCS) and their correlations to previously reported adverse cardiovascular changes in this cohort.

Methods and results

Fifty-seven CCS and 53 healthy controls (age 20-30 years) were studied. Plasma long-chain ceramides, known to be cardiotoxic (C16:0, C18:0, C24:0, and C24:1), were analysed by mass spectrometry. The coronary event risk test 2 (CERT2) score was calculated from the ceramide data. Cardiac and carotid artery ultrasound data and lipid data available from previous studies of this cohort were used to study partial correlations with ceramide and CERT2 score data. All four analysed ceramides were elevated in CCS compared with controls (P ≤ 0.012). The greatest difference was noted for C18:0, which was 33% higher in CCS compared with controls adjusted for sex, age, and body mass index (BMI) (P < 0.001). The CERT2 score was higher in CCS compared with controls (P < 0.001). In the CCS group, 35% had a high to very high CERT2 score (7-12) when compared with 9% in the control group (P < 0.001). The CCS subgroup with a CERT2 score ≥ 7 had higher heart rate, systolic blood pressure, and higher levels of apolipoprotein B compared with CCS with a CERT2 score < 6 (P ≤ 0.011). When adjusted for age, sex, and BMI, CERT2 score was significantly correlated with arterial stiffness, growth hormone, and cranial radiotherapy (P < 0.044).

Conclusion

Ceramides could be important biomarkers in understanding the pathophysiology of CVD and in predicting CVD disease risk in young adult CCS.

Emerging roles and therapeutic potentials of sphingolipids in pathophysiology: emphasis on fatty acyl heterogeneity

Sphingolipids not only exert structural roles in cellular membranes, but also act as signaling molecules in various physiological and pathological processes. A myriad of studies have shown that abnormal levels of sphingolipids and their metabolic enzymes are associated with a variety of human diseases. Moreover, blood sphingolipids can also be used as biomarkers for disease diagnosis. This review summarizes the biosynthesis, metabolism, and pathological roles of sphingolipids, with emphasis on the biosynthesis of ceramide, the precursor for the biosynthesis of complex sphingolipids with different fatty acyl chains. The possibility of using sphingolipids for disease prediction, diagnosis, and treatment is also discussed. Targeting endogenous ceramides and complex sphingolipids along with their specific fatty acyl chain to promote future drug development will also be discussed.

Expression of Ceramide Synthases in Mice and Their Roles in Regulating Acyl-Chain Sphingolipids: A Framework for Baseline Levels and Future Implications in Aging and Disease

Sphingolipids are an important class of lipids present in all eukaryotic cells that regulate critical cellular processes. Disturbances in sphingolipid homeostasis have been linked to several diseases in humans. Ceramides are central in sphingolipid metabolism and are largely synthesized by six ceramide synthase (CerS) isoforms (CerS1-6), each with a preference for different fatty acyl chain lengths. Although the tissue distribution of CerS mRNA expression in humans and the roles of CerS isoforms in synthesizing ceramides with different acyl chain lengths are known, it is unknown how CerS expression dictates ceramides and downstream metabolites within tissues. In this study, we analyzed sphingolipid levels and CerS mRNA expression in 3-month-old C57BL/6J mouse brain, heart, kidney, liver, lung, and skeletal muscle. The results showed that CerS expression and sphingolipid species abundance varied by tissue and that CerS expression was a predictor of ceramide species within tissues. Interestingly, although CerS expression was not predictive of complex sphingolipid species within all tissues, composite scores for CerSs contributions to total sphingolipids measured in each tissue correlated to CerS expression. Lastly, we determined that the most abundant ceramide species in mouse tissues aligned with CerS mRNA expression in corresponding human tissues (based on chain length preference), suggesting that mice are relevant preclinical models for ceramide and sphingolipid research. SIGNIFICANCE STATEMENT: The current study demonstrates that ceramide synthase (CerS) expression in specific tissues correlates not only with ceramide species but contributes to the generation of complex sphingolipids as well. As many of the CerSs and/or specific ceramide species have been implicated in disease, these studies suggest the potential for CerSs as therapeutic targets and the use of sphingolipid species as diagnostics in specific tissues.

Mitochondrial electron transport chain, ceramide, and coenzyme Q are linked in a pathway that drives insulin resistance in skeletal muscle

Insulin resistance (IR) is a complex metabolic disorder that underlies several human diseases, including type 2 diabetes and cardiovascular disease. Despite extensive research, the precise mechanisms underlying IR development remain poorly understood. Previously we showed that deficiency of coenzyme Q (CoQ) is necessary and sufficient for IR in adipocytes and skeletal muscle (Fazakerley et al., 2018). Here, we provide new insights into the mechanistic connections between cellular alterations associated with IR, including increased ceramides, CoQ deficiency, mitochondrial dysfunction, and oxidative stress. We demonstrate that elevated levels of ceramide in the mitochondria of skeletal muscle cells result in CoQ depletion and loss of mitochondrial respiratory chain components, leading to mitochondrial dysfunction and IR. Further, decreasing mitochondrial ceramide levels in vitro and in animal models (mice, C57BL/6J) (under chow and high-fat diet) increased CoQ levels and was protective against IR. CoQ supplementation also rescued ceramide-associated IR. Examination of the mitochondrial proteome from human muscle biopsies revealed a strong correlation between the respirasome system and mitochondrial ceramide as key determinants of insulin sensitivity. Our findings highlight the mitochondrial ceramide-CoQ-respiratory chain nexus as a potential foundation of an IR pathway that may also play a critical role in other conditions associated with ceramide accumulation and mitochondrial dysfunction, such as heart failure, cancer, and aging. These insights may have important clinical implications for the development of novel therapeutic strategies for the treatment of IR and related metabolic disorders.

CerS6-dependent ceramide synthesis in hypothalamic neurons promotes ER/mitochondrial stress and impairs glucose homeostasis in obese mice

Dysregulation of hypothalamic ceramides has been associated with disrupted neuronal pathways in control of energy and glucose homeostasis. However, the specific ceramide species promoting neuronal lipotoxicity in obesity have remained obscure. Here, we find increased expression of the C16:0 ceramide-producing ceramide synthase (CerS)6 in cultured hypothalamic neurons exposed to palmitate in vitro and in the hypothalamus of obese mice. Conditional deletion of CerS6 in hypothalamic neurons attenuates high-fat diet (HFD)-dependent weight gain and improves glucose metabolism. Specifically, CerS6 deficiency in neurons expressing pro-opiomelanocortin (POMC) or steroidogenic factor 1 (SF-1) alters feeding behavior and alleviates the adverse metabolic effects of HFD feeding on insulin sensitivity and glucose tolerance. POMC-expressing cell-selective deletion of CerS6 prevents the diet-induced alterations of mitochondrial morphology and improves cellular leptin sensitivity. Our experiments reveal functions of CerS6-derived ceramides in hypothalamic lipotoxicity, altered mitochondrial dynamics, and ER/mitochondrial stress in the deregulation of food intake and glucose metabolism in obesity.

The heat shock protein Hsp27 controls mitochondrial function by modulating ceramide generation

Sphingolipids have key functions in membrane structure and cellular signaling. Ceramide is the central molecule of the sphingolipid metabolism and is generated by ceramide synthases (CerS) in the de novo pathway. Despite their critical function, mechanisms regulating CerS remain largely unknown. Using an unbiased proteomics approach, we find that the small heat shock protein 27 (Hsp27) interacts specifically with CerS1 but not other CerS. Functionally, our data show that Hsp27 acts as an endogenous inhibitor of CerS1. Wild-type Hsp27, but not a mutant deficient in CerS1 binding, inhibits CerS1 activity. Additionally, silencing of Hsp27 enhances CerS1-generated ceramide accumulation in cells. Moreover, phosphorylation of Hsp27 modulates Hsp27-CerS1 interaction and CerS1 activity in acute stress-response conditions. Biologically, we show that Hsp27 knockdown impedes mitochondrial function and induces lethal mitophagy in a CerS1-dependent manner. Overall, we identify an important mode of CerS1 regulation and CerS1-mediated mitophagy through protein-protein interaction with Hsp27.

Mitochondrial electron transport chain, ceramide and Coenzyme Q are linked in a pathway that drives insulin resistance in skeletal muscle

Insulin resistance (IR) is a complex metabolic disorder that underlies several human diseases, including type 2 diabetes and cardiovascular disease. Despite extensive research, the precise mechanisms underlying IR development remain poorly understood. Here, we provide new insights into the mechanistic connections between cellular alterations associated with IR, including increased ceramides, deficiency of coenzyme Q (CoQ), mitochondrial dysfunction, and oxidative stress. We demonstrate that elevated levels of ceramide in the mitochondria of skeletal muscle cells results in CoQ depletion and loss of mitochondrial respiratory chain components, leading to mitochondrial dysfunction and IR. Further, decreasing mitochondrial ceramide levels in vitro and in animal models (under chow and high fat diet) increased CoQ levels and was protective against IR. CoQ supplementation also rescued ceramide-associated IR. Examination of the mitochondrial proteome from human muscle biopsies revealed a strong correlation between the respirasome system and mitochondrial ceramide as key determinants of insulin sensitivity. Our findings highlight the mitochondrial Ceramide-CoQ-respiratory chain nexus as a potential foundation of an IR pathway that may also play a critical role in other conditions associated with ceramide accumulation and mitochondrial dysfunction, such as heart failure, cancer, and aging. These insights may have important clinical implications for the development of novel therapeutic strategies for the treatment of IR and related metabolic disorders.

Lipid remodeling of adipose tissue in metabolic health and disease

Adipose tissue is a dynamic and metabolically active organ that plays a crucial role in energy homeostasis and endocrine function. Recent advancements in lipidomics techniques have enabled the study of the complex lipid composition of adipose tissue and its role in metabolic disorders such as obesity, diabetes, and cardiovascular disease. In addition, adipose tissue lipidomics has emerged as a powerful tool for understanding the molecular mechanisms underlying these disorders and identifying bioactive lipid mediators and potential therapeutic targets. This review aims to summarize recent lipidomics studies that investigated the dynamic remodeling of adipose tissue lipids in response to specific physiological changes, pharmacological interventions, and pathological conditions. We discuss the molecular mechanisms of lipid remodeling in adipose tissue and explore the recent identification of bioactive lipid mediators generated in adipose tissue that regulate adipocytes and systemic metabolism. We propose that manipulating lipid-mediator metabolism could serve as a therapeutic approach for preventing or treating obesity-related metabolic diseases.

Fructose aggravates copper-deficiency-induced non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD), is the most common cause of chronic liver disease, affecting 24% of the global population. Accumulating evidence demonstrates that copper deficiency (CuD) is implicated in the development of NAFLD, besides, high fructose consumption by promoting inflammation contributes to NAFLD. However, how CuD and/or fructose (Fru) causes NAFLD is not clearly delineated. The present study aims to investigate the role of CuD and/or fructose supplement on hepatic steatosis and hepatic injury. We established a CuD rat model by feeding weaning male Sprague-Dawley rats for 4 weeks with CuD diet. Fructose was supplemented in drinking water. We found the promoting role of CuD or Fructose (Fru) in the progress of NAFLD, which was aggravated by combination of the two. Furthermore, we presented the alteration of hepatic lipid profiles (including content, composition, and saturation), especially ceramide (Cer), cardiolipin (CL), phosphatidylcholine (PC) and phosphatidylethanolamine (PE) was closely associated with CuD and/or Fru fed induced-NAFLD in rat models. In conclusion, insufficient copper intake or excessive fructose supplement resulted in adverse effects on the hepatic lipid profile, and fructose supplement causes a further hepatic injury in CuD-induced NAFLD, which illuminated a better understanding of NAFLD.

CERS6-derived ceramides aggravate kidney fibrosis by inhibiting PINK1-mediated mitophagy in diabetic kidney disease

During diabetic kidney disease (DKD), ectopic ceramide (CER) accumulation in renal tubular epithelial cells (RTECs) is associated with interstitial fibrosis and albuminuria. As RTECs are primarily responsible for renal energy metabolism, their function is intimately linked to mitochondrial quality control. The role of CER synthesis in the progression of diabetic renal fibrosis has not been thoroughly investigated. In this study, we observed a significant upregulation of ceramide synthase 6 (Cers6) expression in the renal cortex of db/db mice, coinciding with increased production of CER (d18:1/14:0) and CER (d18:1/16:0) by Cer6. Concurrently, the number of damaged mitochondria in RTECs rose. Cers6 deficiency reduced the abnormal accumulation of CER (d18:1/14:0) and CER (d18:1/16:0) in the kidney cortex, restoring the PTEN-induced kinase 1 (PINK1)-mediated mitophagy in RTECs, and resulting in a decrease in damaged mitochondria and attenuation of interstitial fibrosis in DKD. Automated docking analysis suggested that both CER (d18:1/14:0) and CER (d18:1/16:0) could bind to the PINK1 protein. Furthermore, inhibiting PINK1 expression in CERS6 knockdown HK-2 cells diminished the therapeutic effect of CERS6 deficiency on DKD. In summary, CERS6-derived CER (d18:1/14:0) and CER (d18:1/16:0) inhibit PINK1-regulated mitophagy by possibly binding to the PINK1 protein, thereby exacerbating the progression of renal interstitial fibrosis in DKD.NEW & NOTEWORTHY This article addresses the roles of ceramide synthase 6 (CERS6) and CERS6-derived ceramides in renal tubular epithelial cells of diabetic kidney disease (DKD) associated interstitial fibrosis. Results from knockdown of CERS6 adjusted the ceramide pool in kidney cortex and markedly protected from diabetic-induced kidney fibrosis in vivo and in vitro. Mechanically, CERS6-derived ceramides might interact with PINK1 to inhibit PINK1/Parkin-mediated mitophagy and aggravate renal interstitial fibrosis in DKD.

Deoxysphingolipids: Atypical Skeletal Muscle Lipids Related to Insulin Resistance in Humans That Decrease Insulin Sensitivity In Vitro

Sphingolipids are thought to promote skeletal muscle insulin resistance. Deoxysphingolipids (dSLs) are atypical sphingolipids that are increased in the plasma of individuals with type 2 diabetes and cause β-cell dysfunction in vitro. However, their role in human skeletal muscle is unknown. We found that dSL species are significantly elevated in muscle of individuals with obesity and type 2 diabetes compared with athletes and lean individuals and are inversely related to insulin sensitivity. Furthermore, we observed a significant reduction in muscle dSL content in individuals with obesity who completed a combined weight loss and exercise intervention. Increased dSL content in primary human myotubes caused a decrease in insulin sensitivity associated with increased inflammation, decreased AMPK phosphorylation, and altered insulin signaling. Our findings reveal a central role for dSL in human muscle insulin resistance and suggest dSLs as therapeutic targets for the treatment and prevention of type 2 diabetes.

ARTICLE HIGHLIGHTS

Deoxysphingolipids (dSLs) are atypical sphingolipids elevated in the plasma of individuals with type 2 diabetes, and their role in muscle insulin resistance has not been investigated. We evaluated dSL in vivo in skeletal muscle from cross-sectional and longitudinal insulin-sensitizing intervention studies and in vitro in myotubes manipulated to synthesize higher dSLs. dSLs were increased in the muscle of people with insulin resistance, inversely correlated to insulin sensitivity, and significantly decreased after an insulin-sensitizing intervention; increased intracellular dSL concentrations cause myotubes to become more insulin resistant. Reduction of muscle dSL levels is a potential novel therapeutic target to prevent/treat skeletal muscle insulin resistance.

Inhibiting de novo ceramide synthesis restores mitochondrial and protein homeostasis in muscle aging

Disruption of mitochondrial function and protein homeostasis plays a central role in aging. However, how these processes interact and what governs their failure in aging remain poorly understood. Here, we showed that ceramide biosynthesis controls the decline in mitochondrial and protein homeostasis during muscle aging. Analysis of transcriptome datasets derived from muscle biopsies obtained from both aged individuals and patients with a diverse range of muscle disorders revealed that changes in ceramide biosynthesis, as well as disturbances in mitochondrial and protein homeostasis pathways, are prevalent features in these conditions. By performing targeted lipidomics analyses, we found that ceramides accumulated in skeletal muscle with increasing age across Caenorhabditis elegans, mice, and humans. Inhibition of serine palmitoyltransferase (SPT), the rate-limiting enzyme of the ceramide de novo synthesis, by gene silencing or by treatment with myriocin restored proteostasis and mitochondrial function in human myoblasts, in C. elegans, and in the skeletal muscles of mice during aging. Restoration of these age-related processes improved health and life span in the nematode and muscle health and fitness in mice. Collectively, our data implicate pharmacological and genetic suppression of ceramide biosynthesis as potential therapeutic approaches to delay muscle aging and to manage related proteinopathies via mitochondrial and proteostasis remodeling.

Ceramide analogue C2-cer induces a loss in insulin sensitivity in muscle cells through the salvage/recycling pathway

Ceramides have been shown to play a major role in the onset of skeletal muscle insulin resistance and therefore in the prevalence of type 2 diabetes (T2D). However, many of the studies involved in the discovery of deleterious ceramide actions used a non-physiological cell-permeable short-chain ceramide analogue, the C2-ceramide (C2-cer). In the present study, we determined how C2-cer promotes insulin resistance in muscle cells. We demonstrate that C2-cer enters the salvage/recycling pathway and becomes de-acylated, yielding sphingosine, re-acylation of which depends on the availability of long chain fatty acids provided by the lipogenesis pathway in muscle cells. Importantly, we show these salvaged ceramides are actually responsible for the inhibition of insulin signaling induced by C2-cer. Interestingly, we also show that the exogenous and endogenous mono-unsaturated fatty acid oleate prevents C2-cer to be recycled into endogenous ceramide species in a diacylglycerol O-acyltransferase 1 (DGAT1)-dependent mechanism, which forces free fatty acid metabolism towards triacylglyceride production. Altogether, the study highlights for the first time that C2-cer induces a loss in insulin sensitivity through the salvage/recycling pathway in muscle cells. This study also validates C2-cer as a convenient tool to decipher mechanisms by which long-chain ceramides mediate insulin resistance in muscle cells and suggests that in addition to the de novo ceramide synthesis, recycling of ceramide could contribute to muscle insulin resistance observed in obesity and T2D.

Butyrate mitigates metabolic dysfunctions via the ERα-AMPK pathway in muscle in OVX mice with diet-induced obesity

The higher prevalence of metabolic syndrome (MetS) in women after menopause is associated with a decrease in circulating 17β-oestradiol. To explore novel treatments for MetS in women with oestrogen deficiency, we studied the effect of exogenous butyrate on diet-induced obesity and metabolic dysfunctions using ovariectomized (OVX) mice as a menopause model. Oral administration of sodium butyrate (NaB) reduced the body fat content and blood lipids, increased whole-body energy expenditure, and improved insulin sensitivity. Additionally, NaB induced oestrogen receptor alpha (ERα) expression, activated the phosphorylation of AMPK and PGC1α, and improved mitochondrial aerobic respiration in cultured skeletal muscle cells. In conclusion, oral NaB improves metabolic parameters in OVX mice with diet-induced obesity. Oral supplementation with NaB might provide a novel therapeutic approach to treating MetS in women with menopause. Video Abstract.

The Dark Side of Sphingolipids: Searching for Potential Cardiovascular Biomarkers

Cardiovascular diseases (CVDs) are the leading cause of death and illness in Europe and worldwide, responsible for a staggering 47% of deaths in Europe. Over the past few years, there has been increasing evidence pointing to bioactive sphingolipids as drivers of CVDs. Among them, most studies place emphasis on the cardiovascular effect of ceramides and sphingosine-1-phosphate (S1P), reporting correlation between their aberrant expression and CVD risk factors. In experimental in vivo models, pharmacological inhibition of de novo ceramide synthesis averts the development of diabetes, atherosclerosis, hypertension and heart failure. In humans, levels of circulating sphingolipids have been suggested as prognostic indicators for a broad spectrum of diseases. This article provides a comprehensive review of sphingolipids' contribution to cardiovascular, cerebrovascular and metabolic diseases, focusing on the latest experimental and clinical findings. Cumulatively, these studies indicate that monitoring sphingolipid level alterations could allow for better assessment of cardiovascular disease progression and/or severity, and also suggest them as a potential target for future therapeutic intervention. Some approaches may include the down-regulation of specific sphingolipid species levels in the circulation, by inhibiting critical enzymes that catalyze ceramide metabolism, such as ceramidases, sphingomyelinases and sphingosine kinases. Therefore, manipulation of the sphingolipid pathway may be a promising strategy for the treatment of cardio- and cerebrovascular diseases.

Potential Drug Targets for Ceramide Metabolism in Cardiovascular Disease

Cardiovascular disease poses a significant threat to the quality of human life. Metabolic abnormalities caused by excessive caloric intake have been shown to lead to the development of cardiovascular diseases. Ceramides are structural molecules found in biological membranes; they are crucial for cell survival and lipid metabolism, as they maintain barrier function and membrane fluidity. Increasing evidence has demonstrated that ceramide has a strong correlation with cardiovascular disease progression. Nevertheless, it remains a challenge to develop sphingolipids as therapeutic targets to improve the prognosis of cardiovascular diseases. In this review, we summarize the three synthesis pathways of ceramide and other intermediates that are important in ceramide metabolism. Furthermore, mechanistic studies and therapeutic strategies, including clinical drugs and bioactive molecules based on these intermediates, are discussed.

Targeted Sphingolipid Analysis in Heart, Gizzard, and Breast Muscle in Chickens Reveals Possible New Target Organs of Fumonisins

Alteration of sphingolipid synthesis is a key event in fumonisins toxicity, but only limited data have been reported regarding the effects of fumonisins on the sphingolipidome. Recent studies in chickens found that the changes in sphingolipids in liver, kidney, lung, and brain differed greatly. This study aimed to determine the effects of fumonisins on sphingolipids in heart, gizzard, and breast muscle in chickens fed 20.8 mg FB1 + FB2/kg for 9 days. A significant increase in the sphinganine:sphingosine ratio due to an increase in sphinganine was observed in heart and gizzard. Dihydroceramides and ceramides increased in the hearts of chickens fed fumonisins, but decreased in the gizzard. The dihydrosphingomyelin, sphingomyelin, and glycosylceramide concentrations paralleled those of ceramides, although the effects were less pronounced. In the heart, sphingolipids with fatty acid chain lengths of 20 to 26 carbons were more affected than those with 14-16 carbons; this difference was not observed in the gizzard. Partial least squares-discriminant analysis on sphingolipids in the heart allowed chickens to be divided into two distinct groups according to their diet. The same was the case for the gizzard. Pearson coefficients of correlation among all the sphingolipids assayed revealed strong positive correlations in the hearts of chickens fed fumonisins compared to chickens fed a control diet, as well as compared to gizzard, irrespective of the diet fed. By contrast, no effect of fumonisins was observed on sphingolipids in breast muscle.

Serum dihydroceramides correlate with insulin sensitivity in humans and decrease insulin sensitivity in vitro

Serum ceramides, especially C16:0 and C18:0 species, are linked to CVD risk and insulin resistance, but details of this association are not well understood. We performed this study to quantify a broad range of serum sphingolipids in individuals spanning the physiologic range of insulin sensitivity and to determine if dihydroceramides cause insulin resistance in vitro. As expected, we found that serum triglycerides were significantly greater in individuals with obesity and T2D compared with athletes and lean individuals. Serum ceramides were not significantly different within groups but, using all ceramide data relative to insulin sensitivity as a continuous variable, we observed significant inverse relationships between C18:0, C20:0, and C22:0 species and insulin sensitivity. Interestingly, we found that total serum dihydroceramides and individual species were significantly greater in individuals with obesity and T2D compared with athletes and lean individuals, with C18:0 species showing the strongest inverse relationship to insulin sensitivity. Finally, we administered a physiological mix of dihydroceramides to primary myotubes and found decreased insulin sensitivity in vitro without changing the overall intracellular sphingolipid content, suggesting a direct effect on insulin resistance. These data extend what is known regarding serum sphingolipids and insulin resistance and show the importance of serum dihydroceramides to predict and promote insulin resistance in humans.

Ceramide Acyl Chain Length and Its Relevance to Intracellular Lipid Regulation

Ceramides are a class of sphingolipids which are implicated in skin disorders, obesity, and other metabolic diseases. As a class with pleiotropic effects, recent efforts have centred on discerning specific ceramide species and their effects on atopic dermatitis, obesity, type 2 diabetes, and cardiovascular diseases. This delineation has allowed the identification of disease biomarkers, with long acyl chain ceramides such as C16- and C18-ceramides linked to metabolic dysfunction and cardiac function decline, while ultra-long acyl chain ceramides (>25 carbon acyl chain) were reported to be essential for maintaining a functional skin barrier. Given the intricate link between free fatty acids with ceramides, especially the de novo synthetic pathway, intracellular lipid droplet formation is increasingly viewed as an important mechanism for preventing accumulation of toxic ceramide species. Here, we review recent reports of various ceramide species involved in skin abnormalities and metabolic diseases, and we propose that promotion of lipid droplet biogenesis can be seen as a potential protective mechanism against deleterious ceramides.

The Lard Works in Mysterious Ways: Ceramides in Nutrition-Linked Chronic Disease

Diet influences onset, progression, and severity of several chronic diseases, including heart failure, diabetes, steatohepatitis, and a subset of cancers. The prevalence and clinical burden of these obesity-linked diseases has risen over the past two decades. These metabolic disorders are driven by ectopic lipid deposition in tissues not suited for fat storage, leading to lipotoxic disruption of cell function and survival. Sphingolipids such as ceramides are among the most deleterious and bioactive metabolites that accrue, as they participate in selective insulin resistance, dyslipidemia, oxidative stress and apoptosis. This review discusses our current understanding of biochemical pathways controlling ceramide synthesis, production and action; influences of diet on ceramide levels; application of circulating ceramides as clinical biomarkers of metabolic disease; and molecular mechanisms linking ceramides to altered metabolism and survival of cells. Development of nutritional or pharmacological strategies to lower ceramides could have therapeutic value in a wide range of prevalent diseases.

Contribution of specific ceramides to obesity-associated metabolic diseases

Ceramides are a heterogeneous group of bioactive membrane sphingolipids that play specialized regulatory roles in cellular metabolism depending on their characteristic fatty acyl chain lengths and subcellular distribution. As obesity progresses, certain ceramide molecular species accumulate in metabolic tissues and cause cell-type-specific lipotoxic reactions that disrupt metabolic homeostasis and lead to the development of cardiometabolic diseases. Several mechanisms for ceramide action have been inferred from studies in vitro, but only recently have we begun to better understand the acyl chain length specificity of ceramide-mediated signaling in the context of physiology and disease in vivo. New discoveries show that specific ceramides affect various metabolic pathways and that global or tissue-specific reduction in selected ceramide pools in obese rodents is sufficient to improve metabolic health. Here, we review the tissue-specific regulation and functions of ceramides in obesity, thus highlighting the emerging concept of selectively inhibiting production or action of ceramides with specific acyl chain lengths as novel therapeutic strategies to ameliorate obesity-associated diseases.

Following Roux-en-Y gastric bypass surgery, serum ceramides demarcate patients that will fail to achieve normoglycemia and diabetes remission

BACKGROUND

Obesity is a prevalent health threat and risk factor for type 2 diabetes. In this study, we evaluate the relationship between ceramides, which inhibit insulin secretion and sensitivity, and markers of glucose homeostasis and diabetes remission or recursion in patients who have undergone a Roux-en-Y gastric bypass (RYGB).

METHODS

The Utah Obesity Study is a prospective cohort study, with targeted ceramide and dihydroceramide measurements performed on banked serum samples. The Utah Obesity Study consists of 1,156 participants in three groups: a RYGB surgery group, a non-surgery group denied insurance coverage, and severely obese population controls. Clinical examinations and ceramide assessments were performed at baseline and 2 and 12 years after RYGB surgery.

FINDINGS

Surgery patients (84% female, 42.2 ± 10.6 years of age at baseline) displayed lower levels of several serum dihydroceramides and ceramides at 2 and 12 years after RYGB. By contrast, neither the control group (77% female, 48.7± 6.4 years of age at baseline) nor the non-surgery group (95% female, 43.0± 11.4 years of age at baseline) experienced significant decreases in any species. Using a linear mixed effect model, we found that multiple dihydroceramides and ceramides positively associated with the glycemic control measures HOMA-IR and HbA1c. In surgery group participants with prevalent diabetes, ceramides inversely predict diabetes remission, independent of changes in weight.

CONCLUSIONS

Ceramide decreases may explain the insulin sensitization and diabetes resolution observed in most RYGB surgery patients.

FUNDING

Funded by the National Institutes of health (NIH), The Juvenile Diabetes Research Foundation, and the American Heart Association.

Influence of the Human Lipidome on Epicardial Fat Volume in Mexican American Individuals

Introduction

Cardiovascular disease (CVD) is the leading cause of mortality worldwide and is the leading cause of death in the US. Lipid dysregulation is a well-known precursor to metabolic diseases, including CVD. There is a growing body of literature that suggests MRI-derived epicardial fat volume, or epicardial adipose tissue (EAT) volume, is linked to the development of coronary artery disease. Interestingly, epicardial fat is also actively involved in lipid and energy homeostasis, with epicardial adipose tissue having a greater capacity for release and uptake of free fatty acids. However, there is a scarcity of knowledge on the influence of plasma lipids on EAT volume.

Aim

The focus of this study is on the identification of novel lipidomic species associated with CMRI-derived measures of epicardial fat in Mexican American individuals.

Methods

We performed lipidomic profiling on 200 Mexican American individuals. High-throughput mass spectrometry enabled rapid capture of precise lipidomic profiles, providing measures of 799 unique species from circulating plasma samples. Because of our extended pedigree design, we utilized a standard quantitative genetic linear mixed model analysis to determine whether lipids were correlated with EAT by formally testing for association between each lipid species and the CMRI epicardial fat phenotype.

Results

After correction for multiple testing using the FDR approach, we identified 135 lipid species showing significant association with epicardial fat. Of those, 131 lipid species were positively correlated with EAT, where increased circulating lipid levels were correlated with increased epicardial fat. Interestingly, the top 10 lipid species associated with an increased epicardial fat volume were from the deoxyceramide (Cer(m)) and triacylglycerol (TG) families. Deoxyceramides are atypical and neurotoxic sphingolipids. Triacylglycerols are an abundant lipid class and comprise the bulk of storage fat in tissues. Pathologically elevated TG and Cer(m) levels are related to CVD risk and, in our study, to EAT volume.

Conclusion

Our results indicate that specific lipid abnormalities such as enriched saturated triacylglycerols and the presence of toxic ceramides Cer(m) in plasma of our individuals could precede CVD with increased EAT volume.

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

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

Sphingolipid metabolism plays a key role in diabetic peripheral neuropathy

INTRODUCTION

As the most common chronic complication of diabetes mellitus (DM), diabetic peripheral neuropathy (DPN) seriously affects the quality of life of DM patients. So, it is of great significance for the diagnosis and treatment of DPN. In recent years, there have been numerous studies on pathogenesis and biomarkers of DM, but there are few studies on the biomarkers of DPN.

OBJECTIVES

This research is intended to identify abnormal metabolic pathways, search for potential biomarkers of DPN, and provide a metabolic basis for the diagnosis and mechanism of DPN.

METHODS

Serum samples from 23 healthy controls (HC), 42 DM patients and 30 DPN patients and urine samples from 42 HC, 40 DM patients, and 30 DPN patients were collected. UPLC-Q-TOF/MS was used to analyze the samples. Potential biomarkers were screened from principal component analysis (PCA) to orthogonal partial least squares discriminant analysis (OPLS-DA) and further evaluated by receiver operating characteristic analysis (ROC). The biomarkers were then enriched and pathway analyzed.

RESULTS

12 potential DPN biomarkers were identified from patient's serum. 11 potential DPN biomarkers were identified from the patient's urine. Among them, the diagnostic ability of gluconic acid, lipoic acid, sphinganine, bilirubin, sphingosine and 4-hydroxybenzoic acid was increased by ROC analysis. Potential biomarkers suggest that the disorder of DPN metabolism may be linked to sphingolipid metabolism.

CONCLUSIONS

This research laid a theoretical foundation for the diagnosis and pathogenesis of DPN.

Long-chain ceramides are cell non-autonomous signals linking lipotoxicity to endoplasmic reticulum stress in skeletal muscle

The endoplasmic reticulum (ER) regulates cellular protein and lipid biosynthesis. ER dysfunction leads to protein misfolding and the unfolded protein response (UPR), which limits protein synthesis to prevent cytotoxicity. Chronic ER stress in skeletal muscle is a unifying mechanism linking lipotoxicity to metabolic disease. Unidentified signals from cells undergoing ER stress propagate paracrine and systemic UPR activation. Here, we induce ER stress and lipotoxicity in myotubes. We observe ER stress-inducing lipid cell non-autonomous signal(s). Lipidomics identifies that palmitate-induced cell stress induces long-chain ceramide 40:1 and 42:1 secretion. Ceramide synthesis through the ceramide synthase 2 de novo pathway is regulated by UPR kinase Perk. Inactivation of CerS2 in mice reduces systemic and muscle ceramide signals and muscle UPR activation. The ceramides are packaged into extracellular vesicles, secreted and induce UPR activation in naïve myotubes through dihydroceramide accumulation. This study furthers our understanding of ER stress by identifying UPR-inducing cell non-autonomous signals.

Ceramides as Mediators of Oxidative Stress and Inflammation in Cardiometabolic Disease

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

Dihydroceramide- and ceramide-profiling provides insights into human cardiometabolic disease etiology

Metabolic alterations precede cardiometabolic disease onset. Here we present ceramide- and dihydroceramide-profiling data from a nested case-cohort (type 2 diabetes [T2D, n = 775]; cardiovascular disease [CVD, n = 551]; random subcohort [n = 1137]) in the prospective EPIC-Potsdam study. We apply the novel NetCoupler-algorithm to link a data-driven (dihydro)ceramide network to T2D and CVD risk. Controlling for confounding by other (dihydro)ceramides, ceramides C18:0 and C22:0 and dihydroceramides C20:0 and C22:2 are associated with higher and ceramide C20:0 and dihydroceramide C26:1 with lower T2D risk. Ceramide C16:0 and dihydroceramide C22:2 are associated with higher CVD risk. Genome-wide association studies and Mendelian randomization analyses support a role of ceramide C22:0 in T2D etiology. Our results also suggest that (dh)ceramides partly mediate the putative adverse effect of high red meat consumption and benefits of coffee consumption on T2D risk. Thus, (dihydro)ceramides may play a critical role in linking genetic predisposition and dietary habits to cardiometabolic disease risk.

Among lipid classes, ceramides are linked to impaired cardiometabolic health. Here the authors report the association of specific ceramides and dihydroceramides on the risk of developing type 2 diabetes and or cardiovascular disease in a prospective population cohort.

Fibroblast growth factor-21 improves insulin action in nonlactating ewes

During metabolically demanding physiological states, ruminants and other mammals coordinate nutrient use among tissues by varying the set point of insulin action. This set point is regulated in part by metabolic hormones with some antagonizing (e.g., growth hormone and TNFα) and others potentiating (e.g., adiponectin) insulin action. Fibroblast growth factor-21 (FGF21) was recently identified as a sensitizing hormone in rodent and primate models of defective insulin action. FGF21 administration, however, failed to improve insulin action in dairy cows during the naturally occurring insulin resistance of lactation, raising the possibility that ruminants as a class of animals or lactation as a physiological state are unresponsive to FGF21. To start addressing this question, we asked whether FGF21 could improve insulin action in nonlactating ewes. Gene expression studies showed that the ovine FGF21 system resembles that of other species, with liver as the major site of FGF21 expression and adipose tissue as a target tissue based on high expression of the FGF21 receptor complex and activation of p44/42 extracellular signal-regulated kinase (ERK1/2) following exogenous FGF21 administration. FGF21 treatment for 13 days reduced plasma glucose and insulin over the entire treatment period and improved glucose disposal during a glucose tolerance test. FGF21 increased plasma adiponectin by day 3 of treatment but had no effect on the plasma concentrations of total, C16:0-, or C18:0-ceramide. Overall, these data confirm that the insulin-sensitizing effects of FGF21 are conserved in ruminants and raise the possibility that lactation is an FGF21-resistant state.

Emerging Roles of Ceramide in Cardiovascular Diseases

Ceramide is a core molecule of sphingolipid metabolism that causes selective insulin resistance and dyslipidemia. Research on its involvement in cardiovascular diseases has grown rapidly. In resting cells, ceramide levels are extremely low, while they rapidly accumulate upon encountering external stimuli. Recently, the regulation of ceramide levels under pathological conditions, including myocardial infarction, hypertension, and atherosclerosis, has drawn great attention. Increased ceramide levels are strongly associated with adverse cardiovascular risks and events while inhibiting the synthesis of ceramide or accelerating its degradation improves a variety of cardiovascular diseases. In this article, we summarize the role of ceramide in cardiovascular disease, investigate the possible application of ceramide as a new diagnostic biomarker and a therapeutic target for cardiovascular disorders, and highlight the remaining problems.

Inhibition of sphingolipid de novo synthesis counteracts muscular dystrophy

Duchenne muscular dystrophy (DMD), the most common muscular dystrophy, is a severe muscle disorder, causing muscle weakness, loss of independence, and premature death. Here, we establish the link between sphingolipids and muscular dystrophy. Transcripts of sphingolipid de novo biosynthesis pathway are up-regulated in skeletal muscle of patients with DMD and other muscular dystrophies, which is accompanied by accumulation of metabolites of the sphingolipid pathway in muscle and plasma. Pharmacological inhibition of sphingolipid synthesis by myriocin in the mdx mouse model of DMD ameliorated the loss in muscle function while reducing inflammation, improving Ca²⁺ homeostasis, preventing fibrosis of the skeletal muscle, heart, and diaphragm, and restoring the balance between M1 and M2 macrophages. Myriocin alleviated the DMD phenotype more than glucocorticoids. Our study identifies inhibition of sphingolipid synthesis, targeting multiple pathogenetic pathways simultaneously, as a strong candidate for treatment of muscular dystrophies.

Exploration of Potential Biomarkers for Type 2 Diabetes by UPLC-QTOF-MS and WGCNA of Skin Surface Lipids

BACKGROUND

Diabetes has become popular and has become one of the most important global health care challenges. Patients with diabetes have a high incidence of skin diseases. Cell and animal models are often used to study the skin conditions of people with diabetes.

METHODS

In this study, a volunteer questionnaire survey, skin lipomics analysis based on ultra-high performance liquid chromatography-quadrupole tandem time-of-flight mass spectrometry (UPLC-QTOF-MS), and weighted gene co-expression network analysis (WGCNA) were used to study the differences in skin conditions and skin lipids of participants with type 2 diabetes mellitus (Group D) versus healthy individuals (Group H) and the correlation between these groups. The questionnaire was used to investigate personal basic, diabetes, and facial skin status information of 77 female volunteers aged 55-65 years old from the Peking University Shougang Hospital. The facial skin lipids of all volunteers were analysed by UPLC-QTOF-MS technique; the differential lipids between groups D and H were analysed by partial least-squared discriminant and univariate analysis.

RESULTS

In total, 23 kinds of differential lipids were identified, all of which belonged to sphingolipids. The use of WGCNA combined clinical information with lipid analysis to study the relationship between glycosylated haemoglobin, skin pigmentation/non-pigmentation, and skin lipids. Two types of lipids were identified to distinguish between hub lipids of high and low glycosylated haemoglobin; 12 types of lipids were identified that could distinguish between the hub lipids of pigmented and non-pigmented participants (PLS-DA).

CONCLUSION

The experimental results not only provide a reference for the diagnosis and classification of diabetes via analysing the skin lipids of patients, but also provides a theoretical basis for further study on the effects of diabetes on the skin of patients.

CerS1 but Not CerS5 Gene Silencing, Improves Insulin Sensitivity and Glucose Uptake in Skeletal Muscle

Skeletal muscle is perceived as a major tissue in glucose and lipid metabolism. High fat diet (HFD) lead to the accumulation of intramuscular lipids, including: long chain acyl-CoA, diacylglycerols, and ceramides. Ceramides are considered to be one of the most important lipid groups in the generation of skeletal muscle insulin resistance. So far, it has not been clearly established whether all ceramides adversely affect the functioning of the insulin pathway, or whether there are certain ceramide species that play a pivotal role in the induction of insulin resistance. Therefore, we designed a study in which the expression of CerS1 and CerS5 genes responsible for the synthesis of C18:0-Cer and C16:0-Cer, respectively, was locally silenced in the gastrocnemius muscle of HFD-fed mice through in vivo electroporation-mediated shRNA plasmids. Our study indicates that HFD feeding induced both, the systemic and skeletal muscle insulin resistance, which was accompanied by an increase in the intramuscular lipid levels, decreased activation of the insulin pathway and, consequently, a decrease in the skeletal muscle glucose uptake. CerS1 silencing leads to a reduction in C18:0-Cer content, with a subsequent increase in the activity of the insulin pathway, and an improvement in skeletal muscle glucose uptake. Such effects were not visible in case of CerS5 silencing, which indicates that the accumulation of C18:0-Cer plays a decisive role in the induction of skeletal muscle insulin resistance.

The Changes of Lipidomic Profiles Reveal Therapeutic Effects of Exenatide in Patients With Type 2 Diabetes

OBJECTIVE

Exenatide has been demonstrated beneficial effects on patients with type 2 diabetes mellitus (T2DM) regarding lipid metabolism. However, the potential mechanism remains unclear. We used a lipidomic approach to evaluate lipid changes in response to treatment with exenatide in T2DM patients.

METHODS

Serum lipidomic profiles of 35 newly diagnosed T2DM patients (before and after exenatide treatment) and 20 age-matched healthy controls were analyzed by ultrahigh-performance liquid chromatography-tandem quadrupole time-of-flight mass spectrometry.

RESULTS

A total of 45 lipid species including sphingomyelins (SMs), ceramides (CERs), lysophosphatidylcholines (LPCs), phosphatidylethanolamines (PEs), lysophosphatidylethanolamines (LPEs) and phosphatidylcholines (PCs) were identified in all participants. Compared to the healthy controls, 13 lipid species [SM (d18:1/18:0, d18:1/18:1), Cer (d18:1/18:0, d18:1/16:0, d18:1/20:0, d18:1/24:1), LPC (15:0, 16:0, 17:0), PC (19:0/19:0), LPE (18:0) and PE (16:0/22:6, 18:0/22:6)] were markedly increased in the T2DM group, while PE (17:0/17:0) and PC (18:1/18:0) were decreased (P < 0.05). The serum SM (d18:1/18:0, d18:1/18:1), LPC (16:0), and LPE (18:0) were significantly decreased after exenatide treatment, which was accompanied by the amelioration of lipids and glycemic parameters (TC, LDL-C, ApoA-I, FCP and HbA_1c) in T2DM patients. The chord diagrams showed distinct correlation patterns between lipid classes and subclasses among healthy controls, T2DM patients before and after exenatide treatment.

CONCLUSION

Our results revealed that the therapeutic benefits of exenatide on T2DM might be involved in the improved lipid metabolism, especially SM, LPC, and LPE.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov, identifier NCT03297879.

Lipidomic approaches to dissect dysregulated lipid metabolism in kidney disease

Dyslipidaemia is a hallmark of chronic kidney disease (CKD). The severity of dyslipidaemia not only correlates with CKD stage but is also associated with CKD-associated cardiovascular disease and mortality. Understanding how lipids are dysregulated in CKD is, however, challenging owing to the incredible diversity of lipid structures. CKD-associated dyslipidaemia occurs as a consequence of complex interactions between genetic, environmental and kidney-specific factors, which to understand, requires an appreciation of perturbations in the underlying network of genes, proteins and lipids. Modern lipidomic technologies attempt to systematically identify and quantify lipid species from biological systems. The rapid development of a variety of analytical platforms based on mass spectrometry has enabled the identification of complex lipids at great precision and depth. Insights from lipidomics studies to date suggest that the overall architecture of free fatty acid partitioning between fatty acid oxidation and complex lipid fatty acid composition is an important driver of CKD progression. Available evidence suggests that CKD progression is associated with metabolic inflexibility, reflecting a diminished capacity to utilize free fatty acids through β-oxidation, and resulting in the diversion of accumulating fatty acids to complex lipids such as triglycerides. This effect is reversed with interventions that improve kidney health, suggesting that targeting of lipid abnormalities could be beneficial in preventing CKD progression.

Involvement of Ceramides in Non-Alcoholic Fatty Liver Disease (NAFLD) Atherosclerosis (ATS) Development: Mechanisms and Therapeutic Targets

Non-alcoholic fatty liver disease (NAFLD) and atherosclerosis (ATS) are worldwide known diseases with increased incidence and prevalence. These two are driven and are interconnected by multiple oxidative and metabolic functions such as lipotoxicity. A gamut of evidence suggests that sphingolipids (SL), such as ceramides, account for much of the tissue damage. Although in humans they are proving to be accurate biomarkers of adverse cardiovascular disease outcomes and NAFLD progression, in rodents, pharmacological inhibition or depletion of enzymes driving de novo ceramide synthesis prevents the development of metabolic driven diseases such as diabetes, ATS, and hepatic steatosis. In this narrative review, we discuss the pathways which generate the ceramide synthesis, the potential use of circulating ceramides as novel biomarkers in the development and progression of ATS and related diseases, and their potential use as therapeutic targets in NAFDL-ATS development which can further provide new clues in this field.

Ovarian insufficiency impairs glucose-stimulated insulin secretion through activation of hypothalamic de novo ceramide synthesis

Oestrogens regulate body weight through their action on hypothalamus to modulate food intake and energy expenditure. Hypothalamic de novo ceramide synthesis plays a central role on obesity induced by oestrogen deficiency. Depletion in oestrogens is also known to be associated with glucose intolerance, which favours type 2 diabetes (T2D). However, the implication of hypothalamic ceramide in the regulation of glucose homeostasis by oestrogen is unknown. Here, we studied glucose homeostasis and insulin secretion in ovariectomized (OVX) female rats. OVX induces body weight gain associated with a hypothalamic inflammation and impaired glucose homeostasis. Genetic blockade of ceramide synthesis in the ventromedial nucleus of the hypothalamus (VMH) reverses hypothalamic inflammation and partly restored glucose tolerance induced by OVX. Furthermore, glucose-stimulated insulin secretion (GSIS) is increased in OVX rats due to a raise of insulin secretion second phase, a characteristic of early stage of T2D. In contrast, GSIS from isolated islets of OVX rats is totally blunted. Inhibition of ceramide synthesis in the VMH restores GSIS from isolated OVX islets and represses the second phase of insulin secretion. Stimulation of oestrogen receptor α (ERα) by oestradiol (E2) down-regulates ceramide synthesis in hypothalamic neuronal GT1-7 cells but no in microglial SIM-A9 cells. In contrast, genetic inactivation of ERα in VMH upregulates ceramide synthesis. These results indicate that hypothalamic neuronal de novo ceramide synthesis triggers the OVX-dependent impairment of glucose homeostasis which is partly mediated by a dysregulation of GSIS.

Effects of serine palmitoyltransferase inhibition by myriocin in ad libitum-fed and nutrient-restricted ewes

The fungal isolate myriocin inhibits serine palmitoyltransferase and de novo ceramide synthesis in rodents; however, the effects of myriocin on ceramide concentrations and metabolism have not been previously investigated in ruminants. In our study, 12 non-lactating crossbred ewes received an intravenous bolus of myriocin (0, 0.1, 0.3, or 1.0 mg/kg/body weight [BW]; CON, LOW, MOD, or HIGH) every 48 h for 17 d. Ewes consumed a high-energy diet from day 1 to 14 and were nutrient-restricted (straw only) from day 15 to 17. Blood was collected preprandial and at 1, 6, and 12 h relative to bolus and nutrient restriction. Tissues were collected following euthanasia on day 17. Plasma was analyzed for free fatty acids (FFAs), glucose, and insulin. Plasma and tissue ceramides were quantified using mass spectrometry. HIGH selectively decreased metabolizable energy intake, BW, and plasma insulin, and increased plasma FFA (Dose, P < 0.05). Myriocin linearly decreased plasma very-long-chain (VLC) ceramide and dihydroceramide (DHCer) by day 13 (Linear, P < 0.05). During nutrient restriction, fold-change in FFA was lower with increasing dose (P < 0.05). Nutrient restriction increased plasma C16:0-Cer, an effect suppressed by MOD and HIGH (Dose × Time, P < 0.05). Myriocin linearly decreased most ceramide and DHCer species in the liver and omental and mesenteric adipose, VLC ceramide and DHCer in the pancreas, and C18:0-Cer in skeletal muscle and subcutaneous adipose tissue (Linear, P ≤ 0.05). We conclude that the intravenous delivery of 0.3 mg of myriocin/kg of BW/48 h decreases circulating and tissue ceramide without modifying energy intake in ruminants.

Beneficial Metabolic Effects of TREM2 in Obesity Are Uncoupled From Its Expression on Macrophages

Obesity-induced white adipose tissue (WAT) hypertrophy is associated with elevated adipose tissue macrophage (ATM) content. Overexpression of the triggering receptor expressed on myeloid cells 2 (TREM2) reportedly increases adiposity, worsening health. Paradoxically, using insulin resistance, elevated fat mass, and hypercholesterolemia as hallmarks of unhealthy obesity, a recent report demonstrated that ATM-expressed TREM2 promoted health. Here, we identified that in mice, TREM2 deficiency aggravated diet-induced insulin resistance and hepatic steatosis independently of fat and cholesterol levels. Metabolomics linked TREM2 deficiency with elevated obesity-instigated serum ceramides that correlated with impaired insulin sensitivity. Remarkably, while inhibiting ceramide synthesis exerted no influences on TREM2-dependent ATM remodeling, inflammation, or lipid load, it restored insulin tolerance, reversing adipose hypertrophy and secondary hepatic steatosis of TREM2-deficient animals. Bone marrow transplantation experiments revealed unremarkable influences of immune cell-expressed TREM2 on health, instead demonstrating that WAT-intrinsic mechanisms impinging on sphingolipid metabolism dominate in the systemic protective effects of TREM2 on metabolic health.

Association of plasma ceramides with prevalent and incident type 2 diabetes mellitus in middle and older aged adults

AIMS

The role of ceramides in the pathogenesis of type 2 diabetes mellitus (T2DM) is incompletely characterized. Given that ceramides represent therapeutic targets to disrupt the euglycemia-T2DM transition, we aimed to characterize their association with prevalent and incident T2DM in a novel cohort.

METHODS

We examined the cross-sectional and longitudinal association of baseline ceramides with prevalent and incident T2DM among 1423 adults (47% women; median (range) baseline age 72 (51-95) years) in the Mayo Clinic Study of Aging cohort. We examined the associations of ceramides with prevalent T2DM (adjusted odds ratio [95% confidence interval]) at baseline and incident T2DM (adjusted hazard ratio [95% confidence interval]) during median follow-up of 6.2 years, after adjusting for demographic and metabolic factors.

RESULTS

Among 1423 adults, there were 222 prevalent and 37 incident cases of T2DM. In cross-sectional analyses, higher levels of ceramide C16:0 were associated with lower odds of prevalent T2DM (aOR 0.84 [0.71-0.99];P = 0.03) whereas C18:0 (aOR 1.27 [1.06-1.42];P = 0.01), C18:0/16:0 (aOR 1.41 [1.22-1.62]; P < 0.001) and C18:0/24:0 (aOR 1.22 [1.05-1.41]; P = 0.01) were associated with higher odds. In Cox hazard regression models, C18:0/16:0 (aHR 1.63 [1.26-2.10];P < 0.001) and C18:0 (aHR 1.53 [1.12-2.08];P = 0.01) were associated with increased risk of incident T2DM.

CONCLUSIONS

In this prospective population-based cohort, ceramides were associated with prevalent T2DM (C16:0,C18:0, C18:0/C16:0 ratio, C18:0/C24:0 ratio) and incident T2DM (C18:0, C18:0/C16:0 ratio) and could suggest targets for the primary and secondary prevention of T2DM.

Appearance of Intact Molecules of Dietary Ceramides Prepared from Soy Sauce Lees and Rice Glucosylceramides in Mouse Plasma

Although the beneficial effects of dietary sphingolipids have recently been reported, the mechanism of their intestinal absorption has yet to be fully elucidated. In this study, the absorption and metabolism of dietary ceramides and glucosylceramides were evaluated by liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis in the plasma of mice after a single oral administration. Ceramide molecules prepared from soy sauce lees (mainly composed of phytosphingosine and its derivatives) were undetectable or minor compounds in the plasma of control mice but appeared 1-6 h after administration. Rice glucosylceramide (mainly composed of sphingadienine) was endogenously detected in mouse plasma and showed a tendency to increase 1-6 h after administration by LC-MS/MS analysis. In addition, the ceramide molecules, which are hydrolysates of dietary glucosylceramide, were significantly increased in the plasma after administration. These findings strongly suggest that dietary ceramides and glucosylceramides are partly absorbed as intact molecules or hydrolysates.