Sphingolipids and insulin resistance: the five Ws

Metabolomics: a new frontier in neurodegenerative disease biomarker discovery

BACKGROUND

Neurodegenerative disorders are a group of debilitating diseases affecting the central nervous system, and are characterized by the progressive loss of neurons, leading to declines in cognitive function, movement, and overall quality of life. While the exact causes remain elusive, it's believed that a combination of genetic, environmental, and lifestyle factors contribute to their development. Metabolites, the end products of cellular processes, reflect the physiological state of an organism. By analysing these molecules, researchers can gain a deeper understanding of the underlying metabolic changes associated with neurodegenerative disorders.

AIM OF REVIEW

This review aims to explore the possibilities between metabolites and their association with neurodegenerative disorders such as amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), Parkinson's disease (PD), Multiple sclerosis (MS) and Huntington's disease (HD).

KEY SCIENTIFIC CONCEPTS OF REVIEW

Metabolomic studies could potentially illuminate altered biochemical pathways, facilitating earlier detection and treatment of these conditions. Metabolomic investigations have revealed the role of oxidative stress, alterations in glucose and fat metabolism, mitochondrial dysfunction, apoptosis, glutamate excitotoxicity and alterations in myelin composition in neurodegenerative disorders. The common metabolic biomarkers identified includes glutamate, taurine, uric acid, branched chain amino acids, acylcarnitine, creatinine, choline, with some more amino acids and lipids. Metabolomics offers valuable insights into disease mechanisms and potential therapeutic targets by identifying biochemical and metabolic alterations, but still there are several aspects to be explored for accurate mapping of metabolites with specific pathway involved in the disease.

The role of sphingolipids in heart failure

Advanced heart failure (HF) is characterized by changes in the structure, function, and metabolism of cardiac muscle. As the disease progresses, cardiomyocytes shift their ATP production from fatty acid oxidation to glycolysis. This shift results in an accumulation of lipid metabolites, particularly sphingolipids, which can disrupt normal cellular function and contribute to cardiac dysfunction. In animal models of obesity, accumulation of toxic sphingolipid metabolites in the heart has been described as cardiac lipotoxicity. In humans, HF is classified into two groups based on ejection fraction (EF): HF with reduced EF of less than 40% (HFrEF) and HF with preserved EF of greater than 50% (HFpEF). Despite shared risk factors and comorbidities, the structural and cellular differences between HFrEF and HFpEF distinguish them as separate conditions. Ceramides (Cer), a type of sphingolipid, have gained significant attention for their involvement in the development and prognosis of atherosclerotic disease and myocardial infarction, while sphingosine-1-phosphate, a downstream product of Cer, has shown cardioprotective properties. The aim of this review is to describe the role of sphingolipids in HF with reduced and preserved EF. By understanding the role of sphingolipids through animal and human studies, this review aims to pave the way for developing strategies that target abnormal signalling pathways in the failing heart, ultimately bridging the gap between scientific research and clinical applications.

Metabolomic signatures of cognitive function in a type 2 Diabetes-Enriched cohort

To understand the relationship between type 2 diabetes (T2D) and risk for developing cognitive impairment, this study is the first to examine association between metabolites measured at mid-life and cognitive performance assessed later in life (8-10 years) in a T2D-enriched cohort. The discovery set included metabolomics from European Americans (EAs; n = 137) and African Americans (AAs; n = 134) from the Diabetes Heart Study (DHS) and the African American-DHS (AA-DHS). The cognitive testing battery included measures of executive function, memory, attention, language, and global cognition. Ancestry-specific linear regression analyses were performed and a false discovery rate (FDR)-corrected p-value was used to assess significance. Overall, fewer significant metabolites were associated with cognitive performance in AAs (n = 19) as compared to EAs (n = 118) suggesting racial differences. There was a positive association between sphingomyelins and cognitive performance, consistent with prior reports. Novel findings implicated partially characterized metabolites linked to oxidative breakdown of bilirubin to multiple cognitive domains suggesting further exploration of this class of metabolites towards improving pathophysiologic understanding and early intervention. Cross-ancestry replication identified four metabolites that generalized to both populations. Replication was performed among additional study participants, i.e. 421 EAs and 167 AAs, followed by a formal meta-analysis. Replication bolstered the association of multiple metabolites with cognitive function. Among these, cortisol was associated in AAs suggesting a link between stress and risk for reduced cognitive function. Further work is needed to provide insight into the pathophysiologic mechanisms and highlight metabolites for inclusion in risk stratification models of cognitive performance.

Metabolic disorders, inter-organ crosstalk, and inflammation in the progression of metabolic dysfunction-associated steatotic liver disease

Metabolic dysfunction-associated steatotic liver disease (MASLD) represents a global health concern, affecting over 30 % of adults. It is a principal driver in the development of cirrhosis and hepatocellular carcinoma. The complex pathogenesis of MASLD involves an excessive accumulation of lipids, subsequently disrupting lipid metabolism and prompting inflammation within the liver. This review synthesizes the recent research progress in understanding the mechanisms contributing to MASLD progression, with particular emphasis on metabolic disorders and interorgan crosstalk. We highlight the molecular mechanisms linked to these factors and explore their potential as novel targets for pharmacological intervention. The insights gleaned from this article have important implications for both the prevention and therapeutic management of MASLD.

Lipid metabolism in MASLD and MASH: From mechanism to the clinic

Metabolic dysfunction-associated steatotic liver disease/steatohepatitis (MASLD/MASH) is recognised as a metabolic disease characterised by excess intrahepatic lipid accumulation due to lipid overflow and synthesis, alongside impaired oxidation and/or export of these lipids. But where do these lipids come from? The main pathways related to hepatic lipid accumulation are de novo lipogenesis and excess fatty acid transport to the liver (due to increased lipolysis, adipose tissue insulin resistance, as well as excess dietary fatty acid intake, in particular of saturated fatty acids). Not only triglycerides but also other lipids are secreted by the liver and are associated with a worse histological profile in MASH, as shown by lipidomics. Herein, we review the role of lipid metabolism in MASLD/MASH and discuss the impact of weight loss (diet, bariatric surgery, GLP-1RAs) or other pharmacological treatments (PPAR or THRβ agonists) on hepatic lipid metabolism, lipidomics, and the resolution of MASH.

Online Direct Infusion Mass Spectrometry of Liquid-Liquid Extraction Phases for Metabolite and Lipid Profiling with the Direct Infusion Probe

Background/Objectives: Profiling of metabolites and lipids in biological samples can provide invaluable insights into life-sustaining chemical processes. The ability to detect both metabolites and lipids in the same sample can enhance these understandings and connect cellular dynamics. However, simultaneous detection of metabolites and lipids is generally hampered by chromatographic systems tailored to one molecular type. This void can be filled by direct infusion mass spectrometry (MS), where all ionizable molecules can be detected simultaneously. However, in direct infusion MS, the high chemical complexity of biological samples can introduce limitations in detectability due to matrix effects causing ionization suppression. Methods: Decreased sample complexity and increased detectability and molecular coverage was provided by combining our direct infusion probe (DIP) with liquid-liquid extraction (LLE) and directly sampling the different phases for direct infusion. Three commonly used LLE methods for separating lipids and metabolites were evaluated. Results: The butanol-methanol (BUME) method was found to be preferred since it provides high molecular coverage and have low solvent toxicity. The established BUME DIP-MS method was used as a fast and sensitive analysis tool to study chemical changes in insulin-secreting cells upon glucose stimulation. By analyzing the metabolome at distinct time points, down to 1-min apart, we found high dynamics of the intracellular metabolome. Conclusions: The rapid workflow with LLE DIP-MS enables higher sensitivity of phase separated metabolites and lipids. The application of BUME DIP-MS provides novel information on the dynamics of the intracellular metabolome of INS-1 during the two phases of insulin release for both metabolite and lipid classes.

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.

Adiponectin overexpression improves metabolic abnormalities caused by acid ceramidase deficiency but does not prolong lifespan in a mouse model of Farber Disease

Farber Disease is a debilitating and lethal childhood disease of ceramide accumulation caused by acid ceramidase deficiency. The potent induction of a ligand-gated neutral ceramidase activity promoted by adiponectin may provide sufficient lowering of ceramides to allow for the treatment of Farber Disease. In vitro, adiponectin or adiponectin receptor agonist treatments lowered total ceramide concentrations in human fibroblasts from a patient with Farber Disease. However, adiponectin overexpression in a Farber Disease mouse model did not improve lifespan or immune infiltration. Intriguingly, mice heterozygous for the Farber Disease mutation were more prone to glucose intolerance and insulin resistance when fed a high-fat diet, and adiponectin overexpression protected from these metabolic perturbations. These studies suggest that adiponectin evokes a ceramidase activity that is not reliant on the functional expression of acid ceramidase, but indicates that additional strategies are required to ameliorate outcomes of Farber Disease.

Sphingolipids in mitochondria-from function to disease

Sphingolipids are not only structural components of cellular membranes but also play vital roles in cell signaling and modulation of cellular processes. Within mitochondria, sphingolipids exert diverse effects on mitochondrial dynamics, energy metabolism, oxidative stress, and cell death pathways. In this review, we summarize literature addressing the crucial role of sphingolipids in mitochondria, highlighting their impact on mitochondrial dynamics, cellular bioenergetics, and important cell processes including apoptosis and mitophagy.

Integrative metabolomics science in Alzheimer's disease: Relevance and future perspectives

Alzheimer's disease (AD) is determined by various pathophysiological mechanisms starting 10-25 years before the onset of clinical symptoms. As multiple functionally interconnected molecular/cellular pathways appear disrupted in AD, the exploitation of high-throughput unbiased omics sciences is critical to elucidating the precise pathogenesis of AD. Among different omics, metabolomics is a fast-growing discipline allowing for the simultaneous detection and quantification of hundreds/thousands of perturbed metabolites in tissues or biofluids, reproducing the fluctuations of multiple networks affected by a disease. Here, we seek to critically depict the main metabolomics methodologies with the aim of identifying new potential AD biomarkers and further elucidating AD pathophysiological mechanisms. From a systems biology perspective, as metabolic alterations can occur before the development of clinical signs, metabolomics - coupled with existing accessible biomarkers used for AD screening and diagnosis - can support early disease diagnosis and help develop individualized treatment plans. Presently, the majority of metabolomic analyses emphasized that lipid metabolism is the most consistently altered pathway in AD pathogenesis. The possibility that metabolomics may reveal crucial steps in AD pathogenesis is undermined by the difficulty in discriminating between the causal or epiphenomenal or compensatory nature of metabolic findings.

Noncanonical Regulation of cAMP-Dependent Insulin Secretion and Its Implications in Type 2 Diabetes

Impaired glucose tolerance (IGT) and β-cell dysfunction in insulin resistance associated with obesity lead to type 2 diabetes (T2D). Glucose-stimulated insulin secretion (GSIS) from β-cells occurs via a canonical pathway that involves glucose metabolism, ATP generation, inactivation of K ATP channels, plasma membrane depolarization, and increases in cytosolic concentrations of [Ca 2+ ] c . However, optimal insulin secretion requires amplification of GSIS by increases in cyclic adenosine monophosphate (cAMP) signaling. The cAMP effectors protein kinase A (PKA) and exchange factor activated by cyclic-AMP (Epac) regulate membrane depolarization, gene expression, and trafficking and fusion of insulin granules to the plasma membrane for amplifying GSIS. The widely recognized lipid signaling generated within β-cells by the β-isoform of Ca 2+ -independent phospholipase A 2 enzyme (iPLA 2 β) participates in cAMP-stimulated insulin secretion (cSIS). Recent work has identified the role of a G-protein coupled receptor (GPCR) activated signaling by the complement 1q like-3 (C1ql3) secreted protein in inhibiting cSIS. In the IGT state, cSIS is attenuated, and the β-cell function is reduced. Interestingly, while β-cell-specific deletion of iPLA 2 β reduces cAMP-mediated amplification of GSIS, the loss of iPLA 2 β in macrophages (MØ) confers protection against the development of glucose intolerance associated with diet-induced obesity (DIO). In this article, we discuss canonical (glucose and cAMP) and novel noncanonical (iPLA 2 β and C1ql3) pathways and how they may affect β-cell (dys)function in the context of impaired glucose intolerance associated with obesity and T2D. In conclusion, we provide a perspective that in IGT states, targeting noncanonical pathways along with canonical pathways could be a more comprehensive approach for restoring β-cell function in T2D. © 2023 American Physiological Society. Compr Physiol 13:5023-5049, 2023.

Hepatocyte-derived GDF15 suppresses feeding and improves insulin sensitivity in obese mice

Growth differentiation factor 15 (GDF15) is a stress-induced secreted protein whose circulating levels are increased in the context of obesity. Recombinant GDF15 reduces body weight and improves glycemia in obese models, which is largely attributed to the central action of GDF15 to suppress feeding and reduce body weight. Despite these advances in knowledge, the tissue-specific sites of GDF15 production during obesity are unknown, and the effects of modulating circulating GDF15 levels on insulin sensitivity have not been evaluated directly. Here, we demonstrate that hepatocyte Gdf15 expression is sufficient for changes in circulating levels of GDF15 during obesity and that restoring Gdf15 expression specifically in hepatocytes of Gdf15 knockout mice results in marked improvements in hyperinsulinemia, hepatic insulin sensitivity, and to a lesser extent peripheral insulin sensitivity. These data support that liver hepatocytes are the primary source of circulating GDF15 in obesity.

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.

Advances in Proteomic and Metabolomic Profiling of Neurodegenerative Diseases

Proteomics and metabolomics are two emerging fields that hold promise to shine light on the molecular mechanisms causing neurodegenerative diseases. Research in this area may reveal and quantify specific metabolites and proteins that can be targeted by therapeutic interventions intended at halting or reversing the neurodegenerative process. This review aims at providing a general overview on the current status of proteomic and metabolomic profiling in neurodegenerative diseases. We focus on the most common neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. We discuss the relevance of state-of-the-art metabolomics and proteomics approaches and their potential for biomarker discovery. We critically review advancements made so far, highlighting how metabolomics and proteomics may have a significant impact in future therapeutic and biomarker development. Finally, we further outline technologies used so far as well as challenges and limitations, placing the current information in a future-facing context.

Possible Gender Influence in the Mechanisms Underlying the Oxidative Stress, Inflammatory Response, and the Metabolic Alterations in Patients with Obesity and/or Type 2 Diabetes

The number of patients afflicted by type 2 diabetes and its morbidities has increased alarmingly, becoming the cause of many deaths. Normally, during nutrient intake, insulin secretion is increased and glucagon secretion is repressed, but when plasma glucose concentration increases, a state of prediabetes occurs. High concentration of plasma glucose breaks the redox balance, inducing an oxidative stress that promotes chronic inflammation, insulin resistance, and impaired insulin secretion. In the same context, obesity is one of the most crucial factors inducing insulin resistance, inflammation, and contributing to the onset of type 2 diabetes. Measurements of metabolites like glucose, fructose, amino acids, and lipids exhibit significant predictive associations with type 2 diabetes or a prediabetes state and lead to changes in plasma metabolites that could be selectively affected by gender and age. In terms of gender, women and men have biological dissimilarities that might have an important role for the development, diagnosis, therapy, and prevention of type 2 diabetes, obesity, and relevant hazards in both genders, for type 2 diabetes. Therefore, the present review attempts to analyze the influence of gender on the relationships among inflammatory events, oxidative stress, and metabolic alterations in patients undergoing obesity and/or type 2 diabetes.

ANGPTL8 in metabolic homeostasis: more friend than foe?

ANGPTL8 is an important cytokine, which is significantly increased in type 2 diabetes mellitus (T2DM), obesity and metabolic syndrome. Many studies have shown that ANGPTL8 can be used as a bio-marker of these metabolic disorders related diseases, and the baseline ANGPTL8 level has also been found to be positively correlated with retinopathy and all-cause mortality in patients with T2DM. This may be related to the inhibition of lipoprotein lipase activity and the reduction of circulating triglyceride (TG) clearance by ANGPTL8. Consistently, inhibition of ANGPTL8 seems to prevent or improve atherosclerosis. However, it is puzzling that ANGPTL8 seems to have a directing function for TG uptake in peripheral tissues; that is, ANGPTL8 specifically enhances the reserve and buffering function of white adipose tissue, which may alleviate the ectopic lipid accumulation to a certain extent. Furthermore, ANGPTL8 can improve insulin sensitivity and inhibit hepatic glucose production. These contradictory results lead to different opinions on the role of ANGPTL8 in metabolic disorders. In this paper, the correlation between ANGPTL8 and metabolic diseases, the regulation of ANGPTL8 and the physiological role of ANGPTL8 in the process of glucose and lipid metabolism were summarized, and the physiological/pathological significance of ANGPTL8 in the process of metabolic disorder was discussed.

Associations of plasma sphingolipid profiles with insulin response during oral glucose testing in Icelandic horses

BACKGROUND

Sphingolipids modulate insulin sensitivity in mammals. Increased synthesis of ceramides is linked to decreased insulin sensitivity of tissues. Conversely, activation of the insulin signaling pathway can downregulate ceramide synthesis. Elucidating the association between sphingolipid metabolism and insulin response during oral glucose testing may help explain the pathophysiology of insulin dysregulation in horses.

HYPOTHESES

Horses with insulin dysregulation will have a plasma sphingolipid profile characterized by increased ceramide concentrations. The plasma sphingolipid profile will have decreased ceramide concentrations after acute activation of the insulin signaling pathway by oral glucose testing.

ANIMALS

Twelve Icelandic horses.

METHODS

Horses were subjected to an oral glucose test (0.5 g/kg body weight glucose), with plasma insulin concentrations measured at 0, 30, 60, 120, 180, and 240 minutes postglucose administration. Plasma samples were collected at 0 and 120 minutes for sphingolipid profiling using a liquid chromatography-mass spectrometry-based metabolomics analysis. Eighty-three species of sphingolipids were detected, including 3-ketosphinganines, dihydroceramides, ceramides, dihydrosphingomyelins, sphingomyelins, galatosylceramides, glucosylceramides, lactosylceramides, and ceramide-1-phosphates.

RESULTS

Glucose administration did not significantly alter plasma sphingolipid profiles. C22:0-ceramide, C24:1-ceramide, C23:0-ceramide, C16:1-sphingomyelin, C22:0-dihydroceramide, and C24:0-ceramide were positively correlated with the insulin response (area under the curve).

CONCLUSION AND CLINICAL IMPORTANCE

Positive correlation between the insulin response and sphingolipid concentrations implies upregulated sphingolipid metabolism in insulin dysregulated horses. A high plasma ceramide concentration can indicate insulin dysregulation in horses.

Onset of Senescence and Steatosis in Hepatocytes as a Consequence of a Shift in the Diacylglycerol/Ceramide Balance at the Plasma Membrane

Ceramide and diacylglycerol (DAG) are bioactive lipids and mediate many cellular signaling pathways. Sphingomyelin synthase (SMS) is the single metabolic link between the two, while SMS2 is the only SMS form located at the plasma membrane. SMS2 functions were investigated in HepG2 cell lines stably expressing SMS2. SMS2 overexpression did not alter sphingomyelin (SM), phosphatidylcholine (PC), or ceramide levels. DAG content increased by approx. 40% and led to downregulation of DAG-dependent protein kinase C (PKC). SMS2 overexpression also induced senescence, characterized by positivity for β-galactosidase activity and heterochromatin foci. HepG2-SMS2 cells exhibited protruded mitochondria and suppressed mitochondrial respiration rates. ATP production and the abundance of Complex V were substantially lower in HepG2-SMS2 cells as compared to controls. SMS2 overexpression was associated with inflammasome activation based on increases in IL-1β and nlpr3 mRNA levels. HepG2-SMS2 cells exhibited lipid droplet accumulation, constitutive activation of AMPK based on elevated ¹⁷²Thr phosphorylation, increased AMPK abundance, and insensitivity to insulin suppression of AMPK. Thus, our results show that SMS2 regulates DAG homeostasis and signaling in hepatocytes and also provide proof of principle for the concept that offset in bioactive lipids’ production at the plasma membrane can drive the senescence program in association with steatosis and, seemingly, by cell-autonomous mechanisms.

Targeted lipidomics reveals associations between serum sphingolipids and insulin sensitivity measured by the hyperinsulinemic-euglycemic clamp

AIMS

Sphingolipids(SPs) and their substrates and constituents, fatty acids (FAs), are implicated in the pathogenesis of various metabolic diseases associated. This study aimed to systematically investigate the associations between serum sphingolipids and insulin sensitivity as well as insulin secretion.

METHODS

We conducted a lipidomics evaluation of molecularly distinct SPs in the serum of 86 consecutive Chinese adults using LC/MS. The glucose infusion rate over 30 min (GIR₃₀) was measured under steady conditions to assess insulin sensitivity by the gold standard hyperinsulinemic-euglycemic clamp. We created the ROC curves to detect the serum SMs diagnostic value.

RESULTS

Total and subspecies of serum SMs and globotriaosyl ceramides (Gb3s) were positively related to GIR₃₀, free FAs (FFA 16:1, FFA20:4), some long chain GM3 and complex ceramide GluCers showed strong negative correlations with GIR₃₀. Notably, ROC curves showed that SM/Cer and SM d18:0/26:0 may be good serum lipid predictors of diagnostic indicators of insulin sensitivity close to conventional clinical indexes such as 1/HOMA-IR (areas under the curve > 0.80) based on GIR₃₀ as standard diagnostic criteria, and (SM/Cer)/(BMI*LDLc) areas under the curve = 0.93) is the best.

CONCLUSIONS

These results provide novel associations between serum sphingolipid between insulin sensitivity measured by the hyperinsulinemic-euglycemic clamp and identify two specific SPs that may represent prognostic biomarkers for insulin sensitivity.

Inhibition of Sphingosine-1-Phosphate-Induced Th17 Cells Ameliorates Alcohol-Associated Steatohepatitis in Mice

BACKGROUND AND AIMS

Chronic alcohol consumption is accompanied by intestinal inflammation. However, little is known about how alterations to the intestinal immune system and sphingolipids contribute to the pathogenesis of alcohol-associated liver disease (ALD).

APPROACH AND RESULTS

We used wild-type mice, retinoid-related orphan receptor gamma t (RORγt)-deficient mice, sphingosine kinase-deficient mice, and local gut anti-inflammatory, 5-aminosalicyclic acid-treated mice in a chronic-binge ethanol feeding model. Targeted lipidomics assessed the sphingolipids in gut and liver samples. Gut immune cell populations, the amounts of sphingolipids, and the level of liver injury were examined. Alcohol intake induces a pro-inflammatory shift in immune cell populations in the gut, including an increase in Th17 cells. Using RORγt-deficient mice, we found that Th17 cells are required for alcohol-associated gut inflammation and the development of ALD. Treatment with 5-aminosalicyclic acid decreases alcohol-induced liver injury and reverses gut inflammation by the suppression of CD4⁺ /RORγt⁺ /interleukin-17A⁺ cells. Increased Th17 cells were due to up-regulation of sphingosine kinase 1 activity and RORγt activation. We found that S1P/S1PR1 signaling is required for the development of Th17 cell-mediated ALD. Importantly, in vivo intervention blocking of S1P/S1PR1 signaling markedly attenuated alcohol-induced liver inflammation, steatosis, and damage.

CONCLUSIONS

Gut inflammation is a functional alteration of immune cells in ALD. Reducing gut Th17 cells leads to reduced liver damage. S1P signaling was crucial in the pathogenesis of ALD in a Th17 cell-dependent manner. Furthermore, our findings suggest that compounds that reduce gut inflammation locally may represent a unique targeted approach in the treatment of ALD.

Role of Sphingolipid Signaling in Glomerular Diseases: Focus on DKD and FSGS

Sphingolipids are well-recognized as major players in the pathogenesis of many human diseases, including chronic kidney disease. The kidney is a very sensitive organ to alterations in sphingolipid metabolism. The critical issues to be addressed in this review relate to the role of sphingolipids and enzymes involved in sphingolipid metabolism in the pathogenesis of glomerular diseases with a special focus on podocytes, a key cellular component of the glomerular filtration barrier. Among several sphingolipids, we will highlight the role of ceramide, sphingosine, sphingosine-1-phosphate and ceramide-1-phosphate. Additionally, we will summarize the current knowledge with regard to the use of sphingolipids as therapeutic agents for the treatment of podocyte injury in kidney disease.

Liver-specific Prkn knockout mice are more susceptible to diet-induced hepatic steatosis and insulin resistance

Objective

PARKIN is an E3 ubiquitin ligase that regulates mitochondrial quality control through a process called mitophagy. Recent human and rodent studies suggest that loss of hepatic mitophagy may occur during the pathogenesis of obesity-associated fatty liver and contribute to changes in mitochondrial metabolism associated with this disease. Whole-body Prkn knockout mice are paradoxically protected against diet-induced hepatic steatosis; however, liver-specific effects of Prkn deficiency cannot be discerned in this model due to pleotropic effects of germline Prkn deletion on energy balance and subsequent protection against diet-induced obesity. We therefore generated the first liver-specific Prkn knockout mouse strain (LKO) to directly address the role of hepatic Prkn.

Methods

Littermate control (WT) and LKO mice were fed regular chow (RC) or high-fat diet (HFD) and changes in body weight and composition were measured over time. Liver mitochondrial content was assessed using multiple, complementary techniques, and mitochondrial respiratory capacity was assessed using Oroboros O₂K platform. Liver fat was measured biochemically and assessed histologically, while global changes in hepatic gene expression were measured by RNA-seq. Whole-body and tissue-specific insulin resistance were assessed by hyperinsulinemic-euglycemic clamp with isotopic tracers.

Results

Liver-specific deletion of Prkn had no effect on body weight or adiposity during RC or HFD feeding; however, hepatic steatosis was increased by 45% in HFD-fed LKO compared with WT mice (P < 0.05). While there were no differences in mitochondrial content between genotypes on either diet, mitochondrial respiratory capacity and efficiency in the liver were significantly reduced in LKO mice. Gene enrichment analyses from liver RNA-seq results suggested significant changes in pathways related to lipid metabolism and fibrosis in HFD-fed Prkn knockout mice. Finally, whole-body insulin sensitivity was reduced by 35% in HFD-fed LKO mice (P < 0.05), which was primarily due to increased hepatic insulin resistance (60% of whole-body effect; P = 0.11).

Conclusions

These data demonstrate that PARKIN contributes to mitochondrial homeostasis in the liver and plays a protective role against the pathogenesis of hepatic steatosis and insulin resistance.

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Mitochondrial respiratory capacity is reduced in liver-specific Prkn knockout mice.

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Liver-specific Prkn knockout mice develop more severe steatosis during high-fat diet feeding.

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Pathogenesis of NAFLD, including insulin resistance and markers of fibrosis, is enhanced in liver-specific Prkn knockout mice.

Lipid mediators of insulin signaling in diabetic kidney disease

Diabetic kidney disease (DKD) affects ∼40% of patients with diabetes and is associated with high mortality rates. Among different cellular targets in DKD, podocytes, highly specialized epithelial cells of the glomerular filtration barrier, are injured in the early stages of DKD. Both clinical and experimental data support the role of preserved insulin signaling as a major contributor to podocyte function and survival. However, little is known about the key modulators of podocyte insulin signaling. This review summarizes the novel knowledge that intracellular lipids such as cholesterol and sphingolipids are major determinants of podocyte insulin signaling. In particular, the implications of these lipids on DKD development, progression, and treatment will be addressed.

Therapeutic effect and autophagy regulation of myriocin in nonalcoholic steatohepatitis

BACKGROUND

Ceramide plays pathogenic roles in nonalcoholic fatty liver disease (NAFLD) via multiple mechanisms, and as such inhibition of ceramide de novo synthesis in the liver may be of therapeutically beneficial in patients with NAFLD. In this study, we aimed to explore whether inhibition of ceramide signaling by myriocin is beneficial in animal model of NAFLD via regulating autophagy.

METHODS

Sprague Dawley rats were randomly divided into three groups: standard chow (n = 10), high-fat diet (HFD) (n = 10) or HFD combined with oral administration of myriocin (0.3 mg/kg on alternate days for 8 weeks) (n = 10). Liver histology and autophagy function were measured. HepG2 cells were incubated with fatty acid with or without myriocin treatment. Lipid accumulation and autophagy markers in the HepG2 cells were analyzed. Serum ceramide changes were studied in 104 subjects consisting healthy adults, liver biopsy-proven patients with NAFLD and liver biopsy-proven patients with chronic hepatitis B (CHB).

RESULTS

Myriocin reversed the elevated body weight and serum transaminases and alleviated dyslipidemia in HFD fed rats. Myriocin treatment significantly attenuated liver pathology including steatosis, lobular inflammation and ballooning. By qPCR analysis, it was revealed that myriocin corrected the expression pattern of fatty acid metabolism associated genes including Fabp1, Pparα, Cpt-1α and Acox-2. Further, myriocin also restored the impaired hepatic autophagy function in rats with HFD-induced NASH, and this has been verified in HepG2 cells. Among the sphingolipid species that we screened in lipidomic profiles, significantly increased ceramide was observed in NASH patients as compared to the controls and non-NASH patients, regardless of whether or not they have active CHB.

CONCLUSIONS

Ceramide may play an important regulatory role in the autophagy function in the pathogenesis of NASH. Hence, blockade of ceramide signaling by myriocin may be of therapeutically beneficial in NASH.

TRIAL REGISTRATION

Registration ID: ChiCTR-DDT-13003983 . Data of registration: 13 May, 2013, retrospectively registered.

SMPDL3b modulates insulin receptor signaling in diabetic kidney disease

Sphingomyelin phosphodiesterase acid-like 3b (SMPDL3b) is a lipid raft enzyme that regulates plasma membrane (PM) fluidity. Here we report that SMPDL3b excess, as observed in podocytes in diabetic kidney disease (DKD), impairs insulin receptor isoform B-dependent pro-survival insulin signaling by interfering with insulin receptor isoforms binding to caveolin-1 in the PM. SMPDL3b excess affects the production of active sphingolipids resulting in decreased ceramide-1-phosphate (C1P) content as observed in human podocytes in vitro and in kidney cortexes of diabetic db/db mice in vivo. Podocyte-specific Smpdl3b deficiency in db/db mice is sufficient to restore kidney cortex C1P content and to protect from DKD. Exogenous administration of C1P restores IR signaling in vitro and prevents established DKD progression in vivo. Taken together, we identify SMPDL3b as a modulator of insulin signaling and demonstrate that supplementation with exogenous C1P may represent a lipid therapeutic strategy to treat diabetic complications such as DKD.

Plasma Protein and MicroRNA Biomarkers of Insulin Resistance: A Network-Based Integrative -Omics Analysis

Although insulin resistance (IR) is a key pathophysiologic condition underlying various metabolic disorders, impaired cellular glucose uptake is one of many manifestations of metabolic derangements in the human body. To study the systems-wide molecular changes associated with obesity-dependent IR, we integrated information on plasma proteins and microRNAs in eight obese insulin-resistant (OIR, HOMA-IR > 2.5) and nine lean insulin-sensitive (LIS, HOMA-IR < 1.0) normoglycemic males. Of 374 circulating miRNAs we profiled, 65 species increased and 73 species decreased in the OIR compared to the LIS subjects, suggesting that the overall balance of the miRNA secretome is shifted in the OIR subjects. We also observed that 40 plasma proteins increased and 4 plasma proteins decreased in the OIR subjects compared to the LIS subjects, and most proteins are involved in metabolic and endocytic functions. We used an integrative -omics analysis framework called iOmicsPASS to link differentially regulated miRNAs with their target genes on the TargetScan map and the human protein interactome. Combined with tissue of origin information, the integrative analysis allowed us to nominate obesity-dependent and obesity-independent protein markers, along with potential sites of post-transcriptional regulation by some of the miRNAs. We also observed the changes in each -omics platform that are not linked by the TargetScan map, suggesting that proteins and microRNAs provide orthogonal information for the progression of OIR. In summary, our integrative analysis provides a network of elevated plasma markers of OIR and a global shift of microRNA secretome composition in the blood plasma.

Glycosylated sphingolipids and progression to kidney dysfunction in type 1 diabetes

BACKGROUND

Glycosphingolipids are important components of cell membranes, modulators of cell-cell interactions and cell recognition, and have recently emerged as bioactive molecules and important players in nearly all cell biological processes. We previously have shown that decreased plasma levels of long and very long species of ceramides were able to predict the development of macroalbuminuria (MA) in type 1 diabetes.

OBJECTIVE

This study proposed to examine whether plasma glycosphingolipids could predict development of diabetic nephropathy, assessed as MA or chronic kidney disease (CKD).

METHODS

Measurement of plasma hexosylceramides (H) and lactosylceramides (L) were conducted in the Lipidomics Core Facility of our Institution in a subcohort of 432 patients from the DCCT/Epidemiology of Diabetes Interventions and Complications cohort in plasma collected at entry into the study. Inverse probability weighted Cox proportional hazards regression models were used to assess the effect of glycosphingolipids levels on the risk of developing MA (albumin excretion rate ≥300 mg/24 hours) or CKD (glomerular filtration rate <60 mL/min) over a period of 21 to 28 years.

RESULTS

Decreases of several long and very long chain lactosylceramides were significantly associated with increased risk of progression to MA but not CKD. Among the hexosylceramides, the only significant association observed was between one of its minor species C18:1-H and CKD.

CONCLUSION

Our findings showed that decreased levels of long and very long lactosylceramides were able to predict the development of MA in type 1 diabetes. This finding is similar to previous findings showing that low levels of long and very long ceramides were also able to predict development of MA in the same cohort. Further studies are needed to determine the changes in sphingolipid metabolism leading to the development of complications.

Inhibiting Ceramide Synthesis Attenuates Hepatic Steatosis and Fibrosis in Rats With Non-alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) is one of the most common metabolic disorder diseases, which include a histological spectrum of conditions ranging from simple steatosis to non-alcoholic steatohepatitis (NASH). Dysregulated metabolism of sphingomyelin in the liver plays a critical role in the pathogenesis of NAFLD. Ceramides are central molecules of sphingolipid biosynthesis and catabolism and play an important role in insulin resistance, apoptosis, and inflammation. In addition, apoptosis is a main contributor to the development of NAFLD. This study detected whether the inhibition of ceramide synthesis ameliorated hepatic steatosis and fibrosis in rats with NAFLD. Sprague-Dawley rats were used to establish the NAFLD model. Here, we showed that hepatic ceramide, steatosis, and fibrosis increased in liver tissue from rats with NAFLD. Chronic treatment with myriocin inhibited ceramide and lipid accumulation and improved fibrosis in liver tissue samples of high fat diet (HFD)-fed rats. In addition, hepatic inflammation and apoptosis were markedly ameliorated in HFD-fed rats treated with myriocin. Furthermore, myriocin treatment regulated the expression of pro-apoptosis and anti-apoptosis proteins by inactivating the c-Jun N-terminal kinase (JNK) signaling pathway in the liver of HFD-fed rats. Collectively, ceramide plays an important role in the pathogenesis of NASH and may represent a potential therapeutic strategy to prevent NAFLD.

Sphk2-/- mice are protected from obesity and insulin resistance

Sphingosine kinases phosphorylate sphingosine to sphingosine 1‑phosphate (S1P), which functions as a signaling molecule. We have previously shown that sphingosine kinase 2 (Sphk2) is important for insulin secretion. To obtain a better understanding of the role of Sphk2 in glucose and lipid metabolism, we have characterized 20- and 52-week old Sphk2^-/- mice using glucose and insulin tolerance tests and by analyzing metabolic gene expression in adipose tissue. A detailed metabolic characterization of these mice revealed that aging Sphk2^-/- mice are protected from metabolic decline and obesity compared to WT mice. Specifically, we found that 52-week old male Sphk2^-/- mice had decreased weight and fat mass, and increased glucose tolerance and insulin sensitivity compared to control mice. Indirect calorimetry studies demonstrated an increased energy expenditure and food intake in 52-week old male Sphk2^-/- versus control mice. Furthermore, expression of adiponectin gene in adipose tissue was increased and the plasma levels of adiponectin elevated in aged Sphk2^-/- mice compared to WT. Analysis of lipid metabolic gene expression in adipose tissue showed increased expression of the Atgl gene, which was associated with increased Atgl protein levels. Atgl encodes for the adipocyte triglyceride lipase, which catalyzes the rate-limiting step of lipolysis. In summary, these data suggest that mice lacking the Sphk2 gene are protected from obesity and insulin resistance during aging. The beneficial metabolic effects observed in aged Sphk2^-/- mice may be in part due to enhanced lipolysis by Atgl and increased levels of adiponectin, which has lipid- and glucose-lowering effects.

Genome-wide association study identifies novel recessive genetic variants for high TGs in an Arab population

Abnormal blood lipid levels are influenced by genetic and lifestyle/dietary factors. Although many genetic variants associated with blood lipid traits have been identified in Europeans, similar data in Middle Eastern populations are limited. We performed a genome-wide association study with Arab individuals (discovery cohort: 1,353; replication cohort: 1,176) from Kuwait to identify possible associations of genetic variants with high lipid levels. We used Illumina HumanOmniExpress BeadChip and candidate SNP genotyping in the discovery and replication phases, respectively. For association tests, we used genetic models that were based on additive and recessive modes of inheritance. High triglycerides (TGs) were recessively associated with six risk variants (rs1002487/RPS6KA1, rs11805972/LAD1) rs7761746/Or5v1, rs39745/CTTNBP2-LSM8, rs2934952/PGAP3, and rs9626773/RP11-191L9.4-CERK) at genome-wide significance (P  6.12E-09), and another six variants (rs10873925/ST6GALNAC5, rs4663379/SPP2-ARL4C, rs10033119/NPY1R, rs17709449/LINC00911-FLRT2, rs11654954/CDK12-NEUROD2, and rs9972882/STARD3) were associated at borderline significance (P  5.0E-08). High TG was also additively associated with rs11654954. All of the 12 identified markers are novel and are harbored in runs of homozygosity. Literature evidence supports the involvement of these gene loci in lipid-related processes. This study in an Arab population augments international efforts to identify genetic regulation of lipid traits.

Sphingolipid metabolism in non-alcoholic fatty liver diseases

Non-alcoholic fatty liver disease (NAFLD) involves a panel of pathologies starting with hepatic steatosis and continuing to irreversible and serious conditions like steatohepatitis (NASH) and hepatocarcinoma. NAFLD is multifactorial in origin and corresponds to abnormal fat deposition in liver. Even if triglycerides are mostly associated with these pathologies, other lipid moieties seem to be involved in the development and severity of NAFLD. That is the case with sphingolipids and more particularly ceramides. In this review, we explore the relationship between NAFLD and sphingolipid metabolism. After providing an analysis of complex sphingolipid metabolism, we focus on the potential involvement of sphingolipids in the different pathologies associated with NAFLD. An unbalanced ratio between ceramides and terminal metabolic products in the liver and plasma promotes weight gain, inflammation, and insulin resistance. In the etiology of NAFLD, some sphingolipid species such as ceramides may be potential biomarkers for NAFLD. We review the clinical relevance of sphingolipids in liver diseases.

Lipid phosphate phosphatase 3 regulates adipocyte sphingolipid synthesis, but not developmental adipogenesis or diet-induced obesity in mice

Dephosphorylation of phosphatidic acid (PA) is the penultimate step in triglyceride synthesis. Adipocytes express soluble intracellular PA-specific phosphatases (Lipins) and broader specificity membrane-associated lipid phosphate phosphatases (LPPs) that can also dephosphorylate PA. Inactivation of lipin1 causes lipodystrophy in mice due to defective developmental adipogenesis. Triglyceride synthesis is diminished but not ablated by inactivation of lipin1 in differentiated adipocytes implicating other PA phosphatases in this process. To investigate the possible role of LPPs in adipocyte lipid metabolism and signaling we made mice with adipocyte-targeted inactivation of LPP3 encoded by the Plpp3(Ppap2b) gene. Adipocyte LPP3 deficiency resulted in blunted ceramide and sphingomyelin accumulation during diet-induced adipose tissue expansion, accumulation of the LPP3 substrate sphingosine 1- phosphate, and reduced expression of serine palmitoyl transferase. However, adiposity was unaffected by LPP3 deficiency on standard, high fat diet or Western diets, although Western diet-fed mice with adipocyte LPP3 deficiency exhibited improved glucose tolerance. Our results demonstrate functional compartmentalization of lipid phosphatase activity in adipocytes and identify an unexpected role for LPP3 in the regulation of diet-dependent sphingolipid synthesis that may impact on insulin signaling.

Sphingolipids at the Crossroads of NAFLD and Senescence

Nonalcoholic fatty liver disease (NAFLD) is a group of liver disorders encompassing simple hepatic steatosis and its more aggressive forms of nonalcoholic steatohepatitis and cirrhosis. It is a rapidly growing health concern and the major cause for the increasing incidence of primary liver tumors. Unequivocal evidence shows that sphingolipid metabolism is altered in the course of the disease and these changes might contribute to NAFLD progression. Recent data provide solid support to the notion that deregulated ceramide and sphingosine-1-phosphate metabolism are present at all stages of NAFLD, i.e., steatosis, nonalcoholic steatohepatitis, advanced fibrosis, and hepatocellular carcinoma (HCC). Insulin sensitivity, de novo lipogenesis, and the resulting lipotoxicity, fibrosis, and angiogenesis are all seemingly regulated in a manner that involves either ceramide and/or sphingosine-1-phosphate. Sphingolipids might also participate in the onset of hepatocellular senescence. The latter has been shown to contribute to the advancement of cirrhosis to HCC in the classical cases of end-stage liver disease, i.e., viral- or alcohol-induced; however, emerging evidence suggests that senescence is also involved in the pathogenicity of NAFLD possibly via changes in ceramide metabolism.

Changes in Serum Sphingomyelin After Roux-en-Y Gastric Bypass Surgery Are Related to Diabetes Status

Metabolic surgery is superior to lifestyle intervention in reducing weight and lowering glycemia and recently suggested as treatment for type 2 diabetes mellitus. Especially Roux-en-Y gastric bypass (RYGB) has been focus for much research, but still the mechanisms of action are only partly elucidated. We suggest that several mechanisms might be mediated by sphingolipids like sphingomyelin. We measured serum sphingomyelin before and up to 2 years after RYGB surgery in 220 patients, divided before surgery in one non-diabetic subgroup and two diabetic subgroups, one of which contained patients obtaining remission of type 2 diabetes after RYGB, while patients in the other still had diabetes after RYGB. Pre- and postoperative sphingomyelin levels were compared within and between groups. Sphingomyelin levels were lower in diabetic patients than in non-diabetic patients before surgery. Following RYGB, mean sphingomyelin concentration fell significantly in the non-diabetic subgroup and the preoperative difference between patients with and without diabetes disappeared. Changes in diabetic subgroups were not significant. Relative to bodyweight, an increase in sphingomyelin was seen in all subgroups, irrespective of diabetes status. We conclude that RYGB has a strong influence on sphingomyelin metabolism, as seen reflected in changed serum levels. Most significantly, no differences between the two diabetic subgroups were detected after surgery, which might suggest that patients in both groups still are in a "diabetic state" using the non-diabetic subgroup as a reference.

Adapting to obesity with adipose tissue inflammation

Adipose tissue not only has an important role in the storage of excess nutrients but also senses nutrient status and regulates energy mobilization. An overall positive energy balance is associated with overnutrition and leads to excessive accumulation of fat in adipocytes. These cells respond by initiating an inflammatory response that, although maladaptive in the long run, might initially be a physiological response to the stresses obesity places on adipose tissue. In this Review, we characterize adipose tissue inflammation and review the current knowledge of what triggers obesity-associated inflammation in adipose tissue. We examine the connection between adipose tissue inflammation and the development of insulin resistance and catecholamine resistance and discuss the ensuing state of metabolic inflexibility. Finally, we review the current and potential new anti-inflammatory treatments for obesity-associated metabolic disease.

Retinol saturase modulates lipid metabolism and the production of reactive oxygen species

Retinol saturase (RetSat) catalyzes the saturation of double bonds of all-trans-retinol leading to the production of dihydroretinoid metabolites. Beside its role in retinoid metabolism, there is evidence that RetSat modulates the cellular response to oxidative stress and plays critical roles in adipogenesis and the accumulation of lipids. Here, we explore the relationship between RetSat, lipid metabolism and oxidative stress using in vitro and in vivo models with altered expression of RetSat. Our results reveal that RetSat is a potent modulator of the cellular response to oxidative stress and the generation of reactive oxygen species (ROS). The levels of reactive aldehydes products of lipid peroxidation, as measured based on thiobarbituric acid reactivity, are increased in RetSat overexpressing cells and, conversely, reduced in cells and tissues with reduced or absent expression of RetSat compared to controls. Despite increased weight gain, neutral lipid accumulation and alterations in hepatic lipid composition, RetSat^-/- mice exhibit normal responses to insulin. In conclusion, our findings further expand upon the role of RetSat in oxidative stress and lipid metabolism and could provide insight in the significance of alterations of RetSat expression as observed in metabolic disorders.

The expanding role of sphingolipids in lipid droplet biogenesis

Sphingolipids are a diverse class of lipids that have regulatory, structural, and metabolic functions. Although chemically distinct from the neutral lipids and the glycerophospholipids, which are the main lipid components of the lipid droplets, sphingolipids have nonetheless been shown to influence lipid droplet formation. The goal of this article is to review the available information and provide a cohesive picture of the role sphingolipids play in lipid droplet biogenesis. The following topics are discussed: (i) the abundance of sphingolipids in lipid droplets and their functional significance; (ii) cross-talk between the synthetic pathways of sphingolipids, glycerophospholipids, and neutral lipids; (iii) the impact of bioactive sphingolipids on TAG synthesis and degradation; (iv) interactions between sphingolipids and other lipid droplet components, like cholesterol esters and proteins; (v) inhibition/genetic deletion of specific sphingolipid metabolic enzymes and the resulting effects on lipid droplet formation in mouse models. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

Lipid droplets and liver disease: from basic biology to clinical implications

Lipid droplets are dynamic organelles that store neutral lipids during times of energy excess and serve as an energy reservoir during deprivation. Many prevalent metabolic diseases, such as the metabolic syndrome or obesity, often result in abnormal lipid accumulation in lipid droplets in the liver, also called hepatic steatosis. Obesity-related steatosis, or NAFLD in particular, is a major public health concern worldwide and is frequently associated with insulin resistance and type 2 diabetes mellitus. Here, we review the latest insights into the biology of lipid droplets and their role in maintaining lipid homeostasis in the liver. We also offer a perspective of liver diseases that feature lipid accumulation in these lipid storage organelles, which include NAFLD and viral hepatitis. Although clinical applications of this knowledge are just beginning, we highlight new opportunities for identifying molecular targets for treating hepatic steatosis and steatohepatitis.

Lipidomic and metabolic changes in the P4-type ATPase ATP10D deficient C57BL/6J wild type mice upon rescue of ATP10D function

Background

Sequence variants near the human gene for P4-type ATPase, class V, type 10D (ATP10D) were shown to significantly associate with circulating hexosylceramide d18:1/16:0 and d18:1/24:1 levels, obesity, insulin resistance, plasma high density lipoprotein (HDL), coronary stenotic index and intracranial atherosclerotic index. In mice Atp10d is associated with HDL modulation and C57BL/6 mice expressing a truncated, non-functional form of ATP10D easily develop obesity and insulin resistance on high-fat diet.

Results

We analyzed metabolic differences of ATP10D deficient C57BL/6J wild type and ATP10D transgenic C57BL/6J BAC129 mice. ATP10D transgenic mice gain 25% less weight on high-fat diet concomitant with a reduced increase in fat cell mass but independent of adipocyte size change. ATP10D transgenic mice also had 26% lower triacylglycerol levels with approximately 76% bound to very low density lipoprotein while in ATP10D deficient wild type mice 57% are bound to low density lipoprotein. Furthermore increased oxygen consumption and CO₂ production, 38% lower glucose and 69% lower insulin levels and better insulin sensitivity were observed in ATP10D transgenic mice. Besides decreased hexosylceramide species levels were detected. Part of these effects may be due to reduced hepatic stearoyl-CoA desaturase 1 (SCD1) expression in ATP10D transgenic mice, which was reflected by altered fatty acid and lipid species patterns. There was a significant decrease in the hepatic 18:1 to 18:0 free fatty acid ratio in transgenic mice. The ratio of 16:1 to 16:0 was not significantly different. Interestingly both ratios were significantly reduced in plasma total fatty acids.

Summary

In summary we found that ATP10D reduces high-fat diet induced obesity and improves insulin sensitivity. ATP10D transgenic mice showed altered hepatic expression of lipid-metabolism associated genes, including Scd1, along with changes in hepatic and plasma lipid species and plasma lipoprotein pattern.

Englerin A induces an acute inflammatory response and reveals lipid metabolism and ER stress as targetable vulnerabilities in renal cell carcinoma

Renal cell carcinoma (RCC) is among the top ten most common forms of cancer and is the most common malignancy of the kidney. Clear cell renal carcinoma (cc-RCC), the most common type of RCC, is one of the most refractory cancers with an incidence that is on the rise. Screening of plant extracts in search of new anti-cancer agents resulted in the discovery of englerin A, a guaiane sesquiterpene with potent cytotoxicity against renal cancer cells and a small subset of other cancer cells. Though a few cellular targets have been identified for englerin A, it is still not clear what mechanisms account for the cytotoxicity of englerin A in RCC, which occurs at concentrations well below those used to engage the targets previously identified. Unlike any prior study, the current study used a systems biology approach to explore the mechanism(s) of action of englerin A. Metabolomics analyses indicated that englerin A profoundly altered lipid metabolism by 24 h in cc-RCC cell lines and generated significant levels of ceramides that were highly toxic to these cells. Microarray analyses determined that englerin A induced ER stress signaling and an acute inflammatory response, which was confirmed by quantitative PCR and Western Blot analyses. Additionally, fluorescence confocal microscopy revealed that englerin A at 25 nM disrupted the morphology of the ER confirming the deleterious effect of englerin A on the ER. Collectively, our findings suggest that cc-RCC is highly sensitive to disruptions in lipid metabolism and ER stress and that these vulnerabilities can be targeted for the treatment of cc-RCC and possibly other lipid storing cancers. Furthermore, our results suggest that ceramides may be a mediator of some of the actions of englerin A. Lastly, the acute inflammatory response induced by englerin A may mediate anti-tumor immunity.

Inflammatory mechanisms linking obesity and metabolic disease

There are currently over 1.9 billion people who are obese or overweight, leading to a rise in related health complications, including insulin resistance, type 2 diabetes, cardiovascular disease, liver disease, cancer, and neurodegeneration. The finding that obesity and metabolic disorder are accompanied by chronic low-grade inflammation has fundamentally changed our view of the underlying causes and progression of obesity and metabolic syndrome. We now know that an inflammatory program is activated early in adipose expansion and during chronic obesity, permanently skewing the immune system to a proinflammatory phenotype, and we are beginning to delineate the reciprocal influence of obesity and inflammation. Reviews in this series examine the activation of the innate and adaptive immune system in obesity; inflammation within diabetic islets, brain, liver, gut, and muscle; the role of inflammation in fibrosis and angiogenesis; the factors that contribute to the initiation of inflammation; and therapeutic approaches to modulate inflammation in the context of obesity and metabolic syndrome.

The suppression of hepatic glucose production improves metabolism and insulin sensitivity in subcutaneous adipose tissue in mice

AIMS/HYPOTHESIS

Despite the strong correlation between non-alcoholic fatty liver disease and insulin resistance, hepatic steatosis is associated with greater whole-body insulin sensitivity in several models. We previously reported that the inhibition of hepatic glucose production (HGP) protects against the development of obesity and diabetes despite severe steatosis, thanks to the secretion of specific hepatokines such as fibroblast growth factor 21 (FGF21) and angiopoietin-related growth factor. In this work, we focused on adipose tissue to assess whether liver metabolic fluxes might, by interorgan communication, control insulin signalling in lean animals.

METHODS

Insulin signalling was studied in the adipose tissue of mice lacking the catalytic subunit of glucose 6-phosphatase, the key enzyme in endogenous glucose production, in the liver (L-G6pc ^-/- mice). Morphological and metabolic changes in the adipose tissues were characterised by histological analyses, gene expression and protein content.

RESULTS

Mice lacking HGP exhibited improved insulin sensitivity of the phosphoinositide 3-kinase/Akt pathway in the subcutaneous adipose tissue associated with a browning of adipocytes. The suppression of HGP increased FGF21 levels in lean animals, and increased FGF21 was responsible for the metabolic changes in the subcutaneous adipose tissue but not for its greater insulin sensitivity. The latter might be linked to an increase in the ratio of monounsaturated to saturated fatty acids released by the liver.

CONCLUSIONS

Our work provides evidence that HGP controls subcutaneous adipose tissue browning and insulin sensitivity through two pathways: the release of beneficial hepatokines and changes in hepatic fatty acids profile.

Lipid profiling of the therapeutic effects of berberine in patients with nonalcoholic fatty liver disease

BACKGROUND

We recently demonstrated a positive effect of berberine on nonalcoholic fatty liver disease patients after 16 weeks of treatment by comparing mere lifestyle intervention in type 2 diabetes patients with berberine treatment, which decreased the content of hepatic fat. However, the potential mechanisms of the clinical effects are unclear. We used a lipidomic approach to characterize the state of lipid metabolism as reflected in the circulation of subjects with nonalcoholic fatty liver disease (NAFLD) before and after berberine treatment.

METHODS

Liquid chromatography-mass spectrometry evaluated the various lipid metabolites in serum samples obtained from the participants (41 patients in the berberine group and 39 patients in the mere lifestyle intervention group) before and after treatment.

RESULTS

A total of 256 serum lipid molecular species were identified and quantified. Both treatments regulated various types of lipids in metabolic pathways, such as free fatty acids, phosphoglycerides and glycerides, in metabolic pathways, but berberine induced a substantially greater change in serum lipid species compared with mere lifestyle intervention after treatment. Berberine also caused obvious differences on ceramides. Berberine treatment markedly decreased serum levels of ceramide and ceramide-1-phosphate.

CONCLUSIONS

Berberine altered circulating ceramides, which may underlie the improvement in fatty liver disease. ClinicalTrials.gov NCT00633282, Registered March 3, 2008.

Lipid-Induced Insulin Resistance in Skeletal Muscle: The Chase for the Culprit Goes from Total Intramuscular Fat to Lipid Intermediates, and Finally to Species of Lipid Intermediates

The skeletal muscle is the largest organ in the body. It plays a particularly pivotal role in glucose homeostasis, as it can account for up to 40% of the body and for up to 80%-90% of insulin-stimulated glucose disposal. Hence, insulin resistance (IR) in skeletal muscle has been a focus of much research and review. The fact that skeletal muscle IR precedes β-cell dysfunction makes it an ideal target for countering the diabetes epidemic. It is generally accepted that the accumulation of lipids in the skeletal muscle, due to dietary lipid oversupply, is closely linked with IR. Our understanding of this link between intramyocellular lipids (IMCL) and glycemic control has changed over the years. Initially, skeletal muscle IR was related to total IMCL. The inconsistencies in this explanation led to the discovery that particular lipid intermediates are more important than total IMCL. The two most commonly cited lipid intermediates for causing skeletal muscle IR are ceramides and diacylglycerol (DAG) in IMCL. Still, not all cases of IR and dysfunction in glycemic control have shown an increase in either or both of these lipids. In this review, we will summarise the latest research results that, using the lipidomics approach, have elucidated DAG and ceramide species that are involved in skeletal muscle IR in animal models and human subjects.

Susceptibility of Different Mouse Wild Type Strains to Develop Diet-Induced NAFLD/AFLD-Associated Liver Disease

Although non-alcoholic and alcoholic fatty liver disease have been intensively studied, concerning pathophysiological mechanisms are still incompletely understood. This may be due to the use of different animal models and resulting model-associated variation. Therefore, this study aimed to compare three frequently used wild type mouse strains in their susceptibility to develop diet-induced features of non-alcoholic/alcoholic fatty liver disease. Fatty liver disease associated clinical, biochemical, and histological features in C57BL/6, CD-1, and 129Sv WT mice were induced by (i) high-fat diet feeding, (ii) ethanol feeding only, and (iii) the combination of high-fat diet and ethanol feeding. Hepatic and subcutaneous adipose lipid profiles were compared in CD-1 and 129Sv mice. Additionally hepatic fatty acid composition was determined in 129Sv mice. In C57BL/6 mice dietary regimens resulted in heterogeneous hepatic responses, ranging from pronounced steatosis and inflammation to a lack of any features of fatty liver disease. Liver-related serum biochemistry showed high deviations within the regimen groups. CD-1 mice did not exhibit significant changes in metabolic and liver markers and developed no significant steatosis or inflammation as a response to dietary regimens. Although 129Sv mice showed no weight gain, this strain achieved most consistent features of fatty liver disease, apparent from concentration alterations of liver-related serum biochemistry as well as moderate steatosis and inflammation as a result of all dietary regimens. Furthermore, the hepatic lipid profile as well as the fatty acid composition of 129Sv mice were considerably altered, upon feeding the different dietary regimens. Accordingly, diet-induced non-alcoholic/alcoholic fatty liver disease is most consistently promoted in 129Sv mice compared to C57BL/6 and CD-1 mice. As a conclusion, this study demonstrates the importance of genetic background of used mouse strains for modeling diet-induced non-alcoholic/alcoholic fatty liver disease.

The Potential Protective Action of Vitamin D in Hepatic Insulin Resistance and Pancreatic Islet Dysfunction in Type 2 Diabetes Mellitus

Vitamin D deficiency (i.e., hypovitaminosis D) is associated with increased insulin resistance, impaired insulin secretion, and poorly controlled glucose homeostasis, and thus is correlated with the risk of metabolic diseases, including type 2 diabetes mellitus (T2DM). The liver plays key roles in glucose and lipid metabolism, and its dysregulation leads to abnormalities in hepatic glucose output and triglyceride accumulation. Meanwhile, the pancreatic islets are constituted in large part by insulin-secreting β cells. Consequently, islet dysfunction, such as occurs in T2DM, produces hyperglycemia. In this review, we provide a critical appraisal of the modulatory actions of vitamin D in hepatic insulin sensitivity and islet insulin secretion, and we discuss the potential roles of a local vitamin D signaling in regulating hepatic and pancreatic islet functions. This information provides a scientific basis for establishing the benefits of the maintenance, or dietary manipulation, of adequate vitamin D status in the prevention and management of obesity-induced T2DM and non-alcoholic fatty liver disease.

Upregulation of S1P1 and Rac1 receptors in the pulmonary vasculature of nitrofen-induced congenital diaphragmatic hernia

PURPOSE

Sphingolipids play a crucial role in pulmonary development. The sphingosine kinase 1 (SphK1) modulates the synthesis of sphingolipid sphingosine-1-phosphate (S1P). S1P regulates cell proliferation and angiogenesis via different receptors, S1P1, S1P2 and S1P3, which all influence the expression of Ras-related C3 botulinum toxin substrate 1 (Rac1). We designed this study to test the hypothesis that the S1P/Rac1 pathway is altered in the nitrofen-induced CDH model.

METHODS

Pregnant rats received nitrofen or vehicle on D9. On D21, fetuses were killed and divided into nitrofen and control group (n = 12). QRT-PCR, western blotting and confocal-immunofluorescence microscopy were performed to reveal pulmonary gene and protein expression levels of SphK1, S1P1, S1P2, S1P3 and Rac1.

RESULTS

Pulmonary gene expression of S1P1 and Rac1 was significantly increased in the CDH group compared to controls, whereas S1P2 and S1P3 expression was decreased. These results were confirmed by western blotting and confocal microscopy. SphK1 expression was not found to be altered.

CONCLUSION

The increased expression of S1P1 and Rac1 in the pulmonary vasculature of nitrofen-induced CDH lungs suggests that S1P1 and Rac1 are important mediators of PH in this model.