Plasma 1-deoxysphingolipids are early predictors of incident type 2 diabetes mellitus

The fatty acid omega hydroxylase genes (CYP4 family) in the progression of metabolic dysfunction-associated steatotic liver disease (MASLD): An RNA sequence database analysis and review

Fatty acid omega hydroxylase P450s consist of enzymes that hydroxylate various chain-length saturated and unsaturated fatty acids (FAs) and bioactive eicosanoid lipids. The human cytochrome P450 gene 4 family (CYP4) consists of 12 members that are associated with several human diseases. However, their role in the progression of metabolic dysfunction-associated fatty liver disease (MASLD) remains largely unknown. It has long been thought that the induction of CYP4 family P450 during fasting and starvation prevents FA-related lipotoxicity through FA metabolism to dicarboxylic acids that are chain-shortened in peroxisomes and then transported to the mitochondria for complete oxidation. Several studies have revealed that peroxisome succinate transported to the mitochondria is used for gluconeogenesis during fasting and starvation, and recent evidence suggests that peroxisome acetate can be utilized for lipogenesis and lipid droplet formation as well as epigenetic modification of gene transcription. In addition, omega hydroxylation of the bioactive eicosanoid arachidonic acid to 20-Hydroxyeicosatetraenoic acid (20-HETE) is essential for activating the GPR75 receptor, leading to vasoconstriction and cell proliferation. Several mouse models of diet-induced MASLD have revealed the induction of selective CYP4A members and the suppression of CYP4F during steatosis and steatohepatitis, suggesting a critical metabolic role in the progression of fatty liver disease. Thus, to further investigate the functional roles of CYP4 genes, we analyzed the differential gene expression of 12 members of CYP4 gene family in datasets from the Gene Expression Omnibus (GEO) from patients with steatosis, steatohepatitis, fibrosis, cirrhosis, and hepatocellular carcinoma. We also observed the differential expression of various CYP4 genes in the progression of MASLD, indicating that different CYP4 members may have unique functional roles in the metabolism of specific FAs and eicosanoids at various stages of fatty liver disease. These results suggest that targeting selective members of the CYP4A family is a viable therapeutic approach for treating and managing MASLD.

Occult hypoperfusion and changes of systemic lipid levels after severe trauma: an analysis in a standardized porcine polytrauma model

BACKGROUND

Occult hypoperfusion describes the absence of sufficient microcirculation despite normal vital signs. It is known to be associated with prolonged elevation of serum lactate and later complications in severely injured patients. We hypothesized that changes in circulating lipids are related to responsiveness to resuscitation. The purpose of this study is investigating the relation between responsiveness to resuscitation and lipidomic course after poly trauma.

METHODS

Twenty-five male pigs were exposed a combined injury of blunt chest trauma, liver laceration, controlled haemorrhagic shock, and femoral shaft fracture. After 1 h, animals received resuscitation and fracture stabilization. Venous blood was taken regularly and 233 specific lipids were analysed. Animals were divided into two groups based on serum lactate level at the end point as an indicator of responsiveness to resuscitation (<2 mmol/L: responder group (R group), ≧2 mmol/L: occult hypoperfusion group (OH group)).

RESULTS

Eighteen animals met criteria for the R group, four animals for the OH group, and three animals died. Acylcarnitines showed a significant increase at 1 h compared to baseline in both groups. Six lipid subgroups showed a significant increase only in R group at 2 h. There was no significant change at other time points.

CONCLUSIONS

Six lipid groups increased significantly only in the R group at 2 h, which may support the idea that they could serve as potential biomarkers to help us to detect the presence of occult hypoperfusion and insufficient resuscitation. We feel that further study is required to confirm the role and mechanism of lipid changes after trauma.

Hepatocyte ABCA1 deficiency is associated with reduced HDL sphingolipids

ATP binding cassette transporter A1 (ABCA1) limits the formation of high density lipoproteins (HDL) as genetic loss of ABCA1 function causes virtual HDL deficiency in patients with Tangier disease. Mice with a hepatocyte-specific ABCA1 knockout (Abca1 HSKO) have 20% of wild type (WT) plasma HDL-cholesterol levels, suggesting a major contribution of hepatic ABCA1 to the HDL phenotype. Whether plasma sphingolipids are reduced in Tangier disease and to what extent hepatic ABCA1 contributes to plasma sphingolipid (SL) levels is unknown. Here, we report a drastic reduction of total SL levels in plasma of a Tangier patient with compound heterozygosity for mutations in ABCA1. Compared to mutation-free controls, heterozygous mutations in ABCA1 had no significant effect on total SLs in plasma; however, apoB-depleted plasma showed a reduction in total SL also in het carriers. Similarly, liver specific Abca1 KO mice (Abca1 HSKO) showed reduced total sphingolipids in plasma and liver. In parallel, apoM and sphingosine-1-phosphate (S1P) levels were reduced in plasma of Abca1 HSKO mice. Primary hepatocytes from Abca1 HSKO mice showed a modest, but significant reduction in total SLs concentration compared to WT hepatocytes, although SL de novo synthesis and secretion were slightly increased in Abca1 HSKO hepatocytes. We conclude that hepatic ABCA1 is a signficant contributor to maintaining total plasma pool of HDL sphingolipids, including sphingomyelins and S1P.

Imbalanced unfolded protein response signaling contributes to 1-deoxysphingolipid retinal toxicity

The accumulation of atypical, cytotoxic 1-deoxysphingolipids (1-dSLs) has been linked to retinal diseases such as diabetic retinopathy and Macular Telangiectasia Type 2. However, the molecular mechanisms by which 1-dSLs induce toxicity in retinal cells remain poorly understood. Here, we integrate bulk and single-nucleus RNA-sequencing to define biological pathways that modulate 1-dSL toxicity in human retinal organoids. Our results demonstrate that 1-dSLs differentially activate signaling arms of the unfolded protein response (UPR) in photoreceptor cells and Müller glia. Using a combination of pharmacologic activators and inhibitors, we show that sustained PERK signaling through the integrated stress response (ISR) and deficiencies in signaling through the protective ATF6 arm of the UPR are implicated in 1-dSL-induced photoreceptor toxicity. Further, we demonstrate that pharmacologic activation of ATF6 mitigates 1-dSL toxicity without impacting PERK/ISR signaling. Collectively, our results identify new opportunities to intervene in 1-dSL linked diseases through targeting different arms of the UPR.

Effect of L-serine on oxidative stress markers in the kidney of streptozotocin-induced diabetic mice

Oxidative stress is critical in the occurrence and development of diabetes and its related complications. L-serine has recently been shown to reduce oxidative stress, the incidence of autoimmune diabetes and improve glucose homeostasis. The aim of this study was to investigate the effects of daily L-serine administration on blood glucose, renal function and oxidative stress markers in the kidney of streptozotocin-induced diabetic mice. Eighteen C57BL/6 male mice were randomly divided into three groups (n = 6 per group). Streptozotocin was used to induce diabetes and a group of diabetic mice was treated with 280 mg/day of L-serine dissolved in drinking water for 4 weeks. The level of blood glucose, biochemical markers of renal function (total protein, urea, creatinine and albumin) and oxidative stress markers (protein carbonyls, malondialdehyde, glutathione peroxidase, superoxide dismutase and catalase) were measured using spectrophotometry. The results indicated that L-serine significantly decreased the glucose level in diabetic mice (188.6 ± 22.69 mg/dL, P = 0.02). Moreover, treatment of diabetic mice with L-serine reduced protein carbonyls (3.249 ± 0.9165 nmol/mg protein, P < 0.05) and malondialdehyde levels (1.891 ± 0.7696 μM/mg protein, P = 0.051). However, L-serine showed no significant effects on renal function, and a slight reduction in histopathological changes was observed in mice receiving L-serine. This study revealed that L-serine effectively ameliorates oxidative stress in kidney tissue and reduces the blood glucose concentration in diabetic mice.

Deoxy-sphingolipids, oxidative stress, and vitamin C correlate with qualitative and quantitative patterns of small fiber dysfunction and degeneration

ABSTRACT

Defined by dysfunction or degeneration of Aδ and C fibers, small fiber neuropathies (SFNs) entail a relevant health burden. In 50% of cases, the underlying cause cannot be identified or treated. In 100 individuals (70% female individuals; mean age: 44.8 years) with an idiopathic, skin biopsy-confirmed SFN, we characterized the symptomatic spectrum and measured markers of oxidative stress (vitamin C, selenium, and glutathione) and inflammation (transforming growth factor beta, tumor necrosis factor alpha), as well as neurotoxic 1-deoxy-sphingolipids. Neuropathic pain was the most abundant symptom (95%) and cause of daily life impairment (72%). Despite the common use of pain killers (64%), the painDETECT questionnaire revealed scores above 13 points in 80% of patients. In the quantitative sensory testing (QST), a dysfunction of Aδ fibers was observed in 70% and of C fibers in 44%, affecting the face, hands, or feet. Despite normal nerve conduction studies, QST revealed Aβ fiber involvement in 46% of patients' test areas. Despite absence of diabetes mellitus or mutations in SPTLC1 or SPTLC2 , plasma 1-deoxy-sphingolipids were significantly higher in the sensory loss patient cluster when compared with those in patients with thermal hyperalgesia ( P < 0.01) or those in the healthy category ( P < 0.1), correlating inversely with the intraepidermal nerve fiber density (1-deoxy-SA: P < 0.05, 1-deoxy-SO: P < 0.01). Patients with arterial hypertension, overweight (body mass index > 25 kg/m 2 ), or hyperlipidemia showed significantly lower L-serine (arterial hypertension: P < 0.01) and higher 1-deoxy-sphingolipid levels (arterial hypertension: P < 0.001, overweight: P < 0.001, hyperlipidemia: P < 0.01). Lower vitamin C levels correlated with functional Aβ involvement ( P < 0.05). Reduced glutathione was lower in patients with Aδ dysfunction ( P < 0.05). Idiopathic SFNs are heterogeneous. As a new pathomechanism, plasma 1-deoxy-sphingolipids might link the metabolic syndrome with small fiber degeneration.

Genetic pain loss disorders

Genetic pain loss includes congenital insensitivity to pain (CIP), hereditary sensory neuropathies and, if autonomic nerves are involved, hereditary sensory and autonomic neuropathy (HSAN). This heterogeneous group of disorders highlights the essential role of nociception in protecting against tissue damage. Patients with genetic pain loss have recurrent injuries, burns and poorly healing wounds as disease hallmarks. CIP and HSAN are caused by pathogenic genetic variants in >20 genes that lead to developmental defects, neurodegeneration or altered neuronal excitability of peripheral damage-sensing neurons. These genetic variants lead to hyperactivity of sodium channels, disturbed haem metabolism, altered clathrin-mediated transport and impaired gene regulatory mechanisms affecting epigenetic marks, long non-coding RNAs and repetitive elements. Therapies for pain loss disorders are mainly symptomatic but the first targeted therapies are being tested. Conversely, chronic pain remains one of the greatest unresolved medical challenges, and the genes and mechanisms associated with pain loss offer new targets for analgesics. Given the progress that has been made, the coming years are promising both in terms of targeted treatments for pain loss disorders and the development of innovative pain medicines based on knowledge of these genetic diseases.

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.

1-deoxysphingolipids bind to COUP-TF to modulate lymphatic and cardiac cell development

Identification of physiological modulators of nuclear hormone receptor (NHR) activity is paramount for understanding the link between metabolism and transcriptional networks that orchestrate development and cellular physiology. Using libraries of metabolic enzymes alongside their substrates and products, we identify 1-deoxysphingosines as modulators of the activity of NR2F1 and 2 (COUP-TFs), which are orphan NHRs that are critical for development of the nervous system, heart, veins, and lymphatic vessels. We show that these non-canonical alanine-based sphingolipids bind to the NR2F1/2 ligand-binding domains (LBDs) and modulate their transcriptional activity in cell-based assays at physiological concentrations. Furthermore, inhibition of sphingolipid biosynthesis phenocopies NR2F1/2 deficiency in endothelium and cardiomyocytes, and increases in 1-deoxysphingosine levels activate NR2F1/2-dependent differentiation programs. Our findings suggest that 1-deoxysphingosines are physiological regulators of NR2F1/2-mediated transcription.

Lipidomics in Biomarker Research

Lipids are natural substances found in all living organisms and involved in many biological functions. Imbalances in the lipid metabolism are linked to various diseases such as obesity, diabetes, or cardiovascular disease. Lipids comprise thousands of chemically distinct species making them a challenge to analyze because of their great structural diversity.Thanks to the technological improvements in the fields of chromatography, high-resolution mass spectrometry, and bioinformatics over the last years, it is now possible to perform global lipidomics analyses, allowing the concomitant detection, identification, and relative quantification of hundreds of lipid species. This review shall provide an insight into a general lipidomics workflow and its application in metabolic biomarker research.

1-Deoxysphingolipids, Early Predictors of Type 2 Diabetes, Compromise the Functionality of Skeletal Myoblasts

Metabolic dysfunction, dysregulated differentiation, and atrophy of skeletal muscle occur as part of a cluster of abnormalities associated with the development of Type 2 diabetes mellitus (T2DM). Recent interest has turned to the attention of the role of 1-deoxysphingolipids (1-DSL), atypical class of sphingolipids which are found significantly elevated in patients diagnosed with T2DM but also in the asymptomatic population who later develop T2DM. In vitro studies demonstrated that 1-DSL have cytotoxic properties and compromise the secretion of insulin from pancreatic beta cells. However, the role of 1-DSL on the functionality of skeletal muscle cells in the pathophysiology of T2DM still remains unclear. This study aimed to investigate whether 1-DSL are cytotoxic and disrupt the cellular processes of skeletal muscle precursors (myoblasts) and differentiated cells (myotubes) by performing a battery of in vitro assays including cell viability adenosine triphosphate assay, migration assay, myoblast fusion assay, glucose uptake assay, and immunocytochemistry. Our results demonstrated that 1-DSL significantly reduced the viability of myoblasts in a concentration and time-dependent manner, and induced apoptosis as well as cellular necrosis. Importantly, myoblasts were more sensitive to the cytotoxic effects induced by 1-DSL rather than by saturated fatty acids, such as palmitate, which are critical mediators of skeletal muscle dysfunction in T2DM. Additionally, 1-DSL significantly reduced the migration ability of myoblasts and the differentiation process of myoblasts into myotubes. 1-DSL also triggered autophagy in myoblasts and significantly reduced insulin-stimulated glucose uptake in myotubes. These findings demonstrate that 1-DSL directly compromise the functionality of skeletal muscle cells and suggest that increased levels of 1-DSL observed during the development of T2DM are likely to contribute to the pathophysiology of muscle dysfunction detected in this disease.

Nervonic acid limits weight gain in a mouse model of diet-induced obesity

Lipid perturbations contribute to detrimental outcomes in obesity. We previously demonstrated that nervonic acid, a C24:1 ω-9 fatty acid, predominantly acylated to sphingolipids, including ceramides, are selectively reduced in a mouse model of obesity. It is currently unknown if deficiency of nervonic acid-sphingolipid metabolites contribute to complications of obesity. Mice were fed a standard diet, a high fat diet, or these diets supplemented isocalorically with nervonic acid. The primary objective was to determine if dietary nervonic acid content alters the metabolic phenotype in mice fed a high fat diet. Furthermore, we investigated if nervonic acid alters markers of impaired fatty acid oxidation in the liver. We observed that a nervonic acid-enriched isocaloric diet reduced weight gain and adiposity in mice fed a high fat diet. The nervonic acid enrichment led to increased C24:1-ceramides and improved several metabolic parameters including blood glucose levels, and insulin and glucose tolerance. Mechanistically, nervonic acid supplementation increased PPARα and PGC1α expression and improved the acylcarnitine profile in liver. These alterations indicate improved energy metabolism through increased β-oxidation of fatty acids. Taken together, increasing dietary nervonic acid improves metabolic parameters in mice fed a high fat diet. Strategies that prevent deficiency of, or restore, nervonic acid may represent an effective strategy to treat obesity and obesity-related complications.

Serine administration as a novel prophylactic approach to reduce the severity of acute pancreatitis during diabetes in mice

AIMS/HYPOTHESIS

Compared with the general population, individuals with diabetes have a higher risk of developing severe acute pancreatitis, a highly debilitating and potentially lethal inflammation of the exocrine pancreas. In this study, we investigated whether 1-deoxysphingolipids, atypical lipids that increase in the circulation following the development of diabetes, exacerbate the severity of pancreatitis in a diabetic setting.

METHODS

We analysed whether administration of an L-serine-enriched diet to mouse models of diabetes, an established method for decreasing the synthesis of 1-deoxysphingolipids in vivo, reduced the severity of acute pancreatitis. Furthermore, we elucidated the molecular mechanisms underlying the lipotoxicity exerted by 1-deoxysphingolipids towards rodent pancreatic acinar cells in vitro.

RESULTS

We demonstrated that L-serine supplementation reduced the damage of acinar tissue resulting from the induction of pancreatitis in diabetic mice (average histological damage score: 1.5 in L-serine-treated mice vs 2.7 in the control group). At the cellular level, we showed that L-serine decreased the production of reactive oxygen species, endoplasmic reticulum stress and cellular apoptosis in acinar tissue. Importantly, these parameters, together with DNA damage, were triggered in acinar cells upon treatment with 1-deoxysphingolipids in vitro, suggesting that these lipids are cytotoxic towards pancreatic acinar cells in a cell-autonomous manner. In search of the initiating events of the observed cytotoxicity, we discovered that 1-deoxysphingolipids induced early mitochondrial dysfunction in acinar cells, characterised by ultrastructural alterations, impaired oxygen consumption rate and reduced ATP synthesis.

CONCLUSIONS/INTERPRETATION

Our results suggest that 1-deoxysphingolipids directly damage the functionality of pancreatic acinar cells and highlight that an L-serine-enriched diet may be used as a promising prophylactic intervention to reduce the severity of pancreatitis in the context of diabetes.

The mitochondrial-derived peptide MOTS-c is a regulator of plasma metabolites and enhances insulin sensitivity

MOTS‐c is an exercise mimetic and improves insulin sensitivity in aged and diet‐induced obese mice. Although plasma markers are good markers for the metabolic condition, whether MOTS‐c changes plasma markers in diet‐induced obese mice has not been examined. Here, we used an unbiased metabolomics approach to examine the effect of MOTS‐c on plasma markers of metabolic dysfunction. We found that three pathways – sphingolipid metabolism, monoacylglycerol metabolism, and dicarboxylate metabolism – were reduced in MOTS‐c–injected mice. Interestingly, these pathways are upregulated in obese and T2D models. MOTS‐c improves insulin sensitivity and increases beta‐oxidation to prevent fat accumulation in DIO mice through these pathways. These results provide us a better understanding of the mechanism of how MOTS‐c improves insulin sensitivity and reduces the body weight and fatty liver and opens a new venue for further study.

Resolving Sphingolipid Isomers Using Cryogenic Infrared Spectroscopy

1‐Deoxysphingolipids are a recently described class of sphingolipids that have been shown to be associated with several disease states including diabetic and hereditary neuropathy. The identification and characterization of 1‐deoxysphingolipids and their metabolites is therefore highly important. However, exact structure determination requires a combination of sophisticated analytical techniques due to the presence of various isomers, such as ketone/alkenol isomers, carbon–carbon double‐bond (C=C) isomers and hydroxylation regioisomers. Here we demonstrate that cryogenic gas‐phase infrared (IR) spectroscopy of ionized 1‐deoxysphingolipids enables the identification and differentiation of isomers by their unique spectroscopic fingerprints. In particular, C=C bond positions and stereochemical configurations can be distinguished by specific interactions between the charged amine and the double bond. The results demonstrate the power of gas‐phase IR spectroscopy to overcome the challenge of isomer resolution in conventional mass spectrometry and pave the way for deeper analysis of the lipidome.

Contributions of amino acid, acylcarnitine and sphingolipid profiles to type 2 diabetes risk among South-Asian Surinamese and Dutch adults

INTRODUCTION

People of South Asian origin are at high risk of type 2 diabetes (T2D), but the underpinning mechanisms are not fully understood. We determined ethnic differences in acylcarnitine, amino acid and sphingolipid concentrations and determined the associations with T2D.

RESEARCH DESIGN AND METHODS

Associations between these metabolites and incident T2D among Dutch and South-Asian Surinamese were determined in participants from the Healthy Life in an Urban Setting (HELIUS) study (Amsterdam, the Netherlands) using Prentice-weighted Cox regression. The HELIUS study includes 95 incident T2D cases and a representative subcohort of 700 people from a cohort of 5977 participants with a mean follow-up of 4 years.

RESULTS

Concentrations of acylcarnitines were comparable between both ethnic groups. Amino acid and lactosylceramide concentrations were higher among South-Asian Surinamese than Dutch (eg, isoleucine 65.7 (SD 16.3) vs 60.7 (SD 15.6) µmol/L). Ceramide concentrations were lower among South-Asian Surinamese than Dutch (eg, Cer d18:1 8.48 (SD 2.04) vs 9.08 (SD 2.29) µmol/L). Metabolic dysregulation preceded T2D without evidence for a multiplicative interaction by ethnicity. Most amino acids and (dihydro)ceramides were associated with increased risk (eg, Cer d18:1 HR 2.38, 95% CI 1.81 to 3.12) while acylcarnitines, glycine, glutamine and lactosylceramides were associated with decreased risk for T2D (eg, LacCer d18:2 HR 0.56, 95% CI 0.42 to 0.77).

CONCLUSIONS

Overall, these data suggest that the disturbances underlying amino acid and sphingolipid metabolism may be predictive of T2D risk in populations of both South Asian and European background. These observations may be used as starting point to unravel the underlying metabolic disturbances.

Farnesoid X receptor activation induces the degradation of hepatotoxic 1-deoxysphingolipids in non-alcoholic fatty liver disease

BACKGROUND & AIMS

Patients with non-alcoholic fatty liver disease (NAFLD) exhibit higher levels of plasma 1-deoxysphingolipids than healthy individuals. The aim of this study was to investigate the role of farnesoid X receptor (FXR) in 1-deoxysphingolipid de novo synthesis and degradation.

METHODS

Mice were fed with a high-fat diet (HFD) to induce obesity and NAFLD, and then treated with the FXR ligand obeticholic acid (OCA). Histology and gene expression analysis were performed on liver tissue. Sphingolipid patterns from NAFLD patients and mouse models were assessed by liquid chromatography-mass spectrometry. The molecular mechanism underlying the effect of FXR activation on sphingolipid metabolism was studied in Huh7 cells and primary cultured hepatocytes, as well as in a 1-deoxysphinganine-treated mouse model.

RESULTS

1-deoxysphingolipids were increased in both NAFLD patients and mouse models. FXR activation by OCA protected the liver against oxidative stress, apoptosis, and reduced 1-deoxysphingolipid levels, both in a HFD-induced mouse model of obesity and in 1-deoxysphinganine-treated mice. In vitro, FXR activation lowered intracellular 1-deoxysphingolipid levels by inducing Cyp4f-mediated degradation, but not by inhibiting de novo synthesis, thereby protecting hepatocytes against doxSA-induced cytotoxicity, mitochondrial damage, and apoptosis. Overexpression of Cyp4f13 in cells was sufficient to ameliorate doxSA-induced cytotoxicity. Treatment with the Cyp4f pan-inhibitor HET0016 or FXR knock-down fully abolished the protective effect of OCA, indicating that OCA-mediated 1-deoxysphingolipid degradation is FXR and Cyp4f dependent.

CONCLUSIONS

Our study identifies FXR-Cyp4f as a novel regulatory pathway for 1-deoxysphingolipid metabolism. FXR activation represents a promising therapeutic strategy for patients with metabolic syndrome and NAFLD.

From Association to Causality: the Role of the Gut Microbiota and Its Functional Products on Host Metabolism

Many genomic studies have revealed associations between the gut microbiota composition and host metabolism. These observations led to the idea that a causal relationship could exist between the microbiota and metabolic diseases, a concept supported by studies showing compositional changes in the microbial community in metabolic diseases and transmissibility of host phenotype via microbiota transfer. Accumulating data suggest that the microbiota may affect host metabolic phenotypes through the production of metabolites. These bioactive microbial metabolites, sensitive fingerprints of microbial function, can act as inter-kingdom signaling messengers via penetration into host blood circulation and tissues. These fingerprints may be used for diagnostic purposes, and increased understanding of strain specificity in producing microbial metabolites can identify bacterial strains or specific metabolites that can be used for therapeutic purposes. Here, we will review data supporting the causal role of the gut microbiota in metabolism and discuss mechanisms and potential clinical implications.

FADS3 is a Δ14Z sphingoid base desaturase that contributes to gender differences in the human plasma sphingolipidome

Sphingolipids (SLs) are structurally diverse lipids that are defined by the presence of a long-chain base (LCB) backbone. Typically, LCBs contain a single Δ4E double bond (DB) (mostly d18:1), whereas the dienic LCB sphingadienine (d18:2) contains a second DB at the Δ14Z position. The enzyme introducing the Δ14Z DB is unknown. We analyzed the LCB plasma profile in a gender-, age-, and BMI-matched subgroup of the CoLaus cohort (n = 658). Sphingadienine levels showed a significant association with gender, being on average ∼30% higher in females. A genome-wide association study (GWAS) revealed variants in the fatty acid desaturase 3 (FADS3) gene to be significantly associated with the plasma d18:2/d18:1 ratio (p = -log 7.9). Metabolic labeling assays, FADS3 overexpression and knockdown approaches, and plasma LCB profiling in FADS3-deficient mice confirmed that FADS3 is a bona fide LCB desaturase and required for the introduction of the Δ14Z double bond. Moreover, we showed that FADS3 is required for the conversion of the atypical cytotoxic 1-deoxysphinganine (1-deoxySA, m18:0) to 1-deoxysphingosine (1-deoxySO, m18:1). HEK293 cells overexpressing FADS3 were more resistant to m18:0 toxicity than WT cells. In summary, using a combination of metabolic profiling and GWAS, we identified FADS3 to be essential for forming Δ14Z DB containing LCBs, such as d18:2 and m18:1. Our results unravel FADS3 as a Δ14Z LCB desaturase, thereby disclosing the last missing enzyme of the SL de novo synthesis pathway.

Druggable Sphingolipid Pathways: Experimental Models and Clinical Opportunities

Intensive research in the field of sphingolipids has revealed diverse roles in cell biological responses and human health and disease. This immense molecular family is primarily represented by the bioactive molecules ceramide, sphingosine, and sphingosine 1-phosphate (S1P). The flux of sphingolipid metabolism at both the subcellular and extracellular levels provides multiple opportunities for pharmacological intervention. The caveat is that perturbation of any single node of this highly regulated flux may have effects that propagate throughout the metabolic network in a dramatic and sometimes unexpected manner. Beginning with S1P, the receptors for which have thus far been the most clinically tractable pharmacological targets, this review will describe recent advances in therapeutic modulators targeting sphingolipids, their chaperones, transporters, and metabolic enzymes.

Analysis of 1-Deoxysphingoid Bases and Their N-Acyl Metabolites and Exploration of Their Occurrence in Some Food Materials

Most common sphingolipids are comprised of "typical" sphingoid bases (sphinganine, sphingosine, and structurally related compounds) and are produced via the condensation of l-serine with a fatty acyl-CoA by serine palmitoyltransferase. Some organisms, including mammals, also produce "atypical" sphingoid bases that lack a 1-hydroxyl group as a result of the utilization of l-alanine or glycine instead of l-serine, resulting in the formation of 1-deoxy- or 1-desoxymethylsphingoid bases, respectively. Elevated production of "atypical" sphingolipids has been associated with human disease, but 1-deoxysphingoid bases have also been found to have potential as anticancer compounds, hence, the importance of knowing more about the occurrence of these compounds in food. Most of the "typical" and "atypical" sphingoid bases are found as the N-acyl metabolites (e.g., ceramides and 1-deoxyceramides) in mammals, but this has not been uniformly assessed in previous studies nor determined in consumed food. Therefore, we developed a method for the quantitative analysis of "typical" and "atypical" sphingoid bases and their N-acyl derivatives by reverse-phase liquid chromatography coupled to electrospray ionization tandem mass spectrometry. On the basis of these analyses, there was considerable variability in the amounts and molecular subspecies of atypical sphingoid bases and their N-acyl metabolites found in different edible sources. These findings demonstrate that a broader assessment of the types of sphingolipids in foods is needed because some diets might contain sufficient amounts of atypical as well as typical sphingolipids that could have beneficial or possibly deleterious effects on human health.

Metabolic Messengers: ceramides

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

Analysis of sphingolipid composition in human vitreous from control and diabetic individuals

OBJECTIVE

Sphingolipids have a fundamental role in many cellular processes, and they have been implicated in insulin resistance and Diabetes Mellitus (DM) and its complications, including diabetic retinopathy (DR). Little is known about how bioactive sphingolipids relate to retinopathies in human DM. In this study, we analyzed the sphingolipid composition of type 2 diabetic (T2DM) and non-diabetic human vitreous samples.

METHODS

We conducted an observational study on post-mortem human vitreous samples from non-diabetic (Controls; n = 4; age: 71.6 ± 11.0 years, mean ± SD) and type 2 diabetic (T2DM; n = 9; age: 67.0 ± 9.2 years) donors to identify changes in sphingolipid composition. Samples were analyzed by a triple quadrupole mass spectrometer and individual sphingolipid species were identified and quantified using established protocols.

RESULTS

The total quantity (pmol/mg) of ceramide (Cer), lactosylceramide (Lac-Cer), and sphingomyelin (SM) were increased in type 2 diabetic vitreous samples. Among individual species, we found a general trend of increase in the longer chain species of ceramides, hexosylceramides (Hex-Cer), Lac-Cer, and SM.

CONCLUSIONS

This study shows the presence of measurable levels of sphingolipids in human vitreous. The results indicate changes in sphingolipid composition in the vitreous due to type 2 diabetes, which could be connected to the disease pathologies of the retina, retinal vessels, vitreous and the surrounding tissues.

Plasma metabolites predict both insulin resistance and incident type 2 diabetes: a metabolomics approach within the Prevención con Dieta Mediterránea (PREDIMED) study

BACKGROUND

Insulin resistance is a complex metabolic disorder and is often associated with type 2 diabetes (T2D).

OBJECTIVES

The aim of this study was to test whether baseline metabolites can additionally improve the prediction of insulin resistance beyond classical risk factors. Furthermore, we examined whether a multimetabolite model predicting insulin resistance in nondiabetics can also predict incident T2D.

METHODS

We used a case-cohort study nested within the Prevención con Dieta Mediterránea (PREDIMED) trial in subsets of 700, 500, and 256 participants without T2D at baseline and 1 and 3 y. Fasting plasma metabolites were semiquantitatively profiled with liquid chromatography-tandem mass spectrometry. We assessed associations between metabolite concentrations and the homeostasis model of insulin resistance (HOMA-IR) through the use of elastic net regression analysis. We subsequently examined associations between the baseline HOMA-IR-related multimetabolite model and T2D incidence through the use of weighted Cox proportional hazard models.

RESULTS

We identified a set of baseline metabolites associated with HOMA-IR. One-year changes in metabolites were also significantly associated with HOMA-IR. The area under the curve was significantly greater for the model containing the classical risk factors and metabolites together compared with classical risk factors alone at baseline [0.81 (95% CI: 0.79, 0.84) compared with 0.69 (95% CI: 0.66, 0.73)] and during a 1-y period [0.69 (95% CI: 0.66, 0.72) compared with 0.57 (95% CI: 0.53, 0.62)]. The variance in HOMA-IR explained by the combination of metabolites and classical risk factors was also higher in all time periods. The estimated HRs for incident T2D in the multimetabolite score (model 3) predicting high HOMA-IR (median value or higher) or HOMA-IR (continuous) at baseline were 2.00 (95% CI: 1.58, 2.55) and 2.24 (95% CI: 1.72, 2.90), respectively, after adjustment for T2D risk factors.

CONCLUSIONS

The multimetabolite model identified in our study notably improved the predictive ability for HOMA-IR beyond classical risk factors and significantly predicted the risk of T2D.

Sphingolipid biosynthesis in man and microbes

A new review covering up to 2018 Sphingolipids are essential molecules that, despite their long history, are still stimulating interest today. The reasons for this are that, as well as playing structural roles within cell membranes, they have also been shown to perform a myriad of cell signalling functions vital to the correct function of eukaryotic and prokaryotic organisms. Indeed, sphingolipid disregulation that alters the tightly-controlled balance of these key lipids has been closely linked to a number of diseases such as diabetes, asthma and various neuropathologies. Sphingolipid biogenesis, metabolism and regulation is mediated by a large number of enzymes, proteins and second messengers. There appears to be a core pathway common to all sphingolipid-producing organisms but recent studies have begun to dissect out important, species-specific differences. Many of these have only recently been discovered and in most cases the molecular and biochemical details are only beginning to emerge. Where there is a direct link from classic biochemistry to clinical symptoms, a number a drug companies have undertaken a medicinal chemistry campaign to try to deliver a therapeutic intervention to alleviate a number of diseases. Where appropriate, we highlight targets where natural products have been exploited as useful tools. Taking all these aspects into account this review covers the structural, mechanistic and regulatory features of sphingolipid biosynthetic and metabolic enzymes.

The phenotype of adult versus pediatric patients with inborn errors of metabolism

Until recently, inborn errors of metabolism (IEM) were considered a pediatric specialty, as emphasized by the term "inborn," and the concept of adult onset IEM has only very recently reached the adult medical community. Still, an increasing number of adult onset IEM have now been recognized, as new metabolomics and molecular diagnostic techniques have become available. Here, we discuss possible mechanisms underlying phenotypic variability in adult versus children with IEM. Specifically, phenotypic severity and age of onset are expected to be modulated by differences in residual protein activity possibly driven by various genetic factors. Phenotypic variability may also occur in the context of similar protein expression, which suggests the intervention of environmental, ontogenic, and aging factors.

Vitamin D Supplementation Enhances C18(dihydro)ceramide Levels in Type 2 Diabetes Patients

Sphingolipids are characterized by a broad range of bioactive properties. Particularly, the development of insulin resistance, a major pathophysiological hallmark of Type 2 Diabetes mellitus (T2D), has been linked to ceramide signaling. Since vitamin D supplementation may slow down T2D progression by improving glucose concentrations and insulin sensitivity, we investigated whether vitamin D supplementation impacts on plasma sphingolipid levels in T2D patients. Thus, plasma samples of 59 patients with non-insulin-requiring T2D from a placebo-controlled, randomized, and double-blind study were retrospectively analyzed. Once per week, patients received either 20 drops of Vigantol oil, corresponding to a daily dose of 1904 IU/d vitamin D (verum: n = 31), or a placebo oil consisting of medium chain triglycerides (placebo: n = 28). Blood samples were taken from all of the participants at three different time points: 1) at the beginning of the study (baseline), 2) after 6 months supplementation, and 3) after an additional 6 months of follow-up. Plasma sphingolipids were measured by high-performance liquid chromatography tandem mass spectrometry. At baseline and 6 months follow-up, no significant differences in plasma sphingolipid species were detected between the placebo and verum groups. After 6 months, vitamin D supplementation significantly enhanced plasma C18dihydroceramide (dhCer; N-stearoyl-sphinganine (d18:0/18:0)) and C18ceramide (Cer; N-stearoyl-sphingosine (d18:1/18:0)) levels were observed in the verum group compared to the placebo group. This was accompanied by significantly higher 25-hydroxyvitamin D₃ (25(OH)D₃) blood levels in patients receiving vitamin D compared to the placebo group. Taken together, vitamin D supplementation induced changes of the C18 chain-length-specific dhCer and Cer plasma levels in patients with T2D. The regulation of sphingolipid signaling by vitamin D may thus unravel a novel mechanism by which vitamin D can influence glucose utilization and insulin action. Whether this acts favorably or unfavorably for the progression of T2D needs to be clarified.