CerS6-Derived Sphingolipids Interact with Mff and Promote Mitochondrial Fragmentation in Obesity

Ceramide microdomains: the major influencers of the sphingolipid media platform

Like 'influencers' who achieve fame and power through social media, ceramides are low abundance members of communication platforms that have a mighty impact on their surroundings. Ceramide microdomains form within sphingolipid-laden lipid rafts that confer detergent resistance to cell membranes and serve as important signaling hubs. In cells exposed to excessive amounts of saturated fatty acids (e.g. in obesity), the abundance of ceramide-rich microdomains within these rafts increases, leading to concomitant alterations in cellular metabolism and survival that contribute to cardiometabolic disease. In this mini-review, we discuss the evidence supporting the formation of these ceramide microdomains and describe the spectrum of harmful ceramide-driven metabolic actions under the context of an evolutionary theory. Moreover, we discuss the proximal 'followers' of these ceramide media stars that account for the diverse intracellular actions that allow them to influence obesity-linked disease.

High-fat feeding drives the intestinal production and assembly of C16:0 ceramides in chylomicrons

Consumption of a diet rich in saturated fat increases lipid absorption from the intestine, assembly into chylomicrons, and delivery to metabolic tissues via the lymphatic and circulatory systems. Accumulation of ceramide lipids, composed of sphingosine and a fatty acid, in metabolic tissues contributes to the pathogenesis of cardiovascular diseases, type 2 diabetes mellitus and cancer. Using a mesenteric lymph duct cannulated rat model, we showed that ceramides are generated by the intestine and assembled into chylomicrons, which are transported via the mesenteric lymphatic system. A lipidomic screen of intestinal-derived chylomicrons identified a diverse range of fatty acid, sphingolipid, and glycerolipid species that have not been previously detected in chylomicrons, including the metabolically deleterious C16:0 ceramide that increased in response to high-fat feeding in rats and human high-lipid meal replacement enteral feeding. In conclusion, high-fat feeding increases the export of intestinal-derived C16:0 ceramide in chylomicrons, identifying a potentially unknown mechanism through which ceramides are transported systemically to contribute to metabolic dysfunction.

The fate of intracellular S1P regulates lipid droplet turnover and lipotoxicity in pancreatic beta-cells

Lipotoxicity has been considered the main cause of pancreatic beta-cell failure during type 2 diabetes development. Lipid droplets (LD) are believed to regulate the beta-cell sensitivity to free fatty acids (FFA), but the underlying molecular mechanisms are largely unclear. Accumulating evidence points, however, to an important role of intracellular sphingosine-1-phosphate (S1P) metabolism in lipotoxicity-mediated disturbances of beta-cell function. In the present study, we compared the effects of an increased irreversible S1P degradation (S1P-lyase, SPL overexpression) with those associated with an enhanced S1P recycling (overexpression of S1P phosphatase 1, SGPP1) on LD formation and lipotoxicity in rat INS1E beta-cells. Interestingly, although both approaches led to a reduced S1P concentration, they had opposite effects on the susceptibility to FFA. Overexpression of SGPP1 prevented FFA-mediated caspase-3 activation by a mechanism involving an enhanced lipid storage capacity and prevention of oxidative stress. In contrast, SPL overexpression limited LD biogenesis, content, and size, while accelerating lipophagy. This was associated with FFA-induced hydrogen peroxide formation, mitochondrial fragmentation, and dysfunction, as well as ER stress. These changes coincided with the upregulation of proapoptotic ceramides but were independent of lipid peroxidation rate. Also in human EndoC-βH1 beta-cells, suppression of SPL with simultaneous overexpression of SGPP1 led to a similar and even more pronounced LD phenotype as that in INS1E-SGPP1 cells. Thus, intracellular S1P turnover significantly regulates LD content and size and influences beta-cell sensitivity to FFA.

ERLIN1/2 scaffolds bridge TMUB1 and RNF170 and restrict cholesterol esterification to regulate the secretory pathway

Complexes of ERLIN1 and ERLIN2 (ER lipid raft-associated 1 and 2) form large ring-like cup-shaped structures on the endoplasmic reticulum (ER) membrane and serve as platforms to bind cholesterol and E3 ubiquitin ligases, potentially defining functional nanodomains. Here, we show that ERLIN scaffolds mediate the interaction between the full-length isoform of TMUB1 (transmembrane and ubiquitin-like domain-containing 1) and RNF170 (RING finger protein 170). We identify a luminal N-terminal conserved region in TMUB1 and RNF170, which is required for this interaction. Three-dimensional modelling shows that this conserved motif binds the stomatin/prohibitin/flotillin/HflKC domain of two adjacent ERLIN subunits at different interfaces. Protein variants that preclude these interactions have been previously linked to hereditary spastic paraplegia. Using omics-based approaches in combination with phenotypic characterization of HeLa cells lacking both ERLINs, we demonstrate a role of ERLIN scaffolds in limiting cholesterol esterification, thereby favouring cholesterol transport from the ER to the Golgi apparatus and regulating Golgi morphology and the secretory pathway.

Ethnicity-related differences in mitochondrial regulation by insulin stimulation in diabetes

Mitochondrial dysfunction has long been implicated in the development of insulin resistance, which is a hallmark of type 2 diabetes. However, recent studies reveal ethnicity-related differences in mitochondrial processes, underscoring the need for nuance in studying mitochondrial dysfunction and insulin sensitivity. Furthermore, the higher prevalence of type 2 diabetes among African Americans and individuals of African descent has brought attention to the role of ethnicity in disease susceptibility. In this review, which covers existing literature, genetic studies, and clinical data, we aim to elucidate the complex relationship between mitochondrial alterations and insulin stimulation by considering how mitochondrial dynamics, contact sites, pathways, and metabolomics may be differentially regulated across ethnicities, through mechanisms such as single nucleotide polymorphisms (SNPs). In addition to achieving a better understanding of insulin stimulation, future studies identifying novel regulators of mitochondrial structure and function could provide valuable insights into ethnicity-dependent insulin signaling and personalized care.

Anti-adipogenesis effect of indole-3-acrylic acid on human preadipocytes and HFD-induced zebrafish

BACKGROUND

Obesity, defined as excessive or abnormal body fat accumulation, which could significantly increase the risk of cardiovascular disease, type 2 diabetes mellitus (T2DM) diseases and seriously affect people's quality of life. More than 2 billion people are overweight, and the incidence of obesity is increasing rapidly worldwide, it has become a widely concerned public health issue in the world. Diverse evidence show that active metabolites are involved in the pathophysiological processes of obesity.

AIMS

However, whether the downstream catabolite of tryptophan, 3-indole acrylic acid (IA), is involved in obesity remains unclear.

METHODS

We collected the samples of serum from peripheral blood of obesity and health controls, and liquid chromatography-mass spectrometry (LC-MS) was performed to identify the plasma levels of IA. Additionally, we verified the potential benefits of IA on human preadipocytes and HFD- induced zebrafish by cell viability assay, flow cytometry assay, Oil red O staining, total cholesterol (T-CHO), triglyceride (TG) and nonesterified free fatty acids (NEFA) measurements and Nile Red staining. RNA-Seq, functional analysis and western blot revealed the mechanisms underlying the function of IA.

RESULTS

We found that the content of IA in peripheral blood serum of overweight people was significantly lower than that of normal people. In addition, supplementation with IA in zebrafish larvae induced by a high fat diet (HFD) dramatically reduced HFD induced lipid accumulation. IA had no effect on proliferation and apoptosis of preadipocytes, but significantly inhibited adipogenesis of preadipocytes by down-regulate CEBPα and PPARγ. RNA-Seq and functional analysis revealed that IA regulated the adipogenesis of preadipocytes through stimulate the phosphorylation of STAT1.

CONCLUSIONS

Taken together, IA has been identified as a potent metabolite for the prevention or treatment of obesity.

Ceramides in the central control of metabolism

Central ceramides regulate energy metabolism by impacting hypothalamic neurons. This allows ceramides to integrate endocrine signals - such as leptin, ghrelin, thyroid hormones, or estradiol - and to modulate the central control of puberty. In this forum article we discuss recent evidence suggesting that specific ceramide species and neuronal populations are involved in these effects.

Analysis and Quantification of the Mitochondrial-ER Lipidome

Mitochondria are vital organelles essential for cellular functions, but their lipid composition and response to stressors are not fully understood. Recent advancements in lipidomics reveal insights into lipid functions, especially their roles in metabolic perturbations and diseases. Previous methods have focused on the protein composition of mitochondria and mitochondrial-associated membranes. The advantage of our technique is that it combines organelle isolation with targeted lipidomics, offering new insights into the composition and dynamics of these organelles in pathological conditions. We developed a mitochondria isolation protocol for L6 myotubes, enabling lipidomics analysis of specific organelles without interference from other cellular compartments. This approach offers a unique opportunity to dissect lipid dynamics within mitochondria and their associated ER compartments under cellular stress. Key features • Analysis and quantification of lipids in mitochondria-ER fraction through liquid chromatography-tandem mass spectrometry-based lipidomics (LC-MS/MS lipidomics). • LC-MS/MS lipidomics provide precise and unbiased information on the lipid composition in in vitro systems. • LC-MS/MS lipidomics facilitates the identification of lipid signatures in mammalian cells.

Plasma 16:0 ceramide as a marker of cardiovascular risk estimated by carotid intima-media thickness in people with type 2 diabetes

AIM

New tools are required to better assess cardiovascular risk in individuals with type 2 diabetes mellitus (T2DM). Plasma ceramides emerge as promising candidates, given their substantial influence on the pathogenesis of both T2DM and atherosclerosis. The current study aimed to investigate whether plasma ceramides in patients with T2DM are a predictive factor for carotid intima-media thickness (CIMT), a well-established noninvasive marker for atherosclerosis that predicts adverse cardiovascular outcomes.

METHODS

A lipidomic analysis was carried out on the circulating ceramides of a large cohort consisting of 246 patients with T2DM who underwent a high-resolution real-time B ultrasonography to measure CIMT.

RESULTS

Both plasma 16:0 ceramide and the 16:0/24:0 ceramide ratio were positively associated with CIMT, even after adjustment for traditional cardiovascular risk factors [standardized β ± standard error: 0.168 ± 0.072 (P = 0.020) and 0.180 ± 0.068 (P = 0.009), respectively]. Similar independent associations were found with respect to the prediction of CIMT ≥ 0.80 mm [β = 8.07 ± 3.90 (P = 0.038) and 16.5 ± 7.0 (P = 0.019), respectively]. The goodness-of-fit for multivariate models in predicting CIMT was 5.7 and 7.6 times higher when plasma 16:0 ceramide or the 16:0/24:0 ceramide ratio were included in combination with traditional cardiovascular risk factors (P = 0.020 and 0.015, respectively). This reached a 3.1 and 10.0-fold increase regarding the ability to predict CIMT ≥ 0.80 mm (P = 0.039 and 0.008, respectively).

CONCLUSIONS

Our findings suggest that 16:0 ceramide and the 16:0/24:0 ceramide ratio may serve as plasma biomarkers to improve cardiovascular risk assessment in individuals with T2DM.

PAQR4 regulates adipocyte function and systemic metabolic health by mediating ceramide levels

PAQR4 is an orphan receptor in the PAQR family with an unknown function in metabolism. Here, we identify a critical role of PAQR4 in maintaining adipose tissue function and whole-body metabolic health. We demonstrate that expression of Paqr4 specifically in adipocytes, in an inducible and reversible fashion, leads to partial lipodystrophy, hyperglycaemia and hyperinsulinaemia, which is ameliorated by wild-type adipose tissue transplants or leptin treatment. By contrast, deletion of Paqr4 in adipocytes improves healthy adipose remodelling and glucose homoeostasis in diet-induced obesity. Mechanistically, PAQR4 regulates ceramide levels by mediating the stability of ceramide synthases (CERS2 and CERS5) and, thus, their activities. Overactivation of the PQAR4-CERS axis causes ceramide accumulation and impairs adipose tissue function through suppressing adipogenesis and triggering adipocyte de-differentiation. Blocking de novo ceramide biosynthesis rescues PAQR4-induced metabolic defects. Collectively, our findings suggest a critical function of PAQR4 in regulating cellular ceramide homoeostasis and targeting PAQR4 offers an approach for the treatment of metabolic disorders.

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.

Comprehensive profiling of lipid metabolic reprogramming expands precision medicine for HCC

BACKGROUND AND AIMS

Liver HCC is the second leading cause of cancer-related deaths worldwide. The heterogeneity of this malignancy is driven by a wide range of genetic alterations, leading to a lack of effective therapeutic options. In this study, we conducted a systematic multi-omics characterization of HCC to uncover its metabolic reprogramming signature.

APPROACH AND RESULTS

Through a comprehensive analysis incorporating transcriptomic, metabolomic, and lipidomic investigations, we identified significant changes in metabolic pathways related to glucose flux, lipid oxidation and degradation, and de novo lipogenesis in HCC. The lipidomic analysis revealed abnormal alterations in glycerol-lipids, phosphatidylcholine, and sphingolipid derivatives. Machine-learning techniques identified a panel of genes associated with lipid metabolism as common biomarkers for HCC across different etiologies. Our findings suggest that targeting phosphatidylcholine with saturated fatty acids and long-chain sphingolipid biosynthesis pathways, particularly by inhibiting lysophosphatidylcholine acyltransferase 1 ( LPCAT1 ) and ceramide synthase 5 ( CERS5 ) as potential therapeutic strategies for HCC in vivo and in vitro. Notably, our data revealed an oncogenic role of CERS5 in promoting tumor progression through lipophagy.

CONCLUSIONS

In conclusion, our study elucidates the metabolic reprogramming nature of lipid metabolism in HCC, identifies prognostic markers and therapeutic targets, and highlights potential metabolism-related targets for therapeutic intervention in HCC.

Therapeutic implications for sphingolipid metabolism in metabolic dysfunction-associated steatohepatitis

The prevalence of metabolic dysfunction-associated steatotic liver disease (MASLD), a leading cause of chronic liver disease, has increased worldwide along with the epidemics of obesity and related dysmetabolic conditions characterized by impaired glucose metabolism and insulin signaling, such as type 2 diabetes mellitus (T2D). MASLD can be defined as an excessive accumulation of lipid droplets in hepatocytes that occurs when the hepatic lipid metabolism is totally surpassed. This metabolic lipid inflexibility constitutes a central node in the pathogenesis of MASLD and is frequently linked to the overproduction of lipotoxic species, increased cellular stress, and mitochondrial dysfunction. A compelling body of evidence suggests that the accumulation of lipid species derived from sphingolipid metabolism, such as ceramides, contributes significantly to the structural and functional tissue damage observed in more severe grades of MASLD by triggering inflammatory and fibrogenic mechanisms. In this context, MASLD can further progress to metabolic dysfunction-associated steatohepatitis (MASH), which represents the advanced form of MASLD, and hepatic fibrosis. In this review, we discuss the role of sphingolipid species as drivers of MASH and the mechanisms involved in the disease. In addition, given the absence of approved therapies and the limited options for treating MASH, we discuss the feasibility of therapeutic strategies to protect against MASH and other severe manifestations by modulating sphingolipid metabolism.

Regulation of cellular and systemic sphingolipid homeostasis

One hundred and fifty years ago, Johann Thudichum described sphingolipids as unusual "Sphinx-like" lipids from the brain. Today, we know that thousands of sphingolipid molecules mediate many essential functions in embryonic development and normal physiology. In addition, sphingolipid metabolism and signalling pathways are dysregulated in a wide range of pathologies, and therapeutic agents that target sphingolipids are now used to treat several human diseases. However, our understanding of sphingolipid regulation at cellular and organismal levels and their functions in developmental, physiological and pathological settings is rudimentary. In this Review, we discuss recent advances in sphingolipid pathways in different organelles, how secreted sphingolipid mediators modulate physiology and disease, progress in sphingolipid-targeted therapeutic and diagnostic research, and the trans-cellular sphingolipid metabolic networks between microbiota and mammals. Advances in sphingolipid biology have led to a deeper understanding of mammalian physiology and may lead to progress in the management of many diseases.

Chemoproteomic Profiling of Signaling Metabolite Fructose-1,6-Bisphosphate Interacting Proteins in Living Cells

Fructose-1,6-bisphosphate (FBP), a cellular endogenous sugar metabolite in the glycolytic pathway, has recently been reported to act as a signaling molecule to regulate various cellular events through the engagement of important proteins. Though tremendous progress has been made in identifying specific FBP-protein interactions, the comprehensive identification of FBP-interacting proteins and their regulatory mechanisms remains largely unexplored. Here, we describe a concise synthetic approach for the scalable preparation of a photoaffinity FBP probe that enables the quantitative chemoproteomic profiling of FBP-protein interactions based on photoaffinity labeling (PAL) directly in living cells. Using such a protocol, we captured known FBP targets including PKM2 and MDH2. Furthermore, among unknown FBP-interacting proteins, we identified a mitochondrial metabolic enzyme aldehyde dehydrogenase 2 (ALDH2), against which FBP showed inhibitory activity and resulted in cellular ROS upregulation accompanied by mitochondrial fragmentation. Our findings disclosed a new mode of glucose signaling mediating by the FBP-ALDH2-ROS axis.

Transcriptomic insights into the lipotoxicity of high-fat high-fructose diet in rat and mouse

Along with the increasing prevalence of obesity worldwide, the deleterious effects of high-calorie diet are gradually recognized through more and more epidemiological studies. However, the concealed and chronic causality whitewashes its unhealthy character. Given an ingenious mechanism orchestrates the metabolic adaptation to high-fat high-fructose (HFF) diet and connive its lipotoxicity, in this study, an experimental rat/mouse model of obesity was induced and a comparative transcriptomic analysis was performed to probe the mystery. Our results demonstrated that HFF diet consumption altered the transcriptomic pattern as well as different high-calorie diet fed rat/mouse manifested distinct hepatic transcriptome. Validation with RT-qPCR and Western blotting confirmed that SREBP1-FASN involved in de novo lipogenesis partly mediated metabolic self-adaption. Moreover, hepatic ACSL1-CPT1A-CPT2 pathway involved in fatty acids β-oxidation, played a key role in the metabolic adaption to HFF. Collectively, our findings enrich the knowledge of the chronic adaptation mechanisms and also shed light on future investigations. Meanwhile, our results also suggest that efforts to restore the fatty acids metabolic fate could be a promising avenue to fight against obesity and associated steatosis and insulin resistance challenged by HFF diet.

Lipotoxicity: The missing link between diabetes and periodontitis?

Lipotoxicity refers to the accumulation of lipids in tissues other than adipose tissue (body fat). It is one of the major pathophysiological mechanisms responsible for the progression of diabetes complications such as non-alcoholic fatty liver disease and diabetic nephropathy. Accumulating evidence indicates that lipotoxicity also contributes significantly to the toxic effects of diabetes on periodontitis. Therefore, we reviewed the current in vivo, in vitro, and clinical evidence of the detrimental effects of lipotoxicity on periodontitis, focusing on its molecular mechanisms, especially oxidative and endoplasmic reticulum stress, inflammation, ceramides, adipokines, and programmed cell death pathways. By elucidating potential therapeutic strategies targeting lipotoxicity and describing their associated mechanisms and clinical outcomes, including metformin, statins, liraglutide, adiponectin, and omega-3 PUFA, this review seeks to provide a more comprehensive and effective treatment framework against diabetes-associated periodontitis. Furthermore, the challenges and future research directions are proposed, aiming to contribute to a more profound understanding of the impact of lipotoxicity on periodontitis.

Loss of the ceramide synthase HYL-2 from Caenorhabditis elegans impairs stress responses and alters sphingolipid composition

Sphingolipids, essential membrane components and signaling molecules in cells, have ceramides at the core of their metabolic pathways. Initially termed as "longevity assurance genes", the encoding genes of ceramide synthases are closely associated with individual aging and stress responses, although the mechanisms remain unclear. This study aims to explore the alterations and underlying mechanisms of three ceramide synthases, HYL-1, HYL-2, and LAGR-1, in the aging and stress responses of Caenorhabditis elegans. Our results showed the knockdown of HYL-1 extends the lifespan and enhance stress resistance in worms, whereas the loss of HYL-2 function significantly impairs tolerances to heat, oxidation, and ultraviolet stress. Stress intolerance induced by HYL-2 deficiency may result from intracellular mitochondrial dysfunction, accumulation of reactive oxygen species, and abnormal nuclear translocation of DAF-16 under stress conditions. Loss of HYL-2 led to a significant reduction of predominant ceramides (d17:1/C20∼C23) as well as corresponding complex sphingolipids. Furthermore, the N-acyl chain length composition of sphingolipids underwent dramatic modifications, characterized by a decrease in C22 sphingolipids and an increase in C24 sphingolipids. Extra d18:1-ceramides resulted in diminished stress resilience in wild-type worms, while supplementation of d18:1/C16 ceramide to HYL-2-deficient worms marginally improved stress tolerance to heat and oxidation. These findings indicate the importance of appropriate ceramide content and composition in maintaining subcellular homeostasis and nuclear-cytoplasmic signal transduction during healthy aging and stress responses.

N-Acetylcysteine Alleviates Impaired Muscular Function Resulting from Sphingosine Phosphate Lyase Functional Deficiency-Induced Sphingoid Base and Ceramide Accumulation in Caenorhabditis elegans

Sphingosine-1-phosphate lyase (SPL) resides at the endpoint of the sphingolipid metabolic pathway, catalyzing the irreversible breakdown of sphingosine-1-phosphate. Depletion of SPL precipitates compromised muscle morphology and function; nevertheless, the precise mechanistic underpinnings remain elusive. Here, we elucidate a model of SPL functional deficiency in Caenorhabditis elegans using spl-1 RNA interference. Within these SPL-deficient nematodes, we observed diminished motility and perturbed muscle fiber organization, correlated with the accumulation of sphingoid bases, their phosphorylated forms, and ceramides (collectively referred to as the "sphingolipid rheostat"). The disturbance in mitochondrial morphology was also notable, as SPL functional loss resulted in heightened levels of reactive oxygen species. Remarkably, the administration of the antioxidant N-acetylcysteine (NAC) ameliorates locomotor impairment and rectifies muscle fiber disarray, underscoring its therapeutic promise for ceramide-accumulation-related muscle disorders. Our findings emphasize the pivotal role of SPL in preserving muscle integrity and advocate for exploring antioxidant interventions, such as NAC supplementation, as prospective therapeutic strategies for addressing muscle function decline associated with sphingolipid/ceramide metabolism disruption.

METTL5 promotes gastric cancer progression via sphingomyelin metabolism

BACKGROUND

The treatment of gastric cancer (GC) has caused an enormous social burden worldwide. Accumulating studies have reported that N6-methyladenosine (m6A) is closely related to tumor progression. METTL5 is a m6A methyltransferase that plays a pivotal role in maintaining the metabolic stability of cells. However, its aberrant regulation in GC has not been fully elucidated.

AIM

To excavate the role of METTL5 in the development of GC.

METHODS

METTL5 expression and clinicopathological characteristics were analyzed via The Cancer Genome Atlas dataset and further verified via immunohistochemistry, western blotting and real-time quantitative polymerase chain reaction in tissue microarrays and clinical samples. The tumor-promoting effect of METTL5 on HGC-27 and AGS cells was explored in vitro by Cell Counting Kit-8 assays, colony formation assays, scratch healing assays, transwell assays and flow cytometry. The tumor-promoting role of METTL5 in vivo was evaluated in a xenograft tumor model. The EpiQuik m6A RNA Methylation Quantification Kit was used for m6A quantification. Next, liquid chromatography-mass spectrometry was used to evaluate the association between METTL5 and sphingomyelin metabolism, which was confirmed by Enzyme-linked immunosorbent assay and rescue tests. In addition, we investigated whether METTL5 affects the sensitivity of GC cells to cisplatin via colony formation and transwell experiments.

RESULTS

Our research revealed substantial upregulation of METTL5, which suggested a poor prognosis of GC patients. Increased METTL5 expression indicated distant lymph node metastasis, advanced cancer stage and pathological grade. An increased level of METTL5 correlated with a high degree of m6A methylation. METTL5 markedly promotes the proliferation, migration, and invasion of GC cells in vitro. METTL5 also promotes the growth of GC in animal models. METTL5 knockdown resulted in significant changes in sphingomyelin metabolism, which implies that METTL5 may impact the development of GC via sphingomyelin metabolism. In addition, high METTL5 expression led to cisplatin resistance.

CONCLUSION

METTL5 was found to be an oncogenic driver of GC and may be a new target for therapy since it facilitates GC carcinogenesis through sphingomyelin metabolism and cisplatin resistance.

Spatial mapping of hepatic ER and mitochondria architecture reveals zonated remodeling in fasting and obesity

The hepatocytes within the liver present an immense capacity to adapt to changes in nutrient availability. Here, by using high resolution volume electron microscopy, we map how hepatic subcellular spatial organization is regulated during nutritional fluctuations and as a function of liver zonation. We identify that fasting leads to remodeling of endoplasmic reticulum (ER) architecture in hepatocytes, characterized by the induction of single rough ER sheet around the mitochondria, which becomes larger and flatter. These alterations are enriched in periportal and mid-lobular hepatocytes but not in pericentral hepatocytes. Gain- and loss-of-function in vivo models demonstrate that the Ribosome receptor binding protein1 (RRBP1) is required to enable fasting-induced ER sheet-mitochondria interactions and to regulate hepatic fatty acid oxidation. Endogenous RRBP1 is enriched around periportal and mid-lobular regions of the liver. In obesity, ER-mitochondria interactions are distinct and fasting fails to induce rough ER sheet-mitochondrion interactions. These findings illustrate the importance of a regulated molecular architecture for hepatocyte metabolic flexibility.

Ceramides and mitochondrial homeostasis

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

Bile acid metabolism and signaling in health and disease: molecular mechanisms and therapeutic targets

Bile acids, once considered mere dietary surfactants, now emerge as critical modulators of macronutrient (lipid, carbohydrate, protein) metabolism and the systemic pro-inflammatory/anti-inflammatory balance. Bile acid metabolism and signaling pathways play a crucial role in protecting against, or if aberrant, inducing cardiometabolic, inflammatory, and neoplastic conditions, strongly influencing health and disease. No curative treatment exists for any bile acid influenced disease, while the most promising and well-developed bile acid therapeutic was recently rejected by the FDA. Here, we provide a bottom-up approach on bile acids, mechanistically explaining their biochemistry, physiology, and pharmacology at canonical and non-canonical receptors. Using this mechanistic model of bile acids, we explain how abnormal bile acid physiology drives disease pathogenesis, emphasizing how ceramide synthesis may serve as a unifying pathogenic feature for cardiometabolic diseases. We provide an in-depth summary on pre-existing bile acid receptor modulators, explain their shortcomings, and propose solutions for how they may be remedied. Lastly, we rationalize novel targets for further translational drug discovery and provide future perspectives. Rather than dismissing bile acid therapeutics due to recent setbacks, we believe that there is immense clinical potential and a high likelihood for the future success of bile acid therapeutics.

Food perception promotes phosphorylation of MFFS131 and mitochondrial fragmentation in liver

Liver mitochondria play a central role in metabolic adaptations to changing nutritional states, yet their dynamic regulation upon anticipated changes in nutrient availability has remained unaddressed. Here, we found that sensory food perception rapidly induced mitochondrial fragmentation in the liver through protein kinase B/AKT (AKT)-dependent phosphorylation of serine 131 of the mitochondrial fission factor (MFFS131). This response was mediated by activation of hypothalamic pro-opiomelanocortin (POMC)-expressing neurons. A nonphosphorylatable MFFS131G knock-in mutation abrogated AKT-induced mitochondrial fragmentation in vitro. In vivo, MFFS131G knock-in mice displayed altered liver mitochondrial dynamics and impaired insulin-stimulated suppression of hepatic glucose production. Thus, rapid activation of a hypothalamus-liver axis can adapt mitochondrial function to anticipated changes of nutritional state in control of hepatic glucose metabolism.

Docosahexaenoic Acid Modulates Nonalcoholic Fatty Liver Disease by Suppressing Endocannabinoid System

SCOPE

Endocannabinoid signaling regulates energy homeostasis, and is tightly associated with nonalcoholic fatty liver disease (NAFLD). The study previously finds that supplementation of docosahexaenoic acid (DHA) has superior function to ameliorate NAFLD compared with eicosapentaenoic acid (EPA), however, the underlying mechanism remains elusive. The present study aims to investigate whether DHA intervention alleviates NAFLD via endocannabinoid system.

METHODS AND RESULTS

In a case-control study, the serum endocannabinoid ligands in 60 NAFLD and 60 healthy subjects are measured. Meanwhile, NAFLD model is established in mice fed a high-fat and -cholesterol diet (HFD) for 9 weeks. DHA or EPA is administrated for additional 9 weeks. Serum primary endocannabinoid ligands, namely anandamide (AEA) and 2-arachidoniylglycerol (2-AG), are significantly higher in individuals with NAFLD compared with healthy controls. NAFLD model shows that serum 2-AG concentrations and adipocyte cannabinoid receptor 1 expression levels are significantly lower in DHA group compared with HFD group. Lipidomic and targeted ceramide analyses further confirm that endocannabinoid signaling inhibition has exerted deletion of hepatic C16:0-ceramide contents, resulting in down-regulation of de novo fatty acid synthesis and up-regulation of fatty acid β-oxidation related protein expression levels.

CONCLUSIONS

This work elucidates that DHA has improved NAFLD by suppressing endocannabinoid system.

The use of click chemistry in sphingolipid research

Sphingolipid dysregulation is involved in a range of rare and fatal diseases as well as common pathologies including cancer, infectious diseases or neurodegeneration. Gaining insights into how sphingolipids are involved in these diseases would contribute much to our understanding of human physiology, as well as the pathology mechanisms. However, scientific progress is hampered by a lack of suitable tools that can be used in intact systems. To overcome this, efforts have turned to engineering modified lipids with small clickable tags and to harnessing the power of click chemistry to localize and follow these minimally modified lipid probes in cells. We hope to inspire the readers of this Review to consider applying existing click chemistry tools for their own aspects of sphingolipid research. To this end, we focus here on different biological applications of clickable lipids, mainly to follow metabolic conversions, their visualization by confocal or superresolution microscopy or the identification of their protein interaction partners. Finally, we describe recent approaches employing organelle-targeted and clickable lipid probes to accurately follow intracellular sphingolipid transport with organellar precision.

The effects of restraint stress on ceramide metabolism disorders in the rat liver: the role of CerS6 in hepatocyte injury

BACKGROUND

Stress is implicated in various pathological conditions leading to liver injury. Existing evidence suggests that excessive stress can induce mitochondrial damage in hepatocytes, yet the underlying mechanism remains unclear. Ceramide synthase 6 (CerS6)-derived C16:0 ceramide is recognised as a lipotoxic substance capable of causing mitochondrial damage. However, the role of CerS6 in stress has received insufficient attention. This study aimed to explore the involvement of CerS6 in stress-induced hepatic damage and its associated mechanisms.

METHODS

The rat restraint stress model and a corticosterone (CORT)-induced hepatocyte stress model were employed for in vivo and in vitro experimental analyses, respectively. Changes in mitochondrial damage and ceramide metabolism in hepatocytes induced by stress were evaluated. The impact of CORT on mitochondrial damage and ceramide metabolism in hepatocytes was assessed following CerS6 knockdown. Mitochondria were isolated using a commercial kit, and ceramides in liver tissue and hepatocytes were detected by LC-MS/MS.

RESULTS

In comparison to the control group, rats subjected to one week of restraint exhibited elevated serum CORT levels. The liver displayed significant signs of mitochondrial damage, accompanied by increased CerS6 and mitochondrial C16:0 ceramide, along with activation of the AMPK/p38 MAPK pathway. In vitro studies demonstrated that CORT treatment of hepatocytes resulted in mitochondrial damage, concomitant with elevated CerS6 and mitochondrial C16:0 ceramide. Furthermore, CORT induced sequential phosphorylation of AMPK and p38 MAPK proteins, and inhibition of the p38 MAPK pathway using SB203580 mitigated the CORT-induced elevation in CerS6 protein. Knocking down CerS6 in hepatocytes inhibited both the increase in C16:0 ceramide and the release of mitochondrial cytochrome c induced by CORT.

CONCLUSIONS

CerS6-associated C16:0 ceramide plays a mediating role in stress-induced mitochondrial damage in hepatocytes. The molecular mechanism is linked to CORT-induced activation of the AMPK/p38 MAPK pathway, leading to upregulated CerS6.

The relevance between abnormally elevated serum ceramide and cognitive impairment in Alzheimer's disease model mice and its mechanism

RATIONALE

The plasma ceramide levels in Alzheimer's disease (AD) patients are found abnormally elevated, which is related to cognitive decline.

OBJECTIVES

This research was aimed to investigate the mechanisms of aberrant elevated ceramides in the pathogenesis of AD.

RESULTS

The ICR mice intracerebroventricularly injected with Aβ1-42 and APP/PS1 transgenic mice were employed as AD mice. The cognitive deficiency, impaired episodic and spatial memory were observed without altered spontaneous ability. The serum levels of p-tau and ceramide were evidently elevated. The modified expressions and activities of glycogen synthase kinase-3β (GSK-3β) and protein phosphatase 2A (PP2A) influenced the serum content of p-tau. The levels of ceramide synthesis-related genes including sptlc1, sptlc2, cers2, and cers6 in the liver of AD mice were increased, while the ceramide degradation-related gene asah2 did not significantly change. The regulations of these genes were conducted by activated nuclear factor kappa-B (NF-κB) signaling. NF-κB, promoted by free fatty acid (FFA), also increased the hepatic concentrations of proinflammatory cytokines. The FFA amount was modulated by fatty acid synthesis-related genes acc1 and srebp-1c. Besides, the decreased levels of pre-proopiomelanocortin (pomc) mRNA and increased agouti-related protein (agrp) mRNA were found in the hypothalamus without significant alteration of melanocortin receptor 4 (MC4R) mRNA. The bioinformatic analyses proved the results using GEO datasets and AlzData.

CONCLUSIONS

Ceramide was positively related to the increased p-tau and impaired cognitive function. The increased generation of ceramide and endoplasmic reticulum stress in the hypothalamus was positively related to fatty acid synthesis and NF-κB signaling via brain-liver axis.

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

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

Aberrant accumulation of ceramides in mitochondria triggers cell death by inducing autophagy in Arabidopsis

Sphingolipids are membrane lipids and play critical roles in signal transduction. Ceramides are central components of sphingolipid metabolism that are involved in cell death. However, the mechanism of ceramides regulating cell death in plants remains unclear. Here, we found that ceramides accumulated in mitochondria of accelerated cell death 5 mutant (acd5), and expression of mitochondrion-localized ceramide kinase (ACD5) suppressed mitochondrial ceramide accumulation and the acd5 cell death phenotype. Using immuno-electron microscopy, we observed hyperaccumulation of ceramides in acer acd5 double mutants, which are characterized by mutations in both ACER (alkaline ceramidase) and ACD5 genes. The results confirmed that plants with specific ceramide accumulation exhibited localization of ceramides to mitochondria, resulting in an increase in mitochondrial reactive oxygen species production. Interestingly, when compared with the wild type, autophagy-deficient mutants showed stronger resistance to ceramide-induced cell death. Lipid profiling analysis demonstrated that plants with ceramide accumulation exhibited a significant increase in phosphatidylethanolamine levels. Furthermore, exogenous ceramide treatment or endogenous ceramide accumulation induces autophagy. When exposed to exogenous ceramides, an increase in the level of the autophagy-specific ubiquitin-like protein, ATG8e, associated with mitochondria, where it directly bound to ceramides. Taken together, we propose that the accumulation of ceramides in mitochondria can induce cell death by regulating autophagy.

A review of the mechanisms of abnormal ceramide metabolism in type 2 diabetes mellitus, Alzheimer's disease, and their co-morbidities

The global prevalence of type 2 diabetes mellitus (T2DM) and Alzheimer's disease (AD) is rapidly increasing, revealing a strong association between these two diseases. Currently, there are no curative medication available for the comorbidity of T2DM and AD. Ceramides are structural components of cell membrane lipids and act as signal molecules regulating cell homeostasis. Their synthesis and degradation play crucial roles in maintaining metabolic balance in vivo, serving as important mediators in the development of neurodegenerative and metabolic disorders. Abnormal ceramide metabolism disrupts intracellular signaling, induces oxidative stress, activates inflammatory factors, and impacts glucose and lipid homeostasis in metabolism-related tissues like the liver, skeletal muscle, and adipose tissue, driving the occurrence and progression of T2DM. The connection between changes in ceramide levels in the brain, amyloid β accumulation, and tau hyper-phosphorylation is evident. Additionally, ceramide regulates cell survival and apoptosis through related signaling pathways, actively participating in the occurrence and progression of AD. Regulatory enzymes, their metabolites, and signaling pathways impact core pathological molecular mechanisms shared by T2DM and AD, such as insulin resistance and inflammatory response. Consequently, regulating ceramide metabolism may become a potential therapeutic target and intervention for the comorbidity of T2DM and AD. The paper comprehensively summarizes and discusses the role of ceramide and its metabolites in the pathogenesis of T2DM and AD, as well as the latest progress in the treatment of T2DM with AD.

Disruption of CerS6-mediated sphingolipid metabolism by FTO deficiency aggravates ulcerative colitis

BACKGROUND AND AIMS

Deregulation of RNA N6-methyladenosine (m6A) modification in intestinal epithelial cells (IECs) influences intestinal immune cells and leads to intestinal inflammation. We studied the function of fat mass-and obesity-associated protein (FTO), one of the m6A demethylases, in patients with ulcerative colitis (UC).

METHODS

We analysed colon tissues of Ftoflox/flox; Villin-cre mice and their Ftoflox/flox littermates with dextran sulfate sodium (DSS) using real-time PCR and 16s rRNA sequencing. RNA and methylated RNA immunoprecipitation sequencing were used to analyse immunocytes and IECs. Macrophages were treated with conditioned medium of FTO-knockdown MODE-K cells or sphingosine-1-phosphate (S1P) and analysed for gene expression. Liquid chromatograph mass spectrometry identified C16-ceramide.

RESULTS

FTO downregulation was identified in our in-house cohort and external cohorts of UC patients. Dysbiosis of gut microbiota, increased infiltration of proinflammatory macrophages, and enhanced differentiation of Th17 cells were observed in Ftoflox/flox;Villin-cre mice under DSS treatment. FTO deficiency resulted in an increase in m6A modification and a decrease in mRNA stability of CerS6, the gene encoding ceramide synthetase, leading to the downregulation of CerS6 and the accumulation of S1P in IECs. Subsequentially, the secretion of S1P by IECs triggered proinflammatory macrophages to secrete serum amyloid A protein 1/3, ultimately inducing Th17 cell differentiation. In addition, through bioinformatic analysis and experimental validation, we identified UC patients with lower FTO expression might respond better to vedolizumab treatment.

CONCLUSIONS

FTO downregulation promoted UC by decreasing CerS6 expression, leading to increased S1P accumulation in IECs and aggravating colitis via m6A-dependent mechanisms. Lower FTO expression in UC patients may enhance their response to vedolizumab treatment.

Mitochondrial Stress and Mitokines: Therapeutic Perspectives for the Treatment of Metabolic Diseases

Mitochondrial stress and the dysregulated mitochondrial unfolded protein response (UPRmt) are linked to various diseases, including metabolic disorders, neurodegenerative diseases, and cancer. Mitokines, signaling molecules released by mitochondrial stress response and UPRmt, are crucial mediators of inter-organ communication and influence systemic metabolic and physiological processes. In this review, we provide a comprehensive overview of mitokines, including their regulation by exercise and lifestyle interventions and their implications for various diseases. The endocrine actions of mitokines related to mitochondrial stress and adaptations are highlighted, specifically the broad functions of fibroblast growth factor 21 and growth differentiation factor 15, as well as their specific actions in regulating inter-tissue communication and metabolic homeostasis. Finally, we discuss the potential of physiological and genetic interventions to reduce the hazards associated with dysregulated mitokine signaling and preserve an equilibrium in mitochondrial stress-induced responses. This review provides valuable insights into the mechanisms underlying mitochondrial regulation of health and disease by exploring mitokine interactions and their regulation, which will facilitate the development of targeted therapies and personalized interventions to improve health outcomes and quality of life.

Hypoxia induced deregulation of sphingolipids in colon cancer is a prognostic marker for patient outcome

Sphingolipids are important for the physicochemical properties of cellular membranes and deregulated in tumors. In human colon cancer tissue ceramide synthase (CerS) 4 and CerS5 are reduced which correlates with a reduced survival probability of late-stage colon cancer patients. Both enzymes are reduced after hypoxia in advanced colorectal cancer (CRC) cells (HCT-116, SW620) but not in non-metastatic CRC cells (SW480, Caco-2). Downregulation of CerS4 or CerS5 in advanced CRC cells enhanced tumor formation in nude mice and organoid growth in vitro. This was accompanied by an enhanced proliferation rate and metabolic changes leading to a shift towards the Warburg effect. In contrast, CerS4 or CerS5 depletion in Caco-2 cells reduced tumor growth in vivo. Lipidomic and proteomic analysis of membrane fractions revealed significant changes in tumor-promoting cellular pathways and cellular transporters. This study identifies CerS4 and CerS5 as prognostic markers for advanced colon cancer patients and provides a comprehensive overview about the associated cellular metabolic changes. We propose that the expression level of CerS4 and CerS5 in colon tumors could serve as a basis for decision-making for personalized treatment of advanced colon cancer patients. Trial registration: The study was accredited by the study board of the Deutsche Krebsgesellschaft (Registration No: St-D203, 2017/06/30, retrospectively registered).

Overexpression of ORMDL3 confers sexual dimorphism in diet-induced non-alcoholic steatohepatitis

OBJECTIVE

The bioactive sphingolipid metabolites ceramide and sphingosine-1-phosphate (S1P) accumulate with overnutrition and have been implicated in non-alcoholic steatohepatitis (NASH) development. ORMDL3, a negative regulator of the rate-limiting step in ceramide biosynthesis, has been identified as an obesity-related gene. Therefore, we assessed the role of ORMDL3 in diet-induced obesity and development of NASH.

METHODS

Globally overexpressing Ormdl3-Flag transgenic mice (ORMDL3TG) were fed a western high-fat, carbohydrate and cholesterol enriched diet, with high fructose-glucose drinking water. Physiological, biochemical and sphingolipidomic analyses were employed to measure the effect of ORMDL3 overexpression on NASH development.

RESULTS

ORMDL3TG male but not female mice fed a western high-fat diet and sugar water had exacerbated adipocyte hypertrophy together with increased severity of white adipose inflammation and fibrosis. Hepatic steatosis, dyslipidemia, impaired glucose homeostasis, hyperinsulinemia, and insulin resistance were significantly more severe only in obese ORMDL3TG male mice that accompanied dramatic liver fibrosis, inflammation, and formation of hepatic crown-like structures, which are unique features of human and murine NASH. Obesogenic diet induces ORMDL expression in male mice but reduces it in females. Mechanistically, overexpression of Ormdl3 lowered the levels of S1P and ceramides only in obese female mice and antithetically increased them in tissues of obese males. ORMDL3TG male mice exhibited a much greater induction of the UPR, propagating ER stress that contributed to their early development of NASH.

CONCLUSIONS

This study uncovered a previously unrecognized role for ORMDL3 in sexual dimorphism important for the development and progression of NASH.

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

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

Ceramide synthase 6 (CerS6) is upregulated in alcohol-associated liver disease and exhibits sex-based differences in the regulation of energy homeostasis and lipid droplet accumulation

OBJECTIVE

Alcohol-associated liver disease (ALD) is the leading cause of liver-related mortality worldwide. Current strategies to manage ALD focus largely on advanced stage disease, however, metabolic changes such as glucose intolerance are apparent at the earliest stage of alcoholic steatosis and increase the risk of disease progression. Ceramides impair insulin signaling and accumulate in ALD, and metabolic pathways involving ceramide synthase 6 (CerS6) are perturbed in ALD during hepatic steatosis. In this study, we aimed to investigate the role of CerS6 in ALD development and the relevance of CerS6 to human ALD.

METHODS

C57BL/6 WT and CerS6 KO mice of both sexes were fed either a Lieber-DeCarli control (CON) or 15% ethanol (EtOH) diet for six weeks. In vivo metabolic tests including glucose and insulin tolerance tests (GTT and ITT) and energy expenditure were performed. The mice were euthanized, and serum and liver lipids and liver histology were examined. For in vitro studies, CerS6 was deleted in human hepatocytes, VL17A and cells were incubated with EtOH and/or C16:0-ceramides. RNAseq analysis was performed in livers from mice and human patients with different stages of ALD and diseased controls.

RESULTS

After six weeks on an EtOH diet, CerS6 KO mice had reduced body weight, food intake, and %fat mass compared to WT mice. Energy expenditure increased in both male and female KO mice, however, was only statistically significant in male mice. In response to EtOH, WT mice developed mild hepatic steatosis, while steatosis was ameliorated in KO mice as determined by H&E and ORO staining. KO mice showed significantly decreased long-chain ceramide species, especially C16:0-ceramides, in the serum and liver tissues compared to WT mice. CerS6 deletion decreased serum TG and NEFA only in male not female mice. CerS6 deletion improved glucose tolerance and insulin resistance in EtOH-fed mice of both sexes. RNAseq analysis revealed that 74 genes are significantly upregulated and 66 genes are downregulated by CerS6 deletion in EtOH-fed male mice, with key network pathways including TG biosynthetic process, positive regulation of lipid localization, and fat cell differentiation. Similar to RNAseq results, absence of CerS6 significantly decreased mRNA expression of lipid droplet associated proteins in EtOH-fed mice. In vitro, EtOH stimulation significantly increased PLIN2 protein expression in VL17A cells while CerS6 deletion inhibited EtOH-mediated PLIN2 upregulation. C16:0-ceramide treatment significantly increased PLIN2 protein expression compared to CON. Notably, progression of ALD in humans was associated with increased hepatic CerS6 expression.

CONCLUSIONS

Our findings demonstrate that CerS6 deletion improves glucose homeostasis in alcohol-fed mice and exhibits sex-based differences in the attenuation of EtOH-induced weight gain and hepatic steatosis. Additionally, we unveil that CerS6 plays a major role as a regulator of lipid droplet biogenesis in alcohol-induced intra-hepatic lipid droplet formation, identifying it as a putative target for early ALD management.

Metabolic bias: Lipid structures as determinants of their metabolic fates

Cellular lipids have an enormous diversity in their chemical structures, which affect the physicochemical properties of lipids and membranes, as well as their regulatory roles on protein functions. Here, I review additional roles of lipid structures. Multiple studies show that structural differences affect how lipids, even from the same class, are metabolically converted via distinct pathways. I propose the name "structure-guided metabolic bias" for this phenomenon, and discuss its biological relevance. This metabolic bias seems implicated in the buildup of basic cellular lipid compositions, as well as genetic predisposition to diseases. Thus, guiding metabolic biases is an important function of lipid structures, while having the characteristic of being difficult to study by in vitro biochemical reconstitutions.

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

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

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.

Ceramides in peripheral arterial plaque lead to endothelial cell dysfunction

Background

Peripheral arterial atheroprogression is increasingly prevalent, and is a risk factor for major limb amputations in individuals with risk factors such as diabetes. We previously demonstrated that bioactive lipids are significantly altered in arterial tissue of individuals with diabetes and advanced peripheral arterial disease.

Methods

Here we evaluated whether sphingolipid ceramide 18:1/16:0 (C16) is a cellular regulator in endothelial cells and peripheral tibial arterial tissue in individuals with diabetes.

Results

We observed that C16 is the single most elevated ceramide in peripheral arterial tissue from below the knee in individuals with diabetes (11% increase, P < .05). C16 content in tibial arterial tissue positively correlates with sphingomyelin (SPM) content in patients with and without diabetes (r2 = 0.5, P < .005; r2 = 0.17, P < .05; respectively). Tibial arteries of individuals with diabetes demonstrated no difference in CERS6 expression (encoding ceramide synthase 6; the predominate ceramide synthesis enzyme), but higher SMPD expression (encoding sphingomyelin phosphodiesterase that catalyzes ceramide synthesis from sphingomyelins; P < .05). SMPD4, but not SMPD2, was particularly elevated in maximally diseased (Max) tibial arterial segments (P < .05). In vitro, exogenous C16 caused endothelial cells (HUVECs) to have decreased proliferation (P < .03), increased apoptosis (P < .003), and decreased autophagy (P < .008). Selective knockdown of SMPD2 and SMPD4 decreased native production of C16 (P < .01 and P < .001, respectively), but only knockdown of SMPD4 rescued cellular proliferation (P < .005) following exogenous supplementation with C16.

Conclusions

Our findings suggest that C16 is a tissue biomarker for peripheral arterial disease severity in the setting of diabetes, and can impact endothelial cell viability and function.

Clinical relevance

Peripheral arterial disease and its end-stage manifestation known as chronic limb-threatening ischemia (CLTI) represent ongoing prevalent and intricate medical challenges. Individuals with diabetes have a heightened risk of developing CLTI and experiencing its complications, including wounds, ulcers, and major amputations. In the present study, we conducted a comprehensive examination of the molecular lipid composition within arterial segments from individuals with CLTI, and with and without diabetes. Our investigations unveiled a striking revelation: the sphingolipid ceramide 18:1/16:0 emerged as the predominant ceramide species that was significantly elevated in the peripheral arterial intima below the knee in patients with diabetes. Moreover, this heightened ceramide presence is associated with a marked impairment of endothelial cell function and viability. Additionally, our study revealed a concurrent elevation in the expression of sphingomyelin phosphodiesterases, enzymes responsible for catalyzing ceramide synthesis from sphingomyelins, within maximally diseased arterial segments. These findings underscore the pivotal role of ceramides and their biosynthesis enzymes in the context of CLTI, offering new insights into potential therapeutic avenues for managing this challenging disease process.

Enrofloxacin (ENR) exposure induces lipotoxicity by promoting mitochondrial fragmentation via dephosphorylation of DRP1 at S627 site

Enrofloxacin (ENR) is a kind of widespread hazardous pollutant on aquatic ecosystems and causes toxic effects, such as disorders of metabolism, on aquatic animals. However, its potential mechanisms at an environmental concentration on metabolic disorders of aquatic organisms remain unclear. Herin, we found that hepatic lipotoxicity was induced by ENR exposure, which led to ENR accumulation, oxidative stress, mitochondrial fragmentation, and fatty acid transfer blockage from lipid droplets into fragmented mitochondria. ENR-induced lipotoxicity and mitochondrial β-oxidation down-regulation were mediated by reactive oxygen species (ROS). Moreover, dynamin-like protein 1 (DRP1) mediated ENR-induced mitochondrial fragmentation and changes of lipid metabolism. Mechanistically, ENR induced increment of DRP1 mitochondrial localization via dephosphorylating DRP1 at S627 and promoted its interaction with mitochondrial fission factor (MFF), leading to mitochondria fragmentation. For the first time, our study provides an innovative mechanistic link between hepatic lipotoxicity and mitochondrial fragmentation under ENR exposure, and thus identifies previously unknown mechanisms for the direct relationship between environmental ENR concentration and lipotoxicity in aquatic animals. Our study provides innovative insights for toxicological mechanisms and environmental risk assessments of antibiotics in aquatic environment.

Ceramides Increase Fatty Acid Utilization in Intestinal Progenitors to Enhance Stemness and Increase Tumor Risk

BACKGROUND & AIMS

Cancers of the alimentary tract, including esophageal adenocarcinomas, colorectal cancers, and cancers of the gastric cardia, are common comorbidities of obesity. Prolonged, excessive delivery of macronutrients to the cells lining the gut can increase one's risk for these cancers by inducing imbalances in the rate of intestinal stem cell proliferation vs differentiation, which can produce polyps and other aberrant growths. We investigated whether ceramides, which are sphingolipids that serve as a signal of nutritional excess, alter stem cell behaviors to influence cancer risk.

METHODS

We profiled sphingolipids and sphingolipid-synthesizing enzymes in human adenomas and tumors. Thereafter, we manipulated expression of sphingolipid-producing enzymes, including serine palmitoyltransferase (SPT), in intestinal progenitors of mice, cultured organoids, and Drosophila to discern whether sphingolipids altered stem cell proliferation and metabolism.

RESULTS

SPT, which diverts dietary fatty acids and amino acids into the biosynthetic pathway that produces ceramides and other sphingolipids, is a critical modulator of intestinal stem cell homeostasis. SPT and other enzymes in the sphingolipid biosynthesis pathway are up-regulated in human intestinal adenomas. They produce ceramides, which serve as prostemness signals that stimulate peroxisome-proliferator activated receptor-α and induce fatty acid binding protein-1. These actions lead to increased lipid utilization and enhanced proliferation of intestinal progenitors.

CONCLUSIONS

Ceramides serve as critical links between dietary macronutrients, epithelial regeneration, and cancer risk.

Sphingolipid metabolism in brain insulin resistance and neurological diseases

Sphingolipids, as members of the large lipid family, are important components of plasma membrane. Sphingolipids participate in biological signal transduction to regulate various important physiological processes such as cell growth, apoptosis, senescence, and differentiation. Numerous studies have demonstrated that sphingolipids are strongly associated with glucose metabolism and insulin resistance. Insulin resistance, including peripheral insulin resistance and brain insulin resistance, is closely related to the occurrence and development of many metabolic diseases. In addition to metabolic diseases, like type 2 diabetes, brain insulin resistance is also involved in the progression of neurodegenerative diseases including Alzheimer's disease and Parkinson's disease. However, the specific mechanism of sphingolipids in brain insulin resistance has not been systematically summarized. This article reviews the involvement of sphingolipids in brain insulin resistance, highlighting the role and molecular biological mechanism of sphingolipid metabolism in cognitive dysfunctions and neuropathological abnormalities of the brain.

Ceramide d18:1/24:1 as a potential biomarker to differentiate obesity subtypes with unfavorable health outcomes

BACKGROUND

The criteria for metabolically healthy obesity (MHO) and metabolically unhealthy obesity (MUO) remain controversial. This research aimed to identify a potential biomarker to differentiate the subtypes of obesity.

METHODS

The study conducted a lipidomic evaluation of ceramide in the serum of 77 Chinese adults who had undergone hyperinsulinemic-euglycemic clamps. These adults were divided into three groups according to the clinical data: normal weight control group (N = 21), MHO (N = 20), and MUO (N = 36).

RESULTS

The serum Cer d18:1/24:1 level in the MHO group was lower than that in the MUO group. As the Cer d18:1/24:1 level increased, insulin sensitivity decreased, and the unfavorable parameters increased in parallel. Multivariate logistic regression analysis revealed that serum Cer d18:1/24:1 levels were independently correlated with MUO in obesity. Individuals with higher levels of Cer d18:1/24:1 also had an elevated risk of cardiovascular disease. Most ceramide subtype levels increased in obesity compared to normal-weight individuals, but the levels of serum Cer d18:0/18:0 and Cer d18:1/16:0 decreased in obesity.

CONCLUSIONS

The relationships between ceramide subtypes and metabolic profiles might be heterogeneous in populations with different body weights. Cer d18:1/24:1 could be a biomarker that can be used to differentiate MUO from MHO, and to better predict who will develop unfavorable health outcomes among obese individuals.

TRIAL REGISTRATION

The First Affiliated Hospital of Nanjing Medical University's Institutional Review Board authorized this study protocol, and all participants provided written informed consent (2014-SR-003) prior to study entry.

The Sphingolipid-Signaling Pathway as a Modulator of Infection by SARS-CoV-2

Ceramides and other related sphingolipids, important cellular components linked to metabolic homeostasis and cardiometabolic diseases, have been found to be involved in different steps of the SARS-CoV-2 life cycle. Hence, changes in their physiological levels are identified as predictors of COVID-19 severity and prognosis, as well as potential therapeutic targets. In this review, an overview of the SARS-CoV-2 life cycle is given, followed by a description of the sphingolipid metabolism and its role in viral infection, with a particular focus on those steps required to finalize the viral life cycle. Furthermore, the use and development of pharmaceutical strategies to target sphingolipids to prevent and treat severe and long-term symptoms of infectious diseases, particularly COVID-19, are reviewed herein. Finally, research perspectives and current challenges in this research field are highlighted. Although many aspects of sphingolipid metabolism are not fully known, this review aims to highlight how the discovery and use of molecules targeting sphingolipids with reliable and selective properties may offer new therapeutic alternatives to infectious and other diseases, including COVID-19.

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

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

Circulating Sphingolipids in Insulin Resistance, Diabetes and Associated Complications

Sphingolipids play an important role in the development of diabetes, both type 1 and type 2 diabetes, as well as in the development of both micro- and macro-vascular complications. Several reviews have been published concerning the role of sphingolipids in diabetes but most of the emphasis has been on the possible mechanisms by which sphingolipids, mainly ceramides, contribute to the development of diabetes. Research on circulating levels of the different classes of sphingolipids in serum and in lipoproteins and their importance as biomarkers to predict not only the development of diabetes but also of its complications has only recently emerged and it is still in its infancy. This review summarizes the previously published literature concerning sphingolipid-mediated mechanisms involved in the development of diabetes and its complications, focusing on how circulating plasma sphingolipid levels and the relative content carried by the different lipoproteins may impact their role as possible biomarkers both in the development of diabetes and mainly in the development of diabetic complications. Further studies in this field may open new therapeutic avenues to prevent or arrest/reduce both the development of diabetes and progression of its complications.

The transcription factor ChREBP links mitochondrial lipidomes to mitochondrial morphology and progression of diabetic kidney disease

A substantial body of evidence has established the contributions of both mitochondrial dynamics and lipid metabolism to the pathogenesis of diabetic kidney disease (DKD). However, the precise interplay between these two key metabolic regulators of DKD is not fully understood. Here, we uncover a link between mitochondrial dynamics and lipid metabolism by investigating the role of carbohydrate-response element-binding protein (ChREBP), a glucose-responsive transcription factor and a master regulator of lipogenesis, in kidney podocytes. We find that inducible podocyte-specific knockdown of ChREBP in diabetic db/db mice improves key biochemical and histological features of DKD in addition to significantly reducing mitochondrial fragmentation. Because of the critical role of ChREBP in lipid metabolism, we interrogated whether and how mitochondrial lipidomes play a role in ChREBP-mediated mitochondrial fission. Our findings suggest a key role for a family of ether phospholipids in ChREBP-induced mitochondrial remodeling. We find that overexpression of glyceronephosphate O-acyltransferase, a critical enzyme in the biosynthesis of plasmalogens, reverses the protective phenotype of ChREBP deficiency on mitochondrial fragmentation. Finally, our data also points to Gnpat as a direct transcriptional target of ChREBP. Taken together, our results uncover a distinct mitochondrial lipid signature as the link between ChREBP-induced mitochondrial dynamics and progression of DKD.