Upregulation of the serine palmitoyltransferase subunit SPTLC2 by endoplasmic reticulum stress inhibits the hepatic insulin response

N-acetylcysteine influence on PI3K/Akt/mTOR and sphingolipid pathways in rats with MASLD induced by HFD: a promising new therapeutic purpose

Sphingolipid and glucose metabolism play important roles in the induction and progression of severe liver disorders like metabolic dysfunction-associated steatotic liver disease (MASLD). The perturbation in sphingolipid formation may improve the liver structure and functioning and may constitute the potential therapeutic options for the development of simple steatosis and its progression to steatohepatitis. This study aims to assess the influence of N-acetylcysteine (NAC) on the sphingolipid and insulin signaling pathways in rats subjected to standard or high-fat diets. Sphingolipid level was measured using high-performance liquid chromatography (HPLC). A multiplex assay kit determined the level of phosphorylated form of proteins included in the PI3K/Akt/mTOR pathway. The immunoblotting estimated the expression of proteins from sphingolipid and insulin transduction pathways. A histological Oil red O staining was used to assess the hepatic accumulation of lipid droplets. Molecular docking was applied to showcase NAC interaction with PI3K/Akt/mTOR pathway proteins. NAC decreased dihydroceramide and ceramide levels and increased phosphorylation of sphingosine and sphinganine. This antioxidant also enhanced phosphorylated Akt, GSK3α/β, and P70 S6 kinase and decreased phosphorylated S6RP. In silico docking analysis of insulin signaling molecules evidenced the higher binding affinity of NAC with all tested proteins, i.e., IRS1, PTEN, Akt, GSK3α/β, P70 S6 kinase, and S6RP, suggesting a potential protective influence on insulin resistance development, which is one of the criteria for MASLD diagnosing. Based on these data, NAC improved the hepatic insulin sensitivity and sphingolipid synthesis and storage, improving and restoring glucose homeostasis.

Renshenjian decoction alleviates diabetes via HIF-1α/SPTLC2 pathway by targeting ceramide synthesis

ETHNOPHARMACOLOGICAL RELEVANCE

Renshenjian decoction (RSJ) is a traditional recipe for treating diabetes in an ancient Chinese medical book, composed of Puerariae lobatae Radix and Ginseng Radix. Despite its traditional use, the mechanism of RSJ in improving diabetes has not been elucidated.

AIM OF THE STUDY

This research aimed to investigate the effects and underlying mechanisms of RSJ in treating diabetes.

MATERIALS AND METHODS

Diabetic (T2DM) mice were induced using a high-fat diet coupled with streptozotocin injections. The effect of RSJ was assessed through biochemical and histopathological analyses of blood and liver samples to evaluate liver function and tissue damage in T2DM mice. The mechanism of RSJ in treating diabetes was explored using molecular docking, metabolomics, and molecular biology techniques. Additionally, the role of RSJ was further validated in T2DM mice treated with the HIF-1α stabilizer DMOG.

RESULTS

RSJ could enhance insulin sensitivity and lower blood glucose and lipids in T2DM mice. These effects may be related to various components in RSJ, including puerarin, ginsenoside Rb1, and ginsenoside Re. Liver metabonomics revealed 57 differential metabolites in the model and RSJ groups, with 22 enriched pathways, predominantly related to sphingolipid metabolism. Furthermore, qPCR and Western blot results demonstrated that RSJ dramatically decreases the expression of HIF-1α and SPTLC2, as well as the ceramide content in the liver of diabetic mice. These results were partially reversed by the DMOG.

CONCLUSION

RSJ can inhibit the expression of ceramide through the HIF-1α/SPTLC2 pathway, which helps to improve diabetes and diabetes-related dyslipidemia and liver injury.

Puerarin Targets HIF-1α to Modulate Hypoxia-Related Sphingolipid Metabolism in Diabetic Hepatopathy via the SPTLC2/Ceramide Pathway

Background and Objectives: Diabetic hepatopathy, characterized by hepatic hypoxia and metabolic dysregulation, has a rising global incidence and prevalence, with limited effective treatments. Hepatic hypoxia activates hypoxia-inducible factor-1 alpha (HIF-1α), regulating sphingolipid metabolism and elevating ceramide, a key factor in insulin resistance. Puerarin (Pue), a flavonoid derived from Pueraria lobata, exhibits therapeutic effects in diabetes, but its effects on hypoxia-related hepatic metabolism are unclear. This study investigates Pue's mechanisms in modulating hepatic metabolism, focusing on HIF-1α and sphingolipid metabolism. Methods: Using bioinformatics and molecular docking, HIF-1α was identified as a key target in diabetic liver disease, confirmed via drug affinity responsive target stability. In vitro experiments utilized insulin-resistant HepG2 cells to assess glucose intake and HIF-1α expression. In vivo, type 2 diabetes mellitus (T2DM) was induced in mice using a high-fat diet and streptozotocin injections. Pue administration was evaluated for its effects on fasting blood glucose, oral glucose tolerance, and hepatoprotective effects. Liver metabolomics and qPCR/Western blot analyses were conducted to assess metabolic pathways. Results: Pue increased glucose uptake in HepG2 cells and bound HIF-1α. Pue reduced HIF-1α expression in HepG2 cells, an effect attenuated by the HIF-1α stabilizer DMOG. Pue improved fasting blood glucose, oral glucose tolerance, and hepatoprotective effects in T2DM mice, which DMOG reversed. Metabolomics revealed that Pue modulates sphingolipid metabolism, decreasing ceramide content. qPCR and Western blot results confirmed that Pue dramatically decreases HIF-1α and SPTLC2 expression. Conclusions: Pue improves diabetic hepatopathy by reducing ceramide expression through the HIF-1α/SPTLC2 pathway, offering a novel therapeutic strategy for diabetes management.

Transcriptomics, lipidomics, and single-nucleus RNA sequencing integration: exploring sphingolipids in MASH-HCC progression

BACKGROUND & AIMS

Metabolic dysfunction-associated steatotic liver disease (MASLD) encompasses various conditions, ranging from simple steatosis to metabolic dysfunction-associated steatohepatitis (MASH) and cirrhosis. MASLD is a significant risk factor for hepatocellular carcinoma (HCC) and is rapidly becoming the primary cause of liver transplantation. Dysregulated sphingolipid metabolism has been linked to the development of MASH-HCC. However, detailed insight into the sphingolipid profiles and cell type-specific changes in key genes involved in sphingolipid metabolism remains limited and forms the primary focus of this study.

APPROACHES & RESULTS

This study used the well-characterized diet-induced MASH-HCC mouse model (DIAMOND). Total RNA sequencing data, NanoString nCounter® Gene profiling, and single-nucleus RNA sequencing (snRNA-seq) GEO data (GSE225381) were used in characterizing gene regulation in MASH-HCC progression. Sphingolipids in the serum and liver were profiled using targeted lipidomics. RNA data analysis showed dysregulation of key genes involved in sphingolipid metabolism, including ceramide synthase 6 (Cers6), serine palmitoyltransferase long chain base subunit 2 (Sptlc2), sphingosine kinase 2 (SphK2), and sphingosine-1-phosphate receptor 1-3 (S1pr1-3) which paralleled significant changes in sphingolipid composition and levels in both serum and liver. Furthermore, TCGA-LIHC patient data were analyzed and potential prognostic genes for MASH-HCC were identified using univariate and multivariate Cox analysis. The multivariate Cox analysis underscored the prognostic significance of several genes related to sphingolipid metabolism, including CERS6, SPTLC2, and S1PR1.

CONCLUSION

Our findings provided valuable insights into the role of sphingolipids in the progression of MASH to HCC. Specific serum and liver sphingolipid profiles may serve as valuable biomarkers for diagnosis and prognosis in MASH-HCC.

Regulation mechanism of endoplasmic reticulum stress on metabolic enzymes in liver diseases

The endoplasmic reticulum (ER) plays a pivotal role in protein folding and secretion, Ca2+ storage, and lipid synthesis in eukaryotic cells. When the burden of protein synthesis and folding required to be handled exceeds the processing capacity of the ER, the accumulation of misfolded/unfolded proteins triggers ER stress. In response to short-term ER stress, the unfolded protein response (UPR) is activated to allow cells to survive. When ER stress is severe and sustained, it typically provokes cell death through multiple approaches. It is well documented that ER stress and metabolic deregulation are functionally intertwined, both are considered contributing factors to the pathogenesis of liver diseases, including non-alcoholic fatty liver disease (NAFLD), alcoholic liver disease (ALD), ischemia/reperfusion (I/R) injury, viral hepatitis, liver fibrosis, and hepatocellular carcinoma (HCC). Hepatocytes are rich in smooth and rough ER, which harbor metabolic enzymes that are capable of sensing alterations in various nutritional status and external stimuli. Extensive research has focused on the molecular mechanism linking ER stress with metabolic enzymes. The purpose of this review is to summarize the current knowledge regarding the effects of ER stress on metabolic enzymes in various liver diseases and to provide potential therapeutic strategies for chronic liver diseases via targeting UPR.

Fumonisins alone or mixed with other fusariotoxins increase the C22-24:C16 sphingolipid ratios in chicken livers, while deoxynivalenol and zearalenone have no effect

Poultry feed is often contaminated with fumonisins, deoxynivalenol, and zearalenone, which can result in oxidative damage, inflammation and change in lipid metabolism. Although sphingolipids play key roles in cells, only the effects of fumonisins on the sphingolipidome are well-documented. In chickens, fumonisins have been shown to increase the sphinganine to sphingosine ratio and the C22-24:C16 sphingolipid ratio, which has been proposed as a new biomarker of toxicity. In this study, we used UHPLC-MSMS targeted analysis to measure the effect of fusariotoxins on sphingolipids in the livers of chickens fed with diets containing fusariotoxins administered individually and in combination, at the maximum levels recommended by the European Commission. Chickens were exposed from hatching until they reached 35 days of age. This study revealed for the first time that fumonisins, deoxynivalenol, and zearalenone alone and in combination have numerous effects on the sphingolipidome in chicken livers. A 30-50 % decrease in ceramide, dihydroceramide, sphingomyelin, dihydrosphingomyelin, monohexosylceramide and lactosylceramide measured at the class level was observed when fusariotoxins were administered alone, whereas a 30-100 % increase in dihydroceramide, sphingomyelin, dihydrosphingomyelin, and monohexosylceramide was observed when the fusariotoxins were administered in combination. For these different variables, strong significant interactions were observed between fumonisins and zearalenone and between fumonisins and deoxynivalenol, whereas interactions between deoxynivalenol and zearalenone were less frequent and less significant. Interestingly, an increase in the C22-24:C16 ratio of ceramides, sphingomyelins, and monohexosylceramides was observed in chickens fed the diets containing fumonisins only, and this increase was close when the toxin was administered alone or in combination with deoxynivalenol and zearalenone. This effect mainly corresponded to a decrease in sphingolipids with a fatty acid chain length of 16 carbons, whereas C22-24 sphingolipids were unaffected or increased. In conclusion the C22-24:C16 ratio emerged as a specific biomarker, with variations dependent only on the presence of fumonisins.

Roles and therapeutic targeting of ceramide metabolism in cancer

BACKGROUND

Ceramides are sphingolipids that act as signaling molecules involved in regulating cellular processes including apoptosis, proliferation, and metabolism. Deregulation of ceramide metabolism contributes to cancer development and progression. Therefore, regulation of ceramide levels in cancer cells is being explored as a new approach for cancer therapy.

SCOPE OF THE REVIEW

This review discusses the multiple roles of ceramides in cancer cells and strategies to modulate ceramide levels for cancer therapy. Ceramides attenuate cell survival signaling and metabolic pathways, while activating apoptotic mechanisms, making them tumor-suppressive. Approaches to increase ceramide levels in cancer cells include using synthetic analogs, inhibiting ceramide degradation, and activating ceramide synthesis. We also highlight combination therapies such as use of ceramide modulators with chemotherapies, immunotherapies, apoptosis inducers, and anti-angiogenics, which offer synergistic antitumor effects. Additionally, we also describe ongoing clinical trials evaluating ceramide nanoliposomes and analogs. Finally, we discuss the challenges of these therapeutic approaches including the complexity of ceramide metabolism, targeted delivery, cancer heterogeneity, resistance mechanisms, and long-term safety.

MAJOR CONCLUSIONS

Ceramide-based therapy is a potentially promising approach for cancer therapy. However, overcoming hurdles in pharmacokinetics, specificity, and resistance is needed to optimize its efficacy and safety. This requires comprehensive preclinical/clinical studies into ceramide signaling, formulations, and combination therapies. Ceramide modulation offers opportunities for developing novel cancer treatments, but a deeper understanding of ceramide biology is vital to advance its clinical applications.

Impacts of glutamate, an exercise-responsive metabolite on insulin signaling

AIMS

Disruption of the insulin signaling pathway leads to insulin resistance (IR). IR is characterized by impaired glucose and lipid metabolism. Elevated levels of circulating glutamate are correlated with metabolic indicators and may potentially predict the onset of metabolic diseases. Glutamate receptor antagonists have significantly enhanced insulin sensitivity, and improved glucose and lipid metabolism. Exercise is a well-known strategy to combat IR. The aims of our narrative review are to summarize preclinical and clinical findings to show the correlations between circulating glutamate levels, IR and metabolic diseases, discuss the causal role of excessive glutamate in IR and metabolic disturbance, and present an overview of the exercise-induced alteration in circulating glutamate levels.

MATERIALS AND METHODS

A literature search was conducted to identify studies on glutamate, insulin signaling, and exercise in the PubMed database. The search covered articles published from December 1955 to January 2024, using the search terms of "glutamate", "glutamic acid", "insulin signaling", "insulin resistance", "insulin sensitivity", "exercise", and "physical activity".

KEY FINDINGS

Elevated levels of circulating glutamate are correlated with IR. Excessive glutamate can potentially hinder the insulin signaling pathway through various mechanisms, including the activation of ectopic lipid accumulation, inflammation, and endoplasmic reticulum stress. Glutamate can also modify mitochondrial function through Ca2+ and induce purine degradation mediated by AMP deaminase 2. Exercise has the potential to decrease circulating levels of glutamate, which can be attributed to accelerated glutamate catabolism and enhanced glutamate uptake.

SIGNIFICANCE

Glutamate may act as a mediator in the exercise-induced improvement of insulin sensitivity.

Ceramide enhanced the hepatic glucagon response through regulation of CREB activity

BACKGROUND & AIMS

Hyperglycemia is associated with lipid disorders in patients with diabetes. Ceramides are metabolites involved in sphingolipid metabolism that accumulate during lipid disorders and exert deleterious effects on glucose and lipid metabolism. However, the effects of ceramide on glucagon-mediated hepatic gluconeogenesis remain largely unknown. This study was designed to investigate the impact of ceramides on gluconeogenesis in the context of the hepatic glucagon response, with the aim of finding new pharmacological interventions for hyperglycemia in diabetes.

METHODS

Liquid chromatography-mass spectrometry was used to quantify ceramide content in the serum of patients with diabetes. Primary hepatocytes were isolated from male C57BL/6J mice to study the effects of ceramide on hepatic glucose production. Immunofluorescence staining was performed to view cAMP-responsive element-binding protein (CREB)- regulated transcription co-activator 2 (CRTC2) nuclear translocation in hepatocytes. Serine palmitoyl-transferase, long chain base subunit 2 (Sptlc2) knockdown mice were generated using an adeno-associated virus containing shRNA, and hepatic glucose production was assessed glucagon tolerance and pyruvate tolerance tests in mice fed a normal chow diet and high-fat diet.

RESULTS

Increased ceramide levels were observed in the serum of patients newly diagnosed with type 2 diabetes. De novo ceramide synthesis was activated in mice with metabolic disorders. Ceramide enhanced hepatic glucose production in primary hepatocytes. In contrast, genetic silencing of Sptlc2 prevented this process. Mechanistically, ceramides de-phosphorylate CRTC2 (Ser 171) and facilitate its translocation into the nucleus for CREB activation, thereby augmenting the hepatic glucagon response. Hepatic Sptlc2 silencing blocked ceramide generation in the liver and thus restrained the hepatic glucagon response in mice fed a normal chow diet and high-fat diet.

CONCLUSIONS

These data indicate that ceramide serves as an intracellular messenger that augments hepatic glucose production by regulating CRTC2/CREB activity in the context of the hepatic glucagon response, suggesting that CRTC2 phosphorylation might be a potential node for pharmacological interventions to restrain the hyperglycemic response during fasting in diabetes.

Mysterious sphingolipids: metabolic interrelationships at the center of pathophysiology

Metabolic pathways are complex and intertwined. Deficiencies in one or more enzymes in a given pathway are directly linked with genetic diseases, most of them having devastating manifestations. The metabolic pathways undertaken by sphingolipids are diverse and elaborate with ceramide species serving as the hubs of sphingolipid intermediary metabolism and function. Sphingolipids are bioactive lipids that serve a multitude of cellular functions. Being pleiotropic in function, deficiency or overproduction of certain sphingolipids is associated with many genetic and chronic diseases. In this up-to-date review article, we strive to gather recent scientific evidence about sphingolipid metabolism, its enzymes, and regulation. We shed light on the importance of sphingolipid metabolism in a variety of genetic diseases and in nervous and immune system ailments. This is a comprehensive review of the state of the field of sphingolipid biochemistry.

2,8-Dihydroxyadenine-induced nephropathy causes hexosylceramide accumulation with increased mTOR signaling, reduced levels of protective SirT3 expression and impaired renal mitochondrial function

AIM

Chronic kidney disease (CKD) is accompanied by increased cardiovascular risk and heart failure (HF). In rodents, 2,8-dihydroxyadenine (DHA)-induced nephropathy is a frequently used CKD model. Cardiac and kidney tubular cells share high energy demand to guarantee constant contractive force of the heart or reabsorption/secretion of primary filtrated molecules and waste products by the kidney. Here we analyze time-dependent mechanisms of kidney damage and cardiac consequences under consideration of energetic pathways with the focus on mitochondrial function and lipid metabolism in mice.

METHODS AND RESULTS

CKD was induced by alternating dietary adenine supplementation (0.2 % or 0.05 % of adenine) in C57BL/6J mice for 9 weeks. Progressive kidney damage led to reduced creatinine clearance, kidney fibrosis and renal inflammation after 3, 6, and 9 weeks. No difference in cardiac function, mitochondrial respiration nor left ventricular fibrosis was observed at any time point. Investigating mechanisms of renal damage, protective SirT3 was decreased in CKD, which contrasted an increase in protein kinase B (AKT) expression, mechanistic target of rapamycin (mTOR) downstream signaling, induction of oxidative and endoplasmic reticulum (ER) stress. This occurred together with impaired renal mitochondrial function and accumulation of hexosylceramides (HexCer) as an established mediator of inflammation and mitochondrial dysfunction in the kidney.

CONCLUSIONS

2,8-DHA-induced CKD results in renal activation of the mTOR downstream signaling, endoplasmic reticulum stress, tubular injury, fibrosis, inflammation, oxidative stress and impaired kidney mitochondrial function in conjunction with renal hexosylceramide accumulation in C57BL/6J mice.

Functional roles of sphingolipids in immunity and their implication in disease

Sphingolipids, which are components of cellular membranes and organ tissues, can be synthesized or degraded to modulate cellular responses according to environmental cues, and the balance among the different sphingolipids is important for directing immune responses, regardless of whether they originate, as intra- or extracellular immune events. Recent progress in multiomics-based analyses and methodological approaches has revealed that human health and diseases are closely related to the homeostasis of sphingolipid metabolism, and disease-specific alterations in sphingolipids and related enzymes can be prognostic markers of human disease progression. Accumulating human clinical data from genome-wide association studies and preclinical data from disease models provide support for the notion that sphingolipids are the missing pieces that supplement our understanding of immune responses and diseases in which the functions of the involved proteins and nucleotides have been established. In this review, we analyze sphingolipid-related enzymes and reported human diseases to understand the important roles of sphingolipid metabolism. We discuss the defects and alterations in sphingolipid metabolism in human disease, along with functional roles in immune cells. We also introduce several methodological approaches and provide summaries of research on sphingolipid modulators in this review that should be helpful in studying the roles of sphingolipids in preclinical studies for the investigation of experimental and molecular medicines.

ORMDL in metabolic health and disease

Obesity is a key risk factor for the development of metabolic disease. Bioactive sphingolipid metabolites are among the lipids increased in obesity. Obesogenic saturated fatty acids are substrates for serine palmitoyltransferase (SPT) the rate-limiting step in de novo sphingolipid biosynthesis. The mammalian orosomucoid-like protein isoforms ORMDL1-3 negatively regulate SPT activity. Here we summarize evidence that dysregulation of sphingolipid metabolism and SPT activity correlates with pathogenesis of obesity. This review also discusses the current understanding of the function of SPT and ORMDL in obesity and metabolic disease. Gaps and limitations in current knowledge are highlighted together with the need to further understand how ORMDL3, which has been identified as an obesity-related gene, contributes to the pathogenesis of obesity and development of metabolic disease related to its physiological functions. Finally, we point out the needs to move this young field of research forward.

Plasma Sphingoid Base Profiles of Patients Diagnosed with Intrinsic or Idiosyncratic Drug-induced Liver Injury

Sphingolipids are exceptionally diverse, comprising hundreds of unique species. The bulk of circulating sphingolipids are synthesized in the liver, thereby plasma sphingolipid profiles represent reliable surrogates of hepatic sphingolipid metabolism and content. As changes in plasma sphingolipid content have been associated to exposure to drugs inducing hepatotoxicity both in vitro and in rodents, in the present study the translatability of the preclinical data was assessed by analyzing the plasma of patients with suspected drug-induced liver injury (DILI) and control subjects. DILI patients, whether intrinsic or idiosyncratic cases, had no alterations in total sphingoid base levels and profile composition compared to controls, whereby cardiovascular disease (CVD) was a confounding factor. Upon exclusion of CVD individuals, elevation of 1-deoxysphingosine (1-deoxySO) in the DILI group emerged. Notably, 1-deoxySO values did not correlate with ALT values. While 1-deoxySO was elevated in all DILI cases, only intrinsic DILI cases concomitantly displayed reduction of select shorter chain sphingoid bases. Significant perturbation of the sphingolipid metabolism observed in this small exploratory clinical study is discussed and put into context, in the consideration that sphingolipids might contribute to the onset and progression of DILI, and that circulating sphingoid bases may function as mechanistic markers to study DILI pathophysiology.

Mitochondrial Complex I Disruption Causes Broad Reorchestration of Plant Lipidome Including Chloroplast Lipids

Mitochondrial complex I (CI) plays a crucial role in oxidising NADH generated by the metabolism (including photorespiration) and thereby participates in the mitochondrial electron transfer chain feeding oxidative phosphorylation that generates ATP. However, CI mutations are not lethal in plants and cause moderate phenotypes, and therefore CI mutants are instrumental to examine consequences of mitochondrial homeostasis disturbance on plant cell metabolisms and signalling. To date, the consequences of CI disruption on the lipidome have not been examined. Yet, in principle, mitochondrial dysfunction should impact on lipid synthesis through chloroplasts (via changes in photorespiration, redox homeostasis, and N metabolism) and the endoplasmic reticulum (ER) (via perturbed mitochondrion-ER crosstalk). Here, we took advantage of lipidomics technology (by LC-MS), phospholipid quantitation by ³¹P-NMR, and total lipid quantitation to assess the impact of CI disruption on leaf, pollen, and seed lipids using three well-characterised CI mutants: CMSII in N. sylvestris and both ndufs4 and ndufs8 in Arabidopsis. Our results show multiple changes in cellular lipids, including galactolipids (chloroplastic), sphingolipids, and ceramides (synthesised by ER), suggesting that mitochondrial homeostasis is essential for the regulation of whole cellular lipidome via specific signalling pathways. In particular, the observed modifications in phospholipid and sphingolipid/ceramide molecular species suggest that CI activity controls phosphatidic acid-mediated signalling.