Roles of ceramide and sphingolipids in pancreatic β-cell function and dysfunction

Targeted Lipidomics and Transcriptomics Unveil Aberrant Lipid Metabolic Remodeling in Visceral and Subcutaneous Adipose Tissue under ER Stress

Endoplasmic reticulum (ER) stress is known to impair the function of visceral adipose tissue (VAT) and subcutaneous adipose tissue (SAT), disrupting lipid metabolism. Despite the crucial role lipid plays in regulating adipose tissue function, the specific lipidomic alterations in VAT and SAT under ER stress remain unclear. In this study, ER stress was induced in VAT and SAT, and targeted lipidomic and transcriptomic approaches were used to analyze lipid metabolism and gene expression profiles. The results revealed that VAT exhibited a stronger ER stress response, characterized by a significant increase in binding immunoglobulin protein (BiP) expression and notable lipidomic disruptions, especially in glycerides and sterols. These disruptions were marked by a decrease in protective polyunsaturated fatty acyl species and the accumulation of lipotoxic molecules. In contrast, SAT displayed less severe lipidomic alterations. Transcriptomic analysis indicated that VAT was more susceptible to immune activation, inflammation, and metabolic dysfunction, while SAT primarily showed alterations in protein folding processes. These findings underscore the tissue-specific mechanisms of ER stress adaptation in VAT and SAT. In conclusion, VAT appears to be a critical target for addressing metabolic dysfunction in obesity and related disorders, with potential therapeutic implications for managing ER stress-induced metabolic diseases.

A specific metabolomic and lipidomic signature reveals the postpartum resolution of gestational diabetes mellitus or its evolution to type 2 diabetes in rat

Gestational diabetes mellitus (GDM) represents a major public health concern due to adverse maternal postpartum and long-term outcomes. Current strategies to manage GDM fail to reduce the maternal risk to develop later impaired glucose tolerance (IGT) and type 2 diabetes (T2D). In a rodent model of diet-induced GDM without obesity, we explored the perinatal metabolic adaptations in dams with gestational IGT followed by either persistent or resolved postpartum IGT. Female Sprague-Dawley rats were fed a high-fat high-sucrose (HFHS) or a chow [control group (CTL)] diet, 1 wk before mating and throughout gestation (G). Following parturition, HFHS dams were randomized to two subgroups: one switched to a chow diet and the other one maintained on an HFHS diet throughout lactation (L). Oral glucose tolerance tests (OGTTs) were performed, and plasma metabolome-lipidome were characterized at G12 and L12. We found that 1) in GDM-pregnant dams, IGT was associated with incomplete fatty acid oxidation (FAO), enhanced gluconeogenesis, altered insulin signaling, and oxidative stress; 2) improved glucose tolerance postpartum seemed to restore complete FAO along with elevation of nervonic acid-containing sphingomyelins, assumed to impart β-cell protection; and 3) persistence of IGT after delivery was associated with metabolites known to predict the early onset of insulin and leptin resistance, with maintained liver dysfunction. Our findings shed light on the impact of postpartum IGT evolution on maternal metabolic outcome after an episode of GDM. They suggest innovative strategies, implemented shortly after delivery and targeted on these biomarkers, should be explored to curb or delay the transition from GDM to T2D in these mothers.NEW & NOTEWORTHY Specific metabolomic/lipidomic features are associated with GDM postpartum outcomes. GDM-pregnant dams exhibit partial fatty acid oxidation and boosted gluconeogenesis. Resolution of postpartum IGT relies on nervonic acid-sphingomyelin, a β-cell protector. Postpartum IGT persistence suggests muscle insulin resistance and liver dysfunction.

Decrypting the Possible Mechanistic Role of Fenofibrate in Alzheimer's Disease and Type 2 Diabetes: The Truth and Mystery

Alzheimer's disease (AD) is a neurodegenerative disease caused by the progressive deposition of extracellular amyloid beta (Aβ) and intracellular neurofibrillary tangles (NFTs). Of note, metabolic disorders such as insulin resistance (IR) and type 2 diabetes (T2D) are associated with the development of brain IR and associated neurodegeneration. In addition, AD neuropathology and linked cognitive impairment accelerate the development of peripheral IR and the progression of T2D. Therefore, there is a bidirectional relationship between T2D and AD. It has been demonstrated that AD and T2D induce dysregulation of peroxisome proliferator-activated receptor alpha (PPAR-α) leading to the central and peripheral metabolic disturbances. Hence, dysregulated PPAR-α could be a shared mechanism in both AD and T2D, and restoration of PPAR-α signalling by PPAR-α agonist fenofibrate (FN) may alleviate T2D and AD. Therefore, this review aims to shed light on the potential involvement of PPAR-α in T2D and AD, and how FN could be effective in the management of AD. FN seems to be effective in both AD and T2D by dual neuroprotective and antidiabetic effects that can mitigate AD neuropathology and T2D-related complications by modulating various cellular processes and inflammatory signalling pathways. In conclusion, FN could be a possible candidate in the management of AD and T2D by modulating different signalling pathways involved in the pathogenesis of these conditions.

PPARs: modulating lipotoxicity and thus inhibiting fibrosis

Peroxisome proliferator-activated receptors (PPARs) belong to the nuclear hormone receptor family of ligand-activated receptors and are known for their roles as key factors in the regulation of lipid metabolism. In the more than three decades since their discovery, most reports on PPARs have focused on their roles in lipid metabolism, and a portion of the new research has also focused on the relationship between PPARs and fibrosis. Interestingly, lipid metabolism disorders and fibrosis are also inextricably linked. This implies that PPARs, lipid metabolism and fibrosis are interrelated. On this basis, we have summarized the molecular mechanisms of PPARs regulating fibrosis through lipid metabolism and PPARγ directly regulating fibrosis, and pointed out the contradictions and enigmas that need to be further explored in the processes of PPARs regulating lipid metabolism and fibrosis. The aim of the present review is to provide new ideas for PPARs for the treatment of lipid metabolism disorders and fibrosis.

Sphingolipids modulate redox signalling during human sperm capacitation

STUDY QUESTION

What role do sphingolipids have in mediating human sperm capacitation?

SUMMARY ANSWER

Sphingosine 1-phosphate (S1P) mediates the acquisition of fertilizing competency in human spermatozoa by engaging with its Gi-coupled receptor S1PR1 and promoting production of reactive oxygen species such as nitric oxide and superoxide anion.

WHAT IS KNOWN ALREADY

Bioactive sphingolipids, such as S1P, are fundamental for regulating numerous physiological domains and processes, such as cell membranes and signalling, cell death and proliferation, cell migration and invasiveness, inflammation, and central nervous system development.

STUDY DESIGN, SIZE, DURATION

Semen samples were obtained from a cohort of 10 healthy non-smoking volunteers (18-30 years old) to investigate the role of S1P in sperm.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Percoll-selected human spermatozoa were incubated at 37°C for 3.5 h in BWW media with or without foetal cord serum ultrafiltrate (FCSu), sphingosine (Sph), or ceramide (Cer). Spermatozoa were also incubated with or without pharmacological inhibitors of sphingolipid metabolism. Protein tyrosine phosphorylation was determined by immunoblotting. The acrosome reaction was determined by PSA-FTIC labelling of the acrosome and analysed using fluorescence microscopy. Intracellular nitric oxide (NO•) production was determined using a DAF-2DA probe. Immunocytochemistry was performed to localize and assess the functional relationship of key components of lipid signalling in spermatozoa. Sperm viability and motility of the samples were evaluated by the hypo-osmotic swelling (HOS) test and computer-aided sperm analysis (CASA). Statistical differences between groups were determined using ANOVA and Tukey's test. Normal distribution of the data and variance homogeneity were assessed using Shapiro-Wilk and Levene's test, respectively. A difference was considered significant when the P-value was ≤0.05.

MAIN RESULTS AND THE ROLE OF CHANCE

S1P mediates the acquisition of fertilizing competency in human spermatozoa by engaging with its Gi-coupled receptor S1PR1. We found that S1PR1 redistributes to the post-acrosomal region upon induction of capacitation. S1P signalling promotes the activation of the PI3K-AKT pathway, leading to NO• production during sperm capacitation. L-NAME, an nitric oxide synthase inhibitor, impaired the Sph- and Cer-dependent capacitation. Additionally, Sph and Cer promote superoxide anion (O2•-) production, and the extracellular addition of superoxide dismutase (SOD) prevented Sph- and Cer-dependent capacitation, suggesting that Sph and Cer stimulate O2•- production during sperm capacitation. Protein kinase type R (PKR), ceramide kinase (CERK), and protein kinase C (PKC) are responsible for translocating and activating sphingosine kinase 1 (SphK1), which is necessary to promote S1P production for sperm capacitation.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

The utilization and actions of sphingolipids may differ in spermatozoa of different species.

WIDER IMPLICATIONS OF THE FINDINGS

Sphingolipid metabolites such as Sph, Cer, S1P, and ceramide 1-phosphate (C1P) play a crucial role in inducing human sperm capacitation. Our research has provided new insights into fundamental sphingolipid processes in human sperm, including the importance of C1P in translocating and activating SphK1 as well as the S1P signalling to regulate the PI3K/AKT/NOS pathway to generate NO• for sperm capacitation. We are the first to identify the presence of PKR in human spermatozoa and its role in the phosphorylation activities of SphK1 with the subsequent activation of S1P signalling. Furthermore, our study has identified that S1PR1 and S1PR3 are involved in capacitation and the acrosome reaction, respectively. These findings shed light on a novel mechanism by which sphingolipids drive capacitation in human sperm and pave the way for further exploration of the role of bioactive sphingolipid metabolites in this process. Lastly, our studies lay the foundation for examining the lipid profile of infertile males, as potential discrepancies can affect the functional capacity of spermatozoa to reach fertilizing potential.

STUDY FUNDING/COMPETING INTEREST(S)

This research was funded by the Canadian Institutes of Health Research (CIHR), grant number PJT-165962 to C.O.F. S.S. was awarded a Research Institute-MUHC Desjardins Studentship. There are no competing interests to report.

Sphingolipid Analysis in Clinical Research

Sphingolipids are the most diverse class of lipids due to the numerous variations in their structural components. This diversity is also reflected in their extremely different functions. Sphingolipids are not only constituents of cell membranes but have emerged as key signaling molecules involved in a variety of cellular functions, such as cell growth and differentiation, proliferation and apoptotic cell death. Lipidomic analyses in clinical research have identified pathways and products of sphingolipid metabolism that are altered in several human pathologies. In this article, we describe how to properly design a lipidomic experiment in clinical research, how to handle plasma and serum samples for this purpose, and how to measure sphingolipids using liquid chromatography-mass spectrometry.

Sphingolipid profiling as a biomarker of type 2 diabetes risk: evidence from the MIDUS and PREDIMED studies

BACKGROUND

Type 2 diabetes (T2D) has become a worldwide pandemic. While ceramides may serve as intermediary between obesity-related lipotoxicity and T2D, the relationship with simple glycosphingolipids remains uncertain. The aim of this study was to characterize the associations between blood glycosphingolipid and ceramide species with T2D and to identify a circulating sphingolipid profile that could serve as novel biomarker for T2D risk.

METHODS

Cross-sectional relationship between sphingolipid levels, insulin resistance, and T2D prevalence were evaluated in 2,072 American adults from MIDUS cohort. Prospectively, the association between sphingolipid species and the incidence of T2D was analyzed using a case-cohort design nested within the PREDIMED trial (250 cases and a random sample of 692 participants, with 3.8 years of median follow-up). Circulating levels of sphingolipid species in both populations were measured using LC/MS. Hazard ratios were estimated with weighted Cox regression models using Barlow weights.

RESULTS

In American adults, only CER18:0 and CER22:0 were linked to insulin resistance and a higher prevalence of T2D. Conversely, three lactosylceramides (LCER 14:0, 16:0, and 24:1) showed a strong inverse relationship with both insulin resistance and T2D. These findings led to development of two sphingolipid scores. In the prospective analysis, these scores consistently predicted a reduced risk of T2D incidence in PREDIMED (HR: 0.64, 95% CI 0.44 to 0.94 and 0.58, 0.40 to 0.85 respectively) between extreme quartiles, with 5-year absolute risk differences of 9.6% (95% CI: 0.3-20.5%) and 11.4% (1.0-21.6%). They were validated in the same trial with samples obtained after 1 year of follow-up.

CONCLUSIONS

Our findings support the potential usefulness of circulating sphingolipid profiles as novel biomarkers for T2D risk. Moreover, this study opens the door for future research on the predictive value and possible protective roles of lactosylceramides in T2D.

Pathology and Treatments of Alzheimer's Disease Based on Considering Changes in Brain Energy Metabolism Due to Type 2 Diabetes

Alzheimer's disease (AD) is a progressive neurodegenerative disorder with cognitive dysfunction, memory decline, and behavioral disturbance, and it is pathologically characterized by the accumulation of amyloid plaques and neurofibrillary tangles in the brain. Although various hypotheses have been proposed to explain the pathogenesis of AD, including the amyloid beta hypothesis, oxidative stress hypothesis, and abnormal phosphorylation of tau proteins, the exact pathogenic mechanisms underlying AD remain largely undefined. Furthermore, effective curative treatments are very limited. Epidemiologic studies provide convincing evidence for a significant association between type 2 diabetes and AD. Here, we showed energy metabolism using glucose, lactate, ketone bodies, and lipids as energy substrates in a normal brain, and changes in such energy metabolism due to type 2 diabetes. We also showed the influences of such altered energy metabolism due to type 2 diabetes on the pathology of AD. Furthermore, we comprehensively searched for risk factors related with type 2 diabetes for AD and showed possible therapeutic interventions based on considering risk factors and altered brain energy metabolism due to type 2 diabetes for the development of AD.

Sulfation pathways in the maintenance of functional beta-cell mass and implications for diabetes

Diabetes Type 1 and Type 2 are widely occurring diseases. In spite of a vast amount of biomedical literature about diabetic processes in general, links to certain biological processes are only becoming evident these days. One such area of biology is the sulfation of small molecules, such as steroid hormones or metabolites from the gastrointestinal tract, as well as larger biomolecules, such as proteins and proteoglycans. Thus, modulating the physicochemical propensities of the different sulfate acceptors, resulting in enhanced solubility, expedited circulatory transit, or enhanced macromolecular interaction. This review lists evidence for the involvement of sulfation pathways in the maintenance of functional pancreatic beta-cell mass and the implications for diabetes, grouped into various classes of sulfated biomolecule. Complex heparan sulfates might play a role in the development and maintenance of beta-cells. The sulfolipids sulfatide and sulfo-cholesterol might contribute to beta-cell health. In beta-cells, there are only very few proteins with confirmed sulfation on some tyrosine residues, with the IRS4 molecule being one of them. Sulfated steroid hormones, such as estradiol-sulfate and vitamin-D-sulfate, may facilitate downstream steroid signaling in beta-cells, following de-sulfation. Indoxyl sulfate is a metabolite from the intestine, that causes kidney damage, contributing to diabetic kidney disease. Finally, from a technological perspective, there is heparan sulfate, heparin, and chondroitin sulfate, that all might be involved in next-generation beta-cell transplantation. Sulfation pathways may play a role in pancreatic beta-cells through multiple mechanisms. A more coherent understanding of sulfation pathways in diabetes will facilitate discussion and guide future research.

Role of fatty acids in the pathogenesis of ß-cell failure and Type-2 diabetes

Pancreatic ß-cells are glucose sensors in charge of regulated insulin delivery to the organism, achieving glucose homeostasis and overall energy storage. The latter function promotes obesity when nutrient intake chronically exceeds daily expenditure. In case of ß-cell failure, such weight gain may pave the way for the development of Type-2 diabetes. However, the causal link between excessive body fat mass and potential degradation of ß-cells remains largely unknown and debated. Over the last decades, intensive research has been conducted on the role of lipids in the pathogenesis of ß-cells, also referred to as lipotoxicity. Among various lipid species, the usual suspects are essentially the non-esterified fatty acids (NEFA), in particular the saturated ones such as palmitate. This review describes the fundamentals and the latest advances of research on the role of fatty acids in ß-cells. This includes intracellular pathways and receptor-mediated signaling, both participating in regulated glucose-stimulated insulin secretion as well as being implicated in ß-cell dysfunction. The discussion extends to the contribution of high glucose exposure, or glucotoxicity, to ß-cell defects. Combining glucotoxicity and lipotoxicity results in the synergistic and more deleterious glucolipotoxicity effect. In recent years, alternative roles for intracellular lipids have been uncovered, pointing to a protective function in case of nutrient overload. This requires dynamic storage of NEFA as neutral lipid droplets within the ß-cell, along with active glycerolipid/NEFA cycle allowing subsequent recruitment of lipid species supporting glucose-stimulated insulin secretion. Overall, the latest studies have revealed the two faces of the same coin.

Inhibition of de novo ceramide synthesis by sirtuin-1 improves beta-cell function and glucose metabolism in type 2 diabetes

AIMS

Obesity and type 2 diabetes (T2D) are major risk factors for cardiovascular (CV) diseases. Dysregulated pro-apoptotic ceramide synthesis reduces β-cell insulin secretion, thereby promoting hyperglycaemic states that may manifest as T2D. Pro-apoptotic ceramides modulate insulin sensitivity and glucose tolerance while being linked to poor CV outcomes. Sirtuin-1 (SIRT1) is a NAD + -dependent deacetylase that protects against pancreatic β-cell dysfunction; however, systemic levels are decreased in obese-T2D mice and may promote pro-apoptotic ceramide synthesis and hyperglycaemia. Herein, we aimed to assess the effects of restoring circulating SIRT1 levels to prevent metabolic imbalance in obese and diabetic mice.

METHODS AND RESULTS

Circulating SIRT1 levels were reduced in obese-diabetic mice (db/db) as compared to age-matched non-diabetic db/+ controls. Restoration of SIRT1 plasma levels with recombinant murine SIRT1 for 4 weeks prevented body weight gain and improved glucose tolerance, insulin sensitivity, and vascular function in mice models of obesity and T2D. Untargeted lipidomics revealed that SIRT1 restored insulin secretory function of β-cells by reducing synthesis and accumulation of pro-apoptotic ceramides. Molecular mechanisms involved direct binding to and deacetylation of Toll-like receptor 4 (TLR4) by SIRT1 in β-cells, thereby decreasing the rate-limiting enzymes of sphingolipid synthesis SPTLC1/2 via AKT/NF-κB. Among patients with T2D, those with high baseline plasma levels of SIRT1 prior to metabolic surgery displayed restored β-cell function (HOMA2-β) and were more likely to have T2D remission during follow-up.

CONCLUSION

Acetylation of TLR4 promotes β-cell dysfunction via ceramide synthesis in T2D, which is blunted by systemic SIRT1 replenishment. Hence, restoration of systemic SIRT1 may provide a novel therapeutic strategy to counteract toxic ceramide synthesis and mitigate CV complications of T2D.

Brain lipid sensing and the neural control of energy balance

The central nervous system continuously detects circulating concentrations of lipids such as fatty acids and troglycerides. Once information has been detected, the central nervous system can in turn participate in the control of energy balance and blood sugar levels and in particular regulate the secretion and action of insulin. Neurons capable of detecting circulating lipid variations are located in the hypothalamus and in other regions such as the nucleus accumbens, the striatum or the hippocampus. An excess of lipids will have deleterious effects and may induce central lipotoxicity, in particular following local production of ceramides and the appearance of neuroinflammation which may lead to metabolic diseases such as obesity and type 2 diabetes.

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.

Tauroursodeoxycholic Acid (TUDCA) Relieves Streptozotocin (STZ)-Induced Diabetic Rat Model via Modulation of Lipotoxicity, Oxidative Stress, Inflammation, and Apoptosis

Tauroursodeoxycholic acid (TUDCA) is approved for the treatment of liver diseases. However, the antihyperglycemic effects/mechanisms of TUDCA are still less clear. The present study aimed to evaluate the antidiabetic action of TUDCA in streptozotocin (STZ)-induced type 2 diabetes mellitus (T2DM) in rats. Fifteen adult Wistar albino male rats were randomly divided into three groups (n = five in each): control, diabetic (STZ), and STZ+TUDCA. The results showed that TUDCA treatment significantly reduced blood glucose, HbA1c%, and HOMA-IR as well as elevated the insulin levels in diabetic rats. TUDCA therapy increased the incretin GLP-1 concentrations, decreased serum ceramide synthase (CS), improved the serum lipid profile, and restored the glycogen content in the liver and skeletal muscles. Furthermore, serum inflammatory parameters (such as TNF-α, IL-6, IL-1ß, and PGE-2) were substantially reduced with TUDCA treatment. In the pancreas, STZ+TUDCA-treated rats underwent an obvious enhancement of enzymatic (CAT and SOD) and non-enzymatic (GSH) antioxidant defense systems and a marked decrease in markers of the lipid peroxidation rate (MDA) and nitrosative stress (NO) compared to STZ-alone. At the molecular level, TUDCA decreased the pancreatic mRNA levels of iNOS and apoptotic-related factors (p53 and caspase-3). In conclusion, TUDCA may be useful for diabetes management and could be able to counteract diabetic disorders via anti-hyperlipidemic, antioxidant, anti-inflammatory, and anti-apoptotic actions.

Remodeling ceramide homeostasis promotes functional maturation of human pluripotent stem cell-derived β cells

Human pluripotent stem cell-derived β cells (hPSC-β cells) show the potential to restore euglycemia. However, the immature functionality of hPSC-β cells has limited their efficacy in application. Here, by deciphering the continuous maturation process of hPSC-β cells post transplantation via single-cell RNA sequencing (scRNA-seq) and single-cell assay for transposase-accessible chromatin sequencing (scATAC-seq), we show that functional maturation of hPSC-β cells is an orderly multistep process during which cells sequentially undergo metabolic adaption, removal of negative regulators of cell function, and establishment of a more specialized transcriptome and epigenome. Importantly, remodeling lipid metabolism, especially downregulating the metabolic activity of ceramides, the central hub of sphingolipid metabolism, is critical for β cell maturation. Limiting intracellular accumulation of ceramides in hPSC-β cells remarkably enhanced their function, as indicated by improvements in insulin processing and glucose-stimulated insulin secretion. In summary, our findings provide insights into the maturation of human pancreatic β cells and highlight the importance of ceramide homeostasis in function acquisition.

Proinflammatory stress activates neutral sphingomyelinase 2 based generation of a ceramide-enriched β cell EV subpopulation

β cell extracellular vesicles (EVs) play a role as paracrine effectors in islet health, yet mechanisms connecting β cell stress to changes in EV cargo and potential impacts on diabetes remain poorly defined. We hypothesized that β cell inflammatory stress engages neutral sphingomyelinase 2 (nSMase2)-dependent EV formation pathways, generating ceramide-enriched EVs that could impact surrounding β cells. Consistent with this, proinflammatory cytokine treatment of INS-1 β cells and human islets concurrently increased β cell nSMase2 and ceramide expression, as well as EV ceramide staining. Direct chemical activation or genetic knockdown of nSMase2, or treatment with a GLP-1 receptor agonist also modulated cellular and EV ceramide. Small RNA sequencing of ceramide-enriched EVs identified a distinct set of miRNAs linked to β cell function and identity. Coculture experiments using CD9-GFP tagged INS-1 cell EVs demonstrated that either cytokine treatment or chemical nSMase2 activation increased EV transfer to recipient cells. Children with recent-onset T1D showed no abnormalities in circulating ceramide-enriched EVs, suggesting a localized, rather than systemic phenomenon. These findings highlight nSMase2 as a regulator of β cell EV cargo and identify ceramide-enriched EV populations as a contributor to EV-related paracrine signaling under conditions of β cell inflammatory stress.

Article Highlights

a. Why did we undertake this study?: Mechanisms connecting β cell stress to changes in extracellular vesicle (EV) cargo and potential impacts on diabetes are poorly defined.b. What is the specific question we wanted to answer?: Does β cell inflammatory stress engage neutral sphingomyelinase 2 (nSMase2)-dependent EV formation pathways to generate ceramide-enriched EVs.c. What did we find?: Proinflammatory cytokine treatment of β cells increased β cell ceramide expression, along with EV ceramide in part via increases in nSMase2. Ceramide-enriched EVs housed a distinct set of miRNAs linked to insulin signaling. Both cytokine treatment and nSMase2 activation increase EV transfer to other β cells.d. What are the implications of our findings?: Our findings highlight nSMase2 as a regulator of β cell EV cargo and identify ceramide-enriched EV populations as a contributor to EV-related paracrine signaling under conditions of β cell inflammatory stress.

Plasma Sphingolipid Profile of Healthy Black and White Adults Differs Based on Their Parental History of Type 2 Diabetes

CONTEXT

Ceramides and sphingolipids have been linked to type 2 diabetes (T2D). The Ceramides and Sphingolipids as Predictors of Incident Dysglycemia (CASPID) study is designed to determine the association of plasma sphingolipids with the pathophysiology of human T2D.

OBJECTIVE

A comparison of plasma sphingolipids profiles in Black and White adults with (FH+) and without (FH-) family history of T2D.

DESIGN

We recruited 100 Black and White FH- (54 Black, 46 White) and 140 FH+ (75 Black, 65 White) adults. Fasting plasma levels of 58 sphingolipid species, including 18 each from 3 major classes (ceramides, monohexosylceramides, and sphingomyelins, all with 18:1 sphingoid base) and 4 long-chain sphingoid base-containing species, were measured by liquid chromatography/mass spectrometry.

RESULTS

Sphingomyelin was the most abundant sphingolipid in plasma (89% in FH-), and was significantly elevated in FH+ subjects (93%). Ceramides and monohexosylceramides comprised 5% and 6% of total sphingolipids in the plasma of FH- subjects, and were reduced significantly in FH+ subjects (3% and 4%, respectively). In FH+ subjects, most ceramide and monohexosylceramide species were decreased but sphingomyelin species were increased. The level of C18:1 species of all 3 classes was elevated in FH+ subjects.

CONCLUSION

Elevated levels of sphingomyelin, the major sphingolipids of plasma, and oleic acid-containing sphingolipids in healthy FH+ subjects compared with healthy FH- subjects may reflect heritable elements linking sphingolipids and the development of T2D.

Mechanism of Obesity-Related Lipotoxicity and Clinical Perspective

The link between cellular exposure to fatty acid species and toxicity phenotypes remains poorly understood. However, structural characterization and functional profiling of human plasma free fatty acids (FFAs) analysis has revealed that FFAs are located either in the toxic cluster or in the cluster that is transcriptionally responsive to lipotoxic stress and creates genetic risk factors. Genome-wide short hairpin RNA screen has identified more than 350 genes modulating lipotoxicity. Hypertrophic adipocytes in obese adipose are both unable to expand further to store excess lipids in the diet and are resistant to the antilipolytic action of insulin. In addition to lipolysis, the inability of packaging the excess lipids into lipid droplets causes circulating fatty acids to reach toxic levels in non-adipose tissues. Deleterious effects of accumulated lipid in non-adipose tissues are known as lipotoxicity. Although triglycerides serve a storage function for long-chain non-esterified fatty acid and their products such as ceramide and diacylglycerols (DAGs), overloading of palmitic acid fraction of saturated fatty acids (SFAs) raises ceramide levels. The excess DAG and ceramide load create harmful effects on multiple organs and systems, inducing chronic inflammation in obesity. Thus, lipotoxic inflammation results in β cells death and pancreatic islets dysfunction. Endoplasmic reticulum stress stimuli induce lipolysis by activating cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) and extracellular signal-regulated kinase (Erk) 1/2 signaling in adipocytes. However, palmitic acid-induced endoplasmic reticulum stress-c-Jun N-terminal kinase (JNK)-autophagy axis in hypertrophic adipocytes is a pro-survival mechanism against endoplasmic reticulum stress and cell death induced by SFAs. Endoplasmic reticulum-localized acyl-coenzyme A (CoA): glycerol-3-phosphate acyltransferase (GPAT) enzymes are mediators of lipotoxicity, and inhibiting these enzymes has therapeutic potential for lipotoxicity. Lipotoxicity increases the number of autophagosomes, which engulf palmitic acid, and thus suppress the autophagic turnover. Fatty acid desaturation promotes palmitate detoxification and storages into triglycerides. As therapeutic targets of glucolipotoxicity, in addition to caloric restriction and exercise, there are four different pharmacological approaches, which consist of metformin, glucagon-like peptide 1 (GLP-1) receptor agonists, peroxisome proliferator-activated receptor-gamma (PPARγ) ligands thiazolidinediones, and chaperones are still used in clinical practice. Furthermore, induction of the brown fat-like phenotype with the mixture of eicosapentanoic acid and docosahexaenoic acid appears as a potential therapeutic application for treatment of lipotoxicity.

Endoplasmic Reticulum Stress and Obesity

In recent years, the world has seen an alarming increase in obesity and is closely associated with insulin resistance, which is a state of low-grade inflammation, the latter characterized by elevated levels of proinflammatory cytokines in blood and tissues. A shift in energy balance alters systemic metabolic regulation and the important role that chronic inflammation, endoplasmic reticulum (ER) dysfunction, and activation of the unfolded protein response (UPR) plays in this process.Why obesity is so closely associated with insulin resistance and inflammation is not understood well. This suggests that there are probably many causes for obesity-related insulin resistance and inflammation. One of the faulty mechanisms is protein homeostasis, protein quality control system included protein folding, chaperone activity, and ER-associated degradation leading to endoplasmic reticulum (ER) stress.The ER is a vast membranous network responsible for the trafficking of a wide range of proteins and plays a central role in integrating multiple metabolic signals critical in cellular homeostasis. Conditions that may trigger unfolded protein response activation include increased protein synthesis, the presence of mutant or misfolded proteins, inhibition of protein glycosylation, imbalance of ER calcium levels, glucose and energy deprivation, hypoxia, pathogens, or pathogen-associated components and toxins. Thus, characterizing the mechanisms contributing to obesity and identifying potential targets for its prevention and treatment will have a great impact on the control of associated conditions, particularly T2D.

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.

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

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

Gestational Diabetes Mellitus, Postpartum Lipidomic Signatures, and Subsequent Risk of Type 2 Diabetes: A Lipidome-Wide Association Study

OBJECTIVE

To identify a postpartum lipidomic signature associated with gestational diabetes mellitus (GDM) and investigate the role of the identified lipids in the progression to type 2 diabetes (T2D).

RESEARCH DESIGN AND METHODS

This prospective cohort study enrolled 1,409 women at 24-72 h after delivery of a singleton baby and followed them prospectively at the Boston Medical Center. The lipidome was profiled by liquid chromatography-tandem mass spectrometry. Diagnoses of GDM and incident T2D were extracted from medical records and verified using plasma glucose levels.

RESULTS

Mean (SD) age of study women at baseline was 28.5 (6.6) years. A total of 219 (16.4%) women developed incident diabetes over a median follow-up of 11.8 (interquartile range 8.2-14.8) years. We identified 33 postpartum lipid species associated with GDM, including 16 inverse associations (primarily cholesterol esters and phosphatidylcholine plasmalogens), and 17 positive associations (primarily diacyglycerols and triacyglycerols). Of these, four were associated with risk of incident T2D and mediated ∼12% of the progression from GDM to T2D. The identified lipid species modestly improved the predictive performance for incident T2D above classical risk factors when the entire follow-up period was considered.

CONCLUSIONS

GDM was associated with a wide range of lipid metabolic alterations at early postpartum, among which some lipid species were also associated with incident T2D and mediated the progression from GDM to T2D. The improvements attained by including lipid species in the prediction of T2D provides new insights regarding the early detection and prevention of progression to T2D.

Associations between Per- and Polyfluoroalkyl Substances Exposures and Blood Lipid Levels among Adults-A Meta-Analysis

BACKGROUND

Associations between per- and polyfluoroalkyl substances (PFAS) and blood lipid levels in humans were mixed.

OBJECTIVES

The objective of this meta-analysis was to summarize associations between PFAS and blood lipids in adults.

METHODS

A literature search was conducted on PubMed and Web of Science for articles published through 13 May 2022 that examined associations between PFAS and blood lipids, including total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), and triacylglycerols (TGs). Inclusion criteria included the presence of associations between five PFAS (PFOA, PFOS, PFHxS, PFDA, and PFNA) and four blood lipid measures (TC, HDL-C, LDL-C, and TGs) in adults. Data on study characteristics and PFAS-lipid associations were extracted. Assessments of individual study quality were performed. Associations of changes of blood lipid levels corresponding to 1 interquartile range (IQR)-unit increase of blood PFAS levels were pooled using random effects models. Dose-response relationships were examined.

RESULTS

Twenty-nine publications were included in the present analyses. Every IQR increase of PFOA was significantly associated with a 2.1 -mg / dL increase in TC (95% CI: 1.2, 3.0), a 1.3 -mg / dL increase in TGs (95% CI: 0.1, 2.4), and a 1.4 -mg / dL increase in LDL-C (95% CI: 0.6, 2.2). PFOS was also significantly associated with TC and LDL-C levels, and the corresponding values were 2.6 (95% CI: 1.5, 3.6) and 1.9 (95% CI: 0.9, 3.0), respectively. Associations of PFOS and PFOA with HDL-C levels were largely null. For minor PFAS species, PFHxS was significantly associated with higher levels of HDL-C [0.8 (95% CI: 0.5, 1.2)]. Inverse associations were observed between PFDA and TGs [- 5.0 (95% CI: - 8.1 , - 1.9 )] and between PFNA and TGs [- 1.7 (95% CI: - 3.5 , - 0.02 )], whereas a positive association was observed between PFDA and HDL-C [1.4 (95% CI: 0.1, 2.7)]. Nonsignificant nonlinear dose-response relationships were identified for associations of PFOA and PFOS with certain blood lipids.

DISCUSSION

PFOA and PFOS were significantly associated with TC and LDL-C levels in adults. Whether these findings may translate into an elevated cardiovascular disease risk associated with PFAS exposure warrants further investigation. https://doi.org/10.1289/EHP11840.

Relationship of Sulfatides Physiological Function and Peroxisome Proliferator-Activated Receptor α

Sulfatides are unique sphingolipids present in the serum and the plasma membrane. Sulfatides exert important functions in a number of systems in the human body, including the nervous, immune, cardiovascular, and coagulation systems.Furthermore, it is closely related to tumor occurrence, development, and metastasis. Peroxisome proliferators-activated receptor α (PPARα) is a class of the nuclear receptor superfamily of transcription factors, which is a potential regulator of sulfatides. This review not only summarizes the current knowledge on the physiological functions of sulfatides in various systems, but also discusses the possible PPARα regulatory mechanisms in sulfatide metabolism and functions. The results of the present analysis provide deep insights and further novel ideas for expanding the research on the physiological function and clinical application of sulfatides.

Discovering Gummadiol and Isoarboreol as potential inhibitors of sphingosine kinase 1: virtual screening and MD simulation studies

Sphingosine kinase 1 (SphK1) dysfunction is well-known to be linked to various severe diseases, including breast, lung, prostate, and hematological cancers. Due to its crucial function in the onset of cancer and its progression, it is considered a notable drug target for anticancer therapy. Small molecule inhibitors with high specificity and efficacy towards SphK1 are needed for their therapeutic use. In order to find possible SphK1 inhibitors, we conducted a stepwise structure-based virtual screening of plant-based molecules available from the IMPPAT library. A multi-step virtual screening, including physicochemical and ADMET evaluation, PAINS, molecular docking, PASS analysis followed by molecular dynamics (MD) simulation and principal component analysis, identifies two compounds, Gummadiol and Isoarboreol, against SphK1. All-atom MD simulations were performed for 100 ns which examined the structural changes and stability of the docked complexes in the aqueous environment. The time evolution data of structural deviations and compactness, PCA and free energy landscapes suggested that the binding of Gummadiol and Isoarboreol with SphK1 is considerably stable throughout the trajectory. The study highlighted the use of phytochemicals in anticancer therapeutics and presented Gummadiol and Isoarboreol as promising inhibitors of SphK1.Communicated by Ramaswamy H. Sarma.

Type 2 Diabetes and Alzheimer's Disease: The Emerging Role of Cellular Lipotoxicity

Type 2 diabetes (T2D) and Alzheimer's diseases (AD) represent major health issues that have reached alarming levels in the last decades. Although growing evidence demonstrates that AD is a significant comorbidity of T2D, and there is a ~1.4-2-fold increase in the risk of developing AD among T2D patients, the involvement of possible common triggers in the pathogenesis of these two diseases remains largely unknown. Of note, recent mechanistic insights suggest that lipotoxicity could represent the missing ring in the pathogenetic mechanisms linking T2D to AD. Indeed, obesity, which represents the main cause of lipotoxicity, has been recognized as a major risk factor for both pathological conditions. Lipotoxicity can lead to inflammation, insulin resistance, oxidative stress, ceramide and amyloid accumulation, endoplasmic reticulum stress, ferroptosis, and autophagy, which are shared biological events in the pathogenesis of T2D and AD. In the current review, we try to provide a critical and comprehensive view of the common molecular pathways activated by lipotoxicity in T2D and AD, attempting to summarize how these mechanisms can drive future research and open the way to new therapeutic perspectives.

Sphingolipid subtypes differentially control proinsulin processing and systemic glucose homeostasis

Impaired proinsulin-to-insulin processing in pancreatic β-cells is a key defective step in both type 1 diabetes and type 2 diabetes (T2D) (refs. 1,2), but the mechanisms involved remain to be defined. Altered metabolism of sphingolipids (SLs) has been linked to development of obesity, type 1 diabetes and T2D (refs. 3-8); nonetheless, the role of specific SL species in β-cell function and demise is unclear. Here we define the lipid signature of T2D-associated β-cell failure, including an imbalance of specific very-long-chain SLs and long-chain SLs. β-cell-specific ablation of CerS2, the enzyme necessary for generation of very-long-chain SLs, selectively reduces insulin content, impairs insulin secretion and disturbs systemic glucose tolerance in multiple complementary models. In contrast, ablation of long-chain-SL-synthesizing enzymes has no effect on insulin content. By quantitatively defining the SL-protein interactome, we reveal that CerS2 ablation affects SL binding to several endoplasmic reticulum-Golgi transport proteins, including Tmed2, which we define as an endogenous regulator of the essential proinsulin processing enzyme Pcsk1. Our study uncovers roles for specific SL subtypes and SL-binding proteins in β-cell function and T2D-associated β-cell failure.

Differences in the Distribution of Ceramides and Sphingosine among Lipoprotein and Lipoprotein-Depleted Fractions in Patients with Type 2 Diabetes Mellitus

AIM

In addition to the quantity and quality, the carriers, such as lipoproteins and albumin, can affect the physiological properties and clinical significance of lipids. This study aimed to elucidate the modulation of the levels of ceramides and sphingosine, which are considered as proatherosclerotic lipids, in lipoproteins and lipoprotein-depleted fractions in subjects with type 2 diabetes.

METHODS

We separated the serum samples collected from healthy subjects (n=22) and subjects with type 2 diabetes (n=39) into Triglyceride (TG)-rich lipoproteins (TRL), low-density lipoprotein (LDL), high-density lipoprotein (HDL), and lipoprotein-depleted fractions via ultracentrifugation. Then, we measured the levels of six species of ceramides, sphingosine, and dihydrosphingosine via LC-MS/MS and statistically analyzed them to identify the sphingolipids in each fraction, which are associated with diabetes as well as cardiovascular and renal complications.

RESULTS

In subjects with diabetes, the levels of sphingosine and dihydrosphingosine in the TRL, LDL, and lipoprotein-depleted fractions were higher, whereas those in the HDL were lower. In addition, the ceramide levels in HDL were lower, whereas those in lipoprotein-depleted fractions were higher. Furthermore, The levels of ceramides in lipoproteins, especially LDL, were negatively associated with the presence of cardiovascular diseases and stage 4 diabetic nephropathy.

CONCLUSIONS

The contents of ceramides and sphingosine in lipoproteins and lipoprotein-depleted fractions were differently modulated in diabetes and associated with cardiovascular diseases and diabetic nephropathy. The carrier might be an important factor for the biological properties and clinical significance of these sphingolipids.

Alterations of the gut microbiota and metabolomics in children with e-waste lead exposure

BACKGROUND

A lead (Pb) exposure can alter the composition and metabolites of gut microbiota. However, few studies investigated this association in the children.

METHODS

A total of 551 children aged 3-7 years were recruited from Guiyu (the e-waste dismantling area) and Haojiang (the reference area). There were finally 70 subjects met the inclusive criteria. Blood and urinary Pb concentrations were detected by GFAAS and ICP-MS techniques. The microbiota and metabolites were measured in stool samples using 16 S rRNA MiSeq sequencing technology and gas chromatography-mass spectrometry (GC-MS), respectively.

RESULTS

Average Pb concentrations in the blood and urine of children were higher in Guiyu than in Haojiang. There were 58 kinds of differential genera and 19 types of discrepant metabolites between the two groups, and wide and significant correlations were found between them. Exposure to Pb caused the most significant differences in microbiota, metabolites, and physical development parameters between the two groups in terms of microbiota, metabolites, and physical development indicators. Sphingolipid metabolism and ion transport may also be altered by Pb exposure.

CONCLUSIONS

Exposure to Pb is associated with significant alterations in the gut microbiota and metabolome in children. More research is needed to confirm the findings of this study.

Contribution of specific ceramides to obesity-associated metabolic diseases

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

Stimulating the expression of sphingosine kinase 1 (SphK1) is beneficial to reduce acrylamide-induced nerve cell damage

Sphingosine kinase 1 (SphK1) is an important signaling molecule for cell proliferation and survival. However, the role of SphK1 in acrylamide (ACR)-induced nerve injury remains unclear. The purpose of this study was to investigate the role and potential mechanism of SphK1 in ACR-induced nerve injury. Liquid chromatography triple quadrupole tandem mass spectrometry (LC-MS/MS) and reverse transcription-quantitative PCR (RT-qPCR) were used to detect sphingosine 1-phosphate (S1P) content in serum and SphK1 content in whole blood from an occupational work group exposed to ACR compared to a non-exposed group. For in vitro experiments, SphK1 in human SH-SY5Y neuroblastoma cells was activated using SphK1-specific activator phorbol 12-myristate 13-acetate (PMA). Our research also utilized cell viability assays, flow cytometry, western blots, RT-qPCR and related protein detection to assess activity of the mitogen activated protein kinase (MAPK) signaling pathway. The results of the population study showed that the contents of SphK1 and S1P in the ACR-exposed occupational contact group were lower than in the non-exposed group. The results of in vitro experiments showed that expression of SphK1 decreased with the increase in ACR concentration. Activating SphK1 improved the survival rate of SH-SY5Y cells and decreased the apoptosis rate. Activating SphK1 in SH-SY5Y cells also regulated MAPK signaling, including enhancing the phosphorylation of extracellular signal-regulated protein kinases (ERK) and inhibiting the phosphorylation of c-Jun N-terminal kinase (JNK) and p38. These results suggest that activating SphK1 can protect against nerve cell damage caused by ACR.

Elucidating the metabolic characteristics of pancreatic β-cells from patients with type 2 diabetes (T2D) using a genome-scale metabolic modeling

Diabetes is a global health problem caused primarily by the inability of pancreatic β-cells to secrete adequate insulin. Despite extensive research, the identity of factors contributing to the dysregulated metabolism-secretion coupling in the β-cells remains elusive. The present study attempts to capture some of these factors responsible for the impaired β-cell metabolism-secretion coupling that contributes to diabetes pathogenesis. The metabolic-flux profiles of pancreatic β-cells were predicted using genome-scale metabolic modeling for ten diabetic patients and ten control subjects. Analysis of these flux states shows reduction in the mitochondrial fatty acid oxidation and mitochondrial oxidative phosphorylation pathways, that leads to decreased insulin secretion in diabetes. We also observed elevated reactive oxygen species (ROS) generation through peroxisomal fatty acid β-oxidation. In addition, cellular antioxidant defense systems were found to be attenuated in diabetes. Our analysis also uncovered the possible changes in the plasma metabolites in diabetes due to the β-cells failure. These efforts subsequently led to the identification of seven metabolites associated with cardiovascular disease (CVD) pathogenesis, thus establishing its link as a secondary complication of diabetes.

To Be or Not to Be: The Divergent Action and Metabolism of Sphingosine-1 Phosphate in Pancreatic Beta-Cells in Response to Cytokines and Fatty Acids

Sphingosine-1 phosphate (S1P) is a bioactive sphingolipid with multiple functions conveyed by the activation of cell surface receptors and/or intracellular mediators. A growing body of evidence indicates its important role in pancreatic insulin-secreting beta-cells that are necessary for maintenance of glucose homeostasis. The dysfunction and/or death of beta-cells lead to diabetes development. Diabetes is a serious public health burden with incidence growing rapidly in recent decades. The two major types of diabetes are the autoimmune-mediated type 1 diabetes (T1DM) and the metabolic stress-related type 2 diabetes (T2DM). Despite many differences in the development, both types of diabetes are characterized by chronic hyperglycemia and inflammation. The inflammatory component of diabetes remains under-characterized. Recent years have brought new insights into the possible mechanism involved in the increased inflammatory response, suggesting that environmental factors such as a westernized diet may participate in this process. Dietary lipids, particularly palmitate, are substrates for the biosynthesis of bioactive sphingolipids. Disturbed serum sphingolipid profiles were observed in both T1DM and T2DM patients. Many polymorphisms were identified in genes encoding enzymes of the sphingolipid pathway, including sphingosine kinase 2 (SK2), the S1P generating enzyme which is highly expressed in beta-cells. Proinflammatory cytokines and free fatty acids have been shown to modulate the expression and activity of S1P-generating and S1P-catabolizing enzymes. In this review, the similarities and differences in the action of extracellular and intracellular S1P in beta-cells exposed to cytokines or free fatty acids will be identified and the outlook for future research will be discussed.

Crisis of the Asian gut: associations among diet, microbiota, and metabolic diseases

The increase of lifestyle-related diseases in Asia has recently become remarkably serious. This has been associated with a change in dietary habits that may alter the complex gut microbiota and its metabolic function in Asian people. Notably, the penetration of modern Western diets into Asia, which has been accompanied by an increase in fat content and decrease in plant-derived dietary fiber, is restructuring the Asian gut microbiome. In this review, we introduce the current status of obesity and diabetes in Asia and discuss the links of changes in dietary style with gut microbiota alterations which may predispose Asian people to metabolic diseases.

Lipid-Induced Adaptations of the Pancreatic Beta-Cell to Glucotoxic Conditions Sustain Insulin Secretion

Over the last decades, lipotoxicity and glucotoxicity emerged as established mechanisms participating in the pathophysiology of obesity-related type 2 diabetes in general, and in the loss of β-cell function in particular. However, these terms hold various potential biological processes, and it is not clear what precisely they refer to and to what extent they might be clinically relevant. In this review, we discuss the basis and the last advances of research regarding the role of free fatty acids, their metabolic intracellular pathways, and receptor-mediated signaling related to glucose-stimulated insulin secretion, as well as lipid-induced β-cell dysfunction. We also describe the role of chronically elevated glucose, namely, glucotoxicity, which promotes failure and dedifferentiation of the β cell. Glucolipotoxicity combines deleterious effects of exposures to both high glucose and free fatty acids, supposedly provoking synergistic defects on the β cell. Nevertheless, recent studies have highlighted the glycerolipid/free fatty acid cycle as a protective pathway mediating active storage and recruitment of lipids. Finally, we discuss the putative correspondence of the loss of functional β cells in type 2 diabetes with a natural, although accelerated, aging process.

Predictive values of serum metabolites in early pregnancy and their possible pathways for gestational diabetes: A nested case-control study in Tianjin, China

AIMS

To investigate the associations and predictive values of serum metabolites in early pregnancy for later development of gestational diabetes mellitus (GDM), and further explore their metabolic pathways to GDM.

METHODS

We conducted a 1:1 nested case-control study including 486 pregnant women from Tianjin, China, and collected blood samples at their first registration (median at 10th gestational week). Liquid chromatography-tandem mass spectrometry was used to measure serum metabolites. Orthogonal partial least squares discriminant analysis was used to select specific metabolites associated with GDM, and pathway analysis was used to identify the metabolic pathways related to GDM.

RESULTS

A total of 64 serum metabolites were included in this analysis, 17 of which were identified as specific metabolites associated with GDM. Ten metabolites increased and seven metabolites decreased GDM risk. Inclusion of these specific metabolites to the model of traditional risk factors greatly increased the predictive value from 0.69 (95% confidence interval: 0.64-0.74) to 0.92 (0.90-0.95). In addition, we found that glycerophospholipid metabolism, sphingolipid metabolism and primary bile acid biosynthesis were main metabolic pathways related to GDM.

CONCLUSION

We identified a set of serum metabolites and their metabolic pathways in early pregnancy associated with GDM, which provided a theoretical basis for further research on the molecular pathways to GDM and early identification of GDM.

Alterations in β-Cell Sphingolipid Profile Associated with ER Stress and iPLA2β: Another Contributor to β-Cell Apoptosis in Type 1 Diabetes

Type 1 diabetes (T1D) development, in part, is due to ER stress-induced β-cell apoptosis. Activation of the Ca²⁺-independent phospholipase A₂ beta (iPLA₂β) leads to the generation of pro-inflammatory eicosanoids, which contribute to β-cell death and T1D. ER stress induces iPLA₂β-mediated generation of pro-apoptotic ceramides via neutral sphingomyelinase (NSMase). To gain a better understanding of the impact of iPLA₂β on sphingolipids (SLs), we characterized their profile in β-cells undergoing ER stress. ESI/MS/MS analyses followed by ANOVA/Student’s t-test were used to assess differences in sphingolipids molecular species in Vector (V) control and iPLA₂β-overexpressing (OE) INS-1 and Akita (AK, spontaneous model of ER stress) and WT-littermate (AK-WT) β-cells. As expected, iPLA₂β induction was greater in the OE and AK cells in comparison with V and WT cells. We report here that ER stress led to elevations in pro-apoptotic and decreases in pro-survival sphingolipids and that the inactivation of iPLA₂β restores the sphingolipid species toward those that promote cell survival. In view of our recent finding that the SL profile in macrophages—the initiators of autoimmune responses leading to T1D—is not significantly altered during T1D development, we posit that the iPLA₂β-mediated shift in the β-cell sphingolipid profile is an important contributor to β-cell death associated with T1D.

Plasma triacylglycerols are biomarkers of β-cell function in mice and humans

Objectives

To find plasma biomarkers prognostic of type 2 diabetes, which could also inform on pancreatic β-cell deregulations or defects in the function of insulin target tissues.

Methods

We conducted a systems biology approach to characterize the plasma lipidomes of C57Bl/6J, DBA/2J, and BALB/cJ mice under different nutritional conditions, as well as their pancreatic islet and liver transcriptomes. We searched for correlations between plasma lipids and tissue gene expression modules.

Results

We identified strong correlation between plasma triacylglycerols (TAGs) and islet gene modules that comprise key regulators of glucose- and lipid-regulated insulin secretion and of the insulin signaling pathway, the two top hits were Gck and Abhd6 for negative and positive correlations, respectively. Correlations were also found between sphingomyelins and islet gene modules that overlapped in part with the gene modules correlated with TAGs. In the liver, the gene module most strongly correlated with plasma TAGs was enriched in mRNAs encoding fatty acid and carnitine transporters as well as multiple enzymes of the β-oxidation pathway. In humans, plasma TAGs also correlated with the expression of several of the same key regulators of insulin secretion and the insulin signaling pathway identified in mice. This cross-species comparative analysis further led to the identification of PITPNC1 as a candidate regulator of glucose-stimulated insulin secretion.

Conclusion

TAGs emerge as biomarkers of a liver-to-β-cell axis that links hepatic β-oxidation to β-cell functional mass and insulin secretion.

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Plasma triacylglycerols correlated with genes controlling β-cell mass and function.

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Plasma triacylglycerols correlated with genes controlling liver β-oxidation.

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In humans, triacylglycerols also correlated with key regulators of insulin secretion.

•

Mouse and human data identified PITPNC1 as a candidate regulator of insulin secretion.

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Triacylglycerols are biomarkers of the liver-to-β-cell axis and β-cell function.

Sphingosine-1 Phosphate Lyase Regulates Sensitivity of Pancreatic Beta-Cells to Lipotoxicity

Elevated levels of free fatty acids (FFAs) have been related to pancreatic beta-cell failure in type 2 diabetes (T2DM), though the underlying mechanisms are not yet fully understood. FFAs have been shown to dysregulate formation of bioactive sphingolipids, such as ceramides and sphingosine-1 phosphate (S1P) in beta-cells. The aim of this study was to analyze the role of sphingosine-1 phosphate lyase (SPL), a key enzyme of the sphingolipid pathway that catalyzes an irreversible degradation of S1P, in the sensitivity of beta-cells to lipotoxicity. To validate the role of SPL in lipotoxicity, we modulated SPL expression in rat INS1E cells and in human EndoC-βH1 beta-cells. SPL overexpression in INS1E cells (INS1E-SPL), which are characterized by a moderate basal expression level of SPL, resulted in an acceleration of palmitate-mediated cell viability loss, proliferation inhibition and induction of oxidative stress. SPL overexpression affected the mRNA expression of ER stress markers and mitochondrial chaperones. In contrast to control cells, in INS1E-SPL cells no protective effect of oleate was detected. Moreover, Plin2 expression and lipid droplet formation were strongly reduced in OA-treated INS1E-SPL cells. Silencing of SPL in human EndoC-βH1 beta-cells, which are characterized by a significantly higher SPL expression as compared to rodent beta-cells, resulted in prevention of FFA-mediated caspase-3/7 activation. Our findings indicate that an adequate control of S1P degradation by SPL might be crucially involved in the susceptibility of pancreatic beta-cells to lipotoxicity.

Metachromatic Leukodystrophy: Diagnosis and Treatment Challenges

Metachromatic leukodystrophy is a neurological disease of the lysosomal deposit that has a significant impact given the implications for the neurodegenerative deterioration of the patient. Currently, there is no treatment available that reverses the development of characteristic neurological and systemic symptoms. Objective. Carry out an updated bibliographic search on the most critical advances in the treatment and diagnosis for LDM. A retrospective topic review published in English and Spanish in the Orphanet and Pubmed databases. Current treatment options, such as enzyme replacement therapy and hematopoietic stem cell transplantation aimed at decreasing the rapid progression of the disease, improving patient survival; however, these are costly. The pathophysiological events of intracellular signaling related to the deficiency of the enzyme Arylsulfatase A and subsequent accumulation of sulphatides and glycosylated ceramides have not yet been established. Recently, the accumulation of C16 sulphatides has been shown to inhibit glycolysis and insulin secretion in pancreatic cells. The significant advance in technology has allowed timely diagnosis in patients suffering from LDM; however, they still do not have an effective treatment.

Neutral sphingomyelinase-2 and cardiometabolic diseases

Sphingolipids, in particular ceramides, play vital role in pathophysiological processes linked to metabolic syndrome, with implications in the development of insulin resistance, pancreatic ß-cell dysfunction, type 2 diabetes, atherosclerosis, inflammation, nonalcoholic steatohepatitis, and cancer. Ceramides are produced by the hydrolysis of sphingomyelin, catalyzed by different sphingomyelinases, including neutral sphingomyelinase 2 (nSMase2), whose dysregulation appears to underlie many of the inflammation-related pathologies. In this review, we discuss the current knowledge on the biochemistry of nSMase2 and ceramide production and its regulation by inflammatory cytokines, with particular reference to cardiometabolic diseases. nSMase2 contribution to pathogenic processes appears to involve cyclical feed-forward interaction with proinflammatory cytokines, such as TNF-α and IL-1ß, which activate nSMase2 and the production of ceramides, that in turn triggers the synthesis and release of inflammatory cytokines. We elaborate these pathogenic interactions at the molecular level and discuss the potential therapeutic benefits of inhibiting nSMase2 against inflammation-driven cardiometabolic diseases.

CD36 Signal Transduction in Metabolic Diseases: Novel Insights and Therapeutic Targeting

The cluster of differentiation 36 (CD36) is a scavenger receptor present on various types of cells and has multiple biological functions that may be important in inflammation and in the pathogenesis of metabolic diseases, including diabetes. Here, we consider recent insights into how the CD36 response becomes deregulated under metabolic conditions, as well as the therapeutic benefits of CD36 inhibition, which may provide clues for developing strategies aimed at the treatment or prevention of diabetes associated with metabolic diseases. To facilitate this process further, it is important to pinpoint regulatory mechanisms that are relevant under physiological and pathological conditions. In particular, understanding the mechanisms involved in dictating specific CD36 downstream cellular outcomes will aid in the discovery of potent compounds that target specific CD36 downstream signaling cascades.

MicroRNA Sequences Modulated by Beta Cell Lipid Metabolism: Implications for Type 2 Diabetes Mellitus

Alterations in lipid metabolism within beta cells and islets contributes to dysfunction and apoptosis of beta cells, leading to loss of insulin secretion and the onset of type 2 diabetes. Over the last decade, there has been an explosion of interest in understanding the landscape of gene expression which influences beta cell function, including the importance of small non-coding microRNA sequences in this context. This review sought to identify the microRNA sequences regulated by metabolic challenges in beta cells and islets, their targets, highlight their function and assess their possible relevance as biomarkers of disease progression in diabetic individuals. Predictive analysis was used to explore networks of genes targeted by these microRNA sequences, which may offer new therapeutic strategies to protect beta cell function and delay the onset of type 2 diabetes.

Molecular mechanisms of lipotoxicity-induced pancreatic β-cell dysfunction

Type 2 diabetes (T2D), a heterogeneous disorder derived from metabolic dysfunctions, leads to a glucose overflow in the circulation due to both defective insulin secretion and peripheral insulin resistance. One of the critical risk factor for T2D is obesity, which represents a global epidemic that has nearly tripled since 1975. Obesity is characterized by chronically elevated free fatty acid (FFA) levels, which cause deleterious effects on glucose homeostasis referred to as lipotoxicity. Here, we review the physiological FFA roles onto glucose-stimulated insulin secretion (GSIS) and the pathological ones affecting many steps of the mechanisms and modulation of GSIS. We also describe in vitro and in vivo experimental evidences addressing lipotoxicity in β-cells and the role of saturation and chain length of FFA on the potency of GSIS stimulation. The molecular mechanisms underpinning lipotoxic-β-cell dysfunction are also reviewed. Among them, endoplasmic reticulum stress, oxidative stress and mitochondrial dysfunction, inflammation, impaired autophagy and β-cell dedifferentiation. Finally therapeutic strategies for the β-cells dysfunctions such as the use of metformin, glucagon-like peptide 1, thiazolidinediones, anti-inflammatory drugs, chemical chaperones and weight are discussed.

Dietary Fat and Protein Intake in Relation to Plasma Sphingolipids as Determined by a Large-Scale Lipidomic Analysis

Sphingolipid concentrations have been associated with risk of type 2 diabetes and cardiovascular diseases. Because sphingolipids can be synthesized de novo from saturated fatty acids (SFA), dietary fatty acids may affect plasma sphingolipid concentrations. We aimed to evaluate dietary fat and protein intakes in relation to circulating sphingolipid levels. We used cross-sectional data from 2860 ethnic Chinese Singaporeans collected from 2004–2007. Nutrient intakes were estimated on the basis of a validated 159-item food frequency questionnaire. We quantified 79 molecularly distinct sphingolipids in a large-scale lipidomic evaluation from plasma samples. Higher saturated fat intake was associated with higher concentrations of 16:1;O2 sphingolipids including ceramides, monohexosylcermides, dihexosylceramides, sphingomyelins, and sphingosine 1-phosphates. Higher polyunsaturated fat intake was associated with lower plasma long-chain ceramides and long-chain monohexosylcermide concentrations. Protein intake was inversely associated with concentrations of most subclasses of sphingolipids, with the exception of sphingolipids containing a 16:1;O2 sphingoid base. Lower intake of saturated fat and higher intake of polyunsaturated fat and protein may decrease plasma concentrations of several sphingolipid classes. These findings may represent a novel biological mechanism for the impact of nutrient intakes on cardio-metabolic health.

Emerging role of protein kinases in diabetes mellitus: From mechanism to therapy

Diabetes mellitus has emerged as a severe burden on the medical health system across the globe. Presently, around 422 million people are suffering from diabetes which is speculated to be expanded to about 600 million by 2035. Patients with type 2 diabetes are at increased risk of developing detrimental metabolic and cardiovascular complications. The scientific understanding of this chronic disease and its underlying root cause is not yet fully unraveled. Protein kinases are well known to regulate almost every cellular process through phosphorylation of target protein in diverse signaling pathways. The important role of several protein kinases including AMP-activated protein kinase, IκB kinase and protein kinase C have been well demonstrated in various animal models. They modulate glucose tolerance, inflammation and insulin resistance in the cells via acting on diverse downstream targets and signaling pathways. Thus, modulating the activity of potential human kinases which are significantly involved in diabetes by targeting with small molecule inhibitors could be an attractive therapeutic strategy to tackle diabetes. In this chapter, we have discussed the potential role of protein kinases in glucose metabolism and insulin sensitivity, and in the pathogenesis of diabetes mellitus. Furthermore, the small molecules reported in the literature that can be potentially used for the treatment of diabetes have been discussed in detail.

Ceramides in Metabolism: Key Lipotoxic Players

The global prevalence of metabolic diseases such as type 2 diabetes mellitus, steatohepatitis, myocardial infarction, and stroke has increased dramatically over the past two decades. These obesity-fueled disorders result, in part, from the aberrant accumulation of harmful lipid metabolites in tissues not suited for lipid storage (e.g., the liver, vasculature, heart, and pancreatic beta-cells). Among the numerous lipid subtypes that accumulate, sphingolipids such as ceramides are particularly impactful, as they elicit the selective insulin resistance, dyslipidemia, and ultimately cell death that underlie nearly all metabolic disorders. This review summarizes recent findings on the regulatory pathways controlling ceramide production, the molecular mechanisms linking the lipids to these discrete pathogenic events, and exciting attempts to develop therapeutics to reduce ceramide levels to combat metabolic disease.

Sphingolipids in Type 1 Diabetes: Focus on Beta-Cells

Type 1 diabetes (T1DM) is a chronic autoimmune disease, with a strong genetic background, leading to a gradual loss of pancreatic beta-cells, which secrete insulin and control glucose homeostasis. Patients with T1DM require life-long substitution with insulin and are at high risk for development of severe secondary complications. The incidence of T1DM has been continuously growing in the last decades, indicating an important contribution of environmental factors. Accumulating data indicates that sphingolipids may be crucially involved in T1DM development. The serum lipidome of T1DM patients is characterized by significantly altered sphingolipid composition compared to nondiabetic, healthy probands. Recently, several polymorphisms in the genes encoding the enzymatic machinery for sphingolipid production have been identified in T1DM individuals. Evidence gained from studies in rodent islets and beta-cells exposed to cytokines indicates dysregulation of the sphingolipid biosynthetic pathway and impaired function of several sphingolipids. Moreover, a number of glycosphingolipids have been suggested to act as beta-cell autoantigens. Studies in animal models of autoimmune diabetes, such as the Non Obese Diabetic (NOD) mouse and the LEW.1AR1-iddm (IDDM) rat, indicate a crucial role of sphingolipids in immune cell trafficking, islet infiltration and diabetes development. In this review, the up-to-date status on the findings about sphingolipids in T1DM will be provided, the under-investigated research areas will be identified and perspectives for future studies will be given.

Differential Expression of Ormdl Genes in the Islets of Mice and Humans with Obesity

The orosomucoid-like (Ormdl) proteins play a critical role in sphingolipid homeostasis, inflammation, and ER stress, all of which are associated with obesity and βcell dysfunction. However, their roles in β cells and obesity remain unknown. Here, we show that islets from overweight/obese human donors displayed marginally reduced ORMDL1-2 expression, whereas ORMDL3 expression was significantly downregulated compared with islets from lean donors. In contrast, Ormdl3 was substantially upregulated in the islets of leptin-deficient obese (ob/ob) mice compared with lean mice. Treatment of ob/ob mice and their islets with leptin markedly reduced islet Ormld3 expression. Ormdl3 knockdown in a β cell line induced expression of pro-apoptotic markers, which was rescued by ceramide synthase inhibitor fumonisin B1. Our results reveal differential expression of Ormdl3 in the islets of a mouse model and humans with obesity, highlight the potential effect of leptin in this differential regulation, and suggest a role for Ormdl3 in β cell apoptosis.

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Islets of overweight/obese human donors display markedly reduced ORMDL3 expression

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Ormdl3 expression was significantly upregulated in the islets of ob/ob mice

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Leptin treatment markedly reduced Ormld3 expression in the islets of ob/ob mice

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Fumonisin B1 restores increased apoptotic marker levels induced by Ormdl3 silencing