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

L-serine: a neglected amino acid with a potential therapeutic role in diabetes

L-serine is classified as a non-essential amino acid; however, L-serine is indispensable having a central role in a broad range of cellular processes. Growing evidence suggests a role for L-serine in the development of diabetes mellitus and its related complications, with L-serine being positively correlated to insulin secretion and sensitivity. L-serine metabolism is altered in type 1, type 2, and gestational diabetes, and L-serine supplementations improve glucose homeostasis and mitochondrial function, and reduce neuronal death. Additionally, L-serine lowers the incidence of autoimmune diabetes in NOD mice. Dietary supplementations of L-serine are generally regarded as safe (GRAS) by the FDA. Therefore, we believe that L-serine should be considered as an emerging therapeutic option in diabetes, although work remains in order to fully understand the role of L-serine in diabetes.

Sphingolipids and the unfolded protein response

The unfolded protein response (UPR) is a response by the endoplasmic reticulum to stress, classically caused by any disruption to cell homeostasis that results in an accumulation in unfolded proteins. However, there is an increasing body of research demonstrating that the UPR can also be activated by changes in lipid homeostasis, including changes in sphingolipid metabolism. Sphingolipids are a family of bioactive lipids with important roles in both the formation and integrity of cellular membranes, and regulation of key cellular processes, including cell proliferation and apoptosis. Bi-directional interactions between sphingolipids and the UPR have now been observed in a range of diseases, including cancer, diabetes and liver disease. Determining how these two key cellular components influence each other could play an important role in deciphering the causes of these diseases and potentially reveal new therapeutic approaches.

Imaging mass spectrometry enables molecular profiling of mouse and human pancreatic tissue

AIMS/HYPOTHESIS

The molecular response and function of pancreatic islet cells during metabolic stress is a complex process. The anatomical location and small size of pancreatic islets coupled with current methodological limitations have prevented the achievement of a complete, coherent picture of the role that lipids and proteins play in cellular processes under normal conditions and in diseased states. Herein, we describe the development of untargeted tissue imaging mass spectrometry (IMS) technologies for the study of in situ protein and, more specifically, lipid distributions in murine and human pancreases.

METHODS

We developed matrix-assisted laser desorption/ionisation (MALDI) IMS protocols to study metabolite, lipid and protein distributions in mouse (wild-type and ob/ob mouse models) and human pancreases. IMS allows for the facile discrimination of chemically similar lipid and metabolite isoforms that cannot be distinguished using standard immunohistochemical techniques. Co-registration of MS images with immunofluorescence images acquired from serial tissue sections allowed accurate cross-registration of cell types. By acquiring immunofluorescence images first, this serial section approach guides targeted high spatial resolution IMS analyses (down to 15 μm) of regions of interest and leads to reduced time requirements for data acquisition.

RESULTS

MALDI IMS enabled the molecular identification of specific phospholipid and glycolipid isoforms in pancreatic islets with intra-islet spatial resolution. This technology shows that subtle differences in the chemical structure of phospholipids can dramatically affect their distribution patterns and, presumably, cellular function within the islet and exocrine compartments of the pancreas (e.g. 18:1 vs 18:2 fatty acyl groups in phosphatidylcholine lipids). We also observed the localisation of specific GM3 ganglioside lipids [GM3(d34:1), GM3(d36:1), GM3(d38:1) and GM3(d40:1)] within murine islet cells that were correlated with a higher level of GM3 synthase as verified by immunostaining. However, in human pancreas, GM3 gangliosides were equally distributed in both the endocrine and exocrine tissue, with only one GM3 isoform showing islet-specific localisation.

CONCLUSIONS/INTERPRETATION

The development of more complete molecular profiles of pancreatic tissue will provide important insight into the molecular state of the pancreas during islet development, normal function, and diseased states. For example, this study demonstrates that these results can provide novel insight into the potential signalling mechanisms involving phospholipids and glycolipids that would be difficult to detect by targeted methods, and can help raise new hypotheses about the types of physiological control exerted on endocrine hormone-producing cells in islets. Importantly, the in situ measurements afforded by IMS do not require a priori knowledge of molecules of interest and are not susceptible to the limitations of immunohistochemistry, providing the opportunity for novel biomarker discovery. Notably, the presence of multiple GM3 isoforms in mouse islets and the differential localisation of lipids in human tissue underscore the important role these molecules play in regulating insulin modulation and suggest species, organ, and cell specificity. This approach demonstrates the importance of both high spatial resolution and high molecular specificity to accurately survey the molecular composition of complex, multi-functional tissues such as the pancreas.

Perfluoroalkyl substances, metabolomic profiling, and alterations in glucose homeostasis among overweight and obese Hispanic children: A proof-of-concept analysis

OBJECTIVE

To examine the prospective associations between exposure to perfluoroalkyl substances (PFASs) and longitudinal measurements of glucose metabolism in high-risk overweight and obese Hispanic children.

METHODS

Forty overweight and obese Hispanic children (8-14 years) from urban Los Angeles underwent clinical measures and 2-hour oral glucose tolerance tests (OGTT) at baseline and a follow-up visit (range: 1-3 years after enrollment). Baseline plasma perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS), perfluorohexane sulfonic acid (PFHxS), and the plasma metabolome were measured by liquid-chromatography with high-resolution mass spectrometry. Multiple linear regression models were used to assess the association between baseline PFASs and changes in glucose homeostasis over follow-up. A metabolome-wide association study coupled with pathway enrichment analysis was performed to evaluate metabolic dysregulation associated with plasma PFASs concentrations. We performed a structural integrated analysis aiming to characterize the joint impact of all factors and to identify latent clusters of children with alterations in glucose homeostasis, based on their exposure and metabolomics profile.

RESULTS

Each ln (ng/ml) increase in PFOA and PFHxS concentrations was associated with a 30.6 mg/dL (95% CI: 8.8-52.4) and 10.2 mg/dL (95% CI: 2.7-17.7) increase in 2-hour glucose levels, respectively. A ln (ng/ml) increase in PFHxS concentrations was also associated with 17.8 mg/dL increase in the glucose area under the curve (95% CI: 1.5-34.1). Pathway enrichment analysis showed significant alterations of lipids (e.g., glycosphingolipids, linoleic acid, and de novo lipogenesis), and amino acids (e.g., aspartate and asparagine, tyrosine, arginine and proline) in association to PFASs exposure. The integrated analysis identified a cluster of children with increased 2-h glucose levels over follow up, characterized by increased PFAS levels and altered metabolite patterns.

CONCLUSIONS

This proof-of-concept analysis shows that higher PFAS exposure was associated with dysregulation of several lipid and amino acid pathways and longitudinal alterations in glucose homeostasis in Hispanic youth. Larger studies are needed to confirm these findings and fully elucidate the underlying biological mechanisms.

Connecting pancreatic islet lipid metabolism with insulin secretion and the development of type 2 diabetes

Obesity is the major contributing factor for the increased prevalence of type 2 diabetes (T2D) in recent years. Sustained positive influx of lipids is considered to be a precipitating factor for beta cell dysfunction and serves as a connection between obesity and T2D. Importantly, fatty acids (FA), a key building block of lipids, are a double-edged sword for beta cells. FA acutely increase glucose-stimulated insulin secretion through cell-surface receptor and intracellular pathways. However, chronic exposure to FA, combined with elevated glucose, impair the viability and function of beta cells in vitro and in animal models of obesity (glucolipotoxicity), providing an experimental basis for the propensity of beta cell demise under obesity in humans. To better understand the two-sided relationship between lipids and beta cells, we present a current view of acute and chronic handling of lipids by beta cells and implications for beta cell function and health. We also discuss an emerging role for lipid droplets (LD) in the dynamic regulation of lipid metabolism in beta cells and insulin secretion, along with a potential role for LD under nutritional stress in beta cells, and incorporate recent advancement in the field of lipid droplet biology.

Targeting Type 1 Diabetes: Selective Approaches for New Therapies

The clinical onset of type 1 diabetes is characterized by the destruction of the insulin-producing β cells of the pancreas and is caused by autoantigen-induced inflammation (insulitis) of the islets of Langerhans. The current standard of care for type 1 diabetes mellitus patients allows for management of the disease with exogenous insulin, but patients eventually succumb to many chronic complications such as limb amputation, blindness, and kidney failure. New therapeutic approaches now on the horizon are looking beyond glycemic management and are evaluating new strategies from protecting and regenerating endogenous islets to treating the underlying autoimmunity through selective modulation of key immune cell populations. Currently, there are no effective treatments for the autoimmunity that causes the disease, and strategies that aim to delay or prevent the onset of the disease will play an important role in the future of diabetes research. In this review, we summarize many of the key efforts underway that utilize molecular approaches to selectively modulate this disease and look at new therapeutic paradigms that can transform clinical treatment.

The Role of S1P and the Related Signaling Pathway in the Development of Tissue Fibrosis

Tissue fibrosis, including pulmonary fibrosis, hepatic fibrosis, and cardiac fibrosis, is an important stage in the development of many diseases. It can lead to structural damage and dysfunction and even severe carcinogenesis or death. There is currently no effective method for the treatment of fibrosis. At present, the molecular mechanism of tissue fibrosis has not yet been fully elucidated, but many studies have demonstrated that it is involved in conveying the complex messages between fibroblasts and various cytokines. Sphingosine 1-phosphate (S1P) is a naturally bioactive sphingolipid. S1P and the related signaling pathways are important intracellular metabolic pathways involved in many life activities, including cell proliferation, differentiation, apoptosis, and cellular signal transduction. Increasing evidence suggests that S1P and its signaling pathways play an important role in the development of tissue fibrosis; however, the mechanisms of these effects have not yet been fully elucidated, and even the role of S1P and its signaling pathways are still controversial. This article focuses on the role of S1P and the related signaling pathways in the development of fibrosis of lung, liver, heart, and other tissues, with emphasis on the application of inhibitors of some of molecules in the pathway in clinical treatment of fibrosis diseases.

MANF Is Required for the Postnatal Expansion and Maintenance of Pancreatic β-Cell Mass in Mice

Global lack of mesencephalic astrocyte-derived neurotropic factor (MANF) leads to progressive postnatal loss of β-cell mass and insulin-dependent diabetes in mice. Similar to Manf^-/- mice, embryonic ablation of MANF specifically from the pancreas results in diabetes. In this study, we assessed the importance of MANF for the postnatal expansion of pancreatic β-cell mass and for adult β-cell maintenance in mice. Detailed analysis of Pdx-1Cre^+/- ::Manf^fl/fl mice revealed mosaic MANF expression in postnatal pancreata and a significant correlation between the number of MANF-positive β-cells and β-cell mass in individual mice. In vitro, recombinant MANF induced β-cell proliferation in islets from aged mice and protected from hyperglycemia-induced endoplasmic reticulum (ER) stress. Consequently, excision of MANF from β-cells of adult MIP-1Cre^ERT::Manf^fl/fl mice resulted in reduced β-cell mass and diabetes caused largely by β-cell ER stress and apoptosis, possibly accompanied by β-cell dedifferentiation and reduced rates of β-cell proliferation. Thus, MANF expression in adult mouse β-cells is needed for their maintenance in vivo. We also revealed a mechanistic link between ER stress and inflammatory signaling pathways leading to β-cell death in the absence of MANF. Hence, MANF might be a potential target for regenerative therapy in diabetes.

Plasma lipid species at type 1 diabetes onset predict residual beta-cell function after 6 months

INTRODUCTION

The identification of metabolomic dysregulation appears promising for the prediction of type 1 diabetes and may also reveal metabolic pathways leading to beta-cell destruction. Recent studies indicate that regulation of multiple phospholipids precede the presence of autoantigens in the development of type 1 diabetes.

OBJECTIVES

We hypothesize that lipid biomarkers in plasma from children with recent onset type 1 diabetes will reflect their remaining beta-cell function and predict future changes in beta-cell function.

METHODS

We performed targeted lipidomic profiling by electrospray ionization tandem mass spectrometry to acquire comparative measures of 354 lipid species covering 25 lipid classes and subclasses in plasma samples from 123 patients < 17 years of age followed prospectively at 1, 3, 6 and 12 months after diagnosis. Lipidomic profiles were analysed using liner regression to investigate the relationship between plasma lipids and meal stimulated C-peptide levels at each time point. P-values were corrected for multiple comparisons by the method of Benjamini and Hochberg.

RESULTS

Linear regression analysis showed that the relative levels of cholesteryl ester, diacylglycerol and triacylglycerol at 1 month were associated to the change in c-peptide levels from 1 to 6 months (corrected p-values of 4.06E-03, 1.72E-02 and 1.72E02, respectively). Medium chain saturated and monounsaturated fatty acids were the major constituents of the di- and triacylglycerol species suggesting a link with increased lipogenesis.

CONCLUSION

These observations support the hypothesis of lipid disturbances as explanatory factors for residual beta-cell function in children with new onset type 1 diabetes.

Emerging Roles for Mesencephalic Astrocyte-Derived Neurotrophic Factor (MANF) in Pancreatic Beta Cells and Diabetes

Mesencephalic astrocyte-derived neurotrophic factor (MANF) was originally identified as a secreted trophic factor for dopamine neurons in vitro. It protects and restores damaged cells in rodent models of Parkinson's disease, brain and heart ischemia, spinocerebellar ataxia and retina in vivo. However, its exact mechanism of action is not known. MANF is widely expressed in most human and mouse organs with high levels in secretory tissues. Intracellularly, MANF localizes to the endoplasmic reticulum (ER) and ER stress increases it's expression in cells and tissues. Furthermore, increased MANF levels has been detected in the sera of young children with newly diagnosed Type 1 (T1D) diabetes and Type 2 (T2D) diabetic patients. ER stress is caused by the accumulation of misfolded and aggregated proteins in the ER. It activates a cellular defense mechanism, the unfolded protein response (UPR), a signaling cascade trying to restore ER homeostasis. However, if prolonged, unresolved ER stress leads to apoptosis. Unresolved ER stress contributes to the progressive death of pancreatic insulin-producing beta cells in both T1D and T2D. Diabetes mellitus is characterized by hyperglycemia, caused by the inability of the beta cells to maintain sufficient levels of circulating insulin. The current medications, insulin and antidiabetic drugs, alleviate diabetic symptoms but cannot reconstitute physiological insulin secretion which increases the risk of devastating vascular complications of the disease. Thus, one of the main strategies in improving current diabetes therapy is to define and validate novel approaches to protect beta cells from stress as well as activate their regeneration. Embryonic deletion of the Manf gene in mice led to gradual postnatal development of insulin-deficient diabetes caused by reduced beta cell proliferation and increased beta cell death due to increased and sustained ER stress. In vitro, recombinant MANF partly protected mouse and human beta cells from ER stress-induced beta cell death and potentiated mouse and human beta cell proliferation. Importantly, in vivo overexpression of MANF in the pancreas of T1D mice led to increased beta cell proliferation and decreased beta cell death, suggesting that MANF could be a new therapeutic candidate for beta cell protection and regeneration in diabetes.

Abnormal islet sphingolipid metabolism in type 1 diabetes

AIMS/HYPOTHESIS

Sphingolipids play important roles in beta cell physiology, by regulating proinsulin folding and insulin secretion and in controlling apoptosis, as studied in animal models and cell cultures. Here we investigate whether sphingolipid metabolism may contribute to the pathogenesis of human type 1 diabetes and whether increasing the levels of the sphingolipid sulfatide would prevent models of diabetes in NOD mice.

METHODS

We examined the amount and distribution of sulfatide in human pancreatic islets by immunohistochemistry, immunofluorescence and electron microscopy. Transcriptional analysis was used to evaluate expression of sphingolipid-related genes in isolated human islets. Genome-wide association studies (GWAS) and a T cell proliferation assay were used to identify type 1 diabetes related polymorphisms and test how these affect cellular islet autoimmunity. Finally, we treated NOD mice with fenofibrate, a known activator of sulfatide biosynthesis, to evaluate the effect on experimental autoimmune diabetes development.

RESULTS

We found reduced amounts of sulfatide, 23% of the levels in control participants, in pancreatic islets of individuals with newly diagnosed type 1 diabetes, which were associated with reduced expression of enzymes involved in sphingolipid metabolism. Next, we discovered eight gene polymorphisms (ORMDL3, SPHK2, B4GALNT1, SLC1A5, GALC, PPARD, PPARG and B4GALT1) involved in sphingolipid metabolism that contribute to the genetic predisposition to type 1 diabetes. These gene polymorphisms correlated with the degree of cellular islet autoimmunity in a cohort of individuals with type 1 diabetes. Finally, using fenofibrate, which activates sulfatide biosynthesis, we completely prevented diabetes in NOD mice and even reversed the disease in half of otherwise diabetic animals.

CONCLUSIONS/INTERPRETATION

These results indicate that islet sphingolipid metabolism is abnormal in type 1 diabetes and suggest that modulation may represent a novel therapeutic approach.

DATA AVAILABILITY

The RNA expression data is available online at https://www.dropbox.com/s/93mk5tzl5fdyo6b/Abnormal%20islet%20sphingolipid%20metabolism%20in%20type%201%20diabetes%2C%20RNA%20expression.xlsx?dl=0 . A list of SNPs identified is available at https://www.dropbox.com/s/yfojma9xanpp2ju/Abnormal%20islet%20sphingolipid%20metabolism%20in%20type%201%20diabetes%20SNP.xlsx?dl=0 .

Fatty Acid-Stimulated Insulin Secretion vs. Lipotoxicity

Fatty acid (FA)-stimulated insulin secretion (FASIS) is reviewed here in contrast to type 2 diabetes etiology, resulting from FA overload, oxidative stress, intermediate hyperinsulinemia, and inflammation, all converging into insulin resistance. Focusing on pancreatic islet β-cells, we compare the physiological FA roles with the pathological ones. Considering FAs not as mere amplifiers of glucose-stimulated insulin secretion (GSIS), but as parallel insulin granule exocytosis inductors, partly independent of the K_ATP channel closure, we describe the FA initiating roles in the prediabetic state that is induced by retardations in the glycerol-3-phosphate (glucose)-promoted glycerol/FA cycle and by the impaired GPR40/FFA1 (free FA1) receptor pathway, specifically in its amplification by the redox-activated mitochondrial phospholipase, iPLA2γ. Also, excessive dietary FAs stimulate intestine enterocyte incretin secretion, further elevating GSIS, even at low glucose levels, thus contributing to diabetic hyperinsulinemia. With overnutrition and obesity, the FA overload causes impaired GSIS by metabolic dysbalance, paralleled by oxidative and metabolic stress, endoplasmic reticulum stress and numerous pro-apoptotic signaling, all leading to decreased β-cell survival. Lipotoxicity is exerted by saturated FAs, whereas ω-3 polyunsaturated FAs frequently exert antilipotoxic effects. FA-facilitated inflammation upon the recruitment of excess M1 macrophages into islets (over resolving M2 type), amplified by cytokine and chemokine secretion by β-cells, leads to an inevitable failure of pancreatic β-cells.

RACking up ceramide-induced islet β-cell dysfunction

The International Diabetes Federation predicts that by 2045 the number of individuals afflicted with diabetes will increase to 629 million. Furthermore, ∼352 million individuals with impaired glucose tolerance are at increased risk for developing diabetes. Several mechanisms have been proposed for the onset of metabolic dysfunction and demise of the islet β-cell leading to the pathogenesis of diabetes. It is widely accepted that the onset of type 2 diabetes is due to an intricate interplay between genetic expression of the disease and a multitude of factors including increased oxidative and endoplasmic reticulum stress consequential to glucolipotoxicity and inflammation. Compelling experimental evidence from in vitro and in vivo studies implicates intracellular generation of ceramide (CER), a biologically-active sphingolipid, as a trigger in the onset of β-cell demise under above pathological conditions. Recent pharmacological and molecular biological evidence affirms regulatory roles for Ras-related C3 botulinum toxin substrate 1 (Rac1), a small G protein, in the islet β-cell function in health and diabetes. In this Commentary, we overviewed the emerging evidence implicating potential cross-talk between Rac1 and ceramide signaling pathways in the onset of metabolic dysregulation of the islet β-cell culminating in impaired physiological insulin secretion, loss of β-cell mass and the onset of diabetes. Further, we propose a model depicting contributory roles of defective protein lipidation (prenylation) pathway in the induction of metabolic defects in the β-cell under metabolic stress conditions. Potential avenues for the identification of novel therapeutic targets for the prevention/treatment of diabetes and its associated complications are highlighted.

L-serine supplementation lowers diabetes incidence and improves blood glucose homeostasis in NOD mice

Sphingolipids are a diverse group of lipids with important roles in beta-cell biology regulating insulin folding and controlling apoptosis. Sphingolipid biosynthesis begins with the condensation of L-serine and palmitoyl-CoA. Here we tested the effect of L-serine supplementation on autoimmune diabetes development and blood glucose homeostasis in female NOD mice. We found that continuous supplementation of L-serine reduces diabetes incidence and insulitis score. In addition, L-serine treated mice had an improved glucose tolerance test, reduced HOMA-IR, and reduced blood glucose levels. L-serine led to a small reduction in body weight accompanied by reduced food and water intake. L-serine had no effect on pancreatic sphingolipids as measured by mass spectrometry. The data thus suggests that L-serine could be used as a therapeutic supplement in the treatment of Type 1 Diabetes and to improve blood glucose homeostasis.

Pancreatic Beta Cell Death: Novel Potential Mechanisms in Diabetes Therapy

PURPOSE OF REVIEW

Describing the diverse molecular mechanisms (particularly immunological) involved in the death of the pancreatic beta cell in type 1 and type 2 diabetes mellitus.

RECENT FINDINGS

Beta cell death is the final event in a series of mechanisms that, up to date, have not been entirely clarified; it represents the pathophysiological mechanism in the natural history of diabetes mellitus. These mechanisms are not limited to an apoptotic process only, which is characteristic of the immune-mediated insulitis in type 1 diabetes mellitus. They also include the action of proinflammatory cytokines, the production of reactive oxygen species, DNA fragmentation (typical of necroptosis in type 1 diabetic patients), excessive production of islet amyloid polypeptide with the consequent endoplasmic reticulum stress, disruption in autophagy mechanisms, and protein complex formation, such as the inflammasome, capable of increasing oxidative stress produced by mitochondrial damage.

SUMMARY

Necroptosis, autophagy, and pyroptosis are molecular mechanisms that modulate the survival of the pancreatic beta cell, demonstrating the importance of the immune system in glucolipotoxicity processes and the potential role for immunometabolism as another component of what once known as the "ominous octet."

Omega-3 PUFA ameliorates hyperhomocysteinemia-induced hepatic steatosis in mice by inhibiting hepatic ceramide synthesis

Hyperhomocysteinemia (HHcy) is a key risk factor in hepatic steatosis. In this study, we applied a metabolomic approach to investigate the changes in the metabolite profile due to HHcy-induced hepatic steatosis and the effects of omega-3 PUFA (polyunsaturated fatty acid) supplementation in mice. HHcy was induced in mice by giving DL-Hcy (1.8 g/L) in drinking water for 6 weeks, then the mice were sacrificed, and the metabolic profiles of the liver and plasma were analyzed through UPLC-ESI-QTOFMS-based lipidomics. Hepatic triglycerides and cholesterol were further assayed. The expression of ceramide metabolism-related genes was measured by quantitative PCR. Compared with control mice, HHcy mice exhibited hepatic steatosis with a notable increase in ceramide-related metabolites and subsequent upregulation of ceramide synthesis genes such as Sptlc3, Degs2, Cer4 and Smpd4. Omega-3 PUFA was simultaneously administered in HHcy mice through chow diet containing 3.3% omega-3 PUFA supplement for 6 weeks, which significantly ameliorated Hcy-induced hepatic steatosis. The decrease in hepatic lipid accumulation was mainly due to reduced hepatic levels of ceramides, which was partly the result of the lower expression of ceramide synthesis genes, Sptlc3 and Degs2. Similar beneficial effects of DHA were observed in Hcy-stimulated primary hepatocytes in vitro. In summary, Hcy-induced ceramide elevation in hepatocytes might contribute to the development of hepatic steatosis. Furthermore, downregulation of ceramide levels through omega-3 PUFA supplementation ameliorates hepatic lipid accumulation. Thus, ceramide is a potential therapeutic target for the treatment of hepatic steatosis.

Possible Role of Phosphatidylcholine and Sphingomyelin on Fumonisin B1-mediated Toxicity

A major corn-related mycotoxin, fumonisin B1 (FB1), continues to attract attention of researchers as well as risk-assessors due to the diverse toxicological characteristics, including distinct target tissues in different animal species and opposite susceptibility in males and females in mice and rats. More than thirty years passed since the structure identification as a sphingoid-like chemical, but the causal mechanism of the toxicity remains obscure in spites of extensive studies. Considerable amounts of knowledge have been accumulated on the biochemical/toxicological actions of FB1, but the influence on lipid dynamics and mobilization in the body has not been focused well in relation to the FB1-mediated toxicity. Considerable influences of this toxin on mobilization of sphingolipids and phospholipids and also on adaptive changes in their compositions in tissues are implicated from recent studies on FB1-interacting ceramide synthases. Accumulated patho-physiological data also suggest a possible role of hepatic phospholipid on FB1-mediated toxicity. Thus, a mechanism of FB1-mediated toxicity is discussed in relation to the mobilization of phospholipids and sphingolipids in the body in this context.

Overview of immune abnormalities in lysosomal storage disorders

The critical relevance of the lysosomal compartment for normal cellular function can be proved by numbering the clinical phenotypes that arise in lysosomal storage disorders (LSDs), a group of around 70 different monogenic autosomal or X-linked syndromes, caused by specific lysosomal enzyme deficiencies: all LSDs are characterized by progressive accumulation of heterogeneous biologic materials in the lysosomes of various parts of the body such as viscera, skeleton, skin, heart, and central nervous system. At least a fraction of LSDs has been associated with mixed abnormalities involving the immune system, while some patients with LSDs may result more prone to autoimmune phenomena. A large production of proinflammatory cytokines has been observed in Gaucher and Fabry diseases, and wide different autoantibody production has been also reported in both. Many immune-mediated reactions are crucial to the pathogenesis of different inflammatory signs in mucopolysaccharidoses, and subverted heparan sulphate catabolism might dysregulate cellular homeostasis in the brain of these patients. Furthermore, an inappropriate activation of microglia is implicated in the neurodegenerative foci of Niemann-Pick disease, in which abnormal signalling pathways are activated by impaired sphingolipid metabolism. In addition, not the simple impaired catabolism of gangliosides per se, but also the production of anti-ganglioside autoantibodies contributes to the neurological disease of gangliosidoses. Even if the exact relationship between the modification of lysosomal activities and modulation of the immune system remains obscure, there is emerging evidence of different impaired immunity responses in a variety of LSDs: in this review we investigate and summarize the immune abnormalities and/or clinical data about immune system irregularities which have been described in a subset of LSDs.

Local Sphingosine Kinase 1 Activity Improves Islet Transplantation

Pancreatic islet transplantation is a promising clinical treatment for type 1 diabetes, but success is limited by extensive β-cell death in the immediate posttransplant period and impaired islet function in the longer term. Following transplantation, appropriate vascular remodeling is crucial to ensure the survival and function of engrafted islets. The sphingosine kinase (SK) pathway is an important regulator of vascular beds, but its role in the survival and function of transplanted islets is unknown. We observed that donor islets from mice deficient in SK1 (Sphk1 knockout) contain a reduced number of resident intraislet vascular endothelial cells. Furthermore, we demonstrate that the main product of SK1, sphingosine-1-phosphate, controls the migration of intraislet endothelial cells in vitro. We reveal in vivo that Sphk1 knockout islets have an impaired ability to cure diabetes compared with wild-type controls. Thus, SK1-deficient islets not only contain fewer resident vascular cells that participate in revascularization, but likely also a reduced ability to recruit new vessels into the transplanted islet. Together, our data suggest that SK1 is important for islet revascularization following transplantation and represents a novel clinical target for improving transplant outcomes.

An Intestinal Farnesoid X Receptor-Ceramide Signaling Axis Modulates Hepatic Gluconeogenesis in Mice

Increasing evidence supports the view that intestinal farnesoid X receptor (FXR) is involved in glucose tolerance and that FXR signaling can be profoundly impacted by the gut microbiota. Selective manipulation of the gut microbiota-FXR signaling axis was reported to significantly impact glucose intolerance, but the precise molecular mechanism remains largely unknown. Here, caffeic acid phenethyl ester (CAPE), an over-the-counter dietary supplement and an inhibitor of bacterial bile salt hydrolase, increased levels of intestinal tauro-β-muricholic acid, which selectively suppresses intestinal FXR signaling. Intestinal FXR inhibition decreased ceramide levels by suppressing expression of genes involved in ceramide synthesis specifically in the intestinal ileum epithelial cells. The lower serum ceramides mediated decreased hepatic mitochondrial acetyl-CoA levels and pyruvate carboxylase (PC) activities and attenuated hepatic gluconeogenesis, independent of body weight change and hepatic insulin signaling in vivo; this was reversed by treatment of mice with ceramides or the FXR agonist GW4064. Ceramides substantially attenuated mitochondrial citrate synthase activities primarily through the induction of endoplasmic reticulum stress, which triggers increased hepatic mitochondrial acetyl-CoA levels and PC activities. These results reveal a mechanism by which the dietary supplement CAPE and intestinal FXR regulates hepatic gluconeogenesis and suggest that inhibiting intestinal FXR is a strategy for treating hyperglycemia.

The role of sphingolipid signalling in diabetes‑associated pathologies (Review)

Sphingosine kinase (SphK) is an important signalling enzyme that catalyses the phosphorylation of sphingosine (Sph) to form sphingosine‑1‑phosphate (S1P). The multifunctional lipid, S1P binds to a family of five G protein-coupled receptors (GPCRs). As an intracellular second messenger, S1P activates key signalling cascades responsible for the maintenance of sphingolipid metabolism, and has been implicated in the progression of cancer, and the development of other inflammatory and metabolic diseases. SphK and S1P are critical molecules involved in the regulation of various cellular metabolic processes, such as cell proliferation, survival, apoptosis, adhesion and migration. There is strong evidence supporting the critical roles of SphK and S1P in the progression of diabetes mellitus, including insulin sensitivity and insulin secretion, pancreatic β‑cell apoptosis, and the development of diabetic inflammatory state. In this review, we summarise the current state of knowledge for SphK/S1P signalling effects, associated with the development of insulin resistance, pancreatic β‑cell death and the vascular complications of diabetes mellitus.

Sphingolipid regulators of cellular dysfunction in Type 2 diabetes mellitus: a systems overview

Climbing obesity rates have contributed to worldwide increases in obesity-associated diseases, including the metabolic syndrome and Type 2 diabetes mellitus (T2DM). Sphingolipids, an important class of structural and signaling lipids, have emerged as key players in the development and pathogenesis of insulin resistance and T2DM. More specifically, sphingolipids have been demonstrated to play integral roles in lipotoxicity and other aspects of pathogenesis in T2DM, although the cellular mechanisms by which this occurs and by which sphingolipid metabolism is dysregulated in T2DM remain under investigation. This review summarizes current knowledge of sphingolipid metabolism and signaling in key organs and tissues affected by T2DM, including the pancreas, adipose tissue, skeletal muscle, cardiovascular system and liver, and highlights areas that ripe for future investigation.

Poor glycemic control in type 2 diabetes enhances functional and compositional alterations of small, dense HDL3c

High-density lipoprotein (HDL) possesses multiple biological activities; small, dense HDL3c particles displaying distinct lipidomic composition exert potent antiatherogenic activities which can be compromised in dyslipidemic, hyperglycemic insulin-resistant states. However, it remains indeterminate (i) whether such functional HDL deficiency is related to altered HDL composition, and (ii) whether it originates from atherogenic dyslipidemia, dysglycemia, or both. In the present work we analyzed compositional characteristics of HDL subpopulations and functional activity of small, dense HDL3c particles in treatment-naïve patients with well-controlled (n=10) and poorly-controlled (n=8) type 2 diabetes (T2D) and in normolipidemic age- and sex-matched controls (n=11). Our data reveal that patients with both well- and poorly-controlled T2D displayed dyslipidemia and low-grade inflammation associated with altered HDL composition. Such compositional alterations in small, dense HDL subfractions were specifically correlated with plasma HbA1c levels. Further analysis using a lipidomic approach revealed that small, dense HDL3c particles from T2D patients with poor glycemic control displayed additional modifications of their chemical composition. In parallel, antioxidative activity of HDL3c towards oxidation of low-density lipoprotein was diminished. These findings indicate that defective functionality of small, dense HDL particles in patients with T2D is not only affected by the presence of atherogenic dyslipidemia, but also by the level of glycemic control, reflecting compositional alterations of HDL.

An Intestinal Microbiota-Farnesoid X Receptor Axis Modulates Metabolic Disease

The gut microbiota is associated with metabolic diseases including obesity, insulin resistance, and nonalcoholic fatty liver disease, as shown by correlative studies and by transplant of microbiota from obese humans and mice into germ-free mice. Modification of the microbiota by treatment of high-fat diet (HFD)-fed mice with tempol or antibiotics resulted in decreased adverse metabolic phenotypes. This was owing to lower levels of the genera Lactobacillus and decreased bile salt hydrolase (BSH) activity. The decreased BSH resulted in increased levels of tauro-β-muricholic acid (MCA), a substrate of BSH and a potent farnesoid X receptor (FXR) antagonist. Mice lacking expression of FXR in the intestine were resistant to HFD-induced obesity, insulin resistance, and nonalcoholic fatty liver disease, thus confirming that intestinal FXR is involved in the potentiation of metabolic disease. A potent intestinal FXR antagonist, glycine-β-MCA (Gly-MCA), which is resistant to BSH, was developed, which, when administered to HFD-treated mice, mimics the effect of the altered microbiota on HFD-induced metabolic disease. Gly-MCA had similar effects on genetically obese leptin-deficient mice. The decrease in adverse metabolic phenotype by tempol, antibiotics, and Gly-MCA was caused by decreased serum ceramides. Mice lacking FXR in the intestine also have lower serum ceramide levels, and are resistant to HFD-induced metabolic disease, and this was reversed by injection of C16:0 ceramide. In mouse ileum, because of the presence of endogenous FXR agonists produced in the liver, FXR target genes involved in ceramide synthesis are activated and when Gly-MCA is administered they are repressed, which likely accounts for the decrease in serum ceramides. These studies show that ceramides produced in the ileum under control of FXR influence metabolic diseases.

Fatty acid-related modulations of membrane fluidity in cells: detection and implications

Metabolic homeostasis of fatty acids is complex and well-regulated in all organisms. The biosynthesis of saturated fatty acids (SFA) in mammals provides substrates for β-oxidation and ATP production. Monounsaturated fatty acids (MUFA) are products of desaturases that introduce a methylene group in cis geometry in SFA. Polyunsaturated fatty acids (n-6 and n-3 PUFA) are products of elongation and desaturation of the essential linoleic acid and α-linolenic acid, respectively. The liver processes dietary fatty acids and exports them in lipoproteins for distribution and storage in peripheral tissues. The three types of fatty acids are integrated in membrane phospholipids and determine their biophysical properties and functions. This study was aimed at investigating effects of fatty acids on membrane biophysical properties under varying nutritional and pathological conditions, by integrating lipidomic analysis of membrane phospholipids with functional two-photon microscopy (fTPM) of cellular membranes. This approach was applied to two case studies: first, pancreatic beta-cells, to investigate hormetic and detrimental effects of lipids. Second, red blood cells extracted from a genetic mouse model defective in lipoproteins, to understand the role of lipids in hepatic diseases and metabolic syndrome and their effect on circulating cells.

Increased sphingosine 1-phosphate mediates inflammation and fibrosis in tubular injury in diabetic nephropathy

Hyperglycemia induces all isoforms of transforming growth factor β (TGFβ), which in turn play key roles in inflammation and fibrosis that characterize diabetic nephropathy. Sphingosine 1-phosphate (S1P) is a signaling sphingolipid, derived from sphingosine by the action of sphingosine kinase (SK). S1P mediates many biological processes, which mimic TGFβ signaling. To determine the role of SK1 and S1P in inducing fibrosis and inflammation, and the interaction with TGFβ-1, 2 and 3 signalling in diabetic nephropathy, human proximal tubular cells (HK2 cells) were exposed to normal (5 mmol/L) or high (30 mmol/L) glucose or TGFβ-1, -2, -3 ± an SK inhibitor (SKI-II) or SK1 siRNA. Control and diabetic wild type (WT) and SK1(-/-) mice were studied. Fibrotic and inflammatory markers, and relevant downstream signalling pathways were assessed. SK1 mRNA and protein expression was increased in HK2 cells exposed to high glucose or TGFβ1,-2,-3. All TGFβ isoforms induced fibronectin, collagen IV and macrophage chemoattractant protein 1 (MCP1), which were reversed by both SKI-II and SK1 siRNA. Exposure to S1P increased phospho-p44/42 expression, AP-1 binding and NFkB phosphorylation. WT diabetic mice exhibited increased renal cortical S1P, fibronectin, collagen IV and MCP1 mRNA and protein expression compared to SK1(-/-) diabetic mice. In summary, this study demonstrates that inhibiting the formation of S1P reduces tubulointerstitial renal inflammation and fibrosis in diabetic nephropathy.

Ceranib-2-induced suicidal erythrocyte death

Ceramide is known to trigger apoptosis of nucleated cells and eryptosis of erythrocytes. Eryptosis is characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine translocation to the erythrocyte surface. Besides ceramide, stimulators of eryptosis include increase of cytosolic Ca(2+) -activity ([Ca(2+) ]i ) and oxidative stress. Ceramide is degraded by acid ceramidase and inhibition of the enzyme similarly triggers apoptosis. The present study explored, whether ceramidase inhibitor Ceranib-2 induces eryptosis. Flow cytometry was employed to quantify phosphatidylserine-exposure at the cell surface from annexin-V-binding, cell volume from forward scatter, [Ca(2+) ]i from Fluo3-fluorescence, reactive oxygen species (ROS) from DCF dependent fluorescence, and ceramide abundance utilizing specific antibodies. Hemolysis was estimated from hemoglobin concentration in the supernatant. A 48 h exposure of human erythrocytes to Ceranib-2 significantly increased the percentage of annexin-V-binding cells (≥50 μM) and the percentage of hemolytic cells (≥10 μM) without significantly modifying forward scatter. Ceranib-2 significantly increased Fluo3-fluorescence, DCF fluorescence and ceramide abundance. The effect of Ceranib-2 on annexin-V-binding was not significantly blunted by removal of extracellular Ca(2+) . Ceranib-2 triggers phospholipid scrambling of the erythrocyte cell membrane, an effect at least in part due to increase of ceramide abundance and induction of oxidative stress, but not dependent on Ca(2+) entry. Copyright © 2016 John Wiley & Sons, Ltd.

A comprehensive lipidomic screen of pancreatic β-cells using mass spectroscopy defines novel features of glucose-stimulated turnover of neutral lipids, sphingolipids and plasmalogens

Objective

Glucose promotes lipid remodelling in pancreatic β-cells, and this is thought to contribute to the regulation of insulin secretion, but the metabolic pathways and potential signalling intermediates have not been fully elaborated.

Methods

Using mass spectrometry (MS) we quantified changes in approximately 300 lipid metabolites in MIN6 β-cells and isolated mouse islets following 1 h stimulation with glucose. Flux through sphingolipid pathways was also assessed in ³H-sphinganine-labelled cells using TLC.

Results

Glucose specifically activates the conversion of triacylglycerol (TAG) to diacylglycerol (DAG). This leads indirectly to the formation of 18:1 monoacylglycerol (MAG), via degradation of saturated/monounsaturated DAG species, such as 16:0_18:1 DAG, which are the most abundant, immediate products of glucose-stimulated TAG hydrolysis. However, 16:0-containing, di-saturated DAG species are a better direct marker of TAG hydrolysis since, unlike the 18:1-containing DAGs, they are predominately formed via this route. Using multiple reaction monitoring, we confirmed that in islets under basal conditions, 18:1 MAG is the most abundant species. We further demonstrated a novel site of glucose to enhance the conversion of ceramide to sphingomyelin (SM) and galactosylceramide (GalCer). Flux and product:precursor analyses suggest regulation of the enzyme SM synthase, which would constitute a separate mechanism for localized generation of DAG in response to glucose. Phosphatidylcholine (PC) plasmalogen (P) species, specifically those containing 20:4, 22:5 and 22:6 side chains, were also diminished in the presence of glucose, whereas the more abundant phosphatidylethanolamine plasmalogens were unchanged.

Conclusion

Our results highlight 18:1 MAG, GalCer, PC(P) and DAG/SM as potential contributors to metabolic stimulus-secretion coupling.

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Using mass spectroscopy lipidomics we have defined new aspects of glucose simulated lipid turnover in pancreatic beta cells.

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Glucose directly stimulates triacylglycerol hydrolysis, of which di-saturated diacylglycerol species are excellent markers.

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C18:1 is the most abundant monacylglycerol, and the one most obviously increased by glucose.

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Phosphatidylcholine plasmalogens with poly-unsaturated side chains are preferentially decreased by glucose.

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Glucose specifically enhances the conversion of ceramide to both sphingomyelin and galactosylceramide.

Stress-impaired transcription factor expression and insulin secretion in transplanted human islets

Type 2 diabetes is characterized by insulin resistance, hyperglycemia, and progressive β cell dysfunction. Excess glucose and lipid impair β cell function in islet cell lines, cultured rodent and human islets, and in vivo rodent models. Here, we examined the mechanistic consequences of glucotoxic and lipotoxic conditions on human islets in vivo and developed and/or used 3 complementary models that allowed comparison of the effects of hyperglycemic and/or insulin-resistant metabolic stress conditions on human and mouse islets, which responded quite differently to these challenges. Hyperglycemia and/or insulin resistance impaired insulin secretion only from human islets in vivo. In human grafts, chronic insulin resistance decreased antioxidant enzyme expression and increased superoxide and amyloid formation. In human islet grafts, expression of transcription factors NKX6.1 and MAFB was decreased by chronic insulin resistance, but only MAFB decreased under chronic hyperglycemia. Knockdown of NKX6.1 or MAFB expression in a human β cell line recapitulated the insulin secretion defect seen in vivo. Contrary to rodent islet studies, neither insulin resistance nor hyperglycemia led to human β cell proliferation or apoptosis. These results demonstrate profound differences in how excess glucose or lipid influence mouse and human insulin secretion and β cell activity and show that reduced expression of key islet-enriched transcription factors is an important mediator of glucotoxicity and lipotoxicity.

Disruption of sphingolipid metabolism augments ceramide-induced autophagy in preeclampsia

Bioactive sphingolipids including ceramides are involved in a variety of pathophysiological processes by regulating cell death and survival. The objective of the current study was to examine ceramide metabolism in preeclampsia, a serious disorder of pregnancy characterized by oxidative stress, and increased trophoblast cell death and autophagy. Maternal circulating and placental ceramide levels quantified by tandem mass spectrometry were elevated in pregnancies complicated by preeclampsia. Placental ceramides were elevated due to greater de novo synthesis via high serine palmitoyltransferase activity and reduced lysosomal breakdown via diminished ASAH1 expression caused by TGFB3-induced E2F4 transcriptional repression. SMPD1 activity was reduced; hence, sphingomyelin degradation by SMPD1 did not contribute to elevated ceramide levels in preeclampsia. Oxidative stress triggered similar changes in ceramide levels and acid hydrolase expression in villous explants and trophoblast cells. MALDI-imaging mass spectrometry localized the ceramide increases to the trophophoblast layers and syncytial knots of placentae from pregnancies complicated by preeclampsia. ASAH1 inhibition or ceramide treatment induced autophagy in human trophoblast cells via a shift of the BOK-MCL1 rheostat toward prodeath BOK. Pharmacological inhibition of ASAH1 activity in pregnant mice resulted in increased placental ceramide content, abnormal placentation, reduced fetal growth, and increased autophagy via a similar shift in the BOK-MCL1 system. Our results reveal that oxidative stress-induced reduction of lysosomal hydrolase activities in combination with elevated de novo synthesis leads to ceramide overload, resulting in increased trophoblast cell autophagy, and typifies preeclampsia as a sphingolipid storage disorder.

miRNA-375 a Sensor of Glucotoxicity Is Altered in the Serum of Children with Newly Diagnosed Type 1 Diabetes

Background. The use of miRNAs as biomarkers for Type 1 Diabetes (T1D) risk is attractive as T1D is usually diagnosed in front of acute symptoms. As miR-375 is highly expressed in the endocrine pancreas, we postulated that its circulating level might reflect beta cell alterations and might be altered in the blood of T1D patients recently diagnosed. Methods. Sera were obtained from 22 T1D children at onset of the disease, before subcutaneous insulin treatment, and from 10 nondiabetic pediatric controls. MiR-375 seric level was quantified by stem-loop RT-PCR-based assay. MiRNAs regulations in isolated human islets in response to high glucose concentrations were determined by TaqMan Low-Density Array. Results. The abundance of miR-375, among the 410 miRNAs detected in human islets, mirrored its well-established role in rodent islet biology. Upregulated miRNAs targeted genes involved in islet homeostasis and regulation of beta cell mass. Downregulated miRNAs, including miR-375, were involved in pancreas secretion and protein turnover. Seric level of miR-375 was lower in T1D children versus age-matched controls, without any correlations with HbA1c, glycaemia, and number of autoantibodies. Conclusion. Altered circulating level of miR-375 at onset of T1D might be a general biomarker of metabolic alterations and inflammation associated with the disease.

Lipidomic and metabolomic characterization of a genetically modified mouse model of the early stages of human type 1 diabetes pathogenesis

The early mechanisms regulating progression towards beta cell failure in type 1 diabetes (T1D) are poorly understood, but it is generally acknowledged that genetic and environmental components are involved. The metabolomic phenotype is sensitive to minor variations in both, and accordingly reflects changes that may lead to the development of T1D. We used two different extraction methods in combination with both liquid- and gas chromatographic techniques coupled to mass spectrometry to profile the metabolites in a transgenic non-diabetes prone C57BL/6 mouse expressing CD154 under the control of the rat insulin promoter (RIP) crossed into the immuno-deficient recombination-activating gene (RAG) knockout (-/-) C57BL/6 mouse, resembling the early stages of human T1D. We hypothesized that alterations in the metabolomic phenotype would characterize the early pathogenesis of T1D, thus metabolomic profiling could provide new insight to the development of T1D. Comparison of the metabolome of the RIP CD154 × RAG^-/- mice to RAG^-/- mice and C57BL/6 mice revealed alterations of >100 different lipids and metabolites in serum. Low lysophosphatidylcholine levels, accumulation of ceramides as well as methionine deficits were detected in the pre-type 1 diabetic mice. Additionally higher lysophosphatidylinositol levels and low phosphatidylglycerol levels where novel findings in the pre-type 1 diabetic mice. These observations suggest that metabolomic disturbances precede the onset of T1D.

Uncovering Factors Related to Pancreatic Beta-Cell Function

AIM

The incidence of type 2 diabetes has increased rapidly on a global scale. Beta-cell dysfunction contributes to the overall pathogenesis of type 2 diabetes. However, factors contributing to beta-cell function are not clear. The aims of this study were (i) to identify factors related to pancreatic beta-cell function and (ii) to perform mechanistic studies in vitro.

METHODS

Three specific measures of beta-cell function were assessed for 110 participants who completed an oral glucose tolerance test as part of the Metabolic Challenge Study. Anthropometric and biochemical parameters were assessed as potential modulators of beta-cell function. Subsequent in vitro experiments were performed using the BRIN-BD11 pancreatic beta-cell line. Validation of findings were performed in a second human cohort.

RESULTS

Waist-to-hip ratio was the strongest anthropometric modulator of beta-cell function, with beta-coefficients of -0.33 (p = 0.001) and -0.30 (p = 0.002) for beta-cell function/homeostatic model assessment of insulin resistance (HOMA-IR), and disposition index respectively. Additionally, the resistin-to-adiponectin ratio (RA index) emerged as being strongly associated with beta-cell function, with beta-coefficients of -0.24 (p = 0.038) and -0.25 (p = 0.028) for beta-cell function/HOMA-IR, and disposition index respectively. Similar results were obtained using a third measure for beta-cell function. In vitro experiments revealed that the RA index was a potent regulator of acute insulin secretion where a high RA index (20ng ml-1 resistin, 5nmol l-1 g-adiponectin) significantly decreased insulin secretion whereas a low RA index (10ng ml-1 resistin, 10nmol l-1 g-adiponectin) significantly increased insulin secretion. The RA index was successfully validated in a second human cohort with beta-coefficients of -0.40 (p = 0.006) and -0.38 (p = 0.008) for beta-cell function/ HOMA-IR, and disposition index respectively.

CONCLUSIONS

Waist-to-hip ratio and RA index were identified as significant modulators of beta-cell function. The ability of the RA index to modulate insulin secretion was confirmed in mechanistic studies. Future work should identify strategies to alter the RA index.

Squalene is lipotoxic to yeast cells defective in lipid droplet biogenesis

The toxic effect of overloaded lipids on cell physiology and viability was described in various organisms. In this study we focused on the potential lipotoxicity of squalene, a linear triterpene synthesized in eukaryotic cells as an intermediate in sterol biosynthesis. Squalene toxicity was studied in the yeast Saccharomyces cerevisiae, a model unicellular eukaryote established in lipotoxicity studies. Squalene levels in yeast are typically low but its accumulation can be induced under specific conditions, e.g. by inhibition of squalene monooxygenase with the antimycotic terbinafine. At higher levels squalene is stored in lipid droplets. We demonstrated that low doses of terbinafine caused severe impairment of growth and loss of viability of the yeast mutant dga1Δ lro1Δ are1Δ are2Δ unable to form lipid droplets and that these defects were linked to squalene accumulation. The hypersensitivity of the lipid droplet-less mutant to terbinafine was alleviated by decreasing squalene accumulation with low doses of squalene synthase inhibitor zaragozic acid. Our results proved that accumulated squalene is lipotoxic to yeast cells if it cannot be efficiently sequestered in lipid droplets. This supports the hypothesis about the role of squalene in the fungicidal activity of terbinafine. Squalene toxicity may represent also a limiting factor for production of this high-value lipid in yeast.

Effect of pioglitazone on plasma ceramides in adults with metabolic syndrome

BACKGROUND

Metabolic syndrome (MetS) appears closely linked with ceramide accumulation, inducing insulin resistance and toxicity to multiple cell types. Animal studies demonstrate that thiazolidinediones (TZDs) reduce ceramide concentrations in plasma and skeletal muscle and support lowering of ceramide levels as a potential mediator of TZDs' mechanism of action in reducing insulin resistance; however, studies in humans have yet to be reported. This study investigated the effects of pioglitazone therapy on plasma ceramides to understand the mechanism by which TZDs improve insulin resistance in MetS.

METHODS

Thirty-seven subjects with MetS were studied in a single-centre, randomized, double-blind, placebo-controlled trial comparing pioglitazone to placebo. Data were collected at baseline and after 6 months of therapy. The primary endpoint was the change from baseline in plasma ceramide concentrations.

RESULTS

Treatment with pioglitazone for 6 months, compared with placebo, significantly reduced multiple plasma ceramide concentrations: C18:0 (p = 0.001), C20:0 (p = 0.0004), C24 : 1 (p = 0.009), dihydroceramide C18 :0 (p = 0.005), dihydroceramide C24:1 (p = 0.004), lactosylceramide C16:0 (p = 0.02) and the hexosylceramides C16:0 (p = 0.0003), C18 : 0 (p = 0.00001), C22:0 (p = 0.00002) and C24:1 (p = 0.0006). Additionally, significant reductions were found when ceramides were grouped by species: ceramides (p = 0.03), dihydroceramides (p = 0.02), hexosylceramides (p = 0.00001) and lactosylceramides (p = 0.02). The total of all measured ceramides was also significantly reduced (p = 0.001). Following treatment with pioglitazone, the decrease in some ceramide species correlated negatively with the change in insulin sensitivity (dihydroceramide C16:0, r = -0.54; p = 0.02) and positively with total (lactosylceramide C24:0, r = 0.53; p = 0.02) and high molecular weight (lactosylceramide C24:0, r = 0.48; p = 0.05) adiponectin measurements; however, significant associations with changes in liver fat and glycemic control reduction were not found.

CONCLUSIONS

Pioglitazone in individuals with MetS induces a potent decrease in plasma ceramides, and some of the changes correlate with changes in insulin resistance and adiponectin levels.

The prediction of type 2 diabetes in women with previous gestational diabetes mellitus using lipidomics

AIMS/HYPOTHESIS

The risk of developing diabetes is greater for women with previous gestational diabetes mellitus (GDM). In the general population, plasma lipidomic analysis can identify individuals at risk of developing type 2 diabetes. The aim of this study was to determine whether circulating lipid levels 12 weeks following a GDM pregnancy were associated with an increased risk of developing type 2 diabetes.

METHODS

Plasma lipid profiles containing >300 lipids were measured in 104 normal glucose-tolerant women 12 weeks following an index GDM pregnancy using electrospray-ionisation tandem mass spectrometry. Women were assessed for 10 years for development of overt type 2 diabetes.

RESULTS

Among the 104 women with previous GDM, 21 (20%) developed diabetes during the median follow-up period of 8.5 years. Three lipid species, the cholesteryl ester species CE 20:4, the alkenylphosphatidylethanolamine species PE(P-36:2) and the phosphatidylserine species PS 38:4, were independently and positively associated with the development of type 2 diabetes. In a clinical model of prediction of type 2 diabetes that included age, BMI, and levels of pregnancy fasting glucose, postnatal fasting glucose, triacylglycerol and total cholesterol, the addition of these three lipid species resulted in an improvement in the net reclassification index of 22.3%.

CONCLUSIONS/INTERPRETATION

The lipid species CE 20:4, PE(P-36:2) and PS 38:4 were significant risk factors for the development of type 2 diabetes in women with a previous history of GDM. This report is the first to use plasma lipidomic analysis to identify individual lipids as potential biomarkers for the prediction of type 2 diabetes in women with a history of GDM.

Metabolomics Analysis Reveals that AICAR Affects Glycerolipid, Ceramide and Nucleotide Synthesis Pathways in INS-1 Cells

AMPK regulates many metabolic pathways including fatty acid and glucose metabolism, both of which are closely associated with insulin secretion in pancreatic β-cells. Insulin secretion is regulated by metabolic coupling factors such as ATP/ADP ratio and other metabolites generated by the metabolism of nutrients such as glucose, fatty acid and amino acids. However, the connection between AMPK activation and insulin secretion in β-cells has not yet been fully elucidated at a metabolic level. To study the effect of AMPK activation on glucose stimulated insulin secretion, we applied the pharmacological activator 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) to an INS-1 (832/13) β-cell line. We measured the change in 66 metabolites in the presence or absence of AICAR using different stable isotopic labeled nutrients to probe selected pathways. AMPK activation by AICAR increased basal insulin secretion and reduced the glucose stimulation index. Although ATP/ADP ratios were not strongly affected by AICAR, several other metabolites and pathways important for insulin secretion were affected by AICAR treatment including long-chain CoAs, malonyl-CoA, 3-hydroxy-3 methylglutaryl CoA, diacylglycerol, and farnesyl pyrophosphate. Tracer studies using 13C-glucose revealed lower glucose flux in the purine and pyrimidine pathway and in the glycerolipid synthesis pathway. Untargeted metabolomics revealed reduction in ceramides caused by AICAR that may explain the beneficial role of AMPK in protecting β-cells from lipotoxicity. Taken together, the results provide an overall picture of the metabolic changes associated with AICAR treatment and how it modulates insulin secretion and β-cell survival.

Lipidomic profiling at the interface of metabolic surgery and cardiovascular disease

Bariatric surgery has helped patients attain not only significant and sustained weight loss but has also proved to be an effective means of mitigating or reversing various obesity-related comorbidities. The impressive rates of remission or resolution of type 2 diabetes mellitus (T2D) following bariatric surgery are well documented and have rightly received great attention. Less understood are the effects of bariatric surgery on cardiovascular disease (CVD) and its underlying risk factors. Thanks to the availability of increasingly sensitive laboratory tools, the emerging science of lipidomics and metagenomics is poised to offer significant contributions to our understanding of metabolically induced vascular diseases. They are set to identify novel mechanisms explaining how the varied approaches of bariatric surgery produce the remarkable improvements in multiple organs observed during patient follow-up. This article reviews recent and novel findings in patients through the lens of lipidomics with an emphasis on CVD.

Beta cell response to nutrient overload involves phospholipid remodelling and lipid peroxidation

AIMS/HYPOTHESIS

Membrane phospholipids are the major intracellular source for fatty acid-derived mediators, which regulate myriad cell functions. We showed previously that high glucose levels triggered the hydrolysis of polyunsaturated fatty acids from beta cell phospholipids. These fatty acids were subjected to free radical-catalysed peroxidation to generate the bioactive aldehyde 4-hydroxy-2E-nonenal (4-HNE). The latter activated the nuclear peroxisome proliferator-activated receptor-δ (PPARδ), which in turn augmented glucose-stimulated insulin secretion. The present study aimed at investigating the combined effects of glucose and fatty acid overload on phospholipid turnover and the subsequent generation of lipid mediators, which affect insulin secretion and beta cell viability.

METHODS

INS-1E cells were incubated with increasing glucose concentrations (5-25 mmol/l) without or with palmitic acid (PA; 50-500 μmol/l) and taken for fatty acid-based lipidomic analysis and functional assays. Rat isolated islets of Langerhans were used similarly.

RESULTS

PA was incorporated into membrane phospholipids in a concentration- and time-dependent manner; incorporation was highest at 25 mmol/l glucose. This was coupled to a rapid exchange with saturated, mono-unsaturated and polyunsaturated fatty acids. Importantly, released arachidonic acid and linoleic acid were subjected to peroxidation, resulting in the generation of 4-HNE, which further augmented insulin secretion by activating PPARδ in beta cells. However, this adaptive increase in insulin secretion was abolished at high glucose and PA levels, which induced endoplasmic reticulum stress, apoptosis and cell death.

CONCLUSIONS/INTERPRETATION

These findings highlight a key role for phospholipid remodelling and fatty acid peroxidation in mediating adaptive and cytotoxic interactions induced by nutrient overload in beta cells.

Sphingosine 1-phosphate counteracts insulin signaling in pancreatic β-cells via the sphingosine 1-phosphate receptor subtype 2

Glucolipotoxic stress has been identified as a key player in the progression of pancreatic β-cell dysfunction contributing to insulin resistance and the development of type 2 diabetes mellitus (T2D). It has been suggested that bioactive lipid intermediates, formed under lipotoxic conditions, are involved in these processes. Here, we show that sphingosine 1-phosphate (S1P) levels are not only increased in palmitate-stimulated pancreatic β-cells but also regulate β-cell homeostasis in a divergent manner. Although S1P possesses a prosurvival effect in β-cells, an enhanced level of the sphingolipid antagonizes insulin-mediated cell growth and survival via the sphingosine 1-phosphate receptor subtype 2 (S1P2) followed by an inhibition of Akt-signaling. In an attempt to investigate the role of the S1P/S1P2 axis in vivo, the New Zealand obese (NZO) diabetic mouse model, characterized by β-cell loss under high-fat diet (HFD) conditions, was used. The occurrence of T2D was accompanied by an increase of plasma S1P levels. To examine whether S1P contributes to the morphologic changes of islets via S1P2, the receptor antagonist JTE-013 was administered. Most interestingly, JTE-013 rescued β-cell damage clearly indicating an important role of the S1P2 in β-cell homeostasis. Therefore, the present study provides a new therapeutic strategy to diminish β-cell dysfunction and the development of T2D.

Islet β-cell failure in type 2 diabetes--Within the network of toxic lipids

Obesity-related type 2 diabetes develops in individuals with the onset of β-cell dysfunction. Pancreatic islet lipotoxicity is now recognized as a primary reason for the onset and progression of the disease. Such dysfunction is reflected by the aberrant secretory capacity and detrimental loss of β-cell mass and survival. Elevated circulating serum fatty acid levels and disordered lipid metabolism management are particularly interesting in the search for biologically relevant triggers of β-cell demise. Herein, we review various types of toxic lipid metabolites that may play a significant role in pancreatic islet failure. The lipotoxic effect on β-cells depends on the type of lipid mediator (e.g., long-chain fatty acids, diacylglycerols, ceramides, phospholipids), cellular location of its action (e.g., endoplasmic reticulum, mitochondria), and associated-organelle conditions (e.g., membranes, vesicles). We also discuss various aspects of lipid action in β-cells, including effects on metabolic pathways, stress responses (e.g., oxidative stress, endoplasmic reticulum stress, and autophagy), and gene expression.

Evidence of contribution of iPLA2β-mediated events during islet β-cell apoptosis due to proinflammatory cytokines suggests a role for iPLA2β in T1D development

Type 1 diabetes (T1D) results from autoimmune destruction of islet β-cells, but the underlying mechanisms that contribute to this process are incompletely understood, especially the role of lipid signals generated by β-cells. Proinflammatory cytokines induce ER stress in β-cells and we previously found that the Ca(2+)-independent phospholipase A2β (iPLA2β) participates in ER stress-induced β-cell apoptosis. In view of reports of elevated iPLA2β in T1D, we examined if iPLA2β participates in cytokine-mediated islet β-cell apoptosis. We find that the proinflammatory cytokine combination IL-1β+IFNγ, induces: a) ER stress, mSREBP-1, and iPLA2β, b) lysophosphatidylcholine (LPC) generation, c) neutral sphingomyelinase-2 (NSMase2), d) ceramide accumulation, e) mitochondrial membrane decompensation, f) caspase-3 activation, and g) β-cell apoptosis. The presence of a sterol regulatory element in the iPLA2β gene raises the possibility that activation of SREBP-1 after proinflammatory cytokine exposure contributes to iPLA2β induction. The IL-1β+IFNγ-induced outcomes (b-g) are all inhibited by iPLA2β inactivation, suggesting that iPLA2β-derived lipid signals contribute to consequential islet β-cell death. Consistent with this possibility, ER stress and β-cell apoptosis induced by proinflammatory cytokines are exacerbated in islets from RIP-iPLA2β-Tg mice and blunted in islets from iPLA2β-KO mice. These observations suggest that iPLA2β-mediated events participate in amplifying β-cell apoptosis due to proinflammatory cytokines and also that iPLA2β activation may have a reciprocal impact on ER stress development. They raise the possibility that iPLA2β inhibition, leading to ameliorations in ER stress, apoptosis, and immune responses resulting from LPC-stimulated immune cell chemotaxis, may be beneficial in preserving β-cell mass and delaying/preventing T1D evolution.

Lipotoxic endoplasmic reticulum stress, β cell failure, and type 2 diabetes mellitus

Failure of the unfolded protein response (UPR) to maintain optimal folding of pro-insulin in the endoplasmic reticulum (ER) leads to unresolved ER stress and β cell death. This contributes not only to some rare forms of diabetes, but also to type 2 diabetes mellitus (T2DM). Many key findings, elaborated over the past decade, are based on the lipotoxicity model, entailing chronic exposure of β cells to elevated levels of fatty acids (FAs). Here, we update recent progress on how FAs initiate ER stress, particularly via disruption of protein trafficking, and how this leads to apoptosis. We also highlight differences in how β cells are impacted by the classic UPR, versus the more selective UPR that arises as part of a broader response to lipotoxicity.

Roles of Sphingolipid Metabolism in Pancreatic β Cell Dysfunction Induced by Lipotoxicity

Pancreatic β cells secrete insulin in order to maintain glucose homeostasis. However, various environmental stresses such as obesity have been shown to induce loss of secretory responsiveness in pancreatic β cells and pancreatic β cell apoptosis which can favor the development of type 2 diabetes (T2D). Indeed, elevated levels of free fatty acids (FFAs) have been shown to induce β cell apoptosis. Importantly, the chronic adverse effects of FFAs on β cell function and viability are potentiated in the presence of hyperglycaemia, a phenomenon that has been termed gluco-lipotoxicity. The molecular mechanisms underlying the pathogenesis of gluco-lipotoxicity in pancreatic β cells are not completely understood. Recent studies have shown that sphingolipid metabolism plays a key role in gluco-lipotoxicity induced apoptosis and loss of function of pancreatic β cells. The present review focuses on how the two main sphingolipid mediators, ceramides and sphingoid base-1-phosphates, regulate the deleterious effects of gluco-lipotoxicity on pancreatic β cells. The review highlights the role of a sphingolipid biostat on the dysregulation of β cell fate and function induced by gluco-lipotoxicity, offering the possibility of new therapeutic targets to prevent the onset of T2D.