Inhibition of PKB/Akt1 by C2-ceramide involves activation of ceramide-activated protein phosphatase in PC12 cells

shRNA-mediated down-regulation of Acsl1 reverses skeletal muscle insulin resistance in obese C57BL6/J mice

Prolonged consumption of diet rich in fats is regarded as the major factor leading to the insulin resistance (IR) and type 2 diabetes (T2D). Emerging evidence link excessive accumulation of bioactive lipids such as diacylglycerol (DAG) and ceramide (Cer), with impairment of insulin signaling in skeletal muscle. Until recently, little has been known about the involvement of long-chain acyl-CoAs synthetases in the above mechanism. To examine possible role of long-chain acyl-coenzyme A synthetase 1 (Acsl1) (a major muscular ACSL isoform) in mediating HFD-induced IR we locally silenced Acsl1 in gastrocnemius of high-fat diet (HFD)-fed C57BL/6J mice through electroporation-delivered shRNA and compared it to non-silenced tissue within the same animal. Acsl1 down-regulation decreased the content of muscular long-chain acyl-CoA (LCACoA) and both the Cer (C18:1-Cer and C24:1-Cer) and DAG (C16:0/18:0-DAG, C16:0/18:2-DAG, C18:0/18:0-DAG) and simultaneously improved insulin sensitivity and glucose uptake as compared with non-silenced tissue. Acsl1 down-regulation decreased expression of mitochondrial β-oxidation enzymes, and the content of both the short-chain acylcarnitine (SCA-Car) and short-chain acyl-CoA (SCACoA) in muscle, pointing towards reduction of mitochondrial FA oxidation. The results indicate, that beneficial effects of Acsl1 partial ablation on muscular insulin sensitivity are connected with inhibition of Cer and DAG accumulation, and outweigh detrimental impact of decreased mitochondrial fatty acids metabolism in skeletal muscle of obese HFD-fed mice.

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.

Ceramide Is Involved in Temozolomide Resistance in Human Glioblastoma U87MG Overexpressing EGFR

Glioblastoma multiforme (GBM) is the most frequent and deadly brain tumor. Many sphingolipids are crucial players in the regulation of glioma cell growth as well as in the response to different chemotherapeutic drugs. In particular, ceramide (Cer) is a tumor suppressor lipid, able to induce antiproliferative and apoptotic responses in different types of tumors including GBM, most of which overexpress the epidermal growth factor receptor variant III (EGFRvIII). In this paper, we investigated whether Cer metabolism is altered in the U87MG human glioma cell line overexpressing EGFRvIII (EGFR+ cells) to elucidate their possible interplay in the mechanisms regulating GBM survival properties and the response to the alkylating agent temozolomide (TMZ). Notably, we demonstrated that a low dose of TMZ significantly increases Cer levels in U87MG cells but slightly in EGFR+ cells (sensitive and resistant to TMZ, respectively). Moreover, the inhibition of the synthesis of complex sphingolipids made EGFR+ cells sensitive to TMZ, thus involving Cer accumulation/removal in TMZ resistance of GBM cells. This suggests that the enhanced resistance of EGFR+ cells to TMZ is dependent on Cer metabolism. Altogether, our results indicate that EGFRvIII expression confers a TMZ-resistance phenotype to U87MG glioma cells by counteracting Cer increase.

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.

Ceramide analogue C2-cer induces a loss in insulin sensitivity in muscle cells through the salvage/recycling pathway

Ceramides have been shown to play a major role in the onset of skeletal muscle insulin resistance and therefore in the prevalence of type 2 diabetes (T2D). However, many of the studies involved in the discovery of deleterious ceramide actions used a non-physiological cell-permeable short-chain ceramide analogue, the C2-ceramide (C2-cer). In the present study, we determined how C2-cer promotes insulin resistance in muscle cells. We demonstrate that C2-cer enters the salvage/recycling pathway and becomes de-acylated, yielding sphingosine, re-acylation of which depends on the availability of long chain fatty acids provided by the lipogenesis pathway in muscle cells. Importantly, we show these salvaged ceramides are actually responsible for the inhibition of insulin signaling induced by C2-cer. Interestingly, we also show that the exogenous and endogenous mono-unsaturated fatty acid oleate prevents C2-cer to be recycled into endogenous ceramide species in a diacylglycerol O-acyltransferase 1 (DGAT1)-dependent mechanism, which forces free fatty acid metabolism towards triacylglyceride production. Altogether, the study highlights for the first time that C2-cer induces a loss in insulin sensitivity through the salvage/recycling pathway in muscle cells. This study also validates C2-cer as a convenient tool to decipher mechanisms by which long-chain ceramides mediate insulin resistance in muscle cells and suggests that in addition to the de novo ceramide synthesis, recycling of ceramide could contribute to muscle insulin resistance observed in obesity and T2D.

Food Polyphenols and Type II Diabetes Mellitus: Pharmacology and Mechanisms

Type II diabetes mellitus and its related complications are growing public health problems. Many natural products present in our diet, including polyphenols, can be used in treating and managing type II diabetes mellitus and different diseases, owing to their numerous biological properties. Anthocyanins, flavonols, stilbenes, curcuminoids, hesperidin, hesperetin, naringenin, and phenolic acids are common polyphenols found in blueberries, chokeberries, sea-buckthorn, mulberries, turmeric, citrus fruits, and cereals. These compounds exhibit antidiabetic effects through different pathways. Accordingly, this review presents an overview of the most recent developments in using food polyphenols for managing and treating type II diabetes mellitus, along with various mechanisms. In addition, the present work summarizes the literature about the anti-diabetic effect of food polyphenols and evaluates their potential as complementary or alternative medicines to treat type II diabetes mellitus. Results obtained from this survey show that anthocyanins, flavonols, stilbenes, curcuminoids, and phenolic acids can manage diabetes mellitus by protecting pancreatic β-cells against glucose toxicity, promoting β-cell proliferation, reducing β-cell apoptosis, and inhibiting α-glucosidases or α-amylase. In addition, these phenolic compounds exhibit antioxidant anti-inflammatory activities, modulate carbohydrate and lipid metabolism, optimize oxidative stress, reduce insulin resistance, and stimulate the pancreas to secrete insulin. They also activate insulin signaling and inhibit digestive enzymes, regulate intestinal microbiota, improve adipose tissue metabolism, inhibit glucose absorption, and inhibit the formation of advanced glycation end products. However, insufficient data are available on the effective mechanisms necessary to manage diabetes.

Scutellaria incarnata Vent. root extract and isolated phenylethanoid glycosides are neuroprotective against C2-ceramide toxicity

ETHNOPHARMACOLOGICAL RELEVANCE

Neuronal death is a central process in neurodegenerative diseases and represents a therapeutic challenge for their prevention and treatment. Scutellaria incarnata Vent. roots are used traditionally in Colombia for central nervous system conditions including those affecting cognitive functions, but their chemistry and neuroprotective action remain to be explored to understand the scientific basis for their medicinal uses. In this study, S. incarnata roots are investigated to assess whether they have neuroprotective effects that could provide some explanation for their traditional use in neurodegenerative diseases.

AIM OF THE STUDY

To evaluate the neuroprotective effect of S. incarnata roots and its chemical constituents against C₂-ceramide-induced cell death in Cath.-a-differentiated (CAD) cells.

MATERIALS AND METHODS

S. incarnata root ethanol extract was fractionated and compounds were isolated by column chromatography; their structures were elucidated by nuclear magnetic resonance spectroscopy, mass spectrometry and infrared spectroscopy. The cytotoxic and neuroprotective effects against C₂-ceramide of S. incarnata root extract, fractions and isolated compounds were assessed in CAD cells.

RESULTS

S. incarnata root extract and its n-butanol fraction were not cytotoxic but showed neuroprotective effects against C₂-ceramide toxicity in CAD cells. The phenylethanoid glycosides incarnatoside (isolated for the first time) and stachysoside C (12.5, 25 and 50 μg/mL) from S. incarnata roots also protected CAD cells against C₂-ceramide without inducing cytotoxic effects.

CONCLUSION

The observed neuroprotective effects of S. incarnata root extract and isolated phenylethanoid glycosides in CAD cells provide an ethnopharmacological basis for the traditional use of this species in Colombia for central nervous system disorders.

Pathophysiology of type 2 diabetes and the impact of altered metabolic interorgan crosstalk

Diabetes is a complex and multifactorial disease that affects millions of people worldwide, reducing the quality of life significantly, and results in grave consequences for our health care system. In type 2 diabetes (T2D), the lack of β-cell compensatory mechanisms overcoming peripherally developed insulin resistance is a paramount factor leading to disturbed blood glucose levels and lipid metabolism. Impaired β-cell functions and insulin resistance have been studied extensively resulting in a good understanding of these pathways but much less is known about interorgan crosstalk, which we define as signaling between tissues by secreted factors. Besides hormones and organokines, dysregulated blood glucose and long-lasting hyperglycemia in T2D is associated with changes in metabolism with metabolites from different tissues contributing to the development of this disease. Recent data suggest that metabolites, such as lipids including free fatty acids and amino acids, play important roles in the interorgan crosstalk during the development of T2D. In general, metabolic remodeling affects physiological homeostasis and impacts the development of T2D. Hence, we highlight the importance of metabolic interorgan crosstalk in this review to gain enhanced knowledge of the pathophysiology of T2D, which may lead to new therapeutic approaches to treat this disease.

The Impact of Phytochemicals in Obesity-Related Metabolic Diseases: Focus on Ceramide Metabolism

The prevalence of obesity and related metabolic diseases has increased dramatically worldwide. As obesity progresses, various lipid species accumulate in ectopic tissues. Amongst them, ceramides-a deleterious sphingolipid species-accumulate and cause lipotoxicity and metabolic disturbances. Dysregulated ceramide metabolism appears to be a key feature in the pathogenesis of obesity-related metabolic diseases. Notably, dietary modification might have an impact on modulating ceramide metabolism. Phytochemicals are plant-derived compounds with various physiological properties, which have been shown to protect against obesity-related metabolic diseases. In this review, we aim to examine the impact of a myriad of phytochemicals and their dietary sources in altering ceramide deposition and ceramide-related metabolism from in vitro, in vivo, and human clinical/epidemiological studies. This review discusses how numerous phytochemicals are able to alleviate ceramide-induced metabolic defects and reduce the risk of obesity-related metabolic diseases via diverse mechanisms.

Reducing Akt2 in retinal pigment epithelial cells causes a compensatory increase in Akt1 and attenuates diabetic retinopathy

The retinal pigment epithelium (RPE) plays an important role in the development of diabetic retinopathy (DR), a leading cause of blindness worldwide. Here we set out to explore the role of Akt2 signaling-integral to both RPE homeostasis and glucose metabolism-to DR. Using human tissue and genetically manipulated mice (including RPE-specific conditional knockout (cKO) and knock-in (KI) mice), we investigate whether Akts in the RPE influences DR in models of diabetic eye disease. We found that Akt1 and Akt2 activities were reciprocally regulated in the RPE of DR donor tissue and diabetic mice. Akt2 cKO attenuated diabetes-induced retinal abnormalities through a compensatory upregulation of phospho-Akt1 leading to an inhibition of vascular injury, inflammatory cytokine release, and infiltration of immune cells mediated by the GSK3β/NF-κB signaling pathway; overexpression of Akt2 has no effect. We propose that targeting Akt1 activity in the RPE may be a novel therapy for treating DR.

The Lard Works in Mysterious Ways: Ceramides in Nutrition-Linked Chronic Disease

Diet influences onset, progression, and severity of several chronic diseases, including heart failure, diabetes, steatohepatitis, and a subset of cancers. The prevalence and clinical burden of these obesity-linked diseases has risen over the past two decades. These metabolic disorders are driven by ectopic lipid deposition in tissues not suited for fat storage, leading to lipotoxic disruption of cell function and survival. Sphingolipids such as ceramides are among the most deleterious and bioactive metabolites that accrue, as they participate in selective insulin resistance, dyslipidemia, oxidative stress and apoptosis. This review discusses our current understanding of biochemical pathways controlling ceramide synthesis, production and action; influences of diet on ceramide levels; application of circulating ceramides as clinical biomarkers of metabolic disease; and molecular mechanisms linking ceramides to altered metabolism and survival of cells. Development of nutritional or pharmacological strategies to lower ceramides could have therapeutic value in a wide range of prevalent diseases.

Ceramide changes in abdominal subcutaneous and visceral adipose tissue among diabetic and nondiabetic patients

BACKGROUND

This study profiles ceramides extracted from visceral and subcutaneous adipose tissue of human subjects by liquid chromatography-mass spectrometry to determine a correlation with status of diabetes and gender.

METHODS

Samples of visceral and abdominal wall subcutaneous adipose tissue (n = 36 and n = 31, respectively) were taken during laparoscopic surgery from 36 patients (14 nondiabetic, 22 diabetic and prediabetic) undergoing bariatric surgery with a body mass index (BMI) >35 kg/m² with ≥1 existing comorbidity or BMI ≥40 kg/m² . Sphingolipids were extracted and analyzed using liquid chromatography-mass spectrometry.

RESULTS

After logarithm 2 conversion, paired analysis of visceral to subcutaneous tissue showed differential accumulation of Cer(d18:1/16:0), Cer(d18:1/18:0), and Cer(d18:1/24:1) in visceral tissue of prediabetic/diabetic female subjects, but not in males. Within-tissue analysis showed higher mean levels of ceramide species linked to insulin resistance, such as Cer(d18:1/18:0) and Cer(d18:1/16:0), in visceral tissue of prediabetic/diabetic patients compared with nondiabetic subjects and higher content of Cer(d18:1/14:0) in subcutaneous tissue of insulin-resistant female patients compared with prediabetic/diabetic males. Statistically significant differences in mean levels of ceramide species between insulin-resistant African American and insulin-resistant Caucasian patients were not evident in visceral or subcutaneous tissue.

CONCLUSIONS

Analysis of ceramides is important for developing a better understanding of biological processes underlying type 2 diabetes, metabolic syndrome, and obesity. Knowledge of the accumulated ceramides/dihydroceramides may reflect on the prelipolytic state that leads the lipotoxic phase of insulin resistance and may shed light on the predisposition to insulin resistance by gender.

Serine Palmitoyltransferase Gene Silencing Prevents Ceramide Accumulation and Insulin Resistance in Muscles in Mice Fed a High-Fat Diet

Skeletal muscles account for ~80% of insulin-stimulated glucose uptake and play a key role in lipid metabolism. Consumption of a high-fat diet (HFD) contributes to metabolic changes in muscles, including the development of insulin resistance. The studies carried out to date indicate that the accumulation of biologically active lipids, such as long-chain acyl-CoA, diacylglycerols and ceramides, play an important role in the development of insulin resistance in skeletal muscles. Unfortunately, it has not yet been clarified which of these lipid groups plays the dominant role in inducing these disorders. In order to explore this topic further, we locally silenced the gene encoding serine palmitoyltransferase (SPT) in the gastrocnemius muscle of animals with HFD-induced insulin resistance. This enzyme is primarily responsible for the first step of de novo ceramide biosynthesis. The obtained results confirm that the HFD induces the development of whole-body insulin resistance, which results in inhibition of the insulin pathway. This is associated with an increased level of biologically active lipids in the muscles. Our results also demonstrate that silencing the SPT gene with the shRNA plasmid reduces the accumulation of ceramides in gastrocnemius muscle, which, in turn, boosts the activity of the insulin signaling pathway. Furthermore, inhibition of ceramide synthesis does not significantly affect the content of other lipids, which suggests the leading role of ceramide in the lipid-related induction of skeletal muscle insulin resistance.

Blood glucose regulation in context of infection

The immune and endocrine systems collectively control homeostasis in the body. The endocrine system ensures that values of essential factors and nutrients such as glucose, electrolytes and vitamins are maintained within threshold values. The immune system resolves local disruptions in tissue homeostasis, caused by pathogens or malfunctioning cells. The immediate goals of these two systems do not always align. The immune system benefits from optimal access to nutrients for itself and restriction of nutrient availability to all other organs to limit pathogen replication. The endocrine system aims to ensure optimal nutrient access for all organs, limited only by the nutrients stores that the body has available. The actual state of homeostatic parameters such as blood glucose levels represents a careful balance based on regulatory signals from the immune and endocrine systems. This state is not static but continuously adjusted in response to changes in the current metabolic needs of the body, the amount of resources it has available and the level of threats it encounters. This balance is maintained by the ability of the immune and endocrine systems to interact and co-regulate systemic metabolism. In context of metabolic disease, this system is disrupted, which impairs functionality of both systems. The failure of the endocrine system to retain levels of nutrients such as glucose within threshold values impairs functionality of the immune system. In addition, metabolic stress of organs in context of obesity is perceived by the immune system as a disruption in local homeostasis, which it tries to resolve by the excretion of factors which further disrupt normal metabolic control. In this chapter, we will discuss how the immune and endocrine systems interact under homeostatic conditions and during infection with a focus on blood glucose regulation. In addition, we will discuss how this system fails in the context of metabolic disease.

Ceramide Metabolism and Parkinson's Disease-Therapeutic Targets

Ceramide is a bioactive sphingolipid involved in numerous cellular processes. In addition to being the precursor of complex sphingolipids, ceramides can act as second messengers, especially when they are generated at the plasma membrane of cells. Its metabolic dysfunction may lead to or be a consequence of an underlying disease. Recent reports on transcriptomics and electrospray ionization mass spectrometry analysis have demonstrated the variation of specific levels of sphingolipids and enzymes involved in their metabolism in different neurodegenerative diseases. In the present review, we highlight the most relevant discoveries related to ceramide and neurodegeneration, with a special focus on Parkinson's disease.

Sphingolipids as a Culprit of Mitochondrial Dysfunction in Insulin Resistance and Type 2 Diabetes

Insulin resistance is defined as a complex pathological condition of abnormal cellular and metabolic response to insulin. Obesity and consumption of high-fat diet lead to ectopic accumulation of bioactive lipids in insulin-sensitive tissues. Intracellular lipid accumulation is regarded as one of the major factors in the induction of insulin resistance and type 2 diabetes (T2D). A significant number of studies have described the involvement of ceramides and other sphingolipids in the inhibition of insulin-signaling pathway in both skeletal muscles and the liver. Adverse effects of sphingolipid accumulation have recently been linked to the activation of protein kinase Cζ (PKCζ) and protein phosphatase 2A (PP2A), which, in turn, negatively affect phosphorylation of serine/threonine kinase Akt [also known as protein kinase B (PKB)], leading to decreased glucose uptake in skeletal muscles as well as increased gluconeogenesis and glycogenolysis in the liver. Sphingolipids, in addition to their direct impact on the insulin signaling pathway, may be responsible for other negative aspects of diabetes, namely mitochondrial dysfunction and deficiency. Mitochondrial health, which is characterized by appropriate mitochondrial quantity, oxidative capacity, controlled oxidative stress, undisturbed respiratory chain function, adenosine triphosphate (ATP) production and mitochondrial proliferation through fission and fusion, is impaired in the skeletal muscles and liver of T2D subjects. Recent findings suggest that impaired mitochondrial function may play a key role in the development of insulin resistance. Mitochondria stay in contact with the endoplasmic reticulum (ER), Golgi membranes and mitochondria-associated membranes (MAM) that are the main places of sphingolipid synthesis. Moreover, mitochondria are capable of synthesizing ceramide though ceramide synthase (CerS) activity. Recently, ceramides have been demonstrated to negatively affect mitochondrial respiratory chain function and fission/fusion activity, which is also a hallmark of T2D. Despite a significant correlation between sphingolipids, mitochondrial dysfunction, insulin resistance and T2D, this subject has not received much attention compared to the direct effect of sphingolipids on the insulin signaling pathway. In this review, we focus on the current state of scientific knowledge regarding the involvement of sphingolipids in the induction of insulin resistance by inhibiting mitochondrial function.

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.

Novel Insights and Mechanisms of Lipotoxicity-Driven Insulin Resistance

A large number of studies reported an association between elevated circulating and tissue lipid content and metabolic disorders in obesity, type 2 diabetes (T2D) and aging. This state of uncontrolled tissue lipid accumulation has been called lipotoxicity. It was later shown that excess lipid flux is mainly neutralized within lipid droplets as triglycerides, while several bioactive lipid species such as diacylglycerols (DAGs), ceramides and their derivatives have been mechanistically linked to the pathogenesis of insulin resistance (IR) by antagonizing insulin signaling and action in metabolic organs such as the liver and skeletal muscle. Skeletal muscle and the liver are the main sites of glucose disposal in the body and IR in these tissues plays a pivotal role in the development of T2D. In this review, we critically examine recent literature supporting a causal role of DAGs and ceramides in the development of IR. A particular emphasis is placed on transgenic mouse models with modulation of total DAG and ceramide pools, as well as on modulation of specific subspecies, in relation to insulin sensitivity. Collectively, although a wide number of studies converge towards the conclusion that both DAGs and ceramides cause IR in metabolic organs, there are still some uncertainties on their mechanisms of action. Recent studies reveal that subcellular localization and acyl chain composition are determinants in the biological activity of these lipotoxic lipids and should be further examined.

Ceramides in Adipose Tissue

Adipose tissue is a key nutrient-sensing depot that regulates excess energy storage and consumption. Adipocytes, the key components of the adipose tissue, have unique ability to store excess energy in the form of triglycerides, sense systemic energy demands, and secrete factors (lipids, peptides, cytokines, and adipokines) to regulate other metabolic tissues. The presence of various types of adipocytes (white, brown, and beige) characterized by the number/size of lipid droplets, mitochondrial density, and thermogenic capacity, further highlights how intricate is the communication of these cell-types with other metabolic tissues to sense nutrients. In obesity the inherent capacity of adipose tissue to store and sense nutrients is compromised, causing spillover of the intermediate lipid metabolites into circulation and resulting in their ectopic deposition in tissues not suitable for lipid storage, a phenomenon known as lipotoxicity. This results in a spectrum of cellular dysfunction, that underlies various metabolic diseases. Of the numerous lipid classes implicated in eliciting lipotoxicity, sphingolipid: ceramides are among the most deleterious as they modulate signaling pathways involved in regulating glucose metabolism, triglyceride synthesis, apoptosis, and fibrosis. Notably, recent experimental studies have strongly implicated ceramides in the development of numerous metabolic diseases such as insulin resistance, diabetes, cardiomyopathy, hepatic-steatosis, and atherosclerosis. Herein we discuss and summarizes recent findings that implicate ceramides as a key contributor to adipocyte dysfunction underlying metabolic diseases and how depletion of ceramides can be exploited to improve metabolic health.

Altered mitochondrial metabolism in the insulin-resistant heart

Obesity-induced insulin resistance and type 2 diabetes mellitus can ultimately result in various complications, including diabetic cardiomyopathy. In this case, cardiac dysfunction is characterized by metabolic disturbances such as impaired glucose oxidation and an increased reliance on fatty acid (FA) oxidation. Mitochondrial dysfunction has often been associated with the altered metabolic function in the diabetic heart, and may result from FA-induced lipotoxicity and uncoupling of oxidative phosphorylation. In this review, we address the metabolic changes in the diabetic heart, focusing on the loss of metabolic flexibility and cardiac mitochondrial function. We consider the alterations observed in mitochondrial substrate utilization, bioenergetics and dynamics, and highlight new areas of research which may improve our understanding of the cause and effect of cardiac mitochondrial dysfunction in diabetes. Finally, we explore how lifestyle (nutrition and exercise) and pharmacological interventions can prevent and treat metabolic and mitochondrial dysfunction in diabetes.

Ceramides: Nutrient Signals that Drive Hepatosteatosis

Ceramides are minor components of the hepatic lipidome that have major effects on liver function. These products of lipid and protein metabolism accumulate when the energy needs of the hepatocyte have been met and its storage capacity is full, such that free fatty acids start to couple to the sphingoid backbone rather than the glycerol moiety that is the scaffold for glycerolipids (e.g., triglycerides) or the carnitine moiety that shunts them into mitochondria. As ceramides accrue, they initiate actions that protect cells from acute increases in detergent-like fatty acids; for example, they alter cellular substrate preference from glucose to lipids and they enhance triglyceride storage. When prolonged, these ceramide actions cause insulin resistance and hepatic steatosis, 2 of the underlying drivers of cardiometabolic diseases. Herein the author discusses the mechanisms linking ceramides to the development of insulin resistance, hepatosteatosis and resultant cardiometabolic disorders.

The Cross-Talk Between Sphingolipids and Insulin-Like Growth Factor Signaling: Significance for Aging and Neurodegeneration

Bioactive sphingolipids: sphingosine, sphingosine-1-phosphate (S1P), ceramide, and ceramide-1-phosphate (C1P) are increasingly implicated in cell survival, proliferation, differentiation, and in multiple aspects of stress response in the nervous system. The opposite roles of closely related sphingolipid species in cell survival/death signaling is reflected in the concept of tightly controlled sphingolipid rheostat. Aging has a complex influence on sphingolipid metabolism, disturbing signaling pathways and the properties of lipid membranes. A metabolic signature of stress resistance-associated sphingolipids correlates with longevity in humans. Moreover, accumulating evidence suggests extensive links between sphingolipid signaling and the insulin-like growth factor I (IGF-I)-Akt-mTOR pathway (IIS), which is involved in the modulation of aging process and longevity. IIS integrates a wide array of metabolic signals, cross-talks with p53, nuclear factor κB (NF-κB), or reactive oxygen species (ROS) and influences gene expression to shape the cellular metabolic profile and stress resistance. The multiple connections between sphingolipids and IIS signaling suggest possible engagement of these compounds in the aging process itself, which creates a vulnerable background for the majority of neurodegenerative disorders.

The triacylglycerol, hydroxytriolein, inhibits triple negative mammary breast cancer cell proliferation through a mechanism dependent on dihydroceramide and Akt

The plasma membrane is an attractive target for new anticancer drugs, not least because regulating its lipid structure can control multiple signaling pathways involved in cancer cell proliferation, differentiation and survival. Accordingly, the novel anticancer drug hydroxytriolein (HTO) was designed to interact with and regulate the composition and structure of the membrane, which in turn controls the interaction of amphitropic signaling membrane proteins with the lipid bilayer. Changes in signaling provoked by HTO impair the growth of triple negative breast cancer (TNBC) cells, aggressive breast tumor cells that have a worse prognosis than other types of breast cancers and for which there is as yet no effective targeted therapy. HTO alters the lipid composition and structure of cancer cell membranes, inhibiting the growth of MDA-MB-231 and BT-549 TNBC cells in vitro. Depending on the cellular context, HTO could regulate two pathways involved in TNBC cell proliferation. On the one hand, HTO might stimulate ERK signaling and induce TNBC cell autophagy, while on the other, it could increase dihydroceramide and ceramide production, which would inhibit Akt independently of EGFR activation and provoke cell death. In vivo studies using a model of human TNBC show that HTO and its fatty acid constituent (2-hydroxyoleic acid) impair tumor growth, with no undesired side effects. For these reasons, HTO appears to be a promising anticancer molecule that targets the lipid bilayer (membrane-lipid therapy). By regulating membrane lipids, HTO controls important signaling pathways involved in cancer cell growth, the basis of its pharmacological efficacy and safety.

AKT/protein kinase B associates with β-actin in the nucleus of melanoma cells

The serine-threonine kinase AKT/PKB is a critical regulator of various essential cellular processes, and dysregulation of AKT has been implicated in many diseases, including cancer. Despite AKT action is known to function mainly in the cytoplasm, AKT has been reported to translocate to the nucleus. However, very little is known about the mechanism required for the nuclear import of AKT as well as its function in this cellular compartment. In the present study, we characterized the presence of endogenous nuclear AKT in human melanoma cells and addressed the possible role of AKT by exploring its potential association with key interaction nuclear partners. Confocal and Western blot analyses showed that both phosphorylated and non-phosphorylated forms of AKT are present in melanoma cells nuclei. Using mass spectrometry in combination with protein-crosslinking and co-immunoprecipitation, we identified a series of putative protein partners of nuclear AKT, including heterogeneous nuclear ribonucleoprotein (hnRNP), cytoskeleton proteins β-actin, γ-actin, β-actin-like 2 and vimentin. Confocal microscopy and biochemical analyses validated β-actin as a new nuclear AKT-interacting partner. Cofilin and active RNA Polymerase II, two proteins that have been described to interact and work in concert with nuclear actin in transcription regulation, were also found associated with nuclear AKT. Overall, the present study uncovered a yet unrecognized nuclear coupling of AKT and provides insights into the involvement of AKT in the interaction network of nuclear actin.

A role for phosphatidylcholine and phosphatidylethanolamine in hepatic insulin signaling

Phosphatidylethanolamine N-methyltransferase (PEMT) is an important enzyme in hepatic phosphatidylcholine (PC) biosynthesis. Pemt^-/- mice fed a high-fat diet are protected from obesity and whole-body insulin resistance. However, Pemt^-/- mice develop severe nonalcoholic steatohepatitis (NASH). Because NASH is often associated with hepatic insulin resistance, we investigated whether the increased insulin sensitivity in Pemt^-/- mice was restricted to nonhepatic tissues or whether the liver was also insulin sensitive. Strikingly, the livers of Pemt^-/- mice compared with those of Pemt^+/+ mice were not insulin resistant, despite elevated levels of hepatic triacylglycerols and diacylglycerols, as well as increased hepatic inflammation and fibrosis. Endogenous glucose production was lower in Pemt^-/- mice under both basal and hyperinsulinemic conditions. Experiments in primary hepatocytes and hepatoma cells revealed improved insulin signaling in the absence of PEMT, which was not due to changes in diacylglycerols, ceramides, or gangliosides. On the other hand, the phospholipid composition in hepatocytes seems critically important for insulin signaling such that lowering the PC:phosphatidylethanolamine (PE) ratio improves insulin signaling. Thus, treatments to reduce the PC:PE ratio in liver may protect against the development of hepatic insulin resistance.-Van der Veen, J. N., Lingrell, S., McCloskey, N., LeBlond, N. D., Galleguillos, D., Zhao, Y. Y., Curtis, J. M., Sipione, S., Fullerton, M. D., Vance, D. E., Jacobs, R. L. A role for phosphatidylcholine and phosphatidylethanolamine in hepatic insulin signaling.

The Role of Ceramide and Sphingosine-1-Phosphate in Alzheimer's Disease and Other Neurodegenerative Disorders

Bioactive sphingolipids-ceramide, sphingosine, and their respective 1-phosphates (C1P and S1P)-are signaling molecules serving as intracellular second messengers. Moreover, S1P acts through G protein-coupled receptors in the plasma membrane. Accumulating evidence points to sphingolipids' engagement in brain aging and in neurodegenerative disorders such as Alzheimer's, Parkinson's, and Huntington's diseases and amyotrophic lateral sclerosis. Metabolic alterations observed in the course of neurodegeneration favor ceramide-dependent pro-apoptotic signaling, while the levels of the neuroprotective S1P are reduced. These trends are observed early in the diseases' development, suggesting causal relationship. Mechanistic evidence has shown links between altered ceramide/S1P rheostat and the production, secretion, and aggregation of amyloid β/α-synuclein as well as signaling pathways of critical importance for the pathomechanism of protein conformation diseases. Sphingolipids influence multiple aspects of Akt/protein kinase B signaling, a pathway that regulates metabolism, stress response, and Bcl-2 family proteins. The cross-talk between sphingolipids and transcription factors including NF-κB, FOXOs, and AP-1 may be also important for immune regulation and cell survival/death. Sphingolipids regulate exosomes and other secretion mechanisms that can contribute to either the spread of neurotoxic proteins between brain cells, or their clearance. Recent discoveries also suggest the importance of intracellular and exosomal pools of small regulatory RNAs in the creation of disturbed signaling environment in the diseased brain. The identified interactions of bioactive sphingolipids urge for their evaluation as potential therapeutic targets. Moreover, the early disturbances in sphingolipid metabolism may deliver easily accessible biomarkers of neurodegenerative disorders.

Roles of Ceramides and Other Sphingolipids in Immune Cell Function and Inflammation

Ceramides are bioactive sphingolipids that support the structure of the plasma membrane and mediate numerous cell-signaling events in eukaryotic cells. The finding that ceramides act as second messengers transducing cellular signals has attracted substantial attention in several fields of Biology. Since all cells contain lipid plasma membranes, the impact of various ceramides, ceramide synthases, ceramide metabolites, and other sphingolipids has been implicated in a vast range of cellular functions including, migration, proliferation, response to external stimuli, and death. The roles of lipids in these functions widely differ among the diverse cell types. Herein, we discuss the roles of ceramides and other sphingolipids in mediating the function of various immune cells; particularly dendritic cells, neutrophils, and macrophages. In addition, we highlight the main studies describing effects of ceramides in inflammation, specifically in various inflammatory settings including insulin resistance, graft-versus-host disease, immune suppression in cancer, multiple sclerosis, and inflammatory bowel disease.

Sphingolipids in adipose tissue: What's tipping the scale?

Adipose tissue lies at the heart of obesity, mediating its many effects upon the rest of the body, with its unique capacity to expand and regenerate, throughout the lifespan of the organism. Adipose is appreciated as an endocrine organ, with its myriad adipokines that elicit both physiological and pathological outcomes. Sphingolipids, bioactive signaling molecules, affect many aspects of obesity and the metabolic syndrome. While sphingolipids are appreciated in the context of these diseases in other tissues, there are many discoveries yet to be uncovered in the adipose tissue. This review focuses on the effects of sphingolipids on various aspects of adipose function and dysfunction. The processes of adipogenesis, metabolism and thermogenesis, in addition to inflammation and insulin resistance are intimately linked to sphingolipids as discussed below.

Overexpression of acid ceramidase (ASAH1) protects retinal cells (ARPE19) from oxidative stress

Over 11 million people in the United States alone have some form of age-related macular degeneration (AMD). Oxidative stress, cell death, and the degeneration of retinal pigment epithelial (RPE) cells contribute to AMD pathology. Recent evidence suggests that ceramide (Cer), a cellular sphingolipid mediator that acts as a second messenger to induce apoptosis, might play a role in RPE cell death. The lysosomal breakdown of Cer by acid ceramidase [N-acylsphingosine amidohydrolase (ASAH)1] into sphingosine (Sph) is the major source for Sph 1-phosphate production, which has an opposing role to Cer and provides cytoprotection. Here, we investigated the role of Cer in human RPE-derived ARPE19 cells under hydrogen peroxide-induced oxidative stress, and show that Cer and hexosyl-Cer levels increase in the oxidatively stressed ARPE19 cells, which can be prevented by overexpression of lysosomal ASAH1. This study demonstrates that oxidative stress generates sphingolipid death mediators in retinal cells and that induction of ASAH1 could rescue retinal cells from oxidative stress by hydrolyzing excess Cers.

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

Ethanol and C2 ceramide activate fatty acid oxidation in human hepatoma cells

Obesogenic lipids and the sphingolipid ceramide have been implicated as potential cofactors in alcoholic liver disease (ALD) patients. However, the mechanisms by which these lipids modulate lipid trafficking in ethanol-treated human liver cells to promote steatosis, an early stage of ALD, are poorly understood. We measured fatty acid (FA) uptake, triglyceride export, FA synthesis and FA oxidation in human hepatoma (VL-17A) cells in response to ethanol and the exogenous lipids oleate, palmitate and C2 ceramide. We found that in combination with ethanol, both oleate and palmitate promote lipid droplet accumulation while C2 ceramide inhibits lipid droplet accumulation by enhancing FA oxidation. Further, using both a pharmacologic and siRNA approach to reduce peroxisome proliferator-activated receptors α (PPARα) gene expression, we demonstrate that C2 ceramide abrogates ethanol-mediated suppression of FA oxidation through an indirect PPARα mechanism. Together, these data suggest that lipids interact differentially with ethanol to modulate hepatocellular lipid droplet accumulation and may provide novel targets for preventing the earliest stage of alcoholic liver disease, alcoholic steatosis.

Defining lipid mediators of insulin resistance - controversies and challenges

Essential elements of all cells, lipids play important roles in energy production, signalling and as structural components. Despite these critical functions, excessive availability and intracellular accumulation of lipid is now recognised as a major factor contributing to many human diseases, including obesity and diabetes. In the context of these metabolic disorders, ectopic deposition of lipid has been proposed to have deleterious effects of insulin action. While this relationship has been recognised for some time now, there is currently no unifying mechanism to explain how lipids precipitate the development of insulin resistance. This review summarises the evidence linking specific lipid molecules to the induction of insulin resistance, describing some of the current controversies and challenges for future studies in this field.

The ceramide activated protein phosphatase Sit4 impairs sphingolipid dynamics, mitochondrial function and lifespan in a yeast model of Niemann-Pick type C1

The Niemann-Pick type C is a rare neurodegenerative disease that results from loss-of-function point mutations in NPC1 or NPC2, which affect the homeostasis of sphingolipids and sterols in human cells. We have previously shown that yeast lacking Ncr1, the orthologue of human NPC1 protein, display a premature ageing phenotype and higher sensitivity to oxidative stress associated with mitochondrial dysfunctions and accumulation of long chain bases. In this study, a lipidomic analysis revealed specific changes in the levels of ceramide species in ncr1Δ cells, including decreases in dihydroceramides and increases in phytoceramides. Moreover, the activation of Sit4, a ceramide-activated protein phosphatase, increased in ncr1Δ cells. Deletion of SIT4 or CDC55, its regulatory subunit, increased the chronological lifespan and hydrogen peroxide resistance of ncr1Δ cells and suppressed its mitochondrial defects. Notably, Sch9 and Pkh1-mediated phosphorylation of Sch9 decreased significantly in ncr1Δsit4Δ cells. These results suggest that phytoceramide accumulation and Sit4-dependent signaling mediate the mitochondrial dysfunction and shortened lifespan in the yeast model of Niemann-Pick type C1, in part through modulation of the Pkh1-Sch9 pathway.

Roles of Diacylglycerols and Ceramides in Hepatic Insulin Resistance

Although ample evidence links hepatic lipid accumulation with hepatic insulin resistance, the mechanistic basis of this association is incompletely understood and controversial. Diacylglycerols (DAGs) and ceramides have emerged as the two best-studied putative mediators of lipid-induced hepatic insulin resistance. Both lipids were first associated with insulin resistance in skeletal muscle and were subsequently hypothesized to mediate insulin resistance in the liver. However, the putative roles for DAGs and ceramides in hepatic insulin resistance have proved more complex than originally imagined, with various genetic and pharmacologic manipulations yielding a vast and occasionally contradictory trove of data to sort. In this review we examine the state of this field, turning a critical eye toward both DAGs and ceramides as putative mediators of lipid-induced hepatic insulin resistance.

Myriocin treatment affects lipid metabolism in skeletal muscles of rats with streptozotocin-induced type 1 diabetes

PURPOSE

The aim of this work was to assess the effect(s) of de novo ceramide synthesis inhibition on lipid metabolism in skeletal muscle tissue of type 1 diabetic rats. The latter seems to be of vital importance, since previous works have shown its positive influence on lipid metabolism and glucose homeostasis in the case of its counterpart - type 2 diabetes.

MATERIALS/METHODS

The animals were randomly assigned to one of the following groups: C - control, M - myriocin (ceramide de novo synthesis inhibitor), D - diabetes (induced by streptozotocin injections); D+M - diabetes+myriocin. We have evaluated intracellular concentration of key sphingolipid species, via chromatography (GC and HPLC), and the activity of their most important enzymes, using radiometric approach. The aforementioned assessments were evaluated in respect to the three different types of muscle tissue representing different spectra of muscle metabolism (soleus - oxidative, red gastrocnemious - oxidative-glycolytic, white gastrocnemious - glycolytic).

RESULTS

Interestingly, our therapeutic intervention not only lowered the level of ceramide, its precursors (sphinganine) and derivatives (sphingosine and sphingosine-1-phosphate), but also reduced other lipid species (triacylglycerols, diacylglycerols and free fatty acids) content, thus improving glucose homeostasis in type 1 diabetic animals.

CONCLUSIONS

In the light of the results ensuing from this study, it seems conceivable that the reduction of intramuscular ceramide production and accumulation could bestow an insulin-sensitizing effect. If so, then SPT inhibition could find potential future applications as a therapeutic intervention aimed to mitigate the effects of insulin resistance.

Role of Ceramide in Apoptosis and Development of Insulin Resistance

This review presents data on the functional biochemistry of ceramide, one of the key sphingolipids with properties of a secondary messenger. Molecular mechanisms of the involvement of ceramide in apoptosis in pancreatic β-cells and its role in the formation of insulin resistance in pathogenesis of type 2 diabetes are reviewed. One of the main predispositions for the development of insulin resistance and diabetes is obesity, which is associated with ectopic fat deposition and significant increase in intracellular concentrations of cytotoxic ceramides. A possible approach to the restoration of tissue sensitivity to insulin in type 2 diabetes based on selective reduction of the content of cytotoxic ceramides is discussed.

Ceramides and mitochondrial fatty acid oxidation in obesity

Obesity is an epidemic, complex disease that is characterized by increased glucose, lipids, and low-grade inflammation in the circulation, among other factors. It creates the perfect scenario for the production of ceramide, the building block of the sphingolipid family of lipids, which is involved in metabolic disorders such as obesity, diabetes, and cardiovascular disease. In addition, obesity causes a decrease in fatty acid oxidation (FAO), which contributes to lipid accumulation within the cells, conferring more susceptibility to cell dysfunction. C16:0 ceramide, a specific ceramide species, has been identified recently as the principal mediator of obesity-derived insulin resistance, impaired fatty acid oxidation, and hepatic steatosis. In this review, we have sought to cover the importance of the ceramide species and their metabolism, the main ceramide signaling pathways in obesity, and the link between C16:0 ceramide, FAO, and obesity.-Fucho, R., Casals, N., Serra, D., Herrero, L. Ceramides and mitochondrial fatty acid oxidation in obesity.

Lipid profiling of the therapeutic effects of berberine in patients with nonalcoholic fatty liver disease

BACKGROUND

We recently demonstrated a positive effect of berberine on nonalcoholic fatty liver disease patients after 16 weeks of treatment by comparing mere lifestyle intervention in type 2 diabetes patients with berberine treatment, which decreased the content of hepatic fat. However, the potential mechanisms of the clinical effects are unclear. We used a lipidomic approach to characterize the state of lipid metabolism as reflected in the circulation of subjects with nonalcoholic fatty liver disease (NAFLD) before and after berberine treatment.

METHODS

Liquid chromatography-mass spectrometry evaluated the various lipid metabolites in serum samples obtained from the participants (41 patients in the berberine group and 39 patients in the mere lifestyle intervention group) before and after treatment.

RESULTS

A total of 256 serum lipid molecular species were identified and quantified. Both treatments regulated various types of lipids in metabolic pathways, such as free fatty acids, phosphoglycerides and glycerides, in metabolic pathways, but berberine induced a substantially greater change in serum lipid species compared with mere lifestyle intervention after treatment. Berberine also caused obvious differences on ceramides. Berberine treatment markedly decreased serum levels of ceramide and ceramide-1-phosphate.

CONCLUSIONS

Berberine altered circulating ceramides, which may underlie the improvement in fatty liver disease. ClinicalTrials.gov NCT00633282, Registered March 3, 2008.

Hydrogen peroxide/ceramide/Akt signaling axis play a critical role in the antileukemic potential of sanguinarine

Dysregulation of apoptosis is a prime hallmark of leukemia. Therefore, drugs which restore the sensitivity of leukemic cells to apoptotic stimuli are promising candidates in the treatment of leukemia. Recently, we have demonstrated that sanguinarine (SNG), a benzophenanthridine alkaloid, isolated from Sanguinaria canadensis induces ROS-dependent ERK1/2 activation and autophagic cell death in human malignant glioma cells (Pallichankandy et al., 2015; [43]). In this study, we investigated the antileukemic potential of SNG in vitro, and further examined the molecular mechanisms of SNG-induced cell death. In human leukemic cells, SNG activated apoptotic cell death pathway characterized by activation of caspase cascade, DNA fragmentation and down-regulation of anti-apoptotic proteins. Importantly, we have identified a crucial role for hydrogen peroxide (H2O2)-dependent ceramide (Cer) generation in the facilitation of SNG-induced apoptosis. Additionally, we have found that SNG inhibits Akt, a key anti-apoptotic protein kinase by dephosphorylating it at Ser(473), leading to the dephosphorylation of its downstream targets, GSK3β and mTOR. Interestingly, inhibition of Cer generation, using acid sphingomyelinase inhibitor, significantly reduced the SNG-induced Akt dephosphorylation and apoptosis, whereas, activation of Cer generation using inhibitors of acid ceramidase and glucosylceramide synthase enhanced it. Furthermore, using a group of ceramide activated protein phosphatases (CAPPs) inhibitor (calyculin A, Okadaic acid, and phosphatidic acid), the involvement of protein phosphatase 1 form of CAPP in SNG-induced Akt dephosphorylation and apoptosis was demonstrated. Altogether, these results underscore a critical role for H2O2-Cer-Akt signaling axis in the antileukemic action of SNG.

Tumor suppressive functions of ceramide: evidence and mechanisms

Studies over the past two decades have identified ceramide as a multifunctional central molecule in the sphingolipid biosynthetic pathway. Given its diverse tumor suppressive activities, molecular understanding of ceramide action will produce fundamental insights into processes that limit tumorigenesis and may identify key molecular targets for therapeutic intervention. Ceramide can be activated by a diverse array of stresses such as heat shock, genotoxic damage, oxidative stress and anticancer drugs. Ceramide triggers a variety of tumor suppressive and anti-proliferative cellular programs such as apoptosis, autophagy, senescence, and necroptosis by activating or repressing key effector molecules. Defects in ceramide generation and metabolism in cancer contribute to tumor cell survival and resistance to chemotherapy. The potent and versatile anticancer activity profile of ceramide has motivated drug development efforts to (re-)activate ceramide in established tumors. This review focuses on our current understanding of the tumor suppressive functions of ceramide and highlights the potential downstream targets of ceramide which are involved in its tumor suppressive action.

A reflection of the lasting contributions from Dr. Robert Bittman to sterol trafficking, sphingolipid and phospholipid research

With the passing of Dr. Robert Bittman from pancreatic cancer on the 1st October 2014, the lipid research field lost one of the most influential and significant personalities. Robert Bittman's genius was in chemical design and his contribution to the lipid research field was truly immense. The reagents and chemicals he designed and synthesised allowed interrogation of the role of lipids in constituting complex biophysical membranes, sterol transfer and in cellular communication networks. Here we provide a review of these works which serve as a lasting memory to his life.

Ceramides in Alzheimer's Disease: Key Mediators of Neuronal Apoptosis Induced by Oxidative Stress and Aβ Accumulation

Alzheimer's disease (AD), the most common chronic and progressive neurodegenerative disorder, is characterized by extracellular deposits of amyloid β-peptides (Aβ) and intracellular deposits of hyperphosphorylated tau (phospho-tau) protein. Ceramides, the major molecules of sphingolipid metabolism and lipid second messengers, have been associated with AD progression and pathology via Aβ generation. Enhanced levels of ceramides directly increase Aβ through stabilization of β-secretase, the key enzyme in the amyloidogenic processing of Aβ precursor protein (APP). As a positive feedback loop, the generated oligomeric and fibrillar Aβ induces a further increase in ceramide levels by activating sphingomyelinases that catalyze the catabolic breakdown of sphingomyelin to ceramide. Evidence also supports important role of ceramides in neuronal apoptosis. Ceramides may initiate a cascade of biochemical alterations, which ultimately leads to neuronal death by diverse mechanisms, including depolarization and permeabilization of mitochondria, increased production of reactive oxygen species (ROS), cytochrome c release, Bcl-2 depletion, and caspase-3 activation, mainly by modulating intracellular signalling, particularly along the pathways related to Akt/PKB kinase and mitogen-activated protein kinases (MAPKs). This review summarizes recent findings related to the role of ceramides in oxidative stress-driven neuronal apoptosis and interplay with Aβ in the cascade of events ending in neuronal degeneration.

Mitochondrial fission mediates ceramide-induced metabolic disruption in skeletal muscle

Ceramide is a sphingolipid that serves as an important second messenger in an increasing number of stress-induced pathways. Ceramide has long been known to affect the mitochondria, altering both morphology and physiology. We sought to assess the impact of ceramide on skeletal muscle mitochondrial structure and function. A primary observation was the rapid and dramatic division of mitochondria in ceramide-treated cells. This effect is likely to be a result of increased Drp1 (dynamin-related protein 1) action, as ceramide increased Drp1 expression and Drp1 inhibition prevented ceramide-induced mitochondrial fission. Further, we found that ceramide treatment reduced mitochondrial O2 consumption (i.e. respiration) in cultured myotubes and permeabilized red gastrocnemius muscle fibre bundles. Ceramide treatment also increased H2O2 levels and reduced Akt/PKB (protein kinase B) phosphorylation in myotubes. However, inhibition of mitochondrial fission via Drp1 knockdown completely protected the myotubes and fibre bundles from ceramide-induced metabolic disruption, including maintained mitochondrial respiration, reduced H2O2 levels and unaffected insulin signalling. These data suggest that the forced and sustained mitochondrial fission that results from ceramide accrual may alter metabolic function in skeletal muscle, which is a prominent site not only of energy demand (via the mitochondria), but also of ceramide accrual with weight gain.

Involvement of the Na+/Ca2+ exchanger isoform 1 (NCX1) in neuronal growth factor (NGF)-induced neuronal differentiation through Ca2+-dependent Akt phosphorylation

NGF induces neuronal differentiation by modulating [Ca(2+)]i. However, the role of the three isoforms of the main Ca(2+)-extruding system, the Na(+)/Ca(2+) exchanger (NCX), in NGF-induced differentiation remains unexplored. We investigated whether NCX1, NCX2, and NCX3 isoforms could play a relevant role in neuronal differentiation through the modulation of [Ca(2+)]i and the Akt pathway. NGF caused progressive neurite elongation; a significant increase of the well known marker of growth cones, GAP-43; and an enhancement of endoplasmic reticulum (ER) Ca(2+) content and of Akt phosphorylation through an early activation of ERK1/2. Interestingly, during NGF-induced differentiation, the NCX1 protein level increased, NCX3 decreased, and NCX2 remained unaffected. At the same time, NCX total activity increased. Moreover, NCX1 colocalized and coimmunoprecipitated with GAP-43, and NCX1 silencing prevented NGF-induced effects on GAP-43 expression, Akt phosphorylation, and neurite outgrowth. On the other hand, the overexpression of its neuronal splicing isoform, NCX1.4, even in the absence of NGF, induced an increase in Akt phosphorylation and GAP-43 protein expression. Interestingly, tetrodotoxin-sensitive Na(+) currents and 1,3-benzenedicarboxylic acid, 4,4'-[1,4,10-trioxa-7,13-diazacyclopentadecane-7,13-diylbis(5-methoxy-6,12-benzofurandiyl)]bis-, tetrakis[(acetyloxy)methyl] ester-detected [Na(+)]i significantly increased in cells overexpressing NCX1.4 as well as ER Ca(2+) content. This latter effect was prevented by tetrodotoxin. Furthermore, either the [Ca(2+)]i chelator(1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) (BAPTA-AM) or the PI3K inhibitor LY 294002 prevented Akt phosphorylation and GAP-43 protein expression rise in NCX1.4 overexpressing cells. Moreover, in primary cortical neurons, NCX1 silencing prevented Akt phosphorylation, GAP-43 and MAP2 overexpression, and neurite elongation. Collectively, these data show that NCX1 participates in neuronal differentiation through the modulation of ER Ca(2+) content and PI3K signaling.