Ceramide induces the dephosphorylation and inhibition of constitutively activated Akt in PTEN negative U87mg cells

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.

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.

The Dark Side of Sphingolipids: Searching for Potential Cardiovascular Biomarkers

Cardiovascular diseases (CVDs) are the leading cause of death and illness in Europe and worldwide, responsible for a staggering 47% of deaths in Europe. Over the past few years, there has been increasing evidence pointing to bioactive sphingolipids as drivers of CVDs. Among them, most studies place emphasis on the cardiovascular effect of ceramides and sphingosine-1-phosphate (S1P), reporting correlation between their aberrant expression and CVD risk factors. In experimental in vivo models, pharmacological inhibition of de novo ceramide synthesis averts the development of diabetes, atherosclerosis, hypertension and heart failure. In humans, levels of circulating sphingolipids have been suggested as prognostic indicators for a broad spectrum of diseases. This article provides a comprehensive review of sphingolipids' contribution to cardiovascular, cerebrovascular and metabolic diseases, focusing on the latest experimental and clinical findings. Cumulatively, these studies indicate that monitoring sphingolipid level alterations could allow for better assessment of cardiovascular disease progression and/or severity, and also suggest them as a potential target for future therapeutic intervention. Some approaches may include the down-regulation of specific sphingolipid species levels in the circulation, by inhibiting critical enzymes that catalyze ceramide metabolism, such as ceramidases, sphingomyelinases and sphingosine kinases. Therefore, manipulation of the sphingolipid pathway may be a promising strategy for the treatment of cardio- and cerebrovascular diseases.

Depletion of ALMS1 affects TGF-β signalling pathway and downstream processes such as cell migration and adhesion capacity

Background: ALMS1 is a ubiquitous gene associated with Alström syndrome (ALMS). The main symptoms of ALMS affect multiple organs and tissues, generating at last, multi-organic fibrosis in the lungs, kidneys and liver. TGF-β is one of the main pathways implicated in fibrosis, controlling the cell cycle, apoptosis, cell migration, cell adhesion and epithelial-mesenchymal transition (EMT). Nevertheless, the role of ALMS1 gene in fibrosis generation and other implicated processes such as cell migration or cell adhesion via the TGF- β pathway has not been elucidated yet. Methods: Initially, we evaluated how depletion of ALMS1 affects different processes like apoptosis, cell cycle and mitochondrial activity in HeLa cells. Then, we performed proteomic profiling with TGF-β stimuli in HeLa ALMS1 -/- cells and validated the results by examining different EMT biomarkers using qPCR. The expression of these EMT biomarkers were also studied in hTERT-BJ-5ta ALMS1 -/-. Finally, we evaluated the SMAD3 and SMAD2 phosphorylation and cell migration capacity in both models. Results: Depletion of ALMS1 generated apoptosis resistance to thapsigargin (THAP) and C2-Ceramide (C2-C), and G2/M cell cycle arrest in HeLa cells. For mitochondrial activity, results did not show significant differences between ALMS1 +/+ and ALMS1 -/-. Proteomic results showed inhibition of downstream pathways regulated by TGF-β. The protein-coding genes (PCG) were associated with processes like focal adhesion or cell-substrate adherens junction in HeLa. SNAI1 showed an opposite pattern to what would be expected when activating the EMT in HeLa and BJ-5ta. Finally, in BJ-5ta model a reduced activation of SMAD3 but not SMAD2 were also observed. In HeLa model no alterations in the canonical TGF-β pathway were observed but both cell lines showed a reduction in migration capacity. Conclusion: ALMS1 has a role in controlling the cell cycle and the apoptosis processes. Moreover, the depletion of ALMS1 affects the signal transduction through the TGF-β and other processes like the cell migration and adhesion capacity.

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.

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.

Ceramide and Sphingosine 1-Phosphate in Liver Diseases

The liver is an important organ in the regulation of glucose and lipid metabolism. It is responsible for systemic energy homeostasis. When energy need exceeds the storage capacity in the liver, fatty acids are shunted into nonoxidative sphingolipid biosynthesis, which increases the level of cellular ceramides. Accumulation of ceramides alters substrate utilization from glucose to lipids, activates triglyceride storage, and results in the development of both insulin resistance and hepatosteatosis, increasing the likelihood of major metabolic diseases. Another sphingolipid metabolite, sphingosine 1-phosphate (S1P) is a bioactive signaling molecule that acts via S1P-specific G protein coupled receptors. It regulates many cellular and physiological events. Since an increase in plasma S1P is associated with obesity, it seems reasonable that recent studies have provided evidence that S1P is linked to lipid pathophysiology, including hepatosteatosis and fibrosis. Herein, we review recent findings on ceramides and S1P in obesity-mediated liver diseases and the therapeutic potential of these sphingolipid metabolites.

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 Full Spectrum of Alzheimer's Disease Is Rooted in Metabolic Derangements That Drive Type 3 Diabetes

The standard practice in neuropathology is to diagnose Alzheimer's disease (AD) based on the distribution and abundance of neurofibrillary tangles and Aβ deposits. However, other significant abnormalities including neuroinflammation, gliosis, white matter degeneration, non-Aβ microvascular disease, and insulin-related metabolic dysfunction require further study to understand how they could be targeted to more effectively remediate AD. This review addresses non-Aβ and non-pTau AD-associated pathologies, highlighting their major features, roles in neurodegeneration, and etiopathic links to deficits in brain insulin and insulin-like growth factor signaling and cognitive impairment. The discussion delineates why AD with its most characteristic clinical and pathological phenotypic profiles should be regarded as a brain form of diabetes, i.e., type 3 diabetes, and entertains the hypothesis that type 3 diabetes is just one of the categories of insulin resistance diseases that can occur independently or overlap with one or more of the others, including type 2 diabetes, metabolic syndrome, and nonalcoholic fatty liver disease.

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.

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.

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.

Direct analysis of PI(3,4,5)P3 using liquid chromatography electrospray ionization tandem mass spectrometry

Phosphatidylinositol (3,4,5) trisphosphate (PIP₃) is a biologically active membrane phospholipid that is essential for the growth and survival of all eukaryotic cells. We describe a new method that directly measures PIP₃ and describe the HPLC separation and measurement of the positional isomers of phosphatidylinositol bisphosphate, PI(3,5)P₂, PI(3,4)P₂ and PI(4,5)P₂. Mass spectrometric analyses were performed online using ultra-high performance liquid chromatography (UHPLC)-electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) in the negative multiple-reaction monitoring (MRM) modes. Rapid separation of PIP₃ from PI, phosphatidylinositol phosphate (PIP) and PIP₂ was accomplished by C18 reverse phase chromatography with the addition of the ion pairing reagents diisopropylethanolamine (DiiPEA) and ethylenediamine tetraacetic acid tetrasodium salt dihydrate (EDTA) to the samples and mobile phase with a total run time, including equilibration, of 12 min. Importantly, these chromatography conditions result in no carryover of PIP, PIP₂, and PIP₃ between samples. To validate the new method, U87MG cancer cells were serum starved and treated with PDGF to stimulate PIP3 biosynthesis in the presence or absence of the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002. Results generated with the LC/MS method were in excellent agreement with results generated using [³³P] phosphate radiolabeled U87MG cells and anion exchange chromatography analysis, a well validated method for measuring PIP₃. To demonstrate the usefulness of the new method, we generated reproducible IC₅₀ data for several well-characterized PI3K small molecule inhibitors using a U87MG cell-based assay as well as showing PIP₃ can be measured from additional cancer cell lines. Together, our results demonstrate this novel method is sensitive, reproducible and can be used to directly measure PIP₃ without radiolabeling or complex lipid derivatization.

Sphingolipids and glycerophospholipids - The "ying and yang" of lipotoxicity in metabolic diseases

Sphingolipids in general and ceramides in particular, contribute to pathophysiological mechanisms by modifying signalling and metabolic pathways. Here, we present the available evidence for a bidirectional homeostatic crosstalk between sphingolipids and glycerophospholipids, whose dysregulation contributes to lipotoxicity induced metabolic stress. The initial evidence for this crosstalk originates from simulated models designed to investigate the biophysical properties of sphingolipids in plasma membrane representations. In this review, we reinterpret some of the original findings and conceptualise them as a sort of "ying/yang" interaction model of opposed/complementary forces, which is consistent with the current knowledge of lipid homeostasis and pathophysiology. We also propose that the dysregulation of the balance between sphingolipids and glycerophospholipids results in a lipotoxic insult relevant in the pathophysiology of common metabolic diseases, typically characterised by their increased ceramide/sphingosine pools.

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.

From Molecules to the Clinic: Linking Schizophrenia and Metabolic Syndrome through Sphingolipids Metabolism

Metabolic syndrome (MS) is a prevalent and severe comorbidity observed in schizophrenia (SZ). The exact nature of this association is controversial and very often accredited to the effects of psychotropic medications and disease-induced life-style modifications, such as inactive lifestyle, poor dietary choices, and smoking. However, drug therapy and disease-induced lifestyle factors are likely not the only factors contributing to the observed converging nature of these conditions, since an increased prevalence of MS is also observed in first episode and drug-naïve psychosis populations. MS and SZ share common intrinsic susceptibility factors and etiopathogenic mechanisms, which may change the way we approach clinical management of SZ patients. Among the most relevant common pathogenic pathways of SZ and MS are alterations in the sphingolipids (SLs) metabolism and SLs homeostasis. SLs have important structural functions as they participate in the formation of membrane “lipid rafts.” SLs also play physiological roles in cell differentiation, proliferation, and inflammatory processes, which might be part of MS/SZ common pathophysiological processes. In this article we review a plausible mechanism to explain the link between MS and SZ through a disruption in SL homeostasis. Additionally, we provide insights on how this hypothesis can lead to the developing of new diagnostic/therapeutic technologies for SZ patients.

Locking PDK1 in DFG-out conformation through 2-oxo-indole containing molecules: Another tools to fight glioblastoma

The phosphoinositide-dependent kinase-1 (PDK1) is one of the main components of the PI3K/Akt pathway. Also named the "master kinase" of the AGC family, PDK1 plays a critical role in tumorigenesis, by enhancing cell proliferation and inhibiting apoptosis, as well as in cell invasion and metastasis formation. Although there have been done huge efforts in discovering specific compounds targeting PDK1, nowadays no PDK1 inhibitor has yet entered the clinic. With the aim to pick out novel and potent PDK1 inhibitors, herein we report the design and synthesis of a new class of molecules obtained by merging the 2-oxo-indole nucleus with the 2-oxo-pyridonyl fragment, two moieties with high affinity for the PDK1 hinge region and its DFG-out binding site, respectively. To this purpose, a small series of compounds were synthesised and a tandem application of docking and Molecular Dynamic (MD) was employed to get insight into their mode of binding. The OXID-pyridonyl hybrid 8, possessing the lower IC50 (IC50 = 112 nM), was also tested against recombinant kinases involved in the PI3K/PDK1/Akt pathway and was subjected to vitro studies to evaluate the cytotoxicity and the inhibition of tumour cell migration. All together the results let us to consider 8, as a lead compound of a new generation of PDK1 inhibitors and encourage us to further studies in this direction.

Proteomic analysis of human glioblastoma cell lines differently resistant to a nitric oxide releasing agent

Glioblastoma multiforme is the most aggressive astrocytoma characterized by the development of resistant cells to various cytotoxic stimuli. Nitric oxide (NO) is able to overcome tumor resistance in PTEN mutated rat C6 glioma cells due to its ability to inhibit cell growth by influencing the intracellular distribution of ceramide. The aim of this study is to monitor the effects of NO donor PAPANONOate on ceramide trafficking in human glioma cell lines, CCF-STTG1 (PTEN-mutated, p53-wt) and T98G (PTEN-harboring, p53-mutated), together with the assessment of their differential molecular signature by 2D-DIGE and MALDI mass spectrometry. In the CCF-STTG1 cell line, the results indicate that treatment with PAPANONOate decreased cell proliferation (<50%) and intracellular trafficking of ceramide, assessed by BODIPY-C5Cer, while these events were not observed in the T98G cell line. Proteomic results suggest that CCF-STTG1 cells are characterized by an increased expression of proteins involved in NO-associated ER stress (i.e. protein disulfide-isomerase A3, calreticulin, 78 kDa glucose-regulated protein), which could compromise ceramide delivery from ER to Golgi, leading to ceramide accumulation in ER and partial growth arrest. Conversely, T98G cell lines, resistant to NO exposure, are characterized by increased levels of cytosolic antioxidant proteins (i.e. glutathione-S-transferase P, peroxiredoxin 1), which might buffer intracellular NO. By providing differential ceramide distribution after NO exposure and differential protein expression of two high grade glioma cell lines, this study highlights specific proteins as possible markers for tumor aggressiveness. This study demonstrates that, in two different high grade glioma cell lines, NO exposure results in a different ceramide distribution and protein expression. Furthermore, this study highlights specific proteins as possible markers for tumor aggressiveness.

Inhibition of ceramide de novo synthesis with myriocin affects lipid metabolism in the liver of rats with streptozotocin-induced type 1 diabetes

Nowadays diabetes is one of the most common metabolic diseases. Sphingolipids, which are vitally important constituents of intracellular signal transduction pathways, may be among the most pathogenic lipid moieties intermingled in the origin and development of diabetes. It is now well established that inhibition of de novo ceramide synthesis with myriocin exerts positive effects on lipid metabolism and glucose homeostasis in type 2 diabetes mellitus animal models. However, its influence on type I diabetes still remains unknown. Therefore, the scope of this paper is to fulfill that particular gap in our knowledge.

Role of ceramide in diabetes mellitus: evidence and mechanisms

Diabetes mellitus is a metabolic disease with multiple complications that causes serious diseases over the years. The condition leads to severe economic consequences and is reaching pandemic level globally. Much research is being carried out to address this disease and its underlying molecular mechanism. This review focuses on the diverse role and mechanism of ceramide, a prime sphingolipid signaling molecule, in the pathogenesis of type 1 and type 2 diabetes and its complications. Studies using cultured cells, animal models, and human subjects demonstrate that ceramide is a key player in the induction of β-cell apoptosis, insulin resistance, and reduction of insulin gene expression. Ceramide induces β-cell apoptosis by multiple mechanisms namely; activation of extrinsic apoptotic pathway, increasing cytochrome c release, free radical generation, induction of endoplasmic reticulum stress and inhibition of Akt. Ceramide also modulates many of the insulin signaling intermediates such as insulin receptor substrate, Akt, Glut-4, and it causes insulin resistance. Ceramide reduces the synthesis of insulin hormone by attenuation of insulin gene expression. Better understanding of this area will increase our understanding of the contribution of ceramide to the pathogenesis of diabetes, and further help in identifying potential therapeutic targets for the management of diabetes mellitus and its complications.

A ceramide-centric view of insulin resistance

The recent implementation of genomic and lipidomic approaches has produced a large body of evidence implicating the sphingolipid ceramide in a diverse range of physiological processes and as a critical modulator of cellular stress. In this review, we discuss from a historical perspective the most important discoveries produced over the last decade supporting a role for ceramide and its metabolites in the pathogenesis of insulin resistance and other obesity-associated metabolic diseases. Moreover, we describe how a ceramide-centric view of insulin resistance might be reconciled in the context of other prominent models of nutrient-induced insulin resistance.

Imipramine blocks ethanol-induced ASMase activation, ceramide generation, and PP2A activation, and ameliorates hepatic steatosis in ethanol-fed mice

Our previous data showed the inhibitory effect of ethanol on AMP-activated protein kinase phosphorylation, which appears to be mediated, in part, through increased levels of hepatic ceramide and activation of protein phosphatase 2A (Liangpunsakul S, Sozio MS, Shin E, Zhao Z, Xu Y, Ross RA, Zeng Y, Crabb DW. Am J Physiol Gastrointest Liver Physiol 298: G1004-G1012, 2010). The effect of ethanol on AMP-activated protein kinase phosphorylation was reversed by imipramine, suggesting that the generation of ceramide via acid sphingomyelinase (ASMase) is stimulated by ethanol. In this study, we determined the effects of imipramine on the development of hepatic steatosis, the generation of ceramide, and downstream effects of ceramide on inflammatory, insulin, and apoptotic signaling pathways, in ethanol-fed mice. The effect of ethanol and imipramine (10 μg/g body wt ip) on ceramide levels, as well as inflammatory, insulin, and apoptotic signaling pathways, was studied in C57BL/6J mice fed the Lieber-DeCarli diet. Ethanol-fed mice developed the expected steatosis, and cotreatment with imipramine for the last 2 wk of ethanol feeding resulted in improvement in hepatic steatosis. Ethanol feeding for 4 wk induced impaired glucose tolerance compared with controls, and this was modestly improved with imipramine treatment. There was a significant decrease in total ceramide concentrations in response to imipramine in ethanol-fed mice treated with and without imipramine (287 ± 11 vs. 348 ± 12 pmol/mg tissue). The magnitude and specificity of inhibition on each ceramide species differed. A significant decrease was observed for C16 (28 ± 3 vs. 33 ± 2 pmol/mg tissue) and C24 (164 ± 9 vs. 201 ± 4 pmol/mg tissue) ceramide. Ethanol feeding increased the levels of the phosphorylated forms of ERK slightly and increased phospho-p38 and phospho-JNK substantially. The levels of phospho-p38 and phospho-JNK were reduced by treatment with imipramine. The activation of ASMase and generation of ceramide in response to ethanol feeding may underlie several effects of ethanol. ASMase inhibitors may be considered as a therapeutic target for alcohol-induced hepatic steatosis and activation of stress kinases.

New developments on the TNFα-mediated signalling pathways

TNFα (tumour necrosis factor α) is an extensively studied pleiotropic cytokine associated with the pathogenesis of a variety of inflammatory diseases. It elicits a wide spectrum of cellular responses which mediates and regulates inflammation, immune response, cell survival, proliferation and apoptosis. TNFα initiates its responses by binding to its receptors. TNFα-induced effector responses are mediated by the actions and interactions among the various intracellular signalling mediators in the cell. TNFα induces both survival and apoptotic signal in a TRADD (TNF receptor-associated DD)-dependent and -independent way. The signals are further transduced via a variety of signalling mediators, including caspases, MAPKs (mitogen-activated protein kinases), phospholipid mediators and miRNA/miR (microRNA), whose roles in specific functional responses is not fully understood. Elucidating the complexity and cross talks among signalling mediators involved in the TNFα-mediated responses will certainly aid in the identification of molecular targets, which can potentially lead to the development of novel therapeutics to treat TNFα-associated disorders and in dampening inflammation.

Sphingolipids and insulin resistance: the five Ws

PURPOSE OF REVIEW

Inhibition of sphingolipid synthesis increases insulin sensitivity, resolves hepatic steatosis, and prevents the onset of diabetes in obese rodents. I herein review these interventional studies, aiming to summarize the five Ws - the 'Who, What, Where, When, and Why' questions that need to be addressed to understand roles of sphingolipids in the pathogenesis of diabetes.

RECENT FINDINGS

Who: ceramides and glucosylceramides are likely to be independent antagonists of insulin action. Where: recent data suggest that ceramides may inhibit insulin action in skeletal muscle, whereas glucosylceramides may be more efficacious in adipose tissue. In contrast, sphingolipid accumulation in the liver appears to be insufficient to induce insulin resistance. What: ceramides and glucosylceramides inhibit different insulin signaling events, but it is unclear whether these actions account for the broad spectrum of therapeutic benefits resulting from sphingolipid depletion. When: recent data suggest that obesity-induced inflammation is important for the induction of sphingolipid synthesis. Why: sphingolipids have an evolutionarily conserved role to starve cells of nutrients, and the inhibition of insulin action is possibly a component of this broader action.

SUMMARY

Despite considerable attention to the question of how sphingolipids induce metabolic disease, there exist enormous gaps in knowledge. Further elucidation of these molecular details will be essential for the development of new therapeutic strategies for inhibiting sphingolipid action and ameliorating metabolic diseases.

Phosphatidylinositol 3-kinase/AKT pathway regulates the endoplasmic reticulum to golgi traffic of ceramide in glioma cells: a link between lipid signaling pathways involved in the control of cell survival

Different lines of evidence indicate that both aberrant activation of the phosphatidylinositol 3-OH kinase (PI3K)/Akt survival pathway and down-regulation of the death mediator ceramide play a critical role in the aggressive behavior, apoptosis resistance, and adverse clinical outcome of glioblastoma multiforme. Furthermore, the inhibition of the PI3K/Akt pathway and the up-regulation of ceramide have been found functional to the activity of many cytotoxic treatments against glioma cell lines and glioblastomas as well. A reciprocal control between PI3K/Akt and ceramide signaling in glioma cell survival/death is suggested by data demonstrating a protective role of PI3K/Akt on ceramide-induced cell death in glial cells. In this study we investigated the role of the PI3K/Akt pathway in the regulation of the ceramide metabolism in C6 glioma cells, a cell line in which the PI3K/Akt pathway is constitutively activated. Metabolic experiments performed with different radioactive metabolic precursors of sphingolipids and microscopy studies with fluorescent ceramides demonstrated that the chemical inhibition of PI3K and the transfection with a dominant negative Akt strongly inhibited ceramide utilization for the biosynthesis of complex sphingolipids by controlling the endoplasmic reticulum (ER) to Golgi vesicular transport of ceramide. These findings constitute the first evidence for a PI3K/Akt-dependent regulation of vesicle-mediated movements of ceramide in the ER-Golgi district. Moreover, the findings also suggest the activation of the PI3K/Akt pathway as crucial to coordinate the biosynthesis of membrane complex sphingolipids with cell proliferation and growth and/or to maintain low ceramide levels, especially as concerns those treatments that promote ceramide biosynthesis in the ER.

Sphingolipids, insulin resistance, and metabolic disease: new insights from in vivo manipulation of sphingolipid metabolism

Obesity and dyslipidemia are risk factors for metabolic disorders including diabetes and cardiovascular disease. Sphingolipids such as ceramide and glucosylceramides, while being a relatively minor component of the lipid milieu in most tissues, may be among the most pathogenic lipids in the onset of the sequelae associated with excess adiposity. Circulating factors associated with obesity (e.g., saturated fatty acids, inflammatory cytokines) selectively induce enzymes that promote sphingolipid synthesis, and lipidomic profiling reveals relationships between tissue sphingolipid levels and certain metabolic diseases. Moreover, studies in cultured cells and isolated tissues implicate sphingolipids in certain cellular events associated with diabetes and cardiovascular disease, including insulin resistance, pancreatic beta-cell failure, cardiomyopathy, and vascular dysfunction. However, definitive evidence that sphingolipids contribute to insulin resistance, diabetes, and atherosclerosis has come only recently, as researchers have found that pharmacological inhibition or genetic ablation of enzymes controlling sphingolipid synthesis in rodents ameliorates each of these conditions. Herein we will review the role of ceramide and other sphingolipid metabolites in insulin resistance, beta-cell failure, cardiomyopathy, and vascular dysfunction, focusing on these in vivo studies that identify enzymes controlling sphingolipid metabolism as therapeutic targets for combating metabolic disease.

Ceramide inhibits LPS-induced production of IL-5, IL-10, and IL-13 from mast cells

Mast cells are central regulators of allergic inflammation through production of various chemical mediators and cytokines. Bacterial infection occasionally worsens allergic inflammation. Although the exact mechanism of this phenomenon remains unclear, we have previously reported that LPS stimulates mast cells to produce not only pro-inflammatory cytokines, such as IL-6 and TNF-alpha, but also Th2-type cytokines, such as IL-5 and IL-13, and a regulatory cytokine, IL-10. In the present study, we have studied the effect of ceramide on LPS-mediated cytokine production from mast cells, as ceramide modulates various cellular functions in many cell types. Administration of cell-permeable C8 ceramide reduced production of IL-5, IL-10, and IL-13 from LPS-stimulated mouse bone marrow-derived mast cells (BMMCs) apparently through transcriptional inhibition, but did not affect IL-6 or TNF-alpha production. Consistently, LPS-stimulated production of IL-5, IL-10, and IL-13 from BMMCs is significantly enhanced in the presence of fumonisin B1, a de novo ceramide synthesis inhibitor. Interestingly, the same C8 ceramide treatment showed opposite effects on cytokine production from LPS-stimulated macrophages, reducing IL-6 and TNF-alpha while not affecting IL-10 production. C8 ceramide pretreatment significantly reduced LPS-induced Akt phosphorylation in BMMCs, but not in macrophages. Furthermore, pretreatment of BMMCs by wortmannin, a specific inhibitor of PI3 kinase, inhibited LPS-stimulated expression of IL-5, IL-10, and IL-13, but not that of TNF-alpha or IL-6. Thus, ceramide appeared to down-regulate LPS-stimulated production of IL-5, IL-10, and IL-13 from mast cells by inhibiting PI3 kinase-Akt pathway in a cell type-specific manner.

Dietary sphingolipids suppress a subset of preneoplastic rat liver lesions exhibiting high PTEN, low phospho-Akt and high levels of ceramide species

Rat liver glutathione-S-transferase Pi-(GST-P)-positive enzyme-altered foci (EAF) are preneoplastic lesions that develop in response to carcinogenic stress. They are often used as endpoints in e.g. chemopreventive studies. In this study we characterize a pAkt-negative/ceramide-positive (pAkt-/cer+) EAF phenotype, as defined by immunohistochemistry for pAkt and ceramide species, in diethylnitrosamine(DEN)-, phenobarbital- or aflatoxinB1-treated rats. There was a close to 100% overlap for the pAkt and the ceramide marker. Furthermore, serial sections stained for PTEN indicated a close correlation between PTEN-positive and pAkt-negative lesions in DEN-treated rats. Experiments with DEN-treated rats given sphingomyelin in the diet suggested that sphingomyelin selectively targeted these lesions. In in vitro experiments sphingosine rapidly decreased pAkt levels in hepatocytes, and in experiments with hepatocytes from DEN-treated rats sphingosine selectively killed EAF cells. Furthermore, pretreatment with antisense Akt oligonucleotides in vitro sensitized non-EAF hepatocytes, so that EAF and non-EAF cells became equally sensitive to sphingosine. It is concluded that rat liver, in response to carcinogenic stress, develops a distinct EAF phenotype exhibiting low pAkt levels and concomitant alterations in sphingolipid metabolism. Our data also suggest that pAkt-/cer+ EAF are selectively targeted by sphingolipids in the diet and that lesions with this phenotype should be of particular interest for future studies on chemopreventive effects that may affect sphingolipid metabolism.

The pathophysiology of the growth plate in juvenile idiopathic arthritis

Children with chronic inflammatory diseases, such as juvenile idiopathic arthritis (JIA), suffer from a variety of growth disorders. These range from general growth retardation to local acceleration of growth in the affected limb. These disorders are associated with the increased production of proinflammatory cytokines, which may influence growth through a local effect in the growth plates of long bones and/or systemic effects throughout the whole body. In this article we review these aspects and also discuss the evidence for interaction between the inflammatory cytokine and growth-signalling pathways.

Ceramide regulation of the tumor suppressor phosphatase PTEN in rafts isolated from neurotumor cell lines

The neutral sphingolipid ceramide has been implicated in the apoptotic death of cells by a number of different mechanisms, including activation of protein kinase B (Akt) phosphatase. Here we present evidence that ceramide recruits the tumor suppressor PTEN (phosphatase and tensin homolog deleted from chromosome 10) into membrane microdomains (rafts), where it could act to reduce the levels of polyphosphoinositides necessary for the activation of Akt. A PTEN construct with a red-fluorescent protein (RFP) tag was overexpressed in both a human cell line derived from oligodendroglioma (HOG) and a rat pheochromocytoma cell line (PC12) by means of an inducible promoter system (Tet-Off). Induction of PTEN by removal of doxycycline enhanced both capsase-3 and cell death with staurosporine, wortmannin, or C2-ceramide, whereas antisense PTEN had the reverse effect. Overexpression of PTEN also increased acid sphingomyelinase (ASMase) activity. PTEN normally has a generalized (cytosolic/membrane) distribution, but treatment with C2-ceramide translocated a fraction of the PTEN to the plasma membrane, showing a plasma membrane distribution similar to that observed for a prenylated green-fluorescent (GFP) construct. PTEN was then shown to translocate to the detergent-resistant membrane microdomain fraction (raft) of the plasma membrane. The colocalization of sphingomyelinases, ceramide, polyphosphoinositides, and PTEN in the raft fraction further suggests that the association of these lipids is critical for regulating cell death.

TRAIL-induced apoptosis in gliomas is enhanced by Akt-inhibition and is independent of JNK activation

Patients with malignant gliomas have a poor prognosis and new treatment paradigms are needed against this disease. TRAIL/Apo2L selectively induces apoptosis in malignant cells sparing normal cells and is hence of interest as a potential therapeutic agent against gliomas. To determine the factors that modulate sensitivity to TRAIL, we examined the differences in TRAIL-activated signaling pathways in glioma cells with variable sensitivities to the agent. Apoptosis in response to TRAIL was unrelated to DR5 expression or endogenous p53 status in a panel of 8 glioma cell lines. TRAIL activated the extrinsic (cleavage of caspase-8, caspase-3 and PARP) and mitochondrial apoptotic pathways and reduced FLIP levels. It also induced caspase-dependent JNK activation, which did not influence TRAIL-induced apoptosis. Because the pro-survival PI3K/Akt pathway is highly relevant to gliomas, we assessed whether Akt could protect against TRAIL-induced apoptosis. Pretreatment with SH-6, a novel Akt inhibitor, enhanced TRAIL-induced apoptosis, suggesting a protective role for Akt. Conversely, TRAIL induced caspase-dependent cleavage of Akt neutralizing its anti-apoptotic effects. These results demonstrate that TRAIL-induced apoptosis in gliomas involves both activation of death pathways and downregulation of survival pathways. Additional studies are warranted to determine the therapeutic potential of TRAIL against gliomas.

A role for sphingolipids in producing the common features of type 2 diabetes, metabolic syndrome X, and Cushing's syndrome

Metabolic syndrome X and type 2 diabetes share many metabolic and morphological similarities with Cushing's syndrome, a rare disorder caused by systemic glucocorticoid excess. Pathologies frequently associated with these diseases include insulin resistance, atherosclerosis, susceptibility to infection, poor wound healing, and hypertension. The similarity of the clinical profiles associated with these disorders suggests the influence of a common molecular mechanism for disease onset. Interestingly, numerous studies identify ceramides and other sphingolipids as potential contributors to these sequelae. Herein we review studies demonstrating that aberrant ceramide accumulation contributes to the development of the deleterious clinical manifestations associated with these diseases.

Sphingolipids as coenzymes in anion transfer and tumor death

Many kinds of natural sphingolipids and their analogs stimulate or inhibit a wide assortment of biochemical phenomena and enzymes. The puzzle considered here is: how can these lipids control so many different kinds of processes? In almost every study in which a structural comparison was made, an allylic alcohol moiety [-CH=CH-CH(OH)-] was found to be an essential feature of the sphingolipid. Many of those stimulations lead to cell death, emphasizing the importance of allylic sphingolipid structure in the design of chemotherapeutic agents. The proposal offered here is that these lipids function as coenzymes, in which the allylic moiety acts as an anion transferring agent, forming transient phosphate or acyl or peptidyl esters for the synthesis or hydrolysis of phosphoproteins, proteins, and phospholipids. Sphingolipids that inhibit these reactions may simply displace the active sphingolipids from their sites in the enzymes' active regions, or bind to the enzymes' allosteric region. This kind of competition could act as a major homeostatic control mechanism. Some of the allylic sphingolipids also generate reactive oxygen, possibly by oxidation of the allylic alcohol group. This explains the need to control redox-controlling metabolites in sphingolipid-controlled processes (e.g., glutathione). Many anticancer drugs that produce apoptosis in tumors possess an allylic alcohol residue, affect protein phosphorylation, and produce reactive oxygen species. They may be therapeutically useful because they control the action of sphingolipids as anion transfer agonists or inhibitors.

Involvement of ceramide in the mechanism of Cr(VI)-induced apoptosis of CHO cells

Mitochondria reduce Cr(VI) to Cr(V) with concomitant generation of reactive oxygen species, thereby exhibiting cytotoxic effects leading to apoptosis in various types of cells. To clarify the mechanism by which Cr(VI) induces apoptosis, we examined the effect of Cr(VI) on Chinese hamster ovary (CHO) cells. Cr(VI) increased cellular levels of ceramide by activating acid sphingomyelinase (ASMase) and inhibiting the phosphorylation of pleckstrin homology domain-containing protein kinase B (Akt). Cr(VI) also induced cyclosporin A- and trifluoperazine-sensitive depolarization of mitochondria and activated caspase-3, 8 and 9, thereby causing fragmentation of cellular DNA. The presence of desipramine, an inhibitor of ASMase, and membrane permeable pCPT-cAMP suppressed the Cr(VI)-induced activation of caspases and DNA fragmentation. These results suggested that accumulation of ceramide play an important role in the Cr(VI)-induced apoptosis of CHO cells through activation of mitochondrial membrane permeability transition.

Sphingolipid metabolites in neural signalling and function

Sphingolipid metabolites, such as ceramide, sphingosine, sphingosine-1-phosphate (S1P) and complex sphingolipids (gangliosides), are recognized as molecules capable of regulating a variety of cellular processes. The role of sphingolipid metabolites has been studied mainly in non-neuronal tissues. These studies have underscored their importance as signals transducers, involved in control of proliferation, survival, differentiation and apoptosis. In this review, we will focus on studies performed over the last years in the nervous system, discussing the recent developments and the current perspectives in sphingolipid metabolism and functions.

Regulation of insulin action by ceramide: dual mechanisms linking ceramide accumulation to the inhibition of Akt/protein kinase B

The sphingolipid ceramide negatively regulates insulin action by inhibiting Akt/protein kinase B (PKB), a serine/threonine kinase that is a central regulator of glucose uptake and anabolic metabolism. Despite considerable attention, the molecular mechanism accounting for this action of ceramide has remained both elusive and controversial. Herein we utilized deletion constructs encoding two different functional domains of Akt/PKB to identify which region of the enzyme conferred responsiveness to ceramide. Surprisingly the findings obtained with these separate domains reveal that ceramide blocks insulin stimulation of Akt/PKB by two independent mechanisms. First, using the isolated pleckstrin homology domain, we found that ceramide specifically blocks the translocation of Akt/PKB, but not its upstream activator phosphoinositide-dependent kinase-1, to the plasma membrane. Second, using a construct lacking this pleckstrin homology domain, which does not require translocation for activation, we found that ceramide stimulates the dephosphorylation of Akt/PKB by protein phosphatase 2A. Collectively these findings identify at least two independent mechanisms by which excessive ceramide accumulation in peripheral tissues could contribute to the development of insulin resistance. Moreover the results obtained provide a unifying theory to account for the numerous dissenting reports investigating the actions of ceramide toward Akt/PKB.

Peroxisome Proliferator Activator Receptors (PPAR), Insulin Resistance, and Cardiomyopathy

Diabetes is a risk factor for coronary atherosclerosis, myocardial infarction, and ischemic cardiomyopathy. Insulin resistance is associated with left ventricular (LV) hypertrophy and hypertensive cardiomyopathy. Even in the absence of coronary artery disease or hypertension, "diabetic cardiomyopathy" can develop because of myocardial autonomic dysfunction or impaired coronary flow reserve. The relationship between insulin resistance and cardiomyopathy is bidirectional. Systemic and myocardial glucose uptake is compromised in heart failure independent of etiology. These abnormalities are associated with cellular deficits of insulin signaling. Insulin resistance in heart failure can be detrimental, because transcriptional shifts in metabolic gene expression favor glucose over fat as a substrate for high-energy phosphate production. Although preexisting diabetes accelerates this process of "metabolic death," insulin resistance can also develop secondary to cardiomyopathy-associated overabundance of neurohormones and cytokines. Insulin resistance and fatty acid excess are potential therapeutic targets in heart failure, striving for efficient myocardial substrate utilization. Peroxisome proliferator activator receptor gamma (PPARgamma) agonists are antidiabetic agents with antilipemic and insulin-sensitizing activity. Experimental studies suggest salutary effects in limiting infarct size, attenuating myocardial reperfusion injury, inhibiting hypertrophic signaling and vascular antiinflammatory actions through cytokine inhibition. However, clinical applicability in diabetic patients experiencing heart failure has been hampered because of increased edema and even fewer reports of exacerbation associated with these compounds. Evidence to date argues for peripheral mechanisms of edema unrelated to central hemodynamics. Nevertheless, they are currently contraindicated in New York Heart Association (NYHA) III-IV patients, particularly in combination with insulin. Investigations are underway to decipher mechanisms, risks, and benefits of PPARgamma agonists, as well as the role of the structurally related PPARalpha receptor on cardiovascular metabolism and function.

Akt1 protects against inflammatory microglial activation through maintenance of membrane asymmetry and modulation of cysteine protease activity

In several cell systems, protein kinase B (Akt1) can promote cell growth and development, but the "antiapoptotic" pathways of this kinase that may offer protection against cellular inflammatory demise have not been defined. Given that early cellular membrane phosphatidylserine exposure is a critical component of apoptosis, we investigated the role of Akt1 during neuronal apoptotic injury. By employing differentiated SH-SY5Y neuronal cells that overexpress a constitutively active form of Akt1 (myristoylated Akt1), free radical-induced cell injury was assessed through trypan blue dye exclusion, DNA fragmentation, membrane phosphatidylserine exposure, protein kinase B phosphorylation, cysteine protease activity, and mitochondrial membrane potential. Membrane phosphatidylserine exposure was both necessary and sufficient for microglial activation, insofar as cotreatment with an antiphosphatidylserine receptor-neutralizing antibody could prevent microglial activity following neuronal loss of membrane asymmetry. Furthermore, expression of myristoylated Akt1 not only prevented cell injury through the prevention of membrane phosphatidylserine exposure and genomic DNA fragmentation but also inhibited microglial activation and proliferation that required the inhibition of caspase 9-, caspase 3-, and caspase 1-like activities linked to cytochrome c release. Interestingly, Akt1 modulation of membrane phosphatidylserine exposure was primarily through caspase 1 activity. Removal of Akt1 activity abolished neuronal protection, suggesting that Akt1 functions as a critical pathway for the maintenance of cellular integrity and the prevention of phagocytic cellular removal during neurodegenerative insults.

Regulation of the glutamate transporter EAAT1 by the ubiquitin ligase Nedd4-2 and the serum and glucocorticoid-inducible kinase isoforms SGK1/3 and protein kinase B

Surface expression of the glial glutamate transporter EAAT1 is stimulated by insulin-like growth factor 1 through activation of phosphatidylinositol-3-kinase. Downstream targets include serum and glucocorticoid-sensitive kinase isoforms SGK1, SGK2 and SGK3, and protein kinase B. SGK1 regulates Nedd4-2, a ubiquitin ligase that prepares cell membrane proteins for degradation. To test whether Nedd4-2, SGK1, SGK3 and protein kinase B regulate EAAT1, cRNA encoding EAAT1 was injected into Xenopus oocytes with or without additional injection of wild-type Nedd4-2, constitutively active S422DSGK1, inactive K127NSGK1, wild-type SGK3 and/or constitutively active T308D,S473DPKB. Glutamate induces a current in Xenopus oocytes expressing EAAT1, but not in water-injected oocytes, which is decreased by co-expression of Nedd4-2, an effect reversed by additional co-expression of S422DSGK1, SGK3 and T308D,S473DPKB, but not K127NSGK1. Site-directed mutagenesis of the SGK1 phosphorylation sites in the Nedd4-2 protein (S382A,S468ANedd4-2) and in the EAAT1 protein (T482AEAAT1, T482DEAAT1) significantly blunts the effect of S422DSGK1. Moreover, the current is significantly larger in T482DEAAT1- than in T482AEAAT1-expressing oocytes, indicating that a negative charge mimicking phosphorylation at T482 increases transport. The experiments reveal a powerful novel mechanism that regulates the activity of EAAT1. This mechanism might participate in the regulation of neuronal excitability and glutamate transport in other tissues.