Ceramide directly activates protein kinase C zeta to regulate a stress-activated protein kinase signaling complex

Ceramide microdomains: the major influencers of the sphingolipid media platform

Like 'influencers' who achieve fame and power through social media, ceramides are low abundance members of communication platforms that have a mighty impact on their surroundings. Ceramide microdomains form within sphingolipid-laden lipid rafts that confer detergent resistance to cell membranes and serve as important signaling hubs. In cells exposed to excessive amounts of saturated fatty acids (e.g. in obesity), the abundance of ceramide-rich microdomains within these rafts increases, leading to concomitant alterations in cellular metabolism and survival that contribute to cardiometabolic disease. In this mini-review, we discuss the evidence supporting the formation of these ceramide microdomains and describe the spectrum of harmful ceramide-driven metabolic actions under the context of an evolutionary theory. Moreover, we discuss the proximal 'followers' of these ceramide media stars that account for the diverse intracellular actions that allow them to influence obesity-linked disease.

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-1-phosphate is a regulator of Golgi structure and is co-opted by the obligate intracellular bacterial pathogen Anaplasma phagocytophilum

Many intracellular pathogens structurally disrupt the Golgi apparatus as an evolutionarily conserved promicrobial strategy. Yet, the host factors and signaling processes involved are often poorly understood, particularly for Anaplasma phagocytophilum, the agent of human granulocytic anaplasmosis. We found that A. phagocytophilum elevated cellular levels of the bioactive sphingolipid, ceramide-1-phosphate (C1P), to promote Golgi fragmentation that enables bacterial proliferation, conversion from its non-infectious to infectious form, and productive infection. A. phagocytophilum poorly infected mice deficient in ceramide kinase, the Golgi-localized enzyme responsible for C1P biosynthesis. C1P regulated Golgi morphology via activation of a PKCα/Cdc42/JNK signaling axis that culminates in phosphorylation of Golgi structural proteins, GRASP55 and GRASP65. siRNA-mediated depletion of Cdc42 blocked A. phagocytophilum from altering Golgi morphology, which impaired anterograde trafficking of trans-Golgi vesicles into and maturation of the pathogen-occupied vacuole. Cells overexpressing phosphorylation-resistant versions of GRASP55 and GRASP65 presented with suppressed C1P- and A. phagocytophilum-induced Golgi fragmentation and poorly supported infection by the bacterium. By studying A. phagocytophilum, we identify C1P as a regulator of Golgi structure and a host factor that is relevant to disease progression associated with Golgi fragmentation.IMPORTANCECeramide-1-phosphate (C1P), a bioactive sphingolipid that regulates diverse processes vital to mammalian physiology, is linked to disease states such as cancer, inflammation, and wound healing. By studying the obligate intracellular bacterium Anaplasma phagocytophilum, we discovered that C1P is a major regulator of Golgi morphology. A. phagocytophilum elevated C1P levels to induce signaling events that promote Golgi fragmentation and increase vesicular traffic into the pathogen-occupied vacuole that the bacterium parasitizes. As several intracellular microbial pathogens destabilize the Golgi to drive their infection cycles and changes in Golgi morphology is also linked to cancer and neurodegenerative disorder progression, this study identifies C1P as a potential broad-spectrum therapeutic target for infectious and non-infectious diseases.

A high-fat diet supplemented with medium-chain triglycerides ameliorates hepatic steatosis by reducing ceramide and diacylglycerol accumulation in mice

Non-alcoholic fatty liver disease (NAFLD) is projected to be the most common chronic liver disease worldwide and is closely linked to obesity, insulin resistance and type 2 diabetes. Currently, no pharmacological treatments are available to treat NAFLD, and lifestyle modification, including dietary interventions, is the only remedy. Therefore, we conducted a study to determine whether supplementation with medium-chain triglycerides (MCTs), containing a mixture of C8 and C10 (60/40), attenuates NAFLD in obese and insulin-resistant mice. To achieve that, we fed C57BL/6 male mice a high-fat diet (HFD) for 12 weeks to induce obesity and hepatic steatosis, after which obese mice were assigned randomly either to remain on the HFD or to transition to an HFD supplemented with MCTs (HFD + MCTs) or a low-fat diet (LFD) for 6 weeks as another dietary intervention model. Another group of mice was kept on an LFD throughout the study and used as a lean control group. Obese mice that transitioned to HFD + MCTs exhibited improvement in glucose and insulin tolerance tests, and the latter improvement was independent of changes in adiposity when compared with HFD-fed mice. Additionally, supplementation with MCTs significantly reduced hepatic steatosis, improved liver enzymes and decreased hepatic expression of inflammation-related genes to levels similar to those observed in obese mice transitioned to an LFD. Importantly, HFD + MCTs markedly lowered hepatic ceramide and diacylglycerol content and prevented protein kinase C-ε translocation to the plasma membrane. Our study demonstrated that supplementation with MCTs formulated mainly from C8 and C10 effectively ameliorated NAFLD in obese mice.

AMPK and Beyond: The Signaling Network Controlling RabGAPs and Contraction-Mediated Glucose Uptake in Skeletal Muscle

Impaired skeletal muscle glucose uptake is a key feature in the development of insulin resistance and type 2 diabetes. Skeletal muscle glucose uptake can be enhanced by a variety of different stimuli, including insulin and contraction as the most prominent. In contrast to the clearance of glucose from the bloodstream in response to insulin stimulation, exercise-induced glucose uptake into skeletal muscle is unaffected during the progression of insulin resistance, placing physical activity at the center of prevention and treatment of metabolic diseases. The two Rab GTPase-activating proteins (RabGAPs), TBC1D1 and TBC1D4, represent critical nodes at the convergence of insulin- and exercise-stimulated signaling pathways, as phosphorylation of the two closely related signaling factors leads to enhanced translocation of glucose transporter 4 (GLUT4) to the plasma membrane, resulting in increased cellular glucose uptake. However, the full network of intracellular signaling pathways that control exercise-induced glucose uptake and that overlap with the insulin-stimulated pathway upstream of the RabGAPs is not fully understood. In this review, we discuss the current state of knowledge on exercise- and insulin-regulated kinases as well as hypoxia as stimulus that may be involved in the regulation of skeletal muscle glucose uptake.

Syntheses of the ω-pyridinium-containing very-long-chain ceramides PyrCer(24:1(15Z)) and PyrCer(24:0) and their anticancer activity

Ceramides, crucial sphingolipids in cellular biology, play various roles ranging from structural membrane integrity to signaling pathway regulation. Structurally, a ceramide consists of a fatty acid connected to a sphingoid base. The characteristics of the fatty acid chain, including length and saturation, determine the physiological properties of the ceramide. Ceramides typically fall into the following categories based on chain length: medium, long, very-long, and ultra-long. Among them, two very-long-chain ceramides, Cer(24:1(15Z)) and Cer(24:0), have been extensively studied, and they are known for their regulatory functions. However, the hydrophobic natures of ceramides, arising from their long hydrocarbon chain impedes their solubilities and levels of cellular delivery. Although ω-pyridinium ceramide analogs (ω-PyrCers) have been developed to address this issue, ω-PyrCers with very-long fatty acid chains or unsaturation have not been developed, presumably due to limited access to the corresponding ω-bromo fatty acids required in their syntheses. In this study, we prepared the ω-PyrCers of Cer(24:1(15Z)) and Cer(24:0), PyrCer(24:1(15Z)) and PyrCer(24:0), respectively. The key in the synthesis is the Wittig reaction to prepare the ω-bromo fatty acid with an appropriate chain length and (Z)-double bond position. Preliminary evaluation of the PyrCer(24:1(15Z)) and PyrCer(24:0) revealed their potential in hepatocellular carcinoma treatment.

Sphingolipids and Atherosclerosis: The Dual Role of Ceramide and Sphingosine-1-Phosphate

Sphingolipids are bioactive molecules that play either pro- and anti-atherogenic roles in the formation and maturation of atherosclerotic plaques. Among SLs, ceramide and sphingosine-1-phosphate showed antithetic properties in regulating various molecular mechanisms and have emerged as novel potential targets for regulating the development of atherosclerosis. In particular, maintaining the balance of the so-called ceramide/S1P rheostat is important to prevent the occurrence of endothelial dysfunction, which is the trigger for the entire atherosclerotic process and is strongly associated with increased oxidative stress. In addition, these two sphingolipids, together with many other sphingolipid mediators, are directly involved in the progression of atherogenesis and the formation of atherosclerotic plaques by promoting the oxidation of low-density lipoproteins (LDL) and influencing the vascular smooth muscle cell phenotype. The modulation of ceramide and S1P levels may therefore allow the development of new antioxidant therapies that can prevent or at least impair the onset of atherogenesis, which would ultimately improve the quality of life of patients with coronary artery disease and significantly reduce their mortality.

Mechanisms underlying Nrf2 nuclear translocation by non-lethal levels of hydrogen peroxide: p38 MAPK-dependent neutral sphingomyelinase2 membrane trafficking and ceramide/PKCζ/CK2 signaling

Hydrogen peroxide is an aerobic metabolite playing a central role in redox signaling and oxidative stress. H₂O₂ could activate redox sensitive transcription factors, such as Nrf2, AP-1 and NF-κB by different manners. In some cells, treatment with non-lethal levels of H₂O₂ induces rapid activation of Nrf2, which upregulates expression of a set of genes involved in glutathione (GSH) synthesis and defenses against oxidative damage. It depends on two steps, the rapid translational activation of Nrf2 and facilitation of Nrf2 nuclear translocation. We review the molecular mechanisms by which H₂O₂ induces nuclear translocation of Nrf2 in cultured cells by highlighting the role of neutral sphingomyelinase 2 (nSMase2), a GSH sensor. H₂O₂ enters cells through aquaporin channels in the plasma membrane and is rapidly reduced to H₂O by GSH peroxidases to consume cellular GSH, resulting in nSMase2 activation to generate ceramide. H₂O₂ also activates p38 MAP kinase, which enhances transfer of nSMase2 from perinuclear regions to plasma membrane lipid rafts to accelerate ceramide generation. Low levels of ceramide activate PKCζ, which then activates casein kinase 2 (CK2). These protein kinases are able to phosphorylate Nrf2 to stabilize and activate it. Notably, Nrf2 also binds to caveolin-1 (Cav1), which protects Nrf2 from Keap1-mediated degradation and limits Nrf2 nuclear translocation. We propose that Cav1serves as a signaling hub for the control of H₂O₂-mediated phosphorylation of Nrf2 by kinases, which results in release of Nrf2 from Cav1 to facilitate nuclear translocation. In summary, H₂O₂ induces GSH depletion which is recovered by Nrf2 activation dependent on p38/nSMase2/ceramide signaling.

Restoration of atypical protein kinase C ζ function in autosomal dominant polycystic kidney disease ameliorates disease progression

Autosomal dominant polycystic kidney disease (ADPKD) affects more than 500,000 individuals in the United States alone. In most cases, ADPKD is caused by a loss-of-function mutation in the PKD1 gene, which encodes polycystin-1 (PC1). Previous studies reported that PC1 interacts with atypical protein kinase C (aPKC). Here we show that PC1 binds to the ζ isoform of aPKC (PKCζ) and identify two PKCζ phosphorylation sites on PC1's C-terminal tail. PKCζ expression is down-regulated in patients with ADPKD and orthologous and nonorthologous PKD mouse models. We find that the US Food and Drug Administration-approved drug FTY720 restores PKCζ expression in in vitro and in vivo models of polycystic kidney disease (PKD) and this correlates with ameliorated disease progression in multiple PKD mouse models. Importantly, we show that FTY720 treatment is less effective in PKCζ null versions of these PKD mouse models, elucidating a PKCζ-specific mechanism of action that includes inhibiting STAT3 activity and cyst-lining cell proliferation. Taken together, our results reveal that PKCζ down-regulation is a hallmark of PKD and that its stabilization by FTY720 may represent a therapeutic approach to the treat the disease.

A Rheostat of Ceramide and Sphingosine-1-Phosphate as a Determinant of Oxidative Stress-Mediated Kidney Injury

Reactive oxygen species (ROS) modulate sphingolipid metabolism, including enzymes that generate ceramide and sphingosine-1-phosphate (S1P), and a ROS-antioxidant rheostat determines the metabolism of ceramide-S1P. ROS induce ceramide production by activating ceramide-producing enzymes, leading to apoptosis, while they inhibit S1P production, which promotes survival by suppressing sphingosine kinases (SphKs). A ceramide-S1P rheostat regulates ROS-induced mitochondrial dysfunction, apoptotic/anti-apoptotic Bcl-2 family proteins and signaling pathways, leading to apoptosis, survival, cell proliferation, inflammation and fibrosis in the kidney. Ceramide inhibits the mitochondrial respiration chain and induces ceramide channel formation and the closure of voltage-dependent anion channels, leading to mitochondrial dysfunction, altered Bcl-2 family protein expression, ROS generation and disturbed calcium homeostasis. This activates ceramide-induced signaling pathways, leading to apoptosis. These events are mitigated by S1P/S1P receptors (S1PRs) that restore mitochondrial function and activate signaling pathways. SphK1 promotes survival and cell proliferation and inhibits inflammation, while SphK2 has the opposite effect. However, both SphK1 and SphK2 promote fibrosis. Thus, a ceramide-SphKs/S1P rheostat modulates oxidant-induced kidney injury by affecting mitochondrial function, ROS production, Bcl-2 family proteins, calcium homeostasis and their downstream signaling pathways. This review will summarize the current evidence for a role of interaction between ROS-antioxidants and ceramide-SphKs/S1P and of a ceramide-SphKs/S1P rheostat in the regulation of oxidative stress-mediated kidney diseases.

Revisiting the contribution of mitochondrial biology to the pathophysiology of skeletal muscle insulin resistance

While the etiology of type 2 diabetes is multifaceted, the induction of insulin resistance in skeletal muscle is a key phenomenon, and impairments in insulin signaling in this tissue directly contribute to hyperglycemia. Despite the lack of clarity regarding the specific mechanisms whereby insulin signaling is impaired, the key role of a high lipid environment within skeletal muscle has been recognized for decades. Many of the proposed mechanisms leading to the attenuation of insulin signaling - namely the accumulation of reactive lipids and the pathological production of reactive oxygen species (ROS), appear to rely on this high lipid environment. Mitochondrial biology is a central component to these processes, as these organelles are almost exclusively responsible for the oxidation and metabolism of lipids within skeletal muscle and are a primary source of ROS production. Classic studies have suggested that reductions in skeletal muscle mitochondrial content and/or function contribute to lipid-induced insulin resistance; however, in recent years the role of mitochondria in the pathophysiology of insulin resistance has been gradually re-evaluated to consider the biological effects of alterations in mitochondrial content. In this respect, while reductions in mitochondrial content are not required for the induction of insulin resistance, mechanisms that increase mitochondrial content are thought to enhance mitochondrial substrate sensitivity and submaximal adenosine diphosphate (ADP) kinetics. Thus, this review will describe the central role of a high lipid environment in the pathophysiology of insulin resistance, and present both classic and contemporary views of how mitochondrial biology contributes to insulin resistance in skeletal muscle.

Ceramides in Skin Health and Disease: An Update

Ceramides are a class of sphingolipid that is the backbone structure for all sphingolipids, such as glycosphingolipids and phosphosphingolipids. While being a minor constituent of cellular membranes, ceramides are the major lipid component (along with cholesterol, free fatty acid, and other minor components) of the intercellular spaces of stratum corneum that forms the epidermal permeability barrier. These stratum corneum ceramides consist of unique heterogenous molecular species that have only been identified in terrestrial mammals. Alterations of ceramide molecular profiles are characterized in skin diseases associated with compromised permeability barrier functions, such as atopic dermatitis, psoriasis and xerosis. In addition, hereditary abnormalities of some ichthyoses are associated with an epidermal unique ceramide species, omega-O-acylceramide. Ceramides also serve as lipid modulators to regulate cellular functions, including cell cycle arrest, differentiation, and apoptosis, and it has been demonstrated that changes in ceramide metabolism also cause certain diseases. In addition, ceramide metabolites, sphingoid bases, sphingoid base-1-phosphate and ceramide-1-phosphate are also lipid mediators that regulate cellular functions. In this review article, we describe diverse physiological and pathological roles of ceramides and their metabolites in epidermal permeability barrier function, epidermal cell proliferation and differentiation, immunity, and cutaneous diseases. Finally, we summarize the utilization of ceramides as therapy to treat cutaneous disease.

Characterization and regulation of extracellular vesicles in the lumen of the ovine uterus†

Secretions of the endometrium are vital for peri-implantation growth and development of the sheep conceptus. Extracellular vesicles (EVs) are present in the uterine lumen, emanate from both the endometrial epithelia of the uterus and trophectoderm of the conceptus, and hypothesized to mediate communication between those cell types during pregnancy establishment in sheep. Size-exclusion chromatography and nanoparticle tracking analysis determined that total EV number in the uterine lumen increased from days 10 to 14 of the cycle but was lower on days 12 and 14 of pregnancy in sheep. Intrauterine infusions of interferon tau (IFNT) did not affect total EV number in the uterine lumen. Quantitative mass spectrometric analyses defined proteins and lipids in EVs isolated from the uterine lumen of day 14 cyclic and pregnant sheep. In vitro analyses found that EVs decreased ovine trophectoderm cell proliferation and increased IFNT production without effects on gene expression as determined by RNA-seq. Collective results support the idea EVs impact conceptus growth during pregnancy establishment via effects on trophectoderm cell growth.

Insights into the lipidome and primary metabolome of the uterus from day 14 cyclic and pregnant sheep†

In ruminants, conceptus elongation requires the endometrium and its secretions. The amino acid, carbohydrate, and protein composition of the uterine lumen during early pregnancy has been defined in sheep; however, a comprehensive understanding of metabolomic changes in the uterine lumen is lacking, particularly with respect to lipids. Here, the lipidome and primary metabolome of the uterine lumen, endometrium, and/or conceptus was determined on day 14 of the estrous cycle and pregnancy. Lipid droplets and select triglycerides were depleted in the endometrium of pregnant ewes. In contrast, select ceramides, diglycerides, and non-esterified fatty acids as well as several phospholipid classes (phosphatidylcholine, phosphatidylinositol, phosphatidylglycerols, and diacylglycerols) were elevated in the uterine lumen of pregnant ewes. Lipidomic analysis of the conceptus revealed that triglycerides are particularly abundant within the conceptus. Primary metabolite analyses found elevated amino acids, carbohydrates, and energy substrates, among others, in the uterine lumen of pregnant ewes. Collectively, this study supports the hypothesis that lipids are important components of the uterine lumen that govern conceptus elongation and growth during early pregnancy.

Sphingolipids as multifaceted mediators in ovarian cancer

Ovarian cancer is the most lethal gynaecological malignancy. It is commonly diagnosed at advanced stage when it has metastasised to the abdominal cavity and treatment becomes very challenging. While current standard therapy involving debulking surgery and platinum + taxane-based chemotherapy is associated with high response rates initially, the large majority of patients relapse and ultimately succumb to chemotherapy-resistant disease. In order to improve survival novel strategies for early detection and therapeutics against treatment-refractory disease are urgently needed. A promising new target against ovarian cancer is the sphingolipid pathway which is commonly hijacked in cancer to support cell proliferation and survival and has been shown to promote chemoresistance and metastasis in a wide range of malignant neoplasms. In particular, the sphingosine kinase 1-sphingosine 1-phosphate receptor 1 axis has been shown to be altered in ovarian cancer in multiple ways and therefore represents an attractive therapeutic target. Here we review the roles of sphingolipids in ovarian cancer progression, metastasis and chemoresistance, highlighting novel strategies to target this pathway that represent potential avenues to improve patient survival.

Identification of Key Phospholipids That Bind and Activate Atypical PKCs

PKCζ and PKCι/λ form the atypical protein kinase C subgroup, characterised by a lack of regulation by calcium and the neutral lipid diacylglycerol. To better understand the regulation of these kinases, we systematically explored their interactions with various purified phospholipids using the lipid overlay assays, followed by kinase activity assays to evaluate the lipid effects on their enzymatic activity. We observed that both PKCζ and PKCι interact with phosphatidic acid and phosphatidylserine. Conversely, PKCι is unique in binding also to phosphatidylinositol-monophosphates (e.g., phosphatidylinositol 3-phosphate, 4-phosphate, and 5-phosphate). Moreover, we observed that phosphatidylinositol 4-phosphate specifically activates PKCι, while both isoforms are responsive to phosphatidic acid and phosphatidylserine. Overall, our results suggest that atypical Protein kinase C (PKC) localisation and activity are regulated by membrane lipids distinct from those involved in conventional PKCs and unveil a specific regulation of PKCι by phosphatidylinositol-monophosphates.

Inhibitors of Ceramide- and Sphingosine-Metabolizing Enzymes as Sensitizers in Radiotherapy and Chemotherapy for Head and Neck Squamous Cell Carcinoma

In the treatment of advanced head and neck squamous cell carcinoma (HNSCC), including oral SCC, radiotherapy is a commonly performed therapeutic modality. The combined use of radiotherapy with chemotherapy improves therapeutic effects, but it also increases adverse events. Ceramide, a central molecule in sphingolipid metabolism and signaling pathways, mediates antiproliferative responses, and its level increases in response to radiotherapy and chemotherapy. However, when ceramide is metabolized, prosurvival factors, such as sphingosine-1-phosphate (S1P), ceramide-1-phosphate (C1P), and glucosylceramide, are produced, reducing the antitumor effects of ceramide. The activities of ceramide- and sphingosine-metabolizing enzymes are also associated with radio- and chemo-resistance. Ceramide analogs and low molecular-weight compounds targeting these enzymes exert anticancer effects. Synthetic ceramides and a therapeutic approach using ultrasound have also been developed. Inhibitors of ceramide- and sphingosine-metabolizing enzymes and synthetic ceramides can function as sensitizers of radiotherapy and chemotherapy for HNSCC.

Lipidomic Phenotyping Reveals Extensive Lipid Remodeling during Adipogenesis in Human Adipocytes

Differentiation of preadipocytes into mature adipocytes is a highly complex cellular process. At lipidome level, the adipogenesis remains poorly characterized. To investigate the lipidomic changes during human adipogenesis, we used the Lipidyzer^TM assay, which quantified 743 lipid species from 11 classes. The undifferentiated human SGBS cell strain showed a heterogeneous lipid class composition with the most abundant classes, phosphatidylethanolamines (PE), phosphatidylcholines (PC), and sphingomyelins (SM). The differentiation process was accompanied by increased ceramide concentrations. After completion of differentiation around day 4, massive lipid remodeling occurred during maturation, characterized by substantial synthesis of diacylglycerols (DAG), lysophosphatidylethanolamines (LPE), PC, PE, SM, and triacylglycerols (TAG). Lipid species composition became more homogeneous during differentiation to highly concentrated saturated and monounsaturated long-chain fatty acids (LCFA), with the four most abundant being C16:0, C16:1, C18:0, and C18:1. Simultaneously, the amount of polyunsaturated and very long-chain fatty acids (VLCFA) markedly decreased. High negative correlation coefficients between PE and PC species containing VLCFA and TAG species as well as between ceramides and SM imply that PE, PC, and ceramides might have served as additional sources for TAG and SM synthesis, respectively. These results highlight the enormous remodeling at the lipid level over several lipid classes during adipogenesis.

PKCζ mediated anti-proliferative effect of C2 ceramide on neutralization of the tumor microenvironment and melanoma regression

Immunotherapy, which has advantages over chemotherapy due to lesser toxicity and higher specificity, is on the rise to treat cancer. Recently, pro-apoptotic glycolipid, ceramide has emerged as a key regulator in cancer immunotherapy. The present study elucidated the potential anti-melanoma efficacy of cell-permeable, exogenous C2 ceramide on cell death and amelioration of tumor microenvironment (TME). We, for the first time, demonstrated that C2 ceramide triggered apoptosis of melanoma cells by augmenting PKCζ along with pro-inflammatory cytokines and signaling factors. C2 ceramide showed a PKCζ-mediated tumor-suppressive role in melanoma without exhibiting hepatotoxicity and nephrotoxicity. Moreover, PKCζ was revealed as one of the key regulators of Akt and ceramide during C2 ceramide-mediated apoptosis. C2 ceramide was effective in repolarization of M2 macrophage phenotype and reduction of angiogenic factors such as VEGF, VEGFR1, VEGFR2, HIF1α. Interestingly, PKCζ knockdown attenuated C2 ceramide-mediated inhibition of melanoma progression. Restoration of the Th1 type TME by C2 ceramide enhanced cytotoxic T cell-mediated killing of melanoma cells. Altogether, the study unraveled that C2 ceramide-induced PKCζ was associated with favorable immune cell functioning in TME leading to melanoma regression. Thus, our findings explored a novel mechanistic insight into C2 ceramide as a promising immunotherapeutic agent in melanoma treatment.

Primary cilia control cell alignment and patterning in bone development via ceramide-PKCζ-β-catenin signaling

Intraflagellar transport (IFT) proteins are essential for cilia assembly and function. IFT protein mutations lead to ciliopathies, which manifest as variable skeletal abnormalities. However, how IFT proteins regulate cell alignment during bone development is unknown. Here, we show that the deletion of IFT20 in osteoblast lineage using Osterix-Cre and inducible type I Collagen-CreERT cause a compromised cell alignment and a reduced bone mass. This finding was validated by the disorganized collagen fibrils and decreased bone strength and stiffness in IFT20-deficient femurs. IFT20 maintains cilia and cell alignment in osteoblasts, as the concentric organization of three-dimensional spheroids was disrupted by IFT20 deletion. Mechanistically, IFT20 interacts with the ceramide-PKCζ complex to promote PKCζ phosphorylation in cilia and induce the apical localization of β-catenin in osteoblasts, both of which were disrupted in the absence of IFT20. These results reveal that IFT20 regulates polarity and cell alignment via ceramide-pPKCζ-β-catenin signaling during bone development.

Sphingolipids in Alzheimer's disease, how can we target them?

Altered levels of sphingolipids and their metabolites in the brain, and the related downstream effects on neuronal homeostasis and the immune system, provide a framework for understanding mechanisms in neurodegenerative disorders and for developing new intervention strategies. In this review we will discuss: the metabolites of sphingolipids that function as second messengers; and functional aberrations of the pathway resulting in Alzheimer's disease (AD) pathophysiology. Focusing on the central product of the sphingolipid pathway ceramide, we describ approaches to pharmacologically decrease ceramide levels in the brain and we argue on how the sphingolipid pathway may represent a new framework for developing novel intervention strategies in AD. We also highlight the possible use of clinical and non-clinical drugs to modulate the sphingolipid pathway and sphingolipid-related biological cascades.

Reactive oxygen species as mediators of oxygen signaling during fetal-to-neonatal circulatory transition

Reactive oxygen species (ROS) are frequently seen as pathological agents of oxidative stress. However, ROS are not always deleterious and can also act as cell signaling molecules. Vascular oxygen sensing and signaling during fetal-to-neonatal circulatory transition is a remarkable example of the physiological regulatory actions of ROS. The fetal relative hypoxic environment induces hypoxic pulmonary vasoconstriction (HPV) and ductus arteriosus (DA) relaxation favoring the presence of high pulmonary vascular resistance and right-to-left ductal shunt. At birth, the increase in oxygen tension causes relaxation of pulmonary arteries (PAs) and normoxic DA vasoconstriction (NDAV), thus diverting blood flow to the lungs. Although the response to changes in oxygen tension is diametrically opposite, the mechanisms responsible for HPV and NDAV appear to be the result of a similar interaction between triggering and modulating factors that lead to an increase in cytosolic Ca²⁺ concentration and Ca²⁺ sensitization of the contractile apparatus. Growing evidence points to an increase in ROS (mitochondria- and/or NADPH-derived superoxide and/or H₂O₂), leading to inhibition of voltage-gated K⁺ channels, membrane depolarization, and activation of voltage-gated L-type Ca²⁺ channels as critical events in the signaling pathway of both HPV and NDAV. Several groups of investigators have completed this pathway adding other elements such as neutral sphingomyelinase-derived ceramide, the sarcoplasmic/endoplasmic reticulum (through ryanodine and inositol 1,4,5-trisphosphate receptors), Rho kinase-mediated Ca²⁺ sensitization, or transient receptor potential channels. The present review focus on the role of ROS as mediators of the homeostatic oxygen sensing system during fetal and neonatal life not only in the PAs and DA but also in systemic arteries.

Anti-inflammatory and Regulatory Effects of Huanglian Jiedu Decoction on Lipid Homeostasis and the TLR4/MyD88 Signaling Pathway in LPS-Induced Zebrafish

Huanglian Jiedu decoction (HLJDD) has been used in the clinical treatment of inflammatory conditions. To clarify the mechanism of its comprehensive anti-inflammatory activities, the correlation between lipid homeostasis and the TLR4/MyD88 signaling pathway in zebrafish was established in the present study. In the lipopolysaccharide (LPS)-induced inflammation in zebrafish model, RT-PCR assays of five inflammatory cytokines and six targeted proteins were measured. Lipidomics analysis was conducted to identify potential lipid markers. HLJDD displayed strong efficacies, with a 61% anti-inflammatory rate at a concentration of 50 μg/mL. The activation of TLR4/MyD88 played an essential role in the inflammatory process. All protein indexes in the HLJDD group exhibited a tendency to reverse back to normal levels. Moreover, 79 potential pathological lipid biomarkers were identified. Compared with the model group, 61 therapeutic lipid biomarkers were detected in HLJDD group. Most perturbations of lipids were ameliorated by HLJDD, mainly through the glycerophospholipid metabolic pathway. In the visual network study, the corresponding lipoproteins such as PLA2, SGMS, and SMDP were observed as important intermediates between lipid homeostasis and the TLR4/MyD88 signaling pathway.

aPKC in neuronal differentiation, maturation and function

The atypical Protein Kinase Cs (aPKCs)—PRKCI, PRKCZ and PKMζ—form a subfamily within the Protein Kinase C (PKC) family. These kinases are expressed in the nervous system, including during its development and in adulthood. One of the aPKCs, PKMζ, appears to be restricted to the nervous system. aPKCs are known to play a role in a variety of cellular responses such as proliferation, differentiation, polarity, migration, survival and key metabolic functions such as glucose uptake, that are critical for nervous system development and function. Therefore, these kinases have garnered a lot of interest in terms of their functional role in the nervous system. Here we review the expression and function of aPKCs in neural development and in neuronal maturation and function. Despite seemingly paradoxical findings with genetic deletion versus gene silencing approaches, we posit that aPKCs are likely candidates for regulating many important neurodevelopmental and neuronal functions, and may be associated with a number of human neuropsychiatric diseases.

Biochemistry of very-long-chain and long-chain ceramides in cystic fibrosis and other diseases: The importance of side chain

Ceramides, the principal building blocks of all sphingolipids, have attracted the attention of many scientists around the world interested in developing treatments for cystic fibrosis, the most common genetic disease of Caucasians. Many years of fruitful research in this field have produced some fundamentally important, yet controversial results. Here, we aimed to summarize the current knowledge on the role of long- and very-long- chain ceramides, the most abundant species of ceramides in animal cells, in cystic fibrosis and other diseases. We also aim to explain the importance of the length of their side chain in the context of stability of transmembrane proteins through a concise synthesis of their biophysical chemistry, cell biology, and physiology. This review also addresses several remaining riddles in this field. Finally, we discuss the technical challenges associated with the analysis and quantification of ceramides. We provide the evaluation of the antibodies used for ceramide quantification and we demonstrate their lack of specificity. Results and discussion presented here will be of interest to anyone studying these enigmatic lipids.

Drugs That Modify Cholesterol Metabolism Alter the p38/JNK-Mediated Targeted and Nontargeted Response to Alpha and Auger Radioimmunotherapy

PURPOSE

For the development of new anticancer therapeutic radiopharmaceuticals, including alpha particle emitters, it is important to determine the contribution of targeted effects in irradiated cells, and also of nontargeted effects in nonirradiated neighboring cells, because they may affect the therapeutic efficacy and contribute to side effects.

EXPERIMENTAL DESIGN

Here, we investigated the contribution of nontargeted cytotoxic and genotoxic effects in vitro and in vivo (in xenografted mice) during alpha (²¹²Pb/²¹²Bi, ²¹³Bi) and Auger (¹²⁵I) radioimmunotherapy (RIT).

RESULTS

Between 67% and 94% (alpha RIT) and 8% and 15% (Auger RIT) of cancer cells were killed by targeted effects, whereas 7% to 36% (alpha RIT) and 27% to 29% (Auger RIT) of cells were killed by nontargeted effects. We then demonstrated that the nontargeted cell response to alpha and Auger RIT was partly driven by lipid raft-mediated activation of p38 kinase and JNK. Reactive oxygen species also played a significant role in these nontargeted effects, as demonstrated by NF-κB activation and the inhibitory effects of antioxidant enzymes and radical scavengers. Compared with RIT alone, the use of RIT with ASMase inhibitor (imipramine) or with a lipid raft disruptor (e.g., methyl-beta-cyclodextrin or filipin) led to an increase in clonogenic cell survival in vitro and to larger tumors and less tissue DNA damage in vivo. These results were supported by an inhibitory effect of pravastatin on Auger RIT.

CONCLUSIONS

Cell membrane-mediated nontargeted effects play a significant role during Auger and alpha RIT, and drugs that modulate cholesterol level, such as statins, could interfere with RIT efficacy.

Understanding mechanisms of oocyte development by follicular fluid lipidomics

PURPOSE

The present study aimed to provide a non-invasive approach to studying mechanisms responsible for oocyte development.

METHODS

To this end, follicular fluid (FF) from 62 patients undergoing in vitro fertilization (IVF) cycles was split into two groups depending on the pregnancy outcome: pregnant (n = 28) and non-pregnant (n = 34) groups. Data were acquired by the MALDI-TOF mass spectrometry. Principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA) were applied to the data set. A ROC curve, to predict success rate, was constructed, and the lipids were attributed.

RESULTS

Six ions were differentially represented in FF of pregnant and non-pregnant patients, with an area under the curve of 0.962. Phosphatidic acid, phosphatidylglycerol, and triacylglycerol were hyper-represented in the pregnant group, while glucosylceramide was hyper-represented in the non-pregnant group. Enriched functions related to these lipids are steroidogenesis, cellular response, signal transduction, cell cycle, and activation of protein kinase C for the pregnant group and apoptosis inhibition for the non-pregnant group.

CONCLUSION

Human FF fingerprinting can both improve the understanding concerning mechanisms responsible for oocyte development and its effect on embryo implantation potential and assist in the management of IVF cycles.

Biochemistry of very-long-chain and long-chain ceramides in cystic fibrosis and other diseases: The importance of side chain

Ceramides, the principal building blocks of all sphingolipids, have attracted the attention of many scientists around the world interested in developing treatments for cystic fibrosis, the most common genetic disease of Caucasians. Many years of fruitful research in this field have produced some fundamentally important, yet controversial results. Here, we aimed to summarize the current knowledge on the role of long- and very-long- chain ceramides, the most abundant species of ceramides in animal cells, in cystic fibrosis and other diseases. We also aim to explain the importance of the length of their side chain in the context of stability of transmembrane proteins through a concise synthesis of their biophysical chemistry, cell biology, and physiology. This review also addresses several remaining riddles in this field. Finally, we discuss the technical challenges associated with the analysis and quantification of ceramides. We provide the evaluation of the antibodies used for ceramide quantification and we demonstrate their lack of specificity. Results and discussion presented here will be of interest to anyone studying these enigmatic lipids.

Ceramide and Regulation of Vascular Tone

In addition to playing a role as a structural component of cellular membranes, ceramide is now clearly recognized as a bioactive lipid implicated in a variety of physiological functions. This review aims to provide updated information on the role of ceramide in the regulation of vascular tone. Ceramide may induce vasodilator or vasoconstrictor effects by interacting with several signaling pathways in endothelial and smooth muscle cells. There is a clear, albeit complex, interaction between ceramide and redox signaling. In fact, reactive oxygen species (ROS) activate different ceramide generating pathways and, conversely, ceramide is known to increase ROS production. In recent years, ceramide has emerged as a novel key player in oxygen sensing in vascular cells and mediating vascular responses of crucial physiological relevance such as hypoxic pulmonary vasoconstriction (HPV) or normoxic ductus arteriosus constriction. Likewise, a growing body of evidence over the last years suggests that exaggerated production of vascular ceramide may have detrimental effects in a number of pathological processes including cardiovascular and lung diseases.

Activation of atypical protein kinase C by sphingosine 1-phosphate revealed by an aPKC-specific activity reporter

Atypical protein kinase C (aPKC) isozymes are unique in the PKC superfamily in that they are not regulated by the lipid second messenger diacylglycerol, which has led to speculation about whether a different second messenger acutely controls their function. Here, using a genetically encoded reporter that we designed, aPKC-specific C kinase activity reporter (aCKAR), we found that the lipid mediator sphingosine 1-phosphate (S1P) promoted the cellular activity of aPKC. Intracellular S1P directly bound to the purified kinase domain of aPKC and relieved autoinhibitory constraints, thereby activating the kinase. In silico studies identified potential binding sites on the kinase domain, one of which was validated biochemically. In HeLa cells, S1P-dependent activation of aPKC suppressed apoptosis. Together, our findings identify a previously undescribed molecular mechanism of aPKC regulation, a molecular target for S1P in cell survival regulation, and a tool to further explore the biochemical and biological functions of aPKC.

Sphingolipids in neurodegeneration (with focus on ceramide and S1P)

For many decades, research on sphingolipids associated with neurodegenerative disease focused on alterations in glycosphingolipids, particularly glycosylceramides (cerebrosides), sulfatides, and gangliosides. This seemed quite natural since many of these glycolipids are constituents of myelin and accumulated in lipid storage diseases (sphingolipidoses) resulting from enzyme deficiencies in glycolipid metabolism. With the advent of recognizing ceramide and its derivative, sphingosine-1-phosphate (S1P), as key players in lipid cell signaling and regulation of cell death and survival, research focus shifted toward these two sphingolipids. Ceramide and S1P are invoked in a plethora of cell biological processes participating in neurodegeneration such as ER stress, autophagy, dysregulation of protein and lipid transport, exosome secretion and neurotoxic protein spreading, neuroinflammation, and mitochondrial dysfunction. Hence, it is timely to discuss various functions of ceramide and S1P in neurodegenerative disease and to define sphingolipid metabolism and cell signaling pathways as potential targets for therapy.

Targeted and Off-Target (Bystander and Abscopal) Effects of Radiation Therapy: Redox Mechanisms and Risk/Benefit Analysis

SIGNIFICANCE

Radiation therapy (from external beams to unsealed and sealed radionuclide sources) takes advantage of the detrimental effects of the clustered production of radicals and reactive oxygen species (ROS). Research has mainly focused on the interaction of radiation with water, which is the major constituent of living beings, and with nuclear DNA, which contains the genetic information. This led to the so-called target theory according to which cells have to be hit by ionizing particles to elicit an important biological response, including cell death. In cancer therapy, the Poisson law and linear quadratic mathematical models have been used to describe the probability of hits per cell as a function of the radiation dose. Recent Advances: However, in the last 20 years, many studies have shown that radiation generates "danger" signals that propagate from irradiated to nonirradiated cells, leading to bystander and other off-target effects.

CRITICAL ISSUES

Like for targeted effects, redox mechanisms play a key role also in off-target effects through transmission of ROS and reactive nitrogen species (RNS), and also of cytokines, ATP, and extracellular DNA. Particularly, nuclear factor kappa B is essential for triggering self-sustained production of ROS and RNS, thus making the bystander response similar to inflammation. In some therapeutic cases, this phenomenon is associated with recruitment of immune cells that are involved in distant irradiation effects (called "away-from-target" i.e., abscopal effects).

FUTURE DIRECTIONS

Determining the contribution of targeted and off-target effects in the clinic is still challenging. This has important consequences not only in radiotherapy but also possibly in diagnostic procedures and in radiation protection.

Sphingolipids and lipid rafts: Novel concepts and methods of analysis

About twenty years ago, the functional lipid raft model of the plasma membrane was published. It took into account decades of research showing that cellular membranes are not just homogenous mixtures of lipids and proteins. Lateral anisotropy leads to assembly of membrane domains with specific lipid and protein composition regulating vesicular traffic, cell polarity, and cell signaling pathways in a plethora of biological processes. However, what appeared to be a clearly defined entity of clustered raft lipids and proteins became increasingly fluid over the years, and many of the fundamental questions about biogenesis and structure of lipid rafts remained unanswered. Experimental obstacles in visualizing lipids and their interactions hampered progress in understanding just how big rafts are, where and when they are formed, and with which proteins raft lipids interact. In recent years, we have begun to answer some of these questions and sphingolipids may take center stage in re-defining the meaning and functional significance of lipid rafts. In addition to the archetypical cholesterol-sphingomyelin raft with liquid ordered (Lo) phase and the liquid-disordered (Ld) non-raft regions of cellular membranes, a third type of microdomains termed ceramide-rich platforms (CRPs) with gel-like structure has been identified. CRPs are "ceramide rafts" that may offer some fresh view on the membrane mesostructure and answer several critical questions for our understanding of lipid rafts.

MAPK/ERK and JNK pathways regulate lipid synthesis and cell growth of Chlamydomonas reinhardtii under osmotic stress, respectively

Microalgae have great potential for the production of biofuels due to the ability of the organism to accumulate large quantities of storage lipids under stress conditions. Mitogen activated protein kinase (MAPK) signaling cascades are widely recognized for their role in stress response signal transduction in eukaryotes. To assess the correlation between MAPK activation and lipid productivity, Chlamydomonas reinhardtii was studied under various concentrations of NaCl. The results demonstrated that C. reinhardtii exhibits elevated levels of extracellular-signal regulated kinase (ERK) and c-Jun N-terminal kinase (JNK) activities after undergoing osmotic stress, as well as an increase in cellular lipid content. To establish a more direct causal link between both kinases and lipid productivity, C. reinhardtii was subjected to biochemically induced regulation of ERK and JNK pathways. Activating the MEK-ERK pathway via C6 ceramide treatment increased ERK activation and lipid production simultaneously, while PD98059 mediated inhibition of the pathway yielded opposite results. Interestingly, suppression of the JNK pathway with SP600125 resulted in a substantial decrease in cell viability under osmotic stress. These results suggest that ERK and JNK MAP kinases have important roles in microalgal lipid accumulation and cell growth under osmotic stress, respectively.

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.

Sphingolipid metabolism in non-alcoholic fatty liver diseases

Non-alcoholic fatty liver disease (NAFLD) involves a panel of pathologies starting with hepatic steatosis and continuing to irreversible and serious conditions like steatohepatitis (NASH) and hepatocarcinoma. NAFLD is multifactorial in origin and corresponds to abnormal fat deposition in liver. Even if triglycerides are mostly associated with these pathologies, other lipid moieties seem to be involved in the development and severity of NAFLD. That is the case with sphingolipids and more particularly ceramides. In this review, we explore the relationship between NAFLD and sphingolipid metabolism. After providing an analysis of complex sphingolipid metabolism, we focus on the potential involvement of sphingolipids in the different pathologies associated with NAFLD. An unbalanced ratio between ceramides and terminal metabolic products in the liver and plasma promotes weight gain, inflammation, and insulin resistance. In the etiology of NAFLD, some sphingolipid species such as ceramides may be potential biomarkers for NAFLD. We review the clinical relevance of sphingolipids in liver diseases.

Ceramide Signaling and p53 Pathways

Ceramides, important players in signal transduction, interact with multiple cellular pathways, including p53 pathways. However, the relationship between ceramide and p53 is very complex, and mechanisms underlying their coregulation are diverse and not fully characterized. The role of p53, an important cellular regulator and a transcription factor, is linked to its tumor suppressor function. Ceramides are involved in the regulation of fundamental processes in cancer cells including cell death, proliferation, autophagy, and drug resistance. This regulation, however, can be pro-death or pro-survival depending on cancer type, the balance between ceramide species, the rate of their synthesis and utilization, and the availability of a specific array of downstream targets. This chapter highlights the central role of ceramide in sphingolipid metabolism, its role in cancer, specific effectors in ceramide pathways controlled by p53, and coregulation of ceramide and p53 signaling. We discuss the recent studies, which underscore the function of p53 in the regulation of ceramide pathways and the reciprocal regulation of p53 by ceramide. This complex relationship is based on several molecular mechanisms including the p53-dependent transcriptional regulation of enzymes in sphingolipid pathways, the activation of mutant p53 through ceramide-mediated alternative splicing, as well as modulation of the p53 function through direct and indirect effects on p53 coregulators and downstream targets. Further insight into the connections between ceramide and p53 will allow simultaneous targeting of the two pathways with a potential to yield more efficient anticancer therapeutics.

Lysosphingolipids and sphingolipidoses: Psychosine in Krabbe's disease

Until recently, lipids were considered inert building blocks of cellular membranes. This changed three decades ago when lipids were found to regulate cell polarity and vesicle transport, and the "lipid raft" concept took shape. The lipid-driven membrane anisotropy in form of "rafts" that associate with proteins led to the view that organized complexes of lipids and proteins regulate various cell functions. Disturbance of this organization can lead to cellular, tissue, and organ malfunction. Sphingolipidoses, lysosomal storage diseases that are caused by enzyme deficiencies in the sphingolipid degradation pathway, were found to be particularly detrimental to the brain. These enzyme deficiencies result in accumulation of sphingolipid metabolites in lysosomes, although it is not yet clear how this accumulation affects the organization of lipids in cellular membranes. Krabbe's disease (KD), or globoid cell leukodystrophy, was one of the first sphingolipidosis for which the raft concept offered a potential mechanism. KD is caused by mutations in the enzyme β-galactocerebrosidase; however, elevation of its substrate, galactosylceramide, is not observed or considered detrimental. Instead, it was found that a byproduct of galactosylceramide metabolism, the lysosphingolipid psychosine, is accumulated. The "psychosine hypothesis" has been refined by showing that psychosine disrupts lipid rafts and vesicular transport critical for the function of glia and neurons. The role of psychosine in KD is an example of how the disruption of sphingolipid metabolism can lead to elevation of a toxic lysosphingolipid, resulting in disruption of cellular membrane organization and neurotoxicity. © 2016 Wiley Periodicals, Inc.

Synthesis of Chrysogeside B from Halotolerant Fungus Penicillium and Its Antimicrobial Activities Evaluation

Chrysogeside B, a natural cerebroside, was efficiently synthesized from commercial feedstocks. The bioassays showed that compounds 4, 5 and 6 exhibited enhanced biological activities compared Chrysogeside B. Further studies revealed that free hydroxyl groups and glycosidic bond have significant impact on the antimicrobial activities. The synthesis of Chrysogeside B and analogues designed to allow identification of the features of this glycolipid required for recognition by tested bacteria and Hela cells is described.

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.

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.

Molecular Control of Atypical Protein Kinase C: Tipping the Balance between Self-Renewal and Differentiation

Complex organisms are faced with the challenge of generating and maintaining diverse cell types, ranging from simple epithelia to neurons and motile immune cells [1-3]. To meet this challenge, a complex set of regulatory pathways controls nearly every aspect of cell growth and function, including genetic and epigenetic programming, cytoskeleton dynamics, and protein trafficking. The far reach of cell fate specification pathways makes it particularly catastrophic when they malfunction, both during development and for tissue homeostasis in adult organisms. Furthermore, the therapeutic promise of stem cells derives from their ability to deftly navigate the multitude of pathways that control cell fate [4]. How the molecular components making up these pathways function to specify cell fate is beginning to become clear. Work from diverse systems suggests that the atypical Protein Kinase C (aPKC) is a key regulator of cell fate decisions in metazoans [5-7]. Here, we examine some of the diverse physiological outcomes of aPKC's function in differentiation, along with the molecular pathways that control aPKC and those that are responsive to changes in its catalytic activity.

Proteome-wide Identification of Novel Ceramide-binding Proteins by Yeast Surface cDNA Display and Deep Sequencing

Although the bioactive sphingolipid ceramide is an important cell signaling molecule, relatively few direct ceramide-interacting proteins are known. We used an approach combining yeast surface cDNA display and deep sequencing technology to identify novel proteins binding directly to ceramide. We identified 234 candidate ceramide-binding protein fragments and validated binding for 20. Most (17) bound selectively to ceramide, although a few (3) bound to other lipids as well. Several novel ceramide-binding domains were discovered, including the EF-hand calcium-binding motif, the heat shock chaperonin-binding motif STI1, the SCP2 sterol-binding domain, and the tetratricopeptide repeat region motif. Interestingly, four of the verified ceramide-binding proteins (HPCA, HPCAL1, NCS1, and VSNL1) and an additional three candidate ceramide-binding proteins (NCALD, HPCAL4, and KCNIP3) belong to the neuronal calcium sensor family of EF hand-containing proteins. We used mutagenesis to map the ceramide-binding site in HPCA and to create a mutant HPCA that does not bind to ceramide. We demonstrated selective binding to ceramide by mammalian cell-produced wild type but not mutant HPCA. Intriguingly, we also identified a fragment from prostaglandin D2synthase that binds preferentially to ceramide 1-phosphate. The wide variety of proteins and domains capable of binding to ceramide suggests that many of the signaling functions of ceramide may be regulated by direct binding to these proteins. Based on the deep sequencing data, we estimate that our yeast surface cDNA display library covers ∼60% of the human proteome and our selection/deep sequencing protocol can identify target-interacting protein fragments that are present at extremely low frequency in the starting library. Thus, the yeast surface cDNA display/deep sequencing approach is a rapid, comprehensive, and flexible method for the analysis of protein-ligand interactions, particularly for the study of non-protein ligands.

Unraveling the role of the Target of Rapamycin signaling in sphingolipid metabolism

Sphingolipids are important bioactive molecules that regulate basic aspects of cellular metabolism and physiology, including cell growth, adhesion, migration, senescence, apoptosis, endocytosis, and autophagy in yeast and higher eukaryotes. Since they have the ability to modulate the activation of several proteins and signaling pathways, variations in the relative levels of different sphingolipid species result in important changes in overall cellular functions and fate. Sphingolipid metabolism and their route of synthesis are highly conserved from yeast to mammalian cells. Studies using the budding yeast Saccharomyces cerevisiae have served in many ways to foster our understanding of sphingolipid dynamics and their role in the regulation of cellular processes. In the past decade, studies in S. cerevisiae have unraveled a functional association between the Target of Rapamycin (TOR) pathway and sphingolipids, showing that both TOR Complex 1 (TORC1) and TOR Complex 2 (TORC2) branches control temporal and spatial aspects of sphingolipid metabolism in response to physiological and environmental cues. In this review, we report recent findings in this emerging and exciting link between the TOR pathway and sphingolipids and implications in human health and disease.

Autophagic flux and autophagosome morphogenesis require the participation of sphingolipids

Apoptosis and autophagy are two evolutionary conserved processes that exert a critical role in the maintenance of tissue homeostasis. While apoptosis is a tightly regulated cell program implicated in the removal of damaged or unwanted cells, autophagy is a cellular catabolic pathway that is involved in the lysosomal degradation and recycling of proteins and organelles, and is thereby considered an important cytoprotection mechanism. Sphingolipids (SLs), which are ubiquitous membrane lipids in eukaryotes, participate in the generation of various membrane structures, including lipid rafts and caveolae, and contribute to a number of cellular functions such as cell proliferation, apoptosis and, as suggested more recently, autophagy. For instance, SLs are hypothesized to be involved in several intracellular processes, including organelle membrane scrambling, whilst at the plasma membrane lipid rafts, acting as catalytic domains, strongly contribute to the ignition of critical signaling pathways determining cell fate. In particular, by targeting several shared regulators, ceramide, sphingosine-1-phosphate, dihydroceramide, sphingomyelin and gangliosides seem able to differentially regulate the autophagic pathway and/or contribute to the autophagosome formation. This review illustrates recent studies on this matter, particularly lipid rafts, briefly underscoring the possible implication of SLs and their alterations in the autophagy disturbances and in the pathogenesis of some human diseases.

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.

Regulation of Chlamydomonas flagella and ependymal cell motile cilia by ceramide-mediated translocation of GSK3

Cilia are important organelles formed by cell membrane protrusions; however, little is known about their regulation by membrane lipids. A novel, evolutionarily conserved activation mechanism for GSK3 by the sphingolipid (phyto)ceramide is characterized that is critical for ciliogenesis in Chlamydomonas and murine ependymal cells.

Cilia are important organelles formed by cell membrane protrusions; however, little is known about their regulation by membrane lipids. We characterize a novel activation mechanism for glycogen synthase kinase-3 (GSK3) by the sphingolipids phytoceramide and ceramide that is critical for ciliogenesis in Chlamydomonas and murine ependymal cells, respectively. We show for the first time that Chlamydomonas expresses serine palmitoyl transferase (SPT), the first enzyme in (phyto)ceramide biosynthesis. Inhibition of SPT in Chlamydomonas by myriocin led to loss of flagella and reduced tubulin acetylation, which was prevented by supplementation with the precursor dihydrosphingosine. Immunocytochemistry showed that (phyto)ceramide was colocalized with phospho–Tyr-216-GSK3 (pYGSK3) at the base and tip of Chlamydomonas flagella and motile cilia in ependymal cells. The (phyto)ceramide distribution was consistent with that of a bifunctional ceramide analogue UV cross-linked and visualized by click-chemistry–mediated fluorescent labeling. Ceramide depletion, by myriocin or neutral sphingomyelinase deficiency (fro/fro mouse), led to GSK3 dephosphorylation and defective flagella and cilia. Motile cilia were rescued and pYGSK3 localization restored by incubation of fro/fro ependymal cells with exogenous C24:1 ceramide, which directly bound to pYGSK3. Our findings suggest that (phyto)ceramide-mediated translocation of pYGSK into flagella and cilia is an evolutionarily conserved mechanism fundamental to the regulation of ciliogenesis.

DHA at nutritional doses restores insulin sensitivity in skeletal muscle by preventing lipotoxicity and inflammation

Skeletal muscle plays a major role in the control of whole body glucose disposal in response to insulin stimulus. Excessive supply of fatty acids to this tissue triggers cellular and molecular disturbances leading to lipotoxicity, inflammation, mitochondrial dysfunctions, impaired insulin response and decreased glucose uptake. This study was conducted to analyze the preventive effect of docosahexaenoic acid (DHA), a long-chain polyunsaturated n-3 fatty acid, against insulin resistance, lipotoxicity and inflammation in skeletal muscle at doses compatible with nutritional supplementation. DHA (30 μM) prevented insulin resistance in C2C12 myotubes exposed to palmitate (500 μM) by decreasing protein kinase C (PKC)-θ activation and restoring cellular acylcarnitine profile, insulin-dependent AKT phosphorylation and glucose uptake. Furthermore, DHA protected C2C12 myotubes from palmitate- or lipopolysaccharide-induced increase in Ptgs2, interleukin 6 and tumor necrosis factor-α mRNA level, probably through the inhibition of p38 MAP kinase and c-Jun amino-terminal kinase. In LDLR -/- mice fed a high-cholesterol-high-sucrose diet, supplementation with DHA reaching up to 2% of daily energy intake enhanced the insulin-dependent AKT phosphorylation and reduced the PKC-θ activation in skeletal muscle. Therefore, DHA used at physiological doses participates in the regulation of muscle lipid and glucose metabolisms by preventing lipotoxicity and inflammation.