Differential roles of de novo sphingolipid biosynthesis and turnover in the "burst" of free sphingosine and sphinganine, and their 1-phosphates and N-acyl-derivatives, that occurs upon changing the medium of cells in culture

Don't Be Surprised When These Surprise You: Some Infrequently Studied Sphingoid Bases, Metabolites, and Factors That Should Be Kept in Mind During Sphingolipidomic Studies

Sphingolipidomic mass spectrometry has provided valuable information-and surprises-about sphingolipid structures, metabolism, and functions in normal biological processes and disease. Nonetheless, many noteworthy compounds are not routinely determined, such as the following: most of the sphingoid bases that mammals biosynthesize de novo other than sphingosine (and sometimes sphinganine) or acquire from exogenous sources; infrequently considered metabolites of sphingoid bases, such as N-(methyl)n-derivatives; "ceramides" other than the most common N-acylsphingosines; and complex sphingolipids other than sphingomyelins and simple glycosphingolipids, including glucosyl- and galactosylceramides, which are usually reported as "monohexosylceramides". These and other subspecies are discussed, as well as some of the circumstances when they are likely to be seen (or present and missed) due to experimental conditions that can influence sphingolipid metabolism, uptake from the diet or from the microbiome, or as artifacts produced during extraction and analysis. If these compounds and factors are kept in mind during the design and interpretation of lipidomic studies, investigators are likely to be surprised by how often they appear and thereby advance knowledge about them.

The function of sphingolipids in different pathogenesis of Alzheimer's disease: A comprehensive review

Sphingolipids (SPLs) represent a highly diverse and structurally complex lipid class. The discussion of SPL metabolism-related issues is of importance in understanding the neuropathological progression of Alzheimer's disease (AD). AD is characterized by the accumulation of extracellular deposits of the amyloid β-peptide (Aβ) and intraneuronal aggregates of the microtubule-associated protein tau. Critical roles of Aβ oligomer deposited and ganglioside GM1 could be formed as "seed" from insoluble GAβ polymer in initiating the pathogenic process, while tau might also mediate SPLs and their toxicity. The interaction between ceramide and α-Synuclein (α-Syn) accelerates the aggregation of ferroptosis and exacerbates the pathogenesis of AD. For instance, reducing the levels of SPLs can mitigate α-Syn accumulation and inhibit AD progression. Meanwhile, loss of SPLs may inhibit the expression of APOE4 and confer protection against AD, while the loss of APOE4 expression also disrupts SPLs homeostasis. Moreover, the heightened activation of sphingomyelinase promotes the ferroptosis signaling pathway, leading to exacerbated AD symptoms. Ferroptosis plays a vital role in the pathological progression of AD by influencing Aβ, tau, APOE, and α-Syn. Conversely, the development of AD also exacerbates the manifestation of ferroptosis and SPLs. We are compiling the emerging techniques (Derivatization and IM-MS) of sphingolipidomics, to overcome the challenges of AD diagnosis and treatment. In this review, we examined the intricate neuro-mechanistic interactions between SPLs and Aβ, tau, α-Syn, APOE, and ferroptosis, mediating the onset of AD. Furthermore, our findings highlight the potential of targeting SPLs as underexplored avenue for devising innovative therapeutic strategies against AD.

Diverse Roles of Ceramide in the Progression and Pathogenesis of Alzheimer’s Disease

Alzheimer’s disease (AD) is the most common neurodegenerative disorder, and is associated with several pathophysiological features, including cellular dysfunction, failure of neurotransmission, cognitive impairment, cell death, and other clinical consequences. Advanced research on the pathogenesis of AD has elucidated a mechanistic framework and revealed many therapeutic possibilities. Among the mechanisms, sphingolipids are mentioned as distinctive mediators to be associated with the pathology of AD. Reportedly, alteration in the metabolism of sphingolipids and their metabolites result in the dysfunction of mitochondria, autophagy, amyloid beta regulation, and neuronal homeostasis, which exacerbates AD progression. Considering the importance of sphingolipids, in this review, we discuss the role of ceramide, a bioactive sphingolipid metabolite, in the progression and pathogenesis of AD. Herein, we describe the ceramide synthesis pathway and its involvement in the dysregulation of homeostasis, which eventually leads to AD. Furthermore, this review references different therapeutics proposed to modulate the ceramide pathway to maintain ceramide levels and prevent the disease progression.

Glycosphingolipids in Filamentous Fungi: Biological Roles and Potential Applications in Cosmetics and Health Foods

Filamentous fungi are a group of economically important fungi used in the production of fermented foods, industrial enzymes, and secondary metabolites. Glycosphingolipids (GSLs) as constituents of lipid rafts are involved in growth, differentiation, and response to environment stress in filamentous fungi. In addition to these key roles, GSLs are also important in the barrier function of skin to retain moisture as a moisturizing ingredient in cosmetics or health products for their strong biological activity as a functional component. GSLs found in filamentous fungi are divided in two major classes: neutral GSLs (glycosylceramides), glucosylceramides (GlcCers), and/or galactosylceramides (GalCers) and acidic GSLs, mannosylinositol phosphorylceramide (MIPC) and mannosyldiinositol phosphorylceramide [M(IP)₂C]. Glycosylceramides are one of the abundant GSLs in Aspergillus and known to improve skin-barrier function and prevent intestinal impairment as a prebiotic. Some filamentous fungi of Aspergillus spp., synthesizing both GlcCer and GalCer, would be an amenable source to exploit glycosylceramides that wildly adding in cosmetics as moisturizing ingredients or health food as dietary supplements. In this minireview, the types, structures, and biosynthetic pathways of GSLs in filamentous fungi, and the relevance of GSLs in fungal growth, spore formation, and environmental stress response are explained. Furthermore, the advantage, potential development, and application of GlcCer and GalCer from filamentous fungi Aspergillus spp. are also investigate based on the use of plant GlcCer in health foods and cosmetics.

Sphingolipid Metabolism and Signaling in Skeletal Muscle: From Physiology to Physiopathology

Sphingolipids represent one of the major classes of eukaryotic lipids. They play an essential structural role, especially in cell membranes where they also possess signaling properties and are capable of modulating multiple cell functions, such as apoptosis, cell proliferation, differentiation, and inflammation. Many sphingolipid derivatives, such as ceramide, sphingosine-1-phosphate, and ganglioside, have been shown to play many crucial roles in muscle under physiological and pathological conditions. This review will summarize our knowledge of sphingolipids and their effects on muscle fate, highlighting the role of this class of lipids in modulating muscle cell differentiation, regeneration, aging, response to insulin, and contraction. We show that modulating sphingolipid metabolism may be a novel and interesting way for preventing and/or treating several muscle-related diseases.

Metabolomics facilitates the discovery of metabolic biomarkers and pathways for ischemic stroke: a systematic review

INTRODUCTION

Ischemic stroke (IS) is a major contributor to the global disease burden, and effective biomarkers for IS management in clinical practice are urgently needed. Metabolomics can detect metabolites that are small enough to cross the blood-brain barrier in a high-throughput manner, and thus represents a powerful tool for discovering biomarkers of IS.

OBJECTIVES

In this study, we conducted a systematic review to identify potential metabolic biomarkers and pathways that might facilitate risk predictions, clinical diagnoses, the recognition of complications, predictions of recurrence and an understanding of the pathogenesis of IS.

METHODS

The PubMed and Web of Science databases were searched for relevant studies published between January 2000 and July 2019. The study objectives, study designs and reported metabolic biomarkers were systematically examined and compared. Pathway analysis was performed using the MetaboAnalyst online software.

RESULTS

Twenty-eight studies were included in this systematic review. Many consistent metabolites, including isoleucine, leucine, valine, glycine, lysine, glutamate, LysoPC(16:0), LysoPC(18:2), serine, uric acid, citrate and palmitic acid, possess potential as biomarkers of IS. Metabolic pathways and dysregulations that are implicated in excitotoxicity, inflammation, apoptosis, oxidative stress, neuroprotection, energy failure, and elevation of intracellular Ca²⁺ levels, were indicated as playing important roles in the development and progression of IS.

CONCLUSIONS

This systematic review summarizes potential metabolic biomarkers and pathways related to IS, which may provide opportunities for the construction of diagnostic or predictive models for IS and the discovery of novel therapeutic targets.

Invited review: Sphingolipid biology in the dairy cow: The emerging role of ceramide

The physiological control of lactation through coordinated adaptations is of fundamental importance for mammalian neonatal life. The putative actions of reduced insulin sensitivity and responsiveness and enhanced adipose tissue lipolysis spare glucose for the mammary synthesis of milk. However, severe insulin antagonism and body fat mobilization may jeopardize hepatic health and lactation in dairy cattle. Interestingly, lipolysis- and dietary-derived fatty acids may impair insulin sensitivity in cows. The mechanisms are undefined yet have major implications for the development of postpartum fatty liver disease. In nonruminants, the sphingolipid ceramide is a potent mediator of saturated fat-induced insulin resistance that defines in part the mechanisms of type 2 diabetes mellitus and nonalcoholic fatty liver disease. In ruminants including the lactating dairy cow, the functions of ceramide had remained virtually undescribed. Through a series of hypothesis-centered studies, ceramide has emerged as a potential antagonist of insulin-stimulated glucose utilization by adipose and skeletal muscle tissues in dairy cattle. Importantly, bovine data suggest that the ability of ceramide to inhibit insulin action likely depends on the lipolysis-dependent hepatic synthesis and secretion of ceramide during early lactation. Although these mechanisms appear to fade as lactation advances beyond peak milk production, early evidence suggests that palmitic acid feeding is a means to augment ceramide supply. Herein, we review a body of work that focuses on sphingolipid biology and the role of ceramide in the dairy cow within the framework of hepatic and fatty acid metabolism, insulin function, and lactation. The potential involvement of ceramide within the endocrine control of lactation is also considered.

Necroptosis in primate luteolysis: a role for ceramide

The corpus luteum (CL) is a transient endocrine organ, yet molecular mechanisms resulting in its demise are not well known. The presence of phosphorylated mixed lineage kinase domain-like pseudokinase pMLKL(T357/S358) in human and nonhuman primate CL samples (Macaca mulatta and Callithrix jacchus) implied that necroptosis of luteal cells may be involved. In M. mulatta CL, pMLKL positive staining became detectable only from the mid-late luteal phase onwards, pointing to necroptosis during regression of the CL. Cell death, including necroptosis, was previously observed in cultures of human luteal granulosa cells (GCs), an apt model for the study of the human CL. To explore mechanisms of necroptotic cell death in GCs during culture, we performed a proteomic analysis. The levels of 50 proteins were significantly altered after 5 days of culture. Interconnectivity analysis and immunocytochemistry implicated specifically the ceramide salvage pathway to be enhanced. M. mulatta CL transcriptome analysis indicated in vivo relevance. Perturbing endogenous ceramide generation by fumonisin B1 (FB1) and addition of soluble ceramide (C2-CER) yielded opposite actions on viability of GCs and therefore supported the significance of the ceramide pathway. Morphological changes indicated necrotic cell death in the C2-CER treated group. Studies with the pan caspase blocker zVAD-fmk or the necroptosis blocker necrosulfonamid (NSA) further supported that C2-CER induced necroptosis. Our data pinpoint necroptosis in a physiological process, namely CL regression. This raises the possibility that the primate CL could be rescued by pharmacological inhibition of necroptosis or by interaction with ceramide metabolism.

Sphingolipid biosynthesis in man and microbes

A new review covering up to 2018 Sphingolipids are essential molecules that, despite their long history, are still stimulating interest today. The reasons for this are that, as well as playing structural roles within cell membranes, they have also been shown to perform a myriad of cell signalling functions vital to the correct function of eukaryotic and prokaryotic organisms. Indeed, sphingolipid disregulation that alters the tightly-controlled balance of these key lipids has been closely linked to a number of diseases such as diabetes, asthma and various neuropathologies. Sphingolipid biogenesis, metabolism and regulation is mediated by a large number of enzymes, proteins and second messengers. There appears to be a core pathway common to all sphingolipid-producing organisms but recent studies have begun to dissect out important, species-specific differences. Many of these have only recently been discovered and in most cases the molecular and biochemical details are only beginning to emerge. Where there is a direct link from classic biochemistry to clinical symptoms, a number a drug companies have undertaken a medicinal chemistry campaign to try to deliver a therapeutic intervention to alleviate a number of diseases. Where appropriate, we highlight targets where natural products have been exploited as useful tools. Taking all these aspects into account this review covers the structural, mechanistic and regulatory features of sphingolipid biosynthetic and metabolic enzymes.

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.

Analysis of Sphingolipid Synthesis and Transport by Metabolic Labeling of Cultured Cells with [³H]Serine

Analysis of lipid biosynthesis by radioactive precursor incorporation provides information on metabolic rates and the identity of rate-limiting enzymes and transporters. The biosynthesis of sphingolipids in cultured cells is initiated in the endoplasmic reticulum (ER) by the formation of a sphingoid base from serine and palmitoyl-CoA. N-acylation of the sphingoid base produces ceramide, which is transported to the Golgi apparatus where phosphocholine or carbohydrate headgroups are added to form sphingomyelin (SM) and complex glycosphingolipids (GSLs), respectively. Herein is described a protocol to measure ceramide and SM biosynthesis in cultured cells based on [(3)H]serine incorporation at the first step in the pathway. The method can be used to assay the effect of pharmacological and genetic manipulations on ceramide synthesis and transport to the Golgi apparatus.

Determining the Turnover of Glycosphingolipid Species by Stable-Isotope Tracer Lipidomics

In this study, we used water-soluble stable mass isotope precursors to measure the turnover of endogenous ceramide (Cer) and glycosphingolipids (GSLs) in HEp-2 cells. Cells incubated in the presence of [¹³C-U]glucose showed rapid incorporation of hexose residues with an increased mass of 6Da into GSLs. Different turnover rates of GSL classes and their molecular species were observed. Approximately 30% of the glucosylceramide, 50% of the lactosylceramide, and 50% of the globotriaosylceramide species showed a much slower turnover than the rest. This demonstrates the existence of different lipid pools, where a certain fraction of species survived for a long time in the cells. The species with the shortest N-amidated fatty acyl groups (C16:0 and C18:0) showed a more rapid turnover than those with the longest N-amidated fatty acids (C24:0 and C24:1). Experiments with addition of [¹³C-U]serine were performed to study de novo synthesis of Cer from serine and palmitoyl-CoA. These experiments revealed that de novo synthesis contributes to a minor extent to the total synthesis of new sphingolipids and showed that there is a more rapid formation of the longest Cer species (C24:0 and C24:1) than of the shortest species (C16:0), that is, the opposite as observed for the GSLs in the experiments with [¹³C-U]glucose. In conclusion, this FLUX lipidomics experimental approach with the addition of [¹³C-U]glucose to cells allows us to not only study the total turnover but also permit observations of lipid intermediates and metabolic flow of endogenous GSL species at the molecular lipid level.

Streptococcal serum opacity factor promotes cholesterol ester metabolism and bile acid secretion in vitro and in vivo

Plasma high density lipoprotein-cholesterol (HDL-C) concentrations negatively correlate with atherosclerotic cardiovascular disease. HDL is thought to have several atheroprotective functions, which are likely distinct from the epidemiological inverse relationship between HDL-C levels and risk. Specifically, strategies that reduce HDL-C while promoting reverse cholesterol transport (RCT) may have therapeutic value. The major product of the serum opacity factor (SOF) reaction versus HDL is a cholesteryl ester (CE)-rich microemulsion (CERM), which contains apo E and the CE of ~400,000 HDL particles. Huh7 hepatocytes take up CE faster when delivered as CERM than as HDL, in part via the LDL-receptor (LDLR). Here we compared the final RCT step, hepatic uptake and subsequent intracellular processing to cholesterol and bile salts for radiolabeled HDL-, CERM- and LDL-CE by Huh7 cells and in vivo in C57BL/6J mice. In Huh7 cells, uptake from LDL was greater than from CERM (2-4X) and HDL (5-10X). Halftimes for [(14)C]CE hydrolysis were 3.0±0.2, 4.4±0.6 and 5.4±0.7h respectively for HDL, CERM and LDL-CE. The fraction of sterols secreted as bile acids was ~50% by 8h for all three particles. HDL, CERM and LDL-CE metabolism in mice showed efficient plasma clearance of CERM-CE, liver uptake and metabolism, and secretion as bile acids into the gall bladder. This work supports the therapeutic potential of the SOF reaction, which diverts HDL-CE to the LDLR, thereby increasing hepatic CE uptake, and sterol disposal as bile acids.

Ceramide synthases in biomedical research

Sphingolipid metabolism consists of multiple metabolic pathways that converge upon ceramide, one of the key molecules among sphingolipids (SLs). In mammals, ceramide synthesis occurs via N-acylation of sphingoid backbones, dihydrosphingosine (dhSo) or sphingosine (So). The reaction is catalyzed by ceramide synthases (CerS), a family of enzymes with six different isoforms, with each one showing specificity towards a restricted group of acyl-CoAs, thus producing ceramides (Cer) and dihydroceramides (dhCer) with different fatty acid chain lengths. A large body of evidence documents the role of both So and dhSo as bioactive molecules, as well as the involvement of dhCer and Cer in physiological and pathological processes. In particular, the fatty acid composition of Cer has different effects in cell biology and in the onset and progression of different diseases. Therefore, modulation of CerS activity represents an attractive target in biomedical research and in finding new treatment modalities. In this review, we discuss functional, structural and biochemical features of CerS and examine CerS inhibitors that are currently available.

Activation of S1P2 receptor, a possible mechanism of inhibition of adipogenic differentiation by sphingosine 1‑phosphate

Sphingosine 1‑phosphate (S1P) belongs to a significant group of signaling sphingolipids and exerts most of its activity as a ligand of G‑protein‑coupled receptors. In our previous study, S1P demonstrated a novel biological activity with the anti‑adipogenesis of 3T3‑L1 preadipocytes. In the present study, we identified a possible mechanism of S1P‑mediated anti‑adipogenic effects, particularly in target pathways of the S1P receptors, including S1P1 and S1P2. The mRNA levels of S1P1 and S1P2 receptors were increased by MDI media treatment, whereas S1P treatment highly induced S1P2 but not S1P1 receptor protein in adipocytes. Triglyceride accumulation assay using an agonist and antagonist of S1P receptors revealed that S1P2 receptor was only involved in S1P‑mediated anti‑adipogenic effects. Furthermore, pharmacological inhibition of S1P2 signals completely retrieved S1P‑mediated downregulation of the transcriptional levels of peroxisome proliferator‑activated receptor γ, CCAAT/enhancer binding protein α and adiponectin, which are markers of adipogenic differentiation. This study demonstrated that S1P2 receptor signals may regulate the S1P‑mediated anti‑adipogenic differentiation and also identifies the S1P2 receptor as a possible mechanism of anti‑adipogenic differentiation.

Ceramide function in the brain: when a slight tilt is enough

Ceramide, the precursor of all complex sphingolipids, is a potent signaling molecule that mediates key events of cellular pathophysiology. In the nervous system, the sphingolipid metabolism has an important impact. Neurons are polarized cells and their normal functions, such as neuronal connectivity and synaptic transmission, rely on selective trafficking of molecules across plasma membrane. Sphingolipids are abundant on neural cellular membranes and represent potent regulators of brain homeostasis. Ceramide intracellular levels are fine-tuned and alteration of the sphingolipid-ceramide profile contributes to the development of age-related, neurological and neuroinflammatory diseases. The purpose of this review is to guide the reader towards a better understanding of the sphingolipid-ceramide pathway system. First, ceramide biology is presented including structure, physical properties and metabolism. Second, we describe the function of ceramide as a lipid second messenger in cell physiology. Finally, we highlight the relevance of sphingolipids and ceramide in the progression of different neurodegenerative diseases.

Ceramide synthases at the centre of sphingolipid metabolism and biology

Sphingolipid metabolism in metazoan cells consists of a complex interconnected web of numerous enzymes, metabolites and modes of regulation. At the centre of sphingolipid metabolism reside CerSs (ceramide synthases), a group of enzymes that catalyse the formation of ceramides from sphingoid base and acyl-CoA substrates. From a metabolic perspective, these enzymes occupy a unique niche in that they simultaneously regulate de novo sphingolipid synthesis and the recycling of free sphingosine produced from the degradation of pre-formed sphingolipids (salvage pathway). Six mammalian CerSs (CerS1-CerS6) have been identified. Unique characteristics have been described for each of these enzymes, but perhaps the most notable is the ability of individual CerS isoforms to produce ceramides with characteristic acyl-chain distributions. Through this control of acyl-chain length and perhaps in a compartment-specific manner, CerSs appear to regulate multiple aspects of sphingolipid-mediated cell and organismal biology. In the present review, we discuss the function of CerSs as critical regulators of sphingolipid metabolism, highlight their unique characteristics and explore the emerging roles of CerSs in regulating programmed cell death, cancer and many other aspects of biology.

Glycosphingolipid requirements for endosome-to-Golgi transport of Shiga toxin

Shiga toxin binds to globotriaosylceramide (Gb3) receptors on the target cell surface. To enter the cytosol, Shiga toxin is dependent on endocytic uptake, retrograde transport to the Golgi apparatus and further to the endoplasmic reticulum before translocation of the enzymatically active moiety to the cytosol. Here, we have investigated the importance of newly synthesized glycosphingolipids for the uptake and intracellular transport of Shiga toxin in HEp-2 cells. Inhibition of glycosphingolipid synthesis by treatment with either PDMP or Fumonisin B(1) for 24-48 h strongly reduced the transport of Gb3-bound Shiga toxin from endosomes to the Golgi apparatus. This was associated with a change in localization of sorting nexins 1 and 2, and accompanied by a protection against the toxin. In contrast, there was no effect on transport or toxicity of the plant toxin ricin. High-resolution mass spectrometry revealed a 2-fold reduction in Gb3 at conditions giving a 10-fold inhibition of Shiga toxin transport to the Golgi. Furthermore, mass spectrometry showed that the treatment with PDMP (DL-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol) and Fumonisin B(1) among other changes of the lipidome, affected the relative content of the different glycosphingolipid species. The largest depletion was observed for the hexosylceramide species with the N-amidated fatty acid 16:0, whereas hexosylceramide species with 24:1 were less affected. Quantitative lipid profiling with mass spectrometry demonstrated that PDMP did not influence the content of sphingomyelins, phospholipids and plasmalogens. In contrast, Fumonisin B(1) affected the amount and composition of sphingomyelin and glycolipids and altered the profiles of phospholipids and plasmalogens.

Involvement of acid beta-glucosidase 1 in the salvage pathway of ceramide formation

Activation of protein kinase C (PKC) promotes the salvage pathway of ceramide formation, and acid sphingomyelinase has been implicated, in part, in providing substrate for this pathway (Zeidan, Y. H., and Hannun, Y. A. (2007) J. Biol. Chem. 282, 11549-11561). In the present study, we examined whether acid beta-glucosidase 1 (GBA1), which hydrolyzes glucosylceramide to form lysosomal ceramide, was involved in PKC-regulated formation of ceramide from recycled sphingosine. Glucosylceramide levels declined after treatment of MCF-7 cells with a potent PKC activator, phorbol 12-myristate 13-acetate (PMA). Silencing GBA1 by small interfering RNAs significantly attenuated acid glucocerebrosidase activity and decreased PMA-induced formation of ceramide by 50%. Silencing GBA1 blocked PMA-induced degradation of glucosylceramide and generation of sphingosine, the source for ceramide biosynthesis. Reciprocally, forced expression of GBA1 increased ceramide levels. These observations indicate that GBA1 activation can generate the source (sphingosine) for PMA-induced formation of ceramide through the salvage pathway. Next, the role of PKCdelta, a direct effector of PMA, in the formation of ceramide was determined. By attenuating expression of PKCdelta, cells failed to trigger PMA-induced alterations in levels of ceramide, sphingomyelin, and glucosylceramide. Thus, PKCdelta activation is suggested to stimulate the degradation of both sphingomyelin and glucosylceramide leading to the salvage pathway of ceramide formation. Collectively, GBA1 is identified as a novel source of regulated formation of ceramide, and PKCdelta is an upstream regulator of this pathway.

Modulation of sphingolipid metabolism by the phosphatidylinositol-4-phosphate phosphatase Sac1p through regulation of phosphatidylinositol in Saccharomyces cerevisiae

Sphingolipids and phosphoinositides both play signaling roles in Saccharomyces cerevisiae. Although previous data indicate independent functions for these two classes of lipids, recent genetic studies have suggested interactions between phosphatidylinositol (PtdIns) phosphate effectors and sphingolipid biosynthetic enzymes. The present study was undertaken to further define the effects of phosphatidylinositol 4-phosphate (PtdIns(4)P) metabolism on cell sphingolipid metabolism. The data presented indicate that deletion of SAC1, a gene encoding a PtdIns(4)P phosphatase, increased levels of most sphingolipid species, including sphingoid bases, sphingoid base phosphates, and phytoceramide. In contrast, sac1Delta dramatically reduced inositol phosphosphingolipids, which result from the addition of a PtdIns-derived phosphoinositol head group to ceramides through Aur1p. Deletion of SAC1 decreased PtdIns dramatically in both steady-state and pulse labeling studies, suggesting that the observed effects on sphingolipids may result from modulation of the availability of PtdIns as a substrate for Aur1p. Supporting this hypothesis, acute attenuation of PtdIns(4)P production through Stt4p immediately increased PtdIns and subsequently reduced sphingoid bases. This reduction was overcome by the inhibition of Aur1p. Moreover, modulation of sphingoid bases through perturbation of PtdIns(4)P metabolism initiated sphingolipid-dependent biological effects, supporting the biological relevance for this route of regulating sphingolipids. These findings suggest that, in addition to potential signaling effects of PtdInsP effectors on sphingolipid metabolism, PtdIns kinases may exert substantial effects on cell sphingolipid profiles at a metabolic level through modulation of PtdIns available as a substrate for complex sphingolipid synthesis.

Effects of Mycoplasma pneumoniae infection on sphingolipid metabolism in human lung carcinoma A549 cells

The role of sphingolipids in bacterial pathogenesis has been gradually recognized. In an effort to identify the possible involvement of sphingolipids during Mycoplasma pneumoniae (M. pneumoniae) infection, we first adopted a lipidomic approach to achieve the profiles of major sphingolipid species of M. pneumoniae as well as human lung carcinoma A549 cells, and further evaluated the effects of M. pneumoniae infection on sphingolipid metabolism in A549 cells. It was shown that M. pneumoniae and A549 cells share many common sphingolipid species, however, M. pneumoniae possesses certain specific molecular species that are not found in A549 cells. On the other hand, M. pneumoniae infection could alter sphingolipid metabolism in A549 cell, including the generation of new ceramide and sphingomyelin species, or the increase/decrease of intensities, which varies depending on the different infection doses and times. The effects of M. pneumoniae infection on two key enzymes in sphingolipid metabolism, serine palmitoyltransferase (SPT) and acid sphingomyelinase (ASM), were also examined. It was found that M. pneumoniae infection could affect the expression of SPT or the distribution of ASM at certain concentrations. These data suggest that M. pneumoniae infection could influence sphingolipid metabolism of its host, which might be related to its pathogenicity.

The sphingolipid salvage pathway in ceramide metabolism and signaling

Sphingolipids are important components of eukaryotic cells, many of which function as bioactive signaling molecules. Of these, ceramide is a central metabolite and plays key roles in a variety of cellular responses, including regulation of cell growth, viability, differentiation, and senescence. Ceramide is composed of the long-chain sphingoid base, sphingosine, in N-linkage to a variety of acyl groups. Sphingosine serves as the product of sphingolipid catabolism, and it is mostly salvaged through reacylation, resulting in the generation of ceramide or its derivatives. This recycling of sphingosine is termed the "salvage pathway", and recent evidence points to important roles for this pathway in ceramide metabolism and function. A number of enzymes are involved in the salvage pathway, and these include sphingomyelinases, cerebrosidases, ceramidases, and ceramide synthases. Recent studies suggest that the salvage pathway is not only subject to regulation, but it also modulates the formation of ceramide and subsequent ceramide-dependent cellular signals. This review focuses on the salvage pathway in ceramide metabolism, its regulation, its experimental analysis, and emerging biological functions.

Effect of exercise duration on ceramide metabolism in the rat heart

AIM

We aimed at gaining more insight into the mechanisms underlying exercise-induced alterations in myocardial ceramide (CER) content by employing physical activity of various durations and examining all key pathways of CER metabolism.

METHODS

The experiments were carried out on male Wistar rats divided into four groups (n = 6 in each case): control, exercised for 30 and 90 min and until exhaustion on the electrically driven treadmill moving with a speed of 1200 m h(-1) and set at +10 degrees incline. The animals were anaesthetized and samples of the heart's left ventricle were excised.

RESULTS

Thirty-minute exercise decreased the level of CER in the heart by 15%. However, after 90 min of running it returned to the baseline and at the point of exhaustion it exceeded that of the control animals by 26%. The initial reduction in the content of CER was probably a result of its augmented degradation, as a concomitant elevation in the activity of acid ceramidase and the level of sphingosine was observed. The transition from reduction in CER content after 30 min of exercise to its accumulation at the point of exhaustion was a consequence of gradual reduction in the activity of acid ceramidase and simultaneous increase in the rate of de novo CER synthesis, as evidenced by progressive activation of serine palmitoyltransferase and accumulation of sphinganine.

CONCLUSION

We conclude that the effect of physical effort on myocardial CER content and metabolism depends to a large extent on exercise duration.

Thematic review series: sphingolipids. New insights into sphingolipid metabolism and function in budding yeast

Our understanding of sphingolipid metabolism and functions in the baker's yeast Saccharomyces cerevisiae has progressed substantially in the past 2 years. Yeast sphingolipids contain a C26-acyl moiety, all of the genes necessary to make these long-chain fatty acids have been identified, and a mechanism for how chain length is determined has been proposed. Advances in understanding how the de novo synthesis of ceramide and complex sphingolipids is regulated have been made, and they demonstrate that the Target Of Rapamycin Complex 2 (TORC2) controls ceramide synthase activity. Other work shows that TORC2 regulates the level of complex sphingolipids in a pathway using the Slm1 and Slm2 proteins to control the protein phosphatase calcineurin, which regulates the breakdown of complex sphingolipids. The activity of Slm1 and Slm2 has also been shown to be regulated during heat stress by phosphoinositides and TORC2, along with sphingoid long-chain bases and the Pkh1 and Pkh2 protein kinases, to control the actin cytoskeleton, the trafficking of nutrient transporters, and cell viability. Together, these results provide the first molecular insights into understanding previous genetic interaction data that indicated a connection between sphingolipids and the TORC2 and phosphoinositide signaling networks. This new knowledge provides a foundation for greatly advancing our understanding of sphingolipid biology in yeast.

Interactions between neural membrane glycerophospholipid and sphingolipid mediators: A recipe for neural cell survival or suicide

The neural membranes contain phospholipids, sphingolipids, cholesterol, and proteins. Glycerophospholipids and sphingolipids are precursors for lipid mediators involved in signal transduction processes. Degradation of glycerophospholipids by phospholipase A(2) (PLA(2)) generates arachidonic acid (AA) and docosahexaenoic acids (DHA). Arachidonic acid is metabolized to eicosanoids and DHA is metabolized to docosanoids. The catabolism of glycosphingolipids generates ceramide, ceramide 1-phosphate, sphingosine, and sphingosine 1-phosphate. These metabolites modulate PLA(2) activity. Arachidonic acid, a product derived from glycerophospholipid catabolism by PLA(2), modulates sphingomyelinase (SMase), the enzyme that generates ceramide and phosphocholine. Furthermore, sphingosine 1-phosphate modulates cyclooxygenase, an enzyme responsible for eicosanoid production in brain. This suggests that an interplay and cross talk occurs between lipid mediators of glycerophospholipid and glycosphingolipid metabolism in brain tissue. This interplay between metabolites of glycerophospholipid and sphingolipid metabolism may play an important role in initiation and maintenance of oxidative stress associated with neurologic disorders as well as in neural cell proliferation, differentiation, and apoptosis. Recent studies indicate that PLA(2) and SMase inhibitors can be used as neuroprotective and anti-apoptotic agents. Development of novel inhibitors of PLA(2) and SMase may be useful for the treatment of oxidative stress, and apoptosis associated with neurologic disorders such as stroke, Alzheimer disease, Parkinson disease, and head and spinal cord injuries.

Selective Substrate Supply in the Regulation of Yeast de Novo Sphingolipid Synthesis

The heat stress response of Saccharomyces cerevisiae is characterized by transient cell cycle arrest, altered gene expression, degradation of nutrient permeases, trehalose accumulation, and translation initiation of heat shock proteins. Importantly heat stress also induces de novo sphingolipid synthesis upon which many of these subprograms of the heat stress response depend. Despite extensive data addressing the roles for sphingolipids in heat stress, the mechanism(s) by which heat induces sphingolipid synthesis remains unknown. This study was undertaken to determine the events and/or factors required for heat stress-induced sphingolipid synthesis. Data presented indicate that heat does not directly alter the in vitro activity of serine palmitoyltransferase (SPT), the enzyme responsible for initiating de novo sphingolipid synthesis. Moreover deletion of the small peptide Tsc3p, which is thought to maximize SPT activity, specifically reduced production of C(20) sphingolipid species by over 70% but did not significantly decrease overall sphingoid base production. In contrast, the fatty-acid synthase inhibitor cerulenin nearly completely blocked sphingoid base production after heat, indicating a requirement for endogenous fatty acids for heat-mediated sphingoid base synthesis. Consistent with this, genetic studies show that fatty acid import does not contribute to heat-induced de novo synthesis under normal conditions. Interestingly the absence of medium serine also ameliorated heat-induced sphingoid base production, indicating a requirement for exogenous serine for the response, and consistent with this finding, disruption of synthesis of endogenous serine did not affect heat-induced sphingolipid synthesis. Serine uptake assays indicated that heat increased serine uptake from medium by 100% during the first 10 min of heat stress. Moreover treatments that increase serine uptake in the absence of heat including acute medium acidification and glucose treatment also enhanced de novo sphingoid base synthesis equivalent to that induced by heat stress. These data agree with findings from mammalian systems that availability of substrates is a key determinant of flux through sphingolipid synthesis. Moreover data presented here indicate that SPT activity can be driven by several factors that increase serine uptake in the absence of heat. These findings may provide insights into the many systems in which de novo synthesis is increased in the absence of elevated in vitro SPT activity.

Phospholipid scramblase-3 regulates cardiolipin de novo biosynthesis and its resynthesis in growing HeLa cells

PLS3 (phospholipid scramblase-3) is a new member of the family of phospholipid scramblases and transports CL (cardiolipin) from the inner to the outer mitochondrial membrane. In the present paper we examined whether changing the levels of functional PLS3 in HeLa cells altered de novo CL biosynthesis and its resynthesis. HeLa cells overexpressing PLS3 or expressing a disrupted PLS3 (F258V) or control were incubated with [1,3-3H]glycerol and radioactivity incorporated into CL was determined. CL biosynthesis from [1,3-3H]glycerol was increased 1.8-fold in PLS3 cells and 2.1-fold in F258V cells compared with control. This was due to a 64% (P<0.05) and 2.6-fold (P<0.05) elevation in CL synthase activity in PLS3 and F258V cells respectively, compared with control, and not due to changes in phosphatidylglycerolphosphate synthase activity. The increase in CL synthase activity in these cells was due to an increase in its mRNA expression. In contrast, resynthesis of CL from [1-14C]linoleic acid was reduced 52% (P<0.05) in PLS3 and 45% (P<0.05) in F258V cells compared with control and this was due to a reduction in mitochondrial monolysocardiolipin acyltransferase activity. Although protein levels of mitochondrial monolysocardiolipin acyltransferase were unaltered, activity and mRNA expression of endoplasmic reticulum monolysocardiolipin acyltransferase was upregulated in PLS3 and F258V cells compared with controls. These data indicate that the CL resynthesis in HeLa cells is sensitive to the mitochondrial localization of CL and not the level of the reacylating enzymes. Alterations in functional PLS3 levels in PLS3 or F258V cells did not affect the mitochondrial decarboxylation of phosphatidylserine to phosphatidylethanolamine indicating that the biosynthetic changes to CL were specific for this mitochondrial phospholipid. We hypothesize that the cardiolipin resynthesis machinery in the cell 'senses' altered levels of CL on mitochondrial membranes and that de novo CL biosynthesis is up-regulated in HeLa cells as a compensatory mechanism in response to altered movement of mitochondrial CL. The results identify PLS3 as a novel regulator of CL de novo biosynthesis and its resynthesis.

Ceramides and other bioactive sphingolipid backbones in health and disease: lipidomic analysis, metabolism and roles in membrane structure, dynamics, signaling and autophagy

Sphingolipids are comprised of a backbone sphingoid base that may be phosphorylated, acylated, glycosylated, bridged to various headgroups through phosphodiester linkages, or otherwise modified. Organisms usually contain large numbers of sphingolipid subspecies and knowledge about the types and amounts is imperative because they influence membrane structure, interactions with the extracellular matrix and neighboring cells, vesicular traffic and the formation of specialized structures such as phagosomes and autophagosomes, as well as participate in intracellular and extracellular signaling. Fortunately, "sphingolipidomic" analysis is becoming feasible (at least for important subsets such as all of the backbone "signaling" subspecies: ceramides, ceramide 1-phosphates, sphingoid bases, sphingoid base 1-phosphates, inter alia) using mass spectrometry, and these profiles are revealing many surprises, such as that under certain conditions cells contain significant amounts of "unusual" species: N-mono-, di-, and tri-methyl-sphingoid bases (including N,N-dimethylsphingosine); 3-ketodihydroceramides; N-acetyl-sphingoid bases (C2-ceramides); and dihydroceramides, in the latter case, in very high proportions when cells are treated with the anticancer drug fenretinide (4-hydroxyphenylretinamide). The elevation of DHceramides by fenretinide is befuddling because the 4,5-trans-double bond of ceramide has been thought to be required for biological activity; however, DHceramides induce autophagy and may be important in the regulation of this important cellular process. The complexity of the sphingolipidome is hard to imagine, but one hopes that, when partnered with other systems biology approaches, the causes and consequences of the complexity will explain how these intriguing compounds are involved in almost every aspect of cell behavior and the malfunctions of many diseases.

Amelioration of fumonisin B1 hepatotoxicity in mice by depletion of T cells with anti-Thy-1.2

Fumonisin B1 is a mycotoxin produced by Fusarium verticillioides, frequently associated with corn. It produces species-specific and organ-specific toxicity, including equine leukoencephalomalacia, porcine pulmonary edema, and hepatic or renal damage in most animal species. Fumonisin B1 perturbs sphingolipid metabolism by inhibiting ceramide synthase. Our previous studies in male mice indicated that fumonisin B1-induced hepatotoxicity is modulated by the localized activation of cytokines in liver macrophages and other cell types. In the current study, male athymic nude mice and their wild type counterparts (WT), the latter with or without depletion of T cells, were treated subcutaneously with fumonisin B1 at 2.25 mg/kg/day for 5 days and sampled 24 h after the last injection. Depletion of T cells in WT was achieved by a single intravenous injection of 50 microg monoclonal antibody against Thy-1.2 surface antigen of mature peripheral T lymphocytes 24 h before the first fumonisin B1 treatment. The depletion of T cells nearly abolished fumonisin B1-mediated liver toxicity as indicated by the near normal concentrations of circulating liver enzymes and by enumeration of apoptotic hepatocytes. There was no difference in the fumonisin B1-induced elevation in circulating liver enzymes between WT and nude mice. Fumonisin B1-induced mRNA expression of tumor necrosis factor alpha and interleukin-1alpha was observed in nude and WT mice but not in T cell-depleted mice. Hepatotoxic response to fumonisin B1 was unaltered in mice lacking natural killer cells. This study suggested that T cells and corresponding proinflammatory cytokines have a vital role in mediating fumonisin B1-induced hepatic toxicity.

Effects of fumonisins on liver and kidney sphinganine and the sphinganine to sphingosine ratio during chronic exposure in ducks

Sa and the Sa/So ratio are very sensitive biomarkers of exposure to fumonisins in several species. We previously demonstrated that increases in Sa and in the Sa/So ratio in serum were less pronounced when ducks ingested fumonisins for more than 7 weeks than when animals were exposed for only 1-2 weeks [S.T. Tran, D. Tardieu, A. Auvergne, J.D. Bailly, R. Babilé, S. Durand, G. Benard, P. Guerre, Serum sphinganine and the sphinganine to sphingosine ratio as biomarker of dietary fumonisins during chronic exposure in ducks, Chem. Biol. Interact., in press]. The aim of this study was to investigate the kinetics of Sa and of the Sa/So in both liver and kidney of ducks that have been previously tested for Sa and the Sa/So ratio in serum. Analysis were performed on treatment days 0, 7, 14, 28 and 77 in five groups of ducks fed fumonisins obtained from an extract of Fusarium verticillioides culture material by daily gavage to obtain an exposure equal to 0, 2, 8, 32 and 128 mg FB1/kg feed. Sa and the Sa/So ratio in tissues were then correlated with Sa and the Sa/So ratio previously obtained in serum. The amounts on sphinganine 1-phosphate (Sa1P) and sphingosine1-phosphate (So1P) in the liver were also investigated. On day 7 of treatment, 2mg/kg FB1 in the feed were sufficient to increase Sa and the Sa/So ratio in liver (by 165 and 148%, respectively) and kidney (by 193 and 104%, respectively). At a rate of 128 mg/kg FB1 in the feed, a very high increase in Sa concentration was observed in both liver and kidney without mortality and/or signs of necrosis (respective increase of 2034 and 3768%). Although the precise mechanism of the resistance of ducks to fumonisin-induced hepatotoxicity is still uncertain, it might be linked to the rate at which the sphingoid bases sphinganine and sphingosine are converted to their 1-phosphate or other metabolite and eliminated from target tissues.

Myriocin prevents fumonisin B1-induced sphingoid base accumulation in mice liver without ameliorating hepatotoxicity

Fumonisin B(1) (FB(1)), a mycotoxin produced by Fusarium verticillioides present on corn and corn-based products, causes species- and organ-specific diseases. The hepatotoxic effects of FB(1) in mice have been closely correlated with the accumulation of free sphinganine, a marker for ceramide synthase inhibition, and reduced biosynthesis of more complex sphingolipids. It has been shown that FB(1) modulates expression of many cell signaling factors. In the current study we used myriocin, a specific inhibitor of serine palmitoyltransferase, to investigate the role of free sphinganine accumulation in FB(1)-induced hepatotoxicity and increased expression of selected signaling genes in BALB/c mice. The mice were pretreated daily with intraperitoneal injection of 1.0 mg/kg myriocin 30 min before subcutaneous injections of 2.25 mg/kg of FB(1) for 3 days. Results showed that myriocin alone was not hepatotoxic and the combination of myriocin plus FB(1) completely prevented the FB(1)-induced elevation of hepatic free sphinganine and prevented the FB(1)-induced induction of selected cell signaling genes, suggesting that accumulation of free sphinganine and/or its metabolites contribute to the FB(1)-modulation of the cell signaling factors. However, the combination of myriocin and FB(1) did not prevent FB(1)-increased concentration of plasma alanine aminotransferase and only slightly attenuated aspartate aminotransferase; it did not affect the FB(1)-induced hepatocyte apoptosis or increased cell proliferation. A longer combined treatment of myriocin and FB(1) was highly toxic. The hepatotoxic effects in mice seen in this study are most likely due to a combination of factors including accumulation of free sphinganine, depletion of more complex sphingolipids and sphingomyelin, or other unknown mechanisms.

2n-fatty acids from phosphatidylcholine label sphingolipids—A novel role of phospholipase A2?

In order to find out whether there is a phospholipase A2 (PLA2)-mediated link between glycerophospholipids and sphingolipids, L929 cells were labeled with 1n-palmitoyl-2n-[1-14C]palmitoyl phosphatidylcholine for 16-18 h or 90 min. After labeling for 16-18 h, 14C-sphingomyelin (SM), 14C-ceramide and 14C-sphingosine were demonstrated on autoradiograms of thin layer chromatograms of untreated or mildly hydrolyzed lipid extracts in different chromatographic systems. Strong hydrolysis of labeled SM proved that both possible moieties of SM, sphingosine and acyl moiety, had been labeled. The identity of SM and its enzymatic degradation product, ceramide, was verified by mass spectrometry. The label in SM-derived ceramide was demonstrated on an autoradiogram after thin layer chromatography. The inhibitor of (dihydro)ceramide synthase fumonisin B1 suppressed the label in sphingolipids significantly during 16-18 h (ceramide and SM), as well as during 90-min labeling (SM). The presence of inhibitors of PLA2 (bromoenol lactone, aristolochic acid and quinacrine dihydrochloride) diminished the label in SM significantly during the 90-min labeling. These results demonstrate a close metabolic relationship between glycerophospholipids and sphingolipids and give evidence for a novel role of PLA2.

Inhibition of sphingolipid biosynthesis decreases phosphorylated ERK2 in LLC-PK1 cells

Fumonisin B(1) (FB(1)) is a fungal toxin produced by Fusarium verticillioides that inhibits ceramide synthase (CS), a key enzyme in the de novo sphingolipid biosynthesis pathway. In LLC-PK(1) cells, FB(1) inhibits cell proliferation and induces apoptosis, which can be prevented by inhibitors of serine palmitoyltransferase (SPT). Inhibition of SPT prevents the FB(1)-induced accumulation of free sphinganine, a precursor of ceramide biosynthesis. However, not all of the effects of FB(1) in LLC-PK(1) cells can be explained solely by the increase in free sphingoid bases. The downstream signaling pathways that are affected by FB(1)-induced disruption of sphingolipid biosynthesis are not well understood. This study determined, in LLC-PK(1) cells, changes in p42 MAP kinase (phosphorylated ERK2 [pERK2]) phosphorylation in response to various inhibitors of key enzymes of the de novo sphingolipid biosynthesis pathway (CS, SPT, and glucosylceramide synthase [GlcCer synthase]). The results show that inhibition of any of the three enzymes caused a similar decrease in the extent of phosphorylation of ERK2 with no reduction in total ERK2. The co-treatment of FB(1) (CS inhibitor) with SPT inhibitors or the GlcCer synthase inhibitor had no effect on the FB(1)-induced reduction in pERK2 phosphorylation, indicating that FB(1)-mediated changes in phosphorylation of pERK2 was independent of increases in free sphinganine or its metabolites or a reduction in ceramide. Nonetheless, the decrease in pERK2 phosphorylation was dependent on inhibition of de novo sphingolipid biosynthesis. Decreased pERK2 activity could contribute to the physiological effects of FB(1) in LLC-PK(1) cells that are not due to alteration in pathways modulated by free sphingoid bases and their metabolites but are sensitive to inhibition of glycosphingolipid biosynthesis.

Sphingosine kinase activity confers resistance to apoptosis by fumonisin B1 in human embryonic kidney (HEK-293) cells

Fumonisin B1 induces cytotoxicity in sensitive cells by inhibiting ceramide synthase due to its structural similarity to the long-chain backbones of sphingolipids. The resulting accumulation of sphingoid bases has been established as a mechanism for fumonisin B1 cytotoxicity. We found that despite the accumulation of sphinganine, human embryonic kidney (HEK-293) cells are resistant to fumonisin B1 toxicity; 25 microM fumonisin B1 exposure for 48 h did not increase apoptosis in these cells, while it did so in sensitive porcine kidney epithelial (LLC-PK1) cells. In this study, DL-threo-dihydrosphingosine, the sphingosine kinase inhibitor (SKI), considerably increased the sensitivity of HEK-293 cells to fumonisin B1. Treatment of these cells with 25 microM fumonisin B1 and 2.5 microM SKI increased apoptosis. Sphingoid bases, sphinganine or sphingosine, added to cell cultures induced apoptosis by themselves and their effects were potentiated by SKI or fumonisin B1. Addition of physiological amounts of sphingosine-1-phosphate prevented the toxic effects induced by SKI inhibition and fumonisin B1. Results indicated that HEK-293 cells are resistant to fumonisin B1 due to rapid formation of sphingosine-1-phosphate that imparts survival properties. Taken together, these findings suggest that sphingoid base metabolism by sphingosine kinase may be a critical event in rendering the HEK-293 cells relatively resistant to fumonisin B1-induced apoptosis.

Angiotensin II-stimulated cortisol secretion is mediated by phospholipase D

Angiotensin II (Ang-II) regulates a variety of cellular functions including cortisol secretion. In the present report, we demonstrate that Ang-II activates phospholipase D (PLD) in zona fasciculata (ZF) cells of bovine adrenal glands, and that this effect is associated to the stimulation of cortisol secretion by this hormone. PLD activation was dependent upon extracellular Ca2+, and was blocked by inhibition of protein kinase C (PKC). Using the reverse transcription-polymerase chain reaction technique, we demonstrated that ZF cells express both PLD-1 and PLD-2 isozymes. Primary alcohols, which attenuate the formation of phosphatidate (the product of PLD), and cell-permeable ceramides, which inhibit PLD potently, blocked Ang-II-stimulated cortisol secretion. Furthermore, propranolol or chlorpromazine, which are potent inhibitors of phosphatidate phosphohydrolase (PAP) (the enzyme that produces diacylglycerol from phosphatidate), also blocked cortisol secretion. These data suggest that the PLD/PAP pathway plays an important role in the regulation of cortisol secretion by Ang-II in ZF cells.

De novo ceramide accumulation due to inhibition of its conversion to complex sphingolipids in apoptotic photosensitized cells

The oxidative stress induced by photodynamic therapy (PDT) with the photosensitizer phthalocyanine 4 is accompanied by increases in ceramide mass. To assess the regulation of de novo sphingolipid metabolism during PDT-induced apoptosis, Jurkat human T lymphoma and Chinese hamster ovary cells were labeled with [14C]serine, a substrate of serine palmitoyltransferase (SPT), the enzyme catalyzing the initial step in the sphingolipid biosynthesis. A substantial elevation in [14C]ceramide with a concomitant decrease in [14C]sphingomyelin was detected. The labeling of [14C]ceramide was completely abrogated by the SPT inhibitor ISP-1. In addition, ISP-1 partly suppressed PDT-induced apoptosis. Pulse-chase experiments showed that the contribution of sphingomyelin degradation to PDT-initiated increase in de novo ceramide was absent or minor. PDT had no effect on either mRNA amounts of the SPT subunits LCB1 and LCB2, LCB1 protein expression, or SPT activity in Jurkat cells. Moreover in Chinese hamster ovary cells LCB1 protein underwent substantial photodestruction, and SPT activity was profoundly inhibited after treatment. We next examined whether PDT affects conversion of ceramide to complex sphingolipids. Sphingomyelin synthase, as well as glucosylceramide synthase, was inactivated by PDT in both cell lines in a dose-dependent manner. These results are the first to show that in the absence of SPT up-regulation PDT induces accumulation of de novo ceramide by inhibiting its conversion to complex sphingolipids.

Quantification of sphingosine and sphinganine from crude lipid extracts by HPLC electrospray ionization tandem mass spectrometry

Sphingosine (SPH) comprises the backbone of sphingolipids and is known as a second messenger involved in the modulation of cell growth, differentiation, and apoptosis. The currently available methods for the quantification of SPH are, in part, complicated, time-consuming, insensitive, or unselective. Therefore, a fast and convenient methodology for the quantification of SPH and the biosynthetic intermediate sphinganine (SPA) was developed. The method is based on an HPLC separation coupled to electrospray ionization tandem mass spectrometry (MS/MS). Quantitation is achieved by the use of a constant concentration of a non-naturally occurring internal standard, 17-carbon chain SPH (C17-SPH), together with a calibration curve established by spiking different concentrations of naturally occurring sphingoid bases. SPH and SPA coeluted with C17-SPH, which allows an accurate correction of the analyte response. Interference of the SPH+2 isotope with SPA quantification was corrected by an experimentally determined factor. The limits of detection were 9 fmol for SPH and 21 fmol for SPA. The overall coefficients of variation were 8% and 13% for SPH and SPA, respectively. The developed HPLC-tandem mass spectrometry methodology, with an analysis time of 3.5 min, simple sample preparation, and automated data analysis, allows high-throughput quantification of sphingoid bases from crude lipid extracts and is a valuable tool for studies of cellular sphingolipid metabolism and signaling.

Diacylglycerol generated during sphingomyelin synthesis is involved in protein kinase C activation and cell proliferation in Madin-Darby canine kidney cells

We have investigated the effects of inhibiting sphingomyelin (SM) biosynthesis on cellular diacylglycerol (DAG) content and protein kinase C (PKC) activation during growth initiation in Madin-Darby canine kidney cells. We utilized beta-chloroalanine (BCA) to inactivate serine C -palmitoyltransferase, the first enzyme in the sphingolipid biosynthesis pathway. This inactivation prevented growth, but did not affect viability. When the inhibitor was replaced with fresh culture medium, the cells continued their proliferation in a normal way. BCA (2 mM) inhibited [(32)P]P(i), [(3)H]palmitic acid and [ methyl -(3)H]choline incorporation into SM, but did not influence the synthesis of other major phospholipids. SM synthesis and DAG generation were decreased by 51% and 47.6% respectively. Particulate PKC activity was not observed in cells incubated with BCA, in contrast with a 5-fold increase in control cells. BCA inhibited 75% of the [(3)H]thymidine incorporation, and the cells were arrested before the S phase of the cell cycle. Moreover, exogenous D-erythrosphingosine restored SM synthesis, DAG generation and cell proliferation. These data indicate that the contribution of DAG generated during SM synthesis plays an important role in PKC activation and cell proliferation.

Increased susceptibility of renal epithelial cells to TNFalpha-induced apoptosis following treatment with fumonisin B1

Previous studies have shown that tumor necrosis factor alpha (TNFalpha) is involved in the pathogenic events following exposure to fumonisin B(1) (FB(1)), a potent inhibitor of ceramide synthase and sphingolipid biosynthesis. The intimate role of sphingolipid mediators in TNFalpha signaling and cellular death suggests that FB(1) may alter the sensitivity of cells to TNFalpha-induced apoptosis. We tested the hypothesis that FB(1) treatment will increase the sensitivity of porcine renal epithelial cells to TNFalpha. Porcine renal epithelial cells (LLC-PK(1)) were treated with FB(1) for 48 h prior to treatment with TNFalpha. A dose-dependent increase in TNFalpha-induced apoptosis was observed in cells pretreated with FB(1). Cells treated with FB(1) showed increased DNA fragmentation and terminal uridine nucleotide end labeling in response to TNFalpha treatment. FB(1) increased DNA synthesis and resulted in cell cycle arrest in the G(2)/M phase of the cell cycle. Flow cytometric analysis of the cell cycle indicated that TNFalpha predominantly killed cells in the G(2)/M phase. The activation of JNK, a mitogen-activated protein kinase (MAPK), was increased following 48 h exposure to FB(1). Phosphorylation of p38 and ERK remained unchanged following treatment with FB(1). FB(1) also increased free sphingoid base levels under identical treatment conditions. Results suggest that FB(1) increased free sphingoid base levels and the population of cells in the G(2)/M phase. This population was shown to be most susceptible to TNFalpha-induced apoptosis. Phosphorylation of pro-apoptotic JNK may play an important role in these effects.

Sphingosine-1-phosphate stimulates cortisol secretion

We show here for the first time that sphingosine-1-phosphate (Sph-1-P) stimulates cortisol secretion in zona fasciculata cells of bovine adrenal glands. This effect was dependent upon protein kinase C (PKC) and extracellular Ca2+, and was inhibited by pertussis toxin. Sph-1-P activated phospholipase D (PLD) through a pertussis toxin-sensitive mechanism, also involving extracellular Ca2+ and PKC. Primary alcohols, which attenuate formation of phosphatidic acid (the product of PLD), and cell-permeable ceramides, which inhibit PLD, blocked Sph-1-P-induced cortisol secretion. In conclusion, Sph-1-P stimulates cortisol secretion through a mechanism involving Gi/o protein-coupled receptors, extracellular Ca2+, PKC and PLD.

Simultaneous quantitative determination method for ceramide species from crude cellular extracts by high-performance liquid chromatography-thermospray mass spectrometry

I have developed a simple method which enabled simultaneous analysis of ceramides in the subcellular fractions from cultured cells by HPLC-thermospray mass spectrometry. The HPLC-thermospray mass spectra from ceramide standards were characterized by the high intensity of the MNa(+) and MH(+)-H(2)O ions. As the other minor ions, MK(+), MH(+) and m/z 282 ions were detected. Although the preponderance of MNa(+) ions compared with the MH(+)-H(2)O ions was detected in non-hydroxy fatty acid-ceramides, the preponderance of MH(+)-H(2)O ions based on the elimination of the hydroxyl group introduced at the alpha-position of acyl-portion compared with the MNa(+) ions was detected in alpha-hydroxy fatty acid-ceramides. In calibrations for authentic ceramides using N-octanoylsphingosine as an internal standard, an approximately linear relationship existed between the ratios of peak-areas of each ceramide to that of the internal standard and the known amounts of each ceramide. The factor (f) of each ceramide was calculated as follows; N-oleoyl-D-sphingosine (f=0.45), N-palmitoyl-D-sphingosine (f=0.40), N-stearoyl-D-sphingosine (f=0.39), N-nervonoyl-D-sphingosine (f=0.39) and N-lignoceroyl-D-sphingosine (f=0.35). In subcellular fractions from A549 and HepG2 cells, although ceramide species content per mg protein was high in the nuclear envelope fractions, the 7000 g pellet fractions and the 100000 g pellet fractions, a large portion of the ceramide species was concentrated in the nuclear envelope fraction. In addition, this method was applied to a mild alkaline hydrolyzate of total ceramides from pig stratum corneum, and MNa(+)/MH(+)-H(2)O ions corresponding to several omega-hydroxyacyl-ceramides were detected.

Serine palmitoyltransferase: role in apoptotic de novo ceramide synthesis and other stress responses

Serine palmitoyltransferase is the first and rate-limiting enzyme of sphingolipid synthesis. As such, it is a central control point in the synthesis of bioactivate sphingolipids, and it plays an important role in mediating cellular stress responses. In this review, its role in mediating these responses is discussed within the context of de novo ceramide synthesis. Furthermore, a discussion is provided of its regulation as discerned from both yeast and mammalian studies.

Fumonisins and fumonisin analogs as inhibitors of ceramide synthase and inducers of apoptosis

Sphingoid bases are growth inhibitory and pro-apoptotic for many types of cells when added to cells exogenously, and can be elevated to toxic amounts endogenously when cells are exposed to inhibitors of ceramide synthase. An important category of naturally occurring inhibitors are the fumonisins, which inhibit ceramide synthase through structural similarities with both the sphingoid base and fatty acyl-CoA co-substrates. Fumonisins cause a wide spectrum of disease (liver and renal toxicity and carcinogenesis, neurotoxicity, induction of pulmonary edema, and others), and most-possibly all-of the pathophysiologic effects of fumonisins are attributable to disruption of the sphingolipid metabolism. The products of alkaline hydrolysis of fumonisins (which occurs during the preparation of masa flour for tortillas) are aminopentols that also inhibit ceramide synthase, but more weakly. Nonetheless, the aminopentols (and other 1-deoxy analogs of sphinganine) are acylated to derivatives that inhibit ceramide synthase, perhaps as product analogs, elevate sphinganine, and kill the cells. Somewhat paradoxically, fumonisins sometimes stimulate growth and inhibit apoptosis, possibly due to elevation of sphinganine 1-phosphate, which is known to have these cellular effects. These findings underscore the complexity of sphingolipid metabolism and the difficulty of identifying the pertinent mediators unless a full profile of the potentially bioactive species is evaluated.

Upstream of growth and differentiation factor 1 (uog1), a mammalian homolog of the yeast longevity assurance gene 1 (LAG1), regulates N-stearoyl-sphinganine (C18-(dihydro)ceramide) synthesis in a fumonisin B1-independent manner in mammalian cells

The longevity assurance gene (LAG1) and its homolog (LAC1) are required for acyl-CoA-dependent synthesis of ceramides containing very long acyl chain (e.g. C26) fatty acids in yeast, and a homolog of LAG1, ASC1, confers resistance in plants to fumonisin B(1), an inhibitor of ceramide synthesis. To understand further the mechanism of regulation of ceramide synthesis, we now characterize a mammalian homolog of LAG1, upstream of growth and differentiation factor-1 (uog1). cDNA clones of uog1 were obtained from expression sequence-tagged clones and sub-cloned into a mammalian expression vector. Transient transfection of human embryonic kidney 293T cells with uog1 followed by metabolic labeling with [4,5-(3)H]sphinganine or L-3-[(3)H]serine demonstrated that uog1 conferred fumonisin B(1) resistance with respect to the ability of the cells to continue to produce ceramide. Surprisingly, this ceramide was channeled into neutral glycosphingolipids but not into gangliosides. Electrospray tandem mass spectrometry confirmed the elevation in sphingolipids and revealed that the ceramides and neutral glycosphingolipids of uog1-transfected cells contain primarily stearic acid (C18), that this enrichment was further increased by FB(1), and that the amount of stearic acid in sphingomyelin was also increased. UOG1 was localized to the endoplasmic reticulum, demonstrating that the fatty acid selectivity and the fumonisin B(1) resistance are not due to a subcellular localization different from that found previously for ceramide synthase activity. Furthermore, in vitro assays of uog1-transfected cells demonstrated elevated ceramide synthase activity when stearoyl-CoA but not palmitoyl-CoA was used as substrate. We propose a role for UOG1 in regulating C18-ceramide (N-stearoyl-sphinganine) synthesis, and we note that not only is this the first case of ceramide formation in mammalian cells with such a high degree of fatty acid specificity, but also that the N-stearoyl-sphinganine produced by UOG1 most significantly impacts neutral glycosphingolipid synthesis.