The molecular mechanism of phytosphingosine binding to FFAR4/GPR120 differs from that of other fatty acids

Free fatty acid receptor 4 (FFAR4)/GPR120 comprises a receptor for medium‐ and long‐chain fatty acids. We previously identified phytosphingosine (PHS) as a novel ligand of FFAR4. Although many natural FFAR4 ligands have carboxyl groups, PHS does not, thus suggesting that binding to FFAR4 is driven by a completely different mechanism than other natural ligands such as α‐linolenic acid (ALA). To test this hypothesis, we performed docking simulation analysis using a FFAR4 homology model based on a protein model derived from the crystal structure of activated turkey beta‐1 adrenoceptor. The docking simulation revealed that the probable hydrogen bonds to FFAR4 differ between various ligands. In particular, binding was predicted between R264 of the FFAR4 and the oxygen of the carboxylate group in ALA, as well as between E249 of the FFAR4 and the oxygen of the hydroxy group at the C4‐position in PHS. Alanine substitution at E249 (E249A) dramatically reduced PHS‐induced FFAR4 activation but demonstrated a weaker effect on ALA‐induced FFAR4 activation. Kinetic analysis and K_m values clearly demonstrated that the E249A substitution resulted in reduced affinity for PHS but not for ALA. Additionally, we observed that sphingosine, lacking a hydroxyl group at C4‐position, could not activate FFAR4. Our data show that E249 of the FFAR4 receptor is crucial for binding to the hydroxy group at the C4‐position in PHS, and this is a completely different molecular mechanism of binding from ALA. Because GPR120 agonists have attracted attention as treatments for type 2 diabetes, our findings may provide new insights into their development.

Teadenol A in microbial fermented tea acts as a novel ligand on GPR120 to increase GLP-1 secretion

Post-fermented teas, produced by microbial fermentation, are attracting attention due to their health benefits that reduce the risk of hyperlipidemia and atherosclerosis. Although several novel polyphenols have been identified from post-fermented teas, their biological activities have not yet been fully elucidated. In this study, we found that teadenol A, a polyphenol recently isolated from Japanese post-fermented tea, acts as a novel ligand on a long-chain fatty acid receptor, GPR120. Teadenol A activated GPR120 was over-expressed in 293T cells, and this activation was inhibited by the GPR120 antagonist AH7614. Additionally, teadenol A induced Erk1/2 phosphorylation and increased the intracellular Ca²⁺ concentration in 293T cells, and these effects were completely dependent on GPR120 expression. Our results suggest that teadenol A binds and activates GPR120 directly. Furthermore, teadenol A enhanced the secretion of GLP-1 from intestinal endocrine STC-1 cells. GLP-1 suppresses appetite and increases insulin secretion, exhibiting anti-diabetic effects. GPR120/GLP-1 signaling is attracting attention as a potential target for pharmaceuticals against type 2 diabetes. Our results suggest that teadenol A is a key molecule in post-fermented tea responsible for beneficial effects on metabolic syndrome.

Koji glycosylceramide commonly contained in Japanese traditional fermented foods alters cholesterol metabolism in obese mice

Koji, which is manufactured by proliferating non-pathogenic fungus Aspergillus oryzae on steamed rice, is the base for Japanese traditional fermented foods. We have revealed that koji and related Japanese fermented foods and drinks such as amazake, shio-koji, unfiltered sake and miso contain abundant glycosylceramide. Here, we report that feeding of koji glycosylceramide to obese mice alters the cholesterol metabolism . Liver cholesterol was significantly decreased in obese mice fed with koji glycosylceramide. We hypothesized that their liver cholesterol was decreased because it was converted to bile acids. Consistent with the hypothesis, many bile acids were increased in the cecum and feces of obese mice fed with koji glycosylceramide. Expressions of CYP7A1 and ABCG8 involved in the metabolism of cholesterol were significantly increased in the liver of mice fed with koji glycosylceramide. Therefore, it was considered that koji glycosylceramide affects the cholesterol metabolism in obese mice.

Sphingomyelin in microdomains of the plasma membrane regulates amino acid-stimulated mTOR signal activation

Sphingomyelin (SM) is required for cells to proliferate, but the reason is not fully understood. In order to asses this question, we employed a cell line, ZS, which lacks both SMS1 and SMS2, isolated from mouse embryonic fibroblasts in SMS1 and 2 double knockout mouse, and SMS1 or SMS2 re-expressing cells, ZS/SMS1 or ZS/SMS2, respectively. We investigated regulation of SM in activating the mammalian target of rapamycin (mTOR) signal induced by essential amino acids (EAA), using these cells. EAA-stimulated mTOR signal was more activated in ZS/SMS1 and ZS/SMS2 cells than in controls. Treatment with methyl-b-cyclodextrin dramatically inhibited the activation. Interestingly, we found that the expression of CD98, LAT-1 and ASCT-2, amino acid transporters concerned with mTOR activation, was down-regulated in ZS cells. Transporters localized in microdomains and formed a functional complex. Our results indicate that SM affect proliferation through the transport of amino acids via SM-enriched microdomains.

Structure-inspired design of a sphingolipid mimic sphingosine-1-phosphate receptor agonist from a naturally occurring sphingomyelin synthase inhibitor

A monophosphate derivative of ginkgolic acid binds to sphingosine 1-phosphate (S1P) receptors to perform similar functions to the lipid mediator S1P.

Possible roles of long-chain sphingomyelines and sphingomyelin synthase 2 in mouse macrophage inflammatory response

To evaluate the precise role of sphingomyelin synthase 2 (SMS2) in sphingomyelin (SM) metabolism and their anti-inflammatory properties, we analyzed species of major SM and ceramide (Cer) (18:1, 18:0 sphingoid backbone, C14 - C26 N-acyl part) in SMS2 knockout and wild-type mouse plasma and liver using HPLC-MS. SMS2 deficiency significantly decreased very long chain SM (SM (d18:1/22:0) and SM (d18:1/24:0 or d18:0/24:1)) and increased very long chain Cer (Cer (d18:1/24:0 or d18:0/24:1) and Cer (d18:1/24:1)), but not long chain SM (SM (d18:1/16:0), SM (d18:1/18:0 or d18:0/18:1) and SM (d18:1/18:1)) in plasma. To examine the effects of SM on inflammation, we studied the role of very long chain SM in macrophage activation. Addition of SM (d18:1/24:0) strongly upregulated several macrophage activation markers, SM (d18:1/6:0) and Cer (d18:1/24:0) however, did not. It was suggested that very long chain SM but not long chain SM were decreased in SMS2-deficient mice liver and plasma. And the exogenously added very long chain SM (d18:1/24:0) could activate macrophages directly, suggesting a novel role of plasma very long chain SM in modulating macrophage activation and resulting inflammation.

Japanese traditional dietary fungus koji Aspergillus oryzae functions as a prebiotic for Blautia coccoides through glycosylceramide: Japanese dietary fungus koji is a new prebiotic

Background

The Japanese traditional cuisine, Washoku, considered to be responsible for increased longevity among the Japanese, comprises various foods fermented with the non-pathogenic fungus Aspergillus oryzae (koji). We have recently revealed that koji contains an abundant amount of glycosylceramide. Intestinal microbes have significant effect on health. However, the effects of koji glycosylceramide on intestinal microbes have not been studied.

Materials and methods

Glycosylceramide was extracted and purified from koji. C57BL/6N mice were fed a diet containing 1 % purified koji glycosylceramide for 1 week. Nutritional parameters and faecal lipid constituents were analyzed. The intestinal microbial flora of mice on this diet was investigated.

Results

Ingested koji glycosylceramide was neither digested by intestinal enzymes nor was it detected in the faeces, suggesting that koji glycosylceramide was digested by the intestinal microbial flora. Intestinal microbial flora that digested koji glycosylceramide had an increased ratio of Blautia coccoides. Stimulation of B. coccoides growth by pure koji glycosylceramide was confirmed in vitro.

Conclusions

Koji functions as a prebiotic for B. coccoides through glycosylceramide. Since there are many reports of the effects of B. coccoides on health, an increase in intestinal B. coccoides by koji glycosylceramide might be the connection between Japanese cuisine, intestinal microbial flora, and longevity.

Sphingomyelin synthase 2, but not sphingomyelin synthase 1, is involved in HIV-1 envelope-mediated membrane fusion

Membrane fusion between the viral envelope and plasma membranes of target cells has previously been correlated with HIV-1 infection. Lipids in the plasma membrane, including sphingomyelin, may be crucially involved in HIV-1 infection; however, the role of lipid-metabolic enzymes in membrane fusion remains unclear. In this study, we examined the roles of sphingomyelin synthase (SMS) in HIV-1 Env-mediated membrane fusion using a cell-cell fusion assay with HIV-1 mimetics and their target cells. We employed reconstituted cells as target cells that stably express Sms1 or Sms2 in Sms-deficient cells. Fusion susceptibility was ∼5-fold higher in Sms2-expressing cells (not in Sms1-expressing cells) than in Sms-deficient cells. The enhancement of fusion susceptibility observed in Sms2-expressing cells was reversed and reduced by Sms2 knockdown. We also found that catalytically nonactive Sms2 promoted membrane fusion susceptibility. Moreover, SMS2 co-localized and was constitutively associated with the HIV receptor·co-receptor complex in the plasma membrane. In addition, HIV-1 Env treatment resulted in a transient increase in nonreceptor tyrosine kinase (Pyk2) phosphorylation in Sms2-expressing and catalytically nonactive Sms2-expressing cells. We observed that F-actin polymerization in the region of membrane fusion was more prominent in Sms2-expressing cells than Sms-deficient cells. Taken together, our research provides insight into a novel function of SMS2 which is the regulation of HIV-1 Env-mediated membrane fusion via actin rearrangement.

Decreased amyloid-β pathologies by intracerebral loading of glycosphingolipid-enriched exosomes in Alzheimer model mice

Elevated levels of amyloid-β peptide (Aβ) in the human brain are linked to the pathogenesis of Alzheimer disease. Recent in vitro studies have demonstrated that extracellular Aβ can bind to exosomes, which are cell-secreted nanovesicles with lipid membranes that are known to transport their cargos intercellularly. Such findings suggest that the exosomes are involved in Aβ metabolism in brain. Here, we found that neuroblastoma-derived exosomes exogenously injected into mouse brains trapped Aβ and with the associated Aβ were internalized into brain-resident phagocyte microglia. Accordingly, continuous intracerebral administration of the exosomes into amyloid-β precursor protein transgenic mice resulted in marked reductions in Aβ levels, amyloid depositions, and Aβ-mediated synaptotoxicity in the hippocampus. In addition, we determined that glycosphingolipids (GSLs), a group of membrane glycolipids, are highly abundant in the exosomes, and the enriched glycans of the GSLs are essential for Aβ binding and assembly on the exosomes both in vitro and in vivo. Our data demonstrate that intracerebrally administered exosomes can act as potent scavengers for Aβ by carrying it on the exosome surface GSLs and suggest a role of exosomes in Aβ clearance in the central nervous system. Improving Aβ clearance by exosome administration would provide a novel therapeutic intervention for Alzheimer disease.

Improved high-fat diet-induced glucose intolerance by an oral administration of phytosphingosine

We have previously reported that phytoceramide and phytosphingosine (PHS) stimulated the transcriptional activity of peroxisome proliferator-activated receptor γ (PPARγ) in cells. PPARγ is a therapeutic target for type 2 diabetes. We found in this study that an oral administration of PHS improved diet-induced glucose intolerance in mice. Since PHS is highly expressed in yeast, PHS in fermented foods may improve diabetes.

A sensitive cell-based method to screen for selective inhibitors of SMS1 or SMS2 using HPLC and a fluorescent substrate

Recent studies have revealed that sphingomyelin (SM) is involved in metabolic syndrome and is a new target of an anti-metabolic syndrome drug. Deficiencies in the enzyme SM synthase 1 (SMS1) result in severe abnormalities, whereas deficiencies in SMS2 do not. SMS1 and SMS2 synthesize SM under similar conditions, so their respective activities cannot be measured separately. We report here on a sensitive, high-throughput and reliable cell-based method to separately measure each SMS activity and to screen for SMS-specific inhibitors, using HPLC and fluorescent ceramide (Cer) analogs. We isolated SMS-null cells and stably transfected them with SMS1 or SMS2. Using these cells, individual SMS activities could be measured separately. Fluorescent Cer, SM, and glucosylceramide analogs could be separated within 4 min by HPLC using an NH(2) column. SMS activities of SMS1- or SMS2-expressing cells seeded in a single well of a 96-well plate could be measured using HPLC and fluorescent Cer analogs. This method clearly demonstrated that treatment of the cells with their respective siRNA or D609, an inhibitor of SMS, resulted in a significant decrease in each SMS activity. These results indicate that our newly developed method can be utilized for screening therapeutics against metabolic syndrome that target SMS2.

Regulation of autophagy and its associated cell death by "sphingolipid rheostat": reciprocal role of ceramide and sphingosine 1-phosphate in the mammalian target of rapamycin pathway

The role of "sphingolipid rheostat" by ceramide and sphingosine 1-phosphate (S1P) in the regulation of autophagy remains unclear. In human leukemia HL-60 cells, amino acid deprivation (AA(-)) caused autophagy with an increase in acid sphingomyleinase (SMase) activity and ceramide, which serves as an autophagy inducing lipid. Knockdown of acid SMase significantly suppressed the autophagy induction. S1P treatment counteracted autophagy induction by AA(-) or C(2)-ceramide. AA(-) treatment promoted mammalian target of rapamycin (mTOR) dephosphorylation/inactivation, inducing autophagy. S1P treatment suppressed mTOR inactivation and autophagy induction by AA(-). S1P exerts biological actions via cell surface receptors, and S1P(3) among five S1P receptors was predominantly expressed in HL-60 cells. We evaluated the involvement of S1P(3) in suppressing autophagy induction. S1P treatment of CHO cells had no effects on mTOR inactivation and autophagy induction by AA(-) or C(2)-ceramide. Whereas S1P treatment of S1P(3) overexpressing CHO cells resulted in activation of the mTOR pathway, preventing cells from undergoing autophagy induced by AA(-) or C(2)-ceramide. These results indicate that S1P-S1P(3) plays a role in counteracting ceramide signals that mediate mTOR-controlled autophagy. In addition, we evaluated the involvement of ceramide-activated protein phosphatases (CAPPs) in ceramide-dependent inactivation of the mTOR pathway. Inhibition of CAPP by okadaic acid in AA(-)- or C(2)-ceramide-treated cells suppressed dephosphorylation/inactivation of mTOR, autophagy induction, and autophagy-associated cell death, indicating a novel role of ceramide-CAPPs in autophagy induction. Moreover, S1P(3) engagement by S1P counteracted cell death. Taken together, these results indicated that sphingolipid rheostat in ceramide-CAPPs and S1P-S1P(3) signaling modulates autophagy and its associated cell death through regulation of the mTOR pathway.

4,8-Sphingadienine and 4-hydroxy-8-sphingenine activate ceramide production in the skin

Background

Ingestion of glucosylceramide improves transepidermal water loss (TEWL) from the skin, but the underlying mechanism by which a small amount of dietary glucosylceramide can vastly improve skin conditions remains unclear. In a previous report, glucosylceramides were shown to be digested to sphingoids, which were shown to be absorbed through the intestinal epithelium. Based on these observations, we hypothesized that sphingoids are the key molecules facilitating endogenous ceramide production. In this study, we assessed the effect of 4,8-sphingadienine (d18:2) and 4-hydroxy-8-sphingenine (t18:1), derived from konjac glucosylceramide, on stimulating ceramide production.

Methods

Konjac glucosylceramide acidolysis was performed using hydrochloric acid; the resulting d18:2 and t18:1 were fractionated by column chromatography. Real-time quantitative RT-PCR was performed to assess the effect of d18:2 and t18:1 on gene expression in normal human epidermal keratinocytes, while their effect on the nuclear receptor, peroxisome proliferator-activated receptor (PPAR)γ, was measured using a receptor-cofactor assay system. The effect of d18:2 and t18:1 on stimulating ceramide production was evaluated using HPTLC analysis in a 3-dimensional human skin model.

Results

We noted the upregulation of genes related to de novo ceramide synthesis as well as of those encoding the elongases of very long-chain fatty acids by d18:2 and t18:1, but not by glucosylceramide and 4-sphingenine. Both these sphingoids also facilitated the expression of PPARβ/δ and PPARγ; moreover, they also demonstrated ligand activity for PPARγ. These results indicated that d18:2 and t18:1 promote the differentiation of keratinocytes. Analysis of the lipids within the 3-dimensional human skin model indicated that treatment with d18:2 and t18:1 not only upregulated gene expression but also increased ceramide production.

Conclusions

The sphingoids d18:2 and t18:1 activated genes related to de novo ceramide synthesis and increased ceramide production, whereas glucosylceramide and 4-sphingenine could not. These results suggest that the effect of dietary glucosylceramides on the skin is mediated by d18:2 and t18:1.

Sphingolipid-modulated exosome secretion promotes clearance of amyloid-β by microglia

Amyloid β-peptide (Aβ), the pathogenic agent of Alzheimer disease, is a physiological metabolite whose levels are constantly controlled in normal brain. Recent studies have demonstrated that a fraction of extracellular Aβ is associated with exosomes, small membrane vesicles of endosomal origin, although the fate of Aβ in association with exosome is largely unknown. In this study, we identified novel roles for neuron-derived exosomes acting on extracellular Aβ, i.e. exosomes drive conformational changes in Aβ to form nontoxic amyloid fibrils and promote uptake of Aβ by microglia. The Aβ internalized together with exosomes was further transported to lysosomes and degraded. We also found that blockade of phosphatidylserine on the surface of exosomes by annexin V not only prevented exosome uptake but also suppressed Aβ incorporation into microglia. In addition, we demonstrated that secretion of neuron-derived exosomes was modulated by the activities of sphingolipid-metabolizing enzymes, including neutral sphingomyelinase 2 (nSMase2) and sphingomyelin synthase 2 (SMS2). In transwell experiments, up-regulation of exosome secretion from neuronal cells by treatment with SMS2 siRNA enhanced Aβ uptake into microglial cells and significantly decreased extracellular levels of Aβ. Our findings indicate a novel mechanism responsible for clearance of Aβ through its association with exosomes. The modulation of the vesicle release and/or elimination may alter the risk of AD.

Qualitative and quantitative cellular glycomics of glycosphingolipids based on rhodococcal endoglycosylceramidase-assisted glycan cleavage, glycoblotting-assisted sample preparation, and matrix-assisted laser desorption ionization tandem time-of-flight mass spectrometry analysis

Glycosphingolipids (GSLs) are crucially important components of the cellular membrane, where they comprise microdomains with many critical biological functions. Despite this fact, qualitative and quantitative techniques for the analysis of GSLs still lag behind the needs of researchers. In this study, a reliable procedure for the elucidation of cellular GSL-glycomes was established based on (a) enzymatic glycan cleavage by endoglycosylceramidases derived from Rhodococcus sp. in combination with (b) glycoblotting-assisted sample preparation. The mixture of endoglycosylceramidase I and II was employed to maximize the release of glycan moieties from the major classes of GSLs (i.e. ganglio-, (neo)lacto- and globo-series GSLs). The glycoblotting technique enabled the quantitative detection of GSL-glycans using as few as 2 × 10(5) cells. Thirty-seven different kinds of cellular GSL glycans were successfully observed in 11 kinds of cells, including Chinese hamster ovary cells and their lectin-resistant mutants as well as murine and human embryonic carcinoma cells. Furthermore, in-depth structural clarification in terms of discrimination of isomers was achieved by MALDI-TOF/TOF mass spectrometry analysis and/or linkage-specific glycosidase digestion. These novel analytical techniques were shown to be capable of delineating cell-specific GSL-glycomes. Thus, they are anticipated to have a broad range of applications for the characterization, description, and comparison of various cellular/tissue samples in the fields of drug discovery and regenerative medicine.

Phytoceramide and sphingoid bases derived from brewer's yeast Saccharomyces pastorianus activate peroxisome proliferator-activated receptors

Background

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that regulate lipid and glucose metabolism. PPARα is highly expressed in the liver and controls genes involved in lipid catabolism. We previously reported that synthetic sphingolipid analogs, part of which contains shorter-length fatty acid chains than natural sphingolipids, stimulated the transcriptional activities of PPARs. Sphingosine and dihydrosphingosine (DHS) are abundant sphingoid bases, and ceramide and dihydroceramide are major ceramide species in mammals. In contrast, phytosphingosine (PHS) and DHS are the main sphingoid bases in fungi. PHS and phytoceramide exist in particular tissues such as the epidermis in mammals, and involvement of ceramide species in PPARβ activation in cultured keratinocytes has been reported. The purpose of the present study is to investigate whether natural sphingolipids with C18 fatty acid and yeast-derived sphingoid bases activate PPARs as PPAR agonists.

Method

Lipids of brewer's yeast contain PHS- and DHS-based sphingolipids. To obtain the sphingoid bases, lipids were extracted from brewer's yeast and acid-hydrolyzed. The sphingoid base fraction was purified and quantified. To assess the effects of sphingolipids on PPAR activation, luciferase reporter assay was carried out. NIH/3T3 and human hepatoma (HepG2) cells were transfected with expression vectors for PPARs and retinoid × receptors, and PPAR responsive element reporter vector. When indicated, the PPAR/Gal4 chimera system was performed to enhance the credibility of experiments. Sphingolipids were added to the cells and the dual luciferase reporter assay was performed to determine the transcriptional activity of PPARs.

Results

We observed that phytoceramide increased the transcriptional activities of PPARs significantly, whereas ceramide and dihydroceramide did not change PPAR activities. Phytoceramide also increased transactivation of PPAR/Gal4 chimera receptors. Yeast-derived sphingoid base fraction, which contained PHS and DHS, or authentic PHS or DHS increased PPAR-dependent transcription. Additionally, phytoceramide stimulated PPARα activity in HepG2 hepatocytes, suggesting that phytoceramide activates genes regulated by PPARα.

Conclusions

Phytoceramide and yeast-derived sphingoid bases activate PPARs, whereas ceramide and dihydroceramide do not change the PPAR activity. The present findings suggest that phytoceramide acts as a PPAR ligand that would regulate PPAR-targeted genes.

Dynamic modification of sphingomyelin in lipid microdomains controls development of obesity, fatty liver, and type 2 diabetes

Lipid microdomains or caveolae, small invaginations of plasma membrane, have emerged as important elements for lipid uptake and glucose homeostasis. Sphingomyelin (SM) is one of the major phospholipids of the lipid microdomains. In this study, we investigated the physiological function of sphingomyelin synthase 2 (SMS2) using SMS2 knock-out mice, and we found that SMS2 deficiency prevents high fat diet-induced obesity and insulin resistance. Interestingly, in the liver of SMS2 knock-out mice, large and mature lipid droplets were scarcely observed. Treatment with siRNA for SMS2 also decreased the large lipid droplets in HepG2 cells. Additionally, the siRNA of SMS2 decreased the accumulation of triglyceride in liver of leptin-deficient (ob/ob) mice, strongly suggesting that SMS2 is involved in lipid droplet formation. Furthermore, we found that SMS2 exists in lipid microdomains and partially associates with the fatty acid transporter CD36/FAT and with caveolin 1, a scaffolding protein of caveolae. Because CD36/FAT and caveolin 1 exist in lipid microdomains and are coordinately involved in lipid droplet formation, SMS2 is implicated in the modulation of the SM in lipid microdomains, resulting in the regulation of CD36/FAT and caveolae. Here, we established new cell lines, in which we can completely distinguish SMS2 activity from SMS1 activity, and we demonstrated that SMS2 could convert ceramide produced in the outer leaflet of the plasma membrane into SM. Our findings demonstrate the novel and dynamic regulation of lipid microdomains via conformational changes in lipids on the plasma membrane by SMS2, which is responsible for obesity and type 2 diabetes.

Ceramide kinase is not essential but might act as an Ca2+-sensor for mast cell activation

Ceramide kinase (CerK) catalyzes the conversion of ceramide to ceramide 1-phosphate (C1P). We previously revealed that CerK is involved in the activation of mast cells. In this study, we performed an advanced investigation into the role of CerK on the activation of mast cells using CERK-/- mice. Although CERK-/- mice were less prone to exhibiting a passive cutaneous anaphylactic shock (PCA)-reaction compared to wild type (WT) mice, the differences were not significant. In bone marrow-derived mast cells (BMMC) activated by cross-linking antigen (Ag)/IgE, not high, but low concentrations of Ag had a reduced effect on degranulation in BMMC from CERK-/- mice compared to effects on BMMC from WT mice. Similarly, when the BMMCs were activated with calcium ionophore to focus on the downstream signaling of Ca(2+)-elevation, only a low concentration of ionophore had a reduced effect on degranulation in the BMMC from CERK-/- mice compared to the effect on BMMC from WT mice. Furthermore, the CerK inhibitor K1 reduced the differences in degranulation observed between the BMMC from CERK-/- and WT mice in a dose-dependent manner, demonstrating a contribution for CerK and its product C1P in degranulation. Although CerK is not essential for activation of mast cells, especially a potent and acute activation such as a PCA reaction, CerK might act as an modulator for mild and chronic activation of mast cells, thus increasing sensitivity to cytoplasmic Ca(2+).

Ceramide biosynthesis in keratinocyte and its role in skin function

The enucleate layer of the epidermis, i.e. the stratum corneum, is responsible for certain critical protective functions, such as epidermal permeability barrier function. Within the epidermal membrane lamella component, ceramides are the dominant lipid class by weight (over 50%) and exhibit the greatest molecular heterogeneity in terms of sphingoid base and fatty acid composition. It is now evermore important to understand how ceramide production and functions are controlled in the epidermis, since decreased epidermal ceramide content has been linked to water loss and barrier dysfunction. During the past several years, critical enzymes in ceramide biosynthesis have been identified, including ceramide synthases (CerS) and ceramide hydroxylase/desaturase. In this review, we describe the molecular heterogeneity of ceramides synthesized in the epidermis and their possible roles in epidermal permeability barrier functions. We also describe recent studies that identified the family of CerS (CerS1-CerS6) in mammals. We further focus on the roles of specific isoforms of these enzymes in synthesizing the epidermal ceramides, especially in relation to chain-length specificity. In addition, we provide experimental information, including our recent findings, as to how applying ceramide or ceramide-containing substances to skin, orally or directly, can benefit skin health.

Role of ceramide kinase in peroxisome proliferator-activated receptor beta-induced cell survival of mouse keratinocytes

Ceramide (Cer) is known to be a lipid mediator in apoptosis and to have an important role in cell fate, via control of intracellular Cer levels. Recently, ceramide kinase (CerK) was identified as an enzyme that converts Cer to ceramide 1-phosphate (C1P). We examined potential functions of CerK in the regulation of keratinocyte survival, and the possible involvement of peroxisome proliferator-activated receptor beta (PPARbeta). PPARbeta is known to be a nuclear receptor acting as a ligand-inducible transcription factor and has been implicated in the control of keratinocyte survival. In the mouse keratinocyte cell line SP1, serum starvation induced cell death and the accumulation of intracellular Cer, an apoptotic event. However, apoptosis was inhibited by activation of PPARbeta. Interestingly, activation of PPARbeta enhanced the mRNA expression of CerK and CerK activity. Furthermore, the cell survival effect of PPARbeta was greatly diminished in keratinocytes isolated from CerK-null mice. Chromatin immunoprecipitation revealed that, in vivo, PPARbeta binds to the CerK gene via a sequence located in the first intron. Electrophoretic mobility-shift assays confirmed that PPARbeta associates with this sequence in vitro. These findings indicated that CerK gene expression was directly regulated by PPARbeta. In conclusion, our results demonstrate that PPARbeta-mediated upregulation of CerK gene expression is necessary for keratinocyte survival against serum starvation-induced apoptosis.

The generation and behavioral analysis of ceramide kinase-null mice, indicating a function in cerebellar Purkinje cells

The discovery of ceramide kinase (CerK), which phosphorylates ceramide (Cer) to ceramide 1-phisphate (C1P), established a new pathway for Cer metabolism. Among mouse tissues, brain contains the highest CerK activity. In this study, we found that CerK is highly expressed in cerebellar Purkinje cells. Since Purkinje cells are important for motor-related behaviors, we generated CerK-null mice and performed behavioral analyses. The CerK-null mice were healthy, and displayed no histological abnormalities. The mice lost CerK activity completely, suggesting that CerK is the only enzyme that phosphorylate Cer. However, cellular C1P levels were not different between the CerK-null and wild-type mice, indicating the presence of other C1P-producing pathway. The general motor-coordination was not impaired in the CerK-null mice, but emotional behavior was slightly affected. Our findings suggest that CerK is not necessary for survival at an individual level, but might be involved in higher brain function related to emotion.

Transbilayer movement of ceramide in the plasma membrane of live cells

Ceramide (Cer) is the precursor for sphingolipids and functions as a second messenger in a variety of cellular processes including apoptosis. However, no direct target of Cer leading to apoptosis has been identified. Understanding the movement and trafficking of Cer is important for fully understanding Cer signaling. In this study, we identified, for the first time, the transbilayer movement of Cer in the plasma membrane (PM) of living cells. We developed a new method to monitor transbilayer Cer movement using ceramide kinase activity. To produce Cer on the extracellular leaflet of the PM, bacterial sphingomyelinase (SMase) was added to rat basophilic leukemia cells. Interestingly, the dramatic elevation of ceramide 1-phosphate (C1P), the product of CerK, was observed following the increase of Cer induced by SMase treatment. Since we determined that both the protein and catalytic activity of CerK exists in the intracellular compartment, the all conversion of Cer to C1P by CerK should be occur intracellularly. This result indicates the rapid transbilayer movement of Cer from the outer leaflet to the inner leaflet of the PM of living cells. Furthermore, protease digestion of membrane proteins, inhibition of ABC transporters (by glibencramide) and of cation channels (by carbonyl cyanide m-chlorophenylhydrozone), and modification of cholesterol content did not affect the transbilayer movement of Cer. Thus, this movement might occur spontaneously. Our findings indicate not only Cer movement in the PM, but also identify an intrinsic property of Cer enabling Cer signaling.

Metabolism and biological functions of two phosphorylated sphingolipids, sphingosine 1-phosphate and ceramide 1-phosphate

Sphingolipids are major lipid constituents of the eukaryotic plasma membrane. Without certain sphingolipids, cells and/or embryos cannot survive, indicating that sphingolipids possess important physiological functions that are not substituted for by other lipids. One such role may be signaling. Recent studies have revealed that some sphingolipid metabolites, such as long-chain bases (LCBs; sphingosine (Sph) in mammals), long-chain base 1-phosphates (LCBPs; sphingosine 1-phosphate (S1P) in mammals), ceramide (Cer), and ceramide 1-phosphate (C1P), act as signaling molecules. The addition of phosphate groups to LCB/Sph and Cer generates LCBP/S1P and C1P, respectively. These phospholipids exhibit completely different functions than those of their precursors. In this review, we describe recent advances in understanding the functions of LCBP/S1P and C1P in mammals and in the yeast Saccharomyces cerevisiae. Since LCB/Sph, LCBP/S1P, Cer, and C1P are mutually convertible, regulation of not only the total amount of the each lipid but also of the overall balance in cellular levels is important. Therefore, we describe in detail their metabolic pathways, as well as the genes involved in each reaction.

Apoptosis occurs via the ceramide recycling pathway in human HaCaT keratinocytes

Keratinocytes contain abundant ceramides compared to other cells. However, studies on these cells have mainly focused on the barrier function of ceramide, while their other roles, such as those in apoptosis or cell cycle arrest, have not been well addressed. In this study, we investigated the apoptosis-inducing effect of exogenously added cell-permeable ceramides in HaCaT keratinocytes. We found that N-hexanoyl sphingosine (C6-ceramide) induced apoptosis efficiently through the accumulation of long chain ceramides. On the other hand, N-acetyl sphingosine (C2-ceramide) induced neither apoptosis nor accumulation of long chain ceramides. We also found that exogenously added C6-ceramide was hydrolyzed to sphingosine and then reacylated in long chain ceramides (ceramide recycling pathway), but that C2-ceramide was not hydrolyzed and thus not recycled. We propose that this is the basis for the chain length-specific heterogeneity observed in ceramide-induced apoptosis in these cells. These results also imply that keratinocytes utilize exogenous sphingolipids or ceramides to coordinate their own ceramide compositions.

Dietary glucosylceramide improves skin barrier function in hairless mice

BACKGROUND

Sphingolipids are known to play an important role in both water retention and epidermal permeability barrier function in mammalian stratum corneum. However, little is known about the effects on epidermal function of orally administered sphingolipids.

OBJECTIVE

We examined the effect of dietary glucosylceramide (GluCer) on the maintenance and recovery of epidermal barrier function.

METHODS

Hairless mice were fed a particular diet (HR-AD) for 4 weeks to induce chronic skin perturbation. Subsequently, a normal diet supplemented with GluCer (from rice bran and germ) was provided for the next 4 weeks. Transepidermal water loss (TEWL) and stratum corneum flexibility were measured throughout this recovery phase. Additional hairless mice were fed a diet with or without a maize-extracted GluCer supplement for 5 weeks, then their skin was acutely perturbed with repeated tape-stripping, and the TEWL was measured.

RESULTS

Although skin functions were generally lower following chronic perturbation, in GluCer-fed mice the TEWL was significantly reduced at 2 weeks and the stratum corneum flexibility was increased at 3 weeks compared to controls. Following acute barrier perturbation by tape-stripping, mice an HR-AD fed a GluCer diet exhibited enhanced recovery compared with the control diet group.

CONCLUSION

These results demonstrate that in hairless mice skin barrier functions impaired by chronic or acute perturbations were improved by dietary GluCer. The oral administration of GluCer may be useful for the preservation and recovery of epidermal barrier functions an HR-AD.

Identification of a nuclear localization signal in the retinitis pigmentosa-mutated RP26 protein, ceramide kinase-like protein

Retinitis pigmentosa (RP) is a genetically heterogeneous disease characterized by degeneration of the retina. A mutation in a new ceramide kinase (CERK) homologous gene, named CERK-like protein (CERKL), was found to cause autosomal recessive retinitis pigmentosa (RP26). Here, we show a point mutation of one of two putative nuclear localization signal (NLS) sequences inhibited the nuclear localization of the protein. Furthermore, the tetra-GFP-tagged NLS, which cannot passively enter the nucleus, was observed not only in the nucleus but also in the nucleolus. Our results provide the first evidence of the active nuclear import of CERKL and suggest that the identified NLS might be responsible for nucleolar retention of the protein. As recent studies have shown other RP-related proteins are localized in the nucleus or the nucleolus, our identification of NLS in CERKL suggests that CERKL likely plays important roles for retinal functions in the nucleus and the nucleolus.

The interaction between the pleckstrin homology domain of ceramide kinase and phosphatidylinositol 4,5-bisphosphate regulates the plasma membrane targeting and ceramide 1-phosphate levels

Ceramide kinase (CERK) converts ceramide to ceramide-1-phosphate (C1P), which has recently emerged as a new bioactive molecule capable of regulating diverse cellular functions. The N-terminus of the CERK protein encompasses a sequence motif known as a pleckstrin homology (PH) domain. Although the PH domain was previously demonstrated to be an important domain for the subcellular localization of CERK, the precise properties of this domain remained unclear. In this study, we reveal that the PH domain of CERK exhibits high affinity for phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)), among other lipids. Furthermore, in COS7 cells, GFP-fused CERK translocated rapidly from the cytoplasm to the plasma membrane in response to hyper-osmotic stress, which is known to increase the intracellular PI(4,5)P(2) levels, whereas a PH domain deletion mutant did not. Additionally, in [(32)P]orthophosphate-labeled COS7 cells, the translocation of CERK to the plasma membrane induced a 2.8-fold increase in C1P levels. The study presented here provides insight into the crucial role of the CERK-PH domain in plasma membrane targeting, through its binding to PI(4,5)P(2), and subsequent induction of C1P production in the vicinity of the membrane.

Suppression of mast cell degranulation by a novel ceramide kinase inhibitor, the F-12509A olefin isomer K1

Antigen-induced degranulation of mast cells plays a pivotal role in allergic and inflammatory responses. Recently, ceramide kinase (CERK) and its phosphorylated product ceramide 1-phosphate (C1P) have emerged as important players in mast cell degranulation. Here, we describe the synthesis of a novel F-12509A olefin isomer, K1, as an effective CERK inhibitor. In vitro kinase assays demonstrated that K1 effectively inhibits CERK without inhibiting sphingosine kinase and diacylglycerol kinase. Treating RBL-2H3 cells with K1 reduced cellular C1P levels to 40% yet had no effect on cell growth. Furthermore, treatment with K1 significantly suppressed both calcium ionophore- and IgE/antigen-induced degranulation, indicating that K1 interferes with signals that happen downstream of Ca(2+) mobilization. Finally, we show that K1 affects neither IgE/antigen-induced global tyrosine phosphorylation nor subsequent Ca(2+) elevation, suggesting a specificity for CERK-mediated signals. Our novel CERK inhibitor provides a useful tool for studying the biological functions of CERK and C1P. Moreover, to our knowledge, this is the first report demonstrating that inhibition of CERK suppresses IgE/antigen-induced mast cell degranulation. This finding suggests that CERK inhibitors might be a potential therapeutic tool in the treatment of allergic diseases.

Calmodulin Is Involved in the Ca2+-dependent Activation of Ceramide Kinase as a Calcium Sensor

We recently demonstrated that the activation of ceramide kinase (CERK) and the formation of its product, ceramide 1-phosphate (C1P), are necessary for the degranulation pathway in mast cells and that the kinase activity of this enzyme is completely dependent on the intracellular concentration of Ca(2+) (Mitsutake, S., Kim, T.-J., Inagaki, Y., Kato, M., Yamashita, T., and Igarashi, Y. (2004) J. Biol. Chem. 279, 17570-17577). Despite the demonstrated importance of Ca(2+) as a regulator of CERK activity, there are no apparent binding domains in the enzyme and the regulatory mechanism has not been well understood. In the present study, we found that calmodulin (CaM) is involved in the Ca(2+)-dependent activation of CERK. The CaM antagonist W-7 decreased both CERK activity and intracellular C1P formation. Additionally, exogenously added CaM enhanced CERK activity even at low concentrations of Ca(2+). The CERK protein was co-immunoprecipitated with an anti-CaM antibody, indicating formation of intracellular CaM.CERK complexes. An in vitro CaM binding assay also demonstrated Ca(2+)-dependent binding of CaM to CERK. These results strongly suggest that CaM acts as a Ca(2+) sensor for CERK. Furthermore, a CaM binding assay using various mutants of CERK revealed that the binding site of CERK is located within amino acids 422-435. This region appears to include a type 1-8-14B CaM binding motif and is predicted to form an amphipathic helical wheel, which is utilized in CaM recognition. The expression of a deletion mutant of CERK that contained the CaM binding domain but lost CERK activity inhibited the Ca(2+)-dependent C1P formation. These results suggest that this domain could saturate the CaM and hence block Ca(2+)-dependent activation of CERK. Finally, we reveal that in mast cell degranulation CERK acts downstream of CaM, similar to CaM-dependent protein kinase II, which had been assumed to be the main target of CaM in mast cells.

The leucine 10 residue in the pleckstrin homology domain of ceramide kinase is crucial for its catalytic activity

Ceramide kinase (CERK) converts ceramide (Cer) to ceramide-1-phosphate (C1P), a newly recognized bioactive molecule capable of regulating diverse cellular functions. The N-terminus of the CERK protein encompasses a sequence motif known as a pleckstrin homology (PH) domain. However, little is known regarding the functional roles of this domain in CERK. In this study, we have demonstrated that the PH domain of CERK is essential for its enzyme activity. Using site-directed mutagenesis, we have further determined that Leu10 in the PH domain has an important role in CERK activity. Replacing this residue with a neutral alanine or isoleucine, caused a dramatic decrease in CERK activity to 1% and 29%, respectively, compared to CERK, but had no effect on substrate affinity. The study presented here suggests that the PH domain of CERK is not only indispensable for its activity but also act as a regulator of CERK activity.

Synthesis and evaluation of sphingoid analogs as inhibitors of sphingosine kinases

Sphingosine 1-phosphate (S1P), a product of sphingosine kinases (SphK), mediates diverse biological processes such as cell differentiation, proliferation, motility, and apoptosis. In an effort to search and identify specific inhibitors of human SphK, the inhibitory effects of synthetic sphingoid analogs on kinase activity were examined. Among the analogs tested, we found two, SG12 and SG14, that have specific inhibitory effects on hSphK2. N,N-Dimethylsphingosine (DMS), a well-known SphK inhibitor, displayed inhibitory effects for both SphK1 and SphK2, as well as protein kinase C. In contrast, SG12 and SG14 exhibited selective inhibitory effects on hSphK2. Furthermore, SG14 did not affect PKC. In isolated platelets, SG14 blocked the conversion of sphingosine into sphingosine 1-phosphate significantly. This is the first report on the identification of a hSphK2-specific inhibitor, which may provide a useful tool for studying the biological functions of hSphK2.

Asp177 in C4 domain of mouse sphingosine kinase 1a is important for the sphingosine recognition

Sphingosine kinase (SK) is the enzyme that catalyzes the formation of sphingosine 1-phosphate (S1P). Although diverse biological functions have been reported for SK, its recognition site for its substrate sphingosine (Sph) is still unclear. We constructed various mutants of mouse sphingosine kinase 1a (mSK1a), carrying mutations in the C4 domain, which we had expected to encompass the Sph-binding site. We analyzed the influence of these mutations on the SK activity and substrate kinetics. One mutation, Asp177-->Asn177, caused a dramatic decrease in SK activity (to approximately 6% of wild type) and an increase in the Km value for Sph (10.1-->108 microM), with no change in the affinity for ATP. This result suggests that the C4 domain, especially the Asp177, is involved in the specific recognition of Sph. In this report, we are able, for the first time, to provide an account of the Sph-binding site of SK.

Ceramide kinase is a mediator of calcium-dependent degranulation in mast cells

Ceramide kinase (CERK) catalyzes the conversion of ceramide to ceramide 1-phosphate (C1P) and is known to be activated by calcium. Although several groups have examined the functions of CERK and its product C1P, the functions of C1P and CERK are not understood. We studied the RBL-2H3 cell line, a widely used model for mast cells, and found that CERK and C1P are required for activation of the degranulation process in mast cells. We found that C1P formation was enhanced during activation induced by IgE/antigen or by Ca(2+) ionophore A23187. The formation of C1P required the intracellular elevation of Ca(2+). We generated RBL-2H3 cells that stably express CERK, and when these cells were treated with A23187, a concomitant C1P formation was observed and degranulation increased 4-fold, compared with mock transfectants. The cell-permeable N-acetylsphingosine (C(2)-ceramide), a poor substrate of CERK, inhibited both the formation of C1P and degranulation, indicating that C1P formation was necessary for degranulation. Exogenous introduction of CERK into permeabilized RBL-2H3 cells caused degranulation. We identified a cytosolic localization of CERK that provides exposure to cytosolic Ca(2+). Taken together, these results indicate that C1P formation is a necessary step in the degranulation pathway in RBL-2H3 cells.

Identification of functional nuclear export sequences in human sphingosine kinase 1

Sphingosine kinase (SPHK) is an enzyme that phosphorylates sphingosine to form sphingosine 1-phosphate (S1P). Human SPHK1 (hSPHK1) was localized predominantly in the cytoplasm when transiently expressed in Cos7 cells. In this study, we have found two functional nuclear export signal (NES) sequences in the middle region of hSPHK1. Deletion and mutagenesis studies revealed that the cytoplasmic localization of SPHK1 depends on its nuclear export, directed by the NES. Furthermore, upon treatment with leptomycin B, a specific inhibitor of the nuclear export receptor CRM1, a marked nuclear accumulation of hSPHK1 was observed, indicating that hSPHK1 shuttles between the cytoplasm and the nucleus. Our results provide the first evidence of the active nuclear export of SPHK1 and suggest it is mediated by a CRM1-dependent pathway.

Molecular cloning and characterization of sphingolipid ceramide N-deacylase from a marine bacterium, Shewanella alga G8

Recently, lyso-sphingolipids have been identified as ligands for several orphan G protein-coupled receptors, although the molecular mechanism for their generation has yet to be clarified. Here, we report the molecular cloning of the enzyme, which catalyzes the generation of lyso-sphingolipids from various sphingolipids (sphingolipid ceramide N-deacylase). The 75-kDa enzyme was purified from the marine bacterium, Shewanella alga G8, and its gene was cloned from a G8 genomic library using sequences of the purified enzyme. The cloned enzyme was composed of 992 amino acids, including a signal sequence of 35 residues, and its molecular weight was estimated to be 109,843. Significant sequence similarities were found with an unknown protein of Streptomyces fradiae Y59 and a Lumbricus terrestris lectin but not other known functional proteins. The 106-kDa recombinant enzyme expressed in Escherichia coli hydrolyzed various glycosphingolipids and sphingomyelin, although it seems to be much less active than the native 75-kDa enzyme. In vitro translation using wheat germ extract revealed the activity of a 75-kDa deletion mutant lacking a C terminus to be much stronger than that of the full-length enzyme, suggesting that C-terminal processing is necessary for full activity.