Stereospecificity of sphingosine-induced intracellular calcium mobilization and cellular proliferation

Quantification of enantiomers and blind identification of erythro-sphingosine non-racemates by cold ion spectroscopy

Enantiomers of a lipid erythro-sphingosine have been quantified with ≈4% accuracy by UV cold ion spectroscopy of their non-covalent complexes with a chiral aromatic molecule. The diastereomeric configuration of such complexes enables the quantification using just a single enantiomeric lipid standard and the identification of non-racemic solutions with no standards at all.

Role of bioactive sphingolipids in physiology and pathology

Sphingolipids are a class of complex lipids containing a backbone of sphingoid bases, namely the organic aliphatic amino alcohol sphingosine (Sph), that are essential constituents of eukaryotic cells. They were first described as major components of cell membrane architecture, but it is now well established that some sphingolipids are bioactive and can regulate key biological functions. These include cell growth and survival, cell differentiation, angiogenesis, autophagy, cell migration, or organogenesis. Furthermore, some bioactive sphingolipids are implicated in pathological processes including inflammation-associated illnesses such as atherosclerosis, rheumatoid arthritis, inflammatory bowel disease (namely Crohn's disease and ulcerative colitis), type II diabetes, obesity, and cancer. A major sphingolipid metabolite is ceramide, which is the core of sphingolipid metabolism and can act as second messenger, especially when it is produced at the plasma membrane of cells. Ceramides promote cell cycle arrest and apoptosis. However, ceramide 1-phosphate (C1P), the product of ceramide kinase (CerK), and Sph 1-phosphate (S1P), which is generated by the action of Sph kinases (SphK), stimulate cell proliferation and inhibit apoptosis. Recently, C1P has been implicated in the spontaneous migration of cells from some types of cancer, and can enhance cell migration/invasion of malignant cells through interaction with a Gi protein-coupled receptor. In addition, CerK and SphK are implicated in inflammatory responses, some of which are associated with cancer progression and metastasis. Hence, targeting these sphingolipid kinases to inhibit C1P or S1P production, or blockade of their receptors might contribute to the development of novel therapeutic strategies to reduce metabolic alterations and disease.

The Many Facets of Sphingolipids in the Specific Phases of Acute Inflammatory Response

This review provides an overview on components of the sphingolipid superfamily, on their localization and metabolism. Information about the sphingolipid biological activity in cell physiopathology is given. Recent studies highlight the role of sphingolipids in inflammatory process. We summarize the emerging data that support the different roles of the sphingolipid members in specific phases of inflammation: (1) migration of immune cells, (2) recognition of exogenous agents, and (3) activation/differentiation of immune cells.

Mouse Sphingosine Kinase 1a Is Negatively Regulated through Conventional PKC-Dependent Phosphorylation at S373 Residue

Sphingosine kinase is a lipid kinase that converts sphingosine into sphingosine-1-phosphate, an important signaling molecule with intracellular and extracellular functions. Although diverse extracellular stimuli influence cellular sphingosine kinase activity, the molecular mechanisms underlying its regulation remain to be clarified. In this study, we investigated the phosphorylation-dependent regulation of mouse sphingosine kinase (mSK) isoforms 1 and 2. mSK1a was robustly phosphorylated in response to extracellular stimuli such as phorbol ester, whereas mSK2 exhibited a high basal level of phosphorylation in quiescent cells regardless of agonist stimulation. Interestingly, phorbol ester-induced phosphorylation of mSK1a correlated with suppression of its activity. Chemical inhibition of conventional PKCs (cPKCs) abolished mSK1a phosphorylation, while overexpression of PKCα, a cPKC isoform, potentiated the phosphorylation, in response to phorbol ester. Furthermore, an in vitro kinase assay showed that PKCα directly phosphorylated mSK1a. In addition, phosphopeptide mapping analysis determined that the S373 residue of mSK1a was the only site phosphorylated by cPKC. Interestingly, alanine substitution of S373 made mSK1a refractory to the inhibitory effect of phorbol esters, whereas glutamate substitution of the same residue resulted in a significant reduction in mSK1a activity, suggesting the significant role of this phosphorylation event. Taken together, we propose that mSK1a is negatively regulated through cPKC-dependent phosphorylation at S373 residue.

Roles for sphingolipids in Saccharomyces cerevisiae

Studies using Saccharomyces cerevisiae, the common baker's or brewer's yeast, have progressed over the past twenty years from knowing which sphingolipids are present in cells and a basic outline of how they are made to a complete or nearly complete directory of the genes that catalyze their anabolism and catabolism. In addition, cellular processes that depend upon sphingolipids have been identified including protein trafficking/exocytosis, endocytosis and actin cytoskeleton dynamics, membrane microdomains, calcium signaling, regulation of transcription and translation, cell cycle control, stress resistance, nutrient uptake and aging. These will be summarized here along with new data not previously reviewed. Advances in our knowledge of sphingolipids and their roles in yeast are impressive but molecular mechanisms remain elusive and are a primary challenge for further progress in understanding the specific functions of sphingolipids.

Disruption of the nucleolus mediates stabilization of p53 in response to DNA damage and other stresses

p53 protects against cancer through its capacity to induce cell cycle arrest or apoptosis under a large variety of cellular stresses. It is not known how such diversity of signals can be integrated by a single molecule. However, the literature reveals that a common denominator in all p53-inducing stresses is nucleolar disruption. We thus postulated that the impairment of nucleolar function might stabilize p53 by preventing its degradation. Using micropore irradiation, we demonstrate that large amounts of nuclear DNA damage fail to stabilize p53 unless the nucleolus is also disrupted. Forcing nucleolar disruption by anti-upstream binding factor (UBF) microinjection (in the absence of DNA damage) also causes p53 stabilization. We propose that the nucleolus is a stress sensor responsible for maintenance of low levels of p53, which are automatically elevated as soon as nucleolar function is impaired in response to stress. Our model integrates all known p53-inducing agents and also explains cell cycle-related variations in p53 levels which correlate with established phases of nucleolar assembly/disassembly through the cell cycle.

Neutral ceramidase secreted by endothelial cells is released in part associated with caveolin-1

Neutral ceramidase (CDase) is a key enzyme of sphingomyelin (SM) metabolism implicated in cell signaling triggered by a variety of extracellular ligands. Previously it was shown that in murine endothelial cells a portion of neutral CDase is localized in detergent-resistant light membranes. In this study subcellular distribution of neutral CDase was further investigated. In accordance with the previous finding, the enzyme was identified in caveolae. Moreover, the same protein was detected in medium-speed supernatant of cell-conditioned medium, accounting for CDase activity measurable in the medium at neutral pH. Notably, these cells released also the caveolae-scaffolding protein caveolin-1 (cav-1). Interestingly, secreted neutral CDase and cav-1 coimmunoprecipitated. In addition, acid sphingomyelinase (SMase) activity was detectable in cav-1 immunocomplexes. These findings are consistent with the view that neutral CDase is released, in part, in association with cav-1 together with acid SMase. It remains to be established whether the here-identified secreted cav-1-enriched complex acts as platform to facilitate extracellular metabolism of SM.

Metabolism of the unnatural anticancer lipid safingol, L-threo-dihydrosphingosine, in cultured cells

We studied the metabolism of radioactively labeled safingol (l-threo-dihydrosphingosine) in primary cultured neurons, B104 neuroblastoma cells, and Swiss 3T3 fibroblasts, and compared it to that of its natural stereoisomer d-erythro-dihydrosphingosine. Both sphingoid bases are used as biosynthetic precursors for complex sphingolipids, albeit to different rates. Whereas a considerable amount of the natural sphingoid base is also directed to the catabolic pathway (20-66%, cell type dependent), only a minor amount of the nonnatural safingol is subjected to catabolic cleavage, most of it being N-acylated to the respective stereochemical variant of dihydroceramide. Interestingly, N-acylation of safingol to l-threo-dihydroceramide is less sensitive to fumonisin B1 than the formation of the natural d-erythro-dihydroceramide. In addition, safingol-derived l-threo-dihydroceramide, unlike its physiologic counterpart, is not desaturated. Most of it either accumulates in the cells (up to 50%) or is used as a biosynthetic precursor of the respective dihydrosphingomyelin (up to 45%). About 5% is, however, glucosylated and channeled into the glycosphingolipid biosynthetic pathway. Our results demonstrate that, despite its nonnatural stereochemistry, safingol is recognized and metabolized preferentially by enzymes of the sphingolipid biosynthetic pathway. Furthermore, our data suggest that the cytotoxic potential of safingol is reduced rather than enhanced via its metabolic conversion.

Total syntheses of symbioramide derivatives from l-serine and their antileukemic activities

Naturally occurring symbioramide, (2S,3R,2'R,3'E)-N-(2'-hydroxy-3'-octadecenoyl)-dihydrosphingosine 1a, was synthesized from d-erythro-dihydrosphingosine (amino part, 2) and (2R,3E)-2-hydroxy-3-octadecenoic acid (acid part, 3a), both of which were prepared from l-serine. Its diastereomer, (2S,3R,2'S,3'E)-1b, having an enantiomer of the unnatural-type acid part that was prepared from d-mannitol, and its corresponding (Z)-isomers, (2S,3R,2'R,3'Z)-1c and (2S,3R,2'S,3'Z)-1d, were also prepared. The antileukemic activities of 1a-d against HL-60 and L-1210 cells were appreciated by a MTT assay. None of the four symbioramide derivatives showed antileukemic activities in HL-60 cells. In L-1210 cells, all the symbioramide derivatives showed moderate antileukemic activities. Compound 1d had the most effective activity against L-1210 cells among the four derivatives. The data suggest that unnatural types of (2'S)-isomers of acid parts are more active than those of (2'R)-isomers.

Glycoinositol phospholipids from Trypanosoma cruzi transmit signals to the cells of the host immune system through both ceramide and glycan chains

Chagas' disease is a chronic disease affecting millions of people in Latin America. The cell surface of Trypanosoma cruzi, the etiological agent, is covered by a glycocalyx whose components play important roles in parasite survival and infectivity. The most abundant surface component is a glycolipid (glycoinositol phospholipid, GIPL) related in structure to glycosylphosphatidyl inositol anchors. In this review, we describe the biological effects of highly purified native GIPLs and their glycan or lipid moities on cells of the host immune system.

Dichotomy of Ca2+ signals triggered by different phospholipid pathways in antigen stimulation of human mast cells

Mast cell activation triggers Ca(2+) signals and the release of enzyme-containing granules, events that play a major role in allergic/hypersensitivity reactions. However, the precise molecular mechanisms that regulate antigen-triggered degranulation and Ca(2+) fluxes in human mast cells are still poorly understood. Here we show, for the first time, that a receptor can trigger Ca(2+) via two separate molecular mechanisms. Using an antisense approach, we show that IgE-antigen stimulation of human bone marrow-derived mast cells triggers a sphingosine kinase (SPHK) 1-mediated fast and transient Ca(2+) release from intracellular stores. However, phospholipase C (PLC) gamma1 triggers a second (slower) wave of calcium release from intracellular stores, and it is this PLCgamma1-generated signal that is responsible for Ca(2+) entry. Surprisingly, FcepsilonRI (a high affinity receptor for IgE)-triggered mast cell degranulation depends on the first, sphingosine kinase-mediated Ca(2+) signal. These two pathways act independently because antisense knock down of either enzyme does not interfere with the activity of the other enzyme. Of interest, similar to PLCgamma1, SPHK1 translocates rapidly to the membrane after FcepsilonRI cross-linking. Here we also show that SPHK1 activity depends on phospholipase D1 and that FcepsilonRI-triggered mast cell degranulation depends primarily on the activation of both phospholipase D1 and SPHK1.

Fumonisin B(1) induces apoptosis in LLC-PK(1) renal epithelial cells via a sphinganine- and calmodulin-dependent pathway

Fumonisins are a family of mycotoxins produced by Fusarium moniliforme, which is the most common mold found on corn throughout the world. These compounds are both toxic and carcinogenic for animals, and perhaps humans, with the kidney being the most sensitive organ to fumonisin toxicity. The molecular mechanism of fumonisin toxicity appears to involve disruption of de novo biosynthesis of sphingolipids and accumulation of sphinganine. The goals of this study were to determine whether fumonisin B(1) kills LLC-PK(1) renal kidney epithelial cells by inducing apoptosis and to identify genes affected by sphinganine that mediate fumonisin B(1)-induced cell death. Fumonisin B(1) produced morphological changes (i.e., cell shrinkage, membrane blebbing) and time-dependent increases in DNA fragmentation demonstrating that the toxin induces apoptosis. Simultaneously, fumonisin B(1) blocked sphingolipid biosynthesis and caused accumulation of sphinganine. To further investigate the role of sphinganine in fumonisin B(1)-induced apoptosis, beta-fluoroalanine (betaFA) was used to inhibit serine palmitoyltransferase, which catalyzes an earlier step in the sphingolipid biosynthetic pathway. betaFA blocked sphinganine accumulation and prevented fumonisin B(1)-induced DNA fragmentation, confirming that apoptosis induced by fumonisin B(1) is dependent upon accumulation of sphinganine. To examine gene expression, differential display reverse transcriptase polymerase chain reaction (DDRT-PCR) was applied to RNA isolated after 16 h of exposure to fumonisin B(1). Differential expression in response to fumonisin B(1) of a gene identified as calmodulin has been verified by Northern analysis. Sphinganine appears to mediate the effect because betaFA reduces induction of calmodulin mRNA by fumonisin B(1). Fumonisin B(1) also increases calmodulin protein in a concentration-dependent manner and the calmodulin antagonist W7 blocks fumonisin B(1)-induced DNA fragmentation, supporting a role for calmodulin in fumonisin B(1)-induced apoptosis. In contrast, fumonisin B(1) had no effect on expression of bcl-2 family genes (bax, bcl-2, and bcl-x). These findings demonstrate that fumonisin B(1) kills LLC-PK(1) kidney cells by inducing apoptosis. Further, the results establish a sequence of events for fumonisin B(1)-induced apoptosis involving initial disruption of sphingolipid metabolism and accumulation of sphinganine (or a metabolite), which, in turn, induces expression of calmodulin.

Interleukin-1beta induces chronic activation and de novo synthesis of neutral ceramidase in renal mesangial cells

The lipid signaling molecule ceramide is formed by the action of acid and neutral sphingomyelinases and degraded by acid and neutral ceramidases. Short-term stimulation of mesangial cells with the pro-inflammatory cytokine interleukin-1beta (IL-1beta) leads to a rapid and transient increase in neutral sphingomyelinase activity (Kaszkin, M., Huwiler, A., Scholz, K., van den Bosch, H., and Pfeilschifter, J. (1998) FEBS Lett. 440, 163-166). In this study, we report on a second delayed peak of activation occurring after hours of IL-1beta treatment. This second phase of activation was first detectable after 2 h of treatment and steadily increased over the next 2 h, reaching maximal values after 4 h. In parallel, a pronounced increase in neutral ceramidase activity was observed, accounting for a constant or even decreased level of ceramide after long-term IL-1beta treatment, despite continuous sphingomyelinase activation. The increase in neutral ceramidase activity was due to expressional up-regulation, as detected by an increase in mRNA levels and enhanced de novo protein synthesis. The increase in neutral ceramidase protein levels and activity could be blocked dose- dependently by the p38 MAPK inhibitor SB 202190, whereas the classical MAPK pathway inhibitor U0126 and the protein kinase C inhibitor Ro 318220 were ineffective. Moreover, cotreatment of cells for 24 h with IL-1beta and SB 202190 led to an increase in ceramide formation. Interestingly, IL-1beta-stimulated neutral ceramidase activation was not reduced in mesangial cells isolated from mice deficient in MAPK-activated protein kinase-2, which is a downstream substrate of p38 MAPK, thus suggesting that the p38 MAPK-mediated induction of neutral ceramidase occurs independently of the MAPK-activated protein kinase-2 pathway. In summary, our results suggest a biphasic regulation of sphingomyelin hydrolysis in cytokine-treated mesangial cells with delayed de novo synthesis of neutral ceramidase counteracting sphingomyelinase activity and apoptosis. Neutral ceramidase may thus represent a novel cytoprotective enzyme for mesangial cells exposed to inflammatory stress conditions.

The total syntheses of D-erythro-sphingosine, N-palmitoylsphingosine (ceramide), and glucosylceramide (cerebroside) via an azidosphingosine analog

The total synthesis of D-erythro-sphingosine (9) was performed by a chirospecific method starting from D-galactose via an azidosphingosine intermediate to give highly homogeneous (>99.9% C18:1) sphingosine base (9) which contained no observable olefin isomerization by product and was demonstrated to be optically pure by a novel method utilizing Mosher's acid. Ceramide (10) was prepared from this sphingosine (9) with highly homogeneous (99.8% C16:0) palmitic acid by two methods. The cerebroside glucosylceramide (23) was the next sphingolipid in this series to be synthesized in a highly homogeneous form. These three sphingolipids are currently being used for biophysical studies of the structures of their hydrated bio-molecular assemblies.

Activation of a p53-independent, sphingolipid-mediated cytolytic pathway in p53-negative mouse fibroblast cells treated with N-methyl-N-nitro-N-nitrosoguanidine

Sphingolipids such as ceramide are important mediators of apoptosis and growth arrest triggered by ligands such as tumor necrosis factor and Fas-L binding to their receptors. When LM (expressing p53) and LME6 (lacking p53) cells were exposed to the genotoxin N-methyl-N-nitro-N-nitrosoguanidine (MNNG), both cell lines underwent cytolysis in a very similar manner, suggesting the presence of a p53-independent apoptotic response to this genotoxic stress. To determine whether sphingolipids such as ceramide might serve as mediators in this system, the responses of these cells to exogenous sphingolipids as well as their changes in endogenous sphingolipid levels after DNA damage were examined. Treatment with exogenous C2-ceramide and sphingosine led to cell death in both LM and LME6, and treatment of the LME6 cells with MNNG resulted in a transient increase in intracellular ceramide of approximately 50% over a period of 3 h. Finally, treatment with the de novo inhibitor of ceramide synthesis ISP-1 protected LME6 cells from MNNG-triggered cell death. This MNNG-triggered induction of ceramide was not observed in the p53-expressing LM cells, suggesting that it may be down-regulated by p53. Although ceramide-mediated cell death can proceed in the absence of p53, exogenously added C2-ceramide increased the cellular p53 level in LM cells, suggesting that the two pathways do interact.

Sphingosine 1-phosphate: synthesis and release

Sphingosine 1-phosphate (Sph-1-P) is a bioactive sphingolipid, acting both as an intracellular second messenger and extracellular mediator, in mammalian cells. In cell types where Sph-1-P acts as an intracellular messenger, stimulation-dependent synthesis of Sph-1-P, possibly resulting from sphingosine (Sph) kinase activation, is essential. Since this important kinase has recently been cloned, precise regulation of intracellular Sph-1-P synthesis will be clarified in the near future. As an intercellular mediator, elucidation of sources for extracellular Sph-1-P is important, in addition to identification of the cell surface receptors for this phospholipid. Blood platelets are very unique in that they store Sph-1-P abundantly (possibly due to the existence of highly active Sph kinase and a lack of Sph-1-P lyase) and release this bioactive lipid extracellularly upon stimulation. It is likely that platelets are an important source for extracellular Sph-1-P, especially for plasma and serum Sph-1-P. Platelet-derived Sph-1-P seems to play an important role in vascular biology.

Role of sphingosine in the tumor necrosis factor alpha stimulatory effect on lactate dehydrogenase A expression and activity in porcine Sertoli cells

In this study, the intracellular signaling mechanisms through which TNFalpha increases LDH(A4) activity/expression in primary cultures of porcine testicular Sertoli cells were investigated. Studies were focused on sphingomyelin hydrolysis pathway. Treatment of [(14)C]serine-labeled cells with TNFalpha (15 ng/ml, 0.8 nM) resulted in a transient decrease (approximately 20%) in cellular [(14)C]sphingomyelin and in an increase (approximately 27%) in [(14)C]sphingosine that remained elevated for at least 75 min. In the same experiments, no significant changes were detected in ceramide levels. Exogenous sphingosine stimulated LDH(A4) activity and LDHA expression in a dose-dependent manner (ED(50) = 8 microM of sphingosine). Such an increase in LDHA messenger RNA levels and LDH(A4) activity was detected at 24 h and was maximal after 48 h of treatment. Kinetically, the increase in LDH(A4) activity was similar whether Sertoli cells were treated with sphingosine (12 microM) or with TNFalpha (20 ng/ml). Although sphingosine mimicked the action of TNFalpha on Sertoli cells LDH(A4) activity and expression, the maximal stimulatory effect represented about 30% of TNFalpha maximal activity. Sphingomyelinase, C2 ceramide, sphingosine 1-phosphate, N, N-dimethylsphingosine, and phosphorylcholine had no significant effect on LDHA expression/LDH(A4) activity. Exogenous C2 ceramide increased LDH(A4) activity only in cytokine-treated cells, suggesting its involvement as sphingosine precursor in TNFalpha-stimulated LDH(A4) activity via the sphingomyelin hydrolysis pathway. The LDH(A4) activity stimulated by TNFalpha was decreased by 36.2% by an inhibitor of sphingosine formation, NH4Cl (4 mM), supporting a role of sphingosine in the TNFalpha effect. Moreover, bisindolylmaleimide (100 nM), a protein kinase C (PKC) inhibitor decreased significantly by 28.7% the TNFalpha effect on LDH(A4) activity but had no effect on the stimulating action of sphingosine, suggesting that if PKC is involved in TNFalpha action, the sphingosine effect on LDH(A4) is unrelated to the PKC activity or inhibition. Together, the present data suggest that in primary Sertoli cell cultures, TNFalpha stimulating action on LDHA expression is partly exerted via sphingomyelin hydrolysis pathway, sphingosine being the active metabolite.

Separation and purification of sphingomyelin diastereomers by high-performance liquid chromatography

All naturally occurring sphingomyelins have the d-erythro-(2S,3R) configuration of the sphingoid base. We have developed a normal-phase HPLC method for the separation of this natural stereoisomer from the l-threo-sphingomyelin, which is the other stereoisomer commonly present in semisynthetic preparations of acyl-chain defined sphingomyelins. The chromatographic method was developed by modification of a previously reported method for phospholipid separation on a normal-phase diol column. The separation was accomplished by a binary gradient of solvent mixtures (A) hexane:isopropanol:acetic acid (82:17:1.0 by vol) and (B) isopropanol:water:acetic acid (85:14:1.0 by vol) with 0.08 vol% triethylamine added to both solvent mixtures. The program of gradient elution was optimized for maximal separation of sphingomyelin diastereomers. For detection of the lipids, a light-scattering detector was used. This analytical scale HPLC method was also used for purification of the stereoisomers (up to 0.5 mg of N-oleoyl-sphingomyelin in a single injection). The purified stereoisomers were at least 99% pure according to high-performance thin-layer chromatography and analytical HPLC.

Sphingomyelin metabolites in vascular cell signaling and atherogenesis

The atherosclerotic lesion most probably develops through a number of cellular events which implicate all vascular cell types and include synthesis of extracellular proteins, cell proliferation, differentiation and death. Sphingolipids and sphingolipid metabolizing enzymes may play important roles in atherogenesis, not only because of lipoprotein alterations but also by mediating a number of cellular events which are believed to be crucial in the development of the vascular lesions such as proliferation or cell death. Exogenous sphingolipids may mediate various biological effects such as apoptosis, mitogenesis or differentiation depending on the cell type. Moreover, several molecules present in the atherogenic lesion, such as oxidized LDL, growth factors or cytokines, which activate intracellular signaling pathways leading to vascular cell modifications, can stimulate sphingomyelin hydrolysis and generation of ceramide (and other metabolites as sphingosine-1-phosphate). Here we review the potential implication of the sphingomyelin/ceramide cycle in vascular cell signaling related to atherosclerosis, and more generally the role of sphingolipids in the events observed during the atherosclerotic process as cell differentiation, migration, adhesion, retraction, proliferation and death.

Tumor necrosis factor-alpha-mediated signal transduction in human neutrophils: involvement of sphingomyelin metabolites in the priming effect of TNF-alpha on the fMLP-stimulated superoxide production

We investigated the mechanism underlying the priming effect of TNF-alpha on fMLP-stimulated superoxide production in human neutrophils. TNF-alpha enhanced fMLP-stimulated superoxide production in a concentration-dependent manner. TNF-alpha also induced sphingomyelin (SM) hydrolysis and increased the formation of its metabolite, sphingosine-1-phosphate (SP-1-P). The treatment of neutrophils with sphingomyelinase also resulted in a similar priming effect. C2 ceramide produced a concentration-dependent inhibition of fMLP-stimulated superoxide production within the concentration range of 1-30 microM. Sphingosine had a dual effect on fMLP-stimulated superoxide generation, exhibiting a priming effect at lower concentrations (0.2-1 microM), but an inhibitory effect at higher concentrations (1-30 microM). SP-1-P (1-30 microM), showed a concentration-dependent enhancement of fMLP stimulated superoxide production. Furthermore, after treating neutrophils with DL-threo-dihydro-sphingosine, a competitive inhibitor of sphingosine kinase, TNF-alpha produced a similar dual effect as observed with sphingosine. These results strongly suggest that SM hydrolysis plays a key role in the intracellular signal transduction mediating the TNF-alpha-mediated priming effect.

Advances in the signal transduction of ceramide and related sphingolipids

Recently, the sphingolipid metabolites ceramide, sphingosine, ceramide 1-P, and sphingosine 1-P have been implicated as second messengers involved in many different cellular functions. Publications on this topic are appearing at a rapidly increasing rate and new developments in this field are also appearing rapidly. It is thus important to summarize the results obtained from many different laboratories and from different fields of research to obtain a clearer picture of the importance of sphingolipid metabolites. This article reviews the studies from the last few years and includes the effects of a variety of extracellular agents on sphingolipid signal transduction pathways in different tissues and cells and on the mechanisms of regulation. Sphingomyelin exists in a number of functionally distinct pools and is composed of distinct molecular species. Sphingomyelin metabolites may be formed by many different pathways. For example, the generation of ceramide from sphingomyelin can be catalyzed by at least five different sphingomyelinases. A large variety of stimuli can induce the generation of ceramide, leading to activation or inhibition of various cellular events such as proliferation, differentiation, apoptosis, and inflammation. The effect of ceramide on these physiological processes is due to its many different downstream targets. It can activate ceramide-activated protein kinases and ceramide-activated protein phosphatases. It also activates or inhibits PKCs, PLD, PLA2, PC-PLC, nitric oxide synthase, and the ERK and SAPK/JNK signaling cascades. Ceramide activates or inhibits transcription factors, modulates calcium homeostasis and interacts with the retinoblastoma protein to regulate cell cycle progression. Most of the work in this field has involved the study of ceramide effects, but the roles of the other three sphingomyelin metabolites is now attracting much attention. The complex interactions between signaling components and ceramide and the controls regulating these interactions are now being identified and are presented in this review.

Analysis of natural and synthetic sphingomyelins using high-performance thin-layer chromatography

The chromatographic behaviour of molecular species of sphingomyelin on HPTLC was investigated. Sphingomyelin gave a double band pattern on HPTLC plates developed using chloroform/methanol/acetic acid/water (25 : 15 : 4 : 2, v/v) or chloroform/methanol/water (25 : 10 : 1.1, v/v). HPTLC analysis of acyl chain-defined sphingomyelins showed that the Rf values increased linearly with the length of the N-linked acyl chain. A double-banded pattern was therefore seen for natural sphingomyelins with a bimodal fatty acid composition. Racemic sphingomyelins also gave a double band pattern on HPTLC, where the lower band represented the Derythro and the upper band the Lthreo isomer. We also showed that Derythro-N-16:0-dihydrosphingomyelin migrated faster on HPTLC than Derythro-N-16:0-sphingomyelin. The upper and lower band sphingomyelins from two different cell lines (human skin fibroblasts and baby hamster kidney cells) were separately scraped off the HPTLC plates and the fatty acid and long-chain base profiles were studied using GC-MS. The lower bands contained short-chain fatty acids and most of the fatty acids in the upper bands were long. The predominant long-chain base was sphingosine, which was found in both upper and lower bands, but sphinganine was found only in the upper bands. To conclude, there are at least three possible reasons for the sphingomyelin double bands on HPTLC; acyl chain length, long-chain base composition and stereochemistry. These reasons might sometimes overlap and, therefore, HPTLC alone is insufficient for complete analysis of the molecular species of sphingomyelin.

Metabolism and functional effects of sphingolipids in blood cells

We examined the sphingolipid metabolism of peripheral blood cells, i. e. platelets, erythrocytes, neutrophils and mononuclear cells. A distinguishing characteristic of sphingolipid metabolism in these highly differentiated cells was their high sphingosine (Sph) kinase activity. The occurrence of [3H]sphingosine 1-phosphate (Sph-1-P) from [3H]Sph (actively incorporated from the outside) in the blood cells was strong, long-lasting, and independent of cell activation. Hence, the possibility of Sph-1-P playing a second messenger role is remote in these cells. About 40% of platelet Sph-1-P could be released extracellularly by 12-O-tetradecanoylphorbol 13-acetate, possibly through mediation by protein kinase C. On the other hand, in erythrocytes, neutrophils and mononuclear cells a significant percentage of Sph-1-P formed inside the cell was discharged without stimulation, whereas the stimulation-dependent release was marginal. In contrast to active [3H]Sph conversion to [3H]Sph-1-P, formation of [3H]sphingomyelin was barely detectable in the blood cells; this was especially true for anucleate platelets and erythrocytes. The Sph --> Sph-1-P pathway may become predominant over the Sph --> Cer --> sphingomyelin pathway during late-stage differentiation into platelets or erythrocytes. Sph and its methylated derivative, N, N-dimethylsphingosine, induced apoptosis not only in neutrophils but also in mononuclear cells, whereas Sph-1-P elicited Ca2+ mobilization in platelets. Our results suggest that all blood cells may remove plasma Sph, which is harmful or suppressive to cellular functions, and change it into Sph-1-P, acting as the source of plasma Sph-1-P, which may play a variety of important roles in blood vessels.

The proximal portion of the COOH terminus of the oxytocin receptor is required for coupling to g(q), but not g(i). Independent mechanisms for elevating intracellular calcium concentrations from intracellular stores

As the oxytocin receptor plays a key role in parturition and lactation, there is considerable interest in defining its structure/functional relationships. We previously showed that the rat oxytocin receptor transfected into Chinese hamster ovary cells was coupled to both G(q/11) and G(i/o), and that oxytocin stimulated ERK-2 phosphorylation and prostaglandin E(2) synthesis via protein kinase C activity. In this study, we show that deletion of 51 amino acid residues from the carboxyl terminus resulted in reduced affinity for oxytocin and a corresponding rightward shift in the dose-response curve for oxytocin-stimulated [Ca(2+)](i). However, oxytocin-stimulated ERK-2 phosphorylation and prostaglandin E(2) synthesis did not occur in cells expressing the truncated receptor. Oxytocin also failed to increase phospholipase A activity or activate protein kinase C, indicating that the mutant receptor is uncoupled from G(q)-mediated pathways. The Delta51 receptor is coupled to G(i), as oxytocin-stimulated Ca(2+) transients were inhibited by pertussis toxin, and a Gbetagamma sequestrant. Preincubation of Delta51 cells with the tyrosine kinase inhibitor, genistein, also blocked the oxytocin effect. A Delta39 mutant had all the activities of the wild type oxytocin receptor. These results show that the portion between 39 and 51 residues from the COOH terminus of the rat oxytocin receptor is required for interaction with G(q/11), but not G(i/o). Furthermore, an increase in intracellular calcium was generated via a G(i)betagamma-tyrosine kinase pathway from intracellular stores that are distinct from G(q)-mediated inositol trisphosphate-regulated stores.

Costimulatory action of glycoinositolphospholipids from Trypanosoma cruzi: increased interleukin 2 secretion and induction of nuclear translocation of the nuclear factor of activated T cells 1

The effects of the glycoinositolphospholipids (GIPLs) fromTrypanosoma cruzi on T lymphocyte activation were investigated in a mouse T cell hybridoma (DO-11.10). Purified GIPLs from T. cruzi strains Y and G markedly increased IL-2 mRNA transcripts and IL-2 secretion induced by mitogenic anti-CD3 and anti-Thy1 mAbs. This costimulatory function was also revealed by the induction of IL-2 secretion after the simultaneous addition of the T. cruzi GIPLs and either the calcium ionophore A23187 or phorbol ester. The capacity of the GIPL molecule to induce an increase in cytoplasmic calcium levels was also demonstrated. After exposure of T cell hybridoma to GIPL, the nuclear transcription factor NFAT1 became partially dephosphorylated, and its nuclear localization was demonstrated both in the T cell hybridoma and in Balb/c CD3(+) cells. These results demonstrate that T. cruzi GIPL molecules are capable of signaling to T cells and therefore could be valuable tools for the study of T cell activation, besides playing a potential role in subverting the T lymphocyte immune response during T. cruzi infection.

Formyl peptide receptor signaling in HL-60 cells through sphingosine kinase

Sphingosine-1-phosphate (SPP) produced from sphingosine by sphingosine kinase has recently been reported to act as intracellular second messenger for a number of plasma membrane receptors. In the present study, we investigated whether the sphingosine kinase/SPP pathway is involved in cellular signaling of the Gi protein-coupled formyl peptide receptor in myeloid differentiated human leukemia (HL-60) cells. Receptor activation resulted in rapid and transient production of SPP by sphingosine kinase, which was abolished after pertussis toxin treatment. Direct activation of heterotrimeric G proteins by AlF4- also rapidly increased SPP formation in intact HL-60 cells. In cytosolic preparations of HL-60 cells, sphingosine kinase activity was stimulated by the stable GTP analog, guanosine 5'-O-(3-thiotriphosphate). Inhibition of sphingosine kinase by DL-threo-dihydrosphingosine and N,N-dimethylsphingosine did not affect phospholipase C stimulation and superoxide production but markedly inhibited receptor-stimulated Ca2+ mobilization and enzyme release. We conclude that the formyl peptide receptor stimulates through Gi-type G proteins SPP production by sphingosine kinase, that the enzyme is also stimulated by direct G protein activation, and that the sphingosine kinase/SPP pathway apparently plays an important role in chemoattractant signaling in myeloid differentiated HL-60 cells.

Ca2+ signaling induced by sphingosylphosphorylcholine and sphingosine 1-phosphate via distinct mechanisms in rat glomerular mesangial cells

BACKGROUND

To elucidate the molecular mechanism underlying sphingosine 1-phosphate (S1P) and sphingosylphosphorylcholine (SPC) mediated signaling, we compared their effects with those of adenosine triphosphate (ATP) and angiotensin II (Ang II) on the cytosolic free Ca2+ concentration ([Ca2+]i), inositol 1,4, 5-trisphosphate (IP3) generation and arachidonic acid release in rat glomerular mesangial cells.

METHODS

The fluorescent Ca2+ indicator, Fura-2, was used to measure the [Ca2+]i changes in cultured rat glomerular mesangial cells either in suspension or attached to the coverslips.

RESULTS

SPC 5 microM, S1P 5 microM, ATP 100 microM and Ang II 90 nM all induced increases in the [Ca2+]i, and the effect showed marked homologous desensitization, while heterologous desensitization was less. After the initial exposure of the cells to SPC, the increase in [Ca2+]i induced by subsequent addition of ATP or Ang II was only reduced by about 14.3% and 4.8%, respectively. After the initial exposure to S1P, a greater reduction was seen (42. 1% and 47.7%, respectively). Both arachidonic acid release and IP3 generation were activated by all four agonists with an identical rank order of effectiveness of SPC >> S1P > ATP = Ang II; both were pertussis toxin-sensitive and cholera toxin-resistant. The arachidonic acid release induced by all four agonists showed identical susceptibility to removal of extracellular Ca2+, whereas IP3 generation displayed differential extracellular Ca2+ dependence. Only SPC-induced IP3 generation was highly sensitive to extracellular Ca2+ level, and this Ca2+ dependence was abolished after pretreatment of cells with arachidonyl trifluoromethyl ketone (AACOCF3), a phospholipase A2 inhibitor. Furthermore, the Mn2+ influx was markedly greater in SPC-stimulated cells than in either control or other agonist-stimulated cells, and was decreased by prior exposure of cells to AACOCF3. After phospholipase A2 was inhibited or in the absence of extracellular Ca2+, SPC displayed identical effectiveness as S1P on desensitizing the action of ATP or Ang II on the increase in [Ca2+]i. Conclusions. Our results indicate that all four agents primarily activate phospholipase C through their receptor occupancies, but that SPC alone also induces further significant Mn2+ influx and IP3 generation attributable to its primary stimulatory effect on arachidonic acid release. Thus, the heterologous desensitization to ATP or Ang II induced by SPC was less profound than that induced by S1P, since SPC induced a Ca2+ influx.

Phosphorylated cis-4-methylsphingosine mimics the mitogenic effect of sphingosine-1-phosphate in Swiss 3T3 fibroblasts

The phosphorylated derivative of sphingosine, sphingosine-1-phosphate, is a short-living metabolite of ultimate ceramide degradation and was shown to act as an intracellular signaling molecule, stimulating cell proliferation in quiescent Swiss 3T3 fibroblasts and inducing the release of calcium from intracellular stores (Zhang, H., Desai, N. N., Olivera, A., Seki, T., Brooker, G., and Spiegel, S. (1991) J. Cell. Biol. 114, 155-167). In the present study, 24-h treatment of Swiss 3T3 fibroblasts with the synthetic sphingosine analogue cis-4-methylsphingosine resulted in proliferation of quiescent Swiss 3T3 fibroblasts that was 3-fold stronger than that of equimolar sphingosine-1-phosphate. The phosphorylated derivative of cis-4-methylsphingosine accumulated drastically in the cells. Simultaneous treatment with the sphingosine kinase inhibitor L-threo-sphinganine reduced both the amount of phosphorylated cis-4-methylsphingosine and cell proliferation induced by this compound by about 50%, indicating that the phosphorylated derivative mediated the proliferative stimulus. The mitogenic effect of cis-4-methylsphingosine was associated with a mobilization of intracellular calcium in Swiss 3T3 fibroblasts that was similar to that induced by sphingosine-1-phosphate. The results demonstrate that the phosphorylated derivative of cis-4-methylsphingosine mimics the previously reported mitogenic action of sphingosine-1-phosphate in Swiss 3T3 cells, and the stronger effect most likely corresponds to the unusual accumulation of this compound.

Discrimination between plasma membrane and intracellular target sites of sphingosylphosphorylcholine

On the background of the emerging concept of G protein-coupled sphingolipid receptors, Ca2+ mobilization by sphingosylphosphorylcholine (SPPC) in intact cells and SPPC-induced Ca2+ release in permeabilized cells, both occurring at similar, micromolar concentrations, were characterized and compared. In intact human embryonic kidney (HEK-293) cells, SPPC rapidly increased [Ca2+]i by mobilization of Ca2+ from thapsigargin-sensitive stores. In saponin-permeabilized HEK-293 cells, SPPC released stored Ca2+, in a manner similar to but independent of inositol 1,4,5-trisphosphate. Only the action of SPPC on intact cells, but not that in permeabilized cells, was, at least in part, sensitive to pertussis toxin. In addition and most important, Ca2+ release by SPPC in permeabilized cells was not stereoselective, whereas in intact cells only the naturally occurring D-erythro-SPPC, but not L-threo-SPPC, increased [Ca2+]i. Stereoselectivity of SPPC-induced [Ca2+]i increase was also demonstrated in bovine aortic endothelial cells. In conclusion, Ca2+ mobilization by SPPC in intact cells is independent of the previously described SPPC-gated Ca2+ channel on endoplasmic reticulum but probably mediated by a membrane sphingolipid receptor. Thus, SPPC can regulate Ca2+ homeostasis by acting apparently at two cellular targets, which exhibit clearly distinct recognition patterns.

Purification and characterization of rat kidney sphingosine kinase

Sphingosine kinase catalyzes the formation of the bioactive sphingolipid metabolite sphingosine 1-phosphate, which plays important roles in numerous physiological processes, including growth, survival, and motility. We have purified rat kidney sphingosine kinase 6 x 10(5)-fold to apparent homogeneity. The purification procedure involved ammonium sulfate precipitation followed by chromatography on an anion exchange column. Partially purified sphingosine kinase was found to be stabilized by the presence of high salt, and thus, a scheme was developed to purify sphingosine kinase using sequential dye-ligand chromatography steps (since the enzyme bound to these matrices even in the presence of salt) followed by EAH-Sepharose chromatography. This 385-fold purified sphingosine kinase bound tightly to calmodulin-Sepharose and could be eluted in high yield with EGTA in the presence of 1 M NaCl. After concentration, the calmodulin eluate was further purified by successive high pressure liquid chromatography separations on hydroxylapatite, Mono Q, and Superdex 75 gel filtration columns. Purified sphingosine kinase has an apparent molecular mass of approximately 49 kDa under denaturing conditions on SDS-polyacrylamide gel, which is similar to the molecular mass determined by gel filtration, suggesting that the active form is a monomer. Sphingosine kinase shows substrate specificity for D-erythro-sphingosine and does not catalyze the phosphorylation of phosphatidylinositol, diacylglycerol, ceramide, DL-threo-dihydrosphingosine, or N,N-dimethylsphingosine. However, the latter two sphingolipids were potent competitive inhibitors. With sphingosine as substrate, the enzyme had a broad pH optimum of 6.6-7.5 and showed Michaelis-Menten kinetics, with Km values of 5 and 93 microM for sphingosine and ATP, respectively. This study provides the basis for molecular characterization of a key enzyme in sphingolipid signaling.

FcgammaRI coupling to phospholipase D initiates sphingosine kinase-mediated calcium mobilization and vesicular trafficking

Aggregation of receptors specific for the constant region of immunoglobulin G activates a repertoire of monocyte responses that can lead ultimately to targeted cell killing via antibody-directed cellular cytotoxicity. The high affinity receptor, FcgammaRI, contains no recognized signaling motif in its cytoplasmic tail but rather utilizes the gamma-chain of FcepsilonRI as an accessory molecule to recruit tyrosine kinases for signal transduction. We show here that, in a human monocytic cell line primed with interferon-gamma, FcgammaRI mobilizes intracellular calcium stores using a novel pathway that involves tyrosine kinase coupling to phospholipase D and resultant downstream activation of sphingosine kinase. Moreover, FcgammaRI is not coupled to phospholipase C; hence, calcium release from intracellular stores occurred in the absence of any measurable rise in inositol triphosphate. Finally, as this novel activation pathway is also shown to be responsible for mediating the vesicular trafficking of internalized immune complexes for degradation, it is likely to play a key role in controlling intracellular events triggered by FcgammaRI.

A GTPase-independent mechanism of p21-activated kinase activation. Regulation by sphingosine and other biologically active lipids

p21-activated kinases (PAKs) are serine/threonine kinases that have been identified as targets for the small GTPases Rac and Cdc42. PAKs have been implicated in cytoskeletal regulation, stimulation of mitogen-activated protein kinase cascades, and in control of the phagocyte NADPH oxidase. Membrane targeting of PAK1 induced increased kinase activity in a GTPase-independent manner, suggesting that other mechanisms for PAK regulation exist. We observed concentration- and time-dependent activation of PAK1 by sphingosine and several related long chain sphingoid bases but not by ceramides or a variety of other lipids. Although phospholipids were generally ineffective, phosphatidic acid and phosphatidylinositol also had stimulatory effects on PAK1. Lipid stimulation induced a similar level of PAK1 activity as did stimulation by GTPases, and the patterns of PAK1 autophosphorylation determined after partial tryptic digestion and two-dimensional peptide analysis were similar with each class of activator. Lipid stimulation of PAK1 activity was dependent upon intact PAK kinase activity, as indicated by studies with a kinase-dead PAK1 mutant. Treatment of COS-7 cells expressing wild type PAK1 with sphingosine, fumonisin B, or sphingomyelinase, all of which are able to elevate the levels of free sphingosine, induced increased activity of PAK1 as determined using a p47(phox) peptide substrate. Studies using PAK1 mutants suggest that lipids act at a site overlapping or identical to the GTPase-binding domain on PAK. The inactive sphingosine derivative N,N-dimethylsphingosine was an effective inhibitor of PAK1 activation in response to either sphingosine or Cdc42. Our results demonstrate a novel GTPase-independent mechanism of PAK activation and, additionally, suggest that PAK(s) may be important mediators of the biological effects of sphingolipids.

Differential effects of sphingomyelinase and cell-permeable ceramide analogs on proliferation of Swiss 3T3 fibroblasts

Metabolites of sphingomyelin, ceramide and sphingosine, have previously been implicated in cell growth regulation. Here we show that cell-permeable ceramide analogs and treatment with sphingomyelinase, which hydrolyzes sphingomyelin located on the outer leaflet of the bilayer, increase the progression of quiescent Swiss 3T3 fibroblasts through the S phase of the cell cycle leading to an increase in cell division. Although both potentiate the mitogenic effects of several growth factors [14], sphingomyelinase treatment antagonized the mitogenic effect of the tumor promoter, 12-O-tetradecanoylphorbol 13-acetate (TPA), while ceramide analogs had no effect, and sphingosine, a further metabolite of ceramide, potentiated the mitogenic effect of TPA. Concomitantly, sphingomyelinase, but not ceramide analogs, blunted the rapid increase in membrane-associated protein kinase C (PKC) activity induced by TPA without affecting the translocation of PKC alpha, delta, epsilon or zeta isoforms. Moreover, in contrast to sphingosine which activates phospholipase D (PLD) leading to an increase in phosphatidic acid levels, sphingomyelinase, but not ceramide analogs, reduced TPA-stimulated PLD activity. Our results suggest that the signaling pathways utilized by sphingomyelinase differ from those of cell-permeable ceramide analogs, and both act differently than sphingosine. The differential effects of exogenous short-chain ceramide analogs and sphingomyelinase call for caution in using these analogs as tools to study the role of ceramide in diverse cellular functions.

cis-4-Methylsphingosine decreases sphingolipid biosynthesis by specifically interfering with serine palmitoyltransferase activity in primary cultured neurons

The effect of six different structurally modified sphingosine analogues on biosynthesis of sphingolipids was studied in primary cultured murine cerebellar neurons. Treatment of cells with cis-4-methylsphingosine at micromolar levels resulted in a markedly decreased sphingolipid biosynthesis, whereas the other compounds examined, trans-4-methylsphingosine, cis-5-methylsphingosine, trans-5-methylsphingosine, cis-sphingosine, and 1-deoxysphingosine, inhibited sphingolipid biosynthesis less efficiently. The inhibition of sphingolipid biosynthesis by the various compounds was paralleled by a decrease of serine palmitoyltransferase activity in situ. For cis-4-methylsphingosine the inhibitory effect on serine palmitoyltransferase activity was shown to be concentration- and time-dependent. Half-maximal reduction of enzyme activity occurred after 24 h of treatment with 10 microM of the compound. The activity of other enzymes of sphingolipid biosynthesis as well as phospholipid and protein biosynthesis was not affected. Analysis of the sphingoid moiety of cellular sphingolipids suggests that the sphingosine analogues listed above were subject to degradation rather than being utilized as precursors for sphingolipid biosynthesis by cultured neurons. Except of 1-deoxysphingosine, the other five sphingosine analogues were shown to be substrates for sphingosine kinase in vitro. After 24 h of treatment of primary cerebellar neurons with the various sphingosine analogues the relative percentage of the respective intracellular 1-phosphate derivatives paralleled exactly the inhibitory effect on serine palmitoyltransferase activity observed when cells were treated with the unphosphorylated compounds. In contrast to the respective 1-phosphate derivatives of the other methyl-branched sphingosine analogues examined, cis-4-methylsphingosine 1-phosphate showed an intracellular accumulation suggesting a delayed turnover rate in cultured murine neurons for this compound. These results suggest that the inhibitory effect of the sphingosine analogues on serine palmitoyltransferase is mediated by their respective 1-phosphate derivatives and that the pronounced effect of cis-4-methylsphingosine is caused by a high intracellular concentration of cis-4-methylsphingosine 1-phosphate. cis-4-Methylsphingosine, in addition, caused drastic changes in cell morphology of primary cerebellar neurons, which were not observed when these cells were treated with one of the other sphingosine analogues examined.

Modulation of skeletal muscle Ca2(+)-release channel activity by sphingosine

The effect of D-erythro-C18-sphingosine (sphingosine) and related compounds on the Ca(2+)-release channel (ryanodine binding protein) was examined on rabbit skeletal muscle membranes, on the purified ryanodine binding protein, and on the channel reconstituted into planar lipid bilayers. Sphingosine inhibited [3H]ryanodine binding to sarcoplasmic reticulum (SR) membranes in a dose-dependent manner similar to published results (R. A. Sabbadini, R. Betto, A. Teresi, G. Fachechi-Cassano, and G. Salviati. J. Biol. Chem. 267: 15475-15484, 1992). The sphingolipid also inhibited [3H]ryanodine binding to the purified ryanodine binding protein. Our results demonstrate that the inhibition of [3H]ryanodine binding by sphingosine is due to an increased rate of dissociation of bound [3H]ryanodine from SR membranes and a decreased rate of association of [3H]ryanodine to the high-affinity site. Unlike other modulators of the Ca(2+)-release channel, sphingosine can remove bound [3H]ryanodine from the high-affinity site within minutes. Sphingosine increased the rate of dissociation of [3H]ryanodine bound to a solubilized proteolytic fragment derived from the carboxy terminus of the ryanodine binding protein (cleavage at Arg4475). Sphingosine also inhibited the activity of the Ca(2+)-release channel incorporated into planar lipid bilayers. Taken together, the data provide evidence for a direct effect of sphingosine on the Ca(2+)-release channel. Sphingosine is a noncompetitive inhibitor at the high-affinity ryanodine binding site, and it interacts with a site between Arg4475 and the carboxy terminus of the Ca(2+)-release channel.

Direct effect of ceramide on the mitochondrial electron transport chain leads to generation of reactive oxygen species. Role of mitochondrial glutathione

Ceramide is a sphingolipid that is generated in the signaling of inflammatory cytokines such as tumor necrosis factor (TNF), which exerts many functional roles depending on the cell type where it is produced. Since TNF cytotoxicity is mediated by overproduction of reactive oxygen species from mitochondria, we have examined the role of ceramide in generation of oxidative stress in isolated rat liver mitochondria. The present studies demonstrate that addition of N-acetylsphingosine (C2-ceramide) to mitochondria led to an increase of fluorescence of dihydrorhodamine 123 or dichlorofluorescein-stained mitochondria, indicating formation of hydrogen peroxide. Such effect was significant at 0.25 microM and maximal at 1-5 microM C2, decreasing at greater concentrations. This inductive effect of ceramide was mimicked by N-hexanoylsphingosine at the same concentration range, whereas the immediate precursor of C2, C2-dihydroceramide increased hydrogen peroxide at 1-5 microM. Sphingosine generated hydrogen peroxide at concentrations >/=10 microM, whereas diacylglycerol failed to increase hydrogen peroxide. The increase in hydrogen peroxide induced by C2 was not triggered by mitochondrial permeability transition as C2 did not induce mitochondrial swelling. Blocking electron transport chain at complex I and II prevented the increase in hydrogen peroxide induced by C2; however, interruption of electron flow at complex III by antimycin A potentiated the inductive effect of C2. Depletion of matrix GSH prior to exposure to ceramide resulted in a potentiated increase (2-fold) of hydrogen peroxide generation, leading to lipid peroxidation and loss of activity of respiratory chain complex IV compared with GSH-repleted mitochondria. Mitochondria isolated from TNF-treated cells showed an increase (2-3-fold) in the amount of ceramide compared with mitochondria from untreated cells. These results suggest that mitochondria are a target of ceramide produced in the signaling of TNF whose effect on mitochondrial electron transport chain leads to overproduction of hydrogen peroxide and consequently this phenomena may account for the generation of reactive oxygen species during TNF cytotoxicity.

Divergence in signal transduction pathways of platelet-derived growth factor (PDGF) and epidermal growth factor (EGF) receptors. Involvement of sphingosine 1-phosphate in PDGF but not EGF signaling

Platelet-derived growth factor (PDGF) and serum, but not epidermal growth factor (EGF), stimulated sphingosine kinase activity in Swiss 3T3 fibroblasts and increased intracellular concentrations of sphingosine 1-phosphate (SPP), a sphingolipid second messenger (Olivera, A., and Spiegel, S. (1993) Nature 365, 557-560). We report herein that DL-threo-dihydrosphingosine (DHS), a competitive inhibitor of sphingosine kinase that prevents PDGF-induced SPP formation, specifically inhibited the activation of two cyclin-dependent kinases (p34(cdc2) kinase and Cdk2 kinase) induced by PDGF, but not by EGF. SPP reversed the inhibitory effects of DHS on PDGF-stimulated cyclin-dependent kinases and DNA synthesis, demonstrating that the DHS effects were mediated via inhibition of sphingosine kinase. DHS also markedly reduced PDGF-stimulated but not EGF-stimulated mitogen-activated protein kinase activity and DNA binding activity of activator protein-1. Examination of the early signaling events of PDGF action revealed that DHS did not affect PDGF-induced autophosphorylation of the growth factor receptor or phosphorylation of the SH2/SH3 adaptor protein Shc and its association with Grb2. This sphingosine kinase inhibitor did not abrogate activation of phosphatidylinositol 3-kinase by PDGF. In agreement, treatment with SPP had no effect on these responses but did, however, potently stimulate phosphorylation of Crk, another SH2/SH3 adaptor protein. Moreover, DHS inhibited PDGF-stimulated, but not EGF-stimulated, Crk phosphorylation. Thus, regulation of sphingosine kinase activity defines divergence in signal transduction pathways of PDGF and EGF receptors leading to mitogen-activated protein kinase activation.

Sphingosine 1-phosphate induces platelet activation through an extracellular action and shares a platelet surface receptor with lysophosphatidic acid

Sphingosine 1-phosphate (Sph-1-P) has been implicated as an intracellular second messenger in many studies. We investigated the metabolism of Sph-1-P and the mechanism by which Sph-1-P induces activation in enucleated and highly differentiated platelets. Platelets lack Sph-1-P lyase activity, possess persistently active sphingosine (Sph) kinase, and abundantly store Sph-1-P. Although exogenous Sph-1-P activated platelets, intracellular Sph-1-P, formed from exogenously added Sph by cytosolic Sph kinase, failed to do so. To support the notion that exogenous Sph-1-P stimulates platelets from outside, contact of platelet surfaces with immobilized Sph-1-P covalently linked to glass particles resulted in platelet activation. Furthermore, we detected the specific binding sites for radiolabeled Sph-1-P on the platelet surface, suggesting extracellular effects of Sph-1-P on plasma membrane receptors. This specific Sph-1-P binding was inhibited not by other sphingolipids but by lysophosphatidic acid (LPA), and platelet aggregation response to LPA was specifically desensitized by prior addition of Sph-1-P. Finally, internally stored Sph-1-P is released extracellularly upon stimulation, and the release correlated well with protein kinase C activation in intact platelets. These results suggest that Sph-1-P acts not intracellularly but intercellularly, following discharge from activated platelets, and shares a platelet surface receptor with LPA.

Picomole scale stereochemical analysis of sphingosines and dihydrosphingosines

We have developed a simple picomole (low nanogram) scale HPLC scheme which can separate all eight isomers of sphingosine and dihydrosphingosine thus leading to the identification of their relative and absolute configurations. The amino group of the sample is derivatized to its fluorescent N-naphthimide which is analyzed by normal and chiral phase HPLC, coupled with fluorescence peak detection. If necessary, the results of this HPLC method can be further corroborated by measurements of circular dichroic (CD) spectra of the N-naphthimido-derivatives and/or N,O-chromophoric derivatives.

Ceramide signalling and the immune response

Ceramide, produced through either the induction of SM hydrolysis or synthesized de novo transduces signals mediating differentiation, growth, growth arrest, apoptosis, cytokine biosynthesis and secretion, and a variety of other cellular functions. A generalized ceramide signal transduction scheme is shown in Fig. 2 in which ceramide is generated through the activation of distinct SMases residing in separate subcellular compartments in response to specific stimuli. Clearly, specificity of cellular responses to ceramide depends upon many factors which include the nature of the stimulus, co-stimulatory signals and the cell type involved. Ceramide derived from neutral SMase activation is thought to be involved in modulating CAPK and MAP kinases, PLA2 (arachidonic acid mobilization), and CAPP while ceramide generated through acid SMase activation appears to be primarily involved in NF-kappa B activation. While there is no apparent cross-talk between these two ceramide-mediated signalling pathways, there is likely to be significant cross-talk between ceramide signalling and other signal transduction pathways (e.g., the PKC and MAP kinase pathways). Other downstream targets for ceramide action include Cox, IL-6 and IL-2 gene expression, PKC zeta, Vav, Rb, c-Myc, c-Fos, c-Jun and other transcriptional regulators. Many, if not all, of these ceramide-mediated signalling events have been identified in the various cells comprising the immune system and are integral to the optimal functioning of the immune system. Although the role of the SM pathway and the generation of ceramide in T and B lymphocytes have only recently been recognized, it is clear from these studies that signal transduction through SM and ceramide can strongly affect the immune response, either directly through cell signalling events, or indirectly through cytokines produced by other cells as the result of signalling through the SM pathway. An overview of the signalling mechanisms coupling ceramide to the modulation of the immune response is depicted in Fig. 3 and shows how ceramide may play pivotal roles in regulating a number of complex processes. The SM pathway represents a potentially valuable focal point for therapeutic control of immune responses, perhaps for either enhancement of the activity of T cells in the elimination of tumors, or the down-regulation of lymphocyte function in instances of autoimmune disease. The recent explosion of knowledge regarding ceramide signalling notwithstanding, a number of critical questions need to be answered before a comprehensive, mechanistic understanding can be formulated relative to the incredibly varied effects of ceramide on cell function. For example, (i) how is a structurally simple molecule like ceramide able to mediate so many different, and sometimes paradoxical, physiological responses ranging from cell proliferation and differentiation to inhibition of cell growth and apoptosis, (ii) what are the molecular identities and modes of activation of the various SMase isoforms, (iii) what determines the distribution of the unique isoforms of SMase in cells of different lineages or at different stages of differentiation, (iv) what is the relative contribution of ceramide generated through SM hydrolysis versus de novo synthesis, and (v) by what means does ceramide interact with specific intracellular targets? Although a number of ceramide-activatable kinases, phosphatases, and their protein substrates have been identified, a more extensive search for additional cellular targets will be indispensable in determining the phosphorylation cascades linking the activation of the SM pathway to the regulation of nuclear events. Clearly, cross-talk between ceramide-induced signal transduction cascades and other signalling pathways adds to the inherent difficulty in distinguishing the specific effects of complex, intertwining signalling pathways.

Sphingoid bases and phospholipase D activation

There is increased interest in physiological functions and mechanisms of action of sphingolipids metabolites, ceramide, sphingosine, and sphingosine-1-phosphate (SPP), members of a new class of lipid second messengers. This review summarizes current knowledge regarding the role of these sphingolipids metabolites in the actions of growth factors and focuses on the second messenger roles of sphingosine and its metabolite, SPP, in the regulation of cell growth. We also discuss possible interactions with intermediates of the well known glycerophospholipid cycle. Sphingosine and SPP generally provide positive mitogenic signals whereas ceramide has been reported to induce apoptosis and cell arrest in several mammalian cell lines. Stimulation of phospholipase D leading to an increase in phosphatidic acid, a positive regulator of cell growth, by sphingosine and SPP, and its inhibition by ceramide, might be related to their opposite effects on cell growth. This also indicates that sphingolipid turnover could regulate the diacylglycerol cycle. Cross-talk between sphingolipid turnover pathways and the diacylglycerol cycle increases complexity of signaling pathways leading to cellular proliferation and adds additional sites of regulation.

Signal transduction through lipid second messengers

This review emphasizes the generation of glycerolipid and sphingolipid second messengers, and their molecular targets. The role of the phosphatidylinositol transfer protein and phospholipase D in signal transmission, and the structures of the 1, 2-diacylglycerol and calcium-binding sites of protein kinase C are discussed. Further, ceramide signaling through protein kinases and the role of cross-talk in the signaling of apoptosis and inflammation are addressed.