Ceramide 1-phosphate increases intracellular free calcium concentrations in thyroid FRTL-5 cells: evidence for an effect mediated by inositol 1,4,5-trisphosphate and intracellular sphingosine 1-phosphate

The pair ceramide 1-phosphate/ceramide kinase regulates intracellular calcium and progesterone-induced human sperm acrosomal exocytosis

Before fertilization, spermatozoa must undergo calcium-regulated acrosome exocytosis in response to physiological stimuli such as progesterone and zona pellucida. Our laboratory has elucidated the signaling cascades accomplished by different sphingolipids during human sperm acrosomal exocytosis. Recently, we established that ceramide increases intracellular calcium by activating various channels and stimulating the acrosome reaction. However, whether ceramide induces exocytosis on its own, activation of the ceramide kinase/ceramide 1-phosphate (CERK/C1P) pathway or both is still an unsolved issue. Here, we demonstrate that C1P addition induces exocytosis in intact, capacitated human sperm. Real-time imaging in single-cell and calcium measurements in sperm population showed that C1P needs extracellular calcium to induce [Ca2+]i increase. The sphingolipid triggered the cation influx through voltage-operated calcium (VOC) and store-operated calcium (SOC) channels. However, it requires calcium efflux from internal stores through inositol 3-phosphate receptors (IP3R) and ryanodine receptors (RyR) to achieve calcium rise and the acrosome reaction. We report the presence of the CERK in human spermatozoa, the enzyme that catalyzes C1P synthesis. Furthermore, CERK exhibited calcium-stimulated enzymatic activity during the acrosome reaction. Exocytosis assays using a CERK inhibitor demonstrated that ceramide induces acrosomal exocytosis, mainly due to C1P synthesis. Strikingly, progesterone required CERK activity to induce intracellular calcium increase and acrosome exocytosis. This is the first report, implicating the bioactive sphingolipid C1P in the physiological progesterone pathway leading to the sperm acrosome reaction.

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

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

Sphingosine 1-phosphate and cancer: lessons from thyroid cancer cells

Sphingomyelin is found in the cell membrane of all eukaryotic cells, and was for a long time considered merely as a structural component. However, during the last two decades, metabolites of sphingomyelin, especially sphingosine 1-phosphate (S1P), have proven to be physiologically significant regulators of cell function. Through its five different G protein-coupled receptors, S1P regulates a wide array of cellular processes, ranging from stimulating cellular proliferation and migration, to the inhibition of apoptosis and induction of angiogenesis and modulation of cellular calcium homeostasis. Many of the processes regulated by S1P are important for normal cell physiology, but may also induce severe pathological conditions, especially in malignancies like cancer. Thus, understanding S1P signaling mechanisms has been the aim of a multitude of investigations. Great interest has also been shown in understanding the action of sphingosine kinase (SphK), i.e., the kinase phosphorylating sphingosine to S1P, and the interactions between S1P and growth factor signaling. In the present review, we will discuss recent findings regarding the possible importance of S1P and SphK in the etiology of thyroid cancer. Although clinical data is still scarce, our in vitro findings suggest that S1P may function as a “double-edged sword”, as the receptor profile of thyroid cancer cells largely determines whether S1P stimulates or blocks cellular migration. We will also discuss the interactions between S1P- and VEGF-evoked signaling, and the importance of a S1P₁-VEGF receptor 2 complex in thyroid cancer cells.

Ceramide kinase: the first decade

It has been some 20 years since the initial discovery of ceramide 1-phosphate (C1P) and nearly a decade since ceramide kinase (CERK) was cloned. Many studies have shown that C1P is important for membrane biology and for the regulation of membrane-bound proteins, and the CERK enzyme has appeared to be tightly regulated in order to control both ceramide levels and production of C1P. Furthermore, C1P made by CERK has emerged as a genuine signalling entity. However, it represents only part of the C1P pool that is available in the cell, therefore suggesting that alternative unknown C1P-producing mechanisms may also play a role. Recent technological developments for measuring complex sphingolipids in biological samples, together with the availability of Cerk-deficient animals as well as potent CERK inhibitors, have now provided new grounds for investigating C1P biology further. Here, we will review the current understanding of CERK and C1P in terms of biochemistry and functional implications, with particular attention to C1P produced by CERK.

The ceramide-1-phosphate analogue PCERA-1 modulates tumour necrosis factor-alpha and interleukin-10 production in macrophages via the cAMP-PKA-CREB pathway in a GTP-dependent manner

The synthetic phospho-ceramide analogue-1 (PCERA-1) down-regulates production of the pro-inflammatory cytokine tumour necrosis factor-alpha (TNF-alpha) and up-regulates production of the anti-inflammatory cytokine interleukin-10 (IL-10) in lipopolysaccharide (LPS) -stimulated macrophages. We have previously reported that PCERA-1 increases cyclic adenosine monophosphate (cAMP) levels. The objective of this study was to delineate the signalling pathway leading from PCERA-1 via cAMP to modulation of TNF-alpha and IL-10 production. We show here that PCERA-1 elevates intra-cellular cAMP level in a guanosine triphosphate-dependent manner in RAW264.7 macrophages. The cell-permeable dibutyryl cAMP was able to mimic the effects of PCERA-1 on cytokine production, whereas 8-chloro-phenylthio-methyladenosine-cAMP, which specifically activates the exchange protein directly activated by cAMP (EPAC) but not protein kinase A (PKA), failed to mimic PCERA-1 activities. Consistently, the PKA inhibitor H89 efficiently blocked PCERA-1-driven cytokine modulation as well as PCERA-1-stimulated phosphorylation of cAMP response element binding protein (CREB) on Ser-133. Finally, PCERA-1 activated cAMP-responsive transcription of a luciferase reporter, in synergism with the phosphodiesterase (PDE)-4 inhibitor rolipram. Our results suggest that PCERA-1 activates a G(s) protein-coupled receptor, leading to elevation of cAMP, which acts via the PKA-CREB pathway to promote TNF-alpha suppression and IL-10 induction in LPS-stimulated macrophages. Identification of the PCERA-1 receptor is expected to set up a new target for development of novel anti-inflammatory drugs.

Control of metabolism and signaling of simple bioactive sphingolipids: Implications in disease

Simple bioactive sphingolipids include ceramide, sphingosine and their phosphorylated forms sphingosine 1-phosphate and ceramide 1-phosphate. These molecules are crucial regulators of cell functions. In particular, they play important roles in the regulation of angiogenesis, apoptosis, cell proliferation, differentiation, migration, and inflammation. Decoding the mechanisms by which these cellular functions are regulated requires detailed understanding of the signaling pathways that are implicated in these processes. Most importantly, the development of inhibitors of the enzymes involved in their metabolism may be crucial for establishing new therapeutic strategies for treatment of disease.

Ceramide-1-phosphate in phagocytosis and calcium homeostasis

Sphingolipids are well established sources of important signaling molecules. For example, ceramide (Cer) has been described as a potent inhibitor of cell growth and inducer of apoptosis. In contrast, ceramide-1-phosphate (C1P) has been reported to have mitogenic properties and to inhibit apoptosis. Our understanding of the distinct biological roles of C1P in the regulation of DNA synthesis, inflammation, membrane fusion and intracellular Ca2+ increase has rapidly expanded. C1P is a bioactive sphingolipid formed by the phosphorylation of ceramide catalyzed by ceramide kinase (CERK). This chapter specifically focuses on the role of C1P in phagocytosis and Ca2+ homeostasis. Studies of the metabolism of C1P during phagocytosis, may lead to a better understanding of its role in signaling. Potentially, the inhibition of CERK and C1P formation may be a therapeutic target for inflammation.

Activation of protein kinase C-alpha is essential for stimulation of cell proliferation by ceramide 1-phosphate

We previously demonstrated that ceramide-1-phosphate (C1P) stimulates fibroblast and macrophage proliferation, but the mechanisms involved in this action have only been partially described. Here we demonstrate that C1P induces translocation of protein kinase C-alpha (PKC-alpha) from the soluble to the membrane fraction of bone marrow-derived macrophages. Translocation of this enzyme was accompanied by its phosphorylation on Ser 657 residue. Activation of PKC-alpha was independent of prior stimulation of phosphatidylinositol-dependent or phosphatidylcholine-dependent phospholipase C activities, but required activation of sphingomyelin synthesis. Inhibition of PKC-alpha activation also blocked C1P-stimulated macrophage proliferation indicating that this enzyme is essential for the mitogenic effect of C1P.

Sphingosine kinase as a regulator of calcium entry through autocrine sphingosine 1-phosphate signaling in thyroid FRTL-5 cells

Calcium entry is one of the main regulators of intracellular signaling. Here, we have described the importance of sphingosine, sphingosine kinase 1 (SK1), and sphingosine 1-phosphate (S1P) in regulating calcium entry in thyroid FRTL-5 cells. In cells incubated with the phosphatase inhibitor calyculin A, which evokes calcium entry without mobilizing sequestered intracellular calcium, sphingosine inhibited calcium entry in a concentration-dependent manner. Furthermore, inhibiting SK1 or the ATP-binding cassette ABCC1 multidrug transporter attenuated calcium entry. The addition of exogenous S1P restored calcium entry. Neither sphingosine nor inhibition of SK1 attenuated thapsigargin-evoked calcium entry. Blocking S1P receptor 2 or phospholipase C attenuated calcium entry, whereas blocking S1P receptor 3 did not. Overexpression of wild-type SK1, but not SK2, enhanced calyculin-evoked calcium entry compared with mock-transfected cells, whereas calcium entry was decreased in cells transfected with the dominant-negative G82D SK1 mutant. Exogenous S1P restored calcium entry in G82D cells. Our results suggest that the calcium entry pathway is blocked by sphingosine and that activation of SK1 and the production of S1P, through an autocrine mechanism, facilitate calcium entry through activation of S1P receptor 2. This is a novel mechanism by which the sphingosine-S1P rheostat regulates cellular calcium homeostasis.

Chain length specificity for activation of cPLA2alpha by C1P: use of the dodecane delivery system to determine lipid-specific effects

Previously, our laboratory demonstrated that ceramide-1-phosphate (C1P) specifically activated group IVA cytosolic phospholipase A(2) (cPLA(2)alpha) in vitro. In this study, we investigated the chain length specificity of this interaction. C1P with an acyl-chain of >or=6 carbons efficiently activated cPLA(2)alpha in vitro, whereas C(2)-C1P, was unable to do so. Delivery of C1P to cells via the newly characterized ethanol/dodecane system demonstrated a lipid-specific activation of cPLA(2)alpha, AA release, and PGE(2) synthesis (EC(50) = 400 nM) when compared to structurally similar lipids. C1P delivered as vesicles in water also induced a lipid-specific increase in AA release. Mass spectrometric analysis demonstrated that C1P delivered via ethanol/dodecane induced a 3-fold increase in endogenous C1P with little metabolism to ceramide. C1P was also more efficiently delivered (>3-fold) to internal membranes by ethanol/dodecane as compared to vesiculated C1P. Using this now established delivery method for lipids, C(2)-C1P was shown to be ineffective in the induction of AA release as compared with C(6)-C1P, C(16)-C1P, and C(18:1) C1P. Here, we demonstrate that C1P requires >or=6 carbon acyl-chain to activate cPLA(2)alpha. Thus, published reports on the biological activity of C(2)-C1P are not via eicosanoid synthesis. Furthermore, this study demonstrates that the alcohol/dodecane system can be used to efficiently deliver exogenous phospholipids to cells for the examination of specific biological effects.

Implication of ceramide, ceramide 1-phosphate and sphingosine 1-phosphate in tumorigenesis

In the last two decades there has been considerable progress in our understanding of the role of sphingolipids in controlling signal transduction processes, particularly in the mechanisms leading to regulation of cell growth and death. Ceramide is a well-characterized sphingolipid metabolite and second messenger that can be produced by cancer cells in response to a variety of stimuli, including therapeutic drugs, leading to cell cycle arrest and apoptosis. Although this is a promising aspect when thinking of treating cancer, it should be borne in mind that ceramide production may not always be a growth inhibitory or pro-apoptotic signal. In fact, ceramide can be readily converted to sphingosine 1-phosphate (S1P) by the concerted actions of ceramidases and sphingosine kinases, or to ceramide 1-phosphate (C1P) by the action of ceramide kinase. In general, S1P and C1P have opposing effects to ceramide, acting as pro-survival or mitogenic signals in most cell types. This review will address our current understanding of the many roles of ceramide, S1P and C1P in the regulation of cell growth and survival with special emphasis to the emerging role of these molecules and their metabolizing enzymes in controlling tumor progression and metastasis.

Ceramide kinase and ceramide-1-phosphate

It has been over a decade since the sphingolipid ceramide-1-phosphate (C1P) was described. Until recently, only sparse reports on possible biological functions for this lipid have been published. A large number of reports have now surfaced demonstrating distinct biological mechanisms regulated by C1P produced from ceramide kinase (CERK). In the following methods chapter, the methodologies for examining CERK function in vitro and in cells are outlined in detail. The methodologies for examining C1P levels and the use of exogenous C1P on cells to observe lipid specific effects on a particular biology are also detailed.

Ceramide 1-phosphate stimulates macrophage proliferation through activation of the PI3-kinase/PKB, JNK and ERK1/2 pathways

Ceramide 1-phosphate (C1P) was first shown to be mitogenic for fibroblasts, but the mechanisms whereby it stimulated cell proliferation have remained largely unknown. Here we demonstrate that C1P stimulates DNA synthesis and cell division in murine bone marrow-derived macrophages. C1P caused rapid phosphorylation of protein kinase B (PKB, also known as Akt), a downstream target of phosphatidylinositol 3-kinase (PI3-K). Selective inhibition of PI3-K blocked both DNA synthesis and cell growth. C1P induced phosphorylation of GSK-3beta, which is a major target of PKB, and this effect was also abolished by inhibition of PI3-K. In addition, C1P upregulated the expression of cyclin D1 and c-Myc, two major targets of GSK-3beta, which are important regulators of cell proliferation. C1P stimulated the activity of NF-kappaB, and inhibitors of this transcription factor completely blocked macrophage proliferation. Lastly, C1P induced phosphorylation of the mitogen activated protein kinases (MAPK) extracellularly regulated kinases 1 and 2 (ERK1/2), and c-Jun N-terminal kinase (JNK). Inhibition of ERK1/2 and JNK also blocked C1P-induced macrophage proliferation. It can be concluded that C1P stimulates macrophage proliferation through activation of the PI3-K/PKB, ERK and JNK pathways, and that GSK-3beta, c-Myc, cyclin D1, and NF-kappaB are important downstream effectors in this action.

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.

DNA polymerase beta catalytic efficiency mirrors the Asn279-dCTP H-bonding strength

Ternary complexes of wild type or mutant form of human DNA polymerase beta (pol beta) bound to DNA and dCTP substrates were studied by molecular dynamics (MD) simulations. The occurrences of contact configurations (CC) of structurally important atom pairs were sampled along the MD trajectories, and converted into free-energy differences, DeltaG(CC). DeltaG(CC) values were correlated with the experimental binding and catalytic free energies for the wild type pol beta and its Arg183Ala, Tyr271Ala, Asp276Val, Lys280Gly, Arg283Ala, and Glu295Ala mutants. The correlation coefficients show that the strength of the H-bond between dCTP and Asn279 is a strong predictor of the mutation-induced changes in the catalytic efficiency of pol beta. This finding is consistent with the view that enzyme preorganization plays a major role in controlling DNA polymerase specific activity.

On the presence of C2-ceramide in mammalian tissues: possible relationship to etherphospholipids and phosphorylation by ceramide kinase

C(2)-ceramide (N-acetyl-sphingenine) is often used as an analog to study ceramide-mediated cellular processes. According to Lee et al. [J. Biol. Chem. 271 (1996), 209-217], C(2)-ceramide is formed by an acetyl transfer from platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) to sphingenine. To substantiate these unconfirmed findings, we (i) developed a method to quantify C(2)-ceramide and (ii) analyzed C(2)-ceramide levels in Pex5(-/-) mice, a model for Zellweger syndrome, in which the synthesis of ether lipids such as PAF is impaired. The presence of C(2)-ceramide could be established in brain (+/-10 pmol/g) and liver (+/-25 pmol/g) from control mice, and was approximately 5000-fold less than the main long-chain ceramide species. In Pex5(-/-) mice, C(2)-ceramide levels did not differ significantly compared to control tissues. Given the presence of a ceramide kinase in mammals, phosphorylation of C(2)-ceramide by human ceramide kinase (HsCERK) was tested. C(2)-ceramide appears to be a good substrate when albumin is used as carrier. In CHO cells overexpressing HsCERK, phosphorylation of exogenously added C(2)-ceramide could also be demonstrated. Our data indicate that C(2)-ceramide is present in mammalian tissues and can be converted to C(2)-ceramide-1-phosphate, in addition to other documented metabolic alterations, but does not seem to be linked to ether lipid metabolism.

A critical beta6-beta7 loop in the pleckstrin homology domain of ceramide kinase

CerK (ceramide kinase) produces ceramide 1-phosphate, a sphingophospholipid with recognized signalling properties. It localizes to the Golgi complex and fractionates essentially between detergent-soluble and -insoluble fractions; however, the determinants are unknown. Here, we made a detailed mutagenesis study of the N-terminal PH domain (pleckstrin homology domain) of CerK, based on modelling, and identified key positively charged amino acid residues within an unusual motif in the loop interconnecting beta-strands 6 and 7. These residues are critical for CerK membrane association and polyphosphoinositide binding and activity. Their mutagenesis results in increased thermolability, sensitivity to proteolysis, reduced apparent molecular mass as well as propensity of the recombinant mutant protein to aggregate, indicating that this loop impacts the overall conformation of the CerK protein. This is in contrast with most PH domains whose function strongly relies on charges located in the beta1-beta2 loop.

Effects of ceramide-1-phosphate on cultured cells: dependence on dodecane in the vehicle

Ceramide-1-phosphate (C1P), the product of ceramide kinase, is a sphingophospholipid with recently recognized signaling properties. In particular, it was reported to be mitogenic and capable of direct stimulation of cytosolic phospholipase A(2alpha). Much of the present knowledge has relied on the use of C1P of various acyl chain lengths, together with diverse protocols to deliver it to cultured cells. A mixture of ethanol (or methanol) with dodecane, as the vehicle, has become popular. However, the contribution of this solvent to the observed effects of C1P has not been documented. Here, we show that addition of C1P in ethanol-dodecane to culture medium leads to irreversible cytotoxic effects. These culminate in mitochondrial swelling, vacuole formation, and cell death. Not only the toxicity of C1P, but also its ability to trigger prostaglandin E2 release, is fully dependent upon addition of a premade C1P-dodecane mixture. Furthermore, we show that these effects are not restricted to C1P. They result from the capacity of dodecane to interact with phospholipids; hence, they go undetected with a vehicle control. This study should raise awareness about the use of dodecane for phospholipid delivery and, in turn, help in unraveling C1P signaling, which is still poorly understood.

Ceramide 1-phosphate/ceramide, a switch between life and death

Ceramide is a well-characterized sphingolipid metabolite and second messenger that participates in numerous biological processes. In addition to serving as a precursor to complex sphingolipids, ceramide is a potent signaling molecule capable of regulating vital cellular functions. Perhaps its major role in signal transduction is to induce cell cycle arrest, and promote apoptosis. In contrast, little is known about the metabolic or signaling pathways that are regulated by the phosphorylated form of ceramide. It was first demonstrated that ceramide-1-phosphate (C1P) had mitogenic properties, and more recently it has been described as potent inhibitor of apoptosis and inducer of cell survival. C1P and ceramide are antagonistic molecules that can be interconverted in cells by kinase and phosphatase activities. An appropriate balance between the levels of these two metabolites seems to be crucial for cell and tissue homeostasis. Switching this balance towards accumulation of one or the other may result in metabolic dysfunction, or disease. Therefore, the activity of the enzymes that are involved in C1P and ceramide metabolism must be efficiently coordinated to ensure normal cell functioning.

Ceramide-1-phosphate activates cytosolic phospholipase A2α directly and by PKC pathway

Ceramide-1-phosphate (C1P), a novel bioactive sphingolipid, is implicated in the vital cellular processes such as cell proliferation and inflammation. The role of C1P on activity of cytosolic phospholipase A2alpha (cPLA2alpha), a key enzyme for the release of arachidonic acid (AA) and prostanoids, has not been well elucidated. In this study, we investigated the effect of C1P on the release of AA from L929 cells and a variant, which lacks cPLA2alpha expression, C12 cells. C1P at 30 microM alone induced AA release from L929 cells without an increase in intracellular Ca2+ concentration. C1P-induced AA release was marginal in C12 cells, and treatment with an intracellular Ca2+ chelator (BAPTA-AM) or an inhibitor of cPLA2alpha (2 microM pyrrophenone) decreased C1P-induced AA release in L929 cells. C1P increased the enzymatic activity of cPLA2alpha over two-fold in the presence of Ca2+. C1P triggered the translocation of cPLA2alpha and its C2 domain from the cytosol to the perinuclear region in CHO-K1 cells. Interestingly, C1P at 10 microM synergistically enhanced ionomycin-induced AA release from L929 cells. The AA release induced by C1P with and without ionomycin decreased by treatment with protein kinase C (PKC) inhibitor (10 microM GF109203X) and in the PKC-depleted cells. C1P at 10 microM stimulated the translocation of PKC (alpha and delta) from the soluble to the membrane fractions. We propose that C1P stimulates AA release via two mechanisms; direct activation of cPLA2alpha, and the PKC-dependent pathway.

Dopamine release in PC12 cells is mediated by Ca2+-dependent production of ceramide via sphingomyelin pathway

A presynaptic membrane disturbance is an essential process for the release of various neurotransmitters. Ceramide, which is a tumor suppressive lipid, has been shown to act as a channel-forming molecule and serve as a precursor of ceramide-1-phosphate, which can disturb the cellular membrane. This study found that while permeable ceramide increases the rate of dopamine release in the presence of a Ca(2+)-ionophore, A23187, permeable ceramide-1-phosphate provoked its release even without the ionophore. The treatment of PC12 cells with the ionophore at concentrations < 2 microM produced ceramide via the sphingomyelin (SM) pathway with a concomitant release of dopamine, and no cell damage was observed. The addition of a Ca(2+) chelator, EGTA, to the medium inhibited the increase in the release of both the ceramide and dopamine. This suggests that ceramide might be produced by Ca(2+) and is implicated in the membrane disturbance associated with the release of dopamine as a result of its conversion to ceramide-1-phosphate. Consistent with these results, this study detected a membrane-associated and neutral pH optimum sphingomyelinase (SMase) whose activity was increased by Ca(2+). Together, these results demonstrate that ceramide can be produced via the activation of a neutral form of SMase through Ca(2+), and is involved in the dopamine release in concert with Ca(2+).

Ceramide-1-P induces Ca2+ mobilization in Jurkat T-cells by elevation of Ins(1,4,5)-P3 and activation of a store-operated calcium channel

Sphingolipids comprise a very important class of second messengers involved in cell growth, differentiation, and apoptosis, among other different functions. Recently, these lipids have been implicated in calcium mobilization in different cell lines, including Jurkat T-lymphocytes. However, the effect of each particular sphingolipid appears to be cell-line specific. Among them, the least studied is ceramide-1-P (Cer-1-P). Here, we show that Cer-1-P increased the intracellular Ca(2+) concentration in Jurkat T-cells. Furthermore, laser-scanning confocal microscopy indicated that Ca(2+) is released from the endoplasmic reticulum. An effect on store-operated Ca(2+) channels was evidenced by whole-cell "patch clamp" measurements after Cer-1-P induced Ca(2+) store depletion. The mechanism of action of Cer-1-P resembles that of the Jurkat anti-TCR antibody, but differs from that of ceramide, since Cer-1-P induced an increase in Ins(1,4,5)-P(3).

Ceramide-1-phosphate promotes cell survival through activation of the phosphatidylinositol 3-kinase/protein kinase B pathway

In this report, we show for the first time that ceramide-1-phosphate (C1P) stimulates the phosphatidylinositol 3-kinase (PI3-K)/protein kinase B (PKB) pathway, which is a major mechanism whereby growth factors promote cell survival. Also, C1P induced IkappaB phosphorylation, and enhanced the DNA binding activity of the transcription factor NF-kappaB. Apoptotic macrophages showed a marked reduction of Bcl-X(L) levels, and this was prevented by C1P. These findings suggest that C1P blocks apoptosis, at least in part, by stimulating the PI3-K/PKB/NF-kappaB pathway and maintaining the production of antiapoptotic Bcl-X(L). Based on these and our previous observations, we propose a working model for C1P in which inhibition of acid sphingomyelinase and the subsequent decrease in ceramide levels would allow cell signaling through stimulation of the PI3-K/PKB pathway to promote cell survival.

Enhancement of intracellular sphingosine-1-phosphate production by inositol 1,4,5-trisphosphate-evoked calcium mobilisation in HEK-293 cells: endogenous sphingosine-1-phosphate as a modulator of the calcium response

Sphingosine-1-phosphate (S1P) regulates many cellular functions, such as migration, differentiation and growth. The effects of S1P are thought to be primarily mediated by G-protein coupled receptors, but an intracellular function as a calcium releasing second messenger has also been proposed. Here we show that in HEK-293 cells, exogenous S1P mobilises sequestered calcium by a mechanism primarily dependent on the phospholipase C (PLC)/inositol 1,4,5-trisphosphate (IP3) pathway, and secondarily on the subsequent synthesis of intracellular S1P. Stimulating HEK-293 cells exogenously with S1P increased the production of both inositol phosphates and intracellular S1P. The calcium response was inhibited in cells treated with 2-APB, caffeine or U73122, showing that the PLC/IP3 pathway for calcium release is activated in response to exogenous S1P. The calcium response was partially inhibited in cells treated with the sphingosine kinase inhibitor DMS and in cells expressing a catalytically inactive sphingosine kinase, showing that endogenously produced S1P is also involved. Importantly, 2-APB and U73122 inhibited the S1P-evoked production of intracellular S1P. S1P is therefore not likely a major calcium releasing second messenger in HEK-293 cells, but rather a secondary regulator of calcium mobilisation.

Effects of synthetic sphingosine-1-phosphate analogs on arachidonic acid metabolism and cell death

Sphingolipid metabolites such as sphingosine regulate cell functions including cell death and arachidonic acid (AA) metabolism. D-erythro-C18-Sphingosine-1-phosphate (D-e-S1P), a sphingolipid metabolite, acts as an intracellular messenger in addition to being an endogenous ligand of some cell surface receptors. The development of S1P analogs may be useful for studying and/or regulating S1P-mediated cellular responses. In the present study, we found that several synthetic S1P analogs at pharmacological concentrations stimulated AA metabolism and cell death in PC12 cells. D-erythro-N,O,O-Trimethyl-C18-S1P (D-e-TM-S1P), L-threo-O,O-dimethyl-C18-S1P (L-t-DM-S1P) and L-threo-O,O-dimethyl-3O-benzyl-C18-S1P (L-t-DMBn-S1P) at 100 microM stimulated [(3)H]AA release from the prelabeled PC12 cells. L-t-DMBn-S1P at 20 microM increased prostanoid formation in PC12 cells. L-t-DMBn-S1P-induced AA release was inhibited by D-e-sphingosine, but not by the tested PLA(2) inhibitors. L-t-DMBn-S1P did not stimulate the activity of cytosolic phospholipase A(2alpha) (cPLA(2alpha)) in vitro and the translocation of cPLA(2alpha) in the cells, and caused AA release from the cells lacking cPLA(2alpha). These findings suggest that L-t-DMBn-S1P stimulated AA release in a cPLA(2alpha)-independent manner. In contrast, D-e-S1P and D-erythro-N-monomethyl-C18-S1P caused cell death without AA release in PC12 cells, and the effects of D-e-TM-S1P, L-t-DM-S1P and L-t-DMBn-S1P on cell death were limited. Synthetic S1P analogs may be useful tools for studying AA metabolism and cell death in cells.

Ceramide 1-phosphate enhances calcium entry through voltage-operated calcium channels by a protein kinase C-dependent mechanism in GH4C1 rat pituitary cells

Sphingomyelin derivatives modulate a multitude of cellular processes, including the regulation of [Ca2+]i (the intracellular free calcium concentration). Previous studies have shown that these metabolites often inhibit calcium entry through VOCCs (voltage-operated calcium channels). In the present study, we show that, in pituitary GH4C1 cells, C1P (C2-ceramide 1-phosphate) enhances calcium entry in a dose-dependent manner. The phospholipase C inhibitor U73122 attenuated the response. C1P invoked a small, but significant, increase in the formation of inositol phosphates. Pre-treatment of the cells with pertussis toxin was without an effect on the C1P-evoked increase in [Ca2+]i. The effect of C1P was critically dependent on extracellular calcium, since no increase in [Ca2+]i was observed when cells in a calcium-free buffer were stimulated with C1P. Furthermore, if the cells were retreated with 300 nM of the VOCC inhibitor nimodipine, the effect of C1P was almost totally abolished. In addition, ceramide C8-1-phosphate evoked an increase in [Ca2+]i, but the onset of the response was slow compared with that of C1P. In cells treated with 1 mM thapsigargin for 15 min, C1P still evoked an increase in [Ca2+]i. In patch-clamp experiments in the whole-cell mode, C1P enhanced calcium entry through the VOCCs compared with vehicle-treated cells. Dialysis of the cells with C1P did not enhance the calcium current. On-cell patch-clamp experiments showed an enhanced probability of the VOCCs being open (P(open)) in the presence of C1P. Inhibition of PKC (protein kinase C) with GF109203X and down-regulation of PKC with PMA attenuated the C1P-evoked increase in [Ca2+]i. Furthermore, down-regulation of PKC abolished the effect of C1P on P(open). This is the first report showing that a sphingomyelin derivative enhances calcium entry through VOCCs.

Ceramide-1-phosphate: a novel regulator of cell activation

Ceramide-1-phosphate (CER-1-P) is emerging as a novel bioactive sphingolipid. It is formed by phosphorylation of ceramide catalyzed by ceramide kinase, and has been implicated in different cellular processes. Cer-1-P is mitogenic for fibroblasts, blocks apoptosis in macrophages, controls phagocytosis in neutrophils, and mediates inflammatory responses. Only recently have we started to uncover the signaling pathways that cytosolic phospholipase A2 and acid sphingomyelinase are direct intracellular targets of Cer-1-P, and that it may also induce phosphorylation of extracellular signal-regulated kinase-2 and calcium mobilization. These actions of Cer-1-P seem to be cell type-specific.

Ceramide 1-phosphate is a direct activator of cytosolic phospholipase A2

Recently, we demonstrated that ceramide kinase, and its product, ceramide 1-phosphate (Cer-1-P), were mediators of arachidonic acid released in cells in response to interleukin-1beta and calcium ionophore (Pettus, B. J., Bielawska, A., Spiegel, S., Roddy, P., Hannun, Y. A., and Chalfant, C. E. (2003) J. Biol. Chem. 278, 38206-38213). In this study, we demonstrate that down-regulation of cytosolic phospholipase A(2) (cPLA(2)) using RNA interference technology abolished the ability of Cer-1-P to induce arachidonic acid release in A549 cells, demonstrating that cPLA(2) is the key phospholipase A(2) downstream of Cer-1-P. Treatment of A549 cells with Cer-1-P (2.5 microm) induced the translocation of full-length cPLA(2) from the cytosol to the Golgi apparatus/perinuclear regions, which are known sites of translocation in response to agonists. Cer-1-P also induced the translocation of the CaLB/C2 domain of cPLA(2) in the same manner, suggesting that this domain is responsive to Cer-1-P either directly or indirectly. In vitro studies were then conducted to distinguish these two possibilities. In vitro binding studies disclosed that Cer-1-P interacts directly with full-length cPLA(2) and with the CaLB domain in a calcium- and lipid-specific manner with a K(Ca) of 1.54 microm. Furthermore, Cer-1-P induced a calcium-dependent increase in cPLA(2) enzymatic activity as well as lowering the EC(50) of calcium for the enzyme from 191 to 31 nm. This study identifies Cer-1-P as an anionic lipid that translocates and directly activates cPLA(2), demonstrating a role for this bioactive lipid in the mediation of inflammatory responses.

Ceramide kinase mediates cytokine- and calcium ionophore-induced arachidonic acid release

Despite the importance of prostaglandins, little is known about the regulation of prostanoid synthesis proximal to the activation of cytosolic phospholipase A2, the initial rate-limiting step. In this study, ceramide-1-phosphate (C-1-P) was shown to be a specific and potent inducer of arachidonic acid (AA) and prostanoid synthesis in cells. This study also demonstrates that two well established activators of AA release and prostanoid synthesis, the cytokine, interleukin-1beta (IL-1beta), and the calcium ionophore, A23187, induce an increase in C-1-P levels within the relevant time-frame of AA release. Furthermore, the enzyme responsible for the production of C-1-P in mammalian cells, ceramide kinase, was activated in response to IL-1beta and A23187. RNA interference targeted to ceramide kinase specifically down-regulated ceramide kinase mRNA and activity with a concomitant decrease of AA release in response to IL-1beta and A23187. Down-regulation of ceramide kinase had no effect on AA release induced by exogenous C-1-P. Collectively, these results indicate that ceramide kinase, via the formation of C-1-P, is an upstream modulator of phospholipase A2 activation. This study identifies previously unknown roles for ceramide kinase and its product, C-1-P, in AA release and production of eicosanoids and provides clues for potential new targets to block inflammatory responses.

Ceramide-1-phosphate blocks apoptosis through inhibition of acid sphingomyelinase in macrophages

It was reported previously that ceramide-1-phosphate (Cer-1-P) is mitogenic for fibroblasts (Gómez-Muñoz, A., P. A. Duffy, A. Martin, L. O'Brien, H-S. Byun, R. Bittman, and D. N. Brindley. 1995. Mol. Pharmacol. 47: 883-889; Gómez-Muñoz, A., L. M. Frago, L. Alvarez, and I. Varela-Nieto. 1997. Biochem. J. 325: 435-440). We now show that Cer-1-P prevents cell death in bone-marrow-derived macrophages (BMDMs) after withdrawal of macrophage colony-stimulating factor (M-CSF). Removal of M-CSF is known to induce apoptosis in these cells. Cer-1-P blocked activation of the caspase-9/caspase-3 pathway and prevented DNA fragmentation, indicating that the enhancement of cell survival was due to inhibition of apoptosis. M-CSF deprivation resulted in activation of acid sphingomyelinase (A-SMase), increased ceramide levels, and a decrease in intracellular Cer-1-P. Exogenously added Cer-1-P inhibited A-SMase in intact BMDMs at concentrations that also prevented apoptosis. Cer-1-P also inhibited A-SMase in cell homogenates, suggesting a possible direct physical interaction of Cer-1-P with the enzyme. In conclusion, these data demonstrate that Cer-1-P blocks apoptosis in BMDMs through inhibition of A-SMase, thereby reducing ceramide generation. This adds a new dimension to the understanding of the metabolic interrelationship of ceramides and Cer-1-P, and shows how altering the balance of intracellular levels of these mediators can affect cell survival.