Activation of sphingosine kinase by tumor necrosis factor-alpha inhibits apoptosis in human endothelial cells

Sphingosine kinase regulates hepatoma cell differentiation: roles of hepatocyte nuclear factor and retinoid receptor

In hepatoma Huh-7 cells, inhibition of sphingosine kinase (SphK) activity by N,N-dimethylsphingosine (DMS) resulted in up-regulated production of liver-specific serum proteins including albumin and alpha-fetoprotein (AFP). The changes in these protein levels coincided well with those of two liver-enriched transcription factors, hepatocyte nuclear factor (HNF)-1 and -4, which regulate a number of liver-specific genes at the transcriptional level. Moreover, DMS induced the expression of retinoic acid receptor-alpha and retinoid X receptor-alpha. In DMS-treated cells, 9-cis retinoic acid (RA) further enhanced HNF-4alpha and albumin expression but it inhibited AFP accumulation. These results suggest that activation of SphK disengages cells from their liver-specific phenotype, and that 9-cis RA further induces differentiation of hepatoma cells when SphK activity is inhibited.

Sphingosine 1-phosphate may be a major component of plasma lipoproteins responsible for the cytoprotective actions in human umbilical vein endothelial cells

Sphingosine 1-phosphate (S1P), a novel lipid mediator, is concentrated in the fraction of lipoproteins that include high density lipoprotein (HDL) and low density lipoprotein (LDL) in human plasma. Here, we show that oxidation of LDL resulted in a marked reduction in the S1P level in association with a marked accumulation of lysophosphatidylcholine (LPC). We therefore investigated the role of the lipoprotein-associated lipids especially S1P in the lipoprotein-induced cytoprotective or cytotoxic actions in human umbilical vein endothelial cells. The viability of the cells gradually decreased in the absence of serum or growth factors in the culture medium. The addition of oxidized LDL (ox-LDL) accelerated the decrease in the cell viability. LPC and 7-ketocholesterol mimicked ox-LDL actions. On the other hand, HDL and LDL almost completely reversed the serum deprivation- or ox-LDL-induced cytotoxicity. Exogenous S1P mimicked cytoprotective actions. Moreover, the S1P-rich fraction and chromatographically purified S1P from HDL exerted cytoprotective actions, but the rest of the fractions did not. The cytoprotective actions of HDL and S1P were associated with extracellular signal-regulated kinase (ERK) activation and were almost completely inhibited by pertussis toxin and PD98059, an ERK kinase inhibitor. The HDL-induced action was specifically desensitized in the S1P-pretreated cells. Taken together, these results indicate that the lipoprotein-associated S1P and the lipid receptor-mediated signal pathways may be responsible for the lipoprotein-induced cytoprotective actions. Furthermore, the decrease in the S1P content, in addition to the accumulation of cytotoxic substances such as LPC, may be important for the acquisition of the cytotoxic property to ox-LDL.

TNF-alpha-induced sphingosine 1-phosphate inhibits apoptosis through a phosphatidylinositol 3-kinase/Akt pathway in human hepatocytes

Human hepatocytes usually are resistant to TNF-alpha cytotoxicity. In mouse or rat hepatocytes, repression of NF-kappaB activation is sufficient to induce TNF-alpha-mediated apoptosis. However, in both Huh-7 human hepatoma cells and Hc human normal hepatocytes, when infected with an adenovirus expressing a mutated form of IkappaBalpha (Ad5IkappaB), which almost completely blocks NF-kappaB activation, >80% of the cells survived 24 h after TNF-alpha stimulation. Here, we report that TNF-alpha activates other antiapoptotic factors, such as sphingosine kinase (SphK), phosphatidylinositol 3-kinase (PI3K), and Akt kinase. Pretreatment of cells with N,N-dimethylsphingosine (DMS), an inhibitor of SphK, or LY 294002, an inhibitor of PI3K that acts upstream of Akt, increased the number of apoptotic cells induced by TNF-alpha in Ad5IkappaB-infected Huh-7 and Hc cells. TNF-alpha-induced activations of PI3K and Akt were inhibited by DMS. In contrast, exogenous sphingosine 1-phosphate, a product of SphK, was found to activate Akt and partially rescued the cells from TNF-alpha-induced apoptosis. Although Akt has been reported to activate NF-kappaB, DMS and LY 294002 failed to prevent TNF-alpha-induced NF-kappaB activation, suggesting that the antiapoptotic effects of SphK and Akt are independent of NF-kappaB. Furthermore, apoptosis mediated by Fas ligand (FasL) involving Akt activation also was potentiated by DMS pretreatment in Hc cells. Sphingosine 1-phosphate administration partially protected cells from FasL-mediated apoptosis. These results indicate that not only NF-kappaB but also SphK and PI3K/Akt are involved in the signaling pathway(s) for protection of human hepatocytes from the apoptotic action of TNF-alpha and probably FasL.

Inhibition of phosphatidylinositol-3 kinase/Akt or mitogen-activated protein kinase signaling sensitizes endothelial cells to TNF-alpha cytotoxicity

Bovine carotid artery endothelial (BAE) cells are resistant to tumor necrosis factor-alpha (TNF), like most other cells. We examined if mitogen-activated protein (MAP) kinase and phosphatidylinositol-3 (PI3) kinase/Akt pathways are involved in this effect. In BAE cells, TNF activates MAP kinase in a MAP kinase kinase 1 (MEK1) manner and Akt in PI3-kinase-dependent manner. Pretreatment with either the MEK1 inhibitor U0126 or PI3-kinase inhibitor LY294002 sensitized BAE cells to TNF-induced apoptosis. Neither U0126 nor LY294002 pretreatment affected TNF-induced activation of NF-kappaB, suggesting that the MAP kinase or PI3-kinase/Akt-mediated anti-apoptotic effect induced by TNF was not relevant to NF-kappaB activation. Both MAP kinase and PI3-kinase/Akt -mediated signaling could prevent cytochrome c release and mitochondrial transmembrane potential (Deltapsi) decrease. PI3-kinase/Akt signaling attenuated caspase-8 activity, whereas MAP kinase signaling impaired caspase-9 activity. These results suggest that TNF-induced MAP kinase and PI3-kinase/Akt signaling play important roles in protecting BAE cells from TNF cytotoxicity.

Sphingosine 1-phosphate protects human umbilical vein endothelial cells from serum-deprived apoptosis by nitric oxide production

Sphingosine 1-phosphate (S1P) can prevent endothelial cell apoptosis. We investigated the molecular mechanisms and signaling pathways by which S1P protects endothelial cells from serum deprivation-induced apoptosis. We show here that human umbilical vein endothelial cells (HUVECs) undergo apoptosis associated with increased DEVDase activity, caspase-3 activation, cytochrome c release, and DNA fragmentation after 24 h of serum deprivation. These apoptotic markers were suppressed by the addition of S1P, the NO donor S-nitroso-N-acetylpenicillamine (100 micrometer), or caspase-3 inhibitor z-VAD-fmk. The protective effects of S1P were reversed by the nitric-oxide synthase (NOS) inhibitor N-monomethyl-l-arginine, but not by the soluble guanylyl cyclase inhibitor 1H-(1,2,4)oxadiazolo[4,3-a]-quanoxaline-1-one, suggesting that NO, but not cGMP, is responsible for S1P protection from apoptosis. Furthermore, S1P increased NO production by enhancing Ca(2+)-sensitive NOS activity without changes in the eNOS protein level. S1P-mediated cell survival and NO production were suppressed significantly by pretreatment with antisense oligonucleotide of EDG-1 and partially by EDG-3 antisense. S1P-mediated NO production was suppressed by the addition of pertussis toxin, an inhibitor of G(i) proteins, the specific inhibitor of phospholipase C (PLC), and the Ca(2+) chelator BAPTA-AM. These findings indicate that S1P protects HUVECs from apoptosis through the activation of eNOS activity mainly through an EDG-1 and -3/G(i)/PLC/Ca(2+) signaling pathway.

Receptor-mediated control of regulatory volume decrease (RVD) and apoptotic volume decrease (AVD)

A fundamental property of animal cells is the ability to regulate their own cell volume. Even under hypotonic stress imposed by either decreased extracellular or increased intracellular osmolarity, the cells can re-adjust their volume after transient osmotic swelling by a mechanism known as regulatory volume decrease (RVD). In most cell types, RVD is accomplished mainly by KCl efflux induced by parallel activation of K+ and Cl- channels. We have studied the molecular mechanism of RVD in a human epithelial cell line (Intestine 407). Osmotic swelling results in a significant increase in the cytosolic Ca2+ concentration and thereby activates intermediate-conductance Ca2+-dependent K+ (IK) channels. Osmotic swelling also induces ATP release from the cells to the extracellular compartment. Released ATP stimulates purinergic ATP (P2Y2) receptors, thereby inducing phospholipase C-mediated Ca2+ mobilization. Thus, RVD is facilitated by stimulation of P2Y2 receptors due to augmentation of IK channels. In contrast, stimulation of another G protein-coupled Ca2+-sensing receptor (CaR) enhances the activity of volume-sensitive outwardly rectifying Cl- channels, thereby facilitating RVD. Therefore, it is possible that Ca2+ efflux stimulated by swelling-induced and P2Y2 receptor-mediated intracellular Ca2+ mobilization activates the CaR, thereby secondarily upregulating the volume-regulatory Cl- conductance. On the other hand, the initial process towards apoptotic cell death is coupled to normotonic cell shrinkage, called apoptotic volume decrease (AVD). Stimulation of death receptors, such as TNF receptor and Fas, induces AVD and thereafter biochemical apoptotic events in human lymphoid (U937), human epithelial (HeLa), mouse neuroblastoma x rat glioma hybrid (NG108-15) and rat phaeochromocytoma (PC12) cells. In those cells exhibiting AVD, facilitation of RVD is always observed. Both AVD induction and RVD facilitation as well as succeeding apoptotic events can be abolished by prior treatment with a blocker of volume-regulatory K+ or Cl- channels, suggesting that AVD is caused by normotonic activation of ion channels that are normally involved in RVD under hypotonic conditions. Therefore, it is likely that G protein-coupled receptors involved in RVD regulation and death receptors triggering AVD may share common downstream signals which should give us key clues to the detailed mechanisms of volume regulation and survival of animal cells. In this Topical Review, we look at the physiological ionic mechanisms of cell volume regulation and cell death-associated volume changes from the facet of receptor-mediated cellular processes.

Sphingosine kinase: a mediator of vital cellular functions

Ample evidence indicates that sphingosine-1-phosphate (SPP) can serve as an intracellular second messenger regulating calcium mobilization, and cell growth and survival. Moreover, the dynamic balance between levels of the sphingolipids metabolites, ceramide and SPP, and consequent regulation of opposing signaling pathways, is an important factor that determines whether a cell survives or dies. SPP has also recently been shown to be the ligand for the EDG-1 family of G protein-coupled receptors, which now includes EDG-1, -3, -5, -6, and -8. SPP is thus a lipid mediator that has novel dual actions signaling inside and outside of the cell. This review is focussed on sphingosine kinase, the enzyme that regulates levels of SPP and thus plays a critical role in diverse biological processes.

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.

An oncogenic role of sphingosine kinase

Sphingosine kinase (SphK) is a highly conserved lipid kinase that phosphorylates sphingosine to form sphingosine-1-phosphate (S1P). S1P/SphK has been implicated as a signalling pathway to regulate diverse cellular functions [1-3], including cell growth, proliferation and survival [4-8]. We report that cells overexpressing SphK have increased enzymatic activity and acquire the transformed phenotype, as determined by focus formation, colony growth in soft agar and the ability to form tumours in NOD/SCID mice. This is the first demonstration that a wild-type lipid kinase gene acts as an oncogene. Using a chemical inhibitor of SphK, or an SphK mutant that inhibits enzyme activation, we found that SphK activity is involved in oncogenic H-Ras-mediated transformation, suggesting a novel signalling pathway for Ras activation. The findings not only point to a new signalling pathway in transformation but also to the potential of SphK inhibitors in cancer therapy.

Sphingosine 1-phosphate signalling via the endothelial differentiation gene family of G-protein-coupled receptors

Sphingosine 1-phosphate (S1P) is stored in and released from platelets in response to cell activation. However, recent studies show that it is also released from a number of cell types, where it can function as a paracrine/autocrine signal to regulate cell proliferation, differentiation, survival, and motility. This review discusses the role of S1P in cellular regulation, both at the molecular level and in terms of health and disease. The main biochemical routes for S1P synthesis (sphingosine kinase) and degradation (S1P lyase and S1P phosphatase) are described. The major focus is on the ability of S1P to bind to a novel family of G-protein-coupled receptors (endothelial differentiation gene [EDG]-1, -3, -5, -6, and -8) to elicit signal transduction (via G(q)-, G(i)-, G(12)-, G(13)-, and Rho-dependent routes). Effector pathways regulated by S1P are divergent, such as extracellular signal-regulated kinase, p38 mitogen-activated protein kinase, phospholipases C and D, adenylyl cyclase, and focal adhesion kinase, and occur in multiple cell types, such as immune cells, neurones, smooth muscle, etc. This provides a molecular basis for the ability of S1P to act as a pleiotropic bioactive lipid with an important role in cellular regulation. We also give an account of the expanding role for S1P in health and disease; in particular, with regard to its role in atherosclerosis, angiogenesis, cancer, and inflammation. Finally, we describe future directions for S1P research and novel approaches whereby S1P signalling can be manipulated for therapeutic intervention in disease.

Expression of a catalytically inactive sphingosine kinase mutant blocks agonist-induced sphingosine kinase activation. A dominant-negative sphingosine kinase

Sphingosine kinase (SK) catalyzes the formation of sphingosine 1-phosphate (S1P), a lipid messenger that plays an important role in a variety of mammalian cell processes, including inhibition of apoptosis and stimulation of cell proliferation. Basal levels of S1P in cells are generally low but can increase rapidly when cells are exposed to various agonists through rapid and transient activation of SK activity. To date, elucidation of the exact signaling pathways affected by these elevated S1P levels has relied on the use of SK inhibitors that are known to have direct effects on other enzymes in the cell. Furthermore, these inhibitors block basal SK activity, which is thought to have a housekeeping function in the cell. To produce a specific inhibitor of SK activation we sought to generate a catalytically inactive, dominant-negative SK. This was accomplished by site-directed mutagenesis of Gly(82) to Asp of the human SK, a residue identified through sequence similarity to the putative catalytic domain of diacylglycerol kinase. This mutant had no detectable SK activity when expressed at high levels in HEK293T cells. Activation of endogenous SK activity by tumor necrosis factor-alpha (TNFalpha), interleukin-1beta, and phorbol esters in HEK293T cells was blocked by expression of this inactive sphingosine kinase (hSK(G82D)). Basal SK activity was unaffected by expression of hSK(G82D). Expression of hSK(G82D) had no effect on TNFalpha-induced activation of protein kinase C and sphingomyelinase activities. Thus, hSK(G82D) acts as a specific dominant-negative SK to block SK activation. This discovery provides a powerful tool for the elucidation of the exact signaling pathways affected by elevated S1P levels following SK activation. To this end we have employed the dominant-negative SK to demonstrate that TNFalpha activation of extracellular signal-regulated kinases 1 and 2 (ERK1,2) is dependent on SK activation.

Agonist-modulated targeting of the EDG-1 receptor to plasmalemmal caveolae. eNOS activation by sphingosine 1-phosphate and the role of caveolin-1 in sphingolipid signal transduction

Plasmalemmal caveolae are membrane microdomains that are specifically enriched in sphingolipids and contain a wide array of signaling proteins, including the endothelial isoform of nitric-oxide synthase (eNOS). EDG-1 is a G protein-coupled receptor for sphingosine 1-phosphate (S1P) that is expressed in endothelial cells and has been implicated in diverse vascular signal transduction pathways. We analyzed the subcellular distribution of EDG-1 in COS-7 cells transiently transfected with cDNA constructs encoding epitope-tagged EDG-1. Subcellular fractionation of cell lysates resolved by ultracentrifugation in discontinuous sucrose gradients revealed that approximately 55% of the EDG-1 protein was recovered in fractions enriched in caveolin-1, a resident protein of caveolae. Co-immunoprecipitation experiments showed that EDG-1 could be specifically precipitated by antibodies directed against caveolin-1 and vice versa. The targeting of EDG-1 to caveolae-enriched fractions was markedly increased (from 51 +/- 11% to 93 +/- 14%) by treatment of transfected cells with S1P (5 microm, 60 min). In co-transfection experiments expressing EDG-1 and eNOS cDNAs in COS-7 cells, we found that S1P treatment significantly and specifically increased nitric-oxide synthase activity, with an EC(50) of 30 nm S1P. Overexpression of transfected caveolin-1 cDNA together with EDG-1 and eNOS markedly diminished S1P-mediated eNOS activation; caveolin overexpression also attenuated agonist-induced phosphorylation of EDG-1 receptor by >90%. These results suggest that the interaction of the EDG-1 receptor with caveolin may serve to inhibit signaling through the S1P pathway, even as the targeting of EDG-1 to caveolae facilitates the interactions of this receptor with ligands and effectors that are also targeted to caveolae. The agonist-modulated targeting of EDG-1 to caveolae and its dynamic inhibitory interactions with caveolin identify new points for regulation of sphingolipid-dependent signaling in the vascular wall.

Neutral/alkaline and acid ceramidase activities are actively released by murine endothelial cells

Ceramidases (CDase(s)) play a key role in sphingolipid metabolism by hydrolyzing ceramide into sphingosine. Here we report that murine endothelial cells, macrophages, and human fibroblasts are all able to release acid as well as neutral/alkaline CDase activities in the culture medium. Endothelial cells were characterized by the highest specific activity of cellular as well as secreted CDases. The release of both enzymatic activities was reduced by protein synthesis inhibitor cycloheximide but was unaffected by the blocking of RNA transcription with actinomycin D. The discharge of acid and neutral/alkaline CDases was also diminished by brefeldin A, a fungal metabolite which disrupts Golgi apparatus. Remarkably, treatment of endothelial cells with bradykinin resulted in a significant increase of neutral/alkaline but not acid CDase release. This report represents the first evidence for the existence of constitutive and regulated release of CDase activities by endothelial cells. In view of the known ability of these cells to secrete sphingomyelinase, this finding suggests that CDase may participate in extracellular sphingomyelin metabolism which is presently known to have a role in atherogenesis and could be involved in other physiological or pathological events.

Sphingosine 1-phosphate signalling in mammalian cells

Sphingosine 1-phosphate is formed in cells in response to diverse stimuli, including growth factors, cytokines, G-protein-coupled receptor agonists, antigen, etc. Its production is catalysed by sphingosine kinase, while degradation is either via cleavage to produce palmitaldehyde and phosphoethanolamine or by dephosphorylation. In this review we discuss the most recent advances in our understanding of the role of the enzymes involved in metabolism of this lysolipid. Sphingosine 1-phosphate can also bind to members of the endothelial differentiation gene (EDG) G-protein-coupled receptor family [namely EDG1, EDG3, EDG5 (also known as H218 or AGR16), EDG6 and EDG8] to elicit biological responses. These receptors are coupled differentially via G(i), G(q), G(12/13) and Rho to multiple effector systems, including adenylate cyclase, phospholipases C and D, extracellular-signal-regulated kinase, c-Jun N-terminal kinase, p38 mitogen-activated protein kinase and non-receptor tyrosine kinases. These signalling pathways are linked to transcription factor activation, cytoskeletal proteins, adhesion molecule expression, caspase activities, etc. Therefore sphingosine 1-phosphate can affect diverse biological responses, including mitogenesis, differentiation, migration and apoptosis, via receptor-dependent mechanisms. Additionally, sphingosine 1-phosphate has been proposed to play an intracellular role, for example in Ca(2+) mobilization, activation of non-receptor tyrosine kinases, inhibition of caspases, etc. We review the evidence for both intracellular and extracellular actions, and extensively discuss future approaches that will ultimately resolve the question of dual action. Certainly, sphingosine 1-phosphate will prove to be unique if it elicits both extra- and intra-cellular actions. Finally, we review the evidence that implicates sphingosine 1-phosphate in pathophysiological disease states, such as cancer, angiogenesis and inflammation. Thus there is a need for the development of new therapeutic compounds, such as receptor antagonists. However, identification of the most suitable targets for drug intervention requires a full understanding of the signalling and action profile of this lysosphingolipid. This article describes where the research field is in relation to achieving this aim.

Functional characterization of human sphingosine kinase-1

Sphingosine kinase catalyzes the phosphorylation of sphingosine to form sphingosine 1-phosphate (SPP), a novel lipid mediator with both intra- and extracellular functions. Based on sequence identity to murine sphingosine kinase (mSPHK1a), we cloned and characterized the first human sphingosine kinase (hSPHK1). The open reading frame of hSPHK1 encodes a 384 amino acid protein with 85% identity and 92% similarity to mSPHK1a at the amino acid level. Similar to mSPHK1a, when HEK293 cells were transfected with hSPHK1, there were marked increases in sphingosine kinase activity resulting in elevated SPP levels. hSPHK1 also specifically phosphorylated D-erythro-sphingosine and to a lesser extent sphinganine, but not other lipids, such as D,L-threo-dihydrosphingosine, N, N-dimethylsphingosine, diacylglycerol, ceramide, or phosphatidylinositol. Northern analysis revealed that hSPHK1 was widely expressed with highest levels in adult liver, kidney, heart and skeletal muscle. Thus, hSPHK1 belongs to a highly conserved unique lipid kinase family that regulates diverse biological functions.

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.

Sphingosine-1-phosphate is a ligand for the G protein-coupled receptor EDG-6

EDG-6 is a recently cloned member of the endothelial differentiation gene (EDG) G protein-coupled receptor family that is expressed in lymphoid and hematopoietic tissue and in the lung. Homology of EDG-6 to the known sphingosine-1-phosphate (SPP) receptors EDG-1, EDG-3, and EDG-5 and lysophosphatidic acid (LPA) receptors EDG-2 and EDG-4 suggested that its ligand may be a lysophospholipid or lysosphingolipid. We examined the binding of [32P]SPP to HEK293 cells, transiently transfected with cDNA encoding EDG-6. Binding of [32P]SPP was saturable, demonstrating high affinity (KD = 63 nmol/L). Binding was also specific for SPP, as only unlabeled SPP and sphinganine-1-phosphate, which lacks the trans double bond at the 4 position, potently displaced radiolabeled SPP. LPA did not compete for binding of SPP at any concentration tested, whereas sphingosylphosphorylcholine competed for binding to EDG-6, but only at very high concentrations. In addition, SPP activated extracellular signal-regulated kinase (Erk) in EDG-6 transfected cells in a pertussis toxin-sensitive manner. These results indicate that EDG-6 is a high affinity receptor for SPP, which couples to a Gi/o protein, resulting in the activation of growth-related signaling pathways.

Sphingosine-1-phosphate is a ligand for the G protein-coupled receptor EDG-6

EDG-6 is a recently cloned member of the endothelial differentiation gene (EDG) G protein-coupled receptor family that is expressed in lymphoid and hematopoietic tissue and in the lung. Homology of EDG-6 to the known sphingosine-1-phosphate (SPP) receptors EDG-1, EDG-3, and EDG-5 and lysophosphatidic acid (LPA) receptors EDG-2 and EDG-4 suggested that its ligand may be a lysophospholipid or lysosphingolipid. We examined the binding of [32P]SPP to HEK293 cells, transiently transfected with cDNA encoding EDG-6. Binding of [32P]SPP was saturable, demonstrating high affinity (KD = 63 nmol/L). Binding was also specific for SPP, as only unlabeled SPP and sphinganine-1-phosphate, which lacks the trans double bond at the 4 position, potently displaced radiolabeled SPP. LPA did not compete for binding of SPP at any concentration tested, whereas sphingosylphosphorylcholine competed for binding to EDG-6, but only at very high concentrations. In addition, SPP activated extracellular signal-regulated kinase (Erk) in EDG-6 transfected cells in a pertussis toxin-sensitive manner. These results indicate that EDG-6 is a high affinity receptor for SPP, which couples to a Gi/o protein, resulting in the activation of growth-related signaling pathways.