Edg-6 as a putative sphingosine 1-phosphate receptor coupling to Ca(2+) signaling pathway

Sphingosylphosphocholine is a naturally occurring lipid mediator in blood plasma: a possible role in regulating cardiac function via sphingolipid receptors

Blood plasma and serum contain factors that activate inwardly rectifying GIRK1/GIRK4 K+ channels in atrial myocytes via one or more non-atropine-sensitive receptors coupled to pertussis-toxin-sensitive G-proteins. This channel is also the target of muscarinic M(2) receptors activated by the physiological release of acetylcholine from parasympathetic nerve endings. By using a combination of HPLC and TLC techniques with matrix-assisted laser desorption ionization-time-of-flight MS, we purified and identified sphingosine 1-phosphate (SPP) and sphingosylphosphocholine (SPC) as the plasma and serum factors responsible for activating the inwardly rectifying K+ channel (I(K)). With the use of MS the concentration of SPC was estimated at 50 nM in plasma and 130 nM in serum; those concentrations exceeded the 1.5 nM EC(50) measured in guinea-pig atrial myocytes. With the use of reverse-transcriptase-mediated PCR and/or Western blot analysis, we detected Edg1, Edg3, Edg5 and Edg8 as well as OGR1 sphingolipid receptor transcripts and/or proteins. In perfused guinea-pig hearts, SPC exerted a negative chronotropic effect with a threshold concentration of 1 microM. SPC was completely removed after perfusion through the coronary circulation at a concentration of 10 microM. On the basis of their constitutive presence in plasma, the expression of specific receptors, and a mechanism of ligand inactivation, we propose that SPP and SPC might have a physiologically relevant role in the regulation of the heart.

Sphingosine 1-phosphate receptors

Sphingosine-1-phosphate (SPP) is a bioactive lipid produced from the metabolism of sphingomyelin. It is an important constituent of serum and regulates cell growth, survival, migration, differentiation and gene expression. Its mode of action has been enigmatic; however, recent findings have shown that a family of G-protein-coupled receptors (GPCR) of the endothelial differentiation gene (EDG) family serve as plasma membrane-localized receptors for SPP. Furthermore, the EDG receptors appear to be SPP receptor subtypes with distinct signaling characteristics. In vascular endothelial cells, SPP acts on EDG-1 and EDG-3 subtypes of receptors to induce cell survival and morphogenesis. Such pathways appear to be critical for SPP-induced angiogenic response in vivo. In addition, the EDG-1 gene is essential for vascular maturation in development. Moreover, developmental studies in Zebrafish have indicated that SPP signaling via the EDG-5 like receptor Miles Apart (Mil) is essential for heart development. These data strongly suggest that a physiological role of SPP is in the formation of the cardiovascular system. Despite these recent findings, much needs to be clarified with respect to the physiological role of SPP synthesis and action. This review will focus on the recent findings on SPP receptors and the effects on the cardiovascular system.

Nrg-1 belongs to the endothelial differentiation gene family of G protein-coupled sphingosine-1-phosphate receptors

The previously cloned rat nerve growth factor-regulated G protein-coupled receptor NRG-1 (Glickman, M., Malek, R. L., Kwitek-Black, A. E., Jacob, H. J., and Lee N. H. (1999) Mol. Cell. Neurosci. 14, 141-52), also known as EDG-8, binds sphingosine-1-phosphate (S1P) with high affinity and specificity. In this paper we examined the signal transduction pathways regulated by the binding of S1P to EDG-8. In Chinese hamster ovary cells heterologously expressing EDG-8, S1P inhibited forskolin-induced cAMP accumulation and activated c-Jun NH2-terminal kinase. Surprisingly, S1P inhibited serum-induced activation of extracellular regulated protein kinase 1 and 2 (ERK1/2). Treatment with pertussis toxin, which ADP-ribosylates and inactivates G(i), blocked S1P-mediated inhibition of cAMP accumulation, but had no effect on c-Jun NH2-terminal kinase activation or inhibition of ERK1/2. The inhibitory effect of S1P on ERK1/2 activity was abolished by treatment with orthovanadate, suggesting the involvement of a tyrosine phosphatase. A subunit selective [35S] guanosine 5'-3-O-(thio)triphosphate binding assay demonstrates that EDG-8 activated G(i/o) and G12 but not Gs and G(q/11) in response to S1P. In agreement, EDG-8 did not stimulate phosphoinositide turnover or cAMP accumulation. The ability of S1P to induce mitogenesis in cells expressing the EDG-1 subfamily of G protein-coupled receptors is well characterized. In contrast, S1P inhibited proliferation in Chinese hamster ovary cells expressing EDG-8 but not empty vector. The antiproliferative effect, like S1P-mediated ERK1/2 inhibition, was orthovanadate-sensitive and pertussis toxin-insensitive. Our results indicate that EDG-8, a member of the EDG-1 subfamily, couples to unique signaling pathways.

Lysophosphatidic acid-induced mitogenesis is regulated by lipid phosphate phosphatases and is Edg-receptor independent

Lysophosphatidic acid (LPA) is an extracellular signaling mediator with a broad range of cellular responses. Three G-protein-coupled receptors (Edg-2, -4, and -7) have been identified as receptors for LPA. In this study, the ectophosphatase lipid phosphate phosphatase 1 (LPP1) has been shown to down-regulate LPA-mediated mitogenesis. Furthermore, using degradation-resistant phosphonate analogs of LPA and stereoselective agonists of the Edg receptors we have demonstrated that the mitogenic and platelet aggregation responses to LPA are independent of Edg-2, -4, and -7. Specifically, we found that LPA degradation is insufficient to account for the decrease in LPA potency in mitogenic assays, and the stereoselectivity observed at the Edg receptors is not reflected in mitogenesis. Additionally, RH7777 cells, which are devoid of Edg-2, -4, and -7 receptor mRNA, have a mitogenic response to LPA and LPA analogs. Finally, we have determined that the ligand selectivity of the platelet aggregation response is consistent with that of mitogenesis, but not with Edg-2, -4, and -7.

Differential roles of Edg-1 and Edg-5, sphingosine 1-phosphate receptors, in the signaling pathways in C6 glioma cells

Several cDNA encoding G-protein-coupled receptors, i.e. Edg-1,-3,-5,-6 and -8, have recently been identified as sphingosine 1-phosphate (S1P) receptors. However, the role of the respective receptor subtype has not been well defined. In C6 glioma cells, exogenous S1P induced expression of fibroblast growth factor-2 (FGF-2), a potent neurotrophic factor, which was associated with the stimulation of extracellular signal-regulated kinase (ERK) and the expression of early growth response-1 (Egr-1). S1P also stimulated phospholipase C (PLC)/Ca(2+) system and phospholipase D (PLD). In this study, we sought to identify S1P receptors responsible for these S1P-induced actions. Of five S1P receptor subtypes, Edg-1 and Edg-5 are expressed in the glioma cells, as evidenced by Northern blotting. We therefore prepared the cells overexpressing these S1P receptor subtypes and compared the intrinsic activities to stimulate these signaling pathways and their sensitivity to pertussis toxin (PTX). The potency of S1P and dihydrosphingosine 1-phosphate (DHS1P), another S1P receptor agonist, to stimulate the Edg-1 and Edg-5 receptors was also examined. We found that the intrinsic activity that stimulated ERK/Egr-1/FGF-2 system was much higher in Edg-1 than in Edg-5. Furthermore, DHS1P was as potent as S1P in activating ERK in control C6 cells, a pattern also observed in cells overexpressing Edg-1. On the other hand, the stimulation of the PLC/Ca(2+) system and PLD induced by S1P was PTX-insensitive, and the potency of S1P in activating PLD was roughly one order higher than that of DHS1P in control C6 cells; similar responsiveness to such pharmacological tools were observed in Edg-5-overexpressing cells. Taken together, these results suggest that Edg-1 may be the main receptor mediating the stimulation of ERK/Egr-1/FGF-2 system but that Edg-5 may be responsible for the stimulation of PLC-Ca(2+) system and PLD in native C6 glioma cells.

Characterization of a zebrafish (Danio rerio) sphingosine 1-phosphate receptor expressed in the embryonic brain

Sphingosine 1-phosphate elicits a variety of responses in mammals via at least five G protein-coupled Edg receptors. We cloned zebrafish edg1 and expressed it in Rh7777 cells. In these cultures, S1P inhibited forskolin-driven rises in cAMP and this response was eliminated by pretreatment of the cultures with pertussis toxin. In Rh7777 membranes, S1P stimulated GTPgamma[(35)S] binding 2-3 fold. Zebrafish edg1 is expressed in embryonic brain, particularly ventral diencephalon, optic stalks, and anterior hindbrain. Our findings suggest that nonmammalian vertebrates use S1P to signal during embryogenesis and that the properties of Edg1 receptor have been conserved for 400 million years.

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.

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.

Mammalian PSP24s (alpha and beta isoforms) are not responsive to lysophosphatidic acid in mammalian expression systems

Xenopus PSP24 (xPSP24) is a G-protein-coupled receptor which was originally identified as a functional lysophosphatidic acid (LPA) receptor. We obtained two different types (alpha and beta) of mammalian homologues of xPSP24 and found that these receptors are highly expressed in the brain (Kawasawa et al., Biochem. Biophys. Res. Commun. 276, 952-956, 2000). These receptor did not respond to LPA by GTPgammaS binding assays, while Edg2 or Edg4 showed responses to LPA under the same assay conditions. Furthermore, a sensitive reporter gene assay using PC12 cells with serum response element promoter failed to detect any response of mammalian PSP24s to various concentrations of LPA. Thus, unlike xPSP24, we conclude that PSP24s are not functional LPA receptors in mammalian systems.

Potential role of EDG receptors and lysophospholipids as their endogenous ligands in the respiratory tract

The role of lipid mediators derived from membrane glycerophospholipids and sphingolipids as intracellular messenger has been studied intensively during the last two decades, but with the recent discovery of high affinity G-protein coupled receptors for the lysophospholipids lysophosphatidic acid (LPA), sphingosine-1-phosphate (S1P) and sphingosylphosphorylcholine (SPC), increasing attention has been paid to the role of these lipid mediators as extracellular mediators. This review will summarize the biosynthesis and metabolism of lysophospholipids and describe the family of endothelial differentiation gene (EDG) receptors as high affinity receptors for lysophospholipids. Furthermore, an overview of the numerous biological effects of lysophospholipids which might be mediated by EDG receptors will be given together with an outlook on the potential role of such mechanisms in pulmonary physiology and pathophysiology.

Expression and characterization of Edg-1 receptors in rat cardiomyocytes: calcium deregulation in response to sphingosine 1-phosphate

Recent evidence indicates that sphingolipids are produced by the heart during hypoxic stress and by blood platelets during thrombus formation. It is therefore possible that sphingolipids may influence heart cell function by interacting with G-protein-coupled receptors of the Edg family. In the present study, it was found that sphingosine 1-phosphate (Sph1P), the prototypical ligand for Edg receptors, produced calcium overload in rat cardiomyocytes. The cDNA for Edg-1 was cloned from rat cardiomyocytes and, when transfected in an antisense orientation, effectively blocked Edg-1 protein expression and reduced the Sph1P-mediated calcium deregulation. Taken together, these results demonstrate that cardiomyocytes express an extracellular lipid-sensitive receptorsystem that can respond to sphingolipid mediators. Because the major source of Sph1P is from blood platelets, we speculate that Edg-mediated Sph1P negative inotropic and cardiotoxic effects may play important roles in acute myocardial ischemia where Sph1P levels are probably elevated in response to thrombus.

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.

Sphingosylphosphorylcholine induces endothelial cell migration and morphogenesis

Sphingosylphosphorylcholine (SPC) is one of the biologically active phospholipids that may act as extracellular messengers. Particularly important is the role of these lipids in the angiogenic response, a complex process involving endothelial cell migration, proliferation, and morphologic differentiation. Here we demonstrate that SPC and its hydrolytic product, sphingosine, induce chemotactic migration of human and bovine endothelial cells. The response is approximately equal to that elicited by vascular endothelial cell growth factor. The effect of SPC and sphingosine was associated with a rapid down-regulation of Edg1, a sphingosine 1-phosphate (SPP)-specific receptor involved in endothelial cell chemotaxis. Both SPC and sphingosine induced differentiation of endothelial cells into capillary-like structures in vitro. Thus, SPC and sphingosine join SPP among the biologically active lipids with angiogenic potential. Since neuronal abnormalities accompany pathological accumulation of SPC in brain tissue, it is possible that SPC is a modulator of angiogenesis in neural tissue upon its release from brain cells following trauma or neoplastic growth.