Molecular cloning and characterization of a novel human G-protein-coupled receptor, EDG7, for lysophosphatidic acid

Fatty acid composition of lysophosphatidic acid and lysophosphatidylinositol in plasma from patients with ovarian cancer and other gynecological diseases

OBJECTIVE

We previously reported that plasma levels of total lysophosphatidic acid (LPA) represented a potential biomarker for ovarian cancer and other gynecological cancers [1]. However, total LPA is composed of different LPA species with distinct fatty acid chains. The major objective of the current study, therefore, was to determine whether one or more specific fatty acid LPA species was associated with disease or disease staging. If this was determined, these species could be useful in further improving the sensitivity and/or specificity of this biomarker for the diagnosis and/or prognosis of the disease. Because lysophosphatidylinositol (LPI) co-migrates with LPA, this study represents the analysis of combined molecular species from both lysolipid classes.

METHODS

The patient population, sample collection, and analyses have been reported previously [1]. Lipids were hydrolyzed from the LPA band on thin-layer chromatography plates. The following individual fatty acid species were analyzed by gas chromatography: palmitic acid (16:0), stearic acid (18:0), oleic acid (18:1), linoleic acid (18:2), arachidonic acid (20:4), and docosahexaenoic acid (22:6). The LPA/LPI fatty acid composition levels were analyzed and compared with disease status.

RESULTS

Distinct plasma LPA/LPI fatty acid chain species were not associated with ovarian or other gynecological cancers, compared to patients with benign gynecological disease or healthy controls. However, an increased presence of unsaturated fatty acids in plasma LPA/LPI was found in patients with late-stage or recurrent ovarian cancer and possibly with other gynecological cancers.

CONCLUSIONS

Analysis of individual fatty acid species present in plasma LPA/LPI do not appear to enhance the sensitivity or specificity of total LPA/LPI as a marker for gynecological cancer detection. However, our results suggest that increased LPA/LPI species with unsaturated fatty acid chains may be associated with late-stage or recurrent ovarian cancer.

Neurobiology of the Edg2 lysophosphatidic acid receptor

Lysophosphatidic acid (LPA, 1-acyl-sn-glycerol-3-phosphate) is a well-known lipid growth factor that is found widely in various tissues including brain and is reported to drive different intracellular signaling pathways. In the nervous system, LPA studies have drawn many neuroscientists' attention because it has some actions related to neurogenesis such as cell rounding and proliferation. Remarkable advances in this field have been obtained along with the discovery of the cDNA clone for its receptor, vzg1/edg2, a member of the seven transmembrane-type edg family. Successive studies have revealed that edg2 activation by LPA mediates several neurobiological actions related to neurogenesis, neuronal excitability and survival activity on developing and postnatal neurons. Here we focused their molecular basis of signaling through G proteins and in vivo roles of edg2 in such neurobiological events.

Ceramide inhibition of mammalian phospholipase D1 and D2 activities is antagonized by phosphatidylinositol 4,5-bisphosphate

Ceramides inhibit phospholipase D (PLD) activity in several mammalian cell types. These effects have been related to preventing activation by ARF1, RhoA, and protein kinase C-alpha and -beta and therefore indicate that PLD1 is inhibited. In the present work, we investigated the effects of ceramides in inhibiting both PLD1 and PLD2 and the interaction with another activator, phosphatidylinositol 4,5-bisphosphate (PIP2). PLD1 and PLD2 were overexpressed separately in Sf9 insect cells using baculovirus vectors. In our cell-free system, PLD1 activity was inhibited completely by C2-ceramide at sub-optimum concentrations of PIP2 (3 and 6 microM), whereas at supra-optimum PIP2 concentrations (18 and 24 microM) C2-ceramide did not inhibit PLD1 activity. Partially purified PLD2 exhibited an absolute requirement for PIP2 when the activity was measured using Triton X-100 micelles. Ceramides inhibited PLD2 activity, and this inhibition was decreased as PIP2 concentrations increased. However, C2-ceramide also reversibly inhibited the activity of PLD1 and PLD2 mutants in which binding of PIP2 was decreased, indicating that ceramides are interacting with the catalytic core of the mammalian PLDs. By contrast, C2-ceramide failed to produce a significant inhibition of PLDs from bacteria and plants. Our results provide a novel demonstration that ceramides reversibly inhibit mammalian PLD2 as well as PLD1 activities and that both of these actions are more pronounced when PIP2 concentrations are rate-limiting.

Phenotypic modulation of vascular smooth muscle cells induced by unsaturated lysophosphatidic acids

The phenotypic modulation of vascular smooth muscle cells (VSMCs) from the differentiated state to the dedifferentiated one is critically involved in the development and progression of atherosclerosis. Although many cytokines and growth factors have been reported as atherogenic factors, the critical pathogens for inducing atherosclerosis remain unknown, largely because proper examining systems of them have not been developed. We recently established primary culture systems for visceral SMCs and VSMCs in which both SMCs, when cultured on laminin with insulin-like growth factor-I, show a differentiated phenotype, as indicated by a spindle-like shape, ligand-induced contractility, and a high level of SMC differentiation marker gene expression. In this study, we searched for critical dedifferentiation factors for these SMCs using our culture system. We found that polar lipids extracted from human serum markedly induced VSMC dedifferentiation, and this activity was solely present in the lysophosphatidic acid (LPA) fraction. Among several LPA species detected in human serum lipids, unsaturated LPAs were identified as major contributors to the induction of VSMC dedifferentiation. Signaling and phenotype analyses revealed that unsaturated LPA-induced VSMC dedifferentiation is mediated through the coordinated activation of extracellular signal-regulated kinase and p38 mitogen-activated protein kinase. Thus, this report demonstrates the first finding that unsaturated LPAs, but not saturated LPAs, specifically induce VSMC phenotypic modulation, suggesting that these molecules could function as atherogenic factors.

Expression and function of lysophosphatidic acid receptors in cultured rodent microglial cells

Microglia are the resident tissue macrophages of the central nervous system. They are rapidly activated by a variety of insults; and recently, receptors linked to cytoplasmic Ca(2+) signals have been implicated in such events. One potential class of receptors are those recognizing lysophosphatidic acid (LPA). LPA is a phospholipid signaling molecule that has been shown to cause multiple cellular responses, including increases in cytoplasmic calcium. We examined whether any of the known LPA receptor genes (lp(A1)/Edg2, lp(A2)/Edg4, and lp(A3)/Edg7) are expressed by cultured mouse or rat microglia. Reverse transcriptase-polymerase chain reaction indicated that mouse microglia predominantly expressed the lp(A1) gene, whereas rat microglia predominantly expressed lp(A3). Although LPA induced increases in the cytoplasmic Ca(2+) concentration in both microglial preparations, the responses differed substantially. The Ca(2+) signal in rat microglia occurred primarily through Ca(2+) influx via the plasma membrane, whereas the Ca(2+) signal in mouse microglia was due to release from intracellular stores. Only at high concentrations was an additional influx component recruited. Additionally, LPA induced increased metabolic activity in mouse (but not rat) microglial cells. Our findings provide evidence for functional LPA receptors on microglia. Thus, LPA might play an important role as a mediator of microglial activation in response to central nervous system injury.

Lysophospholipid receptors

Lysophospholipids (LPs), including lysophosphatidic acid and sphingosine 1-phosphate, produce many cellular effects. However, the prolonged absence of any cloned and identified LP receptor has left open the question of how these lipids actually bring about these effects. The cloning and functional identification of the first LP receptor, lp(A1)/vzg-1, has led rapidly to the identification and classification of multiple orphan receptors/expression sequence tags known by many names (e.g. edg, mrec1.3, gpcr26, H218, AGR16, nrg-1) as members of a common cognate G protein-coupled receptor family. We review features of the LP receptor family, including molecular characteristics, genomics, signaling properties, and gene expression. A major question for which only partial answers are available concerns the biological significance of receptor-mediated LP signaling. Recent studies that demonstrate the role of receptor-mediated LP signaling in the nervous system, cardiovascular system, and other organ systems indicate the importance of this signaling in development, function, and pathophysiology and portend an exciting time ahead for this growing field.

Identification of serum factor inducing ectodomain shedding of proHB-EGF and sStudies of noncleavable mutants of proHB-EGF

The ectodomain of the transmembrane form of HB-EGF (proHB-EGF) is cleaved at the cell surface by proteases, yielding a soluble growth factor. A number of stimuli, including TPA, accelerate this cleavage. However, proHB-EGF is shed constitutively under normal culture conditions without any particular stimuli. We demonstrate here that constitutive cleavage resulted largely from factor(s) contained in supplemented FCS in a culture medium. Analysis of serum factors, including digestion with enzymes, separation by thin layer chromatography, and shedding assay with purified phospholipids, revealed that lysophosphatidic acid (LPA) is a major factor in FCS for stimulation of proHB-EGF shedding. We also studied here ectodomain shedding of two kinds of mutant form of proHB-EGF which have a single amino acid substitution around the putative cleavage sites. These mutant forms showed resistance to stimuli of both TPA and LPA, suggesting that proHB-EGF is cleaved at the similar site by stimulation with TPA and LPA.

Subtype-specific, differential activities of the EDG family receptors for sphingosine-1-phosphate, a novel lysophospholipid mediator

The lysosphingolipid sphingosine-1-phosphate (S1P) and the structurally related lipid lysophosphatidic acid (LPA) elicit a wide spectrum of biological responses in a variety of cell types, including mitogenesis, cell-shape changes, migration and contraction. Recent studies have unveiled the existence of the G protein-coupled heptahelical receptor subfamily for the biologically active lysophospholipids, which consists of the two receptor subgroups specific for S1P and LPA, respectively. The S1P receptor subgroup comprises four members, i.e. EDG-1, EDG-3, EDG-5/AGR16 and EDG-6, with considerable amino acid similarity among them. The S1P receptor subtypes are coupled to different heterotrimeric G proteins, leading to the activation of a unique set of multiple intracellular signaling pathways. The expression of transcripts of the S1P receptor subtypes is wide-spread, except for EDG-6 which exhibits lymphoid tissue-specific expression. Plasma contains substantial concentrations of S1P as well as LPA. Activated platelets appear to be a major source of S1P and LPA in blood. In addition, accumulating evidence demonstrates that S1P and LPA are released from a variety of cell types in response to various extracellular stimuli. These observations demonstrate the existence of the novel signaling system comprising the lysosphingolipids and their cognate receptors, suggesting physiological and pathological roles.

Two novel Xenopus homologs of mammalian LP(A1)/EDG-2 function as lysophosphatidic acid receptors in Xenopus oocytes and mammalian cells

Lysophosphatidic acid (LPA) induces diverse biological responses in many types of cells and tissues by activating its specific G protein-coupled receptors (GPCRs). Previously, three cognate LPA GPCRs (LP(A1)/VZG-1/EDG-2, LP(A2)/EDG-4, and LP(A3)/EDG-7) were identified in mammals. By contrast, an unrelated GPCR, PSP24, was reported to be a high affinity LPA receptor in Xenopus laevis oocytes, raising the possibility that Xenopus uses a very different form of LPA signaling. Toward addressing this issue, we report two novel Xenopus genes, xlp(A1)-1 and xlp(A1)-2, encoding LP(A1) homologs (approximately 90% amino acid sequence identity with mammalian LP(A1)). Both xlp(A1)-1 and xlp(A1)-2 are expressed in oocytes and the nervous system. Overexpression of either gene in oocytes potentiated LPA-induced oscillatory chloride ion currents through a pertussis toxin-insensitive pathway. Injection of antisense oligonucleotides designed to inhibit xlp(A1)-1 and xlp(A1)-2 expression in oocytes eliminated their endogenous response to LPA. Furthermore, retrovirus-mediated heterologous expression of xlp(A1)-1 or xlp(A1)-2 in B103 rat neuroblastoma cells that are unresponsive to LPA conferred LPA-induced cell rounding and adenylyl cyclase inhibition. These results indicate that XLP(A1)-1 and XLP(A1)-2 are functional Xenopus LPA receptors and demonstrate the evolutionary conservation of LPA signaling over a range of vertebrate phylogeny.

The mouse lp(A3)/Edg7 lysophosphatidic acid receptor gene: genomic structure, chromosomal localization, and expression pattern

The extracellular signaling molecule, lysophosphatidic acid (LPA), mediates proliferative and morphological effects on cells and has been proposed to be involved in several biological processes including neuronal development, wound healing, and cancer progression. Three mammalian G protein-coupled receptors, encoded by genes designated lp (lysophospholipid) receptor or edg (endothelial differentiation gene), mediate the effects of LPA, activating similar (e.g. Ca(2+) release) as well as distinct (neurite retraction) responses. To understand the evolution and function of LPA receptor genes, we characterized lp(A3)/Edg7 in mouse and human and compared the expression pattern with the other two known LPA receptor genes (lp(A1)/Edg2 and lp(A2)/Edg4non-mutant). We found mouse and human lp(A3) to have nearly identical three-exon genomic structures, with introns upstream of the coding region for transmembrane domain (TMD) I and within the coding region for TMD VI. This structure is similar to lp(A1) and lp(A2), indicating a common ancestral gene with two introns. We localized mouse lp(A3) to distal Chromosome 3 near the varitint waddler (Va) gene, in a region syntenic with the human lp(A3) chromosomal location (1p22.3-31.1). We found highest expression levels of each of the three LPA receptor genes in adult mouse testes, relatively high expression levels of lp(A2) and lp(A3) in kidney, and moderate expression of lp(A2) and lp(A3) in lung. All lp(A) transcripts were expressed during brain development, with lp(A1) and lp(A2) transcripts expressed during the embryonic neurogenic period, and lp(A3) transcript during the early postnatal period. Our results indicate both overlapping as well as distinct functions of lp(A1), lp(A2), and lp(A3).

Endothelial differentiation gene-2 receptor is involved in lysophosphatidic acid-dependent control of 3T3F442A preadipocyte proliferation and spreading

EDG-2, EDG-4, EDG-7, and PSP24 genes encode distinct lysophosphatidic acid (LPA) receptors. The aim of the present study was to determine which receptor subtype is involved in the biological responses generated by LPA in preadipocytes. Growing 3T3F442A preadipocytes express EDG-2 and EDG-4 mRNAs, with no expression of EDG-7 or PSP24 mRNAs. Quantitative reverse transcriptase-polymerase chain reaction revealed that EDG-2 transcripts were 10-fold more abundant than that of EDG-4. To determine the involvement of the EDG-2 receptor in the responses of growing preadipocytes to LPA, stable transfection of antisense EDG-2 cDNA was performed in growing 3T3F442A preadipocytes. This procedure, led to a significant and specific reduction in EDG-2 mRNA and protein. This was associated with a significant alteration in the effect of LPA on both cell proliferation and cell spreading. Finally, the differentiation of growing preadipocytes into quiescent adipocytes led to a strong reduction in the level of EDG-2 transcripts. Results demonstrate the significant contribution of the EDG-2 receptor in the biological responses generated by LPA in 3T3F442A preadipocytes.

Sphingosine 1-phosphate and activation of endothelial nitric-oxide synthase. differential regulation of Akt and MAP kinase pathways by EDG and bradykinin receptors in vascular endothelial cells

Sphingosine 1-phosphate (S1P) is a platelet-derived sphingolipid that elicits numerous biological responses in endothelial cells mediated by a family of G protein-coupled EDG receptors. Stimulation of EDG receptors by S1P has been shown to activate the endothelial isoform of nitric-oxide synthase (eNOS) in heterologous expression systems (Igarashi, J., and Michel, T. (2000) J. Biol. Chem. 275, 32363-32370). However, the signaling pathways that modulate eNOS regulation by S1P/EDG in vascular endothelial cells remain less well understood. We now report that S1P treatment of bovine aortic endothelial cells (BAEC) acutely increases eNOS enzyme activity; the EC(50) for S1P activation of eNOS is approximately 10 nm. The magnitude of eNOS activation by S1P in BAEC is equivalent to that elicited by the agonist bradykinin. S1P treatment activates Akt, a protein kinase implicated in phosphorylation of eNOS. S1P treatment of BAEC leads to eNOS phosphorylation at Ser(1179), a residue phosphorylated by Akt; an eNOS mutant in which this Akt phosphorylation site is inactivated shows attenuated S1P-induced eNOS activation. S1P-induced activation both of Akt and of eNOS is inhibited by pertussis toxin, by the phosphoinositide 3-kinase inhibitor wortmannin, and by the intracellular calcium chelator BAPTA (1,2-bis(aminophenoxy)ethane-N,N,N',N'-tetraacetic acid). By contrast to S1P, activation of G protein-coupled bradykinin B2 receptors neither activates kinase Akt nor promotes Ser(1179) eNOS phosphorylation despite robustly activating eNOS enzyme activity. Understanding the differential regulation of protein kinase pathways by S1P and bradykinin may lead to the identification of new points for eNOS regulation in vascular endothelial cells.

PKD in intestinal epithelial cells: rapid activation by phorbol esters, LPA, and angiotensin through PKC

Protein kinase C (PKC) is implicated in the regulation of multiple important functions in intestinal epithelial cells, but the downstream signaling targets of PKCs in these cells remain poorly characterized. Here we report that treatment of normal rat intestinal cell lines IEC-6 and IEC-18 with phorbol 12,13-dibutyrate (PDBu) led to a rapid and striking PKC-dependent activation of protein kinase D (PKD; also known as PKCmu). Unlike conventional and novel PKCs, PKD did not undergo downregulation in response to prolonged (24 h) exposure of IEC-6 or IEC-18 cells to PDBu. PKD was also rapidly activated in these cells by lysophosphatidic acid (LPA) or angiotensin in a concentration-dependent fashion via a PKC-dependent pathway. EC(50) values were 0.1 microM and 2 nM for LPA and angiotensin II, respectively. LPA-induced PKD activation was prevented selectively by treatment with pertussis toxin. PKD activation was tightly associated with an increase in PKD autophosphorylation at serine 916. Our results identify PKD as a novel early point of convergence and integration of G(i) and G(q) signaling in intestinal epithelial cells.

The LPA receptors

Lysophosphatidic acid (LPA) is a growth factor-like lipid that produces many cellular responses. These responses, including actin cytoskeletal rearrangements, cell proliferation and inhibition of gap junction communication, have been documented in many cell types over the last 2 decades. Both non-receptor and receptor-mediated mechanisms had been implicated to explain these responses. A clear advance in this field was the cloning and functional identification of LPA receptors, and there are currently three high-affinity members, LPA1, LPA2 and LPA3 (synonymous with orphan receptor names edg-2, edg-4 and edg-7, respectively). Here we review the gene structure, expression and functions of LPA receptors. We also discuss the in vivo roles mediated by a single LPA receptor type, based on studies of the nervous system, a major locus of LPA receptor expression.

Lipid phosphate phosphatase-1 dephosphorylates exogenous lysophosphatidate and thereby attenuates its effects on cell signalling

The serum-derived phospholipid growth factor, lysophosphatidate (LPA), activates cells through a family of G-protein-coupled EDG receptors. The present article examines the role of lipid phosphate phosphatase-1 (LPP-1, or phosphatidate phosphate 2A) in regulating cell activation by LPA. Overexpressing LPP-1 approximately doubled the rate of dephosphorylation of exogenous LPA by Rat2 fibroblasts. The amount of LPA dephosphorylation was restricted to less than 10% of the total exogenous LPA. Over-expression of LPP-1 attenuated cell activation as indicated by diminished responses including cAMP, Ca2+, activation of phospholipase D and ERK, DNA synthesis and cell division. LPP-1 therefore provides a novel level of regulation for controlling cell signalling by exogenous LPA.

Hen egg yolk and white contain high amounts of lysophosphatidic acids, growth factor-like lipids: distinct molecular species compositions

Hen egg yolk and white were found to contain high amounts of lysophosphatidic acid (acyl LPA) in addition to small amounts of lysoplasmanic acid (alkyl LPA). The levels of acyl LPA in hen egg yolk (44.23 nmol/g tissue) and white (8.81 nmol/g tissue) were on the same order as or higher than the levels of acyl LPA known to be required to elicit biological responses in various animal tissues. Noticeably, there is a marked difference between the fatty acid composition of egg yolk acyl LPA and of egg white acyl LPA; egg yolk acyl LPA predominantly contains saturated fatty acids as the acyl moiety, whereas egg white acyl LPA primarily contains polyunsaturated fatty acids. We found that the level of acyl LPA, especially polyunsaturated fatty acid-containing acyl LPA, in egg white was augmented markedly during the incubation at 37 degrees C, while there was no change in egg yolk. We confirmed that egg white contains both the substrate, i.e., polyunsaturated fatty acid-containing lysophosphatidylcholine (LPC), and the enzyme activity catalyzing the hydrolysis of polyunsaturated fatty acid-containing LPC to the corresponding acyl LPA. Egg yolk LPA and egg white LPA may play separate physiological roles in the development, differentiation, and growth of embryos.

Physiological responses to lysophosphatidic acid and related glycero-phospholipids

1-Acyl-2-hydroxy(lyso)-sn-glycero-3-phosphate (lysophosphatidic acid, LPA) has attracted a lot of attention in recent years due to the wide range of its biological effects that span the phylogenetic tree from slime mold to human. LPA can be viewed as a pleiotropic phospholipid growth factor that utilizes the same signal transduction mechanisms as traditional polypeptide growth factors; however, LPA activates these mechanism via specific G protein-coupled receptors. The concentration of LPA in serum is in the high micromolar range, making it the most abundant mitogen/survival factor present in serum, one that is often unknowingly utilized in tissue culture. The present review gives a historical perspective and a critical analysis of the LPA literature with a special emphasis on the physiological implications of its effects.

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.

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.

Lysophosphatidic acid-induced Ca2+ mobilization requires intracellular sphingosine 1-phosphate production. Potential involvement of endogenous EDG-4 receptors

Lysophosphatidic acid (LPA)-mediated Ca(2+) mobilization in human SH-SY5Y neuroblastoma cells does not involve either inositol 1,4, 5-trisphosphate (Ins(1,4,5)P(3))- or ryanodine-receptor pathways, but is sensitive to inhibitors of sphingosine kinase. This present study identifies Edg-4 as the receptor subtype involved and investigates the presence of a Ca(2+) signaling cascade based upon the lipid second messenger molecule, sphingosine 1-phosphate. Both LPA and direct G-protein activation increase [(3)H]sphingosine 1-phosphate levels in SH-SY5Y cells. Measurements of (45)Ca(2+) release in premeabilized SH-SY5Y cells indicates that sphingosine 1-phosphate, sphingosine, and sphingosylphosphorylcholine, but not N-acetylsphingosine are capable of mobilizing intracellular Ca(2+). Furthermore, the effect of sphingosine was attenuated by the sphingosine kinase inhibitor dimethylsphingosine, or removal of ATP. Confocal microscopy demonstrated that LPA stimulated intracellular Ca(2+) "puffs," which resulted from an interaction between the sphingolipid Ca(2+) release pathway and Ins(1,4,5)P(3) receptors. Down-regulation of Ins(1,4,5)P(3) receptors uncovered a Ca(2+) response to LPA, which was manifest as a progressive increase in global cellular Ca(2+) with no discernible foci. We suggest that activation of an LPA-sensitive Edg-4 receptor solely utilizes the production of intracellular sphingosine 1-phosphate to stimulate Ca(2+) mobilization in SH-SY5Y cells. Unlike traditional Ca(2+) release processes, this novel pathway does not require the progressive recruitment of elementary Ca(2+) events.

Lysophosphatidic acid receptors

Lysophosphatidic acid (LPA) is a simple bioactive phospholipid with diverse physiological actions on many cell types. LPA induces proliferative and/or morphological effects and has been proposed to be involved in biologically important processes including neurogenesis, myelination, angiogenesis, wound healing, and cancer progression. LPA acts through specific G protein-coupled, seven-transmembrane domain receptors. To date, three mammalian cognate receptor genes, lp(A1)/vzg-1/Edg2, lp(A2)/Edg4, and lp(A3)/Edg7, have been identified that encode high-affinity LPA receptors. Here, we review current knowledge on these LPA receptors, including their isolation, function, expression pattern, gene structure, chromosomal location, and possible physiological or pathological roles.

Requirement for the lpA1 lysophosphatidic acid receptor gene in normal suckling behavior

Although extracellular application of lysophosphatidic acid (LPA) has been extensively documented to produce a variety of cellular responses through a family of specific G protein-coupled receptors, the in vivo organismal role of LPA signaling remains largely unknown. The first identified LPA receptor gene, lp(A1)/vzg-1/edg-2, was previously shown to have remarkably enriched embryonic expression in the cerebral cortex and dorsal olfactory bulb and postnatal expression in myelinating glia including Schwann cells. Here, we show that targeted deletion of lp(A1) results in approximately 50% neonatal lethality, impaired suckling in neonatal pups, and loss of LPA responsivity in embryonic cerebral cortical neuroblasts with survivors showing reduced size, craniofacial dysmorphism, and increased apoptosis in sciatic nerve Schwann cells. The suckling defect was responsible for the death among lp(A1)((-/-)) neonates and the stunted growth of survivors. Impaired suckling behavior was attributable to defective olfaction, which is likely related to developmental abnormalities in olfactory bulb and/or cerebral cortex. Our results provide evidence that endogenous lysophospholipid signaling requires an lp receptor gene and indicate that LPA signaling through the LP(A1) receptor is required for normal development of an inborn, neonatal behavior.

Functional comparisons of the lysophosphatidic acid receptors, LP(A1)/VZG-1/EDG-2, LP(A2)/EDG-4, and LP(A3)/EDG-7 in neuronal cell lines using a retrovirus expression system

Lysophosphatidic acid (LPA) is a potent lipid mediator with diverse physiological actions on a wide variety of cells and tissues. Three cognate G-protein-coupled receptors have been identified as mammalian LPA receptors: LP(A1)/VZG-1/EDG-2, LP(A2)/EDG-4, and LP(A3)/EDG-7. The mouse forms of these genes were analyzed in rodent cell lines derived from nervous system cells that can express these receptors functionally. An efficient retrovirus expression system was used, and each receptor was heterologously expressed in B103 rat neuroblastoma cells that neither express these receptors nor respond to LPA in all assays tested. Comparative analyses of signaling pathways that are activated within minutes of ligand delivery were carried out. LPA induced cell rounding in LP(A1)- and LP(A2)-expressing cells. By contrast, LP(A3) expression resulted in neurite elongation in B103 cells and inhibited LPA-dependent cell rounding in TR mouse neuroblast cells that endogenously express LP(A1) and LP(A2) but not LP(A3). Each of the receptors could couple to multiple G-proteins and induced LPA-dependent inositol phosphate production, mitogen-activated protein kinase activation, and arachidonic acid release while inhibiting forskolin-induced cAMP accumulation, although the efficacy and potency of LPA varied from receptor to receptor. These results indicate both shared and distinct functions among the three mammalian LPA receptors. The retroviruses developed in this study should provide tools for addressing these functions in vivo.

Lysophosphatidic acid enhances collagen gel contraction by hepatic stellate cells: association with rho-kinase

We studied the effect of lysophosphatidic acid (LPA) on collagen gel contraction by cultured rat hepatic stellate cells (HSCs) in association with the function of Rho-kinase, one of the target molecules of small GTPase Rho. Binding studies showed a single class-binding site of LPA on HSCs. LPA enhanced the contraction of a collagen lattice seeded with HSCs. LPA increased the number of HSCs with polygonal morphology that contained actin stress fibers, and enhanced the phosphorylation of myosin light chain and the assembly of focal adhesion kinase and RhoA around fibronectin-coated beads seeded on HSCs. The electric cell-substrate impedance sensor system showed that LPA enhanced adhesion of HSC to extracellular substrate. All the effects of LPA were suppressed by Y-27632, Rho-kinase inhibitor. These data support the notion that LPA is involved in modulating HSC morphology, its attachment to surrounding extracellular matrix and its contraction by a mechanism involving Rho-kinase.

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.

Lipid phosphate phosphatase-1 and Ca2+ control lysophosphatidate signaling through EDG-2 receptors

The serum-derived phospholipid growth factor, lysophosphatidate (LPA), activates cells through the EDG family of G protein-coupled receptors. The present study investigated mechanisms by which dephosphorylation of exogenous LPA by lipid phosphate phosphatase-1 (LPP-1) controls cell signaling. Overexpressing LPP-1 decreased the net specific cell association of LPA with Rat2 fibroblasts by approximately 50% at 37 degrees C when less than 10% of LPA was dephosphorylated. This attenuated cell activation as indicated by diminished responses, including cAMP, Ca(2+), activation of phospholipase D and ERK, DNA synthesis, and cell division. Conversely, decreasing LPP-1 expression increased net LPA association, ERK stimulation, and DNA synthesis. Whereas changing LPP-1 expression did not alter the apparent K(d) and B(max) for LPA binding at 4 degrees C, increasing Ca(2+) from 0 to 50 micrometer increased the K(d) from 40 to 900 nm. Decreasing extracellular Ca(2+) from 1.8 mm to 10 micrometer increased LPA binding by 20-fold, shifting the threshold for ERK activation to the nanomolar range. Hence the Ca(2+) dependence of the apparent K(d) values explains the long-standing discrepancy of why micromolar LPA is often needed to activate cells at physiological Ca(2+) levels. In addition, the work demonstrates that LPP-1 can regulate specific LPA association with cells without significantly depleting bulk LPA concentrations in the extracellular medium. This identifies a novel mechanism for controlling EDG-2 receptor activation.

Lysophosphatidic acid (LPA) receptors of the EDG family are differentially activated by LPA species. Structure-activity relationship of cloned LPA receptors

We examined the structure-activity relationship of cloned lysophosphatidic acid (LPA) receptors (endothelial cell differentiation gene (EDG) 2, EDG4, and EDG7) by measuring [Ca(2+)](i) in Sf9 insect cells expressing each receptor using LPA with various acyl chains bound at either the sn-1 or the sn-2 position of the glycerol backbone. For EDG7 the highest reactivity was observed with LPA with Delta9-unsaturated fatty acid (oleic (18:1), linoleic (18:2), and linolenic (18:3)) at sn-2 followed by 2-palmitoleoyl (16:1) and 2-arachidonoyl (20:4) LPA. In contrast, EDG2 and EDG4 showed broad ligand specificities, although EDG2 and EDG4 discriminated between 14:0 (myristoyl) and 16:0 (palmitoyl), and 12:0 (lauroyl) and 14:0 LPAs, respectively. EDG7 recognizes the cis double bond at the Delta9 position of octadecanoyl residues, since 2-elaidoyl (18:1, trans) and 2-petroselinoyl (18:1, cis-Delta12) LPA were poor ligands for EDG7. In conclusion, the present study demonstrates that each LPA receptor can be activated differentially by the LPA species.

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.

Identification of an EDG7 variant, HOFNH30, a G-protein-coupled receptor for lysophosphatidic acid

We have identified a cDNA, designated HOFNH30, which encodes a 354 amino acid G-protein-coupled receptor (GPCR). This receptor has 96% amino acid identity to the Jurkat-T cell-derived EDG7 and could be a splice variant. RT-PCR analysis demonstrated that HOFNH30 mRNA is expressed in placenta whereas EDG7 mRNA shows highest expression in prostate. The HOFNH30 gene is localized to human chromosome 1p22. 3-1p31.1. When HOFNH30 was expressed in RBL-2H3 cells, LPA and phosphatidic acid (PA) induced a calcium mobilization response with EC(50) values of 13 nM and 3 microM, respectively. LPA also induced phosphorylation of mitogen-activated protein kinase (p42(MAPK) and p44(MAPK)) in HOFNH30-transfected but not vector-transfected RBL-2H3 cells. In the present study, we have identified a novel variant from the EDG receptor family, a GPCR for which LPA is a high-affinity endogenous ligand.

The role of sphingosine kinase in the signaling initiated at the high-affinity receptor for IgE (FcepsilonRI) in mast cells

Over the last few years, sphingolipids have emerged as an additional class of lipids participating in signaling events in various cell types. The best-investigated examples so far are ceramide and sphingosine-1-phosphate. Ceramide-activated protein kinase and sphingosine kinase are two enzymes which respond to and generate these mediators. In particular, sphingosine kinase, its substrate sphingosine and the product sphingosine-1-phosphate have recently been implicated in the signaling cascades initiated at the FcepsilonRI of mast cells. High intracellular levels of sphingosine seem to serve as an 'intracellular' inhibitor which is 'deactivated' by the action of sphingosine kinase, due to the conversion to sphingosine-1-phosphate. One mode of action of the inhibitory process in this cell type is prevention of the activation of the mitogen-activated protein (MAP) kinase pathway. Sphingosine-1-phosphate itself, the product of this enzymatic reaction, is believed to lead to Ca(2+) mobilization and to stimulate the MAP kinase pathway. The existence and function of this second messenger explains the 'IP3 gap' described in mast cells after FcepsilonRI activation. Therefore, a picture emerges whereby the balance of these two lipid molecules seems to be decisive for the activation of mast cells by IgE plus antigen, with sphingosine kinase acting as a permissive switch for stimulation.

Cutting edge: differential constitutive expression of functional receptors for lysophosphatidic acid by human blood lymphocytes

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) from platelets and macrophages mediate T cell functions. Endothelial differentiation gene-encoded G protein-coupled receptors (Edg Rs) are specific for S1P (Edg-1, -3, -5, and -8 Rs) and LPA (Edg-2, -4, and -7 Rs). Human T cell tumors express many Edg Rs for both LPA and S1P. In contrast, human blood CD4+ T cells express predominantly Edg-4, and CD8+ T cells show only traces of Edg-2 and -5, by quantification of mRNA and Edg R Ags. LPA at 10-10-10-6 M suppressed significantly the secretion of IL-2 from anti-CD3 plus anti-CD28 Ab-challenged CD4+ T cells, but not CD8+ T cells. Monoclonal anti-Edg-4 R Ab, like LPA, suppressed stimulated IL-2 secretion from CD4+ T cells, but not CD8+ T cells. Constitutive expression of Edg-4 by CD4+, but not CD8+, human T cells accounts for differential functional responsiveness of the T cell subsets to LPA.

Gelsolin binding and cellular presentation of lysophosphatidic acid

Lysophosphatidic acid (LPA) in biological fluids binds to serum albumin and other proteins that enhance its effects on cellular functions. The actin-severing protein gelsolin binds LPA with an affinity (K(d) = 6 nm) similar to that of the G protein-coupled LPA receptors encoded by endothelial differentiation genes 2, 4, and 7 (Edg-2, -4, and -7 receptors) and greater than that of serum albumin (K(d) = 360 nm). At concentrations of 10% or less of that in plasma, which are observed in fluids of injured tissues, purified and recombinant gelsolin augment LPA stimulation of nuclear signals and protein synthesis in rat cardiac myocytes (RCMs) that express Edg-2 and -4 receptors. At concentrations of 20% or more of that in plasma, gelsolin suppresses LPA stimulation of RCMs. The lack of effect of gelsolin on RCM responses to monoclonal anti-Edg-4 receptor antibody plus a phorbol ester without LPA attests to its specificity for LPA delivery and the absence of post-receptor effects. Inhibition of gelsolin binding and cellular delivery of LPA by l-alpha-phosphatidylinositol-4,5-bisphosphate (PIP2) and peptides constituting the two PIP2 binding domains of gelsolin suggests competition between LPA and PIP2 for the same sites. Thus, delivery of LPA to RCMs is affinity-coupled to Edg receptors by gelsolin in a PIP2-regulated process.

Expression of lysophosphatidic acid receptor in rat astrocytes: mitogenic effect and expression of neurotrophic genes

Lysophosphatidic acid (LPA) is a phospholipid mediator with a variety of biological activities. It remains unknown, however, which cells in the brain express the LPA receptor. The present study was undertaken to identify cells in the rat brain expressing functional LPA receptors, and to explore biological roles of LPA in these cells. We found that the LPA receptor was most dominantly expressed in rat astrocytes, determined by LPA-induced Ca2+ imaging, and by Northern blot analyses. LPA induced a mitogenic response and expression of immediate early genes in astrocytes, through pertussis-toxin sensitive G-protein(s). LPA also stimulated the expression of various cytokine genes, including nerve growth factor, interleukin (IL)-1beta, IL-3 and IL-6. Thus, astrocytes are the major target of LPA in the brain. We propose that LPA may play important roles in neuronal development, gliosis and wound-healing process in the brain.

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.

Molecular identification and characterization of G protein-coupled receptors for lysophosphatidic acid and sphingosine 1-phosphate

Lysophospholipid mediators, lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), play important roles in diverse biological processes, including cell proliferation, survival, cytoskeleton changes, migration, wound healing, angiogenesis, tumor invasion, and embryonic development. The recent breakthrough in cloning and identification of several G-protein-coupled receptors for LPA and S1P has provided tools to dissect the complex mechanisms by which these lysophospholipids exert their (patho)physiological functions. Here, strategies and efforts in cloning and identifying the Edg family receptors for LPA and S1P are reviewed. Reporter gene assays used to take advantage of signaling properties of LPA and S1P, and to overcome intrinsic difficulties unique to this system, are described. From these experimental approaches, important lessons can be learned and applied to future studies of signal transduction of lysophospholipid receptors.

Lipid phosphate phosphatase-1 in the regulation of lysophosphatidate signaling

Mammalian lipid phosphate phosphatases (LPPs, or Type 2 phosphatidate phosphohydrolases) constitute a family of enzymes that belongs to a phosphatase superfamily. The LPPs dephosphorylate a variety of bioactive lipid phosphates including phosphatidate, lysophosphatidate, sphingosine 1-phosphate, and ceramide 1-phosphate. Mouse LPP-1 was stably expressed in rat2 fibroblasts to determine its structural and functional properties. Transduced cells showed increased dephosphorylation of exogenous lysophosphatidate. This result is compatible with mutational studies that show the active site of LPP-1 to be located on the external surface of the plasma membrane. Elevated LPP-1 activity attenuated the ability of lysophosphatidate to stimulate mitogen-activated protein kinase (ERK1 and 2) activities and DNA synthesis. It is concluded that one function of LPP-1 is to dephosphorylate exogenous lysophosphatidate, thereby attenuating cell signaling through endothelial cell differentiation gene (EDG) receptors.

Sphingosine-1-phosphate signaling via the EDG-1 family of G-protein-coupled receptors

The bioactive lipid sphingosine-1-phosphate (SPP) is abundantly formed and released during the activation of platelets by thrombotic stimuli. Once exported, SPP interacts with the G-protein-coupled receptors (GPCR) of the EDG-1 family. SPP binds to EDG-1 with the dissociation constant of approximately 8 nM and induces signal transduction events such as mitogen-activated protein kinase (MAP kinase) activation, decrease of cAMP levels, remodeling of the actin cytoskeleton, among others. EDG-1 is a prototypical member of a large family of GPCRs that interact with glycero- and sphingolysolipid phosphates, namely, SPP and lysophosphatidic acid (LPA). Three other GPCRs, trivially termed EDG-3, EDG-5, and EDG-8, are also high-affinity receptors for SPP. The four SPP receptor subtypes regulate different intracellular signal transduction pathways. In vascular endothelial cells, cooperative signaling between EDG-1 and EDG-3 subtypes of SPP receptors results in adherens junction assembly, cell survival, morphogenesis into capillary-like networks, and angiogenesis. SPP acts distinctly, albeit cooperatively, with polypeptide angiogenic factors, resulting in the formation of mature neovessels. Thus SPP signaling as an extracellular mediator via the EDG-1 family of GPCRs may be a heretofore unrecognized mechanism for the regulation of angiogenesis and vascular endothelial cell function.

Pharmacological characterization of phospholipid growth-factor receptors

The phospholipid growth-factor (PLGE) terminology is proposed to describe a group of endogenous glycerol- and sphingolipid mediators that regulate cell proliferation through plasma membrane receptors. In addition to LPA and SPP, multiple PLGFs are present in blood plasma and serum. PLGF activity is regulated by its stimulus-coupled production and by endogenous inhibitors. In addition to LPA and SPP, alkenyl-glycerophosphate, cyclic-phosphatidic acid, and sphingosylphosphorylcholine were detected in biological fluids using mass spectrometry. Heterologous desensitization studies indicate the expression of multiple LPA-activated receptors in a variety of cell types, which are differentially activated by the different PLGFs. Northern blot and RT-PCR results reinforce the coexpression of PSP24 alpha and different members of the EDG1-7 receptors in the same cell. Stable heterologous expression of the PSP24 alpha, EDG2, and EDG4 receptors in HEK293 cells show distinct PLGF specificities and dose-response properties for each receptor subtype. Thus, both the controlled availability of the different agonists/inhibitors and the regulated expression of their receptors regulate the biological effects of PLGFs.

Identification of lysophospholipid receptors in human platelets: the relation of two agonists, lysophosphatidic acid and sphingosine 1-phosphate

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (Sph-1-P) are known as structurally related bio-active lipids activating platelets through their respective receptors. Although the receptors for LPA and Sph-1-P have been recently identified in various cells, the identification and characterization of ones in platelets have been reported only preliminarily. In this report, we first investigated the distinct modes of LPA and Sph-1-P actions in platelet activation and found that LPA functioned as a much stronger agonist than Sph-1-P, and high concentrations of Sph-1-P specifically desensitized LPA-induced intracellular Ca(2+) mobilization. In order to identify the responsible receptors underlying these observations, we analyzed the LPA and Sph-1-P receptors which might be expressed in human platelets, by RT-PCR. We found for the first time that Edg2, 4, 6 and 7 mRNA are expressed in human platelets.

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

The endothelial differentiation gene-6 (Edg-6) was recently identified as an orphan G-protein-coupled receptor. Its predicted amino acid sequence is very close to Edg family of receptor proteins whose ligand is supposed to be lysophosphatidic acid (LPA) or lysosphingolipid such as sphingosine 1-phosphate (S1P) and sphingosylphosphorylcholine (SPC). Transfection of the Edg-6 into Chinese hamster ovary (CHO) cells and K562 cells resulted in the appearance of high-affinity [(3)H]S1P binding activity. Among lipids employed, S1P and, even though less potent, SPC, displaced the [(3)H]S1P binding, but LPA was inactive. In Edg-6-transfected CHO cells, an increase in cytosolic Ca(2+) concentration in response to S1P or SPC was clearly enhanced without change in the LPA-induced action as compared with the vector-transfected cells. The enhancement of the Ca(2+) response was associated with a significant accumulation of inositol phosphate, reflecting activation of phospholipase C. Similar enhancement of Ca(2+) response to S1P or SPC was also observed in Edg-6-expressing K562 cells. These lipid-induced actions in CHO cells and K562 cells expressing Edg-6 were markedly suppressed by pertussis toxin treatment. We conclude that Edg-6 is one of S1P or lysosphingolipid receptors that couple to phospholipase C-Ca(2+) system through pertussis toxin-sensitive G-proteins.

Enhancement of the migration of metastatic human breast cancer cells by phosphatidic acid

Phosphatidic acid (PA), lysophosphatidic acid (LPA), and sphingosine 1-phosphate (SPP) are naturally occurring phospholipids which induce a variety of effects as extracellular messengers. In this study, we compared the effects of these phospholipid signaling molecules on the migration of invasive and noninvasive breast cancer cell lines, an index of the metastatic potential of these cells. As previously demonstrated, invasive MDA-MB-231 breast cancer cells exhibited increased constitutive (nonstimulated) migration in comparison to poorly invasive MCF-7 cells. Phosphatidic acid employed at nanomolar concentrations markedly potentiated migration of the invasive cells but had no effect on migration of either the noninvasive MCF-7 cells or nonneoplastic human epithelial cells. Lysophosphatidic acid and sphingosine 1-phosphate inhibited both the directed (chemotactic) and random (chemokinetic) migration of MDA-MB-231 cells. Experiments were undertaken to characterize the signaling pathway involved in constitutive and PA-stimulated migration of MDA-MB-231 cells. The tyrosine kinase inhibitors staurosporine and genistein inhibited constitutive and PA-induced migration in a dose-dependent manner, consistent with a role for tyrosine phosphorylation in the migratory response. In addition, the phosphatidylinositol (PI) 3' kinase inhibitors wortmannin and LY294002 strongly inhibited both the constitutive and PA-stimulated migration of the invasive breast cancer cells, indicating that PI-3' kinase plays an important role in the metastatic migration of breast cancer cells. Finally, PA-induced migration of MDA-MB-231 was markedly attenuated by pretreatment of cells with Clostridium difficile Toxin B, pertussis toxin and suramin, implying a role for a Gi receptor-dependent process involving activation of the small GTP-binding protein Rho. Since an enhanced ability to migrate heightens the metastatic potential of cells within solid tumors, our results suggest that the metastatic capabilities of breast cancer cells may be enhanced by a receptor-driven cellular process initiated by phosphatidic acid or related lipid phosphate messengers.