BBA - Molecular and Cell Biology of Lipids (v.1582, #1-3)
Lysolipid mediators in cell signaling and disease by Gabor Tigyi; Edward J Goetzl (vii).
Biological functions of a novel lipid mediator, cyclic phosphatidic acid by Kimiko Murakami-Murofushi; Ayako Uchiyama; Yuko Fujiwara; Tetsuyuki Kobayashi; Susumu Kobayashi; Mutsuko Mukai; Hiromu Murofushi; Gabor Tigyi (1-7).
A novel bioactive lipid, cyclic phosphatidic acid (cPA), was isolated originally from myxoamoebae of a true slime mold, Physarum polycephalum, and has now been detected in a wide range of organisms from slime molds to humans. It has a cyclic phosphate at the sn-2 and sn-3 positions of the glycerol carbons, and this structure is absolutely necessary for its activities. This substance shows specific biological functions, including antimitogenic regulation of the cell cycle, regulation of actin stress fiber formation and rearrangement, inhibition of cancer cell invasion and metastasis, regulation of differentiation and viability of neuronal cells, and mobilization of intracellular calcium. Although the structure of cPA is similar to that of lysophosphatidic acid (LPA), its biological activities are apparently distinct from those of LPA. In the present review, we focus mainly on the enzymatic formation of cPA, the antimitogenic regulation of the cell cycle, the inhibition of cancer cell invasion and metastasis, and the neurotrophic effect of cPA.
Keywords: Cyclic phosphatidic acid; Cell cycle; Cancer cell invasion; Tumor metastasis; Neurotrophic effect; Phospholipase D;
Sphingosine-1-phosphate and lipid phosphohydrolases by Hervé Le Stunff; Courtney Peterson; Hong Liu; Sheldon Milstien; Sarah Spiegel (8-17).
Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that acts as both an extracellular ligand for the endothelial differentiation gene-1 (EDG-1) G-protein coupled receptor (GPCR) family and as an intracellular messenger. Cellular levels of S1P are low and tightly regulated in a spatial-temporal manner not only by sphingosine kinase (SPHK) but also by degradation catalyzed by S1P lyase, specific S1P phosphohydrolases, and by general lipid phosphate phosphohydrolases (LPPs). LPPs are characterized as magnesium-independent, insensitive to inhibition by N-ethylmaleimide (NEM) and possessing broad substrate specificity with a variety of phosphorylated lipids, including S1P, phosphatidic acid (PA), and lysophosphatidic acid (LPA). LPPs contain three highly conserved domains that define a phosphohydrolase superfamily. Recently, several specific S1P phosphohydrolases have been identified in yeast and mammalian cells. Phylogenetic and biochemical analyses indicate that these enzymes constitute a new subset of the LPP family. As further evidence, S1P phosphohydrolases exhibit high specificity for phosphorylated sphingoid bases. Enforced expression of S1P phosphohydrolase alters the cellular levels of sphingolipid metabolites in yeast and mammalian cells, increasing sphingosine and ceramide, bioactive sphingolipids that often have opposing biological actions to S1P. By regulating the cellular ratio between ceramide/sphingosine and S1P, S1P phosphohydrolase is poised to be a critical factor in cell survival/cell death decisions. Indeed, expression of S1P phosphohydrolase in mammalian cells increases apoptosis, whereas deletion of S1P phosphohydrolases in yeast correlates with resistance to heat stress. In this review, we discuss the role of phosphohydrolases in the metabolism of S1P and how turnover of S1P can regulate sphingolipid metabolites signaling.
Keywords: Sphingolipid; Sphingosine-1-phosphate; Sphingosine; Ceramide; Lipid phosphate phosphohydrolase; S1P phosphohydrolase; Sphingosine kinase;
Physiological and pathophysiological roles of lysophosphatidic acids produced by secretory lysophospholipase D in body fluids by Akira Tokumura (18-25).
Recently, a family of phospholipid mediators has received much attention because of its variety of biological activities. Lysophosphatidic acid (LPA) is a central member of the phospholipid autacoid family that exerts diverse effects through binding to and activation of several specific receptors coupled to G-proteins. In accordance with its function as a receptor agonist, there are pathways for extracellular generation of LPA in vivo. One pathway involves a novel lysophospholipase D activity that was originally found in rat plasma. LPA is also produced in significant amounts after incubation of various plasma-derived body fluids such as human follicular fluid at 25–37°C. In animal models, LPA was shown to stimulate oocyte maturation, embryonic development and transport in the oviduct. An increase in serum lysophospholipase D activity was observed during pregnancy in human. These results suggest that LPA generated by lysophospholipase D is likely to play an important role in reproductive biology. LPA produced by lysophospholipase D activity in body fluids has also been observed under pathophysiological conditions: serum and ascitic fluid from ovarian cancer patients and serum from hypercholesterolemic rabbits. Hence, excess generation of LPA by lysophospholipase D activity in body fluids has been suggested to be relevant to the pathogenesis of cancer and atherosclerosis.
Keywords: Lysophosphatidic acid; Lysophosphatidylcholine; Lysophospholipase D; Plasma; Serum; Body fluid; Oocyte maturation;
Structure and function of phosphatidylserine-specific phospholipase A1 by Junken Aoki; Yuki Nagai; Hiroyuki Hosono; Keizo Inoue; Hiroyuki Arai (26-32).
Phospholipase A1 (PLA1) is an enzyme that hydrolyzes the sn-1 fatty acids from phospholipids and produces 2-acyl-lysophospholipids. Although PLA1 activities are detected in many tissues and cell lines, a limited number of PLA1s have been purified and cloned so far. These include phosphatidylserine (PS)-specific PLA1 (PS-PLA1) from rat platelets, PLA1 from vespid venom, and phosphatidic acid (PA)-preferential PLA1 (PA-PLA1). Structurally, the former two PLA1s belong to the lipase family, where they form a subfamily among the lipase family. An alignment of the PLA1s with other members of the lipase family revealed two molecular characteristics of PLA1: the presence of extremely short lids and deleted β9 loops. The two surface loops have been implicated in the ligand recognition in human pancreatic lipase (PL) and guinea pig PL-related protein 2. Under physiological conditions, accessibility of PS-PLA1 to its substrate is limited as it is a secreted enzyme and PS is normally located in the inner leaflet of the lipid bilayer. However, PS-PLA1 efficiently hydrolyzes PS exposed on the surface of cells such as apoptotic cells and activated platelets, and produces 2-acyl-lysophosphatidylserine (lysoPS), which is a lipid mediator for mast cells, T cells and neural cells. Identification of PS-PLA1 reveals the presence of PLA1 subfamily within the lipase family and suggests that PLA1 has a role in the production of lysophospholipid mediators.
Keywords: Phospholipase A1; Phosphatidylserine; Lysophosphatidylserine; Mast cell;
Lipid phosphate phosphatases regulate signal transduction through glycerolipids and sphingolipids by David N. Brindley; Denis English; Carlos Pilquil; Katherine Buri; Zong-Chao Ling (33-44).
Lipid phosphate esters including lysophosphatidate (LPA), phosphatidate (PA), sphingosine 1-phosphate (S1P) and ceramide 1-phosphate (C1P) are bioactive in mammalian cells and serve as mediators of signal transduction. LPA and S1P are present in biological fluids and activate cells through stimulation of their respective G-protein-coupled receptors, LPA1–3 and S1P1–5. LPA stimulates fibroblast division and is important in wound repair. It is also active in maintaining the growth of ovarian cancers. S1P stimulates chemotaxis, proliferation and differentiation of vascular endothelial and smooth muscle cells and is an important participant in the angiogenic response and neovessel maturation. PA and C1P are believed to act primarily inside the cell where they facilitate vesicle transport. The lipid phosphates are substrates for a family of lipid phosphate phosphatases (LPPs) that dramatically alter the signaling balance between the phosphate esters and their dephosphorylated products. In the case of PA, S1P and C1P, the products are diacylglycerol (DAG), sphingosine and ceramide, respectively. These latter lipids are also bioactive and, thus, the LPPs change signals that the cell receives. The LPPs are integral membrane proteins that act both inside and outside the cell. The “ecto-activity” of the LPPs regulates the circulating and locally effective concentrations of LPA and S1P. Conversely, the internal activity controls the relative accumulation of PA or C1P in response to stimulation by various agonists thereby affecting cell signaling downstream of EDG and other receptors. This article will review the various LPPs and discuss how these enzymes could regulate signal transduction by lipid mediators.
Keywords: Ceramide; Diacylglycerol; EDG receptor; Lysophosphatidate; Phosphatidate; Sphingosine 1-phosphate;
Roles for lipid phosphate phosphatases in regulation of cellular signaling by Vicki A Sciorra; Andrew J Morris (45-51).
Lipid phosphate phosphatases (LPPs) are a family of integral membrane glycoproteins that catalyze the dephosphorylation of a number of bioactive lipid mediators including lysophosphatidic acid (LPA), sphingosine 1-phosphate (S1P) and phosphatidic acid (PA). These mediators exert complex effects on cell function through both actions at cell surface receptors and on intracellular targets. The LPP-catalyzed dephosphorylation of these substrates can both terminate their signaling actions and itself generate further molecules with biological activity. Recent advances have revealed that a family of structurally related genes is responsible for LPP activities in species from yeast to mammals. These genes exhibit distinct but overlapping expression patterns and their products appear to be heterogeneous with respect to their posttranslational modification and subcellular localizations. Here we review the structure and catalytic properties of the LPPs and consider recent developments in understanding their cellular biology and functions.
Keywords: Lysophosphatidic acid; Sphingosine phosphate; Lipid phosphate phosphatase;
Biological activities and metabolism of the lysophosphoinositides and glycerophosphoinositols by Daniela Corda; Cristiano Iurisci; Christopher P Berrie (52-69).
The lysophospholipids are integral components of the plasma membrane that have often been considered as side products of the phospholipase A2 (PLA2)-dependent production of arachidonic acid and the deacylation/reacylation processes involved in phospholipid homeostasis. Data indicating roles of these lipid derivatives in hormone responses and cell transformation have now led to a different view, and the understanding of their involvement in the modulation of cell function is building up. Here, we will summarise the current knowledge concerning the biological roles of the lysophosphoinositides and the glycerophosphoinositols (their fully deacylated counterparts) in the framework of their known effects, and those of the other lysophospholipids and glycerophospholipids.
Keywords: Lysophospholipid; Phosphoinositide metabolism; Lipid signalling; Phospholipase A; Lysophospholipase; Cell transformation;
Lysophospholipid receptor nomenclature by Kevin R Lynch (70-71).
Keywords: Lysophospholipid; Sphingosine 1-phosphate; Endothelial differentiation gene;
Signaling of sphingosine-1-phosphate via the S1P/EDG-family of G-protein-coupled receptors by Michael J Kluk; Timothy Hla (72-80).
The sphingosine-1-phosphate/Endothelial Differentiation Gene (S1P/EDG) family of G-protein-coupled receptors (GPCR) currently includes five different isoforms, which differentially regulate fundamental cellular processes such as migration, proliferation, cytoskeletal organization, adherens junction assembly and morphogenesis. Additionally, specific S1P/EDG isoforms can regulate important physiological processes such as blood vessel maturation, cardiac development and angiogenesis in vivo. Herein, we review the current state of knowledge of the expression patterns, signaling pathways and functional characteristics of the different S1P receptors. Further investigation in this field will likely improve our understanding of cardiovascular development as well as vascular diseases and may lead to novel therapeutic approaches.
Keywords: Sphingosine-1-phosphate (S1P); Lysophospholipid; Endothelial Differentiation Gene (EDG); G-protein-coupled receptor; Signal transduction; Angiogenesis;
Sphingosylphosphorylcholine and lysophosphatidylcholine: G protein-coupled receptors and receptor-mediated signal transduction by Yan Xu (81-88).
In recent years, certain lysophospholipids (lyso-PLs) have been recognized as important cell signaling molecules. Among them, two phosphorylcholine-containing lyso-PLs, sphingosylphosphorylcholine (SPC) and lysophosphatidylcholine (LPC), have been shown to be involved in many cellular processes and are produced under physiological and pathological conditions. Although signaling properties of SPC and LPC have been studied in a variety of cellular systems, specific cell membrane receptors for SPC and LPC have not been identified previously. Recently, ovarian cancer G protein-coupled receptor 1 (OGR1, also known as GPR68), G protein-coupled receptor 4 (GPR4), and G2A have been identified as receptors for SPC and LPC. The signaling and ligand-binding properties of these receptors are reviewed here. These discoveries provide an intriguing opportunity and a novel approach in studying the pathophysiological roles of SPC and LPC and their receptors.
Keywords: G protein-coupled receptor; Sphingosylphosphorylcholine; Lysophosphatidylcholine; Ovarian cancer G protein-coupled receptor 1; G2A; G protein-coupled receptor 4;
Homodimerization and heterodimerization of S1P/EDG sphingosine-1-phosphate receptors by James R Van Brocklyn; Babak Behbahani; Norman H Lee (89-93).
Sphingosine-1-phosphate (S1P) binds to and signals through several members of a group of G protein-coupled receptors (GPCRs) known as the S1P/EDG family. Several of these receptors are coexpressed in various cell types and recent reports have shown that biological effects of S1P often require more than one S1P receptor subtype. Recent evidence indicates that many GPCRs exist as dimers. We show that S1P receptors form both homodimers as well as heterodimers with other members of the S1P subfamily of receptors. We also discuss the role that GPCR dimers play in receptor function and what this may mean for S1P signaling.
Keywords: Sphingolipid; Lysophospholipid; G protein-coupled receptor; Dimerization;
Pathways of transduction engaged by sphingosine 1-phosphate through G protein-coupled receptors by Sandra Siehler; David R Manning (94-99).
Pathways of transduction employed by receptors for sphingosine 1-phosphate (S1P) are identified by the nature of second messengers and/or downstream targets regulated and, more formally, by direct assays of heterotrimeric G protein activation. The different methods generally agree. S1P1 couples to members of the Gi family, apparently selectively, although reported pertussis toxin (PTX)-insensitive actions make categorical statements regarding exclusivity difficult. S1P2 and S1P3 couple to members of the Gi, Gq, and G12/13 families. S1P4 couples to Gi and possibly G12/13, while S1P5 couples to Gi and G12/13 but not to Gq. In virtually all circumstances, coupling of S1P receptors to Gi is reflected in PTX-sensitive inhibition of adenylyl cyclase, activation of extracellular-regulated kinases (ERKs), and, depending on the cell, activation of phospholipase C (PLC). Coupling to Gq is reflected in PTX-insensitive activation of phospholipase C. Coupling to G12/13 is reflected in activation of Rho and subsequent activation of serum response factor (SRF). Specific linkages have been verified in almost all instances by receptor-promoted [35S]GTPγS/GDP exchange on identified G proteins.
Keywords: Sphingosine 1-phosphate; G protein; Transduction; Adenylyl cyclase; Phospholipase C; Rho; Sf9;
Trans-regulation of epidermal growth factor receptor by lysophosphatidic acid and G protein-coupled receptors by Jie Wu; Jess M Cunnick (100-106).
Lysophosphatidic acid (LPA) is known to induce protein tyrosine phosphorylation and has growth factor-like effects. In the last several years, the epidermal growth factor (EGF) receptor has been recognized as a protein tyrosine kinase that plays a central role in mediating LPA-induced tyrosine phosphorylation and Erk MAP kinase activation. In this article, we review recent progress in the study of trans-regulation of EGF receptor by LPA and G protein-coupled receptors (GPCR) and discuss the gap in our knowledge of the mechanism by which LPA induces EGF receptor activation.
Keywords: Lysophosphatidic acid; EGF receptor; G protein-coupled receptor; Tyrosine kinase; MAP kinase; HB-EGF;
Phosphoinositide 3-kinases in lysophosphatidic acid signaling: regulation and cross-talk with the Ras/mitogen-activated protein kinase pathway by Armelle Yart; Hugues Chap; Patrick Raynal (107-111).
Recent reports have shown that phosphoinositide 3-kinases (PI3Ks) mediate various biological activities of lysophosphatidic acid (LPA), including cell proliferation or survival. In addition, these enzymes have been proposed to be early intermediates of mitogen-activated protein kinase (MAPK) activation. Here we summarize our current knowledge of the mechanisms underlying these observations. p110γ is an isoform of PI3K that can be activated in vitro by Gβγ subunits and was therefore considered as the logical candidate to mediate responses induced by G protein-coupled receptor (GPCR) agonists. In agreement with this, p110γ has been involved in different biochemical models linking Gβγ to MAPK activation. Nevertheless, its apparent tissue-specific distribution has raised questions regarding the physiological relevance of these models. In addition, LPA can activate p110β, a member of the phosphotyrosine-dependent PI3K subfamily that participates in the mitogenic effect of LPA. Its activation is thought to involve a synergistic effect of Gβγ and phosphotyrosine motifs provided by a transactivated EGF receptor/Gab1 pathway. We are currently studying a possible role of p110β upstream from Ras, suggesting that this protein could provide a novel connection between βγ and the MAPK pathway.
Keywords: Lysophosphatidic acid; Phosphoinositide 3-kinase; p110β; Gab1; Ras; ERK1/2;
Subtype-specific differential regulation of Rho family G proteins and cell migration by the Edg family sphingosine-1-phosphate receptors by Yoh Takuwa (112-120).
One of the striking activities of the Edg family sphingosine-1-phosphate (S1P) receptors includes receptor isotype-specific, bimodal regulatory activity on cell migration. While Edg1 and Edg3 act as typical chemotactic receptors, Edg5 uniquely acts as a chemorepellant receptor. Consistent with this, Edg1 and Edg3, and Edg5 regulate the activity of the Rho family GTPase Rac positively and negatively, respectively. Thus, Edg isotype-specific, differential regulatory activities on Rac seem to be important as mechanisms underlying the bimodal regulation of cell migration by S1P. Edg5-mediated Rac inhibition involves stimulation of Rac-GTPase-activating protein (GAP) activity, rather than inhibition of Rac-guanine nucleotide exchange factor (GEF) activity. Many cell types including vascular smooth muscle and endothelial cells express more than a single S1P receptor isotype. In these cells, it appears that an integration of the Edg isotype-selective, positive and negative signals on cellular Rac activity is a critical determinant for eventual direction of regulation on cell motility by S1P. Physiological and pathological roles for the repulsive activity of Edg5 receptor remain to be clarified.
Keywords: Sphingosine-1-phosphate; Edg; Cell migration; Chemorepellant; Rac; Rho;
Sphingosine 1-phosphate signalling and termination at lipid phosphate receptors by Susan Pyne; Nigel J Pyne (121-131).
Sphingosine 1-phosphate (S1P) is a polar lysophospholipid metabolite that is stored in platelets and released upon their activation. However, diverse stimuli such as growth factors, cytokines, G-protein coupled receptor (GPCR) agonists and antigens have been shown to increase sphingosine kinase activity and S1P formation in other cell types, such as smooth muscle. Indeed, S1P has been implicated in the regulation of several important cellular processes, such as proliferation, differentiation, apoptosis and migration in these cells. Over the past few years, there has been a major advance in our understanding of how S1P can act as an intercellular mediator by binding to a new class of G-protein coupled receptors to regulate cell function. This review focuses on the enzymatic regulation of S1P formation and degradation and its interaction with a novel tethered receptor complex containing the S1P receptor (S1P1) and the platelet-derived growth factor (PDGF) β receptor. This tethered receptor complex enables coincident integrative signalling to p42/p44 MAPK. This is compared with a sequential model in which PDGF promotes S1P release, which in turn acts on S1P1 to promote Rac signalling.
Keywords: Sphingosine 1-phosphate; S1P receptor; Sphingosine kinase; Sphingosine 1-phosphate phosphatase; Lipid phosphate phosphatase; Growth factor receptor; Cell motility; Proliferation;
Plasma lipoproteins behave as carriers of extracellular sphingosine 1-phosphate: is this an atherogenic mediator or an anti-atherogenic mediator? by Fumikazu Okajima (132-137).
Sphingosine 1-phosphate (S1P) concentration in plasma and serum has been estimated to be within 200–900 nM. Among plasma and serum components, S1P is concentrated in lipoprotein fractions with a rank order of high-density lipoprotein (HDL)>low-density lipoprotein (LDL)>very low-density lipoprotein (VLDL)>lipoprotein-deficient plasma (LPDP) when expressed as the per unit amount of protein. It is well known that LDL, especially oxidized LDL, is closely correlated and HDL is inversely correlated, with the risk of cardiovascular disease, such as atherosclerosis. Evidence was presented that a part of HDL-induced actions previously reported are mediated by the lipoprotein-associated S1P. Furthermore, S1P content in LDL was markedly decreased during its oxidation. This paper will discuss whether S1P is an atherogenic mediator or an anti-atherogenic mediator.
Keywords: Sphingosine 1-phosphate; Edg family receptor; High-density lipoprotein; Low-density lipoprotein; Oxidized low-density lipoprotein; Atherosclerosis;
Sphingolipids involved in the induction of multinuclear cell formation by Yasunori Kozutsumi; Takayuki Kanazawa; Yidi Sun; Toshiyuki Yamaji; Harumi Yamamoto; Hiromu Takematsu (138-143).
In this review, we focus on sphingolipids as potential regulators of the induction of multinuclear cell formation through the inhibition of cytokinesis. A sphingolipid, psychosine (Psy) (galactosylsphingosine), was demonstrated to be a trigger lipid for the inhibition of cytokinesis and the induction of multinuclear giant cells associated with a sphingolipid metabolic disease, globoid cell leukodystrophy (GLD). Indeed, Psy is known to accumulate in the patients' brains. Interestingly, inhibition of sphingolipid biosynthesis also induced multinuclear cells. When cells were treated with a new immunosuppressant, ISP-1/myriocin, which inhibits serine palmitoyltransferase, the first step enzyme of sphingolipid biosynthesis, the cells underwent multinucleation and apoptosis. At present, a definitive model of the function of sphingolipids as to the induction of multinuclear cell formation is not available due to the rudimentary information but possible mechanisms are discussed.
Keywords: Multinuclear cell; Globoid cell leukodystrophy; Psychosine; ISP-1/myriocin; Serine palmitoyltransferase; Sphingolipid;
Regulation of neuronal cytoskeleton by lysophosphatidic acid: role of GSK-3 by C.L Sayas; J Ávila; F Wandosell (144-153).
Neurite retraction is a crucial process during nervous system development and neurodegeneration. This process implies reorganization of the neuronal cytoskeleton. Some bioactive lipids such as lysophosphatidic acid (LPA) induce neurite retraction. The reorganization of the actin cytoskeleton during neurite retraction is one of the best-characterized effects of LPA. However, less information is available regarding the reorganization of the microtubule (MT) network in response to LPA in neuronal cells. Here, we first give an overview of the roles of cytoskeleton during neurite outgrowth, and subsequently, we review some of the data from different laboratories concerning LPA-induced cytoskeletal rearrangement in neuronal cells. We also summarize our own recent results about modifications of MTs during LPA-induced neurite retraction. We have shown that LPA induces changes in tubulin pools and increases in the phosphorylation levels of microtubule-associated proteins (MAPs), such as Tau. Tau hyperphosphorylation in response to LPA is mediated by the activation of glycogen synthase kinase-3 (GSK-3). The upregulation of GSK-3 activity by LPA seems to be a general process as it occurs in diverse neuronal cells of different species in correlation with the neurite retraction process.
Keywords: Lysophosphatidic acid; Neurite retraction; Microtubule reorganization; GSK-3 activation; Tau hyperphosphorylation; Gα12/13;
Multiple astrocyte responses to lysophosphatidic acids by Marion R Steiner; Jan R Urso; Jennifer Klein; Sheldon M Steiner (154-160).
Lysophosphatidic acid (LPA) and LPA receptors are enriched in the brain. Moreover, the levels of these receptors and ligand are modulated during brain development and injury, respectively, suggesting multiple roles for LPA in the brain. In cultured astrocytes and glioma-derived cells, LPA increases intracellular calcium concentrations and causes morphological changes. LPA also induces glioma cell migration. In normal astrocytes, LPA stimulates reactive oxygen species synthesis, activation of multiple protein kinases and expression of c-fos and c-jun. It is noteworthy that LPA-induced astrocyte responses vary as a function of the specific brain region of origin of the astrocytes. This may be one factor in the finding of LPA-stimulated proliferation in some, but not all, astrocyte studies. The species and/or developmental stage also differed in many of the astrocyte proliferation analyses. Micromolar LPA is required to elicit some astrocyte responses, including the stimulation of cytokine expression and inhibition of glutamate uptake. These events could significantly impact on survival of injured neurons and micromolar LPA concentrations are likely in diverse brain pathologies. There are important aspects of astrocyte LPA responses still to be fully evaluated, including functions in development and activation, synergy between LPA and other biomediators, and astrocyte interactions with other cells.
Keywords: Lysophosphatidic acid; Astrocyte; Glioma; Brain;
Lysophospholipid mediators of immunity and neoplasia by Mei-Chuan Huang; Markus Graeler; Geetha Shankar; Juliet Spencer; Edward J Goetzl (161-167).
Lysophosphatidic acid (LPA), sphingosine 1-phosphate (S1P) and some other structurally related lysophospholipids are active growth factors and stimuli for diverse cellular functions. LPA and S1P promote early T cell migration to tissue sites of immune responses and regulate T cell proliferation and secretion of numerous cytokines. Edg-4 (LPA2) LPA receptors, which are constitutively expressed by helper T cells, and Edg-2 (LPA1) LPA receptors, which are expressed only by activated helper T cells, transduce opposite effects of LPA on some T cell responses. A similar mechanism is observed for fine regulation of Edg R-mediated effects of LPA on ovarian cancer cells. Edg-4 (LPA2) R transduces proliferative responses, recruitment of autocrine protein growth factors, and migration of ovarian cancer cells, whereas Edg-2 (LPA1) R transduces inhibition of Edg-4 (LPA2) R-mediated responses and concurrently elicits apoptosis and anoikis of ovarian cancer cells. Edg-4 (LPA2) R is a distinctive functional marker for ovarian carcinoma, and is expressed both as the wild-type and a carboxyl-terminally extended gain-of-function mutant. Newly discovered non-lipid agonists and antagonists for individual Edg receptors will permit more sophisticated analyses of their respective contributions in human biology and pathophysiology, and may represent novel therapeutic modalities in immune disorders and cancer.
Keywords: T cell; G protein-coupled receptor; Cytokine; Growth factor; Serum response element; Ovarian cancer;
Lysophospholipids and their G protein-coupled receptors in inflammation and immunity by Markus H Gräler; Edward J Goetzl (168-174).
Among the family of lipid-derived mediators, the group of lysophospholipids including lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) have growth-related and -unrelated effects on diverse cell types including lymphocytes, macrophages, smooth muscle cells, endothelial cells, and neuronal cells. This review summarizes the known effects of lysophospholipids and their G protein-coupled receptors (GPCRs) in inflammation and immunity. Lysophospholipids have the capacity to evoke and modulate immune responses by attracting and activating T-cells, B-cells and macrophages directly and influencing their interactions with other cell types. Immune cells express multiple subsets of lysophospholipid receptors, which are critical for specific cellular responses such as proliferation and migration that are fundamental to immunity. Investigation of the expression pattern of EDG-receptors on human T-cells revealed a dynamic transcriptional regulation influenced by both developmental stages and activation states. Other lipid mediators like psychosine and other GPCRs for lipid mediators like G2A also may be involved in the development of normal immune and inflammatory reactions and diseases. These observations suggest that agonists and antagonists for lysophospholipid receptors may influence immune responses.
Keywords: Lymphocyte; Macrophage; Lysophospholipid; G protein-coupled receptor; Apoptosis; Wound healing;
Lysophospholipid regulation of mononuclear phagocytes by Hsinyu Lee; Jia-Jun Liao; Markus Graeler; Mei-Chuan Huang; Edward J Goetzl (175-177).
Blood monocytes and tissue macrophages derived from monocyte differentiation in tissues are central elements of innate immunity in host defense against numerous pathogens and other challenges. These mononuclear phagocytes also participate in wound healing and normal tissue remodeling in development and growth. Pathological perversion of their physiological roles leads to participation of mononuclear phagocytes in fibrosing diseases including granulomatous disorders, chronic inflammation typical of arthritis, and atherosclerosis. Lysophospholipids, including lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), are platelet-derived lipid growth factors considered to participate in leukocyte differentiation and activation. This section summarizes our recent observations of the effects of lysophospholipids on mononuclear phagocytes.
Keywords: Lysophosphatidic acid; Sphingosine 1-phosphate; Macrophage; Wound healing; Atherosclerosis;
Sphingosylphosphorylcholine—biological functions and mechanisms of action by Dagmar Meyer zu Heringdorf; Herbert M Himmel; Karl H Jakobs (178-189).
Compared to the lysophospholipid mediators, sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA), little information is available regarding the molecular mechanisms of action, metabolism and physiological significance of the related sphingosylphosphorylcholine (SPC). S1P and LPA have recently been established as agonists at several G-protein-coupled receptors of the EDG family, S1P additionally serves an intracellular second messenger function. Several cellular effects of SPC can be explained by low-affinity binding to and activation of S1P-EDG receptors. However, certain cellular and subcellular actions of SPC are not shared by S1P, suggesting that SPC, which has been identified in normal blood plasma, ascites and various tissues, is a lipid mediator in its own right. This concept was corroborated by the recent discovery of specific high-affinity G-protein-coupled SPC receptors. In this article, our present knowledge on cellular actions and biological functions of SPC will be reviewed.
Keywords: Lysophospholipid; Lipid signalling; Sphingosylphosphorylcholine; Sphingosine-1-phosphate; Lysophosphatidylcholine; G-protein-coupled receptor;
Differential effects of sphingosine 1-phosphate and lysophosphatidic acid on endothelial cells by Tracee Scalise Panetti (190-196).
This review discusses multiple effects of sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) on endothelial cells and proposes that S1P and LPA are important regulators of the vascular system. Two physiologic sources of S1P and LPA are platelets and lipoproteins. S1P is an inducer of angiogenesis in vivo whereas LPA is not. S1P and LPA act through endothelial cell surface Edg receptors. S1P stimulates endothelial cell migration, but inhibits migration of most nonendothelial cells. Edg1 and Edg3 receptors, working through Gi, play an important role in regulation of S1P-stimulated endothelial cell migration. LPA effects on endothelial cells are more restricted than the effects of S1P on endothelial cells. LPA stimulates migration of certain endothelial cells on certain extracellular matrix proteins. However, LPA acts like S1P in its effects on the endothelial cell cytoskeleton, proliferation, cell–cell adhesion molecule expression, and vascular permeability. LPA receptors on endothelial cells are likely Edg2 and Edg4. Future studies should better delineate the roles of Edg receptors and downstream pathways on effects of extracellular S1P and LPA and the contributions of intracellularly generated S1P and nitric oxide (NO).
Keywords: Angiogenesis; Edg receptor; Endothelial cell; Lysophosphatidic acid; Migration; Sphingosine 1-phosphate;
In vivo roles of lysophospholipid receptors revealed by gene targeting studies in mice by Amy H Yang; Isao Ishii; Jerold Chun (197-203).
Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are extracellular ligands for a family of G protein-coupled receptors (GPCRs), LPA1/2/3 and S1P1/2/3/4/5. Through coupling to multiple classes of G proteins and activating multiple signaling pathways, LPA/S1P receptors have been shown to be integral players for many essential cellular and physiological processes. Generation and analysis of mice deficient in each of LPA1, LPA2, S1P1, S1P2, and S1P3 have provided valuable information on the in vivo roles of these receptors. This review is focussed on expression patterns of each receptor gene in wild-type mice, targeted deletion approaches for generating mutant animals, main phenotypes of receptor-null mice, and alterations in signaling characteristics in receptor-deficient primary cells. Altogether, these data give insights to the importance of LPA/S1P receptors at the cellular and organismal level.
Keywords: Lysophospholipid; Lysophosphatidic acid; Sphingosine 1-phosphate; LPA receptor; S1P receptor; Signal transduction;
Athero- and thrombogenic actions of lysophosphatidic acid and sphingosine-1-phosphate by Wolfgang Siess (204-215).
Lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are potent bioactive phospholipids with specific and multiple effects on blood cells and cells of the vessel wall. Released by activated platelets, LPA and S1P mediate physiological wound healing processes such as vascular repair. Evidence is accumulating that these lipid mediators can, however, under certain conditions become athero- and thrombogenic molecules that might aggravate cardiovascular disease. For example, LPA present in minimally modified LDL and within the intima of atherosclerotic lesions may play a role in the early phase of atherosclerosis by inducing barrier dysfunction and increased monocyte adhesion of the endothelium, as well as in the late phase by triggering platelet activation and intra-arterial thrombus formation upon rupture of the atherosclerotic plaque. Moreover, LPA and S1P, by stimulating the proliferation of fibroblasts and by enhancing the survival of inflammatory cells are likely to play a central role in the excessive fibroproliferative and inflammatory response to vascular injury that characterizes the progression of atherosclerosis. Furthermore, LPA can cause the phenotypic dedifferentiation of medial vascular smooth muscle cells, and S1P is able to stimulate the migration and proliferation of intimal vascular smooth muscle cells; both processes ultimately lead to the formation of the neointima. Most importantly, as LPA and S1P bind to and activate multiple G-protein receptors, it emerges that the beneficial or harmful action of LPA and S1P are critically dependent on the expression profile of their receptor subtypes and their coupling to different signal transduction pathways in the target cells. By targeting specific subtypes of LPA and S1P receptors in selective cells of the vascular wall and blood, new strategies for the prevention and therapy of cardiovascular diseases can be envisioned.
Keywords: Lysophosphatidic acid; Sphingosine-1-phosphate; Atherosclerosis; Thrombosis; Vascular tone; Plaque stability; Endothelial dysfunction; Neointima; Platelet activation;
Lysophospholipids and the cardiovascular system by Joel S Karliner (216-221).
The lysophospholipids sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA) have varied effects on the cardiovascular system. S1P is necessary for normal vascular development and may play an important role in angiogenesis. These molecules may exert potentially detrimental effects. Both S1P and LPA are released from activated platelets and can in turn stimulate platelet aggregation. These thrombogenic effects would further enhance ischemia in acute coronary syndromes and myocardial infarction. LPA is a major component of the lipid core of human atherosclerotic plaques and can stimulate vascular smooth muscle proliferation. Both LPA and S1P cause cardiac myocyte hypertrophy in vitro. Beneficial effects include cardioprotection both in vitro and during ischemia/reperfusion in an ex vivo whole heart mouse model. Understanding both the acute and the chronic physiologic and pathophysiologic roles of the lysophospholipids and their cognate receptors and signaling pathways in the cardiovascular system, which are likely to be species-, tissue-, and cell-specific, may allow the development of molecules that can be targeted to stimulate or inhibit a specific function.
Keywords: Sphingosine-1-phosphate; Vascular development; Cardioprotection; Lysophosphatidic acid; Atherosclerosis; G-protein-coupled receptors; Ischemia/reperfusion;
Sphingosine-1-phosphate receptors and the development of the vascular system by Maria Laura Allende; Richard L Proia (222-227).
Extracellular sphingolipid signaling has been implicated as an essential event in vascular development. Sphingosine-1-phosphate (S1P), through interactions with G protein-coupled receptors, regulates functions of endothelial and smooth muscle cells (SMCs)—the major cell types of the vasculature. The knockout of the gene encoding the S1P1 receptor (formally known as Edg-1) in mice blocks vascular maturation, the process where SMCs and pericytes envelop nascent endothelial tubes. The question that remains is how stimulation of S1P receptors controls this critical event in the developmental sequence leading to the formation of functional blood vessels.
Keywords: Angiogenesis; Sphingosine-1-phosphate; G protein-coupled receptor; Sphingolipid; Vascular smooth muscle; Endothelial cell;
Lipid mediators of angiogenesis and the signalling pathways they initiate by Denis English; David N Brindley; Sarah Spiegel; Joe G.N Garcia (228-239).
Investigations carried out over the past 3 years have implicated a key role for sphingosine 1-phosphate (SPP) in angiogenesis and blood vessel maturation. SPP is capable of inducing almost every aspect of angiogenesis and vessel maturation in vitro, including endothelial cell chemotaxis, survival, proliferation, capillary morphogenesis and adherence antigen deployment, as well as stabilizing developing endothelial cell monolayers and recruitment of smooth muscle cells to maturing vessels. Acting in conjunction with protein angiogenic factors, SPP induces prolific vascular development in many established models of angiogenesis in vivo. Thus, SPP is a unique, potent and multifaceted angiogenic agent. While SPP induces angiogenic effects by ligating members of the endothelial differentiation gene (EDG) G-protein-coupled family of receptors, recent studies suggest that endogenously produced SPP may also account for the ability of tyrosine kinase receptors to induce cell migration. Thus, SPP provides a clear link between tyrosine kinase and G-protein-coupled receptor agonists involved in the angiogenic response. However, the mechanisms by which SPP exerts its effects on vascular cells remain unclear, conflicting and controversial. Precise definition of the signalling pathways by which SPP induces specific aspects of the angiogenic response promises to lead to new and effective therapeutic approaches to regulate angiogenesis at sites of tissue damage, neoplastic transformation and inflammation. This review will trace the discovery of SPP as a novel angiogenic factor as it outlines present information on the signalling pathways by which SPP induces its effects on cells of the developing vascular bed.
Keywords: Sphingosine 1-phosphate; Angiogenesis; Vascular development; Lipid mediator; Endothelial cell; Smooth muscle cell;
Lysophosphatidic acid in airway function and disease by Myron L Toews; Tracy L Ediger; Debra J Romberger; Stephen I Rennard (240-250).
Lysophosphatidic acid (LPA) is a bioactive lipid mediator and important component of serum. Studies over the past several years which have documented diverse effects of LPA on multiple types of airway cells and which suggest possible involvement of LPA in lung disease are reviewed here. LPA enhances contractility of airway smooth muscle. It also stimulates proliferation of cultured airway smooth muscle cells and exhibits a striking synergism with epidermal growth factor (EGF) for stimulating mitogenesis. Recent studies of the molecular components and signaling pathways mediating synergism are described, including LPA-induced upregulation of EGF receptors and activation of multiple transcription factors by both LPA and EGF. A model for the effects of LPA and EGF on mitogenesis that includes EGF receptor upregulation and synergism between Ras and Rho for activation of the transcription factor AP-1 is presented. LPA stimulates fibronectin secretion and filopodia extension in airway epithelial cells as well as proliferation and collagen gel contraction by lung fibroblasts. A hypothesis for LPA involvement in the airway repair and remodeling, which contribute to the pathology of asthma and other airway diseases, is presented, and future directions for research into the roles of LPA in airway function and disease are suggested.
Keywords: Lysophosphatidic acid; Airway smooth muscle; Mitogenesis; Contraction; Asthma; Lung;
EDG receptors and hepatic pathophysiology of LPA and S1P: EDG-ology of liver injury by Stanislav I Svetlov; Yuri Y Sautin; James M Crawford (251-256).
The biological roles of phospholipid growth factors lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) have been broadly investigated. The cellular effects of LPA and S1P are mediated predominantly via endothelial differentiation gene (EDG) receptors. Yet, the biological significance of LPA, S1P and their EDG receptors in cells of the liver remains unclear. Recent data demonstrate the presence of EDG2 and EDG4 mRNA for LPA receptor in a murine hepatocyte cell line transformed with human TGF-α, and in primary mouse hepatocytes. EDG2 receptor protein is expressed in mouse liver, where it appears to be located in nonparenchymal cells. Moreover, we have obtained data suggesting that proliferation of small hepatocyte-progenitors and stem (oval) cells during liver injury is associated with the expression of EDG2 and EDG4 receptors. LPA, and possibly S1P, appear to be essential factors that control proliferation and motility of hepatic stellate cells (HSC) and hepatoma cells. It is proposed that LPA, S1P and their respective EDG receptors play important roles in pathophysiology of chronic liver injury and fibrogenesis. The underlying mechanisms recruited by LPA and S1P in pathogenesis of liver injury remain to be investigated.
Keywords: Lysophosphatidic acid; Sphingosine-1-phosphate; EDG receptor; Liver injury; Hepatic stem cell;
Lysophosphatidic acid is a bioactive mediator in ovarian cancer by Xianjun Fang; Michel Schummer; Muling Mao; Shuangxing Yu; Fazal Haq Tabassam; Ramona Swaby; Yutaka Hasegawa; Janos L Tanyi; Ruthie LaPushin; Astrid Eder; Robert Jaffe; Jim Erickson; Gordon B Mills (257-264).
Lysophosphatidic acid (LPA) is a naturally occurring phospholipid that exhibits pleiotrophic biological activities, ranging from rapid morphological changes to long-term cellular effects such as induction of gene expression and stimulation of cell proliferation and survival on a wide spectrum of cell types. LPA binds and activates distinct members of the Edg/LP subfamily of G protein-coupled receptors that link to multiple G proteins including G(i), G(q) and G(12/13) to elicit cellular responses. LPA plays a critical role as a general growth, survival and pro-angiogenic factor, in the regulation of physiological and pathophysiological processes in vivo and in vitro. Our previous work indicates that abnormalities in LPA metabolism and function in ovarian cancer patients may contribute to the initiation and progression of the disease. Thus, LPA could be a potential target for cancer therapy. This review summarizes evidence that implicates LPA in the pathophysiology of human ovarian cancer and likely other types of human malignancies.
Keywords: Lysophosphatidic acid; Edg receptor; Cell survival; Ovarian cancer; Cancer therapy;
Mitogenic action of LPA in prostate by Yehia Daaka (265-269).
The lipid growth factor lysophosphatidic acid (LPA) elicits multiple cellular responses, including cell growth and survival. LPA acts upon target cells by activating its cognate receptors, which belong to the G protein-coupled endothelial differentiation gene (EDG) family. To date, three known LPA receptors, termed LPA1, LPA2 and LPA3, have been molecularly characterized and cloned. Here, we review recent data describing the molecular steps involved in the LPA receptor-mediated activation of mitogenic extracellular signal-regulated kinase (ERK) pathway in prostate cancer. Induction of ERK by LPA proceeds via Gβγ-dependent activation of tyrosine kinases, including the epidermal growth factor (EGF) receptor and c-Src. Further, LPA-induced ERK activation involves matrix metalloproteinases (MMPs), which cause the release of active EGFR ligands. Finally, we present data demonstrating a correlation between the mitogenic effects of LPA and expression of the lpA1 gene in the prostate cancer cells.
Keywords: LPA; EDG; MAP kinase; Cell proliferation; Prostate cancer;
Lysophosphatidic acid as an autocrine and paracrine mediator by Yuhuan Xie; Terra C. Gibbs; Kathryn E. Meier (270-281).
Recent studies have established that lysophosphatidic acid (LPA) is produced by a wide variety of cell types, and that most mammalian cells express receptors for LPA. These findings raise the hypothesis that LPA acts as an autocrine mediator to initiate signaling in the cells where it is produced, as well as a paracrine mediator to affect neighboring cells. The extent to which these scenarios occur will depend on the species of LPA generated, the LPA receptors expressed, and the ability of these receptors to bind to the LPA produced. The enzymes involved in LPA synthesis and their cellular localization in relationship to LPA receptors are also likely to be important. Studies addressing these issues with respect to the potential roles of LPA as an autocrine and paracrine mediator are reviewed, with examples.
Keywords: Lysophosphatidic acid; Phospholipase; G-protein-coupled receptor; Prostate cancer; Ovarian cancer; Adipocyte;
Modulation of gastrointestinal wound repair and inflammation by phospholipids by Andreas Sturm; Axel U Dignass (282-288).
The mucosal surface of the digestive tract is a critical barrier between a broad spectrum of noxious and immunogenic substances present in the gastrointestinal lumen and the underlying mucosal immune system. Its preservation following various forms of injury or physiological damage is essential to prevent the invasion of harmful luminal factors into the host, which subsequently may lead to inflammation, uncontrolled immune response, and a disequilibrium of the homeostasis of the host. The preservation of this barrier following injuries is regulated by a broad spectrum of structurally distinct regulatory molecules, including phospholipids. Phospholipids play a pivotal role in the modulation of intestinal inflammation. They have been demonstrated to both promote and inhibit inflammation, and their overall impact in an individual setting seems to be dependent on several factors, including the level of immune cell activation and the presence of other mediators. Modulation of lipid mediators through administration of lysophosphatidic acid (LPA) or lisofylline (LSF), inhibitors of phospholipase A2 (PLA2) biosynthesis or monoclonal antibodies against thromboxane (TBX) or platelet-activating factor (PAF) as a therapeutic approach have been used in several models of inflammation; however, beneficial effects were not always convincing and further studies are warranted.
Keywords: Phospholipid; Lysophospholipid; Lisofylline; Lysophosphatidic acid; Intestine; Wound healing; Inflammation; Restitution;
Structure–activity relationships of lysophosphatidic acid analogs by Kevin R Lynch; Timothy L Macdonald (289-294).
The physiologic effects of lysophosphatidic acid (LPA) remain poorly understood. Our ignorance is due in part to lack of medicinal chemistry focussed on this pleiotropic lipid mediator. Beginning with commercially available phospholipids tested on whole cells or tissues and continuing with synthetic analogs tested at recombinant LPA receptors, the features of the LPA pharmacophore have become visible. An active LPA mimetic has a long aliphatic chain terminating in a phosphate monoester; bulky substitutions at the second carbon (relative to the phosphate) are tolerated poorly and a dissociable proton near the phosphate group seems required for optimal activity. These requirements are met by substituting ethanolamine for the glyceryl group in LPA. Substitutions at the second carbon of the N-acyl ethanolamide phosphoric acid (NAEPA) result in highly active agonists, including some receptor type selective compounds, if the substituent is small (e.g. methyl, methylene amino, methylene hydroxy). However, bulky hydrophobic substituents lead to compounds with decreased agonist, or even antagonist, activities. Examination of naturally occurring plant lipids led to the discovery of another LPA receptor antagonist, di-octyl glyceryl pyrophosphate. An unexplained result obtained in testing the LPA mimetics is the strong stereoselectivity exhibited by some responses (e.g. calcium mobilization) and the lack of stereoselectivity of other responses (e.g. platelet aggregation).
Keywords: Structure–activity relationship; Lysophosphatidic acid; Dissociable proton;
Stereochemical properties of lysophosphatidic acid receptor activation and metabolism by Kazuaki Yokoyama; Daniel L Baker; Tamas Virag; Karoly Liliom; Hoe-Sup Byun; Gabor Tigyi; Robert Bittman (295-308).
Ligand recognition by G protein-coupled receptors (GPCR), as well as substrate recognition by enzymes, almost always shows a preference for a naturally occurring enantiomer over the unnatural one. Recognition of lysophosphatidic acid (LPA) by its receptors is an exception, as both the natural l (R) and unnatural d (S) stereoisomers of LPA are equally active in bioassays. In contrast to the enantiomers of LPA, analogs of N-acyl-serine phosphoric acid (NASPA) and N-acyl-ethanolamine phosphoric acid (NAEPA), which contain a serine and an ethanolamine backbone, respectively, in place of glycerol, are recognized in a stereoselective manner. This stereoselective interaction may lead to the development of receptor subtype-selective antagonists. In the present study, we review the stereochemical aspects of LPA pharmacology and describe the chemical synthesis of pure LPA enantiomers together with their ligand-binding properties toward the LPA1, LPA2, and LPA3 receptors and their metabolism by lipid phosphate phosphatase 1 (LPP1). Finally, we evaluate the concept of stereopharmacology in developing novel ligands for LPA receptors.
Keywords: LPA receptor; Lipid phosphate phosphatase 1; LPA antagonist; Stereoselectivity; Lipid synthesis;
Molecular basis for lysophosphatidic acid receptor antagonist selectivity by Vineet M Sardar; Debra L Bautista; David J Fischer; Kazuaki Yokoyama; Nora Nusser; Tamas Virag; De-an Wang; Daniel L Baker; Gabor Tigyi; Abby L Parrill (309-317).
Recent characterization of lysophosphatidic acid (LPA) receptors has made possible studies elucidating the structure–activity relationships (SAR) for agonist activity at individual receptors. Additionally, the availability of these receptors has allowed the identification of antagonists of LPA-induced effects. Two receptor-subtype selective LPA receptor antagonists, one selective for the LPA1/EDG2 receptor (a benzyl-4-oxybenzyl N-acyl ethanolamide phosphate, NAEPA, derivative) and the other selective for the LPA3/EDG7 receptor (diacylglycerol pyrophosphate, DGPP, 8:0), have recently been reported. The receptor SAR for both agonists and antagonists are reviewed, and the molecular basis for the difference between agonism and antagonism as well as for receptor-subtype antagonist selectivity identified by molecular modeling is described. The implications of the newly available receptor-subtype selective antagonists are also discussed.
Keywords: G protein-coupled receptor; Molecular modeling; Endothelial differentiation gene receptor; Diacylglycerol pyrophosphate; Homology modeling; Lysophosphatidic acid;