Two pathways for lysophosphatidic acid production

Lysophosphatidic acid (LPA, 1- or 2-acyl-sn-glycerol 3-phosphate) is a simple phospholipid but displays an intriguing cell biology that is mediated via interactions with G protein-coupled seven transmembrane receptors (GPCRs). So far, five GPCRs, designated LPA(1-5), and, more recently, two additional GPCRs, GPR87 and P2Y5, have been identified as receptors for LPA. These LPA receptors can be classified into two families, the EDG and P2Y families, depending on their primary structures. Recent studies on gene targeting mice and family diseases of these receptors revealed that LPA is involved in both pathological and physiological states including brain development (LPA(1)), neuropathy pain (LPA(1)), lung fibrosis (LPA(1)), renal fibrosis (LPA(1)) protection against radiation-induced intestinal injury (LPA(2)), implantation (LPA(3)) and hair growth (P2Y5). LPA is produced both in cells and biological fluids, where multiple synthetic reactions occur. There are at least two pathways for LPA production. In serum or plasma, LPA is predominantly produced by a plasma enzyme called autotaxin (ATX). ATX is a multifunctional ectoenzyme and is involved in many patho-physiological conditions such as cancer, neuropathy pain, lymphocyte tracking in lymph nodes, obesity, diabetes and embryonic blood vessel formation. LPA is also produced from phosphatidic acid (PA) by its deacylation catalyzed by phospholipase A (PLA)-type enzymes. However, the physiological roles of this pathway as well as the enzymes involved remained to be solved. A number of phospholipase A(1) and A(2) isozymes could be involved in this pathway. One PA-selective PLA(1) called mPA-PLA(1)alpha/LIPH is specifically expressed in hair follicles, where it has a critical role in hair growth by producing LPA through a novel LPA receptor called P2Y5.     

Lysophosphatidic acid: A potential mediator of osteoblast–osteoclast signaling in bone

Osteoclasts (bone resorbing cells) and osteoblasts (bone forming cells) play essential roles in skeletal development, mineral homeostasis and bone remodeling. The actions of these two cell types are tightly coordinated, and imbalances in bone formation and resorption can result in disease states, such as osteoporosis. Lysophosphatidic acid (LPA) is a potent bioactive phospholipid that influences a number of cellular processes, including proliferation, survival and migration. LPA is also involved in wound healing and pathological conditions, such as tumor metastasis and autoimmune disorders. During trauma, activated platelets are likely a source of LPA in bone. Physiologically, osteoblasts themselves can also produce LPA, which in turn promotes osteogenesis. The capacity for local production of LPA, coupled with the proximity of osteoblasts and osteoclasts, leads to the intriguing possibility that LPA acts as a paracrine mediator of osteoblast–osteoclast signaling. Here we summarize emerging evidence that LPA enhances the differentiation of osteoclast precursors, and regulates the morphology, resorptive activity and survival of mature osteoclasts. These actions arise through stimulation of multiple LPA receptors and intracellular signaling pathways. Moreover, LPA is a potent mitogen implicated in promoting the metastasis of breast and ovarian tumors to bone. Thus, LPA released from osteoblasts is potentially an important autocrine and paracrine mediator — physiologically regulating skeletal development and remodeling, while contributing pathologically to metastatic bone disease. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.     

Lysophosphatidic acid (LPA) receptors: Signaling properties and disease relevance

Lysophosphatidic acid (LPA), a water-soluble phospholipid, has gained significant attention in recent years since the discovery that it acts as a potent signaling molecule with wide-ranging effects on many different target tissues. There are currently five identified G protein-coupled receptors for LPA and more are undergoing validation. The complexity of the expression pattern and signaling properties of LPA receptors results in multiple influences on developmental, physiological, and pathological processes. This review provides a summary of LPA receptor signaling and current views on the potential involvement of this pathway in human diseases that include cardiovascular, cancer, neuropathic pain, neuropsychiatric disorders, reproductive disorders, and fibrosis. The involvement of LPA signaling in these processes implicates multiple, potential drug targets including LPA receptor subtypes and LPA metabolizing enzymes. Modulation of LPA signaling may thus provide therapeutic inroads for the treatment of human disease.     

Methods for quantifying lysophosphatidic acid in body fluids: A review

Lysophosphatidic acid (LPA) is a bioactive lipid involved in cellular signal transduction. LPA plays a role in both physiological and pathological processes. Elevated levels of LPA are observed in the plasma of patients with epithelial ovarian cancer, indicating its potential as a diagnostic marker. Quantification of total LPA can be performed by radioenzymatic, fluorometric, colorimetric, or immunoezymatic assay. Determination of individual LPA molecular species requires the use of capillary electrophoresis, gas chromatography, thin layer chromatography, liquid chromatography, or a matrix-assisted laser desorption/ionization time-of-flight method connected to an appropriate detection system.     

The ins and outs of lysophosphatidic acid signaling

Lysophosphatidic acid (LPA) is a lipid mediator with a wide variety of biological actions, particularly as an inducer of cell proliferation, migration and survival. LPA binds to specific G-protein-coupled receptors and thereby activates multiple signal transduction pathways, including those initiated by the small GTPases Ras, Rho, and Rac. LPA signaling has been implicated in such diverse processes as wound healing, brain development, vascular remodeling and tumor progression. Knowledge of precisely how and where LPA is produced has long proved elusive. Excitingly, it has recently been discovered that LPA is generated from precursors by 'autotaxin', a once enigmatic exo-phosphodiesterase implicated in tumor cell motility. Exogenous phospholipases D can also produce LPA, which may contribute to their toxicity. Here we review recent progress in our understanding of LPA bioactivity, signaling and synthesis.     

Sensitizing effect of lysophosphatidic acid on Ca2+ response to hypotonic stress in cultured lens epithelial cells.

The effects of lysophosphatidic acid (LPA), a bioactive phospholipid, on the response of the cytosolic free Ca2+ concentration ([Ca2+]i) to hypotonic stress were studied in cultured bovine lens epithelial cells, to test whether LPA affects cellular swelling-mediated increase in [Ca2+]i, which may relate to formation of sugar cataracts. Exposure of the cells to a 30% hypotonic stress caused only a slight increase in [Ca2+]i. Pretreatment with LPA (10 microM) significantly augmented the hypotonic stress-induced [Ca2+]i response, whereas addition of LPA to the cells did not affect [Ca2+]i. The hypotonic stress-induced increase in [Ca2+]i in the presence of LPA was inhibited by Gd3+, a blocker of mechanosensitive cation channels, but not by nicardipine, a L-type Ca2+ channel blocker, or thapsigargin, an inhibitor of endoplasmic reticulum-ATPase pump. These results show that LPA sensitizes the response to hypotonic stress via increase in Ca2+ influx through Gd3+-sensitive stretch-activated ion channels, and not via Ca2+ release from intracellular stores. On the other hand, LPA did not affect the [Ca2+]i response to ATP, a Ca2+ mobilizing agonist. Therefore, LPA sensitizes the hypotonic stress-induced [Ca2+]i response in lens epithelial cells, suggesting that LPA potentiates the development of cataracts induced by cellular swelling such as sugar cataract.     

Lysophosphatidic acid is a lipid mediator with wide range of biological activities. Biosynthetic pathways and mechanism of action

Lysophosphatidic acid (LPA) is a lipid mediator required for maintaining homeostasis of numerous physiological functions and also involved in development of some pathological processes through interactions with G protein-coupled receptors. Recently many data have appeared about the role of this phospholipid in humans, but pathways of LPA biosynthesis and mechanisms of its action remain unclear. This review presents modern concepts about biosynthesis, reception, and biological activity of LPA in humans. Natural and synthetic LPA analogs are considered in the view of their possible use in pharmacology as agonists and/or antagonists of G protein-coupled receptors of LPA.     

Non-Edg family LPA receptors: the cutting edge of LPA research

Lysophosphatidic acid (LPA) is a bioactive lipid mediator with diverse physiological and pathological actions on many types of cells. Originally, LPA was thought to elicit its biological functions through three subtypes of endothelial differentiation gene (Edg) family G protein-coupled receptors (LPA1, LPA2 and LPA3) until our group identified a fourth subtype, LPA4. The discovery of this receptor, which is structurally distinct from the Edg family LPA receptors, led to the identification of two additional LPA receptors, LPA5 and LPA6, homologous to LPA4. These 'non-Edg family' LPA receptors now provide a new framework for understanding the diverse functions of LPA, including vascular development, platelet activation and hair growth. In this review, we summarize the identification, intracellular signalling and biological functions of this novel cluster of LPA receptors.     

Autotaxin protects microglial cells against oxidative stress

Oxidative stress occurs when antioxidant defenses are overwhelmed by oxygen-reactive species and can lead to cellular damage, as seen in several neurodegenerative disorders. Microglia are specialized cells in the central nervous system that act as the first and main form of active immune defense in the response to pathological events. Autotaxin (ATX) plays an important role in the modulation of critical cellular functions, through its enzymatic production of lysophosphatidic acid (LPA). In this study, we investigated the potential role of ATX in the response of microglial cells to oxidative stress. We show that treatment of a microglial BV2 cell line with hydrogen peroxide (H(2)O(2)) stimulates ATX expression and LPA production. Stable overexpression of ATX inhibits microglial activation (CD11b expression) and protects against H(2)O(2)-treatment-induced cellular damage. This protective effect of ATX was partially reduced in the presence of the LPA-receptor antagonist Ki16425. ATX overexpression was also associated with a reduction in intracellular ROS formation, carbonylated protein accumulation, proteasomal activity, and catalase expression. Our results suggest that up-regulation of ATX expression in microglia could be a mechanism for protection against oxidative stress, thereby reducing inflammation in the nervous system.     

The Edg family G protein-coupled receptors for lysophospholipids: their signaling properties and biological activities

Sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA) are blood-borne lysophospholipids with a wide spectrum of biological activities, which include stimulation of cell growth, prevention of apoptosis, regulation of actin cytoskeleton, and modulation of cell shape, cell migration, and invasion. Activated platelets appear to be a major source of both S1P and LPA in blood. Despite the diversity of their biosynthetic origins, they are considered to share substantial structural similarity. Indeed, recent investigation has revealed that S1P and LPA act via a single family of G protein-coupled receptors designated as Edg. Thus, the Edg isoforms, Edg1 (also called S1P(1)), Edg5 (S1P(2)), Edg3 (S1P(3)), Edg6 (S1P(4)), and Edg8 (S1P(5)), are specific receptors for S1P (and SPC with a lower affinity), whereas Edg2 (LPA(1)), Edg4 (LPA(2)), and Edg7 (LPA(3)) serve as receptors specific for LPA. Each receptor isoform displays a unique tissue expression pattern and coupling to a distinct set of heterotrimeric G proteins, leading to the activation of an isoform-specific panel of multiple intracellular signaling pathways. Recent studies on knockout mice have unveiled non-redundant Edg receptor functions that are essential for normal development and vascular maturation. In addition, the Edg lysophospholipid signaling system may play a role in modulating cell motility under such pathological conditions as inflammation, tumor cell dissemination and vascular remodeling.     

Lysophosphatidic acid and its role in reproduction

Lysophosphatidic acid (LPA) belongs to a new family of lipid mediators that are endogenous growth factors and that elicit diverse biological effects, usually via the activation of G protein-coupled receptors. LPA can be generated after cell activation through the hydrolysis of preexisting phospholipids in the membranes of stimulated cells. A dramatic elevation of LPA levels was found in serum of patients suffering from ovarian carcinoma. Because these high LPA amounts can be detected as early as stage I of the disease, LPA has been introduced as a new marker for ovarian cancer. Progression of the malignancy is correlated with a differential expression of various LPA receptor subtypes. The presence of LPA in the follicular fluid of healthy individuals implicates that this biological mediator may be relevant to normal ovarian physiology. LPA induces proliferation and mitogenic signaling of prostate cancer cells, and a novel LPA receptor isoform has been recognized in healthy prostate tissues. This evidence indicates multiple roles for LPA in both male and female reproductive physiology and pathology. In this review, we summarize the literature on LPA generation, the way it is degraded, and the mechanisms by which signals are transduced by various LPA receptors in reproductive tissues, and we discuss possible future research directions in these areas.     

Lysophosphatidic acid as a novel cell survival/apoptotic factor

Lysophosphatidic acid (LPA) activates its cognate G protein-coupled receptors (GPCRs) LPA(1-3) to exert diverse cellular effects, including cell survival and apoptosis. The potent survival effect of LPA on Schwann cells (SCs) is mediated through the pertussis toxin (PTX)-sensitive G(i/o)/phosphoinositide 3-kinase (PI3K)/Akt signaling pathways and possibly enhanced by the activation of PTX-insensitive Rho-dependent pathways. LPA promotes survival of many other cell types mainly through PTX-sensitive G(i/o) proteins. Paradoxically, LPA also induces apoptosis in certain cells, such as myeloid progenitor cells, hippocampal neurons, and PC12 cells, in which the activation of the Rho-dependent pathways and caspase cascades has been implicated. The effects of LPA on both cell survival and apoptosis underscore important roles for this lipid in normal development and pathological processes.     

Lysophosphatidic acid antagonizes the morphoregulatory effects of the luteinizing hormone on luteal cells: possible role of small Rho-G-proteins.

Lysophosphatidic acid (LPA) is a biologically active phospholipid recently introduced as a new marker for ovarian cancer. Because high concentrations of LPA have also been found in the follicular fluid from healthy subjects, one can presume that this biological mediator may have relevance for normal ovarian physiology as well. We have reported earlier that luteal cells possess specific binding sites for LPA. Using these cells as a model, we show now that LPA is able to modulate the morphological cell shape changes induced by LH in that it inhibits the formation of stellate processes induced by LH. This morphoregulatory effect of LPA is mimicked by cytotoxic necrotizing factor 1, a bacterial toxin known to activate small G-proteins from the Rho family. On the other hand, C3-exotransferase that acts mainly through the inhibition of Rho A mimics the effects of LH. Furthermore, we report here that the morphoregulatory effects of LPA are accompanied by the translocation of Rho proteins from the cytosol to cell membrane, an effect generally considered to be an indicator for the activation of Rho-GTPases. During the development and rescue of the corpus luteum, major morphoregulatory effects are exerted by LH that appear to be modulated by LPA via an activation of Rho proteins.     

Roles for lysophosphatidic acid signaling in vascular development and disease

The bioactive lipid lysophosphatidic acid (LPA) is emerging as an important mediator of inflammation in cardiovascular diseases. Produced in large part by the secreted lysophospholipase D autotaxin (ATX), LPA acts on a series of G protein-coupled receptors and may have action on atypical receptors such as RAGE to exert potent effects on vascular cells, including the promotion of foam cell formation and phenotypic modulation of smooth muscle cells. The signaling effects of LPA can be terminated by integral membrane lipid phosphate phosphatases (LPP) that hydrolyze the lipid to receptor inactive products. Human genetic variants in PLPP3, that predict lower levels of LPP3, associate with risk for premature coronary artery disease, and reductions of LPP3 expression in mice promote the development of experimental atherosclerosis and enhance inflammation in the atherosclerotic lesions. Recent evidence also supports a role for ATX, and potentially LPP3, in calcific aortic stenosis. In summary, LPA may be a relevant inflammatory mediator in atherosclerotic cardiovascular disease and heightened LPA signaling may explain the cardiovascular disease risk effect of PLPP3 variants.     

Oxytocin and lysophosphatidic acid induce stress fiber formation in human myometrial cells via a pathway involving Rho-kinase

The actin cytoskeleton is important for stress fiber formation and contributes to the initiation and maintenance of smooth muscle contraction. To determine if oxytocin and lysophosphatidic acid (LPA) induce stress fiber formation, cultured human myometrial cells were exposed to oxytocin (10(-5) M) or LPA (10(-6) M), and filamentous (F) and globular (G) actin pools were stained with fluorescein isothiocyanate-phalloidin and Texas red DNase I, respectively. The F- to G-actin fluorescent-staining ratio was measured by fluorescence microscopy. Oxytocin and LPA increased stress fiber formation, as indicated by an increase in the F- to G-actin fluorescent-staining ratio. The Rho-kinase inhibitor Y-27632 markedly attenuated this increase. Oxytocin-induced stress fiber formation was completely inhibited in the presence of the oxytocin antagonist compound VI. Tyrosine kinase inhibition with tyrphostin A23 partially blocked the increase induced by oxytocin but had no effect on LPA-induced stress fiber formation. Stress fiber formation was not blocked by pertussis toxin, mitogen-activated protein kinase, or protein kinase C inhibitors. Our results show that human myometrial cells respond to oxytocin and LPA with the formation of stress fibers that may be involved in the maintenance of uterine contractions. Rho-kinase appears to be a key signaling factor in this pathway.     

Lysophosphatidic Acid Sensitises Ca Influx through Mechanosensitive Ion Channels in Cultured Lens Epithelial Cells

We investigated the effect of lysophosphatidic acid (LPA), a bioactive phospholipid, on the response in cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) to mechanical stress in cultured bovine lens epithelial cells. Spritzing of bath solution onto cells as mechanical stress caused marked increase in [Ca²⁺]_i in the presence of LPA and this increase was concentration-dependent (1–10 μM), whereas neither addition of LPA alone nor the mechanical stress in the absence of LPA affected [Ca²⁺]_i. The mechanical stress-induced increase in [Ca²⁺]_i in the presence of LPA was inhibited by removing extracellular Ca²⁺ or by addition of Gd³⁺, a blocker of mechanosensitive cation channels, but not by nicardipine, thapsigargin, an inhibitor of endoplasmic reticulum-ATPase pump, or U73122, a phospholipase C inhibitor. These results show that LPA sensitises Ca²⁺ influx through cation-selective mechanosensitive channels, but does not sensitise Ca²⁺ release from intracellular stores, triggered by changes in mechanical stress. On the other hand, phosphatidic acid had less of a sensitising effect than LPA, and neither lysophosphatidylcholine nor chlorpromazine had any effect. Also Ca²⁺ mobilising agonists, ATP, histamine and carbachol, did not sensitise Ca²⁺ response to the mechanical stress. These results show that LPA sensitises mechanoreceptor-linked response in lens epithelial cells, suggesting that it plays a role in the development of cataracts due to increases in [Ca²⁺]_i induced by mechanical stress.     

Lysophospholipids and the cardiovascular system

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.     

Novel bioactive glycerol-based lysophospholipids: New data – New insight into their function

Based on the results of research conducted over last two decades, lysophospholipids (LPLs) were observed to be not only structural components of cellular membranes but also biologically active molecules influencing a broad variety of processes such as carcinogenesis, neurogenesis, immunity, vascular development or regulation of metabolic diseases. With a growing interest in the involvement of extracellular lysophospholipids in both normal physiology and pathology, it has become evident that those small molecules may have therapeutic potential. While lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) have been studied in detail, other LPLs such as lysophosphatidylglycerol (LPG), lysophosphatidylserine (LPS), lysophosphatidylinositol (LPI), lysophosphatidylethanolamine (LPE) or even lysophosphatidylcholine (LPC) have not been elucidated to such a high degree. Although information concerning the latter LPLs is sparse as compared to LPA and S1P, within the last couple of years much progress has been made. Recently published data suggest that these compounds may regulate fundamental cellular activities by modulating multiple molecular targets, e.g. by binding to specific receptors and/or altering the structure and fluidity of lipid rafts. Therefore, the present review is devoted to novel bioactive glycerol-based lysophospholipids and recent findings concerning their functions and possible signaling pathways regulating physiological and pathological processes.     

LPA4/p2y9/GPR23 mediates rho-dependent morphological changes in a rat neuronal cell line

Lysophosphatidic acid (LPA) is a potent lipid mediator that evokes a variety of biological responses in many cell types via its specific G protein-coupled receptors. In particular, LPA affects cell morphology, cell survival, and cell cycle progression in neuronal cells. Recently, we identified p2y(9)/GPR23 as a novel fourth LPA receptor, LPA(4) (Noguchi, K., Ishii, S., and Shimizu, T. (2003) J. Biol. Chem. 278, 25600-25606). To assess the functions of LPA(4) in neuronal cells, we used rat neuroblastoma B103 cells that lack endogenous responses to LPA. In B103 cells stably expressing LPA(4), we observed G(q/11)-dependent calcium mobilization, but LPA did not affect adenylyl cyclase activity. In LPA(4) transfectants, LPA induced dramatic morphological changes, i.e. neurite retraction, cell aggregation, and cadherin-dependent cell adhesion, which involved Rho-mediated signaling pathways. Thus, our results demonstrated that LPA(4) as well as LPA(1) couple to G(q/11) and G(12/13), whereas LPA(4) differs from LPA(1) in that it does not couple to G(i/o). Through neurite retraction and cell aggregation, LPA(4) may play a role in neuronal development such as neurogenesis and neuronal migration.     

Lysophosphatidic acid-induced membrane ruffling and brain-derived neurotrophic factor gene expression are mediated by ATP release in primary microglia

We examined the effects of lysophosphatidic acid (LPA) on microglia, which may play an important role in the development and maintenance of neuropathic pain. LPA caused membrane ruffling as detected by scanning electron microscopy, and increased the expression of brain-derived neurotrophic factor (BDNF) in a primary culture of rat microglia, which express LPA(3), but not LPA(1) or LPA(2) receptors. These actions were inhibited by a Galpha(q/11)-antisense oligodeoxynucleotide (AS-ODN), U73122, an inhibitor of phospholipase C (PLC), and apyrase, which specifically degrades ATP and ADP. When ATP release was measured using a luciferin-luciferase bioluminescence assay, LPA was shown to increase it in an LPA(3) and PLC inhibitor-reversible manner. However, LPA-induced ATP release was also blocked by the Galpha(q/11) AS-ODN, but not by pertussis toxin. These results suggest that LPA induces the release of ATP from rat primary cultured microglia via the LPA(3) receptor, Galpha(q/11) and PLC, and that the released ATP or ectopically converted ADP may in turn cause membrane ruffling via P2Y(12) receptors and Galpha(i/o) activation, and BDNF expression via activation of P2X(4) receptors.