Recombinant human G protein-coupled lysophosphatidic acid receptors mediate intracellular calcium mobilization

Regulation of CD8 T-cell signaling, metabolism, and cytotoxic activity by extracellular lysophosphatidic acid

Lysophosphatidic acid (LPA) is an endogenous bioactive lipid that is produced extracellularly and signals to cells via cognate LPA receptors, which are G-protein coupled receptors (GPCRs). Mature lymphocytes in mice and humans express three LPA receptors, LPA2 , LPA5, and LPA6 , and work from our group has determined that LPA5 signaling by T lymphocytes inhibits specific antigen-receptor signaling pathways that ultimately impair lymphocyte activation, proliferation, and function. In this review, we discuss previous and ongoing work characterizing the ability of an LPA-LPA5 axis to serve as a peripheral immunological tolerance mechanism that restrains adaptive immunity but is subverted during settings of chronic inflammation. Specifically, LPA-LPA5 signaling is found to regulate effector cytotoxic CD8 T cells by (at least) two mechanisms: (i) regulating the actin-microtubule cytoskeleton in a manner that impairs immunological synapse formation between an effector CD8 T cell and antigen-specific target cell, thus directly impairing cytotoxic activity, and (ii) shifting T-cell metabolism to depend on fatty-acid oxidation for mitochondrial respiration and reducing metabolic efficiency. The in vivo outcome of LPA5 inhibitory activity impairs CD8 T-cell killing and tumor immunity in mouse models providing impetus to consider LPA5 antagonism for the treatment of malignancies and chronic infections.

G Protein-Coupled Receptors in Osteoarthritis: A Novel Perspective on Pathogenesis and Treatment

G protein-coupled receptors (GPCRs) are transmembrane receptor proteins that trigger numerous intracellular signaling pathways in response to the extracellular stimuli. The GPCRs superfamily contains enormous structural and functional diversity and mediates extensive biological processes. Until now, critical roles have been established in many diseases, including osteoarthritis (OA). Existing studies have shown that GPCRs play an important role in some OA-related pathogenesis, such as cartilage matrix degradation, synovitis, subchondral bone remodeling, and osteophyte formation. However, current pharmacological treatments are mostly symptomatic and there is a paucity of disease-modifying OA drugs so far. Targeting GPCRs is capable of inhibiting cartilage matrix degradation and synovitis and up-regulating cartilage matrix synthesis, providing a new therapeutic strategy for OA. In this review, we have comprehensively summarized the structures, biofunctions, and the novel roles of GPCRs in the pathogenesis and treatment of OA, which is expected to lay the foundation for the development of novel therapeutics against OA. Even though targeting GPCRs may ameliorate OA progression, many GPCRs-related therapeutic strategies are still in the pre-clinical stage and require further investigation.

G-protein coupled receptor, PI3K and Rho signaling pathways regulate the cascades of Tau and amyloid-β in Alzheimer's disease

Alzheimer's disease is a progressive neurodegenerative disease characterized by the presence of amyloid-β plaques in the extracellular environment and aggregates of Tau protein that forms neurofibrillary tangles (NFTs) in neuronal cells. Along with these pathological proteins, the disease shows neuroinflammation, neuronal death, impairment in the immune function of microglia and synaptic loss, which are mediated by several important signaling pathways. The PI3K/Akt-mediated survival-signaling pathway is activated by many receptors such as G-protein coupled receptors (GPCRs), triggering receptor expressed on myeloid cells 2 (TREM2), and lysophosphatidic acid (LPA) receptor. The signaling pathway not only increases the survival of neurons but also regulates inflammation, phagocytosis, cellular protection, Tau phosphorylation and Aβ secretion as well. In this review, we focused on receptors, which activate PI3K/Akt pathway and its potential to treat Alzheimer's disease. Among several membrane receptors, GPCRs are the major drug targets for therapy, and GPCR signaling pathways are altered during Alzheimer's disease. Several GPCRs are involved in the pathogenic progression, phosphorylation of Tau protein by activation of various cellular kinases and are involved in the amyloidogenic pathway of amyloid-β synthesis. Apart from various GPCR signaling pathways, GPCR regulating/ interacting proteins are involved in the pathogenesis of Alzheimer's disease. These include several small GTPases, Ras homolog enriched in brain, GPCR associated sorting proteins, β-arrestins, etc., that play a critical role in disease progression and has been elaborated in this review.

The Role of Lysophosphatidic Acid in Adult Stem Cells

Stem cells are undifferentiated multipotent precursor cells that are capable both of perpetuating themselves as stem cells (self-renewal) and of undergoing differentiation into one or more specialized types of cells. And these stem cells have been reported to reside within distinct anatomic locations termed “niches”. The long-term goals of stem cell biology range from an understanding of cell-lineage determination and tissue organization to cellular therapeutics for degenerative diseases. Stem cells maintain tissue function throughout an organism’s lifespan by replacing differentiated cells. To perform this function, stem cells provide a unique combination of multilineage developmental potential and the capacity to undergo self-renewing divisions. The loss of self-renewal capacity in stem cells underlies certain degenerative diseases and the aging process. This self-renewal regulation must balance the regenerative needs of tissues that persist throughout life. Recent evidence suggests lysophosphatidic acid (LPA) signaling pathway plays an important role in the regulation of a variety of stem cells. In this review, we summarize the evidence linking between LPA and stem cell regulation. The LPA-induced signaling pathway regulates the proliferation and survival of stem cells and progenitors, and thus are likely to play a role in the maintenance of stem cell population in the body. This lipid mediator regulatory system can be a novel potential therapeutics for stem cell maintenance.

Arginine 313 of the putative 8th helix mediates Gαq/14 coupling of human CC chemokine receptors CCR2a and CCR2b

In man, two CC chemokine receptor isoforms, CCR2a and CCR2b, are present that belong to the rhodopsin-like G protein-coupled receptor family, and couple to G_i and G_q family members. The CCR2 receptors are known to regulate canonical functions of chemokines such as directed migration of leukocytes, and to potentially control non-canonical functions such as differentiation, proliferation, and gene transcription of immune and non-immune cells. We recently reported on the activation of phospholipase C isoenzymes and RhoA GTPases by coupling of the two CCR2 receptors to members of the G_q family, in particular Gα_q and Gα₁₄. So far little is known about the structural requirements for the CCR2/G_q/14 interaction. Interestingly, the CCR2 receptor isoforms are identical up to arginine 313 (R³¹³) that is part of the putative 8th helix in CCR2 receptors, and the 8th helix has been implicated in the interaction of rhodopsin-like G protein-coupled receptors with Gα_q. In the present work we describe that the 8th helix of both CCR2a and CCR2b is critically involved in selectively activating Gα_q/14-regulated signaling. Refined analysis using various CCR2a and CCR2b mutants and analyzing their cellular signaling, e.g. ligand-dependent (i) activation of phospholipase C isoenzymes, (ii) stimulation of serum response factor-mediated gene transcription, (iii) activation of mitogen-activated protein kinases, (iv) internalization, and (v) changes in intracellular calcium concentrations, identified arginine 313 within the amino terminal portion of helix 8 to play a role for the agonist-mediated conformational changes and the formation of a Gα_q/14 binding surface. We show that R³¹³ determines Gα_q/14 protein-dependent but not G_i protein-dependent cellular signaling, and plays no role in G_q/G_i-independent receptor internalization, indicating a role of R³¹³ in biased signaling of CCR2 receptors.

Convergent regulation of neuronal differentiation and Erk and Akt kinases in human neural progenitor cells by lysophosphatidic acid, sphingosine 1-phosphate, and LIF: specific roles for the LPA1 receptor

The bioactive lysophospholipids lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) have diverse effects on the developing nervous system and neural progenitors, but the molecular basis for their pleiotropic effects is poorly understood. We previously defined LPA and S1P signaling in proliferating human neural progenitor (hNP) cells, and the current study investigates their role in neuronal differentiation of these cells. Differentiation in the presence of LPA or S1P significantly enhanced cell survival and decreased expression of neuronal markers. Further, the LPA receptor antagonist Ki16425 fully blocked the effects of LPA, and differentiation in the presence of Ki16425 dramatically enhanced neurite length. LPA and S1P robustly activated Erk, but surprisingly both strongly suppressed Akt activation. Ki16425 and pertussis toxin blocked LPA activation of Erk but not LPA inhibition of Akt, suggesting distinct receptor and G-protein subtypes mediate these effects. Finally, we explored cross talk between lysophospholipid signaling and the cytokine leukemia inhibitory factor (LIF). LPA/S1P effects on neuronal differentiation were amplified in the presence of LIF. Similarly, the ability of LPA/S1P to regulate Erk and Akt was impacted by the presence of LIF; LIF enhanced the inhibitory effect of LPA/S1P on Akt phosphorylation, while LIF blunted the activation of Erk by LPA/S1P. Taken together, our results suggest that LPA and S1P enhance survival and inhibit neuronal differentiation of hNP cells, and LPA1 is critical for the effect of LPA. The pleiotropic effects of LPA may reflect differences in receptor subtype expression or cross talk with LIF receptor signaling.

LPA receptor signaling: pharmacology, physiology, and pathophysiology

Lysophosphatidic acid (LPA) is a small ubiquitous lipid found in vertebrate and nonvertebrate organisms that mediates diverse biological actions and demonstrates medicinal relevance. LPA's functional roles are driven by extracellular signaling through at least six 7-transmembrane G protein-coupled receptors. These receptors are named LPA1-6 and signal through numerous effector pathways activated by heterotrimeric G proteins, including Gi/o, G12/13, Gq, and Gs LPA receptor-mediated effects have been described in numerous cell types and model systems, both in vitro and in vivo, through gain- and loss-of-function studies. These studies have revealed physiological and pathophysiological influences on virtually every organ system and developmental stage of an organism. These include the nervous, cardiovascular, reproductive, and pulmonary systems. Disturbances in normal LPA signaling may contribute to a range of diseases, including neurodevelopmental and neuropsychiatric disorders, pain, cardiovascular disease, bone disorders, fibrosis, cancer, infertility, and obesity. These studies underscore the potential of LPA receptor subtypes and related signaling mechanisms to provide novel therapeutic targets.

Melanoma biomolecules: independently identified but functionally intertwined

The majority of patients diagnosed with melanoma present with thin lesions and generally these patients have a good prognosis. However, 5% of patients with early melanoma (<1 mm thick) will have recurrence and die within 10 years, despite no evidence of local or metastatic spread at the time of diagnosis. Thus, there is a need for additional prognostic markers to help identify those patients that may be at risk of recurrent disease. Many studies and several meta-analyses have compared gene and protein expression in melanocytes, naevi, primary, and metastatic melanoma in an attempt to find informative prognostic markers for these patients. However, although a large number of putative biomarkers have been described, few of these molecules are informative when used in isolation. The best approach is likely to involve a combination of molecules. We believe one approach could be to analyze the expression of a group of interacting proteins that regulate different aspects of the metastatic pathway. This is because a primary lesion expressing proteins involved in multiple stages of metastasis may be more likely to lead to secondary disease than one that does not. This review focuses on five putative biomarkers – melanoma cell adhesion molecule (MCAM), galectin-3 (gal-3), matrix metalloproteinase 2 (MMP-2), chondroitin sulfate proteoglycan 4 (CSPG4), and paired box 3 (PAX3). The goal is to provide context around what is known about the contribution of these biomarkers to melanoma biology and metastasis. Although each of these molecules have been independently identified as likely biomarkers, it is clear from our analyses that each are closely linked with each other, with intertwined roles in melanoma biology.

Migration of gastric cancer cells in response to lysophosphatidic acid is mediated by LPA receptor 2

Lysophosphatidic acid (LPA), a natural phospholipid, is able to modulate diverse cellular responses through LPA receptors (LPARs). Several studies have reported that LPAR2 gene expression is increased in a variety of cancer cells, suggesting that LPAR2 is involved in gastric cancer. The present study investigated the expression profiles of the LPAR and involvement of the receptor subtypes in the LPA-induced migration of gastric cancer cells using cell migration assays, RNA interference, quantitative real-time PCR and western blotting. LPAR2 was observed to be highly expressed in SGC-7901 cells, a human gastric cancer cell line, while LPAR1 and LPAR3 were not. Transient transfection with LPAR2 siRNA was observed to reduce LPAR2 mRNA in SGC-7901 cells and eliminate the LPA-induced cell migration. It was also observed that LPA-induced SGC-7901 cell migration was inhibited by the inhibitor for Gq/11 protein and p38. The results suggest that the LPAR2/Gq/11/p38 pathway regulates LPA-induced SGC-7901 cell migration. The present findings suggest that LPAR2 may be a potential target for the clinical treatment of gastric cancer.

Galectin-3 contributes to melanoma growth and metastasis via regulation of NFAT1 and autotaxin

Melanoma is the deadliest form of skin cancer in which patients with metastatic disease have a 5-year survival rate of less than 10%. Recently, the overexpression of a β-galactoside binding protein, galectin-3 (LGALS3), has been correlated with metastatic melanoma in patients. We have previously shown that silencing galectin-3 in metastatic melanoma cells reduces tumor growth and metastasis. Gene expression profiling identified the protumorigenic gene autotaxin (ENPP2) to be downregulated after silencing galectin-3. Here we report that galectin-3 regulates autotaxin expression at the transcriptional level by modulating the expression of the transcription factor NFAT1 (NFATC2). Silencing galectin-3 reduced NFAT1 protein expression, which resulted in decreased autotaxin expression and activity. Reexpression of autotaxin in galectin-3 silenced melanoma cells rescues angiogenesis, tumor growth, and metastasis in vivo. Silencing NFAT1 expression in metastatic melanoma cells inhibited tumor growth and metastatic capabilities in vivo. Our data elucidate a previously unidentified mechanism by which galectin-3 regulates autotaxin and assign a novel role for NFAT1 during melanoma progression.

Identification and pharmacological characterization of multiple allosteric binding sites on the free fatty acid 1 receptor

Activation of FFA1 (GPR40), a member of G protein-coupling receptor family A, is mediated by medium- and long-chain fatty acids and leads to amplification of glucose-stimulated insulin secretion, suggesting a potential role for free fatty acid 1 (FFA1) as a target for type 2 diabetes. It was assumed previously that there is a single binding site for fatty acids and synthetic FFA1 agonists. However, using members of two chemical series of partial and full agonists that have been identified, radioligand binding interaction studies revealed that the full agonists do not bind to the same site as the partial agonists but exhibit positive heterotropic cooperativity. Analysis of functional data reveals positive functional cooperativity between the full agonists and partial agonists in various functional assays (in vitro and ex vivo) and also in vivo. Furthermore, the endogenous fatty acid docosahexaenoic acid (DHA) shows negative or neutral cooperativity with members of both series of agonists in binding assays but displays positive cooperativity in functional assays. Another synthetic agonist is allosteric with members of both agonist series, but apparently competitive with DHA. Therefore, there appear to be three allosterically linked binding sites on FFA1 with agonists specific for each of these sites. Activation of free fatty acid 1 receptor (FFAR1) by each of these agonists is differentially affected by mutations of two arginine residues, previously found to be important for FFAR1 binding and activation. These ligands with their high potencies and strong positive functional cooperativity with endogenous fatty acids, demonstrated in vitro and in vivo, have the potential to deliver therapeutic benefits.

Pharmacological tools for lysophospholipid GPCRs: development of agonists and antagonists for LPA and S1P receptors

Previous studies on lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) using various approaches have shown that both the molecules can act as intercellular signaling molecules. The discovery of the Edg subfamily of G-protein-coupled receptors (GPCRs) (later renamed LPA(1-3) and S1P(1-5)) for these molecules has opened up a new avenue for pathophysiological research on lysophospholipids. Genetic and molecular studies on lysophospholipid GPCRs have elucidated pathophysiological impacts and roles in cellular signaling pathways. Recently, lysophospholipid GPCR genes have been used to develop receptor subtype-selective agonists and antagonists. The discovery of FTY720, a novel immune modulator, along with other chemical tools, has provided a means of elucidating the functions of each lysophospholipid GPCR on an organ and the whole body level. This communication attempts to retrospectively review the development of agonists and antagonists for lysophospholipid GPCRs, provide integrated information on pharmacological tools for lysophospholipid GPCR signaling, and speculate on future drug development.

Propofol and aminophylline antagonize each other during the mobilization of intracellular calcium in human umbilical vein endothelial cells

This study examined whether propofol and aminophylline affect the mobilization of intracellular calcium in human umbilical vein endothelial cells. Intracellular calcium was measured using laser scanning confocal microscopy. Cultured and serum-starved cells on round coverslips were incubated with propofol or aminophylline for 30 min, and then stimulated with lysophosphatidic acid, propofol and aminophylline. The results were expressed as relative fluorescence intensity and fold stimulation. Propofol decreased the concentration of intracellular calcium, whereas aminophylline caused increased mobilization of intracellular calcium in a concentration-dependent manner. Propofol suppressed the lysophosphatidic acid-induced mobilization of intracellular calcium in a concentration-dependent manner. Propofol further prevented the aminophylline-induced increase of intracellular calcium at clinically relevant concentrations. However, aminophylline reversed the inhibitory effect of propofol on the elevation of intracellular calcium by lysophosphatidic acid. Our results suggest that propofol and aminophylline antagonize each other on the mobilization of intracellular calcium in human umbilical vein endothelial cells at clinically relevant concentrations. Serious consideration should be given to how this interaction affects mobilization of intracellular calcium when these two drugs are used together.

Diversity of lysophosphatidic acid receptor-mediated intracellular calcium signaling in early cortical neurogenesis

Lysophosphatidic acid (LPA) is a membrane-derived lysophospholipid that can induce pleomorphic effects in neural progenitor cells (NPCs) from the cerebral cortex, including alterations in ionic conductance. LPA-induced, calcium-mediated conductance changes have been reported; however, the underlying molecular mechanisms have not been determined. We show here that activation of specific cognate receptors accounts for nearly all intracellular calcium responses evoked by LPA in acutely cultured nestin-positive NPCs from the developing mouse cerebral cortex. Fast-onset changes in intracellular calcium levels required release from thapsigargin-sensitive stores by a pertussis toxin-insensitive mechanism. The influx of extracellular calcium through Cd(2+)/Ni(2+)-insensitive influx pathways, approximately one-half of which were Gd(3+) sensitive, contributed to the temporal diversity of responses. Quantitative reverse transcription-PCR revealed the presence of all five known LPA receptors in primary NPCs, with prominent expression of LPA(1), LPA(2), and LPA(4). Combined genetic and pharmacological studies indicated that NPC responses were mediated by LPA(1) (approximately 30% of the cells), LPA(2) (approximately 30%), a combination of receptors on single cells (approximately 30%), and non-LPA(1,2,3) pathways (approximately 10%). LPA responsivity was significantly reduced in more differentiated TuJ1(+) cells within cultures. Calcium transients in a large proportion of LPA-responsive NPCs were also initiated by the closely related signaling lipid S1P (sphingosine-1-phosphate). These data demonstrate for the first time the involvement of LPA receptors in mediating surprisingly diverse NPC calcium responses involving multiple receptor subtypes that function within a single cell. Compared with other known factors, lysophospholipids represent the major activator of calcium signaling identified within NPCs at this early stage in corticogenesis.

Biological roles of lysophosphatidic acid signaling through its production by autotaxin

Lysophosphatidic acid (LPA) exhibits a wide variety of biological functions as a bio-active lysophospholipid through G-protein-coupled receptors specific to LPA. Currently at least six LPA receptors are identified, named LPA(1) to LPA(6), while the existence of other LPA receptors has been suggested. From studies on knockout mice and hereditary diseases of these LPA receptors, it is now clear that LPA is involved in various biological processes including brain development and embryo implantation, as well as patho-physiological conditions including neuropathic pain and pulmonary and renal fibrosis. Unlike sphingosine 1-phosphate, a structurally similar bio-active lysophospholipid to LPA and produced intracellularly, LPA is produced by multiple extracellular degradative routes. A plasma enzyme called autotaxin (ATX) is responsible for the most of LPA production in our bodies. ATX converts lysophospholipids such as lysophosphatidylcholine to LPA by its lysophospholipase D activity. Recent studies on ATX have revealed new aspects of LPA. In this review, we highlight recent advances in our understanding of LPA functions and several aspects of ATX, including its activity, expression, structure, biochemical properties, the mechanism by which it stimulates cell motility and its pahto-physiological function through LPA production.

Direct voltage control of endogenous lysophosphatidic acid G-protein-coupled receptors in Xenopus oocytes

Lysophosphatidic acid (LPA) G-protein-coupled receptors (GPCRs) play important roles in a variety of physiological and pathophysiological processes, including cell proliferation, angiogenesis, central nervous system development and carcinogenesis. Whilst many ion channels and transporters are recognized to be controlled by a change in cell membrane potential, little is known about the voltage dependence of other proteins involved in cell signalling. Here, we show that the InsP(3)-mediated Ca(2+) response stimulated by the endogenous LPA GPCR in Xenopus oocytes is potentiated by membrane depolarization. Depolarization was able to repetitively stimulate transient [Ca(2+)](i) increases after the initial agonist-evoked response. In addition, the initial rate and amplitude of the LPA-dependent Ca(2+) response were significantly modulated by the steady holding potential over the physiological range, such that the response to LPA was potentiated at depolarized potentials and inhibited at hyperpolarized potentials. Enhancement of LPA receptor-evoked Ca(2+) mobilization by membrane depolarization was observed over a wide range of agonist concentrations. Importantly, the amplitude of the depolarization-evoked intracellular Ca(2+) increase displayed an inverse relationship with agonist concentration such that the greatest effect of voltage was observed at near-threshold levels of agonist. Voltage-dependent Ca(2+) release was not induced by direct elevation of InsP(3) or by activation of heterotrimeric G-proteins in the absence of agonist, indicating that the LPA GPCR itself represents the primary site of action of membrane voltage. This novel modulation of LPA signalling by membrane potential may have important consequences for control of Ca(2+) signals both in excitable and non-excitable tissues.

Proteomic analysis of bone tissues of patients with osteonecrosis of the femoral head

Osteonecrosis of the femoral head (ONFH) is a devastating disease that can result in a femoral head collapse. By proteomics analysis, we identified 1,967 proteins with two or more unique peptides from ONFH and from control bones with a false discovery rate of 4.8%. Using spectral counting, we identified 141 overexpressed and 56 underexpressed proteins comparing ONFH bones to the controls. GSEA (gene set enrichment analysis) revealed that proteins overexpressed in ONFH are enriched for gene sets related to multiple myeloma and adult T-cell lymphoma (ATL), and to JAK2-dependent genes. We confirmed the underexpression of CHST2 (isoform 1 of carbohydrate sulfotransferase 2), a key protein involved in biosynthesis of chondroitin sulfate proteoglycans, and the underexpression of GPCR26 (G-protein coupled receptor 26), a protein that mediates intracellular calcium mobilization, in ONFH bones compared to controls. Taken together, our data suggest that biosynthesis of chondroitin sulfate proteoglycans and cation transport and mobilization may be a key process involved in the pathogenesis of ONFH. Our analysis sheds new light on the understanding of the pathogenesis of ONFH.

Lysophosphatidic acid-induced platelet shape change revealed through LPA(1-5) receptor-selective probes and albumin

Lysophosphatidic acid (LPA), a component of mildly-oxidized LDL and the lipid rich core of atherosclerotic plaques, elicits platelet activation. LPA is the ligand of G protein-coupled receptors (GPCR) of the EDG family (LPA(1-3)) and the newly identified LPA(4-7) subcluster. LPA(4), LPA(5) and LPA(7) increase cellular cAMP levels that would induce platelet inhibition rather than activation. In the present study we quantified the mRNA levels of the LPA(1-7) GPCR in human platelets and found a rank order LPA(4) = LPA(5) > LPA(7) > LPA(6) = LPA(2) >> LPA(1) > LPA(3). We examined platelet shape change using a panel of LPA receptor subtype-selective agonists and antagonists and compared them with their pharmacological profiles obtained in heterologous LPA(1-5) receptor expression systems. Responses to different natural acyl and alkyl species of LPA, and octyl phosphatidic acid analogs, alpha-substituted phosphonate analogs, N-palmitoyl-tyrosine phosphoric acid, N-palmitoyl-serine phosphoric acid were tested. All of these compounds elicited platelet activation and also inhibited LPA-induced platelet shape change after pre-incubation, suggesting that receptor desensitization is likely responsible for the inhibition of this response. Fatty acid free albumin (10 microM) lacking platelet activity completely inhibited platelet shape change induced by LPA with an IC(50) of 1.1 microM but had no effect on the activation of LPA(1,2,3,&5) expressed in endogenously non-LPA-responsive RH7777 cells. However, albumin reduced LPA(4) activation and shifted the dose-response curve to the right. LPA(5) transiently expressed in RH7777 cells showed preference to alkyl-LPA over acyl-LPA that is similar to that in platelets. LPA did not increase cAMP levels in platelets. In conclusion, our results with the pharmacological compounds and albumin demonstrate that LPA does not induce platelet shape change simply through activation of LPA(1-5), and the receptor(s) mediating LPA-induced platelet activation remains elusive.

Identification and functional characterization of allosteric agonists for the G protein-coupled receptor FFA2

FFA2 (GPR43) has been identified as a receptor for short-chain fatty acids (SCFAs) that include acetate and propionate. FFA2 is highly expressed in islets, a subset of immune cells, and adipocytes. Although the potential roles of FFA2 activation in these tissues have previously been described, the physiological functions are still unclear. The potency for SCFAs on FFA2 is low, in the high micromolar to millimolar concentrations. To identify better pharmacological tools to study receptor function, we used high-throughput screening (HTS) to discover a series of small molecule phenylacetamides as novel and more potent FFA2 agonists. This series is specific for FFA2 over FFA1 (GPR40) and FFA3 (GPR41), and it is able to activate both the Galpha(q) and Galpha(i) pathways in vitro on Chinese hamster ovary cells stably expressing FFA2. Treatment of adipocytes with these compounds also resulted in Galpha(i)-dependent inhibition of lipolysis similar to that of endogenous ligands (SCFAs). It is noteworthy that these compounds not only acted as FFA2 agonists but also exhibited positive cooperativity with acetate or propionate. The observed allosteric modulation was consistent in all the functional assays that we have explored, including cAMP, calcium mobilization, guanosine 5'-[gamma-thio]triphosphate binding, and lipolysis. Molecular modeling analysis of FFA2 based on human beta(2)-adrenergic receptor structure revealed potential nonoverlapping binding sites for the endogenous and synthetic ligands, further providing insight into the binding pocket for the allosteric interactions. This is the first report describing the identification of novel allosteric modulators with agonist activity for FFA2, and these compounds may serve as tools for further unraveling the physiological functions of the receptor and its involvement in various diseases.

Lysophosphatidic acid induced nuclear translocation of nuclear factor-κB in Panc-1 cells by mobilizing cytosolic free calcium

AIM: To clarify whether Lysophosphatidic acid (LPA) activates the nuclear translocation of nuclear factor-κB (NF-κB) in pancreatic cancer.

METHODS: Panc-1, a human pancreatic cancer cell line, was used throughout the study. The expression of LPA receptors was confirmed by reverse-transcript polymerase chain reaction (RT-PCR). Cytosolic free calcium was measured by fluorescent calcium indicator fura-2, and the localization of NF-κB was visualized by immunofluorescent method with or without various agents, which effect cell signaling.

RESULTS: Panc-1 expressed LPA receptors, LP_A1, LP_A2 and LP_A3. LPA caused the elevation of cytosolic free calcium dose-dependently. LPA also caused the nuclear translocation of NF-κB. Cytosolic free calcium was attenuated by pertussis toxin (PTX) and U73122, an inhibitor of phospholipase C. The translocation of NF-κB was similarly attenuated by PTX and U73122, but phorbol ester, an activator of protein kinase C, alone did not translocate NF-κB. Furthermore, the translocation of NF-κB was completely blocked by Ca²⁺ chelator BAPTA-AM. Thapsigargin, an endoplasmic-reticulum Ca²⁺-ATPase pump inhibitor, also promoted the translocation of NF-κB. Staurosporine, a protein kinase C inhibitor, attenuated translocation of NF-κB induced by LPA.

CONCLUSION: These findings suggest that protein kinase C is activated endogenously in Panc-1, and protein kinase C is essential for activating NF-κB with cytosolic calcium and that LPA induces the nuclear translocation of NF-κB in Panc-1 by mobilizing cytosolic free calcium.

Dioleoyl phosphatidic acid induces morphological changes through an endogenous LPA receptor in C6 glioma cells

Previously, we suggested that dioleoyl phosphatidic acid (PA) and lysophosphatidic acid (LPA) increased [Ca(2+)](i) through endogenous LPA receptors coupled to pertussis toxin-sensitive G proteins in rat C6 glioma cells. In the present report, we investigated morphological changes and cytotoxicity induced by PA and LPA in C6 glioma cells. Isoproterenol treatment led to changes in the cell morphology of rat C6 glioma cells, which were reverted by the addition of PA and LPA. PA-and LPA-induced morphological reversions were inhibited by treatment with Ki16425, an LPA(1)/LPA(3) receptor antagonist. VPC32183, another LPA(1)/LPA(3) receptor antagonist with a different structure, only inhibited PA-induced morphological reversion but not LPA-induced reversion. However, the reversions were not inhibited by treatment with pertussis toxin, a specific inhibitor of G(i/o) proteins. In addition, cytotoxicity was only induced by LPA but not by PA in C6 glioma cells. Our results suggest that PA may act as a partial agonist at endogenous LPA receptors, which are sensitive to Ki16425 and coupled to PTX-insensitive G proteins, to evoke morphological changes in C6 glioma cells.

Lysophosphatidylserine induces calcium signaling through Ki16425/VPC32183-sensitive GPCR in bone marrow-derived mast cells and in C6 glioma and colon cancer cells

Lysophosphatidylserine (LPS) can be generated following phosphatidylserine-specific phospholipase A2 activation. The effects of LPS on cellular activities and the identities of its target molecules, however, have not been fully elucidated. In this study, we observed that LPS stimulated intracellular calcium increased in mouse bone marrow-derived mast cells (BMMC), and rat C6 glioma and human HCT116 colon cancer cells and compared the LPS-induced Ca2+ increases with the response by lysophosphatidic acid (LPA), a structurally related bioactive lysolipid. In order to test involvement of signaling molecules in the LPS-induced Ca2+ signaling, we used pertussis toxin (PTX), U73122, and 2-APB, which are specific inhibitors for G proteins, phospholipase C (PLC), and IP3 receptors, respectively. The increases due to LPS and LPA were inhibited by PTX, U-73122 and 2-APB, suggesting that both lipids stimulate calcium signaling via G proteins (Gi/o types), PLC activation, and subsequent IP3 production, although the sensitivity to pharmacological inhibitors varied from complete inhibition to partial inhibition depending on cell type and lysolipid. Furthermore, we observed that Ki16425 completely inhibited an LPS-induced Ca2+ response in three cell types, but that the effect of VPC32183 varied from complete inhibition in BMMC and C6 glioma cells to partial inhibition in HCT116 cells. Therefore, we conclude that LPS increases [Ca2+]i through Ki16425/VPC32183-sensitive G protein-coupled receptors (GPCR), G protein, PLC, and IP3 in mouse BMMC, rat C6, and human HCT116 cells.

Subtype-specific residues involved in ligand activation of the endothelial differentiation gene family lysophosphatidic acid receptors

Lysophosphatidic acid (LPA) is a ligand for three endothelial differentiation gene family G protein-coupled receptors, LPA(1-3). We performed computational modeling-guided mutagenesis of conserved residues in transmembrane domains 3, 4, 5, and 7 of LPA(1-3) predicted to interact with the glycerophosphate motif of LPA C18:1. The mutants were expressed in RH7777 cells, and the efficacy (E(max)) and potency (EC(50)) of LPA-elicited Ca(2+) transients were measured. Mutation to alanine of R3.28 universally decreased both the efficacy and potency in LPA(1-3) and eliminated strong ionic interactions in the modeled LPA complexes. The alanine mutation at Q3.29 decreased modeled interactions and activation in LPA(1) and LPA(2) more than in LPA(3). The mutation W4.64A had no effect on activation and modeled LPA interaction of LPA(1) and LPA(2) but reduced the activation and modeled interactions of LPA(3). The R5.38A mutant of LPA(2) and R5.38N mutant of LPA(3) showed diminished activation by LPA; however, in LPA(1) the D5.38A mutation did not, and mutation to arginine enhanced receptor activation. In LPA(2), K7.36A decreased the potency of LPA; in LPA(1) this same mutation increased the E(max). In LPA(3), R7.36A had almost no effect on receptor activation; however, the mutation K7.35A increased the EC(50) in response to LPA 10-fold. In LPA(1-3), the mutation Q3.29E caused a modest increase in EC(50) in response to LPA but caused the LPA receptors to become more responsive to sphingosine 1-phosphate (S1P). Surprisingly micromolar concentrations of S1P activated the wild type LPA(2) and LPA(3) receptors, indicating that S1P may function as a weak agonist of endothelial differentiation gene family LPA receptors.

Discovery of potent LPA2 (EDG4) antagonists as potential anticancer agents

The LPA(2) protein is overexpressed in many tumor cells. We report the optimization of a series of LPA(2) antagonists using calcium mobilization assay (aequorin assay) that led to the discovery of the first reported inhibitors selective for LPA(2). Key compounds were evaluated in vitro for inhibition of LPA(2) mediated Erk activation and proliferation of HCT-116 cells. These compounds could be used to evaluate the benefits of LPA(2) inhibition both in vitro and in vivo.

Embryo spacing and implantation timing are differentially regulated by LPA3-mediated lysophosphatidic acid signaling in mice

In polytocous animals, blastocysts are evenly distributed along each uterine horn and implant. The molecular mechanisms underlying these precise events remain elusive. We recently showed that lysophosphatidic acid (LPA) has critical roles in the establishment of early pregnancy by affecting embryo spacing and subsequent implantation through its receptor, LPA3. Targeted deletion of Lpa3 in mice resulted in delayed implantation and embryo crowding, which is associated with a dramatic decrease in the prostaglandins and prostaglandin-endoperoxide synthase 2 expression levels. Exogenous administration of prostaglandins rescued the delayed implantation but did not rescue the defects in embryo spacing, suggesting the role of prostaglandins in implantation downstream of LPA3 signaling. In the present study, to know how LPA3 signaling regulates the embryo spacing, we determined the time course distribution of blastocysts during the preimplantation period. In wild-type (WT) uteri, blastocysts were distributed evenly along the uterine horns at Embryonic Day 3.8 (E3.8), whereas in the Lpa3-deficient uteri, they were clustered in the vicinity of the cervix, suggesting that the mislocalization and resulting crowding of the embryos are the cause of the delayed implantation. However, embryos transferred singly into E2.5 pseudopregnant Lpa3-deficient uterine horns still showed delayed implantation but on-time implantation in WT uteri, indicating that embryo spacing and implantation timing are two segregated events. We also found that an LPA3-specific agonist induced rapid uterine contraction in WT mice but not in Lpa3-deficient mice. Because the uterine contraction is critical for embryo spacing, our results suggest that LPA3 signaling controls embryo spacing via uterine contraction around E3.5.

Lysophospholipid signaling: beyond the EDGs

As our understanding of the myriads of biological effects caused by lysophospholipids expands, we become witnesses to another miracle of nature that has endowed the simplest lysophospholipids with functions seemingly ubiquitous to every mammalian cell. A decade after the discovery of the EDG family lysophospholipid receptors, the field has gained unimaginable impetus explaining the biological effects of sphingosine-1-phosphate and lysophosphatidic acid (LPA). The discovery of LPA receptors in the purinergic G-protein-coupled receptor (GPCR) gene cluster refined this picture and added complexity to our concepts of lysophospholipid cell signaling. The intracellular lysophospholipid targets - identified and not yet identified - make us realize the dual mediator and second messenger roles of lysophospholipids. In this paper we provide new data obtained concerning LPA-elicited responses using cell lines naturally lacking or intentionally knocked out of many of the known LPA GPCR, widely used by investigators in the field as cells with LPA receptor "null background." Our observations raise caution about the lack of LPA responsiveness in these cells and underline the unprecedented complexity and redundancy of lysophospholipid-evoked cellular responses.

Dioleoyl phosphatidic acid increases intracellular Ca2+ through endogenous LPA receptors in C6 glioma and L2071 fibroblasts

Phosphatidic acid (PA) increased intracellular Ca(2+) concentration ([Ca(2+)](i)) in C6 rat glioma and L2071 mouse fibroblast cells. Dioleoyl PA (PA, 18:1) was the most efficacious, followed by dipalmitoyl PA (16:0 PA) and dimyristoyl PA (14:0 PA). Lysophosphatidic acid (LPA) also increased the [Ca(2+)](i) in the both cells. PA desensitized LPA-induced Ca(2+) response completely in C6 cells, but partly in L2071 cells. Treatment of pertussis toxin (PTX), a specific inhibitor of G(i/o)-type G proteins, completely ameliorated LPA- and PA-induced Ca(2+) response in C6 cells. However, in L2071 cells, PTX inhibited PA-induced Ca(2+) increase by 80% and LPA-induced one by 20%. Ki16425, a specific inhibitor of LPA(1)/LPA(3) receptors, completely inhibited both LPA- and PA-induced Ca(2+) responses in C6 cells. On the other hand, in L2071 cells, Ki16425 completely inhibited PA-induced Ca(2+) response, but partly LPA-induced one. VPC32183, another specific inhibitor of LPA(1)/LPA(3) receptors, completely inhibited LPA- and PA-induced Ca(2+) responses in both C6 and L2071 cells. Therefore, PA and LPA appear to increase [Ca(2+)](i) through Ki16425/VPC32183-sensitive LPA receptor coupled to PTX-sensitive G proteins in C6 cells. In L2071 cells, however, LPA increases [Ca(2+)](i) through Ki16425-insensitive LPA receptor coupled to PTX-insensitive G proteins and Ki16425-sensitive LPA receptor coupled to PTX-sensitive G protein, whereas PA utilized only the latter pathway. Our results suggest that PA acts as a partial agonist on endogenous LPA receptors, which are sensitive to Ki16425 and coupled to PTX-sensitive G protein, but not on LPA receptors, which are not sensitive to Ki16425 and coupled to PTX-insensitive G protein.

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.

Role of protein kinase C-dependent A-kinase anchoring proteins in lysophosphatidic acid-induced cAMP signaling in human diploid fibroblasts

Previously, we reported that lysophosphatidic acid (LPA)-induced adenosine 3',5'-cyclic monophosphate (cAMP) production by human diploid fibroblasts depends on the age of the fibroblasts. In this study, we examined the role of A-kinase anchoring proteins (AKAP) in the regulation of LPA-stimulated cAMP production in senescent fibroblasts. We found that levels of protein kinase C (PKC)-dependent AKAPs, such as Gravin and AKAP79, were elevated in senescent cells. Co-immunoprecipitation experiments revealed that Gravin and AKAP79 do not associate with adenylyl cyclase type 2 (AC2) but bind to AC4/6, which interacts with calcium-dependent PKCs alpha/beta both in young and senescent fibroblasts. When the expression of Gravin and AKAP79 was blocked by small interference RNA transfection, the basal level of cAMP was greatly reduced and the cAMP status after LPA treatment was also reversed. Protein kinase A showed a similar pattern in terms of its basal activity and LPA-dependent modulation. These data suggest that Gravin and to a lesser extent, AKAP79, may play important roles in maintaining the basal AC activity and in coupling the AC systems to inhibitory signals such as Gialpha in young cells, and to stimulatory signals such as PKCs in senescent cells. This study also demonstrates that Gravin is especially important for the long-term activation of PKC by LPA in senescent cells. We conclude that LPA-dependent increased level of cAMP in senescent human diploid fibroblasts is associated with increases in Gravin levels resulting in its increased binding with and activation of calcium-dependent PKC alpha/beta and AC4/6.

Lysophosphatidic acid induces endothelial cell death by modulating the redox environment

Oxidant stress plays a significant role in hypoxic-ischemic injury to the susceptible microvascular endothelial cells. During oxidant stress, lysophosphatidic acid (LPA) concentrations increase. We explored whether LPA caused cytotoxicity to neuromicrovascular cells and the potential mechanisms thereof. LPA caused a dose-dependent death of porcine cerebral microvascular as well as human umbilical vein endothelial cells; cell death appeared oncotic rather than apoptotic. LPA-induced cell death was mediated via LPA(1) receptor, because the specific LPA(1) receptor antagonist THG1603 fully abrogated LPA's effects. LPA decreased intracellular GSH levels and induced a p38 MAPK/JNK-dependent inducible nitric oxide synthase (NOS) expression. Pretreatment with the antioxidant GSH precursor N-acetyl-cysteine (NAC), as well as with inhibitors of NOS [N(omega)-nitro-l-arginine (l-NNA); 1400W], significantly prevented LPA-induced endothelial cell death (in vitro) to comparable extents; as expected, p38 MAPK (SB203580) and JNK (SP-600125) inhibitors also diminished cell death. LPA did not increase indexes of oxidation (isoprostanes, hydroperoxides, and protein nitration) but did augment protein nitrosylation. Endothelial cytotoxicity by LPA in vitro was reproduced ex vivo in brain and in vivo in retina; THG1603, NAC, l-NNA, and combined SB-203580 and SP600125 prevented the microvascular rarefaction. Data implicate novel properties for LPA as a modulator of the cell redox environment, which partakes in endothelial cell death and ensued neuromicrovascular rarefaction.

Inhibition of transcellular tumor cell migration and metastasis by novel carba-derivatives of cyclic phosphatidic acid

Cyclic phosphatidic acid (1-acyl-sn-glycerol-2,3-cyclic phosphate; cPA) is a naturally occurring analog of lysophosphatidic acid (LPA) with a variety of distinctly different biological activities from those of LPA. In contrast to LPA, a potent inducer of tumor cell invasion, palmitoyl-cPA inhibits FBS- and LPA-induced transcellular migration and metastasis. To prevent the conversion of cPA to LPA we synthesized cPA derivatives by stabilizing the cyclic phosphate ring; to prevent the cleavage of the fatty acid we generated alkyl ether analogs of cPA. Both sets of compounds were tested for inhibitory activity on transcellular tumor cell migration. Carba derivatives, in which the phosphate oxygen was replaced with a methylene group at either the sn-2 or the sn-3 position, showed much more potent inhibitory effects on MM1 tumor cell transcellular migration and the pulmonary metastasis of B16-F0 melanoma than the natural pal-cPA. The antimetastatic effect of carba-cPA was accompanied by the inhibition of RhoA activation and was not due to inhibition of the activation of LPA receptors.

Expression and function of lysophosphatidic acid LPA1 receptor in prostate cancer cells

The bioactive phospholipid lysophosphatidic acid (LPA) promotes cell proliferation, survival, and migration by acting on cognate G protein-coupled receptors named LPA(1), LPA(2), and LPA(3). We profiled gene expression of LPA receptors in androgen-dependent and androgen-insensitive prostate cancer cells and found that LPA(1) gene is differentially expressed in androgen-insensitive and LPA-responsive but not androgen-dependent and LPA-resistant cells. In human prostate specimens, expression of LPA(1) gene was significantly higher in the cancer compared with the benign tissues. The androgen-dependent LNCaP cells do not express LPA(1) and do not proliferate in response to LPA stimulation, implying LPA(1) transduces cell growth signals. Accordingly, stable expression of LPA(1) in LNCaP cells rendered them responsive to LPA-induced cell proliferation and decreased their doubling time in serum. Implantation of LNCaP-LPA(1) cells resulted in increased rate of tumor growth in animals compared with those tumors that developed from the wild-type cells. Growth of LNCaP cells depends on androgen receptor activation, and we show that LPA(1) transduces Galphai-dependent signals to promote nuclear localization of androgen receptor and cell proliferation. In addition, treatment with bicalutamide inhibited LPA-induced cell cycle progression and proliferation of LNCaP-LPA(1) cells. These results suggest the possible utility of LPA(1) as a drug target to interfere with progression of prostate cancer.

Nitric oxide signaling via nuclearized endothelial nitric-oxide synthase modulates expression of the immediate early genes iNOS and mPGES-1

Stimulation of freshly isolated rat hepatocytes with lysophosphatidic acid (LPA) resulted in LPA1 receptor-mediated and nitricoxide-dependent up-regulation of the immediate early genes iNOS (inducible nitric-oxide synthase (NOS)) and mPGES-1 (microsomal prostaglandin E synthase-1). Because LPA is a ligand for both cell surface and intracellular receptor sites and a potent endothelial NOS (eNOS) activator, we hypothesized that NO derived from activated nuclearized eNOS might participate in gene regulation. Herein we show, by confocal microscopy performed on porcine cerebral endothelial cells expressing native LPA1-receptor and eNOS and on HTC4 rat hepatoma cells co-transfected with recombinant human LPA1-receptor and fused eNOS-GFP cDNA, a dynamic eNOS translocation from peripheral to nuclear regions upon stimulation with LPA. Nuclear localization of eNOS and its downstream effector, soluble guanylate cyclase, were demonstrated in situ in rat liver specimens by immunogold labeling using specific antibodies. Stimulation of this nuclear fraction with LPA and the NO donor sodium nitroprusside resulted, respectively, in increased production of nitrite (and eNOS phosphorylation) and cGMP; these separate responses were also correspondingly blocked by NOS inhibitor L-NAME and soluble guanylate cyclase inhibitor ODQ. In addition, sodium nitroprusside evoked a sequential increase in nuclear Ca2+ transients, activation of p42 MAPK, NF-kappaB binding to DNA consensus sequence, and dependent iNOS RNA. This study describes a hitherto unrecognized molecular mechanism by which nuclear eNOS through ensuing NO modulates nuclear calcium homeostasis involved in gene transcription-associated events. Moreover, our findings strongly support the concept of the nucleus as an autonomous signaling compartment.

Lysophosphatidic acid-induced changes in cAMP profiles in young and senescent human fibroblasts as a clue to the ageing process

This study attempts to elucidate the molecular mechanisms underlying the ageing-dependent cAMP profiles in human diploid fibroblasts stimulated by lysophosphatidic acid (LPA). In senescent cells, LPA-dependent Gialpha activation was reduced, with a consequent reduction in Gi-suppressed cAMP levels, without alterations in the levels of Gialpha proteins. In young cells, when Gialpha activity was inhibited by pertussis toxin pretreatment, or when its expression was blocked by siRNA, the pattern of changes in cAMP levels in response to LPA was similar to that seen in senescent cells. An increase in protein kinase C (PKC)-dependent isoforms of adenylyl cyclase (AC) types II, IV, and VI was also observed in these senescent fibroblasts. In senescent cells treated with PKC-specific inhibitors, bis-indolylmaleimide, Gö6976, rottlerin, and PKCvarepsilonV1, LPA-induced cAMP accumulation was inhibited, indicating that increased ACs in response to LPA occur via the activation of protein kinase Cs. When the expression of AC II, IV, and VI was blocked by siRNA in senescent fibroblasts, LPA-induced cAMP accumulation was also blocked. These results suggest that the senescence-associated increase of cAMP levels after LPA treatment is associated with reduced Gialpha, increased AC II, IV, and VI proteins, and PKC-dependent stimulation of their activities and provide an explanation for the age-dependent differences in cAMP-related physiological responses.

The signaling pathways linking to lysophosphatidic acid-promoted meiotic maturation in mice

The signaling pathways linking to lysophosphatidic acid-promoted meiotic maturation in mice were studied. When mouse oocyte-cumulus cells complexes were cultured with 10(-5) M lysophosphatidic acid (the LPA group), the rate of oocyte nuclear maturation was significantly increased. Additions of pertussis toxin, genistein, U73122, Ro320432, PD98059 or SB203580 significantly suppressed the increase in lysophosphatidic acid-stimulated nuclear maturation rate. These results suggested that Gi/o-coupled lysophosphatidic acid receptors activate phosphatidylinositol-specific phospholipase C, and result in ERK and MAP kinase activation, which is triggered by diacylglycerol-dependent protein kinase C. When intracellular cAMP concentrations of oocytes in the LPA and control groups were measured using the acetylation assay, the intracellular cAMP concentration of an oocyte in the LPA group was significantly lower than the control oocyte (0.117+/-0.04 fmol/oocyte vs. 0.176+/-0.036 fmol/oocyte, p<0.05). In conclusion, our results suggested that lysophosphatidic acid stimulates phospholipase C through a Gi-protein linked receptor on the surface of mouse cumulus cells and stimulates both extracellular signal-regulated kinase and p38 mitogen-activated kinase, resulting in the closure or loose of gap junctions between cumulus cells and the oocyte. The resultant early decrease of oocyte cAMP levels may promote nuclear maturation of mouse oocytes in vitro.

Lysophospholipids as mediators of immunity

Lysophospholipids (LPLs) are lipid-derived signaling molecules exemplified by lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P). Originally identified as serum-associated growth factors, these mediators now are known to signal through a family of diverse G protein-coupled receptors (GPCRs). Virtually all cells that participate in the immune response express multiple receptors for LPLs. The development of antibody reagents that recognize the receptors for each LPL and the derivation of receptor-selective agonists and receptor-null mouse strains have provided insights into the widely diverse functions of LPLs in immune responses, particularly the role of S1P in lymphocyte trafficking. This review focuses on the biology of the LPLs as these molecules relate to functional regulation of immune cells in vitro and to the regulation of integrated immune responses in vivo.

Lysophosphatidylserine Stimulates L2071 Mouse Fibroblast Chemotactic Migration via a Process Involving Pertussis Toxin-Sensitive Trimeric G-Proteins

Lysophosphatidylserine (LPS) may be generated after phosphatidylserine-specific phospholipase A2 activation. However, the effects of LPS on cellular activities and the identities of its target molecules have not been fully elucidated. In this study, we observed that LPS stimulates an intracellular calcium increase in L2071 mouse fibroblast cells, and that this increase was inhibited by 1-[6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H-pyrrole-2,5-dione (U-73122) but not by pertussis toxin, suggesting that LPS stimulates calcium signaling via G-protein coupled receptor-mediated phospholipase C activation. Moreover, LPS-induced calcium mobilization was not inhibited by the lysophosphatidic acid receptor antagonist, (S)-phosphoric acid mono-{2-octadec-9-enoylamino-3-[4-(pyridine-2-ylmethoxy)-phenyl]-propyl} ester (VPC 32183), thus indicating that LPS binds to a receptor other than lysophosphatidic acid receptors. It was also found that LPS stimulates two types of mitogen-activated protein kinase [i.e., extracellular signal-regulated protein kinase (ERK) and p38 kinase] in L2071 cells. Furthermore, these LPS-induced ERK and p38 kinase activations were inhibited by pertussis toxin, which suggests the role of pertussis toxin-sensitive G-proteins in the process. In terms of functional issues, LPS stimulated L2071 cell chemotactic migration, which was completely inhibited by pertussis toxin, indicating the involvement of pertussis toxin-sensitive G(i) protein(s). This chemotaxis of L2071 cells induced by LPS was also dramatically inhibited by 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002) and by 2'-amino-3'-methoxyflavone (PD98059). This study demonstrates that LPS stimulates at least two different signaling cascades, one of which involves a pertussis toxin-insensitive but phospholipase C-dependent intracellular calcium increase, and the other involves a pertussis toxin-sensitive chemotactic migration mediated by phosphoinositide 3-kinase and ERK.

IL-4 regulates MEK expression required for lysophosphatidic acid-mediated chemokine generation by human mast cells

IL-4 and mast cells (MCs) mediate mucosal defense against helminths and are central to allergic inflammation. Lysophosphatidic acid (LPA), an abundant, potent lipid growth factor, stimulates the growth of cultured human MCs (hMCs) in vitro through a pathway involving LPA receptors 1 and 3 (termed the LPA(1) and LPA(3) receptors, respectively) and peroxisome proliferator-activated receptor-gamma. We now report that LPA potently induces the generation of proinflammatory chemokines (MIP-1beta, IL-8, and MCP-1) by hMCs by a mechanism that absolutely requires IL-4. The de novo expression of chemokine mRNA and protein generation involves synergistic actions of calcium flux-dependent NFAT transcription factors and ERK. ERK phosphorylation and chemokine production in response to LPA require IL-4-dependent up-regulation of MEK-1 expression by a pathway involving PI3K. Although receptor-selective agonists for both the LPA(2) and LPA(3) receptors induce calcium fluxes by hMCs, only the LPA(2) receptor-selective agonist fatty alcohol phosphate-12 mimics the IL-4-dependent effect of LPA on chemokine generation. The fact that LPA, an endogenous lipid mediator, activates hMCs by an LPA(2) receptor-dependent pathway indicates functional distinctions between different LPA receptor family members that are expressed constitutively by cells of a single hemopoietic lineage. Moreover, the regulation of MEK-dependent signaling is a mechanism by which IL-4 could amplify inflammation in mucosal immune responses through receptor systems for endogenous ligands such as LPA.

Lysophosphatidic acid stimulates cell proliferation in rat chondrocytes

Rat primary chondrocytes express the lysophosphatidic acid (LPA) receptor, LPA1, LPA3, but not LPA2. When chondrocytes were stimulated with LPA, phospholipase C-mediated cytosolic calcium increase was dramatically induced. LPA also stimulated two kinds of mitogen-activated protein kinase, extracellular signal-regulated kinase (ERK) and p38 kinase in chondrocytes. In terms of the LPA-mediated functional modulation of chondrocytes, LPA stimulated cellular proliferation. We examined the signaling pathways involved in LPA-mediated cellular proliferation. LPA-induced chondrocyte proliferation was almost completely blocked by 2'-amino-3'-methoxyflavone (PD98059) but not by 4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole (SB203580), suggesting that ERK activity is essentially required for the process. Pertussis toxin almost completely inhibited the LPA-induced cellular proliferation and ERK activation, indicating the role of G(i/o) protein(s) in the processes. This study demonstrates the physiological role of LPA on the modulation of rat primary chondrocyte proliferation, and the crucial role played by ERK in the process.

Insights into seven and single transmembrane-spanning domain receptors and their signaling pathways in human natural killer cells

Human natural killer (NK) cells are important cells of the innate immune system. These cells perform two prominent functions: the first is recognizing and destroying virally infected cells and transformed cells; the second is secreting various cytokines that shape up the innate and adaptive immune re-sponses. For these cells to perform these activities, they express different sets of receptors. The receptors used by NK cells to extravasate into sites of injury belong to the seven transmembrane (7TM) family of receptors, which characteristically bind heterotrimeric G proteins. These receptors allow NK cells to sense the chemotactic gradients and activate second messengers, which aid NK cells in polarizing and migrating toward the sites of injured tissues. In addition, these receptors determine how and why human resting NK cells are mainly found in the bloodstream, whereas activated NK cells extravasate into inflammatory sites. Receptors for chemokines and lysophospholipids belong to the 7TM family. On the other hand, NK cells recognize invading or transformed cells through another set of receptors that belong to the single transmembrane-spanning domain family. These receptors are either inhibitory or activating. Inhibitory receptors contain the immune receptor tyrosine-based inhibitory motif, and activating receptors belong to either those that associate with adaptor molecules containing the immune receptor tyrosine-based activating motif (ITAM) or those that associate with adaptor molecules containing motifs other than ITAM. This article will describe the nature of these receptors and examine the intracellular signaling pathways induced in NK cells after ligating both types of receptors. These pathways are crucial for NK cell biology, development, and functions.