A dual role of lysophosphatidic acid type 2 receptor (LPAR2) in nonsteroidal anti-inflammatory drug-induced mouse enteropathy

Lysophosphatidic acid (LPA) is a bioactive phospholipid mediator that has been found to ameliorate nonsteroidal anti-inflammatory drug (NSAID)-induced gastric injury by acting on lysophosphatidic acid type 2 receptor (LPAR2). In this study, we investigated whether LPAR2 signaling was implicated in the development of NSAID-induced small intestinal injury (enteropathy), another major complication of NSAID use. Wild-type (WT) and Lpar2 deficient (Lpar2-/-) mice were treated with a single, large dose (20 or 30 mg/kg, i.g.) of indomethacin (IND). The mice were euthanized at 6 or 24 h after IND treatment. We showed that IND-induced mucosal enteropathy and neutrophil recruitment occurred much earlier (at 6 h after IND treatment) in Lpar2-/- mice compared to WT mice, but the tissue levels of inflammatory mediators (IL-1β, TNF-α, inducible COX-2, CAMP) remained at much lower levels. Administration of a selective LPAR2 agonist DBIBB (1, 10 mg/kg, i.g., twice at 24 h and 30 min before IND treatment) dose-dependently reduced mucosal injury and neutrophil activation in enteropathy, but it also enhanced IND-induced elevation of several proinflammatory chemokines and cytokines. By assessing caspase-3 activation, we found significantly increased intestinal apoptosis in IND-treated Lpar2-/- mice, but it was attenuated after DBIBB administration, especially in non-obese diabetic/severe combined immunodeficiency (NOD/SCID) mice. Finally, we showed that IND treatment reduced the plasma activity and expression of autotaxin (ATX), the main LPA-producing enzyme, and also reduced the intestinal expression of Lpar2 mRNA, which preceded the development of mucosal damage. We conclude that LPAR2 has a dual role in NSAID enteropathy, as it contributes to the maintenance of mucosal integrity after NSAID exposure, but also orchestrates the inflammatory responses associated with ulceration. Our study suggests that IND-induced inhibition of the ATX-LPAR2 axis is an early event in the pathogenesis of enteropathy.

Enhancement of Sphingomyelinase-Induced Endothelial Nitric Oxide Synthase-Mediated Vasorelaxation in a Murine Model of Type 2 Diabetes

Sphingolipids are important biological mediators both in health and disease. We investigated the vascular effects of enhanced sphingomyelinase (SMase) activity in a mouse model of type 2 diabetes mellitus (T2DM) to gain an understanding of the signaling pathways involved. Myography was used to measure changes in the tone of the thoracic aorta after administration of 0.2 U/mL neutral SMase in the presence or absence of the thromboxane prostanoid (TP) receptor antagonist SQ 29,548 and the nitric oxide synthase (NOS) inhibitor L-NAME. In precontracted aortic segments of non-diabetic mice, SMase induced transient contraction and subsequent weak relaxation, whereas vessels of diabetic (Lepr^db/Lepr^db, referred to as db/db) mice showed marked relaxation. In the presence of the TP receptor antagonist, SMase induced enhanced relaxation in both groups, which was 3-fold stronger in the vessels of db/db mice as compared to controls and could not be abolished by ceramidase or sphingosine-kinase inhibitors. Co-administration of the NOS inhibitor L-NAME abolished vasorelaxation in both groups. Our results indicate dual vasoactive effects of SMase: TP-mediated vasoconstriction and NO-mediated vasorelaxation. Surprisingly, in spite of the general endothelial dysfunction in T2DM, the endothelial NOS-mediated vasorelaxant effect of SMase was markedly enhanced.

Revisiting the role of lysophosphatidic acid in stem cell biology

Stem cells possess unique biological characteristics such as the ability to self-renew and to undergo multilineage differentiation into specialized cells. Whereas embryonic stem cells (ESC) can differentiate into all cell types of the body, somatic stem cells (SSC) are a population of stem cells located in distinct niches throughout the body that differentiate into the specific cell types of the tissue in which they reside in. SSC function mainly to restore cells as part of normal tissue homeostasis or to replenish cells that are damaged due to injury. Cancer stem-like cells (CSC) are said to be analogous to SSC in this manner where tumor growth and progression as well as metastasis are fueled by a small population of CSC that reside within the corresponding tumor. Moreover, emerging evidence indicates that CSC are inherently resistant to chemo- and radiotherapy that are often the cause of cancer relapse. Hence, major research efforts have been directed at identifying CSC populations in different cancer types and understanding their biology. Many factors are thought to regulate and maintain cell stemness, including bioactive lysophospholipids such as lysophosphatidic acid (LPA). In this review, we discuss some of the newly discovered functions of LPA not only in the regulation of CSC but also normal SSC, the similarities in these regulatory functions, and how these discoveries can pave way to the development of novel therapies in cancer and regenerative medicine.

Dysregulation of lysophospholipid signaling by p53 in malignant cells and the tumor microenvironment

The TP53 gene has been widely studied for its roles in cell cycle control, maintaining genome stability, activating repair mechanisms upon DNA damage, and initiating apoptosis should repair mechanisms fail. Thus, it is not surprising that mutations of p53 are the most common genetic alterations found in human cancer. Emerging evidence indicates that dysregulation of lipid metabolism by p53 can have a profound impact not only on cancer cells but also cells of the tumor microenvironment (TME). In particular, intermediates of the sphingolipid and lysophospholipid pathways regulate many cellular responses common to p53 such as cell survival, migration, DNA damage repair and apoptosis. The majority of these cellular events become dysregulated in cancer as well as cell senescence. In this review, we will provide an account on the seminal contributions of Prof. Lina Obeid, who deciphered the crosstalk between p53 and the sphingolipid pathway particularly in modulating DNA damage repair and apoptosis in non-transformed as well as transformed cells. We will also provide insights on the integrative role of p53 with the lysophosphatidic acid (LPA) signaling pathway in cancer progression and TME regulation.

The LPA2 receptor agonist Radioprotectin-1 spares Lgr5-positive intestinal stem cells from radiation injury in murine enteroids

Rapidly proliferating cells are highly sensitive to ionizing radiation and can undergo apoptosis if the oxidative and genotoxic injury exceed the defensive and regenerative capacity of the cell. Our earlier work has established the antiapoptotic action of the growth factor-like lipid mediator lysophosphatidic acid (LPA). Activation of the LPA₂ GPCR has been hypothesized to elicit antiapoptotic and regenerative actions of LPA. Based on this hypothesis we developed a novel nonlipid agonist of LPA₂, which we designated Radioprotectin-1 (RP-1). We tested RP-1 at the six murine LPA GPCR subtypes using the transforming growth factor alpha shedding assay and found that it had a 25 nM EC₅₀ that is similar to that of LPA18:1 at 32 nM. RP-1 effectively reduced apoptosis induced by γ-irradiation and the radiomimetic drug Adriamycin only in cells that expressed LPA₂ either endogenously or after transfection. RP-1 reduced γ-H2AX levels in irradiated mouse embryonic fibroblasts transduced with the human LPA₂ GPCR but was ineffective in vector transduced MEF control cells and significantly increased clonogenic survival after γ-irradiation. γ-Irradiation induced the expression of lpar2 transcripts that was further enhanced by RP-1 exposure within 30 min after irradiation. RP-1 decreased the mortality of C57BL/6 mice in models of the hematopoietic and gastrointestinal acute radiation syndromes. Using Lgr5-EGFP-CreER;Tdtomato^flox transgenic mice, we found that RP-1 increased the survival and growth of intestinal enteroids via the enhanced survival of Lgr5⁺ intestinal stem cells. Taken together, our results suggest that the LPA₂-specific agonist RP-1 exerts its radioprotective and radiomitigative action through specific activation of the upregulated LPA₂ GPCR in Lgr5⁺ stem cells.

LPA1 receptor-mediated thromboxane A2 release is responsible for lysophosphatidic acid-induced vascular smooth muscle contraction

Lysophosphatidic acid (LPA) has been recognized recently as an endothelium-dependent vasodilator, but several lines of evidence indicate that it may also stimulate vascular smooth muscle cells (VSMCs), thereby contributing to vasoregulation and remodeling. In the present study, mRNA expression of all 6 LPA receptor genes was detected in murine aortic VSMCs, with the highest levels of LPA₁, LPA₂, LPA₄, and LPA₆ In endothelium-denuded thoracic aorta (TA) and abdominal aorta (AA) segments, 1-oleoyl-LPA and the LPA_1-3 agonist VPC31143 induced dose-dependent vasoconstriction. VPC31143-induced AA contraction was sensitive to pertussis toxin (PTX), the LPA_1&3 antagonist Ki16425, and genetic deletion of LPA₁ but not that of LPA₂ or inhibition of LPA₃, by diacylglycerol pyrophosphate. Surprisingly, vasoconstriction was also diminished in vessels lacking cyclooxygenase-1 [COX1 knockout (KO)] or the thromboxane prostanoid (TP) receptor (TP KO). VPC31143 increased thromboxane A₂ (TXA₂) release from TA of wild-type, TP-KO, and LPA₂-KO mice but not from LPA₁-KO or COX1-KO mice, and PTX blocked this effect. Our findings indicate that LPA causes vasoconstriction in VSMCs, mediated by LPA₁-, G_i-, and COX1-dependent autocrine/paracrine TXA₂ release and consequent TP activation. We propose that this new-found interaction between the LPA/LPA₁ and TXA₂/TP pathways plays significant roles in vasoregulation, hemostasis, thrombosis, and vascular remodeling.-Dancs, P. T., Ruisanchez, E., Balogh, A., Panta, C. R., Miklós, Z., Nüsing, R. M., Aoki, J., Chun, J., Offermanns, S., Tigyi, G., Benyó, Z. LPA₁ receptor-mediated thromboxane A₂ release is responsible for lysophosphatidic acid-induced vascular smooth muscle contraction.

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.

Lysophosphatidic acid 2 receptor-mediated supramolecular complex formation regulates its antiapoptotic effect

The G protein-coupled lysophosphatidic acid 2 (LPA(2)) receptor elicits prosurvival responses to prevent and rescue cells from apoptosis. However, G protein-coupled signals are not sufficient for the full protective effect of LPA(2). LPA(2) differs from other LPA receptor subtypes in the C-terminal tail, where it contains a zinc finger-binding motif for the interactions with LIM domain-containing TRIP6 and proapoptotic Siva-1, and a PDZ-binding motif through which it complexes with the NHERF2 scaffold protein. In this report, we identify a unique CXXC motif of LPA(2) responsible for the binding to TRIP6 and Siva-1, and demonstrate that disruption of these macromolecular complexes or knockdown of TRIP6 or NHERF2 expression attenuates LPA(2)-mediated protection from chemotherapeutic agent-induced apoptosis. In contrast, knockdown of Siva-1 expression enhances this effect. Furthermore, a PDZ-mediated direct interaction between TRIP6 and NHERF2 facilitates their interaction with LPA(2). Together, these results suggest that in addition to G protein-activated signals, the cooperation embedded in the LPA(2)-TRIP6-NHERF2 ternary complex provides a novel ligand-dependent signal amplification mechanism that is required for LPA(2)-mediated full activation of antiapoptotic signaling.

Serine phosphorylation differentially affects RhoA binding to effectors: implications to NGF-induced neurite outgrowth

Activation of RhoA prevents NGF-induced outgrowth and causes retraction of neurites in neuronal cells, including PC12 cells. Despite its inhibitory effect on neurite outgrowth, NGF activates GTP loading of and effector binding to RhoA, setting up an apparent contradiction. According to the molecular switch hypothesis of GTPase function GTP-loading of RhoA should be sufficient to activate its effectors uniformly. However, when monitoring NGF-induced binding of GTP-RhoA to multiple targets, we noted differential interactions with its effectors. We found that NGF elicits a protein kinase A-mediated phosphorylation of RhoA on serine(188), which renders it unable to bind to Rho-associated kinase (ROK), whereas it retains the ability to interact with other RhoA targets including rhotekin, mDia-1 and PKN. We show in vitro and in vivo that phosphorylation of serine(188) represents an additional switch, capable of directing signals among effector pathways. In the context of PC12 cell differentiation, NGF-induced phosphorylation of RhoA on serine(188) prevents it from interacting with ROK, which would otherwise block neurite outgrowth. Transfection of RhoA(S188A) mutant into PC12 cells prevents NGF-induced neurite outgrowth, just like constitutively activated RhoA(14V) does, indicating the requirement of this phosphorylation site. Replacement of serine(188) with the phosphomimetic glutamate residue in RhoA(V14/S188E) selectively impairs interaction with ROK and when transfected into PC12 cells restores NGF-induced neurite outgrowth. Therefore, phosphorylation of serine(188) may serve as a novel secondary switch of RhoA capable of overriding GTP-binding-elicited effector activation to a subset of targets such as ROK, which interact with the C-terminus of RhoA.

A single amino acid determines lysophospholipid specificity of the S1P1 (EDG1) and LPA1 (EDG2) phospholipid growth factor receptors

The phospholipid growth factors sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA) are ligands for the related G protein-coupled receptors S1P(1)/EDG1 and LPA(1)/EDG2, respectively. We have developed a model of LPA(1) that predicts interactions between three polar residues and LPA. One of these, glutamine 125, which is conserved in the LPA receptor subfamily (LPA(1)/EDG2, LPA(2)/EDG4, and LPA(3)/EDG7), hydrogen bonds with the LPA hydroxyl group. Our previous S1P(1) study identified that the corresponding glutamate residue, conserved in all S1P receptors, ion pairs with the S1P ammonium. These two results predict that this residue might influence ligand recognition and specificity. Characterization of glutamate/glutamine interchange point mutants of S1P(1) and LPA(1) validated this prediction as the presence of glutamate was required for S1P recognition, whereas LPA recognition was possible with either glutamine or glutamate. The most likely explanation for this dual specificity behavior is a shift in the equilibrium between the acid and conjugate base forms of glutamic acid due to other amino acids surrounding that position in LPA(1), producing a mixture of receptors including those having an anionic glutamate that recognize S1P and others with a neutral glutamic acid that recognize LPA. Thus, computational modeling of these receptors provided valid information necessary for understanding the molecular pharmacology of these receptors.

Short-chain phosphatidates are subtype-selective antagonists of lysophosphatidic acid receptors

Lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are members of the phospholipid growth factor family. A major limitation in the field to date has been a lack of receptor subtype-specific agonists and antagonists. Here, we report that dioctylglycerol pyrophosphate and dioctylphosphatidic acid are selective antagonists of the LPA(1) and LPA(3) receptors, but prefer LPA(3) by an order of magnitude. Neither molecule had an agonistic or antagonistic effect on LPA(2) receptor. Consistent with this receptor subtype selectivity, dioctylglycerol pyrophosphate inhibited cellular responses to LPA in NIH3T3 fibroblasts, HEY ovarian cancer cells, PC12 pheochromocytoma cells, and Xenopus laevis oocytes. Responses elicited by S1P in these cell lines that endogenously express S1P(1), S1P(2), S1P(3), and S1P(5) receptors were unaffected by dioctylglycerol pyrophosphate. Responses evoked by the G protein-coupled receptor ligands acetylcholine, serotonin, ATP, and thrombin receptor-activating peptide were similarly unaffected, suggesting that the short-chain phosphatidates are receptor subtype-specific lysophosphatidate antagonists.

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

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

Physiological responses to lysophosphatidic acid and related glycero-phospholipids

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

Topical application of the phospholipid growth factor lysophosphatidic acid promotes wound healing in vivo

The lipid mediator lysophosphatidic acid (LPA) regulates cell proliferation and enhances cell motility in vitro, both of which are important events in wound healing. To evaluate the effects of LPA in vivo, it was applied to a full-thickness wound of rat skin. LPA in micromolar concentrations, or solvent, was applied daily. Animals were killed at 1, 3, 6, and 9 days after wounding and processed for histological evaluation, including hematoxylin-eosin staining and histochemical markers for macrophage-histiocytes, proliferating cells, and capillary endothelial cells. LPA treatment accelerated wound closing and increased neoepithelial thickness. Cytological evaluation showed no evidence for a secondary inflammation-mediated injury, infection, or increased keloid formation. Whereas LPA caused only a modest dose-dependent increase in proliferating cells, a marked increase in the immigration of histiocyte-macrophage cells was observed as early as day 1. The peaks of several cytological features and immunohistological markers preceded those of the untreated side. Our data suggest that exogenously applied LPA in this model promotes healing and that macrophage-histiocytes are the primary LPA-responsive cells in vivo.

Identification of Edg1 receptor residues that recognize sphingosine 1-phosphate

Originating from its DNA sequence, a computational model of the Edg1 receptor has been developed that predicts critical interactions with its ligand, sphingosine 1-phosphate. The basic amino acids Arg(120) and Arg(292) ion pair with the phosphate, whereas the acidic Glu(121) residue ion pairs with the ammonium moiety of sphingosine 1-phosphate. The requirement of these interactions for specific ligand recognition has been confirmed through examination of site-directed mutants by radioligand binding, ligand-induced [(35)S]GTPgammaS binding, and receptor internalization assays. These ion-pairing interactions explain the ligand specificity of the Edg1 receptor and provide insight into ligand specificity differences within the Edg receptor family. This computational map of the ligand binding pocket provides information necessary for understanding the molecular pharmacology of this receptor, thus underlining the potential of the computational method in predicting ligand-receptor interactions.

Sphingosine 1-phosphate as a major bioactive lysophospholipid that is released from platelets and interacts with endothelial cells

The serum-borne lysophospholipid mediators sphingosine 1-phosphate (Sph-1-P) and lysophosphatidic acid (LPA) have been shown to be released from activated platelets and to act on endothelial cells. In this study, we employed the repeated lipid extraction (under alkaline and acidic conditions), capable of detecting Sph-1-P, LPA, and possibly structurally similar lysophospholipids, whereby a marked formation of [(32)P]Sph-1-P, but not [(32)P]LPA, was observed in [(32)P]orthophosphate-labeled platelets. Platelet Sph-1-P release, possibly mediated by protein kinase C, was greatly enhanced in the presence of albumin, which formed a complex with Sph-1-P. This finding suggests that platelet Sph-1-P may become accessible to depletion by albumin when its transbilayer movement (flipping) across the plasma membrane is enhanced by protein kinase C. Although human umbilical vein endothelial cells expressed receptors for both Sph-1-P and LPA, Sph-1-P acted much more potently than LPA on the cells in terms of intracellular Ca(++) mobilization, cytoskeletal reorganization, and migration. The results suggest that Sph-1-P, rather than LPA, is a major bioactive lysophospholipid that is released from platelets and interacts with endothelial cells, under the conditions in which critical platelet-endothelial interactions (including thrombosis, angiogenesis, and atherosclerosis) occur. Furthermore, albumin-bound Sph-1-P may account for at least some of the serum biological activities on endothelial cells, which have been ascribed to the effects of albumin-bound LPA, based on the similarities between LPA and serum effects.

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

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

Pharmacological characterization of phospholipid growth-factor receptors

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

Localization of the PAK1-, WASP-, and IQGAP1-specifying regions of Cdc42

The Rho family small GTPase Cdc42 transmits divergent intracellular signals through multiple effector proteins to elicit cellular responses such as cytoskeletal reorganization. Potential effectors of Cdc42 implicated in mediating its cytoskeletal effect in mammalian cells include PAK1, WASP, and IQGAP1. To investigate the determinants of Cdc42-effector specificity, we utilized recombinant Cdc42 mutants and chimeras made between Cdc42 and RhoA to map the regions of Cdc42 contributing to specific effector p21-binding domain (PBD) interaction. Site-directed mutants of the switch I domain and neighboring regions of Cdc42 demonstrated differential binding patterns toward the PBDs of PAK1, WASP, and IQGAP1, suggesting that switch I provides essential determinants for the effector binding, but recognition of each effector by Cdc42 involves a distinct mechanism. Differing from Rac1, the switch I domain and the surrounding region (amino acids 29 to 55) of Cdc42 appeared to be sufficient for specific binding to PAK1, whereas determinants outside the switch I domain, residues 157-191 and 84-120 in particular, were necessary and sufficient to confer specificity to WASP and IQGAP1, respectively. In addition, IQGAP1, but not PAK1 nor WASP, required the unique "insert region," residues 122-134, of Cdc42 to achieve high affinity binding. Microinjection of the constitutively active Cdc42/RhoA chimeras into serum-starved Swiss 3T3 cells showed that although preserving PAK1- and WASP-binding activity could retain the peripheral actin microspike (PAM)-inducing activity of Cdc42, interaction with PAK1 or WASP was not required for this activity. Moreover, IQGAP1-binding alone by Cdc42 was insufficient for PAM-induction. Thus, Cdc42 utilizes multiple distinct structural determinants to specify different effector recognition and to elicit PAM-inducing effect.

Structurally similar oxidized phospholipids differentially regulate endothelial binding of monocytes and neutrophils

We previously have demonstrated that oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (OxPAPC), a component of minimally modified low density lipoprotein (MM-LDL), activates endothelial cells to bind monocytes. 1-Palmitoyl-2- (5-oxovaleroyl)-sn-glycero-3-phosphorylcholine (POVPC) and 1- palmitoyl-2-glutaroyl-sn-glycero-3-phosphorylcholine (PGPC), which are present in OxPAPC, MM-LDL, and atherosclerotic lesions, were shown to have a major role in the activation of endothelial cells. We now demonstrate that these two highly similar molecules have dramatically different effects on leukocyte endothelial interactions. POVPC is a potent regulator of monocyte-specific endothelial interactions. Treatment of endothelial cells with POVPC increased monocyte binding by inducing the surface expression of the connecting segment 1 domain of fibronectin; no increase in neutrophil binding was observed. In addition, POVPC strongly inhibited lipopolysaccharide-mediated induction of neutrophil binding and expression of E-selectin protein and mRNA. This inhibition was mediated by a protein kinase A-dependent pathway, resulting in down-regulation of NF-kappaB-dependent transcription. In contrast, PGPC induced both monocyte and neutrophil binding and expression of E-selectin and vascular cell adhesion molecule 1. We present evidence to suggest that the two phospholipids act by different novel receptors present in Xenopus laevis oocytes and that POVPC, but not PGPC, stimulates a cAMP-mediated pathway. At concentrations equal to that present in MM-LDL, the effect of POVPC dominates and inhibits PGPC-induced neutrophil binding and E-selectin expression in endothelial cells. In summary, our data provide evidence that both POVPC and PGPC are important regulators of leukocyte-endothelial interactions and that POVPC may play a dominant role in a number of chronic inflammatory processes where oxidized phospholipids are known to be present.

Lysophosphatidic acid as a regulator of endothelial/leukocyte interaction

Lysophosphatidic acid (LPA) is produced by a variety of activated cell types and acts as an intercellular mediator of processes associated with inflammation and repair including platelets aggregation, and smooth muscle and fibroblast proliferation. However no previous studies have examined the effects of LPA on endothelial cell leukocyte interactions. We have examined the ability of LPA to activate human aortic endothelial cells (HAEC) to bind monocytes, neutrophils, and HL60 cells (a neutrophil surrogate). Treatment of HAEC for 4 hours with 10 microM LPA caused an increase in the binding of monocytes, neutrophils, and HL60. LPA but not phosphatidic acid dose-dependently increased E-selectin and vascular cell adhesion molecule-1 (VCAM-1) cell surface expression. We performed several studies to characterize the receptor mediating the LPA effect. We demonstrate that at least five potential LPA receptors are expressed by HAEC: Edg-1, -3, -4, and -5 as well as PSP24. Cyclic phosphate-containing phosphatidic acid analogue, an agonist for the type 3 low affinity LPA receptor, was not effective in activating HAEC to bind leukocytes, excluding a role for this receptor. The selective receptor antagonists N-palmitoyl-serine and N-palmitoyl-tyrosine (which inhibits PSP24) completely inhibited LPA-induced VCAM expression; however these antagonists inhibited E-selectin expression by only 30%, suggesting a role for at least one additional LPA receptor mediating E-selectin expression. We propose that Edg-1 might be the second receptor, because this receptor, when expressed in HEK293 cells, similarly to the PSP24 receptor, caused ERK activation to nanomolar concentration of LPA. Exposure of HAEC to sphingosine-1-phosphate, another Edg-1 receptor agonist, increased surface expression of E-selectin and to a much smaller extent VCAM-1. The effects of both LPA and sphingosine-1-phosphate on the induction of both VCAM-1 and E-selectin expression was abolished by pretreatment with pertussis toxin suggesting that both LPA receptors in HAEC couple to a Gi pathway. These findings reveal an important and novel role for LPA and its receptors in inflammatory processes.

Different roles for RhoA during neurite initiation, elongation, and regeneration in PC12 cells

The goal of the present study was to characterize the effects of RhoA at different stages of nerve growth factor (NGF)-induced neuronal differentiation in the PC12 model. This comparative analysis was prompted by previous studies that reported apparently opposite effects for Rho in different models of neuronal differentiation and regeneration. PC12 cells were transfected with activated V14RhoA or dominant negative N19RhoA under the control of either a constitutive or a steroid-regulated promoter. Upon exposure to NGF, V14RhoA cells continued to proliferate and did not extend neurites; however, they remained responsive to NGF, as indicated by the activation of extracellular signal-regulated kinases. This inability to differentiate was reversed by C3 toxin and activation of cyclic AMP signaling, which inactivate RhoA. N19RhoA expression led to an increase in neurite initiation and branching. In contrast, when the RhoA mutants were expressed after NGF priming, only the rate of neurite extension was altered; V14RhoA clones had neurites approximately twice as long, whereas neurites of N19RhoA cells were approximately 50% shorter than those of appropriate controls. The effects of Rho in neurite regeneration mimicked those observed during the initial stages of morphogenesis; activation inhibited, whereas inactivation promoted, neurite outgrowth. Our results indicate that RhoA function changes at different stages of NGF-induced neuronal differentiation and neurite regeneration.

Inactivation of Rho signaling pathway promotes CNS axon regeneration

Regeneration in the CNS is blocked by many different growth inhibitory proteins. To foster regeneration, we have investigated a strategy to block the neuronal response to growth inhibitory signals. Here, we report that injured axons regrow directly on complex inhibitory substrates when Rho GTPase is inactivated. Treatment of PC12 cells with C3 enzyme to inactivate Rho and transfection with dominant negative Rho allowed neurite growth on inhibitory substrates. Primary retinal neurons treated with C3 extended neurites on myelin-associated glycoprotein and myelin substrates. To explore regeneration in vivo, we crushed optic nerves of adult rat. After C3 treatment, numerous cut axons traversed the lesion to regrow in the distal white matter of the optic nerve. These results indicate that targeting signaling mechanisms converging to Rho stimulates axon regeneration on inhibitory CNS substrates.

Lysophosphatidic acid mediates the rapid activation of platelets and endothelial cells by mildly oxidized low density lipoprotein and accumulates in human atherosclerotic lesions

Oxidized low density lipoprotein (LDL) is a key factor in the pathogenesis of atherosclerosis and its thrombotic complications, such as stroke and myocardial infarction. It activates endothelial cells and platelets through mechanisms that are largely unknown. Here, we show that lysophosphatidic acid (LPA) was formed during mild oxidation of LDL and was the active compound in mildly oxidized LDL and minimally modified LDL, initiating platelet activation and stimulating endothelial cell stress-fiber and gap formation. Antagonists of the LPA receptor prevented platelet and endothelial cell activation by mildly oxidized LDL. We also found that LPA accumulated in and was the primary platelet-activating lipid of atherosclerotic plaques. Notably, the amount of LPA within the human carotid atherosclerotic lesion was highest in the lipid-rich core, the region most thrombogenic and most prone to rupture. Given the potent biological activity of LPA on platelets and on cells of the vessel wall, our study identifies LPA as an atherothrombogenic molecule and suggests a possible strategy to prevent and treat atherosclerosis and cardiocerebrovascular diseases.

Naturally occurring analogs of lysophosphatidic acid elicit different cellular responses through selective activation of multiple receptor subtypes

Lysophosphatidic acid (LPA), plasmalogen-glycerophosphate (alkenyl-GP) and, cyclic-phosphatidic acid (cyclic-PA) are naturally occurring phospholipid growth factors (PLGFs). PLGFs elicit diverse biological effects via the activation of G protein-coupled receptors in a variety of cell types. In NIH3T3 fibroblasts, LPA and alkenyl-GP both induced proliferation, whereas cyclic-PA was antiproliferative. LPA and alkenyl-GP decreased cAMP in a pertussis toxin-sensitive manner, whereas cyclic-PA caused cAMP to increase. LPA and alkenyl-GP both stimulated the activity of the mitogen-actived protein kinases extracellular signal regulated kinases 1 and 2 and c-Jun NH2-terminal kinase, whereas cyclic-PA did not. All three PLGFs induced the formation of stress fibers in NIH3T3 fibroblasts. To determine whether these lipids activated the same or different receptors, heterologous desensitization patterns were established among the three PLGFs by monitoring changes in intracellular Ca2+ in NIH3T3 fibroblasts. LPA cross-desensitized both the alkenyl-GP and cyclic-PA responses. Alkenyl-GP cross-desensitized the cyclic-PA response, but only partially desensitized the LPA response. Cyclic-PA only partially desensitized both the alkenyl-GP and LPA responses. We propose that pharmacologically distinct subsets of PLGF receptors exist that distinguish between cyclic-PA and alkenyl-GP, but are all activated by LPA. We provide evidence that the PSP24 receptor is selective for LPA and not activated by the other two PLGFs. RT-PCR and Northern blot analysis indicate the co-expression of mRNAs encoding the EDG-2, EDG-4, and PSP24 receptors in a variety of cell lines and tissues. However, the lack of mRNA expression for these three receptors in the LPA-responsive Rat-1 and Sp2-O-Ag14 cells suggests that a number of PLGF receptor subtypes remain unidentified.

Identification of a novel growth factor-like lipid, 1-O-cis-alk-1'-enyl-2-lyso-sn-glycero-3-phosphate (alkenyl-GP) that is present in commercial sphingolipid preparations

Lysophosphatidic acid, a member of the acidic phospholipid autacoid (APA) family of lipid mediators, elicits diverse cellular effects that range from mitogenesis to the prevention of programmed cell death. Sphingosine 1-phosphate and sphingosylphosphorylcholine have also been proposed to be ligands of the APA receptors. However, key observations that provide the foundation of this hypothesis have not been universally reproducible, leading to a controversy in the field. We provide evidence that 1-O-cis-alk-1'-enyl-2-lyso-sn-glycero-3-phosphate (alkenyl-GP) is present in some commercial sphingolipid preparations and is responsible for many of their APA-like effects, which were previously attributed to sphingosylphosphorylcholine. Alkenyl-GP was generated by acidic and basic methanolysis from ethanolamine lysoplasmalogen, which was present in the sphingomyelin fraction that is used to manufacture sphingosylphosphorylcholine. We present the structural identification of alkenyl-GP, using 1H and 13C NMR, Fourier transform infrared spectrometry, and mass spectrometry. Alkenyl-GP was a potent activator of the mitogen-activated protein kinases ERK1/2 and elicited a mitogenic response in Swiss 3T3 fibroblasts. In contrast, sphingosylphosphorylcholine at a concentration of 10 microM was only a weak mitogen and only weakly activated the extracellular signal-regulated protein kinases. Alkenyl-GP has recently been detected as an injury-induced component in the anterior chamber of the eye (Liliom, K., Guan, Z., Tseng, H., Desiderio, D. M., Tigyi, G., and Watsky, M. (1998) Am. J. Physiol. 274, C1065-C1074), indicating that this lipid is a naturally occurring member of the APA mediator family.

Growth factor-like phospholipids generated after corneal injury

The present study provides evidence that growth factor-like glycerophosphate mediators of the lysophosphatidic acid (LPA) family are present in the aqueous humor and the lacrimal gland fluid of the rabbit eye. By use of a combination of HPLC, two-dimensional TLC, mass spectrometry, and the Xenopus oocyte bioassay, the LPA-like phospholipids LPA, cyclic PA, alkenyl-glycerophosphate (GP), lysophosphatidylserine, and phosphatidic acid were detected as physiological constituents of the fluids bathing the cornea. Corneal injury resulted in an increased production of some of these mediators. Alkenyl-GP, a novel member of the LPA family, has been identified in postinjury aqueous humor, establishing that it is generated endogenously. LPA and its homologues were found to be mitogenic in freshly dissociated keratocytes from uninjured corneas. There appears to be a link between the occurrence of LPA responsiveness in keratocytes activated by injury and the increase in LPA-like activity in aqueous humor. These data suggest that LPA and its homologues are involved in maintaining the integrity of the normal cornea and in promoting cellular regeneration of the injured cornea.

Edg-2/Vzg-1 couples to the yeast pheromone response pathway selectively in response to lysophosphatidic acid

We have functionally expressed the human cDNA encoding the putative lysophosphatidic acid (LPA) receptor Edg-2 (Vzg-1) in Saccharomyces cerevisiae in an attempt to determine the agonist specificity of this G-protein-coupled receptor. LPA activated the pheromone response pathway in S. cerevisiae expressing Edg-2 in a time- and dose-dependent manner as determined by induction of a pheromone-responsive FUS1::lacZ reporter gene. LPA-mediated activation of the pheromone response pathway was dependent on mutational inactivation of the SST2 gene, the GTPase-activating protein for the yeast G alpha protein (the GPA1 gene product). This indicates that, in sst2 delta yeast cells, Edg-2 can efficiently couple to the yeast heterotrimeric G-protein in response to LPA and activate the yeast mitogen-activated protein kinase pathway. The Edg-2 receptor showed a high degree of specificity for LPA; other lyso-glycerophospholipids, sphingosine 1-phosphate, and diacyl-glycerophospholipids did not activate FUS1::lacZ. LPA analogs including a cyclic phosphoester form and ether-linked forms of LPA activated FUS1::lacZ, although fatty acid chains of 6 and 10 carbons did not activate FUS1::lacZ, suggesting a role for the side chain in ligand binding or receptor activation. These results indicate that Edg-2 encodes a highly specific LPA receptor.

Rho proteins play a critical role in cell migration during the early phase of mucosal restitution

In the intestine, several growth factors stimulate migration of epithelial cells, contributing to the maintenance of tissue integrity. The Ras-like GTPase Rho regulates a signal transduction pathway linking growth factor receptors to the formation of actin stress fibers and focal adhesions, presumed to be important for motility. Using an in vitro wound-induced migration assay, we have examined the role of Rho GTPases in the migration of IEC-6 and Caco-2 cells, and provide evidence that the Rho GTPases play an essential role in the initial phase of mucosal wound healing. Treatment of the cells with Clostridium difficile toxins A and B, inhibitors of the Rho family GTPases inhibited migration in a dose-dependent fashion. Microinjection of the inhibitory exchange factor Rho-guanine nucleotide dissociation inhibitor (GDI), or Clostridium botulinum C3 ADP-ribosyl transferase (C3) toxin, a Rho-ADP-ribosylating exoenzyme, potently inhibited migration. Microinjection of RhoT19N, a dominant negative form of RhoA, or in vitro ADP-ribosylated RhoA impaired the ability of cells to migrate. Rho-GDI and C3 exoenzyme also inhibited EGF-induced migration of IEC-6 cells. These results demonstrate that Rho is required for endogenous and EGF-induced migration of small intestinal crypt cells, and that Rho proteins are essential elements of a mechanism by which growth factors induce cell migration to restitute mucosal integrity.

Local anesthetics inhibit receptors coupled to phosphoinositide signaling in Xenopus oocytes

Effects and the mechanism of action of quaternary amine local anesthetics on ligand- and voltage-activated ion currents were studied using voltage-clamped ovarian follicles and oocytes from Xenopus laevis. The fast inward and slow outward currents in response to acetylcholine were unaltered by procaine, whereas the oscillatory and smooth inward chloride currents (ICl) were abolished. Potassium currents (IK) elicited by norepinephrine and oscillatory ICl elicited by lysophosphatidic acid were blocked. Procaine caused a noncompetitive inhibition of oscillatory ICl mediated by heterologously expressed neurotransmitter receptors from the rat brain. Threefold differences were found in the procaine sensitivity of the 5-HT2a and 5-HT2c receptors. The rank order of intrinsic inhibitory activity of local anesthetics was: procaine > lidocaine > dibucaine > tetracaine. Extra- or intracellular application of procaine did not alter the Ca2+-activated Cl- current, indicating that neither the endogenous voltage-gated Ca2+ nor the Ca2+-activated Cl- channels account for the inhibition. Procaine caused only a slight reduction in ICl elicited by photolysis of caged inositol 1,4,5-trisphosphate (InsP3) and did not abolish ICl triggered by GTP[gamma-S]-induced direct activation of G proteins. For receptors coupling to the phosphoinositide/Ca2+ signal transduction pathway, the primary and physiologically relevant site of procaine action appears to be on the extracellular surface, upstream from the G protein, presumably on the receptor.

The faciogenital dysplasia gene product FGD1 functions as a Cdc42Hs-specific guanine-nucleotide exchange factor

The Rho family of small GTP-binding proteins plays important roles in the regulation of actin cytoskeleton organization and cell growth. Activation of these GTPases involves the replacement of bound GDP with GTP, a process catalyzed by the Dbl-like guanine-nucleotide exchange factors, all of which seem to share a putative catalytic motif termed the Dbl homology (DH) domain, followed by a pleckstrin homology (PH) domain. Here we have examined the role of a Dbl-like molecule, the faciogenital dysplasia gene product (FGD1), which when mutated in its Dbl homology domain, cosegregates with the developmental disease Aarskog-Scott syndrome. We report that a polypeptide of FGD1 encompassing the DH and PH domains can bind specifically to the Rho family GTPase Cdc42Hs and stimulates the GDP-GTP exchange of the isoprenylated form of Cdc42Hs. Microinjection of this FGD1 polypeptide into Swiss 3T3 fibroblast cells induces the formation of peripheral actin microspikes, similar to that previously observed when cells were injected with a constitutively active form of Cdc42Hs. This effect of FGD1 on actin organization is readily inhibited by coinjection of a dominant-negative mutant of Cdc42Hs. Examination of NIH 3T3 cells expressing the FGD1 fragment revealed that similar to cells expressing Dbl, two independent elements downstream of Cdc42Hs, the Jun NH2-terminal kinase and the p70 S6 kinase, became activated. Hence, our results indicate that FGD1, through its DH and PH domains, acts as a Cdc42Hs-specific guanine-nucleotide exchange factor and suggest that the Cdc42Hs GTPase may have a role in mammalian development.

Molecular cloning of a high-affinity receptor for the growth factor-like lipid mediator lysophosphatidic acid from Xenopus oocytes

Lysophosphatidic acid (1-acyl-2-lyso-snglycero-3-phosphate, LPA) is a multifunctional lipid mediator found in a variety of organisms that span the phylogenetic tree from humans to plants. Although its physiological function is not clearly understood, LPA is a potent regulator of mammalian cell proliferation; it is one of the major mitogens found in blood serum. In Xenopus laevis oocytes, LPA elicits oscillatory Cl- currents. This current, like other effects of LPA, is consistent with a plasma membrane receptor-mediated activation of G protein-linked signal transduction pathways. Herein we report the identification of a complementary DNA from Xenopus that encodes a functional high-affinity LPA receptor. The predicted structure of this protein of 372 amino acids contains features common to members of the seven transmembrane receptor superfamily with a predicted extracellular amino and intracellular carboxyl terminus. An antisense oligonucleotide derived from the first 5-11 predicted amino acids, selectively inhibited the expression of the endogenous high-affinity LPA receptors in Xenopus oocytes, whereas the same oligonucleotide did not affect the low-affinity LPA receptor. Expression of the full-length cRNA in oocytes led to an increase in maximal Cl- current due to increased expression of the high-affinity LPA receptor, but activation of the low-affinity receptor was, again, unaffected. Oocytes expressing cRNA prepared from this clone showed no response to other lipid mediators including prostaglandins, leukotrienes, sphingosine 1-phosphate, sphingosylphosphorylcholine, and platelet-activating factor, suggesting that the receptor is highly selective for LPA.

A novel membrane receptor with high affinity for lysosphingomyelin and sphingosine 1-phosphate in atrial myocytes

Activation of IK(ACh) is the major effect of the vagal neutrotransmitter acetylcholine in the heart. We report that both lysosphingomyelin (D-erythro-sphingosyl-phosphorylcholine; SPC) and sphingosine 1-phosphate (SPP) activate IK(ACh) in guinea pig atrial myocytes through the same receptor with an EC50 of 1.5 and 1.2 nM, respectively. Pertussis toxin abolished the activation of IK(ACh) by either lipid. The putative receptor showed an exquisite stereoselectivity for the naturally occurring D-erythro-(2S,3R)-SPC stereoisomer, the structure of which was confirmed by mass spectroscopy and NMR. These lipids caused complete homologous and heterologous desensitization with each other but not with ACh, indicating that both act on the same receptor. This receptor displays a distinct structure-activity relationship: it requires an unsubstituted amino group because N-acetyl-SPC, lysophosphatidic acid and lysophosphatidylcholine were inactive. Because SPP and SPC are naturally occurring products of membrane lipid metabolism, it appears that these compounds might be important extracellular mediators acting on a family of bona fide G protein-coupled receptors. Expression of these receptors in the heart raises the possibility that sphingolipids may be a part of the physiological and/or pathophysiological regulation of the heart. Based on their ligand selectivity we propose a classification of the sphingolipid receptors.

N-palmitoyl-serine and N-palmitoyl-tyrosine phosphoric acids are selective competitive antagonists of the lysophosphatidic acid receptors

Lysophosphatidic acid is the best characterized member of a lipid mediator family with growth factor-like activities that act through a class of G protein-coupled plasma membrane receptors. In Xenopus laevis oocytes, lysophosphatidate activates at least two pharmacologically distinct receptor subtypes distinguished by 1-acyl-sn-glycero-2,3-cyclic phosphate. Both of these naturally occurring ligands elicit oscillatory Cl- currents in the oocyte through G protein-coupled activation of the phosphoinositide/Ca2+ second messenger system, which in turn leads to the opening of Ca(2+)-activated Cl- channels. We developed an improved chemical synthesis and purification procedure for two N-acylated amino acid phosphates. N-Palmitoyl-serine and N-palmitoyl-tyrosine phosphoric acids inhibited the lysophosphatidate-activated Cl- currents with IC50 values of 5.4 +/- 0.7 and 6.5 +/- 1.5 nM at the high affinity site and 805 +/- 97 and 172 +/- 36 nM at the low affinity receptor site, respectively. In selective activation of the cyclic lysophosphatidate receptor, IC50 values of 330 +/- 30 and 490 +/- 40 nM were obtained, respectively. The D- and L-stereoisomers were equally effective when applied extracellularly. In contrast, they were ineffective when microinjected into the oocyte, indicating an extracellular site of inhibition. The inhibitors did not alter currents elicited by the different acetylcholine, serotonin, and glutamate receptors expressed heterologously in the oocyte. Pharmacological analysis of the results indicates that N-palmitoyl-serine and N-palmitoyl-tyrosine phosphoric acids are potent and specific competitive inhibitors of the lysophosphatidate receptors in the X. laevis oocyte.

Xenopus oocytes express multiple receptors for LPA-like lipid mediators

In Xenopus laevis oocytes, both lysophosphatidic acid (LPA) and a cyclic phosphate-containing analogue 1-acyl-sn-glycero-2,3-cyclic phosphate (cLPA) isolated from Physarum polycephalum activated oscillatory Cl- currents. cLPA elicited oscillatory currents only when applied extracellularly and, similarly to LPA, evoked homologous desensitization. cLPA applied to oocytes previously desensitized b y LPA failed to elicit a current, indicating that LPA completely desensitized the cLPA receptors. In contrast, when oocytes were desensitized by cLPA, LPA still evoked large currents. The lack of heterologous desensitization between cLPA and LPA indicates that the former acts on a distinct receptor subpopulation(s), which is also activated by LPA. The alkyl-ether analogue 1-hexadecyl-2-lyso-sn-glycero-3-phosphate (16:0-GP) and dioleoyl-phosphatidic acid (18:1-PA) showed heterologous desensitization patterns similar to that of LPA with regard to cLPA. Complete heterologous desensitization was obtained between LPA and 16:0-GP or 18:1-PA. These observations demonstrate the simultaneous expression of at least two different types of receptors for LPA-like lipid mediators on Xenopus oocytes and that these receptors show different pharmacological properties in their selectivity to cLPA.