Lysophospholipid receptors

Cannabinoid pharmacology in the cardiovascular system: potential protective mechanisms through lipid signalling

Cannabinoids include not only plant-derived compounds (of which delta9-tetrahydrocannabinol is the primary psychoactive ingredient of cannabis), but also synthetic agents and endogenous substances termed endocannabinoids which include anandamide (2-arachidonoylethanolamide) and 2-arachidonoylglycerol. Cannabinoids act on specific, G-protein-coupled, receptors which are currently divided into two types, CB1 and CB2. Relatively selective agonists and antagonists for these receptors have been developed, although one agent (SR141716A) widely used as an antagonist at CB1 receptors has non-cannabinoid receptor-mediated effects at concentrations which are often used to define the presence of the CB1 receptor. Both cannabinoid receptors are primarily coupled to Gi/o proteins and act to inhibit adenylyl cyclase. Stimulation of CB1 receptors also modulates the activity of K+ and Ca2+ channels and of protein kinase pathways including protein kinase B (Akt) which might mediate effects on apoptosis. CB, receptors may activate the extracellular signal-regulated kinase cascade through ceramide signalling. Cannabinoid actions on the cardiovascular system have been widely interpreted as being mediated by CB1 receptors although there are a growing number of observations, particularly in isolated heart and blood vessel preparations, that suggest that other cannabinoid receptors may exist. Interestingly, the currently identified cannabinoid receptors appear to be related to a wider family of lipid receptor, those for the lysophospholipids, which are also linked to Gi/o protein signalling. Anandamide also activates vanilloid VR1 receptors on sensory nerves and releases the vasoactive peptide, calcitonin gene-related peptide (CGRP), which brings about vasodilatation through its action on CGRP receptors. Current evidence suggests that endocannabinoids have important protective roles in pathophysiological conditions such as shock and myocardial infarction. Therefore, their cardiovascular effects and the receptors mediating them are the subject of increasing investigative interest.

Lysophosphatidic acid modulates the regenerative responses of human gingival fibroblasts and enhances the actions of platelet-derived growth factor

BACKGROUND

Platelet-derived growth factor (PDGF) has been used to promote healing in many in vitro and in vivo models of periodontal regeneration. PDGF is known to interact extensively with another platelet mediator, lysophosphatidic acid (LPA), to enhance regenerative responses in non-oral systems. PDGF and LPA are both liberated by platelets in the blood clot, which is known to be critical in stabilizing early periodontal wound healing. The purpose of this study was to evaluate the basic interactions of LPA with primary human gingival fibroblasts (GF) alone and with PDGF-BB for promoting GF growth and migration, as well as their effects in an in vitro oral wound-healing model.

METHODS

GF regenerative responses were measured using 1 and 10 microM LPA in the absence or presence of 1 or 10 ng/ml PDGF-BB. Cell growth was determined using [3H]thymidine incorporation and cell counting. Migration responses were measured using a microchemotaxis chamber. For the in vitro wound-healing experiments, GF were grown to confluence on glass slides, and a 3 mm wide wound was mechanically inflicted. Percent wound fill on days 4, 6, and 9 was analyzed using computer-assisted histomorphometry.

RESULTS

GF exhibited proliferative and chemotactic responses to LPA. These responses were synergistic when LPA and PDGF-BB were present together. LPA on its own did not stimulate statistically significant wound fill, but when combined with PDGF-BB, wound fill was equivalent to the 10% serum positive control group by day 6 (5.5-fold of negative control, [P<0.001]) and again on day 9 (6-fold of negative control, [P<0.001]).

CONCLUSIONS

These studies provide the first evidence that LPA stimulates human GF regenerative responses and that it interacts positively with PDGF-BB to regulate these actions. The results suggest that LPA needs to be further investigated in the oral system as a factor that should be considered for incorporation when designing new periodontal wound-healing therapies using PDGF.

Expression of the sphingosine 1-phosphate receptor, S1P1, on T-cells controls thymic emigration

S1P(1) is a widely distributed G protein-coupled receptor whose ligand, sphingosine 1-phosphate, is present in high concentrations in the blood. The sphingosine 1-phosphate receptor-signaling pathway is believed to have potent effects on cell trafficking in the immune system. To determine the precise role of the S1P(1) receptor on T-cells, we established a T-cell-specific S1P(1) knock-out mouse. The mutant mice showed a block in the egress of mature T-cells into the periphery. The expression of the S1P(1) receptor was up-regulated in mature thymocytes, and its deletion altered the chemotactic responses of thymocytes to sphingosine 1-phosphate. The results indicated that the expression of the S1P(1) receptor on T-cells controls their exit from the thymus and entry into the blood and, thus, has a central role in regulating the numbers of peripheral T-cells.

Pharmacological characterization of lysophospholipid receptor signal transduction pathways in rat cerebrocortical astrocytes

Lysophosphatidic acid (1-acyl-2-lyso-sn-glycero-3-phosphate; LPA) and sphingosine-1-phosphate (S1P) are bioactive phospholipids which respectively act as agonists for the G-protein-coupled lpA receptors (LPA1, LPA2, and LPA3) and s1p receptors (S1P1, S1P2, S1P3, S1P4, and S1P5), collectively referred to as lysophospholipid receptors (lpR). Since astrocytes are responsive to LPA and S1P, we examined mechanisms of lpR signaling in rat cortical secondary astrocytes. Rat cortical astrocyte mRNA expression by quantitative TaqMan polymerase chain reaction (PCR) analysis revealed the following order of relative expression of lpR mRNAs: s1p3>s1p1>lpa1>s1p2=lpa3>>s1p5. Activation of lpRs by LPA or S1P led to multiple pharmacological effects, including the influx of calcium, phosphoinositide (PI) hydrolysis, phosphorylation of extracellular receptor regulated kinase (ERK) and release of [3H]-arachidonic acid (AA). These signalling events downstream of lpR activation were inhibited to varying degrees by pertussis toxin (PTX) pretreatment or by the inhibition of sphingosine kinase (SK), a rate-limiting enzyme in the biosynthesis of S1P from sphingosine. These results suggest that astrocyte lpR signalling mechanisms likely involve both Gi- and Gq-coupled GPCRs and that receptor-mediated activation of SK leads to intracellular generation of S1P, which in turn amplifies the lpR signalling in a paracrine/autocrine manner.

Genetics and cell biology of lysophosphatidic acid receptor-mediated signaling during cortical neurogenesis

Lysophosphatidic acid (LPA) is a small lysophospholipid that signals through G-protein coupled receptors (GPCRs) to mediate diverse cellular responses. Two LPA receptors, LPA(1) and LPA(2), show gene expression profiles in mouse embryonic cerebral cortex, suggesting roles for LPA signaling in cerebral cortical development. Here, we review loss-of-function and gain-of-function models that have been used to examine LPA signaling. Genetic deletion of lpa(1) or both lpa(1) and lpa(2) in mice results in 50-65% neonatal lethality, but not obvious cortical phenotypes in survivors, suggesting that compensatory signaling systems exist for regulating cortical development. A gain-of-function model, approached by increasing receptor activation through exogenous delivery of LPA, shows that LPA signaling regulates cerebral cortical growth and anatomy by affecting proliferation, differentiation and cell survival during embryonic development.

Metabolic pathways and physiological and pathological significances of lysolipid phosphate mediators

Lysophosphatidic acid and sphingosine 1-phosphate are structurally simple and physiologically very important lysophospholipids. Because they possess distinct structural backbones (glycerol and sphingosine, respectively), there are different metabolic pathways for their intracellular production. Recently, several key enzymes that produce or degrade these lysolipid phosphate mediators extracellularly have been characterized. This review focuses on the physiological and pathophysiological significances of the extracellular metabolic pathways involving recently characterized exo-type lysophospholipase D, ecto-type phospholipase A, and ecto-type lipid phosphate phosphatase.

The endogenous cannabinoid system and its role in nociceptive behavior

The analgesic properties of exogenous cannabinoids have been recognized for many years and suggest a regulatory role for the endogenous cannabinoid ("endocannabinoid") system in mammalian nociceptive pathways. The endocannabinoid system includes: (1) at least two families of lipid signaling molecules, the N-acyl ethanolamines (e.g., anandamide) and the monoacylglycerols (e.g., 2-arachidonoyl glycerol); (2) multiple enzymes involved in the biosynthesis and degradation of these lipids, including the integral membrane enzyme fatty acid amide hydrolase; and (3) two G-protein coupled receptors, CB1 and CB2, which are primarily localized to the nervous system and immune system, respectively. Here, we review recent genetic, behavioral, and pharmacological studies that have tested the function of the endocannabinoid system in pain sensation. Collectively, these investigations support a role for endocannabinoids in modulating behavioral responses to acute, inflammatory, and neuropathic pain stimuli.

Lysophospholipid receptor-dependent and -independent calcium signaling

Changes in cellular Ca(2+) concentrations form a ubiquitous signal regulating numerous processes such as fertilization, differentiation, proliferation, contraction, and secretion. The Ca(2+) signal, highly organized in space and time, is generated by the cellular Ca(2+) signaling toolkit. Lysophospholipids, such as sphingosine-1-phosphate (S1P), sphingosylphosphorylcholine (SPC), or lysophosphatidic acid (LPA) use this toolkit in a specific manner to initiate their cellular responses. Acting as agonists at G protein-coupled receptors, S1P, SPC, and LPA increase the intracellular free Ca(2+) concentration ([Ca(2+)](i)) by using the classical, phospholipase C (PLC)-dependent pathway as well as PLC-independent pathways such as sphingosine kinase (SphK)/S1P. The S1P(1) receptor, via protein kinase C, inhibits the [Ca(2+)](i) transients caused by other receptors. Both S1P and SPC also act intracellularly to regulate [Ca(2+)](i). Intracellular S1P mobilizes Ca(2+) in intact cells independently of G protein-coupled S1P receptors, and Ca(2+) signaling by many agonists requires SphK-mediated S1P production. As shown for the FcepsilonRI receptor, PLC and SphK may contribute specific components to the overall [Ca(2+)](i) transient. Of the many open questions, identification of the intracellular S1P target site(s) appears to be of particular importance.

T cell chemotaxis to lysophosphatidylcholine through the G2A receptor

G2A is an immunoregulatory G protein-coupled receptor predominantly expressed in lymphocytes and macrophages. Ectopic overexpression studies have implicated G2A as a receptor for the bioactive lysophospholipid, lysophosphatidylcholine (LPC). However, the functional consequences of LPC-G2A interaction at physiological levels of receptor expression, and in a cellular context relevant to its immunological role, remain largely unknown. Here, we show impaired chemotaxis to LPC of a T lymphoid cell line in which G2A expression was chronically down-regulated by RNA interference technology. Rescuing this phenotype by reconstitution of the physiological level of receptor expression further supports a functional connection between LPC-G2A interaction and cellular motility. Overexpression of G2A in the T lymphoid cell line significantly enhanced chemotaxis to LPC. It also modified migration toward the LPC-related molecule, lysophosphatidic acid, indicating the possibility of crosstalk between G2A and endogenous lysophosphatidic acid receptors. The role of G2A in LPC-mediated cell migration may be relevant to the autoimmune syndrome associated with genetic inactivation of this G protein-coupled receptor in mice. The experimental system described here can be useful for understanding the structural requirements for LPC recognition by G2A and the signaling pathways regulated by this ligand-receptor pair.

S1P 3 ‐mediated Akt activation and crosstalk with platelet‐derived growth factor receptor (PDGFR)

Akt plays a pivotal role in cell survival and tumorigenesis. We investigated the potential interaction between sphingosine-1-phosphate (S1P) and platelet-derived growth factor (PDGF) in the Akt signaling pathway. Using mouse embryonic fibroblasts (MEFs) from S1P receptor knockout mice, we show here that S1P3 was required for S473 phosphorylation of Akt by S1P. In addition, S1P-stimulated activation of Akt, but not ERK, was blocked by a PDGF receptor (PDGFR)-specific inhibitor, AG1296, suggesting a S1P3-mediated specific crosstalk between the Akt signaling pathways of S1P and PDGFR in MEFs. We investigated this crosstalk under different conditions and found that both Akt and ERK activation induced by S1P, but not lysophosphatidic acid (LPA), in HEY ovarian cancer cells required PDGFR but not epidermal growth factor receptor (EGFR) or insulin-like growth factor-I receptor (IGFR). Importantly, S1P induced a Gi-dependent tyrosine phosphorylation of PDGFR in HEY cells. This dependence on PDGFR in S1P-induced Akt activation was also observed in A2780, T47D, and HMEC-1 cells (which express S1P3), but not in PC-3 or GI-101A cells (which do not express S1P3), further supporting that S1P3 mediates the crosstalk between S1P and PDGFR. This is the first report demonstrating a unique interaction between S1P3 and PDGFR, in addition to demonstrating a specific role for S1P3 in S1P-induced Akt activation.

Sphingosine kinase type 1 induces G12/13-mediated stress fiber formation, yet promotes growth and survival independent of G protein-coupled receptors

Sphingosine 1-phosphate (S1P) is the ligand for a family of specific G protein-coupled receptors (GPCRs) that regulate a wide variety of important cellular functions, including growth, survival, cytoskeletal rearrangements, and cell motility. However, whether it also has an intracellular function is still a matter of great debate. Overexpression of sphingosine kinase type 1, which generated S1P, induced extensive stress fibers and impaired formation of the Src-focal adhesion kinase signaling complex, with consequent aberrant focal adhesion turnover, leading to inhibition of cell locomotion. We have dissected biological responses dependent on intracellular S1P from those that are receptor-mediated by specifically blocking signaling of Galphaq, Galphai, Galpha12/13, and Gbetagamma subunits, the G proteins that S1P receptors (S1PRs) couple to and signal through. We found that intracellular S1P signaled "inside out" through its cell-surface receptors linked to G12/13-mediated stress fiber formation, important for cell motility. Remarkably, cell growth stimulation and suppression of apoptosis by endogenous S1P were independent of GPCRs and inside-out signaling. Using fibroblasts from embryonic mice devoid of functional S1PRs, we also demonstrated that, in contrast to exogenous S1P, intracellular S1P formed by overexpression of sphingosine kinase type 1 promoted growth and survival independent of its GPCRs. Hence, exogenous and intracellularly generated S1Ps affect cell growth and survival by divergent pathways. Our results demonstrate a receptor-independent intracellular function of S1P, reminiscent of its action in yeast cells that lack S1PRs.

Non-proliferative effects of lysophosphatidic acid enhance cortical growth and folding

Lysophosphatidic acid (LPA) is a phospholipid that has extracellular signaling properties mediated by G protein-coupled receptors. Two LPA receptors, LPA(1) and LPA(2), are expressed in the embryonic cerebral cortex, suggesting roles for LPA signaling in cortical formation. Here we report that intact cerebral cortices exposed to extracellular LPA ex vivo rapidly increased in width and produced folds resembling gyri, which are not normally present in mouse brains and are absent in LPA(1) LPA(2) double-null mice. Mechanistically, growth was not due to increased proliferation but rather to receptor-dependent reduced cell death and increased terminal mitosis of neural progenitor cells (NPCs). Our results implicate extracellular lipid signals as new influences on brain formation during embryonic development.

Common signaling pathways link activation of murine PAR-1, LPA, and S1P receptors to proliferation of astrocytes

Receptors for the serine protease thrombin and for lysophospholipids are coupled to G proteins and control a wide range of cellular functions, including mitogenesis. Activators of these receptors are present in blood, and can enter the brain during central nervous system (CNS) injury. Reactive astrogliosis, a prominent component of CNS injury with potentially harmful consequences, may involve proliferation of astrocytes. In this study, we have examined the expression and activation of protease activated receptors (PARs), lysophosphatidic acid (LPA) receptors, and sphingosine-1-phosphate (S1P) receptors on murine astrocytes. We show that activation of these three receptor classes can lead to astrogliosis in vivo and proliferation of astrocytes in vitro. Cultured murine cortical astrocytes express mRNA for multiple receptor subtypes of PAR (PAR-1-4), LPA (LPA-1-3) and S1P (S1P-1, -3, -4, and -5) receptors. Comparison of the intracellular signaling pathways of glial PAR-1, LPA, and S1P receptors indicates that each receptor class activates multiple downstream signaling pathways, including Gq/11-directed inositol lipid/Ca2+ signaling, Gi/o activation of mitogen-activated protein kinases (MAPK) (extracellular signal-regulated kinase 1/2 and stress activated protein kinase/c-jun N-terminal kinase, but not p38), and activation of Rho pathways. Furthermore, activation of these different receptor classes can differentially regulate two transcription factor pathways, serum response element and nuclear factor of activated T cells. Blockade of Gi/o signaling with pertussis toxin, MAPK activation with 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophynyltio)butadiene (U0126), or Rho kinase signaling with R-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexane carboxamide (Y27632) can markedly reduce the proliferative response of glial cells to PAR-1, LPA, or S1P receptor activation, suggesting that each of these pathways is important in coupling of receptor activation to glial proliferation.

Lipid phosphate phosphatases regulate lysophosphatidic acid production and signaling in platelets: studies using chemical inhibitors of lipid phosphate phosphatase activity

Blood platelets play an essential role in ischemic heart disease and stroke contributing to acute thrombotic events by release of potent inflammatory agents within the vasculature. Lysophosphatidic acid (LPA) is a bioactive lipid mediator produced by platelets and found in the blood and atherosclerotic plaques. LPA receptors on platelets, leukocytes, endothelial cells, and smooth muscle cells regulate growth, differentiation, survival, motility, and contractile activity. Definition of the opposing pathways of synthesis and degradation that control extracellular LPA levels is critical to understanding how LPA bioactivity is regulated. We show that intact platelets and platelet membranes actively dephosphorylate LPA and identify the major enzyme responsible as lipid phosphate phosphatase 1 (LPP1). Localization of LPP1 to the platelet surface is increased by exposure to LPA. A novel receptor-inactive sn-3-substituted difluoromethylenephosphonate analog of phosphatidic acid that is a potent competitive inhibitor of LPP1 activity potentiates platelet aggregation and shape change responses to LPA and amplifies LPA production by agonist-stimulated platelets. Our results identify LPP1 as a pivotal regulator of LPA signaling in the cardiovascular system. These findings are consistent with genetic and cell biological evidence implicating LPPs as negative regulators of lysophospholipid signaling and suggest that the mechanisms involve both attenuation of lysophospholipid actions at cell surface receptors and opposition of lysophospholipid production.

Photolysis of intracellular caged sphingosine-1-phosphate causes Ca2+mobilization independently of G-protein-coupled receptors

Sphingosine-1-phosphate (S1P), the product of sphingosine kinase, activates several widely expressed G-protein-coupled receptors (GPCR). S1P might also play a role as second messenger, but this hypothesis has been challenged by recent findings. Here we demonstrate that intracellular S1P can mobilize Ca(2+) in intact cells independently of S1P-GPCR. Within seconds, S1P generated by the photolysis of caged S1P raised the intracellular free Ca(2+) concentration in HEK-293, SKNMC and HepG2 cells, in which the response to extracellularly applied S1P was either blocked or absent. Ca(2+) transients induced by photolysis of caged S1P were caused by Ca(2+) mobilization from thapsigargin-sensitive stores. These results provide direct evidence for a true intracellular action of S1P.

Modulation of pro-inflammatory gene expression by nuclear lysophosphatidic acid receptor type-1

Lysophosphatidic acid (LPA) is a bioactive molecule involved in inflammation, immunity, wound healing, and neoplasia. Its pleiotropic actions arise presumably by interaction with their cell surface G protein-coupled receptors. Herein, the presence of the specific nuclear lysophosphatidic acid receptor-1 (LPA1R) was revealed in unstimulated porcine cerebral microvascular endothelial cells (pCMVECs), LPA1R stably transfected HTC4 rat hepatoma cells, and rat liver tissue using complementary approaches, including radioligand binding experiments, electron- and cryomicroscopy, cell fractionation, and immunoblotting with three distinct antibodies. Coimmunoprecipitation studies in enriched plasmalemmal fractions of unstimulated pCMVEC showed that LPA1Rs are dually sequestrated in caveolin-1 and clathrin subcompartments, whereas in nuclear fractions LPA1R appeared primarily in caveolae. Immunofluorescent assays using a cell-free isolated nuclear system confirmed LPA1R and caveolin-1 co-localization. In pCMVEC, LPA-stimulated increases in cyclooxygenase-2 and inducible nitric-oxide synthase RNA and protein expression were insensitive to caveolea-disrupting agents but sensitive to LPA-generating phospholipase A2 enzyme and tyrosine kinase inhibitors. Moreover, LPA-induced increases in Ca2+ transients and/or iNOS expression in highly purified rat liver nuclei were prevented by pertussis toxin, phosphoinositide 3-kinase/Akt inhibitor wortmannin and Ca2+ chelator and channel blockers EGTA and SK&F96365, respectively. This study describes for the first time the nucleus as a potential organelle for LPA intracrine signaling in the regulation of pro-inflammatory gene expression.

Egress: a receptor-regulated step in lymphocyte trafficking

Blood lymphocyte numbers, which are maintained by recirculation through secondary lymphoid organs, are essential for the efficient development of immune responses. Recirculating populations of B and T lymphocytes are regulated by the sphingosine-1-phosphate (S1P) receptor-dependent control of lymphocyte egress. T-cell egress from thymus into blood, egress from lymph node and Peyer's patch into lymph, and B-cell egress into lymph are rapidly and completely inhibited by agonism of S1P receptors. Mesenteric lymph nodes show log-jamming of lymphocytes subjacent to sinus-lining endothelium. Agonism of S1P receptors produces rapid peripheral blood lymphopenia, which is maintained in the presence of receptor agonist. Effector CD4+ and CD8+ T cells, produced by clonal expansion in draining lymph node in response to antigen, are sequestered in lymph node and fail to reach the peripheral blood. The S1P receptor system may represent an early physiological link between the non-specific inflammatory response and the alteration of lymphocyte traffic through draining lymph nodes. Pharmacological subversion of the S1P receptor system, through systemic S1P agonist-induced inhibition of lymphocyte egress, suppresses antigenic responses to peripheral, but not to systemically, delivered antigen. This inhibition induces significant immunosuppression in models of transplantation and autoimmune tissue damage that may prove to be of clinical benefit.

Lysophosphatidic acid regulates the proliferation and migration of olfactory ensheathing cells in vitro

Olfactory ensheathing cells (OECs) are a unique type of macroglia with axonal growth-promoting properties. However, our understanding of the factors that regulate OECs is at early stages. Lysophosphatidic acid (LPA) is a lipid that influences diverse functions in the nervous system. Recent studies suggest that glial cells, including astrocytes and Schwann cells, are important targets of LPA. However, the influences of LPA on OECs are not known. To address if LPA can influence OECs, we examined effects of LPA on the proliferation and migration of OECs and intracellular effector events. Initially, we observed that OECs expressed the genes for LPA1, LPA2, and LPA3 receptors. When OECs were treated with LPA, we observed stimulated proliferation and migration of OECs. Treatment of OECs with LPA also induced actin cytoskeleton reorganization and focal adhesion assembly. These effects of LPA were blocked by treatment with C3 exoenzyme or Y-27632, which inhibit Rho-GTPase and Rho-associated kinase, respectively. Effects of LPA on OEC proliferation were blocked by the MEK inhibitors PD098059 and U0126 and by the phosphotidylinositol 3-kinase (PI 3-K) inhibitors LY0294002 and wortmannin. These results show that LPA acts via Rho-GTPase, MAPK, and PI 3-K signaling cascades to influence OEC proliferation, migration, and cytoskeleton assembly.

Rapid induction of medullary thymocyte phenotypic maturation and egress inhibition by nanomolar sphingosine 1-phosphate receptor agonist

Only a small number of T cells generated in the thymus each day are selected to replenish the peripheral T cell pool. Much is known about thymic selection; however, little is known of the mechanisms regulating medullary maturation and the release of mature T cells into the blood. Here we demonstrate a rapid acceleration of medullary thymocyte phenotypic maturation through loss of CD69 induced by sphingosine 1-phosphate (S1P) receptor agonist. Low nanomolar agonist concentrations selectively induce changes in CD69(int) CD62L(high) single positive T cells, resulting in down-modulation of CD69 within 2 h. While CD69 loss is accelerated, egress of mature T cells into blood is inhibited >95% within 2 h. Both processes exhibit parallel sensitivities and dose-responses. Together, these data reveal a potent means for rapidly regulating thymic export where S1P receptor agonism alters both phenotypic maturation and egress of thymocytes into blood during late thymic maturation. The S1P system is now shown to acutely regulate both thymic and lymph node egress. Inhibition of lymphocyte egress from thymus and lymph node can contribute synergistically to clinically useful immunosupression by disrupting recirculation of peripheral T cells.

Sphingosine-1-phosphate receptor subtype-specific positive and negative regulation of Rac and haematogenous metastasis of melanoma cells

We have recently reported that S1P (sphingosine-1-phosphate) differentially regulates cellular Rac activity and cell migration in either a positive or a negative direction via distinct G-protein-coupled receptor subtypes, i.e. S1P1/Edg1 (endothelial differentiation gene) and S1P2/Edg5 respectively, when each of the S1P receptor subtypes is expressed in CHO (Chinese-hamster ovary) cells. In B16F10 mouse melanoma cells, in which S1P2, but not the other S1P receptor subtypes, is endogenously expressed, S1P inhibited cell migration with concomitant inhibition of Rac and stimulation of RhoA in dose-dependent manners. Overexpression of S1P2 in the melanoma cells resulted in potentiation of S1P inhibition of both Rac and cell migration. In contrast, overexpression of S1P1 led to stimulation of cell migration, particularly at the lower S1P concentrations. Treatment of B16F10 cells with S1P inhibited lung metastasis 3 weeks after injection into mouse tail veins. Intriguingly, overexpression of S1P2 greatly potentiated the inhibition of metastasis by S1P, whereas that of S1P1 resulted in aggravation of metastasis. Suppression of cellular Rac activity by adenovirus-transduced expression of N17Rac, but not N19RhoA, strongly inhibited cell migration in vitro and lung metastasis in vivo. These results provide the first evidence that G-protein-coupled receptors could participate in the regulation of metastasis, in which ligand-dependent, subtype-specific regulation of the cellular Rac activity is probably critically involved as a mechanism.

Selective blockade of lysophosphatidic acid LPA3 receptors reduces murine renal ischemia-reperfusion injury

Lysophosphatidic acid (LPA) released during ischemia has diverse physiological effects via its G protein-coupled receptors, LPA1, LPA2, and LPA3 (formerly Edg-2, -4, and -7). We tested the hypothesis that selective blockade of LPA receptors affords protection from renal ischemia-reperfusion (I/R) injury. By real-time PCR, LPA1-3 receptor mRNAs were expressed in mouse renal cortex, outer medulla, and inner medulla with the following rank order LPA3 = LPA2 > LPA1. In C57BL/6 mice whose kidneys were subjected to ischemia and reperfusion, treatment with a selective LPA3 agonist, oleoyl-methoxy phosphothionate (OMPT), enhanced injury. In contrast, a dual LPA1/LPA3-receptor antagonist, VPC-12249, reduced I/R injury, but this protective effect was lost when the antagonist was coadministered with OMPT. Interestingly, delaying administration of VPC-12249 until 30 min after the start of reperfusion did not alter its efficacy significantly. We conclude that VPC-12249 reduces renal I/R injury predominantly by LPA3 receptor blockade and could serve as a novel compound in the treatment of ischemia acute renal failure.

Ligand-dependent inhibition of B16 melanoma cell migration and invasion via endogenous S1P2 G protein-coupled receptor. Requirement of inhibition of cellular RAC activity

We investigated mechanisms for inhibition of B16 melanoma cell migration and invasion by sphingosine-1-phosphate (S1P), which is the ligand for the Edg family G protein-coupled receptors and also implicated as an intracellular second messenger. S1P, dihydro-S1P, and sphingosylphosphorylcholine inhibited B16 cell migration and invasion with the relative potencies expected as S1P2 receptor agonists. The S1P2-selective antagonist JTE013 completely abolished the responses to these agonists. In addition, JTE013 abrogated the inhibition by sphingosine, which is the S1P precursor but not an agonist for S1P receptors, indicating that the sphingosine effects were mediated via S1P2 stimulation, most likely by S1P that was converted from sphingosine. S1P induced inhibition and activation, respectively, of Rac and RhoA in B16 cells, which were abrogated by JTE013. Adenovirus-mediated expression of N17Rac mimicked S1P inhibition of migration, whereas C3 toxin pretreatment, but not Rho kinase inhibitors, reversed the S1P inhibition. Overexpression of S1P2 sensitized, and that of either S1P1 or S1P3 desensitized, B16 cells to S1P inhibition of Rac and migration. In JTE013-pretreated, S1P3-overexpressing B16 cells, S1P stimulated cellular RhoA but failed to inhibit either Rac or migration, indicating that RhoA stimulation itself is not sufficient for inhibition of migration. These results provide compelling evidence that endogenously expressed S1P2 negatively regulates cell motility and invasion through ligand-dependent reciprocal regulation of cellular Rac and RhoA activities. In the presence of JTE013, S1P instead stimulated Rac and migration in B16 cells that overexpress either S1P1 or S1P3, unveiling counteractions between S1P2 and S1P1 or S1P3 chemotactic receptor.

Fatty acid amide hydrolase: an emerging therapeutic target in the endocannabinoid system

The medicinal properties of exogenous cannabinoids have been recognized for centuries and can largely be attributed to the activation in the nervous system of a single G-protein-coupled receptor, CB1. However, the beneficial properties of cannabinoids, which include relief of pain and spasticity, are counterbalanced by adverse effects such as cognitive and motor dysfunction. The recent discoveries of anandamide, a natural lipid ligand for CB1, and an enzyme, fatty acid amide hydrolase (FAAH), that terminates anandamide signaling have inspired pharmacological strategies to augment endogenous cannabinoid ('endocannabinoid') activity with FAAH inhibitors, which might exhibit superior selectivity in their elicited behavioral effects compared with direct CB1 agonists.

Receptor-dependent RhoA activation in G12/G13-deficient cells: genetic evidence for an involvement of Gq/G11

The small GTPase RhoA is involved in the regulation of various cellular functions like the remodeling of the actin cytoskeleton and the induction of transcriptional activity. G-protein-coupled receptors (GPCRs), which are able to activate Gq/G11 and G12/G13 are major upstream regulators of RhoA activity, and G12/G13 have been shown to couple GPCRs to the activation of Rho by regulating the activity of a subfamily of RhoGEF proteins. However, the possible contribution of Gq/G11 to the regulation of RhoA activity via GPCRs is controversial. We have used a genetic approach to study the role of heterotrimeric G-proteins in the activation of RhoA via endogenous GPCRs. In pertussis toxin-treated Galpha12/Galpha13-deficient as well as in Galphaq/Galpha11-deficient mouse embryonic fibroblasts (MEFs), in which coupling of receptors is restricted to Gq/G11 and G12/G13, respectively, receptor activation results in Rho activation. Rho activation induced by receptor agonists via Gq/G11 occurs with lower potency than Rho activation via G12/G13. Activation of RhoA via Gq/G11 is not affected by the phospholipase-C blocker U73122 or the Ca2+-chelator BAPTA, but can be blocked by a dominant-negative mutant of the RhoGEF protein LARG. Our data clearly show that G12/G13 as well as Gq/G11 alone can couple GPCRs to the rapid activation of RhoA. Gq/G11-mediated RhoA activation occurs independently of phospholipase C-beta and appears to involve LARG.

Identification of p2y9/GPR23 as a novel G protein-coupled receptor for lysophosphatidic acid, structurally distant from the Edg family

Lysophosphatidic acid (LPA) is a bioactive lipid mediator with diverse physiological and pathological actions on many types of cells. LPA has been widely considered to elicit its biological functions through three types of G protein-coupled receptors, Edg-2 (endothelial cell differentiation gene-2)/LPA1/vzg-1 (ventricular zone gene-1), Edg-4/LPA2, and Edg-7/LPA3. We identified an orphan G protein-coupled receptor, p2y9/GPR23, as the fourth LPA receptor (LPA4). Membrane fractions of RH7777 cells transiently expressing p2y9/GPR23 displayed a specific binding for 1-oleoyl-LPA with a Kd value of around 45 nm. Competition binding and reporter gene assays showed that p2y9/GPR23 preferred structural analogs of LPA with a rank order of 1-oleoyl- > 1-stearoyl- > 1-palmitoyl- > 1-myristoyl- > 1-alkyl- > 1-alkenyl-LPA. In Chinese hamster ovary cells expressing p2y9/GPR23, 1-oleoyl-LPA induced an increase in intracellular Ca2+ concentration and stimulated adenylyl cyclase activity. Quantitative real-time PCR demonstrated that mRNA of p2y9/GPR23 was significantly abundant in ovary compared with other tissues. Interestingly, p2y9/GPR23 shares only 20-24% amino acid identities with Edg-2/LPA1, Edg-4/LPA2, and Edg-7/LPA3, and phylogenetic analysis also shows that p2y9/GPR23 is far distant from the Edg family. These facts suggest that p2y9/GPR23 has evolved from different ancestor sequences from the Edg family.

Inhibition of Ca(2+) signalling by the sphingosine 1-phosphate receptor S1P(1)

The lysophospholipid, sphingosine 1-phosphate (S1P), regulates a multitude of cellular functions by activating specific G protein-coupled receptors (GPCRs) (S1P(1-5), plus three newly identified S1P receptors). The G(i)-coupled S1P(1) receptor inhibits adenylyl cyclase, stimulates mitogen-activated protein kinases (MAP kinases) and cell migration, and is required for blood vessel maturation. Here, we report that S1P(1) inhibits Ca(2+) signalling in a number of cell types. In HEK-293 cells, which endogenously express S1P(1-3), overexpression of S1P(1) reduced intracellular free Ca(2+) concentration ([Ca(2+)](i)) increases induced by various receptor agonists as well as thapsigargin. The inhibitory Ca(2+) signalling of S1P(1) was blocked by pertussis toxin (PTX) and the protein kinase C (PKC) inhibitor, Gö6976, and imitated by phorbol ester and overexpression of classical PKC isoforms. Activation of S1P(1) stably expressed in RH7777 cells, which endogenously do not express S1P receptors, also inhibited Ca(2+) signalling, without mediating Ca(2+) mobilization on its own. It is concluded that the widely expressed S1P receptor S1P(1) inhibits Ca(2+) signalling, most likely via G(i) proteins and classical PKC isoforms. Co-expression of S1P(1) with S1P(3), but not S1P(2), reversed the inhibitory effect of S1P(1), furthermore suggesting a specific interplay of S1P receptor subtypes usually found within a single cell type.

Pleiotropic actions of sphingosine-1-phosphate

Sphingosine-1-phosphate (SIP) is a bioactive sphingolipid metabolite that regulates diverse cellular responses including, growth, survival, cytoskeleton rearrangements and movement. SIP plays an important role during development, particularly in vascular maturation and has been implicated in pathophysiology of cancer, wound healing, and atherosclerosis. This review summarizes the evidence showing that signaling induced by SIP is complex and involves both intracellular and extracellular actions. The intracellular effects of SIP remain speculative awaiting the identification of specific targets whereas the extracellular effects of SIP are clearly mediated through the activation of five specific G protein coupled receptors, called S1P1-5. Recent studies demonstrate that intracellular generated SIP can act in a paracrine or autocrine manner to activate its cell surface receptors.

Lipid receptors in cardiovascular development

To most people, concerns over the link between lipids and cardiovascular health most likely end with monitoring their daily consumption of dietary fats. However, it has become increasingly clear that, in addition to effects on adult cardiovascular physiology, lipids also play key roles in the formation of a functioning cardiovascular system. The lysophospholipids, lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), have come to the forefront as developmental and physiological regulators of the cardiovascular system. In this review, we discuss the function of the G protein-coupled receptors responsible for transducing LPA and S1P signals during development of the vertebrate cardiovascular system, focusing first on their role in angiogenesis and then on their function during embryonic development.

Lysophosphatidic acid stimulates CREB through mitogen- and stress-activated protein kinase-1

Lysophosphatidic acid (LPA) is a growth factor-like phospholipid that elicits a variety of cellular responses in numerous cell types, including neurons, immune cells, and fibroblasts. In this report, we investigated the possibility that LPA activates the transcription factor cAMP response element-binding protein, CREB, in Rat-2 fibroblast cells. CREB is activated in many cells downstream of signaling events, such as growth factor and neurotrophin stimulation. We found that LPA rapidly stimulated phosphorylation of CREB at Ser133 in a time- and dose-dependent manner, as revealed by immunoblot analysis with a phospho-specific antibody recognizing CREB on Ser133. LPA-induced phosphorylation of CREB was dependent on the activation of extracellular signal-regulated kinase 1/2 (ERK1/2) and p38 mitogen-activated protein kinase (p38 MAPK). Inhibition of ERK1/2 with PD98059 and of p38 MAPK with SB203580 efficiently blocked LPA-mediated phosphorylation of CREB. The LPA-induced CREB phosphorylation was abolished by H89, an inhibitor of mitogen- and stress-activated protein kinase-1 (MSK1). Together, these data suggest that LPA stimulates nuclear transcription factor CREB via mitogen-activated protein kinase signaling components, ERK1/2, p38 MAPK, and MSK1 in Rat-2 fibroblast cells.

Atheroprotective effects of high-density lipoproteins

Observational studies provide overwhelming evidence that a low high-density lipoprotein (HDL)-cholesterol level increases the risk of coronary events, both in healthy subjects and in patients with coronary heart disease. Based on in vitro experiments, several mechanistic explanations for the atheroprotective function of HDL have been suggested. However, few of these were verified in vivo in humans or in experiments with transgenic animals. The HDL functions currently most widely held to account for the antiatherogenic effect include participation in reverse cholesterol transport, protection against endothelial dysfunction, and inhibition of oxidative stress. This review summarizes current views on the molecular mechanism underlying these atheroprotective effects of HDL.

Edg-8 receptors are preferentially expressed in oligodendrocyte lineage cells of the rat CNS

The messenger RNA for endothelial differentiation gene 8 receptors is known to be expressed almost exclusively in the rat CNS, but the nature of the expressing cells has not been defined. Using an antibody specific for endothelial differentiation gene 8, we investigated the immunohistochemical localization of endothelial differentiation gene 8 receptors in the rat CNS. Immunopositive staining was detected in a subset of glial cells distributed throughout the brain and spinal cord, including both gray and white matter, but not in the dorsal root ganglion. The distribution and morphological similarity in comparative immunostaining for endothelial differentiation gene 8 and various glial markers suggested that endothelial differentiation gene 8 is preferentially expressed in NG2-positive oligodendrocyte progenitor cells in adult rat brains. Counts of endothelial differentiation gene 8-positive cells and NG2-positive cells in the forebrain revealed that a subset of NG2-positive cells was endothelial differentiation gene 8-positive, and that the ratio of endothelial differentiation gene 8-positive cells to NG2-positive cells varied from region to region. In 17-day-old embryonic brains, the endothelial differentiation gene 8 distribution was similar to that of an oligodendrocytic marker, 2',3'-cyclic nucleotide 3'-phosphodiesterase. These data suggest that endothelial differentiation gene 8 receptors are preferentially expressed in oligodendrocyte lineage cells including oligodendrocyte progenitor cells and immature/maturating oligodendrocytes in rat CNS, and that they might have important functions in oligodendrocytic maturation and myelination.

Cell migration and programmed cell death of Drosophila germ cells

Cell migration and programmed cell death are essential components of animal development and homeostasis, and the germ cells of Drosophila provide a simple genetic system to study the molecular mechanisms that govern these important cellular processes. Detailed descriptions of germ cell migration in Drosophila were accomplished long ago, but most genetic and molecular analyses of the process have occurred within the past 10 years. A few of the genes required for germ cell migration have been identified, and a very interesting picture is emerging. However, a process as complex as cell migration must involve the functions of many more molecules. In addition, cell migration and cell death mechanisms are often linked, as it is important to eliminate cells that are misplaced and could present a danger to the organism. In Drosophila, genes involved in germ cell migration can also affect programmed cell death. Currently, very little is known about how germ cells ectopic to the gonads are eliminated. To date, only four genes have been reported with roles in germ cell death, and three of these have additional functions in germ cell pathfinding. The nature of the cell death program has not been elucidated. Here, I provide a brief review of Drosophila germ cell migration and programmed cell death at both the descriptive and molecular levels. Many questions remain to be answered, but advances made in recent years are providing useful insights into these critical biological phenomena.

Lysophosphatidic acid promotes the proliferation of adult Schwann cells isolated from axotomized sciatic nerve

We have previously found that adult Schwann cells express receptors for lysophosphatidic acid (EDG2, EDG7) and sphingosine-1-phosphate (EDG5) and that expression of these receptors is significantly upregulated in injured sciatic nerve coincident with postaxotomy Schwann cell proliferation. Based on these observations, we hypothesized that lysophosphatidic acid and/or sphingosine-1-phosphate promote Schwann cell mitogenesis in injured adult nerve. We found that both saturated and unsaturated forms of lysophosphatidic acid, but not sphingosine-1-phosphate, induce DNA synthesis in adult Schwann cells isolated from surgically transected sciatic nerve. Lysophosphatidic acid induces adult Schwann cell DNA synthesis in a dose-dependent manner, acting at 0.1- to 10-microM concentrations. Lysophosphatidic acid-mediated stimulation of adult Schwann cell DNA synthesis occurs via a signaling pathway involving a pertussis toxin-sensitive (G(i)/G(o)) G-protein. Activation of phosphatidylinositol-3-kinase, cAMP-dependent protein kinase A and mitogen-activated protein kinase kinase is also required for lysophosphatidic acid-induced Schwann cell mitogenesis. These findings demonstrate that lysophosphatidic acid promotes proliferation of adult Schwann cells isolated from injured nerve and are consistent with the hypothesis that lysophosphatidic acid promotes in vivo Schwann cell mitogenesis in regenerating peripheral nerve.

Sphingosine-1-phosphate and its potentially paradoxical effects on critical parameters of cutaneous wound healing

The sphingolipid metabolite sphingosine-1-phosphate has emerged as a new bioactive molecule involved in the regulation of cell growth, differentiation, survival, and chemotaxis as well as angiogenesis and embryogenesis. These effects are mediated either via G-protein-coupled receptors or through intracellular actions. The most prominent sources of sphingosine-1-phosphate are human platelets suggesting its potential role in wound healing. In agreement with a positive function on reconstruction of wounded skin, we identified sphingosine-1-phosphate as a potent chemoattractant for keratinocytes as well as an activator of extracellular matrix production by fibroblasts. An unexpected finding is a strong cell growth arrest of keratinocytes after exposure to sphingosine-1-phosphate, as keratinocyte proliferation is critical for re-epithelialization of the wound. Most interestingly, the anti-proliferative effect of sphingosine-1-phosphate is not a result of cytotoxicity or apoptosis as sphingosine-1-phosphate even protects these cells from programmed cell death. Moreover, sphingosine-1-phosphate enhances differentiation of keratinocytes. To investigate further by which signaling pathway cell growth inhibition is mediated expression of the mRNA of all sphingosine-1-phosphate receptors (S1P1-5) was identified. 1 (Edg 1), 2 (Edg 5), 3 (Edg 3), 4 (Edg 6), and 5 (Edg 8) mRNA in keratinocytes was identified. As demonstrated in guanosine 5-[gamma-35S] triphosphate-gammaS binding assays, these G-protein-coupled receptors are functional at nanomolar concentrations. As the anti-proliferative effect of sphingosine-1-phosphate is only partially inhibited in the presence of pertussis toxin, it was investigated if intracellular actions are also involved. Microinjections of sphingosine-1-phosphate in keratinocytes also reduce proliferation suggesting that both sphingosine-1-phosphate receptors as well as intracellular actions mediate sphingosine-1-phosphate- induced cell growth arrest.

Sphingosine 1-phosphate and dioleoylphosphatidic acid are low affinity agonists for the orphan receptor GPR63

Five high affinity G-protein-coupled receptors for sphingosine 1-phosphate (S1P) have been characterised so far (S1P(1,2,3,4,5) formerly referred to as edg1,5,3,6,8). In this study, we show that S1P, dihydro-sphingosine 1-phosphate (dihydro-S1P) and dioleoylphosphatidic acid (doPA) are agonists for the orphan receptor GPR63. All three phospholipids mobilise intracellular calcium in CHO cells transiently transfected with GPR63. Calcium signals required cotransfection of a chimeric Galpha(q/i) protein in a fluorometric imaging plate reader (FLIPR) assay but did not require overexpressed G proteins in an aequorin assay, using a green fluorescent protein (GFP)-aequorin fusion protein as a bioluminescent Ca(2+) reporter. GPR63 expression in CHO cells confers proliferative responses to S1P in a pertussis toxin (PTX)-insensitive manner. Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) indicated highest expression in brain, especially in the thalamus and the nucleus caudatus. In peripheral tissues, highest expression was observed in thymus, stomach and small intestine; lower abundance of transcripts was detected in kidney, spleen, pancreas and heart. The discovery that S1P, dihydro-S1P and dioleoylphosphatidic acid activate GPR63 will facilitate the identification of agonists and antagonists, and help to unravel the biological function of this receptor.

Expression of lysophosphatidic acid receptors and vascular endothelial growth factor mediating lysophosphatidic acid in the development of human ovarian cancer

Lysophosphatidic acid (LPA) receptors including LPA(1), LPA(2), and LPA(3) mediate lysophosphatidic acid signals. We analyzed the expression of LPA receptors, vascular endothelial growth factor (VEGF), and interleukin-8 in 97 patients from normal ovary to ovarian cancer, using reverse transcription polymerase chain reaction. LPA(2), LPA(3), and VEGF expression ratios significantly increased in cancer, compared to those in non-cancerous state (P<0.05). A significant correlation in the expression ratios between LPA(2) or LPA(3) and VEGF was found (gamma=0.617, P<0.0001; gamma=0.431, P<0.001) in patients with cancer. These results suggested that LPA(2) and LPA(3) may be involved in VEGF expression mediated by LPA signals in human ovarian oncogenesis.

Altered cAMP signaling induced by lysophosphatidic acid in senescent human diploid fibroblasts

Lysophosphatidic acid (LPA) is a lipid mitogen that acts through G-protein-coupled receptors. LPA responsiveness has been reported to be dependent on the senescent state of the cells. To solve the mechanism underlying, we observed LPA-dependent cAMP status and found its age-dependent contrasting profile such as high level of cAMP in the senescent cells vs its low level in the young cells. In order to clarify the molecular mechanism of the ageing effect, we examined various molecular species involved in the cAMP signaling pathway by semi-quantitative RT-PCR. EDG-1 and EDG-4 were unchanged, but EDG-2 and EDG-7 were reduced with age. Senescent cells showed a partial reduction of Gi1, Gi2, and Gi3, but no change in the level of Gq. Decreased Gis and Gi-coupled LPA receptors may reduce the inhibitory effect of Gi alpha on adenylyl cyclases (ACs), resulting in cAMP accumulation via activation of adenylyl cyclase in senescent fibroblasts. We also observed an age-dependent increase in some of AC isoforms: II, IV, and VI. In conclusion, multiple changes in the cAMP signaling pathway of the senescent cells might explain the altered responsiveness to the mitogenic stimuli.

A G protein-coupled receptor responsive to bile acids

So far some nuclear receptors for bile acids have been identified. However, no cell surface receptor for bile acids has yet been reported. We found that a novel G protein-coupled receptor, TGR5, is responsive to bile acids as a cell-surface receptor. Bile acids specifically induced receptor internalization, the activation of extracellular signal-regulated kinase mitogen-activated protein kinase, the increase of guanosine 5'-O-3-thio-triphosphate binding in membrane fractions, and intracellular cAMP production in Chinese hamster ovary cells expressing TGR5. Our quantitative analyses for TGR5 mRNA showed that it was abundantly expressed in monocytes/macrophages in human and rabbit. Treatment with bile acids was found to suppress the functions of rabbit alveolar macrophages including phagocytosis and lipopolysaccharide-stimulated cytokine productions. We prepared a monocytic cell line expressing TGR5 by transfecting a TGR5 cDNA into THP-1 cells that did not express TGR5 originally. Treatment with bile acids suppressed the cytokine productions in the THP-1 cells expressing TGR5, whereas it did not influence those in the original THP-1 cells, suggesting that TGR5 is implicated in the suppression of macrophage functions by bile acids.

Mechanisms of lysophosphatidic acid-induced DNA synthesis in vascular smooth muscle cells

In order to investigate the signal transduction mechanisms of lysophosphatidic acid (LPA)-induced vascular smooth muscle (VSM) DNA synthesis, rat aortic A10 cells were used as an experimental model and [ H]-thymidine incorporation was used as an index of DNA synthesis. LPA caused dose- and time-dependent increase in DNA synthesis in A10 VSM cells. LPA (10 microM) also stimulated the activity of casein kinase II (CKII) in a time-dependent manner. The inhibitors of CKII, daidzein and 5,6-dichlorobenzimidazole riboside, diminished the LPA-induced increase in CKII activity and DNA synthesis. The LPA-stimulated activities of extracellularly regulated kinases (ERK) and p38 kinases as well as the stimulatory effects of LPA on DNA synthesis were blocked by ERK inhibitor, PD98059, and p38 kinase inhibitor, SB203580. The LPA-induced increase in intracellular free Ca and the LPA-induced DNA synthesis were not affected by Ca channel blockers, verapamil and diltiazem, as well as a Ca -dependent protein phosphatase (calcineurin) inhibitor, cyclosporine A. These data suggest that the LPA-induced DNA synthesis in VSM cells may be mediated by a signal transduction mechanism involving CKII, ERK, and p38 K.

Role of the G-protein-coupled receptor GPR12 as high-affinity receptor for sphingosylphosphorylcholine and its expression and function in brain development

Lysophospholipids are bioactive molecules influencing numerous cellular processes such as proliferation, differentiation, and motility. As extracellular ligands, they interact with specific members of the G-protein-coupled receptor family. We show in this paper that the lysophospholipid sphingosylphosphorylcholine is a high-affinity ligand for the orphan G-protein-coupled receptor GPR12. Heterologous expression of GPR12 in Chinese hamster ovary cells and in frog oocytes revealed a high-affinity interaction with sphingosylphosphorylcholine in the nanomolar range. Blockade of its action by pertussis toxin was taken as evidence that GPR12 is coupled to an inhibitory G-protein. In the adult mouse brain, GPR12 was expressed in the limbic system. During mouse embryonal development, GPR12 transcripts were detected in the CNS, especially in areas where neuronal differentiation occurs. Consistent with this we found that cultures of embryonal cerebral cortical neurons responded to sphingosylphosphorylcholine with an increase in synaptic contacts. The GPR12-expressing hippocampal cell line HT22 reacted to sphingosylphophorylcholine with an increase in cell proliferation and cell clustering. Other receptors known to interact at nanomolar concentrations with sphingosylphosphorycholine were expressed neither in the developing cerebral cortex nor in the HT22 cell line. We therefore hypothesize that sphingosylphosphorylcholine, most likely by interaction with GPR12, has positive effects on the differentiation and maturation of postmitotic neurons and that it may also influence the proliferation of neuronal precursor cells.

Regulation of oxytocin receptor expression in cultured human myometrial cells by fetal bovine serum and lysophospholipids

Oxytocin receptor (OTR) expression in human myometrium increases over 150-fold from the beginning of pregnancy to the end. In the present studies, we examined potential mechanisms of OTR up-regulation, using myometrial cells in primary culture from women in late gestation. OTR ligand-binding sites and steady-state mRNA levels were down regulated by serum starvation, and up-regulated by restoration of fetal bovine serum (FBS). Transcriptional activity of the OTR gene was the same with or without FBS treatment, but FBS increased OTR mRNA half-life about 5-fold. Lysophospholipids (lysophosphatidic acid and sphingosine 1-phosphate), which are present in serum, had similar effects as FBS. Lysophospholipid receptor mRNAs of the endothelial differentiation gene (Edg) family (Edgs 1, 3, 4, and 5) were demonstrated in myometrial cells by RT-PCR. These G protein-coupled receptors have been shown to be coupled to G(i/o) and to mediate activation of phosphoinositol 3-phosphate kinase. Indeed, the effects of the lysophospholipids and FBS were completely blocked by pertussis toxin, a G(i/o) inhibitor. Likewise, inhibition of G(i/o) signaling by elevation of intracellular cAMP or inhibition of phosphoinositol 3-phosphate kinase blocked FBS effects on OTR mRNA stability. We do not presently understand the mechanisms of OTR up-regulation in human myometrium in vivo, but the present studies might lead to the description of mRNA-stabilizing factors whose activity can be quantified in tissue samples during pregnancy to elucidate the process of OTR up-regulation.

Lysophosphatidic acid as a novel cell survival/apoptotic factor

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

Lysophosphatidic acid in neural signaling

The physiological and pathological importance of lysophosphatidic acid (LPA) in the nervous system is underscored by its presence, as well as the expression of its receptors in neural tissues. In fact, LPA produces responses in a broad range of cell types related to the function of the nervous system. These cell types include neural cell lines, neural progenitors, primary neurons, oligodendrocytes, Schwann cells, astrocytes, microglia, and brain endothelial cells. LPA-induced cell type-specific effects include changes in cell morphology, promotion of cell proliferation and cell survival, induction of cell death, changes in ion conductance and Ca2+ mobilization, induction of pain transmission, and stimulation of vasoconstriction. These effects are mediated through a number of G protein-coupled LPA receptors that activate various downstream signaling cascades. This review provides a current summary of LPA-induced effects in neural cells in vitro or in vivo in combination with our current understanding of the signaling pathways responsible for these effects.

The phospholipids sphingosine-1-phosphate and lysophosphatidic acid prevent apoptosis in osteoblastic cells via a signaling pathway involving G(i) proteins and phosphatidylinositol-3 kinase

The naturally occurring phospholipids lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) have recently emerged as bioactive compounds that exert mitogenic effects in many cell types, including osteoblasts. In the current study, we examined the ability of each of these compounds to influence osteoblast survival. Using terminal deoxynucleotidyl transferase-mediated deoxyuridine 5'-triphosphate nick-end labeling and DNA fragmentation assays, we found that both LPA and S1P dose-dependently inhibited (by at least 50% and 40%, respectively) the apoptosis induced by serum withdrawal in cultures of primary calvarial rat osteoblasts and SaOS-2 cells. The antiapoptotic effects were inhibited by pertussis toxin, wortmannin, and LY294002, implicating G(i) proteins and phosphatidylinositol-3 kinase (PI-3 kinase) in the signaling pathway that mediates phospholipid-induced osteoblast survival. Specific inhibitors of p42/44 MAPK signaling did not block LPA- or S1P-induced osteoblast survival. LPA and S1P induced PI-3 kinase-dependent activation of p70 S6 kinase, but rapamycin, a specific inhibitor of p70 S6 kinase activation, did not prevent phospholipid-induced osteoblast survival. LPA and S1P also inhibited apoptosis in Swiss 3T3 fibroblastic cells in a G(i) protein-dependent fashion. In fibroblastic cells, however, the antiapoptotic effects of S1P were sensitive to inhibition of both PI-3 kinase and p42/44 MAPK signaling, whereas those of LPA were partially abrogated by inhibitors of p42/44 MAPK signaling but not by PI-3 kinase inhibitors. These data demonstrate that LPA and S1P potently promote osteoblast survival in vitro, and that cell-type specificity exists in the antiapoptotic signaling pathways activated by phospholipids.

Neurobiology of lysophosphatidic acid signaling

Lysophosphatidic acid (LPA), a growth factor-like lysophospholipid, induces diverse cellular responses. The identification of the first LPA receptor gene, through studies of neuroproliferative regions within the embryonic cerebral cortex, has led to the classification of a family of at least eight lysophospholipid receptors with diverse roles in organismal development and function. A growing body of literature has identified roles for LPA signaling under physiological and pathological conditions, particularly within the developing nervous system. Here the authors review features of the LPA receptor family and cellular responses of nervous system-derived cells, and discuss developmental and pathological roles for LPA signaling in the nervous system.

Characterization of the expression of PDZ-RhoGEF, LARG and G(alpha)12/G(alpha)13 proteins in the murine nervous system

Small GTPases of the Rho-family, like Rho, Rac and Cdc42, are involved in neuronal morphogenesis by regulating growth cone morphology or dendritic spine formation. G-proteins of the G12-family, G12 and G13, couple G-protein-coupled receptors (GPCRs) to the activation of RhoA. Recently, two novel Rho-specific guanine nucleotide exchange factors (RhoGEFs), PDZ-RhoGEF and LARG, have been identified to interact with the activated alpha-subunits of G12/G13 and are thus believed to mediate GPCR-induced Rho activation. Although studies in neuronal cell lines have shown that G12/G13 and PDZ-RhoGEF mediate GPCR-induced neurite retraction, the role, as well as the expression of this signalling pathway, in intact brain has not been adequately studied. In the present study, we have characterized systematically the expression of G(alpha)12, G(alpha)13, PDZ-RhoGEF and LARG in various murine tissues as well as their subcellular localization in the central and peripheral nervous systems. By performing immunohistochemistry, using polyclonal antibodies raised against the above proteins, we observed that G(alpha)12, G(alpha)13 and their RhoGEF-effectors are distributed widely in the mammalian nervous system. Moreover, these proteins localize to distinct morphological compartments within neurons. While LARG and G(alpha)12 were mainly found in somata of the neurons, PDZ-RhoGEF and G(alpha)13 were predominantly localized in the neuropil of central neurons. Interestingly, PDZ-RhoGEF is a neural-specific protein, whereas LARG is nearly ubiqoutous. Our data provide evidence that the G12/13-RhoGEF-mediated pathway is present throughout the adult brain and may be involved in regulation of neuronal morphogenesis and function via GPCRs.

Structural adaptations in a membrane enzyme that terminates endocannabinoid signaling

Cellular communication in the nervous system is mediated by chemical messengers that include amino acids, monoamines, peptide hormones, and lipids. An interesting question is how neurons regulate signals that are transmitted by membrane-embedded lipids. Here, we report the 2.8 angstrom crystal structure of the integral membrane protein fatty acid amide hydrolase (FAAH), an enzyme that degrades members of the endocannabinoid class of signaling lipids and terminates their activity. The structure of FAAH complexed with an arachidonyl inhibitor reveals how a set of discrete structural alterations allows this enzyme, in contrast to soluble hydrolases of the same family, to integrate into cell membranes and establish direct access to the bilayer from its active site.

Synthesis of chiral (alpha,alpha-difluoroalkyl)phosphonate analogues of (lyso)phosphatidic acid via hydrolytic kinetic resolution

The hydrolytic kinetic resolution of 1,1-difluoro-3,4-epoxy-butylphosphonate using a chiral salen-Co complex was employed as a key step to obtain enantiomeric diols in 99% ee as key intermediates. The enantiomerically homogeneous (alpha,alpha-difluoroalkyl)phosphonates were obtained after selective esterification and deprotection of the corresponding phosphonates. These compounds are novel phosphatase-resistant analogues of lysophosphatidic acid and phosphatidic acid. [reaction: see text]

Involvement of phospholipases D1 and D2 in sphingosine 1-phosphate-induced ERK (extracellular-signal-regulated kinase) activation and interleukin-8 secretion in human bronchial epithelial cells

Sphingosine 1-phosphate (S1P), a metabolite of sphingomyelin degradation, stimulates interleukin-8 (IL-8) secretion in human bronchial epithelial (Beas-2B) cells. The molecular mechanisms regulating S1P-mediated IL-8 secretion are yet to be completely defined. Here we provide evidence that activation of phospholipases D1 and D2 (PLD1 and PLD2) by S1P regulates the phosphorylation of extracellular-signal-regulated kinase (ERK) and IL-8 secretion in Beas-2B cells. S1P, in a time- and dose-dependent manner, enhanced the threonine/tyrosine phosphorylation of ERK. The inhibition of S1P-induced ERK phosphorylation by pertussis toxin and PD 98059 indicated coupling of S1P receptors to G(i) and the ERK signalling cascade respectively. Treatment of Beas-2B cells with butan-1-ol, but not butan-3-ol, abrogated the S1P-induced phosphorylation of Raf-1 and ERK, suggesting that PLD is involved in this activation. The roles of PLD1 and PLD2 in ERK activation and IL-8 secretion activated by S1P were investigated by infecting cells with adenoviral constructs of wild-type and catalytically inactive mutants of PLD1 and PLD2. Infection of Beas-2B cells with the wild-type constructs resulted in the activation of PLD1 and PLD2 by S1P and PMA. Also, the enhanced production of [(32)P]phosphatidic acid and [(32)P]phosphatidylbutanol in the presence of butan-1-ol and the increased phosphorylation of ERK by S1P were blocked by the catalytically inactive mutants hPLD1-K898R and mPLD2-K758R. Transient transfection of Beas-2B cells with human PLD1 and mouse PLD2 cDNAs potentiated S1P-mediated IL-8 secretion compared with vector controls. In addition, PD 98059 attenuated IL-8 secretion induced by S1P in a dose-dependent fashion. These results demonstrate that both PLD1 and PLD2 participate in S1P stimulation of ERK phosphorylation and IL-8 secretion in bronchial epithelial cells.