Lysophosphatidic acid: G-protein signalling and cellular responses

Exposure of platelet membrane phosphatidylserine regulates blood coagulation

This article addresses the role of platelet membrane phosphatidylserine (PS) in regulating the production of thrombin, the central regulatory molecule of blood coagulation. PS is normally located on the cytoplasmic face of the resting platelet membrane but appears on the plasma-oriented surface of discrete membrane vesicles that derive from activated platelets. Thrombin, the central molecule of coagulation, is produced from prothrombin by a complex ("prothrombinase") between factor Xa and its protein cofactor (factor V(a)) that forms on platelet-derived membranes. This complex enhances the rate of activation of prothrombin to thrombin by roughly 150,000 fold relative to factor X(a) in solution. It is widely accepted that the negatively charged surface of PS-containing platelet-derived membranes is at least partly responsible for this rate enhancement, although there is not universal agreement on mechanism by which this occurs. Our efforts have led to an alternative view, namely that PS molecules bind to discrete regulatory sites on both factors X(a) and V(a) and allosterically alter their proteolytic and cofactor activities. In this view, exposure of PS on the surface of activated platelet vesicles is a key regulatory event in blood coagulation, and PS serves as a second messenger in this regulatory process. This article reviews our knowledge of the prothrombinase reaction and summarizes recent evidence leading to this alternative viewpoint. This viewpoint suggests a key role for PS both in normal hemostasis and in thrombotic disease.

Protein kinase C epsilon dependence of the recovery from down-regulation of S1P1 G protein-coupled receptors of T lymphocytes

Sphingosine 1-phosphate (S1P) from mononuclear phagocytes and platelets signals T cells predominantly through S1P1 G protein-coupled receptors (Rs) to enhance survival, stimulate and suppress migration, and inhibit other immunologically relevant responses. Cellular S1P1 Rs and their signaling functions are rapidly down-regulated by S1P, through a protein kinase C (PKC)-independent mechanism, but characteristics of cell-surface re-expression of down-regulated S1P1 Rs have not been elucidated. T cell chemotactic responses (CT) to 10 and 100 nm S1P and inhibition of T cell chemotaxis to chemokines (CI) by 1 and 3 microm S1P were suppressed after 1 h of preincubation with 100 nm S1P, but recovered fully after 12-24 h of exposure to S1P. Late recovery of down-regulated CT and CI, but not early down-regulation, was suppressed by PKC and PKCepsilon-selective inhibitors and was absent in T cells from PKCepsilon-null mice. The same PKCepsilon inhibitors blocked S1P-evoked increases in T cell nuclear levels of c-Fos and phosphorylated c-Jun and JunD after 24 h, but not 1 h. A mixture of c-Fos plus c-Jun antisense oligonucleotides prevented late recovery of down-regulated CT and CI, without affecting S1P induction of down-regulation. Similarly, S1P-elicited threonine phosphorylation of S1P1 Rs was suppressed by a selective inhibitor of PKCepsilon after 24 h, but not 1 h. Biochemical requisites for recovery of down-regulated S1P1 Rs thus differ from those for S1P induction of down-regulation.

Bombesin, lysophosphatidic acid, and epidermal growth factor rapidly stimulate focal adhesion kinase phosphorylation at Ser-910: requirement for ERK activation

A rapid increase in the tyrosine phosphorylation of focal adhesion kinase (FAK) has been extensively documented in cells stimulated by multiple signaling molecules, but virtually nothing is known about the regulation of FAK phosphorylation at serine residues. Stimulation of Swiss 3T3 cells with bombesin promoted a striking increase ( approximately 13-fold) in the phosphorylation of FAK at Ser-910, as revealed by site-specific antibodies that recognized the phosphorylated state of this residue. Lysophosphatidic acid and epidermal growth factor (EGF) also stimulated FAK phosphorylation at Ser-910. Direct activation of protein kinase C isoforms with phorbol-12,13-dibutyrate (PDB) also promoted striking phosphorylation of FAK at Ser-910. Treatment with the protein kinase C inhibitor GF I or Ro 31-8220 or chronic exposure to PDB prevented the increase in FAK phosphorylation at Ser-910 induced by bombesin or PDB but not by EGF. Treatment with the ERK inhibitors U0126 and PD98059 prevented FAK phosphorylation at Ser-910 in response to all of the stimuli tested. Furthermore, incubation of activated ERK2 with FAK immunocomplexes leads to FAK phosphorylation at Ser-910 in vitro. Our results demonstrate, for the first time, that stimulation with bombesin, lysophosphatidic acid, PDB, or EGF induces phosphorylation of endogenous FAK at Ser-910 via an ERK-dependent pathway in Swiss 3T3 cells.

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.

A new phospholipid phosphatase, PRG-1, is involved in axon growth and regenerative sprouting

Outgrowth of axons in the central nervous system is governed by specific molecular cues. Molecules detected so far act as ligands that bind to specific receptors. Here, we report a new membrane-associated lipid phosphate phosphatase that we have named plasticity-related gene 1 (PRG-1), which facilitates axonal outgrowth during development and regenerative sprouting. PRG-1 is specifically expressed in neurons and is located in the membranes of outgrowing axons. There, it acts as an ecto-enzyme and attenuates phospholipid-induced axon collapse in neurons and facilitates outgrowth in the hippocampus. Thus, we propose a novel mechanism by which axons are able to control phospholipid-mediated signaling and overcome the growth-inhibiting, phospholipid-rich environment of the extracellular space.

Dual mechanisms for lysophosphatidic acid stimulation of human ovarian carcinoma cells

BACKGROUND

Lysophosphatidic acid (LPA), at concentrations present in ascitic fluid, indirectly stimulates the growth of malignant ovarian tumors by increasing the expression of vascular endothelial growth factor (VEGF) in ovarian cancer cells. We investigated whether LPA could also directly promote ovarian tumor growth by increasing the level of cyclin D1, a key G1-phase checkpoint regulator, which thereby increases cell proliferation.

METHODS

Expression of cyclin D1 and LPA receptors (EDG4 and EDG7) was determined in six ovarian cancer cell lines (including OVCAR-3 cells) and immortalized ovarian surface epithelial cells (IOSE-29). Cyclin D1 promoter activity was measured in LPA-treated OVCAR-3 cells cotransfected with cyclin D1 promoter-driven luciferase constructs and cDNA expression plasmids for IkappaBalphaM (a nuclear factor kappaB [NFkappaB] super-repressor).

RESULTS

Four of six cancer cell lines, including OVCAR-3, overexpressed cyclin D1 protein relative to levels in IOSE-29 cells. LPA treatment increased cyclin D1 protein in a dose- and time-dependent manner in OVCAR-3 cells but not in IOSE-29 cells. LPA stimulated cyclin D1 promoter activity (3.0-fold, 95% confidence interval [CI] = 2.7-fold to 3.3-fold). Mutation of the NFkappaB-binding site in the cyclin D1 promoter to block NFkappaB binding and expression of IkappaBalphaM, which binds NFkappaB and inhibits its binding to the promoter, markedly diminished LPA stimulation of cyclin D1 promoter activity (activity stimulated only 1.4-fold, 95% CI = 1.1-fold to 1.7-fold, and 0.7-fold, 95% CI = 0.6-fold to 0.8-fold, respectively). EDG4 was overexpressed in all cancer cell lines studied relative to that in IOSE-29 cells, but EDG7 was overexpressed in only two lines.

CONCLUSIONS

Dual mechanisms are probably involved in LPA stimulation of ovarian tumor growth in vivo. In addition to the previously characterized indirect mechanism that increases angiogenesis via VEGF, LPA may directly increase the level of cyclin D1 in ovarian cancer cells, increasing their proliferation.

Agonist-induced endocytosis of lysophosphatidic acid-coupled LPA1/EDG-2 receptors via a dynamin2- and Rab5-dependent pathway

Lysophosphatidic acid (LPA) is a serum-borne phospholipid that exerts a pleiotropic range of effects on cells through activation of three closely related G-protein-coupled receptors termed LPA1/EDG-2, LPA2/EDG-4 and LPA3/EDG-7. Of these receptors, the LPA1 receptor is the most widely expressed. In this study, we investigated the agonist-induced endocytosis of the human LPA1 receptor, bearing an N-terminal FLAG epitope tag, in stably transfected HeLa cells. Treatment with LPA induced the rapid endocytosis of approximately 40% of surface LPA1 within 15 minutes. Internalization was both dose dependent and LPA specific since neither lysophophatidylcholine nor sphingosine-1-phosphate induced LPA1 endocytosis. Removal of agonist following 30 minutes incubation resulted in recycling of LPA1 back to the cell surface. LPA1 internalization was strongly inhibited by dominant-inhibitory mutants of both dynamin2 (K44A) and Rab5a (S34N). In addition, both dynamin2 K44A and Rab5 S34N mildly inhibited LPA1-dependent activation of serum response factor. Finally, our results also indicate that LPA1 exhibits basal, LPA-dependent internalization in the presence of serum-containing medium.

Lysophosphatidic acid stimulates brush border Na+/H+ exchanger 3 (NHE3) activity by increasing its exocytosis by an NHE3 kinase A regulatory protein-dependent mechanism

Na(+)/H(+) exchanger 3 (NHE3) kinase A regulatory protein (E3KARP) has been implicated in cAMP- and Ca(2+)-dependent inhibition of NHE3. In the current study, a new role of E3KARP is demonstrated in the stimulation of NHE3 activity. Lysophosphatidic acid (LPA) is a mediator of the restitution phase of inflammation but has not been studied for effects on sodium absorption. LPA has no effect on NHE3 activity in opossum kidney (OK) proximal tubule cells, which lack expression of endogenous E3KARP. However, in OK cells exogenously expressing E3KARP, LPA stimulated NHE3 activity. Consistent with the stimulatory effect on NHE3 activity, LPA treatment increased the surface NHE3 amount, which occurred by accelerating exocytic trafficking (endocytic recycling) to the apical plasma membrane. These LPA effects only occurred in OK cells transfected with E3KARP. The LPA-induced increases of NHE3 activity, surface NHE3 amounts, and exocytosis were completely inhibited by pretreatment with the PI 3-kinase inhibitor, LY294002. LPA stimulation of the phosphorylation of Akt was used as an assay for PI 3-kinase activity. LY294002 completely prevented the LPA-induced increase in Akt phosphorylation, which is consistent with the inhibitory effect of LY294002 on the LPA stimulation of NHE3 activity. The LPA-induced phosphorylation of Akt was the same in OK cells with and without E3KARP. These results show that LPA stimulates NHE3 in the apical surface of OK cells by a mechanism that is dependent on both E3KARP and PI 3-kinase. This is the first demonstration that rapid stimulation of NHE3 activity is dependent on an apical membrane PDZ domain protein.

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.

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.

Cooperation of Gq, Gi, and G12/13 in protein kinase D activation and phosphorylation induced by lysophosphatidic acid

To examine the contribution of different G-protein pathways to lysophosphatidic acid (LPA)-induced protein kinase D (PKD) activation, we tested the effect of LPA on PKD activity in murine embryonic cell lines deficient in Galpha(q/11) (Galpha(q/11) KO cells) or Galpha(12/13) (Galpha(12/13) KO cells) and used cells lacking rhodopsin kinase (RK cells) as a control. In RK and Galpha(12/13) KO cells, LPA induced PKD activation through a phospholipase C/protein kinase C pathway in a concentration-dependent fashion with maximal stimulation (6-fold for RK cells and 4-fold for Galpha(12/13) KO cells in autophosphorylation activity) achieved at 3 microm. In contrast, LPA did not induce any significant increase in PKD activity in Galpha(q/11) KO cells. However, LPA induced a significantly increased PKD activity when Galpha(q/11) KO cells were transfected with Galpha(q). LPA-induced PKD activation was modestly attenuated by prior exposure of RK cells to pertussis toxin (PTx) but abolished by the combination treatments of PTx and Clostridium difficile toxin B. Surprisingly, PTx alone strikingly inhibited LPA-induced PKD activation in a concentration-dependent fashion in Galpha(12/13) KO cells. Similar results were obtained when activation loop phosphorylation at Ser-744 was determined using an antibody that detects the phosphorylated state of this residue. Our results indicate that G(q) is necessary but not sufficient to mediate LPA-induced PKD activation. In addition to G(q), LPA requires additional G-protein pathways to elicit a maximal response with G(i) playing a critical role in Galpha(12/13) KO cells. We conclude that LPA induces PKD activation through G(q), G(i), and G(12) and propose that PKD activation is a point of convergence in the action of multiple G-protein pathways.

Lysophosphatidic acid induction of tissue factor expression in aortic smooth muscle cells

OBJECTIVE

Tissue factor (TF), the initiator of the coagulation cascade, is expressed by cells in atherosclerotic lesions. Lysophosphatidic acid (LPA) is a component of oxidized lipoproteins and an agent released by activated platelets. The present study investigated whether and how TF expression is regulated by LPA.

METHODS AND RESULTS

Northern blotting, Western blotting, and TF activity assays demonstrated that LPA markedly induced TF mRNA, protein, and activity in vascular smooth muscle cells. LPA-induced TF expression is primarily controlled at the transcriptional level. Phosphorylation of mitogen-activated protein kinase kinase (MEK) and extracellular signaling-regulated kinases (ERK1/2) was rapidly and markedly induced by LPA. MEK inhibitors U0126 and PD98059 blocked both ERK activation and the increase in TF mRNA. In contrast, the specific p38 MAP kinase inhibitor SB203580 had no effect on LPA-induced TF mRNA increase. The Galpha(i) protein inhibitor, pertussis toxin, abolished LPA-induced phosphorylation of MEKs and ERKs, as well as the induction of TF mRNA.

CONCLUSIONS

Our data demonstrate that a Galpha(i) protein and activation of MEKs and ERKs mediate LPA-induced TF expression. Our data suggest that elevated LPA could be a thrombogenic risk factor by upregulating TF expression. These results may have important implications in vascular remodeling and vascular diseases.

Rac activation by lysophosphatidic acid LPA1 receptors through the guanine nucleotide exchange factor Tiam1

Lysophosphatidic acid (LPA) is a serum-borne phospholipid that activates its own G protein-coupled receptors present in numerous cell types. In addition to stimulating cell proliferation, LPA also induces cytoskeletal changes and promotes cell migration in a RhoA- and Rac-dependent manner. Whereas RhoA is activated via Galpha(12/13)-linked Rho-specific guanine nucleotide exchange factors, it is unknown how LPA receptors may signal to Rac. Here we report that the prototypic LPA(1) receptor (previously named Edg2), when expressed in B103 neuroblastoma cells, mediates transient activation of RhoA and robust, prolonged activation of Rac leading to cell spreading, lamellipodia formation, and stimulation of cell migration. LPA-induced Rac activation is inhibited by pertussis toxin and requires phosphoinositide 3-kinase activity. Strikingly, LPA fails to activate Rac in cell types that lack the Rac-specific exchange factor Tiam1; however, enforced expression of Tiam1 restores LPA-induced Rac activation in those cells. Tiam1-deficient cells show enhanced RhoA activation, stress fiber formation, and cell rounding in response to LPA, consistent with Tiam1/Rac counteracting RhoA. We conclude that LPA(1) receptors couple to a G(i)-phosphoinositide 3-kinase-Tiam1 pathway to activate Rac, with consequent suppression of RhoA activity, and thereby stimulate cell spreading and motility.

A possible role of RhoA/Rho-kinase in experimental spinal cord injury in rat

Secondary injury following traumatic spinal cord injury is induced by the activation of a number of cellular and molecular changes. RhoA, a small GTPase, regulates the organization of the actin cytoskeleton, gene expression, cell proliferation, and has been implicated in the regenerative process. This study was undertaken to investigate the involvement of the RhoA signaling pathway in the secondary injury that follows traumatic spinal cord injury in rats. RhoA mRNA and protein expressions were enhanced significantly in the injured spinal cord 1 week after surgery (P<0.05, ANOVA). C3 exozyme (RhoA inhibitor), Y-27632 (selective Rho kinase inhibitor), and Fasudil (non-selective protein kinase inhibitor) were administered after spinal cord injury, and the subjects were evaluated for 5 weeks as per BBB locomotor score. Poor rat response interrupted the C3 experiment. Y-27632 slightly, but significantly (P<0.05, ANOVA), delayed the recovery. Fasudil significantly improved the BBB score (P<0.05, ANOVA). In conclusion, spinal cord injury activates the RhoA/Rho-kinase alpha, beta associated pathway. However, their role in secondary injury or in the improvement of functional recovery remains unclear. Fasudil might exert a cytoprotective effect by mechanisms other than inhibiting Rho-kinase alpha, beta.

Lysophosphatidic acid inhibits C-type natriuretic peptide activation of guanylyl cyclase-B

C-type natriuretic peptide (CNP), found in endothelial cells, chondrocytes, and neurons, binds its cognate transmembrane receptor, natriuretic peptide receptor-B (NPR-B/GC-B), and stimulates the synthesis of the intracellular signaling molecule, cGMP. The known physiologic consequences of this binding event are vasorelaxation, inhibition of cell proliferation, and the stimulation of long bone growth. Here we report that 10% fetal bovine serum markedly reduced CNP-dependent cGMP elevations in NIH3T3 fibroblast. The purified serum components platelet-derived growth factor and lysophosphatidic acid (LPA) mimicked the effect of serum on CNP-dependent cGMP elevations, but the latter factor resulted in the most dramatic reductions. The LPA-dependent inhibition was rapid and dose dependent, having t(1/2) and IC(50) values of approximately 5 min and 3.0 micro M LPA, respectively. The decreased cGMP concentrations resulted from reduced CNP-dependent NPR-B guanylyl cyclase activity that did not require losses in receptor protein or activation of protein kinase C, indicating a previously undescribed desensitization pathway. These data suggest that NPR-B is repressed by LPA and that one mechanism by which LPA exerts its effects is through the heterologous desensitization of the CNP/NPR-B/cGMP pathway. We hypothesize that cross-talk between the LPA and CNP signaling pathway maximizes the response of fibroblasts in the wound-healing process.

Sphingosine 1-phosphate induces cell contraction via calcium-independent/Rho-dependent pathways in undifferentiated skeletal muscle cells

We have previously shown that sphingosine 1-phosphate (S1P) can induce intracellular Ca(2+) mobilization and cell contraction in C2C12 myoblasts and that the two phenomena are temporally unrelated. Although Ca(2+)-independent mechanisms of cell contraction have been the focus of numerous studies on Ca(2+) sensitization of smooth muscle, comparatively less studies have focused on the role that these mechanisms play in the regulation of skeletal muscle contractility. Phosphorylation and activation of myosin by Rho-dependent kinase mediate most of Ca(2+)-independent contractile responses. In the present study, we examined the potential role of Rho/Rho-kinase cascade activation in S1P-induced C2C12 cell contraction. First, we showed that depletion of Ca(2+), by pre-treatment with BAPTA, did not affect S1P-induced myoblastic contractility, whereas it abolished S1P-induced Ca(2+) transients. These results correlated with the absence of troponin C and with the immature cytoskeletal organization of these cells. Experimental evidence demonstrating the involvement of Rho pathway in S1P-stimulated myoblast contraction included: the activation/translocation of RhoA to the membrane in response to agonist-stimulation in cells depleted of Ca(2+) and the inhibition of dynamic changes of the actin cytoskeleton in cells where Rho functions had been inhibited either by overexpression of RhoGDI, a physiological inhibitor of GDP dissociation from Rho proteins, or by pretreatment with Y-27632, a specific Rho kinase inhibitor. Contribution of protein kinase C in this cytoskeletal rearrangement was also evaluated. However, the pretreatment with Gö6976 or rottlerin, specific inhibitors of PKC alpha and PKC delta, respectively, failed to inhibit the agonist-induced myoblastic contraction. Single particle tracking of G-actin fluorescent probe was performed to statistically evaluate actin cytoskeletal dynamics in response to S1P. Stimulation with S1P was also able to increase the phosphorylation level of myosin light chain II. In conclusion, our results strongly suggest that Ca(2+)-independent/Rho-Rho kinase-dependent pathways may exert an important role in S1P-induced myoblastic cell contraction.

Cdc42 and Rac1 are necessary for autotaxin-induced tumor cell motility in A2058 melanoma cells

Autotaxin (ATX) is a strong motogen that can increase invasiveness and angiogenesis. In the present study, we investigated the signal transduction mechanism of ATX-induced tumor cell motility. Unlike N19RhoA expressing cells, the cells expressing N17Cdc42 or N17Rac1 showed reduced motility against ATX. ATX activated Cdc42 and Rac1 and increased complex formation between these small G proteins and p21-activated kinase (PAK). Furthermore, ATX phosphorylated focal adhesion kinase (FAK) that was not shown in cells expressing dominant negative mutants of Cdc42 or Rac1. Collectively, these data strongly indicate that Cdc42 and Rac1 are essential for ATX-induced tumor cell motility in A2058 melanoma cells, and that PAK and FAK might be also involved in the process.

Activation-regulated expression and chemotactic function of sphingosine 1-phosphate receptors in mouse splenic T cells

Sphingosine 1-phosphate (S1P) from platelets and macrophages stimulates migration and enhances survival of T cells. Mouse spleen CD4 and CD8 T cells are shown to express predominantly S1P1 (Edg-1) and S1P4 (Edg-6) G-protein-coupled receptors with only minimal representation of S1P2, S1P3, and S1P5. At and below plasma concentrations of healthy mammals (1 nM-1 microM), S1P evokes trans-Matrigel chemotaxis of mouse CD4 and CD8 T cells and recruits T cells into subcutaneous air pouches. T cell receptor-mediated activation of CD4 T cells suppresses expression of S1P1 and S1P4 receptors and eliminates their chemotactic responses to S1P. The immunoregulator FTY720, a structural homologue of S1P, lacks T cell chemotactic activity and competitively inhibits T cell chemotactic responses to S1P in vitro and in vivo. S1P may be a distinctive contributor to compartmental immunity by attracting naïve and memory T cells preferentially over activated effector T cells.

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.

A lysophosphatidic acid analogue is revealed as a potent inhibitor of phosphatidylcholine synthesis, inducing apoptosis

A previous study demonstrated that cross-desensitization experiments performed with the lysophosphatidic acid (LPA) analogues (R)- and (S)-N-palmitoyl-norleucinol 1-phosphate (PNPAs) inhibited LPA-induced platelet aggregation without any stereospecificity. Here we report opposite biological effects of the two enantiomers on mitogenesis of IMR-90 fibroblasts in relation to their respective metabolism. (R)PNPA was proliferative, while (S)PNPA induced apoptosis by specifically inhibiting phosphatidylcholine biosynthesis at the last step of the CDP-choline pathway controlled by cholinephosphotransferase. This effect was not direct but required dephosphorylation of PNPAs by ecto-lipid phosphate phosphatase before cellular uptake of the generated N-palmitoyl-norleucinols (PNOHs). Inhibition of cholinephosphotransferase by the derivative (S)PNOH was confirmed by an in vitro assay. (S)PNPA proapoptotic effects led us to clarify the mechanism linking cholinephosphotransferase inhibition to apoptosis. Three proapoptotic responses were observed: the activation of caspase-3, the production of ceramides from newly synthesized pools (as demonstrated by the inhibitor Fumonisin B1) and finally the activation of stress-activated protein kinase, p38 and c-Jun N-terminal kinases 1/2, as a result of ceramide increase. Thus our data demonstrate that synthetic analogues of LPA might display stereospecific effects leading to apoptosis independently of classical LPA-activated pathways.

Purification and properties of a phospholipase A2/lipase preferring phosphatidic acid, bis(monoacylglycerol) phosphate, and monoacylglycerol from rat testis

Phospholipase A(2) (PLA(2)) was purified to homogeneity from the supernatant fraction of rat testis homogenate. The purified 63-kDa enzyme did not require Ca(2+) ions for activity and exhibited both phosphatidic acid-preferring PLA(2) and monoacylglycerol lipase activities with a modest specificity toward unsaturated acyl chains. Anionic detergents enhanced these activities. Serine-modifying irreversible inhibitors, (p-amidinophenyl) methanesulfonyl fluoride and methylarachidonyl fluorophosphonate, inhibited both activities to a similar extent, indicating a single active site is involved in PLA(2) and lipase activities. The sequence of NH(2)-terminal 12 amino acids of purified enzyme was identical to that of a carboxylesterase from rat liver. The optimal pH for PLA(2) activity (around 5.5) differed from that for lipase activity (around 8.0). At pH 5.5 the enzyme also hydrolyzed bis(monoacylglycerol) phosphate, or lysobisphosphatidic acid (LBPA), that has been hitherto known as a secretory PLA(2)-resistant phospholipid and a late endosome marker. LBPA-enriched fractions were prepared from liver lysosome fractions of chloroquine-treated rats, treated with excess of pancreatic PLA(2), and then used for assaying LBPA-hydrolyzing activity. LBPA and the reaction products were identified by microbore normal phase high performance liquid chromatography/electrospray ionization ion-trap mass spectrometry. These enzymatic properties suggest that the enzyme can metabolize phosphatidic and lysobisphosphatidic acids in cellular acidic compartments.

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]

Sphingosine 1-phosphate is a ligand of the human gpr3, gpr6 and gpr12 family of constitutively active G protein-coupled receptors

Five G protein-coupled receptors (GPCRs) for the lysophospholipid sphingosine 1-phosphate (S1P) have been cloned and characterized so far. We report here about the identification of gpr3, gpr6 and gpr12 as additional members of the S1P-GPCR family. When expressed transiently in HEK293 cells, gpr3, gpr6 and gpr12 confer constitutive activation of adenylate cyclase (AC) similar in amplitude to that seen with fully activated G(alpha)(s)-coupled receptors. Culturing the transfected cells in medium with charcoal-stripped serum (devoid of lipids) significantly reduces cyclic adenosine monophosphate (cAMP) levels, suggesting a lipid-like ligand. A library containing 200 bioactive lipids was applied in functional assays recording intracellular Ca(2+) mobilization. S1P and dihydrosphingosine 1-phosphate (DHS1P) were identified as functional activators exhibiting nanomolar EC(50) values. In the presence of the S1P and LPA receptor antagonist suramin, gpr3-, gpr6- and gpr12-mediated intracellular Ca(2+) mobilization via S1P is enhanced. Besides constitutive activation of G(alpha)(s) type of G proteins, all three receptors are capable of constitutively activating inhibitory G(alpha)(i/o) proteins: (i) in the presence of pertussis toxin, gpr3-, gpr6- and gpr12-mediated stimulation of AC is enhanced; and (ii) overexpression of G(alpha)(i) significantly reduces the stimulatory action on intracellular cAMP levels. Agonist (S1P)-mediated internalization can be visualized in intact HEK293 cells using a gpr6 green fluorescent protein (GFP) fusion protein. In summary, our data suggest that gpr3, gpr6 and gpr12 are a family of constitutively active receptors with dual coupling to G(alpha)(s) and G(alpha)(i) type of G proteins. Constitutive activation of AC and mobilization of [Ca(2+)](i) can be modulated by the sphingophospholipids S1P and DHS1P, adding three additional members to the family of S1P receptors.

Cutting edge: suppression of T cell chemotaxis by sphingosine 1-phosphate

Murine CD4 and CD8 T cells express predominantly types 1 and 4 sphingosine 1-phosphate (S1P) G protein-coupled receptors (designated S1P1 and S1P4 or previously endothelial differentiation gene-encoded 1 and 6) for S1P, which has a normal plasma concentration of 0.1-1 microM. S1P now is shown to enhance chemotaxis of CD4 T cells to CCL-21 and CCL-5 by up to 2.5-fold at 10 nM to 0.1 microM, whereas 0.3-3 microM S1P inhibits this chemotaxis by up to 70%. Chemotaxis of S1P(1), but not S1P(4), transfectants to CXCL1 and CXCL4 was similarly affected by S1P. Activation of CD4 T cells, which decreases S1P receptor expression, suppressed effects of S1P on chemotaxis. Pretreatment of labeled CD4 T cells with S1P before reintroduction into mice inhibited by a maximum of 75% their migration into chemokine-challenged s.c. air pouches. The S1P-S1P(1) receptor axis thus controls recruitment of naive T cells by maintaining their response threshold to diverse lymphotactic factors.

Comparative analysis of human and rat S1P(5) (edg8): differential expression profiles and sensitivities to antagonists

Five guanine nucleotide-binding protein-coupled receptors (S1P(1-5)) for the lysophospholipid mediator sphingosine 1-phosphate (S1P) have thus far been described. Whereas tissue distribution and functional properties of the human S1P(1-4) genes are well characterized, only limited functional and expression data are available for S1P(5), todate. Northern blot analysis indicated that human S1P(5) (hS1P(5)) is an alternatively spliced gene, with a 5.4-kb transcript that is predominantly expressed in peripheral tissues, and a 2.4-kb transcript expressed in brain, spleen, and peripheral blood leucocytes. In contrast, rat S1P(5) (rS1P(5)) was exclusively detected in brain and skin. Expression of hS1P(5) and rS1P(5) in mammalian CHO-K1 or HEK293 cells conferred onto the cells the ability to mobilize intracellular calcium as determined by a functional Fluorometric Imaging Plate Reader assay, when challenged with S1P and dihydro S1P, respectively. Applying a lipid library with 200 bioactive lipids in a functional Fluorometric Imaging Plate Reader assay did not reveal additional agonists. However, both receptors exhibited differential sensitivity towards the S1P- and lysophosphatidic acid-receptor antagonist, suramin: rS1P(5)-mediated intracellular calcium mobilization was partly inhibited by suramin (IC(50): 5800 microM), whereas hS1P(5) was completely antagonized (IC(50): 130 microM). Both receptors were sensitive towards inhibition with the related drug (8,8'-(carbonylbis(imino-3,1-phenylene))bis(1,3,5-naphthalenetrisulfonic acid)) but IC(50) values differed significantly (340 microM for hS1P(5), 4000 microM for rS1P(5)). In addition, rS1P(5) displayed antiproliferative effects in transfected CHO-K1 and HEK293 cells in contrast to hS1P(5). Taken together, our data imply that differences between hS1P(5) and rS1P(5) will be an important point to be considered in the development of selective receptor antagonists.

Concise synthesis of acyl migration-blocked 1,1-difluorinated analogues of lysophosphatidic acid

Lysophosphatidic acid (LPA, 1- or 2-acyl-sn-glycerol 3-phosphate) is an important phospholipid mediator produced by activated platelets and by ovarian cancer cells. Efforts to understand LPA signaling through G-protein-coupled receptors are hampered by the facile acyl migration that results in equilibration to a mixture of the 1- or 2-acyl species under physiological conditions. We describe a new and efficient route to enantiomerically homogeneous lysophospholipid analogues from D-mannitol 1,2:5,6-bis-acetonide to give two 1,1-difluorodeoxy analogues of (2R)-acyl-sn-glycerol 3-phosphate. These compounds are migration-blocked analogues of the labile sn-2 LPA species. The (19)F NMR of diastereotopic fluorines of the difluoromethyl group shows an unexpected solvent dependence.

Characterization of lpa(2) (Edg4) and lpa(1)/lpa(2) (Edg2/Edg4) lysophosphatidic acid receptor knockout mice: signaling deficits without obvious phenotypic abnormality attributable to lpa(2)

Lysophosphatidic acid (LPA), a bioactive lipid produced by several cell types including postmitotic neurons and activated platelets, is thought to be involved in various biological processes, including brain development. Three cognate G protein-coupled receptors encoded by lpa(1)/lp(A1)/Edg-2/Gpcr26, lpa(2)/lp(A2)/Edg-4, and lpa(3)/lp(A3)/Edg-7 mediate the cellular effects of LPA. We have previously shown that deletion of lpa(1) in mice results in craniofacial dysmorphism, semilethality due to defective suckling behavior, and generation of a small fraction of pups with frontal hematoma. To further investigate the role of these receptors and LPA signaling in the organism, we deleted lpa(2) in mice. Homozygous knockout (lpa(2)((-/-))) mice were born at the expected frequency and displayed no obvious phenotypic abnormalities. Intercrosses allowed generation of lpa(1)((-/-)) lpa(2)((-/-)) double knockout mice, which displayed no additional phenotypic abnormalities relative to lpa(1)((-/-)) mice except for an increased incidence of perinatal frontal hematoma. Histological analyses of lpa(1)((-/-)) lpa(2)((-/-)) embryonic cerebral cortices did not reveal obvious differences in the proliferating cell population. However, many LPA-induced responses, including phospholipase C activation, Ca(2+) mobilization, adenylyl cyclase activation, proliferation, JNK activation, Akt activation, and stress fiber formation, were absent or severely reduced in embryonic fibroblasts derived from lpa(1)((-/-)) lpa(2)((-/-)) mice. Except for adenylyl cyclase activation [which was nearly abolished in lpa(1)((-/-)) fibroblasts], these responses were only partially affected in lpa(1)((-/-)) and lpa(2)((-/-)) fibroblasts. Thus, although LPA(2) is not essential for normal mouse development, it does act redundantly with LPA(1) to mediate most LPA responses in fibroblasts.

Astrocyte stellation in saline media lacking bicarbonate: possible relation to intracellular pH and tyrosine phosphorylation

Primary cultures of astrocytes exhibit a polygonal morphology, but on treatment with agents that increase cAMP they change to stellate cells. We found that astrocyte stellation also occurred on replacing the culture medium with saline buffered with HEPES. However, stellation did not occur when the medium was replaced with saline buffered with bicarbonate/CO(2) provided Ca(2+) was present. Since exposure of astrocytes to media lacking bicarbonate results in a decrease in intracellular pH (pH(i)) we sought evidence for an association between pH(i) and morphology. Astrocytic pH(i) was monitored for 60 min after transferring the cells to HEPES or bicarbonate-buffered saline. HEPES-induced stellation was associated with transient acidification which coincided with the morphological changes. Acidification was not observed in cells transferred to bicarbonate-saline. However when cytoplasmic acidification of cells in bicarbonate-saline was induced pharmacologically, rapid stellation occurred. Stellation induced by cAMP is reversed by activation of the RhoA pathway with lysophosphatidic acid (LPA). Here we found that LPA inhibited HEPES-induced stellation, but only with Ca(2+) present. Inhibition of stellation by LPA+Ca(2+) was associated with transient acidification followed by modest alkanization. A close association of tyrosine phosphorylation with stellation and pH(i) was observed. Thus incubation of astrocytes in HEPES-saline with orthovanadate to inhibit dephosphorylation abolished stellation and acidification; conversely incubation of cells in bicarbonate-saline with genistein to inhibit tyrosine kinases caused stellation and major acidification. Acidification may be one of several factors resulting in stellation, but it is not a necessary factor since stellation without acidification was observed in bicarbonate-saline lacking Ca(2+).

Dual regulation of actin rearrangement through lysophosphatidic acid receptor in neuroblast cell lines: actin depolymerization by Ca(2+)-alpha-actinin and polymerization by rho

Lysophosphatidic acid (LPA) is a potent lipid mediator with actions on many cell types. Morphological changes involving actin polymerization are mediated by at least two cognate G protein-coupled receptors, LPA(1)/EDG-2 or LPA(2)/EDG-4. Herein, we show that LPA can also induce actin depolymerization preceding actin polymerization within single TR mouse immortalized neuroblasts. Actin depolymerization resulted in immediate loss of membrane ruffling, whereas actin polymerization resulted in process retraction. Each pathway was found to be independent: depolymerization mediated by intracellular calcium mobilization, and alpha-actinin activity and polymerization mediated by the activation of the small Rho GTPase. alpha-Actinin-mediated depolymerization seems to be involved in growth cone collapse of primary neurons, indicating a physiological significance of LPA-induced actin depolymerization. Further evidence for dual regulation of actin rearrangement was found by heterologous retroviral transduction of either lpa(1) or lpa(2) in B103 cells that neither express LPA receptors nor respond to LPA, to confer both forms of LPA-induced actin rearrangements. These results suggest that diverging intracellular signals from a single type of LPA receptor could regulate actin depolymerization, as well as polymerization, within a single cell. This dual actin rearrangement may play a novel, important role in regulation of the neuronal morphology and motility during brain development.

LPA is a novel lipid regulator of mesangial cell hexokinase activity and HKII isoform expression

The prototypical extracellular phospholipid mediator, lysophosphatidic acid (LPA), exhibits growth factor-like properties and represents an important survival factor in serum. This potent mesangial cell mitogen is increased in conditions associated with glomerular injury. It is also a known activator of the classic mitogen-activated protein kinase (MAPK) pathway, which plays an important role in the regulation of mesangial cell hexokinase (HK) activity. To better understand the mechanisms coupling metabolism to injury, we examined the ability of LPA to regulate HK activity and expression in cultured murine mesangial cells. LPA increased total HK activity in a concentration- and time-dependent manner, with maximal increases of >50% observed within 12 h of exposure to LPA concentrations > or =25 microM (apparent ED(50) 2 microM). These effects were associated with increased extracellular signal-regulated kinase (ERK) activity and were prevented by the pharmacological inhibition of either MAPK/ERK kinase or protein kinase C (PKC). Increased HK activity was also associated with increased glucose (Glc) utilization and lactate accumulation, as well as selectively increased HKII isoform abundance. The ability of exogenous LPA to increase HK activity was both Ca2+ independent and pertussis toxin insensitive and was mimicked by LPA-generating phospholipase A2. We conclude that LPA constitutes a novel lipid regulator of mesangial cell HK activity and Glc metabolism. This regulation requires sequential activation of both Ca2+-independent PKC and the classic MAPK pathway and culminates in increased HKII abundance. These previously unrecognized metabolic consequences of LPA stimulation have both physiological and pathophysiological implications. They also suggest a novel mechanism whereby metabolism may be coupled to cellular injury via extracellular lipid mediators.

Interaction of G alpha(12) with G alpha(13) and G alpha(q) signaling pathways

The G(12) subfamily of heterotrimeric G-proteins consists of two members, G(12) and G(13). Gene-targeting studies have revealed a role for G(13) in blood vessel development. Mice lacking the alpha subunit of G(13) die around embryonic day 10 as the result of an angiogenic defect. On the other hand, the physiological role of G(12) is still unclear. To address this issue, we generated G alpha(12)-deficient mice. In contrast to the G alpha(13)-deficient mice, G alpha(12)-deficient mice are viable, fertile, and do not show apparent abnormalities. However, G alpha(12) does not seem to be entirely redundant, because in the offspring generated from G alpha(12)+/- G alpha(13)+/- intercrosses, at least one intact G alpha(12) allele is required for the survival of animals with only one G alpha(13) allele. In addition, G alpha(12) and G alpha(13) showed a difference in mediating cell migratory response to lysophosphatidic acid in embryonic fibroblast cells. Furthermore, mice lacking both G alpha(12) and G alpha(q) die in utero at about embryonic day 13. These data indicate that the G alpha(12)-mediated signaling pathway functionally interacts not only with the G alpha(13)- but also with the G alpha(q/11)-mediated signaling systems.

Lysophosphatidic acid protects and rescues intestinal epithelial cells from radiation- and chemotherapy-induced apoptosis

BACKGROUND & AIMS

We have investigated whether the phospholipid growth factor lysophosphatidic acid (LPA) could prevent intestinal epithelial cells-6 (IEC-6) from apoptosis elicited by 4 different mechanisms. The antiapoptotic effect of LPA was also tested in a mouse model of radiation-induced apoptosis.

METHODS

Apoptosis was elicited by serum withdrawal, exposure to camptothecin, gamma-irradiation, or rat tumor necrosis factor alpha and evaluated by DNA fragmentation enzyme-linked immunosorbent assay (ELISA) and annexin V staining. Caspase-3/CPP32 activity and activation was measured by ELISA and Western blotting, respectively. Reverse-transcription polymerase chain reaction (RT-PCR) was applied to examine the expression of LPA-receptor transcripts. Mice were treated with 250 microL of 1 mmol/L LPA and exposed to whole-body gamma-irradiation with a dose of 12 or 15 Gy and the number and localization of apoptotic bodies along the crypt were recorded.

RESULTS

LPA pretreatment reduced DNA fragmentation induced in all models of apoptosis. LPA rescued cells from apoptosis when applied up to 1 hour after camptothecin treatment or 2 hours after irradiation. LPA inhibited the activation of caspase-3/CPP32 and attenuated its activity. Blocking LPA1 receptors by pertussis toxin and the inhibition of epithelial growth factor receptor tyrosine kinase significantly attenuated the protective effect. In irradiated mice, oral LPA significantly reduced the number of apoptotic bodies in the crypt.

CONCLUSIONS

(1) LPA prevents and rescues IEC-6 from apoptosis elicited by 4 different mechanisms. (2) This antiapoptotic activity is mediated through LPA1 and LPA2 receptors through the inhibition of caspase-3/CPP32 activation. (3) LPA protects enterocytes against radiation-induced apoptosis. This study suggests that in patients undergoing cancer therapy, dietary LPA might have therapeutically useful antiapoptotic capacity in the intestinal epithelium.

5-Hydroxytryptamine 4(a) receptor is coupled to the Galpha subunit of heterotrimeric G13 protein

Serotonin (5-hydroxytryptamine (5-HT)) is an important neurotransmitter that regulates multiple events in the central nervous system. Many of the 5-HT functions are mediated via G protein-coupled receptors that are coupled to multiple heterotrimeric G proteins, including G(s), G(i), and G(q) subfamilies (Martin, G. R., Eglen, R. M., Hamblin, M. W., Hoyer, D., and Yocca, F. (1998) Trends Pharmacol. Sci. 19, 2-4). Here we show for the first time that the 5-hydroxytryptamine 4(a) receptor (5-HT(4(a))) is coupled not only to heterotrimeric G(s) but also to G(13) protein, as assessed both by biochemical and functional assays. Using reconstitution of 5-HT(4(a)) receptor with different G proteins in Spodoptera frugiperda (Sf.9) cells, we have proved that agonist stimulation of receptor-induced guanosine 5'-(3-O-thio)triphosphate binding to Galpha(13) protein. We then determined that expression of 5-HT(4(a)) receptor in mammalian cells induced constitutive- as well as agonist-promoted activation of a transcription factor, serum response element, through the activation of Galpha(13) and RhoA. Finally, we have determined that expression of 5-HT(4(a)) receptor in neuroblastoma x glioma NIE-115 cells cause RhoA-dependent neurite retraction and cell rounding under basal conditions and after agonist stimulation. These data suggest that by activating 5-HT(4(a)) receptor-G(13) pathway, serotonin plays a prominent role in regulating neuronal architecture in addition to its classical role in neurotransmission.

Sphingosine 1-phosphate induces Ca2+ transients and cytoskeletal rearrangement in C2C12 myoblastic cells

In many cell systems, sphingosine 1-phosphate (SPP) increases cytosolic Ca2+ concentration ([Ca2+]i) by acting as intracellular mediator and extracellular ligand. We recently demonstrated (Meacci E, Cencetti F, Formigli L, Squecco R, Donati C, Tiribilli B, Quercioli F, Zecchi-Orlandini S, Francini F, and Bruni P. Biochem J 362: 349-357, 2002) involvement of endothelial differentiation gene (Edg) receptors (Rs) specific for SPP in agonist-mediated Ca2+ response of a mouse skeletal myoblastic (C2C12) cell line. Here, we investigated the Ca2+ sources of SPP-mediated Ca2+ transients in C2C12 cells and the possible correlation of ion response to cytoskeletal rearrangement. Confocal fluorescence imaging of C2C12 cells preloaded with Ca2+ dye fluo 3 revealed that SPP elicited a transient Ca2+ increase propagating as a wave throughout the cell. This response required extracellular and intracellular Ca2+ pool mobilization. Indeed, it was significantly reduced by removal of external Ca2+, pretreatment with nifedipine (blocker of L-type plasma membrane Ca2+ channels), and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]-mediated Ca2+ pathway inhibitors. Involvement of EdgRs was tested with suramin (specific inhibitor of Edg-3). Fluorescence associated with Ins(1,4,5)P3Rs and L-type Ca2+ channels was evident in C2C12 cells. SPP also induced C2C12 cell contraction. This event, however, was unrelated to [Ca2+]i increase, because the two phenomena were temporally shifted. We propose that SPP may promote C2C12 cell contraction through Ca2+-independent mechanisms.

Lysophosphatidic acid influences the morphology and motility of young, postmitotic cortical neurons

Lysophosphatidic acid (LPA) is a bioactive lysophospholipid that produces process retraction and cell rounding through its cognate receptors in neuroblastoma cell lines. Although the expression profile of LPA receptors in developing brains suggests a role for LPA in central nervous system (CNS) development, how LPA influences the morphology of postmitotic CNS neurons remains to be determined. Here we have investigated the effects of exogenous LPA on the morphology of young, postmitotic neurons in primary culture. When treated with LPA, these neurons responded by not only retracting processes but also producing retraction fiber "caps" characterized by fine actin filaments emanating from a dense core. Retraction fiber caps gradually vanished due to the outward spread of regrowing membranes along the fibers, suggesting a role for caps as scaffolds for regrowth of retracted processes. Furthermore, LPA also affects neuronal migration in vitro and in vivo. Taken together, these results implicate LPA as an extracellular lipid signal affecting process outgrowth and migration of early postmitotic neurons during development.

ANG II and LPA induce Pyk2 tyrosine phosphorylation in intestinal epithelial cells: role of Ca2+, PKC, and Rho kinase

The G protein-coupled receptor agonists angiotensin II (ANG II) and lysophosphatidic acid (LPA) rapidly induce tyrosine phosphorylation of the cytosolic proline-rich tyrosine kinase 2 (Pyk2) in IEC-18 intestinal epithelial cells. The combined Pyk2 tyrosine phosphorylation induced by phorbol 12,13-dibutyrate, a direct agonist of protein kinase C (PKC), and ionomycin, a Ca2+ ionophore, was equal to that induced by ANG II. Inhibition of either PKC or Ca2+ signaling attenuated the effect of ANG II and LPA, although simultaneous inhibition of both pathways failed to completely abolish Pyk2 tyrosine phosphorylation. Cytochalasin D, which disrupts stress fibers, strongly inhibited the response of Pyk2 to ANG II or LPA. The distinct Rho-associated kinase (ROK) inhibitors HA-1077 and Y-27632, as well as the Rho inhibitor Clostridium botulinum C3 exoenzyme, also significantly attenuated ANG II- and LPA-stimulated Pyk2 tyrosine phosphorylation. Simultaneous inhibition of PKC, Ca2+, and either actin assembly or ROK completely abolished the Pyk2 response. Together, these results show that ANG II and LPA rapidly induce Pyk2 tyrosine phosphorylation in intestinal epithelial cells via separate Ca2+-, PKC-, and Rho-mediated pathways.

Molecular basis for lysophosphatidic acid receptor antagonist selectivity

Recent characterization of lysophosphatidic acid (LPA) receptors has made possible studies elucidating the structure-activity relationships (SAR) for agonist activity at individual receptors. Additionally, the availability of these receptors has allowed the identification of antagonists of LPA-induced effects. Two receptor-subtype selective LPA receptor antagonists, one selective for the LPA1/EDG2 receptor (a benzyl-4-oxybenzyl N-acyl ethanolamide phosphate, NAEPA, derivative) and the other selective for the LPA3/EDG7 receptor (diacylglycerol pyrophosphate, DGPP, 8:0), have recently been reported. The receptor SAR for both agonists and antagonists are reviewed, and the molecular basis for the difference between agonism and antagonism as well as for receptor-subtype antagonist selectivity identified by molecular modeling is described. The implications of the newly available receptor-subtype selective antagonists are also discussed.

G protein-coupled receptor signalling and cross-talk: achieving rapidity and specificity

Activation of a given type of G protein-coupled receptor (GPCR) triggers a limited set of signalling events in a very rapid and specific manner. The classical paradigm of GPCR signalling was rather linear and sequential. Emerging evidence, however, has revealed that this is only a part of the complex signalling mediated by GPCR. Propagation of GPCR signalling involves cross-regulation of many but specific pathways, including cross-talks between different GPCRs as well as with other signalling pathways. Moreover, it is increasingly apparent that GPCRs can activate both heterotrimeric G protein-dependent and G protein-independent signalling pathways. In this review, we discuss how the signallings initiated by GPCRs achieve rapidity as well as specificity, and how the GPCRs can cross-regulate other specific signalling pathways at the same time. New concepts regarding GPCR signalling have been arising to address this issue, which include multiprotein signalling complex and signalling compartment in microdomain concepts that enable close colocalization or even contact among the proteins engaged in the specific signal transduction. The final outcome of a stimulation of GPCR will thus be the sum of its own specific set of intracellular signalling pathways it regulates.

Inhibition of rho-kinase-induced myristoylated alanine-rich C kinase substrate (MARCKS) phosphorylation in human neuronal cells by H-1152, a novel and specific Rho-kinase inhibitor

The functions of small G protein Rho-associated kinase (Rho-kinase) have been determined in muscle and non-muscle cells, but, particularly in neuronal cells, its effector(s) has not been well known. Recently, we preliminarily reported that Rho-kinase phosphorylates the Ser159 residue in myristoylated alanine-rich C kinase substrate (MARCKS) in vitro, but it remains obscure in vivo. To further clarify this point, we developed an isoquinolinesulfonamide derivative, H-1152, that is a more specific, stronger and membrane-permeable inhibitor of Rho-kinase with a Ki value of 1.6 nM, but poor inhibitor of other serine/threonine kinases. H-1152 dose-dependently inhibited the phosphorylation of MARCKS in human neuroteratoma (NT-2) cells stimulated by Rho-activator lysophosphatidic acid (LPA), which was determined by phosphorylation site-specific antibody against phospho-Ser159 in MARCKS, whereas it hardly inhibited the phosphorylation stimulated by phorbol-12,13-dibutyrate (PDBu). In contrast, two other Rho-kinase inhibitors, HA-1077 at 30 microM and Y-27632 at 10-30 microM, inhibited the phosphorylation of MARCKS in the cells stimulated by LPA and PDBu. A PKC inhibitor Ro-31-8220 selectively inhibited PDBu-induced phosphorylation of MARCKS. Taken together with our previous results, the present findings strongly suggest that Rho/Rho-kinase phosphorylates MARCKS at Ser159 residue in neuronal cells in response to LPA stimulation and that H-1152 is a useful tool to confirm Rho-kinase function(s) in cells and tissues.

Lysophosphatidic acid stimulates proliferation of cultured smooth muscle cells from human BPH tissue: sildenafil and papaverin generate inhibition

BACKGROUND

The endogenous substance lysophosphatidic acid (LPA) has been found to generate proliferation of cultured smooth muscle cells (SMC). Therefore, the effect of LPA on human benign prostate hyperplasia (BPH) could be of interest.

METHODS

The proliferative effect of LPA on cultured human prostatic SMC from specimens obtained at trans-urethral resection of the prostate (TURP) because of BPH, was analyzed by [3H]-thymidine and [35S]-methionine incorporation. In addition, LPA stimulated BPH SMC were treated with papaverin, forskolin, sildenafil or zaprinast, well known to increase the intracellular level of cAMP or cGMP.

RESULTS

LPA produced a dose-dependent increase in BPH SMC, both regarding DNA- and protein-synthesis with EC50 values of 3 and 10 microM, respectively. Furthermore, both papaverin, a general phosphodiesterase inhibitor regarding cAMP hydrolyzes, and forskolin, an adenylyl cyclase stimulating agent, inhibited the LPA-stimulated DNA replication in a dose dependent manner with IC50 = 2.5, and 0.35 microM, respectively. cGMP increasing agents, such as the NO-donors SIN-1 and SNAP, produced a weak anti-proliferative response. However, both phosphodiesterase 5 inhibitors sildenafil (Viagra) and zaprinast efficiently blocked DNA replication. In addition, when the protein synthesis was examined, we found that the LPA response was significantly inhibited by forskolin and papaverin.

CONCLUSIONS

The major conclusion of this investigation is that the endogenous serum component LPA, is able to promote human BPH SMC growth. In addition, our study indicates that cyclic nucleotides can inhibit this effect. Future clinical studies will be needed to determine if different specific phosphodiesterase inhibitors per se or in combination could represent a new therapeutic possibility for the treatment of BPH.

Protein kinases as targets for anticancer agents: from inhibitors to useful drugs

Many components of mitogenic signaling pathways in normal and neoplastic cells have been identified, including the large family of protein kinases, which function as components of signal transduction pathways, playing a central role in diverse biological processes, such as control of cell growth, metabolism, differentiation, and apoptosis. The development of selective protein kinase inhibitors that can block or modulate diseases caused by abnormalities in these signaling pathways is widely considered a promising approach for drug development. Because of their deregulation in human cancers, protein kinases, such as Bcr-Abl, those in the epidermal growth factor-receptor (HER) family, the cell cycle regulating kinases such as the cyclin-dependent kinases, as well as the vascular endothelial growth factor-receptor kinases involved in the neo-vascularization of tumors, are among the protein kinases considered as prime targets for the development of selective inhibitors. These drug-discovery efforts have generated inhibitors and low-molecular weight therapeutics directed against the ATP-binding site of various protein kinases that are in various stages of development (up to Phase II/III clinical trials). Three examples of inhibitors of protein kinases are reviewed, including low-molecular weight compounds targeting the cell cycle kinases; a potent and selective inhibitor of the HER1/HER2 receptor tyrosine kinase, the pyrollopyrimidine PKI166; and the 2-phenyl-aminopyrimidine STI571 (Glivec(R), Gleevec) a targeted drug therapy directed toward Bcr-Abl, the key player in chronic leukemia (CML). Some members of the HER family of receptor tyrosine kinases, in particular HER1 and HER2, have been found to be overexpressed in a variety of human tumors, suggesting that inhibition of HER signaling would be a viable antiproliferative strategy. The pyrrolo-pyrimidine PKI166 was developed as an HER1/HER2 inhibitor with potent in vitro antiproliferative and in vivo antitumor activity. Based upon its clear association with disease, the Bcr-Abl tyrosine kinase in CML represents the ideal target to validate the clinical utility of protein kinase inhibitors as therapeutic agents. In a preclinical model, STI571 (Glivec(R), Gleevec) showed potent in vitro and in vivo antitumor activity that was selective for Abl, c-Kit, and the platelet-derived growth factor-receptor. Phase I/II studies demonstrated that STI571 is well tolerated, and that it showed promising hematological and cytogenetic responses in CML and clinical responses in the c-Kit-driven gastrointestinal tumors.

The novel and specific Rho-kinase inhibitor (S)-(+)-2-methyl-1-[(4-methyl-5-isoquinoline)sulfonyl]-homopiperazine as a probing molecule for Rho-kinase-involved pathway

We have developed several kinds of protein kinase inhibitors, which are classified as isoquinolinesulfonamides and characterized as ATP competitive inhibitors of Ser/Thr protein kinases. These include H9, H89, KN62, and 1-(5-isoquinolinesulfonyl)-homopiperazine (HA-1077) against protein kinase C (PKC), protein kinase A, Ca(2+)/calmodulin-dependent protein kinase II, and Rho-kinase, respectively, and they have been used widely to confirm the involvement of the target protein kinase in biological or physiological reaction(s). In some cases, inhibitors have predicted the involvement of the target protein kinase in cell or tissue before its precise mechanism or its effector was defined. On a clinical level, we developed the Rho-kinase inhibitor HA-1077 as an anti-spastic that effectively suppresses the spasm of cerebral arteries after subarachnoid hemorrhage. We have improved HA-1077 to obtain (S)-(+)-2-methyl-1-[(4-methyl-5-isoquinoline)sulfonyl]-homopiperazine (H-1152P), which is a more selective inhibitor of Rho-kinase, with a K(i) value of 1.6 nM for Rho-kinase, 630 nM for protein kinase A, and 9270 nM for PKC. This inhibitor suppressed the phosphorylation of myristoylated alanine-rich C-kinase substance (MARCKS) in neuronal cells stimulated with lysophosphatidic acid, whose phosphorylation site was confirmed to be the Ser159 residue, using a phosphorylation site-specific antibody. In contrast, phorbol 12-myristate 13-acetate-induced phosphorylation of MARCKS was scarcely inhibited by H-1152P. Furthermore, lysophosphatidic acid-stimulated phosphorylation in neuronal cells was characterized as a C3 toxin-sensitive event. Our results show that the Rho-kinase inhibitor targets a protein with a well-known function, MARCKS in neuronal cells. Although MARCKS is widely recognized as a substrate of PKC, our results raise the possibility that MARCKS is a target protein of Rho-kinase in neuronal cells. In this review, we address the possible role of Rho-kinase in neuronal functions, using the Rho-kinase specific inhibitor H-1152P.

Phospholipase D stimulation is required for sphingosine-1-phosphate activation of actin stress fibre assembly in human airway epithelial cells

In human airway epithelial cells, sphingosine-1-phosphate (SPP) and lysophosphatidic acid (LPA) stimulated the production of phosphatidic acid (PA), which was inhibited by the primary alcohol butan-1-ol, but not by the inactive butan-2-ol, clearly indicating phospholipase D (PLD) involvement. Both SPP and LPA stimulated actin stress fibre formation, which was also butan-2-ol-insensitive and inhibited by butan-1-ol. SPP-induced PLD activation and cytoskeletal remodelling were insensitive to brefeldin A and toxin B from Clostridium difficile, which conversely blocked the effect of LPA, suggesting that the monomeric GTPases ADP ribosylation factor (ARF) and Rho are involved in LPA, but not in SPP responses. Pertussis toxin inhibited SPP- but not LPA-induced effects. PLD activation and stress fibre formation by both lysolipids were abolished by the tyrosine kinase inhibitor genistein. Addition of PA to cells caused a massive stress fibre assembly. In conclusion, PLD is one of the signalling components linking SPP-receptor activation to assembly of actin stress fibres.

Proteolytic inhibition of Salmonella enterica serovar typhimurium-induced activation of the mitogen-activated protein kinases ERK and JNK in cultured human intestinal cells

Bromelain, a mixture of cysteine proteases from pineapple stems, blocks signaling by the mitogen-activated protein (MAP) kinases extracellular regulated kinase 1 (ERK-1) and ERK-2, inhibits inflammation, and protects against enterotoxigenic Escherichia coli infection. In this study, we examined the effect of bromelain on Salmonella enterica serovar Typhimurium infection, since an important feature of its pathogenesis is its ability to induce activation of ERK-1 and ERK-2, which leads to internalization of bacteria and induction of inflammatory responses. Our results show that bromelain dose dependently blocks serovar Typhimurium-induced ERK-1, ERK-2, and c-Jun NH(2)-terminal kinase (JNK) activation in Caco-2 cells. Bromelain also blocked signaling induced by carbachol and anisomycin, pharmacological MAP kinase agonists. Despite bromelain inhibition of serovar Typhimurium-induced MAP kinase signaling, it did not prevent subsequent invasion of the Caco-2 cells by serovar Typhimurium or alter serovar Typhimurium -induced decreases in resistance across Caco-2 monolayers. Surprisingly, bromelain also did not block serovar Typhimurium-induced interleukin-8 (IL-8) secretion but synergized with serovar Typhimurium to enhance IL-8 production. We also found that serovar Typhimurium does not induce ERK phosphorylation in Caco-2 cells in the absence of serum but that serovar Typhimurium-induced invasion and decreases in monolayer resistance are unaffected. Collectively, these data indicate that serovar Typhimurium-induced invasion of Caco-2 cells, changes in the resistance of epithelial cell monolayers, and IL-8 production can occur independently of the ERK and JNK signaling pathways. Data also confirm that bromelain is a novel inhibitor of MAP kinase signaling pathways and suggest a novel role for proteases as inhibitors of signal transduction pathways in intestinal epithelial cells.