Lysophospholipids--receptor revelations

Point-counterpoint of sphingosine 1-phosphate metabolism

Sphingosine 1-phosphate (S1P), an evolutionarily conserved bioactive lipid mediator, is now recognized as a potent modulator of cell regulation. In vertebrates, S1P interacts with cell surface G protein-coupled receptors of the EDG family and induces profound effects in a variety of organ systems. Indeed, an S1P receptor agonist is undergoing clinical trials to combat immune-mediated transplant rejection. Recent information on S1P receptor biology suggests potential utility in the control of cardiovascular processes, including angiogenesis, vascular permeability, arteriogenesis, and vasospasm. However, studies from diverse invertebrates, such as yeast, Dictyostelium, Drosophila, and Caenorhabditis elegans have shown that S1P is involved in important regulatory functions in the apparent absence of EDG S1P receptor homologues. Metabolic pathways of S1P synthesis, degradation, and release have recently been described at the molecular level. Genetic and biochemical studies of these enzymes have illuminated the importance of S1P signaling systems both inside and outside of cells. The revelation of receptor-dependent pathways, as well as novel metabolic/intracellular pathways has provided new biological insights and may ultimately pave the way for the development of novel therapeutic approaches for cardiovascular diseases.

PAM mediates sustained inhibition of cAMP signaling by sphingosine-1-phosphate

PAM (Protein Associated with Myc) is an almost ubiquitously expressed protein that is one of the most potent inhibitors of adenylyl cyclase activity known so far. Here we show that PAM is localized at the endoplasmic reticulum in HeLa cells and that upon serum treatment PAM is recruited to the plasma membrane, causing an inhibition of adenylyl cyclase activity. We purified the serum factor that induced PAM translocation and identified it as sphingosine-1-phosphate (S1P). Within 15 min after incubation with S1P, PAM appeared at the plasma membrane and was detectable for up to 120 min. Sphingosine-1-phosphate induced adenylyl cyclase inhibition in two phases: an initial (1-10 min) and a late (20-240 min) phase. The initial adenylyl cyclase inhibition was Gi-mediated and PAM independent. In the late phase, adenylyl cyclase inhibition was PAM dependent and attenuated cyclic AMP (cAMP) signaling by various cAMP-elevating signals. This makes PAM the longest lasting nontranscriptional regulator of adenylyl cyclase activity known to date and presents a novel mechanism for the temporal regulation of cAMP signaling.

FTY720: sphingosine 1-phosphate receptor-1 in the control of lymphocyte egress and endothelial barrier function

The novel immunomodulator FTY720 is effective in experimental models of transplantation and autoimmunity, and is currently undergoing Phase III clinical trials for prevention of kidney graft rejection. In contrast to conventional immunosuppressants, FTY720 does not impair T- and B-cell activation, proliferation and effector function, but interferes with cell traffic between lymphoid organs and blood. The molecular basis for the mode of action of the drug has only recently been established. FTY720, after phosphorylation, acts as a high-affinity agonist at the G protein-coupled sphingosine 1-phosphate receptor-1 (S1P(1)) on thymocytes and lymphocytes, thereby inducing aberrant internalization of the receptor. This renders the cells unresponsive to the serum lipid sphingosine 1-phosphate (S1P), depriving them from an obligatory signal to egress from lymphoid organs. As a consequence, lymphocytes are unable to recirculate to peripheral inflammatory tissues and graft sites but remain functional in the lymphoid compartment. In addition to the effects on lymphocyte recirculation, the drug acts on endothelial cells and preserves vascular integrity by enhancing adherens junction assembly and endothelial barrier function. The available data establish S1P(1) as a key target for FTY720, and further point to therapeutically relevant effects of the drug on lymphocytes and vascular endothelium.

Translocation of lysophosphatidic acid phosphatase in response to gonadotropin-releasing hormone to the plasma membrane in ovarian cancer cell

OBJECTIVE

Lysophosphatidic acid mediates proliferative and/or morphologic effects on multiple cell lineages, which include ovarian cancer cells. Lysophosphatidic acid hydrolysis limits the duration of lysophosphatidic acid's action. We examined hormonal translocation of lipid phosphate phosphatase type 3 to the plasma membrane in gonadotropin-releasing hormone-responsive ovarian cancers.

STUDY DESIGN

Ovarian cancers that were removed surgically and the ovarian cancer cell lines Caov-3 and SK-OV-3 were examined. Lipid phosphate phosphatase type 3 protein and activity in plasma membranes were assessed by immunohistochemical staining with lipid phosphate phosphatase type 3-specific antibodies and by the measurement of the conversion of exogenous [(3)H-oleoyl]lysophosphatidic acid to mono[(3)H-oleoyl]glycerol, respectively.

RESULTS

In ovarian cancers that were removed surgically, the cell surface staining and activity measurements indicated that a portion of the enzyme was localized to the plasma membrane. In Caov-3 cells and SK-OV-3 cells, lipid phosphate phosphatase type 3 protein was present both in the cytoplasm and at the plasma membrane. Treatment of the cells with a gonadotropin-releasing hormone agonist buserelin produced a rapid and progressive translocation of lipid phosphate phosphatase type 3 protein to the plasma membrane, with a concomitant loss of cytoplasmic staining. The enzyme activity in plasma membrane was also increased when the cell lines were exposed to the gonadotropin-releasing hormone agonist in intact cells before the assay of the cell membranes.

CONCLUSION

These findings support the presence of lipid phosphate phosphatase type 3 in plasma membrane of ovarian cancers and provide for the ability of agonists (such as gonadotropin-releasing hormone) to induce the translocation of lipid phosphate phosphatase type 3 to plasma membrane in ovarian cancer cells.

Reactions of the diiron enzyme stearoyl-acyl carrier protein desaturase

Stearoyl-acyl carrier protein Delta(9) desaturase (Delta9D) produces oleic acid, a nutritionally valuable fatty acid containing a cis double bond between C-9 and C-10. This multiprotein diiron enzyme complex reacts with stearoyl-acyl carrier protein, reduced [2Fe-2S] ferredoxin, and O(2) to complete the highly regiospecific and stereoselective desaturation reaction. Interactions with the acyl chain provide stability to the enzyme-substrate complex, give an energetic contribution to catalytic selectivity, and help to order the electron transfer, O(2) binding, and C-H bond cleavage steps of catalysis. Reactions with natural acyl chains indicate the involvement of a highly reactive diiron intermediate capable of oxidizing secondary C-H bonds (bond dissociation energy approximately 95 kcal/mol), but also capable of diagnostic O-atom transfer reactions with the appropriate substrate analogues. For soluble Delta9D, the natural reaction may initiate at the C-10 position, in contrast to the well-established initial reactivity of the membrane enzyme homologue stearoyl-coenzyme A (CoA) Delta(9) desaturase at the C-9 position.

Molecular cloning and expression regulation of PRG-3, a new member of the plasticity-related gene family

Phospholipid-mediated signalling on neurons provokes diverse responses such as neurogenesis, pattern formation and neurite remodelling. We have recently uncovered a novel set of molecules in the mammalian brain, named plasticity-related genes (PRGs), which mediate lipid phosphate phosphatase activity and provide evidence for their involvement in mechanisms of neuronal plasticity. Here, we report on a new member of the vertebrate-specific PRG family, which we have named plasticity-related gene-3 (PRG-3). PRG-3 is heavily expressed in the brain and shows a specific expression pattern during brain development where PRG-3 expression is found predominantly in neuronal cell layers and is already expressed at embryonic day 16. In the mature brain, strongest PRG-3 expression occurs in the hippocampus and cerebellum. Overexcitation of neurons induced by kainic acid leads to a transient down-regulation of PRG-3. Furthermore, PRG-3 is expressed on neurite extensions and promotes neurite growth and a spreading-like cell body in neuronal cells and COS-7 cells. In contrast to previously described members of the PRG family, PRG-3 does not perform its function through enzymatic phospholipid degradation. In summary, our findings feature a new member of the PRG family which shows dynamic expression regulation during brain development and neuronal excitation.

Regioselective cis-trans isomerization of arachidonic double bonds by thiyl radicals: the influence of phospholipid supramolecular organization

Trans unsaturated fatty acids in humans may be originated by two different contributions. The exogenous track is due to dietary supplementation of trans fats and the endogenous path deals with free-radical-catalyzed cis-trans isomerization of fatty acids. Arachidonic acid residue (5c,8c,11c,14c-20:4), which has only two out of the four double bonds deriving from the diet, was used to differentiate the two paths and to assess the importance of a radical reaction. A detailed study on the formation of trans phospholipids catalyzed by the HOCH2CH2S* radical was carried out on L-alpha-phosphatidylcholine from egg lecithin and 1-stearoyl-2-arachidonoyl-L-alpha-phosphatidylcholine (SAPC) in homogeneous solution or in large unilamellar vesicles (LUVET). Thiyl radicals were generated from the corresponding thiol by either gamma-irradiation or UV photolysis, and the reaction course was followed by GC, Ag/TLC, and 13C NMR analyses. The isomerization was found to be independent of cis double bond location (random process) in i-PrOH solution. In the case of vesicles, the supramolecular organization of lipids produced a dramatic change of the isomerization outcome: (i) in egg lecithin, the reactivity of arachidonate moieties is higher than that of oleate and linoleate residues, (ii) in the linoleate residues of egg lecithin, the 9t,12c-18:2 isomer prevailed on the 9c,12t-18:2 isomer (3:1 ratio), and (iii) a regioselective isomerization of SAPC arachidonate residues occurred in the 5 and 8 positions. This effect of "positional preference" indicates that thiyl radicals entering the hydrophobic region of the membrane bilayer start to isomerize polyunsaturated fatty acid residues having the double bonds nearest to the membrane surfaces. We propose that arachidonic acid and its trans isomers can function as biomarkers in membranes for distinguishing the two trans fatty acid-forming pathways.

Immune cell regulation and cardiovascular effects of sphingosine 1-phosphate receptor agonists in rodents are mediated via distinct receptor subtypes

Sphingosine 1-phosphate (S1P) is a bioactive lysolipid with pleiotropic functions mediated through a family of G protein-coupled receptors, S1P(1,2,3,4,5). Physiological effects of S1P receptor agonists include regulation of cardiovascular function and immunosuppression via redistribution of lymphocytes from blood to secondary lymphoid organs. The phosphorylated metabolite of the immunosuppressant agent FTY720 (2-amino-2-(2-[4-octylphenyl]ethyl)-1,3-propanediol) and other phosphonate analogs with differential receptor selectivity were investigated. No significant species differences in compound potency or rank order of activity on receptors cloned from human, murine, and rat sources were observed. All synthetic analogs were high-affinity agonists on S1P(1), with IC(50) values for ligand binding between 0.3 and 14 nM. The correlation between S1P(1) receptor activation and the ED(50) for lymphocyte reduction was highly significant (p < 0.001) and lower for the other receptors. In contrast to S1P(1)-mediated effects on lymphocyte recirculation, three lines of evidence link S1P(3) receptor activity with acute toxicity and cardiovascular regulation: compound potency on S1P(3) correlated with toxicity and bradycardia; the shift in potency of phosphorylated-FTY720 for inducing lymphopenia versus bradycardia and hypertension was consistent with affinity for S1P(1) relative to S1P(3); and toxicity, bradycardia, and hypertension were absent in S1P(3)(-/-) mice. Blood pressure effects of agonists in anesthetized rats were complex, whereas hypertension was the predominant effect in conscious rats and mice. Immunolocalization of S1P(3) in rodent heart revealed abundant expression on myocytes and perivascular smooth muscle cells consistent with regulation of bradycardia and hypertension, whereas S1P(1) expression was restricted to the vascular endothelium.

Ceramide synthesis and metabolism as a target for cancer therapy

Sphingolipids, which include ceramides and sphingosine, are essential structural components of cell membranes that also have messenger functions that regulate the proliferation, survival, and death of cells. Exogenous application of ceramide is cytotoxic, and exposure of cells to radiation or chemotherapy is associated with increased ceramide levels due to enhanced de novo synthesis, catabolism of sphingomyelin, or both. Ceramide can be metabolized to less toxic forms by glycosylation, acylation, or by catabolism to sphingosine, which is then phosphorylated to the anti-apoptotic sphingosine 1-phosphate. Glucosylceramide synthase overexpression has been shown to enhance resistance to doxorubicin, suggesting that inhibition of ceramide metabolism or catabolism might enhance cancer chemotherapy. Several anticancer agents, including the cytotoxic retinoid, fenretinide (4-HPR), have been shown to act, at least in part, by increasing tumor cell ceramide via de novo synthesis. Combinations of 4-HPR and modulators of ceramide action and/or metabolism demonstrated increased anti-tumor activity in pre-clinical models with minimal toxicity for non-malignant cells, and were effective in a p53-independent manner against tumor cell lines resistant to standard cytotoxic agents. Phase I trials of ceramide metabolism inhibitors in combination with 4-HPR and with other cytotoxic agents are in development. Thus, pharmacological manipulation of sphingolipid metabolism to enhance tumor cell ceramide is being realized and offers a novel approach to cancer chemotherapy.

Genetic basis of congenital generalized lipodystrophy

Congenital generalized lipodystrophy (CGL) is an autosomal recessive disorder characterized by extreme lack of body fat and severe insulin resistance since birth. Recently, mutations have been reported in 1-acylglycerol-3-phosphate-O-acyltransferase 2 (AGPAT2) and Berardinelli-Seip congenital lipodystrophy 2 (BSCL2 or Seipin) genes in affected subjects from pedigrees linked to chromosomes 9q34 and 11q13, respectively. The AGPAT2 catalyses the acylation of the lysophosphatidic acid at the sn-2 position to form phosphatidic acid, a key intermediate in the biosynthesis of triacylglycerol and glycerophospholipids. High expression of AGPAT2 mRNA in adipose tissue compared to other isoforms suggests that the mutations might affect the adipose tissue the most. The function of BSCL2 remains unknown. Several CGL pedigrees reveal no mutation in either of the above genes and are not linked to these loci, suggesting additional genetic loci for CGL. Thus, several distinct mechanisms can lead to extreme lack of adipose tissue in humans and cause CGL.

Regulation of limb development by the sphingosine 1-phosphate receptor S1p1/EDG-1 occurs via the hypoxia/VEGF axis

Angiogenesis, also known as new blood vessel formation, is regulated coordinately with other tissue differentiation events during limb development. Although vascular endothelial cell growth factor (VEGF) is important in the regulation of angiogenesis, chondrogenesis and osteogenesis during limb development, the role of other angiogenic factors is not well understood. Sphingosine 1-phosphate, a platelet-derived lipid mediator, regulates angiogenesis and vascular maturation via its action on the G-protein-coupled receptor S1P(1) (also known as EDG-1). In addition to vascular defects, abnormal limb development was also observed in S1p(1)(-/-) mice. Here we show that strong induction of S1P(1) expression is observed in the blood vessels and the interdigital mesenchymal cells during limb development. Deletion of S1P(1) results in aberrant chondrocyte condensation and defective digit morphogenesis. Interestingly, the vasculature in the S1p(1)(-/-) limbs was hyperplastic and morphologically altered. In addition, the hypoxia inducible factor (HIF)-1 alpha and its response gene VEGF were induced in S1p(1)(-/-) limbs. However, aberrant regulation of HIF-1 alpha and VEGF were not observed in embryonic fibroblasts derived from S1p(1)(-/-) mice, suggesting a non-cell autonomous effect of S1P(1) on VEGF expression. Indeed, similar limb defects were observed in endothelium-specific S1P(1) null mice in vivo. These data suggest that the function of S1P(1) in the developing vasculature is essential for proper limb development.

Sphingolipids as Bioactive Regulators of Thrombin Generation

Sphingolipids contribute to modulation of two opposing cell processes, cell growth and apoptotic cell death; ceramide and sphingosine promote the latter and sphingosine-1-phosphate triggers the former. Thrombin, a pro-inflammatory protease that is regulated by the blood coagulation cascade, exerts similar effects depending on cell type. Here we report a new mechanism for cross-talk between sphingolipid metabolism and thrombin generation. Sphingosine and sphinganine, but not ceramide or sphingosine-1-phosphate, down-regulated thrombin generation on platelet surfaces (IC(50) = 2.4 and 1.4 microm for sphingosine and sphinganine, respectively) as well as in whole plasma clotting assays. Thrombin generation was also inhibited by glucosylsphingosine, lysosphingomyelin, phytosphingosine, and primary alkylamines with >10 carbons. Acylation of the amino group ablated anticoagulant activities. Factor Va was required for the anticoagulant property of sphingosine because prothrombin activation was inhibited by sphingosine, sphinganine, and stearylamine in the presence but not in the absence of factor Va. Sphingosine did not inhibit thrombin generation when Gla-domainless factor Xa was used in prothrombinase assays, whereas sphingosine inhibited activation of Gla-domainless prothrombin by factor Xa/factor Va in the absence of phospholipids (IC(50) = 0.49 microm). Fluorescence spectroscopy studies showed that sphingosine binds to fluorescein-labeled factor Xa and that this interaction required the Gla domain. These results imply that sphingosine disrupts interactions between factor Va and the Gla domain of factor Xa in the prothrombinase complex. Thus, certain sphingolipids may be bioactive lipid mediators of thrombin generation such that certain sphingolipid metabolites may modulate proteases that affect cell growth and death, blood coagulation, and inflammation.

Evidence for tight coupling of Gi protein-mediated lysophosphatidic acid receptor to stimulated cytokine production in ovarian cancer cell

OBJECTIVE

Lysophosphatidic acid stimulates the proliferation of ovarian cancer cell through specific members of the guanosine triphosphate-binding protein-coupled receptor family. We attempted to identify the guanosine triphosphate-binding protein subtypes that are linked to lysophosphatidic acid receptor-stimulated production of cytokine, which are involved potentially in ovarian cancer development.

STUDY DESIGN

Cytokine assay kits were used to determine interleukin-6, interleukin-8, and tumor necrotic factor-alpha that were produced from Caov-3 and SK-OV3 ovarian cancer cell lines. Thealpha-subunit of Gi was detected by pertussis toxin-catalyzed adenosine diphosphate ribosylation from nicotinamide adenine dinucleotide in isolated plasma membrane.

RESULTS

Pertussis toxin, but not cholera toxin, brought about adenosine diphosphate ribosylation of Galphai of 41 kd in the plasma membrane. Incubation with lysophosphatidic acid and nonhydrolyzable guanosine triphosphate analog decreased the adenosine diphosphate-ribosylation activity in a dose-dependent manner; a one half-maximal effect occurred with 10 micromol/L lysophosphatidic acid. The apparent inhibition by lysophosphatidic acid of the adenosine diphosphate ribosylation demonstrated that lysophosphatidic acid resolved the alpha-subunit of the Gi to guanosine triphosphate-bound form in the membranes. Pretreatment of the ovarian cancer cells with the pertussis toxin completely inhibited lysophosphatidic acid-stimulated production of interleukin-6, interleukin-8 and tumor necrosis factor-alpha. The lysophosphatidic acid-stimulated cytokine production was dose-dependent with a one half-maximal effect at 10 micromol/L. Phosphatidic acid and ceramide 1-phosphate had no effect on the lysophosphatidic acid action on cytokine expression.

CONCLUSION

These data demonstrate the coupling of lysophosphatidic acid receptor to Gi protein subfamily in ovarian cancer cell. The Gi that couples lysophosphatidic acid receptor to the effector may define the differences in the signaling pathways of lysophosphatidic acid-activated cytokine expression and proliferation.

The sphingosine 1-phosphate receptor agonist FTY720 supports CXCR4-dependent migration and bone marrow homing of human CD34+ progenitor cells

The novel immunosuppressant FTY720 activates sphingosine 1-phosphate receptors (S1PRs) that affect responsiveness of lymphocytes to chemokines such as stromal cell-derived factor 1 (SDF-1), resulting in increased lymphocyte homing to secondary lymphoid organs. Since SDF-1 and its receptor CXCR4 are also involved in bone marrow (BM) homing of hematopoietic stem and progenitor cells (HPCs), we analyzed expression of S1PRs and the influence of FTY720 on SDF-1/CXCR4-mediated effects in human HPCs. By reverse transcriptase-polymerase chain reaction (RT-PCR), S1PRs were expressed in mobilized CD34+ HPCs, particularly in primitive CD34+/CD38- cells. Incubation of HPCs with FTY720 resulted in prolonged SDF-1-induced calcium mobilization and actin polymerization, and substantially increased SDF-1-dependent in vitro transendothelial migration, without affecting VLA-4, VLA-5, and CXCR4 expression. In nonobese diabetic-severe combined immunodeficient (NOD/SCID) mice, the number of CD34+/CD38- cells that homed to the BM after 18 hours was significantly raised by pretreatment of animals and cells with FTY720, tending to result in improved engraftment. In addition, in vitro growth of HPCs (week-5 cobblestone area-forming cells [CAFCs]) was 2.4-fold increased. We conclude that activation of S1PRs by FTY720 increases CXCR4 function in HPCs both in vitro and in vivo, supporting homing and proliferation of HPCs. In the hematopoietic microenvironment, S1PRs are involved in migration and maintenance of HPCs by modulating the effects of SDF-1.

Interaction between non-heme iron of lipoxygenases and cumene hydroperoxide: basis for enzyme activation, inactivation, and inhibition

Lipoxygenase catalysis depends in a critical fashion on the redox properties of a unique mononuclear non-heme iron cofactor. The isolated enzyme contains predominantly, if not exclusively, iron(II), but the catalytically active form of the enzyme has iron(III). The activating oxidation of the iron takes place in a reaction with the hydroperoxide product of the catalyzed reaction. In a second peroxide-dependent process, lipoxygenases are also inactivated. To examine the redox activation/inactivation dichotomy in lipoxygenase chemistry, the interaction between lipoxygenase-1 (and -3) and cumene hydroperoxide was investigated. Cumene hydroperoxide was a reversible inhibitor of the reaction catalyzed by lipoxygenase-1 under standard assay conditions at high substrate concentrations. Reconciliation of the data with the currently held kinetic mechanism requires simultaneous binding of substrate and peroxide. The enzyme also was both oxidized and largely inactivated in a reaction with the peroxide in the absence of substrate. The consequences of this reaction for the enzyme included the hydroxylation at C beta of two amino acid side chains in the vicinity of the cofactor, Trp and Leu. The modifications were identified by mass spectrometry and X-ray crystallography. The peroxide-induced oxidation of iron was also accompanied by a subtle rearrangement in the coordination sphere of the non-heme iron atom. Since the enzyme retains catalytic activity, albeit diminished, after treatment with cumene hydroperoxide, the structure of the iron site may reflect the catalytically relevant form of the cofactor.

Identification of functional nuclear export sequences in human sphingosine kinase 1

Sphingosine kinase (SPHK) is an enzyme that phosphorylates sphingosine to form sphingosine 1-phosphate (S1P). Human SPHK1 (hSPHK1) was localized predominantly in the cytoplasm when transiently expressed in Cos7 cells. In this study, we have found two functional nuclear export signal (NES) sequences in the middle region of hSPHK1. Deletion and mutagenesis studies revealed that the cytoplasmic localization of SPHK1 depends on its nuclear export, directed by the NES. Furthermore, upon treatment with leptomycin B, a specific inhibitor of the nuclear export receptor CRM1, a marked nuclear accumulation of hSPHK1 was observed, indicating that hSPHK1 shuttles between the cytoplasm and the nucleus. Our results provide the first evidence of the active nuclear export of SPHK1 and suggest it is mediated by a CRM1-dependent pathway.

Sphingosine 1-phosphate (S1P) receptor subtypes S1P1 and S1P3, respectively, regulate lymphocyte recirculation and heart rate

Sphingosine 1-phosphate (S1P) influences heart rate, coronary artery caliber, endothelial integrity, and lymphocyte recirculation through five related high affinity G-protein-coupled receptors. Inhibition of lymphocyte recirculation by non-selective S1P receptor agonists produces clinical immunosuppression preventing transplant rejection but is associated with transient bradycardia. Understanding the contribution of individual receptors has been limited by the embryonic lethality of the S1P(1) knock-out and the unavailability of selective agonists or antagonists. A potent, S1P(1)-receptor selective agonist structurally unrelated to S1P was found to activate multiple signals triggered by S1P, including guanosine 5'-3-O-(thio)triphosphate binding, calcium flux, Akt and ERK1/2 phosphorylation, and stimulation of migration of S1P(1)- but not S1P(3)-expressing cells in vitro. The agonist also alters lymphocyte trafficking in vivo. Use of selective agonism together with deletant mice lacking S1P(3) receptor reveals that agonism of S1P(1) receptor alone is sufficient to control lymphocyte recirculation. Moreover, S1P(1) receptor agonist plasma levels are causally associated with induction and maintenance of lymphopenia. S1P(3), and not S1P(1), is directly implicated in sinus bradycardia. The sustained bradycardia induced by S1P receptor non-selective immunosuppressive agonists in wild-type mice is abolished in S1P(3)-/- mice, whereas S1P(1)-selective agonist does not produce bradycardia. Separation of receptor subtype usage for control of lymphocyte recirculation and heart rate may allow the identification of selective immunosuppressive S1P(1) receptor agonists with an enhanced therapeutic window. S1P(1)-selective agonists will be of broad utility in understanding cell functions in vitro, and vascular physiology in vivo, and the success of the chemical approach for S1P(1) suggests that selective tools for the resolution of function across this broad lipid receptor family are now possible.

Activation of sphingosine kinase 1 by ERK1/2-mediated phosphorylation

Sphingosine kinase 1 is an agonist-activated signalling enzyme that catalyses the formation of sphingosine 1-phosphate, a lipid second messenger that has been implicated in a number of agonist-driven cellular responses, including stimulation of cell proliferation, inhibition of apoptosis and expression of inflammatory molecules. Although agonist-induced stimulation of sphingosine kinase activity is critical in a number of signalling pathways, nothing has been known of the molecular mechanism of this activation. Here we show that this activation results directly from phosphorylation of sphingosine kinase 1 at Ser225, and present several lines of evidence to show compellingly that the activating kinase is ERK1/2 or a close relative. Furthermore, we show that phosphorylation of sphingosine kinase 1 at Ser225 results not only in an increase in enzyme activity, but is also necessary for translocation of the enzyme from the cytosol to the plasma membrane. Thus, these studies have elucidated the mechanism of agonist-mediated sphingosine kinase activation, and represent a key finding in understanding the regulation of sphingosine kinase/sphingosine 1-phosphate-controlled signalling pathways.

Blood lipid mediator sphingosine 1-phosphate potently stimulates platelet-derived growth factor-A and -B chain expression through S1P1-Gi-Ras-MAPK-dependent induction of Kruppel-like factor 5

Platelet-derived growth factors (PDGFs), potent mitogens and chemoattractants for mesenchymal cell types, play essential roles in development of several organs including blood vessels, kidney, and lung, and are also implicated in the pathogenesis of atherosclerosis and malignancies. Blood lipid mediator sphingosine 1-phosphate (S1P) regulates migration, proliferation, and apoptosis in a variety of cell types through multiple G protein-coupled receptors of the Edg family, and is necessary for vascular formation at the developmental stage. We found in the present study that S1P induced severalfold increases in the mRNA and protein levels of PDGF-A and -B chains in vascular smooth muscle cells and neointimal cells. S1P stimulation of PDGF mRNA and protein expression was abolished by the small interfering RNA duplexes targeting S1P(1)/Edg1 receptor subtype. S1P stimulated the small GTPase Ras in a G(i)-dependent manner, and activated ERK and p38 MAPK in G(i)- and Ras-dependent manners. Pertussis toxin pretreatment, adenovirus-mediated Asn(17)Ras expression, the MEK inhibitor PD98059, or the p38 MAPK inhibitor SB203580 markedly suppressed PDGF mRNA and protein up-regulation, indicating the involvement of G(i)-Ras-ERK/p38 MAPK in S1P stimulation of PDGF expression. S1P stimulated expression of the transcription factor KLF5 in manners dependent on G(i), Ras, and ERK/p38 MAPK. Down-regulation of KLF5 by small interfering RNA duplexes abolished S1P-induced PDGF-A and -B chain expression. On the other hand, overexpression of KLF5 stimulated basal and S1P-induced PDGF expression. Either S1P stimulation or KLF5 overexpression increased the PDGF-B promoter activity in a cis-element-dependent manner. These results reveal the S1P(1)-triggered, G(i)-Ras-ERK/p38 MAPK-KLF5-dependent, stimulatory regulation of PDGF gene transcription in vascular smooth muscle 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.

The Action Mode of Lysophosphatidylcholine in Human Monocytes

To elucidate the action and signal transduction of lysophosphatidylcholine (LPC), we challenged a set of LPC on U937 human monocytes and found that LPC mobilized Ca(2+). The Ca(2+) response was not blocked by pertussis toxin, an inhibitor of G(i/o) proteins, or by U73122, a phospholipase C inhibitor. Furthermore, the response was totally blocked by addition of EGTA to the extracellular media, suggesting that Ca(2+) influx across the plasma membrane was the only source of LPC-induced Ca(2+) response in the U937 cells. 16:0 and 18:0 LPC induced similar responses. Recently it has been suggested that two G protein-coupled receptors function as LPC receptors in the plasma membrane. RT-PCR analysis indicated that neither the G2A receptor nor the GPR4 receptor is expressed in the U937 monocytes. Our data suggests that another action mechanism of LPC may be involved in the LPC response in the U937 cells.

Discovery of new G protein-coupled receptors for lipid mediators

Successful sequencing of the human genome has opened a new era in the life sciences and has greatly accelerated biomedical research. Among various research endeavors benefiting from established genomic information, one of the most fruitful areas is the research on orphan G protein-coupled receptors (GPCRs). Many intercellular mediators, including peptides, lipids, and other small molecules, have found their GPCRs in the plasma membrane, e.g., relaxin and tyramine. In the past 14 months, more than one dozen papers have been published reporting the finding of intercellular lipid mediators acting on rhodopsin family GPCRs. This review focuses primarily on intercellular lipid mediators and their recently discovered GPCRs.

Phosphorylation and action of the immunomodulator FTY720 inhibits vascular endothelial cell growth factor-induced vascular permeability

FTY720, a potent immunosuppressive agent, is phosphorylated in vivo into FTY720-P, a high affinity agonist for sphingosine 1-phosphate (S1P) receptors. The effects of FTY720 on vascular cells, a major target of S1P action, have not been addressed. We now report the metabolic activation of FTY720 by sphingosine kinase-2 and potent activation of vascular endothelial cell functions in vitro and in vivo by phosphorylated FTY720 (FTY720-P). Incubation of endothelial cells with FTY720 resulted in phosphorylation by sphingosine kinase activity and formation of FTY720-P. Sphingosine kinase-2 effectively phosphorylated FTY720 in the human embryonic kidney 293T heterologous expression system. FTY720-P treatment of endothelial cells stimulated extracellular signal-activated kinase and Akt phosphorylation and adherens junction assembly and promoted cell survival. The effects of FTY720-P were inhibited by pertussis toxin, suggesting the requirement for Gi-coupled S1P receptors. Indeed, transmonolayer permeability induced by vascular endothelial cell growth factor was potently reversed by FTY720-P. Furthermore, oral FTY720 administration in mice potently blocked VEGF-induced vascular permeability in vivo. These findings suggest that FTY720 or its analogs may find utility in the therapeutic regulation of vascular permeability, an important process in angiogenesis, inflammation, and pathological conditions such as sepsis, hypoxia, and solid tumor growth.

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.

Sphingosine kinase 2 is a nuclear protein and inhibits DNA synthesis

Sphingosine kinase-1 (SPHK1) is a key enzyme catalyzing the formation of an important bioactive lipid messenger, sphingosine 1-phosphate, and is implicated in the regulation of cell proliferation and antiapoptotic processes. Biological features of another isozyme SPHK2, however, remain unclear. The present studies were undertaken to characterize SPHK2 by comparison with SPHK1. When SPHK2 was transiently expressed in various cell lines, it was localized in the nuclei as well as in the cytosol, whereas SPHK1 was distributed in the cytosol but not in the nucleus. We have mapped a functional nuclear localization signal (NLS) to the N-terminal region of SPHK2. We have observed that the expression of SPHK2 in various cell types causes inhibition of DNA synthesis, resulting in the cell cycle arrest at G1/S phase. We have also demonstrated that an NLS mutant of SPHK2, SPHK2R93E/R94E, failed to enter the nucleus and to inhibit DNA synthesis. Moreover, a fusion protein, NLS-SPHK1, where SPHK1 was fused to the NLS sequence of SPHK2 acquired the ability to enter nuclei and inhibited DNA synthesis. These results indicate that SPHK2 localizes in the nuclei and causes inhibition of DNA synthesis, and this may affect subsequent cellular events.

G-protein-coupled receptor S1P1 acts within endothelial cells to regulate vascular maturation

Sphingosine-1-phosphate (S1P) stimulates signaling pathways via G-protein-coupled receptors and triggers diverse cellular processes, including growth, survival, and migration. In S1P1 receptor-deficient embryos, blood vessels were incompletely covered by vascular smooth muscle cells (VSMCs), indicating the S1P1 receptor regulates vascular maturation. Because S1P1 receptor expression is not restricted to a particular cell type, it was not known whether the S1P1 receptor controlled VSMC coverage of vessels in a cell-autonomous fashion by functioning directly in VSMCs or indirectly through its activity in endothelial cells (ECs). By using the Cre/loxP system, we disrupted the S1P1 gene solely in ECs. The phenotype of the conditional mutant embryos mimicked the one obtained in the embryos globally deficient in S1P1. Thus, vessel coverage by VSMCs is directed by the activity of the S1P1 receptor in ECs.

Sphingosine-1-phosphate stimulates contraction of human airway smooth muscle cells

The bioactive sphingolipid sphingosine-1-phosphate (S1P) that is increased in airways of asthmatic subjects markedly induced contraction of human airway smooth muscle (HASM) cells embedded in collagen matrices in a Gi-independent manner. Dihydro-S1P, which binds to S1P receptors, also stimulated contractility. S1P induced formation of stress fibers, contraction of individual HASM cells, and stimulated myosin light chain phosphorylation, which was inhibited by the Rho-associated kinase inhibitor Y-27632. S1P-stimulated HASM cell contractility was independent of the ERK1/2 and PKC signaling pathways, important regulators of airway smooth muscle contraction. However, removal of extracellular calcium completely blocked S1P-mediated contraction and Y-27632 reduced it. S1P also induced calcium mobilization that was not desensitized by repeated additions. Pretreatment with thapsigargin to deplete InsP3-sensitive calcium stores partially blocked increases in [Ca2+]i induced by S1P, yet did not inhibit S1P-stimulated contraction. In sharp contrast, the L-type calcium channel blocker verapamil markedly decreased S1P-induced HASM cell contraction, supporting a role for calcium influx from extracellular sources. Collectively, our results suggest that S1P may regulate HASM contractility, important in the pathobiology of asthma.

p38 Mitogen-activated protein kinase is crucially involved in osteoclast differentiation but not in cytokine production, phagocytosis, or dendritic cell differentiation of bone marrow macrophages

We previously reported that p38 MAPK signaling is required for osteoclast differentiation but not osteoclast function. Here we further investigated the role of p38 MAPK in the function and differentiation of mouse bone marrow macrophages (BMM phi), common precursors of osteoclasts and dendritic cells. Lipopolysaccharide (LPS) activated the p38 MAPK signaling pathway in BMM phi by sequential phosphorylation of MAPK kinase 3/6, p38 MAPK, and activating transcription factor-2. Treatment of BMM phi with SB203580, a p38 MAPK inhibitor, suppressed LPS-induced phosphorylation of activating transcription factor-2. LPS stimulated production of IL-1 beta, TNF alpha, and IL-6 in BMM phi, and SB203580 failed to inhibit the LPS-induced cytokine production. BMM phi incorporated latex beads via phagocytosis, and SB203580 had no effect on this phagocytosis. BMM phi differentiated into dendritic cells when treated with granulocyte macrophage colony-stimulating factor together with CD40 ligand, TNF alpha, or LPS, and SB203580 failed to inhibit this differentiation. Thus, p38 MAPK-mediated signals are not involved in either BMM phi function or BMM phi differentiation into dendritic cells. The differentiation of BMM phi into osteoclasts in response to receptor activator of nuclear factor-kappa B ligand or TNF alpha was strongly inhibited by SB203580. These findings emphasize the crucial roles of p38 MAPK-mediated signaling in osteoclast differentiation.

Galardin (GM 6001), a broad-spectrum matrix metalloproteinase inhibitor, blocks bombesin- and LPA-induced EGF receptor transactivation and DNA synthesis in rat-1 cells

Matrix metalloproteinases (MMPs) have been implicated in the transactivation of the epidermal growth factor receptor (EGFR) induced by G-protein coupled receptor (GPCR) agonists. Although EGFR phosphorylation and downstream signaling have been shown to be dependent on MMP activity in many systems, a role for MMPs in GPCR-induced DNA synthesis has not been studied in any detail. In this study we utilized the broad-spectrum matrix metalloproteinase inhibitor, galardin (Ilomastat, GM 6001), to study the mechanism of bombesin- or LPA-induced EGFR transactivation and the role of MMPs in early and late response mitogenic signaling in Rat-1 cells stably transfected with the bombesin/GRP receptor (BoR-15 cells). Addition of galardin to cells stimulated with bombesin or LPA specifically inhibited total EGFR phosphorylation, as well as site-specific phosphorylation of tyrosine 845, a putative Src phosphorylation site, and tyrosine 1068, a typical autophosphorylation site. Galardin treatment also inhibited extracellular signal-regulated kinase (ERK) activation induced by bombesin or LPA, but not by EGF. In addition, galardin inhibited bombesin- or LPA-induced DNA synthesis in a dose dependent manner, when stimulated by increasing concentrations of bombesin, and when added after bombesin stimulation. Furthermore, addition of galardin post-bombesin stimulation indicated that by 3 h sufficient accumulation of EGFR ligands had occurred to continue to induce transactivation despite an inhibition of MMP activity. Taken together, our results suggest that MMPs act as early as 5 min, and up to around 3 h, to mediate GPCR-induced EGFR transactivation, ERK activation, and stimulation of DNA synthesis.

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.

Lysophospholipids synergistically promote primitive hematopoietic cell chemotaxis via a mechanism involving Vav 1

Hematopoiesis is sustained by the proliferation and development of an extremely low number of hematopoietic stem cells resident in the bone marrow. These stem cells can migrate from their bone marrow microenvironment and can be found at low levels in the peripheral blood. The factors that regulate egress or ingress of the stem cells from the marrow include cytokines and chemokines. This process of stem cell trafficking is fundamental to both stem cell biology and stem cell transplantation. We show that primitive hematopoietic cells with cobblestone area-forming cell activity express receptors for and display enhanced motility in response to a new class of stem cell agonists, namely lysophospholipids. These agents synergistically promote chemokine-stimulated cell chemotaxis, a process that is crucial in stem cell homing. The response to lysophospholipids is mediated by Rac, Rho, and Cdc42 G proteins and the hematopoietic-specific guanyl nucleotide exchange factor Vav 1. Inhibitor studies also show a critical role for phosphatidylinositol 3 kinase (PI3K). Lipid mediators, therefore, regulate the critical process of primitive hematopoietic cell motility via a PI3K- and Vav-dependent mechanism and may govern stem cell movement in vivo. These results are of relevance to understanding stem cell trafficking during bone marrow transplantation.

Sphingosine kinase type 2 is a putative BH3-only protein that induces apoptosis

There are two isoforms of sphingosine kinase (SphK) that catalyze the formation of sphingosine 1-phosphate, a potent sphingolipid mediator. Whereas SphK1 stimulates growth and survival, here we show that SphK2 enhanced apoptosis in diverse cell types and also suppressed cellular proliferation. Apoptosis was preceded by cytochrome c release and activation of caspase-3. SphK2-induced apoptosis was independent of activation of sphingosine 1-phosphate receptors. Sequence analysis revealed that SphK2 contains a 9-amino acid motif similar to that present in BH3-only proteins, a pro-apoptotic subgroup of the Bcl-2 family. As with other BH3-only proteins, co-immunoprecipitation demonstrated that SphK2 interacted with Bcl-xL. Moreover, site-directed mutation of Leu-219, the conserved leucine residue present in all BH3 domains, markedly suppressed SphK2-induced apoptosis. Hence, the apoptotic effect of SphK2 might be because of its putative BH3 domain.

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.

Sphingosine kinase transmits estrogen signaling in human breast cancer cells

Current understanding of cytoplasmic signaling pathways that mediate estrogen action in human breast cancer is incomplete. Here we report that treatment with 17beta-estradiol (E2) activates a novel signaling pathway via activation of sphingosine kinase (SphK) in MCF-7 breast cancer cells. We found that E2 has dual actions to stimulate SphK activity, i.e. a rapid and transient activation mediated by putative membrane G protein-coupled estrogen receptors (ER) and a delayed but prolonged activation relying on the transcriptional activity of ER. The E2-induced SphK activity consequently activates downstream signal cascades including intracellular Ca2+ mobilization and Erk1/2 activation. Enforced expression of human SphK type 1 gene in MCF-7 cells resulted in increases in SphK activity and cell growth. Moreover, the E2-dependent mitogenesis were highly promoted by SphK overexpression as determined by colony growth in soft agar and solid focus formation. In contrast, expression of SphKG82D, a dominant-negative mutant SphK, profoundly inhibited the E2-mediated Ca2+ mobilization, Erk1/2 activity and neoplastic cell growth. Thus, our data suggest that SphK activation is an important cytoplasmic signaling to transduce estrogen-dependent mitogenic and carcinogenic action in human breast cancer cells.

Role of phospholipase D in agonist-stimulated lysophosphatidic acid synthesis by ovarian cancer cells

Lysophosphatidic acid (LPA) is a receptor-active lipid mediator with a broad range of biological effects. Ovarian cancer cells synthesize LPA, which promotes their motility, growth, and survival. We show that a murine homolog of a human protein previously reported to hydrolyze LPA is a highly selective detergent-stimulated LPA phosphatase that can be used to detect and quantitate LPA. Use of this protein in novel enzymatic assay demonstrates that SK-OV-3 ovarian cancer cells release physiologically relevant levels of biologically active LPA into the extracellular space. LPA release is markedly increased by nucleotide agonists acting through a P2Y4 purinergic receptor. Promotion of LPA formation by nucleotides is accompanied by stimulation of phospholipase D (PLD) activity. Overexpression of both PLD1 and PLD2 in SK-OV-3 cells produces active enzymes, but only overexpression of PLD2 results in significant amplification of both nucleotide-stimulated PLD activity and LPA production. SK-OV-3 cells express and secrete a phospholipase A2 activity that can generate LPA from the lipid product of PLD, phosphatidic acid. Our results identify a novel role for nucleotides in the regulation of ovarian cancer cells and suggest an indirect but critical function for PLD2 in agonist-stimulated LPA production.

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.

VEGF induces S1P1 receptors in endothelial cells: Implications for cross-talk between sphingolipid and growth factor receptors

Sphingosine 1-phosphate (S1P) is a platelet-derived sphingolipid that binds to S1P1 (EDG-1) receptors and activates the endothelial isoform of NO synthase (eNOS). S1P and the polypeptide growth factor vascular endothelial growth factor (VEGF) act independently to modulate angiogenesis and activate eNOS. In these studies, we explored the cross-talk between S1P and VEGF signaling pathways. When cultured bovine aortic endothelial cells were treated with VEGF (10 ng/ml), the expression of S1P1 protein and mRNA increased by approximately 4-fold. S1P1 up-regulation by VEGF was seen within 30 min of VEGF addition and reached a maximum after 1.5 h. By contrast, expression of neither bradykinin B2 receptors nor the scaffolding protein caveolin-1 was altered by VEGF treatment. The EC50 for VEGF-promoted induction of S1P1 expression was approximately 2 ng/ml, within its physiological concentration range. S1P1 induction by VEGF was attenuated by the tyrosine kinase inhibitor genistein and by the PKC inhibitor calphostin C. Preincubation of bovine aortic endothelial cells with VEGF (10 ng/ml for 90 min) markedly enhanced subsequent S1P-dependent eNOS activation. VEGF pretreatment of cultured endothelial cells also markedly potentiated S1P-promoted eNOS phosphorylation at Ser-1179, as well as S1P-mediated activation of kinase Akt. In isolated rat arteries, VEGF pretreatment markedly potentiated S1P-mediated vasorelaxation and eNOS Ser-1179 phosphorylation. Taken together, these data indicate that VEGF specifically induces expression of S1P1 receptors, associated with enhanced intracellular signaling responses to S1P and the potentiation of S1P-mediated vasorelaxation. We suggest that VEGF acts to sensitize the vascular endothelium to the effects of lipid mediators by promoting the induction of S1P1 receptors, representing a potentially important point of cross-talk between receptor-regulated eNOS signaling pathways in the vasculature.

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.

Distribution of sphingosine kinase activity in mouse tissues: contribution of SPHK1

Sphingosine kinase (SPHK) phosphorylates sphingosine to form a bioactive lipid mediator, sphingosine 1-phosphate (S1P). S1P mediates such diverse biological processes as regulation of cell differentiation, motility, and apoptosis both extracellularly, via S1P (Edg) family receptors, and intracellularly, through unidentified targets. In cells S1P is short-lived, so the synthetic process catalyzed by sphingosine kinase may be important in maintaining the cellular levels of the compound. Thus far, two sphingosine kinases have been reported, with SPHK1 exhibiting the higher activity. However, several studies suggest the existence of unidentified sphingosine kinases. Therefore, to further elucidate the role of SPHK1 in the formation of S1P, we investigated its contribution to the total sphingosine kinase activity in mouse tissues. We found that SPHK1 is a major sphingosine kinase in many tissues, especially in brain, heart, and colon. However, some tissues such as spleen, small intestine, and lung contained sphingosine kinase activity that was not attributable to SPHK1 or SPHK2, as determined by immunodepletion assays. Furthermore, the presence of other sphingosine kinases with different properties, i.e., higher activity toward phytosphingosine and a different subcellular distribution, is suggested.

Neutral ceramidase secreted by endothelial cells is released in part associated with caveolin-1

Neutral ceramidase (CDase) is a key enzyme of sphingomyelin (SM) metabolism implicated in cell signaling triggered by a variety of extracellular ligands. Previously it was shown that in murine endothelial cells a portion of neutral CDase is localized in detergent-resistant light membranes. In this study subcellular distribution of neutral CDase was further investigated. In accordance with the previous finding, the enzyme was identified in caveolae. Moreover, the same protein was detected in medium-speed supernatant of cell-conditioned medium, accounting for CDase activity measurable in the medium at neutral pH. Notably, these cells released also the caveolae-scaffolding protein caveolin-1 (cav-1). Interestingly, secreted neutral CDase and cav-1 coimmunoprecipitated. In addition, acid sphingomyelinase (SMase) activity was detectable in cav-1 immunocomplexes. These findings are consistent with the view that neutral CDase is released, in part, in association with cav-1 together with acid SMase. It remains to be established whether the here-identified secreted cav-1-enriched complex acts as platform to facilitate extracellular metabolism of SM.

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.

Subtype-selective antagonists of lysophosphatidic Acid receptors inhibit platelet activation triggered by the lipid core of atherosclerotic plaques

BACKGROUND

Lysophosphatidic acid (LPA) is a platelet-activating component of mildly oxidized LDL (mox-LDL) and lipids isolated from human atherosclerotic plaques. Specific antagonists of platelet LPA receptors could be useful inhibitors of thrombus formation in patients with cardiovascular disease.

METHODS AND RESULTS

Short-chain analogs of phosphatidic acid (PA) were examined for their effect on two initial platelet responses, platelet shape change and Ca2+ mobilization. Dioctylglycerol pyrophosphate [DGPP(8:0)] and dioctylphosphatidic acid [PA(8:0)], recently described selective antagonists of the LPA1 and LPA3 receptors, inhibited platelet activation evoked by LPA but not by other platelet stimuli. DGPP(8:0) was more potent than PA(8:0). DGPP(8:0) also inhibited platelet shape change induced by mox-LDL and lipid extracts from human atherosclerotic plaques. Notably, we demonstrate for the first time that the lipid-rich core isolated from soft plaques was able to directly induce shape change. This effect was completely abrogated by prior incubation of platelets with DGPP(8:0). Moreover, coapplication of the lipid-rich core or LPA together with subthreshold concentrations of ADP or epinephrine synergistically induced platelet aggregation; this effect was inhibited by DGPP(8:0). Analysis by liquid chromatography-mass spectrometry revealed the presence of LPA alkyl- and acyl-molecular species with high platelet-activating potency (16:0-alkyl-LPA, 20:4-acyl-LPA).

CONCLUSIONS

LPA molecules present in the core region of atherosclerotic plaques trigger rapid platelet activation through the stimulation of LPA1 and LPA3 receptors. Antagonists of platelet LPA receptors might provide a new strategy to prevent thrombus formation in patients with cardiovascular diseases.

Lysophosphatidic acid signals through mitogen-activated protein kinase-extracellular signal regulated kinase in ovarian theca cells expressing the LPA1/edg2-receptor: involvement of a nonclassical pathway?

We investigated the mechanism of lysophosphatidic acid (LPA) signaling in ovarian theca cells and observed that stimulation with this bioactive lipid markedly enhanced Thr/Tyr phosphorylation of the MAPK ERK1/2. Activation of ERK was transient, showing a peak at 5 min that declined thereafter, and was not associated with a concomitant nuclear translocation of the enzyme, suggesting that a cytosolic tyrosine phosphatase may be responsible for switching off the signal. Epidermal growth factor (EGF)-induced activation of the enzyme in the same cell system was more rapid (peaking at 1 min), sustainable for at least 60 min, and could be suppressed by prior treatment with either pertussis toxin or a noncompetitive inhibitor of Ras acceptor protein, manumycin A. This functional inhibition of either Gi or Ras failed, however, to affect the LPA-induced ERK-phosphorylation. Surprisingly, functional inhibition of Rho-GTPase, in C3-exotoxin-lipofected cells, markedly reduced LPA-stimulated phosphorylation of ERK, without affecting the EGF-induced stimulation of MAPK. Theca cells labeled with anti-LPA1/edg2-type antibody showed a distinct cell surface labeling, which is reflected in the expression of (LPA1)-type LPA receptors at both mRNA and protein levels. The findings indicate that LPA transiently stimulates MAPK ERK in LPA1/edg2-expressing theca cells and suggest an alternative mechanism regulating the activation of ERK that differs from the canonical EGF-Ras-MAPK kinase pathway.