Lysophosphatidic acid and sphingosine 1-phosphate biology: the role of lipid phosphate phosphatases

Discovery of Potent, Orally Bioavailable Sphingosine-1-Phosphate Transporter (Spns2) Inhibitors

Targeting the S1P pathway has resulted in the development of S1P1 receptor modulators for the treatment of multiple sclerosis and ulcerative colitis. We hypothesize that targeting an upstream node of the S1P pathway may provide an improved adverse event profile. In this report, we performed a structure-activity relationship study focusing on the benzoxazole scaffold in SLB1122168, which lead to the discovery of 11i (SLF80821178) as a potent inhibitor of S1P release from HeLa cells (IC50: 51 ± 3 nM). Administration of SLF80821178 to mice induced ∼50% reduction in circulating lymphocyte counts, recapitulating the lymphopenia characteristic of Spns2 null animals. Molecular modeling studies suggest that SLF80821178 binds Spns2 in its occluded inward-facing state and forms hydrogen bonds with Asn112 and Ser211 and π stacking with Phe234. Taken together, SLF80821178 can serve as a scaffold for future inhibitor development and represents a chemical tool to study the therapeutic implication of inhibiting Spns2.

Assessing Spns2-dependent S1P Transport as a Prospective Therapeutic Target

S1P (sphingosine 1-phosphate) receptor modulator (SRM) drugs interfere with lymphocyte trafficking by downregulating lymphocyte S1P receptors. While the immunosuppressive activity of SRM drugs has proved useful in treating autoimmune diseases such as multiple sclerosis, that drug class is beset by on-target liabilities such as initial dose bradycardia. The S1P that binds to cell surface lymphocyte S1P receptors is provided by S1P transporters. Mice born deficient in one of these, spinster homolog 2 (Spns2), are lymphocytopenic and have low lymph S1P concentrations. Such observations suggest that inhibition of Spns2-mediated S1P transport might provide another therapeutically beneficial method to modulate immune cell positioning. We report here results using a novel S1P transport blocker (STB), SLF80821178, to investigate the consequences of S1P transport inhibition in rodents. We found that SLF80821178 is efficacious in a multiple sclerosis model but - unlike the SRM fingolimod - neither decreases heart rate nor compromises lung endothelial barrier function. Notably, although Spns2 null mice have a sensorineural hearing defect, mice treated chronically with SLF80821178 have normal hearing acuity. STBs such as SLF80821178 evoke a dose-dependent decrease in peripheral blood lymphocyte counts, which affords a reliable pharmacodynamic marker of target engagement. However, the maximal reduction in circulating lymphocyte counts in response to SLF80821178 is substantially less than the response to SRMs such as fingolimod (50% vs. 90%) due to a lesser effect on T lymphocyte sub-populations by SLF80821178. Finally, in contrast to results obtained with Spns2 deficient mice, lymph S1P concentrations were not significantly changed in response to administration of STBs at doses that evoke maximal lymphopenia, which indicates that current understanding of the mechanism of action of S1P transport inhibitors is incomplete.

Decreased Lipid Phosphate Phosphatase 1/3 and Increased Lipid Phosphate Phosphatase 2 Expression in the Human Breast Cancer Tumor Microenvironment Promotes Tumor Progression and Immune System Evasion

The LPP family is comprised of three enzymes that dephosphorylate bioactive lipid phosphates both intracellularly and extracellularly. Pre-clinical breast cancer models have demonstrated that decreased LPP1/3 with increased LPP2 expression correlates to tumorigenesis. This though has not been well verified in human specimens. In this study, we correlate LPP expression data to clinical outcomes in over 5000 breast cancers from three independent cohorts (TCGA, METABRIC, and GSE96058), investigate biological function using gene set enrichment analysis (GSEA) and the xCell cell-type enrichment analysis, and confirm sources of LPP production in the tumor microenvironment (TME) using single-cell RNA-sequencing (scRNAseq) data. Decreased LPP1/3 and increased LPP2 expression correlated to increased tumor grade, proliferation, and tumor mutational burden (all p < 0.001), as well as worse overall survival (hazard ratios 1.3-1.5). Further, cytolytic activity was decreased, consistent with immune system invasion. GSEA data demonstrated multiple increased inflammatory signaling, survival, stemness, and cell signaling pathways with this phenotype across all three cohorts. scRNAseq and the xCell algorithm demonstrated that most tumor LPP1/3 was expressed by endothelial cells and tumor-associated fibroblasts and LPP2 by cancer cells (all p < 0.01). Restoring the balance in LPP expression levels, particularly through LPP2 inhibition, could represent novel adjuvant therapeutic options in breast cancer treatment.

Metabolomics and biochemical assays reveal the metabolic responses to hypo-salinity stress and osmoregulatory role of cAMP-PKA pathway in Mercenaria mercenaria

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Metabolomics reveals the metabolic responses of hard clam to hypo-salinity stress.

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cAMP-PKA pathway and NKA play osmoregulatory roles in hard clams.

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Activated antioxidant responses and reorganized membrane lipids occurred at 5 d.

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Alaine and lactate accumulation suggest the onset of anaerobic metabolism at 1 d.

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Fatty acids β-oxidation is promoted to provide energy for osmoregulation.

Hypo-salinity events frequently occur in marine ecosystem due to persistent rainfall and freshwater inflow, reducing the cytosol osmolarity and triggering cellular stress responses in aquatic organisms. Euryhaline bivalves have developed sophisticated regulatory mechanisms to adapt to salinity fluctuations over a long period of evolution. In this study, we performed multiple biochemical assays, widely targeted metabolomics, and gene expression analysis to investigate the comprehensive metabolic responses to hypo-salinity stress and osmoregulation mechanisms in hard clam Mercenaria mercenaria, which is a euryhaline bivalve species widely cultured in China. During hypo-salinity stress, increased vacuoles appeared in gill filaments. The Na⁺ and Cl^- concentrations in gills significantly decreased because of the up-regulation of Na⁺/K⁺-ATPase (NKA) activity. The cAMP content dramatically decreased at 5 d post hypo-salinity stress. Meanwhile, the gene expression levels of adenylate cyclase, proteinkinase A, and sodium and calcium channel proteins were evidently down-regulated, suggesting that cAMP-PKA pathway was inhibited to prevent ambient inorganic ions from entering the gill cells. Antioxidant metabolites, such as serine and Tyr-containing dipeptides, were significantly up-regulated to resist oxidative stress. Glycerolipid metabolism was strengthened to stabilize membrane structure when hypo-salinity stress was prolonged to 5 days. At 1 d post hypo-salinity stress, an increase in alanine and lactate contents marked the initiation of anaerobic metabolism. Acylcarnitines accumulation indicated that fatty acids β-oxidation was promoted to provide energy for osmoregulation. The potential biomarkers of hypo-salinity stress were identified in hard clams. This study provides novel insights into the metabolic regulatory mechanisms to hypo-salinity stress in euryhaline bivalves.

Sphingosine 1-Phosphate Metabolism and Signaling

Sphingosine 1-phosphate (S1P) is a well-defined bioactive lipid molecule derived from membrane sphingolipid metabolism. In the past decades, a series of key enzymes involved in generation of S1P have been identified and characterized in detail, as well as enzymes degrading S1P. S1P requires transporter to cross the plasma membrane and carrier to deliver to its cognate receptors and therefore transduces signaling in autocrine, paracrine, or endocrine fashions. The essential roles in regulation of development, metabolism, inflammation, and many other aspects of life are mainly executed when S1P binds to receptors provoking the downstream signaling cascades in distinct cells. This chapter will review the synthesis, degradation, transportation, and signaling of S1P and try to provide a comprehensive view of the biology of S1P, evoking new enthusiasms and ideas into the field of the fascinating S1P.

Plasticity-Related Gene 5 Is Expressed in a Late Phase of Neurodifferentiation After Neuronal Cell-Fate Determination

During adult neurogenesis, neuronal stem cells differentiate into mature neurons that are functionally integrated into the existing network. One hallmark during the late phase of this neurodifferentiation process is the formation of dendritic spines. These morphological specialized structures form the basis of most excitatory synapses in the brain, and are essential for neuronal communication. Additionally, dendritic spines are affected in neurological disorders, such as Alzheimer’s disease or schizophrenia. However, the mechanisms underlying spinogenesis, as well as spine pathologies, are poorly understood. Plasticity-related Gene 5 (PRG5), a neuronal transmembrane protein, has previously been linked to spinogenesis in vitro. Here, we analyze endogenous expression of the PRG5 protein in different mouse brain areas, as well as on a subcellular level. We found that native PRG5 is expressed dendritically, and in high abundance in areas characterized by their regenerative capacity, such as the hippocampus and the olfactory bulb. During adult neurogenesis, PRG5 is specifically expressed in a late phase after neuronal cell-fate determination associated with dendritic spine formation. On a subcellular level, we found PRG5 not to be localized at the postsynaptic density, but at the base of the synapse. In addition, we showed that PRG5-induced formation of membrane protrusions is independent from neuronal activity, supporting a possible role in the morphology and stabilization of spines.

Metabolic response of Mercenaria mercenaria under heat and hypoxia stress by widely targeted metabolomic approach

In the context of global climatic changes, marine organisms have been exposed to environmental stressors including heat and hypoxia. This calls for the design of multi-stressors to uncover the impact of oceanic factors on aquatic organisms. So far, little is known about the metabolic response of marine organisms, especially bivalves, to the combined effects of heat and hypoxia. In this study, we employed widely targeted metabolomic analysis to study the metabolic response of gills in hard clam, a heat- and hypoxia-tolerant bivalve. A total of 810 metabolites were identified. Results showed that the heat group (HT) and heat plus hypoxia group (HL) had a higher number of differential metabolites than the hypoxia group (LO). Glycolysis was affected by the heat and heat plus hypoxia stress. Moreover, anaerobic metabolic biomarkers were accumulated marking the onset of anaerobic metabolism. Environmental stresses may affect Tricarboxylic acid (TCA) cycle. Accumulation of carnitine and glycerophospholipid may promote fatty acid β oxidation and maintain cell membrane stability, respectively. The high content of oxidized lipids (i.e., Leukotriene) in HL and HT groups implied that the organisms were under ROS stress. The significantly differential metabolites of organic osmolytes and vitamins might relieve ROS stress. Moreover, accumulation of thermoprotective osmolytes (monosaccharide, Trimethylamine N-oxide (TMAO)) accumulation was helpful to maintain protein homeostasis. This investigation provided new insights into the adaptation mechanisms of hard clam to heat, hypoxia and combined stress at the metabolite level and highlighted the roles of molecules and protectants.

Ceramide Metabolism Enzymes-Therapeutic Targets against Cancer

Sphingolipids are both structural molecules that are essential for cell architecture and second messengers that are involved in numerous cell functions. Ceramide is the central hub of sphingolipid metabolism. In addition to being the precursor of complex sphingolipids, ceramides induce cell cycle arrest and promote cell death and inflammation. At least some of the enzymes involved in the regulation of sphingolipid metabolism are altered in carcinogenesis, and some are targets for anticancer drugs. A number of scientific reports have shown how alterations in sphingolipid pools can affect cell proliferation, survival and migration. Determination of sphingolipid levels and the regulation of the enzymes that are implicated in their metabolism is a key factor for developing novel therapeutic strategies or improving conventional therapies. The present review highlights the importance of bioactive sphingolipids and their regulatory enzymes as targets for therapeutic interventions with especial emphasis in carcinogenesis and cancer dissemination.

A UPLC-Q-TOF/MS-based plasma metabolomics approach reveals the mechanism of Compound Kushen Injection-based intervention against non-small cell lung cancer in Lewis tumor-bearing mice

BACKGROUND

Compound Kushen Injection (CKI), a well-known Chinese Medicine preparation, has been used to treat non-small cell lung cancer (NSCLC) for more than 15 years, and its clinical curative effect is considered to be beneficial.

HYPOTHESIS/PURPOSE

This study was designed to evaluate the effects and underlying mechanisms of CKI against NSCLC using an ultra-high-performance liquid chromatography quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF/MS)-based plasma metabolomics approach.

METHODS

4',6-diamidino-2-phenylindole (DAPI) staining and 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) dye reduction assay were employed to assess apoptosis and the viability of A549 cells with and without CKI treatment. The weight/volume of Lewis lung carcinoma (LLC) sarcomas and histopathological examinations were used to evaluate the anti-tumor effects of CKI against NSCLC. A UPLC-Q-TOF/MS method combined with multivariate data analysis was developed to characterize metabolomic fingerprinting and to screen functional biomarkers that are linked to the CKI treatment of LLC mice, and then metabolic pathway analysis was used to investigate the therapeutic mechanism of CKI.

RESULTS

DAPI staining and MTT dye reduction assays indicated that CKI-induced apoptosis and inhibited the proliferation of A549 cells, respectively, in a concentration-dependent manner. The sarcoma volumes and weights in LLC tumor-bearing mice in CKI-dosed groups were significantly lower than those in a model group, which was treated with physiological saline. Histopathological analysis of sections of sarcomas and left pulmonary lobes indicated that CKI exerts an ameliorative effect against LLC. Fourteen functional biomarkers that are related to the therapeutic effects of CKI on LLC were screened and identified using a metabolomics study. Analysis of metabolic pathways revealed that the therapeutic effects of CKI on LLC mainly involved glycerophospholipid metabolism, amino acid metabolism and sphingolipid metabolism. As glycerophospholipid metabolism is a crucial feature of cancer-specific metabolism, the enzymes that are involved in 1-acyl-sn-glycero-3-phosphoinositol biosynthesis were further evaluated. Western blotting results indicated that CKI modulated the abnormal biosynthesis pathway of 1-acyl-sn-glycero-3-phosphoinositol by activation of cytidine diphosphate-diacylglycerol-inositol 3-phosphatidyltransferase (CDIPT) and cytosolic phospholipase A2 (cPLA2), and by inhibition of lysophosphatidic acid acyltransferase gamma (AGPAT3).

CONCLUSION

This study demonstrated that CKI has a favorable anti-tumor effect and that a UPLC-Q-TOF/MS-based metabolomics method in conjunction with further verifications at the biochemical level is a promising approach for investigating its underlying mechanisms.

Lysophosphatidic Acid and Hematopoiesis: From Microenvironmental Effects to Intracellular Signaling

Vertebrate hematopoiesis is a complex physiological process that is tightly regulated by intracellular signaling and extracellular microenvironment. In recent decades, breakthroughs in lineage-tracing technologies and lipidomics have revealed the existence of numerous lipid molecules in hematopoietic microenvironment. Lysophosphatidic acid (LPA), a bioactive phospholipid molecule, is one of the identified lipids that participates in hematopoiesis. LPA exhibits various physiological functions through activation of G-protein-coupled receptors. The functions of these LPARs have been widely studied in stem cells, while the roles of LPARs in hematopoietic stem cells have rarely been examined. Nonetheless, mounting evidence supports the importance of the LPA-LPAR axis in hematopoiesis. In this article, we have reviewed regulation of hematopoiesis in general and focused on the microenvironmental and intracellular effects of the LPA in hematopoiesis. Discoveries in these areas may be beneficial to our understanding of blood-related disorders, especially in the context of prevention and therapy for anemia.

The migrations of Drosophila muscle founders and primordial germ cells are interdependent

Caudal visceral mesoderm (CVM) cells migrate from posterior to anterior of the Drosophila embryo as two bilateral streams of cells to support the specification of longitudinal muscles along the midgut. To accomplish this long-distance migration, CVM cells receive input from their environment, but little is known about how this collective cell migration is regulated. In a screen we found that wunen mutants exhibit CVM cell migration defects. Wunens are lipid phosphate phosphatases known to regulate the directional migration of primordial germ cells (PGCs). PGC and CVM cell types interact while PGCs are en route to the somatic gonadal mesoderm, and previous studies have shown that CVM impacts PGC migration. In turn, we found here that CVM cells exhibit an affinity for PGCs, localizing to the position of PGCs whether mislocalized or trapped in the endoderm. In the absence of PGCs, CVM cells exhibit subtle changes, including more cohesive movement of the migrating collective, and an increased number of longitudinal muscles is found at anterior sections of the larval midgut. These data demonstrate that PGC and CVM cell migrations are interdependent and suggest that distinct migrating cell types can coordinately influence each other to promote effective cell migration during development.

Untargeted metabolomic analysis using LC-TOF/MS and LC-MS/MS for revealing metabolic alterations linked to alcohol-induced hepatic steatosis in rat serum and plasma

Untargeted LC-MS metabolomics to screen differential metabolites in rat serum and plasma, and reveal metabolic alterations linked to AHS.

Recent advances in targeting the autotaxin-lysophosphatidate-lipid phosphate phosphatase axis in vivo

Extracellular lysophosphatidate (LPA) is a potent bioactive lipid that signals through six G-protein-coupled receptors. This signaling is required for embryogenesis, tissue repair and remodeling processes. LPA is produced from circulating lysophosphatidylcholine by autotaxin (ATX), and is degraded outside cells by a family of three enzymes called the lipid phosphate phosphatases (LPPs). In many pathological conditions, particularly in cancers, LPA concentrations are increased due to high ATX expression and low LPP activity. In cancers, LPA signaling drives tumor growth, angiogenesis, metastasis, resistance to chemotherapy and decreased efficacy of radiotherapy. Hence, targeting the ATX-LPA-LPP axis is an attractive strategy for introducing novel adjuvant therapeutic options. In this review, we will summarize current progress in targeting the ATX-LPA-LPP axis with inhibitors of autotaxin activity, LPA receptor antagonists, LPA monoclonal antibodies, and increasing low LPP expression. Some of these agents are already in clinical trials and have applications beyond cancer, including chronic inflammatory diseases.

Inhibitors of sphingosine-1-phosphate metabolism (sphingosine kinases and sphingosine-1-phosphate lyase)

Sphingolipids (SLs) are essential structural and signaling molecules of eukaryotic cells. Among them, sphingosine 1 phosphate (S1P) is a recognized promoter of cell survival, also involved, inter alia, in inflammation and tumorigenesis processes. The knowledge and modulation of the enzymes implicated in the biosynthesis and degradation of S1P are capital to control the intracellular levels of this lipid and, ultimately, to determine the cell fate. Starting with a general overview of the main metabolic pathways involved in SL metabolism, this review is mainly focused on the description of the most relevant findings concerning the development of modulators of S1P, namely inhibitors of the enzymes regulating S1P synthesis (sphingosine kinases) and degradation (sphingosine 1 phosphate phosphatase and lyase). In addition, a brief overview of the most significant agonists and antagonists at the S1P receptors is also addressed.

Lipid phosphate phosphatases and their roles in mammalian physiology and pathology

Lipid phosphate phosphatases (LPPs) are a group of enzymes that belong to a phosphatase/phosphotransferase family. Mammalian LPPs consist of three isoforms: LPP1, LPP2, and LPP3. They share highly conserved catalytic domains and catalyze the dephosphorylation of a variety of lipid phosphates, including phosphatidate, lysophosphatidate (LPA), sphingosine 1-phosphate (S1P), ceramide 1-phosphate, and diacylglycerol pyrophosphate. LPPs are integral membrane proteins, which are localized on plasma membranes with the active site on the outer leaflet. This enables the LPPs to degrade extracellular LPA and S1P, thereby attenuating their effects on the activation of surface receptors. LPP3 also exhibits noncatalytic effects at the cell surface. LPP expression on internal membranes, such as endoplasmic reticulum and Golgi, facilitates the metabolism of internal lipid phosphates, presumably on the luminal surface of these organelles. This action probably explains the signaling effects of the LPPs, which occur downstream of receptor activation. The three isoforms of LPPs show distinct and nonredundant effects in several physiological and pathological processes including embryo development, vascular function, and tumor progression. This review is intended to present an up-to-date understanding of the physiological and pathological consequences of changing the activities of the different LPPs, especially in relation to cell signaling by LPA and S1P.

Lysophosphatidic acid as a lipid mediator with multiple biological actions

Lysophosphatidic acid (LPA) is one of the simplest glycerophospholipids with one fatty acid chain and a phosphate group as a polar head. Although LPA had been viewed just as a metabolic intermediate in de novo lipid synthetic pathways, it has recently been paid much attention as a lipid mediator. LPA exerts many kinds of cellular processes, such as cell proliferation and smooth muscle contraction, through cognate G protein-coupled receptors. Because lipids are not coded by the genome directly, it is difficult to know their patho- and physiological roles. However, recent studies have identified several key factors mediating the biological roles of LPA, such as receptors and producing enzymes. In addition, studies of transgenic and gene knockout animals for these LPA-related genes, have revealed the biological significance of LPA. In this review we will summarize recent advances in the studies of LPA production and its roles in both physiological and pathological conditions.

Caveolae regulate the nanoscale organization of the plasma membrane to remotely control Ras signaling

Caveolae transduce mechanical stress into plasma membrane lipid alterations that disrupt Ras organization in an isoform-specific manner and modulate downstream signal transduction.

The molecular mechanisms whereby caveolae exert control over cellular signaling have to date remained elusive. We have therefore explored the role caveolae play in modulating Ras signaling. Lipidomic and gene array analyses revealed that caveolin-1 (CAV1) deficiency results in altered cellular lipid composition, and plasma membrane (PM) phosphatidylserine distribution. These changes correlated with increased K-Ras expression and extensive isoform-specific perturbation of Ras spatial organization: in CAV1-deficient cells K-RasG12V nanoclustering and MAPK activation were enhanced, whereas GTP-dependent lateral segregation of H-Ras was abolished resulting in compromised signal output from H-RasG12V nanoclusters. These changes in Ras nanoclustering were phenocopied by the down-regulation of Cavin1, another crucial caveolar structural component, and by acute loss of caveolae in response to increased osmotic pressure. Thus, we postulate that caveolae remotely regulate Ras nanoclustering and signal transduction by controlling PM organization. Similarly, caveolae transduce mechanical stress into PM lipid alterations that, in turn, modulate Ras PM organization.

Phospholipids: "greasing the wheels" of humoral immunity

Phospholipids are major structural components of all cellular membranes. In addition, certain phospholipids execute regulatory activities that affect cell behavior, function and fate in critically important physiological settings. The influence of phospholipids is especially obvious in the adaptive immune system, where these macromolecules mediate both intrinsic and extrinsic effects on B and T lymphocytes. This review article highlights the action of lysophospholipid sphingosine-1-phosphate as a lymphocyte chemoattractant, the function of phosphatidylinositol phosphates as signaling conduits in lymphocytes and the role of phospholipids as raw materials for membrane assembly and organelle biogenesis in activated B lymphocytes. Special emphasis is placed on the means by which these three processes push humoral immune responses forward. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Lipids and eicosanoids in fibrosis: emerging targets for therapy

PURPOSE OF REVIEW

Lipid mediators including the lysophospholipids, sphingolipids and eicosanoids have long been implicated in inflammation, cancer and numerous other diseases. Over the last decade, new research suggests a role for these mediators in fibrosis.

RECENT FINDINGS

Recent developments in the study of fibrotic mediators have centered on lysophospholipids and eicosanoids. New research is evaluating metabolic-profiling strategies to quantitatively measure lipid mediators in human plasma. Lysophosphatidic acid receptor antagonists are currently under development with early phase trials scheduled for idiopathic pulmonary fibrosis and scleroderma dermal fibrosis. Eicosanoids have long been implicated in maintaining tissue homeostasis, and the balance of profibrotic and antifibrotic effects has drawn attention in recent years. Targeting the prostanoids, specifically PGE2 and PGI2, as well as the leukotrienes is now being considered for antifibrotic therapies.

SUMMARY

Lipid mediators have significant roles in many disease processes. Significant research now suggests a critical role for these mediators in the pathogenesis of fibrosis. Targeting these mediators is a promising area of drug discovery.

Role of the autotaxin-lysophosphatidate axis in cancer resistance to chemotherapy and radiotherapy

High expression of autotaxin in cancers is often associated with increased tumor progression, angiogenesis and metastasis. This is explained mainly since autotaxin produces the lipid growth factor, lysophosphatidate (LPA), which stimulates cell division, survival and migration. It has recently become evident that these signaling effects of LPA also produce resistance to chemotherapy and radiation-induced cell death. This results especially from the stimulation of LPA(2) receptors, which depletes the cell of Siva-1, a pro-apoptotic signaling protein and stimulates prosurvival kinase pathways through a mechanism mediated via TRIP-6. LPA signaling also increases the formation of sphingosine 1-phosphate, a pro-survival lipid. At the same time, LPA decreases the accumulation of ceramides, which are used in radiation therapy and by many chemotherapeutic agents to stimulate apoptosis. The signaling actions of extracellular LPA are terminated by its dephosphorylation by a family of lipid phosphate phosphatases (LPP) that act as ecto-enzymes. In addition, lipid phosphate phoshatase-1 attenuates signaling downstream of the activation of both LPA receptors and receptor tyrosine kinases. This makes many cancer cells hypersensitive to the action of various growth factors since they often express low LPP1/3 activity. Increasing our understanding of the complicated signaling pathways that are used by LPA to stimulate cell survival should identify new therapeutic targets that can be exploited to increase the efficacy of chemo- and radio-therapy. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

Mouse SPNS2 functions as a sphingosine-1-phosphate transporter in vascular endothelial cells

Sphingosine-1-phosphate (S1P), a sphingolipid metabolite that is produced inside the cells, regulates a variety of physiological and pathological responses via S1P receptors (S1P1–5). Signal transduction between cells consists of three steps; the synthesis of signaling molecules, their export to the extracellular space and their recognition by receptors. An S1P concentration gradient is essential for the migration of various cell types that express S1P receptors, such as lymphocytes, pre-osteoclasts, cancer cells and endothelial cells. To maintain this concentration gradient, plasma S1P concentration must be at a higher level. However, little is known about the molecular mechanism by which S1P is supplied to extracellular environments such as blood plasma. Here, we show that SPNS2 functions as an S1P transporter in vascular endothelial cells but not in erythrocytes and platelets. Moreover, the plasma S1P concentration of SPNS2-deficient mice was reduced to approximately 60% of wild-type, and SPNS2-deficient mice were lymphopenic. Our results demonstrate that SPNS2 is the first physiological S1P transporter in mammals and is a key determinant of lymphocyte egress from the thymus.

Lipid phosphate phosphatase inhibitors locally amplify lysophosphatidic acid LPA1 receptor signalling in rat brain cryosections without affecting global LPA degradation

Background

Lysophosphatidic acid (LPA) is a signalling phospholipid with multiple biological functions, mainly mediated through specific G protein-coupled receptors. Aberrant LPA signalling is being increasingly implicated in the pathology of common human diseases, such as arteriosclerosis and cancer. The lifetime of the signalling pool of LPA is controlled by the equilibrium between synthesizing and degradative enzymatic activity. In the current study, we have characterized these enzymatic pathways in rat brain by pharmacologically manipulating the enzymatic machinery required for LPA degradation.

Results

In rat brain cryosections, the lifetime of bioactive LPA was found to be controlled by Mg²⁺-independent, N-ethylmaleimide-insensitive phosphatase activity, attributed to lipid phosphate phosphatases (LPPs). Pharmacological inhibition of this LPP activity amplified LPA₁ receptor signalling, as revealed using functional autoradiography. Although two LPP inhibitors, sodium orthovanadate and propranolol, locally amplified receptor responses, they did not affect global brain LPA phosphatase activity (also attributed to Mg²⁺-independent, N-ethylmaleimide-insensitive phosphatases), as confirmed by P_i determination and by LC/MS/MS. Interestingly, the phosphate analog, aluminium fluoride (AlF_x^-) not only irreversibly inhibited LPP activity thereby potentiating LPA₁ receptor responses, but also totally prevented LPA degradation, however this latter effect was not essential in order to observe AlF_x^--dependent potentiation of receptor signalling.

Conclusions

We conclude that vanadate- and propranolol-sensitive LPP activity locally guards the signalling pool of LPA whereas the majority of brain LPA phosphatase activity is attributed to LPP-like enzymatic activity which, like LPP activity, is sensitive to AlF_x^- but resistant to the LPP inhibitors, vanadate and propranolol.

Simultaneous quantitation of sphingoid bases and their phosphates in biological samples by liquid chromatography/electrospray ionization tandem mass spectrometry

We developed a liquid chromatography/electrospray ionization tandem mass spectrometry method for the simultaneous quantitative determination of C18 sphingosine (Sph), C18 dihydrosphingosine (dhSph), C18 phytosphingosine (pSph), C18 sphingosine-1-phosphate (S1P), C18 dihydrosphingosine-1-phosphate (dhS1P), and C18 phytosphingosine-1-phosphate (pS1P). Samples were prepared by simple methanol deproteinization and analyzed in selected reaction monitoring modes. No peak tailing was observed on the chromatograms using a Capcell Pak ACR column (1.5 mm i.d. × 250 mm, 3 μm, Shiseido). The calibration curves of the sphingoids showed good linearity (r > 0.996) over the range of 0.050-5.00 pmol per injection. The accuracy and precision of this method were demonstrated using four representative biological samples (serum, brain, liver, and spleen) from mice that contained known amounts of the sphingoids. Samples of mice tissue such as plasma, brain, eye, testis, liver, kidney, lung, spleen, lymph node, and thymus were examined for their Sph, dhSph, pSph, S1P, dhS1P, and pS1P composition. The results confirmed the usefulness of this method for the physiological and pathological analysis of the composition of important sphingoids.

The emerging role of lysophosphatidic acid (LPA) in skeletal biology

Lysophosphatidic acid (LPA) is the simplest signalling lipid eliciting pleiotropic actions upon most mammalian cell types. Although LPA has an established role in many biological processes, particularly wound healing and cancer, the participation of LPA in skeletal biology is just beginning to emerge. Early studies, identified in this review, gave a solid indication that LPA, via binding to one of several cell surface receptors, activated multiple intracellular systems culminating in altered cell morphology, growth, motility and survival. More recently the ablation of murine LPA1 and 4 receptors implies that this lipid has a role in skeletal development and post natal bone accrual. Greater understanding of the ability of LPA to influence, for example, osteoblast growth, maturation and survival could be advantageous in developing novel strategies aimed at improving skeletal tissue repair and regeneration. Herein this review provides an insight into the diversity of studies exploring the actions of a small lipid on those major cell types key to skeletal tissue health and homeostasis.

Metabolomics study of alcohol-induced liver injury and hepatocellular carcinoma xenografts in mice

Alcohol abuse is one of the major causes of liver injury and a promoter for hepatocellular carcinoma (HCC). To understand the disease-associated metabolic changes, we investigated and compared the profiles of metabolites in nude mice with alcohol-induced liver injury or bearing a HCC xenograft (HCCX). Alcohol-induced liver injury was achieved by daily administration of grain liquor, and HCC xenografts were generated by subcutaneous inoculation of HepG2 cells in nude mice. Metabolites in serum samples were profiled by ultra-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UPLC/Q-TOF MS). The acquired data was analyzed by principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA) to identify potential disease-specific biomarkers. Results showed that the phosphatidylcholine (PC) levels were significantly higher in both liver injury and HCCX mice compared with the control. Interestingly, lysophosphatidylcholines (LPCs) that contain saturated or monounsaturated fatty acids were reduced in both liver injury and HCCX mice, but polyunsaturated fatty acids LPCs were elevated in liver injury mice only. These data delineated the disease-related metabolic alterations of LPCs in liver injury and HCC, suggesting that the LPC profile in serum may be biomarkers for these two common liver diseases.

Lipid phosphate phosphatase 3 enables efficient thymic egress

Lipid phosphate phosphatase 3 in endothelial and epithelial cells promotes efficient T cell emigration from the thymus to the periphery.

The signaling lipid sphingosine-1-phosphate (S1P) stabilizes the vasculature, directs lymphocyte egress from lymphoid organs, and shapes inflammatory responses. However, little is known about how S1P distribution is controlled in vivo, and it is not clear how a ubiquitously made lipid functions as a signal that requires precise spatial and temporal control. We have found that lipid phosphate phosphatase 3 (LPP3) enables efficient export of mature T cells from the thymus into circulation, and several lines of evidence suggest that LPP3 promotes exit by destroying thymic S1P. Although five additional S1P-degrading enzymes are expressed in the thymus, they cannot compensate for the loss of LPP3. Moreover, conditional deletion of LPP3 in either epithelial cells or endothelial cells is sufficient to inhibit egress. These results suggest that S1P generation and destruction are tightly regulated and that LPP3 is essential to establish the balance.

Genetic evidence for antagonism between Pak protein kinase and Rho1 small GTPase signaling in regulation of the actin cytoskeleton during Drosophila oogenesis

During Drosophila oogenesis, basally localized F-actin bundles in the follicle cells covering the egg chamber drive its elongation along the anterior-posterior axis. The basal F-actin of the follicle cell is an attractive system for the genetic analysis of the regulation of the actin cytoskeleton, and results obtained in this system are likely to be broadly applicable in understanding tissue remodeling. Mutations in a number of genes, including that encoding the p21-activated kinase Pak, have been shown to disrupt organization of the basal F-actin and in turn affect egg chamber elongation. pak mutant egg chambers have disorganized F-actin distribution and remain spherical due to a failure to elongate. In a genetic screen to identify modifiers of the pak rounded egg chamber phenotype several second chromosome deficiencies were identified as suppressors. One suppressing deficiency removes the rho1 locus, and we determined using several rho1 alleles that removal of a single copy of rho1 can suppress the pak phenotype. Reduction of any component of the Rho1-activated actomyosin contractility pathway suppresses pak oogenesis defects, suggesting that Pak counteracts Rho1 signaling. There is ectopic myosin light chain phosphorylation in pak mutant follicle cell clones in elongating egg chambers, probably due at least in part to mislocalization of RhoGEF2, an activator of the Rho1 pathway. In early egg chambers, pak mutant follicle cells have reduced levels of myosin phosphorylation and we conclude that Pak both promotes and restricts myosin light chain phosphorylation in a temporally distinct manner during oogenesis.

Regulation of phosphatidic Acid metabolism by sphingolipids in the central nervous system

This paper explores the way ceramide, sphingosine, ceramide 1-phosphate, and sphingosine 1-phosphate modulate the generation of second lipid messengers from phosphatidic acid in two experimental models of the central nervous system: in vertebrate rod outer segments prepared from dark-adapted retinas as well as in rod outer segments prepared from light-adapted retinas and in rat cerebral cortex synaptosomes under physiological aging conditions. Particular attention is paid to lipid phosphate phosphatase, diacylglycerol lipase, and monoacylglycerol lipase. Based on the findings reported in this paper, it can be concluded that proteins related to phototransduction phenomena are involved in the effects derived from sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide and that age-related changes occur in the metabolism of phosphatidic acid from cerebral cortex synaptosomes in the presence of either sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide. The present paper demonstrates, in two different models of central nervous system, how sphingolipids influence phosphatidic acid metabolism under different physiological conditions such as light and aging.

LPA(3), a unique G protein-coupled receptor for lysophosphatidic acid

Lysophosphatidic acid (LPA; 1- or 2-acyl-sn-glycerol-3-phosphate) is a phospholipid that is involved in numerous normal physiological and pathological processes such as brain development, blood vessel formation, embryo implantation, hair growth, neuropathic pain, lung fibrosis and colon cancer. Most of these functions are mediated by G protein-coupled receptors (GPCRs) specific to LPA. So far, six GPCRs for LPA have been identified: LPA(1)/Edg2, LPA(2)/Edg4, LPA(3)/Edg7, LPA(4)/GPR23/P2Y9, LPA(5)/GPR92 and LPA(6)/P2Y5. An intracellular target of LPA has also been proposed. Among the LPA receptors, LPA(3) is unique in that it is activated significantly by a specific form of LPA (2-acyl LPA with unsaturated fatty acids) and is expressed in a limited number of tissues such as the reproductive organs. Recent studies have shown that LPA(3)-mediated LPA signaling is essential for proper embryo implantation and have revealed an unexpected genetic linkage between LPA and prostaglandin signaling. Here we review recent advances in the study of LPA(3), especially studies using LPA(3)-deficient mice. In addition, we focus on the agonists and antagonists that are specific to each LPA receptor as important tools for the functional study of LPA signaling.

Role of alkaline ceramidases in the generation of sphingosine and its phosphate in erythrocytes

Plasma sphingosine-1-phosphate (S1P) has been suggested to mainly originate from erythrocytes; however, within the erythrocyte, how sphingosine (SPH) generation--the precursor to S1P--is controlled is unknown. SPH is only generated from the hydrolysis of ceramides via ceramidases. Five human ceramidases have been identified: 1 acid, 1 neutral, and 3 alkaline ceramidases (ACER1, ACER2, and ACER3). Here, we demonstrate that only alkaline ceramidase activity is expressed in erythrocytes and that it is instrumental for SPH generation. Erythrocytes have alkaline but not acid or neutral ceramidase activity on D-e-C(18:1)-ceramide, a common substrate of ceramidases. Not only alkaline ceramidase activity but also the generation of SPH and S1P are increased during erythroid differentiation in K562 erythroleukemic cells. Such SPH and S1P increases were inhibited by the alkaline ceramidase inhibitor D-e-MAPP, suggesting that alkaline ceramidases have a role in the generation of SPH and S1P in erythroid cells. Alkaline ceramidase activity is highly expressed in mouse erythrocytes, and intravenous administration of D-e-MAPP decreased both SPH and S1P in erythrocytes and plasma. Collectively, these results suggest that alkaline ceramidase activity is important for the generation of SPH, the S1P precursor in erythrocytes.

Recent advances in understanding the role of lysophosphatidic acid in the pathogenesis of gastrointestinal tumors

Lysophosphatidic acid is an important intercellular lipid mediator that acts through G-protein coupled receptors to mediate the development and progression of a variety of tumors. In this article, we will summarize the structure and physiological function of lysophosphatidic acid and review its role in the pathogenesis of colon cancer, gastric cancer, liver cancer and other gastrointestinal tumors.

An overview of sphingolipid metabolism: from synthesis to breakdown

Sphingolipids constitute a class of lipids defined by their eighteen carbon amino-alcohol backbones which are synthesized in the ER from nonsphingolipid precursors. Modification of this basic structure is what gives rise to the vast family of sphingolipids that play significant roles in membrane biology and provide many bioactive metabolites that regulate cell function. Despite the diversity of structure and function of sphingolipids, their creation and destruction are governed by common synthetic and catabolic pathways. In this regard, sphingolipid metabolism can be imagined as an array of interconnected networks that diverge from a single common entry point and converge into a single common breakdown pathway. In their simplest forms, sphingosine, phytosphingosine and dihydrosphingosine serve as the backbones upon which further complexity is achieved. For example, phosphorylation of the C1 hydroxyl group yields the final breakdown products and/or the important signaling molecules sphingosine-1-phosphate, phytosphingosine-1-phosphate and dihydrosphingosine-1-phosphate, respectively. On the other hand, acylation of sphingosine, phytosphingosine, or dihydrosphingosine with one of several possible acyl CoA molecules through the action of distinct ceramide synthases produces the molecules defined as ceramide, phytoceramide, or dihydroceramide. Ceramide, due to the differing acyl CoAs that can be used to produce it, is technically a class of molecules rather than a single molecule and therefore may have different biological functions depending on the acyl chain it is composed of. At the apex of complexity is the group of lipids known as glycosphingolipids (GSL) which contain dozens of different sphingolipid species differing by both the order and type of sugar residues attached to their headgroups. Since these molecules are produced from ceramide precursors, they too may have differences in their acyl chain composition, revealing an additional layer of variation. The glycosphingolipids are divided broadly into two categories: glucosphingolipids and galactosphingolipids. The glucosphingolipids depend initially on the enzyme glucosylceramide synthase (GCS) which attaches glucose as the first residue to the C1 hydroxyl position. Galactosphingolipids, on the other hand, are generated from galactosylceramide synthase (GalCerS), an evolutionarily dissimilar enzyme from GCS. Glycosphingolipids are further divided based upon further modification by various glycosyltransferases which increases the potential variation in lipid species by several fold. Far more abundant are the sphingomyelin species which are produced in parallel with glycosphingolipids, however they are defined by a phosphocholine headgroup rather than the addition of sugar residues. Although sphingomyelin species all share a common headgroup, they too are produced from a variety of ceramide species and therefore can have differing acyl chains attached to their C-2 amino groups. Whether or not the differing acyl chain lengths in SMs dictate unique functions or important biophysical distinctions has not yet been established. Understanding the function of all the existing glycosphingolipids and sphingomyelin species will be a major undertaking in the future since the tools to study and measure these species are only beginning to be developed (see Fig 1 for an illustrated depiction of the various sphingolipid structures). The simple sphingolipids serve both as the precursors and the breakdown products of the more complex ones. Importantly, in recent decades, these simple sphingolipids have gained attention for having significant signaling and regulatory roles within cells. In addition, many tools have emerged to measure the levels of simple sphingolipids and therefore have become the focus of even more intense study in recent years. With this thought in mind, this chapter will pay tribute to the complex sphingolipids, but focus on the regulation of simple sphingolipid metabolism.

Lipid second messengers and related enzymes in vertebrate rod outer segments

Rod outer segments (ROSs) are specialized light-sensitive organelles in vertebrate photoreceptor cells. Lipids in ROS are of considerable importance, not only in providing an adequate environment for efficient phototransduction, but also in originating the second messengers involved in signal transduction. ROSs have the ability to adapt the sensitivity and speed of their responses to ever-changing conditions of ambient illumination. A major contributor to this adaptation is the light-driven translocation of key signaling proteins into and out of ROS. The present review shows how generation of the second lipid messengers from phosphatidylcholine, phosphatidic acid, and diacylglycerol is modulated by the different illumination states in the vertebrate retina. Findings suggest that the light-induced translocation of phototransduction proteins influences the enzymatic activities of phospholipase D, lipid phosphate phosphatase, diacylglyceride lipase, and diacylglyceride kinase, all of which are responsible for the generation of the second messenger molecules.

Inducible capillary formation in lymphatic endothelial cells by blocking lipid phosphate phosphatase-3 activity

Lymphangiogenesis plays critical roles under normal and/or pathological conditions; however, the molecular contributors to this event were unknown until recently. In the present study, we first employed gene chip analysis and confirmed that lipid phosphate phosphatase-3 (LPP3) expression was increased until capillary formation in the conditionally immortalized rat lymphatic endothelial cell line. Signaling responses occur when several lipids induce acute biological functions; further, lipid phosphate phosphatases (LPPs) control their functions via dephosphorylation; however, there is no report on the association between LPP3 and lymphangiogenesis. siRNA-targeted LPP3 significantly increased capillary formation of human lymphatic endothelial cells; in contrast, it decreased cell adhesion to the basement membrane matrix. Furthermore, the inducible effect of the LPP inhibitor on capillary formation was observed. For the first time, we report that LPP3 abolishes accelerated abnormal lymphangiogenesis. Blocking LPP3 activities may aid in the development of novel therapy for lymph vessel defects.

Sphingolipids in inflammation: pathological implications and potential therapeutic targets

Sphingolipids are formed via the metabolism of sphingomyelin, a constituent of the plasma membrane, or by de novo synthesis. Enzymatic pathways result in the formation of several different lipid mediators, which are known to have important roles in many cellular processes, including proliferation, apoptosis and migration. Several studies now suggest that these sphingolipid mediators, including ceramide, ceramide 1-phosphate and sphingosine 1-phosphate (S1P), are likely to have an integral role in inflammation. This can involve, for example, activation of pro-inflammatory transcription factors in different cell types and induction of cyclooxygenase-2, leading to production of pro-inflammatory prostaglandins. The mode of action of each sphingolipid is different. Increased ceramide production leads to the formation of ceramide-rich areas of the membrane, which may assemble signalling complexes, whereas S1P acts via high-affinity G-protein-coupled S1P receptors on the plasma membrane. Recent studies have demonstrated that in vitro effects of sphingolipids on inflammation can translate into in vivo models. This review will highlight the areas of research where sphingolipids are involved in inflammation and the mechanisms of action of each mediator. In addition, the therapeutic potential of drugs that alter sphingolipid actions will be examined with reference to disease states, such as asthma and inflammatory bowel disease, which involve important inflammatory components. A significant body of research now indicates that sphingolipids are intimately involved in the inflammatory process and recent studies have demonstrated that these lipids, together with associated enzymes and receptors, can provide effective drug targets for the treatment of pathological inflammation.

Lipid phosphate phosphohydrolase type 1 (LPP1) degrades extracellular lysophosphatidic acid in vivo

LPA (lysophosphatidic acid) is a lipid mediator that stimulates cell proliferation and growth, and is involved in physiological and pathological processes such as wound healing, platelet activation, angiogenesis and the growth of tumours. Therefore defining the mechanisms of LPA production and degradation are of interest in understanding the regulation of these processes. Extracellular LPA synthesis is relatively well understood, whereas the mechanisms of its degradation are not. One route of LPA degradation is dephosphorylation. A candidate enzyme is the integral membrane exophosphatase LPP1 (lipid phosphate phosphohydrolase type 1). In the present paper, we report the development of a mouse wherein the LPP1 gene (Ppap2a) was disrupted. The homozygous mice, which are phenotypically unremarkable, generally lack Ppap2a mRNA, and multiple tissues exhibit a substantial (35-95%) reduction in LPA phosphatase activity. Compared with wild-type littermates, Ppap2a(tr/tr) animals have increased levels of plasma LPA, and LPA injected intravenously is metabolized at a 4-fold lower rate. Our results demonstrate that LPA is rapidly metabolized in the bloodstream and that LPP1 is an important determinant of this turnover. These results indicate that LPP1 is a catabolic enzyme for LPA in vivo.

Phosphatidate degradation: phosphatidate phosphatases (lipins) and lipid phosphate phosphatases

Three lipid phosphate phosphatases (LPPs) regulate cell signaling by modifying the concentrations of a variety of lipid phosphates versus their dephosphorylated products. In particular, the LPPs are normally considered to regulate signaling by the phospholipase D (PLD) pathway by converting phosphatidate (PA) to diacylglycerol (DAG). LPP activities do modulate the accumulations of PA and DAG following PLD activation, but this could also involve an effect upstream of PLD activation. The active sites of the LPPs are on the exterior surface of plasma membranes, or on the luminal surface of internal membranes. Consequently, the actions of the LPPs in metabolizing PA formed by PLD1 or PLD2 should depend on the access of this substrate to the active site of the LPPs. Alternatively, PA generated on the cytosolic surface of membranes should be readily accessible to the family of specific phosphatidate phosphatases, namely the lipins. Presently, there is only indirect evidence for the lipins participating in cell signaling following PLD activation. So far, we know relatively little about how individual LPPs and specific phosphatidate phosphatases (lipins) modulate cell signaling through controlling the turnover of bioactive lipids that are formed after PLD activation.

Lipid phosphate phosphatases and signaling

Lipid phosphate phosphatases (LPPs) regulate cell signaling by modifying the concentrations of lipid phosphates versus their dephosphorylated products. The ecto-activity regulates the availability of extracellular lysophosphatidate (LPA) and sphingosine 1-phosphate (S1P) and thereby signaling by their respective receptors. LPP products (monoacylglycerol or sphingosine) are taken up by cells and rephosphorylated to produce LPA and S1P, respectively, which activate intracellular signaling cascades. The proposed integrin binding domain on the external surface of LPP3 modifies cell/cell interactions. Expression of LPPs on internal membranes controls signaling depending on the access of lipid phosphates to their active sites. Different LPPs perform distinct functions, probably based on integrin binding, their locations, and their abilities to metabolize different lipid phosphates in vivo.

LPA modulates monocyte migration directly and via LPA-stimulated endothelial cells

Lysophosphatidic acid (LPA) is a bioactive lysophospholipid ligand present in oxidized low-density lipoprotein. The effects of LPA were investigated, first separately on endothelial cells (EC) and monocytes. Using Ki16425 (an LPA(1) and LPA(3) receptor antagonist), GW9662 [a peroxisome proliferator-activator receptor (PPARgamma) antagonist], and pertussis toxin (that inhibits G(i/o)), we demonstrate that LPA enhances IL-8 and monocyte chemoattractant protein-1 expression through a LPA(1)-, LPA(3)-, G(i/o)- and PPARgamma-dependent manner in the EAhy926 cells. The effect of LPA on chemokine overexpression was confirmed in human umbilical vein endothelial cells. LPA was able to enhance monocyte migration at concentrations <1 microM and to inhibit their migration at LPA concentrations >1 microM, as demonstrated by using a chemotaxis assay. We then investigated the effects of LPA on the cross-talk between EC and monocytes by evaluating the chemotactic activity in the supernatants of LPA-treated EC. At 1 microM LPA, both cell types respond cooperatively, favoring monocyte migration. At higher LPA concentration (25 microM), the chemotactic response varies as a function of time. After 4 h, the chemotactic effect of the cytokines secreted by the EC is counteracted by the direct inhibitory effect of LPA on monocytes. For longer periods of time (24 h), we observe a monocyte migration, probably due to lowered concentrations of bioactive LPA, given the induction of lipid phosphate phosphatase-2 in monocytes that may inactivate LPA. These results suggest that LPA activates EC to secrete chemokines that in combination with LPA itself might favor or not favor interactions between endothelium and circulating monocytes.

Identification of the orphan GPCR, P2Y(10) receptor as the sphingosine-1-phosphate and lysophosphatidic acid receptor

Phylogenetic analysis of transmembrane regions of GPCRs using PHYLIP indicated that the orphan receptor P2Y(10) receptor was classified into the cluster consisting nucleotide and lipid receptors. Based on the results, we studied the abilities of nucleotides and lipids to activate the P2Y(10) receptors. As a result, sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA) evoked intracellular Ca(2+) increases in the CHO cells stably expressing the P2Y(10) fused with a G(16alpha) protein. These Ca(2+) responses were inhibited by S1P receptor and LPA receptor antagonists. The introduction of siRNA designed for P2Y(10) receptor into the P2Y(10)-CHO cells effectively blocked both S1P- and LPA-induced Ca(2+) increases. RT-PCR analysis showed that the mouse P2Y(10) was expressed in reproductive organs, brain, lung and skeletal muscle, suggesting the receptor plays physiological roles throughout the whole body. In conclusion, the P2Y(10) receptor is the first receptor identified as a dual lysophospholipid receptor.

Plasma sphingosine 1-phosphate metabolism and analysis

The importance of sphingosine 1-phosphate (Sph-1-P) as an intercellular sphingolipid mediator has been established in various systems, and this is especially true in the areas of vascular biology and immunology. Blood platelets store Sph-1-P abundantly and release this bioactive lysophospholipid extracellularly upon stimulation, while vascular endothelial cells and smooth muscle cells respond dramatically to this platelet-derived bioactive lipid. Most of the responses elicited by extracellular Sph-1-P are believed to be mediated by G protein-coupled cell surface receptors, i.e., S1Ps. It is likely that regulation of Sph-1-P biological activity could be important for therapeutics, including but not limited to control of vascular disorders. Furthermore, elucidation of the mechanisms by which the levels of Sph-1-P in the blood are regulated seems important. Accordingly, the application of Sph-1-P analysis to laboratory medicine may be an important task in clinical medicine. In this review, Sph-1-P-related metabolism in the plasma will be summarized. Briefly, the levels and bioactivities of plasma Sph-1-P in vivo may be regulated by various factors, including Sph-1-P release from platelets (and red blood cells, based upon the recent reports), Sph-1-P distribution between albumin and lipoproteins, and S1P expression and lipid phosphate phosphatase activity on the cell surface. Then, application of Sph-1-P analysis to laboratory medicine will be discussed.

Targeting the conversion of ceramide to sphingosine 1-phosphate as a novel strategy for cancer therapy

Sphingolipids not only function as structural components of cell membranes but also act as signaling molecules to regulate fundamental cellular responses, such as cell death and differentiation, proliferation and certain types of inflammation. Particularly the cellular balance between ceramide and sphingosine 1-phosphate seems to be crucial for a cell's decision to either undergo apoptosis or proliferate, two events which are implicated in tumor development and growth. Whereas ceramide possesses proapoptotic capacity in many cell types, sphingosine 1-phosphate acts as a counterplayer able to induce cell proliferation and protect cells from undergoing apoptosis. Therefore, tipping the balance in favour of ceramide production, i.e. by inhibiting ceramidase or sphingosine kinase activities has potential to support its proapoptotic action and hence represents a promising rational approach to effective cancer therapy. This review highlights most recent data on the regulation of cellular sphingolipid formation and their potential implication in tumor development, and provides perspectives for their use as targets in molecular intervention therapy.