Sphingosine 1-phosphate regulates melanoma cell motility through a receptor-coupled extracellular action and in a pertussis toxin-insensitive manner

The status of the lysophosphatidic acid receptor type 1 (LPA1R)

The current status of the LPA₁receptor and its ligands in the drug development pipeline is reviewed.

Therapeutic potential of targeting SK1 in human cancers

Sphingosine kinase 1 (SK1) is a lipid enzyme with oncogenic properties that converts the proapoptotic lipids ceramide and sphingosine into the antiapoptotic lipid sphingosine-1-phosphate and activates the signal transduction pathways that lead to cell proliferation, migration, the activation of the inflammatory response, and the impairment of apoptosis. There is compelling evidence that SK1 activation contributes to cancer progression leading to increased oncogenic transformation, tumor growth, resistance to therapies, tumor neovascularization, and metastatic spread. High levels of SK1 expression or activity have been associated with a poor prognosis in several human cancers. Recent studies using cancer cell and mouse models demonstrate a significant potential for SK1-targeting therapies to synergize with the effects of chemotherapy and radiotherapy; however, until recently the absence of clinically applicable SK1 inhibitors has limited the translation of these findings into patients. With the recent discovery of SK1 inhibiting properties of a clinically approved drug FTY720 (Fingolimod), SK1 has gained significant attention from both clinicians and the pharmaceutical industry and it is hoped that trials of newly developed SK1 inhibitors may follow soon. This review provides an overview of the SK1 signaling, its relevance to cancer progression, and the potential clinical significance of targeting SK1 for improved local or systemic control of human cancers.

Regulation of cancer cell migration and invasion by sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that has been implicated in regulation of a number of cancer cell malignant behaviors, including cell proliferation, survival, chemotherapeutic resistance and angiogenesis. However, the effects of S1P on cancer cell migration, invasion and metastasis, are perhaps its most complex, due to the fact that, depending upon the S1P receptors that mediate its responses and the crosstalk with other signaling pathways, S1P can either positively or negatively regulate invasion. This review summarizes the effects of S1P on cancer cell invasion and the mechanisms by which it affects this important aspect of cancer cell behavior.

Identification of the hydrophobic ligand binding pocket of the S1P1 receptor

Sphingosine 1-phosphate (S1P), a naturally occurring sphingolipid mediator and also a second messenger with growth factor-like actions in almost every cell type, is an endogenous ligand of five G protein-coupled receptors (GPCRs) in the endothelial differentiation gene family. The lack of GPCR crystal structures sets serious limitations to rational drug design and in silico searches for subtype-selective ligands. Here we report on the experimental validation of a computational model of the ligand binding pocket of the S1P1 GPCR surrounding the aliphatic portion of S1P. The extensive mutagenesis-based validation confirmed 18 residues lining the hydrophobic ligand binding pocket, which, combined with the previously validated three head group-interacting residues, now complete the mapping of the S1P ligand recognition site. We identified six mutants (L3.43G/L3.44G, L3.43E/L3.44E, L5.52A, F5.48G, V6.40L, and F6.44G) that maintained wild type [32P]S1P binding with abolished ligand-dependent activation by S1P. These data suggest a role for these amino acids in the conformational transition of S1P1 to its activated state. Three aromatic mutations (F5.48Y, F6.44G, and W6.48A) result in differential activation, by S1P or SEW2871, indicating that structural differences between the two agonists can partially compensate for differences in the amino acid side chain. The now validated ligand binding pocket provided us with a pharmacophore model, which was used for in silico screening of the NCI, National Institutes of Health, Developmental Therapeutics chemical library, leading to the identification of two novel nonlipid agonists of S1P1.

Trichostatin A and 5-aza-2'-deoxycytidine switch S1P from an inhibitor to a stimulator of motility through epigenetic regulation of S1P receptors

The histone deacetylase inhibitor, trichostatin A (TSA), and the DNA methyltransferase inhibitor, 5-aza-2'-deoxycytidine (Aza-dC), induced epigenetic regulation of sphingosine-1-phosphate (S1P) receptors in human melanoma cells, switching S1P from motility inhibitor to stimulator. Quantitative PCR revealed increased expression of S1P(1) and S1P(3), associated with S1P-induced chemotaxis, and decreased expression of S1P(2), associated with motility inhibition. Expression of lysophosphatidic acid (LPA) receptors was less affected. The TSA effect was reversible suggesting no mutational change, and Aza-dC treatment resulted in demethylation of a putative S1P(1) promoter. S1P receptors, therefore, appear to be susceptible to epigenetic regulation, accompanied by altered cellular functionality.

Sphingosine 1-phosphate receptor expression profile and regulation of migration in human thyroid cancer cells

S1P (sphingosine 1-phosphate) receptor expression and the effects of S1P on migration were studied in one papillary (NPA), two follicular (ML-1, WRO) and two anaplastic (FRO, ARO) thyroid cancer cell lines, as well as in human thyroid cells in primary culture. Additionally, the effects of S1P on proliferation, adhesion and calcium signalling were addressed in ML-1 and FRO cells. All cell types expressed multiple S1P receptors. S1P evoked intracellular calcium signalling in primary cultures, ML-1 cells and FRO cells. Neither proliferation nor migration was affected in primary cultures, whereas S1P partly inhibited proliferation in ML-1 and FRO cells. Low nanomolar concentrations of S1P inhibited migration in FRO, WRO and ARO cells, but stimulated ML-1 cell migration. Consistently, S1P1 and S1P3, which mediate migratory responses, were strongly expressed in ML-1 cells, and S1P2, which inhibits migration, was the dominating receptor in the other cell lines. The migratory effect in ML-1 cells was mediated by G(i) and phosphatidylinositol 3-kinase. Both S1P and the S1P1-specific agonist SEW-2871 induced Akt phosphorylation at Ser473. However, SEW-2871 failed to stimulate migration, whereas the S1P1/S1P3 antagonist VPC 23019 inhibited S1P-induced migration. The results suggest that aberrant S1P receptor expression may enhance thyroid cancer cell migration and thus contribute to the metastatic behaviour of some thyroid tumours.

Sphingosine 1-phosphate regulates cytoskeleton dynamics: implications in its biological response

The bioactive sphingolipid sphingosine 1-phosphate (S1P) elicits robust cytoskeletal rearrangement in a large variety of cell systems, mainly acting through a panel of specific cell surface receptors, named S1P receptors. Recent studies have begun to delineate the molecular mechanisms involved in the complex process responsible for cytoskeletal rearrangement following S1P ligation to its receptors. Notably, changes of cell shape and/or motility induced by S1P via cytoskeletal remodelling are functional to the biological action exerted by S1P which appears to be highly cell-specific. This review focuses on the current knowledge of the regulatory mechanisms of cytoskeleton dynamics elicited by S1P, with special emphasis on the relationship between cytoskeletal remodelling and the biological effects evoked by the sphingolipid in various cell types.

Sphingosine 1-phosphate inhibits cell migration in C2C12 myoblasts

This study shows that sphingosine 1-phosphate (S1P) exerts an anti-migratory action in C2C12 myoblasts by reducing directional cell motility and fully abrogating the chemotactic response to insulin-like growth factor-1. The anti-migratory response to S1P required ligation to S1P(2), being attenuated in myoblasts where the receptor was down-regulated by specific antisense oligodeoxyribonucleotides or small interfering RNA (siRNA) and conversely potentiated in S1P(2)-overexpressing myoblasts. The investigation of RhoA and Rac GTPases, critically implicated in cell motility regulation, demonstrated that RhoA was rapidly activated by S1P, while Rac1 was unaffected within the first 5 min but stimulated thereafter. RhoA, but not Rac activation, was identified as a S1P(2)-dependent pathway in experiments in which receptor expression was attenuated by siRNA treatment or up-regulated by S1P(2)-encoding plasmid transfection. Finally, by expression of the dominant negative mutant of RhoA, the GTPase was found implicated in the anti-migratory action of S1P, whereas modulation of Rac1 functionality unaffected the anti-chemotactic effect of S1P, ruling out a role for this protein in the biological response. Since S1P was previously shown to inhibit myoblast proliferation and stimulate myogenesis, the here identified novel biological activity is in favour of a complex physiological role of the sphingolipid in the process of muscle repair.

Extracellular release of newly synthesized sphingosine-1-phosphate by cerebellar granule cells and astrocytes

Sphingosine-1-phosphate (S1P) is a potent biomediator that can act as either an intracellular or an intercellular messenger. In the nervous system it exerts a wide range of actions, and specific membrane receptors for it have been identified in various regions. However, the physiological origin of extracellular S1P in the nervous system is largely unknown. We investigated cerebellar granule cells at different stages of differentiation and astrocytes in primary cultures as possible origins of extracellular S1P. Although these cells show marked differences in S1P metabolism, we found that they can all release S1P and express mRNAs for S1P specific receptors. Extracellular S1P derives from the export of newly synthesized intracellular S1P, and not from the action of a released sphingosine kinase. S1P release is rapid, efficient, and can be regulated by exogenous stimuli. Phorbol ester treatment resulted in an increase in sphingosine kinase 1 activity in the membranes, accompanied by a significant increase in extracellular S1P. S1P release in cells from the cerebellum emerges as a regulated mechanism, possibly related to a specific pool of newly synthesized S1P. To our knowledge, this is the first evidence of the extracellular release of S1P by primary cells from the CNS, which supports a role of S1P as autocrine/paracrine physiological messenger in the cerebellum.

Sphingosine-1-phosphate plays a role in the suppression of lateral pseudopod formation during Dictyostelium discoideum cell migration and chemotaxis

Sphingosine-1-phosphate (S-1-P) is a bioactive lipid that plays a role in diverse biological processes. It functions both as an extracellular ligand through a family of high-affinity G-protein-coupled receptors, and intracellularly as a second messenger. A growing body of evidence has implicated S-1-P in controlling cell movement and chemotaxis in cultured mammalian cells. Mutant D. discoideum cells, in which the gene encoding the S-1-P lyase had been specifically disrupted by homologous recombination, previously were shown to be defective in pseudopod formation, suggesting that a resulting defect might exist in motility and/or chemotaxis. To test this prediction, we analyzed the behavior of mutant cells in buffer, and in both spatial and temporal gradients of the chemoattractant cAMP, using computer-assisted 2-D and 3-D motion analysis systems. Under all conditions, S-1-P lyase null mutants were unable to suppress lateral pseudopod formation like wild-type control cells. This resulted in a reduction in velocity in buffer and spatial gradients of cAMP. Mutant cells exhibited positive chemotaxis in spatial gradients of cAMP, but did so with lowered efficiency, again because of their inability to suppress lateral pseudopod formation. Mutant cells responded normally to simulated temporal waves of cAMP but mimicked the temporal dynamics of natural chemotactic waves. The effect must be intracellular since no homologs of the S-1-P receptors have been identified in the Dictyostelium genome. The defects in the S-1-P lyase null mutants were similar to those seen in mutants lacking the genes for myosin IA, myosin IB, and clathrin, indicating that S-1-P signaling may play a role in modulating the activity or organization of these cytoskeletal elements in the regulation of lateral pseudopod formation.

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.

Endothelial differentiation gene receptors in pancreatic islets and INS-1 cells

The endothelial differentiation gene (EDG) receptors are a class of G protein-coupled receptors. EDG-1, -3, -5, -6, and -8 bind the bioactive lipid sphingosine-1-phosphate (SPP) as the primary signaling ligand. EDG-2, -4, and -7 bind the ligand lysophosphatidic acid. EDG-1, -2, -3, -5, -6, and -7, but not -8, mRNAs were expressed in isolated rat pancreatic islets, whereas INS-1 insulinoma cells expressed only EDG-1, -2, -3, and -5 mRNAs. EDG-4 mRNA was expressed in mouse islets. EDG-1 mRNA but not EDG-3 mRNA was rapidly induced relative to 18S rRNA after stimulation of isolated islets with phorbol 12-myristate 13-acetate (PMA) or cholecystokinin-8S for 2 h. The protein kinase C inhibitor GF 109203X blocked the EDG-1 induction by PMA. Similarly, in islets stimulated for 2 h with 17 mmol/l glucose, the relative EDG-1 mRNA levels increased almost twofold compared with levels in control islets at 5.5 mmol/l glucose. In contrast, after 11 mmol/l glucose stimulation for 7 days, the relative levels of rat islet EDG-1 mRNA were significantly reduced to 54% below that of islets cultured at 5.5 mmol/l glucose. There was no change in relative EDG-3 mRNA levels. Stimulation of EDG receptors in islets and INS-1 cells with SPP inhibited glucagon-like peptide 1 (GLP-1)-stimulated cAMP production and insulin secretion in a concentration-dependent manner. Pertussis toxin antagonized the SPP effects on insulin release. Thus, EDG receptors are expressed in pancreatic islet beta-cells and G(i) seems to mediate the inhibition by SPP of adenylyl cyclase and cAMP formation and inhibition of the stimulation of insulin secretion by GLP-1.

ERK and RhoA differentially regulate pseudopodia growth and retraction during chemotaxis

Nonmotile cells extend and retract pseudopodia-like structures in a random manner, whereas motile cells establish a single dominant pseudopodium in the direction of movement. This is a critical step necessary for cell migration and occurs prior to cell body translocation, yet little is known about how this process is regulated. Here we show that myosin II light chain (MLC) phosphorylation at its regulatory serine 19 is elevated in growing and retracting pseudopodia. MLC phosphorylation in the extending pseudopodium was associated with strong and persistent amplification of extracellular-regulated signal kinase (ERK) and MLC kinase activity, which specifically localized to the leading pseudopodium. Interestingly, inhibition of ERK or MLC kinase activity prevented MLC phosphorylation and pseudopodia extension but not retraction. In contrast, inhibition of RhoA activity specifically decreased pseudopodia retraction but not extension. Importantly, inhibition of RhoA activity specifically blocked MLC phosphorylation associated with retracting pseudopodia. Inhibition of either ERK or RhoA signals prevents chemotaxis, indicating that both pathways contribute to the establishment of cell polarity and migration. Together, these findings demonstrate that ERK and RhoA are distinct pathways that control pseudopodia extension and retraction, respectively, through differential modulation of MLC phosphorylation and contractile processes.

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

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

Sphingosine-1-phosphate lyase has a central role in the development of Dictyostelium discoideum

Sphingosine-1-phosphate, a product of sphingomyelin degradation, is an important element of signal transduction pathways that regulate cell proliferation and cell death. We have demonstrated additional roles for sphingosine-1-phosphate in growth and multicellular development. The specific disruption in Dictyostelium discoideum of the sphingosine-1-phosphate lyase gene, which encodes the enzyme that catalyzes sphingosine-1-phosphate degradation, results in a mutant strain with aberrant morphogenesis, as well as an increase in viability during stationary phase. The absence of sphingosine-1-phosphate lyase affects multiple stages throughout development, including the cytoskeletal architecture of aggregating cells, the ability to form migrating slugs, and the control of cell type-specific gene expression and terminal spore differentiation. This pleiotropic effect, which is due to the loss of sphingosine-1-phosphate lyase, establishes sphingolipids as pivotal regulatory molecules in a wide range of processes in multicellular development.

CpG island of rat sphingosine kinase-1 gene: tissue-dependent DNA methylation status and multiple alternative first exons

It is generally recognized that CpG islands are not methylated in normal tissues. SPHK1 is a key enzyme catalyzing the production of sphingosine 1-phosphate, a novel signaling molecule for the proliferation and differentiation of various cells, including neural cells. Sequencing of genomic DNA and cDNA reveals that rat Sphk1a consists of six exons encoding 383 amino acids. Furthermore, we identified six alternative first exons for mRNA subtypes (Sphk1a, -b, -c, -d, -e, and -f) within a 3.7-kb CpG island. The CpG island contains a tissue-dependent, differentially methylated region (T-DMR; approximately 200 bp), which is located - 800 bp upstream of the first exon of Sphk1a. T-DMR is hypomethylated in the adult brain where Sphk1a is expressed, whereas it is hypermethylated in the adult heart where the gene is not expressed. In fetal tissues, hypomethylation of T-DMR is not associated with expression of Sphk1a, which suggests that differential availability of transcription factors is also likely to be involved in the mechanism of its expression. Here, we identify rat Sphk1, using multiple alternative first exons for the subtypes, and demonstrate that there is a CpG island bearing T-DMR.

Lysophospholipid receptors

Lysophospholipids (LPs), including lysophosphatidic acid and sphingosine 1-phosphate, produce many cellular effects. However, the prolonged absence of any cloned and identified LP receptor has left open the question of how these lipids actually bring about these effects. The cloning and functional identification of the first LP receptor, lp(A1)/vzg-1, has led rapidly to the identification and classification of multiple orphan receptors/expression sequence tags known by many names (e.g. edg, mrec1.3, gpcr26, H218, AGR16, nrg-1) as members of a common cognate G protein-coupled receptor family. We review features of the LP receptor family, including molecular characteristics, genomics, signaling properties, and gene expression. A major question for which only partial answers are available concerns the biological significance of receptor-mediated LP signaling. Recent studies that demonstrate the role of receptor-mediated LP signaling in the nervous system, cardiovascular system, and other organ systems indicate the importance of this signaling in development, function, and pathophysiology and portend an exciting time ahead for this growing field.

Sphingosine 1-phosphate activates Akt, nitric oxide production, and chemotaxis through a Gi protein/phosphoinositide 3-kinase pathway in endothelial cells

Sphingosine 1-phosphate (SPP) binds to members of the endothelial differentiation gene family (EDG) of receptors and leads to diverse signaling events including cell survival, growth, migration and differentiation. However, the mechanisms of how SPP activates these proangiogenic pathways are poorly understood. Here we show that SPP signals through the EDG-1 receptor to the heterotrimeric G protein G(i), leading to activation of the serine/threonine kinase Akt and phosphorylation of the Akt substrate, endothelial nitric-oxide synthase (eNOS). Inhibition of G(i) signaling, and phosphoinositide 3-kinase (PI 3-kinase) activity resulted in a decrease in SPP-induced endothelial cell chemotaxis. SPP also stimulates eNOS phosphorylation and NO release and these effects are also attenuated by inhibition of G(i) signaling, PI 3-kinase, and Akt. However, inhibition of NO production did not influence SPP-induced chemotaxis but effectively blocked the chemotactic actions of vascular endothelial growth factor. Thus, SPP signals through G(i) and PI 3-kinase leading to Akt activation and eNOS phosphorylation.

Subtype-specific, differential activities of the EDG family receptors for sphingosine-1-phosphate, a novel lysophospholipid mediator

The lysosphingolipid sphingosine-1-phosphate (S1P) and the structurally related lipid lysophosphatidic acid (LPA) elicit a wide spectrum of biological responses in a variety of cell types, including mitogenesis, cell-shape changes, migration and contraction. Recent studies have unveiled the existence of the G protein-coupled heptahelical receptor subfamily for the biologically active lysophospholipids, which consists of the two receptor subgroups specific for S1P and LPA, respectively. The S1P receptor subgroup comprises four members, i.e. EDG-1, EDG-3, EDG-5/AGR16 and EDG-6, with considerable amino acid similarity among them. The S1P receptor subtypes are coupled to different heterotrimeric G proteins, leading to the activation of a unique set of multiple intracellular signaling pathways. The expression of transcripts of the S1P receptor subtypes is wide-spread, except for EDG-6 which exhibits lymphoid tissue-specific expression. Plasma contains substantial concentrations of S1P as well as LPA. Activated platelets appear to be a major source of S1P and LPA in blood. In addition, accumulating evidence demonstrates that S1P and LPA are released from a variety of cell types in response to various extracellular stimuli. These observations demonstrate the existence of the novel signaling system comprising the lysosphingolipids and their cognate receptors, suggesting physiological and pathological roles.

Small GTP-binding protein, Rho, both increased and decreased cellular motility, activation of matrix metalloproteinase 2 and invasion of human osteosarcoma cells

Rho, a member of the small GTP-binding proteins, and one of its downstream effectors ROCK (Rho-associated coiled-coil forming protein kinase) play an important role in the invasion of tumor cells. Lysophosphatidic acid (LPA) activates Rho and ROCK and promotes the organization of stress fibers and focal adhesions. However, the effect of LPA on tumor cell invasion is still controversial. In the present study, human osteosarcoma cells treated with a high concentration of LPA (high LPA) showed considerable formation of stress fibers and focal adhesions compared to the cells treated with a low concentration of LPA (low LPA). C3 (inhibitor of Rho) or Y27632 (an inhibitor of ROCK) inhibited the effects of LPA, indicating that LPA activates the Rho-ROCK pathway in the cells. In addition, Rho activation assay showed that the activation level of Rho can be altered by changing the concentration of LPA. Low LPA stimulated the motility and invasion of the cells, while high LPA reduced both. The disruption of extracellular matrix (ECM) by matrix metalloproteinase 2 (MMP2) is also critical for tumor cell invasion. MMP2 is activated by membranous type-1 MMP (MT1-MMP) and type-2 tissue inhibitor of MMP (TIMP2). High LPA suppressed the activation of MMP2 through down-regulation of MT1-MMP and TIMP2. C3 and Y27632 reversed the suppression of the activation of MMP2 and expression of MT1-MMP and TIMP2, suggesting the involvement of the Rho-ROCK pathway in ECM degradation. Tyrosine phosphorylation of focal adhesion kinase (FAK) was also required for the invasion of tumor cells to occur. Low LPA enhanced the tyrosine phosphorylation of FAK whereas high LPA reduced it. In conclusion, we suggest that Rho has a dual effect on the invasion of osteosarcoma cells by modulating the motility, the ability to degrade ECM and tyrosine phosphorylation of FAK.

The LPA receptors

Lysophosphatidic acid (LPA) is a growth factor-like lipid that produces many cellular responses. These responses, including actin cytoskeletal rearrangements, cell proliferation and inhibition of gap junction communication, have been documented in many cell types over the last 2 decades. Both non-receptor and receptor-mediated mechanisms had been implicated to explain these responses. A clear advance in this field was the cloning and functional identification of LPA receptors, and there are currently three high-affinity members, LPA1, LPA2 and LPA3 (synonymous with orphan receptor names edg-2, edg-4 and edg-7, respectively). Here we review the gene structure, expression and functions of LPA receptors. We also discuss the in vivo roles mediated by a single LPA receptor type, based on studies of the nervous system, a major locus of LPA receptor expression.

Modulation of cell interactions with extracellular matrix by lysophosphatidic acid and sphingosine 1-phosphate

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (SPP) are lipid mediators released upon platelet activation. The concentration of LPA in serum is estimated at 1-10 microM whereas the concentration in plasma is considerably less. The SPP concentration in serum is 0.5 microM, approximately two-fold higher than the plasma concentration. The lipids are present during tissue injury and promote cellular processes involved in wound repair. LPA and SPP have multiple effects on cells, many of which are pertinent to wound healing and require that the cells interact in some fashion with components of the extracellular matrix. This review focuses on modulation of cell adhesion, cell migration, collagen gel contraction, and fibronectin matrix assembly by LPA and SPP.

Sphingosine 1-phosphate: synthesis and release

Sphingosine 1-phosphate (Sph-1-P) is a bioactive sphingolipid, acting both as an intracellular second messenger and extracellular mediator, in mammalian cells. In cell types where Sph-1-P acts as an intracellular messenger, stimulation-dependent synthesis of Sph-1-P, possibly resulting from sphingosine (Sph) kinase activation, is essential. Since this important kinase has recently been cloned, precise regulation of intracellular Sph-1-P synthesis will be clarified in the near future. As an intercellular mediator, elucidation of sources for extracellular Sph-1-P is important, in addition to identification of the cell surface receptors for this phospholipid. Blood platelets are very unique in that they store Sph-1-P abundantly (possibly due to the existence of highly active Sph kinase and a lack of Sph-1-P lyase) and release this bioactive lipid extracellularly upon stimulation. It is likely that platelets are an important source for extracellular Sph-1-P, especially for plasma and serum Sph-1-P. Platelet-derived Sph-1-P seems to play an important role in vascular biology.

Stimulation of intracellular sphingosine-1-phosphate production by G-protein-coupled sphingosine-1-phosphate receptors

Recently, a family of G-protein-coupled receptors named endothelial differentiation gene (Edg) receptor family has been identified, which are specifically activated by the two serum lipids, sphingosine-1-phosphate and lysophosphatidic acid. Sphingosine-1-phosphate can also act intracellularly to release Ca2+ from intracellular stores. Since in several cell types, G-protein-coupled lysophosphatidic acid or sphingosine-1-phosphate receptors mobilize Ca2+ in the absence of a measurable phospholipase C stimulation, it was analysed here whether intracellular sphingosine-1-phosphate production was the signalling mechanism used by extracellular sphingosine-1-phosphate for mobilization of stored Ca2+. Sphingosine-1-phosphate and the low affinity sphingosine-1-phosphate receptor agonist, sphingosylphosphorylcholine, induced a rapid, transient and nearly complete pertussis toxin-sensitive Ca2+ mobilization in human embryonic kidney (HEK-293) cells. The G-protein-coupled sphingosine-1-phosphate receptors, Edg-1, Edg-3 and Edg-5, were found to be endogenously expressed in these cells. Most interestingly, sphingosine-1-phosphate and sphingosylphosphorylcholine did not induce a measurable production of inositol-1,4,5-trisphosphate or accumulation of inositol phosphates. Instead, sphingosine-1-phosphate and sphingosylphosphorylcholine induced a rapid and transient increase in production of intracellular sphingosine-1-phosphate with a maximum of about 1.4-fold at 30 s. Stimulation of sphingosine-1-phosphate formation by sphingosine-1-phosphate and sphingosylphosphorylcholine was fully blocked by pertussis toxin, indicating that extracellular sphingosine-1-phosphate via endogenously expressed G(i)-coupled receptors induces a stimulation of intracellular sphingosine-1-phosphate production. As sphingosine-1-phosphate- and sphingosylphosphorylcholine-induced increases in intracellular Ca2+ were blunted by sphingosine kinase inhibitors, this sphingosine-1-phosphate production appears to mediate Ca2+ signalling by extracellular sphingosine-1-phosphate and sphingosylphosphorylcholine in HEK-293 cells.

Inhibitory regulation of Rac activation, membrane ruffling, and cell migration by the G protein-coupled sphingosine-1-phosphate receptor EDG5 but not EDG1 or EDG3

Sphingosine-1-phosphate (S1P) is a bioactive lysophospholipid that induces a variety of biological responses in diverse cell types. Many, if not all, of these responses are mediated by members of the EDG (endothelial differentiation gene) family G protein-coupled receptors EDG1, EDG3, and EDG5 (AGR16). Among prominent activities of S1P is the regulation of cell motility; S1P stimulates or inhibits cell motility depending on cell types. In the present study, we provide evidence for EDG subtype-specific, contrasting regulation of cell motility and cellular Rac activity. In CHO cells expressing EDG1 or EDG3 (EDG1 cells or EDG3 cells, respectively) S1P as well as insulin-like growth factor I (IGF I) induced chemotaxis and membrane ruffling in phosphoinositide (PI) 3-kinase- and Rac-dependent manners. Both S1P and IGF I induced a biphasic increase in the amount of the GTP-bound active form of Rac. In CHO cells expressing EDG5 (EDG5 cells), IGF I similarly stimulated cell migration; however, in contrast to what was found for EDG1 and EDG3 cells, S1P did not stimulate migration but totally abolished IGF I-directed chemotaxis and membrane ruffling, in a manner dependent on a concentration gradient of S1P. In EDG5 cells, S1P stimulated PI 3-kinase activity as it did in EDG1 cells but inhibited the basal Rac activity and totally abolished IGF I-induced Rac activation, which involved stimulation of Rac-GTPase-activating protein activity rather than inhibition of Rac-guanine nucleotide exchange activity. S1P induced comparable increases in the amounts of GTP-RhoA in EDG3 and EDG5 cells. Neither S1P nor IGF I increased the amount of GTP-bound Cdc42. However, expression of N(17)-Cdc42, but not N(19)-RhoA, suppressed S1P- and IGF I-directed chemotaxis, suggesting a requirement for basal Cdc42 activity for chemotaxis. Taken together, the present results demonstrate that EDG5 is the first example of a hitherto-unrecognized type of receptors that negatively regulate Rac activity, thereby inhibiting cell migration and membrane ruffling.

Lysophosphatidic acid (LPA) is a novel extracellular regulator of cortical neuroblast morphology

During cerebral cortical neurogenesis, neuroblasts in the ventricular zone (VZ) undergo a shape change termed "interkinetic nuclear migration" whereby cells alternate between fusiform and rounded morphologies. We previously identified lp(A1), the first receptor gene for a signaling phospholipid called lysophosphatidic acid (LPA) and showed its enriched expression in the VZ. Here we report that LPA induces changes in neuroblast morphology from fusiform to round in primary culture, accompanied by nuclear movements, and formation of f-actin retraction fibers. These changes are mediated by the activation of the small GTPase, Rho. In explant cultures, where the cerebral cortical architecture remains intact, LPA not only induces cellular and nuclear rounding in the VZ, but also produces an accumulation of rounded nuclei at the ventricular surface. Consistent with a biological role for these responses, utilization of a sensitive and specific bioassay indicates that postmitotic neurons can produce extracellular LPA. These results implicate LPA as a novel factor in cortical neurogenesis and further implicate LPA as an extracellular signal from postmitotic neurons to proliferating neuroblasts.

Evidence for Edg-3 receptor-mediated activation of I(K.ACh) by sphingosine-1-phosphate in human atrial cardiomyocytes

Sphingosine-1-phosphate (SPP) and sphingosylphosphorylcholine (SPPC) have been reported to activate muscarinic receptor-activated inward rectifier K(+) current (I(K.ACh)) in cultured guinea pig atrial myocytes with similar nanomolar potency. Members of the endothelial differentiation gene (Edg) receptor family were recently identified as receptors for SPP; however, these receptors respond only to micromolar concentrations of SPPC. Here we investigated the sphingolipid-induced activation of I(K.ACh) in freshly isolated guinea pig, mouse, and human atrial myocytes. SPP activated I(K.ACh) in atrial myocytes from all three species with a similar nanomolar potency (EC(50) values: 4-8 nM). At these low concentrations, SPPC also activated I(K.ACh) in guinea pig myocytes. In contrast, SPPC was almost ineffective in mouse and human myocytes, thus resembling the pharmacology of the Edg receptors. Transcripts of Edg-1, Edg-3, and Edg-5 were detected in human atrial cells. Moreover, activation of I(K.ACh) by SPP was blocked by the Edg-3-selective antagonist suramin, which did not affect basal or carbachol-stimulated K(+) currents. In conclusion, these data indicate that I(K.ACh) activation by SPP and SPPC exhibits large species differences. Furthermore, they suggest that SPP-induced I(K.ACh) activation in human atrial myocytes is mediated by the Edg-3 subtype of SPP receptors.

Molecular cloning and characterization of a lipid phosphohydrolase that degrades sphingosine-1- phosphate and induces cell death

Sphingosine and sphingosine-1-phosphate (SPP) are interconvertible sphingolipid metabolites with opposing effects on cell growth and apoptosis. Based on sequence homology with LBP1, a lipid phosphohydrolase that regulates the levels of phosphorylated sphingoid bases in yeast, we report here the cloning, identification, and characterization of a mammalian SPP phosphatase (mSPP1). This hydrophobic enzyme, which contains the type 2 lipid phosphohydrolase conserved sequence motif, shows substrate specificity for SPP. Partially purified Myc-tagged mSPP1 was also highly active at dephosphorylating SPP. When expressed in yeast, mSPP1 can partially substitute for the function of LBP1. Membrane fractions from human embryonic kidney HEK293 cells transfected with mSPP1 markedly degraded SPP but not lysophosphatidic acid, phosphatidic acid, or ceramide-1-phosphate. Enforced expression of mSPP1 in NIH 3T3 fibroblasts not only decreased SPP and enhanced ceramide levels, it also markedly diminished survival and induced the characteristic traits of apoptosis. Collectively, our results suggest that SPP phosphohydrolase may regulate the dynamic balance between sphingolipid metabolite levels in mammalian cells and consequently influence cell fate.

Sphingosine 1-phosphate: a ligand for the EDG-1 family of G-protein-coupled receptors

Ample evidence indicates that sphingosine-1-phosphate (SPP) can serve as an intracellular second messenger regulating calcium mobilization, cell growth, and survival. Moreover, the dynamic balance between levels of the sphingolipids metabolites, ceramide, and SPP, and consequent regulation of opposing signaling pathways, is an important factor that determines whether a cell survives or dies. This ceramide/SPP rheostat is an evolutionarily conserved stress regulatory mechanism influencing growth and survival of yeast. In addition, SPP also has been identified as the ligand for the G-protein-coupled receptors EDG-1, -3, -5, and -6. Binding of SPP to EDG-1 regulates chemotaxis and in vitro angiogenesis of endothelial cells, whereas EDG-5, and possibly EDG-3, are likely the cell surface receptors responsible for cell rounding and neurite retractions induced by SPP. Hence, the studies identify a family of highly specific SPP receptors that are capable of mediating different biological responses. Thus, it is suggested that SPP is a prototype for a novel class of lipid mediators that act both extracellularly as ligands for cell surface receptors and intracellularly as second messengers. Recently, sphingosine kinase was purified to homogeneity and the first mammalian sphingosine kinase, the enzyme responsible for the formation of SPP, was cloned. The studies should provide the necessary tools to develop insight into the biological roles of this important bioactive sphingolipid.

Sphingosine-1-phosphate activates phospholipase D in human airway epithelial cells via a G protein-coupled receptor

Sphingosine-1-phosphate (SPP) acts as a first messenger in immortalized human airway epithelial cells (CFNPE9o(-)), possibly interacting with an Edg family receptor. Expression of the SPP receptors Edg-1 and Edg-3, as well as a low level of Edg-5/H218, was detected in these cells, in agreement with their ability to specifically bind SPP. The related lipids, lysophosphatidic acid and sphingosylphosphorylcholine, were unable to displace SPP from its high affinity binding sites, suggesting that the biological responses to these different lysolipids are mediated by distinct receptors. SPP markedly inhibited forskolin-stimulated cAMP accumulation in a dose-dependent manner and caused a remarkable elevation of intracellular calcium, both effects being sensitive to pertussis toxin treatment. Most importantly, SPP stimulated phosphatidic acid formation, which was maximal after 2 min and decreased within 8-10 min. In the presence of butan-1-ol, suppression of SPP-induced phosphatidic acid formation and production of phosphatidylbutanol were found, clearly indicating activation of phospholipase D (PLD). This finding was also confirmed by analysis of the fatty acid composition of phosphatidic acid, showing an increase in the monounsaturated oleic acid only. The decrease of phosphatidic acid level after 8-10 min incubation with SPP was accompanied by a parallel increase of diacylglycerol production, which was abolished in the presence of butan-1-ol. This result indicates that activation of phospholipase D is followed by stimulation of phosphatidate phosphohydrolase activity. Phosphatidic acid formation was insensitive to protein kinase C inhibitors and almost completely inhibited by pertussis toxin treatment, suggesting that SPP activates phospholipase D via a G(i/o) protein-coupled receptor.

Identification of lysophospholipid receptors in human platelets: the relation of two agonists, lysophosphatidic acid and sphingosine 1-phosphate

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (Sph-1-P) are known as structurally related bio-active lipids activating platelets through their respective receptors. Although the receptors for LPA and Sph-1-P have been recently identified in various cells, the identification and characterization of ones in platelets have been reported only preliminarily. In this report, we first investigated the distinct modes of LPA and Sph-1-P actions in platelet activation and found that LPA functioned as a much stronger agonist than Sph-1-P, and high concentrations of Sph-1-P specifically desensitized LPA-induced intracellular Ca(2+) mobilization. In order to identify the responsible receptors underlying these observations, we analyzed the LPA and Sph-1-P receptors which might be expressed in human platelets, by RT-PCR. We found for the first time that Edg2, 4, 6 and 7 mRNA are expressed in human platelets.

Enhancement of the migration of metastatic human breast cancer cells by phosphatidic acid

Phosphatidic acid (PA), lysophosphatidic acid (LPA), and sphingosine 1-phosphate (SPP) are naturally occurring phospholipids which induce a variety of effects as extracellular messengers. In this study, we compared the effects of these phospholipid signaling molecules on the migration of invasive and noninvasive breast cancer cell lines, an index of the metastatic potential of these cells. As previously demonstrated, invasive MDA-MB-231 breast cancer cells exhibited increased constitutive (nonstimulated) migration in comparison to poorly invasive MCF-7 cells. Phosphatidic acid employed at nanomolar concentrations markedly potentiated migration of the invasive cells but had no effect on migration of either the noninvasive MCF-7 cells or nonneoplastic human epithelial cells. Lysophosphatidic acid and sphingosine 1-phosphate inhibited both the directed (chemotactic) and random (chemokinetic) migration of MDA-MB-231 cells. Experiments were undertaken to characterize the signaling pathway involved in constitutive and PA-stimulated migration of MDA-MB-231 cells. The tyrosine kinase inhibitors staurosporine and genistein inhibited constitutive and PA-induced migration in a dose-dependent manner, consistent with a role for tyrosine phosphorylation in the migratory response. In addition, the phosphatidylinositol (PI) 3' kinase inhibitors wortmannin and LY294002 strongly inhibited both the constitutive and PA-stimulated migration of the invasive breast cancer cells, indicating that PI-3' kinase plays an important role in the metastatic migration of breast cancer cells. Finally, PA-induced migration of MDA-MB-231 was markedly attenuated by pretreatment of cells with Clostridium difficile Toxin B, pertussis toxin and suramin, implying a role for a Gi receptor-dependent process involving activation of the small GTP-binding protein Rho. Since an enhanced ability to migrate heightens the metastatic potential of cells within solid tumors, our results suggest that the metastatic capabilities of breast cancer cells may be enhanced by a receptor-driven cellular process initiated by phosphatidic acid or related lipid phosphate messengers.

Sphingosine 1-phosphate stimulates tyrosine phosphorylation of focal adhesion kinase and chemotactic motility of endothelial cells via the G(i) protein-linked phospholipase C pathway

We have previously shown that sphingosine 1-phosphate (S1P) stimulates motility of human umbilical vein endothelial cells (HUVECs) (O.-H. Lee et al., Biochem. Biophys. Res. Commun. 264, 743-750, 1999). To investigate the molecular mechanisms by which S1P stimulates HUVEC motility, we examined tyrosine phosphorylation of p125 focal adhesion kinase (p125(FAK)) which is important for cell migration. S1P induces a rapid increase in tyrosine phosphorylation of p125(FAK). Compared with other structurally related lipid metabolites such as sphingosine, C2-ceramide, and lysophosphatidic acid, S1P uniquely stimulated p125(FAK) tyrosine phosphorylation and migration of HUVECs. The effect of S1P on p125(FAK) tyrosine phosphorylation was markedly reduced by treatment with pertussis toxin or U73122, a phospholipase C (PLC) inhibitor. As a downstream signal of PLC, p125(FAK) tyrosine phosphorylation in response to S1P was totally blocked by depletion of the intracellular calcium pool. However, protein kinase C (PKC) inhibitor had no effect on the response to S1P. Finally, chemotaxis assays revealed that inhibition of PLC but not PKC significantly abrogated S1P-stimulated HUVEC migration. These results suggest that the G(i)-coupled receptor-mediated PLC-Ca(2+) signaling pathway may be importantly involved in S1P-stimulated focal adhesion formation and migration of endothelial cells.

Sphingosine 1-phosphate stimulates cell migration through a G(i)-coupled cell surface receptor. Potential involvement in angiogenesis

Sphingosine 1-phosphate (SPP) has been shown to inhibit chemotaxis of a variety of cells, in some cases through intracellular actions, while in others through receptor-mediated effects. Surprisingly, we found that low concentrations of SPP (10-100 nM) increased chemotaxis of HEK293 cells overexpressing the G protein-coupled SPP receptor EDG-1. In agreement with previous findings in human breast cancer cells (Wang, F., Nohara, K., Olivera, O., Thompson, E. W., and Spiegel, S. (1999) Exp. Cell Res. 247, 17-28), SPP, at micromolar concentrations, inhibited chemotaxis of both vector- and EDG-1-overexpressing HEK293 cells. Nanomolar concentrations of SPP also induced a marked increase in chemotaxis of human umbilical vein endothelial cells (HUVEC) and bovine aortic endothelial cells (BAEC), which express the SPP receptors EDG-1 and EDG-3, while higher concentrations of SPP were less effective. Treatment with pertussis toxin, which ADP-ribosylates and inactivates G(i)-coupled receptors, blocked SPP-induced chemotaxis. Checkerboard analysis indicated that SPP stimulates both chemotaxis and chemokinesis. Taken together, these data suggest that SPP stimulates cell migration by binding to EDG-1. Similar to SPP, sphinganine 1-phosphate (dihydro-SPP), which also binds to this family of SPP receptors, enhanced chemotaxis; whereas, another structurally related lysophospholipid, lysophosphatidic acid, did not compete with SPP for binding nor did it have significant effects on chemotaxis of endothelial cells. Furthermore, SPP increased proliferation of HUVEC and BAEC in a pertussis toxin-sensitive manner. SPP and dihydro-SPP also stimulated tube formation of BAEC grown on collagen gels (in vitro angiogenesis), and potentiated tube formation induced by basic fibroblast growth factor. Pertussis toxin treatment blocked SPP-, but not bFGF-stimulated in vitro angiogenesis. Our results suggest that SPP may play a role in angiogenesis through binding to endothelial cell G(i)-coupled SPP receptors.

Advances in the signal transduction of ceramide and related sphingolipids

Recently, the sphingolipid metabolites ceramide, sphingosine, ceramide 1-P, and sphingosine 1-P have been implicated as second messengers involved in many different cellular functions. Publications on this topic are appearing at a rapidly increasing rate and new developments in this field are also appearing rapidly. It is thus important to summarize the results obtained from many different laboratories and from different fields of research to obtain a clearer picture of the importance of sphingolipid metabolites. This article reviews the studies from the last few years and includes the effects of a variety of extracellular agents on sphingolipid signal transduction pathways in different tissues and cells and on the mechanisms of regulation. Sphingomyelin exists in a number of functionally distinct pools and is composed of distinct molecular species. Sphingomyelin metabolites may be formed by many different pathways. For example, the generation of ceramide from sphingomyelin can be catalyzed by at least five different sphingomyelinases. A large variety of stimuli can induce the generation of ceramide, leading to activation or inhibition of various cellular events such as proliferation, differentiation, apoptosis, and inflammation. The effect of ceramide on these physiological processes is due to its many different downstream targets. It can activate ceramide-activated protein kinases and ceramide-activated protein phosphatases. It also activates or inhibits PKCs, PLD, PLA2, PC-PLC, nitric oxide synthase, and the ERK and SAPK/JNK signaling cascades. Ceramide activates or inhibits transcription factors, modulates calcium homeostasis and interacts with the retinoblastoma protein to regulate cell cycle progression. Most of the work in this field has involved the study of ceramide effects, but the roles of the other three sphingomyelin metabolites is now attracting much attention. The complex interactions between signaling components and ceramide and the controls regulating these interactions are now being identified and are presented in this review.

Bioactive lysophospholipids and their G protein-coupled receptors

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are serum-borne lysophospholipids that signal through their cognate G protein-coupled receptors to evoke a great variety of responses in numerous cell types. In addition to stimulating cell proliferation and survival, LPA and S1P induce profound cytoskeletal changes through Rho-mediated signaling pathways, leading to such diverse responses as cell rounding, neurite retraction, and modulation of tumor cell invasiveness (transcellular migration). A major recent advance is the identification of a subfamily of heptahelical receptors for LPA and S1P.

Sphingosine 1-phosphate induces angiogenesis: its angiogenic action and signaling mechanism in human umbilical vein endothelial cells

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid metabolite abundantly stored in platelets and released upon platelet activation. Recently, S1P has been postulated for its potential roles in angiogenesis. In this study, we provided several lines of evidence showing that S1P has angiogenic activity. In vitro, S1P stimulated DNA synthesis and chemotactic motility of human umbilical vein endothelial cells (HUVECs) in a dose-dependent manner, reaching a near maximum at 1 microM. S1P also significantly induced tube formation of HUVECs on Matrigel. Matrigel plug assay in mice revealed that S1P promotes angiogenesis in vivo. In addition, exposure of HUVECs to S1P led to rapid activation of extracellular signal-regulated kinases (ERKs) and p38 mitogen-activated protein kinase (p38 MAPK) in a pertussis toxin (PTX)-sensitive manner. Notably, HUVEC migration and tube formation in response to S1P were completely blocked by pretreatment with PTX. Further, the MEK inhibitor U0126 markedly inhibited S1P-induced tube formation but S1P-induced migration was not affected by inhibition of ERK and p38 MAPK. Taken together, these results indicate that S1P induces angiogenesis predominantly via G(i) protein-coupled receptors in endothelial cells and suggest that S1P may act as an important modulator of platelet-induced angiogenesis.

Sphingosine kinase expression increases intracellular sphingosine-1-phosphate and promotes cell growth and survival

Sphingosine-1-phosphate (SPP) is a bioactive lipid that has recently been identified as the ligand for the EDG family of G protein-coupled cell surface receptors. However, the mitogenic and survival effects of exogenous SPP may not correlate with binding to cell-surface receptors (Van Brocklyn, J.R., M.J. Lee, R. Menzeleev, A. Olivera, L. Edsall, O. Cuvillier, D.M. Thomas, P.J.P. Coopman, S. Thangada, T. Hla, and S. Spiegel. 1998. J. Cell Biol. 142:229-240). The recent cloning of sphingosine kinase, a unique lipid kinase responsible for the formation of SPP, has provided a new tool to investigate the role of intracellular SPP. Expression of sphingosine kinase markedly increased SPP levels in NIH 3T3 fibroblasts and HEK293 cells, but no detectable secretion of SPP into the medium was observed. The increased sphingosine kinase activity in NIH 3T3 fibroblasts was sufficient to promote growth in low- serum media, expedite the G(1)/S transition, and increase DNA synthesis and the proportion of cells in the S phase of the cell cycle with a concomitant increase in cell numbers. Transient or stable overexpression of sphingosine kinase in NIH 3T3 fibroblasts or HEK293 cells protected against apoptosis induced by serum deprivation or ceramide elevation. N,N-Dimethylsphingosine, a competitive inhibitor of sphingosine kinase, blocked the effects of sphingosine kinase overexpression on cell proliferation and suppression of apoptosis. In contrast, pertussis toxin did not abrogate these biological responses. In Jurkat T cells, overexpression of sphingosine kinase also suppressed serum deprivation- and ceramide-induced apoptosis and, to a lesser extent, Fas-induced apoptosis, which correlated with inhibition of DEVDase activity, as well as inhibition of the executionary caspase-3. Taken together with ample evidence showing that growth and survival factors activate sphingosine kinase, our results indicate that SPP functions as a second messenger important for growth and survival of cells. Hence, SPP belongs to a novel class of lipid mediators that can function inside and outside cells.

The electrostatics of lipid surfaces

Charged lipids constitute a substantial fraction of all membrane lipids. Their charges vary in quantity and distribution within their headgroup regions. In long range interactions, their charges' value and electrostatic potential in the vicinity of the membrane surface can be approximated by the Guy-Chapman theory. This theory treats the interface as a charged structureless plain surrounded by uniform environments. However, if one considers intermolecular interactions, such assumptions need to be revised. The interface is in reality a thick region containing the residual charges of lipid headgroups. Their arrangement depends on the type of lipid present in the membrane. The variety of lipids and their biological functions suggests that charge distribution determines the extent and type of interaction with surface associated molecules. Numerous examples show that protein behavior at the lipid bilayer surface is determined by the type of lipid present, indicating protein specificity towards certain surface locations and local properties (determined by lipid composition) of a particular type. Such specificity is achieved by a combination of electrostatic, hydrophobic and enthropic effects. Comparing lipid biological activity, it can be stated that residual charge distribution is one of the factors of intermolecular recognition leading to the specific interaction of lipid molecules and selected proteins in various processes, particularly those involved with signal transduction pathways. Such specificity enables a variety of processes occurring simultaneously on the same membrane surface to function without cross-reaction interference.

Enzymatic measurement of sphingosine 1-phosphate

Sphingosine 1-phosphate (SPP) is a sphingolipid metabolite which has novel dual actions acting as both an intracellular second messenger and a ligand for a family of G protein-coupled receptors. This paper describes a rapid enzymatic method to quantify mass levels of SPP in serum, mammalian tissues, and cultured cells. The assay utilizes an alkaline lipid extraction to selectively separate SPP from other phospholipids and sphingolipids, including sphingosine. Extracted SPP is efficiently converted to sphingosine by alkaline phosphatase treatment. Sphingosine thus formed is then quantitatively phosphorylated to [(32)P]SPP using recombinant sphingosine kinase and [gamma-(32)P]ATP. With this procedure we were able to obtain reproducible measurements of SPP over a broad range from 0.25 pmol to 2.5 nmol. In various rat tissues, levels of SPP varied between 0. 5 and 6 pmol/mg wet wt. The lowest levels were found in heart and testes, while brain contained the highest levels. The method was adapted easily to measure minute amounts of SPP present in various cultured cell types. The amount of SPP in cell extracts was proportional to the cell number and varied between 0.04 and 2 pmol/10(6) cells. Concurrent measurements of sphingosine levels revealed that its concentration was significantly higher than SPP in most cells and tissues. Furthermore, with this assay we were able to measure increases in intracellular SPP levels in rat pheochromocytoma PC12 cells after treatment with exogenous sphingosine or with nerve growth factor which stimulates sphingosine kinase activity.

Involvement of focal adhesion kinase in inhibition of motility of human breast cancer cells by sphingosine 1-phosphate

Sphingosine 1-phosphate (SPP), a bioactive sphingolipid metabolite, inhibits chemoinvasiveness of the aggressive, estrogen-independent MDA-MB-231 human breast cancer cell line. As in many other cell types, SPP stimulated proliferation of MDA-MB-231 cells, albeit to a lesser extent. Treatment of MDA-MB-231 cells with SPP had no significant effect on their adhesiveness to Matrigel, and only high concentrations of SPP partially inhibited matrix metalloproteinase-2 activation induced by Con A. However, SPP at a concentration that strongly inhibited invasiveness also markedly reduced chemotactic motility. To investigate the molecular mechanisms by which SPP interferes with cell motility, we examined tyrosine phosphorylation of focal adhesion kinase (FAK) and paxillin, which are important for organization of focal adhesions and cell motility. SPP rapidly increased tyrosine phosphorylation of FAK and paxillin and of the paxillin-associated protein Crk. Overexpression of FAK and kinase-defective FAK in MDA-MB-231 cells resulted in a slight increase in motility without affecting the inhibitory effect of SPP, whereas expression of FAK with a mutation of the major autophosphorylation site (F397) abolished the inhibitory effect of SPP on cell motility. In contrast, the phosphoinositide 3'-kinase inhibitor, wortmannin, inhibited chemotactic motility in both vector and FAK-F397-transfected cells. Our results suggest that autophosphorylation of FAK on Y397 may play an important role in SPP signaling leading to decreased cell motility.

Sphingosine 1-phosphate-induced cell rounding and neurite retraction are mediated by the G protein-coupled receptor H218

Sphingosine 1-phosphate (SPP) is a lipid second messenger that also acts as a first messenger through the G protein-coupled receptor Edg-1. Here we show that SPP also binds to the related receptors H218 and Edg-3 with high affinity and specificity. SPP and sphinganine 1-phosphate bind to these receptors, whereas neither sphingosylphosphorylcholine nor lysophosphatidic acid compete with SPP for binding to either receptor. Transfection of HEK293 cells with H218 or edg-3, but not edg-1, induces rounded cell morphology in the presence of serum, which contains high levels of SPP. SPP treatment of cells overexpressing H218 cultured in delipidated serum causes cell rounding. A similar but less dramatic effect was observed in cells overexpressing Edg-3 but not with Edg-1. Cell rounding was correlated with apoptotic cell death, probably as a result of loss of attachment. Nerve growth factor-induced neuritogenesis in PC12 cells was inhibited by overexpression of H218 and to a lesser extent Edg-3. SPP treatment rapidly enhanced neurite retraction in PC12 cells overexpressing Edg-1, Edg-3, or H218. Thus, H218, and possibly Edg-3, may be the cell surface receptors responsible for cell rounding and neurite retraction induced by SPP. Moreover, the identification of these two additional SPP receptors indicates that a family of highly specific receptors exists that mediate different responses to SPP.

Comparative analysis of three murine G-protein coupled receptors activated by sphingosine-1-phosphate

The cloning and analysis of the first identified lysophosphatidic acid (LPA) receptor gene, lpA1 (also referred to as vzg-1 or edg-2), led us to identify homologous murine genes that might also encode receptors for related lysophospholipid ligands. Three murine genomic clones (designated lpB1, lpB2, and lpB3) were isolated, corresponding to human/rat Edg-1, rat H218/AGR16, and human edg-3, respectively. Based on the amino acid similarities of their predicted proteins (44-52% identical), the three lpB genes could be grouped into a separate G-protein coupled receptor subfamily, distinct from that containing the LPA receptor genes lpA1 and lpA2. Unlike lpA1 and lpA2, which contain multiple coding exons, all lpB members contained a single coding exon. Heterologous expression of individual lpB members in a hepatoma cell line (RH7777), followed by 35S-GTPgammaS incorporation assays demonstrated that each of the three LPB receptors conferred sphingosine-1-phosphate-dependent, but not lysophosphatidic acid-dependent, G-protein activation. Northern blot and in situ hybridization analyses revealed overlapping as well as distinct expression patterns in both embryonic and adult tissues. This comparative characterization of multiple sphingosine-1-phosphate receptor genes and their spatiotemporal expression patterns will aid in understanding the biological roles of this enlarging lysophospholipid receptor family.

Invasion of T-lymphoma cells: cooperation between Rho family GTPases and lysophospholipid receptor signaling

Rho-like GTPases orchestrate distinct cytoskeletal changes in response to receptor stimulation. Invasion of T-lymphoma cells into a fibroblast monolayer is induced by Tiam1, an activator of the Rho-like GTPase Rac, and by constitutively active V12Rac1. Here we show that activated V12Cdc42 can also induce invasion of T-lymphoma cells. Activated RhoA potentiates invasion, but fails by itself to mimic Rac and Cdc42. However, invasion is inhibited by the Rho-inactivating C3 transferase. Thus, RhoA is required but not sufficient for invasion. Invasion of T-lymphoma cells is critically dependent on the presence of serum. Serum can be replaced by the serum-borne lipids lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) (10(-7)-10(-6) M), which act on distinct G protein-linked receptors to activate RhoA and phospholipase C (PLC)-Ca2+ signaling. LPA- and S1P-induced invasion is preceded by Rho-dependent F-actin redistribution and pseudopodia formation. However, expression of both V14RhoA and V12Rac1 does not bypass the LPA/S1P requirement for invasion, indicating involvement of an additional signaling pathway independent of RhoA. The PLC inhibitor U-73122, but not the inactive analog U-73343, abolishes invasion. Our results indicate that T-lymphoma invasion is driven by Tiam1/Rac or Cdc42 activation, and is dependent on LPA/S1P receptor-mediated RhoA and PLC signaling pathways which lead to pseudopod formation and enhanced infiltration.