Edg-1, the G protein-coupled receptor for sphingosine-1-phosphate, is essential for vascular maturation

Sply regulation of sphingolipid signaling molecules is essential for Drosophila development

Sphingosine-1-phosphate is a sphingolipid metabolite that regulates cell proliferation, migration and apoptosis through specific signaling pathways. Sphingosine-1-phosphate lyase catalyzes the conversion of sphingosine-1-phosphate to ethanolamine phosphate and a fatty aldehyde. We report the cloning of the Drosophila sphingosine-1-phosphate lyase gene (Sply) and demonstrate its importance for adult muscle development and integrity, reproduction and larval viability. Sply expression is temporally regulated, with onset of expression during mid-embryogenesis. Sply null mutants accumulate both phosphorylated and unphosphorylated sphingoid bases and exhibit semi-lethality, increased apoptosis in developing embryos, diminished egg-laying, and gross pattern abnormalities in dorsal longitudinal flight muscles. These defects are corrected by restoring Sply expression or by introduction of a suppressor mutation that diminishes sphingolipid synthesis and accumulation of sphingolipid intermediates. This is the first demonstration of novel and complex developmental pathologies directly linked to a disruption of sphingolipid catabolism in metazoans.

S1P3 receptors mediate the potent constriction of cerebral arteries by sphingosine-1-phosphate

We characterized the effect of Sphingosine-1-phosphate (S1P) on vascular tone. S1P selectively constricted isolated cerebral, but not peripheral arteries, despite ubiquitous expression of S1P(1), S1P(2), S1P(3) and S1P(5) receptor mRNA. Clostridium B and C3 toxins and the rho-kinase inhibitor Y27632 (trans-N-(4-pyridyl)-4-(l-aminoethyl)-cyclohexane carboxamide) reduced this vasoconstriction to S1P, indicating that the response was mediated through Rho. Pertussis toxin displayed only weak inhibition, suggesting minor involvement of G(i/o) protein. The S1P effect was specifically reduced by adenovirus bearing a s1p(3) but not s1p(2), antisense construct. Furthermore, suramin, which selectively blocks S1P(3) receptors, inhibited the vasoconstrictor effect of S1P, indicating that S1P(3) receptors account for at least part of S1P-mediated vasoconstriction in cerebral arteries. In vivo, intracarotid injection of S1P decreased cerebral blood flow, an effect prevented by suramin treatment. Because S1P constricts cerebral blood vessels and is released from platelets during clotting, the S1P/S1P(3) system constitutes a novel potential target for cerebrovascular disease therapy.

The aorta and heart differentially express RGS (regulators of G-protein signalling) proteins that selectively regulate sphingosine 1-phosphate, angiotensin II and endothelin-1 signalling

Normal cardiovascular development and physiology depend in part upon signalling through G-protein-coupled receptors (GPCRs), such as the angiotensin II type 1 (AT(1)) receptor, sphingosine 1-phosphate (S1P) receptors and endothelin-1 (ET-1) receptor. Since regulator of G-protein signalling (RGS) proteins function as GTPase-activating proteins for the G alpha subunit of heterotrimeric G-proteins, these proteins undoubtedly have functional roles in the cardiovascular system. In the present paper, we show that human aorta and heart differentially express RGS1, RGS2, RGS3S (short-form), RGS3L (long-form), PDZ-RGS3 (PDZ domain-containing) and RGS4. The aorta prominently expresses mRNAs for all these RGS proteins except PDZ-RGS3. Various stimuli that are critical for both cardiovascular development and function regulate dynamically the mRNA levels of several of these RGS proteins in primary human aortic smooth muscle cells. Both RGS1 and RGS3 inhibit signalling through the S1P(1) (formerly known as EDG-1), S1P(2) (formerly known as EDG-5) and S1P(3) (formerly known as EDG-3) receptors, whereas RGS2 and RGS4 selectively attenuate S1P(2)-and S1P(3)-receptor signalling respectively. All of the tested RGS proteins inhibit AT(1)-receptor signalling, whereas only RGS3 and, to a lesser extent, RGS4 inhibit ET(A)-receptor signalling. The conspicuous expression of RGS proteins in the cardiovascular system and their selective effects on relevant GPCR-signalling pathways provide additional evidence that they have functional roles in cardiovascular development and physiology.

Sphingosine-1-phosphate: an enigmatic signalling lipid

The evolutionarily conserved actions of the sphingolipid metabolite, sphingosine-1-phosphate (S1P), in yeast, plants and mammals have shown that it has important functions. In higher eukaryotes, S1P is the ligand for a family of five G-protein-coupled receptors. These S1P receptors are differentially expressed, coupled to various G proteins, and regulate angiogenesis, vascular maturation, cardiac development and immunity, and are important for directed cell movement.

Inhibitory and stimulatory regulation of Rac and cell motility by the G12/13-Rho and Gi pathways integrated downstream of a single G protein-coupled sphingosine-1-phosphate receptor isoform

The G protein-coupled receptors S1P2/Edg5 and S1P3/Edg3 both mediate sphingosine-1-phosphate (S1P) stimulation of Rho, yet S1P2 but not S1P3 mediates downregulation of Rac activation, membrane ruffling, and cell migration in response to chemoattractants. Specific inhibition of endogenous Galpha12 and Galpha13, but not of Galphaq, by expression of respective C-terminal peptides abolished S1P2-mediated inhibition of Rac, membrane ruffling, and migration, as well as stimulation of Rho and stress fiber formation. Fusion receptors comprising S1P2 and either Galpha12 or Galpha13, but not Galphaq, mediated S1P stimulation of Rho and also inhibition of Rac and migration. Overexpression of Galphai, by contrast, specifically antagonized S1P2-mediated inhibition of Rac and migration. The S1P2 actions were mimicked by expression of V14Rho and were abolished by C3 toxin and N19Rho, but not Rho kinase inhibitors. In contrast to S1P2, S1P3 mediated S1P-directed, pertussis toxin-sensitive chemotaxis and Rac activation despite concurrent stimulation of Rho via G12/13. Upon inactivation of Gi by pertussis toxin, S1P3 mediated inhibition of Rac and migration just like S1P2. These results indicate that integration of counteracting signals from the Gi- and the G12/13-Rho pathways directs either positive or negative regulation of Rac, and thus cell migration, upon activation of a single S1P receptor isoform.

Expression and functional characterization of SCaMPER: a sphingolipid-modulated calcium channel of cardiomyocytes

Calcium channels are important in a variety of cellular events including muscle contraction, signaling, proliferation, and apoptosis. Sphingolipids have been recognized as mediators of intracellular calcium release through their actions on a calcium channel, sphingolipid calcium release-mediating protein of the endoplasmic reticulum (SCaMPER). The current study investigates the expression and function of SCaMPER in cardiomyocytes. Northern analyses and RT-PCR cloning and sequencing revealed SCaMPER expression in both human and rat cardiac tissue. Immunofluorescence and Western blot analyses demonstrated that SCaMPER is abundant in cardiac tissue and is localized to the sarcotubular junction. This was confirmed by the colocalization of SCaMPER with dihydropyridine and ryanodine receptors by confocal microscopy. Purified T tubules were shown to contain SCaMPER and immunoelectron micrographs suggested that SCaMPER is located to the junctional T tubules, but a junctional SR localization cannot be ruled out. The sphingolipid ligand for SCaMPER, sphingosylphosphorylcholine (SPC), initiated calcium release from the cardiomyocyte SR. Importantly, antisense knockdown of SCaMPER mRNA produced a substantial reduction of sphingolipid-induced calcium release, suggesting that SCaMPER is a potentially important calcium channel of cardiomyocytes.

[Analyses of Ca-related ion channel currents and their involvement in Ca mobilization in smooth muscle and endothelial cells]

Changes in intracellular Ca concentration ([Ca2+]i) play dominant roles in the regulation of ion channel activity. Thus, analyses of Ca-related ion channels, whose activation is responsible for and/or dependent on the changes in [Ca2+]i, are important to understand the physiological and pharmacological characteristics of smooth muscle cells (SMCs) and endothelial cells (ECs). We have clarified that, in SMCs, Ca mobilization by membrane depolarization and bioactive substances affects the activity of Ca-activated K (IK-Ca) and Cl channel currents. On the other hand, by measuring IK-Ca as an indicator of Ca mobilization, we found that palmitoylcarnitine (PC), a lipid released under ischemic conditions, mobilizes Ca in ECs via stimulation of endothelial differential gene (Edg) receptors. Moreover, sphingosine-1-phosphate, which is a lipid mediator and has a similar structure to PC, elevated [Ca2+]i in ECs via the activation of cation channels through Edg1 receptors. A myo-endothelial interaction is another regulatory factor of Ca mobilization in ECs as well as in SMCs. Nifedipine and levcromakalim, which have no effects on ion channels in ECs themselves, changed the membrane potential of ECs via a myo-endothelial pathway. These integral analyses provide better understanding of the functional roles of Ca-related ion channels and their involvement in Ca mobilization in SMCs and ECs.

Pericyte-specific expression of Rgs5: implications for PDGF and EDG receptor signaling during vascular maturation

RGS proteins finely tune heterotrimeric G-protein signaling. Implying the need for such fine-tuning in the developing vascular system, in situ hybridization revealed a striking and extensive expression pattern of Rgs5 in the arterial walls of E12.5-E17.5 mouse embryos. The distribution and location of the Rgs5-positive cells typified that of pericytes and strikingly overlapped the known expression pattern of platelet-derived growth factor receptor (PDGFR)-beta. Both E14.5 PDGFR-beta- and platelet-derived growth factor (PDGF)-B-deficient mice exhibited markedly reduced levels of Rgs5 in their vascular plexa and small arteries. This likely reflects the loss of pericytes in the mutant mice. RGS5 acts as a potent GTPase activating protein for Gi(alpha) and Gq(alpha) and it attenuated angiotensin II-, endothelin-1-, sphingosine-1-phosphate-, and PDGF-induced ERK-2 phosphorylation. Together these results indicate that RGS5 exerts control over PDGFR-beta and GPCR-mediated signaling pathways active during fetal vascular maturation.

Rules for gene usage inferred from a comparison of large-scale gene expression profiles of T and B lymphocyte development

Ribonucleic acid expression profiles of seven consecutive stages of mouse thymocyte development were generated on high density oligonucleotide arrays. Previously known expression patterns of several genes were confirmed. Ten percent (1,304 of more than 13,000) of the monitored genes were found with 99% confidence to be differentially expressed across all T cell developmental stages. When compared with 1,204 genes differentially expressed in five consecutive B lineage developmental stages of bone marrow, >40% (546 genes) appeared to be shared by both lineages. However, when four pools of functionally distinct cell stages were compared between B and T cell development, DJ-rearranged precursor cells and resting immature precursor cells before and after surface Ag receptor expression shared less than 10%, mature resting lymphocytes between 15 and 20%, and only cycling precursors responding to precursor lymphocyte receptor deposition shared >50% of these differentially expressed genes. Three general rules emerge from these results: 1) proliferation of cells at comparable stages is in majority executed by the same genes; 2) intracellular signaling and intercellular communication are effected largely by different genes; and 3) most genes are not used strictly at comparable, but rather at several, stages, possibly in different functional contexts.

Sphingosine kinases: a novel family of lipid kinases

Sphingosine kinase (SPHK) catalyzes the formation of sphingosine-1-phosphate (S11). S1P plays an important role in regulation of a variety of biological processes through intracellular and extracellular actions. S1P has recently been shown to be the ligand for the EDG-1 family of G-protein-coupled receptors. To date, seven cloned SPHKs have been reported with confirmed SPHK activity, including human, mouse, yeast, and plant. A computer search of various databases suggests that a new SPHK family is emerging. The cloning and manipulation of SPHK genes will no doubt provide us with important information about the functions of S1P in a wide range of organisms.

Platelet-derived growth factor (PDGF)-induced chemotaxis does not require the G protein-coupled receptor S1P1 in murine embryonic fibroblasts and vascular smooth muscle cells

Sphingosine 1-phosphate (S1P), a bioactive lipid mediator, signals via G protein-coupled receptors (GPCR). The prototypical S1P receptor, S1P1 (also known as EDG-1), a Gi-linked receptor, is critical for vascular maturation during development. Recent work suggested that platelet-derived growth factor (PDGF)-induced cell migration required the S1P1 receptor, representing a novel mechanism for cross-talk between receptor tyrosine kinases and GPCRs. Since both S1P and PDGF are implicated in vascular smooth muscle cell (VSMC) pathobiology and development, we investigated this issue in rat VSMC and in embryonic fibroblasts derived from S1P1 null mice. Our data suggest that the S1P1 receptor is critical for S1P-induced, Gi-dependent migration but not for PDGF-BB-induced, receptor tyrosine kinase-dependent chemotaxis in VSMC. In addition, lack of S1P1 receptor in mouse embryonic fibroblasts did not significantly affect PDGF-induced cell migration. These data question the generality of the concept that S1P1 GPCR is a critical downstream component of PDGF-induced chemotaxis.

Pulmonary phosphatidic acid phosphatase and lipid phosphate phosphohydrolase

The lung contains two distinct forms of phosphatidic acid phosphatase (PAP). PAP1 is a cytosolic enzyme that is activated through fatty acid-induced translocation to the endoplasmic reticulum, where it converts phosphatidic acid (PA) to diacylglycerol (DAG) for the biosynthesis of phospholipids and neutral lipids. PAP1 is Mg(2+) dependent and sulfhydryl reagent sensitive. PAP2 is a six-transmembrane-domain integral protein localized to the plasma membrane. Because PAP2 degrades sphingosine-1-phosphate (S1P) and ceramide-1-phosphate in addition to PA and lyso-PA, it has been renamed lipid phosphate phosphohydrolase (LPP). LPP is Mg(2+) independent and sulfhydryl reagent insensitive. This review describes LPP isoforms found in the lung and their location in signaling platforms (rafts/caveolae). Pulmonary LPPs likely function in the phospholipase D pathway, thereby controlling surfactant secretion. Through lowering the levels of lyso-PA and S1P, which serve as agonists for endothelial differentiation gene receptors, LPPs regulate cell division, differentiation, apoptosis, and mobility. LPP activity could also influence transdifferentiation of alveolar type II to type I cells. It is considered likely that these lipid phosphohydrolases have critical roles in lung morphogenesis and in acute lung injury and repair.

Sphingosine kinase, sphingosine-1-phosphate, and apoptosis

The sphingolipid metabolites ceramide (Cer), sphingosine (Sph), and sphingosine-1-phosphate (S1P) play an important role in the regulation of cell proliferation, survival, and cell death. Cer and Sph usually inhibit proliferation and promote apoptosis, while the further metabolite S1P stimulates growth and suppresses apoptosis. Because these metabolites are interconvertible, it has been proposed that it is not the absolute amounts of these metabolites but rather their relative levels that determines cell fate. The relevance of this "sphingolipid rheostat" and its role in regulating cell fate has been borne out by work in many labs using many different cell types and experimental manipulations. A central finding of these studies is that Sph kinase (SphK), the enzyme that phosphorylates Sph to form S1P, is a critical regulator of the sphingolipid rheostat, as it not only produces the pro-growth, anti-apoptotic messenger S1P, but also decreases levels of pro-apoptotic Cer and Sph. Given the role of the sphingolipid rheostat in regulating growth and apoptosis, it is not surprising that sphingolipid metabolism is often found to be disregulated in cancer, a disease characterized by enhanced cell growth, diminished cell death, or both. Anticancer therapeutics targeting SphK are potentially clinically relevant. Indeed, inhibition of SphK has been shown to suppress gastric tumor growth [Cancer Res. 51 (1991) 1613] and conversely, overexpression of SphK increases tumorigenicity [Curr. Biol. 10 (2000) 1527]. Moreover, S1P has also been shown to regulate angiogenesis, or new blood vessel formation [Cell 99 (1999) 301], which is critical for tumor progression. Furthermore, there is intriguing new evidence that S1P can act in an autocrine and/or paracrine fashion [Science 291 (2001) 1800] to regulate blood vessel formation [J. Clin. Invest. 106 (2000) 951]. Thus, SphK may not only protect tumors from apoptosis, it may also increase their vascularization, further enhancing growth. The cytoprotective effects of SphK/S1P may also be important for clinical benefit, as S1P has been shown to protect oocytes from radiation-induced cell death in vivo [Nat. Med. 6 (2000) 1109]. Here we review the growing literature on the regulation of SphK and the role of SphK and its product, S1P, in apoptosis.

Enhancement of sphingosine 1-phosphate-induced migration of vascular endothelial cells and smooth muscle cells by an EDG-5 antagonist

Sphingosine 1-phosphate (Sph-1-P), a bioactive lysophospholipid capable of inducing a wide spectrum of biological responses, acts as an intercellular mediator, through interaction with the endothelial differentiation gene (EDG)/S1P family of G protein-coupled receptors. In this study, the effects of JTE-013, a specific antagonist of the migration-inhibitory receptor EDG-5, on Sph-1-P-elicited responses were examined in human umbilical vein endothelial cells (HUVECs) and vascular smooth muscle cells (SMCs), which expressed EDG-5 protein weakly and abundantly, respectively. This pyrazolopyridine compound reversed the inhibitory effect of Sph-1-P on SMC migration and further enhanced Sph-1-P-stimulated HUVEC migration. In contrast, its effect on Sph-1-P-induced intracellular Ca(2+) mobilization was marginal. Our results indicate that specific regulation of Sph-1-P-modulated migration responses in vascular cells can be achieved by EDG-5 antagonists and that manipulation of Sph-1-P biological activities by each EDG antagonist may lead to a therapeutical application to control vascular diseases.

Targets for pharmacological intervention of endothelial hyperpermeability and barrier function

Many diseases share the common feature of vascular leakage, and endothelial barrier dysfunction is often the underlying cause. The subsequent stages of endothelial barrier dysfunction contribute to endothelial hyperpermeability. Vasoactive agents induce loss of junctional integrity, a process that involves actin-myosin interaction. Subsequently, the interaction of leukocytes amplifies leakage by the leukocyte-derived mediators. The processes mainly occur at the postcapillary venules. The whole microvascular bed, including the capillaries, becomes involved in vascular leakage by the induction of angiogenesis. Plasma leakage results from gaps between endothelial cells as well as by the induction of transcellular transport pathways. Several mechanisms can improve endothelial barrier function, depending on the tissue affected and the cause of hyperpermeability. They include blockade of specific receptors and elevation of cyclic AMP (cAMP) by agents such as beta(2)-adrenergic agents. However, current therapies based on these principles often fail. Recent research has identified several new promising targets for pharmacological therapy. Endogenous compounds were also found with barrier-improving characteristics. Important insights were obtained in the different pathways involved in barrier dysfunction. Such insights regard the regulation of endothelial contraction and endothelial junction integrity: inhibitors of RhoA activation and Rho kinase represent a potentially valuable group of agents with endothelial hyperpermeability reducing properties, and strategies to target vascular endothelial growth factor (VEGF)-mediated edema are under current investigation. In clinical practice, not only tools to improve an impaired endothelial barrier function are necessary. Sometimes, a controlled, temporal, and local increase in permeability can also be desired, for example, with the aim to enhance drug delivery. Therefore, vessel leakiness is also being exploited to enable tissue access of liposomes, viral vectors, and other therapeutic agents that do not readily cross healthy endothelium. This review discusses strategies for targeting signaling molecules in therapies for diseases involving altered endothelial permeability.

Sphingosine-1-phosphate induces G(alphai)-coupled, PI3K/ras-dependent smooth muscle cell migration

BACKGROUND

Sphingolipids such as sphingosine-1-phosphate (S-1-P) are potent extracellular mediators released in response to vessel injury. S-1-P binds to G-protein-coupled receptors, which can be either G(alphai) or G(alphaq) linked. This study examines the signaling pathways involved in vascular smooth muscle cell migration after stimulation by S-1-P. We hypothesized that S-1-P stimulates migration of smooth muscle cells that is dependent upon a G(alphai)-coupled receptor, ras, phosphoinositol 3-kinase (PI3-K), and ERK 1/2.

METHODS

Vascular smooth muscle cells were cultured in vitro. A linear wound assay and Boyden chamber assay of migration were employed in the presence of S-1-P and inhibitors of G(alphai) [pertussis toxin (PTx), 100 ng/ml], G(alphaq) (GP-2A, 10 microM), ras [manumycin A (MA), 10 microM], PI3-K [Wortmannin (Wn), 10 microM], and MEK1 [PD98059 (PD), 25 microM]. Western blotting was performed separately to examine p42/p44 MAP kinase (ERK 1/2) activation in response to S-1-P with these inhibitors.

RESULTS

S-1-P induced vascular smooth muscle cell migration. This response was decreased by preincubation with PTx, suggesting a receptor linked, G(alphai)-mediated response. Application of a G(alphaq) inhibitor did not affect this response. S-1-P induced ERK 1/2 phosphorylation in a time-dependent manner. This S-1-P-induced cell migration was PD-sensitive in the Boyden chamber assay, confirming that it is MEK1- and ERK1/2-dependent. Inhibition of ras with MA and PI3-K with Wn also reduced ERK phosphorylation and smooth muscle cell migration in response to S-1-P.

CONCLUSIONS

S-1-P induces smooth muscle cell migration through a G(alphai)-linked, ras- and PI3-K-coupled, ERK 1/2-dependent process. Understanding signal transduction will allow targeted molecular interventions to treat the response of a vessel to injury.

Molecular basis of endothelial cell morphogenesis in three-dimensional extracellular matrices

Although many studies have focused on blood vessel development and new blood vessel formation associated with disease processes, the question of how endothelial cells (ECs) assemble into tubes in three dimensions (i.e., EC morphogenesis) remains unanswered. EC morphogenesis is particularly dependent on a signaling axis involving the extracellular matrix (ECM), integrins, and the cytoskeleton, which regulates EC shape changes and signals the pathways necessary for tube formation. Recent studies reveal that genes regulating this matrix-integrin-cytoskeletal (MIC) signaling axis are differentially expressed during EC morphogenesis. The Rho GTPases represent an important class of molecules involved in these events. Cdc42 and Rac1 are required for the process of EC intracellular vacuole formation and coalescence that regulates EC lumen formation in three-dimensional (3D) extracellular matrices, while RhoA appears to stabilize capillary tube networks. Once EC tube networks are established, supporting cells, such as pericytes, are recruited to further stabilize these networks, perhaps by regulating EC basement membrane matrix assembly. Furthermore, we consider recent work showing that EC morphogenesis is balanced by a tendency for newly formed tubes to regress. This morphogenesis-regression balance is controlled by differential gene expression of such molecules as VEGF, angiopoietin-2, and PAI-1, as well as a plasmin- and matrix metalloproteinase-dependent mechanism that induces tube regression through degradation of ECM scaffolds that support EC-lined tubes. It is our hope that this review will stimulate increased interest and effort focused on the basic mechanisms regulating capillary tube formation and regression in 3D extracellular matrices.

Embryonic brain expression analysis of lysophospholipid receptor genes suggests roles for s1p(1) in neurogenesis and s1p(1-3) in angiogenesis

In a comparison of embryonic brain expression patterns of lysophosphatidic acid and sphingosine 1-phosphate receptor genes (lpa(1-3) and s1p(1-5), respectively), transcripts detected by Northern blot were subsequently localized using in situ hybridization. We found striking s1p(1) expression adjacent to several ventricles. Near the lateral ventricle, s1p(1) expression was temporally and spatially coincident with neurogenesis and overlapped with lpa(1) in the neocortical area. We also observed a widespread diffuse pattern for lpa(2-3) and a scattered punctate pattern for s1p(1-3). The punctate pattern colocalized with vascular endothelial markers. Together, these results suggest that s1p(1) influences neurogenesis and s1p(1-3) influence angiogenesis in the developing brain.

Sphingosine-1-phosphate: dual messenger functions

The sphingolipid metabolite sphingosine-1-phosphate (S1P) is a serum-borne lipid that regulates many vital cellular processes. S1P is the ligand of a family of five specific G protein-coupled receptors that are differentially expressed in different tissues and regulate diverse cellular actions. Much less is known of the intracellular actions of S1P. It has been suggested that S1P may also function as an intracellular second messenger to regulate calcium mobilization, cell growth and suppression of apoptosis in response to a variety of extracellular stimuli. Dissecting the dual actions and identification of intracellular targets of S1P has been challenging, but there is ample evidence to suggest that the balance between S1P and ceramide and/or sphingosine levels in cells is an important determinant of cell fate.

Sphingosine 1-phosphate signaling in atherosclerosis and vascular biology

PURPOSE OF REVIEW

Sphingosine 1-phosphate is a novel lipid mediator which exerts various actions on endothelial cells and vascular smooth muscle cells. In this review, we discuss the latest findings about the molecule in vascular biology.

RECENT FINDINGS

It has been demonstrated that most sphingosine 1-phosphate-induced actions are mediated by the Edg-family of its receptors. Sphingosine 1-phosphate stimulates the migration and proliferation of endothelial cells and is cytoprotective towards them. The involvement of phosphoinositide 3-kinase and nitric oxide in sphingosine 1-phosphate downstream signaling in endothelial cells was recently reported, as was the enhancement of endothelial barrier integrity induced by the molecule. Sphingosine 1-phosphate inhibits migration of vascular smooth muscle cells and this inhibition was reported to be mediated by inhibition of Rac. Sphingosine 1-phosphate is concentrated in the lipoprotein fraction in plasma, and high-density lipoprotein exerted endothelial cytoprotection through its component of this molecule.

SUMMARY

Sphingosine 1-phosphate might play a critical role in the development of atherosclerosis.

Cloning and characterization of a protein kinase A anchoring protein (AKAP)-related protein that interacts with and regulates sphingosine kinase 1 activity

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid metabolite that has novel dual actions. S1P is the ligand for a family of G protein-coupled receptors known as S1PRs that mediate various physiological functions. Growth factors rapidly activate sphingosine kinase type 1 (SPHK1) resulting in phosphorylation of sphingosine to form S1P, which plays important roles in cell growth regulation and protection from apoptosis. However, little is known of the mechanism(s) by which SPHK activity is regulated. Using a yeast two-hybrid screening approach, we cloned a 3-kb cDNA encoding a SPHK1-interacting protein (SKIP). BLAST analysis revealed that SKIP corresponded to the C-terminal region of a larger ( approximately 7 kb) cDNA that encoded a protein with a high degree of similarity to a family of protein kinase A anchor proteins (AKAP). In confirmation of the yeast two-hybrid assay, glutathione S-transferase (GST)-SPHK1 specifically pulled down SKIP, whereas GST did not. Moreover, immunoprecipitation of in vitro translated SPHK1 and SKIP revealed that SKIP and SPHK1 are tightly associated. Furthermore, SKIP overexpression in NIH 3T3 fibroblasts reduced SPHK1 activity and interfered with its biological functions. The apoptotic-sparing effect of SPHK1 against serum deprivation was reduced when co-transfected with SKIP. In addition, SPHK1-enhanced cell proliferation was also abolished by SKIP, with a corresponding decrease in activation of ERK. Taken together, these results indicate that SKIP is a novel protein likely to play a regulatory role in the modulation of SPHK1 activity.

Sphingosine 1-phosphate as a therapeutic agent

The bioactive sphingolipid metabolite sphingosine-1-phosphate (S1P), formed by activation of sphingosine kinase in response to diverse stimuli, is an important lipid mediator that has novel dual actions - both inside and outside of cells. S1P is the ligand for a family of five G protein-coupled receptors. Activation of these GPCRs by S1P or dihydro-S1P regulates diverse processes, including cell migration, angiogenesis, vascular maturation, heart development, and neurite retraction. There is also abundant evidence that S1P can function as a second messenger important for regulation of calcium homeostasis, cell growth, and suppression of apoptosis. In many cases, the intracellular level of S1P and ceramide, another important sphingolipid metabolite associated with cell death and cell growth arrest, coordinately determine cell fate. Changes in S1P and ceramide have been implicated in a number of pathological conditions in which apoptosis plays an important role. Importantly, radiation-induced oocyte loss in adult female mice, the event that drives premature ovarian failure and infertility in female cancer patients, was completely prevented by in vivo therapy with S1P. Understanding the biosynthesis, metabolism and functions of S1P can uncover new targets for the pharmaceutical and therapeutic applications of S1P.

The roles of sphingosine-1-phosphate in asthma

Asthma is a complex condition in which exposure to environmental antigens induces inflammatory reactions in the airway characterized by activation of mast cells and eosinophils. Mast cells are known to be the main effector cells in eliciting IgE-mediated allergic response. These cells secrete various substances that perpetuate inflammation and provoke airway smooth muscle (ASM) contraction. A newly recognized addition to the repertoire of FcepsilonRI-mediated signaling events is the activation of sphingosine kinase leading to the generation of the potent sphingolipid mediator, sphingosine-1-phosphate (S1P) from sphingosine. S1P secretion by the lung significantly increases after challenge with an allergen, adding this sphingolipid metabolite to the variety of mediators that are released during an allergic reaction [FASEB J. 15 (2001) 1212]. Indeed, similar to previous reports, we found that FcepsilonRI cross-linking not only increased cellular levels of S1P, it also markedly enhanced its secretion from rat basophilic leukemia RBL-2H3 cells. Moreover, S1P induced degranulation of RBL and bone marrow derived mast cells (BMMCs) cells as determined by hexosaminidase release. Treatment of BMMCs with the sphingosine kinase inhibitors, DL-threo-dihydrosphingosine and dimethylsphingosine, reduced IgE/Ag stimulated histamine release. RT-PCR analysis demonstrated that these mast cells express S1P receptors EDG-1 and EDG-5 but not EDG-3, EDG-6 or EDG-8 transcripts. Further studies are needed to determine whether IgE triggering results in transactivation of EDG-1 or EDG-5 present on mast cells and whether this is a critical event for mast cell activation.

Phospholipase D activation by sphingosine 1-phosphate regulates interleukin-8 secretion in human bronchial epithelial cells

Sphingosine 1-phosphate (S1P), a potent bioactive sphingolipid, has been implicated in many critical cellular events, including a regulatory role in the pathogenesis of airway inflammation. We investigated the participation of S1P as an inflammatory mediator by assessing interleukin-8 (IL-8) secretion and phospholipase D (PLD) activation in human bronchial epithelial cells (Beas-2B). S1P(1), S1P(3), S1P(4), S1P(5), and weak S1P(2) receptors were detected in Beas-2B and primary human bronchial epithelial cells. S1P stimulated a rapid activation of PLD, which was nearly abolished by pertussis toxin (PTX) treatment, consistent with S1P receptor/G(i) protein coupling. S1P also markedly induced Beas-2B secretion of IL-8, a powerful neutrophil chemoattractant and activator, in a PTX-sensitive manner. This S1P-mediated response was dependent on transcription as indicated by a strong induction of IL-8 promoter-mediated luciferase activity in transfected Beas-2B cells and a complete inhibition by actinomycin D. Beas-2B exposure to 1-butanol, which converts the PLD-generated phosphatidic acid (PA) to phosphatidylbutanol by a transphosphatidylation reaction, significantly attenuated the S1P-induced IL-8 secretion, indicating the involvement of PLD-derived PA in the signaling pathway. Inhibition of 12-O-tetradecanoyl-phorbol-13-acetate-stimulated IL-8 production by 1-butanol further strengthened this observation. Blocking protein kinase C and Rho kinase also attenuated S1P-induced IL-8 secretion. Our data suggest that PLD-derived PA, protein kinase C, and Rho are important signaling components in S1P-mediated IL-8 secretion by human bronchial epithelial cells.

Endothelium-specific platelet-derived growth factor-B ablation mimics diabetic retinopathy

Loss of pericytes from the capillary wall is a hallmark of diabetic retinopathy, however, the pathogenic significance of this phenomenon is unclear. In previous mouse gene knockout models leading to pericyte deficiency, prenatal lethality has so far precluded analysis of postnatal consequences in the retina. We now report that endothelium-restricted ablation of platelet-derived growth factor-B generates viable mice with extensive inter- and intra-individual variation in the density of pericytes throughout the CNS. We found a strong inverse correlation between pericyte density and the formation of a range of retinal microvascular abnormalities strongly reminiscent of those seen in diabetic humans. Proliferative retinopathy invariably developed when pericyte density was <50% of normal. Our data suggest that a reduction of the pericyte density is sufficient to cause retinopathy in mice, implying that pericyte loss may also be a causal pathogenic event in human diabetic retinopathy.

Lysophospholipid receptors in the nervous system

The lysophospholipid mediators, lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P), are responsible for cell signaling in diverse pathways including survival, proliferation, motility, and differentiation. Most of this signaling occurs through an eight-member family of G-protein coupled receptors once known as the endothelial differentiation gene (EDG) family. More recently, the EDG receptors have been divided into two subfamilies: the lysophosphatidic acid subfamily, which includes LPA1, (EDG-2/VZG-1), LPA2 (EDG-4), and LPA3 (EDG-7), and the sphingosine-1-phosphate receptor subfamily, which includes S1P1 (EDG-1), S1P2 (EDG-5/H218/AGR16), S1P3 (EDG-3), S1P4 (EDG-6), and S1P5 (EDG-8/NRG-1). The ubiquitous expression of these receptors across species, coupled with their diverse cellular functions, has made lysophospholipid receptors an important focus of signal transduction research. Neuroscientists have recently begun to explore the role of lysophospholipid receptors in a number of cell types; this research has implicated these receptors in the survival, migration, and differentiation of cells in the mammalian nervous system.

Comparison of sphingosine 1-phosphate-induced intracellular signaling pathways in vascular smooth muscles: differential role in vasoconstriction

Sphingosine 1-phosphate (S1P), a lipid released from activated platelets, influences physiological processes in the cardiovascular system via activation of the endothelial differentiation gene (EDG/S1P) family of 7 transmembrane G protein-coupled receptors. In cultured vascular smooth muscle (VSM) cells, S1P signaling has been shown to stimulate proliferative responses; however, its role in vasoconstriction has not been examined. In the present study, the effects of S1P and EDG/S1P receptor expression were determined in rat VSM from cerebral artery and aorta. S1P induced constriction of cerebral artery, which was partly dependent on activation of p160(ROCK) (Rho-kinase). S1P also induced activation of RhoA in cerebral artery with a similar time course to contraction. In aorta, S1P did not produce a constriction or RhoA activation. In VSM myocytes from cerebral arteries, stimulation with S1P gives rise to a global increase in [Ca2+]i, initially generated via Ca2+ release from the sarcoplasmic reticulum by an inositol 1,4,5-trisphosphate-dependent pathway. In aorta VSM, a small increase in [Ca2+]i was observed after stimulation at higher concentrations of S1P. S1P induced activation of p42/p44(mapk) in aorta and cerebral artery VSM. Subtype-specific S1P receptor antibodies revealed that the expression of S1P3/EDG-3 and S1P2/EDG-5 receptors is 4-fold higher in cerebral artery compared with aorta. S1P(1)/EDG-1 receptor expression was similar in both types of VSM. Therefore, the ability of S1P to act as a vasoactive mediator is dependent on the activation of associated signaling pathways and may vary in different VSM. This differential signaling may be related to the expression of S1P receptor subtypes.

A noncoding RNA regulates human protease-activated receptor-1 gene during embryogenesis

Activation of the human protease-activated receptor-1 (PAR-1) by thrombin leads to myriad functions essential for maintaining vascular integrity. Upregulation of PAR-1 expression is considered important in atherosclerosis, angiogenesis and tumor metastasis. In vitro analysis of the human PAR-1 promoter function revealed a positive regulatory element between -4.2 and -3.2 kb of the transcription start site. This element was examined in transgenic mice containing either 4.1 or 2.9 kb of the 5' flanking sequence driving a LacZ reporter gene. Only the 4.1 kb PAR-1 transgene was expressed in vivo and only during embryonic development. The transgene expression was observed only in developing arteries and not in veins. Further examination of this putative regulatory sequence identified a novel noncoding RNA (ncR-uPAR:noncoding RNA upstream of the PAR-1) gene at -3.4 kb. The ncR-uPAR upregulated PAR-1-core promoter-driven luciferase activity and mRNA expression in vitro in a Pol II-dependent manner. This noncoding RNA appears to act in trans, albeit locally at the adjacent PAR-1 promoter. These data suggest that an untranslated RNA plays a role in PAR-1 gene expression during embryonic growth.

In silico data filtering to identify new angiogenesis targets from a large in vitro gene profiling data set

The objective of this study was to use gene expression data from well-defined cell culture models, in combination with expression data from diagnostic samples of human diseased tissues, to identify potential therapeutic targets and markers of disease. Using Affymetrix oligonucleotide array technology, we identified a common profile of genes upregulated during endothelial morphogenesis into tubelike structures in three in vitro models of angiogenesis. Rigorous data selection criteria were used to identify a list of over 1,000 genes whose expression was increased more than twofold over baseline at either 4, 8, 24, 40 or 50 h. To further refine and prioritize this list, we used standard bioinformatic algorithms to identify potential transmembrane and secreted proteins. We then overlapped this gene set with genes upregulated in colon tumors vs. normal colon, resulting in a subset of 128 genes in common with our endothelial list. We removed from this list those genes expressed in 6 different colon tumor lines, resulting in a list of 24 putative, vascular-specific angiogenesis-associated genes. Three genes, gp34, stanniocalcin-1 (STC-1), and GA733-1, were expressed at levels 10-fold or more in colon tumors compared with normal mucosa. We validated the vascular-specific expression of one of these genes, STC-1, by in situ hybridization. The ability to combine in vitro and in vivo data sets should permit one to identify putative angiogenesis target genes in various tumors, chronic inflammation, and other disorders where therapeutic manipulation of angiogenesis is a desirable treatment modality.

Marked perinatal lethality and cellular signaling deficits in mice null for the two sphingosine 1-phosphate (S1P) receptors, S1P(2)/LP(B2)/EDG-5 and S1P(3)/LP(B3)/EDG-3

Five cognate G protein-coupled receptors (S1P(1-5)) have been shown to mediate various cellular effects of sphingosine 1-phosphate (S1P). Here we report the generation of mice null for S1P(2) and for both S1P(2) and S1P(3). S1P(2)-null mice were viable and fertile and developed normally. The litter sizes from S1P(2)S1P(3) double-null crosses were remarkably reduced compared with controls, and double-null pups often did not survive through infancy, although double-null survivors lacked any obvious phenotype. Mouse embryonic fibroblasts (MEFs) were examined for the effects of receptor deletions on S1P signaling pathways. Wild-type MEFs were responsive to S1P in activation of Rho and phospholipase C (PLC), intracellular calcium mobilization, and inhibition of forskolin-activated adenylyl cyclase. S1P(2)-null MEFs showed a significant decrease in Rho activation, but no effect on PLC activation, calcium mobilization, or adenylyl cyclase inhibition. Double-null MEFs displayed a complete loss of Rho activation and a significant decrease in PLC activation and calcium mobilization, with no effect on adenylyl cyclase inhibition. These data extend our previous findings on S1P(3)-null mice and indicate preferential coupling of the S1P(2) and S1P(3) receptors to Rho and PLC/Ca(2+) pathways, respectively. Although either receptor subtype supports embryonic development, deletion of both produces marked perinatal lethality, demonstrating an essential role for combined S1P signaling by these receptors.

Hepatocyte growth factor enhances endothelial cell barrier function and cortical cytoskeletal rearrangement: potential role of glycogen synthase kinase-3beta

The stabilization of endothelial cell (EC) barrier function within newly formed capillaries is a critical feature of angiogenesis. We examined human lung EC barrier regulation elicited by hepatocyte growth factor (HGF), a recognized angiogenic factor and EC chemoattractant. HGF rapidly and dose-dependently elevated transendothelial electrical resistance (TER) of EC monolayers (>50% increase at 100 ng/ml), with immunofluorescence microscopic evidence of both cytoplasmic actin stress fiber dissolution and strong augmentation of the cortical actin ring. HGF rapidly stimulated phosphatidylinositol 3'-kinase, ERK, p38 mitogen-activated protein kinase, and protein kinase C activities. Pharmacological inhibitor studies demonstrated each pathway to be intimately involved in HGF-induced increases in TER, cortical actin thickening, and phosphorylation of the Ser/Thr glycogen synthase kinase-3beta (GSK-3beta), a potential target for the HGF barrier-promoting response. GSK-3beta phosphorylation was strongly correlated with reductions in both HGF-induced TER and enhanced beta-catenin immunoreactivity observed at cell-cell junctions. Our data suggest a model in which HGF-mediated EC cytoskeletal rearrangement and barrier enhancement depend critically on the activation of a complex kinase cascade that converges at GSK-3beta to increase the availability of beta-catenin, thereby enhancing endothelial junctional integrity and vascular barrier function.

Ceramide kinase, a novel lipid kinase. Molecular cloning and functional characterization

Ceramide-1-phosphate is a sphingolipid metabolite that has been implicated in membrane fusion of brain synaptic vesicles and neutrophil phagolysosome formation. Ceramide-1-phosphate can be produced by ATP-dependent ceramide kinase activity, although little is known of this enzyme because it has not yet been highly purified or cloned. Based on sequence homology to sphingosine kinase type 1, we have now cloned a related lipid kinase, human ceramide kinase (hCERK). hCERK encodes a protein of 537 amino acids that has a catalytic region with a high degree of similarity to the diacylglycerol kinase catalytic domain. hCERK also has a putative N-myristoylation site on its NH(2) terminus followed by a pleckstrin homology domain. Membrane but not cytosolic fractions from HEK293 cells transiently transfected with a hCERK expression vector readily phosphorylated ceramide but not sphingosine or other sphingoid bases, diacylglycerol or phosphatidylinositol. This activity was clearly distinguished from those of bacterial or human diacylglycerol kinases. With natural ceramide as a substrate, the enzyme had a pH optimum of 6.0-7.5 and showed Michaelis-Menten kinetics, with K(m) values of 187 and 32 microm for ceramide and ATP, respectively. Northern blot analysis revealed that hCERK mRNA expression was high in the brain, heart, skeletal muscle, kidney, and liver. A BLAST search analysis using the hCERK sequence revealed that putative ceramide kinases (CERKs) exist widely in diverse multicellular organisms including plants, nematodes, insects, and vertebrates. Phylogenetic analysis revealed that CERKs are a new class of lipid kinases that are clearly distinct from sphingosine and diacylglycerol kinases. Cloning of CERK should provide new molecular tools to investigate the physiological functions of ceramide-1-phosphate.

The immune modulator FTY720 targets sphingosine 1-phosphate receptors

Immunosuppressant drugs such as cyclosporin have allowed widespread organ transplantation, but their utility remains limited by toxicities, and they are ineffective in chronic management of autoimmune diseases such as multiple sclerosis. In contrast, the immune modulating drug FTY720 is efficacious in a variety of transplant and autoimmune models without inducing a generalized immunosuppressed state and is effective in human kidney transplantation. FTY720 elicits a lymphopenia resulting from a reversible redistribution of lymphocytes from circulation to secondary lymphoid tissues by unknown mechanisms. Using FTY720 and several analogs, we show now that FTY720 is phosphorylated by sphingosine kinase; the phosphorylated compound is a potent agonist at four sphingosine 1-phosphate receptors and represents the therapeutic principle in a rodent model of multiple sclerosis. Our results suggest that FTY720, after phosphorylation, acts through sphingosine 1-phosphate signaling pathways to modulate chemotactic responses and lymphocyte trafficking.

Cell type-specific localization of human cardiac S1P receptors

Sphingosine 1-phosphate (S1P), which derives from the metabolism of sphingomyelin, is mainly synthesized, stored, and released from platelets after activation by physiological and pathophysiological events. S1P acts in cardiovascular tissues through cell surface G-protein-coupled receptors of the endothelial differentiation gene (EDG) family, i.e., EDG1, EDG3 and EDG5. The aim of the present study was to assess the precise distribution of EDG1, EDG3, and EDG5 receptors expressed in human cardiovascular tissues to investigate their respective physiological implication. When assessed by Northern blots, EDG1, EDG3, and EDG5 displayed wide expression levels in decreasing order, respectively. In particular, EDG3 was mainly detected in the aorta. Detailed analysis by in situ hybridization (ISH) and immunohistochemistry (IHC) revealed strong EDG1 expression in cardiomyocytes and in endothelial cells of cardiac vessels. In cardiomyocytes, the EDG1 receptor is likely to be co-expressed with EDG3 and EDG5, although EDG1 exhibits the most prominent expression pattern. Unlike EDG3 and EDG5, which are expressed in the smooth muscle cell layer of the human aorta, no signal corresponding to EDG1 expression could be detected in the aorta. Moreover, only EDG3 expression was also found in smooth muscle cells of cardiac vessels. The present results provide new insight into the expression pattern of S1P receptors in human cardiovascular tissues, indicating a differential pattern of expression for these receptors in human vessels.

A mouse orthologue of puromycin-insensitive leucyl-specific aminopeptidase is expressed in endothelial cells and plays an important role in angiogenesis

Using polymerase chain reaction-coupled subtractive hybridization, we have isolated genes expressed during the in vitro differentiation of murine embryonic stem cells into endothelial cells (ECs). Among the genes obtained, we identified one gene that was inducible by vascular endothelial growth factor (VEGF) in the murine EC line MSS31. Analysis of the nucleotide and deduced amino acid sequences revealed that the protein was composed of 930 amino acids, including an HEXXH(X)18E consensus sequence of the M1 aminopeptidase family, and is thought to be a mouse orthologue of puromycin-insensitive leucyl-specific aminopeptidase (mPILSAP). The recombinant protein hydrolyzed N-terminal leucyl and methionyl residues from synthetic substrates. Immunohistochemical analysis revealed that mPILSAP was expressed in ECs during postnatal angiogenesis. Specific elimination of mPILSAP expression by antisense oligodeoxynucleotide (AS-ODN) attenuated VEGF-stimulated proliferation, migration, and network formation of ECs in vitro. Moreover, AS-ODN to mPILSAP inhibited angiogenesis in vivo. These results suggest a novel function of mPILSAP, which is expressed in ECs and plays an important role in angiogenesis.

Apoptotic effect of sphingosine 1-phosphate and increased sphingosine 1-phosphate hydrolysis on mesangial cells cultured at low cell density

The lipid mediator sphingosine 1-phosphate (S1P) may alter the proliferation of mesangial cells during pathophysiological processes. Here, S1P stimulated proliferation of rat mesangial cells and phosphorylation of MAPKs at subconfluent cell density. Both effects were inhibited by pertussis toxin treatment. Mesangial cells expressed several S1P receptors of the endothelial differentiation gene family: EDG-1, -3, -5, and -8. Conversely, S1P induced apoptosis at low cell density (2 x 10(4) cells/cm(2)), which was demonstrated by flow cytometry and Hoechst staining. Apoptosis was observed also in quiescent or growing cells and was not reverted by lysophosphatidic acid or platelet-derived growth factor. S1P enhanced phosphorylation of SAPKs. Incubation with [(33)P]S1P, [(3)H]S1P, and [(3)H]sphingosine demonstrated increased S1P hydrolysis, resulting in enhanced intracellular sphingosine levels and decreased S1P levels. A rise in total ceramide levels was also observed; however, ceramide did not originate from [(3)H]sphingosine, and S1P-induced apoptosis was not inhibited by fumonisin B, precluding involvement of de novo ceramide synthesis in apoptosis. Therefore, we suggest that sphingosine accumulation and decreased S1P are primarily responsible for S1P-induced apoptosis. In conclusion, incubation of low-density mesangial cells with S1P results in apoptosis, presumably due to increased S1P hydrolysis.

Extracellular export of sphingosine kinase-1 enzyme. Sphingosine 1-phosphate generation and the induction of angiogenic vascular maturation

The enzyme sphingosine kinase (SK) catalyzes the formation of sphingosine 1-phosphate (S1P), a bioactive lipid that acts extracellularly on G protein-coupled receptors of the S1P(1)/EDG-1 subfamily. Although S1P is formed in the cytosol of various cells, S1P release is not understood and is controversial because this lipid mediator is also regarded as a second messenger. In this report, we describe the existence of an extracellular S1P-generating system in vascular endothelial cells. Endothelial cells release SK constitutively and form S1P in the range of receptor stimulation. Levels of sphingosine but not ATP in the extracellular environment are rate-limiting. Treatment of endothelial cells with small interfering RNA for SK-1 transcript specifically inhibited SK export, and SK-1-transfected human embryonic kidney 293 cells exhibited enhanced release of SK-1. The export of SK-1 is constitutive and is inhibited by cytochalasin D and treatment at 4 degrees C but not by brefeldin A or nocodazole, suggesting that a nonclassical secretory pathway that requires the actin cytoskeleton dynamics is involved. Because S1P regulates angiogenesis and vascular maturation, we overexpressed SK-1 using an adenoviral vector in vivo in the Matrigel system of angiogenesis. Overexpression of SK-1 resulted in enhanced release of SK activity and induced angiogenesis and vascular maturation. These findings suggest that S1P is made in the extracellular milieu and that extracellular export of SK contributes to the action of S1P in the vascular system.

Dual regulation of EDG1/S1P(1) receptor phosphorylation and internalization by protein kinase C and G-protein-coupled receptor kinase 2

Here we demonstrate that phosphorylation of the sphingosine 1-phosphate (SSP) receptor "endothelial differentiation gene 1" (EDG1 or S1P(1)) receptor is increased in response to either SSP or phorbol 12-myristate 13-acetate (PMA) exposure but not lysophosphatidic acid. Phosphoamino acid analysis demonstrated that SSP stimulated the accumulation of phosphoserine and phosphothreonine but not phosphotyrosine. An inhibitor of PMA-stimulated EDG1 phosphorylation failed to block SSP-stimulated phosphorylation. Additionally, removal of 12 amino acids from the carboxyl terminus of EDG1 specifically reduced SSP- but not PMA-stimulated phosphorylation, suggesting that SSP and PMA increase EDG1 phosphorylation via distinct mechanisms. In vitro assays revealed that G-protein-coupled receptor kinase 2 may be at least partially responsible for SSP-stimulated EDG1 phosphorylation observed in intact cells. In addition, phosphorylation by PMA and SSP were associated with a loss of EDG1 from the cell surface by distinct mechanisms. Removal of 12 residues from the carboxyl terminus of EDG1 completely inhibited SSP-mediated internalization, suggesting that this domain dictates susceptibility to receptor internalization while retaining sensitivity to SSP-stimulated phosphorylation. Thus, we conclude that (a) EDG1 phosphorylation and internalization are controlled via independent mechanisms by agonist occupation of the receptor and protein kinase C activation, and (b) although determinants within the receptor's carboxyl-terminal tail conferring EDG1 sensitivity to agonist-mediated internalization and G-protein-coupled receptor kinase phosphorylation exhibit a degree of overlap, the two phenomena are separable.

Sphingolipids and the regulation of the immune response

Over the last decade evidence has accumulated that sphingolipids are important and specific signalling molecules for cell-to-cell communication (mediator function) as well as for intracellular signalling processes (second messenger function). In addition, glycosylated sphingolipids are essential building blocks of rafts thereby participating in the initiation of receptor mediated signalling events. In immunology, processes such as T cell apoptosis, Th1 versus Th2 T cell differentiation, phagocytosis, and allergic excitability are either influenced or directly regulated by this class of lipids. Models such as the 'dual function concept' (differentiation of structural components versus signalling molecules) and the 'rheostat concept' (the balance of two or more sphingolipids is essential for the biological function) describe the multiple properties of these signalling molecules.

Organogenesis--heart and blood formation from the zebrafish point of view

Organs are specialized tissues used for enhanced physiology and environmental adaptation. The cells of the embryo are genetically programmed to establish organ form and function through conserved developmental modules. The zebrafish is a powerful model system that is poised to contribute to our basic understanding of vertebrate organogenesis. This review develops the theme of modules and illustrates how zebrafish have been particularly useful for understanding heart and blood formation.

EDG-1 links the PDGF receptor to Src and focal adhesion kinase activation leading to lamellipodia formation and cell migration

Sphingosine-1-phosphate (SPP), formed by sphingosine kinase, is the ligand for EDG-1, a GPCR important for cell migration and vascular maturation. Here we show that cytoskeletal rearrangements, lamellipodia extensions, and cell motility induced by platelet-derived growth factor (PDGF) are abrogated in EDG-1 null fibroblasts. However, EDG-1 appears to be dispensable for mitogenicity and survival effects, even those induced by its ligand SPP and by PDGF. Furthermore, PDGF induced focal adhesion formation and activation of FAK, Src, and stress-activated protein kinase 2, p38, were dysregulated in the absence of EDG-1. In contrast, tyrosine phosphorylation of the PDGFR and activation of extracellular signal regulated kinase (ERK1/2), important for growth and survival, were unaltered. Our results suggest that EDG-1 functions as an integrator linking the PDGFR to lamellipodia extension and cell migration. PDGF, which stimulates sphingosine kinase, leading to increased SPP levels in many cell types, also induces translocation of sphingosine kinase to membrane ruffles. Hence, recruitment of sphingosine kinase to the cell's leading edge and localized formation of SPP may spatially and temporally stimulate EDG-1, resulting in activation and integration of downstream signals important for directional movement toward chemoattractants, such as PDGF. These results may also shed light on the vital role of EDG-1 in vascular maturation.

A novel function of sphingosine-1-phosphate to activate a non-selective cation channel in human endothelial cells

1. The Ca2+ entry pathway activated by sphingosine-1-phosphate (S1P) was examined in primary cultured vascular endothelial cells dispersed from human umbilical vein (HUVECs) by measuring intracellular Ca2+ concentration ([Ca2+]i), whole-cell membrane currents and single channel activity. 2. Application of S1P to HUVECs induced a slowly developing, sustained increase in [Ca2+]i. When Ca2+ was absent from the bathing solution, no S1P-induced changes in [Ca2+]i were observed. Tert-butylhydroquinone (BHQ), an inhibitor of Ca2+ pumps in endoplasmic reticulum, and histamine induced a transient elevation of [Ca2+]i in HUVECs. 3. Pretreatment of HUVECs with 100 ng x ml(-1) pertussis toxin (PTX) for 15 h almost abolished the S1P effect on [Ca2+]i and reduced the histamine effect to 40% of the control. The BHQ-induced elevation of [Ca2+]i was insensitive to PTX. 4. When whole-cell membrane currents were recorded using the amphotericin B-perforated-patch clamp technique while monitoring [Ca2+]i, application of S1P induced a tiny inward current (I(S1P)) which was followed by the elevation of [Ca2+]i. I(S1P) reversed at +20.0 +/- 2.7 mV under these experimental conditions. 5. When S1P was included in the pipette solution in the excised inside-out patch clamp configuration, single channel activity with a conductance of 17 pS was activated. This channel activity depended on the presence of intracellular GTP. 6. In summary, these results show that S1P has a novel effect in mammalian cardiovascular endothelium to activate a non-selective cation (NSC) channel in a GTP-dependent manner via a PTX-sensitive G-protein. This S1P-sensitive NSC channel acts as a Ca2+ entry pathway in endothelium.

Lysophospholipids--receptor revelations

Upon cell activation, membrane phospholipids are metabolized into potent lysophospholipid (LP) mediators, such as sphingosine 1-phosphate and lysophosphatidic acid. LPs fulfill signaling roles in organisms as diverse as yeast and humans. The recent discovery of G protein-coupled receptors for LPs in higher eukaryotes, and their involvement in regulating diverse processes such as angiogenesis, cardiac development, neuronal survival, and immunity, has stimulated growing interest in these lipid mediators. LP receptor biology has generated insights into fundamental cellular mechanisms and may provide therapeutic targets for drug development.

Sphingolipid mediators in cardiovascular cell biology and pathology

Sphingolipids have emerged as a new class of lipid mediators. In response to various extracellular stimuli, sphingolipid turnover can be stimulated in vascular cells and cardiac myocytes. Subsequent generation of sphingolipid molecules such as ceramide, sphingosine, and sphingosine-1-phosphate, is followed by regulation of ion fluxes and activation of various signaling pathways leading to smooth muscle cell proliferation, endothelial cell differentiation or apoptotic cell death, cell contraction, retraction, or migration. The importance of sphingolipids in cardiovascular signaling is illustrated by recent observations implicating them in physiological processes such as vasculogenesis as well as in frequent pathological conditions, including atherosclerosis and its complications.

Recovery and expression of messenger RNA from postmortem human brain tissue

The Bryan Alzheimer's Disease Research Center Rapid Autopsy Program at Duke University Medical Center obtains postmortem human brain tissue for experimental investigations. We evaluated 19 brains for RNA integrity and mRNA gene expression. Nine were from patients diagnosed with Alzheimer's disease, and ten were from nondemented controls. In all cases, the following variables were recorded: postmortem procurement delay (range, 1 hour and 10 minutes to 14 hours), pH of cerebrospinal fluid, premortem fever or sepsis, provision of supplemental oxygen in the agonal period, and temporal relation to time of death (either sudden death or protracted illness). Total RNA was extracted, quantified, and evaluated by agarose gel electrophoresis and quantitative gene expression analysis of 18S rRNA and edg-1 using TaqMan technology. All samples appeared to yield intact RNA without significant degradation, and expression of the edg-1 gene was detected by the real time reverse transcriptase polymerase chain reaction in all cases. We conclude that intact RNA can be obtained from postmortem human brain tissue, even in patients with severe premortem illnesses and delayed postmortem tissue procurement intervals. However, we caution that the successful expression of certain genes from postmortem brain tissue may require enhanced procurement efforts to maximize RNA integrity.

Role of the sphingosine 1-phosphate receptor EDG-1 in vascular smooth muscle cell proliferation and migration

Sphingosine 1-phosphate (S1P), a platelet-derived ligand for the EDG-1 family of G protein-coupled receptors (GPCRs), has recently emerged as a regulator of vascular development. Although S1P has potent effects on endothelial cells and vascular smooth muscle cells (VSMCs), the functions of the specific S1P receptors in the latter cell type are not known. Here we show that pup-intimal VSMCs express higher levels of EDG-1 mRNA than adult-medial VSMCs. Stable transfection of EDG-1 into adult-medial VSMCs enhanced their proliferative response to S1P, concomitant with induction of p70 S6 kinase activity and expression of cyclin D1. Pertussis toxin treatment inhibited S1P-induced p70 S6 kinase activation, cyclin D1 expression and proliferation, suggesting that EDG-1-coupling to the G(i) pathway is critical. Furthermore, blocking p70 S6 kinase phosphorylation with rapamycin inhibited cyclin D1 expression and proliferation, suggesting that activation of p70 S6 kinase is critical in EDG-1/G(i)-mediated cell proliferation. EDG-1 expression also profoundly enhanced the migratory response of adult-medial VSMCs to S1P. S1P-induced migration of adult-medial VSMCs expressing exogenous EDG-1 required G(i) activation but not p70 S6 kinase. These results suggest that enhanced expression of EDG-1 in VSMCs dramatically stimulates both the proliferative and migratory responses to S1P. Since EDG-1 is expressed in the pup-intimal phenotype of VSMCs, S1P signaling via EDG-1 may play a role in vascular diseases in which the proliferation and migration of VSMCs are dysregulated.