Sphingosine 1-phosphate: synthesis and release

Effects of lysophospholipids on tumor microenvironment

The effects of lysophospholipids (LPLs) on cancer microenvironment is a vast and growing field. These lipids are secreted physiologically by various cell types. They play highly important roles in the development, activation and regulation of the immune system. They are also secreted by cancerous cells and there is a strong association between LPLs and cancer. It is clear that these lipids and in particular sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) play major roles in regulating the growth of tumor cells, and in manipulating the immune system. These activities can be divided into two parts; the first involves the ability of S1P and LPA to either directly or through some of the enzymes that generate them such as sphingosine kinases or phospholipases, induce the motility and invasiveness of tumor cells. The second mechanism involves the recently discovered effects of these lipids on the anti-tumor effector natural killer (NK) cells. Whereas S1P and LPA induce the recruitment of these effector cells, they also inhibit their cytolysis of tumor cells. This may support the environment of cancer and the ability of cancer cells to grow, spread and metastasize. Consequently, LPLs or their receptors may be attractive targets for developing drugs in the treatment of cancer where LPLs or their receptors are up-regulated.

Protection of LPS-induced murine acute lung injury by sphingosine-1-phosphate lyase suppression

A defining feature of acute lung injury (ALI) is the increased lung vascular permeability and alveolar flooding, which leads to associated morbidity and mortality. Specific therapies to alleviate the unremitting vascular leak in ALI are not currently clinically available; however, our prior studies indicate a protective role for sphingosine-1-phosphate (S1P) in animal models of ALI with reductions in lung edema. As S1P levels are tightly regulated by synthesis and degradation, we tested the hypothesis that inhibition of S1P lyase (S1PL), the enzyme that irreversibly degrades S1P via cleavage, could ameliorate ALI. Intratracheal instillation of LPS to mice enhanced S1PL expression, decreased S1P levels in lung tissue, and induced lung inflammation and injury. LPS challenge of wild-type mice receiving 2-acetyl-4(5)-[1(R),2(S),3(R),4-tetrahydroxybutyl]-imidazole to inhibit S1PL or S1PL(+/-) mice resulted in increased S1P levels in lung tissue and bronchoalveolar lavage fluids and reduced lung injury and inflammation. Moreover, down-regulation of S1PL expression by short interfering RNA (siRNA) in primary human lung microvascular endothelial cells increased S1P levels, and attenuated LPS-mediated phosphorylation of p38 mitogen-activated protein kinase and I-κB, IL-6 secretion, and endothelial barrier disruption via Rac1 activation. These results identify a novel role for intracellularly generated S1P in protection against ALI and suggest S1PL as a potential therapeutic target.

Roles of sphingosine-1-phosphate signaling in angiogenesis

Sphingosine-1-phosphate (S1P) is a blood-borne lipid mediator with pleiotropic biological activities. S1P acts via the specific cell surface G-protein-coupled receptors, S1P_1-5. S1P₁ and S1P₂ were originally identified from vascular endothelial cells (ECs) and smooth muscle cells, respectively. Emerging evidence shows that S1P plays crucial roles in the regulation of vascular functions, including vascular formation, barrier protection and vascular tone via S1P₁, S1P₂ and S1P₃. In particular, S1P regulates vascular formation through multiple mechanisms; S1P exerts both positive and negative effects on angiogenesis and vascular maturation. The positive and negative effects of S1P are mediated by S1P₁ and S1P₂, respectively. These effects of S1P₁ and S1P₂ are probably mediated by the S1P receptors expressed in multiple cell types including ECs and bone-marrow-derived cells. The receptor-subtype-specific, distinct effects of S1P favor the development of novel therapeutic tactics for antitumor angiogenesis in cancer and therapeutic angiogenesis in ischemic diseases.

Erythrocytes serve as a reservoir for cellular and extracellular sphingosine 1-phosphate

Sphingosine 1-phosphate (S1P) in blood is phosphorylated, stored, and transported by red blood cells (RBC). Release of S1P from RBC into plasma is a regulated process that does not occur in plasma- or serum-free media. Plasma fractionation and incubations with isolated and recombinant proteins identified high density lipoprotein (HDL) and serum albumin (SA) as non-redundant endogenous triggers for S1P release from RBC. S1P bound to SA and HDL was able to stimulate the S1P(1) receptor in calcium flux experiments. The binding capability of acceptor molecules triggers S1P release, as demonstrated with the anti-S1P antibody Sphingomab. More S1P was extracted from RBC membranes by HDL than by SA. Blood samples from anemic patients confirmed a reduced capacity for S1P release in plasma. In co-cultures of RBC and endothelial cells (EC), we observed transcellular transportation of S1P as a second function of RBC-associated S1P in the absence of SA and HDL and during tight RBC-EC contact, mimicking conditions in tissue interstitium and capillaries. In contrast to S1P bound to SA and HDL, RBC-associated S1P was significantly incorporated by EC after S1P lyase (SGPL1) inhibition. RBC-associated S1P, therefore, has two functions: (1) It contributes to the cellular pool of SGPL1-sensitive S1P in tissues after transcellular transportation and (2) it helps maintain extracellular S1P levels via SA and HDL independently from SGPL1 activity.

Tumor necrosis factor-induced neutrophil adhesion occurs via sphingosine kinase-1-dependent activation of endothelial {alpha}5{beta}1 integrin

Leukocyte recruitment plays a major role in the immune response to infectious pathogens, as well as during inflammatory and autoimmune disorders. The process of leukocyte extravasation from the blood requires a complex cascade of adhesive events between the leukocytes and the endothelium, including initial leukocyte rolling, adhesion, and finally transendothelial migration. Current research in this area aims to identify the key leukocyte subsets that initiate a given disease and to identify the trafficking molecule(s) that will most specifically inhibit those cells. Herein we demonstrate that tumor necrosis factor (TNF)alpha activates the integrin alpha(5)beta(1) without altering total expression levels of beta(1) integrin on human umbilical vein endothelial cells. Moreover, our studies suggest that TNFalpha-induced beta(1) activation is dependent on sphingosine kinase-1, but independent of the sphingosine-1-phosphate family of G protein-coupled receptors. We also show, using a parallel plate flow chamber assay, that neutrophil adhesion to TNFalpha-activated endothelium can be attenuated by blocking alpha(5)beta(1) or its ligand angiopoietin-2. These observations add new complexities that broaden the accepted concept of cellular trafficking with neutrophil adhesion to TNFalpha activated endothelial cells being sphingosine kinase-1, alpha(5)beta(1), and angiopoietin-2 dependent. Moreover, this work supports the notion that sphingosine kinase-1 may be the single target required for an effective broad spectrum approach to combat inflammation and immune disorders.

Role of sphingosine kinase-1 in paracrine/transcellular angiogenesis and lymphangiogenesis in vitro

Sphingosine-1-phosphate (S1P) is an important bioactive sphingolipid involved in angiogenesis and lymphangiogenesis, 2 important processes that influence the growth, survival, and spread of tumors. S1P acts as an extracellular mediator through binding to 5 highly specific S1P receptors, S1P(1-5). Sphingosine kinase-1 (SK1), one of 2 known sphingosine kinase enzymes responsible for S1P production, appears to be overexpressed in many tumors. Although a role for S1P in angiogenesis and lymphangiogenesis has been established, it is unclear whether S1P secreted from cancer cells has a paracrine function in a tumor environment. Here we investigated whether modulation of cellular SK1 could initiate a paracrine angiogenic and lymphangiogenic switch. We found that SK1 overexpression in HEK cells or its down-regulation in glioma or breast cancer cells modulated extracellular S1P levels accordingly, which in turn increased or decreased both migration and tube formation in cocultured vascular or lymphatic endothelial cells. In contrast, down-regulation of sphingosine kinase 2 in both glioma and breast cancer cells had no appreciable effect on cellular or secreted S1P levels. In addition, vascular endothelial growth factors VEGF and VEGF-C down-regulation in cancer cells appeared insufficient to block the angiogenic and lymphangiogenic switch triggered by these cells. Moreover, S1P initiated endothelial cell sprouting in 3-dimensional collagen matrices, which is representative of a multistep angiogenic process. Our data collectively demonstrate for the first time that SK1 plays an essential role in regulating in vitro paracrine angiogenesis and lymphangiogenesis.

Sphingosine 1-phosphate induces vascular endothelial growth factor expression in endothelial cells

Angiogenesis is essential for tumor growth and vascular endothelial cell growth factor (VEGF) plays a key role in this process. Conversely, sphingosine 1-phosphate (S1P) is a biologically active sphingolipid known to play a key role in cancer progression by regulating endothelial cell proliferation and migration. In this study, the authors found that S1P increases the level of VEGF mRNA in human umbilical vein endothelial cells (HUVECs) and immortalized HUVECs (iHUVECs). Additionally, S1P was found to increase VEGF promoter activity in MS-1 mouse pancreatic islet endothelial cells. Furthermore, a pharmacological inhibitory study revealed that G(alpha i/o)-mediated phospholipase C, Akt, Erk, and p38 MAPK signaling are involved in this S1P-induced expression of VEGF. A component of AP1 transcription factor is important for S1P-induced VEGF expression. Taken together, these findings suggest that S1P enhances endothelial cell proliferation and migration by upregulating the expression of VEGF mRNA.

Sphingosine kinase-1 associates with integrin {alpha}V{beta}3 to mediate endothelial cell survival

Sphingosine kinase (SK)-1 promotes endothelial cell (EC) survival through the cell junction molecule CD31 (platelet endothelial cell adhesion molecule-1). The integrin alpha(v)beta(3) is also essential for EC survival; inhibition of alpha(v)beta(3) ligation promotes apoptosis. Herein we demonstrate that under basal conditions, SK-1, alpha(v)beta(3), and CD31 exist as a heterotrimeric complex. Under conditions that affect EC survival such as loss of contact with the extracellular matrix or growth factor activation, more of this heterotrimeric complex forms. Overexpression studies demonstrate a requirement for SK-1 phosphorylation at serine 225 for increased heterotrimeric complex formation, activation of alpha(v)beta(3), and EC survival signals, including Bcl-X and nuclear factor-kappaB pathways. Moreover, beta(3) integrin depletion confirmed the requirement for this heterotrimeric complex in SK-1-mediated EC survival. Thus, with alpha(v)beta(3) integrin being identifiable primarily on angiogenic ECs and SK-1 being highly expressed in tumors, targeting SK-1 may affect multiple survival pathways, and its inhibition may be highly efficacious in controlling pathological EC survival.

Sphingosine 1-phosphate enhances Fc gamma receptor-mediated neutrophil activation and recruitment under flow conditions

Sphingosine 1-phosphate (S1P) is a bioactive phospholipid that is released by platelets and endothelial cells and has been implicated in diverse biological functions. We hypothesized that S1P may influence immune complex-mediated polymorphonuclear neutrophil activation. Using flow cytometry and fluorescence spectrometry, we found that exogenous addition of S1P led to an enhanced polymorphonuclear neutrophil Fcgamma receptor-mediated rise in intracellular Ca(2+) and reactive oxygen species generation in a pertussis toxin-independent manner, while having only a small effect by itself. Thus, S1P amplifies a positive feedback loop where Fcgamma receptor-mediated rises in Ca(2+) and reactive oxygen species are interdependent, with reactive oxygen species acting to increase tyrosine phosphorylation and activity of upstream signaling intermediates. S1P augmentation of Fcgamma receptor signaling translates to downstream functional consequences, including shape change and recruitment to endothelial surfaces coated with suboptimal levels of immune complexes. Taken together, S1P from activated platelets or endothelial cells may serve to amplify leukocyte recruitment and tissue injury at sites of immune complex deposition in vasculitis.

S1P(5) is required for sphingosine 1-phosphate-induced autophagy in human prostate cancer PC-3 cells

Sphingosine 1-phosphate (S1P) is a platelet- and endothelial cell-released lysophospholipid that regulates various cellular functions through activating a specific family of G protein-coupled receptors. Both platelet activation and angiogenesis play important roles in cancer development, implying that cancer cells might encounter a large amount of S1P during these processes. Cancer cells, in the meantime, may experience nutrient deprivation and rely on autophagy for early development. Whether extracellular S1P regulates autophagy remains to be tested. In the present work, we investigated whether autophagy is regulated by S1P in PC-3 cells. Through monitoring the modification patterns of LC3 by Western blotting, we demonstrated that autophagy was induced by exogenously applied S1P in PC-3 cells. This observation was further confirmed by fluorescence microscopy using PC-3 cells stably expressing enhanced green fluorescent protein-LC3. By applying small interfering RNA and dihydro-S1P, S1P(5) activation was found to be involved in this process. Besides, mammalian target of rapamycin signaling was inhibited upon S1P treatment. Taken together, our results suggest that, under serum-starved conditions, S1P further upregulates autophagic activity through S1P(5)-dependent pathways in PC-3 cells.

Endothelial-mural cell signaling in vascular development and angiogenesis

Mural cells are essential components of blood vessels and are necessary for normal development, homeostasis, and organ function. Alterations in mural cell density or the stable attachment of mural cells to the endothelium is associated with several human diseases such as diabetic retinopathy, venous malformation, and hereditary stroke. In addition mural cells are implicated in regulating tumor growth and have thus been suggested as potential antiangiogenic targets in tumor therapy. In recent years our knowledge of mural cell function and endothelial-mural cell signaling has increased dramatically, and we now begin to understand the mechanistic basis of the key signaling pathways involved. This is mainly thanks to sophisticated in vivo experiments using a broad repertoire of genetic technologies. In this review, we summarize the five currently best understood signaling pathways implicated in mural cell biology. We discuss PDGFB/PDGFRbeta- dependent pericyte recruitment, as well as the role of angiopoietins and Tie receptors in vascular maturation. In addition, we highlight the effects of sphingosine-1-phosphate signaling on adherens junction assembly and vascular stability, as well as the role of TGF-beta-signaling in mural cell differentiation. We further reflect recent data suggesting an important function for Notch3 signaling in mural cell maturation.

Targeting sphingosine-1-phosphate signalling for cardioprotection

Sphingosine-1-phosphate (S1P) is a bioactive lysophospholipid generated by the sphingosine kinase (SK1 or SK2)-catalysed phosphorylation of sphingosine. Plasma S1P is carried in high-density lipoprotein (HDL) or bound to albumin and is reported to arise from activated platelets and erythrocytes. In addition, extracellular SK1 released from vascular endothelial cells may also contribute to plasma S1P levels. S1P exerts its effects through a family of five high affinity S1P-specific G protein-coupled receptors (GPCRs), S1P(1-5). Various S1P receptors are present in the cardiovascular system, including cardiac tissue. Additionally, intracellular S1P may have a second messenger action. Since S1P is recognised as a survival factor in many tissues, there has been much interest in S1P as a cardioprotective agent. Recent evidence indicates that S1P can pre-condition and post-condition the heart and that the cardioprotective effect of HDL may be because of its S1P content. In addition, evidence is emerging that the cardioprotective effects of cannabinoids and S1P may be linked.

Activation of sphingosine kinase by muscarinic receptors enhances NO-mediated and attenuates EDHF-mediated vasorelaxation

Local formation of the sphingomyelin metabolite sphingosine-1-phosphate (S1P) within the vascular wall has been shown to modulate vascular reactivity. In this study we investigated whether sphingosine kinase, the enzyme responsible for S1P synthesis, plays a role in muscarinic receptor-mediated NO production and vascular relaxation in different blood vessel types. For this purpose, sphingosine kinase translocation and sphingolipid-dependent NO-production after muscarinic receptor stimulation were assessed in an endothelial cell line. Furthermore, we used the sphingosine kinase inhibitor N,N-dimethylsphingosine (DMS) to investigate the role of sphingosine kinase in the relaxant responses to the muscarinic agonist methacholine (MCh) in isolated rat aorta and mesenteric arteries. Activation of M(3)-receptors in an endothelial cell line induced a fast translocation of YFP-tagged sphingosine kinase-1 from the cytosol to the plasma membrane. Concomitant NO-production in this cell line was partially inhibited by DMS. Accordingly, in rat aorta the relaxant responses to MCh were attenuated in the presence of DMS, while the responses to the NO-donor sodium nitroprusside were unaltered. In contrast, DMS enhanced the relaxant responses to MCh in mesenteric artery preparations. This effect could also be observed in the presence of NO synthase and cyclooxygenase inhibitors, indicating that sphingosine kinase inhibition specifically enhanced endothelium-derived hyperpolarizing factor-mediated (i.e. non-NO and non-prostacyclin-dependent) relaxation. We conclude that sphingosine kinase differentially regulates vascular tone in different vessel types, enhancing NO-dependent vasorelaxation but counteracting EDHF-dependent vasorelaxation. This observation enhances our understanding of the complex mechanisms by which sphingolipids regulate vascular homeostasis. Moreover, a disturbed regulation of sphingolipid metabolism in the vascular wall may therefore play a role in the aetiology/pathology of disease states characterized by endothelial dysfunction.

Biological roles of lysophospholipid receptors revealed by genetic null mice: an update

Two lysophospholipids (LPs), lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), are known to affect various cellular events. Their actions are mediated by binding to at least ten bona fide high-affinity G protein-coupled receptors referred to as LPA1-5 and S1P1-5. These LPs are expressed throughout the body and are involved in a range of biological activities including normal development, as well as functioning in most organ systems. A growing number of biological functions have been uncovered in vivo using single- or multiple-null mice for each LP receptor. This review will focus on findings from in vivo as well as in vitro studies using genetic null mice for the LP receptors, LPA1,2,3 and S1P1,2,3,5, and for the LP producing enzymes, autotaxin and sphingosine kinase 1/2.

Targeting human mast cells expressing g-protein-coupled receptors in allergic diseases

The G-protein-coupled receptors (GPCRs) are the largest known group of integral membrane receptor proteins and are the most common targets of pharmacotherapy. Mast cells (MCs) have been reported to play an important role in allergic diseases, such as urticaria and bronchial asthma. There is an increasing body of clinical evidence that MCs are recruited into allergic reactions by non-IgE-dependent mechanisms. Human MCs are activated and secrete histamine in response to neuropeptides, such as substance P and somatostatin, mediated by a GPCR, MRGX2. The microenvironment surrounding MCs in their resident tissues is likely to contain multiple factors that modify antigen-dependent MC activation. MCs express various GPCRs, and since the function of human MCs is modulated by various GPCR ligands, such as adenosine and sphingosine-1-phosphate, which are present in high levels in the bronchial alveolar lavage fluid of asthmatic patients, the GPCRs expressed on MCs may play an important role in human allergic diseases. The GPCRs expressed on MCs may serve as drug targets for the treatment of allergic diseases.

Sphingolipids in the lungs

Sphingolipids such as sphingosine-1-phosphate (S1P), ceramide, or sphingomyelin are essential constituents of plasma membranes and regulate many (patho)physiological cellular responses inducing apoptosis and cell survival, vascular permeability, mast cell activation, and airway smooth muscle functions. The complexity of sphingolipid biology is generated by a great variety of compounds, diverse receptors, and often antagonistic functions of different sphingolipids. For instance, apoptosis is promoted by ceramide and prevented by S1P, and pulmonary vascular permeability is increased by S1P2/3 receptors and by ceramide, whereas S1P1 receptors stabilize barrier integrity. Several enzymes of the sphingolipid metabolism respond to external stimuli such as sphingomyelinase isoenzymes that are activated by many stress stimuli and the sphingosine kinase isoenzymes that are activated by allergens. The past years have provided increasing evidence that these processes contribute to pulmonary disorders including asthma, chronic obstructive pulmonary disease, acute lung injury, and cystic fibrosis. Sphingolipid metabolism offers several novel therapeutic targets for the treatment of lung diseases such as emphysema, asthma, cystic fibrosis, respiratory tract infection, sepsis, and acute lung injury.

HDL: bridging past and present with a look at the future

Clinical and epidemiological studies have shown that HDLs, a class of plasma lipoproteins, heterogeneous in size and density, have an atheroprotective role attributed, for years, to their capacity to promote the efflux of cholesterol from activated cholesterol-loaded arterial macrophages. Recent studies, however, have recognized that the physical heterogeneity of HDLs is associated with multiple functions that involve both the protein and the lipid components of these particles. ApoA-I, quantitatively the major protein constituent, has an amphipathic structure suited for transport of lipids. It readily interacts with the ATP-binding cassette transporter ABCA1, the SR-B1 scavenger receptor; activates the enzyme lecithin-cholesterol acyl transferase (LCAT), which is critical for HDL maturation. It also has antioxidant and antiinflammatory properties, along with the HDL-associated enzymes paraoxonase, platelet activating factor acetylhydrolase (PAF), and glutathione peroxidase. Regarding the lipid moiety, an atheroprotective role has been recognized for lysosphingolipids, particularly sphingosine-1-phosphate (S1P). All of these atheroprotective functions are lost in the post-translational dependent dysfunctional plasma HDLs of subjects with systemic inflammation, coronary heart disease, diabetes, and chronic renal disease. The emerging notion that particle quality has more predictive power than quantity has stimulated further exploration of the HDL proteome, already revealing unsuspected pro- or antiatherogenic proteins/peptides associated with HDL.

The vascular S1P gradient-cellular sources and biological significance

Sphingosine 1-phosphate (S1P), a product of sphingomyelin metabolism, is enriched in the circulatory system whereas it is estimated to be much lower in interstitial fluids of tissues. This concentration gradient, termed the vascular S1P gradient appears to form as a result of substrate availability and the action of metabolic enzymes. S1P levels in blood and lymph are estimated to be in the muM range. In the immune system, the S1P gradient is needed as a spatial cue for lymphocyte and hematopoietic cell trafficking. During inflammatory reactions in which enhanced vascular permeability occurs, a burst of S1P becomes available to its receptors in the extravascular compartment, which likely contributes to the tissue reactions. Thus, the presence of the vascular S1P gradient is thought to contribute to physiological and pathological conditions. From an evolutionary perspective, S1P receptors may have co-evolved with the advent of a closed vascular system and the trafficking paradigms for hematopoietic cells to navigate in and out of the vascular system.

The role of sphingosine-1-phosphate and its receptors in asthma

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid metabolite that plays important roles in allergic responses, including asthma and anaphylaxis, the incidence of which is rising worldwide especially in industrialized urban populations. In this review, we will discuss how S1P is formed and released, and how it acts at many cellular levels, including mast cells, the airway epithelium, airway smooth muscle and many immune cells. Since the actions of S1P on all of these cells could exacerbate allergic responses, the proteins that synthesize, release and respond to S1P offer plausible targets for a new generation of antiinflammatory therapeutics.

Lysophospholipid signaling in the function and pathology of the reproductive system

BACKGROUND

Lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are two prominent signaling lysophospholipids (LPs) exerting their functions through a group of G protein-coupled receptors (GPCRs). This review covers current knowledge of the LP signaling in the function and pathology of the reproductive system.

METHODS

PubMed was searched up to May 2008 for papers on lysophospholipids/LPA/S1P/LPC/SPC in combination with each part of the reproductive system, such as testis/ovary/uterus.

RESULTS

LPA and SIP are found in significant amounts in serum and other biological fluids. To date, 10 LP receptors have been identified, including LPA(1-5) and S1P(1-5). In vitro and in vivo studies from the past three decades have demonstrated or suggested the physiological functions of LP signaling in reproduction, such as spermatogenesis, male sexual function, ovarian function, fertilization, early embryo development, embryo spacing, implantation, decidualization, pregnancy maintenance and parturition, as well as pathological roles in ovary, cervix, mammary gland and prostate cancers.

CONCLUSIONS

Receptor knock-out and other studies indicate tissue-specific and receptor-specific functions of LP signaling in reproduction. More comprehensive studies are required to define mechanisms of LP signaling and explore the potential use as a therapeutic target.

"Inside-out" signaling of sphingosine-1-phosphate: therapeutic targets

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid metabolite involved in many critical cellular processes including proliferation, survival, and migration, as well as angiogenesis and allergic responses. S1P levels inside cells are tightly regulated by the balance between its synthesis by sphingosine kinases and degradation. S1P is interconvertible with ceramide, which is a critical mediator of apoptosis. It has been postulated that the ratio between S1P and ceramide determines cell fate. Activation of sphingosine kinase by a variety of agonists increases intracellular S1P, which in turn can function intracellularly as a second messenger or be secreted out of the cell and act extracellularly by binding to and signaling through S1P receptors in autocrine and/or paracrine manners. Recent studies suggest that this "inside-out" signaling by S1P may play a role in many human diseases, including cancer, atherosclerosis, inflammation, and autoimmune disorders such as multiple sclerosis. In this review we summarize metabolism of S1P, mechanisms of sphingosine kinase activation, and S1P receptors and their downstream signaling pathways and examine relationships to multiple disease processes. In particular, we describe recent preclinical and clinical trials of therapies targeting S1P signaling, including 2-amino-2-propane-1,3-diol hydrochloride (FTY720, fingolimod), S1P receptor agonists, sphingosine kinase inhibitors, and anti-S1P monoclonal antibody.

Enhanced release of sphingosine-1-phosphate from hypercholesterolemic platelets: role in development of hypercholesterolemic atherosclerosis

Although it is well known that sphingosine-1-phosphate (S1P), which induces many biological responses, is present in plasma and is mainly released from activated platelets, little is known whether the release of S1P is increased when platelets are activated in the hypercholesterolemic condition, and what are the roles of increased S1P generation in the development or progression of the atherosclerosis. Results show that 0.5% cholesterol diet for 16 weeks induces platelet hyperaggregability to low doses of agonists as well as development of hypercholesterolemic atherosclerosis in the rabbits. The generation and released level of S1P were significantly increased in the hypersensitized platelets and blood plasma in hypercholesterolemic rabbits. We also demonstrated that S1P increased VSMC proliferation via endothelial differentiation gene (EDG)-1 receptor dependent pathway. Our results indicate that release of S1P from activated platelets was increased by enhanced platelet sensitivity in hypercholesterolemia, which potentiated the ox-LDL-induced VSMC proliferation via EDG-1 receptor pathway.

G12/13 and Gq mediate S1P2-induced inhibition of Rac and migration in vascular smooth muscle in a manner dependent on Rho but not Rho kinase

AIMS

The lysophospholipid mediator sphingosine-1-phosphate (S1P) activates G protein-coupled receptors (GPCRs) to induce potent inhibition of platelet-derived growth factor (PDGF)-induced Rac activation and, thereby, chemotaxis in rat vascular smooth muscle cells (VSMCs). We explored the heterotrimeric G protein and the downstream mechanism that mediated S1P inhibition of Rac and cell migration in VSMCs.

METHODS AND RESULTS

S1P inhibition of PDGF-induced cell migration and Rac activation in VSMCs was abolished by the selective S1P(2) receptor antagonist JTE-013. The C-terminal peptides of Galpha subunits (Galpha-CTs) act as specific inhibitors of respective G protein-GPCR coupling. Adenovirus-mediated expression of Galpha(12)-CT, Galpha(13)-CT, and Galpha(q)-CT, but not that of Galpha(s)-CT or LacZ or pertussis toxin treatment, abrogated S1P inhibition of PDGF-induced Rac activation and migration, indicating that both G(12/13) and G(q) classes are necessary for the S1P inhibition. The expression of Galpha(q)-CT as well as Galpha(12)-CT and Galpha(13)-CT also abolished S1P-induced Rho stimulation. C3 toxin, but not a Rho kinase inhibitor or a dominant negative form of Rho kinase, abolished S1P inhibition of PDGF-induced Rac activation and cell migration. The angiotensin II receptor AT(1), which robustly couples to G(q), did not mediate either Rho activation or inhibition of PDGF-induced Rac activation or migration, suggesting that activation of G(q) alone was not sufficient for Rho activation and resultant Rac inhibition. However, the AT(1) receptor fused to Galpha(12) was able to induce not only Rho stimulation but also inhibition of PDGF-induced Rac activation and migration. Phospholipase C inhibition did not affect S1P-induced Rho activation, and protein kinase C activation by a phorbol ester did not mimic S1P action, suggesting that S1P inhibition of migration or Rac was not dependent on the phospholipase C pathway.

CONCLUSION

These observations together suggest that S1P(2) mediates inhibition of Rac and migration through the coordinated action of G(12/13) and G(q) for Rho activation in VSMCs.

Sphingosine-1-phosphate signaling and biological activities in the cardiovascular system

The plasma lysophospholipid mediator sphingosine-1-phosphate (S1P) is produced exclusively by sphingosine kinase (SPHK) 1 and SPHK2 in vivo, and plays diverse biological and pathophysiological roles by acting largely through three members of the G protein-coupled S1P receptors, S1P1, S1P2 and S1P3. S1P1 expressed on endothelial cells mediates embryonic vascular maturation and maintains vascular integrity by contributing to eNOS activation, inhibiting vascular permeability and inducing endothelial cell chemotaxis via Gi-coupled mechanisms. By contrast, S1P2, is expressed in high levels on vascular smooth muscle cells (VSMCs) and certain types of tumor cells, inhibiting Rac and cell migration via a G(12/13)-and Rho-dependent mechanism. In rat neointimal VSMCs, S1P1 is upregulated to mediate local production of platelet-derived growth factor, which is a key player in vascular remodeling. S1P3 expressed on endothelial cells also mediates chemotaxis toward S1P and vasorelaxation via NO production in certain vascular bed, playing protective roles for vascular integrity. S1P3 expressed on VSMCs and cardiac sinoatrial node cells mediates vasopressor and negative chronotropic effect, respectively. In addition, S1P3, together with S1P2 and SPHK1, is suggested to play a protective role against acute myocardial ischemia. However, our recent work indicates that overexpressed SPHK1 is involved in cardiomyocyte degeneration and fibrosis in vivo, in part through S1P activation of the S1P3 signaling. We also demonstrated that exogenously administered S1P accelerates neovascularization and blood flow recovery in ischemic limbs, suggesting its usefulness for angiogenic therapy. These results provide evidence for S1P receptor subtype-specific pharmacological intervention as a novel therapeutic approach to cardiovascular diseases and cancer.

Sphingosine 1-phosphate induces platelet/endothelial cell adhesion molecule-1 phosphorylation in human endothelial cells through cSrc and Fyn

Sphingosine 1-phosphate (S1P) is a multifunctional phospholipid which acts through a specific family of G protein-coupled receptors. Platelet/endothelial cell adhesion molecule-1 (PECAM-1) form trans-homophilic binding at lateral cell border. Upon stimulation, its cytoplasmic tyrosine residues could be phosphorylated and interact with various downstream signaling molecules. In this study, we demonstrated that S1P induced PECAM-1 tyrosine phosphorylation in human umbilical cord vein cells (HUVECs). By pharmacological inhibitors, it was suggested that G(i) and Src family kinases were involved in PECAM-1 phosphorylation. Moreover, cSrc and Fyn siRNA significantly suppressed S1P-induced PECAM-1 phosphorylation. These results suggested that S1P-induced PECAM-1 phosphorylation through G(i) and subsequent cSrc and Fyn. Our findings provide further understanding of S1P and PECAM-1 signaling as well as their functions in endothelial cells.

Vascular endothelium as a contributor of plasma sphingosine 1-phosphate

Sphingosine 1-phosphate (S1P), an abundant lipid mediator in plasma, regulates vascular and immune cells by activating S1P receptors. In this report, we investigated the mechanisms by which high plasma S1P levels are maintained in mice. We found that plasma S1P turns over rapidly with a half-life of approximately 15 minutes, suggesting the existence of a high-capacity biosynthetic source(s). Transplantation of bone marrow from wild-type to Sphk1(-/-)Sphk2(+/-) mice restored plasma S1P levels, suggesting that hematopoietic cells are capable of secreting S1P into plasma. However, plasma S1P levels were not appreciably altered in mice that were thrombocytopenic, anemic, or leukopenic. Surprisingly, reconstitution of Sphk1(-/-)Sphk2(+/-) bone marrow cells into wild-type hosts failed to reduce plasma S1P, suggesting the existence of an additional, nonhematopoietic source for plasma S1P. Adenoviral expression of Sphk1 in the liver of Sphk1(-/-) mice restored plasma S1P levels. In vitro, vascular endothelial cells, but not hepatocytes, secreted S1P in a constitutive manner. Interestingly, laminar shear stress downregulated the expression of S1P lyase (Sgpl) and S1P phosphatase-1 (Sgpp1) while concomitantly stimulating S1P release from endothelial cells in vitro. Modulation of expression of endothelial S1P lyase with small interfering RNA and adenoviral expression altered S1P secretion, suggesting an important role played by this enzyme. These data suggest that the vascular endothelium, in addition to the hematopoietic system, is a major contributor of plasma S1P.

Thymic microenvironments for T-cell repertoire formation

Functionally competent immune system includes a functionally competent T-cell repertoire that is reactive to foreign antigens but is tolerant to self-antigens. The repertoire of T cells is primarily formed in the thymus through positive and negative selection of developing thymocytes. Immature thymocytes that undergo V(D)J recombination of T-cell antigen receptor (TCR) genes and that express the virgin repertoire of TCRs are generated in thymic cortex. The recent discovery of thymoproteasomes, a molecular complex specifically expressed in cortical thymic epithelial cells (cTEC), has revealed a unique role of cTEC in cuing the further development of immature thymocytes in thymic cortex, possibly by displaying unique self-peptides that induce positive selection. Cortical thymocytes that receive TCR-mediated positive selection signals are destined to survive for further differentiation and are induced to express CCR7, a chemokine receptor. Being attracted to CCR7 ligands expressed by medullary thymic epithelial cells (mTEC), CCR7-expressing positively selected thymocytes relocate to thymic medulla. The medullary microenvironment displays another set of unique self-peptides for trimming positively selected T-cell repertoire to establish self-tolerance, via promiscuous expression of tissue-specific antigens by mTEC and efficient antigen presentation by dendritic cells. Recent results demonstrate that tumor necrosis factor (TNF) superfamily ligands, including receptor activating NF-kappaB ligand (RANKL), CD40L, and lymphotoxin, are produced by positively selected thymocytes and pivotally regulate mTEC development and thymic medulla formation.

HDL serves as a S1P signaling platform mediating a multitude of cardiovascular effects

The lysosphingolipid sphingosine 1-phosphate (S1P) is a component of HDL. Findings from a growing number of studies indicate that S1P is a mediator of many of the cardiovascular effects of HDL, including the ability to promote vasodilation, vasoconstriction, and angiogenesis, protect against ischemia/reperfusion injury, and inhibit/reverse atherosclerosis. These latter cardioprotective effects are being shown to involve the S1P-mediated suppression of inflammatory processes, including reduction of the endothelial expression of monocyte and lymphocyte adhesion molecules, decreased recruitment of polymorphonuclear cells to sites of infarction, and blocking of cardiomyocyte apoptosis after myocardial infarction. This review article summarizes the evidence that S1P as a component of HDL serves to regulate vascular cell and lymphocyte behaviors associated with cardiovascular (patho)physiology.

The potential roles of sphingolipids in vascular smooth-muscle function

Increasing experimental evidence has demonstrated that sphingolipids are likely to have an important regulatory function in the cardiovascular system. Two sphingolipids released from activated platelets, and therefore of particular relevance, are S1P (sphingosine 1-phosphate) and SPC (sphingosylphosphocholine). Both S1P and SPC can act as vasoconstrictors and may modulate VSMC (vascular smooth muscle cell) phenotype, as observed during the pathogenesis of vascular disease. Recent research has suggested that SPC may act as a pro-inflammatory mediator in VSMCs and, in some circumstances, may also contribute to the development of vascular disease.

Plasma sphingosine 1-phosphate metabolism and analysis

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

Autocrine and paracrine roles of sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid metabolite that has been implicated in many biological processes, including cell migration, survival, proliferation, angiogenesis and immune and allergic responses. S1P levels inside cells are regulated tightly by the balance between its synthesis by sphingosine kinases and degradation by S1P lyases and S1P phosphatases. Activation of sphingosine kinase by any of a variety of agonists increases S1P levels, which in turn can function intracellularly as a second messenger or in an autocrine and/or paracrine fashion to activate and signal through S1P receptors present on the surface of the cell. This review summarizes recent findings on the roles of S1P as a mediator of the actions of cytokines, growth factors and hormones.

Stem cell regulation by lysophospholipids

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) regulate a diverse range of mammalian cell processes, largely through engaging multiple G protein-coupled receptors specific for these lysophospholipids. LPA and S1P have been clearly identified to have widespread physiological and pathophysiological actions, controlling events within the reproductive, gastrointestinal, vascular, nervous and immune systems, and also having a prominent role in cancer. Here we review the recent literature showing the additional emerging role for LPA and S1P in the regulation of stem cells and their progenitors. We discuss the role of these lysophospholipids in regulating the proliferation, survival, differentiation and migration of a range of adult and embryonic stem cells and progenitors, and thus are likely to play a substantial role in the maintenance, generation, mobilisation and homing of stem cell and progenitor populations in the body.

S1P1 receptor expression regulates emergence of NKT cells in peripheral tissues

The S1P1 receptor, on the surface of lymphocytes and endothelial cells, regulates the unique trafficking behavior of certain lymphocyte populations. We have examined whether the S1P1 receptor also dictates the distinctive tissue distribution of V alpha14-J alpha18 natural killer T (NKT) cells, whose trafficking pattern is not well understood. Mice (TCS1P1 KO) were established with a conditional deletion of the S1P1 receptor in thymocytes that included precursors of NKT cells. Within the thymus, NKT cells were found at normal or increased levels, indicating that S1P1 receptor expression was dispensable for NKT cell development. However, substantially reduced numbers of NKT cells were detected in the peripheral tissues of the TCS1P1 KO mice. Short-term S1P1 deletion after NKT cells had established residence in the periphery did not substantially alter their distribution in tissues, except for a partial decrease in the spleen. FTY720, a S1P1 receptor ligand that has potent effects on the trafficking of conventional T cells, did not alter the preexisting distribution of NKT cells within peripheral tissues of wild-type mice. Our results indicate that the S1P1 receptor expression on NKT cells is dispensable for development within thymus but is essential for the establishment of their tissue residency in the periphery.

Sphingosine-1-phosphate in allergic responses, asthma and anaphylaxis

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid metabolite involved in many cellular processes, acting not only as an extracellular ligand to its specific G protein-coupled receptors, but also as a putative intracellular messenger with yet unidentified targets. Mast cells are tissue-dwelling pivotal early effectors of allergic responses, which produce and secrete S1P that can bind to its receptors present on mast cells to influence their activation and functions. In this review, we will first discuss the current knowledge of S1P production by two isozymes of sphingosine kinase (SphK). Mechanisms of SphK activation will be discussed, with an emphasis on experimental approaches developed to study their differential activation and biological roles in the context of mast cells. The relevance of mast cells in the etiology of allergic disorders, asthma and anaphylaxis is well established. In this review, this concept will be revisited, focusing on the contribution of S1P production and secretion to the symptoms associated with dysregulated inflammatory responses. To conclude, counteracting the proinflammatory effects of S1P could be envisioned as a therapeutic strategy to treat allergic disorders, exacerbated airway inflammation, and anaphylactic reactions, and various options will be discussed, such as the development of pharmacological tools to inhibit SphKs, S1P neutralizing monoclonal antibody, and S1P receptor antagonists.

The suppressive effect of sphingosine 1-phosphate on monocyte-endothelium adhesion may be mediated by the rearrangement of the endothelial integrins alpha(5)beta(1) and alpha(v)beta(3)

BACKGROUND

Sphingosine 1-phosphate (S1P), known to play important roles in vascular biology, is a bioactive lysophospholipid mediator that maintains endothelial integrity via its cell-surface receptors (S1Ps). In this in vitro study, we aimed to examine the role of S1P in monocyte-endothelium adhesion, which is an important event in the pathophysiology of atherosclerosis.

METHODS AND RESULTS

S1P pretreatment of human umbilical vein endothelial cells (ECs), but not U937 cells, effectively suppressed U937-EC adhesion independently from the expression of adhesion molecules, namely ICAM-1, VCAM-1, and E-selectin. This S1P-induced suppressive effect was inhibited by the blockage of S1P(1) and S1P(3) receptors and the specific inhibitors of G(i) protein, Src family proteins, phosphatidylinositol 3-kinase, and Rac1, indicating involvement of these key downstream pathways. Moreover, the RGD peptide and antibodies, which neutralize adhesion via alpha(5)beta(1) and alpha(v)beta(3), effectively inhibited U937-EC adhesion with a degree similar to S1P pretreatment. Both an adhesion assay and flow-cytometric analysis demonstrated that U937 cells adhered through integrins alpha(5)beta(1) and alpha(v)beta(3) expressed on the apical surface of monolayer ECs, and S1P shifted the localization of these integrins from the apical surface to the basal surface.

CONCLUSIONS

From the present results, we propose that S1P may contribute to the maintenance of vascular integrity and the regulation of atherogenesis through the rearrangement of endothelial integrins.

Vascular effects of sphingolipids

The sphingomyelin metabolites ceramide, sphingosine 1-phosphate (S1P) and sphingosylphosphorylcholine (SPC) are emerging modulators of vascular tone. While ceramide appears to act primarily intracellularly, S1P and SPC appear to mainly work via specific receptors, although those for SPC have not yet been defined unequivocally. Each of the sphingomyelin metabolites can induce both vasoconstriction and vasodilatation and, in some cases--ceramide on the one hand, and S1P and SPC on the other hand--have opposite effects on vascular tone. The differences in effects between vessels may relate to the relative roles of endothelial and smooth muscle cells in mediating them, as well as to the distinct expression patterns of S1P receptors among vascular beds and among endothelial and smooth muscle cells. Recent evidence suggests that vascular tone is not only modulated by sphingomyelin metabolites which are exogenously added or reach the vessel wall via the bloodstream but also by those formed locally by cells in the vessel wall. Such local formation can be induced by known vasoactive agents such as angiotensin II and may serve a signalling function.

CONCLUSION

We conclude that sphingomyelin metabolites are important endogenous modulators of vascular function, which may contribute to the pathophysiology of some diseases and be targets for therapeutic interventions.

Promotion of lymphocyte egress into blood and lymph by distinct sources of sphingosine-1-phosphate

Lymphocytes require sphingosine-1-phosphate (S1P) receptor-1 to exit lymphoid organs, but the source(s) of extracellular S1P and whether S1P directly promotes egress are unknown. By using mice in which the two kinases that generate S1P were conditionally ablated, we find that plasma S1P is mainly hematopoietic in origin, with erythrocytes a major contributor, whereas lymph S1P is from a distinct radiation-resistant source. Lymphocyte egress from thymus and secondary lymphoid organs was markedly reduced in kinase-deficient mice. Restoration of S1P to plasma rescued egress to blood but not lymph, and the rescue required lymphocyte expression of S1P-receptor-1. Thus, separate sources provide S1P to plasma and lymph to help lymphocytes exit the low-S1P environment of lymphoid organs. Disruption of compartmentalized S1P signaling is a plausible mechanism by which S1P-receptor-1 agonists function as immunosuppressives.

Sphingosine 1-phosphate protects rat liver sinusoidal endothelial cells from ethanol-induced apoptosis: Role of intracellular calcium and nitric oxide

In alcoholic liver disease, ethanol-induced damage to sinusoidal endothelial cells (SECs) appears to be important in the progression of liver damage. However, little is known about the mechanisms responsible for protection of SECs against ethanol-induced injury. To elucidate the role of sphingosine 1-phosphate (S1P), which is stored in platelets and may be released from them on their activation, we investigated the effect of S1P on rat liver SECs in primary culture. Pretreatment of cells with 1 mumol/L S1P attenuated ethanol-induced apoptosis. Electron microscopy confirmed this protective effect of S1P on damaged SECs in liver tissues after perfusion of ethanol. In the absence of ethanol, S1P increased DNA synthesis as determined via incorporation of bromodeoxyuridine. S1P also ameliorated the decreased DNA synthesis of cells induced by ethanol. Addition of S1P to cells induced an increase in intracellular calcium concentrations and NO production in cells. Western blotting revealed that S1P significantly induced the activation of endothelial NO synthase (eNOS), but not Akt, and that S1P-induced activation of eNOS was blocked by trifluoperazine, a calmodulin inhibitor. Furthermore, N(G)-nitro-L-arginine methyl ester, a NO synthase inhibitor, cancelled the effect of S1P on DNA synthesis, apoptosis, and NO production in vitro as well as the protective effect of S1P on cell damage in situ. In conclusion, the biological effect of S1P is at least partially mediated by Ca(2+)-sensitive eNOS activation and subsequent NO formation; extracellular S1P could contribute to sinusoidal protection and remodeling in alcoholic liver injury.

LC-MS/MS-analysis of sphingosine-1-phosphate and related compounds in plasma samples

Sphingosine-1-phosphate (S1P) and related compounds are important signaling molecules and are normal constituents of human plasma. So far, only a few methods exist for their determination specifically in plasma demanding radioactive agents, more or less time consuming extraction or derivatization procedures. Here, we describe a very simple, reliable, sensitive standard-addition method for the simultaneous determination of S1P, sphingosine (SPH), sphinganine (SAPH) and sphinganine-1-phosphate (SA1P) in human and rat plasma samples. After methanol precipitation of plasma samples the supernatants were directly assessed by liquid chromatography-electrospray ionisation-tandem mass spectrometry (LC-ESI-MS/MS). HPLC analysis was done under gradient conditions using a C18 reversed phase column. The lower limit of quantification (LLOQ) was <10.2, <4.6, <1.9 and 0.57ng/ml for S1P, SPH, SAPH and SA1P, respectively. Variations in accuracy and intraday and interday precision were <15% over the range of calibration. All analytes were normal constituents both in human and rat plasma although the SA1P concentrations in a few rat plasma samples were below the lower limit of quantification. This validated method is suitable to generate new pharmacological findings by monitoring plasma concentrations of S1P and related compounds especially when low amounts of plasma samples are present (e.g. plasma samples from rodents).

Essential roles of sphingosine 1-phosphate/S1P1 receptor axis in the migration of neural stem cells toward a site of spinal cord injury

Neural stem/progenitor cells (NSPCs) migrate toward a damaged area of the central nervous system (CNS) for the purpose of limiting and/or repairing the damage. Although this migratory property of NSPCs could theoretically be exploited for cell-based therapeutics of CNS diseases, little is known of the mechanisms responsible for migratory responses of NSPCs. Here, we found that sphingosine 1-phosphate (Sph-1-P), a physiological lysophospholipid mediator, had a potent chemoattractant activity for NSPCs, in which, of Sph-1-P receptors, S1P(1) was abundantly expressed. Sph-1-P-induced NSPC migration was inhibited by the pretreatment with pertussis toxin, Y-27632 (a Rho kinase inhibitor), and VPC23019 (a competitive inhibitor of S1P(1) and S1P(3)). Sph-1-P does not act as intracellular mediator or in an autocrine manner, because [(3)H]sphingosine, incorporated into NSPCs, was mainly converted to ceramide and sphingomyeline intracellularly, and the stimulation-dependent formation and extracellular release of Sph-1-P were not observed. Further, Sph-1-P concentration in the spinal cord was significantly increased at 7 days after a contusion injury, due to accumulation of microglia and reactive astrocytes in the injured area. This locally increased Sph-1-P concentration contributed to the migration of in vivo transplanted NSPCs through its receptor S1P(1), given that lentiviral transduction of NSPCs with a short hairpin RNA interference for S1P(1) abolished in vivo NSPC migration toward the injured area. This is the first report to identify a physiological role for a lipid mediator in NSPC migration toward a pathological area of the CNS and further indicates that the Sph-1-P/S1P(1) pathway may have therapeutic potential for CNS injuries.

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.

Extracellular export of sphingosine kinase-1a contributes to the vascular S1P gradient

Sphingosine 1-phosphate (S1P), produced by Sphks (sphingosine kinases), is a multifunctional lipid mediator that regulates immune cell trafficking and vascular development. Mammals maintain a large concentration gradient of S1P between vascular and extravascular compartments. Mechanisms by which S1P is released from cells and concentrated in the plasma are poorly understood. We recently demonstrated [Ancellin, Colmont, Su, Li, Mittereder, Chae, Stefansson, Liau and Hla (2002) J. Biol. Chem. 277, 6667-6675] that Sphk1 activity is constitutively secreted by vascular endothelial cells. In the present study, we show that among the five Sphk isoforms expressed in endothelial cells, the Sphk-1a isoform is selectively secreted in HEK-293 cells (human embryonic kidney cells) and human umbilical-vein endothelial cells. In sharp contrast, Sphk2 is not secreted. The exported Sphk-1a isoform is enzymatically active and produced sufficient S1P to induce S1P receptor internalization. Wild-type mouse plasma contains significant Sphk activity (179 pmol x min(-1) x g(-1)). In contrast, Sphk1-/- mouse plasma has undetectable Sphk activity and approx. 65% reduction in S1P levels. Moreover, human plasma contains enzymatically active Sphk1 (46 pmol x min(-1) x g(-1)). These results suggest that export of Sphk-1a occurs under physiological conditions and may contribute to the establishment of the vascular S1P gradient.

Negative regulation of endothelial morphogenesis and angiogenesis by S1P2 receptor

We speculated that the sphingosine-1-phosphate (S1P) receptor S1P(2), which uniquely inhibits cell migration, might mediate inhibitory effects on endothelial cell migration and angiogenesis, different from S1P(1) and S1P(3). Mouse vascular endothelial cells, which endogenously express S1P(2) and S1P(3), but not S1P(1), responded to S1P and epidermal growth factor (EGF) with stimulation of Rac, migration, and the formation of tube-like structures on the Matrigel. The S1P(3)-antagonist VPC-23019 abolished S1P-induced, G(i)-dependent Rac stimulation, cell migration, and tube formation, whereas the S1P(2)-antagonist JTE-013 enhanced these S1P-induced responses, suggesting that S1P(2) exerts inhibitory effects on endothelial Rac, migration, and angiogenesis. S1P(2) overexpression markedly augmented S1P-induced, G(i)-independent inhibition of EGF-induced migration and tube formation. Finally, the blockade of S1P(2) by JTE-013 potentiated S1P-induced stimulation of angiogenesis in vivo in the Matrigel implant assay. These observations indicate that in contrast to S1P(1) and S1P(3), S1P(2) negatively regulates endothelial morphogenesis and angiogenesis most likely through down-regulating Rac.

Essential requirement for sphingosine kinase activity in eNOS-dependent NO release and vasorelaxation

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that acts both as an extracellular ligand for endothelial differentiation gene receptor family and as an intracellular second messenger. Cellular levels of S1P are low and tightly regulated by sphingosine kinase (SPK). Recent studies have suggested that eNOS pathway may function as a downstream target for the biological effects of receptor-mediated S1P. Here we have studied the possible interplay between intracellular SIP generation and the eNOS activation pathway. S1P causes an endothelium-dependent vasorelaxation in rat aorta that is PTX sensitive, inhibited by L-NAME that involves eNOS phosphorylation, and mainly dependent on hsp90. When rat aorta rings were incubated with the SPK inhibitor DL-threo-dihydrosphingosine (DTD), there was a concentration-dependent reduction of Ach-induced vasorelaxation, implying a consistent contribution of sphingolipid pathway through intracellular sphingosine release and phosphorylation. Co-immunoprecipitation experiments consistently showed increased association of hsp90 with eNOS after exposure of cells to S1P as well to BK or calcium ionophore A-23187. Interestingly, as opposite to A-23187, BK and S1P effect were significantly inhibited by pretreatment with the SPK inhibitor DTD. In conclusion, our data demonstrate that an interplay exists among eNOS, hsp90, and intracellularly generated S1P where eNOS coupling to hsp90 is a major determinant for NO release as confirmed by our functional and molecular studies.

Biological effects of lysophospholipids

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are potent biologically active lipid mediators that exert a wide range of cellular effects through specific G protein-coupled receptors. To date, four LPA receptors and five S1P receptors have been identified. These receptors are expressed in a large number of tissues and cell types, allowing for a wide variety of cellular responses to lysophospholipid signaling, including cell adhesion, cell motility, cytoskeletal changes, proliferation, angiogenesis, process retraction, and cell survival. In addition, recent studies in mice show that specific lysophospholipid receptors are required for proper cardiovascular, immune, respiratory, and reproductive system development and function. Lysophospholipid receptors may also have specific roles in cancer and other diseases. This review will cover identification and expression of the lysophospholipid receptors, as well as receptor signaling properties and function. Additionally, phenotypes of mice deficient for specific lysophospholipid receptors will be discussed to demonstrate how these animals have furthered our understanding of the role lysophospholipids play in normal biology and disease.

Sphingosine-1-Phosphate Acts via Rho-Associated Kinase and Nitric Oxide to Regulate Human Placental Vascular Tone1

Sphingosine-1-phosphate (S1P), a bioactive lipid released from activated platelets, has been demonstrated in animal models to regulate vascular tone through receptor-mediated activation of Rho-associated kinase 1 and nitric oxide synthase 3. The role of S1P in regulation of human vascular tone (particularly during pregnancy, with its unique vascular adaptations and localized platelet activation) is unknown. We hypothesized that S1P would constrict small placental arteries through activation of Rho-associated kinases with modulation by nitric oxide. Reverse transcription-polymerase chain reaction of chorionic plate artery preparations detected mRNAs encoding all five receptors for S1P, and S1P induced dose-dependent vasoconstriction of both chorionic plate and stem villous isobarically mounted arteries, which at 10 micromol/L was 32.9% +/- 3.86% (mean +/- SEM) and 34.6% +/- 7.01%, respectively. In stem villous arteries, S1P-induced vasoconstriction was enhanced significantly following inhibition of nitric oxide synthases with N(G)-nitro-L-arginine methyl ester (100 micromol/L, 52.6% +/- 6.28%, P < 0.05). The S1P-induced vasoconstriction was reversed by Y27632, an inhibitor of Rho-associated kinases (10 micromol/L) in both chorionic plate (to 14.9% +/- 4.95%) and stem villous arteries (to 2.71% +/- 6.13%). The S1P added to alpha-toxin-permeabilized, isometrically mounted chorionic plate arteries bathed in submaximal Ca(2+)-activating solution induced Ca(2+)-sensitization of constriction, which was 47.7% +/- 10.0% of that occurring to maximal Ca(2+)-activating solution. This was reduced by Y27632 to 18.4% +/- 18.4%. Interestingly, S1P-induced vasoconstriction occurred in all isobarically mounted arteries but was inconsistent in isometrically mounted chorionic plate arteries. In summary, S1P-induced vasoconstriction in human placental arteries is mediated by increased Ca(2+)-sensitization through activation of Rho-associated kinases, and this vasoconstriction also is modulated by nitric oxide. Identification of these actions of S1P in the placental vasculature is important for understanding both normal and potentially abnormal vascular adaptations with pregnancy.