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.     

Sphingosine 1-phosphate receptors

Sphingosine-1-phosphate (SPP) is a bioactive lipid produced from the metabolism of sphingomyelin. It is an important constituent of serum and regulates cell growth, survival, migration, differentiation and gene expression. Its mode of action has been enigmatic; however, recent findings have shown that a family of G-protein-coupled receptors (GPCR) of the endothelial differentiation gene (EDG) family serve as plasma membrane-localized receptors for SPP. Furthermore, the EDG receptors appear to be SPP receptor subtypes with distinct signaling characteristics. In vascular endothelial cells, SPP acts on EDG-1 and EDG-3 subtypes of receptors to induce cell survival and morphogenesis. Such pathways appear to be critical for SPP-induced angiogenic response in vivo. In addition, the EDG-1 gene is essential for vascular maturation in development. Moreover, developmental studies in Zebrafish have indicated that SPP signaling via the EDG-5 like receptor Miles Apart (Mil) is essential for heart development. These data strongly suggest that a physiological role of SPP is in the formation of the cardiovascular system. Despite these recent findings, much needs to be clarified with respect to the physiological role of SPP synthesis and action. This review will focus on the recent findings on SPP receptors and the effects on the cardiovascular system.     

Sphingosine 1-phosphate receptors

Sphingosine-1-phosphate (SPP) is a bioactive lipid produced from the metabolism of sphingomyelin. It is an important constituent of serum and regulates cell growth, survival, migration, differentiation and gene expression. Its mode of action has been enigmatic; however, recent findings have shown that a family of G-protein-coupled receptors (GPCR) of the endothelial differentiation gene (EDG) family serve as plasma membrane-localized receptors for SPP. Furthermore, the EDG receptors appear to be SPP receptor subtypes with distinct signaling characteristics. In vascular endothelial cells, SPP acts on EDG-1 and EDG-3 subtypes of receptors to induce cell survival and morphogenesis. Such pathways appear to be critical for SPP-induced angiogenic response in vivo. In addition, the EDG-1 gene is essential for vascular maturation in development. Moreover, developmental studies in Zebrafish have indicated that SPP signaling via the EDG-5 like receptor Miles Apart (Mil) is essential for heart development. These data strongly suggest that a physiological role of SPP is in the formation of the cardiovascular system. Despite these recent findings, much needs to be clarified with respect to the physiological role of SPP synthesis and action. This review will focus on the recent findings on SPP receptors and the effects on the cardiovascular system.     

Sphingosine 1-phosphate, lysophosphatidic acid and growth factor signaling and termination

Sphingosine 1 phosphate (S1P) and lysophosphatidic acid (LPA) are bioactive lipid phosphates that bind to cell surface G-protein coupled receptors (GPCR) and, in addition, exhibit intracellular actions. We have summarised herein, an important functional interaction between lipid phosphate GPCR and receptor tyrosine kinases (RTK) that enables growth factors to spatially regulate effectors, thereby governing the nature of the biological response. For instance, we describe how the formation of functional complexes between the S1P(1) receptor and PDGFbeta receptor may effectively re-programme platelet-derived growth factor from a mitogenic to a migratory stimulus. This is achieved by integration of RTK- and GPCR-specific signals that results in spatial regulation of a cytoplasmic retained pool of extracellular signal regulated kinase-1/2 linked to myosin light chain kinase, myosin light chain phosphorylation and migration. We therefore suggest that the lipid phosphate receptor is a major determinant in regulating growth factor-dependent biology. Growth factors can also increase S1P inside cells, and we discuss the concept of spatial/temporal aspects of compartmentalised intracellular signaling of S1P in relation to defined interactions between, for instance, sphingosine kinase, phospholipase D1 and lipid phosphate phosphatases and regulation of cell survival.     

Sphingosine 1-phosphate chemical biology

A dozen years ago, the term 'S1P' (sphingosine 1-phosphate) was not in the lexicons of scientific literature databases. By early 2008, this query term retrieved well over 1000 citations from PubMed - about 225 of these appeared in 2007. Indeed, S1P is arguably the most heavily studied lipid molecule at present. What happened to distinguish S1P among many other signaling lipids? We believe that the seminal event was the linking of the investigational drug, FTY720 (fingolimod), to S1P signaling. This realization profoundly altered understanding of S1P biology, revealing both that S1P is prominent in lymphocyte trafficking and that mimicking S1P signaling with an agonist drug can modulate the immune system to considerable therapeutic benefit. Neither fact was known prior to FTY720; indeed, this molecule is testament to the power of chemical biology. In this communication, we attempt to summarize progress to date in S1P chemical biology.     

Sphingosine 1-phosphate signaling: providing cells with a sense of direction

Sphingosine 1-phosphate (S1P) is a sphingolipid metabolite that regulates diverse biological functions. S1P has been identified as a high-affinity ligand for a family of five G-protein-coupled receptors, known as the S1P receptors. The physiological role of the S1P receptor S1P(1) in vascular maturation was recently revealed by gene disruption in mice. In addition to other cellular processes, the binding of S1P to its receptors regulates motility and directional migration of a variety of cell types, including endothelial cells and vascular smooth muscle cells. This review focuses on the important role of S1P and its receptors in cell migration and describes a new paradigm for receptor cross-communication in which transactivation of S1P(1) by a receptor tyrosine kinase (PDGFR) is crucial for cell motility.     

Sphingosine 1-phosphate signalling in cancer

There is an increasing body of evidence demonstrating a critical role for the bioactive lipid S1P (sphingosine 1-phosphate) in cancer. S1P is synthesized and metabolized by a number of enzymes, including sphingosine kinase, S1P lyase and S1P phosphatases. S1P binds to cell-surface G-protein-coupled receptors (S1P1-S1P5) to elicit cell responses and can also regulate, by direct binding, a number of intracellular targets such as HDAC (histone deacetylase) 1/2 to induce epigenetic regulation. S1P is involved in cancer progression including cell transformation/oncogenesis, cell survival/apoptosis, cell migration/metastasis and tumour microenvironment neovascularization. In the present paper, we describe our research findings regarding the correlation of sphingosine kinase 1 and S1P receptor expression in tumours with clinical outcome and we define some of the molecular mechanisms underlying the involvement of sphingosine kinase 1 and S1P receptors in the formation of a cancer cell migratory phenotype. The role of sphingosine kinase 1 in the acquisition of chemotherapeutic resistance and the interaction of S1P receptors with oncogenes such as HER2 is also reviewed. We also discuss novel aspects of the use of small-molecule inhibitors of sphingosine kinase 1 in terms of allosterism, ubiquitin-proteasomal degradation of sphingosine kinase 1 and anticancer activity. Finally, we describe how S1P receptor-modulating agents abrogate S1P receptor-receptor tyrosine kinase interactions, with potential to inhibit growth-factor-dependent cancer progression.     

Sphingosine 1-phosphate: synthesis and release

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

Sphingosine 1-phosphate breakdown in platelets

We examined the formation of sphingolipid mediators in platelets, which abundantly store, and release extracellularly, sphingosine 1-phosphate (Sph-1-P). Challenging [(3)H]Sph-labeled platelet suspensions with thrombin or 12-O-tetradecanoylphorbol 13-acetate (TPA) resulted in a decrease in Sph-1-P formation and an increase in sphingosine (Sph), ceramide (Cer), and sphingomyelin formation. Sph conversion into Cer, and Cer conversion into sphingomyelin were not affected upon activation, suggesting that Sph-1-P dephosphorylation may initiate the formation of sphingolipid signaling molecules. In fact, Sph-1-P phosphatase (but not lyase) activity was detected in platelets, but this activity was not enhanced by thrombin or TPA. When quantified with [(3)H]acetic anhydride acetylation, followed by HPLC separation, the amounts of Sph-1-P and Sph decreased and increased, respectively, upon stimulation with thrombin or TPA, and these changes were attenuated by staurosporine. Under these TPA treatment conditions, over half of the [(3)H]Sph-1-P (formed in platelets incubated with [(3)H]Sph) was detected extracellularly, possibly due to its release from platelets, which was completely inhibited by staurosporine pretreatment. Furthermore, when TPA-induced Sph-1-P release was blocked by staurosporine after the stimulation, the extracellular [(3)H]Sph-1-P radioactivity decreased, suggesting that the Sph-1-P released may undergo dephosphorylation extracellularly. To support this, [(32)P]Sph-1-P, when added extracellularly to platelet suspensions, was rapidly degraded, possibly due to the ecto-phosphatase activity. Our results suggest the presence in anucleate platelets of a transmembrane cycling pathway starting with Sph-1-P dephosphorylation and leading to the formation of other sphingolipid mediators.     

Sphingosine 1-phosphate: synthesis and release

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

Modulators of the Sphingosine 1-phosphate receptor 1

The Sphingosine 1-phosphate receptor (S1P-R) signaling system has proven to be of biological and medical importance in autoimmune settings. S1P₁-R is a validated drug target for multiple sclerosis (MS) for which FTY720 (Fingolimod), a S1P_1,3–5-R pan-agonist, was recently approved as the first orally active drug for the treatment of relapsing-remitting MS. Transient bradycardia and long half-life are the FTY720 critical pitfalls. This review provides the latest advances on next-generation S1P₁-R modulators from 2012 up to date, with an overview of the chemical structures, structure–activity relationships, and relevant biological and clinical properties.     

Sphingosine kinase and sphingosine 1-phosphate in asthma

Sphingolipids are amphiphatic molecules ubiquitously expressed in all eukaryotic cell membranes. Initially characterized as structural components of cell membranes, sphingolipids have emerged as sources of important signalling molecules over the past decade. Sphingolipid metabolites, such as ceramide and S1P (sphingosine 1-phosphate), have been demonstrated to have roles as potent bioactive messengers involved in cell differentiation, proliferation, apoptosis, migration and angiogenesis. The importance of SphK (sphingosine kinase) and S1P in inflammation has been demonstrated extensively. The prevalence of asthma is increasing in many developed nations. Consequently, there is an urgent need for the development of new agents for the treatment of asthma, especially for patients who respond poorly to conventional therapy. Recent studies have demonstrated the important role of SphK and S1P in the development of asthma by regulating pro-inflammatory responses. These novel pathways represent exciting potential therapeutic targets in the treatment of asthma and are described in the present review.     

Sphingosine 1-phosphate and control of vascular tone

Sphingosine-1-phosphate (S1P) is a platelet-derived lipid mediator that activates the endothelial isoform of nitric oxide synthase (eNOS) in endothelial cells. However, the role of S1P in endothelium-dependent vasodilation and the signaling pathways elicited by S1P in intact vessels are largely unknown. We found that S1P induces dose-dependent transient relaxation of isolated pressurized mesenteric arterioles (EC(50) 10 +/- 3 nM); maximal vasodilation (55 +/- 8%) is seen approximately 2 min after S1P addition and returns to baseline by 5 min. S1P promotes comparable responses in arterioles from wild-type but not eNOS(null) mice. S1P-induced vasodilation is abrogated by removal of endothelium or by the addition of the NOS inhibitor N(omega)-monomethyl-l-arginine but is not affected by the cyclooxygenase inhibitor indomethacin, nor by the blockade of K(+) channels by using 4-aminopyridine. S1P-induced vasodilation is attenuated by pertussis toxin, by the phosphoinositide 3-kinase (PI3-kinase) inhibitor wortmannin, and by the calcium chelator BAPTA. With the use of high-sensitivity protein immunoblots in extracts from single pressurized vessels, we found that S1P, but not bradykinin, promotes the phosphorylation of eNOS at Ser(1179). Maximum S1P-induced eNOS Ser(1179) phosphorylation was reached at the time of maximum vasorelaxation, but enzyme phosphorylation persisted for several minutes after vasodilation had resolved. Thus regulatory pathways distinct from eNOS Ser(1179) dephosphorylation serve to terminate agonist-promoted vasorelaxation. Taken together, our findings demonstrate that S1P, an important intercellular mediator of platelet-vessel wall interactions, is a effective arteriolar vasodilator that acts via G protein-dependent, calcium-sensitive, and PI3-kinase-modulated signaling pathways.     

Sphingosine kinases, sphingosine-1-phosphate and sphingolipidomics

It has become abundantly clear over the past decade that sphingolipids and their metabolites are key signaling molecules. Ceramide, the backbone of all sphingolipids, predominantly inhibits cell growth and induces apoptosis, while its metabolite, sphingosine-1-phosphate promotes growth and survival. Given the interconvertibility of these two opposing signaling molecules, it is essential that any study that examines the effects of one also look at the other. The newly available technology of liquid chromatography-tandem mass spectroscopy (LC-MS/MS) is increasingly being applied for this purpose, as it can quickly identify and measure many different sphingolipids simultaneously. An added benefit of LC-MS/MS is that it is several orders of magnitude more sensitive than enzymatic methods or more traditional chromatographic techniques, allowing smaller sample sizes and increased throughput. Here, we briefly discuss the importance of LC-MS/MS for measuring sphingolipid metabolites and some future directions researchers may take given the increasingly accessibility to this technology.     

Sphingosine 1-phosphate analogs as receptor antagonists

Sphingosine 1-phosphate (S1P) is a lysophospholipid mediator that evokes a variety of cell and tissue responses via a set of cell surface receptors. The recent development of S1P receptor agonists, led by the immunomodulatory pro-drug FTY720, has revealed that S1P signaling is an important regulator of lymphocyte trafficking. With the twin goals of understanding structure-activity relationships of S1P ligands and developing tool compounds to explore S1P biology, we synthesized and tested numerous S1P analogs. We report herein that a subset of our aryl amide-containing compounds are antagonists at the S1P(1) and S1P(3) receptors. The lead compound in series, VPC23019, was found in broken cell and whole cell assays to behave as a competitive antagonist at the S1P(1) and S1P(3) receptors. The structure-activity relationship of this series is steep; for example, a slight modification of the lead compound resulted in VPC25239, which was one log order more potent at the S1P(3) receptor. These new chemical entities will enable further understanding of S1P signaling and provide leads for further S1P receptor antagonist development.     

Sphingosine 1-phosphate, a key mediator of the cytokine network: juxtacrine signaling

Sphingosine 1-phosphate (S1P) is a sphingolipid metabolite, which has emerged as an important signaling mediator participating in the regulation of multiple cellular processes. The discovery of a family of S1P receptors, together with the more recently identified intracellular targets, has provided fundamental understanding of the multi-faceted actions of S1P. Evidence from both in vitro and in vivo studies has implicated the S1P signaling system in the control of immunity, inflammation and many associated diseases. Enigmatically, S1P appears to have both pro- and anti-inflammatory effects depending on the cell context. Here, we review this emerging area and argue for a pivotal role for S1P, as a key mediator of the cytokine network, acting through juxtacrine signaling in the immune system.