Involvement of a pertussis toxin-sensitive G protein in the mitogenic signaling pathways of sphingosine 1-phosphate

Renal Medullary Overexpression of Sphingosine-1-Phosphate Receptor 1 Transgene Attenuates Deoxycorticosterone Acetate (DOCA)-Salt Hypertension

BACKGROUND

Our previous studies showed that renal medullary sphingosine-1-phosphate receptor 1 (S1PR1) mediated sodium excretion, high salt intake increased S1PR1 level, deoxycorticosterone acetate (DOCA) blocked high salt-induced S1PR1 in the renal medulla, and that conditional knockout of S1PR1 in the collecting duct aggravated DOCA-salt hypertension. The present study tested the hypothesis that overexpression of S1PR1 transgene in the renal medulla attenuates the sodium retention and hypertension in DOCA-salt mouse model.

METHODS

Male C57BL/6J mice received renal medullary transfection of control or S1PR1-expressing plasmids and then DOCA-salt treatment. Renal sodium excretion and arterial pressure were compared between control and S1PR1-overexpressed mice in response to high salt loading or pressure natriuresis.

RESULTS

S1PR1-transfected mice showed significantly enhanced urinary sodium excretion in response to acute sodium loading (0.93 ± 0.27 in control vs. 4.72 ± 1.12 µmol/min/gKW in S1PR1-overexpressed mice, P < 0.05) and the pressure natriuresis (3.58 ± 1.77 vs. 9.52 ± 1.38, P < 0.05), less positive sodium balance in response to chronic high-salt intake (3.05 ± 0.39 vs. 1.65 ± 0.39 mmol/72 hr, P < 0.05), and consequently, the attenuation of DOCA-salt hypertension (134.2 ± 6.79 vs. 109.8 ± 3.54 mm Hg, P < 0.05). The αENaC protein amount in the renal medulla was not changed, however, the βENaC was significantly decreased and the γENaC was significantly increased in S1PR1-overexpressed mice. The immunostaining showed apical membrane translocation of γENaC, while no change of αENaC and βENaC in control mice, and that the apical membrane translocation of γENaC was blocked in S1PR1-treasffected mice.

CONCLUSIONS

These results suggested that activation of S1PR1 in the renal medulla attenuates DOCA-induced sodium retention and salt-sensitive hypertension associated with inhibition of ENaC.

Sphingosine 1-Phosphate Signaling at the Skin Barrier Interface

Sphingosine 1-phosphate (S1P) is a product of membrane sphingolipid metabolism. S1P is secreted and acts via G-protein-coupled receptors, S1PR1-5, and is involved in diverse cellular functions, including cell proliferation, immune suppression, and cardiovascular functions. Recent studies have shown that the effects of S1P signaling are extended further by coupling the different S1P receptors and their respective downstream signaling pathways. Our group has recently reported that S1P inhibits cell proliferation and induces differentiation in human keratinocytes. There is a growing understanding of the connection between S1P signaling, skin barrier function, and skin diseases. For example, the activation of S1PR1 and S1PR2 during bacterial invasion regulates the synthesis of inflammatory cytokines in human keratinocytes. Moreover, S1P-S1PR2 signaling is involved in the production of inflammatory cytokines and can be triggered by epidermal mechanical stress and bacterial invasion. This review highlights how S1P affects human keratinocyte proliferation, differentiation, immunoreaction, and mast cell immune response, in addition to its effects on the skin barrier interface. Finally, studies targeting S1P-S1PR signaling involved in inflammatory skin diseases are also presented.

Collecting duct-specific knockout of sphingosine-1-phosphate receptor 1 aggravates DOCA-salt hypertension in mice

OBJECTIVE

We have previously reported that renal medullary sphingosine-1-phosphate (S1P) regulates sodium excretion via the S1P type-1 receptor (S1PR1). As S1PR1 is predominantly expressed in collecting ducts (CD), the present study tested the hypothesis that the CD-S1PR1 pathway plays a critical role in sodium excretion and contributes to salt-sensitive hypertension.

METHODS

CD-specific S1PR1 knockout mice were generated by crossing aquaporin-2-Cre mice with S1PR1-floxed mice. Renal sodium excretion and arterial pressure were compared between wild type and KO mice in response to high-salt challenges and treatment of deoxycorticosterone acetate (DOCA) salt.

RESULTS

Protein levels of renal medullary S1PR1 were increased by 100% after high-salt intake, whereas DOCA treatment with high-salt intake blocked the increase of S1PR1 levels. Urinary sodium excretions in knockout mice were decreased by 60% compared with wild type mice after acute intravenous sodium loading (0.84 ± 0.16 vs. 2.22 ± 0.62 μmole/min per g kwt). The pressure natriuresis was impaired in knockout mice compared with wild type mice (4.32 ± 1.04 vs. 8.73 ± 0.19 μmole/min per g kwt). The chronic high-salt intake-induced positive sodium balance was enhanced in knockout mice compared with wild type mice (5.27 ± 0.39 vs. 2.38 ± 1.04 mmol/100 g BW per 24 h). After 10-day DOCA-salt treatment, knockout mice developed more severe hypertension than wild type mice (SBP 142 ± 8 vs. 115 ± 4 mmHg).

CONCLUSION

The deletion of CD-S1PR1 reduced sodium excretion, promoted sodium retention, and accelerated DOCA-salt-induced salt-sensitive hypertension, suggesting that the CD-S1PR1 signaling is an important antihypertensive pathway by promoting sodium excretion and that impairment of renal medullary S1PR1 may represent a novel mechanism for salt-sensitive hypertension.

Sphingosine-1-Phosphate Metabolism and Signaling in Kidney Diseases

In the past few decades, sphingolipids and sphingolipid metabolites have gained attention because of their essential role in the pathogenesis and progression of kidney diseases. Studies in models of experimental and clinical nephropathies have described accumulation of sphingolipids and sphingolipid metabolites, and it has become clear that the intracellular sphingolipid composition of renal cells is an important determinant of renal function. Proper function of the glomerular filtration barrier depends heavily on the integrity of lipid rafts, which include sphingolipids as key components. In addition to contributing to the structural integrity of membranes, sphingolipid metabolites, such as sphingosine-1-phosphate (S1P), play important roles as second messengers regulating biologic processes, such as cell growth, differentiation, migration, and apoptosis. This review will focus on the role of S1P in renal cells and how aberrant extracellular and intracellular S1P signaling contributes to the pathogenesis and progression of kidney diseases.

Sphingosine-1-phosphate: From insipid lipid to a key regulator

It is a great honor to be asked to write a "Reflections" article by one of the true icons of biochemistry, Herb Tabor. I felt humbled, especially since it follows many written by biochemists I admire and whose contributions have shaped major advances in biochemistry and molecular biology in the last century. Here I present my personal reflections on my adventure with the bioactive sphingolipid metabolite sphingosine-1-phosphate intertwined with those of my family life as a wife, mother, and grandmother. These reflections brought back many memories of events in my early career that played significant roles in determining the path I have taken for more than 40 years and that brought much fun and satisfaction into my life. It has been an exciting journey so far, with many surprises along the way, that still continues.

Sphingosine-1-phosphate (S1P) analog phytosphingosine-1-phosphate (P1P) improves the in vitro maturation efficiency of porcine oocytes via regulation of oxidative stress and apoptosis

Phytosphingosine-1-phosphate (P1P) is a signaling sphingolipid that regulates various physiological activities. However, little is known about the effect of P1P in the context of reproduction. Thus, we aimed to investigate the influence of P1P on oocyte maturation during porcine in vitro maturation (IVM). Here, we report the expression of S1PR1-3 among P1P receptors (S1PR1-4) in cumulus cells and oocytes. When P1P was administered at concentrations of 10, 50, 100, and 1,000 nM during IVM, the metaphase II rate was significantly increased in the 1,000 nM (1 μM) P1P treatment group. Maturation rate improvement by P1P supplementation was observed only in the presence of epidermal growth factor (EGF). Oocytes under the influence of P1P showed decreased intracellular reactive oxygen species levels but no significant differences in glutathione levels. In our molecular studies, P1P treatment upregulated gene expression involved in cumulus expansion (Has2 and EGF), antioxidant enzymes (SOD3 and Cat), and developmental competence (Oct4) while activating extracellular signal-regulated kinase1/2 and Akt signaling. P1P treatment also influenced oocyte survival by shifting the ratio of Bcl-2 to Bax while inactivating JNK signaling. We further demonstrated that oocytes matured with P1P displayed significantly higher developmental competence (cleavage and blastocyst [BL] formation rate) and greater BL quality (total cell number and the ratio of apoptotic cells) when activated via parthenogenetic activation (PA) and in vitro fertilization. Despite the low levels of endogenous P1P found in animals, exogenous P1P influenced animal reproduction, as shown by increased porcine oocyte maturation as well as preimplantation embryo development. This study and its findings are potentially relevant for both human and animal-assisted reproduction.

'Crystal' Clear? Lysophospholipid Receptor Structure Insights and Controversies

Lysophospholipids (LPLs), particularly sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA), are bioactive lipid modulators of cellular homeostasis and pathology. The discovery and characterization of five S1P- and six LPA-specific G protein-coupled receptors (GPCRs), S1P1-5 and LPA1-6, have expanded their known involvement in all mammalian physiological systems. Resolution of the S1P1, LPA1, and LPA6 crystal structures has fueled the growing interest in these receptors and their ligands as targets for pharmacological manipulation. In this review, we have attempted to provide an integrated overview of the three crystallized LPL GPCRs with biochemical and physiological structure-function data. Finally, we provide a novel discussion of how chaperones for LPLs may be considered when extrapolating crystallographic and computational data toward understanding actual biological interactions and phenotypes.

Evidence Suggests Sphingosine 1-Phosphate Might Be Actively Generated, Degraded, and Transported to Extracellular Spaces With Increased S1P2 and S1P3 Expression in Colon Cancer

BACKGROUND

A pivotal role of sphingosine 1-phosphate (S1P) in cancer has been suggested based on the ceramide-S1P rheostat theory that the intracellular balance between prosurvival S1P and proapoptotic ceramide determines cell fate. Upregulation of S1P-generating sphingosine kinases (SKs) and downregulation of S1P-degrading S1P lyase (SPL) might increase intracellular S1P levels to exert a prosurvival effect in cancer in general, such as colon cancer. However, we recently observed a distinct S1P metabolism in hepatocellular carcinoma tissues that increased SPL mRNA levels with reduced S1P levels. Thus, we investigated S1P metabolism in colon cancer.

PATIENTS AND METHODS

We enrolled 26 consecutive colon cancer patients, who had undergone surgical treatment.

RESULTS

Not only SK, but also SPL, mRNA levels were increased in colon cancer tissues compared with the adjacent nontumorous tissues. Furthermore, the mRNA levels of another S1P degrading enzyme, S1P phosphatase 1, S1P transporters, spinster homolog 2, adenosine triphosphate-binding cassette subfamily C member 1, and S1P receptors, S1P₂ and S1P₃ were also increased, but the S1P levels were not increased in the colon cancer tissues. The reduction of SPL expression by silencing led to reduced proliferation and invasion, and overexpression of SPL caused enhanced proliferation in colon cancer cell lines.

CONCLUSION

In human colon cancer tissues, mRNA levels of S1P-generating and S1P-degrading enzymes, transporters from inside to outside the cells, and S1P receptors, S1P₂ and S1P₃ were elevated, suggesting active S1P metabolism and movement. This altered S1P metabolism might play a role in colon cancer pathophysiology.

Sphingosine Kinases and Sphingosine 1-Phosphate Receptors: Signaling and Actions in the Cardiovascular System

The sphingosine kinases 1 and 2 (SphK1 and 2) catalyze the phosphorylation of the lipid, sphingosine, generating the signal transmitter, sphingosine 1-phosphate (S1P). The activation of such kinases and the subsequent S1P generation and secretion in the blood serum of mammals represent a major checkpoint in many cellular signaling cascades. In fact, activating the SphK/S1P system is critical for cell motility and proliferation, cytoskeletal organization, cell growth, survival, and response to stress. In the cardiovascular system, the physiological effects of S1P intervene through the binding and activation of a family of five highly selective G protein-coupled receptors, called S1PR_1-5. Importantly, SphK/S1P signal is present on both vascular and myocardial cells. S1P is a well-recognized survival factor in many tissues. Therefore, it is not surprising that the last two decades have seen a flourishing of interest and investigative efforts directed to obtain additional mechanistic insights into the signaling, as well as the biological activity of this phospholipid, and of its receptors, especially in the cardiovascular system. Here, we will provide an up-to-date account on the structure and function of sphingosine kinases, discussing the generation, release, and function of S1P. Keeping the bull's eye on the cardiovascular system, we will review the structure and signaling cascades and biological actions emanating from the stimulation of different S1P receptors. We will end this article with a summary of the most recent, experimental and clinical observations targeting S1PRs and SphKs as possible new therapeutic avenues for cardiovascular disorders, such as heart failure.

Unveiling the participation of avian kinin ornithokinin and its receptors in the chicken inflammatory response

Vasoactive peptides are key early mediators of inflammation released through activation of different enzymatic systems. The mammalian kinin-kallikrein (K-KLK) system produces bradykinin (BK) through proteolytic cleavage of a kininogen precursor by enzymes named kallikreins. BK acts through specific ubiquitous G-protein coupled receptors (B1R and B2R) to participate in physiological processes and inflammatory responses, such as activation of mononuclear phagocytes. In chickens, the BK-like nonapeptide ornithokinin (OK) has been shown to promote intracellular calcium increase in embryonic fibroblasts and to be vasodilatory in vivo. Also, one of its receptors (B2R) was already cloned. However, the participation of chicken K-KLK system components in the inflammatory response remains unknown and was therefore investigated. We first showed that B1R, B2R and kininogen 1 (KNG1) are expressed in unstimulated chicken tissues and macrophages. We next showed that chicken B1R and B2R are expressed at transcript and protein levels in chicken macrophages and are upregulated by E. coli LPS or avian pathogenic E. coli (APEC) infection. Interestingly, exogenous OK induced internalization and degradation of OK receptors protein, notably B2R. Also, OK induced intracellular calcium increase and potentiated zymosan-induced ROS production and Dextran-FITC endocytosis by chicken macrophages. Exogenous OK itself did not promote APEC killing and had no pro-inflammatory effect. However, when combined with LPS or APEC, OK upregulated cytokine/chemokine gene expression and NO production by chicken macrophages. This effect was not blocked by canonical non-peptide B1R or B2R receptor antagonists but was GPCR- and PI3K/Akt-dependent. In vivo, pulmonary colibacillosis led to upregulation of OK receptors expression in chicken lungs and liver. Also, colibacillosis led to significant upregulation of OK precursor KNG1 expression in liver and in cultured hepatocytes (LMH). We therefore provide hitherto unknown information on how OK and its receptors are involved in inflammation and infection in chickens.

Sphingosine Kinases and Sphingosine 1-Phosphate Receptors: Signaling and Actions in the Cardiovascular System

The sphingosine kinases 1 and 2 (SphK1 and 2) catalyze the phosphorylation of the lipid, sphingosine, generating the signal transmitter, sphingosine 1-phosphate (S1P). The activation of such kinases and the subsequent S1P generation and secretion in the blood serum of mammals represent a major checkpoint in many cellular signaling cascades. In fact, activating the SphK/S1P system is critical for cell motility and proliferation, cytoskeletal organization, cell growth, survival, and response to stress. In the cardiovascular system, the physiological effects of S1P intervene through the binding and activation of a family of five highly selective G protein-coupled receptors, called S1PR_1-5. Importantly, SphK/S1P signal is present on both vascular and myocardial cells. S1P is a well-recognized survival factor in many tissues. Therefore, it is not surprising that the last two decades have seen a flourishing of interest and investigative efforts directed to obtain additional mechanistic insights into the signaling, as well as the biological activity of this phospholipid, and of its receptors, especially in the cardiovascular system. Here, we will provide an up-to-date account on the structure and function of sphingosine kinases, discussing the generation, release, and function of S1P. Keeping the bull's eye on the cardiovascular system, we will review the structure and signaling cascades and biological actions emanating from the stimulation of different S1P receptors. We will end this article with a summary of the most recent, experimental and clinical observations targeting S1PRs and SphKs as possible new therapeutic avenues for cardiovascular disorders, such as heart failure.

Sphingosine 1-Phosphate Receptor Modulators and Drug Discovery

Initial discovery on sphingosine 1-phosphate (S1P) as an intracellular second messenger was faced unexpectedly with roles of S1P as a first messenger, which subsequently resulted in cloning of its G protein-coupled receptors, S1P_1–5. The molecular identification of S1P receptors opened up a new avenue for pathophysiological research on this lipid mediator. Cellular and molecular in vitro studies and in vivo studies on gene deficient mice have elucidated cellular signaling pathways and the pathophysiological meanings of S1P receptors. Another unexpected finding that fingolimod (FTY720) modulates S1P receptors accelerated drug discovery in this field. Fingolimod was approved as a first-in-class, orally active drug for relapsing multiple sclerosis in 2010, and its applications in other disease conditions are currently under clinical trials. In addition, more selective S1P receptor modulators with better pharmacokinetic profiles and fewer side effects are under development. Some of them are being clinically tested in the contexts of multiple sclerosis and other autoimmune and inflammatory disorders, such as, psoriasis, Crohn’s disease, ulcerative colitis, polymyositis, dermatomyositis, liver failure, renal failure, acute stroke, and transplant rejection. In this review, the authors discuss the state of the art regarding the status of drug discovery efforts targeting S1P receptors and place emphasis on potential clinical applications.

Increased mRNA Levels of Sphingosine Kinases and S1P Lyase and Reduced Levels of S1P Were Observed in Hepatocellular Carcinoma in Association with Poorer Differentiation and Earlier Recurrence

Although sphingosine 1-phosphate (S1P) has been reported to play an important role in cancer pathophysiology, little is known about S1P and hepatocellular carcinoma (HCC). To clarify the relationship between S1P and HCC, 77 patients with HCC who underwent surgical treatment were consecutively enrolled in this study. In addition, S1P and its metabolites were quantitated by LC-MS/MS. The mRNA levels of sphingosine kinases (SKs), which phosphorylate sphingosine to generate S1P, were increased in HCC tissues compared with adjacent non-HCC tissues. Higher mRNA levels of SKs in HCC were associated with poorer differentiation and microvascular invasion, whereas a higher level of SK2 mRNA was a risk factor for intra- and extra-hepatic recurrence. S1P levels, however, were unexpectedly reduced in HCC compared with non-HCC tissues, and increased mRNA levels of S1P lyase (SPL), which degrades S1P, were observed in HCC compared with non-HCC tissues. Higher SPL mRNA levels in HCC were associated with poorer differentiation. Finally, in HCC cell lines, inhibition of the expression of SKs or SPL by siRNA led to reduced proliferation, invasion and migration, whereas overexpression of SKs or SPL enhanced proliferation. In conclusion, increased SK and SPL mRNA expression along with reduced S1P levels were more commonly observed in HCC tissues compared with adjacent non-HCC tissues and were associated with poor differentiation and early recurrence. SPL as well as SKs may be therapeutic targets for HCC treatment.

Trafficking and Functions of Bioactive Sphingolipids: Lessons from Cells and Model Membranes

Ceramide and sphingosine and their phosphorylated counterparts are recognized as "bioactive sphingolipids" and modulate membrane integrity, the activity of enzymes, or act as ligands of G protein-coupled receptors. The subcellular distribution of the bioactive sphingolipids is central to their function as the same lipid can mediate diametrically opposite effects depending on its location. To ensure that these lipids are present in the right amount and in the appropriate organelles, cells employ selective lipid transport and compartmentalize sphingolipid-metabolizing enzymes to characteristic subcellular sites. Our knowledge of key mechanisms involved in sphingolipid signaling and trafficking has increased substantially in the past decades-thanks to advances in biochemical and cell biological methods. In this review, we focus on the bioactive sphingolipids and discuss how the combination of studies in cells and in model membranes have contributed to our understanding of how they behave and function in living organisms.

A reflection of the lasting contributions from Dr. Robert Bittman to sterol trafficking, sphingolipid and phospholipid research

With the passing of Dr. Robert Bittman from pancreatic cancer on the 1st October 2014, the lipid research field lost one of the most influential and significant personalities. Robert Bittman's genius was in chemical design and his contribution to the lipid research field was truly immense. The reagents and chemicals he designed and synthesised allowed interrogation of the role of lipids in constituting complex biophysical membranes, sterol transfer and in cellular communication networks. Here we provide a review of these works which serve as a lasting memory to his life.

Role of Sphingosine-1-Phosphate in Transplant Vasculopathy Evoked by Anti-HLA Antibody

Transplant vasculopathy (TV) represents the main cause of late graft failure and limits the long-term success of organ transplantation. Cellular and humoral immune responses contribute to the pathogenesis of the concentric and diffuse intimal hyperplasia of arteries of the grafted organ. We recently reported that the mitogenic signaling, evoked in human vascular smooth muscle cells (hmSMC) by the anti-HLA class I monoclonal antibody W6/32, implicates neutral sphingomyelinase-2, suggesting a role for sphingolipids in intimal hyperplasia of TV. Here, we investigated whether the mitogenic sphingolipid, sphingosine-1-phosphate (S1P), is involved in intimal hyperplasia elicited by W6/32. Studies were done on cultured hmSMC and on an in vivo model of TV, consisting of human mesenteric arteries grafted into SCID/beige mice, injected weekly with W6/32. hmSMC migration and DNA synthesis elicited by W6/32 were inhibited by the sphingosine kinase-1 (SK1) inhibitor dimethylsphingosine, the anti-S1P antibody Sphingomab and the S1PR1/R3 inhibitor VPC23019. W6/32 stimulated SK1 activity, while siRNA silencing SK1, S1PR1 and S1PR3 inhibited hmSMC migration. In vivo, Sphingomab significantly reduced the intimal thickening induced by W6/32. These data emphasize the role of S1P in intimal hyperplasia elicited by the humoral immune response, and open perspectives for preventing TV with S1P inhibitors.

Roles of sphingosine-1-phosphate in reproduction

Sphingosine-1-phosphate (S1P) plays crucial roles in the regulation of cell growth, proliferation, differentiation, cell survival, migration, and angiogenesis. In the reproductive system, S1P protects mammalian germ cells from irradiation or chemotherapy-induced cell death in vivo and in vitro. Moreover, S1P could improve the survival rate of thawed ovary and transplanted ovary. Furthermore, S1P could improve the developmental potential of oocyte and preimplantation embryo. In conclusion, S1P plays important roles in reproduction.

ETS-1-mediated transcriptional up-regulation of CD44 is required for sphingosine-1-phosphate receptor subtype 3-stimulated chemotaxis

Sphingosine-1-phosphate (S1P)-regulated chemotaxis plays critical roles in various physiological and pathophysiological conditions. S1P-regulated chemotaxis is mediated by the S1P family of G-protein-coupled receptors. However, molecular details of the S1P-regulated chemotaxis are incompletely understood. Cultured human lung adenocarcinoma cell lines abundantly express S1P receptor subtype 3 (S1P3), thus providing a tractable in vitro system to characterize molecular mechanism(s) underlying the S1P3 receptor-regulated chemotactic response. S1P treatment enhances CD44 expression and induces membrane localization of CD44 polypeptides via the S1P3/Rho kinase (ROCK) signaling pathway. Knockdown of CD44 completely diminishes the S1P-stimulated chemotaxis. Promoter analysis suggests that the CD44 promoter contains binding sites of the ETS-1 (v-ets erythroblastosis virus E26 oncogene homolog 1) transcriptional factor. ChIP assay confirms that S1P treatment stimulates the binding of ETS-1 to the CD44 promoter region. Moreover, S1P induces the expression and nuclear translocation of ETS-1. Knockdown of S1P3 or inhibition of ROCK abrogates the S1P-induced ETS-1 expression. Furthermore, knockdown of ETS-1 inhibits the S1P-induced CD44 expression and cell migration. In addition, we showed that S1P3/ROCK signaling up-regulates ETS-1 via the activity of JNK. Collectively, we characterized a novel signaling axis, i.e., ROCK-JNK-ETS-1-CD44 pathway, which plays an essential role in the S1P3-regulated chemotactic response.

Sphingosine-1-phosphate receptor 2

Sphingosine-1-phosphate (S1P) is a potent bioactive sphingolipid involved in cell proliferation, angiogenesis, inflammation and malignant transformation among other functions. S1P acts either directly on intracellular targets or activates G protein-coupled receptors, specifically five S1P receptors (S1PRs). The identified S1PRs differ in cellular and tissue distribution, and each is coupled to specific G proteins, which mediate unique functions. Here, we describe functional characteristics of all five receptors, emphasizing S1PR2, which is critical in the immune, nervous, metabolic, cardiovascular, musculoskeletal, and renal systems. This review also describes the role of this receptor in tumor growth and metastasis and suggests potential therapeutic avenues that exploit S1PR2.

Roles of sphingosine 1-phosphate on tumorigenesis

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid with a variety of biological activities. It is generated from the conversion of ceramide to sphingosine by ceramidase and the subsequent conversion of sphingosine to S1P, which is catalyzed by sphingosine kinases. Through increasing its intracellular levels by sphingolipid metabolism and binding to its cell surface receptors, S1P regulates several physiological and pathological processes, including cell proliferation, migration, angiogenesis and autophagy. These processes are responsible for tumor growth, metastasis and invasion and promote tumor survival. Since ceramide and S1P have distinct functions in regulating in cell fate decision, the balance between the ceramide/sphingosine/S1P rheostat becomes a potent therapeutic target for cancer cells. Herein, we summarize our current understanding of S1P signaling on tumorigenesis and its potential as a target for cancer therapy.

Pharmacological tools for lysophospholipid GPCRs: development of agonists and antagonists for LPA and S1P receptors

Previous studies on lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) using various approaches have shown that both the molecules can act as intercellular signaling molecules. The discovery of the Edg subfamily of G-protein-coupled receptors (GPCRs) (later renamed LPA(1-3) and S1P(1-5)) for these molecules has opened up a new avenue for pathophysiological research on lysophospholipids. Genetic and molecular studies on lysophospholipid GPCRs have elucidated pathophysiological impacts and roles in cellular signaling pathways. Recently, lysophospholipid GPCR genes have been used to develop receptor subtype-selective agonists and antagonists. The discovery of FTY720, a novel immune modulator, along with other chemical tools, has provided a means of elucidating the functions of each lysophospholipid GPCR on an organ and the whole body level. This communication attempts to retrospectively review the development of agonists and antagonists for lysophospholipid GPCRs, provide integrated information on pharmacological tools for lysophospholipid GPCR signaling, and speculate on future drug development.

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.

Plasma concentration of bioactive lipid mediator sphingosine 1-phosphate is reduced in patients with chronic hepatitis C

BACKGROUND

Bioactive lipid mediator S1P has been suggested to play pathophysiological roles in various fields of clinical science as a circulating paracrine mediator. We previously established a reliable method of measuring plasma S1P concentration, and reported that the one in healthy subjects has a gender difference and a correlation with red blood cell (RBC)-parameters, however, the reports of S1P measurements in the blood in patients with a specific disease have been scarce. Because our previous evidence suggests that S1P is involved in liver pathophysiology, we examined plasma S1P concentration in chronic hepatitis C patients.

METHODS

S1P assay was performed using a high-performance liquid chromatography system.

RESULTS

Plasma S1P concentrations were reduced in chronic hepatitis C patients compared with in healthy subjects with the same hemoglobin concentration, irrespective of gender. Among the blood parameters, serum hyaluronic acid concentration, a surrogate marker for liver fibrosis, was most closely and inversely correlated with plasma S1P concentration. Furthermore, plasma S1P concentration decreased throughout the progression of carbon tetrachloride-induced liver fibrosis in rats.

CONCLUSIONS

Plasma S1P concentration was reduced in chronic hepatitis C patients, and liver fibrosis might be involved, at least in part, in the mechanism responsible for this reduction.

FTY720 prevents ischemia/reperfusion injury-associated arrhythmias in an ex vivo rat heart model via activation of Pak1/Akt signaling

Recent studies demonstrated a role of sphingosine-1-phosphate (S1P) in the protection against the stress of ischemia/reperfusion (I/R) injury. In experiments reported here, we have investigated the signaling through the S1P cascade by FTY720, a sphingolipid drug candidate displaying structural similarity to S1P, underlying the S1P cardioprotective effect. In ex vivo rat heart and isolated sinoatrial node models, FTY720 significantly prevented arrhythmic events associated with I/R injury including premature ventricular beats, VT, and sinus bradycardia as well as A-V conduction block. Real-time PCR and Western blot analysis demonstrated the expression of the S1P receptor transcript pools and corresponding proteins including S1P1, S1P2, and S1P3 in tissues dissected from sinoatrial node, atrium and ventricle. FTY720 (25 nM) significantly blunted the depression of the levels of phospho-Pak1 and phospho-Akt with ischemia and with reperfusion. There was a significant increase in phospho-Pak1 levels by 35%, 199%, and 205% after 5, 10, and 15 min of treatment with 25 nM FTY720 compared with control nontreated myocytes. However, there was no significant difference in the levels of total Pak1 expression between nontreated and FTY720 treated. Phospho-Akt levels were increased by 44%, 63%, and 61% after 5, 10, and 15 min of treatment with 25 nM FTY720, respectively. Our data provide the first evidence that FTY720 prevents I/R injury-associated arrhythmias and indicate its potential significance as an important and new agent protecting against I/R injury. Our data also indicate, for the first time, that the cardioprotective effect of FTY720 is likely to involve activation of signaling through the Pak1.

FcepsilonRI-mediated mast cell migration: signaling pathways and dependence on cytosolic free Ca2+ concentration

IgE-sensitized rat basophilic leukemia (RBL)-2H3 mast cells have been shown to migrate towards antigen. In the present study we tried to identify the mechanism by which antigen causes mast cell migration. Antigen caused migration of RBL-2H3 cells at the concentration ranges of 1000-fold lower than those required for degranulation and the dose response was biphasic. This suggests that mast cells can detect very low concentration gradients of antigen (pg/ml ranges), which initiate migration until they degranulate near the origin of antigen, of which concentration is in the ng/ml ranges. Similar phenomenon was observed in human mast cells (HMCs) derived from CD34(+) progenitors. As one mechanism of mast cell migration, we tested the involvement of sphingosine 1-phosphate (S1P). Fc epsilon RI-mediated cell migration was dependent on the production of S1P but independent of a S1P receptor or its signaling pathways as determined with S1P receptor antagonist VPC23019 and Gi protein inhibitor pertussis toxin (PTX). This indicated that the site of action of S1P produced by antigen stimulation was intracellular. However, S1P-induced mast cell migration was dependent on S1P receptor activation and inhibited by both VPC23019 and PTX. Cell migration towards antigen or extracellular S1P was dependent on the activation of the phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) pathways, while only migration towards antigen was inhibited by the inhibitors of sphingosine kinase and phospholipase C (PLC) and intracellular calcium chelator BAPTA. In summary, our data suggest that the high affinity receptor for IgE (Fc epsilon RI)-mediated mast cell migration is dependent on the production of S1P but independent of S1P receptors. Cell migration mediated by either Fc epsilon RI or S1P receptors involves activation of both PI3K and MAPK.

The influence of sphingosine-1-phosphate receptor signaling on lymphocyte trafficking: how a bioactive lipid mediator grew up from an "immature" vascular maturation factor to a "mature" mediator of lymphocyte behavior and function

Since the initial observations that highlighted the importance of lymphocyte trafficking for immune responses, the pathways utilized by B and T lymphocytes to recirculate and properly position themselves have been intensely studied. Most of the chemoattractants along with their cognate receptors that affect lymphocyte trafficking have been identified. Some of their functions are promotion of lymphocyte ingress into immune organs, localization of cells to specific regions within those organs, maintenance of lymphocyte basal motility in immune organs, facilitation of lymphocyte egress from these organs, and control of migration and homing of lymphocytes in the periphery. Since the seminal discovery that agonism of sphingosine-1-phosphate receptors evokes changes in lymphocyte homing and trafficking, considerable effort has been undertaken to characterize the mechanism utilized by these receptors to influence lymphocyte behavior. This review will focus on the influence of sphingosine-1-phosphate signaling system on lymphocyte localization, egress from lymph organs, and its effects on the lymphatic vasculature.

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.

Ceramide 1-phosphate (C1P) promotes cell migration Involvement of a specific C1P receptor

Ceramide 1-phosphate (C1P) is a bioactive sphingolipid that is implicated in the regulation of cell homeostasis and the control of inflammation. It is mitogenic for fibroblasts and macrophages, and has been described as potent inhibitor of apoptosis. Using RAW 264.7 macrophages we have now discovered a new biological activity of C1P: stimulation of cell migration. This novel action can only be observed when C1P is applied exogenously to the cells in culture, and not by increasing the intracellular levels of C1P. This fact led to identify a specific receptor through which C1P stimulates cell migration. The receptor is coupled to G(i) proteins and causes phosphorylation of extracellularly regulated kinases 1 and 2, and protein kinase B (also known as Akt) upon ligation with C1P. Inhibition of either of these pathways completely abolished C1P-stimulated macrophage migration. In addition, C1P stimulated the DNA binding activity of nuclear factor kappa B, and blockade of this transcription factor resulted in complete inhibition of macrophage migration. This newly identified receptor could be an important drug target for treatment of illnesses that are associated to inflammatory processes, or to diseases in which cell migration is a major cause of pathology, as it occurs in metastatic tumors.

Sphingosine 1-phosphate regulates regeneration and fibrosis after liver injury via sphingosine 1-phosphate receptor 2

Sphingosine 1-phosphate (S1P), a bioactive lipid mediator, stimulates proliferation and contractility in hepatic stellate cells, the principal matrix-producing cells in the liver, and inhibits proliferation via S1P receptor 2 (S1P(2)) in hepatocytes in rats in vitro. A potential role of S1P and S1P(2) in liver regeneration and fibrosis was examined in S1P(2)-deficient mice. Nuclear 5-bromo-2'-deoxy-uridine labeling, proliferating cell nuclear antigen (PCNA) staining in hepatocytes, and the ratio of liver weight to body weight were enhanced at 48 h in S1P(2)-deficient mice after a single carbon tetrachloride (CCl(4)) injection. After dimethylnitrosamine (DMN) administration with a lethal dose, PCNA staining in hepatocytes was enhanced at 48 h and survival rate was higher in S1P(2)-deficient mice. Serum aminotransferase level was unaltered in those mice compared with wild-type mice in both CCl(4)- and DMN-induced liver injury, suggesting that S1P(2) inactivation accelerated regeneration not as a response to enhanced liver damage. After chronic CCl(4) administration, fibrosis was less apparent, with reduced expression of smooth-muscle alpha-actin-positive cells in the livers of S1P(2)-deficient mice, suggesting that S1P(2) inactivation ameliorated CCl(4)-induced fibrosis due to the decreased accumulation of hepatic stellate cells. Thus, S1P plays a significant role in regeneration and fibrosis after liver injury via S1P(2).

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.

Lysophosphatidic acid (LPA) and angiogenesis

Lysophosphatidic acid (LPA) is a simple lipid with many important biological functions such as the regulation of cellular proliferation, cellular migration, differentiation, and suppression of apoptosis. Although a direct angiogenic effect of LPA has not been reported to date, there are indications that LPA promotes angiogenesis. In addition, LPA is a chemoattractant for cultured endothelial cells and promotes barrier function in such cultures. To test the hypothesis that LPA is angiogenic, we used the chicken chorio-allantoic membrane (CAM) assay. Sequence analysis of the cloned, full-length chicken LPA receptor cDNAs revealed three receptor types that are orthologous to the mammalian LPA(1), LPA(2), and LPA(3) receptors. We document herein that LPA is angiogenic in the CAM system and further that synthetic LPA receptor agonists and antagonists mimic or block this response, respectively. Our results predict that LPA receptor antagonists are a possible therapeutic route to interdicting angiogenesis.

Lysophospholipid receptors in cell signaling

There is increasing evidence that different phospholipids are involved in regulation of various cell processes and cell-cell interactions. Lysophospholipids (lysophosphatidic acid, lysophosphatidylcholine) and a number of lysosphingolipids play particular roles in these regulations. Their effects are mediated by specific G-protein-coupled receptors. G-Protein coupled signal transduction to the cell nucleus involving a chain of intracellular protein kinases induces the main effects in cells--growth, proliferation, survival, or apoptosis. This review summarizes recent data on various groups of lysophospholipid receptors and their cell signal transduction pathways.

Sphingosine-1-phosphate activates BKCa channels independently of G protein-coupled receptor in human endothelial cells

The effect of sphingosine-1-phosphate (S1P) on large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels was examined in primary cultured human umbilical vein endothelial cells by measuring intracellular Ca(2+) concentration ([Ca(2+)](i)), whole cell membrane currents, and single-channel activity. In nystatin-perforated current-clamped cells, S1P hyperpolarized the membrane and simultaneously increased [Ca(2+)](i). [Ca(2+)](i) and membrane potentials were strongly correlated. In whole cell clamped cells, BK(Ca) currents were activated by increasing [Ca(2+)](i) via cell dialysis with pipette solution, and the activated BK(Ca) currents were further enhanced by S1P. When [Ca(2+)](i) was buffered at 1 microM, the S1P concentration required to evoke half-maximal activation was 403 +/- 13 nM. In inside-out patches, when S1P was included in the bath solution, S1P enhanced BK(Ca) channel activity in a reversible manner and shifted the relationship between Ca(2+) concentration in the bath solution and the mean open probability to the left. In whole cell clamped cells or inside-out patches loaded with guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS; 1 mM) using a patch pipette, GDPbetaS application or pretreatment of cells with pertussis toxin (100 ng/ml) for 15 h did not affect S1P-induced BK(Ca) current and channel activation. These results suggest that S1P enhances BK(Ca) channel activity by increasing Ca(2+) sensitivity. This channel activation hyperpolarizes the membrane and thereby increases Ca(2+) influx through Ca(2+) entry channels. Inasmuch as S1P activates BK(Ca) channels via a mechanism independent of G protein-coupled receptors, S1P may be a component of the intracellular second messenger that is involved in Ca(2+) mobilization in human endothelial cells.

Enhancement of intracellular sphingosine-1-phosphate production by inositol 1,4,5-trisphosphate-evoked calcium mobilisation in HEK-293 cells: endogenous sphingosine-1-phosphate as a modulator of the calcium response

Sphingosine-1-phosphate (S1P) regulates many cellular functions, such as migration, differentiation and growth. The effects of S1P are thought to be primarily mediated by G-protein coupled receptors, but an intracellular function as a calcium releasing second messenger has also been proposed. Here we show that in HEK-293 cells, exogenous S1P mobilises sequestered calcium by a mechanism primarily dependent on the phospholipase C (PLC)/inositol 1,4,5-trisphosphate (IP3) pathway, and secondarily on the subsequent synthesis of intracellular S1P. Stimulating HEK-293 cells exogenously with S1P increased the production of both inositol phosphates and intracellular S1P. The calcium response was inhibited in cells treated with 2-APB, caffeine or U73122, showing that the PLC/IP3 pathway for calcium release is activated in response to exogenous S1P. The calcium response was partially inhibited in cells treated with the sphingosine kinase inhibitor DMS and in cells expressing a catalytically inactive sphingosine kinase, showing that endogenously produced S1P is also involved. Importantly, 2-APB and U73122 inhibited the S1P-evoked production of intracellular S1P. S1P is therefore not likely a major calcium releasing second messenger in HEK-293 cells, but rather a secondary regulator of calcium mobilisation.

Sphingosine-1-phosphate inhibits human keratinocyte proliferation via Akt/protein kinase B inactivation

Although sphingosine-1-phosphate (S1P) is a well-known mitogen, it has only recently been demonstrated that S1P is able to inhibit cell proliferation in human epidermal keratinocytes and hepatic myofibroblasts. In the present study, we investigated the possible signalling pathways involved in the growth inhibition of human keratinocytes. Our results show that S1P potently inhibits keratinocyte proliferation, and that this leads to the inhibition of DNA synthesis. Interestingly, the prolonged activation of extracellular signal-regulated protein kinase (ERK) and the transient inactivation of Akt/protein kinase B (PKB) were also observed in concert with the inhibition of keratinocyte proliferation by S1P. To verify further the antiproliferative action of S1P, we examined changes in cell cycle-related proteins. S1P inhibited cyclin D(2) synthesis but stimulated p21(WAF1/CIP1) (p21) and p27(KIP1) (p27) synthesis; all are inhibitors of cyclin-dependent kinase. Furthermore, we found that the growth inhibition by S1P was in part abolished by pertussis toxin (PTX) treatment, but that ERK activation and Akt/PKB inhibition were not abrogated, suggesting that S1P functions both intracellularly, as a second messenger, and extracellularly, as a ligand for cell surface receptors. Insulin-like growth factor I (IGF-I) is a well-established human keratinocyte mitogen and is known to stimulate Akt/PKB in various cell types. In the present study, S1P was found to inhibit the keratinocyte proliferation and Akt/PKB activation induced by IGF-I. Our results suggest that S1P may play an important role in the negative regulation of keratinocyte proliferation by inhibiting the Akt/PKB pathway.

Signaling events during induction of plasminogen activator inhibitor-1 expression by sphingosylphosphorylcholine in cultured human dermal fibroblasts

Sphingosylphosphorylcholine (SPC) is a bioactive sphingolipid metabolite that can enhance wound healing. In a search for effectors downstream of SPC in the wound-healing process, we found that the expression of the gene for plasminogen activator inhibitor-1 (PAI-1) was significantly affected. ELISA and western blot analyses showed that SPC markedly induced PAI-1 production in human dermal fibroblasts cultured in vitro. Inhibition by pre-treatment with pertussis toxin (PTx), but not by tyrphostin A47 (a receptor tyrosine kinase inhibitor), indicated that PTx-sensitive G proteins were involved in SPC-induced PAI-1 expression. SPC elicited a rapid and transient increase in intracellular calcium levels ([Ca2+]i), measured using laser scanning confocal microscopy, which was partly mediated through PTx-sensitive G proteins. Pre-treatment with thapsigargin, but not with EGTA, abolished SPC-induced PAI-1 expression, indicating the importance of Ca2+ release from internal stores. Phorbol-12-myristate-13-acetate (PMA) induced the expression of PAI-1, and pre-treatment with Ro 31-8220 (a PKC inhibitor) markedly suppressed SPC-induced PAI-1 expression. SPC-induced PAI-1 expression was also significantly suppressed by PD98059 (a specific MAPK kinase 1/2 inhibitor). Consistent with this result, SPC stimulated the phosphorylation of p42/44 extracellular signal-regulated kinase (ERK). Together, these results suggest that SPC induces PAI-1 production through a G protein-coupled calcium increase and downstream kinase signaling events in cultured human dermal fibroblasts.

Sphingosine 1-phosphate enhances portal pressure in isolated perfused liver via S1P2 with Rho activation

Although structural changes are most important to determine vascular resistance in portal hypertension, vasoactive mediators also contribute to its regulation. Hepatic stellate cells (HSCs) are assumed to play a role in modulating intrahepatic vascular resistance based on their residence in the space of Disse and capacity to contract. Because sphingosine 1-phosphate (S1P) has been shown to stimulate HSC contractility, we wondered if S1P could regulate portal pressure. S1P at 0.5-5 microM increased portal pressure in isolated rat perfused liver. This effect was abrogated in the presence of a binding antagonist for S1P2, JTE-013. Perfusion of isolated rat liver with 5 microM S1P increased Rho activity in the liver, and co-perfusion with JTE-013 cancelled S1P-induced Rho activation. Because S1P is present in human plasma at approximately 0.2 microM, S1P might readily regulate portal vascular tone in physiological and pathological status. The antagonist for S1P2 merits consideration for treatment of portal hypertension.

Sphingosine-1-phosphate mediates calcium signaling in guinea pig enteroglial cells

The enteric nervous system, which regulates multiple aspects of digestive activity, is composed of two major cell types, neurons and glial cells. Enteric glia, but not enteric neurons, respond to bioactive lipids with calcium signaling. The sphingomyelin metabolite sphingosine-1-phosphate (S1P) caused dose-dependent calcium (Ca(2+)) signaling using extracellular and intracellular Ca(2+). The signal transduction cascade was pertussis toxin-insensitive and involved an extracellular receptor since repetitive exposure yielded diminished responsiveness. Inhibition of either phospholipase C or the inositol 1,4,5-trisphosphate receptor abolished S1P effects. RT-PCR analysis demonstrated the presence of S1P-coupled endothelial differentiation gene (EDG) receptor mRNAs (EDG-1, EDG-3, and EDG-5) within the enteric nervous system. Immunocytochemical analysis demonstrated strong expression of both EDG-1 and EDG-3 and weak expression of EDG-5 in enteric glial cells. Other sphingomyelin cycle components, including sphingomyelin, sphingomyelinase, and sphingosine caused Ca(2+) transients in enteric glia. Related lipids lysophosphatidic acid and sphingosylphosphorylcholine also induced Ca(2+) signaling in enteric glia, suggesting that multiple lipid-activated signaling mechanisms exist in these cells.

Sphingosine kinase type 1 induces G12/13-mediated stress fiber formation, yet promotes growth and survival independent of G protein-coupled receptors

Sphingosine 1-phosphate (S1P) is the ligand for a family of specific G protein-coupled receptors (GPCRs) that regulate a wide variety of important cellular functions, including growth, survival, cytoskeletal rearrangements, and cell motility. However, whether it also has an intracellular function is still a matter of great debate. Overexpression of sphingosine kinase type 1, which generated S1P, induced extensive stress fibers and impaired formation of the Src-focal adhesion kinase signaling complex, with consequent aberrant focal adhesion turnover, leading to inhibition of cell locomotion. We have dissected biological responses dependent on intracellular S1P from those that are receptor-mediated by specifically blocking signaling of Galphaq, Galphai, Galpha12/13, and Gbetagamma subunits, the G proteins that S1P receptors (S1PRs) couple to and signal through. We found that intracellular S1P signaled "inside out" through its cell-surface receptors linked to G12/13-mediated stress fiber formation, important for cell motility. Remarkably, cell growth stimulation and suppression of apoptosis by endogenous S1P were independent of GPCRs and inside-out signaling. Using fibroblasts from embryonic mice devoid of functional S1PRs, we also demonstrated that, in contrast to exogenous S1P, intracellular S1P formed by overexpression of sphingosine kinase type 1 promoted growth and survival independent of its GPCRs. Hence, exogenous and intracellularly generated S1Ps affect cell growth and survival by divergent pathways. Our results demonstrate a receptor-independent intracellular function of S1P, reminiscent of its action in yeast cells that lack S1PRs.

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.

Edg-8 receptors are preferentially expressed in oligodendrocyte lineage cells of the rat CNS

The messenger RNA for endothelial differentiation gene 8 receptors is known to be expressed almost exclusively in the rat CNS, but the nature of the expressing cells has not been defined. Using an antibody specific for endothelial differentiation gene 8, we investigated the immunohistochemical localization of endothelial differentiation gene 8 receptors in the rat CNS. Immunopositive staining was detected in a subset of glial cells distributed throughout the brain and spinal cord, including both gray and white matter, but not in the dorsal root ganglion. The distribution and morphological similarity in comparative immunostaining for endothelial differentiation gene 8 and various glial markers suggested that endothelial differentiation gene 8 is preferentially expressed in NG2-positive oligodendrocyte progenitor cells in adult rat brains. Counts of endothelial differentiation gene 8-positive cells and NG2-positive cells in the forebrain revealed that a subset of NG2-positive cells was endothelial differentiation gene 8-positive, and that the ratio of endothelial differentiation gene 8-positive cells to NG2-positive cells varied from region to region. In 17-day-old embryonic brains, the endothelial differentiation gene 8 distribution was similar to that of an oligodendrocytic marker, 2',3'-cyclic nucleotide 3'-phosphodiesterase. These data suggest that endothelial differentiation gene 8 receptors are preferentially expressed in oligodendrocyte lineage cells including oligodendrocyte progenitor cells and immature/maturating oligodendrocytes in rat CNS, and that they might have important functions in oligodendrocytic maturation and myelination.

Sphingosine-1-phosphate and its potentially paradoxical effects on critical parameters of cutaneous wound healing

The sphingolipid metabolite sphingosine-1-phosphate has emerged as a new bioactive molecule involved in the regulation of cell growth, differentiation, survival, and chemotaxis as well as angiogenesis and embryogenesis. These effects are mediated either via G-protein-coupled receptors or through intracellular actions. The most prominent sources of sphingosine-1-phosphate are human platelets suggesting its potential role in wound healing. In agreement with a positive function on reconstruction of wounded skin, we identified sphingosine-1-phosphate as a potent chemoattractant for keratinocytes as well as an activator of extracellular matrix production by fibroblasts. An unexpected finding is a strong cell growth arrest of keratinocytes after exposure to sphingosine-1-phosphate, as keratinocyte proliferation is critical for re-epithelialization of the wound. Most interestingly, the anti-proliferative effect of sphingosine-1-phosphate is not a result of cytotoxicity or apoptosis as sphingosine-1-phosphate even protects these cells from programmed cell death. Moreover, sphingosine-1-phosphate enhances differentiation of keratinocytes. To investigate further by which signaling pathway cell growth inhibition is mediated expression of the mRNA of all sphingosine-1-phosphate receptors (S1P1-5) was identified. 1 (Edg 1), 2 (Edg 5), 3 (Edg 3), 4 (Edg 6), and 5 (Edg 8) mRNA in keratinocytes was identified. As demonstrated in guanosine 5-[gamma-35S] triphosphate-gammaS binding assays, these G-protein-coupled receptors are functional at nanomolar concentrations. As the anti-proliferative effect of sphingosine-1-phosphate is only partially inhibited in the presence of pertussis toxin, it was investigated if intracellular actions are also involved. Microinjections of sphingosine-1-phosphate in keratinocytes also reduce proliferation suggesting that both sphingosine-1-phosphate receptors as well as intracellular actions mediate sphingosine-1-phosphate- induced cell growth arrest.

Assessment of the role of sphingosine 1-phosphate and its receptors in high-density lipoprotein-induced stimulation of astroglial cell function

It has been suggested that lipoproteins in the central nervous system are involved in the regulation of several neural functions independent of cholesterol metabolism as well as those related to lipid metabolism. We recently demonstrated that lipoproteins are carriers for sphingosine 1-phosphate (S1P). This raised the possibility that S1P mediates the neural cell functions induced by lipoproteins. In the current study, we examined the effects of plasma high-density lipoprotein (HDL) on astroglial cell functions, focusing especially on the role of the lipoprotein-associated S1P. In rat type I astrocytes or C6 glioma cells, similar to S1P, HDL stimulated DNA synthesis and mRNA expression of fibroblast growth factor-2, a potent neurotrophic factor, which was associated with the activation of extracellular signal-regulated kinase (ERK) in a pertussis toxin-sensitive manner. The data from fractionation studies of HDL indicated that S1P may be a major component for the activation of ERK. In C6 glioma cells, HDL also induced phospholipase C-dependent intracellular Ca(2+) mobilization. Desensitization of the C6 glioma cells with S1P abolished these HDL-induced actions. Furthermore, overexpression of S1P receptors in C6 glioma cells led to a significant enhancement of HDL-induced ERK activation and Ca(2+) mobilization. Thus, at least some HDL-induced actions may be mediated by cell-surface S1P receptors in astroglial cells. These results imply that S1P might partially mediate lipoprotein-induced cholesterol metabolism-independent neural cell functions in the central nervous system.

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.

Does the coronary risk factor low density lipoprotein alter growth and signaling in vascular smooth muscle cells?

There is increasing evidence that hypertension promotes low density lipoprotein (LDL) transportation into the subendothelial space of the vascular wall. Vascular smooth muscle cell (VSMC) proliferation plays an important role in the development and progression of cardiovascular diseases. Recently, several studies have demonstrated that LDL acts as a classic growth factor promoting VSMC growth via mitogenic signals normally elicited by classic growth factors. The present work summarizes current nontraditional concepts regarding possible cellular mechanisms through which hypertension and LDL may promote the development of atherosclerosis. Especially addressed are the possible effects of an elevated blood pressure in combination with LDL on VSMC growth. The new research concept concerning LDL as a growth factor and carrier for biological active phospholipids such as sphingosine-1-phosphate and sphingosylphosphorylcholine may contribute to an understanding of the pathogenesis of atherosclerosis by elevated high blood pressure.

D-erythro-N,N-dimethylsphingosine inhibits bFGF-induced proliferation of cerebral, aortic and coronary smooth muscle cells

The role of sphingosine kinase (SphK) on basic fibroblast growth factor (bFGF)-induced proliferation of cerebral, aortic and coronary smooth muscle cells (SMC) was addressed using D-erythro-N,N-dimethylsphingosine (DMS), an inhibitor of SphK which blocks conversion of sphingosine to sphingosine-1-phosphate (S1P). DMS concentration-dependently reduced the bFGF-induced proliferation of rat cerebral and aortic, and human coronary SMC. This suggests that SphK is one of the key enzymes in the mitogenic response to bFGF in vascular SMC as supported by the finding that S1P stimulated proliferation of SMC. Fumonisin B1, a dihydroceramidesynthase inhibitor which blocks the conversion of dihydrosphingosine to seramide, did not affect SMC proliferation induced by bFGF. Staurosporine, an inhibitor of protein kinase C (PKC), inhibited proliferation of SMC induced by bFGF, and both bFGF- and S1P-induced proliferation of SMC was sensitive to pertussis toxin (PTX), an inhibitor of Gi-protein activity. The present study thus demonstrates that SphK, PKC and Gi-protein activities are required for bFGF-mitogenic signaling in SMC. The bFGF mitogenic effect in vascular SMC might at least in part act via the SphK pathway and a Gi-protein.

Two forms of membrane-bound sphingosine kinase in Tetrahymena and activity changes during growth and the cell cycle

Sphingosine kinase is responsible for the formation of sphingosine-1-phosphate, a sphingolipid mediator with important roles in numerous physiological processes. The sphingosine kinase activity of Tetrahymena pyriformis was recovered predominantly in the particulate fraction and it could be solubilised in 1% beta-octylglucoside. Anion-exchange chromatography resolved the beta-octylglucoside-solubilised sphingosine kinase activity into two peaks corresponding to proteins of Mr 140,000 and 80,000 respectively, as determined by subsequent size exclusion chromatography on Superdex 200. N,N-dimethylsphingosine did not inhibit the sphingosine kinase activity in either fraction, whereas D,L-threo-dihydrosphingosine inhibited sphingosine phosphorylation by the Mr 80,000 kinase but had no effect on the Mr 140,000 kinase activity. The activities also showed different stimulatory responses to Triton X-100 or NaCl. Overall, the results suggest the existence in Tetrahymena of two distinct membrane-associated sphingosine kinases. The kinase activity determined at the different culture stages showed a transient elevation at the mid-logarithmic phase. Further, the sphingosine kinase activity was examined during the synchronous cell division induced by cyclic heat treatments in T. pyriformis. We report for the first time that the sphingosine kinase activity greatly increased at 30 to 45 min after the end of heat treatment prior to the synchronous cell division (75 min), suggesting that the activity changes were associated with the cell cycle and that the up-regulated sphingosine kinase activity would be required for the initiation of the oncoming synchronous cell division in Tetrahymena cells.