Sphingosine-1-phosphate metabolism: A structural perspective

Sphingolipids represent an important class of bioactive signaling lipids which have key roles in numerous cellular processes. Over the last few decades, the levels of bioactive sphingolipids and/or their metabolizing enzymes have been realized to be important factors involved in disease development and progression, most notably in cancer. Targeting sphingolipid-metabolizing enzymes in disease states has been the focus of many studies and has resulted in a number of pharmacological inhibitors, with some making it into the clinic as therapeutics. In order to better understand the regulation of sphingolipid-metabolizing enzymes as well as to develop much more potent and specific inhibitors, the field of sphingolipids has recently taken a turn toward structural biology. The last decade has seen the structural determination of a number of sphingolipid enzymes and effector proteins. In these terms, one of the most complete arms of the sphingolipid pathway is the sphingosine-1-phosphate (S1P) arm. The structures of proteins involved in the function and regulation of S1P are being used to investigate further the regulation of said proteins as well as in the design and development of inhibitors as potential therapeutics.

Periodontal inflammation and alveolar bone loss induced by Aggregatibacter actinomycetemcomitans is attenuated in sphingosine kinase 1-deficient mice

BACKGROUND AND OBJECTIVE

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid, which is generated by activation of sphingosine kinase (SK) 1 and/or 2 in most mammalian cells with various stimuli, including the oral pathogen Aggregatibacter actinomycetemcomitans. S1P signaling has been shown to regulate the migration of monocytes and macrophages (osteoclast precursors) from the circulation to bone tissues and affect bone homeostasis. We aimed to determine the effects of SK1 deficiency on S1P generation, proinflammatory cytokine production, chemotaxis of monocytes and macrophages, and periodontitis induced by A. actinomycetemcomitans.

MATERIAL AND METHODS

Murine bone marrow-derived monocytes and macrophages (BMMs) from SK1 knockout (KO) mice or wild-type (WT) mice were either untreated or exposed to A. actinomycetemcomitans. The mRNA levels of SK1, SK2 and intracellular sphingolipid levels were quantified. In addition, murine WT BMMs were treated with vehicle, S1P, with or without A. actinomycetemcomitans and the mRNA levels of cyclooxygenase 2 (COX-2), interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF) were quantified. The protein levels of prostaglandin E2, IL-1β, IL-6 and TNF-α were quantified in the cell media of SK1 KO BMMs or WT BMMs with or without bacterial stimulation. Furthermore, a transwell migration assay was performed and the number of migrated WT BMMs in the presence of vehicle, bacteria-stimulated media, with or without S1P was quantified. Finally, in vivo studies were performed on SK1 KO and WT mice by injecting either phosphate-buffered saline or A. actinomycetemcomitans in the periodontal tissues. The mice maxillae were scanned by micro-computed tomography, and alveolar bone volume was analyzed. The number of periodontal leukocytes and osteoclasts were quantified in maxillary tissue sections.

RESULTS

SK1 mRNA levels significantly increased after A. actinomycetemcomitans stimulation in murine WT BMMs, but were undetectable in SK1 KO BMMs. Deficiency of SK1 in murine BMMs resulted in decreased S1P generation induced by A. actinomycetemcomitans as compared with WT BMMs. Additionally, low levels of S1P (≤ 1 μM) did not have a significant impact on the mRNA production of COX-2, IL-1β, IL-6 and TNF in murine BMMs with or without the presence of A. actinomycetemcomitans. There were no significant differences in prostaglandin E2 , IL-1β, IL-6 and TNF-α protein levels in the media between SK1 KO BMMs and WT BMMs with or without bacterial stimulation. Importantly, low levels of S1P (≤ 1 μM) dose-dependently promoted the chemotaxis of BMMs. The bacteria-stimulated media derived from SK1 BMMs significantly reduced the chemotaxis response compared with WT control. Finally, SK1 KO mice showed significantly attenuated alveolar bone loss stimulated by A. actinomycetemcomitans compared with WT mice treated with A. actinomycetemcomitans. Histological analysis of periodontal tissue sections revealed that SK1 KO mice treated with A. actinomycetemcomitans significantly reduced the number of infiltrated periodontal leukocytes and mature osteoclasts attached on the alveolar bone compared with WT mice.

CONCLUSION

Our studies support that SK1 and S1P play an important role in the inflammatory bone loss response induced by the oral pathogen A. actinomycetemcomitans. Reducing S1P generation by inhibiting SK1 has the potential as a novel therapeutic strategy for periodontitis and other inflammatory bone loss diseases.

The effect of the sphingosine-1-phosphate analogue FTY720 on atrioventricular nodal tissue

The sphingosine‐1‐phosphate (S1P) receptor modulator, fingolimod (FTY720), has been used for the treatment of patients with relapsing forms of multiple sclerosis, but atrioventricular (AV) conduction block have been reported in some patients after the first dose. The underlying mechanism of this AV node conduction blockade is still not well‐understood. In this study, we hypothesize that expression of this particular arrhythmia might be related to a direct effect of FTY720 on AV node rather than a parasympathetic mimetic action. We, therefore, investigated the effect of FTY720 on AV nodal, using in vitro rat model preparation, under both basal as well as ischaemia/reperfusion conditions. We first look at the expression pattern of S1P receptors on the AV node using real‐time PCR. Although all three S1P receptor isoforms were expressed in AVN tissues, S1P1 receptor isoform expression level was higher than S1P2 and S1P3. The effect of 25 nM FTY720 on cycle length (CL) was subsequently studied via extracellular potentials recordings. FTY720 caused a mild to moderate prolongation in CL by an average 9% in AVN (n = 10, P < 0.05) preparations. We also show that FTY720 attenuated both ischaemia and reperfusion induced AVN rhythmic disturbance. To our knowledge, these remarkable findings have not been previously reported in the literature, and stress the importance for extensive monitoring period in certain cases, especially in patients taking concurrently AV node blocker agents.

Sphingolipid metabolism in colorectal adenomas varies depending on histological architecture of polyps and grade of nuclear dysplasia

Incidence of colorectal cancer (CRC) is growing worldwide. Pathogenetic mechanisms responsible for its onset and progression need further clarification. Colorectal adenomatous polyps are precancerous lesions with malignant potential dependent on histological architecture and grade of nuclear dysplasia. One of the factors conditioning CRC development are abnormalities in sphingolipid metabolism. The aim of this study was to assess the levels of sphingolipids in human colorectal adenomas. The control group (C, n = 12) consisted of patients with no colonic polyps. The examined group consisted of patients with prior diagnosed colonic polyps, qualified to endoscopic polypectomy. This group was further divided due to histological architecture into tubular adenomas group (TA, n = 10), tubulovillous adenomas with low-grade dysplasia (LGD-TVA, n = 10), and tubulovillous adenomas group with high-grade dysplasia (HGD-TVA, n = 11). In tissue samples, sphingolipd metabolite contents were measured using high performance liquid chromatography (HPLC). In cases of polypoid lesions with low malignancy potential (tubular adenomas), concentration of ceramide, which is characterized by proapoptotic and anti-proliferative properties, increases compared with control group (p < 0.05), whereas content of sphingosine-1-phosphate with anti-apoptotic and stimulating cellular proliferation properties is reduced in comparison with control group (p < 0.05). On the contrary, in cases of more advanced form of adenomatous polyps (tubulovillous adenomas with high-grade dysplasia), the ceramide level decreases compared with control group (p < 0.05) while sphingosine-1-phosphate concentration is elevated (p < 0.05). We found that concentrations of pro-apoptotic ceramide are decreased and pro-proliferative S1P levels are increased in polypoid lesions with high malignancy potential, and it was the opposite in those with low malignancy potential.

Targeting sphingolipid metabolism in the treatment of obesity/type 2 diabetes

INTRODUCTION

Obesity is a major factor that is linked to the development of type 2 diabetes (T2D). Excess circulating fatty acids (FAs), which characterize obesity, induce insulin resistance, steatosis, β cells dysfunction and apoptosis. These deleterious effects have been defined as lipotoxicity.

AREAS COVERED

FAs are metabolized to different lipid species, including ceramides which play a crucial role in lipotoxicity. The action of ceramides on tissues, such as muscle, liver, adipose tissue and pancreatic β cells, during the development of T2D will also be reviewed. In addition, the potential antagonist action of other sphingolipids, namely sphingoid base phosphates, on lipotoxicity in skeletal muscle and β cells will be addressed.

EXPERT OPINION

Ceramide is a critical mediator to the development of T2D linked to obesity. Targeting proteins involved in ceramide's deleterious action has not been possible due to their involvement in many other intracellular signaling pathways. A possible means of counteracting ceramide action would be to prevent the accumulation of the specific ceramide species involved in both insulin resistance and β-cell apoptosis/dysfunction. Another possibility would be to adjust the dynamic balance between ceramide and sphingoid base phosphate, both known to display opposing properties on the development of T2D-linked obesity.

Actin polymerization-enhancing drugs promote ovarian follicle growth mediated by the Hippo signaling effector YAP

Hippo signaling pathway consists of conserved serine/threonine kinases to maintain optimal organ sizes. Studies have demonstrated that fragmentation of murine ovaries increases actin polymerization and disrupts Hippo signaling, leading to nuclear translocation of Hippo signaling effector Yes-associated protein (YAP) in ovarian follicles and follicle growth. For patients with polycystic ovarian syndrome showing follicle arrest, ovarian wedge resection and laser drilling promote follicle growth. Because these damaging procedures likely involve actin polymerization, we tested whether actin polymerization-promoting drugs could promote YAP translocation and stimulate follicle growth. Treatment of murine ovaries with μM Jasplakinolide (JASP), an actin polymerization-promoting cyclic peptide, or sphingosine-1-phosphate (S1P), a follicular fluid constituent known to promote actin polymerization, increased the conversion of globular actin to the filamentous form, followed by increased nuclear YAP and expression of downstream connective tissue growth factor (CCN2). After short-term treatments with JASP or S1P, in vitro cultured and in vivo grafted ovaries showed follicle growth. Furthermore, induction of constitutively active YAP in ovarian grafts of transgenic mice enhanced follicle development, whereas treatment of human ovarian cortices with JASP or S1P increased CCN2 expression. Thus, JASP and S1P stimulate follicle growth and are potential therapeutic agents for treating polycystic ovarian syndrome and other ovarian disorders.

Overview and safety of fingolimod hydrochloride use in patients with multiple sclerosis

INTRODUCTION

Fingolimod (Gilenya®, FTY720) is an oral sphingosine-1-phosphate analogue that was approved by the FDA in 2010 for the treatment of relapsing forms of multiple sclerosis (MS). Fingolimod's mechanism of action is primarily related to lymphocyte sequestration in primary and secondary lymphoid tissues. Phase III trials demonstrated a reduction in annualized relapse rate and MRI progression in fingolimod-treated subjects compared with both placebo and IFN-β-treated subjects. Frequent adverse effects include fatigue, gastrointestinal disturbance, headache and upper respiratory tract infection. More serious, but rare, adverse events associated with fingolimod include atrioventricular block, symptomatic bradycardia, herpetic viral infections and macular edema.

AREAS COVERED

We discuss the mechanism of action, pharmacokinetics, clinical efficacy and safety profile of fingolimod in patients with relapsing MS.

EXPERT OPINION

Fingolimod is an effective treatment for relapsing MS and its oral route of administration may be preferred by some. Fingolimod is generally well tolerated but requires diligence in patient selection and monitoring. Additional information is needed regarding risk of infection, malignancy and rebound disease with long-term use of fingolimod.

Patient experience and practice trends in multiple sclerosis - clinical utility of fingolimod

Targeting sphingosine-1-phosphate pathway with orally available immune-modulatory fingolimod (Gilenya™) therapy ameliorates relapsing-remitting multiple sclerosis (RRMS) by decreasing relapse rate as shown in FREEDOMS and TRANSFORMS. Fingolimod has also been shown to be superior to interferon-beta therapy as evidenced by TRANSFORMS. Albeit multiple benefits in treatment of multiple sclerosis including high efficacy and ease of administration, potential untoward effects such as cardiotoxicity, risk of infection, and cancer exist, thus mandating careful screening and frequent monitoring of patients undergoing treatment with fingolimod. This review outlines mechanism of action, observations, side effects, and practice guidelines on use of fingolimod in treatment of RRMS.

Human T-Cell Development and Thymic Egress: An Infectious Disease Perspective

Emigration of mature naïve CD4 SP T cells from the human thymus to the periphery is not fully understood, although elucidation of the mechanisms that govern egress of T cells is crucial to understanding both basic immunology and the immune response in diseases such as HIV infection. Recent work has brought to light the requirement for sphingosine-1-phosphate (S1P) and its receptors in a variety of fields including mature naïve T-cell egress from the thymus of mice. We are examining the expression and function of this novel requisite T-cell egress receptor within the human thymus, characterizing changes observed in the expression and function of this receptor in infectious diseases. To perform this work, we use a variety of humanized murine models reviewed in this article. Future work in the field of T-cell egress, especially as it pertains to S1P receptors, should advance the fields of basic T-cell immunology and immunopathology and open new avenues for exploration into novel therapeutics.

Sphingosine-1-phosphate Receptor Agonism Reduces Bordetella pertussis-mediated Lung Pathology

Recent pertussis resurgence represents a major public health concern. Currently, there are no effective treatments for critical pertussis in infants. Recent data have demonstrated the potential of sphingosine-1-phosphate receptor (S1PR) agonism in the treatment of infectious diseases. We used the murine Bordetella pertussis model to test the hypothesis that treatment with S1PR agonist AAL-R reduces pulmonary inflammation during infection. AAL-R treatment resulted in reduced expression of inflammatory cytokines and chemokines and attenuated lung pathology in infected mice. These results demonstrate a role for sphingosine-1-phosphate (S1P) signaling in B. pertussis-mediated pathology and highlight the possibility of host-targeted therapy for pertussis.

The sphingosine-1-phosphate/sphingosine-1-phosphate receptor 2 axis regulates early airway T-cell infiltration in murine mast cell-dependent acute allergic responses

BACKGROUND

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid produced by mast cells (MCs) on cross-linking of their high-affinity receptors for IgE by antigen that can amplify MC responses by binding to its S1P receptors. An acute MC-dependent allergic reaction can lead to systemic shock, but the early events of its development in lung tissues have not been investigated, and S1P functions in the onset of allergic processes remain to be examined.

OBJECTIVE

We used a highly specific neutralizing anti-S1P antibody (mAb) and the sphingosine-1-phosphate receptor 2 (S1PR2) antagonist JTE-013 to study the signaling contributions of S1P and S1PR2 to MC- and IgE-dependent airway allergic responses in mice within minutes after antigen challenge.

METHODS

Allergic reaction was triggered by a single intraperitoneal dose of antigen in sensitized mice pretreated intraperitoneally with anti-S1P, isotype control mAb, JTE-013, or vehicle before antigen challenge.

RESULTS

Kinetics experiments revealed early pulmonary infiltration of mostly T cells around blood vessels of sensitized mice 20 minutes after antigen exposure. Pretreatment with anti-S1P mAb inhibited in vitro MC activation, as well as in vivo development of airway infiltration and MC activation, reducing serum levels of histamine, cytokines, and the chemokines monocyte chemoattractant protein 1/CCL2, macrophage inflammatory protein 1α/CCL3, and RANTES/CCL5. S1PR2 antagonism or deficiency or MC deficiency recapitulated these results. Both in vitro and in vivo experiments demonstrated MC S1PR2 dependency for chemokine release and the necessity for signal transducer and activator of transcription 3 activation.

CONCLUSION

Activation of S1PR2 by S1P and downstream signal transducer and activator of transcription 3 signaling in MCs regulate early T-cell recruitment to antigen-challenged lungs through chemokine production.

Benzoxazolone carboxamides: potent and systemically active inhibitors of intracellular acid ceramidase

The ceramides are a family of bioactive lipid-derived messengers involved in the control of cellular senescence, inflammation, and apoptosis. Ceramide hydrolysis by acid ceramidase (AC) stops the biological activity of these substances and influences survival and function of normal and neoplastic cells. Because of its central role in the ceramide metabolism, AC may offer a novel molecular target in disorders with dysfunctional ceramide-mediated signaling. Here, a class of benzoxazolone carboxamides is identified as the first potent and systemically active inhibitors of AC. Prototype members of this class inhibit AC with low nanomolar potency by covalent binding to the catalytic cysteine. Their metabolic stability and high in vivo efficacy suggest that these compounds may be used as probes to investigate the roles of ceramide in health and disease, and that this scaffold may represent a promising starting point for the development of novel therapeutic agents.

Harnessing the power of yeast to elucidate the role of sphingolipids in metabolic and signaling processes pertinent to psychiatric disorders

The development of therapies for neuropsychiatric disorders is hampered by the lack of understanding of the mechanisms underlying their pathologies. While aberrant sphingolipid metabolism is associated with psychiatric illness, the role of sphingolipids in these disorders is not understood. The genetically tractable yeast model can be exploited in order to elucidate the cellular consequences of sphingolipid perturbation. Hypotheses generated from studies in yeast and tested in mammalian cells may contribute to our understanding of the role of sphingolipids in psychiatric disorders and to the development of new treatments. Here, we compare sphingolipid metabolism in yeast and mammalian cells, discuss studies implicating sphingolipids in psychiatric disorders and propose approaches that utilize yeast in order to elucidate sphingolipid function and identify drugs that target sphingolipid synthesis.

Sphingosine-1-phosphate induces pro-remodelling response in airway smooth muscle cells

Background

Increased proliferation of airway smooth muscle (ASM) cells leading to hyperplasia and increased ASM mass is one of the most characteristic features of airway remodelling in asthma. A bioactive lipid, sphingosine-1-phosphate (S1P), has been suggested to affect airway remodelling by stimulation of human ASM cell proliferation.

Objective

To investigate the effect of S1P on signalling and regulation of gene expression in ASM cells from healthy and asthmatic individuals.

Methods

Airway smooth muscle cells grown from bronchial biopsies of healthy and asthmatic individuals were exposed to S1P. Gene expression was analysed using microarray, real-time PCR and Western blotting. Receptor signalling and function were determined by mRNA knockdown and intracellular calcium mobilization experiments.

Results

S1P potently regulated the expression of more than 80 genes in human ASM cells, including several genes known to be involved in the regulation of cell proliferation and airway remodelling (HBEGF, TGFB3, TXNIP, PLAUR, SERPINE1, RGS4). S1P acting through S1P₂ and S1P₃ receptors activated intracellular calcium mobilization and extracellular signal-regulated and Rho-associated kinases to regulate gene expression. S1P-induced responses were not inhibited by corticosteroids and did not differ significantly between ASM cells from healthy and asthmatic individuals.

Conclusion

S1P induces a steroid-resistant, pro-remodelling pathway in ASM cells. Targeting S1P or its receptors could be a novel treatment strategy for inhibiting airway remodelling in asthma.

Emerging Role of Sphingosine-1-phosphate in Inflammation, Cancer, and Lymphangiogenesis. Biomolecules. 2013;3. [PMID: 24286034 PMCID: PMC3839861 DOI: 10.3390/biom3030408]

The main function of the lymphatic system is to control and maintain fluid homeostasis, lipid transport, and immune cell trafficking. In recent years, the pathological roles of lymphangiogenesis, the generation of new lymphatic vessels from preexisting ones, in inflammatory diseases and cancer progression are beginning to be elucidated. Sphingosine-1-phosphate (S1P), a bioactive lipid, mediates multiple cellular events, such as cell proliferation, differentiation, and trafficking, and is now known as an important mediator of inflammation and cancer. In this review, we will discuss recent findings showing the emerging role of S1P in lymphangiogenesis, in inflammation, and in cancer.

Roles of Sphingolipid Metabolism in Pancreatic β Cell Dysfunction Induced by Lipotoxicity

Pancreatic β cells secrete insulin in order to maintain glucose homeostasis. However, various environmental stresses such as obesity have been shown to induce loss of secretory responsiveness in pancreatic β cells and pancreatic β cell apoptosis which can favor the development of type 2 diabetes (T2D). Indeed, elevated levels of free fatty acids (FFAs) have been shown to induce β cell apoptosis. Importantly, the chronic adverse effects of FFAs on β cell function and viability are potentiated in the presence of hyperglycaemia, a phenomenon that has been termed gluco-lipotoxicity. The molecular mechanisms underlying the pathogenesis of gluco-lipotoxicity in pancreatic β cells are not completely understood. Recent studies have shown that sphingolipid metabolism plays a key role in gluco-lipotoxicity induced apoptosis and loss of function of pancreatic β cells. The present review focuses on how the two main sphingolipid mediators, ceramides and sphingoid base-1-phosphates, regulate the deleterious effects of gluco-lipotoxicity on pancreatic β cells. The review highlights the role of a sphingolipid biostat on the dysregulation of β cell fate and function induced by gluco-lipotoxicity, offering the possibility of new therapeutic targets to prevent the onset of T2D.

Albumin modulates S1P delivery from red blood cells in perfused microvessels: mechanism of the protein effect

Removal of plasma proteins from perfusates increases vascular permeability. The common interpretation of the action of albumin is that it forms part of the permeability barrier by electrostatic binding to the endothelial glycocalyx. We tested the alternate hypothesis that removal of perfusate albumin in rat venular microvessels decreased the availability of sphingosine-1-phosphate (S1P), which is normally carried in plasma bound to albumin and lipoproteins and is required to maintain stable baseline endothelial barriers (Am J Physiol Heart Circ Physiol 303: H825-H834, 2012). Red blood cells (RBCs) are a primary source of S1P in the normal circulation. We compared apparent albumin permeability coefficients [solute permeability (Ps)] measured using perfusates containing albumin (10 mg/ml, control) and conditioned by 20-min exposure to rat RBCs with Ps when test perfusates were in RBC-conditioned protein-free Ringer solution. The control perfusate S1P concentration (439 ± 46 nM) was near the normal plasma value at 37 °C and established a stable baseline Ps (0.9 ± 0.4 × 10(-6) cm/s). Ringer solution perfusate contained 52 ± 8 nM S1P and increased Ps more than 10-fold (16.1 ± 3.9 × 10(-6) cm/s). Consistent with albumin-dependent transport of S1P from RBCs, S1P concentrations in RBC-conditioned solutions decreased as albumin concentration, hematocrit, and temperature decreased. Protein-free Ringer solution perfusates that used liposomes instead of RBCs as flow markers failed to maintain normal permeability, reproducing the "albumin effect" in these mammalian microvessels. We conclude that the albumin effect depends on the action of albumin to facilitate the release and transport of S1P from RBCs that normally provide a significant amount of S1P to the endothelium.

Multiple sclerosis: current and emerging disease-modifying therapies and treatment strategies

Multiple sclerosis (MS) is a chronic inflammatory demyelinating central nervous system disease that typically strikes young adults, especially women. The pathobiology of MS includes inflammatory and neurodegenerative mechanisms that affect both white and gray matter. These mechanisms underlie the relapsing, and often eventually progressive, course of MS, which is heterogeneous; confident prediction of long-term individual prognosis is not yet possible. However, because revised MS diagnostic criteria that incorporate neuroimaging data facilitate early diagnosis, most patients are faced with making important long-term treatment decisions, most notably the use and selection of disease-modifying therapy (DMT). Currently, there are 10 approved MS DMTs with varying degrees of efficacy for reducing relapse risk and preserving neurological function, but their long-term benefits remain unclear. Moreover, available DMTs differ with respect to the route and frequency of administration, tolerability and likelihood of treatment adherence, common adverse effects, risk of major toxicity, and pregnancy-related risks. Thorough understanding of the benefit-risk profiles of these therapies is necessary to establish logical and safe treatment plans for individuals with MS. We review the available evidence supporting risk-benefit profiles for available and emerging DMTs. We also assess the place of individual DMTs within the context of several different MS management strategies, including those currently in use (sequential monotherapy, escalation therapy, and induction and maintenance therapy) and others that may soon become feasible (combination approaches and "personalized medicine"). We conducted this review using a comprehensive search of MEDLINE, PubMed, EMBASE, Cochrane Database of Systematic Reviews, and Cochrane Central Register of Controlled Trials, from January 1, 1990, to August 31, 2013. The following search terms were used: multiple sclerosis, randomized controlled trials, interferon-beta, glatiramer acetate, mitoxantrone, natalizumab, fingolimod, teriflunomide, dimethyl fumarate, BG-12, alemtuzumab, rituximab, ocrelizumab, daclizumab, neutralizing antibodies, progressive multifocal leukoencephalopathy.

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.

RP-HPLC-fluorescence analysis of aliphatic aldehydes: application to aldehyde-generating enzymes HACL1 and SGPL1

Long-chain aldehydes are commonly produced in various processes, such as peroxisomal α-oxidation of long-chain 3-methyl-branched and 2-hydroxy fatty acids and microsomal breakdown of phosphorylated sphingoid bases. The enzymes involved in the aldehyde-generating steps of these processes are 2-hydroxyacyl-CoA lyase (HACL1) and sphingosine-1-phosphate lyase (SGPL1), respectively. In the present work, nonradioactive assays for these enzymes were developed employing the Hantzsch reaction. Tridecanal (C13-al) and heptadecanal (C17-al) were selected as model compounds and cyclohexane-1,3-dione as 1,3-diketone, and the fluorescent derivatives were analyzed by reversed phase (RP)-HPLC. Assay mixture composition, as well as pH and heating, were optimized for C13-al and C17-al. Under optimized conditions, these aldehydes could be quantified in picomolar range and different long-chain aldehyde derivatives were well resolved with a linear gradient elution by RP-HPLC. Aldehydes generated by recombinant enzymes could easily be detected via this method. Moreover, the assay allowed to document activity or deficiency in tissue homogenates and fibroblast lysates without an extraction step. In conclusion, a simple, quick, and cheap assay for the study of HACL1 and SGPL1 activities was developed, without relying on expensive mass spectrometric detectors or radioactive substrates.

3-amino-4-(3-hexylphenylamino)-4-oxobutyl phosphonic acid (W146), a Selective Antagonist of Sphingosine-1-phospahte Receptor Subtype 1, Enhances AMD3100-stimulated Mobilization of Hematopoietic Stem Progenitor Cells in Animals

Sphingosine-1-phosphate (S1P), a serum-borne bioactive lipid, regulates various physiological functions. We observed that the S1P receptor subtype 1 (S1P₁), a high affinity G-protein coupled receptor of S1P, is the major S1P receptor expressed in the Kit⁺/Sca-1⁺/Lin^- (KSL) hematopoietic stem progenitor cells (HSPCs, KSL-HSPCs). In this study, we investigate function of S1P₁ receptors in the regulation of HSPC mobilization in animals. Treatment with SEW2871, a specific agonist of S1P₁, had no effect on KSL-HSPC mobilization. In addition, mice pretreated with SEW2871 followed by AMD3100, a well-known activator of KSL-HSPC mobilization by antagonizing the stromal-derived factor-1 (SDF-1)/C-X-C chemokine receptor type 4 (CXCR4) signaling axis, did not enhance the AMD3100-induced KSL-HSPC mobilization. In contrast, pretreatment of (R)-3-amino-4-(3-hexylphenylamino)-4-oxobutyl phosphonic acid (W146), a selective antagonist of S1P₁, significantly augments AMD3100-induced KSL-HSPC mobilization into peripheral blood. The inactive enantiomer W140 was incapable of enhancing the AMD3100-induced KSL-HSPC mobilization. Moreover, treatment with selective antagonists for S1P₂ and S1P₃ had no effects on AMD3100-mediated KSL-HSPC mobilization. Collectively, our data suggest that S1P/S1P₁ signaling regulates the SDF-1/CXCR4-mediated retention of KSL-HSPCs in bone marrow microenvironment.

Activation of Src and release of intracellular calcium by phosphatidic acid during Xenopus laevis fertilization

We report a new step in the fertilization in Xenopus laevis which has been found to involve activation of Src tyrosine kinase to stimulate phospholipase C-γ (PLC-γ) which increases inositol 1,4,5-trisphosphate (IP3) to release intracellular calcium ([Ca](i)). Molecular species analysis and mass measurements suggested that sperm activate phospholipase D (PLD) to elevate phosphatidic acid (PA). We now report that PA mass increased 2.7 fold by 1 min after insemination and inhibition of PA production by two methods inhibited activation of Src and PLCγ, increased [Ca](i) and other fertilization events. As compared to 14 other lipids, PA specifically bound Xenopus Src but not PLCγ. Addition of synthetic PA activated egg Src (an action requiring intact lipid rafts) and PLCγ as well as doubling the amount of PLCγ in rafts. In the absence of elevated [Ca](i), PA addition elevated IP3 mass to levels equivalent to that induced by sperm (but twice that achieved by calcium ionophore). Finally, PA induced [Ca](i) release that was blocked by an IP3 receptor inhibitor. As only PLD1b message was detected, and Western blotting did not detect PLD2, we suggest that sperm activate PLD1b to elevate PA which then binds to and activates Src leading to PLCγ stimulation, IP3 elevation and [Ca](i) release. Due to these and other studies, PA may also play a role in membrane fusion events such as sperm-egg fusion, cortical granule exocytosis, the elevation of phosphatidylinositol 4,5-bisphosphate and the large, late increase in sn 1,2-diacylglycerol in fertilization.

Systemic treatment with the sphingosine-1-phosphate analog FTY720 does not improve fracture healing in mice

Sphingosine-1-phosphate (S1P) has recently been recognized as a crucial coupling molecule of osteoclast and osteoblast activity provoking osteoanabolic effects. Targeting S1P receptors could, therefore, be a potential strategy to support bone formation in osteopenic diseases or in fracture repair. Here we investigated whether systemic treatment with the S1P analog FTY720 (Fingolimod) could improve fracture healing. Twelve-week-old, female C57BL/6 mice received an osteotomy of the femur, which was stabilized using an external fixator. The mice received a daily subcutaneous injection of either FTY720 (6 mg/kg) or vehicle from the third postoperative day. Fracture healing was evaluated after 10 and 21 days using biomechanical testing, µ-computed tomography, and histomorphometry. Because FTY720 is supposed to influence osteoclast recruitment, osteoclasts were identified in the fracture callus by staining for tartrate resistant acid phosphatase (TRAP). There were no significant differences in callus mechanical properties, tissue composition and osteoclast number between the groups, suggesting that systemically applied FTY720 did not influence bone regeneration in this model of regular fracture healing. Even if further studies should test the potency of FTY720 under unfavorable healing conditions, we conclude that the effect of systemically applied FTY720 on fracture healing might be inferior compared to other anabolic treatments. © 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31:1845-1850, 2013.

Sphingosine-1-phosphate stimulated connective tissue growth factor expression in human buccal fibroblasts: Inhibition by epigallocatechin-3-gallate

BACKGROUND/PURPOSE

Connective tissue growth factor (CCN2) has been associated with the pathogenesis of various fibrotic diseases, including oral submucous fibrosis (OSF). The chemical constituents of areca nut along with the mechanical trauma cause OSF. The coarse fibers of areca nut injure the mucosa and hence sphingosine-1-phosphate (S1P) is released at the wounded sites. Recent studies have shown that S1P is involved in wound healing and the development of fibrosis. The aims of this study were to investigate the effects of S1P on CCN2 expression in human buccal fibroblasts (HBFs) and identify the potential targets for drug intervention or chemoprevention of OSF.

METHODS

Western blot analyses were used to study the effects of S1P on CCN2 expression and its signaling pathways in HBFs and whether epigallocatechin-3-gallate (EGCG), the main and most significant polyphenol in green tea, could inhibit this pathway.

RESULTS

S1P significantly enhanced CCN2 synthesis in HBFs. This effect can be inhibited by c-Jun NH2-terminal kinase (JNK) inhibitor and extracellular signal-regulated kinase inhibitor but not by P38 mitogen-activated protein kinase inhibitor. Interestingly, EGCG completely blocked S1P-induced CCN2 expression via suppressing S1P-induced JNK phosphorylation.

CONCLUSION

S1P released by repetitive mechanical trauma during AN chewing may contribute to the pathogenesis of OSF through upregulating CCN2 expression in HBFs. EGCG could be an adjuvant to the current offered therapy options or the prevention of OSF through suppression of JNK activation.

Targeting ovarian cancer and chemoresistance through selective inhibition of sphingosine kinase-2 with ABC294640

ABC294640, a selective inhibitor of sphingosine kinase-2, inhibits the formation of sphingosine 1-phosphate (S1P), a signaling lipid implicated in promoting tumor survival. We investigated the anticancer activity of ABC294640 in two ovarian cancer cell lines, BG-1 and Caov-3. ABC294640 dose-dependently inhibited clonogenic survival and cell viability of both ovarian cancer lines in vitro. Using enzyme-linked immunosorbant assays and western blot detection in chemoresistant Caov-3 cells, treatment with ABC294640 alone also potentiated bcl-2-associated X-protein and caspase-9 transcription levels, although it did not significantly increase apoptotic cell death. Interestingly, ABC294640 administered to Caov-3 ovarian cancer cells in conjunction with paclitaxel induced apoptotic cell death through activation of caspase-9. Induction of apoptosis may mediate the anticancer effect of ABC294640 in ovarian cancer, although its precise antitumor mechanism is unclear. Ultimately, through its inhibition of S1P formation and subsequent effects on critical survival signaling cascades, ABC294640 may prove to be a useful adjunct to help re-sensitize tumors to standard therapy.

Decoding cell death signals in liver inflammation

Inflammation can be either beneficial or detrimental to the liver, depending on multiple factors. Mild (i.e., limited in intensity and destined to resolve) inflammatory responses have indeed been shown to exert consistent hepatoprotective effects, contributing to tissue repair and promoting the re-establishment of homeostasis. Conversely, excessive (i.e., disproportionate in intensity and permanent) inflammation may induce a massive loss of hepatocytes and hence exacerbate the severity of various hepatic conditions, including ischemia-reperfusion injury, systemic metabolic alterations (e.g., obesity, diabetes, non-alcoholic fatty liver disorders), alcoholic hepatitis, intoxication by xenobiotics and infection, de facto being associated with irreversible liver damage, fibrosis, and carcinogenesis. Both liver-resident cells (e.g., Kupffer cells, hepatic stellate cells, sinusoidal endothelial cells) and cells that are recruited in response to injury (e.g., monocytes, macrophages, dendritic cells, natural killer cells) emit pro-inflammatory signals including - but not limited to - cytokines, chemokines, lipid messengers, and reactive oxygen species that contribute to the apoptotic or necrotic demise of hepatocytes. In turn, dying hepatocytes release damage-associated molecular patterns that-upon binding to evolutionary conserved pattern recognition receptors-activate cells of the innate immune system to further stimulate inflammatory responses, hence establishing a highly hepatotoxic feedforward cycle of inflammation and cell death. In this review, we discuss the cellular and molecular mechanisms that account for the most deleterious effect of hepatic inflammation at the cellular level, that is, the initiation of a massive cell death response among hepatocytes.

Decoding cell death signals in liver inflammation

Inflammation can be either beneficial or detrimental to the liver, depending on multiple factors. Mild (i.e., limited in intensity and destined to resolve) inflammatory responses have indeed been shown to exert consistent hepatoprotective effects, contributing to tissue repair and promoting the re-establishment of homeostasis. Conversely, excessive (i.e., disproportionate in intensity and permanent) inflammation may induce a massive loss of hepatocytes and hence exacerbate the severity of various hepatic conditions, including ischemia-reperfusion injury, systemic metabolic alterations (e.g., obesity, diabetes, non-alcoholic fatty liver disorders), alcoholic hepatitis, intoxication by xenobiotics and infection, de facto being associated with irreversible liver damage, fibrosis, and carcinogenesis. Both liver-resident cells (e.g., Kupffer cells, hepatic stellate cells, sinusoidal endothelial cells) and cells that are recruited in response to injury (e.g., monocytes, macrophages, dendritic cells, natural killer cells) emit pro-inflammatory signals including - but not limited to - cytokines, chemokines, lipid messengers, and reactive oxygen species that contribute to the apoptotic or necrotic demise of hepatocytes. In turn, dying hepatocytes release damage-associated molecular patterns that-upon binding to evolutionary conserved pattern recognition receptors-activate cells of the innate immune system to further stimulate inflammatory responses, hence establishing a highly hepatotoxic feedforward cycle of inflammation and cell death. In this review, we discuss the cellular and molecular mechanisms that account for the most deleterious effect of hepatic inflammation at the cellular level, that is, the initiation of a massive cell death response among hepatocytes.

Sphingosine-1-phosphate: Driver of NFκB and STAT3 persistent activation in chronic intestinal inflammation and colitis-associated cancer

Inflammatory bowel disease is associated with an increased risk of colorectal cancer. Colitis-associated cancer is a classical inflammation-driven cancer in which constitutive NFκB and STAT3 activation drive tumorigenesis. Recent findings published by Liang et al. in Cancer Cell demonstrate that sphingosine kinase 1 (SphK1)-mediated upregulation of sphingosine-1-phosphate (S1P) drives a persistent NFκB/IL-6/STAT3/sphingosine-1-phosphate receptor 1 (S1PR1) amplification loop that is critical to the development of chronic colitis and colitis-associated cancer. FTY720, an antagonist of S1PR1, abolished persistent NFκB/IL-6/STAT3 signaling and reduced the development and progression of colitis-associated cancer. Targeting SphK1/S1P/S1PR1 may provide a therapeutic option to prevent the progression of colitis to cancer.

Sphingosine kinase and sphingosine-1-phosphate: regulators in autoimmune and inflammatory disease

Sphingolipids and their metabolizing enzymes are beginning to be recognized as critical mediators in biological processes, specifically in inflammation and autoimmunity. Sphingosine kinases (SKs) and their lipid product sphingosine-1-phosphate (S1P) play essential roles in inflammatory signaling processes, as well as disease development and progression. SKs can be activated by numerous growth factors and cytokines, including TNF-α and IL-1β, leading to the generation of S1P. S1P exerts its biological effects on intracellular and extracellular targets, such as S1P receptors. In addition to roles in inflammatory signaling pathways SKs, S1P and S1P receptors have been implicated in immune cell function and trafficking, specifically in lymphocytes. This review will discuss the contribution of the bioactive sphingolipid S1P, its generating enzyme SK, and its cell surface receptors in the inflammatory and autoimmune diseases systemic lupus erythematosus, arthritis and inflammatory bowel disease.

Sphingosine Phosphate Lyase Regulates Murine Embryonic Stem Cell Proliferation and Pluripotency through an S1P2/STAT3 Signaling Pathway. Biomolecules. 2013;3:351-368. [PMID: 24619572 PMCID: PMC3947625 DOI: 10.3390/biom3030351]

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that activates a family of G protein coupled-receptors (GPCRs) implicated in mammalian development, angiogenesis, immunity and tissue regeneration. S1P functions as a trophic factor for many cell types, including embryonic stem cells (ESCs). Sphingosine phosphate lyase (SPL) is an intracellular enzyme that catalyzes the irreversible degradation of S1P. We found SPL to be highly expressed in murine ESCs (mESCs). To investigate the role of SPL in mESC biology, we silenced SPL in mESCs via stable transfection with a lentiviral SPL-specific short hairpin RNA (shRNA) construct. SPL-knockdown (SPL-KD) mESCs showed a 5-fold increase in cellular S1P levels, increased proliferation rates and high expression of cell surface pluripotency markers SSEA1 and OCT4 compared to vector control cells. Compared to control mESCs, SPL-KD cells showed robust activation of STAT3 and a 10-fold increase in S1P₂ expression. Inhibition of S1P₂ or STAT3 reversed the proliferation and pluripotency phenotypes of SPL-KD mESCs. Further, inhibition of S1P₂ attenuated, in a dose-dependent fashion, the high levels of OCT4 and STAT3 activation observed in SPL-KD mESCs. Finally, we showed that SPL-KD cells are capable of generating embryoid bodies from which muscle stem cells, called satellite cells, can be isolated. These findings demonstrate an important role for SPL in ESC homeostasis and suggest that SPL inhibition could facilitate ex vivo ESC expansion for therapeutic purposes.

Antiapoptotic agent sphingosine-1-phosphate protects vitrified murine ovarian grafts

Significant follicle loss from frozen ovarian grafts is unavoidable. The authors evaluated the protective effects of the antiapoptotic agent sphingosine-1-phosphate (S1P) on vitrified ovarian grafts. Three-week-old sexually immature female FVB mice were divided into 4 groups, fresh, control without S1P, 0.5 mmol/L S1P, and 2 mmol/L S1P. The ovaries were pretreated with S1P for 1 hour and then cryopreserved by modified vitrification. The frozen-thawed ovaries were autotransplanted under the back muscles of mice for 10 days. Expression of apoptosis-related genes encoding caspase 3 and c-Myc was analyzed in the vitrified ovaries and 10 days after transplantation using real-time quantitative polymerase chain reaction. To quantify the ovarian reserve, anti-Müllerian hormone (AMH) levels and follicles were measured in the 10-day vitrified ovarian grafts. Caspase 3 and c-Myc messenger RNA did not differ significantly in the 4 groups after vitrification but was significantly upregulated in the control group after transplantation. The AMH levels and primordial follicle pool were significantly higher in the S1P-treated groups than in the control group but lower than that in the fresh group. The S1P protects vitrified ovarian grafts from ischemic reperfusion injury rather than from vitrification-associated process.

Sphingosine-1-phosphate receptor-2 mediated NFκB activation contributes to tumor necrosis factor-α induced VCAM-1 and ICAM-1 expression in endothelial cells

Sphingosine-1-phosphate (S1P) regulates a wide array of biological functions in endothelial cells. We previously showed that S1P receptor subtype 2 (S1P2) is significantly up-regulated in the atherosclerotic endothelium (J. Biol. Chem. 283:30363, 2008). In this study, we investigated the roles of S1P2-mediated signaling in the proinflammatory responses of endothelial cells. Treatment with tumor necrosis factor-α (TNFα), a proinflammatory cytokine, increased the expression of S1P2 receptors in endothelial cells. TNFα treatment also enhanced sphingosine kinase 1 expression and increased S1P production. Pharmacological inhibition or knockdown of S1P2 receptors completely abrogated the TNFα-induced VCAM-1 (vascular cell adhesion molecule 1) and ICAM-1 (intercellular adhesion molecule 1) expression in endothelial cells. In contrast, pharmacological inhibition or knockdown of other S1P receptor subtypes had no effect on the TNFα-stimulated ICAM-1 and VCAM-1 expression. Moreover, ectopic expression of S1P2 receptors increased VCAM-1 and ICAM-1 expression in endothelial cells in response to S1P stimulation. Mechanistically, we show that antagonizing S1P2 signaling markedly inhibited the TNFα-stimulated NFκB activation. Utilizing the NFκB reporter luciferase assay, the S1P/S1P2 signaling was shown to stimulate NFκB activation. Moreover, the S1P/S1P2-stimulated VCAM-1/ICAM-1 expression was completely abolished by the pharmacological inhibitor of NFκB. Collectively, our data suggest that TNFα treatment activates autocrine S1P/S1P2 signaling, which subsequently activates NFκB and leads to the proinflammatory responses in endothelial cells.

Roles, regulation and inhibitors of sphingosine kinase 2

The bioactive sphingolipids ceramide, sphingosine and sphingosine-1-phosphate (S1P) are important signalling molecules that regulate a diverse array of cellular processes. Most notably, the balance of the levels of these three sphingolipids in cells, termed the 'sphingolipid rheostat', can dictate cell fate, where ceramide and sphingosine enhance apoptosis and S1P promotes cell survival and proliferation. The sphingosine kinases (SKs) catalyse the production of S1P from sphingosine and are therefore central regulators of the sphingolipid rheostat and attractive targets for cancer therapy. Two SKs exist in humans: SK1 and SK2. SK1 has been extensively studied and there is a large body of evidence to demonstrate its role in promoting cell survival, proliferation and neoplastic transformation. SK1 is also elevated in many human cancers which appears to contribute to carcinogenesis, chemotherapeutic resistance and poor patient outcome. SK2, however, has not been as well characterized, and there are contradictions in the key physiological functions that have been proposed for this isoform. Despite this, many studies are now emerging that implicate SK2 in key roles in a variety of diseases, including the development of a range of solid tumours. Here, we review the literature examining SK2, its physiological and pathophysiological functions, the current knowledge of its regulation, and recent developments in targeting this complex enzyme.

19F/18F exchange synthesis for a novel [18F]S1P3-radiopharmaceutical

(19)F/(18)F isotope exchange is a useful method to label drug molecules containing (19)F-fluorine with (18)F without modifying the drug molecule itself. Sphingosine-1-phosphate (S1P) is an important cellular mediator that functions by signaling through cell surface receptors. S1P is involved in several cell responses and may be related to many central nervous system disorders, including neural malfunction in Alzheimer's disease. In this study, [(18)F]1-benzyl-N-(3,4-difluorobenzyl)-2-isopropyl-6-(2-methoxyethoxy)-1H-indole-3-carboxamide, a novel (18)F-labeled positron emission tomography tracer for the S1P3 receptor, was successfully synthesized using the (19)F/(18)F isotope exchange reaction. Parameters of the reaction kinetics were studied, and correlations between the initial (18)F-activity, the amount of precursor, radiochemical yield and specific activity (SA) were determined. Contrary to expectations, high initial (18)F-activity decreased the radiochemical yield, and only a minor increase of SA occurred. This is most probably due to the complexity of the molecule and the subsequent susceptibility to radiolytic bond disruption. On the basis of the present results, a convenient condition for the (19)F/(18)F exchange reaction is the use of 2 µmol precursor with 20 GBq of (18)F-activity. This afforded a radiochemical yield of ~10% with an SA of 0.3 GBq/µmol. Results from this study are of interest for new tracer development where high initial (18)F-activity and (19)F/(18)F isotope exchange is used.

Advances in the treatment of relapsing-remitting multiple sclerosis - critical appraisal of fingolimod

Multiple sclerosis (MS) is a chronic inflammatory disorder of the central nervous system, traditionally considered to be an autoimmune, demyelinating disease. Based on this understanding, initial therapeutic strategies were directed at immune modulation and inflammation control. At present, there are five licensed first-line disease-modifying drugs for MS in Europe, and two second-line treatments. Currently available MS therapies have shown significant efficacy throughout many trials, but they produce different side effects. Despite disease-modifying drugs being well known and safe, they require regular and frequent parenteral administration and are associated with limited long-term treatment adherence. Therefore, the development of new therapeutic strategies is warranted. Several oral compounds are in late stages of development for treating MS. fingolimod is an oral sphingosine-1-phosphate receptor modulator that has demonstrated superior efficacy compared with placebo and interferon β-1a in phase III studies. It has already been approved in the treatment of MS. This review focuses on advances in current and novel oral treatment approaches in MS. We summarily review the oral compounds in this study, focusing on the recent development, approval, and the clinical experience with fingolimod.

Spns2, a transporter of phosphorylated sphingoid bases, regulates their blood and lymph levels, and the lymphatic network

Sphingosine-1-phosphate (S1P), a ligand for 5 specific receptors, is a potent lipid mediator that plays important roles in lymphocyte trafficking and immune responses. S1P is produced inside cells and therefore must be secreted to exert its effects through these receptors. Spinster 2 (Spns2) is one of the cell surface transporters thought to secrete S1P. We have shown that Spns2 can export endogenous S1P from cells and also dihydro-S1P, which is active at all cell surface S1P receptors. Moreover, Spns2 mice have decreased levels of both of these phosphorylated sphingoid bases in blood, accompanied by increases in very long chain ceramide species, and have defective lymphocyte trafficking. Surprisingly, levels of S1P and dihydro-S1P were increased in lymph from Spns2 mice as well as in specific tissues, including lymph nodes, and interstitial fluid. Moreover, lymph nodes from Spns2 mice have aberrant lymphatic sinus that appeared collapsed, with reduced numbers of lymphocytes. Our data suggest that Spns2 is an S1P transporter in vivo that plays a role in regulation not only of blood S1P but also lymph node and lymph S1P levels and consequently influences lymphocyte trafficking and lymphatic vessel network organization.

Effects of sphingosine-1-phosphate on pacemaker activity of interstitial cells of Cajal from mouse small intestine

Interstitial cells of Cajal (ICC) are the pacemaker cells that generate the rhythmic oscillation responsible for the production of slow waves in gastrointestinal smooth muscle. Spingolipids are known to present in digestive system and are responsible for multiple important physiological and pathological processes. In this study, we are interested in the action of sphingosine 1-phosphate (S1P) on ICC. S1P depolarized the membrane and increased tonic inward pacemaker currents. FTY720 phosphate (FTY720P, an S1P(1,3,4,5) agonist) and SEW 2871 (an S1P(1) agonist) had no effects on pacemaker activity. Suramin (an S1P(3) antagonist) did not block the S1P-induced action on pacemaker currents. However, JTE-013 (an S1P(2) antagonist) blocked the S1P-induced action. RT-PCR revealed the presence of the S1P(2) in ICC. Calphostin C (a protein kinase C inhibitor), NS-398 (a cyclooxygenase-2 inhibitor), PD 98059 (a p42/44 inhibitor), or SB 203580 (a p38 inhibitor) had no effects on S1P-induced action. However, c-jun NH(2)-terminal kinase (JNK) inhibitor II suppressed S1P-induced action. External Ca(2+)-free solution or thapsigargin (a Ca(2+)-ATPase inhibitor of endoplasmic reticulum) suppressed action of S1P on ICC. In recording of intracellular Ca(2+) ([Ca(2+)](i)) concentration using fluo-4/AM S1P increased intensity of spontaneous [Ca(2+)](i) oscillations in ICC. These results suggest that S1P can modulate pacemaker activity of ICC through S1P(2) via regulation of external and internal Ca(2+) and mitogenactivated protein kinase activation.

Erythrocyte-derived sphingosine-1-phosphate stabilizes basal hydraulic conductivity and solute permeability in rat microvessels

Exogenous sphingosine-1-phosphate (S1P), a lipid mediator in blood, attenuates acute microvascular permeability increases via receptor S1P1 to stabilize the endothelium. To evaluate the contribution of erythrocytes as an endogenous source of S1P to the regulation of basal permeability, we measured permeability coefficients in intact individually perfused venular microvessels of rat mesentery. This strategy also enabled the contributions of other endogenous S1P sources to be evaluated. Apparent permeability coefficients (P(S)) to albumin and α-lactalbumin and the hydraulic conductivity of mesenteric microvessels were measured in the presence or absence of rat erythrocytes or rat erythrocyte-conditioned perfusate. Rat erythrocytes added to the perfusate were the principal source of S1P in these microvessels. Basal P(S) to albumin was stable and typical of blood-perfused microvessels (mean 0.5 × 10(-6) cm/s) when erythrocytes or erythrocyte-conditioned perfusates were present. When they were absent, P(S) to albumin or α-lactalbumin increased up to 40-fold (over 10 min). When exogenous S1P was added to perfusates, permeability returned to levels comparable with those seen in the presence of erythrocytes. Addition of SEW 2871, an agonist specific for S1P1, in the absence of red blood cells reduced P(S)(BSA) (40-fold reduction) toward basal. The specific S1P1 receptor antagonist (W-146) reversed the stabilizing action of erythrocytes and increased permeability (27-fold increase) in a manner similar to that seen in the absence of erythrocytes. Erythrocytes are a primary source of S1P that maintains normal venular microvessel permeability. Absence of erythrocytes or conditioned perfusate in in vivo and in vitro models of endothelial barriers elevates basal permeability.

S1P(4) receptor mediates S1P-induced vasoconstriction in normotensive and hypertensive rat lungs

This study aimed to identify receptors mediating sphingosine-1-phosphate (S1P)-induced vasoconstriction in the normotensive and chronic hypoxia-induced hypertensive rat pulmonary circulation. In isolated perfused lungs from normoxic rats, infusion of S1P caused a sustained vasoconstriction, which was not reduced by combinational pretreatment with the dual S1P_{1 and 3} receptor antagonist VPC23019 and the S1P₂ receptor antagonist JTE013. The S1P₄ receptor agonists phytosphingosine-1-phospate and VPC23153, but not the dual S1P_{1 and 3} receptor agonist VPC24191, caused dose-dependent vasoconstrictions. In hypertensive lungs from chronically hypoxic rats, the vasoconstrictor responses to S1P and VPC23153 were markedly enhanced. The S1P₄ receptor agonist VPC 23153 caused contraction of isolated pulmonary but not of renal or mesenteric arteries from chronically hypoxic rats. S1P₄ receptor protein as well as mRNA were detected in both normotensive and hypertensive pulmonary arteries. In contrast to what has been reported in the systemic circulation and mouse lung, our findings raise the possibility that S1P₄ receptor plays a significant role in S1P-induced vasoconstriction in the normotensive and hypertensive rat pulmonary circulation.

Dendritic cells lower the permeability of endothelial monolayers

The permeability of cultured endothelial monolayers is higher than the permeability of endothelium in vivo. Co-culture with astrocytes can induce a tight, blood-brain-barrier phenotype in aortic endothelium in vitro. We hypothesised that dendritic cells, which reside in the intima of non-cerebral arteries and have features in common with astrocytes, may also reduce the permeability of cultured aortic endothelium. The permeability of porcine aortic endothelial monolayers was reduced by non-contact co-culture with dendritic cells (but not with the peripheral blood monocytes from which they were derived) and by dendritic cell conditioned medium, indicating a soluble mediator. The reduction in permeability was similar to that obtained by co-culture with astrocytes; however, dendritic cells did not up-regulate P-glycoprotein and there was no synergy with the effect of chronic shear stress on permeability, contrary to observations with astrocytes. Endothelial permeability was reduced by sphingosine-1-phosphate, which mediates the barrier-tightening effect of platelets, but inhibitors of sphingosine-1-phosphate receptors did not block the effect of dendritic cells. Rates of endothelial mitosis and apoptosis were also unaffected by co-culture. Hence dendritic cells reduce permeability by different mechanisms from those mediating barrier-tightening effects of astrocytes and platelets, although factors mediating the permeability-lowering effects of chronic shear stress may be involved. We speculate that dendritic cells influence endothelial permeability in vivo.

Lysophosphatidic acid and sphingosine-1-phosphate promote morphogenesis and block invasion of prostate cancer cells in three-dimensional organotypic models

Normal prostate and some malignant prostate cancer (PrCa) cell lines undergo acinar differentiation and form spheroids in three-dimensional (3-D) organotypic culture. Acini formed by PC-3 and PC-3M, less pronounced also in other PrCa cell lines, spontaneously undergo an invasive switch, leading to the disintegration of epithelial structures and the basal lamina, and formation of invadopodia. This demonstrates the highly dynamic nature of epithelial plasticity, balancing epithelial-to-mesenchymal transition against metastable acinar differentiation. This study assessed the role of lipid metabolites on epithelial maturation. PC-3 cells completely failed to form acinar structures in delipidated serum. Adding back lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) rescued acinar morphogenesis and repressed invasion effectively. Blocking LPA receptor 1 (LPAR1) functions by siRNA (small interference RNA) or the specific LPAR1 inhibitor Ki16425 promoted invasion, while silencing of other G-protein-coupled receptors responsive to LPA or S1P mainly caused growth arrest or had no effects. The G-proteins Gα(12/13) and Gα(i) were identified as key mediators of LPA signalling via stimulation of RhoA and Rho kinases ROCK1 and 2, activating Rac1, while inhibition of adenylate cyclase and accumulation of cAMP may be secondary. Interfering with these pathways specifically impeded epithelial polarization in transformed cells. In contrast, blocking the same pathways in non-transformed, normal cells promoted differentiation. We conclude that LPA and LPAR1 effectively promote epithelial maturation and block invasion of PrCa cells in 3-D culture. The analysis of clinical transcriptome data confirmed reduced expression of LPAR1 in a subset of PrCa's. Our study demonstrates a metastasis-suppressor function for LPAR1 and Gα(12/13) signalling, regulating cell motility and invasion versus epithelial maturation.

Defining a role for sphingosine kinase 1 in p53-dependent tumors

p53 is a crucial tumor suppressor that is mutated or deleted in a majority of cancers. Exactly how p53 prevents tumor progression has proved elusive for many years; however, this information is crucial to define targets for chemotherapeutic development that can effectively restore p53 function. Bioactive sphingolipids have recently emerged as important regulators of proliferative, apoptotic and senescent cellular processes. In this study, we demonstrate that the enzyme sphingosine kinase 1 (SK1), a critical enzyme in the regulation of the key bioactive sphingolipids ceramide, sphingosine and sphingosine-1-phosphate (S1P), serves as a key downstream target for p53 action. Our results show that SK1 is proteolysed in response to genotoxic stress in a p53-dependent manner. p53 null mice display elevation of SK1 levels and a tumor-promoting dysregulation of bioactive sphingolipids in which the anti-growth sphingolipid ceramide is decreased and the pro-growth sphingolipid S1P is increased. Importantly, deletion of SK1 in p53 null mice completely abrogated thymic lymphomas in these mice and prolonged their life span by ~30%. Deletion of SK1 also significantly attenuated the formation of other cancers in p53 heterozygote mice. The mechanism of p53 tumor suppression by loss of SK1 is mediated by elevations of sphingosine and ceramide, which in turn were accompanied by increased expression of cell cycle inhibitors and tumor cell senescence. Thus, targeting SK1 may restore sphingolipid homeostasis in p53-dependent tumors and provide insights into novel therapeutic approaches to cancer.

Circulating sphingosine-1-phosphate and erythrocyte sphingosine kinase-1 activity as novel biomarkers for early prostate cancer detection

BACKGROUND

Current markers available for screening normal populations and for monitoring prostate cancer (PCa) treatment lack sensitivity and selectivity. Sphingosine-1-phosphate (S1P) is a circulating lipid second messenger involved in cell growth and migration, the immune response, angiogenesis, and malignant transformation.

METHODS

Eighty-eight patients with localised, locally advanced, or metastatic PCa were recruited into this prospective single-centre study. Plasma S1P levels were measured and compared with age-matched controls with benign prostate hyperplasia (BPH) (n=110) or with young healthy males with the very small chance of having PCa foci (n=20).

RESULTS

Levels of circulating S1P were significantly higher in healthy subjects (10.36 ± 0.69 pmol per mg protein, P<0.0001) and patients with BPH (9.39 ± 0.75, P=0.0013) than in patients with PCa (6.89 ± 0.58, ANOVA, P=0.0019). Circulating S1P levels were an early marker of PCa progression to hormonal unresponsiveness and correlated with prostate-specific antigen (PSA) levels and lymph node metastasis. During the course of the study, nine patients have died of PCa. Importantly, their circulating S1P levels were significantly lower (5.11 ± 0.75) than in the surviving patients (7.02 ± 0.22, n=79, P=0.0439). Our data suggest that the decrease in circulating S1P during PCa progression may stem from a highly significant downregulation of erythrocyte sphingosine kinase-1 (SphK1) activity (2.14 ± 0.17 pmol per mg protein per minute in PCa patients vs 4.7 ± 0.42 in healthy individuals, P<0.0001), which may be a potential mechanism of cancer-induced anaemia.

CONCLUSION

This current study has provided a potential mechanism for cancer-related anaemia and the first evidence that plasma S1P and erythrocyte SphK1 activity are the potential markers for the diagnosis, monitoring, and predicating for PCa mortality.

Analysis of sphingosine-1-phosphate signaling mutants reveals endodermal requirements for the growth but not dorsoventral patterning of jaw skeletal precursors

Development of the head skeleton involves reciprocal interactions between cranial neural crest cells (CNCCs) and the surrounding pharyngeal endoderm and ectoderm. Whereas elegant experiments in avians have shown a prominent role for the endoderm in facial skeleton development, the relative functions of the endoderm in growth versus regional identity of skeletal precursors have remained unclear. Here we describe novel craniofacial defects in zebrafish harboring mutations in the Sphingosine-1-phospate (S1P) type 2 receptor (s1pr2) or the S1P transporter Spinster 2 (spns2), and we show that S1P signaling functions in the endoderm for the proper growth and positioning of the jaw skeleton. Surprisingly, analysis of s1pr2 and spns2 mutants, as well as sox32 mutants that completely lack endoderm, reveals that the dorsal-ventral (DV) patterning of jaw skeletal precursors is largely unaffected even in the absence of endoderm. Instead, we observe reductions in the ectodermal expression of Fibroblast growth factor 8a (Fgf8a), and transgenic misexpression of Shha restores fgf8a expression and partially rescues the growth and differentiation of jaw skeletal precursors. Hence, we propose that the S1P-dependent anterior foregut endoderm functions primarily through Shh to regulate the growth but not DV patterning of zebrafish jaw precursors.

Sphingosine 1-phosphate-mediated α1B-adrenoceptor desensitization and phosphorylation. Direct and paracrine/autocrine actions

Sphingosine-1-phosphate-induced α1B-adrenergic receptor desensitization and phosphorylation were studied in rat-1 fibroblasts stably expressing enhanced green fluorescent protein-tagged adrenoceptors. Sphingosine-1-phosphate induced adrenoceptor desensitization and phosphorylation through a signaling cascade that involved phosphoinositide 3-kinase and protein kinase C activities. The autocrine/paracrine role of sphingosine-1-phosphate was also studied. It was observed that activation of receptor tyrosine kinases, such as insulin growth factor-1 (IGF-I) and epidermal growth factor (EGF) receptors increased sphingosine kinase activity. Such activation and consequent production of sphingosine-1-phosphate appear to be functionally relevant in IGF-I- and EGF-induced α1B-adrenoceptor phosphorylation and desensitization as evidenced by the following facts: a) expression of a catalytically inactive (dominant-negative) mutant of sphingosine kinase 1 or b) S1P1 receptor knockdown markedly reduced this growth factor action. This action of sphingosine-1-phosphate involves EGF receptor transactivation. In addition, taking advantage of the presence of the eGFP tag in the receptor construction, we showed that S1P was capable of inducing α1B-adrenergic receptor internalization and that its autocrine/paracrine generation was relevant for internalization induced by IGF-I. Four distinct hormone receptors and two autocrine/paracrine mediators participate in IGF-I receptor-α1B-adrenergic receptor crosstalk.

Sphingosine-1-phosphate rapidly increases cortisol biosynthesis and the expression of genes involved in cholesterol uptake and transport in H295R adrenocortical cells

In the acute phase of adrenocortical steroidogenesis, adrenocorticotrophin (ACTH) activates a cAMP/PKA-signaling pathway that promotes the transport of free cholesterol to the inner mitochondrial membrane. We have previously shown that ACTH rapidly stimulates the metabolism of sphingolipids and the secretion of sphingosine-1-phosphate (S1P) in H295R cells. In this study, we examined the effect of S1P on genes involved in the acute phase of steroidogenesis. We show that S1P increases the expression of steroidogenic acute regulatory protein (StAR), 18-kDa translocator protein (TSPO), low-density lipoprotein receptor (LDLR), and scavenger receptor class B type I (SR-BI). S1P-induced StAR mRNA expression requires Gα(i) signaling, phospholipase C (PLC), Ca(2+)/calmodulin-dependent kinase II (CamKII), and ERK1/2 activation. S1P also increases intracellular Ca(2+), the phosphorylation of hormone sensitive lipase (HSL) at Ser(563), and cortisol secretion. Collectively, these findings identify multiple roles for S1P in the regulation of glucocorticoid biosynthesis.

Ceramide and sphingosine-1-phosphate signaling in embryonic stem cell differentiation

Recent studies show that bioactive lipids are important regulators for stem cell survival and differentiation. The sphingolipid ceramide and its derivative, sphingosine-1-phosphate (S1P), can act synergistically on embryonic stem (ES) cell differentiation. We show here simple methods to analyze sphingolipids in differentiating ES cells and to use ceramide and S1P analogs for the guided differentiation of mouse ES cells toward neuronal and glial lineage.

Sphingosine kinase-2 inhibition improves mitochondrial function and survival after hepatic ischemia-reperfusion

BACKGROUND & AIMS

The mitochondrial permeability transition (MPT) and inflammation play important roles in liver injury caused by ischemia-reperfusion (IR). This study investigated the roles of sphingosine kinase-2 (SK2) in mitochondrial dysfunction and inflammation after hepatic IR.

METHODS

Mice were gavaged with vehicle or ABC294640 (50 mg/kg), a selective inhibitor of SK2, 1 h before surgery and subjected to 1 h-warm ischemia to ~70% of the liver followed by reperfusion.

RESULTS

Following IR, hepatic SK2 mRNA and sphingosine-1-phosphate (S1P) levels increased ~25- and 3-fold, respectively. SK2 inhibition blunted S1P production and liver injury by 54-91%, and increased mouse survival from 28% to 100%. At 2 h after reperfusion, mitochondrial depolarization was observed in 74% of viable hepatocytes, and mitochondrial voids excluding calcein disappeared, indicating MPT onset in vivo. SK2 inhibition decreased mitochondrial depolarization and prevented MPT onset. Inducible nitric oxide synthase, phosphorylated NFκB-p65, TNFα mRNA, and neutrophil infiltration, all increased markedly after hepatic IR, and these increases were blunted by SK2 inhibition. In cultured hepatocytes, anoxia/re-oxygenation resulted in increases of SK2 mRNA, S1P levels, and cell death. SK2 siRNA and ABC294640 each substantially decreased S1P production and cell death in cultured hepatocytes.

CONCLUSIONS

SK2 plays an important role in mitochondrial dysfunction, inflammation responses, hepatocyte death, and survival after hepatic IR and represents a new target for the treatment of IR injury.

High-throughput assays to measure intracellular Ca²⁺ mobilization in cells that express recombinant S1P receptor subtypes

Intracellular Ca(2+) mobilization is a useful readout to screen for agonists or antagonists of G-protein -coupled receptors (GPCRs). Here, we describe methods to conduct high-throughput screening of stably or transiently transfected HTC4 cells expressing the individual S1P1-5 receptor subtypes. The cells are grown in 96-well plates and loaded with the cell permeable fluorescent Ca(2+) indicator dye Fura-2-AM. Changes in intracellular Ca(2+) levels in response to S1P or test compounds are detected using a FlexStation II scanning fluorometer with integrated fluidics transfer capabilities.

Targeting NFĸB mediated breast cancer chemoresistance through selective inhibition of sphingosine kinase-2

Resistance to chemotherapy remains a significant obstacle in the treatment of hormone- independent breast cancer. Recent evidence suggests that altered sphingolipid signaling through increased sphingosine kinase activity may be an important mediator of breast cancer drug resistance. Sphingosine kinase-1 (Sphk1) is a proposed key regulator of breast cancer tumorigenesis, proliferation and resistance. There is, however, conflicting data on the role of sphingosine kinase-2 (Sphk2) in cancer biology and resistance, with some suggesting that Sphk2 has an opposing role to that of Sphk1. Here, we studied the effects of the novel selective Sphk2 inhibitor, ABC294640 (3-(4-chlorophenyl)-adamantane-1-carboxylic acid (pyridin-4-ylmethyl) amide), on human breast cancer. ABC294640 blocked both viability and survival at low micromolar IC(50) concentrations in the endocrine therapy-resistant MDA-MB-231 and chemoresistant MCF-7TN-R cell systems. Treatment with the inhibitor significantly reduced proliferation, as seen in immunofluorescence staining of Ki-67 in vitro. Interestingly, pharmacological inhibition of Sphk2 induced apoptosis through the intrinsic programmed cell death pathway. Furthermore, ABC294640 also diminished NF-ĸB survival signaling, through decreased activation of the Ser536 phosphorylation site on the p65 subunit. Xenografts of MCF-7TN-R cells growing in immunocompromised mice were utilized to validate the therapeutic efficacy of the sphingosine kinase-2 inhibitor. Treatment with 50 mg of ABC294640/kg completely blocked tumor volume in this model. These results indicate that pharmacological inhibition of Sphk2 with the orally bioavailable selective inhibitor, ABC294640, has therapeutic potential in the treatment of chemo- and endocrine therapy- resistant breast cancer.