The role of sphingosine kinase and sphingosine-1-phosphate in the regulation of endothelial cell biology

Pancreatic Acinar Cells-Derived Sphingosine-1-Phosphate Contributes to Fibrosis of Chronic Pancreatitis via Inducing Autophagy and Activation of Pancreatic Stellate Cells

BACKGROUND & AIMS

Studies have demonstrated that activated pancreatic stellate cells (PSCs) play a crucial role in pancreatic fibrogenesis in chronic pancreatitis (CP); however, the precise mechanism for PSCs activation has not been fully elucidated. We analyzed the role of injured pancreatic acinar cells (iPACs) in the activation of PSCs of CP.

METHODS

Sphingosine kinase 1 (SPHK1)/sphingosine-1-phosphate (S1P) signaling was evaluated in experimental CP induced by cerulein injection or pancreatic duct ligation, as well as in PACs injured by cholecystokinin. The activation of PSCs and pancreatic fibrosis in CP samples was evaluated by immunohistochemical and immunofluorescence analyses. In vitro coculture assay of iPACs and PSCs was created to evaluate the effect of the SPHK1/S1P pathway and S1P receptor 2 (SIPR2) on autophagy and activation of PSCs. The pathogenesis of CP was assessed in SPHK1-/- mice or PACs-specific SPHK1-knockdown mice with recombinant adeno-associated virus serotypes 9-SPHK1-knockdown, as well as in mice treated with inhibitor of SPHK1 and S1P receptor 2 (S1PR2).

RESULTS

SPHK1/S1P was remarkably increased in iPACs and acinar cells in pancreatic tissues of CP mice. Meanwhile, the pathogenesis, fibrosis, and PSCs activation of CP was significantly prevented in SPHK1-/- mice and recombinant adeno-associated virus serotypes 9-SPHK1-knockdown mice. Meanwhile, iPACs obviously activated PSCs, which was prevented by SPHK1 knockdown in iPACs. Moreover, iPACs-derived S1P specifically combined to S1PR2 of PSCs, by which modulated 5' adenosine monophosphate-activated protein kinase/mechanistic target of rapamycin pathway and consequently induced autophagy and activation of PSCs. Furthermore, hypoxia-inducible factor 1-α and -2α promoted SPHK1 transcription of PACs under hypoxia conditions, which is a distinct characteristic of the CP microenvironment. Coincidently, inhibition of SPHK1 and S1PR2 activity with inhibitor PF-543 and JTE-013 obviously impeded pancreatic fibrogenesis of CP mice.

CONCLUSIONS

The activated SPHK1/S1P pathway in iPACs induces autophagy and activation of PSCs by regulating the S1PR2/5' adenosine monophosphate-activated protein kinase/mammalian target of rapamycin pathway, which promotes fibrogenesis of CP. The hypoxia microenvironment might contribute to the cross talk between PACs and PSCs in pathogenesis of CP.

Effects of sphingolipid metabolism disorders on endothelial cells

Many cardiovascular disorders, including atherosclerosis, hypertension, coronary heart disease, diabetes, etc., are characterized by endothelial cell dysfunction. Endothelial cell function is closely related to sphingolipid metabolism, and normal sphingolipid metabolism is critical for maintaining endothelial cell homeostasis. Sphingolipid metabolites or key enzymes in abnormal situation, including sphingosine, ceramide (Cer), sphingosine-1-phosphate (S1P), serine, sphingosine kinase (SPHK), ceramide kinase (Cerk), sphingosine-1-phosphate lyase (S1PL) etc., may have a protective or damaging effect on the function of endothelial cells. This review summarizes the effects of sphingolipid metabolites and key enzymes disordering in sphingolipid metabolism on endothelial cells, offering some insights into further research on the pathogenesis of cardiovascular diseases and corresponding therapeutic targets.

Extracellular Vesicles and Thrombogenicity in Atrial Fibrillation

Extracellular vesicles (EVs) are defined as a heterogenic group of lipid bilayer vesicular structures with a size in the range of 30–4000 nm that are released by all types of cultured cells. EVs derived from platelets, mononuclears, endothelial cells, and adipose tissue cells significantly increase in several cardiovascular diseases, including in atrial fibrillation (AF). EVs are engaged in cell-to-cell cooperation, endothelium integrity, inflammation, and immune response and are a cargo for several active molecules, such as regulatory peptides, receptors, growth factors, hormones, and lipids. Being transductors of the intercellular communication, EVs regulate angiogenesis, neovascularization, coagulation, and maintain tissue reparation. There is a large amount of evidence regarding the fact that AF is associated with elevated levels of EVs derived from platelets and mononuclears and a decreased number of EVs produced by endothelial cells. Moreover, some invasive procedures that are generally performed for the treatment of AF, i.e., pulmonary vein isolation, were found to be triggers for elevated levels of platelet and mononuclear EVs and, in turn, mediated the transient activation of the coagulation cascade. The review depicts the role of EVs in thrombogenicity in connection with a risk of thromboembolic complications, including ischemic stroke and systemic thromboembolism, in patients with various forms of AF.

Activation of SphK2 contributes to adipocyte-induced EOC cell proliferation

Epithelial ovarian cancer (EOC) is the leading cause of deaths due to cancer in women. Adipocytes have been suggested to play a key role in the stimulation of EOC growth. However, the mechanisms underlying the adipocyte-induced EOC proliferation remain undefined. Here, we provide the first evidence that adipocytes induce the activation of sphingosine kinase (SphK) 2 in EOC, which represents a novel pathway that mediates the adipocyte-induced EOC growth. SphK2 inhibition in EOC cells led to a remarkable inhibition of the adipocyte-induced cell proliferation. Moreover, the adipocyte-induced SphK2 activation in EOC cells was extracellular signal-regulated protein kinases (ERK) dependent. Furthermore, silencing SphK2 in EOC significantly inhibited the adipocyte-induced expression of phospho-ERK and c-Myc, two crucial players in EOC growth. Collectively, the current study unraveled a previously unrecognized role of SphK2 in the adipocyte-induced growth-promoting action in EOC, suggesting a novel target for EOC treatment.

Sphingosine-1-Phosphate in the Tumor Microenvironment: A Signaling Hub Regulating Cancer Hallmarks

As a key hub of malignant properties, the cancer microenvironment plays a crucial role intimately connected to tumor properties. Accumulating evidence supports that the lysophospholipid sphingosine-1-phosphate acts as a key signal in the cancer extracellular milieu. In this review, we have a particular focus on glioblastoma, representative of a highly aggressive and deleterious neoplasm in humans. First, we highlight recent advances and emerging concepts for how tumor cells and different recruited normal cells contribute to the sphingosine-1-phosphate enrichment in the cancer microenvironment. Then, we describe and discuss how sphingosine-1-phosphate signaling contributes to favor cancer hallmarks including enhancement of proliferation, stemness, invasion, death resistance, angiogenesis, immune evasion and, possibly, aberrant metabolism. We also discuss the potential of how sphingosine-1-phosphate control mechanisms are coordinated across distinct cancer microenvironments. Further progress in understanding the role of S1P signaling in cancer will depend crucially on increasing knowledge of its participation in the tumor microenvironment.

Glucocorticoid resistance of allogeneic T cells alters the gene expression profile in the inflamed small intestine of mice suffering from acute graft-versus-host disease

Glucocorticoids (GCs) play an important role in controlling acute graft-versus-host disease (aGvHD), a frequent complication of allogeneic hematopoietic stem cell transplantation. The anti-inflammatory activity of GCs is mainly ascribed to the modulation of T cells and macrophages, for which reason a genetically induced GC resistance of either of these cell types causes aggravated aGvHD. Since only a few genes are currently known that are differentially regulated under these conditions, we analyzed the expression of 54 candidate genes in the inflamed small intestine of mice suffering from aGvHD when either allogeneic T cells or host myeloid cells were GC resistant using a microfluidic dynamic array platform for high-throughput quantitative PCR. The majority of genes categorized as cytokines (e.g. Il2, Il6), chemokines (e.g. Ccl2, Cxcl1), cell surface receptors (e.g. Fasl, Ctla4) and intracellular molecules (e.g. Dusp1, Arg1) were upregulated in mice transplanted with GC resistant allogeneic T cells. Moreover, the expression of several genes linked to energy metabolism (e.g. Glut1) was altered. Surprisingly, mice harboring GC resistant myeloid cells showed almost no changes in gene expression despite their fatal disease course after aGvHD induction. To identify additional genes in the inflamed small intestine that were affected by a GC resistance of allogeneic T cells, we performed an RNAseq analysis, which uncovered more than 500 differentially expressed transcripts (e.g. Cxcr6, Glut3, Otc, Aoc1, Il1r1, Sphk1) that were enriched for biological processes associated with inflammation and tissue disassembly. The changes in gene expression could be confirmed during full-blown disease but hardly any of them in the preclinical phase using high-throughput quantitative PCR. Further analysis of some of these genes revealed a highly selective expression pattern in T cells, intestinal epithelial cells and macrophages, which correlated with their regulation during disease progression. Collectively, we identified an altered gene expression profile caused by GC resistance of transplanted allogeneic T cells, which could help to define new targets for aGvHD therapy.

A bidirectional crosstalk between glioblastoma and brain endothelial cells potentiates the angiogenic and proliferative signaling of sphingosine-1-phosphate in the glioblastoma microenvironment

Glioblastoma is one of the most malignant, angiogenic, and incurable tumors in humans. The aberrant communication between glioblastoma cells and tumor microenvironment represents one of the major factors regulating glioblastoma malignancy and angiogenic properties. Emerging evidence implicates sphingosine-1-phosphate signaling in the pathobiology of glioblastoma and angiogenesis, but its role in glioblastoma-endothelial crosstalk remains largely unknown. In this study, we sought to determine whether the crosstalk between glioblastoma cells and brain endothelial cells regulates sphingosine-1-phosphate signaling in the tumor microenvironment. Using human glioblastoma and brain endothelial cell lines, as well as primary brain endothelial cells derived from human glioblastoma, we report that glioblastoma-co-culture promotes the expression, activity, and plasma membrane enrichment of sphingosine kinase 2 in brain endothelial cells, leading to increased cellular level of sphingosine-1-phosphate, and significant potentiation of its secretion. In turn, extracellular sphingosine-1-phosphate stimulates glioblastoma cell proliferation, and brain endothelial cells migration and angiogenesis. We also show that, after co-culture, glioblastoma cells exhibit enhanced expression of S1P₁ and S1P₃, the sphingosine-1-phosphate receptors that are of paramount importance for cell growth and invasivity. Collectively, our results envision glioblastoma-endothelial crosstalk as a multi-compartmental strategy to enforce pro-tumoral sphingosine-1-phosphate signaling in the glioblastoma microenvironment.

Reduced cerebral vascularization in experimental neuronopathic Gaucher disease

The glycosphingolipidosis, Gaucher disease, in which a range of neurological manifestations occur, results from a deficiency of acid β-glucocerebrosidase, with subsequent accumulation of β-glucocerebroside, its upstream substrates, and the non-acylated congener β-glucosylsphingosine. However, the mechanisms by which end-organ dysfunction arise are poorly understood. Here, we report strikingly diminished cerebral microvascular density in a murine model of disease, and provide a detailed analysis of the accompanying cerebral glycosphingolipidome in these animals, with marked elevations of β-glucosylsphingosine. Further in vitro studies confirmed a concentration-dependent impairment of endothelial cytokinesis upon exposure to quasi-pathological concentrations of β-glucosylsphingosine. These findings support a premise for pathogenic disruption of cerebral angiogenesis as an end-organ effect, with potential for therapeutic modulation in neuronopathic Gaucher disease. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Sphingosine 1-phosphate is a ligand for peroxisome proliferator-activated receptor-γ that regulates neoangiogenesis

Sphingosine 1-phosphate (S1P) is a bioactive lipid that can function both extracellularly and intracellularly to mediate a variety of cellular processes. Using lipid affinity matrices and a radiolabeled lipid binding assay, we reveal that S1P directly interacts with the transcription factor peroxisome proliferator-activated receptor (PPAR)γ. Herein, we show that S1P treatment of human endothelial cells (ECs) activated a luciferase-tagged PPARγ-specific gene reporter by ∼12-fold, independent of the S1P receptors. More specifically, in silico docking, gene reporter, and binding assays revealed that His323 of the PPARγ ligand binding domain is important for binding to S1P. PPARγ functions when associated with coregulatory proteins, and herein we identify that peroxisome proliferator-activated receptor-γ coactivator 1 (PGC1)β binds to PPARγ in ECs and their progenitors (nonadherent endothelial forming cells) and that the formation of this PPARγ:PGC1β complex is increased in response to S1P. ECs treated with S1P selectively regulated known PPARγ target genes with PGC1β and plasminogen-activated inhibitor-1 being increased, no change to adipocyte fatty acid binding protein 2 and suppression of CD36. S1P-induced in vitro tube formation was significantly attenuated in the presence of the PPARγ antagonist GW9662, and in vivo application of GW9662 also reduced vascular development in Matrigel plugs. Interestingly, activation of PPARγ by the synthetic ligand troglitazone also reduced tube formation in vitro and in vivo. To support this, Sphk1(-/-)Sphk2(+/-) mice, with low circulating S1P levels, demonstrated a similar reduction in vascular development. Taken together, our data reveal that the transcription factor, PPARγ, is a bona fide intracellular target for S1P and thus suggest that the S1P:PPARγ:PGC1β complex may be a useful target to manipulate neovascularization.

Sphingosine kinase 1 expressed by endothelial colony-forming cells has a critical role in their revascularization activity

AIMS

Cell therapy based on endothelial colony-forming cells (ECFCs) is a promising option for ischaemic cardiovascular diseases. A better understanding of the mechanisms by which these cells promote revascularization remains a critical challenge to improving their therapeutic potential. We aimed to identify the critical mechanisms involved in the revascularization activity of ECFCs by using the paracrine properties of mesenchymal stem cells (MSC).

METHODS AND RESULTS

Conditioned medium from human bone marrow-derived MSCs (MSC-CM) increased the angiogenic activity of cord blood ECFCs in vitro (proliferation, migration, and pseudo-tube formation), the survival of ECFCs in mice (Matrigel Plug assay), and the capacity of ECFCs to promote the recovery of blood perfusion in mice with hindlimb ischaemia. Furthermore, the capillary density in ischaemic gastrocnemius muscle was significantly increased in mice transplanted with the ECFCs pre-treated with the MSC-CM. The enhancement of ECFCs activity involved the up-regulation of sphingosine kinase 1 (SphK1) expression and activity. The inhibition of SphK1 in ECFCs by using an inhibitor or a siRNA knockdown of SphK1 prevented the stimulation of the ECFCs induced by the MSC-CM. The improvement of ECFC activity by MSC-CM also involved the up-regulation of sphingosine-1-phosphate receptor 1 (S1P1) and a S1P/S1P1/3-dependent mechanism. Finally, we showed that the stimulation of ECFCs with exogenous S1P increased angiogenesis and promoted blood perfusion in hindlimb ischaemia.

CONCLUSION

The up-regulation of SphK1 and S1P-dependent pathways is critical for the angiogenic/vasculogenic activity of ECFCs. The identification of this pathway provides attractive targets to optimize cell-based therapy for revascularization in ischaemic diseases.

Sphingosine kinase 1 deficiency confers protection against hyperoxia-induced bronchopulmonary dysplasia in a murine model: role of S1P signaling and Nox proteins

Bronchopulmonary dysplasia of the premature newborn is characterized by lung injury, resulting in alveolar simplification and reduced pulmonary function. Exposure of neonatal mice to hyperoxia enhanced sphingosine-1-phosphate (S1P) levels in lung tissues; however, the role of increased S1P in the pathobiological characteristics of bronchopulmonary dysplasia has not been investigated. We hypothesized that an altered S1P signaling axis, in part, is responsible for neonatal lung injury leading to bronchopulmonary dysplasia. To validate this hypothesis, newborn wild-type, sphingosine kinase1(-/-) (Sphk1(-/-)), sphingosine kinase 2(-/-) (Sphk2(-/-)), and S1P lyase(+/-) (Sgpl1(+/-)) mice were exposed to hyperoxia (75%) from postnatal day 1 to 7. Sphk1(-/-), but not Sphk2(-/-) or Sgpl1(+/-), mice offered protection against hyperoxia-induced lung injury, with improved alveolarization and alveolar integrity compared with wild type. Furthermore, SphK1 deficiency attenuated hyperoxia-induced accumulation of IL-6 in bronchoalveolar lavage fluids and NADPH oxidase (NOX) 2 and NOX4 protein expression in lung tissue. In vitro experiments using human lung microvascular endothelial cells showed that exogenous S1P stimulated intracellular reactive oxygen species (ROS) generation, whereas SphK1 siRNA, or inhibitor against SphK1, attenuated hyperoxia-induced S1P generation. Knockdown of NOX2 and NOX4, using specific siRNA, reduced both basal and S1P-induced ROS formation. These results suggest an important role for SphK1-mediated S1P signaling-regulated ROS in the development of hyperoxia-induced lung injury in a murine neonatal model of bronchopulmonary dysplasia.

Inhibition of sphingosine-1-phosphate lyase rescues sphingosine kinase-1-knockout phenotype following murine cardiac arrest

AIMS

To test the role of sphingosine-1-phosphate (S1P) signaling system in the in vivo setting of resuscitation and survival after cardiac arrest.

MAIN METHODS

A mouse model of potassium-induced cardiac arrest and resuscitation was used to test the importance of S1P homeostasis in resuscitation and survival. C57BL/6 and sphingosine kinase-1 knockout (SphK1-KO) female mice were arrested for 8 min then subjected to 5 minute CPR with epinephrine bolus given at 90s after the beginning of CPR. Animal survival was monitored for 4h post-resuscitation. Upregulation of tissue and circulatory S1P levels were achieved via inhibition of S1P lyase by 2-acetyl-5-tetrahydroxybutyl imidazole (THI). Plasma and heart tissue S1P and ceramide levels were quantified by targeted ESI-LC/MS/MS.

KEY FINDINGS

Lack of SphK1 and low tissue/circulatory S1P levels in SphK1-KO mice led to poor animal resuscitation after cardiac arrest and to impaired survival post-resuscitation. Inhibition of S1P lyase in SphK1-KO mice drastically improved animal resuscitation and survival. Improved resuscitation and survival of THI-treated SphK1-KO mice were better correlated with cardiac dihydro-S1P (DHS1P) than S1P levels. The lack of SphK1 and the inhibition of S1P lyase by THI were accompanied by modulation in cardiac S1PR1 and S1PR2 expression and by selective changes in plasma N-palmitoyl- and N-behenoyl-ceramide levels.

SIGNIFICANCE

Our data provide evidence for the crucial role for SphK1 and S1P signaling system in resuscitation and survival after cardiac arrest, which may form the basis for development of novel therapeutic strategy to support resuscitation and long-term survival of cardiac arrest patients.

Sphingosine kinase/sphingosine 1-phosphate (S1P)/S1P receptor axis is involved in liver fibrosis-associated angiogenesis

BACKGROUND & AIMS

Sphingosine kinase (SphK)/sphingosine 1-phosphate (S1P)/S1P receptor (S1PR) axis is involved in multiple biological processes, including liver fibrosis. Angiogenesis is an important pathophysiological process closely associated with liver fibrosis; however, the functional role of SphK/S1P/S1PR in this process remains incompletely defined.

METHODS

Bile duct ligation or carbon tetrachloride was used to induce liver fibrosis in mice. Human fibrotic samples were obtained from livers of patients undergoing liver transplantation. S1P levels in the liver were examined by HPLC. Expression of angiogenic markers, including angiopoietin 1, CD31, vascular cell adhesion molecule-1, and von Willebrand factor, was characterized by immunofluorescence, real-time RT-PCR, and Western blot in the fibrotic liver and primary mouse hepatic stellate cells (HSCs). SphK inhibitor (SKI) or S1PR antagonists were administered intraperitoneally in mice.

RESULTS

S1P levels in the liver were closely correlated with mRNA expression of angiogenic markers. Ang1 is expressed in activated HSCs of the fibrotic liver and in primary HSCs. In HSCs, by using specific antagonists or siRNAs, we demonstrated S1P stimulation induced Ang1 expression via S1PR1 and S1PR3. In vivo, S1P reduction by SKI inhibited angiogenesis in fibrotic mice. Furthermore, S1PR1/3 antagonist significantly blocked upregulation of angiogenic markers in the injured liver, and attenuated the extent of liver fibrosis, while S1PR2 antagonist had no effect on angiogenesis, supporting the key role of S1PR1 and S1PR3 in angiogenesis underlying liver fibrosis process.

CONCLUSIONS

SphK1/S1P/S1PR1/3 axis plays a crucial role in the angiogenic process required for fibrosis development, which may represent an effective therapeutic strategy for liver fibrosis.

Paracrine proangiopoietic effects of human umbilical cord blood-derived purified CD133+ cells--implications for stem cell therapies in regenerative medicine

CD133+ cells purified from hematopoietic tissues are enriched mostly for hematopoietic stem/progenitor cells, but also contain some endothelial progenitor cells and very small embryonic-like stem cells. CD133+ cells, which are akin to CD34+ cells, are a potential source of stem cells in regenerative medicine. However, the lack of convincing donor-derived chimerism in the damaged organs of patients treated with these cells suggests that the improvement in function involves mechanisms other than a direct contribution to the damaged tissues. We hypothesized that CD133+ cells secrete several paracrine factors that play a major role in the positive effects observed after treatment and tested supernatants derived from these cells for the presence of such factors. We observed that CD133+ cells and CD133+ cell-derived microvesicles (MVs) express mRNAs for several antiapoptotic and proangiopoietic factors, including kit ligand, insulin growth factor-1, vascular endothelial growth factor, basic fibroblast growth factor, and interleukin-8. These factors were also detected in a CD133+ cell-derived conditioned medium (CM). More important, the CD133+ cell-derived CM and MVs chemoattracted endothelial cells and display proangiopoietic activity both in vitro and in vivo assays. This observation should be taken into consideration when evaluating clinical outcomes from purified CD133+ cell therapies in regenerative medicine.

Mouse SPNS2 functions as a sphingosine-1-phosphate transporter in vascular endothelial cells

Sphingosine-1-phosphate (S1P), a sphingolipid metabolite that is produced inside the cells, regulates a variety of physiological and pathological responses via S1P receptors (S1P1–5). Signal transduction between cells consists of three steps; the synthesis of signaling molecules, their export to the extracellular space and their recognition by receptors. An S1P concentration gradient is essential for the migration of various cell types that express S1P receptors, such as lymphocytes, pre-osteoclasts, cancer cells and endothelial cells. To maintain this concentration gradient, plasma S1P concentration must be at a higher level. However, little is known about the molecular mechanism by which S1P is supplied to extracellular environments such as blood plasma. Here, we show that SPNS2 functions as an S1P transporter in vascular endothelial cells but not in erythrocytes and platelets. Moreover, the plasma S1P concentration of SPNS2-deficient mice was reduced to approximately 60% of wild-type, and SPNS2-deficient mice were lymphopenic. Our results demonstrate that SPNS2 is the first physiological S1P transporter in mammals and is a key determinant of lymphocyte egress from the thymus.

Pharmacological relevance and potential of sphingosine 1-phosphate in the vascular system

Sphingosine-1-phosphate (S1P) was identified as a crucial molecule for regulating immune responses, inflammatory processes as well as influencing the cardiovascular system. S1P mediates differentiation, proliferation and migration during vascular development and homoeostasis. S1P is a naturally occurring lipid metabolite and is present in human blood in nanomolar concentrations. S1P is not only involved in physiological but also in pathophysiological processes. Therefore, this complex signalling system is potentially interesting for pharmacological intervention. Modulation of the system might influence inflammatory, angiogenic or vasoregulatory processes. S1P activates G-protein coupled receptors, namely S1P(1-5) , whereas only S1P(1-3) is present in vascular cells. S1P can also act as an intracellular signalling molecule. This review highlights the pharmacological potential of S1P signalling in the vascular system by giving an overview of S1P-mediated processes in endothelial cells (ECs) and vascular smooth muscle cells (VSMCs). After a short summary of S1P metabolism and signalling pathways, the role of S1P in EC and VSMC proliferation and migration, the cause of relaxation and constriction of arterial blood vessels, the protective functions on endothelial apoptosis, as well as the regulatory function in leukocyte adhesion and inflammatory responses are summarized. This is followed by a detailed description of currently known pharmacological agonists and antagonists as new tools for mediating S1P signalling in the vasculature. The variety of effects influenced by S1P provides plenty of therapeutic targets currently under investigation for potential pharmacological intervention.

Vascular biology: the role of sphingosine 1-phosphate in both the resting state and inflammation

The vascular and immune systems of mammals are closely intertwined: the individual components of the immune system must move between various body compartments to perform their function effectively. Sphingosine 1-phosphate (S1P), a bioactive lipid mediator, exerts effects on the two organ systems and influences the interaction between them. In the resting state, the vascular S1P gradient contributes to control of lymphocyte recirculation through the blood, lymphoid tissue and lymphatic vasculature. The high level of S1P in blood helps maintain endothelial barrier integrity. During the inflammatory process, both the level of S1P in different immune compartments and S1P receptor expression on lymphocytes and endothelial cells are modified, resulting in functionally important changes in endothelial cell and lymphocyte behaviour. These include transient arrest of lymphocytes in secondary lymphoid tissue, crucial for generation of adaptive immunity, and subsequent promotion of lymphocyte recruitment to sites of inflammation. This review begins with an outline of the basic biochemistry of S1P. S1P receptor signalling is then discussed, followed by an exploration of the roles of S1P in the vascular and immune systems, with particular focus on the interface between them. The latter part concerns crosstalk between S1P and other signalling pathways, and concludes with a look at therapies targeting the S1P-S1P receptor axis.

An association between gene expression and better survival in female mice following myocardial infarction

Following myocardial infarction, the prognosis for females is better than males. Estrogen is thought to be protective, but clinical trials with hormone replacement failed to show protection. Here, we sought to identify novel mechanisms that might explain this sex-based difference. By diverging from the traditional focus on sex hormones, we employed a conceptually novel approach to this question by using a non-biased approach to measure global changes in gene expression following infarction. We hypothesized that specific gene programs are initiated in the heart following infarction that might account for this sex-based difference. We induced small, medium, and large infarcts in male and female mice and measured changes in gene expression by microarray following infarction. Regardless of infarct size, survival was better in females, while mortality occurred 3-10 days following infarction in males. Two days following infarction, males developed significant ventricular dilation, the best predictor of mortality in humans. Three days following infarction, we measured gene expression by microarray, comparing male versus female and sham versus surgery/infarction. In general, our results indicate a higher relative level of gene induction in females versus males and identified programs for angiogenesis, extracellular matrix remodeling, and immune response. This pattern of gene expression was linked to less pathologic remodeling in female hearts, including increased capillary density and decreased fibrosis. In summary, our results suggest an association between improved survival and less pathologic remodeling and the relative induction of gene expression in females following myocardial infarction.

Sphingosine kinase-1 (SphK-1) regulates Mycobacterium smegmatis infection in macrophages

Sphingosine kinase-1 is known to mediate Mycobacterium smegmatis induced inflammatory responses in macrophages, but its role in controlling infection has not been reported to date. We aimed to unravel the significance of SphK-1 in controlling M. smegmatis infection in RAW 264.7 macrophages. Our results demonstrated for the first time that selective inhibition of SphK-1 by either D, L threo dihydrosphingosine (DHS; a competitive inhibitor of Sphk-1) or Sphk-1 siRNA rendered RAW macrophages sensitive to M. smegmatis infection. This was due to the reduction in the expression of iNOs, p38, pp-38, late phagosomal marker, LAMP-2 and stabilization of the RelA (pp-65) subunit of NF-kappaB. This led to a reduction in the generation of NO and secretion of TNF-alpha in infected macrophages. Congruently, overexpression of SphK-1 conferred resistance in macrophages to infection which was due to enhancement in the generation of NO and expression of iNOs, pp38 and LAMP-2. In addition, our results also unraveled a novel regulation of p38MAPK by SphK-1 during M. smegmatis infection and generation of NO in macrophages. Enhanced NO generation and expression of iNOs in SphK-1++ infected macrophages demonstrated their M-1(bright) phenotype of these macrophages. These findings thus suggested a novel antimycobacterial role of SphK-1 in macrophages.

The sphingosine kinase inhibitor N,N-dimethylsphingosine inhibits neointimal hyperplasia

BACKGROUND AND PURPOSE

Sphingosine-1-phosphate and its receptors may be involved in vascular smooth muscle cell (VSMC) proliferation following vascular injury. Here, we evaluate the effect of d-erythro-N,N-dimethylsphingosine (DMS), a sphingosine kinase (SK) inhibitor, on VSMC proliferation, apoptosis and neointimal formation.

EXPERIMENTAL APPROACH

Growth responses in vitro to fetal calf serum (FCS) were measured by [(3)H]-thymidine incorporation and extracellular signal-regulated kinase-1/2 (ERK-1/2) activation in quiescent primary cultures of porcine VSMC in the presence and absence of various concentrations of the SK inhibitor DMS. In vivo treatment with DMS was delivered with a local endoluminal catheter, following balloon injury of coronary arteries. The artery intimal formation was investigated by angiography, myography and histomorphometry.

KEY RESULTS

In vitro experiments indicated that DMS induced a dose-dependent reduction in [(3)H]-thymidine incorporation and ERK-1/2 activation via a protein kinase C (PKC) independent mechanism with an IC(50) value of 12 +/- 6 and 15 +/- 10 microM respectively. DMS also reduced Akt signalling. Four weeks following in vivo delivery of DMS, complete functional endothelial regeneration was observed in all treatment groups, with significant reduction of intimal formation (vehicle 23.7 +/- 4.6% vs. DMS infusion 8.92 +/- 2.9%, P < 0.05).

CONCLUSIONS AND IMPLICATIONS

Taken together, these results demonstrate that local administration of the SK inhibitor, DMS, reduced neointimal formation, and this effect could involve inhibition of ERK-1/2 and Akt signalling, and modulation of smooth muscle growth.

Sphingosine kinase-1: a potential mediator of renal fibrosis

Renal fibrosis contributes to glomerulosclerosis and tubulointerstitial damage in chronic kidney disease. A well-established pathway implicated in the progression of fibrosis is the induction of connective tissue growth factor by transforming growth factor-beta, resulting in the accumulation of extracellular matrix proteins. Ren and colleagues demonstrate that sphingosine kinase-1 is involved in the regulation of this pathway in the glomerulus. This raises the possibility of targeting sphingosine kinase-1 to prevent fibrosis in chronic kidney disease patients.

Differential gene expression in a coculture model of angiogenesis reveals modulation of select pathways and a role for Notch signaling

Communication between endothelial and mural cells (smooth muscle cells, pericytes, and fibroblasts) can dictate blood vessel size and shape during angiogenesis, and control the functional aspects of mature blood vessels, by determining things such as contractile properties. The ability of these different cell types to regulate each other's activities led us to ask how their interactions directly modulate gene expression. To address this, we utilized a three-dimensional model of angiogenesis and screened for genes whose expression was altered under coculture conditions. Using a BeadChip array, we identified 323 genes that were uniquely regulated when endothelial cells and mural cells (fibroblasts) were cultured together. Data mining tools revealed that differential expression of genes from the integrin, blood coagulation, and angiogenesis pathways were overrepresented in coculture conditions. Scans of the promoters of these differentially modulated genes identified a multitude of conserved C promoter binding factor (CBF)1/CSL elements, implicating Notch signaling in their regulation. Accordingly, inhibition of the Notch pathway with gamma-secretase inhibitor DAPT or NOTCH3-specific small interfering RNA blocked the coculture-induced regulation of several of these genes in fibroblasts. These data show that coculturing of endothelial cells and fibroblasts causes profound changes in gene expression and suggest that Notch signaling is a critical mediator of the resultant transcription.