In vitro and in vivo modulation of vascular barrier integrity by sphingosine 1-phosphate: mechanistic insights

Quantitative proteomics analysis of human endothelial cell membrane rafts: evidence of MARCKS and MRP regulation in the sphingosine 1-phosphate-induced barrier enhancement

Endothelial cell barrier dysfunction results in the increased vascular permeability observed in inflammation, tumor metastasis, angiogenesis, and atherosclerosis. Sphingosine 1-phosphate (S1P), a biologically active phosphorylated lipid growth factor released from activated platelets, enhances the endothelial cell barrier integrity in vitro and in vivo. To begin to identify the molecular mechanisms mediating S1P induced endothelial barrier enhancement, quantitative proteomics analysis (iTRAQ) was performed on membrane rafts isolated from human pulmonary artery endothelial cells in the absence or presence of S1P stimulation. Our results demonstrated that S1P mediates rapid and specific recruitment (1 microM, 5 min) of myristoylated alanine-rich protein kinase C substrate (MARCKS) and MARCKS-related protein (MRP) to membrane rafts. Western blot experiments confirmed these findings with both MARCKS and MRP. Finally, small interfering RNA-mediated silencing of MARCKS or MRP or both attenuates S1P-mediated endothelial cell barrier enhancement. These data suggest the regulation of S1P-mediated endothelial cell barrier enhancement via the cell specific localization of MARCKS and MRP and validate the utility of proteomics approaches in the identification of novel molecular targets.

Pharmacophore-based design of sphingosine 1-phosphate-3 receptor antagonists that include a 3,4-dialkoxybenzophenone scaffold

Sphingosine 1-phosphate (S1P) receptors are G-protein-coupled receptors. Among the five identified subtypes S1P1-5, the S1P3 receptor expressed on vascular endothelial cells has been shown to play an important role in cell proliferation, migration, and inflammation. A pharmacophore-based database search was used to identify a potent scaffold for an S1P3 receptor antagonist by common feature-based alignment and further validated using the Güner-Henry (GH) scoring method. Assumed excluded volumes were merged into this model to evaluate the steric effect with the S1P3 receptor. Three commercially available compounds were identified as S1P3 receptor antagonists, with IC50 values <5 microM. The synthesis of further derivatives revealed that the 3,4-dialkoxybenzophenone scaffold is a potent component of an S1P3 receptor antagonist. Our results indicate that pharmacophore-based design of S1P3 receptor antagonists can be used to expand the possibility of structural modification through scaffold-hopping based on a database search.

Tipping the gatekeeper: S1P regulation of endothelial barrier function

The lysophospholipid sphingosine 1-phosphate (S1P) is a pleiotropic signaling lipid present constitutively in plasma, and secreted locally at elevated concentrations at sites of inflammation. S1P maintains essential variable homeostatic functions in addition to inducing pathophysiology through the activation of five specific high-affinity G-protein-coupled receptors. Therefore, S1P can function as an extracellular rheostat regulating tonic and acutely evoked functions. Although S1P receptors can regulate lymphoid development and lymphocyte trafficking, and different opinions exist on the roles of receptor agonism and functional antagonism in regulating lymphocyte recirculation, this personal perspective highlights the pivotal control points regulated by constitutive and induced S1P receptor tone at vascular endothelial and lymphatic endothelial barriers, through which S1P agonism impacts on both innate and adaptive immunity. We also emphasize how specific, proof-of-concept chemical tools complement genetic approaches by enabling reversible perturbation of the S1P-S1P(1) receptor axis and, thus, clarifying in vivo mechanisms in the absence of developmental compensations.

Erythrocytes store and release sphingosine 1-phosphate in blood

The blood constituent sphingosine 1-phosphate (S1P) is a specific ligand for five G-protein-coupled receptors designated S1P(1-5). Expression of the S1P1 receptor on lymphocytes is required for their exit from secondary lymphoid organs, suggesting that S1P serves as a stimulus for maintaining lymphocyte circulation in blood. Despite its potential role in immune surveillance, the regulatory system that controls blood S1P levels is not well understood. This report reveals that erythrocytes constitute a buffer system for S1P in blood. They efficiently incorporated and stored S1P, and protected it from cellular degradation. They also released S1P into plasma, but not into other serum-free media, indicating that S1P release was controlled by a plasma factor. Erythrocytes did not generate S1P since an increase in plasma S1P levels was always accompanied by a decrease in cellular S1P levels. Thrombocytes that were reported to generate and release S1P after activation did not contribute to the observed S1P release in blood. The amount of erythrocytes as well as the proportion of plasma in the medium determined the magnitude of S1P release. Adoptively transferred S1P-loaded and unloaded mouse erythrocytes displayed a normal life span and similar S1P levels 24 h after recovery, indicating that S1P incorporation and release are dynamically regulated in vivo.

Constrained azacyclic analogues of the immunomodulatory agent FTY720 as molecular probes for sphingosine 1-phosphate receptors

Constrained azacyclic analogues of FTY720 were prepared starting with d- and l-pyroglutamic acids. One enantiomer was shown to be a substrate for sphingosine kinase 2, being phosphorylated 4-fold more efficiently than FTY720. Among the corresponding phosphates, two were found to have unusual specificity in binding to S1P receptors: while being inactive on S1P1 and S1P3, they acted as potent agonists on S1P4 and S1P5. The phosphates may be useful to explore the biology and binding site of these receptors.

Lysophospholipid receptors: signalling, pharmacology and regulation by lysophospholipid metabolism

The lysophospholipids, sphingosine-1-phosphate (S1P), lysophosphatidic acid (LPA), sphingosylphosphorylcholine (SPC) and lysophosphatidylcholine (LPC), activate diverse groups of G-protein-coupled receptors that are widely expressed and regulate decisive cellular functions. Receptors of the endothelial differentiation gene family are activated by S1P (S1P(1-5)) or LPA (LPA(1-3)); two more distantly related receptors are activated by LPA (LPA(4/5)); the GPR(3/6/12) receptors have a high constitutive activity but are further activated by S1P and/or SPC; and receptors of the OGR1 cluster (OGR1, GPR4, G2A, TDAG8) appear to be activated by SPC, LPC, psychosine and/or protons. G-protein-coupled lysophospholipid receptors regulate cellular Ca(2+) homoeostasis and the cytoskeleton, proliferation and survival, migration and adhesion. They have been implicated in development, regulation of the cardiovascular, immune and nervous systems, inflammation, arteriosclerosis and cancer. The availability of S1P and LPA at their G-protein-coupled receptors is regulated by enzymes that generate or metabolize these lysophospholipids, and localization plays an important role in this process. Besides FTY720, which is phosphorylated by sphingosine kinase-2 and then acts on four of the five S1P receptors of the endothelial differentiation gene family, other compounds have been identified that interact with more ore less selectivity with lysophospholipid receptors.

Normal acute and chronic inflammatory responses in sphingosine kinase 1 knockout mice

Sphingosine-1-phosophate, generated from the phosphorylation of sphingosine by sphingosine kinase enzymes, is suggested to function as an intracellular second messenger for inflammatory mediators, including formyl peptide, C5a, and Fc. More recently, a role for sphingosine kinases during inflammation has also been proposed. Here we show that sphingosine kinase 1 knockout mice exhibit normal inflammatory cell recruitment during thioglycollate-induced peritonitis and that sphingosine kinase 1-null neutrophils respond normally to formyl peptide. In the collagen-induced arthritis model of rheumatoid arthritis, sphingosine kinase 1 knockout mice developed arthritis with normal incidence and severity. Our findings show that sphingosine kinase 1 is dispensable for inflammatory responses and support the need for more extensive studies of sphingosine kinases in inflammation.

Alphastatin downregulates vascular endothelial cells sphingosine kinase activity and suppresses tumor growth in nude mice bearing human gastric cancer xenografts

AIM: To investigate whether alphastatin could inhibit human gastric cancer growth and furthermore whether sphingosine kinase (SPK) activity is involved in this process.

METHODS: Using migration assay, MTT assay and Matrigel assay, the effect of alphastatin on vascular endothelial cells (ECs) was evaluated in vitro. SPK and endothelial differentiation gene (EDG)-1, -3, -5 mRNAs were detected by reverse transcription-polymerase chain reaction (RT-PCR). SPK activity assay was used to evaluate the effect of alphastatin on ECs. Matrigel plug assay in nude mice was used to investigate the effect of alphastatin on angiogenesis in vivo. Female nude mice were subcutaneously implanted with human gastric cancer cells (BGC823) for the tumor xenografts studies. Micro vessel density was analyzed in Factor VIII-stained tumor sections by the immunohistochemical SP method.

RESULTS: In vitro, alphastatin inhibited the migration and tube formation of ECs, but had no effect on proliferation of ECs. RT-PCR analysis demonstrated that ECs expressed SPK and EDG-1, -3, -5 mRNAs. In vivo, alphastatin sufficiently suppressed neovascularization of the tumor in the nude mice. Daily administration of alphastatin produced significant tumor growth suppression. Immunohistochemical studies of tumor tissues revealed decreased micro vessel density in alphastatin-treated animals as compared with controls.

CONCLUSION: Downregulating ECs SPK activity may be one of the mechanisms that alphastatin inhibits gastric cancer angiogenesis. Alphastatin might be a useful and relatively nontoxic adjuvant therapy in the treatment of gastric cancer.

Enhancement of capillary leakage and restoration of lymphocyte egress by a chiral S1P1 antagonist in vivo

Sphingosine 1-phosphate (S1P, 1) regulates vascular barrier and lymphoid development, as well as lymphocyte egress from lymphoid organs, by activating high-affinity S1P1 receptors. We used reversible chemical probes (i) to gain mechanistic insights into S1P systems organization not accessible through genetic manipulations and (ii) to investigate their potential for therapeutic modulation. Vascular (but not airway) administration of the preferred R enantiomer of an in vivo-active chiral S1P1 receptor antagonist induced loss of capillary integrity in mouse skin and lung. In contrast, the antagonist did not affect the number of constitutive blood lymphocytes. Instead, alteration of lymphocyte trafficking and phenotype required supraphysiological elevation of S1P1 tone and was reversed by the antagonist. In vivo two-photon imaging of lymph nodes confirmed requirements for obligate agonism, and the data were consistent with the presence of a stromal barrier mechanism for gating lymphocyte egress. Thus, chemical modulation reveals differences in S1P-S1P1 'set points' among tissues and highlights both mechanistic advantages (lymphocyte sequestration) and risks (pulmonary edema) of therapeutic intervention.

Delivery of sphingosine 1-phosphate from poly(ethylene glycol) hydrogels

While protein growth factors promote therapeutic angiogenesis, delivery of lipid factors such as sphingosine 1-phosphate (S1P) may provide better stabilization of newly formed vessels. We developed a biomaterial for the controlled delivery of S1P, a bioactive lipid released from activated platelets. Multiarm poly(ethylene glycol)-vinyl sulfone was cross-linked with albumin, a lipid-transporting protein, to form hydrogels. The rate of S1P release from the materials followed Fickian kinetics and was dependent upon the presence of lipid carriers in the release solution. Delivery of S1P from RGD-modified hydrogels increased the cell migration speed of endothelial cells growing on the materials. The materials also induced angiogenesis in the chorioallantoic membrane assay. Our data demonstrate that the storage and release of lipid factors provides a new route for the induction of angiogenesis by artificial materials.

Focal adhesions: paradigm for a signaling nexus

The vascular wall contains intimal endothelium and medial smooth muscle that act as contiguous tissues with tight spatial and functional coordination in response to tonic and episodic input from the bloodstream and the surrounding parenchyma. Focal adhesions are molecular bridges between the intracellular and extracellular spaces that integrate a variety of environmental stimuli and mediate 2-way crosstalk between the extracellular matrix and the cytoskeleton. Focal adhesion components are targets for biochemical and mechanical stimuli that evoke crucial developmental and injury response mechanisms including cell growth, movement, and differentiation, and tailoring of the extracellular microenvironment. Focal adhesions provide the vascular wall constituents with flexible and specific tools for exchanging cues in a complex system. The molecular mechanisms that underlie these vital communications are detailed in this review with the goal of defining future targets for vascular tissue engineering and for the therapeutic modulation of disordered vascular growth, inflammation, thrombosis, and angiogenesis.

Efficacy of mycophenolic acid combined with KRP-203, a novel immunomodulator, in a rat heart transplantation model

BACKGROUND

To explore a more effective and less toxic immunosuppressive strategy in organ transplantation, we recently developed the novel sphingosine-1-phosphate receptor agonist KRP-203. This study examined the efficacy of KRP-203 combined with mycophenolic acid (MPA), an active metabolite of mycophenolate mofetil, in rat heart allografts.

METHODS

Heterotopic heart transplantation was performed in a rat combination of DA (MHC haplotype: RT1(a)) to Lewis (RT1). The recipients were divided into 12 groups (n = 5-7): Syngeneic (Lewis to Lewis), Vehicle, KRP-203 (0.3 and 1 mg/kg), MPA (10 and 20 mg/kg), 10 mg/kg MPA with KRP-203 (0.03, 0.3, 1, and 3 mg/kg), and 20 mg/kg MPA with KRP-203 (0.3 and 1 mg/kg). MPA, KRP-203, and vehicle were given orally.

RESULTS

The mean days of survival were 5.8 (vehicle), 7 and 7.9 (0.3 and 1 mg/kg KRP-203, respectively), 12.7 and >54.4 (10 and 20 mg/kg MPA), >39.6 and >30.5 (10 mg/kg MPA with 1 and 3 mg/kg KRP-203), >100 and >87.8 (20 mg/kg MPA with 0.3 and 1 mg/kg KRP-203). Histologic and immunohistochemical analysis revealed that diffuse mononuclear cell infiltration (macrophages and T cells), hemorrhage, myocardial necrosis and fibrosis, and expression of endothelin-1, transforming growth factor-beta1, monocyte chemoattractant protein-1, interleukin-8, and E-selectin were markedly diminished in the allografts treated with MPA combined with KRP-203. Pharmacokinetic experiments indicated no interaction between MPA and KRP-203, and both combination regimens were well tolerated.

CONCLUSIONS

Combination therapy of MPA with KRP-203 has a therapeutic potential as a novel immunosuppressant strategy in clinical transplantation.

Emerging medicinal roles for lysophospholipid signaling

The two lysophospholipids (LPs) lysophosphatidic acid and sphingosine 1-phosphate (S1P) regulate diverse biological processes. Over the past decade, it has become clear that medically relevant LP activities are mediated by specific G protein-coupled receptors, implicating them in the etiology of a growing number of disorders. A new class of LP agonists shows promise for drug therapy: the experimental drug FTY720 is phosphorylated in vivo to produce a potent S1P receptor agonist (FTY720-P) and is currently in Phase III clinical trials for kidney transplantation and Phase II for multiple sclerosis. Recent genetic and pharmacological studies on LP signaling in animal disease models have identified new areas in which interventions in LP signaling might provide novel therapeutic approaches for the treatment of human diseases.

CD69 down-modulation and inhibition of thymic egress by short- and long-term selective chemical agonism of sphingosine 1-phosphate receptors

Thymic development requires proliferation, selection, maturation and release of mature single-positive CD4 and CD8 T cells into the periphery. In mice, non-selective sphingosine-1 phosphate (S1P) receptor agonists, active on four of the five known S1P receptors, alter thymocyte phenotype and egress. Here, we show that down-modulation of CD69 occurs acutely and transiently at a discrete and late stage of medullary development after a single-dose administration of S1P(1) receptor-selective agonist, which induces long-term tonic receptor activation in the absence of receptor degradation. In addition, agonist acutely inhibited egress of mature thymocytes into peripheral lymphoid organs, suggesting that both the phenotype and migration of medullary thymocytes are regulated simultaneously and coordinately by agonism of S1P(1) alone. Long-term dosing shifted the early/late medullary thymocyte ratio with an expansion of the late medullary compartment, as mature CD69(-) thymocytes were retained within the thymus. Therefore, chemical agonism of S1P(1) accelerates medullary phenotypic maturation and inhibits egress, leading to the expansion and accumulation of the recent thymocyte emigrant population in the medulla. However, chemical agonism fails to replicate the S1P(1)-null CD69(hi) late medullary phenotype, suggesting that agonism and gene deletion operate by distinct mechanisms, and that functional receptor antagonism may not be required for lymphocyte sequestration.

Activation of Rac-1 and Cdc42 stabilizes the microvascular endothelial barrier

We have demonstrated previously that the Rho family GTPase Rac-1 is required for maintenance of endothelial barrier functions in mouse microvascular myocardial endothelial (MyEnd) cells in vitro as well as in rat mesenteric microvessels in vivo. In this study, we tested the hypothesis that specific activation of Rac-1 would stabilize microvascular endothelial barrier functions. For this purpose we used Escherichia coli Cytotoxic necrotizing factor (CNF-1) under conditions (300 ng/ml, 120 min) where it strongly activated Rac-1 and Cdc42 but not Rho A in MyEnd cells. Under these conditions, CNF-1 induced translocation of the actin-binding proteins cortactin and vasodilator-stimulated phosphoprotein (VASP) to cell junctions, increased the junction-associated actin filament belt, and reduced monolayer permeability. We also tested the effect of CNF-1 on endothelial barrier properties in vivo using single-perfused mesenteric microvessels. In contrast to cultured microvascular monolayers, CNF-1 did not reduce baseline barrier functions assayed as hydraulic conductivity (Lp). However, following 120 min pretreatment, CNF-1 significantly attenuated the peak Lp increase in response to platelet-activating factor (PAF, 10 nM) to 12.6+/-4 x 10(-7) cm/(s cmH(2)O) compared to 46.2+/-10 x 10(-7) cm/(s cmH(2)O) in experiments using PAF alone. These experiments indicate that activation of Rac-1 and Cdc42 stabilizes microvascular endothelial barrier functions in vitro and in vivo, likely by increasing the junction-associated actin cytoskeleton.

Sphingosine 1-phosphate and its receptors: an autocrine and paracrine network

Sphingosine 1-phosphate (S1P) is a biologically active lysophospholipid that transmits signals through a family of G-protein-coupled receptors to control cellular differentiation and survival, as well as the vital functions of several types of immune cell. In this Review article, we discuss recent results that indicate that S1P and its receptors are required for the emigration of thymocytes from the thymus, the trafficking of lymphocytes in secondary lymphoid organs and the migration of B cells into splenic follicles. In an autocrine manner, through interactions with different G-protein-coupled receptors, S1P also enhances optimal mast-cell migration and release of pro-inflammatory mediators in allergic reactions. S1P-S1P-receptor regulatory systems might therefore be novel targets for the therapy of diverse immunological diseases.

S1P3 receptor-induced reorganization of epithelial tight junctions compromises lung barrier integrity and is potentiated by TNF

Pulmonary pathologies including adult respiratory distress syndrome are characterized by disruption of pulmonary integrity and edema compromising respiratory function. Sphingosine 1-phosphate (S1P) is a lipid mediator synthesized and/or stored in mast cells, platelets, and epithelial cells, with production up-regulated by the proinflammatory cytokines IL-1 and TNF. S1P administration via the airways but not via the vasculature induces lung leakage. Using receptor-null mice, we show that S1P, acting on S1P3 receptor expressed on both type I and type II alveolar epithelial cells but not vascular endothelium, induces pulmonary edema by acute tight junction opening. WT but not S1P3-null mice showed disruption of pulmonary epithelial tight junctions and the appearance of paracellular gaps between epithelial cells by electron microscopy within 1 h of airways exposure to S1P. We further show by fluorescence microscopy that S1P induced rapid loss of ZO-1 reactivity, an essential component of the cytoplasmic plaque associated with tight junctions, as well as of the tetraspannin Claudin-18, an integral membrane organizer of tight junctions. S1P shows synergistic activity with the proinflammatory cytokine TNF, showing both pulmonary edema and mortality at subthreshold S1P doses. Specifically, preexposure of mice to subthreshold doses of TNF, which alone induced no lung edema, exacerbated S1P-induced edema and impaired survival. S1P, acting through S1P3, regulates epithelial integrity and acts additively with TNF in compromising respiratory barrier function. Because S1P3-null mice are resistant to S1P-induced pulmonary leakage, either alone or in the presence of TNF, S1P3 antagonism may be useful in protecting epithelial integrity in pulmonary disease.

Immunohistochemical Distribution of Sphingosine Kinase 1 in Normal and Tumor Lung Tissue

Sphingosine kinase 1 (SK1) is a key enzyme critical to the sphingolipid metabolic pathway responsible for catalyzing the formation of the bioactive lipid sphingosine-1-phosphate. SK1-mediated production of sphingosine-1-phosphate has been shown to stimulate such biological processes as cell growth, differentiation, migration, angiogenesis, and inhibition of apoptosis. In this study, cell type–specific immunolocalization of SK1 was examined in the bronchus/terminal bronchiole of the lung. Strong immunopositive staining was evident at the apical surface of pseudostratified epithelial cells of the bronchus and underlying smooth muscle cells, submucosal serous glands, immature chondrocytes, type II alveolar cells, foamy macrophages, endothelial cells of blood vessels, and neural bundles. Immunohistochemical screening for SK1 expression was performed in 25 samples of normal/tumor patient matched non–small-cell lung cancer tissue and found that 25 of 25 tumor samples (carcinoid [5 samples], squamous [10 samples], and adenocarcinoma tumors [10 samples]), exhibited overwhelmingly positive immunostaining for SK1 as compared with patient-matched normal tissue. In addition, an approximately 2-fold elevation of SK1 mRNA expression was observed in lung cancer tissue versus normal tissue, as well as in several other solid tumors. Taken together, these findings define the localization of SK1 in lung and provide clues as to how SK1 may play a role in normal lung physiology and the pathophysiology of lung cancer.