Stable knock-down of the sphingosine 1-phosphate receptor S1P1 influences multiple functions of human endothelial cells

Ascotricins A and B, novel antagonists of sphingosine-1-phosphate receptor 1 from Ascotricha chartarum Berk. SANK 14186

Ascotricins A and B were isolated as novel sphingosine-1-phosphate receptor 1 (S1P(1)) antagonists from a cultured broth of a fungus identified as Ascotricha chartarum Berk. SANK 14186. The two compounds were purified by solvent extraction, reversed-phase (RP) column chromatography and a preparative RP-HPLC. The structures were determined by various NMR experiments and by LC/MS and GC/MS analyses. The S1P(1) antagonist activities were measured by a cyclic AMP assay using S1P(1)-expressing cells and the IC(50) values were 8.2 and 1.8 microM, respectively. In a [(33)P]sphingosine-1-phosphate/S1P(1)-binding assay, those values were 120 and 39 microM, and in a migration assay using human umbilical vein endothelial cells (HUVECs), they were 94 and 28 microM, respectively. Thus, ascotricins A and B are novel S1P(1) antagonists showing an inhibition activity toward HUVEC migration.

Arsenic requires sphingosine-1-phosphate type 1 receptors to induce angiogenic genes and endothelial cell remodeling

Arsenic in drinking water is a major public health concern as it increases risk and incidence of cardiovascular disease and cancer. Arsenic exposure affects multiple vascular beds, promoting liver sinusoidal capillarization and portal hypertension, ischemic heart disease, peripheral vascular disease, and tumor angiogenesis. While Rac1-GTPase and NADPH oxidase activities are essential for arsenic-stimulated endothelial cell signaling for angiogenesis or liver sinusoid capillarization, the mechanism for initiating these effects is unknown. We found that arsenic-stimulated cell signaling and angiogenic gene expression in human microvascular endothelial cells were Pertussis toxin sensitive, indicating a G-protein coupled signaling pathway. Incubating human microvascular endothelial cells with the sphingosine-1-phosphate type 1 receptor (S1P(1)) inhibitor VPC23019 or performing small interfering RNA knockdown of S1P(1) blocked arsenic-stimulated HMVEC angiogenic gene expression and tube formation, but did not affect induction of either HMOX1 or IL8. Liver sinusoidal endothelial cells (LSECs) defenestrate and capillarize in response to aging and environmental oxidant stresses. We found that S1P(1) was enriched on LSECs in vivo and in primary cell culture and that VPC23019 inhibited both sphingosine-1-phosphate-stimulated and arsenic-stimulated LSEC oxidant generation and defenestration. These studies identified novel roles for S1P(1) in mediating arsenic stimulation of both angiogenesis and pathogenic LSEC capillarization, as well as demonstrating a role for S1P(1) in mediating environmental responses in the liver vasculature, providing possible mechanistic insight into arsenic-induced vascular pathogenesis and disease.

Molecular profile of endothelial invasion of three-dimensional collagen matrices: insights into angiogenic sprout induction in wound healing

Sprouting angiogenesis is a multistep process consisting of basement membrane degradation, endothelial cell (EC) activation, proliferation, invasion, lumen formation, and sprout stabilization. Such complexity is consistent with a requirement for orchestration of individual gene expression alongside multiple signaling pathways. To better understand the mechanisms that direct the transformation of adherent ECs on the surface of collagen matrices to develop multicellular invading sprouts, we analyzed differential gene expression with time using a defined in vitro model of EC invasion driven by the combination of sphingosine-1-phosphate, basic FGF, and VEGF. Gene expression changes were confirmed by real-time PCR and Western blot analyses. A cohort of cell adhesion molecule genes involved in adherens junction and cell-extracellular matrix (ECM) interactions were upregulated, whereas a set of genes associated with tight junctions were downregulated. Numerous genes encoding ECM proteins and proteases were induced, indicating that biosynthesis and remodeling of ECM is indispensable for sprouting angiogenesis. Knockdown of a highly upregulated gene, a disintegrin and metalloproteinase with thrombospondin-type repeats-1 (ADAMTS1), decreased invasion responses, confirming a role for ADAMTS1 in mediating EC invasion. Furthermore, differential expression of multiple members of the Wnt and Notch pathways was observed. Functional experiments indicated that inhibition and activation of the Notch signaling pathway stimulated and inhibited EC invasion responses, respectively. This study has enhanced the molecular road map of gene expression changes that occur during endothelial invasion and highlighted the utility of three-dimensional models to study EC morphogenesis.

Regulation of vascular permeability by sphingosine 1-phosphate

A significant and sustained increase in vascular permeability is a hallmark of acute inflammatory diseases such as acute lung injury (ALI) and sepsis and is an essential component of tumor metastasis, angiogenesis, and atherosclerosis. Sphingosine 1-phosphate (S1P), an endogenous bioactive lipid produced in many cell types, regulates endothelial barrier function by activation of its G-protein coupled receptor S1P(1). S1P enhances vascular barrier function through a series of profound events initiated by S1P(1) ligation with subsequent downstream activation of the Rho family of small GTPases, cytoskeletal reorganization, adherens junction and tight junction assembly, and focal adhesion formation. Furthermore, recent studies have identified transactivation of S1P(1) signaling by other barrier-enhancing agents as a common mechanism for promoting endothelial barrier function. This review summarizes the state of our current knowledge about the mechanisms through which the S1P/S1P(1) axis reduces vascular permeability, which remains an area of active investigation that will hopefully produce novel therapeutic agents in the near future.

Immunohistochemical detection of sphingosine-1-phosphate receptor 1 in vascular and lymphatic endothelial cells

Sphingosine-1-phosphate receptor 1 (S1P(1)), a receptor for sphingosine-1-phosphate, has been shown to play an important role in the migration, proliferation, and survival of several types of cell including endothelial cells. Given that S1P(1) signaling could serve as a therapeutic target, we evaluate the expression of S1P(1) in formalin-fixed and paraffin-embedded sections from human tissues, using automated immunostainers (Ventana). The specificity of the polyclonal rabbit anti-human S1P(1) antibody used in this study was defined by immunostaining of the vasculature in S1P ( 1 ) ( -/- ) and S1P ( 1 ) ( +/- ) mouse embryos. The antibody stained the newly formed vasculatures ex vivo in a serum-free matrix culture model using rat aortic rings. In human specimens, S1P(1) was strongly expressed on the cell surface membrane of endothelial cells of blood and lymphatic vessels in all tissues examined. The expression of S1P(1) was confirmed by the flow cytometric analysis and real time RT-PCR of an angiosarcoma cell line. This study indicates that S1P(1) can be used as an immunohistochemical marker for human tissue endothelial cells.

Amicus or adversary: platelets in lung biology, acute injury, and inflammation

Platelets are the chief effector cells in hemostasis and have additional major functions in inflammation, vascular integrity, and tissue repair. Platelets and the lungs have interrelated activities. Previous studies provide evidence that platelets contribute to pulmonary vascular barrier function and are required for defense against pulmonary hemorrhage, and that the lungs can influence platelet number and distribution. There is also evidence that platelets contribute to pathologic syndromes of pulmonary inflammation and thrombosis. Thus, platelets have an "amicus or adversary" relationship with the lung. Recent observations and discoveries have established new paradigms relevant to influences of platelets on lung cell and molecular biology. These new findings are in a variety of areas including thrombopoieis, nontraditional activities of platelets, new synthetic capabilities and mechanisms of post-translational gene expression, interactions of platelets with endothelial cells and contributions to alveolar capillary barrier permeability, interactions of platelets with myeloid leukocytes, and platelet involvement in stem cell signaling and vascular repair. These issues are considered in a translational approach, with an emphasis on acute lung injury and the acute respiratory distress syndrome.

Targeting SphK1 as a new strategy against cancer

Sphingolipid metabolites have emerged as critical players in a number of fundamental biological processes. Among them, sphingosine-1-phosphate (S1P) promotes cell survival and proliferation, in contrast to ceramide and sphingosine, which induce cell growth arrest and apoptosis. These sphingolipids with opposing functions are interconvertible inside cells, suggesting that a finely tuned balance between them can determine cell fate. Sphingosine kinases (SphKs), which catalyze the phosphorylation of sphingosine to S1P, are critical regulators of this balance. Of the two identified SphKs, sphingosine kinase type 1 (SphK1) has been shown to regulate various processes important for cancer progression and will be the focus of this review, since much less is known of biological functions of SphK2, especially in cancer. SphK1 is overexpressed in various types of cancers and upregulation of SphK1 has been associated with tumor angiogenesis and resistance to radiation and chemotherapy. Many growth factors, through their tyrosine kinase receptors (RTKs), stimulate SphK1 leading to a rapid increase in S1P. This S1P in turn can activate S1P receptors and their downstream signaling. Conversely, activation of S1P receptors can induce transactivation of various RTKs. Thus, SphK1 may play important roles in S1P receptor RTK amplification loops. Here we review the role of SphK1 in tumorigenesis, hormonal therapy, chemotherapy resistance, and as a prognostic marker. We will also review studies on the effects of SphK inhibitors in cells in vitro and in animals in vivo and in some clinical trials and highlight the potential of SphK1 as a new target for cancer therapeutics.

FTY720 inhibits S1P-mediated endothelial healing: relationship to S1P1-receptor surface expression

The phosphorylated derivative of the immunosuppressant FTY720 interacts with and modulates the function of sphingosine-1-phosphate (S1P)-receptors. We observed a significant reduction of endothelial surface binding of a S1P(1)-specific antibody after FTY720 treatment of 6h and longer, which was associated with a reduced healing after mechanic injury, impaired angiogenesis and enhanced adhesion molecule expression. FTY720 (5h) had no impact on the expression of S1P(1)- or S1P(3)-encoding transcripts. Notably, pre-treatment of cells with FTY720 for only 30min, which did not reduce S1P(1) surface expression, inhibited the rapid S1P- and SEW2871- (a S1P(1) agonist) induced cortical actin formation and cell migration. FTY720 was effective at concentrations as low as 5nM. FTY720 at therapeutic concentrations may be harmful by impairing important endothelial functions. Interestingly, FTY720 inhibited endothelial actin remodelling and cell migration without decreasing S1P(1) surface expression.

Sphingosine-1-phosphate receptors: biology and therapeutic potential in kidney disease

The major sphingolipid metabolite, sphingosine-1-phosphate (S1P), has important biological functions. S1P is the ligand for a family of five G-protein-coupled receptors with distinct signaling pathways that regulate angiogenesis, vascular maturation, immunity, chemotaxis, and other important biological pathways. Recently, clinical trials have targeted S1P receptors (S1PRs) for autoimmune diseases and transplantation and have generated considerable interest in developing additional, more selective compounds. This review summarizes current knowledge on the biology of S1P and S1PRs that forms the basis for future drug development and the treatment of kidney disease.

VE-cadherin: the major endothelial adhesion molecule controlling cellular junctions and blood vessel formation

Vascular endothelial (VE)-cadherin is a strictly endothelial specific adhesion molecule located at junctions between endothelial cells. In analogy of the role of E-cadherin as major determinant for epithelial cell contact integrity, VE-cadherin is of vital importance for the maintenance and control of endothelial cell contacts. Mechanisms that regulate VE-cadherin-mediated adhesion are important for the control of vascular permeability and leukocyte extravasation. In addition to its adhesive functions, VE-cadherin regulates various cellular processes such as cell proliferation and apoptosis and modulates vascular endothelial growth factor receptor functions. Consequently, VE-cadherin is essential during embryonic angiogenesis. This review will focus on recent new developments in understanding the role of VE-cadherin in controlling endothelial cell contacts and influencing endothelial cell behavior by various outside-in signaling processes.

Variability in RNA interference in neuroendocrine PC12 cell lines stably transfected with an shRNA plasmid

RNA interference (RNAi) has quickly become a very powerful technique for specifically suppressing or knocking down the expression of any desired gene. Many fields of research, including neuroscience, have benefitted from RNAi methods. It has been well documented that different small interfering RNAs (siRNAs) and small hairpin RNAs (shRNAs) vary greatly in terms of their effectiveness, and much attention has been focused on guidelines and algorithms for the selection of effective siRNAs. However, it has not been widely appreciated that a single shRNA-expressing plasmid can also produce widely varying levels of knockdown in different stably transfected cell lines derived from the same transfection. Here we report that knockdown of three distinct target proteins varies from minimal to almost complete in independent, stably transfected PC12 cell lines. This variability in knockdown among cell lines emphasizes the importance of characterizing a number of cell lines when attempting to establish stable knockdown cell lines, but also offers the possibility of studying the effects of graded levels of protein expression.

Vascular effects of sphingolipids

The sphingomyelin metabolites ceramide, sphingosine 1-phosphate (S1P) and sphingosylphosphorylcholine (SPC) are emerging modulators of vascular tone. While ceramide appears to act primarily intracellularly, S1P and SPC appear to mainly work via specific receptors, although those for SPC have not yet been defined unequivocally. Each of the sphingomyelin metabolites can induce both vasoconstriction and vasodilatation and, in some cases--ceramide on the one hand, and S1P and SPC on the other hand--have opposite effects on vascular tone. The differences in effects between vessels may relate to the relative roles of endothelial and smooth muscle cells in mediating them, as well as to the distinct expression patterns of S1P receptors among vascular beds and among endothelial and smooth muscle cells. Recent evidence suggests that vascular tone is not only modulated by sphingomyelin metabolites which are exogenously added or reach the vessel wall via the bloodstream but also by those formed locally by cells in the vessel wall. Such local formation can be induced by known vasoactive agents such as angiotensin II and may serve a signalling function.

CONCLUSION

We conclude that sphingomyelin metabolites are important endogenous modulators of vascular function, which may contribute to the pathophysiology of some diseases and be targets for therapeutic interventions.

Attenuation of vascular permeability by methylnaltrexone: role of mOP-R and S1P3 transactivation

Endothelial cell (EC) barrier dysfunction (i.e., increased vascular permeability) is observed in inflammatory states, tumor angiogenesis, atherosclerosis, and both sepsis and acute lung injury. Therefore, agents that preserve vascular integrity have important clinical therapeutic implications. We examined the effects of methylnaltrexone (MNTX), a mu opioid receptor (mOP-R) antagonist, on human pulmonary EC barrier disruption produced by edemagenic agents including morphine, the endogenous mOP-R agonist DAMGO, thrombin, and LPS. Pretreatment of EC with MNTX (0.1 muM, 1 h) or the uncharged mOP-R antagonist naloxone attenuated morphine- and DAMGO-induced barrier disruption in vitro. However, MNTX, but not naloxone, pretreatment of EC inhibited thrombin- and LPS-induced barrier disruption, indicating potential mOP-R-independent effects of MNTX. In addition, intravenously delivered MNTX attenuated LPS-induced vascular hyperpermeability in the murine lung. We next examined the mechanistic basis for this MNTX barrier protection and observed that silencing of mOP-R attenuated the morphine- and DAMGO-induced EC barrier disruption, but not the permeability response to either thrombin or LPS. Because activation of the sphingosine 1-phosphate receptor, S1P(3), is key to a number of barrier-disruptive responses, we examined the role of this receptor in the permeability response to mOP-R ligation. Morphine, DAMGO, thrombin, and LPS induced RhoA/ROCK-mediated threonine phosphorylation of S1P(3), which was blocked by MNTX, suggesting S1P(3) transactivation. In addition, silencing of S1P(3) receptor expression (siRNA) abolished the permeability response to each edemagenic agonist. These results indicate that MNTX provides barrier protection against edemagenic agonists via inhibition of S1P(3) receptor activation and represents a potentially useful therapeutic agent for syndromes of increased vascular permeability.

Sphingosine-1-phosphate and FTY720 as anti-atherosclerotic lipid compounds

All stages of atherosclerosis have been identified as a chronic vascular inflammatory disease. In the last few years there is increasing evidence that endogenous lysophospholipids such as sphingosine-1-phosphate (S1P) have potent anti-inflammatory properties. The S1P analogue FTY720 that has been developed as a potent, orally active, immunosuppressant in the field of transplantation and autoimmune disease has interesting effects on inflammatory processes in the arterial vessel wall. S1P targets five specific S1P receptors (S1P(1-5)), which are ubiquitously expressed. S1P(1-3) receptor expression is identified in arterial vessels. S1P and FTY720 show potent silencing effects on some vascular proinflammatory mechanisms in endothelial and vascular smooth muscle cells. In addition, the interaction of monocytes with the vessel wall is inhibited. As shown recently, FTY720 can effectively reduce the progression of atherosclerosis in apolipoprotein E-deficient mice having a high-cholesterol diet. It is not entirely clear which S1P receptor subtype is mainly involved in this process. However, it is currently speculated that the S1P(3) and probably the S1P(1) is involved in the anti-atherosclerotic effects of FTY720. This review summarizes the current knowledge about S1P- and FTY720-effects on mechanisms of vascular inflammatory disease. In addition S1P receptor subtypes are identified which might be interesting for molecular drug targeting.

Sphingosine 1-phosphate regulates inflammation-related genes in human endothelial cells through S1P1 and S1P3

Sphingosine 1-phosphate (S1P) is a bioactive lysophospholipid (LPL) ligand that binds endothelial differentiation gene (Edg) family G-protein-coupled receptors and has been implicated as an important regulator in endothelial cells during inflammation processes. In this study, we attempt to determine which S1P receptors mediating the inflammatory response in human endothelial cells. Our results indicated that introduction of siRNA against S1P(1) significantly suppressed S1P-induced ICAM-1 mRNA, total protein, and cell surface expressions in human umbilical vein endothelial cells (HUVECs). Moreover, U937 cells adhesion to S1P-treated HUVECs was profoundly reduced by knock-down of S1P(1) in HUVECs. By knock-down of S1P(1) or S1P(3) in HUVECs, S1P-enhanced IL-8, MCP-1 mRNA expression, and THP-1 cell chemotaxis toward S1P-treated HUVEC-conditioned media was profoundly reduced. These results suggested that S1P-induced inflammatory response genes expression is mediated through S1P(1) and S1P(3). Our findings suggest the possible utilization of S1P(1) or S1P(3) as drug targets to treat severe inflammation.

Dual Roles of Tight Junction-associated Protein, Zonula Occludens-1, in Sphingosine 1-Phosphate-mediated Endothelial Chemotaxis and Barrier Integrity

In this report, sphingosine-1-phosphate (S1P), a serum-borne bioactive lipid, is shown to activate tight-junction-associated protein Zonula Occludens-1 (ZO-1), which in turn plays a critical role in regulating endothelial chemotaxis and barrier integrity. After S1P stimulation, ZO-1 was redistributed to the lamellipodia and cell-cell junctions via the S1P1/G(i)/Akt/Rac pathway. Similarly, both endothelial barrier integrity and cell motility were significantly enhanced in S1P-treated cells through the G(i)/Akt/Rac pathway. Importantly, S1P-enhanced barrier integrity and cell migration were abrogated in ZO-1 knockdown cells, indicating ZO-1 is functionally indispensable for these processes. To investigate the underlying mechanisms, we demonstrated that cortactin plays a critical role in S1P-induced ZO-1 redistribution to the lamellipodia. In addition, S1P significantly induced the formation of endothelial tight junctions. ZO-1 and alpha-catenin polypeptides were colocalized in S1P-induced junctional structures; whereas, cortactin was not observed in these regions. Together, these results suggest that S1P induces the formation of two distinct ZO-1 complexes to regulate two different endothelial functions: ZO-1/cortactin complexes to regulate chemotactic response and ZO-1/alpha-catenin complexes to regulate endothelial barrier integrity. The concerted operation of these two ZO-1 complexes may coordinate two important S1P-mediated functions, i.e. migration and barrier integrity, in vascular endothelial cells.

Sphingosine kinase: biochemical and cellular regulation and role in disease

Sphingolipids have emerged as molecules whose metabolism is regulated leading to generation of bioactive products including ceramide, sphingosine, and sphingosine-1-phosphate. The balance between cellular levels of these bioactive products is increasingly recognized to be critical to cell regulation; whereby, ceramide and sphingosine cause apoptosis and growth arrest phenotypes, and sphingosine-1-phosphate mediates proliferative and angiogenic responses. Sphingosine kinase is a key enzyme in modulating the levels of these lipids and is emerging as an important and regulated enzyme. This review is geared at mechanisms of regulation of sphingosine kinase and the coming to light of its role in disease.

S1P1-selective agonist, SEW2871, ameliorates ischemic acute renal failure

The pathogenesis of renal ischemia/reperfusion (I/R) injury involves activating several signal transduction cascade systems in endothelial cells. Sphingosine 1-phospate (S1P) maintains endothelial cell integrity and inhibits lymphocyte egress via the specific S1P(1) receptor, and may play a role in reducing ischemic renal injury. We examined the protective effects of a newly identified S1P(1)-selective agonist, SEW2871, on mouse renal I/R injury. Kidneys were harvested 1-4 days after I/R injury for histopathology, immunofluorescence studies, and quantitative real-time reverse transcriptase-polymerase chain reaction analyses to assess the change in gene expression profiles of inflammation-associated cytokines and adhesion molecules. SEW2871 improved renal function with a 40% reduction in plasma creatinine levels (P<0.01) and a significant reduction in tubular necrosis scores (I/R only: 4.3+/-0.2 vs I/R+SEW2871: 2.5+/-0.4, P<0.05) 24 h after ischemia. These changes were accompanied by 69% reduction in circulating lymphocytes, and 77 and 66% reduction in infiltrating neutrophils and macrophages in renal outer medulla, respectively (all P<0.01). The mRNA abundance of tumor necrotic factor-alpha (TNF-alpha), P-selectin, E-selectin, and intercellular adhesion molecule-1 (ICAM-1) was markedly increased by I/R injury (3.5-, 4.1-, 3.5-, and 4.8-folds, respectively, all P<0.05 vs sham). SEW2871 treatment partially reversed the upregulation of TNF-alpha, P-selectin, and ICAM-1 (47, 59, 54%, respectively, vs I/R control: 100%, all P<0.05). The reduction in protein expression of TNF-alpha, P-selectin, and ICAM-1 was further confirmed with immunofluorescence studies. These results suggest that SEW2871 ameliorates renal I/R injury by inhibiting lymphocyte egress and reducing pro-inflammatory molecules. This new class of renoprotective agent shows promise as a novel approach in preventing/treating ischemic acute renal failure.

Lentiviral siRNA silencing of sphingosine-1-phosphate receptors S1P1 and S1P2 in smooth muscle

Sphingosine-1-phosphate regulates diverse biological processes through five receptor types, S1P(1-5). Two or more S1P receptors are usually co-expressed on target cells. We have previously shown that smooth muscle cells of the gut co-express S1P(1) and S1P(2) receptors that could mediate distinct functions. In the absence of selective agonists and antagonists, we developed siRNA constructs to silence each receptor separately. The constructs were based on identical sequences in several mammalian species. A lentiviral vector-based system was used to deliver siRNA into HEK293T cells and smooth muscle cells. One S1P(1) and two S1P(2) siRNA constructs specifically inhibited ectopic expression of S1P(1) and S1P(2) receptors, respectively, as determined by immunocytochemistry and Western blot, and endogenous expression of S1P(1) and S1P(2) receptors in smooth muscle cells, as determined by RT-PCR. Measurement of PLC-beta and Rho kinase activities, which mediate initial and sustained muscle contraction, confirmed receptor silencing and showed that contraction is mediated exclusively by S1P(2) receptors.