Independence of tumor necrosis factor-alpha-induced adhesion molecule expression from sphingosine 1-phosphate signaling in vascular endothelial cells

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.

Sphingosine 1-phosphate (S1P) induces expression of E-selectin and adhesion of monocytes via intracellular signalling pathways in vascular endothelial cells

Sphingosine 1-phosphate (S1P) - a constitutive component of human plasma - is implicated as a signalling molecule in the regulation of cell adhesion molecules (CAM) in vascular endothelial cells (EC), but the degree of the S1P-induced expression of CAM and the involvement of the S1P(1) receptor are still ambiguous. Here, we report that S1P, when added to vascular EC in the absence of other stimuli, induced a strictly proportional and concentration-dependent expression of E-selectin mRNA, of E-selectin protein and of the number of adhering THP-1 monocytes to EC. Experiments with exogenous [(3)H]S1P showed a multi-exponential influx kinetic of intracellular uptake of [(3)H]S1P up to a steady state level over 2h. This process could be inhibited or enhanced by various synthetic modulators targeting both, S1P(1) receptor-dependent (Akt, ERK1/2) as well as independent DMS-sensitive pathways. The S1P(1) receptor signalling was shown to drive the sphingosine kinase - the rate limiting enzyme for the formation of S1P - to a higher or lower activity. Furthermore, S1P as an intracellular messenger induced the phosphorylation and nuclear translocation of the p65 subunit of NF-kappaB and in turn the expression of E-selectin and monocyte adhesion. Taken together, these results suggest that the physiologically controlled variation in intracellular S1P concentrations may represent a novel not yet known mechanism of fine-tuning the expression of proinflammatory and atherogenic E-selectin cell adhesion molecule by vascular endothelial cells.

Sphingosine-1-phosphate as a mediator of high-density lipoprotein effects in cardiovascular protection

Sphingosine-1-phosphate (S1P) has gained special attention in the high-density lipoprotein (HDL) field because HDL is the most prominent plasma carrier of S1P and because the S1P content of HDL may be responsible for many of the pleiotropic functions of HDL. This revelation has come from the evidence that HDL employ S1P receptors and signalling pathways to implement several HDL-ascribed biological effects as diverse as endothelial nitric oxide production, vasodilation, survival, and cardioprotection. This review focuses on HDL effects that are completely or partially mediated by the S1P content of the HDL particle and differentiates them from genuine HDL effects that are S1P-independent. In addition, the functional properties of 'free', HDL-unbound S1P are sometimes different from or even contrary to those of HDL-associated S1P. The nature of the physical interactions between HDL and local and systemic S1P production will be discussed as well as their consequences for organ function. Finally, we will elucidate the potential benefits and limitations of S1P analogues as a new class of functional HDL mimetics for cardiovascular therapy.

The suppressive effect of sphingosine 1-phosphate on monocyte-endothelium adhesion may be mediated by the rearrangement of the endothelial integrins alpha(5)beta(1) and alpha(v)beta(3)

BACKGROUND

Sphingosine 1-phosphate (S1P), known to play important roles in vascular biology, is a bioactive lysophospholipid mediator that maintains endothelial integrity via its cell-surface receptors (S1Ps). In this in vitro study, we aimed to examine the role of S1P in monocyte-endothelium adhesion, which is an important event in the pathophysiology of atherosclerosis.

METHODS AND RESULTS

S1P pretreatment of human umbilical vein endothelial cells (ECs), but not U937 cells, effectively suppressed U937-EC adhesion independently from the expression of adhesion molecules, namely ICAM-1, VCAM-1, and E-selectin. This S1P-induced suppressive effect was inhibited by the blockage of S1P(1) and S1P(3) receptors and the specific inhibitors of G(i) protein, Src family proteins, phosphatidylinositol 3-kinase, and Rac1, indicating involvement of these key downstream pathways. Moreover, the RGD peptide and antibodies, which neutralize adhesion via alpha(5)beta(1) and alpha(v)beta(3), effectively inhibited U937-EC adhesion with a degree similar to S1P pretreatment. Both an adhesion assay and flow-cytometric analysis demonstrated that U937 cells adhered through integrins alpha(5)beta(1) and alpha(v)beta(3) expressed on the apical surface of monolayer ECs, and S1P shifted the localization of these integrins from the apical surface to the basal surface.

CONCLUSIONS

From the present results, we propose that S1P may contribute to the maintenance of vascular integrity and the regulation of atherogenesis through the rearrangement of endothelial integrins.

Role of Scavenger Receptor Class B Type I and Sphingosine 1-Phosphate Receptors in High Density Lipoprotein-induced Inhibition of Adhesion Molecule Expression in Endothelial Cells

We characterized the molecular mechanisms by which high density lipoprotein (HDL) inhibits the expression of adhesion molecules, including vascular cell adhesion molecule-1 and intercellular adhesion molecule-1, induced by sphingosine 1-phosphate (S1P) and tumor necrosis factor (TNF) alpha in endothelial cells. HDL inhibited S1P-induced nuclear factor kappaB activation and adhesion molecule expression in human umbilical vein endothelial cells. The inhibitory HDL actions were associated with nitric-oxide synthase (NOS) activation and were reversed by inhibitors for phosphatidylinositol 3-kinase and NOS. The HDL-induced inhibitory actions were also attenuated by the down-regulation of scavenger receptor class B type I (SR-BI) and its associated protein PDZK1. When TNFalpha was used as a stimulant, the HDL-induced NOS activation and the inhibitory action on adhesion molecule expression were, in part, attenuated by the down-regulation of the expression of S1P receptors, especially S1P(1), in addition to SR-BI. Reconstituted HDL composed mainly of apolipoprotein A-I and phosphatidylcholine mimicked the SR-BI-sensitive part of HDL-induced actions. Down-regulation of S1P(3) receptors severely suppressed the stimulatory actions of S1P. Although G(i/o) proteins may play roles in either stimulatory or inhibitory S1P actions, as judged from pertussis toxin sensitivity, the coupling of S1P(3) receptors to G(12/13) proteins may be critical to distinguish the stimulatory pathways from the inhibitory ones. In conclusion, even though S1P alone stimulates adhesion molecule expression, HDL overcomes S1P(3) receptor-mediated stimulatory actions through SR-BI/PDZK1-mediated signaling pathways involving phosphatidylinositol 3-kinase and NOS. In addition, the S1P component of HDL plays a role in the inhibition of TNFalpha-induced actions through S1P receptors, especially S1P(1).

Sphingosine 1-phosphate receptors mediate stimulatory and inhibitory signalings for expression of adhesion molecules in endothelial cells

Sphingosine 1-phosphate (S1P) stimulates expression of vascular cell adhesion molecule-1 and intercellular adhesion molecule-1 in human umbilical vein endothelial cells. S1P-induced actions were associated with nuclear factor kappa-B activation and inhibited by pertussis toxin as well as by antisense oligonucleotides specific to S1P receptors, especially, S1P(3). S1P also stimulated endothelial nitric oxide synthase (eNOS) and its activation was markedly inhibited by the antisense oligonucleotide for the S1P(1) receptor rather than that for the S1P(3) receptor. The dose-response curve of S1P to stimulate adhesion molecule expression was shifted to the left in the presence of the phosphatidylinositol 3-kinase inhibitor wortmannin and the NOS inhibitor Nomega-nitro-l-arginine methyl ester. NO donor S-nitroso-N-acetylpenicillamine inhibited S1P-induced adhesion molecule expression. Moreover, tumor necrosis factor-alpha-induced adhesion molecule expression was markedly inhibited by S1P in a manner sensitive to inhibitors for PI3-K and NOS. These results suggest that S1P receptors are coupled to both stimulatory and inhibitory pathways for adhesion molecule expression. The stimulatory pathway involves nuclear factor kappa-B and inhibitory one does phosphatidylinositol 3-kinase and NOS.

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

OBJECTIVES

Sphingosine 1-phosphate (S1P) is a bioactive phospholipid acting both as a ligand for the G protein-coupled receptors S1P1-5 and as a second messenger. Because S1P1 knockout is lethal in the transgenic mouse, an alternative approach to study the function of S1P1 in endothelial cells is needed.

METHODS AND RESULTS

All human endothelial cells analyzed expressed abundant S1P1 transcripts. We permanently silenced (by RNA interference) the expression of S1P1 in the human endothelial cell lines AS-M.5 and ISO-HAS.1. The S1P1 knock-down cells manifested a distinct morphology and showed neither actin ruffles in response to S1P nor an angiogenic reaction. In addition, these cells were more sensitive to oxidant stress-mediated injury. New S1P1-dependent gene targets were identified in human endothelial cells. S1P1 silencing decreased the expression of platelet-endothelial cell adhesion molecule-1 and VE-cadherin and abolished the induction of E-selectin after cell stimulation with lipopolysaccharide or tumor necrosis factor-alpha. Microarray analysis revealed downregulation of further endothelial specific transcripts after S1P1 silencing.

CONCLUSIONS

Long-term silencing of S1P1 enabled us for the first time to demonstrate the involvement of S1P1 in key functions of endothelial cells and to identify new S1P1-dependent gene targets.