Abstract
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).
Highlights
The plasma level of HDL2 has been shown to be inversely correlated with the risk of atherosclerosis and associated cardiovascular disease [1, 2]
Materials—sphingosine 1-phosphate (S1P) was purchased from Cayman Chemical Co.; wortmannin was from Calbiochem-Novabiochem; L-NG-nitroarginine methyl ester hydrochloride (L-NAME), and D-NGnitroarginine methyl ester hydrochloride (D-NAME) were from BIOMOL Research Laboratories Inc.; anti-endothelial nitricoxide synthase antibody, anti-Ser(P)1177 eNOS antibody, and anti--actin antibody were from Cell Signaling Technology Inc.; primary mouse antibodies for vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) were from Chemicon International; and anti-scavenger receptor class B type I (SR-BI) and anti-PDZK1 were from Santa Cruz Biotechnology, Inc
We first characterized in detail the action of high density lipoprotein (HDL) on exogenous S1P-induced adhesion molecule expression to exclude the possible role of the S1P component of HDL
Summary
Other groups have demonstrated that the S1P3 receptor mediates HDL-induced relaxation through NO synthase (NOS) activation and NO synthesis in mouse artery [11]. S1P action on adhesion molecule expression is not simple; S1P alone stimulates the expression of VCAM-1 and ICAM-1 through NF-B activation [12,13,14,15], whereas the lysolipid inhibits the TNF␣-induced adhesion of monocytes [15, 16]. These results suggest that both S1P and HDL have opposing potentials for atherogenesis. As expected, we demonstrated the involvement of S1P receptors, in addition to SR-BI, in the HDL-induced inhibition of adhesion molecule expression when TNF␣ was used as a pro-atherogenic stimulant
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