Research ArticleVASCULAR BIOLOGY

HDL-bound sphingosine 1-phosphate acts as a biased agonist for the endothelial cell receptor S1P1 to limit vascular inflammation

Sci. Signal.  11 Aug 2015:
Vol. 8, Issue 389, pp. ra79
DOI: 10.1126/scisignal.aaa2581

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Maintaining vascular health with HDL

Flow through blood vessels subjects endothelial cells to abnormal shear forces at specific locations that trigger inflammation, which contributes to atherosclerotic plaque formation. Galvani et al. found that vascular inflammation and atherosclerosis in mice were suppressed by the endothelial cell receptor S1P1, which is activated by S1P, a lipid mediator that is abundant in blood bound to different chaperone proteins. S1P suppressed inflammation in cultured endothelial cells when bound to the lipoprotein ApoM+HDL. Thus, S1P bound to different chaperones triggered distinct or “biased” signaling pathways, which may also contribute to the protective effect of HDL, commonly called “good cholesterol,” in atherosclerosis.

Abstract

The sphingosine 1-phosphate receptor 1 (S1P1) is abundant in endothelial cells, where it regulates vascular development and microvascular barrier function. In investigating the role of endothelial cell S1P1 in adult mice, we found that the endothelial S1P1 signal was enhanced in regions of the arterial vasculature experiencing inflammation. The abundance of proinflammatory adhesion proteins, such as ICAM-1, was enhanced in mice with endothelial cell–specific deletion of S1pr1 and suppressed in mice with endothelial cell–specific overexpression of S1pr1, suggesting a protective function of S1P1 in vascular disease. The chaperones ApoM+HDL (HDL) or albumin bind to sphingosine 1-phosphate (S1P) in the circulation; therefore, we tested the effects of S1P bound to each chaperone on S1P1 signaling in cultured human umbilical vein endothelial cells (HUVECs). Exposure of HUVECs to ApoM+HDL-S1P, but not to albumin-S1P, promoted the formation of a cell surface S1P1–β-arrestin 2 complex and attenuated the ability of the proinflammatory cytokine TNFα to activate NF-κB and increase ICAM-1 abundance. Although S1P bound to either chaperone induced MAPK activation, albumin-S1P triggered greater Gi activation and receptor endocytosis. Endothelial cell–specific deletion of S1pr1 in the hypercholesterolemic Apoe−/− mouse model of atherosclerosis enhanced atherosclerotic lesion formation in the descending aorta. We propose that the ability of ApoM+HDL to act as a biased agonist on S1P1 inhibits vascular inflammation, which may partially explain the cardiovascular protective functions of HDL.

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