Elevating Endogenous Sphingosine-1-Phosphate (S1P) Levels Improves Endothelial Function and Ameliorates Atherosclerosis in Low Density Lipoprotein Receptor-Deficient (LDL-R−/−) Mice

Background  Sphingosine-1-phosphate (S1P) is a bioactive lysosphingolipid and a constituent of high-density lipoprotein (HDL) exerting several atheroprotective effects in vitro. However, the few studies addressing anti-atherogenic effects of S1P in vivo have led to disparate results. We here examined atherosclerosis development in low-density lipoprotein receptor (LDL-R)-deficient (LDL-R −/− ) mice with elevated endogenous S1P levels. Methods and Results  Sub-lethally irradiated LDL-R −/− mice were transplanted with bone marrow deficient in sphingosine kinase 2 (SphK2), which led to the elevation of S1P concentrations in erythrocytes, plasma and HDL by approximately 1.5- to 2.0-fold in SphK2 −/− /LDL-R −/− mice. Afterwards, mice were fed a Western diet for 14 weeks. Elevation of endogenous S1P significantly reduced atherosclerotic lesion formation by approximately half without affecting the plasma lipid profile. Furthermore, the macrophage content of atherosclerotic lesions and lipopolysaccharide-induced monocyte recruitment to the peritoneal cavity were reduced in SphK2 −/− /LDL-R −/− mice. Studies using intra-vital microscopy revealed that endogenous S1P lowered leukocyte adhesion to capillary wall and decreased endothelial permeability to fluorescently labelled LDL. Moreover, SphK2 −/− /LDL-R −/− mice displayed decreased levels of vascular cell adhesion molecule 1 in atherosclerotic lesions and in plasma. Studies in vitro demonstrated reduced monocyte adhesion and transport across an endothelial layer exposed to increasing S1P concentrations, murine plasma enriched in S1P or plasma obtained from SphK2-deficient animals. In addition, decreased permeability to fluorescence-labelled dextran beads or LDL was observed in S1P-treated endothelial cells. Conclusion  We conclude that raising endogenous S1P levels exerts anti-atherogenic effects in LDL-R −/− mice that are mediated by favourable modulation of endothelial function.