High Density Lipoprotein-associated Sphingosine 1-Phosphate Promotes Endothelial Barrier Function

Abstract

High density lipoproteins (HDL) are major plasma carriers of sphingosine 1-phosphate (S1P). Here we show that HDL increases endothelial barrier integrity as measured by electric cell substrate impedance sensing. S1P was implicated as the mediator in this process through findings showing that pertussis toxin, an inhibitor of G i -coupled S1P receptors, as well as antagonists of the S1P receptor, S1P1, inhibited barrier enhancement by HDL. Additional findings show that HDL stimulates endothelial cell activation of Erk1/2 and Akt, signaling pathway intermediates that have been implicated in S1P-dependent endothelial barrier activity. HDL was also found to promote endothelial cell motility, a process that may also relate to endothelial barrier function in the context of a vascular injury response. The effects of HDL on endothelial cell Erk1/2 and Akt activation and motility were suppressed by pertussis toxin and S1P1 antagonists. However, both HDL-induced barrier enhancement and HDL-induced motility showed a greater dependence on Akt activation as compared with Erk1/2 activation. Together, the findings indicate that HDL has endothelial barrier promoting activities, which are attributable to its S1P component and signaling through the S1P1/Akt pathway.Sphingosine 1-phosphate (S1P) 2 is a plasma-borne lysosphingolipid that has been shown to regulate endothelial barrier integrity (1). For example, treatment of cultured endothelial cells with S1P associated with the carrier albumin acts to increase endothelial barrier activity as indicated by increased transendothelial electrical resistance (2, 3). Moreover, S1P administration greatly reduces lung capillary leakage induced in mice by lipopolysaccharide treatment (4). Mechanistically, S1P acts to enhance tight junction formation in neighboring endothelial cells by influencing subcellular distributions of tight junction components including ZO-1 and claudin-5 (5). In addition, S1P induces endothelial cortical actin assembly (1) and relocation of endothelial cell junctional adhesion molecules including platelet endothelial cell adhesion molecule and vascular endothelial-cadherin to cell-cell junctional areas (5).In plasma, S1P is found associated with multiple lipoproteins including low density lipoproteins, very low density lipoproteins, and high density lipoproteins (HDL). However, the bulk of the lipoprotein particle-associated S1P (54%) is bound to HDL (6). A number of recent studies point to the S1P cargo of HDL as being a mediator of many of the cardiovascular effects of HDL including the ability to promote vasodilation, vasoconstriction, and angiogenesis, protect against ischemia/reperfusion injury, and inhibit/reverse atherosclerosis (7). One important cardiovascular-related effect of S1P that has not yet been attributed to HDL is regulation of endothelial barrier activity, a major physiological function of the endothelium. Here we investigate the role of HDL as a regulator of endothelial barrier integrity processes known to be dependent on S1P signaling. EXPERIMEN...

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