Sphingosine-1-phosphate receptor 3 regulates the transendothelial transport of high-density lipoproteins and low-density lipoproteins in opposite ways

The entry of lipoproteins from blood into the arterial wall is a rate-limiting step in atherosclerosis. It is controversial whether this happens by filtration or regulated transendothelial transport.

Because sphingosine-1-phosphate (S1P) preserves the endothelial barrier, we investigated in vivo and in vitro, whether S1P and its cognate S1P-receptor 3 (S1P₃) regulate the transendothelial transport of lipoproteins.

Compared to apoE-haploinsufficient mice (CTRL), apoE-haploinsufficient mice with additional endothelium-specific knock-in of S1P₃ (S1P₃-iECKI) showed decreased transport of LDL and Evan’s Blue but increased transport of HDL from blood into the peritoneal cave. After 30 weeks of high-fat diet feeding, S1P₃-iECKI mice had lower levels of non-HDL-cholesterol and less atherosclerosis than CTRL mice. In vitro stimulation with an S1P₃ agonist increased the transport of ¹²⁵I-HDL but decreased the transport of ¹²⁵I-LDL through human aortic endothelial cells (HAECs). Conversely, inhibition or knock-down of S1P₃ decreased the transport of ¹²⁵I-HDL but increased the transport of ¹²⁵I-LDL. Silencing of SCARB1 encoding scavenger receptor B1 (SR-BI) abrogated the stimulation of ¹²⁵I-HDL transport by the S1P₃ agonist. The transendothelial transport of ¹²⁵I-LDL was decreased by silencing of SCARB1 or ACVLR1 encoding activin-like kinase 1 but not by interference with LDLR. None of the three knock-downs prevented the stimulatory effect of S1P₃ inhibition on transendothelial ¹²⁵I-LDL transport.

S1P₃ regulates the transendothelial transport of HDL and LDL oppositely by SR-BI-dependent and SR-BI-independent mechanisms, respectively. This divergence supports a contention that lipoproteins pass the endothelial barrier by specifically regulated mechanisms rather than passive filtration.