Apolipoprotein A1 and high-density lipoprotein limit low-density lipoprotein transcytosis by binding scavenger receptor B1

Atherosclerosis results from the deposition and oxidation of low-density lipoprotein (LDL) and immune cell infiltration in the sub-arterial space leading to arterial occlusion. Numerous studies have shown that transcytosis transports circulating LDL across endothelial cells lining the blood vessels. LDL transcytosis is initiated by binding to either Scavenger Receptor B1 (SR-B1) or Activin A receptor-like kinase 1 (ALK1) on the apical side of endothelial cells leading to its transit and release on the basolateral side. Individuals with elevated levels of circulating high-density lipoprotein (HDL) are partly protected from atherosclerosis due to its ability to remove excess cholesterol and act as an antioxidant. Apolipoprotein A1 (APOA1), an HDL constituent, can bind to SR-B1, raising the possibility that APOA1/HDL may also compete with LDL for SR-B1 binding and thereby limit LDL deposition in the sub-arterial space. To examine this possibility, we used in vitro approaches to quantify the internalization and transcytosis of fluorescent LDL in coronary endothelial cells. Using microscale thermophoresis and affinity capture, we find that SR-B1 and APOA1 directly interact, and that binding is enhanced when using the cardioprotective variant of APOA1 termed Milano (APOA1-Milano). In a murine model, transiently increasing the levels of HDL reduced the acute deposition of fluorescently labeled LDL in the atheroprone inner curvature of the aorta. Reduced LDL deposition was also observed when increasing circulating wild-type APOA1 or the APOA1-Milano variant, with a more robust inhibition from the APOA1-Milano. The results suggest that HDL may limit SR-B1-mediated LDL transcytosis and deposition, adding to the mechanisms by which it can act as an atheroprotective particle.