Abstract 229: Endothelial Cell Handling Of Atherogenic ApoB-carrying Lipoproteins

Atherosclerosis is triggered by the subendothelial retention of cholesterol-rich, apoB -containing LDL (which carries apoB100) and chylomicron remnants (which carry apoB48, the 48% N terminal sequence of apoB). In addition, multiple studies have established elevated postprandial TRLs as an independent risk factor for CVD. However, the mechanisms whereby postprandial TRLs (which circulate mostly within chylomicrons) exert their atherogenic effects remain unknown. Two receptors - SR-BI and ALK1 - have been shown to bind apoB and mediate transcytosis of LDL across the endothelium. We have shown that SR-BI also mediates EC uptake of nascent chylomicrons. RNAseq analysis revealed that ECs exposed to chylomicrons express a host of inflammatory and immune response genes, which does not occur with knockdown of SR-BI. In addition, following chylomicron uptake ECs release small extracellular vesicles that induce lipid accumulation in macrophages and lead to reduced eNOS phosphorylation in naïve ECs. Conversely, although binding of ALK1 to apoB is believed to occur through the N-terminus of apoB, the receptor is not involved in chylomicron uptake. Using a series of N terminal apoB peptides (apoB8-18, with sequences comprising the 8-18% N terminus of apoB), we found that chylomicron uptake was inhibited by apoB15 and apoB18, and LDL uptake was additionally inhibited by apoB12. These observations suggest that a) the binding sequence to SR-BI is between residues 547-684 of apoB, and b) an additional sequence between residues 365-547 participates in the binding to ALK1. Molecular modeling of apoB interactions identified an additional sequence in the C terminus of apoB potentially involved in the binding to ALK1. This sequence is absent in chylomicrons, which may explain why ALK1 inhibition and competition with apoB12 fail to significantly inhibit chylomicron uptake. Consistently, uptake of apoB100-carrying chylomicrons from Apobec -/- mice was inhibited by both ALK1 siRNA and competition with apoB12. Our observations suggest that a) the N terminus of apoB harbors distinct binding sequences to SR-BI and ALK1, and b) chylomicron uptake by ECs contributes to atherogenesis by inducing endothelial dysfunction, inflammation, and macrophage foam cell formation.