Exchangeable APOA1 on HDL inhibits LDL binding to proteoglycans

The entrapment of LDLs by proteoglycans (PGs) in the extracellular matrix of the arterial intima is a key initial step in the development of atherosclerosis. HDLs can interfere with this process, but the underlying mechanism is not fully understood. The aim of this study was to investigate the mechanisms by which HDL inhibits LDL binding to PG. An In-Cell ELISA was used to measure the binding of LDL to PGs in the extracellular matrix synthesized by mouse vascular smooth muscle cells. Fast-protein liquid chromatography, immunoprecipitation, SDS-PAGE, and immunoblotting analysis were performed to characterize how HDL and its apolipoproteins inhibit LDL binding to PGs. HDL and APOA1 inhibited LDL binding to PGs in a dose-dependent manner. Competition experiments showed that HDL did not compete directly with LDL for PG binding. Instead, APOA1 dissociated from HDL and associated with LDL, reducing the ability of LDL to bind PGs. This was demonstrated by separating HDL and LDL using porous filters of different sizes and tracking the movement of either HDL or APOA1. When APOA1 was solidly anchored to HDL particles, HDL lost the ability to affect LDL-PG binding. HDL inhibits LDL binding to PGs through an interaction with its main apolipoprotein, APOA1, specifically, a pool of loosely attached, exchangeable, lipid-free APOA1 on the HDL surface. These findings identify lipid-free APOA1 as a critical mediator of the ability of HDL to reduce LDL retention in the arterial wall and provide new insights into the antiatherogenic properties of HDL.