CHOLESTEROL EFFLUX FROM FIBROBLASTS TO DISCOIDAL LIPOPROTEINS WITH APOLIPOPROTEIN-A-I (LPA-I) INCREASES WITH PARTICLE-SIZE BUT CHOLESTEROL TRANSFER FROM LPA-I TO LIPOPROTEINS DECREASES WITH SIZE

To understand the role of different discoidal lipoproteins in cellular cholesterol efflux, defined discoidal lipoproteins containing 2, 3, or 4 apolipoproteins (apo) A-I per particle (Lp2A-I, Lp3A-I, and Lp4A-I) were prepared from mixtures of apoA-I and phospholipids with or without cholesterol. Each particle had a slow prebeta migration on agarose gel electrophoresis which further decreased as the number of apoA-I increased. Incubation of cholesterol-labeled human fibroblasts with the different LpA-I at an equimolar concentration in apoA-I showed that the best acceptors of cellular cholesterol were Lp4A-I, followed by Lp3A-I and Lp2A-I. Cholesterol efflux to these particles was positively correlated to the number of apoA-I, to the ratio of phospholipids to apoA-I, and to the size of particles, three interrelated parameters. To follow the subsequent movement of cellular cholesterol after it became associated with LpA-I, cholesterol- and apoA-I-labeled LpA-I were incubated with plasma which resulted in parallel modifications of each labels electrophoretic migration with time. However, [H-3]cholesterol-labeled LpA-I transferred from prebeta to alpha migration with a precursor-product relationship while I-125-LpA-I progressively shifted from prebeta to alpha migration. The change in electrophoretic migration of I-125-LpA-I is independent of cholesterol and appears related only to a modification of apoA-I charge. Lp2A-I was fastest in changing its electrophoretic migration to alpha, followed by Lp3A-I and then Lp4A-I. The large discoidal particles containing four or three apoA-I thus display a greater stability, and we propose that both this stability and their greater capacity to bind cholesterol can make them the most efficient lipoprotein acceptor of cellular cholesterol in the milieu where they are produced.