Transfer of cholesterol from Ob1771 cells or LDL to reconstituted, defined high density lipoproteins

We used defined, reconstituted high density lipoproteins (rHDL) to study the effects of structure and composition of these particles on their role as cholesterol acceptors from cell membranes or from low density lipoproteins (LDL). Three discoidal rHDL and one spherical rHDL with distinct apolipoprotein A-I conformations, diameters and compositions were used in conjunction with Ob1771 cells to measure the rate of [3H]cholesterol efflux from the cells, direct binding to the cells, and competition with native HDL3 for binding. In addition, the same rHDL particles were used to study the kinetics of cholesterol mass transfer from LDL. The results show that the rates of cholesterol transfer depend on the nature of the donor (t1/2 11-19 min from LDL, and t1/2 5 h from the cells), on the phosphatidylcholine/cholesterol ratio in the acceptors (the closer this ratio is to the equilibrium value, the slower is the rate), and on the diameter of the acceptors (the smallest particles have the lowest t1/2 for cholesterol uptake from LDL, and are the most effective acceptors of [3H]cholesterol from cells after their phospholipid content is taken into account). The cholesterol uptake by the rHDL, both from the cells and from LDL, is determined mostly by the phospholipid pool available in the acceptors. Binding to the cells was equivalent for all the rHDL (Kd = 38-67 micrograms/ml) and comparable to HDL3, suggesting that the differences in apoA-I conformation have no effect on the binding to cells. Finally we observed that exposure of rHDL to cells may lead to remodeling of some of the lipoprotein particles.