Effect of the apolipoprotein A-I and surface lipid composition of reconstituted discoidal HDL on cholesterol efflux from cultured fibroblasts.

Five series of reconstituted discoidal HDL (LpA-I) particles have been prepared, and their constituents, apolipoprotein A-I (apoA-I), 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), unesterified cholesterol (UC), phosphatidylinositol (PI), or sphingomyelin (SM), have been systematically varied to elucidate the relationship between HDL composition and cholesterol efflux from non-cholesterol-loaded human skin fibroblasts. The physical properties, such as hydrodynamic diameters, alpha-helix contents, and surface potentials, of these LpA-I have been measured and related to the ability of the LpA-I to accept cellular cholesterol. The results show that for LpA-I particles containing 2, 3, or 4 apoA-I per particle, Lp4A-I are the best acceptors of cellular cholesterol, followed by Lp3A-I and then Lp2A-I particles. Discoidal Lp2A-I with variations in POPC content, from 121 to 266 mol/particle; show no difference in their abilities to promote cholesterol efflux. Similarly, inclusion of 7 and 15 mol of free cholesterol to Lp2A-I also does not affect their ability to accept cellular cholesterol. However, increasing the content of either PI or SM, up to 20 mol/particle, is associated with significantly increased abilities of the LpA-I to promote cholesterol efflux. The efflux of cellular cholesterol to discoidal LpA-I particles is independent of specific changes in apoA-I conformation and charge, but appears to be positively related to major changes in the size of the lipoprotein particle. The study suggests that in contrast to interlipoprotein cholesterol transfers, the efflux of cholesterol from cultured fibroblasts is less sensitive to factors that affect the frequency of molecular collisions and more dependent on the ability of an HDL particle to absorb and retain cholesterol molecules. Since SM and PI appear to modulate this adsorption/desorption of cholesterol to HDL, variations in the concentration of these lipids within HDL would be expected to affect plasma cholesterol homeostasis.