The function of albumin and lipoproteins containing apoA-I in promoting cellular cholesterol efflux from human skin fibroblasts.

Abstract

This study is designed to elucidate the factors that control the transport of cholesterol molecules from the plasma membrane of cultured human cells to lipoprotein particles. We are particularly interested in determining how the composition of HDL particles and their related physico-chemical parameters control the ability of these lipoproteins to release cellular cholesterol. We have investigated the ability of both serum albumin and well defined, homogeneous reconstituted HDL particles (LpA-I) to receive and retain cholesterol molecules from cultured human fibroblasts. Human serum albumin induces a time-dependent bi-phasic efflux of cholesterol from non-cholesterol loaded human skin fibroblasts. Although the magnitude of transfer to albumin is lower than that to reconstituted discoidal LpA-I, it is specific and occurs within a normal physiological concentration range. The reverse transfer of albumin-bound cholesterol back to fibroblasts is very rapid and is inhibited by both LDL and LpA-I, which appear to be able to compete for the transferred lipids. Therefore, albumin appears to be able to contribute to the net efflux of cellular cholesterol and its transfer to lipoprotein particles. Model discoidal and spherical LpA-I particles have been prepared in vitro by the complexation of apoA-I and pure lipids using cholate dialysis or cosonication method respectively. LpA-I surface lipid composition; phosphatidylcholine (PC), cholesterol (UC), sphingomyelin (SM) and phosphatidylinositol (PI), and core neutral lipid composition; cholesteryl esters and triglycerides, have been varied systematically to characterize the effect of each constituent on the release of cellular cholesterol and on LpA-I physico-chemical properties. The efflux to discoidal LpA-I is independent of PC or UC content, but is related to the molecule number of apoA-I per LpA-I particle and to the accompanying changes in particle sizes. The addition of PI and SM into Lp2A-I significantly increases the efflux of cholesterol to these particles, apparently by modifying the interfacial lipid packing and surface charge. In sonicated LpA-I complexes, the association of few molecules of PC and SM transforms lipid-free apoA-I into small distinct lipoprotein-like complexes with only one apoA-I (Lp1A-I), which are significantly better acceptors of cellular cholesterol than lipid-free apoA-I. The progressive increase of PC from 5 to 35.5 moles per apoA-I alters the $\alpha$-helix content of apoA-I and the negative surface charge of the LpA-I, and significantly increases the ability to receive and retain cholesterol molecules from the cultured cells. In contrast, at high PC/apoA-I ratio, inclusion of either SM or PI or core neutral lipids into a sonicated Lp2A-I has no effect on cholesterol efflux. In conclusion, this study shows that efflux of cellular cholesterol is mediated differently by albumin, discoidal or spherical LpA-I. While particle size and surface lipid physical properties appear to be critical to the efflux of cholesterol to discoidal LpA-I, the ability of sonicated LpA-I to receive and retain cellular cholesterol is dependent on factors that modify the conformation and stability of apoA-I. The study shows that the progressive association of PC and SM with apoA-I is critical to transform this apolipoprotein into an optimal recipient of cholesterol molecules.