Interaction of plasma-derived lipid transfer protein with macrophages in culture

This study investigates the ability of human plasmaderived lipid transfer protein to facilitate lipid transfer to and from intact viable cells in culture. Mouse peritoneal macrophages or 5774 macrophages were preincubated with acetylated low density lipoprotein and [3H]oleate/albumin to promote the intracellular synthesis and accumulation of cholesteryl [ 3H]oleate and 3H-labeled triglyceride. The addition of partially purified lipid transfer protein to cultures of lipid-loaded macrophages resulted in a time and concentration-dependent transfer of radiolabeled cholesteryl ester and triglyceride from macrophages to the medium. At 48 hr, lipid transfer protein facilitated the net transfer of 16 and 11% of cellular cholesteryl ester and triglyceride radioactivity, respectively, to the medium; transfer in the absence of the lipid transfer protein was <2%. The transfer of cholesteryl ester radioactivity was accompanied by a similar decrease in cellular cholesteryl ester mass indicating a net transfer event. Lipid transfer from cells was not dependent on the presence of a lipoprotein acceptor in the medium; however, low and high density lipoproteins present at 200 pg cholesterol/ml did significantly stimulate the transfer protein-facilitated efflux of these lipids. Lipid transfer protein did not appear capable of transferring radiolabeled lipid from low density or high density lipoprotein to macrophages. Radiolabeled cholesteryl ester and triglyceride transferred from cells to the medium by lipid transfer protein were associated with large molecular weight (> 2 x lo6) components in the medium with an average density greater than 1.21 g/ml; these lipids were not associated with lipid transfer protein itself. However, these radiolabeled lipids were readily incorporated into low or high density lipoproteins when these lipoproteins were added to the medium either during or after its incubation with cells. It is concluded that lipid transfer protein can facilitate the net efflux of cholesteryl esters from intact, living macrophages. These studies suggest a novel and potentially antiatherogenic role for lipid transfer protein. Morton, R. E. Interaction of‘ plasma-derived lipid transfer protein with maqophages in culture. J. Lipid Res. 1988. 29: 1367-1377. Supplementary key words cholesteryl esters triglyceride high density lipoproteins atherogenesis Several investigators have purified a protein from human lipoprotein-deficient plasma that is responsible for the majority of the cholesteryl ester and triglyceride transfer activity in this plasma fraction (1, 2). This protein, designated LTP, is characterized as a hydrophobic glycoprotein of G 74,000 molecular weight. LTP facilitates the transfer of cholesteryl ester, triglyceride, and phospholipid, as well as retinyl ester and cholesteryl ether, but not unesterified cholesterol, between lipoprotein particles (3). Although the transfer mechanism is incompletely understood, it appears that the physical binding of LTP to the lipoprotein surface, perhaps through interaction with phospholipid head groups (4), is an essential event in the transfer process (5). Consistent with the aforementioned interaction of LTP with lipoproteins through surface phospholipids, it appears that LTP can utilize a wide variety of phospholipidcontaining membrane surfaces as substrate. For example, in addition to the well-recognized transfer of apolar lipids among very low, low, and high density lipoproteins (6-8), LTP also catalyzes the transfer of lipids between lecithincholesterol liposomes and low density lipoproteins (3), between lipoproteins and Intralipid (9), and between isolated rough and smooth endoplasmic reticulum (10). Furthermore, Stein et al. (11) have shown that partially purified LTP can facilitate the transfer of cholesteryl ester, which is associated with the extracellular matrix of cultured smooth muscle cells, to the cell culture medium. More recently, these authors have demonstrated that partially purified LTP can promote the efflux of cholesteryl ester from “disintegrating“ (Le., chemically fixed and permeabilized) smooth muscle cells and macrophages to the culture medium (12). The results described above collectively suggest that LTP may also interact with intact viable cells. Indeed, Granot, Tabas, and Tall (13) recently demonstrated that LTP can promote the transfer of cholesteryl ester from high density lipoprotein into cultured hepatocytes and Abbreviations: LTP, lipid transfer protein; BSA, bovine serum albumin; DMEM, Dulbecco’s Modified Eagle’s Medium; LDL, low density lipoprotein; HDL, high density lipoprotein; PBS, phosphate-buffered saline. Journal of Lipid Research Volume 29, 1988 1367 by gest, on O cber 9, 2017 w w w .j.org D ow nladed fom smooth muscle cells. In light of the central role of the macrophage in lipid metabolism in the atherosclerotic lesion, we have presently examined the capacity of LTP to promote lipid transfer to and from cultured cholesteryl ester-loaded macrophages. EXPERIMENTAL PROCEDURES