-HDL involves plasma membrane microsolubilization

High density lipoprotein (HDL) is able to remove unesterified cholesterol from peripheral cells in the process of reverse cholesterol transport by an aqueous diffusion mechanism as well as by an apolipoprotein (apo)-mediated process. The aqueous diffusion mechanism is understood but the molecular mechanism of lipid-poor preb -HDL(apo-) mediated cholesterol removal is not known. Measurements of the initial rates of efflux of unesterified cholesterol and phospholipid from human fibroblasts to lipid-free, human apoA-I showed that both lipids are released from the cells during a 10-min incubation with apoA-I. The concentration-dependence of efflux of the lipids is the same ( K m 5 0.4 and 0.6 m g apoA-I/ml for cholesterol and phospholipid flux, respectively), suggesting a membrane microsolubilization process. A finite pool of about 1% of the plasma membrane cholesterol is accessible for release by solubilization; the limited size of this cholesterol pool is not due to a lack of availability of apoA-I, but rather to the restricted amount of phospholipid that is removed from the plasma membrane. Plasma membrane domains may be involved in membrane microsolubilization, but caveolar cholesterol seems not to be specifically accessed in this process. Membrane microsolubilization is the process by which preb 1HDL removes cell cholesterol in the first step of reverse cholesterol transport. When apoA-I is present in the extracellular space, the relative contributions of cholesterol efflux by membrane microsolubilization and by aqueous diffusion are determined by the degree of lipidation of the apoA-I molecules.— Gillotte, K. L., W. S. Davidson, S. LundKatz, G. H. Rothblat, and M. C. Phillips. Removal of cellular cholesterol by preb -HDL involves plasma membrane microsolubilization. J. Lipid Res. 1998. 39: 1918–1928. Supplementary key words apolipoprotein A-I • high density lipoprotein • cellular cholesterol efflux High density lipoproteins (HDL) mediate the process of reverse cholesterol transport (RCT) in which excess peripheral cholesterol is carried to sites of metabolism (1, 2). This lipoprotein class is heterogeneous in nature and consists of subspecies that are defined on the basis of their apolipoprotein and lipid composition, charge, shape, and size (3, 4). For instance, HDL species containing only apolipoprotein (apo) A-I (Lp A-I) or both apoA-I and apoA-II (Lp A-I 1 A-II) behave differently as substrates for lecithin:cholesterol acyltransferase (LCAT) (5). In the last decade, attention has been drawn to a lipid-poor subspecies of HDL, preb 1-HDL. Preb 1-HDL contains apoA-I as its sole protein moiety and is understood to be an efficient initial acceptor of cellular unesterified (free) cholesterol (FC), in the first step of the RCT course (6–10). There is some suggestion that the lipid-poor preb 1-HDL may act as a shuttle of FC from the plasma membrane to fully lipidated HDL species (10–12). However, at this point, the mechanisms giving rise to the proficiency of this HDL species in stimulating the efflux of plasma membrane FC in the initial stages of RCT are not known. Cholesterol efflux from the cell plasma membrane to lipidated apoA-I, such as mature HDL 3 particles, occurs by the so-called aqueous diffusion mechanism (2, 13, 14). In this process, FC molecules desorb from the plasma membrane, diffuse through the extracellular aqueous phase, and incorporate into lipidated acceptors encountered by collision (for a review, see ref. 13). This diffusional mechanism is well defined and it can contribute significantly to FC efflux from cells. The presence of scavenger receptor type B class I (SR-BI) in the plasma membrane of cells can facilitate efflux of FC to HDL (15). Several studies have described that under particular conditions leading to a reduction in particle size, apoA-I molecules can dissociate Abbreviations: apo, apolipoprotein; CETP, cholesteryl ester transfer protein; FBS, fetal bovine serum; FC, free (unesterified) cholesterol; GLC, gas–liquid chromatography; HDL, high density lipoprotein; LCAT, lecithin:cholesterol acyltransferase; LDL, low density lipoprotein; MEM, minimal essential medium; PBS, phosphate-buffered salt solution; PL, phospholipid; POPC, 1-palmitoyl, 2-oleoyl phosphatidylcholine; RCT, reverse cholesterol transport; rHDL, reconstituted HDL; TLC, thin-layer chromatography. 1 Current address: Department of Medicine, University of California, San Diego, La Jolla, CA 92093. 2 Current address: Department of Pathology and Laboratory Medicine, University of Cincinnati College of Medicine, Cincinnati, OH 45267-0529. 3 To whom correspondence should be addressed. by gest, on A uust 7, 2017 w w w .j.org D ow nladed fom