APOLIPOPROTEIN A-I-MEDIATED EFFLUX OF STEROLS FROM OXIDIZED LDL-LOADED MACROPHAGES

Although oxidized low-density lipoprotein (OxLDL) can accumulate in ma crophages in vitro, generating cholesterol-loaded cells, little attent ion has been paid to the capacity of such macrophages loaded with OxLD L to export cholesterol and oxidized sterol moieties. In vitro lipid-l oaded cells were generated by incubating primary cultures of mouse per itoneal macrophages with acetylated LDL (AcLDL) or OxLDL for 24 hours. The cellular content of native cholesterol, individual cholesteryl es ters, and 7-ketocholesterol was determined by high-performance liquid chromatography. These cells were then incubated with medium containing apolipoprotein (ape) A-I and albumin or albumin alone for up to 24 ho urs; cholesterol and oxidized sterol efflux were measured both in term s of intracellular depletion and extracellular accumulation. Macrophag es loaded with AcLDL accumulated cholesterol and large quantities of c holesteryl esters, whereas OxLDL-loaded cells accumulated cholesterol, a number of oxidized compounds (predominantly 7-ketocholesterol), and a relatively small quantity of cholesteryl esters. AcLDL-derived cell s released approximately 50% of their total cholesterol (unesterified and esterified) to apo A-I-containing medium over 24 hours in the form of unesterified cholesterol, whereas OxLDL-derived cells released app roximately 30% of their total cholesterol and 7% of their total conten t of 7-ketocholesterol over the same period. There was minimal efflux of any sterol in the absence of apo A-I. The proportions of cholestero l and 7-ketocholesterol released by either AcLDL- or OxLDL-loaded cell s were not reduced by inhibiting cellular acyl-CoA:cholesterol acyl tr ansferase using Sandoz 58-035, despite substantial alterations in the proportions of both free cholesterol and (in OxLDL-loaded cells) free 7-ketocholesterol in these cells. Furthermore, the subcellular distrib utions of both cholesterol and 7-ketocholesterol in individual subcell ular organelle fractions were identical to that of free cholesterol in nonloaded cells, indicating that these sterols in OxLDL-loaded cells are not selectively sequestered in lysosomes. 7-Ketocholesterol is rel eased much less efficiently than cholesterol from OxLDL-loaded cells. In addition, OxLDL-loaded cells release cholesterol less efficiently t han do cells derived from AcLDL. It is possible that this impairment o f efflux from OxLDL-loaded cells influences the generation and persist ence of the foam cell phenotype in vivo and may therefore contribute t o the atherogenicity of OxLDL.