Production of cholesterol-enriched nascent high density lipoproteins by human monocyte-derived macrophages is a mechanism that contributes to macrophage cholesterol efflux

Atherosclerotic lesions have a lipid core containing crystals and liposomes enriched in unesterified cholesterol as well as numerous monocyte-macrophages enriched in cholesteryl ester. Sufficient amounts of plasma-derived high density lipoproteins (HDL) may not reach and efficiently remove the cholesterol deposited in lesion macrophages or in the lipid core of lesions. We examined the potential of human monocyte-macrophages to produce nascent HDL and to solubilize cholesterol derived from interaction of monocyte-macrophages with lipoprotein and non-lipoprotein sources of cholesterol. Monocyte-macrophages produced discoidal (25 +/- 6 nm long and 6 +/- 1 nm wide (mean +/- S.D.)) and vesicular (89 +/- 41 nm in diameter) lipoprotein particles following and during enrichment of macrophages with cholesterol from acetylated low density lipoprotein (AcLDL) or cholesterol crystals. During cholesterol enrichment, discoidal particles progressively accumulated in the medium for up to 6 days. In contrast, vesicles did not increase past 2 days of incubation. Both the discoidal and vesicular lipoprotein particles had a peak density of about 1.09-1.10 g/ml. The discoidal particles contained apolipoprotein E (apoE), whereas the vesicles contained a major protein constituent with a molecular mass of 22,000 daltons. The vesicles did not contain detectable apoE and the 22,000-dalton protein was not the 22,000-dalton thrombolytic fragment of apoE. Following cholesterol enrichment of macrophages with AcLDL or cholesterol crystals, macrophages excreted much of their accumulated cholesterol, even in the absence of exogenously added cholesterol acceptors. Most of this excreted cholesterol was recovered from the culture medium and was carried in the apoE discoidal particles that showed cholesterol enrichment up to a 2:1 unesterified cholesterol to phospholipid molar ratio. The findings suggest that sufficient production of these nascent HDL by macrophages within atherosclerotic lesions should facilitate removal of cellular and extracellular cholesterol, even in the absence of plasma-derived HDL.