Macrophage Foam Cell Formation with Native Low Density Lipoprotein

Abstract

This investigation has elucidated a mechanism for development of macrophage foam cells when macrophages are incubated with native low density lipoprotein (LDL). LDL is believed to be the main source of cholesterol that accumulates in monocyte-derived macrophages within atherosclerotic plaques, but native LDL has not previously been shown to cause substantial cholesterol accumulation when incubated with macrophages. We have found that activation of human monocyte-derived macrophages with phorbol 12-myristate 13-acetate ( Engorgement of macrophages with cholesterol is the defining pathological characteristic of atherosclerotic plaques, the cause of most heart attacks and strokes. Cholesterol accumulation in macrophages not only contributes to cholesterol retention within the vessel wall, but also alters macrophage biology. Cholesterol-loaded macrophages secrete plaque-disrupting matrix metalloproteinases, and produce tissue factor that promotes thrombosis when plaques rupture (1-3). Thus, how macrophages accumulate cholesterol and become foam cells has been the subject of intense investigation.Low density lipoprotein (LDL), 1 the main carrier of plasma cholesterol, enters the vessel wall and then by some mechanism enters macrophages. Previously, native LDL could not be shown to cause foam cell formation because the cellular receptor that binds LDL is poorly expressed on differentiated macrophages and down-regulates during cholesterol uptake, limiting total cholesterol accumulation (4 -7). Moreover, the LDL receptor is not expressed in human atherosclerotic plaques (8). Thus, most previous studies of macrophage foam cell formation have focused on modifying LDL in some way that increases its binding to macrophages. Increased macrophage binding of LDL has been achieved with chemical modifications to the apoB component of the LDL, aggregation of LDL induced by either vortexing or treatment of LDL with lipases, and complexing of LDL with other molecules, for example, glycosaminoglycans or antibodies, which bind macrophages and promote LDL uptake by piggyback endocytosis (9). Macrophages take up modified LDL by receptormediated endocytosis in pinocytotic vesicles, phagocytic vacuoles, or patocytic surface-connected compartments (9).One popular hypothesis of foam cell formation involves LDL oxidation. LDL oxidation promotes macrophage LDL uptake that is mediated by various macrophage scavenger receptors (10). Although oxidation of LDL has important biological effects that could influence atherosclerotic plaque development (11), oxidation of LDL does not readily explain foam cell formation. Incubation of human monocyte-derived macrophages with oxidized LDL, even strongly oxidized with artificial chemical systems, produces little macrophage cholesterol accumulation (12, 13). Also, oxidized LDL is poorly metabolized within lysosomes of macrophages because of partial inactivation by oxidized LDL of the lysosomal enzymes that degrade LDL (14 -16). This limits the capacity of oxidized LDL to induce acyl-CoA:cholesterol acyltra...

Different cellular traffic of LDL-cholesterol and acetylated LDL-cholesterol leads to distinct reverse cholesterol transport pathways

“…Increasing LDL concentration resulted in a linear and nonsaturating increase in uptake, which at 250 mg/ml doubled total cellular cholesterol and increased cholesteryl ester level to z5% (Fig. 3A, B), indicating that a non-receptor-independent mechanism contributed to macrophage LDL uptake, as reported by others (13,14). In contrast, increasing AcLDL concentration up to 50 mg/ml rapidly increased cellular cholesterol, which plateaued at a 3-fold increase, and was accompanied by an increase in cholesteryl ester to z40% (Fig.…”

“…The cell can also export excess cholesterol to appropriate extracellular acceptors by transfer mechanisms through cholesterol gradients that involve mostly HDL, mediated via scavenger receptor class B type I (SR-BI) (4) and the ABCG1/G4 transporter (5,6) or the apolipoprotein A-I (apoA-I)-mediated pathway that operates through ABCA1 (7)(8)(9)(10). In addition to the canonical receptor pathway for regulated LDL uptake by the LDL receptor (LDLr) (11,12), the macrophage can take up LDL via receptor-independent pathways, such as macropinocytosis, which, in a dosedependent manner, can lead to macrophage foam cell formation under certain conditions in vitro (13)(14)(15).…”

Endothelial Lipase is Increased by Inflammation and Promotes LDL Uptake in Macrophages

“…It has been recently postulated, however, that nLDL plays a role in the accumulation of cholesterol in macrophages independent of the LDL receptor's activity, particularly when cells are stimulated with, for example, inflammatory cytokines. Kruth, et al reported that the stimulation of human monocyte/macrophages with PMA resulted in the accumulation of cholesterol from nLDL, and that this pathway is involved in macropinocytosis 37) . From these observations, we speculate that the EL on macrophages acts as a molecular bridge between circulating nLDL and macrophages in blood vessel walls, and promotes the uptake of LDL by macrophages.…”

Class B Scavenger Receptors CD36 and SR-BI Are Receptors for Hypochlorite-modified Low Density Lipoprotein

“…The radioactivity and protein content of the cell lysate was measured in the same aliquot. Cell-free wells were incubated with [ 3 H]sucrose in parallel incubations and gave a background count less than 5% of the [ 3 H]sucrose radioactivity detected in macrophages (44).…”