Cholesteryl ester uptake and flow in Paramecium primaurelia
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Cholesteryl ester-rich particles extracted from human atherosclerotic plaques were shown to increase the rate of incorporation of [14C]oleate into cholesteryl [14C]oleate and to cause massive accumulation of cholesteryl esters in monolayers of mouse peritoneal macrophages. This stimulation showed saturation kinetics and susceptibility to competition by polyanions (polyinosinic acid, fucoidin, dextran sulfate), suggesting that cell surface binding was required. Cellular uptake and lysosomal hydrolysis of the cholesteryl esters were also required, as indicated by the finding that stimulation of cholesteryl ester formation was prevented by the lysosomal inhibitor, chloroquine. The cholesterol esterification-stimulating activity of the aortic extracts was excluded on a 2% agarose column and floated in the density range of 1.006 to 1.063 g/ml. Cholesterol-rich extracts from human adrenal glands and liver did not stimulate cholesteryl ester formation in macrophages. The aortic extracts did not stimulate cholesteryl ester synthesis in human fibroblasts. Complexes of 125I-labeled albumin and cholesteryl linoleate formed in vitro were taken up and degraded in macrophages, but not in fibroblasts, by a process resembling the uptake of the aortic extracts. The current data suggest that macrophages express mechanisms for internalizing certain types of cholesteryl ester-rich lipid/protein complexes, including those present in atherosclerotic plaques.
Cholesteryl ester
Cholesterylester transfer protein
Foam cell
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Reversible accumulation of cholesteryl esters in macrophages incubated with acetylated lipoproteins.
Mouse peritoneal macrophages accumulate large amounts of cholesteryl ester when incubated with human low-density lipoprotein that has been modified by chemical acetylation (acetyl-LDL). This accumulation is related to a high-affinity cell surface binding site that mediates the uptake of acetyl-LDL by adsorptive endocytosis and its delivery to lysosomes. The current studies demonstrate that the cholesteryl ester accumulation can be considered in terms of a two-compartment model: (a) the incoming cholesteryl esters of acetyl-LDL are hydrolyzed in lysosomes, and (b) the resultant free cholesterol is re-esterified in the cytosol where the newly formed esters are stored as lipid droplets. The following biochemical and morphologic evidence supports the hydrolysis-re-esterification mechanism: (a) Incubation of macrophages with acetyl-LDL markedly increased the rate of cholesteryl ester synthesis from [14C]oleate, and this was accompanied by an increase in the acyl-CoA:cholesteryl acyltransferase activity of cell-free extracts. (b) When macrophages were incubated with reconstituted acetyl-LDL in which the endogenous cholesterol was replaced with [3H]-cholesteryl linoleate, the [3H]cholesteryl linoleate was hydrolyzed, and at least one-half of the resultant [3H]cholesterol was re-esterified to form [3H]cholesteryl oleate, which accumulated within the cell. The lysosomal enzyme inhibitor chloroquine inhibited the hydrolysis of the [3H]cholesteryl linoleate, thus preventing the formation of [3H]cholesteryl oleate and leading to the accumulation of unhydrolyzed [3H]cholesteryl linoleate within the cells. (c) In the electron microscope, macrophages incubated with acetyl-LDL had numerous cytoplasmic lipid droplets that were not surrounded by a limiting membrane. The time course of droplet accumulation was similar to the time course of cholesteryl ester accumulation as measured biochemically. (d) When acetyl-LDL was removed from the incubation medium, biochemical and morphological studies showed that cytoplasmic cholesteryl esters were rapidly hydrolyzed and that the resultant free cholesterol was excreted from the cell.
Cholesteryl ester
Sterol O-acyltransferase
Foam cell
Lipid droplet
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Cholesteryl ester
Chylomicron
Sterol O-acyltransferase
Cholesterylester transfer protein
Reverse cholesterol transport
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Triolein
Cholesteryl ester
Sterol O-acyltransferase
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Cholesteryl ester
Linolenate
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Cholesteryl ester
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Cholesteryl ester
High-density lipoprotein
Hep G2
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The cholesteryl oleate-POPC dispersions (1:3, mol/mol, mean particle size 110+/-20 nm) were taken up by the human hepatoma line Hep G2 cells via endocytosis. Internalization of the cholesteryl oleate-POPC dispersions by Hep G2 cells was dependent on the incubation time and dispersion concentration. At the cholesteryl oleate concentration 100 microM, its total uptake and internalization were found to be 1.5 nmol and 0.8 nmol per 1 mg of cell protein/24 h, respectively. Intracellular cleavage of the cholesteryl oleate incorporated in dispersions resulted in accumulation of free cholesterol capable of being released into the medium and metabolized to water-soluble polar products, presumably bile acids; oleic acid released is, apparently, involved in biosynthesis of triacylglycerides. The low-density lipoprotein receptor is not involved in internalization of lipid dispersions, and the presence of the cholesteryl oleate-POPC dispersions has no effect on the receptor-dependent internalization of cholesteryl esters of the low-density lipoproteins. The obtained data allow us to consider nonspecific internalization of cholesteryl esters by hepatocytes as a substantial part of the nonpolar lipid clearance.
Internalization
Cholesteryl ester
POPC
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Cholesteryl ester
Cholesterylester transfer protein
Efflux
Bovine serum albumin
Sterol O-acyltransferase
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Cholesteryl ester
Cholesterylester transfer protein
Sterol O-acyltransferase
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