ABSTRACT
Macrophage or macrophage-derived foam cell death is one of the characteristic events in the development of cell-poor lipid-rich cores of the advanced atherosclerotic plaques. Although the in vivo mechanism for the death of macrophages is unclear, one possible candidate for the agent which induces macrophage cell death is oxidized low density lipoprotein (Ox-LDL). To investigate the mechanism of Ox-LDL-induced macrophage cell death, we have recently employed macrophage cell genetics and isolated mutant cells resistant to the cytotoxic effect of Ox-LDL from mutagenized populations of murine macrophage-derived J774 cells (Hakamata, H., Miyazaki, A., Sakai, M., Matsuda, H., Suzuki, H., Kodama, T., and Horiuchi, S. (1998) J. Lipid Res. 39, 482-494). The results obtained showed that one mutant form, JO21b cells, was characterized by reduced expression of type I and type II class A macrophage scavenger receptors (MSR-AI/AII) with a concomitant decrease in the uptake of Ox-LDL. Moreover, peritoneal macrophages obtained from MSR-AI/AII-knockout mice showed a higher resistance to the cytotoxic effect of Ox-LDL compared to those of their wild-type littermates. From these results, we have concluded that Ox-LDL cytotoxicity to macrophages is enhanced by effective endocytic uptake of Ox-LDL through MSR-AI/AII. These findings imply a possibility that formation of the cell-poor lipid-rich core is also enhanced by MSR-AI/AII-mediated uptake of Ox-LDL and subsequent macrophage cell death in atherosclerotic lesions.
Subject(s)
Arteriosclerosis/etiology , Lipoproteins, LDL/metabolism , Macrophages/metabolism , Animals , Arteriosclerosis/pathology , Cell Death/genetics , Endothelium, Vascular/metabolism , Endothelium, Vascular/pathology , Humans , Ketocholesterols/pharmacology , Lipoproteins, LDL/pharmacology , Lysophosphatidylcholines/pharmacology , Macrophages/drug effects , Mice , Mutation , Oxidation-Reduction , Receptors, Immunologic/genetics , Receptors, Immunologic/metabolism , Receptors, Scavenger , Scavenger Receptors, Class A , tert-Butylhydroperoxide/pharmacologyABSTRACT
Oxidized low density lipoprotein (ox-LDL) was incubated with discoidal complexes of apolipoprotein A-I (apo A-I) and dimyristoylphosphatidylcholine (DMPC) (DMPC/apo A-I) in a cell-free system and re-isolated on Sephacryl S-400 gel filtration chromatography. Analyses of re-isolated ox-LDL showed that apo A-I was transferred from DMPC/apo A-I to ox-LDL, which accounted for 10% of the total protein of ox-LDL. Re-isolated ox-LDL also showed a 2.2-fold increase in phospholipid and a 14% decrease in cholesterol content on an apo B basis. The electrophoretic mobility of re-isolated ox-LDL was markedly reduced almost to that of native LDL. Moreover, the amounts of re-isolated ox-LDL to be degraded by mouse peritoneal macrophages as well as the capacity of re-isolated ox-LDL to accumulate cholesteryl esters (CE) in these cells were markedly reduced (60% and 80% reduction, respectively), suggesting that the ligand activity of ox-LDL for the scavenger receptor was significantly reduced upon treatment with DMPC/apo A-I. Parallel incubation of ox-LDL with free apo A-I led to a similar incorporation of apo A-I into ox-LDL. However, it had no effects on the ligand activity of ox-LDL. Thus, it is likely that the reduction in the ligand activity of ox-LDL by DMPC/apo A-I is explained by the change in the lipid moiety (mainly phospholipid) of ox-LDL. Since discoidal high density lipoprotein (HDL) is known to occur in vivo, this phenomenon might explain one of the anti-atherogenic functions of HDL.
Subject(s)
Cholesterol Esters/metabolism , Lipoproteins, LDL/metabolism , Lipoproteins, LDL/pharmacology , Macrophages, Peritoneal/drug effects , Membrane Proteins , Receptors, Lipoprotein , Animals , Apolipoprotein A-I/metabolism , Arteriosclerosis/metabolism , Arteriosclerosis/pathology , Cell-Free System , Chemical Phenomena , Chemistry, Physical , Chromatography, Gel , Dimyristoylphosphatidylcholine/metabolism , Endocytosis , Foam Cells/metabolism , Lipoproteins, LDL/chemistry , Lipoproteins, LDL/physiology , Liposomes , Macrophages, Peritoneal/metabolism , Male , Mice , Mice, Mutant Strains , Oxidation-Reduction , Receptors, Immunologic/metabolism , Receptors, Scavenger , Scavenger Receptors, Class BABSTRACT
When rat macrophages were converted to foam cells with acetylated low density lipoprotein (acetyl-LDL) and then reacted with high density lipoprotein (HDL) and an inhibitor of acyl-coenzyme A:cholesterol acyltransferase (58-035) (sequential incubation system), 58-035 did not enhance HDL-induced cholesterol efflux. In contrast, when macrophages were exposed to acetyl-LDL in the presence of both HDL and 58-035 (simultaneous incubation system), HDL-induced cholesterol efflux was enhanced 1.6-fold by 58-035. Cholesterol efflux with HDL alone was 2-fold greater in simultaneous incubation than in sequential incubation. These results suggest the presence of an efficient cholesterol efflux pathway in simultaneous incubation which is not available in sequential incubation. This pathway, which we refer to as the neutral cholesterol ester hydrolase-independent pathway, is characterized by the efflux of lysosome-derived cholesterol without re-esterification.
