Lysosomal accumulation of oxidized phosphatidylcholine-apolipoprotein B complex in macrophages: intracellular fate of oxidized low density lipoprotein

Oxidized phosphatidylcholine (OxPC) formed in oxidized low density lipoprot ein (OxLDL) is thought to be involved in the development of atherosclerosis . OxPC has been found in foam cells in atherosclerotic lesions and suggeste d to be the epitope for OxLDL recognition by macrophages. OxPC is present a s a complex with apolipoprotein B (apoB) in OxLDL, since some OxPC call bin d with proteins. In the current study, the intracellular fate of OxPC-apoB complexes after internalization of OxLDL by macrophages was investigated. M urine macrophage cell line J774.1 was incubated with either OxLDL or acetyl ated LDL for 24 h, then the cells were further incubated for up to 24 h in new medium without lipoprotein. Modified apoB in the cells was quantitated by sandwich ELISA using monoclonal antibodies against OxPC and apoB. Intrac ellular OxLDL decreased rapidly for the first 4 h to approx. 20% of that be fore medium change, with the apparent metabolism of OxPC-apoB complex ceasi ng. OxPC-apoB complexes that remained in the cells after 24 h chasing incre ased as the period of OxLDL loading in macrophages prolongs. Acetylated LDL in the cells decreased quickly and disappeared after 4 h of chasing. Subce llular fractionation using sucrose density gradient ultracentrifugation of macrophages, which had already accumulated OxPC-apoB complexes by 24 h of i ncubation with OxLDL and further 24 h chasing, showed that the complex was co-localized with endosomal and lysosomal markers. Immunohistochemical doub le staining studies demonstrated that OxPC and apoB co-localize in foam cel ls in early atherosclerotic lesions obtained from human coronary artery. Th ese results suggest that OxPC-apoB complexes originating from OxLDL accumul ate in foam cells in human atherosclerotic lesions as well as in macrophage s in vitro. (C) 2000 Elsevier Science B.V, All rights reserved.