Immunological and functional properties of in vitro oxidized low density lipoprotein

We studied the effect of in vitro moderate oxidation on low density lipoprotein (LDL) conformation and metabolism. LDL was modified with either copper ions or phospholipase A2 plus lipoxygenase and, in both cases, mild oxidative conditions were used. The resulting conformational changes were investigated by studying immunological and biological properties of oxidized LDL. The immunoreactivity of apolipoprotein (apo) B-100 was examined using seven monoclonal antibodies. The biological implications of conformational changes were provided by cell-lipoprotein interaction studies using human fibroblasts and mouse peritoneal macrophages. Enzymatically treated LDL presented a relatively less oxidative degree of modification because it generated lower levels of TBARS, and displayed a lower electronegativity and more comparable cellular interactions with those of native LDL. Nevertheless, dramatic immunological changes were measured on both forms of LDL, i.e., a significant increase in the immunoreactivity of an epitope located in the B/E receptor binding domain, but also at epitopes far from this site and located in the N-terminal part of the apoB-100 molecule. The immunoreactivity of the C-terminal region was in contrast, decreased. Yet, as compared with enzymatically oxidized LDL, much more dramatic structural changes with chemically modified LDL were observed, resulting in such a particular conformation of lipoprotein that its interaction with the macrophagic scavenger receptor was favored, but its recognition by the B/E receptor of fibroblast was abolished. In contrast, despite a lower interaction between enzymatically modified LDL and the B/E receptor, the metabolism of this lipoprotein was quite comparable with that of native LDL and its degradation with cultured macrophages was poor. The use of in vitro models is common for study of the relationship between oxidized LDL and atherogenesis in humans. The choice of the more appropriate way to modify lipoproteins is of interest and is discussed.