Phospholipids in oxidized low density lipoproteins perturb the ability of macrophages to degrade internalized macromolecules and reduce intracellular cathepsin B activity

Previous studies showed that pre-treatment of mouse peritoneal macrophages (MPM) with oxidized low density lipoprotein (oxLDL) repressed subsequent degradation of oxLDL following uptake. Parallel studies on the activity of the lysosomal protease, cathepsin B in MPM and in vitro indicate that oxLDL also induces a reduction in this activity. We now report that pre-treatment of MPM with the lipid portion of oxLDL induced a reduction both in the degradation of internalized small macromolecules such as maleylated (mal) BSA (30%) or larger ones such as aggregated LDL (100%), and in cellular cathepsin B activity (42%). Binding and uptake of malBSA were not affected. Pre-treatment of MPM for 2 h with oxidized phosphatidylcholine (oxPC) isolated from oxLDL or generated from Cu2+-treated 1-palmitoyl-2-linoleoyl phosphatidylcholine (oxPLPC), also inhibited 125I-malBSA degradation and reduced cathepsin B activity in MPM and in vitro. Further separation of oxPLPC and oxPC from oxLDL by thin layer chromatography led to the isolation of a polar lipid fraction possessing most of the biological activity in oxPC. Partial characterization of this fraction from oxPLPC using liquid chromatography/electrospray ionization/mass spectrometry indicated that this polar fraction containing fragmentation products of linoleate, was still comprised of multiple bioactive molecular ions. Collectively, these results suggest that specific oxPC fractions in oxLDL are partially responsible for the alterations in MPM metabolism under study induced by oxLDL.