Loading with oxidised low density lipoprotein alters endocytic and secretory activities of murine macrophages

Lipid-loaded macrophages were produced in vitro by incubation with acetylated or copper-oxidized LDL. In order to establish whether cellular membrane traffic is generally perturbed by such loading, we assessed endocytosis of fluid; cell surface binding, internalisation and degradation of a soluble ligand and of a particulate preparation; and exocytosis of lysosmal enzymes. Fluid-phase pinocytosis of sucrose was unaffected by either form of loading. Binding, uptake and degradation of soluble (mannosylated-BSA) and particulate (zymosan) ligands by these lipid-loaded and by non-loaded cells were compared. Loading with oxidized LDL decreased the processing of both ligands, while loading with acetylated LDL had little effect. Loading with oxidized LDL (Ox-LDL) also decreased zymosan binding at 4°C; and the internalisation and degradation of ligands in Ox-LDL loaded and non-loaded cells reflected the extent of surface binding. Changes in binding release of lysosomal β-N-acetyl-D-glucosaminidase from the cells. Loading with Ox- but not Ac-LDL decreased β-N-acetyl-D-glucosaminidase secretion. After incubation with zymosan, intracellular levels of the enzyme were increased in the Ox-LDL loaded cells. Zymosan uptake and β-N-acetyl-D-glucosaminidase secretion were correlated, but enzyme-activity per culture rose more in the absence than in the presence of zymosan. We conclude that membrane traffic is perturbed in model foam cells, particularly those loaded with Ox-LDL.