UPTAKE OF OXIDIZED LDL BY MACROPHAGES RESULTS IN PARTIAL LYSOSOMAL-ENZYME INACTIVATION AND RELOCATION

The cytotoxicity of oxidized LDL (oxLDL) to several types of artery wa ll cells might contribute to atherosclerosis by causing cell death, pr esumably by both apoptosis and necrosis. After its uptake into macroph age lysosomes by receptor-mediated endocytosis, oxLDL is poorly degrad ed, resulting in ceroid-containing loam cells. We studied the influenc e of oxLDL on lysosomal enzyme activity and, in particular, on lysosom al membrane stability and the modulation of these cellular characteris tics by HDL and vitamin E (vit-E). Unexposed cells and cells exposed t o acetylated LDL (AcLDL) were used as controls. The lysosomal marker e nzymes cathepsin L, and N-acetyl-beta-glucosaminidase (NA beta Gase) w ere biochemically assayed in J-774 cells after fractionation. Lysosoma l integrity in living cells was assayed by the acridine orange (AO) re location test. Cathepsin D was immunocytochemically demonstrated in J- 774 cells and human significantly decreased, whereas their-relative cy tosolic activities were enhanced, alter oxLDL exposure. Labilization o f the lysosomal membranes was further proven by decreased lysosomal AO uptake and relocation to the cytosol of cathepsin D, as estimated by light and electron microscopic immunocytochemistry. HDL and vit-E dimi nished the cytotoxicity of oxLDL by decreasing the lysosomal damage. T he results indicate that endocytosed oxLDL not only partially inactiva tes lysosomal enzymes but also destabilizes the acidic vacuolar compar tment, causing relocation of lysosomal enzymes to the cytosol. Exposur e to AcLDL resulted in its uptake with enlargement of the lysosomal ap paratus, but the stability of the lysosomal membranes was not changed.