The toxicity to macrophages of oxidized low-density lipoprotein is mediated through lysosomal damage

Oxidized low-density lipoprotein (ox-LDL) has been shown to degrade poorly within the secondary lysosomes of macrophages but its possible effect on lysosomal integrity has received less attention. The effect of ultraviolet-C oxidized LDL (UVox-LDL) on cellular viability, and lysosomal membrane stability, was examined on cultured murine J-774 cells and human monocyte-derived macrophages (HMDMs). The acridine orange (AO) relocalization test was applied to study the lysosomal integrity of living cells. UVox-LDL dramatically reduced J-774 cell proliferation at a concentration of 25 microg/ml. Incubation with 5 microM copper alone, normally used to induce LDL oxidation, was also toxic. In contrast to the effects of ox-LDL, in concentrations up to 75 microg/ml, native LDL (nLDL) rather stimulated J-774 cell replication. Incubation with UVox-LDL (25-75 microg/ml) also altered cellular AO uptake, depending on time and dose: its lysosomal accumulation decreased and its cytosolic accumulation increased. This shift indicates damaged lysosomal membranes with decreased intralysosomal, and increased cytosolic, H+ concentration. Many J-774 cells exposed to UVox-LDL initially transformed into foam cells and then assumed an apoptotic-type morphology with TUNEL-positive nuclei. We conclude that ox-LDL is cytotoxic to macrophages due to oxidative damage of lysosomal membranes, with ensuing destabilization and leakage to the cytosol of lysosomal contents, such as hydrolytic enzymes, causing degeneration of apoptotic type.