Uptake of oxidized LDL by macrophages differs from that of acetyl LDL and leads to expansion of an acidic endolysosomal compartment

Accumulation of cholesterol by macrophage foam cells in atherosclerotic les ions is thought to involve the uptake of modified low density lipoproteins (LDLs). Previous studies have shown that there is impaired degradation of o xidized LDL in macrophages. The present study was done to determine whether the differences in intracellular metabolism of oxidized LDL and acetyl LDL were associated with delivery to different intracellular compartments. Mou se peritoneal macrophages were incubated with 1,1'-dioctadecyl-3,3,3',3'-te tramethylindocarbocyanine perchlorate-labeled oxidized LDL or 3,3'-dioctade cyloxacapbocyanine perchlorate-labeled acetyl LDL and examined by fluoresce nce microscopy. Deconvolution image analysis showed <10% colocalization of the 2 lipoproteins at incubation times ranging from 30 minutes to 6 hours. Subcellular: fractionation of macrophages after incubation with Tc-99m-labe led oxidized LDL revealed accumulation of the tracer in a compartment with a d=1.042 g/mL, consistent with endosomes. Surprisingly, there was a concur rent dramatic shift of the density of lysosomal marker enzymes from d = 1.1 g/mL to the same fractions that contained Tc-99m, indicating that this com partment was formed after fusion with primary lysosomes. Parallel experimen ts in J774 cells, a murine macrophage-like cell line, did not show a simila r density shift, perhaps because of the slower rate of accumulation of oxid ized LDL by these cells. Fluorescence microscopy of macrophages labeled wit h a lysosomotropic dye revealed a marked expansion of the acidic compartmen t after exposure of cells to oxidized LDL. We conclude that oxidized LDL an d acetyl LDI, are internalized by morphologically distinct pathways. Furthe rmore, because of its impaired lysosomal degradation, oxidized LDL causes e xpansion of and a decrease in the density of the lysosomal compartment in m acrophages.