Uptake of Oxidized LDL by Macrophages Differs From That of Acetyl LDL and Leads to Expansion of an Acidic Endolysosomal Compartment

Abstract

Accumulation of cholesterol by macrophage foam cells in atherosclerotic lesions is thought to involve the uptake of modified low density lipoproteins (LDLs). Previous studies have shown that there is impaired degradation of oxidized 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. Mouse peritoneal macrophages were incubated with 1,1'-dioctadecyl-3, 3,3',3'-tetramethylindocarbocyanine perchlo- rate-labeled oxidized LDL or 3,3'-dioctadecyloxacarbocyanine perchlorate-labeled acetyl LDL and examined by fluorescence 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 (99m)Tc-labeled oxidized LDL revealed accumulation of the tracer in a compartment with a d=1.042 g/mL, consistent with endosomes. Surprisingly, there was a concurrent dramatic shift of the density of lysosomal marker enzymes from d=1.1 g/mL to the same fractions that contained (99m)Tc, indicating that this compartment was formed after fusion with primary lysosomes. Parallel experiments in J774 cells, a murine macrophage-like cell line, did not show a similar density shift, perhaps because of the slower rate of accumulation of oxidized LDL by these cells. Fluorescence microscopy of macrophages labeled with a lysosomotropic dye revealed a marked expansion of the acidic compartment after exposure of cells to oxidized LDL. We conclude that oxidized LDL and acetyl LDL are internalized by morphologically distinct pathways. Furthermore, because of its impaired lysosomal degradation, oxidized LDL causes expansion of and a decrease in the density of the lysosomal compartment in macrophages.