Oxidized low density lipoprotein leads to macrophage accumulation of unesterified cholesterol as a result of lysosomal trapping of the lipoprotein hydrolyzed cholesteryl ester.

The early atherosclerotic lesion is comprised of foam cell macrophages filled with cholesteryl ester (CE), unesterified cholesterol (UC), and cholesterol oxides. Upon incubation of macrophages with oxidized low density lipoprotein (Ox-LDL), they accumulate UC rather than CE, which was shown to accumulate after incubation of cells with acetylated LDL (Ac-LDL). Using lipoproteins that were doubly labeled in their CE as well as in their protein moieties, we have demonstrated that lysosomal hydrolysis of the Ox-LDL CE was similar to the hydrolysis of the CE in Ac-LDL or native LDL whereas, as shown previously, a markedly impaired degradation of the protein moiety of Ox-LDL was observed. Cell fractionation revealed that the UC was derived from the hydrolyzed CE in Ox-LDL and was trapped in the macrophage lysosomal fraction, whereas in cells incubated with Ac-LDL, the lipoprotein UC was rapidly transported to the microsomal and cytosolic compartments. Lysosomal accumulation of Ox-LDL-derived UC could be related to the effect of the oxysterols in Ox-LDL, as oxidation of Ac-LDL or incubation of macrophages with Ac-LDL in the presence of oxysterols, in comparison to cell incubation with Ac-LDL, resulted in lysosomal accumulation of unesterified cholesterol. As a consequence of lysosomal trapping of Ox-LDL-derived UC, its availability to esterification was markedly impaired (by 6-fold), in comparison to the cholesterol esterification rate of Ac-LDL-derived UC. However, when the cholesterol esterification was expressed per lysosomal released UC, cellular cholesterol esterification rate of Ox-LDL-derived UC was found to be similar to that of Ac-LDL-derived UC. High density lipoprotein (HDL)-mediated efflux of the Ox-LDL-derived cholesterol from macrophages was similar to that found for Ac-LDL-derived cholesterol after 24 h of cell incubation with HDL3. Major defects in the cellular metabolism of Ox-LDL-derived 7-ketocholesterol were also found and could be related to its lysosomal trapping (together with the UC), its limited capacity to be esterified, and a 40% reduction in its HDL-mediated efflux from macrophages, in comparison to the efflux of the Ox-LDL-derived UC. We conclude that upon incubation of macrophages with Ox-LDL, lysosomal hydrolysis of the lipoprotein CE is not impaired but cellular accumulation of the Ox-LDL-derived UC occurs as a result of trapping of the hydrolyzed CE in the macrophage lysosomal compartment which may be related to the effect of oxysterols in Ox-LDL.