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Lysosomal Cholesterol Accumulation in Model Atherosclerotic Foam Cells

Atherosclerosis (hardening of the arteries) remains a major cause of death in the United States. Massive cholesterol accumulation within macrophage lysosomes is a key event in atherosclerosis. The accumulation produces “foamy” appearing cells. The mechanism for this foam cell formation is unknown. In previous studies we showed that cultured macrophages incubated with oxidized low density lipoprotein (oxLDL) show a similar lysosomal cholesterol accumulation. This accumulation occurred in two stages. At first, the cholesteryl esters (CE) in the oxLDL were hydrolyzed to free cholesterol but the resultant cholesterol was unable to leave the lysosome. Following this, hydrolysis of oxLDL CE was inhibited resulting in lysosomes that contained significant free and esterified cholesterol. In contrast, incubation of cells with acetylated LDL (acLDL) did not produce significant lysosomal cholesterol accumulation. The cells stored equivalent amounts of cholesterol to those treated with oxLDL but the cholesterol was stored as cytoplasmic cholesteryl ester droplets. This indicates that CE from acLDL was hydrolyzed normally in lysosomes and the resulting free cholesterol was delivered to the cytoplasm for reesterification to CE.