Apolipoprotein E is the determinant that mediates the receptor uptake of β-very low density lipoproteins by mouse macrophages

Abstract

Beta very low density lipoproteins (β-VLDL) from cholesterol-fed animals and from patients with Type III hyperlipoproteinemia are internalized by a receptor-mediated process in mouse macrophages. Once internalized, the cholesteryl esters of β-VLDL are hydrolyzed in lysosomes, and the released cholesterol is re-esterified, resulting in a massive accumulation of cholesteryl esters. In the present study, competitive binding experiments demonstrated that canine apo E HDL(c) (lipoproteins that contain almost exclusively apolipoprotein E) inhibited the receptor-mediated uptake of 125l-β-VLDL. The incorporation of human apo E into β-VLDL was also shown to modulate binding. Reductively methylated β-VLDL (methyl β-VLDL) were not taken up by macrophages and did not stimulate cholesteryl ester synthesis. When unmodified human apo E-3 was incorporated into the lipoprotein in place of the canine methyl apo E, these hybrid β-VLDL (methyl β-VLDL [E-3]) were internalized and degraded and were as effective as native β-VLDL in stimulating cholesteryl ester synthesis in macrophages. In the reverse experiment, the incorporation of methyl apo E-3 into native canine β-VLDL (β-VLDL [methyl E-3]) reduced the binding activity of the β-VLDL and abolished their ability to stimulate cellular cholesteryl ester synthesis. Canine β-VLDL into which apo E-2(Arg158→Cys) had been incorporated had less ability to stimulate cholesteryl ester synthesis (20%) than did native β-VLDL, but they were more active than β-VLDL [methyl E-2] or β-VLDL [methyl E-3], which had virtually no activity. These results demonstrate that apo E is the determinant mediating the receptor binding and uptake of β-VLDL by mouse macrophages.