Uptake of canine beta-very low density lipoproteins by mouse peritoneal macrophages is mediated by a low density lipoprotein receptor.

Abstract

The receptor on mouse peritoneal macrophages that mediates the uptake of canine beta-very low density lipoproteins (beta-VLDL) has been identified in this study as an unusual apolipoprotein (apo-) B,E(LDL) receptor. Ligand blots of Triton X-100 extracts of mouse peritoneal macrophages using 125I-beta-VLDL identified a single protein. This protein cross-reacted with antibodies against bovine apo-B,E(LDL) receptors, but its apparent Mr was approximately 5,000 less than that of the human apo-B,E(LDL) receptor. Binding studies at 4 degrees C demonstrated specific and saturable binding of low density lipoproteins (LDL), beta-VLDL, and cholesterol-induced high density lipoproteins in plasma that contain apo-E as their only protein constituent (apo-E HDLc) to mouse macrophages. Apolipoprotein E-containing lipoproteins (beta-VLDL and apo-E HDLc) bound to mouse macrophages and human fibroblasts with the same high affinity. However, LDL bound to mouse macrophages with an 18-fold lower affinity than to human fibroblasts. Mouse fibroblasts also bound LDL with a similar low affinity. Compared with the apo-B,E(LDL) receptors on human fibroblasts, the apo-B,E(LDL) receptors on mouse macrophages were resistant to down-regulation by incubation of the cells with LDL or beta-VLDL. There are three lines of evidence that an unusual apo-B,E(LDL) receptor on mouse peritoneal macrophages mediates the binding and uptake of beta-VLDL: LDL with residual apo-E removed displaced completely the 125I-beta-VLDL binding to mouse macrophages, preincubation of the mouse macrophages with apo-B,E(LDL) receptor antibody inhibited both the binding of beta-VLDL and LDL to the cells and the formation of beta-VLDL- and LDL-induced cholesteryl esters, and binding of 125I-beta-VLDL to the cells after down-regulation correlated directly with the amount of mouse macrophage apo-B,E(LDL) receptor as determined on immunoblots. This unusual receptor binds LDL poorly, but binds apo-E-containing lipoproteins with normal very high affinity and is resistant to down-regulation by extracellular cholesterol.