J. Biol. Chem., Vol. 261, Issue 24, 11194-11201, Aug, 1986
Uptake of canine beta-very low density lipoproteins by mouse peritoneal macrophages is mediated by a low density lipoprotein receptor
C Koo, ME Wernette-Hammond and TL Innerarity
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.