Apolipoprotein B-48 or Its Apolipoprotein B-100 Equivalent Mediates the Binding of Triglyceride-Rich Lipoproteins to Their Unique Human Monocyte-Macrophage Receptor

Studies in animals and humans have demonstrated uptake of plasma chylomicrons (triglyceride-rich lipoprotein [TGRLP] of Sf>400) by accessible macrophages in vivo. One potential mechanism is via a unique receptor pathway we previously identified in human blood and THP-1 monocytes and macrophages for the lipoprotein lipase (LpL)- and apolipoprotein (apo) E-independent, high-affinity, specific binding of plasma chylomicrons and hypertriglyceridemic VLDL (HTG-VLDL) to cell-surface membrane-binding proteins (MBP 200, 235; apparent Mr 200, 235 kD on SDS-PAGE) that leads to lipid accumulation in vitro. Competitive binding studies reported here demonstrate that anti-apoB antibodies specifically block the high-affinity binding of TGRLP to this receptor on THP-1 cells and on ligand blots. LpL, which binds to an N-terminal domain of apoB, also inhibits TGRLP binding both to this site on THP-1s and to MBP 200, 235 by binding to apoB. Chylomicrons of Sf>1100 that contain apoB-48, but not apoB-100, bind specifically to MBP 200, 235, and this binding is blocked by anti-apoB IgG. In contrast, lactoferrin and heparin do not inhibit TGRLP binding. We conclude that the receptor-binding domain is within apoB-48 (or an equivalent in apoB-100) near the LpL-binding domain, but not a heparin-binding domain. Uptake of TGRLP by this mechanism could provide essential nutrients or, in HTG, cause excess lipid accumulation and foam cell formation.