Human Monocyte–Derived Macrophages Express an ≈120-kD Ox-LDL Binding Protein With Strong Identity to CD68

Abstract

A protein that specifically binds oxidized LDL (Ox-LDL) has recently been characterized in mouse peritoneal macrophages and identified as macrosialin, a protein with a molecular weight of 95 kD. First, the present work shows that human monocyte–derived macrophages express a membrane protein with a molecular weight of ≈120 kD that selectively binds Ox-LDL. Second, we tested whether this ≈120-kD Ox-LDL binding protein had any relation to CD68, the human homologue of macrosialin. The following evidence was obtained to support the role of CD68 as an Ox-LDL binding protein: (1) Ligand blots with Ox-LDL and Western blots with Ki-M6, an anti–human CD68 monoclonal antibody, revealed a single band with a molecular weight of ≈120 kD under reducing and nonreducing condition. (2) The expression patterns of the ≈120-kD Ox-LDL binding membrane protein and of CD68 paralleled each other during monocyte/macrophage differentiation. (3) Digestion withN-glycosidase F demonstrated that both CD68 and the Ox-LDL binding protein are glycoproteins; both showed a similar shift of ≈18 kD in apparent molecular weight. (4) CD68, probed with monoclonal antibody Ki-M6, and the ≈120-kD Ox-LDL binding protein were coprecipitated with EBM11, another anti-CD68 antibody. About 5000 molecules of CD68 are expressed on the cell surface of human macrophages. Ligation of¹²⁵I–Ki-M6 to cells leads to its internalization and degradation. This capacity would be sufficient to allow for the specific uptake and degradation of Ox-LDL. Taken together, these data support a role for CD68 as a specific Ox-LDL binding protein in human monocyte–derived macrophages.