Binding Characteristics of Human Lactoferrin to the Human Monocytic Leukemia Cell Line THP-1 Differentiated into macrophages

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The characteristics of the binding of human lactoferrin (LF) to the cells of a human monocytic leukemia cell line, THP-1, were investigated. ¹²⁵I-Labeled LF (¹²⁵I-LF) bound to THP-1 cells, and the binding increased markedly as the cells matured into macrophages (THP-1 macrophages) by stimulation with phorbol 12-myristate 13-acetate. Scatchard analysis of the binding of ¹²⁵I-LF to THP-1 macrophages indicated that high and low affinity receptor sites (K_d=0.57×10^-6 and 3.7×10^-6M, respectively) are present on the cells. The number of these high and low affinity receptor sites were 2.4×10⁶, and 2.5×10⁶ per cell, respectively. Removal of iron from ¹²⁵I-LF did not affect its binding to THP-1 macrophages, indicating that the binding is not dependent on Fe (III) ion. The binding of the labeled LF to THP-1 macrophages was markedly decreased following acetylation, suggesting that the amino residues of the polypeptide portion of LF play a major role in the binding. The binding of labeled LF was partially inhibited by the isolated whole oligosaccharides of LF, and by the isolated whole oligosaccharides of band 3 glycoprotein of human erythrocyte membrane which contain poly-N-acetyllactosaminyl saccharide chains, like the LF oligosaccharides. Their inhibitory activity did not depend on the terminal sialyl residues of the saccharide chains. Lacto-N-fucopentaose III and lacto-N-neotetraose, and analogous structure being present in the poly-N-acetyllactosaminyl chains of LF, also partially inhibited the binding of ¹²⁵I-LF to the THP-1 macrophages. When poly-N-acetyllactosaminyl saccharide chains of ¹²⁵I-LF were cleaved by endo β-galactosidase, the binding of ¹²⁵I-LF was partially reduced. These results suggest that binding of LF to THP-1 macrophages is primarily mediated by its protein component, but a short oligosaccharide structure, possibly Galβ1-4GlcNAcβ1-3Gal, which is contained in the nonreducing terminal region of poly-N-acetyllactosaminyl saccharide chains of LF and band 3, and in lacto-N-fucopentaose III and lacto-N-neotetraose is also recognized by THP-1 macrophages, and this recognition partly contributes to the binding of LF to cells.