Generation of Fibronectin Receptors on Macrophages by Wheat Germ Lectin

Abstract

A chymotrypsin-derived and 125I-labelled 125-kDa fragment of human plasma fibronectin which contained the cell binding site, was only weakly bound by peritoneal macrophages of guinea pigs and binding was not saturable. In presence of wheat germ lectin binding increased proportionally to the logarithm of the lectin concentration. Association of 125I-fragment with cells was partially prevented by non-labelled fragment indicating a saturable receptor-ligand interaction. An apparent affinity constant of about 2--4 x 10(-5) M was evaluated. A considerable fraction of the cell-bound 125I-fragment resisted removal by proteases suggesting that it was internalized. In order to investigate an influence of wheat germ lectin on the binding of 125I-fibronectin by the cells the macrophages were preincubated with the lectin followed by washing and evaluation of 125I-fibronectin binding. A simultaneous incubation of the cells with 125I-fibronectin and lectin was impractical due to partial interaction of the two proteins giving rise to some unspecific precipitates. Preincubation with wheat germ lectin considerably improved the capacity of the macrophages for binding of 125I-fibronectin. Again the binding of 125I-labelled protein could be restricted by unlabelled one. N-acetyl-glucosamine inhibited the binding of 125I-fibronectin by wheat germ lectin-treated cells if applied in the preincubation phase and more effectively, if applied in the final 125I-fibronectin binding assay. N-Acetylneuraminic acid also inhibited this step. In addition to wheat germ lectin concanavalin A was capable of generating fibronectin receptors on the cell surface. Soy bean lectin, however, was ineffective.