α2‐macroglobulin stimulation of protein tyrosine phosphorylation in macrophages via the mannose receptor for Fcγ receptor‐mediated phagocytosis activation

Abstract

Macrophage phagocytic activity has previously been shown to be increased by binding of modified α₂-macroglobulin (α₂M) with mannose residues at termini of sugar chains to mannose receptors on macrophages, with a subsequent increase in the number of Fcγ receptors at the cell surface. In the present study, an examination was made of the association of protein tyrosine kinase with the increase in number of Fcγ receptors following binding of modified α₂M to mannose receptors. The phagocytosis of IgG-opsonized sheep red blood cells through the action of the Fcγ receptor by modified α₂M was inhibited by mannose and herbimycin A and slightly so by genistein. The mannose receptor would thus appear to be associated with tyrosine kinase activity. By Western blotting, tyrosine phosphorylated proteins with molecular weights of 32 000, 34 000, 36 000, 65 000, 85 000 and 110 000 appeared or increased upon treating macrophages with modified α₂M. The degree of tyrosine phosphorylated proteins was the same for control macrophages following incubation in the presence of mannose and herbimycin A. Genistein treatment affected only tyrosine phosphorylated proteins of 65 000 and 110 000. The binding of modified α₂M to mannose receptors was demonstrated by the inducement of tyrosine kinase activation that was sensitive to herbimycin A, followed by an increase in Fcγ receptors and consequently greater phagocytosis.