| Abstract: |
Fcγ receptor (FcγR)-mediated phagocytosis is known to require tyrosine kinases (TKs). We identified c-Cbl and Cbl-b as proteins that undergo tyrosine phosphorylation during phagocytosis. Cbl-deficient macrophages displayed enhanced FcγR-mediated signaling and phagocytosis. Surprisingly, binding of IgG-coated targets (EIgG) was also enhanced. c-Cbl-deficient macrophages expressed less FcγRIIb, the inhibitory Fcγ receptor; however, this did not account for enhanced target binding. We isolated the function of one Fc receptor isoform, FcγRI, using IgG2a-coated targets (EIgG2a). Cbl-deficient macrophages demonstrated a disproportionate increase in binding EIgG2a, suggesting that signal strength regulates binding efficiency toward opsonized targets. In resting cells, FcγRI colocalized with the Src family TK Hck in F-actin-rich structures, which was enhanced in Cbl-deficient macrophages. Target binding was sensitive to TK inhibitors, profoundly inhibited following depletion of cholesterol, and ablated at 4°C or in the presence of inhibitors of actin polymerization. Sensitivity of EIgG binding to cytoskeletal disruption was inversely proportional to opsonin density. These findings challenge the view that FcγR-mediated binding is a passive event. They suggest that dynamic engagement of TKs and the cytoskeleton enables macrophages to serve as cellular "Venus fly traps", with the capacity to capture phagocytic targets under conditions of limiting opsonin density. Copyright © 2009 by The American Association of Immunologists, Inc. |
| Keywords: |
signal transduction; controlled study; protein phosphorylation; genetics; nonhuman; binding affinity; protein localization; animal cell; mouse; animal; metabolism; mouse mutant; animals; mice; mice, knockout; actin; bone marrow cells; cells, cultured; cell line; protein binding; down-regulation; protein tyrosine kinase; tyrosine; phosphorylation; mice, inbred c57bl; physiology; c57bl mouse; immunology; drug antagonism; regulatory mechanism; immunoglobulin g; cell culture; fc receptor; receptors, igg; down regulation; cholesterol; bone marrow cell; cbl protein; f actin; fc receptor iib; hematopoietic cell kinase; opsonin; cbl protein, mouse; density; macrophage; phagocytosis; protein family; actins; macrophages; proto-oncogene proteins c-cbl
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