Differential kinase requirements in human and mouse Fc-gamma receptor phagocytosis and endocytosis

Fe gamma receptors (Fc gamma Rs) contribute to the internalization of large and small immune complexes through phagocytosis and endocytosis, respectively. The molecular processes underlying these internalization mechanisms differ dramatically and have distinct outcomes in inumme clearance and modulation of cell function. However, it is unclear how the same receptors (Fc gamma R) binding to identical ligands (IgG) can elicit such distinct responses. We and others have shown that Syk kinase, Sre-related tyrosine kinases (SRTKs) and phosphatidyl inositol 3-kinases (PI3K) play important roles in Fc gamma R phagocytosis. Herein, we demonstrate that these kinases are not required for Fc gamma R endocytosis. Endocytosis of heat-aggregated IgG (HA-IgG) by COS-1 cells stably transfected with Fc-gamma RIIA or chimeric Fe gamma RI-gamma-gamma (EC-TM-CYT) was not significantly altered by PP2, piceatannol, or wortmannin. In contrast, phagocytosis of large opsonized particles (IgGsensitized sheep erythrocytes, EA) was markedly reduced by these inhibitors. These results were confirmed in primary mouse bone marrow-derived macrophages and freshly isolated human monocytes. Levels of receptor phosphorylation were similar when Fc gamma RIIA was cross-liuked using HA-IgG or EA. However, inhibition of FcyR phosphorylation prevented only Fc gamma R phagocytosis. Finally, biochemical analyses of PI3K(p85)-Syk binding indicated that direct interactions between native Syk and PI3K proteins are differentially regulated during Fc gamma R phagocytosis and endocytosis. Overall, our results indicate that Fc gamma R endocytosis and phagocytosis differ dramatically in their requirement for Syk, SRTKs, and PI3K, pointing to striking differences in their signal transduction mechanisms. We propose a competitive inhihition-based model in which PI3K and c-Cbl play contrasting roles in the induction of phagocytosis or endocytosis signaling cascades.