Syk-dependent phosphorylation of Shc. A potential link between Fc epsilon RI and the Ras/mitogen-activated protein kinase signaling pathway through Sos and Grb2

Antigen receptors on T- and B-cells activate Ras through a signaling pathway that results in the tyrosine phosphorylation of Shc and the formation of a complex of Shc with the Grb2 adaptor protein. The high affinity receptor for immunoglobulin E (Fc epsilon RI) in cultured mast (RBL-2H3) cells has been reported to function differently. Here we show to the contrary that engagement of Fc epsilon RI with antigen leads to increased tyrosine phosphorylation of Shc and the association of Shc with Grb2 and other proteins (p120 and p140). Like the Fc epsilon RI-mediated activation of the mitogen-activated protein kinase cascade, these responses are dependent on the tyrosine kinase Syk; they are enhanced by overexpression of Syk and are blocked by expression of dominant-negative Syk. Sos is constitutively associated with Grb2 in these cells but dissociates from Shc on stimulation with antigen. These reactions are rapid, reversible, and associated with the activation of Ras. Therefore, the Syk-dependent tyrosine phosphorylation of Shc and its association with Grb2 may provide a pathway through Sos for activation of Ras by Fc epsilon RI.