Correlations among tyrosine phosphorylation of Shc, p72^syk, PLC-γ1, and [Ca²⁺]_i flux in FcγRIIA signaling

Tyrosine phosphorylation plays a critical role in FcγRIIA signaling. In a mouse macrophage cell line transfected with human FcγRIIA, cross-linking FcγRIIA led to the transient generation of inositol 1, 4, 5-trisphosphate (IP3), [Ca²⁺]_i flux, and rapid tyrosine phosphorylation of cellular substrates, including Shc, PLC-γ1, and a tyrosine kinase p72^syk. In addition, tyrosine phosphorylated FcγRIIA was co-precipitated with activated PLC-γ1. In contrast, no tyrosine phosphorylation of Shc or PLC-γ1 was detected in cells transfected with mutant receptors that failed to trigger [Ca²⁺]_i flux. PMA inhibits both tyrosine phosphorylation of She and IP3 production leading to [Ca²⁺]_i flux. However, PMA does not affect tyrosine phosphorylation of PLC-γ1 and p72^syk. These results suggest that tyrosine phosphorylation of Shc and PLC-γ1 is important for the initiation of [Ca²⁺]_i flux, and that activation of protein kinase C may modulate the activity of PLC-γ1 through serine/threonine phosphorylation.