No abstract

] i increase is essential for many cellular functions and is required for the phagocyte Fc␥R-induced oxidative burst (6, 7).Many lines of evidence in both human and murine systems indicate that tyrosine phosphorylation events are critical for phagocyte Fc␥R functions, including phagocytosis (8 -10). In addition, in many systems examined, tyrosine kinase activity is required for the receptor-induced rise in [Ca 2ϩ ] i (presumably through tyrosine phosphorylation of phospholipase C␥1 and generation of inositol 1,4,5-trisphosphate). However, the role of [Ca 2ϩ ] i in Fc␥ receptor-mediated phagocytosis has been controversial. For example, work in murine macrophage cell lines suggests that transients in [Ca 2ϩ ] i are not essential for phagocytosis of antibody-opsonized erythrocytes (EA) (11-13). In contrast, phagocytosis of EA by human neutrophils is significantly impaired by chelation of intracellular calcium and abrogation of [Ca 2ϩ ] i transients (14,15 ] i -dependent or independent fashion (19). These observations, coupled with the recent data of Stendahl and co-workers (20) that [Ca 2ϩ ] i storage organelles accumulate at contact sites during phagocytosis in human neu-

Tyrosine phosphorylation plays a critical role in Fc gamma RIIA signaling. In a mouse macrophage cell line transfected with human Fc gamma RIIA, cross-linking Fc gamma RIIA 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-gamma 1, and a tyrosine kinase p72syk. In addition, tyrosine phosphorylated Fc gamma RIIA was co-precipitated with activated PLC-gamma 1. In contrast, no tyrosine phosphorylation of Shc or PLC-gamma 1 was detected in cells transfected with mutant receptors that failed to trigger [Ca2+]i flux. PMA inhibits both tyrosine phosphorylation of Shc and IP3 production leading to [Ca2+]i flux. However, PMA does not affect tyrosine phosphorylation of PLC-gamma 1 and p72syk. These results suggest that tyrosine phosphorylation of Shc and PLC-gamma 1 is important for the initiation of [Ca2+]i flux, and that activation of protein kinase C may modulate the activity of PLC-gamma 1 through serine/threonine phosphorylation.