The Phospholipase Cγ1-dependent Pathway of FcεRI-mediated Mast Cell Activation Is Regulated Independently of Phosphatidylinositol 3-Kinase

Abstract

Mast cell degranulation following FcεRI aggregation is generally believed to be dependent on phosphatidylinositide 3-kinase (PI 3-kinase)-mediated phospholipase C (PLC)γ activation. Here we report evidence that the PLCγ1-dependent pathway of FcεRI-mediated activation of mast cells is independent of PI 3-kinase activation. In primary cultures of human mast cells, FcεRI aggregation induced a rapid translocation and phosphorylation of PLCγ1, and subsequent inositol trisphosphate (IP3) production, which preceded PI 3-kinase-related signals. In addition, although PI 3-kinase-mediated responses were completely inhibited by wortmannin, even at high concentrations, this PI 3-kinase inhibitor had no effect on parameters of FcεRI-mediated PLCγ activation, and had little effect on the initial increase in intracellular calcium levels that correlated with PLCγ activation. Wortmannin, however, did produce a partial (∼50%) concentration-dependent inhibition of FcεRI-mediated degranulation in human mast cells and a partial inhibition of the later calcium response at higher concentrations. Further studies, conducted in mast cells derived from the bone marrow of mice deficient in the p85α and p85β subunits of PI 3-kinase, also revealed no defects in FcεRI-mediated PLCγ 1 activation. These data are consistent with the conclusion that the PLCγ-dependent component of FcεRI-mediated calcium flux leading to degranulation of mast cells is independent of PI 3-kinase. However, PI 3-kinase may contribute to the later phase of FcεRI-mediated degranulation in human mast cells.