The phospholipase Cγ1-dependent pathway of FcεRI-mediated mast cell activation is regulated independently of phosphatidylinositol 3-kinase

Mast cell degranulation following FcepsilonRI aggregation is generally believed to be dependent on phosphatidylinositide 3-kinase (PI 3-kinase)-mediated phospholipase C (PLC) gamma activation. Here we report evidence that the PLCgamma(1)-dependent pathway of FcepsilonRI-mediated activation of mast cells is independent of PI 3-kinase activation. In primary cultures of human mast cells, FcepsilonRI aggregation induced a rapid translocation and phosphorylation of PLCgamma(1), 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 FcepsilonRI-mediated PLCgamma activation, and had little effect on the initial increase in intracellular calcium levels that correlated with PLCgamma activation. Wortmannin, however, did produce a partial ( similar to 50%) concentration-dependent inhibition of FcepsilonRI-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 p85alpha and p85beta subunits of PI 3-kinase, also revealed no defects in FcepsilonRI-mediated PLCgamma(1) activation. These data are consistent with the conclusion that the PLCgamma-dependent component of FcepsilonRI-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 FcepsilonRI-mediated degranulation in human mast cells.