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Cross-linking the IgE-bound Fc⑀RI with polyvalent antigen leads to Ca 2؉ -dependent degranulation from mast cells and basophils, initiating the allergic response. This overview addresses novel roles for PI 3-kinase in the regulation of signaling events that lie downstream of Fc⑀RI-mediated tyrosine kinase activation. The first novel role for PI 3-kinase is in the regulation of PLC␥ activity and is demonstrated by a dramatic inhibition of Fc⑀RI-induced Ins(1,4,5)P 3 production after treatment of RBL-2H3 cells with wortmannin, a PI 3-kinase inhibitor. We show that PI 3-kinase lipid products support Ins(1,4,5)P 3 production in at least two ways: by promoting translocation and phosphorylation of PLC␥1 and by direct stimulation of both PLC␥ isoforms. In vitro stimulation of PLC␥ activity by PtdIns(3,4,5)P 3 synergizes with activation by in vivo tyrosine phosphorylation for maximal enzymatic activity. A second novel role for PI 3-kinase is in the regulation of antigen-stimulated Ca 2؉ influx. Compared with control cells, Ca 2؉ responses are markedly diminished in antigen-stimulated cells after wortmannin pretreatment. Differences include both a longer lag time to the initial elevation in Ca 2؉ after antigen and an inhibition of the sustained Ca 2؉ influx phase. However, thapsigargin challenge during the sustained phase demonstrates no difference in the state of the Ca 2؉ stores in antigen-stimulated cells in the presence or absence of wortmannin. These data suggest that sufficient Ins(1,4,5)P 3 is synthesized in wortmannintreated cells to mobilize intracellular calcium stores and, furthermore, that the affected phase of Ca 2؉ influx is unlikely to be attributed to capacitative mechanisms. These data are consistent with a model where at least two pathways mediate Ca 2؉ influx in antigen-stimulated RBL-2H3 cells, one that is dependent on signals from empty stores (capacitative influx) and another that is downstream of PI 3-kinase. J. Leukoc. Biol. 65: 321-329; 1999.

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