P110β and p110δ phosphatidylinositol 3-kinases up-regulate FcεRI-activated Ca2+ influx by enhancing inositol 1,4,5-trisphosphate production

Fc is an element of RI-induced Ca2+ signaling in mast cells is initiated by activation of cytosolic tyrosine kinases, Here, in vitro phospholipase assays establish that the phosphatidylinositol S-kinase (PI 3-kinase) lipid product, phosphatidylinositol 3,4,5-triphosphate, further stimulates phospholipase Cy2 that has been activated by conformational changes associated with tyrosine phosphorylation or low pH, A microinjection approach is used to directly assess the consequences of inhibiting class IA PI 3-kinases on Ca2+ responses after Fc is an element of RI cross-linking in RBL-2H3 cells. Injection of antibodies to the p110 beta or p110 delta catalytic isoforms of PI 3-kinase, but not antibodies to p110 alpha, lengthens the lag time to release of Ca2+ stores and blunts the sustained phase of the calcium response. Ca2+ responses are also inhibited in cells microinjected with recombinant inositol polyphosphate B-phosphatase I, which degrades inositol 1,4,5-trisphosphate (Ins(1,4,5)P-3), or heparin, a competitive inhibitor of the Ins(1,4,5)P-3 receptor. This indicates a requirement for Ins(1,4,5)P-3 to initiate and sustain Ca2+ responses even when PI 3-kinase is fully active. Antigen-induced cell ruffling, a calcium-independent event, is blocked by injection of p110 beta and p110 delta antibodies, but not by injection of B-phosphatase I, heparin, or anti-p110 alpha antibodies. These results suggest that the p110 beta and p110 delta isoforms of PI 3-kinase support Fc is an element of RI-induced calcium signaling by modulating Ins(1,4,5)P-3 production, not by directly regulating the Ca2+ influx channel.