p110β and p110δ Phosphatidylinositol 3-Kinases Up-regulate FcεRI-activated Ca2+ Influx by Enhancing Inositol 1,4,5-Trisphosphate Production

Abstract

FcεRI-induced Ca2+ signaling in mast cells is initiated by activation of cytosolic tyrosine kinases. Here, in vitro phospholipase assays establish that the phosphatidylinositol 3-kinase (PI 3-kinase) lipid product, phosphatidylinositol 3,4,5-triphosphate, further stimulates phospholipase Cγ2 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 Ca 2+ responses after FcεRI cross-linking in RBL-2H3 cells. Injection of antibodies to the p110β or p110δ catalytic isoforms of PI 3-kinase, but not antibodies to p110α, 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 5-phosphatase I, which degrades inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), or heparin, a competitive inhibitor of the Ins(1,4,5)P3 receptor. This indicates a requirement for Ins(1,4,5)P3 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 p110β and p110δ antibodies, but not by injection of 5-phosphatase I, heparin, or anti-p110α antibodies. These results suggest that the p110β and p110δ isoforms of PI 3-kinase support FcεRI-induced calcium signaling by modulating Ins(1,4,5)P3 production, not by directly regulating the Ca2+ influx channel.