PI3Kγ Adaptor Subunits Define Coupling to Degranulation and Cell Motility by Distinct PtdIns(3,4,5)P3 Pools in Mast Cells

Phosphoinositide 3-kinase gamma (PI3K gamma) plays a major role in chronic inflammation and allergy. It is a heterodimer of a catalytic p110 gamma subunit and an adaptor protein, either p101 or the p101 homolog p84 (p87(PIKAP)). It is unclear whether both PI3K gamma complexes specifically modulate responses such as chemotaxis and degranulation. In mast cells, the p84:p110 gamma complex synergizes with immunoglobulin E (IgE)- and antigen-clustered Fc epsilon RI receptor signaling and is required to achieve maximal degranulation. During this process, PI3K gamma is activated by ligands of heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs), in particular adenosine receptors, through autocrine and paracrine pathways. Here, we show that p110 gamma needs p84 to relay signals from GPCRs to formation of phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P-3], phosphorylation of Akt, migration of cells, and synergistic adenosine-enforced degranulation. Furthermore, the absence of adaptor subunits could not be compensated for by increased p110 gamma abundance. Differentiated, p110 gamma null cells also lost adaptor proteins. Complementation of p110 gamma null mast cells with p101 and p110 gamma restored the activation of Akt and cell migration, but failed to support degranulation. Lack of degranulation was attributed to a change in the spatiotemporal localization of PI3K gamma-derived PtdIns(3,4,5)P-3; although both p84:p110 gamma and p101:p110 gamma complexes initially deposited PtdIns(3,4,5)P-3 at the plasma membrane, p101:p110 gamma-derived PtdIns(3,4,5)P-3 was rapidly endocytosed to motile, microtubule-associated vesicles. In addition, p84:p110 gamma, but not p101:p110 gamma signaling was sensitive to disruption of lipid rafts. Our results demonstrate a nonredundant function for the p101 and p84 PI3K gamma adaptor proteins and show that distinct pools of PtdIns(3,4,5)P-3 at the plasma membrane can elicit specific cell responses.