Dual Phosphorylation of Phosphoinositide 3-Kinase Adaptor Grb2-Associated Binder 2 Is Responsible for Superoxide Formation Synergistically Stimulated by Fcγ and Formyl-Methionyl-Leucyl-Phenylalanine Receptors in Differentiated THP-1 Cells

Abstract

The class Ia phosphoinositide (PI) 3-kinase consisting of p110 catalytic and p85 regulatory subunits is activated by Tyr kinase-linked membrane receptors such as FcγRII through the association of p85 with the phosphorylated receptors or adaptors. The heterodimeric PI 3-kinase is also activated by G protein-coupled chemotactic fMLP receptors, and activation of the lipid kinase plays an important role in various immune responses, including superoxide formation in neutrophils. Although fMLP-induced superoxide formation is markedly enhanced in FcγRII-primed neutrophils, the molecular mechanisms remain poorly characterized. In this study, we identified two Tyr-phosphorylated proteins, c-Cbl (Casitas B-lineage lymphoma) and Grb2-associated binder 2 (Gab2), as PI 3-kinase adaptors that are Tyr phosphorylated upon the stimulation of FcγRII in differentiated neutrophil-like THP-1 cells. Interestingly, Gab2 was, but c-Cbl was not, further Ser/Thr phosphorylated by fMLP. Thus, the adaptor Gab2 appeared to be dually phosphorylated at the Ser/Thr and Tyr residues through the two different types of membrane receptors. The Ser/Thr phosphorylation of Gab2 required the activation of extracellular signal-regulated kinase, and fMLP receptor stimulation indeed activated extracellular signal-regulated kinase in the cells. Enhanced superoxide formation in response to Fcγ and fMLP was markedly attenuated when the Gab2 Ser/Thr phosphorylation was inhibited. These results show the importance of the dual phosphorylation of PI 3-kinase adaptor Gab2 for the enhanced superoxide formation in neutrophil-type cells.