Two distinct kinases responsible for the phosphorylation of tyrosine residues of Gab2 in neutrophil precursor cell upon G-CSF stimulation.

Abstract Gab2 is one of the most heavily tyrosine-phosphorylated protein in granulocyte precursor cell, GM-I62-1, upon G-CSF stimulation. Effects of several pharmacological inhibitors to tyrosine kinases were examined on the tyrosine phosphorylation of Gab2 upon G-CSF stimulation. Specific Src kinase inhibitor, PP1, had marginal effects, if any, on the Gab2 tyrosine phosphorylation. In contrast, Syk specific inhibitors, Piceatannol and GS-9973, had partial but distinct inhibitory effects on the Gab2 tyrosine phosphorylation upon G-CSF stimulation. When the tyrosine residues are phosphorylated, downstream effector molecules, SHP2 and p85 subunit of PI3K, are known to bind to these phosphorylated residues of Gab2 through their SH2 domains. Pretreatments of cells with Syk specific inhibitors reduced the SHP2 binding to Gab2. However, no inhibitory effect of these inhibitors on p85 binding to Gab2 was observed. These observations indicated that Syk phosphorylates tyrosine residues of SHP2 binding sites of Gab2, and that the tyrosine residues of Gab2 to which p85 subunit of PI3K binds upon G-CSF stimulation are phosphorylated by other tyrosine kinase(s).