Signaling by the Granulocyte-Colony Stimulating Factor Receptor Involves Jak2-Dependent Phosphorylation of Gab2.

Granulocyte colony stimulating factor (G-CSF) is the major cytokine involved in neutrophil production. G-CSF has pleiotropic effects on myeloid cells, initially stimulating proliferation but later promoting differentiation. The specific signaling pathways that mediate the diverse effects of G-CSF remain incompletely understood. Recently, the scaffolding molecule Grb2-associated binder protein 2 (Gab2) was shown to play an important role in G-CSF induced myeloid differentiation (Zhu et al. Blood 2004). Ligand stimulation of the G-CSF receptor results in the rapid phosphorylation of Gab2, but the identity of the responsible kinases and the molecular events dependent on Gab2 phosphorylation remain unclear. Because Janus kinases (Jaks) play a central role in G-CSF signaling, we investigated the involvement of Jaks in G-CSF-stimulated Gab2 phosphorylation using the hematologic DT40 cell line stably transduced with the human G-CSF receptor (DT40GR). Antisense Jak1 and Jak2 constructs expressed in DT40GR cells each produced a marked reduction in their target Jak protein, but only antisense Jak2 reduced G-CSF-stimulated Gab2 phosphorylation. To determine whether Gab2 phosphorylation required Jak2 kinase activity, dominant negative Jak2 mutants lacking catalytic activity were expressed in the DT40GR cells. Expression of dominant negative Jak2 inhibited Gab2 phosphorylation in response to G-CSF. Similarly, treatment with the Jak2-selective kinase inhibitor AG490 markedly reduced G-CSF-dependent Gab2 phosphorylation. Co-immunoprecipitation studies further demonstrated a G-CSF- and Gab2 phosphorylation-dependent association of Jak2 with Gab2 in vivo, which was detectable by 30 seconds after G-CSF stimulation. To determine whether Gab2 was a direct substrate of Jak2, we performed in vitro phosphorylation studies using Gab2-GST fusion protein substrates. Jak2 immunoprecipitated from G-CSF-stimulated cells, but not from control cells, phosphorylated the Gab2 fusion protein. To identify potential Jak2 tyrosine phosphorylation sites in Gab2, we used site-directed mutagenesis to produce three Gab2 tyrosine mutants. Tyrosines 409, 452, and 476 were each replaced by phenylalanine (Y409F, Y452F, and Y476F). The Y452F and Y476F mutations of Gab2 each inhibited G-CSF-stimulated Jak2-dependent phosphorylation of Gab2, both in stably-transfected DT40GR cells and in transiently-transfected 293 cells also transduced with the G-CSF receptor. In contrast, G-CSF-stimulated Gab2 phosphorylation appeared unaffected by the Y409F mutation. We also evaluated downstream events in G-CSF signaling in cells expressing these Gab2 tyrosine- mutants. Akt and Erk phosphorylation following G-CSF stimulation was inhibited by both the Y452F and Y476F Gab2 mutations, but was unaffected by the Y409F mutation. These results suggest that Jak2 may mediate G-CSF differentiation signals through Stat-independent mechanisms.