G-CSF Receptor Activates Lyn through an Essential Temporal-Spatial Recruitment of Gab2 and Shp2.

The Hematopoietin/Cytokine Receptors, such as the Granulocyte Colony-Stimulating Factor (G-CSF) Receptor, signal through the Src and Janus kinases. Src activation involves the sequential regulation of positive and negative tyrosine phosphorylation sites. The C-terminal phosphotyrosine serves as a negative phosphorylation site via its interaction with an endogenous SH2 domain. How Lyn, the predominant Src kinase expressed in granulocytes, becomes activated following G-CSF Receptor engagement is not known. Because Lyn contains a negative phosphotyrosine site (pTyr507), we hypothesize that Shp2, an activating tyrosine phosphatase, might be involved. We report that G-CSF stimulation induces the dissociation of Lyn from the scaffolding protein Gab2, which leads to the dephosphorylation of pTyr507 and autophosphorylation of Tyr396. The dephosphorylation of Lyn pTyr507 was impaired in Shp2-deficient murine embryonic fibroblasts transfected with the G-CSF Receptor in response to G-CSF stimulation. Phosphorylation of Lyn Tyr 396 was impaired in cells treated with Gab2 siRNA. Gab2 constitutively associated with Shp2, and mutations of Tyr614 and Tyr643 in Gab2 abolished the recruitment of Shp2. The constitutively activated Shp2 E76A directed the dephosphorylation of Lyn pTyr507 in vitro. Based on these data, we propose that Gab2 forms a complex with Lyn and following G-CSF stimulation, Gab2 recruits Shp2, which dephosphorylates Lyn pTyr507, leading to Lyn activation. Because gain-of-function mutation in Shp2 or recruitment of Lyn in imatinib-resistant Bcr-Abl+ disease occurs in myeloid and lymphoid leukemias, aberrant pathways involving Src can now be targeted through small molecule inhibitors, such as dasatinib.