Fps/Fes protein-tyrosine kinase regulates mast cell adhesion and migration downstream of Kit and β1 integrin receptors

Activation of Kit receptor protein-tyrosine kinase (PTK) by its ligand Stem Cell Factor (SCF) is required for the development of mast cells, and for the regulation of mast cell proliferation, migration and modulation of inflammatory mediator release. Recent studies have implicated the non-receptor PTK Fps/Fes (hereafter referred to as Fes) in signaling downstream of oncogenic Kit, however, the potential role of Fes in regulating Kit signaling is not well defined. In this study, we show that SCF induces transient tyrosine phosphorylation of wild-type Fes as well as kinase-dead Fes in bone marrow-derived mast cells (BMMCs). The latter finding implicates an upstream kinase acting on Fes, which we identified as Fyn PTK. SCF treatment of BMMCs promoted recruitment of Fes to Kit, potentially via direct interaction of the Fes SH2 domain with phosphorylated Kit. While Fes was not required for SCF-induced signaling to Akt and Erk kinases, Fes-deficient (fes−/−) BMMCs displayed a defect in sustained p38 kinase activation, compared to control cells. SCF-treated Fes-deficient BMMCs also displayed elevated β1 integrin-mediated cell adhesion and spreading on fibronectin, compared to control cells, and a reduction in cell polarization at later times of SCF treatment. Restoring Fes expression in fes−/− BMMCs by retroviral transduction was sufficient to rescue cell spreading and polarization defects. Interestingly, SCF-induced chemotaxis of BMMCs was also defective in Fes-deficient BMMCs, and restored in Fes-rescue BMMCs. Overall, these results implicate Fes in regulating cross-talk between Kit and β1 integrins to promote cytoskeletal reorganization and motility of mast cells.