GTK, a Src-related tyrosine kinase, induces nerve growth factor-independent neurite outgrowth in PC12 cells through activation of the Rap1 pathway - Relationship to Shb tyrosine phosphorylation and elevated levels of focal adhesion kinase

The rat pheochromocytoma cell line PC12 is extensively used as a model for studies of neuronal cell differentiation. These cells develop a sympathetic neuronlike phenotype when cultured in the presence of nerve growth factor. The present study was performed in or der to assess the role of mouse GTK (previously named BSK/IYK), a cytoplasmic tyrosine kinase belonging to the Src family, for neurite outgrowth in PC12 cells. We report that PC12 cells stably overexpressing GTK exhibit a larger fraction of cells with neurites as compared with control cells, and this response is not accompanied by an increased ERK activity. Treatment of the cells with the MEK inhibitor PD980 59 did not reduce the GTK-dependent increased in neurite outgrowth. GTK exp ression induces a nerve growth factor-independent Rap1 activation, probably through altered CrkII signaling. We observe increased CrkII complex format ion with p130(Cas), focal adhesion kinase (FAK), and Shb in PC12-GTK cells. The expression of GTK also correlates with a markedly increased content of FAK, phosphorylation of the adaptor protein Shb, and an association betwee n these two proteins. Transient transfection of GTK-over-expressing cells w ith RalGDS-RBD or Rap1GAP, inhibitors of the Rap1 pathway, reduces the GTK- dependent neurite outgrowth. These data suggest that GTK participates in a signaling pathway, perhaps involving Shb, FAK and Rap1, that induces neurit e outgrowth in PC12 cells.