Gastrin/CCKB G protein-coupled receptors have been shown to mediate proliferative effects of their endogenous ligands. In the present study, we examined the signal transduction mechanisms linked to the G/CCKB receptor occupancy. We report here that gastrin stimulates MAP kinase activation in a dose- and time-dependent manner, a pathway known to play a key role in cell proliferation. We also characterized the molecular events, upstream of p21-Ras, that may link the MAP kinase pathway to G/CCKB receptors. Gastrin induced a rapid and transient increase in tyrosine phosphorylation of several proteins including the 2 isoforms (46 and 52 kDa) of the adaptor protein She. Phosphorylated Shc subsequently associated with a complex that includes Grb2 and the p21-Ras activator, Sos. Our results also indicate that Sos becomes phosphorylated in response to gastrin as shown by a reduction in electrophoretic mobility of the protein. Tyrosine phosphorylation of Shc and subsequent complex formation with Grb2 and Sos appear to be a common mechanism by which tyrosine kinase receptors and the G/CCKB G protein-coupled receptor stimulate the Ras-dependent MAP kinase pathway.
