The role of signalling pathways downstream from the Grb2 adaptor protein in Met receptor and Tpr-Met oncoprotein biological activities /

Activation of the Met receptor tyrosine kinase by its ligand, Hepatocyte Growth Factor (HGF), leads to mitogenesis, cell motility, morphogenesis, and angiogenesis. Mutational analysis has demonstrated the requirement of a single tyrosine within the carboxy-terminus (Y1356) of the Met receptor for the recruitment and activation of all Met-dependent signalling pathways and for the transformation of fibroblasts by the Tpr-Met oncogene. The selective abolishment of Grb2 from the Tpr-Met oncoprotein, by generating an asparagine to histidine mutation two amino acids downstream from Y1356 (N1358H), led to a reduction in Tpr-Met-mediated transformation of fibroblasts. Moreover, Met receptor studies demonstrate that while a Grb2 binding site is not required for epithelial cell motility, it is critical for the formation of branching tubules when cells are suspended in a collagen matrix. This suggests that Grb2-dependent pathways are involved in the organization and polarization of epithelial cells following Met receptor stimulation.Grb2 associated molecules, Gab1 and Cbl, are highly phosphorylated following stimulation of the Met receptor. Moreover, signaling pathways associated with Gab1 are critical for branching tubulogenesis in epithelial cells. Expression of a constitutively active version of Cbl, 70z-Cbl, results in an epithelial-mesenchymal transition, leading to the breakdown of cellular junctions and reorganization of the actin cytoskeleton. The amino-terminal SH2 domain is the minimal region required to induce morphological changes, which may be mediated through its interaction with the Met receptor, and/or an unidentified protein of 150 kDa