Abstract
Cooperation between the Neu/ErbB-2 and transforming growth factor beta (TGF-beta) signaling pathways enhances the invasive and metastatic capabilities of breast cancer cells; however, the underlying mechanisms mediating this synergy have yet to be fully explained. We demonstrate that TGF-beta induces the migration and invasion of mammary tumor explants expressing an activated Neu/ErbB-2 receptor, which requires signaling from autophosphorylation sites located in the C terminus. A systematic analysis of mammary tumor explants expressing Neu/ErbB-2 add-back receptors that couple to distinct signaling molecules has mapped the synergistic effect of TGF-beta-induced motility and invasion to signals emanating from tyrosine residues 1226/1227 and 1253 of Neu/ErbB-2. Given that the ShcA adaptor protein is known to interact with Neu/ErbB-2 through these residues, we investigated the importance of this signaling molecule in TGF-beta-induced cell motility and invasion. The reduction of ShcA expression rendered cells expressing activated Neu/ErbB-2, or add-back receptors signaling specifically through tyrosines 1226/1227 or 1253, unresponsive to TGF-beta-induced motility and invasion. In addition, a dominant-negative form of ShcA, lacking its three known tyrosine phosphorylation sites, completely abrogates the TGF-beta-induced migration and invasion of breast cancer cells expressing activated Neu/ErbB-2. Our results implicate signaling through the ShcA adaptor as a key component in the synergistic interaction between these pathways.