| The survival of squamous epithelial cells requires signals generated by integrin-matrix interactions. Following conversion to squamous cell carcinoma, the cells remain sensitive to detachment-induced anoikis, yet in tumor cell aggregates, which are matrix deficient, these cells are capable of suprabasal survival. Their survival is enhanced through a process we call synoikis , whereby junctional adhesions between neighboring cells generate specific downstream survival signals. Here we tested the hypothesis that activation of EGFR and downstream signaling pathway(s) are required for E-cadherin-mediated cell survival.; We found that in squamous cell carcinoma cells, both c-erbB family members EGFR and erbB2 were expressed. By using immunofluorescence staining and immunostaining, we have demonstrated the colocalization of EGFR and E-cadherin at cell-cell junctions in SCC cells grown as multicellular aggregates. E-cadherin-mediated cell-cell adhesion specifically induced activation of EGFR, but not erbB2 or another tyrosine kinase receptor c-Met.; Next we demonstrated that EGFR activation in turn triggered the ERK/MAPK signaling module, leading to elevation of anti-apoptotic Bcl-2. Using pharmaceutical inhibitors, we found that EGFR and the downstream ERK/MAPK activation were required for E-cadherin-mediated cell survival. Ectopic expression of E-cadherin in a receptor-negative cell line was sufficient to confer resistance to anoikis.; In additional studies, we investigated the mechanism of EGFR activation and found that after intercellular adhesion, formation of adherens junctions triggers the recruitment of E-cadherin-EGFR complexes to the sites of cell-cell contacts, correlating with EGFR transactivation. E-cadherin-mediated EGFR activation was also dependent on cytoskeleton organization. Analysis of the process with a dominant-negative EGFR mutant indicates that activation of EGFR was ligand independent.; Our data implicate cell-cell adhesion-induced activation of EGFR as a cooperative mechanism that generates compensatory survival signaling, protecting cells from detachment-induced death. |
