c-erbB2-induced epithelial-mesenchymal transition in mammary epithelial cells is suppressed by cell-cell contact and initiated prior to E-cadherin downregulation

Prolonged signalling from the growth factor receptor subunit and proto-oncogene c-erbB2 has been shown to cause epithelial-mesenchymal transition (EMT) in mammary epithelial cells. Using a system where c-erbB2 homodimer signalling can be induced in human mammary epithelial cells, we characterised the properties of c-erbB2-induced EMT. The cells resulting from this transdifferentiation showed a pronounced and stable fibroblastic phenotype with spindle-like morphology, homogeneous high expression of vimentin, N-cadherin, and integrin α5 as well as loss of E-cadherin and desmoplakin. However, the rate at which EMT occurred was very slow compared to other reported systems, as complete conversion was not seen until after 12-15 weeks of c-erbB2 signalling. This time delay was however not due to the presence of long-lived intermediate cell types as measured by expression of combinations of markers. By studying morphological time-courses of individual colonies of epithelial cells subjected to c-erbB2 signalling, we could conclude that apart from EMT, c-erbB2 could also cause transition to very large cells retaining cell-cell contact but with little or no proliferative capacity. EMT preferentially occurred in small colonies, suggesting that extensive cell-cell contact inhibits EMT. When testing this conclusion by exposing cells to c-erbB2 signalling at different cell densities, we concluded that only cells kept at low density would undergo EMT. High cell density also prevented the proliferative decrease associated with prolonged c-erbB2 signalling. Immunofluorescence microscopy revealed that disruption of cell-cell contact was preceded by partial relocalisation of β-catenin to the cytoplasm whereas downregulation of E-cadherin appeared to occur after initiation of cell scattering.