Constitutive overexpression of c-erbB2 in the mammary epithelial cell line MTSV1-7 has been shown to result in epithelial-mesenchymal conversion, anchorage-independent growth and loss of organized morphogenesis in collagen. To elucidate the events leading to this drastic change, MTSV1-7 cells and its subclone HB2 (which shows a more strictly epithelial phenotype) were transfected with the hybrid trk-neu receptor consisting of the extracellular domain of the trkA nerve growth factor (NGF) receptor and the transmembrane and cytoplasmic domains of c-erbB2 (neu). In cells expressing this construct, c-erbB2 homodimerization can be mimicked by addition of NGF. In trk-neu transfectants of HB2 cells, modest expression led to increased cell proliferation upon NGF treatment. When clones with higher expression levels were grown in collagen, NGF instead induced cell scattering, diminished viability and dramatically increased apoptosis. Interestingly, both the dissociation of colonies and loss of cell viability could be completely reversed by treatment of the cells with antibodies that activate the adhesive capacity of the α 2 β 1 integrin. Long-term NGF treatment of high-expressing transfectants generated fibroblastic clones displaying a reduced expression of integrin α 2 and E-cadherin, and extensive apoptosis in collagen. These results, which indicate that strong c-erbB2 signalling may lead to downregulation and/or inactivation of the α 2 β 1 integrin, promoting apoptosis in collagen, provide one possible explanation to the increased apoptosis frequently seen in early tumour development.