The intracellular signaling events causing tumor cells to become metastatic are not well understood. N-cadherin and FGF-2 synergistically increase migration, invasion, and secretion of extracellular proteases in breast tumor cells. Here, we define a metastatic signaling cascade activated by N-cadherin and FGF-2. In the presence of N-cadherin, FGF-2 caused sustained activation of the MAPK-ERK pathway, leading to MMP-9 gene transcription and cellular invasion. N-cadherin prevented the FGF receptor (FGFR) from undergoing ligand-induced internalization, resulting in increased FGFR-1 stability. Association of FGFR-1 with N-cadherin was mediated by the first two Ig-like domains of FGFR-1. These results suggest that protection of the FGFR-1 from ligand-induced downregulation by N-cadherin enhances receptor signaling and provides a mechanism by which tumor cells can acquire metastatic properties.

The loss of E-cadherin expression or function in epithelial carcinomas has long been thought as a primary reason for disruption of tight epithelial cell-cell contacts and release of invasive tumor cells from the primary tumor. Indeed, E-cadherin serves as a widely acting suppressor of invasion and growth of epithelial cancers, and its functional elimination represents a key step in the acquisition of the invasive phenotype for many tumors. Recent evidence indicates, however, that in addition to the loss of the "invasion-suppressor" E-cadherin, another adhesion molecule, N-cadherin, becomes upregulated in invasive tumor cell lines. N-cadherin was shown to be present in the most invasive and dedifferentiated breast cancer cell lines, and its exogenous expression in tumor cells induces a scattered morphology and heightened motility, invasion, and metastasis. N-cadherin cooperates with the FGF receptor, resulting in signals that lead to the up-modulation of MMP-9 and, hence, cellular invasion. In addition to a signaling function in metastasis, N-cadherin probably also supports the systemic dissemination of tumor cells by enabling circulating tumor cells to associate with the stroma and the endothelium at distant sites. Here, we summarize the various aspects of the E- to N-cadherin switching in epithelial carcinomas and its potential impact on metastatic progression.

N-cadherin is up-regulated in aggressive breast carcinomas, but its mechanism of action in vivo remains unknown. Transgenic mice coexpressing N-cadherin and polyomavirus middle T antigen (PyVmT) in the mammary epithelium displayed increased pulmonary metastasis, with no differences in tumor onset or growth relative to control PyVmT mice. PyVmT-N-cadherin tumors contained higher levels of phosphorylated extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK) than PyVmT controls, and phosphorylated ERK staining was further increased in pulmonary metastases. Tumor cell isolates from PyVmT-N-cadherin mice exhibited enhanced ERK activation, motility, invasion, and matrix metalloproteinase-9 (MMP-9) expression relative to PyVmT controls. MAPK/ERK kinase 1 inhibition in PyVmT-N-cadherin cells reduced MMP-9 production and invasion but not motility. Furthermore, inactivation of fibroblast growth factor receptor in PyVmT-N-cadherin cells reduced motility, invasion, and ERK activation but had no effect on PyVmT cells. Thus, de novo expression of N-cadherin in mammary ducts enhances metastasis of breast tumors via enhanced ERK signaling. [Cancer Res 2007;67(7):3106-16]

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