MEK1 signaling mediates transformation and metastasis of EpH4 mammary epithelial cells independent of an epithelial to mesenchymal transition.

Activation of the mitogen-activated protein/extracellular signal-regulated kinase kinase (MEK)-mitogen-activated protein kinase (MAPK) pathway is a frequent event in tumorigenesis, and analysis of human breast carcinomas demonstrates that 25-50% of these tumors express elevated levels of activated MAPK1/2. However, a direct role for MEK1 in regulating the invasive and metastatic potential of mammary epithelial cells remains to be established. To directly address the role of constitutive MEK1 signaling in transformation, we have selected the murine mammary epithelial cell line, EpH4, as a model system. EpH4 cells expressing constitutively activated MEK1 display invasive growth in 3-dimensional collagen gels and enhanced motility, and metastatic potential in modified Boyden chamber assays. Furthermore, analysis of markers of normal epithelial morphology by immunofluorescence revealed reorganization of the actin cytoskeleton, and mislocalization of beta-catenin and ZO-1 away from sites of cell-cell contact. However, in contrast to expectations, these changes occurred independently of an epithelial to mesenchymal transition, a change seen frequently in transformed epithelial cells. Moreover, transplantation of EpH4 cells expressing constitutively activated MEK1 into the cleared mammary fat pads of immune-competent hosts rapidly produced tumors that were highly invasive, well vascularized, and readily metastasized to distant organs. Gene expression profiling was performed to identify the downstream targets of MEK1 signaling. Constitutive MEK1 induced the expression of genes involved in proliferation and of matrix metalloproteinases, which regulate invasion and metastasis. These results demonstrate that constitutively activated MEK1 brings about robust tumorigenic changes in murine mammary epithelial cells, and mediates their invasiveness and metastasis in vivo without a requirement for epithelial to mesenchymal transition.