Separation and Characterization of Epithelial and Mesenchymal-like Murine Mammary Tumor Cells Reveals Epithelial Cell Differentiation Plasticity and Enhanced Tumorigenicity of Epithelial-enriched Tumor Cells

Tumors are composed of heterogeneous populations of cells including tumor-initiating cells (TICs) and metastatic precursors. While the origin of these cells is unknown, there is evidence that tumor cells can transdifferentiate from an epithelial to a mesenchymal phenotype, a property referred to as epithelial-to-mesenchymal transition (EMT). This cellular plasticity may explain the heterogeneous nature of tumors and differences in the tumorigenic and invasive properties of cells. Understanding the origin of these cells and the contribution of external factors that influence the acquisition of cellular properties is critical for the development of therapeutics to eradicate cancer. In this study, we show that primary murine tumor cells harvested from FVB/N Tg (MMTV/Neu) spontaneous mammary tumors possess differentiation plasticity and can be enriched to be epithelial or mesenchymal-like using selected culture media conditions, and we show evidence of EMT in a clonal population of primary epithelial tumor cells when cultured in fibroblast growth factor-1 (FGF-1) or transforming growth factor-β (TGF-β). We also determined that in contrast to the identification of mesenchymal-like tumor cells as TICs in orthotopic xenograph models of tumorigenicity, epithelial-enriched murine mammary tumor cells were more tumorigenic as compared to mesenchymal-enriched cells when transplanted back subcutaneously into syngeneic immune competent mice. Together, these data suggest that EMT plasticity can be induced in primary murine mammary tumor cells, and that tumorigenicity of epithelial or mesenchymal-like cells may be influenced by factors such as the site of tumor inoculation or the immune state of the host (xenogenic immune compromised versus syngeneic immune competent).