Epithelial to mesenchymal transition in breast cancer is associated with resistance to therapy and a cancer stem cell-like phenotype

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Epithelial to mesenchymal transition (EMT) and acquisition of cancer stemness are the two key steps for tumor progression, metastases development and relapse of the disease. While self-renewal of cancer stem cells (CSCs) is responsible for tumor maintenance, EMT confers an invasive and migratory phenotype. Both EMT and the CSCs phenotype have been linked to the development of chemoresistance (Shah et al, Anti-Cancer Drugs 2007, 18:371-375; Dean et al, Nature Reviews 2005, 5:275-284). Since relapse of breast cancer is explained by resistance to conventional therapies, the goal of this study was to determine if mesenchymal breast cancer cell lines shared a CSC phenotype and the ability to resist chemo- and radiotherapy. The established mesenchymal cell lines (M1, M2, M3 and M4) were from neu-antigen-negative variant relapse breast tumors that had developed following immune-mediated rejection of neu-positive epithelial tumors within parental non-transgenic FVB/N mice through EMT (Knutson et al, J Immunol 2006,177:1526-1533). Quantitative flow cytometric analysis with MESF (molecules of equivalent soluble fluorochrome) beads showed that mesenchymal cell lines were CD24^lowCD44^highESA+, as observed in human breast CSCs (Al-Hajj et al, 2003; PNAS, 100:3983-3988). Furthermore, all of the mesenchymal cell lines were highly tumorigenic as compared to the originating epithelial cell lines. Western blot analysis showed a higher expression of breast cancer resistance protein BCRP (ABCG2) in mesenchymal cell lines. BCRP has been associated with efflux of mitoxantrone, topoisomerase-I-inhibitors, methotrexate and some ErbB1 inhibitors. Chemoresistance assays with mitoxantrone and lapatinib (1uM) and clonogenic radiation assays after short courses of fractionated irradiation (0, 4, 8, 16 and 32 Gy) showed that mesenchymal cell lines were highly resistant to treatment compared to the parental epithelial cell line. Therefore, these results suggest that EMT may lead to breast tumors with a CSC-like phenotype and ability to resist conventional breast cancer therapies. Understanding the cellular and molecular characteristics of tumorigenicity and resistance to therapy could potentially lead to identification of new therapeutic targets.