Epithelial-to-mesenchymal transition : what is the impact on breast cancer stem cells and drug resistance

Abstract

Breast cancer is fatal mainly due to the development of metastatic disease and the resistance to current in use chemotherapeutic options. The role of cancer stem cells (CSCs) has become one of increasing interest over the last few years, as they seem to be involved in the acquisition of drug resistant and more aggressive cell phenotypes. Side population (SP) cells are a putative CSC population identified by their ability to efflux the DNA binding dye Hoechst 33342 due to the expression of members of the ABC transporter family of proteins. This efflux ability is also thought to confer on these cells the ability to efflux chemotherapeutic reagents. Epithelial-to-Mesenchymal Transition (EMT) has been shown to regulate the function of several CSC populations and it is a process that is predominantly associated with metastasis. Hypoxia has been shown to activate several EMT related signalling pathways. The aims of the present study were to investigate the effects of Transforming Growth Factor-β1 (TGF-β1) and hypoxia driven EMT on the breast cancer stem cells (BCSCs), including SP and CD44+ cells of the MDA-MB-231 (ER-/PR-/HER2-) and MCF-7 (ER+/PR+/HER2-) breast cancer cell lines. The TGF-β receptor expression was also assessed in both cells lines and I confirmed the activation of the TGF-β signalling pathway when these were induced to undergo EMT. Both MDA-MB-231 and MCF-7 cells were found to express TGFB-RI, but MCF-7 cells had low expression of TGFB-RII. Interestingly, both TGF-β1 and hypoxia-induced EMT resulted in the loss of MDA-MB-231 SP phenotype, while TGF-β1 significantly reduced and hypoxia significantly increased the MCF-7 SP population. Changes on the CD44+ cells were found to be non significant. My data suggest that autocrine TGF-β1 production might be responsible for the reduction of the SP population in both cell lines and that targeting the SP population through the TGF-β signalling pathway in hormonal responsive breast cancer patients may be promoted by the cooperative effect of hypoxia and TGF-β treatment. Most importantly, I concluded that resistance to chemotherapeutic treatment due to SP presence can be further induced by hypoxia as seen by the increased MCF-7 SP numbers compared to hypoxia alone. Understanding the regulatory mechanisms of SP cells by EMT could enable the identification of new therapeutic targets in certain groups of breast cancer patients for overcoming metastasis and drug resistance.