Hypoxia-Driven TGFβ Modulation of Side Population Cells in Breast Cancer: The Potential Role of ERα

SIMPLE SUMMARY: A process termed epithelial-to-mesenchymal transition can allow cancer cells to acquire or increase stem cell-like characteristics, which include increased potentials for migration and tissue invasion. This study focused on the effects of transforming growth factor-beta or hypoxia, which are both drivers of epithelial-to-mesenchymal transition, on rare, stem cell-like “side population” cells within the well-characterised human breast cancer cell lines MDA-MB-231 (a model for hormone non-responsive breast cancer) and MCF7 (a model for hormone responsive breast cancer). Both cell lines responded similarly to transforming growth factor-β, whereas hypoxia decreased the side-population cells in MDA-MB-231 but increased the cells in MCF7. Further work performed at the single-cell level, showed that hypoxia specifically increased multi-drug resistance of MCF7 side population cells. Taken together, these data suggest that a better understanding of the regulation of epithelial-to-mesenchymal transition is needed and might allow the identification of new therapeutic targets for the treatment of breast cancer. ABSTRACT: Epithelial-to-mesenchymal transition (EMT) is known to be important in regulating the behaviour of cancer cells enabling them to acquire stem cell characteristics or by enhancing the stem cell characteristics of cancer stem cells, resulting in these cells becoming more migratory and invasive. EMT can be driven by a number of mechanisms, including the TGF-β1 signalling pathway and/or by hypoxia. However, these drivers of EMT differ in their actions in regulating side population (SP) cell behaviour, even within SPs isolated from the same tissue. In this study we examined CoCl(2) exposure and TGF-β driven EMT on SP cells of the MDA-MB-231 and MCF7 breast cancer cell lines. Both TGF-β1 and CoCl(2) treatment led to the depletion of MDA-MB-231 SP. Whilst TGF-β1 treatment significantly reduced the MCF7 SP cells, CoCl(2) exposure led to a significant increase. Single cell analysis revealed that CoCl(2) exposure of MCF7 SP leads to increased expression of ABCG2 and HES1, both associated with multi-drug resistance. We also examined the mammosphere forming efficiency in response to CoCl(2) exposure in these cell lines, and saw the same effect as seen with the SP cells. We suggest that these contrasting effects are due to ERα expression and the inversely correlated expression of TGFB-RII, which is almost absent in the MCF7 cells. Understanding the EMT-mediated mechanisms of the regulation of SP cells could enable the identification of new therapeutic targets in breast cancer.