A biological and pharmacological investigation of the unfolded protein response in models of breast cancer
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Breast cancer is the most common malignancy in women worldwide. At the molecular level, breast cancer is a heterogeneous disease. For example, the claudin low subtype of breast cancer is characterised by epithelial-mesenchymal transition (EMT). EMT is a process by which epithelial cells lose cell-cell adhesion and their cell polarity, and gain migratory and invasive properties to become mesenchymal cells. The claudin-low subtype is also driven by specific oncogenes, such as MYC or KRAS. Both EMT and oncogene-driven tumorigenesis have been associated with the unfolded protein response (UPR). The UPR is a cellular stress response pathway which activates an adaptive mechanisms to overcome stress and restore endoplasmic reticulum (ER) homeostasis. The UPR comprises three signalling pathways which are activated by three transmembrane sensors of the ER: inositol requiring enzyme 1 (IRE1), protein kinase RNA‐activated (PKR)‐like ER kinase (PERK) and activating transcription factor 6 (ATF6). In breast cancer, IRE1, PERK and ATF6 contribute to cancer cell proliferation, drug resistance and EMT. Chapter III shows that the pharmacological block of IRE1 has no effect on EMT in breast cancer. The knockout of XBP1 was also not sufficient to induce an epithelial phenotype in the MDA-MB-231 claudin low breast cancer cell line. However, Chapter IV shows that plakoglobin, a component of the cell cell contacts, was down regulated by IRE1 in MDA-MB-231 cell line thus affecting cell migration and adhesion in an EMT independent manner. In Chapter V, the relationship between UPR and oncogenes in the regulation of apoptosis was also investigated by overexpressing the oncogenes MYC and KRASG12V in the MCF10A non-tumorigenic mammary epithelial cell line. We observed that the UPR was not activated by MYC and KRASG12V although these oncoproteins decreased cell viability and induced the activation of caspase-3. Finally, in Chapter VI, a cell free drug screening for IRE1 identified compound NUIG10 as a promising inhibitor of IRE1. Altogether, this thesis has evaluated and broadened our knowledge of the role that the UPR and more specifically IRE1 plays in breast cancer.