Epithelial to mesenchymal transition in breast cancer : a novel murine model system and the regulatory role of tead transcription factors

Cancer is a leading cause of death worldwide, accounting for 7.6 million, or ~13% of all deaths in 2008. The majority of cancers arise from epithelia. Breast cancer, originating from the mammary epithelium, is the most frequent cancer in women worldwide. Breast cancer detection and treatment at early stages is an effective measure of counteracting the number of deaths, however at later stages, cancer cells may spread from the primary tumor to secondary organs, a multistage process called metastasis. This process involves the dissemination of cancer cells from the primary tumor, entrance into the vascular system, extravasation and re-growth (colonization) in the target organ. Metastasis is the actual cause of death in 90% of cancer patients.
Breast cancer treatment is complicated by the existence of substantial biological heterogeneity between and within tumors: At least five different subtypes of breast cancer with variable response to treatment and outcome have been proposed. The biological differences between these tumor subtypes are mainly determined by the nature of the oncogenic hit(s) and the cell type in which transformation originally occurred. In addition to different tumor types, progressing tumors (and also their metastatic outgrowth) consist of individual tumor cells with varying features, which can be evoked by acquisition of cumulative genetic/epigenetic alterations and/or by differential stimulation by components of the nearby tumor microenvironment.
These circumstances call for a better understanding of the underlying mechanisms that provide cancer cells with malignant features, such as the acquisition of a metastatic behavior and treatment resistance.
One mechanism that endows cancer cells with several pro-metastatic features and treatment resistance is the epithelial-mesenchymal transition (EMT). EMT is a latent embryonic program that can be aberrantly reactivated in epithelial tumor cells during tumor progression. Activation of EMT-like programs in tumor cells leads to dissolution of cell-cell adhesions, a loss of polarity and an acquisition of migratory, invasive and stem-cell-like traits.
Studies investigating the role of EMT in cancer have mainly employed a combination of different model systems for in vitro and in vivo experiments. Due to the lack of model systems that allow the study of breast cancer associated EMT in vitro and in vivo using the same cell line, I have established a stable cell line (Py2T) from a breast tumor of an MMTV-PyMT transgenic mouse. I show here that this epithelial cell line undergoes bona fide EMT under cell culture conditions when stimulated with the well-known EMT-inducer transforming growth factor β (TGFβ). TGFβ treatment of Py2T cells leads to downregulated expression of the epithelial marker E-cadherin and an upregulation of mesenchymal markers, concomitant with the induction of migratory and invasive properties. When orthotopically injected into mice, Py2T cells generate tumors that are highly invasive and display a mesenchymal phenotype characterized by the absence of E-cadherin expression, suggesting that these cells undergo spontaneous EMT-like changes in vivo. Interestingly, Py2T cells overexpressing a dominant-negative TGFβ-receptor, leading to a block of TGFβ responsiveness, also form tumors upon fat-pad transplantation, yet in these tumors a partial re-expression of E-cadherin can be observed, suggesting that TGFβ signaling contributes to the EMT phenotype in vivo. Together, my results show that the Py2T model system is a versatile tool to study EMT both in vitro and in vivo.
The second project presented in this thesis aimed at the identification of critical transcription factors (TFs) that mediate the widespread changes in gene expression observed during EMT. A genome-wide bioinformatics analysis has uncovered that the DNA-binding motif of Tead transcription factors (MCAT motif) is present in a large number of promoters of EMT-regulated genes. Here I show that Tead transcriptional activity is increased during EMT. Moreover, the expression levels of several Tead family members are also upregulated during EMT, and my results demonstrate that elevated transcriptional activity of Tead2 is sufficient to induce EMT. Furthermore, inhibition or depletion of Teads attenuates the EMT process. Moreover, Tead2 levels also can control the subcellular localization of the Tead co-activators Yap and Taz, a mechanism that possibly contributes to the increased Tead activity observed during EMT. I further demonstrate that Zyxin, a focal adhesion component and regulator of actin remodeling, which has previously been shown to be required for EMT-induced migration, is a direct target gene of Tead2. Collectively, these results demonstrate an important regulatory role of Tead transcription factors in the EMT process.