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Epithelial-to-mesenchymal transition (EMT) is a dynamic process controlling the transition of cells between epithelial and mesenchymal states in various physiological or pathological conditions. In cancer, EMT promotes cell dissemination and metastatic colonization, enriches tumors with stem cell populations and confers resistance to anticancer therapy. Instigators of EMT activate a cohort of transcription factors (EMT-TF) which regulate the expression of each other and confer dynamic chromatin modifications to transcriptionally repress epithelial and induce mesenchymal genes. In this respect, transforming growth factor-β (TGF-β) is a potent inducer of EMT in different types of cancer.

In this study we first identified a link between the EMT-TF SNAI1 that is highly expressed in aggressive triple-negative breast cancers (TNBC), with the establishment of an intermediate epithelial-mesenchymal phenotype and the dual transcriptional induction of FOXA1 and androgen receptor which define mammary epithelial cell differentiation towards the luminal subtype. Studying additional phenotypes of SNAI1 mutant TNBCs, we showed that SNAI1 through TGF-β/SMAD signaling and repression of FOXA1, induces the guanine exchange factor PSD4/EFA6B, driving a vesicular trafficking program that promotes cell-matrix interactions and invasiveness.

Tumor-derived extracellular vesicles (EV) are important mediators of intercellular communication and of microenvironment formation where tumors develop. In this study we showed that MEK/ERK signaling, drives TGF-β promoting EV secretion by regulating cholesterol homeostasis in TNBC cells. Additionally, TGF-β ligands and matrix metalloproteases identified as EV protein contents, conferred pro-invasive attributes and resistance to chemotherapeutic drugs in recipient cells.

Metabolism has a well-documented role in tumor progression and EMT maintenance and here we propose that a hybrid epithelial-mesenchymal state upon knockout of the EMT-TF SNAI2 in TNBC cells, associated with altered expression of genes involved in metabolic pathways. This perturbed cell cycle progression in the mutant cells presumably via the transcription and stem cell factor SOX4.

In conclusion, this study provides insights into the contribution of the SNAIL family EMT-TFs, in the dynamic EMT process and the mechanisms by which this manifests the development of aggressive breast carcinomas. Furthermore, it provides means on the way TGF-β impacts on the biogenesis, secretion, and functional transfer of EV cargo molecules in the context of cancer.