Epithelial-mesenchymal transition (EMT) is a morphogenetic program essential for embryonic development and wound healing, but can adversely cause fibrosis and metastatic cancer progression when deregulated. Here, we established a model of efficient EMT induction in normal finite lifespan human mammary epithelial cells (HMEC) using transforming growth factor beta (TGFbeta). We demonstrate that EMT in HMEC occurs in three distinctive phases that are governed by a hierarchy of EMT-activating transcription factors (TFs). Loss of epithelial cell polarity (ZO-1), and acquisition of mesenchymal marker (Vimentin, Fibronectin) expression are immediate-early events, whereas switching from E-cadherin to N-cadherin protein expression occurs only after EMT-like morphological changes become apparent. The kinetics of TF induction suggests that ZEB1 and SNAIL mediate early EMT induction reinforced by ZEB2, while GOOSECOID and FOXC2 may play a role in EMT maintenance. TWIST and SLUG were not significantly induced in this system. Furthermore, we show for the first time that normal HMEC acquire a CD44(+)/CD24(-/low) stem cell phenotype during a third phase of EMT that is characterized by maximum TF expression levels. Our results may have important implications for understanding potential changes that might occur in normal breast epithelium under pathological conditions triggering elevated TGFbeta levels.
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