MET after transient Twist activation results in de novo gain of malignant traits

During Epithelial-Mesenchymal Transition (EMT), apical-basal polarized epithelial cells are converted to front-to-back polarized mesenchymal cells that only form loose cell-cell adhesions. These phenotypic changes are accompanied by acquisition of increased motility and invasiveness. EMT programs are orchestrated by pleiotropic transcription factors (TFs), such as Twist1 and Snail1 and effect morphogenetic steps during embryogenesis, including mesoderm formation and neural crest migration. EMTs have also been implicated in the acquisition of aggressive traits by carcinoma cells, including the ability to complete several steps of the metastatic cascade as well as propagation of the tumor by single cells (clonogenicity), a defining trait of tumor-initiating or cancer stem cells. However, the molecular links between the expression of EMT-TFs, the process of EMT and acquisition of clonogenicity remain obscure. Using inducible Twist1 or Snail1 expressed in CD24-positive mammary epithelial cells, we show that clonal growth in anchorage independence and EMT are induced sequentially and independently: clonogenic potential is induced prior to EMT and requires transient TF-activity. By contrast, EMT depends on continuous TF-activation over a longer period. In 3D-collagen assays, continuous Twist1 activity suppresses colony formation, whereas transient activation induces highly invasive growth independently of EMT. In conclusion, our results demonstrate that transient Twist1 activation suffices to drive tumor progression of CD24-positive breast epithelial cells, assessed by invasive as well as anchorage-independent clonal growth, whereas chronic Twist1 exposure can suppress these traits of aggressive tumor cells. We performed gene expression microarray analysis on CD24high and CD24negative populations derived from HMLE-Twist1-ER or HMLE-Snail1-ER cell lines upon various culture conditions