Epithelial–mesenchymal transition (EMT) and metastasis: yes, no, maybe?

An epithelial to mesenchymal transition (EMT) is a process of cell remodeling critical during embryonic development and organogenesis. During an EMT, epithelial cells lose their polarized organization and acquire migratory and invasive capabilities. While a plethora of experimental results have indicated that manipulating an EMT also affects cancer metastasis, its reverse process, a mesenchymal to epithelial transition (MET), seems to support metastatic outgrowth in distant organs. Moreover, recent reports investigating cancer cells circulating in the blood stream or employing genetic lineage-tracing have questioned a critical role of an EMT in metastasis formation. Hence, we need to better understand the molecular networks underlying the cell plasticity conferred by an EMT or a MET and its functional contribution to malignant tumor progression.