Mechanisms of Disease: epithelial–mesenchymal transition—does cellular plasticity fuel neoplastic progression?

Abstract
Epithelial–mesenchymal transition (EMT) is a phenotypic conversion that facilitates organ morphogenesis and tissue remodeling. The authors of this Review discuss the phenomenon of EMT in relation to tumor development, and the function of EMT in promoting invasion and metastasis. The roles of ERK1, ERK2 and PI3-kinase, as microenvironmental responsive regulators of EMT are also highlighted. Epithelial–mesenchymal transition (EMT) is a phenotypic conversion that facilitates organ morphogenesis and tissue remodeling in physiological processes, such as embryonic development and wound healing. A similar phenotypic conversion is also detected in fibrotic diseases and neoplasia, and is associated with disease progression. EMT in cancer epithelial cells often seems to be an incomplete and bidirectional process. In this Review, we discuss the phenomenon of EMT as it pertains to tumor development, focusing on exceptions to the commonly held rule that EMT promotes invasion and metastasis. We also highlight the role of RAS-controlled signaling mediators, ERK1, ERK2 and phosphatidylinositol 3-kinase, as microenvironmental responsive regulators of EMT.