The role of epithelial-mesenchymal transition (EMT) in metastasis remains controversial. EMT has been postulated as an absolute requirement for tumor invasion and metastasis. Three different models including incomplete EMT, mesenchymal-epithelial transition (MET), and collective migration have been proposed for the role of EMT in cancer invasion and metastasis. However, skepticism remains about whether EMT truly occurs during cancer progression, and if it does, whether it plays an indispensible role in metastasis. Our recent findings suggest that EMT cells are responsible for degrading the surrounding matrix to enable invasion and intravasation of both EMT and non-EMT cells. Only non-EMT cells that have entered the blood stream are able to reestablish colonies in the secondary sites. Here, we discuss an alternative model for the role of EMT in cancer metastasis in which EMT and non-EMT cells cooperate to complete the entire process of spontaneous metastasis. [Cancer Res 2009;69(18):7135-9]
Epithelial-Mesenchymal Transition and Cancer MetastasisEpithelial-mesenchymal transition (EMT) was first recognized as a central differentiation process in early embryogenic morphogenesis (1). It is a coordinated molecular and cellular change defined as a reduction in cell-cell adhesion, apical-basolateral polarity, and epithelial markers, as well as an acquisition of motility, spindle-cell shape, and mesenchymal markers. The definition of EMT in embryo development, which includes an ordered series of transcriptional events and a switch in cell fate, has not been strictly followed in cancer research, in which this term has been more liberally referred to as a recognizable change in cellular phenotype characterized as loss of cell junctions and gain of migratory behaviors (2).This more inclusive EMT process has been proposed and supported by numerous publications to be a potent mechanism that enhances the detachment of cancer cells from primary tumors. However, it is still controversial whether transformation of a noninvasive tumor into a metastatic tumor truly represents an EMT, and if it is, how important it is in the process of cancer metastasis (3, 4). The main argument for the lack of a role of EMT in cancer is that metastases seem histopathologically similar to the primary tumors from which they are derived. To resolve this apparent contradiction, a mesenchymal-epithelial transition (MET) process in the metastatic sites has been postulated as part of the process of metastatic tumor formation (5). MET is an attractive hypothesis that can explain the histopathological similarity between primary and metastatic tumors. In support of the MET hypothesis, dynamic expression of E-cadherin (CDH1) in cancer progression has been documented. However, direct experimental data supporting MET in cancer metastasis are still lacking. For example, Graff and colleagues showed that the DNA methylation status of the CDH1 promoter varies at different stages of the metastatic process (6). In primary breast cancers, the tumor cells ...