Partial Mesenchymal to Epithelial Reverting Transition in Breast and Prostate Cancer Metastases

Epithelial to mesenchymal transition (EMT) is an oft-studied mechanism for the initiation of metastasis. We have recently shown that once cancer cells disseminate to a secondary organ, a mesenchymal to epithelial reverting transition (MErT) may occur, which we postulate is to enable metastatic colonization. Despite a wealth of in vitro and in vivo studies, evidence supportive of MErT in human specimens is rare and difficult to document because clinically detectable metastases are typically past the micrometastatic stage at which this transition is most likely evident. We obtained paired primary and metastatic tumors from breast and prostate cancer patients and evaluated expression of various epithelial and mesenchymal markers by immunohistochemistry. The metastases exhibited increased expression of membranous E-cadherin compared to primary tumors, consistent with EMT at the primary site and MErT at the metastatic site. However, the re-emergence of the epithelial phenotype was only partial or incomplete. Expression of epithelial markers connexins 26 and/or 43 was also increased on the majority of metastases, particularly those to the brain. Despite the upregulation of epithelial markers in metastases, expression of mesenchymal markers vimentin and FSP1 was mostly unchanged. We also examined prostate carcinoma metastases of varied sizes and found that while E-cadherin expression was increased compared to the primary lesion, the expression inversely correlated with size of the metastasis. This not only suggests that a second EMT may occur in the ectopic site for tumor growth or to seed further metastases, but also provides a basis for the failure to discern epithelial phenotypes in clinically examined macrometastases. In summary, we report increased expression of epithelial markers and persistence of mesenchymal markers consistent with a partial MErT that readily allows for a second EMT at the metastatic site. Our results suggest that cancer cells continue to display phenotypic plasticity beyond the EMT that initiates metastasis.