The highly metastatic 4T1 breast carcinoma model possesses features of a hybrid epithelial/mesenchymal phenotype

Epithelial-mesenchymal transitions (EMTs) are thought to promote metastasis via downregulation of E-cadherin (also known as Cdh1) and upregulation of mesenchymal markers such as N-cadherin (Cdh2) and vimentin (Vim). Contrary to this, E-cadherin is retained in many invasive carcinomas and promotes collective cell invasion. To investigate how E-cadherin regulates metastasis, we examined the highly metastatic, E-cadherin-positive murine 4T1 breast cancer model, together with the less metastatic, 4T1-related cell lines 4T07, 168FARN and 67NR. We found that 4T1 cells display a hybrid epithelial/mesenchymal phenotype with co-expression of epithelial and mesenchymal markers, whereas 4T07, 168FARN, and 67NR cells display progressively more mesenchymal phenotypes in vitro that relate inversely to their metastatic capacity in vivo. Using RNA interference and constitutive expression, we demonstrate that the expression level of E-cadherin does not determine 4T1 or 4T07 cell metastatic capacity in mice. Mechanistically, 4T1 cells possess highly dynamic, unstable cell-cell junctions and can undergo collective invasion without E-cadherin downregulation. However, 4T1 orthotopic tumors in vivo also contain subregions of EMT-like loss of E-cadherin. Thus, 4T1 cells function as a model for carcinomas with a hybrid epithelial/mesenchymal phenotype that promotes invasion and metastasis.