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 and upregulation of mesenchymal markers such as N-cadherin and vimentin. 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-E/M phenotype with co-expression of epithelial and mesenchymal markers, while 4T07, 168FARN, and 67NR 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-E/M phenotype that promotes invasion and metastasis.