Promotion of epithelial–mesenchymal transition and tumor growth by 17β‐estradiol in an ER+/HER2+ cell line derived from human breast epithelial stem cells

A tumorigenic cell line with estrogen receptor and HER2 expression (ER/HER2+), R2N1d, was developed from a human breast epithelial cell type with stem cell characteristics in a growth factor/hormone-deprived cell culture condition. This study was undertaken to test whether tumor growth and other biological effects could be induced by estrogen in this cell line. The results clearly show that estrogen treatment greatly promoted the tumor growth of R2N1d cells in immune-deficient mice. Estrogen treatment of R2N1d cells in vitro was also found to induce other phenotypic changes related to breast carcinogenesis, that is, 1) the induction of epithelial–mesenchymal transition (EMT) shown by molecular and functional marker changes; 2) a significant increase of the CD44high/CD24−/low stem cell population; 3) the enhancement of cell growth rate and colony-forming ability; and 4) the acquisition of metastatic ability, that is, increased cell migration and invasiveness. From these results, we conclude that 1) estrogen could induce EMT and cancer stem cells and promote tumor growth in ER+/HER2+ cells known to be derived from human breast epithelial stem cells, and 2) normal stem cells could give rise to cancer stem cells.