TGF-β-induced c-Myb affects the expression of EMT-associated genes and promotes invasion of ER+ breast cancer cells.

Advanced breast cancer cells acquire metastatic properties in response to TGF-β. We show here that the expression of c-Myb increases in TGF-β-treated ER+ breast cancer cells by protein stabilization, transcription activation and release from miR200-dependent down-regulation. In particular, we mapped 2 sites for miR200b, miR200c and miR429 binding in the 3' UTR of the human c-myb gene. These microRNAs decreased the expression of c-Myb when transfected in MCF-7 cells. In addition, luciferase activity from a vector containing the 3' UTR of the c-myb gene was inhibited by miR200s through a binding-dependent mechanism. siRNA- and shRNA-mediated down-regulation was used to investigate the role of c-Myb for the effects induced by TGF- β in ER+ breast cancer MCF-7 and ZR-75.1 cells. Transfection with c-Myb siRNAs blocked the increase of Slug (SNAI2) and Bcl-2 expression and reversed the decrease in E-cadherin expression induced by TGF-β treatment. Conversely, c-Myb down-regulation decreased invasion and anchorage-independent growth of breast cancer cells expressing a constitutively active TGF-β receptor I. Finally, apoptosis induced by etoposide increased in c-Myb-silenced TGF-β-treated ER+ cell lines. In summary, exposure of ER+ breast cancer cells to TGF-β induces an increase of c-Myb expression which is required for expression of EMT-associated markers, in vitro invasion and anchorage-independent growth. Furthermore, our findings suggest a potentially detrimental effect of TGF-β and c-Myb co-expression in breast cancer.