TGFβ-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 beta. We show here that the expression of c-Myb increases in TGF beta-treated ER+ breast cancer cells by protein stabilization, transcription activation and release from miR200-dependent downregulation. In particular, we mapped two 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 downregulation was used to investigate the role of c-Myb for the effects induced by TGF beta 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 beta treatment. Conversely, c-Myb downregulation decreased invasion and anchorage-independent growth of breast cancer cells expressing a constitutively active TGF beta receptor I. Finally, apoptosis induced by etoposide increased in c-Myb-silenced TGF beta-treated ER+ cell lines. In summary, exposure of ER+ breast cancer cells to TGF beta 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 beta and c-Myb co-expression in breast cancer.