Isobavachalcone sensitizes cells to E2-induced paclitaxel resistance by down-regulating CD44 expression in ER+ breast cancer cells.

Oestrogen receptor (ER) is expressed in approximately 60%-70% of human breast cancer. Clinical trials and retrospective analyses have shown that ER-positive (ER+) tumours are more tolerant to chemotherapeutic drug resistance than ER-negative (ER-) tumours. In addition, isobavachalcone (IBC) is known as a kind of phytoestrogen with antitumour effect. However, the underlying mechanism of IBC in ER+ breast cancer needs to be elucidated further. Our in vitro experiments showed that IBC could attenuate 17β-estradiol (E2 )-induced paclitaxel resistance and that E2 could stimulate CD44 expression in ER+ breast cancer cells but not in ER- cells. Meanwhile, E2 could promote ERα expression to render ER+ breast cancer cells resistant to paclitaxel. Furthermore, we established paclitaxel-resistant breast cancer cell lines and determined the function of ERα in the enhancement of paclitaxel resistance via the regulation of CD44 transcription. IBC down-regulated ERα and CD44 expression and thus inhibited tumour growth in paclitaxel-resistant xenograft models. Overall, our data demonstrated for the first time that IBC could decrease CD44 expression level via the ERα pathway and make ER+ breast cancer cells sensitive to paclitaxel treatment.