ShRNA targeting Notch1 sensitizes breast cancer stem cell to paclitaxel

Breast cancer is currently the most lethal gynecologic malignancy in many countries, and paclitaxel is a cornerstone in the treatment of this malignancy. Unfortunately, the efficacy of paclitaxel is limited due to the development of drug resistance. Evidence has suggested that cancer stem cells (CSCs) are involved in resistance to various forms of therapies, including chemotherapy. However, the interaction between paclitaxel resistance and CSCs and its underlying mechanisms have not been previously explored. In this study, we confirmed that paclitaxel enriched breast CSCs (CD44+/CD24−) in a dose-dependent manner in MCF-7 human breast cancer cell line. We then demonstrated that Notch1 was overexpressed in breast CSCs isolated from paclitaxel-treated MCF-7 cells compared to non-CSCs. The short hairpin RNA (shRNA) mediated knock-down of Notch1 inhibited MCF-7 cell proliferation and induced cell apoptosis. The anti-apoptosis protein NF-κB was decreased significantly when treated with shRNA-Notch1, and this effect was sharply improved by combination with paclitaxel. Paclitaxel decreased CD44+/CD24− cell population in MCF-7 cells and reduced the size and number of primary mammospheres after down-regulating the Notch1. Furthermore, shRNA-Notch1 inhibited the growth of tumor xenografts in nude mice noticeably. RT-PCR and Western blotting analysis showed that the expressions of ALDH1, NICD, Hes-1 and the drug transporter ABCG2 were decreased both in vitro and in vivo. These results suggest that Notch1 might play a critical role in the resistance to paclitaxel, and targeting Notch1 may have important clinical applications in cancer therapy.