Reduced miR-128 in Breast Tumor-Initiating Cells Induces Chemotherapeutic Resistance via Bmi-1 and ABCC5

Purpose: Tumor-initiating cells are resistant to chemotherapy, but how microRNAs play a role in regulating drug resistance of breast tumor-initiating cells (BT-IC) needs to be clarified. Experimental Design: Lentivirus-mediated miR-128 transduction was done in BT-ICs, enriched by mammosphere cultures or CD44(+)CD24(-) fluorescence-activated cell sorting. Apoptosis and DNA damage were determined upon treatment with doxorubicin. Expression of miR-128 in breast cancer tissues was examined by in situ hybridization and correlated with breast tumor response to neoadjuvant chemotherapy and patient survival. Results: MiR-128 was significantly reduced in chemoresistant BT-ICs enriched from breast cancer cell lines and primary breast tumors (P < 0.01), accompanied by an overexpression of Bmi-1 and ABCC5, which were identified as targets of miR-128. Ectopic expression of miR-128 reduced the protein levels of Bmi-1 and ABCC5 in BT-ICs, along with decreased cell viability (P < 0.001) and increased apoptosis (P < 0.001) and DNA damage (P < 0.001) in the presence of doxorubicin. Reduced miR-128 expression in breast tumor tissues was associated with chemotherapeutic resistance (P < 0.001) and poor survival of breast cancer patients (P < 0.05; n = 57). Conclusions: Reduction in miR-128 leading to Bmi-1 and ABCC5 overexpression is a stem cell-like feature of BT-ICs, which contributes to chemotherapeutic resistance in breast cancers. Ectopic expression of miR-128 sensitizes BT-ICs to the proapoptotic and DNA-damaging effects of doxorubicin, indicating therapeutic potential. Clin Cancer Res; 17(22); 7105-15. (C) 2011 AACR.