Activation of STAT3 and Bcl-2 and reduction of reactive oxygen species (ROS) promote radioresistance in breast cancer and overcome of radioresistance with niclosamide.

Radiotherapy significantly improves the therapeutic outcomes and survival of breast cancer patients. However, the acquired resistance to this therapeutic modality is a major clinical challenge. Here we show that ionizing irradiation (IR)-induced phosphorylation of signal transducer and activator of transcription 3 (STAT3) at the Tyr705 residue and the induction of reactive oxygen species (ROS) in wild-type and radioresistant MDA-MB-231 and MDA-MB-468 triple-negative breast cancer (TNBC) cell lines. Comparing with radiosensitive parental TNBC cells, significantly low levels of ROS and higher protein levels of phospho-STAT3 and Bcl-2 were observed in TNBC cells with acquired radioresistance. Moreover, knockdown of STAT3 by shRNA sensitized the TNBC cells to IR. Niclosamide, a potent inhibitor of STAT3, overcame the radioresistance in TNBC cells via inhibition of STAT3 and Bcl-2 and induction of ROS. In combination with radiation, niclosamide treatment resulted in significant increase of ROS generation and induction of apoptosis in parental and radioresistant TNBC cells in vitro and TNBC xenograft tumors in vivo. These findings demonstrate that activation of STAT3 and Bcl-2 and reduction of ROS contribute to the development of radioresistance in TNBC, and niclosamide acts as a potent radiosensitizer via inhibiting STAT3 and Bcl-2 and increasing ROS generation in TNBC cells and xenograft tumors. Our findings suggest that niclosamide in combination with irradiation may offer an effective alternative approach for restoring the sensitivity of radioresistant TNBC cells to IR for improved therapeutic efficacy and outcomes.