The miR-15 Family Enhances the Radiosensitivity of Breast Cancer Cells by Targeting G2 Checkpoints

Enhancing radiosensitivity is an important area of investigation for improving breast cancer therapy outcomes. The aim of this study was to assess the role of the miR-15 family in the radiosensitivity of breast cancer cells. MicroRNAs (miRNAs) encoded by the miR-15 cluster are known to induce G(1) arrest and apoptosis by targeting G(1) checkpoints and the anti-apoptotic B cell lymphoma 2 (BCL-2) gene. However, the effect of the miR-15 family on G(2)/M arrest and radiosensitivity remains poorly understood. In the current study, cells transfected with miR-15a/15b/16 mimic or inhibitor were irradiated and examined by: clonogenic assays, phosphorylated H2AX assay, flow cytometry, 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), real-time PCR and Western blot. Real-time PCR was also used to monitor time-dependent changes of miR-15a/15b/16 expression after irradiation. A putative target site for miR-15a/15b/16 within the Chk1 and Wee1 30 UTRs was confirmed using luciferase reporter assays. Additionally, siRNA was used to validate the effect of Chk1 and Wee1 on radiosensitivity in breast cancer cells. In our study, we investigated the effects of radiation on the miR-15 family and found a time-dependent change in the expression of miR-15a/15b/16 in breast cancer cells postirradiation, as well as an increase in miR-15 family-mediated sensitization of breast cancer cells to radiation. The increase in radiosensitivity induced by the miR-15 family was associated with persistent unrepaired DNA damage, abrogation of radiation-induced G(2) arrest and suppressed cell proliferation, and appear to involve both the checkpoint kinase 1 (Chk1) and Wee1. In addition, we found that inhibition of the miR-15 family could not induce cell resistance to radiation. These findings suggest that the expression of the miR-15 family contributes to increased radiosensitivity of breast cancer cells by influencing G(2)/M checkpoint proteins. (C) 2015 by Radiation Research Society