Abstract. The tumor-suppressive let-7 family of microRNAs (miRNAs) has been previously identified to induce cell apoptosis, proliferation-inhibition and suppression of the self-renewal capacities of cancer stem cells (CSCs). However, let-7-mediated sensitization of tumors to radiation treatment rema

The tumor-suppressive let-7 family of microRNAs (miRNAs) has been previously identified to induce cell apoptosis, proliferation-inhibition and suppression of the self-renewal capacities of cancer stem cells (CSCs). However, let-7-mediated sensitization of tumors to radiation treatment remains to be investigated fully in triple negative breast cancer (TNBC), of which the clinical treatment is challenging. The inhibitory effect of let-7 miRNAs on the self-renewal ability of CSCs from TNBC was investigated. It was identified that radiation inhibited the self-renewal ability of TNBC stem cells by inhibiting cyclin D1 and protein kinase B (Akt1) phosphorylation. Let-7d stimulates radiation-induced tumor repression, exerting synergistic effects with radiotherapy on stem cell renewal. Through western blotting, immunofluorescence and a luciferase assay, it was identified that reduced cyclin D1/Akt1/wingless type MMTV integration site family member 1 (Wnt1) signaling activity accounts for the let-7-induced radiation sensitization. Let-7 directly inhibits cyclin D1 expression, resulting in low phosphorylation of Akt1, which is critical for the let-7-induced inhibition of mammosphere numbers. The let-7d-induced Akt1 inhibition contributed to tumor repression, with similar results to those obtained with Akt inhibitors. Furthermore, it was identified that the inhibition of Wnt1 is critical for the functioning of let-7d, and that addition of recombinant Wnt1 abolished the effects of let-7d on sensitization to radiotherapy. Let-7d is suggested to be a promising therapeutic agent in the treatment of TNBC by targeting CSCs and sensitizing tumors to radiotherapy via inhibition of cyclin D1/Akt1/Wnt1 signaling. Introduction Breast cancer is the most common malignancy observed amongst women, accounting for nearly a quarter of all cases, and outcomes vary depending on cancer type, extent of disease the and patient's age (1). Four main subtypes of breast tumor have been identified based on patterns of gene expression, including luminal A/B, human epidermal growth factor receptor 2 (HER2)+ [estrogen receptor (ER)-, progesterone receptor (PR)-] and triple negative tumors (ER-, PRand HER‐), which influence the therapeutic strategies used and the clinical prognosis (2). Endocrine therapies have been identified to be effective in treating breast cancer of luminal A and luminal B subtypes, and when used together with chemotherapy, the prognosis of patients was observed to be improved (3,4). Breast cancer in young women is more likely to be an aggressive triple negative or HER2+ sub type, and is more likely to be identified at an advanced stage, due to the aggressive subtype (5). Trastuzumab has been demonstrated to improve prognosis of patients with overexpressed HER-2; however, effective treatment strategies for triple negative breast cancer (TNBC), which is resistant to common chemotherapies and does not respond well to endocrine therapies, remain to be identified. Radiotherapy is used as a remedial strategy, therefore increasing the sensitization of TNBC cells to radiation may improve the clinical prognosis and quality of life of patients (6). MicroRNAs (miRNAs) are deregulated in the majority of malignancies, serving roles in tumor development and progression, and numerous miRNAs have been reported to influence the therapeutic response of cancer to clinical treatments (7,8). In breast cancer, miR-17/20, miR-190, miR-200, miR-34, the let‐7 family and additional miRNAs have been identified to be involved in the pathogenesis of tumor biology (7-9). The let‐7 family was the earliest identified family of non‐translational RNAs, of which the functions remain to be fully elucidated. Let-7 miRNAs act as suppressive genes through directly binding to the 3'untranslated region (UTR) of Ras, interleukin-6, cyclin D1, mitogen-activated protein kinase, LIN28-A/B, c-Myc, DICER1 and numerous other oncogenes in ER-positive breast cancer (7,10). However, the role of let-7 in TNBC, and the let-7-induced sensitization of tumor repression Let‐7 miRNAs sensitize breast cancer stem cells to radiation‐induced repression through inhibition of the cyclin D1/Akt1/Wnt1 signaling pathway HUIFANG SUN, CHANGMAO DING, HUIYU ZHANG and JIANBO GAO Department of Radiology, The First Affiliated Hospital of Zhengzhou University, Zhengzhou, Henan 450052, P.R. China Received April 18, 2015; Accepted July 12, 2016 DOI: 10.3892/mmr.2016. 5656 Correspondence to: Professor Jianbo Gao, Department of Radiology, The First Affiliated Hospital of Zhengzhou University, 1 Jianshe East Road, Erqi, Zhengzhou, Henan 450052, P.R. China E-mail: cjr.gaojianbo@vip.163. com