MiR-99a Enhances the Radiation Sensitivity of Non-Small Cell Lung Cancer by Targeting mTOR

Background/Aims: Radiation therapy is an important and effective modality for the treatment of non-small cell lung cancer (NSCLC). MicroRNAs (miRNAs) are crucial post-transcriptional regulators that are involved in numerous important biologic processes. However, their potential involvement in radiation sensitivity remains unknown. Materials: We performed integrated analysis of miRNA expression in NSCLC using The Cancer Genome Atlas datasets. miR-99a was found to be significantly upregulated in cancer tissue and regulated cell survival. Cell culture was used to assess the role of miR-99a in radiation sensitivity. We then used flow cytometry to examine the effects of miR-99a on the cell cycle and apoptosis in cells exposed to radiation. To identify gene targets of miR-99a, a bioinformatics approach was adopted, and the findings of this analysis were verified using luciferase reporter assays. Finally, an in vivo study was conducted to examine the effect of miR-99a on tumor volume in an NSCLC mouse model undergoing radiation therapy. Results: miR-99a was significantly upregulated in radiation-sensitive A549 cells compared with radiation-resistant A549 cells. miR-99a overexpression was shown to enhance radiosensitivity, while inhibition of miR-99a resulted in radioresistance of NSCLC cell lines in vitro and in vivo. In addition, by bioinformatics software analysis and luciferase assays, mammalian target of rapamycin (mTOR) was identified as a direct target of miR-99a. Furthermore, AZD2014, an inhibitor of mTOR, enhanced radiosensitivity and apoptosis in NSCLC cell lines, while mTOR overexpression resulted in radioresistance and cell survival from miR-99a-induced cell apoptosis. Moreover, miR-99a overexpression further increased the efficacy of radiation therapy in an NSCLC xenograft mouse model, and miR-99a and mTOR expression was significantly inversely correlated. Conclusions: Altogether, these data suggested miR-99a functions as a tumor suppressor that has a critical role in regulating radiosensitivity of NSCLC by targeting the mTOR signaling pathway.