MicroRNA-21 inhibitor potentiates anti-tumor effect of radiation therapy in vitro and in vivo

MicroRNA-21 (miR-21) plays important roles in carcinogenesis and is highly expressed in diverse human cancers. We evaluated the potential of targeting miRNA-21 to overcome the radioresistance of human cancer cells having an activated EGFR2-associated signaling and also aimed to elucidate the mechanisms of radiosensitization, and the effect on epithelial- mesenchymal transition (EMT). Ectopic overexpression of miR-21 up-regulated EGFR/HER2-associated signaling and increased radioresistance of a panel of human cancer cells (U251, U87, and A549 cells). In contrast, a specific inhibitor of miR-21 attenuated this signaling and radiosensitized a panel of human cancer cells. Inhibition of miR-21 was associated with persistent γH2AX foci formation. Inhibition of miR-21 decreased the typical features of EMT, such as invasion and migration and vascular tube formation. Treatment with anti-miR-21 decreased tumor burden in nude mice bearing intracranial U251 xenografts compared to controls. Combined treatment of anti-miR-21 and radiation further decreased tumor burden compared to each treatment alone. In summary, miR-21 is an important onco-miR, which confers radioresistance and diverse features of EMT. Inhibition of miR-21 could be a potential strategy for improving the efficacy of radiation therapy via unique modulation of pro-survival signaling implicated in radiation response and EMT.