MiR-495 functions as an adjuvant to radiation therapy by reducing the radiation-induced bystander effect

The radiation-induced bystander effect (RIBE) is an important factor in tumor radiation therapy because it may increase the probability of normal cellular injury and the likelihood of secondary cancers after radiotherapy. Here, we identified the role of miR-495 in alleviating RIBEs during radiotherapy. Luciferase reporter assay results confirmed that miR-495 regulated endothelial nitric oxide synthase (eNOS) by targeting the Sp1 3'-untranslated region. Consequently, after radiation, tumor cells expressed less eNOS and Sp1 than controls. In vitro cell irradiation data based on flow-cytometric analysis and enzymed linked immunosorbent assay confirmed that nitric oxide (NO) and its downstream product transforming growth factor β1 (TGF-β1) were critical signaling factors contributing to RIBEs. Fewer normal LO₂ liver cells were injured and fewer micronuclei were observed when treated with the medium of the miR-495 overexpressing HepG2 and ZR75-1 tumor cells. Accordingly, treatment with the miR-495 antagomir led to higher NO and TGF-β1 levels and more injured LO₂ cells. In vivo experiments indicated that local irradiation of tumors overexpressing miR-495 produced fewer necrotic foci in non-irradiated liver tissue compared with controls. miR-495 was upregulated in clinical cancer tissues compared with adjacent non-cancerous tissues, and radiation significantly reduced the expression level of miR-495 in carcinoma cell lines. In summary, miR-495 may have promise as an adjuvant for tumor radiation therapy to decrease RIBEs involving the Sp1/eNOS pathway.