MicroRNA-145 Impairs Classical Non-Homologous End-Joining in Response to Ionizing Radiation-Induced DNA Double-Strand Breaks via Targeting DNA-PKcs.

DNA double-strand breaks (DSBs) are one of the most lethal types of DNA damage due to the fact that unrepaired or mis-repaired DSBs lead to genomic instability or chromosomal aberrations, thereby causing cell death or tumorigenesis. The classical non-homologous end-joining pathway (c-NHEJ) is the major repair mechanism for rejoining DSBs, and the catalytic subunit of DNA-dependent protein kinase (DNA-PK) is a critical factor in this pathway; however, regulation of DNA-PK expression remains unknown. In this study, we demonstrate that miR-145 directly suppresses DNA-PK by binding to the 3'-UTR and inhibiting translation, thereby causing an accumulation of DNA damage, impairing c-NHEJ, and rendering cells hypersensitive to ionizing radiation (IR). Of note, miR-145-mediated suppression of DNA damage repair and enhanced IR sensitivity were both reversed by either inhibiting miR-145 or overexpressing DNA-PK. In addition, we show that the levels of Akt1 phosphorylation in cancer cells are correlated with miR-145 suppression and DNA-PK upregulation. Furthermore, the overexpression of miR-145 in Akt1-suppressed cells inhibited c-NHEJ by downregulating DNA-PK. These results reveal a novel miRNA-mediated regulation of DNA repair and identify miR-145 as an important regulator of c-NHEJ.