MiR-122-5p increases radiosensitivity and aggravates radiation-induced rectal injury through CCAR1

Radiation-induced rectal injury is a major side-effect observed in patients with pelvic malignancies who receive radiotherapy. MicroRNA (miRNA), involved in many cellular biological processes, can be disturbed by ionizing radiation (IR). In this study, we have investigated the function of microRNA-122-5p (miR-122-5p) in radiation-induced rectal injury. MiR-122-5p levels in the serum of rectal cancer patients or in the rectal tissues of C57BL/6 mice before and after IR were detected by quantitative real-time PCR (qRT-PCR). We found that the miR-122-5p levels were significantly up-regulated in patients' serum or in mice rectal tissues after IR. Elevation of miR-122-5p levels sensitized human intestinal epithelial crypt (HIEC) cells to IR both in vitro and in vivo. MiR-122-5p mimic was transfected to HIEC cells and the downstream targets were predicted by bioinformatic analysis. Two putative target sites of miR-122-5p in the 3'UTR of the cell cycle and apoptosis regulator 1 (CCAR1) mRNA were found and verified by luciferase reporter assay. Overexpression of miR-122-5p or silencing CCAR1 combined with IR significantly inhibited cell survival, enhanced radiosensitivity, and increased cell apoptosis compared to that in the negative control group in vitro. In vivo injection of miR-122-5p antagomir after IR significantly alleviated radiation-induced rectal injury in mice. These results suggest that miR-122-5p aggravates radiation-induced rectal injury through targeting CCAR1.