miR-1226-5p is involved in radioresistance of colorectal cancer by activating M2 macrophages through suppressing IRF1

Although the representative treatment for colorectal cancer (CRC) is radiotherapy, cancer cells survive due to inherent radioresistance or resistance acquired after radiation treatment, accelerating tumor malignancy and causing local recurrence and metastasis. However, the detailed mechanisms of malignancy induced after radiotherapy are not well understood. To develop more effective and improved radiotherapy and diagnostic methods, it is necessary to clearly identify the mechanisms of radioresistance and discover related biomarkers. To analyze the expression pattern of miRNAs in radioresistant CRC, sequence analysis was performed in radioresistant HCT116 cells using Gene Expression Omnibus, and then miR-1226-5p, which had the highest expression in resistant cells compared to parental cells, was selected. To confirm the effect of miR-1226-5 on tumorigenicity, Western blot, qRT-PCR, transwell migration, and invasion assays were performed to confirm the expression of EMT factors, cell mobility and invasiveness. Additionally, the tumorigenic ability of miR-1226-5p was confirmed in organoids derived from colorectal cancer patients. In CRC cells, IRF1, a target gene of miR-1226-5p, and circSLC43A1, which acts as a sponge for miR-1226-5p, were discovered and the mechanism was analyzed by confirming the tumorigenic phenotype. To analyze the effect of tumor-derived miR-1226-5p on macrophages, the expression of M2 marker in co-cultured cells and CRC patient tissues were confirmed by qRT-PCR and immunohistochemical (IHC) staining analyses. This study found that overexpressed miR-1226-5p in radioresistant CRC dramatically promoted epithelial-mesenchymal transition (EMT), migration, invasion, and tumor growth by suppressing the expression of its target gene, IRF1. Additionally, we discovered circSLC43A1, a factor that acts as a sponge for miR-1226-5p and suppresses its expression, and verified that EMT, migration, invasion, and tumor growth are suppressed by circSLC43A1 in radioresistant CRC cells. Resistant CRC cells-derived miR-1226-5p was transferred to macrophages and contributed to tumorigenicity by inducing M2 polarization and secretion of TGF-β. This study showed that the circSLC43A1/miR-1226-5p/IRF1 axis is involved in radioresistance and cancer aggressiveness in CRC. It was suggested that the discovered signaling factors could be used as potential biomarkers for diagnosis and treatment of radioresistant CRC.