MiR-221Down-regulate RAD51and Sensitizes Tumor Cells To Chemotherapy

[Background]DNA double-strand breaks (DSBs) are one of the most lethal types of DNA damage. DSBs that cannot be repaired may lead to cell death, whereas improper repair may cause chromosomal rearrangements, such as deletion and translocation, which may drive cancer initiation and progression. DSBs are mainly repaired by homologous recombination repair (HRR) and non-homologous end joining (NHEJ). In eukaryotic cells, RAD51plays a key role in the homologous recombination repair. Recent studies have revealed that RAD51overexpression may cause tumor resistance when over-expressed in human tumor cells. Therefore, to understand how RAD51expression is regulated in cancer cells and to effectively decrease intracellular levels of RAD51are critical for cancer therapy.MicroRNAs (miRNAs) are small, single-stranded non-coding RNA found in eukaryotic cells and viruses. MicroRNAs can bind to their target mRNAs, thereby regulating the expression of genes post transcription. MicroRNAs have been found to target genes that are involved at multiple stages of DNA repair; therefore, finding new miRNAs that target DNA repair genes like RAD51is of fundamental and practical interests.We identified miR-221as a potential inhibitor of human RAD51through bioformatic analysis. We then confirmed, using inhibitors or mimic of miR-221, that miR-221can indeed downregulate RAD51and render cancer cells sensitive to ionizing radiation and chemotherapy drugs.[Objective] The purpose of our study is to test whether miR-221can downregulate RAD51in cancer cells and to determine whether it can impair DNA repair and render cells sensitive to ionizing radiation and chemotherapy drugs.[Materials and Methods]U2OS cells were transfected with lentivarial vectors carring miR-221and control plasmids respectively. The levels of RAD51protein in U2OS were determined by Western blotting. Cells were treated by DNA damage-inducing agents. y-H2AX and RAD51foci were determined by immunofluorescent staining. Cell survival rate was determined by MTT assay.[Results]1. Overexpression of miR-221decreased the expression of RAD51.2. Inhibition of miR-221led to increase in the level of RAD51.3. Overexpression of miR-221reduced RAD51foci formation.4. Overexpression of miR-221impaired the repair of double-strand breaks and led to increased genomic instabilitu.5. A luciferase reporter containing the3’UTR of RAD51indicated a negative regulation of its expression by miR-221.6. Overexpression of miR-221led to increased sensitivity to ionizing radiation and chemotherapy drugs.[Conclusions]miR-221can significantly downregulate RAD51, impair DNA repair and confer sensitivity to ionizing radiation and chemotherapy drugs.