miR‑340‑5p: A potential direct regulator of Nrf2 expression in the post‑exercise skeletal muscle of mice.

Nuclear factor erythroid 2‑related factor 2 (Nrf2) is a key transcription factor that serves a critical role in protecting against cellular stress induced by exercise. The effects of exercise training on Nrf2 expression have been widely studied; however, the post‑transcriptional/translational regulation of Nrf2 is poorly understood. The aim of the present study was to identify the exercise‑induced microRNAs (miRNAs/miRs) targeting Nrf2. A total of 20 C57BL/6J mice were divided into the control (n=10) and exercise (n=10) groups. Following eight weeks of aerobic exercise training, Nrf2 mRNA expression in the hind‑limb muscles was significantly increased in the exercise group, while that of Nrf2 protein remained unchanged. In addition, 58 differentially expressed miRNAs have been detected; among them, miR‑101a‑3p and miR‑340‑5p were predicted to target the 3' untranslated region of Nrf2 mRNA by analysis with three bioinformatics tools. The binding affinity of the two miRNAs were verified via a dual luciferase reporter assay; only miR‑340‑5p was determined to bind directly to Nrf2 mRNA. Additionally, miR‑340‑5p mimics and inhibitors were transfected into C2C12 cells to investigate the biological effects of endogenous miR‑340‑5p on the expression of Nrf2. The results revealed that the expression levels of Nrf2 protein in C2C12 cells were significantly decreased in the miR‑340‑5p mimics group and significantly increased in the miR‑340‑5p inhibitors group, while Nrf2 mRNA expression levels were unchanged. The results indicated that miR‑340‑5p serves a role in the post‑transcriptional regulation of Nrf2 expression. In conclusion, the novel findings of the present study were that miR‑340‑5p may be a potential direct regulator of Nrf2 gene expression and might be involved in the regulation of Nrf2 protein expression in mouse skeletal muscles following aerobic exercise. These results may provide insight into the underlying regulatory mechanisms of Nrf2 protein expression in skeletal muscle during aerobic exercise.