Abstract: Objective: To investigate the effect of microRNA-9-5p (miR-9-5p) targeting the myocyte enhancer factor 2C (MEF2C) on the biologicals behaviors of the alveolar rhabdomyosarcoma (ARMS) cells, and to provide the basis for the molecular diagnosis and targeted therapy of ARMS. Methods: The expression levels of miR-9-5p and MEF2C mRNA in ARMS tissue and cells were detected by qRT-PCR method, the proliferation rate of cells was detected by CCK-8 method, the apoptotic rate was detected by flow cytometry, the numbers of invasion and migration cells were detected by Transwell chamber assay, the luciferase activity in 293T cells was detected by double luciferase reporter gene, and the expression level of MEF2C protein in the cells was detected by Western blotting method. Results: The expression levels of miR-9-5p in ARMS tissue and cells were higher than those in normal skeletal muscle tissue and HSKMC cells (P<0.05). Compared with miR-NC group, the expression level of miR-9-5p in RH30 cells in miR-9-5p inhibitor group was decreased (P<0.01),the proliferation rate of RH30 cells in miR-9-5p inhibitor group was decreased (P<0.05), the apoptotic rate was increased (P<0.05), and the numbers of invasion and migration cells were decreased (P<0.05). The luciferase activity in the 293T cels after co-transfection with MEF2C-3'-UTR-WT and added with miR-9-5p was decreased (P<0.01);compared with miR-NC group, the expression levels of MEF2C mRNA and protein in the RH30 cells in miR-9-5p inhibitor group were increased (P<0.05).The expression level of MEF2C mRNA in the ARMS tissue was lower than that in normal skeletal muscle tissue (P<0.01), which was negatively correlated with the expression of miR-9-5p in ARMS tissue (r=-0.5420, P<0.05);the expression level of MEF2C mRNA in the RH30 cells and PLA802 cells was lower than that in the HSKMC cells (P<0.01).Compared with transfection with control plasmid(EV), the expression level of MEF2C mRNA in the RH30 cells after transfection with MEF2C over-expression plasmid was increased (P<0.01), the proliferation rate was decreased (P<0.01), the apoptotic rate was increased (P<0.05), the number of invasion cells was decreased (P<0.05), and the number of migration cells was decreased (P<0.01).Compared with transfection with si-NC, the expression level of MEF2C mRNA in the RH30 cells after transfection with MEF2C siRNA was decreased (P<0.01), the proliferation rate was increased (P<0.05), the apoptotic rate was decreased (P<0.01), the number of invasion cells was increased (P<0.01),and the number of migration cells was increased (P<0.01). Conclusion: MiR-9-5p can directly target the 3'-UTR of MEF2C mRNA to induce mRNA degradation and inhibit the expression of MEF2C, and promote the proliferation, invasion, migration and anti-apoptosis ability of the RH30 cells.

Key words: alveolar rhabdomyosarcoma, miR-9-5p, myocyte enhancer 2C, cell proliferation, apoptosis