MicroRNA-338-3p inhibits tumor growth and metastasis in osteosarcoma cells by targeting RUNX2/CDK4 and inhibition of MAPK pathway

Osteosarcoma (OS) is one of the most aggressive bone tumors. MicroRNAs (miRNAs) have been found to implicate in the pathogenesis of different types of cancers, including OS. This study aimed to explore the roles of miR-338-3p in OS and investigate the underlying mechanism. Human OS cell lines (MG-63 and U2OS) and osteoblast (hFOB) cell line were used in the study. The expression levels of miR-338-3p, runt-related transcription factor 2 (RUNX2) and cyclin-dependent kinase 4 (CDK4) were altered by transient transfection and determined by quantitative real-time polymerase chain reaction/Western blot analysis. Cell viability, colony numbers, migration, and invasion, and apoptotic cells were measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, colony formation assay, transwell assay, and flow cytometry assay, respectively. Dual luciferase reporter assay was performed to identify the target gene of miR-338-3p. Western blot assay was carried to measure the protein expression levels involved in cell apoptosis, migration, and mitogen-activated protein kinases (MAPK) pathway. We found that the expression of miR-338-3p was downregulated in MG-63 cell and U2OS cells, compared with hFOB cells. MiR-338-3p suppression significantly increased cell viability and colony numbers, promoted cell migration, and invasion, but suppressed cell apoptosis in MG-63 and U2OS cells. Opposite results were observed in the miR-338-3p overexpression. Interestingly, RUNX2 and CDK4 were direct target genes of miR-338-3p. RUNX2 inhibition shared a similar effect of miR-338-3p mimic on MG-63 cells. Furthermore, miR-338-3p inhibited the activation of MAPK pathway in MG-63 cells. To conclude, these findings suggested that miR-338-3p functioned as a tumor suppressor in OS cells by targeting RUNX2 and CDK4, as well as inhibition of the MAPK pathway.