MicroRNA-22 mediates the cisplatin resistance of osteosarcoma cells by inhibiting autophagy via the PI3K/Akt/mTOR pathway

Osteosarcoma (OS) is the most common primary malignant tumor of the bone affecting children and adolescents. Chemotherapy is now considered as a standard component of OS treatment, not only for children, but also for adults. However, chemoresistance continues to pose a challenge to therapy. Inhibition of autophagy has been demonstrated to decrease chemoresistance in OS. Moreover, microRNA-22 (miR-22) inhibits autophagy, leading to an improvement in the sensitivity of cisplatin (CDDP) in OS. The aim of the present study was therefore to investigate whether miR-22 could mediate the CDDP resistance of OS cells by inhibiting autophagy via the phosphoinositide 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway. Cell proliferation assay, LC3 flow cytometry assay and monodansylcadaverine staining in MG63 cells and CDDP resistance cells (MG63/CDDP) were performed to explore to role of miR-22 and CDDP in OS chemoresistance. Inoculation of tumor cells in an in vivo model, reverse transcription-quantitative PCR (RT-qPCR) assay, western blot analysis, and immunohistochemistry analysis were performed to investigate the role of miR-22 and CDDP in the PI3K/Akt/mTOR pathway as it is affected by autophagy. The results revealed that miR-22 inhibited the proliferation of MG63 and MG63/CDDP cells, and enhanced the anti-proliferative ability of CDDP in vivo and in vitro. miR-22 mediated the CDDP resistance of OS cells by inhibiting autophagy and decreasing CDDP-induced autophagy via downregulation of the expression of PI3K, Akt, and mTOR at the mRNA level, and the expression of PI3K, phosphorylated (p)-Akt, and p-mTOR at the protein level. It was also convincingly demonstrated that miR-22 mediates the CDDP resistance of OS by inhibiting autophagy via the PI3K/Akt/mTOR pathway. Furthermore, in the MG63 cells that were affected by CDDP, the role of miR-22 was shown to be similar to that of the investigated inhibitor of PI3K (wortmannin) in terms of regulating the PI3K/Akt/mTOR pathway, and wortmannin could also promote the effect of miR-22. Interestingly, CDDP was demonstrated to induce autophagy by inhibiting the PI3K/Akt/mTOR pathway, whereas the pathway was upregulated in the state of chemoresistance. In conclusion, downregulation of the PI3K/Akt/mTOR pathway was shown to assist in the process of preventing chemoresistance.