miR-221/222-Mediated Inhibition of Autophagy Promotes Dexamethasone Resistance in Multiple Myeloma

Inherent or acquired resistance to chemotherapeutic drugs is still an obstacle for the treatment of multiple myeloma (MM). MicroRNA dysregulation is related to the development of chemoresistance in cancers. However, its role in chemoresistance of MM is largely unknown. Here we demonstrated that miR-221/222 were upregulated in plasma cells from patients with MM, especially those with relapsed or refractory disease. Moreover, expression levels of miR-221/222 were inversely correlated with dexamethasone (Dex) sensitivity of human MM cell lines. Importantly, we found that Dex induced pro-death autophagy in MM cells and the inhibition of autophagy significantly decreased Dex-induced cell death. Mechanistically, autophagy-related gene 12 (ATG12) was identified as a novel target gene of miR-221/222, and miR-221/222 overexpression inhibited autophagy by directly targeting ATG12 and the p27kip (p27)-mammalian target of rapamycin (mTOR) pathway. Indeed, Dex treatment decreased the expression of miR-221/222, thereby activating the ATG12/p27mTOR autophagy-regulatory axis and inducing cell death in Dex-sensitive MM cells. Furthermore, both in vitro and in vivo results showed that the inhibitions of miR-221/222 increased the expression of ATG12 and p27 and functionally induced extended autophagy and cell death of MM cells. In conclusion, our findings demonstrated the crucial role of the miR-221/222ATG12/ p27-mTOR autophagy-regulatory axis in Dex resistance of MM, and they suggest potential prediction and treatment strategies for glucocorticoid resistance.