Up-regulation of miRNA-221 inhibits hypoxia/reoxygenation-induced autophagy through the DDIT4/mTORC1 and Tp53inp1/p62 pathways

Timely reperfusion in acute myocardial infarction has improved clinical outcomes but the benefits are partially offset by ischemia-reperfusion injury (I/R). MiRNA regulates mRNA of multiple effectors within injury and survival cell signaling pathways. We have previously reported the protective effects of miRNA-221 in I/R injury. The purpose of this study was to explore the mechanisms underlying cardioprotection of miR-221. Myoblast H9c2 and neonatal rat ventricular myocytes (NRVM) were subjected to 0.2% O2 hypoxia followed by 2 h of re-oxygenation (H/R). In gain-and-loss function studies through transfections of miR-221 mimic (miR-221) and inhibitor (miR-221-i), the protective effects of miR-221 were confirmed as assessed by increased cell metabolic activity (WST-1) and decreased LDH release. Autophagy was assessed by GFP-LC3 labeling of autophagosome formation, LC3 and p62 measurements. Co-immuno-precipitation and specific gene cloning and function were used to identify the pathways underpinning miR-221 effects. MiR-221 significantly reduced H/R injury in association with inhibition of autophagy. Underlying mechanisms include (1) down-regulation of Ddit4 (disinhibiting the mTORC1/p-4EBP1 pathway) which inhibits autophagosome formation (2) down-regulation of Tp53inp1 (with reduced Tp53inp1/p62 complex formation) which inhibits autophagosome degradation. In conclusion, miRNA-221 exerts cytoprotective effects in hypoxia-reoxygenation injury in association with alterations in autophagic cell injury. Mir-221 may constitute is a novel therapeutic target in the treatment of cardiac I/R injury.