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MicroRNAs (miRNAs) have been demonstrated to regulate more than 60% of human genes by complementary base pairing between their seed sequences and the targeted mRNAs, resulting in decay and/or translational repression of the mRNAs. A unique mechanistic action of miRNAs is that a single miRNA can simultaneously target a cluster of genes that are involved in functionally associated signalling cascades to generate synergistic impact on cellular process. The objective of this study is to define miR-125a and miR-125b as key determinants in cardiac mitochondrial injury upon myocardial infarction (MI). Approaches and Results: To this aim, a MI mouse model was established by ligation of the left anterior descending coronary artery. MicroRNA profiling of heart tissues from MI and Sham mice revealed that miR-125a/b were robustly upregulated in the MI hearts. TargetScan analysis identified multiple putative targeting sites of miR-125a/b within the 3’-UTR of genes involved in inflammatory signaling, mitochondrial and endoplasmic reticulum (ER) stress, mitochondrial fusion and fatty acid β-oxidation. These genes include A20 in NFκB-mediated inflammation, ITPR1 in ER calcium channel, PACS2 and MFN1 in mitochondrial membrane remodeling as well as PGC1á/å and PPARα that regulate mitochondrial biogenesis and energy homeostasis. Q-RT-PCR analysis validated the significantly reduced expression of these genes, which confirmed the specific-interaction between miR-125a/b and the genes. Immunoblotting analysis further revealed activation of ER stress in the MI heart, indicated by the activation of ER markers, Grp78 and phosphor-JNK, in the MI cardiomyocytes. The adverse consequences from multiple compromised signalling led to the development of cardiac apoptosis and heart failure. In vitro, exposure the human cardiomyocytes AC16 to hypoxia or Lipopolysaccharides treatment upregulated miR-125a/b expression which was accompanied with impaired mitochondrial biosynthesis and oxidative metabolism, leading to apoptotic cell death. Conclusion: This novel finding defines miR-125a/b as key regulators of cardiac mitochondrial and ER homeostasis, which may lend support to the development of therapeutic strategy for MI by manipulating miR-125a/b abundance.

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