MicroRNA Expression Signature and the Role of MicroRNA-21 in the Early Phase of Acute Myocardial Infarction

Abstract

Several recent reports have suggested that microRNAs (miRNAs) might play critical roles in acute myocardial infarction (AMI). However, the miRNA expression signature in the early phase of AMI has not been identified. In this study, the miRNA expression signature was investigated in rat hearts 6 h after AMI. Compared with the expression signature in the noninfarcted areas, 38 miRNAs were differentially expressed in infarcted areas and 33 miRNAs were aberrantly expressed in the border areas. Remarkably, miR-21 expression was significantly down-regulated in infarcted areas, but was up-regulated in border areas. The down-regulation of miR-21 in the infarcted areas was inhibited by ischemic preconditioning, a known cardiac protective method. Overexpression of miR-21 via adenovirus expressing miR-21 (Ad-miR-21) decreased myocardial infarct size by 29% at 24 h and decreased the dimension of left ventricles at 2 weeks after AMI. Using both gain-of-function and loss-offunction approaches in cultured cardiac myocytes, we identified that miR-21 had a protective effect on ischemia-induced cell apoptosis that was associated with its target gene programmed cell death 4 and activator protein 1 pathway. The protective effect of miR-21 against ischemia-induced cardiac myocyte damage was further confirmed in vivo by decreased cell apoptosis in the border and infarcted areas of the infarcted rat hearts after treatment with Ad-miR-21. The results suggest that miRNAs such as miR-21 may play critical roles in the early phase of AMI. MicroRNAs (miRNAs)3 are endogenous, noncoding, singlestranded RNAs of ϳ22 nucleotides and constitute a novel class of gene regulators (1-3). Analogous to the first RNA revolution in the 1980s, when Zaug and Cech (4) discovered the enzymatic activity of RNA, the more recent discoveries of RNA interference and miRNA may represent the second RNA revolution (5).Although the first miRNA, lin-4, was discovered in 1993 (6, 7), their presence in vertebrates was confirmed only in 2001 (8). miRNAs are initially transcribed in the nucleus by RNA polymerase II or III to form large pri-miRNA transcripts (9). These pri-miRNAs are then processed by the RNase III enzymes, Drosha, Pasha, and Dicer, to generate 18-to 24-nucleotide mature miRNAs. In addition to this miRNA biogenesis pathway, some miRNA precursors are able to bypass Drosha processing to produce miRNAs via Dicer, possibly representing an alternative pathway for miRNA biogenesis (10, 11). The mature miRNAs bind to the 3Ј-untranslated region of their mRNA targets and negatively regulate gene expression via degradation or translational inhibition.Currently, about 600 miRNAs have been cloned and sequenced in humans, and the estimated number of miRNA genes is as high as 1,000 in the human genome (12, 13). Functionally, an individual miRNA is as important as a transcription factor because it is able to regulate the expression of its multiple target genes. As a group, miRNAs are estimated to regulate over 30% of the genes in a cell (14). It is thus not surprising th...

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“…miR-21 had a protective effect on ischemia-induced cell apoptosis by targeting PDCD4 and AP-1, which might play critical roles in the early phase of AMI. Importantly, some miRNAs in the non-infarcted area were also differentially expressed 6 h after AMI, suggesting that in addition to dysregulated miRNAs in infarcted tissue and border zone, some miRNAs dysregulated in the remote myocardium might also contribute in the pathophysiological response to AMI [47] . Another potential role of miR-21 in the atrial fibrillation (AF) resulted from experimental HF after MI.…”

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