Introduction: Mounting evidence indicates that microRNAs (miRs) play a fundamental role in cardiovascular disease. We and others have reported the upregulation of several miRs in acute myocardial infarction (AMI) and heart failure (HF). However, despite increased levels of specific miRs, the expression of their target proteins is not always observed. The molecular mechanisms underlying such discrepancy are unknown.
Hypothesis: Our hypothesis is that the action of miRs (i.e. translational inhibition and messenger RNA degradation) relies on the actual expression of the protein "trinucleotide repeat-containing gene 6a" (TNRC6a), a componenet of the post-transcriptional gene silencing miR machinery.
Methods: To verify our hypothesis we measured at different time points TNRC6a protein levels in murine hearts following coronary artery ligation or sham surgery. We also tested the mechanistic role of TNRC6a in vitro.
Results: In the in vivo experiments, we observed a significant (p<.01) upregulation of TNRC6a after AMI, accompanied by increased levels of several miRs - including miR-15b, miR-21, miR-26, miR-34a, miR-140 - and significant downregulation of their target proteins Sirtuin-1 (SIRT1), Phosphatase and tensin homolog (PTEN), Small mothers against decapentaplegic-1 (SMAD1), Aldehyde dehydrogenase -2 (ALDH2), and Mitofusin-1 (MFN1). After TNRC6a knockdown, the upregulation of miRs was comparable to the response observed in untreated and scramble-treated rodents, but strikingly, their target proteins were not downregulated, resulting in an amelioration of cardiac function, mitochondrial dynamics, and overall attenuation of both apoptotic and fibrotic responses. Our in vitro data in cardiomyocytes confirmed that hypoxic stress induced TNRC6a upregulation, augmented expression of miR-15b, miR-21, miR-26, miR-34a, and miR-140, and downregulation (protein levels) of SIRT1, PTEN, SMAD1, ALDH2, and MFN1. As seen in the in vivo setting, silencing TNRC6a led to hypoxia-induced upregulation of miRs, not mirrored by downregulation of their target proteins.
Conclusions: Our data indicate that TNRC6a is a pivotal regulator of miR function following AMI, representing a novel therapeutic target to treat ischemic cardiovascular disorders.