The present study was designed to decipher molecular mechanisms underlying nicotine's promoting atrial fibrillation (AF) by inducing atrial structural remodelling. The canine model of AF was successfully established by nicotine administration and rapid pacing. The atrial fibroblasts isolated from healthy dogs were treated with nicotine. The role of microRNAs (miRNAs) on the expression and regulation of transforming growth factor-β1 (TGF-β1), TGF-β receptor type II (TGF-βRII), and collagen production was evaluated in vivo and in vitro. Administration of nicotine for 30 days increased AF vulnerability by ∼eight- to 15-fold in dogs. Nicotine stimulated remarkable collagen production and atrial fibrosis both in vitro in cultured canine atrial fibroblasts and in vivo in atrial tissues. Nicotine produced significant upregulation of expression of TGF-β1 and TGF-βRII at the protein level, and a 60–70% decrease in the levels of miRNAs miR-133 and miR-590. This downregulation of miR-133 and miR-590 partly accounts for the upregulation of TGF-β1 and TGF-βRII, because our data established TGF-β1 and TGF-βRII as targets for miR-133 and miR-590 repression. Transfection of miR-133 or miR-590 into cultured atrial fibroblasts decreased TGF-β1 and TGF-βRII levels and collagen content. These effects were abolished by the antisense oligonucleotides against miR-133 or miR-590. The effects of nicotine were prevented by an α7 nicotinic acetylcholine receptor antagonist. We conclude that the profibrotic response to nicotine in canine atrium is critically dependent upon downregulation of miR-133 and miR-590.

The present study was designed to investigate whether the beneficial effects of β-blocker propranolol are related to regulation of microRNA miR-1. We demonstrated that propranolol reduced the incidence of arrhythmias in a rat model of myocardial infarction by coronary artery occlusion. Overexpression of miR-1 was observed in ischaemic myocardium and strikingly, administration of propranolol reversed the up-regulation of miR-1 nearly back to the control level. In agreement with its miR-1-reducing effect, propranolol relieved myocardial injuries during ischaemia, restored the membrane depolarization and cardiac conduction slowing, by rescuing the expression of inward rectifying K⁺ channel subunit Kir2.1 and gap junction channel connexin 43. Our results further revealed that the β-adrenoceptor–cAMP–Protein Kinase A (PKA) signalling pathway contributed to the expression of miR-1, and serum response factor (SRF), which is known as one of the transcriptional enhancers of miR-1, was up-regulated in ischaemic myocardium. Moreover, propranolol inhibited the β-adrenoceptor–cAMP–PKA signalling pathway and suppressed SRF expression. We conclude that the β-adrenergic pathway can stimulate expression of arrhythmogenic miR-1, contributing to ischaemic arrhythmogenesis, and β-blockers produce their beneficial effects partially by down-regulating miR-1, which might be a novel strategy for ischaemic cardioprotection.

Rationale: Cardiac fibrosis is associated with various cardiovascular diseases and can eventually lead to heart failure. Dysregulation of long non-coding RNAs (lncRNAs) has recently been recognized as one of the key mechanisms involved in cardiac diseases. However, the potential roles and underlying mechanisms of lncRNAs in cardiac fibrosis have not been explicitly delineated.

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive, and high-lethality fibrotic lung disease characterized by excessive fibroblast proliferation, extracellular matrix accumulation, and, ultimately, loss of lung function. Although dysregulation of some microRNAs (miRs) has been shown to play important roles in the pathophysiological processes of IPF, the role of miRs in fibrotic lung diseases is not well understood. In this study, we found downregulation of miR-26a in the lungs of mice with experimental pulmonary fibrosis and in IPF, which resulted in posttranscriptional derepression of connective tissue growth factor (CTGF), and induced collagen production. More importantly, inhibition of miR-26a in the lungs caused pulmonary fibrosis in vivo, whereas overexpression of miR-26a repressed transforming growth factor (TGF)-β1-induced fibrogenesis in MRC-5 cells and attenuated experimental pulmonary fibrosis in mice. Our study showed that miR-26a was downregulated by TGF-β1-mediated phosphorylation of Smad3. Moreover, miR-26a inhibited the nuclear translocation of p-Smad3 through directly targeting Smad4, which determines the nuclear translocation of p-Smad2/Smad3. Taken together, our experiments demonstrated the antifibrotic effects of miR-26a in fibrotic lung diseases and suggested a new strategy for the prevention and treatment of IPF using miR-26a. The current study also uncovered a novel positive feedback loop between miR-26a and p-Smad3, which is involved in pulmonary fibrosis.

More evidence is emerging of the roles long non-coding RNAs (lncRNAs) play as regulatory factors in a variety of biological processes, but the mechanisms underlying the function of lncRNAs in acute myocardial infarction (AMI) have not been explicitly delineated. The present study identified the lncRNA 2810403D21Rik/AK007586/Mirf (myocardial infarction-regulatory factor), that inhibited macroautophagy/autophagy by modulating Mir26a (microRNA 26a). Inhibition of Mir26a led to cardiac injury both in vitro and in vivo, whereas overexpression of Mir26a attenuated ischemic stress-induced cell death by activating autophagy through targeting Usp15 (ubiquitin specific peptidase 15). More importantly, 2810403D21Rik/Mirf acted as a competitive endogenous RNA (ceRNA) of Mir26a; forced expression of 2810403D21Rik/Mirf downregulated Mir26a to inhibit autophagy. In contrast, loss of 2810403D21Rik/Mirf resulted in upregulation of Mir26a to promote autophagy and alleviate cardiac injury, which in turn improved cardiac function in MI mice. This study identified a lncRNA 2810403D21Rik/Mirf that functions as an anti-autophagic molecule via ceRNA activity toward Mir26a. Our findings suggest that knockdown of 2810403D21Rik/Mirf might be a novel therapeutic approach for cardiac diseases associated with autophagy. Abbreviations: 3-MA: 3-methyladenine; AAV-9: adenovirus associated virus-9; agoMir26a: cholesterol-conjugated Mir26a mimic; AMI: acute myocardial infarction; AMO-26a: Mir26a inhibitor; ATG: autophagy related; BECN1: beclin 1; ceRNA: competitive endogenous RNAs; EF: ejection fraction; f-2810403D21Rik/Mirf: fragment encompassing the Mir26a binding site; FS: fraction shortening; GFP-mRFP: a plasmid expressing green fluorescent protein-monomeric red fluorescent protein; lncRNA: long non-coding RNA; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; Mirf: myocardial infarction-regulatory factor; miRNAs: microRNAs; NC: negative control; NMCMs: neonatal mice cardiomyocytes; shRNA: short hairpin RNA; siRNA: small interfering RNA; SQSTM1/p62: sequestosome 1; TEM: transmission electron microscopy; Usp15: ubiquitin specific peptidase 15.

Cardiac hypertrophy accompanied by maladaptive cardiac remodeling is the uppermost risk factor for the development of heart failure. Long non-coding RNAs (lncRNAs) and microRNAs (miRNAs) have various biological functions, and their vital role in the regulation of cardiac hypertrophy still needs to be explored. In this study, we demonstrated that lncRNA Plscr4 was upregulated in hypertrophic mice hearts and in angiotensin II (Ang II)-treated cardiomyocytes. Next, we observed that overexpression of Plscr4 attenuated Ang II-induced cardiomyocyte hypertrophy. Conversely, the inhibition of Plscr4 gave rise to cardiomyocyte hypertrophy. Furthermore, overexpression of Plscr4 attenuated TAC (transverse aortic constriction)-induced cardiac hypertrophy. Finally, we demonstrated that Plscr4 acted as an endogenous sponge of miR-214 and forced expression of Plscr4 downregulated miR-214 expression to promote Mfn2 and attenuate hypertrophy. In contrast, knockdown of Plscr4 upregulated miR-214 to induce cardiomyocyte hypertrophy. Additionally, luciferase assay showed that miR-214 was the direct target of Plscr4, and overexpression of miR-214 counteracted the anti-hypertrophy effect of Plscr4. Collectively, these findings identify Plscr4 as a negative regulator of cardiac hypertrophy in vivo and in vitro due to its regulation of the miR-214-Mfn2 axis, suggesting that Plscr4 might act as a therapeutic target for the treatment of cardiac hypertrophy and heart failure.