Acute myocardial infarction (MI) due to coronary artery occlusion is accompanied by a pathological remodeling response that includes hypertrophic cardiac growth and fibrosis, which impair cardiac contractility. Previously, we showed that cardiac hypertrophy and heart failure are accompanied by characteristic changes in the expression of a collection of specific microRNAs (miRNAs), which act as negative regulators of gene expression. Here, we show that MI in mice and humans also results in the dysregulation of specific miRNAs, which are similar to but distinct from those involved in hypertrophy and heart failure. Among the MI-regulated miRNAs are members of the miR-29 family, which are downregulated in the region of the heart adjacent to the infarct. The miR-29 family targets a cadre of mRNAs that encode proteins involved in fibrosis, including multiple collagens, fibrillins, and elastin. Thus, down-regulation of miR-29 would be predicted to derepress the expression of these mRNAs and enhance the fibrotic response. Indeed, down-regulation of miR-29 with anti-miRs in vitro and in vivo induces the expression of collagens, whereas over-expression of miR-29 in fibroblasts reduces collagen expression. We conclude that miR-29 acts as a regulator of cardiac fibrosis and represents a potential therapeutic target for tissue fibrosis in general.

Rationale: Myocardial infarction (MI) results in loss of cardiac myocytes in the ischemic zone of the heart, followed by fibrosis and scar formation, which diminish cardiac contractility and impede angiogenesis and repair. Myofibroblasts, a specialized cell type that switches from a fibroblast-like state to a contractile, smooth muscle-like state, are believed to be primarily responsible for fibrosis of the injured heart and other tissues, although the transcriptional mediators of fibrosis and myofibroblast activation remain poorly defined. Myocardin-related transcription factors (

The epicardium encapsulates the heart and functions as a source of multipotent progenitor cells and paracrine factors essential for cardiac development and repair. Injury of the adult heart results in reactivation of a developmental gene program in the epicardium, but the transcriptional basis of epicardial gene expression has not been delineated. We established a mouse embryonic heart organ culture and gene expression system that facilitated the identification of epicardial enhancers activated during heart development and injury. Epicardial activation of these enhancers depends on a combinatorial transcriptional code centered on CCAAT/enhancer binding protein (C/EBP) transcription factors. Disruption of C/EBP signaling in the adult epicardium reduced injury-induced neutrophil infiltration and improved cardiac function. These findings reveal a transcriptional basis for epicardial activation and heart injury, providing a platform for enhancing cardiac regeneration.

Hypoxic heart disease is a predominant cause of disability and death worldwide. Since adult mammalian hearts are incapable of regeneration after hypoxia, attempts to modify this deficiency are critical. As demonstrated in zebrafish, recall of the embryonic developmental program may be the key to success. Because thymosin β4 (TB4) is beneficial for myocardial cell survival and essential for coronary development in embryos, we hypothesized that it reactivates the embryonic developmental program and initiates epicardial progenitor mobilization in adult mammals. We found that TB4 stimulates capillary-like tube formation of adult coronary endothelial cells and increases embryonic endothelial cell migration and proliferation in vitro. The increase of blood vessel/epicardial substance (Bves) expressing cells accompanied by elevated VEGF, Flk-1, TGF-β, FGFR-2, FGFR-4, FGF-17 and β-Catenin expression and increase of Tbx-18 and Wt-1 positive myocardial progenitors suggested organ-wide recall of the embryonic program in the adult epicardium. TB4 also positively regulated the expression and phosphorylation of myristoylated alanine-rich C-kinase substrate (Marcks), a direct substrate and indicator of protein kinase C (PKC) activity in vitro and in vivo. PKC inhibition significantly reduced TB4 initiated epicardial thickening, capillary growth and the number of myocardial progenitors. Our results demonstrate that TB4 is the first known molecule capable of organ-wide activation of the embryonic coronary developmental program in the adult mammalian heart after systemic administration and that PKC plays a significant role in the process.

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