Diffuse myocardial fibrosis: mechanisms, diagnosis and therapeutic approaches

Over the past decade, myocardial structure, cardiomyocyte function, and intramyocardial signaling were shown to be specifically altered in heart failure with preserved ejection fraction (HFPEF). A new paradigm for HFPEF development is therefore proposed, which identifies a systemic proinflammatory state induced by comorbidities as the cause of myocardial structural and functional alterations. The new paradigm presumes the following sequence of events in HFPEF: 1) a high prevalence of comorbidities such as overweight/obesity, diabetes mellitus, chronic obstructive pulmonary disease, and salt-sensitive hypertension induce a systemic proinflammatory state; 2) a systemic proinflammatory state causes coronary microvascular endothelial inflammation; 3) coronary microvascular endothelial inflammation reduces nitric oxide bioavailability, cyclic guanosine monophosphate content, and protein kinase G (PKG) activity in adjacent cardiomyocytes; 4) low PKG activity favors hypertrophy development and increases resting tension because of hypophosphorylation of titin; and 5) both stiff cardiomyocytes and interstitial fibrosis contribute to high diastolic left ventricular (LV) stiffness and heart failure development. The new HFPEF paradigm shifts emphasis from LV afterload excess to coronary microvascular inflammation. This shift is supported by a favorable Laplace relationship in concentric LV hypertrophy and by all cardiac chambers showing similar remodeling and dysfunction. Myocardial remodeling in HFPEF differs from heart failure with reduced ejection fraction, in which remodeling is driven by loss of cardiomyocytes. The new HFPEF paradigm proposes comorbidities, plasma markers of inflammation, or vascular hyperemic responses to be included in diagnostic algorithms and aims at restoring myocardial PKG activity. Copyright © 2013 American College of Cardiology Foundation. Published by Elsevier Inc. All rights reserved.

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 down-regulated 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.