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Right ventricular failure predicts adverse outcome in patients with pulmonary hypertension (PH), and in subjects with left ventricular heart failure and is associated with interstitial fibrosis. This review manuscript discusses the cellular effectors and molecular mechanisms implicated in right ventricular fibrosis. The right ventricular interstitium contains vascular cells, fibroblasts, and immune cells, enmeshed in a collagen-based matrix. Right ventricular pressure overload in PH is associated with the expansion of the fibroblast population, myofibroblast activation, and secretion of extracellular matrix proteins. Mechanosensitive transduction of adrenergic signalling and stimulation of the renin-angiotensin-aldosterone cascade trigger the activation of right ventricular fibroblasts. Inflammatory cytokines and chemokines may contribute to expansion and activation of macrophages that may serve as a source of fibrogenic growth factors, such as transforming growth factor (TGF)-β. Endothelin-1, TGF-βs, and matricellular proteins co-operate to activate cardiac myofibroblasts, and promote synthesis of matrix proteins. In comparison with the left ventricle, the RV tolerates well volume overload and ischemia; whether the right ventricular interstitial cells and matrix are implicated in these favourable responses remains unknown. Expansion of fibroblasts and extracellular matrix protein deposition are prominent features of arrhythmogenic right ventricular cardiomyopathies and may be implicated in the pathogenesis of arrhythmic events. Prevailing conceptual paradigms on right ventricular remodelling are based on extrapolation of findings in models of left ventricular injury. Considering the unique embryologic, morphological, and physiologic properties of the RV and the clinical significance of right ventricular failure, there is a need further to dissect RV-specific mechanisms of fibrosis and interstitial remodelling.

The phenylephrine-induced complex-1 (PEX1) transcription factor, also known as zinc-finger protein 260 (Zfp260), is an effector of endothelin-1 and alpha(1)-adrenergic signaling in cardiac hypertrophy. However, the role of PEX1 in transcriptional regulation of myocardial remodeling remains largely unknown. In the present study, we used PEX1 gain- and loss-of-function to examine the effects of PEX1 on left ventricular remodeling. Adenoviral constructs expressing PEX1, antisense PEX1, or LacZ were delivered by local injection into the anterior wall of the left ventricle in Sprague-Dawley rats. PEX1 overexpression led to induction of hypertrophic gene program and increased fibrosis. In agreement with this, the expression of genes involved in the fibrotic process, such as collagens I and III, matrix metalloproteinases (MMPs), fibronectin-1, transforming growth factor beta-1 and connective tissue growth factor, were significantly up-regulated following PEX1 overexpression, whereas silencing of PEX1 significantly inhibited the expression of pro-fibrotic genes and increased left ventricular ejection fraction and fractional shortening. In vitro luciferase reporter assays showed that PEX1 regulates the expression of MMP-9 by activating promoter. Furthermore, PEX1 gain- and loss-of-function experiments in rat neonatal cardiac fibroblasts and myocytes revealed that MMP-9 gene expression was affected by PEX1 predominantly in fibroblasts. Our results indicate that PEX1 is involved in regulating cardiac fibrosis and extracellular matrix turnover, particularly fibroblasts being responsible for the fibrosis-associated changes in gene expression. Furthermore, PEX1 activation of the MMP-9 promoter triggers the pro-fibrotic response directed by PEX1.

Hypoxic injury to the heart results in cardiac fibrosis that leads to cardiac dysfunction and heart failure. SNAIL1 is a zinc finger transcription factor implicated in fibrosis following organ injury and cancer. To determine if the action of SNAIL1 contributed to cardiac fibrosis following hypoxic injury, we used an endogenous SNAIL1 bioluminescence reporter mice, and SNAIL1 knockout mouse models. Here we report that SNAIL1 expression is upregulated in the infarcted heart, especially in the myofibroblasts. Utilizing primary cardiac fibroblasts in ex vivo cultures we find that pro-fibrotic factors and collagen I increase SNAIL1 protein level. SNAIL1 is required in cardiac fibroblasts for the adoption of myofibroblast fate, collagen I expression and expression of fibrosis-related genes. Taken together this data suggests that SNAIL1 expression is induced in the cardiac fibroblasts after hypoxic injury and contributes to myofibroblast phenotype and a fibrotic scar formation. Resultant collagen deposition in the scar can maintain elevated SNAIL1 expression in the myofibroblasts and help propagate fibrosis.

The majority of cardiac fibroblasts in a mature mammalian heart are derived from the epicardium during prenatal development and reactivate developmental programs during the progression of fibrotic disease. In addition, epicardial activation, proliferation, and fibrosis occur with ischemic, but not hypertensive injury. Here we review cellular and molecular mechanisms that control epicardium-derived cell lineages during development and disease with a focus on cardiac fibroblasts. This article is part of a special issue entitled "Fibrosis and Myocardial Remodeling".