Therapeutic targets for cardiac fibrosis: from old school to next-gen

Abstract

Cardiovascular diseases remain the leading cause of death worldwide, with pathological fibrotic remodeling mediated by activated cardiac myofibroblasts representing a unifying theme across etiologies. Despite the profound contributions of myocardial fibrosis to cardiac dysfunction and heart failure, there currently exist limited clinical interventions that effectively target the cardiac fibroblast and its role in fibrotic tissue deposition. Exploration of novel strategies designed to mitigate or reverse myofibroblast activation and cardiac fibrosis will likely yield powerful therapeutic approaches for the treatment of multiple diseases of the heart, including heart failure with preserved or reduced ejection fraction, acute coronary syndrome, and cardiovascular disease linked to type 2 diabetes. In this Review, we provide an overview of classical regulators of cardiac fibrosis and highlight emerging, next-generation epigenetic regulatory targets that have the potential to revolutionize treatment of the expanding cardiovascular disease patient population.

Figure 1. Classical signaling pathways regulating CF activation and approaches for targeting fibrosis of the heart.

Numerous signaling pathways have been implicated in the regulation of CF activation and fibrotic remodeling. Therapeutic targeting of these pathways is of intense scientific and clinical interest. TGF-β stimulation of the TGF-β receptor (TGFβR) drives fibroblast activation canonically through SMAD2/3 activation and nuclear translocation, or non-canonically by inducing TGF-β–activated kinase 1–mediated (TAK1-mediated) phosphorylation of p38. While activation of the β₂-adrenergic receptor (β₂-AR) is thought to be antifibrotic through induction of cAMP production and activation of exchange protein directly activated by cAMP (EPAC), this signaling can be uncoupled through GPCR kinase 2–mediated (GRK2-mediated) or GRK5-mediated receptor phosphorylation. β₃-AR agonists, such as mirabegron, may ameliorate fibroblast activation through yet unknown mechanisms. Angiotensin II (Ang II) mediates fibroblast activation through stimulation of the Ang II type 1 receptor (AT1R), by both promoting TGF-β production and inducing systemic release of the mineralocorticoid aldosterone from the adrenal cortex. Induction of cGMP-dependent protein kinase (PKG), through either B-type natriuretic peptide–mediated (BNP-mediated) activation of type A and B natriuretic peptide receptors (NPR-A/B) or stimulation of soluble guanylate cyclase (sGC) by nitric oxide (NO), has also demonstrated antifibrotic properties. Established and investigatory therapeutic strategies targeting these pathways are listed below. ACE, angiotensin-converting enzyme; ARB, angiotensin receptor blocker.

Figure 2. Epigenetic regulation of CF activation and next-generation therapeutic strategies.

(A) The most notable histone acetyltransferase (HAT) in the control of cardiac fibrosis is p300, which mediates acetylation of histone tail lysine residues in enhancers and super-enhancers that control expression of profibrotic genes. p300 has a bromodomain, which mediates binding of the enzyme to acetyl-histones in chromatin. Bromodomain-containing protein 4 (BRD4) also binds acetyl-histones and initiates a profibrotic gene program by activating RNA polymerase II (RNA Pol II). (B) The small-molecule acetyl-lysine mimic JQ1 binds the bromodomains of BRD4 to displace it from chromatin, thereby attenuating profibrotic gene expression. Similarly, CBP30 inhibits the p300 bromodomain, while A-485 inhibits p300 catalytic activity. Pharmacological inhibition of histone deacetylases (HDACs) using compounds such as ITF2357/givinostat creates spurious acetyl-histone marks, resulting in mislocalization of p300 and BRD4 in the cardiac fibroblast genome, with resulting disruption of the profibrotic gene program. (C) In activated CFs, BRD4 associates with an enhancer element approximately 65 kb downstream of the gene encoding a homeobox transcription factor, Meox1. Enhancer-bound BRD4 loops to associate with the Meox1 promoter, resulting in stimulation of its expression and initiation of a profibrotic gene expression cascade.

Figure 3. Discovering the next generation of antifibrotic epigenetic inhibitors for the heart.

Proposed model for expeditiously uncovering novel epigenetics-based therapeutics targeting fibroblast activation and cardiac fibrosis. The recent development of highly selective and potent inhibitors of myriad epigenetic targets has laid a strong foundation for therapeutic investigation using ex vivo, imaging-based phenotypic screening and subsequent exploration in in vivo models of cardiac fibrosis and HF. Given the existence of “hidden fibrosis,” histological assessment of fibrotic remodeling of the heart should be complemented with techniques such as ECM mass spectrometry, atomic force microscopy (AFM), and single-cell RNA sequencing. We envision that these approaches will allow desperately needed therapeutic strategies targeting myofibroblast activation and fibrotic remodeling to finally bridge the gap to the clinical realm.