Dynamic Chromatin Targeting of BRD4 Stimulates Cardiac Fibroblast Activation.

Abstract

RATIONALE\nSmall molecule inhibitors of the acetyl-histone binding protein BRD4 have been shown to block cardiac fibrosis in pre-clinical models of heart failure (HF). However, since the inhibitors target BRD4 ubiquitously, it is unclear whether this chromatin reader protein functions in cell type-specific manner to control pathological myocardial fibrosis. Furthermore, the molecular mechanisms by which BRD4 stimulates the transcriptional program for cardiac fibrosis remain unknown.\n\n\nOBJECTIVE\nWe sought to test the hypothesis that BRD4 functions in a cell-autonomous and signal-responsive manner to control activation of cardiac fibroblasts, which are the major extracellular matrix (ECM)-producing cells of the heart.\n\n\nMETHODS AND RESULTS\nRNA-sequencing (RNA-seq), mass spectrometry and cell-based assays employing primary adult rat ventricular fibroblasts (ARVFs) demonstrated that BRD4 functions as an effector of TGF-β signaling to stimulate conversion of quiescent cardiac fibroblasts into Periostin (Postn)-positive cells that express high levels of ECM. These findings were confirmed in vivo through whole-transcriptome analysis of cardiac fibroblasts from mice subjected to transverse aortic constriction (TAC) and treated with the small molecule BRD4 inhibitor, JQ1. Chromatin immunoprecipitation-sequencing (ChIP-seq) revealed that BRD4 undergoes stimulus-dependent, genome-wide redistribution in cardiac fibroblasts, becoming enriched on a subset of enhancers and super-enhancers, and leading to RNA polymerase II activation and expression of downstream target genes. Employing the SERTA domain-containing protein 4 (Sertad4) locus as a prototype, we demonstrate that dynamic chromatin targeting of BRD4 is controlled, in part, by p38 mitogen-activated protein kinase (MAPK) and provide evidence of a critical function for Sertad4 in TGF-β-mediated cardiac fibroblast activation.\n\n\nCONCLUSIONS\nThese findings define BRD4 as a central regulator of the pro-fibrotic cardiac fibroblast phenotype, establish a p38-dependent signaling circuit for epigenetic reprogramming in HF, and uncover a novel role for Sertad4. The work provides a mechanistic foundation for the development of BRD4 inhibitors as targeted anti-fibrotic therapies for the heart.