Dynamic chromatin targeting of BRD4 stimulates cardiac fibroblast activation [ChIP-seq]

Small 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, the mechanisms by which BRD4 promotes pathological myocardial fibrosis remain unclear. Here, we demonstrate that BRD4 functions as an effector of TGF-b signaling to stimulate conversion of quiescent cardiac fibroblasts into Periostin (Postn)-positive cells that express high levels of extracellular matrix. 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, and provide evidence of a novel function for Sertad4 in TGF-b-mediated cardiac fibroblast activation. These findings define BRD4 as a central regulator of the pro-fibrotic cell state of cardiac fibroblasts, and establish a signaling circuit for epigenetic reprogramming in HF.乙酰组蛋白结合蛋白BRD4（acetyl-histone binding protein BRD4）的小分子抑制剂已被证实可在心力衰竭（heart failure，HF）的临床前模型中阻断心肌纤维化。然而，BRD4促进病理性心肌纤维化（pathological myocardial fibrosis）的具体机制仍不明确。本研究证实，BRD4可作为转化生长因子-β（transforming growth factor-β，TGF-b）信号通路的效应分子，促进静息态心脏成纤维细胞（quiescent cardiac fibroblasts）转化为高表达细胞外基质（extracellular matrix）的骨桥蛋白（Periostin，Postn）阳性细胞。BRD4在心脏成纤维细胞（cardiac fibroblasts）中呈现刺激依赖的全基因组重分布，在部分增强子（enhancers）和超级增强子（super-enhancers）区域发生富集，进而激活RNA聚合酶Ⅱ（RNA polymerase II）并上调下游靶基因的表达。本研究以含SERTA结构域蛋白4（SERTA domain-containing protein 4，Sertad4）基因座为模型，证实BRD4的动态染色质靶向（chromatin targeting）调控部分依赖于p38丝裂原活化蛋白激酶（p38 mitogen-activated protein kinase），并揭示了Sertad4在TGF-b介导的心脏成纤维细胞活化中的全新功能。上述研究结果将BRD4确定为心脏成纤维细胞促纤维化细胞状态的核心调控因子，并确立了心力衰竭中表观遗传重编程（epigenetic reprogramming）的信号通路环路。