Differential activation of P-TEFb complexes in the development of cardiomyocyte hypertrophy following activation of distinct GPCRs

Pathological cardiac hypertrophy is driven by neurohormonal activation of specific G protein-coupled receptors (GPCRs) in cardiomyocytes and is accompanied by large-scale changes in cardiomyocyte gene expression. These transcriptional changes require activity of positive transcription elongation factor b (P-TEFb), which is recruited to target genes by the bromodomain protein Brd4 or the Super Elongation Complex (SEC). Here we describe GPCR-specific regulation of these P-TEFb complexes and a novel mechanism for activating Brd4 in primary neonatal rat cardiomyocytes. The SEC was required for the hypertrophic response downstream of either the α1-adrenergic receptor (α1-AR) or the endothelin receptor (ETR). In contrast, Brd4 inhibition selectively impaired the α1-AR response. This was corroborated by the finding that activation of α1-AR, but not ETR, increased Brd4 occupancy at promoters and super enhancers of hypertrophic genes. Transcriptome analysis demonstrated that activation of both receptors initiated similar gene expression programs, but that Brd4 inhibition attenuated hypertrophic genes more robustly following α1-AR activation. Finally, we show that protein kinase A (PKA) is required for α1-AR stimulation of Brd4 chromatin occupancy. The differential role of the Brd4/P-TEFb complex in response to distinct GPCR pathways has potential clinical implications as therapies targeting this complex are currently being explored for heart failure.病理性心肌肥厚由心肌细胞中特定G蛋白偶联受体（G protein-coupled receptors, GPCRs）的神经激素激活所驱动，并伴随心肌细胞基因表达的大规模改变。此类转录变化需要正性转录延伸因子b（positive transcription elongation factor b, P-TEFb）的活性，该因子可通过溴结构域蛋白Brd4或超级延伸复合物（Super Elongation Complex, SEC）被招募至靶基因。本研究阐明了上述P-TEFb复合物的GPCR特异性调控机制，以及在原代新生大鼠心肌细胞中激活Brd4的全新机制。超级延伸复合物是α1肾上腺素能受体（α1-adrenergic receptor, α1-AR）或内皮素受体（endothelin receptor, ETR）下游肥厚反应的必需组分。与之相反，Brd4抑制可选择性损伤α1-AR介导的肥厚反应。这一结论得到了以下发现的佐证：激活α1-AR（而非ETR）可增加Brd4在肥厚基因启动子和超级增强子区域的染色质占据水平。转录组分析显示，两种受体的激活均可引发相似的基因表达程序，但Brd4抑制在α1-AR激活后能更显著地减弱肥厚相关基因的表达。本研究最终证实，蛋白激酶A（protein kinase A, PKA）是α1-AR刺激Brd4染色质占据所必需的信号分子。Brd4/P-TEFb复合物在不同GPCR通路响应中的差异化作用具有潜在临床价值，因为靶向该复合物的治疗策略目前正被探索用于心力衰竭的治疗。