BRD4-PRC2 Represses Transcription of T-helper 2-Specific Negative Regulators during T-cell Differentiation

BRD4 is a well-recognized transcriptional activator, but how it regulates gene transcriptional repression in a cell-type specific manner has remained elusive. In this study, we report that BRD4 works with Polycomb repressive complex 2 (PRC2) to repress transcriptional expression of the T-helper 2 (Th2)-negative regulators Foxp3 and E3-ubiqutin ligase Fbxw7 during lineage-specific differentiation of Th2 cells from mouse primary naïve CD4+ T cells. Brd4 binds to the lysine-acetylated-EED subunit of the PRC2 complex via its second bromodomain (BD2) to facilitate histone H3 lysine 27 trimethylation (H3K27me3) at target gene loci and thereby transcriptional repression. We found that Foxp3 represses transcription of Th2-specific transcription factor Gata3, while Fbxw7 promotes its ubiquitination-directed protein degradation. BRD4-mediated repression of Foxp3 and Fbxw7 in turn promotes BRD4- and Gata3-mediated transcriptional activation of Th2 cytokines including Il4, Il5 and Il13. Chemical inhibition of the BRD4 BD2 induces transcriptional de-repression of Foxp3 and Fbxw7, and thus transcriptional down-regulation of Il4, Il5 and Il13, resulting in inhibition of Th2 cell lineage differentiation. Our study presents a previously unappreciated mechanism of BRD4's role in orchestrating a Th2-specific transcriptional program that coordinates gene repression and activation, and safeguards cell lineage differentiation.

BRD4是公认的转录激活因子，但其如何以细胞类型特异性方式调控基因转录抑制的机制迄今仍难以阐明。本研究发现，BRD4可与多梳蛋白抑制复合体2（Polycomb repressive complex 2, PRC2）协同，在小鼠原代初始CD4+ T细胞向T辅助细胞2（T-helper 2, Th2）细胞的谱系特异性分化过程中，抑制Th2负调控因子叉头框蛋白P3（Foxp3）以及E3泛素连接酶Fbxw7的转录表达。BRD4可通过其第二个溴结构域（BD2）结合PRC2复合体中经赖氨酸乙酰化修饰的EED亚基，以促进靶基因位点处的组蛋白H3赖氨酸27三甲基化（H3K27me3），进而介导转录抑制。本研究发现，Foxp3可抑制Th2特异性转录因子Gata3的转录，而Fbxw7则可通过泛素化途径介导其蛋白质降解。BRD4介导的Foxp3与Fbxw7转录抑制，反过来可促进BRD4及Gata3介导的Th2细胞因子（包括白细胞介素4（Il4）、白细胞介素5（Il5）及白细胞介素13（Il13））的转录激活。化学抑制BRD4的BD2结构域可诱导Foxp3与Fbxw7的转录去抑制，进而导致Il4、Il5及Il13的转录下调，最终抑制Th2细胞的谱系分化。本研究揭示了一种此前未被认知的BRD4功能机制：BRD4可协同调控Th2特异性转录程序，同时协调基因的转录抑制与激活，从而保障细胞谱系分化的正常进程。