Chen Yen-Ju, Chen Yu-Lian, Chang Yao, Wu Chung-Chun, Ko Ying-Chieh, Tsao Sai Wah, Chen Jen-Yang, Lin Su-Fang
National Institute of Cancer Research, National Health Research Institutes, Miaoli County, Taiwan.
National Institute of Infectious Diseases and Vaccinology, National Health Research Institutes, Tainan, Taiwan.
J Virol. 2017 Jul 12;91(15). doi: 10.1128/JVI.00736-17. Print 2017 Aug 1.
Rta, an Epstein-Barr virus (EBV) immediate-early protein, reactivates viral lytic replication that is closely associated with tumorigenesis. In previous studies, we demonstrated that in epithelial cells Rta efficiently induced cellular senescence, which is an irreversible G arrest likely to provide a favorable environment for productive replications of EBV and Kaposi's sarcoma-associated herpesvirus (KSHV). To restrict progression of the cell cycle, Rta simultaneously upregulates CDK inhibitors and downregulates MYC, CCND1, and JUN, among others. Rta has long been known as a potent transcriptional activator, thus its role in gene repression is unexpected. analysis revealed that the promoter regions of , , and are common in (i) the presence of CpG islands, (ii) strong chromatin immunoprecipitation (ChIP) signals of CCCTC-binding factor (CTCF), and (iii) having at least one Rta binding site. By combining ChIP assays and DNA methylation analysis, here we provide evidence showing that Rta binding accumulated CpG methylation and decreased CTCF occupancy in the regulatory regions of , , and , which were associated with downregulated gene expression. Stable residence of CTCF in the viral latency and reactivation control regions is a hallmark of viral latency. Here, we observed that Rta-mediated decreased binding of CTCF in the viral genome is concurrent with virus reactivation. Via interfering with CTCF binding, in the host genome Rta can function as a transcriptional repressor for gene silencing, while in the viral genome Rta acts as an activator for lytic gene loci by removing a topological constraint established by CTCF. CTCF is a multifunctional protein that variously participates in gene expression and higher-order chromatin structure of the cellular and viral genomes. In certain loci of the genome, CTCF occupancy and DNA methylation are mutually exclusive. Here, we demonstrate that the Epstein-Barr virus (EBV) immediate-early protein, Rta, known to be a transcriptional activator, can also function as a transcriptional repressor. Via enriching CpG methylation and decreasing CTCF reloading, Rta binding efficiently shut down the expression of , , and , thus impeding cell cycle progression. Rta-mediated disruption of CTCF binding was also detected in the latency/reactivation control regions of the EBV genome, and this in turn led to viral lytic cycle progression. As emerging evidence indicates that a methylated EBV genome is a preferable substrate for EBV Zta, the other immediate-early protein, our results suggest a mechanistic link in understanding the molecular processes of viral latent-lytic switch.
Rta是一种爱泼斯坦-巴尔病毒(EBV)即刻早期蛋白,可重新激活与肿瘤发生密切相关的病毒裂解复制。在先前的研究中,我们证明,在上皮细胞中Rta能有效诱导细胞衰老,这是一种不可逆的G期停滞,可能为EBV和卡波西肉瘤相关疱疹病毒(KSHV)的有效复制提供有利环境。为了限制细胞周期进程,Rta同时上调细胞周期蛋白依赖性激酶(CDK)抑制剂并下调MYC、CCND1和JUN等。长期以来,Rta一直被认为是一种强大的转录激活因子,因此其在基因抑制中的作用出乎意料。分析显示, 、 和 的启动子区域在以下方面具有共性:(i)存在CpG岛;(ii)CCCTC结合因子(CTCF)的强烈染色质免疫沉淀(ChIP)信号;(iii)至少有一个Rta结合位点。通过结合ChIP分析和DNA甲基化分析,我们在此提供证据表明,Rta结合导致 、 和 调控区域的CpG甲基化积累并降低CTCF占有率,这与基因表达下调相关。CTCF在病毒潜伏和再激活控制区域的稳定驻留是病毒潜伏的一个标志。在此,我们观察到Rta介导的病毒基因组中CTCF结合减少与病毒再激活同时发生。通过干扰CTCF结合,在宿主基因组中Rta可作为转录抑制因子实现基因沉默,而在病毒基因组中Rta通过消除CTCF建立的拓扑限制,作为裂解基因位点的激活因子发挥作用。CTCF是一种多功能蛋白,以多种方式参与细胞和病毒基因组的基因表达及高级染色质结构。在基因组的某些位点,CTCF占有率和DNA甲基化相互排斥。在此,我们证明,已知作为转录激活因子的爱泼斯坦-巴尔病毒(EBV)即刻早期蛋白Rta也可作为转录抑制因子发挥作用。通过富集CpG甲基化并减少CTCF重新加载,Rta结合有效关闭了 、 和 的表达,从而阻碍细胞周期进程。在EBV基因组的潜伏/再激活控制区域也检测到Rta介导的CTCF结合破坏,这进而导致病毒裂解周期进程。由于新出现的证据表明,甲基化的EBV基因组是EBV另一种即刻早期蛋白Zta的优选底物,我们的结果提示了在理解病毒潜伏-裂解转换分子过程中的一种机制联系。