Primers used in RTqPCR.

The Human gamma-herpesviruses Kaposi’s sarcoma herpesvirus (KSHV) and Epstein-Barr virus (EBV) are causally associated to a wide range of cancers. While the default infection program for these viruses is latent, sporadic lytic reactivation supports virus dissemination and oncogenesis. Despite its relevance, the repertoire of host factors governing the transition from latent to lytic phase is not yet complete, leaving much of this complex process unresolved. Here we show that heat shock factor 2 (HSF2), a transcription factor involved in regulation of stress responses and specific cell differentiation processes, promotes gamma-herpesvirus lytic gene expression. In lymphatic endothelial cells infected with KSHV and in gastric cancer cells positive for EBV, ectopic HSF2 enhances the expression of lytic genes; While knocking down HSF2 significantly decreases their expression. HSF2 overexpression is accompanied by decreased levels of repressive histone marks at the promoters of the lytic regulators KSHV ORF50 and EBV BZLF1, both characterized by poised chromatin features. Our results demonstrate that endogenous HSF2 binds to the promoters of KSHV ORF50 and EBV BZLF1 genes and shifts the bivalent chromatin state towards a more transcriptionally permissive state. We detected HSF2 binding to the ORF50 promoter in latent cells, in contrast, in lytic cells, HSF2 occupancy at the ORF50 promoter is lost in conjunction with its proteasomal degradation. These findings identify HSF2 as a regulator of gamma-herpesvirus lytic gene expression in latency and offer new insights on the function of this transcription factors at poised gene promoters, improving our understanding of its role in differentiation and development.

人类γ疱疹病毒科的卡波西肉瘤疱疹病毒（Kaposi’s sarcoma herpesvirus, KSHV）与EB病毒（Epstein-Barr virus, EBV）可引发多种恶性肿瘤。尽管这类病毒的默认感染程序为潜伏感染，偶发的裂解性激活却可支持病毒播散与肿瘤发生。尽管该过程具有重要研究价值，但调控潜伏态向裂解态转变的宿主因子完整集合目前仍未完全明确，使得这一复杂过程的诸多环节尚未被阐明。本研究证实，热休克因子2（heat shock factor 2, HSF2）——一种参与应激反应调控与特定细胞分化过程的转录因子——可促进γ疱疹病毒的裂解基因表达。在感染KSHV的淋巴管内皮细胞以及EBV阳性胃癌细胞中，异位表达HSF2可增强裂解基因的表达；而敲低HSF2则会显著降低其表达水平。HSF2过表达伴随裂解调控因子KSHV ORF50与EBV BZLF1启动子区域的抑制性组蛋白修饰水平降低，这两个基因均以 poised染色质（poised chromatin）为特征。本研究结果表明，内源性HSF2可结合KSHV ORF50与EBV BZLF1基因的启动子区域，并将二价染色质状态转变为转录更具活性的状态。我们在潜伏感染细胞中检测到HSF2结合至ORF50启动子区域；与之相反，在裂解感染细胞中，HSF2在ORF50启动子处的结合会随着其蛋白酶体降解而消失。上述发现将HSF2鉴定为潜伏感染期γ疱疹病毒裂解基因表达的调控因子，并为该转录因子在 poised染色质基因启动子处的功能提供了全新见解，加深了我们对其在细胞分化与发育中作用的理解。