核受体RXRalpha在EB病毒潜伏感染与鼻咽癌细胞去分化中的作用及机制研究

Undifferentiated nasopharyngeal carcinoma (NPC) is an endemic type of cancer in Southern China. Epstein-Barr virus infection is believed to be an etiological factor for its pathogenesis. Previous studies have shown that ectopic overexpression of EBV oncoproteins would lead to the inhibition of epithelial cell differentiation; however, so far no direct evidence shows the regulatory roles of EBV infection per se on the differentiation properties of nasopharyngeal epithelial (NPE) cells...Our preliminary findings revealed that infection of M81, an EBV strain derived from NPC, contributed to the differentiation-resistant growth phenotype in immortalized NPE cells in vitro. Tumors in vivo grown by EBV-negative tumorigenic NPE cells exhibited well-differentiated histological features, as compared to the EBV-positive counterparts, suggesting the inhibitory functions of EBV in epithelial cell differentiation. We also found a consistently downregulated expression pattern of retinoid X receptor alpha (RXRalpha) upon M81 infection in a panel of immortalized NPE cell lines, as well as in EBV-positive NPC cell lines and xenografts, as compared to the EBV-negative counterpart, suggesting its functional roles in regulating cell differentiation properties. Interestingly, overexpression of RXRalpha in a newly established NPC cell line NPC43 enhanced the expression of EBV lytic genes, indicating the regulatory roles of RXRalpha in maintaining EBV latency in NPE cells...In this proposed study, we will further confirm the functional significance of RXRalpha in mediating EBV infection-induced cell de-differentiation phenotype. We will also examine the expression pattern of RXRalpha in NPC tissues and explore its clinical relevance in NPC. Mechanism study will be conducted to examine the regulatory network between EBV-encoded genes, RXRalpha, and other host differentiation-regulation determinants. Furthermore, we will evaluate the therapeutic potential of targeting RXRalpha in the treatment of NPC using multiple preclinical models. The findings in this pilot study will provide molecular and translational insights in EBV infection-induced differentiation resistance in NPC.

未分化型鼻咽癌高发于中国华南地区，且与EB病毒感染密切相关，迄今尚无直接证据表明EB病毒感染可影响鼻咽上皮细胞的分化表型。本课题的前期研究结果表明，M81-EB病毒株的感染可使永生化鼻咽上皮细胞呈分化耐受性生长表型；与病毒阳性组相比，EB病毒阴性的异种移植瘤组织表现出高分化的组织学特性，进一步证实EB病毒感染对鼻咽上皮细胞分化的抑制作用。核受体RXRalpha在多种EB病毒阳性模型中低表达；与正常组织相比，其在鼻咽肿瘤组织中也呈低表达；过表达RXRalpha可诱导EB病毒进入裂解性复制阶段，这些结果提示RXRalpha在维持EB病毒潜伏感染与调节宿主细胞分化特性中具有功能学意义。本项目拟进一步明确RXRalpha在介导EB病毒感染抑制鼻咽上皮细胞分化中的关键作用及分子机制，为临床上靶向RXRalpha诱导细胞分化及EB病毒裂解性感染、开发鼻咽癌治疗新手段提供新的理论基础。

鼻咽癌高发于中国华南地区。病理学特征分析结果发现，该地区的鼻咽癌绝大多数为未分化型，且与EBV（Epstein-Barr virus）感染密切相关。虽然多项基于单基因的功能学研究结果表明EBV的表达产物（包括蛋白、lncRNA及miRNA等）可对鼻咽癌的发生发展起促进作用，但是，与EBV感染可直接导致B细胞转化为LCL（lymphoblastoid cell line，淋巴母细胞系）的实验结果不同，迄今未知，仍无直接证据证实EBV感染可对鼻咽上皮细胞起到恶性转化的作用。本课题采用全新建立的来源于鼻咽癌的M81-EBV株进行研究，结果表明：（1）M81-EBV感染可使永生化鼻咽上皮细胞在小鼠体内生长成瘤；（2）M81-EBV感染的鼻咽上皮细胞呈分化耐受性生长表型；（3）与阴性组相比，EBV阳性的移植瘤组织呈低分化组织学特性，免疫组化分析显示p63与Ki-67高表达；（4）核受体RXRalpha在多种EBV阳性的鼻咽癌模型中低表达；（5）在鼻咽癌细胞中过表达RXRalpha可诱导EBV裂解性复制基因的表达。本课题的研究结果首次证明了EBV感染可直接对鼻咽上皮细胞产生恶性转化作用，并明确了EBV对于鼻咽癌细胞低分化或未分化表型的功能学意义。同时，也证明了RXRalpha在维持EBV潜伏感染与调节宿主细胞分化特性中的作用。本项目的研究结果将为后续相关研究的开展提供实验基础与模型，并为鼻咽癌的临床治疗提供理论指导意义。