EB病毒對EphA4的調控及其對EB病毒相關之淋巴增殖疾病的致病性之影響

Abstract

EB病毒 (Epstein-Barr virus, EBV)，為致癌性之疱疹病毒，與許多淋巴性細胞異常之疾病有高度相關性，包括柏金氏淋巴瘤 (Burkitt’s lymphoma)、霍金氏症 (Hodgkin disease)、瀰漫性大B細胞淋巴瘤 (diffuse large B-cell lymphoma，DLBCL)、移植後淋巴增生症 (post-transplant lymphoproliferative diseases，PTLD)、等疾病，在體外實驗中具備使人類B細胞 (primary B cell) 轉形成不朽化淋巴母細胞株 (lymphoblastoid cell line, LCL) 的能力。近年來，在EB病毒相關之腫瘤形成 (neoplasia) 中發現，受體酪胺酸激酶 (Receptor tyrosine kinase, RTK) 扮演了一個很重要的角色，然而EB病毒是如何藉由調控B細胞內受體酪胺酸激酶，進而導致B細胞惡性腫瘤形成的機制，目前並未十分清楚。本篇我們發現人類B細胞受到EB病毒感染會造成受體酪胺酸激酶中隸屬於最大Eph家族的EphA4在轉錄以及轉譯的表現量下降，從外送 (overexpression) 與抑制 (knockdown) 的實驗證明LMP1造成EphA4降低。在細胞調控機制方面，LMP1透過ERK路徑去促進Sp1來達到抑制EphA4啟動子之活性 (promoter activity)；在功能上面，外送EphA4會抑制LCL的細胞生長；在病理上，我們發現EB病毒陰性的扁桃腺檢體可偵測到EphA4之表現，而EB病毒陽性的PTLD檢體則無EphA4表現，除此之外，利用免疫化學染色 (Immunochemical staining) 之方法，在有無感染EB病毒的DLBCL檢體中發現，EphA4的表現與EB病毒的感染呈負相關；從資料庫分析DLBCL病人的存活率，發現低度表現EphA4的病人，其存活率較差。我們的發現為B細胞中的EphA4如何被致癌蛋白LMP1調控，提供了一個嶄新的機制，並發掘EphA4在B細胞中的功能，這些研究成果能提供EphA4在EB病毒陽性之PTLD及DLBCL所扮演的角色一些新的觀點。

Epstein-Barr virus (EBV), an oncogenic human virus, is associated with several lymphoproliferative disorders, including Burkitt’s lymphoma, Hodgkin’s disease, diffuse large B-cell lymphoma (DLBCL) and post-transplant lymphoproliferative disorder (PTLD). In vitro, EBV transforms primary B cells into lymphoblastoid cell lines (LCLs). Recently, several studies have shown that receptor tyrosine kinases (RTKs) play important roles in EBV-associated neoplasia. However, details of the involvement of RTKs in EBV-regulated B cell neoplasia and malignancies remain largely unclear. Here, we found that EphA4, which belongs to the largest RTK Eph family, was downregulated in primary B cells post-EBV infection at the transcriptional and translational levels. Overexpression and knockdown experiments confirmed that EBV-encoded latent membrane protein 1 (LMP1) was responsible for this EphA4 suppression. Mechanistically, LMP1 triggered the ERK pathway and promoted Sp1 to suppress EphA4 promoter activity. Functionally, overexpression of EphA4 prevented LCLs from proliferation. Pathologically, the expression of EphA4 was detected in EBV-negative tonsils but not in EBV-positive PTLD. In addition, an inverse correlation of EphA4 expression and EBV presence was verified by immunochemical staining of EBV-positive and EBV-negative DLBCL, suggesting EBV infection was associated with reduced EphA4 expression. Analysis of a public dataset showed that lower EphA4 expression was correlated with a poor survival rate of DLBCL patients. Our findings provide a novel mechanism by which EphA4 can be regulated by an oncogenic LMP1 protein and explore its possible function in B cells. The results provide new insights into the role of EphA4 in EBV-positive PTLD and DLBCL.