BGLF4蛋白質激酶對NF-κB抑制EB病毒溶裂期啟動子之調控

Abstract

EB 病毒 (Epstein-Barr virus, EBV) 為普遍感染人類的γ型疱疹病毒，研究發現其與許多人類惡性腫瘤的發生有相關性。BGLF4 是EB 病毒目前唯一已知的serine/threonine 蛋白質激酶 (protein kinase)，是在溶裂期早期表現的基因產物，具自我磷酸化的能力。研究也指出BGLF4 可藉對細胞或病毒的轉錄因子進行磷酸化修飾而調控其轉活化活性。實驗室之前觀察到BGLF4 可抑制細胞內重要的轉錄因子NF-κB 的活性。研究也指出NF-κB 的活性對於EB 病毒潛伏期的維持極為重要。NF-κB 可抑制EB 病毒前早期轉活化因子 (transactivators) Zta 與Rta 活化溶裂期基因啟動子 (lytic promoter) BHLF1 promoter 的活性，影響EB 病毒溶裂期的進行。推測NF-κB 的活性對於EB 病毒潛伏期與溶裂期的轉換扮演重要的調控角色。本研究因此欲探討BGLF4 抑制NF-κB 活性的機制及對EB 病毒溶裂期進行的影響。利用轉染方式外送BGLF4進入細胞表現，分別以poly (I:C) 與TNF-α 活化NF-κB，再利用螢光酵素報導基因檢測法 (Luciferase assay) 檢測細胞內NF-κB 活性高低，發現BGLF4 可明顯地抑制NF-κB 的活性。本篇研究亦觀察到NF-κB 會抑制Zta、Rta 或Zta 與Rta 共同活化之BHLF1 promoter 的活性，且BGLF4 對NF-кB 抑制Zta 的現象有拮抗能力。進一步探討BGLF4 抑制NF-κB 活性的機制，觀察BGLF4是否影響NF-κB 訊息傳導。結果顯示BGLF4 並不影響IκBα 的降解與NF-κB 的入核。NF-κB 常會與其他轉錄因子，如CBP/p300 及RNA 聚合酶等形成巨大的強化體 (enhanceosome) 活化基因啟動。近年來在NF-κB 強化體中發現一個新的輔活化子UXT，具穩定NF-κB 強化體的能力。實驗室之前亦觀察到UXT 能與BGLF4 產生交互作用，在試管內可被BGLF4 磷酸化。進一步以共同免疫沉澱實驗結果證明BGLF4 會干擾NF-κB 與輔活化子UXT 之間的交互作用。由上述實驗結果推測BGLF4 可能藉由破壞NF-κB 強化體的完整性而降低NF-κB 的活性，以幫助EB病毒溶裂期順利進行。

Epstein-Barr virus (EBV), which is widely spread in general population throughout the world, is a gammaherpesvirus linked to human malignant diseases. BGLF4, expressed in the early stage of EBV lytic cycle, is a Ser/Thr protein kinase which can autophosphorylate and phosphorylate cellular and viral transcription activators to modulate their transactivation activities. Our investigation indicated BGLF4 suppresses the transcriptional activity of NF-κB, a major transcription factor in cells. It was known that NF-κB activity is important for the maintenance of EBV latency through down-regulation of the transactivation activities of EBV immediate early transactivators, Zta and Rta, on BHLF1 promoter. It is suggested that NF-кB plays an important role in the switch between latency and lytic cycle of EBV replication. The specific aim of this study is to reveal how BGLF4 inhibits NF-κB activity and its effects on EBV lytic replication. By luciferase assay, we found that overexpression of BGLF4 dramatically represses cellular NF-кB activity induced by poly(I:C) and TNF-α. This study also proved that NF-кB inhibits the transactivation activities of Zta and Rta respectively, and BGLF4 can rescue the NF-κB-mediated inhibition of Zta. To reveal the mechanism of how BGLF4 inhibits NF-кB activity, Western blot was performed to show that BGLF4 neither affects the degradation of IκBα nor impedes NF-кB nuclear translocation. Recently, a novel coactivator, UXT, was found in NF-κB enhanceosome. It is proved that UXT can stabilize NF-κB enhanceosome to promote NF-κB nuclear function. Becauese BGLF4 was found to interact with and phosphorylate UXT in our previous study, possible effects of BGLF4 on UXT and NF-κB p65 interaction was investigated. Coimmunoprcipitation data show that BGLF4 interrupts the interaction between NF-кB and its coactivator, UXT, suggesting that BGLF4 may interrupt the stability and the integrity of NF-кB enhanceosome to down-regulate NF-кB activity.