EB病毒小RNA抑制FASTK表現之探討

Abstract

EB病毒（Epstein-Barr virus, EBV）感染全球超過90%人口，為致癌性之疱疹病毒，且體外實驗中具備使初代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。EB病毒小RNA（Epstein-Barr virus encoded small RNAs, EBERs）分為EBER1與EBER2，是EB病毒潛伏期之轉錄產物，文獻指出EBERs具備中止PKR引起的細胞凋亡與誘發生長因子幫助細胞生長之能力，顯示EBERs於協助EB病毒感染細胞的存活甚至癌化過程扮演重要角色。FASTK（Fas-activated serine/threonine kinase）是一細胞內持續磷酸化之蛋白質，FASTK會透過磷酸化TIA-1和促使IAP家族表現抵抗細胞凋亡，亦參與Fas和FGFR2 mRNA選擇性剪接，此外小鼠模式中還發現FASTK與幫助促發炎細胞激素分泌有關。已有研究顯示FASTK基因表現於鼻咽癌檢體中顯著下降，本篇研究則觀察到FASTK之蛋白質表現在EB病毒不朽化的LCL中顯著下降，但其mRNA表現卻不受影響；此外於感染EB病毒之KMH2細胞株KMH2-EBV內同樣發現只有FASTK蛋白質表現受到調控；而進一步在KMH2細胞株單獨轉染EB病毒基因後得知EBER1與EBER2可以抑制FASTK蛋白質表現但不影響其mRNA總量。綜合上述EBERs與FASTK皆具影響細胞激素表現功能，進一步探討EBERs是否透過抑制FASTK調控細胞激素表現。首先觀察IL-10 mRNA表現在KMH2-EBV、KMH2細胞株轉染EBERs或剔減FASTK後皆有顯著上升，反之若在LCL轉染FASTK則會抑制IL-10 mRNA表現量。接著更證明EBERs是透過IRF3磷酸化方式引發IL-10 mRNA表現，而FASTK會受MAVS調控並具抑制IRF3磷酸化之能力。IL-10已被報導能夠協助病毒逃避免疫攻擊與幫助細胞生長，上述結果暗示EBERs可能透過抑制FASTK促使IL-10細胞激素表現，進而協助EB病毒感染細胞之存活。

Epstein-Barr virus (EBV) is an oncogenic herpesvirus that persistently infects over 90% human population. EBV infects primary B cells in vitro and transforms them into immortalized lymphoblastoid cell line (LCL). Epstein-Barr virus encoded small RNAs (EBERs), EBER1 and EBER2, are viral transcripts in latent stage. It is well documented that EBERs can prevent PKR mediated apoptosis and induce growth cytokines to promote cell growth. Therefore EBERs play an important role in survival and oncogenesis of EBV infected cells. Fas-activated serine/threonine kinase (FASTK) is a cellular continuous phosphorylation phosphoprotein. FASTK is considered as a survival protein via TIA-1 phosphorylation and IAP family induction. FASTK also involves in Fas and FGFR2 mRNA alternative splicing. Besides many proinflammatory cytokines secretion are supported by FASTK in mice model. It has been found that FASTK mRNA is down-regulated in nasopharyngeal carcinoma (NPC) tissue. This study finds that FASTK protein but not mRNA is down-regulated in EBV transformed LCLs and KMH2-EBV. Transfection of EBER1 and EBER2 in KMH2 are down-regulated FASTK protein but not mRNA significantly. Owing to EBERs and FASTK are able to regulate cytokines expression, we begin to examine whether EBERs regulate cytokines through FASTK. We find that IL-10 mRNA is up-regulated in KMH2-EBV, EBERs transfected or FASTK knockdown KMH2. Moreover, IL-10 mRNA is down-regulated in FASTK transfected LCLs. We further determine that IL-10 expression is through EBERs mediated IRF3 phosphorylation. FASTK is regulated by MAVS and inhibits IRF3 phosphorylation as well. IL-10 is an essential factor for immune evasion and growth promotion. The results indicate that EBERs may down-regulate FASTK to facilitate IL-10 expression and promote EBV infected cells survival.