STAT3在抗病毒反應中所扮演的角色的研究

Abstract

在先天性的免疫力(innate immunity)中的第一型干擾素(type I IFN)是一種用來對抗病毒入侵的強效細胞素(cytokine)。目前已知第一型干擾素的刺激會活化細胞中的三種訊號傳導與轉錄子(STAT)蛋白：分別是STAT1、STAT2和STAT3。利用基因剔除小鼠的方式已證實STAT1和STAT2在第一型干擾素引起的抗病毒反應扮演非常重要的正調控角色，但是STAT3的角色並不清楚。為了研究這個議題,我們利用STAT3基因剔除的小鼠胚胎纖維母細胞株(MEF cell line)和小鼠骨髓衍生的初代巨噬細胞(primary BMM)證實了第一型干擾素的訊號在STAT3基因剔除的狀況下會增強。經由微陣列分析(microarray)和反轉錄即時聚合酶鏈式反應(RT-QPCR)的分析結果，我們發現第一型干擾素下游的抗病毒基因的表現量在STAT3基因剔除的小鼠細胞中比對照組的細胞高。我們也進一步地利用腦心肌炎病毒（EMCV）或疱疹性口腔炎病毒（VSV）直接感染細胞。實驗結果發現，STAT3基因剔除的小鼠細胞表現出較強的抗病毒能力，因此存活率比對照組細胞高且病毒效價(viral titer)比對照組細胞低。在將具有正常功能的STAT3回復到STAT3基因剔除的小鼠肧胎纖維母細胞株或利用介白素6 (IL-6) 的刺激來高度活化細胞中的STAT3，我們發現STAT3會抑制第一型干擾素的訊號以及其引起的抗病毒反應。證實了STAT3活性的增加的確可以負調控第一型干擾素的訊號和抗病毒反應。令人驚訝的是，我們發現不具有轉錄能力的STAT3蛋白質N端1-134 a.a. 就足以具有抑制效果。我們同時也發現了無論是腦心肌病毒的感染或是類鐸受體作用劑(TLR agonist)的刺激下,STAT3基因剔除的小鼠細胞會產生比較高量的第一型干擾素。我們進一步利用報導基因表現分析（reporter assay）證實了STAT3可能具有抑制聚肌胞苷酸 (poly (I:C))或干擾素轉接蛋白（MAVS）下游的第一型干擾素基因表現能力。以上的研究結果證實了STAT3負調控抗病毒反應的作用機制可能有兩種。第一，STAT3可以直接負調控第一型干擾素的訊息傳導以及其誘發的抗病毒能力。第二，STAT3也具有負調控第一型干擾素基因表現的能力，因而逹到抑制抗病毒反應的效果。以上的實驗結果證實了STAT3在第一型干擾素或是Toll樣受體所誘發的抗病毒反應中扮演著負調控的角色。

Type I interferons (IFNs) are potent cytokines for innate immunity to combat viral infections. It has been well documented that signal transducer and activator of transcription (STAT) proteins such as STAT1, STAT2, and STAT3 are activated upon IFN-α/β stimulation. While essential roles of STAT1 and STAT2 in type I IFN-mediated antiviral responses are demonstrated in gene-targeting mice, the role of STAT3 remains unclear. Using STAT3KO mouse fibroblasts (MEFs) and primary bone-marrow-derived macrophages (BMMs) lacking STAT3, we demonstrated that IFN-α signals in STAT3KO MEFs or BMMs were enhanced. Both microarray and RT-QPCR analysis revealed that the induction of several IFN-a-inducible antiviral-associated genes was higher in STAT3KO cells than that in wild-type cells in response to IFN-α. Moreover, STAT3KO cells also displayed increased antiviral responses to encephalomyocarditis virus (EMCV) or vesicular stomatitis virus (VSV) infections with increased survival levels and decreased viral titers. STAT3 restoration to STAT3KO MEFs or STAT3 hyper-activation by IL-6 stimulation attenuated the enhanced type I IFN signals and antiviral responses, suggesting that STAT3 negatively regulates these two responses. Surprisingly, STAT3 1-134 a.a. was sufficient to reverse the hyper-antiviral activity. We also found that EMCV infection or TLR agonists stimulation induces the increased level of IFN-α/β in the absence of STAT3 in vitro or in vivo. Additionally, STAT3 might suppress the IFN-β promoter activity upon poly (I:C) treatment and mitochondrial antiviral signaling (MAVS)-mediated signaling pathway. Therefore, at least, two mechanisms might contribute to the enhanced antiviral activity, namely enhanced signaling of type I IFNs and increased IFN-α/β production, in the cells lacking STAT3. Taken together, these results suggest an important and yet previously uncharacterized role of STAT3 in IFN-α/β and TLR-mediated antiviral responses.