The RNA-Specific Adenosine Deaminase ADAR1 Inhibits Human Protein Kinase R Activation.
The RNA-Specific Adenosine Deaminase ADAR1 Inhibits Human Protein Kinase R Activation.
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RNA 特异性腺苷脱氨酶 ADAR1 抑制人蛋白激酶 R 激活。
DOI：
10.1089/vim.2018.0056
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发表时间：
2018
期刊：
影响因子：
2.2
通讯作者：
Wang,Qingde
中科院分区：
文献类型：
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作者：
Wang,TonyT;Li,ZGalvin;Wang,Qingde
ADAR 1是一种RNA编辑酶，通过嘌呤C-6的水解脱氨基作用催化双链RNA（dsRNA）中腺苷（A）转化为肌苷（I）。A-to-I编辑是核苷酸取代编辑的一种形式，因为I在翻译过程中被核糖体解码为鸟苷（G）而不是A，在RNA依赖性RNA复制过程中被聚合酶解码为鸟苷（G）。此外，A-to-I编辑可以改变RNA结构的稳定性，因为I：U错配比A：U碱基对更不稳定。ADAR 1含有三个dsRNA结合基序，随后是高度保守的C-末端催化结构域和锌指结合结构域。人ADAR 1的两种主要变体由于不同的启动子使用而存在，并且发现于不同的细胞内位置。全长蛋白质ADAR 1 p150由干扰素（IFN）诱导型启动子表达并定位于细胞质，而N-末端截短变体p110定位于细胞核。ADAR 1的RNA编辑功能依赖于其RNA结合结构域和脱氨催化结构域的活性。大多数经过ADAR1编辑的RNA转录物存在于转录RNA的非编码区，特别是重复序列，如占人类基因组10%的Alu元件，而ADAR1也修饰信使RNA（mRNA）和microRNA“种子”序列上的蛋白质密码子和剪接位点。ADAR 1的全基因敲除（KO）在小鼠中是胚胎致死的，由于IFN的大量产生、胚胎肝造血细胞的损失和广泛的细胞凋亡，胎儿在性交后11.5 - 12.0天死亡，尽管细胞死亡的分子原因仍然未知。我们之前报道过ADAR1阻断内源性dsRNA激活视黄酸诱导基因I样受体依赖性先天免疫途径（6）。对来自ADAR 1 −/−胚胎肝造血干细胞的微阵列数据的全基因组转录组分析显示，ADAR 1基因缺失与I型和II型干扰素刺激基因（ISG）的基因表达特征密切相关。有趣的是，在dsRNA诱导的RNA结合蛋白基因表达中，只有IFN诱导的基因在ADAR 1 −/−细胞中上调。在胚胎组织中，IFN-a和IFN-b的蛋白质水平也显著增加，尽管未检测到III型IFN（即IFN-c）。这些发现表明ADAR 1参与造血细胞中IFN和ISGs的调节。事实上，随后的研究表明，ADAR 1在抑制"内源性" RNA激活MDA5-MAVS介导的先天性免疫途径中起重要作用; MDA5 KO挽救了ADAR 1 KO诱导的I型IFN产生，MDA5-ADAR 1双敲除（DKO）也挽救了胚胎致死性（2）。令人惊讶的是，IFNAR1 KO未能拯救ADAR1 KO动物免于死亡，这意味着ADAR1 KO诱导的细胞死亡不仅仅归因于IFN的过度产生（3）。在这些dsRNA结合蛋白中，蛋白激酶R（PKR）在病毒感染期间被激活，然后磷酸化真核生物翻译起始因子2（eIF2）的α亚基，导致翻译和病毒复制的抑制。最近的一份报告显示，人ADAR 1通过阻断PKR活化，在I型IFN应答期间防止翻译关闭（1）。作者观察到，ADAR1 KO神经元祖细胞表现出MDA5依赖性IFN产生、PKR激活和细胞死亡，这提出了一种有趣的可能性，即可能在没有ADAR1的情况下，内源性RNA参与并激活PKR，导致细胞死亡。如果是真的，这可能提供了一个机械的解释...
ADAR1, AN RNA EDITINg ENzYME, catalyzes the conversion of adenosine (A) to inosine (I) in double-stranded RNA (dsRNA) by hydrolytic deamination of purine C-6. A-to-I editing is a form of nucleotide substitution editing, because I is decoded as guanosine (G) instead of A by ribosomes during translation and by polymerases during RNA-dependent RNA replication. In addition, A-to-I editing can alter RNA structure stability as I: U mismatches are less stable than A: U base pairs. ADAR1 contains three dsRNA binding motifs, followed by a highly conserved C-terminal catalytic domain and a zinc finger binding domain. Two major variants of human ADAR1 exist due to different promoter usage and are found in distinct intracellular locations. The full-length protein, ADAR1 p150, is expressed from an interferon (IFN) inducible promoter and localizes to the cytoplasm, whereas the N-terminal truncation variant, p110, localizes to the nucleus. The RNA editing function of ADAR1 is dependent on the activities of its RNA binding domain and the deamination catalytic domain. Most RNA transcripts subjected to ADAR1 editing were found in noncoding regions of transcribed RNAs, especially repeated sequences, such as Alu elements that compose 10% of the human genome, whereas ADAR1 also modifies the protein codon and splicing sites on messenger RNAs (mRNAs) and microRNA ‘‘seed’’sequences. Global knockout (KO) of ADAR1 is embryonic lethal in mice, with fetal demise at 11.5–12.0 days after coitus due to massive production of IFN, loss of embryonic liver hematopoietic cells, and widespread apoptosis, although the molecular cause of cell death remains unknown. We have previously reported that ADAR1 blocks endogenous dsRNA from activating the retinoic acid-inducible gene I-like receptor-dependent innate immune pathway (6). A genome-wide transcriptome analysis of the microarray data from ADAR1−/− embryonic liver hematopoietic stem cells revealed that ADAR1 gene deletion was strongly associated with a gene expression signature of types I and II interferon-stimulated genes (ISGs). Interestingly, among the dsRNA-induced gene expression of RNA binding proteins, only IFN-inducible genes were upregulated in ADAR1−/− cells. Protein levels of IFN-a and IFN-b were also dramatically increased in the embryonic tissue, although type IIIFN (ie, IFN-c) was not detectable. These findings implicated ADAR1 in the regulation of IFN and ISGs in hematopoietic cells. Indeed, subsequent studies demonstrated that ADAR1 plays an essential role in suppressing ‘‘endogenous’’RNA from activating the MDA5-MAVS-mediated innate immune pathway; MDA5 KO rescues ADAR1 KO-induced type I IFN production and MDA5-ADAR1 double knockout (DKO) also rescued the embryonic lethality (2). Surprisingly, IFNAR1 KO failed to rescue ADAR1 KO animals from dying, implying that ADAR1 KO-induced cell death is not solely attributed to overproduction of IFN (3). Among those dsRNA binding proteins, protein kinase R (PKR) is activated during viral infections and then phosphorylates the a subunit of eukaryotic translation initiation factor 2 (eIF2), leading to inhibition of translation and viral replication. A recent report showed that human ADAR1 prevents translational shutdown during the type I IFN response by blocking PKR activation (1). The authors observed that ADAR1 KO neuronal progenitor cells exhibit MDA5-dependent IFN production, PKR activation, and cell death, raising an interesting possibility that perhaps without ADAR1, endogenous RNA engages and activates PKR, resulting in cell death. If true, this could provide a mechanistic explanation of …