RNA干擾(RNA interference, RNAi)現象，是一種雙股RNA (double-stranded RNA, dsRNA)引起的基因沉默現象。在許多生物中都已證實有RNAi現象發生，包括阿拉伯芥、菸草、線蟲、果蠅、斑馬魚以及老鼠等。由線蟲(C. elegans)的研究發現Dicer基因缺失的幼體會使得線蟲體內一種類似siRNA (small interfering RNA)，具有調控發育功能的stRNA (small temporal RNA)無法產生，導致幼蟲不能正常發育為成蟲。也有研究指出RNAi所造成的基因沉默現象屬於植物抗病毒機制之一種，因此RNAi可能與生物抵抗病毒感染或是與跳躍子(transposon)的調控有關。本實驗以人類肺癌細胞株(A549)及野生型腺病毒(adenovirus)第五型(Ad5)為模式探討RNAi與哺乳動物細胞對抗病毒感染機制之關係。首先建構表現雙股E1A (dsE1A)基因的載體，並轉染至A549細胞中；將Ad5感染表現dsE1A基因的細胞(A549/β actin-dsE1A-9)時，A549/β actin-dsE1A-9細胞在相同病毒濃度時對病毒感染的細胞病變程度明顯比A549細胞低。為了避免干擾素造成的非專一性基因沉默，在培養細胞的培養液中加入RNase L抑制劑(ZnCl2)，發現A549/β actin-dsE1A-9細胞對病毒感染的細胞病變程度更為降低；顯示此抗病毒機制有干擾素引起的非專一性基因沈默參與。為了進一步證明實驗中對病毒的抗性與RNAi有關，有序列專一性的特性，改用與Ad5不同群組的Ad3病毒感染細胞，A549/β actin-dsE1A-9與A549細胞對Ad3的細胞病變程度並無差異，顯示細胞病變具有序列專一性，並且siRNA的分析實驗結果也印證RNAi現象確實發生。因此RNAi現象不但可在人類細胞株中發現，亦為哺乳動物細胞抵抗病毒感染的機制之一。

RNA interference (RNAi) phenomenon is a double-stranded RNA induced sequence specific gene silencing. RNAi was also observed in Arabidopsis, tobacco, nematode, Drosophila, zebra fish, and mice. Recent studies in C. elegans have identified a protein, Dicer, that specifically cleaves double stranded RNA into siRNA (small interfering RNA). The dicer deficient larvae could not produce a siRNA like RNA, stRNA (small temporal RNA), to transform from larvae to adult. Genetic studies have extended RNAi to either one of anti-viral effects in plant or transposon regulation. In this study, we used A549 human lung carcinoma cell line and wild type adenovirus type 5 (Ad5) as a model to characterize the relationships between anti-viral effect and RNAi in mammalian cells. A549/β actin-dsE1A-9 cells, which carried partial Ad5 E1A gene, and parental A549 cells exhibited distinct responses when they were infected with Ad5. A549/β actin-dsE1A-9 cells were more resistant to Ad5 infection at the same titer compared with A549 cells. Interferon responses may be induced by virus infection in mammalian cells, resulting in non-sequence-specific gene silencing or even apoptosis. Further experiment by addition of RNase L inhibitor ZnCl2 showed promotion of the anti-viral effect in A549/β actin-dsE1A-9 cells, therefore excluding the effect of IFN-induced non-sequence-specific gene silencing. To confirm that the anti-viral effect was sequence-specific to RNAi, Ad3, which belongs to a different group of adenovirus, was used to infect A549/β actin-dsE1A-9 cells in comparison to Ad5. The susceptibilities of A549/β actin-dsE1A-9 and A549 cells to Ad3-induced cytopathic effect were similar. Further detection of the Ad5 E1A gene siRNA showed RNAi phenomenon happened, demonstrating that the anti-viral effect is sequence-specific and induced by RNAi.

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