C型肝炎病毒非結構性蛋白質NS5A調節細胞端粒酶基因之分子機制

Abstract

C型肝炎病毒（hepatitis C virus, HCV）屬於黃熱病毒科（Flaviviridae），已知自然界中唯一的宿主是人類，主要感染肝臟細胞。目前估計全球3%的人口，約兩億多人是C型肝炎的帶原者。病患一經感染C型肝炎則有約八成的比例會變成慢性的帶原者，這些帶原者中又有相當高的比例會發展成肝硬化或肝纖維化，最終形成肝癌。根據統計，C型肝炎已是美國導致病患進行換肝手術排名第一的因素，在世界各地都對人力及資源造成非常大的傷害。 目前對於C型肝炎病毒致病機制的了解尚未十分透徹。根據研究，C型肝炎病毒在宿主細胞內會利用宿主的轉譯工具來合成病毒自身的蛋白質，主要分為結構性蛋白質及非結構性蛋白質(structural and non-structural proteins)兩種。其中結構性蛋白質的核心蛋白質(core protein)及屬於非結構性蛋白質的非結構性蛋白質NS3及NS5A都被認為具有導致細胞癌化的能力。核心蛋白及非結構性蛋白質3在肝細胞中的功能已有許多研究，但非結構性蛋白質5A尚有許多待釐清的部分。目前已知NS5A會參予雙股核醣核酸活化蛋白質磷酸酶(dsRNA-activated protein kinase, PKR)的抑制作用，科學家對於其抑制的詳細機制還有爭論，但可以確定的是一旦PKR被抑制則會降低病人對干擾素(interferon)的敏感度，使C型肝炎的臨床治療效果下降。

最近的研究指出C型肝炎的患者其肝細胞染色體兩端的端粒(telomere)較未受C型肝炎病毒感染的細胞為短，而端粒的縮短已被認為會造成細胞分裂時染色質的不穩定，發生染色質分配錯誤且斷裂的情況而加速癌化。本篇實驗利用luciferase assay、RT-PCR等實驗證實NS5A會抑制人類細胞端粒酶基因(hTERT)的轉錄能力，而藉PKR結合序列突變的NS5A表現質體及磷酸激酶抑制藥物，發現NS5A對PKR活化的抑制是調控hTERT轉錄作用的主要路徑。另外hTERT的啟動序列中Sp1 transcription factor的表現量也會被NS5A所抑制，可能藉此造成hTERT的轉錄下降而影響telomerase的活性。本論文對於此調控機制的證實或許可對C型肝炎致病原因做更進一步的解釋。

Hepatitis C virus (HCV) mainly utilizes human hepatocytes as its natural host. Approximately 200 million people, 3% of the world's population, are infected with HCV nowadays. Once infection established, HCV has a high propensity to yield chronic infection that often leads to liver fibrosis, cirrhosis, and ultimately hepatocellular carcinoma (HCC). End-stage liver disease due to HCV infection has become the leading indication for liver transplantation in the United States, causing a vital threat to human health worldwild. The genome of HCV is a positive-sense 9.6 kb RNA molecule, comprising 5' and 3' untranslated regions (UTRs) flanking a single open reading frame with approximately 3000 amino acid residues that encodes four structural and six nonstructural proteins. Among them, nonstructural protein 5A (NS5A) has earned increasing attentions for its cell transforming potential and determining role of interferon (IFN) sensitivity. NS5A has previously been shown to interact with and inhibit normal signaling pathway of IFN-induced protein kinase (PKR). Once PKR is repressed, it can result in insensitivity of host hepatocytes to interferon treatment and unregulated expression of PKR downstream signaling proteins. However, cell effects exerted by NS5A through repression of the PKR pathway are not totally clearified yet. Previous studies indicated that patients with chronic HCV infection often have shorter telomeres in their hepatocytes compared to that of normal ones. In fact, shortening telomere was observed in about 80% liver cancer tissues, but the underlying mechanism remained to be undetermined. In this study, interactions between NS5A and human telomerase gene hTERT were examined in order to investigate possible reasons of telomere shortening in HCV patients. From results obtained by RT-PCR and luciferase reporter assay, NS5A repressed transcription activity of telomerase gene in a dose-dependent manner. In addition, by appling a muatnt NS5A construct and different kinase inhibitors, NS5A was shown to inhibit hTERT trough the NS5A-PKR pathway. The expression of transcription factor Sp1, an activator of hTERT, was also repressed in cells expressing NS5A but not the mutant. These results showed that the hepatitis C virus NS5A protein down-regulates hTERT transcriptional activity in hepatoma cells, possibly through interacting with PKR to repress Sp1 expression.