Date Published: July 1, 2016
Publisher: Public Library of Science
Author(s): Geon-Woo Kim, Seung-Hoon Lee, Hee Cho, Minwoo Kim, Eui-Cheol Shin, Jong-Won Oh, Timothy L. Tellinghuisen.
The liver-specific microRNA miR-122, which has essential roles in liver development and metabolism, is a key proviral factor for hepatitis C virus (HCV). Despite its crucial role in the liver and HCV life cycle, little is known about the molecular mechanism of miR-122 expression regulation by HCV infection. Here, we show that the HCV core protein downregulates the abundance of miR-122 by promoting its destabilization via the inhibition of GLD-2, a non-canonical cytoplasmic poly(A) polymerase. The decrease in miR-122 expression resulted in the dysregulation of the known functions of miR-122, including its proviral activity for HCV. By high-throughput sequencing of small RNAs from human liver biopsies, we found that the 22-nucleotide (nt) prototype miR-122 is modified at its 3′ end by 3′-terminal non-templated and templated nucleotide additions. Remarkably, the proportion of miR-122 isomers bearing a single nucleotide tail of any ribonucleotide decreased in liver specimens from patients with HCV. We found that these single-nucleotide-tailed miR-122 isomers display increased miRNA activity and stability over the 22-nt prototype miR-122 and that the 3′-terminal extension is catalyzed by the unique terminal nucleotidyl transferase activity of GLD-2, which is capable of adding any single ribonucleotide without preference of adenylate to the miR-122 3′ end. The HCV core protein specifically inhibited GLD-2, and its interaction with GLD-2 in the cytoplasm was found to be responsible for miR-122 downregulation. Collectively, our results provide new insights into the regulatory role of the HCV core protein in controlling viral RNA abundance and miR-122 functions through miR-122 stability modulation.
Hepatitis C virus (HCV), a positive-sense single stranded RNA virus, causes chronic hepatitis and liver cirrhosis, often leading to the development of hepatocellular carcinoma. The HCV genome is composed of a long open reading frame (ORF) that is flanked by untranslated regions (UTRs) at both the 5′ and 3′ ends. The ORF encodes a polyprotein of approximately 3010 amino acids that is processed by cellular and viral proteases into 10 polypeptides, including structural (core protein and envelope proteins E1 and E2) and non-structural (NS) proteins .
In this study, we examined the mechanism by which the cellular abundance of miR-122 is downregulated by HCV. Diverse regulatory mechanisms, which include the transcription regulation and post-transcriptional turn-over control of mature miRNA, might explain the differential expression of miRNAs in HCV-infected cells. We demonstrated that the HCV core protein inhibits GLD-2 and thereby promotes miR-122 destabilization, which leads to the downregulation of HCV RNA abundance.