Double-stranded RNA-activated protein kinase (PKR), an interferon (IFN)-stimulated gene, is activated by binding with double-stranded RNA, a putative replicative intermediate of the hepatitis C virus (HCV). Activated PKR phosphorylates the alpha subunit of eukaryotic initiation factor-2 to inhibit the translation of viral protein.
We established stable PKR knockdown Huh7 cells using RNA interference and investigated the effect of PKR against HCV replication using a subgenomic replicon that expressed luciferase reporter protein and the JFH1 full-length HCV genome.
In stable PKR knockdown cells that harboured a subgenomic replicon, luciferase activity was approximately three times higher than that of control cells, indicating that the subgenomic replicon replicated with a higher efficiency in stable PKR knockdown cells than that in control cells. Furthermore, stable PKR knockdown cells secreted significantly more HCV particles than did control cells after transfection with the full-length HCV genome. The replication of the subgenomic replicon was suppressed by the addition of IFN-alpha in both cells. Although the extent of suppression was significantly lower in stable PKR knockdown than control cells using a low concentration (2.5-5 U/ml) of IFN-alpha, even 10 U/ml IFN-alpha suppressed the replication of subgenomic replicon by >98% in both cells.
Double-stranded RNA-activated protein kinase plays an important role in suppressing HCV replication in an innate state, but may not be essential in IFN therapy.
[Show abstract][Hide abstract] ABSTRACT: Persistent hepatitis C virus infection is associated with progressive hepatic fibrosis and liver cancer. Acute infection evokes several distinct innate immune responses, but these are partially or completely countered by the virus. Hepatitis C virus proteins serve dual functions in replication and immune evasion, acting to disrupt cellular signaling pathways leading to interferon synthesis, subvert Jak-STAT signaling to limit expression of interferon-stimulated genes, and block antiviral activities of interferon-stimulated genes. The net effect is a multilayered evasion of innate immunity, which negatively influences the subsequent development of antigen-specific adaptive immunity, thereby contributing to virus persistence and resistance to therapy.
[Show abstract][Hide abstract] ABSTRACT: Interferon alpha (IFN-α) therapy is widely used to treat patients with chronic hepatitis B (CHB) but the sustained response rate is low, and the molecular mechanisms for the ineffectiveness of IFN-α treatments are not known. We screened differentially expressed genes between responders (Rs) and nonresponders (NRs) in patients with CHB treated with IFN-α to explore the molecular basis for treatment failure. Expression profiling was performed on percutaneous needle liver biopsy specimens taken before therapy. Gene expression levels were compared between seven patients who did not respond to therapy (NR) and six who did respond (R). Gene ontology category and KEGG pathway were analysed for differentially expressed genes, and the selected differentially expressed genes were confirmed using real-time polymerase chain reaction. We identified 3592 genes whose expression levels differed significantly between all Rs and NRs (P < 0.05); many of these genes are IFN-stimulated genes (ISGs) and immune-related genes. The ISGs were more highly expressed, while immune-related genes were inhibited in NRs before IFN-α treatment. Two ISGs (CEB1 and USP18) that are linked in an IFN inhibitory pathway are highly expressed in NRs, and a potential antiviral gene ISG20 was inhibited in NRs, suggesting a possible rationale for treatment nonresponse. Patients who do or do not respond to IFN have different liver gene expression profiles before IFN-α treatment. Preactivation of the IFN signalling pathway leading to the increased expression of inhibitory ISGs and inhibition of immune response in the pretreatment livers was associated with treatment failure.
[Show abstract][Hide abstract] ABSTRACT: Hepatitis C virus (HCV) is a major causative agent of chronic hepatitis and hepatocellular carcinoma worldwide and thus poses a significant public health threat. A hallmark of HCV infection is the extraordinary ability of the virus to persist in a majority of infected people. Innate immune responses represent the front line of defense of the human body against HCV immediately after infection. They also play a crucial role in orchestrating subsequent HCV-specific adaptive immunity that is pivotal for viral clearance. Accumulating evidence suggests that the host has evolved multifaceted innate immune mechanisms to sense HCV infection and elicit defense responses, while HCV has developed elaborate strategies to circumvent many of these. Defining the interplay of HCV with host innate immunity reveals mechanistic insights into hepatitis C pathogenesis and informs approaches to therapy. In this review, we summarize recent advances in understanding innate immune responses to HCV infection, focusing on induction and effector mechanisms of the interferon antiviral response as well as the evasion strategies of HCV.
Seminars in Immunopathology 08/2012; 35(1). DOI:10.1007/s00281-012-0332-x · 7.75 Impact Factor
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