Abstract
Hepatitis C virus is a poor inducer of interferon (IFN), although its structured viral RNA can bind the RNA helicase RIG-I, and activate the IFN-induction pathway. Low IFN induction has been attributed to HCV NS3/4A protease-mediated cleavage of the mitochondria-adapter MAVS. Here, we have investigated the early events of IFN induction upon HCV infection, using the cell-cultured HCV JFH1 strain and the new HCV-permissive hepatoma-derived Huh7.25.CD81 cell subclone. These cells depend on ectopic expression of the RIG-I ubiquitinating enzyme TRIM25 to induce IFN through the RIG-I/MAVS pathway. We observed induction of IFN during the first 12 hrs of HCV infection, after which a decline occurred which was more abrupt at the protein than at the RNA level, revealing a novel HCV-mediated control of IFN induction at the level of translation. The cellular protein kinase PKR is an important regulator of translation, through the phosphorylation of its substrate the eIF2α initiation factor. A comparison of the expression of luciferase placed under the control of an eIF2α-dependent (IRESEMCV) or independent (IRESHCV) RNA showed a specific HCV-mediated inhibition of eIF2α-dependent translation. We demonstrated that HCV infection triggers the phosphorylation of both PKR and eIF2α at 12 and 15 hrs post-infection. PKR silencing, as well as treatment with PKR pharmacological inhibitors, restored IFN induction in JFH1-infected cells, at least until 18 hrs post-infection, at which time a decrease in IFN expression could be attributed to NS3/4A-mediated MAVS cleavage. Importantly, both PKR silencing and PKR inhibitors led to inhibition of HCV yields in cells that express functional RIG-I/MAVS. In conclusion, here we provide the first evidence that HCV uses PKR to restrain its ability to induce IFN through the RIG-I/MAVS pathway. This opens up new possibilities to assay PKR chemical inhibitors for their potential to boost innate immunity in HCV infection.
Highlights
In response to invasion with bacterial or viral pathogens, cells are able to mount an immediate immune response through their ability to use specialized cellular molecules, referred to as pattern recognition receptors or PRRs, to detect unusual DNA, ssRNA or dsRNA structures
Ectopic expression of TRIM25 in Huh7.25/CD81 cells had no effect alone, but resulted in a strong induction of IFN when the cells were infected with Sendai virus, indicating that these cells present a defect in the RIG-I/MAVS pathway at the level of TRIM25
Whatever the nature of this defect, ectopic expression of TRIM25 allows the use of Huh7.25/CD81 cells as a model to examine the effect of Hepatitis C virus (HCV) infection on the IFN induction pathway
Summary
In response to invasion with bacterial or viral pathogens, cells are able to mount an immediate immune response through their ability to use specialized cellular molecules, referred to as pattern recognition receptors or PRRs, to detect unusual DNA, ssRNA or dsRNA structures. Among these PRRs, are the CARD-containing DexD/H RNA helicases RIG-I and MDA5, which are activated upon binding either to both 59-triphosphate ssRNA and short double-stranded RNA structures (RIG-I) or to long dsRNA and higher-ordered RNA structures (MDA5) [1]. This leads to induction of the interferons and pro-inflammatory cytokines that are involved in the innate immune response [4]
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