ISG15, a ubiquitin-like interferon-stimulated gene, promotes hepatitis C virus production in vitro: implications for chronic infection and response to treatment

Abstract

Upregulation of interferon (IFN)-stimulated genes (ISGs), including IFN-stimulated gene 15 (ISG15) and other members of the ISG15 pathway, in pre-treatment liver tissue of patients chronically infected with hepatitis C virus (HCV) is associated with subsequent treatment failure (pegylated IFN-a/ribavirin). This study assessed the effect of ISG15 on HCV production in vitro. The levels of ISG15 and of its conjugation to target proteins (ISGylation) were increased by plasmid transfection, but ISGylation was inhibited by small interfering RNA directed against the E1 activating enzyme, Ube1L, in Huh7.5 cells. Cells were infected with HCV FL-J6/JFH virus, and HCV RNA and viral titres were determined. Levels of both HCV RNA and virus increased when levels of ISG15 and ISGylation were increased, and decreased when ISGylation was inhibited. The effects of ISGylation on HCV were independent of upstream IFN signalling: IFN-a-induced ISG expression was not altered by Ube1L knockdown. Thus, although ISG15 has antiviral activity against most viruses, ISG15 promotes HCV production. HCV might exploit ISG15 as a host immune evasion mechanism, and this may in part explain how increased expression of ISGs, especially ISG15, correlates with subsequent IFN-based treatment failure. INTRODUCTIONHepatitis C virus (HCV) is adept at evading host antiviral mechanisms and is often resistant to the current standard of care -combination treatment with pegylated interferon (IFN)-a and ribavirin (PegIFN/Rib). This regimen eradicates the virus in only 50 % of cases. A number of mechanisms contribute to evasion and treatment resistance, including cleavage of the RIG-I adaptor protein IPS1/ MAVS/Cardif by the HCV NS3/NS4A protease and modulation of the host response by the HCV core protein Loo et al., 2006). However, none of these mechanisms has consistently been demonstrated to play a role in the clinical disease and thus cannot explain the ability of the virus to escape the host response in patients.Response to treatment can be predicted by levels of expression of IFN-stimulated genes (ISGs) in the liver prior to initiation of PegIFN/Rib treatment. Non-responders have increased expression of a number of ISGs (Chen et al., 2005;Feld et al., 2007; Asahina et al., 2008;Asselah et al., 2008;Sarasin-Filipowicz, et al., 2008). Three of these ISGs are components of the ISG15 ubiquitin-like pathway. ISG15 was the first ubiquitin-like protein to be described and, like its homologue ubiquitin, is conjugated to proteins in a tightly regulated process called ISGylation. The ISG15 E1 activating protein, Ube1L, coordinates with the E2 conjugating enzyme (UbcH8) and the E3 ligase (CEB1) to join the C terminus of ISG15 to a wide variety of proteins (Loeb & Haas, 1992). ISG15 can be removed from its target proteins by USP18, an ISG15 protease (Kim et al., 2008b). Thus, ISG15 inhibits virus production for many viruses, but may promote production of some.In this study, we examined the role of ISG15 and ISGylation in HCV production in vitro, using the FL-J6/JFH...

Inhibition of Hepatitis C Virus Replication by IFN-Mediated ISGylation of HCV-NS5A

“…6). Similarly, ISG15 was recently reported to promote HCV replication in the HCV J6/JFH1-infected Huh-7.5 cells (47). There are several possible explanations for this difference.…”

Gene Expression Profiling To Predict and Assess the Consequences of Therapy-Induced Virus Eradication in Chronic Hepatitis C Virus Infection

“…Previously, it was shown that the expression levels of ISG mRNA in hepatocytes or in peripheral blood are associated with the outcome of IFN-based therapy for chronic HCV infection (11,22,23). However, the predictive signature (21-probe set) did not include any ISG, indicating that baseline expression of ISG cannot predict viral relapse after successful IFN-based therapy.…”

Toll Like Receptors in Chronic Viral Hepatitis – Friend and Foe

“…ISGylated proteins are not degraded by the proteasome as ubiquitinylated proteins are (Malakhov et al 2002;Ritchie & Zhang, 2004). Recently published data indicate that ISGylation negatively modulates interferon signaling (Chua et al 2009;Broering et al 2010), in addition ISGylation and ISG15 itself directly promote HCV replication (Chen et al 2010;Broering et al 2010). These observations may explain why elevated expression of ISGs, and ISG15 in particular, during HCV infection is beneficial for the hepatitis C virus (Figure 5).…”