Modulation of alpha interferon anti-hepatitis C virus activity by ISG15

Abstract

ISG15 has recently been reported to possess antiviral properties against viruses, both in vivo and in vitro. Knock-down of ISG15 gene expression by small interfering RNA followed by alpha interferon (IFN-a) treatment in Huh-7 cells resulted in an increased phenotypic sensitivity to IFN-a, as determined by measuring hepatitis C virus (HCV) RNA replication inhibition in stably transfected HCV replicon cells and in cells infected with genotype 1a HCVcc (infectious HCV). This IFN-a-specific effect, which was not observed with IFN-c, correlated with an increase in expression of the IFN-a-inducible genes IFI6, IFITM3, OAS1 and MX1, whereas the expression of the non-IFN-a-inducible genes PTBP-1 and JAK1 remained unchanged. It has previously been reported that, unlike ISG15 knock-down, increased sensitivity to IFN-a after knock-down of USP18 occurs through the prolonged phosphorylation of STAT-1. Combination knock-down of ISG15 and USP18 resulted in a moderate increase in IFN-a-inducible gene expression compared with single ISG15 or USP18 knock-down. Furthermore, the phenotype of increased gene expression after ISG15 knock-down and IFN-a treatment was also observed in non-hepatic cell lines A549 and HeLa. Taken together, these results reveal a novel function for ISG15 in the regulation of the IFN-a pathway and its antiviral effect.

Knockdown of USP18 Increases Alpha 2a Interferon Signaling and Induction of Interferon-Stimulating Genes but Does Not Increase Antiviral Activity in Huh7 Cells

“…Our data broadly agree with those reported previously by Chua et al (4), who also showed a modest effect of the USP18 knockdown on AV potency, whereas the results reported by Randall et al (28) suggest a more potent effect. In an attempt to understand why one study observed a Ͼ1-log effect on AV potency but we and Chua et al saw only a Ͻ1-log effect, we can point to a few differences in experimental conditions which may be significant.…”

“…Two groups have previously reported the effect of reduced human USP18 gene expression levels on IFN activity in HCV replication systems in vitro and found either modest (Ͻ10-fold) or substantial (Ͼ10-fold) increases in AV activity (4,28), leaving this question unresolved. Our studies shed further light on the potential clinical utility of a USP18 antagonist as an HCV therapeutic.…”

“…As a result, around 300 ISGs are transcriptionally upregulated, including canonical antiviral (AV) genes such as 2Ј,5Ј-oligoadenylate synthetase (2-5 OAS) and the ubiquitin-like protein ISG15 (5). ISG15 is one of the genes most strongly induced after IFN treatment and is known to play a role in the AV activity of alphaviruses and HCV (4,20,21,36). The multiple upregulated ISG products then initiate multiple secondary effector pathways to further execute the AV activity.…”

Identification of proteins that mediate the pro-viral functions of the interferon stimulated gene 15 in hepatitis C virus replication

“…Furthermore, ISG15 expression can be located in hepatocytes or nonparenchymal liver cells (NPC) predicting non-response or response to treatment, respectively (Chen et al, 2010a). ISG15 has been identified as a pro-viral host factor, which directly promotes HCV replication and additionally inhibits the IFN response (Chen et al, 2010b;Broering et al, 2010;Chua et al, 2009). Therefore, therapeutic strategies based on the suppression of ISG15 might overcome non-response to standard combination treatment in HCV-infected patients.…”

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).…”