Antiapoptotic and Oncogenic Potentials of Hepatitis C Virus Are Linked to Interferon Resistance by Viral Repression of the PKR Protein Kinase

Abstract

Hepatitis C virus (HCV) is prevalent worldwide and has become a major cause of liver dysfunction and hepatocellular carcinoma. The high prevalence of HCV reflects the persistent nature of infection and the large frequency of cases that resist the current interferon (IFN)-based anti-HCV therapeutic regimens. HCV resistance to IFN has been attributed, in part, to the function of the viral nonstructural 5A (NS5A) protein. NS5A from IFN-resistant strains of HCV can repress the PKR protein kinase, a mediator of the IFN-induced antiviral and apoptotic responses of the host cell and a tumor suppressor. Here we examined the relationship between HCV persistence and resistance to IFN therapy. When expressed in mammalian cells, NS5A from IFN-resistant HCV conferred IFN resistance to vesicular stomatitis virus (VSV), which normally is sensitive to the antiviral actions of IFN. NS5A blocked viral double-stranded RNA (dsRNA)-induced PKR activation and phosphorylation of eIF-2α in IFN-treated cells, resulting in high levels of VSV mRNA translation. Mutations within the PKR-binding domain of NS5A restored PKR function and the IFN-induced block to viral mRNA translation. The effects due to NS5A inhibition of PKR were not limited to the rescue of viral mRNA translation but also included a block in PKR-dependent host signaling pathways. Cells expressing NS5A exhibited defective PKR signaling and were refractory to apoptosis induced by exogenous dsRNA. Resistance to apoptosis was attributed to an NS5A-mediated block in eIF-2α phosphorylation. Moreover, cells expressing NS5A exhibited a transformed phenotype and formed solid tumors in vivo. Disruption of apoptosis and tumorogenesis required the PKR-binding function of NS5A, demonstrating that these properties may be linked to the IFN-resistant phenotype of HCV.

High Amino Acid Variability Within the Ns5a of Hepatitis C Virus (Hcv) Is Associated With Hepatocellular Carcinoma in Patients With Hcv–1B-Related Cirrhosis

“…These residues have been described as quite heterogeneous among different HCV-1b isolates. 16,20,39,49,50 On the other hand, serine residues, that are probably implicated in the hyperphosphorylation of NS5A 61,62 were highly conserved in the majority of viral isolates analyzed in this study.…”

“…[60][61][62]69 It has been reported that HCV-NS5A protein may repress cellular PKR function by a direct interaction via the PKR-bd, which can be abrogated by the presence of mutations within the ISDR. 20 Nevertheless, HCV-interference of the antiviral activity of interferon by mechanisms unrelated to PKR-bd function is also possible, as recently observed. 67,70 Regulation of PKR probably implicates various cellular pathways that may be different depending on the cell type and stimulatory events.…”

“…18 Cellular PKR is an interferon-inducible protein-kinase that mediates the antiviral and antiproliferative effects of interferon by phosphorylating eukaryotic initiation factor-2 alpha (eIF2-␣). 19,20 A 12-aa sequence stretch within the E2 protein, named PePHD, may also be a viral determinant of the susceptibility of HCV to interferon. 17 This region shares sequence similarity with the cellular PKR auto phosphorylation site and its target, the eIF2-␣ phosphorylation site, and can bind to and inhibit PKR in vitro.…”

Intracellular Events and Cell Fate in Filovirus Infection

“…Intriguingly, PKR is not activated in ZEBOV-infected cells, and VP35 has been shown to actively block PKR activation [160,161]. It has been reported for various viruses that they inhibit PKR-mediated eIF2α phosphorylation to prevent host cell apoptosis [162][163][164]. However, it is not known if the inhibition of PKR in EBOV-infected cells blocks the induction of apoptosis.…”

Hyper-activated IRF-1 and STAT1 contribute to enhanced Interferon stimulated gene (ISG) expression by Interferon α and γ co-treatment in human hepatoma cells

“…On the other hand, viruses also encode various genes that can antagonize the pro-apoptotic signaling. For example, NS5A protein of HCV is well known for its anti-apoptotic activity although the exact mechanism is still controversial [35,36]; EBV (Epstein-Barr virus)-encoded poly(A) RNAs (EBERs) exhibit striking resistance of IFN-α induced apoptosis in Burkitt's lymphoma cell lines [37].…”