Inhibition of Alpha Interferon Signaling by Hepatitis B Virus

Abstract

Alpha interferon (IFN-␣) and pegylated IFN-␣ (pegIFN-␣) are used for the treatment of chronic hepatitis B (CHB). Unfortunately, only a minority of patients can be cured. The mechanisms responsible for hepatitis B virus (HBV) resistance to pegIFN-␣ treatment are not known. pegIFN-␣ is also used to treat patients with chronic hepatitis C (CHC). As with chronic hepatitis B, many patients with chronic hepatitis C cannot be cured. In CHC, IFN-␣ signaling has been found to be inhibited by an upregulation of protein phosphatase 2A (PP2A). PP2A inhibits protein arginine methyltransferase 1 (PRMT1), the enzyme that catalyzes the methylation of the important IFN-␣ signal transducer STAT1. Hypomethylated STAT1 is less active because it is bound by its inhibitor, PIAS1. In the present work, we investigated whether similar molecular mechanisms are also responsible for the IFN-␣ resistance found in many patients with chronic hepatitis B. We analyzed the expression of PP2A, the enzymatic activity of PRMT1 (methylation assays), the phosphorylation and methylation of STAT1, the association of STAT1 with PIAS1 (via coimmunoprecipitation assays), the binding of activated STAT1 to interferon-stimulated response elements (via electrophoretic mobility shift assays), and the induction of interferon target genes (via real-time RT-PCR) in human hepatoma cells expressing HBV proteins as well as in liver biopsies from patients with chronic hepatitis B and from controls. We found an increased expression of PP2A and an inhibition of IFN-␣ signaling in cells expressing HBV proteins and in liver biopsies of patients with CHB. The molecular mechanisms involved are similar to those found in chronic hepatitis C.More than 350 million people worldwide are chronically infected with hepatitis B virus (HBV) (19,21). Chronic hepatitis B (CHB) can progress to cirrhosis and hepatocellular carcinoma. Approved treatments for CHB include a few nucleos(t)ide analogues, such as lamivudine and adefovir, or alpha interferon (IFN-␣), recently in pegylated form (pegIFN-␣) (19). pegIFN-␣2a given for 48 weeks can induce the seroconversion of hepatitis B e antigen (HBeAg) in 32% of patients (20). However, over 60% of patients will continue to suffer from chronic active hepatitis B despite pegIFN-␣2a therapy. The molecular mechanisms responsible for the ineffectiveness of IFN-␣ treatments in CHB are not known. pegIFN-␣ (in combination with ribavirin) is also the current standard therapy for chronic hepatitis C (CHC). Interestingly, as with CHB, pegIFN-␣ is not effective in many patients with CHC. Over the last years, several molecular mechanisms responsible for viral evasion of the type I IFN system have been studied (10,14). One of these mechanisms involved in the evasion of hepatitis C virus (HCV) has been elucidated in our laboratory over the last years: HCV proteins interfere with IFN-␣-induced signaling through the Jak-STAT pathway (3,8,15).The interferon system is an important component of the host response against viruses, and mice with deficiencies of IFN re...

Mechanism of inhibiting type I interferon induction by hepatitis B virus X protein

“…In agreement with our data, a recent study has shown that IFNβ promoter activation induced by SeV was attenuated in HepG2.2.15 cells (Wang and Ryu, 2010). Replication of HBV does not have an effect on phosphorylation of STAT1 in cells stimulated with IFN (Christen et al, 2007), and we confirmed this result. However, we also found that signaling events downstream of STAT1 was not affected by HBV (Fig.…”

“…However, we also found that signaling events downstream of STAT1 was not affected by HBV (Fig. 5F), which are contradictive to published studies that show nuclear translocation and DNA binding ability of STAT1 are impaired by HBV polymerase (Wu et al, 2007) and surface proteins and/or HBx (Christen et al, 2007) in Huh7 cells, respectively. Cell type specific differences might explain the apparent discrepancy.…”

“…For example, HBx enhances HBV replication and secretion of surface and precore protein in HepG2 cells but not in Huh7 cells (Melegari et al, 1998). On the other hand, the decreased function of transcriptional coregulator PRMT1 on STAT1 methylation, which was reported to account for the reduction of transcriptional activity of STAT1 (Christen et al, 2007), is challenged by the finding that PRMT1 mediates arginine methylation of protein inhibitor of activated STAT1 (PIAS1) instead, leading to repression of IFN-inducible transcription via promoting the release of STAT1 from target gene promoter (Weber et al, 2009). …”

“…Therefore, strategies may have been adapted by HBV to suppress the initial defenses of innate immunity, including type I IFN response. In line with the hypothesis, several HBV proteins, when overexpressed in cells, interfere with JAK-STAT signaling and ISGs expression: HBV surface proteins and/or X protein (HBx) upregulates protein phosphatase 2A (PP2A), which reduces the transcriptional activity of ISGF3 through inhibiting the enzyme activity of protein arginine methyltransferase 1 (PRMT1) (Christen et al, 2007); HBV polymerase inhibits nuclear translocation of STAT1 (Wu et al, 2007); HBV precore/core proteins prevent MXA gene expression via their interaction with the MXA promoter (Fernández et al, 2003). It should be noted that a virus may not be capable of spreading rapidly within a host's body because of the generation of antiviral state in neighboring cells, if the virus antagonizes IFN signaling but fails to limit IFN production (Randall and Goodbourn, 2008).…”

MicroRNA-146a Feedback Suppresses T Cell Immune Function by Targeting Stat1 in Patients with Chronic Hepatitis B

“…Inhibition of the key IFN-a signal transducer Stat1 by HBV has been suggested to antagonize the IFN response (35,36). In this study, we found that miR-146a, which targets Stat1, is significantly upregulated in the T cells of patients with CHB, which could shed light on the mechanism of the limited effectiveness of therapeutic IFN-a in CHB.…”