Hepatitis B virus X protein inhibits extracellular IFN-α-mediated signal transduction by downregulation of type I IFN receptor

Abstract

Abstract.We have previously shown that hepatitis B virus (HBV) protein X (HBX), a regulatory protein of HBV, activates Stat1, leading to type I interferon (IFN) production. Type I IFN secreted from HBX-expressing hepatic cells enforces antiviral signals through its binding to the cognate type I IFN receptor. We therefore investigated how cells handle this detrimental situation. Interestingly, compared to Chang cells stably expressing an empty vector (Chang-Vec), Chang cells stably expressing HBX (Chang-HBX) showed lower levels of IFN-α receptor 1 (IFNAR1) protein, a subunit of type I IFN receptor. The levels of IFNAR1 transcripts detected in Chang-HBX cells were lower than the levels in Chang-Vec cells, indicating that HBX regulates IFNAR1 at the transcriptional level. Moreover, we observed that HBX induced the translocation of IFNAR1 to the cytoplasm. Consistent with these observations, HBX also downregulated Tyk2, which is required for the stable expression of IFNAR1 on the cell surface. Eventually, Chang-HBX cells consistently maintained a lower level of IFNAR1 expression and displayed no proper response to IFN-α, while Chang-Vec cells exhibited a proper response to IFN-α treatment. Taken together, we propose that HBX downregulates IFNAR1, leading to the avoidance of extracellular IFN-α signal transduction. IntroductionThe type I interferon (IFN) receptor consists of 2 subunits, IFN-α receptor 1 (IFNAR1) and IFNAR2, which belong to the type II cytokine receptor superfamily (1). This heterodimeric complex is able to interact with IFN-α and IFN-β, resulting in the phosphorylation of Tyk2 and Jak1 that are bound to IFNAR1 and IFNAR2, respectively (2). Subsequently, the Stat proteins, Stat1 and Stat2 are phosphorylated at specific tyrosine residues, which allows the 2 proteins to form a Stat1/2 heterodimer based on SH2/phosphotyrosine interactions. The formation of this heterodimer facilitates its association with IFN regulatory factor (IRF)9 to form an active heterotrimeric transcription factor called IFN-stimulated gene factor (ISGF)3. ISGF3 targets specific sequences such as IFN-stimulated response element and IFN-γ-activated sequence in the promoters of IFN-stimulated genes, leading to the establishment of antiviral status (3).After type I IFN binds to its cognate type I IFN receptor, the receptor is downregulated by endocytosis mediated by cargospecific clathrin machinery, and degraded via the lysosomal and proteosomal pathways in order to limit the magnitude and duration of IFN signaling (4,5). The adaptin protein 2 complex, a component of the cargo-specific clathrin machinery recognizes the Tyr-based endocytic motif of IFNAR1, leading to the efficient endocytosis of IFNAR1 (5). The Skip, Cullin, F-box containing complex β-TrCP E3 ubiquitin ligase mediates the ubiquitination of IFNAR1 in a phosphorylation-dependent manner, eventually designating IFNAR1 for lysosomal degradation (5). Catalytic activation of Tyk2 is required for these events but is not essential for IFNAR1 internalization (6). Conversely, it ...

Identification of novel susceptibility loci associated with hepatitis B surface antigen seroclearance in chronic hepatitis B

“…This pathway represents an alternative mechanism for the induction of inflammatory and apoptotic genes in the absence of the IFN-α/β receptor during viral infection. CHB might occur during an immunosuppressed state that is mediated by several actions of HBV protein X, such as, down-regulation of type I IFN receptor and the inhibition of extracellular IFN-α-mediated signal transduction [ 33 ]. Thus, the inflammatory pathway via HOXA13, an alternative mechanism functioning in the absence of type I IFN receptor, may play a crucial role in the elimination of HBV in patients with CHB.…”

Pathogenesis of Hepatitis B Virus Associated Chronic Liver Disease

“…As mentioned above, sequestration and reverse transcription of pregenomic HBV RNA in immature nucleocapsids (Figure 2) may block the induction of innate immunity. In addition, although HBV replication is exquisitely sensitive to inhibition by IFNs, HBx appears to block IFN expression and signaling [72][73][74], suggesting that both innate and adaptive immunity could be compromised, thereby permitting virus persistence. Under these circumstances, CLD would continue to damage the liver while being unable to resolve the virus infection.…”

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“…For example, the EBV-encoded latent membrane proteins, LMP2A and LMP2B, attenuate signaling by Type 1 IFN by stimulating the intracellular turnover of the receptors precursors (69). Among other viruses implicated in accelerating the turnover of IFNAR1 are herpes simpex virus (63), hepatitis C (3, 15, 48, 60) and B (12) viruses, vesicular stomatitis virus (3, 48), and SARS coronavirus (52). …”