ABSTRACT
Previous studies have shown that both hepatitis A virus and hepatitis C virus inhibit innate immunity by cleaving the mitochondrial antiviral signaling (MAVS) protein, an essential component of the virus-activated signaling pathway that activates NF-kappaB and IFN regulatory factor-3 to induce the production of type I IFN. For human hepatitis B virus (HBV), hepatitis B s-Ag, hepatitis B e-Ag, or HBV virions have been shown to suppress TLR-induced antiviral activity with reduced IFN-beta production and subsequent induction of IFN-stimulated genes. However, HBV-mediated suppression of the RIG-I-MDA5 pathway is unknown. In this study, we found that HBV suppressed poly(deoxyadenylate-thymidylate)-activated IFN-beta production in hepatocytes. Specifically, hepatitis B virus X (HBX) interacted with MAVS and promoted the degradation of MAVS through Lys(136) ubiquitin in MAVS protein, thus preventing the induction of IFN-beta. Further analysis of clinical samples revealed that MAVS protein was downregulated in hepatocellular carcinomas of HBV origin, which correlated with increased sensitivities of primary murine hepatocytes isolated from HBX knock-in transgenic mice upon vesicular stomatitis virus infections. By establishing a link between MAVS and HBX, this study suggests that HBV can target the RIG-I signaling by HBX-mediated MAVS downregulation, thereby attenuating the antiviral response of the innate immune system.
Subject(s)
Adaptor Proteins, Signal Transducing/immunology , Immunity, Innate/immunology , Mitochondrial Proteins/immunology , Trans-Activators/immunology , Adaptor Proteins, Signal Transducing/genetics , Adaptor Proteins, Signal Transducing/metabolism , Animals , Cell Line , Chlorocebus aethiops , DEAD Box Protein 58 , DEAD-box RNA Helicases/genetics , DEAD-box RNA Helicases/immunology , DEAD-box RNA Helicases/metabolism , Down-Regulation , Hep G2 Cells , Humans , Immunoblotting , Interferon-gamma/genetics , Interferon-gamma/immunology , Interferon-gamma/metabolism , Kinetics , Mitochondrial Proteins/genetics , Mitochondrial Proteins/metabolism , Poly dA-dT/genetics , Poly dA-dT/immunology , Poly dA-dT/metabolism , Protein Binding , Receptors, Immunologic , Reverse Transcriptase Polymerase Chain Reaction , Signal Transduction/immunology , Trans-Activators/genetics , Trans-Activators/metabolism , Transfection , Vero Cells , Viral Regulatory and Accessory ProteinsABSTRACT
Innate immunity to viruses involves receptors such as Retinoic Acid Induced Gene-1 (RIG-I), which senses viral RNA and triggers a signaling pathway involving the outer mitochondrial membrane protein mitochondrial antiviral signaling (MAVS). Recent work has identified that NLRX1, a member of another class of innate immune receptors, sequesters MAVS away from RIG-I and thereby prevents mitochondrial antiviral immunity. In this study, we demonstrate that the proteasome PSMA7 (alpha4) subunit associates with MAVS in vivo and in vitro. Expression of PSMA7 results in a potent inhibition of RIG-1 and MAVS-mediated IFN-beta promoter activity; conversely, depletion of PSMA7 with small interference RNA enhances virus-induced type I IFN production, with consequent reduction of virus replication. Furthermore, a striking reduction in the abundance of endogenous MAVS with overexpressed PSMA7 was found and virus infection leads to transient increase in the endogenous PSMA7 protein level. Cumulatively, these results suggest that PSMA7 is a negative regulator of the MAVS-mediated innate immunity that probably serves to attenuate the establishment of an antiviral state during viral infection, highlighting the biological significance of PSMA7-MAVS association as an important cellular regulatory control.
