GP73 represses host innate immune response to promote virus replication by facilitating MAVS and TRAF6 degradation.

Fang Liu,Kailang Wu,Jianguo Wu,Lishan Su,Hui Jiang,Xuewu Zhang,Chengliang Zhu,Qi Zhang,Yingle Liu,Tianci Wang,Yahui Ren

doi:10.1371/journal.ppat.1006321

Abstract

Hepatitis C virus (HCV) infection is a leading cause of chronic liver diseases and hepatocellular carcinoma (HCC) and Golgi protein 73 (GP73) is a serum biomarker for liver diseases and HCC. However, the mechanism underlying GP73 regulates HCV infection is largely unknown. Here, we revealed that GP73 acts as a novel negative regulator of host innate immunity to facilitate HCV infection. GP73 expression is activated and correlated with interferon-beta (IFN-β) production during HCV infection in patients’ serum, primary human hepatocytes (PHHs) and human hepatoma cells through mitochondrial antiviral signaling protein (MAVS), TNF receptor-associated factor 6 (TRAF6) and mitogen-activated protein kinase kinase/extracellular regulated protein kinase (MEK/ERK) pathway. Detailed studies revealed that HCV infection activates MAVS that in turn recruits TRAF6 via TRAF-interacting-motifs (TIMs), and TRAF6 subsequently directly recruits GP73 to MAVS via coiled-coil domain. After binding with MAVS and TRAF6, GP73 promotes MAVS and TRAF6 degradation through proteasome-dependent pathway. Moreover, GP73 attenuates IFN-β promoter, IFN-stimulated response element (ISRE) and nuclear factor κB (NF-κB) promoter and down-regulates IFN-β, IFN-λ1, interleukin-6 (IL-6) and IFN-stimulated gene 56 (ISG56), leading to the repression of host innate immunity. Finally, knock-down of GP73 down-regulates HCV infection and replication in Huh7-MAVSR cells and primary human hepatocytes (PHHs), but such repression is rescued by GP73m4 (a mutant GP73 resists to GP73-shRNA#4) in Huh7-MAVSR cells, suggesting that GP73 facilitates HCV infection. Taken together, we demonstrated that GP73 acts as a negative regulator of innate immunity to facilitate HCV infection by interacting with MAVS/TRAF6 and promoting MAVS/TRAF6 degradation. This study provides new insights into the mechanism of HCV infection and pathogenesis, and suggests that GP73 is a new potential antiviral target in the prevention and treatment of HCV associated diseases.

Highlights

Innate immune response is critical for host defense against microbial infection including bacteria, fungi and viruses
Golgi protein 73 (GP73) attenuates IFN-β promoter, IFN-stimulated response element (ISRE) and nuclear factor κB (NF-κB) promoter and down-regulates IFN-β, IFN-λ1, interleukin-6 (IL-6) and IFN-stimulated gene 56 (ISG56), leading to the repression of host innate immunity and the facilitation of virus infection. These results reveal a novel mechanism by which GP73 acts as a novel negative regulator of host innate immunity to facilitate virus infection and provide new insights into the therapeutic design of anti-Hepatitis C virus (HCV) drugs
Serum GP73 protein was significantly higher in HCV infected patients compared to healthy individuals (Fig 1A), suggesting that GP73 is activated in infected patients

Summary

Introduction

Innate immune response is critical for host defense against microbial infection including bacteria, fungi and viruses. Pathogen-associated molecular patterns (PAMPs) are recognized by pattern recognition receptors (PRRs), which lead to the production of type I interferons (IFNs), proinflammatory cytokines and downstream effectors [1]. While RLRs, including retinoic acid inducible gene I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5), sense viral RNA in the cytoplasm, which in turn recruit and activate mitochondrial antiviral signaling protein (MAVS) [4,5,6,7]. MAVS further recruits TNF receptor associated factors (TRAF2/3/5/6) to activate nuclear factor κB (NF-κB) and interferon regulatory factors (IRF3/7) leading to the production of IFNs and cytokines [1, 8, 9]

Results

Discussion

Conclusion