Interferon signaling suppresses the unfolded protein response and induces cell death in hepatocyte accumulating hepatitis B surface antigen
Virus infection, such as hepatitis B virus (HBV), often causes endoplasmic reticulum (ER) stress. The unfolded protein response (UPR) is counteractive machinery to ER stress, and the failure of UPR to cope with ER stress results in cell death. Mechanisms that regulate the balance between ER stress and UPR in HBV infection is poorly understood. Type 1 and type 2 interferons have been implicated in hepatic flares during chronic HBV infection. Here, we examined the interplay between ER stress, UPR, and IFNs using transgenic mice that express hepatitis B surface antigen (HBsAg) (HBs-Tg mice) and humanized-liver chimeric mice infected with HBV. IFN causes severe and moderate liver injury in HBs-Tg mice and HBV infected chimeric mice, respectively. The degree of liver injury is directly correlated with HBsAg levels in the liver, and reduction of HBsAg in the transgenic mice alleviates IFN mediated liver injury. Analyses of total gene expression and UPR biomarkers protein expression in the liver revealed that UPR is induced in HBs-Tg mice and HBV infected chimeric mice, indicating that HBsAg accumulation causes ER stress. Notably, IFN administration transiently suppressed UPR biomarkers before liver injury without affecting intrahepatic HBsAg levels. Furthermore, UPR upregulation by glucose-regulated protein 78 (GRP78) suppression or low dose tunicamycin alleviated IFN mediated liver injury. These results suggest that IFN induces ER stress-associated cell death by reducing UPR. IFN{gamma} uses the same mechanism to exert cytotoxicity to HBsAg accumulating hepatocytes. Collectively, our data reveal a previously unknown mechanism by which IFNs selectively induce cell death in virus-infected cells. This study also identifies UPR as a potential target for regulating ER stress-associated cell death. Author summaryHepatitis B virus (HBV) causes acute and chronic infections that kill over 600,000 people every year from severe hepatitis, liver cirrhosis, and cancer. Mechanisms of chronic liver injury remain largely unknown. Both type 1 and type 2 interferons (IFNs) have been implicated in hepatic flares during chronic HBV infection, although HBV per se is a poor IFN inducer. In addition, while IFN, a type 1 IFN, used to be the first-line treatment for chronic hepatitis B (CHB) patients, adverse side effects, including hepatic flares, severely limit their therapeutic effectiveness. These clinical observations suggest a pathogenic role of IFNs in HBV infection. Here, we demonstrate that IFN-1s cause severe and moderate hepatitis in transgenic mice expressing hepatitis B surface antigen (HBs-Tg mice) and human hepatocyte chimeric mice infected with HBV, respectively. HBsAg accumulation appears to cause ER stress because a counteractive response to ER stress, namely, unfolded protein response (UPR), was induced in both HBs-Tg mice and HBV infected chimeric mice. Our results indicate that IFN-1s suppress UPR before causing liver injury. UPR was also suppressed by IFN{gamma}. Induction of UPR in HBs-Tg mice before treatment with IFN and IFN{gamma} significantly alleviated liver injury. We suggest that IFNs exert cytotoxicity to ER stress accumulating cells by suppressing UPR.