Multiple Functions of Cellular FLIP Are Essential for Replication of Hepatitis B Virus

Abstract

Hepatitis B virus (HBV) infection is a leading cause of liver diseases; however, the host factors which facilitate the replication and persistence of HBV are largely unidentified. Cellular FLICE inhibitory protein (c-FLIP) is a typical antiapoptotic protein. In many cases of liver diseases, the expression level of c-FLIP is altered, which affects the fate of hepatocytes. We previously found that c-FLIP and its cleaved form interact with HBV X protein (HBx), which is essential for HBV replication, and regulate diverse cellular signals. In this study, we investigated the role of endogenous c-FLIP in HBV replication and its underlying mechanisms. The knockdown of endogenous c-FLIP revealed that this protein regulates HBV replication through two different mechanisms. (i) c-FLIP interacts with HBx and protects it from ubiquitin-dependent degradation. The N-terminal DED1 domain of c-FLIP is required for HBx stabilization. (ii) c-FLIP regulates the expression or stability of hepatocyte nuclear factors (HNFs), which have critical roles in HBV transcription and maintenance of hepatocytes. c-FLIP regulates the stability of HNFs through physical interactions. We verified our findings in three HBV infection systems: HepG2-NTCP cells, differentiated HepaRG cells, and primary human hepatocytes. In conclusion, our results identify c-FLIP as an essential factor in HBV replication. c-FLIP regulates viral replication through its multiple effects on viral and host proteins that have critical roles in HBV replication. Although the chronic hepatitis B virus (HBV) infection still poses a major health concern, the host factors which are required for the replication of HBV are largely uncharacterized. Our studies identify cellular FLICE inhibitory protein (c-FLIP) as an essential factor in HBV replication. We found the dual roles of c-FLIP in regulation of HBV replication: c-FLIP interacts with HBx and enhances its stability and regulates the expression or stability of hepatocyte nuclear factors which are essential for transcription of HBV genome. Our findings may provide a new target for intervention in persistent HBV infection.

Fatal cytokine release syndrome by an aberrant FLIP/STAT3 axis

“…Viruses have evolved a myriad of ways to escape host apoptotic process and thereby preserve infected cells from early death, which eliminates the replicative niche [ 64 ]. Several viruses including hepatitis C virus [ 65 ], hepatitis B virus [ 66 ], human T-cell leukemia virus-1 [ 67 ], human immunodeficiency virus 1 [ 68 ], Epstein Barr Virus [ 69 ], and influenza A virus [ 70 ] induce the anti-apoptotic protein c-FLIP, which blocks caspase-mediated cell death [ 40 ]. Poxviruses and herpesviruses encode proteins with high homology to c-FLIP, such as the Kaposi’s herpesvirus K13/vFLIP and the herpesvirus saimiri orf71, which harbor DED domains responsible for blocking procaspase cleavage, preventing apoptosis, and favoring viral latency [ 36 ].…”

Suppression of Hepatocyte Nuclear Factor 4 α by Long-term Infection of Hepatitis B Virus Contributes to Tumor Cell Proliferation

“…HBx protein plays an important role in HBV replication and liver pathogenesis through regulation of different host proteins that are involved in cell differentiation and proliferation [6,[26][27][28][29][30]. It has been reported that HBx modifies the epigenetic regulation of HBV by interaction with cccDNA, which serves as a template for HBV replication [31].…”

“…The constructs for WT HBV 1.2mer (1.2(+)), X-null HBV 1.2mer (1.2(-)), HBV 1.3mer (1.3(+)) and X-null HBV 1.3mer (1.3(-)) were kindly provided by Wang Shick Ryu [38,39] (Yonsei University, Seoul, South Korea). The construct of HBx-HA was described in a previous study [28]. The plasmids for pAAV control vector and pAAV-HBV genotype A, B, and C were kindly provided by Professor Pei-Jer Chen (National Taiwan University, Taipei, Taiwan) [40].…”

Suppression of Hepatocyte Nuclear Factor 4 Alpha by Long-Term Infection of Hepatitis B Virus Contributes to Tumor Cell Proliferation

“…HBx protein plays an important role in HBV replication and liver pathogenesis through regulation of different host protein that are involved in cell differentiation and proliferation [6,[26][27][28][29][30]. It has been reported that HBx modifies the epigenetic regulation of HBV by interaction with cccDNA, which serves as a template for HBV replication [31].…”

“…The constructs for WT HBV 1.2mer (1.2(+)), X-null HBV 1.2mer (1.2(-)), HBV 1.3mer (1.3(+)) and X-null HBV 1.3mer (1.3(-)) were kindly provided by Wang Shick Ryu [38,39] (Yonsei University, Seoul, South Korea). The construct of HBx-HA was described in previous study [28]. The plasmids for pAAV control vector and pAAV-HBV genotype A, B, and C were kindly provided by Professor Pei-Jer Chen (National Taiwan University, Taipei, Taiwan) [40].…”