The hepatitis B virus X protein enhances AP-1 activation through interaction with Jab1

Abstract

Hepatitis B virus X protein (HBx) has many cellular functions and is a major factor in hepatitis and hepatocellular carcinoma caused by HBV infection. A proteomic approach was used to search for HBx-interacting proteins in order to elucidate the molecular mechanism of hepatocarcinogenesis. HBx was attached to myc and flag tags (MEF tags) and expressed in 293T cells; the protein complex formed within the cells was purified and characterized by mass spectrometry. COP9 signalosome (CSN) subunits 3 and 4 were subsequently identified as HBx-interacting proteins. In addition, CSN subunit 5, Jun activation domain-binding protein 1 (Jab1), was shown to be a novel cellular target of HBx. In vivo and in vitro interactions between HBx and Jab1 were confirmed by standard immunoprecipitation and GST pull-down assays. An analysis of HBx deletion constructs showed that amino acids 30-125 of HBx were responsible for binding to Jab1. Confocal laser microscopy demonstrated that HBx was mainly localized in the cytoplasm, while Jab1 was found mainly in the nucleus and partially in the cytoplasm, and that the two proteins colocalized in the cytoplasm. The cotransfection of HBx and Jab1 resulted in substantial activator protein 1 (AP-1) activation and knockdown of endogenous Jab1 attenuated AP-1 activation caused by HBx. In addition, the coexpression of HBx and Jab1 potentiated phosphorylation of JNK, leading to the subsequent phosphorylation of c-Jun, whereas the level of c-Jun and JNK phosphorylation induced by HBx was decreased in Jab1 knockdown cells. These results suggest that the interaction between HBx and Jab1 enhances HBx-mediated AP-1 activation.

The transcription factor c-JUN/AP-1 promotes HBV-related liver tumorigenesis in mice

“…Increased hepatocyte proliferation may occur in response to HBV-mediated liver damage or through the hepatoviral transactivator HBx or truncated preS2 proteins, all of which are expressed in HBV + mice and have been shown to promote c-Jun expression. 22,23 Impaired hepatocyte proliferation in the absence of c-Jun correlated with increased expression of the cell cycle inhibitor p21, which is consistent with previous findings in c-Jun -/-mouse embryonic fibroblasts and in c-Jun Δli mice following 2/3 partial hepatectomy. 13,24 Expression of the cytokine Opn is an early predictor of HCC formation and predicts its recurrence upon surgical resection, in particular in HBV-infected patients.…”

C-terminal-truncated hepatitis B virus X protein enhances the development of diethylnitrosamine-induced hepatocellular carcinogenesis

“…It could therefore be hypothesized that the observed increase in IL-1b and TNFa transcripts might account for JNK activation. Furthermore, in a hepatoma cell line, it has been reported that HBx protein potentiated the phosphorylation of JNK by interacting with Jab1 (Tanaka et al, 2006). Along the same line of evidence, we have observed increased expression of IL-6 in both transgenic mice.…”

Geminiviruses Subvert Ubiquitination by Altering CSN-Mediated Derubylation of SCF E3 Ligase Complexes and Inhibit Jasmonate Signaling inArabidopsis thaliana

“…Although interactions with components of the CRLs have been described previously for DNA and RNA viruses, only few examples of interactions with the CSN complex have been reported, all limited to animal viruses (Mahalingam et al, 1998;Oh et al, 2006;Tanaka et al, 2006;Hsieh et al, 2007). Even though, as for C2-CSN5, those interactions seem to play a role during virus infection (Oh et al, 2006;Tanaka et al, 2006;Hsieh et al, 2007), the mechanisms proposed point to a redirection of proteosomal degradation rather than to an effect on the derubylating activity of the CSN complex itself.…”

“…Even though, as for C2-CSN5, those interactions seem to play a role during virus infection (Oh et al, 2006;Tanaka et al, 2006;Hsieh et al, 2007), the mechanisms proposed point to a redirection of proteosomal degradation rather than to an effect on the derubylating activity of the CSN complex itself.…”