Chronic hepatitis B virus infection is a major risk factor for hepatocellular carcinoma (HCC). Hepatitis B virus X protein (HBx) is a hepatitis B virus protein that has multiple cellular functions, but its role in HCC pathogenesis has been controversial. Farnesoid X receptor (FXR) is a nuclear receptor with activities in anti-inflammation and inhibition of hepatocarcinogenesis. However, whether or how FXR can impact hepatitis B virus/HBx-induced hepatocarcinogenesis remains unclear. In this study, we showed that HBx can interact with FXR and function as a coactivator of FXR. Expression of HBx in vivo enhanced FXR-responsive gene regulation. HBx also increased the transcriptional activity of FXR in a luciferase reporter gene assay. The HBx-FXR interaction was confirmed by coimmunoprecipitation and glutathione S-transferase pull-down assays, and the FXR activation function 1 domain was mapped to bind to the third α helix in the C terminus of HBx. We also found that the C-terminally truncated variants of HBx, which were found in clinical HCC, were not effective at transactivating FXR. Interestingly, recruitment of the full-length HBx, but not the C-terminally truncated HBx, enhanced the binding of FXR to its response element. In vivo, FXR ablation markedly sensitized mice to HBx-induced hepatocarcinogenesis.
Conclusions: We propose that transactivation of FXR by full-length HBx may represent a protective mechanism to inhibit HCC and that this inhibition may be compromised upon the appearance of C-terminally truncated HBx or when the expression and/or activity of FXR is decreased. (Hepatology 2017;65:893-906).
© 2016 by the American Association for the Study of Liver Diseases.