TGF-β suppression of HBV RNA through AID-dependent recruitment of an RNA exosome complex

Transforming growth factor (TGF)-β inhibits hepatitis B virus (HBV) replication although the intracellular effectors involved are not determined. Here, we report that reduction of HBV transcripts by TGF-β is dependent on AID expression, which significantly decreases both HBV transcripts and viral DNA, resulting in inhibition of viral replication. Immunoprecipitation reveals that AID physically associates with viral P protein that binds to specific virus RNA sequence called epsilon. AID also binds to an RNA degradation complex (RNA exosome proteins), indicating that AID, RNA exosome, and P protein form an RNP complex. Suppression of HBV transcripts by TGF-β was abrogated by depletion of either AID or RNA exosome components, suggesting that AID and the RNA exosome involve in TGF-β mediated suppression of HBV RNA. Moreover, AID-mediated HBV reduction does not occur when P protein is disrupted or when viral transcription is inhibited. These results suggest that induced expression of AID by TGF-β causes recruitment of the RNA exosome to viral RNP complex and the RNA exosome degrades HBV RNA in a transcription-coupled manner.
Author Summary HBV is one of the causative factors of hepatocellular carcinoma. Recent studies have shown that the members of the APOBEC deaminase family are antiviral factors that suppress the replication of viruses, such as HIV-1 and HBV. APOBEC3G suppresses viral replication by either hypermutation of nascent DNA or inhibition of reverse transcription. Recent studies have been suggested that AID, another APOBEC family member, restricts viruses and retrotransposons that use reverse transcription for their replication. However, little is known about the antiviral mechanisms of AID. TGF-β is a pleiotropic cytokine involved in the suppression of HBV replication, but the mechanism underlying its anti-HBV activity is unclear. In this study, we found that AID plays a role in the anti-HBV activity of TGF-β. Further study revealed that AID physically associates with a viral RNP complex containing reverse transcriptase and recruits the RNA degradosome (RNA exosome) to the RNP complex to degrade the viral RNA. To the best of our knowledge, this study is the first to reveal a novel antiviral pathway in which AID triggers viral RNA degradation by tethering the RNA exosome to the viral reverse transcriptase/RNA complex. Viral RNA may be another target for APOBEC antiviral activity.