Interferon alpha-inducible protein 27 (IFI27) inhibits hepatitis B virus (HBV) transcription through downregulating cellular transcription factor C/EBPα.

Interferon alpha (IFNα) is the only approved immunomodulatory drug for chronic hepatitis B treatment, exerting its antiviral effects through the induction of interferon-stimulated genes (ISGs). To identify key antiviral ISGs that inhibit hepatitis B virus (HBV) replication, we performed transcriptome analysis of IFNα-treated HepG2-NTCP cells and found that IFI27 was among the most differentially expressed genes. The high inducibility of IFI27 by IFNα was further validated in primary human hepatocytes. Overexpression of IFI27 significantly suppressed HBV replication in both HBV-transfected and -infected cells, primarily by reducing HBV RNA transcription. Conversely, IFI27 knockdown markedly diminished the antiviral effect of IFNα in HBV-infected cells. IFI27 is predominantly localized in the cytoplasm, and RNA-seq analysis revealed that IFI27 inhibits HBV transcription without drastically altering the host transcriptome, indicating that IFI27 does not inhibit HBV transcription directly or through altering the transcription of cellular transcription factors or inducing antiviral signaling pathways. Instead, we found that IFI27 suppresses HBV transcription by promoting the ubiquitination-dependent proteasomal degradation of C/EBPα in the cytoplasm, a cellular transcription factor critical for HBV RNA transcription. Further investigation identified the E3 ubiquitin ligase SKP2 as a key mediator of this process, facilitating IFI27-induced C/EBPα ubiquitination and degradation. Notably, SKP2 knockdown abrogated IFI27's antiviral activity against HBV. Taken together, our findings reveal that IFI27 contributes to IFNα-mediated antiviral activity against HBV by targeting C/EBPα for SKP2-dependent ubiquitination and proteasomal degradation. This study thus sheds new light on the antiviral mechanism of IFNα-based therapy for chronic hepatitis B.IMPORTANCEChronic hepatitis B virus (HBV) infection affects approximately 250 million people worldwide with limited treatment options. Interferon alpha (IFNα) remains the only approved immunomodulatory treatment for chronic hepatitis B, working in HBV-infected liver cells through inducing antiviral genes. To identify key interferon-inducible genes involved in HBV suppression, we performed transcriptome analysis of IFNα-treated liver cells and identified IFI27 as one of the most upregulated genes. Functional studies demonstrated that IFI27 inhibits HBV replication by reducing viral RNA transcription, and its knockdown significantly impaired the antiviral effect of IFNα. Mechanistically, IFI27 suppresses HBV transcription by promoting the ubiquitin-proteasome-mediated degradation of C/EBPα, a transcription factor critical for HBV RNA synthesis. This process is dependent on the E3 ubiquitin ligase SKP2, as SKP2 knockdown abolished IFI27-mediated antiviral activity. These findings reveal IFI27 as a critical mediator of IFNα-induced antiviral responses against HBV and provide new insights into host-directed antiviral mechanisms with potential therapeutic implications.