Hepatitis B Virus Polymerase Impairs Interferon-α-Induced STAT Activation Through Inhibition of Importin-α5 and Protein Kinase C-δ

Treatment with exogenous interferon (IFN)-alpha is not effective in the majority of patients with chronic hepatitis B virus (HBV) infection. Recent evidence suggests that HBV has evolved strategies to block the nuclear translocation of signal transducer and activator of transcription (STAT) 1 to limit IFN-alpha-induced cellular antiviral responses. However, it remains unclear whether STAT1 translocation is impaired in chronic hepatitis B patients and what mechanisms are involved. Here we report that the expression of HBV polymerase (Pol) in human hepatic cell lines inhibited induction of IFN-stimulated genes and resulted in a weakened antiviral activity of IFN-alpha. Ectopic expression of Pol suppressed IFN-alpha-induced STAT1 serine 727 phosphorylation and STAT1/2 nuclear accumulation, whereas STAT1 tyrosine 701 phosphorylation, and STAT1-STAT2 heterodimer formation were not affected. Further studies demonstrated that Pol interacted with the catalytic domain of protein kinase C-delta (PKC-delta) and perturbed PKC-delta phosphorylation and its association with STAT1, which resulted in the suppression of STAT1 Ser727 phosphorylation. Moreover, Pol was found to interfere with nuclear transportation of STAT1/2 by competitively binding to the region of importin-alpha 5 required for STAT1/2 recruitment. Truncation analysis suggested that the terminal protein and RNase H domains of Pol were able to bind to PKC-delta and importin-alpha 5, respectively, and were responsible for the inhibition of IFN-alpha signaling. More importantly, the inhibition of STAT1 and PKC-delta phosphorylation were confirmed in a hydrodynamic-based HBV mouse model, and the blockage of IFN-alpha-induced STAT1/2 nuclear translocation was observed in HBV-infected cells from liver biopsies of chronic HBV patients. Conclusions: These results demonstrate a role for Pol in HBV-mediated antagonization of IFN-alpha signaling and provide a possible molecular mechanism by which HBV resists the IFN therapy and maintains its persistence. (HEPATOLOGY 2013;57:470-482)