Induction of hepatitis D virus large antigen translocation to the cytoplasm by hepatitis B virus surface antigens correlates with endoplasmic reticulum stress and NF-κB activation

Abstract

It is known that hepatitis D virus (HDV) requires hepatitis B virus (HBV) for supplying envelope proteins (HBsAgs) to produce mature virions, and the HDV large antigen (LDAg) is responsible for interacting with HBsAgs. However, the signal molecules involved in the cross-talk between HBsAgs and LDAg have never been reported. It has been previously demonstrated that the small form of HBsAg can facilitate the translocation of HDV large antigen green fluorescent protein (GFP) fusion protein (GFP–LD) from the nucleus to the cytoplasm. In this study, it was confirmed that the small form of HBsAg can facilitate both GFP–LD and authentic LDAg for nuclear export. It was also shown that the three forms of HBsAgs (large, middle and small) induced various rates (from 35.4 to 57.2 %) of GFP–LD nuclear export. Since HBsAgs are localized inside the endoplasmic reticulum (ER), this suggests that ER stress possibly initiates the signal for inducing LDAg translocation. This supposition is supported by results that show that around 9 % of cells appear with GFP–LD in the cytoplasm after treatment with the ER stress inducers, brefeldin A (BFA) and tunicamycin, in the absence of HBsAg. Western blot and immunofluorescence microscopy results further showed that the activation of NF-κB is linked to the ER stress that induces GFP–LD translocation. Combining this with results showing that tumour necrosis factor alpha (TNF-α) can also induce GFP–LD translocation, it was concluded that LDAg translocation correlates with ER stress and activation of NF-κB. Nevertheless, TNF-α-induced GFP–LD translocation was independent of new protein synthesis, suggesting that a post-translational event occurs to GFP–LD to allow translocation.