Background: Hepatitis B virus (HBV) causes both acute and chronic liver injury. Viral proteins are involved in the pathological progress. Hepatitis B core antigen (HBcAg), a component of viral nucleocapsid, is not only essential for HBV lifecycle, but also exhibits strong immunogenicity. The cytoplasmic location of HBcAg in liver biopsy is associated with liver injury and inflammation, but the exact mechanisms remain to be elaborated. Methods: Huh7, SMMC-7721 and L-02 cells were transfected with pEGFP-N1-HBcAg to establish an intracellular HBcAg expression model. The mRNA and protein levels of Interleukin (IL)-6 were detected by qPCR and ELISA respectively. The signaling pathway-related proteins were investigated by western blot and immunofluorescence assay. Results: HBcAg increased the expression and secretion of IL-6 through activating extracellular signal-related kinase (ERK), p38 mitogen-activated protein kinase (p38 MAPK) and nuclear factor-kappa B (NF-κB). These activations can be blocked by specific inhibitors of the three pathways. Conclusions: HBcAg actives p38, ERK1/2 and NF-κB to enhance the production of IL-6 in hepatocytes. This provides a molecular mechanism to explain the association of cytoplasmic HBcAg with severe liver injury and inflammation.

During hepatitis B virus (HBV) infection, three viral envelope proteins of HBV are overexpressed in the endoplasmic reticulum (ER). The large S protein (LHBs) and truncated middle S protein (MHBst) have been documented to play roles in regulating host gene expression and contribute to hepatic disease development. As a predominant protein at the ultrastructural level in biopsy samples taken from viremic patients, the role of the middle S protein (MHBs) remains to be understood despite its high immunogenicity. When we transfected hepatocytes with an enhanced green fluorescent protein (EGFP)-tagged MHBs expressing plasmid, the results showed that expression of MHBs cause an upregulation of IL-6 at the message RNA and protein levels through activating the p38 mitogen-activated protein kinase (p38 MAPK) and nuclear factor-kappa B (NF-κB) pathways. The use of specific inhibitors of the signaling pathways can diminish this upregulation. The use of BAPTA-AM attenuated the stimulation caused by MHBs. We further found that MHBs accumulated in the endoplasmic reticulum and increased the amount of glucose regulated protein 78 (GRP78/BiP). Our results provide a possibility that MHBs could be involved in liver disease progression.