Hepatitis B small surface protein hijacking Bip is initial and essential to promote lipid synthesis

To date, the molecular pathogenic mechanisms between HBsAg and liver metabolic disorders have not been fully understood. To explore the overall effects of HBsAg on liver tissues from HBV transgenic mice, proteome, interactome, and signal pathway analysis were employed to uncover the underlying mechanisms. Bioinformatics analysis of 191 differentially expressed proteins suggested that HBV upregulated the expression of multiple enzymes involved in lipid synthesis, and small HBs (SHBs) caused lipid accumulation in cells. Further studies showed that SHBs bound to binding immunoglobulin protein (Bip), which normally functions in cell homeostasis against the unfolded protein response (UPR) signaling via occupying inositol-requiring enzyme 1 (IRE1). Hijacking Bip by SHBs alleviated the inhibition of post-endoplasmic reticulum (ER) signaling and sequential activation of the IRE1 downstream transcription factors involved in lipid synthesis, such as spliced X-box binding protein 1 (sXBP1) and sterol regulatory element-binding protein 1 (SREBP1), leading to lipid metabolism disorder. The restoration of Bip can alleviate ER stress, and block the sequential post-ER signaling caused by SHBs. This study revealed a new pathway through which SHBs promote lipid disorder, and suggests that Bip may serve as a novel target for intervention in HBV related liver diseases. SIGNIFICANCE: In this study, we found a new pathway promoting the lipid disorder by SHBs through quantitative proteomics studies, and Bip may serve as a novel target for intervention in HBV related liver diseases. These findings highlight a novel role of SHBs in regulating cell lipid metabolism and provide an insight into the relationship between HBV infection and liver fatty disorders, which may serve as a potential therapeutic target for intervention of HBV related liver diseases.