The emerging role of hepatitis B virus pre-S2 deletion mutant proteins in HBV tumorigenesis

Abstract

Chronic hepatitis B virus (HBV) infection can cause hepatocellular carcinoma (HCC). Several hypotheses have been proposed to explain the mechanisms of HBV tumorigenesis, including inflammation and liver regeneration associated with cytotoxic immune injuries and transcriptional activators of mutant HBV gene products. The mutant viral oncoprotein-driven tumorigenesis is prevailed at the advanced stage or anti-HBe-positive phase of chronic HBV infection. Besides HBx, the pre-S2 (deletion) mutant protein represents a newly recognized oncoprotein that is accumulated in the endoplasmic reticulum (ER) and manifests as type II ground glass hepatocytes (GGH). The retention of pre-S2 mutant protein in ER can induce ER stress and initiate an ER stress-dependent VEGF/Akt/mTOR and NFκB/COX-2 signal pathway. Additionally, the pre-S2 mutant large surface protein can induce an ER stress-independent pathway to transactivate JAB-1/p27/RB/cyclin A,D pathway, leading to growth advantage of type II GGH. The pre-S2 mutant protein-induced ER stress can also cause DNA damage, centrosome overduplication, and genomic instability. In 5-10% of type II GGHs, there is co-expression of pre-S2 mutant protein and HBx antigen which exhibited enhanced oncogenic effects in transgenic mice. The mTOR signal cascade is consistently activated throughout the course of pre-S2 mutant transgenic livers and in human HCC tissues, leading to metabolic disorders and HCC tumorigenesis. Clinically, the presence of pre-S2 deletion mutants in sera frequently develop resistance to nucleoside analogues anti-virals and predict HCC development. The pre-S2 deletion mutants and type II GGHs therefore represent novel biomarkers of HBV-related HCCs. A versatile DNA array chip has been developed to detect pre-S2 mutants in serum. Overall, the presence of pre-S2 mutants in serum has implications for anti-viral treatment and can predict HCC development. Targeting at pre-S2 mutant protein-induced, ER stress-dependent, mTOR signal cascade and metabolic disorders may offer potential strategy for chemoprevention or therapy in high risk chronic HBV carriers.

Figure 1

Two types of ground glass hepatocytes and the corresponding deletions at pre-S regions of large surface protein. Ground glass hepatocytes (GGH) and the expression patterns of hepatitis B surface antigens. Type I GGH are usually scattered singly and express an inclusion-like pattern of the surface antigen, while type II GGH consistently cluster in nodules and express a marginal pattern of the surface antigen (A). The profile of deletions over the pre-S regions in sera of HBV-related HCC patients. A complex combination of deletions at pre-S1 and pre-S2 regions may occur in patients of chronic HBV carriers (B). Hematoxylin–Eosin stain, HE; Immunohistochemical stain, IHC.

Figure 2

Schematic depiction of the potential signals induced by pre-S mutants and the candidate targets for chemoprevention. Both types of pre-S mutants can induce endoplasmic reticulum (ER) stress signals, which may lead to oxidative stress and DNA damage, leading to genomic instability. The pre-S mutants may also activate two signal pathways to protect the hepatocytes from apoptosis, one involving nuclear factor (NF)-κB to upregulate cyclooxygenase-2 (COX-2) and the other vascular endothelial growth factor to activate Akt/mammalian target of Rapamy- cin (mTOR) signaling. Pre-S2 mutant can additionally induce an ER stress-independent c-Jun activation domain binding protein 1 (JAB1)/ p27/retinoblastoma (Rb)/adenovirus E2 promoter binding factor/cyclin A signal to initiate cell cycle progression. Combined effects of genomic instability and cell proliferation will potentially result in carcinogenesis. Resveratrol and Silymarin are two nature products could be used to target PPAR-α/γ and mTOR signal cascade for chemoprevention in high risk HBV carriers. Cdk2, cyclin-dependent kinase 2; HBV, hepatitis B virus; ROI, reactive oxygen intermediate.

Figure 3

Proposed model of HBV pre-S2 mutant-induced in metabolic disturbance and tumorigenesis. HBV pre-S2 mutant activated mTOR through ER stress-dependent VEGF-A/Akt signal cascade. The activated mTOR induced glycolysis via YY1/c-Myc/Glut1 signaing and promoted de novo lipogenesis through the activation of SREBP-1 and ACLY signals. The combined effects of aerobic glycolysis and de novo lipogenesis contributed to growth advantages of hepatocytes and subsequent HCC development.