Hepatitis B spliced protein (HBSP) promotes the carcinogenic effects of benzo [alpha] pyrene by interacting with microsomal epoxide hydrolase and enhancing its hydrolysis activity

Alternative splicing of viral transcripts: the dark side of HBV

Regulation of alternative splicing is one of the most efficient mechanisms to enlarge the proteomic diversity in eukaryotic organisms. Many viruses hijack the splicing machinery following infection to accomplish their replication cycle. Regarding the HBV, numerous reports have described alternative splicing events of the long viral transcript (pregenomic RNA), which also acts as a template for viral genome replication. Alternative splicing of HBV pregenomic RNAs allows the synthesis of at least 20 spliced variants. In addition, almost all these spliced forms give rise to defective particles, detected in the blood of infected patients. HBV-spliced RNAs have long been unconsidered, probably due to their uneasy detection in comparison to unspliced forms as well as for their dispensable role during viral replication. However, recent data highlighted the relevance of these HBV-spliced variants through (1) the trans-regulation of the alternative splicing of viral transcripts along the course of liver disease; (2) the ability to generate defective particle formation, putative biomarker of the liver disease progression; (3) modulation of viral replication; and (4) their intrinsic propensity to encode for novel viral proteins involved in liver pathogenesis and immune response. Altogether, tricky regulation of HBV alternative splicing may contribute to modulate multiple viral and cellular processes all along the course of HBV-related liver disease.

Aberrant Splicing Events and Epigenetics in Viral Oncogenomics: Current Therapeutic Strategies

Global cancer incidence and mortality are on the rise. Although cancer is fundamentally a non-communicable disease, a large number of cancers are known to have a viral aetiology. A high burden of infectious agents (Human immunodeficiency virus (HIV), human papillomavirus (HPV), hepatitis B virus (HBV)) in certain Sub-Saharan African countries drives the rates of certain cancers. About one-third of all cancers in Africa are attributed to infection. Seven viruses have been identified with carcinogenic characteristics, namely the HPV, HBV, Hepatitis C virus (HCV), Epstein–Barr virus (EBV), Human T cell leukaemia virus 1 (HTLV-1), Kaposi’s Sarcoma Herpesvirus (KSHV), and HIV-1. The cellular splicing machinery is compromised upon infection, and the virus generates splicing variants that promote cell proliferation, suppress signalling pathways, inhibition of tumour suppressors, alter gene expression through epigenetic modification, and mechanisms to evade an immune response, promoting carcinogenesis. A number of these splice variants are specific to virally-induced cancers. Elucidating mechanisms underlying how the virus utilises these splice variants to maintain its latent and lytic phase will provide insights into novel targets for drug discovery. This review will focus on the splicing genomics, epigenetic modifications induced by and current therapeutic strategies against HPV, HBV, HCV, EBV, HTLV-1, KSHV and HIV-1.

Benzo(a)pyrene affects proliferation with reference to metabolic genes and ROS/HIF-1α/HO-1 signaling in A549 and MCF-7 cancer cells

Benzo(a)pyrene (BaP) is a representative polycyclic aromatic hydrocarbon (PAH) compound, which has been implicated in cancer initiation and promotion. Although BaP is one of the most extensively studied pollutants, the underlying mechanisms through which BaP affects reactive oxygen species (ROS)/hypoxia-inducible factor 1α (HIF-1α)/heme oxygenase 1(HO-1) signaling during lung or breast carcinogenesis are not yet fully understood. In this study, we analyzed the effects of 0 (control), 1, 5, or 25 µM BaP exposure on A549 and MCF-7 cancer cells, by evaluating cell viability, cell cycle, and regulatory protein expression, metabolic gene expression, and ROS/HIF-1α/HO-1 signaling. Cell viability increased following exposure to 1 and 5 µM BaP in A549 cells but decreased following exposure to all concentrations of BaP in MCF-7 cells. BaP significantly increased the proportions of cells in S and G2/M phases, with concomitant reductions in the proportions of cells in G0/G1 phase, following 5 and 25 µM exposure, which was accompanied by the upregulation of the regulatory proteins cyclin A, cyclin B, cyclin-dependent kinase (CDK)1, and CDK2. The subsequent upregulation of cytochrome p450 (CYP)1A1, CYP1B1, CYP3A4, epoxide hydrolase (EH), aldo-keto reductase (AKRC1) expression, and the attenuation of multi-drug resistance protein 4 (MRP4), glutathione-S-transferase (GST)1A1, and GST1B1 were also observed in both cell lines. Moreover, the induction of ROS and the modulation of HIF-1α and HO-1 were observed after BaP exposure. Taken together, these findings suggest that BaP affects proliferation with reference to metabolic genes and ROS/HIF-1α/HO-1 signaling in A549 and MCF-7 cancer cells.

HBV Genome and Life Cycle

Chronic hepatitis B virus (HBV) infection remains to be a serious threat to public health and is associated with many liver diseases including chronic hepatitis B (CHB), liver cirrhosis, and hepatocellular carcinoma. Although nucleos(t)ide analogues (NA) and pegylated interferon-α (Peg-IFNα) have been confirmed to be efficient in inhibiting HBV replication, it is difficult to eradicate HBV and achieve the clinical cure of CHB. Therefore, long-term therapy has been recommended to CHB treatment under the current antiviral therapy. In this context, the new antiviral therapy targeting one or multiple critical steps of viral life cycle may be an alternative approach in future. In the last decade, the functional receptor [sodium-taurocholate cotransporting polypeptide (NTCP)] of HBV entry into hepatocytes has been discovered, and the immature nucleocapsids containing the non- or partially reverse-transcribed pregenomic RNA, the nucleocapsids containing double-strand linear DNA (dslDNA), and the empty particles devoid of any HBV nucleic acid have been found to be released into circulation, which have supplemented the life cycle of HBV. The understanding of HBV life cycle may offer a new instruction for searching the potential antiviral targets, and the new viral markers used to monitor the efficacy of antiviral therapy for CHB patients in the future.

Hepatitis B virus core protein promotes hepatocarcinogenesis by enhancing Src expression and activating the Src/PI3K/Akt pathway

Hepatitis B virus core protein (HBc) is expressed preferentially in hepatitis B virus (HBV)-associated hepatocellular carcinoma (HCC). HBc can function as an oncogene arising from its gene regulatory properties, but how it contributes functionally to hepatocarcinogenesis remains unclear. In this study, we determined the molecular and functional roles of HBc during HBV-associated hepatocellular tumorigenesis. HBc increased tumor formation of hepatoma cells. Moreover, expression of HBc specifically promoted proliferation of hepatoma cells in vitro. Mechanistic investigations revealed that these effects were caused by activation of the Src/PI3K/Akt pathway through proximal switch from inactive Src to the active form of the kinase by HBc. HBc-mediated sarcoma (Src) kinase activation was associated with down-regulation of C-terminal Src kinase (Csk). In addition, HBc enhances Src expression by activation of alternative Src 1A promoter in an Sp1 transcription factor-dependent manner. Proliferation induced by stable HBc expression was associated with increased G₁-S cell cycle progression mediated by Src kinase activation. HBc-induced cellular proliferation and tumor formation were reversed by administration of the Src inhibitor saracatinib. Together, our findings suggest that HBc promotes tumorigenesis of hepatoma cells by enhancing the expression of total Src and the active form of the kinase and subsequently activates Src/PI3K/Akt signaling pathway, revealing novel insights into the underlying mechanisms of HBV-associated hepatocarcinogenesis.-Liu, W., Guo, T.-F., Jing, Z.-T., Yang, Z., Liu, L., Yang, Y.-P., Lin, X., Tong, Q.-Y. Hepatitis B virus core protein promotes hepatocarcinogenesis by enhancing Src expression and activating the Src/PI3K/Akt pathway.

Identification of PA28β as a potential novel biomarker in human esophageal squamous cell carcinoma

Esophageal squamous cell carcinoma (ESCC) is one of the most common and serious malignancies in China. However, the exact mechanisms of tumor formation and progression are unclear. As late diagnosis and poor therapeutic efficacy result in lower survival rates, identifying biomarkers for early detection, prognostic evaluation, and recurrence monitoring of ESCC is necessary. Here we analyzed 10 protein expression profiles of ESCC core tissues and paired normal esophageal epithelial tissues using two-dimensional gel electrophoresis. We excised 29 protein spots with two-fold or greater differential expression between cancer and normal tissues and identified them using matrix-assisted laser desorption/ionization-time-of-flight/time-of-flight mass spectrometry. The role of PA28β in ESCC cell was confirmed using cell growth, colony formation and soft agar in TE-1 cells pre- and post- PA28β transfection. Compared to their expression in the adjacent normal epithelia, 12 proteins, including transgelin (TAGLN), were upregulated in ESCC tissues; 17 proteins, including proteasome activator 28-beta subunit (PA28β), were downregulated (p < 0.05). Western blotting and immunohistochemistry confirmed that PA28β was significantly underexpressed in ESCC tissues. The functional assays demonstrate that PA28β inhibited cell growth, proliferation and malignancy of TE-1 cells. Among the differentially expressed proteins, PA28β is a potential tumor inhibitor.

Alternative splicing of hepatitis B virus: A novel virus/host interaction altering liver immunity

BACKGROUND & AIMS

Hepatitis B virus (HBV) RNA can undergo alternative splicing, but the relevance of this post-transcriptional regulation remains elusive. The mechanism of HBV alternative splicing regulation and its impact on liver pathogenesis were investigated.

METHODS

HBV RNA-interacting proteins were identified by RNA pull-down, combined with mass spectrometry analysis. HBV splicing regulation was investigated in chemically and surgically induced liver damage, in whole HBV genome transgenic mice and in hepatoma cells. Viral and endogenous gene expression were quantified by quantitative reverse transcription polymerase chain reaction, Western blot and enzyme-linked immunosorbent assay. Resident liver immune cells were studied by fluorescence-activated cell sorting.

RESULTS

HBV pregenomic RNA-interacting proteins were identified and 15% were directly related to the splicing machinery. Expression of these splicing factors was modulated in HBV transgenic mice with liver injuries and contributed to an increase of the HBV spliced RNA encoding for HBV splicing-generated protein (HBSP). HBSP transgenic mice with chemically induced liver fibrosis exhibited attenuated hepatic damage. The protective effect of HBSP resulted from a decrease of inflammatory monocyte/macrophage recruitment through downregulation of C-C motif chemokine ligand 2 (CCL2) expression in hepatocytes. In human hepatoma cells, the ability of HBSP to control CCL2 expression was confirmed and maintained in a whole HBV context. Finally, viral spliced RNA detection related to a decrease of CCL2 expression in the livers of HBV chronic carriers underscored this mechanism.

CONCLUSION

The microenvironment, modified by liver injury, increased HBSP RNA expression through splicing factor regulation, which in turn controlled hepatocyte chemokine synthesis. This feedback mechanism provides a novel insight into liver immunopathogenesis during HBV infection. Lay summary: Hepatitis B virus persists for decades in the liver of chronically infected patients. Immune escape is one of the main mechanisms developed by this virus to survive. Our study highlights how the crosstalk between virus and liver infected cells may contribute to this immune escape.

Hepatocellular carcinoma and hepatitis B surface protein

The tumorigenesis of hepatitis B virus (HBV)-associated hepatocellular carcinoma (HCC) has been widely studied. HBV envelope proteins are important for the structure and life cycle of HBV, and these proteins are useful for judging the natural disease course and guiding treatment. Truncated and mutated preS/S are produced by integrated viral sequences that are defective for replication. The preS/S mutants are considered “precursor lesions” of HCC. Different preS/S mutants induce various mechanisms of tumorigenesis, such as transactivation of transcription factors and an immune inflammatory response, thereby contributing to HCC. The preS2 mutants and type II “Ground Glass” hepatocytes represent novel biomarkers of HBV-associated HCC. The preS mutants may induce the unfolded protein response and endoplasmic reticulum stress-dependent and stress-independent pathways. Treatments to inhibit hepatitis B surface antigen (HBsAg) and damage secondary to HBsAg or the preS/S mutants include antivirals and antioxidants, such as silymarin, resveratrol, and glycyrrhizin acid. Methods for the prevention and treatment of HCC should be comprehensive.

Microsomal epoxide hydrolase 1 (EPHX1): Gene, structure, function, and role in human disease

Microsomal epoxide hydrolase (EPHX1) is an evolutionarily highly conserved biotransformation enzyme for converting epoxides to diols. Notably, the enzyme is able to either detoxify or bioactivate a wide range of substrates. Mutations and polymorphic variants in the EPHX1 gene have been associated with susceptibility to several human diseases including cancer. This review summarizes the key knowledge concerning EPHX1 gene and protein structure, expression pattern and regulation, and substrate specificity. The relevance of EPHX1 for human pathology is especially discussed.