Hepatitis B virus X protein differentially affects the ubiquitin-mediated proteasomal degradation of beta-catenin depending on the status of cellular p53

Conserved Functions of Orthohepadnavirus X Proteins to Inhibit Type-I Interferon Signaling

Orthohepadnavirus causes chronic hepatitis in a broad range of mammals, including primates, cats, woodchucks, and bats. Hepatitis B virus (HBV) X protein inhibits type-I interferon (IFN) signaling, thereby promoting HBV escape from the human innate immune system and establishing persistent infection. However, whether X proteins of Orthohepadnavirus viruses in other species display a similar inhibitory activity remains unknown. Here, we investigated the anti-IFN activity of 17 Orthohepadnavirus X proteins derived from various hosts. We observed conserved activity of Orthohepadnavirus X proteins in inhibiting TIR-domain-containing adaptor protein inducing IFN-β (TRIF)-mediated IFN-β signaling pathway through TRIF degradation. X proteins from domestic cat hepadnavirus (DCH), a novel member of Orthohepadnavirus, inhibited mitochondrial antiviral signaling protein (MAVS)-mediated IFNβ signaling pathway comparable with HBV X. These results indicate that inhibition of IFN signaling is conserved in Orthohepadnavirus X proteins.

Identification of Serum CMTM2 as a Potential Biomarker for HBV-Related Disorders

Substantial advance supports that CMTM2 serve as an important performer in physiological and pathological processes. However, very little is clear about the relationship between CMTM2 and HBV-related disorders. Here, for the first time, we explore that whether or not serum CMTM2 is involved in HBV-related diseases. We found that CMTM2 values were significantly lower in patients compared to healthy control (p <0.001), using ELISA assay. Furthermore, serum CMTM2 levels were negatively correlated with HBV DNA levels in CHB patients but not correlated with the serum levels of ALT and AST. Serum CMTM2 concentrations were not correlated with the serum levels of ALT, AST and HBV DNA load in HBLC and HCC patients. In addition, analysis of the ROC curve indicated that CMTM2 levels were significantly associated with the diagnostic value of HBV-related disorders. Finally, downregulation of CMTM2 was observed in HBV-infected cell model. CMTM2 degradation could be attributed to HBx-activated Lys48 (K48)-linked polyubiquitination, which was abolished by treatment with the proteasome inhibitor MG132. HBV infection suppresses CMTM2 expression by activating ubiquitin-proteasome system. Serum CMTM2 levels can be adopted as an effective indicator of the pathogenesis of HBV-related disorders.

HBx natural variants containing Ser-101 instead of Pro-101 evade ubiquitin-dependent proteasomal degradation by activating proteasomal activator 28 gamma expression

Proteasomal activator 28 gamma (PA28γ) is frequently overexpressed in hepatocellular carcinoma; however, its underlying mechanism and role in hepatitis B virus (HBV) replication are largely unknown. Here, we found that HBV X protein (HBx) natural variants containing Ser-101 instead of Pro-101 increase reactive oxygen species levels in the mitochondria and activate the ataxia telangiectasia mutated/checkpoint kinase 2 pathway in the nucleus, resulting in the phosphorylation of p53 at Ser-15 and Ser-20 and the subsequent upregulation of its protein levels. Therefore, HBx variants containing Ser-101 induced p53-dependent activation of PA28γ expression in human hepatoma cells. The elevated PA28γ levels upregulated HBx levels through the inhibition of seven in absentia homologue 1-dependent proteasomal degradation. The self-amplifying ability of HBx variants containing Ser-101 via a positive feedback loop involving p53 and PA28γ was accurately reproduced in both a 1.2-mer HBV replicon and in vitro HBV infection systems, which also provided evidence for the stimulation of HBV replication by these HBx variants. In conclusion, the ability of HBx to upregulate PA28γ levels via p53 activation, in a Ser-101-dependent pathway, is critical for the stimulation of HBV replication.

Complexity on modulation of NF-κB pathways by hepatitis B and C: A double-edged sword in hepatocarcinogenesis

Nuclear factor-κB (NF-κB), a family of master regulated dimeric transcription factors, signaling transduction pathways are active players in the cell signaling that control vital cellular processes, including cell growth, proliferation, differentiation, apoptosis, morphogenesis, angiogenesis, and immune responses. Nevertheless, aberrant regulation of the NF-κB signaling pathways has been associated with a significant number of human cancers. In fact, NF-κB acts as a double-edged sword in the vital cellular processes and carcinogenesis. This review provides an overview on the modulation of the NF-κB signaling pathways by proteins of hepatitis B and C viruses. One of the major NF-κB events that are modulated by these viruses is the induction of hepatocellular carcinoma. Given the central function of NF-κB in carcinogenesis, it has turned out to be a considerable therapeutic target for cancer therapy.

Downregulation of SHIP2 by Hepatitis B Virus X Promotes the Metastasis and Chemoresistance of Hepatocellular Carcinoma through SKP2

Hepatitis B virus (HBV)-encoded X protein (HBx) plays an important role in the development of hepatocellular carcinoma (HCC). The protein SH2 domain containing inositol 5-phosphatase 2 (SHIP2) belongs to the family of enzymes that dephosphorylate the 5 position of PI(3,4,5)P3 to produce PI(3,4)P2. Expression of SHIP2 has been associated with several cancers including HCC. However, its role in the development of HBV-related HCC remains elusive. In this study, we performed tissue microarray analysis using 49 cases of HCC to explore SHIP2 expression changes and found that SHIP2 was downregulated in HBV-positive HCC. In addition, S-phase kinase-associated protein 2 (SKP2), a component of the E3 ubiquitin–ligase complex, was increased in HCC cell lines that overexpressed HBx, which also showed a notable accumulation of polyubiquitinated SHIP2. Moreover, HCC cells with silenced SHIP2 had increased expression of mesenchymal markers, which promotes cell migration, enhances glucose uptake, and leads to resistance to the chemotherapy drug (5-Fluorouracil, 5-FU). Taken together, our results demonstrate that HBx downregulates SHIP2 through SKP2 and suggest a potential role for SHIP2 in HBx-mediated HCC migration.

The interaction of hepatitis B virus with the ubiquitin proteasome system in viral replication and associated pathogenesis

Background

The ubiquitin proteasome system (UPS) regulates the expression levels of cellular proteins by ubiquitination of protein substrates followed by their degradation via the proteasome. As a highly conserved cellular degradation mechanism, the UPS affects a variety of biological processes and participates in viral propagation.

Main body

During hepatitis B virus (HBV) infection, the UPS is shown to act as a double-edged sword in viral pathogenesis. On the one hand, the UPS acts as a host defense mechanism to selectively recognize HBV proteins as well as special cellular proteins that favor the viral life cycle and induces their ubiquitin-dependent proteasomal degradation to limit HBV infection. On the other hand, the HBV has evolved to subvert the UPS function for its own advantage. Moreover, in the infected hepatocytes, certain cellular proteins that are dependent on the UPS are involved in abnormal biological processes which are mediated by HBV.

Conclusion

The molecular interaction of HBV with the UPS to modulate viral propagation and pathogenesis is summarized in the review. Considering the important role of the UPS in HBV infection, a better understanding of the HBV-UPS interaction could provide novel insight into the mechanisms that are involved in viral replication and pathogenesis and help to develop potential treatment strategies targeting the UPS.

CGK062, a small chemical molecule, inhibits cancer upregulated gene 2‑induced oncogenesis through NEK2 and β‑catenin

The mechanisms through which cancer‑upregulated gene 2 (CUG2), a novel oncogene, affects Wnt/β‑catenin signaling, essential for tumorigenesis, are unclear. In this study, we aimed to elucidate some of these mechanisms in A549 lung cancer cells. Under the overexpression of CUG2, the protein levels and activity of β‑catenin were evaluated by western blot analysis and luciferase assay. To examine a biological consequence of β‑catenin under CUG2 overexpression, cell migration, invasion and sphere formation assay were performed. The upregulation of β‑catenin induced by CUG2 overexpression was also accessed by xenotransplantation in mice. We first found that CUG2 overexpression increased β‑catenin expression and activity. The suppression of β‑catenin decreased cancer stem cell (CSC)‑like phenotypes, indicating that β‑catenin is involved in CUG2‑mediated CSC‑like phenotypes. Notably, CUG2 overexpression increased the phosphorylation of β‑catenin at Ser33/Ser37, which is known to recruit E3 ligase for β‑catenin degradation. Moreover, CUG2 interacted with and enhanced the expression and kinase activity of never in mitosis gene A‑related kinase 2 (NEK2). Recombinant NEK2 phosphorylated β‑catenin at Ser33/Ser37, while NEK2 knockdown decreased the phosphorylation of β‑catenin, suggesting that NEK2 is involved in the phosphorylation of β‑catenin at Ser33/Ser37. Treatment with CGK062, a small chemical molecule, which promotes the phosphorylation of β‑catenin at Ser33/Ser37 through protein kinase C (PKC)α to induce its degradation, reduced β‑catenin levels and inhibited the CUG2‑induced features of malignant tumors, including increased cell migration, invasion and sphere formation. Furthermore, CGK062 treatment suppressed CUG2‑mediated tumor formation in nude mice. Taken together, the findings of this study suggest that CUG2 enhances the phosphorylation of β‑catenin at Ser33/Ser37 by activating NEK2, thus stabilizing β‑catenin. CGK062 may thus have potential for use as a therapeutic drug against CUG2‑overexpressing lung cancer cells.

Hepatitis B virus X protein suppresses all-trans retinoic acid-induced apoptosis in human hepatocytes by repressing p14 expression via DNA methylation

All-trans retinoic acid (ATRA), the most biologically active metabolite of vitamin A, is known to activate p14 expression via promoter hypermethylation to induce p53-dependent apoptosis in human hepatocytes. In this study, we found that the oncogenic hepatitis B virus (HBV) X protein (HBx) of HBV, derived from both overexpression and 1.2-mer replicon systems, suppresses ATRA-induced apoptosis in p53-positive human hepatocytes. For this effect, HBx upregulated both protein and enzyme activity levels of DNA methyltransferase 1, 3a and 3b, in the presence of ATRA and thereby inhibited p14 expression via promoter hypermethylation, resulting in inactivation of the p14-mouse double minute 2 pathway and subsequent downregulation of p53 levels. As a result, HBx was able to impair the potential of ATRA to activate apoptosis-related molecules, including Bax, p53-upregulated modulator of apoptosis, caspase-9, caspase-3 and poly (ADP-ribose) polymerase. In conclusion, the present study provides a new oncogenic action mechanism of HBx, namely by suppressing the anticancer potential of ATRA to induce p53-dependent apoptosis in HBV-infected hepatocytes.

Modulation of Wnt signaling pathway by hepatitis B virus

Hepatitis B virus (HBV) has a global distribution and is one of the leading causes of hepatocellular carcinoma. The precise mechanism of pathogenicity of HBV-associated hepatocellular carcinoma (HCC) is not yet fully understood. Viral-related proteins are known to take control of several cellular pathways like Wnt/β-catenin, TGF-β, Raf/MAPK and ROS for the virus's own replication. This affects cellular persistence, multiplication, migration, alteration and genomic instability. The Wnt/FZD/β-catenin signaling pathway plays a significant role in the pathology and physiology of the liver and has been identified as a main factor in HCC development. The role of β-catenin is linked mainly to the canonical pathway of the signaling system. Progression of liver diseases is known to be accompanied by disturbances in β-catenin expression (mainly overexpression), with its cytoplasmic or nuclear translocation. In recent years, studies have documented that the HBV X protein and hepatitis B surface antigen (HBsAg) can act as pathogenic factors that are involved in the modulation and induction of canonical Wnt signaling pathway. In the present review we explore the interaction of HBV genome products with components of the Wnt/β-catenin signaling pathway that results in the enhancement of the pathway and leads to hepatocarcinogenesis.

Hepatitis B virus X protein activates E3 ubiquitin ligase Siah-1 to control virus propagation via a negative feedback loop

The seven in absentia homologue 1 (Siah-1) protein is an E3 ubiquitin ligase that induces ubiquitin-dependent proteasomal degradation of HBx, the principal regulatory protein of hepatitis B virus (HBV); however, its role in HBV propagation remains unknown. Here, we found that HBx upregulates Siah-1 levels in HepG2 but not in Hep3B cells, in which p53 is absent. For this effect, HBx sequentially activated ataxia telangiectasia mutated kinase and checkpoint kinase 2 via phosphorylation at the Ser-1981 and Thr-68 residues, respectively, which led to the activation of p53 via phosphorylation at the Ser-15 and Ser-20 residues. As a result, HBx was heavily ubiquitinated by Siah-1 and degraded by the ubiquitin-proteasome system in HepG2 cells, whereas this effect was marginal or undetectable in Hep3B cells. Knock-down of p53 in HepG2 cells downregulated Siah-1 levels and subsequently upregulated HBx levels, whereas ectopic p53 expression in Hep3B cells upregulated Siah-1 levels and subsequently downregulated HBx levels. In addition, Siah-1 knock-down impaired the ubiquitination and proteasomal degradation of HBx in HepG2 cells, whereas ectopic Siah-1 expression induced ubiquitin-dependent proteasomal degradation of HBx in Hep3B cells. The effects of HBx on p53 and Siah-1 were exactly reproduced in a 1.2-mer HBV replicon system, mimicking the natural course of HBV infection. In particular, Siah-1 knock-down upregulated the levels of HBx derived from the HBV replicon, resulting in an increase in HBV production. In conclusion, HBx modulates its own protein level via a negative feedback loop involving p53 and Siah-1 to control HBV propagation.

Peptidyl-Prolyl cis/trans Isomerase NIMA-Interacting 1 as a Therapeutic Target in Hepatocellular Carcinoma

Phosphorylation of proteins on serine or threonine residues preceding proline is a pivotal signaling mechanism regulating cell proliferation. The recent identification and characterization of the enzyme peptidyl-prolyl cis/trans isomerase never in mitosis A (NIMA)-interacting 1 (PIN1) has led to the discovery of a new mechanism regulating phosphorylation in cell signaling. PIN1 specifically binds phosphorylated serine or threonine residues immediately preceding proline (pSer/Thr-Pro) and then regulates protein functions, including catalytic activity, phosphorylation status, protein interactions, subcellular location, and protein stability, by promoting cis/trans isomerization of the peptide bond. Recent results have indicated that such conformational changes following phosphorylation represent a novel signaling mechanism in the regulation of many cellular functions. Understanding this mechanism also provides new insight into the pathogenesis and treatment of human hepatocellular carcinoma. A better understanding of the role of PIN1 in the pathogenesis of hepatocellular carcinoma may lead to the identification of molecular targets for prevention and therapeutic intervention.

HBx interacted with Smad4 to deprive activin a growth inhibition function in hepatocyte HL7702 on CRM1 manner

Hepatitis B virus (HBV) is implicated in the pathogenesis of hepatocellular carcinoma, which has been found to be associated with TGF-beta signaling. Activin A is a TGF-β family cytokine that exhibits cell proliferation inhibition on normal hepatocyte. How HBV-encoded X oncoprotein play in activin's activity on hepatocyte has not been developed. In this study, a nontumor hepatic cell line HL7702 with HBX ectogenic expression has been established. MTT and BrdU assays showed that HBx promoted growth of HL7702 cells in vitro and downregulated activin signaling. Deregulated activin signaling pathway by HBX failed to activate target gene p21/waf1 and p15 transcription. In addition, mammalian two-hybrid and coimmunoprecipitation assays revealed that HBX could directly interact with activin signaling transduction protein Smad4, making activated Smad2/3/4 nucleus translocation suppressed. Furthermore, we detected that leptomycin B, the inhibitor of CRM1 protein, could recover nuclear translocation of endogenous Smads complex in HL7702 with HBX expression, indicating that HBX antagonized Smads nucleus translocation, at least partially, on CRM1-dependent manner. Leptomycin B was found to have antigrowth activity on HBX-expressed HL7702, according to its antitumor function in previous study. Above all, HBX antagonized activin signaling in normal human liver cells by interacting with Smad4 might one of the considerable causes of HBX-induced hepatocyte transformation, which deprived activin's cell growth inhibition function at an early stage of tumorigenesis.

Hepatitis B Virus X Protein Upregulates hELG1/ ATAD5 Expression through E2F1 in Hepatocellular Carcinoma

The precise mechanism by which HBx protein of hepatitis B virus (HBV) impacts on hepato-carcinogenesis remain largely elusive despite strong evidences for its' involvement in the process. Here, we have investigated the role of HBx on expression of a novel gene hELG1/ATAD5, which is required for genome maintenance and its' importance in hepatocarcinogenesis. This study has for the first time showed that the expression of this gene was significantly higher in human cancer such as HBV-associated hepatocellular carcinoma (HCC) and in different HCC cell lines compared to normal liver. In addition, a significant elevation in ATAD5 expression was also found in HBx transfected HCC cell lines implicating HBx mediated transcriptional regulation on ATAD5. Using different deletion mutant constructs of putative promoter, the active promoter region was first identified here and subsequently the regulatory region of HBx was mapped by promoter-luciferase assay. But ChIP assay with anti-HBx antibody revealed that HBx was not physically present in ATAD5 transcription machinery whereas anti-E2F1 antibody showed the presence of E2F1 in the complex. Luciferase assay with E2F1 binding site mutant had further confirmed it. Moreover, both loss-and gain-of-function studies of ATAD5 showed that ATAD5 could enhance HBV production in transfected cells whereas knock down of ATAD5 increased the sensitivity of HCC cell line to chemotherapeutics 5-fluorouracil. Overall, this data suggests that a positive feedback loop regulation between ATAD5 and HBV contributed to both viral replication and chemo-resistance of HCC cells.

Innate immune evasion by hepatitis B virus-mediated downregulation of TRIF

Hepatocytes are the target host cells during hepatitis B virus (HBV) infection. Recent studies indicate that the innate immune response of hepatocytes plays an important role in inhibiting HBV replication. TIR-domain-containing adaptor inducing interferon-beta (TRIF) is a key component in innate immune signaling pathways. In this study, we found that the TRIF protein was downregulated in human hepatoma cell lines and liver tissue samples harboring HBV. Hepatitis B virus X protein (HBX) reduced TRIF protein expression in a dose-dependent manner via the proteasomal pathway. HBX appeared to not directly interact with TRIF as these proteins failed to co-immunoprecipitate when overexpressed in hepatoma cells. TRIF upregulation in hepatoma cell lines dramatically inhibited HBV transcription and expression of its viral proteins. Cellular apoptosis induced by TRIF was also confirmed in hepatoma cell lines. Taken together, these findings reveal a new mechanism for HBV evasion of the cellular innate immunity by HBX-mediated degradation of TRIF protein in hepatocytes.

The hepatitis B virus (HBV) HBx protein activates AKT to simultaneously regulate HBV replication and hepatocyte survival

Chronic infection with hepatitis B virus (HBV) is a risk factor for developing liver diseases such as hepatocellular carcinoma (HCC). HBx is a multifunctional protein encoded by the HBV genome; HBx stimulates HBV replication and is thought to play an important role in the development of HBV-associated HCC. HBx can activate the phosphatidylinositol 3-kinase (PI3K)/AKT signaling pathway in some cell lines; however, whether HBx regulates PI3K/AKT signaling in normal hepatocytes has not been evaluated. In studies described here, we assessed HBx activation of PI3K/AKT signaling in an ex vivo model of cultured primary hepatocytes and determined how this HBx activity affects HBV replication. We report that HBx activates AKT in primary hepatocytes and that the activation of AKT decreases HBV replication and HBV mRNA and core protein levels. We show that the transcription factor hepatocyte nuclear factor 4α (HNF4α) is a target of HBx-regulated AKT, and we link HNF4α to HBx-regulated AKT modulation of HBV transcription and replication. Although we and others have shown that HBx stimulates and is likely required for HBV replication, we now report that HBx also activates signals that can diminish the overall level of HBV replication. While this may seem counterintuitive, we show that an important effect of HBx activation of AKT is inhibition of apoptosis. Consequently, our studies suggest that HBx balances HBV replication and cell survival by stimulating signaling pathways that enhance hepatocyte survival at the expense of higher levels of HBV replication.

IMPORTANCE

Chronic hepatitis B virus (HBV) infection is a common cause of the development of liver cancer. Regulation of cell signaling pathways by the HBV HBx protein is thought to influence the development of HBV-associated liver cancer. HBx stimulates, and may be essential for, HBV replication. We show that HBx activates AKT in hepatocytes to reduce HBV replication. While this seems contradictory to an essential role of HBx during HBV replication, HBx activation of AKT inhibits hepatocyte apoptosis, and this may facilitate persistent, noncytopathic HBV replication. AKT regulates HBV replication by reducing the activity of the transcription factor hepatocyte nuclear factor 4α (HNF4α). HBx activation of AKT may contribute to the development of liver cancer by facilitating persistent HBV replication, augmenting the dedifferentiation of hepatocytes by inhibiting HNF4α functions, and activating AKT-regulated oncogenic pathways. AKT-regulated factors may provide therapeutic targets for inhibiting HBV replication and the development of HBV-associated liver cancer.

The HBx oncoprotein of hepatitis B virus deregulates the cell cycle by promoting the intracellular accumulation and re-compartmentalization of the cellular deubiquitinase USP37

The HBx oncoprotein of hepatitis B Virus has been accredited as one of the protagonists in driving hepatocarcinogenesis. HBx exerts its influence over the cell cycle progression by potentiating the activity of cyclin A/E-CDK2 complex, the Cyclin A partner of which is a well-known target of cellular deubiquitinase USP37. In the present study, we observed the intracellular accumulation of cyclin A and USP37 proteins under the HBx microenvironment. Flow cytometry analysis of the HBx-expressing cells showed deregulation of cell cycle apparently due to the enhanced gene expression and stabilization of USP37 protein and deubiquitination of Cyclin A by USP37. Our co-immunoprecipitation and confocal microscopic studies suggested a direct interaction between USP37 and HBx. This interaction promoted the translocation of USP37 outside the nucleus and prevented its association and ubiquitination by E3 ubiquitin ligases - APC/CDH1 and SCF/β-TrCP. Thus, HBx seems to control the cell cycle progression via the cyclin A-CDK2 complex by regulating the intracellular distribution and stability of deubiquitinase USP37.

Are human polyomaviruses co-factors for cancers induced by other oncoviruses?

Presently, 12 human polyomaviruses are known: BK polyomavirus (BKPyV), JCPyV, KIPyV, WUPyV, Merkel cell polyomavirus (MCPyV), HPyV6, HPyV7, Trichodysplasia spinulosa-associated polyomavirus, HPyV9, HPyV10, STLPyV and HPyV12. In addition, the non-human primate polyomavirus simian virus 40 (SV40) seems to circulate in the human population. MCPyV was first described in 2008 and is now accepted to be an etiological factor in about 80% of the rare but aggressive skin cancer Merkel cell carcinoma. SV40, BKPyV and JCPyV or part of their genomes can transform cells, including human cells, and induce tumours in animal models. Moreover, DNA and RNA sequences and proteins of these three viruses have been discovered in tumour tissue. Despite these observations, their role in cancer remains controversial. So far, an association between cancer and the other human polyomaviruses is lacking. Because human polyomavirus DNA has been found in a broad spectrum of cell types, simultaneous dwelling with other oncogenic viruses is possible. Co-infecting human polyomaviruses may therefore act as a co-factor in the development of cancer, including those induced by other oncoviruses. Reviewing studies that report co-infection with human polyomaviruses and other tumour viruses in cancer tissue fail to detect a clear link between co-infection and cancer. Directions for future studies to elaborate on a possible auxiliary role of human polyomaviruses in cancer are suggested, and the mechanisms by which human polyomaviruses may synergize with other viruses in oncogenic transformation are discussed.

Hepatitis B virus X upregulates HuR protein level to stabilize HER2 expression in hepatocellular carcinoma cells

Hepatitis B virus- (HBV-) associated hepatocellular carcinoma (HCC) is the most common type of liver cancer. However, the underlying mechanism of HCC tumorigenesis is very complicated and HBV-encoded X protein (HBx) has been reported to play the most important role in this process. Activation of downstream signal pathways of epidermal growth factor receptor (EGFR) family is known to mediate HBx-dependent HCC tumor progression. Interestingly, HER2 (also known as ErbB2/Neu/EGFR2) is frequently overexpressed in HBx-expressing HCC patients and is associated with their poor prognosis. However, it remains unclear whether and how HBx regulates HER2 expression. In this study, our data showed that HBx expression increased HER2 protein level via enhancing its mRNA stability. The induction of RNA-binding protein HuR expression by HBx mediated the HER2 mRNA stabilization. Finally, the upregulated HER2 expression promoted the migration ability of HBx-expressing HCC cells. These findings deciphered the molecular mechanism of HBx-mediated HER2 upregulation in HBV-associated HCC.

Epigenetic silencing of NAD(P)H:quinone oxidoreductase 1 by hepatitis B virus X protein increases mitochondrial injury and cellular susceptibility to oxidative stress in hepatoma cells

NAD(P)H:quinone oxidoreductase 1 (NQO1) is a phase II enzyme that participates in the detoxification of dopamine-derived quinone molecules and reactive oxygen species. Our prior work using a proteomic approach found that NQO1 protein levels were significantly decreased in stable hepatitis B virus (HBV)-producing hepatoma cells relative to the empty-vector-transfected controls. However, the mechanism and biological significance of the NQO1 suppression remain elusive. In this study we demonstrate that HBV X protein (HBx) induces epigenetic silencing of NQO1 in hepatoma cells through promoter hypermethylation via recruitment of DNA methyltransferase DNMT3A to the promoter region of the NQO1 gene. In HBV-related hepatocellular carcinoma (HCC) specimens, HBx expression was correlated negatively to NQO1 transcripts but positively to NQO1 promoter hypermethylation. Downregulation of NQO1 by HBx reduced intracellular glutathione levels, impaired mitochondrial function, and increased susceptibility of hepatoma cells to oxidative stress-induced cell injury. These results suggest a novel mechanism for HBV-mediated pathogenesis of chronic liver diseases, including HCC.

Using proteomics to identify the HBx interactome in hepatitis B virus: how can this inform the clinic?

Hepatitis B virus (HBV) is a small and enveloped DNA virus, of which chronic infection is the main risk factor of liver cirrhosis and hepatocellular carcinoma. Hepatitis B virus X protein (HBx) is a multifunctional protein encoded by HBV genome, which have significant effects on HBV replication and pathogenesis. Through directly interacting with cellular proteins, HBx is capable to promote HBV replication, regulate transcription of host genes, disrupt protein degradation, modulate signaling pathway, manipulate cell death and deregulate cell cycle. In this review, we briefly discuss the diversified effects of HBx-interactome and their potential clinical significances.

Differential expression of hepatic apurinic/apyrimidinic endonuclease 1, a DNA repair enzyme, in chronic hepatitis

AIM: To determine hepatic expression of apurinic/apyrimidinic endonuclease 1 (APE-1) and 8-hydroxydeoxyguanosine (8-OHdG) in patients with chronic hepatitis B and C.

METHODS: Liver biopsies were obtained from 27 patients with chronic hepatitis B virus (HBV), 30 with chronic hepatitis C virus (HCV), 6 with autoimmune hepatitis (AIH), and 6 with primary biliary cirrhosis (PBC). Normal liver tissue was obtained from surgical resection specimens of four patients. Hepatic APE-1 protein and mRNA expression were assayed by Western blot and by real-time polymerase chain reaction, respectively. Hepatocellular APE-1 and 8-OHdG expression were determined by immunohistochemistry.

RESULTS: The staining intensity of hepatocellular nuclear APE-1 was lower in the HBV group than in the other groups (P < 0.05). Hepatic APE-1 protein levels were reduced in the HBV group relative to the other groups. Hepatic APE-1 mRNA levels were also lower in the HBV group. The proportion of hepatocytes with 8-OHdG-positive nuclei was increased in the HCV, AIH and PBC groups (P < 0.05), but not in the HBV group. Hepatocellular nuclear APE-1 levels were positively correlated with hepatocellular 8-OHdG levels in both the HBV and HCV groups (HBV, r = 0.34, P < 0.05; HCV, r = 0.54, P < 0.01).

CONCLUSION: An imbalance between oxidative DNA damage and APE-1 expression may contribute to hepatocarcinogenesis in chronic viral hepatitis.

Modulation of apoptotic signaling by the hepatitis B virus X protein

Worldwide, an estimated 350 million people are chronically infected with the Hepatitis B Virus (HBV); chronic infection with HBV is associated with the development of severe liver diseases including hepatitis and cirrhosis. Individuals who are chronically infected with HBV also have a significantly higher risk of developing hepatocellular carcinoma (HCC) than uninfected individuals. The HBV X protein (HBx) is a key regulatory HBV protein that is important for HBV replication, and likely plays a cofactor role in the development of HCC in chronically HBV-infected individuals. Although some of the functions of HBx that may contribute to the development of HCC have been characterized, many HBx activities, and their putative roles during the development of HBV-associated HCC, remain incompletely understood. HBx is a multifunctional protein that localizes to the cytoplasm, nucleus, and mitochondria of HBV‑infected hepatocytes. HBx regulates numerous cellular signal transduction pathways and transcription factors as well as cell cycle progression and apoptosis. In this review, we will summarize reports in which the impact of HBx expression on cellular apoptotic pathways has been analyzed. Although various effects of HBx on apoptotic pathways have been observed in different model systems, studies of HBx activities in biologically relevant hepatocyte systems have begun to clarify apoptotic effects of HBx and suggest mechanisms that could link HBx modulation of apoptotic pathways to the development of HBV-associated HCC.

The viral transactivator HBx protein exhibits a high potential for regulation via phosphorylation through an evolutionarily conserved mechanism

BACKGROUND

Hepatitis B virus (HBV) encodes an oncogenic factor, HBx, which is a multifunctional protein that can induce dysfunctional regulation of signaling pathways, transcription, and cell cycle progression, among other processes, through interactions with target host factors. The subcellular localization of HBx is both cytoplasmic and nuclear. This dynamic distribution of HBx could be essential to the multiple roles of the protein at different stages during HBV infection. Transactivational functions of HBx may be exerted both in the nucleus, via interaction with host DNA-binding proteins, and in the cytoplasm, via signaling pathways. Although there have been many studies describing different pathways altered by HBx, and its innumerable binding partners, the molecular mechanism that regulates its different roles has been difficult to elucidate.

METHODS

In the current study, we took a bioinformatics approach to investigate whether the viral protein HBx might be regulated via phosphorylation by an evolutionarily conserved mechanism.

RESULTS

We found that the phylogenetically conserved residues Ser25 and Ser41 (both within the negative regulatory domain), and Thr81 (in the transactivation domain) are predicted to be phosphorylated. By molecular 3D modeling of HBx, we further show these residues are all predicted to be exposed on the surface of the protein, making them easily accesible to these types of modifications. Furthermore, we have also identified Yin Yang sites that might have the potential to be phosphorylated and O-β-GlcNAc interplay at the same residues.

CONCLUSIONS

Thus, we propose that the different roles of HBx displayed in different subcellular locations might be regulated by an evolutionarily conserved mechanism of posttranslational modification, via phosphorylation.

Signaling networks in human hepatocarcinogenesis--novel aspects and therapeutic options

Hepatocellular carcinoma (HCC) represents one of the most common human malignancies with poor prognosis. Because therapeutic strategies are insufficient for most HCC patients, there is a great need to determine the central molecular mechanisms and pathways in order to derive novel targets for systemic therapy. There is vast evidence that not only the dysregulation of distinct signaling cascades, but also their interactions at different levels, affect tumor cell function. Through these interactions, the effects of pathways can be increased, and even new tumor-supporting qualities acquired that further facilitate HCC progression. Although several approaches for the modulation of these relevant pathways are under development, future therapeutic strategies should take into account that oncogenic stimuli cannot be understood in a monodimensional manner. In order to avoid escape mechanisms during therapy, strategies based on comprehensive knowledge of the interactive regulatory network in hepatocarcinogenesis are necessary.

p53 Plays an important role in cell fate determination after exposure to microcystin-LR

BACKGROUND

Microcystin-LR, a cyclic heptapeptide, possesses the ability to inhibit the serine/threonine protein phosphatases PP1 and PP2A and, consequently, exhibits acute hepatocytotoxicity. Moreover, microcystin-LR induces cellular proliferation, resulting in tumor-promoting activity in hepatocytes. However, mechanisms that regulate the balance between cell death and proliferation after microcystin-LR treatment remain unclear.

OBJECTIVE

We examined the contribution of the transcription factor p53, as well as that of the hepatic uptake transporter for microcystin-LR, organic anion transporting polypeptide 1B3 (OATP1B3), to the cellular response to microcystin-LR exposure.

METHODS

We analyzed intracellular signaling responses to microcystin-LR by immunoblotting and real-time reverse-transcriptase polymerase chain reaction techniques using HEK293 human embryonic kidney cells stably transfected with SLCO1B3 (HEK293-OATP1B3). In addition, we analyzed the effect of attenuation of p53 function, via the p53 inhibitor pifithrin-alpha, and knockdown of p53 mRNA on the cytotoxicity of microcystin-LR using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay.

RESULTS

Microcystin-LR induced the phosphorylation and accumulation of p53 in HEK293-OATP1B3 cells, which resulted in up-regulation of the expression of p53 transcript targets, including p21 and seven in absentia homolog 1 (siah-1). In addition, microcystin-LR activated Akt signaling through the phosphorylation of Akt and glycogen synthase kinase 3beta. Although Akt signaling was activated, the accumulation of p53 led cells to apoptosis after treatment with 50 nM microcystin-LR for 24 hr. Both pharmacological inhibition of transcription factor activity of p53 by pifithrin-alpha and knockdown of p53 with small hairpin RNA attenuated the susceptibility of HEK293-OATP1B3 cells to microcystin-LR.

CONCLUSIONS

This study demonstrates the importance of p53 in the regulation of cell fate after exposure to microcystin-LR. Our results suggest that, under conditions of p53 inactivation (including p53 mutation), chronic exposure to low doses of microcystin-LR may lead to cell proliferation through activation of Akt signaling. Results of this study may contribute to the development of chemoprevention and chemotherapeutic approaches to microcystin-LR poisoning.

Hepatitis B virus X protein prevents apoptosis of hepatocellular carcinoma cells by upregulating SATB1 and HURP expression

Protein X from hepatitis B virus (HBV) appears to play a critical role in the development of hepatocellular carcinoma (HCC). The hepatoma upregulated protein (HURP) is also upregulated in a majority of HCC cases, therefore suggesting that HURP represents an oncogene. In this study, we describe a link between the viral protein HBx, HURP, and the establishment of cisplatin chemoresistance in HCC cells. Hep3B cells which express HBx displayed increased levels of HURP mRNA and protein, and showed resistance to cisplatin-induced apoptosis. Knockdown of HURP in HBx-expressing cells reversed this effect and sensitized Hep3B cells to cisplatin. Interestingly, SATB1, a global gene regulator which is often overexpressed in malignant breast cancer, was also induced following expression of HBx. The anti-apoptotic effect of HBx was shown to require activation of the p38/MAPK pathway in Hep3B cells. In addition, the expression of survivin, an anti-apoptotic protein, was also upregulated by HBx in an HURP-dependent manner. Taken together, these results indicate that HBx activates the expression of HURP via the p38/MAPK pathway and the SATB1 protein, culminating with the accumulation of the anti-apoptotic protein survivin. Our findings illustrate the role of the viral protein HBx in preventing apoptosis during cancer progression and establishment of chemoresistance.

The proteasome and its role in the degradation of oxidized proteins

The generation of free radicals and the resulting oxidative modification of cell structures are omnipresent in mammalian cells. This includes the permanent oxidation of proteins leading to the disruption of the protein structure and an impaired functionality. In consequence, these oxidized proteins have to be removed in order to prevent serious metabolic disturbances. The most important cellular proteolytic system responsible for the removal of oxidized proteins is the proteasomal system. For normal functioning, the proteasomal system needs the coordinated interaction of numerous components. This review describes the fundamental functions of the 20S "core" proteasome, its regulators, and the roles of the proteasomal system beyond the removal of oxidized proteins in mammalian cells.

HBV X protein: elucidating a role in oncogenesis

Chronic HBV infection is associated with the development of hepatocellular carcinoma (HCC). HBV contributes to tumorigenesis by encoding hepatitis B x antigen (HBxAg), which is a trans-regulatory protein that appears to contribute to HCC by altering patterns of host gene expression. In this review, recent data is presented that outlines some of the putative mechanisms whereby HBxAg contributes to HCC. With the development of animal models of HBxAg-mediated HCC, the relevance and temporal order of putative steps in this process can now be dissected to elucidate what is rate limiting and when. This will have a profound impact on the design of novel and specific therapeutics for HCC.

Hepatocellular carcinoma and the ubiquitin-proteasome system

Hepatocellular carcinoma is one of the largest causes of cancer-related deaths worldwide for which there are very limited treatment options that are currently effective. The ubiquitin-proteasome system has rapidly become acknowledged as both critical for normal cellular function and a frequent target of de-regulation leading to disease. This review appraises the evidence linking the ubiquitin-proteasome system with this devastatingly intractable cancer and asks whether it may prove to be fertile ground for the development of novel therapeutic interventions against hepatocellular carcinoma.

SIAH-1 interacts with the Kaposi's sarcoma-associated herpesvirus-encoded ORF45 protein and promotes its ubiquitylation and proteasomal degradation

Kaposi's sarcoma-associated herpesvirus (KSHV), also referred to as human herpesvirus 8, is a potentially tumorigenic virus implicated in the etiology of Kaposi's sarcoma, primary effusion lymphoma, and some forms of multicentric Castleman's disease. The open reading frame 45 (ORF45) protein, encoded by the KSHV genome, is capable of inhibiting virus-dependent interferon induction and appears to be essential for both early and late stages of infection. In the present study, we show, both in yeast two-hybrid assays and in mammalian cells, that the ORF45 protein interacts with the cellular ubiquitin E3 ligase family designated seven in absentia homologue (SIAH). We provide evidence that SIAH-1 promotes the degradation of KSHV ORF45 through a RING domain-dependent mechanism and via the ubiquitin-proteasome system. Furthermore, our data indicate the involvement of SIAH-1 in the regulation of the expression of ORF45 in KSHV-infected cells. Since the availability of KSHV ORF45 is expected to influence the course of KSHV infection, our findings identify a novel biological role for SIAH proteins as modulators of virus infection.