Figure 1: Mechanisms underlying how HBX protein transcriptionally transactivates various cellular genes leading to disruption in multiple biological processes that are critical for the development of HBV-induced hepatocellular carcinoma. (A) Production of TGFβ2 and VEGF via activation of several pathways including PI3K, mTOR, IκB kinase β (IKKβ), NF-κB, ribosomal protein S6 kinase 1 and Rac. (B) Regulation of IL-8 and IL-29 through ERK and JNK signalling pathways. (C) Suppression of p53 protein-mediated apoptosis via activation of HBx-survivin-HBXIP complex. (D) Induction of apoptosis or cell proliferation associated with aberrant chromosome segregation through interaction of HBx and DDB1 protein
Mankgopo M Kgatle1* Asgar A Kalla2 C Wendy Spearman1 Henry N Hairwadzi11Division of Hepatology and Liver Research, Department of Medicine, Faculty of Health Sciences, University of Cape Town, Groote Schuur Hospital, Western Cape, South Africa.
2Division of Rheumatology, Department of Medicine, Faculty of Health Sciences, University of Cape Town, Groote Schuur Hospital, Western Cape, South Africa.
*Corresponding author: Mankgopo M Kgatle, Division of Hepatology, Department of Medicine, Faculty of Health Sciences, University of Cape Town, Groote Schuur Hospital, Cape Town, South Africa, E-mail: firstname.lastname@example.org
Hepatitis B x (HBx) is the smallest protein of 17kDa that is encoded by hepatitis B virus (HBV). HBx protein is more abundantly expressed in the cytoplasm than in the nucleus of HBV-infected hepatocytes. Considerable data suggest that HBx protein exploits the entire body of cellular signalling pathways by exerting its transcriptional transactivation activities and epigenetic alterations for viral survival and propagation. In this review, we intend to highlight the transcriptional activities of HBx protein and its role in inducing epigenetic anomalies that may lead to tumour initiation, aggressiveness and metastases.
Epigenetics alterations; HBV; HBx; HCC; DNA Methylation; Histone Modification; Transcriptional activities
HBV is a non-cytopathic hepadnavirus transmitted prenatally, sexually and percutaneously. More than 350 million people are chronically infected worldwide and 1million die from infection annually as a result of hepatic cirrhosis and hepatocellular carcinoma (HCC) [1,2]. Amongst four HBV partially overlapping open reading frames is the smallest hepatitis B virus X (HBx) gene that encodes 154 amino acid regulatory protein . HBx protein is a transcriptional transactivator that is required to initiate and maintain virus replication . It promotes viral propagation and ultimately HBV-related malignant transformation by abnormally regulating several cellular pathways, which are involved in DNA repair, cell growth, differentiation, adhesion, proliferation and apoptosis [5-9]. This review highlights various molecular mechanisms which HBx protein utilises in promoting HBV-induced HCC.
Transcriptional Transactivation Activities of HBx Protein and Hepatocarcinogenesis
Dysregulated apoptosis is a phenotypic feature of HBV-related hepatocarcinogenesis. HBV-induced DNA damage often triggers localised apoptotic-related signals resulting in tissue necrosis. This process enables the elimination of damaged, unwanted and redundant hepatocytes that may otherwise lead to uncontrolled cell growth, proliferation and liver disease [10,11]. Aberrant regulation of cell proliferation and apoptosis has been identified as the consequence of abnormal inactivation or activation of gene transcription in HBV-induced HCC . Although the underlying mechanism remains unknown, HBx protein may inhibit apoptosis by blocking the transactivation of caspase cascade 3, nuclear factor kappa B (NF-κB) and phosphatidylinositol 3-kinase (PI3K) signal transduction pathways [12-14]. For instance, HBx protein inhibits Fasmediated apoptosis of hepatoma cells by upregulating mFAS/FasL, sFas and NF-κB (Figure 1) [10,15]. On the other hand, interaction of HBV x-associated protein with protein kinase C was shown to induce apoptosis by switching on the transcription factor NF-κB [16,17]. Being localised in the cytoplasm, HBx protein also has the ability to sensitize HBV-infected cells towards FLICE-like inhibitory protein (FLIP), jun amino-terminal kinases (JNK), caspases 3 and 9 pro-apoptotic pathways and induce apoptosis . HBx exerts its oncogenic properties and causes HCC in nude mice by transforming the non-transformed immortalized liver cell line QSG7701 . HBx also communicates with mitogen activated protein kinases/extracellular signal regulated kinases (MAPK/ERK) signalling and activates PI3K/Akt pathway to transform cells leading to c-myc-mediated cell survival through inhibition of HBx-induced apoptosis (Figure 1A) [20,21]. This may influence HBV replication by abnormally regulating various cellular processes such as DNA repair, cell growth, differentiation, adhesion, and proliferation, which may promote hepatocarcinogenesis [10,15,22-24].
HBx and transforming growth factor beta 1
Transforming growth factor beta (TGF-β), encoded by TGF-β gene, is required for wound healing and hepatic tissue repair. It regulates several cellular functions including cell growth, differentiation, apoptosis and homoeostasis . TGF-β belongs to the TGF super family that includes various isoforms: TGF-β1, TGF-β2 and TGF-β3 . TGF-β1 is a cytokine that is produced in response to liver injury by activated hepatocytes, platelets and Kupffer cells . As a transcriptional transactivator, HBx protein suppresses TGF-β-induced apoptosis through activation of PI3K pathway that contributes to hepatocarcinogenesis by cross-talking with other pathways such as AKT/mTOR and Ras/MAPK (Figure 1A) . HBx-mediated upregulation of TGF-β1 and downregulation of α2- macroglobulin promotes proliferated hepatic stellate cells leading to HBV-related fibrosis [11,27,29]. Upregulation of TGF-β1 correlates with the mutation and loss of mannose-6-phosphate/IGF-II receptor that mediates TGF-β1 activation leading to HBV-induced HCC [30-32]. Liu and co-authors have shown that HBV promotes hepatocarcinogenesis in BALB/c mice through upregulation of SMAD7 and inhibition of TGF- β-induced apoptosis . In human hepatic stellate cells, upregulation of TGF-β1 and its downstream mediator of fibrogenic action known as connective tissue growth factor lead to enhanced cell proliferation and progressive fibrosis .
HBx and vascular endothelial growth factor
HCC was recently labelled a hypervascular tumour due to its association with vascular endothelial growth factor (VEGF)-mediated activities that promotes vasculogenesis and angiogenesis . Activation of COX-2-mediated PGE2 enhances the expression of VEGF and tumour angiogenesis in HBV-related HCC . In dysplastic nodules of hepatocarcinogenesis, upregulation of VEGF-A and its co-operating receptors FIK-1 and hypoxia-inducible factor-1 alpha (HIF-1α) promote angiogenesis and support sustained growth of these precursor lesions contributing to the formation of hepatic cancer and metastases . Production of VEGF occurs via activation of several pathways including mTOR, IκB kinase β (IKKβ), NF-κB, ribosomal protein S6 kinase 1 and Rac (Figures 1A and 1B) [37-41]. HBx-induced expression of VEGFR-3 splice variant in HCC patients correlates with tumour aggressiveness, tumour relapse and poor prognosis .
HBx and interleukin-8
Interleukin-8 (IL-8) is a leukocyte chemotactic activating cytokine secreted in response to an inflammatory stimulus by macrophages, endothelial and epithelial cells. IL-8 may function as a regulatory factor within the tumour environment and it is implicated in various cellular signalling including cell growth, proliferation, angiogenesis and migration. Upregulation of IL-8 in chronic HBV infection has been observed, and it was found to correlate with interferon-alpha therapy resistance, advanced liver inflammation and fibrosis [43,44]. Previous studies have shown that HBx protein increases the expression of IL-8 by interacting with NF-κB and CCAAT enhancer-binding protein (C/EBP)-like cis elements . C/ EBP-like cis element regulates the expression of COX-2, another protein implicated in HBx transcriptional transactivation activity. Overexpression of COX-2 significantly correlates with increased HBx protein in HCC, suggesting that COX-2 may be hijacked in influencing HCC-related micro angiogenesis and metastases . This may be explained by the co-operative network and triple effects of IL-8, IL-29 and COX-2 when upregulated in response to increased viral replication in chronic HBVinfected patients and hepatoma cultured cells. It appears that HBV induces a differential regulatory network of inflammatory responses in which IL-29, IL-8 and COX-2 regulate one another. In this way, upregulation of IL-29 by HBV activates IL-8 that in turn suppresses IL-29 production (Figure 1B). This enhances the translocation of cAMP response element binding (CREB) and C/EBP transcription factors from cytosol to nucleus by stimulating ERK and JNK signalling pathways, which activate COX- 2 and PGE2 production leading to enhanced HBV replication associated with severe inflammation and tumorigenesis. COX-2 also represses the production of IL-8, and IL-29 induces antiviral factors protein kinase R and 2’-5’ oligoadenylate synthetase leading to suppressed HBV replication .
HBx and p53 protein
HBx protein contributes to hepatocarcinogenesis by blocking p53- mediated cellular processes that are important for maintaining the genomic integrity of hepatocytes [48,49]. In normal circumstances, p53 regulates apoptosis by interacting with cytoplasmic transcription factors such as repair cross-complement in grodent-repair deficiency group 2, xanthoma pigmentosa B, fatty acid synthase, p21CIPWARF1, and ankyrinrepeat containing and proline-rich region-containing proteins (ASPP). These transcription factors are implicated in the p53-mediated nucleotide excision repair and enhance the binding of p53 gene to proapoptotic stimuli [13,50-52]. Suppression of p53 protein-mediated apoptosis also occurs via activation of cyclooxygenase 2 (COX-2)-prostaglandin E2 and McI-1 anti-apoptotic pathways (Figure 1C). In cultured hepatic oval cells, HBx protein promotes cell proliferation by enhancing the expression of Let-7a-microRNA, signal transducer and activator of transcription 3 (STAT3) and cyclin D1 though activation of MAPK/ERK and P13Kdependent signalling pathways . Interaction of HBx protein and p53 mutant correlates with progressive tumour formation driven by the activation of MYC, JNK, VEGF and phosphatase and tensin homolog (PTEN) through PI3K/AKT pathway . Inhibiting the transcription activities of HBx protein could lead to suppressed tumour initiation, growth and metastases.
HBx protein and survivin-HBXIP complex
Several studies demonstrate that anti-apoptotic protein survivin forms a complex with hepatitis B-X interacting protein (HBXIP), and that this complex interacts with HBx protein in contributing to hepatocarcinogenesis. Survivin is an anti-apoptosis gene expressed in various human malignancies, and its upregulation is implicated in HBxassociated HCC [54-56]. HBXIP is a conserved 18 kDa ubiquitous protein that was first discovered as a binding partner for HBx protein, and it negatively regulates HBx protein activity leading to disruption in the HBV replicative cycle . Mouse studies have shown that HBXIP is required for hepatocyte growth and survival; it functions as a binding partner for survivin . HBx protein interacts with survivin-HBXIP complex and correlates with dysregulated centromere dynamics and mitotic spindle formation (Figure 1C) . This interaction was found to supress caspase activation, modulate p53 checkpoints and control spindle formation and proper kinetochore attachment cell division in a survivin-dependent manner that exacerbates hepatocarcinogenesis. HBx protein interacts with survivin-HBXIP complex and promotes cell cycle arrest and suppressed hepatoma cell growth through mechanisms that modulate oncoprotein HBXIP and tumour suppressor miR-520b [57,59,60]. Hu and co-authors  have shown that miR-520b impedes breast cancer cell migration by targeting andregulating the expression of IL-8 and HBXIP. Following this line of evidence, deactivation of mitogen-activated protein kinase kinase 2 (MEKK2) and cyclin D1 resulted in the downregulation of miR-520b and hepatoma cell proliferation in HCC, suggesting potential therapeutic targets . In contrast, HBx protein upregulates MEKK2 through activation of transcription factor AP-2 alpha (TFAP2A) and sphingosine kinase 1 (SPHK1), and this leads to tumour aggressiveness [63,64]. MEKK2 is a member of the MAPK signalling pathway that activates the JNK/MAPK pathway and ERK5 leading to regulation in tumour growth and metastasis .
HBx and damage-specific DNA binding protein 1
Damage-specific DNA binding protein 1 (DDB1) is the wellcharacterised binding partner of HBx protein that may contribute towards promoting viral replication, and their interaction was found to be conserved in all mammalian hepadnaviruses including woodchucks [65,66]. DDB1 is a 127kDa protein that binds to DDB2, a protein that facilitates its transportation in the nucleus. This binding forms a heterodimeric DNAdamage binding complex that functions in nucleotide-excision repair pathway and recognises the DDB2 ultraviolet-induced DNA damage. DDB1 also serves as an adaptor for the culin 4 (CUL4)-DDB1 ubiquitin E3 ligase complex that ubiquitinates and degrades substrate proteins by the proteasome contributing to hepatocarcinogenesis [67,68]. Although they enter the nuclear compartment separately, HBx protein interacts with DDB1 to activate viral replication by interfering with hepatocyte viability in cell culture. Interaction of HBx and DDB1 protein was thought to interfere with CUL4-DDB1 ubiquitin E3 ligase complex and cause genome instability by inducing apoptosis or cell proliferation associated with aberrant chromosome segregation (Figure 1D) [69,70]. However, a recent study has shown that there were no differences in the levels of HBV DNA replication in cells transfected with DDB1 expression plasmid carrying a wild-type HBx replicon as compared to the one with X-null HBx replicon . This study suggested that DDB1 interacts with HBV covalently closed circular DNA (cccDNA) and promotes viral replication via an unknown mechanism that does not involve interaction with HBx protein. Interaction of DDB1 with DDB2 could also induce transcriptional activation either by being directly recruited to the cccDNA by the p300/ CBP histone acetyltransferase (HAT) or by acting as a transcriptional factor that interacts with E2F1 and Sp1 transcription factors which disrupt cell cycle control [72-74].
Effects of HBx-induced Epigenetic Alterations in Hepatocarcinogenesis
Epigenetics is a non-mutational alteration of gene expression that occurs through epigenetic marks or tags such as methylation of DNA and covalent modification of histones proteins [75,76]. DNA methylation attaches the methyl groups to the nucleotide sequence via catalysis by several DNA methyltransferases (DNMTs). Histone modifications either add or remove chemical groups to or from histones via histone modifying enzymes such as HATs and histone deacetylases (HDACs). HBx protein has been shown to trigger aberrant epigenetic signatures that influence HBV-induced hepatocarcinogenesis [77-79]. However, the precise mechanisms of action are still being elucidated.
Tumour suppressor genes are often aberrantly repressed due to DNA hypermethylation. This type of epigenetic lesion describes an addition of methyl group in the 5’-methylcytosine of gene promoter regions . HBx gene has been repeatedly reported to be frequently integrated and preferentially maintained in patients with HBV-related HCC. HBx protein induces the hypermethylation of several tumour suppressor genes by modulating the transcriptional activation of DNMTs that result in the loss of gene expression and normal functions leading to hepatocarcinogenesis [81-88]. Several promoter regions encoding ASPP, retinoic acid receptor β2 (RARβ2), insulin-like growth factor binding 3 (IGFB3), caveolin- 1(Cav1), long interspersed nuclear elements-1(LINE1), retinoblastoma (pRB), E-cadherin, glutathione S-transferase P1(GSTP1) and human telomerase reverse transcriptase (hTERT) tumour suppressor genes have been shown to be repressed via HBx-induced DNMT1 and DNMT3A hypermethylation (Figure 2) [81-88]. HBx-induced hypermethylation may disrupt cellular signalling pathways such as ubiquitination, DNA repair, transcription, proliferation and apoptosis accompanied by tumour development, aggressiveness and metastases (Figure 2) [81-88]. HBxmediated downregulation of secreted frizzled-related protein (SFRP)- 1 and SFRP5 in HBV-related HCC tissues was significantly associated with upregulation of DNMT1 that led to poor tumour differentiation by disrupting Wnt pathway. Silencing the expression of DNMT1 with methylation inhibitor restored SFRP1 and SFRP5 expression leading to inhibition in HCC growth and regression of HBx-induced EMT . DNA hypomethylation signifies the loss of methylation that affects mostly repeated sequences and it is accountable for the global DNA hypomethylation that is frequently observed in several malignancies. In contrast to DNA hypermethylation, hypomethylation correlates with activation of proto-oncogenes. Upregulation of insulin-like growth factor 2 (IGF2) oncogene via HBx-induced hypermethylation coincides with poor clinical outcome in HCC patients .
DNA methylation can collaborate with histone modifications and alters hepatic gene expression synergistically. Silence in DLEC 1 gene expression is mediated by both DNA hypermethylation and histone acetylation . HBx-induced expression of deleted in lung and esophageal cancer 1 (DLEC 1) gene via activation of HATs leads to suppression of tumour progression . Through the activation of DNMT1 expression mediated by the pRB-E2F pathway, HBx protein induces DNA hypermethylation of DLEC 1 gene and suppresses its transcriptional activities . HBx protein also allows the establishment of active chromatin by interacting with lysine specific histone demethylase 1 (KDM1A) and su(var) 3-9, enhancer of zeste, trithorax 1A (SET1A) enzymes which trimethylate H3K4, an epigenetic mark associated with active transcription . HBV cccDNA is tightly packed in the nucleus as an episomal DNA, and it is required for viral persistence and replication. HBV cccDNA is packaged into minichromosomes by histone and non-histone proteins [93,94]. It has recently been shown that HBx protein activates SET domain bifurcated 1 and recruits heterochromatin protein 1 leading to silence in the transcription of HBV cccDNA via trimethylation of H3 on lysine 9 (H3K9me3) . It is evident that HBx protein regulates HBV replication and transcription by remodelling minichromosomes.
Cooperative networking between DNA methylation and noncoding miRNA (miR) in regulating gene expression and promoting hepatocarcinogenesis has also been repeatedly reported [62,96]. HBx protein promotes hepatocarcinogenesis by inducing hypermethylation via activation of DNMTs and silencing gene transcription in the promoter region of tumour suppressor genes such as miR-21, miR-101, miR-152 and miR-205 (Figure 2) [61,62,97,98]. Suppressed expression of miR- 205 was associated with HBx-enhanced hepatocyte transformation and proliferation that favour malignant transformation and subsequently cancer development . Upregulation of HBx-induced miR-21 leads to the loss of programmed cell death 4 gene expression and normal function in HCC patients, providing another novel insight into mechanisms underlying HBV-related HCC pathogenesis .
Figure 2: Aberrant HBx-induced epigenetic mechanisms involved in the alteration of gene transcription and normal cellular processes that are important in hepatocarcinogenesis
As a multifunctional protein, HBx exerts its actions by either interacting with key transcriptional factors or epigenetically regulating tumour suppressor genes that are critical for HBV-related hepatocarcinogenesis and metastases. Current literature shows HBx protein triggers epigenetic abnormalities and disrupts cellular signalling pathways that favour uncontrolled hepatocyte proliferation, development of HBV-induced inflammation, fibrosis and cancer. Despite the reported evidence, the role of HBx protein in the pathogenesis of HBV-related HCC still remains enigmatic and therefore warrants further investigation that will characterise the structure and functions of this protein. This will provide insight into molecular mechanisms underlying the role of HBx protein as an epigenetic and gene transcription regulator.
This work was supported by the National Research Foundation, Medical Research Foundation and Oppenheimer Memorial Trust.
The authors declare no conflict of interest.
- Gerlich WH (2013) Medical virology of hepatitis B: how it began and where we are now. Virol J 10: 239. [Ref.]
- McMahon BJ (2014) Chronic hepatitis B virus infection. Med Clin North Am 98: 39-54. [Ref.]
- Seeger C, Mason WS (2000) Hepatitis B virus biology. Microbiol Mol Biol Rev 64: 51-68. [Ref.]
- Lucifora J, Arzberger S, Durantel D, Belloni L, Strubin M, et al. (2011) Hepatitis B virus X protein is essential to initiate and maintain virus replication after infection. J Hepatol 55: 996-1003. [Ref.]
- Kekule AS, Lauer U, Weiss L, Luber B, Hofschneider PH (1993) Hepatitis B virus transactivator HBx uses a tumour promoter signalling pathway. Nature 361: 742-745. [Ref.]
- Klein NP, Schneider RJ (1997) Activation of Src family kinases by hepatitis B virus HBx protein and coupled signaling to Ras. Mol Cell Biol 17: 6427-6436. [Ref.]
- Benn J, Su F, Doria M, Schneider RJ (1996) Hepatitis B virus HBx protein induces transcription factor AP-1 by activation of extracellular signal-regulated and c-Jun N-terminal mitogen-activated protein kinases. J Virol 70: 4978-4985. [Ref.]
- Cross JC, Wen P, Rutter WJ (1993) Transactivation by hepatitis B virus X protein is promiscuous and dependent on mitogen-activated cellular serine/threonine kinases. Proc Natl Acad Sci 90: 8078-8082. [Ref.]
- Lee Y, Yun Y (1998) HBx protein of hepatitis B virus activates Jak1- STAT signaling. J Biol Chem 273: 25510-25515. [Ref.]
- Pan J, Duan L, Sun BS, Feitelson MA (2001) Hepatitis B virus X protein protects against anti-Fas-mediated apoptosis in human liver cells by inducing NF- kappa B. J Gen Virol 82: 171-182. [Ref.]
- Pan J, Clayton M, Feitelson MA (2004) Hepatitis B virus X antigen promotes transforming growth factor-β1 (TGF-β1) activity by upregulation of TGF-β1 and down-regulation of α2 -macroglobulin. J Gen Virol 85: 275-282. [Ref.]
- Hsieh YH, Su IJ, Wang HC, Chang WW, Lei HY, et al. (2004) Pre-S mutant surface antigens in chronic hepatitis B virus infection induce oxidative stress and DNA damage. Carcinogenesis 25 2023-2032. [Ref.]
- Gottlob K, Fulco M, Levrero M, Graessmann A (1998) The hepatitis B virus HBx protein inhibits caspase 3 activity. J Biol Chem 273: 33347- 33353. [Ref.]
- Suzuki K, Hayashi N, Miyamoto Y, Yamamoto M, Ohkawa K, et al. (1996) Expression of vascular permeability factor/vascular endothelial growth factor in human hepatocellular carcinoma. Cancer Res 56: 3004-3009. [Ref.]
- Liu W, Lin YT, Yan XL, Ding YL, Wu YL, et al. (2015) Hepatitis B virus core protein inhibits Fas-mediated apoptosis of hepatoma cells via regulation of mFas/FasL and sFas expression. FASEB J 29: 1113- 1123. [Ref.]
- Gearhart TL, Bouchard MJ (2010) Replication of the hepatitis B virus requires a calcium-dependent HBx-induced G1 phase arrest of hepatocytes. Virology 407: 14-25. [Ref.]
- Oh JC, Jeong D, Kim I, Oh S (2003) Activation of calcium signaling by hepatitis B virus-X protein in liver cells. Expo Mol Med 35: 301-309. [Ref.]
- Kim KH, Seong BL (2003) Pro-apoptotic function of HBV X protein is mediated by interaction with c-FLIP and enhancement of deathinducing signal. EMBO J 22: 2104-2116. [Ref.]
- Zheng F, Liu GZ, Li T, Tan DM (2009) Study on tumor formation of hepatocyte transformed by hepatitis B virus X gene in nude mice. Zhonghua Gan Zang Bing Za Zhi 17: 117-119. [Ref.]
- Kim YC, Song K, Yoon G, Nam M, Ryu W (2001) Activated ras oncogene collaborates with HBx gene of hepatitis B virus to transform cells by suppressing HBx-mediated apoptosis. Oncogene 20: 16-23. [Ref.]
- Wei W, Huang W, Pan Y, Zhu F, Wu J (2006) Functional switch of viral protein HBx on cell apoptosis, transformation, and tumorigenesis in association with oncoprotein Ras. Cancer Lett 244: 119-128. [Ref.]
- Wang Y, Lu Y, Toh ST, Sung WK, Tan P, et al. (2010) Lethal-7 is down-regulated by the hepatitis B virus x protein and targets signal transducer and activator of transcription 3. J Hepatol 53: 57-66. [Ref.]
- Jung S, Kim Y (2013) C-terminal region of HBx is crucial for mitochondrial DNA damage. Cancer Lett 331: 76-83. [Ref.]
- Wang H, Yang S, Liang H, Li C (2014) HBx protein promotes oval cell proliferation by up-regulation of cyclin D1 via activation of the MEK/ ERK and PI3K/Akt pathways. Int J Mol Sci 15: 3507-3518. [Ref.]
- Verrecchia F, Mauviel A (2007) Transforming growth factor-beta and fibrosis. World J Gastroenterol 13: 3056-3062. [Ref.]
- Gressner AM, Weiskirchen R, Breitkopf K, Dooley S (2002) Roles of TGF-beta in hepatic fibrosis. Front Biosci 7: d793-d807. [Ref.]
- Martín-Vílchez S, Sanz-Cameno P, Rodríguez-Muñoz Y, Majano PL, Molina-Jiménez F, et al. (2008) The hepatitis B virus X protein induces paracrine activation of human hepatic stellate cells. Hepatology 47: 1872-1883. [Ref.]
- Shih WL, Kuo ML, Chuang SE, Cheng AL, Doong SL (2000) Hepatitis B virus X protein inhibits transforming growth factor-beta -induced apoptosis through the activation of phosphatidylinositol 3-kinase pathway. J Biol Chem 275: 25858-25864. [Ref.]
- Chen H, Chen Z, Huang R, Lin N, Wang X (2014) Hepatitis B virus X protein activates human hepatic stellate cells through upregulating TGFβ1. Genet Mol Res 13: 8645-8656. [Ref.]
- Castilla A, Prieto J, Fausto N (1991) Transforming growth factors β1 and α in chronic liver disease effects of interferon alfa therapy. N Engl J Med 324: 933-940. [Ref.]
- Ito N, Kawata S, Tamura S, Takaishi K, Shirai Y, et al. (1991) Elevated levels of transforming growth factor beta messenger RNA and its polypeptide in human hepatocellular carcinoma. Cancer Res 51: 4080-4083. [Ref.]
- Song BC, Chung YH, Kim JA, Choi WB, Suh DD, et al. (2002) Transforming growth factor‐beta1 as a useful serologic marker of small hepatocellular carcinoma. Cancer 94: 175-180. [Ref.]
- Liu N, Jiao T, Huang Y, Liu W, Li Z, et al. (2015) Hepatitis B virus regulates apoptosis and tumorigenesis through the microRNA-15aSmad7-transforming growth factor beta pathway. J Virol 89: 2739- 2749. [Ref.]
- Guo G, Tan D, Zhu P, Liu F (2009) Hepatitis B virus X protein promotes proliferation and upregulates TGF-beta1 and CTGF in human hepatic stellate cell line, LX-2. Hepatobiliary Pancreat Dis Int 8: 59-64. [Ref.]
- Muto J, Shirabe K, Sugimachi K, Maehara Y (2015) Review of angiogenesis in hepatocellular carcinoma. Hepatol Res 45: 1-9. [Ref.]
- Cheng AS, Chan HL, To KF, Leung WK, Chan KK, et al. (2004) Cyclooxygenase-2 pathway correlates with vascular endothelial growth factor expression and tumor angiogenesis in hepatitis B virusassociated hepatocellular carcinoma. Int J Oncol 24: 853-860. [Ref.]
- Nakamura K, Zen Y, Sato Y, Kozaka K, Matsui O, et al. (2007) Vascular endothelial growth factor, its receptor Flk-1, and hypoxia inducible factor-1α are involved in malignant transformation in dysplastic nodules of the liver. Hum Pathol 38: 1532-1546. [Ref.]
- Yen CJ, Lin YJ, Yen CS, Tsai HW, Tsai TF, et al. (2012) Hepatitis B virus X protein upregulates mTOR signaling through IKKβ to increase cell proliferation and VEGF production in hepatocellular carcinoma. PloS One 7: e41931. [Ref.]
- Yang JC, Teng CF, Wu HC, Tsai HW, Chuang HC, et al. (2009) Enhanced expression of vascular endothelial growth factor‐A in ground glass hepatocytes and its implication in hepatitis B virus hepatocarcinogenesis. Hepatology 49: 1962-1971. [Ref.]
- Moon EJ, Jeong CH, Jeong JW, Kim KR, Yu DY, et al. (2004) Hepatitis B virus X protein induces angiogenesis by stabilizing hypoxia-inducible factor-1alpha. FASEB J 18: 382-384. [Ref.]
- Lee TK, Poon RT, Yuen AP, Man K, Yang ZF, et al. (2006) Rac activation is associated with hepatocellular carcinoma metastasis by up-regulation of vascular endothelial growth factor expression. Clin Cancer Res 12: 5082-5089. [Ref.]
- Lian Z, Liu J, Wu M, Wang HY, Arbuthnot P, et al. (2007) Hepatitis B x antigen up‐regulates vascular endothelial growth factor receptor 3 in hepatocarcinogenesis. Hepatology 45: 1390-1399. [Ref.]
- Yang P, Markowitz GJ, Wang X (2014) The hepatitis B virus-associated tumor microenvironment in hepatocellular carcinoma. Nat Sci Rev 1: 396-412. [Ref.]
- Pollicino T, Bellinghieri L, Restuccia A, Raffa G, Musolino C, et al. (2013) Hepatitis B virus (HBV) induces the expression of interleukin-8 that in turn reduces HBV sensitivity to interferon-alpha. Virology 444: 317-328. [Ref.]
- Mahé Y, Mukaida N, Kuno K, Akiyama M, Ikeda N, et al. (1991) Hepatitis B virus X protein transactivates human interleukin-8 gene through acting on nuclear factor kB and CCAAT/enhancer-binding protein-like cis-elements. J Biol Chem 266: 13759-13763. [Ref.]
- Liu KG, Shao XL, Xie HH, Xu L, Zhao H, et al. (2010) The expression of hepatitis B virus X protein and cyclooxygenase-2 in hepatitis B virusrelated hepatocellular carcinoma: correlation with microangiogenesis and metastasis, and what is the possible mechanism. Zhonghua Gan Zang Bing Za Zhi 18: 831-836. [Ref.]
- Yu Y, Gong R, Mu Y, Chen Y, Zhu C, et al. (2011) Hepatitis B virus induces a novel inflammation network involving three inflammatory factors, IL-29, IL-8, and cyclooxygenase-2. J Immunol 187: 4844- 4860. [Ref.]
- Lee TH, Elledge SJ, Butel JS (1995) Hepatitis B virus X protein interacts with a probable cellular DNA repair protein. J Virol 69: 1107- 1114. [Ref.]
- Smith ML, Chen IT, Zhan Q, O’Connor PM, Fornace AJ (1995) Involvement of the p53 tumor suppressor in repair of u.v.-type DNA damage. Oncogene 10: 1053-1059. [Ref.]
- Qadri I, Conaway JW, Conaway RC, Schaack J, Siddiqui A (1996) Hepatitis B virus transactivator protein, HBx, associates with the components of TFIIH and stimulates the DNA helicase activity of TFIIH. Proc Natl Acad Sci 93: 10578-10583. [Ref.]
- Samuels-Lev Y, O’Connor DJ, Bergamaschi D, Trigiante G, Hsieh JK, et al. (2001) ASPP proteins specifically stimulate the apoptotic function of p53. Mol Cell 8: 781-794.
- Harris CC (1996) Structure and function of the p53 tumor suppressor gene: clues for rational cancer therapeutic strategies. J Natl Cancer Inst 88: 1442-1455. [Ref.]
- Lu J, Yang W, Tsai S, Lin Y, Chang P, et al. (2013) Liverspecific expressions of HBx and src in the p53 mutant trigger hepatocarcinogenesis in zebrafish. PLoS One 8: e76951. [Ref.]
- Ambrosini G, Adida C, Altieri DC (1997) A novel anti-apoptosis gene, survivin, expressed in cancer and lymphoma. Nat Med 3: 917-921. [Ref.]
- Li F, Ambrosini G, Chu EY, Plescia J, Tognin S, et al. (1998) Control of apoptosis and mitotic spindle checkpoint by survivin. Nature 396: 580-584. [Ref.]
- Zhang X, Dong N, Yin L, Cai N, Ma H, et al. (2005) Hepatitis B virus X protein upregulates survivin expression in hepatoma tissues. J Med Virol 77: 374-381. [Ref.]
- Marusawa H, Matsuzawa S, Welsh K, Zou H, Armstrong R, et al. (2003) HBXIP functions as a cofactor of survivin in apoptosis suppression. EMBO J 22: 2729-2740. [Ref.]
- Fujii R, Zhu C, Wen Y, Marusawa H, Bailly-Maitre B, et al. (2006) HBXIP, cellular target of hepatitis B virus oncoprotein, is a regulator of centrosome dynamics and cytokinesis. Cancer Res 66: 9099-9107. [Ref.]
- Zhang W, Lu Z, Kong G, Gao Y, Wang T, et al. (2014) Hepatitis B virus X protein accelerates hepatocarcinogenesis with partner survivin through modulating miR-520b and HBXIP. Mol Cancer 13: 128. [Ref.]
- Garcia-Saez I, Lacroix F, Blot D, Gabel F, Skoufias D (2011) Structural characterization of HBXIP: the protein that interacts with the antiapoptotic protein survivin and the oncogenic viral protein HBx. J Mol Biol 405: 331-340. [Ref.]
- Hu N, Zhang J, Cui W, Kong G, Zhang S, et al. (2011) miR-520b regulates migration of breast cancer cells by targeting hepatitis B X-interacting protein and interleukin-8. J Biol Chem 286: 13714- 13722. [Ref.]
- Zhang W, Kong G, Zhang J, Wang T, Ye L, et al. (2012) MicroRNA- 520b inhibits growth of hepatoma cells by targeting MEKK2 and cyclin D1. PLoS One 7: e31450. [Ref.]
- Lu ZP, Xiao ZL, Yang Z, Li J, Feng GX, et al. (2015) Hepatitis B virus X protein promotes human hepatoma cell growth via upregulation of transcription factor AP2α and sphingosine kinase 1. Acta Pharmacol Sin 36: 1228-1236. [Ref.]
- Kong GY, Zhang JP, Zhang S, Shan CL, Ye LH, et al. (2011) Hepatitis B virus X protein promotes hepatoma cell proliferation via upregulation of MEKK2. Acta Pharmacol Sin 32: 1173-1180. [Ref.]
- Sitterlin D, Bergametti F, Tiollais P, Tennant BC, Transy C (2000) Correct binding of viral X protein to UVDDB-p127 cellular protein is critical for efficient infection by hepatitis B viruses. Oncogene 19: 4427-4431. [Ref.]
- Sitterlin D, Lee TH, Prigent S, Tiollais P, Butel JS, et al. (1997) Interaction of the UV-damaged DNA-binding protein with hepatitis B virus X protein is conserved among mammalian hepadnaviruses and restricted to transactivation-proficient X-insertion mutants. J Virol 71: 6194-6199. [Ref.]
- Yamaji S, Zhang M, Zhang J, Endo Y, Bibikova E, et al. (2010) Hepatocyte-specific deletion of DDB1 induces liver regeneration and tumorigenesis. Proc Natl Acad Sci 107: 22237-22242. [Ref.]
- Pan Y, Wang B, Yang X, Bai F, Xu Q, et al. (2015) CUL4A facilitates hepatocarcinogenesis by promoting cell cycle progression and epithelial-mesenchymal transition. Sci Rep 5: 17006. [Ref.]
- Lin-Marq N, Bontron S, Leupin O, Strubin M (2001) Hepatitis B virus X protein interferes with cell viability through interaction with the p127- kDa UV-damaged DNA-binding protein. Virology 287: 266-274. [Ref.]
- Martin‐Lluesma S, Schaeffer C, Robert EI, Van Breugel PC, Leupin O, et al. (2008) Hepatitis B virus X protein affects S phase progression leading to chromosome segregation defects by binding to damaged DNA binding protein 1. Hepatology 48: 1467-1476. [Ref.]
- Kim W, Lee S, Son Y, Ko C, Ryu WS (2016) DDB1 Stimulates Viral Transcription of Hepatitis B Virus via HBx-Independent Mechanisms. JVirol 90: 9644-9653. [Ref.]
- Decorsière A, Mueller H, Van Breugel PC, Abdul F, Gerossier L, et al. (2016) Hepatitis B virus X protein identifies the Smc5/6 complex as a host restriction factor. Nature 531: 386-380. [Ref.]
- Lin SY, Black AR, Kostic D, Pajovic S, Hoover CN, et al. (1996) Cell cycle-regulated association of E2F1 and Sp1 is related to their functional interaction. Mol Cell Biol 16: 1668-1675. [Ref.]
- Slagle BL, Bouchard MJ (2016) Hepatitis B Virus X and Regulation of Viral Gene Expression. Cold Spring Harb Perspect Med 6: a021402. [Ref.]
- Jones PA, Baylin SB (2002) The fundamental role of epigenetic events in cancer. Nat Reb Genet 3: 415-428. [Ref.]
- Jaenisch R, Bird A (2003) Epigenetic regulation of gene expression: how the genome integrates intrinsic and environmental signals. Nat Genet 33: 245-254. [Ref.]
- Park IY, Sohn BH, Yu E, Suh DJ, Chung Y, et al. (2007) Aberrant epigenetic modifications in hepatocarcinogenesis induced by hepatitis B virus X protein. Gastroenterology 132: 1476-1494. [Ref.]
- Zhang X, Zhang H, Ye L (2006) Effects of hepatitis B virus X protein on the development of liver cancer. J Lab Clin Med 147: 58-66. [Ref.]
- Zhou Y, Cao L, Li B, Zhang X, Yin Z (2012) Expression of HBx protein in hepatitis B virus-infected intrahepatic cholangiocarcinoma. Hepatobiliary Pancreat Dis Int 11: 532-535. [Ref.]
- Esteller M (2002) CpG island hypermethylation and tumor suppressor genes: a booming present, a brighter future. Oncogene 21: 5427- 5440. [Ref.]
- Zheng D, Zhang L, Cheng N, Xu X, Deng Q, et al. (2009) Epigenetic modification induced by hepatitis B virus X protein via interaction with de novo DNA methyltransferase DNMT3A. J Hepatol 50: 377-387. [Ref.]
- Jung JK, Arora P, Pagano JS, Jang KL (2007) Expression of DNA methyltransferase 1 is activated by hepatitis B virus X protein via a regulatory circuit involving the p16INK4a-cyclin D1-CDK 4/6-pRbE2F1 pathway. Cancer Res 67: 5771-5778. [Ref.]
- Zhu Y, Zhu R, Fan J, Pan Q, Li H, et al. (2010) Hepatitis B virus X protein induces hypermethylation of p16(INK4A) promoter via DNA methyltransferases in the early stage of HBV‐associated hepatocarcinogenesis. J Viral Hepat 17: 98-107. [Ref.]
- Lee JO1, Kwun HJ, Jung JK, Choi KH, Min DS, et al. (2005) Hepatitis B virus X protein represses E-cadherin expression via activation of DNA methyltransferase 1. Oncogene 24: 6617-6625. [Ref.]
- Liu J, Lian Z, Han S, Waye M, Wang H, et al. (2006) Downregulation of E-cadherin by hepatitis B virus X antigen in hepatocellullar carcinoma. Oncogene 25: 1008-1017. [Ref.]
- Kwong J, Chow LS, Wong AY, Hung W, Chung GT, et al. (2007) Epigenetic inactivation of the deleted in lung and esophageal cancer 1 gene in nasopharyngeal carcinoma. Genes Chrom Cancer 46: 171-180. [Ref.]
- Tse EY, Ko FC, Tung EK, Chan LK, Lee TK, et al. (2012) Caveolin‐1 overexpression is associated with hepatocellular carcinoma tumourigenesis and metastasis. J Pathol 226: 645-653. [Ref.]
- Kgatle MM, Spearman CW, Wayne M, Ramesar R, Kalla AA, et al. (2016) Genome-Wide Analysis of core Promoter Region CytosinePhosphate-Guanine Islands Hypermethylation Profiles In Chronic Hepatitis B Virus Patients In South Africa. IJMHS 6: 3-14. [Ref.]
- Xie Q, Chen L, Shan X, Shan X, Tang J, et al. (2014) Epigenetic silencing of SFRP1 and SFRP5 by hepatitis B virus X protein enhances hepatoma cell tumorigenicity through Wnt signaling pathway. Int J Cancer 135: 635-646. [Ref.]
- Tang S, Hu W, Hu J, Wu S, Li J, et al. (2015) Hepatitis B virus X protein promotes P3 transcript expression of the insulin‐like growth factor 2 gene via inducing hypomethylation of P3 promoter in hepatocellular carcinoma. Liver Int 35: 608-619. [Ref.]
- Niu D, Feng H, Chen WN (2010) DLEC1 Expression Is Modulated by Epigenetic Modifications in Hepatocelluar Carcinoma Cells: Role of HBx Genotypes. Cancers (Basel) 2: 1689-1704. [Ref.]
- Alarcon V, Hernandez S, Rubio L, Alvarez F, Flores Y, et al. (2016) The enzymes LSD1 and Set1A cooperate with the viral protein HBx to establish an active hepatitis B viral chromatin state. Sci Rep 6: 25901. [Ref.]
- Pollicino T, Belloni L, Raffa G, Pediconi N, Squadrito G, et al. (2006) Hepatitis B virus replication is regulated by the acetylation status of hepatitis B virus cccDNA-bound H3 and H4 histones. Gastroenterology 130: 823-837. [Ref.]
- Luo L, Chen S, Gong Q, Luo N, Lei Y, et al. (2013) Hepatitis B virus X protein modulates remodelling of minichromosomes related to hepatitis B virus replication in HepG2 cells. Int J Mol Med 31: 197-204. [Ref.]
- Rivière L, Gerossier L, Ducroux A, Dion S, Deng Q, et al. (2015) HBx relieves chromatin-mediated transcriptional repression of hepatitis B viral cccDNA involving SETDB1 histone methyltransferase. J Hepatol 63: 1093-1102. [Ref.]
- Qiu X, Dong S, Qiao F, Lu S, Song Y, et al. (2013) HBx-mediated miR-21 upregulation represses tumor-suppressor function of PDCD4 in hepatocellular carcinoma. Oncogene 32: 3296-3305. [Ref.]
- Wei X, Xiang T, Ren G, Tan C, Liu R, et al. (2013) miR-101 is downregulated by the hepatitis B virus x protein and induces aberrant DNA methylation by targeting DNA methyltransferase 3A. Cell Signal 25: 439-446. [Ref.]
- Wei X, Tan C, Tang C, Ren G, Xiang T, et al. (2013) Epigenetic repression of miR-132 expression by the hepatitis B virus x protein in hepatitis B virus-related hepatocellular carcinoma. Cell Signal 25: 1037-1043. [Ref.]
- Zhang T, Zhang J, Cui M, Liu F, You X, et al. (2013) Hepatitis B virus X protein inhibits tumor suppressor miR-205 through inducing hypermethylation of miR-205 promoter to enhance carcinogenesis. Neoplasia 15: 1282-1291. [Ref.]
Download Provisional PDF Here
Aritcle Type: Review Article
Citation: Kgatle MM, Kalla AA, Spearman CW, Hairwadzi HN (2016) The Role of HBx-mediated Transcriptional Activities and Epigenetic Alterations in Hepatitis B Virus-induced Hepatocellular Carcinoma. J Emerg Dis Virol 3(1): doi http://dx.doi. org/10.16966/2473-1846.126
Copyright: © 2016 Kgatle MM, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.