Conserved transactivating and pro-apoptotic functions of hepadnaviral X protein in ortho- and avihepadnaviruses

Hepatitis B Viral Protein HBx: Roles in Viral Replication and Hepatocarcinogenesis

Hepatitis B virus (HBV) infection remains a major public health concern worldwide, with approximately 296 million individuals chronically infected. The HBV-encoded X protein (HBx) is a regulatory protein of 17 kDa, reportedly responsible for a broad range of functions, including viral replication and oncogenic processes. In this review, we summarize the state of knowledge on the mechanisms underlying HBx functions in viral replication, the antiviral effect of therapeutics directed against HBx, and the role of HBx in liver cancer development (including a hypothetical model of hepatocarcinogenesis). We conclude by highlighting major unanswered questions in the field and the implications of their answers.

Early mesozoic coexistence of amniotes and hepadnaviridae

Hepadnaviridae are double-stranded DNA viruses that infect some species of birds and mammals. This includes humans, where hepatitis B viruses (HBVs) are prevalent pathogens in considerable parts of the global population. Recently, endogenized sequences of HBVs (eHBVs) have been discovered in bird genomes where they constitute direct evidence for the coexistence of these viruses and their hosts from the late Mesozoic until present. Nevertheless, virtually nothing is known about the ancient host range of this virus family in other animals. Here we report the first eHBVs from crocodilian, snake, and turtle genomes, including a turtle eHBV that endogenized >207 million years ago. This genomic “fossil” is >125 million years older than the oldest avian eHBV and provides the first direct evidence that Hepadnaviridae already existed during the Early Mesozoic. This implies that the Mesozoic fossil record of HBV infection spans three of the five major groups of land vertebrates, namely birds, crocodilians, and turtles. We show that the deep phylogenetic relationships of HBVs are largely congruent with the deep phylogeny of their amniote hosts, which suggests an ancient amniote–HBV coexistence and codivergence, at least since the Early Mesozoic. Notably, the organization of overlapping genes as well as the structure of elements involved in viral replication has remained highly conserved among HBVs along that time span, except for the presence of the X gene. We provide multiple lines of evidence that the tumor-promoting X protein of mammalian HBVs lacks a homolog in all other hepadnaviruses and propose a novel scenario for the emergence of X via segmental duplication and overprinting of pre-existing reading frames in the ancestor of mammalian HBVs. Our study reveals an unforeseen host range of prehistoric HBVs and provides novel insights into the genome evolution of hepadnaviruses throughout their long-lasting association with amniote hosts.

Viruses are not known to leave physical fossil traces, which makes our understanding of their evolutionary prehistory crucially dependent on the detection of endogenous viruses. Ancient endogenous viruses, also known as paleoviruses, are relics of viral genomes or fragments thereof that once infiltrated their host's germline and then remained as molecular “fossils” within the host genome. The massive genome sequencing of recent years has unearthed vast numbers of paleoviruses from various animal genomes, including the first endogenous hepatitis B viruses (eHBVs) in bird genomes. We screened genomes of land vertebrates (amniotes) for the presence of paleoviruses and identified ancient eHBVs in the recently sequenced genomes of crocodilians, snakes, and turtles. We report an eHBV that is >207 million years old, making it the oldest endogenous virus currently known. Furthermore, our results provide direct evidence that the Hepadnaviridae virus family infected birds, crocodilians and turtles during the Mesozoic Era, and suggest a long-lasting coexistence of these viruses and their amniote hosts at least since the Early Mesozoic. We challenge previous views on the origin of the oncogenic X gene and provide an evolutionary explanation as to why only mammalian hepatitis B infection leads to hepatocellular carcinoma.

Redox regulation of T-cell turnover by the p13 protein of human T-cell leukemia virus type 1: distinct effects in primary versus transformed cells

The present study investigated the function of p13, a mitochondrial protein of human T-cell leukemia virus type 1 (HTLV-1). Although necessary for viral propagation in vivo, the mechanism of function of p13 is incompletely understood. Drawing from studies in isolated mitochondria, we analyzed the effects of p13 on mitochondrial reactive oxygen species (ROS) in transformed and primary T cells. In transformed cells (Jurkat, HeLa), p13 did not affect ROS unless the cells were subjected to glucose deprivation, which led to a p13-dependent increase in ROS and cell death. Using RNA interference we confirmed that expression of p13 also influences glucose starvation-induced cell death in the context of HTLV-1–infected cells. ROS measurements showed an increasing gradient from resting to mitogen-activated primary T cells to transformed T cells (Jurkat). Expression of p13 in primary T cells resulted in their activation, an effect that was abrogated by ROS scavengers. These findings suggest that p13 may have a distinct impact on cell turnover depending on the inherent ROS levels; in the context of the HTLV-1 propagation strategy, p13 could increase the pool of “normal” infected cells while culling cells acquiring a transformed phenotype, thus favoring lifelong persistence of the virus in the host.

Analysis of hepatitis B virus X gene phylogeny, genetic variability and its impact on pathogenesis: implications in Eastern Indian HBV carriers

HBx genetic variability was explored in the Eastern Indian population with low HCC incidence. DNase I sensitive HBV DNA was detected in 53% samples, which differed significantly between clinical groups (P<0.001). HBV genotypes A (Aa/A1), C (Cs/C1) and D (D1, D2, D3, D5) were detected in 37.5%, 18.7% and 43.7% samples respectively. Population specific signature HBx residues A(36), V(88), S(101) in Aa/A1 and residues P(41), Q(110) in D5 were detected. Mutations T(127), M(130) and I(131) were detected in 66.7%, 91% and 75% of genotype A, C and D5 samples respectively. Very low occurrence of HCC associated mutations (V(5)M/L, P(38)S, and H(94)Y) and absence of C-terminal deletions were observed. Our study shows that HBV genotype associated clinically important HBx variations may evolve and act distinctly in different geo-ethnic populations. Further studies on HBx functions from the perspective of genetic variability are essential for the better understanding of the clinical significance of HBV.

Host-range and pathogenicity of hepatitis B viruses

Hepatitis B viruses are small enveloped DNA viruses referred to as Hepadnaviridae that cause transient or persistent (chronic) infections of the liver. This family is divided into two genera, orthohepadnavirus and avihepadnavirus, which infect mammals or birds as natural hosts, respectively. They possess a narrow host range determined by the initial steps of viral attachment and entry. Hepatitis B virus is the focus of biomedical research owing to its medical significance. Approximately 2 billion people have serological evidence of hepatitis B, and of these approximately 350 million people have chronic infections (World Health Organisation, Fact Sheet WHO/204, October 2000). Depending on viral and host factors, the outcomes of infection with hepatitis B virus vary between acute hepatitis, mild or severe chronic hepatitis or cirrhosis. Chronic infections are associated with an increased risk for the development of hepatocellular carcinoma.

Molecular functions and biological roles of hepatitis B virus x protein

Chronic infection of hepatitis B virus (HBV) is one of the major causes of hepatocellular carcinoma (HCC) in the world. Hepatitis B virus X protein (HBx) has been long suspected to be involved in hepatocarcinogenesis, although its oncogenic role remains controversial. HBx is a multifunctional regulator that modulates transcription, signal transduction, cell cycle progress, protein degradation pathways, apoptosis, and genetic stability by directly or indirectly interacting with host factors. This review focuses on the biological roles of HBx in HBV replication and cellular transformation in terms of the molecular functions of HBx. Using the transient HBV replication assay, ectopically expressed HBx could stimulate HBV transcription and replication with the X-defective replicon to the level of those with the wild one. The transcription coactivation is mainly contributing to the stimulatory role of HBx on HBV replication although the other functions may affect HBV replication. Effect of HBx on cellular transformation remains controversial and was never addressed with human primary or immortal cells. Using the human immortalized primary cells, HBx was found to retain the ability to overcome active oncogene RAS-induced senescence that requires full-length HBx. At least two functions of HBx, the coactivation function and the ability to overcome oncogene-induced senescence, may be cooperatively involved in HBV-related hepatocarcinogenesis.

Differential effects on apoptosis induction in hepatocyte lines by stable expression of hepatitis B virus X protein

AIM: Hepatitis B virus protein X (HBx) has been shown to be weakly oncogenic in vitro. The transforming activities of HBx have been linked with the inhibition of several functions of the tumor suppressor p53. We have studied whether HBx may have different effects on p53 depending on the cell type.

METHODS: We used the human hepatoma cell line HepG2 and the immortalized murine hepatocyte line AML12 and analyzed stably transfected clones which expressed physiological amounts of HBx. P53 was induced by UV irradiation.

RESULTS: The p53 induction by UV irradiation was unaffected by stable expression of HBx. However, the expression of the cyclin kinase inhibitor p21^waf/cip/sdi which gets activated by p53 was affected in the HBx transformed cell line AML12-HBx9, but not in HepG2. In AML-HBx9 cells, p21^waf/cip/sdi-protein expression and p21^waf/cip/sdi transcription were deregulated. Furthermore, the process of apoptosis was affected in opposite ways in the two cell lines investigated. While stable expression of HBx enhanced apoptosis induced by UV irradiation in HepG2-cells, apoptosis was decreased in HBx transformed AML12-HBx9. P53 repressed transcription from the HBV enhancer I, when expressed from expression vectors or after induction of endogenous p53 by UV irradiation. Repression by endogenous p53 was partially reversible by stably expressed HBx in both cell lines.

CONCLUSION: Stable expression of HBx leads to deregulation of apoptosis induced by UV irradiation depending on the cell line used. In an immortalized hepatocyte line HBx acted anti-apoptotic whereas expression in a carcinoma derived hepatocyte line HBx enhanced apoptosis.

Hepatocellular carcinoma cells containing hepatitis B virus X protein have enhanced invasive potential conditionally

AIMS

To establish a sustaining hepatitis B virus X protein expressed Chang liver cell line and to explore their biological behaviours of invasive potential induced by hepatitis B virus X protein.

METHODS

Polymerase chain reaction was used to amplify the HBx gene from the whole hepatitis B virus genome. The gene was then subcloned into the eukaryotic expression vector pcDNA3.1 to construct the pcDNA3.1-HBx plasmid. Gene transfection mediated by Lipofectamine was used to introduce the plasmid into the human liver cell line Chang, and stable expression of the HBx gene was detected.

RESULTS

HBx gene was cloned from the transfected Chang liver cells by reverse transcription-polymerase chain reaction, and confirmed by electrophoresis. The stably transfected Chang cells expressing HBx with malignant characteristics were verified and compared with control cells in terms of their growth curves, clonogenicity, wound healing abilities, migration and metastasis.

CONCLUSION

The stabilising human liver cell lines Chang liver containing HBx gene expression have been established successfully. The invasive potential of Chang cells was conditionally enhanced by HBx transfection.

Functional switch of viral protein HBx on cell apoptosis, transformation, and tumorigenesis in association with oncoprotein Ras

The X protein (HBx) of hepatitis B virus (HBV) plays important roles in hepatitis, cirrhosis, and hepatocellular carcinoma (HCC) during viral infection. In this study, we demonstrated that co-transfection of mouse embryo fibroblasts (STO) with HBx and activated Ras triggered apoptotic cell death, while HBx or activated Ras individually failed to induce apoptosis. In addition, STO cells were able to form colonies on soft agar after transfected with HBx or Ras, and cells co-transfected with both genes failed to transform. Moreover, nude mice injected with STO cells carrying either HBx or Ras could develop tumor, but tumor growth was inhibited by the injection of both STO cells harboring HBx and carrying Ras. These results suggested that HBx plays a role as a tumor inducer and stimulates neoplastic transformation of normal cells, but shifts its function to the induction of apoptosis in association with Ras.

Animal models for the study of HBV replication and its variants

Enormous progresses in hepatitis B virus research have been made through the identification of avian and mammalian HBV related viruses, which offer ample opportunities for studies in naturally occurring hosts. However, none of these natural hosts belongs to the commonly used laboratory animals, and the development of various mouse strains carrying HBV transgenes offered unique opportunities to investigate some mechanisms of viral pathogenesis. Furthermore, the need to perform infection studies in a system harbouring HBV-permissive hepatocytes has lately led researchers to create new challenging human mouse chimera models of HBV infection. In this review, we will overview the type of animal models currently available in hepadnavirus research.

Involvement of extracellular signal-regulated kinase/mitogen-activated protein kinase pathway in multidrug resistance induced by HBx in hepatoma cell line

AIM: To investigate the molecular mechanism of the influence of HBx protein on multidrug resistance associated genes: multidrug resistance 1 (MDR-1), multidrug related protein (MRP-1), lung resistance related protein (LRP) in hepatoma cells and the potential role of extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) pathway in this process.

METHODS: A cell model stably expressing the HBx protein was established by liposome-mediated transfection of HBx gene into HepG2 cell line. The expression of multidrug resistance associated genes and proteins was detected by RT-PCR and Western blot. AnnexinV-FITC/PI assay was used to confirm the multidrug resistance (MDR) phenotype of transfected cells by fluorescence cytometry (FACS). The ERK/MAPK pathway activation was measured by Western blot through comparing the ratio of phosphorylation of ERK/MAPK to total ERK/MAPK protein. After treated with the ERK/MAPK pathway inhibitor U0126, the HBx-expressing cells were harvested. Then RT-PCR, Western blot and FACS were used to analyze the alterations in the expression of multidrug resistance associated genes and the MDR phenotype after exposure.

RESULTS: Compared with the control group, the transfected cells showed a higher expression of MDR associated genes and proteins. Marked elevations in MDR-1 (64.3%), MRP-1 (87.5%) and LRP (90.8%) were observed in the transfected cells (P < 0.05). RT-PCR revealed that the over-expression of MDR associated proteins was due to amplification of such genes (MDR1 2.9 fold, MRP1 1.67 fold, LRP1.95 fold). Furthermore, we found that the ERK/MAPK activity was remarkably high in the HBx-expressing cells. The activation of ERK/MAPK, as measured by the ratio of phosphorylated ERK bands normalized to the total ERK bands, was increased by 2.3-fold in HBx-transfected cells compared with cells transfected with the empty vector. After treated with the ERK/MAPK pathway inhibitor, the level of MDR associated genes and proteins in the transfected cells decreased to some extent. Compared with controls, a significant decrease in MDR-1 mRNA (53.3%), MRP-1 mRNA (59.7%) as well as LRP mRNA (56.4%) was observed in the U0126 treated transfected cells after 12 h. Western blot also demonstrated that the protein expression of these MDR associated genes slightly reduced after treated with U0126 for 12 h (MDR-1 40.1%, MRP-1 29.4%, LRP35.7%). This change was accompanied with the rise of cell apoptosis ratio confirmed by Annexin V-PI detection. The apoptosis index of U0126-treated cells increased by 1.28 fold, compared with that of transfected cells. Obviously, the MDR phenotype of these cells was obviously related with increased activities of the ERK/MAPK pathway.

CONCLUSION: HBx protein might be one of the causes for the occurrence of MDR in HCC, and ERK/MAPK pathway might be involved in this change.

A duck hepatitis B virus strain with a knockout mutation in the putative X ORF shows similar infectivity and in vivo growth characteristics to wild-type virus

Hepadnaviruses including human hepatitis B virus (HBV) and duck hepatitis B virus (DHBV) express X proteins, HBx and DHBx, respectively. Both HBx and DHBx are transcriptional activators and modulate cellular signaling in in vitro assays. To test whether the DHBx protein plays a role in virus infection, we compared the in vivo infectivity and growth characteristics of a DHBV3 strain with a stop codon in the X-like ORF (DHBV3-X-K.O.) to those of the wild-type DHBV3 strain. Here we report that the two strains showed no significant difference in (i). their ability to induce infection that resulted in stable viraemia measured by serum surface antigen (DHBsAg) and DHBV DNA, and detection of viral proteins and replicative DNA intermediates in the liver; (ii). the rate of spread of infection in liver and extrahepatic sites after low-dose virus inoculation; and (iii). the ability to produce transient or persistent infection under balanced age/dose conditions designed to detect small differences between the strains. Thus, none of the infection parameters assayed were detectably affected by the X-ORF knockout mutation, raising the question whether DHBx expression plays a physiological role during in vivo infection with wild-type DHBV.

Activating mechanism of transcriptor NF-kappaB regulated by hepatitis B virus X protein in hepatocellular carcinoma

AIM: To investigate the mechanism and significance of NF-κB activation regulated by hepatitis B virus X protein (HBx) in hepatitis B virus (HBV)-associated hepatocellular carcinoma (HCC).

METHODS: The expression levels of HBx, p65, IκB-α and ubiquitin were detected by immunohistochemistry in HCC tissue microarrays (TMA) respectively, and IκB-α was detected by Western blot in HCC and corresponding liver tissues.

RESULTS: The percentage of informative TMA samples was 98.8% in 186 cases with a total of 367 samples. Compared with corresponding liver tissues (60.0%), the HBx expression was obviously decreased in HBV-associated HCC (47.9%, u = 2.24, P < 0.05). On the contrary, the expressions of p65 (20.6% vs 45.3%, u = 4.85, P < 0.01) and ubiquitin (8.9% vs 59.0%, u = 9.68, P < 0.01) were notably elevated in HCC. In addition, IκB-α had a tendency to go up. Importantly, positive relativity was observed between HBx and p65 (χ² = 10.26, P < 0.01), p65 and IκB-α (χ² = 16.86, P < 0.01), IκB-α and ubiquitin (χ² = 8.90, P < 0.01) in HCC, respectively.

CONCLUSION: Both active and non-active forms of NF-κB are increased in HBV-associated HCC. Variant HBx is the major cause of the enhancement of NF-κB activity. The activation always proceeds in nucleus and the proteasome complexes play an important role in the activation.

Cloning and identification of human gene 1 transactivated by hepatitis B virus X antigen

AIM

To study the transactivation effects of HBxAg, and clone the target genes of HBxAg transactivating effects, in order to help understand the mechanism of pathogenesis of HBxAg.

METHODS

Polymerase chain reaction (PCR) was employed to amplify the coding sequence of HBxAg. The hepatoblastoma cell HepG2 was transfected by pcDNA3.1(-) and pcDNA3.1(-)-X, respectively. Total mRNA was purified from the HepG2 cells transfected and suppression subtractive hybridization(SSH) method was used to analyze the differentially expressed DNA sequence between the two groups. The sequences were searched for homologous DNA sequence from GenBank. The new DNA sequence was confirmed and the full-length coding sequence was identified according to the Kozak rule and the existence of polyadenyl signal sequences. Reverse transcription PCR (RT-PCR)was used to amplify the new gene by using mRNA from HepG2 cell as the template. The coding sequence for the new gene was deduced according to the nucleotide sequence.

RESULTS

PCR technique was employed to amplify the coding sequence for HBxAg by using pCP10 plasmid containing whole HBV genome as the template. The recombinant plasmid expressing HBxAg was confirmed by restriction enzyme digestion and sequencing. HepG2 cells were transfected with pcDNA3.1(-) and pcDNA3.1(-)-X by lipofectamine, respectively. Total mRNA was purified from transfected HepG2 cell, and suppression subtractive hybridization method was used for the screening and identification of differentially expressed genes by these two cell groups. After sequencing, each DNA sequence was compared with the genes deposited in the GenBank and the new gene with no homology with known genes in this database was identified. Electric polymerase chain reaction was conducted for the cloning of the full-length DNA of the new gene and in conjunction with Kozak rule and the existence of polyadenyl signal sequence. RT-PCR technique was used to amplify the new gene, named as XTP1, from the mRNA of HepG2 cells. The sequence for the XTP1 gene was deposited into GenBank, and the accession number is AF488828.

CONCLUSION

A new gene named XTP1 which is transac-tivated by hepatitis B virus X protein has been successfully cloned by molecular biological methods. These results pave the way for the study of the molecular mechanism of HBxAg transactivating effects and the development of new therapy for chronic hepatitis B.

Reconstitution of gene expression from a regulatory-protein-deficient hepatitis B virus genome by cell-permeable HBx protein

Various functions are ascribed to the HBx regulatory protein of the hepatitis B virus (HBV). Due to the low expression level of HBx, it has been difficult to correlate spatial and temporal HBx expression levels with specific functions. Based on a novel cell-permeable peptide, known as the translocation motif (TLM), cell-permeable HBx fusion proteins were generated. The TLM-HBx fusion protein is rapidly internalized from the medium into almost all cells, whereas no significant internalization was seen with wild-type HBx. The major fraction of internalized HBx protein moves from the cytoplasm to the nucleus. The cytosolic fraction, however, activates c-RAF1/extracellular-signal-related kinase 2 signalling and causes activation of activator protein 1 (AP1) and nuclear factor-kappaB. The TLM-HBx protein rescues HBV gene expression from an activator-deficient HBV genome. These results indicate that cell-permeable regulatory proteins provide a novel, efficient tool for a clearly defined, dose-dependent analysis of regulatory protein function, without affecting the integrity of the cell, and can be used for the safe reconstitution of virus production from a regulatory-protein-deficient virus genome.

An infectious clone of woolly monkey hepatitis B virus

Members of the Hepadnaviridae family have been isolated from birds, rodents, and primates. A new hepadnavirus isolated from the woolly monkey, a New World primate, is phylogenetically distinct from other primate isolates. An animal model has been established for woolly monkey hepatitis B virus (WMHBV) by using spider monkeys, since woolly monkeys are endangered. In this study, a greater-than-genome length construct was prepared without amplification by using covalently closed circular DNA extracted from the liver of an infected woolly monkey. Transfection of the human liver cell line Huh7 with WMHBV DNA resulted in the production of viral transcripts, DNA replicative intermediates, and secreted virions at levels similar to those obtained with an infectious human HBV clone, demonstrating that the host range restriction of WMHBV is not at the level of genome replication. WMHBV particles from the medium of transfected cultures initiated an infection in a spider monkey similar to that obtained with virions derived from woolly monkey serum. In an attempt to adapt the virus for higher levels of replication in spider monkeys, immunosuppressed and newborn animals were inoculated. Neither procedure produced persistent infections, and the level of viral replication remained several logs lower than that observed in persistently infected woolly monkeys. These data demonstrate the production of an infectious clone for WMHBV and extend the characterization of the spider monkey animal model.

Transcomplementation of core and polymerase functions of the woolly monkey and human hepatitis B viruses

Woolly monkey hepatitis B virus (WMHBV) is a new member of Hepadnaviridae that was isolated from a New World monkey and is phylogenetically distinct from the HBV family. In this study, we explored the functional significance of sequence divergence in the HBV and WMHBV genomes. Independently expressed TP and RT domains of the WMHBV reverse transcriptase (Pol) formed a complex functional for in vitro nucleotide priming, consistent with previous results from priming reactions conducted with HBV. Transcomplementation assays between HBV and WMHBV TP and RT components for in vitro priming demonstrated functional compatibility, although priming with the combination of WMHBV RT and HBV TP was reduced. Examination of cross-species protein-protein interactions revealed that WMHBV core coprecipitated with HBV TP and RT, as well as with WMHBV TP and RT. Analysis in Huh7 cells revealed that WMHBV core and Pol complemented core-negative and Pol-negative HBV mutant genomes for replication. These results highlight the conservation of function despite significant sequence divergence in these viruses.