Hepatitis B virus X gene expression is activated by X protein but repressed by p53 tumor suppressor gene product in the transient expression system

Viral cis-regulatory elements as sensors of cellular states and environmental cues

To withstand a hostile cellular environment and replicate, viruses must sense, interpret, and respond to many internal and external cues. Retroviruses and DNA viruses can intercept these cues impinging on host transcription factors via cis-regulatory elements (CREs) in viral genomes, allowing them to sense and coordinate context-specific responses to varied signals. Here, we explore the characteristics of viral CREs, the classes of signals and host transcription factors that regulate them, and how this informs outcomes of viral replication, immune evasion, and latency. We propose that viral CREs constitute central hubs for signal integration from multiple pathways and that sequence variation between viral isolates can rapidly rewire sensing mechanisms, contributing to the variability observed in patient outcomes.

Virus-host interplay in hepatitis B virus infection and epigenetic treatment strategies

Worldwide, chronic hepatitis B virus (HBV) infection is a major health problem and no cure exists. Importantly, hepatocyte intrusion by HBV particles results in a complex deregulation of both viral and host cellular genetic and epigenetic processes. Among the attempts to develop novel therapeutic approaches against HBV infection, several options targeting the epigenomic regulation of HBV replication are gaining attention. These include the experimental treatment with 'epidrugs'. Moreover, as a targeted approach, the principle of 'epigenetic editing' recently is being exploited to control viral replication. Silencing of HBV by specific rewriting of epigenetic marks might diminish viral replication, viremia, and infectivity, eventually controlling the disease and its complications. Additionally, epigenetic editing can be used as an experimental tool to increase our limited understanding regarding the role of epigenetic modifications in viral infections. Aiming for permanent epigenetic reprogramming of the viral genome without unspecific side effects, this breakthrough may pave the roads for an ambitious technological pursuit: to start designing a curative approach utilizing manipulative molecular therapies for viral infections in vivo.

Nucleotide changes related to hepatocellular carcinoma in the enhancer 1/x-promoter of hepatitis B virus subgenotype C2 in cirrhotic patients

Hepatocellular carcinoma (HCC) is widely known to develop more frequently in cirrhotic patients with a high expression of Hepatitis B virus X protein (HBx), which is controlled by the enhancer 1 (Enh1)/X-promoter. To examine the effect of the mutations in the Enh1/X-promoter region in hepatitis B virus (HBV) genomes on the development of HCC, we investigated the differences in HBV isolated from cirrhotic patients with or without HCC along with the promoter activities of certain specific mutations within the Enh1/X-promoter. We examined 160 hepatitis B surface antigen (HBsAg)-positive cirrhotic patients (80 HCC patients, 80 non-HCC patients) by evaluating the biochemical, virological, and molecular characteristics. We evaluated the functional differences in certain specific mutations within the Enh1/X-promoter. The isolated sequences included all of the subgenotypes C2. The sites that showed higher mutation rates in the HCC group were G1053A and G1229A, which were found to be independent risk factors through multiple logistic analysis (P < 0.05). Their promoter activities were elevated 2.38- and 4.68-fold, respectively, over that of the wild type in the HepG2 cells. Similarly, both the mRNA and protein levels of HBx in these two mutants were much higher than that in wild type-transfected HepG2 cells. Mutated nucleotides of the Enh1/X-promoter, especially G1053A and G1229A mutations in the HBV subgenotype C2 of patients with cirrhosis, can be risk factors for hepatocarcinogenesis, and this might be due to an increase in the HBx levels through the transactivation of the Enh1/X-promoter.

Hepatitis B virus X gene is implicated in liver carcinogenesis

Hepatitis B virus (HBV) is a small hepatotropic and highly species-specific enveloped DNA virus. The carcinogenicity of this virus has become focused on the X gene and its coded X protein. The X protein itself is unable to bind to DNA directly, but works as a potent transcriptional activator through multiple cis-acting elements and mediates several signal transduction cascades. Two regions of the X protein, aa.61-69 and aa.105-140, are found essential for the viral replication and expression as well. These functions interacting with transcription factors and signaling cascades are acting cooperatively to cause cell proliferation. Furthermore, the association of X protein with mitochondria causes loss of the mitochondrial membrane potential and subsequently causes cell death, the function of which is attributed to the aa.68-104 region of X protein. As a result, the X protein has two independent proliferative and cell death-promoting activities. Liver cancer has been shown to result from a series of mutations in specific oncogenes and tumor suppressor genes. In a recent study, X protein stimulates ROS generation in the mitochondria due to collapse of the membrane potential and increases the mutation frequency, that evokes malignant transformation. Inflammation as a result of HBV infection is concerned to cause DNA damage. In the past 10years, the possibility that several viral proteins directly engaged in the DNA damage has increased to some extent. From an evolutionary viewpoint, it is noteworthy that several arrangement proteins have been found in viruses. Thus, there is some clue that a small amount of X protein acts as an arrangement protein for HBV replication dependent upon cellular DNA damage due to generated ROS as an amplified signal.

Characterization of transcription from TATA-less promoters: identification of a new core promoter element XCPE2 and analysis of factor requirements

BACKGROUND

More than 80% of mammalian protein-coding genes are driven by TATA-less promoters which often show multiple transcriptional start sites (TSSs). However, little is known about the core promoter DNA sequences or mechanisms of transcriptional initiation for this class of promoters.

METHODOLOGY/PRINCIPAL FINDINGS

Here we identify a new core promoter element XCPE2 (X core promoter element 2) (consensus sequence: A/C/G-C-C/T-C-G/A-T-T-G/A-C-C/A(+1)-C/T) that can direct specific transcription from the second TSS of hepatitis B virus X gene mRNA. XCPE2 sequences can also be found in human promoter regions and typically appear to drive one of the start sites within multiple TSS-containing TATA-less promoters. To gain insight into mechanisms of transcriptional initiation from this class of promoters, we examined requirements of several general transcription factors by in vitro transcription experiments using immunodepleted nuclear extracts and purified factors. Our results show that XCPE2-driven transcription uses at least TFIIB, either TFIID or free TBP, RNA polymerase II (RNA pol II) and the MED26-containing mediator complex but not Gcn5. Therefore, XCPE2-driven transcription can be carried out by a mechanism which differs from previously described TAF-dependent mechanisms for initiator (Inr)- or downstream promoter element (DPE)-containing promoters, the TBP- and SAGA (Spt-Ada-Gcn5-acetyltransferase)-dependent mechanism for yeast TATA-containing promoters, or the TFTC (TBP-free-TAF-containing complex)-dependent mechanism for certain Inr-containing TATA-less promoters. EMSA assays using XCPE2 promoter and purified factors further suggest that XCPE2 promoter recognition requires a set of factors different from those for TATA box, Inr, or DPE promoter recognition.

CONCLUSIONS/SIGNIFICANCE

We identified a new core promoter element XCPE2 that are found in multiple TSS-containing TATA-less promoters. Mechanisms of promoter recognition and transcriptional initiation for XCPE2-driven promoters appear different from previously shown mechanisms for classical promoters that show single "focused" TSSs. Our studies provide insight into novel mechanisms of RNA Pol II transcription from multiple TSS-containing TATA-less promoters.

The new core promoter element XCPE1 (X Core Promoter Element 1) directs activator-, mediator-, and TATA-binding protein-dependent but TFIID-independent RNA polymerase II transcription from TATA-less promoters

The core promoter is a critical DNA element required for accurate transcription and regulation of transcription. Several core promoter elements have been previously identified in eukaryotes, but those cannot account for transcription from most RNA polymerase II-transcribed genes. Additional, as-yet-unidentified core promoter elements must be present in eukaryotic genomes. From extensive analyses of the hepatitis B virus X gene promoter, here we identify a new core promoter element, XCPE1 (the X gene core promoter element 1), that drives RNA polymerase II transcription. XCPE1 is located between nucleotides -8 and +2 relative to the transcriptional start site (+1) and has a consensus sequence of G/A/T-G/C-G-T/C-G-G-G/A-A-G/C(+1)-A/C. XCPE1 shows fairly weak transcriptional activity alone but exerts significant, specific promoter activity when accompanied by activator-binding sites. XCPE1 is also found in the core promoter regions of about 1% of human genes, particularly in poorly characterized TATA-less genes. Our in vitro transcription studies suggest that the XCPE1-driven transcription can be highly active in the absence of TFIID because it can utilize either free TBP or the complete TFIID complex. Our findings suggest the possibility of the existence of a TAF1 (TFIID)-independent transcriptional initiation mechanism that may be used by a category of TATA-less promoters in higher eukaryotes.

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.

Clinical aspects of hepatocellular carcinoma in Japan

HCC in Japan has very different characteristics from that in other Asian countries. Because, among the Japanese HCC patients approximately 80% of the patients are HCV positive and they are aged over 60 years old. On the other hand, in many Asian countries HBVpositive HCC patients are dominant and their age is younger than the Japanese patients. Early diagnosis of HCC is mainly performed by means of imaging diagnostic technique such as abdominal ultrasonography, dynamic CT, dynamic MRI and CT angiography. If small HCC less than 3 cm in diameter is found and liver function is well preserved, local ablation therapy or surgical treatment promises better than 5 years survival (over 60%). While, TAE or TACE is performed in cases of HCC larger than 3 cm in size, if liver failure is not complicated. In advanced HCC cases with multiple tumors, arterial infusion of anticancer drug has been applied. However, its efficacy is not estimated. Chemoprevention is another modality for HCC. Eradication of HCV with an antiviral agent has proven to prevent hepatocarcinogenesis. As for chemoprevention of HCC, some trials are on going in Japan.

Nuclear respiratory factor 1 plays an essential role in transcriptional initiation from the hepatitis B virus x gene promoter

The X gene of hepatitis B virus (HBV) is one of the major factors in HBV-induced hepatocarcinogenesis and is essential for the establishment of productive HBV replication in vivo. Recent studies have shown that the X gene product targets mitochondria and induces calcium flux, thereby activating Ca(+)-dependent signal transduction pathways. However, regulatory mechanisms of X gene expression have remained unclear. Previous studies had localized a minimal promoter activity to a 21-bp GC-rich sequence located 130 bp upstream of the X protein coding region and showed that there was a cellular protein bound to this DNA. Interestingly, the 21-bp sequence identified as an X gene minimal promoter does not contain any previously identified core promoter elements, such as a TATA box. To better understand the mechanisms of transcriptional initiation of the X gene, we set out to biochemically purify the binding protein(s) for the 21-bp DNA. We report here the identification of the X gene minimal promoter-binding activity as nuclear respiratory factor 1 (NRF1), a previously known transcription factor that activates the majority of nucleus-encoded mitochondrial genes and various housekeeping genes. Primer extension analyses of the X mRNAs show that mutations at the binding site specifically inactivate transcription from this promoter and that a dominant-negative NRF1 mutant and short interfering RNAs inhibit transcription from this promoter. Therefore, NRF1 specifically binds the 21-bp minimal promoter and positively contributes to transcription of the X gene. Simultaneous activation of the X gene and mitochondrial genes by NRF1 may allow the X protein to target mitochondria most efficiently.

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Regulatory elements of hepatitis B virus transcription

The precise modulation of hepatitis B virus (HBV) gene expression is essential for replication of the virus. HBV sequences are transcribed under the control of the preC/pregenomic, S1, S2 and X promoters. With the exception of S1, all the HBV promoters lack the orthodox TATA box motifs required for the formation of the transcription initiation complex, and as such they represent a unique model of transcription initiation elements. The presence of two enhancer sequences and negative regulatory elements in the HBV genome further augments the controlled synthesis of HBV- RNA. All these transcription cis-elements are embedded within protein coding regions of the genome. This feature demonstrates the remarkable ability of the virus to maximize the function of its small genome. HBV transcription control elements also display a preference for liver-specific or liver-enriched trans-factors, which contributes to the liver tropism of the virus. This review outlines the major HBV transcription regulatory elements and highlights the reliance of accurate HBV gene modulation on the complex interplay between several trans-acting factors and their corresponding cis- motifs in the viral genome.

P53-responsive genes and the potential for cancer diagnostics and therapeutics development

P53 protein regulates cell responses to DNA damage to keep genomic stability by transactivation and trans-repression of its downstream target genes. P53 protein also has activators, inactivators, or co-factors via interaction with other proteins. Both the p53-regulated genes and interacted proteins form a huge network. As tumors usually escape from proliferating controls by means of accumulation of genetic alterations, p53 is one of the most important tumor suppressor genes that can be targeted for diagnosis, prognosis, and therapeutic intervention. Reviewing the p53-network is of great importance. In this review, we are focusing on cancer-related p53 downstream-regulated genes. Various methods dealing with the discovery of p53-regulated genes by the detection of gene expression have been applied. Recently high throughput functional genomics methods, such as DNA microarray, serial analysis of gene expression (SAGE), differential display, and protein two-dimensional gel electrophoresis, have provided a wealth of information on the dynamics of cell context responses. Hundreds of genes have been discovered whose transcriptions are regulated by p53 protein. They were grouped, based on their functions, into sub-classes including cell-cycle regulation, DNA repair, angiogenesis, metastasis, and multidrug resistance. P53 plays a pivotal role in keeping genomic stability and tumor suppression. The deeper we investigate the cell responses as mediated by p53, the more complex p53-network becomes. However, understanding p53-network, offers great opportunities to develop more sensitive and accurate diagnostic/prognostic tools, as well as more efficient therapies for cancer.

Microinjection technique used to study functional interaction between p53 and hepatitis B virus X gene in apoptosis

Microinjection of expression vectors into cultured cells has been utilized to study functional interaction of p53 and the hepatitis B virus HBx gene in apoptosis. This approach allows us to determine protein-protein interactions in primary cultured human cells at a single cell level, including fibroblasts, mammary epithelial cells, renal epithelial cells, and hepatocytes. In principle, this approach can be used to study functional interaction of p53 and any gene that is either pro- or anti-apoptotic. The use of primary cultured human cells minimizes ambiguous results associated with immortalized or tumorigenic cell lines. Moreover, it is an easy and effective way to introduce genes of interests into primary human cells with defined genetic defects, thereby facilitating the delineation of genetic pathways.

Interaction of Hepatitis B virus with cellular processes in liver carcinogenesis

Hepatitis B infection is strongly linked epidemiologically to hepatocellular carcinoma development. This article reviews the molecular mechanisms by which hepatitis B encoded proteins such as hepatitis B x and hepatitis B surface transactivators may interact with gene transcription, tumor suppression, apoptosis, and signalling pathways of the liver cell with the possible consequence of tumor induction. Data on the interaction between hepatitis B proteins and cellular processes are often conflicting indicating a non-specific simultaneous interaction with antagonistic cellular processes that result in the formation of escape mutants that are not subject to these selective pressures.

Hepatocellular carcinoma in Sweden: its association with viral hepatitis, especially with hepatitis C viral genotypes

Viral markers of chronic hepatitis were tested for in 95 frozen serum samples from 299 patients from Malmö, Sweden, with hepatocellular carcinoma (HCC), diagnosed between 1977 and 1994. Hepatitis B analysis included anti-HBc, HBsAg and, if anti-HBc positive, HBV DNA. Hepatitis C infection analysis included anti-HCV screening, RIBA, HCV RNA and HCV genotyping. HCV genotyping was also carried out in 9 HCV-viraemic HCC-patients from Gothenburg. HCV genotype distribution in HCC cases was compared with Swedish HCV-infected blood donors. Among the 95 patients from Malmö, 28 (29%) had anti-HBc, but only 5 (5%) were chronic HBV carriers, compared with 16 (17%) with chronic hepatitis C (p = 0.021). HCV-related HCC was more common among immigrants (8/16 vs. 8/79; p < 0.001). Genotyping of 25 HCV-infected cases showed genotype 1a in 6 (24%), genotype 1b in 13 (52%), genotype 2b in 4 (16%), and genotype 3a in 2 (8.0%) patients. Genotype 1b was more common among HCC patients than among blood donors (p < 0.001), but 8 of 13 genotype 1b-infected patients were from countries where genotype 1b is predominant. Among native Swedes there was no difference between the HCV genotypes infecting blood donors and those found in HCC patients.

HBV transcription repression in response to genotoxic stress is p53-dependent and abrogated by pX

Transcription of hepatitis B Virus (HBV), an important risk factor of hepatocellular carcinoma (HCC), is controlled by cellular transcription activators including some of the cellular signaling targets. Consequently, HBV transcription rate changes in response to the cellular physiological conditions. In this report we investigated HBV gene expression and the role of physiological levels of the viral X protein (pX) under cisplatin induced genotoxic stress. We show that under these conditions the RNA level of an HBV mutant which does not express pX is sharply reduced. Studies revealed that transcription repression is responsible for the observed reduction in HBV RNA level. Repression of HBV transcription was obtained only in the p53 proficient cells. Furthermore, HBV transcription rate is recovered by the cotransfected p53 dominant negative plasmid, indicating that p53 is directly responsible for HBV transcription repression. Unexpectedly, p73, the recent p53 homologue, does not repress but rather activates HBV transcription. Interestingly, pX produced either by the HBV genome or by a cotransfected plasmid, relieves the p53 mediated repression. Collectively, these results attribute a physiological role to p53-inactivation by pX, and explain how pX may support HCC development.

Post-transcriptional control of the level of mRNA by hepatitis B virus X gene in the transient expression system using human hepatic cells

BACKGROUND

Hepatitis B virus (HBV) infection is closely related to the development of not only acute or chronic hepatitis, but also hepatocellular carcinoma. Among the HBV genes, the X gene has been implicated in the carcinogenicity of this virus as a major causative factor by its ability to activate viral and cellular genes in trans via protein-protein interaction with cellular factors without binding to DNA.

RESULTS

To explore the possibility of other functions of the X gene, we examined the effect of X protein on the transient expression system of simian virus 40 (SV40) large T-antigen or chloramphenicol acetyltransferase (CAT) mRNA using SV40 promoter or EF-1alpha (human elongation factor 1alpha) promoter, by co-transfecting an X gene expression plasmid to human hepatic cell lines, HepG2 and Huh7. In contrast to the SV40 promoter-mediated expression, the level of both T-antigen and CAT mRNAs expressed from the EF-1alpha promoter was strikingly decreased by X protein in both hepatic cells. The nuclear run-on assay and the mRNA decay experiment using actinomycin D, indicated that the effect of X protein on the lowering of the level of chimeric mRNA was due to the degradation of mRNA, but not repression of transcriptional initiation. Moreover, this effect was dependent on the 22 bp sequence in the 5' untranslated region of mRNA derived from the EF-1alpha promoter.

CONCLUSION

The present data suggest a new function of the X gene to post-transcriptionally control the stability of mRNA through the 5' untranslated region derived from the EF-1alpha promoter in human hepatic cells.

Characterization of a specific region in the hepatitis B virus enhancer I for the efficient expression of X gene in the hepatic cell

Hepatitis B virus (HBV) enhancer I has been shown to consist of several cis-acting sequences for the HBV gene expression efficiently in certain types of cells. Transcriptional regulation of HBV X gene mediated by enhancer I might be one of the mechanisms by which HBV obtains hepatotropism. By mutagenesis analysis of enhancer I function in the enhancer I/X gene promoter complex, we characterized a specific transcriptional regulatory region (designated as a LSR element, nt 989-1030) of enhancer I for the X gene promoter by means of the transient transfection technique using hepatic and nonhepatic cells. Based on the analysis of protein factors interacting with the LSR element, liver-enriched transcriptional factors, HNF3 and HNF4 or retinoid X receptor alpha (RXR alpha), are probably implicated in the activity of enhancer I for the efficient expression of X gene through their interaction with the LSR element in the hepatic cell. Furthermore, the isolated LSR element was demonstrated to function alone as a specific cis-acting element and to be able to activate transcription from the X gene promoter efficiently in the hepatic cell in an orientation-independent manner.

Mutations of the X region of hepatitis B virus and their clinical implications

Nucleotide (nt) sequences of the X region of more than 130 hepatitis B virus (HBV) isolates were determined and derived from patients with a variety of clinical features. Correlation of nt substitutions with clinicopathological characteristics was attempted. The X region (465nt) is crucial for the replication and expression of HBV because the X protein transactivates the HBV genes and this region contains the core promoter, enhancer II, and two direct repeats. There are several mutational hotspots, some of which seem to relate to immunological epitopes of the X protein. Two kinds of mutations which have important clinical significances were found. One is an 8-nt deletion between nt 1770 and 1777, which truncates 20 amino acids from the carboxyl terminus of the X protein. This deletion leads to the suppression of replication and expression of HBV DNA, resulting in immunoserological marker (HBsAg) negativity. This silent HBV infection is responsible for the majority of non-A to non-E hepatitis. The other mutation substituting T for C (nt 1655), T for A (nt 1764) and A for G (nt 1766) seems to relate to fulminant hepatitis. Further sequencing studies and in vitro mutagenesis experiments will clarify the significance of other mutations of the X region.