Activating transcription factor 2 (ATF2) down-regulates hepatitis B virus X promoter activity by the competition for the activating protein 1 binding site and the formation of the ATF2-Jun heterodimer

SIAH2 suppresses c-JUN pathway by promoting the polyubiquitination and degradation of HBx in hepatocellular carcinoma

As an important protein encoded by hepatitis B virus (HBV), HBV X protein (HBx) plays an important role in the development of hepatocellular carcinoma (HCC). It has been shown that seven in absentia homologue 1 (SIAH1) could regulates the degradation of HBx through the ubiquitin-proteasome pathway. However, as a member of SIAH family, the regulatory effects of SIAH2 on HBx remain unclear. In this study, we first confirmed that SIAH2 could reduce the protein levels of HBx depending on its E3 ligase activity. Moreover, SIAH2 interacted with HBx and induced its K48-linked polyubiquitination and proteasomal degradation. Furthermore, we provided evidence that SIAH2 inhibits HBx-associated HCC cells proliferation by regulating HBx. In conclusion, our study identified a novel role for SIAH2 in promoting HBx degradation and SIAH2 exerts an inhibitory effect in the proliferation of HBx-associated HCC through inducing the degradation of HBx. Our study provides a new idea for the targeted degradation of HBx and may have great huge significance into providing novel evidence for the targeted therapy of HBV-infected HCC.

Forthcoming Developments in Models to Study the Hepatitis B Virus Replication Cycle, Pathogenesis, and Pharmacological Advancements

Hepatitis, liver cirrhosis, and hepatocellular carcinoma are all manifestations of chronic hepatitis B. Its pathogenesis and molecular mechanism remain mysterious. As medical science progresses, different models are being used to study the disease from the physiological and molecular levels. Animal models have played an unprecedented role in achieving in-depth knowledge of the disease while posing no risk of harming humans throughout the study. The scarcity of acceptable animal models has slowed progress in hepatitis B virus (HBV) research and preclinical testing of antiviral medicines since HBV has a narrow species tropism and exclusively infects humans and higher primates. The development of human chimeric mice was supported by a better understanding of the obstacles to interspecies transmission, which has substantially opened the way for HBV research in vivo and the evaluation of possible chronic hepatitis B therapeutics. Animal models are cumbersome to handle, not accessible, and expensive. Hence, it is herculean to investigate the HBV replication cycle in animal models. Therefore, it becomes essential to build a splendid in vitro cell culture system to demonstrate the mechanisms attained by the HBV for its multiplication and sustenance. We also addressed the advantages and caveats associated with different models in examining HBV.

HBV cccDNA-A Culprit and Stumbling Block for the Hepatitis B Virus Infection: Its Presence in Hepatocytes Perplexed the Possible Mission for a Functional Cure

Hepatitis B virus infection (HBV) is still a big health problem across the globe. It has been linked to the development of liver cirrhosis and hepatocellular carcinoma and can trigger different types of liver damage. Existing medicines are unable to disable covalently closed circular DNA (cccDNA), which may result in HBV persistence and recurrence. The current therapeutic goal is to achieve a functional cure, which means HBV-DNA no longer exists when treatment stops and the absence of HBsAg seroclearance. However, due to the presence of integrated HBV DNA and cccDNA functional treatment is now regarded to be difficult. In order to uncover pathways for potential therapeutic targets and identify medicines that could result in large rates of functional cure, a thorough understanding of the virus' biology is required. The proteins of the virus and episomal cccDNA are thought to be critical for the management and support of the HBV replication cycle as they interact directly with the host proteome to establish the best atmosphere for the virus while evading immune detection. The breakthroughs of host dependence factors, cccDNA transcription, epigenetic regulation, and immune-mediated breakdown have all produced significant progress in our understanding of cccDNA biology during the past decade. There are some strategies where cccDNA can be targeted either in a direct or indirect way and are presently at the point of discovery or preclinical or early clinical advancement. Editing of genomes, techniques targeting host dependence factors or epigenetic gene maintenance, nucleocapsid modulators, miRNA, siRNA, virion secretory inhibitors, and immune-mediated degradation are only a few examples. Though cccDNA approaches for direct targeting are still in the early stages of development, the assembly of capsid modulators and immune-reliant treatments have made it to the clinic. Clinical trials are currently being conducted to determine their efficiency and safety in patients, as well as their effect on viral cccDNA. The influence of recent breakthroughs in the development of new treatment techniques on cccDNA biology is also summarized in this review.

Identification and functional characterization of miR-451a as a novel plasma-based biomarker for occult hepatitis B virus infection

BACKGROUND

Numerous studies have indicated that miRNAs might play significant roles in the development of hepatitis B virus (HBV) infection. while the miRNAs in occult HBV infection (OBI) are still largely unknown.

METHODS

Initially, 15 HBV infection-related miRNAs in plasma of 10 OBI and 10 healthy controls (HCs) was analyzed by qRT-PCR. Significantly dysregulated miRNAs were subsequently validated in another 64 OBI, 20HCs, 31 chronic hepatitis B (CHB) and 20 asymptomatic HBsAg carriers (ASC). Furthermore, the potential biological functions and molecular mechanisms of miR-451a in HBV infection were investigated using HBV-expressing hepatoma cell lines.

RESULTS

Compared to HCs, plasma miR-451a and miR-340-3p were significantly up-regulated in OBI, ASC and CHB patients, while no significant difference was found among OBI, ASC and CHB patients. ROC curve analysis indicated that both plasma miR-451a and miR-340-3p could moderately distinguish OBI from HCs, with AUCs of 0.76 and 0.78, respectively. When combined, the differentiation efficiency of this miRNA panel was better, with an AUC of 0.82. While, they both could not specifically separate the stage of chronic HBV infection. Functional experiments showed that overexpression of miR-451a might suppress HBV replication and gene expression in hepatoma cell lines. Mechanistically, miR-451a might inhibit HBV replication and gene expression by directly targeting ATF2.

CONCLUSIONS

A plasma panel, including miR-340-3p and miR-451a that might suppress HBV replication by targeting ATF2, has the potential as biomarkers for HBV infection. In the setting of blood donations, this panel would be more practical to moderately differentiate OBI in HBsAg-negative donors.

The Hepatitis B Virus Interactome: A Comprehensive Overview

Despite the availability of a prophylactic vaccine, chronic hepatitis B (CHB) caused by the hepatitis B virus (HBV) is a major health problem affecting an estimated 292 million people globally. Current therapeutic goals are to achieve functional cure characterized by HBsAg seroclearance and the absence of HBV-DNA after treatment cessation. However, at present, functional cure is thought to be complicated due to the presence of covalently closed circular DNA (cccDNA) and integrated HBV-DNA. Even if the episomal cccDNA is silenced or eliminated, it remains unclear how important the high level of HBsAg that is expressed from integrated HBV DNA is for the pathology. To identify therapies that could bring about high rates of functional cure, in-depth knowledge of the virus' biology is imperative to pinpoint mechanisms for novel therapeutic targets. The viral proteins and the episomal cccDNA are considered integral for the control and maintenance of the HBV life cycle and through direct interaction with the host proteome they help create the most optimal environment for the virus whilst avoiding immune detection. New HBV-host protein interactions are continuously being identified. Unfortunately, a compendium of the most recent information is lacking and an interactome is unavailable. This article provides a comprehensive review of the virus-host relationship from viral entry to release, as well as an interactome of cccDNA, HBc, and HBx.

Silencing hepatitis B virus covalently closed circular DNA: The potential of an epigenetic therapy approach

Global prophylactic vaccination programmes have helped to curb new hepatitis B virus (HBV) infections. However, it is estimated that nearly 300 million people are chronically infected and have a high risk of developing hepatocellular carcinoma. As such, HBV remains a serious health priority and the development of novel curative therapeutics is urgently needed. Chronic HBV infection has been attributed to the persistence of the covalently closed circular DNA (cccDNA) which establishes itself as a minichromosome in the nucleus of hepatocytes. As the viral transcription intermediate, the cccDNA is responsible for producing new virions and perpetuating infection. HBV is dependent on various host factors for cccDNA formation and the minichromosome is amenable to epigenetic modifications. Two HBV proteins, X (HBx) and core (HBc) promote viral replication by modulating the cccDNA epigenome and regulating host cell responses. This includes viral and host gene expression, chromatin remodeling, DNA methylation, the antiviral immune response, apoptosis, and ubiquitination. Elimination of the cccDNA minichromosome would result in a sterilizing cure; however, this may be difficult to achieve. Epigenetic therapies could permanently silence the cccDNA minichromosome and promote a functional cure. This review explores the cccDNA epigenome, how host and viral factors influence transcription, and the recent epigenetic therapies and epigenome engineering approaches that have been described.

Host Transcription Factors in Hepatitis B Virus RNA Synthesis

The hepatitis B virus (HBV) chronically infects over 250 million people worldwide and is one of the leading causes of liver cancer and hepatocellular carcinoma. HBV persistence is due in part to the highly stable HBV minichromosome or HBV covalently closed circular DNA (cccDNA) that resides in the nucleus. As HBV replication requires the help of host transcription factors to replicate, focusing on host protein–HBV genome interactions may reveal insights into new drug targets against cccDNA. The structural details on such complexes, however, remain poorly defined. In this review, the current literature regarding host transcription factors’ interactions with HBV cccDNA is discussed.

HBx-related long non-coding RNA 01152 promotes cell proliferation and survival by IL-23 in hepatocellular carcinoma

Accumulating evidence suggests that long-noncoding RNA (lncRNA) plays important roles in hepatitis B virus (HBV) infections. However, the mechanism underlying how lncRNA regulate hepatocellular carcinoma process remains largely unknown. In this study we found that the expression of LINC01152 was significantly increased in HBV positive HCC tissues and cells and was induced by HBx in vitro. The overexpression of LINC01152 could increases HCC cell proliferation and promotes tumor formation in nude mice. Mechanistically, HBx could increase the transcription of LINC01152. Elevated LINC01152 binds to the promoter region of IL-23, promoting its transcriptional activity and upregulating the levels of Stat3 and p-Stat3. Our findings suggest that LINC01152 plays an important role in HBV-related hepatocellular carcinoma development and may serve as a therapeutic marker for hepatocellular carcinoma.

The regulatory role of activating transcription factor 2 in inflammation

Activating transcription factor 2 (ATF2) is a member of the leucine zipper family of DNA-binding proteins and is widely distributed in tissues including the liver, lung, spleen, and kidney. Like c-Jun and c-Fos, ATF2 responds to stress-related stimuli and may thereby influence cell proliferation, inflammation, apoptosis, oncogenesis, neurological development and function, and skeletal remodeling. Recent studies clarify the regulatory role of ATF2 in inflammation and describe potential inhibitors of this protein. In this paper, we summarize the properties and functions of ATF2 and explore potential applications of ATF2 inhibitors as tools for research and for the development of immunosuppressive and anti-inflammatory drugs.

Hepatitis B virus X protein impedes the DNA repair via its association with transcription factor, TFIIH

Background

Hepatitis B virus (HBV) infections play an important role in the development of hepatocellular carcinoma (HCC). HBV X protein (HBx) is a multifunctional protein that can modulate various cellular processes and plays a crucial role in the pathogenesis of HCC. HBx is known to interact with DNA helicase components of TFIIH, a basal transcriptional factor and an integral component of DNA excision repair.

Results

In this study, the functional relevance of this association was further investigated in the context to DNA repair. By site-directed mutagenesis HBx's critical residues for interaction with TFIIH were identified. Similarly, TFIIH mutants lacking ATPase domain and the conserved carboxyl-terminal domain failed to interact with HBx. Yeast and mammalian cells expressing HBx^wt conferred hypersensitivity to UV irradiation, which is interpreted as a basic deficiency in nucleotide excision repair. HBx^mut120 (Glu to Val) was defective in binding to TFIIH and failed to respond to UV.

Conclusions

We conclude that HBx may act as the promoting factor by inhibiting DNA repair causing DNA damage and accumulation of errors, thereby contributing to HCC development.

Antiviral effect of Curcuma longa Linn extract against hepatitis B virus replication

ETHNOPHARMACOLOGICAL RELEVANCE

A medicinal herb Curcuma longa Linn has been used for treating various liver diseases caused by hepatitis B virus (HBV) in Asia.

AIM OF THE STUDY

The study was performed in order to investigate the antiviral activity of Curcuma longa Linn against HBV replication in liver cells.

MATERIALS AND METHODS

Aqueous extract of Curcuma longa Linn (CLL) was prepared and used to analyze its antiviral activity against HBV replication in HepG 2.2.15 cells, which contain HBV genomes. The inhibitory effect of CLL on HBV replication was examined by testing the levels of secreted HBV surface antigens (HBsAg), HBV DNAs, and HBV RNAs in HepG 2.2.15 cells using ELISA, Southern blot, and Northern blot analyses. Cytotoxic activities of CLL extract on various liver cells were analyzed by MTT assay. To dissect the inhibitory mechanism of CLL extract on HBV replication, the levels of p53 protein and p53 mRNAs were analyzed by Western blot and RT-PCR in HepG 2.2.15 cells. The repression of CLL extract on HBV transcription was analyzed by RT-PCR and CAT assay.

RESULTS

CLL extract repressed the secretion of HBsAg from HepG 2.2.15 cells. CLL extract also suppressed the production of HBV particles and the level of intracellular HBV RNAs in HepG 2.2.15 cells, suggesting that CLL extract inhibits HBV replication. We found that the anti-HBV activity of CLL extract is mediated through enhancing the cellular accumulation of p53 protein by transactivating the transcription of p53 gene as well as increasing the stability of p53 protein. It turned out that CLL extract repressed the transcription of HBx gene by suppressing HBV enhancer I and X promoter through p53 protein. In addition, CLL extract did not have any cytotoxic effects on liver cells.

CONCLUSION

These data showed that CLL extract represses HBV replication through enhancing the level of p53 protein. CLL extract can be used as a safe and specific drug for patients with liver diseases caused by HBV infection.

Prospero-related homeobox protein (Prox1) inhibits hepatitis B virus replication through repressing multiple cis regulatory elements

Hepatitis B virus (HBV) gene transcription is controlled by viral promoters and enhancers, the activities of which are regulated by a number of cellular factors as well as virally encoded proteins. Negative regulation of HBV cis-element activities by cellular factors has been reported less widely than their activation. In this study, we report that nuclear factor Prospero-related homeobox protein (Prox1) represses HBV antigen expression and genome replication in cultured hepatocytes. By using reporter-gene analysis, three of the four HBV promoters, namely the enhancer II/core promoter (ENII/Cp), preS1 promoter (Sp1) and enhancer I/X promoter, were identified as targets for Prox1-mediated repression. Mechanistic analysis then revealed that, for ENII/Cp, Prox1 serves as a corepressor of liver receptor homologue 1 (LRH-1) and downregulates LRH-1-mediated activation of ENII/Cp, whereas for Sp1, Prox1 partially represses Sp1 activity by interacting directly with hepatocyte nuclear factor 1. Identification of Prox1 as an HBV repressor will help in the understanding of detailed interactions between viral cis elements and host cellular factors and may also form the basis for new anti-HBV intervention therapeutics.

Retinoic acid decreases ATF-2 phosphorylation and sensitizes melanoma cells to taxol-mediated growth inhibition

Cutaneous melanoma is often resistant to chemo- and radiotherapy. This resistance has recently been demonstrated to be due, at least in part, to high activating transcription factor 2 (ATF-2) activity in these tumors. In concordance with these reports, we found that B16 mouse melanoma cells had higher levels of ATF-2 than immortalized, but non-malignant mouse melanocytes. In addition, the melanoma cells had a much higher amount of phosphorylated (active) ATF-2 than the immortalized melanocytes. In the course of determining how retinoic acid (RA) stimulates activating protein-1 (AP-1) activity in B16 melanoma, we discovered that this retinoid decreased the phosphorylation of ATF-2. It appears that this effect is mediated through p38 MAPK, because RA decreased p38 phosphorylation, and a selective inhibitor of p38 MAPK (SB203580) also inhibited the phosphorylation of ATF-2. Since ATF-2 activity appears to be involved in resistance of melanoma to chemotherapy, we tested the hypothesis that treatment of the melanoma cells with RA would sensitize them to the growth-inhibitory effect of taxol. We found that pretreatment of B16 cells with RA decreased the IC50 from 50 nM to 1 nM taxol. On the basis of these findings and our previous work on AP-1, we propose a model in which treatment of B16 cells with RA decreases the phosphorylation of ATF-2, which results in less dimer formation with Jun. The "freed-up" Jun can then form a heterodimer with Fos, resulting in the increased AP-1 activity observed in RA-treated B16 cells. Shifting the balance from predominantly ATF-2:Jun dimers to a higher amount of Jun:Fos dimers could lead a change in target gene expression that reduces resistance to chemotherapeutic drugs and contributes to the pathway by which RA arrests proliferation and induces differentiation.

Liver-derived cell lines QSG-7701 and HepG2 support different HBV replication patterns

Hepatitis B virus (HBV) infection is currently still a worldwide heath concern. In our study, we compared HBV replication patterns in two liver-derived cell lines, QSG-7701 and HepG2. Viral markers of HBV replication in culture medium and cells were analyzed after transfection of these cells with plasmid pUC18-HBV1.2 into. We showed that QSG-7701 cells could support more stable and a higher level of HBV replication than HepG2 cells. Gene expression profiles of QSG-7701 and HepG2 cells by microarray analysis showed that many genes were differentially expressed between these two cell lines, including those that are related to the HBV life cycle. The global gene expression profile of these two cell types provides some clues to explain how different HBV replication is achieved. QSG-7701 cells offer a new opportunity for basic research on HBV virus-host interactions.

Effects of hepatitis B virus X protein on the development of liver cancer

Hepatitis B virus (HBV) infections play an important role in the development of cirrhosis and hepatocellular carcinoma (HCC). The pathogenesis of HBV-related HCC, however, has not been fully described. Evidence suggests that the HBV X protein (HBx) plays a crucial role in the pathogenesis of HCC. The high occurrence of anti-HBx antibody in the serum of HCC patients indicates that it could be a prognostic marker of HBV infection and HCC. HBx stimulates and influences signal transduction pathways within cells. HBx also binds to such protein targets as p53, proteasome subunits, and UV-damaged DNA binding proteins. It also interacts with the cyclic AMP-responsive element binding protein, ATF-2, NFkappaB, and basal transcription factors. HBx is primarily localized to the cytoplasm, where it interacts with and stimulates protein kinases, including protein kinase C, Janus kinase/STAT, IKK, PI-3-K, stress-activated protein kinase/Jun N-terminal kinase, and protein kinase B/Akt. It is also found in the mitochondrion, where it influences the Bcl-2 family. This review examines the role of HBx in the life cycle of HBV as well as the various signal transduction pathways involved in the pathogenesis of HBV-induced hepatocarcinogenesis.

Evaluation of transcriptional efficiency of hepatitis B virus covalently closed circular DNA by reverse transcription-PCR combined with the restriction enzyme digestion method

Virus persistence in chronic hepatitis B patients is due to the sustaining level of covalently closed circular DNA (cccDNA) within the nuclei of infected hepatocytes. In this study, we used a modified 1.3-fold hepatitis B virus (HBV) genome, with a BclI genetic marker embedded in the redundancy region, to examine the transcriptional activity of cccDNA and the effect of the HBx protein on transcriptional regulation. After harvesting total RNA from transfected cells or stable lines, we specifically identified and monitored the transcripts from cccDNA by using reverse transcription-PCR (RT-PCR) combined with the restriction enzyme digestion method. In this approach, we have found that (i) RT-PCR combined with detection of the BclI marker is a highly specific method for distinguishing cccDNA-derived transcripts from the original integrated viral genome, (ii) the transcriptional ability of cccDNA was less efficient than that from the integrated viral genome, and (iii) the transcriptional activity of cccDNA was significantly regulated by the HBx protein, a potential transcription activator. In conclusion, we provided a tool with which to elucidate the transcriptional regulation of cccDNA and clarified the transcriptional regulation mechanism of HBx on cccDNA. The results obtained may be helpful in the development of a clinical intervention for patients with chronic HBV infections.

Transcriptional Activation of Gammaherpesviral Oncogene Promoters by the Hepatitis B Viral X Protein (HBx)

The latent membrane protein-1 (LMP1) of Epstein-Barr Virus (EBV), saimiri transformation protein (STP) of Herpesvirus saimiri (HVS), and K1 protein of Kaposi's sarcoma-associated herpesvirus (KSHV) are potent gammaherpesvirus oncogenes. To study the possible effects of double viral infection, we investigated the effects of oncogenic early proteins of DNA viruses E1A and E1B (adenovirus-5), E6 and E7 (human papillomavirus-16), HBx (hepatitis B virus), Tag (SV40), and gammaherpesviral oncogene during co-infection in human B-lymphoma (Ramos) and human T-cell leukemia (Jurkat) cell lines. HBx transactivated the promoters of LMP1, STP, and K1 the most, by about six-, three-, and twofold, respectively. Analyses of site-directed mutation and the heterologous promoter system showed that HBx activated the promoter activity of these genes via the NF-kappaB site. These results suggest that HBV (HBx) infection of cells previously infected by gammaherpesviruses transactivates their oncogenes, resulting in possible virus-related disease pathogenesis.

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Tyrphostin AG 555 inhibits bovine papillomavirus transcription by changing the ratio between E2 transactivator/repressor function

The tyrosine kinase inhibitor (tyrphostin) AG 555 selectively interferes with viral transcription in bovine papillomavirus type 1 (BPV-1)-transformed fibroblasts and induces suppression of cyclin-dependent kinase activity and cell cycle arrest. Concomitant with inhibition of viral transcription, c-Jun was strongly up-regulated, which was consistent with the observation that AG 555 treatment also led to an activation of the mitogen-activated protein kinase pathway by enhancing phosphorylation of JNK and p38. Increased JNK and p38 activity resulted in higher phosphorylation of the AP-1 family members c-Jun and activating transcription factor 2. Scanning the BPV-1 genome for potential binding sequences, an intragenic AP-1 site (BAP-1) within the E7 open reading frame was detected. Enhanced dimerization of phosphorylated activating transcription factor 2 together with c-Jun and binding to BAP-1 seem to be responsible for viral dysregulation because both suppression of BPV-1 and induction of c-Jun mRNA could be almost entirely abrogated by simultaneous treatment with SB 203580, an inhibitor of p38 mitogen-activated protein kinase activity. Moreover, dissecting the complex transcriptional pattern of episomal BPV-1 with specific primer sets for reverse transcription-PCR analysis, the repressive effect could be attributed to a selective down-regulation of the mRNA encoding the E2 transactivator function in favor of the E2 repressor, whose mRNA level remained constant during AG 555 treatment. These data indicate that tyrphostin AG 555 disturbs the balance of negative and positive regulatory factors necessary to maintain the homeostasis of a virus-transformed phenotype.

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Transcriptional activation of the promoter of human cytomegalovirus immediate early gene (CMV-IE) by the hepatitis B viral X protein (HBx) through the NF-kappaB site

The reactivation of latent cytomegalovirus (CMV) in a human by another viral infection may induce virus-related symptoms. Based on this presumption, we investigated the effect of HBx on the activation of the CMV-IE, which is also known as a transactivator and potential oncogene. The HBx transactivated the CMV-IE promoter by up to 4- and 18-fold factors in human liver HepG2 and lung fibroblast MRC-5 cells, respectively. Cotransfection of HBx with each transcription factor presented in the CMV-IE promoter showed that only NF-kappaB synergistically activated the promoter by up to a 14-fold factor. Serial deletion assays and point mutation analysis showed that the third NF-kappaB site (nt -267 to -258) and the second one (nt -162 to -153) appeared as the major responsible site and minor one, respectively, for the transactivation. These results suggest the possibility that the HBV infection of a cell previously infected by CMV would exert influence on the reactivation of the latent cytomegalovirus in a human to induce virus-related symptoms.

Activation of the human transforming growth factor alpha (TGF-alpha) gene by the hepatitis B viral X protein (HBx) through AP-2 sites

The HBx protein is known as a transactivator and potential oncogene, and TGF-alpha as a potent mitogen in hepatocellular carcinoma. By assays of serial deletion of the promoter of TGF-alpha gene and the cotransfection of HBx and AP-2 expression vectors, we observed that the HBx significantly activated the promoter activity through AP-2 sites located in the proximal region of the TGF-alpha promoter (-136 to -30). This effect was also observed in the heterologous promoter assay system containing AP-2 sites. The mutation analyses of three AP-2 sites in the promoter revealed that all three AP-2 sites contributed to the activation of the TGF-a gene in the presence of HBx. Accordingly, the mRNA level of TGF-alpha was significantly elevated in the HBx-expressing cell, HepG2-HBx and the HBV-producing cell, HepG2-K8. These results suggest that the HBx protein could increase the mitogenic effect of TGF-alpha by the transactivation of the gene through AP-2 binding sites and consequently, these interactions may accelerate the process of hepatocarcinogenesis.

Transcriptional repression of p21(waf1) promoter by hepatitis B virus X protein via a p53-independent pathway

The X-gene product of hepatitis B virus (HBx) has been implicated in hepatitis B virus (HBV)-mediated hepatocellular carcinoma through its ability to induce liver cancer in some transgenic mice and to transactivate a variety of viral and cellular promoters. In this study, we demonstrated that the level of p21(waf1) RNA was decreased in the HBx-expressing cells and this effect was due to the transcriptional repression of the p21(waf1) gene by HBx via a p53-independent pathway. As the Sp1 binding sites of the p21(waf1) promoter were sufficient to confer HBx responsiveness to a previously non-responsive promoter, we suggested that HBx represses the transcription of p21(waf1) by downregulating the activity of Sp1. Because the tumor repressor p21(waf1) protein is a universal inhibitor of cyclin-CDK complexes and DNA replication that induces cell cycle arrest at the G1-S checkpoint, the repression of p21(waf1) by HBx might play an important role in a HBV-mediated pathogenesis.

CREB/ATF proteins enhance the basal and CD154- and IL-4-induced transcriptional activity of the human Igamma1 proximal promoter

To understand the underlying basis for the strong IL-4- and CD154-mediated Igamma1 promoter activity in Ramos 2G6 B cells, we carried out transient transfection assays with luciferase-based constructs containing approximately 2.2 kb and 500 bp of the human Igamma1 proximal promoter region. As a comparison, the corresponding regions of the human Igamma3 promoter were tested under identical conditions. We found that both Igamma1 and Igamma3 promoter constructs were activated upon transfection into Ramos B cells and that activity was significantly up-regulated by CD154 and IL-4 signals. However, the Igamma1 promoter was measurably stronger than the Igamma3 promoter with respect to both basal and induced responses. Sequence comparison revealed a divergent 36-bp region containing multiple putative transcription factor binding sites in the Igamma1 but not the Igamma3 promoter. A mutational "swap" of this sequence resulted in a marked decrease and increase in Igamma1 and Igamma3 basal and induced promoter activity, respectively. Gel retardation assays with Igamma1-specific probes revealed CREB-containing complexes that were not observed with the corresponding Igamma3 probes. Mutation of a single nucleotide in overlapping CREB sites in the Igamma1 sequence resulted in a significant decrease in basal activity with a corresponding reduction in the level of IL-4- and CD154-mediated transcription.

The Pit-1beta domain dictates active repression and alteration of histone acetylation of the proximal prolactin promoter

A critical problem in current molecular biology is to gain a detailed understanding of the molecular mechanisms by which related transcription factor isoforms with identical DNA sequence specificity mediate distinct transcription responses. Pit-1 and Pit-1beta constitute such a pair of transcription factor isoforms. Pit-1 enhances the Ras signaling pathway to the prolactin promoter, and Pit-1beta represses basal prolactin promoter activity as well as Ras signaling to the prolactin promoter in pituitary cells. We have previously demonstrated that the beta-domain amino acid sequence dictates the transcriptional properties of Pit-1beta. Here, we show that five hydrophobic beta-domain residues are required for Pit-1 isoform-specific repression of Ras signaling, and we demonstrate that sodium butyrate and trichostatin A, pharmacological inhibitors of histone deacetylation, as well as viral Ski protein, a dominant-negative inhibitor of recruitment of N-CoR/mSin3 histone deacetylase complexes, specifically reverse beta isoform-specific repression of Ras signaling. Moreover, we directly demonstrate, with a chromatin immunoprecipitation assay, that the Pit-1beta isoform alters the histone acetylation state of the proximal prolactin promoter. This differential analysis of Pit-1/Pit-1beta isoform function provides significant insights into the structural determinants that govern how different transcription factors with identical DNA sequence specificity can display opposite effects on target gene activity.

Selective transcriptional regulations in the human liver cell by hepatitis B viral X protein

The hepatitis B viral X protein (HBx) is known as a transcription factor and potential oncogene. To gain a better view of the effect of HBx on the transcriptional regulation in the human liver cell, we constructed a HepG2 cell line stably expressing HBx (HepG2-HBx), and performed cDNA microarray analysis on 588 cellular cDNAs comparing with untransformed control cells. Two genes (IGFR-2, RhoA) of oncogenes, one gene (p55CDC) of cell cycle regulators, three genes (thrombin receptor, MLK-3, MacMARCKS) of intracellular transducers, one gene (HSP27) of stress response proteins, two genes (FAST kinase, Bak) of apoptosis response proteins, one gene (p21(WAF)) of transcription factors were highly up-regulated; one gene (transcription elongation factor SII) of transcription factors and two genes (monocyte chemotactic protein 1, T-lymphocyte-secreted protein I-309) of growth factors were highly down-regulated. These results showed selective transcriptional regulation by HBx in the human liver cell.

Transcriptional repression of the human p53 gene by hepatitis B viral X protein

Hepatitis B viral X protein (HBx) and the human p53 protein (p53) have been known as a transactivator and as a tumor suppressor, respectively. These two proteins have also been known to interact with each other to neutralize their authentic functions and the p53 represses the HBV enhancer/X promoter activity. Here we report that the promoter activity of the human p53 gene was strongly repressed by the HBx using the chloramphenicol acetyl transferase (CAT) assay. Analyses of serial deletion, site-directed mutagenesis and the heterologous promoter system showed that the site responsible for the repression was the E-box element in the promoter of the p53 gene. In addition, HBx as expected also repressed the activation of the p53 promoter by c-Myc through the E-box element. Northern blot analyses also showed that the expression of the p53 gene in the HepG2-K8 cell line, which expresses HBV genes including HBx, was much more repressed than that of the control cell HepG2. These results with previous data suggest that the shift of the reciprocal inhibitory activities at the levels of protein-protein interaction and transcription between HBx and p53 may play a decisive role in the HBV-related hepatocarcinogenesis.

The synergistic transactivation of the hepatitis B viral (HBV) pregenomic promoter by the E6 protein of human papillomavirus type 16 (HPV-16 E6) with HBV X protein was mediated through the AP1 site of E element in the enhancer I (EnI) in human liver cell

Infection by HBV of a cell already infected with other viral species or vice versa has been suggested as being involved in hepatocellular carcinoma. Using the CAT assay method, we investigated the interactive roles of HBx and potentially oncogenic and transactivating viral early proteins such as Ad5 E1A, HPV-16 E6, and SV40 T ag. In the presence of HBx, only HPV-16 E6 showed significant synergistic transactivation of EnI. We further investigated the function of the HPV-16 E6 using deletion, heterologous promoter, and mutation analyses on the EnI promoter. The results showed that the synergistic effect was mediated through the AP1 site of the E element in EnI by the direct activation of AP1 and support the idea that the infection by HBV of the cell with other viral species such as HPV-16 could increase the transcription activity of the HBV and other oncogenes containing an AP1 site in the promoter.

Down-regulation of tumor necrosis factor alpha expression by activating transcription factor 2 increases UVC-induced apoptosis of late-stage melanoma cells

To identify mechanisms whereby activating transcription factor 2 (ATF2) alters the radiation resistance of human melanoma cells, we examined the possible role of ATF2 in UVC-induced apoptosis. Forced expression of full-length or truncated (Delta1-195 amino acids) forms of ATF2 in LU1205, a late-stage human melanoma cell line, elevated the levels of UVC-induced apoptosis. At the same time, either truncated or full-length forms of ATF2 reduced UVC-induced activation of the tumor necrosis factor-alpha (TNFalpha) promoter and decreased expression of TNFalpha. Forced expression of c-Jun in ATF2-expressing melanoma cells restored TNFalpha expression, suggesting that both forms of ATF2 sequestered transcription factors that positively regulate TNFalpha expression in response to UV irradiation. Antagonistic antibodies to Fas, but not to TNFR1, efficiently suppressed UVC-induced apoptosis, suggesting that the Fas pathway mediates the primary apoptotic signal in melanoma cells whereas the TNFR1 pathway elicits a survival signal. Indeed, treatment of melanoma cells with TNFalpha before UVC irradiation partially suppressed UVC-induced apoptosis, further supporting the protective role of TNFalpha in UVC-treated melanoma cells. Taken together, our findings suggest that ATF2 contributes to UVC-induced apoptosis through transcriptional silencing of TNFalpha, which balances Fas-mediated cell death in melanoma.

Interaction of hepatitis B viral X protein and CCAAT/ enhancer-binding protein alpha synergistically activates the hepatitis B viral enhancer II/pregenomic promoter

The hepatitis B viral X protein (HBx) is known to exert its transactivation activity by the interaction with several cellular transcription factors. Here we report the interaction of HBx and CCAAT/enhancer-binding protein alpha (C/EBPalpha) and their effects on the enhancer/promoters of hepatitis B virus (HBV). A chloramphenicol acetyltransferase assay showed that the cotransfection of HBx and C/EBPalpha strongly activated the enhancer II/pregenomic promoter of HBV in a synergistic manner. This effect was also observed in the heterologous expression system with promoters of SV40 and herpes simplex virus thymidine kinase genes. Serial deletion analysis of the enhancer II/pregenomic promoter identified the responsible region (nucleotides 1639-1679), in which two C/EBP-binding sites are located. An in vitro interaction assay and electrophoretic mobility shift assay showed that HBx augmented the DNA binding activity of C/EBPalpha by direct interaction with it, and its basic leucine zipper domain was responsible for the interaction with HBx. Domain analysis of HBx showed that the central region (amino acids 78-103) was necessary for direct interaction with C/EBPalpha. However, the complete form of HBx was necessary for the synergistic activation of the HBV pregenomic promoter. These results suggest that the interaction of HBx and C/EBPalpha enhances the transcription of the HBV pregenomic promoter for the effective life cycle of HBV in hepatocytes.

Insulin activates the hepatitis B virus X gene through the activating protein-1 binding site in HepG2 cells

Insulin stimulates cellular oncogenic activators such as c-jun, c-fos, and c-myc; and hepatitis B virus (HBV) X, a viral transactivator, is known to induce liver cancer in transgenic mice. In this respect, the effect of insulin on the expression of HBx protein was investigated in HepG2 cells. Insulin-stimulated transcription from the HBV X promoter in a dose-dependent manner was assessed by chloramphenicol acetyltransferase (CAT) assay. A mutation preventing AP-1 binding to the E element abolished the activation of the HBV X promoter by insulin. In addition, insulin stimulated the minimal thymidine kinase (tk) gene promoter activity through both the HBV E element and the consensus AP-1 binding site in HepG2 cells. An electrophoretic mobility shift assay (EMSA) using insulin-treated HepG2 nuclear extracts showed that insulin actually enhanced the binding of nuclear proteins to the HBV E element as well as to the consensus AP-1 binding site. Both HBV E and AP-1 oligonucleotides were effective competitors for this binding. These results showed that insulin elevated the expression of HBx protein through the AP-1 binding site of HBV EnI. We suggest that insulin can augment the role of HBx in the development of hepatocellular carcinoma (HCC) in HBV-infected liver, probably through interaction with other cellular oncogenes.

Hepatitis B virus replication in human HepG2 cells mediated by hepatitis B virus recombinant baculovirus

A novel transient mechanism for studying hepatitis B virus (HBV) gene expression and replication using recombinant HBV baculovirus to deliver the HBV genome to HepG2 cells was generated. In HBV baculovirus infected HepG2 cells, HBV transcripts, and intracellular and secreted HBV antigens are produced; replication occurs as evidenced by the presence of high levels of intracellular replicative intermediates and protected HBV DNA in the medium. Density-gradient analysis of extracellular HBV DNA indicated that the DNA was contained predominantly in enveloped HBV virions. Covalently closed circular (CCC) DNA is present indicating that, in this system, HBV core particles are capable of delivering newly synthesized HBV genomes back into the nuclei of infected cells. HBV gene expression is driven exclusively from endogenous promoters. Levels of HBV gene expression and replication can be achieved in HBV baculovirus-infected HepG2 cells which far exceed levels found in HepG2 2.2.15 cells. HBV baculovirus infection of HepG2 cells lends itself readily to experimental manipulation as follows: 1) HBV expression can be initiated any time relative to seeding of HepG2 cells; 2) levels of HBV replication can be regulated over a wide range simply by changing the baculovirus multiplicity of infection; 3) HBV replication is readily detectable by one day post infection with HBV baculovirus and persists at least through day eleven post infection; and (4) the transient nature of the infection can be extended and/or enhanced by superinfecting the cultures. We conclude that infection of HepG2 cells by HBV recombinant baculovirus represents a simple to use and highly flexible system for studying the effects of antivirals and/or cytokines on HBV production and for understanding HBV replication and pathogenesis at the molecular level.

p53CP, a putative p53 competing protein that specifically binds to the consensus p53 DNA binding sites: a third member of the p53 family?

p53 tumor suppressor protein negatively regulates cell growth, mainly through the transactivation of its downstream target genes. As a sequence-specific DNA binding transcription factor, p53 specifically binds to a 20-bp consensus motif 5'-PuPuPuC(A/T) (T/A)GPyPyPyPuPuPuC(A/T)(T/A)GPyPyPy-3'. We have now identified, partially purified, and characterized an additional approximately 40-kDa nuclear protein, p53CP (p53 competing protein), that specifically binds to the consensus p53 binding sites found in several p53 downstream target genes, including Waf-1, Gadd45, Mdm2, Bax, and RGC. The minimal sequence requirement for binding is a 14-bp motif, 5'-CTTGCTTGAACAGG-3' [5'-C(A/T)(T/A)GPyPyPyPuPuPuC(A/T)(T/A)G-3'], which includes the central nucleotides of the typical p53 binding site with one mismatch. p53CP and p53 (complexed with antibody) showed a similar binding specificity to Waf-1 site but differences in Gadd45 and T3SF binding. Like p53, p53CP also binds both double- and single-stranded DNA oligonucleotides. Important to note, cell cycle blockers and DNA damaging reagents, which induce p53 binding activity, were found to inhibit p53CP binding in p53-positive, but not in p53-negative, cells. This finding suggested a p53-dependent coordinate regulation of p53 and p53CP in response to external stimuli. p53CP therefore could be a third member of the p53 family, in addition to p53 and p73, a newly identified p53 homolog. p53CP, if sequestering p53 from its DNA binding sites through competitive binding, may provide a novel mechanism of p53 inactivation. Alternatively, p53CP may have p53-like functions by binding and transactivating p53 downstream target genes. Cloning of the p53CP gene ultimately will resolve this issue.