Differential immediate early gene expression in conditional hepatitis B virus pX-transforming versus nontransforming hepatocyte cell lines

ATF3 induction prevents precocious activation of skeletal muscle stem cell by regulating H2B expression

Skeletal muscle stem cells (also called satellite cells, SCs) are important for maintaining muscle tissue homeostasis and damage-induced regeneration. However, it remains poorly understood how SCs enter cell cycle to become activated upon injury. Here we report that AP-1 family member ATF3 (Activating Transcription Factor 3) prevents SC premature activation. Atf3 is rapidly and transiently induced in SCs upon activation. Short-term deletion of Atf3 in SCs accelerates acute injury-induced regeneration, however, its long-term deletion exhausts the SC pool and thus impairs muscle regeneration. The Atf3 loss also provokes SC activation during voluntary exercise and enhances the activation during endurance exercise. Mechanistically, ATF3 directly activates the transcription of Histone 2B genes, whose reduction accelerates nucleosome displacement and gene transcription required for SC activation. Finally, the ATF3-dependent H2B expression also prevents genome instability and replicative senescence in SCs. Therefore, this study has revealed a previously unknown mechanism for preserving the SC population by actively suppressing precocious activation, in which ATF3 is a key regulator.

EpCAM-regulated intramembrane proteolysis induces a cancer stem cell-like gene signature in hepatitis B virus-infected hepatocytes

BACKGROUND & AIMS

Hepatocytes in which the hepatitis B virus (HBV) is replicating exhibit loss of the chromatin modifying polycomb repressive complex 2 (PRC2), resulting in re-expression of specific, cellular PRC2-repressed genes. Epithelial cell adhesion molecule (EpCAM) is a PRC2-repressed gene, normally expressed in hepatic progenitors, but re-expressed in hepatic cancer stem cells (hCSCs). Herein, we investigated the functional significance of EpCAM re-expression in HBV-mediated hepatocarcinogenesis.

METHODS

Employing molecular approaches (transfections, fluorescence-activated cell sorting, immunoblotting, qRT-PCR), we investigated the role of EpCAM-regulated intramembrane proteolysis (RIP) in HBV replicating cells in vitro, and in liver tumors from HBV X/c-myc mice and chronically HBV infected patients.

RESULTS

EpCAM undergoes RIP in HBV replicating cells, activating canonical Wnt signaling. Transfection of Wnt-responsive plasmid expressing green fluorescent protein (GFP) identified a GFP ⁺ population of HBV replicating cells. These GFP⁺/Wnt⁺ cells exhibited cisplatin- and sorafenib-resistant growth resembling hCSCs, and increased expression of pluripotency genes NANOG, OCT4, SOX2, and hCSC markers BAMBI, CD44 and CD133. These genes are referred as EpCAM RIP and Wnt-induced hCSC-like gene signature. Interestingly, this gene signature is also overexpressed in liver tumors of X/c-myc bitransgenic mice. Clinically, a group of HBV-associated hepatocellular carcinomas was identified, exhibiting elevated expression of the hCSC-like gene signature and associated with reduced overall survival post-surgical resection.

CONCLUSIONS

The hCSC-like gene signature offers promise as prognostic tool for classifying subtypes of HBV-induced HCCs. Since EpCAM RIP and Wnt signaling drive expression of this hCSC-like signature, inhibition of these pathways can be explored as therapeutic strategy for this subtype of HBV-associated HCCs.

LAY SUMMARY

In this study, we provide evidence for a molecular mechanism by which chronic infection by the hepatitis B virus results in the development of poor prognosis liver cancer. Based on this mechanism our results suggest possible therapeutic interventions.

RNA helicase DEAD box protein 5 regulates Polycomb repressive complex 2/Hox transcript antisense intergenic RNA function in hepatitis B virus infection and hepatocarcinogenesis

Chronic hepatitis B virus (HBV) infection is a major factor in hepatocellular carcinoma (HCC) pathogenesis by a mechanism not yet understood. Elucidating mechanisms of HBV-mediated hepatocarcinogenesis is needed to gain insights into classification and treatment of HCC. In HBV replicating cells, including virus-associated HCCs, suppressor of zeste 12 homolog (SUZ12), a core subunit of Polycomb repressive complex2 (PRC2), undergoes proteasomal degradation. This process requires the long noncoding RNA, Hox transcript antisense intergenic RNA (HOTAIR). Intriguingly, HOTAIR interacts with PRC2 and also binds RNA-binding E3 ligases, serving as a ubiquitination scaffold. Herein, we identified the RNA helicase, DEAD box protein 5 (DDX5), as a regulator of SUZ12 stability and PRC2-mediated gene repression, acting by regulating RNA-protein complexes formed with HOTAIR. Specifically, knockdown of DDX5 and/or HOTAIR enabled reexpression of PRC2-repressed genes epithelial cell adhesion molecule (EpCAM) and pluripotency genes. Also, knockdown of DDX5 enhanced transcription from the HBV minichromosome. The helicase activity of DDX5 stabilized SUZ12- and PRC2-mediated gene silencing, by displacing the RNA-binding E3 ligase, Mex-3 RNA-binding family member B (Mex3b), from HOTAIR. Conversely, ectopic expression of Mex3b ubiquitinated SUZ12, displaced DDX5 from HOTAIR, and induced SUZ12 down-regulation. In G2 phase of cells expressing the HBV X protein (HBx), SUZ12 preferentially associated with Mex3b, but not DDX5, resulting in de-repression of PRC2 targets, including EpCAM and pluripotency genes. Significantly, liver tumors from HBx/c-myc bitransgenic mice and chronically HBV-infected patients exhibited a strong negative correlation between DDX5 messenger RNA levels, pluripotency gene expression, and liver tumor differentiation. Notably, chronically infected HBV patients with HCC expressing reduced DDX5 exhibited poor prognosis after tumor resection, identifying DDX5 as an important player in poor prognosis HCC.

CONCLUSION

The RNA helicase DDX5, and E3 ligase Mex3b, are important cellular targets for the design of novel, epigenetic therapies to combat HBV infection and poor prognosis HBV-associated liver cancer. (Hepatology 2016;64:1033-1048).

PLK1 and HOTAIR Accelerate Proteasomal Degradation of SUZ12 and ZNF198 during Hepatitis B Virus-Induced Liver Carcinogenesis

Elucidating mechanisms of hepatitis B virus (HBV)-mediated hepatocarcinogenesis is needed to gain insights into the etiology and treatment of liver cancer. Cells where HBV is replicating exhibit increased expression of Plk1 kinase and reduced levels of two transcription repression factors, SUZ12 and ZNF198. SUZ12 is an essential subunit of the transcription repressive complex PRC2. ZNF198 stabilizes the transcription repressive complex composed of LSD1, Co-REST, and HDAC1. These two transcription repressive complexes are held together by binding the long noncoding RNA HOTAIR. In this study, we linked these regulatory events mechanistically by showing that Plk1 induces proteasomal degradation of SUZ12 and ZNF198 by site-specific phosphorylation. Plk1-dependent ubiquitination of SUZ12 and ZNF198 was enhanced by expression of HOTAIR, significantly reducing SUZ12 and ZNF198 stability. In cells expressing the HBV X protein (HBx), downregulation of SUZ12 and ZNF198 mediated global changes in histone modifications. In turn, HBx-expressing cells propagated an altered chromatin landscape after cell division, as exemplified by changes in histone modifications of the EpCAM promoter, a target of PRC2 and LSD1/Co-REST/HDAC1 complexes. Notably, liver tumors from X/c-myc bitransgenic mice exhibited downregulation of SUZ12 and ZNF198 along with elevated expression of Plk1, HOTAIR, and EpCAM. Clinically, similar effects were documented in a set of HBV-related liver tumors consistent with the likelihood that downregulation of SUZ12 and ZNF198 leads to epigenetic reprogramming of infected hepatocytes. Because both Plk1 and HOTAIR are elevated in many human cancers, we propose that their combined effects are involved in epigenetic reprogramming associated broadly with oncogenic transformation.

Hepatitis B virus X protein induces EpCAM expression via active DNA demethylation directed by RelA in complex with EZH2 and TET2

Chronic hepatitis B virus (HBV) infection is a major risk factor for developing hepatocellular carcinoma (HCC), and HBV X protein (HBx) acts as cofactor in hepatocarcinogenesis. In liver tumors from animals modeling HBx- and HBV-mediated hepatocarcinogenesis, downregulation of chromatin regulating proteins SUZ12 and ZNF198 induces expression of several genes, including epithelial cell adhesion molecule (EpCAM). EpCAM upregulation occurs in HBV-mediated HCCs and hepatic cancer stem cells, by a mechanism not understood. Herein we demonstrate HBx induces EpCAM expression via active DNA demethylation. In hepatocytes, EpCAM is silenced by polycomb repressive complex 2 (PRC2) and ZNF198/LSD1/Co-REST/HDAC1 chromatin-modifying complexes. Cells with stable knockdown of SUZ12, an essential PRC2 subunit, upon HBx expression demethylate a CpG dinucleotide located adjacent to NF-κB/RelA half-site. This NF-κB/RelA site is in a CpG island downstream from EpCAM transcriptional start site (TSS). Chromatin immunoprecipitation (ChIP) assays demonstrate HBx-dependent RelA occupancy of NF-κB half-site, whereas RelA knockdown suppresses CpG demethylation and EpCAM expression. Tumor necrosis factor-α activates RelA, propagating demethylation to nearby CpG sites, shown by sodium bisulfite sequencing. RelA-dependent demethylation occurring upon HBx expression requires methyltrasferase EZH2, TET2 a key factor in cytosine demethylation and inactive DNMT3L, shown by knockdown assays and sodium bisulfite sequencing. Co-immunoprecipitations and sequential ChIP assays demonstrate that RelA in the presence of HBx forms a complex with EZH2, TET2 and DNMT3L, although the role of DNMT3L remains to be understood. Interestingly, the human EpCAM gene also has a CpG island downstream from its TSS, and a NF-κB-binding site flanked by CpGs. HepG2 cells derived from human HCC exhibit demethylation of these NF-κB-flanking CpG sites, and HBV replication propagates demethylation to nearby CpG sites. DLK1, another PRC2 target gene, also upregulated in HBV-mediated HCCs, is demethylated in liver tumors at CpG dinucleotides flanking the NF-κB-binding sequence, supporting that this active DNA demethylation mechanism functions during oncogenic transformation.

Technical standards for hepatitis B virus X protein (HBx) research

Chronic infection with hepatitis B virus (HBV) is a risk factor for developing hepatocellular carcinoma (HCC). The life cycle of HBV is complex and has been difficult to study because HBV does not infect cultured cells. The HBV regulatory X protein (HBx) controls the level of HBV replication and possesses an HCC cofactor role. Attempts to understand the mechanism(s) that underlie HBx effects on HBV replication and HBV-associated carcinogenesis have led to many reported HBx activities that are likely influenced by the assays used. This review summarizes experimental systems commonly used to study HBx functions, describes limitations of these experimental systems that should be considered, and suggests approaches for ensuring the biological relevance of HBx studies.

Cell cycle regulation during viral infection

To replicate their genomes in cells and generate new progeny, viruses typically require factors provided by the cells that they have infected. Subversion of the cellular machinery that controls replication of the infected host cell is a common activity of many viruses. Viruses employ different strategies to deregulate cell cycle checkpoint controls and modulate cell proliferation pathways. A number of DNA and RNA viruses encode proteins that target critical cell cycle regulators to achieve cellular conditions that are beneficial for viral replication. Many DNA viruses induce quiescent cells to enter the cell cycle; this is thought to increase pools of deoxynucleotides and thus, facilitate viral replication. In contrast, some viruses can arrest cells in a particular phase of the cell cycle that is favorable for replication of the specific virus. Cell cycle arrest may inhibit early cell death of infected cells, allow the cells to evade immune defenses, or help promote virus assembly. Although beneficial for the viral life cycle, virus-mediated alterations in normal cell cycle control mechanisms could have detrimental effects on cellular physiology and may ultimately contribute to pathologies associated with the viral infection, including cell transformation and cancer progression and maintenance. In this chapter, we summarize various strategies employed by DNA and RNA viruses to modulate the replication cycle of the virus-infected cell. When known, we describe how these virus-associated effects influence replication of the virus and contribute to diseases associated with infection by that specific virus.

Transection of preganglionic axons leads to CNS neuronal plasticity followed by survival and target reinnervation

The goals of the present study were to investigate the changes in sympathetic preganglionic neurons following transection of distal axons in the cervical sympathetic trunk (CST) that innervate the superior cervical ganglion (SCG) and to assess changes in the protein expression of brain derived neurotrophic factor (BDNF) and its receptor TrkB in the thoracic spinal cord. At 1 week, a significant decrease in soma volume and reduced soma expression of choline acetyltransferase (ChAT) in the intermediolateral cell column (IML) of T1 spinal cord were observed, with both ChAT-ir and non-immunoreactive neurons expressing the injury marker activating transcription factor 3. These changes were transient, and at later time points, ChAT expression and soma volume returned to control values and the number of ATF3 neurons declined. No evidence for cell loss or neuronal apoptosis was detected at any time point. Protein levels of BDNF and/or full length TrkB in the spinal cord were increased throughout the survival period. In the SCG, both ChAT-ir axons and ChAT protein remained decreased at 16 weeks, but were increased compared to the 10 week time point. These results suggest that though IML neurons show reduced ChAT expression and cell volume at 1 week following CST transection, at later time points, the neurons recovered and exhibited no significant signs of neurodegeneration. The alterations in BDNF and/or TrkB may have contributed to the survival of the IML neurons and the recovery of ChAT expression, as well as to the reinnervation of the SCG.

Subset of Suz12/PRC2 target genes is activated during hepatitis B virus replication and liver carcinogenesis associated with HBV X protein

Chronic hepatitis B virus (HBV) infection is a major risk factor for developing liver cancer, and the HBV X protein (pX) has been implicated as a cofactor in hepatocyte transformation. We have shown that HBV replication as well as in vitro transformation by pX are associated with induction of the mitotic polo-like kinase 1 (Plk1) and down-regulation of the chromatin remodeling components Suz12 and Znf198. Herein, we demonstrate the same inverse relationship between Plk1 and Suz12/Znf198 in liver tumors from X/c-myc bitransgenic mice and woodchuck hepatitis virus (WHV)-infected woodchucks. Employing these animal models and the HBV replicating HepAD38 cells we examined the effect of Suz12/Znf198 down-regulation on gene expression. Genes analyzed include hepatic cancer stem cell markers BAMBI, DKK1,2, DLK1, EpCAM, MYC, and proliferation genes CCNA1, CCND2, IGFII, MCM4-6, PLK1, RPA2, and TYMS. Suz12 occupancy at the promoters of BAMBI, CCND2, DKK2, DLK1, EpCAM, and IGFII was demonstrated by chromatin immunoprecipitation in untransformed hepatocytes, but was markedly reduced in pX-transformed and Suz12 knockdown cells. Accordingly, we refer to these genes as "Suz12 repressed" genes in untransformed hepatocytes. The Suz12 repressed genes and proliferation genes were induced in HBV-replicating HepAD38 cells and, interestingly, they exhibited distinct expression profiles during hepatocellular carcinoma (HCC) progression in X/c-myc bitransgenics. Specifically, CCND2, EpCAM, and IGFII expression was elevated at the proliferative and preneoplastic stages in X/c-myc bitransgenic livers, whereas BAMBI and PLK1 were overexpressed in hepatic tumors from X/c-myc bitransgenics and WHV-infected woodchucks. Importantly, most of these genes were selectively up-regulated in HBV-induced HCCs.

CONCLUSION

The distinct expression profile of the identified Suz12 repressed genes in combination with the proliferation genes hold promise as biomarkers for progression of chronic HBV infection to HCC.

MK3 controls Polycomb target gene expression via negative feedback on ERK

Background

Gene-environment interactions are mediated by epigenetic mechanisms. Polycomb Group proteins constitute part of an epigenetic cellular transcriptional memory system that is subject to dynamic modulation during differentiation. Molecular insight in processes that control dynamic chromatin association and dissociation of Polycomb repressive complexes during and beyond development is limited. We recently showed that MK3 interacts with Polycomb repressive complex 1 (PRC1). The functional relevance of this interaction, however, remained poorly understood. MK3 is activated downstream of mitogen- and stress-activated protein kinases (M/SAPKs), all of which fulfill crucial roles during development. We here use activation of the immediate-early response gene ATF3, a bona fide PRC1 target gene, as a model to study how MK3 and its effector kinases MAPK/ERK and SAPK/P38 are involved in regulation of PRC1-dependent ATF3 transcription.

Results

Our current data show that mitogenic signaling through ERK, P38 and MK3 regulates ATF3 expression by PRC1/chromatin dissociation and epigenetic modulation. Mitogenic stimulation results in transient P38-dependent H3S28 phosphorylation and ERK-driven PRC1/chromatin dissociation at PRC1 targets. H3S28 phosphorylation by itself appears not sufficient to induce PRC1/chromatin dissociation, nor ATF3 transcription, as inhibition of MEK/ERK signaling blocks BMI1/chromatin dissociation and ATF3 expression, despite induced H3S28 phosphorylation. In addition, we establish that concomitant loss of local H3K27me3 promoter marking is not required for ATF3 activation. We identify pERK as a novel signaling-induced binding partner of PRC1, and provide evidence that MK3 controls ATF3 expression in cultured cells via negative regulatory feedback on M/SAPKs. Dramatically increased ectopic wing vein formation in the absence of Drosophila MK in a Drosophila ERK gain-of-function wing vein patterning model, supports the existence of MK-mediated negative feedback regulation on pERK.

Conclusion

We here identify and characterize important actors in a PRC1-dependent epigenetic signal/response mechanism, some of which appear to be nonspecific global responses, whereas others provide modular specificity. Our findings provide novel insight into a Polycomb-mediated epigenetic mechanism that dynamically controls gene transcription and support a direct link between PRC1 and cellular responses to changes in the microenvironment.

Akt augments the oncogenic potential of the HBx protein of hepatitis B virus by phosphorylation

Hepatitis B virus X protein (HBx) is a putative viral oncoprotein that plays an important role in various cellular processes, including modulation of the phosphatidylinositol 3-kinase/Akt signalling pathway. However, the molecular mechanism of Akt activation remains elusive. Here we show that HBx interacts with Akt1 kinase and is phosphorylated at serine 31 as indicated by mutational analysis of the Akt recognition motif (creating the HBxS31A mutant) or immunoblotting of HBx immunoprecipitates using Akt motif-specific antibody. The Akt-dependent phosphorylation of HBx was abrogated in the presence of the phosphatidylinositol 3-kinase inhibitor LY294002 or Akt1 gene silencing by specific siRNA. Co-immunoprecipitation studies provided evidence for HBx-Akt interaction in a cellular environment. This interaction was also confirmed in hepatoma HepG2.2.15 cells in which HBx was expressed at physiological levels from the integrated hepatitis B viral genome. The HBx-Akt interaction was essential for Akt signalling, and involved displacement of the Akt-bound negative regulator 'C-terminal modulator protein' by HBx. HBx-activated Akt phosphorylated its downstream target glycogen synthase kinase 3β, leading to stabilization of β-catenin, while p65 phosphorylation resulted in enhanced promoter recruitment and expression of target genes encoding cyclin D1 and Bcl-XL. Further, the oncogenic potential of HBx was significantly augmented in the presence of Akt in a soft agar colony formation assay. Together, these results suggest that oncogenic co-operation between HBx and Akt may be important for cell proliferation, abrogation of apoptosis and tumorigenic transformation of cells.

Proteins ZNF198 and SUZ12 are down-regulated in hepatitis B virus (HBV) X protein-mediated hepatocyte transformation and in HBV replication

Chronic hepatitis B virus (HBV) infection is a major etiologic factor in hepatocellular carcinoma (HCC) pathogenesis, involving effects of chronic liver inflammation and of the weakly oncogenic HBV X protein (pX). pX-mediated hepatocyte transformation requires Polo-like kinase1 (Plk1) activity, but the mechanism is not fully understood. We identified by a genome-wide short hairpin RNA (shRNA) library screen the genes zinc finger, MYM-type 2 (ZNF198) and suppressor of zeste 12 homolog (Drosophila) (SUZ12) whose protein depletion rescues pX-expressing cells from DNA damage-induced apoptosis. ZNF198 and SUZ12 are components of chromatin remodeling complexes and associate with promyelocytic leukemia (PML) nuclear bodies. Knockdown of ZNF198 and SUZ12 by small interfering RNA (siRNA) reduced p53 stability and DNA repair, rescued pX-expressing hepatocytes from DNA damage-induced apoptosis, and increased pX-induced polyploidy and oncogenic transformation, suggesting ZNF198 and SUZ12 have a role in pX-mediated transformation. Interestingly, during pX-mediated transformation the protein but not messenger RNA (mRNA) levels of ZNF198 and SUZ12 progressively decreased, whereas Plk1 levels increased. Inhibition of Plk1 activity restored protein levels of ZNF198 and SUZ12. In addition, transfected Polo-box-domain (PBD) of Plk1 coimmunoprecipitated with ZNF198 and SUZ12, suggesting that these proteins are Plk1 substrates. Elevated Plk1 and reduced protein levels of ZNF198 and SUZ12 were also observed in human liver cancer cell lines derived from HBV-related tumors and in the presence of HBV replication. Importantly, knockdown by siRNA of ZNF198 and SUZ12 enhanced HBV replication.

CONCLUSION

Reduced protein levels of ZNF198 and SUZ12 and elevated Plk1 occur during pX-mediated hepatocyte transformation in human liver cancer cell lines, as well as during HBV replication, underscoring the significance of these genes both in HBV-mediated HCC pathogenesis and HBV replication. We propose Plk1 activity down-regulates ZNF198 and SUZ12, thereby enhancing both HBV replication and pX-mediated oncogenic transformation.

Polo-like kinase 1 activated by the hepatitis B virus X protein attenuates both the DNA damage checkpoint and DNA repair resulting in partial polyploidy

Hepatitis B virus X protein (pX), implicated in hepatocarcinogenesis, induces DNA damage because of re-replication and allows propagation of damaged DNA, resulting in partial polyploidy and oncogenic transformation. The mechanism by which pX allows cells with DNA damage to continue proliferating is unknown. Herein, we show pX activates Polo-like kinase 1 (Plk1) in the G(2) phase, thereby attenuating the DNA damage checkpoint. Specifically, in the G(2) phase of pX-expressing cells, the checkpoint kinase Chk1 was inactive despite DNA damage, and protein levels of claspin, an adaptor of ataxia telangiectasia-mutated and Rad3-related protein-mediated Chk1 phosphorylation, were reduced. Pharmacologic inhibition or knockdown of Plk1 restored claspin protein levels, Chk1 activation, and p53 stabilization. Also, protein levels of DNA repair protein Mre11 were decreased in the G(2) phase of pX-expressing cells but not with Plk1 knockdown. Interestingly, in pX-expressing cells, Mre11 co-immunoprecipitated with transfected Plk1 Polo-box domain, and inhibition of Plk1 increased Mre11 stability in cycloheximide-treated cells. These results suggest that pX-activated Plk1 by down-regulating Mre11 attenuates DNA repair. Importantly, concurrent inhibition of Plk1, p53, and Mre11 increased the number of pX-expressing cells with DNA damage entering mitosis, relative to Plk1 inhibition alone. By contrast, inhibition or knockdown of Plk1 reduced pX-induced polyploidy while increasing apoptosis. We conclude Plk1, activated by pX, allows propagation of DNA damage by concurrently attenuating the DNA damage checkpoint and DNA repair, resulting in polyploidy. We propose this novel Plk1 mechanism initiates pX-mediated hepatocyte transformation.

Cooperation of tumor-derived HBx mutants and p53-249(ser) mutant in regulating cell proliferation, anchorage-independent growth and aneuploidy in a telomerase-immortalized normal human hepatocyte-derived cell line

Hepatocellular carcinoma (HCC) is a common cancer, and hepatitis B virus (HBV) is a major etiological agent. Convincing epidemiological and experimental evidence also links HCC to aflatoxin, a naturally occurring mycotoxin that produces a signature p53-249(ser) mutation. Recently, we have reported that tumor-derived HBx variants encoded by HBV exhibited attenuated transactivation and proapoptotic functions but retained their ability to block p53-mediated apoptosis. These results indicate that mutations in HBx may contribute to the development of HCC. In this study, we determined whether tumor-derived HBx mutants along, or in cooperation with p53-249(ser), could alter cell proliferation and chromosome stability of normal human hepatocytes. To test this hypothesis, we established a telomerase immortalized normal human hepatocycte line HHT4 that exhibited a near diploid karyotype and expressed many hepatocyte-specific genes. We found that overexpression one of the tumor-derived HBx mutants, CT, significantly increased colony forming efficiency (CFE) while its corresponding wild-type allele CNT significantly decreased CFE in HHT4 cells. p53-249(ser) rescued CNT-mediated inhibition of colony formation. Although HHT4 cells lacked an anchorage independent growth capability as they did not form any colonies in soft agar, the CT-expressing HHT4 cells could form colonies, which could be significantly enhanced by p53-249(ser). Induction of aneuploidy could be observed in HHT4 cells expressing CT, but additionally recurring chromosome abnormalities could only be detected in cells coexpressing CT and p53-249(ser). Our results are consistent with the hypothesis that certain mutations in HBx and p53 at codon 249 may cooperate in contributing to liver carcinogenesis.

Lethal-7 is down-regulated by the hepatitis B virus x protein and targets signal transducer and activator of transcription 3

BACKGROUND & AIMS

The pleiotropic hepatitis B virus (HBV) x protein (HBx), associated with hepatocellular carcinoma (HCC), has been implicated in the deregulation of cellular gene expression at the transcriptional level. To date, it remains unknown if HBx regulates the expression of miRNAs which play important roles in gene-regulation at the post-transcriptional and/or translational level.

METHODS

miRNA microarrays were employed to compare the expression of cellular miRNAs in HBx-versus control-HepG2 cells. Reverse-transcription Taqman realtime-PCR was used to examine let-7a expression in normal liver as well as paired HCC-tumor and adjacent non-tumorous liver. Let-7a miRNA was functionally characterized in cells with transiently altered let-7a expression. The direct target of let-7a was identified in silico and validated using 3'UTR-reporter assay.

RESULTS

HBx up-regulates 7 and down-regulates 11 miRNAs, including the let-7 family. HBx expression was found to have a significant inverse correlation with the expression of the highly-expressed members of the let-7 family in HCC patients, highlighting the clinical relevance of our observations. Further characterization of let-7a, the most highly expressed let-7 family member, revealed that it negatively regulates cellular proliferation partly through targeting signal transducer and activator of transcription 3 (STAT3). HBx-mediated down-regulation of let-7a and up-regulation of STAT3 supports cell proliferation in HBx cells.

CONCLUSION

This study thus represents the first demonstration of HBx's ability to deregulate cellular miRNA expression. The deregulation of the expression of the let-7 family of miRNAs by HBx may represent a potential novel pathway through which HBx acts to deregulate cell proliferation leading to hepatocarcinogenesis.

Bee venom protects hepatocytes from tumor necrosis factor-alpha and actinomycin D

Honeybee (Apis mellifera) venom (BV) has a broad array of therapeutic applications in traditional medicine to treat variety of diseases. It is also known that BV possesses anti-inflammatory and anticancer effect and that it can inhibit proliferation and induces apoptosis in cancer cells, but there is no evidence of information regarding anti-apoptosis of BV on hepatocytes. In the present study, we investigated the anti-apoptotic effect of BV on tumor necrosis factor (TNF)-alpha with actinomycin (Act) D induces apoptosis in hepatocytes. TNF-alpha/Act D-treated hepatocytes were exposed to different low concentration (1, 10 and 100 ng/mL) of BV. Our results showed statistically significant inhibition in DNA damage caused by BV treatment compared to corresponding TNF-alpha/Act D-treated hepatocytes. BV suppressed TNF-alpha/Act Dtreated activation of bcl-2 family and caspase family, which resulted in inhibition of cytochrome c release and PARP cleavage. These results demonstrate that low concentration BV possess a potent suppressive effect on anti-apoptotic responses of TNF-alpha/Act D-treated hepatocytes and suggest that these compounds may contribute substantial therapeutic potential for the treatment of liver diseases.

Bee Venom Inhibits Hepatic Fibrosis Through Suppression of Pro-Fibrogenic Cytokine Expression

Bee venom (BV) has a long tradition of use for the control of pain and inflammation in various chronic diseases. Carbon tetrachloride ( CCl4) is known to induce hepatotoxicity after being metabolized to the highly reactive trichloromethyl free radical and its peroxy radical. The purpose of the current study was to examine whether BV regulates the pro-inflammation and fibrosis related genes against a mouse model of hepatic fibrosis induced by CCl4and ethanol-treated hepatocytes (ETH). Test mice were administered with CCl4(2 ml/mg) and hepatocytes were treated with 25 mM ethanol. BV was added to the final concentration of 0.05–0.5 mg/kg and 1–100 ng/ml for in vivo and in vitro testing, respectively. Fibrotic livers and ETH were used for the measurement of hepatocyte necrosis, pro-inflammatory cytokines and fibrogenic genes. BV suppressed CCl4-induced hepatocyte necrosis markers of serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT). It also inhibited the secretion of interleukin (IL)-1β and tumor necrosis factor (TNF)-α. Moreover, BV inhibited CCl4-induced expression of transforming growth factor (TGF)-β1, α-smooth muscle actin (SMA) and fibronectin. Similarly, ETH exhibited significant suppression of IL-1β, TNF-α, TGF-β1 and fibronectin when cultured with BV. These results suggest that BV possesses anti-fibrogenic properties that are mediated by the suppression of pro-inflammatory cytokines and fibrogenic gene expression. BV has substantial therapeutic potential for the treatment of fibrotic diseases.

Transformation of human liver L-O2 cells mediated by stable HBx transfection

AIM

To explore the mechanism of hepatocarcinogenesis associated with the hepatitis B virus X protein (HBx), we investigated the role of HBx in transformation using human liver L-O2 cells stably transfected with HBx as a model.

METHODS

Plasmids encoding HBx were stably transfected into immortalized human liver L-O2 cells and rodent fibroblast NIH/3T3 cells. The expression of alfa-fetoprotein (AFP), c-Myc, HBx, and survivin in the engineered cells was examined by Western blotting. The malignant phenotype of the cells was demonstrated by anchorage-independent colony formation and tumor formation in nude mice. RNA interference assays, Western blotting, luciferase reporter gene assays and flow cytometry analysis were performed. The number of centrosomes in the L-O2-X cells was determined by gamma-tubulin immunostaining. The effect of HBx on the transcriptional activity of human telomerase reverse transcriptase (hTERT) and hTERT activity in L-O2-X cells and/or 3T3-X cells was detected by the luciferase reporter gene assay and telomerase repeat amplification protocol (TRAP).

RESULTS

Stable HBx transfection resulted in a malignant phenotype in the engineered cells in vivo and in vitro. Meanwhile, HBx was able to increase the transcription of the NF-kappaB, AP-1, and survivin genes and to upregulate the expression levels of c-Myc and survivin. Abnormal centrosome duplication and activated hTERT were responsible for the transformation.

CONCLUSION

Stable HBx transfection leads to genomic instability of host cells, which is responsible for hepatocarcinogenesis; meanwhile, transactivation by the HBx protein contributes to the development of hepatocellular carcinoma (HCC). The L-O2-X cell line is an ideal model for investigating the mechanism of HBx-mediated transformation.

Polo-like kinase 1 inhibition suppresses hepatitis B virus X protein-induced transformation in an in vitro model of liver cancer progression

Chronic hepatitis B virus (HBV) infection is linked to development of hepatocellular carcinoma (HCC). The HBV X protein (pX) is implicated in HCC pathogenesis acting as a weak oncogene or a cofactor in hepatocarcinogenesis. pX induces DNA re-replication, DNA damage, and partial polyploidy in a poorly differentiated, immortalized hepatocyte cell line. In this study we employed sorted, pX-induced polyploid cells to investigate their growth and oncogenic transformation potential over the course of 70 cell doublings. Immediately after live cell-sorting, nearly 40% of pX-induced polyploid cells undergo apoptosis, whereas the surviving cells exhibit proliferation sensitive to p53. After 40 cell generations the pX-expressing polyploid cultures exhibit loss of p53 function and become growth factor- and anchorage-independent, indicative of oncogenic transformation. The pX-induced polyploid cultures in the course of 70 cell generations undergo progressively increasing DNA damage, propagate damaged DNA to daughter cells, and display increased expression of a cluster of proliferation genes shown to be elevated in human HCC, including HBV-HCC. One of these genes is the mitotic kinase Polo-like kinase 1 (Plk1). Oncogenic transformation is suppressed in the absence of pX expression, and significantly, by inhibition of Plk1. These results identify Plk1 as crucial in pX-mediated oncogenic transformation.

CONCLUSION

Partial polyploidy induced by pX is not immediately associated with oncogenic transformation. Continued DNA damage for 40 cell generations is reproducibly associated with loss of p53 function, enhanced expression of Plk1, and oncogenic transformation. Because Plk1 expression is also elevated in HBV-HCC tumors, this in vitro cellular model simulates liver cancer progression and pathogenesis in chronic HBV patients. Inhibition of Plk1 activity suppresses pX-mediated oncogenic transformation, identifying Plk1 as a promising therapeutic target for HBV-mediated HCC.

Hepatitis B virus HBx protein localized to the nucleus restores HBx-deficient virus replication in HepG2 cells and in vivo in hydrodynamically-injected mice

Identifying the requirements for the regulatory HBx protein in hepatitis B virus (HBV) replication is an important goal. A plasmid-based HBV replication assay was used to evaluate whether HBx subcellular localization influences its ability to promote virus replication, as measured by real time PCR quantitation of viral capsid-associated DNA. HBx targeted to the nucleus by a nuclear localization signal (NLS-HBx) was able to restore HBx-deficient HBV replication, while HBx containing a nuclear export signal (NES-HBx) was not. Both NLS-HBx and NES-HBx were expressed at similar levels (by immunoprecipitation and Western blotting), and proper localization of the signal sequence-tagged proteins was confirmed by deconvolution microscopy using HBx, NLS-HBx, and NES-HBx proteins fused to GFP. Importantly, these findings were confirmed in vivo by hydrodynamic injection into mice. Our results demonstrate that in these HBV replication assays, at least one function of HBx requires its localization to the nucleus.

Hepatitis B virus X protein increases the Cdt1-to-geminin ratio inducing DNA re-replication and polyploidy

Hepatitis B virus X protein (pX) is implicated in hepatocellular carcinoma pathogenesis by an unknown mechanism. Employing the tetracycline-regulated pX-expressing 4pX-1 cell line, derived from the murine AML12 hepatocyte cell line, we demonstrate that pX induces partial polyploidy (>4N DNA). Depletion of p53 in 4pX-1 cells increases by 5-fold the polyploid cells in response to pX expression, indicating that p53 antagonizes pX-induced polyploidy. Dual-parameter flow cytometric analyses show pX-dependent bromodeoxyuridine (BrdUrd) incorporation in 4pX-1 cells containing 4N and >4N DNA, suggesting pX induces DNA re-replication. Interestingly, pX increases expression of endogenous replication initiation factors Cdc6 and Cdtl while suppressing geminin expression, a negative regulator of rereplication. In comparison to a geminin knockdown 4pX-1 cell line used as DNA re-replication control, the Cdt1/geminin ratio is greater in 4pX-1 cells expressing pX, indicating that pX promotes DNA re-replication. In support of this conclusion, pX-expressing 4pX-1 cells, similar to the geminin knockdown 4pX-1 cells, continue to incorporate BrdUrd in the G2 phase and exhibit nuclear Cdc6 and MCM5 co-localization and the absence of geminin. In addition, pX expression activates the ATR kinase, the sensor of DNA re-replication, which in turn phosphorylates RAD17 and H2AX. Interestingly, phospho-H2AX-positive and BrdUrd -positive cells progress through mitosis, demonstrating a link between pX-induced DNA re-replication and polyploidy. Our studies high-light a novel function of pX that likely contributes to hepatocellular carcinoma pathogenesis.

Hepatitis B virus X protein via the p38MAPK pathway induces E2F1 release and ATR kinase activation mediating p53 apoptosis

Hepatitis B virus (HBV) X protein (pX) is implicated in hepatocellular carcinoma (HCC) pathogenesis by an unknown mechanism. Deletions or mutations of genes involved in the p53 pathway are often associated with HBV-mediated HCC, indicating rescue from p53 apoptosis is a likely mechanism in HBV-HCC pathogenesis. Herein, we determined the mechanism by which pX sensitizes hepatocytes to p53-mediated apoptosis. Although it is well established that the Rb/E2F/ARF pathway stabilizes p53, and the DNA damage-activated ATM/ATR kinases activate p53, the mechanism that coordinates these two pathways has not been determined. We demonstrate that the p38MAPK pathway activated by pX serves this role in p53 apoptosis. Specifically, the activated p38MAPK pathway stabilizes p53 via E2F1-mediated ARF expression, and also activates the transcriptional function of p53 by activating ATR. Knockdown of p53, E2F1, ATR, or p38MAPKalpha abrogates pX-mediated apoptosis, demonstrating that E2F1, ATR, and p38MAPKalpha are all essential in p53 apoptosis in response to pX. Specifically, in response to pX expression, the p38MAPK pathway activates Cdk4 and Cdk2, leading to phosphorylation of Rb, release of E2F1, and transcription of ARF. The p38MAPK pathway also activates ATR, leading to phosphorylation of p53 on Ser-18 and Ser-23, transcription of pro-apoptotic genes Bax, Fas, and Noxa, and apoptosis. In conclusion, pX sensitizes hepatocytes to p53 apoptosis via activation of the p38MAPK pathway, which couples p53 stabilization and p53 activation, by E2F1 induction and ATR activation, respectively.

Hepatitis B virus X protein mutants exhibit distinct biological activities in hepatoma Huh7 cells

The role of the hepatitis B virus X protein (HBx) in hepatocarcinogenesis remains controversial. To investigate the biological impact of hepatitis B virus x gene (HBx) mutation on hepatoma cells, plasmids expressing the full-length HBx or HBx deletion mutants were constructed. The biological activities in these transfectants were analyzed by a series of assays. Results showed that HBx3'-20 and HBx3'-40 amino acid deletion mutants exhibited an increase in cellular proliferation, focus formation, tumorigenicity, and invasive growth and metastasis through promotion of the cell cycle from G0/G1 to the S phase, when compared with the full-length HBx. In contrast, HBx3'-30 amino acid deletion mutant repressed cell proliferation by blocking in G1 phase. The expression of P53, p21(WAF1), p14(ARF), and MDM2 proteins was regulated by expression of HBx mutants. In conclusions, HBx variants showed different effects and functions on cell proliferation and invasion by regulation of the cell cycle progression and its associated proteins expression.

Mutations in the C-terminus of the X protein of hepatitis B virus regulate Wnt-5a expression in hepatoma Huh7 cells: cDNA microarray and proteomic analyses

Background: The hepatitis B virus x gene (HBx) is a promiscuous transactivator implicated in the development of hepatocellular carcinoma (HCC). The present study was designed to investigate the molecular events regulated by HBx. Methods: Genomic and proteomic expression profiling was performed in Huh7 HCC cells transfected with HBx mutants with a C-terminal deletion. The gene and protein expression of wingless-type murine-mammary-tumour virus (MMTV) integration site family, member 5A (Wnt-5a) was validated by analyses of reverse transcription–polymerase chain reaction (RT–PCR), real-time RT–PCR, western blot and immunohistochemistry. Results: Differentially expressed genes and proteins were found in the transfected Huh7 HCC cells; most of them were involved in transcriptional regulation, although others including oncogenes or tumor suppressor genes, and molecules involved in cell junctions, signal transduction pathways, metabolism or the immune response were also observed. The expression of the Wnt-5a gene was elevated >10-fold in Huh7 cells transfected with the HBx3′-30 amino acid deletion mutant. However, the expression was downregulated by the transfection with the HBx3′-40 amino acid deletion mutant. The changes in Wnt-5a expression were also observed in human HCC tissues, compared with corresponding non-cancerous liver tissues. A negative correlation was found between the expression of Wnt-5a and HBx COOH mutations in HCC tissues. Conclusions: HBx mutants may participate in the development and progression of HCC, at least in part through the Wnt-5a pathway.

Whole genome expression profiling of hepatitis B virus-transfected cell line reveals the potential targets of anti-HBV drugs

Hepatitis B virus (HBV) infection is a major health concern world wide, and few effective treatments have been developed. It has recently been reported that inhibiting host-cell proteins can prevent viral infection. The human genome may contain more genes required for HBV infection and replication than the viral genome. A systematic approach to find these potential antiviral targets is by host gene expression analysis using DNA microarrays. The aim of this study was to identify and validate novel cellular anti-HBV targets. The Human Whole Genome Bioarray was used to analyze differentially expressed genes in HepG2.2.15 cells and HepG2 cells. Altered gene expression in HepG2.2.15 cells was studied following treatment with the anti-HBV drug, lamivudine. Genes that were differentially expressed during HBV infection and reversed with anti-HBV drugs were validated by semiquantitative reverse transcription-PCR. Bioinformatics analysis revealed ABHD2, EREG, ACVR2B, CDC34, KHDRBS3 and RORA as potential cellular anti-HBV targets. Antisense oligodeoxynucleotides were used to test the antiviral activity of these potential targets. Results strongly suggested that inhibition of ABHD2 or EREG significantly blocked HBV propagation in HepG2.2.15 cells. This study demonstrates that ABHD2 and EREG are essential for HBV propagation and provides strong evidence that these proteins could be used as potential targets for anti-HBV drugs.

Hepatitis B virus X protein enhances androgen receptor-responsive gene expression depending on androgen level

Persistent hepatitis B virus (HBV) infection is a major risk of hepatocellular carcinoma (HCC). One intriguing feature of HBV-related HCC is the male predominance, with a male to female ratio of 5-7:1. This dominance has been attributed to the elevated androgen level and the enhanced androgen receptor (AR)-mediated activity in the host. How HBV infection and AR signaling modulate HCC is unknown. We investigated whether the HBV nonstructural protein, X protein (HBx) could cooperate with the AR signaling pathway to enhance carcinogenesis. We found that HBx increased the anchorage-independent colony-formation potency of AR in a nontransformed mouse hepatocyte cell line. We also found that HBx functioned as a positive transcriptional coregulator to increase AR-mediated transcriptional activity. This transcription enhancement was increased in the presence of androgen in a concentration-responsive manner, thus explaining a more prominent effect in males. HBx did not physically associate with ligand-bound AR in the nucleus, and it likely augmented AR activity by increasing the phosphorylation of AR through HBx-mediated activation of the c-Src kinase signaling pathway. Our study documents HBx as a previously undescribed class of noncellular positive coregulators for AR. The results reveal a mechanism for the vulnerability of males to microbial infections and the subsequent development of cancer.

Homeodomain transcription factor Phox2a, via cyclic AMP-mediated activation, induces p27Kip1 transcription, coordinating neural progenitor cell cycle exit and differentiation

Mechanisms coordinating neural progenitor cell cycle exit and differentiation are incompletely understood. The cyclin-dependent kinase inhibitor p27(Kip1) is transcriptionally induced, switching specific neural progenitors from proliferation to differentiation. However, neuronal differentiation-specific transcription factors mediating p27(Kip1) transcription have not been identified. We demonstrate the homeodomain transcription factor Phox2a, required for central nervous system (CNS)- and neural crest (NC)-derived noradrenergic neuron differentiation, coordinates cell cycle exit and differentiation by inducing p27(Kip1) transcription. Phox2a transcription and activation in the CNS-derived CAD cell line and primary NC cells is mediated by combined cyclic AMP (cAMP) and bone morphogenetic protein 2 (BMP2) signaling. In the CAD cellular model, cAMP and BMP2 signaling initially induces proliferation of the undifferentiated precursors, followed by p27(Kip1) transcription, G(1) arrest, and neuronal differentiation. Small interfering RNA silencing of either Phox2a or p27(Kip1) suppresses p27(Kip1) transcription and neuronal differentiation, suggesting a causal link between p27(Kip1) expression and differentiation. Conversely, ectopic Phox2a expression via the Tet-off expression system promotes accelerated CAD cell neuronal differentiation and p27(Kip1) transcription only in the presence of cAMP signaling. Importantly, endogenous or ectopically expressed Phox2a activated by cAMP signaling binds homeodomain cis-acting elements of the p27(Kip1) promoter in vivo and mediates p27(Kip1)-luciferase expression in CAD and NC cells. We conclude that developmental cues of cAMP signaling causally link Phox2a activation with p27(Kip1) transcription, thereby coordinating neural progenitor cell cycle exit and differentiation.

The mitogenic function of hepatitis B virus X protein resides within amino acids 51 to 140 and is modulated by N- and C-terminal regulatory regions

The hepatitis B virus (HBV) X protein (pX) is implicated in hepatocarcinogenesis by an unknown mechanism. pX variants encoded by HBV genomes found integrated in genomic DNA from liver tumors of patients with hepatocellular carcinoma (HCC) generally lack amino acids 134 to 154. Since deregulation of mitogenic pathways is linked to oncogenic transformation, herein we define the pX region required for mitogenic pathway activation. A series of pX deletions was used to construct tetracycline-regulated pX-expressing cell lines. The activation of the mitogenic pathways by these pX deletions expressed in the constructed cell lines was measured by transient transreporter assays, effects on endogenous cyclin A expression, and apoptosis. Conditional expression of pX51-140 in AML12 clone 4 cell line activates the mitogenic pathways, induces endogenous cyclin A expression, and sensitizes cells to apoptosis, similar to wild-type (WT) pX. By contrast, pX1-115 is inactive, supporting the idea that amino acids 116 to 140 are required for mitogenic pathway activation. Moreover, this pX deletion analysis demonstrates that WT pX function is modulated by two regions spanning amino acids 1 to 78 and 141 to 154. The N-terminal X1-78, expressed via a retroviral vector in WT pX-expressing 4pX-1 cells, coimmunoprecipitates with WT pX, indicating this pX region participates in protein-protein interactions leading to pX oligomerization. Interestingly, pX1-78 interferes with WT pX in mediating mitogenic pathway activation, endogenous gene expression, and apoptosis. The C-terminal pX region spanning amino acids 141 to 154 decreases pX stability, determined by pulse-chase studies of WT pX and pX1-140, suggesting that increased stability of naturally occurring pX variants lacking amino acids 134 to 154 may play a role in HCC development.

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.

Gene expression analysis in HBV transgenic mouse liver: a model to study early events related to hepatocarcinogenesis

Hepatitis B virus (HBV) is one of the major etiological factors responsible for the development of hepatocellular carcinoma (HCC). We used a transgenic mouse, containing HBV sequences, as a model system to unravel the molecular mechanisms of hepatocarcinogenesis induced by HBV. We chose this animal model because it consistently develops liver cancer after intermediate steps that mimic the natural history of HBV infection in humans. In this study, we focus our attention on the early events leading to liver cancer. We compared the gene expression profile of 3-month-old transgenic mice with that of 3-month-old wild-type (wt) animals. In the transgenic mouse, microarray data analysis showed a total of 45 significantly differentially expressed genes, 25 highly expressed (fold change > or =2; P = 0.0025), and 20 downregulated (fold change < or =0.5; P = 0.0025). These genes belong to several different functional categories such as the regulation of immunological response, transcription, intracellular calcium ion mobilization, regulation of cell cycle and proliferation, NF-kappab signal transduction cascades, and apoptosis. In particular, the upregulation of the antiapoptotic gene NuprI and the downregulation of the proapoptotic gene Bnip3 were found. This observation was supported by an in vitro apoptosis assay that showed downregulation of apoptosis in hepatocytes of HBV transgenic mouse compared with wt mice treated with staurosporine. In conclusion, our experimental approach allowed identification of new genes modulated by HBV and showed that the apoptotic process was deregulated in transgenic mouse hepatocytes. These data shed light on one possible mechanism by which HBV induces hepatocarcinogenesis.

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.

Sustained activation of p38 mitogen-activated protein kinase and c-Jun N-terminal kinase pathways by hepatitis B virus X protein mediates apoptosis via induction of Fas/FasL and tumor necrosis factor (TNF) receptor 1/TNF-alpha expression

Activation of the cellular stress pathways (c-Jun N-terminal kinase [JNK] and p38 mitogen-activated protein [MAP] kinase) is linked to apoptosis. However, whether both pathways are required for apoptosis remains unresolved. Hepatitis B virus X protein (pX) activates p38 MAP kinase and JNK pathways and, in response to weak apoptotic signals, sensitizes hepatocytes to apoptosis. Employing hepatocyte cell lines expressing pX, which was regulated by tetracycline, we investigated the mechanism of apoptosis by p38 MAP kinase and JNK pathway activation. Inhibition of the p38 MAP kinase pathway rescues by 80% the initiation of pX-mediated apoptosis, whereas subsequent apoptotic events involve both pathways. pX-mediated activation of p38 MAP kinase and JNK pathways is sustained, inducing the transcription of the death receptor family genes encoding Fas/FasL and tumor necrosis factor receptor 1 (TNFR1)/TNF-alpha and the p53-regulated Bax and Noxa genes. The pX-dependent expression of Fas/FasL and TNFR1/TNF-alpha mediates caspase 8 activation, resulting in Bid cleavage. In turn, activated Bid, acting with pX-induced Bax and Noxa, mediates the mitochondrial release of cytochrome c, resulting in the activation of caspase 9 and apoptosis. Combined antibody neutralization of FasL and TNF-alpha reduces by 70% the initiation of pX-mediated apoptosis. These results support the importance of the pX-dependent activation of both the p38 MAP kinase and JNK pathways in pX-mediated apoptosis and suggest that this mechanism of apoptosis occurs in vivo in response to weak apoptotic signals.

cDNA microarray analysis of early gene expression profiles associated with hepatitis B virus X protein-mediated hepatocarcinogenesis

Chronic hepatitis B virus (HBV) infection is one of the major causes of hepatocellular carcinoma. HBV encodes an oncogenic hepatitis B virus X protein (HBx), which can transactivate host cell transcriptional machinery and mediate cellular transformation. To disclose the early genetic response in HBx-mediated transformation process, we constructed a conditional HBx-expressing hepatocyte cell line, which allows us to compare the gene expression profiles under controllable HBx induction. A cDNA microarray containing more than 8700 mouse genes and ESTs was utilized to examine the gene expression profiles. We identified 260 candidate genes and 259 ESTs which have shown aberrant expression under HBx induction. Most of them are involved in signal transduction pathway, cell cycle control, metastasis, transcriptional regulation, immune response, and metabolism. These results provide additional insight into early cellular targets of HBx, which could give us a better understanding of the function of HBx and their progressive changes during HBx-mediated hepatocarcinogenesis.

Cytochrome C oxidase III interacts with hepatitis B virus X protein in vivo by yeast two-hybrid system

AIM: To screen and identify the proteins which interact with hepatitis B virus (HBV) X protein in hepatocytes by yeast two-hybrid system and to explore the effects of X protein in the development of hepatocellular carcinoma (HCC).

METHODS: With HBV X gene amplified by polymerase chain reaction (PCR), HBV X bait plasmid, named pAS2-1-X, was constructed by yeast-two hybridization system3 and verified by auto-sequencing assay. pAS2-1-X was transformed into the yeast AH109, and X-BD fusion protein expressed in the yeast cells was detected by Western blotting. The yeast cells cotransformed with pAS2-1-X and normal human liver cDNA library were grown in selective SC/-trp-leu-his-ade medium. The second screen was performed with β-gal activity detection, and false positive clones were eliminated by segregation analysis, true positive clones were amplified, sequenced and analyzed with bioinformatics. Mating experiment was peformed to confirm the binding of putative proteins to X protein in the yeast cells.

RESULTS: Bait plasmid pAS2-1-X was successfully constructed and pAS2-1-X correctly expressed BD-X fusion protein in yeast AH109. One hundred and three clones grew in the selective SC/-trp-leu-his-ade medium, and only one clone passed through β-gal activity detection and segregation analysis. The inserted cDNA fragment showed high homology with Homo sapiens cytochrome C oxidase III (cox III). Furthermore, mating experiment identified that the binding of cox III to X protein was specific.

CONCLUSION: cox III protein is a novel protein that can interact with X protein in vivo by yeast two-hybrid system, and may contribute to the development of HCC through the interaction with X protein.

Stabilized beta-catenin promotes hepatocyte proliferation and inhibits TNFalpha-induced apoptosis

The human hepatocellular carcinoma (HCC) is one of the most common malignant tumors worldwide. The mechanisms of liver cell oncogenic transformation are still unknown. The beta-catenin mutations are identified in up to 30% of HCC and 80% of hepatoblastoma, suggesting a potential role of beta-catenin in the pathogenesis of liver cancers. To define the biological role of the stabilized beta-catenin in liver cell growth and transformation, we examined the effect of mutant beta-catenin on an immortalized murine hepatocyte cell line, AML12. A cell line that stably expresses mutant beta-catenin was established. The cell proliferation, apoptosis, and cell transformation of this cell line were characterized. Our data indicate that the stabilized beta-catenin enhances hepatocyte proliferation, suppresses TNFalpha/Act D-induced cell apoptosis, and causes weak anchorage-independent cell growth. The stabilized beta-catenin-containing cells did not develop tumor in immune-deficient mice. The target genes, c-myc and cyclin D1, were activated by beta-catenin in the hepatocytes. Our study suggests that mutant beta-catenin can promote cell proliferation and cell survival ability, but the stabilized beta-catenin alone is insufficient for completely oncogenic transformation.

Molecular aspects of hepatitis B viral infection and the viral carcinogenesis

Of many viral causes of human cancer, few are of greater global importance than the hepatitis B virus (HBV). Over 250 million people worldwide are persistently infected with HBV. A significant minority of these develop severe pathologic consequences, including chronic hepatitis, cirrhosis, and hepatocellular carcinoma (HCC). Earlier epidemiological evidence suggested a link between chronic HBV infection and HCC. Further, the existence of related animal viruses that induce acute and chronic infections of the liver, and eventually HCC, confirms the concept that HBV belongs to one of the few human oncogenic viruses. Although it is clear that chronic HBV infections are major risk factors, relatively little is understood about how the viral factors contribute to hepatocarcinogenesis. This review will introduce molecular aspects of the viral infection, and highlight recent findings on the viral contribution to hepatocarcinogenesis.

A critical evaluation of DNA adducts as biological markers for human exposure to polycyclic aromatic compounds

The causative role of polycyclic aromatic hydrocarbons (PAH) in human carcinogenesis is undisputed. Measurements of PAH-DNA adduct levels in easily accessible white blood cells therefore represent useful early endpoints in exposure intervention or chemoprevention studies. The successful applicability of DNA adducts as early endpoints depends on several criteria: i. adduct levels in easily accessible surrogate tissues should reflect adduct levels in target-tissues, ii. toxicokinetics and the temporal relevance should be properly defined. iii. sources of interand intra-individual variability must be known and controllable, and finally iv. adduct analyses must have advantages as compared to other markers of PAHexposure. In general, higher DNA adduct levels or a higher proportion of subjects with detectable DNA adduct levels were found in exposed individuals as compared with nonexposed subjects, but saturation may occur at high exposures. Furthermore, DNA adduct levels varied according to changes in exposure, for example smoking cessation resulted in lower DNA adduct levels and adduct levels paralleled seasonal variations of air-pollution. Intraindividual variation during continuous exposure was low over a short period of time (weeks), but varied significantly when longer time periods (months) were investigated. Inter-individual variation is currently only partly explained by genetic polymorphisms in genes involved in PAH-metabolism and deserves further investigation. DNA adduct measurements may have three advantages over traditional exposure assessment: i. they can smooth the extreme variability in exposure which is typical for environmental toxicants and may integrate exposure over a longer period of time. Therefore, DNA adduct assessment may reduce the monitoring effort. ii. biological monitoring of DNA adducts accounts for all exposure routes. iii. DNA adducts may account for inter-individual differences in uptake, elimination, distribution, metabolism and repair amongst exposed individuals. In conclusion, there is now a sufficiently large scientific basis to justify the application of DNA adduct measurements as biomarkers in exposure assessment and intervention studies. Their use in risk-assessment, however, requires further investigation.

The hepatitis B virus X protein promotes tumor cell invasion by inducing membrane-type matrix metalloproteinase-1 and cyclooxygenase-2 expression

Hepatocellular carcinoma is strongly associated with chronic infection by the hepatitis B virus (HBV) and has poor prognosis due to intrahepatic metastasis. HBx is often the only HBV protein detected in hepatic tumor cells; however, its contribution to tumor invasion and metastasis has not been established so far. In this work, we show that HBx enhances tumor cell invasion, both in vivo and in vitro. The increased invasive capacity induced by HBx is mediated by an upregulation of membrane-type 1 matrix metalloproteinase (MT1-MMP) expression, which in turn activates matrix metalloproteinase-2. Induction of both MT1-MMP expression and cell invasion by HBx is dependent on cyclooxygenase-2 (COX-2) activity. In addition, HBx upregulates the expression of COX-2, which is mediated by the transcriptional activation of the COX-2 gene promoter in a nuclear factor of activated T cell-dependent (NF-AT-dependent) manner. These results demonstrate the ability of HBx to promote tumor cell invasion by a mechanism involving the upregulation of MT1-MMP and COX-2 and provide new insights into the mechanism of action of this viral protein and its involvement in tumor metastasis and recurrence of hepatocellular carcinoma.

The hepatitis B virus X protein promotes tumor cell invasion by inducing membrane-type matrix metalloproteinase-1 and cyclooxygenase-2 expression

Hepatocellular carcinoma is strongly associated with chronic infection by the hepatitis B virus (HBV) and has poor prognosis due to intrahepatic metastasis. HBx is often the only HBV protein detected in hepatic tumor cells; however, its contribution to tumor invasion and metastasis has not been established so far. In this work, we show that HBx enhances tumor cell invasion, both in vivo and in vitro. The increased invasive capacity induced by HBx is mediated by an upregulation of membrane-type 1 matrix metalloproteinase (MT1-MMP) expression, which in turn activates matrix metalloproteinase-2. Induction of both MT1-MMP expression and cell invasion by HBx is dependent on cyclooxygenase-2 (COX-2) activity. In addition, HBx upregulates the expression of COX-2, which is mediated by the transcriptional activation of the COX-2 gene promoter in a nuclear factor of activated T cell-dependent (NF-AT-dependent) manner. These results demonstrate the ability of HBx to promote tumor cell invasion by a mechanism involving the upregulation of MT1-MMP and COX-2 and provide new insights into the mechanism of action of this viral protein and its involvement in tumor metastasis and recurrence of hepatocellular carcinoma.

Hepatitis B virus X protein activates the p38 mitogen-activated protein kinase pathway in dedifferentiated hepatocytes

Hepatitis B virus X protein (pX) is implicated in hepatocarcinogenesis by an unknown mechanism. Employing a cellular model linked to pX-mediated transformation, we investigated the role of the previously reported Stat3 activation by pX in hepatocyte transformation. Our model is composed of a differentiated hepatocyte (AML12) 3pX-1 cell line that undergoes pX-dependent transformation and a dedifferentiated hepatocyte (AML12) 4pX-1 cell line that does not exhibit transformation by pX. We report that pX-dependent Stat3 activation occurs only in non-pX-transforming 4pX-1 cells and conclude that Stat3 activation is not linked to pX-mediated transformation. Maximum Stat3 transactivation requires Ser727 phosphorylation, mediated by mitogenic pathway activation. Employing dominant negative mutants and inhibitors of mitogenic pathways, we demonstrate that maximum, pX-dependent Stat3 transactivation is inhibited by the p38 mitogen-activated protein kinase (MAPK)-specific inhibitor SB 203580. Using transient-transreporter and in vitro kinase assays, we demonstrate for the first time that pX activates the p38 MAPK pathway only in 4pX-1 cells. pX-mediated Stat3 and p38 MAPK activation is Ca(2+) and c-Src dependent, in agreement with the established cellular action of pX. Importantly, pX-dependent activation of p38 MAPK inactivates Cdc25C by phosphorylation of Ser216, thus initiating activation of the G(2)/M checkpoint, resulting in 4pX-1 cell growth retardation. Interestingly, pX expression in the less differentiated hepatocyte 4pX-1 cells activates signaling pathways known to be active in regenerating hepatocytes. These results suggest that pX expression in the infected liver effects distinct mitogenic pathway activation in less differentiated versus differentiated hepatocytes.

Hepatitis B virus X protein differentially regulates cell cycle progression in X-transforming versus nontransforming hepatocyte (AML12) cell lines

Hepatitis B virus (HBV) X protein (pX) is implicated in hepatocarcinogenesis of chronically infected HBV patients. To understand mechanism(s) of pX-mediated cellular transformation, we employed two tetracycline-regulated, pX-expressing cell lines, constructed in AML12 immortalized hepatocytes: one a differentiated (3pX-1) and the other a de-differentiated (4pX-1) hepatocyte cell line. Only 3pX-1 cells undergo pX-mediated transformation, via sustained Ras-Raf-mitogen-activated protein kinase pathway activation. pX-nontransforming 4pX-1 cells display sustained, pX-dependent JNK pathway activation. To understand how pX mediates different growth characteristics in 3pX-1 and 4pX-1 cells, we report, herein, comparative cell cycle analyses. pX-transforming 3pX-1 cells display pX-dependent G(1), S, and G(2)/M progression evidenced by cyclin D(1), A, and B(1) induction, and Cdc2 kinase activation. pX-nontransforming 4pX-1 cells display pX-dependent G(1) and S phase entry, followed by S phase pause and absence of Cdc2 kinase activation. Interestingly, 4pX-1 cells exhibit selective pX-induced expression of cyclin-dependent kinase inhibitor p21(Cip1), tumor suppressor p19(ARF), and proapoptotic genes bax and IGFBP-3. Despite the pX-mediated induction of growth arrest and apoptotic genes and the absence of pX-dependent Cdc2 activation, 4pX-1 cells do not undergo pX-dependent G(2)/M arrest or apoptosis. Nocodazole-treated, G(2)/M-arrested 4pX-1 cells exhibit pX-dependent formation of multinucleated cells, similar to human T-cell lymphotropic virus type I Tax-expressing cells. We propose that in 4pX-1 cells, pX deregulates the G(2)/M checkpoint, thus rescuing cells from pX-mediated apoptosis.

Sustained activation of mitogen-activated protein kinases and activator protein 1 by the hepatitis B virus X protein in mouse hepatocytes in vivo

Transcriptional activation of diverse cellular genes by the X protein (HBx) of hepatitis B virus (HBV) has been suggested as one of the mechanisms for HBV-associated hepatocellular carcinoma. However, such functions of HBx have been studied using transformed cells in culture and have not been examined in the normal adult hepatocytes, a natural host of HBV. Using an efficient hepatocyte-specific virus-based gene delivery system developed in our laboratory earlier, we studied the HBx action in vivo. We demonstrate that following virosome-mediated delivery of HBx DNA, a large population (>50%) of hepatocytes express the HBx protein in a dose-dependent manner, which induces a significant increase in the activity of extracellular signal-regulated kinases (ERKs) in the livers of HBx-transfected mice. Inhibition of HBx-induced ERK activation following intravenous administration of PD98059, a mitogen-activated protein kinase kinase kinase (MEK) inhibitor, confirmed the requirement for MEK in the activation of ERKs by HBx. Induction of ERK activity by HBx was sustained for up to 30 days. Interestingly, sustained activation of c-Jun N-terminal kinases (JNKs) for up to 30 days was also noted. Such constitutive ERK and JNK activation as a consequence of continued HBx expression also led to sustained stimulation of further downstream events, such as increased levels of c-Jun and c-Fos proteins along with the persistent induction of activator protein 1 binding activity. Taken together, our data suggest a critical role of these molecules in HBx-mediated cell transformation.

Hepatitis B virus X protein differentially activates RAS-RAF-MAPK and JNK pathways in X-transforming versus non-transforming AML12 hepatocytes

The hepatitis B virus (HBV) X protein (pX) is implicated in hepatocarcinogenesis of chronic HBV patients by an unknown mechanism. Activities of pX likely relevant to hepatocyte transformation include activation of the mitogenic RAS-RAF-MAPK and JNK pathways. To assess the importance of mitogenic pathway activation by pX in transformation, we employed a cellular model system composed of two tetracycline-regulated, pX-expressing cell lines, constructed in AML12-immortalized hepatocytes. This system includes the differentiated 3pX-1 and the de-differentiated 4pX-1 hepatocytes. Our studies have demonstrated that conditional pX expression transforms only 3pX-1 cells. Here, comparative in vitro kinase assays and various in vivo analyses demonstrate that pX affects an inverse activation of RAS-RAF-MAPK and JNK pathways in 3pX-1 versus 4pX-1 cells. Sustained pX-dependent RAS-RAF-MAPK pathway activation is observed in pX-transforming 3pX-1 cells, whereas sustained pX-dependent JNK pathway activation is observed in pX non-transforming 4pX-1 cells. This differential, pX-dependent mitogenic pathway activation affects differential activation of cAMP-response element-binding protein and c-Jun and determines the proliferative response of 3pX-1 and 4pX-1 cells. Furthermore, tetracycline-regulated, pX-NLS-expressing cell lines demonstrate that expression of the nuclear pX-NLS variant minimally activates the RAS-RAF-MAPK pathway and results in markedly reduced transformation. These results link sustained, pX-mediated activation of RAS-RAF-MAPK pathway to hepatocyte transformation.

The molecular biology and nomenclature of the activating transcription factor/cAMP responsive element binding family of transcription factors: activating transcription factor proteins and homeostasis

The mammalian ATF/CREB family of transcription factors represents a large group of basic region-leucine zipper (bZip) proteins which was originally defined in the late 1980s by their ability to bind to the consensus ATF/CRE site 'TGACGTCA'. Over the past decade, cDNA clones encoding identical or homologous proteins have been isolated by different laboratories and given different names. These proteins can be grouped into subgroups according to their amino acid similarity. In this review, we will briefly describe the classification of these proteins with a historical perspective of their nomenclature. We will then review three members of the ATF/CREB family of proteins: ATF3, ATF4 and ATF6. We will address four issues for each protein: (a) homologous proteins and alternative names, (b) dimer formation with other bZip proteins, (c) transcriptional activity, and (d) potential physiological functions. Although the name Activating Transcription Factor (ATF) implies that they are transcriptional activators, some of these proteins are transcriptional repressors. ATF3 homodimer is a transcriptional repressor and ATF4 has been reported to be either an activator or a repressor. We will review the reports on the transcriptional activities of ATF4, and propose potential explanations for the discrepancy. Although the physiological functions of these proteins are not well understood, some clues can be gained from studies with different approaches. When the data are available, we will address the following questions. (a) How is the expression (at the mRNA level or protein level) regulated? (b) How are the transcriptional activities regulated? (c) What are the interacting proteins (other than bZip partners)? (d) What are the consequences of ectopically expressing the gene (gain-of-function) or deleting the gene (loss-of-function)? Although answers to these questions are far from being complete, together they provide clues to the functions of these ATF proteins. Despite the diversity in the potential functions of these proteins, one common theme is their involvement in cellular responses to extracellular signals, indicating a role for these ATF proteins in homeostasis.

The hepatitis B virus HBx protein induces adherens junction disruption in a src-dependent manner

Chronic hepatitis B virus infection is strongly associated with the development of hepatocellular carcinoma (HCC). Epithelial tumors are frequently characterized by loss of cadherin expression or function. Cadherin-dependent adhesion prevents the acquisition of a migratory and invasive phenotype, and loss of its function is itself enough for the progression from adenoma to carcinoma. The HBx protein of hepatitis B virus is thought to contribute to the development of the carcinoma, however, its role in the oncogenic and metastatic processes is far from being fully understood. We report herein the ability of HBx to disrupt intercellular adhesion in three different cell lines stably transfected with an inducible HBx expression vector. The linkage between the actin cytoskeleton and cadherin complex, which is essential for its function, is disrupted in the presence of HBx, as indicated by detergent solubility and immunoprecipitation experiments. In addition, beta-catenin was tyrosine phosphorylated in HBx-expressing cells. Inhibition of the src family of tyrosine kinases resulted in the prevention of the disruption of adherens junctions. These results suggest that HBx is able to disrupt intercellular adhesion in a src-dependent manner, and provide a novel mechanism by which HBx may contribute to the development of HCC.

X protein of hepatitis B virus modulates cytokine and growth factor related signal transduction pathways during the course of viral infections and hepatocarcinogenesis

Hepatitis B virus produces chronic infections of the liver leading to cirrhosis and hepatocellular carcinoma. The X protein of hepatitis B virus (HBx) is a multifunctional protein that can interact with p53 but can also influence a variety of signal transduction pathways within the cell. In most instances this small viral protein favors cell survival and probably initiates hepatocarcinogenesis. HBx upregulates the activity of a number of transcription factors including NF-kappa B, AP-1, CREB, and TBP. However, the majority of HBx is localized to the cytoplasm where it interacts with and stimulates protein kinases such as protein kinase C, Janus kinase/STAT, IKK, PI-3-K, stress-activated protein kinase/Jun N-terminal kinase, and protein kinase B/Akt. This small viral protein can localize to the mitochondrion. HBx may act as an adaptor or kinase activator to influence signal transduction pathways. This review will attempt to analyze the involvement of HBx in signal transduction pathways during hepatitis B viral infections and hepatocellular carcinoma development.

Activating transcription factor 3 induces DNA synthesis and expression of cyclin D1 in hepatocytes

Activating transcription factor 3 (ATF3) is an early response gene that is induced rapidly during in vivo situations of cellular growth such as liver regeneration. However, neither the physiological function nor the potential target genes of this transcription factor related to cellular proliferation have been identified in the liver or other tissues. We demonstrate here that endogenous ATF3 mRNA expression is rapidly induced up to 4-fold upon mitogenic stimulation of quiescent Hepa 1-6 mouse hepatoma cells. Overexpression of exogenous ATF3 results in a significant, dose-dependent increase in DNA synthesis of up to 140% over control cells. ATF3-transfected cells also display significantly higher rates of [(3)H]thymidine incorporation in comparison with nontransfected controls in the presence of serum. Northern blot analysis and co-transfection experiments demonstrate that overexpression of ATF3 enhances cyclin D1 mRNA expression and activates the cyclin D1 promoter 2.5-fold when activating protein-1 (AP-1) and cyclic AMP response element (CRE) sites within the promoter are intact. ATF3-mediated promoter activation is reduced to 1.3-fold and 1.6-fold respectively when the AP-1 or CRE sites are mutated, and mutation of both sites simultaneously leads to the complete abrogation of promoter activation. Furthermore, DNA-binding studies demonstrate that ATF3 binds directly to the AP-1 site within the cyclin D1 promoter. These results indicate that ATF3 expression stimulates hepatocellular proliferation, suggesting that this effect is mediated, at least in part, by the ATF3-dependent activation of cyclin D1 transcription.

Overexpression of LAT1/CD98 light chain is sufficient to increase system L-amino acid transport activity in mouse hepatocytes but not fibroblasts

l-amino acid transporter-1 (LAT1) is a highly conserved gene identified as a light chain of the CD98 amino acid transporter and cellular activation marker. In our previous studies we found increased expression of LAT1 in primary human cancers. We have demonstrated also that LAT1 response to arginine availability is lost in transformed and tumorigenic cells such that expression is constitutively high. System l-amino acid transport activity correlates with changes in LAT1. To assess the functional relevance of increased LAT1 expression and the requirement for 4F2 heavy chain, we overexpressed these CD98 subunits together and separately in nontransformed hepatocytes and fibroblasts. Antigen tags in the expression constructs confirmed that expressed proteins were localized to both cytoplasmic and plasma membrane locations within the cells. Overexpression of LAT1 alone in mouse hepatocytes, but not fibroblasts, was sufficient to increase system l transport, and these cells displayed a growth advantage in conditions of limited arginine. Our results suggest that loss of regulation leading to constitutive expression of LAT1 can contribute to oncogenesis. We hypothesize that the altered LAT1 expression observed in hepatocarcinogenesis gives cells a growth or survival advantage through increased transport activity in a tumor microenvironment of limited amino acid availability.