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

Wild type HBx and truncated HBx: Pleiotropic regulators driving sequential genetic and epigenetic steps of hepatocarcinogenesis and progression of HBV-associated neoplasms

Hepatitis B virus (HBV) is one of the causative agents of hepatocellular carcinoma. The molecular mechanisms of tumorigenesis are complex. One of the host factors involved is apparently the long-lasting inflammatory reaction which accompanies chronic HBV infection. Although HBV lacks a typical viral oncogene, the HBx gene encoding a pleiotropic regulatory protein emerged as a major player in liver carcinogenesis. Here we review the tumorigenic functions of HBx with an emphasis on wild type and truncated HBx variants, and their role in the transcriptional dysregulation and epigenetic reprogramming of the host cell genome. We suggest that HBx acquired by the HBV genome during evolution acts like a cellular proto-onc gene that is activated by deletion during hepatocarcinogenesis. The resulting viral oncogene (v-onc gene) codes for a truncated HBx protein that facilitates tumor progression. Copyright © 2015 John Wiley & Sons, Ltd.

Construction of a lentiviral expression vector of HBx and its stable expression in human Chang liver cell line

AIM: To construct a lentiviral expression vector carrying the hepatitis B virus x (HBx) gene and to test its expression in the human Chang liver cell line.

METHODS: The HBx gene was amplified from plasmid pcDNA3.1-HBx by PCR. The purified HBx gene fragment was inserted into the pEB-3xflag-GP-Puro lentiviral vector, and the inserted fragment was identified by PCR, restriction endonuclease digestion and DNA sequencing. The recombinant vector was then transfected into Chang liver cells. Puromycin was applied to screen stable cell clones, and the expression of HBx in Chang liver cells was examined by immunofluorescence and Western blot.

RESULTS: Restriction enzyme digestion and DNA sequencing showed that the HBx gene had been successfully subcloned into the pEB-3xflag-GP-Puro vector. The titer of purified recombinant lentivirus was 1 × 10⁸ TU/mL. HBx was produced after transfection with the recombinant lentivirus vector and screening with puromycin. Stable expression of HBx protein was present in Chang liver-HBx cells.

CONCLUSION: The Chang liver-HBx cell line that can stably express the HBx gene has been successfully generated; this model cell may be applied to study the role of HBx in the malignant transformation of liver cells.

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.

Premature cell cycle entry induced by hepatitis B virus regulatory HBx protein during compensatory liver regeneration

The cycles of cell death and compensatory regeneration that occur during chronic hepatitis B virus (HBV) infection are central to viral pathogenesis and are a risk factor for the development of liver cancer. The HBV genome encodes one regulatory protein, HBx, which is required for virus replication, although its precise role in replication and pathogenesis is unclear. Because HBx can induce the G(0)-G(1) transition in cultured cells, the purpose of this study was to examine the effect of HBx during liver regeneration. Transgenic mice expressing HBx (ATX) and their wild-type (WT) littermates were used in the partial hepatectomy (PH) model for compensatory regeneration. Liver tissues collected from ATX and WT mice at varying sacrifice time points after PH were examined for markers of cell cycle progression. When compared with WT liver tissues, ATX livers had evidence of premature cell cycle entry as assessed by several variables (BrdUrd incorporation, proliferating cell nuclear antigen and mitotic indices, and reduced steady-state p21 protein levels). However, HBx did not affect apoptosis, glycogen storage, or PH-induced steatosis. Together, these results show that HBx expression can induce cell cycle progression within the regenerating liver. Our data are consistent with a model in which HBx expression contributes to liver disease and cancer formation by affecting early steps in liver regeneration.

HBx genotype D represses GSTP1 expression and increases the oxidative level and apoptosis in HepG2 cells

Epigenetics has been implicated in human cancer development. Epigenetic factors include HBx protein, which is able to induce hypermethylation and suppresses tumor suppressor genes. One of such tumor suppressor genes, GSTP1, shows reduced expression in many human cancers. Hypermethylation of GSTP1 is the most studied mechanism of its silence. In the present study, we reported that GSTP1 expression was completely depleted in HBV integrated HepG2.2.15 cells due to the hypermethylation in its promoter region. And it was HBx, especially HBx genotype D, that played the key role in repressing GSTP1 expression. Further functional studies like ROS assay and apoptosis detection were also used to confirm this repression. Our findings should facilitate the understanding of HBV and their influences on the epigenetic modulations for epigenetic tumorigenesis during HBV-mediated hepatocellular carcinogenesis.

Change of choline compounds in sodium selenite-induced apoptosis of rats used as quantitative analysis by in vitro 9.4T MR spectroscopy

AIM: To study liver cell apoptosis caused by the toxicity of selenium and observe the alteration of choline compounds using in vitro 9.4T high resolution magnetic resonance spectroscopy.

METHODS: Twenty male Wistar rats were randomly divided into two groups. The rats in the treatment group were intraperitoneally injected with sodium selenite and the control group with distilled water. All rats were sacrificed and the livers were dissected. ¹H-MRS data were collected using in vitro 9.4T high resolution magnetic resonance spectrometer. Spectra were processed using XWINNMR and MestRe-c 4.3. HE and TUNEL staining was employed to detect and confirm the change of liver cells.

RESULTS: Good ¹H-MR spectra of perchloric acid extract from liver tissue of rats were obtained. The conventional metabolites were detected and assigned. Concentrations of different ingredient choline compounds in treatment group vs control group were as follows: total choline compounds, 5.08 ± 0.97 mmol/L vs 3.81 ± 1.16 mmol/L (P = 0.05); and free choline, 1.07 ± 0.23 mmol/L vs 0.65 ± 0.20 mmol/L (P = 0.00). However, there was no statistical significance between the two groups. The hepatic sinus and cellular structure of hepatic cells in treatment group were abnormal. Apoptosis of hepatic cells was confirmed by TUNEL assay.

CONCLUSION: High dose selenium compounds can cause the rat liver lesion and induce cell apoptosis in vivo. High resolution ¹H-MRS in vitro can detect diversified metabolism. The changing trend for different ingredient of choline compounds is not completely the same at early period of apoptosis.

Progressive changes in hepatoma cells stably transfected with hepatitis B virus X gene

OBJECTIVE

The aim of this study is to investigate the molecular mechanism of hepatocellular carcinoma (HCC) development induced by hepatitis B virus X protein (HBx).

METHODS

We previously established a H7402-X cell line that constitutively expresses HBx protein. In the present study, H7402-X gene expression profiles and proteins were examined using cDNA microarrays and Western blot analysis. Apoptosis was induced by adriamycin in H7402-X cells. The transcriptional activities of NF-kappaB and AP-1 were examined using a luciferase reporter gene.

RESULTS

The DNA expression profiles identified candidate genes showing aberrant expression in cells overexpressing HBx. Western blot analysis showed that cyclin D, cyclin E, survivin, Bcl-2, and PCNA were up-regulated, whereas p27 was down-regulated in H7402-X cells. Treatment with RNAi targeting HBx mRNA led to the down-regulation of these genes. H7402-X cells were resistant to adriamycin-induced apoptosis. Luciferase reporter gene analysis revealed that HBx induces the transcriptional activities of NF-kappaB and AP-1.

CONCLUSION

Our data provide additional insight into cellular targets of HBx, which allows a better understanding of HBx function and the progressive changes during HBx-mediated hepatocarcinogenesis.

Hepatitis B virus X protein induces apoptosis and cell cycle deregulation through interfering with DNA repair and checkpoint responses

AIM

To investigate the effects of hepatitis B virus X (HBx) gene on apoptosis and cell cycle in hepatocyte line HL-7702 and to discuss the possible mechanisms in the pathway.

METHODS

The recombinant plasmid pcDNA3-X and vector pcDNA3 were transfected into HL-7702 cells and selected by G418 to construct two new cell lines, which were named HL-7702-HBx and HL-7702-con, respectively. Reverse transcription polymerase chain reaction (RT-PCR) and western blot analysis were used to confirm that HBx gene was expressed steadily in the HL-7702-HBx cells. Then apoptosis and cell cycle of the two cells were detected by DNA ladder, flow cytometric analysis, and electronic microscope observation. Apoptosis and cell cycle gene expressions in the two cells were subsequently evaluated by using gene arrays. Some of results were further confirmed by real-time PCR and western blot analysis.

RESULTS

RT-PCR and the western blot analysis showed that HL-7702-HBx expressed the HBx gene steadily. Comparedwith the HL-7702-con cells, there was increased apoptosis and accumulation of the S phase in the HL-7702-HBx cells. The gene array analysis indicated that some DNA repair genes (XRCC1, DDB1, etc.) and DNA damage checkpoint-related genes (Cdc47, RAD17, etc.) played roles in the HBx-mediated imbalance of apoptosis and cell cycle. Both cDNA array analysis and real-time RT-PCR showed that mRNA of XRCC1, Cdc47 and RAD17 were upregulated by HBx. Unexpectedly, the western blot analysis revealed that HBx inhibited their protein expression.

CONCLUSION

The expression of HBx in HL-7702 cells promoted apoptosis and accumulation of the S phase through the inhibition of DNA repair and checkpoints via post-transcriptional mechanisms.