trans-Acting factors, detoxication enzymes and hepatitis B virus replication in a novel set of human hepatoma cell lines

Expression of Cytochromes P-450 2E1, 3A4 and 1A1/1A2 in Growing and Confluent Human HepG2 Hepatoma Cells-Effect of Ethanol

In cultured human hepatoma HepG2 cells, cytochrome (CYP) 1A-associated 7-ethoxyresorufin-O-deethylase (EROD), CYP 3A-associated benzyloxyresorufin O-debenzylase (BROD) and CYP 2E1-associated p-nitrophenol-hydroxylase (PNPH) decreased during time in culture. The enzyme activities in cells at confluence were 35-60% of the activities in cells 24 hours after seeding. Similarly, CYP 3A and CYP 2E1 proteins were present at higher concentrations in growing (G) than in confluent (C) HepG2 cells. CYP 1A1/1A2 protein was not detected, neither in G nor in C HepG2 cells but was strongly induced by 3-methylcholanthrene (3-MC) treatment. Ethanol (EtOH) was shown to increase CYP 2E1 and CYP 3A proteins and CYP 1A1/1A2-, CYP 2E1- and CYP 3A-associated mixed-function oxidase activities (MFOs) in HepG2 cells, as has been previously reported for primary cultures of human hepatocytes. These effects were observed only at the beginning of culture, in growing HepG2 cells, demonstrating the influence of the growth stage of HepG2 cells on their response to EtOH treatment. This is, to our knowledge, the first report on increases in CYP proteins and associated MFOs by EtOH in HepG2 cells. It suggests that growing HepG2 cells provide a useful in vitro model system in which to study the regulation of human CYPs by EtOH.

Transforming growth factor-β1 suppresses hepatitis B virus replication by the reduction of hepatocyte nuclear factor-4α expression

Several studies have demonstrated that cytokine-mediated noncytopathic suppression of hepatitis B virus (HBV) replication may provide an alternative therapeutic strategy for the treatment of chronic hepatitis B infection. In our previous study, we showed that transforming growth factor-beta1 (TGF-β1) could effectively suppress HBV replication at physiological concentrations. Here, we provide more evidence that TGF-β1 specifically diminishes HBV core promoter activity, which subsequently results in a reduction in the level of viral pregenomic RNA (pgRNA), core protein (HBc), nucleocapsid, and consequently suppresses HBV replication. The hepatocyte nuclear factor 4alpha (HNF-4α) binding element(s) within the HBV core promoter region was characterized to be responsive for the inhibitory effect of TGF-β1 on HBV regulation. Furthermore, we found that TGF-β1 treatment significantly repressed HNF-4α expression at both mRNA and protein levels. We demonstrated that RNAi-mediated depletion of HNF-4α was sufficient to reduce HBc synthesis as TGF-β1 did. Prevention of HNF-4α degradation by treating with proteasome inhibitor MG132 also prevented the inhibitory effect of TGF-β1. Finally, we confirmed that HBV replication could be rescued by ectopic expression of HNF-4α in TGF-β1-treated cells. Our data clarify the mechanism by which TGF-β1 suppresses HBV replication, primarily through modulating the expression of HNF-4α gene.

IL-4-mediated transcriptional regulation of human CYP2E1 by two independent signaling pathways

Cytochrome P450 2E1 (CYP2E1), the alcohol-inducible member of the cytochrome P450 super family, plays an important role in both physiological and pathophysiological processes. The present study focused on the induction of human CYP2E1 transcription by the anti-inflammatory cytokine interleukin-4 (IL-4) in human hepatoma B16A2 cells and revealed that this regulation is mediated by two independent pathways. RNA interference and overexpression of STAT6, indicated that the JAK-STAT signaling pathway is involved in IL-4-dependent induction and mutagenesis revealed the presence of a STAT6 binding site in CYP2E1 proximal promoter region (-583/-574-bp). However, inhibition of the JAK-STAT6 pathway using JAK1 siRNA constructs could only partially inhibit the induction of CYP2E1 promoter constructs indicating the presence of a second IL-4 responsive element. Indeed by using a series of truncated CYP2E1 promoter constructs a second more distal IL-4 responsive element (-1604/-1428-bp) was identified, which was further shown to involve the activation of IRS1/2. This induction was dependent on the transcription factor NFATc1 as IL-4-induced CYP2E1 expression was altered by silencing or overexpressing NFATc1. A NFATc1 binding site was identified in the second distal IL-4 responsive element (-1551/-1545-bp) by chromatin immunoprecipitation (ChIP) analysis. Finally simultaneous siRNA-mediated down-regulation of both STAT6 and NFATc1 or mutation of both STAT6 and NFATc1 binding sites abolished the IL-4-dependent transcriptional induction of CYP2E1, demonstrating that both pathways are required for maximal activation. In conclusion, the present study indicates that the induction of CYP2E1 transcription by IL-4 is mediated through two independent parallel pathways, involving JAK-STAT6 and IRS1/2 and NFATc1.

Control of hepatitis B virus at the level of transcription

Hepatitis B virus (HBV) is tightly controlled by a number of noncytotoxic mechanisms. This control occurs within the host hepatocyte at different steps of the HBV replication cycle. HBV persists by establishing a nuclear minichromosome, HBV cccDNA, serving as a transcription template for the viral pregenome and viral mRNAs. Nucleoside/nucleotide analogues widely used for antiviral therapy as well as most antiviral cytokines act at steps after transcription of HBV RNAs and thus can control virus replication but do not directly affect its gene expression. Control of HBV at the level of transcription in contrast is able to restrict both, HBV replication and gene expression. In the review, we focus on how HBV is controlled at the level of transcription. We discuss how the composition of transcription factors determines HBV gene expression and replication and how this may be influenced by antivirally active substances, e.g. the cytokine IL-6 or helioxanthin analogues, or by the differentiation state of the hepatocyte.

Control of hepatitis B virus at the level of transcription

Hepatitis B virus (HBV) is tightly controlled by a number of noncytotoxic mechanisms. This control occurs within the host hepatocyte at different steps of the HBV replication cycle. HBV persists by establishing a nuclear minichromosome, HBV cccDNA, serving as a transcription template for the viral pregenome and viral mRNAs. Nucleoside/nucleotide analogues widely used for antiviral therapy as well as most antiviral cytokines act at steps after transcription of HBV RNAs and thus can control virus replication but do not directly affect its gene expression. Control of HBV at the level of transcription in contrast is able to restrict both, HBV replication and gene expression. In the review, we focus on how HBV is controlled at the level of transcription. We discuss how the composition of transcription factors determines HBV gene expression and replication and how this may be influenced by antivirally active substances, e.g. the cytokine IL-6 or helioxanthin analogues, or by the differentiation state of the hepatocyte.

Evolving concepts in liver tissue modeling and implications for in vitro toxicology

The development of human cell models stably expressing functional properties of the in vivo cells they are derived from for predicting toxicity of chemicals is a major challenge. For mimicking the liver, a major target of toxic chemicals, primary hepatocytes represent the most pertinent model. Their use is limited by interdonor functional variability and early phenotypic changes although their lifespan can be extended not only by culturing in a 2D dimension under sophisticated conditions but also by the use of synthetic and natural scaffolds as 3D supporting templates that allow cells to have a more stable microenvironment. Hepatocytes derived from stem cells could be the most appropriate alternative but up to now only liver progenitors/hepatoblasts are obtained in vitro. A few hepatocyte cell lines have retained a variable set of liver-specific functions. Among them are the human hepatoma HepaRG cells that express drug metabolism capacity at levels close to those found in primary hepatocytes making them a suitable model for both acute and chronic toxicity studies. New screening strategies are now proposed based on miniaturized and automated systems; they include the use of microfluidic chips and cell chips coupled with high content imaging analysis. Toxicogenomics technologies (particularly toxicotranscriptomics) have emerged as promising in vitro approaches for better identification and discrimination of cellular responses to chemicals. They should allow to discriminate compounds on the basis of the identification of a set of markers and/specific signaling pathways.

A cryptic frizzled module in cell surface collagen 18 inhibits Wnt/beta-catenin signaling

Collagens contain cryptic polypeptide modules that regulate major cell functions, such as cell proliferation or death. Collagen XVIII (C18) exists as three amino terminal end variants with specific amino terminal polypeptide modules. We investigated the function of the variant 3 of C18 (V3C18) containing a frizzled module (FZC18), which carries structural identity with the extracellular cysteine-rich domain of the frizzled receptors. We show that V3C18 is a cell surface heparan sulfate proteoglycan, its topology being mediated by the FZC18 module. V3C18 mRNA was expressed at low levels in 21 normal adult human tissues. Its expression was up-regulated in fibrogenesis and in small well-differentiated liver tumors, but decreased in advanced human liver cancers. Low FZC18 immunostaining in liver cancer nodules correlated with markers of high Wnt/β−catenin activity. V3C18 (M_r = 170 kD) was proteolytically processed into a cell surface FZC18-containing 50 kD glycoprotein precursor that bound Wnt3a in vitro through FZC18 and suppressed Wnt3a-induced stabilization of β−catenin. Ectopic expression of either FZC18 (35 kD) or its 50 kD precursor inhibited Wnt/β−catenin signaling in colorectal and liver cancer cell lines, thus downregulating major cell cycle checkpoint gatekeepers cyclin D1 and c-myc and reducing tumor cell growth. By contrast, full-length V3C18 was unable to inhibit Wnt signaling. In summary, we identified a cell-surface signaling pathway whereby FZC18 inhibits Wnt/β−catenin signaling. The signal, encrypted within cell-surface C18, is released by enzymatic processing as an active frizzledcysteine-rich domain (CRD) that reduces cancer cell growth. Thus, extracellular matrix controls Wnt signaling through a collagen-embedded CRD behaving as a cell-surface sensor of proteolysis, conveying feedback cues to control cancer cell fate.

Phenotype–genotype variability in the human CYP3A locus as assessed by the probe drug quinine and analyses of variant CYP3A4 alleles

The human cytochrome P450 3A (CYP3A) enzymes, which metabolize 50% of currently used therapeutic drugs, exhibit great interindividual differences in activity that have a major impact on drug treatment outcome, but hitherto no genetic background importantly contributing to this variation has been identified. In this study we show that CYP3A4 mRNA and hnRNA contents with a few exceptions vary in parallel in human liver, suggesting that mechanisms affecting CYP3A4 transcription, such as promoter polymorphisms, are relevant for interindividual differences in CYP3A4 expression. Tanzanian (n=143) healthy volunteers were phenotyped using quinine as a CYP3A probe and the results were used for association studies with CYP3A4 genotypes. Carriers of CYP3A4*1B had a significantly lower activity than those with CYP3A4*1 whereas no differences were seen for five other SNPs investigated. Nuclear proteins from the B16A2 hepatoma cells were found to bind with less affinity to the CYP3A4*1B element around -392 bp as compared to CYP3A4*1. The data indicate the existence of a genetic CYP3A4 polymorphism with functional importance for interindividual differences in enzyme expression.

Multiparametric characterization by flow cytometry of flow-sorted subpopulations of a human hepatoma cell line useful for drug research

BACKGROUND

Primary cultured hepatocytes are the closest model to the liver for drug research. However, to overcome its limited availability, the search for hepatic cell lines as an alternative to primary cultures is a matter of current interest. In particular, highly differentiated hepatocellular carcinomas have been proposed as in vitro tools for routine experiments in hepatotoxicity and drug metabolism.

METHODS

Cell populations were selected by fluorescence-activated cell sorting based on low and high relative expressions of P-glycoprotein. These cell lines were characterized after 21 days in culture by multiparametric analysis with flow cytometry providing direct information on key cellular functions (stability in culture, intracellular ionic homeostasis, plasmatic and mitochondrial membrane-related parameters, red-ox status, drug transport, and metabolism).

RESULTS

Two subpopulations (ADV-1 and ADV-2) from the differentiated and well-characterized human hepatoma BC2 cell line showed increased activity of drug transport and drug biotransformation capability (cytochrome P450 [CYP] 1A2, CYP2B6, CYP3A4, and CYP2Cs). These subpopulations were characterized extensively by multiparametric flow cytometric analysis.

CONCLUSION

ADV-1 subpopulation showed greater stability in culture, better efficiency regarding intracellular pH maintenance through the operation of Na+/H+ exchange antiporter, and significantly greater CYP-dependent biotransformation activity than the BC2 parental cells and ADV-2 cells.

Transcriptional Regulation of the Human CYP2A6 Gene

Nicotine C-oxidation is primarily catalyzed by CYP2A6 in humans. This enzymatic activity exhibits a large interindividual variability, which to a great extent is caused by genetic polymorphisms in the CYP2A6 gene. There are large interindividual differences in CYP2A6 mRNA and protein levels, but little is known about the transcriptional regulation of CYP2A6, which can, e.g., explain such differences. Using transient transfections of 5'-deleted CYP2A6 promoter constructs in human hepatoma B16A2 cells, we show that maximal promoter activity was harbored in the sequence spanning from -112 to -61. Putative response elements for the transcription factors hepatocyte nuclear factor-4 (HNF-4)alpha, CCAAT-box/enhancer binding protein (C/EBP)alpha, C/EBPbeta, and octamer transcription factor-1 (Oct-1) were identified in this region, and electrophoretic mobility shift assays showed that these transcription factors bind to the predicted elements. To determine the relevance of these sites, expression vectors for these transcription factors were cotransfected with CYP2A6 promoter constructs in HepG2 cells. HNF-4alpha, C/EBPalpha, and Oct-1 exerted an activating effect, whereas overexpression of C/EBPbeta reduced CYP2A6 promoter activity. To confirm the importance of these sites in vivo, mutated CYP2A6 reporter constructs were injected into mouse liver. Mutation of either HNF-4 or C/EBP-Oct-1 motifs significantly decreased promoter activity, 52 and 26% of wildtype, respectively, whereas when both motifs were mutated the activity in mice decreased to 14% of wild type. In conclusion, the data indicate that the constitutive hepatic expression of CYP2A6 is governed by an interplay between the transcription factors HNF-4alpha, C/EBPalpha, C/EBPbeta, and Oct-1. These results will be important for the identification of new polymorphisms affecting CYP2A6 gene expression.

Establishment and characterization of human hepatocellular carcinoma cell line FHCC-98

AIM: To establish a novel human hepatocellular carcinoma (HCC) cell line FHCC-98 from HCC tissue and to provide a suitable model for studying HCC occurrence, progress and metastasis.

METHODS: Serially passaged cells were cultured and their morphologies were observed under light and electron microscope. Cytogenetic study was conducted by using flow cytometry and chromosome analysis. Expressions of tumor markers such as α-fetoprotein (AFP), cytokeratin (CK) and hepatoma metastasis-associated factor HAb18G/CD147 on the FHCC-98 cells were detected by immunocytochemistry or Western blotting. Lactic dehydrogenase (LDH) isoenzymes were detected by polyacrylamide gel electrophoresis (PAGE). Xenograft was performed by inoculating FHCC-98 cells into the flanks of nude mice.

RESULTS: Morphology of FHCC-98 cells was the same as that of other malignant cells. The expressions of the cells were positive for HAb18G/CD147 and CK, and negative for AFP. Its population doubling time was 21.4 h. The cell DNA was tetraploid and the major chromosomes were triploid by cytogenetics analysis. The tumorigenicity in nude mice was 100%. PAGE showed four bands representing LDH₂, LDH₃, LDH₄ and LDH₅.

CONCLUSION: FHCC-98 is a novel HCC cell line and an ideal cell model for further exploring the mechanism of hepatocellular carcinoma invasion and metastasis.

Differentiation of human hepatoma cells during confluence as revealed by gene expression profiling

Certain human hepatocarcinoma cells undergo differentiation when grown at confluence. In order to understand the basis for this differentiation, we investigated the phenotypic changes occurring during confluent growth of the human hepatoma B16A2 cell line. The global gene expression profile of B16A2 cells grown during confluence for 5 weeks was investigated using microarrays containing complementary sequences corresponding to approximately 10,000 genes, and compared with profiles of adult human liver and HepG2 cells. The major part of gene products detected were shared by all three systems and the hepatoma cell lines expressed surprisingly high levels of liver-enriched transcription factors. During confluence of B16A2 cells, the majority of transcriptional changes monitored were directed towards the phenotype of adult human liver in vivo, although the changes accounted for less than 10% of those necessary to acquire a native hepatic phenotype. Several markers of liver differentiation and regeneration were changed in similar manner as observed in developing liver and during liver regeneration. In conclusion, the data indicate that differentiation in vitro of the B16A2 cell line during confluence partially resembles that of hepatic differentiation and regeneration in vivo, implying a partial normalization of a low differentiated phenotype.

A nicotine C-oxidase gene (CYP2A6) polymorphism important for promoter activity

In humans, several polymorphic variants have been described for the gene encoding the major nicotine C-oxidase, cytochrome P450 2A6 (CYP2A6), which is to a great extent responsible for the large interindividual differences seen at the enzymatic and activity levels. Hitherto, mainly polymorphic variants in the open reading frame have been identified. In the present study, we identified a novel single nucleotide polymorphism (SNP) located in the 5' flanking region of the CYP2A6 gene. Sequencing of 1.4 kb of the 5'-upstream region of the CYP2A6 gene from eight individuals revealed a c.-1013A>G polymorphism defining two new alleles, CYP2A6*1D and CYP2A6*1E, lacking or having also the CYP2A7 3'-UTR. Analysis of genomic DNA from 32 Swedish and 109 Turkish subjects by dynamic allele-specific hybridization (DASH) showed that, in both groups, the variants carrying the c.-1013A>G SNP represent approximately 70% of the total number of alleles. Transfection of HepG2 cells with luciferase reporter constructs containing 1019 bp of the CYP2A6 5'-regulatory sequence showed that the region between c.-1005 and c.-1019 elicited a strong enhancer effect and that the CYP2A6*1D promoter had significantly reduced expression as compared to CYP2A6*1A carrying c.-1013A. Electrophoretic mobility shift assays (EMSA) showed that nuclear proteins from HepG2 and B16A2 cells exhibited a higher binding affinity to the probe harboring c.-1013A as compared to the c.-1013G probe, although the transcription factor(s) responsible for this binding could not be identified. In conclusion, our results indicate the presence of a strong enhancer or promoter responsive element between c.-1005 and c.-1019 in the CYP2A6 gene and that a c.-1013A>G polymorphism in this region affects CYP2A6 transcription.

Human drug metabolising cytochrome P450 enzymes: properties and polymorphisms

The cytochrome P450s are responsible for about 75% of phase I dependent drug metabolism and for the metabolism of a huge amount of dietary constituents and endogenous chemicals. The human has 59 active genes, and 6 of those encode important drug metabolising enzymes. About 40% of cytochrome P450 dependent drug metabolism is catalysed by polymorphic enzymes and such drug P450 interactions are frequently seen in adverse drug reaction reports. In this contribution an update of human cytochrome P450 enzymology and pharmacogenetics is given with particular emphasis on CYP1B1, CYP2B6, CYP2E1 and CYP3As.

Genetic polymorphism of CYP1A2 in Ethiopians affecting induction and expression: characterization of novel haplotypes with single-nucleotide polymorphisms in intron 1

CYP1A2 polymorphism has been well studied in white persons and Asians but not in Africans. We performed CYP1A2 genotype and phenotype analysis using caffeine in Ethiopians living in Ethiopia (n = 100) or in Sweden (n = 73). We sequenced the CYP1A2 gene using genomic DNA from 12 subjects, which revealed a novel intron 1 single-nucleotide polymorphism (SNP), -730C>T. We developed SNP-specific polymerase chain reaction-restriction fragment length polymorphism genotyping and molecular haplotyping methods for the intron 1 SNPs, and four different haplotypes were identified: CYP1A2*1A (wild-type for all SNPs), CYP1A2*1F (-164A), CYP1A2*1J (-740G and -164A), and CYP1A2*1K (-730T, -740G, and -164A), having frequencies of 39.9, 49.6, 7.5, and 3.0%, respectively. The frequency of CYP1A2*1J and CYP1A2*1K among Saudi Arabians (n = 136) was 5.9% and 3.6%, and among Spaniards (n = 117) 1.3% and 0.5%, respectively. Functional significance of the different intron 1 haplotypes was analyzed. Subjects with CYP1A2*1K had significantly decreased CYP1A2 activity in vivo, and reporter constructs with this haplotype had significantly less inducibility with 2,3,7,8-tetrachlorodibenzo-p-dioxin in human B16A2 hepatoma cells. Electrophoretic mobility shift assay using nuclear extracts from B16A2 cells revealed a specific DNA binding protein complex to an Ets element. Efficient competition was obtained using oligonucleotide probes carrying the wt sequence and Ets consensus probe, whereas competition was abolished using probes with the -730C>T SNP alone or in combination with -740T>G (CYP1A2*1K). The results indicate a novel polymorphism in intron 1 of importance for Ets-dependent CYP1A2 expression in vivo and inducibility of the enzyme, which might be of critical importance for determination of interindividual differences in drug metabolism and sensitivity to carcinogens activated by CYP1A2.

CYP3A4 inducible model for in vitro analysis of human drug metabolism using a bioartificial liver

CYP3A is responsible for approximately 50% of the therapeutic drug-metabolizing activity in the liver. The present study was undertaken to establish the CYP3A4 inducible model for analysis of human drug metabolism using a bioartificial liver composed of the functional hepatocellular carcinoma cell (HCC) line FLC-5. A radial-flow bioreactor (RFB), which is a carrier-filled type bioreactor, was used for 3-dimensional perfusion culture of FLC-5 cells. The CYP3A4 messenger RNA (mRNA) expression level 48 hours after rifampicin treatment in the RBF was approximately 100 times higher than that in a monolayer culture. Western blot analysis also demonstrated an increase in expression of the CYP3A protein. When testosterone, a substrate for CYP3A4, was added to the rifampicin-treated cell culture, 6 beta-hydroxy testosterone as a metabolite was formed. Electrophoretic mobility shift assay (EMSA) with a CYP3A4 ER6 probe demonstrated that relatively high molecular weight complex containing pregnane X receptor (PXR)/retinoid X receptor alpha(RXR alpha), compared with that in the monolayer culture, is possibly generated in the RFB culture of FLC-5 treated with rifampicin. Similarly, the assay with a probe of HNF-4 alpha-binding motif indicated the formation of a large protein complex in the RFB culture. Because it is known that PXR transactivates CYP3A4 gene via its response element and expression of PXR is regulated by HNF-4 alpha, the large complexes binding to response elements of PXR or HNF-4 alpha in the RFB culture may contribute to up-regulation of CYP3A4 mRNA. In conclusion, the bioartificial liver composed of human functional HCC cell line was useful in studying drug interactions during induction of human CYP3A4.

Metastatic human hepatocellular carcinoma models in nude mice and cell line with metastatic potential

Metastatic human HCC model is needed for the studies on mechanism and interven tion of metastatic recurrence. By using orthotopic implantation of histologically intact tissues of 30 surgical specimens, a patient-like metastatic model of hu man HCC in nude mice (LCI-D20) and a low metastatic model of human HCC in nude mice (LCI-D35) have been established. All mice with transplanted LCI-D20 tumors exhibited extremely high metastatic ability including spontaneous metasta sis to liver, lungs, lymph nodes and peritoneal seeding. Remarkable difference was also found in expression of some of the invasiveness related genes and growth factors between the LCI-D20 and LCI-D35 tumors. PAI-1 increased gradually following tumor progression in LCI-D20 model, and correlated with tumor size and AFP level. Phasic expression of tissue intercellular adhesio nmolecule-1 in this model was also observed. Using corneal micropocket model, it was demonstrated that the vascular response induced by LCI-D20 tumor was stronger than that induced by LCI-D35 tumor. Similar report on metastatic human HCC model in nude mice and human HCC cell line with metastatic potential was rarely found in the literature. This LCI-D20 model has been widely used for the studies on intervention of metastasis, including anti-angiogenesis, antisense approach, metallopro teinase inhibitor, differentiation inducer, etc. It is concluded that the establ ishment of metastatic human HCC model in nude mice and human HCC cell line with metastatic potential will provide important models for the in vivo and in vitro study of HCC invasiveness, angiogenesis as well as intervention of HCC recurrence.

Establishment of cell clones with different metastatic potential from the metastatic hepatocellular carcinoma cell line MHCC97

AIM: To establish clone cells with different metastatic potential for the study of metastasis-related mechanisms.

METHODS: Cloning procedure was performed on parental hepatocellular carcinoma (HCC) cell line MHCC97, and biological characteristics of the target clones selected by in vivo screening were studied.

RESULTS: Two clones with high (MHCC97-H) and low (MHCC97-L) metastatic potential were isolated from the parent cell line. Compared with MHCC97-L, MHCC97-H had smaller cell size (average cell diameter 43 μm vs 50 μm) and faster in vitro and in vivo growth rate (tumor cell doubling time was 34.2 h vs 60.0 h). The main ranges of chromosomes were 55-58 in MHCC97-H and 57-62 in MHCC97-L. Boyden chamber in vitro invasion assay demonstrated that the number of penetrating cells through the artificial basement membrane was (37.5 ± 11.0) cells/field for MHCC97-H vs (17.7 ± 6.3)/field for MHCC97-L. The proportions of cells in G0-G1 phase, S phase, and G2-M phase for MHCC97-H/MHCC97-L were 0.56/0.65, 0.28/0.25 and 0.16/0.10, respectively, as measured by flow cytometry. The serum AFP levels in nude mice 5 wk after orthotopic implantation of tumor tissue were (246 ± 66) μg•L¯¹ for MHCC97-H and (91 ± 66) μg•L¯¹ for MHCC97-L. The pulmonary metastatic rate was 100% (10/10) vs 40% (4/10).

CONCLUSION: Two clones of the same genetic background but with different biological behaviors were established, which could be valuable models for investigation on HCC metastasis.

Identification of a single nucleotide polymorphism in the TATA box of the CYP2A6 gene: impairment of its promoter activity

Human cytochrome P450 2A6 (CYP2A6) constitutes the major nicotine oxidase, and large interindividual differences are seen in the levels of this enzyme, to a great extent caused by the distribution of several different polymorphic gene variants mainly located in the open reading frame (ORF). In the present study, we report a common polymorphism located in the 5' flanking region of CYP2A6 affecting its expression. DHPLC analysis and complete sequence of the open reading frame of the gene from a Turkish individual revealed a -48T > G substitution disrupting the TATA box. Using dynamic allele-specific hybridization (DASH), genotyping of this novel variant (named CYP2A6*9) was carried out in 116 Swedish, 132 Turkish, and 102 Chinese subjects, and the allele frequencies were found to be 5.2, 7.2, and 15.7%, respectively. The significance of the polymorphism was investigated by the construction of luciferase reporter plasmids containing 135 or 500 bp of the 5'-upstream region of the gene transfected into human hepatoma B16A2 cells. The constructs carrying the -48T > G mutation were only expressed at about 50% of the wild-type alleles. It is concluded that the CYP2A6*9 allele might be one of the most common CYP2A6 variants in Caucasians that alters the levels of enzyme expression.

Expression and induction of a large set of drug-metabolizing enzymes by the highly differentiated human hepatoma cell line BC2

The BC2 cell line derived from the human hepatocarcinoma, HGB, undergoes a spontaneous sharp differentiation process in culture as it becomes confluent, remains stably differentiated for several weeks, and may return to proliferation thereafter under appropriate density conditions. The relevance of the line as an hepatic model has been evaluated. Cells synthesize a large number of plasma proteins, and rates of glycogen and urea synthesis increase with time of confluency and become sensitive to insulin, reflecting the process of differentiation. Differentiated BC2 cells express the most relevant cytochrome P-450 (CYP) isozyme activities (CYP1A1/2, 2A6, 2B6, 2C9, 2E1, and 3A4) and conjugating enzymes (glutathione S-transferase and UDP-glucuronyltransferase) and also respond to model inducers. Methylcholanthrene induced an increase in CYP1A1/2 enzyme activity (eightfold), phenobarbital induced CYP2B6 activity (1.7-fold), and dexamethasone induced CYP3A4 activity (fivefold). In parallel, expression of the most relevant liver-enriched transcription factors, HNF-4, HNF-1, C/EBP-alpha and C/EBP-beta mRNAs, was significantly increased in differentiated cultures. This increase was largest in HNF-1 and HNF-4, which supports the idea that a redifferentiation process towards the hepatic phenotype takes place. BC2 is an hepatic cell line that is able to express most hepatic functions, especially the drug-biotransformation function, far more efficiently than any previously described human hepatoma cell line.

The Human Cytochrome P450 1A1 mRNA Is Rapidly Degraded In HepG2 Cells

The cytochromes P450 are a superfamily of enzymes that can carry out a wide range of oxidative reactions. While the transcriptional control of the cytochrome P450 genes has been relatively well-studied, posttranscriptional regulatory mechanisms that contribute to the regulation of P450s are much less well understood. We followed the decay of CYP1A1, CYP1A2, and CYP1B1 mRNAs after induction by the AH receptor ligand 2,3,7,8,-tetrachlorodibenzo-p-dioxin. CYP1A2 and CYP1B1 mRNAs were long-lived in this cell line (to > 24 h). In contrast, the CYP1A1 mRNA decays remarkably quickly. To determine if this rapid decay was unique to CYP1A1, we assessed the decay of selected human P450 and liver-specific mRNAs in HepG2 cells as a comparison. We analyzed albumin, phosphofructokinase, and GAPDH mRNAs and found that they were long-lived, with half-lives >24 h. We show that CYP2E1 mRNA can be detected in HepG2 cells by RT-PCR and that this mRNA also has a basal half-life of >24 h. Thus the CYP1A1 mRNA with its half-life of 2.4 h was one of the shortest-lived mRNA studied and is the most unstable of the cytochrome P450 mRNAs we have tested. The rapid decay of CYP1A1 mRNA is associated with a rapid loss in poly(A) tail length, suggesting that deadenylation is the first step in the decay pathway. The short half-life appears to be conserved across species, which suggests that this characteristic of the CYP1A1 mRNA is important for its function.

Infection of five human liver cell lines by dengue-2 virus

Elevated serum transaminase levels of dengue patients indicate the possible impact of dengue virus infection on liver function. To elucidate the action of dengue virus infection in liver cells, an in vitro cell line system was established that mimicked the liver status of diverse clinical patients. Briefly, four hepatoma cell lines (HA22T, Huh7, Hep3B, and PLC) and one nonmalignant hepatocyte cell line (Chang liver) were included, representing various levels of tumorigenicity and differentiation. Our data showed that in these five cell lines, dengue-2 virus attached to each cell type equally well; however, this virus had higher replication rates and levels of virion production in differentiated Huh7, PLC, Hep3B, and Chang liver cells. Likewise, a lower replication rate was observed in the de-differentiated HA22T cells. Differentiation-related factors seem to play an important role in dengue virus replication. Further study showed that sodium butyrate (NaB, a differentiation inducer) treatment enhanced dengue virus replication in HA22T cells. Moreover, we found that the severity of morphologic aberration and the increase in aspartate aminotransferase (AST) levels correlated with the virus replication rate in the four infected hepatoma cells. In conclusion, we showed that dengue virus can infect diverse liver cells with differing replication efficiency, which causes cytopathic effects (CPEs) of diverse severity. Among the CPEs, the increased AST levels correlated with the clinical results from 24 dengue fever patients, who showed increased AST levels at the onset of fever. In summary, we find that dengue-2 virus replicates actively and causes severe CPEs in differentiated hepatoma cells. Factors related to differentiation as well as tumorigenicity seem to play critical roles, though the mechanisms of action remain unclear.

Structural and functional characterization of the 5'-flanking region of the rat and human cytochrome P450 2E1 genes: identification of a polymorphic repeat in the human gene

Cytochrome P450 2E1 (CYP2E1) is a toxicologically very important enzyme with a high extent of interindividual variability in expression. We sequenced and characterized the 5'-flanking region of the human and rat CYP2E1 genes. The identity between the human and rat sequences (-3.8 kb to +1 kb) was generally between 35 and 60%, and the most similar regions were found in the proximal part of the sequence. Two more distant regions at -1.6 to -2.0 kb and -2.5 to -2. 8 kb in the human sequence were also found to have high identity to the rat sequence. A polymorphic repeat sequence in the human gene was found between -2178 to -1945 bp. The common allele (CYP2E1*1C) contained 6 repeats (each 42-60 bp long) and the rare allele (CYP2E1*1D) had 8 repeats with an allele frequency of 1% among Caucasians and 23% among Chinese. The CYP2E1 5'-flanking regions of the human (-3712 bp to +10 bp) and rat (-3685 bp to +28 bp) genes were ligated in front of a luciferase reporter gene and transfected into rat hepatoma Fao and human hepatoma B16A2 cells. Important species specificity was noted in the control of gene expression and regions of negative and positive cis-acting elements were localized. No difference was seen in the constitutive expression between the two polymorphic forms. The importance of this repeat polymorphism for high and low inducible CYP2E1 phenotypes is discussed.

Laminin isoforms in non-tumoral and tumoral human livers. Expression of alpha1, alpha2, beta1, beta2 and gamma1 chain mRNA and an alpha chain homologous to the alpha2 chain

BACKGROUND/AIMS

Laminins, the major non-collagenous basement membrane components, are involved in various biological processes. Laminin isoforms have never been characterized in human livers. The expression of five laminin mRNA was investigated in livers with or without cancer and in hepatoma cells and, by comparison, in both rat hepatoma and hepatic stellate cells.

METHODS

Laminin alpha1, alpha2, beta1, beta2 and gamma1 mRNA was detected by northern blot and/or RT-PCR in livers without chronic disease (n=5), in both tumoral and non-tumoral areas of livers with hepatocellular carcinomas (n=13) or metastases (n=18), in human HBGC2 and rat Faza-567 hepatoma cell lines, and in 6-day-old rat hepatic stellate cell cultures.

RESULTS

Laminin alpha1, alpha2 and beta1 mRNA were found in 25-33% and gamma1 mRNA in 58% of the livers, the signal for laminin beta2 mRNA being faint in all the samples. Laminin alpha2, beta1, beta2 and gamma1 mRNA were expressed in hepatoma and stellate cells. The laminin alpha2 cDNA probe recognized a 3.5 kb mRNA different from the expected 9 kb mRNA. Using degenerated oligonucleotides, RT-PCR products from both rat hepatoma and stellate cells revealed 90% identity with the alpha2 chain sequence. Antibodies against peptide deduced from the conserved C-terminal domain of both alpha1 and alpha2 chains recognized polypeptides corresponding to the degradation products of alpha2 chain in liver extracts and both media and cell layers from hepatoma and stellate cells. In addition, a Mr=130000 polypeptide was revealed by these antibodies in liver extracts and cell layers, which was consistent with the expected size deduced from the 3.5 kb mRNA.

CONCLUSIONS

This first report on laminin isoforms in human livers indicates that laminin 1 (alpha1-beta1-gamma1), 2 (alpha2-beta1-gamma1), 3 (alpha1-beta2-gamma1) and 4 (alpha2-beta2-gamma1) mRNA and a polypeptide homologous to the alpha2 isoform, which could correspond to a truncated form of this chain, are usually expressed in non-tumoral and/or tumoral livers.