Quantitative relationship of dioxin-responsive gene expression to dioxin response element in Hep3B and HepG2 human hepatocarcinoma cell lines

Ketoconazole Stereoisomers Differentially Induce Cytochrome P450 1A1 Between Human Hepatoma HepG2 and Mouse Hepatoma Hepa1c1c7 Cells

Ketoconazole (KTZ) has 2 chiral centers with the therapeutically active form being a racemic mixture of 2 cis-enantiomers, namely, (2R,4S)-(+)-KTZ and (2S,4R)-(-)-KTZ. The aims of the present study were to examine the effects of (+)-KTZ, (-)-KTZ, and (±)-KTZ on aryl hydrocarbon receptor activation and subsequently CYP1A1 induction in both human HepG2 and murine Hepa1c1c7 hepatoma cells, and to further test their inhibitory effect using recombinant human and mouse CYP1A1 enzymes. Our results demonstrated that (+)-KTZ induced human CYP1A1 more than (-)-KTZ, whereas on the other hand (-)-KTZ induced murine Cyp1a1 more than (+)-KTZ at the mRNA, and activity levels. Human CYP1A1 showed higher affinity to 7ER compared with murine Cyp1a1 (Km values 13.29 nM for human vs. 168.1 nM for murine). The intrinsic clearance values for human and murine CYP1A1 were 194.1 and 87.6 μL/pmol P450/min, respectively, whereas, Vmax values were 2.58 and 14.73 pmol/pmol P450/min, respectively. (+)-KTZ and (-)-KTZ directly inhibited CYP1A1 activity by noncompetitive mechanism. The affinity of (-)-KTZ to interact with human CYP1A1 and murine Cyp1a1 was significantly different from (+)-KTZ, as the Ki values for human CYP1A1 and murine Cyp1a1 were 199.4 and 413.7 nM, respectively, for (+)-KTZ, and 269.3 and 230.8 nM, respectively, for (-)-KTZ.

Two persistent organic pollutants which act through different xenosensors (alpha-endosulfan and 2,3,7,8 tetrachlorodibenzo-p-dioxin) interact in a mixture and downregulate multiple genes involved in human hepatocyte lipid and glucose metabolism

Individuals, typically, are exposed to mixtures of environmental xenobiotics affecting multiple organs and acting through different xenosensors and pathways in species and cell-type specific manners. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and α-endosulfan are Persistent Organic Pollutants (POPs) and endocrine disruptors which act through different xenosensors and accumulate in the liver. Our objective in this HEALS study was to investigate the effects of the mixture of these POPs on gene expression in a human-derived hepatocyte cell line, HepaRG. We found that, in spite of having largely uncorrelated effects, TCDD and α-endosulfan, when mixed, alter the expression of genes. The combined effects of the mixture of the POPs significantly altered the expression of 100 genes (42 up- and 58 down-regulated) whereas the same concentration of either POP alone did not alter significantly the expression of these genes. For 32 other genes, selective inhibitory crosstalk between TCDD and α-endosulfan was observed. One of the POPs inhibited the effect, on gene expression, of the other in the mixture although, when used alone, that POP did not affect expression. The expression of another 82 genes was significantly altered (up- or down-regulated) by a single POP. The addition of the second POP either increased, in the same direction, the effect on gene expression or had no further effect. At low concentrations (0.2 nM TCDD and 1 μM α-endosulfan), the POPs still had significant effects and the levels of expression of the corresponding proteins were found to be affected for some genes. Particularly striking was the 80-90% inhibition, by the mixture, of the expression of a number of genes of several hepatic intermediary metabolic pathways (glycerolipid metabolism, FXR/RXR activation, glycolysis/gluconeogenesis, retinoid and bile acid biosynthesis), whereas each pollutant alone had only a moderate effect.

Transcriptional modulation of the NAD(P)H:quinone oxidoreductase 1 by mercury in human hepatoma HepG2 cells

NAD(P)H:quinone oxidoreductase (NQO1)-mediated detoxification of quinones plays a critical role in cancer prevention. Heavy metals such as mercury (Hg(2+)) alter the carcinogenicity of aryl hydrocarbon receptor ligands, mainly by modifying various xenobiotic-metabolizing enzymes such as NQO1. Therefore, we examined the effect of Hg(2+) on the expression of NQO1 in human hepatoma HepG2 cells. For this purpose HepG2 cells were incubated with various concentrations of Hg(2+) (2.5, 5, and 10μM) in the presence and absence of two NQO1 inducers, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and isothiocyanate sulforaphane (SUL), as bifunctional and monofunctional inducers, respectively. Analysis of the time-dependent effect of Hg(2+) revealed that Hg(2+) increased the expression of NQO1 mRNA in a time-dependent manner. In addition, Hg(2+) increased NQO1 at the mRNA, protein, and activity levels in the presence and absence of both NQO1 inducers, TCDD and SUL, which coincided with increased nuclear accumulation of Nrf2 protein. Investigating the effect of Hg(2+) at the transcriptional level revealed that Hg(2+) significantly induced the antioxidant-responsive element-dependent luciferase reporter gene expression in the absence and the presence of both NQO1 inducers. NQO1 mRNA and protein decay experiments revealed a lack of posttranscriptional and posttranslational mechanisms. Transfecting HepG2 cells with siRNA for Nrf2 significantly decreased the Hg(2+)-mediated induction of NQO1 mRNA and catalytic activity by approximately 90%. In conclusion, we demonstrated that Hg(2+) regulates the expression of the NQO1 gene through a transcriptional mechanism in human hepatoma HepG2 cells. In addition, Nrf2 is involved in the modulation of NQO1 by Hg(2+).

Gene expression and inducibility of the aryl hydrocarbon receptor-dependent pathway in cultured bovine blood lymphocytes

The exposure to dioxin-like (DL) compounds, an important class of persistent environmental pollutants, results in the altered expression of target genes. This occurs through the binding to the aryl hydrocarbon receptor (AhR), the subsequent dimerization with the AhR nuclear translocator (ARNT), and the binding of the complex to DNA responsive elements. A number of genes are up-regulated, including, among others, the AhR repressor (AHRR) and several biotransformation enzymes, such as the members of CYP1 family and NAD(P)H-quinone oxidoreductase (NOQ1). The expression and the inducibility of the above genes were investigated in mitogen-stimulated cultured blood lymphocytes from cattle, which represent a notable source of DL-compound human exposure through dairy products and meat. As assessed by real-time PCR, all the examined genes except CYP1A2 and NQO1 were detected under basal conditions. Cell exposure to the DL-compounds PCB126 or PCB77 in the 10(-6)-10(-9)M concentration range resulted in a 2-4-fold induction of CYPIA1 and CYP1B1, which was antagonized by α-naphthoflavone or PCB153. This study demonstrates for the first time the presence and inducibility of the AhR pathway in easily accessible cells like bovine peripheral lymphocytes and prompts further investigations to verify whether similar changes could occur under in vivo conditions.

Cell characterization by proteome profiling applied to primary hepatocytes and hepatocyte cell lines Hep-G2 and Hep-3B

Hepatocytes are known to express a large number of characteristic proteins. Transformed and cultured hepatocytes only partially maintain functional cell differentiation characteristics, which can be assessed by proteome profiling. Here, we applied 2D-PAGE analysis in addition to shotgun proteomics to assess the functional cell state of primary human hepatocytes (PHH), HepG2 and Hep3B cells. Out of a total of 1995 proteins identified in the cytoplasm of these cells, we filtered 107 proteins which are characteristic for hepatocytes. A total of 104 of those were identified in primary human hepatocytes, 20 in HepG2, and only 6 in Hep3B. Forty-six out of 72 proteins identified in the secretome of PHH, 55 out of 139 in HepG2, and only 24 out of 72 in Hep3B were plasma proteins characteristic for hepatocytes. Beside other biomarker candidates presently identified, 11 proteins of the HepG2 secretome have been described previously as biomarkers for hepatocellular carcinoma. Because of indications that epithelial to mesenchymal transition (EMT) may have occurred in the cultured hepatoma cells, we included the analysis of fibroblasts representative for mesenchymal cells. Hep3B, but not HepG2, secreted five proteins including follistatin-related protein 1 which are characteristic for mesenchymal cells and may be marker proteins for EMT. Our data demonstrate that HepG2 show more features characteristic for hepatocytes than Hep3B, while Hep3B express more mesenchymal proteins indicative for EMT. Proteome profiling thus proved to enable comprehensive assessment of functional cell states and cell differentiation states of cultured hepatocytes and enabled the identification of numerous biomarkers for hepatocellular carcinoma and EMT.

2,3,7,8-tetrachlorodibenzo-p-dioxin counteracts the p53 response to a genotoxicant by upregulating expression of the metastasis marker agr2 in the hepatocarcinoma cell line HepG2

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental pollutant that binds the aryl hydrocarbon receptor (AhR), a transcription factor that triggers various biological responses. In this study, we show that TCDD treatment counteracts the p53 activation (phosphorylation and acetylation) elicited by a genotoxic compound, etoposide, in the human hepatocarcinoma cell line HepG2 and we delineated the mechanisms of this interaction. Using small interfering RNA knockdown experiments, we found that the newly described metastasis marker, anterior gradient-2 (AGR2), is involved in this effect. Both AGR2 messenger RNA (mRNA) and protein levels were increased (sixfold and fourfold, respectively) by TCDD treatment, and this effect was mediated by the AhR receptor. The half-life of AGR2 mRNA was unchanged by TCDD treatment. Analysis of the promoter of the AGR2 gene revealed three putative xenobiotic-responsive elements (XREs) in the proximal 3.5-kb promoter. Transient transfection of HepG2 cells by the Gaussia luciferase reporter gene driven by various deleted and mutated fragments of the promoter indicated that only the most proximal XRE was active. Binding of the AhR to the endogenous AGR2 promoter was also triggered by TCDD treatment. These results suggest that AhR ligands such as TCDD might contribute to tumor progression by inhibiting p53 regulation (phosphorylation and acetylation) triggered by genotoxicants via the increased expression of the metastasis marker AGR2.

Promoters of the genes encoding the transcription factors regulating the cytokine gene expression in macrophages contain putative binding sites for aryl hydrocarbon receptor

The computer system SITECON was used to study the regulatory regions in the transcription factor genes expressed in the activated macrophage and the genes of the proteins mediating the macrophage involvement in the immune response. Dioxin responsive elements (DREs), the specific sites responsible for expression regulation of the genes involved in the cell response to dioxin, were found in these gene regions. Thus, the role of dioxin in activity regulation of the genes involved in development of the immune response can be regulated both directly, by the transcription complex containing dioxin as a ligand, and indirectly, via intrinsic transcription factors.The double regulation, via DRE and the binding sites for the corresponding transcription factors in the promoter regions of macrophage genes, and the interregulation of the genes providing for the immune response allow the system to rapidly respond to a provocative agent (xenobiotic) and finely tune its function.

Toxicogenomic biomarkers for liver toxicity

Toxicogenomics (TGx) is a widely used technique in the preclinical stage of drug development to investigate the molecular mechanisms of toxicity. A number of candidate TGx biomarkers have now been identified and are utilized for both assessing and predicting toxicities. Further accumulation of novel TGx biomarkers will lead to more efficient, appropriate and cost effective drug risk assessment, reinforcing the paradigm of the conventional toxicology system with a more profound understanding of the molecular mechanisms of drug-induced toxicity. In this paper, we overview some practical strategies as well as obstacles for identifying and utilizing TGx biomarkers based on microarray analysis. Since clinical hepatotoxicity is one of the major causes of drug development attrition, the liver has been the best documented target organ for TGx studies to date, and we therefore focused on information from liver TGx studies. In this review, we summarize the current resources in the literature in regard to TGx studies of the liver, from which toxicologists could extract potential TGx biomarker gene sets for better hepatotoxicity risk assessment.

Modulation of TCDD-mediated induction of cytochrome P450 1A1 by mercury, lead, and copper in human HepG2 cell line

Co-contamination with complex mixtures of heavy metals and polycyclic aromatic hydrocarbons (PAHs) is a common environmental problem with multiple biological consequences. In this study, we tested in human hepatoma HepG2 cells the potential effects of three prominent environmental heavy metals, mercury (Hg(2+)), lead (Pb(2+)), and copper (Cu(2+)), on the induction of cytochrome P450 1A1 (CYP1A1) by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the most potent PAH. Our results show that TCDD in the absence and presence of heavy metals did not significantly affect HepG2 cell viability using MTT and LDH leakage assays. Exposure of HepG2 cells with either Hg(2+) or Pb(2+) significantly decreased, whereas Cu(2+) potentiated the CYP1A1 induction mediated by TCDD at the activity levels. In a manner similar to CYP1A1 activity, both Hg(2+) and Pb(2+) significantly down-regulated, while Cu(2+) up-regulated, the induction of CYP1A1 protein mediated by TCDD, suggesting that the modulations of CYP1A1 by heavy metals are mediated at least in part at the translational level. Based on these results, exposure to metal/PAH mixtures would differentially modulate PAHs-mediated carcinogenicity.