The effects of dimethylsulphoxide and 5-aminolaevulinic acid on the activities of cytochrome P450-dependent mixed function oxidase and UDP-glucuronosyl transferase activities in human Hep G2 hepatoma cells

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.

LIVER TISSUE MODEL FOR DRUG TOXICITY SCREENING

Understanding the mechanisms involved in the biotransformation of new drugs and their toxicological implications is important for drug development. In this regard, a lot of effort has been put into research to recreate the liver tissue in the laboratory for the purpose of drug screening. This has also helped to minimize the use of laboratory animal and reduce incidence of post-market withdrawal of drugs. Despite the progress made so far, cell source remains a major limitation since primary human hepatocytes are scarce and the various cell alternatives do not express all the genes found in the normal liver. In terms of tissue construct, there is a current shift to 3D models since the cell–cell interactions found in the 3D configuration enhance the morphology and function of hepatocytes. Furthermore, the engineered tissue's performance can be optimized by cocultures, perfusion-based systems, and the use of scaffolds. Nanotechnology seems promising in the field of tissue engineering, as it has been proven that cell–matrix interactions at the nano level can influence greatly on the outcome of the tissue. The review explores the various cell sources, the 3D model, flow-based systems, cocultures, and nanoscaffolds use in hepatocytes in vitro drug testing

Modulation by extracellular matrices of monooxygenase and CYP1A1 induction in Hep G2 cells in serum-free culture

The in vitro cellular functions of differentiated cells are influenced by culture conditions. Effects of several extracellular matrices (ECMs) on cytochrome P450-dependent monooxygenases (MFOs) induction and cytochrome P4501A1 (CYP1A1) gene expression were estimated in Hep G2 cells cultured in a serum-free medium. The cells were cultured on collagen type I- and II-, fibronectin-, and matrigel-coated dishes and MFO activities were induced by the addition of 3-methylcholanthrene (MC). The induction of ethoxycoumarin O-deethylase (ECOD) and alkoxyresorufin O-dealkylase activities as well as the expression of CYP1A1 mRNA were also determined. ECOD and methoxy- and ethoxyresorufin O-dealkylase activities in Hep G2 cells were enhanced by culturing the cells using a serum-free medium on fibronectin- or matrigel-coated dishes. ECOD activity on fibronectin-coated dishes was about 3-fold higher than that using a serum-supplemented medium on untreated dishes. Furthermore, both immobilized and soluble fibronectin enhanced the induction of MFOs. The expression of CYP1A1 mRNA using fibronectin-coated dishes was about 2-fold higher than that using a serum-supplemented medium on untreated dishes. These findings suggest that the gene expression in cultured cells is greatly influenced by ECMs. By using fibronectin-coated dishes to cell culture in a serum-free medium, reproducible and highly sensitive results can be obtained in experiments using cultured cells.

Sedanolide, a natural phthalide from celery seed oil: effect on hydrogen peroxide and tert-butyl hydroperoxide-induced toxicity in HepG2 and CaCo-2 human cell lines

Sedanolide is a natural compound occurring in edible umbelliferous plants. Celery seed oil, a significant source of sedanolide, is used as an herbal remedy to treat inflammatory-associated conditions such as gout and rheumatism. The objective of this study was to assess the potential protective properties of sedanolide against hydrogen peroxide (H(2)O(2))- and tert-butyl hydroperoxide (tBOOH)-induced toxicity in HepG2 and CaCo-2 cells. Viability of HepG2 and CaCo-2 cells was unaffected by a 24-h exposure to sedanolide (7-500 microM), however, when the cells were cultured in sedanolide-free medium for a further two cell cycles (72 h), a decrease in cell viability was observed for HepG2 cells previously exposed to 500 microM of the compound. Cells pretreated with sedanolide (100 microM for 24 h) and exposed to either H(2)O(2) or tBOOH did not exhibit statistically significant difference in viability from controls. A significant increase (p < 0.05) in DNA strand breaks, as measured by the comet assay, was observed in HepG2 but not CaCo-2 cells following a 24-h incubation with 500 microM sedanolide. Sedanolide did not modulate H(2)O(2)- and tBOOH-induced DNA damage. Sedanolide is relatively nontoxic to cells in culture, however, the protection it afforded against H(2)O(2)- and tBOOH-induced toxicity was not statistically significant.

Comparison of three different in vitro mutation assays used for the investigation of cytochrome P450-mediated mutagenicity of nitro-polycyclic aromatic hydrocarbons

Three different in vitro mutation assays were used to investigate the involvement of cytochrome P450 enzymes in the activation of the nitro-polycyclic aromatic hydrocarbons (nitroPAHs) 1-nitropyrene and 2-nitrofluorene and their reduced metabolites amino-polycyclic aromatic hydrocarbons (aminoPAHs) 1-aminopyrene and 2-aminofluorene. Mutagenicity was investigated at the HPRT locus in Chinese hamster V79 cells with (V79-NH) or without (V79-MZ) endogenous acetyltransferase activity, stably expressing human cytochrome P450 cDNAs; in NIH/3T3 control or stably expressing human CYP1A2 cells, in combination with a shuttle vector containing a reporter gene; and in Salmonella typhimurium TA98, by inhibition of cytochrome P450 enzymes in rat liver S9 mix. Both the HPRT assay and the Ames test did not show any involvement of CYP3A in the activation of 1-nitropyrene to a mutagenic metabolite. In addition, a clear involvement of CYP1A2 in the activation of the nitroPAH 1-nitropyrene was demonstrated in both mutation assays using eukaryotic cells. However, no activation of 1-nitropyrene was seen in the eukaryotic cell lines when expressing only CYP1A2 (V79-MZ1A2) or acetyltransferase (V79-NH, 3T3-LNCX). The reduced metabolite of 1-nitropyrene, 1-aminopyrene, was also shown to be activated to a mutagenic metabolite by CYP1A2, using 3T3-1A2 cells in combination with a shuttle vector, and the Amestest in combination with the specific CYP1A2 inhibitor furafylline. No clear involvement of cytochrome P450 could be demonstrated for activation of 2-nitrofluorene to a mutagenic metabolite, whereas a role for CYP1A2 in the bioactivation of 2-aminofluorene is suggested. In the present study, we have demonstrated the complementary value of the three in vitro mutation assays in the examination of promutagen activation pathways.

Comparison of HepG2 feeder cells generated by exposure to gamma-rays, X-rays, UV-C light or mitomycin C for ability to activate 7,12-dimethylbenz[a]anthracene in a cell-mediated Chinese hamster V79/HGPRT mutation assay

The cell-mediated Chinese hamster V79/HGPRT mutagenicity assay is an established in vitro testing method. Although gamma-irradiated human HepG2 hepatoma cells have been used recently for chemical activation, an alternative is now needed due to scheduled retirement of the available gamma-source. X-irradiation, 254 nm UV-C light and mitomycin C were examined as possible HepG2 mitotic inhibitors, and treated cells compared for activation of 7, 12-dimethylbenz[a]anthracene (DMBA). In colony-forming assays, V79 and HepG2 cells differed in sensitivity to DMBA, with V79 survival declining sharply between 1-2.5 microM (LD50=1.75 microM) while HepG2 survival decreased gradually, beginning at 0.01 microM DMBA (LD50=0.045 microM). When HepG2 feeder cells generated by each method were included in V79/HGPRT mutation assays, activation of 1 microM DMBA was found to vary according to the mitotic inhibitor used, with mutation frequencies decreasing in the order 4000 rads gamma-rays>25 microg/ml mitomycin C>4000 rads X-rays>25 J/m2 UV-C light. Only assays containing gamma-irradiated HepG2 cells generated an increase (2-3-fold) in mutation frequency when DMBA exposure was extended from 24 to 48 h. The effect of HepG2 preincubation with either Aroclor 1254 or DMBA on feeder cell activation of DMBA was also assessed using concentrations of Aroclor 1254 (10 microg/ml) or DMBA (1.0 microM) which were found to produce optimum induction of ethoxyresorufin-O-deethylase (EROD) activity (3.1-fold and 2-fold increases, respectively). Compared to results obtained with uninduced HepG2 cells, assays incorporating HepG2 cells activated by either Aroclor 1254 or DMBA produced slightly increased V79/HGPRT mutation frequencies after 24 h of exposure to mutagen; however, a 48 h incubation with mutagen in the presence of HepG2 preincubated with either Aroclor 1254 or DMBA resulted in higher mutation frequencies regardless of the mitotic inhibitor treatment. EROD activity was also induced 1.4-fold following exposure of HepG2 cells to mitomycin C alone. Although gamma-irradiation remains the treatment of choice for producing metabolically active HepG2 feeder cells, comparison of the alternatives tested suggests that mitomycin C would be a convenient and suitable replacement.

Carrier effects of dosing the H4IIE cells with 3,3',4,4'-tetrachlorobiphenyl (PCB77) in dimethyl sulfoxide or isooctane

A rat hepatoma cell line, H4IIE, serves as a bioassay tool to assess the potential toxicity of dioxin-like chemicals, including polychlorinated biphenyls (PCB) in environmental samples. PCB exposure to these cells induces cytochrome (CYP) P4501A1 activity in a dose-dependent fashion, thus allowing assessment of mixtures. The objective of this study was to determine the effect of different carriers, dimethyl sulfoxide (DMSO) and isooctane on the concentrations of PCBs in the H4IIE cells and induction of CYP1A1 activity as measured by ethoxyresorufin O-deethylase (EROD) activity. H4IIE cells were dosed with three micrograms of UL-14C-PCB77/plate dissolved in DMSO or isooctane, and were harvested at sequential time periods for 4 days. PCB77 concentration and EROD activity were measured in the cells. EROD activity was greater when using DMSO as compared to isooctane, while there was no difference in the distribution of PCB77-derived radioactivities within the cell culture system based upon the carrier solvent used to deliver PCB77.

Validity and reliability of in vitro systems in safety evaluation

In vitro data could make an important contribution to the application of the proposed scheme for the subdivision of the usual 10-fold safety factors (used in risk assessment for inter-species and inter-individual differences) into two separate aspects of toxicokinetics and toxicodynamics. Whereas toxicokinetics (or delivery of the chemical to its site of action via the general circulation) is amenable to direct in vivo measurement, toxicodynamics (or the assessment of the sensitivity of the target tissue to the presence of the chemical) is open to in vitro investigation. Human risk assessment requires human data to be able to replace any of the default safety (or uncertainty) factors (Renwick, 1993). Because human tissues are of limited availability, it is likely that the main quantitative contribution of in vitro data will be to allow chemical specific inter-species differences in toxicodynamics to replace the proposed default value. Although in vitro data from human tissues could be used to define human variability in target organ sensitivity (toxicodynamics) this would require a large number of specimens and the variability detected in vitro should be representative of that present in vivo.

Monooxygenation, cytochrome P4501A1 and P4501A1-mRNA in rat liver slices exposed to beta-naphthoflavone and dexamethasone in vitro

Precision-cut liver slices (0.5 mm) were incubated at 30 degrees C in a modified William's Medium E for up to 48 hrs. During the incubation, K+ and GSH/GSSG concentrations did not decrease. Cytochrome P450-dependent dealkylation rates of 7-ethoxycoumarin (ECOD), 7-allyloxycoumarin (ACOD) and 7-ethoxyresorufin (EROD) decreased to 1/3, 1/2 or did not change at all, respectively, after a 48 hrs incubation period. Exposure of the slices to 25 microM beta-naphthoflavone (beta NF) resulted in about 3 times higher monooxygenation rates. An exposure to a combination beta NF and dexamethasone (10(-6)M) caused a marked induction (6 times higher rates) after 48 hrs. Simultaneously an increase in P4501A1 content was observed. P4501A1-mRNA expression (measured by RT-PCR) was distinctly increased following beta NF exposure for 6 or 24 hrs. DMSO (0.2%) and dexamethasone alone modified monooxygenation rates, but did not have significant effects on P4501A1 content or, in the case of DMSO, P4501A1 gene expression (for dexamethasone not determined). Liver slices are a useful and simple tool for the detection of a beta NF-like induction within a few hours after preparation of the slices.

Development and validation of alternative metabolic systems for mutagenicity testing in short-term assays

We present here the results obtained within the framework of an EU funded project aimed to develop and validate alternative metabolic activating systems to be used in short-term mutagenicity assays, in order to reduce the use of laboratory animals for toxicology testing. The activating systems studied were established cell lines (Hep G2, CHEL), genetically engineered V79 cell lines expressing specific rat cytochromes P450, erythrocyte-derived systems, CYP-mimetic chemical systems and plant homogenates. The metabolically competent cell lines were used as indicator cells for genotoxic effects as well as for the preparation of external activating systems using other indicator cells. The following endpoints were used: micronuclei, chromosomal aberrations and sister chromatid exchanges, mutations at the hprt locus, gene mutations in bacteria (Ames test), unscheduled DNA synthesis and DNA breaks detected in the comet assay. All metabolic systems employed activated some promutagens. With some of them, promutagens belonging to many different classes of chemicals were activated to genotoxicants, including carcinogens negative in liver S9-mediated assays. In other cases, the use of the new activating systems allowed the detection of mutagens at much lower substrate concentrations than in liver S9-mediated assays. Therefore, the alternative metabolizing systems, which do not require the use of laboratory animals, have a substantial potential in in vitro toxicology, in the basic genotoxicity testing as well as in the elucidation of activation mechanisms. However, since the data basis is much smaller for the new systems than for the activating systems produced from subcellular liver preparations, the overlapping use of both systems is recommended for the present and near future. For example, liver S9 preparations may be used with some indicator systems (e.g., bacterial mutagenicity), and metabolically competent mammalian cell lines may be used with other indicator systems (e.g., a cytogenetic endpoint) in a battery of basic tests.

Induction of cytochrome P450-dependent monooxygenase in serum-free cultured Hep G2 cells

We examined the induction of cytochrome P450-dependent mixed-function monooxygenase (MFO) in the human hepatoma cell line Hep G2 by means of several factors. The MFO activities induced in the cells cultured in medium containing five commercial sera varied significantly, and the activity in the cells cultured in the absence of serum was about twice as high as that in cells supplemented with serum. The activity of ethoxycoumarin O-deethylase was highest 12 hr after adding 3-methylcholanthrene, and it was induced by several polycyclic aromatic hydrocarbons such as benzo(a)anthracene and benzo(a)pyrene, which are usually found in urban air as environmental contaminants. Furthermore, an extract from the total suspended particles collected using a high volume air sampler, which was mutagenic in the Ames assay using Salmonella typhimurium TA98, induced the same enzyme activities in Hep G2 cells. These findings suggest that serum-free culture allows the stable and highly sensitive measurement of induced MFO activity, and that studies of MFO induction by environmental samples using human hepatoma Hep G2 cells should provide helpful information regarding the risk associated with environmental contaminants.

Human cell lines in pharmacotoxicology. An introduction to a panel discussion

Various types of cells lines are used in pharmacotoxicology. Established cell lines are easily available, with few ethical restrictions. Some specific properties are preserved, although they have kept the phenotype of the original tissue, which is frequently a tumor phenotype. They are usually more resistant to toxic compounds than freshly isolated cells. Some drug-metabolizing enzymes are expressed and regulated in these cells. Immortalized cell lines are also of interest in toxicology. They are mainly examined for their potential in mutagenicity testing. These cells and numerous others of animal or human origin can be transfected with cDNA coding for human enzymes. They are used for determination of the individual enzyme involved in a particular metabolic pathway, or, when multiple transfections are successfully achieved, for mutagenicity testing. Regulation studies are also possible in such cells after transfection of DNA elements regulating gene transcription.

Bromobenzene detoxification in the human liver-derived HepG2 cell line

1. The applicability of the human hepatoma cell line, HepG2, as a cell culture model for studying xenobiotic liver toxicity has been investigated using the well-characterized hepatotoxic chemical, bromobenzene. 2. Bromobenzene caused a concentration- (0-10 mM) and time-dependent (0-180 min) decrease in HepG2 cell viability. The degree of toxicity was dependent upon the culture medium composition and the state of cell growth. Toxicity in Modified Earle's and Williams' E Media was maximal at 7 days growth compared with 3 and 10 days, and was greater in Williams' than in Earle's medium. Toxicity in Dulbecco's medium was apparent only at 10 days growth and was less than the maximum toxicity in the other media. 3. Bromobenzene was detoxified by epoxide hydrase. The question of metabolic activation by P450 remained unresolved, but any involvement of P450 was by forms not inhibited by ketoconazole. 4. The mechanism of bromobenzene toxicity did not appear to involve lipid peroxidation, depletion of reduced glutathione, calcium-mediated proteolysis or metabolic activation by prostaglandin synthetase, but may have involved direct solvent-induced cell damage. 5. This study demonstrates the potential usefulness of HepG2 cells in toxicity testing and highlights the importance of standardizing culture conditions.

Direct determination of functional activity of cytochrome P-4501A1 and NADPH DT-diaphorase in hepatoma cell lines using noninvasive scanning laser cytometry

Mammalian organisms possess a variety of enzymes that catalyze the biotransformation of numerous chemicals with diverse structure. The gene superfamily comprising the cytochrome P-450 monooxygenases (P-450) are key participants in these reactions, and certain P-450 genes are highly inducible upon xenobiotic exposure. Many of the standard techniques used in the study of these systems rely on the disruption of tissues and cells, together with the preparation of subcellular particles. We have adopted a sensitive new technique, scanning laser cytometry, to monitor P-450-mediated O-dealkylation activities directly in cultured cells. Metabolism in single cells was quantified by fluorescence detection of resorufin, the P-450-mediated O-dealkylation product of alkoxyresorufin ether substrate probes. Functional activities associated with P-4501A1 and NADPH DT-diaphorase were compared among a human hepatoma (Hep G2) cell line and cells derived from mouse (Hepa 1clc7 wt) and rat (H4-II-E) hepatomas. Pretreating cells with the polyaromatic hydrocarbon inducer beta-naphthoflavone resulted in 50- to 100-fold increases in single cell rates of O-dealkylation of ethoxyresorufin (EROD activity). The use of scanning laser cytometry enabled in situ analysis of both constitutive and inducible biotransformation activities without disruption of cells or intracellular processes that determine the toxicologic fate of exogenous chemicals in vivo.

The effects of inducing agents on cytochrome P450 and UDP-glucuronyltransferase activities in human HEPG2 hepatoma cells

Selective induction in vitro of cytochrome P450-dependent mixed-function oxidase (MFO) and UDP-glucuronyltransferase (GT) activities was observed in the human HepG2 hepatoma cell line. 1,2-Benzanthracene (BA) induced MFO O-dealkylation activities for ethoxyresorufin, methoxyresorufin and benzyloxyresorufin, whereas phenobarbitone (PB) selectively induced pentoxyresorufin O-dealkylation and rifampicin (RIF) selectively induced benzyloxyresorufin O-dealkylation. Antibody inhibition experiments indicated that ethoxyresorufin and methoxyresorufin O-dealkylations were catalysed mainly by the P450 1A subfamily in untreated and BA-induced HepG2 cells, that additional unidentified P450 forms were considerably involved in methoxyresorufin and benzyloxyresorufin O-dealkylations and that the P450 2B subfamily was partially responsible for pentoxyresorufin O-dealkylation in PB-induced cells. Bilirubin GT activity was induced by PB, BA, RIF and dexamethasone, but 1-naphthol, morphine and testosterone GT activities were not induced by any of these treatments.