Interindividual variation in biotransformation and cytotoxicity of bromobenzene as determined in primary hepatocyte cultures derived from monkey and human liver

Role of reactive metabolites in drug-induced hepatotoxicity

Drugs are generally converted to biologically inactive forms and eliminated from the body, principally by hepatic metabolism. However, certain drugs undergo biotransformation to metabolites that can interfere with cellular functions through their intrinsic chemical reactivity towards glutathione, leading to thiol depletion, and functionally critical macromolecules, resulting in reversible modification, irreversible adduct formation, and irreversible loss of activity. There is now a great deal of evidence which shows that reactive metabolites are formed from drugs known to cause hepatotoxicity, such as acetaminophen, tamoxifen, isoniazid, and amodiaquine. The main theme of this article is to review the evidence for chemically reactive metabolites being initiating factors for the multiple downstream biological events culminating in toxicity. The major objectives are to understand those idiosyncratic hepatotoxicities thought to be caused by chemically reactive metabolites and to define the role of toxic metabolites.

Evaluation of in vitro assays for assessing the toxicity of cigarette smoke and smokeless tobacco

BACKGROUND

In vitro toxicology studies of tobacco and tobacco smoke have been used to understand why tobacco use causes cancer and to assess the toxicologic impact of tobacco product design changes. The need for toxicology studies has been heightened given the Food and Drug Administration's newly granted authority over tobacco products for mandating tobacco product performance standards and evaluate manufacturers' health claims about modified tobacco products. The goal of this review is to critically evaluate in vitro toxicology methods related to cancer for assessing tobacco products and to identify related research gaps.

METHODS

PubMed database searches were used to identify tobacco-related in vitro toxicology studies published since 1980. Articles published before 1980 with high relevance also were identified. The data were compiled to examine (a) the goals of the study, (b) the methods for collecting test substances, (c) experimental designs, (d) toxicologic end points, and (e) relevance to cancer risk.

RESULTS

A variety of in vitro assays are available to assess tobacco smoke that address different modes of action, mostly using non-human cell models. However, smokeless tobacco products perform poorly in these assays. Although reliable as a screening tool for qualitative assessments, the available in vitro assays have been poorly validated for quantitative comparisons of different tobacco products. Assay batteries have not been developed, although they exist for nontobacco assessments. Extrapolating data from in vitro studies to human risks remains hypothetical.

CONCLUSIONS

In vitro toxicology methods are useful for screening toxicity, but better methods are needed for today's context of regulation and evaluation of health claims.

Ecotoxicological assessment of bromobenzene using a test battery with five model systems

Bromobenzene (BrB) is used as a solvent for crystallization and as an additive to motor oils and may be released into the environment through various waste streams. However, there is limited available information about the toxic hazard of BrB in the aquatic environment. Consequently, the ecotoxicological effects induced by BrB were investigated using five model systems with representants from four trophic levels. The battery included bioluminescence inhibition of the bacterium Vibrio fischeri, growth inhibition of the alga Chlorella vulgaris and immobilization of the cladoceran Daphnia magna. Total protein content, neutral red uptake and MTS metabolization were reduced, while lysosomal function, succinate dehydrogenase activity, G6PDH activity and leakage, metallothionein levels and EROD activity were stimulated in PLHC-1 and RTG-2 fish cell lines. The most sensitive bioindicator was the bioluminiscence of V. fischeri, with an EC(50) of 0.04mM BrB at 15min and a non-observed adverse effect level of 0.02 mM BrB. There is a large difference in sensitivity to BrB among the model systems probably due to the metabolic capacity of the different species. PLHC-1 cells were more sensitive to BrB than RTG-2 cells. The most prominent morphological effects observed were hydropic degeneration, loss of cells and of the perinuclear pattern of distribution of lysosomes. Therefore, BrB should be classified as toxic to aquatic organisms.

Antagonism of TCDD-induced ethoxyresorufin-O-deethylation activity by polybrominated diphenyl ethers (PBDEs) in primary cynomolgus monkey (Macaca fascicularis) hepatocytes

Polybrominated diphenyl ethers (PBDEs) are widespread environmental pollutants, and the levels of certain congeners have been increasing in biota and abiota in recent decades. Some PBDEs are lipophilic and persistent, resulting in bioaccumulation in the environment. Their structural similarity to other polyhalogenated aromatic hydrocarbons (PHAHs) such as polychlorinated biphenyls (PCBs) has raised concerns that PBDEs might act as agonists for the aryl hydrocarbon receptor (AhR). Recent studies in our laboratory with human and rat cell lines indicated no AhR mediated CYP1A1 induction for PBDEs. However, an earlier in vitro study by Van der Burght et al. (1999) [Van der Burght, A.S., Clijsters, P.J., Horbach, G.J., Andersson, P.L., Tysklind, M., van den Berg, M., 1999. Structure-dependent induction of CYP1A by polychlorinated biphenyls in hepatocytes of cynomolgus monkeys (Macaca fascicularis). Toxicol. Appl. Pharmacol. 155, 13-23] indicated that in cynomolgus monkey (M. fascicularis) hepatocytes PCBs with a non-planar configuration could induce CYP1A. As PBDEs show a structural similarity with non-planar (ortho substituted) PCBs, our present study focused on the possible CYP1A induction by PBDEs (BDE-47, -99, -100, -153, -154, -183, and -77) in individual preparations (n=4) of primary hepatocytes of cynomolgus monkeys (M. fascicularis). 7-Ethoxyresorufin-O-deethylase (EROD) was used as a marker for CYP1A-mediated catalytic activity. Cells were exposed for 48 h to various PBDE concentrations (0.01-10 microM), positive controls 2,3,7,8-TCDD (0.001-2.5 nM) and PCB-126 (0.01-10nM), and negative control (DMSO vehicle alone). No statistically significant induction of CYP1A was observed in the hepatocytes after 48 h of exposure to all environmentally relevant PBDEs. After exposing hepatocytes to PBDEs in combination with TCDD, a concentration-dependent decrease in TCDD-induced EROD activity was observed. All PBDEs tested showed a similar reduction in each of four experiments, though quantitative differences were observed. The observed antagonism of TCDD-induced EROD activity by PBDEs occurred in both male (n=3) and female (n=1) hepatocytes and was not due to catalytic inhibition of EROD activity or cytotoxicity. However, based on the results of this study we do not expect these antagonistic effects of PBDEs on CYP1A induction at environmental relevant levels, since these in vitro interactive effects with TCDD were observed only at relatively high concentrations that are normally not seen, e.g. in the human body.

Spontaneous apoptosis, necrosis, energy status, glutathione levels and biotransformation capacities of isolated rat hepatocytes in suspension: effect of the incubation medium

Isolated hepatocytes in suspension express most of the functional activities of the intact liver and offer an easy-to-handle in vitro system for investigating both the biotransformation and damaging effects induced after a single exposure to xenobiotics upto 3-4h. There is, however, a general lack of consensus with respect to the choice of a suitable suspension medium. This motivated us to perform a comparative study of the effects of five frequently used bicarbonate-based media (Ca(2+)-containing Krebs-Henseleit buffer (KHB) with or without 25mM HEPES, 10mM glucose and 2% (g/v) BSA supplements, and Williams' E culture medium) on the viability (LDH leakage, caspase-3 processing and activity, Bid/Bax expression) and functionality (energy status, glutathione content, phases I and II biotransformation) of freshly isolated rat hepatocytes in suspension upto 3h. Also included was the bicarbonate-free HEPES buffer that does not require carbogen gassing, and is therefore handled more easily. The results clearly demonstrated that the type of incubation medium profoundly affected the functionality of the suspended hepatocytes, changing their sensitivity and response to exogenous damaging effects. While HEPES buffer and Williams' E medium offered the lowest background of spontaneous cell death, bicarbonate-based buffers and media seemed more suitable for obtaining both phases I and II biotransformation. Williams' E medium ensured a constant glutathione content of the cells and a lower level of oxidative stress.

Protective effects of trolox C, vitamin C, and catalase on bromobenzene-induced damage to rat hepatocytes

BACKGROUND/METHODS

The protective effects of trolox C (water-soluble vitamin E), vitamin C, and catalase on bromobenzene (BB)-induced toxicity to isolated rat hepatocytes were evaluated. The glutathione (GSH) content of the hepatocytes exposed to BB was measured.

RESULTS

BB caused acute damage to the cells during 2 h of incubation (short) when BB was added directly to the culture wells, whereas a late-occurring and time-dependent increase in lactate dehydrogenase (LDH) leakage rate was observed during 24 h of incubation (long) when BB was dissolved in a different way. Incubation of the cells with trolox C (0.5-2.0 mM) prevented the hepatocellular damage induced by BB at 2.4 mM during the long-term incubation. Vitamin C (0.1-1.0 mM) had a protective effect on BB-induced toxicity during both the short- (BB, 1.6 mM) and the long- (BB, 2.4 mM) term incubations. Catalase (3200 U/ml) also showed a beneficial effect on the cells during the short-term BB exposure. Trolox C (2.0 mM) and vitamin C (0.5 mM) restored BB-induced GSH depletion in the cells.

CONCLUSIONS

BB induced two patterns of LDH leakage from isolated hepatocytes on the basis of different ways of BB exposure and incubation periods. Trolox C, vitamin C, and catalase exerted protective effects on BB-induced toxicity during short- or/and long-term incubations. The effects were concentration-dependent. Restoration of GSH content in BB-exposed hepatocytes suggests that trolox C and vitamin C could reduce GSH consumption during BB metabolism and exert an antioxidant effect.