Survival Mechanisms and Xenobiotic Susceptibility of Keratinocytes Exposed to Metal-Derived Nanoparticles.

Metal-derived nanoparticles (Mt-NPs) are increasingly used in cosmetology due to their ultraviolet shielding (titanium dioxide [TiO2]), antioxidant (cerium dioxide [CeO2]), and biocidal (silver [Ag]) properties. In the absence of overt toxicity (i.e., cell death), Mt-NPs are considered safe for cosmetic applications. However, there is little understanding about the mechanisms involved in the survival of keratinocytes exposed to subtoxic levels of Mt-NPs. Human keratinocytes (HaCaT) were exposed subacutely to subtoxic concentrations (≤30 µg/mL, 48-72 h) of rutile (r) TiO2 (cylindrical), CeO2 (cubic) and Ag (spherical) with a core/hydrodynamic size of <50/<100 nm and >98% purity. Mt-NP uptake was indirectly quantified by changes in the light side scatter, where the kinetics (time/dose-response) suggested that the three types of Mt-NPs were similarly uptaken by keratinocytes. rTiO2 and CeO2, but not Ag-NPs, increased autophagy, whose inhibition prompted cell death. No increase in the steady-state levels of reactive oxygen species (ROS) was induced by exposure to any of the Mt-NPs tested. Interestingly, intracellular Ag-NP aggregates observed an increased far-red autofluorescence (≥740 nm em), which has been ascribed to their binding to thiol molecules such as glutathione (GSH). Accordingly, inhibition of GSH synthesis, but not the impairment of oxidized GSH recycling, sensitized keratinocytes to Ag-NPs suggesting that GSH homeostasis, and its direct scavenging of Ag-NPs, but not ROS, is essential for keratinocyte survival upon exposure to Ag-NP. rTiO2 and Ag, but not CeO2-NPs, compromised metabolic flux (glycolysis and respiration), but ATP levels were unaltered. Finally, we also observed that exposure to Mt-NPs sensitized keratinocytes to non-UV xenobiotic exposure (arsenite and paraquat). Our results demonstrate the differential contribution of autophagy and GSH homeostasis to the survival of human keratinocytes exposed to subtoxic concentrations of Mt-NPs and highlight the increased susceptibility of keratinocytes exposed to Mt-NPs to a second xenobiotic insult.

Ecologically appropriate xenobiotics induce cytochrome P450s in Apis mellifera.

BACKGROUND: Honey bees are exposed to phytochemicals through the nectar, pollen and propolis consumed to sustain the colony. They may also encounter mycotoxins produced by Aspergillus fungi infesting pollen in beebread. Moreover, bees are exposed to agricultural pesticides, particularly in-hive acaricides used against the parasite Varroa destructor. They cope with these and other xenobiotics primarily through enzymatic detoxificative processes, but the regulation of detoxificative enzymes in honey bees remains largely unexplored. METHODOLOGY/PRINCIPAL FINDINGS: We used several approaches to ascertain effects of dietary toxins on bee susceptibility to synthetic and natural xenobiotics, including the acaricide tau-fluvalinate, the agricultural pesticide imidacloprid, and the naturally occurring mycotoxin aflatoxin. We administered potential inducers of cytochrome P450 enzymes, the principal biochemical system for Phase 1 detoxification in insects, to investigate how detoxification is regulated. The drug phenobarbital induces P450s in many insects, yet feeding bees with phenobarbital had no effect on the toxicity of tau-fluvalinate, a pesticide known to be detoxified by bee P450s. Similarly, no P450 induction, as measured by tau-fluvalinate tolerance, occurred in bees fed xanthotoxin, salicylic acid, or indole-3-carbinol, all of which induce P450s in other insects. Only quercetin, a common pollen and honey constituent, reduced tau-fluvalinate toxicity. In microarray comparisons no change in detoxificative gene expression was detected in phenobarbital-treated bees. However, northern blot analyses of guts of bees fed extracts of honey, pollen and propolis showed elevated expression of three CYP6AS P450 genes. Diet did not influence tau-fluvalinate or imidacloprid toxicity in bioassays; however, aflatoxin toxicity was higher in bees consuming sucrose or high-fructose corn syrup than in bees consuming honey. CONCLUSIONS/SIGNIFICANCE: These results suggest that regulation of honey bee P450s is tuned to chemicals occurring naturally in the hive environment and that, in terms of toxicological capacity, a diet of sugar is not equivalent to a diet of honey.

The Nrf2-Keap1 defence pathway: role in protection against drug-induced toxicity.

The metabolic biotransformation of xenobiotics to chemically reactive metabolites can, in some instances, underlie the pathogenesis of certain adverse drug reactions, due to the development of chemical or oxidative stress. In order to guard against such stresses, mammalian cells have evolved multi-faceted, highly-regulated defence systems, one of the most important being that which is regulated by the transcription factor Nrf2. Through regulating the expression of numerous cytoprotective genes, Nrf2 serves as a critical determinant of a cell's capacity to survive, or succumb, to a toxic insult. The aim of this review is to summarise our current understanding of the biochemistry that underlies the Nrf2 defence pathway, and highlight the important role of this transcription factor in the protection against drug-induced toxicity, primarily through the examination of recent investigations that have demonstrated an increased vulnerability to various toxins in animals lacking Nrf2.

Repression of intestinal drug metabolizing enzymes by the SV40 large T antigen.

Toxic compounds such as carcinogens are removed from the body by the action of a series of detoxifying enzymes and transporters expressed in the liver and the small intestine. We have found that intestinal epithelial cells expressing the SV40 large T antigen (TAg) contain significantly lower levels of mRNAs, encoding several drug metabolizing/detoxifying enzymes and transporters compared to their non-transgenic littermates. In addition, TAg blocks the induction of these mRNAs by xenobiotics. The repression depends on an intact LXCXE motif in TAg, suggesting that inactivation of the retinoblastoma (Rb) family of tumor suppressors plays a role in the process. These results imply that a functional Rb pathway in the intestine is necessary for the expression of the detoxification system used to clear carcinogens, and suggest that loss of this tumor suppressor might alter susceptibility to chemical injury. In addition, the effect of TAg on the detoxification pathway appears to be tissue-specific, as its ectopic expression in the liver failed to suppress the P450 enzymes. The TAg-mediated suppression of drug metabolizing/detoxifying enzymes may have broad implications in the metabolism and mechanism of action of both carcinogens and prescription drugs.

Stem cell fusion as an ultimate line of defense against xenobiotics.

There are several indications that the potential of stem cells to fuse with somatic cells is extremely high and, what's more exciting, in some instances goes as far as reprogramming and/or rescuing altered cells. It remains unclear, however, how frequent this mechanism is and what patho-physiological role it might play in nature. A plausible hypothesis, discussed in this paper, suggests that stem cell niches might provide a safeguard for the intact genome and epigenome. By fusing with somatic de-differentiated cells, stem cells might consent epigenetic reprogramming and/or genetic recovery of genes which otherwise could drive altered cells to malignancy. If the many sophisticated mechanisms of metabolism, cell repair, programmed cell death and tissue regeneration should fail, stem cells might represent a final attempt to recover dedifferentiated cells to avoid inflowing in cancer. In the current reappraisal of the different mechanisms of defense against xenobiotics, even the incidence of cancer itself is considered an evolving mechanism which, through a kind of programmed death of individuals exhibiting defective mutations, favors advancement of the phenotypes which adapt best. Additionally, with regard to the mechanisms of transmitting somatic mutations, based on stem cells' capacity to migrate and to fuse, here it is speculated that stem cells might be capable of carrying acquired somatic mutations from peripheral tissues to the gonads, and transmit that information into the germinal line. If appropriately demonstrated, these mechanisms might delineate a novel therapeutic area to be explored. The use of stem cells to reprogram/recover irreversibly damaged cells or to transmit beneficial mutations might be a valuable therapeutic approach in the future.

Effects of estrogenic xenobiotics on human and mouse spermatozoa.

OBJECTIVE: To investigate human sperm responsiveness to the estrogenic xenobiotic genistein and seek further information regarding the mechanism of action of estrogenic xenobiotics using mouse spermatozoa. METHODS: Uncapacitated human spermatozoa were incubated with genistein and assessed using chlortetracycline (CTC) fluorescence. CTC was also used to evaluate mouse sperm responses to daidzein and combinations of genistein, 8-prenylnaringenin and nonylphenol. Several steroids were tested to determine structure-function relationships, and possible involvement of cAMP and G proteins in responses was also investigated. RESULTS: Genistein significantly accelerated capacitation and acrosome loss in human spermatozoa, with 1, 10 and 100 nmol/l being equally effective. In mouse spermatozoa, daidzein produced significant responses, and combinations of xenobiotics at low concentrations were more effective than used singly. The compounds appear to act at the cell surface, and responses to three different steroids were nonidentical. A protein kinase-A inhibitor blocked responses to xenobiotics, while genistein and nonylphenol significantly stimulated cAMP production. Pertussis toxin and dideoxyadenosine blocked responses, suggesting involvement of inhibitory G proteins and membrane-associated adenylyl cyclases. CONCLUSION: Human and mouse sperm responses to genistein are very similar, but human gametes appear to be even more sensitive. The mechanism of action may involve unregulated stimulation of cAMP production, leading to significant acrosome loss, undesirable because already acrosome-reacted cells are nonfertilizing. Xenobiotics were even more effective in combination. Since simultaneous exposure to low concentrations of multiple xenobiotics is likely to occur in animals and humans, further investigation is needed to determine whether this could impair fertility.

Protective effects of vitamin A and vitamin E succinate against 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced body wasting, hepatomegaly, thymic atrophy, production of reactive oxygen species and DNA damage in C57BL/6J mice.

The protective effect of vitamin A and vitamin E succinate against 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced acute toxicity and measures of oxidative stress was studied. Ten mice were treated with either vitamin A (50 mg/kg every other day for eight days) or vitamin E succiante (150 mg/kg/day followed by a dose of 40 mg/kg/day for five additional days). Half of each of the above groups of animals received TCDD on day 4. Five mice received corn oil or TCDD alone. After five days of TCDD treatment, antioxidant combination treatment with vitamin A and TCDD or vitamin E succinate and TCDD resulted in a significant reduction in indicators of acute toxicity including the decrease in total body and thymus weight as compared to TCDD alone (P<0.05). The combination treatment produced also a significant reduction in the increase in liver weight as compared to TCDD only (P<0.05). Following one day of treatment with 50 microg TCDD/kg, vitamin A and vitamin E succinate produced a significant decrease in the production of superoxide anion by peritoneal lavage cells (P<0.05) and in DNA-single strand breaks in the same cells (P<0.05) as assessed by the reduction of cytochrome c and the alkaline elution technique, respectively. A significant decrease in DNA-single strand breaks in peritoneal lavage cells was observed following 5 days treatment with 50 microg TCDD/kg (P<0.05). The results indicate a potential role for oxidative stress in the acute toxicity of TCDD and a protective effect for vitamin A and vitamin E succinate in the overall toxicity of TCDD including measures of oxidative stress.

Use of a human-derived liver cell line for the detection of cytoprotective, antigenotoxic and cogenotoxic agents.

In this paper, we reviewed the data on the use of HepG2 cells to detect cytoprotective, antigenotoxic and cogenotoxic agents. Owing to their intact and inducible phase I and phase II enzymes, HepG2 cells are able to activate and detoxify xenobiotics and therefore reflect the metabolism of xenobiotics in the human body better than other metabolically incompetent cells used in conventional in vitro assays. Several dietary and non-dietary agents were found to be protective against different groups of cytotoxic and DNA-damaging xenobiotics in HepG2 cells and the mechanism of protection includes scavenging of electrophiles, reactive oxygen species and peroxides, inhibition of phase I activating enzymes, induction of phase II detoxifying enzymes and interactions with DNA-repair and/or replication processes. Additionally, certain non-mutagenic substances were found to enhance the effect of genotoxic agents in HepG2 cells by increasing the metabolic activation of the latter. In conclusion, HepG2 cells are of great relevance to detect cytotoxic and genotoxic substances and by extension cytoprotective, antigenotoxic and cogenotoxic agents.

Simultaneous exposure of rats to dioxin and carbon monoxide reduces the xenobiotic but not the hypoxic response.

Aryl hydrocarbon receptor (AhR) and hypoxia-inducible factor-1alpha (HIF-1alpha) are conditionally regulated transcription factor subunits that form heterodimeric complexes with their common partner, AhR nuclear translocator (ARNT/HIF-1beta). Whereas the environmentally toxic compound 2,3,7,8-tetra-chlorodibenzo-p-dioxin (TCDD) initiates the trans-activation activity of AhR:ARNT/HIF-1beta, hypoxic exposure stabilizes HIF-1alpha and functionally activates the HIF-1alpha:ARNT/HIF-1beta complex. To analyze a possible crosstalk between these two pathways in vivo, rats were given dioxin orally and/or were exposed to carbon monoxide (CO), causing functional anemia. We found that exposure to CO inhibited the xenobiotic response while dioxin application had no significant negative impact on hypoxia-mediated gene transcription.

The nuclear pregnane X receptor regulates xenobiotic detoxification.

The pregnane X receptor (PXR), which is a member of the nuclear receptor family of ligand-activated transcription factors, is an integral component of the body's defense mechanism against toxic xenobiotics. PXR is activated by a broad spectrum of lipophilic xenobiotics including prescription drugs, herbs, pesticides, endocrine disruptors and other environmental contaminants. The promiscuous ligand-binding properties of PXR are facilitated by the large volume and smooth shape of its ligand-binding pocket. PXR binds to DNA as a heterodimer with the 9-cis retinoic acid receptor (RXR) and regulates a large number of genes involved in the detoxification and excretion of toxic substances. Although PXR evolved to protect the body, its activation by various prescription drugs and herbs such as St. John's wort represents the molecular basis for an important class of drug-drug interactions. Assays that detect PXR activation can now be used to predict and prevent these drug-drug interactions.

Detection of drug-induced, superoxide-mediated cell damage and its prevention by antioxidants.

The mode of the cytotoxic activity of three benzo(c)fluorene derivatives was characterized. The observed morphological changes of lysosomes or variations of mitochondrial activity are assumed to be the consequence of cell protection against oxidative damage and/or the part of the damage process. To establish the relationship between the quantity of superoxide (O2*-) generated and the degree of damage resulting from O2*-, a simple system based on measurement of 3-(4-iodophenyl)-2-(4-nitrophenyl)-5-phenyltetrazolium chloride (INT) reductase activity in the presence of superoxide dismutase (SOD) was used. The functionality of the chosen battery of in vitro tests was proved using several known superoxide inducers: cyclosporin A (CsA) and benzo(a)pyrene (BP), as well as noninducers: citrinin (CT) and cycloheximide (CH). From the results followed that the cell growth tests are much better indices of toxicity than the other tests. The model system for the evaluation of the protective capacity of antioxidants against superoxide-induced cytotoxicity included simultaneous exposure of HeLa cells to cytotoxic drugs and to quercetin (Qe), an antioxidant of plant origin. The complete abolishment of the inhibition of cell proliferation and clonogenic survival was concluded to be due to the protective effect of the antioxidant. These observations correlated with the decrease of superoxide content as estimated by the INT-reductase assay in the presence of SOD using the same model system, as well as with the increase of intracellular SOD content and its activity.

Inhibitory effect of rifampicin on the depressive action of interleukin-1 on cytochrome P-450-linked monooxygenase system.

It has been shown that interleukin 1 (IL-1) depresses cytochrome P-450-linked monooxygenases. In the present study, the effects of rifampicin on the depressive action of IL-1 on the activities and gene expression of xenobiotic metabolizing enzymes in liver microsomes were investigated in vivo using Wistar rats. Among the monooxygenases studied, we especially focused on the induction mechanism for CYP2D, known to be depressed by IL-1 and responsible for the oxidation of xenobiotics, debrisoquine, bufuralol, and sparteine. The CYP2D protein and its messenger RNA (mRNA) were quantitated by Western blot and slot blot hybridization analyses in the groups treated with and without rifampicin and IL-1. The results showed that the depressive action of IL-1 on CYP2D was offset by additional administration of rifampicin, and the P-450 (CYP2D-linked monooxygenase system is up-regulated at the mRNA level by rifampicin. These results show that rifampicin has a blocking effect on the depressive action of IL-1 on the CYP2D subfamily.

The use of explant lens culture to assess cataractogenic potential.

Explanted cultures of crystalline lenses have been used to investigate mechanisms of xenobiotic-induced cataract formation. However, very few studies have utilized mechanistic information to predict the cataractogenic potential of structurally diverse xenobiotics. The present investigation outlines how visual assessment of lens clarity, biochemical endpoints of toxicity, and mechanisms of lenticular opacity formation can be used to select compounds with a lower probability of causing cataract formation in vivo. The rat lens explant culture system has been used to screen thiazolidinediones against ciglitazone for their direct cataractogenic potential in vitro. The two compounds that were selected as development candidates (englitazone and darglitazone) did not produce cataracts in rats exposed daily for 3 months. The culture system has also been used to illustrate that the lens is capable of metabolizing compounds to reactive intermediates. In this example, the toxicity of S-(1,2-dichlorovinyl)-L-cysteine (DCVC), a model cataractogen, was attenuated by inhibiting lenticular cysteine conjugate beta-lyase metabolism using aminooxyacetic acid. Finally, this model was used retrospectively to investigate the cataractogenic potential of CJ-12,918 and CJ-13,454 in rats. These compounds showed differences in the incidence of cataract formation in vivo based on differences in hepatic metabolism and penetration of parent drug and metabolites into the lens. The rank order of cataractogenic potential in vitro correlated better with in vivo results when an induced S9 microsomal fraction was added to the culture media. However, the model did not correctly predict the cataractogenic potential of ZD2138, a structurally similar compound. These studies illustrate the use of explant culture to assess mechanisms of cataract formation and outline its use and limitations for predicting cataractogenic potential in vivo.

Relative binding affinity does not predict biological response to xenoestrogens in rat endometrial adenocarcinoma cells.

The possible adverse effects of the so-called environmental estrogens have raised considerable concern. Developmental, endocrine and reproductive disorders in wildlife animals have been linked to high exposure to persistent environmental chemicals with estrogen-like activity (xenoestrogens); yet, the potential impact of environmental estrogens on human health is currently under debate also due to lack of data. A battery of in vitro assays exist for identifying compounds with estrogenic activity, but only a few models are available to assess estrogenic potency in a multiparametric analysis. We have recently established the endometrial adenocarcinoma cell line RUCA-I; it enables us to compare estrogenic effects both in vitro and in vivo as these cells are estrogen responsive in vitro and grow estrogen sensitive tumors if inoculated in syngeneic animals in vivo. Here we report in vitro data concerning (a) the relative binding affinity of the selected synthetic chemicals Bisphenol A, nonylphenol, p-tert-octylphenol, and o,p-DDT to the estrogen receptor of RUCA-I cells and (b) the relative potency of these compounds in inducing increased production of complement C3, an endogenous estrogen-responsive gene. Competitive Scatchard analysis revealed that xenoestrogens bound with an at least 1000-fold lower affinity to the estrogen receptor of RUCA-I cells than estradiol itself, thereby exhibiting the following affinity ranking, estradiol>>>nonylphenol>bisphenol A approximately p-tert-octylphenol>o,p-DDT. Despite these low binding affinities, bisphenol A, nonylphenol and p-tert-octylphenol increased production of complement C3 in a dose dependent manner. Compared with estradiol, only 100-fold higher concentrations were needed for all the compounds to achieve similar levels of induction, except o,p-DDT which was by far less potent. Northern blot analyses demonstrated that the increased production of complement C3 was mediated by an increased transcription. In summary, cultured RUCA-I cells represent a valuable endometrial derived model system to assess the relative potencies and the molecular mode of action of environmental estrogens in vitro. Our results further show that no intimate correlation exists between the relative binding affinity and the biological response of these compounds. Therefore, data obtained from single-parametric analyses may result in misleading conclusions. On the other hand, the presented in vitro data will provide us with tools to study the activity of xenoestrogens in vivo and thus carry risk assessment one step further.

[Study of the effects of pectin substances under conditions of ontogenesis impairment by a complex of chemical factors]. / Izuchenie vliianiia pektinovykh veshchestv v usloviiakh narusheniia ontogeneza kompleksom khimicheskikh faktorov.

In the experimental conditions the use of homogenized fruits and vegetables mashed potatoes with the addition of the water-extracted apple-pectin Classic AF-702 as a protector of a toxic action of xenobiotic for the conservation of reproductive ability of the organism and the optimum development of posterity.

Biomarcadores de susceptibilidad a metales con énfasis en salud humana / Susceptibility biomarkers of metals: emphasis in human health

Los biomarcadores son variables cuantificables que permiten determinar la exposición, el efecto y/o la susceptibilidad a un tóxico. La mayoría de los biomarcadores estudiados son de exposición y efecto. Mediante los biomarcadores de susceptibilidad pueden evaluarse las interacciones entre ciertos factores genéticos (p. ej. polimorfismos enzimáticos) y externos (p. ej. exposición simultánea a otros agentes, estatus nutricional, etc.). Los metales han ido adquiriendo un creciente interés en relación con la salud ambiental y humana debido a su alta toxicidad y los biomarcadores pueden ser útiles en el desarrollo de estrategias de prevención y diagnóstico temprano de sus eventuales efectos adversos. En el presente trabajo se detallan los efectos de la interacción sinérgica y antagonista de metales, valencia del metal, metalotioneínas, glutation-S-transferasas, enfermedades crónicas, nicotina, edad, sexo, etc., en relación con la susceptibilidad a la toxicidad de metales. Por ej., una deficiencia en O1-antitripsina (AAT), un biomarcador de utilidad para detectar individuos hipersensibles a sustancias irritantes del sistema respiratorio (p. ej. Cr, Mn, Pb, Be, etc.) y con elevado riesgo de sufrir de enfisema pulmonar, permite anticipar los riesgos ocupacionales que podrían sufrir individuos con estas características. Los biomarcadores de sustancias potencilamente tóxicas podrían ser de gran utilidad para realizar las reglamentaciones sobre bases científicas, vinculadas con los niveles máximos admisibles de las sustancias en el ambiente (AU)

Biomarcadores de susceptibilidad a metales con énfasis en salud humana / Susceptibility biomarkers of metals: emphasis in human health

Los biomarcadores son variables cuantificables que permiten determinar la exposición, el efecto y/o la susceptibilidad a un tóxico. La mayoría de los biomarcadores estudiados son de exposición y efecto. Mediante los biomarcadores de susceptibilidad pueden evaluarse las interacciones entre ciertos factores genéticos (p. ej. polimorfismos enzimáticos) y externos (p. ej. exposición simultánea a otros agentes, estatus nutricional, etc.). Los metales han ido adquiriendo un creciente interés en relación con la salud ambiental y humana debido a su alta toxicidad y los biomarcadores pueden ser útiles en el desarrollo de estrategias de prevención y diagnóstico temprano de sus eventuales efectos adversos. En el presente trabajo se detallan los efectos de la interacción sinérgica y antagonista de metales, valencia del metal, metalotioneínas, glutation-S-transferasas, enfermedades crónicas, nicotina, edad, sexo, etc., en relación con la susceptibilidad a la toxicidad de metales. Por ej., una deficiencia en Ó1-antitripsina (AAT), un biomarcador de utilidad para detectar individuos hipersensibles a sustancias irritantes del sistema respiratorio (p. ej. Cr, Mn, Pb, Be, etc.) y con elevado riesgo de sufrir de enfisema pulmonar, permite anticipar los riesgos ocupacionales que podrían sufrir individuos con estas características. Los biomarcadores de sustancias potencilamente tóxicas podrían ser de gran utilidad para realizar las reglamentaciones sobre bases científicas, vinculadas con los niveles máximos admisibles de las sustancias en el ambiente

Inhibition of xenobiotic-induced genotoxicity in cultured precision-cut human and rat liver slices.

In this study precision-cut liver slices have been used to evaluate the effects of the flavone tangeretin, the flavonoid glycoside naringin and the flavanone naringenin (the aglycone derived from naringin) on xenobiotic-induced genotoxicity. Liver slices were cultured for 24 h in medium containing [3H]thymidine and the test compounds and then processed for autoradiographic determination of unscheduled DNA synthesis (UDS). The cooked food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) markedly induced UDS in cultured human liver slices and both 2-acetylaminofluorene (2-AAF) and aflatoxin B1 (AFB1) induced UDS in cultured rat liver slices. Tangeretin (20 and 50 microM) was found to be a potent inhibitor of 5 and 50 microM PhIP-induced UDS in human liver slices, whereas 20 and 50 microM naringenin was ineffective and naringin only inhibited genotoxicity at a concentration of 1000 microM. In rat liver slices 50 microM tangeretin inhibited 10 and 50 microM 2-AAF-induced UDS, whereas 50 microM naringenin and 100 and 1000 microM naringin were ineffective. None of the three flavonoids examined inhibited 5 microM AFB1-induced UDS in rat liver slices. The inhibition of PhIP- and 2-AAF-induced UDS by tangeretin is probably attributable to the inhibition of the human and rat cytochrome P-450 isoforms which are responsible for the bioactivation of these two genotoxins. Although flavonoids can modulate xenobiotic-induced genotoxicity in human and rat liver slices, any protective effect is dependent on the particular combination of genotoxin and flavonoid examined. These results demonstrate that cultured precision-cut liver slices may be utilised as an in vitro model system to examine the modulation of xenobiotic-induced genotoxicity by flavonoids and other dietary components.

Toxicological consequences of multiple chemical interactions: a primer.

The capacity to understand and successfully predict the toxicological consequences of multiple chemical interactions is a critical challenge facing the scientific community. This article is designed to provide a broad framework introducing the concept of interaction, use of consistent and meaningful terminology and a descriptive assessment of toxicological foundations within which chemical interactions may be evaluated. The article offers guidance on the need to place a high priority on assessing the mechanistic basis of 'superinteractions', that is, unique interactions far exceeding even those of a multiplicative nature. The final section of the article provides a detailed perspective on how the extensive and successful experience of the pharmaceutical industry in assessing and interpreting any interaction for patients can be useful to the issues and concerns of chemical interactions for the field of environmental toxicology and risk assessment.