Ah Receptor Agonist Activity Research Articles

Identification of benzothiazole derivatives and polycyclic aromatic hydrocarbons as aryl hydrocarbon receptor agonists present in tire extracts.

Leachate from rubber tire material contains a complex mixture of chemicals previously shown to produce toxic and biological effects in aquatic organisms. The ability of these leachates to induce Ah receptor (AhR)-dependent cytochrome P4501A1 expression in fish indicated the presence of AhR active chemicals, but the responsible chemicals and their direct interaction with the AhR signaling pathway were not examined. Using a combination of AhR-based bioassays, we have demonstrated the ability of tire extract to stimulate both AhR DNA binding and AhR-dependent gene expression and confirmed that the responsible chemicals were metabolically labile. The application of CALUX (chemical-activated luciferase gene expression) cell bioassay-driven toxicant identification evaluation not only revealed that tire extract contained a variety of known AhR-active polycyclic aromatic hydrocarbons but also identified 2-methylthiobenzothiazole and 2-mercaptobenzothiazole as AhR agonists. Analysis of a structurally diverse series of benzothiazoles identified many that could directly stimulate AhR DNA binding and transiently activate the AhR signaling pathway and identified benzothiazoles as a new class of AhR agonists. In addition to these compounds, the relatively high AhR agonist activity of a large number of fractions strongly suggests that tire extract contains a large number of physiochemically diverse AhR agonists whose identities and toxicological/biological significances are unknown.

Structural Identification of Diindole Agonists of the Aryl Hydrocarbon Receptor Derived from Degradation of Indole-3-pyruvic Acid

Aerobic incubation of the tryptophan transamination/oxidation product indole-3-pyruvic acid (I3P) at pH 7.4 and 37 degrees C yielded products with activity as Ah receptor (AHR) agonists. The extracts were fractionated using HPLC and screened for AHR agonist activity. Two compounds were identified as agonists: 1,3-di(1H-indol-3-yl)propan-2-one (1) and 1-(1H-indol-3-yl)-3-(3H-indol-3-ylidene) propan-2-one (2), with the potency of 2 being 100-fold > 1 [ Nguyen et al. ( 2009 ) Chem. Res. Toxicol. , DOI: 10.1021/tx900043s . ]. Both 1 and 2 showed UV spectra indicative of indole. The molecular formulas were established by high-resolution mass spectrometry (HRMS), and the structures were determined by a combination of NMR methods, including (1)H, natural abundance (13)C, and two-dimensional methods. An intermediate in the oxidation of I3P to 1 is 3-hydroxy-2,4-di(1H-indol-3-yl)butanal (HRMS established the presence of a compound with the formula C(20)H(19)N(2)O(2)). Compound 1 was converted to 2 in air or (faster) with mild oxidants, and 2 could be further oxidized to 1,3-di(3H-indol-3-ylidene)propan-2-one. Determination of the structures allowed estimation of the molar Ah receptor agonist activity of these natural products, similar in potency to known classical AHR inducers.

Changes in toxicity and Ah receptor agonist activity of suspended particulate matter during flood events at the rivers Neckar and Rhine — a mass balance approach using in vitro methods and chemical analysis

Background, aim, and scope As a consequence of flood events, runoff and remobilized sediments may cause an increase of ecotoxicologically relevant effects from contaminant reservoirs. Aquatic and terrestrial organisms as well as cattle and areas of settlement are exposed to dislocated contaminants during and after flood events. In this study, the impacts of two flood events triggered by intense rain at the rivers Neckar and Rhine (Southern Germany) were studied. Effects in correlation to flood flow were assessed at the river Neckar using samples collected at frequent intervals. River Rhine suspended particulate matter (SPM) was sampled over a longer period at normal flow and during a flood event. Three cell lines (H4L1.1c4, GPC.2D.Luc, RTL-W1) were used to compare Ah receptor agonist activity in different biotest systems. Multilayer fractionation was performed to identify causative compounds, focusing on persistent organic contaminants.

Ah receptor agonist activity in frequently consumed food items

The aryl hydrocarbon receptor (AhR) receives much attention for its role in the toxicity of dioxins and dioxin-like polychlorinated biphenyls. However, many other compounds have also been reported to bind and activate AhR, of which natural food components are of special interest from a human health perspective. Using the dioxin receptor–chemical-activated luciferase gene expression (DR CALUX®) bioassay, extracts from many food items frequently consumed in the Netherlands were screened to estimate the intake of natural AhR agonists (NAhRAs). Using the prototypical AhR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as standard, it was estimated that the daily intake of NAhRAs might be considerably higher than the reported intake of dioxins and dioxin-like polychlorinated biphenyls. Potatoes, cruciferous vegetables, bread, hamburgers, and grapefruit juice contained most NAhRAs. Food preparation and acid treatment can show a significant effect on AhR activation. The interaction of natural and xenobiotic AhR agonists should be taken into account when performing risk–benefit analysis of both types of compounds.

AH receptor agonist activity in human blood measured with a cell-based bioassay: Evidence for naturally occurring AH receptor ligands in vivo

In the present study, an aryl hydrocarbon receptor (AHR)-driven reporter gene bioassay was used to measure the activity, measured as an induction equivalent (IEQ) as compared to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), or IEQ concentration in human blood samples from 10 volunteers under different dietary regimens. Blood concentrations of polychlorinated dibenzo-p-dioxins/furans (PCDD/Fs) and polychlorinated biphenyls (PCBs), as determined by analytical chemistry (HR-GC/MS), and expressed as toxic equivalents (TEQs) with the use of TCDD equivalency factors (TEFs), were within a range that has been reported in the general US population, ranging from 0.022 to 0.119 ppt (whole blood basis). However, the human blood IEQ measured directly via bioassay ranged from 13.4 to 218 ppt (whole blood basis). These order of magnitude greater IEQs compared to the TEQs for dioxins, furans, and certain PCBs suggests that human blood contains a relatively high level of AHR agonists able to activate the CYP1A1 dioxin response element (DRE)-linked reporter gene bioassay and that this AHR activity is not accounted for by PCDDs/Fs and dioxin-like PCBs based on standard HR-GC/MS and TEF analysis. When study participants switched from a "baseline" to a high-vegetable diet, increases in bioassay IEQ were observed that were statistically significant (P<0.05). In addition, IEQ activity was elevated above levels observed following dietary intervention in two subjects given indole-3-carbinol (I3C) supplements. We conclude that a substantial portion of the IEQ activity occurred as a result of the increased intake of natural AHR agonists (NAHRAs) present in many fruits, vegetables. and herbs. Our findings also suggest that dietary NAHRAs constitute a substantial daily dietary intake of AHR-active compounds, and these NAHRAs could influence AHR status in humans and play a role in a basal level of AHR activation.

Induction of CYP1A1 and CYP1B1 by benzo( k)fluoranthene and benzo( a)pyrene in T-47D human breast cancer cells: Roles of PAH interactions and PAH metabolites

The interactions of polycyclic aromatic hydrocarbons (PAH) and cytochromes P450 (CYP) are complex; PAHs are enzyme inducers, substrates, and inhibitors. In T-47D breast cancer cells, exposure to 0.1 to 1 μM benzo( k)fluoranthene (BKF) induced CYP1A1/1B1-catalyzed 17β-estradiol (E 2) metabolism, whereas BKF levels greater than 1 μM inhibited E 2 metabolism. Time course studies showed that induction of CYP1-catalyzed E 2 metabolism persisted after the disappearance of BKF or co-exposed benzo( a)pyrene, suggesting that BKF metabolites retaining Ah receptor agonist activity were responsible for prolonged CYP1 induction. BKF metabolites were shown, through the use of ethoxyresorufin O-deethylase and CYP1A1-promoter-luciferase reporter assays to induce CYP1A1/1B1 in T-47D cells. Metabolites formed by oxidation at the C-2/C-3 region of BKF had potencies for CYP1 induction exceeding those of BKF, whereas C-8/C-9 oxidative metabolites were somewhat less potent than BKF. The activities of expressed human CYP1A1 and 1B1 with BKF as substrate were investigated by use of HPLC with fluorescence detection, and by GC/MS. The results showed that both enzymes efficiently catalyzed the formation of 3-, 8-, and 9-OHBKF from BKF. These studies indicate that the inductive effects of PAH metabolites as potent CYP1 inducers are likely to be additional important factors in PAH-CYP interactions that affect metabolism and bioactivation of other PAHs, ultimately modulating PAH toxicity and carcinogenicity.

Fractionation and Determination of Ah Receptor (AhR) Agonists in Organic Waste After Anaerobic Biodegradation and in Batch Experiments with PCB and decaBDE (8 pp)

Anaerobic digestion of organic household waste can lead to an increase in dioxin-like content, as determined by dioxin-specific bioassays. This may be a result of bioactivation of Ah receptor (AhR) agonists into more potent congeners. Work towards identifying the contributing compound groups is important in order to understand the mechanisms and to assess the relevance behind this increase in dioxin-like toxicity, since the residue can be used as a soil fertilising agent. The aim with the present work was to identify compound groups with AhR agonistic properties that caused the previously reported increase in dioxin-like activity after anaerobic biodegradation Firstly, chemical fractionation combined with dioxin bioassay testing was used to find bioactive classes of compounds. Secondly, batch digestion experiments with an externally added polychlorinated biphenyl (PCB) mixture (Clophen A50) and with decabrominated diphenyl ether (decaBDE), respectively, were studied as a possible process for transformation of precursors into more potent, dioxin-like compounds. Mesophilic (37ºC) and thermophilic (55ºC) anaerobic digestion were studied. Two different dioxin-specific bioassays were used to analyse AhR agonists in the biodegraded material, the CELCAD and the DR-CALUX. AhR agonist activity was detected in both di- and polyaromatic fractions of digestate extracts, which indicated that a diverse mixture of compounds contributed to the bioassay responses. No quantifiable activities were induced by the monoaromatic fractions. Further fractionation based on planarity revealed higher concentrations of AhR agonists than what was detected after the first fractionation, probably due to non-additive biological interactions of compounds in the extract that were removed in the second fractionation. These results showed significant activity in the non-planar diaromatic fractions and in the co-planar fractions of both diaromates and polyaromates. In the batch experiment with externally added PCB, an increase in dioxin-like activity was seen after 21 days of digestion at mesophilic conditions. After completed digestion, the content of AhR agonists was equal to the start concentration. PCB analysis with GC-MS indicated that dehalogenation of PCBs occurred in the digestors. The batch experiment with decaBDE showed no significant changes in TEQ-concentrations over time. The results show that the previously reported increase of AhR agonists during mesophilic anaerobic digestion is probably due to an accumulation of several different groups of AhR agonists, both diaromatic and polyaromatic, and both co-planar and non-planar. Batch experiments with externally added PCBs and decaBDE, respectively, did not result in any accumulation of AhR agonist activity after completed digestion, even though chemical analysis indicate a dechlorination of PCBs. Complex, unfractionated extracts were difficult to test using the bioassay approach. Removal of AhR antagonists or otherwise interacting compounds during fractionation may yield bio-TEQ values that are much higher than in the original extract. . Our results indicate that the environmental risk that AhR agonists may pose concerning large-scale anaerobic digestion of organic household waste probably depends on the efficiency of the digester and the sludge residence time. In order to obtain reliable results with the bioassays, an extensive cleanup and fractionation procedure is necessary. Without clean up and fractionation, there is a risk for false negatives and misleading conclusions. DR-CALUX and CELCAD were both suitable for these kinds of studies, provided that suitable fractionation methods are used.

Validation and use of the CALUX‐bioassay for the determination of dioxins and PCBs in bovine milk

There is a strong need for the development of relatively cheap and rapid bioassays for the determination of dioxins and related compounds in food. A newly developed CALUX (Chemical‐Activated LUciferase gene expression) bioassay was tested for its possible use to determine low levels of dioxins in bovine milk. Data show that this mammalian cell‐based test is very sensitive for 2,3,7,8‐substituted dioxins and related PCBs, thereby reflecting the relative potencies of these compounds in comparison to TCDD (TEF‐values). The limit of detection was about 50 fg of TCDD. Furthermore, the response obtained with a mixture of dioxins was additive, in accordance with the TEF‐principle. Milk fat was isolated by centrifugation followed by clean‐up of the fat with n‐pentane, removal of the fat on a 33% H2SO4 silica column, and determination of Ah receptor agonist activity with the CALUX‐bioassay. An equivalent of 67 mg fat was tested per experimental unit, resulting in a limit of quantification around 1 pg i‐TEQ/g fat. To investigate the performance of the method, butter fat was cleaned and spiked with a mixture of 17 different 2,3,7,8‐substituted PCDD and PCDF congeners at 1, 3, 6, 9, 12 and 15 pg TEQ/g fat, as confirmed by GC/MS. In this concentration range, the method showed a recovery of TEQs around 67% (58–87%). The reproducibility, determined in three independent series showed a CV varying between 4% and 54%, with the exception of the sample spiked at 1 pg i‐TEQ (CV 97%). The repeatability determined with the sample spiked at 6pg i‐TEQ/g showed a CV of 10%. Testing of 22 bovine milk samples, taken at different sites in The Netherlands, in the CALUX‐assay showed combined dioxin and dioxin‐like PCB levels equivalent to 1.6 pg TCDD/g fat (range 0.2–4.6). GC/MS analysis of these samples revealed an average level of 1.7 pg i‐TEQ/g fat, varying between 0.5 and 4.7 pg i‐TEQ/g fat. All five samples showing a GC/MS determined dioxin content of more than 2 pg i‐TEQ/g fat gave a response in the CALUX‐assay corresponding to more than 2 pg TCDD/g fat. These data clearly show that the CALUX‐bioassay is a promising method for the rapid and low cost screening of dioxins in bovine milk.

6-Substituted 3,4-Benzocoumarins: A New Structural Class of Inducers and Inhibitors of CYP1A1-Dependent Activity

A new synthetic route was utilized to prepare 6-substituted 3 ,4-benzocoumarins where the substituents were iodo, fluoro, trifluoromethyl, bromo, chioro, isopropyl, ethyl, t-butyl, methyl, hydrogen, amino, phenyl, or nitro; 3,4-naphthocoumarin was also synthesized. The relative affinities of these congeners for the aryl hydrocarbon (Ah) receptor were determined using rat hepatic cytosol and 2,3,7,8-[ 3H]tetrachlorodibenzo- p-dioxin ([ 3H]TCDD) as the radioligand. In addition, the Ah receptor agonist activity of the 6-substituted 3,4-benzocoumarins was determined from their concentration-dependent induction of ethoxyresorufin O-deethylase (EROD) activity. In contrast with many other structural classes of halogenated aromatics, there was not a correlation between the structure-binding versus structure induction relationships for the 6-substituted 3,4-benzocoumarins. These results suggested that some of these congeners may exhibit partial Ah receptor antagonist activities and this was investigated by determining the inhibitory effects of 6-substituted 3,4-benzocoumarins on TCDD-induced EROD activity in rat hepatoma H4II E cells in culture. Only four compounds (6-isopropyl, 6-phenyl, 6-fluoro, and 6- t-butyl) inhibited the TCDD-induced response (21.7 to 64.4% inhibition) and the mechanism of action of the most active inhibitor, 6- t-butyl-3,4-benzocoumarin, was further investigated. In contrast, with other partial Ah receptor antagonists such as α-naphthoflavone, cotreatment of rat hepatoma H4II E cells with 1 nM TCDD plus 1 and 10 μM 6- t-butyl-3,4-benzocoumanin did not result in decreased levels of the Ah receptor complex (liganded with TCDD). In addition, there was not significant inhibition of TCDD-induced CYPlAl mRNA levels or protein as determined by Northern and Western blot analyses. The results suggest that 6- t-butyl-3,4-benzocoumanin or one of its metabolites is a post-translational inhibitor of CYPlAl-dependent enzyme (EROD) activity in this cell line and thus represents a novel Ah receptor-independent inhibition of CYPlAl.

Effects of polychlorinated dibenzofurans on compounds in hepatic DNA of female Sprague-Dawley rats: Structure dependence and mechanistic considerations

Previous work indicated that covalent age-dependent DNA modifications of endogenous origin termed I-compounds may represent useful biomarkers for tumor promotion/carcinogenesis, as various tumor promoters/carcinogens, including 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) and phenobarbital, reduce rat I-compound levels in liver, the target organ. The present study addressed the question as to whether polychlorinated dibenzofurans (PCDFs), which are related to TCDD and its congeners with regard to their toxic and biochemical properties, would also affect hepatic I-compound patterns and levels, and whether such effects would be chemical structure-dependent. Female Sprague-Dawley rats were treated once a week with a single dose (100 μg/kg) of 1,2,3,7,8-pentachlorodibenzofuran (1,2,3,7,8-PeCDF), 1,2,4,7,8-PeCDF, 2,3,4,7,8-PeCDF, or 2,3,4,6,7,8-hexachlorodibenzofuran (2,3,4,6,7,8-HeCDF) for 4 weeks and liver DNA was analyzed at the end of the last week by 32P-postlabeling assay. No carcinogen-DNA adducts were detected; however, levels of both non-polar and polar I-compounds were reduced in a structure-dependent manner. Potencies increased in the order, control (100%, 122 modifications in 10 9 DNA nucleotides = 1,2,4,7,8-PeCDF (104%) <1,2,3,7,8-PeCDF (80%) <2,3,4,7,8-PeCDF (61%) and 2,3,4,6,7,8-HeCDF (61%). Structure-activity relationships for total I-compounds, therefore, paralleled those reported for Ah receptor agonist activity, i.e., compounds that exhibit high cytosolic Ah receptor binding affinities and are also potent inducers of aryl hydrocarbon hydroxylase activity (1,2,3,7,8-PeCDF, 2,3,4,7,8-PeCDF, and 2,3,4,6,7,8-HeCDF) were active, while 1,2,4,7,8-PeCDF, which is a less potent Ah receptor agonist, was inactive. Polar I-compounds responded to a greater extent than did non-polar ones and, in general, individual I-compounds were affected differentially, thus decreased formation or increased removal of I-compounds played a role in the observed effects of the toxins on DNA. It is proposed that Ah receptor-mediated enzyme induction, particularly of cytochrome P450, is involved in reduced hepatic I-compound formation and that subnormal I-compound levels may contribute to tumor promotion.

Mechanism of Action of α-Naphthoflavone as an Ah Receptor Antagonist in MCF-7 Human Breast Cancer Cells

α-Naphthotlavone (αNF) and 6-methyl-1,3,8-trichlorodiben-zofuran (MCDF) inhibited 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced CYP1A1 gene expression in MCF-7 human breast cancer cells and also decreased the accumulation of the nuclear [3H]TCDD-aryl hydrocarbon (Ah) receptor complex. Nuclear extracts from cells treated with 10−6 M αNF and incubated with a dioxin responsive element (DRE, 26-mer) did not form a retarded band in a gel mobility shift assay. In contrast, incubation of nuclear extracts from cells treated with 10−6 M MCDF and DRE gave a retarded band and this is consistent with the antiestrogenic and Ah receptor agonist activity of MCDF in human breast cancer cells. αNF was further investigated as an Ah receptor antagonist by determining the inhibition by αNF of TCDD-induced antiestrogenicity in MCF-7 cells. TCDD (10−9 M) inhibited the 17β-estradiol-induced proliferation of MCF-7 cells and the secretion of the 52-kDa protein. In cotreatment studies, αNF (10−8 to 10−6 M) caused a concentration-dependent decrease in the antiestrogenic responses elicited by TCDD. In addition, αNF inhibited the TCDD-induced down-regulation of nuclear estrogen receptor levels in MCF-7 cells. αNF (10−6 M) alone was inactive as an estrogen or antiestrogen and in cotreatment studies did not affect 17β-estradiol-induced responses in MCF-7 cells. Tamoxifen (10−7 M), an antiestrogen which acts through the estrogen receptor, also inhibited 17β-estradiol-induced cell proliferation and aNF did not affect the tamoxifen-mediated antiproliferative response. Thus, aNF antagonized TCDD-induced CYP1A1 gene expression in MCF-7 cells and also acted as an anti-antiestrogen for TCDDmediated antiestrogemcity in these cells. These results were consistent with the low levels of DRE binding observed with nuclear extracts from cells treated with 10−9 M TCDD plus αNF (10−8 to 10−6 M) or 10−6 M αNF alone. Thus, αNF appears to act as an Ah receptor antagonist in MCF-7 cells by decreasing the levels of transcriptionally active nuclear Ah receptor complexes.

Toxicology, structure-function relationship, and human and environmental health impacts of polychlorinated biphenyls: progress and problems.

Polychlorinated biphenyls (PCBs) are industrial compounds that have been detected as contaminants in almost every component of the global ecosystem including the air, water, sediments, fish, and wildlife and human adipose tissue, milk, and serum. PCBs in commercial products and environmental extracts are complex mixtures of isomers and congeners that can now be analyzed on a congener-specific basis using high-resolution gas chromatographic analysis. PCBs are metabolized primarily via mixed-function oxidases into a broad spectrum of metabolites. The results indicate that metabolic activation is not required for PCB toxicity, and the parent hydrocarbons are responsible for most of the biochemical and toxic responses elicited by these compounds. Some of these responses include developmental and reproductive toxicity, dermal toxicity, endocrine effects, hepatotoxicity, carcinogenesis, and the induction of diverse phase I and phase II drug-metabolizing enzymes. Many of the effects observed for the commercial PCBs are similar to those reported for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds. Structure-function relationships for PCB congeners have identified two major structural classes of PCBs that elicit "TCDD-like" responses, namely, the coplanar PCBs (e.g., 3,3',4,4'-tetraCB, 3,3'4,4',5-pentaCB and 3,3',4,4',5,5'-hexaCB) and their mono-ortho coplanar derivatives. These compounds competitively bind to the TCDD or aryl hydrocarbon (Ah) receptor and exhibit Ah receptor agonist activity. In addition, other structural classes of PCBs elicit biochemical and toxic responses that are not mediated through the Ah receptor. The shor-term effects of PCBs on occupationally exposed humans appear to be reversible, and no consistent changes in overall mortality and cancer mortality have been reported. Recent studies have demonstrated that some developmental deficits in infants and children correlated with in utero exposure to PCBs; however, the etiologic agent(s) or structural class of PCBs responsible for these effects have not been delineated. In contrast, based on a toxic equivalency factor approach, the reproductive and developmental problems in certain wildlife populations appear to be related to the TCDD-like PCB congeners.

Toxicology, Structure-Function Relationship, and Human and Environmental Health Impacts of Polychlorinated Biphenyls: Progress and Problems

Polychlorinated biphenyls (PCBs) are industrial compounds that have been detected as contaminants in almost every component of the global ecosystem including the air, water, sediments, fish, and wildlife and human adipose tissue, milk, and serum. PCBs in commercial products and environmental extracts are complex mixtures of isomers and congeners that can now be analyzed on a congener-specific basis using high-resolution gas chromatographic analysis. PCBs are metabolized primarily via mixed-function oxidases into a broad spectrum of metabolites. The results indicate that metabolic activation is not required for PCB toxicity, and the parent hydrocarbons are responsible for most of the biochemical and toxic responses elicited by these compounds. Some of these responses include developmental and reproductive toxicity, dermal toxicity, endocrine effects, hepatotoxicity, carcinogenesis, and the induction of diverse phase I and phase II drug-metabolizing enzymes. Many of the effects observed for the commercial PCBs are similar to those reported for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds. Structure-function relationships for PCB congeners have identified two major structural classes of PCBs that elicit "TCDD-like" responses, namely, the coplanar PCBs (e.g., 3,3',4,4'-tetraCB, 3,3'4,4',5-pentaCB and 3,3',4,4',5,5'-hexaCB) and their mono-ortho coplanar derivatives. These compounds competitively bind to the TCDD or aryl hydrocarbon (Ah) receptor and exhibit Ah receptor agonist activity. In addition, other structural classes of PCBs elicit biochemical and toxic responses that are not mediated through the Ah receptor. The shor-term effects of PCBs on occupationally exposed humans appear to be reversible, and no consistent changes in overall mortality and cancer mortality have been reported. Recent studies have demonstrated that some developmental deficits in infants and children correlated with in utero exposure to PCBs; however, the etiologic agent(s) or structural class of PCBs responsible for these effects have not been delineated. In contrast, based on a toxic equivalency factor approach, the reproductive and developmental problems in certain wildlife populations appear to be related to the TCDD-like PCB congeners.

Structure-dependent reduction in DNA I-compound adducts by polychlorinated dibenzofurans in female Sprague-Dawley rat liver

The effects of polychlorinated dibenzofurans (PCDFs) on hepatic I-compound DNA adducts in female Sprague-Dawley rats were determined at 2 dose levels. In the first experiments, rats (4 per group) were treated with each of 3 polychlorinated dibenzofuran (PCDF) congeners at 20 μg/kg/week or 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) at 10 μg/kg/week for 4 weeks. Effects on total I-compound levels when compared with age-matched control animals (corn oil) were as follows: control (116) > 1,2,4,7,8-pentachlorodibenzofuran (PeCDF) (112) > 1,2,3,7,8-PeCDF (93) > 2,3,4,7,8-PeCDF (91) > TCDD (72) where the numbers in parentheses were the number of I-compounds in 10 9 DNA nucleotides. In the second experiment, rats (5 per group) were treated with each of the congeners at a higher dose of 100 μg/kg/week for 4 weeks. The effects were as follows: control (123) ≈ 1,2,4,7,8-PeCDF (125) > 1,2,3,7,8-PeCDF (98) > 2,3,4,7,8-PeCDF (75) ≈ 2,3,4,6,7,8-hexachlorodibenzofuran (HCDF) (75). The active compounds (1,2,3,7,8-PeCDF, 2,3,4,7,8-PeCDF and 2,3,4,6,7,8-HCDF) were those that exhibited high cytosolic Ah receptor binding affinities (1.5 – 7.5 × 10 −8 M) and were also potent AHH inducers. The inactive compound (1,2,4,7,8-PeCDF) was less active in the receptor binding and enzyme induction assays. The results showed that there was a structure-dependent correlation between the Ah receptor agonist activity of a congener and its potency to reduce I-compounds in female Sprague-Dawley rat liver.