Structure-dependent induction of CYP2B by polychlorinated biphenyl congeners in female Sprague-Dawley rats

Assessment of binding potencies of polychlorinated biphenyls and polybrominated diphenyl ethers with Baikal seal and mouse constitutive androstane receptors: Comparisons across species and congeners

The present study evaluated the binding potencies (equilibrium dissociation constant: K_D) of polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) with the constitutive androstane receptor (CAR)_ligand binding domain (LBD) of the Baikal seal (bsCAR_LBD) and mouse (mCAR_LBD) using a surface plasmon resonance (SPR) biosensor. The binding affinities of individual congeners with mCAR_LBD tended to be higher than those with bsCAR_LBD but the differences were within the same order of magnitude. Notably, PBDE congeners showed higher binding affinities for both CAR_LBDs than PCB congeners. In silico docking simulations demonstrated that PBDEs had more non-covalent interactions with specific amino acid residues in both CAR_LBDs than PCBs, supporting the results of their binding affinities. Binding affinity comparisons among congeners revealed the structural requirements for higher binding; mono or di ortho-, tri meta-, and di para‑chlorine substitutions for PCBs, and di or tri ortho-, mono meta-, and di para‑bromine substitutions for PBDEs. The binding potencies of these congeners unlikely accounted for their previously reported CAR-mediated transactivation potencies, implying that their transactivation is regulated in a ligand-dependent, but a distinct manner from ligand binding. Risk assessment analysis showed that the K_D values of individual PCB and PBDE congeners were 1-4 orders of magnitude higher than their respective hepatic concentrations in wild Baikal seal population.

Tissue distribution and characterization of feline cytochrome P450 genes related to polychlorinated biphenyl exposure

Cats have been known to be extremely sensitive to chemical exposures. To understand these model species' sensitivity to chemicals and their toxicities, the expression profiles of xenobiotic-metabolizing enzymes should be studied. Unfortunately, the characterization of cytochrome P450 (CYP), the dominant enzyme in phase I metabolism, in cats has not extensively been studied. Polychlorinated biphenyls (PCBs) are known as CYP inducers in animals, but the information regarding the PCB-induced CYP expression in cats is limited. Therefore, in the present study, we aimed to elucidate the mRNA expression of the CYP1-CYP3 families in the cat tissues and to investigate the CYP mRNA expression related to PCB exposure. In cats, the greatest abundance of CYP1-CYP3 (CYP1A2, CYP2A13, CYP2C41, CYP2D6, CYP2E1, CYP2E2, CYP2F2, CYP2F5, CYP2J2, CYP2U1, and CYP3A132) was expressed in the liver, but some extrahepatic isozymes were found in the kidney (CYP1A1), heart (CYP1B1), lung (CYP2B11 and CYP2S1) and small intestine (CYP3A131). In cats, CYP1A1, CYP1A2 and CYP1B1 were significantly upregulated in the liver as well as in several tissues exposed to PCBs, indicating that these CYPs were distinctly induced by PCBs. The strong correlations between 3,3',4,4'-tetrachlorobiphenyl (CB77) and CYP1A1 and CYP1B1 mRNA expressions were noted, demonstrating that CB77 could be a potent CYP1 inducer. In addition, these CYP isoforms could play an essential role in the PCBs biotransformation, particularly 3-4 Cl-PCBs, because a high hydroxylated metabolite level of 3-4 Cl-OH-PCBs was observed in the liver.

Polychlorinated biphenyls and breast cancer: A congener-specific meta-analysis

The incidence of breast cancer is related to various risk factors, especially that the environmental and lifestyle factors account for major contribution at the rate of 70% to 95% over all. However, there still remains some controversy over the epidemiological evidence regarding the effects of environmental carcinogens on the risk of breast cancer. We conducted a quantitative meta-analysis aiming at full evaluation of the effects of polychlorinated biphenyls (PCBs) on breast cancer in a congener-specific fashion. Four online literature databases were systematically searched before 1st January 2015, for studies stating correlation between PCB congeners and breast cancer. The Newcastle-Ottawa Scale was used to evaluate the quality of the studies that were included in our analysis. Sixteen studies were included in our final meta-analysis after screening based on the priori inclusion criteria. Nine PCB congeners were reported by more than two studies and they were presented in detail. The pooled Odds Ratios (ORs) showed a significant increase in the risk of breast cancer in individuals with higher plasma/fat levels of PCB 99 (OR: 1.36; 95% CI: 1.02 to 1.80), PCB 183 (OR: 1.56; 95% CI: 1.25 to 1.95) and PCB 187 (OR: 1.18; 95% CI: 1.01 to 1.39). Besides, the outcomes did not support a relationship between dioxin-like PCB congeners and the risk of breast cancer. The results of our meta-analysis imply that PCB 99, PCB 183 and PCB 187 would increase the risk of breast cancer. The mechanism of this increased risk may be by the induction of the CYP2B family in cytochrome P450 enzymes.

Subchronic inhalation exposure study of an airborne polychlorinated biphenyl mixture resembling the Chicago ambient air congener profile

Although inhalation of atmospheric polychlorinated biphenyls (PCBs) is the most universal exposure route and has become a substantial concern in urban areas, research is lacking to determine the body burden of inhaled PCBs and consequent health effects. To reflect the Chicago airshed environment and mimic the PCB profile in Chicago air, we generated vapors from a Chicago air mixture (CAM). Sprague-Dawley rats were exposed to the CAM vapor for 1.6 h/day via nose-only inhalation for 4 weeks, 520 ± 10 μg/m(3). Congener-specific quantification in tissue and air samples was performed by gas chromatography-tandem mass spectrometry (GC/MS/MS). In contrast to the lower-chlorinated congener-enriched vapor, body tissues mainly contained tri- to hexachlorobiphenyls. Congener profiles varied between vapor and tissues and among different organs. The toxic equivalence (TEQ) and neurotoxic equivalence (NEQ) were also investigated for tissue distribution. We evaluated a variety of end points to catalogue the effects of long-term inhalation exposure, including immune responses, enzyme induction, cellular toxicity, and histopathologic abnormalities. Glutathione oxidized/reduced ratio (GSSG/GSH) was increased in the blood of exposed animals, accompanied by elevation of hematocrit. This study demonstrated that inhalation contributed to the body burden of mostly tri- to hexachlorobiphenyls and produced a distinct profile of congeners in tissue, yet minimal toxicity was found at this exposure dose, estimated at 134 μg/rat.

Bayesian Analysis of a Lipid-Based Physiologically Based Toxicokinetic Model for a Mixture of PCBs in Rats

A lipid-based physiologically based toxicokinetic (PBTK) model has been developed for a mixture of six polychlorinated biphenyls (PCBs) in rats. The aim of this study was to apply population Bayesian analysis to a lipid PBTK model, while incorporating an internal exposure-response model linking enzyme induction and metabolic rate. Lipid-based physiologically based toxicokinetic models are a subset of PBTK models that can simulate concentrations of highly lipophilic compounds in tissue lipids, without the need for partition coefficients. A hierarchical treatment of population metabolic parameters and a CYP450 induction model were incorporated into the lipid-based PBTK framework, and Markov-Chain Monte Carlo was applied to in vivo data. A mass balance of CYP1A and CYP2B in the liver was necessary to model PCB metabolism at high doses. The linked PBTK/induction model remained on a lipid basis and was capable of modeling PCB concentrations in multiple tissues for all dose levels and dose profiles.

Differences in cytochrome P450 enzyme activities between fish and crustacea: relationship with the bioaccumulation patterns of polychlorobiphenyls (PCBs)

Variations in cytochrome P450 enzyme (CYPs) distribution and function between animal groups could result in differential metabolism and elimination kinetics for certain contaminants. Although a number of studies have suggested that differences in polychlorobiphenyl (PCB) accumulation profiles between crustacea and fish might result from differential CYP patterns, the relationship between PCB bioaccumulation and CYP capacities has not been demonstrated in these organisms. In the present study we investigated the hepatic microsomal catalytic activities in three deep-sea fish species, Alepocephalus rostratus (Alepocephalidae), Coelorinchus mediterraneus (Macrouridae), and Lepidion lepidion (Moridae), and the decapod crustacean Aristeus antennatus (Decapoda), using six fluorescent CYP-mediated substrates, namely ER (7-ethoxyresorufin), PR (7-pentoxyresorufin), BR (7-benzyloxyresorufin), CEC (3-cyano-7-ethoxycoumarin), DBF (dibenzylfluorescein) and BFC (7-benzyloxy-4-trifluoromethylcoumarin). Furthermore, we related the metabolic activities to the accumulation patterns of 41 PCB congeners in the muscle of these organisms. The results indicated a marked difference in the presence and activities of CYP isoforms between fish and the crustacean A. antennatus. Liver microsomes of the three selected fish species were capable of metabolizing all six CYP-mediated substrates and enzymes were identified as primarily belonging to CYP1A and CYP3A subfamilies. In contrast, hepatopancreas microsomes from A. antennatus only showed activity for PR and DBF substrates, generally related to mammalian CYP2-like enzymes. Furthermore, a direct relationship between metabolic activities and PCB accumulation profiles could be established. Results revealed that A. antennatus accumulated significantly higher proportions of PCBs 28, 52, 118, 138, 158 and 169 than fish, which is in accordance with the previously observed lack of CYP1A-like biotransformation capacities. Moreover, A. antennatus exhibited lower levels of PCBs 87, 149, 153, 170, 180, 183, 194 and 206 indicating that this crustacean is able to metabolize congeners considered mammalian CYP2B inducers. Hence, the present findings highlight the role of CYP-mediated metabolism in the congener-specific accumulation of PCBs in aquatic organisms and stress the need to further investigate quantitative and qualitative differences in xenobiotic metabolism among animal groups.

Cytochrome P450 CYP2 genes in the common cormorant: Evolutionary relationships with 130 diapsid CYP2 clan sequences and chemical effects on their expression

Cytochrome P450 CYP2 family enzymes are important in a variety of physiological and toxicological processes. CYP2 genes are highly diverse and orthologous relationships remain clouded among CYP2s in different taxa. Sequence and expression analyses of CYP2 genes in diapsids including birds and reptiles may improve understanding of this CYP family. We sought CYP2 genes in a liver cDNA library of the common cormorant (Phalacrocorax carbo), and in the genomes of other diapsids, chicken (Gallus gallus), zebra finch (Taeniopygia guttata), and anole lizard (Anolis carolinensis), for phylogenetic and/or syntenic analyses. Screening of the cDNA library yielded four CYP2 cDNA clones that were phylogenetically classified as CYP2C45, CYP2J25, CYP2AC1, and CYP2AF1. There are numerous newly identified diapsid CYP2 genes that include genes related to the human CYP2Cs, CYP2D6, CYP2G2P, CYP2J2, CYP2R1, CYP2U1, CYP2W1, CYP2AB1P, and CYP2AC1P. Syntenic relationships show that avian CYP2Hs are orthologous to CYP2C62P in humans, CYP2C23 in rats, and Cyp2c44 in mice, and suggest that avian CYP2Hs, along with human CYP2C62P and mouse Cyp2c44, could be renamed as CYP2C23, based upon the nomenclature rules. Analysis of sequence and synteny identifies cormorant and finch CYPs that are apparent orthologs of phenobarbital-inducible chicken CYP2C45. Transcripts of all four cormorant CYP2 genes were detected in the liver of birds from Lake Biwa, Japan. The transcript levels bore no significant relationship to levels of chlorinated organic pollutants in the liver, including polychlorinated biphenyls and dichlorodiphenyltrichloroethane and its metabolites. In contrast, concentrations of perfluorooctane sulfonate and perfluorononanoic acid were negatively correlated with levels of CYP2C45 and/or CYP2J25, suggesting down-regulation of expression by these environmental pollutants. This study expands our view of the phylogeny and evolution of CYP2s, and provides evolutionary insight into the chemical regulation of CYP2 gene expression in diapsids including birds.

Differential effects of polychlorinated biphenyl congeners on serum thyroid hormone levels in rats

Polychlorinated biphenyls (PCBs) are known to reduce serum thyroxine (T(4)) in rats, but the relative effects of individual PCB congeners on thyroid hormones are not known. Thus, male Sprague-Dawley rats were administered Aroclor 1254, Aroclor 1242 (4, 8, 16, or 32 mg/kg/day), PCB 95 (2,2',3,5',6-pentachlorobiphenyl), PCB 99 (2,2',4,4',5-pentachlorobiphenyl), PCB 118 (2,3',4,4',5-pentachlorobiphenyl) (2, 4, 8, or 16 mg/kg/day), PCB 126 (3,3'4,4',5-pentachlorobiphenyl) (2.5, 5, 10, 20, or 40 microg/kg/day), TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) (0.14, 0.43, 1.3, or 3.9 microg/kg/day), or corn oil via oral gavage for 7 days. Rats were necropsied 24 h after the last dose. Serum thyroid hormone levels were evaluated by radioimmunoassay, and induction of hepatic Cyp1a (a TCDD-inducible protein) and Cyp2b (a phenobarbital [PB]-inducible protein) activity was determined by ethoxyresorufin-O-deethylase and pentoxyresorufin-O-deethylase assays, respectively. Significant increases in Cyp1a activity occurred in response to PCBs, except PCB 95 and PCB 99. Aroclor 1254, PCB 99, and PCB 118 significantly induced Cyp2b activity. Serum total T(4) and free T(4) were dramatically reduced in response to each of the seven treatments in a dose-dependent manner. The marked T(4) reductions occurred in response to Aroclor 1254, PCB 99 (a PB-type congener), and PCB 118 (a mixed-type congener). In contrast, reductions in serum triiodothyronine (total and free) were variable and mild, and serum thyroid-stimulating hormone was not significantly affected by any of the compounds. These data indicate that the PB and mixed-type PCB congeners are more effective than the TCDD-type PCB congeners at reducing serum T(4).

PCB153-elicited hepatic responses in the immature, ovariectomized C57BL/6 mice: comparative toxicogenomic effects of dioxin and non-dioxin-like ligands

Polychlorinated biphenyls (PCBs) are ubiquitous contaminants found as complex mixtures of coplanar and non-coplanar congeners. The hepatic temporal and dose-dependent effects of the most abundant non-dioxin-like congener, 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153), were examined in immature, ovariectomized C57BL/6 mice, and compared to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the prototypical aryl hydrocarbon receptor (AhR) ligand. Animals were gavaged once with 300 mg/kg PCB153 or sesame oil vehicle and sacrificed 4, 12, 24, 72 or 168 h post dose. In the dose-response study, mice were gavaged with 1, 3, 10, 30, 100 or 300 mg/kg PCB153 or sesame oil for 24 h. Significant increases in relative liver weights were induced with 300 mg/kg PCB153 between 24 and 168 h, accompanied by slight vacuolization and hepatocellular hypertrophy. The hepatic differential expression of 186 and 177 genes was detected using Agilent 4 x 44 K microarrays in the time course (|fold change|> or =1.5, P1(t)> or =0.999) and dose-response (|fold change|> or =1.5, P1(t)> or =0.985) studies, respectively. Comparative analysis with TCDD suggests that the differential gene expression elicited by PCB153 was not mediated by the AhR. Furthermore, constitutive androstane and pregnane X receptor (CAR/PXR) regulated genes including Cyp2b10, Cyp3a11, Ces2, Insig2 and Abcc3 were dose-dependently induced by PCB153. Collectively, these results suggest that the hepatocellular effects elicited by PCB153 are qualitatively and quantitatively different from TCDD and suggestive of CAR/PXR regulation.

Transactivation potencies of Baikal seal constitutive active/androstane receptor by persistent organic pollutants and brominated flame retardants

To characterize ligand-dependent transcriptional activation of constitutive active/androstane receptor (CAR) in aquatic mammals, transactivation potentials of the Baikal seal (Pusa sibirica) CAR (bsCAR) by environmental pollutants, including persistent organic pollutants (POPs) and brominated flame retardants (BFRs), were investigated using an in vitro reporter gene assay, and compared with those of the mouse CAR (mCAR). Measurement of luciferase reporter gene activities demonstrated that the seal CAR was activated by POPs, including a technical mixture of PCBs (Kanechlor-500), certain individual PCB congeners, DDT compounds, and trans-nonachlor. No or slight bsCAR-dependent activity was detected in experiments with PBDE congeners and HBCDs. The interspecies comparison of lowest observed effect concentration (LOEC) for CAR transactivation byeach compound revealed that bsCAR responds more sensitively to PCBs than mCAR. In addition, bsCAR was weakly deactivated by PBDE99, whereas mCAR transcriptional activity decreased weakly by PBDE100, PBDE154, and PBDE187. Comparison of reporter gene activities by the congeners with the same IUPAC numbers among PCBs and PBDEs revealed that both bsCAR and mCAR were not activated by PBDE99 and PBDE153, but were activated by PCB99 and PCB153. The small ligand-binding pocket in CAR may contribute to difference in response between PCBs and PBDEs. Given that ethical rationale prevents dosing studies with such organohalogens in aquatic mammals, our in vitro assay system constructed with CAR cDNA from a species of interest provides a useful and realistic alternative approach in ecotoxicology.

Activation of CAR and PXR by Dietary, Environmental and Occupational Chemicals Alters Drug Metabolism, Intermediary Metabolism, and Cell Proliferation

The constitutive androstane receptor (CAR) and the pregnane × receptor (PXR) are activated by a variety of endogenous and exogenous ligands, such as steroid hormones, bile acids, pharmaceuticals, and environmental, dietary, and occupational chemicals. In turn, they induce phase I-III detoxification enzymes and transporters that help eliminate these chemicals. Because many of the chemicals that activate CAR and PXR are environmentally-relevant (dietary and anthropogenic), studies need to address whether these chemicals or mixtures of these chemicals may increase the susceptibility to adverse drug interactions. In addition, CAR and PXR are involved in hepatic proliferation, intermediary metabolism, and protection from cholestasis. Therefore, activation of CAR and PXR may have a wide variety of implications for personalized medicine through physiological effects on metabolism and cell proliferation; some beneficial and others adverse. Identifying the chemicals that activate these promiscuous nuclear receptors and understanding how these chemicals may act in concert will help us predict adverse drug reactions (ADRs), predict cholestasis and steatosis, and regulate intermediary metabolism. This review summarizes the available data on CAR and PXR, including the environmental chemicals that activate these receptors, the genes they control, and the physiological processes that are perturbed or depend on CAR and PXR action. This knowledge contributes to a foundation that will be necessary to discern interindividual differences in the downstream biological pathways regulated by these key nuclear receptors.

Effect of PCB153 on BaP-induced genotoxicity in HepG2 cells via modulation of metabolic enzymes

Benzo(a)pyrene (BaP) is a representative environmental carcinogen and is metabolically activated by several cytochrome P450 (CYP) enzymes to become the ultimate carcinogen. Numerous studies have indicated that 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153) could effectively alter the activity of xenobiotic metabolizing enzymes (XMEs). Therefore, we propose that PCB153 may affect BaP-induced genotoxicity mediated by XMEs. In the present study, we treated HepG2 cells with BaP (50 microM) or PCB153 (0.1, 1, 10 and 100 microM), or pretreated the cells with PCB153 for 48 h followed by treatment with a combination of both BaP and PCB153. CYP1A1 activity was dramatically increased in cells treated with either BaP or PCB153. Glutathione-S-transferase (GST) activity was increased in BaP-treated cells, but decreased in PCB153-treated cells. In parallel to studies on enzyme activity, the micronuclei (MN) assay was used to assess the genotoxic damage caused by BaP and PCB153. BaP and PCB153 at 100 microM enhanced MN formation. In contrast to BaP treatment alone, treatment with both BaP and PCB153 significantly enhanced the activity of CYP1A1 and the formation of MN, but reduced the activity of GST. alpha-Naphthoflavone (ANF), an inhibitor of CYP1A1, inhibited MN formation in the presence of both BaP and PCB153. In addition, there was a positive correlation between CYP1A activity and MN formation (r(2)=0.794, P<0.001). Our observations suggest that co-exposure to BaP and PCB153 may increase BaP-induced genotoxicity, possibly through the induction of CYP1A1 and inhibition of GST.

Effects of in utero exposure to 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153) on somatic growth and endocrine status in rat offspring

Exposure to polychlorobiphenyl (PCB) mixtures at an early stage of development has been reported to affect endocrine glands; however, little is known about the precise toxicological properties of individual PCB. The present study was undertaken to determine whether prenatal exposure to 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153), a di-ortho-substituted non-coplanar congener, affects postnatal development in rat offspring. Pregnant Sprague-Dawley rats (Crj: CD (SD) IGS) were given PCB 153 (0, 16, or 64 mg/kg/day) orally from gestational day (GD) 10 through GD 16, and developmental parameters in the male and female offspring were examined. We found no dose-dependent changes in body weight, body length (nose-anus length), tail length, or the weights of kidneys, testes, ovaries and uterus in offspring at 1 or 3 weeks of age. Liver weights were increased in the PCB 153-treated groups, although we observed a significant difference only in males. Anogenital distance was unaffected in the PCB 153-treated groups. We observed a significant dose-dependent decrease in the plasma concentrations of thyroxine and tri-iodothyronine, whereas those of thyroid-stimulating hormone were not significantly changed. In addition, there were no dose-dependent changes in plasma concentrations of growth hormone and insulin-like growth factor-I in any dose group. These findings suggest that prenatal exposure to PCB 153 (GD 10-16, 16-64 mg/kg/day) may alter the thyroid status in rat offspring to some extent without affecting somatic growth or its related hormonal parameters.

Induction of hepatic cytochrome P450 enzymes: methods, mechanisms, recommendations, and in vitro-in vivo correlations

Induction of drug-clearance pathways (Phase 1 and 2 enzymes and transporters) can have important clinical consequences. Inducers can (1) increase the clearance of other drugs, resulting in a decreased therapeutic effect, (2) increase the activation of pro-drugs, causing an alteration in their efficacy and pharmacokinetics, and (3) increase the bioactivation of drugs that contribute to hepatotoxicity via reactive intermediates. Nuclear receptors are key mediators of drug-induced changes in the expression of drug-clearance pathways. However, species differences in nuclear receptor activation make the prediction of cytochrome P450 (CYP) induction in humans from data derived from animal models problematic. Thus, in vitro human-relevant model systems are increasingly used to evaluate enzyme induction. In this review, the authors' current understanding of the mechanisms of enzyme induction and the in vitro methods for assessing the induction potential of new drugs will be discussed. Relevant issues and considerations surrounding proper study design and the interpretation of in vitro results will be discussed in light of the current US Food and Drug Administration (FDA) recommendations.

Relative potency based on hepatic enzyme induction predicts immunosuppressive effects of a mixture of PCDDS/PCDFS and PCBS

The toxic equivalency factor (TEF) approach was employed to compare immunotoxic potency of mixtures containing polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans and polychlorinated biphenyls relative to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), using the antibody response to sheep erythrocytes (SRBC). Mixture-1 (MIX-1) contained TCDD, 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), 1,2,3,7,8-pentachlorodibenzofuran (1-PeCDF), 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF), and 1,2,3,4,6,7,8,9-octachlorodibenzofuran (OCDF). Mixture-2 (MIX-2) contained MIX-1 and the following PCBs, 3,3',4,4'-tetrachlorobiphenyl (IUPAC No. 77), 3,3',4,4',5-pentachlorobiphenyl (126), 3,3',4,4',5,5N-hexachlorobiphenyl (169), 2,3,3',4,4'-pentachlorobiphenyl (105), 2,3',4,4',5-pentachlorobiphenyl (118), and 2,3,3',4,4',5-hexachlorobiphenyl (156). The mixture compositions were based on relative chemical concentrations in food and human tissues. TCDD equivalents (TEQ) of the mixture were estimated using relative potency factors from hepatic enzyme induction in mice [DeVito, M.J., Diliberto, J.J., Ross, D.G., Menache, M.G., Birnbaum, L.S., 1997. Dose-response relationships for polyhalogenated dioxins and dibenzofurans following subchronic treatment in mice. I .CYP1A1 and CYP1A2 enzyme activity in liver, lung and skin. Toxicol. Appl. Pharmacol. 130, 197-208; DeVito, M.J., Menache, G., Diliberto, J.J., Ross, D.G., Birnbaum L.S., 2000. Dose-response relationships for induction of CYP1A1 and CYP1A2 enzyme activity in liver, lung, and skin in female mice following subchronic exposure to polychlorinated biphenyls. Toxicol. Appl. Pharmacol. 167, 157-172] Female mice received 0, 1.5, 15, 150 or 450 ng TCDD/kg/day or approximately 0, 1.5, 15, 150 or 450 ng TEQ/kg/day of MIX-1 or MIX-2 by gavage 5 days per week for 13 weeks. Mice were immunized 3 days after the last exposure and 4 days later, body, spleen, thymus, and liver weights were measured, and antibody response to SRBCs was observed. Exposure to TCDD, MIX-1, and MIX-2 suppressed the antibody response in a dose-dependent manner. Two-way ANOVA indicated no differences in the response between TCDD and the mixtures for body weight, spleen/body weight and decreased antibody responses. The results support the use of the TEF methodology and suggest that immune suppression by dioxin-like chemicals may be of concern at or near background human exposures.

Associations between prenatal exposure to polychlorinated biphenyls and neonatal thyroid-stimulating hormone levels in a Mexican-American population, Salinas Valley, California

BACKGROUND

Studies have reported that prenatal exposure to polychlorinated biphenyls (PCBs) may alter neurodevelopment in both humans and animals. Furthermore, prenatal exposure to some PCB congeners and commercial mixtures has been shown to decrease free and total thyroxine (T(4)) blood levels in animals. Because thyroid hormones (TH) are essential for normal neurologic development, it has been suggested that the deleterious neurodevelopmental effect of PCBs may occur through TH disruption. PCBs may in turn affect TH levels by inducing the microsomal enzyme uridinediphosphate glucuronosyltransferase (UDP-GT), which is involved in TH elimination.

OBJECTIVES

Our goals were to group PCB congeners based on their potential to induce microsomal enzymes in animals, and to examine the relationship between neonatal TSH levels and prenatal exposure to PCB congeners grouped according to their structure and potential mechanisms of action.

METHODS

We measured the concentration of 34 PCB congeners in serum samples collected from 285 pregnant women and the thyroid-stimulating hormone (TSH) levels in their children's blood collected shortly after birth.

RESULTS

We found no association between the sum of PCB congeners, the toxic equivalents, or structure-based groupings (mono- or di-ortho substituted congeners), and TSH blood concentration. However, we found a positive association between the sum of congeners suspected to be UDP-GT inducers (more specifically cytochrome P450 2B inducers) in animals and neonatal TSH levels. In individual congener analyses, PCBs 99, 138, 153, 180, 183, 187, 194, and 199 were positively associated with neonatal TSH levels after adjustment for covariates. PCBs 194 and 199 remained significant after adjustment for multiple hypothesis testing.

CONCLUSIONS

Our results support grouping PCB congeners based on their potential mechanism of action of enzyme induction when investigating associations with TH. Findings also suggest that PCBs affect TH homeostasis even at the low background level of exposure found in the CHAMA-COS (Center for the Health Assessment of Mothers and Children of Salinas) population.

The environmental estrogen, nonylphenol, activates the constitutive androstane receptor

Nonylphenol (NP) and its parent compounds, the nonylphenol ethoxylates are some of the most prevalent chemicals found in U.S. waterways. NP is also resistant to biodegradation and is a known environmental estrogen, which makes NP a chemical of concern. Our data show that NP also activates the constitutive androstane receptor (CAR), an orphan nuclear receptor important in the induction of detoxification enzymes, including the P450s. Transactivation assays demonstrate that NP increases murine CAR (mCAR) transcriptional activity, and NP treatment can overcome the inhibitory effects of the inverse agonist, androstanol, on mCAR activation. Treatment of wild-type (CAR +/+) mice with NP at 50 or 75 mg/kg/day increases Cyp2b protein expression in a dose-dependent manner as demonstrated by Western blotting, and was confirmed by quantitative reverse transcription-PCR of Cyp2b10 transcript levels. CAR-null (CAR -/-) mice show no increased expression of Cyp2b following NP treatment, indicating that CAR is required for NP-mediated Cyp2b induction. In addition, NP increases the translocation of CAR into the nucleus, which is the key step in the commencement of CAR's transcriptional activity. NP also induced CYP2B6 in primary human hepatocytes, and increased Cyp2b10 messenger RNA and protein expression in humanized CAR mice, indicating that NP is an activator of human CAR as well. In conclusion, NP is a CAR activator, and this was demonstrated in vitro with transactivation assays and in vivo with transgenic CAR mouse models.

Organochlorines and risk of non-Hodgkin lymphoma

Organochlorine chemicals and polychlorinated biphenyls (PCBs) have been suspected as possible risk factors for non-Hodgkin lymphoma (NHL). We investigated PCBs and organochlorine pesticides and risk of NHL in a population-based case-control study in British Columbia, Canada. Congeners of PCBs (including dioxinlike congeners) and pesticides or pesticide metabolites were measured in plasma of 422 pretreatment cases and 460 control subjects. This is so far the largest study to examine organochlorines in plasma to date. Several dioxin-like PCB congeners were associated with increased risk of NHL, including dioxin-like PCB nos. 118 and 156 with odds ratios (OR) for the highest versus lowest quartile between 1.6 and 1.8. Several non-dioxin-like congeners also showed significant associations. The PCB congener with the strongest association was no. 180 with an OR for the highest versus the lowest quartile of 1.83 (95% confidence interval = 1.18-2.84). Six pesticide analytes also showed a significant association with NHL; beta-hexachlorocyclohexane, p,p'-DDE, hexachlorobenzene, mirex, oxychlordane and trans-nonachlor. The strongest association was found for oxychlordane, a metabolite of the pesticide chlordane (highest vs. lowest quartile OR = 2.68, 95% confidence interval = 1.69-4.24). Our results provide further evidence that organochlorines contribute to NHL risk.

Carcinogenicity of "non-dioxinlike" polychlorinated biphenyls

Complex technical mixtures of polychlorinated biphenyls (PCBs) cause liver and thyroid neoplasms in rodents, whereas very few data are available on the carcinogenic potency of single non-dioxinlike (NDL) PCB congeners. In most genotoxicity assays technical PCB mixtures and individual congeners were inactive, suggesting that PCBs act as indirect, nongenotoxic carcinogens. Various mechanisms, including suppression of apoptosis in preneoplastic cells or inhibition of intercellular communication, have been suggested to be active in liver tumor promotion by PCBs. A decrease in thyroid hormone levels after PCB treatment has been suggested to play a role in the development of thyroid neoplasms in rats; however, other mechanisms may also be involved. Results from a chronic carcinogenicity study in rats indicate that not the dose of total PCBs but the total TCDD or toxic equivalents (TEQs) associated with "dioxinlike" (DL) constituents within a technical mixture are mainly if not exclusively responsible for the development of liver neoplasms in female rats. Quantitative comparison reveals almost identical dose-response curves for the total TEQs in various technical PCB mixtures and for TCDD as inducers of hepatic neoplasms in female rats. Tumor promotion experiments have shown, however, that, after initiation with a genotoxic carcinogen, technical PCB mixtures and individual DL-and NDL-PCBs act as liver tumor promoters in rodents. Based on these data, a weak carcinogenic potency of individual NDL-PCB congeners cannot be excluded. In epidemiological studies, increased mortality from cancers of the liver, gallbladder, biliary tract, gastrointestinal tract, and from brain cancer and malignant melanoma were observed in workers exposed to a series of technical PCB mixtures. A significant association between PCB concentrations in adipose tissue and non-Hodgkins lymphoma was found in another study. While in all human studies mixed exposure to DL-and NDL-PCBs occurred, no comprehensive data are available on the relative contribution of NDL-PCBs to the overall external and/or internal PCB exposure in those cohorts.

Induction of cytochromes P450, caspase-3 and DNA damage by PCB3 and its hydroxylated metabolites in porcine ovary

Polychlorinated biphenyl (PCBs) levels of tens and hundreds of pg/ml for individual congeners are measured in human follicular fluid. PCB3 (4-chlorobiphenyl), caused a significant increase in estradiol secretion in porcine granulose-theca cell co-cultures and its two metabolites, 4-OH-PCB3 and 3,4-diOH-PCB3, were even more potent than PCB3 itself [Ptak, A., Ludewig, G., Lehmler, H.J., Wojtowicz, A.K., Robertson, L.W., Gregoraszczuk, E.L. 2005. Comparison of the actions of 4-chlorobiphenyl and its hydroxylated metabolites on estradiol secretion by ovarian follicles in primary cells in culture. Reprod. Toxicol. 20, 57-64]. The question is whether these follicle cells are potentially able to metabolize PCB3 to hydroxylated and genotoxic or cytotoxic intermediates. We report here that granulose-theca co-cultures express xenobiotic-metabolizing cytochrome P450 activities, with CYP1A1>CYP2B>>CYP1A2. A significant increase in CYP1A1 and 2B, but not CYP1A2, activity was seen in cells that were exposed to 6 ng/ml PCB3 or 20 nM 17-beta-estradiol. An increase in caspase-3 activity, indicative for apoptosis, was only observed in PCB3-exposed cells after 24 h exposure. Genotoxicity, determined with the Comet assay, was initially reduced after 24 h exposure to PCB3 and both metabolites compared to untreated controls, followed by a significant transient increase in Comets at the 4 and 24 h time point with PCB3 and 4-OH-PCB3. 3,4-diOH-PCB3 induced a significant increase only after 72 h of recovery. We hypothesize that these biphasic damage kinetics may be due to cross-links caused by adduct formation. These results show for the first time that granulose-theca cells in co-culture express CYP1A1, 2B and 1A2 activities and that PCBs at concentrations that are reached in the environment induce genotoxicity in granulosa cells.

Tumor promoting potency of PCBs 28 and 101 in rat liver

Only few data are available on the carcinogenic potency of individual PCB congeners. In this study, we tested the 'non-dioxinlike' congeners PCB 28 and 101 for their potency as liver tumor promoters in female rats which received diethyl-N-nitrosamine as an initiator. After 8 or 16 weeks of PCB treatment (50 and 150 micromol/kg body weight per week), each congener was recovered in the liver according to the dose levels applied, with PCB 28, at the same dose level, showing nine- to 16-fold higher hepatic levels than PCB 101 (approximately, 44 micromol/kg versus 5 micromol/kg liver at low dose, 145 micromol/kg versus 9 micromol/kg liver at high dose). PCB 28 was found to mildly induce hepatic EROD activity, while both congeners induced PROD activity. With each congener, no significant increase in the number of ATPase-deficient or GSTP-positive preneoplastic foci was obtained, while a significant increase in the relative hepatic volume of ATPase-deficient foci was found in the livers of DEN pre-treated animals having received 50 micromol/kg body weight of PCB 101 per week over 16 weeks. Our results revealed that neither the accumulative PCB 28 nor the more readily metabolisable PCB 101 was an efficacious tumor promoter in the livers of female rats.

Induction of ROS formation, poly(ADP-ribose) polymerase-1 activation, and cell death by PCB126 and PCB153 in human T47D and MDA-MB-231 breast cancer cells

The primary purpose of this research is to investigate whether exposure to polychlorinated biphenyls (PCBs), i.e. PCB153 and PCB126, is associated with induction of reactive oxygen species (ROS), poly(ADP-ribose) polymerase-1 (PARP-1) activation, and cell death in human T47D and MDA-MB-231 breast cancer cells. Results indicated that PCB153 and PCB126 induced concentration- and time-dependent increases in cytotoxic response and ROS formation in both T47D and MDA-MB-231 cells. At non-cytotoxic concentrations both PCB153 and PCB126 induced decreases in intracellular NAD(P)H and NAD+ in T47D and MDA-MB-231 cells where T47D cells were more resistant to PCB-induced reduction in intracellular NAD(P)H than MDA-MB-231 cells. Further investigation indicated that three specific PARP inhibitors completely blocked PCB-induced decreases in intracellular NAD(P)H in both T47D and MDA-MB-231 cells. These results imply that decreases in intracellular NAD(P)H in PCB-treated cells may be, in part, due to depletion of intracellular NAD+ pool mediated by PARP-1 activation through formation of DNA strand breaks. Overall, the extent of cytotoxic response, ROS formation, and PARP-1 activation generated in T47D and MDA-MB-231 cells was greater for PCB153 than for PCB126. In addition, the cytotoxicity induced by PCB153 and PCB126 in both T47D and MDA-MB-231 cells was completely blocked by co-treatment of catalase, dimethylsulfoxide, cupper (I)-/iron (II)-specific chelators, and CYP1A/2B inhibitors. This evidence suggests the involvement of ROS, Cu(I), Fe(II), and CYP1A/2B enzymes in mediating the induction of cell death by PCB153 and PCB126. Further, antagonism was observed between PCB126 and PCB153 for effects on cytotoxic response and ROS formation in T47D and MDA-MB-231 cells. Antagonism was also observed between PCB153 and PCB126 in the induction of NAD(P)H depletion at lower concentration (<10 microM) in T47D cells, but not in MDA-MB-231 cells. In conclusions, results from our investigation suggest that ROS formation induced by PCBs is a significant determinant factor in mediating the DNA damage and cell death in human breast cancer cells. The data also suggests that the status of estrogen receptor alpha may play a role in modulating the PCB-induced oxidative DNA damage and cell death in human breast cancer cells.

Thyroid-hormone-disrupting chemicals: evidence for dose-dependent additivity or synergism

Endocrine disruption from environmental contaminants has been linked to a broad spectrum of adverse outcomes. One concern about endocrine-disrupting xenobiotics is the potential for additive or synergistic (i.e., greater-than-additive) effects of mixtures. A short-term dosing model to examine the effects of environmental mixtures on thyroid homeostasis has been developed. Prototypic thyroid-disrupting chemicals (TDCs) such as dioxins, polychlorinated biphenyls (PCBs), and polybrominated diphenyl ethers have been shown to alter thyroid hormone homeostasis in this model primarily by up-regulating hepatic catabolism of thyroid hormones via at least two mechanisms. Our present effort tested the hypothesis that a mixture of TDCs will affect serum total thyroxine (T4) concentrations in a dose-additive manner. Young female Long-Evans rats were dosed via gavage with 18 different polyhalogenated aromatic hydrocarbons [2 dioxins, 4 dibenzofurans, and 12 PCBs, including dioxin-like and non-dioxin-like PCBs] for 4 consecutive days. Serum total T4 was measured via radioimmunoassay in samples collected 24 hr after the last dose. Extensive dose-response functions (based on seven to nine doses per chemical) were determined for individual chemicals. A mixture was custom synthesized with the ratio of chemicals based on environmental concentrations. Serial dilutions of this mixture ranged from approximately background levels to 100-fold greater than background human daily intakes. Six serial dilutions of the mixture were tested in the same 4-day assay. Doses of individual chemicals that were associated with a 30% TH decrease from control (ED30), as well as predicted mixture outcomes were calculated using a flexible single-chemical-required method applicable to chemicals with differing dose thresholds and maximum-effect asymptotes. The single-chemical data were modeled without and with the mixture data to determine, respectively, the expected mixture response (the additivity model) and the experimentally observed mixture response (the empirical model). A likelihood-ratio test revealed statistically significant departure from dose additivity. There was no deviation from additivity at the lowest doses of the mixture, but there was a greater-than-additive effect at the three highest mixtures doses. At high doses the additivity model underpredicted the empirical effects by 2- to 3-fold. These are the first results to suggest dose-dependent additivity and synergism in TDCs that may act via different mechanisms in a complex mixture. The results imply that cumulative risk approaches be considered when assessing the risk of exposure to chemical mixtures that contain TDCs.

Subchronic exposure to TCDD, PeCDF, PCB126, and PCB153: effect on hepatic gene expression

We employed DNA microarray to identify unique hepatic gene expression patterns associated with subchronic exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other halogenated aromatic hydrocarbons (HAHs). Female Harlan Sprague-Dawley rats were exposed for 13 weeks to toxicologically equivalent doses of four different HAHs based on the toxic equivalency factor of each chemical: TCDD (100 ng/kg/day), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF; 200 ng/kg/day), 3,3',4,4',5-pentachlorobiphenyl (PCB126; 1,000 ng/kg/day), or 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153; 1,000 microg/kg/day). Global gene expression profiles for each exposure, which account for 8,799 gene probe sets contained on Affymetrix RGU34A GeneChips, were compared by principal components analysis. The aryl hydrocarbon receptor (AhR) ligands TCDD, PeCDF, and PCB126 produced very similar global gene expression profiles that were unique from the nonAhR ligand PCB153, underscoring the extensive impact of AhR activation and/or the resulting hepatic injury on global gene expression in female rat liver. Many genes were co-expressed during the 13-week TCDD, PeCDF, or PCB126 exposures, including classical AhR-regulated genes and some genes not previously characterized as being AhR regulated, such as carcinoembryonic-cell adhesion molecule 4 (C-CAM4) and adenylate cyclase-associated protein 2 (CAP2). Real-time reverse-transcriptase polymerase chain reaction confirmed the increased expression of these genes in TCDD-, PeCDF-, and PCB126-exposed rats as well as the up- or down-regulation of several other novel dioxin-responsive genes. In summary, DNA microarray successfully identified dioxin-responsive genes expressed after exposure to AhR ligands (TCDD, PeCDF, PCB126) but not after exposure to the non-AhR ligand PCB153. Together, these findings may help to elucidate some of the fundamental features of dioxin toxicity and may further clarify the biologic role of the AhR signaling pathway.

Environmental exposure to polychlorinated biphenyls (PCBs) and breast cancer: a systematic review of the epidemiological evidence

Some PCB congeners have shown oestrogenic effects, and this has raised concern that they may increase the risk of breast cancer. In this article we provide a quantitative review of the epidemiologic evidence on environmental exposure to PCBs and breast cancer risk. The vast majority of prospective and retrospective studies did not find any association between total PCB concentrations and breast cancer risk. No association was found for congeners in groups I (potentially oestrogenic) and III (biologically persistent phenobarbital-type cytochrome P450 inducers), according to the classification proposed by Wolff and Toniolo, while less consistent results were reported for group II (potentially anti-oestrogenic and immunotoxic, dioxin-like). Two studies found a threefold risk of postmenopausal breast cancer for women with an A2455G base change in exon 7 of the polymorphic CYP1A1 gene (a member of the cytochrome P450 family) and high PCB levels, compared with women with two wild-type alleles and low PCB, based however on very few cases. Thus, the epidemiological evidence does not support the hypothesis of an association of environmental exposure to PCBs in adulthood in the general population and risk of breast cancer, although uncertainties remain for selected subgroups of women or individual PCB congeners.

Spectral interactions of tetrachlorobiphenyls with hepatic microsomal cytochrome p450 enzymes

This study investigated the spectral interactions of hepatic microsomal cytochrome p450 (CYP) enzymes with four symmetrical polychlorinated biphenyls (PCBs): 2,2',4,4'-tetrachlorobiphenyl (PCB 47); 2,2',5,5'-tetrachlorobiphenyl (PCB 52); 2,2',6,6'-tetrachlorobiphenyl (PCB 54); and 3,3',4,4'-tetrachlorobiphenyl (PCB 77). The PCBs were selected to explore structure-activity relationships and the effect of the chlorination pattern on PCB-CYP spectral interactions. To examine CYP enzyme specificity, difference spectra were measured with hepatic microsomes prepared from control, phenobarbital (PB)-, 3-methylcholanthrene (MC)-, and dexamethasone (DEX)-treated rats in the absence and presence of CYP-specific antibodies. The four PCB congeners elicited a type I spectral change with all hepatic microsomal preparations. The binding efficiency of the PCBs was highest with microsomes from PB-treated rats. The largest absorbance change and highest binding efficiency were observed with PCB 54, the most non-coplanar congener tested. Antibody inhibition and CYP immunoquantitation data showed that the PCBs bind to CYP1A, CYP2B, CYP2C and CYP3A enzymes to varying degrees. For example, PCB 47, 52, and 54 bind preferentially to CYP2B and to a lesser extent to CYP3A enzymes in microsomes from PB-treated male rats; PCB 52 binds primarily to CYP3A enzymes in microsomes from DEX-treated female rats; and PCB 54 binds to CYP3A and to CYP2C enzymes in microsomes from control male rats. The study demonstrated that the extent of PCB-CYP binding interaction was dependent on the chlorination pattern of the PCB and on the relative abundance of individual CYP enzymes in hepatic microsomes.

The Belgian PCB/dioxin incident: analysis of the food chain contamination and health risk evaluation

The Belgian PCB incident occurred at the end of January 1999 when a mixture of polychlorinated biphenyls (PCBs) contaminated with dioxins was accidentally added to a stock of recycled fat used in the production of animal feeds. Although signs of poultry poisoning were noticed by February, 1999, the source and the extent of the contamination were discovered only in May 1999, when it appeared that more than 2500 farms could have been supplied with contaminated feeds. This resulted in a major food crisis, which rapidly extended to the whole country and could be resolved only by the implementation of a large PCB/dioxin food monitoring program. Screening for PCB contamination was based on the determination of the seven PCB markers. When PCB concentrations exceeded the tolerance levels of 0.1 (milk), 0.2 (poultry, bovine, and pig meat), or 1 (animal feed) microg/g fat, dioxins (17 PCDD/Fs congeners) were also determined. At the end of December 1999, the database contained the results of more than 55,000 PCB and 500 dioxin analyses. The study of PCB levels and profiles in contaminated feeds delivered to poultry or pig farms confirmed that the Belgian PCB incident was due to a single source of PCB oil introduced into the food chain at the end of January 1999. This PCB oil had a congeners pattern closely matched to a mixture of Aroclor 1260/1254 in the proportion 75/25. The total amount of PCBs added to recycled fats was estimated at 50 kg (sum of the seven markers) or approximately 150 kg total PCBs, which corresponds to about 100 liters of PCB oil. This PCB mixture contained about 1g TEQ dioxins (more than 90- contributed by PCDFs) and about 2g TEQ dioxin-like PCBs. The proportions of PCB 52 and 101 congeners were fairly constant in animal feeds, excluding the possibility of secondary contamination due to fat recycling from contaminated animals. The highest concentrations of PCBs and dioxins were found in poultry and especially in the reproduction animals (hens and chicks), which showed the classical manifestations of chick edema disease. The pigs were also affected but to a lesser extent and no sign of intoxication was observed. The study of PCB/dioxin patterns and of the PCB:dioxin ratios revealed major differences in the metabolism of these compounds by farm animals. Whereas the PCBs:dioxins ratio was fairly constant in all poultry products with a mean value similar to that found in contaminated feeds (50,000), in pigs this ratio was both much higher and more variable (values up to 10,000,000), reflecting a faster elimination of dioxins than PCBs in these animals. These metabolic differences also emerged from the PCB and dioxin patterns which were altered much more in pigs than in poultry. Although the most contaminated food products (chicken meat) had PCB and dioxin levels more than 100 times above maximal recommended values, it is unlikely that this incident could have caused adverse effects in the general population of Belgium. A doubling of the PCB and dioxin burden of the young adult population would require the consumption of, respectively, 10 and 20 highly contaminated meals. In view of the very limited proportion of the poultry chain effectively contaminated during the incident (around 2%), such an extreme scenario was quite improbable for the general population except perhaps for farmers consuming their own products. But even in that case, it would have meant going back to the levels in the 1980s or attaining the body burden of subjects regularly eating contaminated seafood.

Structure dependent induction of CYP1A by polychlorinated biphenyls in hepatocytes of male castrated pigs

Hepatocytes cultures prepared from castrated pig hepatocytes (Great Yorkshire x Dutch Landrace), as a model for human liver, were used to study the effect of twenty polychlorinated biphenyls (PCBs) on CYP1A activity, measured as the dealkylation of either ethoxyresorufin or methoxyresorufin. The selection of the PCBs was based on their differences in physico-chemical properties. The non-ortho and mono-ortho substituted PCBs were the most potent CYP1A inducers in pig hepatocytes. In addition, several multiple-ortho substituted congeners, with five or more chlorine atoms, were inducers of CYP1A activity as well. Their relative effect potencies (REP) were proximately 10,000 times lower than the most potent congener, 3,3',4,4',5 PeCB (PCB#126). Using partial least-squares (PLS) modeling, predictions of CYP1A activity could be made for all tetra to hepta substituted congeners. Several multiple-ortho substituted PCBs, which are highly abundant in the biotic and abiotic environment, have been found to induce CYP1A activity in pig hepatocytes. Because induction of CYP1A activity is used as biomarker for Ah-receptor mediated responses, it is suggested to include these congeners in future risk assessment.

Induction of hepatic CYP2B is a more sensitive indicator of exposure to aroclor 1260 than CYP1A in male rats

To evaluate the effect of exposure to an environmentally relevant polychlorinated biphenyl mixture, adult male rats were treated with Aroclor 1260 for 7 days and levels of several cytochrome P450 (CYP) enzymes were measured in liver microsomes prepared 3 days after the last dose. Treatment with Aroclor 1260 at dosages ranging from 0.5 to 50 mg/kg/day had no effect on body weight, but liver weight was increased significantly in rats treated with the two highest dosages. Of the monooxygenase activities examined, benzyloxyresorufin O-dealkylase and testosterone 16beta-hydroxylase activities were increased to the greatest extent with maximal induction of both activities reached at 5 mg/kg/day. Densitometric quantitation of blots probed with antibody against CYP2B revealed that CYP2B1 and CYP2B2 protein levels were increased approximately 55-fold and 16-fold, respectively, after treatment with Aroclor 1260 at 5 mg/kg/day. Ethoxyresorufin O-deethylase activity and CYP1A1 protein levels displayed linear dose-dependent increases, but the hepatic CYP1A1 content did not exceed 10% that of CYP2B1 at all dosages of Aroclor 1260. Microsomal CYP3A- and CYP2A1-mediated enzyme activities and protein levels were also increased by treatment with Aroclor 1260 but to a lesser extent, whereas CYP2C11-mediated enzyme activities and protein levels were reduced. A separate time-course study showed that induction of CYP2B, but not of CYP1A, enzymes persisted for at least 48 days after treatment with Aroclor 1260 at 10 mg/kg/day. In summary, the results indicate that induction of CYP2B enzymes is a more sensitive biomarker of exposure to Aroclor 1260 than CYP1A.

Structure-dependent induction of CYP1A by polychlorinated biphenyls in hepatocytes of cynomolgus monkeys (Macaca fascicularis)

Until now structure-activity relationships (SARs) for in vitro or in vivo CYP1A induction by polychlorinated biphenyls (PCBs) have only been determined in rodents and birds. This study describes the first development of such a SAR in a primate species by using hepatocyte cultures of cynomolgus monkey (Macaca fascicularis). Hepatocyte cultures of primate species might be a more suitable model for humans than those of rodents. For 20 PCBs, the in vitro induction of CYP1A activity was determined by measuring dealkylation of either methoxyresorufin or ethoxyresorufin. Selection of PCBs was based on multivariate physical-chemical characterization of all tetra- through heptachlorinated congeners. The non-ortho-substituted congeners were found to be the most potent inducers, followed by the mono-ortho-substituted PCBs. Multiple-ortho-substituted congeners, with more than five chlorine atoms, were inducers of CYP1A activity in monkey hepatocytes as well, with EC50 values approximately 10,000 times higher than 3,3',4,4',5 PeCB (PCB 126), the most potent congener. Using partial least-squares (PLS) modeling, predictions of CYP1A activity were established for all other tetra- to hepta-substituted congeners. Several congeners, which are abundant in the (a)biotic environment, were predicted to have CYP1A activity in cynomolgus monkey hepatocytes. Because induction of CYP1A activity is generally used as an early and sensitive biomarker for the Ah-receptor-mediated potential of a chemical, further studies are recommended to determine the possible risks of these multiple-ortho PCBs to humans.

Hydroxylated polychlorinated biphenyls (PCBs) as estrogens and antiestrogens: structure-activity relationships

The effects of structure on the estrogenicity and antiestrogenicity of hydroxylated polychlorinated biphenyls were investigated using the following estrogen-sensitive assays: competitive binding to the rat and mouse cytosolic estrogen receptor (ER); immature rat and mouse uterine wet weight, peroxidase and progesterone receptor (PR) levels; induction of luciferase activity in HeLa cells stably transfected with a Gal4:human ER chimera and a 17mer-regulated luciferase reporter gene; proliferation of MCF-7 human breast cancer cells; induction of chloramphenicol acetyl transferase (CAT) activity in MCF-7 cells transiently transfected with a full-length human ER expression plasmid and a plasmid containing an estrogen-responsive vitellogenin A2 promoter linked to a CAT reporter gene. The chemicals synthesized for this study contained a 4-hydroxy group in one ring, a 2- or 3-chloro substituent meta or ortho to the hydroxyl group, and variable substitution (2',3',4',5'-, 2',3',4',6'-, 2',3',5',6'-tetrachloro and 2',4',6'-trichloro) in the chlorophenyl ring. The compounds included: 2,2',3',4',5'- (A), 2,2',3',4',6'- (B), and 2,2',3',5',6'-pentachloro- (C); 2,2',4',6'-tetrachloro-4-biphenylol (D); 2',3,3',4',5'- (E), 2',3,3',4',6'- (F), and 2',3,3',5',6'-pentachloro (G); and 2',3,4',6'-tetrachloro-4-biphenylol (H). With the exception of 2',3,4',6'-tetrachloro-4-biphenylol (H), all of the compounds competitively bound to the mouse and rat ER with relative binding affinities [compared to 17beta-estradiol (E2)] varying from 1.4 x 10(-3) to 5.3 x 10(-5). The structure-ER binding relationships for the hydroxy-PCB congeners were different in the rat and mouse, and no dose-dependent estrogenic activities were observed in the mouse or rat uterus. Several hydroxy-PCB congeners exhibited antiestrogenic activity (primarily in the mouse uterus) and two compounds, 2,2',3',5',6- and 2,2',3',4',6'-pentachloro-4-biphenylol, inhibited E2-induced uterine wet weight, PR binding, and peroxidase activity in the mouse uterus. 2,2',3',4',5'- and 2,2',3',4',6'-Pentachloro-4-biphenylol induced CAT activity in MCF-7 cells transiently transfected with the Vit-CAT plasmid; the remaining congeners did not induce CAT activity but exhibited antiestrogenic activity in MCF-7 cells cotreated with 10(-9) E2 plus 10(-5) M hydroxy-PCBs. Complementary structure-estrogenicity relationships were observed utilizing the HeLa cell luciferase induction and MCF-7 cell proliferation assays. The placement of the 2- or 3-chloro groups in the phenolic ring had minimal effects on estrogenic activity, whereas 2,4,6-trichloro- and 2,3,4,6-tetrachloro substitution in the chlorophenyl ring (B, D, F, and H) were required for this response. Substitution in the phenolic ring was also not important for structure-antiestrogenicity relationships, and the most active compounds (A, C, E, and G) contained 2',3',4',5'- and 2',3',5',6'-tetrachlorophenyl groups. Thus, structure-estrogenicity/antiestrogenicity relationships for this series of hydroxy-PCBs were complex and response-specific.