Inhibition of human and rat CYP1A2 by TCDD and dioxin-like chemicals

Bioaccumulation of BDE47 in testes by TiO2 nanoparticles aggravates the reproductive impairment of male zebrafish by disrupting intercellular junctions

This study attempts to explore the potential impact of titanium dioxide nanoparticles (n-TiO₂) on bioconcentration and reproductive impairments of male zebrafish in the presence of 2,2',4,4'-tetrabromodiphenyl ether (BDE47), the congener of PBDEs predominant in environment and most abundant in biosamples. n-TiO₂ nanoparticles strongly adsorbed BDE47 to form BDE47/TiO₂ complex, which was taken up into the testes of zebrafish, and increased the tissue burdens of both BDE47 and n-TiO₂. Correspondingly, no observed toxic dose of n-TiO₂ (100 μg/L) was found to aggravate the abnormal histological morphology of the testes and the decrease in egg production, gonadosomatic index, sexual hormone levels and related gene expression in zebrafish in the presence of BDE47 at 5 or 50 μg/L. In addition, n-TiO₂ exacerbated the destruction resulting from the ultrastructural disassembly of intercellular connectivity of germ cells in zebrafish and the decrease in transepithelial electrical resistance in TM4 cells induced by BDE47. Furthermore, n-TiO₂ enhanced BDE47 to initially activate p-JNK MAPK signaling pathway and subsequently triggered the downregulation of junction proteins (i.e., ZO-1, Connexin-43 and N-cadherin), leading to impaired cell-cell junctions in vivo and in vitro. Our results demonstrated that n-TiO₂ should act as a carrier to facilitate the accumulation of BDE47 in zebrafish testes and result in a synergistic effect on BDE47-induced adverse reproductive outcomes via disruption of intercellular connectivity of zebrafish testes. This study is beneficial in providing a scientific basis for improving the health risk assessment of environmental pollutants, particularly those that coexist with nanoparticle contamination in realistic environments.

Mass dose rates of particle-bound organic pollutants in the human respiratory tract: Implications for inhalation exposure and risk estimations

To date, little is known about the effective doses of airborne particulate matter (PM) and PM-bound hazardous organic components to the human respiratory tract (HRT). In the light of this, here we provide particle mass dose rates (dose per hour of exposure) of PM and a suite of PM-bound hazardous organic compounds in the HRT for two population age groups (adults & children). More specifically, the mass dose rates of PM and PM-bound polycyclic aromatic hydrocarbons (PAHs), nitrated-PAH (NPAHs), polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs) and polybrominated diphenyl ethers (PBDEs) were estimated at two urban sites using a multiple path particle dosimetry model. We find that, in most cases, the total mass doses are following similar variations across sites and seasons as their ambient total concentrations, however their distribution in the HRT is a function of the particle size distributions and the physiological parameters of each age group. More specifically, the majority of the deposited mass of PM and all the chemical components investigated was accumulated in the upper airways instead of the lungs. We further show that children, due to their different physiology, are more susceptible and receive larger fraction of the total mass doses in the deepest parts of the lungs compared to the adults' group. Comparing the traditional method for estimating the inhalation risk, which is based on the ambient concentration of pollutants, and a modified version using the mass dose in the HRT, we find that the former may overestimate the reported risks. The results presented here provide a novel dataset composed by previously undetermined doses of hazardous airborne particulate organic components in the HRT and demonstrate that alternative health risk estimation approaches may capture some variabilities that are traditionally overlooked.

Occurrence of polybrominated dibenzo-p-dioxins, dibenzofurans (PBDD/Fs) and polybrominated diphenyl ethers (PBDEs) in pilot whales (Globicephala melas) caught around the Faroe Islands

Blubber from Faroese pilot whales (Globicephala melas) was analysed for brominated dioxins PBDD/Fs, with a subset also analysed for chlorinated dioxins, PCDD/Fs. The studied individuals were restricted to juvenile male whales sampled in the Faroe Islands during the period 1997-2013. Among the PBDD/Fs, the furans were predominant, although the relative abundance of various congeners differed between samples. Furans accounted for, on average, 79% of the ∑PBDD/Fs in the samples, with 1,2,3,4,6,7,8-HpBDF the most abundant congener, found in half of the analysed pilot whales. The concentration range for ∑PBDD/Fs among the samples was 0.080-71 pg/g l.w. (lipid weight), and the sum of toxic equivalents ranged from 0.0039 to 4.7 pg TEQ/g l.w. No relationship was found between PBDD/Fs and PCDD/Fs. In addition, 20 pilot whale samples from the period 2010-2013 were analysed for PBDEs. Several PBDE congeners were found in all of the sampled pilot whales, and at noticeably higher levels than PBDD/Fs and PCDD/Fs. The ∑PBDEs ranged from 140 to 1900 ng/g l.w., with BDE #47 the most abundant congener detected in the samples. Results from the present study were then compared with data from previous studies on pilot wales to investigate temporal trends between 1986 and 2013. The comparison indicated that PBDE concentrations in juvenile males have decreased from 1996 to the latest observations in 2013. No relationship between the concentration levels of PBDD/Fs and PBDEs in the sampled pilot whales could be identified, which indicates possible differences in the metabolism of, or exposure to, PBDEs and PBDD/Fs.

Structural-functional adaptations of porcine CYP1A1 to metabolize polychlorinated dibenzo-p-dioxins

Polychlorinated dibenzo-p-dioxins (PCDDs) are widespread by-products of human industrial activity. They accumulate in tissues of animals and humans, exerting numerous adverse effects on different systems. In living organisms, dioxins are metabolized by enzymes of the cytochrome P450 family, including CYP1A1. Particular dioxin congeners differ in their toxicity level and ability to undergo biodegradation. Since the molecular mechanisms underlying dioxin susceptibility or resistance to biodegradation are unknown, in the present study the molecular interactions between five selected dioxins and porcine CYP1A1 protein were investigated. It was found that the ability of a dioxin to undergo CYP1A1-mediated degradation is associated mainly with the number and position of chlorine atoms in the dioxin molecule. Among all examined congeners, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) demonstrated the highest affinity to CYP1A1 and, at the same time, the greatest distance to the active site of the enzyme. Interestingly, in contrast to other dioxins, the binding of the TCDD molecule to the porcine CYP1A1 active site resulted in a rapid and continuous closure of substrate channels. All the information may help to explain the extended half-life of TCDD in living organisms as well as its high toxicity.

Relative effect potency estimates of dioxin-like activity for dioxins, furans, and dioxin-like PCBs in adults based on cytochrome P450 1A1 and 1B1 gene expression in blood

BACKGROUND

In the risk assessment of PCDDs, PCDFs, and dioxin-like (DL) PCBs, regulatory authorities support the use of the toxic equivalency factor (TEF)-scheme derived from a heterogeneous data set of the relative effect potency (REPs) estimates.

OBJECTIVES

We sought to determine REPs for dioxin-like compounds (DLCs) using expression of cytochrome P450 (CYP) 1A1 and 1B1 mRNA in human peripheral blood mononuclear cells representing two different pathways.

METHODS

We used a sex and age adjusted regression-based approach comparing the strength of association between each DLC and the cytochrome P450 (CYP) 1A1 and 1B1 mRNA expression in 320 adults residing in an organochlorine-polluted area of eastern Slovakia.

RESULTS

We calculated REPs based on CYP1A1 expression for 4 PCDDs, 8 PCDFs, and 1 PCB congener, and based on CYP1B1 expression for 5 PCDFs and 11 PCB congeners. REPs from CYP1A1 correlated with REPs previously derived from thyroid volume (ρ=0.85; p<0.001) and serum FT4 (ρ=0.77; p=0.009). The 13 log REPs from CYP1A1 correlated with log WHO-TEFs (r=0.63; p=0.015) and 11 log PCB REPs with PCB consensus toxicity factors (CTFs) for compounds with WHO-TEFs (r=0.80; p=0.003). The complete set of derived 56 log REPs correlated with the log CTFs (r=0.77; p=0.001) and log WHO-TEFs (r=0.81; p<0.001).

CONCLUSIONS

REPs calculated from thyroid and cytochrome P450 endpoints realistically reflect human exposure scenarios because they are based on human chronic and low-dose exposures. While the CYP 1A1 seems more suitable for toxicity evaluation of PCDD/Fs, the CYP 1B1 is more apt for PCDFs and PCBs and reflects different pathways.

Does dioxin exert toxic effects in humans at or near current background body levels?: an evidence-based conclusion

Evidence-based toxicology like evidence-based medicine, provides scientifically grounded evidence-based conclusions as distinguished from authority-based opinions. As an example, we address a proposition from the US Environmental Protection Agency's (EPA) Draft Dioxin [2,3, 7,8-tetrachlorodibenzo-P-dioxin (TCDD)] reassessment that: ‘dioxin... can produce effects... at or near current background body burdens or intake levels’. Guided by a systematic, objective, and unbiased analysis of the available molecular, physiological, and clinical/epidemiologic data, in accordance with accepted principles of scientific logic, we reach the evidence-based conclusion that the proposition is rejected. When gaps in scientific knowledge necessitate formulation of opinions to meet preventive or precautionary goals, the reversion to authority should be explicitly acknowledged.

Urban Endocrine Disruptors Targeting Breast Cancer Proteins

Humans are exposed to a huge amount of environmental pollutants called endocrine disrupting chemicals (EDCs). These molecules interfere with the homeostasis of the body, usually through mimicking natural hormones leading to activation or blocking of their receptors. Many of these compounds have been associated with a broad range of diseases including the development or increased susceptibility to breast cancer, the most prevalent cancer in women worldwide, according to the World Health Organization. Thus, this article presents a virtual high-throughput screening (vHTS) to evaluate the affinity of proteins related to breast cancer, such as ESR1, ERBB2, PGR, BCRA1, and SHBG, among others, with EDCs from urban sources. A blind docking strategy was employed to screen each protein-ligand pair in triplicate in AutoDock Vina 2.0, using the computed binding affinities as ranking criteria. The three-dimensional structures were previously obtained from EDCs DataBank and Protein Data Bank, prepared and optimized by SYBYL X-2.0. Some of the chemicals that exhibited the best affinity scores for breast cancer proteins in each category were 1,3,7,8-tetrachlorodibenzo-p-dioxin, bisphenol A derivatives, perfluorooctanesulfonic acid, and benzo(a)pyrene, for catalase, several proteins, sex hormone-binding globulin, and cytochrome P450 1A2, respectively. An experimental validation of this approach was performed with a complex that gave a moderate binding affinity in silico, the sex hormone binding globulin (SHBG), and bisphenol A (BPA) complex. The protein was obtained using DNA recombinant technology and the physical interaction with BPA assessed through spectroscopic techniques. BPA binds on the recombinant SHBG, and this results in an increase of its α helix content. In short, this work shows the potential of several EDCs to bind breast cancer associated proteins as a tool to prioritize compounds to perform in vitro analysis to benefit the regulation or exposure prevention by the general population.

My Winding Road: From Microbiology to Toxicology and Environmental Health

I would certainly never have predicted that I would become the director of the National Institute of Environmental Health Sciences (NIEHS) and the National Toxicology Program (NTP) when I was a Jewish girl growing up in Teaneck, New Jersey. My family stressed the importance of education. Yet for a girl there were many not-so-subtle suggestions that the appropriate careers were in teaching or nursing, and the most important thing was to be a wife and mother. Well, I can't disagree with the latter, although I would have to add grandmother to that list of achievements. My parents were both college graduates, but my mom only taught high school English for one year before leaving the field to start our family. My dad returned from World War II and joined his brother in accounting. After my first sister was born, my father joined my mother's family jewelry business and helped to open a second retail store. My mother helped my dad out during the busy times—Christmas and wedding season—but otherwise focused on our growing family of three girls and one boy. This became increasingly challenging when it became clear that my little brother was severely retarded and would require extra care.

Mammalian cytochrome P450-dependent metabolism of polychlorinated dibenzo-p-dioxins and coplanar polychlorinated biphenyls

Polychlorinated dibenzo-p-dioxins (PCDDs) and coplanar polychlorinated biphenyls (PCBs) contribute to dioxin toxicity in humans and wildlife after bioaccumulation through the food chain from the environment. The authors examined human and rat cytochrome P450 (CYP)-dependent metabolism of PCDDs and PCBs. A number of human CYP isoforms belonging to the CYP1 and CYP2 families showed remarkable activities toward low-chlorinated PCDDs. In particular, human CYP1A1, CYP1A2, and CYP1B1 showed high activities toward monoCDDs, diCDDs, and triCDDs but no detectable activity toward 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-tetraCDD). Large amino acids located at putative substrate-recognition sites and the F-G loop in rat CYP1A1 contributed to the successful metabolism of 2,3,7,8-tetraCDD. Rat, but not human, CYP1A1 metabolized 3,3',4,4',5-pentachlorobiphenyl (CB126) to two hydroxylated metabolites. These metabolites are probably less toxic than is CB126, due to their higher solubility. Homology models of human and rat CYP1A1s and CB126 docking studies indicated that two amino acid differences in the CB126-binding cavity were important for CB126 metabolism. In this review, the importance of CYPs in the metabolism of dioxins and PCBs in mammals and the species-based differences between humans and rats are described. In addition, the authors reveal the molecular mechanism behind the binding modes of dioxins and PCBs in the heme pocket of CYPs.

Cytochrome P450 3A1 mediates 2,2',4,4'-tetrabromodiphenyl ether-induced reduction of spermatogenesis in adult rats

Background

2,2′,4,4′-tetrabromodiphenyl ether (BDE47) is the dominant PBDE congener in humans, wildlife, and the environment. It has been reported to be metabolized by cytochrome P450 (CYP) enzymes. Still, the effects of BDE47 on spermatogenesis failure are attracting an increasing amount of attention. However, it is unclear whether CYP-mediated metabolism contributes to BDE47-induced reproductive toxicity.

Methodology and Principal Findings

The role of cytochrome P450 3A1 (CYP3A1) in the formation of oxidative metabolites of BDE47 and its induced spermatogenesis failure was investigated in SD rats. BDE47 significantly increased the expression and activity of CYP3A1 in rat liver, and 3-OH-BDE47, the major oxidative metabolite of BDE47, dose-dependently increased in rat liver, serum, and testis, which was aggravated by dexamethasone (DEX), an inducer of CYP3A1. Additionally, testicular 3-OH-BDE47 and reactive oxygen species (ROS) in seminiferous tubules increased especially when BDE47 was administered in combination with DEX, which was confirmed in GC-1 and GC-2 cells that 3-OH-BDE47 induced more ROS production and cell apoptosis via the upregulation of FAS/FASL, p-p53 and caspase 3. As a result, daily sperm production dose-dependently decreased, consistent with histological observations in giant cells and vacuolar spaces and increase in TUNEL-positive apoptotic germ cells.

Conclusion

CYP3A1-mediated metabolic activation of BDE47 and the active metabolite 3-OH-BDE47 and consequent ROS played an important role in reduction of spermatogenesis by germ cell apoptosis. Our study helps provide new insights into the mechanism of reproductive toxicity of environmental chemicals.

Toxicokinetics of dioxins and other organochlorine compounds in Japanese people: association with hepatic CYP1A2 expression levels

Concentrations of persistent organochlorine compounds (OCs) including polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and polychlorinated biphenyls (PCBs) in the liver and adipose tissue of Japanese cadavers were measured, and their toxicokinetics were examined in association with hepatic cytochrome P450 (CYP) 1A protein expression levels. Total 2,3,7,8-tetrachlorodibenzo-p-dioxin toxic equivalents (TEQs) were 66±74 and 65±57 pg/g lipid weight (mean±S.D.) in the liver and adipose tissue, respectively. Total PCBs (sum of 62 congeners targeted), p,p'-dichlorodiphenyl-dichloroethylene (p,p'-DDE) and β-hexachlorocyclohexane (β-HCH) were detected at concentrations over 1 μg/g lipid in both tissues of some specimens. For most of the dioxin-like congeners, total PCBs, p,p'-DDE, oxychlordane, α- and β-HCH, hexachlorobenzene (HCB), and tris(4-chlorophenyl)methane (TCPMe), age-dependent increases in concentrations were found in the adipose tissue of males. No such age-dependent trend was observed in the liver, suggesting that there are different mechanisms underlying the hepatic concentrations of OCs. Immunoblot analyses indicated detectable expression of hepatic CYP1A2 protein, whereas no CYP1A1 protein was detected. The CYP1A2 expression levels were positively correlated with concentrations (on wet weight basis) of 2,3,4,7,8-P₅CDF, the dominant TEQ-contributed congeners in the liver, indicating the induction of this CYP. Hepatic CYP1A2 protein levels were strongly correlated with the liver to adipose concentration (L/A) ratios of PCDD/F congeners with more than 5 chlorine atoms. Together with higher concentrations of the congeners in the liver than in the adipose tissue, the observation on L/A ratios of highly chlorinated PCDD/Fs suggests that induced hepatic CYP1A2 protein is involved in their sequestration in this human population, as observed in model animals (rodents). Nonetheless, the magnitude of hepatic sequestration (L/A ratio) of PCDD/Fs in this human population was lower than in other mammals and birds, reported previously. This study emphasizes the fact that toxicokinetics of some OCs can be affected at least partly by CYP1A2 protein levels in humans. For the extrapolation of their toxicokinetics from model animals to humans, knowledge on the induction and sequestration potencies of CYP1A is necessary.

RO4938581, a GABAAα5 modulator, displays strong CYP1A2 autoinduction properties in rats

Autoinduction in drug metabolism is a known phenomenon observed when a drug induces the enzymes responsible for its own metabolism. The potency, rate and extent of autoinduction following a given treatment paradigm may have therapeutic implications in clinic as well as for in vivo pharmacological assessments in animals. RO4938581, an imidazo-triazolo-benzodiazepine, is a novel GABAAα5 negative modulator recently pursued for the treatment of cognitive dysfunctions. As circulating plasma levels of RO4938581 were shown to decrease rapidly after repeated dosing in rats, with CYP1A2 being involved in the metabolism of the compound, we examined the potential role of RO4938581-mediated autoinduction of CYP1A2. Incubation of rat hepatocytes with RO4938581 revealed potent CYP1A2 induction with significant increase in enzymatic activity at concentrations of 0.1nM and RO4938581 was shown to be 700-fold more potent than β-napththoflavone. Ex vivo studies revealed a 7-fold increase in metabolic CYP1A2 activity in liver microsomes prepared from rats administered with 0.1mg/kg of RO4938581 24h before. This induction profile was reflected in vivo in pharmacokinetic studies in rats where an 8-fold reduction in plasma exposure was observed after a second dose. The reduction in plasma exposures due to CYP1A2 autoinduction were confirmed functionally in contextual fear conditioning paradigm in rats, where a positive pharmacological effect observed after acute drug administration disappeared completely after sub-chronic dosing. Together, these findings suggest that RO4938581 possesses potent CYP1A2 autoinductive properties in rats and may serve as a tool for mechanistic metabolism or drug-drug interaction studies encircling this enzyme in rats.

2,2',3,5',6-Pentachlorobiphenyl (PCB 95) and its hydroxylated metabolites are enantiomerically enriched in female mice

Epidemiological and laboratory studies link polychlorinated biphenyls and their metabolites to adverse neurodevelopmental outcomes. Several neurotoxic PCB congeners are chiral and undergo enantiomeric enrichment in mammalian species, which may modulate PCB developmental neurotoxicity. This study measures levels and enantiomeric enrichment of PCB 95 and its hydroxylated metabolites (OH-PCBs) in adult female C57Bl/6 mice following subchronic exposure to racemic PCB 95. Tissue levels of PCB 95 and OH-PCBs increased with increasing dose. Dose-dependent enantiomeric enrichment of PCB 95 was observed in brain and other tissues. OH-PCBs also displayed enantiomeric enrichment in blood and liver, but were not detected in adipose and brain. In light of data suggesting enantioselective effects of chiral PCBs on molecular targets linked to PCB developmental neurotoxicity, our observations highlight the importance of accounting for PCB and OH-PCB enantiomeric enrichment in the assessment of PCB developmental neurotoxicity.

Characterization and expression of extrahepatic CYP2S1

BACKGROUND

About one-third of the CYP enzymes identified so far, including several novel CYP enzymes such as CYP2S1, CYP2U1 and CYP2W1, belong to the CYP2 family. As with other recently discovered CYP enzymes, detailed information about the catalytic activity and function of CYP2S1 is lacking.

OBJECTIVE

To review and compare the expression of CYP2S1 mRNA and protein in humans, mice and rats, and to critically examine evidence pertaining to CYP2S1 regulation and its catalytic activity.

METHODS

Information about mouse and human CYP2S1 was summarized from published reports. Data about rat CYP2S1 expression was taken from recent work by the authors.

RESULTS/CONCLUSIONS

CYP2S1 shares molecular characteristics of both CYP1 and CYP2 family enzymes but shows a unique tissue profile of expression. Further studies are needed to identify selective substrates and to measure CYP2S1 protein levels before the role of CYP2S1 in xenobiotic metabolism and its relevance to physiological pathways and disease states can be determined.

The effect of dose on 2,3,7,8-TCDD tissue distribution, metabolism and elimination in CYP1A2 (-/-) knockout and C57BL/6N parental strains of mice

Numerous metabolism studies have demonstrated that the toxic contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is poorly metabolized. A hallmark feature of TCDD exposure is induction of hepatic CYP1A2 and subsequent sequestration leading to high liver-to-fat concentration ratios. This study was initiated to determine whether TCDD was inherently poorly metabolized or unavailable for metabolism because of sequestration to CYP1A2. [(3)H]TCDD was administered as a single, oral dose (0.1 and 10 microg/kg) to 12 male C57BL/6N mice or 12 CYP1A2 (-/-) mice. At 96 h, less than 5% of the dose was eliminated in the urine of all groups, and TCDD detected in urine was bound to mouse major urinary protein (mMUP). Feces were the major elimination pathway (24-31% of dose), and fecal extracts and non-extractables were quantitated by HPLC for metabolites. No great differences in urinary or fecal elimination (% dose) were observed between the high and low dose treatments. TCDD concentrations were the highest in adipose tissue for CYP1A2 knockout mice but in liver for C57BL/6N mice supporting the role of hepatic CYP1A2 in the sequestration of TCDD. Overall metabolism between parental and knockout strains showed no statistical differences at either the high or low doses. The data suggested that metabolism of TCDD is inherently slow, due principally to CYP1A1, and that hepatic CYP1A2 is not an active participant in the metabolism of TCDD in male mice. Rather, CYP1A2 governs the pharmacokinetics of TCDD by making it unavailable for hepatic CYP1A1 through sequestration and attenuating extrahepatic tissue disposition.

Aryl hydrocarbon receptor-dependent induction of flavin-containing monooxygenase mRNAs in mouse liver

Flavin-containing monooxygenases (FMOs) are important in detoxication but generally are considered not to be inducible by xenobiotics. Our recent microarray studies revealed induction of FMO2 and FMO3 mRNAs by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in liver of mice with wild-type aryl hydrocarbon receptor (AHR) but not in Ahr-null mice. The aim of the present study was to delineate mechanisms of FMO regulation. In adult male mice, basal FMO3 mRNA is low but was induced 6-fold at 4 h and 6000-fold at 24 h. The ED50 was approximately 1 microg/kg for FMO2 and FMO3, similar to that for the classic AHR-regulated gene, Cyp1a1. In adult female mice basal FMO3 mRNA is high and was not induced at 4 h but was elevated 8-fold at 24 h. FMO5 mRNA was significantly down-regulated by TCDD in both male and female adult mice. Juvenile mice show no sex difference in response to TCDD; FMO3 was induced 4 to 6-fold by TCDD in both sexes. Chromatin immunoprecipitation demonstrated recruitment of AHR and aryl hydrocarbon nuclear translocator proteins to Fmo3 regulatory regions, suggesting that induction by TCDD is a primary AHR-mediated event. Although FMO2 and FMO3 mRNAs were highly induced by TCDD in adult males, overall FMO catalytic activity increased only modestly. In contrast to the striking up-regulation of FMO2 and FMO3 in mouse liver, TCDD has little effect on FMO mRNA in rat liver. However, FMO2 and FMO3 mRNAs were highly induced in transgenic mice that express wild-type rat AHR, indicating that lack of induction in rat is not due to an incompetent AHR in this species.

Coordinated changes in xenobiotic metabolizing enzyme gene expression in aging male rats

In order to gain insight into the effects of aging on susceptibility to environmental toxins, we characterized the expression of xenobiotic metabolizing enzymes (XMEs) from the livers of male F344 and Brown Norway (BN) rats across the adult lifespan. Using full-genome Affymetrix arrays, principal component analysis showed a clear age-dependent separation between young and old animals in both rat strains. Out of 1135 or 1435 genes altered between the old and young groups in the F344 or BN rats, 7 or 3% were XMEs and included members of the phase I, II, and III classes of genes. There was a 20 or 32% overlap in the gene expression profile between the two strains for F344 or BN, respectively. Lipid, ergosterol, alcohol, and fatty acid metabolism genes were also altered with age in both strains. Some of the genes altered by age exhibited a gender-dependent expression pattern in young adult rats, suggesting an increasingly feminized pattern of gene expression with age in male rats. To examine transcriptional responses across lifespan after challenge with a xenobiotic compound, BN rats were exposed to toluene by oral gavage. Toluene exposure decreased the expression of glutathione synthetase, and dramatically increased the number of phase III genes being downregulated. The expression of CYP2B2 and glutathione-S-transferase decreased with age but increased in all age groups after toluene exposure. Decreased ability to detoxify and transport chemicals out of the body with age could result in increased susceptibility to some classes of chemicals in the aging population.

Catalytic function of avian cytochrome P450 1A4 and 1A5 (CYP1A4 and CYP1A5) enzymes heterologously expressed using in vitro yeast system

The present study clarifies the enzymatic properties of two avian cytochrome P4501A (CYP1A) paralogs, CYP1A4 and 1A5, using a yeast-based vector system. Recombinant CYP1A4 and 1A5 proteins from common cormorant (Phalacrocorax carbo) were expressed in yeast cells, and showed typical reduced CO-difference spectra with a peak at 446 nm. Kinetic analysis of O-dealkylase of methoxy-, ethoxy-, pentoxy- and benzyloxyresorufin catalyzed by the CYP1A enzymes revealed that Vmax value for ethoxyresorufin-O-deethylase (EROD) activity was much higher than that for the other three O-dealkylase activities for both isozymes. Interestingly, remarkable substrate specificity of the CYP1As was observed for O-dealkylation of benzyloxyresorufin and methoxyresorufin; CYP1A4 was highly specific for catalyzing benzyloxyresorufin-O-debenzylase activity, whereas CYP1A5 was more efficient in catalyzing methoxyresorufin-O-demethylase activity. The present study also measured CYP1A-dependent EROD activity in the presence of 2,3,7,8-tetrachlorodibenzofuran (TCDF) to evaluate the ability of this dioxin-like congener to inhibit the EROD activity. One hundred nanomolar TCDF noncompetitively inhibited CYP1A5-dependent EROD activity, although no inhibitory effect was detected for CYP1A4-dependent EROD activity. These results indicate that the avian CYP1A paralogs have different affinities for substrate and inhibitor, thus suggesting their distinct physiological and toxicological roles.

The polybrominated diphenyl ether mixture DE-71 is mildly estrogenic

BACKGROUND

Polybrominated diphenyl ethers (PBDEs) are widely found in the environment, and they may act as endocrine disruptors.

OBJECTIVE

Our goal in this study was to test the PBDE mixture DE-71 for estrogenic activity.

METHODS

We used proliferation of cultured breast cancer cells (MCF-7) and trophic effects in the reproductive tracts of ovariectomized mice as estrogen bioassays. DE-71 was administered to mice by subcutaneous injection (sc) or oral gavage (po), alone or in combination with estradiol, for 3 or 34 days. Liver weights and cytochrome P450 enzyme activities were also measured.

RESULTS

DE-71 increased MCF-7 cell proliferation, and this was prevented by antiestrogen. DE-71 cotreatment reduced the effect of estradiol in MCF-7 cells. In the mouse 3-day assay, DE-71 administered alone had no effect on uterine weight, uterine epithelial height (UEH), or vaginal epithelial thickness (VET); however, when DE-71 was administered as a cotreatment, it potentiated estradiol's effect on uterine weight. DE-71 administered sc to BALB/c mice for 34 days slightly increased UEH and VET, and attenuated the estradiol-induced increase in UEH; these effects were not seen in BALB/c mice treated po or in C57BL/6 mice treated sc. DE-71 increased liver weight in BALB/c, C57BL/6, and estrogen receptor-alpha knockout mice. We also found an increase in liver cytochrome P450 1A (CYP1A) and CYP2B activities when DE-71 was administered po, but only CYP2B increased after sc treatment.

CONCLUSION

DE-71 behaves as a weak estrogen. In mice, the treatment route and duration determined if DE-71 was estrogenic. BALB/c mice are more susceptible to DE-71 effects in estrogen target tissues than C57BL/6 mice. DE-71 increased liver weight independently of estrogen receptor-alpha.

Structure-activity relationships for the inhibition of recombinant human cytochromes P450 by curcumin analogues

Inhibition of cytochrome P450 (CYP) is a major cause of drug-drug interactions. In this work, inhibitory potentials of 33 curcumin analogues, i.e. 2,6-dibenzylidenecyclohexanone (A series), 2,5-dibenzylidenecyclopentanone (B series) and 1,4-pentadiene-3-one (C series) substituted analogues of curcumin towards recombinant human CYP1A2, CYP3A4, CYP2B6, CYP2C9 and CYP2D6, all important for drug metabolism, were studied in vitro. Fluorescence plate reader and high performance liquid chromatography (HPLC) assays were used to evaluate CYP-inhibitory activities. MOE-based Quantitative structure-activity relationship (QSAR) analysis suggested that electrostatic and hydrophobic interactions and lipophilicity are important factors for CYP inhibition. Apart from insights in important molecular properties for CYP inhibition, the present results may also guide further design of curcumin analogues with less susceptibility to drug-drug interactions.

Differential inhibition of hepatic microsomal alkoxyresorufin O-dealkylation activities by tetrachlorobiphenyls

Polychlorinated biphenyls (PCBs) elicit a spectrum of biochemical and toxic effects in exposed animals. In the present study, we assessed the effect of PCB structure, using four symmetrically-substituted PCBs, on cytochrome P450 (CYP)-mediated methoxy-, ethoxy- and benzyloxyresorufin O-dealkylase (MROD, EROD and BROD, respectively) activities. We found that 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) inhibited alkoxyresorufin O-dealkylase activities in hepatic microsomes from 3-methylcholanthrene (MC) or phenobarbital (PB)-treated rats. Measurement of the in vitro inhibitory potencies of the tetrachlorobiphenyls revealed that MROD, EROD and BROD activities were differentially inhibited and the degree of inhibition was determined by the chlorination pattern of the PCB. PCB 77 was more potent than PCB 47 or PCB 52 at inhibiting MROD and EROD activities in hepatic microsomes from MC-treated rats, while no inhibition of either activity was observed with PCB 54. In contrast, BROD activity measured in hepatic microsomes from PB-treated rats was inhibited by PCB 47, PCB 52 and PCB 54 but not by PCB 77. The mode of inhibition for each activity was also evaluated statistically. Inhibition of the alkoxyresorufin O-dealkylase activities could not be discerned in hepatic microsomes from corn oil-treated rats because the activities were inherently too low. No evidence for mechanism-based inhibition of MROD, EROD or BROD activities or an effect via CYP reductase was found. The results demonstrate that relatively coplanar PCBs such as PCB 77 preferentially inhibit EROD and MROD activities, whereas noncoplanar PCBs such as PCB 54 preferentially inhibit BROD activity.

Uncertainties related to the assignment of a toxic equivalency factor for 1,2,3,4,6,7,8,9-octachlorodibenzo-p-dioxin

The Toxic Equivalency Factor (TEF) approach is a methodology that assigns relative toxicity values to structurally related chemicals in comparison to a reference chemical. For the polychlorinated dibenzo-p-dioxins (PCDDs), the reference is the most potent congener, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Here, we critically review the literature on the effects of a weak PCDD, 1,2,3,4,6,7,8,9-octachlorodibenzo-p-dioxin (OCDD), and describe the uncertainties of assigning its TEF. PCDDs, including OCDD, are less potent in human cell models compared to the rat models from which the TEF are estimated. This lack of concordance is even more pronounced with the weaker congeners such as OCDD. Furthermore, OCDD is also likely to compete with TCDD for binding to cytochrome P4501A2 (CYP1A2), effectively decreasing the hepatic tissue/fat ratio of TCDD. Overall, the predictive value of TEFs would be improved by incorporating into this number the relative sensitivity of human cell responses compared to rodent responses, by determining the toxicological effects of altering the tissue distribution of dioxin-like compounds through competition for CYP-binding sites, and by understanding the mechanism of cancer causation of any dioxin and whether this mechanism is conserved in humans and at equivalent doses.

Comparison of TCDD and PCB CYP1A induction sensitivities in fresh hepatocytes from human donors, sprague-dawley rats, and rhesus monkeys and HepG2 cells

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related chemicals induce cytochrome P450 1A (CYP1A) gene expression and, at sufficient exposures, cause toxicity. Human health risks from such exposures are typically estimated from animal studies. We tested whether animal models predict human sensitivity by characterizing CYP1A gene expression in cultures of fresh hepatocytes from human donors, rats, and rhesus monkeys and HepG2 human hepatoma cells. We exposed the cells to three aryl hydrocarbon receptor (AhR) ligands of current environmental interest and measured 7-ethoxyresorufin-O-deethylase (EROD) activity and concentrations of CYP1A1 and CYP1A2 mRNA. We found that human cells are about 10-1000 times less sensitive to TCDD, 3,3',4,4',5-pentachlorobiphenyl (PCB 126), and Aroclor 1254 than rat and monkey cells, that relative potencies among these chemicals are different across species, and that gene expression thresholds exist for these chemicals. Newly calculated rat-human interspecies relative potency factors for PCB 126 were more than 100 times lower than the current rodent-derived value. We propose that human-derived values be used to improve the accuracy of estimates of human health risks.