Assessment of the disposition of chiral polychlorinated biphenyls in female mdr 1a/b knockout versus wild-type mice using multivariate analyses

Networks of placental DNA methylation correlate with maternal serum PCB concentrations and child neurodevelopment

BACKGROUND

Gestational exposure to polychlorinated biphenyls (PCBs) has been associated with elevated risk for neurodevelopmental disorders. Placental epigenetics may serve as a potential mechanism of risk or marker of altered placental function. Prior studies have associated differential placental DNA methylation with maternal PCB exposure or with increased risk of autism spectrum disorder (ASD). However, sequencing-based placental methylomes have not previously been tested for simultaneous associations with maternal PCB levels and child neurodevelopmental outcomes.

OBJECTIVES

We aimed to identify placental DNA methylation patterns associated with maternal PCB levels and child neurodevelopmental outcomes in the high-risk ASD MARBLES cohort.

METHODS

We measured 209 PCB congeners in 104 maternal serum samples collected at delivery. We identified networks of DNA methylation from 147 placenta samples using the Comethyl R package, which performs weighted gene correlation network analysis for whole genome bisulfite sequencing data. We tested placental DNA methylation modules for association with maternal serum PCB levels, child neurodevelopment, and other participant traits.

RESULTS

PCBs 153 + 168, 170, 180 + 193, and 187 were detected in over 50% of maternal serum samples and were highly correlated with one another. Consistent with previous findings, maternal age was the strongest predictor of serum PCB levels, alongside year of sample collection, pre-pregnancy BMI, and polyunsaturated fatty acid levels. Twenty seven modules of placental DNA methylation were identified, including five which significantly correlated with one or more PCBs, and four which correlated with child neurodevelopment. Two modules associated with maternal PCB levels as well as child neurodevelopment, and mapped to CSMD1 and AUTS2, genes previously implicated in ASD and identified as differentially methylated regions in mouse brain and placenta following gestational PCB exposure.

CONCLUSIONS

Placental DNA co-methylation modules were associated with maternal PCBs and child neurodevelopment. Methylation of CSMD1 and AUTS2 could be markers of altered placental function and/or ASD risk following maternal PCB exposure.

Assessment of Polychlorinated Biphenyls and Their Hydroxylated Metabolites in Postmortem Human Brain Samples: Age and Brain Region Differences

Exposure to polychlorinated biphenyls (PCBs) and their hydroxylated metabolites (OH-PCBs) has been implicated in neurodevelopmental disorders. However, the distribution of PCBs and OH-PCBs in the human brain has not been characterized. This study investigated the age-, sex-, and brain region-specific distribution of all 209 PCBs using gaschromatography-tandem mass spectrometry (GC-MS/MS) in neonatal (N = 7) and adult (N = 7) postmortem brain samples. OH-PCB analyses were performed by GC-MS/MS (as methylated derivatives) and, in a subset of samples, by nontarget liquid chromatography high-resolution mass spectrometry (Nt-LCMS). Fourteen higher chlorinated PCB congeners were observed with a detection frequency >50%. Six lower chlorinated PCBs were detected with a detection frequency >10%. Higher chlorinated PCBs were observed with higher levels in samples from adult versus younger donors. PCB congener profiles from adult donors showed more similarities across brain regions and donors than younger donors. We also assess the potential neurotoxicity of the PCB residues in the human brain with neurotoxic equivalency (NEQ) approaches. The median ΣNEQs, calculated for the PCB homologues, were 40-fold higher in older versus younger donors. Importantly, lower chlorinated PCBs made considerable contributions to the neurotoxic potential of PCB residues in some donors. OH-PCBs were identified for the first time in a small number of human brain samples by GC-MS/MS and Nt-LCMS analyses, and all contained four or fewer chlorine.

The disposition of polychlorinated biphenyls (PCBs) differs between germ-free and conventional mice

The disposition of toxicants, such as polychlorinated biphenyls (PCBs), in germ-free (GF) vs. conventional (CV) mice has received little attention to date. Here, we investigate PCB levels in three-month-old female CV and GF mice exposed orally daily for 3 days to 0, 6, or 30 mg/kg body weight of the Fox River Mixture (FRM), an environmental PCB mixture. We euthanized animals 24 h after the final dose. PCB profiles in tissues differed from the FRM profile but were similar in tissues across all 4 PCB exposure groups. PCB levels in CV but not GF mice followed the difference in PCB dose. Importantly, PCB levels were higher in CV than GF mice exposed to the same dose. Hepatic cytochrome P450 enzyme or lipid levels did not explain these trends in PCB tissue levels. Thus, toxicity studies with CV and GF animals need to assess the toxicokinetics of the toxicant investigated. CAPSULE: PCB levels are typically higher in conventional than germ-free mice exposed to the same dose of PCBs.

Metabolic activation mechanism of 2,2',3,3',6,6'-hexachlorobiphenyl (PCB136) by cytochrome P450 2B6: A QM/MM approach

Cytochrome P450 enzymes (CYPs) play an essential role in the bio-transformation of polychlorinated biphenyls (PCBs). The present work implemented quantum mechanic/molecular mechanic methods (QM/MM) and density functional theory (DFT) to study the metabolic activation of 2,2',3,3',6,6'-hexachlorobiphenyl (PCB136) catalyzed by CYP2B6. Electrophilic additions at the C^α and C^β positions generate different active intermediates. The electrophilic addition energy barrier of C^β is 10.9 kcal/mol higher than that of C^α, and C^α is the preferred site for the electrophilic addition reaction. Based on the previous experimental studies, this work investigated the mechanism of converting active intermediates into OH-PCB136, which has high toxicity in a non-enzymatic environment. Structural analysis via the electrostatic and noncovalent interactions indicates that Phe108, Ile114, Phe115, Phe206, Phe297, Ala298, Leu363, Val367, TIP32475 and TIP32667 play crucial roles in substrate recognition and metabolism. The analysis suggests that the halogen-π interactions are important factors for the metabolism of CYP2B6 to halogenated environmental pollutants. This work improved the understanding of the metabolism and activation process of chiral PCBs, and can be used as a guide to improve the microbial degradation efficiency of PCB136.

Fatty liver and impaired hepatic metabolism alter the congener-specific distribution of polychlorinated biphenyls (PCBs) in mice with a liver-specific deletion of cytochrome P450 reductase

Polychlorinated biphenyls (PCBs) are persistent organic pollutants that are linked to adverse health outcomes. PCB tissue levels are determinants of PCB toxicity; however, it is unclear how factors, such as an altered metabolism and/or a fatty liver, affect PCB distribution in vivo. We determined the congener-specific disposition of PCBs in mice with a liver-specific deletion of cytochrome P450 reductase (KO), a model of fatty liver with impaired hepatic metabolism, and wild-type (WT) mice. Eight-week-old male WT (M_WT, n = 3), male KO (M_KO, n = 5), female WT (F_WT, n = 4), and female KO mice (F_KO, n = 4) were exposed orally to Aroclor 1254. PCBs were quantified in adipose, blood, brain, and liver tissues by gas chromatography-mass spectrometry. The ΣPCB levels followed the rank order adipose > liver ∼ brain > blood in WT and adipose ∼ liver > brain > blood in KO mice. PCB levels were much higher in the liver of KO than WT mice, irrespective of the sex. A comparison across exposure groups revealed minor genotype and sex-dependent differences in the PCB congener profiles (cos Θ > 0.92). Within each exposure group, tissue profiles showed small differences between tissues (cos Θ = 0.85 to 0.98). These differences were due to a decrease in metabolically more labile PCB congeners and an increase in congeners resistant to metabolism. The tissue-to-blood ratio of PCBs decreased for adipose, increased for the brain, and remained constant for the liver with an increase in chlorination. While these ratios did not follow the trends predicted using a composition-based model, the agreement between experimental and calculated partition coefficients was reasonable. Although the distribution of PCBs differs between KO and WT mice, the magnitude of the partitioning of PCBs from the blood into tissues can be approximated using composition-based models.

Fatty liver and impaired hepatic metabolism alter the congener-specific distribution of polychlorinated biphenyls (PCBs) in mice with a liver-specific deletion of cytochrome P450 reductase

Polychlorinated biphenyls (PCBs) are persistent organic pollutants that are linked to adverse health outcomes. PCB tissue levels are determinants of PCB toxicity; however, it is unclear how factors, such as an altered metabolism and/or a fatty liver, affect PCB distribution in vivo. We determined the congener-specific disposition of PCBs in mice with a liver-specific deletion of cytochrome P450 reductase (KO), a model of fatty liver with impaired hepatic metabolism, and wild-type (WT) mice. Eight-week-old male WT (M_WT, n = 3), male KO (M_KO, n = 5), female WT (F_WT, n = 4), and female KO mice (F_KO, n = 4) were exposed orally to Aroclor 1254. PCBs were quantified in adipose, blood, brain, and liver tissues by gas chromatography-mass spectrometry. The ΣPCB levels followed the rank order adipose > liver ∼ brain > blood in WT and adipose ∼ liver > brain > blood in KO mice. PCB levels were much higher in the liver of KO than WT mice, irrespective of the sex. A comparison across exposure groups revealed minor genotype and sex-dependent differences in the PCB congener profiles (cos Θ > 0.92). Within each exposure group, tissue profiles showed small differences between tissues (cos Θ = 0.85 to 0.98). These differences were due to a decrease in metabolically more labile PCB congeners and an increase in congeners resistant to metabolism. The tissue-to-blood ratio of PCBs decreased for adipose, increased for the brain, and remained constant for the liver with an increase in chlorination. While these ratios did not follow the trends predicted using a composition-based model, the agreement between experimental and calculated partition coefficients was reasonable. Although the distribution of PCBs differs between KO and WT mice, the magnitude of the partitioning of PCBs from the blood into tissues can be approximated using composition-based models.

Evaluating the Role of the Steroid and Xenobiotic Receptor (SXR/PXR) in PCB-153 Metabolism and Protection against Associated Adverse Effects during Perinatal and Chronic Exposure in Mice

BACKGROUND

Polychlorinated biphenyls (PCBs) are environmental toxicants; PCB exposure has been associated with adverse effects on wildlife and humans. However, the mechanisms underlying these adverse effects are not fully understood. The steroid and xenobiotic receptor [SXR; also known as the pregnane X receptor (PXR) and formally known as NR1I2] is a nuclear hormone receptor that regulates inducible metabolism of drugs and xenobiotics and is activated or inhibited by various PCB congeners.

OBJECTIVES

The aim of this study was to investigate the effects of exposure to PCB-153, the most prevalent PCB congener in human tissues, on SXR knockout mice (SXRKO) and to elucidate the role of SXR in PCB-153 metabolism and promotion of its harmful effects.

METHODS

Wild-type (WT) and SXRKO mice were chronically or perinatally exposed to a low dose (54μg/kg/d) of PCB-153. Blood, livers, and spleens were analyzed using transcriptome sequencing (RNA-seq) and molecular techniques to investigate the impacts of exposure on metabolism, oxidative stress, and hematological parameters.

RESULTS

SXRKO mice perinatally exposed to PCB-153 displayed elevated oxidative stress, symptoms of hemolytic anemia, and premature death. Transcriptomal analysis revealed that expression of genes involved in metabolic processes was altered in SXRKO mice. Elevated levels of the PCB-153 metabolite, 3-OH-PCB-153, were found in exposed SXRKO mice compared to exposed WT mice. Blood hemoglobin (HGB) levels were lower throughout the lifespan, and the occurrence of intestinal tumors was larger in SXRKO mice chronically exposed to PCB-153 compared to vehicle and WT controls.

DISCUSSION

Our results suggest that altered metabolism induced by SXR loss of function resulted in the accumulation of hydroxylated metabolites upon exposure to PCB-153, leading to oxidative stress, hemolytic anemia, and tumor development in a mouse model. These results support a major role for SXR/PXR in protection against xenobiotic-induced oxidative stress by maintaining proper metabolism in response to PCB-153 exposure. This role of SXR could be generally applicable to other environmental toxicants as well as pharmaceutical drugs. https://doi.org/10.1289/EHP6262.

Transplacental transfer mechanism of organochlorine pesticides: An in vitro transcellular transport study

Recent studies show that, even after being banned for agricultural applications for over 30 years, organochlorine pesticides (OCPs), including hexachlorocyclohexanes (HCHs) and dichlorodiphenyltrichloroethane (DDT) and its metabolites (DDXs), can still be found in various biological matrices and pose a potential hazard to the fetus in the womb. This study aimed to investigate the possible transplacental transfer mechanism of OCPs using an in vitro placental model. The results showed that for HCHs and DDXs, the placenta had a potential protection mechanism for the fetus by having higher efflux than intake active transport efficiency to transfer the xenobiotic out of the fetal circulation. No enantiomer-specific transport was observed for the chiral OCPs in vitro, hints simple diffusion played the major role in the transplacental transfer. Metabolic and transporter inhibitors were applied in the transepithelial transport experiment to evaluate the role that major transporting protein played in the active efflux process. The ATP production inhibitors were observed to have significant inhibition on transfer, proving the hypothesis that active transport participates in the transplacental transport of OCPs in humans. Multiple transporters contributed simultaneously in the active transport for the OCPs. In this study, we could confirm that the transplacental transfer of OCPs is a combination of simple diffusion and active transport. ATP-binding cassette (ABC) superfamily transporters on the placenta contribute in the active transport. These findings could improve the understanding of the mechanisms of transplacental transfer of the OCPs.

Atropselective Disposition of 2,2',3,4',6-Pentachlorobiphenyl (PCB 91) and Identification of Its Metabolites in Mice with Liver-Specific Deletion of Cytochrome P450 Reductase

Hepatic cytochrome P450 enzymes metabolize chiral polychlorinated biphenyls (PCBs) to hydroxylated metabolites (OH-PCBs). Animal models with impaired metabolism of PCBs are one approach to study how the atropselective oxidation of PCBs to OH-PCBs contributes to toxic outcomes, such as neurodevelopmental disorders, following PCB exposure. We investigated the disposition of PCB 91, a para-substituted PCB congener, in mice with a liver-specific deletion of the cytochrome P450 reductase (cpr) gene (KO mice). KO mice and wild-type (WT) mice were exposed orally to racemic PCB 91 (30 mg/kg b.w.). Levels and enantiomeric fractions of PCB 91 and its hydroxylated metabolites were determined in tissues 3 days after PCB exposure and in excreta on days 1-3 after PCB exposure. PCB 91, but not OH-PCB levels were higher in KO compared to WT mice. The elevated fat and protein content in the liver of KO mice resulted in the hepatic accumulation of PCB 91. OH-PCBs were detected in blood, liver, and excreta samples of KO and WT mice. 2,2',3,4',6-Pentachlorobiphenyl-5-ol (5-91) was the major metabolite. A considerable percent of the total PCB 91 dose (%TD) was excreted with the feces as 5-91 (23%TD and 31%TD in KO and WT mice, respectively). We tentatively identified glucuronide and sulfate metabolites present in urine samples. The PCB 91 atropisomer eluting first on the chiral column (E₁-PCB 91) displayed genotype-dependent atropisomeric enrichment, with a more pronounced atropisomeric enrichment observed in WT compared to KO mice. E₁-atropisomers of 5-91 and 2,2',3,4',6-pentachlorobiphenyl-4-ol (4-91) were enriched in blood and liver, irrespective of the genotype; however, the extent of the enrichment of E₁-5-91 was genotype dependent. These differences in atropselective disposition are consistent with slower metabolism of PCB 91 in KO compared to WT mice and the accumulation of the parent PCB in the fatty liver of KO mice.

Atropselective Oxidation of 2,2',3,3',4,6'-Hexachlorobiphenyl (PCB 132) to Hydroxylated Metabolites by Human Liver Microsomes and Its Implications for PCB 132 Neurotoxicity

Polychlorinated biphenyls (PCBs) have been associated with neurodevelopmental disorders. Several neurotoxic congeners display axial chirality and atropselectively affect cellular targets implicated in PCB neurotoxicity. Only limited information is available regarding the atropselective metabolism of these congeners in humans and their atropselective effects on neurotoxic outcomes. Here we investigate the hypothesis that the oxidation of 2,2',3,3',4,6'-hexachlorobiphenyl (PCB 132) by human liver microsomes (HLMs) and their effects on dopaminergic cells in culture are atropselective. Racemic PCB 132 was incubated with pooled or single donor HLMs, and levels and enantiomeric fractions of PCB 132 and its metabolites were determined gas chromatographically. The major metabolite was either 2,2',3,4,4',6'-hexachlorobiphenyl-3'-ol (3'-140), a 1,2-shift product, or 2,2',3,3',4,6'-hexachlorobiphenyl-5'-ol (5'-132). The PCB 132 metabolite profiles displayed inter-individual differences and depended on the PCB 132 atropisomer. Computational studies suggested that 3'-140 is formed via a 3,4-arene oxide intermediate. The second eluting atropisomer of PCB 132, first eluting atropisomer of 3'-140, and second eluting atropisomer of 5'-132 were enriched in all HLM incubations. Enantiomeric fractions of the PCB 132 metabolites differed only slightly between the single donor HLM preparations investigated. Reactive oxygen species and levels of dopamine and its metabolites were not significantly altered after a 24 h exposure of dopaminergic cells to pure PCB 132 atropisomers. These findings suggest that there are inter-individual differences in the atropselective biotransformation of PCB 132 to its metabolites in humans; however, the resulting atropisomeric enrichment of PCB 132 is unlikely to affect neurotoxic outcomes associated with the endpoints investigated in the study.

Toxicokinetics of Chiral PCB 136 and Its Hydroxylated Metabolites in Mice with a Liver-Specific Deletion of Cytochrome P450 Reductase

Exposure to polychlorinated biphenyls (PCBs) has been implicated in adverse human health effects, including developmental neurotoxicity. Several neurotoxic PCBs are chiral and undergo atropisomeric enrichment in vivo due to atropselective metabolism by cytochrome P450 enzymes. Here we study how the liver-specific deletion of the cytochrome P450 reductase ( cpr) gene alters the toxicokinetics of 2,2',3,3',6,6'-hexachlorobiphenyl (PCB 136) in mice. Male and female mice with a liver-specific deletion of cpr (KO) and congenic wild-type (WT) mice were exposed to a single oral dose of racemic PCB 136 (6.63 mg/kg). Levels and chiral signatures of PCB 136 and its hydroxylated metabolites were determined 1-48 h after PCB exposure in whole blood. Blood levels of PCB 136 were typically higher in M-WT compared to F-WT mice. At the later time points, F-KO mice had significantly higher PCB 136 levels than F-WT mice. 2,2',3',4,6,6'-Hexachlorobiphenyl-3-ol (3-150), 2,2',3,3',6,6'-hexachlorobiphenyl-4-ol (4-136), 2,2',3,3',6,6'-hexachlorobiphenyl-5-ol (5-136), and 4,5-dihydroxy-2,2',3,3',6,6'-hexachlorobiphenyl (4,5-136) were detected in blood, with 5-136 and 4-136 being major metabolites. At later time points, the sum of HO-PCB (∑HO-PCB) levels exceeded PCB 136 levels in the blood; however, higher ∑HO-PCB than PCB 136 levels were observed later in KO than WT mice. PCB 136 and its major metabolites displayed atropisomeric enrichment in a manner that depended on the time point, sex, and genotype. Toxicokinetic analysis revealed sex and genotype-dependent differences in toxicokinetic parameters for PCB 136 atropisomers and its metabolites. The results suggest that mice with a liver-specific deletion of the cpr gene can potentially be used to assess how an altered metabolism of neurotoxic PCB congeners affects neurotoxic outcomes following exposure of the offspring to PCBs via the maternal diet.

Sources and toxicities of phenolic polychlorinated biphenyls (OH-PCBs)

Polychlorinated biphenyls (PCBs), a group of 209 congeners that differ in the number and position of chlorines on the biphenyl ring, are anthropogenic chemicals that belong to the persistent organic pollutants (POPs). For many years, PCBs have been a topic of interest because of their biomagnification in the food chain and their environmental persistence. PCBs with fewer chlorine atoms, however, are less persistent and more susceptible to metabolic attack, giving rise to chemicals characterized by the addition of one or more hydroxyl groups to the chlorinated biphenyl skeleton, collectively known as hydroxylated PCBs (OH-PCBs). In animals and plants, this biotransformation of PCBs to OH-PCBs is primarily carried out by cytochrome P-450-dependent monooxygenases. One of the reasons for infrequent detection of lower chlorinated PCBs in serum and other biological matrices is their shorter half-lives, and their metabolic transformation, resulting in OH-PCBs or their conjugates, such as sulfates and glucuronides, or macromolecule adducts. Recent biomonitoring studies have reported the presence of OH-PCBs in human serum. The occurrence of OH-PCBs, the size of this group (there are 837 mono-hydroxyl PCBs alone), and their wide spectra of physical characteristics (pKa's and log P's ranging over 5 to 6 orders of magnitude) give rise to a multiplicity of biological effects. Among those are bioactivation to electrophilic metabolites that can form covalent adducts with DNA and other macromolecules, interference with hormonal signaling, inhibition of enzymes that regulate cellular concentrations of active hormones, and interference with the transport of hormones. This new information creates an urgent need for a new perspective on these often overlooked metabolites.

Chiral polychlorinated biphenyls: absorption, metabolism and excretion--a review

Seventy eight out of the 209 possible polychlorinated biphenyl (PCB) congeners are chiral, 19 of which exist under ambient conditions as stable rotational isomers that are non-superimposable mirror images of each other. These congeners (C-PCBs) represent up to 6 % by weight of technical PCB mixtures and undergo considerable atropisomeric enrichment in wildlife, laboratory animals, and humans. The objective of this review is to summarize our current knowledge of the processes involved in the absorption, metabolism, and excretion of C-PCBs and their metabolites in laboratory animals and humans. C-PCBs are absorbed and excreted by passive diffusion, a process that, like other physicochemical processes, is inherently not atropselective. In mammals, metabolism by cytochrome P450 (P450) enzymes represents a major route of elimination for many C-PCBs. In vitro studies demonstrate that C-PCBs with a 2,3,6-trichlorosubstitution pattern in one phenyl ring are readily oxidized to hydroxylated PCB metabolites (HO-PCBs) by P450 enzymes, such as rat CYP2B1, human CYP2B6, and dog CYP2B11. The oxidation of C-PCBs is atropselective, thus resulting in a species- and congener-dependent atropisomeric enrichment of C-PCBs and their metabolites. This atropisomeric enrichment of C-PCBs and their metabolites likely plays a poorly understood role in the atropselective toxicity of C-PCBs and, therefore, warrants further investigation.

Effect of pregnancy on the disposition of 2,2',3,5',6-pentachlorobiphenyl (PCB 95) atropisomers and their hydroxylated metabolites in female mice

Chiral PCBs, such as PCB 95, are developmental neurotoxicants that undergo atropisomeric enrichment in nonpregnant adult mice. Because pregnancy is associated with changes in hepatic cytochrome P450 enzyme activity as well as lipid disposition and metabolism, this study investigates the effect of pregnancy on the maternal disposition of chiral PCBs. Female C57BL/6 mice (8 weeks old) were dosed daily beginning 2 weeks prior to conception and continuing throughout gestation and lactation (56 days total) with racemic PCB 95 (0, 0.1, 1.0, or 6.0 mg/kg body wt/day) in peanut butter. Levels and chiral signatures of PCB 95 and its hydroxylated metabolites (OH-PCBs) were determined in adipose, blood, brain, and liver. Tissue levels of PCB 95 increased 4- to 12-fold with increasing dose, with considerable enrichment of the second eluting atropisomer in all tissues (EF range 0.11 to 0.26). OH-PCBs displayed atropisomeric enrichment in blood and liver but were not detected in adipose and brain. Levels of PCB 95 and its metabolites were 2- to 11-fold lower in pregnant dams relative to those previously reported in nonpregnant age-matched female mice; however, PCB 95 and OH-PCB profiles and chiral signatures were similar between both studies. In contrast, human brain samples contained racemic PCB 95 residues (EF = 0.50). These results demonstrate that changes in cytochrome P450 enzyme activity and lipid disposition during pregnancy reduce the PCB body burden in dams but do not affect metabolite profiles or chiral signatures. The differences in chiral signatures between mice and humans suggest species-specific differences in atropisomeric disposition, the toxicological significance of which remains to be determined.

Hepatic metabolism affects the atropselective disposition of 2,2',3,3',6,6'-hexachlorobiphenyl (PCB 136) in mice

To understand the role of hepatic vs extrahepatic metabolism in the disposition of chiral PCBs, we studied the disposition of 2,2',3,3',6,6'-hexachlorobiphenyl (PCB 136) and its hydroxylated metabolites (HO-PCBs) in mice with defective hepatic metabolism due to the liver-specific deletion of cytochrome P450 oxidoreductase (KO mice). Female KO and congenic wild type (WT) mice were treated with racemic PCB 136, and levels and chiral signatures of PCB 136 and HO-PCBs were determined in tissues and excreta 3 days after PCB administration. PCB 136 tissue levels were higher in KO compared to WT mice. Feces was a major route of PCB metabolite excretion, with 2,2',3,3',6,6'-hexachlorobiphenyl-5-ol being the major metabolite recovered from feces. (+)-PCB 136, the second eluting PCB 136 atropisomers, was enriched in all tissues and excreta. The second eluting atropisomers of the HO-PCBs metabolites were enriched in blood and liver; 2,2',3,3',6,6'-hexachlorobiphenyl-5-ol in blood was an exception and displayed an enrichment of the first eluting atropisomers. Fecal HO-PCB levels and chiral signatures changed with time and differed between KO and WT mice, with larger HO-PCB enantiomeric fractions in WT compared to KO mice. Our results demonstrate that hepatic and, possibly, extrahepatic cytochrome P450 (P450) enzymes play a role in the disposition of PCBs.

Increase in accumulation of polychlorinated biphenyls in offspring mouse brain via maternal coexposure to methylmercury and polychlorinated biphenyls

Methylmercury (MeHg) and polychlorinated biphenyls (PCBs) are environmentally persistent neurodevelopmental toxicants. In Japan, the most common source of human exposure is the consumption of contaminated fish and seafood. We investigated the accumulation of MeHg and PCBs in the brains of dams and offspring mice maternally exposed to MeHg and/or PCBs. Pregnant mice (C57BL/6Cr) were assigned to one of four exposure groups: control, MeHg alone (MeHg in diet at 5 mg/kg as Hg), PCB alone (Aroclor1254 by gavage at the dose of 18 mg/kg body weight/3 days) and MeHg+PCB. Levels of MeHg and PCBs were measured in the whole brains of dams and offspring mice on postnatal day 21 (PND21) and at 9 weeks of age. Total mercury, MeHg and PCB congener concentrations were determined by CVAAS, GC-ECD and HRGC/HRMS, respectively. For the mercury concentrations, there were no significant differences between MeHg alone and MeHg+PCB except for pups at 9 weeks. The maternal PCB levels were not significantly different between MeHg+PCB and PCB alone. In pup brain on PND21, MeHg+PCB resulted in a significantly higher PCB level than PCB alone. Although the levels of lightly chlorinated (= 4CBs) homologues were lower for MeHg+PCB than for PCB alone, those of highly chlorinated (>= 5CBs) homologues were significantly higher on PND21 for MeHg+PCB. The PCB composition in dams and pups shifted to higher chlorinated homologues compared with the composition of administered Aroclor1254. For these reasons, further detailed studies are necessary to clarify the interactional effects of PCB metabolism after coexposure to MeHg and PCBs.

Oxidation of polychlorinated biphenyls by liver tissue slices from phenobarbital-pretreated mice is congener-specific and atropselective

Mouse models are powerful tools to study the developmental neurotoxicity of polychlorinated biphenyls (PCBs); however, studies of the oxidation of chiral PCB congeners to potentially neurotoxic hydroxylated metabolites (OH-PCBs) in mice have not been reported. Here, we investigate the atropselective oxidation of chiral PCB 91 (2,2',3,4',6-pentachlorobiphenyl), PCB 95 (2,2',3,5',6-pentachlorobiphenyl), PCB 132 (2,2',3,3',4,6'-hexachlorobiphenyl), PCB 136 (2,2',3,3',6,6'-hexachlorobiphenyl), and PCB 149 (2,2',3,4',5',6-hexachlorobiphenyl) to OH-PCBs in liver tissue slices prepared from female mice. The metabolite profile of PCB 136 typically followed the rank order 5-OH-PCB > 4-OH-PCB > 4,5-OH-PCB, and metabolite levels increased with PCB concentration and incubation time. A similar OH-PCB profile was observed with the other PCB congeners, with 5-OH-PCB/4-OH-PCB ratios ranging from 2 to 12. More 5-OH-PCB 136 was formed in liver tissue slices obtained from animals pretreated with phenobarbital (P450 2B inducer) or, to a lesser extent, dexamethasone (P450 2B and 3A enzyme inducer) compared to tissue slices prepared from vehicle-pretreated animals. The apparent rate of 5-OH-PCBs formation followed the approximate rank order PCB 149 > PCB 91 > PCB 132 ∼ PCB 136 > PCB 95. Atropselective gas chromatography revealed a congener-specific atropisomeric enrichment of major OH-PCB metabolites. Comparison of our results with published OH-PCB patterns and chiral signatures (i.e., the direction and extent of the atropisomeric enrichment) from rat liver microsomal revealed drastic differences between both species, especially following the induction of P450 2B enzymes. These species differences in the metabolism of chiral PCBs should be considered in developmental neurotoxicity studies of PCBs.

Metabolism of 2,2',3,3',6,6'-hexachlorobiphenyl (PCB 136) atropisomers in tissue slices from phenobarbital or dexamethasone-induced rats is sex-dependent

1. Chiral polychlorinated biphenyls (PCBs) such as PCB 136 enantioselectively sensitize the ryanodine receptor (RyR). In light of recent evidence that PCBs cause developmental neurotoxicity via RyR-dependent mechanisms, this suggests that enantioselective PCB metabolism may influence the developmental neurotoxicity of chiral PCBs. However, enantioselective disposition of PCBs has not been fully characterized. 2. The effect of sex and cytochrome P450 (P450) enzyme induction on the enantioselective metabolism of PCB 136 was studied using liver tissue slices prepared from naïve control (CTL), phenobarbital (PB; CYP2B inducer) or dexamethasone (DEX; CYP3A inducer) pretreated adult Sprague-Dawley rats. PCB 136 metabolism was also examined in hippocampal slices derived from untreated rat pups. 3. In liver tissue slices, hydroxylated PCB (OH-PCB) profiles depended on sex and inducer pretreatment, and OH-PCB levels followed the rank orders male > female and PB > DEX > CTL. In contrast, the enantiomeric enrichment of PCB 136 and its metabolites was independent of sex and inducer pretreatment. Only small amounts of PCB 136 partitioned into hippocampal tissue slices and no OH-PCB metabolites were detected. 4. Our results suggest that enantioselective metabolism, sex and induction status of P450 enzymes in the liver may modulate the neurotoxic outcomes of developmental exposure to chiral PCBs.

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.

Gas chromatographic analysis with chiral cyclodextrin phases reveals the enantioselective formation of hydroxylated polychlorinated biphenyls by rat liver microsomes

Chiral PCB congeners are major components of PCB mixtures and undergo enantioselective biotransformation to hydroxylated (OH-)PCBs by cytochrome P450 enzymes. While it is known that biotransformation results in an enantiomeric enrichment of the parent PCB, it is currently unknown if OH-PCBs are formed enantioselectively. The present study screened seven commercial capillary gas chromatography columns containing modified β- or γ-cyclodextrins for their potential to separate the atropisomers of methylated derivatives of OH-PCB. The atropisomers of 3-, 4- and 5-methoxy derivatives were at least partially separated on one or more columns. A subsequent biotransformation study was performed with rat liver microsomes to assess if hydroxylated metabolites are formed enantioselectively from PCBs 91, 95, 132, and 149. The OH-PCBs were extracted from the microsomal incubations, derivatized with diazomethane and analyzed as the respective methoxylated (MeO-)PCB derivatives using selected columns. The 5-hydroxylated metabolites of PCBs 91, 95, 132, and 149 were the major metabolites, which is consistent with PCB's biotransformation by cytochrome P450 2B enzymes. All 5-hydroxylated metabolites displayed a clear, congener-specific enantiomeric enrichment. Overall, this study demonstrates for the first time that chiral PCBs, such as PCB 91, 95, 132, and 149, are enantioselectively metabolized to OH-PCBs by cytochrome P450 enzymes.

Chiral polychlorinated biphenyl transport, metabolism, and distribution: a review

Chirality can be exploited to gain insight into enantioselective fate processes that may otherwise remain undetected because only biological, but not physical and chemical transport and transformation processes in an achiral environment will change enantiomer compositions. This review provides an in-depth overview of the application of chirality to the study of chiral polychlorinated biphenyls (PCBs), an important group of legacy pollutants. Like other chiral compounds, individual PCB enantiomers may interact enantioselectively (or enantiospecifically) with chiral macromolecules, such as cytochrome P-450 enzymes or ryanodine receptors, leading to differences in their toxicological effects and the enantioselective formation of chiral biotransformation products. Species and congener-specific enantiomer enrichment has been demonstrated in environmental compartments, wildlife, and mammals, including humans, typically due to a complex combination of biotransformation processes and uptake via the diet by passive diffusion. Changes in the enantiomer composition of chiral PCBs in the environment have been used to understand complex aerobic and anaerobic microbial transformation pathways, to delineate and quantify PCB sources and transport in the environment, to gain insight into the biotransformation of PCBs in aquatic food webs, and to investigate the enantioselective disposition of PCBs and their methylsulfonyl PCBs metabolites in rodents. Overall, changes in chiral signatures are powerful, but currently underutilized tools for studies of environmental and biological processes of PCBs.