37 research outputs found
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
Genetic differences in the aryl hydrocarbon receptor and CYP1A2 affect sensitivity to developmental polychlorinated biphenyl exposure in mice: relevance to studies of human neurological disorders
Polychlorinated biphenyls (PCBs) are persistent organic pollutants that remain a human health concern with newly discovered sources of contamination and ongoing bioaccumulation and biomagnification. Children exposed during early brain development are at highest risk of neurological deficits, but highly exposed adults reportedly have an increased risk of Parkinson\u27s disease. Our previous studies found allelic differences in the aryl hydrocarbon receptor and cytochrome P450 1A2 (CYP1A2) affect sensitivity to developmental PCB exposure, resulting in cognitive deficits and motor dysfunction. High-affinity Ahr b Cyp1a2(-/-) mice were most sensitive compared with poor-affinity Ahr d Cyp1a2(-/-) and wild-type Ahr b Cyp1a2(+/+) mice. Our follow-up studies assessed biochemical, histological, and gene expression changes to identify the brain regions and pathways affected. We also measured PCB and metabolite levels in tissues to determine if genotype altered toxicokinetics. We found evidence of AHR-mediated toxicity with reduced thymus and spleen weights and significantly reduced thyroxine at P14 in PCB-exposed pups. In the brain, the greatest changes were seen in the cerebellum where a foliation defect was over-represented in Cyp1a2(-/-) mice. In contrast, we found no difference in tyrosine hydroxylase immunostaining in the striatum. Gene expression patterns varied across the three genotypes, but there was clear evidence of AHR activation. Distribution of parent PCB congeners also varied by genotype with strikingly high levels of PCB 77 in poor-affinity Ahr d Cyp1a2(-/-) while Ahr b Cyp1a2(+/+) mice effectively sequestered coplanar PCBs in the liver. Together, our data suggest that the AHR pathway plays a role in developmental PCB neurotoxicity, but we found little evidence that developmental exposure is a risk factor for Parkinson\u27s disease
Chlordane and Heptachlor Are Metabolized Enantioselectively by Rat Liver Microsomes
Chlordane, heptachlor, and their
metabolites are chiral persistent
organic pollutants that undergo enantiomeric enrichment in the environment.
This study investigated the enantioselective metabolism of both chlordane
isomers and heptachlor, major components of technical chlordane, by
liver microsomes prepared from male rats treated with corn oil (CO)
or inducers of CYP2B (PB; phenobarbital) and CYP3A enzymes (DX; dexamethasone),
isoforms induced by chlordane treatment. The extent of the metabolism
of all three parent compounds was dependent on the microsomal preparation
used and followed the rank order PB > DX > CO. The mass balances
ranged
from 49 to 130% of the parent compound added to the microsomal incubations.
Both <i>cis</i>- and <i>trans-</i>chlordane were
enantioselectively metabolized to oxychlordane (EF = 0.45–0.89)
and 1,2-dichlorochlordene (EF = 0.42–0.90). Heptachlor was
metabolized enantioselectively, with heptachlor epoxide B (EF = 0.44–0.54)
being the only metabolite. Interestingly, the direction on the enrichment
for oxychlordane, 1,2-dichlorochlordene, and heptachlor epoxide differed
depending on the microsomal preparation. These findings demonstrate
that the direction and extent of the enantioselective metabolism of
both chlordane isomers and heptachlor is P450 isoform-dependent and
can be modulated by the induction of P450 enzymes
Cytochrome p450 mRNA expression in the rodent brain: species-, sex-, and region-dependent differences.
Cytochrome P450 (P450) enzymes play a critical role in the activation and detoxication of many neurotoxic chemicals. Although research has largely focused on P450-mediated metabolism in the liver, emerging evidence suggests that brain P450s influence neurotoxicity by modulating local metabolite levels. As a first step toward better understanding the relative role of brain P450s in determining neurotoxic outcome, we characterized mRNA expression of specific P450 isoforms in the rodent brain. Adult mice (male and female) and rats (male) were treated with vehicle, phenobarbital, or dexamethasone. Transcripts for CYP2B, CYP3A, CYP1A2, and the orphan CYP4X1 and CYP2S1 were quantified in the liver, hippocampus, cortex, and cerebellum by quantitative (real-time) polymerase chain reaction. These P450s were all detected in the liver with the exception of CYP4X1, which was detected in rat but not mouse liver. P450 expression profiles in the brain varied regionally. With the exception of the hippocampus, there were no sex differences in regional brain P450 expression profiles in mice; however, there were marked species differences. In the liver, phenobarbital induced CYP2B expression in both species. Dexamethasone induced hepatic CYP2B and CYP3A in mice but not rats. In contrast, brain P450s did not respond to these classic hepatic P450 inducers. Our findings demonstrate that P450 mRNA expression in the brain varies by region, regional brain P450 profiles vary between species, and their induction varies from that of hepatic P450s. These novel data will be useful for designing mechanistic studies to examine the relative role of P450-mediated brain metabolism in neurotoxicity
Subacute nicotine co-exposure has no effect on 2,2',3,5',6- pentachlorobiphenyl disposition but alters hepatic cytochrome P450 expression in the male rat.
Polychlorinated biphenyls (PCBs) are metabolized by cytochrome P450 2B enzymes (CYP2B) and nicotine is reported to alter CYP2B activity in the brain and liver. To test the hypothesis that nicotine influences PCB disposition, 2,2',3,5',6-pentachlorobiphenyl (PCB 95) and its metabolites were quantified in tissues of adult male Wistar rats exposed to PCB 95 (6mg/kg/d, p.o.) in the absence or presence of nicotine (1.0mg/kg/d of the tartrate salt, s.c.) for 7 consecutive days. PCB 95 was enantioselectively metabolized to hydroxylated (OH-) PCB metabolites, resulting in a pronounced enrichment of E1-PCB 95 in all tissues investigated. OH-PCBs were detected in blood and liver tissue, but were below the detection limit in adipose, brain and muscle tissues. Co-exposure to nicotine did not change PCB 95 disposition. CYP2B1 mRNA and CYP2B protein were not detected in brain tissues but were detected in liver. Co-exposure to nicotine and PCB 95 increased hepatic CYP2B1 mRNA but did not change CYP2B protein levels relative to vehicle control animals. However, hepatic CYP2B protein in animals co-exposed to PCB 95 and nicotine were reduced compared to animals that received only nicotine. Quantification of CYP2B3, CYP3A2 and CYP1A2 mRNA identified significant effects of nicotine and PCB 95 co-exposure on hepatic CYP3A2 and hippocampal CYP1A2 transcripts. Our findings suggest that nicotine co-exposure does not significantly influence PCB 95 disposition in the rat. However, these studies suggest a novel influence of PCB 95 and nicotine co-exposure on hepatic cytochrome P450 (P450) expression that may warrant further attention due to the increasing use of e-cigarettes and related products
Stereoselective Formation of Mono- and Dihydroxylated Polychlorinated Biphenyls by Rat Cytochrome P450 2B1
Changes in atropisomer composition
of chiral polychlorinated biphenyls (PCBs) and their mono- and dihydroxylated
metabolites (OH- and diOH-PCBs) via rat cytochrome P450 2B1 (CYP2B1)
mediated biotransformation were investigated <i>in vitro</i>. Rat CYP2B1 could stereoselectively biotransform chiral PCBs to
generate <i>meta</i>-OH-PCBs as the major metabolites after
60 min incubations. Nonracemic enantiomer fractions (EFs: concentration
ratios of the (+)-atropisomer or the first-eluting atropisomer over
the total concentrations of two atropisomers) of 5-OH-PCBs, were 0.17,
0.20, 0.85, 0.77, and 0.41 for incubations with PCBs 91, 95, 132,
136, and 149, respectively. CYP-mediated stereoselective formation
of diOH-PCBs from OH-PCBs was observed for the first time. After 60
min stereoselective biotransformation, the EFs of both 4-OH-PCB 95
and 5-OH-PCB 95 changed from racemic (i.e., 0.50) to 0.62 and 0.46,
respectively. These transformations generated statistically nonracemic
4,5-diOH-PCB 95, with EFs of 0.53 and 0.58 for 4-OH-PCB 95 and 5-OH-PCB
95 incubations, respectively. Biotransformation of PCBs 91 and 136
also generated 4,5-diOH-PCB 91 and 4,5-diOH-PCB 136, respectively.
These <i>in vitro</i> results were consistent with that
observed for stereoselective PCB biotransformation by rat liver microsomes
and <i>in vivo</i>. Biotransformation interference between
two atropisomers of PCB 136 was investigated for the first time in
this study. The biotransformation process of (-)-PCB 136 was significantly
disrupted by the presence of (+)-PCB 136 but not the other way around.
Thus, stereoselective metabolism of chiral PCBs and OH-PCBs by CYPs
is a major mechanism for atropisomer composition change of PCBs and
their metabolites in the environment, with the degree of composition
change dependent, at least in part, on stereoselective interference
of atropisomers with each other at the enzyme level