2,3,7,8-Tetrachlorodibenzo-p-dioxin-mediated oxidative stress in CYP1A2 knockout (CYP1A2-/-) mice

Loss of flavin-containing monooxygenase 3 modulates dioxin-like polychlorinated biphenyl 126-induced oxidative stress and hepatotoxicity

Dioxin-like pollutants (DLPs), such as polychlorinated biphenyl 126 (PCB 126), are synthetic chemicals classified as persistent organic pollutants. They accumulate in adipose tissue and have been linked to cardiometabolic disorders, including fatty liver disease. The toxicity of these compounds is associated with activation of the aryl hydrocarbon receptor (Ahr), leading to the induction of phase I metabolizing enzyme cytochrome P4501a1 (Cyp1a1) and the subsequent production of reactive oxygen species (ROS). Recent research has shown that DLPs can also induce the xenobiotic detoxification enzyme flavin-containing monooxygenase 3 (FMO3), which plays a role in metabolic homeostasis. We hypothesized whether genetic deletion of Fmo3 could protect mice, particularly in the liver, where Fmo3 is most inducible, against PCB 126 toxicity. To test this hypothesis, male C57BL/6 wild-type (WT) mice and Fmo3 knockout (Fmo3 KO) mice were exposed to PCB 126 or vehicle (safflower oil) during a 12-week study, at weeks 2 and 4. Various analyses were performed, including hepatic histology, RNA-sequencing, and quantitation of PCB 126 and F2-isoprostane concentrations. The results showed that PCB 126 exposure caused macro and microvesicular fat deposition in WT mice, but this macrovesicular fatty change was absent in Fmo3 KO mice. Moreover, at the pathway level, the hepatic oxidative stress response was significantly different between the two genotypes, with the induction of specific genes observed only in WT mice. Notably, the most abundant F2-isoprostane, 8-iso-15-keto PGE2, increased in WT mice in response to PCB 126 exposure. The study's findings also demonstrated that hepatic tissue concentrations of PCB 126 were higher in WT mice compared to Fmo3 KO mice. In summary, the absence of FMO3 in mice led to a distinctive response to dioxin-like pollutant exposure in the liver, likely due to alterations in lipid metabolism and storage, underscoring the complex interplay of genetic factors in the response to environmental toxins.

Effect of retinyl acetate on transglutaminase 2 activity in carcinogen treated rat liver

Transglutaminase 2 (TG2) has been implicated in wound healing, cellular differentiation, apoptosis and cell survival. TG2 activity increases following acute and chronic liver injury; however, the role of TG2 in tumors, is controversial. TG2 is a retinoid-inducible enzyme. We investigated the effects of retinyl acetate (RA) on the activity and levels of TG2 during the initiation and promotion stages of liver cancer. p-Dimethylaminoazobenzene (p-DAB) was used as initiator and 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) was used as promoter in our model of carcinogenesis. Rats were divided into four groups of 24: control, corn oil control, p-DAB + TCDD, and p-DAB + TCDD + RA. Six rats from each group were sacrificed at days 30, 60, 90 and 120. TG2 activity decreased in the p-DAB + TCDD treated group, but TG2 immunostaining scores did not change by days 90 and 120. Neither TG2 enzyme activity nor the immunostaining score of TG2 protein changed in the tissues of the p-DAB + TCDD + RA group by days 90 and 120. TG2 activity was not be ameliorated by RA during the initiation or promotion stages of carcinogen induced liver cancer.

Physiologically-based pharmacokinetic modelling of immune, reproductive and carcinogenic effects from contaminant exposure in polar bears (Ursus maritimus) across the Arctic

Polar bears (Ursus maritimus) consume large quantities of seal blubber and other high trophic marine mammals and consequently have some of the highest tissue concentrations of organohalogen contaminants (OHCs) among Arctic biota. In the present paper we carried out a risk quotient (RQ) evaluation on OHC-exposed polar bears harvested from 1999 to 2008 and from 11 circumpolar subpopulations spanning from Alaska to Svalbard in order to evaluate the risk of OHC-mediated reproductive effects (embryotoxicity, teratogenicity), immunotoxicity and carcinogenicity (genotoxicity). This RQ evaluation was based on the Critical Body Residue (CBR) concept and a Physiologically-Based Pharmacokinetic Modelling (PBPK) approach using OHC concentrations measured in polar bear adipose or liver tissue. The range of OHC concentrations within polar bear populations were as follows for adipose, sum polychlorinated biphenyls ∑PCBs (1797-10,537 ng/g lw), sum methylsulphone-PCB ∑MeSO2-PCBs (110-672 ng/g lw), sum chlordanes ∑CHLs (765-3477 ng/g lw), α-hexachlorocyclohexane α-HCH (8.5-91.3 ng/g lw), β-hexachlorocyclohexane β-HCH (65.5-542 ng/g lw), sum chlorbenzenes ∑ClBzs (145-304 ng/g lw), dichlorodiphenyltrichloroethane ∑DDTs (31.5-206 ng/g lw), dieldrin (69-249 ng/g lw), polybrominated diphenyl ethers ∑PBDEs (4.6-78.4 ng/g lw). For liver, the perfluorooctanesulfonic acid (PFOS) concentrations ranged from 231-2792 ng/g ww. The total additive RQ from all OHCs ranged from 4.3 in Alaska to 28.6 in East Greenland bears for effects on reproduction, immune health and carcinogenicity, highlighting the important result that the toxic effect threshold (i.e. RQ>1) was exceeded for all polar bear populations assessed. PCBs were the main contributors for all three effect categories, contributing from 70.6% to 94.3% of the total risk and a RQ between 3.8-22.5. ∑MeSO2-PCBs were the second highest effect contributor for reproductive and immunological effects (0.17<RQ<1.4), whereas PFOS was the second highest effect contributor for carcinogenic (genotoxic) effects (0.35<RQ<2.5). The results from this study corroborate and lend further support to previous assessments of the possible adverse health effects of exposure to known and measured OHCs in polar bears. We therefore suggest that Critical Daily Doses (CDD) should be investigated in "ex vivo" dose-response studies on polar bears to replace laboratory studies on rats (Rattus rattus) to reveal whether high RQs are maintained.

Organochlorine pesticides induce epithelial to mesenchymal transition of human primary cultured hepatocytes

Persistent organic pollutants (POPs) are a group of organic or chemicals that adversely affect human health and are persistent in the environment. These highly toxic compounds include industrial chemicals, pesticides such as organochlorines, and unwanted wastes such as dioxins. Although studies have described the general toxicity effects of organochlorine pesticides, the mechanisms underlying its potential carcinogenic effects in the liver are not well understood. In this study, we analyzed the effect of three organochlorine pesticides (dichlorodiphenyltrichloroethane, heptachlore and endosulfan) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the epithelial to mesenchymal transition (EMT) in primary cultured human hepatocytes. We found that these compounds modified the hepatocyte phenotype, inducing cell spread, formation of lamellipodia structures and reorganization of the actin cytoskeleton in stress fibers. These morphological alterations were accompanied by disruption of cell-cell junctions, E-cadherin repression and albumin down-regulation. Interestingly, these characteristic features of dedifferentiating hepatocytes were correlated with the gain of expression of various mesenchymal genes, including vimentin, fibronectin and its receptor ITGA5. These various results show that organochlorines and TCDD accelerate cultured human hepatocyte dedifferentiation and EMT processes. These events could account, at least in part, for the carcionogenic and/or fibrogenic activities of these POPs.

Aryl hydrocarbon receptor ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin enhances liver damage in bile duct-ligated mice

The environmental pollutant 2,3,7,8-tetracholorodibenzo-p-dioxin (TCDD) is known to cause a wide variety of toxic effects, including hepatotoxicity, by way of the aryl hydrocarbon receptor (AHR). Although inducible expression of cytochrome P450 (CYP) 1A1 and CYP1A2 is associated with liver injury caused by high-dose TCDD, the specific role of the AHR-CYP1 cascade in hepatotoxicity remains unclear. We investigated the effects of AHR activation under conditions of cholestasis. We administered oral TCDD to mice at a dose that can effectively induce Cyp1 gene expression without overt liver toxicity and then ligated their bile ducts. TCDD pretreatment enhanced bile duct ligation (BDL)-induced increases in liver and plasma bile acids, bilirubin, and aminotransferases. Histology of TCDD-pretreated BDL mice revealed massive hepatic necrosis without any increase in number of apoptotic cells. Whereas induction of AHR-target genes by TCDD was observed similarly in sham-operated as well as in BDL mice, TCDD pretreatment of BDL mice altered the expression of hepatic genes involved in bile acid synthesis and transport. Increased plasma proinflammatory cytokines, tumor necrosis factor and interleukin-1β, in BDL mice were further elevated by TCDD pretreatment. Liver injury by TCDD plus BDL, such as increased plasma bile acids, bilirubin and aminotransferases, liver necrosis, and increased tumor necrosis factor production, was exaggerated in Cyp1a1/1a2(-/-) double knockout mice. These findings indicate that TCDD aggravates cholestatic liver damage and that the presence of CYP1A1 and CYP1A2 plays a protective role in liver damage caused by TCDD and BDL.

Dehydroascorbate protection against dioxin-induced toxicity in the beta-cell line INS-1E

Oxidative stress has been proposed as a mechanism of the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The aim of this research was to evaluate the protective effects of increased intracellular ascorbate levels against TCDD acute toxicity in the insulin-secreting beta-cell line INS-1E. Ascorbate is considered a potent antioxidant, but its therapeutic efficacy is greatly limited by its slow achievement of high intracellular levels. This might be circumvented by administration of dehydroascorbate (DHA), which is transported at a much higher rate and undergoes rapid intracellular reduction to ascorbate. Indeed, 30 min incubation of INS-1E cells with various concentrations of DHA caused a remarkable, dose-related increase of the intracellular ascorbate levels. INS-1E cells preincubated with 0.5 and 1.0mM DHA showed a greater viability than control cells after 1h exposition to cytotoxic TCDD concentrations. In our experimental conditions, TCDD surprisingly failed to increase ROS production in INS-1E cells, but induced a dose-related mitochondrial depolarisation which was significantly improved by DHA preincubation. Furthermore, DHA preincubation completely prevented the low dose TCDD-induced inhibition of glucose-stimulated insulin secretion. Thus, our results suggest that DHA preincubation protects INS-1E cells against TCDD acute toxicity by partially preserving mitochondrial function.

Molecular Mechanisms of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin Cardiovascular Embryotoxicity

2,3,7,8 Tetrachlorodibenzo-p-dioxin (TCDD) and related planar halogenated aromatic hydrocarbons are widespread environmental contaminants and potent developmental toxicants. Hallmarks of embryonic exposure include edema, hemorrhage, and mortality. Recent studies in zebrafish and chicken have revealed direct impairment of cardiac muscle growth that may underlie these overt symptoms. TCDD toxicity is mediated by the aryl hydrocarbon receptor, but downstream targets remain unclear. Oxidative stress and growth factor modulation have been implicated in TCDD cardiovascular toxicity. Gene expression profiling is elucidating additional pathways by which TCDD might act. We review our understanding of the mechanism of TCDD embryotoxicity at morphological and molecular levels.

Relative cancer potencies of selected dioxin-like compounds on a body-burden basis: comparison to current toxic equivalency factors (TEFs)

Recent National Toxicology Program (NTP) cancer bioassay data for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF), 3,3',4,4',5-pentachlorobiphenyl (PCB 126), and a mixture of these three compounds offer opportunities to assess the accuracy of current World Health Organization (WHO) 1998 toxic equivalency factors (TEFs) for these compounds under a variety of assumptions. An evaluation of the current TEF values for these compounds using body burden in nanograms per kilogram as the dose metric is presented. Average lifetime body burdens were estimated for all compounds at all dose groups based on measured tissue concentrations at 4 time points during the 2-yr NTP studies. Poly-3 adjusted tumor incidences for hepatocellular adenomas, cholangiocarcinomas, and the two tumors combined were modeled using a quantal multistage model and the Hill model with lifetime average body burden as the dose metric. Benchmark doses for a 10% response (BMD10) for each compound and the mixture were estimated. With TCDD as the reference standard, relative potency (REP) estimates were derived from ratios of the BMD10 estimates for PCB 126, 4-PeCDF, and for the toxic equivalent (TEQ) mixture. On a body-burden basis, PCB 126 and 4-PeCDF were 2- to 3-fold and 10- to 12-fold less potent than predicted based on the WHO TEFs, respectively, while the TEQ mixture was approximately 3- to 5-fold less potent than predicted by the TEFs. The current WHO TEF values, which were derived from data on noncancer endpoints evaluated on an administered dose basis, overpredict the carcinogenic potency of these compounds on a body-burden basis compared to TCDD.

Recommended Relative Potency Factors for 2,3,4,7,8-Pentachlorodibenzofuran: The Impact of Different Dose Metrics

The recent National Toxicology Program (NTP) cancer bioassays for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF) permit a reevaluation of the current TEF value of 4-PeCDF. The data also allow for the derivation of relative potency factors (RPFs) for cancer, which are based not only on administered dose but also on potentially more informative dose metrics, such as liver concentration, area under the liver concentration curve, and lifetime average body burden. Our analyses of these data indicate that chi-squared tests of observed versus predicted liver tumor incidence for 4-PeCDF reject the current TEF value of 0.5 value as too high. 4-PeCDF RPFs were derived using estimation methods that either did or did not assume parallelism of the 4-PeCDF and TCDD dose-response curves. The resulting parallelism-based RPFs for administered dose, liver concentration at terminal sacrifice, liver concentration AUC, and lifetime average body burden are 0.26, 0.014, 0.021, and 0.036, respectively. The administered dose RPF estimate is approximately one-half the current TEF value of 0.5. However, the use of administered dose fails to take into account pharmacokinetic differences between congeners and the generally acknowledged belief that body burden or some other measure of cumulative dose is more appropriate for estimating the health risk posed by persistent chemicals. The other three dose metrics do account for these important factors, and the corresponding RPFs are at least 10-fold lower than the current TEF for 4-PeCDF. In summary, our analyses support an administered dose TEF no greater than 0.25 and one in the 0.05-0.1 range for internal dose metrics such as lifetime average liver concentration or body burden.

Temporal and dose-dependent hepatic gene expression patterns in mice provide new insights into TCDD-Mediated hepatotoxicity

In an effort to further characterize the mechanisms of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated toxicity, comprehensive temporal and dose-response microarray analyses were performed on hepatic tissue from immature ovariectomized C57BL/6 mice treated with TCDD. For temporal analysis, mice were gavaged with 30 microg/kg of TCDD or vehicle and sacrificed after 2, 4, 8, 12, 18, 24, 72, or 168 h. Dose-response mice were gavaged with 0, 0.001, 0.01, 0.1, 1, 10, 100, or 300 microg/kg of TCDD and sacrificed after 24 h. Hepatic gene expression profiles were monitored using custom cDNA microarrays containing 13,362 cDNA clones. Gene expression analysis identified 443 and 315 features which exhibited a significant change at one or more doses or time points, respectively, as determined using an empirical Bayes approach. Functional gene annotation extracted from public databases associated gene expression changes with physiological processes such as oxidative stress and metabolism, differentiation, apoptosis, gluconeogenesis, and fatty acid uptake and metabolism. Complementary histopathology (H&E and Oil Red O stains), clinical chemistry (i.e., alanine aminotransferase [ALT], triglyceride [TG], free fatty acids [FFA], cholesterol) and high-resolution gas chromatography/mass spectrometry assessment of hepatic TCDD levels were also performed in order to phenotypically anchor changes in gene expression to physiological end points. Collectively, the data support a proposed mechanism for TCDD-mediated hepatotoxicity, including fatty liver, which involves mobilization of peripheral fat and inappropriate increases in hepatic uptake of fatty acids.

Zebrafish as a novel experimental model for developmental toxicology

It is widely believed that embryos and infants during development are highly sensitive to chemicals that cause serious damage to growth. However, knowledge on the mechanisms of developmental toxicity is scarce. One reason for this is limited convenient model system other than organ cultures using rodents to study the various aspects of developmental toxicology. Cultured cells are not always adequate for this purpose, since events in morphogenesis are processed through interactions with other tissues. We focused on zebrafish embryo (Danio rerio), one of the most important organisms in developmental biology. Saturation mutagenesis, applied to drosophila and nematode to define the functions of genes, has been carried out in zebrafish but almost no other vertebrate, and several thousand lines are available due to the rapid growth and transparent body of this embryo. Enhanced databases for the genome and ESTs are available at websites with abundant genetic and biological background. By targeted gene knock-down with morpholino-modified antisense oligonucleotieds (morpholinos), the translation of a specific protein can be transiently blocked for several days. Many reporter systems in vivo have been established mainly as GFP-transgenic fish for environmental chemicals. Although several excellent studies have been performed with zebrafish embryos on the effects of chemicals, the developmental toxicology of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been most extensively studied to date. We have found that TCDD induces apoptosis in dorsal midbrain with a concomitant decrease in local blood flow, using developing zebrafish. TCDD seems to produce oxidative stress through CYP1A induction in vascular endothelium, resulting in local circulation failure and apoptosis in the dorsal midbrain. In addition to applications in toxicology, an experimental system with zebrafish embryos could help to clarify the mechanism of congenital anomaly, which arises from genetic mutation.

Induction of cytochrome P450 1A is required for circulation failure and edema by 2,3,7,8-tetrachlorodibenzo-p-dioxin in zebrafish

The mechanism of toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is thought to result from changes in gene expression via the aryl hydrocarbon receptor (AHR). The induction of cytochrome P450 1A (CYP1A) in various organs is a cardinal effect of TCDD. However, whether CYP1A is involved in endpoints of TCDD toxicity is controversial. We investigated the role of CYP1A in TCDD-induced developmental toxicities using gene knock-down with morpholino antisense oligos. Exposure of zebrafish embryos to TCDD, at concentrations eliciting the hallmark endpoints of developmental toxicity, induced CYP1A in the heart and vascular endothelium throughout the body. This induction by TCDD was markedly inhibited by morpholinos to zebrafish arylhydrocarbon receptor 2 (zfAHR2-MO) and to zebrafish CYP1A (zfCYP1A-MO). The zfAHR2-MO but not the zfCYP1A-MO inhibited zfCYP1A mRNA expression, indicating the specificities of these morpholinos. Injection of either zfAHR2-MO or zfCYP1A-MO blocked the representative signs of TCDD developmental toxicity in zebrafish, pericardial edema and trunk circulation failure. The morpholinos appeared do not affect normal development in TCDD-untreated embryos. These results suggest a mediatory role of zfCYP1A induction through zfAHR2 activation in causing circulation failure by TCDD in zebrafish. This is the first molecular evidence demonstrating an essential requirement for CYP1A induction in TCDD-evoked developmental toxicities in any vertebrate species.

Mitochondrial reactive oxygen production is dependent on the aromatic hydrocarbon receptor

2,3,7,8-Tetrachlorodibenzo-p-dioxin (dioxin; TCDD) is a pervasive environmental contaminant that induces hepatic and extrahepatic oxidative stress. We have previously shown that dioxin increases mitochondrial respiration-dependent reactive oxygen production. In the present study we examined the dependence of mitochondrial reactive oxygen production on the aromatic hydrocarbon receptor (AHR), cytochrome P450 1A1 (CYP1A1), and cytochrome P450 1A2 (CYP1A2), proteins believed to be important in dioxin-induced liver toxicity. Congenic Ahr(-/-), Cyp1a1(-/-) and Cyp1a2(-/-) knockout mice, and C57BL/6J inbred mice as their Ahr/Cyp1a1/Cyp1a2(+/+) wild-type (wt) counterparts, were injected intraperitoneally with dioxin (15 microg/kg body weight) or corn-oil vehicle on 3 consecutive days. Liver mitochondria were examined 1 week following the first treatment. The level of mitochondrial H(2)O(2) production in vehicle-treated Ahr(-/-) mice was one fifth that found in vehicle-treated wt mice. Whereas dioxin caused a rise in succinate-stimulated mitochondrial H(2)O(2) production in the wt, Cyp1a1(-/-), and Cyp1a2(-/-) mice, this increase did not occur with the Ahr(-/-) knockout. The lack of H(2)O(2) production in Ahr(-/-) mice was not due to low levels of Mn(2+)-superoxide dismutase (SOD2) as shown by Western immunoblot analysis, nor was it due to high levels of mitochondrial glutathione peroxidase (GPX1) activity. Dioxin decreased mitochondrial aconitase (an enzyme inactivated by superoxide) by 44% in wt mice, by 26% in Cyp1a2(-/-) mice, and by 24% in Cyp1a1(-/-) mice; no change was observed in Ahr(-/-) mice. Dioxin treatment increased mitochondrial glutathione levels in the wt, Cyp1a1(-/-), and Cyp1a2(-/-) mice, but not in Ahr(-/-) mice. These results suggest that both constitutive and dioxin-induced mitochondrial reactive oxygen production is associated with a function of the AHR, and these effects are independent of either CYP1A1 or CYP1A2.

Response of anti-oxidant enzymes mRNA in the neonatal rat liver exposed to 1,2,3,4-tetrachlorodibenzo-p-dioxin via lactation

BACKGROUND

The aim of this study was to assess the response to dioxin-induced oxidative stress in neonates via lactation in the model we have described previously.

METHODS

Maternal rats were treated with a single dose of 50 or 100 micro mol/kg 1,2,3,4-tetrachlorodibenzo-p-dioxin (TCDD) on the first day postpartum (day 1). Messenger RNA levels of the key anti-oxidant enzymes (AOE), phospholipid hydroperoxide-glutathione peroxidase (PH-GPx), cellular-glutathione peroxidase (cell-GPx), copper-zinc superoxide dismutase (CuZn SOD), manganese superoxide dismutase (Mn SOD) and catalase (CAT) in the neonatal and maternal livers were determined by a competitive reverse transcription- polymerase chain reaction method.

RESULTS

Lactational transfer of 1,2,3,4-TCDD induced an inhibition of PH-GPx and cell-GPx mRNA in the neonatal liver on day 2 to 68 (P < 0.01) and 62% (P < 0.05) of the control at 100 micro mol/kg, respectively. Both GPx mRNA returned to control levels on day 6 and thereafter increased to levels higher than the controls on day 10. In the dam rat, 10 days after the treatment, no remarkable change of PH-GPx or cell-GPx mRNA was observed. Copper-zinc superoxide dismutase and CAT mRNA of neonates on day 2 were also suppressed at 100 micro mol/kg and then slightly increased on day 10. However, Mn SOD mRNA was not suppressed, but increased to a 2.1-fold level of the control (P < 0.05) on day 10 with 100 micro mol/kg 1,2,3,4-TCDD.

CONCLUSION

Quantitative analysis of AOE mRNA showed that PH-GPx and cell-GPx mRNA, as well as CuZn SOD and CAT mRNA in the neonatal liver were suppressed for a short period of time by 1,2,3,4-TCDD exposure via lactation. Dioxin induced oxidative stress by lactational transfer may alter pretranslation regulation of protective AOE in neonates.

In vivo effects of Panax ginseng extracts on the cytochrome P450-dependent monooxygenase system in the liver of 2,3,7,8-tetrachlorodibenzo-p-dioxin-exposed guinea pig

The effects of the subchronic administration of Panax ginseng extracts were examined on the hepatic cytochrome P450-dependent monooxygenase system of guinea pigs pre-exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Panax ginseng extracts were intraperitoneally administered to guinea pigs at 100 mg/kg/day for 14 days from 1 week after a single intraperitoneal injection of 1 microg of TCDD/kg of body weight. TCDD treatment increased the total cytochrome P450 content 2.86-fold, and this was remarkably inhibited by the administration of Panax ginseng extracts. Treatment with ginseng extract alone also decreased the contents of cytochrome P450 by 33%, but both TCDD and ginseng extracts had no effect on cytochrome b(5) content. The administration of TCDD resulted in a 1.73-fold increase in microsomal NADPH-cytochrome P450 reductase activity in the guinea pig liver, and this was significantly inhibited by ginseng extracts, but treatment with ginseng extracts alone had no effect on its activity, and no statistical changes in the activity of NADPH-cytochrome b(5) reductase were observed in guinea pig liver due to TCDD and/or ginseng extract administration. Compared to the control, ECOD activity remarkably (1.76-fold) increased after TCDD administration, but this increase was completely inhibited by treatment with ginseng extract. Treatment with ginseng extract alone resulted in a 50% reduction of ECOD activity. TCDD administration remarkably induced benzphetamine demethylation (BPDM) activity, while ginseng extract also slightly increased the enzyme's activity, but the induction attributed to ginseng extracts was not statistically significant. Even though administration of ginseng extracts slightly inhibited TCDD-induced BPDM activity, the inhibition was not statistically significant. These results indicate that ginseng extract exerts different effect on the induction of P450 isozymes. From these results, we suggest that Panax ginseng extracts may act as an inhibitor of CYP1A rather than that of CYP2B.

Aryl hydrocarbon receptor (AhR)-mediated induction of xanthine oxidase/xanthine dehydrogenase activity by 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), an environmental contaminant, induced xanthine oxidase and xanthine dehydrogenase (XO/XDH) activities, in addition to ethoxyresorufin-O-dealkylase and methoxyresorufin-O-dealkylase activities in liver of mice. When TCDD was given to mice as a single oral dose of 40 microg/kg, the activities of XO and XDH increased about threefold within 3 days and the increased levels were maintained for 4 weeks. The treatment of mice with 3-methylcholanthrene also induced XO/XDH activities, but phenobarbital and dexamethasone had no effect. The level of aldehyde oxidase, a molybdenum flavoenzyme related to XO/XDH, in mouse liver was also enhanced about 1.5-fold by TCDD treatment. The inducing effect of TCDD and 3-methylcholanthrene was not observed in null mice (AhR(-/-)), which lack the AhR gene. XO and XDH activities were induced by TCDD in heterozygous mice (AhR(+/-)). The lipid peroxidation in liver was stimulated by TCDD. The induction of XO and XDH, which produces reactive oxygen species, may contribute to the various toxicities of TCDD.

Using cytochrome P-450 gene knock-out mice to study chemical metabolism, toxicity, and carcinogenicity

Cytochrome P-450 (CYP) enzymes are heme-containing proteins that carry out oxidative metabolism of a wide range of structurally diverse exogenous chemicals and therapeutic agents as well as endogenous compounds. For some of these xenobiotics, oxidative metabolism results in the formation of toxic, mutagenic, or carcinogenic metabolites. In the past, the role of CYP enzymes in metabolism and chemical-induced toxicity was studied indirectly through use of specific antibodies or inducers and inhibitors of these enzymes. Progress in molecular biology and the ability to bioengineer animal models that do not express CYP1A2, CYP1A1, CYP1B1, CYP2E1, or both CYP1A2 and CYP2E1 isozymes has allowed for direct investigations of the in vivo role of these enzymes in the metabolism, toxicity, and carcinogenicity of xenobiotics. This article reviews research conducted to date that utilizes these genetically bioengineered mice in metabolism, toxicity, or carcinogenicity studies of chemicals. Some studies showed a positive correlation between in vivo results and in vitro predictions for the role of a specific CYP in chemical-induced effects, whereas other studies did not support in vitro predictions. Work reviewed herein demonstrates the importance of using animal models for investigating the role of specific CYP enzymes in metabolism and chemical-induced toxicity or carcinogenicity rather than relying solely on in vitro techniques. Eventually, studies of this nature will facilitate a more accurate assessment of human risks with regard to chemicals by helping us to understand the relationships between chemical metabolism, carcinogenicity, and polymorphisms in CYP enzymes.