o,p'-DDT elicits PXR/CAR-, not ER-, mediated responses in the immature ovariectomized rat liver

Mitochondrial and Proteasome Dysfunction Occurs in the Hearts of Mice Treated with Triazine Herbicide Prometryn

Prometryn is a methylthio-s-triazine herbicide used to control the growth of annual broadleaf and grass weeds in many cultivated plants. Significant traces of prometryn are documented in the environment, mainly in waters, soil, and plants used for human and domestic consumption. Previous studies have shown that triazine herbicides have carcinogenic potential in humans. However, there is limited information about the effects of prometryn on the cardiac system in the literature, or the mechanisms and signaling pathways underlying any potential cytotoxic effects are not known. It is important to understand the possible effects of exogenous compounds such as prometryn on the heart. To determine the mechanisms and signaling pathways affected by prometryn (185 mg/kg every 48 h for seven days), we performed proteomic profiling of male mice heart with quantitative liquid chromatography-tandem mass spectrometry (LC-MS/MS) using ten-plex tandem mass tag (TMT) labeling. The data suggest that several major pathways, including energy metabolism, protein degradation, fatty acid metabolism, calcium signaling, and antioxidant defense system were altered in the hearts of prometryn-treated mice. Proteasome and immunoproteasome activity assays and expression levels showed proteasome dysfunction in the hearts of prometryn-treated mice. The results suggest that prometryn induced changes in mitochondrial function and various signaling pathways within the heart, particularly affecting stress-related responses.

Dichlorodiphenyltrichloroethane Impairs Amyloid Beta Clearance by Decreasing Liver X Receptor α Expression

Abnormal amyloid beta (Aβ) clearance is a distinctive pathological mechanism for Alzheimer’s disease (AD). ATP-binding cassette transporter A1 (ABCA1), which mediates the lipidation of apolipoprotein E, plays a critical role in Aβ clearance. As an environmental factor for AD, dichlorodiphenyltrichloroethane (DDT) can decrease ATP-binding cassette transporter A1 (ABCA1) expression and disrupt Aβ clearance. Liver X receptor α (LXRα) is an autoregulatory transcription factor for ABCA1 and a target of some environmental pollutants, such as organophosphate pesticides. In this study, we aimed to investigate whether DDT could affect Aβ clearance by targeting LXRα. The DDT-pretreated H4 human neuroglioma cells and immortalized astrocytes were incubated with exogenous Aβ to evaluate Aβ consumption. Meanwhile, cytotoxicity and LXRα expression were determined in the DDT-treated cells. Subsequently, the antagonism of DDT on LXRα agonist T0901317 was determined in vitro. The interaction between DDT and LXRα was predicted by molecular docking and molecular dynamics simulation technology. We observed that DDT could inhibit Aβ clearance and decrease the levels of LXRα mRNA and LXRα protein. Moreover, DDT is supposed to strongly bind to LXRα and exert antagonistic effects on LXRα. In conclusion, this study firstly presented that DDT could inhibit LXRα expression, which would contribute to Aβ clearance decline in vitro. It provides an experimental basis to search for potential therapeutic targets of AD.

Critical evaluation of the human relevance of the mode of action for rodent liver tumor formation by activators of the constitutive androstane receptor (CAR)

Many nongenotoxic chemicals have been shown to produce liver tumors in mice and/or rats by a mode of action (MOA) involving activation of the constitutive androstane receptor (CAR). Studies with phenobarbital (PB) and other compounds have identified the key events for this MOA: CAR activation; increased hepatocellular proliferation; altered foci formation; and ultimately the development of adenomas/carcinomas. In terms of human relevance, the pivotal species difference is that CAR activators are mitogenic agents in mouse and rat hepatocytes, but they do not stimulate increased hepatocellular proliferation in humans. This conclusion is supported by substantial in vitro studies with cultured rodent and human hepatocytes and also by in vivo studies with chimeric mice with human hepatocytes. Examination of the literature reveals many similarities in the hepatic effects and species differences between activators of rodent CAR and the peroxisome proliferator-activated receptor alpha (PPARα), with PPARα activators also not being mitogenic agents in human hepatocytes. Overall, a critical analysis of the available data demonstrates that the established MOA for rodent liver tumor formation by PB and other CAR activators is qualitatively not plausible for humans. This conclusion is supported by data from several human epidemiology studies.

Effects of the organochlorine p,p'-DDT on MCF-7 cells: Investigating metabolic and immune modulatory transcriptomic changes

The organochlorine pesticide dichloro-diphenyl-trichloroethane (DDT) is persistent in the environment and leads to adverse human health effects. High levels in breast milk pose a threat to both breast tissue and nursing infants. The objectives of this study were to investigate DDT-induced transcriptomic alterations in enzymes and transporters involved in xenobiotic metabolism, immune responses, oxidative stress markers, and cell growth in a human breast cancer cell line. MCF-7 cells were exposed to both environmentally-relevant and previously-tested concentrations of p,p'-DDT in a short-term experiment. Significant up-regulation of metabolizing enzymes and transporters (ACHE, GSTO1, NQO1 and ABCC2) and oxidative stress markers (CXCL8, HMOX-1, NFE2L2 and TNF) was clearly observed. Conversely, UGT1A6, AHR and cell growth genes (FGF2 and VEGFA) were severely down-regulated. Identification of these genes helps to identify mechanisms of p,p'-DDT action within cells and may be considered as useful biomarkers for exposure to DDT contamination.

DDT exposure induces cell cycle arrest and apoptosis of skin fibroblasts from Indo-Pacific humpback dolphin via mitochondria dysfunction

Although the global use of the 1,1,1-trichloro-2,2-bis (4-chlorophenyl) ethane (p,p'-DDT) has been prohibited, its persistence in the environment has caused long-lasting exposure on marine mammals. Our previous studies revealed exceedingly high residue levels of DDTs in Indo-Pacific humpback dolphins (Sousa chinensis) from the Pearl River Estuary region, China. However, the molecular mechanisms of p,p'-DDT toxicity on the dolphin are largely unknown. This study conducted the first cytotoxicity effect exploration of p,p'-DDT on the dolphin skin fibroblasts (ScSFs) to enhance the understanding of the cellular and molecular regulation impacts. ScSF cells were exposed to p,p'-DDT (28∼168 μM) for 24, 48 and 72 h. The exposure remarkably decreased viability of ScSF cells, possibly due to the synergetic effects of cell cycle arrest and apoptosis via DNA damage and mitochondria dysfunction. The DNA damage and mitochondria dysfunction were likely triggered by an increase of cellular reactive oxygen species (ROS), alteration in mitochondrial membrane potential, reduction in the cellular ATP levels, decreased expression of the genes CDK1, CDK4, cyclin B1, cyclin D1 and apoptosis regulator Bcl-2, release of cytochrome c, and activation of caspase-3, caspase-8 and caspase-9. Moreover, caspase inhibitor displayed protective activity against p,p'-DDT-induced apoptosis, indicating that caspases played a central role in p,p'-DDT-triggered apoptosis in the ScSF cells. We hypothesize apoptosis likely plays a minor role in cytocidal effects induced by p,p'-DDT exposure, but the mechanisms remain unclear. Overall, this research provides new evidence of the cytotoxic mechanisms underlying p,p'-DDT exposure on humpback dolphin skin cells, and suggests that p,p'-DDT contamination is one of key health concern issues for the protection of this marine mammal.

A case study on the application of an expert-driven read-across approach in support of quantitative risk assessment of p,p'-dichlorodiphenyldichloroethane

Deriving human health risk estimates for environmental chemicals has traditionally relied on in vivo toxicity databases to characterize potential adverse health effects and associated dose-response relationships. In the absence of in vivo toxicity information, new approach methods (NAMs) such as read-across have the potential to fill the required data gaps. This case study applied an expert-driven read-across approach to identify and evaluate analogues to fill non-cancer oral toxicity data gaps for p,p'-dichlorodiphenyldichloroethane (p,p'-DDD), an organochlorine contaminant known to occur at contaminated sites in the U.S. The source analogue p,p'-dichlorodiphenyltrichloroethane (DDT) and its no-observed-adverse-effect level of 0.05 mg/kg-day were proposed for the derivation of screening-level health reference values for the target chemical, p,p'-DDD. Among the primary similarity contexts (structure, toxicokinetics, and toxicodynamics), toxicokinetic considerations were instrumental in separating p,p'-DDT as the best source analogue from other potential candidates (p,p'-DDE and methoxychlor). In vitro high-throughput screening (HTS) assays from ToxCast were used to evaluate similarity in bioactivity profiles and make inferences toward plausible mechanisms of toxicity to build confidence in the read-across approach. This work demonstrated the value of NAMs such as read-across and in vitro HTS in human health risk assessment of environmental contaminants with the potential to inform regulatory decision-making.

Metabolomics Approach to Investigate Estrogen Receptor-Dependent and Independent Effects of o,p'-DDT in the Uterus and Brain of Immature Mice

Previous studies have demonstrated the endocrine disruption of o,p'-DDT. In this study, we used a ¹H NMR based metabolomics approach to investigate the estrogenic effects of o,p'-DDT (300 mg/kg) on the uterus and brain after 3 days of oral gavage administration, and ethynylestradiol (EE, 100 μg/kg) was used as a positive control. A supervised statistical analysis (PLS-DA) indicated that o,p'-DDT exerted both estrogenic receptor-(ER)-dependent and independent effects on the uterus but mainly ER-independent effects on the brain at metabolome levels, which was verified by coexposing with the antiestrogenic ICI 182,780. Four changed metabolites-glycine, choline, fumarate, and phenylalanine-were identified as ER-independent alterations in the uterus, while more metabolites, including γ-aminobutyrate, N-acetyl aspartate, and some amino acids, were disturbed based on the ER-independent mechanism in the brain. Together with biological end points, metabolomics is a promising approach to study potential estrogenic chemicals.

Direct effect of p,p'- DDT on mice liver

ABSTRACT Contact with the pesticide dichlorodiphenyltrichloroethane (p,p′-DDT) can be the cause of various harmful effects in humans, wildlife, and the environment. This pesticide is known to be persistent, lipophilic, resistant to degradation, and bioaccumulive in the environment and to be slowly released into bloodstream. Growing evidence shows that exposure to DDT is linked to type 2 diabetes mellitus. Individuals exposed to elevated levels of DDT and its metabolite have an increased prevalence of diabetes and insulin resistance. To evaluate these possible relationships, experiments were performed on eight-week-old female mice, divided into three groups (n = 10 per group): Group 1 received a vehicle-control intraperitoneal (i.p.) injection of sesame oil; Groups 2 and 3 received an i.p. dose of 50 and 100 µg/g p,p′-DDT respectively, dissolved in sesame oil. All groups were treated once daily for four days. Real-time PCR analysis of several genes was undertaken. Additionally, biochemical parameters and histopathological changes were measured. NQO1, HMOX1, NR1I3 and NR3C1 were up-regulated in DDT-exposed animals compared to the vehicle control group, while only SREBP1 was down-regulated in the 100 µg/g group. MTTP and FABP5, not previously reported for DDT exposure, but involved in regulation of fatty acid fluxes, could also function as biomarkers cross-talking between these signaling pathways. These results suggest that beyond epidemiological data, there is increasing molecular evidence that DDT may mimic different processes involved in diabetes and insulin resistance pathways.

Small-molecule modulators of the constitutive androstane receptor

INTRODUCTION

The constitutive androstane receptor (CAR) induces drug-metabolizing enzymes for xenobiotic metabolism.

AREAS COVERED

This review covers recent advances in elucidating the biological functions of CAR and its modulation by a growing number of agonists and inhibitors.

EXPERT OPINION

Extrapolation of animal CAR function to that of humans should be carefully scrutinized, particularly when rodents are used in evaluating the metabolic profile and carcinogenic properties of clinical drugs and environmental chemicals. Continuous efforts are needed to discover novel CAR inhibitors, with extensive understanding of their inhibitory mechanism, species selectivity, and discriminating power against other xenobiotic sensors.

Clinical application of transcriptional activators of bile salt transporters

Hepatobiliary bile salt (BS) transporters are critical determinants of BS homeostasis controlling intracellular concentrations of BSs and their enterohepatic circulation. Genetic or acquired dysfunction of specific transport systems causes intrahepatic and systemic retention of potentially cytotoxic BSs, which, in high concentrations, may disturb integrity of cell membranes and subcellular organelles resulting in cell death, inflammation and fibrosis. Transcriptional regulation of canalicular BS efflux through bile salt export pump (BSEP), basolateral elimination through organic solute transporters alpha and beta (OSTα/OSTβ) as well as inhibition of hepatocellular BS uptake through basolateral Na(+)-taurocholate cotransporting polypeptide (NTCP) represent critical steps in protection from hepatocellular BS overload and can be targeted therapeutically. In this article, we review the potential clinical implications of the major BS transporters BSEP, OSTα/OSTβ and NTCP in the pathogenesis of hereditary and acquired cholestatic syndromes, provide an overview on transcriptional control of these transporters by the key regulatory nuclear receptors and discuss the potential therapeutic role of novel transcriptional activators of BS transporters in cholestasis.

Dichlorodiphenyltrichloroethane technical mixture regulates cell cycle and apoptosis genes through the activation of CAR and ERα in mouse livers

Dichlorodiphenyltrichloroethane (DDT) is a widely used organochlorine pesticide and a xenoestrogen that promotes rodent hepatomegaly and tumours. A recent study has shown significant correlation between DDT serum concentration and liver cancer incidence in humans, but the underlying mechanisms remain elusive. We hypothesised that a mixture of DDT isomers could exert effects on the liver through pathways instead of classical ERs. The acute effects of a DDT mixture containing the two major isomers p,p'-DDT (85%) and o,p'-DDT (15%) on CAR and ERα receptors and their cell cycle and apoptosis target genes were studied in mouse livers. ChIP results demonstrated increased CAR and ERα recruitment to their specific target gene binding sites in response to the DDT mixture. The results of real-time RT-PCR were consistent with the ChIP data and demonstrated that the DDT was able to activate both CAR and ERα in mouse livers, leading to target gene transcriptional increases including Cyp2b10, Gadd45β, cMyc, Mdm2, Ccnd1, cFos and E2f1. Western blot analysis demonstrated increases in cell cycle progression proteins cMyc, Cyclin D1, CDK4 and E2f1 and anti-apoptosis proteins Mdm2 and Gadd45β. In addition, DDT exposure led to Rb phosphorylation. Increases in cell cycle progression and anti-apoptosis proteins were accompanied by a decrease in p53 content and its transcriptional activity. However, the DDT was unable to stimulate the β-catenin signalling pathway, which can play an important role in hepatocyte proliferation. Thus, our results indicate that DDT treatment may result in cell cycle progression and apoptosis inhibition through CAR- and ERα-mediated gene activation in mouse livers. These findings suggest that the proliferative and anti-apoptotic conditions induced by CAR and ERα activation may be important contributors to the early stages of hepatocarcinogenesis as produced by DDT in rodent livers.

In vitro effect of N-acetylcysteine on hepatocyte injury caused by dichlorodiphenyltrichloroethane and its metabolites

The organochlorine pesticide, dichlorodiphenyltrichloroethane (DDT), is still used to combat the spread of malaria in several developing countries despite its accumulation and known hepatotoxic effects that have been demonstrated both in vitro and in vivo. N-Acetylcysteine (NAC) is a recognized hepatoprotective agent that has been reported to reduce hepatotoxicity initiated by many different compounds. The aim of this study was to determine whether NAC could counter in vitro hepatocyte injury induced by DDT or its two major metabolites, dichlorodiphenyldichloroethylene and dichlorodiphenyldichloroethane. HepG2 cell cultures were used to assess the following parameters of toxicity: cellular viability, intracellular levels of reactive oxygen species (ROS), mitochondrial membrane potential and initiation of apoptosis. None of the three test compounds induced ROS generation, yet exposure to any of the three compounds produced mitochondrial hyperpolarization, which was countered by NAC pretreatment. All three test compounds also induced apoptotic cell death, which was inhibited by NAC. Despite NAC counteracting some adverse intracellular changes due to organochlorine exposure, it appeared to aggravate the cytotoxic effects of the organochlorine compounds at low test concentrations. As the same outcome may also occur in vivo, results from the present study raise concern about the use of NAC as treatment for DDT-induced hepatotoxicity.

Low concentrations of o,p'-DDT inhibit gene expression and prostaglandin synthesis by estrogen receptor-independent mechanism in rat ovarian cells

o,p'-DDT is an infamous xenoestrogen as well as a ubiquitous and persistent pollutant. Biomonitoring studies show that women have been internally exposed to o,p'-DDT at range of 0.3-500 ng/g (8.46×10(-10) M-1.41×10(-6) M) in blood and other tissues. However, very limited studies have investigated the biological effects and mechanism(s) of o,p'-DDT at levels equal to or lower than current exposure levels in human. In this study, using primary cultures of rat ovarian granulosa cells, we determined that very low doses of o,p'-DDT (10(-12)-10(-8) M) suppressed the expression of ovarian genes and production of prostaglandin E2 (PGE2). In vivo experiments consistently demonstrated that o,p'-DDT at 0.5-1 mg/kg inhibited the gene expression and PGE2 levels in rat ovary. The surprising results from the receptor inhibitors studies showed that these inhibitory effects were exerted independently of either classical estrogen receptors (ERs) or G protein-coupled receptor 30 (GPR30). Instead, o,p'-DDT altered gene expression or hormone action via inhibiting the activation of protein kinase A (PKA), rather than protein kinase C (PKC). We further revealed that o,p'-DDT directly interfered with the PKA catalytic subunit. Our novel findings support the hypothesis that exposure to low concentrations of o,p'-DDT alters gene expression and hormone synthesis through signaling mediators beyond receptor binding, and imply that the current exposure levels of o,p'-DDT observed in the population likely poses a health risk to female reproduction.

Ortho-aminoazotoluene activates mouse constitutive androstane receptor (mCAR) and increases expression of mCAR target genes

2'-3-dimethyl-4-aminoazobenzene (ortho-aminoazotoluene, OAT) is an azo dye and a rodent carcinogen that has been evaluated by the International Agency for Research on Cancer (IARC) as a possible (class 2B) human carcinogen. Its mechanism of action remains unclear. We examined the role of the xenobiotic receptor Constitutive Androstane Receptor (CAR, NR1I3) as a mediator of the effects of OAT. We found that OAT increases mouse CAR (mCAR) transactivation in a dose-dependent manner. This effect is specific because another closely related azo dye, 3'-methyl-4-dimethyl-aminoazobenzene (3'MeDAB), did not activate mCAR. Real-time Q-PCR analysis in wild-type C57BL/6 mice revealed that OAT induces the hepatic mRNA expression of the following CAR target genes: Cyp2b10, Cyp2c29, Cyp3a11, Ugt1a1, Mrp4, Mrp2 and c-Myc. CAR-null (Car(-/-)) mice showed no increased expression of these genes following OAT treatment, demonstrating that CAR is required for their OAT dependent induction. The OAT-induced CAR-dependent increase of Cyp2b10 and c-Myc expression was confirmed by Western blotting. Immunohistochemistry analysis of wild-type and Car(-/-) livers showed that OAT did not acutely induce hepatocyte proliferation, but at much later time points showed an unexpected CAR-dependent proliferative response. These studies demonstrate that mCAR is an OAT xenosensor, and indicate that at least some of the biological effects of this compound are mediated by this nuclear receptor.

Association of selected persistent organic pollutants in the placenta with the risk of neural tube defects

Persistent organic pollutants (POPs) have been associated with a wide range of adverse health effects. Our case-control study was performed to explore the association between placental levels of selected POPs and risks for neural tube defects (NTDs) in a Chinese population with a high prevalence of NTDs. Cases included 80 fetuses or newborns with NTDs, whereas the controls were 50 healthy, nonmalformed newborn infants. Placental concentrations of polycyclic aromatic hydrocarbons (PAHs), organochlorine pesticides, polychlorinated biphenyls, and polybrominated diphenyl ethers were analyzed by gas chromatography-mass spectrometry. The medians of PAHs, o,p'-isomers of dichlorodiphenyltrichloroethane (DDT) and metabolites, α- and γ-hexachlorocyclohexane (HCH), and α-endosulfan were significantly higher in case placentas than in controls. PAH concentrations above the median were associated with a 4.52-fold [95% confidence interval (CI), 2.10-9.74) increased risk for any NTDs, and 5.84- (95% CI, 2.28-14.96) and 3.71-fold (95% CI, 1.57-8.79) increased risks for anencephaly and spina bifida, respectively. A dose-response relationship was observed between PAH levels and the risk of NTDs, with odds ratios for the second, third, and fourth quartiles, compared with the first, of 1.77- (95% CI, 0.66-4.76), 3.83- (95% CI, 1.37-10.75), and 11.67-fold (95% CI, 3.28-41.49), respectively. A dose-response relationship was observed for anencephaly and spina bifida subtypes. Similar results were observed for o,p'-DDT and metabolites, α-HCH, γ-HCH, and α-endosulfan, whereas no dose-response relationship was observed for the last two pollutants. Elevated placental concentrations of PAHs, o,p'-DDT and metabolites, and α-HCH were associated with increased risks of NTDs in this population.

Elucidating the 'Jekyll and Hyde' nature of PXR: the case for discovering antagonists or allosteric antagonists

The pregnane X receptor belongs to the nuclear hormone receptor superfamily and is involved in the transcriptional control of numerous genes. It was originally thought that it was a xenobiotic sensor controlling detoxification pathways. Recent studies have shown an increasingly important role in inflammation and cancer, supporting its function in abrogating tissue damage. PXR orthologs and PXR-like pathways have been identified in several non-mammalian species which corroborate a conserved role for PXR in cellular detoxification. In summary, PXR has a multiplicity of roles in vivo and is being revealed as behaving like a "Jekyll and Hyde" nuclear hormone receptor. The importance of this review is to elucidate the need for discovery of antagonists of PXR to further probe its biology and therapeutic applications. Although several PXR agonists are already reported, virtually nothing is known about PXR antagonists. Here, we propose the development of PXR antagonists through chemical, genetic and molecular modeling approaches. Based on this review it will be clear that antagonists of PXR and PXR-like pathways will have widespread utility in PXR biology and therapeutics.

Toxicogenomic biomarkers for liver toxicity

Toxicogenomics (TGx) is a widely used technique in the preclinical stage of drug development to investigate the molecular mechanisms of toxicity. A number of candidate TGx biomarkers have now been identified and are utilized for both assessing and predicting toxicities. Further accumulation of novel TGx biomarkers will lead to more efficient, appropriate and cost effective drug risk assessment, reinforcing the paradigm of the conventional toxicology system with a more profound understanding of the molecular mechanisms of drug-induced toxicity. In this paper, we overview some practical strategies as well as obstacles for identifying and utilizing TGx biomarkers based on microarray analysis. Since clinical hepatotoxicity is one of the major causes of drug development attrition, the liver has been the best documented target organ for TGx studies to date, and we therefore focused on information from liver TGx studies. In this review, we summarize the current resources in the literature in regard to TGx studies of the liver, from which toxicologists could extract potential TGx biomarker gene sets for better hepatotoxicity risk assessment.

Species-specific regulation of PXR/CAR/ER-target genes in the mouse and rat liver elicited by o, p'-DDT

BACKGROUND

Dichlorodiphenyltrichloroethane (DDT) is a persistent estrogenic organochlorine pesticide that is a rodent hepatic tumor promoter, with inconclusive carcinogenicity in humans. We have previously reported that o, p'-DDT elicits primarily PXR/CAR-mediated activity, rather than ER-mediated hepatic responses, and suggested that CAR-mediated effects, as opposed to ER-mediated effects, may be more important in tumor promotion in the rat liver. To further characterize species-specific hepatic responses, gene expression analysis, with complementary histopathology and tissue level analyses were investigated in immature, ovariectomized C57BL/6 mice treated with 300 mg/kg o, p'-DDT, and compared to Sprague-Dawley rat data.

RESULTS

Rats and mice exhibited negligible histopathology with rapid o, p'-DDT metabolism. Gene expression profiles were also similar, exhibiting PXR/CAR regulation with the characteristic induction of Cyp2b10 and Cyp3a11. However, PXR-specific target genes such as Apoa4 or Insig2 exhibited more pronounced induction compared to CAR-specific genes in the mouse. In addition, mouse Car mRNA levels decreased, possibly contributing to the preferential activation of mouse PXR. ER-regulated genes Cyp17a1 and Cyp7b1 were also induced, suggesting o, p'-DDT also elicits ER-mediated gene expression in the mouse, while ER-mediated effects were negligible in the rat, possibly due to the inhibitory effects of CAR on ER activities. In addition, o, p'-DDT induced Gadd45a, Gadd45b and Cdkn1, suggesting DNA damage may be an additional risk factor. Furthermore, elevated blood DHEA-S levels at 12 h after treatment in the mouse may also contribute to the endocrine-related effects of o, p'-DDT.

CONCLUSION

Although DDT is known to cause rodent hepatic tumors, the marked species differences in PXR/CAR structure, expression patterns and ligand preference as well as significant species-specific differences in steroidogenesis, especially CYP17A1 expression and activity, confound the extrapolation of these results to humans. Nevertheless, the identification of potential modes of action as well as species-specific responses may assist in the selection and further development of more appropriate models for assessing the toxicity of DDT to humans and wildlife.