Sage Journals: Discover world-class research

Abstract

TCDD and other polyhalogenated aromatic hydrocarbon ligands of the aryl hydrocarbon receptor (AHR) have been classically considered as non-genotoxic compounds because they fail to be directly mutagenic in either bacteria or most in vitro assay systems. They do so in spite of having repeatedly been linked to oxidative stress and to mutagenic and carcinogenic outcomes. Oxidative stress, on the other hand, has been used as a marker for the toxicity of dioxin and its congeners. We have focused this review on the connection between oxidative stress induction and the toxic effects of fetal and adult dioxin exposure, with emphasis on the large species difference in sensitivity to this agent. We examine the roles that the dioxin-inducible cytochromes P450s play in the cellular and toxicological consequences of dioxin exposure with emphasis on oxidative stress involvement. Many components of the health consequences resulting from dioxin exposure may be attributable to epigenetic mechanisms arising from prolonged reactive oxygen generation.

1. INTRODUCTION

Many polynuclear polyhalogenated aromatic hydrocarbons (PHAHs) are known or suspected environmental carcinogens, toxicants and teratogens in animals and humans (Gatmaitan et al., 1977; Talalay et al., 1988; Hebert et al., 1990; Jiang et al., 1991; Butler et al., 1992; Ralston et al., 1994; Hatae et al., 1996). 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD, dioxin) is prototypical of PHAH compounds, including polyhalogenated dibenzo-p-dioxins, dibenzofurans and coplanar biphenyls, that bind to and activate the cytosolic aryl hydrocarbon receptor (AHR). TCDD is a co-planar polychlorinated biphenyl with among the highest AHR binding affinities and agonistic activities (Poland et al., 1976a). It is this interaction of PHAHs, such as TCDD, with the AHR that mediate most if not all effects of low-concentration TCDD exposures.

The AHR is the only bona fide ligand-activated member of the PAS superfamily of proteins, named for the PER (“period” regulator of circadian rhythm), ARNT (“Ah receptor nuclear translocator”) and SIM (“single minded”, regulator of midline cell differentiation) members of helix-loop-helix transcription factors (Alsharif et al., 1994; Hassoun et al., 2003). Prior to ligand binding the AHR resides in the cytosol, associated with two molecules of HSP90 and HSP90 accessory proteins. Upon TCDD binding, the AHR is released from this cytosolic complex and is translocated into the nucleus where it forms a heterodimeric complex with ARNT (Okey et al., 1989). This complex binds to one or more aryl hydrocarbon response elements (AhRE; also known as xenobiotic response elements, XRE; and dioxin response elements, DRE) that function as cis-acting enhancers in the regulatory domains of a growing number of genes collectively known as the AHR gene battery (Nebert et al., 1993). Battery members include phase I cytochromes P450 (CYP) Cyp1a1 and Cyp1a2, Cyp1b1 and NAD(P)H quinone oxidoreductase (Nqo1), and phase II antioxidant enzymes such as UDP-glucuronosyltransferase (Ugt1a1), glutathione S-transferase (Gst1a1) and aldehyde dehydrogenase (Aldh3a1).

Ligands for the AHR include planar PHAHs and diverse classes of plant-derived chemicals. It has been hypothesized that the AHR/ARNT transcriptional complex evolved for defense against an increasingly diverse array of plant toxins and as a result it is unlikely to serve endogenous physiological functions (Gonzalez et al., 1990). More recently however, the AHR has emerged as an important regulator of physiologic and developmental processes in the absence of an apparent exogenous (xenobiotic) ligand (Fernandez-Salguero et al., 1997; Lahvis et al., 2000). The AHR represents a pivotal upstream event in the apoptosis cascade (Nebert et al., 2000; Slim et al., 2000; Dong et al., 2004), exerts an important level of influence on reproductive success (Abbott et al., 1999) and participates in cell cycle regulation (Puga et al., 2002; Marlowe et al., 2004). Further, acting through the AHR, TCDD has been shown to modulate up- or down-regulation of more than 300 known mRNAs and an equivalent number of expressed sequence tags (Puga et al., 2000b). In part, this effect can be attributed to interactions between the AHR and transcription factors other than ARNT, some of which are involved in the control of complex cellular programs, such as cell division and cell fate (Ge et al., 1998; Kolluri et al., 1999; Tian et al., 1999; Puga et al., 2000a; Elferink et al., 2001; Puga et al., 2002; Marlowe et al., 2004). In light of such studies, it is likely that the AHR has important roles in regulating cellular homeostasis that may be disrupted by environmental chemicals. The diversity of AHR ligand interactions, the complexity of the cellular transcriptome, the persistence of AHR activation, and the nature of agonist exposure determine whether the homeostatic equilibrium is maintained or perturbed.

The toxicologic responses elicited by TCDD differ widely among animal species and strains. These differences are attributable to variations in a number of molecular, tissue specific, biochemical and physiological characteristics. In making inter-model comparisons, TCDD dose can be expressed as a variety of different metrics such as administered dose, average daily dose, tissue concentration, average body burden and area under the curve (AUC). As a result, clear-cut dose-response assessments of TCDD are made difficult by the complexity of biologic responses to TCDD, the variety of tissues affected by TCDD and gaps in our understanding of the mechanisms relating exposure to toxicity. As a result, body burden rather than daily intake (administration) has been suggested as the best dose metric for interspecies comparisons and extrapolation, although the vast majority of studies describing TCDD toxicity express dose in terms of acute, subchronic and chronic exposures (DeVito et al., 1995).

2. FUNCTIONAL ALTERATIONS OF THE AHR

Like many other transcription factors, the AHR has been amenable to dissection into functional domains. The C-terminal half of the AHR, containing a glutamine-rich domain, is responsible for transactivation; whereas the N-terminal half of the AHR, consisting of a basic-region helix-loop-helix domain and two PAS domains, has overlapping functions responsible for DNA binding, ligand binding and dimerization (Hankinson, 1995). Unfortunately, the AHR peptide sequence is not particularly well conserved across species, especially the C-terminal half of the protein, thus complicating risk assessment. Polymorphisms identified within the coding region of the AHR instill differences in AHR-responsive gene induction and toxicologic responses to numerous PHAHs (Nebert, 1989; Swanson et al., 1993; Poland et al., 1994). Interspecies variation notwithstanding, the AHR has been widely studied in mice and rats, which, relative to the human AHR, have high ligand binding affinity.

In mice, differences in TCDD sensitivity have been related to polymorphisms in the AHR that give rise to the commonly studied “responsive” and “nonresponsive” strains (C57BL/6 and DBA/2, respectively). AHR polymorphisms in DBA/2 mice reduce ligand binding affinity approximately 10-fold and thereby diminish TCDD potency for acute lethality (Chapman et al., 1985; Okey et al., 1989). Several groups have sequenced the AHR alleles from inbred strains of mice. These studies have characterized four distinct alleles in mice, referred to as Ahr^b-1, Ahr^b-2, Ahr^b-3 , and Ahr^d. Among these, the “responsive” phenotype in C57BL/6 mice is encoded by the autosomal dominant Ahr^b-1 allele while the “nonresponsive” DBA/2 phenotype is encoded by the Ahr^d allele. The four identified mouse alleles differ by 8 nucleotides in their shared open reading frame. In addition, these AHRs differ by 45 amino acids at their C-terminus as a result of a nucleotide change in the Ahr^d allele that replaces the stop codon in the Ahr^b allele with an arginine (Chang et al., 1993; Poland et al., 1994). Most of the amino acid changes distinguishing these strains occur within the transactivation domain and have little or no known functional consequence (Chang et al., 1993). However, a point mutation at position 375 of the DBA/2 AHR results in an ALA to VAL substitution in the second PAS domain of the C57BL/6 strain that is responsible for the difference in ligand binding affinity and transactivation (Poland et al., 1994; Maier et al., 1998). These findings have been further supported in mice with homozygous loss of functional AHR (Fernandez-Salguero et al., 1995; Schmidt et al., 1996). These Ahr ^−/− knockout strains were refractory to TCDD-mediated CYP1A1 induction and were highly resistant to TCDD-mediated pathologies up to 2000 μg/kg, a 10-fold higher dose than that which induce severe toxicity in functional AHR expressing mice (Fernandez-Salguero et al., 1996).

Rat strains have also been characterized with respect to their TCDD sensitivity. At the extremes of TCDD responsiveness are the “sensitive” Long-Evans rats (L-E) and the “resistant” Hans/Wistar (H/W) substrain of Wistar rats that differ by at least 1000-fold in the acute lethality of TCDD (LD₅₀ between 10 to 20 μg/kg and >9600 μg/kg, respectively) (Pohjanvirta et al., 1994b). Inheritance studies implicate the AHR gene locus and a second uncharacterized gene B in the TCDD resistance of the H/W rats, with the AHR contributing the largest quantitative role (Tuomisto et al., 1999). Unlike the C57BL/6 and DBA/2 mice, TCDD resistance is the dominant trait in rats, segregating with autosomal inheritance (Pohjanvirta et al., 1999). Molecular analysis of the coding region of AHR cDNAs from the H/W and the L-E rat revealed a Val497Ala amino acid change in the transactivation domain and, perhaps more importantly, a single point mutation in the first nucleotide of intron 10, resulting in altered mRNA splicing (Pohjanvirta et al., 1998). The loss of this splice-donor site results in the use of the nearest upstream and two downstream consensus splice sites that yields three different molecular AHR species having either a deletion of 43 amino acids in exon 10, an extra 7-amino acid stretch encoded by intron 10, or no translated contribution from exon 11. The net effect of the exon 10 mutation is a modified AHR transactivation domain that has little or no effect on AHR accumulation, ligand binding affinity, or activation of CYP1A gene expression (Pohjanvirta et al., 1988; Unkila et al., 1993; Pohjanvirta et al., 1999), but which effectively converts the Han/Wistar rat into the most resistant naturally occurring mammals to TCDD toxicity (Simanainen et al., 2003).

3. PRINCIPLES OF TCDD SENSITIVITY

Within a single animal, tissues vary in response to TCDD-receptor binding. It is the coupling of TCDD-receptor interaction to a measured response that accounts for varying tissue sensitivities and thereby target organ toxicity. In general, the maximum response elicited by a receptor agonist for a specific endpoint is defined as the intrinsic efficacy of a ligand. Efficacy is therefore a quantitative measure of the signaling events that couples the formation of a ligand-receptor complex to a biologic response (Hestermann et al., 2000). By measuring nonlethal endpoints, differences in TCDD intrinsic efficacy between L-E and H/W rat strains have been categorized into two classes: Type I endpoints (EROD activities, thymus weight, tooth defect) that showed similar efficacy in both strains; and Type II endpoints (body weight, serum FFA and bilirubin levels, and serum ASAT activity) where the response in H/W rats was less than half that observed in L-E rats (Simanainen et al., 2003). The contribution of the AHR and the product of gene B to these endpoints was investigated by segregating the H/W resistant genes into three different rat lines, designated A, B and C, by congenic crossbreeding with inbread L-E rats (Tuomisto et al., 1999). Line A possessed the original “resistant” H/W AHR allele but with a wild-type gene B allele. Line B possessed a normal AHR allele, but was homozygous for the H/W gene B allele. Line C possessed neither of the H/W resistance alleles. These studies demonstrated that the AHR is the most important factor decreasing TCDD intrinsic efficacy, and that an uncharacterized mechanistic difference exists between type I and II effects that is linked to the altered AHR transactivation domain. Relative to the large difference in acute LD₅₀ values between L-E and H/W rat strains, the potency of TCDD for nonlethal type I endpoints was much less affected by the H/W AHR phenotype (Tuomisto et al., 1999; Simanainen et al., 2003). In rat line B, the mutated B allele had only a minor influence on TCDD efficacy and the dose responses did not clearly fit into either Type I or Type II responses, but were clearly different from lines A and C. Thus the B allele is concluded to contribute modestly to TCDD resistance independent of the AHR (Simanainen et al., 2003).

The combination of efficacy and ligand-binding affinity determine the relative potency of TCDD (Hestermann et al., 2000), which is defined as the dose of TCDD required to achieve a specific endpoint. Both parameters contributing to potency can vary between animal species, strains and tissues to produce net sensitivities. For example experiments investigating the relative potency of various AHR ligands have shown that TCDD, PCB126, PCB156 and PCB105 all bind to the AHR with reported affinity constants equivalent to 0.76, 16, 2500 and 4600 nM respectively, relative to [³H]-TCDD binding. However, the stimulus-response relationship demonstrates that while TCDD and PCB126 have high intrinsic efficacy for CYP1A1 induction, PCB126 is much less efficient at eliciting a response after binding the AHR. PCB105, which binds the AHR at high concentrations, competes for [³H]TCDD binding while eliciting no response qualifying PCB105 as a competitive antagonist. With the exception of PCB105, each of these agents is a full agonist since each elicits the same maximal response. Therefore the potency of these AHR ligands can be expressed in terms of the effective concentration that elicit 50% maximal response for CYP1A1 induction (EC₅₀) and have been shown to be equivalent to 0.015 nM (TCDD), 0.12 nM (PCB126) and 1900 nM (PCB156). In comparison PCB105 has no EC₅₀ since it does not elicit a response (Hestermann et al., 2000).

This same principle has been utilized to describe the biologic responses observed between different animal species and strains. For example, the efficacy of TCDD for lethality ranges among rodent species from the guinea pig (LD₅₀ = 1 μg/kg) to H/W rat (LD₅₀ >9600 μg/kg) (Henck et al., 1981; Poland et al., 1982; Pohjanvirta et al., 1994a). This range of sensitivity has been attributed to a restructured transactivation domain in the hamster AHR, presumably producing a much less responsive ligand-receptor complex (Korkalainen et al., 2004). In comparison, experiments investigating TCDD resistance in mice have found that sensitivity correlates with binding affinity because the dose-response curve is shifted to the right without a reduction in response magnitude (Poland et al., 1976b). Thus, both intrinsic efficacy and ligand-binding contribute to the manifestation of TCDD toxicity and the tremendous variability in dose response that has been reported between animal species.

In vitro modeling of the TCDD dose-response using CYP1A1 induction as a biomarker has demonstrated that TCDD need only occupy a fraction of AHR receptors to elicit maximal response. In PLHC-1 cells, a hepatocellular carcinoma cell line derived from the teleost Poeciliopsis lucida, only 1.9 % of available receptor sites were required to be occupied for 50% maximal response while 28% saturation produced 95% maximal response. These data establish a “spare” receptor relationship for the high-intrinsic efficacy AHR ligands such as TCDD, which contrasts with the low-intrinsic activity of ortho-substituted PCB congeners that fail to elicit maximal response even with AHR saturation (Hestermann et al., 2000).

In addition to the individual contribution of intrinsic efficacy and ligand binding to the wide range of observed species and strain TCDD susceptibilities, receptor density also modulates the response. Regulation of receptor expression levels by its own ligand is a common pharmacologic observation and receptor theory predicts that changes in AHR levels will influence both the potency and the maximal response of TCDD. Up-regulation of a receptor's presence increases the potency of its ligand and is referred to as “sensitization”, while down regulation results in “desensitization” and subsequent tolerance. In such a manner, AHR expression is significantly influenced by dose and duration of TCDD exposure (Pollenz, 2002). Following short-term in vitro exposure of cultured Hepa-1 cells to 2 nM TCDD, AHR levels are reduced to less than 20% of original levels within 6 hours following treatment, and this desensitization persists for at least 72 hours (Giannone et al., 1998). In vivo studies, however, have failed to consistently demonstrate a prominent physiologic effect caused AHR by ligand binding. In Sprague Dawley rats, a single oral dose of 10 or 50 μg/kg produced a pronounced initial reduction of liver and cytosol AHR concentrations, though, in the case of the former, depletion persisted for 14 days, while in the latter, depletion was followed by AHR induction (Pollenz et al., 1998; Franc et al., 2001a). A similar effect was also reported for Hans/Wistar rats, though these rats had lower liver AHR concentrations than either SD or L-E rats regardless of TCDD treatment (Franc et al., 2001a). In contrast to high-dose TDCC-mediated loss of the AHR, acute and chronic low-dose TCDD administration produced either an increase, or no change in AHR concentrations (Sloop et al., 1987; Franc et al., 2001a; Franc et al., 2001b). In addition, when increased receptor presence was observed it did not translate into sensitization as determined by CYP1A1 induction (Franc et al., 2001a; Franc et al., 2001b). These reports suggest that low-dose TCDD, such as a typical environmental exposure, is not likely to produce either TCDD tolerance or sensitivity, while higher doses appear to be associated with a transient desensitization. They also demonstrate that receptor density does not contribute to the variation in TCDD responsiveness associated with the L-E and H/W rat strains.

4. EFFECTS OF GENDER AND SEX HORMONES IN THE TCDD DOSE-RESPONSE

In long-term bioassays, TCDD increased the incidence of liver tumors in female, but not male, rats (Kociba et al., 1978; Huff et al., 1991; Sawyer et al., 1999). In general, female rats have been shown to be more susceptible to TCDD-induced oxidative stress (Stohs, 1990), oxidative DNA damage (Wyde et al., 2001b) and hepatocarcinogenesis (Huff et al., 1994). These TCDD-mediated effects are, at least in part, dependent on the presence of estrogen (Jana et al., 2000; Coumoul et al., 2001; Lai et al., 2004), though the role of the estrogen receptor remains equivocal (Wyde et al., 2000; Wyde et al., 2001a). In an initiation-promotion model, ovariectomy inhibited TCDD-induced preneoplastic foci and reduced TCDD-induced liver tumor formation (Lucier et al., 1991) suggesting involvement of estrogen which was later supported by the observation that supplemental 17β-estradiol (E₂) restored tumorogenic sensitivity of ovariectomized females (Wyde et al., 2001b). The presumption that estrogen mediates oxidative stress and carcinogenesis was confirmed in Syrian hamsters, which show 100% kidney tumor incidence following the administration of 17β-estradiol or estrone (Liehr, 1997). Likewise estradiol was also found to produce oxidative DNA damage in hamster tissues and other biological model systems (Han et al., 1995; Tritscher et al., 1996; Wyllie et al., 1997; Hodgson et al., 1998; Cavalieri et al., 2000; Liehr, 2001; Wyde et al., 2001b).

Oxidative estrogen metabolism results in the formation of two estrogen catechols, 2-hydroxylated and 4-hydroxylated estradiol. Under circumstances where these catechols are excessively produced, or where their metabolism is impaired, catalytic oxidation to semiquinones and quinones can occur. Of particular importance is formation of 4-hydroxyestradiol, the oxidized quinone of which has been associated with oxidative DNA damage and increased cancer risk (Bradlow et al., 1985; Bradlow et al., 1986; Telang et al., 1992; Nebert, 1993; Bradlow et al., 1995; Telang et al., 1997; Liehr, 1999; Jefcoate et al., 2000; Cavalieri et al., 2000). In contrast, 2-hydroxyestradiol formation has not been associated with either DNA damage or increased cancer risk (Bradlow et al., 1996; Telang et al., 1997; Cavalieri et al., 2000). Toxicity of 4-hydroxyestradiol results from two types of reactions; a one electron redox cycling reaction that occurs when 4-hydroxyestradiol is oxidized to estrone 3,4-quinone, and a two electron electrophilic addition reaction (Liehr, 2000; Cavalieri et al., 2000). Redox cycling generates superoxide and ultimately the highly reactive genotoxic hydroxyl radical (Roy et al., 1991; Han and Liehr, 1995). Superoxide produced in this way may further enhance redox cycling by mobilizing iron from ferritin, increasing cellular Fenton chemistry (Wyllie and Liehr, 1997; Liehr et al., 2001). Rearrangement of estrone 3,4-quinone produces a strongly electrophilic carbonium cation that may undergo a Michael addition reaction with cellular sulfhydrals such as (i.e. glutathione, protein thiols) or by electrophilic addition to DNA purine bases resulting in depurinating adducts (Cavalieri et al., 2000), and ultimately procarcinogenic mutations (Liehr, 2001; Embrechts et al., 2003). The relative contributions of redox cycling and electrophilic interactions in the oxidative stress response and toxicity have not been firmly established; however, limited evidence suggests that covalent sulfhydral modification by electrophiles is likely to be a greater cytotoxic hazard than transient quinone formation that facilitates disposal from the cell (Buffinton et al., 1989).

Xenobiotics acting through the AHR may alter the metabolic profile of E₂ and therefore its estrogenic and toxicological profile. Metabolism of E₂ to 2-hydroxyestradiol is predominantly catalyzed by cytochrome P450 CYP1A1 (Roy et al., 1992; Spink et al., 1998) with some contribution by members of the CYP3A family (Hammond et al., 1997), while metabolism of E₂ to the 4-hydroxyestradiol is mainly a result of CYP1B1 activity (Spink et al., 1994; Hayes et al., 1996; Jefcoate et al., 2000). In liver, TCDD increases the levels of CYP1A1 and CYP1A2 relative to CYP1B1 (Walker et al., 1999), hence 2-hydroxylation predominates over 4-hydroxylation. Similar results have been reported in several breast epithelial tumor and nontumor cell lines where TCDD strongly induced CYP1A1 activity with resultant 2-hydroxyestradiol formation as the major E₂ metabolite (Spink et al., 1998). In this regard, increased production of 2-hydroxyestradiol relative to 4-hydroxyestradiol and 16α-hydroxyestrone has been observed after exposure to indole 3-carbinol, a dietary micronutrient and AHR proligand. This finding is of potential clinical importance for cancer, since indole carbinols, that bind AHR as acid condensation products, are in clinical trials as cancer chemoprotective agents (Gillner et al., 1985; Malloy et al., 1997; Michnovicz et al., 1997; Telang et al., 1997; Rosen et al., 1998; Yuan et al., 1999; Bell et al., 2000) (reviewed by (Shertzer et al., 2000)).

5. EFFECTS OF TCDD EXPOSURE ON DEVELOPMENT

AHR in development

In the mammalian fetus and in fish larvae, the AHR plays prominent roles in both resolving vascular structures and mediating cardiovascular toxicities of TCDD (Lahvis et al., 2000; Bello et al., 2004). In mammals, the importance of functional AHR is demonstrated in AHR-null mice by a failure of a fetal vascular structure, the ductus venosus, to close, thus permitting blood from the portal vein to bypass the liver by shunting to the inferior vena cava. Functional AHR is also required for normal vascular “pruning” during fetal development, the absence of which results in the propagation into maturity the highly anastomotic vasculature architecture of the liver, eye and kidney that that are characteristically neonatal (Lahvis et al., 2000).

Because of the involvement of the AHR in resolving fetal vascular structures, it is not surprising that the cardiovascular system has been shown to be an important target of TCDD-mediated toxicity (Jokinen et al., 2003; Karyala et al., 2004). Although the mechanisms underlying cardiovascular risks are undetermined, it has been postulated that TCDD interferes with cardiovascular development by sequestering the AHR or displacing an as yet unidentified ligand, thereby preventing the AHR from carrying out its normal endogenous activity. In support of this hypothesis, knockdown of the TCDD-responsive AHR2 in zebrafish with morpholino-substituted oligonucleotides has specifically demonstrated that TCDD retardation of common cardinal vein (CCV) regression is AHR dependent (Bello et al., 2004). That knockdown of AHR2 expression itself did not inhibit CCV regression in a manner similar to that of the Ahr-null mouse is attributable to the fact that zebrafish possess a second, TCDD refractory ahr locus (ahr1) that may compensate for the loss of ahr2 (Bello et al., 2004). In separate experiments, fish have provided evidence that the vascular endothelium is also a sensitive target for TCDD toxicity, and this may prove important with regards to the human health effects of TCDD. In fish TCDD elicits increased vascular permeability, which in lake trout manifests as yolk sac edema, and in zebrafish as extravascular accumulation of serum proteins in mesencephalic brain tissues (Guiney et al., 2000; Dong et al., 2004). Although the precise nature of this microvascular leakage remains to be determined, decreased cardiac output, increased endothelial vacuolation (Guiney et al., 2000), disruption of peripheral vascular beds (Henry et al., 1997) and/or disruption of angiogenic signaling (Bello et al., 2004) have been suggested. Whether TCDD mediates developmental vascular defects in fish by the same mechanism as those observed in mammals following gestational exposure remains to be determined; however, two separate reports demonstrate that loss of the AHR protects against the teratogenic effects of TCDD (Mimura et al., 1997; Peters et al., 1999b).

In humans there are few reliable studies linking maternal exposure to TCDD and related compounds (e.g., other dioxins, furans, and dioxin-like PCBs) with impaired fetal development. A number of epidemiologic studies have been confounded by the use of indirect estimates of TCDD exposure, such as local soil levels (Stockbauer et al., 1988), estimates of dietary consumption (Svensson et al., 1991; Rylander et al., 2000) and correlation with residential location (Revich et al., 2001). Only a few studies have used biologic measures of exposure, such as dioxin or PCB concentrations in breast milk and serum (Patandin et al., 1998; Eskenazi et al., 2003). In both of these studies birth weight and gestational age did not differ between mothers with higher exposure levels relative to controls, though these findings are somewhat offset by reports that birth weight was negatively correlated with cord plasma PCB and dioxin levels (Patandin et al., 1998; Vartiainen et al., 1998). One mechanistic explanation for the equivocal association between maternal exposure and teratogenesis may relate to the low affinity human AHR, comparable to the nonresponsive DBA/J2 mouse strain (Ramadoss et al., 2004). In experiments with AHR-null mice, oral exposure of pregnant dams (40 μg/kg TCDD) was sufficient to produce cleft palate and hydronephrosis in nearly all wild-type fetuses while no mice with the homologous AHR knockout were sensitive to the teratogenic effects (Mimura et al., 1997; Peters et al., 1999a). This possibility is further supported by the use of humanized mice expressing human AHR rather than mouse AHR. These studies demonstrated that mice expressing human AHR had a weaker response to TCDD than resistant DAB/2 mice, and that the humanized AHR phenotype protected against cleft palate (Moriguchi et al., 2003).

A variety of human epidemiologic studies have suggested a link between TCDD exposure and cardiovascular morbidity following occupational exposure (Bertazzi et al., 1989; Flesch-Janys et al., 1995; Vena et al., 1998; Pesatori et al., 1998; Pesatori et al., 2003). Retrospective analysis of 1189 chemical plant workers exposed to dioxin and furans reported a highly significant 2.5-fold (95% confidence interval-1.3–4.7) increase in relative-risk of death from heart disease due to dioxin exposure (Flesch-Janys et al., 1995; Pelclova et al., 2002). The body burden at which these effects were seen ranges from 110 to 4000 ng/kg of TCDD in blood fat, well below the body burden of TCDD shown to induce cancer in rodents (100–140000 ng/kg) (DeVito et al., 1995). These observations were later supported in hyperlipidemic mice subchronicly treated with TCDD (150 ng/kg, 3 times weekly), resulting in increased blood pressure and atherogenic lipids; the two most important clinical risk factors for atherosclerotic plaque formation. Further, TCDD exposed animals had a trend towards earlier onset and increased severity of atherosclerotic plaques compared to vehicle treated mice (Dalton et al., 2001). Similarly, in female Sprague-Dawley rats treated 5 days per week with up to 100 ng/kg/day TCDD for 2 years, cardiomyopathy and chronic active arteritis increased in a dose dependent manner. However the severity of cardiomyopathy did not increase in a dose-responsive manner and only became evident in the later treatment groups (Jokinen et al., 2003)

6. ROLE OF CYTOCHROME P450 ENZYMES IN TCDD TOXICITY

Several cytochrome P450 genes under the control of the AHR, notably those in the CYP1 family (CYP1A1, CYP1A2 and CYP1B1) have been suggested to contribute to TCDD-induced toxicity (Andersen et al., 1998; Nebert et al., 2004). Because TCDD-associated toxicities are slow to develop, requiring days to weeks, it is likely that the transcriptional events elicited by TCDD-mediated AHR activation must be persistent. Therefore, we believe that the persistent changes in gene expression induced by TCDD disrupt signal transduction homeostasis leading to the accumulation of toxicants (i.e reactive oxygen species, lipid peroxidation products) that in turn lead to pathology. In the liver, one such gene circuit involves the Cyp1a monooxygenase subfamily. Studies utilizing Cyp1a1 ^−/− knockout mice from a C57BL/6J background demonstrate that a single high dose of TCDD (200 μg/kg) is highly lethal to Cyp1a1 ^+/+ males but not to Cyp1a1 ^−/− males or to females of either genotype. This protective effect conveyed by gender, however, is quite limited compared to the protective effect afforded by Ahr knockout, that protected against TCDD doses of up to 2 mg/kg (Fernandez-Salguero et al., 1996). Further, Cyp1a1 ^−/− mice are resistant to TCDD-induced wasting syndrome, which is manifested by weight loss or poor weight gain in conjunction with marked increases in serum AST levels, reflecting rhabdomyolysis. Glycogen depletion and down regulation of phospho-enol-pyruvate carboxykinase, combined with SRC oncoprotein action have been suggested to play a role in this process (Dunlap et al., 2002). Cyp1a1 ^−/− mice, regardless of gender, are more resistant to hepatocyte hypertrophy; likewise, Cyp1a1 ^−/− mice experience decreased accumulation of microvesicular and interstitial lipid accumulation for reasons that are not yet clear. It is of interest to note that the H/W rat, which is resistant to TCDD toxicity for reasons already discussed, shows normal acute induction of CYP1 family genes and uroporphyria following TCDD exposure (Simanainen et al., 2003; Niittynen et al., 2003; Uno et al., 2004; Korkalainen et al., 2004).

The other member of the cytochrome P4501A family, CYP1A2, is emerging as a monooxygenase that dichotomously contributes to both protective and sensitizing effects to TCDD toxicity. The protective effects result from pharmacokinetic and antioxidant activities associated with CYP1A2 expression. Pharmacokinetically, CYP1A2 is the primary hepatic TCDD-binding protein, capable of sequestering significant quantities of dioxin; this is not true of CYP1A1 (Diliberto et al., 1997; Uno et al., 2004). Further, CYP1A2 is stabilized by TCDD extending its half-life and therefore augmenting its phramacokinetic effect (Andersen et al., 1997; Diliberto et al., 1997). Presumably by sequestering TCDD, CYP1A2 acts to reduce the free fraction of TCDD available to mediate gene induction through AHR interaction. Pharmacokinetic studies have shown that levels of both CYP1A1 and CYP1A2 must be considered in predicting tissue concentrations of TCDD from the administered doses (Wang et al., 1997a; Wang et al., 1997b; Santostefano et al., 1998). In terms of antioxidant protection, CYP1A2 enzyme activity is associated with decreased microsomal H₂O₂ production, possibly by acting as an electron transport pathway or electron sink for uncoupled electron transfer by CYP2E1 or other microsomal enzyme systems.

Contrasting with these apparent beneficial effects, CYP1A2, and to some extent CYP1A1, has been demonstrated in mice to mediate the uroporphyrinogenic effect of TCDD. In brief, uroporphyria results from dysfunction of uroporpheryninogen decarboxylase (UROD) during hepatic heme synthesis, leading to significant hepatocellular uroporphyrin accumulation and possibly liver injury (Pohjanvirta and Tuomisto, 1994a; Smith et al., 1998). In TCDD-mediated uroporphyria, CYP1A2 is necessary and sufficient to inhibit UROD metabolism, resulting in accumulation of uroporphyrin isomers (Smith et al., 1998). Knockout mouse experiments have shown that loss of CYP1A2 completely, and CYP1A1 partially, protects against TCDD-mediated uroporphyrin accumulation (Smith et al., 2001; Uno et al., 2004).

In the course of the reaction catalyzed by monooxygenase P450 enzymes, two electrons are sequentially transferred from NADPH-dependent cytochrome P450 oxidoreductase to each atom of bound oxygen, resulting in the production of oxygenated substrate and water (Guengerich et al., 1985; Poulos et al., 1992). This reaction is reversible, a process that is perhaps toxicologically important, since physiologically-derived peroxides can metabolize various xenobiotics, particularly carcinogenic arylamines, via the peroxidase activity of CYP1A2 (Anari et al., 1997). Although tight coupling normally exists between oxygen reduction and monooxygenation, some reactive oxygen may be released as either superoxide or H₂O₂ in the course of electron transfer. The monooxygenase-dependent production of ROS in liver microsomes, supported by NADPH, is a well-known phenomenon (Gillette et al., 1957) that clearly contributes to the total cellular production of reactive oxygen in rat liver, without necessitating enzyme induction (Bondy et al., 1994). Even in the absence of exogenous xenobiotic substrates, endogenous substrates, such as any one of the many arachidonic acid metabolites, may stimulate ROS production (Capdevila et al., 1988; Rifkind et al., 1990; Nakai et al., 1992). In this regard, lipoxin A4, a metabolite of arachidonic acid, may act as an inducing ligand for the AHR (Schaldach et al., 1999). Further, substrate independent ROS production, due to inefficient microsomal electron coupling, has been demonstrated for CYP2E (Ekstrom et al., 1986; Dai et al., 1993), CYP2B, and CYP3A (Ahmed et al., 1995).

While xenobiotic AHR ligands, such as TCDD, can induce microsomal CYP1 expression and ROS production, suppression of CYP1A1 activity has been reported with high dose exposure to several PHAH. This phenomenon has been studied in fish and rodent liver microsomes using the AHR inducing compounds 3,3′,4,4′-tetrachlorodiphenyl (TCB) and 3,3′,4,4′,5-pentachlorobiphenyl (PeCB) (Schlezinger et al., 2001). CYP1A1 enzyme activity was strongly inhibited even though treatment with the halogenated biphenyls increased cyp1a1 mRNA. Since these compounds are poorly metabolized, inhibition by product could not explain the results. The loss of CYP1A1 activity was attributed to the ability of TCB and PeCB to accelerate CYP1A1 electron flow with concomitantly increased ROS production. Although some reactive oxygen species are released by enzyme uncoupling, ROS scavengers were unable to prevent the loss of CYP1A1 activity indicating that the chemistry involved occurs entirely within the enzyme active site. TCB also stimulated ROS production in microsomes from insect cells expressing human CYP1A1, but not in microsomes from cells expressing human CYP1A2 (Schlezinger et al., 1999). These results may explain the previous observation in mice that TCDD produced a sustained elevation of hepatic CYP1A2 activity, while CYP1A1 showed a transient increase, followed by a rapid loss (Shertzer et al., 1998).

7. TCDD-MEDIATED PERTURBATION OF REDOX HOMEOSTASIS

In addition to its involvement in normal physiological processes and signal transduction, the AHR appears to mediate toxicological effects through oxidative stress. As used here, the term oxidative stress refers to any condition that produces an oxidative stress response through an increase in the cellular oxidation state. An oxidative shift in cellular redox homeostasis generally results from increased production of reactive oxygen species relative to cellular antioxidant defenses. Although oxidative stress does not necessarily result in toxicity, it is an important mechanistic component of many toxicologic processes. In this regard, TCDD-mediated activation of the AHR shifts the cellular redox balance to produce an oxidative stress response (Hassoun et al., 1998; Shertzer et al., 1998; Slezak et al., 2000; Senft et al., 2002a; Senft et al., 2002b). For this reason it has become widely hypothesized that the toxicity induced by TCDD involves an oxidative stress component; an observation that has been reported by several laboratories (Stohs, 1990; Alsharif et al., 1994; Shertzer et al., 1998; Slezak et al., 2000).

Several mechanisms have been proposed to explain TCDD-mediated oxidative stress including reduction in expression levels of protective antioxidant enzyme systems (Latchoumycandane et al., 2003) and perturbation of cytochrome P450 levels (Nebert et al., 2000; Lee et al., 2002). The incomplete reduction of O₂ by several enzyme systems, in particular the cytochrome P450 enzymes that are induced by TCDD, is known to result in the generation of superoxide and hydrogen peroxide through poor coupling of electron flow. TCDD has been implicated in the formation of the superoxide anion in rat brain (Hassoun et al., 2003) and hydrogen peroxide in mouse liver (Shertzer et al., 1998; Senft et al., 2002a), with resultant generation of lipid peroxides in rat brain, mouse liver and rat testis (Shertzer et al., 1998; Hassoun et al., 2003; Latchoumycandane et al., 2003). Several lines of evidence support the AHR as a mediator of oxidative stress. It has been observed that peritoneal lavage cells from C57BL/6 mice, which carry the high-affinity Ahr ^b1 allele, demonstrated considerably greater production of superoxide anion in response to TCDD relative to cells from low-affinity DBA/2 mice (Alsharif et al., 1994). Likewise, hepatic lipid peroxidation induced by TCDD occurred at low doses (500 ng/kg) in C57BL/6 mice and only at higher doses (5 μg/kg) in DBA/2 mice (Mohammadpour et al., 1988). In addition, inactivation of aconitase activity, a reliable measure of oxidative stress (Pantopoulos et al., 1995), was documented in C57BL/6 but not in DBA/2 mice following TCDD treatment (Smith et al., 1998).

It should be noted that TCDD dose and tissue concentration do not necessarily correlate with ROS production; the pattern of TCDD exposure also has a prominent effect on ROS production. In liver, an acute oral dose of TCDD (10 and 100 μg/kg) administered to C57BL/6 mice produced a sustained increase in liver superoxide anion and thiobarbituric acid reactive substance (TBARS), attaining to hepatic TCDD concentrations of 55 and 321 ng/g respectively at 13 weeks following exposure. In comparison, subchronic TCDD administration (0.15 to 150 ng/kg; 5 days/week for 13 weeks; po) produced increased superoxide and TBARS only with the highest (150 ng/kg/day) exposure level, corresponding to a hepatic TCDD concentration of 12 ng/g of liver. These data suggest that higher tissue TCDD concentrations are required to elicit oxidative stress following acute exposure than with subchronic TCDD exposure (Slezak et al., 2000). For this reason it is clear that uncharacterized factors remain that can contribute to tissue responses during TCDD exposure.

Enzyme systems catalyzing O₂ reduction must be in balance within the cell because partially reduced oxygen species can be more reactive and deleterious than the parent molecule. Such is the case with hydrogen peroxide, which is generated during superoxide detoxification. Detoxification of superoxide to H₂O requires the sequential action of SOD with catalase or glutathione peroxidase. Three- to six-fold overexpression of Cu/Zn SOD in transgenic mice results in increased production of H₂O₂ and hydroxyl radicals, which accompany enhanced apoptosis of thymocytes and bone marrow cells (Peled-Kamar et al., 1995). This is similar in nature to the enhanced neurotoxicity of kainic acid by SOD overexpression that also occurs through the generation of superoxide (Bar-Peled et al., 1996). Therefore, the consequence of TCDD-induced changes in antioxidant enzyme expression is uncertain, as illustrated by the fact that up-regulation of SOD does not necessarily dictate a decrease in cellular ROS.

Two sites of TCDD-induced reactive oxygen production have been proposed: the microsomes and the mitochondria. Microsomal reactive oxygen production in mouse liver is regulated by at least three forms of cytochrome P450s (Uno et al., 2004; Shertzer et al., 2004b) that clearly contribute to the total cellular production of reactive oxygen in rat liver (Dai et al., 1993; Bondy and Naderi, 1994). CYP1A1 and CYP2E1 generate reactive oxygen in liver microsomes, while CYP1A2 diminishes reactive oxygen production. The stoichiometric ratios of NADPH and O₂ utilized relative to H₂O₂ produced indicate that the pathway of electron flow is short-circuited by TCDD-mediated microsome induction, resulting in increased H₂O₂ production (Shertzer et al., 2004a). CYP1A2 contributes to the time course of the oxidative stress response elicited by AHR ligands by reducing the microsomal oxidative stress response, including lipid peroxidation and decreased membrane fluidity, which is observed following TCDD treatment in mice. CYP1A2 appears to act as an electron sink by accepting electrons generated by CYP1A1 and CYP2E1, preventing the generation of H₂O₂ and the oxidation of microsomal membrane lipids (Shertzer et al., 2004b).

Mitochondria appear to be the major site for reactive oxygen production (Senft et al., 2002b; Latchoumycandane et al., 2003) and as such may represent a target for TCDD-dependent injury. One proposed mechanism by which TCDD may contribute to increased mitochondrially-derived reactive oxygen is inhibition of electron transport at complex III, producing a persistent increase in succinate-dependent superoxide and hydrogen peroxide production (Senft et al., 2002b). Respiratory chain-derived reactive oxygen can result from a decrease in cytochrome c oxidase (complex IV) activity, coupled with an increase in succinate-cytochrome c reductase (complex II), resulting in an increase in the reduction state of cytochrome bc ₁ complex (complex III) to facilitate univalent reduction of oxygen (Senft et al., 2002a; Senft et al., 2002b). Mechanistically, TCDD-dependent electron flow through complex III results in increased electron pressure and increase redox cycling of Coenzyme Q or Fe-S proteins. The physiologic relevance of this mechanism however is not well established, since the concentration of TCDD necessary to act in this manner is far greater than would be expected to be found in naturally-occurring exposures.

Following TCDD treatment, and associated with increases in the production of reactive oxygen, both GSH and GSSG increase in the cytosol and in the mitochondria. However, in the mitochondria, GSH increases to a greater extent relative to the cytosol, while GSSG increases to a lesser extent. These differences resulted in shifts in equilibrium for both type 1 (protein mixed disulfides) and type 2 (protein disulfides) thiol-disulfide switches (Schafer et al., 2001). In the cytosol, TCDD produces an increase in oxidation state, with decreases in type 1 and type 2 switches, as well as an increase (more positive) in the reduction potential (ΔE) of GSSG/2GSH. In sharp contrast, mitochondria display an increase in reduction state, with increases in type 1 and type 2 thiol redox switches, as well as a decrease (more negative) in the ΔE of GSSG/2GSH half reaction (Dalton et al., 2004). These results from the authors' labs are consistent with the hypothesis that TCDD mediates an increase in mitochondrial reactive oxygen result from an overall increase in the reduction state of the mitochondria. As such, the mitochondrial generation of reactive oxygen by TCDD may be considered a form of reductive stress, rather that the clearly defined oxidative stress response that occurs in the cytoplasm.

Although TCDD is not genotoxic in the Ames test, one suggested pathway by which it produces toxic effects involves oxidative DNA damage and increased mutation frequency. A strong relationship has been established between oxidative damage to DNA and chemical carcinogenesis (Cairns et al., 1991). Oxidation of DNA at the 8-position deoxyguanosine produces 8-hydroxydeoxyguanosine (8-OHdG), which represents the major promutagenic lesion produced during oxidative stress. When guanosine base modification is followed by DNA replication G→T and A→C transversions can be produced. In addition, reactive oxygen-induced DNA damage activates error-prone polymerase DNA repair that may in turn produce base mispairing (Cairns et al., 1991). Although exonucleases and glycosylases can repair such oxidative DNA damage, the probability of mutation fixation increases with the duration of exposure to a mutagen and with increases in the mitotic rate (Kasai et al., 1986; Cheng et al., 1992; Aronica et al., 1993; Kamiya et al., 1995), which, given the biologic persistence of TCDD, increase the total probability of a mutational event to a level comparable with that of stronger mutagens. An increase in 8-OHdG that persisted 8 weeks after treatment with TCDD was observed in the urine of C57BL/6 mice (Shen et al., 1995; Shertzer et al., 1998) and in the tissue culture medium of hepatoma Hepa-1c1c7 cells treated with TCDD (Park et al., 1996).

Though TCDD has been repeatedly linked to oxidative stress and the oxidative stress response to mutagenesis, TCDD has not been shown to be directly mutagenic in either bacterial or most in vitro assay systems (Giri, 1986). In this regard, an important negative finding has been that, at a dosing regimen capable of producing an oxidative stress response, TCDD did not alter the mutation frequency or the mutation spectrum of the lacI transgene in male or female Big Blue rats (Thornton et al., 2001). Since oxidative stress and some oxidative stress response genes are induced by TCDD in vivo, it can be concluded with caution that TCDD-mediated oxidative damage may not be a prominent cause of mutations. For this reason, an alternate pathway for enhancing cell proliferation and malignant conversion appears likely. Despite the ability of TCDD to generate oxidative base products (Park et al., 1996), the health implications of such findings must be questioned.

8. CONCLUDING REMARKS

In all likelihood, there are several interdependent AHR-dependent pathways that lead to the increased generation of ROS and to the decreased ability to defend against their action. For example, AHR activation may lead to an increase in superoxide production through increased expression of xanthine dehydrogenase/xanthine oxidase and monooxygenases; to an increase in capacity for superoxide reduction through increases in CuZnSOD; and to inhibition of glutathione peroxidases through the generation of J series prostaglandins. The net effect of all these changes would be an increased production of H₂O₂ and a decreased capacity to detoxify it. Studies aimed at understanding AHR-mediated toxicity have led to the discovery of AHR variants that appear to maintain physiological function and yet confer greatly diminish toxicity; perhaps this is due to the remarkable structural plasticity of the AHR, as shown by work in inbred mouse strains and in rats. The human AHRs thus far studied demonstrate ligand binding affinity characteristics similar to those of the low-affinity mouse strains. This is likely to be an important factor explaining why PHAHs show relatively low toxicity in humans. It is also intriguing to speculate that AHR variants may exist in the human population that confer sensitivity to PHAH pollutants because they behave more like the high-affinity rodent AHR variants. Indeed the studies in the rat and hamster suggest that poorly understood functions of the AHR transactivation domain contribute to toxicity.

One of the greater challenges in Ah receptor research is to identify the connection between toxicity and exposure. While the use of various model systems do serve to elucidate mechanism of TCDD toxicity, it remains difficult to draw broad conclusions given the wide variations in TCDD responses associated with species and strain susceptibilities, exposure models and response endpoints. Are changes in the redox state of cells exposed to PHAHs adaptive or toxic? Are the effects of these changes cumulative? The extent to which the AHR ligands elicit oxidative stress may depend on the duration and nature of the exposure, as well as on the properties of the agonist. At times AHR activation may be so transient that it causes modest and largely unnoticed perturbations to the cellular redox status. At other times or with other AHR ligands, the effects might be much more significant and harmful because of the severity and length of the oxidative stress response. We believe that many of the health consequences resulting from TCDD exposure may likely result from epigenetic mechanisms, including those exerted by cytosolic and mitochondrial reactive oxygen production. In this scenario, resultant toxicity would be related to the non-physiological persistent activation of AHR-dependent signaling pathways due to the long biological half-life of the compound.

Footnotes

ACKNOWLEDGEMENTS

Preparation of this review and the work done in our labs cited herein were supported in part by NIH Grants RO1 ES06273, R01 ES10807, R01 ES10133 and P30 ES06096. J.F.R. is supported by NIEHS Training Grant T32 ES07250.

References

Abbott

Schmid

Pitt

Buckalew

Wood

Held

, and Diliberto

. 1999. Adverse reproductive outcomes in the transgenic Ah receptor-deficient mouse. Toxicol Appl Pharmacol 155: 62–70

Ahmed

Napoli

, and Strobel

. 1995. Oxygen radical formation during cytochrome P450-catalyzed cyclosporine metabolism in rat and human liver microsomes at varying hydrogen ion concentrations. Mol Cell Biochem 151: 131–140

Alsharif

Lawson

, and Stohs

. 1994. Oxidative stress induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin is mediated by the aryl hydrocarbon (Ah) receptor complex. Toxicology 92: 39–51

Anari

Khan

Jatoe

, and O'Brien

. 1997. Cytochrome P450 dependent xenobiotic activation by physiological hydroperoxides in intact hepatocytes. Eur J Drug Metab Pharmacokinet 22: 305–310

Andersen

and Barton

. 1998. The use of biochemical and molecular parameters to estimate dose-response relationships at low levels of exposur. Environ Health Perspect 106 Suppl 1: 349–355

Andersen

Birnbaum

Barton

, and Eklund

. 1997. Regional hepatic CYP1A1 and CYP1A2 induction with 2,3,7,8-tetrachlorodibenzo-p-dioxin evaluated with a multicompartment geometric model of hepatic zonation. Toxicol. Appl Pharmacol. 144: 145–155

Aronica

and Katzenellenbogen

. 1993. Stimulation of estrogen receptor-mediated transcription and alteration in the phosphorylation state of the rat uterine estrogen receptor by estrogen, cyclic adenosine monophosphate, and insulin-like growth factor-I. Mol Endocrinol 7: 743–752

Bar-Peled

Korkotian

Segal

, and Groner

. 1996. Constitutive overexpression of Cu/Zn superoxide dismutase exacerbates kainic acid-induced apoptosis of transgenic-Cu/Zn superoxide dismutase neurons. Proc Natl Acad Sci U S A 93: 8530–8535

Bell

Crowley-Nowick

Bradlow

Sepkovic

Schmidt-Grimminger

Howell

Mayeaux

Tucker

Turbat-Herrera

, and Mathis

. 2000. Placebo-controlled trial of indole-3-carbinol in the treatment of CIN. Gynecol Oncol 78: 123–129

10.

Bello

Heideman

, and Peterson

. 2004. 2,3,7,8-Tetrachlorodibenzo-p-Dioxin Inhibits Regression of the Common Cardinal Vein in Developing Zebrafish. Toxicol Sci 78: 258–266

11.

Bertazzi

Zocchetti

Pesatori

Guercilena

Sanarico

, and Radice

. 1989. Mortality in an area contaminated by TCDD following an industrial incident. Med Lav 80: 316–329

12.

Bondy

and Naderi

. 1994. Contribution of hepatic cytochrome P450 systems to the generation of reactive oxygen species. Biochem Pharmacol 48: 155–159

13.

Bradlow

Davis

Lin

Sepkovic

, and Tiwari

. 1995. Effects of pesticides on the ratio of 16 alpha/2-hydroxyestrone: A biologic marker of breast cancer risk. Environ Health Perspect 103 Suppl 7: 147–150

14.

Bradlow

Hershcopf

Martucci

, and Fishman

. 1986. 16α-hydroxylation of estradiol: A possible risk marker for breast cancer. Ann New York Acad Sci 464: 138–151

15.

Bradlow

Hershcopf

Martucci

, and Fishman

. 1985. Estradiol 16 alpha-hydroxylation in the mouse correlates with mammary tumor incidence and presence of murine mammary tumor virus: A possible model for the hormonal etiology of breast cancer in humans. Proc Natl Acad Sci USA 82: 6295–6299

16.

Bradlow

Telang

Sepkovic

, and Osborne

. 1996. 2-hydroxyestrone: The ‘good’ estrogen. J Endocrinol 150 Suppl: S259–S265

17.

Buffinton

Ollinger

Brunmark

, and Cadenas

. 1989. DT-diaphorase-catalysed reduction of 1,4-naphthoquinone derivatives and glutathionyl-quinone conjugates. Effect of substituents on autoxidation rates Biochem J 257: 561–571

18.

Butler

Lang

Young

Caporaso

Vineis

Hayes

Teitel

Massengill

Lawsen

, and Kadlubar

. 1992. Determination of CYP1A2 and acetyltransferase phenotype in human populations by analysis of caffeine urinary metabolites. Pharmacogenetics 2: 116–127

19.

Cairns

Pongratz

Poellinger

, and Okret

. 1991. Assembly of a glucocorticoid receptor complex prior to DNA binding enhances its specific interaction with a glucocorticoid response element. J Biol Chem 266: 11221–11226

20.

Capdevila

Gil

Orellana

Marnett

Mason

Yadagiri

, and Falck

. 1988. Inhibitors of cytochrome P-450-dependent arachidonic acid metabolism. Arch Biochem Biophys 261: 257–263

21.

Cavalieri

Frenkel

Liehr

Rogan

, and Roy

. 2000. Estrogens as endogenous genotoxic agents—DNA adducts and mutations. J Natl Cancer Inst Monogr 75–93

22.

Chang

Smith

Prasad

Sidman

Nebert

, and Puga

. 1993. Ten nucleotide differences, five of which cause amino acid changes, are associated with the Ah receptor locus polymorphism of C57BL/6 and DBA/2 mice. Pharmacogenetics 3: 312–321

23.

Chapman

and Schiller

. 1985. Dose-related effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in C57BL/6J and DBA/2J mice. Toxicol Appl Pharmacol 78: 147–157

24.

Cheng

Cahill

Kasai

Nishimura

, and Loeb

. 1992. 8-Hydroxyguanine, an abundant form of oxidative DNA damage, causes G->T and A->C substitutions. J Biol Chem 267: 166–172

25.

Coumoul

Diry

Robillot

, and Barouki

. 2001. Differential regulation of cytochrome P450 1A1 and 1B1 by a combination of dioxin and pesticides in the breast tumor cell line MCF-7. Cancer Res 61: 3942–3948

26.

Dai

Rashba-Step

, and Cederbaum

. 1993. Stable expression of human cytochrome P4502E1 in HepG2 cells: Characterization of catalytic activities and production of reactive oxygen intermediates. Biochemistry 32: 6928–6937

27.

Dalton

Kerzee

Wang

Miller

Dieter

Lorenz

Shertzer

Nerbert

, and Puga

. 2001. Dioxin exposure is an environmental risk factor for ischemic heart disease. Cardiovasc Toxicol 1: 285–298

28.

Dalton

Chen

Schneider

Nerbert

, and Shertzer

. 2004. Genetically altered mice to evaluate glutathione homeostasis in health and disease. Free Radic Biol Med 37: 1511–1526

29.

DeVito

Birnbaum

Farland

, and Gasiewicz

. 1995. Comparisons of estimated human body burdens of dioxinlike chemicals and TCDD body burdens in experimentally exposed animals. Environ Health Perspect 103: 820–831

30.

Diliberto

Burgin

, and Birnbaum

. 1997. Role of CYP1A2 in hepatic sequestration of dioxin: Studies using CYP1A2 knock-out mice. Biochem Biophys Res Comm 236: 431–433

31.

Dong

Teraoka

Tsujimoto

Stegeman

, and Hiraga

. 2004. Role of aryl hydrocarbon receptor in mesencephalic circulation failure and apoptosis in zebrafish embryos exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin. Toxicol Sci 77: 109–116

32.

Dunlap

Ikeda

Nagashima

Vogel

, and Matsumura

. 2002. Effects of src-deficiency on the expression of in vivo toxicity of TCDD in a strain of c-src knockout mice procured through six generations of backcrossings to C57BL/6 mice. Toxicology 172: 125–141

33.

Ekstrom

Cronholm

, and Ingelman-Sundberg

. 1986. Hydroxyl-radical production and ethanol oxidation by liver microsomes isolated from ethanol-treated rats. Biochem J 233: 755–761

34.

Elferink

, and Levine

. 2001. Maximal Aryl Hydrocarbon Receptor Activity Depends on an Interaction with the Retinoblastoma Protein. Mol Pharmacol 59: 664–673

35.

Embrechts

Lemiere

Van Dongen

Esmans

Buytaert

Van Marck

Kockx

, and Makar

. 2003. Detection of estrogen DNA-adducts in human breast tumor tissue and healthy tissue by combined nano LC-nano ES tandem mass spectrometry. J Am Soc Mass Spectrom 14: 482–491

36.

Eskenazi

Wyrobek

Sloter

Kidd

Moore

Young

, and Moore

. 2003. The association of age and semen quality in healthy men. Hum Reprod 18: 447–454

37.

Fernandez-Salguero

Pineau

Hilbert

McPhail

Lee

Kimura

Nebert

Rudikoff

Ward

, and Gonzalez

. 1995. Immune system impairment and hepatic fibrosis in mice lacking the dioxin-binding Ah receptor. Science 268: 722–726

38.

Fernandez-Salguero

Hilbert

Rudikoff

Ward

, and Gonzalez

. 1996. Aryl-hydrocarbon receptor-deficient mice are resistant to 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced toxicity. Toxicol Appl Pharmacol 140: 173–179

39.

Fernandez-Salguero

Ward

Sundberg

, and Gonzalez

. 1997. Lesions of aryl-hydrocarbon receptor-deficient mice. Vet Pathol 34: 605–614

40.

Flesch-Janys

Berger

Gurn

Manz

Nagel

Waltsgott

, and Dwyer

. 1995. Exposure to polychlorinated dioxins and furans (PCDD/F) and mortality in a cohort of workers from a herbicide-producing plant in Hamburg, Federal Republic of Germany. Am J Epidemiol 142: 1165–1175

41.

Franc

Pohjanvirta

Tuomisto

, and Okey

. 2001a. In vivo up-regulation of aryl hydrocarbon receptor expression by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in a dioxin-resistant rat model. Biochem Pharmacol 62: 1565–1578

42.

Franc

Pohjanvirta

Tuomisto

, and Okey

. 2001b. Persistent, low-dose 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure: Effect on aryl hydrocarbon receptor expression in a dioxin-resistance model. Toxicol Appl Pharmacol 175: 43–53

43.

Gatmaitan

Lewis

Turchin

, and Arias

. 1977. Premature development of ligandin (GSH transferase B) in mice with an inherited defect in endoplasmic reticulum-golgi structure and function. Biochem Biophys Res Comm 75: 337–337

44.

and Elferink

. 1998. A direct interaction between the aryl hydrocarbon receptor and retinoblastoma protein. Linking dioxin signaling to the cell cycle. J Biol Chem 273: 22708–22713

45.

Giannone

Probst

, and Okey

. 1998. Prolonged depletion of AH receptor without alteration of receptor mRNA levels after treatment of cells in culture with 2,3,7,8-tetrachlorodibenzo-p-dioxin. Biochem Pharmacol 55: 489–497

46.

Gillette

Brodie

, and LaDu

. 1957. The oxidation of drugs by liver microsomes: On the role of TPNH and oxygen. J Pharmacol Exp Ther 119: 532–540

47.

Gillner

Bergman

Cambillau

Fernström

, and Gustafsson

. 1985. Interactions of indoles with specific binding sites for 2,3,7,8-tetrachlorodibenzo-p-dioxin in rat liver. Mol Pharmacol 28: 357–363

48.

Giri

. 1986. Mutagenic and genotoxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, a review. Mut Res 168: 241–248

49.

Gonzalez

and Nebert

. 1990. Evolution of the P450 gene superfamily: Animal-plant ‘warfare’, molecular drive and human genetic differences in drug oxidation. Trends Genet 6: 182–186

50.

Guengerich

and Lieber

. 1985. Enzymatic activation of chemicals to toxic metabolites. CRC Critical Reviews in Toxicology 14: 259–307

51.

Guiney

Walker

Spitsbergen

, and Peterson

. 2000. Hemodynamic dysfunction and cytochrome P4501A mRNA expression induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin during embryonic stages of lake trout development. Toxicol Appl Pharmacol 168: 1–14

52.

Hammond

Zhu

Wang

Ricci

, and Liehr

. 1997. Cytochrome P450 metabolism of estradiol in hamster liver and kidney. Toxicol Appl Pharmacol 145: 54–60

53.

Han

and Liehr

. 1995. Microsome-mediated 8-hydroxylation of guanine bases of DNA by steroid estrogens: Correlation of DNA damage by free radicals with metabolic activation to quinones. Carcinogenesis 16: 2571–2574

54.

Hankinson

. 1995. The aryl hydrocarbon receptor complex. Annu Rev Pharmacol Toxicol 35: 307–340

55.

Hassoun

Al Ghafri

, and Abushaban

. 2003. The role of antioxidant enzymes in TCDD-induced oxidative stress in various brain regions of rats after subchronic exposure. Free Radic Biol Med 35: 1028–1036

56.

Hassoun

Wilt

DeVito

Van Birgelen

Alsharif

Birnbaum

, and Stohs

. 1998. Induction of oxidative stress in brain tissues of mice after subchronic exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin. Toxicol Sci 42: 23–27

57.

Hatae

Hara

Yokoyama

Yabuki

Inoue

Ullrich

, and Tanabe

. 1996. Site-directed mutagenesis of human prostacyclin synthase: Alteration of Cys441 of the Cys-pocket, and Glu347 and Arg350 of the EXXR motif. FEBS Lett 389: 268–272

58.

Hayes

Spink

Cao

Walker

, and Sutter

. 1996. 17 beta-estradiol hydroxylation catalyzed by human cytochrome P450 1B1. Proc Natl Acad Sci USA 93: 9776–9781

59.

Hebert

Harris

Elwell

, and Birnbaum

. 1990. Relative toxicity and tumor-promoting ability of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,4,7,8-pentachlorodibenzofuran (PCDF), and 1,2,3,4,7,8-hexachlorodibenzofuran (HCDF) in hairless mice. Toxicol Appl Pharmacol 102: 362–377

60.

Henck

New

Kociba

, and Rao

. 1981. 2,3,7,8-tetrachlorodibenzo-p-dioxin: Acute oral toxicity in hamsters. Toxicol Appl Pharmacol 59: 405–407

61.

Henry

Spitsbergen

Hornung

Abnet

, and Peterson

. 1997. Early life stage toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin in zebrafish (Danio rerio). Toxicol Appl Pharmacol 142: 56–68

62.

Hestermann

Stegeman

, and Hahn

. 2000. Relative contributions of affinity and intrinsic efficacy to aryl hydrocarbon receptor ligand potency. Toxicol Appl Pharmacol 168: 160–172

63.

Hodgson

Ayala-Torres

Thompson

, and Liehr

. 1998. Estrogen-induced microsatellite DNA alterations are associated with Syrian hamster kidney tumorigenesis. Carcinogenesis 19: 2169–2172

64.

Huff

Lucier

, and Tritscher

. 1994. Carcinogenicity of TCDD: Experimental, mechanistic, and epidemiologic evidence. Annu Rev Pharmacol Toxicol 34: 343–372

65.

Huff

Salmon

Hooper

, and Zeise

. 1991. Long-term carcinogenesis studies on 2,3,7,8-tetrachlorodibenzo-p-dioxin and hexachlorodibenzo-p-dioxins. Cell Biol Toxicol 7: 67–94

66.

Jana

Sarkar

Ishizuka

Yonemoto

Tohyama

, and Sone

. 2000. Comparative effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on MCF-7, RL95–2, and LNCaP cells: Role of target steroid hormones in cellular responsiveness to CYP1A1 induction. Mol Cell Biol Res Commun 4: 174–180

67.

Jefcoate

Liehr

Santen

Sutter

Yager

Yue

Santner

Tekmal

Demers

Pauley

Naftolin

Mor

, and Berstein

. 2000. Tissue-specific synthesis and oxidative metabolism of estrogens. J Natl Cancer Inst Monogr 95–112

68.

Jiang

Epstein

Tomic

Freedberg

, and Blumenberg

. 1991. Functional comparison of the upstream regulatory DNA sequences of four human epidermal keratin gene. J Invest Dermatol 96: 162–167

69.

Jokinen

Walker

Brix

Sells

Haseman

, and Nyska

. 2003. Increase in cardiovascular pathology in female sprague-dawley rats following chronic treatment with 2,3,7,8-tetra-chlorodibenzop- dioxin and 3,3′,4,4′,5-pentachlorobiphenyl. Cardiovasc Toxicol 3: 299–310

70.

Kamiya

Miura

Murata-Kamiya

Ishikawa

Sakaguchi

Inoue

Sasaki

Masutani

Hanaoka

, and Nishimura

. 1995. 8-Hydroxyadenine (7,8-dihydro-8-oxoadenine) induces misincorporation in in vitro DNA synthesis and mutations in NIH 3T3 cells. Nucleic Acids Res 23: 2893–2899

71.

Karyala

Guo

Sartor

Medvedovic

Kann

Puga

Ryan

, and Tomlinson

. 2004. Different Global Gene Expression Profiles in Benzo[a]Pyrene- and Dioxin-Treated Vascular Smooth Muscle Cells of AHR-Knockout and Wild-Type Mice. Cardiovasc Toxicol 4: 47–74

72.

Kasai

Crain

Kuchino

Nishimura

Ootsuyama

, and Tanooka

. 1986. Formation of 8-hydroxyguanine moiety in cellular DNA by agents producing oxygen radicals and evidence for its repair. Carcinogenesis 7: 1849–1851

73.

Kociba

Keyes

Beyer

Carreon

Wade

Dittenber

Kalnins

Frauson

Park

Barnard

Hummel

, and Humiston

. 1978. Results of a two-year chronic toxicity and oncogenicity study of 2,3,7,8-tetrachlorodibenzo-p-dioxin in rats. Toxicol Appl Pharmacol 46: 279–303

74.

Kolluri

Weiss

Koff

, and Gottlicher

. 1999. p27(Kip1) induction and inhibition of proliferation by the intracellular Ah receptor in developing thymus and hepatoma cells. Genes Dev 13: 1742–1753

75.

Korkalainen

Tuomisto

, and Pohjanvirta

. 2004. Primary structure and inducibility by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) of aryl hydrocarbon receptor repressor in a TCDD-sensitive and a TCDD-resistant rat strain. Biochem Biophys Res Commun 315: 123–131

76.

Lahvis

Lindell

Thomas

McCuskey

Murphy

Glover

Bentz

Southard

, and Bradfield

. 2000. Portosystemic shunting and persistent fetal vascular structures in aryl hydrocarbon receptor-deficient mice. Proc Natl Acad Sci USA 97: 10442–10447

77.

Lai

Wong

, and Wong

. 2004. Modulation of AhR-mediated CYP1A1 mRNA and EROD activities by 17beta-estradiol and dexamethasone in TCDD-induced H411E cells. Toxicol Sci 78: 41–49

78.

Latchoumycandane

Chitra

, and Mathur

. 2003. 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) induces oxidative stress in the epididymis and epididymal sperm of adult rats. Arch Toxicol 77: 280–284

79.

Lee

Jin

Park

Han

Kim

Jeong

Huh

, and Kim

. 2002. 2,3,7,8-tetrachlorobenzo-p-dioxin inhibits proliferation of SK-N-SH human neuronal cells through decreased production of reactive oxygen species. Free Radic Res 36: 1283–1289

80.

Liehr

. 1997. Hormone-associated cancer: Mechanistic similarities between human breast cancer and estrogen-induced kidney carcinogenesis in hamsters. Environ Health Perspect 105 Suppl 3: 565–569

81.

Liehr

. 1999. 4-hydroxylation of oestrogens as a marker for mammary tumours. Biochem Soc Trans 27: 318–323

82.

Liehr

. 2000. Role of DNA adducts in hormonal carcinogenesis. Regul Toxicol Pharmacol 32: 276–282

83.

Liehr

. 2001. Genotoxicity of the steroidal oestrogens oestrone and oestradiol: Possible mechanism of uterine and mammary cancer development. Hum Reprod Update 7: 273–281

84.

Liehr

and Jones

. 2001. Role of iron in estrogen-induced cancer. Curr Med Chem 8: 839–849

85.

Lucier

Tritscher

Goldsworthy

Foley

Clark

Goldstein

, and Maronpot

. 1991. Ovarian hormones enhance 2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated increases in cell proliferation and preneoplastic foci in a two-stage model for rat hepatocarcinogenesis. Cancer Res 51: 1391–1397

86.

Maier

Micka

Miller

Denko

Chang

C-Y

Nebert

, and Puga

. 1998. Aromatic hydrocarbon receptor (AHR) polymorphism: Development of new methods to correlate genotype with phenotype. Environ Health Perspect 106: 421–426

87.

Malloy

Bradlow

, and Orentreich

. 1997. Interaction between a semisynthetic diet and indole-3-carbinol on mammary tumor incidence in Balb/cfC3H mice. Anticancer Res 17: 4333–4337

88.

Marlowe

Knudsen

Schwemberger

, and Puga

. 2004. The aryl hydrocarbon receptor displaces p300 from E2F-dependent promoters and represses S-phase specific gene expression. J Biol Chem E-Pub.

89.

Michnovicz

Adlercreutz

, and Bradlow

. 1997. Changes in levels of urinary estrogen metabolites after oral indole-3- carbinol treatment in humans. J Natl Cancer Inst 89: 718–723

90.

Mimura

Yamashita

Nakamura

Morita

Takagi

Nakao

Ema

Sogawa

Yasuda

Katsuki

, and Fujii-Kuriyama

. 1997. Loss of teratogenic response to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in mice lacking the Ah (dioxin) receptor. Genes Cells 2: 645–654

91.

Mohammadpour

Murray

, and Stohs

. 1988. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-induced lipid peroxidation in genetically responsive and non-responsive mice. Arch Environ Contam Toxicol 17: 645–650

92.

Moriguchi

Motohashi

Hosoya

Nakajima

Takahashi

Ohsako

Aoki

Nishimura

Tohyama

Fujii-Kuriyama

, and Yamamoto

. 2003. Distinct response to dioxin in an arylhydrocarbon receptor (AHR)-humanized mouse. Proc Natl Acad Sci U S A 100: 5652–5657

93.

Nakai

Ward

Gannon

, and Rifkind

. 1992. -Naphthoflavone induction of a cytochrome P-450 arachidonic acid epoxygenase in chick embryo liver distinct from the aryl hydrocarbon hydroxylase and from phenobarbital-induced arachidonate epoxygenase. J Biol Chem 267: 19503–19512

94.

Nebert

. 1989. The Ah locus: Genetic differences in toxicity, cancer, mutation, and birth defects. Crit Rev Toxicol 20: 153–174

95.

Nebert

. 1993. Elevated estrogen 16 alpha-hydroxylase activity: Is this a genotoxic or nongenotoxic biomarker in human breast cancer risk? Journal National Cancer Institute 85: 1888–1891

96.

Nebert

Dalton

Okey

, and Gonzalez

. 2004. Role of aryl hydrocarbon receptor-mediated induction of the CYP1 enzymes in environmental toxicity and cancer. J Biol Chem E-Pub.

97.

Nebert

Puga

, and Vasiliou

. 1993. Role of the Ah receptor and the dioxin-inducible [Ah] gene battery in toxicity, cancer, and signal transduction. Ann N Y Acad Sci 685: 624–640

98.

Nebert

Roe

Dieter

Solis

Yang

, and Dalton

. 2000. Role of the aromatic hydrocarbon receptor and [Ah] gene battery in the oxidative stress response, cell cycle control, and apoptosis. Biochem Pharmacol 59: 65–85

99.

Niittynen

Tuomisto

Auriola

Pohjanvirta

Syrjala

Simanainen

Viluksela

, and Tuomisto

. 2003. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced accumulation of biliverdin and hepatic peliosis in rats. Toxicol Sci 71: 112–123

100.

Okey

Vella

, and Harper

. 1989. Detection and characterization of a low affinity form of cytosolic Ah receptor in livers of mice nonresponsive to induction of cytochrome P1–450 by 3-methylcholanthrene. Mol Pharmacol 35: 823–830

101.

Pantopoulos

and Hentze

. 1995. Rapid responses to oxidative stress mediated by iron regulatory protein. EMBO J 14: 2917–2924

102.

Park

Shigenaga

, and Ames

. 1996. Induction of cytochrome P4501A1 by 2,3,7,8-tetrachlorodibenzo-p- dioxin or indolo(3,2-b)carbazole is associated with oxidative DNA damage. Proc Natl Acad Sci USA 19: 2322–2327

103.

Patandin

Koopman-Esseboom

de Ridder

Weisglas-Kuperus

, and Sauer

. 1998. Effects of environmental exposure to polychlorinated biphenyls and dioxins on birth size and growth in Dutch children. Pediatr Res 44: 538–545

104.

Pelclova

Fenclova

Preiss

Prochazka

Spacil

Dubska

Okrouhlik

Lukas

, and Urban

. 2002. Lipid metabolism and neuropsychological follow-up study of workers exposed to 2,3,7,8- tetrachlordibenzo- p-dioxin. Int Arch Occup Environ Health 75 Suppl: S60–S66

105.

Peled-Kamar

Lotem

Okon

Sachs

, and Groner

. 1995. Thymic abnormalities and enhanced apoptosis of thymocytes and bone marrow cells in transgenic mice overexpressing Cu/Zn-superoxide dismutase: Implications for Down syndrome. EMBO J 14: 4985–4993

106.

Pesatori

Consonni

Bachetti

Zocchetti

Bonzini

Baccarelli

, and Bertazzi

. 2003. Short- and long-term morbidity and mortality in the population exposed to dioxin after the “Seveso accident”. Ind Health 41: 127–138

107.

Pesatori

Zocchetti

Guercilena

Consonni

Turrini

, and Bertazzi

. 1998. Dioxin exposure and non-malignant health effects: A mortality study. Occup Environ Med 55: 126–131

108.

Peters

Narotsky

Elizondo

Fernandez-Salguero

Gonzalez

, and Abbott

. 1999a. Amelioration of TCDD-induced teratogenesis in aryl hydrocarbon receptor (AhR)-null mice. Toxicol Sci 47: 86–92

109.

Peters

Narotsky

Elizondo

Fernandez-Salguero

Gonzalez

, and Abbott

. 1999b. Amelioration of TCDD-induced teratogenesis in aryl hydrocarbon receptor (AhR)-null mice. Toxicol Sci 47: 86–92

110.

Pohjanvirta

Juvonen

Karenlampi

Raunio

, and Tuomisto

. 1988. Hepatic Ah-receptor levels and the effect of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) on hepatic microsomal monooxygenase activities in a TCDD-susceptible and -resistant rat strain. Toxicol Appl Pharmacol 92: 131–140

111.

Pohjanvirta

and Tuomisto

. 1994a. Short-term toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin in laboratory animals: Effects, mechanisms, and animal models. Pharmacol Rev 46: 483–549

112.

Pohjanvirta

and Tuomisto

. 1994b. Short-term toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin in laboratory animals: Effects, mechanisms, and animal models. Pharmacol Rev 46: 483–549

113.

Pohjanvirta

Viluksela

Tuomisto

Unkila

Karasinska

Franc

Holowenko

Giannone

Harper

Tuomisto

, and Okey

. 1999. Physicochemical differences in the AH receptors of the most TCDD-susceptible and the most TCDD-resistant rat strains. Toxicol Appl Pharmacol 155: 82–95

114.

Pohjanvirta

Wong

Harper

Tuomisto

, and Okey

. 1998. Point mutation in intron sequence causes altered carboxyl-terminal structure in the aryl hydrocarbon receptor of the most 2,3,7,8-tetrachlorodibenzo-p-dioxin-resistant rat strain. Mol Pharmacol 54: 86–93

115.

Poland

Glover

, and Kende

. 1976a. Stereospecific, high affinity binding of 2,3,7,8- tetrachlorodibenzo-p-dioxin by hepatic cytosol. Evidence that the binding species is receptor for induction of aryl hydrocarbon hydroxylase. J Biol Chem 251: 4936–4946

116.

Poland

Glover

, and Kende

. 1976b. Stereospecific, high affinity binding of 2,3,7,8- tetrachlorodibenzo-p-dioxin by hepatic cytosol. Evidence that the binding species is receptor for induction of aryl hydrocarbon hydroxylase. J Biol Chem 251: 4936–4946

117.

Poland

and Knutson

. 1982. 2,3,7,8,-tetrachlorodibenzo-p-dioxin and related halogenated aromatic hydrocarbons: Examination of the mechanisms of toxicity. Annu Rev Pharmacol Toxicol 22: 517–554

118.

Poland

Palen

, and Glover

. 1994. Analysis of the four alleles of the murine aryl hydrocarbon receptor. Mol Pharmacol 46: 915–921

119.

Pollenz

. 2002. The mechanism of AH receptor protein down-regulation (degradation) and its impact on AH receptor-mediated gene regulation. Chem Biol Interact 141: 41–61

120.

Pollenz

Santostefano

Klett

Richardson

Necela

, and Birnbaum

. 1998. Female Sprague-Dawley rats exposed to a single oral dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin exhibit sustained depletion of aryl hydrocarbon receptor protein in liver, spleen, thymus, and lung. Toxicol Sci 42: 117–128

121.

Poulos

and Raag

. 1992. Cytochrome P450cam: Crystallography, oxygen activation, and electron transfer. FASEB J 6: 674–679

122.

Puga

Barnes

Dalton

Chang

Knudsen

, and Maier

. 2000a. Aromatic hydrocarbon receptor interaction with the retinoblastoma protein potentiates repression of E2F-dependent transcription and cell cycle arrest. J Biol Chem 275: 2943–2950

123.

Puga

Maier

, and Medvedovic

. 2000b. The transcriptional signature of dioxin in human hepatoma HepG2 cells. Biochem Pharmacol 60: 1129–1142

124.

Puga

Xia

, and Elferink

. 2002. Role of the aryl hydrocarbon receptor in cell cycle regulation. Chem Biol Interact 141: 117–130

125.

Ralston

Lau

HHS

Seidel

Luch

Platt

, and Baird

. 1994. The potent carcinogen dibenzo[a,l]pyrene is metabolically activated to fjord-region 11,12-diol 13,14-epoxides in human mammary carcinoma MCF-7 cell cultures. Cancer Res 54: 887–890

126.

Ramadoss

Petrulis

Hollingshead

Kusnadi

, and Perdew

. 2004. Divergent roles of hepatitis B virus X-associated protein 2 (XAP2) in human versus mouse Ah receptor complexes. Biochemistry 43: 700–709

127.

Revich

Aksel

Ushakova

Ivanova

Zhuchenko

Klyuev

Brodsky

, and Sotskov

. 2001. Dioxin exposure and public health in Chapaevsk, Russia. Chemosphere 43: 951–966

128.

Rifkind

Gannon

, and Gross

. 1990. Arachidonic metabolism by dioxin-induced cytochrome P-450: A new hypothesis on the role of P-450 in dioxin toxicity. Biochem Biophys Res Comm 172: 1180–1188

129.

Rosen

Woodson

Thompson

Hengesteg

, and Bradlow

. 1998. Preliminary results of the use of indole-3-carbinol for recurrent respiratory papillomatosis. Otolaryngol Head Neck Surg 118: 810–815

130.

Roy

Bernhardt

Strobel

, and Liehr

. 1992. Catalysis of the oxidation of steroid and stilbene estrogens to estrogen quinone metabolites by the beta-naphthoflavone-inducible cytochrome P450 IA family. Arch Biochem Biophys 296: 450–456

131.

Roy

Strobel

, and Liehr

. 1991. Cytochrome b5-mediated redox cycling of estrogen. Arch Biochem Biophys 285: 331–338

132.

Rylander

and Hagmar

. 2000. Medical and psychometric examinations of conscripts born to mothers with a high intake of fish contaminated with persistent organochlorines. Scand J Work Environ Health 26: 207–212

133.

Santostefano

Wang

Richardson

Ross

DeVito

, and Birnbaum

. 1998. A pharmacodynamic analysis of TCDD-induced cytochrome P450 gene expression in multiple tissues: Dose- and time-dependent effects. Toxicol Appl Pharmacol 151: 294–310

134.

Sawyer

and Van Houten

. 1999. Repair of DNA damage in mitochondria. Mutat Res 434: 161–176

135.

Schafer

and Buettner

. 2001. Redox environment of the cell as viewed through the redox state of the glutathione disulfide/glutathione couple. Free Radic Biol Med 30: 1191–1212

136.

Schaldach

Riby

, and Bjeldanes

. 1999. Lipoxin A4: A new class of ligand for the Ah receptor. Biochemistry 38: 7594–7600

137.

Schlezinger

and Stegeman

. 2001. Induction and suppression of cytochrome P450 1A by 3,3′,4,4′,5- pentachlorobiphenyl and its relationship to oxidative stress in the marine fish scup (Stenotomus chrysops). Aquat Toxicol 52: 101–115

138.

Schlezinger

White

, and Stegeman

. 1999. Oxidative inactivation of cytochrome P-450 1A (CYP1A) stimulated by 3,3′,4,4′-tetrachlorobiphenyl: Production of reactive oxygen by vertebrate CYP1As. Mol Pharmacol 56: 588–597

139.

Schmidt

GH-T

Reddy

Simon

, and Bradfield

. 1996. Characterization of a murine Ahr null allele: Involvement of the Ah receptor in hepatic growth and development. Proc Natl Acad Sci USA 93: 6731–6736

140.

Senft

Dalton

Nebert

Genter

Hutchinson

, and Shertzer

. 2002a. Dioxin increases reactive oxygen production in mouse liver mitochondria. Toxicol Appl Pharmacol 178: 15–21

141.

Senft

Dalton

Nebert

Genter

Puga

Hutchinson

Kerzee

Uno

, and Shertzer

. 2002b. Mitochondrial reactive oxygen production is dependent on the aromatic hydrocarbon receptor. Free Radic Biol Med 33: 1268–1278

142.

Shen

H-M

Ong

C-N

Lee

B-L

, and Shi

C-Y

. 1995. Aflatoxin B{-1}-induced 8-hydroxydeoxyguanosine formation in rat hepatic DNA. Carcinogenesis 16: 419–422

143.

Shertzer

Clay

Genter

Chames

Schneider

Oakley

Nebert

, and Dalton

. 2004a. Uncoupling-mediated generation of reactive oxygen by halogenated aromatic hydrocarbons in mouse liver microsomes. Free Radic Biol Med 36: 618–631

144.

Shertzer

Clay

Genter

Schneider

Nebert

, and Dalton

. 2004b. Cyp1a2 protects against reactive oxygen production in mouse liver microsomes. Free Radic Biol Med 36: 605–617

145.

Shertzer

Nebert

Puga

Ary

Sonntag

Dixon

Robinson

Cianciolo

, and Dalton

. 1998. Dioxin causes a sustained oxidative stress response in the mouse. Biochem Biophys Res Comm 253: 44–48

146.

Shertzer

and Senft

. 2000. The micronutrient indole-3-carbinol: Implications for disease and chemoprevention. Drug Metabol Drug Interact 17: 159–188

147.

Simanainen

Tuomisto

, and Viluksela

. 2003. Dose-response analysis of short-term effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin in three differentially susceptible rat lines. Toxicol Appl Pharmacol 187: 128–136

148.

Slezak

Hatch

DeVito

Diliberto

Slade

Crissman

Hassoun

, and Birnbaum

. 2000. Oxidative stress in female B6C3F1 mice following acute and subchronic exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Toxicol Sci 54: 390–398

149.

Slim

Toborek

Robertson

Lehmler

, and Hennig

. 2000. Cellular glutathione status modulates polychlorinated biphenyl-induced stress response and apoptosis in vascular endothelial cells. Toxicol Appl Pharmacol 166: 36–42

150.

Sloop

and Lucier

. 1987. Dose-dependent elevation of Ah receptor binding by TCDD in rat liver 1. Toxicol Appl Pharmacol 88: 329–337

151.

Smith

Clothier

Carthew

Childs

Sinclair

Nebert

, and Dalton

. 2001. Protection of the Cyp1a2(-/-) null mouse against uroporphyria and hepatic injury following exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin. Toxicol Appl Pharmacol 173: 89–98

152.

Smith

Clothier

Robinson

Scullion

Carthew

Edwards

Luo

Lim

, and Toledano

. 1998. Interaction between iron metabolism and 2,3,7,8-tetrachlorodibenzo-p- dioxin in mice with variants of the Ahr gene: A hepatic oxidative mechanism. Mol Pharmacol 53: 52–61

153.

Spink

Hayes

Young

Christou

Sutter

Jefcoate

, and Gierthy

. 1994. The effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on estrogen metabolism in MCF-7 breast cancer cells: Evidence for induction of a novel 17 beta-estradiol 4-hydroxylase. J Steroid Biochem Mol Biol 51: 251–258

154.

Spink

Cao

DePasquale

Pentecost

Fasco

, and Sutter

. 1998. Differential expression of CYP1A1 and CYP1B1 in human breast epithelial cells and breast tumor cells. Carcinogenesis 19: 291–298

155.

Stockbauer

Hoffman

Schramm

, and Edmonds

. 1988. Reproductive outcomes of mothers with potential exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin. Am J Epidemiol 128: 410–419

156.

Stohs

. 1990. Oxidative stress induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Free Radic Biol Med 9: 79–90

157.

Svensson

Nilsson

Hansson

Rappe

Akesson

, and Skerfving

. 1991. Exposure to dioxins and dibenzofurans through the consumption of fish. N Engl J Med 324: 8–12

158.

Swanson

and Bradfield

. 1993. The AH-receptor: Genetics, structure and function. Pharmacogenetics 3: 213–230

159.

Talalay

De Long

, and Prochaska

. 1988. Identification of a common chemical signal regulating the induction of enzymes that protect against chemical carcinogenesis. Proc Natl Acad Sci USA 85: 8261–8265

160.

Telang

Katdare

Bradlow

Osborne

, and Fishman

. 1997. Inhibition of proliferation and modulation of estradiol metabolism: Novel mechanisms for breast cancer prevention by the phytochemical indole-3-carbinol. Proc Soc Exp Biol Med 216: 246–252

161.

Telang

Suto

Wong

Osborne

, and Bradlow

. 1992. Induction by estrogen metabolite 16α-hydroxyestrone of genotoxic damage and aberrant proliferation in mouse mammary epithelial cells. Journal of the National Cancer Institute 84: 634–638

162.

Thornton

Oda

Stuart

Glickman

, and de Boer

. 2001. Mutagenicity of TCDD in Big Blue transgenic rats. Mutat Res 478: 45–50

163.

Tian

Denison

Rabson

, and Gallo

. 1999. Ah receptor and NF-kappaB interactions, a potential mechanism for dioxin toxicity. J Biol Chem 274: 510–515

164.

Tritscher

Seacat

Yager

Groopman

Miller

Bell

Sutter

, and Lucier

. 1996. Increased oxidative DNA damage in livers of 2,3,7,8-tetrachlorodibenzo-p-dioxin treated intact but not ovariectomized rats. Cancer Letters 98: 219–225

165.

Tuomisto

Viluksela

Pohjanvirta

, and Tuomisto

. 1999. The AH receptor and a novel gene determine acute toxic responses to TCDD: Segregation of the resistant alleles to different rat lines. Toxicol Appl Pharmacol 155: 71–81

166.

Unkila

Tuomisto

Pohjanvirta

MacDonald

Tuomisto

Koulu

, and Tuomisto

. 1993. Effect of a single lethal dose of TCDD on the levels of monoamines, their metabolites and tryptophan in discrete brain nuclei and peripheral tissues of Long-Evans rats. Pharmacol Toxicol 72: 279–285

167.

Uno

Dalton

Sinclair

Gorman

Wang

Smith

Miller

Shertzer

, and Nebert

. 2004. Cyp1a1(-/-) male mice: Protection against high-dose TCDD-induced lethality and wasting syndrome, and resistance to intrahepatocyte lipid accumulation and uroporphyria. Toxicol Appl Pharmacol 196: 410–421

168.

Vartiainen

Jaakkola

Saarikoski

, and Tuomisto

. 1998. Birth weight and sex of children and the correlation to the body burden of PCDDs/PCDFs and PCBs of the mother. Environ Health Perspect 106: 61–66

169.

Vena

Boffetta

Becher

Benn

Bueno-De-Mesquita

Coggon

Colin

Flesch-Janys

Green

Kauppinen

Littorin

Lynge

Mathews

Neuberger

Pearce

Pesatori

Saracci

Steenland

, and Kogevinas

. 1998. Exposure to dioxin and nonneoplastic mortality in the expanded IARC international cohort study of phenoxy herbicide and chlorophenol production workers and sprayers. Environ Health Perspect 106 Suppl 2: 645–653

170.

Walker

Portier

Lax

Crofts

Lucier

, and Sutter

. 1999. Characterization of the dose-response of CYP1B1, CYP1A1, and CYP1A2 in the liver of female Sprague-Dawley rats following chronic exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin. Toxicol Appl Pharmacol 154: 279–286

171.

Wang

Santostefano

DeVito

, and Birnbaum

. 1997a. Extrapolation of a previous PBPK model for TCDD across routes of exposure, gender, and from rats to mice. Toxicological Sciences 38–42

172.

Wang

Santostefano

Evans

Richardson

Diliberto

, and Birnbaum

. 1997b. Determination of parameters responsible for pharmacokinetic behavior of TCDD in female Sprague-Dawley rats. Toxicol Appl Pharmacol 147: 151–168

173.

Wyde

Eldridge

Lucier

, and Walker

. 2001a. Regulation of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced tumor promotion by 17 beta-estradiol in female Sprague-Dawley rats. Toxicol Appl Pharmacol 173: 7–17

174.

Wyde

Seely

Lucier

, and Walker

. 2000. Toxicity of chronic exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin in diethylnitrosamine-initiated ovariectomized rats implanted with subcutaneous 17 beta-estradiol pellets. Toxicol Sci 54: 493–499

175.

Wyde

Wong

Kim

Lucier

, and Walker

. 2001b. Induction of hepatic 8-oxodeoxyguanosine adducts by 2,3,7,8- tetrachlorodibenzo-p-dioxin in Sprague-Dawley rats is female-specific and estrogen-dependent. Chem Res Toxicol 14: 849–855

176.

Wyllie

and Liehr

. 1997. Release of iron from ferritin storage by redox cycling of stilbene and steroid estrogen metabolites: A mechanism of induction of free radical damage by estrogen. Arch Biochem Biophys 346: 180–186

Induction of Oxidative Stress Responses by Dioxin and other Ligands of the Aryl Hydrocarbon Receptor

Abstract

1. INTRODUCTION

2. FUNCTIONAL ALTERATIONS OF THE AHR

3. PRINCIPLES OF TCDD SENSITIVITY

4. EFFECTS OF GENDER AND SEX HORMONES IN THE TCDD DOSE-RESPONSE

5. EFFECTS OF TCDD EXPOSURE ON DEVELOPMENT

AHR in development

6. ROLE OF CYTOCHROME P450 ENZYMES IN TCDD TOXICITY

7. TCDD-MEDIATED PERTURBATION OF REDOX HOMEOSTASIS

8. CONCLUDING REMARKS

Footnotes

ACKNOWLEDGEMENTS

References