In vivo kinetics and DNA-binding properties of the Ah receptor in the golden Syrian hamster

In vivo up-regulation of aryl hydrocarbon receptor expression by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in a dioxin-resistant rat model

The aryl hydrocarbon receptor (AHR) mediates toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and regulates expression of several genes such as CYP1A1. Little is known about what regulates expression of the AHR itself. We tested the ability of TCDD to alter in vivo expression of its own receptor in rat strains that are susceptible to TCDD lethality [Long-Evans (Turku AB) (L-E) and Sprague Dawley (SD)] and in a rat strain that is remarkably resistant to TCDD lethality [Han/Wistar (Kuopio) (H/W)]. Rats were administered a single, intragastric dose of 5 or 50 microg/kg of TCDD. Hepatic cytosol, nuclear extract, and RNA were prepared at 1, 4, and 10 days after TCDD exposure. AHR expression was assessed at three levels: ligand binding function, immunoreactive protein and mRNA. TCDD at 5 microg/kg produced a 2- to 3-fold increase in cytosolic AHR in all strains; 50 microg/kg produced depletion at day 1 followed by recovery in SD and H/W but not L-E rats. Both the increase in AHR above basal levels and the recovery from initial depletion were accompanied by elevations in steady-state AHR mRNA, suggesting a pre-translational mechanism for AHR regulation by its own ligand. This up-regulation in vivo is in contrast to the sustained depletion of AHR caused by TCDD in cell culture. There was no clear relationship between AHR regulation and strain sensitivity; thus, the large inherent strain differences in susceptibility to TCDD lethality probably are not explained by differential regulation of AHR by TCDD.

The existence of the 4S polycyclic aromatic hydrocarbon-protein binding in 14-day-old chick embryo liver

Cytochrome P-450IA1, the isozyme most closely associated with aryl hydrocarbon hydroxylase (AHH), is regulated by two high-affinity binding proteins, the 4S polycyclic aromatic hydrocarbon (PAH)-binding protein which primarily binds PAHs and the 8S Ah (dioxin) receptor which binds 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and like congeners. The present study was conducted to determine whether the 4S protein existed in 14-day-old chick embryo liver when AHH activity is maximal to determine if they are linked as is the 8S Ah receptor and to confirm the existence of the dioxin receptor by investigating their ligand binding characteristics in the presence and absence of sodium molybdate, an agent that stabilizes steroid hormone receptors and partially stabilizes the dioxin receptor. Competitive ligand binding studies were performed with liver cytosol from livers of male 14-day-old chick embryos using [3H]-benzo[a]pyrene (B[a]P) or [3H]-TCDD in the presence and absence of a 200-fold excess of B[a]P, benzo[e]pyrene (B[e]P), 3-methylcholanthrene (3-MC), and tetrachlorodibenzofuran (TCDBF). Specific PAH-binding activity was assayed using sucrose gradient analysis. In the absence of molybdate, the 4S PAH-binding protein had high affinity for B[a]P, B[e]P, 3-MC, but very low affinity for TCDBF; the Ah receptor exhibited high affinity for TCDBF. In the presence of sodium molybdate, the Ah receptor was stabilized while the 4S PAH-binding protein was relatively unaffected. These results affirm the existence of two distinct PAH-binding proteins in 14-day-old chick embryo liver cytosol and suggest a linkage of the 4S protein to AHH.

Transformation of the aryl hydrocarbon receptor to a DNA-binding form is accompanied by release of the 90 kDa heat-shock protein and increased affinity for 2,3,7,8-tetrachlorodibenzo-p-dioxin

The binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to the aryl hydrocarbon receptor (AhR) elicits a sequence of poorly defined molecular events that ultimately yield a heteromeric transformed AhR that is active as a transcription factor. We have previously developed a model of the ligand-initiated transformation of the AhR to the DNA-binding state based on characterization of several forms of the AhR with respect to their physicochemical properties and DNA-binding affinities. The present studies were designed to determine whether, and at what stage, this process of transformation alters the receptor's affinity for TCDD. In rat hepatic cytosol, approx. 10% of the TCDD specifically bound to the AhR rapidly dissociated (t1/2 approximately 1 h), while the remainder was only slowly dissociable (t1/2 approximately 70 h). The isolated DNA-binding forms of the receptor (monomeric and transformed) bound TCDD very tightly (t1/2 > 100 h), whereas TCDD was dissociable from the non-DNA-binding receptor form(s). A lower incubation temperature (0-4 degrees C) and the presence of molybdate partially stabilized the non-DNA-binding fraction of the TCDD.receptor complex and also enhanced TCDD dissociation in crude cytosol. Immunoprecipitation of the different AhR forms with an anti-AhR antibody and immunoblotting with antibody to the 90 kDa heat-shock protein (hsp90) demonstrated that hsp90 was associated with the unoccupied receptor complex as well as with a fraction of the non-DNA-binding TCDD.receptor complex; isolated DNA-binding forms did not contain detectable hsp90. We conclude that while hsp90 remains associated with the AhR, TCDD is readily dissociable; following release of hsp90, however, TCDD becomes very tightly bound, and remains so upon completion of transformation.

Binding of the Ah receptor to receptor binding factors in chromatin

Dioxin induces biological responses through interaction with a specific intracellular receptor, the Ah receptor, and the subsequent interaction of the Ah receptor with chromatin. We report the binding of the Ah receptor, partially purified from rabbit liver, to receptor binding factors in chromatin. Rabbit liver chromatin proteins (CP) were isolated by adsorption of chromatin to hydroxylapatite followed by sequential extraction with 1-8 M GdnHCl. To assay for receptor binding a portion of each CP fraction was reconstituted to rabbit double-stranded DNA using a reverse gradient dialysis of 7.5 to 0 M GdnHCl. These reconstituted nucleoacidic proteins were then examined for binding to [3H]-2,3,7,8-tetrachlorodibenzo-p-dioxin ([3H]TCDD)-receptor complexes by the streptomycin filter assay. Prior to the binding assay, [3H]TCDD-receptor complexes were partially purified by step elution from DEAE-cellulose columns. CP fractions 2, 5, and 7 were found to bind to the Ah receptor with high affinity. Scatchard analysis yielded Kd values in the nanomolar range. Competition with 2-fold excess unlabeled TCDD-receptor complexes was demonstrated, and binding was reduced markedly when the receptor was prepared in the presence of 10 mM molybdate. Such chromatin receptor binding factors (RBFs) may participate in the interaction of receptor with specific DNA sequences resulting in modulation of specific gene expression.

Modeling receptor-mediated processes with dioxin: implications for pharmacokinetics and risk assessment

Dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin; TCDD), a widespread polychlorinated aromatic hydrocarbon, caused tumors in the liver and other sites when administered chronically to rats at doses as low as 0.01 microgram/kg/day. It functions in combination with a cellular protein, the Ah receptor, to alter gene regulation, and this resulting modulation of gene expression is believed to be obligatory for both dioxin toxicity and carcinogenicity. The U.S. EPA is reevaluating its dioxin risk assessment and, as part of this process, will be developing risk assessment approaches for chemicals, such as dioxin, whose toxicity is receptor-mediated. This paper describes a receptor-mediated physiologically based pharmacokinetic (PB-PK) model for the tissue distribution and enzyme-inducing properties of dioxin and discusses the potential role of these models in a biologically motivated risk assessment. In this model, ternary interactions among the Ah receptor, dioxin, and DNA binding sites lead to enhanced production of specific hepatic proteins. The model was used to examine the tissue disposition of dioxin and the induction of both a dioxin-binding protein (presumably, cytochrome P4501A2), and cytochrome P4501A1. Tumor promotion correlated more closely with predicted induction of P4501A1 than with induction of hepatic binding proteins. Although increased induction of these proteins is not expected to be causally related to tumor formation, these physiological dosimetry and gene-induction response models will be important for biologically motivated dioxin risk assessments in determining both target tissue dose of dioxin and gene products and in examining the relationship between these gene products and the cellular events more directly involved in tumor promotion.

Factors affecting the toxicity of dioxin-like toxicants: a molecular approach to risk assessment of dioxins

The numerous toxic responses of dioxin-like compounds are mediated by the intracellular Ah (aryl hydrocarbon) receptor. It has been suggested that the regulation of dioxins and similar substances could be placed on a molecular foundation by considering the proportion of Ah-receptor sites occupied by toxicant molecules. The present work has shown that the following formation not yet available would be needed in order to develop this approach: correlation between dioxin exposure and human tissue levels; accurate determination of the association constants for human Ah-receptor with toxicant, and for human receptor-ligand complex with DNA; and knowledge of the intracellular concentrations of both receptor binding sites and DNA binding sites. Furthermore, since not all dioxin-like substances behave identically, this information would need to be gathered for a wide variety of substances.

Ribonuclease inhibits Ah receptor transformation in vitro

The aryl hydrocarbon (Ah) receptor undergoes a ligand-dependent transformation to a heteromeric structure which has the ability to bind DNA sequence-specifically with high affinity. By this mechanism, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related xenobiotics modify gene expression. We observed that transformation was inhibited in vitro by the presence of ribonuclease A (RNAase) during incubation of rat hepatic cytosol with TCDD. This effect was detected as a decreased ability of the TCDD-receptor complex to bind to calf thymus DNA covalently linked to Sepharose, and to a dioxin-responsive enhancer which is upstream of the cytochrome P450IA1 structural gene. RNAase had no effect on previously transformed TCDD-receptor complexes. These observations indicated that RNAase acted during ligand binding and/or transformation of the Ah receptor. Saturation binding analyses demonstrated that RNAase decreased the receptor affinity for TCDD without changing the total number of binding sites. RNAase also inhibited transformation of the TCDD-bound, partially purified, untransformed, receptor. Thus RNAase does not interfere with ligand binding, but inhibits the subsequent transformation of the receptor monomer to the heteromeric, transcriptionally active, form.

Structure-dependent induction of aryl hydrocarbon hydroxylase activity in C57BL/6 mice by 2,3,7,8-tetrachlorodibenzo-p-dioxin and related congeners: mechanistic studies

The time- and dose-dependent induction of murine hepatic microsomal aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) activities by five polychlorinated dibenzo-p-dioxin and dibenzofuran congeners showed that the order of induction potency was 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) greater than 2,3,7,8-tetrachlorodibenzofuran (TCDF) greater than 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PCDD) greater than 1,2,3,7,8-pentachlorodibenzofuran (PCDF) greater than 2,3,7-trichlorodibenzo-p-dioxin (TrCDD). These structure-induction relationships were comparable to the structure-toxicity and competitive structure-receptor binding relationships previously reported for these compounds. However, using the corresponding radiolabeled congeners, the direct binding Kd values for dissociation of the cytosolic receptor-ligand complexes were 9.52, 7.96, 1.27, 3.10, and 8.31 nM for the 2,3,7,8-TCDD, 2,3,7,8-TCDF, 2,3,7-TrCDD, 1,2,3,7,8-PCDD, and 1,2,3,7,8-PCDF congeners and these data were clearly not structure dependent (i.e., similar to the structure-activity relationships). Some of the molecular properties for several radioligand-receptor complexes were similar; for example, the sedimentation coefficients for the cytosolic and nuclear receptor complexes varied from 8.8-10.4 S and 5.98-7.0 S, respectively, and the nuclear receptor complexes for all the radioligands eluted from a DNA-Sepharose column at salt concentrations of 0.27-0.29 M. Treatment of the mice with a maximum inducing dose of 2,3,7,8-[3H]TCDD resulted in a time-dependent formation of the nuclear receptor complex which was maximized between 16-24 hr and subsequently decreased up to 72 hr after initial exposure. In parallel studies, the nuclear receptor complex levels were determined 16 hr after treatment of the mice with different doses (2.25, 4.5, and 45 micrograms/kg) of all five radioligands. The results showed that at submaximal induction of the monooxygenase enzyme activities there was a linear correlation between the induced AHH or EROD activities (after 32 hr) and the corresponding nuclear receptor complex levels. It was also apparent from the data that the relative levels of nuclear receptor complex were structure dependent and this suggests that the transformation or activation of cytosolic receptor complexes may be a ligand structure-dependent process which correlates with the observed structure-activity relationships for 2,3,7,8-TCDD and related compounds.

Binding characteristics of 4S PAH-binding protein and Ah receptor from rats and mice

Cytochrome P-450IA1 (Cyto-P450IA1) is the isozyme most closely associated with aryl hydrocarbon hydroxylase (AHH). At least two distinct high-affinity binding proteins may regulate its expression, the 4S protein that primarily binds polycyclic aromatic hydrocarbons (PAHs), and the 8S Ah receptor that binds 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and like congeners. The present study was conducted to investigate ligand binding characteristics of the 4S and 8S binding proteins before and after separation from liver cytosol in the presence and absence of sodium molybdate. Liver cytosol and 4S and 8S receptor-enriched fractions from livers of male Sprague-Dawley rats (AHH-responsive), and from C57BL/6N (AHH-responsive) and DBA/2N and AKR/N mice (AHH-nonresponsive) served as sources of these proteins. Competitive binding studies were performed using 10 nM [3H]benzo[a]pyrene (BaP) or [3H]-TCDD in the presence and absence of a 200-fold excess of BaP, 3-methylcholanthrene (3-MC), and tetrachlorodibenzofuran (TCDBF). Specific PAH-binding activity was assayed by using either sucrose density gradient analysis or a hydroxylapatite assay. Our results indicate that before and after the separation of liver cytosol into 4S and 8S fractions, ligand binding characteristics were relatively unaltered for the 4S protein in comparison to that for the Ah receptor, particularly in the presence of molybdate. The 4S protein had high affinity for BaP and 3-MC but very low affinity for TCDBF; the 8S protein had high affinity for TCDBF, lesser affinity for 3-MC, and low affinity for BaP. In the presence of sodium molybdate, the Ah receptor fractions were significantly stabilized, whereas the 4S protein was relatively unaffected. After the separation of Ah receptor fraction from liver cytosol in the presence of molybdate, 3-MC consistently bound to a greater extent. These results affirm the existence of two distinct PAH-binding proteins.