Binding of 3-methylcholanthrene and 2,3,7,8-tetrachlorodibenzo-p-dioxin to a common Ah receptor site in mouse and rat hepatic cytosols

The Differential Selectivity of Aryl Hydrocarbon Receptor (AHR) Agonists towards AHR-Dependent Suppression of Mammosphere Formation and Gene Transcription in Human Breast Cancer Cells

We had previously reported that treatment with the aryl hydrocarbon receptor (AHR) agonist β-naphthoflavone (βNF) suppressed mammosphere formation derived from cancer stem cells in human breast cancer MCF-7 cells (Cancer Lett., 317, 2012, Zhao et al.). Here, using several AHR agonists, we have investigated the association of this suppression with the classical ability to induce AHR-mediated gene transcription in the xenobiotic response element (XRE). The mammosphere formation assays were performed using wild-type and AHR-knockout MCF-7 cells in the presence of AHR agonists including 3-methylcholanthrene (3MC), benzo[a]pyrene (BaP), 7,12-dimethylbenz[a]anthracene (DMBA), 6-formylindolo[3,2-b]carbazole (FICZ), indirubin, indole-3-carbinol (I3C), indole-3-acetic acid (IAA), and kynurenine (KYN), followed by the XRE-reporter gene assays of the agonists. We showed that treatments with 3MC, BaP, and DMBA strongly suppressed mammosphere formation of the stem cells in an AHR-dependent manner, while other agonists showed weaker suppression. In reporter gene assays, the strength or duration of AHR/XRE-mediated gene transcription was found to be dependent on the agonist. Although strong transcriptional activation was observed with 3MC, FICZ, indirubin, I3C, IAA, or KYN after 6 h of treatment, only weak activation was seen with BaP or DMBA. While transcriptional activation was sustained or increased at 24 h with 3MC, BaP, or DMBA, appreciable reduction was observed with the other agonists. In conclusions, the results demonstrated that the suppressive effects of AHR agonists on mammosphere formation do not necessarily correlate with their abilities to induce AHR-mediated gene transcription. Hence, different AHR functions may be differentially induced in an agonist-dependent manner.

Methoxychlor inhibits growth and induces atresia through the aryl hydrocarbon receptor pathway in mouse ovarian antral follicles

Methoxychlor (MXC) is an organochlorine pesticide used against pests that attack crops, vegetables, and livestock. MXC inhibits growth and induces atresia (death) of mouse ovarian antral follicles in vitro. Since several studies indicate that many chemicals act through the aryl hydrocarbon receptor (AHR) pathway, the current study tested the hypothesis that MXC binds to the AHR to inhibit growth and induce atresia of antral follicles. The data indicate that MXC binds to AHR. Further, a relatively high dose of MXC (100μg/ml) inhibits growth and induces atresia in both wild-type (WT) and AHR null (AHRKO) follicles, whereas a lower dose of MXC (10μg/ml) inhibits growth and induces atresia in WT, but not in AHRKO follicles. These data indicate that AHR deletion partially protects antral follicles from MXC induced slow growth and atresia. Collectively, these data show that MXC may act through the AHR pathway to inhibit follicle growth and induce atresia in antral follicles of the ovary.

Flavin-containing monooxygenase-3: induction by 3-methylcholanthrene and complex regulation by xenobiotic chemicals in hepatoma cells and mouse liver

Flavin-containing monooxygenases often are thought not to be inducible but we recently demonstrated aryl hydrocarbon receptor (AHR)-dependent induction of FMO mRNAs in mouse liver by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (Celius et al., Drug Metab Dispos 36:2499, 2008). We now evaluated FMO induction by other AHR ligands and xenobiotic chemicals in vivo and in mouse Hepa1c1c7 hepatoma cells (Hepa-1). In mouse liver, 3-methylcholanthrene (3MC) induced FMO3 mRNA 8-fold. In Hepa-1 cells, 3MC and benzo[a]pyrene (BaP) induced FMO3 mRNA >30-fold. Induction by 3MC and BaP was AHR dependent but, surprisingly, the potent AHR agonist, TCDD, did not induce FMO3 mRNA in Hepa-1 cells nor did chromatin immunoprecipitation assays detect recruitment of AHR or ARNT to Fmo3 regulatory elements after exposure to 3MC in liver or in Hepa-1 cells. However, in Hepa-1, 3MC and BaP (but not TCDD) caused recruitment of p53 protein to a p53 response element in the 5'-flanking region of the Fmo3 gene. We tested the possibility that FMO3 induction in Hepa-1 cells might be mediated by Nrf2/anti-oxidant response pathways, but agents known to activate Nrf2 or to induce oxidative stress did not affect FMO3 mRNA levels. The protein synthesis inhibitor, cycloheximide (which causes "superinduction" of CYP1A1 mRNA in TCDD-treated cells), by itself caused dramatic upregulation (>300-fold) of FMO3 mRNA in Hepa-1 suggesting that cycloheximide prevents synthesis of a labile protein that suppresses FMO3 expression. Although FMO3 mRNA is highly induced by 3MC or TCDD in mouse liver and in Hepa-1 cells, FMO protein levels and FMO catalytic function showed only modest elevation.

Introducing the "TCDD-inducible AhR-Nrf2 gene battery"

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induces genes via the transcription factor aryl hydrocarbon receptor (AhR), including Cyp1a1, NAD(P)H:quinone oxidoreductase 1 (Nqo1), UDP-glucuronosyltransferase 1a6 (Ugt1a6), and glutathione S-transferase a1 (Gsta1). These genes are referred to as the "AhR gene battery." However, Nqo1 is also considered a prototypical target gene of the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2). In mice, TCDD induction of Nrf2 and Nrf2 target, Nqo1, is dependent on AhR, and thus TCDD induction of drug-processing genes may be routed through an AhR-Nrf2 sequence. There has been speculation that Nrf2 may be involved in the TCDD induction of drug-processing genes; however, the data are not definitive. Therefore, to address whether TCDD induction of Nqo1, Ugts, and Gsts is dependent on Nrf2, we conducted the definitive experiment by administering TCDD (50 mug/kg, ip) to Nrf2-null and wild-type (WT) mice and collecting livers 24 h later to quantify the mRNA of drug-processing genes. TCDD induction of Cyp1a1 and Ugt1a1 was similar in WT and Nrf2-null mice, whereas TCDD induction of Ugt1a5 and 1a9 was blunted in Nrf2-null mice. TCDD induced Nqo1, Ugt1a6, 2b34, 2b35, 2b36, UDP-glucuronic acid-synthesizing gene UDP-glucose dehydrogenase, and Gsta1, m1, m2, m3, m6, p2, t2, and microsomal Gst1 in WT mice but not in Nrf2-null mice. Therefore, the present study demonstrates the novel finding that Nrf2 is required for TCDD induction of classical AhR battery genes Nqo1, Ugt1a6, and Gsta1, as well as most Ugt and Gst isoforms in livers of mice.

Probing enhanced cytochrome P450 2B1/2 activity in rat hepatocyte spheroids through confocal laser scanning microscopy

Cytochrome P450 (CYP450) enzymes are essential for xenobiotic metabolism. Although CYP450s are found in many tissues, CYP2B1/2 are primarily expressed in the rat liver. The constitutive expression in vivo of CYP2B1/2 is low but it is induced in the presence of various drugs such as phenobarbital (PB). In this study, CYP2B1/2 activity in cultured hepatocytes was assessed in situ with the introduction of a fluorogenic substrate, pentoxyresorufin. The product of 7-pentoxyresorufin-O-dealkylation (PROD), which is catalyzed specifically by CYP2B1/2, was detected using confocal laser scanning microscopy (CLSM). Primary hepatocytes cultured as monolayers on collagen-coated surfaces exhibited background PROD activity and minimal PB inducibility after 4 days in culture. In contrast, rat hepatocytes organized in compacted aggregates, or spheroids, exhibited higher levels of PROD activity and retained their ability for PB induction. The results from the CLSM analysis were verified by RT-PCR and Western immunoblotting analysis. Furthermore, CLSM in conjunction with image processing techniques and three-dimensional reconstruction revealed the localization of enhanced PROD activity in the center of spheroids. The results support the use of CLSM as a powerful tool for investigating CYP2B1/2 activity in cultured rat hepatocytes.

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.

Transcriptional suppression of estrogen receptor gene expression by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

TCDD, the most potent congener of the polychlorinated dioxins, has been shown to be an antiestrogen. The mechanisms of TCDD-induced antiestrogenicity are still under investigation. In this study, we investigated the effects of TCDD on the expression of the estrogen receptor (ER) gene. We studied the levels of un-spliced ER transcript (hnRNA) as well as the ER mRNA in ovary, uterus and liver of TCDD-treated mice with different genetic backgrounds. To quantitate the ER hnRNA levels, the intron and exon boundary of ER hnRNA was amplified by competitive RT-PCR. The ER mRNA from these mice was quantitated by competitive RT-PCR amplifying exons separated by an intron. ER hnRNA and ER mRNA levels were quantitated 4 days after a single i.p. dose of TCDD (5 microg/kg) in female C57BL/6J (B6) mice, which carry the responsive allele to TCDD. TCDD treatment significantly (p < 0.05) suppressed the levels of ER hnRNA in the ovary (27.4%) and uterus (21.9%). The decreases in ER hnRNA were coordinated with significant (p < 0.01) decreases in ER mRNA in ovary (57.7%) and uterus (37.6%). There was a significant decrease (20.3%, p < 0.05) in liver ER mRNA, however, the changes of ER hnRNA in liver were not significant. The coordinated decreases in ER hnRNA and mRNA in TCDD-treated mice suggest a suppression of transcription of the ER gene. We performed the same study on DBA/2J (D2) mice, which possess the "non-responsive" allele of the aryl hydrocarbon receptor (AhR). These mice demonstrated no significant decrease in either the ER mRNA or hnRNA after TCDD treatment. Overall, these results suggest that TCDD suppresses the gene expression of the ER receptor by decreasing its transcription, and the AhR plays an important role in mediating this response.

Brevetoxin-6 (PbTx-6), a nonaromatic marine neurotoxin, is a ligand of the aryl hydrocarbon receptor

Brevetoxins (PbTx) are a family of marine polyether toxins that exert their toxic action by activating voltage-sensitive sodium channels. Two forms of brevetoxin, PbTx-2 and -3, induce hepatic cytochrome P4501A1, measured as ethoxyresorufin O-deethylase (EROD) activity, in redfish and striped bass. P4501A1 induction is transcriptionally regulated through the binding of a ligand, typically a planar aromatic compound, to the aryl hydrocarbon receptor (AhR) and subsequent complex formation with the dioxin response element (DRE), an upstream regulatory region of the CYP1A1 gene. To determine if PbTx, a nonaromatic compound, induced EROD by this mechanism, two sets of experiments were performed. Initially, saturation binding assays with PbTx-2, -3, and -6 were carried out to determine if PbTx-2, -3, or -6 was an AhR ligand. Results showed that PbTx-6 inhibited specific binding of dioxin to the AhR, whereas PbTx-2 and -3 had no effect. Subsequently, gel retardation assays showed that PbTx-6 caused a concentration-dependent increase in AhR-DRE complex formation. The most abundant and neurotoxic forms of brevetoxin, PbTx-2 and -3, did not appear to be involved in this process. However, PbTx-6, the epoxide which is a likely biotransformation product, is at least one of the forms of PbTx involved in EROD induction.

Role of the aromatic hydrocarbon receptor in the suppression of cytochrome P-450 2C11 by polycyclic aromatic hydrocarbons

The aromatic hydrocarbon (AH) receptor mediates the induction of cytochromes P-450 (CYP) of the CYP1A subfamily caused by polycyclic aromatic hydrocarbons (PAHs). CYP1A induction by PAHs is accompanied by down-regulation of CYP2C11, the predominant CYP expressed constitutively in the liver of male rats. We performed a structure-activity relationship study with a series of PAHs of the anthracene class in order to determine if the AH receptor is involved in CYP2C11 down-regulation. Anthracene, benz[a]anthracene, dibenz[a,c]anthracene, dibenz[a,h]anthracene, 7,12-dimethylbenz[a]anthracene, as well as 2,3,7,8-tetrachlorodibenzo-p-dioxin and 3-methylcholanthrene decreased CYP2C11 immunoreactive protein levels to varying degrees in primary rat hepatocytes cultured on a laminin-rich extracellular matrix. The binding affinity of the PAHs for the rat liver cytosolic AH receptor correlated with the potency for transforming the cytosolic AH receptor to its DNA-binding form. In addition, the ability of the PAHs to suppress CYP2C11 correlated with both the AH receptor binding affinity and the AH receptor transformation potency. These results suggest that the AH receptor plays a role in the down-regulation of CYP2C11 caused by PAHs.

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.

A comparison of the mouse versus human aryl hydrocarbon (Ah) receptor complex: effects of proteolysis

The differences in the molecular properties of the nuclear aryl hydrocarbon (Ah) receptor from human Hep G2 and mouse Hepa 1c1c7 cells were investigated by time-dependent partial proteolysis with chymotrypsin or trypsin followed by column chromatographic and velocity sedimentation analysis. The sedimentation coefficients, Stokes radii and apparent molecular weights of the untreated human and mouse Ah receptor complexes were similar. Treatment of the nuclear Ah receptor complexes from both cell lines with chymotrypsin for 10 or 60 min gave lower molecular weight proteolytic products which also exhibited comparable molecular properties and salt gradient elution profiles from Sepharose columns linked to DNA. Treatment of the human and mouse nuclear Ah receptor complexes with trypsin (5 micrograms/mg protein) for 10 or 60 min gave a minor low molecular weight (29.7- or 25.7-kDa) proteolysis product which was detected only with the mouse Hepa 1c1c7 Ah receptor complex. The time- and concentration-dependent proteolytic digest maps of the human and mouse Ah receptor were determined using receptor preparations which were photoaffinity labeled with [125I]7-iodo-2, 3-dibromodibenzo-p-dioxin. The human Ah receptor was significantly more resistant to proteolysis by trypsin or chymotrypsin than the mouse Ah receptor. At a low concentration of chymotrypsin (1 microgram/mg protein) the Hepa 1c1c7 receptor was degraded to two lower molecular weight fragments with apparent M(r) values at 71- and 48-kDa whereas the Hep G2 Ah receptor was relatively stable under these conditions. Although the human Ah receptor was more slowly hydrolyzed than the mouse receptor by trypsin, the major photolabeled breakdown products for the Ah receptor from both cell lines were observed at M(r) 48- and 45-kDa. The results of this study demonstrate that there were subtle but significant differences in the human and mouse Ah receptor complex; however, the proteolysis studies suggest that there are common structural features in their ligand binding sites.

Characterization of the 4 S polycyclic aromatic hydrocarbon-binding protein in human liver and cells

The 4 S polycyclic aromatic hydrocarbon (PAH)-binding protein (PBP) is a soluble protein that binds PAHs with high affinity in mouse, rat, and rabbit. Until now, this protein had not been detected in human placenta or human cells in culture by cytosol labeling and gradient centrifugation assay. Thanks to a preliminary fractionation of cytosol by sedimentation on sucrose gradient or/and gel permeation chromatography, we found that PBP was present in liver, MCF-7 cell line, and hepatocytes of human. To accurately quantitate PBP binding and determine specific binding parameters, a reduction in the amount of charcoal used to adsorb nonspecifically bound benzo[a]pyrene was required. By saturation analysis, the concentration of specific binding sites for [3H]BP in PBP fraction from human liver was 4.6 pmol/mg of protein compared with 14.7 +/- 1.4 pmol/mg in the same fraction from DBA/2J mouse liver. Kinetic studies analyzed by Scatchard and Woolf plots indicate that human liver and MCF-7 cells contain a low-affinity PBP form: the Kd derived from Woolf plot analysis were 14.2 +/- 1.4 and 26.2 +/- 1.8 nM, respectively. DBA/2J mouse possesses a higher-affinity PBP form, the same analysis indicating a Kd of 6.1 +/- 0.3 nM. These data demonstrate that, by comparison to the mouse liver, a lower-affinity form of PBP is present in reduced concentration in human liver, explaining the impossibility of detecting this protein by sedimentation of human cytosol in sucrose gradient.

Effects of ligand structure on the in vitro transformation of the rat cytosolic aryl hydrocarbon receptor

Incubation of radiolabeled, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF),1,2,3,7,8-pentachlorodibenzo-p-dioxin(PeCDD), 1,2,3,7,8-pentachlorodibenzofuran (PeCDF), 1,2,7,8-TCDF, and 2,3,7-trichlorodibenzo-p-dioxin (TrCDD) with rat hepatic cytosol for 2 h at 0 degrees C gave liganded aryl hydrocarbon (Ah) receptor complexes which were indistinguishable as determined by velocity sedimentation analysis and DNA-Sepharose column chromatography. Incubation of the cytosol plus the different radioligands for 2 h at 20 degrees C resulted in the formation of Ah receptor complexes which exhibited increased retention times on DNA-Sepharose columns. It was apparent that the amount of specifically bound Ah receptor complex or the levels of the transformed Ah receptor complex which eluted from the column with 0.2-0.3 M salt were dependent on the structure of the radioligand. For example, after incubation for 2 h at 20 degrees C the overall yields of the specifically bound transformed Ah receptor complex were 3.4, 2.0, 1.2, 1.9, 0.3, and 0.1%, respectively, using 2,3,7,8-TCDD, 2,3,7,8-TCDF, 1,2,3,7,8-PeCDD, 1,2,3,7,8-PeCDF, 1,2,7,8-TCDF, and 2,3,7,8-TrCDD as radioligands. A more quantitative measure of the structure-dependent transformation of the liganded cytosolic Ah receptor complex was determined using a gel retardation assay with a consensus synthetic dioxin-responsive element (DRE) (26-mer, duplex). The EC50 values obtained for the concentration-dependent formation of the retarded DRE-Ah receptor complex using 2,3,7,8-TCDD, 1,2,3,7,8-PeCDD, 2,3,7,8-TCDF, 1,2,3,7,8-PeCDF, 2,3,7-TrCDD, and 1,2,7,8-TCDF as ligands were 0.26, 0.35, 0.78, 1.75, 27.0, and 220 nM, respectively. The structure-dependent differences in these values were similar to their different potencies as Ah receptor agonists and these data suggest that the structure-dependent transformation of the liganded cytosolic Ah receptor may significantly contribute to the structure-activity relationships observed for 2,3,7,8-TCDD and related compounds.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related compounds as antioestrogens: characterization and mechanism of action

In the female Sprague-Dawley rat uterus 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds exhibited a broad spectrum of antioestrogenic responses. For example 2,3,7,8-TCDD inhibited the 17 beta-oestradiol-induced uterine wet weight increase, peroxidase activity, oestrogen and progesterone receptor levels, epidermal growth factor (EGF) receptor binding, and EGF receptor and c-fos protooncogene mRNA levels. The aryl hydrocarbon (Ah) receptor was identified in the rat uterus and the antioestrogenic activities of TCDD and related compounds were structure-dependent. In parallel studies, the effects of TCDD as an antioestrogen in MCF-7 human breast cancer cells was also investigated. TCDD inhibited the 17 beta-oestradiol-induced proliferation of these cells and the secretion of the 34-, 52- and 160-kDa proteins. Treatment of MCF-7 cells with 1 nM [3H]-17 beta-oestradiol resulted in a rapid accumulation of nuclear oestrogen receptor (ER) complexes. Pretreatment of the cells with TCDD caused a rapid decrease in nuclear ER binding activity and immunoreactive protein; moreover, the structure-dependent potencies of TCDD and related compounds as antioestrogens were similar to their Ah receptor binding affinities. TCDD also caused a decrease in nuclear ER levels in wild-type Ah-responsive Hepa 1c1c7 cells but was inactive in Ah non-responsive mutant Hepa 1c1c7 cells. Moreover, in the wild-type cells, both actinomycin D and cycloheximide blocked the effects of TCDD. 6-Methyl-1,3,8-trichlorodibenzofuran (MCDF) has previously been characterized as a TCDD antagonist in rodents and in transformed rodent cell lines. However, like TCDD, MCDF also exhibited a broad spectrum of antioestrogenic activities in both the female Sprague-Dawley rat uterus and MCF-7 cells. MCDF is relatively non-toxic compared to TCDD and is being investigated as a compound which may be clinically useful for the treatment of mammary cancer.

In situ and in vitro photoaffinity labeling of the nuclear aryl hydrocarbon receptor from transformed rodent and human cell lines

The photoaffinity labeling of the nuclear aryl hydrocarbon (Ah) receptor from mouse Hepa 1c1c7, rat hepatoma H-4-II E, and human liver Hep G2 cells was investigated using two high affinity ligands, namely 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin (TCDD) and 7-[125I]iodo-2,3,-dibenzo-p-dioxin ([125I]DBDD). Irradiation of nuclear [3H]TCDD-Ah receptor complexes from the three cell lines for 5 min gave 47, 38, and 62% yields of trichloroacetic acid-precipitable photoadducts from the Hepa 1c1c7, H-4-II E, and Hep G2 cell lines, respectively; denaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis separation followed by autoradiography gave one major Ah receptor photoadduct for each cell line with apparent molecular masses at 97, 100, and 110 kDa, respectively. [125I]DBDD could also be used as a photoaffinity label for the nuclear Ah receptor from the three cell lines; although the maximum net yield of photoaffinity labeled nuclear Ah receptor from the rodent nuclear Ah receptor preparations was relatively low (0.5-2.5%), a greater than 15% yield of photoadduct was obtained from the human Hep G2 cells. Both [3H]TCDD and [125I]DBDD were utilized to photoaffinity label the nuclear Ah receptor in Hepa 1c1c7 cells in suspension and the net yield of photoadducts with these ligands was 94.6 and 3.0%, respectively. The cytosolic Ah receptor from the three cell lines was photolabeled with [125I]DBDD and the net yield of photoadducts varied from 3.3 to 14.7%. The functional activity of the photoaffinity-labeled nuclear TCDD-Ah receptor complexes from the cell lines was also determined by comparing relative binding affinities of the photolyzed and unphotolyzed complexes with a synthetic dioxin-responsive element (DRE) using a gel retardation assay. The photolyzed and unphotolyzed complexes from the three cell lines all bound with the DRE in the gel shift assay; however, the gel mobilities of the rodent and human nuclear receptor-DRE complexes were different. Quantitative analysis of the DRE binding showed that there were no significant differences between the photolyzed and unphotolyzed nuclear receptor complexes from the rodent cells, whereas there was a significant 27% decrease in the DRE binding of the photolyzed versus the unphotolyzed nuclear receptor complex from the human Hep G2 cells. These studies demonstrate the utility of [3H]TCDD and [125I]DBDD as photoaffinity labels for the Ah receptor and illustrate the structural and photochemical differences between the rodent and the human nuclear Ah receptor complexes.

Presence of the 4 S polycyclic hydrocarbon-binding protein in H4-II-E cells

The 4 S polycyclic aromatic hydrocarbon (PAH)-binding protein had been implicated in regulating the expression of rat cytochrome P450IA1 which is most closely associated with aryl hydrocarbon hydroxylase (AHH). We have now investigated the presence of both the 4 S PAH-binding protein and the 8 S Ah receptor in rat hepatoma H4-II-E cells as well as the induction of P450IA1 upon their exposure to PAH's such as benzo[a]pyrene (BP) and 3-methylcholanthrene (3MC), and halogenated dioxins such as 2,3,7,8-tetrachlordibenzo-p-dioxin (TCDD) and 2,3,7,8-tetrachlorodibenzofuran (TCDBF). Sucrose density gradient analyses and hydroxylapatite assays indicate that, in addition to the 8 S protein, the 4 S PAH binding protein is present in these cells. This protein interacts in a saturable and high affinity manner with BP and 3MC, but not with TCDD or TCDBF. Using a P450IA1 probe, the induction of gene expression was observed by Northern blot analysis of total cellular RNA after exposure of the H4-II-E cells to BP, 3MC, or TCDBF. Since the 4 S protein was observed to interact only with BP and 3MC, these results suggest that this protein may also play a role in the PAH-induced expression of cytochrome P450IA1 gene expression in H4-II-E.

Nuclear Ah receptor from mouse hepatoma cells: effect of partial proteolysis on relative molecular mass and DNA-binding properties

The nuclear Ah receptor from mouse hepatoma (Hepa-1c1c9) cells is a 176-kDa multimeric protein which is stable under conditions of up to 1 M KCl. Under denaturing conditions, the Hepa-1 nuclear receptor can be dissociated into a ligand-binding subunit of Mr approximately 91,000. The identity of subunits that compose the nuclear Ah receptor is currently unknown. We used partial proteolysis under nondenaturing conditions as an approach to study the domain organization of the nuclear form of Ah receptor from Hepa-1c1c9 cells treated with [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in culture. Low concentrations of trypsin (0.5 microgram/mg nuclear protein) generated heterogeneous fragments with the main fragment having a Stokes radius (Rs) approximately 6 nm. More discrete ligand-binding fragments of Mr approximately 84,000 (Rs approximately 4 nm/approximately 5 S) and Mr approximately 16,000 (Rs approximately 2 nm/approximately 2 S) could be generated using higher concentrations of trypsin (5 micrograms/mg nuclear protein). The relative concentration of the 84 and 16-kDa fragment was dependent on duration of protease treatment; formation of the 16-kDa fragment was accompanied by some loss in [3H]TCDD binding. Treatment of nuclear Ah receptor with alpha-chymotrypsin (1 microgram/mg nuclear protein) generated a single, apparently homogeneous ligand-binding fragment of Mr approximately 101,000 (Rs approximately 5 nm/approximately 5 S). When analyzed by DNA-cellulose chromatography, the chymotryptic fragment eluted at a significantly higher KCl concentration (462 mM) compared to native untreated nuclear Ah receptor (385 mM). Despite this increased affinity for DNA-cellulose columns, the ligand-binding fragment generated by chymotrypsin treatment was unable to interact with a dioxin responsive element in a gel retardation assay. DNA-cellulose binding ability, therefore, does not appear to be a reliable indicator of specific DNA interactions for these protease-modified fragments.

Equivalent molecular mass of cytosolic and nuclear forms of Ah receptor from Hepa-1 cells determined by photoaffinity labeling with 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin

The structure of the Ah receptor previously has been extensively characterized by reversible binding of the high affinity ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin. We report the use of [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin as a photoaffinity ligand for Ah receptor from the mouse hepatoma cell line Hepa-1c1c9. Both cytosolic and nuclear forms of Ah receptor could be specifically photoaffinity-labeled, which allowed determination of molecular mass for the two forms under denaturing conditions. After analysis by fluorography of polyacrylamide gels run in the presence of sodium dodecyl sulfate, molecular mass for the cytosolic form of Ah receptor was estimated at 92,000 +/- 4,300 and that for the nuclear form was estimated at 93,500 +/- 3,400. Receptor in mixture of cytosol and nuclear extract (each labeled separately with [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin) migrated as a single band. These results are consistent with the presence of a common ligand-binding subunit of identical molecular mass in both cytosolic and nuclear complexes.

Competitive binding of 7-substituted-2,3-dichlorodibenzo-p-dioxins with human placental ah receptor--a QSAR analysis

The competitive binding affinities of thirteen 7-substituted-2,3-dichlorodibenzo-p-dioxins to the human placental cytosolic aryl hydrocarbon (Ah) receptor were determined using [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin as the radioligand. Multiple parameter linear regression analysis of the competitive binding C50 values for these compounds gave the following equation: pEC50 (M) = 6.246 + 1.632 pi - 1.764 sigma 0m + 1.282 HB where pi, sigma m and HB are the physiochemical parameters for substituent lipophilicity, meta-directing electronegativity, and hydrogen bonding capacity respectively. The 7-t-butyl- and 7-phenyl-2,3-dichlorodibenzo-p-dioxins were treated as outliers for the derivation of this equation, and these results suggest that only substituents with van der Waals' volumes less than 40 cm3/mol were accommodated in the receptor binding site. The equations previously derived from the binding of the 7-substituted-2,3-dichlorodibenzo-p-dioxins to the rat, mouse, guinea pig, and hamster hepatic cytosolic receptor were different than the correlation obtained using human placental receptor and provide further evidence for the interspecies differences in the molecular and binding properties of the Ah receptor protein.

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.

Characterization of the Ah receptor mediating aryl hydrocarbon hydroxylase induction in the human liver cell line Hep G2

The Ah receptor, a soluble cytoplasmic receptor that regulates induction of cytochrome P450IA1 and mediates toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), was detected and characterized in the continuous human liver cell line Hep G2. The mean concentration of specific binding sites for TCDD was 112 +/- 26 (SEM) fmol/mg cytosol protein as determined in eight separate cytosol preparations in the presence of sodium molybdate. This is equivalent to 14,000 binding sites per cell, approximately 40% of the sites per cell found in the mouse hepatoma line Hepa-1. The cytosolic Ah receptor from Hep G2 cells sedimented at 9 S and was specific for those halogenated and nonhalogenated aromatic compounds known to be agonists for the Ah receptor in rodent tissues and cells. Specific binding in the 9 S region was detected with both [3H]TCDD and 3-[3H]methylcholanthrene. 3-[3H]Methylcholanthrene did not bind to any component besides that at approximately 9 S. Phenobarbital, dexamethasone, and estradiol did not compete with [3H]TCDD for binding to the Hep G2 Ah receptor. Specific binding of [3H]triamcinolone acetonide to glucocorticoid receptor could also be demonstrated in Hep G2 cytosol. The apparent equilibrium dissociation constant (Kd) for binding of [3H]TCDD to Hep G2 Ah receptor was 9 nM by Woolf plot analysis, about an order of magnitude weaker than the affinity of [3H]TCDD for the mouse Hepa-1 Ah receptor or for the C57BL/6 murine hepatic Ah receptor. [3H]TCDD.Ah receptor complex, which was extracted from nuclei of Hep G2 cells incubated with [3H]TCDD at 37 degrees C in culture, sedimented at approximately 6 S under conditions of high ionic strength. Aryl hydrocarbon hydroxylase (AHH) activity was significantly induced after 24 h of incubation with polycyclic aromatic hydrocarbons: the EC50 for AHH induction was 5.3 microM for benz(a)anthracene and 1.3 microM for 3-methylcholanthrene. Modification of the preparative technique for cell cytosol, especially inclusion of 20 mM sodium molybdate in homogenizing and other buffers, was necessary to detect cytosolic Hep G2 Ah receptor. Hep G2 cells appear to conserve drug-metabolizing activity associated with cytochrome P450IA1 as well as the receptor mechanism which regulates its induction.

Polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and related compounds: environmental and mechanistic considerations which support the development of toxic equivalency factors (TEFs)

Halogenated aromatic compounds, typified by the polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), biphenyls (PCBs), and diphenylethers (PCDEs), are industrial compounds or byproducts which have been widely identified in the environment and in chemical-waste dumpsites. Halogenated aromatics are invariably present in diverse analytes as highly complex mixtures of isomers and congeners and this complicates the hazard and risk assessment of these compounds. Several studies have confirmed the common receptor-mediated mechanism of action of toxic halogenated aromatics and this has resulted in the development of structure-activity relationships for this class of chemicals. The most toxic halogenated aromatic is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and based on in vivo and in vitro studies the relative toxicities of individual halogenated aromatics have been determined relative to TCDD (i.e., toxic equivalents). The derived toxic equivalents can be used for hazard and risk assessment of halogenated aromatic mixtures; moreover, for more complex mixtures containing congeners for which no standards are available (e.g., bromo/chloro mixtures), several in vitro or in vivo assays can be utilized for hazard or risk assessment.

Characterization of the Ah receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin: use of chemical crosslinking and a monoclonal antibody directed against a 59-kDa protein associated with steroid receptors

The Ah receptor regulates induction of cytochrome P450IA1 (aryl hydrocarbon hydroxylase) by "3-methylcholanthrene-type" compounds and mediates the toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin and related halogenated aromatic hydrocarbons. Hepatic Ah receptor from untreated rodents is localized in the cytosol and has an apparent molecular mass of 250 to 300 kDa. This large form can be dissociated into a smaller ligand-binding subunit upon exposure to high ionic strength. The Ah receptor displays many structural similarities to the receptors for steroid hormones. Two non-ligand-binding proteins have been identified to be associated with the cytosolic forms of the steroid hormone receptors. The first is a 90-kDa heat shock protein (hsp 90); the second is a 59-kDa protein (p59) of unknown function. The cytosolic Ah receptor ligand-binding subunit previously has been shown to be associated with hsp 90. In the present study, we used a monoclonal antibody, KN 382/EC1, generated against the 59-kDa protein which is associated with rabbit steroid receptors to determine if p59 also is a component of the large cytosolic Ah receptor complex. Cytosolic forms of rabbit progesterone receptor, glucocorticoid receptor, and Ah receptor were analyzed by velocity sedimentation on sucrose gradients under low-ionic-strength conditions and in the presence of molybdate. Progesterone receptor from rabbit uterine cytosol and glucocorticoid receptor from rabbit liver each had a sedimentation coefficient of approximately 9 S. In the presence of KN 382/EC1 antibody the progesterone receptor and the glucocorticoid receptor both underwent a shift in sedimentation to a value of approximately 11 S. The increase in sedimentation velocity is an indication that the receptor-protein complexes are interacting with the antibody. Under low-ionic-strength conditions the Ah receptors from rabbit uterine cytosol and liver cytosol had a sedimentation coefficient of approximately 9 S. However, in contrast to the steroid receptors, the Ah receptor showed no change in its sedimentation properties in either tissue in the presence of KN 382/EC1, indicating that the antibody is not interacting with the Ah receptor. Multimeric Ah receptor complexes that were chemically crosslinked still did not show any interaction with KN 382/EC1. These data indicate that the 59-kDa protein either is not associated with the Ah receptor or is present in an altered form which the antibody cannot recognize.

Induction of drug-metabolizing enzymes in Gunn rat liver. Effect of polycyclic aromatic hydrocarbons on cytochrome P-450 regulation

Response of congenitally jaundiced rats (Gunn rats) to administration of polycyclic aromatic hydrocarbons (PAH) was investigated and compared to that of Wistar rats. Unlike Wistar, Gunn males did not exhibit changes in the overall cytochrome P-450 content of hepatic microsomes. The first step in the induction process (i.e. presence of cytosolic receptors for PAH) was found present and functionally similar (number of sites, Kd) to that of Wistar rats from which the Gunn strain is derived. An increase in monooxygenase activities related to P-450c and P-450d isoenzymes specifically induced by PAH was noticed, whereas no effect could be detected on the glucuronidation rate of either 4-nitrophenol, testosterone or estrone. As determined by immunoquantification after Western blotting, the isoenzymatic profile of P-450 from PAH-treated male Gunn rats showed an increase of P-450c and P-450d accompanied by an equivalent decrease in P-4502c (major male-specific isoenzyme). The balance between increase in P-450c and P-450d and decrease in P-4502c may explain the absence of increase in the total P-450 in PAH-treated male Gunn rats. Such a response was not observed in PAH-treated male Wistar rats or in female rats of both strains. In contrast, the response of male Gunn rats to PB treatment was similar to that observed in Wistar rats, i.e. increase in overall cytochrome P-450 content of hepatic microsomes and of specific isoenzyme P-450b/e. A possible regulation of P-450 isoenzyme synthesis by the intracellular haem pool might be involved.

Binding of chlorinated benzidines to the rat hepatic aromatic hydrocarbon receptor

The binding of benzidine, 3,3'-dichlorobenzidine (3,3'-Cl2BZ), and the asymmetrically-substituted chlorinated benzidines 3,5-dichlorobenzidine (3,5-Cl2BZ) and 3,5,3'-trichlorobenzidine (Cl3BZ) to the rat hepatic cytosolic aromatic hydrocarbon (Ah) receptor was measured, in order to assess the mechanism of P-450I induction by 3,3'-Cl2BZ. Cl3BZ is the most mutagenic benzidine derivative in the Ames assay. Binding affinity to the Ah receptor protein was determined by displacement of labelled 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) from the receptor, measured with the sucrose density gradient centrifugation technique. The rank order of affinities and the apparent inhibitor constants were: Cl3BZ (4 microM) greater than 3,5-Cl2BZ (8.4 microM) greater than 3,3'-Cl2BZ (10 microM). Benzidine did not displace TCDD from the receptor protein. 4-Aminobiphenyl a structural link between the benzidine and biphenyl series competed weakly with TCDD. The 50% inhibition concentration was about 150 microM. The results are consistent with the hypothesis that the induction of P-450 enzymes by 3,3'-Cl2BZ in vivo is mediated by the Ah receptor.

Physicochemical characterization of the nuclear form of Ah receptor from mouse hepatoma cells exposed in culture to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Molecular properties of nuclear aromatic hydrocarbon (Ah) receptor from Hepa-1c1c9 (Hepa-1) cells were assessed by velocity sedimentation on sucrose gradients and by gel permeation chromatography on Sephacryl S-300. Nuclear Ah receptor was obtained by exposing intact cells to [3H]-2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for 1 h at 37 degrees C in culture followed by extraction of receptor from nuclei with buffers containing 0.5 M KCl. The nuclear Ah receptor was compared to the cytosolic Ah receptor from the same cells. Under conditions of low ionic strength, the Ah receptor from Hepa-1 cytosol sedimented as a single 9.4 +/- 0.63 S binding peak that had a Stokes radius of 7.1 +/- 0.12 nm and an apparent relative molecular mass of 271,000 +/- 16,000. After prolonged (24 h) exposure to high ionic strength (0.5 M KCl), cytosol labeled with [3H]TCDD exhibited two specific binding peaks. The large form of cytosolic Ah receptor seen under high ionic strength conditions sedimented at 9.4 +/- 0.46 S, had a Stokes radius of 6.9 +/- 0.19 nm, and an apparent Mr 267,000 +/- 15,000. The smaller ligand-binding subunit generated by exposing cytosol to 0.5 M KCl sedimented at 4.9 +/- 0.62 S, had a Stokes radius of 5.0 +/- 0.14 nm, and an apparent Mr 104,000 +/- 12,000. Nuclear Ah receptor, analyzed under high ionic strength conditions, sedimented at 6.2 +/- 0.20 S, had a Stokes radius of 6.8 +/- 0.19 nm, and an apparent Mr 176,000 +/- 7000. Nuclear Ah receptor from rat H4IIE hepatoma cells was analyzed and found to have physicochemical characteristics identical to those of nuclear Ah receptor from the mouse Hepa-1 cells. The molecular mass of Hepa-1 nuclear Ah receptor was found to be statistically different from both the Mr approximately 267,000 cytosolic Ah receptor and the Mr approximately 104,000 subunit which were present in cytosol under high ionic strength conditions. Hepa-1 nuclear Ah receptor could not be converted to a smaller ligand-binding subunit by treatment with alkaline phosphatase, ribonuclease, or sulfhydryl-modifying reagents or prolonged exposure to 1.0 M KCl. Cytosolic Ah receptor from Hepa-1 cells was "transformed" by heating at 25 degrees C in vitro into a form with high affinity for DNA-cellulose. The transformed cytosolic Ah receptor, when analyzed under conditions of high ionic strength, sedimented at approximately 6 S, had a Stokes radius of approximately 6.7 nm, and an apparent Mr approximately 167,000.(ABSTRACT TRUNCATED AT 400 WORDS)

Radioligand-dependent differences in the molecular properties of the mouse and rat hepatic aryl hydrocarbon receptor complexes

A comparison of the molecular properties of the male Long-Evans rat and male C57BL/6 mouse hepatic cytosolic aryl hydrocarbon (Ah) receptor complex was determined using 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,7,8-[3H]tetrachlorodibenzofuran (TCDF) as radioligands. In low salt buffer, the sedimentation coefficients, Stokes radii, relative molecular masses, frictional ratios, axial ratios and gel permeation chromatographic properties of the rat receptor complexes were ligand independent. In contrast, there were several ligand-dependent differences in the mouse Ah receptor complexes formed after incubation in low salt buffer and these include: sucrose density gradient analysis of the 2,3,7,8-[3H]TCDF receptor complex gave a 9.5 S specifically bound peak and a 2.6 S nonspecifically bound peak whereas the corresponding 2,3,7,8-[3H]TCDD receptor complex gave a single 9.6 S specifically bound peak; sucrose density gradient analysis of the two major peaks eluted from a Sephacryl S-300 column chromatographic separation of the 2,3,7,8-[3H]TCDF receptor complex gave two specifically bound peaks at 9.2 and 5.1 S. The molecular properties of the rat hepatic cytosolic receptor complexes incubated in high salt (0.4 M KCl) buffer were ligand independent with one exception, namely the significant difference in the sedimentation coefficient of the specifically bound disaggregated 2,3,7,8-[3H]TCDD receptor complex (6.8 S) and the corresponding 2,3,7,8-[3H]TCDF receptor complex (5.0 S). The major ligand-dependent differences in the mouse receptor complexes incubated in high salt (0.4 M KCl) were associated with the sedimentation coefficients of the complexes derived after direct incubation and after gel permeation chromatography. For example, both ligands gave two specifically bound complexes after chromatography on Sephacryl S-300 column and centrifugation of these fractions gave both the approximately 9 and approximately 5 S peaks; this suggested that there was some equilibration between the aggregated and disaggregated receptor complexes. The behavior of the 2,3,7,8-[3H]TCDF mouse receptor complex was similar after incubation in low or high salt buffer except that sucrose density gradient analysis of the gel permeation chromatographic fractions gave an additional specifically bound peak which sedimented at 7.2 S. These studies demonstrate that the molecular properties of the Ah receptor were dependent on the source of the cytosolic receptor preparation, the ionic strength of the incubation media, and the structure of the radioligand.

Kinetic models for association of 2,3,7,8-tetrachlorodibenzo-p-dioxin with the Ah receptor

Saturation binding studies of the interaction between 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and the Ah receptor obtained from the hepatic cytosol of Wistar rats have been carried out. The conventional Scatchard analysis for determination of the equilibrium constant for ligand-receptor binding has been shown to be inappropriate due to thermal inactivation of the unoccupied receptor. Simulation models of the receptor-ligand binding kinetics which take into account receptor degradation have been developed and the results are consistent with two alternative kinetic models. In Model 1, reversible 2,3,7,8-TCDD-receptor binding occurs in parallel with inactivation of the unbound receptor; analysis of the observed data using this model suggests that the previously determined equilibrium constants (Kass) for association of the ligand with the receptor are orders of magnitude too low and the total initial receptor concentrations are somewhat underestimated. In Model 2, the unbound receptor is converted unimolecularly to an activated state which then undergoes competitive degradation or entrapment by ligand. Experiments have been carried out over the temperature range 4-37 degrees C, enabling activation parameters to be obtained. According to Scheme 1, the activation enthalpies for association of receptor with ligand and for thermal inactivation of the unoccupied receptor are high, and numerically almost identical (delta H++ ca 125 kJ mol-1). These reactions are strongly entropically driven and this is consistent with association being accompanied by a conformational change in the receptor protein, and the previously postulated binding of the ligand to a hydrophobic pocket. According to Scheme 2, there is only one enthalpy of activation because both inactivation and entrapment by 2,3,7,8-TCDD are fast processes which follow the same slow activation step. On the basis of this latter model, a 10(-9) M concentration of 2,3,7,8-TCDD is sufficient to trap roughly two-thirds of the activated receptors.

The 4S binding protein acts as a trans-regulator of the polycyclic hydrocarbon-inducible cytochrome P450

A model has been proposed for the induction of cytochrome P450c in liver by polycyclic hydrocarbons such as benzo(a)pyrene (BaP) and 3-methylcholanthrene (3MC). The polycyclic hydrocarbon interacts in specific, saturable, and high-affinity fashion with a rat liver cytosolic 4s binding protein. The latter enters the nucleus, complexes to 5' upstream regions of the cytochrome P450c gene, and stimulates the transcription. The 4s binding protein has been purified from rat liver and its substrate specificity has been determined. The affinity for 3MC or BaP is 1-2 mM. The binding protein has been demonstrated to complex with specific 5'-upstream regions of the P450c gene by using a filtration assay as well as exonuclease footprinting. In addition, the binding protein stimulates in vitro transcription with upstream regions of the P450c gene as template; these data confirm the hypotheses.

Binding characteristics of Ah receptors from rats and mice before and after separation from hepatic cytosols. 7-Hydroxyellipticine as a competitive antagonist of cytochrome P-450 induction

The Ah receptor, a soluble protein implicated in the mechanism of action of the toxic halogenated aryl hydrocarbons has been examined in rodent livers. Due to the difficulty of making reliable quantitative determinations on binding parameters for hydrophobic compounds in cytosols that contain several components, Ah receptors from livers of Sprague-Dawley rats and C57BL/6 mice have been separated, in a preparative manner, using sucrose density gradient centrifugation in a vertical rotor. The binding characteristics of Ah receptors, before and after separation, were assessed by competition of various chemicals as 2,3,7,8-tetrachlorodibenzo-p-dioxin, 2,3,7,8-tetrachlorodibenzofuran, 3-methylcholanthrene, benzo[a]pyrene, beta-naphthoflavone and ellipticines with [3H]3-methylcholanthrene and 2,3,7,8-tetrachloro[3H]dibenzo-p-dioxin as radioligands. The rationale of this approach is supported by the results obtained and the major conclusions are as follows. 1. The intrinsic binding characteristics of Ah receptors were dependent on the presence or absence of other cytosolic binding components (light-density component and 4-S carcinogen-binding protein). 2. In contrast with many previous unsuccessful attempts, the separation of the C57BL/6 Ah receptor allowed the unambiguous detection of a 9-S binding peak with [3H]benzo[a]pyrene as a radioligand. 3. The intrinsic binding characteristics of the separated Ah receptors of Sprague-Dawley rats and C57BL/6 mice were similar if not identical. 4. A good correlation exists between the competitive potency (IC50) of chemicals and their ability to induce aryl hydrocarbon hydroxylase activity, except for 7-hydroxyellipticine which binds to the Ah receptors of rat and mouse liver (IC50 approximately 5-10 microM) without inducing aryl hydrocarbon hydroxylase. 5. When coadministered with various inducers, 7-hydroxyellipticine antagonizes (from about 20% to 65%) the inducing ability of chemicals displaying similar (ellipticines) or weaker (chlorpromazine, phenothiazine) binding affinities for the Ah receptor.

Synthesis and aryl hydrocarbon receptor binding properties of radiolabeled polychlorinated dibenzofuran congeners

Microchlorination of 1,4,9[3H]dibenzofuran gave several polychlorinated dibenzofuran (PCDF) products and 2,3,7,8-[3H]tetrachlorodibenzofuran (TCDF), 1,2,3,7,8-[3H]pentachlorodibenzofuran (PeCDF), and 1,2,3,6,7,8-/1,2,3,4,7,8-hexachlorodibenzofuran (HCDF) of high specific activity (57, 34, and 32.5 Ci/mmol, respectively) were purified by preparative high-pressure liquid chromatography. These compounds were investigated as radioligands for the rat liver cytosolic aryl hydrocarbon (Ah) receptor protein. Like 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin (TCDD), the radiolabeled PCDF congeners exhibited saturable binding with the receptor protein and sucrose density gradient analysis of the radiolabeled ligand-receptor complexes gave specific binding peaks with comparable sedimentation profiles. The rank order of radioligand binding affinities (Kd values) was 2,3,7,8-TCDD greater than 2,3,7,8-TCDF greater than 1,2,3,6,7,8-HCDF greater than 1,2,3,7,8-PeCDF and the maximum difference in Kd values for the four radioligands was less than 13-fold (0.44-5.9 nM). The interactions of the PCDF radioligands with the cytosolic receptor all exhibited saturable binding curves and linear Scatchard plots and the slopes of their Hill plots were in the range 1.0-1.1, thus indicating that cooperativity was not a factor in these binding interactions. The relative stabilities and dissociation kinetics of the radioligand-receptor complexes were highly dependent on the structure of the radioligand. The dissociation curves of the 2,3,7,8-[3H]TCDD and PCDF receptor complexes were biphasic and this suggests that there may be a temporal shift in ligand binding affinities. However, the rates of dissociation did not correlate with the rank order of ligand binding affinities. The stabilities of the radioligand-receptor complexes were also dependent on the structures of the radioligands; for example, the 2,3,7,8-[3H]TCDD-receptor complex degraded more rapidly than the PCDF-receptor complex and these relative stabilities were clearly not related to the Kd values or the relative in vivo or in vitro biologic potencies of these halogenated aryl hydrocarbons.

Purification and photoaffinity labelling of a rat cytosolic binding protein specific for 3-methylcholanthrene

Rat hepatic cytosolic proteins which sediment at 4-5 S on sucrose gradients exhibit high-affinity saturable binding for the carcinogen 3-methylcholanthrene. A rat liver protein of Stokes' radius 3 nm, Mr by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis of 39,000 and with specific 3-methylcholanthrene-binding activity sedimenting at 4.5 S, has been purified 315-fold to apparent homogeneity by using affinity chromatography on a column of 1-hydroxy-3-methylcholanthrene coupled to epoxy-activated Sepharose 6B, in conjunction with two gel-filtration steps. The protein purified by this technique was shown to be associated with the observed specific 3-methylcholanthrene-binding activity by photoaffinity labelling with 1-oxo-3-methylcholanthrene.

Binding of polynuclear aromatic hydrocarbons to the rat 4S cytosolic binding protein: structure-activity relationships

The relative competitive binding affinities of benzo[a]pyrene (B[a]P), benzo[e]pyrene, benzo[g, h, i]perylene, picene, 7,12-dimethylbenz [a]anthracene, 1,2,3,4-dibenz[a]anthracene, 1,2,5,6-dibenz[a]anthracene, perylene, 4H-cyclopenta[d,e,f]-phenanthrene, benz[a] anthracene, triphenylethylene and triptycene for the rat hepatic cytosolic 4S binding protein were determined using [3H]benzo[a]pyrene as the radioligand. With the exception of triphenlethylene, triptycene and 4H-cyclopenta[d,e,f]phenanthrene, the EC50 values for the remainder of these compounds were between 1.25 X 10(-7) and 2.5 X 10(-8) M with 1,2,5,6-dibenz[a]anthracene being the most active ligand. A comparison of the relative cytosolic Ah (9S) receptor binding affinities and aryl hydrocarbon hydroxylase (AHH) induction potencies of these hydrocarbons with their 4S protein binding affinities demonstrated the following: five compounds, namely 1,2,5,6-dibenz[a]-anthracene, 1,2,3,4-dibenz[a]anthracene, picene, benzo[a]pyrene and 3-methylcholanthrene exhibited high to moderate binding affinities for the 4S and 9S cytosolic proteins (EC50 values less than 10(-6) M) and induced AHH in rat hepatoma cells; three compounds, namely perylene, benzo[e]pyrene and benzo[g,h,i]perylene exhibited high affinities for the 4S binding protein (1.25 X 10(-7), 4.4 X 10(-8) and 2.9 X 10(-8) M, respectively) and low affinities (EC50 values greater than 10(-5) M) for the Ah receptor protein; moreover these three compounds did not induce AHH in rat hepatoma H-4-II E cells in culture. These data suggest that the 4S binding protein may not play a significant role in AHH induction although the results do not rule out a function for this protein in the transregulation of AHH and its associated cytochromes P-450.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on hepatic and uterine estrogen receptor levels in rats

Administration of a single dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, 20 or 80 micrograms/kg) resulted in significantly decreased hepatic and uterine estrogen receptor (ER) levels in 25-day-old female Long-Evans rats. By contrast, estradiol (5 and 15 micrograms/kg) administration increased hepatic and uterine ER levels, while a combination of 2,3,7,8-TCDD plus estradiol resulted in uterine and hepatic ER levels which were similar or lower than those observed after treatment with only 2,3,7,8-TCDD. In addition, 2,3,7,8-TCDD significantly decreased the effects of estradiol on uterine wet weight increase. Competitive binding studies showed that estradiol did not bind to the aryl hydrocarbon (Ah) receptor and that 2,3,7,8-TCDD did not bind to the ER. The effects of structure on the activity of polychlorinated dibenzo-p-dioxin congeners on their activity to down-regulate hepatic and uterine ER levels were determined by using 2,3,7,8-TCDD, 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 1,3,7,8-TCDD, and 1,2,4,7,8-PeCDD. Both 2,3,7,8-TCDD and 1,2,3,7,8-PeCDD exhibit high affinities for the Ah receptor and at dose levels of 80 micrograms/kg the hepatic ER levels were decreased 42 and 41%, respectively, and uterine ER levels were decreased 53 and 49%, respectively. 1,3,7,8-TCDD and 1,2,4,7,8-PeCDD bind less avidly to the Ah receptor and at dose levels of 400 micrograms/kg these compounds decreased hepatic ER levels 36 and 40%, respectively, and uterine ER levels 21 and 24%, respectively. These results support a role for the Ah receptor in the down-regulation of uterine and hepatic ER levels in the female rat by 2,3,7,8-TCDD and this effect may be associated with the decrease in spontaneous mammary and uterine tumors observed in female rats treated with 2,3,7,8-TCDD.

Structure and function of the Ah receptor: sulfhydryl groups required for binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin to cytosolic receptor from rodent livers

Cytosol from rodent liver was exposed to a variety of sulfhydryl-modifying reagents to determine if the cytosolic Ah receptor contained reactive sulfhydryl groups that were essential for preservation of the receptor's ligand binding function. At a 2 mM concentration in rat liver cytosol, all sulfhydryl-modifying reagents tested (except iodoacetamide) both blocked binding of [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to unoccupied receptor and caused release of [3H]TCDD from receptor sites that had been labeled with [3H]TCDD before exposure to the sulfhydryl-modifying reagent. Exposure of cytosol to iodoacetamide before labeling with [3H]TCDD prevented subsequent specific binding of [3H]TCDD, but iodoacetamide was not effective at displacing previously bound [3H]TCDD from the Ah receptor. The mercurial reagents, mersalyl, mercuric chloride, and p-hydroxymercuribenzoate, were more effective at releasing bound [3H]TCDD from previously labeled sites than were alkylating agents (iodoacetamide, N-ethylmaleimide) or the disulfide compound 5,5'-dithiobis(2-nitrobenzoate). Presence of bound [3H]TCDD substantially protected the Ah receptor against loss of ligand binding function when the cytosol was exposed to sulfhydryl-modifying reagents. This may indicate that the critical sulfhydryl groups lie in or near the ligand binding site on the receptor. Subtle differences exist between the Ah receptor and the receptors for steroid hormones in response to a spectrum of sulfhydryl-modifying reagents, but the Ah receptor clearly contains a sulfhydryl group (or groups) essential for maintaining the receptor in a state in which it can bind ligands specifically and with high affinity.

Species and tissue distribution of the 4S carcinogen binding protein and its possible role in cytochrome P-450 induction

A carcinogen binding protein (CBP) that is implicated in controlling the expression of rat cytochrome P-450c which is closely associated with aryl hydrocarbon hydroxylase (AHH) was examined in hepatic and extrahepatic tissues of the neonatal and adult New Zealand White (NZW) rabbits, the hepatic tissue of the BALB/cJ and DBA/2J mice, Brown Norway rat, Golden Syrian hamster and Hartley guinea pig. These animals and tissues were examined in order to determine whether there was a correlation of CBP levels and the reported presence or absence of inducibility of AHH in these tissues. The CBP was found in hepatic and extrahepatic tissue of the NZW rabbit and the hepatic tissues of all animals except the Hartley guinea pig. The Hartley guinea pig may provide a useful animal with which to further examine the role of CBP in cytochrome induction. Since the CBP is not a tissue specific protein and because it is found in both neonatal and adult NZW rabbit tissue, the data suggests that the CBP is not the limiting factor in the tissue specific induction of cytochromes nor in developmentally controlled induction of cytochromes previously reported in the rabbit.

The cytosolic receptor binding affinities and AHH induction potencies of 29 polynuclear aromatic hydrocarbons

The dose-response rat hepatic cytosolic receptor-binding avidities, aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) induction potencies in rat hepatoma H-4-II E cells in culture were determined for 29 polynuclear aromatic hydrocarbons. It was apparent that the magnitude of the EC50 values for these in vitro responses were strongly dependent on structure. Dibenz[a,h]anthracene (1.6 X 10(-8) M), 7-methylbenz[a]anthracene (1.6 X 10(-8) M), 3-methylcholanthrene (2.8 X 10(-8) M) and picene (4.5 X 10(-8) m) exhibited the highest affinity for the receptor protein and these compounds were only 5-fold less active the 2,3,7,8-tetrachlorodibenzo-p-dioxin (1 X 10(-8) M). All of the compounds which were active in the receptor-binding and monooxygenase enzyme-induction assays possessed one common structural feature, namely the presence of a phenanthrene structure fused with at least 1 benzo ring. The results also demonstrated that there was not any apparent correlation between the receptor-binding avidities and in vitro monooxygenase enzyme-induction potencies for the most active polynuclear aromatic hydrocarbons.

Ah receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin: ontogeny in chick embryo liver

Aryl hydrocarbon hydroxylase (AHH, cytochrome P1-450) is induced in chick liver very early during embryonic development if embryos are treated with 3-methylcholanthrene-type compounds such as 3,4,3'4'-tetrachlorobiphenyl. In mammals, AHH induction is known to be mediated by the Ah receptor. Liver from embryonic and newly hatched chicks was found to contain a cytosolic receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) which has properties that are very similar to properties of the Ah receptor previously characterized in mammalian tissues. In chick embryo liver, cytosolic binding sites for TCDD were of high affinity (Kd for [3-H]-TCDD = 0.2 nM) and were specific for 3-methylcholanthrene-type inducers. The specific binding component sedimented at about 9S on sucrose density gradients prepared at low ionic strength. A high level of Ah receptor was detected in chick embryo liver by the fifth day of incubation (5 DI); this is at least 24 hours prior to the onset of AHH inducibility. The Ah receptor concentration increased from 5 DI to 8 DI, the period when chick liver is undergoing early morphological differentiation. After 8 DI, Ah receptor levels dropped substantially and remained low into the posthatching period. In contrast, AHH inducibility was high by 7 DI and remained high throughout embryonic development and into the posthatching period. The discrepancy between Ah receptor levels and the degree of AHH inducibility suggests that only a small fraction of the Ah receptor population is required for maximal AHH induction.

Polychlorinated dibenzofurans as 2,3,7,8-TCDD antagonists: in vitro inhibition of monooxygenase enzyme induction

2,4,6,8- and 1,3,6,8-tetrachlorodibenzofuran (TCDF) competitively displace [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) from the rat cytosolic receptor protein and their EC50 values were 1.5 X 10(-6) and 1.25 X 10(-7) M, respectively. In contrast to their relatively high binding avidities these TCDF isomers were poor inducers of benzo[a]pyrene hydroxylase and ethoxyresorufin O-deethylase in rat hepatoma H-4-II E cells in culture (EC50 greater than 10(-5) M). Coadministration of different concentrations of 2,4,6,8- and 1,3,6,8-TCDF (10(-5), 10(-6) and 10(-7) M) with 2 X 10(-10) M, 2,3,7,8-TCDD (a dose which elicits 80% of the maximal induction response) resulted in significant decreases in the expected (additive) induction of benzo[a]pyrene hydroxylase and ethoxyresorufin O-deethylase by the mixture. Thus the partial agonists, 1,3,6,8- and 2,4,6,8-TCDF, antagonize the receptor-mediated enzyme induction activity of 2,3,7,8-TCDD presumably via competitive displacement of 2,3,7,8-TCDD from the receptor protein. In contrast, coadministration of 2,3,7,8-TCDF and 2,3,7,8-TCDD gave additive enzyme induction responses. The identification of the 2,3,7,8-TCDD antagonists represents a new class of halogenated aryl hydrocarbons.

Ah receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin. Codistribution of unoccupied receptor with cytosolic marker enzymes during fractionation of mouse liver, rat liver and cultured Hepa-1c1 cells

In early experiments Ah receptor appeared to be localized in cytosol when in its unoccupied state and it was thought that the receptor translocated into nuclei only when occupied by its ligands. However, a recent report [Whitlock and Galeazzi (1984) J. Biol. Chem. 259, 980-985] concluded that unoccupied Ah receptor in the intact cell was primarily located within the nucleus and that apparent 'cytosolic' Ah receptor was a redistribution artifact caused by fractionation of cells in large volumes of buffer. We examined the effect of buffer volume and ionic strength on apparent 'cytosolic' versus 'nuclear' distribution of unoccupied Ah receptor in liver from C57BL/6J mice and Sprague-Dawley rats as well as Hepa-1c1c9 cells in culture. In all three systems the Ah receptor appears to shift out of the nuclear fraction and into the cytosolic fraction as the volume of buffer is increased or when the ionic strength of the buffer is increased. In each system, however, the distribution of the Ah receptor was identical to the distribution of each of three standard cytosolic marker enzymes: aldolase, glucose-6-phosphate dehydrogenase and lactate dehydrogenase. Co-distribution of unoccupied Ah receptor with these cytosolic marker enzymes during fractionation at varied buffer volumes and ionic strengths makes it seem unlikely that the unoccupied receptor is predominantly a 'nuclear' component in intact cells. Marker enzyme data favor an interpretation that unoccupied Ah receptor is primarily cytoplasmic or that this soluble protein is in equilibrium between cytoplasm and nucleus.