Purification and N-terminal amino acid sequence of the Ah receptor from the C57BL/6J mouse

The Ah receptor is a presumed member of the superfamily of steroid/thyroid hormone receptors, a trace soluble protein present in a wide variety of vertebrate species that mediates the biological effects of halogenated aromatic hydrocarbons. In this paper, we report the purification to homogeneity of this protein (from the liver of C57BL/6J mice) and its N-terminal amino acid sequence. Selective covalent labeling of the Ah receptor in hepatic cytosol with the photoaffinity ligands 2-azido-3-[125I]iodo-7,8-dibromodibenzo-p-dioxin simplified identification and quantitation of the receptor and permitted purification under denaturing conditions. Photoaffinity-labeled hepatic cytosol was applied to a phosphocellulose column at 80 mM NaCl, and the fraction enriched with the Ah receptor eluted with 225 mM NaCl. The eluate was diluted to 150 mM NaCl and applied to a DEAE-cellulose column, and the enriched fraction eluted with 300 mM. These two ion exchange chromatography steps usually gave approximately 100-fold enrichment and 40-50% recovery of Ah receptor. The dilute protein in the eluate was precipitated with n-propanol/trichloroacetic acid and solubilized in formic acid. The sample was then subjected to three successive rounds of high performance liquid chromatography on C4 reverse phase columns. The final, shallow-gradient chromatography was able to resolve the unlabeled 95-kDa receptor protein from the later eluting 125I-photoaffinity-labeled protein. The pooled high performance liquid chromatography fractions subjected to electrophoresis on sodium dodecyl sulfate-polyacrylamide gels contained only the 95-kDa band upon staining with Coomassie blue R250 or silver. Using the above protocol, the Ah receptor was purified greater than 150,000-fold, to apparent homogeneity, with an overall yield of 3-5%. The N-terminal amino acid sequence of the purified peptide was determined to be ala/asp-ser-Arg-Lys-arg-Lys-Pro-Val-Gln-Lys-Thr-Val-Lys-Pro-Ile-Pro-Ala- Glu-Gly--Ile-Lys-ser-Asn-Pro-ser-Lys- (where the lowercase indicates a residue determined with less confidence).

Characterization of polyclonal antibodies to the Ah receptor prepared by immunization with a synthetic peptide hapten

A synthetic peptide based on the N-terminal amino acid sequence of the Ah receptor purified from C57BL/6J mice, linked to keyhole limpet hemocyanin, proved a remarkably good immunogen. All six rabbits that were immunized produced polyclonal antiserum that reacted with the synthetic peptide and the denatured and undenatured Ah receptor. Western blots were especially useful for antibody characterization; hepatic cytosol from C57BL/6J mice, in which the Ah receptor was photoaffinity labeled with 2-azido-3-[125I]iodo-7,8-dibromodibenzo-p-dioxin, was resolved by gel electrophoresis and electrotransferred to nitrocellulose. Co-incidence of the major band immunochemically stained with immunoaffinity-purified antibodies (with apparent Mr = 95,000) and the radiolabeled band on the autoradiograph indicated the specificity of the antibody. The estimated sensitivity of detection of the Ah receptor on a blot is 60 to 120 pg/200 micrograms of protein/gel lane. On Western blots, the antipeptide antibodies stained the photoaffinity-labeled Ah receptor from all four murine variants and all vertebrate forms examined (chicken, rodents, monkey, human), indicating conservation of these N-terminal epitopes. The immunoaffinity-purified antibodies also immunoprecipitated undenatured photoaffinity-labeled Ah receptor from diluted cytosol.

Characterization and strain distribution pattern of the murine Ah receptor specified by the Ahd and Ahb-3 alleles

Two allelic forms of the Ah receptor have been previously identified by covalent labeling of the hepatic cytosol fractions of inbred strains of mice with the photoaffinity ligand 2-azido-3-[125I] iodo-7,8-dibromodibenzo-p-dioxin and resolution of the labeled protein by denaturing gel electrophoresis: 1) a Mr 95,000 protein encoded by the Ahb-1 allele carried by the C57 and C58 family of mice, and 2) a Mr 104,000 protein encoded by the Ahb-2 allele present in other common inbred strains that are responsive to aromatic hydrocarbons (e.g., C3H/He, BALB/cBy, and A). In this report, 125I-photoaffinity labeling is used to characterize two further murine variants and the strains that carry them: 1) the low affinity Ah receptor (Ahd allele) in strains that are nonresponsive to aromatic hydrocarbons and 2) a newly identified, high affinity variant (Ahb-3) found in several strains recently derived from feral mice. The low affinity Ah receptor has been recently characterized by reversible ligand binding by Okey et al. [Mol. Pharmacol. 35:823-830 (1989)], through the inclusion of sodium molybdate in the buffers during tissue preparation and ligand incubation to stabilize the receptor. Examination of the Ah receptor in hepatic cytosol from 18 strains of mice carrying the Ahd allele, by preparation in molybdate and photoaffinity labeling, revealed that all strains express a Mr 104,000 protein. Tissue preparation in 20 mM sodium molybdate and subsequent dilution of the molybdate to approximately 0.5 mM during ligand incubation was found to enhance photoaffinity labeling of the high and low affinity allelic forms of the Ah receptor. A new variant of the receptor (Ahb-3) expressing a Mr 105,000 protein was detected in Mus molossinus, hortulanus, pahari, spretus, and caroli but was absent from the strains of Mus musculus or domesticus that were examined. Allelic variants were also distinguishable by thermolability, i.e., the half-life of specific ligand binding capacity upon incubation at 35 degrees. For the Ahb-1 allele (Mr 95,000) the t 1/2 (thermostability) is 20-30 min, for the Ahb-2 allele (Mr 104,000) the t 1/2 is 3-6 min, and for the Ahb-3 allele (Mr 105,000) the thermolability is intermediate.

Pharmacokinetics of [125I]-2-iodo-3,7,8-trichlorodibenzo-p-dioxin in mice: analysis with a physiological modeling approach

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent inducer of hepatic microsomal enzymes. The influence of an inducing dose of TCDD on tissue distribution and other pharmacokinetic behavior of a TCDD analog in the mice was examined by employing a high specific activity radioligand. [125I]-2-iodo-3,7,8-trichlorodibenzo-p-dioxin (ITCDD). Female C57BL/6J mice were pretreated with 0.1 mumol/kg of TCDD or the vehicle only, followed by 0.1 nmol ITCDD/kg 3 days later. The control animals had the highest concentration of ITCDD-derived radioactivity in the fat, but the TCDD-pretreated animals had the highest concentration in their livers. Whole-body elimination of ITCDD approximated first-order behavior, and induction by pretreatment with the inducing dose of TCDD almost doubled the rate of excretion (control mice, t1/2 = 14.2 days; pretreated mice, t1/2 = 8.0 days). All disposition results in naive and pretreated mice were satisfactorily described by a consistent physiologically based pharmacokinetic model (Leung et al., 1988a) in which induction increased the amount of microsomal ITCDD-binding protein from 1.75 to 20 nmol/liver and increased the rate constant for metabolism of free ITCDD from 1 to 3/hr/kg liver. The binding affinity of the microsomal ITCDD-binding protein was the same (20 nM) in both induced and noninduced mice. Model simulations indicated a time delay in the elimination of nonparent ITCDD metabolites from the body and a more rapid absorption of the parent ligand in the pretreated mice. Consistent with previous physiological modeling with TCDD in different mouse strains, the primary factor influencing the liver/fat concentration ratio appears to be the affinity and capacity of the microsomal TCDD-binding proteins, which are altered by induction. These dose-dependent pharmacokinetic differences with ITCDD are important considerations for TCDD risk assessment in which data from high dose rodent experiments are extrapolated to predict behavior at much lower environmental concentrations in exposed humans.

Stimulation of in vivo hepatic uptake and in vitro hepatic binding of [125I]2-lodo-3,7,8-trichlorodibenzo-p-dioxin by the administration of agonist for the Ah receptor

[125I]2-lodo-3,7,8-trichlorodibenzo-p-dioxin ([125I]Cl3DpD), a radiolabeled, isosteric, analogue of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), was synthesized and used to study in vivo tissue localization and in vivo tissue binding. Twenty-four hours after the administration of a tracer dose (1 X 10(-10) mol/kg) of [125I] Cl3DpD to C57BL/6J mice, the hepatic concentration of radioactivity was 1-2% of the administered dose, whereas in mice pretreated with TCDD (1 X 10(-7) mol/kg), the hepatic accumulation of radiolabel was 25-30% of that administered. Liver homogenate from TCDD-treated mice bound 4 to 10 times more [125I]Cl3DpD than homogenate from control mice. The enhancement of in vivo uptake and in vitro tissue binding of [125I]Cl3DpD by TCDD administration was confined to liver and was not observed in other tissues examined, kidney, lung, spleen, small intestines, and muscle. The administration of TCDD to C57BL/6J mice produces dose-related stimulation of in vivo hepatic uptake of [125I]Cl3DpD, binding of radioligand to liver homogenate, and hepatic aryl hydrocarbon hydroxylase activity, with the dose for half-maximal stimulation, ED50, varying from 1.5 to 4.0 x 10(-9) mol/kg. In congenic C57BL/6J (Ahd/Ahd) mice, which express the lower affinity Ah receptor, the ED50 values for all three responses were shifted to approximately 10-fold higher doses. 3,3',4,4',5,5'-Hexabromobiphenyl, a weak agonist for the Ah receptor produced a dose-related stimulation of these three responses in C57BL/6J mice (ED50 values of approximately 5 X 10(-7) mol/kg), but was without effect in C57BL/6J (Ahd/Ahd) mice. Stimulation of vivo hepatic uptake and in vitro liver homogenate binding of [125I]Cl3DpD was produced by administration of Ah agonists, such as 2,3,7,8-tetrachlorodibenzofuran and beta-naphthoflavone, but inactive congeners and other compounds that do not act via the Ah receptor, e.g. phenobarbital and pregnenolone-16 alpha-carbonitrile, did not evoke these effects. Thus, TCDD and other Ah agonists act through the Ah receptor to increase a liver binding species that increases the hepatic uptake of [125I]Cl3DpD in vivo and binding of this radioligand to liver homogenate in vitro.

[125I]2-iodo-3,7,8-trichlorodibenzo-p-dioxin-binding species in mouse liver induced by agonists for the Ah receptor: characterization and identification

The admininistration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to C57BL/6J mice produces a dose-related increase in the hepatic uptake of [125I]2-iodo-3,7,8-trichlorodibenzo-p-dioxin ([125I]Cl3DpD) in vivo and the binding of the radioligand to liver homogenate in vitro [Mol. Pharmacol. 36: 121-127 (1989)]. The TCDD-induced hepatic binding species was found to be predominantly in the microsomal fraction and was inactivated by heating at 60 degree, trypsin, and mercurials. The TCDD-induced binding species was found to have an apparent equilibrium dissociation constant, KD, ([125I]Cl3DpD) of 56 +/- 16 nM and a pool size, Bmax, of 22 +/- 5 nmol/g of liver. A number of halogenated dibenzo-p-dioxins, biphenyls, and polycyclic aromatic hydrocarbons compete with [125I]Cl3DpD binding to this species; all are aromatic and planar. The distinctive profile of this binding species, a protein of large pool size induced in the microsomal fraction of liver but not other tissues and induced by agonists for the Ah receptor, suggested that this moiety might be cytochrome P3-450. The coincidence of the major microsomal species covalently labeled with the photoaffinity ligand [125I]2-iodo-3-azido-7,8-dibromodibenzo-p-dioxin and that immunochemically stained with polyclonal antiserum that binds to cytochrome P3-450 confirms this hypothesis. This is a novel role for a cytochrome P-450 isozyme, as an induced sequestration site that enhances the hepatic localization of the agonist drug.

A competitive binding assay for 2,3,7,8-tetrachlorodibenzo-p-dioxin and related ligands of the Ah receptor

A sensitive competitive binding assay for the detection of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other ligands of the Ah receptor was developed using a stable preparation of the Ah receptor, the 40-55% ammonium sulfate fraction of liver cytosol from C57BL/6J mice, and the radioligand [125I]2-iodo-7,8-dibromodibenzo-p-dioxin (specific radioactivity, 2176 Ci/mmol, and binding affinity, KD = 6.5 pM). Conditions are described which maximize assay precision and sensitivity, while minimizing sample counting time, ensuring ligand solubility, and permitting attainment of binding equilibrium for competing ligands. Assay conditions were developed to allow calculation of the binding affinity for competing ligands and to ensure that an unknown competitor could be quantified in terms of "TCDD binding equivalents." Standard assay conditions consisted of incubation of 8 pM radioligand and 18-20 pM Ah receptor with 5-1000 pM TCDD, in a 1-ml volume, for 16 hr at 4 degrees. Statistical analysis of the standard curve of bound radioligand versus the log of the concentration of competing TCDD indicated the minimal detectable concentration of TCDD to be 10 pM (3.2 pg in a 1-ml assay alpha less than or equal to 0.01). The simplicity, sensitivity, and reproducibility of this competitive binding assay should prove useful as a screen to detect planar halogenated aromatic hydrocarbons and other ligands of the Ah receptor. The availability of this 125I-labeled dioxin congener also permitted the characterization of Ah receptor-ligand binding over a range of ligand and receptor concentrations not possible with currently available 3H-ligands.

Kinetic and equilibrium studies of Ah receptor-ligand binding: use of [125I]2-iodo-7,8-dibromodibenzo-p-dioxin

In this report, we have used the radioligand [125I]2-iodo-7,8-dibromo-dibenzo-p-dioxin to describe the kinetics of ligand binding to the Ah receptor prepared from C57BL/6J mouse liver. The higher specific activity of this radioligand (2176 Ci/mmol), compared with the usual tritiated ligand [1,6-3H]2,3,7,8-tetrachloro-dibenzo-p-dioxin (58 Ci/mmol) permitted the study of ligand-receptor interactions at much lower component concentrations. For this radioiodinated ligand, Scatchard analysis of saturation binding curves, determined at six different protein concentrations, indicated that the apparent equilibrium dissociation constant, KD, was directly related to the dilution of the receptor preparation; for example, at 1160 micrograms of protein/ml, KD = 1.6 x 10(-10) M; at 36 micrograms of protein/ml, KD = 1.2 x 10(-11) M. Extrapolation of this function to infinite receptor dilution yielded KD = 6 x 10(-12) M. The addition of 70 micrograms/ml of bovine serum albumin to a receptor preparation of 30 micrograms of protein/ml produced a 10-fold decrease in the slope of the Scatchard plot (i.e., 10-fold increase in the apparent KD). Conversely, enrichment of the receptor by high performance liquid chromatography led to an increased slope and thus decreased estimate of KD. The association rate constant (k1), calculated from the integrated second-order rate equation, was 2.8 x 10(10) M-1 hr-1 and, from the initial velocity equation, had a value of 5.25 x 10(10) M-1 hr-1. The dissociation rate constant was biphasic, consisting of a predominant fast component with a rate constant of 0.36 hr-1 (k-1) and a slower component with a rate constant of 4.2-9.4 x 10(-3) hr-1 (k-2). Higher protein concentrations produced a decrease in estimates of k1 but not k-1 or k-2. The KD determined from the ratio of the kinetic rate constant, k-1/k1 = 6.9 x 10(-12) M, is in excellent agreement with that derived from the results of equilibrium binding experiments extrapolated to infinite dilution, KD = 6 x 10(-12) M. The decrease in KD, observed in equilibrium binding studies upon dilution of the receptor preparation, is best explained by a more accurate classification of "free" radioligand at lower protein concentrations. Finally, ligand binding to the Ah receptor is best described by a two-step process, the formation of an initial complex, characterized by rapid ligand dissociation, which undergoes transformation to a second distinct complex displaying a much slower ligand dissociation rate.

Purification of the Ah receptor from C57BL/6J mouse liver

The photoaffinity ligand for the Ah receptor, [125I]-2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin, previously has been shown to selectively label two peptides in the cytosol fraction of C57BL/6J mouse liver: a 95-kDa peptide, the ligand binding moiety of the Ah receptor, and a 70-kDa proteolytic fragment formed from the larger peptide (Poland, A., Glover, E., Ebetino, F. H., and Kende, A.S. (1986) J. Biol. Chem. 261, 6352-6365). These two peptides were partially purified to an approximately 20,000-fold enrichment with a 15-20% yield by the following scheme: 1) photoaffinity labeling of the 35-55% ammonium sulfate fraction of liver cytosol; 2) chromatography on polyethyleneimine-Sepharose coupled at low charge density and heparin/Mn2+ precipitation of the dilute column eluate; 3) DEAE-Sepharose chromatography to remove heparin; 4) chromatography on heparin-Sepharose; 5) preparative sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis followed by electroelution of the protein and ion pair extraction to remove sodium dodecyl sulfate; and 6) high performance liquid chromatography on a reverse-phase C-4 column. Following initial chromatography on polyethyleneimine Sepharose, it was found that substantial subsequent purification could only be achieved under denaturing conditions.

Ca2+-dependent proteolysis of the Ah receptor

We previously reported (J. Biol. Chem. (1986) 261, 6352-6465) that the photoaffinity ligand for the Ah receptor, [125I]-2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin, upon incubation with the liver cytosol fraction from C57BL/6 mice, labeled in a 1:1 ratio two peptides that had apparent molecular masses of 95 and 70 kDa and similar proteolytic fragmentation patterns. In the cytosolic fraction of Hepa 1 cells, a cloned murine hepatoma cell line, the product of photoaffinity labeling is almost exclusively a 95-kDa peptide which is rapidly hydrolyzed by a Ca2+-dependent proteinase to a 70-kDa peptide as well as other fragments. Thus, the ligand binding unit of the Ah receptor in C57BL/6 mouse liver and Hepa 1 cell is a 95-kDa peptide, and the 70-kDa fragment is a proteolytic artifact. The Ca2+-dependent proteinase which hydrolyzes the 95-kDa peptide has the properties of calpain II: (i) an absolute requirement for Ca2+, with maximal activity at 0.5 to 1.0 mM Ca2+; (ii) a pH optimum of 7.5 to 8.0; (iii) inhibition by EDTA, iodoacetamide, leupeptin and L-trans-epoxysuccinylleucylamido(4-guanidino)butane, but not by soybean trypsin inhibitor, aprotinin, or phenylmethanesufonyl fluoride. Upon chromatographic separation of the liver cytosol of C57BL/6 mice on DEAE-Sephacel, Ca2+-dependent proteinase activity (using casein or the labeled 95-kDa peptide as substrates) elutes with 0.25 M NaCl, and a specific proteinase inhibitor elutes with 0.15 M NaCl. Ca2+-dependent proteinase activity that hydrolyzes the 95-kDa peptide is found in the liver cytosols of several mammalian species.

The murine Ah locus: a new allele and mapping to chromosome 12

The Ah locus in mice, the presumed structural gene for the Ah receptor, is polymorphic in mice: some inbred strains carrying the Ahb allele express a high affinity receptor and are sensitive to receptor agonists, while other strains carrying the Ahd allele express a lower affinity receptor and are less sensitive to agonists. Using the photoaffinity ligand for the Ah receptor, [125I]-2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin, we have identified two allelic forms of the high affinity receptor (Ahb). In mouse strains of the C57 and C58 family and MA/MyJ, the photoaffinity labeled peptide in hepatic cytosol has an apparent molecular mass of 95 kDa (Ahb-1 allele), whereas in other responsive strains (e.g., C3H/HeJ, BALB/cByJ, A/J) the labeled peptide has an apparent mass of 104 kDa (Ahb-2 allele). In genetic crosses and backcrosses between C57BL/6J and C3H/HeJ mice, the expression of these peptides is consistent with codominant inheritance of two alleles at a single locus. From Scatchard plots of the specific binding of the reversible ligand, [3H]-2,3,7,8-tetrachlorodibenzo-p-dioxin, to hepatic cytosol, most strains of mice with the Ahb-1 allele were found to have an Ah receptor with a binding affinity, KD, of 0.4-0.7 nM, and a receptor concentration of 130-160 fmol/mg of protein, whereas most strains carrying the Ahb-2 allele have a slightly lower receptor affinity, KD = 0.8-1.3 nM, and a slightly lower receptor concentration, 80-110 fmol/mg of protein. From analysis of the strain distribution pattern of the Ahb-1 and Ahb-2 alleles in recombinant inbred strains, the Ah locus was linked to several unmapped loci (H-17, H-38, Ltw-2, Ly-18, D1Nyul, D1Nyu2), and this entire group mapped to chromosome 12.

Variation in the molecular mass of the Ah receptor among vertebrate species and strains of rats

The Ah receptor in eight vertebrate species was characterized by labeling the cytosolic fraction of tissue with the photoaffinity ligand, [125I]-2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin, and analysis of the products by denaturing gel electrophoresis. The apparent molecular mass of the dominant labeled peptide showed appreciable species variation: mouse-95 kDa; chicken (embryo)-101 kDa; guinea pig-103 kDa; rabbit-104 kDa; rat-106 kDa; human-106 kDa; monkey-113 kDa, and hamster-124 kDa. Seven inbred strains of rats, had a Ah receptor ligand binding peptide of 106 kDa; however outbred Long-Evans rats were shown to be polymorphic expressing a 101 kDa and/or 106 kDa allelic forms. The notable frequency of structural variation in the Ah receptor is in contrast to the analogous highly conserved steroid hormone receptors.

Photoaffinity labelling of the Ah receptor

A series of halodibenzo-p-dioxins bearing the arylazide photolabile functional group were synthesized and tested as photoaffinity labels for the Ah receptor. 2-Azido-3-iodo-7,8-dibromodibenzo-p-dioxin (KD = 0.76 X 10(-9) M) was selected for radiosynthesis. Analysis of the 125I-photoaffinity-labelled proteins in mouse-liver cytosol by denaturing gel electrophoresis revealed two peptides which had apparent molecular masses of 95,000 and 70,000 daltons respectively, were labelled in an approximately 1:1 ratio and were selectively labelled at low concentrations of the photoaffinity ligand (0.05 KD = 0.04 X 10(-9) M). In addition, their labelling was inhibited by co-incubation with an excess of unlabelled ligand. On chromatographic separation under non-denaturing conditions, these two peptides co-migrated. These studies suggest that the Ah receptor in mouse liver cytosol is a heterodimer composed of two non-covalently bound peptides (95 K and 70 K) which each have a ligand binding site.

Photoaffinity labeling of the Ah receptor

A series of halodibenzo-p-dioxins with the photolabile aryl azide functional group were synthesized and screened as potential photoaffinity labels for the Ah receptor, and 2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin was selected for radiosynthesis with 125I (specific activity 2176 Ci/mmol, equilibrium dissociation constant, KD = 0.76 nM). Following incubation of this 125I-labeled photoaffinity ligand with the protamine sulfate-precipitated fraction of C57BL/6J mouse liver cytosol, and irradiation with long wavelength ultraviolet light, the radiolabeled macromolecules were precipitated with acetone and analyzed by denaturing gel electrophoresis and autoradiography. Among the labeled products, two peptides with apparent molecular masses of 95,000 and 70,000 daltons had the following properties: 1) they were selectively labeled at low ligand concentrations; 2) they were labeled in approximately a 1:1 ratio; 3) co-incubation with receptor agonists inhibited the photoaffinity labeling of both peptides to a similar extent, and structure activity relationship for inhibition of labeling by these agonists corresponded to that for their binding affinity to the Ah receptor; 4) upon nondenaturing chromatographic separation of photoaffinity labeled cytosol on high performance liquid chromatography size exclusion and anion exchange columns, the 95- and 70-kDa peptides coelute; 5) the migration of these peptides upon denaturing electrophoresis is the same in the presence or absence of a thiol reducing agent; and 6) proteolysis of the 95- and 70-kDa peptides produces a similar pattern of cleavage peptides. The simplest structure of the Ah receptor in mouse liver cytosol, appears to be a dimer composed of two noncovalently linked subunits of apparent molecular masses of 95 and 70 kDa, which have homologous structure and similar ligand binding sites, but other possibilities are discussed.

Structure-activity relationship of bispyridyloxybenzene for induction of mouse hepatic aminopyrine N-demethylase activity. Chemical, biological, and X-ray crystallographic studies

1,4-bis-[2-(3,5-Dichloropyridyloxy)]-benzene (TCPOBOP) was previously shown to be an extremely potent phenobarbital-like inducer of hepatic microsomal monooxygenase activity in the mouse. To examine the structure-activity relationship, 31 congeners of TCPOBOP were synthesized and tested for their potency to induce hepatic aminopyrine N-demethylase activity in B6D2F1/J mice. For biological activity, the minimum requirement is a) a central 1,4-dioxygenated benzene ring, b) lateral pyridine rings linked to the central ring by ether bonds, but with other lateral heteroaromatic rings, e.g., quinoline or pyrimidine, also active, c) 5,5'-substituents of Cl, Br, or NO2 on the pyridine rings. For a series of 5,5'-substituted and 3,3'-dichloro,5,5'-substituted bispyridyloxybenzenes, no correlation was observed for Hansch pi and sigma p values. To account for this lack of correlation and conformational variability produced by the two ether bonds, we performed x-ray structure determinations on three compounds: a) TCPOBOP, b) the 5,5'-dichloro analogue, and c) the biologically inactive, 3,3'-dichloro analogue. In the two biologically active congeners the positioning of the pyridine rings is anti to the plane of the central benzene ring, and the dihedral angle between the central ring and the pyridines is approximately 60 degrees. In the inactive analogue the pyridine rings are syn and the dihedral angle is 84 degrees. The x-ray crystallographic data are consistent with the ether oxygen having an sp2-bonding conjugating with the heterodipolar bond of the pyridine C(2)--N(1), which strongly restricts rotation about the ether bonds. The potency of TCPOBOP and other bispyridyloxybenzene analogues to induce a phenobarbital-like pleiotropic response and the sharply defined structure-activity relationship among these congeners support the hypothesis that they act by binding to a specific recognition site.

Histologic changes produced by 2,3,7,8-tetrachlorodibenzo-p-dioxin in the skin of mice carrying mutations that affect the integument

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) produces epidermal hyperplasia and hyperkeratosis, squamous metaplasia of the sebaceous gland, and keratinized cyst formation in 8 strains of mice with the recessive mutation, hairless (hr/hr). The extent of these histologic changes is dependent on the genetic background. No cutaneous lesions are produced in haired (hr/+) mice. In examination of mice with 7 other mutations affecting the integument, TCDD produced similar histologic skin changes in cryptothrix, nude, plucked, and atrichosis; a marginal squamous metaplasia of sebaceous glands in Repeated epilation, and had no effect in fur deficient and Naked mutants. These genetically determined epidermal responses are discussed in light of the mechanism of action of TCDD.

2,3,7,8-Tetrachlorodibenzo-p-dioxin: examination of biochemical effects involved in the proliferation and differentiation of XB cells

XB, a cell line derived from a mouse teratoma, differentiates into stratified squamous epithelium when incubated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). To examine the possible biochemical mediators of this response, we compared the effects produced by TCDD to those elicited by other compounds which stimulate epidermal proliferation and/or differentiation in mice. XB/3T3 cultures keratinize when incubated with cholera toxin, epidermal growth factor (EGF), or TCDD, but not 12-0-tetradecanoylphorbol-13-acetate (TPA). Incubation of XB cells with TCDD (10(-9)M) for 48 hours produces a 20% increase in thymidine incorporation, a response which is neither as large nor as rapid as that produced by cholera toxin, TPA, or EGF. Although both cholera toxin and TCDD stimulate differentiation and thymidine incorporation in XB/3T3 cultures, cholera toxin increases cAMP 30-fold in these cells, while TCDD does not affect cAMP accumulation at any of the times studies (15 min to 120 hours). Inhibitors of arachidonic acid metabolism, which block epidermal proliferative responses to TPA in vivo, do not prevent the differentiation of XB cells in response to TCDD. In XB/3T3 cultures, TPA stimulates arachidonic acid release at all times tested (1,6, and 24 hours) and increases the incorporation of 32Pi into total phospholipids and phosphatidylcholine after 3 hours. In contrast, TCDD affects neither arachidonic acid release nor the turnover of phosphatidylinositol or phosphatidylcholine at any of the times tested. Although we examined biochemical effects which have been suggested as part of the mechanism of TCDD and which are produced by other epidermal proliferative compounds in XB cells, no mediator of the TCDD-produced differentiation of XB/3T3 cultures was observed.

Effect of the brachymorphic trait in mice on xenobiotic sulfate ester formation

Mice carrying the recessive mutation brachymorphic have been shown previously to have a reduced capacity to synthesize 3'-phosphoadenosine-5'-phosphosulfate (PAPS), the required coenzyme in sulfation reactions [K. Sugahara and N. Schwartz, Proc. natn. Acad. Sci. U.S.A. 76, 6615 (1979)]. The capacity of the liver cytosol fractions from brachymorphic (bm/bm) mice or their phenotypically normal littermates (+/+ or +/bm) to catalyze the formation of sulfate esters of [3H]estrone and [14C]p-nitrophenol in vitro was determined. When PAPS was added to the reaction, the rates of sulfate ester formation catalyzed by the two cytosol fractions were similar. In contrast, when PAPS was generated in situ from ATP and SO(4)2-, the rates of sulfate ester formation catalyzed by the brachymorphic cytosol were only 4-22% of the rates catalyzed by the cytosol fraction from normal mice. The hepatic cytosol fraction from brachymorphic mice incorporated less 35SO(4)2- into PAPS than that catalyzed by cytosol of normal mice. [14C]p-Nitrophenol (1.5 mumoles/kg) was eliminated from brachymorphic and normal mice as urinary conjugates; in normal mice, 73% of the urinary radioactivity was p-nitrophenyl sulfate, while in the brachymorphic mice only 33% of the urinary excretion was the sulfate ester. Brachymorphic mice have a reduced capacity for synthesizing sulfate esters of xenobiotics in vitro and in vivo, which is attributable to their reduced synthesis of PAPS.

Strong evidence from studies with brachymorphic mice and pentachlorophenol that 1'-sulfoöxysafrole is the major ultimate electrophilic and carcinogenic metabolite of 1'-hydroxysafrole in mouse liver

The role of sulfation of 1'-hydroxysafrole in the formation of hepatic macromolecular adducts and in hepatic tumor formation in mice given 1'-hydroxysafrole was investigated by the use of: (a) mice treated with the specific sulfotransferase inhibitor pentachlorophenol; and (b) brachymorphic mice, which are characterized by a deficiency in the hepatic synthesis of 3'-phosphoadenosine 5'-phosphosulfate. Cytosolic sulfotransferase activity for 1'-hydroxysafrole in both mouse and rat liver was significantly inhibited by 10 microM pentachlorophenol, usually by greater than 90%. Prior administration of nontoxic amounts of pentachlorophenol, either in the diet of adult female CD-1 mice or by i.p. injection of 12-day-old male C57BL/6 X C3H F1 (hereafter called B6C3F1) mice, resulted in an 85% decrease in the level of adducts formed from 1'-hydroxysafrole in hepatic DNA and RNA as compared to those of non-pentachlorophenol-treated animals. Likewise, the chronic administration of a nontoxic level of pentachlorophenol in the diet of adult female CD-1 mice strongly inhibited hepatic tumor induction by long-term dietary administration of either safrole or 1'-hydroxysafrole. Initiation of hepatic tumors by a single i.p. injection of 1'-hydroxysafrole to 12-day-old male B6C3F1 mice was strongly inhibited by prior treatment with pentachlorophenol. Under these conditions, the hepatocarcinogenicity of diethylnitrosamine was not inhibited by pentachlorophenol. Supplementation with adenosine triphosphate and sulfate of hepatic cytosols from adult female or 12-day-old brachymorphic progeny of a B6C3 background outbred to B6C3F1 mice (B6C3F2), of either sex, resulted in 5- to 10-fold less binding of 1'-hydroxysafrole to added RNA than when cytosols from phenotypically normal B6C3F2 mice were used. On administration of [3H]-1'-hydroxysafrole to adult female or 12-day-old brachymorphic B6C3F2 mice of either sex, the levels of hepatic DNA and RNA adducts were 7- to 12-fold lower than those obtained in phenotypically normal B6C3F2 mice of the same age and sex. Brachymorphic mice were also much less responsive than their phenotypically normal littermates to the induction of liver tumors by 1'-hydroxysafrole; lower incidences were observed both when the carcinogen was fed chronically to adult females and when it was administered to males only prior to weaning. Thus, all of these data strongly support the conclusion that 1'-sulfoöxysafrole is the major ultimate electrophilic and tumor-initiating metabolite of 1'-hydroxysafrole.

Response of murine epidermis to 2,3,7,8-tetrachlorodibenzo-p-dioxin: interaction of the ah and hr loci

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related halogenated aromatic hydrocarbons produce epidermal hyperplasia, hyperkeratosis and sebaceous gland metaplasia in the skin of mice bearing the recessive mutation (hr/hr) hairless. This response is mediated through the cytosol receptor protein: the structure-activity relationship for receptor binding corresponds to that for production of the skin lesion, and these histopathological changes segregate with the genetic polymorphism at the Ah locus, the locus determining the cytosol receptor. In HRS/J mice, an inbred strain segregating for the hr locus, both hairless (hr/hr) and haired (hr/+) mice possess the high-affinity cytosol receptor and respond to TCDD with the induction of epidermal aryl hydrocarbon hydroxylase activity, a receptor-mediated biochemical response; however, only hr/hr mice develop the proliferative/metaplastic skin response. We propose a genetic model for the interaction of the Ah and hr loci, to account for the differential response to TCDD observed in the skin of HRS/J hr/hr and hr/+ mice.

2,3,7,8-tetrachlorodibenzo-p-dioxin and related halogenated aromatic hydrocarbons: examination of the mechanism of toxicity

In this review, we have examined the biochemical and toxic responses produced by halogenated aromatic hydrocarbons and have tried to develop a model for their mechanism of action. These compounds bind to a cellular receptor and evoke a sustained pleiotropic response. In many tissues this response consists of the expression of a battery of enzymes which are, for the most part, involved in drug metabolism, but in other tissues, those which develop toxicity, an additional set of genes is expressed which effects cellular involution, division, and/or differentiation. The toxicity of these compounds appears to be due to the sustained expression of a normal cellular regulatory system, of which we were previously unaware. In future investigations it is hoped that we will learn the nature and physiologic role of this regulatory system. Only then can we hope to understand the mechanism of toxicity of these compounds.