Tohoku
J. Exp.
A Brief
Med., 1992, 168, 83-87
Review
of the
Ah
Locus
ALANPOLAND*and CHRISTOPHER BRADFIELDt *McArdle Laboratory for Cancer Research, University of Wisconsin, Madison, WI, and Deparoment of Pharmacology, Northwestern University School of Medicine, Chicago, IL, USA
POLAND,A, and BRADFIELD, C. A Brief Review of the Ah Locus. Tohoku J. Exp. Med., 1992, 168 (2), 83-87 The Ah locus, first described as a functional polymorphism among inbred strains of mice, encodes the Ah receptor - a ligand dependent transcriptional activator. This paper reviews the work on the Ah receptor and its importance in the expression of cytochrome P-4501A1 and the pleiotropic effects of halogenated aromatic hydrocarbons. Ah locus ; Ah receptor ; cytochrome P-450IA1; halogeneted aromatic hydrocarbons
The Ah locus was first identified as a difference in responsiveness to polycyclic aromatic hydrocarbons (PAH) among inbred strains of mice (Nebert et al. 1972; Thomas et al. 1972). Certain inbred strains (e.g., C57BL/6) when challenged with 3-methylcholanthrene (3-MC) respond with the induction of cytochrome P-4501A1 and associated monooxygenase activity (often measured as aryl hydrocarbon hydroxylase (All) activity) ; while other inbred strains (e.g., DBA/2) fail to respond. In genetic crosses and backcrosses between these strains, the trait of aromatic hydrocarbon responsiveness (i.e., induction of All activity by 3-MC) is inherited in a simple autosomal mode. The locus is designated the Ah locus, and the alleles, Ahb for responsive and Ahd for nonresponsive mice (Thomas et al. 1972). 2, 3, 7, 8-Tetrachlorodibenzo-p-dioxin (TCDD), a more potent agonist, induced All activity in all strains of mice tested, those responsive and nonresponsive to PAH ; but the PAH-nonresponsive mice require a 10 fold greater dose of TODD than the PAH-responsive strains (Poland and Glover 1975). It was postulated that 3-MO and TODD bound to the same receptor, a product of the Ah locus, and in mice strains homozygous for the Ahd allele, the receptor has a diminished affinity for ligands resulting in a virtually complete insensitivity to weak agonists like 3-MO, and a diminished sensitivity to the more potent agonist, TODD. Using high specific activity [3H]-TODD, a high affinity saturable binding species was found in the 100,000 X g supernatant fraction (cytosol) of liver from responsive mice, which had the postulated properties of the Ah receptor (Poland et al. 1976). Addressfor reprints : 1400University Avenue,Madison,Wisconsin,53706USA. 83
84
A. Poland
and C. Bradfield
TCDD serves as the prototype for a large group of halogenated aromatic hydrocarbons (which includes chlorinated and brominated dibenzo-p-dioxin dibenzofuran, azo(xy)benzene, and biphenyl congeners) - which are considered as a class because : (1) they all are approximate isostereomers, planar aromatic compounds with lateral ring substitutions ; (2) they all produce a similar pattern of biochemical and toxic responses ; and (3) they all appear to exert their biological effects by virtue of binding to the Ah receptor (Poland and Knutson 1982). The halogenated aromatic hydrocarbons produce a broad pleiotropic response including (a) induction of cytochromes P-4501A1 and P-450IA2, and other coordinately induced "drug metabolizing enzymes", e.g., r--aldehyde dehydrogenase, glutathione-S-transferase isoforms, (b) immune suppression, (c) proliferation and/ or altered differentiation of a variety of epithelial tissues which are species specific, (d) tumor promotion, (e) a wasting syndrome, and (f) alterations in the concentration of a variety of hormones and hormone receptors. All, or nearly all of the biologic effects of the halogenated aromatic hydrocarbons are mediated through the Ah receptor - as supported by two lines of evidence. First, for a large series of congeners, their rank-ordered binding affinities for the Ah receptor corresponds to their rank ordered potencies to elicit various biologic responses. Second, the biochemical and toxic effects produced by these congeners in mice, segregate in genetic crosses with the high affinity Ah allele. For a long period of time identification and characterization of the Ah receptor was dependent on the reversible binding of [3H]-labeled ligand. Progress in characterization and purification of this protein was retarded by (a) this methodology requiring maintanence of the undenatured state, (b) the low concentration of the protein in tissues (< 1 p g/gm), and (c) chromatographic heterogeneity, presumedly due to aggregation. To facilitate characterization, we synthesized [1251]-2-azido-3-iodo-7, 8-dibromodibenzo-p-dioxin, a photoaffinity label for the Ah receptor (Poland et al. 1986). This compound provides a covalent radiolabeled tag for the receptor permits characterization and isolation of this protein under denaturing conditions ; e.g., sodium dodecylsulfate polyacrylamide gel electrophoresis. Using this technique a high affinity specifically labeled peptide can be found in the hepatic cytosol of all vertebrate species from fish to primates. There is an appreciable variation in apparent molecular weight among different species (95-130 kD) in contrast to the conservation of structure and amino acid sequence among many receptors in vertebrate evolutions. We have identified two allelic forms of the Ah receptor in an outbred strain of rat and four alleles among inbred strains of mice. The latter include the low affinity form of the receptor Ahd-140 kD, and three high affinity forms - Ahb-1(95 kD), Ahb-2 (104 kD) and Ahb-3 (105 kD) (Poland et al. 1987; Poland and Glover 1990). Purification of the Ah receptor to homogeneity (-j 150,000 X) was facilitated by photoaffinity labeling and subsequent denaturing high performance liquid chromatography on reverse phase columns (Bradfield et al. 1991). A synthetic
A Brief Review of the Ah Locus
85
peptide, corresponding to the N-terminal amino acid sequence was linked to keyhole limpet hemocyanin and used to produce rabbit polyclonal antibodies. Analysis of the immunoaffinity-purified anti-peptide antibodies reactivity towards the Ah receptor was determined by Western blots. A standard approach was to photoaffinity label the cytosol fraction of various tissues, resolve the components by denaturing gel electrophoresis, electroblot and compared the immunochemical stain and autoradiograph of the blot. For all species of animals tested (avian to human) the autoradiograph of the [125I]-photoaffinity labeled-Ah receptor band superimposed with the immunochemical stain indicating the preservation of N-terminal epitopes in higher vertebrates. The sensitivity of detection of photoaffinity labeling and immunochemical staining was comparable 60 to 120 pg of receptor. In recent years understanding of the transcriptional activation of CYPIAI by the Ah receptor have been elucidated by the application of molecular biology and somatic cell genetics especially in the laboratories of 0. Hankinson, J. Whitlock, and Y. Fuji-Kuriyama. Following the cloning of the CYPIAI gene, the upstream regulatory sequences (the dioxin or xenobiotic response elements) were identified, shown to act like enhancer elements, and a consensus sequence identified. The activated, liganded-Ah receptor has been shown to bind to this element in gel shift and footprinting experiments (benison et al. 1985, 1988). Mutants of the murine hepatoma cell line Hepa 1, which are defective in P-4501A1 inducibility have been isolated and assigned to four complementation groups - three of which affect the functioning of the Ah receptor. In group C mutants, the Ah receptor is present in normal amounts, but doesn't translocate to the nucleus (or form tight nuclear binding) after binding TCDD. Recently, Hoffmann et al. (1991) have isolated the human gene (termed arnt, Ah receptor nuclear translocator gene) which upon transfector corrects this defect. The Ah receptor bears many similarities to the steroid hormone receptors which are ligand-binding transcriptional activators, members of the erbA family of receptors which all have two zinc fingers to bind to DNA (Evans 1988). The unliganded steroid hormone receptors and the Ah receptor exist as aggeregates with other proteins including the 90 kD heat shock protein, from which they dissociate upon ligand binding. The activated steroid hormone receptors form homodimers and bind to their palindromic responsive elements. In contrast, the Ah receptor is associated with another protein (110 kD) as it binds to its enhancer sequence (Elferink et al. 1990). This protein has not been purified as yet, but may be the arnt protein (Hoffman et al. 1991). Finally, evidence has been presented that phosphorylation is necessary for the DNA binding for of the Ah receptor (Pongratz et al. 1991). More details and an integrated picture of the mechanism of transcriptional activation by the Ah receptor should emerge in the near future. Since the initial description of the Ah locus over 20 years ago, investigations
gg
A. Poland and C. Bradfield
on this gene have continued to interest workers in diverse areas. (a) First PAH/ TCDD induction of P-4501A1 is the paradigm for xenobiotic regulation of P-450 isoforms. The emerging detalis of this mechanism (e.g., enhancer sequence heterodimeric transcriptional activator, and sequence of Ah receptor) are of great interest to workers studying induction by phenobarbital, pregnenolone-16a carbonitrile, etc. (b) The Ah locus was one of the first regulatory genetic polymorphisms observed in mice subject to many types of pharmacologic manipulation including variation in inducing agonists and substrates for regulated enzymes. Despite many studies, it is not established if there is a polymorphism in the Ah locus in humans. (c) The metabolism of PAH to their ultimate electrophilic form, and the induction of this metabolism by PAR administration has always been a subject of active investigation in chemical carcinogenesis, thus implicating the Ah locus in carcinogenesis. (d) A new dimension was added to the Ah locus when it shown to mediate all of the biologic and toxic effects of TODD and related halogenated aromatic hydrocarbons. Much of the toxicity and carcinogenicity of PAH compounds is due to their reactive metabolites ; in contrast TCDD and congeners are poorly metabolized, and their toxic and carcinogenic (i.e., tumor promoting) effects are attributable to altered gene expression. In closing, it is worth mentioning one intriguing and unresolved question concerning the Ah locus. The cloning of the Ah receptor cDNA and its sequence should be available in the near future. This will answer the speculation about the structure of the receptor - a member of the erbA family or another family of transcriptional activators. The Ah receptor controls the expression of cytochrome P-4501A1 (CYPIAI) gene, and other drug metabolizing enzymes - but also the expression of a much larger battery of genes that affect epithelial cell proliferation and/or differentiation, immune suppression, altered carbohydrate/lipid metabolism, and altered concentrations of several hormones and hormone receptors. All the known ligands for the Ah receptor are xenobiotics, planar aromatic hydrocarbons - i.e., there is, as yet, no known endogenous or physiologic ligand. One can rationalize the Ah receptor as responding to foreign chemicals, to increase the enzymes that metabolize these compounds and hastening their elimination from the body. The focus of regulation is drug metabolism. There is no need to invoke an endogenous ligand. However, the Ah receptor also regulates cell proliferation/differentiation etc. - changes which seem to have little apparent purpose as an adaption to foreign chemicals - suggesting that a one time in evolution (or individual development) there was a physiologic ligand. Rationalizing this dichotomy of the Ah receptor, response to foreign chemicals, and regulation of apparently physiologic responses may provide insight into one evolution of this system.
A Brief
Review
of the Ah Locus
87
References 1) Bradfield, CA., Glover, E. & Poland, A. (1991) Purification and N-terminal amino acid sequence of the Ah receptor from C57BL/6J mouse. Mol. Pharmacol., 39,13-19. 2) Denison, M., Fisher, J.M. & Whitlock, J.P. (1985) The DNA-recognition site for the dioxin-Ah receptor complex. J Biol. Chem., 263, 17221-17224. 3) Denison, M., Fisher, J.M. & Whitlock, J.P. (1988) Inducible, receptor-dependent protein-DNA interactions at a dioxin-responsive transcriptional enhancer. Proc. Natl. Acad. Sci., 85, 2528-2532. 4) Elferink, C.J., Gasiewicz, TA. & Whitlock, J.P. (1990) Protein-DNA interactions at a dioxin responsive enhancer. J. Biol. Chem., 265, 20708-20712. 5) Evans, R.M. (1988) The steroid and thyroid hormone superfamily. Science, 240, 889-895. 6) Hoffman, E.C., Reyes, H., Chu, F.-F., Sanders, F., Conley, L.H., Brooks, B.B. & Hankinson, 0. (1991) Cloning of a factor required for activity of the Ah (dioxin) receptor. Science,252, 954-958. 7) Nebert, D.W., Goujon, J.E. & Gielen, J.E. (1972) Aryl hydrocarbon hydroxylase induction by polycyclic aromatic hydrocarbons : Simple autosomal dominant trait in the mouse. Nature, 235, 107-110. 8) Poland, A. & Glover, E. (1975) Genetic expression of aryl hydrocarbon hydroxylase by 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin : Evidence for a receptor mutation in genetically nonresponsive mice. Mol. Pharmacol., 11, 389-398. 9) Poland, A. & Knutson, J.C. (1982) 2, 3, 7, 8-Tetrachlorodibenzo-p-dioxin and related halogenated aromatic hydrocarbons : Examination of the mechanism of toxicity. Annu. Rev. Pharmacol. Toxicol., 22, 517-554. 10) Poland, A. & Glover, E. (1990) Characterization and strain distribution of the murine Ah receptor specified by the Ahd and Ahb-3 alleles. Mol. Pharmacol., 38, 306312. 11) Poland, A., Glover, E. & Kende, AS. (1976) Stereospecific, high affinity binding of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin by hepatic cytosol. J. Biol. Chem., 251, 49364945. 12) Poland, A., Glover, E., Ebetino, F.H. & Kende, A.S. (1986) Photoaffinity labeling of the Ah receptor. J. Biol. Chem., 261, 6352-6365. 13) Poland, A., Glover, E. & Taylor, BA. (1987) The murine Ah locus : A new allele and mapping to chromosome 12. Mol. Pharmacol., 32, 471-478. 14) Pongratz, I., Stromstedt, P.E., Mason, G. & Poellinger, L. (1991) Inhibition of specific DNA binding activity of the dioxin receptor by phosphatase treatment. J. Biol. Chem., 266, 16813-16817. 15) Thomas, P.E., Kouri, RE. & Hutton, J.J. (1972) The genetics of aryl hydrocarbon hydroxylase induction in mice : A single gene difference between C57BL/6J and DBA/2J. Biochem. Genetics,6, 157-168.
J. Exp.
A Brief
Med., 1992, 168, 83-87
Review
of the
Ah
Locus
ALANPOLAND*and CHRISTOPHER BRADFIELDt *McArdle Laboratory for Cancer Research, University of Wisconsin, Madison, WI, and Deparoment of Pharmacology, Northwestern University School of Medicine, Chicago, IL, USA
POLAND,A, and BRADFIELD, C. A Brief Review of the Ah Locus. Tohoku J. Exp. Med., 1992, 168 (2), 83-87 The Ah locus, first described as a functional polymorphism among inbred strains of mice, encodes the Ah receptor - a ligand dependent transcriptional activator. This paper reviews the work on the Ah receptor and its importance in the expression of cytochrome P-4501A1 and the pleiotropic effects of halogenated aromatic hydrocarbons. Ah locus ; Ah receptor ; cytochrome P-450IA1; halogeneted aromatic hydrocarbons
The Ah locus was first identified as a difference in responsiveness to polycyclic aromatic hydrocarbons (PAH) among inbred strains of mice (Nebert et al. 1972; Thomas et al. 1972). Certain inbred strains (e.g., C57BL/6) when challenged with 3-methylcholanthrene (3-MC) respond with the induction of cytochrome P-4501A1 and associated monooxygenase activity (often measured as aryl hydrocarbon hydroxylase (All) activity) ; while other inbred strains (e.g., DBA/2) fail to respond. In genetic crosses and backcrosses between these strains, the trait of aromatic hydrocarbon responsiveness (i.e., induction of All activity by 3-MC) is inherited in a simple autosomal mode. The locus is designated the Ah locus, and the alleles, Ahb for responsive and Ahd for nonresponsive mice (Thomas et al. 1972). 2, 3, 7, 8-Tetrachlorodibenzo-p-dioxin (TCDD), a more potent agonist, induced All activity in all strains of mice tested, those responsive and nonresponsive to PAH ; but the PAH-nonresponsive mice require a 10 fold greater dose of TODD than the PAH-responsive strains (Poland and Glover 1975). It was postulated that 3-MO and TODD bound to the same receptor, a product of the Ah locus, and in mice strains homozygous for the Ahd allele, the receptor has a diminished affinity for ligands resulting in a virtually complete insensitivity to weak agonists like 3-MO, and a diminished sensitivity to the more potent agonist, TODD. Using high specific activity [3H]-TODD, a high affinity saturable binding species was found in the 100,000 X g supernatant fraction (cytosol) of liver from responsive mice, which had the postulated properties of the Ah receptor (Poland et al. 1976). Addressfor reprints : 1400University Avenue,Madison,Wisconsin,53706USA. 83
84
A. Poland
and C. Bradfield
TCDD serves as the prototype for a large group of halogenated aromatic hydrocarbons (which includes chlorinated and brominated dibenzo-p-dioxin dibenzofuran, azo(xy)benzene, and biphenyl congeners) - which are considered as a class because : (1) they all are approximate isostereomers, planar aromatic compounds with lateral ring substitutions ; (2) they all produce a similar pattern of biochemical and toxic responses ; and (3) they all appear to exert their biological effects by virtue of binding to the Ah receptor (Poland and Knutson 1982). The halogenated aromatic hydrocarbons produce a broad pleiotropic response including (a) induction of cytochromes P-4501A1 and P-450IA2, and other coordinately induced "drug metabolizing enzymes", e.g., r--aldehyde dehydrogenase, glutathione-S-transferase isoforms, (b) immune suppression, (c) proliferation and/ or altered differentiation of a variety of epithelial tissues which are species specific, (d) tumor promotion, (e) a wasting syndrome, and (f) alterations in the concentration of a variety of hormones and hormone receptors. All, or nearly all of the biologic effects of the halogenated aromatic hydrocarbons are mediated through the Ah receptor - as supported by two lines of evidence. First, for a large series of congeners, their rank-ordered binding affinities for the Ah receptor corresponds to their rank ordered potencies to elicit various biologic responses. Second, the biochemical and toxic effects produced by these congeners in mice, segregate in genetic crosses with the high affinity Ah allele. For a long period of time identification and characterization of the Ah receptor was dependent on the reversible binding of [3H]-labeled ligand. Progress in characterization and purification of this protein was retarded by (a) this methodology requiring maintanence of the undenatured state, (b) the low concentration of the protein in tissues (< 1 p g/gm), and (c) chromatographic heterogeneity, presumedly due to aggregation. To facilitate characterization, we synthesized [1251]-2-azido-3-iodo-7, 8-dibromodibenzo-p-dioxin, a photoaffinity label for the Ah receptor (Poland et al. 1986). This compound provides a covalent radiolabeled tag for the receptor permits characterization and isolation of this protein under denaturing conditions ; e.g., sodium dodecylsulfate polyacrylamide gel electrophoresis. Using this technique a high affinity specifically labeled peptide can be found in the hepatic cytosol of all vertebrate species from fish to primates. There is an appreciable variation in apparent molecular weight among different species (95-130 kD) in contrast to the conservation of structure and amino acid sequence among many receptors in vertebrate evolutions. We have identified two allelic forms of the Ah receptor in an outbred strain of rat and four alleles among inbred strains of mice. The latter include the low affinity form of the receptor Ahd-140 kD, and three high affinity forms - Ahb-1(95 kD), Ahb-2 (104 kD) and Ahb-3 (105 kD) (Poland et al. 1987; Poland and Glover 1990). Purification of the Ah receptor to homogeneity (-j 150,000 X) was facilitated by photoaffinity labeling and subsequent denaturing high performance liquid chromatography on reverse phase columns (Bradfield et al. 1991). A synthetic
A Brief Review of the Ah Locus
85
peptide, corresponding to the N-terminal amino acid sequence was linked to keyhole limpet hemocyanin and used to produce rabbit polyclonal antibodies. Analysis of the immunoaffinity-purified anti-peptide antibodies reactivity towards the Ah receptor was determined by Western blots. A standard approach was to photoaffinity label the cytosol fraction of various tissues, resolve the components by denaturing gel electrophoresis, electroblot and compared the immunochemical stain and autoradiograph of the blot. For all species of animals tested (avian to human) the autoradiograph of the [125I]-photoaffinity labeled-Ah receptor band superimposed with the immunochemical stain indicating the preservation of N-terminal epitopes in higher vertebrates. The sensitivity of detection of photoaffinity labeling and immunochemical staining was comparable 60 to 120 pg of receptor. In recent years understanding of the transcriptional activation of CYPIAI by the Ah receptor have been elucidated by the application of molecular biology and somatic cell genetics especially in the laboratories of 0. Hankinson, J. Whitlock, and Y. Fuji-Kuriyama. Following the cloning of the CYPIAI gene, the upstream regulatory sequences (the dioxin or xenobiotic response elements) were identified, shown to act like enhancer elements, and a consensus sequence identified. The activated, liganded-Ah receptor has been shown to bind to this element in gel shift and footprinting experiments (benison et al. 1985, 1988). Mutants of the murine hepatoma cell line Hepa 1, which are defective in P-4501A1 inducibility have been isolated and assigned to four complementation groups - three of which affect the functioning of the Ah receptor. In group C mutants, the Ah receptor is present in normal amounts, but doesn't translocate to the nucleus (or form tight nuclear binding) after binding TCDD. Recently, Hoffmann et al. (1991) have isolated the human gene (termed arnt, Ah receptor nuclear translocator gene) which upon transfector corrects this defect. The Ah receptor bears many similarities to the steroid hormone receptors which are ligand-binding transcriptional activators, members of the erbA family of receptors which all have two zinc fingers to bind to DNA (Evans 1988). The unliganded steroid hormone receptors and the Ah receptor exist as aggeregates with other proteins including the 90 kD heat shock protein, from which they dissociate upon ligand binding. The activated steroid hormone receptors form homodimers and bind to their palindromic responsive elements. In contrast, the Ah receptor is associated with another protein (110 kD) as it binds to its enhancer sequence (Elferink et al. 1990). This protein has not been purified as yet, but may be the arnt protein (Hoffman et al. 1991). Finally, evidence has been presented that phosphorylation is necessary for the DNA binding for of the Ah receptor (Pongratz et al. 1991). More details and an integrated picture of the mechanism of transcriptional activation by the Ah receptor should emerge in the near future. Since the initial description of the Ah locus over 20 years ago, investigations
gg
A. Poland and C. Bradfield
on this gene have continued to interest workers in diverse areas. (a) First PAH/ TCDD induction of P-4501A1 is the paradigm for xenobiotic regulation of P-450 isoforms. The emerging detalis of this mechanism (e.g., enhancer sequence heterodimeric transcriptional activator, and sequence of Ah receptor) are of great interest to workers studying induction by phenobarbital, pregnenolone-16a carbonitrile, etc. (b) The Ah locus was one of the first regulatory genetic polymorphisms observed in mice subject to many types of pharmacologic manipulation including variation in inducing agonists and substrates for regulated enzymes. Despite many studies, it is not established if there is a polymorphism in the Ah locus in humans. (c) The metabolism of PAH to their ultimate electrophilic form, and the induction of this metabolism by PAR administration has always been a subject of active investigation in chemical carcinogenesis, thus implicating the Ah locus in carcinogenesis. (d) A new dimension was added to the Ah locus when it shown to mediate all of the biologic and toxic effects of TODD and related halogenated aromatic hydrocarbons. Much of the toxicity and carcinogenicity of PAH compounds is due to their reactive metabolites ; in contrast TCDD and congeners are poorly metabolized, and their toxic and carcinogenic (i.e., tumor promoting) effects are attributable to altered gene expression. In closing, it is worth mentioning one intriguing and unresolved question concerning the Ah locus. The cloning of the Ah receptor cDNA and its sequence should be available in the near future. This will answer the speculation about the structure of the receptor - a member of the erbA family or another family of transcriptional activators. The Ah receptor controls the expression of cytochrome P-4501A1 (CYPIAI) gene, and other drug metabolizing enzymes - but also the expression of a much larger battery of genes that affect epithelial cell proliferation and/or differentiation, immune suppression, altered carbohydrate/lipid metabolism, and altered concentrations of several hormones and hormone receptors. All the known ligands for the Ah receptor are xenobiotics, planar aromatic hydrocarbons - i.e., there is, as yet, no known endogenous or physiologic ligand. One can rationalize the Ah receptor as responding to foreign chemicals, to increase the enzymes that metabolize these compounds and hastening their elimination from the body. The focus of regulation is drug metabolism. There is no need to invoke an endogenous ligand. However, the Ah receptor also regulates cell proliferation/differentiation etc. - changes which seem to have little apparent purpose as an adaption to foreign chemicals - suggesting that a one time in evolution (or individual development) there was a physiologic ligand. Rationalizing this dichotomy of the Ah receptor, response to foreign chemicals, and regulation of apparently physiologic responses may provide insight into one evolution of this system.
A Brief
Review
of the Ah Locus
87
References 1) Bradfield, CA., Glover, E. & Poland, A. (1991) Purification and N-terminal amino acid sequence of the Ah receptor from C57BL/6J mouse. Mol. Pharmacol., 39,13-19. 2) Denison, M., Fisher, J.M. & Whitlock, J.P. (1985) The DNA-recognition site for the dioxin-Ah receptor complex. J Biol. Chem., 263, 17221-17224. 3) Denison, M., Fisher, J.M. & Whitlock, J.P. (1988) Inducible, receptor-dependent protein-DNA interactions at a dioxin-responsive transcriptional enhancer. Proc. Natl. Acad. Sci., 85, 2528-2532. 4) Elferink, C.J., Gasiewicz, TA. & Whitlock, J.P. (1990) Protein-DNA interactions at a dioxin responsive enhancer. J. Biol. Chem., 265, 20708-20712. 5) Evans, R.M. (1988) The steroid and thyroid hormone superfamily. Science, 240, 889-895. 6) Hoffman, E.C., Reyes, H., Chu, F.-F., Sanders, F., Conley, L.H., Brooks, B.B. & Hankinson, 0. (1991) Cloning of a factor required for activity of the Ah (dioxin) receptor. Science,252, 954-958. 7) Nebert, D.W., Goujon, J.E. & Gielen, J.E. (1972) Aryl hydrocarbon hydroxylase induction by polycyclic aromatic hydrocarbons : Simple autosomal dominant trait in the mouse. Nature, 235, 107-110. 8) Poland, A. & Glover, E. (1975) Genetic expression of aryl hydrocarbon hydroxylase by 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin : Evidence for a receptor mutation in genetically nonresponsive mice. Mol. Pharmacol., 11, 389-398. 9) Poland, A. & Knutson, J.C. (1982) 2, 3, 7, 8-Tetrachlorodibenzo-p-dioxin and related halogenated aromatic hydrocarbons : Examination of the mechanism of toxicity. Annu. Rev. Pharmacol. Toxicol., 22, 517-554. 10) Poland, A. & Glover, E. (1990) Characterization and strain distribution of the murine Ah receptor specified by the Ahd and Ahb-3 alleles. Mol. Pharmacol., 38, 306312. 11) Poland, A., Glover, E. & Kende, AS. (1976) Stereospecific, high affinity binding of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin by hepatic cytosol. J. Biol. Chem., 251, 49364945. 12) Poland, A., Glover, E., Ebetino, F.H. & Kende, A.S. (1986) Photoaffinity labeling of the Ah receptor. J. Biol. Chem., 261, 6352-6365. 13) Poland, A., Glover, E. & Taylor, BA. (1987) The murine Ah locus : A new allele and mapping to chromosome 12. Mol. Pharmacol., 32, 471-478. 14) Pongratz, I., Stromstedt, P.E., Mason, G. & Poellinger, L. (1991) Inhibition of specific DNA binding activity of the dioxin receptor by phosphatase treatment. J. Biol. Chem., 266, 16813-16817. 15) Thomas, P.E., Kouri, RE. & Hutton, J.J. (1972) The genetics of aryl hydrocarbon hydroxylase induction in mice : A single gene difference between C57BL/6J and DBA/2J. Biochem. Genetics,6, 157-168.
