title: Ligand selectivity and gene regulation by the human aryl hydrocarbon receptor in transgenic mice.
abstract: The aryl hydrocarbon receptor (AHR) is a ligand-inducible transcription factor that displays interspecies differences with the human and mouse AHR C-terminal region sequences sharing only 58% amino acid sequence identity. Compared with the mouse AHR (mAHR), the human AHR (hAHR) displays approximately 10-fold lower relative affinity for prototypical AHR ligands such as 2,3,7,8-tetrachlorodibenzo-p-dioxin, which has been attributed to the amino acid residue valine 381 (alanine 375 in the mAHR) in the ligand binding domain of the hAHR. We investigated whether the 10-fold difference in ligand-binding affinity between the mAHR and hAHR would be observed with a diverse range of AHR ligands. To test this hypothesis, ligand binding assays were performed using the photo-affinity ligand 2-azido-3-[(125)I]iodo-7,8-dibromodibenzo-p-dioxin and liver cytosol isolated from hepatocyte-specific transgenic hAHR mice and C57BL/6J mice. It is noteworthy that competitive ligand-binding assays revealed that, compared with the mAHR, the hAHR has a higher relative affinity for certain compounds, including indirubin [(2Z)-2,3-biindole-2,3 (1'H,1'H)-dione and quercetin (2-(3,4dihydroxyphenyl)-3,5,7-trihydroxy-4H-chromen-4-one]. Electrophoretic mobility shift assays revealed that indirubin was more efficient at transforming the hAHR compared with the mAHR. Indirubin was also a more potent inducer of Cyp1a1 expression in transgenic hAHR mouse hepatocytes compared with C57BL/6J mouse hepatocytes. These observations suggest that indirubin is a potent hAHR ligand that is able to selectively bind to and activate the hAHR. These discoveries imply that there may be a significant degree of structural divergence between mAHR and hAHR ligands and highlights the importance of the hAHR transgenic mouse as a model to study the hAHR in vivo.
authors.affiliation: Center for Molecular Toxicology and Carcinogenesis and the Department of Veterinary and Biomedical Sciences, the Pennsylvania State University, 16802, USA.
authors.forename: Colin A
authors.initials: CA
authors.lastname: Flaveny
authors.forename: Iain A
authors.initials: IA
authors.lastname: Murray
authors.forename: Chris R
authors.initials: CR
authors.lastname: Chiaro
authors.forename: Gary H
authors.initials: GH
authors.lastname: Perdew
mesh_terms: D000818:Animals; D001562:Benzimidazoles / Q000494:pharmacology; D001667:Binding, Competitive; D019556:COS Cells; D002522:Chlorocebus aethiops; D019363:Cytochrome P-450 CYP1A1 / Q000096:biosynthesis; D004147:Dioxins / Q000494:pharmacology; D005786:Gene Expression Regulation; D022781:Hepatocytes / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D007457:Iodine Radioisotopes; D008024:Ligands; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D023421:Models, Animal; D019920:Photoaffinity Labels; D011794:Quercetin / Q000494:pharmacology; D011869:Radioligand Assay; D018336:Receptors, Aryl Hydrocarbon / Q000819:agonists / Q000378:metabolism*
chemical_list: D001562:Benzimidazoles; D004147:Dioxins; D007211:Indoles; D007457:Iodine Radioisotopes; D008024:Ligands; C506997:M50354; D019920:Photoaffinity Labels; D018336:Receptors, Aryl Hydrocarbon; C056902:2-iodo-7,8-dibromodibenzo-1,4-dioxin; D011794:Quercetin; D019363:Cytochrome P-450 CYP1A1; C027185:indirubin