The Ah receptor (AhR) is a ligand-dependent transcription factor known to regulate the toxic and biological effects of a variety of exogenous chemicals, such as the toxic halogenated aromatic hydrocarbons (HAHs) and polycyclic aromatic hydrocarbons (PAHs), and these effects appear to result from AhR-dependent alterations in gene expression. The AhR is also involved in several endogenous developmental and physiological processes, although the responsible endogenous ligand(s) is unknown. While HAHs and PAHs are the prototypical and highest affinity ligands, the AhR can bind and be activated by a diverse range of structurally dissimilar compounds, even though species- and ligand-specific differences in AhR ligand binding specificity and functionality exist. Site-directed mutagenesis and functional analysis studies based on our 3-dimensional (3D) homology model of the AhR ligand binding domain (LBD) performed so far have allowed initial understanding of aspects of the process by which high affinity HAH ligands like 2,3,7,8-tetrachlorodibenzo-p- dioxin (TCDD) can bind to and activate the AhR. However, these same studies suggest that significant differences exist in the amino acid residues to which structurally unrelated AhR ligands specifically interact. We hypothesize that differences in the binding sites and interactions of structurally diverse AhR ligands within the AhR LBD are primarily responsible for the observed ligand promiscuity of the AhR and that these differences could contribute to ligand-specific alterations in AhR conformational states that lead to differences in AhR functionality. To test this hypothesis, we propose to develop a new homology model of the AhR LBD based on recently released X-ray structures of the HIF-2a template complexed with ligands that also bind to the AhR and use this updated model for docking analysis of AhR ligands. Structurally driven site-directed mutagenesis and AhR functional analysis of the interactions of structurally diverse AhR agonists/antagonists with specific amino acids within the AhR LBD, coupled with similar analyses of chimeric mouse AhRs containing the LBD domain of AhRs which do not bind TCDD, will facilitate further identification of residues and structural characteristics of the LBD required for ligand binding. The molecular mechanisms by which binding of structurally diverse ligands within the LBD stimulates transformation of the AhR into its DNA binding form (loss of hsp90 and binding of Arnt) will be examined through analysis of AhR:hsp90 and AhR:Arnt interactions and ligand selective effects on coactivator binding and AhR-dependent gene expression determined. The residues/regions of both proteins involved in complex formation and functional activity will be defined and modeled and ligand-specific alterations in these mechanisms examined. The studies proposed here will provide detailed analysis of the molecular mechanisms by which structurally diverse ligands bind to and activate the AhR. In addition, they will yield insights into the molecular mechanisms of ligand-dependent AhR transformation, the influence of ligand structure on these processes and the diversity of AhR responsiveness.
Little is known about the molecular details and mechanisms by which structurally diverse exogenous and endogenous chemicals can to bind to and activate/inhibit the Ah (dioxin) receptor (AhR), a chemical-responsive cellular protein responsible for mediating the toxic, biological and developmental effects produced by these substances. The work proposed in this application will increase our understanding of the structure of the AhR and the mechanisms by which diverse chemicals can differentially activate or inhibit the AhR or AhR signaling pathways. These studies will not only lead to the identification and development of inhibitors of AhR-dependent toxicity with therapeutic potential (i.e. as anticancer agents and/or anti-toxins), but will provide avenues in which to gain insights into the normal physiological role(s) of this poorly understood receptor.
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|Bonati, Laura; Corrada, Dario; Tagliabue, Sara Giani et al. (2017) Molecular modeling of the AhR structure and interactions can shed light on ligand-dependent activation and transformation mechanisms. Curr Opin Toxicol 2:42-49|
|Corrada, Dario; Denison, Michael S; Bonati, Laura (2017) Structural modeling of the AhR:ARNT complex in the bHLH-PASA-PASB region elucidates the key determinants of dimerization. Mol Biosyst 13:981-990|
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|Denison, Michael S; Faber, Samantha C (2017) And Now for Something Completely Different: Diversity in Ligand-Dependent Activation of Ah Receptor Responses. Curr Opin Toxicol 2:124-131|
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|Brennan, Jennifer C; Bassal, Arzoo; He, Guochun et al. (2016) Development of a recombinant human ovarian (BG1) cell line containing estrogen receptor ? and ? for improved detection of estrogenic/antiestrogenic chemicals. Environ Toxicol Chem 35:91-100|
|Corrada, Dario; Soshilov, Anatoly A; Denison, Michael S et al. (2016) Deciphering Dimerization Modes of PAS Domains: Computational and Experimental Analyses of the AhR:ARNT Complex Reveal New Insights Into the Mechanisms of AhR Transformation. PLoS Comput Biol 12:e1004981|
|Mexia, Nikitia; Gaitanis, Georgios; Velegraki, Aristea et al. (2015) Pityriazepin and other potent AhR ligands isolated from Malassezia furfur yeast. Arch Biochem Biophys 571:16-20|
|Brennan, Jennifer C; He, Guochun; Tsutsumi, Tomoaki et al. (2015) Development of Species-Specific Ah Receptor-Responsive Third Generation CALUX Cell Lines with Enhanced Responsiveness and Improved Detection Limits. Environ Sci Technol 49:11903-12|
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