Aryl hydrocarbon receptor

19611622ENSG00000106546ENSMUSG00000019256P35869P30561NM_001621NM_013464NM_001314027NP_001612NP_001300956NP_038492The aryl hydrocarbon receptor (AhR or AHR or ahr or ahR) is a protein that in humans is encoded by the AHR gene. The aryl hydrocarbon receptor is a transcription factor that regulates gene expression. It was originally thought to function primarily as a sensor of xenobiotic chemicals and also as the regulator of enzymes such as cytochrome P450s that metabolize these chemicals. The most notable of these xenobiotic chemicals are aromatic (aryl) hydrocarbons from which the receptor derives its name. The aryl hydrocarbon receptor (AhR or AHR or ahr or ahR) is a protein that in humans is encoded by the AHR gene. The aryl hydrocarbon receptor is a transcription factor that regulates gene expression. It was originally thought to function primarily as a sensor of xenobiotic chemicals and also as the regulator of enzymes such as cytochrome P450s that metabolize these chemicals. The most notable of these xenobiotic chemicals are aromatic (aryl) hydrocarbons from which the receptor derives its name. More recently, it has been discovered that AhR is activated (or deactivated) by a number of endogenous indole derivatives such as kynurenine. In addition to regulating metabolism enzymes, the AhR has roles in regulating immunity, stem cell maintenance, and cellular differentiation. The aryl hydrocarbon receptor is a member of the family of basic helix-loop-helix transcription factors. AHR binds several exogenous ligands such as natural plant flavonoids, polyphenolics and indoles, as well as synthetic polycyclic aromatic hydrocarbons and dioxin-like compounds. AhR is a cytosolic transcription factor that is normally inactive, bound to several co-chaperones. Upon ligand binding to chemicals such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the chaperones dissociate resulting in AhR translocating into the nucleus and dimerizing with ARNT (AhR nuclear translocator), leading to changes in gene transcription. The AhR protein contains several domains critical for function and is classified as a member of the basic helix-loop-helix/Per-Arnt-Sim (bHLH/PAS) family of transcription factors. The bHLH motif is located in the N-terminal of the protein and is a common entity in a variety of transcription factors. Members of the bHLH superfamily have two functionally distinctive and highly conserved domains. The first is the basic-region (b), which is involved in the binding of the transcription factor to DNA. The second is the helix-loop-helix (HLH) region, which facilitates protein-protein interactions. Also contained with the AhR are two PAS domains, PAS-A and PAS-B, which are stretches of 200-350 amino acids that exhibit a high sequence homology to the protein domains that were originally found in the Drosophila genes period (Per) and single-minded (Sim) and in AhR’s dimerization partner the aryl hydrocarbon receptor nuclear translocator (ARNT). The PAS domains support specific secondary interactions with other PAS domain containing proteins, as is the case with AhR and ARNT, so that dimeric and heteromeric protein complexes can form. The ligand binding site of AhR is contained within the PAS-B domain and contains several conserved residues critical for ligand binding. Finally, a glutamine-rich (Q-rich) domain is located in the C-terminal region of the protein and is involved in co-activator recruitment and transactivation. AhR ligands have been generally classified into two categories, synthetic or naturally occurring. The first ligands to be discovered were synthetic and members of the halogenated aromatic hydrocarbons (polychlorinated dibenzodioxins, dibenzofurans and biphenyls) and polycyclic aromatic hydrocarbons (3-methylcholanthrene, benzopyrene, benzanthracenes and benzoflavones). Research has focused on naturally occurring compounds with the hope of identifying an endogenous ligand. Naturally occurring compounds that have been identified as ligands of Ahr include derivatives of tryptophan such as indigo dye and indirubin, tetrapyrroles such as bilirubin, the arachidonic acid metabolites lipoxin A4 and prostaglandin G, modified low-density lipoprotein and several dietary carotenoids. One assumption made in the search for an endogenous ligand is that the ligand will be a receptor agonist. However, work by Savouret et al. has shown this may not be the case since their findings demonstrate that 7-ketocholesterol competitively inhibits Ahr signal transduction. Carbidopa is a selective aryl hydrocarbon receptor modulator (SAhRM). ICZ is one of the strongest non-halogenated agonists for AHR in vitro reported. Non-ligand bound Ahr is retained in the cytoplasm as an inactive protein complex consisting of a dimer of Hsp90, prostaglandin E synthase 3 (PTGES3, p23) and a single molecule of the immunophilin-like AH receptor-interacting protein, also known as hepatitis B virus X-associated protein 2 (XAP2), AhR interacting protein (AIP), and AhR-activated 9 (ARA9). The dimer of Hsp90, along with PTGES3 (p23), has a multifunctional role in the protection of the receptor from proteolysis, constraining the receptor in a conformation receptive to ligand binding and preventing the premature binding of ARNT. AIP interacts with carboxyl-terminal of Hsp90 and binds to the AhR nuclear localization sequence (NLS) preventing the inappropriate trafficking of the receptor into the nucleus.