AH RECEPTOR, A NOVEL LIGAND-ACTIVATED TRANSCRIPTION FACTOR

The aryl hydrocarbon receptor (AhR) is widely distributed in vertebrat es and is known to be involved in metabolism of xenobiotics including man-made chemicals, most of which act as a ligand for the receptor, al though no endogeneous ligand has yet been known, Upon binding a ligand , the receptor is activated to translocate to the nuclei, and during t he nuclear translocation process, it is dissociated from the 90 kDa he at shock protein (Hsp90) to forms a heterodimer with Arnt (Ah receptor nuclear translocator), The heterodimer complex binds a DNA response e lement termed xenobiotic responsive element (XRE) localized upstream o f the target genes of many drug-metabolizing enzymes including cytochr ome P4501A1 and glutathione S-transferase to activate their transcript ion, Recent cDNA cloning has revealed that the AhR, like Arnt, possess es characteristic structural motifs of basic helix-loop-helix and PAS domains responsible for DNA recognition, heterodimerization, and ligan d binding, and functions as a novel receptor-type transcription factor .