ARYL-HYDROCARBON RECEPTOR-MEDIATED SIGNAL-TRANSDUCTION

The aryl hydrocarbon (or dioxin) receptor (AhR) is a ligand-activated basic helix-loop-helix (bHLH) protein that heterodimerizes with the bH LH protein ARNT (aryl hydrocarbon nuclear translocator) forming a comp lex that binds to xenobiotic regulatory elements in target gene enhanc ers. Genetic, biochemical, and molecular biology studies have revealed that the AhR mediates the toxic and biological effects of environment ally persistent dioxins and related compounds. Cloning of the receptor and its DNA-binding partner, ARNT, has facilitated detailed efforts t o understand the mechanisms of AhR-mediated signal transduction. These studies have determined that this unique receptor consists of several functional domains and belongs to a subfamily of bHLH proteins that s hare a conserved motif termed the PAS domain. In addition, recent gene tic studies have revealed that expression of the AhR is a requirement for proper embryonal development, which appears to be a common functio n shared by many other bHLH proteins. This review is a summary of rece nt molecular studies of AhR-mediated gene regulation.