Figure 14: The arylhydrocarbon receptor (AhR) belongs to the Ligand activated basic helix-loop-helix transcription factors.
In the cytosol, it occurs as an tetrameric complex with heat shock protein (HSP) 90 and X-associated protein 2 (syn: ARA9). Ligands such as dioxin (TCCD) or polycyclic aromatic hydrocarbons dissolve the 2 HSP molecules from the complex. The remaining complex binds to the Arylhydrocarbon Receptor Nucleus Translocator (ARNT), is translocated in to the nucleus and enhance the transcription of xenobiotic response element dependent genes. The gene expression of several proteins including various detoxification enzymes is increased. In addition, cyclin kinase 2 is activated and TGF-β effects are inhibited, resulting in increased cell division.