A DIRECT INTERACTION BETWEEN THE ARYL-HYDROCARBON RECEPTOR AND RETINOBLASTOMA PROTEIN - LINKING DIOXIN SIGNALING TO THE CELL-CYCLE

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcriptio n factor in eukaryotic cells that alters gene expression in response t o the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (T CDD), In 5L hepatoma cells, TCDD induces a G(1), cell cycle arrest thr ough a mechanism that involves the AhR. The retinoblastoma tumor suppr essor protein (pRb) controls cell cycle progression through G(1), in a ddition to promoting differentiation, We examined whether the human Ah R or its dimerization partner, the AhR nuclear translocator, interacts with pRb as a basis of the TCDD-induced cell cycle arrest. In vivo an d in vitro assays reveal a direct interaction between pRb and the AhR but not the AhR nuclear translocator protein. Binding between the AhR and pRb occurs through two distinct regions in the AhR. A high affinit y site lies within the N-terminal 364 amino acids of the AhR, whereas a lower affinity binding region colocalizes with the glutamine-rich tr ansactivation domain of the receptor. AhR ligand binding is not requir ed for the pRb interaction per se, although immunoprecipitation experi ments in 5L cells reveal that pRb associates preferentially with the l iganded AhR, consistent with a requirement for ligand-induced nuclear translocation. These observations provide a mechanistic insight into A hR-mediated cell cycle arrest and a new perspective on TCDD-induced to xicity.