TCDD (2,3,7,8-teuachlorodibenzo-p-dioxin) is the prototype of a class of structurally related halogenated aromatic hydrocatbons. TCDD elicits a wide range of responses in animals and may pose a threat to human health. The aryl hydrocarbon receptor is a ligand-activated receptor/transcription factor, which, together with Arm and other factors, mediates the biological effects of TCDD. The mechanism of action of AhR involves uanscriptional regulation of target genes. However, current knowledge of the target genes regulated by AhR and their roles in TCDD toxicity is limited. By using the mRNA differential display technique, we identified and cloned a novel target gene of AhR, designated TCDD-inducible gene 1 (TIG1). In this study, we characterize the regulation of TIG1 by TCDD. Genetic analyses using AhR- and Arnt-deficient cells demonstrate AhR and Arnt are required for induction of TIG1. Moreover, the transactivation domains of both AhR and Arnt contribute to the induction of the gene. We next examined the effect of cycloheximide, a potent inhibitor of protein synthesis, on the induction of TIG1 by TCDD. Our results reveal that cycloheximide superinduces TIG1 in the presence of TCDD; the superinduction occurs in both a time- and dose-dependent manner. The mechanism of action of superinduction involves inhibition of protein synthesis. Time course studies of superinduction implicate a labile factor in the induction of TIG1. Our results demonstrate that induction of TIG1 by TCDD occurs through the AhR pathway and that a labile factor controls the transcriptional regulation of the gene by AhR. These findings support the model previously proposed by our laboratoty in which a labile factor, designated AhR degradation promoting factor (ADPF), acts as a negative regulator of agonist-activated AhR by controlling the removal of AhR. Inhibition of synthesis of ADPF enhances the stability of nuclear AhR, resulting in superinduction of TIG1 and other AhR regulated genes.
The Toxicologist. Society of Toxicology 40th Annual Meeting, March 25-29, 2001, San Francisco, California