title: Superinduction of CYP1A1 gene expression. Regulation of 2,3,7, 8-tetrachlorodibenzo-p-dioxin-induced degradation of Ah receptor by cycloheximide.
abstract: Cycloheximide superinduces the transcription of CYP1A1 in the presence of an agonist for the Ah receptor (AhR). To investigate the molecular target for "superinduction," we analyzed the agonist-induced degradation of AhR. Whereas 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), a potent agonist of AhR, induces a rapid reduction of the AhR protein, cycloheximide blocks the down-regulation of steady state AhR. Analyses of the turnover of AhR reveal that cycloheximide blocks the shortening of the half-life of AhR by TCDD. Blocking of the TCDD-induced AhR degradation requires inhibition of protein synthesis, because (a) cycloheximide inhibits protein synthesis at the concentration at which it causes superinduction and inhibition of AhR degradation; and (b) puromycin, an inhibitor of protein synthesis by mimicking aminoacyl-tRNA, also blocks the TCDD-induced AhR degradation. The blocking of the TCDD-induced AhR degradation correlates with the superinduction of CYP1A1 gene expression in a time- and dose-dependent manner. Furthermore, cycloheximide is shown to increase the accumulation of the TCDD-activated AhR and the functional AhR x Arnt complex in nucleus. Collectively, our results reveal a mechanism of superinduction by cycloheximide by enhancing the stability of agonist-activated AhR. The finding that inhibition of protein synthesis blocks the TCDD-induced AhR turnover implicates a cycloheximide-sensitive, labile factor (designated as AhR degradation promoting factor, or ADPF) in controlling the removal of agonist-activated AhR in nucleus.
authors.affiliation: Molecular Toxicology Laboratory, Toxicology and Molecular Biology Branch, Health Effects Laboratory Division, NIOSH, National Institutes of Health, Centers for Disease Control and Prevention, Morgantown, West Virginia 26505, USA.
authors.forename: Q
authors.initials: Q
authors.lastname: Ma
authors.forename: A J
authors.initials: AJ
authors.lastname: Renzelli
authors.forename: K T
authors.initials: KT
authors.lastname: Baldwin
authors.forename: J M
authors.initials: JM
authors.lastname: Antonini
mesh_terms: D000818:Animals; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D003513:Cycloheximide / Q000494:pharmacology*; D019363:Cytochrome P-450 CYP1A1 / Q000187:drug effects / Q000235:genetics*; D015536:Down-Regulation; D005455:Fluorescent Antibody Technique; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D015151:Immunoblotting; D051379:Mice; D018613:Microscopy, Confocal; D000072317:Polychlorinated Dibenzodioxins / Q000494:pharmacology*; D011233:Precipitin Tests; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D011691:Puromycin / Q000494:pharmacology; D012313:RNA / Q000378:metabolism; D018336:Receptors, Aryl Hydrocarbon / Q000378:metabolism*; D015398:Signal Transduction; D013997:Time Factors
chemical_list: D000072317:Polychlorinated Dibenzodioxins; D011500:Protein Synthesis Inhibitors; D018336:Receptors, Aryl Hydrocarbon; D011691:Puromycin; D012313:RNA; D003513:Cycloheximide; D019363:Cytochrome P-450 CYP1A1