Regulatory xenobiotic responsive elements in the distal 5'-flanking region of the mouse Cyp1a2 gene required for transcriptional activation by 3-methylcholanthrene and 2,3,7,8-tetrachlorodibenzo-p-dioxin

Constitutive androstane receptor-responsive elements for mouse Cyp1a2 transcriptional activation induced by constitutive androstane receptor ligands

The mouse cytochrome P450 1A2 (Cyp1a2) gene is one of the constitutive androstane receptor (CAR, NR1I3) activator-inducible genes, and the regions involved in induction were examined herein. A reporter gene assay indicated the involvement of the -0.2-kb region in the induction of transcriptional activation by the mouse CAR agonist ligand 1,4-bis-[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP). Some putative nuclear receptor-binding elements were identified in this region, and mutations in the elements located at -160/-155 or -153/-148 abolished this induction. An electrophoretic mobility shift assay demonstrated that a fragment comprised of three elements was capable of binding to the CAR/retinoid X receptor alpha (RXRα) heterodimer. The three elements comprise the two elements indicated above and one located at -146/-141. A chromatin immunoprecipitation assay confirmed CAR binding to the region including these elements in chromatin after treatment with TCPOBOP. These results indicate that mouse Cyp1a2 is the direct target of CAR, and binding of the CAR/RXRα heterodimer to the newly identified region in the promoter may be involved in transcriptional activation. Binding motifs were estimated as ER1 (everted repeat with a spacing of 1 nucleotide, -160/-155 and -153/-148) and ER8 (everted repeat with a spacing of 8 nucleotides, formed with -160/-155 and -146/-141).

Dioxin Disrupts Thyroid Hormone and Glucocorticoid Induction of klf9, a Master Regulator of Frog Metamorphosis

Frog metamorphosis, the development of an air-breathing froglet from an aquatic tadpole, is controlled by thyroid hormone (TH) and glucocorticoids (GC). Metamorphosis is susceptible to disruption by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an aryl hydrocarbon receptor (AHR) agonist. Krüppel-like factor 9 (klf9), an immediate early gene in the endocrine-controlled cascade of expression changes governing metamorphosis, can be synergistically induced by both hormones. This process is mediated by an upstream enhancer cluster, the klf9 synergy module (KSM). klf9 is also an AHR target. We measured klf9 mRNA following exposures to triiodothyronine (T3), corticosterone (CORT), and TCDD in the Xenopus laevis cell line XLK-WG. klf9 was induced 6-fold by 50 nM T3, 4-fold by 100 nM CORT, and 3-fold by 175 nM TCDD. Cotreatments of CORT and TCDD or T3 and TCDD induced klf9 7- and 11-fold, respectively, whereas treatment with all 3 agents induced a 15-fold increase. Transactivation assays examined enhancers from the Xenopus tropicalis klf9 upstream region. KSM-containing segments mediated a strong T3 response and a larger T3/CORT response, whereas induction by TCDD was mediated by a region ∼1 kb farther upstream containing 5 AHR response elements (AHREs). This region also supported a CORT response in the absence of readily identifiable GC responsive elements, suggesting mediation by protein-protein interactions. A functional AHRE cluster is positionally conserved in the human genome, and klf9 was induced by TCDD and TH in HepG2 cells. These results indicate that AHR binding to upstream AHREs represents an early key event in TCDD's disruption of endocrine-regulated klf9 expression and metamorphosis.