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Regulation of the rat UGT1A6 by glucocorticoids involves a cryptic glucocorticoid response element.
Regulation of the rat UGT1A6 by glucocorticoids involves a cryptic glucocorticoid response element.
- Source :
-
Drug metabolism and disposition: the biological fate of chemicals [Drug Metab Dispos] 2008 Feb; Vol. 36 (2), pp. 409-17. Date of Electronic Publication: 2007 Nov 26. - Publication Year :
- 2008
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Abstract
- Glucocorticoids precociously induce fetal rat UGT1A6 and potentiate polycyclic aromatic hydrocarbon (PAH)-dependent induction of this enzyme in vivo and in isolated rat hepatocytes. To establish whether induction was due to glucocorticoid receptor (GR), luciferase reporter vectors were tested in transfection assays with HepG2 cells. Using a reporter construct containing approximately 2.26 kilobases of the 5'-flanking region of the UGT1A6-noncoding leader exon (A1*), dexamethasone increased basal activity 3- to 7-fold in cells cotransfected with an expression plasmid for GR. PAH increased gene expression 23-fold, but the presence of dexamethasone only induced PAH-dependent expression by 1.5-fold, suggesting interaction between GR and the aryl hydrocarbon (Ah) receptor. Furthermore, the GR antagonist RU 38486 [17beta-hydroxy-11beta-(4-dimethylamino-phenyl)-17alpha-(prop-1-ynyl)-estra-4,9-dien-3-one] was a partial agonist that increased, rather than inhibited, basal activity 3-fold. 5'-deletion analysis defined the 5'-boundary for a functional glucocorticoid-responsive unit between base pairs -141 and -118 relative to the transcription start site. This region contains the Ah receptor response element (AhRE), and both PAH and glucocorticoid-dependent gene activation were lost when this area was deleted. Mutation of a single base pair located in the AhRE region simultaneously reduced induction by PAH and increased glucocorticoid induction. Thus, the sequences of both the AhRE and glucocorticoid response elements seem to overlap, suggesting that Ah receptor binding may decrease glucocorticoid-dependent induction due to interactions of these two cis-acting elements. Mutation of a putative GRE located between base pair -81 and -95 reduced, but did not completely eliminate, glucocorticoid-dependent induction of the reporter, suggesting that a nonclassic mechanism of induction is involved in this response.
- Subjects :
- Animals
Benzo(a)pyrene pharmacology
Cell Line, Tumor
Dexamethasone pharmacology
Gene Expression
Genes, Reporter
Glucuronosyltransferase metabolism
Humans
Luciferases metabolism
Mifepristone pharmacology
Peroxisome Proliferator-Activated Receptors metabolism
Rats
Receptors, Aryl Hydrocarbon metabolism
Receptors, Glucocorticoid metabolism
Transfection
Glucocorticoids pharmacology
Glucuronosyltransferase genetics
Receptors, Aryl Hydrocarbon genetics
Receptors, Glucocorticoid genetics
Response Elements
Subjects
Details
- Language :
- English
- ISSN :
- 1521-009X
- Volume :
- 36
- Issue :
- 2
- Database :
- MEDLINE
- Journal :
- Drug metabolism and disposition: the biological fate of chemicals
- Publication Type :
- Academic Journal
- Accession number :
- 18039810
- Full Text :
- https://doi.org/10.1124/dmd.107.018952