Two-Stage Glucocorticoid Induction of CYP3A23through Both the Glucocorticoid and Pregnane X Receptors
- Department of Oncology and the Environmental Toxicology Program, McArdle Laboratory for Cancer Research, University of Wisconsin, Madison, Wisconsin
Abstract
Glucocorticoid inducibility of the CYP3A23 gene is conferred by a multisite unit comprising binding sites for several members of the nuclear receptor superfamily of transcription factors, including the chicken ovalbumin upstream promoter-transcription factor COUP-TF, pregnane X receptor (PXR), and hepatocyte nuclear factor 4 (HNF-4). The presence of three binding sites, each of which interacts with more than one factor, contributes to the complexity of the CYP3A23 glucocorticoid-responsive region. Despite the glucocorticoid sensitivity of this gene, direct binding of ligand-activated glucocorticoid receptor (GR) to theCYP3A23 dexamethasone-responsive region (DexRE) is not required for induction. This study demonstrates that DexRE-2 is the key element within the CYP3A23 proximal promoter, conferring ligand sensitivity via its interaction with the PXR/RXRα heterodimer. The DexRE-1 and HNF-4 sites are not ligand-responsive, but are essential accessory elements required for full promoter inducibility. In addition to ligand-mediated activation of PXR, the overall induction response involves a GR-mediated stimulation of PXR and RXRα expression. Hence, the induction pathway can be divided into two stages. In stage one, maximal induction requires a GR-dependent increase in PXR and RXRα expression, and stage two is characterized by direct transcriptional activation of CYP3A23, which is dependent on ligand-activated PXR as well as accessory factors bound at the DexRE-1 and HNF-4 sites. Because multiple proteins bind at each element within the glucocorticoid-responsive region, factors not contributing to ligand responsiveness, such as chicken ovalbumin upstream promoter-transcription factor, may modulate the response through competitive interactions.
Footnotes
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Dr. Charles B. Kasper, Department of Oncology, McArdle Laboratory for Cancer Research, 1400 University Ave., Madison, WI 53706. E-mail: kasper{at}oncology.wisc.edu
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↵1 Present address: Center for Cardiovascular Research, Department of Internal Medicine, Washington University School of Medicine, Box 8086, 660 S. Euclid, St. Louis, MO 63110.
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This work was supported by National Institutes of Health Grants CA22484 and CA0920. J.M.H. was supported by National Institutes of Health Grant T32-CA-09135.
- Abbreviations:
- CYP
- cytochrome P450
- CMV
- cytomegalovirus
- COUP-TF
- chicken ovalbumin upstream promoter-transcription factor
- CYP3A
- cytochrome P450 3A subfamily
- ds
- double-stranded
- DexRE-1
- dexamethasone response element 1
- DexRE-2
- dexamethasone response element 2
- FCS
- fetal calf serum
- GR
- glucocorticoid receptor
- HNF-4
- hepatocyte nuclear factor 4
- Me2SO
- dimethyl sulfoxide
- MMTV
- mouse mammary tumor virus
- PCN
- pregnenolone 16α-carbonitrile
- PXR
- pregnane X receptor
- RLU
- relative light units
- SV
- simian virus
- TK
- thymidine kinase
- RXR
- retinoid X receptor
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- Received December 20, 1999.
- Accepted March 20, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
