Dexamethasone Enhances Constitutive Androstane Receptor Expression in Human Hepatocytes: Consequences on Cytochrome P450 Gene Regulation
- Jean-Marc Pascussi1,1,
- Sabine Gerbal-Chaloin1,1,
- Jean-Michel Fabre2,
- Patrick Maurel1 and
- Marie-José Vilarem1
- 1Institut National de la Santé et de la Recherche Médicale U128-IFR24, Centre National de la Recherche Scientifique, Montpellier, France (J.-M.P., S.G.-C., P.M., M.-J.V.) and 2Service de Chirurgie, Hopital Saint Eloi, Montpellier, France (J.-M.F.)
Abstract
The barbiturate phenobarbital induces the transcription of cytochromes P450 (CYPs) 2B through the constitutive androstane receptor (CAR; NR1I3). CAR is a member of the nuclear receptor family (NR1) mostly expressed in the liver, which heterodimerizes with retinoid X receptor (RXR) and was shown to transactivate both the phenobarbital responsive element module of the human CYP2B6 gene and the CYP3A4 xenobiotic response element. Because previous studies in rodent hepatocyte cultures have shown that the phenobarbital-mediated induction of CYP2B genes is potentiated by glucocorticoids, we examined the role of activated glucocorticoid receptor in this process. We show that submicromolar concentrations of dexamethasone enhance phenobarbital-mediated induction of CYP3A4, CYP2B6, and CYP2C8 mRNA in cultured human hepatocytes. In parallel, we observed that glucocorticoid agonists, such as dexamethasone, prednisolone, or hydrocortisone, specifically increase human car (hCAR) mRNA expression. Accumulation of hCAR mRNA parallels that of tyrosine aminotransferase: both mRNAs reach a maximum at a concentration of 100 nM dexamethasone and are down-regulated by concomitant treatment with the glucocorticoid antagonist RU486. Moreover, the effect of dexamethasone on hCAR mRNA accumulation appears to be of transcriptional origin because the addition of protein synthesis inhibitor cycloheximide has no effect, and dexamethasone does not affect the degradation of hCAR mRNA. Furthermore, dexamethasone increases both basal and phenobarbital-mediated nuclear translocation of CAR immunoreactive protein in human hepatocytes. The up-regulation of CAR mRNA and protein in response to dexamethasone explains the synergistic effect of this glucocorticoid on phenobarbital-mediated induction of CYP2B genes and the controversial role of the glucocorticoid receptor on phenobarbital-mediated CYP gene inductions.
Footnotes
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Send reprint requests to: Dr. Marie-Jose Vilarem, INSERM U128-IFR24, Centre National de la Recherche Scientifique, 1919 Route de Mende, 34293 Montpellier Cedex 05, France.
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↵1 These two authors have equally contributed to this work.
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This work was supported by grants from the Institut National de la Santé et de la Recherche Médicale, the Ligue Nationale contre le Cancer (J.M.P.), and Laboratoires Fournier (S.G.-C.).
- Abbreviations:
- CYP
- cytochrome P450s
- PXR
- pregnane X receptor
- CAR
- constitutive androstane receptor or constitutively activated receptor
- hCAR
- human CAR
- mCAR
- mouse CAR
- DMSO
- dimethyl sulfoxide
- PCR
- polymerase chain reaction
- LBD
- ligand-binding domain
- DBD
- DNA-binding domain
- TAT
- tyrosine aminotransferase
- GR
- glucocorticoid receptor
- GRE
- glucocorticoid responsive element
- bp
- base pairs
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- Received June 29, 2000.
- Accepted August 31, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
