Dexamethasone enhances constitutive androstane receptor expression in human hepatocytes: consequences on cytochrome P450 gene regulation.
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.
Pascussi
J M
JM
Institut National de la Santé et de la Recherche Médicale U128-IFR24, Centre National de la Recherche Scientifique, Montpellier, France.
Gerbal-Chaloin
S
S
Fabre
J M
JM
Maurel
P
P
Vilarem
M J
MJ
D021581:Active Transport, Cell Nucleus / Q000187:drug effects; D001189:Aryl Hydrocarbon Hydroxylases; D002478:Cells, Cultured; D000090702:Constitutive Androstane Receptor; D065702:Cytochrome P-450 CYP2B6; D065727:Cytochrome P-450 CYP2C8; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis / Q000235:genetics; D003907:Dexamethasone / Q000494:pharmacology*; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects*; D022781:Hepatocytes / Q000187:drug effects* / Q000502:physiology; D006801:Humans; D008099:Liver / Q000378:metabolism; D006899:Mixed Function Oxygenases / Q000096:biosynthesis / Q000235:genetics; D010089:Oxidoreductases, N-Demethylating / Q000096:biosynthesis / Q000235:genetics; D010634:Phenobarbital / Q000494:pharmacology; D012333:RNA, Messenger / Q000096:biosynthesis / Q000187:drug effects; D018160:Receptors, Cytoplasmic and Nuclear / Q000096:biosynthesis* / Q000235:genetics; D011965:Receptors, Glucocorticoid / Q000502:physiology; D039181:Steroid 16-alpha-Hydroxylase; D013250:Steroid Hydroxylases / Q000096:biosynthesis / Q000235:genetics; D014157:Transcription Factors / Q000096:biosynthesis* / Q000235:genetics; D015533:Transcriptional Activation
D000090702:Constitutive Androstane Receptor; C000716327:NR1I3 protein, human; D012333:RNA, Messenger; D018160:Receptors, Cytoplasmic and Nuclear; D011965:Receptors, Glucocorticoid; D014157:Transcription Factors; D003907:Dexamethasone; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D013250:Steroid Hydroxylases; D001189:Aryl Hydrocarbon Hydroxylases; C585599:CYP2B6 protein, human; C450259:CYP2C8 protein, human; C104464:CYP3A protein, human; D065702:Cytochrome P-450 CYP2B6; D065727:Cytochrome P-450 CYP2C8; D051544:Cytochrome P-450 CYP3A; D039181:Steroid 16-alpha-Hydroxylase; C510163:CYP3A4 protein, human; D010089:Oxidoreductases, N-Demethylating; D010634:Phenobarbital