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 i s a member of the nuclear receptor family (NR1) mostly expressed in the liv er, which heterodimerizes with retinoid X receptor (RXR) and was shown to t ransactivate both the phenobarbital responsive element module of the human CYP2B6 gene and the CYP3A4 xenobiotic response element. Because previous st udies in rodent hepatocyte cultures have shown that the phenobarbital-media ted induction of CYP2B genes is potentiated by glucocorticoids, we examined the role of activated glucocorticoid receptor in this process. We show tha t submicromolar concentrations of dexamethasone enhance phenobarbital-media ted induction of CYP3A4, CYP2B6, and CYP2C8 mRNA in cultured human hepatocy tes. In parallel, we observed that glucocorticoid agonists, such as dexamet hasone, prednisolone, or hydrocortisone, specifically increase human car (h CAR) mRNA expression. Accumulation of hCAR mRNA parallels that of tyrosine aminotransferase: both mRNAs reach a maximum at a concentration of 100 nM d examethasone and are downregulated by concomitant treatment with the glucoc orticoid antagonist RU486. Moreover, the effect of dexamethasone on hCAR mR NA accumulation appears to be of transcriptional origin because the additio n of protein synthesis inhibitor cycloheximide has no effect, and dexametha sone does not affect the degradation of hCAR mRNA. Furthermore, dexamethaso ne 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 effe ct of this glucocorticoid on phenobarbital-mediated induction of CYP2B gene s and the controversial role of the glucocorticoid receptor on phenobarbita l-mediated CYP gene inductions.