Dexamethasone induces pregnane X receptor and retinoid X receptor-alpha expression in human hepatocytes: synergistic increase of CYP3A4 induction by pregnane X receptor activators

In this report we show that submicromolar concentrations of dexamethasone enhance pregnane X receptor (PXR) activator-mediated CYP3A4 gene expression in cultured human hepatocytes. Because this result is only observed after 24 h of cotreatment and is inhibited by pretreatment with cycloheximide, we further investigated which factor(s), induced by dexamethasone, might be responsible for this effect. We report that dexamethasone increases both retinoid X receptor-alpha (RXRalpha) and PXR mRNA expression in cultured human hepatocytes, whereas PXR activators such as rifampicin and clotrimazole do not. Accumulation of RXRalpha and PXR mRNA reaches a maximum at a concentration of 100 nM dexamethasone after treatment for 6 to 12 h and is greatly diminished by RU486. A similar pattern of expression is observed with tyrosine aminotransferase mRNA. Moreover, the effect of dexamethasone on PXR mRNA accumulation seems to be through direct action on the glucocorticoid receptor (GR) because the addition of cycloheximide has no effect, and dexamethasone does not affect the degradation of PXR mRNA. Furthermore, dexamethasone induces the accumulation of a RXRalpha-immunoreactive protein and increases the nuclear level of RXRalpha:PXR heterodimer as shown by gel shift assays with a CYP3A4 ER6 PXRE probe. This accumulation of latent PXR and RXRalpha in the nucleus of hepatocytes explains the synergistic effect observed with dexamethasone and PXR activators together on CYP3A4 induction. These results reveal the existence of functional cross talk between the GR and PXR, and may explain some controversial aspects of the role of the GR in CYP3A4 induction.