Two-stage glucocorticoid induction of CYP3A23 through both the glucocorticoid and pregnane X receptors

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 upstr eam promoter-transcription factor COUP-TF, pregnane X receptor (PXR), and h epatocyte nuclear factor 4 (HNF-4). The presence of three binding sites, ea ch of which interacts with more than one factor, contributes to the complex ity of the CYP3A23 glucocorticoid-responsive region. Despite the glucocorti coid sensitivity of this gene, direct binding of ligand-activated glucocort icoid receptor (GR) to the CYP3A23 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 sensiti vity via its interaction with the PXR/RXR alpha heterodimer. The DexRE-1 an d HNF-4 sites are not ligand-responsive, but are essential accessory elemen ts required for full promoter inducibility. In addition to ligand-mediated activation of PXR, the overall induction response involves a GR-mediated st imulation of PXR and RXR alpha expression. Hence, the induction pathway can be divided into two stages. In stage one, maximal induction requires a GR- dependent increase in PXR and RXR alpha expression, and stage two is charac terized 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 respon siveness, such as chicken ovalbumin upstream promoter-transcription factor, may modulate the response through competitive interactions.