The human pregnane X receptor is a ligand-dependent transcription factor that plays critical roles in regulating detoxification genes such as CYP3A4 by recruiting transcriptional coactivators such as steroid receptor coactivator-1 in a ligand-dependent manner. In a previous study (Pharmacogenetics and Genomics 2005, 15: 337–341), we reported a novel pregnane X receptor single nucleotide polymorphism, Q158K, which impaired transactivation of CYP3A4.
Methods and results
By using DNA affinity precipitation assay and electrophoretic mobility shift assay, we have now shown that Q158K does not alter the binding affinity of pregnane X receptor for the CYP3A4 promoter. Instead, as shown using a mammalian two-hybrid assay, it decreased the interaction of pregnane X receptor with steroid receptor coactivator-1 in the presence of rifampin, clotrimazole, paclitaxel, or nifedipine but not in their absence. Rifampin treatment markedly increased pregnane X receptor protein in the wild-type pregnane X receptor-transfected cells as shown by coimmunoprecipitation but not in Q158K pregnane X receptor-transfected cells. The impaired transactivation of the CYP3A4 promoter was reversed by transfecting steroid receptor coactivator-1 expression plasmids. An additional nine pregnane X receptor variants were isolated and selected by random mutagenesis. Mutations Q158, W223, F257, I346, and L424 also reduced CYP3A4 transactivation and interaction in mammalian two-hybrid assays only in the presence of ligands. Although Q158K did not greatly affect the interaction of pregnane X receptor to silencing mediator of retinoid and thyroid hormone receptor, pregnane X receptor-silencing mediator of retinoid and thyroid hormone receptor interaction was impaired in the F257L and I346T variants with or without the presence of pregnane X receptor ligands.
Our data indicate that the impaired induction by the Q158K variant is probably due to defective steroid receptor coactivator-1 interaction in the presence of a pregnane X receptor ligand. As the whole ligand binding domain of pregnane X receptor is required for the interaction with steroid receptor coactivator-1, we propose that, not only the Q158K variant found in Chinese, but also in native pregnane X receptor variants in other ethnic groups (D163G, A370T, R381W, and I403V) affect CYP3A4 induction by altering steroid receptor coactivator-1 recruitment.