Fig. 4.

PB selectively activates hPXR but not mPXR by increasing the interaction of hPXR with coactivator SRC-1. HepG2 cells were transfected with hPXR (A) or mPXR (B) in the presence of CYP3A4/PXR response element/xenobiotic-responsive enhancer module or tk-Cyp3a23-Luc reporter constructs. Transfected cells were then treated with 0.1, 0.5, and 1 mM PB for 24 hours. RIF (10 µM) and PCN (25 µM) were used as positive controls for hPXR and mPXR, respectively. Luciferase activities were determined and expressed relative to vehicle control (0.1% DMSO). Mammalian two-hybrid assays were performed in HepG2 cells transiently transfected with the reporter gene plasmid pG5luc and expression plasmids encoding GAL4-DBD/SRC-1 in the presence of VP16-hourPXR (C) or VP16-mPXR (D) fusion proteins. Cells were treated with 0.1% DMSO, 0.5 and 1 mM PB, 10 µM RIF, or 25 µM PCN for an additional 24 hours before determination of luciferase activities. Three independent measures from each treatment were analyzed and are expressed as mean ± S.D. (**P < 0.01; ***P < 0.001).