CITCO activates hPXR in HepG2 cells stably expressing FLAG-hPXR-WT and CYP3A4-luciferase. (A) Dose-response curves for CITCO, RIF, and SPA70. (B) Dose-response curves for CITCO in the presence of 0, 100, 500 nM, 1, 2, or 10 µM SPA70. (C) Dose-response curves for rifampicin (RIF) in the presence of 0, 100, 500 nM, 1, 2, or 10 µM SPA70. (D–F) Cytotoxicity assays corresponding to (A–C), respectively. The activity of the hPXR-regulated CYP3A4-luciferase is used to measure the activation of hPXR and is expressed as “CYP3A4 Promoter Activity,” in relative luciferase unit (RLU) as described in Materials and Methods. “%Relative Cytotoxicity” is calculated as described in Materials and Methods. Results are expressed as the means ± S.D. of three independent experiments performed in quadruplicate.