Concentration-response curves of activation of PXRs and VDRs by endogenous ligands or their analogs. The ordinate represents activation of the PXR or VDR, relative to vehicle control, and normalized to the maximal activator (rifampicin for human PXR, 5α-androst-3α-ol for zebrafish PXR, n-butyl-p-aminobenzoate for Xenopus laevis BXRα, n-propyl-p-hydroxybenzoate for Xenopus laevis BXRβ, and calcitriol for human and sea lamprey VDRs; see Materials and Methods for more details). The drugs tested were pregnenolone (●), petromyzonol sulfate (sea lamprey bile salt; ○), scymnol sulfate (cartilaginous fish bile salt; □), 3-ketolithocholic acid (mammalian bile acid metabolite; △), n-propyl-p-hydroxybenzoate (▲), and 1,25-(OH)2-vitamin D3 (calcitriol; ■). (A) Human PXR is activated by micromolar concentrations of the steroid pregenonolone, the early bile salts petromyzonol sulfate and scymnol sulfate, 3-ketolithocholic acid, and the benzoate analog. Calcitriol does not activate human PXR. (B) Similar to human PXR, zebrafish PXR is activated by the steroid pregnenolone, the cartilaginous fish bile salt scymnol sulfate, and the benzoate analog, but not by the other compounds. (C, D) The Xenopus laevis BXRs do not share any ligands with human and zebrafish PXRS other than the benzoate analog n-propyl-p-hydroxybenzoate, which activates BXRα and BXRβ robustly. (E, F) Human and sea lamprey VDRs are both activated robustly by nanomolar concentrations of calcitriol. Similar to human PXR, 3-ketolithocholic acid activates human VDR at micromolar concentrations, with an efficacy of only 15% relative to calcitriol. Sea lamprey was not activated at all by 3-ketolithocholic acid. Weak concentration-dependent activation of sea lamprey VDR by petromyzonol sulfate (○) was observed; however, the efficacy of this bile salt was only ~5–6% relative to that of calcitriol. In panels (A, E, F), full-length receptors for human PXR, human VDR, and sea lamprey VDR were used, with the reporter plasmid being CYP3A4-PXRE-Luc. In panels (B, C, D), GAL4-LBD fusion constructs were used for zebrafish PXR and the Xenopus laevis BXRs, with the reporter plasmid being tk-UAS-Luc.