The phytoestrogen coumestrol is a naturally occurring antagonist of the human pregnane X receptor

Mol Endocrinol. 2008 Apr;22(4):838-57. doi: 10.1210/me.2007-0218. Epub 2007 Dec 20.


Antagonizing the action of the human nuclear xenobiotic receptor pregnane X receptor (PXR) may have important clinical implications in preventing drug-drug interactions and improving therapeutic efficacy. We provide evidence that a naturally occurring phytoestrogen, coumestrol, is an antagonist of the nuclear receptor PXR (NR1I2). In transient transfection assays, coumestrol was able to suppress the agonist effects of SR12813 on human PXR activity. PXR activity was assessed and correlated with effects on the metabolism of the anesthetic tribromoethanol and on gene expression in primary human hepatocytes. We found that coumestrol was able to suppress the effects of PXR agonists on the expression of the known PXR target genes, CYP3A4 and CYP2B6, in primary human hepatocytes as well as inhibit metabolism of tribromoethanol in humanized PXR mice. Coumestrol at concentrations above 1.0 microm competed in scintillation proximity assays with a labeled PXR agonist for binding to the ligand-binding cavity. However, mammalian two-hybrid assays and transient transcription data using ligand-binding-cavity mutant forms of PXR show that coumestrol also antagonizes coregulator recruitment. This effect is likely by binding to a surface outside the ligand-binding pocket. Taken together, these data imply that there are antagonist binding site(s) for coumestrol on the surface of PXR. These studies provide the basis for development of novel small molecule inhibitors of PXR with the ultimate goal of clinical applications toward preventing drug-drug interactions.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Aryl Hydrocarbon Hydroxylases / genetics
  • Aryl Hydrocarbon Hydroxylases / metabolism
  • Cell Line
  • Cells, Cultured
  • Constitutive Androstane Receptor
  • Coumestrol / chemistry
  • Coumestrol / metabolism
  • Coumestrol / pharmacology*
  • Cytochrome P-450 CYP2B6
  • Cytochrome P-450 CYP3A / genetics
  • Cytochrome P-450 CYP3A / metabolism
  • Ethanol / analogs & derivatives
  • Ethanol / metabolism
  • Female
  • Gene Expression / drug effects
  • Histone Acetyltransferases / genetics
  • Histone Acetyltransferases / metabolism
  • Humans
  • Immunohistochemistry
  • Mass Spectrometry
  • Mice
  • Mice, Knockout
  • Mice, Transgenic
  • Microscopy, Confocal
  • Nuclear Receptor Coactivator 1
  • Oxidoreductases, N-Demethylating / genetics
  • Oxidoreductases, N-Demethylating / metabolism
  • Phytoestrogens / chemistry
  • Phytoestrogens / metabolism
  • Phytoestrogens / pharmacology*
  • Pregnane X Receptor
  • Protein Binding
  • Receptors, Cytoplasmic and Nuclear / genetics
  • Receptors, Cytoplasmic and Nuclear / metabolism
  • Receptors, Steroid / antagonists & inhibitors*
  • Receptors, Steroid / genetics
  • Receptors, Steroid / metabolism
  • Transcription Factors / genetics
  • Transcription Factors / metabolism
  • Two-Hybrid System Techniques


  • Constitutive Androstane Receptor
  • NR1I2 protein, human
  • Nr1i2 protein, mouse
  • Phytoestrogens
  • Pregnane X Receptor
  • Receptors, Cytoplasmic and Nuclear
  • Receptors, Steroid
  • Transcription Factors
  • tribromoethanol
  • Ethanol
  • Aryl Hydrocarbon Hydroxylases
  • CYP2B6 protein, human
  • Cytochrome P-450 CYP2B6
  • Cytochrome P-450 CYP3A
  • CYP3A4 protein, human
  • Oxidoreductases, N-Demethylating
  • Histone Acetyltransferases
  • NCOA1 protein, human
  • Ncoa1 protein, mouse
  • Nuclear Receptor Coactivator 1
  • Coumestrol