Estrogens and antiestrogens activate hPXR

The pregnane X receptor (PXR, NR1I2) and the estrogen receptors (ER alpha, NR3A1 and ER beta, NR3A2) bind a large number of compounds, including environmental pollutants and drugs, which exhibit remarkably diverse structural features. This prompted us to investigate if ER ligands could be PXR activators. We focused our attention on known estrogens from various chemical classes: physiological and synthetic estrogens and antiestrogens, plant and fungus estrogens, and other man-made chemicals belonging to phthalate plasticizers, surfactant-derived alkylphenols and cosmetics. Altogether, nearly 50 compounds were thus analyzed for their ability to activate human PXR in stably transfected cells, HGPXR cells, derived from HeLa cells and expressing luciferase under the control of a chimeric hPXR. Some of the newly identified hPXR activators were also checked for their ability to induce cytochrome P450 3A4 and 2136 expressions in a primary culture of human hepatocytes. A significant proportion (54%) of compounds with estrogenic activity or able to bind ER were found to be hPXR activators: in particular, antiestrogens, mycoestrogens and phthalates. An even greater proportion is observed if estrogenic pesticides are included. Altogether, these results raise the question of the meaning and consequences of compounds with double PXR/ER activation ability. (C) 2007 Elsevier Ireland Ltd. All rights reserved.