The nuclear pregnane X receptor regulates xenobiotic detoxification.

The pregnane X receptor (PXR), which is a member of the nuclear receptor family of ligand-activated transcription factors, is an integral component of the body's defense mechanism against toxic xenobiotics. PXR is activated by a broad spectrum of lipophilic xenobiotics including prescription drugs, herbs, pesticides, endocrine disruptors and other environmental contaminants. The promiscuous ligand-binding properties of PXR are facilitated by the large volume and smooth shape of its ligand-binding pocket. PXR binds to DNA as a heterodimer with the 9-cis retinoic acid receptor (RXR) and regulates a large number of genes involved in the detoxification and excretion of toxic substances. Although PXR evolved to protect the body, its activation by various prescription drugs and herbs such as St. John's wort represents the molecular basis for an important class of drug-drug interactions. Assays that detect PXR activation can now be used to predict and prevent these drug-drug interactions.