Nuclear hormone receptors (NRs) are ligand-activated intracellular transcription factors that have profound effects on development, homeostasis and detoxification. Ligands that are recognized by NRs include: endogenous hormones, vitamins A and D, drugs an ...d xenobiotic compounds. Several NRs, called orphan receptors, have no known endogenous ligands. Their low affinity for fatty acids, bile acids and/or sterols (e.g., FXR, LXR and PPAR) suggests that they function as metabolic sensors. Other NRs act as xenobiotic sensors by up-regulating the expression of cytochrome P450 enzymes that are xenobiotic metabolizers. NRs function with co-activators and co-repressors to up- or down-regulate the expression of genes and can regulate the expression of a large number of genes. The NR family is highly conserved in vertebrates. They have a modular domain structure consisting of a DNA-binding domain and a ligand-binding domain. Other domains include a modulatory A/B domain, the hinge D domain, and a variable C-terminal F domain that is missing in some NRs.
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