Static and dynamic insights into the function of the human nuclear xenobiotic receptor PXR

The nuclear xenobiotic receptor PXR is a highly promiscuous protein that binds to a spectrum of structurally distinct endogenous compounds and clinical drugs and regulates the genes that express a variety of metabolizing and transport enzymes. An examination of the mobile regions novel to the nuclear receptor ligand binding domain fold elucidate their role in PXR's ability to respond to a variety of small and large agonists.;Unpredictable PXR activation can mediate a number of drug-drug interactions that can lead to decreases in therapeutic efficacy and multi-drug resistance. We show that colupulone from hops induces the expression of numerous drug metabolism and excretion genes in a PXR dependent manner. The 2.8 A crystal structure of colupulone in complex with the ligand binding domain of PXR elucidated the hydrophobic and hydrogen bonding contacts involved in colupulone binding. Docking of other bitter acids onto the colupulone structure indicates a similar binding conformation for the other analogues.;From static methods, we investigated the molecular dynamics of the monomer and dimer form of PXR bound to the retinoid X receptor to determine how differences in motion affect their differential ability to bind to coactivator. Simulations data reveal highly coherent motions for the dimer especially in the activation function (AF) domain, but only weak correlated or uncorrelated motion for the monomer. Simulations of active forms of other nuclear receptors also show highly correlated motion between the helices in the AF-region. Coherence in the AF-region may be a defining characteristic of functionally active nuclear receptors.;We explored PXR inhibition strategies using ketoconazole. We show through a number of mutations at the AF-surface of the PXR ligand-binding domain that ketoconazole may be interacting with specific residues in the AF-region. We also show initial developments in designing a high throughput screening method for testing out inhibitors using fluorescence polarization.;Overall, this research represents static and dynamic insights into PXR function that can be used to develop strategies to improve the clinical efficacy of drugs and reduce unfavorable drug-drug interactions.