Species differences in the induction and activation of the pregnane X receptor (PXR)

Abstract

The pregnane X receptor (PXR) is regarded as the master regulator of the mammalian xenobiotic response. It has evolved the ability to respond to a variety of chemically unrelated endogenous and exogenous compounds, and regulate a large repertoire of genes involved in their uptake, metabolism and excretion, such as the cytochrome P450 (CYP) 3A gene family. Early studies on the regulation of CYP3A genes highlighted species-specific differences in response to various xenobiotics; upon discovery of PXR, it was hypothesised that the species origin of the receptor dictates the species-specific pattern of CYP3A inducibility. The aims of this study were two-fold; first, to determine the molecular mechanisms underlying the regulation of the PXR genes in humans, primates and rodents; and second, to determine potential differences in the ligand binding domain of PXRs from diverse species, in order to assess their impacts on the species differences of response to changes in chemical flux. Using in vitro reporter gene assays, it was demonstrated that human and rat PXR proximal promoters contain regions of positive and negative regulation, suggesting complex mechanisms of regulation in both species. Further analysis, demonstrated that basal expression is regulated by hepatocyte nuclear factors (HNF) 3? and 4a in an identical manner. Exposing the cell lines to various chemicals identified that species- specific PXR gene activation is dependent on the nature of the PXR promoter, the abundance/ratio and ligand affinity of ligand-activated transcription factors within the host cell. I have also demonstrated that the glucocorticoid receptor (GR?) mediates an increase in human PXR gene expression following treatment with clotrimazole; the same transcription factor also appears to regulate the rat PXR gene. Hence, clotrimazole-mediated GR? induction of PXR genes appears to be a cross-species event. In silica analysis of PXR ligand binding domains from diverse species identified that a species-specific activation profile may be attributed to both the spatial geometry of the ligand binding pocket and the amino acid residues lining the walls of the pocket. Taken together, I conclude that species differences in response to changes in the chemical flux are determined by PXR at both the genomic level and proteomic level.