A systematic analysis of predicted phosphorylation sites within the human pregnane X receptor protein

The pregnane X receptor (PXR, NR1I2) regulates the expression of genes that encode drug-metabolizing enzymes and drug transporter proteins in liver and intestine. Understanding the molecular mechanisms that modulate PXR activity is therefore critical for the development of effective therapeutic strategies. Several recent studies have implicated the activation of kinase signaling pathways in the regulation of PXR biological activity, although direct evidence and molecular mechanisms are currently lacking. We therefore sought to characterize potential phosphorylation sites within the PXR protein by use of a rational, comprehensive, and systematic site-directed mutagenesis approach to generate phosphomimetic mutations (Ser/Thr --> Asp) and phospho-deficient mutations (Ser/Thr --> Ala) at 18 predicted consensus kinase recognition sequences in the human PXR protein. Here, we identify amino acid residues Ser8, Thr57, Ser208, Ser305, Ser350, and Thr408 as being critical for biological activity of the PXR protein. Mutations at positions 57 and 408 abolish ligand-inducible PXR activity. Mutations in the extreme N terminus and in the PXR ligand-binding domain at positions Ser8, Ser305, Ser350, and Thr408 decrease the ability of PXR to form heterodimers with retinoid X receptor alpha. Mutations at positions Ser208, Ser305, Ser350, and Thr408 alter PXR-protein cofactor interactions. Finally, the subcellular localization of the PXR protein is profoundly affected by mutations at position Thr408. These data suggest that PXR activity can potentially be regulated by phosphorylation at specific amino acid residues within several predicted consensus kinase recognition sequences to differentially affect PXR biological activity.