Cytochrome P450 Regulation by α-Tocopherol in Pxr-Null and PXR-Humanized Mice

The pregnane X receptor (PXR) has been postulated to play a role in the metabolism of alpha-tocopherol owing to the up-regulation of hepatic cytochrome P450 (P450) 3A in human cell lines and murine models after alpha-tocopherol treatment. However, in vivo studies confirming the role of PXR in alpha-tocopherol metabolism in humans presents significant difficulties and has not been performed. PXR-humanized (hPXR), wildtype, and Pxr-null mouse models were used to determine whether alpha-tocopherol metabolism is influenced by species-specific differences in PXR function in vivo. No significant difference in the concentration of the major alpha-tocopherol metabolites was observed among the hPXR, wild-type, and Pxr-null mice through mass spectrometry-based metabolomics. Gene expression analysis revealed significantly increased expression of Cyp3a11 as well as several other P450s only in wild-type mice, suggesting species-specificity for alpha-tocopherol activation of PXR. Luciferase reporter assay confirmed activation of mouse PXR by alpha-tocopherol. Analysis of the Cyp2c family of genes revealed increased expression of Cyp2c29, Cyp2c37, and Cyp2c55 in wild-type, hPXR, and Pxr-null mice, which suggests PXR-independent induction of Cyp2c gene expression. This study revealed that alpha-tocopherol is a partial agonist of PXR and that PXR is necessary for Cyp3a induction by alpha-tocopherol. The implications of a novel role for alpha-tocopherol in Cyp2c gene regulation are also discussed.