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

Abstract

The pregnane X receptor (PXR) has been postulated to play a role in the metabolism of a-tocopherol owing to the up-regulation of hepatic cytochrome P450 (P450) 3A in human cell lines and murine models after a-tocopherol treatment. However, in vivo studies confirming the role of PXR in a-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 a-tocopherol metabolism is influenced by species-specific differences in PXR function in vivo. No significant difference in the concentration of the major a-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 a-tocopherol activation of PXR. Luciferase reporter assay confirmed activation of mouse PXR by a-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 a-tocopherol is a partial agonist of PXR and that PXR is necessary for Cyp3a induction by a-tocopherol. The implications of a novel role for a-tocopherol in Cyp2c gene regulation are also discussed.

Complexity of vitamin E metabolism

“…Interestingly, the different vitamin E forms have no effect on the expression of phase Ⅱ enzymes, as in vitro none of the eight vitamin E forms showed an altered expression of UGT1A1 mRNA in human primary hepatocytes [180] . Furthermore, the majority of UGT isoforms were not regulated in mice fed α-TOH deficient or enriched diets [170] , and hepatic UGT activity was not influenced by feeding rats daily with 200 mg/kg body weight α-TOH for two weeks. It is of note that expression of UGT1A1 mRNA was increased 1.7-fold after treatment of wild-type mice with the PXR agonist pregnenolone 16α-carbonitrile compared to untreated control mice [156] .…”

“…However, the induction of Cyp3a11, the mouse homologue of human CYP3A4, by α-TOH and the involvement of PXR was confirmed in wild-type mice by Johnson et al [170] . In this study, mice kept vitamin E-deficient were fed daily for two weeks with 500 mg/kg DL-α-TOH acetate, which is equivalent to a typical supplementation with 400 -600 mg/d α-TOH for a 70 kg human.…”

“…Johnson et al [170] found that only the mouse orthologue of CYP4F2, Cyp4f13, but not Cyp4f14 was upregulated by α-TOH in wild-type mice, while in Pxr-null or humanized PXR mice no influence on the expression of Cyp4f13 and Cyp4f14 was observed. In Ttpa -/-and wild-type mice, Traber et al [165] revealed no influence of γ-TOH on the expression of the Cyp4f13 protein.…”

“…This led to the conclusion that α-TOH is a partial agonist of mouse Pxr and that Pxr is required for the induction of Cyp3a11 by α-TOH in mice [170] . Apart from these, there are further contradictory findings.…”

“…The study revealed that urinary excretion of α-CEHC glucuronide was significantly decreased down to 16% and γ-CEHC glucoside was attenuated down to 40% in pregnenolone 16α-carbonitrile-treated wild-type mice compared with control wild-type mice, while urinary excretion of both metabolites were unaffected in the Pxr-null mice. Johnson et al [170] suggested that these findings are the result of a down-regulation of β-oxidation by pregnenolone 16α-carbonitrile. To sum up, the role of CYP3A4 in the metabolism of vitamin E remains unclear.…”