Metabolomics reveals a novel vitamin E metabolite and attenuated vitamin E metabolism upon PXR activation

Abstract

Pregnane X receptor (PXR) is an important nuclear receptor xenosensor that regulates the expression of metabolic enzymes and transporters involved in the metabolism of xenobiotics and endobiotics. In this study, ultraperformance liquid chromatography (UPLC) coupled with electrospray time-of-flight mass spectrometry (TOFMS), revealed altered urinary metabolomes in both Pxr -null and wild-type mice treated with the mouse PXR activator pregnenolone 16a-carbonitrile (PCN). Multivariate data analysis revealed that PCN significantly attenuated the urinary vitamin E metabolite a-carboxyethyl hydroxychroman (CEHC) glucuronide together with a novel metabolite in wild-type but not Pxr -null mice. Deconjugation experiments with bglucuronidase and b-glucosidase suggested that the novel urinary metabolite was g-CEHC b-D-glucoside (Glc). The identity of g-CEHC Glc was confirmed by chemical synthesis and by comparing tandem mass fragmentation of the urinary metabolite with the authentic standard. The lower urinary CEHC was likely due to PXR-mediated repression of hepatic sterol carrier protein 2 involved in peroxisomal b-oxidation of branched-chain fatty acids (BCFA). Using a combination of metabolomic analysis and a genetically modified mouse model, this study revealed that activation of PXR results in attenuated levels of the two vitamin E conjugates, and identification of a novel vitamin E metabolite, g-CEHC Glc. Activation of PXR results in attenuated levels of the two vitamin E conjugates that may be useful as biomarkers of PXR activation.

Complexity of vitamin E metabolism

“…Usually, trolox or 1-naphthol are used as internal standards for the analysis of SCMs [150,151,154] . The CEHCs have been analyzed in rat and murine liver (α-, γ-, δ-CEHC [136,154] ), in rat plasma and in mice serum (γ-, δ-CEHC [136,141] ), as well as in human plasma and serum (α-, γ-, δ-CEHC [140,136,150,152,155] ), in human and mouse urine (α-, γ-, δ-CEHC [134][135][136]144,151,155,156] ) and in human and mouse feces (α-, γ-, δ-CEHC [136] ) as well as in rat bile (α-, γ-CEHC [149] ) and in vitro in A549 and HepG2 cells (γ-CEHC [140,141,144,137] ). Recently, two new metabolites, namely 12'-hydroxychromanol (12'-OH: γ-and δ-12'-OH) and 11'-hydroxychromanol (11'-OH: γ-and δ-11'-OH) have been identified using GC-MS in fecal pellets of mice fed a diet rich in γ-TOH [157] .…”

“…This pattern seems to also characterize degradation of LCMs as two cycles of peroxisomal β-oxidation (resulting in 11'-COOH and 9'-COOH) with separation of propionyl-CoA or acetylCoA were suggested for TOHs [138] . Cho et al [156] noted that the sterol carrier protein-x (SCP-x) is involved in peroxisomal oxidation of branched-chain lipids, serves as a peroxisomal 3-ketoacyl-CoA thiolase, and shows reduced expression in response to pregnane X receptor (PXR) activation (for further details on PXR and vitamin E interactions, see the section on CYP3A4 in chapter "metabolism of vitamin E"). Based on their data the authors suggested that SCP-x is involved in the formation of vitamin E metabolites.…”

“…Several CYP enzymes, including CYP3A4, are regulated by structurally diverse xenobiotics via PXR, a nuclear receptor that regulates the expression of metabolic enzymes and transporters involved in the metabolism of xenobiotics and endobiotics [156,168] . Landes et al [169] showed that vitamin E acts as an agonist of PXR.…”

“…With respect to PXR, more discrepancies have been reported. Cho et al [156] used Pxr-deficient vs wildtype mice both treated with and without pregnenolone 16α-carbonitrile (an activator of rodent Pxr) to analyze the impact of Pxr on vitamin E degradation. 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.…”

“…In addition, other conjugates are known for α-CEHC, such as taurine, glycine or glycine glucuronide conjugates [176] . In mice, glucuronides and sulfates of α-, γ-, δ-CEHC have been found [136,156,176] .…”