Vitamin E activates gene expression via the pregnane X receptor

doi:10.1016/S0006-2952(02)01520-4

Biochemical Pharmacology

Volume 65, Issue 2, 15 January 2003, Pages 269-273

https://doi.org/10.1016/S0006-2952(02)01520-4 Get rights and content

Abstract

Tocopherols and tocotrienols are metabolized by side chain degradation via initial ω-oxidation and subsequent β-oxidation. ω-Oxidation is performed by cytochrome P450 (CYP) enzymes which are often regulated by their substrates themselves. Results presented here show that all forms of Vitamin E are able to activate gene expression via the pregnane X receptor (PXR), a nuclear receptor regulating a variety of drug metabolizing enzymes. In HepG2 cells transfected with the human PXR and the chloramphenicol acetyl transferase (CAT) gene linked to two PXR responsive elements, CAT activity was most strongly induced by α- and γ-tocotrienol followed by rifampicin, δ-, α- and γ-tocopherol. The inductive efficacy was concentration-dependent; its specificity was underscored by a lower response when cotransfection with PXR was omitted. Up-regulation of endogenous CYP3A4 and CYP3A5 mRNA was obtained by γ-tocotrienol, the most potent activator of PXR, with the same efficacy as with rifampicin. This points to a potential interference of individual forms of Vitamin E with the metabolism and efficacy of drugs.

Introduction

Vitamin E is widely considered as an essential constituent of the antioxidant network. As such it is believed to prevent oxidative-stress related diseases like inflammatory processes, atherosclerosis, and even cancer. However, clinical trials undertaken to prove the potential of Vitamin E to prevent cardiovascular diseases were disappointing. In addition, novel functions of Vitamin E have been detected such as inhibition of cell proliferation [1], protein kinase C activity [2], and NADPH oxidase activation [3], up-regulation of the scavenger receptor CD36 [4] and down-regulation of adhesion molecules [5], functions often restricted to α-tocopherol (for review see [6]). α-Tocopherol is preferentially retained in the body. Degradation of all types of Vitamin E occurs via initial ω-oxidation followed by five cycles of β-oxidation resulting in the respective final products, the carboxyethyl hydroxychromans (CEHCs) [7], [8], [9], [10], [11], [12]. Compared to other forms of Vitamin E, α-tocopherol is only poorly metabolized [11]. Metabolism can, however, be enhanced by rifampicin [13], an inducer of a variety of drug metabolizing enzymes. The degradation of tocopherols and tocotrienols by the cellular drug metabolizing system with a preference for those forms not being α-tocopherol has led to the conclusion that Vitamin E might interfere with drug metabolism [6]. The induction of several CYPs by a large number of structurally diverse xenobiotics that often are substrates of the induced cytochromes themselves, is mediated by the nuclear pregnane X receptor (PXR) [14], [15]. We here show that individual forms of Vitamin E are also able to activate PXR and in consequence xenobiotics metabolizing enzymes.

Section snippets

Materials and methods

HepG2 cells (ATCC HB8065) were grown in 24-well plates in RPMI 1640 containing 5% heat-inactivated fetal calf serum, 2 mM alanyl-glutamine, 100 U/mL penicillin, and 100 μg/mL streptomycin (Gibco) at 37° in 5% CO₂ to about 80% confluency. Cells were cotransfected with the human PXR in the pSG5 expression vector [15] (pSG5hPXR; 0.17 μg/well), a CAT reporter plasmid pCATDR3 (0.17 μg/well) containing two copies of the CYP3A1 PXR binding site (DR3, 5′-TGAACTn3TGAACT-3′) [14], and pCH110 (0.5 μg/well), a

Results and discussion

To study an activation of PXR by Vitamin E the model of HepG2 cells was chosen because these cells (i) metabolize all forms of Vitamin E [11] and (ii) express CYPs under the control of PXR [16]. HepG2 cells were transiently cotransfected with the human PXR, the CAT reporter gene containing two times the CYP3A1 direct repeat response element (DR3) to which PXR–RXR heterodimers can bind [15], and with a β-galactosidase expression plasmid for transfection control. Then cells were incubated with

Acknowledgements

The hPXR expression vector and pCATDR3 were kindly provided by S.A. Kliewer, Glaxo Wellcome, Research Triangle Park, NC, USA. R-α- and R-γ-tocotrienol were generous gifts from P. Hoppe, BASF AG, Ludwigshafen, Germany. We thank Stefanie Deubel and Elvira Krohn for skilful technical assistance. The work was supported by the Deutsche Forschungsgemeinschaft, DFG, Br 778/6-1.

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¹: These authors contributed equally to the work.

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Short communicationVitamin E activates gene expression via the pregnane X receptor

Abstract

Introduction

Section snippets

Materials and methods

Results and discussion

Acknowledgements

J. Biol. Chem.

Am. J. Clin. Nutr.

J. Lipid Res.

Am. J. Clin. Nutr.

J. Nutr.

J. Biol. Chem.

Free Radic. Biol. Med.

Cell

Biochem. Biophys. Res. Commun.

FEBS Lett.

J. Nutr.

FEBS Lett.

J. Lipid Res.

Anal. Biochem.

Biochem. Biophys. Res. Commun.

J. Nutr.

Arch. Biochem. Biophys.

Short communication
Vitamin E activates gene expression via the pregnane X receptor