Vitamin E activates gene expression via the pregnane X receptor.

Area of Science:

• Biochemistry
• Pharmacology
• Nutritional Science

Background:

• Tocopherols and tocotrienols (Vitamin E) undergo metabolism via side-chain degradation, involving omega- and beta-oxidation.
• Cytochrome P450 (CYP) enzymes catalyze omega-oxidation and are often regulated by their substrates.
• The pregnane X receptor (PXR) is a nuclear receptor that controls drug-metabolizing enzymes.

Purpose of the Study:

• To investigate the ability of different Vitamin E forms to activate gene expression via the pregnane X receptor (PXR).
• To determine the potential interference of Vitamin E with drug metabolism and efficacy.

Main Methods:

• HepG2 cells were co-transfected with human PXR and a chloramphenicol acetyl transferase (CAT) reporter gene linked to PXR-responsive elements.
• CAT activity was measured to assess PXR activation by various Vitamin E forms and rifampicin.
• Endogenous CYP3A4 and CYP3A5 mRNA levels were analyzed following treatment with gamma-tocotrienol.

Main Results:

• All tested Vitamin E forms activated gene expression through PXR in a concentration-dependent manner.
• Alpha- and gamma-tocotrienol showed the strongest induction of CAT activity, followed by delta-, alpha-, and gamma-tocopherol.
• Gamma-tocotrienol effectively upregulated endogenous CYP3A4 and CYP3A5 mRNA, comparable to rifampicin.

Conclusions:

• Vitamin E isomers, particularly tocotrienols, can activate PXR, suggesting a role in regulating drug-metabolizing enzymes.
• The findings indicate a potential for Vitamin E to influence the metabolism and therapeutic efficacy of drugs metabolized by PXR-regulated pathways.