Tocotrienols activate the steroid and xenobiotic receptor, SXR, and selectively regulate expression of its target genes.
Vitamin E is an essential nutrient with antioxidant activity. Vitamin E is comprised of eight members, alpha-, beta-, gamma-, and delta-tocopherols and alpha-, beta-, gamma-, and delta-tocotrienols. All forms of vitamin E are initially metabolized by omega-oxidation, which is catalyzed by cytochrome P450 enzymes. The steroid and xenobiotic receptor (SXR) is a nuclear receptor that regulates drug clearance in the liver and intestine via induction of genes involved in drug and xenobiotic metabolism. We show here that all four tocotrienols specifically bind to and activate SXR, whereas tocopherols neither bind nor activate. Surprisingly, tocotrienols show tissue-specific induction of SXR target genes, particularly CYP3A4. Tocotrienols up-regulate expression of CYP3A4 but not UDP-glucuronosyltransferase 1A1 (UGT1A1) or multidrug resistance protein-1 (MDR1) in primary hepatocytes. In contrast, tocotrienols induce MDR1 and UGT1A1 but not CYP3A4 expression in intestinal LS180 cells. We found that nuclear receptor corepressor (NCoR) is expressed at relatively high levels in intestinal LS180 cells compared with primary hepatocytes. The unliganded SXR interacts with NCoR, and this interaction is only partially disrupted by tocotrienols. Expression of a dominant-negative NCoR enhanced the ability of tocotrienols to induce CYP3A4 in LS180 cells, suggesting that NCoR plays an important role in tissue-specific gene regulation by SXR. Our findings provide a molecular mechanism explaining how vitamin supplements affect the absorption and effectiveness of drugs. Knowledge of drug-nutrient interactions may help reduce the incidence of decreased drug efficacy.
Department of Developmental and Cell Biology, University of California, 5205 McGaugh Hall, Irvine, CA 92697-2300, USA.
Changcheng
C
Zhou
Michelle M
MM
Tabb
Asal
A
Sadatrafiei
Felix
F
Grün
Bruce
B
Blumberg
D002460:Cell Line; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis* / Q000235:genetics; D004305:Dose-Response Relationship, Drug; D005786:Gene Expression Regulation / Q000187:drug effects / Q000502:physiology; D018390:Gene Targeting / Q000379:methods*; D006801:Humans; D000077297:Pregnane X Receptor; D011485:Protein Binding / Q000187:drug effects / Q000502:physiology; D012333:RNA, Messenger / Q000378:metabolism; D011987:Receptors, Steroid / Q000235:genetics / Q000378:metabolism*; D024508:Tocotrienols / Q000378:metabolism / Q000494:pharmacology*
D000077297:Pregnane X Receptor; D012333:RNA, Messenger; D011987:Receptors, Steroid; D024508:Tocotrienols; D003577:Cytochrome P-450 Enzyme System; C104464:CYP3A protein, human; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human