Abstract
Pregnane X receptor (PXR; NR1I2) is a ligand-activated transcription factor that plays a role not only in drug metabolism and transport but also in various other biological processes. Ginkgo biloba is a herbal medicine commonly used to manage memory impairment. Treatment of primary cultures of rat hepatocytes with G. biloba extract increases the mRNA expression of CYP3A23, which is a target gene for rat PXR. The present study was conducted to test the hypothesis that G. biloba extract activates PXR. Treatment of mouse PXR (mPXR) or human PXR (hPXR)-transfected HepG2 cells with G. biloba extract at 200 μg/ml increased mPXR and hPXR activation by 3.2- and 9.5-fold, respectively. Dose-response analysis showed a log-linear increase in hPXR activation by the extract over the range of 200 to 800 μg/ml. To determine whether G. biloba extract induces hPXR target gene expression, cultured LS180 human colon adenocarcinoma cells were treated for 72 h with the extract. G. biloba extract at 200, 400, and 800 μg/ml increased CYP3A4 mRNA expression by 1.7-, 2.4-, and 2.5-fold, respectively. The same concentrations of the extract increased CYP3A5 (1.3-3.6-fold) and P-glycoprotein (ABCB) 1 (2.7-6.4-fold) mRNA expression. At concentrations (5 and 10 μM) that did not down-regulate PXR gene expression and were not cytotoxic, l-sulforaphane (an hPXR antagonist) decreased CYP3A4, CYP3A5, and ABCB1 gene expression in cells treated with G. biloba extract. In summary, G. biloba extract activated mPXR and hPXR in a cell-based reporter gene assay and induced CYP3A4, CYP3A5, and ABCB1 gene expression in hPXR-expressing LS180 cells.
Footnotes
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This study was supported by the Canadian Institutes of Health Research (Grant MOP-84581 to T.K.H.C.), by the Dawson Endowment Fund in Pharmaceutical Sciences (a major equipment grant to T.K.H.C.), and by the Intramural Research Program of the National Institutes of Health and the National Institute of Environmental Health Sciences (to T.S., M.N.). E.Y.H.Y. received a Kam Li Ma Scholarship in Pharmaceutical Sciences from the University of British Columbia and a partial graduate student traineeship in drug metabolism from Merck Research Laboratories (United States). T.K.H.C. received an Izaak Walton Killam Faculty Research Fellowship and a Senior Scholar Award from the Michael Smith Foundation for Health Research.
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doi:10.1124/dmd.108.023499.
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ABBREVIATIONS: PXR, pregnane X receptor; AUC, area under the plasma concentration-time curve; ABCB1, P-glycoprotein; LDH, lactate dehydrogenase; hPXR, human PXR; mPXR, mouse PXR; PCN, pregnenolone 16α-carbonitrile; DMSO, dimethyl sulfoxide; PCR, polymerase chain reaction; CAR, constitutive androstane receptor.
- Received July 20, 2008.
- Accepted August 21, 2008.
- The American Society for Pharmacology and Experimental Therapeutics
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