title: Isoform-selective activation of human constitutive androstane receptor by Ginkgo biloba extract: functional analysis of the SV23, SV24, and SV25 splice variants.
abstract: Naturally occurring splice variants of human constitutive androstane receptor (hCAR) exist, including hCAR-SV23 (insertion of amino acids SPTV), hCAR-SV24 (APYLT), and hCAR-SV25 (SPTV and APYLT). An extract of Ginkgo biloba was reported to activate hCAR-SV24 and the wild type (hCAR-WT). However, it is not known whether it selectively affects hCAR splice variants, how it activates hCAR isoforms, and which chemical is responsible for the effects of the extract. Therefore, we evaluated the impact of G. biloba extract on the functionality of hCAR-SV23, hCAR-SV24, hCAR-SV25, and hCAR-WT and compared it with that of phenobarbital, di-(2-ethylhexyl)phthalate (DEHP), 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime (CITCO), and 1,4-bis-[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) in cell-based reporter gene assays. Among the hCAR splice variants investigated, only hCAR-SV23 was activated by G. biloba extract, and this required cotransfection of a retinoid X receptor α (RXRα) expression plasmid. The extract activated hCAR-SV23 to a lesser extent than hCAR-WT, but ginkgolide A, ginkgolide B, ginkgolide C, ginkgolide J, and bilobalide were not responsible for the effects of the extract. CITCO activated hCAR-SV23, hCAR-SV24, and hCAR-WT. By comparison, phenobarbital activated hCAR-WT, whereas DEHP activated hCAR-SV23, hCAR-SV24 (with exogenous RXRα supplementation), and hCAR-WT. TCPOBOP did not affect the activity of any of the isoforms. G. biloba extract and phenobarbital did not bind or recruit coactivators to the ligand-binding domains of hCAR-WT and hCAR-SV23, whereas positive results were obtained with the controls (CITCO for hCAR-WT and DEHP for hCAR-SV23). In conclusion, G. biloba extract activates hCAR in an isoform-selective manner, and hCAR-SV23, hCAR-SV24, and hCAR-WT have overlapping, but distinct, sets of ligands.
authors.affiliation: Faculty of Pharmaceutical Sciences, The University of British Columbia, 2146 East Mall, Vancouver, British Columbia, V6T 1Z3, Canada.
authors.forename: Aik Jiang
authors.initials: AJ
authors.lastname: Lau
authors.forename: Guixiang
authors.initials: G
authors.lastname: Yang
authors.forename: Thomas K H
authors.initials: TK
authors.lastname: Chang
mesh_terms: D000090702:Constitutive Androstane Receptor; D017930:Genes, Reporter; D020441:Ginkgo biloba / Q000737:chemistry*; D046934:Ginkgolides / Q000737:chemistry / Q000494:pharmacology*; D056945:Hep G2 Cells; D006801:Humans; D008024:Ligands; D010936:Plant Extracts / Q000737:chemistry / Q000494:pharmacology*; D010957:Plasmids; D020033:Protein Isoforms / Q000819:agonists / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D018160:Receptors, Cytoplasmic and Nuclear / Q000819:agonists* / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D047490:Retinoid X Receptor alpha / Q000819:agonists / Q000502:physiology; D014162:Transfection; D020798:Two-Hybrid System Techniques
chemical_list: D000090702:Constitutive Androstane Receptor; D046934:Ginkgolides; D008024:Ligands; D010936:Plant Extracts; D020033:Protein Isoforms; D018160:Receptors, Cytoplasmic and Nuclear; D047490:Retinoid X Receptor alpha