Abstract
Both the human pregnane X receptor (hPXR) and constitutive androstane receptor (hCAR) are capable of regulating CYP3A4 and CYP2B6 gene expression. However, the majority of currently identified CYP3A4 and CYP2B6 inducers are confirmed activators of hPXR but not hCAR. To compare these receptors with respect to their chemical selectivities, 16 drugs known to induce CYP3A4 and/or CYP2B expression were evaluated for relative activation of hPXR versus hCAR. Because of the high basal but low chemical-induced activation of hCAR in immortalized cells, alternative methods were used to evaluate hCAR activation potential. Thirteen of the 16 compounds were classified as moderate to strong hPXR activators. In contrast, carbamazepine (CMZ), efavirenz (EFV), and nevirapine (NVP) were classified as negligible or weak hPXR activators at concentrations associated with efficacious CYP2B6 reporter or endogenous gene induction in primary human hepatocytes, suggesting potential activation of hCAR. Subsequent experiments demonstrated that these three drugs efficiently induced nuclear accumulation of in vivo-transfected enhanced yellow fluorescent protein-hCAR and significantly increased expression of a CYP2B6 reporter gene when hCAR was expressed in CAR–/– mice. In addition, using a recently identified, chemically responsive splice variant of hCAR (hCAR3), the hCAR activation profiles of the 16 compounds were evaluated. By combining results from the hPXR- and hCAR3-based reporter gene assays, these inducers were classified as hPXR, hCAR, or hPXR/hCAR dual activators. Our results demonstrate that CMZ, EFV, and NVP induce CYP2B6 and CYP3A4 preferentially through hCAR and that hCAR3 represents a sensitive tool for in vitro prediction of chemical-mediated human CAR activation.
Footnotes
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This work was supported by the National Institutes of Health (Grant DK061652).
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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doi:10.1124/jpet.106.112136.
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ABBREVIATIONS: PXR, pregnane X receptor; CAR, constitutive androstane receptor; hPXR, human pregnane X receptor; hCAR, human constitutive androstane receptor; CITCO, 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenyzl)oxime; PHN, phenytoin; RIF, rifampin; CMZ, carbamazepine; EFV, efavirenz; NVP, nevirapine; LOV, lovastatin; MET, metyrapone; MEV, mevastatin; NIC, nicardipine; NIF, nifedipine; OMP, omeprazole; SIM, simvastatin; ART, artemisinin; CPZ, chlorpromazine; CPA, cyclophosphamide; RES, reserpine; RU486, mifepristone; TGZ, troglitazone; PB, phenobarbital; FBS, fetal bovine serum; PXRE, PXR-response element; XREM, xenobiotic-responsive enhancer module; NR, nuclear receptor binding site; EYFP, enhanced yellow fluorescent protein; DMEM, Dulbecco's modified Eagle's medium; DMSO, dimethyl sulfoxide; CTL, control; PCR, polymerase chain reaction; ITS+, insulin transferrin selenium; PBREM, phenobarbital-responsive enhancer module.
- Received August 7, 2006.
- Accepted October 11, 2006.
- The American Society for Pharmacology and Experimental Therapeutics
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