Differential regulation of hepatic CYP2B6 and CYP3A4 genes by constitutive androstane receptor but not pregnane X receptor.
Accumulated evidence suggests that cross-talk between the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR) results in shared transcriptional activation of CYP2B and CYP3A genes. Although most data imply symmetrical cross-regulation of these genes by rodent PXR and CAR, the actual selectivities of the corresponding human receptors are unknown. The objective of this study was to evaluate the symmetry of human (h) PXR and hCAR cross-talk by comparing the selectivities of these receptors for CYP2B6 and CYP3A4. Human hepatocyte studies revealed nonselective induction of both CYP2B6 and CYP3A4 by hPXR activation but marked preferential induction of CYP2B6 by selective hCAR activation. Gel shift assays demonstrated that hPXR exhibited strong and relatively equal binding to all functional response elements in both CYP2B6 and CYP3A4 genes, whereas hCAR displayed significantly weak binding to the CYP3A4 proximal ER6 motif. In cell-based transfection assays, hCAR displayed greater activation of CYP2B6 reporter gene expression compared with CYP3A4 with constructs containing both proximal and distal regulatory elements. Furthermore, in agreement with binding observations, transfection assays using promoter constructs containing repeats of CYP2B6 DR4 and CYP3A4 ER6 motifs revealed an even greater difference in reporter activation by hCAR. In contrast, hPXR activation resulted in less discernible differences between CYP2B6 and CYP3A4 reporter gene expression. These results suggest asymmetrical cross-regulation of CYP2B6 and CYP3A4 by hCAR but not hPXR in that hCAR exhibits preferential induction of CYP2B6 relative to CYP3A4 because of its weak binding and functional activation of the CYP3A4 ER6.
Faucette
Stephanie R
SR
Division of Molecular Pharmaceutics, School of Pharmacy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7360, USA.
Sueyoshi
Tatsuya
T
Smith
Cornelia M
CM
Negishi
Masahiko
M
Lecluyse
Edward L
EL
Wang
Hongbing
H
D001189:Aryl Hydrocarbon Hydroxylases / Q000235:genetics*; D001483:Base Sequence; D045744:Cell Line, Tumor; D000090702:Constitutive Androstane Receptor; D065702:Cytochrome P-450 CYP2B6; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics*; D024202:Electrophoretic Mobility Shift Assay; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects / Q000502:physiology*; D017930:Genes, Reporter; D022781:Hepatocytes / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D006801:Humans; D008099:Liver / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D008969:Molecular Sequence Data; D010089:Oxidoreductases, N-Demethylating / Q000235:genetics*; D010091:Oximes / Q000494:pharmacology; D000077297:Pregnane X Receptor; D011485:Protein Binding; D020239:Receptor Cross-Talk / Q000502:physiology; D018160:Receptors, Cytoplasmic and Nuclear / Q000819:agonists / Q000378:metabolism / Q000502:physiology*; D011987:Receptors, Steroid / Q000819:agonists / Q000378:metabolism / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012293:Rifampin / Q000494:pharmacology; D013844:Thiazoles / Q000494:pharmacology; D014157:Transcription Factors / Q000819:agonists / Q000378:metabolism / Q000502:physiology*
D000090702:Constitutive Androstane Receptor; D010091:Oximes; D000077297:Pregnane X Receptor; D018160:Receptors, Cytoplasmic and Nuclear; D011987:Receptors, Steroid; D013844:Thiazoles; D014157:Transcription Factors; D003577:Cytochrome P-450 Enzyme System; C475093:6-(4-chlorophenyl)imidazo(2,1-b)(1,3)thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime; D001189:Aryl Hydrocarbon Hydroxylases; C585599:CYP2B6 protein, human; D065702:Cytochrome P-450 CYP2B6; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; D010089:Oxidoreductases, N-Demethylating; D012293:Rifampin