Abstract
Hepatocyte nuclear factor 4α (HNF4α) is a key transcription factor for the constitutive expression of cytochromes P450 (P450s) in the liver. However, human hepatoma HepG2 cells show a high level of HNF4α but express only marginal P450 levels. We found that the HNF4α-mediated P450 transcription in HepG2 is impaired by the low level of coactivators peroxisomal proliferator activated receptor-γ coactivator 1α (PGC1α) and steroid receptor coactivator 1 (SRC1). Reporter assays with a chimeric CYP2C9-LUC construct demonstrated that the sole transfection of coactivators induced luciferase activity in HepG2 cells. In HeLa cells however, CYP2C9-LUC activity only significantly increased when coactivators were cotransfected with HNF4α. A deletion mutant lacking the two proximal HNF4α binding sites in the CYP2C9 promoter did not respond to PGC1α or SRC1, demonstrating that coactivators were acting through HNF4α response elements. Adenovirus-mediated transfection of PGC1α in human hepatoma cells caused a significant dose-dependent increase in CYP2C9, CYP1A1, and CYP1A2 and in the positive control CYP7A1. PGC1α also showed a moderate activating effect on CYP3A4, CYP3A5, and CYP2D6. Adenoviral transfection of SRC1 had a lessened effect on P450 genes. Chromatin immunoprecipitation assay demonstrated in vivo binding of HNF4α and PGC1α to HNF4α response sequences in the CYP2C9 promoter and to three new regulatory regions in the common 23.3 kilobase spacer sequence of the CYP1A1/2 cluster. Insulin treatment of HepG2 and human hepatocytes caused repression of PGC1α and a concomitant down-regulation of P450s. Our results establish the importance of coactivators PGC1α and SRC1 for the hepatic expression of human P450s and uncover a new HNF4α-dependent regulatory mechanism to constitutively control the CYP1A1/2 cluster.
Footnotes
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This research was partly supported by a grant from the Ministry of Science and Technology (SAF 2003-09353) and from an EU Integrated Research Project (PREDICTOMICS. LSHB-CT-2004-504761). C. P. M.-J. received a predoctoral grant from the Consellería de Educación Ciencia y Cultura, Generalitat Valenciana.
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ABBREVIATIONS: P450, cytochrome P450; HNF, hepatocyte nuclear factor; PGC1α, peroxisomal proliferator activated receptor-γ coactivator 1α; SRC, steroid receptor coactivator; m.o.i., multiplicity of infection; Ad, recombinant adenovirus; ChIP, chromatin immunoprecipitation; PCR, polymerase chain reaction; RT-PCR, reverse transcription-polymerase chain reaction; kb, kilobase pair(s); bp, base pair(s); PBGD, porphobilinogen deaminase; PXR, pregnane X receptor; C/EBP, CCAAT/enhancer-binding protein; CAR, constitutive androstane receptor; AHR, aryl hydrocarbon receptor; XRE, xenobiotic response element; PCAF, p300/CBP-associated protein.
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↵ The online version of this article (available at http://molpharm.aspetjournals.org) contains supplemental material.
- Received April 6, 2006.
- Accepted August 1, 2006.
- The American Society for Pharmacology and Experimental Therapeutics
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