Abstract
CYP3A4 is the most abundant cytochrome P450 (P450) in human liver, comprising approximately 30% of the total liver P450 content. This enzyme has an important role in steroid catabolism and metabolism of foreign compounds, with the majority of pharmaceutical compounds being substrates for CYP3A4. The molecular mechanisms that underlie transcriptional activation of CYP3A4 are complex with many steroid hormone nuclear receptors, including glucocorticoid receptor, pregnane X receptor (PXR), vitamin D receptor, and constitutive androstane receptor, playing roles. Nowhere is this more evident than in the induction of CYP3A4 gene expression by glucocorticoids. CYP3A genes lack a consensus glucocorticoid receptor response element and yet are highly induced by classical glucocorticoids such as hydrocortisone and dexamethasone. Recent evidence has demonstrated that glucocorticoids are ligands for the orphan nuclear receptor PXR, and induction of CYP3A genes by glucocorticoids may occur primarily through PXR interactions. In this paper, we present a mutant that disrupts a hepatocyte-nuclear-factor-3/CCAAT-enhancer binding protein α binding site in the CYP3A4 proximal promoter. This mutation disrupts induction of a reporter gene construct by the glucocorticoids dexamethasone and hydrocortisone; yet induction by the potent PXR ligand rifampicin is unaffected. Such data provides strong evidence that glucocorticoids induce CYP3A4 gene expression both through the established PXR-dependent pathway but also through a PXR-independent pathway.
Footnotes
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The first two authors contributed equally to the manuscript.
- Abbreviations used are::
- P450
- cytochrome P450
- bp
- base pair(s)
- HNF
- hepatocyte-nuclear-factor
- GR
- glucocorticoid receptor
- GRE
- glucocorticoid response element
- PXR
- pregnane X receptor
- CAR
- constitutive androstane receptor
- C/EBPα
- CCAAT-enhancer binding protein α
- SEAP
- secreted alkaline phosphatase
- PCR
- polymerase chain reaction
- DTT
- dithiothreitol
- EMSA
- electromobility shift assay
- IA
- inductive ability
- Received February 6, 2002.
- Accepted June 11, 2002.
- The American Society for Pharmacology and Experimental Therapeutics
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