Two CCAAT/enhancer binding protein sites in the cytochrome P4503A1 locus: Potencial role in the glucocorticoid response

Induction of CYP3A genes by the ligand‐activated pregnane‐X‐receptor (PXR) involves the interaction of other as yet unidentified liver transcription factors. Here we show that the CYP3A1 promoter contains two active sites controlled by the CCAAT/enhancer‐binding protein α (C/EBPα), previously shown to regulate a number of liver stress response genes. We have identified two functional C/EBP binding sites at the CYP3A1 promoter that confer luciferase activity to C/EBPα cotransfected CHO cells. When inserted upstream of a thymidine kinase promoter, oligonucleotides corresponding to these elements (−350/−311 and −628/−608), increase reporter gene expression when cotransfected with a C/EBPα expression vector. Point mutations in the most conserved nucleotides in either element prevent binding of C/EBPα and abolish transactivation of the CYP3A1 promoter. Moreover, we demonstrate that C/EBPα accumulates in the rat liver nuclei in response to dexamethasone, and that under these conditions C/EBPα binds to the CYP3A1 promoter elements. Our results suggest a correlation between transcription of C/EBPα, nuclear protein function and induction of CYP3A1 by dexamethasone in the liver. They also support the notion that C/EBPα participates in the up‐regulation of the CYP3A1 gene in response to synthetic glucocorticoids.