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.