Downregulation of cytochromes P450 in growth-stimulated rat hepatocytes: role of c-Myc induction and impaired C/EBP binding to DNA.

BACKGROUND/AIMS Several cytochromes P450 (CYPs) are expressed in differentiated hepatocytes, but downregulated in growth-stimulated cells. We determined the signals involved in CYP downregulation by epidermal growth factor (EGF). METHODS Rat hepatocytes were cultured with or without diverse substances for 72 h and EGF for the last 48 h. RESULTS EGF increased c-myc mRNA and protein, and decreased CYP mRNAs and proteins; both effects were prevented by two agents blocking c-myc transcription (retinoic acid and DMSO) and two antisense c-myc oligomers. Despite unchanged CCAAT-enhancer binding protein alpha (C/EBPalpha) and increased C/EBPbeta levels, nuclear proteins of EGF-treated cells did not bind to a C/EBP DNA probe in a gel mobility shift assay. This binding was restored when cells were co-treated with both EGF and c-myc antisense oligomers (preventing c-Myc induction). The N-terminal c-Myc domain added to control nuclear extracts prevented C/EBP DNA binding. A monoclonal anti-c-Myc antibody co-immunoprecipitated c-Myc, C/EBPalpha and C/EBPbeta from nuclear extracts. In cells not treated with EGF, an antisense C/EBPalpha oligomer decreased CYP expression. CONCLUSIONS EGF overexpresses c-Myc, decreases C/EBP binding to DNA and downregulates CYPs. We suggest that c-Myc may form inactive complexes with C/EBPs, thus decreasing C/EBP-mediated CYP transactivation.

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