The cytochrome P4502C subfamily comprises a group of constitutive microsomal hemoproteins which are expressed primarily in liver. In humans, this subfamily is responsible for metabolism of a variety of therapeutic drugs such as warfarin, mephenytoin, omeprazole, and antiinflammatory drugs. In the present study, we analyzed the promoter activity of the 5'-flanking region of two human CYP2C genes, CYP2C9 and CYP2C18. The ability of the 2.2-kb 5'-flanking region of the CYP2C9 gene to direct expression of a luciferase reporter gene in HepG2 cells was 25 times greater than that of the 1.3-kb 5'-flanking region of CYP2C18. Deletional analysis of CYP2C9 indicated that the minimal promoter was located between the translation start site and nucleotide -155, and an HPF-1 domain consensus sequence was identified in this region. Gel shift analysis demonstrated that nuclear proteins from HepG2 cells had a high binding affinity for a 20-bp oligonucleotide containing the HPF-1 site of CYP2C9. Antiserum to rat HNF-4 supershifted this DNA--protein complex, and an oligonucleotide derived from an HNF-4 motif present in the human apolipoprotein CIII promoter competed for the supershifted complex. Cotransfection with an HNF-4 expression plasmid increased transcriptional activity of the CYP2C9 minimal promoter (approximately 2-fold) in HepG2 cells and elevated activity more substantially in nonhepatic NIH3T3 cells (26-fold) and Cos 1 cells (9-fold).(ABSTRACT TRUNCATED AT 250 WORDS)