Identification of a new P450 subfamily, CYP4F1, expressed in rat hepatic tumors.

The expression of the rat cytochrome P450 CYP4 family was studied in hepatic tumors. In most of the primary and transplantable hepatic tumors studied, lauric acid omega-hydroxylase activity associated with the CYP4A subfamily enzymes decreased. The expression of CYP4A proteins and mRNAs in these tumors as assessed by Western and Northern blot was undetectable. However, while RNA analysis revealed the absence of 4A1, 4A2, and 4A3 mRNAs, the expression of CYP4 gene(s) was detected. A Uni-ZAP cDNA library was constructed from a 2-acetylaminofluorene-induced transplantable rat hepatic tumor and screened with a CYP4 family probe. A full-length sequenced cDNA of 1977 bp isolated contained a 23-bp 5' untranslated region, a 1572-bp open reading frame, and a 382-bp 3' untranslated region. This cDNA sequence deduced amino acid sequence encodes a P450 protein having a conserved region of the CYP4 family exhibiting 44-45% amino acid identity to rat CYP4A subfamily members, 43% to human CYP4B1, 35 and 32% to insect CYP4C1 and CYP4D1, respectively. This new P450 was thus named CYP4F1. RNA blot analysis with CYP4F1 cDNA and CYP4F1-specific oligonucleotide probes revealed the expression of CYP4F1 in all tumors. This is the first example of a P450 constitutively expressed in rat hepatomas at levels exceeding those in the parental liver tissue. These results suggest that there is differential regulation of CYP4 genes during hepatic carcinogenesis.