COEXPRESSION OF HUMAN CYP1A1 AND A HUMAN ANALOG OF CYTOCHROME P450-EFIN RESPONSE TO 2,3,7,8-TETRACHLORO-DIBENZO-P-DIOXIN IN THE HUMAN MAMMARY CARCINOMA-DERIVED MCF-7 CELLS

Cultured human mammary carcinoma (MCF-7) cells exhibited constitutive cytochrome P450-dependent metabolism of 7,12-dimethylbenza[a]anthracen e (DMBA) (45-75 pmol/mg microsomal protein). Exposure of the cells to 2,3,7,8-tetrachloro-dibenzo-p-dioxin (TCDD), which is known to induced CYP1A1, not only resulted in a 30-fold increase in the total microsom al metabolism of DMBA but produced substantial differences in the dist ribution of DMBA metabolites formed. This suggested that different cyt ochrome P450 (P450) forms predominated in untreated and induced cells. Comparative studies with TCDD-induced human hepatoblastoma (HepG2) an d skin cell carcinoma (SCC-13) cells and also recombinantly expressed human CYP1A1, confirmed that the DMBA-metabolite profile in TCDD-induc ed MCF-7 cells was that of human CYP1A1. This distribution, however, d iffered substantially from the regioselectivity of rat CYP1A1 and mous e Cyp1a-1. Rabbit antibodies to rat CYP1A1 completely inhibited the DM BA-metabolizing activity of TCDD-induced MCF-7 cells but had no inhibi tory effect on constitutive DMBA metabolism which was, however, comple tely inhibited by chicken antibodies to the novel P450 in mouse embryo fibroblasts (P450-EF). Anti-P450-EF inhibited only 10% of the DMBA-me tabolizing activity in the TCDD-induced MCF-7 cell microsomes. Microso mes from untreated MCF-7 cells expressed a 52 kDa protein that was imm unodetectable by rabbit anti-P450-EF and failed to express immunodetec table levels of human CYP1A1. DMBA metabolism, therefore, switches fro m P450-EF in uninduced microsomes to CYP1A1 in TCDD-induced microsomes . The mobility of the P450-EF-like protein in MCF-7 cells was higher t han that of P450-EF from C3H/10T1/2CL8 (10T1/2) cells (55 kDa). The 52 kDa protein from MCF-7 cells was induced similar to 8-fold by TCDD wh ile CYP1A1 immunodetectable protein was increased to much higher level s. The SCC-13 cell line exhibited a similar pattern of expression of a 52 kDa P450-EF-like protein and CYP1A1. HepG2 cells expressed the hig hest levels of CYP1A1 in response to TCDD without expression of the 52 kDa protein.