Altered regulation of P-450IA1 expression in a multidrug-resistant MCF-7 human breast cancer cell line

MCF-7 human breast cancer cells, selected for resistance to adriamycin (Adr(R)), exhibit the phenotype of multidrug resistance (MDR). Previous studies have shown that resistance in Adr(R) MCF-7 cells is associated with several biochemical changes that are similar to those induced in rat hyperplastic nodules, preneoplastic liver lesions which display broad spectrum resistance to carcinogens and hepatotoxins. In this report, we show that these changes in the Adr(R) MCF-7 cells are also associated with the development of cross-resistance to the procarcinogen benzo(a)pyrene (BP) and are associated with a marked defect in the conversion of BP to its cytotoxic, carcinogenic metabolites by Adr(R) cells. Since aryl hydrocarbon hydroxylase in the principle enzyme activity which converts benzo(a)pyrene to toxic hydroxylated forms, the regulation of cytochrome P-450IA1 expression, the gene encoding this enzyme activity in MCF-7 cells, was examined. Incubation with 100 nM 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for 24 h results in a marked increase in aryl hydrocarbon hydroxylase activity in wild type (WT) but not Adr(R) MCF-7 cells. The alteration in aryl hydrocarbon hydroxylase expression in the Adr(R) cells is not overcome by incubation either with higher concentrations of TCDD (1 μM) or for longer periods of time (4 days). Northern blot analysis indicates that this defect in Adr(R) MCF-7 cells involves a regulatory defect at the level of P-450IA1 RNA. Following transfection of a construct containing the normal mouse P-450IA1 promoter fused to a reporter gene (bacterial chloramphenicol acetyltransferase) into WT and Adr(R) MCF-7 cells, TCDD induced chloramphenicol acetyltransferase activity in WT MCF-7 cells only. Furthermore, TCDD also induces both DT-diaphorase and UDP-glucuronyltransferase activities in WT, but not Adr(R) cells. These data suggest that the defect in the Adr(R) MCF-7 cells is not due to a structural P-450IA1 gene mutation, but rather involves a product regulating the polycyclic hydrocarbon-inducible expression of several drug-metabolizing enzyme activities. This defect in the Adr(R) MCF-7 cells is also associated with the development of resistance to ellipticine, an anticancer agent which is converted to more toxic hydroxylated species by aryl hydrocarbon hydroxylase or a similar mixed function oxidase. The WT and Adr(R) MCF-7 cells represent a useful model to study the regulation of the P-450IA1 gene in human cells.