Relation between Cytochrome P450IA1 Expression and Estrogen Receptor Content of Human Breast Cancer Cells

Multidrug resistance (MDR) in an MCF-7 human breast cancer cell line (MCF7/AdrR) is associated with decreased drug accumulation and overexpression of P-glycoprotein as well as alterations in the levels of specific drug-metabolizing enzymes, including decreased activity of the phase I drug-metabolizing enzyme aryl hydrocarbon hydroxylase (AHH) and increased expression of the anionic form of the phase II drug-metabolizing enzyme glutathione S-transferase. Since the development of MDR in this MCF-7 cell line is also associated with a loss of estrogen receptors (ER), we have examined the expression of cytochrome P450IA1, the gene encoding AHH activity, in other breast cancer cell lines not selected for drug resistance but expressing various levels of ER. These studies show that a relationship exists between 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-inducible AHH activity and the ER content in a series of breast cancer cell lines. In these cell lines expression of AHH activity is regulated, at least in part, at the level of P450IA1 RNA. While TCDDspecific binding proteins (Ah receptors) were found in each of the breast cancer cell lines, there was no apparent relation between the level of nuclear TCDD-binding proteins and the level of TCDD-inducible P450IA1 expression. Previous studies from our laboratory have described an inverse relationship between levels of the anionic form of glutathione S-transferase and ER in breast cancer. The findings of the present study suggest that the ER status of breast cancer cells is associated with distinct patterns in the expression of both phase I and phase II drug-metabolizing enzymes, and that these biochemical changes result in differential sensitivities of ER-positive and ER-negative cell lines to the procarcinogen benzo(a)pyrene and the antineoplastic agent ellipticine.