INDUCTION AND INHIBITION OF ESTRADIOL HYDROXYLASE-ACTIVITIES IN MCF-7 HUMAN BREAST-CANCER CELLS IN CULTURE

The effects of estradiol, progesterone, and tamoxifen on the activity of estradiol 2- and 16α-hydroxylases were studied in human breast cancer cell cultures using a radiometric assay. After 5 days' exposure to these compounds, incubations in the presence of either [2-3H]estradiol or [16α-3H]estradiol as substrate were carried out. In MCF-7 cells, estradiol (10-8 m), progesterone (10-6 m) and tamoxifen (10-6 m) significantly increased 16α-hydroxylase activity (estradiol; 21% progesterone 10% to 32%; tamoxifen 21% to 31%; P < 0.01). Synergistic effects were observed when the cells were successively exposed to tamoxifen and progesterone. Simultaneous treatment with tamoxifen plus estradiol or estradiol plus progesterone showed no change from estradiol alone. On the other hand, although estradiol had no direct effects on 2-hydroxylase activity, tamoxifen decreased this enzymatic activity significantly at 10-6 m (23% to 37%). Progesterone acted synergistically to further decrease this reaction. Treatment with only progesterone caused an increase in 2-hydroxylation. In contrast, a subline of MCF-7 cells with low estrogen receptor levels showed only minimal enzyme-hormone responses. Likewise, treatment of the estrogen receptor-negative MDA-MB-231 human breast cancer cell line with these compounds showed no effects on either 2- or 16α-hydroxylase activity. In the progesterone receptor-rich T47D cell line, estradiol decreased both activities while progesterone increased both. These results indicate that (1) estradiol and tamoxifen, acting as partial agonists, act in the same manner to influence the metabolism of estradiol in MCF-7 cells, probably through an interaction between these compounds and estrogen receptors, and progesterone increases these reactions via the progesterone receptor; and that (2) the cellular estrogen receptor levels are related to the quantitative response to those compounds but do not affect the basal reaction level. These observations support our previous finding that these two metabolic pathways in MCF-7 cells are independently regulated and that this regulation is controlled in part via the estrogen or progesterone receptor. © 1990.