Estradiol formation from testosterone by continuously cultured human breast cancer cells

Two human breast cancer cell lines (MCF-7 and MDA-MB-231) and one cell line derived from normal human breast (HBL-100) were examined for the presence of aromatase activity by determining the amounts of [3H]estradiol ([3H]E2) formed by cell cultures incubated with [3H]testosterone. Aromatase activity was demonstrable in both breast cancer cell lines, but estradiol synthesis was not observed in HBL-100 cultures. The [3H]E2 content of MCF-7 cultures rose as a function of incubation time and substrate concentration. Furthermore, [3H]E2 formation by this cell line was suppressed by several known inhibitors of human placental aromatase. These observations represent the first evidence that some lines of continuously cultured human breast cancer cells, like some human breast tumors, are capable of forming estrogen from an extracellular precursor steroid. Cultured breast cells may provide model systems for investigating the relative importance of intracellular estrogen formation in the regulation of human breast cancer cell growth.