Hormonal Regulation of Estradiol 17β‐Hydroxysteroid Dehydrogenase Activity in the ZR‐75‐1 Human Breast Cancer Cell Line

Estrogens are known to play a major role in mammary tumor growth.' Among the mechanisms regulating estradiol (E2) action in estrogen target tissue^,^-^ the activity of the nicotinamide adenine dinucleotide (NAD)-dependent enzyme 17P-hydroxysteroid dehydrogenase (17P-HSD) plays an important role, because it converts the most potent natural estrogen 17P-estradiol (E2) into estrone (El), a less active compound.6 Recent elucidation of the complete amino acid sequence of human placental 17P-HSD deduced from its corresponding cDNA should provide detailed information about the properties and regulation of this enzyme, which should help our understanding of the hormone dependency of breast cancer.I The present study investigates the hormonal regulation of estradiol 17P-HSD in an in vitro model of human breast cancer. ZR-75-1 cells were thus preincubated for 10 days with optimal concentrations of selected steroids in the presence or absence of insulin. Seventeen-p-HSD was measured in cells incubated for 4 hours in the presence of [3H]E2 (20 nM). Quantification of the metabolites was achieved after extraction, partition, and separation by thin-layer chromatography. FIGURE 1 shows that the synthetic estrogen ethinylestradiol (EE2) decreases the activity of 17P-HSD by approximately 20 and 45% in the absence and presence of insulin, respectively. The specificity of the effect of ethinylestradiol is supported by an almost complete reversal of the effect of the estrogen by the steroidal antiestrogen ICI 164384. Although the synthetic progestin R5020 present alone does not exert a significant influence on the enzymatic activity of 17P-HSD in ZR-75-1 cells, it prevents the inhibitory action of the estrogen ethinylestradiol. Androgens, on the other hand, cause a threefold increase in 17P-HSD activity through specific interaction with the androgen receptor, as indicated by inhibition of the effect of dihydrotestosterone by the pure antiandrogen hydroxyflutamide (FIG. 2). Although ZR-751 cells possess glucocorticoids receptors and glucocorticoid are known to inhibit cell proliferation, treatment for 10 days with dexamethasone has no effect on 17pHSD activity (FIG. 1). The present data demonstrate that the activity of 17p-HSD is under the control of estrogens and androgens in the human breast cell line ZR75-1. Such activity may have an important influence on the response of breast cancer to hormonal therapy.