Differential effects of progestogens, by type and regimen, on estrogen-metabolizing enzymes in human breast cancer cells.

Objectives: To investigate the in vitro effects of five progestogens commonly used in hormone replacement therapy (HRT) on estrogen-metabolizing enzymes in human breast cancer cells.

Methods: The human hormone-dependent breast cancer cell lines T47D, MCF-7, and MCF-7aro were cultured with estradiol (E(2)) and progestogens. The mRNA levels of estrogen-metabolizing enzymes were determined by RT-PCR or Northern blot, and enzyme activities by radiolabeled substrates. Cell proliferation was measured by bromodeoxyuridine incorporation. In vitro models for continuous combined regimen (CCR) and sequential combined regimen (SCR) were established to mimic the in vivo conditions of HRT.

Results: Medroxyprogesterone acetate (MPA) plus E(2) (10(-8)M) stimulated the mRNA levels and activities of estrogen-activating enzymes aromatase (at 10(-8)M MPA), 17beta-hydroxysteroid dehydrogenase type 1 (17betaHSD1) (at 10(-6)M), and sulfatase (at 10(-8) to 10(-6)M) compared to E(2) only. Progesterone also stimulated enzyme activity, but to a lower magnitude. Levonorgestrel, norethindrone, and dienogest showed no enzyme stimulation. The estrogen-inactivating enzymes 17beta-hydroxysteroid dehydrogenase type 2 and sulfotransferase were not affected by any of the progestogens tested. However, all the progestogens (at 10(-8) to 10(-6)M) inhibited E(2)-stimulated cell proliferation. While increased aromatase and 17betaHSD1 activities were observed in the CCR model, no significant enzyme stimulation was observed in the SCR model.

Conclusions: The present study suggested that progestogens exert different actions on estrogen-metabolizing enzymes in breast cancer cells dependent on the specific progestogen and regimen used. Further studies are needed to elucidate whether MPA, a progestogen currently used in HRT, leads to a higher risk of breast cancer development than other progestogens.