Differences in the mechanism of the inhibitory actions of catecholestrogens, tamoxifen and high concentrations of estrogens on prolactin release by cultured rat pituitary tumor cells.

Chronic administration of the catecholestrogens 2-OH-estrone (2-OH1) and 2-OH-estradiol (2-OHE2), of tamoxifen and its metabolites and of high concentrations of estradiol have been previously shown to inhibit the growth of the estrogen/progesterone receptor-positive transplantable prolactin (PRL)-secreting rat pituitary tumor 7315a. The mechanism of action of these inhibitory effects on tumor growth is unknown. In the present study we investigated the direct effects of these compounds on PRL release by a tumor cell clone derived from the 7315a tumor. E2 stimulated PRL release in FCSABS (10% estrogen-stripped fetal calf serum)-cultured tumor cells in a biphasic manner: at low concentrations (0.1-100 nM) there was a dose-dependent stimulation of PRL release, which decreased in response to 1 microM E2 and which was greatly inhibited by 10 microM E2. Both 2-OHE2 (100 nM and 1 microM) and 2-OHE2 (1 microM) inhibited PRL release by FCS-cultured tumor cells. In FCSABS-cultured tumor cells, 0.1-10 nM 2-OHE1 and 1 microM 2-OHE2 inhibited PRL release, but 1-100 nM 2-OHE2 stimulated PRL release. Tamoxifen (TMX) and its metabolites dihydroxy (di-OH-TMX) and 4-hydroxytamoxifen (4-OH-TMX) inhibited PRL in a dose-dependent manner. The PRL release inhibiting effect of 4-OH-TMX was 100 times more potent that those of TMX and di-OH-TMX, which were similar in their effect. The inhibitory effects of micromolar concentrations of the catecholestrogens on PRL release could be overcome by estradiol, while the inhibitory effects of high concentrations of tamoxifen were not prevented by estradiol. Both "endogenous" (catecholestrogens) and "exogenous" (tamoxifen and its metabolites) antiestrogens and very high concentrations of estradiol directly inhibit PRL secretion by cultured pituitary tumor cells. The mechanism of their anti-tumor effects, however, seems to differ. The catecholestrogens have direct anti-estrogenic effects on cultured tumor cells, which can be antagonized by estradiol. The final effect of their mixed antagonistic/agonistic action depends on the presence or absence of estrogens in the culture medium. Tamoxifen also affects tumor growth probably mainly via a direct effect, partly involving anti-estrogenic and partly direct toxic effects.