The interaction of 14 steroidal and nonsteroidal estrogen agonists and antagonists with the intracellular estrogen receptor system was examined in cell suspensions prepared from the testes of mice that develop malignant Leydig cell tumors after prolonged estrogen administration. The ability of these substances to stimulate DNA synthesis in short-term (3-day) studies and to provoke Leydig cell hyperplasia with prolonged (3-mo) administration was also measured. Our data were consistent with the proposal that, in Leydig cells, the carcinogenic effects of estrogens are mediated through the intracellular receptor complex that results in a localization of hormone bound to chromatin and nuclear matrix.

All tested compounds displaced 17β-[3H]estradiol from the cytosolic estrogen receptor, but to varying degrees; and there was no discernible relationship between their ability to compete for this receptor and their efficacy in stimulating DNA synthesis. The effect of the test compounds on the levels of estrogen receptor in cytosol and in nuclei was measured by [3H]estradiol exchange. 17β-Estradiol, equilin, 17α-ethinylestradiol, diethylstilbestrol, hexestrol, dienestrol, coumestrol, and nafoxidine provoked a complete estrogen receptor response: acutely a decrease in the level of cytosolic estrogen receptor and an increase in the nuclear estrogen receptor. All of these substances acutely stimulated DNA synthesis. Tamoxifen, clomiphene, and nitromifene provoked a decrease in cytosolic receptor but no increase in demonstrable nuclear estrogen receptor. 17α-Estradiol, mestranol, and estriol did not significantly alter the levels of estrogen receptor in cytosol or nuclei. Only those substances that increased measurable nuclear estrogen receptor also acutely stimulated DNA synthesis.

Chronic (3-mo) treatment of 2-mo-old male BALB/c mice with diethylstilbestrol, 17β-estradiol, ethinylestradiol, and nafoxidine led to Leydig cell hyperplasia. Chronic mestranol treatment also provoked Leydig cell hyperplasia; this is most probably due to induction of liver metabolism of mestranol to ethinylestradiol. Chronic treatment with 17α-estradiol, tamoxifen, and clomiphene failed to produce significant histologic al changes in the testes. Only chronic administration of those substances that exhibited a complete estrogen receptor response and acutely stimulated DNA synthesis produced Leydig cell hyperplasia.

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