Estrogen action in the reproductive tract of rhesus monkeys during antiprogestin treatment

We previously reported that RU486 can reverse progesterone (P)-induced suppression of the estrogen receptor (ER) in the uterus of pregnant rhesus monkeys, but we did not determine whether estrogen (E) could act through its receptor in the presence of P and RU486. To pursue this question, we treated spayed rhesus monkeys with various hormonal regimens and evaluated the effects of E in the oviduct and endometrium, with and without RU486 treatment, on ER and progestin receptor (PR) levels, morphology, apoptosis, and degree of proliferation. The latter was assessed immunocytochemically with a monoclonal antibody (Ki-67) against a nuclear antigen present only in proliferating cells. Animals were treated for 2 weeks with 17β-estradiol (E₂) and then for 2 weeks with E₂ plus P to produce a regressed oviduct and a secretory endometrium. The animals were then treated for 2 more weeks with four different treatments, as follows: I) E₂, P, and vehicle; II) E₂ and vehicle; III) E₂, P, and RU486; and IV) E₂ and RU486 (n = 4 each). In group I, menstruation did not occur, and the endometrium exhibited stromal cell enlargement, extensive development of the spiral arteries, few Ki-67-positive cells, and low levels of ER and PR. Oviducts in group I remained regressed, Ki-67-positive epithelial cells were few, and levels of ER and PR were low. In contrast, in groups II, III, and IV, the oviducts had responded to E₂ and were fully ciliated and secretory, with elevated levels of ER and nuclear PR. All animals in these three groups menstruated and then regenerated their endometrium. The regenerated endometria expressed elevations in ER, nuclear PR, and epithelial Ki-67 index. However, the endometria of RU486-treated monkeys in groups III and IV had significantly more epithelial cell death by apoptosis, increased stromal cell compaction, and fewer Ki-67-positive stromal cells than in the E₂ controls (group II). In group IV, RU486 caused a significant decrease in endometrial weight. Thus, RU486 blocked P action and allowed E₂ to act in a normal fashion in the oviduct and endometrium on several end points, but it also had antiproliferative effects that opposed E₂ action, especially in the endometrium.