CATECHOL ESTROGEN FORMATION IN THE MOUSE UTERUS AND ITS ROLE IN IMPLANTATION

Estradiol- 2 4-hydroxylase (E-2/4-H) activity was determined in the mouse uterus during early pregnancy as well as in ovarian steroid hormone-treated ovariectomized uterus. Under the assay conditions used, E-4-H was the predominant catechol estrogen-forming monooxygenase enzyme. The inhibition of E-4-H activity by SKF-525A, metyrapone and α-naphthoflavone suggested involvement of cytochrome P450-dependent monooxygenases. A haloestrogen, 2-fluoroestradiol (2-FL-E2), also inhibited this activity. During the peri-implantation period, no change in uterine E-4-H activity was noted on the morning of days 2 through 5, but the activity significantly (P < 0.01) increased in the afternoon of day 4 of pregnancy. A single injection of estradiol-17β (E2, 100 ng/mouse) to ovariectomized mice significantly (P < 0.01) elevated the level of E-4-H activity at 24 h as did injections of progesterone (P4, 2 mg/mouse) for 2 days. When 2 days of P4 (2 mg/mouse) treatment was combined with a single injection of E2 (20 ng/mouse), E-4-H activity increased 1.3-fold (P < 0.05) by 24 h above that of P4 treatment alone. Dexamethasone (200 μg/mouse) and cholesterol (2 mg/mouse) treatment for 2 days had no effect on E-4-H activity. Thus, the stimulatory effect of P4 and E2 on E-4-H activity appeared to be specific. The increased activity of uterine E-4-H prior to implantation on day 4 evening and the modulation of its activity by P4 and/or E2 suggest an involvement of 4-hydroxyestradiol in embryo implantation. In delayed implanting mice, while E2 as low as 10 ng/mouse induced implantation in all animals, a dose of 2-FL-E2 as high as 100 ng/mouse failed to induce implantation in all animals. Furthermore, administration of 2-FL-E2 (50 ng/mouse) just before E2 (10 ng/mouse) injection completely blocked estradiol-inducible implantation. Interestingly, administration of different doses of prostaglandin E2 (PGE2, 2-5 μg/mouse) when given in combination with 2-FL-E2 as low as 25 ng/mouse induced implantation in most of the animals. These results suggest that at least some of the important actions of estrogen in implantation are mediated via formation of catechol estrogens and PGs. © 1990.