Regulation of the estrogen receptor and its messenger ribonucleic acid in the ovariectomized sheep myometrium and endometrium: The role of estradiol and progesterone

Estrogen receptor (ER) mRNA is dramatically increased in sheep myometrium and endometrium during glucocorticoid-induced premature labor and term spontaneous labor. However, the underlying mechanism for the up- regulation of uterine ER in labor is still unknown. We used ovariectomized (OVX) non-pregnant sheep to analyze the role of estradiol and progesterone in the regulation of myometrial and endometrial ER protein and ER mRNA in vivo. Twenty-one OVX ewes were treated with saline (n = 6), or with estradiol infused i.v. for 2 days (50 μg/day, n = 5), or with an intravaginal progesterone sponge for 10 days (containing 0.3 g progesterone, n = 5), or with an intravaginal progesterone sponge for 10 days with estradiol (50 μg/day) administered on Days 9 and 10 with the progesterone sponge still in place (n = 5). The ER protein concentration in both cytosolic and nuclear compartments, analyzed by Western blot, increased significantly (p < 0.05) in the myometrium after estradiol treatment, while progesterone alone had no detectable effect on ER level. Elevated ER protein was observed only in the nuclear fraction of endometrium. However, when estradiol was given together with progesterone treatment, progesterone antagonized the up-regulatory effect of estradiol on the ER level both at the endometrium and myometrium. The changes in cellular ER mRNA followed the pattern observed at the ER protein level. Estrogen receptor mRNA was elevated significantly (p < 0.01) only in estradiol-treated ewes. Expression of the ER gene in ewes receiving progesterone alone or progesterone combined with estradiol was similar to that of the control group. From these observations we conclude that ER gene expression and active ER synthesis in nonpregnant sheep myometrium and endometrium are estradiol-dependent. Progesterone antagonizes this estrogen action. Progesterone down-regulated the elevated ER mRNA when used together with estradiol. In situ hybridization showed that ER mRNA was evenly distributed in the smooth muscle cells and blood vessels of the myometrium and the epithelial cells of the glands in endometrium. In conclusion, we have observed estradiol-dependent activation of ER gene expression as well as active ER synthesis in the nonpregnant sheep myometrium and endometrium. Progesterone acted as an antagonist of estradiol on ER gene expression.