Effect of estradiol on estrogen receptor expression in rat uterine cell types

In rodent uterus, both up- and down-regulation of estrogen receptor alpha ( ER alpha) messenger ribonucleic acid (mRNA) and protein levels by estradiol has been demonstrated; however, it is not known which of the uterine compa rtments (endometrial epithelium, stroma, myometrium) respond to estradiol w ith autoregulation of ER alpha. The purpose of the present study was to inv estigate and compare the kinetics and cell type-specific effects of estradi ol on uterine ER alpha expression in immature and adult rats. Ovariectomize d female rats were injected s.c, with sesame oil or estradiol-17 beta. Uter i were collected and analyzed far changes in ER alpha mRNA using RNase prot ection assays (RPA) and in situ hybridization using radiolabeled probes spe cific for ER alpha. Immunohistochemical analysis was performed with a polyc lonal antibody specific to ER alpha. Expression of ER alpha in the uterine epithelial cells decreased at 3 and 6 h after estradiol administration to i mmature and adult rats, respectively. At 24 h, ER alpha mRNA levels in the immature and mature rat uterus were higher than pretreatment levels but ret urned to baseline by 72 h. Pretreatment with cycloheximide did not block th e 3-h repressive effect of estradiol, suggesting that the estradiol-induced decrease in ER alpha mRNA occurs independent of new protein synthesis. A d ecrease in ER alpha mRNA and protein was also observed in uterine epithelia at 3 and 6 h after an estradiol injection to immature and adult rats, and intensity of both the in situ hybridization signal and the immunostaining i n the epithelium increased at 24 and 72 h. However, the periluminal stromal cells in the adult uterus and the majority of stromal cells of the immatur e uterus appeared to have increased ER alpha expression. The results indica te that down-regulation of ER alpha in the epithelia and up-regulation of s tromal ER alpha play a role in early events associated with estradiol-induc ed cell proliferation of the uterine epithelia.