Effects of glucocorticoids on estrogen receptor messenger ribonucleic acid in the pregnant ovine myometrium in vivo and in vitro

Abstract

We recently reported that estrogen receptor (ER) mRNA is dramatically increased in the sheep myometrium during cortisol-induced premature labor and term spontaneous labor. In this study, we compared myometrial ER mRNA and ER protein levels in tissues from pregnant sheep during infusions of two different glucocorticoids (dexamethasone and betamethasone). Tissues from glucocorticoid-infused sheep not yet in labor were compared with tissues from sheep in labor. The population of ER mRNA-containing cells in myometrium obtained from sheep in labor was determined by in situ hybridization and compared with that of sheep not in labor in order to understand how the cellular distribution of ER mRNA changed during labor. Finally, in vitro myometrial cell culture and immunolocalization of glucocorticoid receptor (GR) were used to determine 1) the effects of different steroids on ER mRNA content, 2) whether or not the myometrium is a functional site for glucocorticoid action, and 3) whether the change in ER mRNA content in the myometrium might be induced by glucocorticoid acting through GR. Increased ER mRNA and protein were found only in the myometrium associated with labor. In situ hybridization of ER mRNA and immunolocalization of ER protein showed that ER mRNA and protein were mainly located in the smooth muscle cells and endothelial cells of blood vessels. In vitro treatment of cultured myometrial cells with hydrocortisone resulted in a 3.5-fold increment in ER mRNA. In contrast, there was no obvious change in ER mRNA when myometrial cells were treated in vitro with either estradiol (10 nM) or progesterone (100 nM) for 24 h. GR was localized exclusively in the pregnant sheep myometrium. We conclude that 1) increased ER protein and ER mRNA in sheep myornetrium are strongly associated with labor; 2) an increase in the population of ER-positive cells is associated with the increment of ER mRNA during glucocorticoid-induced labor; 3) in the pregnant sheep myometrium, both the smooth muscle cells and the endothelial cells of blood vessels are positive for ER protein ER mRNA; and 4) hydrocortisone is a potent stimulus in vitro for increasing ER mRNA content, presumably acting through the GR present in the pregnant sheep myometrium.