Potential regulation of membrane trafficking by estrogen receptor alpha via induction of Rab11 in uterine glands during implantation

The steroid hormone estrogen profoundly influences the early events in the uterus leading to embryo implantation. It is thought that estrogen triggers the expression of a unique set of genes in the preimplantation endometrium that in turn control implantation. To identify these estrogen-induced gene s, we used a delayed implantation model system in which embryo attachment t o endometrium is dependent on estrogen administration. Using a mRNA differe ntial display (DD) method, we isolated a number of cDNAs representing mRNAs whose expression is either turned on or turned off in response to an impla ntation-inducing dose of estrogen. We identified one of these cDNAs as that encoding rab11, a p21ras-like GTP-binding protein (G protein), which funct ions in the targeting of transport vesicles to the plasma membrane. In norm al pregnant rats, rab11 mRNA was expressed at low levels on days 1-2 of pre gnancy, but its expression was markedly enhanced (similar to 6- to 8-fold) between days 3-5 immediately before implantation. In situ hybridization and immunocytochemistry revealed that rab11 expression in the uterus was predo minantly in the glandular epithelium. In ovariectomized rats, the expressio n of rab11 mRNA was induced in the endometrium in response to estrogen. To determine whether this effect of estrogen was mediated through its nuclear receptors, we examined rab11 expression in a transformed endometrial cell l ine, Ishikawa. In transient transfection experiments, we observed that over expression of estrogen receptor (ER) alpha or beta induced endogenous rab11 mRNA in a hormone-dependent manner. ER bound to an antagonist, ICI 182,780 , failed to activate this gene expression. These findings, together with th e observation that ER alpha but not ER beta is detected in the glands of th e preimplantation uterus, indicate that rab11 is one of the proteins that a re specifically induced by estrogen-complexed ER alpha in rat endometrium a t the onset of implantation. Our results imply that estrogen, which induces the synthesis of many growth factors and their receptors and other secreto ry proteins that are thought to be critical for implantation, may also faci litate their transport to the membrane and/or secretion by stimulating the expression of rab11, a component of the membrane-trafficking pathway. This study therefore provides novel insights into the diverse cellular mechanism s by which estrogen, acting via its nuclear receptors, may influence blasto cyst implantation.