Estrogen responses in bovine fetal uterine cells involve pathways directed by both estrogen response element and activator protein-1

Objectives were to examine possible roles of estrogen receptor (ER) in development of the bovine uterine endometrium in the context of ER type, enhancer type, and ligand-independent activation. Expression vectors producing either ER alpha or ER beta were introduced into fetal uterine cells from Day 110 to 120 of gestation (UBF120 cells) and into rat embryo fibroblasts (Rat-1 cells), neither of which express endogenous ER. Reporter constructs containing either an estrogen response element (ERE) or activator protein-1 (AP-1) response element were cotransfected. These reporters were also transfected into fetal uterine cells from Day 180 to 200 of gestation (UBF180 cells), which express ER, In UBF120 and Rat-1 cells transfected with either ER alpha or ER beta, treatment with estradiol-17 beta (E-2) resulted in increased activity of an ERE reporter construct, but not an AP-1 element reporter construct. The antiestrogen ICI 182,780 (ICI) exhibited E-2 antagonist activity with both ER alpha and ER beta. Thus, all components were present for E-2-dependent transcription from an ERE except ER; however, cells were not competent for E-2-dependent transcription mediated through AP-1. In UBF180 cells, E-2 treatment increased both ERE and AP-1 reporter activity. ICI exhibited E-2 antagonist activity. Treatment with epidermal growth factor resulted in increased ERE reporter activity that was inhibited by ICI, indicative of ligand-independent activation of ER. These data suggest that multiple pathways for ER-mediated gene regulation occur in the developing fetal uterus and that nuclear components necessary for action of both ER alpha and ER beta are present prior to expression of the receptor.