Analysis of ligand dependence and hormone response element synergy in transcription by estrogen receptor

In this work we examined two questions: (1) Is the low, but readily detectable, ability of estrogen receptor (ER) to activate transcription in the absence of added 17beta-estradiol caused by traces of estrogen in the growth medium, or by a weak ligand-independent ability of ER to activate transcription? (2) Does the ER exhibit synergistic activation of transcription on reporter genes containing multiple estrogen response elements (EREs)? To study these questions we developed a powerful new reporter gene, containing four EREs, which achieves inductions of up to 330-fold in the presence of liganded ER. We provided several types of evidence indicating that under standard cell culture conditions unliganded ER is unable to activate transcription. We demonstrated that when cells are grown in serum-free medium, estrogenic compounds may be in the base tissue culture medium. We demonstrated a strong cell and ER-dependence in transcriptional synergy, and suggest that cooperative binding of ER to multiple EREs can be responsible for transcriptional synergy in vivo.