Hormone-induced recruitment of Sp1 mediates estrogen activation of the rabbit uteroglobin gene in endometrial epithelium

Steroid hormones activate gene expression by interaction of their receptors with hormone-responsive DNA elements and tissue-specific or ubiquitous factors. To monitor the molecular changes on the promoter of the rabbit uteroglobin gene during early pseudopregnancy in vitro, we have applied the genomic footprinting methodology to endometrial tissue, Estrogen induction results in the simultaneous occupancy of an estrogen-re sponsive element and an adjacent GC/GT box in the promoter, DNA binding assays demonstrate that the corresponding regulatory factors are the ligand-induced estrogen receptor and the ubiquitous transcription factor Sp1, Both factors functionally synergize in primary endometrial cells, showing that the GC/GT box is an essential part of a composite estrogen-responsive unit, However, the estrogen receptor and Sp1 do not bind cooperatively to their sites in vitro, suggesting that other mechanisms might be responsible for the hormone-dependent binding of Sp1 in vivo. Since hormone treatment leads to the appearance of a distinct DNase I-hypersensitive site over the promoter chromatin, an estrogen-induced change in the local chromatin structure could facilitate binding of Sp1 in vivo.