Hormone-dependent recruitment of NF-Y to the uteroglobin gene enhancer associated with chromatin remodeling in rabbit endometrial epithelium

Expression of the rabbit uteroglobin gene is hormonally induced in cells of the endometrial epithelium during the preimplantation phase of pregnancy. Here we show that progesterone activation of the gene is mediated by two clusters of hormone responsive elements located between 2.4 and 2.7 kilobase pairs upstream of the transcriptional start site. Between these two clusters, genomic footprinting studies in the intact endometrial epithelium reveal the hormone-inducible occupancy of several cis-acting elements. One of the protected elements shows sequence homology to the consensus binding site of the transcription factor NF-Y, which binds to the element in gel shift experiments. This uteroglobin Y box is essential for enhancer activity in transient transfection experiments with endometrial and non-endometrial cell lines, in accordance with the ubiquitous expression of NF-Y. To understand why binding of this ubiquitous factor to the uteroglobin Y box in endometrium depends on hormone induction, we examined the chromatin structure of the relevant gene region. In the uninduced state, the enhancer region appears to be organized into positioned nucleosomes. Upon hormone induction, this nucleosomal pattern is lost and the enhancer region becomes hypersensitive to nucleases, suggesting that a hormone-induced change in the local chromatin structure unmasks previously unaccessible binding sites for transcription factors. Our results emphasize the limitations of using transient transfection assays for the functional analysis of cis-acting elements and underline the need for including the native chromatin organization in this kind of studies.