Estrogen Response Element and the Promoter Context of the Human and Mouse Lactoferrin Genes Influence Estrogen Receptor alpha-Mediated Transactivation Activity in Mammary Gland Cells

Abstract

The purpose of this research has been to determine whether an extended estrogen response element half-site (ERRE) contributes to the differential estrogen responses of the human and mouse lactoferrin estrogen response element (ERE) in the context of their natural promoters. This research utilized molecular biology techniques to evaluate gene activation. Transfections of MCF-7 cells showed that liganded ER-alpha activates transcription of the human lactoferrin ERE 4-fold higher than the mouse lactoferrin ERE in the context of their natural promoters. Since the ERRE of the human lactoferrin gene naturally occurs 18 bp upstream from the ERE and is absent in the mouse lactoferrin gene promoter, we created a chimeric mouse lactoferrin CAT reporter, which now encodes the ERRE in the identical location as in the human lactoferrin gene. The addition of the ERRE in the mouse lactoferrin gene rendered this reporter extremely responsive to estrogen stimulation. We also demonstrated that the conformation of the estrogen receptor bound to the ERE alone or in the presence of ERRE differed and that the ERRE influenced the selectivity of coactivators in liganded ER-alpha-mediated transcriptional activity. Like the lactoferrin gene ERE, most known natural estrogen response elements are imperfect palindromes that differ from the consensus by at least a 1 base pair change and confer different levels of ER transcriptional activation compared to the consensus ERE. In contrast to our transient transfection data showing a lower estrogen response of the mouse lactoferrin ERE compared to the human lactoferrin ERE in the context of their natural promoters, in vivo data showed that the gene is robustly transcribed in response of estrogen in both species. Therefore, this research model can be applied to studies of genes that have different hormone responses in vivo versus in vitro in an attempt to identify non-typical estrogen response elements that influence ER-alpha-mediated transactivation.