Cross-talk between Stat5b and estrogen receptor-alpha and -beta in mammary epithelial cells

Both 17 beta -estradiol and prolactin play important roles in the mammary gland, raising the possibility of functional cross-talk between the two signaling pathways. Here, we demonstrate that estrogen receptor-alpha (ER alpha) and -beta (ER beta) are both able to potentiate transcription from a Stat5-responsive promoter when activated by prolactin. Potentiation was observed not only in the presence of 17 beta -estradiol, but also in the presence of anti-estrogens such as tamoxifen and ICI 182,780. The magnitude of the response was dependent on cell-type: in the HC11 mouse mammary epithelial cell line ER beta potentiates transcription efficiently whereas ER alpha showed low, activity. Conversely, in COS-7 cells, both estrogen receptors were active. We show that activation domains in the N-terminus (AF-1) and the C-terminus (AF-2) of the ERs are dispensable for potentiation. The effects are dependent on the presence of an intact DNA-binding/hinge domain, which we show is capable of interacting with Stat5b in vitro and in HC11 cell extracts. We conclude that ER alpha and ER beta act as coactivators for Stat5b through a mechanism which is independent of AF-1 and AF-2