Segregation of steroid receptor coactivator-1 from steroid receptors in mammary epithelium

Steroid receptor coactivator-1 (SRC-1) family members interact with steroid receptors, including estrogen receptor alpha (ER alpha) and progesterone r eceptor (PR), to enhance ligand-dependent transcription. However, the expre ssion of ER alpha and SRC-1 was found to be segregated in distinct subsets of cells within the epithelium of the estrogen-responsive rat mammary gland . This finding was in contrast to the finding for the stroma, where signifi cant numbers of cells coexpressed ER alpha and SRC-1. Treatment of animals with estrogen induced PR expression in the ER alpha-expressing mammary epit helial cells in the absence of detectable SRC-1 and did not affect the segr egated pattern of SRC-1 and ER alpha expression. PR was neither expressed n or induced by estrogen treatment in stroma, despite the coexpression of ER alpha and SRC-1, These results suggest that SRC-1 is not necessary for ER a lpha-mediated induction of PR in mammary epithelial cells and is also not s ufficient, for PR induction in stromal cells expressing both ER alpha and S RC-1. Furthermore, the expression of SRC-1 in a subpopulation of mammary ep ithelial cells distinct from those expressing ER alpha or PR raises the pos sibility that SRC-1 has cell type-specific functions other than simply to a ct as coactivator for ER alpha or PR in the mammary epithelium.