The expression of oestrogen receptor (ER)-β and its variants, but not ERα, in adult human mammary fibroblasts

Whilst oestrogen receptor (ER)-α and ERβ have been shown to be important in the development of the mammary gland, the cell-specific expression pattern of these two receptors within the human breast is not clear. Although it is well established that in the developing rodent mammary gland stromal ERα mediates the secretion of growth factors which stimulate the proliferation of the ductal epithelium, the expression of ERα in human adult breast stromal fibroblasts is controversial, and the expression of ERβ has not been properly defined. In the present study, we have evaluated the expression of ERα and ERβ by immunohistochemistry in normal tissue samples, and in purified human breast fibroblasts by Western blotting, RT-PCR analysis and ligand-binding sucrose gradient assay. Our data clearly demonstrated that ERβ variants, including ERβ1, ERβ2, ERβ5, ERβδ and ERβins, but not ERα, are expressed in human adult mammary fibroblasts. These results are supported by the findings that an ERβ-selective ligand, BAG, but not the ERα high-affinity ligand oestradiol, can induce fibroblast growth factor-7 release and activate transcription from an oestrogen-responsive element promoter in these adult human mammary fibroblasts. Together, these observations revealed that, in the adult breast and in breast cancer, the proliferative signals derived from the stroma of adult mammary glands in response to oestrogen are not mediated by ERα and provide new insights into the nature of stromal-epithelial interactions in the adult mammary gland. In addition, the expression of these ERβ variants in cells where there is no ERα suggested that these ERβ splice forms may have functions other than that of modulating ERα activity.