Mechanism of action of estrogens and selective estrogen receptor modulators.

Estrogen, one of several sex steroid hormones, mediates its actions through the estrogen receptor. The estrogen receptor (ER) has two subtypes, ER alpha and ER beta, each of which predominates in specific tissues and organs. Cofactor proteins interact with the ER to maximize ligand-dependent transactivation of target-gene promoters. The estrogen response element is the final step in estrogen-mediated gene regulation, and current research is focused on alternate response elements. The resulting biologic action can vary according to the specific type of ER, cofactor milieu, response element, and ligand. Selective estrogen receptor modulators (SERMs) exhibit tissue-specific estrogen agonist or antagonist activity. The SERM raloxifene, which binds to ER and targets a distinct DNA element, may distinguish agonist vs antagonist activity by ER subtype and has unique activity among other SERMs because of its molecular conformation. Phytoestrogens, a potential alternative to hormone replacement therapy and for cancer prevention, do not consistently mimic estrogen's activity. Different types of phytoestrogens have different potencies, and taking high-dose supplements after menopause may not emulate the apparent benefits of lifelong consumption of phytoestrogen-rich diets. In conclusion, the complexity of estrogen action--through different ER subtypes, with various cofactors, on alternate response element--is further enhanced by ligands with selective estrogen activity. Additional research is needed to elucidate these pathways and the resulting biological effects.