Molecular mechanisms of estrogen action: selective ligands and receptor pharmacology

Estrogens exert profound effects on the physiology of diverse target cells and these effects appear to be mediated by two estrogen receptor (ER) subty pes, ER alpha and ER beta. We have investigated how ER ligands, ranging fro m pure agonists to antagonists, interact with ER alpha and ER beta, and reg ulate their transcriptional activity on different genes. Mutational mapping -structure activity studies indicate that different residues of the ER liga nd binding domain are involved in the recognition of structurally distinct estrogens and antiestrogens. We have identified from ligands of diverse str ucture, several particularly interesting ones that are high potency selecti ve agonists via ER alpha and others that are full agonists through ER alpha while being full antagonists through ER beta. Antiestrogens such as hydrox ytamoxifen, which are mixed agonist/antagonists through ER alpha are pure a ntagonists through ER beta at estrogen response element-containing gene sit es. Studies with ER alpha/beta chimeric proteins reveal that tamoxifen agon ism requires the activation function 1 region of ER alpha. Through two-hybr id assays, we have isolated an ER-specific coregulator that potentiates ant iestrogen antagonist effectiveness and represses ER transcriptional activit y. We have also focused on understanding the distinct pharmacologies of ant iestrogen- and estrogen-regulated genes. Although antiestrogens are thought to largely act by antagonizing the actions of estrogens, we have found amo ng several new ER-regulated genes, quinone reductase (QR), a detoxifying ph ase 11 antioxidant enzyme, that has its activity up-regulated by antiestrog ens in an ER-dependent manner in breast cancer cells. This response is anta gonized by estrogens, thus showing 'reversed pharmacology'. Increased QR ac tivity by antiestrogens requires a functional ER (ER alpha or ER beta) and is, interestingly, mediated via the electrophile response element in the QR gene 5' regulatory region. The up-regulation of QR may contribute to the b eneficial effects of tamoxifen, raloxifene, and other antiestrogens in brea st cancer prevention and treatment. Estrogens rapidly up-regulate expressio n of several genes associated with cell cytoarchitectural changes including NHE-RF, the sodium hydrogen exchanger regulatory factor, also known as EBP 50. NHE-RF/EBP50 is enriched in microvilli, and may serve as a scaffold ada ptor protein in regulating early changes in cell architecture and signal tr ansduction events induced by estrogen. Analyses of the regulatory regions o f these primary response genes, and the antioxidant and other signaling pat hways involved, are providing considerable insight into the mechanisms by w hich ligands, that function as selective estrogen receptor modulators or SE RMs, exert their marked effects on the activities and properties of target cells. The intriguing biology of estrogens in its diverse target cells is t hus determined by the structure of the ligand, the ER subtype involved, the nature of the hormone-responsive gene promoter, and the character and bala nce of coactivators and corepressors that modulate the cellular response to the ER-ligand complex. The continuing development of ligands that function as selective estrogens or antiestrogens for ER alpha or ER beta should all ow optimized tissue selectivity of these agents for menopausal hormone repl acement therapy and the treatment and prevention of breast cancer. (C) 2000 Elsevier Science Ltd. All rights reserved.