Implications of the binding of tamoxifen to the coactivator recognition site of the estrogen receptor

    1. Sohaib A Khan
    1. Department of Cancer and Cell Biology, University of Cincinnati College of Medicine, 3125 Eden Avenue, Cincinnati, Ohio 45267-0521, USA1Nuclear Receptor Biology Laboratory, Pennington Biomedical Research Center, Louisiana State University, Baton Rouge, Louisiana 70808, USA
    1. (Correspondence should be addressed to S A Khan; Email: sohaib.khan{at}


    A number of studies have reported on the unusual pharmacological behavior of type I antiestrogens, such as tamoxifen. These agents display mixed agonist/antagonist activity in a dose-, cell-, and tissue-specific manner. Consequently, many efforts have been made to develop so-called ‘pure’ antiestrogens that lack mixed agonist/antagonist activity. The recent report of the structure of estrogen receptor (ER) β with a second molecule of 4-hydroxytamoxifen (HT) bound in the coactivator-binding surface of the ligand-binding domain (LBD) represents the first direct example of a second ER ligand-binding site and provides insight into the possible origin of mixed agonist/antagonist activity of type I antiestrogens. In this review, we summarize the biological reports leading up to the structural conformation of a second ER ligand-binding site, compare the ERβ LBD structure bound with two HT molecules to other ER structures, and discuss the potential for small molecular inhibitors designed to directly inhibit ER-coactivator and, more generally, nuclear receptor (NR)-coactivator interactions. The studies support a departure from the traditional paradigm of drug targeting to the ligand-binding site, to that of a rational approach targeting a functionally important surface, namely the NR coactivator-binding (activation function-2) surface. Furthermore, we provide evidence supporting a reevaluation of the strict interpretation of the agonist/antagonist state with respect to the position of helix 12 in the NR LBD.

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