and the Estrogen Receptor

Ifeoma Archimalo '18 

and Sarah  Kazzaz '18


I. Introduction

Estradiol is a common drug used to treat the symptoms of menopause, such as hot flashes. It functions by increasing the amount of estrogen in the body, activating transcription of the estrogen nuclear receptors. However, by increasing the expression of ER, it puts the user at increased risk of developing breast, ovarian, and uterine cancer (where expression of estrogen increases proliferation of mammary cells and possibility of malignant cell development).

Esr1 encodes for estrogen receptor-alpha, one of the two nuclear receptors that mediates the action of 17B-estradiol (E2). Transcriptional activation of the receptor involves the action of two distinct activation functions, AF1 and AF2, located in the N-terminal. When a ligand, such as E2, binds the receptor undergoes a conformational change, facilitating the recruitment of co-activators and the direct binding to cis-acting elements.

In this conformation of ER-alpha, the nuclear receptor is a complex of three receptors (homo trimer) and was triple mutated. At three sites--Cys-381, Cys-417, and Cys-530--cysteine was changed to serine. By changing these amino acid, a partial agonism structure of ER-alpha was back generated, and the position of helix H12 is at the heart of the mechanism.

II. Estrogen Receptor

ER-alpha belongs to the family of nuclear receptors. All members of this family are composed of six domains (A-F). The E region of the ER receptor is the ligand binding domain (LBD), which contains AF-2, a ligand-dependent transactivation factor. Structurally, the LBD is composed primarily of alpha helix folds.

The estrogen receptor belongs to the class of steroid nuclear receptors, which require a ligand to stabilize the receptor and control gene expression.When the receptor is ligand-bound, it will move from the cytoplasm to the nucleus, the receptor will form a dimer, with if then able to bind to DNA of estrogen response elements.The ligand-receptor complex will then be able to recruit other proteins needed for transcription.

In many cancers, especially of the breast, the relative expression of the receptor is determined and utilized by oncologists to guide their treatment of a patient's specific cancer. Most breast cancer cells are considered ER+, meaning they have an ER expression >50% (determined by immunohistochemistry). With an increased expression of ER, proliferation of mammary cells increases, increasing cell division and DNA replication (and thus the chances of mutation) and disrupting apoptosis. Thus when a patient is diagnosed as ER+, oncologists will prescribe drugs. like tamoxifen, which act as estrogen and bond to ER, but will reduce change ints conformation to inactivate transcription, hindering the expression of estrogen nuclear receptors

III. Helix H12

The primary component of the ER receptor that determines whether transcription will occur is the helix H12 . This helix carries AF2, an activation function.

In the antagonist position (transcriptionally inactive)  conformation, a ligand is not bound to the nuclear receptor and the helix is exposed to the solvent and the receptor is unstable. The antagonist position of H12 is not unique.

In the agonist position, ligand binding is present and triggers H12 to reposition on the core of the ligand binding domain, much like a lid closing the ligand pocket. 

When the ER-alpha receptor was triple mutated (C381S, C417S, C530S) the H12 helix (highlighted in red) moved from the agonist position to the antagonist position, despite the presence of ligand binding, creating a partial agonism structure

IV. Estradiol Binding

Estradiol is a ligand that binds to the ER receptor, changing its conformation and activating transcription. The ligand interacts with His-524 through the 17B-hydroxyl . Then His-524 then forms a hydrogen bond with the peptidic carbonyl group of Glu-419 in the loop 6-7 . Glu-419 forms a hydrogen bond network with Glu-339 of helix H3 and Lys-531 of helix H11 . This network favors the proper position of helix H12, into the agonist position.

In addition, ligand binding, faciliates the activation of AF2, the ligand dependent transactivation function.

V. Tamoxifen Binding

Tamoxifen is structurally similar to estradiol and its other derivatives. Therefore it is able to interact and bind to the same binding sites of the ER receptor. However, the result of tamoxifen binding varies greatly from that estradiol, and in actuality has a range of effects. It may simply inactive the receptor, but it may also activate it, but interacts with other proteins and DNA. It has also been observed that when tamoxifen is bound, H12 remains in an antagonist position, keeping the transcription levels low. Regardless of the mechanism, tamoxifen inhibits the growth of cancer cells, and is therefore used widely in the clinical treatment of breast and uterine cancers. 

VI. References

Billon-Gales, Audree Krust, Coralie Fontaine, Anne Abot, Gilles Flouriot, Celine Toutain, Hortense Berges, Alain-Pierre Gadeau, Francoise Lenfant, Pierre Gourdy ,Pierre Chambon, and Jean-Francois Arnal. "Activation Function (AF2) of Estrogen Receptor-alpha Is Required for the Atheroprotective Action of Estradiol but Not to Accelerate Endothelial Healing." Proceedings of the National Academy of Sciences of the United States of America.  National Academy of Sciences, 25 July 2011.

Gangloff, M., Ruff, M., Eller, S., Duclaud, S., Wurtz, J.M., and Moras, D. 2001.
Crystal Structure of a Mutant hERalpha Ligand-Binding Domain Reveals Key Structural Features for the Mechanism for Partial Agonism. J.Biol.Chem. 276: 15059

Goodsell, David S. "The Molecular Perspective: Tamoxifen and the Estrogen Receptor." The Oncologist: Fundamentals of Cancer Medicine." April 2002.

Hall, Julie M., John H. Couse, and Kenneth S. Korach. "The Multifaceted Mechanisms of Estradiol and Estrogen Receptor Signaling." The Journal of Biological Chemistry, 17 July 2001.