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Tamoxifen, Diethylstilbesterol and the Estrogen Receptor Ligand Binding Region

Sara Pecorak '04 and Tom Susman '04


I. Introduction

Recent studies have found that high estrogen levels combined with a genetic predisposition to breast cancer result in a higher occurance of the disease (3).  Estrogen has many functions throughout the body, many of which are regulated through an activated estrogen receptor transcription factor.  These transcription factors, in disease prone patients, can activate genes which lead to uncontrolled cancerous growth.  Researchers have hypothesized that altering the binding of the estrogen to the estrogen receptors can be used in the prevention and treatment of breast cancer (1).

Estrogen binds to the Estrogen receptor which then allows the receptor to bind DNA at the estrogen response element (ERE), a cis-acting enhancer sequence.  The estrogen-receptor-ERE complex then initiates transcription of genes related to the reproductive cycle.  Tamoxifen has been found to be a competitive inhibitor of estrogen-estrogen receptor a (ERa) binding (3).  This drug has been administered clinically and found to decrease the occurance of breast cancer in disease prone patients.  Patients undergoing treatment with tamoxifen are found to have decreased breast tissue density, an indication of a lowered breast cancer risk (1).  Crystal structures of the ERa in complex with estrogen, a synthetic estrogen, and 4-hydroxytamoxifen provide insights into the structural changes caused by this competitive inhibition.  Tamoxifen binds to the ligand binding domain of ERa and causes a conformational shift of helix 12 into an adjacent coactivator site which in turn prevents ERa from binding a coactivator (NR box peptide) which would then activate a specific DNA sequence, the estrogen response element (ERE) (2, 3).

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II. General Structure of Estrogen/ Estrogen Receptor Complex

A common natural estrogen found in human systems is estradiol, commonly refferred to as E-2. Estradiol and its analogs are characterized by having a 3-hydroxy group as opposed to a 3-keto group found in many other steroids (4). These hydroxyl groups are critical to the overall structre of the receptor complex.

The estrogen receptor featured here is the estrogen receptor a ligand binding domain,  and is complexed with estradiol.  ERa Ligand binding domain (LBD) contains twelve a-helices, two b- sheets, all connected with several short straight amino acid chains (in white) . The ERa LBD has two functionally important regions, the first being the ligand binding pocket (estrogen in white) . The estradiol is oriented in the ligand binding pocket with hydrogen bonding and van der Waals contacts. Functionally important polar amino acids in the binding pocket include Glu-353, Arg-394, and His-524 (4). Glu-353 interacts in a H-bond with the 3-hydroxy group. The sidechain of Glu-353 is itself braced by a water mediated H-bond to Arg-394, which is further braced by a H-bond to the carbonyl of a neighboring phenalyanine (Phe-404) (4). The Phe-404 is involved in a hydrophobic interaction to A-ring of the estradiol, thus making the polar OH group mediated through a hydrophobic interaction (4). Eighteen different amino acid residues, including leucine, alanine, methionine, glycine, phenylalanine, and isoleucine (in white), located between amino acid number 340 and 530 create hydrophobic bonds to the estradiol . These interactions are esstential for stabilizing the nonpolar elements of the estradiol ring structure.

The coactivator site, a necessary binding region for transcriptional activation, is composed of exposed surfaces from helices 3, 4, 12, and a signature loop. It should be noted that the extended helix 12 is an anomaly of the crystallization conditions used to make this chime image (4). The major stabilizing factor of the coactivator site includes a hydrophobic interaction between a glutamate in helix 12 and hydrophobic residues in helix 4 . Also, Lys-362 and Val-364 in the loop between helices 3 and 4 (the signature sequence) are also essential for coactivator interaction (4). Helix 12, and its high degree of mobility in the presence of different ligands, illustrates the major functional element of the estrogen receptor (4).

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III. Structure of the Synthetic Estrogen (DES)/ Estrogen Receptor Complex

(DES) is a synthetic estrogen.  The structures of the two molecules are not the same.

However, DES can mimic the function of a natural estrogen and act as ligands because it has similar interactions with the ERa LBD (DES in red) .   The LBD consists of a hydrophobic cavity formed by residues from helices 3,6,7,8, 11, and 12 (3). DES makes three hydrogen bonds with residues of the LBD, His 524, Glu 353, Arg 394 .   Ring A prime binds to the residues contributed by helices 7, 8, and 9, mimicking the interactions of the estrogen C and D rings  (3). The protruding ethyl groups of DES make 4 additional non-polar contacts to Ala 350, Leu 384, Phe 404 and Leu 428 which are not made by estrogen (3).  DES also contacts Met 421 and Met 528 which are not made by estrogen (3).   The position of helix 12 is also different.  Instead of extending from the protein, it is folded onto the molecule. Click here to see a comparison of helix 12 posistion in the three different complexes.

When DES binds to the ERa , a hydrophobic groove  is formed by helices 3, 4, 5 and 12 and the turn between helices 3 and 4 (3).  This groove is known as the NR box, the binding site of the NR Box peptide which is a coactivator.  The NR box peptide contains a signature motif, LXXLL (X=any residue) (3).

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IV. Structural Changes caused by 4-hydroxytamoxifen

There are two critical differences between the structure of 4-hydroxytamoxifen (OHT) and estrogen-like substances.   It lacks a second hydoxyl group and has an additional tail extending from the A ring containing an oxygen and a nitrogen.  

These differences account for the overall changes in the ERa when 4-hydroxytamoxifen is bound to the LBD.  In the ligand binding pocket, there are several hydrophobic interactions that are similar to those made by DES but there are also significant changes .  The dimethylaminoethyl sidechain which extends from  ring C of OHT extends into a space between helices 3 and 11, causing the formation of several new hydrophObic interactions and a cascade effect changing conformations throughout the entire molecule (3).  As a result of this sidechain, the B ring of OHT is forced more deeply into the binding pocket than the A' ring of DES causing further structural changes in the ERa (3).  

OHT makes makes only 2 hydrogen bonds with the ligand binding pocket, Glu-353 and Arg-394 b as opposed to the 3 made by both DES and estrogen.  These binding differences cause portions of helices 3, 8, and 11 to extend (3).

The most significant change in structure caused by OHT binding is the repositioning of helix 12.  Helix 12 repositions over the NR box coactivator site and prevents binding of the NR box protein .  The conformation of the NR box is unchanged.  Helix 12 proteins Leu-540, Leu-544, and Met-543 replace the NR box protein LXXLL motif (3).  Without this coactivator binding, the ERa  remains unactivated and cannot activate the ERE gene nullifying the function of ERa.

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V. References

1. Atkinson C., R. Warren, S.A. Bingham,  and N.E. Day.  Mamographic patterns as a predictive biomarker of breast cancer risk:  effect of tamoxifen.   Cancer Epidemiology, Biomarkers and Prevention 8:  863-866.

2. Klinge, Carolyn M., Sarah C. Jernigan, Stacy l. Smith, Valentyn V. Tyulmenkov, and Peter C. Kulakosky.  2001.  Estrogen response element sequence impacts the conformation and tranxcriptional activity of estrogen receptor alpha.   Moleculat and Cellular Endocrinology 174: 151-166.

3. Shiau, Andrew K., Dannielle Barstad, Paula M. Loria, Lin Cheng, Peter J. Kushner, David A. Agard, and Geoffrey L. Greene. 1998. The Structural Basis of Estrogen Receptor/Coactivator Recognition and the Antagonism of this Interaction by Tamoxifen. Cell  95:  927-937.

4. Tanenbaum, David M., Yong Wang, Shawn P. Williams, and Paul B. Sigler.  1998. Crystallographic comparison of the Estrogen and Progesterone Receptor's Ligand Binding
Domains.  Proceedings of the national Academy of Sciences of the United States of America 95:  5998-6003.

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