Control of the estrogen-like actions of the tamoxifen-estrogen receptor complex by the surface amino acid at position 351

Rational approaches of drug design for the development of selective estrogen receptor modulators (SERMs), implicated in breast cancer

Drug discovery and development have gained momentum due to the rational drug design by engaging computational tools and bioinformatics methodologies. Bioisosteric replacements and hybrid molecular approaches are the other inventive processes, used by medicinal chemists for the desired modifications of leads for clinical drug candidates. SERMs, ought to produce inhibitory activity in breast, uterus and agonist activity in other tissues, are beneficial for estrogen-like actions. ER subtypes α and β are hormone dependent modulators of intracellular signaling and gene expression, and development of ER selective ligands could be an effective approach for treatment of breast cancer. This report has critically investigated the possible designing considerations of SERMs, their in silico interactions, and potent pharmacophore generation approaches viz. indole, restricted benzothiophene [3, 2-b] indole, carborane, xanthendione, combretastatin A-4, organometallic heterocycles, OBHS-SAHA hybrids, benzopyranones, tetrahydroisoquinolines, Dig G derivatives and their specifications in drug design and development, to rationally improve the understanding in drug discovery. This also includes various strategies for the development of dual inhibitors for the management of antiestrogenic resistance.

Ligand Binding Induces Agonistic-Like Conformational Adaptations in Helix 12 of Progesterone Receptor Ligand Binding Domain

Progesterone receptor (PR) is a member of the nuclear receptor (NR) superfamily and plays a vital role in the female reproductive system. The malfunction of it would lead to several types of cancers. The understanding of conformational changes in its ligand binding domain (LBD) is valuable for both biological function studies and therapeutically intervenes. A key unsolved question is how the binding of a ligand (agonist, antagonist, or a selective modulator) induces conformational changes of PR LBD, especially its helix 12. We applied molecular dynamics (MD) simulations to explore the conformational adaptations of PR LBD with or without a ligand or the co-repressor peptides binding. From the simulations, both the agonist progesterone (P4) and the selective PR modulator (SPRM) asoprisnil induces agonistic-like helix 12 conformations (the "closed" states) in PR LBD and the complex of LBD-SPRM is less stable, comparing to the agonist-liganded PR LBD. The results, therefore, explain the partial agonism of the SPRM, which could induce weak agonistic effects in PR. We also found that co-repressor peptides could be stably associated with the LBD and stabilize the LBD in a "semi-open" state for helix 12. These findings would enhance our understanding of PR structural and functional relationships and would also be useful for future structure and knowledge-based drug discovery.

Structural underpinnings of oestrogen receptor mutations in endocrine therapy resistance

Oestrogen receptor-α (ERα), a key driver of breast cancer, normally requires oestrogen for activation. Mutations that constitutively activate ERα without the need for hormone binding are frequently found in endocrine-therapy-resistant breast cancer metastases and are associated with poor patient outcomes. The location of these mutations in the ER ligand-binding domain and their impact on receptor conformation suggest that they subvert distinct mechanisms that normally maintain the low basal state of wild-type ERα in the absence of hormone. Such mutations provide opportunities to probe fundamental issues underlying ligand-mediated control of ERα activity. Instructive contrasts between these ERα mutations and those that arise in the androgen receptor (AR) during anti-androgen treatment of prostate cancer highlight differences in how activation functions in ERs and AR control receptor activity, how hormonal pressures (deprivation versus antagonism) drive the selection of phenotypically different mutants, how altered protein conformations can reduce antagonist potency and how altered ligand-receptor contacts can invert the response that a receptor has to an agonist ligand versus an antagonist ligand. A deeper understanding of how ligand regulation of receptor conformation is linked to receptor function offers a conceptual framework for developing new anti-oestrogens that might be more effective in preventing and treating breast cancer.

The Impact of ESR1 Mutations on the Treatment of Metastatic Breast Cancer

After nearly 20 years of research, it is now established that mutations within the estrogen receptor (ER) gene, ESR1, frequently occur in metastatic breast cancer and influence response to hormone therapy. Though early studies presented differing results, sensitive sequencing techniques now show that ESR1 mutations occur at a frequency between 20 and 40% depending on the assay method. Recent studies have focused on several "hot spot mutations," a cluster of mutations found in the hormone-binding domain of the ESR1 gene. Throughout the course of treatment, tumor evolution can occur, and ESR1 mutations emerge and become enriched in the metastatic setting. Sensitive techniques to continually monitor mutant burden in vivo are needed to effectively treat patients with mutant ESR1. The full impact of these mutations on tumor response to different therapies remains to be determined. However, recent studies indicate that mutant-bearing tumors may be less responsive to specific hormonal therapies, and suggest that aromatase inhibitor (AI) therapy may select for the emergence of ESR1 mutations. Additionally, different mutations may respond discretely to targeted therapies. The need for more preclinical mechanistic studies on ESR1 mutations and the development of better agents to target these mutations are urgently needed. In the future, sequential monitoring of ESR1 mutational status will likely direct personalized therapeutic regimens appropriate to each tumor's unique mutational landscape.

Flexible small molecular anti-estrogens with N,N-dialkylated-2,5-diethoxy-4-morpholinoaniline scaffold targets multiple estrogen receptor conformations

Estrogen mediates various cellular processes including cell proliferation, differentiation, growth and mammary gland function. Estrogen Receptors (ERs) are expressed in 70% of breast cancers. Consequently, estrogen mediated ER signaling plays a critical role in breast cancer diagnosis, prognosis, and treatment. ERs are ligand-triggered transcription factors. However, in the absence of a cognate estrogenic ligand, ERs can be activated by a variety of other extracellular signals. Tamoxifen, an anti-estrogen that selectively targets ER, induces substantial regression of breast tumors and an increase in disease-free survival. Tamoxifen mimics estrogen effects in other tissues thereby providing some beneficial effects including reduced risk of osteoporosis. However, breast cancers that initially respond well to tamoxifen tend to develop resistance and resume growth despite the continued presence of the antagonist. Library of compounds with substituted morpholinoaniline scaffold, a set of structurally divergent potential ER antagonists that fit the tamoxifen pharmacophore, were designed to target ER Ligand Binding Domain (LBD) and to recruit co-regulator proteins including BRCA1 over a range of conformational changes. Two of the lead compounds in the library, BR46 and BR47, were found to inhibit estrogen induced cell proliferation and cell viability. Discovery of novel lead molecules targeting ligand binding pockets of hER has provided structural clues toward the development of new breed of small molecule therapeutics for tamoxifen-resistant breast cancers and would complement already existent anti-estrogen therapy.

The molecular, cellular and clinical consequences of targeting the estrogen receptor following estrogen deprivation therapy

During the past 20 years our understanding of the control of breast tumor development, growth and survival has changed dramatically. The once long forgotten application of high dose synthetic estrogen therapy as the first chemical therapy to treat any cancer has been resurrected, refined and reinvented as the new biology of estrogen-induced apoptosis. High dose estrogen therapy was cast aside once tamoxifen, from its origins as a failed "morning after pill", was reinvented as the first targeted therapy to treat any cancer. The current understanding of the mechanism of estrogen-induced apoptosis is described as a consequence of acquired resistance to long term antihormone therapy in estrogen receptor (ER) positive breast cancer. The ER signal transduction pathway remains a target for therapy in breast cancer despite "antiestrogen" resistance, but becomes a regulator of resistance. Multiple mechanisms of resistance come into play: Selective ER modulator (SERM) stimulated growth, growth factor/ER crosstalk, estrogen-induced apoptosis and mutations of ER. But it is with the science of estrogen-induced apoptosis that the next innovation in women's health will be developed. Recent evidence suggests that the glucocorticoid properties of medroxyprogesterone acetate blunt estrogen-induced apoptosis in estrogen deprived breast cancer cell populations. As a result breast cancer develops during long-term hormone replacement therapy (HRT). A new synthetic progestin with estrogen-like properties, such as the 19 nortestosterone derivatives used in oral contraceptives, will continue to protect the uterus from unopposed estrogen stimulation but at the same time, reinforce apoptosis in vulnerable populations of nascent breast cancer cells.

Defining the conformation of the estrogen receptor complex that controls estrogen-induced apoptosis in breast cancer

Development of acquired antihormone resistance exposes a vulnerability in breast cancer: estrogen-induced apoptosis. Triphenylethylenes (TPEs), which are structurally similar to 4-hydroxytamoxifen (4OHT), were used for mechanistic studies of estrogen-induced apoptosis. These TPEs all stimulate growth in MCF-7 cells, but unlike the planar estrogens they block estrogen-induced apoptosis in the long-term estrogen-deprived MCF7:5C cells. To define the conformation of the TPE:estrogen receptor (ER) complex, we employed a previously validated assay using the induction of transforming growth factor α (TGFα) mRNA in situ in MDA-MB 231 cells stably transfected with wild-type ER (MC2) or D351G ER mutant (JM6). The assays discriminate ligand fit in the ER based on the extremes of published crystallography of planar estrogens or TPE antiestrogens. We classified the conformation of planar estrogens or angular TPE complexes as "estrogen-like" or "antiestrogen-like" complexes, respectively. The TPE:ER complexes did not readily recruit the coactivator steroid receptor coactivator-3 (SRC3) or ER to the PS2 promoter in MCF-7 and MCF7:5C cells, and molecular modeling showed that they prefer to bind to the ER in an antagonistic fashion, i.e., helix 12 not sealing the ligand binding domain (LBD) effectively, and therefore reduce critical SRC3 binding. The fully activated ER complex with helix 12 sealing the LBD is suggested to be the appropriate trigger to initiate rapid estrogen-induced apoptosis.

X-ray structures of progesterone receptor ligand binding domain in its agonist state reveal differing mechanisms for mixed profiles of 11β-substituted steroids

We present here the x-ray structures of the progesterone receptor (PR) in complex with two mixed profile PR modulators whose functional activity results from two differing molecular mechanisms. The structure of Asoprisnil bound to the agonist state of PR demonstrates the contribution of the ligand to increasing stability of the agonist conformation of helix-12 via a specific hydrogen-bond network including Glu(723). This interaction is absent when the full antagonist, RU486, binds to PR. Combined with a previously reported structure of Asoprisnil bound to the antagonist state of the receptor, this structure extends our understanding of the complex molecular interactions underlying the mixed agonist/antagonist profile of the compound. In addition, we present the structure of PR in its agonist conformation bound to the mixed profile compound Org3H whose reduced antagonistic activity and increased agonistic activity compared with reference antagonists is due to an induced fit around Trp(755), resulting in a decreased steric clash with Met(909) but inducing a new internal clash with Val(912) in helix-12. This structure also explains the previously published observation that 16α attachments to RU486 analogs induce mixed profiles by altering the binding of 11β substituents. Together these structures further our understanding of the steric and electrostatic factors that contribute to the function of steroid receptor modulators, providing valuable insight for future compound design.

Selectively targeting estrogen receptors for cancer treatment

Estrogens regulate growth and development through the action of two distinct estrogen receptors (ERs), ERα and ERβ, which mediate proliferation and differentiation of cells. For decades, ERα mediated estrogen signaling has been therapeutically targeted to treat breast cancer, most notably with the selective estrogen receptor modulator (SERM) tamoxifen. Selectively targeting ERs occurs at two levels: tissue selectivity and receptor subtype selectivity. SERMs have been developed with emphasis on tissue selectivity to target ER signaling for breast cancer treatment. Additionally, new approaches to selectively target the action of ERα going beyond ligand-dependent activity are under current investigation. As evidence of the anti-proliferative role of ERβ accumulates, selectively targeting ERβ is an attractive approach for designing new cancer therapies with the emphasis shifted to designing ligands with subtype selectivity. This review will present the mechanistic and structural features of ERs that determine tissue and subtype selectivity with an emphasis on current approaches to selectively target ERα and ERβ for cancer treatment.

Structure-function relationships of estrogenic triphenylethylenes related to endoxifen and 4-hydroxytamoxifen

Estrogens can potentially be classified into planar (class I) or nonplanar (class II) categories, which might have biological consequences. 1,1,2-Triphenylethylene (TPE) derivatives were synthesized and evaluated against 17beta-estradiol (E2) for their estrogenic activity in MCF-7 human breast cancer cells. All TPEs were estrogenic and, unlike 4-hydroxytamoxifen (4OHTAM) and Endoxifen, induced cell growth to a level comparable to that of E2. All the TPEs increased ERE activity in MCF-7:WS8 cells with the order of potency as followed: E2 > 1,1-bis(4,4'-hydroxyphenyl)-2-phenylbut-1-ene (15) > 1,1,2-tris(4-hydroxyphenyl)but-1-ene (3) > Z 4-(1-(4-hydroxyphenyl)-1-phenylbut-1-en-2-yl)phenol (7) > E 4-(1-(4-hydroxyphenyl)-1-phenylbut-1-en-2-yl)phenol (6) > Z(4-(1-(4-ethoxyphenyl)-1-(4-hydroxyphenyl)but-1-en-2-yl)phenol (12) > 4-OHTAM. Transient transfection of the ER-negative breast cancer cell line T47D:C4:2 with wild-type ER or D351G ER mutant revealed that all of the TPEs increased ERE activity in the cells expressing the wild-type ER but not the mutant, thus confirming the importance of Asp351 for ER activation by the TPEs. The findings confirm E2 as a class I estrogen and the TPEs as class II estrogens. Using available conformations of the ER liganded with 4OHTAM or diethylstilbestrol, the TPEs optimally occupy the 4OHTAM ER conformation that expresses Asp351.

Structural insights into corepressor recognition by antagonist-bound estrogen receptors

Direct recruitment of transcriptional corepressors to estrogen receptors (ER) is thought to contribute to the tissue-specific effects of clinically important ER antagonists. Here, we present the crystal structures of two affinity-selected peptides in complex with antagonist-bound ERalpha ligand-binding domain. Both peptides adopt helical conformations, bind along the activation function 2 coregulator interaction surface, and mimic corepressor (CoRNR) sequence motif binding. Peptide binding is weak in a wild-type context but significantly enhanced by removal of ER helix 12. This region contains a previously unrecognized CoRNR motif that is able to compete with corepressors for binding to activation function 2, thereby providing a structural explanation for the poor ability of ER to directly interact with classical corepressors. Furthermore, the ability of other sequence motifs to mimic corepressor binding raises the possibility that coregulators do not necessarily require CoRNR motifs for direct recruitment to antagonist-bound ER.

Three-dimensional models of non-steroidal ligands: a comparative molecular field analysis

The estrogen receptor, ER, is an important biological target whose inhibition is known to be therapeutically relevant in the treatment of postmenopausal osteoporosis. In the present study, two prediction methods (CoMFA and GRIND (Almond)) were used to describe the binding modes of a set of estrogen receptor ligands. The critical alignment step presented in CoMFA was solved by using the information of the molecular descriptors space generated by grid-independent descriptors (GRIND). Then, it was possible to build robust and high predictive models based on the alignment-independent model. Since the structure of estrogen receptor is solved, the results of the present 3D QSAR models, given by the PLS maps based on molecular interaction fields (MIF) were compared to ligand-binding ER domains and showed good agreement.

Targeted mutation of key residues at the start of helix 12 in the hERalpha ligand-binding domain identifies the role of hydrogen-bonding and hydrophobic interactions in the activity of the protein

Estradiol (E(2)) and tamoxifen exert their effects through two members of the nuclear receptor superfamily, estrogen receptor (ER)-alpha and -beta. We want to identify the key interactions linking ligand-binding and activity of the ERalpha. Asp-351 and Leu-536 participate in hydrogen bond (Asp-351) and hydrophobic (Leu-536) interactions at the start of helix 12 in the ligand-binding domain (LBD) of the ERalpha. Mutations at each position alter ER activity, but we do not know which is more important. We mutated these residues in combination and individually and assessed the activity of the mutated ERs in the absence and presence of E(2) and 4-OHT on an ERE-driven and an AP-1-driven promoter, as well as their ability to interact with coregulators. On an ERE-driven promoter, the residue at position 351 determined whether E(2) stimulated or reduced the activity of the ER, as well as the level of activity in the presence of 4-OHT. Surprisingly, mutation of both residues generally did not produce cumulative deleterious effects, and they exerted counterbalancing effects on the basal activity on both promoters. Our results identify the contributions of specific interactions to the activity of the hERalpha, and support the concept that this region couples ligand-binding with ER activity.

Synthesis and Evaluation of (17α,20Z)-21-(4-Substituted-phenyl)-19-norpregna-1,3,5(10),20-tetraene-3,17β-diols as Ligands for the Estrogen Receptor-α Hormone Binding Domain: Comparison with 20E-Isomers

As part of our ongoing program to develop probes for the hormone binding domain of the estrogen receptor-alpha (ERalpha), we prepared and evaluated a series of 17alpha,Z-(4-substituted-phenyl)vinyl estradiol derivatives. The results indicated that the relative binding affinities (RBAs) at 25 degrees C for the new compounds were significant (RBA = 9-57) although less than that of estradiol (RBA = 100) or of the parent unsubstituted phenylvinyl estradiol (RBA = 66). All of the Z-compounds were full agonists in the uterotrophic assay, indicating that the ligands formed estrogen-like complexes with the estrogen receptor-alpha hormone binding domain (ERalpha-HBD). Comparison of corresponding Z- and E-4-substituted phenylvinyl ligands complexed with the ERalpha-HBD indicated small but significant differences in binding modes that may account for the differing trends seen in the structure-activity relationships for the two series.

Estrogen receptor mutations in human disease

As early as the 1800s, the actions of estrogen have been implicated in the development and progression of breast cancer. The estrogen receptor (ER) was identified in the late 1950s and purified a few years later. However, it was not until the 1980s that the first ER was molecularly cloned, and in the mid 1990s, a second ER was cloned. These two related receptors are now called ERalpha and ERbeta, respectively. Since their discovery, much research has focused on identifying alterations within the coding sequence of these receptors in clinical samples. As a result, a large number of naturally occurring splice variants of both ERalpha and ERbeta have been identified in normal epithelium and diseased or cancerous tissues. In contrast, only a few point mutations have been identified in human patient samples from a variety of disease states, including breast cancer, endometrial cancer, and psychiatric diseases. To elucidate the mechanism of action for these variant isoforms or mutant receptors, experimental mutagenesis has been used to analyze the function of distinct amino acid residues in the ERs. This review will focus on ERalpha and ERbeta alterations in breast cancer.

Gene expression in cancer cells is influenced by contact with bone cells in a novel coculture system that models bone metastasis

Contact between bone cells and cancer cells (heterotypic cell contact) is thought to play a central role in the initial growth and progression of metastatic cells. Attempts at studying heterotypic contact in vitro and in vivo have been confounded by difficulty in controlling how and when heterotypic contact occurs between unlike cells. A novel model, the micropatterned coculture system, is described that quantifies and controls heterotypic contact between cancer cells and bone cells in vitro. The micropatterned coculture system is biocompatible, and is modified easily to accommodate two or more different populations of cells. Immunofluorescence of cocultures of prostate cancer-3 cells and osteoblasts show the precise control of cell interactions. Ribonucleic acid of sufficient quantity and quality is isolated readily from cells cocultured on the micropatterned coculture system. The expression of the metastasis associated genes urokinase plasminogen activator, insulinlike growth factor binding protein-1 and insulinlike growth factor binding protein-3 are regulated in response to heterotypic contact and soluble factors respectively. A model of bone metastasis based on the micropatterned coculture system technology will streamline the process for testing therapeutic agents, so that more molecules can be identified for animal and clinical testing at less cost and in less time than using conventional methods.

Synthesis and evaluation of 17alpha-20E-21-(4-substituted phenyl)-19-norpregna-1,3,5(10),20-tetraene-3,17beta-diols as probes for the estrogen receptor alpha hormone binding domain

As part of our program to develop probes for the hormone binding domain of the estrogen receptor alpha (ERalpha), we prepared a series of 4-para-substituted phenylvinyl estradiol derivatives using a combination of solution and solid-phase Pd(0)-catalyzed methods. The compounds 5a-j were evaluated for their binding affinity using the ERalpha hormone binding domain (HDB) isolated from transfected BL21 cells. The results indicated that although the new compounds were somewhat lower in relative binding affinity (RBA at 25 degrees C is 1-60%) than estradiol (100%), most had higher affinity than the unsubstituted parent phenylvinyl estradiol (RBA = 9%). Because the substituents did not generate a structure-activity relationship directly based on physicochemical properties, the series was evaluated using molecular modeling and molecular dynamics to determine key interactions between the ligand, especially the para substituent, and the protein. The results suggest that the observed relative binding affinities are directly related to the calculated binding energies and that amino acids juxtaposed to the para position play a significant but not dominant role in binding. In conclusion, we have identified the 17alpha-E-(4-substituted phenyl)vinyl estradiols as a class of ligands that retain significant affinity for the ERalpha-HBD. In particular, 4-substitution tends to increase receptor affinity compared to the unsubstituted analogue, as exemplified by 5e (4-COCH(3)), which had the highest RBA value (60%) of the series. Palladium(0)-catalyzed coupling reactions on solid support or in solution using suitably substituted iodo arenes and 17alpha-E-tributylstannylvinyl estradiols offer a flexible approach to their preparation. Molecular modeling studies of the receptor suggest that there exists additional ligand accessible regions within the ERalpha-HBD to generate interactions that may enhance receptor affinity or modify efficacy in developing new therapeutic agents. Studies to undertake modification in the properties and/or position of the aryl substituents in subsequent series to further define that role are in progress.

MICoA, a novel metastasis-associated protein 1 (MTA1) interacting protein coactivator, regulates estrogen receptor-alpha transactivation functions

The transcriptional activity of estrogen receptor-alpha (ER-alpha) is modified by coactivators, corepressors, and chromatin remodeling complexes. We have previously shown that the metastasis-associated protein-1 (MTA1), a component of histone deacetylase and nucleosome remodeling complexes, represses ER-driven transcription by recruiting histone deacetylases to the estrogen receptor element (ERE)-containing target gene chromatin in breast cancer cells. Using a yeast two-hybrid screening to clone MTA1-interacting proteins, we identified a previously uncharacterized molecule, which we named as MTA1-interacting coactivator (MICoA). Our findings suggest that estrogen signaling promotes nuclear translocation of MICoA and that MICoA interacts with MTA1 both in vitro and in vivo. MICoA binds to the C-terminal region of MTA1, whereas MTA1 binds to the N-terminal MICoA containing one nuclear receptor interaction LSRLL motif. We showed that MICoA is an ER coactivator, cooperates with other ER coactivators, stimulates ER-transactivation functions, and associates with the endogenous ER and its target gene promoter chromatin. MTA1 also repressed MICoA-mediated stimulation of ERE-mediated transcription in the presence of ER and ER variants with naturally occurring mutations, such as D351Y and K303R, and that it interfered with the association of MICoA with the ER-target gene chromatin. Because chromatin is a highly dynamic structure and because MTA1 and MICoA could be detected within the same complex, these findings suggest that MTA1 and MICoA might transmodulate functions of each other and any potential deregulation of MTA1 is likely to contribute to the functional inactivation of the ER pathway, presumably by derecruitment of MICoA from ER target promoter chromatin.

Resveratrol acts as an estrogen receptor (ER) agonist in breast cancer cells stably transfected with ER alpha

Resveratrol (Res) is a phytoestrogen found in grapes and present in red wine. Res has been shown to function as an estrogen receptor (ER) agonist, but it remains unclear whether it may also exert antagonist activity. Our aim was to study the effects of Res at both the molecular (TGFalpha gene activation) and the cellular (cell growth) levels in breast cancer cells stably transfected with wild-type (wt) ER(D351) and mutant (mut) ER (D351Y). TGFalpha mRNA induction was used as a specific marker of estradiol (E(2)) responsiveness. Res caused a concentration-dependent (10(-8)-10(-4) M) stimulation of TGFalpha mRNA, indicating that it acts as an estrogen agonist in these cell lines. The pure antiestrogen ICI 182,780 (ICI) blocked Res-induced activation of TGFalpha, consistent with action through an ER-mediated pathway. Further studies that combined treatments with E(2) and Res showed that Res does not act as an antagonist in the presence of various (10(-11)-10(-8) M) concentrations of E(2). To determine whether Res can be classified as a type I or type II estrogen (Jordan et al., Cancer Res 2001;61:6619-23,), we examined Res with the D351G ER in the TGFalpha assay and found that Res belongs to the type I estrogens. Both Res and E(2) had concentration-dependent growth inhibitory effects in cells expressing wtER and D351Y ER. Although the pure antiestrogen ICI blocked the growth inhibitory effects of E(2), it did not block the inhibitory effects of Res, suggesting that the antiproliferative effects of Res also involve ER-independent pathways. Interestingly, Res differentially affected the levels of ER protein in these 2 cell lines: Res down-regulated wtER levels while significantly up-regulating the amount of mutD351Y ER. Co-treatment with ICI resulted in strongly reduced ER levels in both cell lines. Gene array studies revealed Res-induced up-regulation of more than 80 genes, among them a profound activation of p21(CIP1)/WAF1, a gene associated with growth arrest. The p21(CIP1)/WAF1 protein levels measured by Western blotting confirmed Res-induced significant up-regulation of this protein in both cell lines. In summary, Res acts as an ER agonist at low doses but also activates ER-independent pathways, some of which inhibit cell growth.

Design of thyroid hormone receptor antagonists from first principles

It is desirable to obtain TR antagonists for treatment of hyperthyroidism and other conditions. We have designed TR antagonists from first principles based on TR crystal structures. Since agonist ligands are buried in the fold of the TR ligand binding domain (LBD), we reasoned that ligands that resemble agonists with large extensions should bind the LBD, but would prevent its folding into an active conformation. In particular, we predicted that extensions at the 5' aryl position of ligand should reposition helix (H) 12, which forms part of the co-activator binding surface, and thereby inhibit TR activity. We have found that some synthetic ligands with 5' aryl ring extensions behave as antagonists (DIBRT, NH-3), or partial antagonists (GC-14, NH-4). Moreover, one compound (NH-3) represents the first potent TR antagonist with nanomolar affinity that also inhibits TR action in an animal model. However, the properties of the ligands also reveal unexpected aspects of TR behavior. While nuclear receptor antagonists generally promote binding of co-repressors, NH-3 blocks co-activator binding and also prevents co-repressor binding. More surprisingly, many compounds with extensions behave as full or partial agonists. We present hypotheses to explain both behaviors in terms of dynamic equilibrium of H12 position.

Investigations on estrogen receptor binding. The estrogenic, antiestrogenic, and cytotoxic properties of C2-alkyl-substituted 1,1-bis(4-hydroxyphenyl)-2-phenylethenes

C2-Alkyl-substituted 1,1-bis(4-hydroxyphenyl)-2-phenylethenes were synthesized and assayed for estrogen receptor binding in a competition experiment with radiolabeled estradiol ([3H]-E2) using calf uterine cytosol. The relative binding affinity decreased with the length of the side chain R = H (3a: 35.2%) > Me (3b: 32.1%) > Et (3c: 6.20%) approximately CH2CF3 (3d: 5.95%) > n-Pr (3e: 2.09%) > Bu (3f: 0.62%). Agonistic and antagonistic effects were evaluated in the luciferase assay with MCF-7-2a cells stably transfected with the plasmid ERE(wtc)luc. All compounds showed high antiestrogenic activity without significant agonistic potency. The comparison of the IC(50) values for the inhibition of E2 (1 nM) documented the dependence of the antagonistic effects on the kind of the side chain: 3a (IC50 = 150 nM), 3b (IC50 = 30 nM), and 3f (IC50 = 500 nM) were weak antagonists, while 3c (IC50 = 15 nM), 3d (IC50 = 9 nM), and 3e (IC50 = 50 nM) were full antiestrogens and antagonized the effect of E2 completely. The most active compound 3d possessed the same antagonistic potency as 4-hydroxytamoxifen (4OHT: IC50= 7 nM) without bearing a basic side chain. 3d as well as all other 1,1-bis(4-hydroxyphenyl)-2-phenylalkenes were not able to influence the proliferation of hormone dependent MCF-7 cells despite the antagonistic mode of action. In this assay tamoxifen (TAM) and 4OHT reduced the cell growth concentration dependent up to T/C(corr) = 15% and 25%, respectively.

Molecular classification of selective oestrogen receptor modulators on the basis of gene expression profiles of breast cancer cells expressing oestrogen receptor alpha

The purpose of this study was to classify selective oestrogen receptor modulators based on gene expression profiles produced in breast cancer cells expressing either wtERalpha or mutant(351)ERalpha. In total, 54 microarray experiments were carried out by using a commercially available Atlas cDNA Expression Arrays (Clontech), containing 588 cancer-related genes. Nine sets of data were generated for each cell line following 24 h of treatment: expression data were obtained for cells treated with vehicle EtOH (Control); with 10(-9) or 10(-8) M oestradiol; with 10(-6) M 4-hydroxytamoxifen; with 10(-6) M raloxifene; with 10(-6) M idoxifene, with 10(-6) M EM 652, with 10(-6) M GW 7604; with 5 x 10(-5) M resveratrol and with 10(-6) M ICI 182,780. We developed a new algorithm 'Expression Signatures' to classify compounds on the basis of differential gene expression profiles. We created dendrograms for each cell line, in which branches represent relationships between compounds. Additionally, clustering analysis was performed using different subsets of genes to assess the robustness of the analysis. In general, only small differences between gene expression profiles treated with compounds were observed with correlation coefficients ranged from 0.83 to 0.98. This observation may be explained by the use of the same cell context for treatments with compounds that essentially belong to the same class of drugs with oestrogen receptors related mechanisms. The most surprising observation was that ICI 182,780 clustered together with oestrodiol and raloxifene for cells expressing wtERalpha and clustered together with EM 652 for cells expressing mutant(351)ERalpha. These data provide a rationale for a more precise and elaborate study in which custom made oligonucleotide arrays can be used with comprehensive sets of genes known to have consensus and putative oestrogen response elements in their promoter regions.

(4R,5S)/(4S,5R)-4,5-Bis(4-hydroxyphenyl)-2-imidazolines: ligands for the estrogen receptor with a novel binding mode

(4R,5S)/(4S,5R)-4,5-Bis(4-hydroxyphenyl)-2-imidazolines 1-7 were synthesized by the reaction of the methoxy-substituted (1R,2S)/(1S,2R)-1,2-diarylethylenediamines 1b-7b with triethyl orthoformate and subsequent ether cleavage with BBr(3). All compounds were tested for estrogen receptor (ER) binding in a competition experiment with [(3)H]-estradiol and for gene activation in a luciferase assay using ER positive MCF-7-2a breast cancer cells stably transfected with the plasmid ERE(wtc)luc. The relative binding affinities of the 2-imidazolines were very low (RBA < 0.1%). Nevertheless, 4-7 possessed full agonistic activity in the luciferase assay. The relative transcription potency increased in the order 5 (2,2'-I) < 6 (2,6-Cl(2), 2'-F) < 4 (2,2'-Cl) < 7 (2,6-Cl(2), 2'-Cl). These data together with spectroscopic and molecular modeling studies were used to investigate the preferred binding mode adopted by the imidazoline ligands. The 1,2-diarylethane pharmacophor takes a Z-stilbene-like structure with pseudoaxially oriented phenyl rings at the planar heterocyclic ring. Because of this unusual spatial structure, the (4R,5S)/(4S,5R)-4,5-bis(4-hydroxyphenyl)-2-imidazolines have to be assigned to a second class of estrogenically active compounds (type II estrogens). In contrast to type I estrogens, e.g., estradiol (E2), diethylstilbestrol (DES), and meso-hexestrol (HES), which are connected to His 524 in the binding site, type II estrogens are very likely H-bonded to Asp 351 in a hydrophobic side pocket.

The tamoxifen-responsive estrogen receptor alpha mutant D351Y shows reduced tamoxifen-dependent interaction with corepressor complexes

The effects of estrogen and anti-estrogen are mediated through the estrogen receptors ERalpha and beta, which function as ligand-induced transcriptional factors. The nonsteroidal anti-estrogen tamoxifen is the most commonly used endocrine in the treatment of all stages of breast cancer in both pre- and postmenopausal women. Several lines of evidence have indicated that tamoxifen promotes association between ERalpha and corepressors N-CoR or silencing mediator for retinoid and thyroid hormone receptor (SMRT). Our results indicate that N-CoR/SMRT recognize and interact with helices H3 and H5 of the ERalpha ligand-binding domain in a 4-hydroxy tamoxifen-dependent manner. The mutant ERalpha(D351Y), derived from a tamoxifen-stimulated tumor and containing an amino acid substitution at position 351 within H3, showed reduced interaction with N-CoR/SMRT and high tamoxifen-induced activation function-1 (AF-1) activity. While the estradiol-dependent transcriptional activity of ERalpha(D351Y) was almost equal to that of wild-type ERalpha, the mutant exhibited higher levels of transcriptional activity in the presence of both E2 and 4-hydroxy tamoxifen compared with wild-type ERalpha. These results may explain the observation that the growth of tumor cells expressing ERalpha(D351Y) can be stimulated by tamoxifen, E2, or both.