An antiestrogen-responsive estrogen receptor-alpha mutant (D351Y) shows weak AF-2 activity in the presence of tamoxifen

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.

Pharmacogenetics of anti-estrogen treatment of breast cancer

A major effort is underway to select genetic polymorphisms potentially relevant to the clinical efficacy and safety of endocrine treatment of breast cancer. Genetic factors of the host that affect the metabolism of tamoxifen, a widely used drug for the adjuvant treatment of breast cancer, have received particular attention. Cytochrome P450 isoform 2D6 (CYP2D6) is a key step in the metabolism of tamoxifen to its active moiety endoxifen. Women with functionally deficient genetic variants of CYP2D6 who are given drugs that inhibit CYP2D6 are exposed to low endoxifen plasma levels and may enjoy reduced benefits from tamoxifen treatment. Therefore, CYP2D6 status may be an important predictor of the benefits of tamoxifen to an individual; unfortunately, the data are not uniformly concordant, and definitive evidence that would suggest the routine analysis of CYP2D6 before commencing tamoxifen treatment is not yet available. Recent research has focused on the role UDP-glucuronosyltransferases, a family of metabolizing enzymes that play an important role in the metabolic clearance of tamoxifen and of the aromatase inhibitors as well, and how interindividual differences in these enzymes may play a role in the clinical outcome upon administration of anti-estrogen treatment. In conclusion, whether a pharmacogenetic profile should be obtained prior to initiating tamoxifen therapy is currently a matter of debate, although summing up all the scientific evidence available on this issue it appears that the genetic screening would be an useful support for clinical decision making in selected patients.

Xenoestrogen action in breast cancer: impact on ER-dependent transcription and mitogenesis

Several estrogen mimics (xenoestrogens) inappropriately activate the estrogen receptor (ER) in the absence of endogenous ligand. Given the importance of the ER in breast cancer growth and regulation, delineating the impact of these agents under conditions related to tumor treatment is of significant importance. We examined the effect of two prevalent xenoestrogens (bisphenol A and coumestrol) on ER activation and ER-dependent mitogenesis in breast cancer cells. We show that the ability of these agents to induce mitogenesis was restricted to conditions of estrogen depletion, and that these agents failed to cooperate with estradiol to induce MCF-7 breast cancer cell growth. These observations are consistent with the impact of each agent specifically on exogenous ER activation as monitored in HeLa cells, wherein the xenoestrogens activated the receptor in the absence of estradiol but failed to cooperate with estrogen. Tamoxifen blocked bisphenol A and coumestrol-mediated ER activation, indicating that exposure to these agents is unlikely to disrupt such therapeutic intervention. The response of tumor-derived ER alleles to these xenoestrogens was also examined. Although the xenoestrogens failed to alter ER-Y537S function, the ER-D351Y mutant demonstrated an enhanced response to bisphenol A. Moreover, tamoxifen enhanced the agonistic effects of xenoestrogens on ER-D351Y. Lastly, we examined the impact of ER co-activator overexpression on xenoestrogen response. Bisphenol A and coumestrol exhibited differential responses to co-activators with regard to ER activation. However, when using mitogenesis as an endpoint, these co-activators were insufficient to provide a significant growth advantage. Combined, these data demonstrate that bisphenol A and coumestrol can impact ER activity and ER-dependent proliferation in breast cancer cells, but the influence of these agents is restricted to conditions of estrogen depletion, selective mutation of the ER, and expression of specific co-activators.

LIGAND CONTROL OF COREGULATOR RECRUITMENT TO NUCLEAR RECEPTORS

Nuclear receptors modulate transcription through ligand-mediated recruitment of transcriptional coregulator proteins. The structural connection between ligand and coregulator is mediated by a molecular switch, made up of the most carboxy-terminal helix in the ligand-binding domain, helix 12. The dynamics of this switch are thought to underlie ligand specificity of nuclear receptor signaling, but the details of this control mechanism have remained elusive. This review highlights recent structural work on how the ligand controls this molecular switch and the modulation of this signaling pathway by receptor subtype and dimer partner.

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.

Selective estrogen receptor modulators: discrimination of agonistic versus antagonistic activities by gene expression profiling in breast cancer cells

Selective estrogen receptor modulators (SERMs) such as tamoxifen are effective in the treatment of many estrogen receptor-positive breast cancers and have also proven to be effective in the prevention of breast cancer in women at high risk for the disease. The comparative abilities of tamoxifen versus raloxifene in breast cancer prevention are currently being compared in the Study of Tamoxifen and Raloxifene trial. To better understand the actions of these compounds in breast cancer, we have examined their effects on the expression of approximately 12,000 genes, using Affymetrix GeneChip microarrays, with quantitative PCR verification in many cases, categorizing their actions as agonist, antagonist, or partial agonist/antagonist. Analysis of gene stimulation and inhibition by the SERMs trans-hydroxytamoxifen (TOT) and raloxifene (Ral) or ICI 182,780 (ICI) and by estradiol (E2) in estrogen receptor-containing MCF-7 human breast cancer cells revealed that (a) TOT was the most E2-like of the three compounds, (b) all three compounds either partially or fully antagonized the action of E2 on most genes, with the order of antagonist activity being ICI > Ral > TOT, (c) TOT and Ral, but not ICI, displayed partial agonist/partial antagonist activity on a number of E2-regulated genes, (d) several stimulatory cell cycle-related genes were down-regulated exclusively by ICI, (e) the estrogen-like activity of Ral nearly always overlapped with that of TOT, indicating that Ral has little unique agonist activity different from that of TOT, and (f) some genes were specifically up-regulated by TOT but not Ral, ICI, or E2. Hence, gene expression profiling can discern fundamental differences among SERMs and provides insight into the distinct biologies of TOT, Ral, and ICI in breast cancer.

Estrogen receptor ? (ESR1) mutant A908G is not a common feature in benign and malignant proliferations of the breast

Alterations in estrogen responsive pathways are thought to contribute to benign and malignant breast disease. It has been reported previously that more than a third of typical epithelial hyperplasia lesions harbor the missense mutation A908G in the estrogen receptor alpha (ESR1) gene. This substitution of an arginine for a lysine at codon 303 was reported to confer mitogenic hypersensitivity to estrogen. To explore this finding further, we analyzed ESR1 for this mutation in a series of breast tissues ranging from typical hyperplasia to invasive cancer. In contrast to previous studies, no evidence for this mutation was found in 36 invasive cancers, 11 in situ carcinomas, 14 epithelial hyperplasias with atypia, 11 epithelial hyperplasias without atypia, and 11 breast cancer cell lines. These results indicate that ESR1 mutant A908G does not occur with significant frequency in either benign or malignant proliferations of breast epithelia.

Identification of estrogen receptor beta expression in Chinese hamster ovary (CHO) cells and comparison of estrogen-responsive gene transcription in cells adapted to serum-free media

Most cultured cell lines require addition of serum to the medium to maintain their proliferative capacity. For studies examining the cellular effects of estrogens serum is charcoal-stripped to remove steroids. Nonetheless, addition of the selective estrogen receptor modulator (SERM) 4-hydroxytamoxifen (4-OHT) inhibits the basal transcriptional activity of estrogen receptors alpha or beta (ERalpha or ERbeta) in transfected cells. We tested the hypothesis that elimination of serum from the culture medium will block 4-OHT's repression of basal activity. Chinese hamster ovary (CHO-K1) cells adapted to serum-free medium exhibited estrogen responsiveness that was identical with that of the cells grown in serum-containing media. 4-OHT-suppressed basal transcription of an estrogen response element (ERE)-reporter in ERalpha-transfected cells even in the absence of serum, indicating that the 4-OHT suppressive activity is not mediated by blocking ER interaction with serum estrogens. We speculate that 4-OHT-ER recruits co-repressors to suppress basal transcription. We discovered that CHO-K1 cells express ERalpha and ERbeta mRNA. However only ERbeta protein was expressed and use of ERbeta-selective 2,3-bis(4-hydroxy-phenyl)propionitrile (DPN) and ERalpha-selective 4-propyl-1,3,5-tris(4-hydroxy-phenyl)pyrazole) (PPT) revealed that only ERbeta was transcriptionally active. In conclusion, growing CHO-K1 in serum-free medium does not impact the estrogen responsiveness and this cell line expresses functional ERbeta.

Differential SERM effects on corepressor binding dictate ERalpha activity in vivo

Selective estrogen receptor modulators (SERMs) show differential effects upon ERalpha activation function 1 (AF-1). Tamoxifen allows strong ERalpha AF-1 activity, whereas raloxifene allows less and ICI 182,780 (ICI) allows none. Here, we show that blockade of corepressor histone de-acetylase (HDAC) activity reverses the differential inhibitory effect of SERMs upon AF-1 activity in MCF-7 cells. This suggests that differential SERM repression of AF-1 involves HDAC-dependent corepressors. Consistent with this, ICI and raloxifene are more potent than tamoxifen in promoting ERalpha-dependent sequestration of progesterone receptor-associated corepressors. Moreover, ICI and raloxifene are more efficient than tamoxifen in promoting ERalpha binding to the corepressor N-CoR in vivo and in vitro. An ERalpha mutation (537X) that increases N-CoR binding in the presence of all SERMs blocks AF-1 activity. An ERalpha mutation (L379R) that decreases N-CoR binding increases AF-1 activity in the presence of ICI and raloxifene and reverses the effect of the 537X mutation. The 537X and L379R mutations also alter the ligand preference of ERalpha action at AP-1 sites and C3 complement, an action that also involves AF-1. Together, our results suggest that differential SERM effects on corepressor binding can explain differences in SERM effects on ERalpha activity. We propose a model for differential effects of SERMs on N-CoR binding.

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.

Structure-function relationships of the raloxifene-estrogen receptor-alpha complex for regulating transforming growth factor-alpha expression in breast cancer cells

Amino acid Asp-351 in the ligand binding domain of estrogen receptor alpha (ERalpha) plays an important role in regulating the estrogen-like activity of selective estrogen receptor modulator-ERalpha complexes. 4-Hydroxytamoxifen is a full agonist at a transforming growth factor alpha target gene in situ in MDA-MB-231 human breast cancer cells stably transfected with the wild-type ERalpha. In contrast, raloxifene (Ral), which is also a selective estrogen receptor modulator, is a complete antiestrogen in this system. Because D351G ERalpha allosterically silences activation function-1 activity in the 4-hydroxytamoxifen-ERalpha complex with the complete loss of estrogen-like activity, we examined the converse interaction of amino acid 351 and the piperidine ring of the antiestrogen side chain of raloxifene to enhance estrogen-like action. MDA-MB-231 cells were either transiently or stably transfected with Asp-351 (the wild type), D351E, D351Y, or D351F ERalpha expression vectors. Profound differences in the agonist and antagonist actions of Ralcenter dotERalpha complexes were noted only in stable transfectants. The agonist activity of the Ralcenter dotERalpha complex was enhanced with D351E and D351Y ERalpha, but raloxifene lost its agonist activity with D351F ERalpha. The distance between the piperidine nitrogen of raloxifene and the negative charge of amino acid 351 was critical for estrogen-like actions. The role of the piperidine ring in neutralizing Asp-351 was addressed using compound R1h, a raloxifene derivative replacing the nitrogen on its piperidine ring with a carbon to form cyclohexane. The derivative was a potent agonist with wild type ERalpha. These results support the concept that the side chain of raloxifene shields and neutralizes the Asp-351 to produce an antiestrogenic ERalpha complex. Alteration of either the side chain or its relationship with the negative charge at amino acid 351 controls the estrogen-like action at activating function 2b of the selective estrogen receptor modulator ERalpha complex.

Chemoprevention with antiestrogens: the beginning of the end for breast cancer. Daniel G. Miller Lecture

In the 1960s, compounds known as nonsteroidal antiestrogens were identified as potential contraceptives, but the drugs caused the induction of ovulation in subfertile women. Tamoxifen and clomiphene were marketed for this indication. However, tamoxifen was advanced for the treatment of breast cancer in the 1970s through a close cooperation between the laboratory and the clinical trials community. The extensive use of long-term adjuvant tamoxifen has resulted in saving the lives of 400,000 women with breast cancer. Tamoxifen is a selective estrogen receptor modulator (SERM) that produces antiestrogenic actions in the breast but estrogen-like actions in bone and lowers serum cholesterol. These properties not only allowed the application of tamoxifen as the first chemopreventive in high-risk pre- and postmenopausal women but also the development of raloxifene to prevent osteoporosis with the potential to prevent breast cancer in postmenopausal women. The future development of SERMs holds the promise of preventing osteoporosis and coronary heart disease as well as breast and endometrial cancer.

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.