Biophysical studies of the interaction between calmodulin and the R287-T311 region of human estrogen receptor α reveals an atypical binding process

Promising Perspectives of the Antiproliferative GPER Inverse Agonist ERα17p in Breast Cancer

The estrogen receptor α (ERα) corresponds to a large platform in charge of the recruitment of a panel of molecules, including steroids and related heterocyclic derivatives, oligonucleotides, peptides and proteins. Its 295-311 region is particularly targeted by post-translational modifications, suggesting that it could be crucial for the control of transcription. In addition to anionic phospholipids, the ERα 295-311 fragment interacts with Ca²⁺-calmodulin, the heat shock protein 70 (Hsp70), ERα and possibly importins. More recently, we have demonstrated that it is prone to interacting with the G-protein-coupled estrogen receptor (GPER). In light of these observations, the pharmacological profile of the corresponding peptide, namely ERα17p, has been explored in breast cancer cells. Remarkably, it exerts apoptosis through GPER and induces a significant decrease (more than 50%) of the size of triple-negative breast tumor xenografts in mice. Herein, we highlight not only the promising therapeutic perspectives in the use of the first peptidic GPER modulator ERα17p, but also the opportunity to modulate GPER for clinical purposes.

Pathological Maintenance and Evolution of Breast Cancer: The Convergence of Irreversible Biological Actions of ER Alpha

Estrogen receptor alpha (ERα) is a modulator of breast cancer maintenance and evolution. Hence, analysis of underlying mechanisms by which ERα operates is of importance for the improvement of the hormonal therapy of the disease. This review focuses on the irreversible character of the mechanism of action of ERα, which also concerns other members of the steroid hormones receptors family. ERα moves in permanence between targets localized especially at the chromatin level to accomplish gene transcriptions imposed by the estrogenic ligands and specific antagonists. Receptor association as at the plasma membrane, where it interacts with other recruitment sites, extends its regulatory potency to growth factors and related peptides through activation of signal transductions pathways. If the latter procedure is suitable for the transcriptions in which the receptor operates as a coregulator of another transcription factor, it is of marginal influence with regard to the direct estrogenic regulation procedure, especially in the context of the present review. Irreversibility of the successive steps of the underlying transcription cycle guarantees maintenance of homeostasis and evolution according to vital necessities. To justify this statement, reported data are essentially described in a holistic view rather than in the context of exhaustive analysis of a molecular event contributing to a specific function as well as in a complementary perspective to elaborate new therapeutic approaches with antagonistic potencies against those tumors promoting ERα properties.

Reciprocality Between Estrogen Biology and Calcium Signaling in the Cardiovascular System

17β-Estradiol (E₂) is the main estrogenic hormone in the body and exerts many cardiovascular protective effects. Via three receptors known to date, including estrogen receptors α (ERα) and β (ERβ) and the G protein-coupled estrogen receptor 1 (GPER, aka GPR30), E₂ regulates numerous calcium-dependent activities in cardiovascular tissues. Nevertheless, effects of E₂ and its receptors on components of the calcium signaling machinery (CSM), the underlying mechanisms, and the linked functional impact are only beginning to be elucidated. A picture is emerging of the reciprocality between estrogen biology and Ca²⁺ signaling. Therein, E₂ and GPER, via both E₂-dependent and E₂-independent actions, moderate Ca²⁺-dependent activities; in turn, ERα and GPER are regulated by Ca²⁺ at the receptor level and downstream signaling via a feedforward loop. This article reviews current understanding of the effects of E₂ and its receptors on the cardiovascular CSM and vice versa with a focus on mechanisms and combined functional impact. An overview of the main CSM components in cardiovascular tissues will be first provided, followed by a brief review of estrogen receptors and their Ca²⁺-dependent regulation. The effects of estrogenic agonists to stimulate acute Ca²⁺ signals will then be reviewed. Subsequently, E₂-dependent and E₂-independent effects of GPER on components of the Ca²⁺ signals triggered by other stimuli will be discussed. Finally, a case study will illustrate how the many mechanisms are coordinated to moderate Ca²⁺-dependent activities in the cardiovascular system.

Calmodulin Lobes Facilitate Dimerization and Activation of Estrogen Receptor-α

Estrogen receptor α (ER-α) is a nuclear hormone receptor that controls selected genes, thereby regulating proliferation and differentiation of target tissues, such as breast. Gene expression controlled by ER-α is modulated by Ca²⁺ via calmodulin (CaM). Here we present the NMR structure of Ca²⁺-CaM bound to two molecules of ER-α (residues 287-305). The two lobes of CaM bind to the same site on two separate ER-α molecules (residues 292, 296, 299, 302, and 303), which explains why CaM binds two molecules of ER-α in a 1:2 complex and stabilizes ER-α dimerization. Exposed glutamate residues in CaM (Glu-11, Glu-14, Glu-84, and Glu-87) form salt bridges with key lysine residues in ER-α (Lys-299, Lys-302, and Lys-303), which is likely to prevent ubiquitination at these sites and inhibit degradation of ER-α. Transfection of cells with full-length CaM slightly increased the ability of estrogen to enhance transcriptional activation by ER-α of endogenous estrogen-responsive genes. By contrast, expression of either the N- or C-lobe of CaM abrogated estrogen-stimulated transcription of the estrogen responsive genes pS2 and progesterone receptor. These data suggest that CaM-induced dimerization of ER-α is required for estrogen-stimulated transcriptional activation by the receptor. In light of the critical role of ER-α in breast carcinoma, our data suggest that small molecules that selectively disrupt the interaction of ER-α with CaM may be useful in the therapy of breast carcinoma.

Discovery at the interface: Toward novel anti-proliferative agents targeting human estrogen receptor/S100 interactions

Estrogen Receptor Alpha (ER) is expressed in about 70% of breast cancer and mediates various cellular signaling events including cell cycle. The antiestrogen tamoxifen is currently administered to patients in order to induce regression of the tumor growth of estrogen receptor positive (ER+) breast cancer. However, upon continued administration, patients develop resistance to tamoxifen. In addition, calcium binding proteins (EF-hand proteins) such as, Calmodulin and S100, are significantly overexpressed in breast cancer cells, can activate transcription of target genes by directly binding to ER in lieu of estrogen. Calmodulin antagonists (w7 and melatonin) have been shown to significantly inhibit ER mediated activities including cell proliferation and transcriptional activity. Furthermore, S100P is shown to mediate tamoxifen resistance and cell migration capacity in MCF-7 breast cancer cells. Molecules targeting specific ER-EF hand protein interfaces could potentially provide an alternative therapeutic strategy to combat these scenarios. Using theoretical 3D models of ER-S100 protein we identified ER conformation-sensing regions of the interacting EF hand proteins and evaluated their ability to bind to ER in silico and to inhibit breast cancer cell proliferation and viability in vitro. The recognition motif of the binding interface was sensitive to small changes in partner orientation as evidenced by significant anti cell proliferative activity of the short peptide derived from S100P residues 74-78, when compared with a longer peptide with altered orientation of the recognition motif derived from S100P 74-81. Structural clues and pharmacophores from peptide-ER interactions can be used to design novel anti-cancer agents.

Application of Circular Dichroism Spectroscopy to the Analysis of the Interaction Between the Estrogen Receptor Alpha and Coactivators: The Case of Calmodulin

The estrogen receptor α ligand-binding domain (ERα-LBD) binds the natural hormone 17β-estradiol (E2) to induce transcription and cell proliferation. This process occurs with the contribution of protein and peptide partners (also called coactivators) that can modulate the structure of ERα, and therefore its specificity of action. As with most transcription factors, ERα exhibits a high content of α helix, making it difficult to routinely run spectroscopic studies capable of deciphering the secondary structure of the different partners under binding conditions. Ca(2+)-calmodulin, a protein also highly structured in α-helix, is a key coactivator for ERα activity. Here, we show how circular dichroism can be used to study the interaction of ERα with Ca(2+)-calmodulin. Our approach allows the determination not only of the conformational changes induced upon complex formation but also the dissociation constant (K d) of this interaction.

The sequence Pro295-Thr311 of the hinge region of oestrogen receptor α is involved in ERK1/2 activation via GPR30 in leiomyoma cells

The ERα (oestrogen receptor α)-derived peptide ERα17p activates rapid signalling events in breast carcinoma cells under steroid-deprived conditions. In the present study, we investigated its effects in ELT3 leiomyoma cells under similar conditions. We show that it activates ERK1/2 (extracellular-signal-regulated kinase 1/2), the Gαi protein, the trans-activation of EGFR (epidermal growth factor receptor) and, finally, cell proliferation. It is partially internalized in cells and induces membrane translocation of β-arrestins. The activation of ERK1/2 is abolished by the GPR30 (G-protein-coupled receptor 30) antagonist G15 and GPR30 siRNA. When ERα is down-regulated by prolonged treatment with E2 (oestradiol) or specific ERα siRNA, the peptide response is blunted. Thus the simultaneous presence of GPR30 and ERα is required for the action of ERα17p. In addition, its PLM sequence, which interferes with the formation of the ERα-calmodulin complex, appears to be requisite for the phosphorylation of ERK1/2 and cell proliferation. Hence ERα17p is, to our knowledge, the first known peptide targeting ERα-GPR30 membrane cross-talk and the subsequent receptor-mediated biological effects.

Binding of calmodulin changes the calcineurin regulatory region to a less dynamic conformation

Calcineurin (CN) is a Ca(2+)/calmodulin (CaM) activated serine/threonine phosphatase, and its regulatory region (CNRR) plays a critical role in the coupling of Ca(2+) signals to cellular responses. Ca(2+)/CaM binds to the CNRR, resulting in a conformational change that removes an autoinhibitory domain (CN467-486) from the active site of the phosphatase and activates the enzyme. However, almost the entire regulatory region (CN374-521) is not visible in the electron density maps of reported structures. In this study, we produced separate CN fragments corresponding to the CNRR (CNRR381-521, CN residues 381-521) and determined the binding affinity of CNRR381-521 for Ca(2+)/CaM using isothermal titration calorimetry (ITC). The structural change in CNRR381-521 induced by Ca(2+)/CaM binding was also investigated by Fourier transform infrared spectroscopy (FT-IR). The results indicate that Ca(2+)/CaM binding to CNRR381-521 is an exothermic reaction with a dissociation constant of 2.0×10(-6) M. Binding of calmodulin changes the calcineurin regulatory region to a less dynamic conformation.

Structural analysis of calmodulin binding by nNOS inhibitory amphibian peptides

Calmodulin (CaM) is a ubiquitous protein in nature and plays a regulatory role in numerous biological processes, including the upregulation of nitric oxide (NO) synthesis in vivo. Several peptides that prevent NO production by interacting with CaM have been isolated in the cutaneous secretions of Australian amphibians, and are thought to serve as a defense mechanism against predators. In this work, we probe the mechanism by which three of these peptides, namely, caerin 1.8, dahlein 5.6, and a synthetic modification of citropin 1.1, interact with CaM to inhibit NO signaling. Isothermal titration calorimetry was used to determine thermodynamic parameters of the binding interactions and revealed that all the peptides bind to CaM in a similar fashion, with the peptide encapsulated between the two lobes of CaM. Ion mobility-mass spectrometry was used to investigate the changes in collision cross section that occur as a result of complexation, providing additional evidence for this binding mode. Finally, nuclear magnetic resonance spectroscopy was used to track chemical shift changes upon binding. The results obtained confirm that these complexes adopt canonical collapsed structures and demonstrate the strength of the interaction between the peptides and CaM. An understanding of these molecular recognition events provides insights into the underlying mechanism of the amphibian host-defense system.

Concentration-dependent and surface-assisted self-assembly properties of a bioactive estrogen receptor α-derived peptide

We have synthesized a 17-mer peptide (ERα17p) that is issued from the hinge region of the estrogen receptor α and which activates the proliferation of breast carcinoma cells in steroid-deprived conditions. In the present paper, we show that at a concentration of ~50 μM, it rapidly forms amyloid-like fibrils with the assistance of electrostatic interactions and that at higher concentrations, it spontaneously forms a hydrogel. By using biophysical, spectral and rheological techniques, we have explored the structural, biophysical and mechanical characteristics of ERα17p with respect to fibril formation and gelation.

Interactions of isoflavones and other plant derived estrogens with estrogen receptors for prevention and treatment of breast cancer-considerations concerning related efficacy and safety

Phytoestrogens are natural endocrine disruptors that interfere with estrogenic pathways. They insert directly within the hormone-binding domain of ERα and β, with a preference for the β isoform of which the concentration predominates in the normal mammary epithelium. Since ERβ antagonizes the growth promoting effect of ERα, which is mainly expressed in estrogen-sensitive tumor cells, a potential protective action against breast cancer incidence has been ascribed to phytoestrogens. The fact that Asian women living in far-east countries who consume isoflavone-rich food are less subjected to breast cancer emergence than their congeners in the USA as well as Caucasian women has been advocated to justify such a concept. Overview of data concerning the mechanism of action phytoestrogens reveals that such a view is an oversimplification: Such compounds interfere with a huge panel of regulatory proteins, giving rise to both promoting and antagonizing carcinogenic effects. Moreover, various physiological and pathological factors able to amplify these effects are not often sufficiently taken into account, which increases the difficulty to interpret data. Nevertheless, this overview of data established that chemical structures and concentrations modulate such effects: at the micromolar level, isoflavones activate ERα-mediated transcription and breast cancer cell proliferation while flavones fail to induce any significant promoting effects. At higher doses, both classes of compounds may display an antitumor activity. Reasons for such distinct behaviors as well as their potential impact in therapeutic applications are analyzed here. Ability of isoflavones and flavones to antagonize the association of calmodulin to ERα, which is required for its enhanced transcriptional activity is evoked to justify the antitumor activity ascribed to some flavones. Finally, a suspicion that peculiar classes of phytoestrogens may adopt a SERM-like conformation is addressed in a context of selection and synthesis of compounds with non-equivocal therapeutic value. This article is part of a Special Issue entitled "Phytoestrogens".

Identification of polyproline II regions derived from the proline-rich nuclear receptor coactivators PNRC and PNRC2: new insights for ERα coactivator interactions

Protein-protein interactions are crucial for signal transductions required for cell differentiation and proliferation. Their modulation is therefore key to the development of therapeutic alternatives, particularly in the context of cancer. According to literature data, the polyproline-rich nuclear receptor coactivators PNRC and PNRC2 interact with estrogen receptor (ERα) through their PxxP SH3-binding motifs. In a search to identify the molecular features governing this interaction, we explored using electronic circular dichroism (ECD) spectroscopy and molecular dynamics (MD) calculations, the capacity of a range of putative biologically active peptides derived from these proteins and containing this PxxP motif(s) to form polyproline II (PPII) domains. An additional more exhaustive structural study on a lead PPII peptide was also performed using 2D nuclear magnetic resonance (NMR) spectroscopy. With the exception of one of all the investigated peptides (PNRC-D), binding assays failed to detect any affinity for Grb2 SH3 domains, suggesting that PPII motifs issued from Grb2 antagonists have a binding mode distinct from those derived from Grb2 agonists. Instead, the peptides revealed a competitive binding ability against a synthetic peptide (ERα17p) with a putative PPII-cognate domain located within a coregulator recruitment region of ERα (AF-2 site). Our work, which constitutes the first structure-related interaction study concerning PNRC and PNRC2, supports not only the existence of PxxP-induced PPII sequences in these coregulators, but also confirms the presence of a PPII recognition site in the AF-2 of the steroid receptor ERα, a region important for transcription regulation.