Identification and characterization of a novel estrogenic ligand actinopolymorphol A

Exposure, toxicological mechanism of endocrine disrupting compounds and future direction of identification using nano-architectonics

Endocrine-disrupting compounds (EDC) are a group of exogenous chemicals that structurally mimic hormones and interfere with the hormonal signaling cascade. EDC interacts with hormone receptors, transcriptional activators, and co-activators, altering the signaling pathway at both genomic and non-genomic levels. Consequently, these compounds are responsible for adverse health ailments such as cancer, reproductive issues, obesity, and cardiovascular and neurological disorders. The persistent nature and increasing incidence of environmental contamination from anthropogenic and industrial effluents have become a global concern, resulting in a movement in both developed and developing countries to identify and estimate the degree of exposure to EDC. The U.S. Environment Protection Agency (EPA) has outlined a series of in vitro and in vivo assays to screen potential endocrine disruptors. However, the multidisciplinary nature and concerns over the widespread application demand alternative and practical techniques for identifying and estimating EDC. The review chronicles the state-of-art 20 years (1990-2023) of scientific literature regarding EDC's exposure and molecular mechanism, highlighting the toxicological effects on the biological system. Alteration in signaling mechanisms by representative endocrine disruptors such as bisphenol A (BPA), diethylstilbestrol (DES), and genistein has been emphasized. We further discuss the currently available assays and techniques for in vitro detection and propose the prominence of designing nano-architectonic-sensor substrates for on-site detection of EDC in the contaminated aqueous environment.

Antiestrogenic Activity and Possible Mode of Action of Certain New Nonsteroidal Coumarin-4-acetamides

The preparation of certain 2-(2-oxo-2H-chromen-4-yl)-N-substituted acetamides IIIa-h was planned as a step in the development of new modified nonsteroidal antiestrogens. The purity of target compounds IIIa-h was checked by thin-layer chromatography (TLC), and their structures were confirmed using various spectroscopic tools including IR, ¹H-NMR, ¹³C-NMR, and MS spectroscopy. Viability tests were applied using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay to evaluate the cytotoxic effect of the synthesized compounds against two breast cancer cell lines, MCF-7 and MDA-MB-231. Compound IIIb proved the most active against MCF-7 cells, with an IC₅₀ value of 0.32 μM. The results of an analysis of in vitro antiestrogenic activity indicated that only compound IIIb exhibited antiestrogenic activity; its IC₅₀ value of 29.49 μM was about twice as potent as that of the reference compound, MIBP. The aromatase activity was evaluated for the synthesized target compounds IIIa-g and the intermediates Ib and IIa. A significant aromatase inhibition was observed for the intermediate Ib and compound IIIe, with IC₅₀ values of 14.5 and 17.4 μM, respectively. Compound IIIb, namely 7-methoxy-4-(2-oxo-2-(piperidin-1-yl)ethyl)-2H-chromen-2-one, could be used as an antiestrogen and/or cytotoxic agent with selective activity against tumor cells.

Natural product modulators of human sensations and mood: molecular mechanisms and therapeutic potential

Humans perceive physical information about the surrounding environment through their senses. This physical information is registered by a collection of highly evolved and finely tuned molecular sensory receptors. A multitude of bioactive, structurally diverse ligands have evolved in nature that bind these molecular receptors. The complex, dynamic interactions between the ligands and the receptors lead to changes in our sensory perception or mood. Here, we review our current knowledge of natural products and their derived analogues that interact specifically with human G protein-coupled receptors, ion channels, and nuclear hormone receptors to modulate the sensations of taste, smell, temperature, pain, and itch, as well as mood and its associated behaviour. We discuss the molecular and structural mechanisms underlying such interactions and highlight cases where subtle differences in natural product chemistry produce drastic changes in functional outcome. We also discuss cases where a single compound triggers complex sensory or behavioural changes in humans through multiple mechanistic targets. Finally, we comment on the therapeutic potential of the reviewed area of research and draw attention to recent technological developments in genomics, metabolomics, and metabolic engineering that allow us to tap the medicinal properties of natural product chemistry without taxing nature.

A Computational-Based Approach to Identify Estrogen Receptor α/β Heterodimer Selective Ligands

The biologic effects of estrogens are transduced by two estrogen receptors (ERs), ERα and ERβ, which function in dimer forms. The ERα/α homodimer promotes and the ERβ/β inhibits estrogen-dependent growth of mammary epithelial cells; the functions of ERα/β heterodimers remain elusive. Using compounds that promote ERα/β heterodimerization, we have previously shown that ERα/β heterodimers appeared to inhibit tumor cell growth and migration in vitro. Further dissection of ERα/β heterodimer functions was hampered by the lack of ERα/β heterodimer-specific ligands. Herein, we report a multistep workflow to identify the selective ERα/β heterodimer-inducing compound. Phytoestrogenic compounds were first screened for ER transcriptional activity using reporter assays and ER dimerization preference using a bioluminescence resonance energy transfer assay. The top hits were subjected to in silico modeling to identify the pharmacophore that confers ERα/β heterodimer specificity. The pharmacophore encompassing seven features that are potentially important for the formation of the ERα/β heterodimer was retrieved and subsequently used for virtual screening of large chemical libraries. Four chemical compounds were identified that selectively induce ERα/β heterodimers over their respective homodimers. Such ligands will become unique tools to reveal the functional insights of ERα/β heterodimers.

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.

Estrogen receptor agonists/antagonists in breast cancer therapy: A critical review

Estrogens display intriguing tissue selective action that is of great biomedical importance in the development of optimal therapeutics for the prevention and treatment of breast cancer. There are also strong evidences to show that both endogenous and exogenous estrogens are involved in the pathogenesis of breast cancer. Tamoxifen has been the only drug of choice for more than 30years to treat patients with estrogen related (ER) positive breast tumors. There is a need therefore, for identifying newer, potential and novel candidates for breast cancer. Keeping this in view, the present review focuses on selective estrogen receptor modulators and estrogen antagonists such as sulfatase and aromatase inhibitors involved in breast cancer therapy. A succinct and critical overview of the structure of estrogen receptors, their signaling and involvement in breast carcinogenesis are herein described.

Monitoring Ligand-Activated Protein-Protein Interactions Using Bioluminescent Resonance Energy Transfer (BRET) Assay

The bioluminescent resonance energy transfer (BRET) assay has been extensively used in cell-based and in vivo imaging systems for detecting protein-protein interactions in the native environment of living cells. These protein-protein interactions are essential for the functional response of many signaling pathways to environmental chemicals. BRET has been used as a toxicological tool for identifying chemicals that either induce or inhibit these protein-protein interactions. This chapter focuses on describing the toxicological applications of BRET and its optimization as a high-throughput detection system in live cells. Here we review the construction of BRET fusion proteins, describe the BRET methodology, and outline strategies to overcome obstacles that may arise. Furthermore, we describe the advantage of BRET over other resonance energy transfer methods for monitoring protein-protein interactions.

Biosynthetic potential-based strain prioritization for natural product discovery: a showcase for diterpenoid-producing actinomycetes

Natural products remain the best sources of drugs and drug leads and serve as outstanding small-molecule probes to dissect fundamental biological processes. A great challenge for the natural product community is to discover novel natural products efficiently and cost effectively. Here we report the development of a practical method to survey biosynthetic potential in microorganisms, thereby identifying the most promising strains and prioritizing them for natural product discovery. Central to our approach is the innovative preparation, by a two-tiered PCR method, of a pool of pathway-specific probes, thereby allowing the survey of all variants of the biosynthetic machineries for the targeted class of natural products. The utility of the method was demonstrated by surveying 100 strains, randomly selected from our actinomycete collection, for their biosynthetic potential of four classes of natural products, aromatic polyketides, reduced polyketides, nonribosomal peptides, and diterpenoids, identifying 16 talented strains. One of the talented strains, Streptomyces griseus CB00830, was finally chosen to showcase the discovery of the targeted classes of natural products, resulting in the isolation of three diterpenoids, six nonribosomal peptides and related metabolites, and three polyketides. Variations of this method should be applicable to the discovery of other classes of natural products.

Δ(9)-Tetrahydrocannabinol disrupts estrogen-signaling through up-regulation of estrogen receptor β (ERβ)

Δ(9)-Tetrahydrocannabinol (Δ(9)-THC) has been reported as possessing antiestrogenic activity, although the mechanisms underlying these effects are poorly delineated. In this study, we used the estrogen receptor α (ERα)-positive human breast cancer cell line, MCF-7, as an experimental model and showed that Δ(9)-THC exposures markedly suppresses 17β-estradiol (E2)- induced MCF-7 cell proliferation. We demonstrate that these effects result from Δ(9)-THC's ability to inhibit E2-liganded ERα activation. Mechanistically, the data obtained from biochemical analyses revealed that (i) Δ(9)-THC up-regulates ERβ, a repressor of ERα, inhibiting the expression of E2/ERα-regulated genes that promote cell growth and that (ii) Δ(9)-THC induction of ERβ modulates E2/ERα signaling in the absence of direct interaction with the E2 ligand binding site. Therefore, the data presented support the concept that Δ(9)-THC's antiestrogenic activities are mediated by the ERβ disruption of E2/ERα signaling.

Differential action of monohydroxylated polycyclic aromatic hydrocarbons with estrogen receptors α and β

Polycyclic aromatic hydrocarbons (PAHs) are a diverse group of widespread environmental pollutants, some of which have been found to be estrogenic or antiestrogenic. Recent data have shown that hydroxylated PAH metabolites may be responsible for the estrogenic effects of some PAHs. The purpose of this study was to investigate the effects of several PAHs, as well as their monohydroxylated metabolites, on estrogen receptors (ERs), ERα and ERβ. Three parent PAHs and their monohydroxylated metabolites were each evaluated using transcriptional reporter assays in isogenic stable cell lines to measure receptor activation, competitive binding assays to determine ligand binding, and bioluminescence resonance energy transfer assays to assess dimerization. Finally, the estrogenic effects of the hydroxylated metabolites were confirmed by quantitative real-time PCR of estrogen-responsive target genes. Although the parent PAHs did not induce ERα or ERβ transcriptional activity, all of the monohydroxylated PAHs (1-OH naphthanol, 9-OH phenanthrene, 1-OH pyrene) selectively induced ERβ transcriptional activity at the concentrations tested, while not activating ERα. Additionally, the monohydroxylated PAHs were able to competively bind ERβ, induce ERβ homodimers, and regulate ERβ target genes. Although monohydroxylated PAHs appeared to have weak agonist activity to ERβ, our results showed that they can elicit a biologically active response from ERβ in human breast cancer cells and potentially interfere with ERβ signaling pathways.

Identification of estrogen receptor dimer selective ligands reveals growth-inhibitory effects on cells that co-express ERα and ERβ

Estrogens play essential roles in the progression of mammary and prostatic diseases. The transcriptional effects of estrogens are transduced by two estrogen receptors, ERα and ERβ, which elicit opposing roles in regulating proliferation: ERα is proliferative while ERβ is anti-proliferative. Exogenous expression of ERβ in ERα-positive cancer cell lines inhibits cell proliferation in response to estrogen and reduces xenografted tumor growth in vivo, suggesting that ERβ might oppose ERα's proliferative effects via formation of ERα/β heterodimers. Despite biochemical and cellular evidence of ERα/β heterodimer formation in cells co-expressing both receptors, the biological roles of the ERα/β heterodimer remain to be elucidated. Here we report the identification of two phytoestrogens that selectively activate ERα/β heterodimers at specific concentrations using a cell-based, two-step high throughput small molecule screen for ER transcriptional activity and ER dimer selectivity. Using ERα/β heterodimer-selective ligands at defined concentrations, we demonstrate that ERα/β heterodimers are growth inhibitory in breast and prostate cells which co-express the two ER isoforms. Furthermore, using Automated Quantitative Analysis (AQUA) to examine nuclear expression of ERα and ERβ in human breast tissue microarrays, we demonstrate that ERα and ERβ are co-expressed in the same cells in breast tumors. The co-expression of ERα and ERβ in the same cells supports the possibility of ERα/β heterodimer formation at physio- and pathological conditions, further suggesting that targeting ERα/β heterodimers might be a novel therapeutic approach to the treatment of cancers which co-express ERα and ERβ.

Actinopolysporins A-C and tubercidin as a Pdcd4 stabilizer from the halophilic actinomycete Actinopolyspora erythraea YIM 90600

Our current natural product program utilizes new actinomycetes originating from unexplored and underexplored ecological niches, employing cytotoxicity against a selected panel of cancer cell lines as the preliminary screen to identify hit strains for natural product dereplication, followed by mechanism-based assays of the purified natural products to discover potential anticancer drug leads. Three new linear polyketides, actinopolysporins A (1), B (2), and C (3), along with the known antineoplastic antibiotic tubercidin (4), were isolated from the halophilic actinomycete Actinopolyspora erythraea YIM 90600, and the structures of the new compounds were elucidated on the basis of spectroscopic data interpretation. All four compounds were assayed for their ability to stabilize the tumor suppressor programmed cell death protein 4 (Pdcd4), which is known to antagonize critical events in oncogenic pathways. Only 4 significantly inhibited proteasomal degradation of a model Pdcd4-luciferase fusion protein, with an IC50 of 0.88±0.09 μM, unveiling a novel biological activity for this well-studied natural product.

Generation of stable reporter breast cancer cell lines for the identification of ER subtype selective ligands

Estrogen signaling is mediated by two estrogen receptors (ERs), ERα and ERβ, which have unique roles in the regulation of breast cancer cell proliferation. ERα induces proliferation in response to estrogen and ERβ inhibits proliferation in breast cancer cells, suggesting that ERβ selective ligands may be beneficial for promoting the anti-proliferative action of ERβ. Subtype selective ligands can be identified using transcriptional assays, but cell lines in which ERα or ERβ are independently expressed are required. Of the available reporter cell lines, none have been generated in breast cancer cells to identify subtype selective ligands. Here we describe the generation of two isogenic breast cancer cell lines, Hs578T-ERαLuc and Hs578T-ERβLuc, with stable integration of an estrogen responsive luciferase reporter gene. Hs578T-ERαLuc and Hs578T-ERβLuc cell lines are highly sensitive to estrogenic chemicals and ER subtype selective ligands, providing a tool to characterize the transcriptional potency and subtype selectivity of estrogenic ligands in the context of breast cancer cells. In addition to measuring reporter activity, ERβ target gene expression and growth inhibitory effects of ERβ selective ligands can be determined as biological endpoints. The finding that activation of ERβ by estrogen or ERβ selective natural phytoestrogens inhibits the growth of Hs578T-ERβ cells implies therapeutic potential for ERβ selective ligands in breast cancer cells that express ERβ.

Endocrine disrupting chemicals targeting estrogen receptor signaling: identification and mechanisms of action

Many endocrine disrupting chemicals (EDCs) adversely impact estrogen signaling by interacting with two estrogen receptors (ERs): ERα and ERβ. Though the receptors have similar ligand binding and DNA binding domains, ERα and ERβ have some unique properties in terms of ligand selectivity and target gene regulation. EDCs that target ER signaling can modify genomic and nongenomic ER activity through direct interactions with ERs, indirectly through transcription factors such as the aryl hydrocarbon receptor (AhR), or through modulation of metabolic enzymes that are critical for normal estrogen synthesis and metabolism. Many EDCs act through multiple mechanisms as exemplified by chemicals that bind both AhR and ER, such as 3-methylcholanthrene. Other EDCs that target ER signaling include phytoestrogens, bisphenolics, and organochlorine pesticides, and many alter normal ER signaling through multiple mechanisms. EDCs can also display tissue-selective ER agonist and antagonist activities similar to selective estrogen receptor modulators (SERMs) designed for pharmaceutical use. Thus, biological effects of EDCs need to be carefully interpreted because EDCs can act through complex tissue-selective modulation of ERs and other signaling pathways in vivo. Current requirements by the U.S. Environmental Protection Agency require some in vitro and cell-based assays to identify EDCs that target ER signaling through direct and metabolic mechanisms. Additional assays may be useful screens for identifying EDCs that act through alternative mechanisms prior to further in vivo study.

Discovery and total synthesis of a new estrogen receptor heterodimerizing actinopolymorphol A from Actinopolymorpha rutilus

Estrogen receptor ERalpha and ERbeta heterodimerization has been implicated in cancer chemoprevention. The discovery, structural elucidation, and total synthesis of a new natural product, actinopolymorphol A (1), from Actinopolymorpha rutilus (YIM45725) that preferentially induces ERalpha/beta heterodimerization is reported. Total synthesis of 1 has allowed us to determine its absolute stereochemistry and that of a previously known deacetylated congener, and 1 represents the first member of a new class of natural products not previously recognized to modulate ER function.