Recent advances in peptidomimetics antagonists targeting estrogen receptor α-coactivator interaction in cancer therapy

Estrogen receptor α (ERα) is a crucial target for ERα positive breast cancer treatment. Previous drug discovery efforts were focused on developing inhibitors that targeted the canonical ligand binding pockets of the ligand binding domain (LBD) of ERα. However, significant percentage of patients developed cancer relapse with drug-resistance. ERα peptidomimetic modulators have been considered as promising treatments for drug resistant breast cancers as they are targeting ERα-coactivator interacting interface instead of the ligand binding pocket of ERα. Herein, we reviewed the recent development of ERα peptidomimetics antagonists.     

Protein arginine methyltransferase 6 enhances ligand-dependent and -independent activity of estrogen receptor α via distinct mechanisms

Recent studies reported that protein arginine methyltransferase 6 (PRMT6) enhances estrogen-induced activity of estrogen receptor α (ERα) and dysfunction of PRMT6 is associated with overall better survival for ERα-positive breast cancer patients. However, it is unclear how PRMT6 promotes ERα activity. Here we report that PRMT6 specifically interacts with ERα at its ligand-binding domain. PRMT6 also methylates ERα both in vitro and in vivo. In addition to enhancing estrogen-induced ERα activity, PRMT6 over-expression up-regulates estrogen-independent activity of ERα and PRMT6 gene silencing in MCF7 cells inhibits ligand-independent ERα activation. More interestingly, the effect of PRMT6 on the ligand-independent ERα activity does not require its methyltransferase activity. Instead, PRMT6 competes with Hsp90 for ERα binding: PRMT6 and Hsp90 bindings to ERα are mutually exclusive and PRMT6 over-expression reduces ERα interaction with Hsp90. In conclusion, PRMT6 requires its methyltransferase activity to enhance ERα's ligand-induced activity, but its effect on ligand-independent activity is likely mediated through competing with Hsp90 for binding to the C-terminal domain of ERα. PRMT6-ERα interaction would prevent ERα-Hsp90 association. Since Hsp90 and associated chaperones serve to maintain ERα conformation for ligand-binding yet functionally inactive, inhibition of ERα-Hsp90 interaction would relieve ERα from the constraint of chaperone complex.     

Ritonavir binds to and downregulates estrogen receptors: Molecular mechanism of promoting early atherosclerosis

Estrogenic actions are closely related to cardiovascular disease. Ritonavir (RTV), a human immunodeficiency virus (HIV) protease inhibitor, induces atherosclerosis in an estrogen-related manner. However, how RTV induce pathological phenotypes through estrogen pathway remains unclear. In this study, we found that RTV increases thickness of coronary artery walls of Sprague Dawley rats and plasma free fatty acids (FFA) levels. In addition, RTV could induce foam cell formation, downregulate both estrogen receptor α (ERα) and ERβ expression, upregulate G protein-coupled estrogen receptor (GPER) expression, and all of them could be partially blocked by 17β-estradiol (E2), suggesting RTV acts as an antagonist for E2. Computational modeling shows a similar interaction with ERα between RTV and 2-aryl indoles, which are highly subtype-selective ligands for ERα. We also found that RTV directly bound to ERα and selectively inhibited the nuclear localization of ERα, and residue Leu536 in the hydrophobic core of ligand binding domain (LBD) was essential for the interaction with RTV. In addition, RTV did not change the secondary structure of ERα-LBD like E2, which explained how ERα lost the capacity of nuclear translocation under the treatment of RTV. All of the evidences suggest that ritonavir acts as an antagonist for 17β-estradiol in regulating α subtype estrogen receptor function and early events of atherosclerosis.     

In silico evidences for the binding of phthalates onto human estrogen receptor α, β subtypes and human estrogen-related receptor γ

Being lipophilic xenobiotic chemicals, phthalates from the surrounding environments can easily be absorbed into the biological system, thereby causing various health problems including cancer and endocrine disruption in test animals and also in humans. In the present in silico study employing Glide, Schrödinger Suite 2012, we analysed in detail the binding affinities of 12 commonly used diphthalates and their metabolites (corresponding mono ester and phthalic acid) onto the ligand-binding domain (LBD) of the human estrogen receptor α (hERα), human estrogen receptor β (hERβ) and human estrogen related receptor γ (hERRγ). Natural ligand 17β estradiol (E2), known xenoestrogen bisphenol A, the phytoestrogen genistein, the agonists/antagonists 4-hydroxy tamoxifen and raloxifene were also docked onto these receptors as positive controls for comparing the binding efficiencies with that of phthalates and their metabolites. Results revealed that E2 had less binding affinity to the receptors in comparison to certain phthalates, i.e. maximum binding scores (G score, kcal/mol) were diisononyl phthalate ( ? 9.44) to hERα, monophenyl phthalate ( ? 8.66) to hERβ and di(2-ethylhexyl)phthalate ( ? 9.38) to hERRγ. The most concerned monophthalates established additional H bonds with certain surrounding crucial amino acid residues in the LBD, and thus showed more affinity to all the receptors than even the natural ligand and other well-characterised xenoestrogens as demonstrated in this study. Briefly, this study gives an insight into the virtual binding behaviours of commonly used phthalates and their metabolites onto hERs and hERRγ, which would accelerate further in vitro mechanistic, preclinical and clinical studies on real in vitro or in vivo platforms.     

Synthesis and evaluation of 17α-(dimethylphenyl)vinyl estradiols as probes of the estrogen receptor-α ligand binding domain

As part of our program to explore the influence of small structural modifications on the biological response of the estrogen receptor-α (ERα), we prepared and evaluated a series of mono-and di-substituted phenyl vinyl estradiols. The target compounds were prepared in 45-80% yields using the Stille coupling reaction and evaluated using competitive binding analysis with the ERα-ligand binding domain (hERα-LBD) and estrogenic activity (induction of alkaline phosphatase in Ishikawa cells). Results indicated that the 2,4- and 2,5-dimethyl derivatives, 5b and 5c, had the highest relative binding affinity (RBA=20.5 and 37.3%) and relative stimulatory activity (RSA=101.0% and 12.3%) of the di-methyl series.     

Estrogen receptor alpha interacts with 17β-hydroxysteroid dehydrogenase type 10 in mitochondria

Estrogen receptor alpha (ERα) is present in the nucleus, the cytosol and in mitochondria. The rat ERα ligand binding domain was employed as bait in a bacterial two-hybrid screening of a human heart cDNA library to detect novel protein-protein interaction partners of ERα in the heart. 17β-Hydroxysteroid dehydrogenase type 10 (17β-HSD10), which converts potent (17β-estradiol) to less potent estrogens (estrone), co-localized with 17β-HSD10 in the mitochondria of rat cardiac myocytes. GST pull-down experiments confirmed the interaction of ERα and 17β-HSD10. These findings suggest that the ERα estrogen receptor might be involved in regulating intracellular estrogen levels by modulating 17β-HSD10 activity.     

Structural Insights into Estrogen Receptor α Methylation by Histone Methyltransferase SMYD2, a Cellular Event Implicated in Estrogen Signaling Regulation

Estrogen receptor (ER) signaling plays a pivotal role in many developmental processes and has been implicated in numerous diseases including cancers. We recently showed that direct ERα methylation by the multi-specificity histone lysine methyltransferase SMYD2 regulates estrogen signaling through repressing ERα-dependent transactivation. However, the mechanism controlling the specificity of the SMYD2–ERα interaction and the structural basis of SMYD2 substrate binding diversity are unknown. Here we present the crystal structure of SMYD2 in complex with a target lysine (Lys266)-containing ERα peptide. The structure reveals that ERα binds SMYD2 in a U-shaped conformation with the binding specificity determined mainly by residues C-terminal to the target lysine. The structure also reveals numerous intrapeptide contacts that ensure shape complementarity between the substrate and the active site of the enzyme, thereby likely serving as an additional structural determinant of substrate specificity. In addition, comparison of the SMYD2–ERα and SMYD2–p53 structures provides the first structural insight into the diverse nature of SMYD2 substrate recognition and suggests that the broad specificity of SMYD2 is achieved by multiple molecular mechanisms such as distinct peptide binding modes and the intrinsic dynamics of peptide ligands. Strikingly, a novel potentially SMYD2-specific polyethylene glycol binding site is identified in the CTD domain, implicating possible functions in extended substrate binding or protein–protein interactions. Our study thus provides the structural basis for the SMYD2-mediated ERα methylation, and the resulting knowledge of SMYD2 substrate specificity and target binding diversity could have important implications in selective drug design against a wide range of ERα-related diseases.