Fluorescence-based assay of estrogen receptor using 12-oxo-9(11)-dehydroestradiol-17 beta

Steroid-binding G-protein-coupled receptors: new drug discovery targets for old ligands

Steroid-binding receptors have long been a successful target class for the pharmaceutical industry. Clinical applications for steroids range from contraception and hormone replacement therapy to immune regulation and cancer therapy. With the recent demonstration that the orphan GPCR, GPR30 binds and is activated by estrogen, as well as the identification of a GPR30-selective agonist, it is likely that GPR30 represents a novel drug target with many potential clinical applications. This review discusses the role of GPR30 in mediating the effects of estrogen, as well as recent efforts to isolate GPR30-specific ligands using a combination of virtual and biomolecular screening. Finally, comments are made on the future directions regarding GPCRs, steroids and drug discovery.

Binding properties of coumestrol to expressed human estrogen receptor

We have studied the binding of coumestrol, an inherently fluorescent analog of 17 beta-estradiol, to human estrogen receptor (hER) in extracts of transfected cultured cells. The binding of coumestrol to the hER as well as its dissociation from the receptor can be directly determined by the change in fluorescence intensity of the probe. In agreement with previous studies using calf uterine extracts, we find that coumestrol binds to the receptor with a ten-fold lower affinity than 17 beta-estradiol. However, the rate of dissociation appears to be close to that of the native ligand. Coumestrol can accept energy from Trp residues in the excited state and, using a C530W hER mutant, we have confirmed that residue 530 is close to the ligand-binding pocket. We also find that the presence of saturating amounts of the specific DNA binding sites (ERE) did not significantly alter the binding affinity of coumestrol to either wild type hER or the C530W mutant.

Characterization of the spectroscopic properties of a tetrahydrochrysene system containing a rigidified hydroxynitrostilbene chromophore: an inherently fluorescent ligand designed for the estrogen receptor

A tetrahydrochrysene system that embodies a hydroxy- and nitro-substituted stilbene chromophore held rigidly near planarity by the tetracyclic nature of the compound was prepared as a fluorescent ligand for the estrogen receptor. It shows strong solvent-dependent fluorescence at long wavelengths. The solvent polarity dependence suggests that the fluorescence arises from an excited state with much n pi * character in cyclohexane; stronger emission comes from an intramolecular charge transfer state that has lower energy in more polar solvents, and finally progressive quenching of the charge transfer state occurs in solvents of higher polarity. The quenching effect is particularly evident in protic solvents. In water, however, the compound shows fluorescence of unusually high energy for an intramolecular charge transfer state, which suggests that photochemistry may be occurring. In solutions of gamma-cyclodextrin, emission from the nitrotetrahydrochrysene is red shifted and intensified relative to water. Photobleaching occurs in H2O but not in ethanol or gamma-cyclodextrin solution. The change in dipole moment between the ground and excited states for the nitrochrysene is 12.9 D, similar to our previous measurements for related nitrostilbenes. The compound displays red-shifted emission in triethylamine, perhaps due to an excited state hydrogen-bonded complex. The absorption and emission properties of the corresponding nitrophenolate were also studied. The nitrophenolate exhibits reverse solvatochromism in its absorption spectra. In conclusion, the high sensitivity of the emission energy and quantum yield of the title compound make it of potential utility as a fluorescent probe.

A synthesis of 7 alpha-substituted estradiols: synthesis and biological evaluation of a 7 alpha-pentyl-substituted BODIPY fluorescent conjugate and a fluorine-18-labeled 7 alpha-pentylestradiol analog

In an effort to assist in the preparation of ligands for the study of the estrogen receptor (ER), we have developed a new synthesis of 7 alpha-substituted estradiols. The key step in the synthesis involves a copper-catalyzed, alpha-selective, 1,6-conjugate addition of 4-pentenyl magnesium bromide to a suitably protected 6-dehydrotestosterone derivative. Desaturation and then reductive aromatization of the resulting 7 alpha-pentenyl androgen gave the 7 alpha-pentenylestradiol in good yields. The alpha-stereoselectivity of this addition in the testosterone series, compared with the 19-nortestosterone series, is significantly improved by the presence of the C-19 methyl group, which shields the beta face from attack. A key intermediate was functionalized further by substitution with fluorine-18 to provide a potential imaging agent for positron emission tomography, and by conjugation with a BODIPY (Molecular Probes Inc., Eugene, OR, USA) fluorophore to make a fluorescent probe for the estrogen receptor. The synthesis and biological evaluation of these analogs is presented, as well as a discussion of the improvements in the synthetic procedure.

Binding of estradiol-peroxidase conjugate to estrogen receptor

The binding of estradiol-horseradish peroxidase conjugate to rat uterine cytosolic estrogen receptor was studied. The conjugate having a steroid to enzyme ratio of 2.8:1 was allowed to bind to protamine precipitated receptor in presence or absence of 100-fold excess of free estradiol. The bound enzyme activity was measured and the data subjected to Scatchard analysis to obtain the dissociation constant and the number of binding sites. Although the binding parameter so obtained differed from values obtained using radiolabelled estradiol, the method may be used for comparative studies.

Receptor binding of NBD-labeled fluorescent estrogens and progestins in whole cells and cell-free preparations

We have studied the interactions of four fluorescent steroid conjugates with either the estrogen or progesterone receptor, both in whole cells and cell-free receptor preparations. The fluorophore, nitrobenzoxadiazole (NBD), was conjugated with a synthetic progestin, with a steroidal estrogen, a non-steroidal estrogen, and with an antiestrogen. With all compounds, receptor-specific binding could be detected by fluorescence measurements following extraction from the protein into an organic solvent. In the native state, however, the NBD-ligand-receptor complex is essentially non-emissive, although these ligands fluoresce strongly when associated with non-specific binders such as albumin. The binding site concentrations and relative affinities determined by fluorescence (after extraction) correspond well with those determined by [3H]estradiol or [3H]R5020 binding to their respective receptors. In T47D breast cancer cells, the NBD-progestin showed receptor-mediated uptake and nuclear localization. These compounds have provided valuable information about the interactions of low and medium affinity ligands with their receptors; however, the successful use of fluorescent ligands for detecting steroid receptors under native-bound conditions, by "imaging" modalities (fluorescence microscopy and flow cytometry) will require the development of fluorophores that are emissive while receptor bound or assay protocols that enable the environment of ligands associated with the receptor to be controlled.