Structure-activity relationships for a large diverse set of natural, synthetic, and environmental estrogens

The chemical UV-filter 3-benzylidene camphor causes an oestrogenic effect in an in vivo fish assay

Chemical UV-filters are used in sun protection products and various kinds of cosmetics. The lipophilic chemical UV-filter 3-benzylidene camphor was investigated for its capability to cause vitellogenin induction, possibly via oestrogen receptor binding, in a well-established in vivo fish assay (juvenile rainbow trout, Oncorhynchus mykiss, vitellogenin ELISA). A clear relationship was demonstrated between the dose of injected 3-benzylidene camphor and the concentration of plasma vitellogenin with a 105-times induction from 68 mg 3-benzylidene camphor /kg/injection and above compared to the control vitellogenin level. The relationship between the injected dose of 3-benzylidene camphor and the percent of responding fish (vitellogenin) was evaluated by logistic regression analysis and effective dose-values (ED-values) were determined. ED10, ED50 and ED90 of 3-benzylidene camphor after 6 days (2 injections) were 6.4, 16 and 26 mg/kg/ injection, respectively. These ED-values place 3-benzylidene camphor among the more potent xenooestrogens discovered to date and necessitates investigations on the distribution, concentration, persistence and bioaccumulation of 3-benzylidene camphor and other UV-filters in nature.

(4R,5S)/(4S,5R)-4,5-Bis(4-hydroxyphenyl)-2-imidazolines: ligands for the estrogen receptor with a novel binding mode

(4R,5S)/(4S,5R)-4,5-Bis(4-hydroxyphenyl)-2-imidazolines 1-7 were synthesized by the reaction of the methoxy-substituted (1R,2S)/(1S,2R)-1,2-diarylethylenediamines 1b-7b with triethyl orthoformate and subsequent ether cleavage with BBr(3). All compounds were tested for estrogen receptor (ER) binding in a competition experiment with [(3)H]-estradiol and for gene activation in a luciferase assay using ER positive MCF-7-2a breast cancer cells stably transfected with the plasmid ERE(wtc)luc. The relative binding affinities of the 2-imidazolines were very low (RBA < 0.1%). Nevertheless, 4-7 possessed full agonistic activity in the luciferase assay. The relative transcription potency increased in the order 5 (2,2'-I) < 6 (2,6-Cl(2), 2'-F) < 4 (2,2'-Cl) < 7 (2,6-Cl(2), 2'-Cl). These data together with spectroscopic and molecular modeling studies were used to investigate the preferred binding mode adopted by the imidazoline ligands. The 1,2-diarylethane pharmacophor takes a Z-stilbene-like structure with pseudoaxially oriented phenyl rings at the planar heterocyclic ring. Because of this unusual spatial structure, the (4R,5S)/(4S,5R)-4,5-bis(4-hydroxyphenyl)-2-imidazolines have to be assigned to a second class of estrogenically active compounds (type II estrogens). In contrast to type I estrogens, e.g., estradiol (E2), diethylstilbestrol (DES), and meso-hexestrol (HES), which are connected to His 524 in the binding site, type II estrogens are very likely H-bonded to Asp 351 in a hydrophobic side pocket.

Xenoestrogenic gene expression: structural features of active polycyclic aromatic hydrocarbons

Estrogenicity was assessed using the Saccharomyces cerevisiae-based Lac-Z reporter assay and was reported as the logarithm of the inverse of the 50% molar beta-galactosidase activity (log[EC50(-1)]). In an effort to quantify the relationship between molecular structure of polycyclic aromatic hydrocarbons (PAHs) and estrogenic gene expression, a series of PAHs were evaluated. With noted exceptions, the results of these studies indicate that the initial two-dimensional structural warning for estrogenicity, the superpositioning of a hydroxylated aromatic system on the phenolic A-ring of 17-beta-estradiol, can be extended to the PAHs. This two-dimensional-alignment criterion correctly identified estrogenicity of 22 of the 29 PAHs evaluated. Moreover, the estrogenic potency of these compounds was directly related to the size of the hydrophobic backbone. The seven compounds classified incorrectly by this structural feature were either dihydroxylated naphthalenes or aromatic nitrogen-heterocyclic compounds; all such compounds were false positives. Results with dihydroxylated naphthalenes reveal derivatives that were nonestrogenic when superimposed on the phenolic A-ring of 17-beta-estradiol had the second hydroxyl group in the position of the C-ring or were catechol-like in structure. Structural alerts for nitrogen-heterocyclic compounds must take into account the position of the hydroxyl group and the in-ring nitrogen atom; compounds with the hydroxyl group and nitrogen atom involved with the same ring were observed to be nonactive.

Phytoestrogens and mycoestrogens bind to the rat uterine estrogen receptor

Consumption of phytoestrogens and mycoestrogens in food products or as dietary supplements is of interest because of both the potential beneficial and adverse effects of these compounds in estrogen-responsive target tissues. Although the hazards of exposure to potent estrogens such as diethylstilbestrol in developing male and female reproductive tracts are well characterized, less is known about the effects of weaker estrogens including phytoestrogens. With some exceptions, ligand binding to the estrogen receptor (ER) predicts uterotrophic activity. Using a well-established and rigorously validated ER-ligand binding assay, we assessed the relative binding affinity (RBA) for 46 chemicals from several chemical structure classes of potential phytoestrogens and mycoestrogens. Although none of the test compounds bound to ER with the affinity of the standard, 17beta-estradiol (E(2)), ER binding was found among all classes of chemical structures (flavones, isoflavones, flavanones, coumarins, chalcones and mycoestrogens). Estrogen receptor relative binding affinities were distributed across a wide range (from approximately 43 to 0.00008; E(2) = 100). These data can be utilized before animal testing to rank order estimates of the potential for in vivo estrogenic activity of a wide range of untested plant chemicals (as well as other chemicals) based on ER binding.

Polychlorinated biphenyls interfere with androgen-induced transcriptional activation and hormone binding

Polychlorinated biphenyls (PCBs) are ubiquitous highly persistent manufactured chemicals known to bioaccumulate in the food chain. Exposure to PCBs has been implicated in a wide range of human health effects, including altering normal endocrine processes and reproductive function. However, very little is understood regarding the specific mechanisms by which PCBs may exert their effects in biological systems. We have examined the ability of PCBs to interfere with transcriptional activation of the androgen receptor (AR) and glucocorticoid receptor (GR) in an in vitro transcription-based reporter assay system. Four Aroclor PCB mixtures were found to antagonize AR-mediated transcription in the presence of the natural AR ligand dihydrotestosterone (DHT). The antagonistic activity of Aroclor mixtures increased in the following order: 1260 < 1242 < 1254 < 1248. These Aroclor mixtures had no discernible effect on GR activity. Aroclor 1254 in the absence of DHT exhibited weak agonistic responses in a dose-dependent manner with AR. Within a series of individual congeners, congeners 42, 128, and 138 are shown to antagonize AR activity. These congeners all share a common core chlorine substitution pattern. Ligand-binding studies demonstrate that endocrine activities of PCB mixtures and congeners on AR are likely due to direct and specific binding to AR ligand-binding domain.

Ligand-selective interactions of ER detected in living cells by fluorescence resonance energy transfer

Some aspects of ligand-regulated transcription activation by the estrogen receptor (ER) are associated with the estrogen-dependent formation of a hydrophobic cleft on the receptor surface. At least in vitro, this cleft is required for direct interaction of ER with an alpha helix, containing variants of the sequence LXXLL, found in many coactivators. In cells, it is unknown whether ER interactions with the different LXXLL-containing helices are uniformly similar or whether they vary with LXXLL sequence or activating ligand. Using fluorescence resonance energy transfer (FRET), we confirm in the physiological environment a direct interaction between the estradiol (E2)-bound ER and LXXLL peptides expressed in living cells as fusions with spectral variants of the green fluorescent protein. This interaction was blocked by a single amino acid mutation in the hydrophobic cleft. No FRET was detected when cells were incubated with the antiestrogenic ligands tamoxifen and ICI 182,780. E2, diethylstilbestrol, ethyl indenestrol A, and 6,4'-dihydroxyflavone all promoted FRET and activated ER-dependent transcription. Measurement of the level of FRET of ER with different LXXLL-containing peptides suggested that the orientations or affinities of the LXXLL interactions with the hydrophobic cleft were globally similar but slightly different for some activating ligands.

Structure-activity relationships for gene activation oestrogenicity: evaluation of a diverse set of aromatic chemicals

Structure-activity relationships for oestrogenicity were developed based on 120 aromatic chemicals evaluated in the Saccharomyces cerevisiae-based Lac-Z reporter assay. Relative gene activation was compared to 17 beta-estradiol and varied over eight orders of magnitude. Analysis of the data compared to 17 beta-estradiol identified three structural criteria that were related to xenoestrogen activity and potency: (1) the hydrogen-bonding ability of the phenolic ring mimicking the A-ring, (2) a hydrophobic centre similar in size and shape to the B- and C-rings, and (3) a hydrogen-bond donor mimicking the 17 beta-hydroxyl moiety of the D-ring, especially with an oxygen-to-oxygen distance similar to that between the 3- and 17 beta-hydroxyl groups of 17 beta-estradiol. Binding data were segregated into activity clusters including strong, moderate, weak, and detectable gene expression, and those compounds that were inactive. The hydrogen-bonding ability of hydroxy group in the 3-position on 17 beta-estradiol was observed to be essential for gene activation. Compounds with a 4-hydroxyl substituted benzene ring and a hydrophobic moiety of size and shape equivalent to the B-ring of 17 beta-estradiol were generally observed to be weakly active compounds. Moderately active compounds have a 4-hydroxyl substituted benzene ring with a hydrophobic moiety equivalent in size and shape to the B- and C-ring of 17 beta-estradiol, or have a high hydrogen-bond donor capacity owing to the presence of halogens on a nonphenolic ring. Strongly active compounds, similar to 4,4'-diethylethylene bisphenol (DES), possess the same hydrophobic ring structure as described for moderately active compounds and an additional hydroxyl group with an oxygen-to-oxygen distance close to that exhibited by the 3- and 17-hydroxyl groups of 17 beta-estradiol.

Lack of binding to isolated estrogen or androgen receptors, and inactivity in the immature rat uterotrophic assay, of the ultraviolet sunscreen filters Tinosorb M-active and Tinosorb S

The presence of structurally diverse chemicals as contaminants in the environment has led to concerns regarding their possible endocrine disturbing effects. Recently, some ultraviolet absorbing components of sunscreen preparations have given positive responses in assays monitoring estrogen-like activity both in vitro and in vivo. Consequently, two recently developed sunscreen components, Tinosorb M-active and Tinosorb S, were evaluated using the in vitro estrogen and androgen receptor competitive binding assays. Neither compound gave a positive response in either of the assays, consistent with the large molecular dimensions of each chemical disfavoring binding to the hormone receptors. Both of the chemicals were inactive in immature rat uterotrophic assays conducted using the subcutaneous route of administration. It is concluded that neither of these agents possess intrinsic estrogenic/antiestrogenic or androgenic/antiandrogenic activity. The several positive control chemicals evaluated gave the expected positive responses in the assays used.

Diversity-oriented synthesis of multisubstituted olefins through the sequential integration of palladium-catalyzed cross-coupling reactions. 2-pyridyldimethyl(vinyl)silane as a versatile platform for olefin synthesis

A novel strategy for the diversity-oriented synthesis of multisubstituted olefins, where 2-pyridyldimethyl(vinyl)silane functions as a versatile platform for olefin synthesis, is described. The palladium-catalyzed Heck-type coupling of 2-pyridyldimethyl(vinyl)silanes with organic iodides took place in the presence of Pd2(dba)3/tri-2-furylphosphine catalyst to give beta-substituted vinylsilanes in excellent yields. The Heck-type coupling occurred even with alpha- and beta-substituted 2-pyridyldimethyl(vinyl)silanes. The one-pot double Heck coupling of 2-pyridyldimethyl(vinyl)silane took place with two different aryl iodides to afford beta,beta-diarylated vinylsilanes in good yields. The palladium-catalyzed Hiyama-type coupling of 2-pyridyldimethyl(vinyl)silane with organic halides took place in the presence of tetrabutylammonium fluoride to give di- and trisubstituted olefins in high yields. The sequential integration of Heck-type (or double Heck) coupling and Hiyama-type coupling produced the multisubstituted olefins in regioselective, stereoselective, and diversity-oriented fashions. Especially, the one-pot sequential Heck/Hiyama coupling reaction provides an extremely facile entry into a diverse range of stereodefined multisubstituted olefins. Mechanistic considerations of both Heck-type and Hiyama-type coupling reactions are also described.

The computational prediction of toxicity

Recent developments in the prediction of toxicity from chemical structure have been reviewed. Attention has been drawn to some of the problems that can be encountered in the area of predictive toxicology, including the need for a multi-disciplinary approach and the need to address mechanisms of action. Progress has been hampered by the sparseness of good quality toxicological data. Perhaps too much effort has been devoted to exploring new statistical methods rather than to the creation of data sets for hitherto uninvestigated toxicological endpoints and/or classes of chemicals.