Effect of cadmium on the interaction of 17beta-estradiol with the rainbow trout estrogen receptor.

The widely reported negative effects of xenoestrogens on the endocrine system of aquatic organisms gave raise to public concern and led to a number of screening and testing initiatives on the international level. Recent studies indicated that not only organic chemicals but also certain heavy metals, including cadmium, can mimic the effects of the endogenous estrogen receptor agonist 17beta-estradiol (E2) and lead to estrogen receptor activation. While the effects of cadmium on the endocrine system and its potential to harm living organisms are no longer in doubt, the exact mode of action is still essentially unknown. In the present study we utilized the rainbow trout ER alpha ligand binding domain (rtER-LBD) fused to glutathione-S-transferase, to study noncovalent interactions between cadmium and the rtER-LBD. ICP-MS data showed that the Cd uptake by the rtER-LBD was strongly pH-dependent. Previous results showing that Cd shields Cys residues of the rtER-LBD against chemical modification, and competitive binding experiments reported here provide insights into the specificity of the interaction of cadmium with the ER hormone binding cavity. It could, for instance, be shown that most of the cadmium adsorbed to the protein could be released into solution either under denaturing conditions, or by stripping from the protein surface using EDTA at physiological conditions. Competitive binding experiments using radio-labeled estradiol showed that, in contrast to previously published data, E2 has an affinity an order of magnitude higher for the ER than for Cd. ICP-MS experiments showed that, despite its higher affinity, increasing E2 concentrations were unable to replace Cd from the rtER-LBD that had been preequilibrated with Cd. These findings were independently confirmed by the [3H]-E2 binding assay. At the same time both ICP-MS and the [3H]-E2 binding assay showed that increasing Cd concentrations not only lead to a decrease in the specific estradiol binding, but also to the release of E2 from the preequilibrated rtER-LBD. The gradual release of [3H]-E2 from the rtER-LBD following incubation with increasing Cd concentrations indicates either direct competition for the same binding site or Cd-induced conformational changes resulting in the release of estradiol.

Identification of the estrogen receptor Cd-binding sites by chemical modification.

The widely reported interactions of the estrogen receptor (ER) with endocrine disrupting chemicals (EDCs) present in the environment gave raise to public concern and led to a number of screening and testing initiatives on the international level. Recent studies indicated that certain heavy metals, including cadmium, can mimic the effects of the endogenous estrogen receptor agonist 17beta-estradiol, and lead to estrogen receptor activation. Previous studies of the chimeric proteins, which incorporate the ligand-binding domain of the human ER, identified Cys 381, Cys 447, Glu 523, His 524 and Asp 538 as possible sites of interactions with cadmium. In the present study we utilized the rainbow trout ER ligand-binding domain fused to glutathione-S-transferase, and used Cd-shielding against various types of chemical modification of the fusion protein to study non-covalent interactions between the ER and Cd. The distribution of exposed and shielded residues allowed to identify amino acid residues involved in the interaction. Our data indicated preferential protection of Cys groups by cadmium, suggesting their involvement in the interaction. This supports data found in the literature on the strong binding affinity of the thiol group towards metals. However, not all Cys in the fusion protein sequence were protected against chemical modification, illustrating the importance of their chemical environment. In general, the location of rtER-LBD Cys residues implicated in Cd interactions did not confirm assignments made by alanine-scanning mutagenesis for the hER, probably due to differences in experimental setup and fusion proteins used. The involvement of other functional groups such as carboxylic acids in the Cd interactions, though not confirmed, can not be completely ruled out due to the general limitations of the chemical modification approach discussed in detail. Suggestions for an improved experimental setup were made.

Comparative analysis of estrogenic activity in sewage treatment plant effluents involving three in vitro assays and chemical analysis of steroids.

In this study, we assessed and compared the suitability of three in vitro screening tools for the measurement of estrogenic activity in sewage treatment plant effluents (STPEs). These assays were the yeast estrogen screen (YES), production of zona radiata proteins (ZRPs) in trout hepatocytes, and the induction of reporter gene expression in the transfected rainbow trout gonad cell line RTG-2. Data obtained with the YES were additionally compared with calculated estrogenicity, based on steroid analysis data of the effluents. For comparison purposes, the response of the in vitro systems toward the estrogenic chemicals beta-estradiol, ethinyl estradiol, bisphenol-A, nonylphenol, and octylphenol was assessed. All three assays showed sensitivities in the same order of magnitude in response to the steroid compounds tested, with ZRP production being the least sensitive. Regarding the estrogenic environmental chemicals tested, the RTG-2 assay was more than an order of magnitude more sensitive than the other two assays. Despite their different sensitivities toward selected test chemicals, the three in vitro systems indicated estrogenic activity in the same concentration range for the tested STPEs. Calculated estrogenicity (chemical analysis) and measured estrogenicity (YES) were of the same order of magnitude for the STPEs tested. The present study indicates that all three in vitro systems, with the yeast-based system being the easiest and most robust, are applicable for the screening of estrogenic activity in effluent samples.

Combined biological and chemical assessment of estrogenic activities in wastewater treatment plant effluents.

Five wastewater treatment plant effluents were analyzed for known endocrine disrupters and estrogenicity. Estrogenicity was determined by using the yeast estrogen screen (YES) and by measuring the blood plasma vitellogenin (VTG) concentrations in exposed male rainbow trout (Oncorhynchus mykiss). While all wastewater treatment plant effluents contained measurable concentrations of estrogens and gave a positive response with the YES, only at two sites did the male fish have significantly increased VTG blood plasma concentrations after the exposure, compared to pre-exposure concentrations. Estrone (E1) concentrations ranged up to 51 ng L(-1), estradiol (E2) up to 6 ng L(-1), and ethinylestradiol (EE2) up to 2 ng L(-1) in the 90 samples analyzed. Alkylphenols, alkylphenolmonoethoxylates and alkylphenoldiethoxylates, even though found at microg L(-1) concentrations in effluents from wastewater treatment plants with a significant industrial content, did not contribute much to the overall estrogenicity of the samples taken due to their low relative potency. Expected estrogenicities were calculated from the chemical data for each sample by using the principle of concentration additivity and relative potencies of the various chemicals as determined with the yeast estrogen screen. Measured and calculated estradiol equivalents gave the same order of magnitude and correlated rather well (R(2)=0.6).