During the past decade a number of pesticides, industrial by-products, manufactured products such as plastics, and natural chemicals have been shown to disrupt the endocrine system. These chemicals are referred to as endocrine-disrupting chemicals (EDCs). These chemicals have received considerable attention, in part because endocrine disruption is a relatively unstudied area in toxicology and is only recently being taken into account in risk assessment. The focus here is on EDCs with estrogenic activity (EEDCs), which are chemicals that act as hormone mimics via estrogen receptor mechanisms; this is currently the largest group of known endocrine disruptors. The main purpose of this article is to present an overview of the mechanisms of hormone action that provide the basis for understanding how EEDCs have the potential to be biologically active at low, environmentally relevant doses. Our strategy is to discuss the receptor mechanisms mediating responses to a natural hormone, 17β-estradiol (E 2 ), and then to use this information as the basis for describing the low-dose effects of chemicals that disrupt the normal functioning of this hormonal system, either by mimicking, modulating, or antagonizing the activity of the hormone. We have chosen to use estrogen as our example because there is more known about the biology of estrogens and xenoestrogens than other components of the endocrine system for which there is evidence for disruption by environmental chemicals; however, the information presented here is applicable to endocrine disruptors that interfere with other hormonal systems.We will begin by briefly reviewing information concerning the relationship between dose, receptor occupancy, and responses (such as cell proliferation) after binding of E 2 to estrogen receptors (ER-α) in cultured human MCF-7 breast cancer cells. A number of specific factors influence the dose of an EEDC that reaches the target cells to produce a response. These factors include route of administration, absorption, distribution, metabolism, rate of clearance, plasma transport, cell uptake, affinity for estrogen receptor subtype in the cell, and the interaction of the ligand-receptor complex with tissue-specific factors comprising the transcriptional apparatus. This mechanistic information provides the basis for establishing the dose at the target site in cells (nuclear receptors associated with DNA or more recently identified receptors associated with the cell membrane) for an EEDC required to elicit a biological response similar to that produced by a dose of E 2 with equal estrogenic activity. Modeling that takes into account each of these factors would encompass physiologically based pharmacokinetic information (1), as well as quantitative structure-activity relationships (QSAR) (2,3). We have previously discussed the factors that influence access of E 2 and EEDCs from blood to estrogen receptors in cells elsewhere (4-6). Our primary focus in this review is on the latter part of the overall process that occurs once an estrogenic chemic...

Background-Allergic airway diseases are more common in females than in males during early adulthood. A relationship between female hormones and asthma prevalence and severity has been suggested, but the cellular and molecular mechanisms are not understood.Objective-To elucidate the mechanism(s) by which estrogens enhance the synthesis and release of mediators of acute hypersensitivity.Methods-Two mast cell/basophil cell lines (RBL-2H3 and HMC-1) and primary cultures of bone marrow derived mast cells, all of which naturally express estrogen receptor-α, were examined. Cells were incubated with physiological concentrations of 17-β-estradiol with and without IgE and allergens. Intracellular Ca 2+ concentrations and the release of β-hexosaminidase and leukotriene C 4 were quantified.Results-Estradiol alone induced partial release of the preformed, granular protein β-hexosaminidase from RBL-2H3, BMMC and HMC-1, but not from BMMC derived from estrogen receptor-α knock-out mice. The newly synthesized LTC 4 was also released from RBL-2H3. Estradiol also enhanced IgE-induced degranulation and potentiated LTC 4 production. Intracellular Ca 2+ concentration increased prior to and in parallel with mediator release. Estrogen receptor antagonists or Ca 2+ chelation inhibited these estrogenic effects.Conclusion-Binding of physiological concentrations of estradiol to a membrane estrogen receptor-α initiates a rapid onset and progressive influx of extracellular Ca 2+ , which supports the synthesis and release of allergic mediators. Estradiol also enhances IgE-dependent mast cell activation, resulting in a shift of the allergen dose response.

Estrogen receptor (ER) ␣ is mutated (lysine 303 to arginine, K303R) in approximately one third of premalignant breast hyperplasias, which renders breast cancer cells expressing the mutant receptor hypersensitive for proliferation in response to low doses of estrogen. It is known that ER␣ is posttranslationally modified by protein acetylation and phosphorylation by a number of secondary messenger signaling cascades. The K303R ER␣ mutation resides at a major protein acetylation site adjacent to a potential protein kinase A (PKA) phosphorylation site at residue 305 within the hinge domain of the receptor. Mutation of this phosphorylation site to aspartic acid to mimic constitutive phosphorylation blocks acetylation of the K303 ER␣ site and generates an enhanced transcriptional response similar to that seen with the naturally occurring K303R mutant receptor. Activation of PKA signaling by the cell-permeable cyclic AMP (cAMP) analog 8-bromo-cAMP further enhances estrogen sensitivity of the mutant receptor, whereas a specific PKA inhibitor antagonizes this increase. We propose that the hypersensitive ER␣ mutant breast cancer phenotype involves an integration of coupled acetylation and phosphorylation events by upstream signaling molecules.

Alterations in histones, chromatin-related proteins, and DNA methylation contribute to transcriptional silencing in cancer, but the sequence of these molecular events is not well understood. Here we demonstrate that on disruption of estrogen receptor (ER) ␣ signaling by small interfering RNA, polycomb repressors and histone deacetylases are recruited to initiate stable repression of the progesterone receptor (PR) gene, a known ER␣ target, in breast cancer cells. The event is accompanied by acquired DNA methylation of the PR promoter, leaving a stable mark that can be inherited by cancer cell progeny. Reestablishing ER␣ signaling alone was not sufficient to reactivate the PR gene; reactivation of the PR gene also requires DNA demethylation. Methylation microarray analysis further showed that progressive DNA methylation occurs in multiple ER␣ targets in breast cancer genomes. The results imply, for the first time, the significance of epigenetic regulation on ER␣ target genes, providing new direction for research in this classical signaling pathway.

The effects of 17β-estradiol (E2) on immune function have been extensively reported. The effects are dependent on concentration and duration of exposure and potential differences in signaling between the known E2 receptors, estrogen receptors (ER) α and ERβ. Through the use of ER-deficient mice, we and others have begun to demonstrate the role of the two known receptors in modulating immune functional activities. Previous studies have shown that cells of the innate immune system have altered function (bactericidal capacity) and patterns of cytokine expression (increased proinflammatory cytokine expression) through amelioration of ERα signaling. In this study, we extend these studies to analysis of T cell differentiation and proliferation in APC-dependent and APC-independent in vitro assay systems. Our results demonstrate that ERα deficiency in splenic macrophages, but not CD11c+ splenic dendritic cells pulsed with OVA significantly enhances proliferative responses and IFN-γ production by transgenic OVA peptide-specific (OT-II) CD4+ T cells when compared with Ag-pulsed APC from wild-type littermates. The addition of E2 in this culture system did not significantly affect the production of IFN-γ. In addition, when purified CD4+ T cells from ERα-deficient and wild-type littermates were stimulated with anti-CD3/CD28 Ab in the absence of E2, there were no significant differences in IFN-γ or IL-4 production. However, the addition of E2 significantly increased IL-4 secretion, as well as increased GATA-3 mRNA levels from ERα-replete CD4+ T cells, while this effect was abrogated in ERα-deficient CD4+ T cells.