In vitro assessment of effects of persistent organic pollutants on the transactivation of estrogen receptor α and ß (ERα and ERß) from the Baikal seal (Pusa sibirica).

To assess the effect of exposure to persistent organic pollutants (POPs) on the estrogen receptor (ER) signaling pathway in Baikal seals (Pusa sibirica), we investigated the molecular characterizations and functions of two Baikal seal ER (bsER) isoforms, bsERα and bsERß. The bsERα and bsERß cDNA clones isolated have an open reading frame of 595 and 530 amino acid residues, respectively. The tissue distribution analyses of bsER mRNAs showed that bsERα transcripts were primarily found in the ovary and uterus, and bsERß in the muscle in wild Baikal seals. The immunofluorescence staining assay showed that 17ß-estradiol (E2) treatment promoted the nuclear translocation of in vitro-expressed bsERα. Transient transfection of bsERα in U2OS cells enhanced the transcription of pS2, an ER target gene of E2. We then measured bsER-mediated transactivation potencies of POPs in an in vitro reporter gene assay system, in which a bsERα or bsERß expression vector was transfected into COS-1 cells. For comparison, transactivation potencies of POPs on mouse ERs (mERα and mERß) were also evaluated in the same manner. Results showed significant dose-dependent responses of bsERs and mERs when treated with p,p'-dichlorodiphenyltrichloroethane (p,p'-DDT), and p,p'-dichlorodiphenyldichloroethylene (p,p'-DDE). bsERs and mERs showed no response when exposed to polychlorinated biphenyls (PCBs) or 2,3,7,8-tetrachlorodibenzo-p-dioxin. Comparison of the dose-response curves of DDTs across species (bsERs vs. mERs) showed that bsERα had a response similar to mERα, but bsERß was less sensitive than mERß. Comparing the lowest observable effective concentrations of p,p'-DDT (2.8 µM) and p,p'-DDE (10 µM) for in vitro bsERα-mediated transactivation with their hepatic concentrations in wild Baikal seals indicated that some individuals accumulated these compounds at levels comparable to the effective concentrations, suggesting the potential disruption of the bsERα signaling pathway in the wild population by these compounds. Co-transfection experiments with bsER and the aryl hydrocarbon receptor (AHR) suggested that high accumulation of estrogenic compounds exerts a synergistic effect with dioxin-like congeners on ER signaling through AHR activation in the wild seal population.

Adipose Tissue as a Site of Toxin Accumulation.

We examine the role of adipose tissue, typically considered an energy storage site, as a potential site of toxicant accumulation. Although the production of most persistent organic pollutants (POPs) was banned years ago, these toxicants persist in the environment due to their resistance to biodegradation and widespread distribution in various environmental forms (e.g., vapor, sediment, and water). As a result, human exposure to these toxicants is inevitable. Largely due to their lipophilicity, POPs bioaccumulate in adipose tissue, resulting in greater body burdens of these environmental toxicants with obesity. POPs of major concern include polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and furans (PCDDs/PCDFs), and polybrominated biphenyls and diphenyl ethers (PBBs/PBDEs), among other organic compounds. In this review, we (i) highlight the physical characteristics of toxicants that enable them to partition into and remain stored in adipose tissue, (ii) discuss the specific mechanisms of action by which these toxicants act to influence adipocyte function, and (iii) review associations between POP exposures and the development of obesity and diabetes. An area of controversy relates to the relative potential beneficial versus hazardous health effects of toxicant sequestration in adipose tissue. © 2017 American Physiological Society. Compr Physiol 7:1085-1135, 2017.

Dioxin-like rather than non-dioxin-like PCBs promote the development of endometriosis through stimulation of endocrine-inflammation interactions.

Polychlorinated biphenyls (PCBs) contain 209 congeners with various structure-activities. Exposure to PCBs was related to disorders of female reproduction. Endometriosis (EM) is an estrogen- and inflammation-dependent disease with high prevalence and severe health outcomes. Epidemiological studies have shown the effects of PCBs exposure on EM in regard to various structures of PCBs. However, little evidence is available from the toxicology considering the structure of PCBs. In the study, environmentally relevant concentrations of PCBs were used to treat primary cultured endometrial cells and an EM mouse model. Dioxin-like CB126, but not non-dioxin-like CB153, significantly enhanced 17ß-estradiol (E2) biosynthesis in a dose-dependent manner. Among the genes related to estrogen metabolism, the level of 17ß-hydroxysteroid dehydrogenase 7 (HSD17B7) showed significant increase following CB126 exposure. We further found that CB126 exposure decreased the methylation of the HSD17B7 promoter. Elevated expression of HSD17B7 was observed in the eutopic endometrium of EM patients. CB126 rather than CB153 triggered the inflammatory response by directly stimulating the secretion of inflammatory factors and indirectly reducing the level of lipoxin A4 (LXA4). Furthermore, the inflammation enhanced the expression of HSD17B7. Antagonism of the aryl hydrocarbon receptor (AhR) diminished the effects induced by CB126. In vivo, the PCB-treated EM mouse model confirmed that CB126 rather than CB153 increased the levels of both E2 and inflammatory factors in peritoneal fluid and promoted the development of endometriotic lesions. In all, CB126, but not CB153, triggered EM development by stimulating estrogen biosynthesis, inflammation and their interactions and that these effects were mediated by the AhR receptor.

Evaluation of relative effect potencies (REPs) for dioxin-like compounds to derive systemic or human-specific TEFs to improve human risk assessment.

Toxic equivalency factors (TEFs) are generally applied for estimating human risk of dioxins and dioxin-like compounds using systemic (e.g., blood) levels, even though these TEFs are established based on intake doses in rodent studies. This review shows that systemic relative effect potencies (REPs) can deviate substantially from intake REPs, but are similar to in vitro-derived REPs. Interestingly, the in vitro REPs for 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) and 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF) are up to one order of magnitude higher than their in vivo REPs and WHO-TEFs, based on oral intake. In addition, clear species-differences in in vitro REPs were apparent for some congeners. Especially the human-derived REP for polychlorinated biphenyl 126 is one to two orders of magnitude lower than rodent REPs and its current WHO-TEF. Next, suggested adapted systemic or human-specific TEFs for these congeners were applied to calculate changes in systemic TEQ concentrations in studies from the USA, Germany and Japan and compared with either the JECFA TDI or USEPA RfD of TCDD. Overall, the effect of such TEF changes for these three congeners on total TEQ roughly balances each other out in the general population. However, results may be different for situations in which a specific group of congeners dominates. For those congeners that show a distinct deviation between either intake and systemic REPs or between rodent- and human-based in vitro REPs, we propose that especially REPs derived from human-based in vitro models are weighted more heavily in establishing systemic or human-specific TEF values to improve human health risk assessment.

Relative sensitivities among avian species to individual and mixtures of aryl hydrocarbon receptor-active compounds.

Dioxins and dioxin-like compounds (DLCs) are potent toxicants to most vertebrates. Sensitivities to DLCs vary among species. In the present study, the sensitivities of avian species (chicken [Gallus gallus], ring-necked pheasant [Phasianus colchicus], and Japanese quail [Coturnix japonica]) to some polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) were determined by using species-specific, in vitro, transactivation assays based on a luciferase reporter gene under control of species-specific aryl hydrocarbon receptors. In ring-necked pheasant and Japanese quail, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was not the most potent inducer of toxic effects. Especially for Japanese quail, the relative potency values of most of 9 PCDD/Fs tested were greater than for TCDD. The rank order of avian species sensitivities to DLCs was chicken > ring-necked pheasant > Japanese quail. Effects of binary mixtures of TCDD, 2,3,7,8-tetrachlorodibenzofuran, and 2,3,4,7,8-pentachlorodibenzofuran were strictly additive. Moreover, we also found that the primary DLCs that were responsible for most of the potency of the DLC mixtures can be deduced by using ordination in a multidimensional space defined by the avian species sensitivities. Overall, the relative potency and the species sensitivities of these chemicals could guide risk assessments to wild species when exposure to mixtures of DLCs in the environment.