Effects of tributyltin on metamorphosis and gonadal differentiation of Xenopus laevis at environmentally relevant concentrations

Evaluation of Lipids and Lipid-Related Transcripts in Human and Ovine Theca Cells and an in Vitro Mouse Model Exposed to the Obesogen Chemical Tributyltin

BACKGROUND

Exposure to obesogenic chemicals has been reported to result in enhanced adipogenesis, higher adipose tissue accumulation, and reduced ovarian hormonal synthesis and follicular function. We have reported that organotins [tributyltin (TBT) and triphenyltin (TPT)] dysregulate cholesterol trafficking in ovarian theca cells, but, whether organotins also exert lipogenic effects on ovarian cells remains unexplored.

OBJECTIVE

We investigated if environmentally relevant exposures to organotins [TBT, TPT, or dibutyltin (DBT)] induce lipid dysregulation in ovarian theca cells and the role of the liver X receptor (LXR) in this effect. We also tested the effect of TBT on oocyte maturation and neutral lipid accumulation, and lipid-related transcript expression in cumulus cells and preimplantation embryos.

METHODS

Primary theca cell cultures derived from human and ovine ovaries were exposed to TBT, TPT, or DBT (1, 10, or 50   ng / ml ). The effect of these chemical exposures on neutral lipid accumulation, lipid abundance and composition, lipid homeostasis-related gene expression, and cytokine secretion was evaluated using liquid chromatography-mass spectrometry (LC-MS), inhibitor-based methods, cytokine secretion, and lipid ontology analyses. We also exposed murine cumulus-oocyte complexes to TBT and evaluated oocyte maturation, embryo development, and lipid homeostasis-related mRNA expression in cumulus cells and blastocysts.

RESULTS

Exposure to TBT resulted in higher intracellular neutral lipids in human and ovine primary theca cells. In ovine theca cells, this effect was dose-dependent, independent of cell stage, and partially mediated by LXR. DBT and TPT resulted in higher intracellular neutral lipids but to a lesser extent in comparison with TBT. More than 140 lipids and 9 cytokines were dysregulated in TBT-exposed human theca cells. Expression of genes involved in lipogenesis and fatty acid synthesis were higher in theca cells, as well as in cumulus cells and blastocysts exposed to TBT. However, TBT did not impact the rates of oocyte maturation or blastocyst development.

DISCUSSION

TBT induced dyslipidemia in primary human and ovine theca cells, which may be responsible for some of the TBT-induced fertility dysregulations reported in rodent models of TBT exposure. https://doi.org/10.1289/EHP13955.

Assessment of toxicity, genotoxicity and oxidative stress in Fejervarya limnocharis exposed to tributyltin

Tributyltin (TBT) is widely used in various commercial applications due to its biocidal properties. Toxicological and genotoxicological data on TBT exposure to amphibians is insufficient. Our study aimed to determine the acute toxicity and genotoxic potential of TBT in Fejervarya limnocharis tadpoles. Furthermore, oxidative stress was also investigated in TBT-treated tadpoles. Tadpoles of Gosner stage (26-30) were screened and subjected to increasing concentrations of TBT (0, 3, 7, 11, 15, 19, 23 µg/L) for determining the LC50 values for 24 h, 48 h, 72 h, and 96 h. LC50 values of TBT for 24 h, 48 h, 72 h, and 96 h were found to be 19.45, 15.07, 13.12, and 11.84 μg/L respectively. Based on the 96 h LC50 value (11.84 µg/L), tadpoles were exposed to different sub-lethal concentrations of TBT for the evaluation of its genotoxic potential and effects on oxidative balance. The role of TBT on survivability, growth, and time to metamorphosis was also assessed. TBT exposure significantly altered the life history traits measured, increased mortality, and delayed the time taken to metamorphosis. Results indicated significant induction of micronucleus (MN, p < 0.001) and other erythrocytic nuclear aberrations (ENA, p < 0.01) in the TBT-treated groups. Significant alterations in comet parameters and oxidative balance were also observed in the treated groups. The present study findings might add to the cause of the gradual population decline seen in the amphibians. This study also demonstrates the alteration of the life-history traits, oxidative balance, and DNA damage upon TBT exposure which can have long-term consequences for the anuran amphibian F. limnocharis.

Thyroid-like hormone signaling in invertebrates and its potential role in initial screening of thyroid hormone system disrupting chemicals

This review examines the presence and evolution of thyroid-like systems in selected aquatic invertebrates to determine the potential use of these organisms in screens for vertebrate thyroid hormone axis disrupting chemicals (THADCs). Such a screen might support the phasing out of some vertebrate testing. Although arthropods including crustaceans do not contain a functional thyroid signaling system, elements of such a system exist in the aquatic phyla mollusks, echinoderms, tunicates, and cephalochordates. These phyla can synthesize thyroid hormone, which has been demonstrated in some groups to induce the nuclear thyroid hormone receptor (THR). Thyroid hormone may act in these phyla through interaction with a membrane integrin receptor. Thyroid hormone regulates inter alia metamorphosis but, unlike in vertebrates, this does not occur via receptor activation by the ligands triiodothyronine (T3) and thyroxine (T4). Instead, the unliganded nuclear receptor itself controls metamorphosis in mollusks, echinoderms, and tunicates, whereas the T3 derivative tri-iodothyroacetic acid (TRIAC) acts as a THR ligand in cephalochordates. In view of this, it may be possible to develop an invertebrate-based screen that is sensitive to vertebrate THADCs that interfere with thyroid hormone synthesis or metabolism along with interaction with membrane receptors. The review makes some recommendations for the need to develop an appropriate test method. Integr Environ Assess Manag 2023;19:63-82. © 2022 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Harbours as unique environmental sites of multiple anthropogenic stressors on fish hormonal systems

Fish development and acclimation to environmental conditions are strongly mediated by the hormonal endocrine system. In environments contaminated by anthropogenic stressors, hormonal pathway alterations can be detrimental for growth, survival, fitness, and at a larger scale for population maintenance. In the context of increasingly contaminated marine environments worldwide, numerous laboratory studies have confirmed the effect of one or a combination of pollutants on fish hormonal systems. However, this has not been confirmed in situ. In this review, we explore the body of knowledge related to the influence of anthropogenic stressors disrupting fish endocrine systems, recent advances (focusing on thyroid hormones and stress hormones such as cortisol), and potential research perspectives. Through this review, we highlight how harbours can be used as "in situ laboratories" given the variety of anthropogenic stressors (such as plastic, chemical, sound, light pollution, and invasive species) that can be simultaneously investigated in harbours over long periods of time.

Developmental, toxicological effects and recovery patterns in Xenopus laevis after exposure to penconazole-based fungicide during the metamorphosis process

Fungicides are a group of chemicals causing pollution of freshwater ecosystems due to their widespread use in agriculture. However, their endocrine disrupting effects are less studied than herbicides and insecticides. The aim of this study was to evaluate the developmental and toxicological effects and recovery patterns of penconazole-based fungicide (PBF) during Xenopus laevis metamorphosis. For this purpose, firstly, the 96 h median lethal (LC₅₀) and effective (EC₅₀) concentrations and minimum concentration to inhibit growth (MCIG) values of PBF were estimated for X. laevis as 4.97, 3.55 and 2.31 mg/L respectively, using Frog Embryo Teratogenesis Assay-Xenopus (FETAX) on Nieuwkoop-Faber (NF) stage 8 embryos. FETAX results showed PBF formulation was slightly teratogenic with a 1.4 teratogenic index; most recorded malformations were gut, abdominal edema, and tail curvature. The Subacute Amphibian Metamorphosis Assay (AMA) was modified based on acute FETAX results, and used to evaluate toxic effects and recovery patterns of relatively low PBF concentrations on metamorphosis using morphological and biochemical markers. NF Stage 51 tadpoles were exposed to two separate groups of each concentration for seven days in the AMA. Secondly, tadpoles of one group of each concentration continued to be exposed to PBF for the next 7 and 14 days while the other group was kept in a pesticide-free environment (depuration/recovery). Various morphological and biochemical markers were measured homogenate samples of tadpoles from exposure and recovery groups. Continuous exposure to relatively low PBF concentrations caused oxidative stress, toxic, and endocrine disrupting effects in the AMA, leading us to conclude that it has negative effects on frog health and development during the recovery period when PBF exposure is terminated. The glutathione S-transferase, glutathione reductase, catalase, carboxylesterase, and acetylcholinesterase activities were higher than the control group transferred to pesticide-free media for 14 days after the 7 days exposure and indicate persistent PBF impact.

mRNA-miRNA sequencing reveals mechanisms of 2,2'-dipyridyl disulfide-induced thyroid disruption in Japanese flounder (Paralichthys olivaceus)

This study was conducted to evaluate the thyroid-disrupting effects of 2,2'-dipyridyl disulfide using Japanese flounder (Paralichthys olivaceus) as an animal model and to reveal the underlying mechanisms from the perspective of miRNA-mRNA interactions. The results indicated that 2,2'-dipyridyl disulfide exposure decelerated the metamorphic progress of P. olivaceus, suggesting its thyroid-disrupting property as an antagonist. Furthermore, radioimmunoassays, thyroid histological observation, real-time polymerase chain reaction, and mRNA sequencing showed that 2,2'-dipyridyl disulfide exposure exerted its thyroid-disrupting effects on larval and juvenile P. olivaceus by targeting multiple processes and pathways involved in the thyroid system, including peripheral metabolism of thyroid hormones, the thyroid hormone synthesis pathway, and the thyroid hormone/thyroid hormone receptor signaling pathway. In particular, global upregulation of the gene expression of three deiodinases caused decreases in thyroid hormone levels after 2,2'-dipyridyl disulfide exposure that are believed to be responsible for the inhibition of metamorphosis in P. olivaceus. Finally, miRNA sequencing suggested that several evolutionarily conserved miRNAs play important roles in the mechanism of 2,2'-dipyridyl disulfide-induced thyroid disruption. Specifically, overexpression of pny-miR-723a and pny-miR-216a resulted in upregulation of deiodinase 1 mRNA levels in the 2,2'-dipyridyl disulfide exposure group. This study provides the first evidence that 2,2'-dipyridyl disulfide has thyroid-disrupting properties and is also the first study remarking on the roles of miRNA-mRNA interactions in the action mechanisms of thyroid disruptors.

Subacute and low-dose tributyltin exposure disturbs the mammalian hypothalamus-pituitary-thyroid axis in a sex-dependent manner

Tributyltin (TBT) is an endocrine disruptor chemical (EDC) capable of altering the proper function of the hypothalamus-pituitary thyroid (HPT) axis. This study aimed to evaluate the subacute effects of TBT on the HPT axis of male and female rats. A dose of 100 ng/kg/day TBT was used in both sexes over a 15-day period, and the morphophysiology and gene expression of the HPT axis were assessed. TBT exposure increased the body weight in both sexes, while food efficiency increased - only in male rats. It was also possible to note alterations in the thyroid, with the presence of a stratified epithelium, cystic degeneration, and increased interstitial collagen deposition. A reduction in T3 and T4 levels was only observed in TBT male rats. A reduction in TSH levels was observed in TBT female rats. Evaluating mRNA expression, we observed a decrease in hepatic D1 and TRH mRNA levels in TBT female rats. An increase in D2 mRNA expression in the hypothalamus was observed in TBT male rats. Additionally, no significant changes in TRH or hepatic D1 mRNA expression in TBT male rats or in hypothalamic D1 and D2 mRNA expression in TBT female rats were observed. Thus, we can conclude that TBT has different toxicological effects on male and female rats by altering thyroid gland morphophysiology, leading to abnormal HPT axis function, and even at subacute and low doses, it may be involved in complex endocrine and metabolic disorders.

Exposure impacts of Imazapyr formulation on larval development and thyroid histology of Xenopus laevis

There are increased concerns about the thyroidal effects of many anthropogenic substances in the environment. These substances include agricultural pesticides and industrial and pharmaceutical chemicals among others. Their potential thyroidal effects are of serious health and ecological concerns, as thyroid hormones mediate numerous physiological processes, including growth regulation, general metabolism and metamorphosis in metamorphic animals. This study assessed thyroidal activities of Arsenal formulation (Imazapyr) at environmentally relevant concentrations of 0.5, 2.0 and 3.5 mg/L following a Xenopus metamorphosis assay (XEMA). The result shows that the Arsenal formulation significantly delayed the tadpole development, reduced the hind-limb length (HLL) and increased the whole-body mass (WBM) at a concentration of 3.5 mg/L relative to the control exposure. In histopathology, the formulation increased the epithelium height, at all exposure concentrations, but reduced the colloidal area at 0.5 and 2 mg/L, respectively, and the gland area at 2 mg/L relative to the control. Consequently, the Arsenal formulation is thyroid-active at environmentally relevant concentrations and poses a threat to both human and wildlife, especially metamorphic organisms. With this exposure impact, more studies are imperative to further characterise other endocrine-disrupting potential of this formulation, while future applications should be reduced or restricted to less risk environment, if it cannot be stopped from sensitive aquatic systems.

Tributyltin Alters Calcium Levels, Mitochondrial Dynamics, and Activates Calpains Within Dorsal Root Ganglion Neurons

Tributyltin (TBT) remains a global health concern. The primary route of human exposure to TBT is either through ingestion or skin absorption, but TBT's effects on the peripheral nervous system have still not been investigated. Therefore, we exposed in vitro sensory dorsal root ganglion (DRG) neurons to TBT at a concentration of 50-200 nM, which is similar to the observed concentrations of TBT in human blood samples. We observed that TBT causes extensive axon degeneration and neuronal death in the DRG neurons. Furthermore, we discovered that TBT causes an increase in both cytosolic and mitochondrial calcium levels, disrupts mitochondrial dynamics, decreases neuronal ATP levels, and leads to the activation of calpains. Additional experiments demonstrated that inhibition of calpain activation prevented TBT-induced fragmentation of neuronal cytoskeletal proteins and neuronal cell death. Thus, we conclude that calpain activation is the key executioner of TBT-induced peripheral neurodegeneration.

Exposure Impacts of Diquat dibromide herbicide formulation on amphibian larval development

Many anthropogenic chemicals in general, and specifically aquatic herbicide formulations have the potential to modulate the thyroid pathways of the endocrine system of aquatic organisms, because they are normally applied directly into the aquatic system, to manage aquatic weeds. These thyroidal effects have been widely linked with disruption in developmental and reproductive processes. In fact, the exposure impacts of many of these substances on metamorphic organisms could produce a precocious metamorphosis. Using Xenopus Metamorphosis Assay (XEMA) protocol, this study assessed the thyroidal effects of environmentally relevant concentrations of Diquat dibromide at 0.05, 0.11, and 0.14 mg/L on Xenopus laevis metamorphosis. The formulation significantly reduced both the fore and hind limb lengths, and disrupted the developmental stage at concentrations of 0.11 and 0.14 mg/L, with a median at NF-stage 57, while median of NF-stage 60 was recorded in the control. Histopathologically, although there was no significant difference in thyroid gland area, the thyroid colloidal area was significantly reduced at 0.14 mg/L, while the mean height of the thyroid follicle increased at 0.05 mg/L The result indicates an extra-thyroidal pathway, due to the dissociation between stage developmental effects and thyroid histopathology. The role of stress pathway occasioned by oxidative mode of action, involving lipid peroxidation and cell damage observed in this study need further investigation, in order to further characterize the physiological and ecological effects on wildlife.

Relevant dose of the environmental contaminant, tributyltin, promotes histomorphological changes in the thyroid gland of male rats

Organotin compounds, such as tributyltin (TBT), are common environmental contaminants and suspected endocrine-disrupting chemicals. Tributyltin is found in antifouling paints, widely used in ships and other vessels. The present study evaluated whether a 15-day treatment with TBT at a dose of 100 ng/kg/day could induce histomorphological changes in the thyroid gland of rats. TBT promoted relevant alterations in the thyroid architecture, being the most relevant histological findings the presence of increased number of small-size follicles in the treated group. In qualitative analyses, colloid vacuolization, papillary budging structures, cystic degeneration and chronic thyroiditis, were observed. Moreover, histomorphometric analysis showed statistically significant changes in the follicular architecture of TBT-treated rats, mainly a decrease in the follicle area (colloid) and an increased epithelial height that resulted in an increased epithelial height/colloid ratio. Augmented collagen deposition was also seen in the thyroids of treated groups. In immunohistochemical (IHC) analyses, the localization of NIS protein was described and a significant increased proliferation index (evaluated by Ki67 positive cells) in the treated group was reported. As an indirect measurement of oxidative stress, mitochondrial protein SDHA was also analyzed by IHC analysis. Although the cytoplasmic expression of SDHA was observed in both groups, the staining intensity score was higher in TBT-treated group. Our results suggest that besides causing histomorphological changes, environmental relevant dose of TBT treatment can also induce oxidative alterations.

Disruptive effects of two organotin pesticides on the thyroid signaling pathway in Xenopus laevis during metamorphosis

Organotin compounds are the ubiquitous environmental pollutants due to their wide industrial and agricultural applications and unexpected releasing into the environment, which show characteristic of endocrine disruptors to interfere with the synthesis, receptor binding or action of endogenous-hormones. Organotin pesticides (OTPs) are used in agriculture and may impact endocrine functions on organisms. Thyroid hormones (THs) play fundamental roles in regulating the basal metabolism and energy balance, while thyroid function can be impaired by environmental contaminants. Therefore, it is crucial to clarify the effects and mechanisms of OTPs on hypothalamus-pituitary-thyroid (HPT) axis. In this study, Xenopus laevis tadpoles at stage 51 were exposed to fentin hydroxide and fenbutatin oxide (0.04, 0.20 and 1.00 μg·L^-1) for 21 days. It was found that both compounds caused inhibitory effects on metamorphic development of tadpoles (e.g., significant decrease in hindlimb length and retarding development). Triiodothyronine (T₃) significantly decreased in tadpoles exposed to 0.20 μg/L and 1.00 μg/L of the two OTPs for 14 days or 21 days. The expressions of TH responsive genes trβ, bteb and dio2 were down-regulated, while tshβ and slc5a5 were up-regulated. Surface plasmon resonance (SPR) binding assays showed that fentin hydroxide had a moderate affinity to recombinant human thyroid hormone receptor β but fenbutatin oxide did not have. Result of the SPR assay was highly consistent with the luciferase reporter gene assays that fentin hydroxide suppressed the relative luciferase activity in the presence of T₃ while fenbutatin oxide did not, demonstrating fentin hydroxide but not fenbutatin oxide displayed an antagonistic activity against T₃-TR complex mediated transcriptional activation. Overall, the findings elucidated the mechanisms induced by OTPs along HPT axis. These results highlighted the adverse influences of organotin pesticides on thyroid hormone- dependent development in vertebrates and the need for more comprehensive investigations of their potential ecological risks.

Concomitant exposure to benzo[a]pyrene and triclosan at environmentally relevant concentrations induces metabolic syndrome with multigenerational consequences in Silurana (Xenopus) tropicalis

Numerous studies suggest that amphibians are highly sensitive to endocrine disruptors (ED) but their precise role in population decline remains unknown. This study shows that frogs exposed to a mixture of ED throughout their life cycle, at environmentally relevant concentrations, developed an unexpected metabolic syndrome. Female Silurana (Xenopus) tropicalis exposed to a mixture of benzo[a]pyrene and triclosan (50 ng·L^-1 each) from the tadpole stage developed liver steatosis and transcriptomic signature associated with glucose intolerance syndrome, and pancreatic insulin hyper secretion typical of pre-diabetes. These metabolic disorders were associated with delayed metamorphosis and developmental mortality in their progeny, both of which have been linked to reduced adult recruitment and reproductive success. Indeed, F1 females were smaller and lighter and presented reduced reproductive capacities, demonstrating a reduced fitness of ED-exposed Xenopus. Our results confirm that amphibians are highly sensitive to ED even at concentrations considered to be safe for other animals. This study demonstrates that ED might be considered as direct contributing factors to amphibian population decline, due to their disruption of energetic metabolism.

Disruptive Effect of Organotin on Thyroid Gland Function Might Contribute to Hypothyroidism

A considerable increase in endocrine abnormalities has been reported over the last few decades worldwide. A growing exposure to endocrine-disrupting chemicals (EDCs) can be one of the causes of endocrine disorders in populations, and these disorders are not only restricted to the metabolic hormone system but can also cause abnormal functions. Thyroid hormone (TH) disruption is defined as an abnormal change in TH production, transport, function, or metabolism, which results in some degree of impairment in body homeostasis. Many EDCs, including organotin compounds (OTCs), are environmental contaminants that are commonly found in antifouling paints used on ships and in several other industrial procedures. OTCs are obesogenic and can disrupt TH metabolism; however, abnormalities in thyroid function resulting from OTC exposure are less well understood. OTCs, one of the most prevalent EDCs that are encountered on a daily basis, modulate the thyroid axis. In most toxicology studies, it has been reported that OTCs might contribute to hypothyroidism.

The role of pparγ in embryonic development of Xenopus tropicalis under triphenyltin-induced teratogenicity

Evidence has shown that triphenyltin (TPT) triggers severe malformations in Xenopus tropicalis embryos, partly due to activation of PPARγ (peroxisome proliferator activated receptor γ) protein. In the present study, we investigated how abundance of pparγ and TPT exposure interact and affect X. tropicalis embryonic development. We observed pparγ expression signals appeared in the neural crest and neural fold, as well as in the brain, eyes and spinal cord organs. Both pparγ overexpression and its Morpholino (MO) knockdown inhibited pax6 (paired box 6) expression, a marker of eye development, and significantly up- and down-regulated lipid and glucose homeostasis related genes, such as lpl (lipoprotein lipase), slc2a4 (solute carrier family 2 (facilitated glucose transporter), member 4) and pck1 (phosphoenolpyruvate carboxykinase 1, cytosolic), thus inducing eye phenotypes. Overexpression of pparγ induced small eye phenotype, while pparγ MO induced small eye plus turbid eye lens microencephaly and enlarged trunk. In contrast, 5-20μgSn/L (stannum/L) TPT exposure reversed some impacts induced by pparγ overexpression, i.e., no small eye, up-regulation of pax6 and down-regulation of pparγ, lpl, slc2a4 and pck1. Meanwhile, microinjection of pparγ MO combined with exposure to 20μgSn/L TPT caused 85% mortality. In brief, our work clearly indicates that pparγ is essential to eye development and inhibition of its expression combined with TPT exposure can be extremely harmful to X. tropicalis embryo.

The obesogen tributyltin induces features of polycystic ovary syndrome (PCOS): a review

Polycystic ovary syndrome (PCOS) is a heterogeneous syndrome characterized by abnormal reproductive cycles, irregular ovulation, and hyperandrogenism. This complex disorder has its origins both within and outside the hypothalamic-pituitary-ovarian axis. Cardio-metabolic factors, such as obesity and insulin resistance, contribute to the manifestation of the PCOS phenotype. Polycystic ovary syndrome is one of the most common endocrine disorders among women of reproductive age. Growing evidence suggested an association between reproductive and metabolic features of PCOS and exposure to endocrine-disrupting chemicals (EDC), such as bisphenol A. Further, the environmental obesogen tributyltin (TBT) was shown to induce reproductive, metabolic and cardiovascular abnormalities resembling those found in women and animal models of PCOS. However, the causal link between TBT exposure and PCOS development remains unclear. The objective of this review was to summarize the most recent research findings on the potential association between TBT exposure and development of PCOS-like features in animal models and humans.

Frontiers in endocrine disruption: Impacts of organotin on the hypothalamus-pituitary-thyroid axis

Endocrine disruptors (EDs), chemical substances widely used in industry and ubiquitously distributed in the environment, are able to interfere with the synthesis, release, transport, metabolism, receptor binding, action, or elimination of endogenous hormones. EDs affect homeostasis mainly by acting on nuclear and nonnuclear steroid receptors but also on serotonin, dopamine, norepinephrine and orphan receptors in addition to thyroid hormone receptors. Tributyltin (TBT), an ED widely used as a pesticide and biocide in antifouling paints, has well-documented actions that include inhibiting aromatase and affecting the nuclear receptors PPARγ and RXR. TBT exposure in humans and experimental models has been shown to mainly affect reproductive function and adipocyte differentiation. Since thyroid hormones play a fundamental role in regulating the basal metabolic rate and energy homeostasis, it is crucial to clarify the effects of TBT on the hypothalamus-pituitary-thyroid axis. Therefore, we review herein the main effects of TBT on important metabolic pathways, with emphasis on disruption of the thyroid axis that could contribute to the development of endocrine and metabolic disorders, such as insulin resistance and obesity.

Tributyltin: Advancing the Science on Assessing Endocrine Disruption with an Unconventional Endocrine-Disrupting Compound

Tributyltin (TBT) has been recognized as an endocrine disrupting chemical (EDC) for several decades. However, only in the last decade, was its primary endocrine mechanism of action (MeOA) elucidated-interactions with the nuclear retinoid-X receptor (RXR), peroxisome proliferator-activated receptor γ (PPARγ), and their heterodimers. This molecular initiating event (MIE) alters a range of reproductive, developmental, and metabolic pathways at the organism level. It is noteworthy that a variety of MeOAs have been proposed over the years for the observed endocrine-type effects of TBT; however, convincing data for the MIE was provided only recently and now several researchers have confirmed and refined the information on this MeOA. One of the most important lessons learned from years of research on TBT concerns apparent species sensitivity. Several aspects such as the rates of uptake and elimination, chemical potency, and metabolic capacity are all important for identifying the most sensitive species for a given chemical, including EDCs. For TBT, much of this was discovered by trial and error, hence important relationships and important sensitive taxa were not identified until several decades after its introduction to the environment. As recognized for many years, TBT-induced responses are known to occur at very low concentrations for molluscs, a fact that has more recently also been observed in fish species. This review explores the MeOA and effects of TBT in different species (aquatic molluscs and other invertebrates, fish, amphibians, birds, and mammals) according to the OECD Conceptual Framework for Endocrine Disruptor Testing and Assessment (CFEDTA). The information gathered on biological effects that are relevant for populations of aquatic animals was used to construct Species Sensitivity Distributions (SSDs) based on No Observed Effect Concentrations (NOECs) and Lowest Observed Effect Concentrations (LOECs). Fish appear at the lower end of these distributions, showing that they are as sensitive as molluscs, and for some species, even more sensitive. Concentrations in the range of 1 ng/L for water exposure (10 ng/g for whole-body burden) have been shown to elicit endocrine-type responses, whereas mortality occurs at water concentrations ten times higher. Current screening and assessment methodologies as compiled in the OECD CFEDTA are able to identify TBT as a potent endocrine disruptor with a high environmental risk for the original use pattern. If those approaches had been available when TBT was introduced to the market, it is likely that its use would have been regulated sooner, thus avoiding the detrimental effects on marine gastropod populations and communities as documented over several decades.

A review of ecological risk assessment methods for amphibians: Comparative assessment of testing methodologies and available data

Historically, ecological risk assessments have rarely included amphibian species, focusing preferentially on other aquatic (fish, invertebrates, algae) and terrestrial wildlife (birds and mammal) species. Often this lack of consideration is due to a paucity of toxicity data, significant variation in study design, uncertainty with regard to exposure, or a combination of all three. Productive risk assessments for amphibians are particularly challenging, given variations in complex life history strategies. Further consideration is needed for the development of useful laboratory animal models and appropriate experimental test procedures that can be effectively applied to the examination of biological response patterns. Using these standardized techniques, risk estimates can be more accurately defined to ensure adequate protection of amphibians from a variety of stress agents. Patterns in toxicity may help to ascertain whether test results from 1 amphibian group (e.g., Urodela) could be sufficiently protective of another (e.g., Anura) and/or whether some nonamphibian aquatic taxonomic groups (e.g., fish or aquatic invertebrates) may be representative of aquatic amphibian life stages. This scope is intended to be a guide in the development of methods that would yield data appropriate for ecological risk decisions applicable to amphibians. Integr Environ Assess Manag 2017;13:601-613. © 2016 SETAC.

Disrupting effects of azocyclotin to the hypothalamo-pituitary-gonadal axis and reproduction of Xenopus laevis

Over the past few decades, the hazards associated with the extensive use of organictin compounds have become an issue of extreme concern, while at present the effects of these substances on amphibians remain poorly understood. In the present study, we chose azocyclotin, one of common use acaricides in China. We focused on sexual development and steroidogenesis disrupting effects of azocyclotin in the Xenopus laevis. Tadpoles were exposed to azocyclotin (0.05 and 0.5μg/L) for long-term (4 months) study. Results showed that exposure to azocyclotin caused developmental toxicity, including decreased survival, body weight, body length, gonadosomatic index, hepatosomatic index and female phenotype. At the same time, statistical increase in mean age at completion of metamorphosis was observed in azocyclotin treatments in comparison with control group. Furthermore, hormone concentrations, and steroidogenesis genes expression of adult frog were further evaluated in 28 days exposure. Results demonstrated that the key regulating hormones, e.g. testosterone and pregnenolone, were significantly upregulated. The expression levels of selected steroidogenic genes were also significantly altered. Our study demonstrated that azocyclotin could delay the metamorphosis and disrupt the gonadal differentiation of X. laevis. Steroidogenesis and the expression of genes involved in the hypothalamus-pituitary-gonadal-liver axis in frogs were disrupted after azocyclotin exposure. Azocyclotin showed both androgenic and antiestrogenic activity for X. laevis. Those findings emphasized the influence of azocyclotin on non-target species in the context of ecotoxicological risk assessment.

Metabolic and immune impairments induced by the endocrine disruptors benzo[a]pyrene and triclosan in Xenopus tropicalis

Despite numerous studies suggesting that amphibians are highly sensitive to cumulative anthropogenic stresses, the role played by endocrine disruptors (EDs) in the decline of amphibian populations remains unclear. EDs have been extensively studied in adult amphibians for their capacity to disturb reproduction by interfering with the sexual hormone axis. Here, we studied the in vivo responses of Xenopus tropicalis males exposed to environmentally relevant concentrations of each ED, benzo[a]pyrene (BaP) and triclosan (TCS) alone (10 μg L(-1)) or a mixture of the two (10 μg L(-1) each) over a 24 h exposure period by following the modulation of the transcription of key genes involved in metabolic, sexual and immunity processes and the cellular changes in liver, spleen and testis. BaP, TCS and the mixture of the two all induced a marked metabolic disorder in the liver highlighted by insulin resistance-like and non-alcoholic fatty liver disease (NAFLD)-like phenotypes together with hepatotoxicity due to the impairment of lipid metabolism. For TCS and the mixture, these metabolic disorders were concomitant with modulation of innate immunity. These results confirmed that in addition to the reproductive effects induced by EDs in amphibians, metabolic disorders and immune system disruption should also be considered.

Trialkyltin Rexinoid-X Receptor Agonists Selectively Potentiate Thyroid Hormone Induced Programs of Xenopus laevis Metamorphosis

The trialkyltins tributyltin (TBT) and triphenyltin (TPT) can function as rexinoid-X receptor (RXR) agonists. We recently showed that RXR agonists can alter thyroid hormone (TH) signaling in a mammalian pituitary TH-responsive reporter cell line, GH3.TRE-Luc. The prevalence of TBT and TPT in the environment prompted us to test whether they could also affect TH signaling. Both trialkyltins induced the integrated luciferase reporter alone and potentiated TH activation at low doses. Trimethyltin, which is not an RXR agonist, did not. We turned to a simple, robust, and specific in vivo model system of TH action: metamorphosis of Xenopus laevis, the African clawed frog. Using a precocious metamorphosis assay, we found that 1nM TBT and TPT, but not trimethyltin, greatly potentiated the effect of TH treatment on resorption phenotypes of the tail, which is lost at metamorphosis, and in the head, which undergoes extensive remodeling including gill loss. Consistent with these responses, TH-induced caspase-3 activation in the tail was enhanced by cotreatment with TBT. Induction of a transgenic reporter gene and endogenous collagenase 3 (mmp13) and fibroblast-activating protein-α (fap) genes were not induced by TBT alone, but TH induction was significantly potentiated by TBT. However, induction of other TH receptor target genes such as TRβ and deiodinase 3 by TH were not affected by TBT cotreatment. These data indicate that trialkyltins that can function as RXR agonists can selectively potentiate gene expression and resultant morphological programs directed by TH signaling in vivo.

Thyroid endocrine disruption of azocyclotin to Xenopus laevis during metamorphosis

Organotin compounds are ubiquitous contaminants that are frequently detected in the environment and in biota, which raises concern about their risk to wildlife and human health. In the present study, Nieuwkoop & Faber stage 51 Xenopus laevis tadpoles were exposed to different concentrations of azocyclotin (0, 0.02, 0.1 and 0.5μg/L) for 21 days, during which time the tadpoles underwent morphological development. Exposure to azocyclotin caused an inhibitory effect on the pre-metamorphic development of X. laevis (e.g., a shortened hind limb length). Azocyclotin induced an alteration of the triiodothyronine (T3) content, which indicated thyroid endocrine disruption. Real-time PCR was performed to examine the expression levels of the genes involved in the thyroid hormone (TH) signaling pathway. Significant down-regulation of the type 2 deiodinase gene was observed, which may be partially responsible for the decreased T3 concentrations. Furthermore, the expression of T3 responsive genes, including thyroid hormone receptor, basic transcription element binding protein, 2tromelysins-3 and matrix metalloproteinase 2, were down-regulated in tadpoles, suggesting that azocyclotin induced a decrease in the T3 contents and, in turn, affected the mRNA expression of downstream genes involved in multiple physiological responses. Chemical analysis showed that azocyclotin could accumulate in X. laevis after 21 days of exposure. In conclusion, the results of the present study showed that azocyclotin could alter the mRNA expression of genes involved in TH signaling as well as the thyroid hormone concentrations in X. laevis tadpoles, leading to endocrine disruption of thyroid system, and that azocyclotin had obvious inhibitory effects on X. laevis metamorphosis.

Hepatic effects of the clomazone herbicide in both its free form and associated with chitosan-alginate nanoparticles in bullfrog tadpoles

The use of agrochemicals in agriculture is intense and most of them could be carried out to aquatic environment. Nevertheless, there are only few studies that assess the effects of these xenobiotics on amphibians. Clomazone is an herbicide widely used in rice fields, where amphibian species live. Thus, those species may be threatened by non-target exposure. However, nanoparticles are being developed to be used as a carrier system for the agrochemicals. Such nanoparticles release the herbicide in a modified way, and are considered to be more efficient and less harmful to the environment. The aim of this study was to comparatively evaluate the effect of clomazone in its free form and associated with nanoparticles, in the liver of bullfrog tadpoles (Lithobates catesbeianus) when submitted to acute exposure for 96 h. According to semi-quantitative analysis, there was an increase in the frequency of melanomacrophage centres, in the accumulation of eosinophils and in lipidosis in the liver of experimental groups exposed to clomazone - in its free form and associated with nanoparticles - in comparison with the control group, and the nanotoxicity of chitosan-alginate nanoparticles. The increase of melanomacrophage centres in all exposed groups was significant (P < 0.0001) in comparison to control group. Therefore, the results of this research have shown that exposure to sublethal doses of the herbicide and nanoparticles triggered hepatic responses. Moreover, these results provided important data about the effect of the clomazone herbicide and organic nanoparticles, which act as carriers of agrochemicals, on the bullfrog tadpole liver.

The obesogen tributyltin

The obesogen hypothesis postulates the role of environmental chemical pollutants that disrupt homeostatic controls and adaptive mechanisms to promote adipose-dependent weight gain leading to obesity and metabolic syndrome complications. One of the most direct molecular mechanisms for coupling environmental chemical exposures to perturbed physiology invokes pollutants mimicking endogenous endocrine hormones or bioactive dietary signaling metabolites that serve as nuclear receptor ligands. The organotin pollutant tributyltin can exert toxicity through multiple mechanisms but most recently has been shown to bind, activate, and mediate RXR-PPARγ transcriptional regulation central to lipid metabolism and adipocyte biology. Data in support of long-term obesogenic effects on whole body adipose tissue are also reported. Organotins represent an important model test system for evaluating the impact and epidemiological significance of chemical insults as contributing factors for obesity and human metabolic health.