The Mammalian "Obesogen" Tributyltin Targets Hepatic Triglyceride Accumulation and the Transcriptional Regulation of Lipid Metabolism in the Liver and Brain of Zebrafish

Synergistic Steatosis Induction in Mice: Exploring the Interactions and Underlying Mechanisms between PFOA and Tributyltin

This study explores the impact of environmental pollutants on nuclear receptors (CAR, PXR, PPARα, PPARγ, FXR, and LXR) and their heterodimerization partner, the Retinoid X Receptor (RXR). Such interaction may contribute to the onset of non-alcoholic fatty liver disease (NAFLD), which is initially characterized by steatosis and potentially progresses to steatohepatitis and fibrosis. Epidemiological studies have linked NAFLD occurrence to the exposure to environmental contaminants like PFAS. This study aims to assess the simultaneous activation of nuclear receptors via perfluorooctanoic acid (PFOA) and RXR coactivation via Tributyltin (TBT), examining their combined effects on steatogenic mechanisms. Mice were exposed to PFOA (10 mg/kg/day), TBT (5 mg/kg/day) or a combination of them for three days. Mechanisms underlying hepatic steatosis were explored by measuring nuclear receptor target gene and lipid metabolism key gene expressions, by quantifying plasma lipids and hepatic damage markers. This study elucidated the involvement of the Liver X Receptor (LXR) in the combined effect on steatosis and highlighted the permissive nature of the LXR/RXR heterodimer. Antagonistic effects of TBT on the PFOA-induced activation of the Pregnane X Receptor (PXR) and Peroxisome Proliferator-Activated Receptor Gamma (PPARγ) were also observed. Overall, this study revealed complex interactions between PFOA and TBT, shedding light on their combined impact on liver health.

Transcriptomic Integration Analyses Uncover Common Bisphenol A Effects Across Species and Tissues Primarily Mediated by Disruption of JUN/FOS, EGFR, ER, PPARG, and P53 Pathways

Bisphenol A (BPA) is a common endocrine disruptor widely used in the production of electronic, sports, and medical equipment, as well as consumer products like milk bottles, dental sealants, and thermal paper. Despite its widespread use, current assessments of BPA exposure risks remain limited due to the lack of comprehensive cross-species comparative analyses. To address this gap, we conducted a study aimed at identifying genes and fundamental molecular processes consistently affected by BPA in various species and tissues, employing an effective data integration method and bioinformatic analyses. Our findings revealed that exposure to BPA led to significant changes in processes like lipid metabolism, proliferation, and apoptosis in the tissues/cells of mammals, fish, and nematodes. These processes were found to be commonly affected in adipose, liver, mammary, uterus, testes, and ovary tissues. Additionally, through an in-depth analysis of signaling pathways influenced by BPA in different species and tissues, we observed that the JUN/FOS, EGFR, ER, PPARG, and P53 pathways, along with their downstream key transcription factors and kinases, were all impacted by BPA. Our study provides compelling evidence that BPA indeed induces similar toxic effects across different species and tissues. Furthermore, our investigation sheds light on the underlying molecular mechanisms responsible for these toxic effects. By uncovering these mechanisms, we gain valuable insights into the potential health implications associated with BPA exposure, highlighting the importance of comprehensive assessments and awareness of this widespread endocrine disruptor.

A bioanalytical approach for assessing the effects of soil extracts from solid waste dumpsite in Calabar (Nigeria) on lipid and estrogenic signaling of fish Poeciliopsis lucida hepatocellular carcinoma-1 cells in vitro and in vivo African catfish (Clarias gariepinus)

In applying bioanalytical approaches, the aim of this study was to determine the toxicity of contaminants derived from a solid waste dumpsite in Calabar (Nigeria), by investigating the alterations of lipid and estrogen signaling pathways in Poeciliopsis lucida hepatocellular carcinoma-1 (PLHC-1) cells and compared to in vivo African catfish (Clarias gariepinus), using polar, nonpolar and elutriate extraction methods. Cells were exposed for 48 hr period to different concentrations of the contaminant extracts. The PLHC-1 cells were evaluated for lipid responses as follows adipoRed assay, retinoid x receptor (rxr), peroxisome proliferator-activated receptor isoforms (ppar-α and γ), estrogen receptor (er-α) and vitellogenin (vtg) transcripts. The lipid signaling activation was also assessed in vivo using C. gariepinus, where hepatic levels of ppar-α were determined at both transcript and functional proteins levels. Data showed variable-, extract type and concentration-specific elevations in mRNA and protein levels for lipidomic and estrogenic effects. These effects were either biphasic at low and high concentrations, depending upon extract type, or concentration-dependent elevations. In general, these toxicological responses may be attributed to soil organic and inorganic contaminants burden previously derived from the dumpsite. Thus, our data demonstrate a unique lipid and endocrine-disruptive chemical (EDC) effects of each soil extract, suggesting multiple and complex contaminant interactions in the environment and biota. Analysis of numerous soil- or sediment-bound contaminants have numerous limitations and cost implications for developing countries. Our approach provides a bioanalytical protocol and endpoints for measuring the metabolic and EDC effects of complex environmental matrices for ecotoxicological assessment and monitoring.

Toxic effects of triphenyltin on the development of zebrafish (Danio rerio) embryos

Triphenyltin (TPT) is known to be an environmental endocrine disruptor and has adverse effects on aquatic animals. In this study, zebrafish embryos were treated with three different concentrations (12.5, 25, 50 nmol/L) based on the LC₅₀ value at 96 h post fertilization (96 hpf), after TPT exposure. The developmental phenotype and hatchability were observed and recorded. Reactive oxygen species (ROS) levels in zebrafish were detected at 72 hpf and 96 hpf using 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) as a probe. The number of neutrophils after exposure was observed using transgenic zebrafish Tg (lyz: DsRed). RNA-seq analysis was used to compare the gene expression changes in zebrafish embryos at 96 hpf in the control group and 50 nmol/L TPT exposure group. The data revealed that TPT caused a delay in hatching of zebrafish embryos in a time- and dose-dependent manner, as well as causing pericardial edema, spinal curvature and melanin reduction. ROS levels in embryos exposed to TPT increased, and the number of neutrophils increased after TPT exposure to Tg (lyz: DsRed) in transgenic zebrafish. RNA-seq results were also analyzed, and KEGG enrichment analysis showed that significant differential genes were enriched in the PPAR signaling pathway (P < 0.05), and the PPAR signaling pathway mainly affected genes related to lipid metabolism. The RNA-seq results were verified using real-time fluorescence quantitative PCR (RT-qPCR). Oil red O and Nile red staining showed increased lipid accumulation after TPT exposure. These findings suggest that TPT affects the development of zebrafish embryos even at relatively low concentrations.

Marine Pollutant Tributyltin Affects DNA Methylation and Fitness of Banded Murex (Hexaplex trunculus) Populations

Banded murex, Hexaplex trunculus, is a marine gastropod whose reproductive fitness can be severely affected by very low concentrations of antifouling compound tributyltin (TBT). TBT has strong xenoandrogen impacts on snails, causing the development of imposex (e.g., the superimposition of male sexual characteristic in females), thereby affecting the fitness of entire populations. TBT is also known as a DNA-demethylating agent and an obesogenic factor. The aim of this study was to unravel the interactions between TBT bioaccumulation, phenotypic responses, and epigenetic and genetic endpoints in native populations of H. trunculus. Seven populations inhabiting environments along the pollution gradient were sampled in the coastal eastern Adriatic. These included sites of intense marine traffic and boat maintenance activity and sites with low anthropogenic impact. Populations inhabiting intermediately and highly polluted sites exhibited higher TBT burdens, higher incidences of imposex, and higher wet masses of snails than populations in lowly polluted sites. Other morphometric traits and cellular biomarker responses did not show clear differentiation among populations in relation to marine traffic/pollution intensity. An analysis of methylation sensitive amplification polymorphism (MSAP) revealed environmentally driven population differentiation and higher epigenetics than genetic within-population diversity. Moreover, decreases in genome-wide DNA methylation coincided with the imposex level and snail mass, suggesting an epigenetic background of the animal phenotypic response.

Transcriptome reveals overview of Ca2+ dose-dependent metabolism disorders in zebrafish larvae after Cd2+ exposure

Cadmium (Cd), a ubiquitous environmental hazardous heavy metal, poses a significant threat to the health of aquatic organisms, including teleosts. Although the toxic profile of Cd is well recognized, little is known regarding the overall view of toxic responses to varying aquatic environmental parameters (e.g., water hardness) at an individual level. Herein, differences in water hardness were partially mimicked by adjusting Ca²⁺ levels in E3 medium. As an in vivo model, zebrafish embryos were exposed to variable Ca²⁺ levels (NV, normal Ca²⁺; LV, low Ca²⁺; HV, high Ca²⁺) alone or combined with 30.7 µg/L Cd²⁺ (NC, LC, and HC, respectively) until 144 hr post-fertilization. The genome-wide transcriptome revealed differentially expressed genes between groups. Functional enrichment analysis found that biological processes related to metabolism, particularly lipid metabolism, were significantly disrupted in NC and LC treatments, while a remission was observed in the HC group. Biochemical assays confirmed that the decrease in Ca²⁺ enhanced synthesis, inhibited mobilization and increased the storage of lipids in Cd²⁺ treatments. This study suggests that the toxic effect of Cd on biological pathways will be influenced by Ca²⁺, which will improve the toxicological understanding and facilitate accurate assessment of Cd.

Cetyl Alcohol Polyethoxylates Disrupt Metabolic Health in Developmentally Exposed Zebrafish

Alcohol polyethoxylates (AEOs), such as cetyl alcohol ethoxylates (CetAEOs), are high-production-volume surfactants used in laundry detergents, hard-surface cleaners, pesticide formulations, textile production, oils, paints, and other products. AEOs have been suggested as lower toxicity replacements for alkylphenol polyethoxylates (APEOs), such as the nonylphenol and octylphenol polyethoxylates. We previously demonstrated that nonylphenol polyethoxylates induced triglyceride accumulation in several in vitro adipogenesis models and promoted adiposity and increased body weights in developmentally exposed zebrafish. We also demonstrated that diverse APEOs and AEOs were able to increase triglyceride accumulation and/or pre-adipocyte proliferation in a murine pre-adipocyte model. As such, the goals of this study were to assess the potential of CetAEOs to promote adiposity and alter growth and/or development (toxicity, length, weight, behavior, energy expenditure) of developmentally exposed zebrafish (Danio rerio). We also sought to expand our understanding of ethoxylate chain-length dependent effects through interrogation of varying chain-length CetAEOs. We demonstrated consistent adipogenic effects in two separate human bone-marrow-derived mesenchymal stem cell models as well as murine pre-adipocytes. Immediately following chemical exposures in zebrafish, we reported disrupted neurodevelopment and aberrant behavior in light/dark activity testing, with medium chain-length CetAEO-exposed fish exhibiting hyperactivity across both light and dark phases. By day 30, we demonstrated that cetyl alcohol and CetAEOs disrupted adipose deposition in developmentally exposed zebrafish, despite no apparent impacts on standard length or gross body weight. This research suggests metabolic health concerns for these common environmental contaminants, suggesting further need to assess molecular mechanisms and better characterize environmental concentrations for human health risk assessments.

A New In Vivo Zebrafish Bioassay Evaluating Liver Steatosis Identifies DDE as a Steatogenic Endocrine Disruptor, Partly through SCD1 Regulation

Non-alcoholic fatty liver disease (NAFLD), which starts with liver steatosis, is a growing worldwide epidemic responsible for chronic liver diseases. Among its risk factors, exposure to environmental contaminants, such as endocrine disrupting compounds (EDC), has been recently emphasized. Given this important public health concern, regulation agencies need novel simple and fast biological tests to evaluate chemical risks. In this context, we developed a new in vivo bioassay called StAZ (Steatogenic Assay on Zebrafish) using an alternative model to animal experimentation, the zebrafish larva, to screen EDCs for their steatogenic properties. Taking advantage of the transparency of zebrafish larvae, we established a method based on fluorescent staining with Nile red to estimate liver lipid content. Following testing of known steatogenic molecules, 10 EDCs suspected to induce metabolic disorders were screened and DDE, the main metabolite of the insecticide DDT, was identified as a potent inducer of steatosis. To confirm this and optimize the assay, we used it in a transgenic zebrafish line expressing a blue fluorescent liver protein reporter. To obtain insight into DDE's effect, the expression of several genes related to steatosis was analyzed; an up-regulation of scd1 expression, probably relying on PXR activation, was found, partly responsible for both membrane remodeling and steatosis.

Metabolism-Disrupting Chemicals Affecting the Liver: Screening, Testing, and Molecular Pathway Identification

The liver is the central metabolic organ of the body. The plethora of anabolic and catabolic pathways in the liver is tightly regulated by physiological signaling but may become imbalanced as a consequence of malnutrition or exposure to certain chemicals, so-called metabolic endocrine disrupters, or metabolism-disrupting chemicals (MDCs). Among different metabolism-related diseases, obesity and non-alcoholic fatty liver disease (NAFLD) constitute a growing health problem, which has been associated with a western lifestyle combining excessive caloric intake and reduced physical activity. In the past years, awareness of chemical exposure as an underlying cause of metabolic endocrine effects has continuously increased. Within this review, we have collected and summarized evidence that certain environmental MDCs are capable of contributing to metabolic diseases such as liver steatosis and cholestasis by different molecular mechanisms, thereby contributing to the metabolic syndrome. Despite the high relevance of metabolism-related diseases, standardized mechanistic assays for the identification and characterization of MDCs are missing. Therefore, the current state of candidate test systems to identify MDCs is presented, and their possible implementation into a testing strategy for MDCs is discussed.

Environmental concentrations of benzophenone-3 disturbed lipid metabolism in the liver of clown anemonefish (Amphiprion ocellaris)

Benzophenone-3 (BP-3) often used as a UV filter in various products and an endocrine disruptor. In this work, we exposed the clown anemonefish to 10 μg/L and 50 μg/L BP-3 for 7 and 14 days. Liver histological, biochemical analysis, and transcriptome sequencing were used to explore the mechanism of the lipid metabolism disorder in the liver of three-month-old clown anemonefish treated with BP-3. The histological and biochemical analysis showed that BP-3 induces morphological changes and lipid droplet accumulation, and the lipid content, lipase, and antioxidant enzyme activity were abnormal. After treatment with 10 μg/L and 50 μg/L BP-3 for 7 days, the transcriptome analysis further demonstrated that the KEGG analysis revealed that the differentially expressed genes (DEGs) were mainly associated with fat digestion and absorption, PPAR signaling pathway, circadian rhythm, and mineral absorption pathways; After 10 μg/L and 50 μg/L of BP-3 exposure for 14 days, the KEGG analysis were mainly associated with circadian rhythm, circadian rhythm-fly, protein processing in the endoplasmic reticulum, and beta-alanine metabolism pathways. Several key genes were involved in the process of liver lipid metabolism, including CD36, APoA-Ⅰ, FABP, LPL, ACS, and PEPCK. The qRT-PCR validation results showed that eight genes (CYP8B1, FABP1, LPL, MGAT, PEPCK, PER1, PSMB4, PSME2) were significantly down-regulated, and the other two genes (Fbxl3, RXR) were significantly up-regulated after 7 days of BP-3 exposure. Similarly, eleven genes (AMPK, ARNTL, Bmal1, CASP3, CYC, CYP2J, CYP2U1, GSK3A, PEPCK, RAC1, RORA) were significantly up-regulated, and the other four genes (NR1D1, PER1, PTGDS, HLF) were significantly down-regulated after 14 days of BP-3 exposure. In conclusion, our results elucidate the physiological and molecular responses to BP-3 exposure in the liver lipid metabolism of clown anemonefish, and these findings reveal that the regulation of lipid metabolism is disturbed when clown anemonefish is exposed to UV filters.

Early- and whole-life exposures to florfenicol disrupts lipid metabolism and induces obesogenic effects in zebrafish (Danio rerio)

Florfenicol (FF), a widely used veterinary antibiotic, has been frequently detected in both aquatic environments and human body fluids. As a result, there is a growing concern on its health risks. Previous studies have revealed various toxicities of FF on animals, while there are relatively limited researches on its metabolic toxicity. Herein, by employing zebrafish as an in vivo model, endpoints at multiple levels of biological organization were measured to investigate the metabolic toxicity, especially disturbances on lipid metabolism, of this emerging pollutant. Our results indicated that early-life exposure (from 2 h past fertilization (hpf) to 15 days past fertilization (dpf)) to FF significantly increased body mass index (BMI) values, staining areas of visceral lipids, and triacylglycerol (TAG) and total cholesterol (TC) contents of larvae. Further, by analyzing expression patterns of genes encoding key proteins regulating lipid metabolism, our data suggested that promoted intestinal absorption and hepatic de novo synthesis of lipids, suppressed TAG decomposition, and inhibited FFA oxidation all contributed to TAG accumulation in larvae. Following whole-life exposure (from 2 hpf to 120 dpf), BMI values, TAG and TC contents all increased significantly in males, and significant increases of hepatic TAG levels were also observed in females. Moreover, FF exposure interfered with lipid homeostasis of males and females in a gender-specific pattern. Our study revealed the obesogenic effects of FF at environmentally relevant concentrations (1, 10, and 100 μg/L) and therefore will benefit assessment of its health risks. Additionally, our results showed that FF exposure caused a more pronounced obesogenic effect in zebrafish larvae than adults, as suggested by significant increases of all endpoints at individual, tissular, and molecular levels in larvae. Therefore, our study also advances the application of zebrafish larval model in assessing metabolic toxicity of chemicals, due to the higher susceptibility of larvae than adults.

Toxicity of tributyltin chloride on haarder (Liza haematocheila) after its acute exposure: Bioaccumulation, antioxidant defense, histological, and transcriptional analyses

Liza haematocheila is exposed to various chemical contaminants from anthropogenic sources, including tributyltin chloride (TBTC). Yet the toxicity mechanism of TBTC on haarder remains unclear. The haarder was exposed to different doses (0, 10%, 20%, and 50% of LC₅₀-96 h) of TBTC. In this study, the results revealed its high bioaccumulation in the livers and significant alteration for development. The activities of antioxidant enzymes superoxide dismutase, catalase, and glutathione peroxidase decreased after 96-h exposure to TBTC, this accompanied by an increased malondialdehyde level. TBTC exposure caused the intense production of reactive oxygen species, a reduction in total blood cell count in serum, and apoptosis-related alterations in livers, indicating that enhanced oxidative stress occurred in the process of TBTC exposure. Histological results revealed angiorrhexis and infiltration of inflammatory cells, vacuolar degeneration of hepatocytes in the livers, and swelling, fusion, and disintegration of gill organs. Interestingly, the obtained transcriptional profiles indicated that high doses of TBTC caused energy disorder, apoptosis, and adipogenesis restriction mediated by cytokines and adipokines in Jak-STAT and adipocytokine signaling pathways. In summary, acute exposure to high doses of TBTC could impair the antioxidant system and pathways related to energy, apoptosis and adipogenesis, eventually posing a serious challenge to the fitness of haarder individuals and its fish populations as marine resources.

The ecotoxicology of marine tributyltin (TBT) hotspots: A review

Tributyltin (TBT) was widely used as a highly efficient biocide in antifouling paints for ship and boat hulls. Eventually, TBT containing paints became globally banned when TBT was found to cause widespread contamination and non-target adverse effects in sensitive species, with induced pseudohermaphroditism in female neogastropods (imposex) being the best-known example. In this review, we address the history and the status of knowledge regarding TBT pollution and marine TBT hotspots, with a special emphasis on the Norwegian coastline. The review also presents a brief update on knowledge of TBT toxicity in various marine species and humans, highlighting the current understanding of toxicity mechanisms relevant for causing endocrine disruption in marine species. Despite observations of reduced TBT sediment concentrations in many marine sediments over the recent decades, contaminant hotspots are still prevalent worldwide. Consequently, efforts to monitor TBT levels and assessment of potential effects in sentinel species being potentially susceptible to TBT in these locations are still highly warranted.

Obesity II: Establishing causal links between chemical exposures and obesity

Obesity is a multifactorial disease with both genetic and environmental components. The prevailing view is that obesity results from an imbalance between energy intake and expenditure caused by overeating and insufficient exercise. We describe another environmental element that can alter the balance between energy intake and energy expenditure: obesogens. Obesogens are a subset of environmental chemicals that act as endocrine disruptors affecting metabolic endpoints. The obesogen hypothesis posits that exposure to endocrine disruptors and other chemicals can alter the development and function of the adipose tissue, liver, pancreas, gastrointestinal tract, and brain, thus changing the set point for control of metabolism. Obesogens can determine how much food is needed to maintain homeostasis and thereby increase the susceptibility to obesity. The most sensitive time for obesogen action is in utero and early childhood, in part via epigenetic programming that can be transmitted to future generations. This review explores the evidence supporting the obesogen hypothesis and highlights knowledge gaps that have prevented widespread acceptance as a contributor to the obesity pandemic. Critically, the obesogen hypothesis changes the narrative from curing obesity to preventing obesity.

Obesity III: Obesogen assays: Limitations, strengths, and new directions

There is increasing evidence of a role for environmental contaminants in disrupting metabolic health in both humans and animals. Despite a growing need for well-understood models for evaluating adipogenic and potential obesogenic contaminants, there has been a reliance on decades-old in vitro models that have not been appropriately managed by cell line providers. There has been a quick rise in available in vitro models in the last ten years, including commercial availability of human mesenchymal stem cell and preadipocyte models; these models require more comprehensive validation but demonstrate real promise in improved translation to human metabolic health. There is also progress in developing three-dimensional and co-culture techniques that allow for the interrogation of a more physiologically relevant state. While diverse rodent models exist for evaluating putative obesogenic and/or adipogenic chemicals in a physiologically relevant context, increasing capabilities have been identified for alternative model organisms such as Drosophila, C. elegans, zebrafish, and medaka in metabolic health testing. These models have several appreciable advantages, including most notably their size, rapid development, large brood sizes, and ease of high-resolution lipid accumulation imaging throughout the organisms. They are anticipated to expand the capabilities of metabolic health research, particularly when coupled with emerging obesogen evaluation techniques as described herein.

Emerging concepts and opportunities for endocrine disruptor screening of the non-EATS modalities

Endocrine disrupting chemicals (EDCs) are ubiquitous in the environment and involve diverse chemical-receptor interactions that can perturb hormone signaling. The Organization for Economic Co-operation and Development has validated several EDC-receptor bioassays to detect endocrine active chemicals and has established guidelines for regulatory testing of EDCs. Focus on testing over the past decade has been initially directed to EATS modalities (estrogen, androgen, thyroid, and steroidogenesis) and validated tests for chemicals that exert effects through non-EATS modalities are less established. Due to recognition that EDCs are vast in their mechanisms of action, novel bioassays are needed to capture the full scope of activity. Here, we highlight the need for validated assays that detect non-EATS modalities and discuss major international efforts underway to develop such tools for regulatory purposes, focusing on non-EATS modalities of high concern (i.e., retinoic acid, aryl hydrocarbon receptor, peroxisome proliferator-activated receptor, and glucocorticoid signaling). Two case studies are presented with strong evidence amongst animals and human studies for non-EATS disruption and associations with wildlife and human disease. This includes metabolic syndrome and insulin signaling (case study 1) and chemicals that impact the cardiovascular system (case study 2). This is relevant as obesity and cardiovascular disease represent two of the most significant health-related crises of our time. Lastly, emerging topics related to EDCs are discussed, including recognition of crosstalk between the EATS and non-EATS axis, complex mixtures containing a variety of EDCs, adverse outcome pathways for chemicals acting through non-EATS mechanisms, and novel models for testing chemicals. Recommendations and considerations for evaluating non-EATS modalities are proposed. Moving forward, improved understanding of the non-EATS modalities will lead to integrated testing strategies that can be used in regulatory bodies to protect environmental, animal, and human health from harmful environmental chemicals.

Nonylphenol Polyethoxylates Enhance Adipose Deposition in Developmentally Exposed Zebrafish

Alkylphenol polyethoxylates (APEOs), such as nonylphenol ethoxylates (NPEOs), are high-production-volume surfactants used in laundry detergents, hard-surface cleaners, pesticide formulations, textile production, oils, paints, and other products. NPEOs comprise −80% of the total production of APEOs and are widely reported across diverse environmental matrices. Despite a growing push for replacement products, APEOs continue to be released into the environment through wastewater at significant levels. Research into related nonionic surfactants from varying sources has reported metabolic health impacts, and we have previously demonstrated that diverse APEOs and alcohol polyethoxylates promote adipogenesis in the murine 3T3-L1 pre-adipocyte model. These effects appeared to be independent of the base alkylphenol and related to the ethoxylate chain length, though limited research has evaluated NPEO exposures in animal models. The goals of this study were to assess the potential of NPEOs to promote adiposity (Nile red fluorescence quantification) and alter growth and/or development (toxicity, length, weight, and energy expenditure) of developmentally exposed zebrafish (Danio rerio). We also sought to expand our understanding of the ability to promote adiposity through evaluation in human mesenchymal stem cells. Herein, we demonstrated consistent adipogenic effects in two separate human bone-marrow-derived mesenchymal stem cell models, and that nonylphenol and its ethoxylates promoted weight gain and increased adipose deposition in developmentally exposed zebrafish. Notably, across both cell and zebrafish models we report increasing adipogenic/obesogenic activity with increasing ethoxylate chain lengths up to maximums around NPEO-6 and then decreasing activity with the longest ethoxylate chain lengths. This research suggests metabolic health concerns for these common obesogens, suggesting further need to assess molecular mechanisms and better characterize environmental concentrations for human health risk assessments.

Induction of lipid metabolism dysfunction, oxidative stress and inflammation response by tris(1-chloro-2-propyl)phosphate in larval/adult zebrafish

As an important organophosphate flame retardant, tris(1-chloro-2-propyl)phosphate (TCPP) is ubiquitous in the environment leading to inevitable human exposure. However, there is a paucity of information regarding its acute/chronic effects on obesity, lipid homeostasis, and hepatocellular carcinoma, especially regarding the underlying molecular mechanisms in humans. Herein, we investigated the effects of TCPP exposure (5-25 mg/L) on lipid homeostasis in larval and adult zebrafish (Danio rerio). TCPP exposure caused remarkable lipid-metabolism dysfunction, which was reflected in obesity and excessive lipid accumulation in zebrafish liver. Mechanistically, TCPP induced the over-expression of adipogenesis genes and suppressed the expression of fatty-acid β-oxidation genes. Consequently, excess lipid synthesis and deficient expenditure triggered oxidative damage and an inflammation response by disrupting the antioxidant system and over-expressing proinflammatory cytokine. Based on high-throughput transcriptome sequencing, we found that TCPP exposure led to enrichment of several pathways involved in lipid metabolism and inflammation, as well as several genes related to pathways of cancer. Notably, increasing expressions of Ki-67 and 53BP1 proteins, which are reliable biomarkers for recognition and risk prediction of cellular proliferation in cancer cells, were observed in liver tissues of adult zebrafish. These results imply that chronic TCPP exposure triggers a potential risk of hepatocellular carcinogenesis (HCC) progression. Collectively, these findings offer new insights into our mechanistic understanding for the health effects of organophosphorus flame retardants on humans.

Bis(2-ethylhexyl)-tetrabromophthalate induces zebrafish obesity by altering the brain-gut axis and intestinal microbial composition

Multiple environmental stressors, including chemicals termed obesogens, contribute to the susceptibility of organisms to obesity. Bis(2-ethylhexyl)-2,3,4,5-tetrabromophthalate (TBPH), a novel brominated flame retardant, is an environmental contaminant that may disrupt lipid metabolism. However, the risk of TBPH leading to obesity remains unknown. Herein, adult female zebrafish fed a normal-fat diet (NFD) or high-fat diet (HFD) were exposed to 0, 0.02 and 2.0 μM TBPH for 6 weeks. The results showed that chronic TBPH exposure lead to significant weight gain, adipocyte hypertrophy, and subcutaneous fat accumulation, which could be enhanced by HFD feeding. HFD individuals also showed significant visceral fat accumulation. Transcription of the main adipokines regulating lipid metabolism associated with the brain-gut axis were significantly affected by TBPH, especially leptin (brain) and adiponectin (intestine). Additionally, peroxisome proliferator-activated nuclear receptor gamma (PPAR-γ) was significantly upregulated in intestine. TBPH increased the abundance of Firmicutes and Bacteroidetes in the gut microbiota in both NFD and HFD groups, resulting in obesity. Interestingly, population diversity analysis indicated that TBPH alone had a comparable impact on gut microbiota composition to that of HDF controls. Thus, TBPH increased the susceptibility of female zebrafish to obesity by disrupting brain-gut axis regulation and gut microbial composition, leading to enhanced fat accumulation under HFD conditions.

An ancestral nuclear receptor couple, PPAR-RXR, is exploited by organotins

Environmental chemicals have been reported to greatly disturb the endocrine and metabolic systems of multiple animal species. A recent example involves the exploitation of the nuclear receptor (NR) heterodimeric pair composed by PPAR/RXR (peroxisome proliferator-activated receptor/retinoid X receptor), which shows lipid perturbation in mammalian species. While gene orthologues of both of these receptors have been described outside vertebrates, no functional characterization of PPAR has been carried in protostome lineages. We provide the first functional analysis of PPAR in Patella sp. (Mollusca), using model obesogens such as tributyltin (TBT), triphenyltin (TPT), and proposed natural ligands (fatty acid molecules). To gain further insights, we used site-directed mutagenesis to PPAR and replaced the tyrosine 277 by a cysteine (the human homologous amino acid and TBT anchor residue) and an alanine. Additionally, we explored the alterations in the fatty acid profiles after an exposure to the model obesogen TBT, in vivo. Our results show that TBT and TPT behave as an antagonist of Patella sp. PPAR/RXR and that the tyrosine 277 is important, but not essential in the response to TBT. Overall, these results suggest a relation between the response of the mollusc PPAR-RXR to TBT and the lipid profile alterations reported at environmentally relevant concentrations. Our findings highlight the importance of comparative analysis between protostome and deuterostome lineages to decipher the differential impact of environmental chemicals.

Obesogenic and developmental effects of TBT on the gene expression of juvenile Japanese medaka (Oryzias latipes)

The widely used antifoulant tributyltin chloride (TBT) is highly toxic to aquatic organisms. In the present study, four-week-old Japanese medaka (Oryzias latipes) juveniles were orally exposed to TBT at 1 and 10 ng/g bw/d for 1, 2, and 4 weeks, respectively. Half of the tested medaka juveniles showed bone morphology alterations in both 1 and 10 ng/g bw/d TBT 4-week exposure groups. Nile Red (NR) staining showed that the juveniles exposed to 1 ng/g bw/d TBT for 2 and 4 weeks had significantly enlarged adipocyte areas. The mRNA-Seq analysis indicated that 1 ng/g bw/d TBT exposure for 2 weeks affected bone morphology through developmental processes. The GO and KEGG analyses suggested that the adipogenic effect of TBT observed in this study may be induced by metabolic processes, oxidative phosphorylation, and fatty acid degradation and metabolism pathways. Therefore, both morphological observation and mRNA-Seq analysis showed obesogenic effects and developmental toxicity of TBT to juvenile Japanese medaka.

The new kids on the block: Emerging obesogens

The current obesity epidemic is calling for action in the determination of contributing factors. Although social and life-style factors have been traditionally associated with metabolic disruption, a subset of endocrine-disrupting chemicals (EDCs), called obesogens are garnering increasing attention for their ability to promote adipose tissue differentiation and accumulation. For some chemicals, such as tributyltin, there is conclusive evidence regarding their ability to promote adipogenesis and their mechanism of action. In recent years, the list of chemicals that exert obesogenic potential is increasing. In this chapter, we review current knowledge of the most recent developments in the field of emerging obesogens with a specific focus on food additives, surfactants, and sunscreens, for which the mechanism of action remains unclear. We also review new evidence relative to the obesogenic potential of environmentally relevant chemical mixtures and point to potential therapeutic approaches to minimize the detrimental effects of obesogens. We conclude by discussing the available tools to investigate new obesogenic chemicals, strategies to maximize reproducibility in adipogenic studies, and future directions that will help propel the field forward.

Metal load and oxidative stress driven by organotin compounds on rainbow trout

Tributyltin-based (TBT) antifouling paints, widely used for the treatment of flooded surfaces, have been banned in 2008 for their high environmental persistence and bioaccumulation in aquatic organisms. Although it is still present in aquatic ecosystems, oxidative stress driven by TBT has been still poorly investigated in fish. The aim of the study was to examine the time-course stress responses in liver of rainbow trout that received a single intraperitoneal injection of tributyltin chloride (TBTC) or tributyltin ethoxide (TBTE), both at a dose of 0.05 and 0.5 mg/kg. Levels of metallothioneins, total glutathione, malondialdehyde, superoxide dismutase, catalase, glutathione peroxidase and glutathione S-transferase were evaluated at 3 and 6 days post-injection. Tin load was measured in the muscle of the same fish. Differences were observed in the time-course accumulation of tin with a clear dose-response relationship. Although individual oxidative stress biomarkers varied, the biomarker profile indicated different stress mechanisms caused by both TBTC and TBTE. The weak induction of metal-trapping metallothioneins and the changes of oxidative stress biomarkers suggested a stress-pressure in both TBT-treated trout, advising for an ecotoxicological risk for freshwater ecosystems.

Obesity and endocrine-disrupting chemicals

Obesity is now a worldwide pandemic. The usual explanation given for the prevalence of obesity is that it results from consumption of a calorie dense diet coupled with physical inactivity. However, this model inadequately explains rising obesity in adults and in children over the past few decades, indicating that other factors must be important contributors. An endocrine-disrupting chemical (EDC) is an exogenous chemical, or mixture that interferes with any aspect of hormone action. EDCs have become pervasive in our environment, allowing humans to be exposed daily through ingestion, inhalation, and direct dermal contact. Exposure to EDCs has been causally linked with obesity in model organisms and associated with obesity occurrence in humans. Obesogens promote adipogenesis and obesity, in vivo, by a variety of mechanisms. The environmental obesogen model holds that exposure to obesogens elicits a predisposition to obesity and that such exposures may be an important yet overlooked factor in the obesity pandemic. Effects produced by EDCs and obesogen exposure may be passed to subsequent, unexposed generations. This "generational toxicology" is not currently factored into risk assessment by regulators but may be another important factor in the obesity pandemic as well as in the worldwide increases in the incidence of noncommunicable diseases that plague populations everywhere. This review addresses the current evidence on how obesogens affect body mass, discusses long-known chemicals that have been more recently identified as obesogens, and how the accumulated knowledge can help identify EDCs hazards.

Dysregulation of lipid metabolism in PLHC-1 and ZFL cells exposed to tributyltin an all-trans retinoic acid

There is increasing awareness that exposure to endocrine disrupters interferes with lipid homeostasis in vertebrates, including fish. Many of these compounds exert their action by binding to nuclear receptors, such as peroxisome proliferator-activated receptors and retinoid X receptor. This work investigates the use of fish liver cells (PLHC-1 and ZFL cells) for the screening of metabolic and lipid disrupters in the aquatic environment by assessing changes in the cell's lipidome after exposure to the model compounds, tributyltin chloride and all-trans retinoic acid. Lipid extracts, analyzed by FIA-ESI (+/-) Orbitrap, evidenced the intracellular accumulation of triglycerides and diglycerides in both cell models after exposure to 100 and 200 nM tributyltin chloride for 24 h. Exposure to 1 μM all-trans retinoic acid led to a significant accumulation of triglycerides in PLHC-1 cells, while few triglycerides were accumulated in ZFL cells. Retinoic acid (cyp26b1, cyp3a65, lrata) and lipid metabolism (fasn, scd, elovl6) related genes were up-regulated by tributyltin chloride and all-trans retinoic acid, while only all-trans retinoic acid down-regulated the expression of dgat1a. The two cell models show sensitivity and responses to tributyltin chloride and all-trans retinoic acid comparable to those previously reported in mammalian cells. These results support the use of fish liver cells as alternative models for the detection of contaminants that act as lipid disrupters in the aquatic environment.

An environmentally relevant mixture of polychlorinated biphenyls (PCBs) and polybrominated diphenylethers (PBDEs) disrupts mitochondrial function, lipid metabolism and neurotransmission in the brain of exposed zebrafish and their unexposed F2 offspring

Polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) are persistent organic pollutants still present in aquatic environments despite their total or partial ban. Previously, we observed that an environmentally realistic mixture of these compounds affects energy balance, growth, and reproduction in exposed zebrafish (F0), and behavior in their unexposed offspring (F1-F4). In the present work, we performed lipidomic and transcriptomic analyses on brains of zebrafish (F0-F2) from exposed and control lineages to identify molecular changes that could explain the observed phenotypes. The use of both technologies highlighted that F0 zebrafish displayed impaired mitochondrial function and lipid metabolism regulation (depletion in triacylglycerols and phospholipids) which can explain disruption of energy homeostasis. A subset of the regulated biological pathways related to energetic metabolism and neurotransmission were inherited in F2. In addition, there were increasing effects on epigenetic pathways from the F0 to the F2 generation. Altogether, we show that the effects of an environmental exposure to PCBs and PBDEs on energetic metabolism as well as neurotransmission extend over 2 generations of zebrafish, possibly due to transgenerational epigenetic inheritance.

Transcriptomic effects of tributyltin (TBT) in zebrafish eleutheroembryos. A functional benchmark dose analysis

Exposure to the antifouling tributyltin (TBT) has been related to imposex in mollusks and to obesogenicity, adipogenesis and masculinization in fish. To understand the underlying molecular mechanisms, we evaluated dose-response effects of TBT (1.7-56 nM) in zebrafish eleutheroembryos transcriptome exposed from 2 to 5 days post-fertilization. RNA-sequencing analysis identified 3238 differentially expressed transcripts in eleutheroembryos exposed to TBT. Benchmark dose analyses (BMD) showed that the point of departure (PoD) for transcriptomic effects (9.28 nM) was similar to the metabolomic PoD (11.5 nM) and about one order of magnitude lower than the morphometric PoD (67.9 nM) or the median lethal concentration (LC₅₀: 93.6 nM). Functional analysis of BMD transcriptomic data identified steroid metabolism and cholesterol and vitamin D₃ biosynthesis as the most sensitive pathways to TBT (<50% PoD). Conversely, transcripts related to general stress and DNA damage became affected only at doses above the PoD. Therefore, our results indicate that transcriptomes can act as early molecular indicators of pollutant exposure, and illustrates their usefulness for the mechanistic identification of the initial toxic events. As the estimated molecular PoDs are close to environmental levels, we concluded that TBT may represent a substantial risk in some natural environments.

Natural and synthetic retinoid X receptor ligands and their role in selected nuclear receptor action

Important key players in the regulatory machinery within the cells are nuclear retinoid X receptors (RXRs), which compose heterodimers in company with several diverse nuclear receptors, playing a role as ligand inducible transcription factors. In general, nuclear receptors are ligand-activated, transcription-modulating proteins affecting transcriptional responses in target genes. RXR molecules forming permissive heterodimers with disparate nuclear receptors comprise peroxisome proliferator-activated receptors (PPARs), liver X receptors (LXRs), farnesoid X receptor (FXR), pregnane X receptor (PXR) and constitutive androstan receptor (CAR). Retinoid receptors (RARs) and thyroid hormone receptors (TRs) may form conditional heterodimers, and dihydroxyvitamin D₃ receptor (VDR) is believed to form nonpermissive heterodimer. Thus, RXRs are the important molecules that are involved in control of many cellular functions in biological processes and diseases, including cancer or diabetes. This article summarizes both naturally occurring and synthetic ligands for nuclear retinoid X receptors and describes, predominantly in mammals, their role in molecular mechanisms within the cells. A focus is also on triorganotin compounds, which are high affinity RXR ligands, and finally, we present an outlook on human microbiota as a potential source of RXR activators. Nevertheless, new synthetic rexinoids with better retinoid X receptor activity and lesser side effects are highly required.

The Echinodermata PPAR: Functional characterization and exploitation by the model lipid homeostasis regulator tributyltin

The wide ecological relevance of lipid homeostasis modulators in the environment has been increasingly acknowledged. Tributyltin (TBT), for instance, was shown to cause lipid modulation, not only in mammals, but also in fish, molluscs, arthropods and rotifers. In vertebrates, TBT is known to interact with a nuclear receptor heterodimer module, formed by the retinoid X receptor (RXR) and the peroxisome proliferator-activated receptor (PPAR). These modulate the expression of genes involved in lipid homeostasis. In the present work, we isolated for the first time the complete coding region of the Echinodermata (Paracentrotus lividus) gene orthologues of PPAR and RXR and evaluated the ability of a model lipid homeostasis modulator, TBT, to interfere with the lipid metabolism in this species. Our results demonstrate that TBT alters the gonadal fatty acid composition and gene expression patterns: yielding sex-specific responses in fatty acid levels, including the decrease of eicosapentaenoic acid (C20:5 n-3, EPA) in males, and increase of arachidonic acid (20:4n-6, ARA) in females, and upregulation of long-chain acyl-CoA synthetase (acsl), ppar and rxr. Furthermore, an in vitro test using COS-1 cells as host and chimeric receptors with the ligand binding domain (LBD) of P. lividus PPAR and RXR shows that organotins (TBT and TPT (Triphenyltin)) suppressed activity of the heterodimer PPAR/RXR in a concentration-dependent manner. Together, these results suggest that TBT acts as a lipid homeostasis modulator at environmentally relevant concentrations in Echinodermata and highlight a possible conserved mode of action via the PPAR/RXR heterodimer.

Occurrence, ecological and human health risks of phenyltin compounds in the marine environment of Hong Kong

Triphenyltin (TPT) has been known as one of the most toxic compounds being released into the marine environment by anthropogenic means. This study assessed the contamination statuses of TPT and its two major degradants, i.e., monophenyltin and diphenyltin, in seawater, sediment and biota samples from marine environments of Hong Kong, a highly urbanized and densely populated city, and evaluated their ecological and human health risks. The results showed that the Hong Kong's marine environments were heavily contaminated with these chemicals, especially for TPT. Concentration ranges of TPT in seawater, sediment and biota samples were 3.8-11.7 ng/L, 71.8-91.7 ng/g d.w., and 9.6-1079.9 ng/g w.w., respectively. As reflected by high hazard quotients (1.7-5.3 for seawaters; 46.1-59.0 for sediments), TPT exhibited high ecological and human health risks. Our results are essential for the future management and control of anthropogenic TPT use in antifouling paints and as biocides in agriculture.

Mechanisms of action, chemical characteristics, and model systems of obesogens

There is increasing evidence for the role of environmental endocrine disrupting contaminants, coined obesogens, in exacerbating the rising obesity epidemic. Obesogens can be found in everyday items ranging from pesticides to food packaging. Although research shows that obesogens can have effects on adipocyte size, phenotype, metabolic activity, and hormone levels, much remains unknown about these chemicals. This review will discuss what is currently known about the mechanisms of obesogens, including expression of the PPARs, hormone interference, and inflammation. Strategies for identifying obesogenic chemicals and their mechanisms through chemical characteristics and model systems will also be discussed. Ultimately, research should focus on improving models to discern precise mechanisms of obesogenic action and to test therapeutics targeting these mechanisms.

Chronic exposure to environmentally relevant levels of simvastatin disrupts zebrafish brain gene signaling involved in energy metabolism

Simvastatin (SIM), a hypocholesterolaemic drug belonging to the statins group, is a widely prescribed pharmaceutical for prevention of cardiovascular diseases. Several studies showed that lipophilic statins, as SIM, cross the blood-brain barrier and interfere with the energy metabolism of the central nervous system in humans and mammalian models. In fish and other aquatic organisms, the effects of SIM on the brain energy metabolism are unknown, particularly following exposure to low environmentally relevant concentrations. Therefore, the present study aimed at investigating the influence of SIM on gene signaling pathways involved in brain energy metabolism of adult zebrafish (Danio rerio) following chronic exposure (90 days) to environmentally relevant SIM concentrations ranging from 8 ng/L to 1000 ng/L. Real-time PCR was used to determine the transcript levels of several genes involved in different pathways of the brain energy metabolism (glut1b, gapdh, acadm, accα, fasn, idh3a, cox4i1, and cox5aa). The findings here reported integrated well with ecological and biochemical responses obtained in a parallel study. Data demonstrated that SIM modulates transcription of key genes involved in the mitochondrial electron transport chain, in glucose transport and metabolism, in fatty acid synthesis and β-oxidation. Further, SIM exposure led to a sex-dependent transcription profile for some of the studied genes. Overall, the present study demonstrated, for the first time, that SIM modulates gene regulation of key pathways involved in the energy metabolism in fish brain at environmentally relevant concentrations.

Glucocorticoid and mineralocorticoid receptors and corticosteroid homeostasis are potential targets for endocrine-disrupting chemicals

Endocrine-disrupting chemicals (EDCs) have received significant concern, since they ubiquitously exist in the environment and are able to induce adverse health effects on human and wildlife. Increasing evidence shows that the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR), members of the steroid receptor subfamily, are potential targets for EDCs. GR and MR mediate the actions of glucocorticoids and mineralocorticoids, respectively, which are two main classes of corticosteroids involved in many physiological processes. The effects of EDCs on the homeostasis of these two classes of corticosteroids have also gained more attention recently. This review summarized the effects of environmental GR/MR ligands on receptor activity, and disruption of corticosteroid homeostasis. More than 130 chemicals classified into 7 main categories were reviewed, including metals, metalloids, pesticides, bisphenol analogues, flame retardants, other industrial chemicals and pharmaceuticals. The mechanisms by which EDCs interfere with GR/MR activity are primarily involved in ligand-receptor binding, nuclear translocation of the receptor complex, DNA-receptor binding, and changes in the expression of endogenous GR/MR genes. Besides directly interfering with receptors, enzyme-catalyzed synthesis and prereceptor regulation pathways of corticosteroids are also important targets for EDCs. The collected evidence suggests that corticosteroids and their receptors should be considered as potential targets for safety assessment of EDCs. The recognition of relevant xenobiotics and their underlying mechanisms of action is still a challenge in this emerging field of research.

Linking chemical exposure to lipid homeostasis: A municipal waste water treatment plant influent is obesogenic for zebrafish larvae

Obesity, a risk factor for the development of type-2 diabetes, hypertension, cardiovascular disease, hepatic steatosis and some cancers, has been ranked in the top 10 health risk in the world by the World Health Organization. Despite the growing body of literature evidencing an association between the obesity epidemic and specific chemical exposure across a wide range of animal taxa, very few studies assessed the effects of chemical mixtures and environmental samples on lipid homeostasis. Additionally, the mode of action of several chemicals reported to alter lipid homeostasis is still poorly understood. Aiming to fill some of these gaps, we combined an in vivo assay with the model species zebrafish (Danio rerio) to screen lipid accumulation and evaluate expression changes of key genes involved in lipid homeostasis, alongside with an in vitro transactivation assay using human and zebrafish nuclear receptors, retinoid X receptor α and peroxisome proliferator-activated receptor γ. Zebrafish larvae were exposed from 4 th day post-fertilization until the end of the experiment (day 18), to six different treatments: experimental control, solvent control, tributyltin at 100 ng/L Sn and 200 ng/L Sn (positive control), and wastewater treatment plant influent at 1.25% and 2.5%. Exposure to tributyltin and to 2.5% influent led to a significant accumulation of lipids, with white adipose tissue deposits concentrating in the perivisceral area. The highest in vitro tested influent concentration (10%) was able to significantly transactivate the human heterodimer PPARγ/RXRα, thus suggesting the presence in the influent of HsPPARγ/RXRα agonists. Our results demonstrate, for the first time, the ability of complex environmental samples from a municipal waste water treatment plant influent to induce lipid accumulation in zebrafish larvae.

Mechanisms of Environmental Contributions to Fatty Liver Disease

PURPOSE

Fatty liver disease (FLD) affects over 25% of the global population and may lead to liver-related mortality due to cirrhosis and liver cancer. FLD caused by occupational and environmental chemical exposures is termed "toxicant-associated steatohepatitis" (TASH). The current review addresses the scientific progress made in the mechanistic understanding of TASH since its initial description in 2010.

RECENT FINDINGS

Recently discovered modes of actions for volatile organic compounds and persistent organic pollutants include the following: (i) the endocrine-, metabolism-, and signaling-disrupting chemical hypotheses; (ii) chemical-nutrient interactions and the "two-hit" hypothesis. These key hypotheses were then reviewed in the context of the steatosis adverse outcome pathway (AOP) proposed by the US Environmental Protection Agency. The conceptual understanding of the contribution of environmental exposures to FLD has progressed significantly. However, because this is a new research area, more studies including mechanistic human data are required to address current knowledge gaps.

Role of Obesogens in the Pathogenesis of Obesity

Obesity is considered to be a 20th century pandemic, and its prevalence correlates with the increasing global pollution and the presence of chemical compounds in the environment. Excessive adiposity results from an imbalance between energy intake and expenditure, but it is not merely an effect of overeating and lack of physical activity. Recently, several compounds that alter the mechanisms responsible for energy homeostasis have been identified and called "obesogens". This work presents the role of obesogens in the pathogenesis of obesity. We reviewed data from in vitro animal and human studies concerning the role of obesogens in the disturbance of energy homeostasis. We identified (i) the main groups and classes of obesogens, (ii) the molecular mechanisms of their action, (iii) their deleterious effect on adipose tissue function and control of appetite, and (iv) possible directions in limiting their influence on human metabolism. Obesogens have a multifactorial detrimental influence on energy homeostasis. Focusing on limiting exposure to obesogens and improving early life nutrition seems to be the most reasonable direction of action to prevent obesity in future generations.

Tributyltin Affects Retinoid X Receptor-Mediated Lipid Metabolism in the Marine Rotifer Brachionus koreanus

To examine how tributyltin (TBT), a model obesogen, affects the lipid metabolism in the marine rotifer Brachionus koreanus, we carried out life-cycle studies and determined the in vitro and in silico interactions of retinoid X receptor (RXR) with TBT, the transcriptional levels of RXR and lipid metabolic genes, and the fatty acid content. The lethal concentration 10% (LC10) was determined to be 5.12 μg/L TBT, and negative effects on ecologically relevant end points (e.g., decreased lifespan and fecundity) were detected at 5 μg/L TBT. On the basis of these findings, subsequent experiments were conducted below 1 μg/L TBT, which did not show any negative effects on ecologically relevant end points in B. koreanus. Nile red staining analysis showed that after exposure to 1 μg/L TBT, B. koreanus stored neutral lipids and had significantly increased transcriptional levels of RXR and lipid metabolism-related genes compared to the control. However, the content of total fatty acids did not significantly change at any exposure level. In the single fatty acids profile, a significant increase in saturated fatty acids (SFAs) 14:0 and 20:0 was observed, but the contents of omega-3 and omega-6 fatty acids were significantly decreased. Also, a transactivation assay of TBT with RXR showed that TBT is an agonist of Bk-RXR with a similar fold-induction to the positive control. Taken together, these results demonstrate that TBT-modulated RXR signaling leads to increase in transcriptional levels of lipid metabolism-related genes and the synthesis of SFAs but decreases the content of polyunsaturated fatty acids (PUFAs). Our findings support a wider taxonomic scope of lipid perturbation due to xenobiotic exposure that occurs via NRs in aquatic animals.

Long-term bisphenol S exposure induces fat accumulation in liver of adult male zebrafish (Danio rerio) and slows yolk lipid consumption in F1 offspring

Bisphenol S (BPS), as a substitute for bisphenol A, was frequently detected in human urine and blood. It has been reported that BPS could disrupt fat metabolism in vivo and vitro although mechanisms remain unclear. Additionally, there is no study that the disruptive effect of BPS on parental fat metabolism indirectly interferes with the lipid metabolism of offspring. Here, after 120-d exposure to 1, 10, 100, and 1000 μg/L BPS, the transcription level of genes involved in lipid metabolism in liver and feeding regulation of brain-gut axis, as well as the hepatic triacylglycerol (TAG) and plasma lipid levels were investigated in both male and female zebrafish. Results showed that in male liver, fatty acid synthesis and degradation were inhibited by reducing transcription levels of srebp1 and pparα, and the synthesis of TAG was significantly increased using fatty acid as a precursor by elevating agpat4 and dgat2 mRNA expression levels. As a consequence, fat accumulation and the increased TAG levels were observed in male liver, and lipid levels were also elevated in male plasma. In female liver, there was no excessive fat accumulation and BPS exposure had a non-monotonic effect on the gene expression of fasn, dagt2, and pparα. Notably, the unexposed offspring showed a large amount of yolk lipid remain at 5 days post fertilization. This study obviously demonstrated that long-term BPS exposure increases the risk of non-alcoholic fatty liver disease in male zebrafish and life-cycle exposure hazard on offspring is noteworthy.

Workshop on the validation and regulatory acceptance of innovative 3R approaches in regulatory toxicology - Evolution versus revolution

At a joint workshop organized by RIVM and BfR, international experts from governmental institutes, regulatory agencies, industry, academia and animal welfare organizations discussed and provided recommendations for the development, validation and implementation of innovative 3R approaches in regulatory toxicology. In particular, an evolutionary improvement of our current approach of test method validation in the context of defined approaches or integrated testing strategies was discussed together with a revolutionary approach based on a comprehensive description of the physiological responses of the human body to chemical exposure and the subsequent definition of relevant and predictive in vitro, in chemico or in silico methods. A more comprehensive evaluation of biological relevance, scientific validity and regulatory purpose of new test methods and assessment strategies together with case studies that provide practical experience with new approaches were discussed as essential steps to build up the necessary confidence to facilitate regulatory acceptance.

Changes in expression of microRNA potentially targeting key regulators of lipid metabolism in primary gilthead sea bream hepatocytes exposed to phthalates or flame retardants

Metabolism disrupting chemicals (MDCs) belong to the group of endocrine-disrupting chemicals (EDCs) and are known to affect endocrine and metabolic functions of liver. There is growing evidence that MDCs may also act modulating the expression levels of micro ribonucleic acids (miRNAs) and thus affecting post-transcriptional expression of hundreds of target genes. Herein, we used a gilthead sea bream in vitro hepatocyte model for analyzing the effects of an exposure to phthalates (i.e. DiDP) or flame retardants (i.e.TMCP) on the expression levels of three miRNAs (i.e. MiR133, MiR29 and MiR199a) selected on the basis of their regulatory roles in signaling pathways related to lipid metabolism. Following computational identification of genes that are regulated by the selected miRNAs, we identified six miRNA targets to be tested in differential gene expression analysis. To determine whether lipid metabolism was altered we have also measured the intracellular total cholesterol and triglyceride levels. The results of our study show that DiDP/TMCP exposure leads to a general decrease in the expression profiles of each miRNA leading to a corresponding upregulation of almost all their putative targets. In addition, these findings were also associated to a corresponding increased hepatocellular lipid content. The present study thus contributes to support the importance of these small molecules in regulating MDC-induced expression of genes associated with hepatic lipid metabolism and highlights the need for more toxicological studies examining miRNAs transcriptional regulatory networks controlling metabolic alterations in fish.

Is the current-use flame retardant, DBE-DBCH, a potential obesogen? Effects on body mass, fat content and associated behaviors in American kestrels

The current-use brominated flame retardant, 1,2-dibromo-4-(1,2-dibromoethyl)cyclohexane (DBE-DBCH), is capable of perturbing sex steroid and thyroid hormone pathways in vitro and in vivo. Chemicals with this capability may also disrupt metabolic processes and are candidate obesogens, but this potential has not yet been determined for DBE-DBCH. Our objective was to examine gross biomarkers of metabolic disruption in captive American kestrels. Birds were exposed by diet to the β isomer at the environmentally relevant dose of 0.239 ng β-DBE-DBCH/g kestrel/day, from 30 days (d) prior to pairing through until chicks hatched (82 d) (n = 30 breeding pairs) or for 28 d (n = 16 pre-breeding pairs), and were compared with vehicle-only exposed controls. Body mass was assessed throughout the breeding season at biologically relevant time points, flight and feeding behavior was measured in 5-min samples daily, and plasma triglycerides and cholesterol were assessed at d10 of brood rearing. Treated males were heavier than controls at pairing (p = 0.051), the final week of courtship (p = 0.061), and at d10 (p = 0.012) and d20 of brood rearing (p = 0.051); β-DBE-DBCH-exposed breeding females were similar in weight to control females. Treated birds tended to have higher plasma triglycerides (p = 0.078), which for females, was positively associated with body mass (p = 0.019). Heavier breeding males had higher plasma concentrations of testosterone and total thyroxine (p ≤ 0.046). Overall, both sexes exposed to β-DBE-DBCH demonstrated reduced flight behavior and increased feeding behavior during courtship. In the pre-breeding pairs, treated male and female kestrels had a higher percentage of body fat than respective controls (p = 0.045). These results demonstrate that β-DBE-DBCH elicited inappropriate fat and weight gain in adult American kestrels, consistent with their increased feeding, reduced flight activity and endocrine changes, and suggests that DBE-DBCH may be an obesogen warranting further research to test this hypothesis.

The retinoic acid receptor (RAR) in molluscs: Function, evolution and endocrine disruption insights

Retinoid acid receptor (RAR)-dependent signalling pathways are essential for the regulation and maintenance of essential biological functions and are recognized targets of disruptive anthropogenic compounds. Recent studies put forward the inability of mollusc RARs to bind and respond to the canonical vertebrate ligand, retinoic acid: a feature that seems to have been lost during evolution. Yet, these studies were carried out in a limited number of molluscs. Therefore, using an in vitro transactivation assay, the present work aimed to characterize phylogenetically relevant mollusc RARs, as monomers or as functional units with RXR, not only in the presence of vertebrate bone fine ligands but also known endocrine disruptors, described to modulate retinoid-dependent pathways. In general, none of the tested mollusc RARs were able to activate reporter gene transcription when exposed to retinoic acid isomers, suggesting that the ability to respond to retinoic acid was lost across molluscs. Similarly, the analysed mollusc RAR were unresponsive towards organochloride pesticides. In contrast, transcriptional repressions were observed with the RAR/RXR unit upon exposure to retinoids or RXR-specific ligands. Loss-of-function and gain-of-function mutations further corroborate the obtained results and suggest that the repressive behaviour, observed with mollusc and human RAR/RXR heterodimers, is possibly mediated by ligand biding to RXR.

Evolutionary Exploitation of Vertebrate Peroxisome Proliferator-Activated Receptor γ by Organotins

Globally persistent man-made chemicals display ever-growing ecosystemic consequences, a hallmark of the Anthropocene epoch. In this context, the assessment of how lineage-specific gene repertoires influence organism sensitivity toward endocrine disruptors is a central question in toxicology. A striking example highlights the role of a group of compounds known as obesogens. In mammals, most examples involve the modulation of the nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ). To address the structural and biological determinants of PPARγ exploitation by a model obesogen, tributyltin (TBT), in chordates, we employed comparative genomics, transactivation and ligand binding assays, homology modeling, and site-directed-mutagenesis. We show that the emergence of multiple PPARs (α, β and γ) in vertebrate ancestry coincides with the acquisition of TBT agonist affinity, as can be deduced from the conserved transactivation and binding affinity of the chondrichthyan and mammalian PPARγ. The amphioxus single-copy PPAR is irresponsive to TBT; as well as the investigated teleosts, this is a probable consequence of a specific mutational remodeling of the ligand binding pocket. Our findings endorse the modulatory ability of man-made chemicals and suggest an evolutionarily diverse setting, with impacts for environmental risk assessment.

Organotins in obesity and associated metabolic disturbances

The objective of the present study was to review the mechanisms of organotin-induced adipogenesis, obesity, and associated metabolic disturbances. Peroxisome proliferator-activated receptor γ (PPARγ) and retinoid X receptor α (RXRα) activation is considered as the key mechanism of organotin-induced adipogenesis. Particularly, organotin exposure results in increased adipogenesis both in cell and animal models. Moreover, transgenerational inheritance of organotin-induced obese phenotype was demonstrated in vivo. At the same time, the existing data demonstrate that organotin compounds (OTCs) induces aberrant expression of PPARγ-targeted genes, resulting in altered of adipokine, glucose transporter, proinflammatory cytokines levels, and lipid and carbohydrate metabolism. The latter is generally characterized by hyperglycemia and insulin resistance. Other mechanisms involved in organotin-induced obesity may include estrogen receptor and corticosteroid signaling, altered DNA methylation, and gut dysfunction. In addition to cellular effects, organotin exposure may also affect neural circuits of appetite regulation, being characterized by neuropeptide Y (NPY) up-regulation in parallel with of pro-opiomelanocortin (POMC), Agouti-related protein (AgRP), and cocaine and amphetamine regulated transcript (CART) down-regulation in the arcuate nucleus. These changes result in increased orexigenic and reduced anorexigenic signaling, leading to increased food intake. The existing data demonstrate that organotins are potent adipogenic agents, however, no epidemiologic studies have been performed to reveal the association between organotin exposure and obesity and the existing indirect human data are contradictory.

Identifying the gaps: Resources and perspectives on the use of nuclear receptor based-assays to improve hazard assessment of emerging contaminants

Biological control of key processes, such as development and reproduction, is largely ascribed to a superfamily of ligand-dependent and independent transcription factors named Nuclear Receptors (NRs). Given their ability to accommodate ligands, NRs are prime targets of man-made compounds that mimic or antagonise the action of endogenous ligands. Accordingly, NRs occupy a prominent role in OECD and EPA guidelines for testing and assessment of Endocrine disrupting chemicals (EDCs). Although NR assays are already a key instrument in the OECD Conceptual Framework for Testing and Assessment of EDCs, the focus is mostly on vertebrate NRs. Here, we address the chief knowledge gaps in the field. More specifically, we (1) verify the growing availability of genomes/transcriptome projects, (2) highlight gaps in the identification and characterization of metazoan NR and in the establishment of (3) life cycle and (4) toxicity testing protocols. An overall bias towards vertebrates and selected invertebrate groups, notably Arthropoda, Annelida and Mollusca, was observed. Hence, if we aim to improve risk assessment of EDCs and emerging pollutants at an ecosystems scale, and understand their mode of action (MOA), we must establish a framework to include a broad phylogenetic sampling of Metazoans.

Toxicology tailored low density oligonucleotide microarray for the thicklip grey mullets (Chelon labrosus): Biomarker gene transcription profile after caging in a polluted harbour

In aquatic organisms inhabiting polluted waters genes are activated to build an adaptive/compensatory defence against the possible effects of pollutants. Such responses can be used as biomarkers of exposure to chemical compounds, outlining the molecular mechanisms activated under specific pollution scenarios. With the aim of exploiting such approach in environmental health assessment, toxicologically relevant gene fragments were sequenced in the thicklip grey mullet (Chelon labrosus) and a toxicologically tailored low-density (160 genes) oligonucleotide microarray was customised. The tool was validated comparing organ/sex specific gene expression profiles and characterising responses under laboratory exposure to model chemicals. Finally, juvenile mullets were caged in a polluted harbour and hepatic gene expression profiles analysed after 5 and 21 days of deployment. Cages were deployed in the inner (IH) and outer (OH) Pasaia harbour, Bay of Biscay. Mussels (Mytilus galloprovincialis) were also caged as biological matrix for chemical bioaccumulation analysis and stress biomarkers measurements. Slightly higher concentrations of chemicals (metals, tributyltin, PAHs, phthalates) were quantified in IH than in OH, fish bile metabolites also revealing higher availability of PAHs in IH. Lysosome membrane stability in mussels was reduced, indicating stress condition in both sites. The developed microarray discriminated mullets showing distinctive expression profiles depending on site and deployment time. Genes related to immune and hypoxia responses were regulated comparing IH and OH at day 5. Phase I and II biotransformation genes, such as cyp2, cyp3 and ugt, were up-regulated in IH, together with the aryl hydrocarbon receptor 2 (ahr2) and the ahr repressor. Similarly, TBT-binding proteins and genes involved in lipid metabolism (pparγ, cyp7) were up-regulated with deployment time. Even if nowadays higher throughput approaches for gene expression analyses are available, the developed mullet tool constitutes a comprehensive tool to assess molecular responses of mullets exposed to pollutants, although it remains to be explored whether it can be applied to assess pollutant exposure in active pollution monitorings and in environmental health assessment.

Obesogenic Endocrine Disrupting Chemicals: Identifying Knowledge Gaps

Endocrine disrupting chemicals (EDCs) are compounds that are part of everyday consumer products and industrial manufacturing processes. EDCs can interfere with the endocrine system, including the adipose tissue. Accumulating evidence from epidemiological, animal, and in vitro studies demonstrates that EDCs can alter body weight, adipose tissue expansion, circulating lipid profile, and adipogenesis, with some resulting in transgenerational effects. These outcomes appear to be mediated through multiple mechanisms, from nuclear receptor binding to epigenetic modifications. A better understanding of the signaling pathways via which these EDCs contribute to an obesogenic phenotype, the interaction amongst complex mixtures of obesogenic EDCs, and the risks they pose relative to the obesity epidemic are still needed for risk assessment and development of prevention strategies.