Molecular and cellular effects of chemicals disrupting steroidogenesis during early ovarian development of brown trout (Salmo trutta fario)

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.

Transcriptomic and targeted metabolomic analysis revealed the toxic effects of prochloraz on larval zebrafish

Prochloraz (PCZ), an imidazole fungicide, has been extensively used in horticulture and agriculture to protect against pests and diseases. To investigate the potential toxicity of PCZ on aquatic organisms, larval zebrafish, as a model, were exposed to a series of concentrations (0, 20, 100, and 500 μg/L) of PCZ for 7 days. With transcriptomic analysis, we found that exposure to high dose PCZ could produce 76 downregulated and 345 upregulated differential expression genes (DEGs). Bioinformatics analysis revealed that most of the DEGs were characterized in the pathways of glycolipid metabolism, amino acid metabolism and oxidative stress in larval zebrafish. Targeted metabolomic analysis was conducted to verify the effects of PCZ on the levels of acyl-carnitines and some amino acids in larval zebrafish. In addition, biochemical indicators related to glycolipid metabolism were affected obviously, manifested as elevated triglyceride (TG) levels and decreased glucose (Glu) levels in whole larvae. The expression levels of genes associated with glycolipid metabolism were affected in larvae after exposure to PCZ (PK, GK, PEPckc, SREBP, ACO). Interestingly, we further confirmed that PCZ could induce oxidative stress by the changing enzyme activities (T-GSH, GSSG) and upregulating several related genes levels in larval zebrafish. Generally, our results revealed that the endpoints related to glycolipid metabolism, amino acid metabolism and oxidative stress were influenced by PCZ in larval zebrafish.

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.

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.

Lipid accumulation, oxidative stress and immune-related molecules affected by tributyltin exposure in muscle tissues of rare minnow (Gobiocypris rarus)

Tributyltin (TBT) is reported to induce adipogenesis in fish, which might affect nutritional qualities and health status. Muscle tissues account for the majority of body mass, and have been described as a major site of fat deposition and an immunologically active organ. Therefore, the present study aims to evaluate whether chronic exposures of TBT, at environmental concentrations of 1, 10 and 100 ng/L, affects lipid accumulation, oxidative stress and immune status in muscle tissues of rare minnow (Gobiocypris rarus). After 60 d of exposure, TBT increased contents of total lipid, total cholesterol, triglyceride and fatty acids in muscle tissues. Interestingly, TBT exposure disrupted fatty acid composition and increased contents of unsaturated fatty acids (such as eicosapentaenoic acid and docosahexaenoic acid) in muscle tissues, which might be a response to preserve membrane functions from TBT exposure. Meanwhile, the concentrations of hepatic fatty acid desaturase 2 (Δ6-desaturase) and stearoyl-CoA desaturase (Δ9-desaturase) were increased after TBT exposure, which might contribute the increase of unsaturated fatty acids. Furthermore, TBT increased muscle lipid peroxidation products, antioxidant enzymes (superoxide dismutase, catalase and glutathione peroxidase), and the expression of immune-related molecules (tumor necrosis factor alpha, interleukin 1 beta and nuclear factor kappa B) in muscle tissues. The disruption of TBT on the lipid accumulation, oxidative stress and immune-toxic effects in muscle tissues of fish might reduce nutritional qualities, and affect growth and health status, which might pose a constant and serious threat to fish and result in economic loss in aquaculture.

RNA-sequencing and pathway analysis reveal alteration of hepatic steroid biosynthesis and retinol metabolism by tributyltin exposure in male rare minnow (Gobiocypris rarus)

Tributyltin (TBT) is widely spread in aquatic ecosystems. Although adverse effects of TBT on reproduction and lipogenesis are observed in fishes, the underlying mechanisms, especially in livers, are still scarce and inconclusive. Thus, RNA-sequencing runs were performed on the hepatic libraries of adult male rare minnow (Gobiocypris rarus) after TBT exposure for 60d. After differentially expressed genes were identified, enrichment analysis and validation by quantitative real-time PCR were conducted. The results showed that TBT up-regulated the profile of hepatic genes in the steroid biosynthesis pathway and down-regulated the profile of hepatic genes in the retinol metabolism pathway. In the hepatic steroid biosynthesis pathway, TBT might induce biosynthesis of cholesterol, which could affect the bioavailability of steroid hormones. More important, 3beta-hydroxysteroid 3-dehydrogenase, a key enzyme in the biosynthesis of all active steroid hormones, was up-regulated by TBT exposure. In the hepatic retinol metabolism pathway, TBT impaired retinoic acid homeostasis which plays essential roles in both reproduction and lipogenesis. The results of two pathways offered new mechanisms underlying the toxicology of TBT and represented a starting point from which detailed mechanistic links should be explored.

Aromatase changes in depression: A postmortem and animal experimental study

A hyperactive hypothalamo-pituitary-adrenal (HPA) axis is a prominent feature in depression. It has been shown that androgens inhibit HPA activity and that estrogens stimulate it. We have therefore investigated, in human postmortem hypothalamus, whether depression features an increase in aromatase, which is the rate-limiting enzyme for the conversion of androgens to estrogens. In addition, we have tested the effect of an aromatase inhibitor on depression-like symptoms in a frequently used animal model for depression. At first, aromatase immunoreactivity (ir) was quantified in the central part of the hypothalamic paraventricular nucleus (PVN) of 10 major depressive disorder (MDD) patients and 10 well-matched control subjects. Subsequently an animal experimental study was performed using the chronic unpredictable mild stress (CUMS) rats as depression model. The effect of administration of 1,4,6-androstatriene-3,17-dione (ATD), an aromatase inhibitor, was investigated by silastic capsule implantation. In the postmortem study, the amount of PVN aromatase-ir decreased significantly in the MDD group compared to the controls (P=0.029). In the animal study, ATD was found to cause significantly increased testosterone (T) levels, both in plasma and in the hypothalamus. However, ATD administration did not show significant effects on the depression-like behaviors or plasma corticosterone levels in CUMS rats. Based on our observations in human postmortem material and the animal experiment, we have to conclude that alterations in aromatase in adulthood do not seem to play a major role in the pathogenesis of the symptoms of depression.

Joint acute and endocrine disruptive toxicities of malathion, cypermethrin and prochloraz to embryo-larval zebrafish, Danio rerio

It remains a daunting challenge to determine ecotoxicological risks of exposure to mixtures of endocrine disrupting chemicals (EDCs) in environmental toxicology. In the present study, we investigated acute and endocrine disruptive toxicities of cypermethrin (CPM), malathion (MAL), prochloraz (PRO) and their binary mixtures of MAL + CPM and MAL + PRO to the early life stages of zebrafish. In the acute lethal toxicity test, three pesticides exhibited different levels of toxicity to zebrafish larvae, and the order of toxicity was as follows: CPM > PRO > MAL. The binary mixture of MAL + CPM displayed a synergistic effect on zebrafish larvae after exposure for 24, 48, 72 and 96 h. However, binary mixture of MAL + PRO showed an antagonistic effect. To evaluate the estrogenic effect, the expression of genes in the hypothalamic-pituitary-gonadal axis was assessed after zebrafish embryos were exposed to CPM, MAL, PRO and their binary mixtures from blastula stage (1 h post-fertilization, 1 hpf) to 14 dpf (14 d post-fertilization). Our data indicated that the transcription patterns of many key genes (vtg1, vtg2, era, erβ1, erβ2, cyp19a1a and cyp19a1b) were affected in hatched zebrafish after exposure to CPM, MAL and PRO. Moreover, following exposure to binary mixtures of 1000 μg/L MAL +4 μg/L CPM and 1000 μg/L MAL +900 μg/L PRO, the gene expressions were significantly changed compared with the individual pesticides. Our data provided a better understanding of bidirectional interactions of toxic response induced by these pesticides.

Tributyltin affects shoaling and anxiety behavior in female rare minnow (Gobiocypris rarus)

Effects of tributyltin (TBT) on reproduction are well established in many fish species. However, few studies report the effects of TBT on non-reproductive behaviors, which is a novel aspect of endocrine disruption in fish. Thus, the present study used rare minnow (Gobiocypris rarus) to investigate the effects of TBT, at environmental concentrations of 1, 10 and 100ng/L, on shoaling and anxiety behaviors. The results showed that fish exposed to TBT had less group cohesion during the course of the 10-min observation period as compared with the control fish. Further, TBT altered the shoaling in the Novel tank test, where shoaling is determined as the tendency to leave a shoal of littermates trapped behind a Plexiglas barrier at one end of the test tank. Fish exposed to TBT had shorter latency before leaving shoal mates and spent more time away from shoal than control fish. In addition, we also used Novel tanks to study the anxiety behavior as the tendency to stay at the bottom when introduced into an unfamiliar environment. The fish exposed to TBT showed increased anxiety, manifested as increased latency to enter the upper half and decreased time in upper half when compared with the control fish. TBT exposure increased the levels of dopamine and its metabolite 3,4-dihydroxyphenylacetic acid, and decreased the levels of 5-hydroxytryptamine and its metabolite 5-hydroxy indole acetic acid in the brain. Thus, the hypofunction of the dopaminergic system or of the serotoninergic system or the combination of the two may underlie the observed behavioral change, which might affect the fitness of fish in their natural environment.

Linking the response of endocrine regulated genes to adverse effects on sex differentiation improves comprehension of aromatase inhibition in a Fish Sexual Development Test

The Fish Sexual Development Test (FSDT) is a non-reproductive test to assess adverse effects of endocrine disrupting chemicals. With the present study it was intended to evaluate whether gene expression endpoints would serve as predictive markers of endocrine disruption in a FSDT. For proof-of-concept, a FSDT according to the OECD TG 234 was conducted with the non-steroidal aromatase inhibitor fadrozole (test concentrations: 10μg/L, 32μg/L, 100μg/L) using zebrafish (Danio rerio). Gene expression analyses using quantitative RT-PCR were included at 48h, 96h, 28days and 63days post fertilization (hpf, dpf). The selection of genes aimed at finding molecular endpoints which could be directly linked to the adverse apical effects of aromatase inhibition. The most prominent effects of fadrozole exposure on the sexual development of zebrafish were a complete sex ratio shift towards males and an acceleration of gonad maturation already at low fadrozole concentrations (10μg/L). Due to the specific inhibition of the aromatase enzyme (Cyp19) by fadrozole and thus, the conversion of C19-androgens to C18-estrogens, the steroid hormone balance controlling the sex ratio of zebrafish was altered. The resulting key event is the regulation of directly estrogen-responsive genes. Subsequently, gene expression of vitellogenin 1 (vtg1) and of the aromatase cyp19a1b isoform (cyp19a1b), were down-regulated upon fadrozole treatment compared to controls. For example, mRNA levels of vtg1 were down-regulated compared to the controls as early as 48 hpf and 96 hpf. Further regulated genes cumulated in pathways suggested to be controlled by endocrine mechanisms, like the steroid and terpenoid synthesis pathway (e.g. mevalonate (diphospho) decarboxylase (mvd), lanosterol synthase (2,3-oxidosqualene-lanosterol cyclase; lss), methylsterol monooxygenase 1 (sc4mol)) and in lipid transport/metabolic processes (steroidogenic acute regulatory protein (star), apolipoprotein Eb (apoEb)). Taken together, this study demonstrated that the existing Adverse Outcome Pathway (AOP) for aromatase inhibition in fish can be translated to the life-stage of sexual differentiation. We were further able to identify MoA-specific marker gene expression which can be instrumental in defining new measurable key events (KE) of existing or new AOPs related to endocrine disruption.

Endocrine and Local IGF-I in the Bony Fish Immune System

A role for GH and IGF-I in the modulation of the immune system has been under discussion for decades. Generally, GH is considered a stimulator of innate immune parameters in mammals and teleost fish. The stimulatory effects in humans as well as in bony fish often appear to be correlated with elevated endocrine IGF-I (liver-derived), which has also been shown to be suppressed during infection in some studies. Nevertheless, data are still fragmentary. Some studies point to an important role of GH and IGF-I particularly during immune organ development and constitution. Even less is known about the potential relevance of local (autocrine/paracrine) IGF-I within adult and developing immune organs, and the distinct localization of IGF-I in immune cells and tissues of mammals and fish has not been systematically defined. Thus far, IGF-I has been localized in different mammalian immune cell types, particularly macrophages and granulocytes, and in supporting cells, but not in T-lymphocytes. In the present study, we detected IGF-I in phagocytic cells isolated from rainbow trout head kidney and, in contrast to some findings in mammals, in T-cells of a channel catfish cell line. Thus, although numerous analogies among mammals and teleosts exist not only for the GH/IGF-system, but also for the immune system, there are differences that should be further investigated. For instance, it is unclear whether the primarily reported role of GH/IGF-I in the innate immune response is due to the lack of studies focusing on the adaptive immune system, or whether it truly preferentially concerns innate immune parameters. Infectious challenges in combination with GH/IGF-I manipulations are another important topic that has not been sufficiently addressed to date, particularly with respect to developmental and environmental influences on fish growth and health.

Dose-dependent compensation responses of the hypothalamic-pituitary-gonadal-liver axis of zebrafish exposed to the fungicide prochloraz

Compensation responses and adaptability of hypothalamic-pituitary-gonadal (HPG) axis have been reported in fish exposed to model chemicals, however due to its importance in predictive toxicology further study was needed to elucidate details of the integrated responses to model chemicals. Transcriptional profiles of the hypothalamic-pituitary-gonadal (HPG) axis and concentrations of 17β-estradiol (E2) in plasma were measured in male and female zebrafish that had been exposed to one of seven concentrations of the fungicide, prochloraz: low (1, 3 or 10μg/L), medium (30 or 100μg/L) or high concentrations (300 or 1000μg/L) for 4 days. In zebrafish exposed to the low and medium concentrations of prochloraz, compensation responses of the HPG axis through transcription, occurred in brain (up-regulation of gnrh, gnrhr and lhβ) and both brain and gonad (up-regulation of steroidogenic genes), respectively. Concentrations of E2 in plasma and expression of estrogen receptor 1 (er1) and vitellogenins (vtgs) in liver did not change. This result suggested that compensatory responses were successful in maintaining homeostasis. In zebrafish exposed to the two greatest concentrations, compensatory responses occurred in brain, gonad and liver through up-regulation of er2β, but it failed to maintain concentration of E2 in blood plasma and expression of er1 and vtgs in liver. Collectedly, the results observed in this study allowed characterization of dose-dependent compensatory responses along the HPG axis and liver and identified key linkages between compensatory responses occurring in brain, gonad and liver after exposure to prochloraz.

Developmental oestrogen exposure differentially modulates IGF-I and TNF-α expression levels in immune organs of Yersinia ruckeri-challenged young adult rainbow trout (Oncorhynchus mykiss)

Intensified aquaculture has strong impact on fish health by stress and infectious diseases and has stimulated the interest in the orchestration of cytokines and growth factors, particularly their influence by environmental factors, however, only scarce data are available on the GH/IGF-system, central physiological system for development and tissue shaping. Most recently, the capability of the host to cope with tissue damage has been postulated as critical for survival. Thus, the present study assessed the combined impacts of estrogens and bacterial infection on the insulin-like growth factors (IGF) and tumor-necrosis factor (TNF)-α. Juvenile rainbow trout were exposed to 2 different concentrations of 17β-estradiol (E2) and infected with Yersinia ruckeri. Gene expressions of IGF-I, IGF-II and TNF-α were measured in liver, head kidney and spleen and all 4 estrogen receptors (ERα1, ERα2, ERβ1 and ERβ2) known in rainbow trout were measured in liver. After 5 weeks of E2 treatment, hepatic up-regulation of ERα1 and ERα2, but down-regulation of ERß1 and ERß2 were observed in those groups receiving E2-enriched food. In liver, the results further indicate a suppressive effect of Yersinia-infection regardless of E2-treatment on day 3, but not of E2-treatment on IGF-I whilst TNF-α gene expression was not influenced by Yersinia-infection but was reduced after 5 weeks of E2-treatment. In spleen, the results show a stimulatory effect of Yersinia-infection, but not of E2-treatment on both, IGF-I and TNF-α gene expressions. In head kidney, E2 strongly suppressed both, IGF-I and TNF-α. To summarise, the treatment effects were tissue- and treatment-specific and point to a relevant role of IGF-I in infection.