New insights into the mechanism of imposex induction in the dogwhelk Nucella lapillus

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.

Aquatic ecotoxicology of anticancer drugs: A systematic review

Anticancer drugs in the aquatic environment have drawn a lot of attention in the last decade. Since wastewater treatment plants are inefficient at fully eliminating trace concentrations of anticancer drugs, these compounds are continuously discharged into the aquatic environment. Subsequently, non-target organisms such as the aquatic biota are directly exposed to a variety of anticancer drugs. To understand the potential impact on the aquatic organisms, a systematic review was conducted in compliance with the PRISMA guidelines. The results acquired from the 152 included studies were analysed and sorted into four categories: the impact of each included anticancer drug, the effect of metabolites, the effect of a mixture of drugs, and risk assessment. Findings showed that risk to the aquatic biota was unlikely to occur as the concentrations needed to induce effects were much higher than those detected in the environment. However, these data were based on acute toxicity and included only basic toxicity endpoints. The concentrations that produced significant effects were much lower when tested in the long-term or in multi-generational studies. Heterogeneity in results was also observed; this depended on the organism tested, the assessment adopted, and the endpoints selected. In this systematic review, an overall view of the research studies was generated by which all the variability factors to be considered were reported and recommendations to guide future studies were proposed.

Sexual Development of the Hermaphroditic Scallop Argopecten irradians Revealed by Morphological, Endocrine and Molecular Analysis

Simultaneous or functional hermaphrodites possessing both ovary and testis at the same time are good materials for studying sexual development. However, previous research on sex determination and differentiation was mainly conducted in gonochoristic species and studies on simultaneous hermaphrodites are still limited. In this study, we conducted a combined morphological, endocrine and molecular study on the gonadal development of a hermaphroditic scallop Argopecten irradians aged 2–10 month old. Morphological analysis showed that sex differentiation occurred at 6 months of age. By examining the dynamic changes of progesterone, testosterone and estradiol, we found testosterone and estradiol were significantly different between the ovaries and testes almost throughout the whole process, suggesting the two hormones may be involved in scallop sex differentiation. In addition, we identified two critical sex-related genes FoxL2 and Dmrt1L, and investigated their spatiotemporal expression patterns. Results showed that FoxL2 and Dmrt1L were female- and male-biased, respectively, and mainly localized in the germ cells and follicular cells, indicating their feasibility as molecular markers for early identification of sex. Further analysis on the changes of FoxL2 and Dmrt1L expression in juveniles showed that significant sexual dimorphic expression of FoxL2 occurred at 2 months of age, earlier than that of Dmrt1L. Moreover, FoxL2 expression was significantly correlated with estradiol/testosterone ratio (E₂/T). All these results indicated that molecular sex differentiation occurs earlier than morphological sex differentiation, and FoxL2 may be a key driver that functions through regulating sex steroid hormones in the scallop. This study will deepen our understanding of the molecular mechanism underlying sex differentiation and development in spiralians.

Antineoplastic Agents: Environmental Prevalence and Adverse Outcomes in Aquatic Organisms

Cancer is the second leading cause of death worldwide, with 9.6 million cancer-related deaths in 2018. Cancer incidence has increased over time, and so has the prescription rate of chemotherapeutic drugs. These pharmaceuticals, known as antineoplastic agents, enter the aquatic environment via human excretion and wastewater. The objectives of the present critical review were to investigate the risk of antineoplastics to aquatic species and to summarize the current state of knowledge regarding their levels in the environment, because many antineoplastics are not adequately removed during wastewater treatment. We conducted 2 separate literature reviews to synthesize data on the global environmental prevalence and toxicity of antineoplastics. The antineoplastics most frequently detected in the environment included cyclophosphamide, ifosfamide, tamoxifen, methotrexate, and 5-fluorouracil; all were detectable in multiple water sources, including effluent and surface waters. These antineoplastics span 3 different mechanistic classes, with cyclophosphamide and ifosfamide classified as alkylating agents, tamoxifen as a hormonal agent, and methotrexate and 5-fluorouracil as antimetabolites. Studies that characterize the risk of antineoplastics released into aquatic environments are scarce. We summarize the biological impacts of the most environmentally prevalent antineoplastics on aquatic organisms and propose an adverse outcome pathway for cyclophosphamide and ifosfamide, 2 widely prescribed drugs with a similar immunotoxic mode of action. Acute and chronic ecotoxicity studies using aquatic models are needed for risk characterization of antineoplastics. Environ Toxicol Chem 2020;39:967-985. © 2020 SETAC.

Reprotoxicity of 4-nonylphenol to Biomphalaria alexandrina snails at certain temperatures

One of the most common compounds in pesticide formulations, plastics, and papers is 4-nonylphenol (4-NP). It is contained in agricultural, industrial, and wastewater effluents, which when discharged into surface waters affect aquatic fauna. Therefore, the present study aimed to use Biomphalaria alexandrina snails to evaluate the chronic toxicity of 4-NP. Its concentrations in collected water samples from Giza Governorate ranged from 400 to 1600 μg/l. Based on these environmentally relevant concentrations, laboratory experiments were carried out using standard 4-NP to investigate the effect of three concentrations; namely 400, 750, and 1600 μg/l. Survival rate of the exposed snails to 4-NP concentrations was affected after 4 weeks. Reproduction of the exposed snails to 4-NP concentrations was lower than that of the control at 30 °C, while the exposed snails to 400 μg/l of 4-NP showed maximum reproduction at 15 °C. The lowest hatchability percentage was recorded with egg masses laid by the exposed snails to 400 and 1600 μg/l of 4-NP at 15 and 30 °C, respectively. Furthermore, the results showed fluctuated levels of progesterone, estradiol, and testosterone depending upon the concentration and the temperature, which played a key role in determining the degree of 4-NP toxicity.

Size, season and origin of gastropods matter in imposex assessments

Through controlled exposure to tributyltin (TBT), the effects of season, size, and population origin were evaluated on imposex incidence in Stramonita brasiliensis. Four experiments were carried out with organisms collected from three different sites on the Brazilian coast (Torres - Rio Grande do Sul, Farol de Santa Marta - Santa Catarina, and Aracruz - Espírito Santo). S. brasiliensis were anesthetized, sexed, measured, classified by size in small (< 30 mm), medium (≥ 30 to ≤ 40 mm), or big (> 40 mm) and injected in the foot muscle with 0.5 μg g^-1 of TBT. Organisms were maintained during one month in aquariums with clear marine water under controlled laboratory conditions. One month after injection, the imposex assessments showed that the population from the southeast (tropical) region was more sensitive to TBT than the population from the south of Brazil (the temperate region). A greater sensitivity in the small and medium categories was observed. Females were also more susceptible to TBT when exposed during their reproductive period. Thus, the present study highlighted the need to ensure that intrinsic biological factors related to organisms are considered in such biomonitoring studies to avoid misinterpretation of results.

Sex steroid imbalances in the muricid Stramonita haemastoma from TBT contaminated sites

Imposex incidence, organotin tissue levels, and sex steroid (free and esterified testosterone and estradiol) levels were assessed in Stramonita haemastoma from Babitonga Bay (Santa Catarina State, Southern Brazil). The imposex levels showed a reduction when compared to a previous evaluation performed in the same area. In spite of that, the detected imposex incidence indicated the occurrence of tributyltin (TBT) inputs that were still able to produce endocrine disruption in local gastropods. In addition, a high level of organotins was observed in tissues of imposexed females. These females also showed a hormonal imbalance, especially in the total testosterone/total estradiol ratio. These findings obtained under realistic field conditions suggest that the steroid pathway could be responsible by the imposex induction after exposure to TBT. In this case, measurements of sex steroid levels can be an additional evidence for monitoring sites and impose affected gastropod populations.

Effects of Tributyltin and Other Retinoid Receptor Agonists in Reproductive-Related Endpoints in the Zebrafish (Danio rerio)

Both field and experimental data examined the influence of exposure to environmental contaminant tributyltin (TBT) on marine organisms. Although most attention focused on the imposex phenomenon in gastropods, adverse effects were also observed in other taxonomic groups. It has been shown that imposex induction involves modulation of retinoid signaling in gastropods. Whether TBT influences similar pathways in fish is yet to be addressed. In this study, larvae of the model teleost Danio rerio were exposed to natural retinoids, all-trans-retinoic acid, 9-cis-retinoic acid, and all-trans-retinol, as well as to the RXR synthetic pan-agonist methoprene acid (MA) and to TBT. Larvae were exposed to TBT from 5 days post fertilization (dpf) to adulthood, and reproductive capacity was assessed and correlated with mode of action. TBT significantly decreased fecundity at environmentally relevant levels at 1 μg TBT Sn/g in diet. Interestingly, in contrast to previous reports, TBT altered zebrafish sex ratio toward females, whereas MA exposure biased sex toward males. Since fecundity was significantly altered in the TBT-exposed group with up to 62% decrease, the potentially affected pathways were investigated. Significant downregulation was observed in brain mRNA levels of aromatase b (CYP19a1b) in females and peroxisome proliferator activated receptor gamma (PPARg) in both males and females, suggesting an involvement of these pathways in reproductive impairment associated with TBT.

Gene expression profiling in the ovary of Queen conch (Strombus gigas) exposed to environments with high tributyltin in the British Virgin Islands

Queen conch (Strombus gigas) are listed in CITES Appendix II. Populations may be declining due to anthropogenic inputs that include pollutants from boating activity. In the British Virgin Islands (BVI), some conch exhibit imposex, a condition in which male external genitalia are present in female conch. Previous studies suggest that tributyltin (TBT), an antifouling chemical used in boat paint, is correlated to increased incidence of imposex although the mechanisms leading to imposex are not known. The present study utilized a Queen conch microarray to measure the response of the ovarian transcriptome in conch inhabiting polluted environments with high TBT levels in the BVI. The polluted sites, Road Harbour (RH) and Trellis Bay (TB), are areas with high boating activity while the reference sites, Guana Island (GI) and Anegada (AN), are areas with low boating activity. There were 754 and 898 probes differentially expressed in the ovary of conch collected at RH and TB respectively compared to conch collected at GI. Of the genes that were differentially expressed at both sites, >10% were shared suggesting that these sites have additional environmental factors influencing gene expression patterns. Functional enrichment analysis showed that the biological processes of cell proliferation, translation, and oxidative stress were over-represented in the polluted sites. Gene set enrichment analysis revealed that transcripts involved in the biological processes of general metabolism, immune, lipid metabolism, and stress were affected in conch from polluted environments. Interestingly, altered stress genes appeared to be more prevalent in conch collected from RH than TB, corresponding to the higher TBT load at RH compared to TB. Our study shows that stress pathways are affected in conch ovary in environments that experience heavy boating activity in the BVIs, although we are unable to directly link changes at the transcriptomics level to high TBT levels.

Imposex development in Nucella lapillus--evidence for the involvement of retinoid X receptor and androgen signalling pathways in vivo

Imposex in female gastropods is a widely documented masculinisation phenomenon in response to tributyltin (TBT) exposure. Although it is generally accepted that imposex is a case of endocrine disruption the underlying mechanisms are controversially discussed with aromatase inhibition and retinoid X receptor (RXR) signalling pathways as two conflicting hypotheses. Hence, we performed injection experiments with the marine dogwhelk Nucella lapillus. As expected TBT induced imposex in all test specimens while the natural RXR ligand 9 cis-retinoic acid did not cause significant effects. Additionally, TBT effects were suppressed if the organotin compound was simultaneously injected with an androgen receptor inhibitor (cyproterone acetate) but not if co-administered with the synthetic RXR antagonist HX531. In contrast, the injection of the RXR agonist HX630 resulted in imposex development in nearly 100% females. Therefore, the results provide evidence for the involvement of the RXR and the androgen signalling pathway. Further investigations are necessary to resolve the biochemical mechanism of imposex development.

Effects of tributyltin (TBT) on in vitro hormonal and biotransformation responses in Atlantic salmon (Salmo salar)

The mechanisms by which the biocide tributyltin (TBT) and its metabolites affect the hormonal and xenobiotic biotransformation pathways in aquatic species are not well understood. In this study hepatocytes isolated from salmon were used to evaluate the mechanistical effects of TBT on fish hormonal and xenobiotic biotransformation pathways. Cells were exposed to 0.01, 0.1, 1, or 5 microM TBT and samples were collected at 0, 12, 24, or 48 h following exposure. Gene expression patterns were evaluated using quantitative polymerase chain reaction (PCR), and cytochrome P-450 (CYP)-mediated enzyme activities were evaluated by ethoxyresorufin, benzyloxyresorufin, and pentoxyresorufin O-deethylase (EROD, BROD, and PROD, respectively) activity assays. Generally, exposure of hepatocytes to 1 microM (at 48 h) and 5 microM TBT (at 12, 24, and 48 h) consistently produced reductions in all mRNA species investigated. TBT produced significant decreases of vitellogen (Vtg) expression at 48 h and modified the expression patterns of estrogen receptors (ERalpha and ERbeta) and androgen receptor-beta (ARbeta) that were dependent on time and TBT concentration. In the xenobiotic biotransformation pathway, TBT produced differential expression patterns that were dependent on exposure time and concentration for all salmonid AhR2 isoforms (AhR2alpha, AhR2beta, AhR2delta, and AhR2gamma). For CYP1A1, CYP3A, AhRR, and Arnt mRNA, TBT produced exposure- and time-specific modulations. Catalytic CYP activities showed that BROD activity increased in an apparent concentration-specific manner in cells exposed to TBT for 12 h. Interestingly, EROD activity showed a TBT concentration-dependent increase at 24 h and PROD at 12 and 48 h of exposure. In general our data show that TBT differentially modulated hormonal and biotransformation responses in the salmon in vitro system. The apparent and consistent decrease of the studied responses with time in 1 and 5 microM exposed hepatocytes suggest a possible transcription inhibitory effect of TBT.

Anti-androgenic effects of sewage treatment plant effluents in the prosobranch gastropod Nucella lapillus

Recent studies performed under laboratory conditions have shown that single exposure to high levels of several xenoestrogens is able to induce imposex in at least two neogastropod species. In an attempt to evaluate if xenoestrogens, at environmentally relevant conditions, do contribute to imposex induction, we have tested the effects of a mixture containing xenoestrogens (municipal sewage effluents) on imposex development in the dogwhelk Nucella lapillus. Exposure for 3 months to the raw (0.25% and 1%) and the final sewage effluent (12.5% and 50%) rendered no increase in the severity of imposex. Conversely, as exposure to high concentrations of natural steroids, estradiol and estrone, had previously been shown to partially rescue imposex development under laboratory conditions, we have also tested if exposure to the final sewage effluent could ameliorated the severity of imposex induction by tributyltin (TBT). The results demonstrated that co-exposure to the final sewage effluent leads to a decrease trend in the severity of imposex in the presence of TBT. Within the studied imposex parameters, the Relative Penis Size index (RPSI) was the most affected with a 50% decrease in the effluent 12.5% plus TBT exposed group and 25% decrease in the effluent 50% plus TBT, if compared with the TBT alone. Overall, our results give further support to the use of imposex in N. lapillus as a specific biomarker of TBT contamination. However, in areas of high inputs of sewage effluents, the assessment of TBT contamination by the use of the imposex phenomenon should ideally also include data on the tissue levels of butyltins.

The contribution of steroidal androgens and estrogens to reproductive maturation of the eastern mud snail Ilyanassa obsoleta

Molluscs exposed to endocrine-disrupting chemicals (EDCs) have exhibited changes in reproductive tract development that are typically associated with androgen or estrogen signaling in vertebrates. However, a role for androgens and estrogens in molluscan reproductive endocrinology has yet to be established. In this study, we investigated putative roles for steroidal androgens and estrogens in recrudescence of the eastern mud snail Ilyanassa obsoleta. Our objectives were to: (1) identify associations among concentrations of testosterone and 17beta-estradiol, sex, and reproductive status in mud snails that suggest these hormones are involved in recrudescence; and (2) determine whether mud snails express NR3C4-like (androgen receptor) and NR3A-like (estrogen receptor) mRNAs in a manner indicative of a role in recrudescence. Temporal changes in testosterone levels in males were consistent with a positive role in recrudescence. Such a trend was not evident in females or for 17beta-estradiol in either sex. Efforts to identify an androgen receptor from the mud snail using targeted, degenerate RT-PCR were unsuccessful. However, an estrogen receptor (ER) cDNA was identified that is highly similar to known ERs of other molluscs. Studies with the ER of other molluscs have shown that this protein does not actually bind estrogens. We therefore considered the possibility that the mud snail ER may regulate reproductive maturation as a ligand-independent transcription factor based upon its tissue abundance. Males expressed greater levels of ER mRNA than did females over the entire reproductive cycle, and this difference was most evident during recrudescence. ER mRNA levels were significantly elevated during recrudescence in males but not females. In conclusion, testosterone may have a role in male reproductive tract recrudescence; however, this putative activity is independent of a NR3C4-type androgen receptor. The ER also may function in male recrudescence, though apparently independent of 17beta-estradiol. The retinoid signaling pathway is discussed as a possible alternative hormone/receptor-mediated signaling pathway that regulates male recrudescence.

Ecological risk assessments of endocrine disrupting organotin compounds using marine neogastropods in Hong Kong

As active ingredients of anti-fouling paints that are widely used on ship hulls, organotin compounds, in particular tributyltin (TBT), are well-known endocrine disruptors causing sex changes in marine organisms and widespread in coastal waters and sediments worldwide. In this study, a comprehensive ecological risk assessment (ERA) of organotins was conducted in Hong Kong waters through determining the imposex status, sex ratio and tissue burdens of these compounds in the neogastropods, Thais clavigera and Thais luteostoma collected from 29 coastal sites. We also investigated the historical trend of organotin effects on these gastropods, and performed a probabilistic ERA based on tissue burden of TBT in the animals. Our results demonstrated that imposex indices were positively correlated with the body burden of organotins in the gastropods. Across all sites, the sex ratio (female:male) decreased significantly with increasing imposex levels or tissue burden of organotins, implying that such pollutants can result in a male-biased population, potentially leading to local extinction in extreme cases. Based on the ERA, 5.4% of all populations of T. clavigera are at risk due to exposure to TBT; the risks include growth inhibition, impairment of immune functions and reduced fitness. Seriously impacted areas included Aberdeen, Repulse Bay, Butterfly Beach, Mui Wo and Ha Mei Wan. A comparison with historical data revealed that there had been some improvement in the areas with low marine traffic, and distant from the major harbour/port. This could partly be due to the restriction on the use of TBT on small vessels (<25m in length) since 1992. Nevertheless, the organotin contamination still remains severe in areas with high marine traffic or adjacent to large harbours/ports. In particular, the situation in the northeastern waters of Hong Kong has been getting worst since 1996 that is probably associated with the rapid development of the cargo container port at Yantian in China.

Endocrine disrupting chemicals: a new and emerging public health problem?

Coordination of targeted toxicological studies is needed