Toxicity of nonylphenol and nonylphenol ethoxylate on Caenorhabditis elegans
Graphical abstract
Introduction
The commercial formulations of most cleaning products, both liquid and solid, are generally constituted by a mixture of one or more surfactants that improves the detergent action and power. Nonylphenol (NP) is a xenobiotic used in the manufacture of antioxidants, lubricating oil additives and in the production of nonylphenol ethoxylate (U.S. Environmental Protection Agency, 1990), among which is the NP-9, commercially known as Tergitol. Nonylphenol ethoxylate is employed in different industries of detergents, textiles, agriculture products, emulsifiers, wetting agents, dispersants, decontaminants and solubilizes (Torres, 2012; Resnik et al., 2010). The product is synthesized adding a chain of epoxy groups to the NP structure, making it a more soluble compound in water. Upon NP-9 degradation, which usually occurs when the surfactant is discharged into water, NP is produced as a final product (Litwa et al., 2016). This last chemical is an emerging compound that has been found in sediments from different ecosystems around the world, including the Pearl River system, South China (0.03–21.9 μg/g dw) (Gong et al., 2011), Danube River, Germany (below LOQ-1.4 μg/g dw) (Grund et al., 2011); Llobregat basin, Spain (below LOQ-0.08 μg/g dw) (Brix et al., 2010); Minnesota lakes, USA (<0.1–0.1 μg/g dw) (Writer et al., 2010); and lakes of the subtropical China (3.5–32.4 μg/g dw) (Wu et al., 2007), among others. In surface waters, NP levels have been reported from below LOQ to 32.9 μg/L (Brix et al., 2010; Writer et al., 2010; Zhang et al., 2009; Wu et al., 2007); whereas in agricultural soils concentrations varied between 14.2 and 60.3 (ng/g dw) (Chen et al., 2011). The average daily intake of NP has been estimated at 0.5 μg/kg of body weight (Niu et al., 2015), and levels of 0.23–0.65 μg/kg are common in children (Raecker et al., 2011). Urine samples have also been registered containing at least 0.1 ng of NP/mL (Calafat et al., 2005), whereas in breast milk, levels may reach more than 30 ng of NP/mL (Ademollo et al., 2008).
Both NP and NP-9 are considered endocrine disrupting chemicals (Cha et al., 2017). It is emphasized that continuous exposure to NP through the intake of food or water can increase the toxicity of other EDCs (Zein et al., 2015), as well as a noticeable risk of obesity, allergy and breast cancer (Sprague et al., 2013). In general, NP-9 is considered biodegradable, but their final metabolite, NP and nonylphenol short-chain ethoxylates, are strongly toxic, persistent and potent endocrine disruptors (Wu et al., 2010; Kim et al., 2007; Vazquez-Duhalt et al., 2005).
It is known that NP enters the soil through different agricultural practices (Jiang et al., 2018); however, little is known about its toxicity on this environmental matrix. Although there are many soil organisms suitable for toxicity studies (Acevedo et al., 2018), Caenorhabditis elegans has been one of the most utilized (Oral et al., 2019; Kim et al., 2018; Höss et al., 2009) as its genome has been completely characterized and its physiology is highly sensitive to different environmental pollutants (Kim et al., 2018; García et al., 2018; Zhao et al., 2017; Tejeda et al., 2016; Anbalagan et al., 2013). Many of the management protocols for C. elegans are available online (Eisenmann, 2005), providing excellent information on this species, allowing the development of several effective, economic and solid toxicological tests (Tejeda and Olivero, 2016; Moya et al., 2015; Yu et al., 2013a; Anderson et al., 2001), this model ingests the contaminants through a buccal opening (small pumps), passes through the pharynx (two-lobed neuromuscular pump that crushes the food) and finally reaches the intestine, for digestion (Eisenmann, 2005). The use of C. elegans in environmental toxicology is extensive, and includes the evaluation of multiple stressors, including nanoplastic (Dong et al., 2018; Zhao et al., 2017), pesticides (Burchfield et al., 2018; García et al., 2018), particulate matter (Wang et al., 2019; Wu et al., 2017; Sun et al., 2015), among other pollutant found in soils (Fajardo et al., 2019; Rai et al., 2019), with a clear anthropogenic fingerprint (Khan et al., 2010).
A useful feature of C. elegans is the possibility to study biochemical responses and signaling pathways related to many types of environmental stress (Kronberg et al., 2018; Eisenmann, 2005). Toxicity mechanisms evaluated using this biological model include those involving stress-related genes, such as glutathione-S-transferase, glutathione-reductase, glutathione-peroxidase, thioredoxin-reductase, peroxiredoxin, heat shock, superoxide dismutase and metallothionein (Kronberg et al., 2018; García et al., 2018; Tejeda et al., 2016; Back et al., 2012). Expression for several of these genes is controlled by DAF-16 (Roh et al., 2018), the forkhead box O (FoxO) transcription factor that acts through the insulin/IGF-1 mediated signaling pathway. Under normal growth conditions DAF-16 is mainly found in the cytosol; however, when the cell is undergoing oxidative or thermal stress, it is transferred to the nucleus where it regulates the expression of a series of genes to increase stress resistance (Lu et al., 2018; Baumeister et al., 2006), fat metabolism, sexual development, innate immunity, and longevity-related processes (Koch et al., 2019; Lu et al., 2018; Bian et al., 2018; Tullet, 2015; Murphy, 2006). It has been demonstrated that the inactivation of the DAF-16/FoxO factors leads to an intracellular accumulation of ROS, promoting accelerated atherosclerosis, proliferation of transformed cells and the long-term proliferative potential of normal stem cells (Martins et al., 2016; Tsuchiya et al., 2013). The aim of this work was to determine the toxicity of NP and NP-9 in C. elegans, exploring the activation of different stress-related mechanisms that may be activated by these emergent pollutants.
Section snippets
Chemicals
The NP and NP-9 chemical structures are shown in Fig. 1.
The starting solution of NP 10,000 μM (Sigma-Aldrich, 99%) was prepared dissolving the compound in dimethylsulfoxide (DMSO, 99%). K medium (52 mM NaCl and 32 mM KCl in ultra-filtered water) was used to prepare the solutions of NP and NP-9 (Tergitol, Sigma-Aldrich, 97%).
Culture of Caenorhabditis elegans
The wild N2 strain of C. elegans was used in the evaluation of the physiological parameters such as lethality, growth and locomotion. Nematodes were maintained on K agar
Worm mortality
The results of the lethality tests after exposure to NP and NP-9, with their respective LC50, are shown in Fig. 2. In both cases, lethality following exposure to tested xenobiotics was dependent on the concentration. The presence of NP within exposed nematodes was verified employing Gas Chromatography-Mass Spectrometry (See Supplementary Material, Section 1). Nonylphenol toxicity was 1.85 orders or magnitude greater than that elicited by NP-9. The mean lethal concentration value for NP was
Discussion
Results presented here indicate NP and the commercial base NP-9 produce different toxicity in the biological model of Caenorhabditis elegans. These emerging contaminants decreased the survival rate, the number of movements of the nematodes and induced several stress-related response genes. In general, the activity observed for the different treatments presented a concentration-dependent response. However, in some cases the curve was not monotonous but biphasic or unimodal, typical behavior of
Conclusion
Nonylphenol exerts lethality on C. elegans at concentrations 1.8 orders or magnitude lower than those registered by NP-9. NP and NP-9 inhibited locomotion and growth in the nematode, processes that occurred at levels that induce oxidative stress and transcriptional responses mediated by the insulin/IGF-1 signaling pathway. The shapes of the dose-response curves involved in these biochemical effects were typical of endocrine disrupting chemicals. Taken together these data evidenced NP and NP-9
Acknowledgments
The authors wish to thank the Program for Doctoral Studies in Colombia (Colciencias 727. 2015); Dr. David De Pomerai at the University of Nottingham (United Kingdom) for supplying the transgenic strains of C. elegans; Dr. Joel Meyer at Duke University (USA) for providing the N2 wild-type strain; the Institutional Scholarship Program supported by the Universidad del Atlántico; the Program for International Mobility of Students endorsed by the University of Cartagena, 2017; and also to Eny Cerpa
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