Retinoid X receptor activities of source waters in China and their removal efficiencies during drinking water treatment processes

The unexpected teratogenicity of RXR antagonist UVI3003 via activation of PPARγ in Xenopus tropicalis

The RXR agonist (triphenyltin, TPT) and the RXR antagonist (UVI3003) both show teratogenicity and, unexpectedly, induce similar malformations in Xenopus tropicalis embryos. In the present study, we exposed X. tropicalis embryos to UVI3003 in seven specific developmental windows and identified changes in gene expression. We further measured the ability of UVI3003 to activate Xenopus RXRα (xRXRα) and PPARγ (xPPARγ) in vitro and in vivo. We found that UVI3003 activated xPPARγ either in Cos7 cells (in vitro) or Xenopus embryos (in vivo). UVI3003 did not significantly activate human or mouse PPARγ in vitro; therefore, the activation of Xenopus PPARγ by UVI3003 is novel. The ability of UVI3003 to activate xPPARγ explains why UVI3003 and TPT yield similar phenotypes in Xenopus embryos. Our results indicate that activating PPARγ leads to teratogenic effects in Xenopus embryos. More generally, we infer that chemicals known to specifically modulate mammalian nuclear hormone receptors cannot be assumed to have the same activity in non-mammalian species, such as Xenopus. Rather they must be tested for activity and specificity on receptors of the species in question to avoid making inappropriate conclusions.

Chlorination by-products of bisphenol A enhanced retinoid X receptor disrupting effects

There is increasing evidence of activities of chlorinated by-products of bisphenol A (BPA) on retinoic acid system. Their agonistic and antagonistic activities to human retinoid X receptor (RXR) were assessed by a two-hybrid yeast assay. Aqueous solutions of 1mg/L BPA were chlorinated by sodium hypochlorite (NaClO). It showed that chlorination of BPA increased RXRβ antagonistic activity, while no agonistic activity was detected, showing chlorine might act as a toxic potentiator rather than a toxic deactivator in RXRβ disrupting effects. BPA and its byproducts including 2,2',6,6'-tetrachlorobisphenol A (TCBPA) and 2,4,6-trichlorophenol (TCP) were quantitatively determined by gas chromatography/mass spectrometry (GC/MS). BPA rapidly degraded. With the increasing of ICC and reaction time, concentration of formed TCBPA increased initially then decreased, while concentration of formed TCP increased stably. Using the toxic equivalent (TEQ) approach, the main contributors should be mono-, di- and tri- chlorobisphenol A at initial chlorine concentration (ICC) of 1mg/L. At ICC of 2 and 5mg/L, the main contributors were TCBPA and TCP, being 57.7%-70.7% and 45.3%-59.4%. Molecular docking showed BPA chlorination by-products might have the same mode of action with BPA, forming hydrogen bond and pi-pi interaction with their OH group or hydrophobic ring.

Evaluation of genotoxic effects of surface waters using a battery of bioassays indicating different mode of action

With the burgeoning contamination of surface waters threatening human health, the genotoxic effects of surface waters have received much attention. Because mutagenic and carcinogenic compounds in water cause tumors by different mechanisms, a battery of bioassays that each indicate a different mode of action (MOA) is required to evaluate the genotoxic effects of contaminants in water samples. In this study, 15 water samples from two source water reservoirs and surrounding rivers in Shijiazhuang city of China were evaluated for genotoxic effects. Target chemical analyses of 14 genotoxic pollutants were performed according to the Environmental quality standards for surface water of China. Then, the in vitro cytokinesis-block micronucleus (CBMN) assay, based on a high-content screening technique, was used to detect the effect of chromosome damage. The SOS/umu test using strain TA1535/pSK1002 was used to detect effects on SOS repair of gene expression. Additionally, two other strains, NM2009 and NM3009, which are highly sensitive to aromatic amines and nitroarenes, respectively, were used in the SOS/umu test to avoid false negative results. In the water samples, only two of the genotoxic chemicals listed in the water standards were detected in a few samples, with concentrations that were below water quality standards. However, positive results for the CBMN assay were observed in two river samples, and positive results for the induction of umuC gene expression in TA1535/pSK1002 were observed in seven river samples. Moreover, positive results were observed for NM2009 with S9 and NM3009 without S9 in some samples that had negative results using the strain TA1535/pSK1002. Based on the results with NM2009 and NM3009, some unknown or undetected aromatic amines and nitroarenes were likely in the source water reservoirs and the surrounding rivers. Furthermore, these compounds were most likely the causative pollutants for the genotoxic effect of these water samples. Therefore, to identify causative pollutants with harmful biological effects, chemical analyses for the pollutants listed in water quality standards is not sufficient, and single-endpoint bioassays may underestimate adverse effects. Thus, a battery of bioassays based on different MOAs is required for the comprehensive detection of harmful biological effects. In conclusion, for genotoxicity screening of surface waters, the SOS/umu test system by using different strains combined with the CBMN assay was a useful approach.

Occurrence of xenobiotic ligands for retinoid X receptors and thyroid hormone receptors in the aquatic environment of Taiwan

Various synthetic compounds are frequently discharged into the environment via human activities. Among them, certain contaminants may disrupt normal physiological functions of wildlife and humans via interactions with nuclear receptors. To protect human health and the environment, it is important to detect environmental ligands for human nuclear receptors. In this study, yeast-based reporter gene assays were used to investigate the occurrence of xenobiotic ligands for retinoid X receptors (RXR) and thyroid hormone receptors (TR) in the aquatic environment of Taiwan. Experimental results revealed that RXR agonist/antagonist activity was detected in river water and sediment samples. In particular, high RXR agonist/antagonist activity was found in the samples collected near river mouths. Additionally, few samples also elicited significant TR antagonist activity. Our findings show that the aquatic environment of Taiwan was contaminated with RXR and TR ligands. Further study is necessary to identify these xenobiotic RXR and TR agonists and antagonists.

Effects of antagonist of retinoid X receptor (UVI3003) on morphology and gene profile of Xenopus tropicalis embryos

We exposed Xenopus tropicalis embryos to a selective antagonist of retinoid X receptor (UVI3003). UVI3003 induced multiple malformations at the concentrations of 200-1000 μg/L after 48 h exposure. The most prominent malformations affected brains, eyes, cement gland and fins. UVI3003 also induced variable and divergent malformations at 250-1500 μg/L after 0-24 and 24-48 h exposure. Microarray analysis showed that seven genes (rps15, serp2, fmr1, cyp2e1, lrrc9, ugtla6 and LOC100490188) were differentially regulated in all three treatment groups after 0-24h exposure. The most significantly affected pathway was galactose metabolism. In 24-48 h exposure groups, 18 genes were differentially regulated, mainly comprising components of the PPAR signaling pathway. These results suggested that UVI3003 is teratogenic in amphibian embryos. Differential gene expression suggests that galactose metabolism and PPAR signaling pathways may provide underlying mechanistic detail accounting for the observed malformations.