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

Abstract

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 metabolismrelated 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.

Of Retinoids and Organotins: The Evolution of the Retinoid X Receptor in Metazoa

“…Our results demonstrated that the interaction of TBT with RXR is conserved among Metazoa, with the exception of Placozoa (T. adhaerens) ( Figure 6). Moreover, we found that the condition described in rotifers [40] is not unique, since the cnidarian A. aurita RXR lacks the key cysteine and was nonetheless activated by TBT. The bryozoan B. neritina RXRs, on the other hand, yielded puzzling results, as similar responses were found independently of the conservation status of the cysteine residue.…”

“…Furthermore, the evolutionary conservation of RXR-dependent modulation by organotins was suggested among lophotrochozoans, vertebrates, and crustaceans [27,33,65]. Recently, an RXR-dependent signaling pathway was suggested to promote the changes in the fatty acid profiles of the rotifer B. koreanus exposed to TBT [40]. The Organisation for Economic Co-operation and Development (OECD) suggests in vitro assays as a level 2 approach to test and assess endocrine disruption by environmental contaminants and denotes NRs as key players in this framework [17].…”

“…Once again, we found a stronger response for the brachiopod M. detruncata RXR when compared to the human RXRα, and weaker responses for the remaining receptors. Even without conservation of the previously described cysteine residue, suggested to be key for TBT binding and mediation of transactivation [42,56], the cnidarian A. aurita RXR and the bryozoan B. neritina RXR1 were able to promote gene transcription in the presence of TBT, similarly to the rotifer B. koreanus RXR [40]. Curiously, and despite the conservation of the key cysteine residue in the bryozoan B. neritina RXR2, both subtypes responded similarly to TBT (P > 0.05), indicating that TBT possibly interacts with the pocket environment of both receptors in a similar fashion.…”

“…Besides natural ligands, RXRs have also been shown to be exploited by endocrine-disrupting chemicals (EDCs), which are substances with the potential to alter endocrine functions causing physiological imbalances, leading to developmental, reproductive, and metabolic defects [38][39][40]. The organotins tributyltin (TBT) and triphenyltin (TPT) were extensively used as biocides in anti-fouling paints, being now persistent organic pollutants in aquatic environments [41].…”

Expression variations of two retinoid signaling pathway receptors in the rotifer Brachionus calyciflorus exposed to three endocrine disruptors

“…This assumption is corroborated by the fact that concomitant significant increase the relative mRNA levels of RXR and RAR, known to act as heterodimers (Germain et al 2003), have been observed after 24 h of exposure to flutamide or cyproterone acetate. Moreover, Bettin et al (1996) shown in gastropods that cyproterone acetate was able to inhibit imposex induction by TBT, known to act through RXR pathways (Abidli et al 2013;Lee et al 2019;Nishikawa et al 2004;Lima et al 2011;Stange et al 2012). Similar profiles of expression after 24 h of exposure could result from a change in the metabolism of the 9-cis-retinoid acid which can bind to both RAR and RXR.…”