Tributyltin (TBT) and dibutyltin (DBT) differently inhibit the mitochondrial Mg-ATPase activity in mussel digestive gland

Toxicity of tributyltin to the European flat oyster Ostrea edulis: Metabolomic responses indicate impacts to energy metabolism, biochemical composition and reproductive maturation

Tri-Butyl Tin (TBT) remains as a legacy pollutant in the benthic environments. Although the toxic impacts and endocrine disruption caused by TBT to gastropod molluscs have been established, the changes in energy reserves allocated to maintenance, growth, reproduction and survival of European oysters Ostrea edulis, a target species of concerted benthic habitat restoration projects, have not been explored. This study was designed to evaluate the effect of TBT chloride (TBTCl) on potential ions and relevant metabolomic pathways and its association with changes in physiological, biochemical and reproductive parameters in O. edulis exposed to environmental relevant concentrations of TBTCl. Oysters were exposed to TBTCl 20 ng/L (n = 30), 200 ng/L (n = 30) and 2000 ng/L (n = 30) for nine weeks. At the end of the exposure, gametogenic stage, sex, energy reserve content and metabolomic profiling analysis were conducted to elucidate the metabolic alterations that occur in individuals exposed to those compounds. Metabolite analysis showed significant changes in the digestive gland biochemistry in oysters exposed to TBTCl, decreasing tissue ATP concentrations through a combination of the disruption of the TCA cycle and other important molecular pathways involved in homeostasis, mitochondrial metabolism and antioxidant response. TBTCl exposure increased mortality and caused changes in the gametogenesis with cycle arrest in stages G0 and G1. Sex determination was affected by TBTCl exposure, increasing the proportion of oysters identified as males in O. edulis treated at 20ng/l TBTCl, and with an increased proportion of inactive stages in oysters treated with 2000 ng/l TBTCl. The presence and persistence of environmental pollutants, such as TBT, could represent an additional threat to the declining O. edulis populations and related taxa around the world, by increasing mortality, changing reproductive maturation, and disrupting metabolism. Our findings identify the need to consider additional factors (e.g. legacy pollution) when identifying coastal locations for shellfish restoration.

Assessing the Association of Mitochondrial Function and Inflammasome Activation in Murine Macrophages Exposed to Select Mitotoxic Tri-Organotin Compounds

BACKGROUND

Mitochondrial function is implicated as a target of environmental toxicants and found in disease or injury models, contributing to acute and chronic inflammation. One mechanism by which mitochondrial damage can propagate inflammation is via activation of the nucleotide-binding oligomerization domain (NOD)-like receptor (NLR) family, pyrin domain-containing receptor (NLRP)3 inflammasome, a protein complex that processes mature interleukin (IL)-1β. IL-1β plays an important role in the innate immune response and dysregulation is associated with autoinflammatory disorders.

OBJECTIVE

The objective was to evaluate whether mitochondrial toxicants recruit inflammasome activation and IL-1β processing.

METHOD

Murine macrophages (RAW 264.7) exposed to tri-organotins (triethyltin bromide (TETBr), trimethyltin hydroxide (TMTOH), triphenyltin hydroxide (TPTOH), bis(tributyltin)oxide) [Bis(TBT)Ox] were examined for pro-inflammatory cytokine induction. TMTOH and TETBr were examined in RAW 264.7 and bone marrow-derived macrophages for mitochondrial bioenergetics, reactive oxygen species (ROS) production, and inflammasome activation via visualization of aggregate formation, caspase-1 flow cytometry, IL-1β enzyme-linked immunosorbent assay and Western blots, and microRNA (miRNA) and mRNA arrays.

RESULTS

TETBr and TMTOH induced inflammasome aggregate formation and IL-1β release in lipopolysaccharide (LPS)-primed macrophages. Mitochondrial bioenergetics and mitochondrial ROS were suppressed. Il1a and Il1b induction with LPS or LPS+ATP challenge was diminished. Differential miRNA and mRNA profiles were observed. Lower miR-151-3p targeted cyclic adenosine monophosphate (cAMP)-mediated and AMP-activated protein kinase signaling pathways; higher miR-6909-5p, miR-7044-5p, and miR-7686-5p targeted Wnt beta-catenin signaling, retinoic acid receptor activation, apoptosis, signal transducer and activator of transcription 3, IL-22, IL-12, and IL-10 signaling. Functional enrichment analysis identified apoptosis and cell survival canonical pathways.

CONCLUSION

Select mitotoxic tri-organotins disrupted murine macrophage transcriptional response to LPS, yet triggered inflammasome activation. The differential response pattern suggested unique functional changes in the inflammatory response that may translate to suppressed host defense or prolong inflammation. We posit a framework to examine immune cell effects of environmental mitotoxic compounds for adverse health outcomes. https://doi.org/10.1289/EHP8314.

Transcriptomic effects of tributyltin (TBT) in zebrafish eleutheroembryos. A functional benchmark dose analysis

Exposure to the antifouling tributyltin (TBT) has been related to imposex in mollusks and to obesogenicity, adipogenesis and masculinization in fish. To understand the underlying molecular mechanisms, we evaluated dose-response effects of TBT (1.7-56 nM) in zebrafish eleutheroembryos transcriptome exposed from 2 to 5 days post-fertilization. RNA-sequencing analysis identified 3238 differentially expressed transcripts in eleutheroembryos exposed to TBT. Benchmark dose analyses (BMD) showed that the point of departure (PoD) for transcriptomic effects (9.28 nM) was similar to the metabolomic PoD (11.5 nM) and about one order of magnitude lower than the morphometric PoD (67.9 nM) or the median lethal concentration (LC₅₀: 93.6 nM). Functional analysis of BMD transcriptomic data identified steroid metabolism and cholesterol and vitamin D₃ biosynthesis as the most sensitive pathways to TBT (<50% PoD). Conversely, transcripts related to general stress and DNA damage became affected only at doses above the PoD. Therefore, our results indicate that transcriptomes can act as early molecular indicators of pollutant exposure, and illustrates their usefulness for the mechanistic identification of the initial toxic events. As the estimated molecular PoDs are close to environmental levels, we concluded that TBT may represent a substantial risk in some natural environments.

Dibutyltin alters immune cell production of the pro-inflammatory cytokines interleukin (IL) 1β and IL-6: role of mitogen-activated protein kinases and changes in mRNA

Dibutyltin (DBT) is used to stabilize plastics and as a deworming agent in some poultry. It is found in human blood (levels as high as 0.3 μM). Interleukin (IL) 1β (IL-1β) and IL-6 are pro-inflammatory cytokines produced by lymphocytes, monocytes, and other cells. Elevated levels of IL-1β and IL-6 have been associated with pathologies including rheumatoid arthritis and cancers. DBT was shown to decrease IL-1β and IL-6 secretion from immune cells at higher concentrations while causing increases at lower concentrations. However, it was not clear if these changes were due to DBT's alteration of the secretory process or due its ability to change cellular synthesis/production of these proteins. This study addresses this question, as well as mechanisms for any observed changes in synthesis/production. Monocyte-depleted peripheral blood mononuclear cells (MD-PBMCs) were exposed to DBT at concentrations of 5, 2.5, 1, 0.5, 0.25, 0.1, and 0.05 μM for 1, 6, and 24 h and the production (combination of secreted and intracellular levels from the same cells) of both IL-1β and IL-6 were measured. Effects of selected DBT exposures on cytokine production were also examined in PBMCs and DBT's effects were similar when monocytes were present. The 24-h exposures to DBT decreased production of both IL-1β and IL-6 at the two highest concentrations but increased production at lower concentrations. Both decreases and increases in cytokine production appear to be explained by DBT-induced changes in mRNA levels. DBT-induced increases in cellular production of the cytokines appear to require p38 and ERK1/2 MAPK pathways.

Mitochondrial Ca2+ -activated F1 FO -ATPase hydrolyzes ATP and promotes the permeability transition pore

The properties of the mitochondrial F₁ F_O -ATPase catalytic site, which can bind Mg²⁺ , Mn²⁺ , or Ca²⁺ and hydrolyze ATP, were explored by inhibition kinetic analyses to cast light on the Ca²⁺ -activated F₁ F_O -ATPase connection with the permeability transition pore (PTP) that initiates cascade events leading to cell death. While the natural cofactor Mg²⁺ activates the F₁ F_O -ATPase in competition with Mn²⁺ , Ca²⁺ is a noncompetitive inhibitor in the presence of Mg²⁺ . Selective F₁ inhibitors (Is-F₁ ), namely NBD-Cl, piceatannol, resveratrol, and quercetin, exerted different mechanisms (mixed and uncompetitive inhibition) on either Ca²⁺ - or Mg²⁺ -activated F₁ F_O -ATPase, consistent with the conclusion that the catalytic mechanism changes when Mg²⁺ is replaced by Ca²⁺ . In a partially purified F₁ domain preparation, Ca²⁺ -activated F₁ -ATPase maintained Is-F₁ sensitivity, and enzyme inhibition was accompanied by the maintenance of the mitochondrial calcium retention capacity and membrane potential. The data strengthen the structural relationship between Ca²⁺ -activated F₁ F_O -ATPase and the PTP, and, in turn, on consequences, such as physiopathological cellular changes.

Comparative transcriptomics analysis of the river pufferfish (Takifugu obscurus) by tributyltin exposure: Clues for revealing its toxic injury mechanism

TBT residual in water had become a noticeable ecological problem for aquatic ecosystems. The river pufferfish (Takifugu obscurus) is a kind of an anadromous fish species and widely distributed in the East China Sea and the Yellow Sea. Because of the water contamination, the pufferfish wild resource had a sudden decline in recent years. Therefore, the study on the response of pufferfish to the TBT exposure may contribute to reveal toxic injury mechanism of T. obscurus under TBT exposure. In this study, the transcriptional library of T. obscurus liver and gill was constructed and sequenced by an improved Illumina HiseqX10 high-throughput sequencing platform under different concentrations of TBT acute stress. The blood cell numbers distinctly decreased after TBT exposure, showing the adverse effects of TBT invasion and self-adjusting ability of the pufferfish. The production of reactive oxygen species increased, demonstrating the oxidation resistance of T. obscurus when exposed to TBT. The obtained data were compared with the genome data of Takifugu rubripes and transcriptional resource database. On this basis, gene function annotation, analysis and classification were carried out by bioinformatics method, and differential genes related to toxic injury function were screened out. Meanwhile, new toxic related genes and related signal pathways were sought to provide new theoretical guidance for the pathogenesis of T. obscurus exposed to TBT. This study not only enriched the transcriptome data of T. obscurus under environmental stress, but also provided a new research method for the response mechanism of T. obscurus under the stimulation of environmental factors.

The inhibition of the mitochondrial F1FO-ATPase activity when activated by Ca2+ opens new regulatory roles for NAD

The mitochondrial F1FO-ATPase is uncompetitively inhibited by NAD+ only when the natural cofactor Mg2+ is replaced by Ca2+, a mode putatively involved in cell death. The Ca2+-dependent F1FO-ATPase is also inhibited when NAD+ concentration in mitochondria is raised by acetoacetate. The enzyme inhibition by NAD+ cannot be ascribed to any de-ac(et)ylation or ADP-ribosylation by sirtuines, as it is not reversed by nicotinamide. Moreover, the addition of acetyl-CoA or palmitate, which would favor the enzyme ac(et)ylation, does not affect the F1FO-ATPase activity. Consistently, NAD+ may play a new role, not associated with redox and non-redox enzymatic reactions, in the Ca2+-dependent regulation of the F1FO-ATPase activity.

Estrogen-mediated protection of the organotin-degrading strain Metarhizium robertsii against oxidative stress promoted by monobutyltin

Dibutyltin (DBT) is a global pollutant characterized by pro-oxidative properties. The fungal strain Metarhizium robertsii can eliminate high levels of DBT efficiently. In this study, induction of oxidative stress as well as its alleviation through the application of natural estrogens during the elimination of DBT by M. robertsii were evaluated. During the first 24 h of incubation, the initial concentration of DBT (20 mg l^-1) was reduced to 3.1 mg l^-1, with simultaneous formation of a major byproduct - monobutyltin (MBT). In the presence of estrone (E1) or 17β-estradiol (E2), the amounts of dibutyltin residues in the fungal cultures were found to be approximately 2-fold higher compared to cultures without estrogens, which was associated with the simultaneous utilization of the compounds by cytochrome P450 enzymes. On the other hand, MBT levels were approximately 2.5 times lower in the fungal cultures with the addition of one of the estrogens. MBT (not DBT) promotes the generation of O₂^-, H₂O₂, and NO at levels 65.89 ± 18.08, 4.04 ± 3.62, and 27.92 ± 1.95, respectively. Superoxide dismutase and catalase activities did not show any response of the M. robertsii strain against the overproduction of superoxide anion and hydrogen peroxide. Application of E1 as well as E2 ensured non-enzymatic defense against nitrosative and oxidative stress through scavenging of nitrogen and oxygen reactive species, and limited their levels from 1.5-fold to 21-fold, depending on the used estrogen.

Post-translational modifications of the mitochondrial F1FO-ATPase

BACKGROUND

The mitochondrial F₁F_O-ATPase has the main role in synthesizing most of ATP, thus providing energy to living cells, but it also works in reverse and hydrolyzes ATP, depending on the transmembrane electrochemical gradient. Within the same complex the vital role of the enzyme of life coexists with that of molecular switch to trigger programmed cell death. The two-faced vital/lethal role makes the enzyme complex an intriguing biochemical target to fight pathogens resistant to traditional therapies and diseases linked to mitochondrial dysfunctions. A variety of post-translational modifications (PTMs) of selected F₁F_O-ATPase aminoacids have been reported to affect the enzyme function.

SCOPE OF REVIEW

By reviewing the known PTMs of aminoacid side chains of both F₁ and F_O sectors according to the most recent advances, the main aim is to highlight how local chemical changes may constitute the molecular key leading to pathological or physiological events.

MAJOR CONCLUSIONS

PTMs represent the chemical tool to modulate the F₁F_O-ATPase activity in response to different stimuli. Some PTMs are required to ensure the enzyme catalysis or, conversely, to inactivate the enzyme function. Each covalent modification of the F₁F_O-ATPase, which occur in response to local changes, is the result of a selective molecular mechanism which, by translating a chemical modification into a biochemical effect, guarantees the enzyme tuning under changing conditions.

GENERAL SIGNIFICANCE

Once highlighted how the molecular mechanism works, some PTMs may be exploited to modulate the effect of drugs targeting the enzyme complex or constitute promising tools for F₁F_O-ATPase-targeted therapeutic strategies.

Efficient dibutyltin (DBT) elimination by the microscopic fungus Metarhizium robertsii under conditions of intensive aeration and ascorbic acid supplementation

Dibutyltin (DBT) is an environmental pollutant characterized by immunotoxic, neurotoxic, and pro-oxidant properties. In this study, an attempt was made to enhance DBT elimination by the Metarhizium robertsii strain. We observed enhanced fungal growth in the bioreactor (pO₂ ≥ 20%) compared to flask cultures (μ _max increased from 0.061 to 0.086 h^-1). Moreover, under aerated conditions, M. robertsii mycelium with "hairy" morphology biodegraded DBT (20 mg l^-1) 10-fold faster in the bioreactor than in the flask cultures. Monobutyltin (MBT) and a hydroxylated derivative of MBT (OHBuSnH₂) were detected as by-products of dibutyltin debutylation. Simultaneous usage of glucose and butyltins indicates the comatabolic nature of monobutyltin and dibutyltin removal. In order to protect fungal cells from oxidative stress caused by DBT presence, vitamin C (20 mg l^-1) was applied. Supplementation with ascorbic acid (AA) resulted in a 3-fold acceleration of MBT removal during the first 7 h of incubation. Using the HPLC-MS/MS technique, a quantitative analysis of malondialdehyde (MDA), a marker of oxidative stress, was performed. In the AA presence, a decrease in the MDA amount (about 45%) was observed compared to the case with fungal cells exposed to DBT alone.

Investigation on the proteome response of transplanted blue mussel (Mytilus sp.) during a long term exposure experiment at differently impacted field stations in the German Bight (North Sea)

In a pilot field study the proteome response of Mytilus sp. was analyzed in relation to the concentration of different trace metal contaminants. Over a period of eight month test organisms have been exposed at a near-shore station in the anthropogenic impacted estuary of the river Elbe and at an off-shore station in the vicinity of the Island of Helgoland in the German Bight (North Sea). The stations differ in their hydrological as well as chemical characteristics. The physiological biomarkers, such as condition index which have been continuously monitored during the experiment clearly indicate the effects of the different environmental conditions. Multiple protein abundance changes were detected utilizing the techniques of two dimensional gel electrophoresis (2dGE) and consequently proteins arising as potential candidates for ecotoxicological monitoring have been identified by MALDI-ToF and ToF/ToF mass spectrometry. Different cytoskeletal proteins, enzymes of energy metabolism, stress proteins and one protein relevant for metal detoxification have been pointed out.

Tributyltin induces mitochondrial fission through NAD-IDH dependent mitofusin degradation in human embryonic carcinoma cells

Organotin compounds, such as tributyltin (TBT), are well-known endocrine disruptors. TBT acts at the nanomolar level through genomic pathways via the peroxisome proliferator activated receptor (PPAR)/retinoid X receptor (RXR). We recently reported that TBT inhibits cell growth and the ATP content in the human embryonic carcinoma cell line NT2/D1 via a non-genomic pathway involving NAD(+)-dependent isocitrate dehydrogenase (NAD-IDH), which metabolizes isocitrate to α-ketoglutarate. However, the molecular mechanisms by which NAD-IDH mediates TBT toxicity remain unclear. In the present study, we evaluated the effects of TBT on mitochondrial NAD-IDH and energy production. Staining with MitoTracker revealed that nanomolar TBT levels induced mitochondrial fragmentation. TBT also degraded the mitochondrial fusion proteins, mitofusins 1 and 2. Interestingly, apigenin, an inhibitor of NAD-IDH, mimicked the effects of TBT. Incubation with an α-ketoglutarate analogue partially recovered TBT-induced mitochondrial dysfunction, supporting the involvement of NAD-IDH. Our data suggest that nanomolar TBT levels impair mitochondrial quality control via NAD-IDH in NT2/D1 cells. Thus, mitochondrial function in embryonic cells could be used to assess cytotoxicity associated with metal exposure.

The obesogen tributyltin

The obesogen hypothesis postulates the role of environmental chemical pollutants that disrupt homeostatic controls and adaptive mechanisms to promote adipose-dependent weight gain leading to obesity and metabolic syndrome complications. One of the most direct molecular mechanisms for coupling environmental chemical exposures to perturbed physiology invokes pollutants mimicking endogenous endocrine hormones or bioactive dietary signaling metabolites that serve as nuclear receptor ligands. The organotin pollutant tributyltin can exert toxicity through multiple mechanisms but most recently has been shown to bind, activate, and mediate RXR-PPARγ transcriptional regulation central to lipid metabolism and adipocyte biology. Data in support of long-term obesogenic effects on whole body adipose tissue are also reported. Organotins represent an important model test system for evaluating the impact and epidemiological significance of chemical insults as contributing factors for obesity and human metabolic health.

Reproductive impacts of tributyltin (TBT) and triphenyltin (TPT) in the hermaphroditic freshwater gastropod Lymnaea stagnalis

Tributyltin (TBT) and triphenyltin (TPT) are emblematic endocrine disruptors, which have been mostly studied in gonochoric prosobranchs. Although both compounds can simultaneously occur in the environment, they have mainly been tested separately for their effects on snail reproduction. Because large discrepancies in experimental conditions occurred in these tests, the present study aimed to compare the relative toxicity of TBT and TPT under similar laboratory conditions in the range of 0 ng Sn/L to 600 ng Sn/L. Tests were performed on the simultaneous hermaphrodite Lymnaea stagnalis, a freshwater snail in which effects of TPT were unknown. Survival, shell length, and reproduction were monitored in a 21-d semistatic test. Frequency of abnormal eggs was assessed as an additional endpoint. Triphenyltin hampered survival while TBT did not. Major effects on shell solidity and reproduction were observed for both compounds, reproductive outputs being more severely hampered by TBT than by TPT. Considering the frequency of abnormal eggs allowed increasing test sensitivity, because snail responses to TBT could be detected at concentrations as low as 19 ng Sn/L. However, the putative mode of action of the 2 compounds could not be deduced from the structure of the molecules or from the response of apical endpoints. Sensitivity of L. stagnalis to TBT and TPT was compared with the sensitivity of prosobranch mollusks with different habitats and different reproductive strategies.

Structural and functional changes in gill mitochondrial membranes from the Mediterranean mussel Mytilus galloprovincialis exposed to tri-n-butyltin

The use of tributyltin (TBT) as a biocide in antifouling paints leads to a ruinous input of this contaminant in the aquatic environment. Human exposure to TBT mainly occurs through ingestion of contaminated seafood such as filter-feeding mollusks. Tributyltin is known to act as a membrane-active toxicant on several targets, but especially on the mitochondria, and by several mechanisms. The effects of tributyltin on fatty acid composition, on Mg-adenosine triphosphatase (ATPase) activities, and on the membrane physical state were investigated in gill mitochondrial membranes from cultivated mussels Mytilus galloprovincialis exposed to 0.5 µg/L and 1.0 µg/L TBT and unexposed for 120 h. The higher TBT exposure dose induced a decrease in the total and n-3 polyunsaturated fatty acids (PUFAs), especially 22:6 n-3, and an activation of the oligomycin-sensitive Mg-ATPase. Both TBT concentrations decreased mitochondrial membrane polarity detected by Laurdan steady-state fluorescence spectroscopy. These findings may help cast light on the multiple modes of action of this toxicant.