Dibutyltin and tributyltin compounds induce thymus atrophy in rats due to a selective action on thymic lymphoblasts

Tributyltin inhibits autophagy by decreasing lysosomal acidity in SH-SY5Y cells

Tributyltin (TBT) is an environmental pollutant that remains in marine sediments and is toxic to mammals. For example, TBT elicits neurotoxic and immunosuppressive effects on rats. However, it is not entirely understood how TBT causes toxicity. Autophagy plays a pivotal role in protein quality control and eliminates aggregated proteins and damaged organelles. We previously reported that TBT dephosphorylates mammalian target of rapamycin (mTOR), which may be involved in enhancement of autophagosome synthesis, in primary cultures of cortical neurons. Autophagosomes can accumulate due to enhancement of autophagosome synthesis or inhibition of autophagic degradation, and we did not clarify whether TBT alters autophagic flux. Here, we investigated the mechanism by which TBT causes accumulation of autophagosomes in SH-SY5Y cells. TBT inhibited autophagy without affecting autophagosome-lysosome fusion before it caused cell death. TBT dramatically decreased the acidity of lysosomes without affecting lysosomal membrane integrity. TBT decreased the mature protein level of cathepsin B, and this may be related to the decrease in lysosomal acidity. These results suggest that TBT inhibits autophagic degradation by decreasing lysosomal acidity. Autophagy impairment may be involved in the mechanism underlying neuronal death and/or T-cell-dependent thymus atrophy induced by TBT.

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.

Simulated gastric hydrolysis and developmental toxicity of dibutyltin bis(2-ethylhexyl thioglycolate) in rats

Previously, dibutyltin dichloride (DBTC) was the putative toxophore for dibutyltin bis-alkyl and bis-thio esters. Recent chemical and toxicological data on dioctyltin bis(2-ethylhexyl thioglycolate) suggest the thioglycolate esters of alkyltins do not generate the dichloride toxophore. Our results, using ¹¹⁹ Sn-nuclear magnetic resonance (NMR) spectroscopy, demonstrated that dibutyltin bis(2-ethylhexyl thioglycolate) (DBTE) is hydrolyzed to dibutyltin chloro-(2-ethylhexyl thioglycolate) (DBTEC) under simulated gastric conditions. No DBTC was detected. DBTE was administered orally to presumed-pregnant Sprague-Dawley rats in a corn oil vehicle at 2.5, 8.5, and 25.0 mg/kg/day (Gestation Day 5 [GD5] through GD19). There were no maternal deaths, no treatment-related statistically significant reductions in feed consumption, maternal body weight or weight gain, or adverse gestational outcomes. Maternal thymus weight was significantly reduced in rats at 25 mg/kg. There were no effects on fetal growth, no dose-dependent pattern of external, visceral, or skeletal malformations, and no increase in anatomical variations. Based on the obtained experimental data, it is concluded here that DBTE forms DBTEC, not DBTC, in the stomach, and DBTE was not teratogenic nor fetotoxic in rats, a species sensitive to DBTC. The maternal no-observed-adverse-effect level (NOAEL) was 8.5 mg/kg/day, and the developmental NOAEL was 25 mg/kg/day, the high dose. The maternal LOAEL was 25 mg/kg/day based on reduced maternal thymus weight.

Environmental pollutants and the immune response

Environmental pollution is one of the most serious challenges to health in the modern world. Pollutants alter immune responses and can provoke immunotoxicity. In this Review, we summarize the major environmental pollutants that are attracting wide-ranging concern and the molecular basis underlying their effects on the immune system. Xenobiotic receptors, including the aryl hydrocarbon receptor (AHR), sense and respond to a subset of environmental pollutants by activating the expression of detoxification enzymes to protect the body. However, chronic activation of the AHR leads to immunotoxicity. KEAP1-NRF2 is another important system that protects the body against environmental pollutants. KEAP1 is a sensor protein that detects environmental pollutants, leading to activation of the transcription factor NRF2. NRF2 protects the body from immunotoxicity by inducing the expression of genes involved in detoxification, antioxidant and anti-inflammatory activities. Intervening in these sensor-response systems could protect the body from the devastating immunotoxicity that can be induced by environmental pollutants.

RIP1 and RIP3 contribute to Tributyltin-induced toxicity in vitro and in vivo

Tributyltin (TBT), a widely distributed environmental pollutant, is toxic to animals and human beings. Although its toxicity, especially the immunosuppressive effect, has been reported a lot, the underlying molecular mechanisms are still unclear. In this study, we investigated the mechanisms of TBT-induced cytotoxicity both in vitro and in vivo. TBT induced cell death in both J774A.1 macrophages and mouse bone marrow-derived macrophages (BMDMs) as measured by the LDH and Annexin V-FITC/PI dual staining assays. Pretreatment with RIP1 inhibitor Necrostatin-1 (Nec-1) or transfection with Rip1 siRNA significantly suppressed TBT-induced cytotoxicity in J774A.1 macrophages or human embryonic kidney cell line (HEK293 cells). TBT-induced cell death was also markedly inhibited in RIP3^-/- BMDMs. In agreement with in vitro results, TBT-induced in vivo immunotoxic effects including leukocyte depletion and thymus atrophy were significantly attenuated in RIP3^-/- mice or WT mice treated with Nec-1. Notably, the mortality rate induced by TBT was remarkably reduced in RIP3^-/- mice (100% vs. 12.5% lethality) or Nec-1-treated mice (100% vs. 59.2% lethality) respectively. These results reveal a critical role of RIP1 and RIP3 in TBT-induced toxicity both in vitro and in vivo.

Tributyltin oxide exposure impairs mouse oocyte maturation and its possible mechanisms

Tributyltin oxide (TBTO) has been widely used as marine antifouling composition, preservative, biocide, and a stabilizer in plastic industry. Previous studies have indicated that TBTO can cause immunotoxicity as an environmental pollutant. However, little is known about its reproductive toxicity, especially on female oocyte maturation and the underlying mechanisms. In this study, mouse oocytes were cultured with different concentrations of TBTO in vitro, and several crucial events during meiotic maturation were evaluated. We found that the first polar body extrusion rate was significantly reduced, which reflected the disruption of meiotic maturation. The rate of abnormal spindle organization increased significantly, accompanied with a higher rate of chromosome misalignment. In addition, TBTO treatment increased reactive oxygen species generation markedly, which also accelerated the early-stage apoptosis. Moreover, heterogeneous mitochondrial distribution, mitochondrial dysfunction, and higher rate of aneuploidy were detected, which consequently disrupted in vitro fertilization. In conclusion, our results indicated that TBTO exposure could impair mouse oocyte maturation by affecting spindle organization, chromosome alignment, mitochondria functions, oxidative stress, and apoptosis.

Synthesis of interleukin 1 beta and interleukin 6 in human lymphocytes is stimulated by tributyltin

Tributyltin (TBT) is a widespread environmental contaminant that is present in human blood and other tissues. It has been shown to disrupt the immune function of human natural killer (NK) cells and to alter the secretion of a number of pro-inflammatory cytokines from immune cells. Secretion of both interleukin 1β (IL-1β) and interleukin 6 (IL-6) from human lymphocytes can be increased dependent upon the level of TBT exposure. This study shows that the TBT-induced increases in secretion of both cytokines are due to TBT-induced increases in the synthesis of these proteins and not simply because of the release of pre-existing cytokine. Furthermore, the data indicate that these TBT-induced increases in IL-1β and IL-6 synthesis require MAP kinase signaling pathways. Additionally, elevated synthesis of IL-1β and IL-6 seen at the highest exposures to TBT (200, 200, 50 nM) were accompanied by increases in the mRNA for these cytokines. TBT-induced increases in IL-1β and IL-6 mRNAs were also shown to be dependent on MAP kinase signaling. The study suggests that TBT has the capacity to increase immune cell production of these 2 important pro-inflammatory cytokines and that this increase is in part explained by increased mRNA for the cytokines.

Butyltin compounds alter secretion of interleukin 6 from human immune cells

Butyltins (BTs), tributyltin (TBT) and dibutyltin (DBT) are organotin compounds that have been used in a variety of industrial applications; as a result, these compounds have been found in human blood. Interleukin (IL)-6 is a proinflammatory mediator that is produced by T lymphocytes and monocytes. It is responsible for immune response regulation as well as tissue repair and cellular growth. Both BTs decrease the ability of human natural killer cells to destroy tumor cells and alter the secretion of proinflammatory cytokines tumor necrosis factor alpha, interferon gamma and IL-1 beta (β) from human lymphocytes ex vivo. Here, we show that BTs alter the secretion of IL-6 from increasingly reconstituted preparations of human immune cells. IL-6 secretion was examined after 24 hour, 48 hour or 6 day exposures to TBT and DBT in highly enriched human natural killer cells, monocyte-depleted peripheral blood mononuclear cells (PBMCs), PBMCs, granulocytes and a preparation combining both PBMCs and granulocytes (PBMCs + granulocytes). The results indicated that both BTs altered IL-6 secretion from all cell preparations. Significant decreases of IL-6 secretion were seen at the highest concentration of TBT (200 nm) and DBT (5-2.5 μm) while the lower concentrations of DBT (0.05 and 0.1 μm) caused elevation of IL-6 secretion. The data indicate that BT-induced alterations of IL-6 secretion from immune cells may be a significant consequence of BT exposures that may potentially affect immune competence.

Dibutyltin-induced alterations of interleukin 1beta secretion from human immune cells

Dibutyltin (DBT) is used to stabilize polyvinyl chloride plastics (including pipes that distribute drinking water) and as a de-worming agent in poultry. DBT is found in human blood, and DBT exposures alter the secretion of tumor necrosis factor alpha and interferon gamma from lymphocytes. Interleukin (IL)-1β is a proinflammatory cytokine that regulates cellular growth, tissue restoration and immune response regulation. IL-1β plays a role in increasing invasiveness of certain tumors. This study reveals that exposures to DBT (24 h, 48 h and 6 days) modify the secretion of IL-1β from increasingly reconstituted preparations of human immune cells (highly enriched human natural killer cells, monocyte-depleted [MD] peripheral blood mononuclear cells [PBMCs], PBMCs, granulocytes and a preparation combining both PBMCs and granulocytes). DBT altered IL-1β secretion from all cell preparations. Higher concentrations of DBT (5 and 2.5 μm) decreased the secretion of IL-1β, while lower concentrations of DBT (0.1 and 0.05 μm) increased the secretion of IL-1β. Selected signaling pathways were examined in MD-PBMCs to determine if they play a role in DBT-induced elevations of IL-1β secretion. Pathways examined were IL-1β converting enzyme (caspase 1), mitogen-activated protein kinases and nuclear factor kappa B. Caspase 1 and mitogen-activated protein kinase pathways appear to be utilized by DBT in increasing IL-1β secretion. These results indicate that DBT alters IL-1β secretion from human immune cells in an ex. vivo system utilizing several IL-1β regulating signaling pathways. Thus, DBT may have the potential to alter IL-1β secretion in an in vivo system. Copyright © 2016 John Wiley & Sons, Ltd.

Exposure to tributyltin induces endoplasmic reticulum stress and the unfolded protein response in zebrafish

Tributyltin (TBT) is a major marine contaminant and causes endocrine disruption, hepatotoxicity, immunotoxicity, and neurotoxicity. However, the molecular mechanisms underlying the toxicity of TBT have not been fully elucidated. We examined whether exposure to TBT induces the endoplasmic reticulum (ER) stress response in zebrafish, a model organism. Zebrafish-derived BRF41 fibroblast cells were exposed to 0.5 or 1 μM TBT for 0.5-16 h and subsequently lysed and immunoblotted to detect ER stress-related proteins. Zebrafish embryos, grown until 32 h post fertilization (hpf), were exposed to 1 μM TBT for 16 h and used in whole mount in situ hybridization and immunohistochemistry to visualize the expression of ER chaperones and an ER stress-related apoptosis factor. Exposure of the BRF41 cells to TBT caused phosphorylation of the zebrafish homolog of protein kinase RNA-activated-like ER kinase (PERK), eukaryotic translation initiation factor 2 alpha (eIF2α), and inositol-requiring enzyme 1 (IRE1), characteristic splicing of X-box binding protein 1 (XBP1) mRNA, and enhanced expression of activating transcription factor 4 (ATF4) protein. In TBT-exposed zebrafish embryos, ectopic expression of the gene encoding zebrafish homolog of the 78 kDa glucose-regulating protein (GRP78) and gene encoding CCAAT/enhancer-binding protein homologous protein (CHOP) was detected in the precursors of the neuromast, which is a sensory organ for detecting water flow and vibration. Our in vitro and in vivo studies revealed that exposure of zebrafish to TBT induces the ER stress response via activation of both the PERK-eIF2α and IRE1-XBP1 pathways of the unfolded protein response (UPR) in an organ-specific manner.

Toxicogenomics-based identification of mechanisms for direct immunotoxicity

Compounds with direct immunotoxic properties, including metals, mycotoxins, agricultural pesticides, and industrial chemicals, form potential human health risks due to exposure through food, drinking water, and the environment. Insights into the mechanisms of action are currently lacking for the majority of these direct immunotoxicants. Therefore, the present work aimed to gain insights into the molecular mechanisms underlying direct immunotoxicity. To this end, we assessed in vitro the effects of 31 test compounds on the transcriptome of the human Jurkat T-cell line. These compounds included direct immunotoxicants, immunosuppressive drugs with different mode of actions, and nonimmunotoxic control chemicals. Pathway analysis of the microarray data allowed us to identify canonical pathways and Gene Ontology processes that were transcriptionally regulated in common by immunotoxicants (1) with structural similarities, such as tributyltin chloride and tributyltin oxide that activated the retinoic acid/X receptor signaling pathway and (2) without structural similarities, such as As2O3, dibutyltin chloride, diazinon, MeHg, ochratoxin A (OTA), S9-treated OTA, S9-treated cyclophosphamide, and S9-treated benzo[a]pyrene, which activated unfolded protein response, and FTY720, lindane, and propanil, which activated the cholesterol biosynthesis pathway. In addition, processes uniquely affected by individual immunotoxicants were identified, such as the induction of Notch receptor signaling and the downregulation of acute-phase response genes by OTA. These findings were validated by quantitative real-time PCR analysis of genes involved in these processes. Our study indicated that diverse modes of action are involved in direct immunotoxicity and that a set of pathways or genes, rather than one single gene, can be used to screen compounds for direct immunotoxicity.

Molecular mechanisms of environmental organotin toxicity in mammals

Organotins such as tributyltin are suspected of having multiple toxic effects in mammals, in addition to their endocrine-disrupting function. Endogenous organotin concentrations in human blood range from a few to a few hundred nM. In this review, we summarize recent findings on the mechanisms of toxicity of environmental organotins such as tributyltin (TBT) and triphenyltin (TPT) in mammals. TBT and TPT are potent inhibitors of mitochondrial ATP synthase, and a recent study suggests that TBT binds directly to ATP synthase. Organotins disturb steroid biosynthesis and degradation. TBT and TPT are dual agonists of retinoid X receptor (RXR) and peroxisome proliferator-activated receptor γ (PPARγ); they also induce the differentiation of adipocytes in vitro and in vivo, probably through PPARγ activation, suggesting that they may work as obesogens. Environmental organotins are also neurotoxic; they induce behavioral abnormality and are toxic to the developing central nervous system. In vitro studies have shown that organotins induce intracellular Ca(2+) elevation and glutamate excitotoxicity. Recently, it was reported that endogenous levels of TBT decrease expression of 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid (AMPA) receptor subunit GluR2, leading to neuronal vulnerability. Most of the experimental studies have employed organotins at concentrations of µM order, and it remains important to clarify the molecular mechanisms of events induced by endogenous levels of environmental organotins.

Assessment of the usefulness of the murine cytotoxic T cell line CTLL-2 for immunotoxicity screening by transcriptomics

A toxicogenomics approach was applied to assess the usefulness of the mouse cytotoxic T cell line CTLL-2 for in vitro immunotoxicity testing. CTLL-2 cells were exposed for 6 h to two model immunotoxic compounds: (1) the mycotoxin deoxynivalenol (DON, 1 and 2 μM), a ribotoxic stress inducer, and (2) the organotin compound tributyltin oxide (TBTO, 100 and 200 nM), an endoplasmic reticulum (ER) stress inducer. Effects on whole-genome mRNA expression were assessed by microarray analysis. The biological interpretation of the microarray data indicated that TBTO (200 nM) induced genes involved in T cell activation, ER stress, NFκB activation and apoptosis, which agreed very well with results obtained before on TBTO exposed Jurkat cells and mouse primary thymocytes. Remarkably, DON (2 μM) downregulated genes involved in T cell activation, ER stress and apoptosis, which is opposite to results obtained before for DON-exposed Jurkat cells and mouse primary thymocytes. Furthermore, the results for DON in CTLL-2 cells are also opposite to the results obtained for TBTO in CTLL-2 cells. In agreement with the lack of induction of ER stress and apoptosis, viability assays showed that CTLL-2 cells are much more resistant to the toxicity of DON than Jurkat cells and primary thymocytes. We propose that CTLL-2 cells lack the signal transduction that induces ER stress and apoptosis in response to ribotoxic stress. Based on the results for TBTO and DON, the CTLL-2 cell line does not yield an added value for immunotoxicity compared to the human Jurkat T cell line.

Pesticide induced immunotoxicity in humans: a comprehensive review of the existing evidence

The immune system can be the target of many chemicals, with potentially severe adverse effects on the host's health. In Western countries pesticides, together with new and modified patterns of exposure to chemicals, have been implicated in the increasing prevalence of diseases associated with alterations of the immune response, such as hypersensitivity reactions, certain autoimmune diseases and cancers. Xenobiotics may initiate, facilitate or exacerbate pathological immune processes, resulting in immunotoxicity by induction of mutations in genes coding for immunoregulatory factors, modifying immune tolerance and activation pathways. The purpose of this article is to update the evidence of pesticide immunotoxicity. Even if experimental data as well as sporadic human studies indicate that some pesticides can affect the immune system, overall, existing epidemiological studies are inadequate to raise conclusions on the immunotoxic risk associated to pesticide exposure. The available studies on the effects of pesticides on human immune system have several limitations including poor indication on exposure levels, multiple chemical exposures, heterogeneity of the approach, and difficulty in giving a prognostic significance to the slight changes often observed. Further studies are necessary, and they should be preferably carried out through comparison of pre and post-exposure findings in the same group of subjects with a matched control group. Attempt should be made to define the prognostic significance of slight changes often observed. Animal and in vitro studies are also important and necessary to scientifically support epidemiological evidences on pesticide-induced immunotoxicity.

Tributyltin (TBT) increases TNFα mRNA expression and induces apoptosis in the murine macrophage cell line in vitro

OBJECTIVE

Tributyltin (TBT) compounds have been widely used as antifouling agents for shipbottom paint. The immune system is a target of TBT intoxication. We evaluated the effects of TBT chloride in macrophages, which have critical roles in the immune system, using a murine macrophage lineage cell line, J774.1,in vitro.

METHODS

We examined tumor necrosis factor α (TNFα), interleukin-1β (IL-1β) andc-jun mRNA expression in J774.1 cells. The effects of TBT on the apoptosis of J774.1 cells were examined by determining the percentage of TUNEL-positive cells and caspase-3 activity.

RESULTS

The mean values of the viabilities of J774.1 cells exposed to TBT decreased dose-dependently. The relative mRNA expression of TNFα increased dose-dependently, however, that of IL-1β was not significantly different among the groups. The mean percentage of TUNEL-positive cells increased dose-dependently. Increases in the caspase-3 activities of J774.1 cells were observed in the groups exposed to higher concentrations of TBT. The mean value of relative mRNA expression of c-Jun transcription factor increased dose-dependently.

CONCLUSIONS

The increases in the percentage of TUNEL-positive cells and in caspase-3 activity suggested that exposure to TBT induces apoptosis of J774.1 cells. The increases in the mRNA expression of TNFα andc-jun by TBT may be related to apoptosis in macrophages.

Transcriptomics analysis of primary mouse thymocytes exposed to bis(tri-n-butyltin)dioxide (TBTO)

The biocide bis(tri-n-butyltin)oxide (TBTO) causes thymus atrophy in rodents and is toxic to many cell types of which thymocytes are the most sensitive. To obtain insight in the mechanisms of action of TBTO, we exposed primary mouse thymocytes in vitro for 3, 6 and 11 h to 0.1, 0.5, 1 and 2 μM TBTO. Subsequently, the cells were subjected to whole-genome gene expression profiling. Biological interpretation of the gene expression data revealed that TBTO affects a wide range of processes. Cell proliferation related genes were downregulated by all treatments except for 3 and 6 h 0.5 μM TBTO which upregulated these genes. Treatment with TBTO resulted in upregulation of genes involved in endoplasmatic reticulum (ER) stress, NFkB and TNFα pathways, and genes involved in DNA damage, p53 signaling and apoptosis. Remarkably, TBTO also increased the expression of genes that are known to be upregulated during T cell activation or during negative selection of thymocytes. The effect of TBTO on expression of genes involved in ER stress and apoptosis was confirmed by qPCR. Induction of the T cell activation response was corroborated by demonstrating that TBTO exposure resulted in translocation of NFAT to the nucleus, which is an essential event for T cell activation.

Caspase-10 is the key initiator caspase involved in tributyltin-mediated apoptosis in human immune cells

Tributyltin (TBT) is one of the most toxic compounds produced by man and distributed in the environment. A multitude of toxic activities have been described, for example, immunotoxic, neurotoxic, and endocrine disruptive effects. Moreover, it has been shown for many cell types that they undergo apoptosis after treatment with TBT and the cell death of immune cells could be the molecular background of its immunotoxic effect. As low as 200 nM up to 1 μM of TBT induces all signs of apoptosis in Jurkat T cells within 1 to 24 hrs of treatment. When compared to Fas-ligand control stimulation, the same sequence of events occurs: membrane blebbing, phosphatidylserine externalisation, the activation of the “death-inducing signalling complex,” and the following sequence of cleavage processes. In genetically modified caspase-8-deficient Jurkat cells, the apoptotic effects are only slightly reduced, whereas, in FADD-negative Jurkat cells, the TBT effect is significantly diminished. We could show that caspase-10 is recruited by the TRAIL-R2 receptor and apoptosis is totally prevented when caspase-10 is specifically inhibited in all three cell lines.

C-Phycocyanin: an effective protective agent against thymic atrophy by tributyltin

Spirulina platensis, used worldwide as a food supplement, is a natural source of protein, vitamins, carbohydrates and polyunsaturated fatty acids. C-Phycocyanin (C-Pc), its major biliprotein, is known to possess anti-oxidant, anti-inflammatory and radical scavenging properties. Our present study showed that treatment with C-Pc protects the rats from Tributyltin (TBT) induced thymic atrophy. The results reveal TBT-induced oxidative stress mediated apoptosis in rat thymocytes in vivo and its attenuation by C-Pc. This ameliorative effect could be attributed to antioxidant activity of the biliprotein. C-Pc also increased TBTC reduced thymic weight and cellularity as well. TBTC-induced ROS generation and lowered GSH levels were restored by C-Pc, suggesting its radical scavenging properties. The various apoptotic determinants such as mitochondrial membrane potential, Bax/Bcl-2 ratio, caspase-3 activity and apoptotic cell population were effectively modulated by C-Pc treatment. We make this first observation to illustrate the effectiveness of C-Pc in reducing TBTC-induced thymic atrophy. The morphology of thymic tissue was restored to near normal by this biliprotein. The present study, therefore, suggests that C-Pc could serve as an effective natural antioxidant for efficient management of TBTC induced oxidative damage.

Selective inhibition of B lymphocytes in TBTC-treated human bone marrow long-term culture

Tributyltin chloride (TBTC) is well known for its immunotoxic effect, in particular towards immature thymocytes. TBTC is also known to induce adipocyte differentiation in primary human bone marrow cultures, which is reflected in the decrease in a number of adipocyte-derived cytokines, chemokines and the adipocyte-linked hormone leptin. Since adipocytes influence haematopoiesis and lymphopoiesis for instance by these cytokines and hormones, we here investigated whether TBTC has an effect on specific lymphocyte subsets in human bone marrow primary cultures. FACS analysis showed a reduction of CD19/CD22-positive B cells by TBTC, both in the presence or absence of cytokines. The treatment did not cause a toxic effect on mature CD3+CD4+ and CD3+CD8+ T cells, suggesting selective TBTC toxicity on B lymphocytes in the presently used in vitro system.

The unexpected sources of organotin contamination in aquatic toxicological laboratory studies

Unaccounted sources of contamination can be problematic in toxicological studies and can range from the presence of impurities, breakdown products or isoforms of the parent compound to the unexpected compounds leaching from dosing apparatus. As these compounds are not being tested, they may not be measured in the dosed aquaria and hence go undetected, potentially contributing as confounding factors in toxicological assessments. In this paper we report the unexpected detection of butyltin compounds (mono, di and tributyltin) in flow-through aquaria waters of an aquatic toxicological set-up. High and variable leaching rates for dibutyltin of 2.0 and 6.6 microg/h were detected during the first week of each of two separate flow-through studies. Following this initial 'surge' of dibutyltin leachate, a decrease in leachate rate was seen with values of 0.9 and 1.2 microg/h by Day 14 (second week of study). The main source of the butyltin leachates was shown, to be the airline tubing used in the assembly of the air-supply into each flow-through tank. A 24h period of incubation of the airline tubing with clean water led to the leaching of concentrations of 63.8 ng/l TBT-Sn, 1638.8 ng/l DBT-Sn and 4054.6 ng/l MBT-Sn. The concentration of tributyltin detected was within its toxicologically effective range and as such could have potentially confounding effects on the toxicological bioassays being used. These accidental findings could be of enormous relevance to aquatic toxicologists and have an important bearing on the choice of materials used to construct experimental exposure aquaria.

Analysis of mechanisms of cell death of T-lymphocytes induced by organotin agents

Organotin compounds are known to cause thymic atrophy and an accompanying deficiency of cell-mediated immunity. The study reported here focused on cell death in the thymus as a contributing factor in the induction of thymic atrophy following exposure to dibutyltin (DBTC) and tributyltin (TBTC). In an in vivo study, a reversible thymic atrophy was induced in rats by a single intraperitoneal administration (2.0 mg/kg) of DBTC or TBTC; the magnitude of this effect over a 4-d post-treatment period differed between the two agents. In in vitro studies, T-lymphocytes were isolated from thymuses of naïve rats and then exposed to 1 microM DBTC or TBTC for varying periods of time. Analysis by flow cytometry showed that DBTC induced primarily necrosis, while TBTC induced apoptosis, of the cells. Activities of caspase-8, -9, and -3 were also measured; TBTC exposure caused marked increases in the activities, while DBTC exposure did not cause any significant change. TBTC exposure also appeared to induce expression of CAD (which fragments DNA), but had minimal effect on levels of the CAD inhibitor, ICAD. In contrast, DBTC exposure resulted in a larger level of ICAD expression. WST-8 and JC-1 assays were used to evaluate mitochondrial function, since a strong activation of caspase-9 by TBTC suggested mitochondrial involvement. The involvement of caspase in the activation was examined using cytochrome c expression; cytochrome expression and the loss of mitochondrial function occurred within 10 min of TBTC exposure. DBTC exposure affected the mitochondria less. These results indicated that effects on mitochondria likely played an important role in the induction of apoptosis by TBTC. The results of this study show that DBTC and TBTC induce necrosis and apoptosis of T-lymphocytes, respectively, by apparently indicating different mechanisms of cell death. It follows that these increases in cell death induced by these organotin compounds likely contributed to the thymic atrophy observed in the rats here.

Reduction in peripheral lymphocytes and thymus atrophy induced by organotin compounds in vivo

To clarify the involvement of apoptosis in the immunotoxicity of organotin compounds, we examined the induction of apoptosis in the peripheral lymphocytes and thymus of mice treated with triphenyltin (TPT), tributyltin (TBT) or dexamethasone (Dex). Application of TPT or TBT and Dex resulted in a transient reduction in peripheral lymphocytes at 3 to 6 hr, and thymus atrophy was observed at 6 and 24 hr after administration. Lymphocyte subpopulation analysis showed that TPT and TBT induced a greater reduction in B cells than in T cells. The maximum levels of organotin in the blood were about 450 ng TPT/ml in the TPT-treated mice, and 170 ng TBT/ml in the TBT-treated mice. When the isolated peripheral lymphocytes were incubated with the organotins at 500 ng/ml, TPT and TBT induced necrosis in over 70% of cells, while both organotins caused lower percentages of apoptosis as well as necrosis after 3 hr at 100 ng/ml. In the thymus, although in vivo treatment of mice with Dex caused apoptosis, neither apoptotic nor necrotic thymocytes were observed in the TPT- and TBT-treated mice, indicating that the thymus atrophy might be caused by the antiproliferative effects of these organotin compounds. Thus, our results did not support the idea that apoptosis played a decisive part in the immunotoxicity of the organotin compounds in vivo.

Gene expression profiling of Bis(tri-n-butyltin)oxide (TBTO)-induced immunotoxicity in mice and rats

Bis(tri-n-butyltin)oxide (TBTO) is one of the organotin compounds that have been used as biocides and occur as persistent environmental pollutants. Human exposure to these compounds occurs through consumption of meat and fish products in which they accumulate. The most sensitive endpoint of TBTO exposure is immunotoxicity. TBTO causes thymus atrophy and thereby interferes with T-lymphocyte-mediated immune responses. Tributyltin compounds have been found to adversely affect a wide range of cellular components and processes in many species, organ systems, and cell types. Both inhibition of proliferation and induction of apoptosis have been observed in thymocytes. We conducted microarray experiments in mice and rats in order to investigate if the immunosuppressive actions of TBTO could be detected by gene expression profiling, and if so, to elucidate the mechanisms of action. Gene expression changes that were detected in mouse thymuses after exposure to a maximum tolerable dose of TBTO correlated to previously observed effects. Most notably, reduction of expression of cell surface determinants and T-cell receptor chains, suppression of cell proliferation, and a possible involvement of nuclear receptors in interference with lipid metabolism by TBTO were observed. The TBTO-induced thymus involution may therefore primarily be caused by inhibition of thymocyte proliferation. In contrast, in rats only limited effects of a lower dose of TBTO were found at the gene expression level in the thymus, even though thymus involution was observed. Here, most gene expression regulation by TBTO was detected in the liver. These preliminary results indicate that gene expression analysis is able to reveal effects of TBTO and to gain insight into its molecular mechanism of action. It may even be a suitable tool to investigate immunotoxicology in general. However, dose and inter-species differences are apparently clearly reflected in the gene expression profiles.

Dibutyltin disrupts glucocorticoid receptor function and impairs glucocorticoid-induced suppression of cytokine production

Background

Organotins are highly toxic and widely distributed environmental chemicals. Dibutyltin (DBT) is used as stabilizer in the production of polyvinyl chloride plastics, and it is also the major metabolite formed from tributyltin (TBT) in vivo. DBT is immunotoxic, however, the responsible targets remain to be defined. Due to the importance of glucocorticoids in immune-modulation, we investigated whether DBT could interfere with glucocorticoid receptor (GR) function.

Methodology

We used HEK-293 cells transiently transfected with human GR as well as rat H4IIE hepatoma cells and native human macrophages and human THP-1 macrophages expressing endogenous receptor to study organotin effects on GR function. Docking of organotins was used to investigate the binding mechanism.

Principal Findings

We found that nanomolar concentrations of DBT, but not other organotins tested, inhibit ligand binding to GR and its transcriptional activity. Docking analysis indicated that DBT inhibits GR activation allosterically by inserting into a site close to the steroid-binding pocket, which disrupts a key interaction between the A-ring of the glucocorticoid and the GR. DBT inhibited glucocorticoid-induced expression of phosphoenolpyruvate carboxykinase (PEPCK) and tyrosine-aminotransferase (TAT) and abolished the glucocorticoid-mediated transrepression of TNF-α-induced NF-κB activity. Moreover, DBT abrogated the glucocorticoid-mediated suppression of interleukin-6 (IL-6) and TNF-α production in lipopolysaccharide (LPS)-stimulated native human macrophages and human THP-1 macrophages.

Conclusions

DBT inhibits ligand binding to GR and subsequent activation of the receptor. By blocking GR activation, DBT may disturb metabolic functions and modulation of the immune system, providing an explanation for some of the toxic effects of this organotin.

TBTC induces adipocyte differentiation in human bone marrow long term culture

Organotins are widely used in agriculture and the chemical industry, causing persistent and widespread pollution. Organotins may affect the brain, liver and immune system and eventually human health. Recently, it has been shown that tri-butyltin (TBT) interacts with nuclear receptors PPAR gamma (peroxisome proliferator-activated receptor gamma) and RXR (retinoid x receptor) leading to adipocyte differentiation in the 3T3 cell line. Since adipocytes are known to influence haematopoiesis, for instance through the expression of cytokines and adhesion molecules, it was considered of interest to further study the adipocyte-stimulating effect of TBTC in human bone marrow cultures. Nile Red spectrofluorimetric analysis showed a significant increase of adipocytes in TBTC-treated cultures after 14 days of long term culture. Real-time PCR and Western blot analysis confirmed the high expression of the specific adipocyte differentiation marker aP2 (adipocyte-specific fatty acid binding protein). PPAR gamma, but not RXR, mRNA was increased after 24 h and 48 h exposure. TBTC also induced a decrease in a number of chemokines, interleukins, and growth factors. Also the expression of leptin, a hormone involved in haematopoiesis, was down regulated by TBTC treatment. It therefore appears that TBTC induced adipocyte differentiation, whilst reducing a number of haematopoietic factors. This study indicates that TBTC may interfere in the haematopoietic process through an alteration of the stromal layer and cytokine homeostasis.

In vitro immunotoxicity of bis(tri-n-butyltin)oxide (TBTO) studied by toxicogenomics

The biocide and environmental pollutant bis(tri-n-butyltin)oxide (TBTO) causes thymus atrophy in rodents. Whether the depletion of thymic lymphocytes by tributyltin compounds may be the result of inhibition of cell proliferation or induction of apoptosis is subject of debate. We examined gene expression profiles in primary rat thymocytes exposed to TBTO in vitro at dose levels of 0, 0.1, 0.3, 0.5, and 1.0microM. By measuring cell viability and apoptosis, exposure conditions were selected that would provide information on changes in gene expression preceding or accompanying functional effects of TBTO. Several processes related to TBTO-induced toxicity were detected at the transcriptome level. Effects on lipid metabolisms appeared to be the first indication of disruption of cellular function. Many transcriptional effects of TBTO at higher dose levels were related to apoptotic processes, which corresponded to present or subsequent thymocyte apoptosis observed phenotypically. The gene expression profile was, however, not unambiguous since expression of apoptosis-related genes was both increased and decreased. Stimulation of glucocorticoid receptor signaling appeared to be a relevant underlying mechanism of action. These findings suggest that TBTO exerts its toxic effects on the thymus primarily by affecting apoptotic processes, but the possibility is discussed that this may in fact represent an early effect that precedes inhibition of cell proliferation. At the highest dose level tested, TBTO additionally repressed mitochondrial function and immune cell activation. Our in vitro toxicogenomics approach thus identified several cellular and molecular targets of TBTO that may mediate the toxicity towards thymocytes and thereby its immunosuppressive effects.

Disruption of glucocorticoid action by environmental chemicals: potential mechanisms and relevance

Glucocorticoids play an essential role in the regulation of key physiological processes, including immunomodulation, brain function, energy metabolism, electrolyte balance and blood pressure. Exposure to naturally occurring compounds or industrial chemicals that impair glucocorticoid action may contribute to the increasing incidence of cognitive deficits, immune disorders and metabolic diseases. Potentially, "glucocorticoid disruptors" can interfere with various steps of hormone action, e.g. hormone synthesis, binding to plasma proteins, delivery to target cells, pre-receptor regulation of the ratio of active versus inactive hormones, glucocorticoid receptor (GR) function, or export and degradation of glucocorticoids. Several recent studies indicate that such chemicals exist and that some of them can cause multiple toxic effects by interfering with different steps of hormone action. For example, increasing evidence suggests that organotins disturb glucocorticoid action by altering the function of factors that regulate the expression of 11beta-hydroxysteroid dehydrogenase (11beta-HSD) pre-receptor enzymes, by direct inhibition of 11beta-HSD2-dependent inactivation of glucocorticoids, and by blocking GR activation. These observations emphasize on the complexity of the toxic effects caused by such compounds and on the need of suitable test systems to assess their effects on each relevant step.

Tri-n-butyltin-induced blockade of store-operated calcium influx in rat thymocytes

Tri-n-butyltin (TBT), one of environmental pollutants, disturbs intracellular Ca(2+) homeostasis by increasing intracellular Ca(2+) concentration ([Ca(2+)]i). Effect of TBT on oscillatory change in [Ca(2+)]i (Ca(2+) oscillation) of rat thymocytes was examined using a laser microscope with fluo-3-AM in order to further elucidate the TBT toxicity related to intracellular Ca(2+). The Ca(2+) oscillation was completely attenuated by 300nM TBT. Since store-operated Ca(2+) channels are involved in the generation of Ca(2+) oscillation, the action of TBT on an increase in [Ca(2+)]i by Ca(2+) influx through store-operated Ca(2+) channels was examined. The increase in [Ca(2+)]i by the store-operated Ca(2+) influx was not affected by 3nM TBT. However, TBT at 10nM or more significantly reduced the increase in [Ca(2+)]i. It is likely that TBT attenuates the Ca(2+) oscillation by reducing the Ca(2+) influx through store-operated Ca(2+) channels.

Acute toxic effects of dioctyltin on immune system of rats

In the present study, dioctyltin chloride (DOTC: 100mg/kg, BW) was orally administered to immature (30-day-old) male rats, and the acute toxic effects were studied. Di- and monooctyltin (its metabolite) accumulations were mainly detected in the liver, and peaked 48h later. A similar pattern was also found in the kidney, but the levels were low or trace amounts. Significantly low thymus and spleen weights were detected in DOTC-treated animals. Increased apoptotic cell numbers in the thymus and spleen were observed in DOTC-treated animals also. Although the expression of 97 genes involved in apoptosis was studied in the thymus, at least 24h after treatment, we could not detect clearly different expressions between DOTC- and vehicle-treated animals. The present results suggest that DOTC was selectively immunotoxic. One of the mechanisms for its immunotoxicity would be via its stimulation of immune cell apoptosis.

The immune system as a target for environmental chemicals: Xenoestrogens and other compounds

The immune system in higher organisms is under integrated control and has the capacity to rapidly respond to the environment. Recently, there has been a significant increase in the prevalence of allergic diseases. Environmental factors likely play a major role in the explosion of allergy. Although the "hygiene hypothesis" may explain the increase in allergic diseases which are prone to T helper 2 (Th2) immune responses, recent findings highlight the possible involvement of environmental xenobiotic chemicals which can modulate normal immune function. Interestingly, several reports suggest that the prevalence of systemic lupus erythematosus, a Th2-type autoimmune disease, is also increasing, although the development of high-sensitivity immunological tests may be a possible cause. The increased prevalence of autoimmune disease in women, the sexual dimorphism of the immune response, and the immunomodulatory effects of sex steroids, have focused attention on the role of chemicals which influence sex steroids in the development of immune diseases. Moreover, recent reports indicate that some environmental chemicals can work on nuclear hormone receptors, other than sex hormone receptors, and modulate immune reactions. This review focuses on the impact of environmental chemicals on immune system function and pathogenesis of immune diseases, including allergy and autoimmune diseases.

Organotins disrupt the 11beta-hydroxysteroid dehydrogenase type 2-dependent local inactivation of glucocorticoids

Organotins, important environmental pollutants widely used in agricultural and industrial applications, accumulate in the food chain and induce imposex in several marine species as well as neurotoxic and immunotoxic effects in higher animals. Reduced birth weight and thymus involution, observed upon exposure to organotins, can also be caused by excessive glucocorticoid levels. We now demonstrate that organotins efficiently inhibit 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2), converting active 11beta-hydroxyglucocorticoids into inactive 11-ketoglucocorticoids, but not 11beta-HSD1, which catalyzes the reverse reaction. Di- and tributyltin as well as di- and triphenyltin inhibited recombinant and endogenous 11beta-HSD2 in lysates and intact cells with IC50 values between 500 nM and 3 microM. Dithiothreitol protected 11beta-HSD2 from organotin-dependent inhibition, indicating that organotins act by binding to one or more cysteines. Mutational analysis and 3-D structural modeling revealed several important interactions of cysteines in 11beta-HSD2. Cys90, Cys228, and Cys264 were essential for enzymatic stability and catalytic activity, suggesting that disruption of such interactions by organotins leads to inhibition of 11beta-HSD2. Enhanced glucocorticoid concentrations due to disruption of 11beta-HSD2 function may contribute to the observed organotin-dependent toxicity in some glucocorticoid-sensitive tissues such as thymus and placenta.

Environmental chemical tributyltin augments adipocyte differentiation

Scientific attention has been drawn to environmental factors that affect obesity and type II diabetes. Previously, acute organotin toxicosis was reported to induce hyperglycemia without morphological abnormalities in islet tissue, suggesting that these compounds have a direct effect on adipose tissue. Therefore, we investigated the effect of tributyltin (TBT) on adipocyte differentiation. When confluent 3T3-L1 cells were incubated with TBT for 2 days in the presence or absence of isobutyl methylxanthine, dexamethasone and insulin (MDI), the lipid accumulation in adipocytes was greatly enhanced. These morphological changes induced by TBT were accompanied by the expression of a differentiation marker for adipocytes in a dose-dependent manner. Co-treatment with the peroxisome proliferator-activated receptor (PPAR)gamma antagonist GW9662 did not inhibit the effect of TBT, suggesting that the observed effect of TBT may not be PPARgamma-dependent. Although TBT was reported to exert androgenic effects and inhibit the activity of aromatase, treatments with dihydrotestosterone or 17beta-estradiol did not influence the aP2 expression in 3T3-L1 cells, suggesting that the TBT effect does not occur via sex-steroids. These findings indicate that TBT may be one of the environmental chemicals that lead to excessive accumulation of adipose tissue, which can result in obesity.

Organotin Compounds: Toxicokinetic Aspects

Organotin compounds have a broad range of applications. While dialkyltin compounds are used primarily as stabilizers for plastics, trisubstituted organotins are mainly used as biocides e.g., as an active ingredient of marine antifouling paints for boats and ships. Since a number of organotin compounds have been demonstrated to be toxic, there is increasing concern that their widespread use may cause adverse effects within environmental and biological systems. Besides carcinogenic and neurotoxic effects, as well as effects on the reproductive system, the most obvious mammalian effects of both various di- and trisubstituted organotins were found on the immune system. Exposure of humans to organotin compounds can take place through consumption of contaminated fish and seafood. In human liver samples, mainly dibutyltin, the metabolite of tributyltin, could be detected indicating that organotin compounds are bioavailable after dietary exposure. The objective of this short review is to present various toxicokinetic aspects of organotin compounds in more detail. While several studies using in vitro systems investigated their metabolism especially by the monooxygenase system, various aspects of absorption, distribution, metabolism, and excretion (ADME) pathways of different organotin compounds were described by data obtained from several studies with laboratory animals. However, most of these studies were not conducted as full ADME studies but dealt only with some of these aspects. Therefore, for definitive conclusions in some cases, additional information is requested. By reviewing and updating the current literature consideration was given preferentially to those organotin compounds which have relevance with respect to human exposure and/or toxicological effects.

Inhibition of ossification in vivo and differentiation of osteoblasts in vitro by tributyltin

Tributyltin is ubiquitous in the environment and an endocrine disruptor for many wildlife species. However, minimal information is available regarding the effect of this chemical on bone formation. When tributyltin chloride (TBT) (1mg/kg body weight) was administered subcutaneously to pregnant mice at 10, 12, and 14 days post coitus (dpc), fetuses at 17.5 days post coitus revealed the inhibition of calcification of supraoccipital bone. In contrast, 1mg/kg body weight monobutyltin trichloride (MBT) did not affect the fetal skeleton. Therefore, we examined the effects of TBT and its metabolites (dibutyltin dichloride, DBT, and MBT) on bone metabolism using rat calvarial osteoblast-like cells (ROB cells). The viability of ROB cells was not affected by the exposure of the cells to 10(-10) to 10(-7)M TBT. However, TBT reduced the activity of alkaline phosphatase (ALPase) and the rate of deposition of calcium of ROB cells. In addition, the expression levels of mRNA for ALPase and osteocalcin, which are markers of osteoblastic differentiation, were depressed by the treatment with TBT. TBT inhibited ALPase activity and the deposition of calcium to a greater extent than did DBT. MBT had no effect on the osteoblast differentiation of ROB cells. Tributyltin is known to inhibit the activity of aromatase. However, the aromatase inhibitor aminoglutethimide did not reproduce the inhibitory effects of TBT on osteoblast differentiation. Our findings indicate that TBT might have critical effects on the formation of bone both in vivo and in vitro although its action mechanism is not clarified.

Effects of in utero tributyltin chloride exposure in the rat on pregnancy outcome

Tributyltin, an organotin, is ubiquitous in the environment. The consumption of contaminated marine species leads to human dietary exposure to this compound. Tributyltin is an endocrine disruptor in many wildlife species and inhibits aromatase in mammalian placental and granulosa-like tumor cell lines. We investigated the effects of tributyltin chloride exposure on pregnancy outcome in the Sprague-Dawley rat. Timed pregnant rats were gavaged either with vehicle (olive oil) or tributyltin chloride (0.25, 2.5, 10, or 20 mg/kg) from days 0-19 or 8-19 of gestation. On gestational day 20, dams were sacrificed, and pregnancy outcome was determined. Tributyltin and its metabolites (dibutyltin, monobutyltin) were measured in maternal blood by gas chromatography. Both tributyltin and dibutyltin were present in maternal blood at approximately equal concentrations, whereas monobutyltin contributed minimally to total organotins. Organotin concentrations increased in a dose-dependent pattern in dams, independent of the window of exposure. Tributyltin chloride administration significantly reduced maternal weight gain only at the highest dose (20 mg/kg); a significant increase in post-implantation loss and decreased litter sizes, in addition to decreased fetal weights, was observed in this group. Tributyltin chloride exposure did not result in external malformations, nor was there a change in sex ratios. However, exposure to 0.25, 2.5, or 10 mg/kg tributyltin chloride from gestation days (GD) 0-19 resulted in a significant increase in normalized anogenital distances in male fetuses; exposure from days 8-19 had no effect. There was a dramatic increase in the incidence of low weight (< or =0.75 of the mean) fetuses after exposure to 20 mg/kg tributyltin chloride. Delayed ossification of the fetal skeleton was observed after in utero exposure to either 10 mg/kg or 20 mg/kg tributyltin chloride. Serum thyroxine and triiodothyronine levels were reduced significantly in dams exposed to 10 and 20 mg/kg tributyltin chloride throughout gestation; in dams treated with tributyltin from GD 8-19, serum thyroxine concentrations, but not triiodothyronine, were significantly decreased at both the 2.5 and 10 mg/kg exposures. Thus, maternal thyroid hormone homeostasis may be important in mediating the developmental toxicity of organotins.

Tributyltin (TBT) induces ultra-rapid caspase activation independent of apoptosome formation in human platelets

Activation of caspases has been demonstrated to be involved in thrombocytopenia and prolonged storage of platelet concentrates. Platelets represent enucleate cells that comprise all elements of the mitochondrial apoptosis pathway. However, no apoptotic stimuli capable of activating the endogenous caspase cascade have been identified so far. Using tributyltin (TBT) we could identify a compound that is capable of activating caspase-9 and -3 in platelets. Recent studies implicate that TBT induces apoptosis via the mitochondrial signaling pathway that is characterized by the formation of a high-molecular-weight complex (apoptosome) containing the adapter protein Apaf-1 and active caspase-9. Interestingly, addition of TBT induced the activation of caspase-9 in an ultra-rapid kinetic within the first 2 min. In addition, size exclusion chromatography revealed that TBT-mediated processing of caspase-9 occurs in the absence of the apoptosome. Thus, these data implicate that TBT induces the activation of caspase-9 by a mechanism not involving the formation of the apoptosome.

Organotin-induced apoptosis occurs in small CD4(+)CD8(+) thymocytes and is accompanied by an increase in RNA synthesis

The organotin compounds di-n-butyltin dichloride (DBTC) and tri-n-butyltin chloride (TBTC) induce thymus atrophy in rats. At low doses they inhibit immature thymocyte proliferation, whereas at higher doses in particular TBTC induces apoptotic cell death. In vitro, a similar concentration-effect relationship was observed, i.e. low concentrations inhibit DNA and protein synthesis and higher concentrations induce apoptosis. The mechanism of apoptosis by organotins has been partly investigated, but their capacity to inhibit protein synthesis seems to contradict with the idea that macromolecular synthesis is required for organotin-induced apoptosis. Therefore, we aimed to evaluate the relation between apoptosis and the synthesis of RNA and proteins, with a focus on the apoptosis-sensitive thymocyte subset. Results showed that DBTC increases RNA synthesis in particular in the subset of small CD4(+)CD8(+) thymocytes, which normally shows a high incidence of DNA fragmentation. Moreover, the RNA synthesis inhibitor actinomycin D or the protein synthesis inhibitor cycloheximide protected cells from apoptosis by DBTC or TBTC. Although organotin compounds increase synthesis of the heat shock protein HSC73/HSP72, heat shock treatment did not initiate apoptosis in thymocytes, neither antagonized organotin-induced apoptosis. This indicates that synthesis of heat shock proteins is not related to organotin-induced increase of RNA synthesis, and that other RNA-molecules are probably involved.