Bis(tri-n-butyltin)oxide induces programmed cell death (apoptosis) in immature rat thymocytes

Identification of antiparasitic drug targets using a multi-omics workflow in the acanthocephalan model

Background

With the expansion of animal production, parasitic helminths are gaining increasing economic importance. However, application of several established deworming agents can harm treated hosts and environment due to their low specificity. Furthermore, the number of parasite strains showing resistance is growing, while hardly any new anthelminthics are being developed. Here, we present a bioinformatics workflow designed to reduce the time and cost in the development of new strategies against parasites. The workflow includes quantitative transcriptomics and proteomics, 3D structure modeling, binding site prediction, and virtual ligand screening. Its use is demonstrated for Acanthocephala (thorny-headed worms) which are an emerging pest in fish aquaculture. We included three acanthocephalans (Pomphorhynchus laevis, Neoechinorhynchus agilis, Neoechinorhynchus buttnerae) from four fish species (common barbel, European eel, thinlip mullet, tambaqui).

Results

The workflow led to eleven highly specific candidate targets in acanthocephalans. The candidate targets showed constant and elevated transcript abundances across definitive and accidental hosts, suggestive of constitutive expression and functional importance. Hence, the impairment of the corresponding proteins should enable specific and effective killing of acanthocephalans. Candidate targets were also highly abundant in the acanthocephalan body wall, through which these gutless parasites take up nutrients. Thus, the candidate targets are likely to be accessible to compounds that are orally administered to fish. Virtual ligand screening led to ten compounds, of which five appeared to be especially promising according to ADMET, GHS, and RO5 criteria: tadalafil, pranazepide, piketoprofen, heliomycin, and the nematicide derquantel.

Conclusions

The combination of genomics, transcriptomics, and proteomics led to a broadly applicable procedure for the cost- and time-saving identification of candidate target proteins in parasites. The ligands predicted to bind can now be further evaluated for their suitability in the control of acanthocephalans. The workflow has been deposited at the Galaxy workflow server under the URL tinyurl.com/yx72rda7.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08882-1.

Tributyltin Oxide Exposure During in vitro Maturation Disrupts Oocyte Maturation and Subsequent Embryonic Developmental Competence in Pigs

Tributyltin oxide (TBTO), an organotin compound, has been demonstrated to have toxic effects on several cell types. Previous research has shown that TBTO impairs mouse denuded oocyte maturation. However, limited information is available on the effects of TBTO exposure on livestock reproductive systems, especially on porcine oocytes in the presence of dense cumulus cells. In the present research, we evaluated the effects of TBTO exposure on porcine oocyte maturation and the possible underlying mechanisms. Porcine cumulus-oocyte complexes were cultured in maturation medium with or without TBTO for 42 h. We found that TBTO exposure during oocyte maturation prevented polar body extrusion, inhibited cumulus expansion and impaired subsequent blastocyst formation after parthenogenetic activation. Further analysis revealed that TBTO exposure not only induced intracellular reactive oxygen species (ROS) accumulation but also caused a loss of mitochondrial membrane potential and reduced intracellular ATP generation. In addition, TBTO exposure impaired porcine oocyte quality by disrupting cellular iron homeostasis. Taken together, these results demonstrate that TBTO exposure impairs the porcine oocyte maturation process by inducing intracellular ROS accumulation, causing mitochondrial dysfunction, and disrupting cellular iron homeostasis, thus decreasing the quality and impairing the subsequent embryonic developmental competence of porcine oocytes.

The immunosuppressant tributyltin oxide blocks the mTOR pathway, like rapamycin, albeit by a different mechanism

We treated the thymoma cell line (EL4) with two model immunosuppressants, rapamycin and tributyltin oxide (TBTO), and compared their effects on the expression levels of proteins that are downstream targets of mTOR kinase 1 (mammalian target of rapamycin, known also as mechanistic target of rapamycin): p70 ribosomal S6 kinase1 and 4E-binding protein 1, a repressor of the cap-binding protein eIF4E. In addition, we evaluated the levels of ribosomal protein S6, p-eIF4B, substrates of p70S6 kinase1, matrin 3 and ribonucleotide reductase, subunit RRM2. The levels of these proteins were evaluated in cell lysates by immunoblot. We found that both compounds inhibited the phosphorylation state of p70S6 kinase 1 and its substrates; however, TBTO, in contrast to rapamycin, reduced the level of the total p70S6k1. Besides, we detected a band with a molecular weight of c. 32 kDa only in the TBTO-treated lysates. This band was detected with a monoclonal antibody specific for S6k1, suggesting that this band might be a degradation product of the kinase. Further, TBTO and rapamycin differentially affected 4E-binding protein 1; the former compound stimulated its phosphorylation state whereas the latter inhibited it. The two immunosuppressants did not affect the level of ribonucleotide reductase, but TBTO downregulated matrin3, in agreement with a previous report, whereas rapamycin had no effect on the expression level of this latter protein. We conclude that TBTO inhibits, like rapamycin, the p70 S6 kinase 1 pathway, but with a different mechanism. However, in contrast to rapamycin, which inhibits the cap-dependent translation, TBTO increases the phosphorylation of 4E-binding protein1.

Resistance to oxidative damage but not immunosuppression by organic tin compounds in natural populations of Daubenton's bats (Myotis daubentonii)

The acute toxicity of organic tin compounds (OTCs) has been studied in detail. However, due to their complex nature, very little is known about species-specific methods of accumulation and consequences for food-webs. Chironomids, on which e.g. Daubenton's bats feed, may act as vectors for the transport of organic tin compounds from aquatic to terrestrial ecosystems. Bats are prone to environmental toxins because of their longevity and their ecological role as top predators. Organic tin compounds are associated with increased formation of reactive oxygen species and associated oxidative damage as well as suppression of immune function. The present paper investigates whether the OTC, tributyltin (TBT) and its metabolite, dibutyltin (DBT), accumulate in natural populations of Daubenton's bats and whether TBT-associated effects are seen in general body condition, redox balance, redox enzyme activities, associated oxidative damage of red blood cells and complement function. We discovered the concentration of bat fur DBT correlated with local marine sediment TBT concentrations. However, we did not find a correlation between the explanatory factors, bat fur DBT and marine sediment TBT concentrations, and several physiological and physical response variables apart from complement activity. Higher DBT concentrations resulted in weaker complement activity and thus a weaker immune response. Although the observed physiological effects in the present study were not strongly correlated to butyltin concentrations in fur or sediment, the result is unique for natural populations so far and raises interesting questions for future ecotoxicological studies.

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.

Phosphoproteomic analysis of mouse thymoma cells treated with tributyltin oxide: TBTO affects proliferation and energy sensing pathways

We report the results of phosphoproteomic analysis of mouse thymoma cells treated with tributyltin oxide (TBTO), an immunotoxic compound. After cell lysis, phosphoproteins were isolated using Phosphoprotein Purification Kit, separated by SDS-PAGE and subsequently digested with trypsin. Phosphopeptides were enriched employing titanium dioxide, and the obtained fractions were analyzed by nano-LC-MS/MS. A total of 160 phosphoproteins and 328 phosphorylation sites were identified in thymoma cells. Among the differentially phosphorylated proteins identified in TBTO-treated cells were key enzymes, which catalyze rate-limiting steps in pathways that are sensitive to cellular energy status. These proteins included acetyl-CoA carboxylase isoform 1, which catalyzes the rate-limiting step of fatty acid synthesis. Another enzyme was glutamine: fructose-6-phosphate amidotransferase, GFAT1, the first and rate-limiting enzyme for the hexoamine synthesis pathway. Pyruvate dehydrogenase (PDH), a multicomplex enzyme that catalyzes the rate-limiting step of aerobic oxidation of fuel carbohydrates, was identified in both TBTO-treated and control cells; however, phosphorylation at residue S293, known to inhibit PDH activity, was identified only in control cells. A lower expression level of ribosomal protein S6 kinase 1, a downstream kinase of the mammalian target of rapamycin signaling pathway implicated in protein synthesis through phosphorylation of 40 ribosomal S6, was observed in the treated cells. Giant kinases like AMP-activated protein kinase (AMPK) and cAMP-dependent protein kinase (PKAR1A), which are known to mediate the phosphorylation of these enzymes, were identified in TBTO-treated cells. Downregulation of proteins, such as MAPK, matrin-3 and ribonucleotide reductase, subunit RRM2, which are implicated in cell proliferation, was also observed in TBTO-treated cells. Together, the results show that TBTO affects proliferation and energy sensor pathways.

Exposure of Jurkat cells to bis (tri-n-butyltin) oxide (TBTO) induces transcriptomics changes indicative for ER- and oxidative stress, T cell activation and apoptosis

Tributyltin oxide (TBTO) is an organotin compound that is widely used as a biocide in agriculture and as an antifouling agent in paints. TBTO is toxic for many cell types, particularly immune cells. The present study aimed to identify the effects of TBTO on the human T lymphocyte cell line Jurkat. Cells were treated with 0.2 and 0.5μM TBTO for 3, 6, 12 and 24h and then subjected to whole genome gene expression microarray analysis. The biological interpretation of the gene expression profiles revealed that endoplasmic reticulum (ER) stress is among the earliest effects of TBTO. Simultaneously or shortly thereafter, oxidative stress, activation of NFKB and NFAT, T cell activation, and apoptosis are induced. The effects of TBTO on genes involved in ER stress, NFAT pathway, T cell activation and apoptosis were confirmed by qRT-PCR. Activation and nuclear translocation of NFATC1 and the oxidative stress response proteins NRF2 and KEAP1 were confirmed by immunocytology. Taking advantage of previously published microarray data, we demonstrated that the induction of ER stress, oxidative stress, T cell activation and apoptosis by TBTO is not unique for Jurkat cells but does also occur in mouse thymocytes both ex vivo and in vivo and rat thymocytes ex vivo. We propose that the induction of ER stress leading to a T cell activation response is a major factor in the higher sensitivity of immune cells above other types of cells for TBTO.

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.

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.

Effects of prometryne on apoptosis and necrosis in thymus, lymph node and spleen in mice

Prometryne is a methylthio-s-triazine herbicide. Significant traces are documented in environment, mainly waters, soil and plants used for nutrition. The aim of this study was to estimate prometryne immunotoxic properties through induction of apoptotic and/or necrotic changes in thymocytes, splenocytes and lymph node cells after repeated subchronical exposure. Three different doses of prometryne (185, 375, 555mgkg(-1)) were applied per os every 48h, over 28 days. Flow cytometry assay (annexinV-FITC and PI) was conducted to record apoptotic and necrotic damage. In the spleen significant changes in the percentage of apoptotic cells were not detected between treated and control groups respectively. In thymus and lymph node, within the lowest dose group (185mgkg(-)1), an increase in percentage of early apoptosis without any significant increase in necrosis was detected. Medium (375mgkg(-1)) as well as high dose triggered increase in late apoptosis in lymph node while in thymus; late apoptosis was increased only in animals exposed to the highest dose (555mgkg(-1)). The highest applied dose, in thymus and lymph node respectively, caused a general decrease in percentage of vital cells in favour of marked increase of percentages of all types of dying cells (apoptotic, late apoptotic/early necrotic and necrotic). Prometryne caused disbalance in major organs of immune system, markedly lymph nodes and thymus, by induction of early apoptotic changes in dose/time specific manner.

Bis(tributyltin)oxide (TBTO) decreases the food allergic response against peanut and ovalbumin in Brown Norway rats

Other factors than the allergen itself may be of importance in the development of food allergy. This report describes the influence of the immunosuppressive compound bis(tributyltin)oxide (TBTO), present in the food chain, on the development of food allergy to peanut or ovalbumin in Brown Norway (BN) rats. To study these effects BN rats were sensitized to either 1 or 10mg peanut or ovalbumin by daily oral gavage and the TBTO-groups were fed a diet containing 80 mg TBTO per kg diet. Co-exposure to TBTO not only resulted in decreased general immunologic parameters such as weights of mesenteric lymph nodes and Peyer's patches, lymphocyte proliferation rates in splenocytes, but also on allergic parameters. In the peanut allergen-model TBTO decreased allergen-specific Th2 cytokine production by spleen cells, number of eosinophilic and basophilic granulocytes in the blood and production of mast cell protease II after oral food challenge. In the ovalbumin allergen-model TBTO decreased the number of eosinophilic and basophilic granulocytes, allergen-specific IgE and production of mast cell protease II after oral food challenge. The data imply that in the process of risk assessment of food allergy attention should be given to immunomodulating compounds present in the diet.

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.

Reactive oxygen species in in vitro pesticide-induced neuronal cell (SH-SY5Y) cytotoxicity: role of NFkappaB and caspase-3

Oxidative stress has been implicated in pesticide-induced neurotoxicity, based on its role in the cascade of biochemical changes that lead to dopaminergic neuronal cell death. We have, therefore, examined the role of oxidative stress caused by the pesticides endosulfan and zineb in human neuroblastoma cells (SH-SY5Y) in culture. Upon treatment with 50-200 microM concentrations of either of these pesticides, SH-SY5Y cells generated both superoxide anion and hydrogen peroxide in a dose-and time-dependent manner. Mixtures of the pesticides significantly enhanced the production of these reactive oxygen species compared to individual pesticide exposures. Pesticide treatment decreased superoxide dismutase, glutathione peroxidase, and catalase activities in SH-SY5Y cells. Additionally, these pesticides induced lipid peroxide (thiobarbituric acid reactive products) formation in these cells. While both pesticides individually (at 100 microM) increased caspase-3 activity, cells exposed to a mixture of the pesticides exhibited significantly low levels of this enzyme, probably due to excessive necrotic cell death. Furthermore, exposure to these pesticides increased nuclear NFkappaB activity. Taken together, these findings suggest that the cytotoxicity of endosulfan and zineb, both individually and in mixtures may, at least in part, be associated with the generation of reactive oxygen species with concomitant increased expression of NFkappaB.

Exposure to mixtures of endosulfan and zineb induces apoptotic and necrotic cell death in SH-SY5Y neuroblastoma cells,in vitro

A number of epidemiological studies have demonstrated a strong association between the incidence of Parkinson's disease and pesticide exposure. Earlier it was demonstrated that exposure to the pesticides endosulfan and zineb, alone and in combination, caused neurodegeneration in vivo. It was hypothesized that these pesticides cause neurotoxicity, in part, by enhancing apoptotic cell death. SH-SY5Y human neuroblastoma cells, which retain a catecholaminergic phenotype, were exposed to endosulfan, zineb or a combination of these chemicals, in vitro. For mixture studies, concentrations of pesticides (100 microM each) were chosen based on LC(25) (lethal concentration) that would result in minimum cell death. Exposure to a mixture of pesticides exhibited significantly (P < or = 0.05) higher toxicity than each one alone. Both pesticides were found to cause apoptotic cell death that was concentration (50-400 microM) dependent. A flow cytometric (7-aminoactinomycin D) assay was used to distinguish live, early apoptotic and late apoptotic/necrotic populations. Exposure to mixtures of the pesticides enhanced both early apoptosis and late apoptosis/necrosis compared with either chemical alone. Visual evaluation using a DNA ladder assay and a fluorescence Annexin V/PI assay confirmed the contribution of both apoptotic and necrotic processes. These findings suggest that the cytotoxicity of endosulfan and zineb, both individually and in mixtures, is associated with the occurrence of early and late apoptotic/necrotic processes in SH-SY5Y human neuroblastoma cells and support the contention that pesticide-induced neuronal cell death leading to neurodegenerative disease may, at least in part, be associated with early and late apoptosis of dopaminergic neurons.

Biochemical, ultrastructural and molecular characterization of the triphenyltin acetate (TPTA)-induced apoptosis in primary cultures of mouse thymocytes

Triphenyltin acetate (TPTA), a triorganotin compound used in agriculture as a biocide, is immunotoxic in vivo and in vitro. The present study was undertaken to ascertain whether apoptosis might play a role in the TPTA toxicity in vitro. Mouse thymocyte primary cultures were exposed to 0, 4 and 8 micromol/L TPTA; methyl prednisolone (1 micromol/L) was used as a positive control. Cell aliquots were harvested after 0, 1, 2, 4, and 8 h and the presence of early or late apoptotic phenomena was checked by (a) morphological investigations; (b) spectrophotometric quantification of fragmented DNA and agarose gel electrophoresis; (c) cell flow cytofluorometry, using an annexin V-FITC kit; and (d) detection of in situ apoptosis by a colorimetric detection kit (Titer-Tacs). TPTA cytotoxicity was also evaluated using the trypan blue dye exclusion test. Morphological investigation indicated apoptosis and/or necrosis. After 8 h of incubation, cells exposed to 4 micromol/L TPTA showed an increase in DNA fragmentation (on electrophoresis), which was confirmed by spectrophotometry (p < 0.05). Flow cytofluorometry pointed out an early (p < 0.05) increase of annexin V-positive (apoptotic) cells in TPTA-exposed flasks, whereas at least partly contradictory, results were obtained with the Titer-Tacs kit. Overall, these results provide evidence that TPTA, at low concentrations (4 micromol/L) induces early and late apoptotic phenomena, whereas cells exposed to the highest concentrations (8 micromol/L) are likely to undergo necrosis rather than apoptosis.

Thymic atrophy induced by methoxychlor in rat pups

The effect of Methoxychlor (MXC) on the thymus was examined in rat pups that were delivered from dams receiving MXC at a dietary concentration of 0 or 1500 ppm for a period from pregnancy to lactation. The pups of both sexes were euthanized on postnatal days (PNDs) 7, 14, and 21. Histologically, the thymus showed marked depletion of cortical lymphocytes on PND 7 and also had an increase in lymphophagocytosis in the cortical area on PNDs 14 and 21. Morphometrical analysis disclosed that both cortex and medulla of the thymus from treated pups were reduced in size, but the reduction was more evident in the cortex. A significant increase in transferase-mediated dUTP nick end labeling-positive cells was detected in the cortex area, corresponding to the presence of lymphophagocytosis. Flow cytometric analysis revealed a significant decrease in the double positive (CD3(int)CD4(+)CD8(+)) immature cells on PND 21. These results have suggested that MXC may impair maturation of thymic lymphocytes in rat pups, which results in enhancement of apoptosis leading to thymic atrophy during the postnatal period.

Tributyltin causes cytochrome C release from isolated mitochondria by two discrete mechanisms

Using isolated liver mitochondria we show that low concentrations of TBT (0.5 microM) cause the release of mitochondrial cytochrome c, in the presence of Ca(2+). This is reflected in a rapid loss of membrane potential (DeltaPsi(m)), and a large-amplitude swelling characteristic of mitochondrial permeability transition (MPT). Despite this, the inclusion of cyclosporin A could not prevent the release of cytochrome c. Further, in the absence of Ca(2+), low concentrations of TBT (0.5 microM) resulted in a slow sub-maximal shift of DeltaPsi(m), not characteristic of MPT, which was still paralleled by a release of cytochrome c. Further experiments showed that the loss of DeltaPsi(m) in the absence of Ca(2+) was due to a combination of inhibition of respiration and a direct uncoupling effect on the respiratory chain. Under these conditions, rapid swelling of mitochondria could be demonstrated, due to chloride exchange over the inner mitochondrial membrane. Taken together these data suggest that TBT can induce the release of cytochrome c in intact cells by at least two mechanisms. The first and critical mechanism is initiated immediately the mitochondria sense the presence of TBT and involves a slow loss of DeltaPsi(m) and induction of swelling, which allows release of cytochrome c in a relatively non-specific manner and independently from a rise in [Ca(2+)](i). The second mechanism involves the induction of formal MPT as intracellular [Ca(2+)](i) increases. These data help to explain previous observations in intact lymphocytes demonstrating TBT-induced release of mitochondrial cytochrome c in the absence of a rise in [Ca(2+)](i) (Stridh, H., Gigliotti, D., Orrenius, S., and Cotgreave, I. A. (1999) Biochem. Biophys. Res. Commun. 266, 460-465).

Triphenyltin acetate-induced cytotoxicity and CD4(+) and CD8(+) depletion in mouse thymocyte primary cultures

Organotin compounds (OTs) find application worldwide as catalysts, stabilizers and biocides. Triphenyltin derivatives (TPs), including the fungicide triphenyltin acetate (TPTA), are OTs mostly used in our country. Some OTs were proved to be immunotoxic and in this paper the cytotoxicity, the possible selective activity upon definite lymphocyte subsets as well as the antiproliferative effect of TPTA was investigated in vitro by using primary cultures of mouse thymocytes. TPTA (5, 10 and 25 microM) was cytotoxic to these cells, as demonstrated by the significant (P<0.05) reduction of the cell viability percentage (trypan blue dye exclusion test), the neutral red uptake and the reduction of tetrazolium salts to formazan products (MTT assay). These overt effects were already noticed after 4 h of exposure to TPTA. The fungicide otherwise significantly reduced, after 24 h of incubation, the percentage of mature single positive thymocytes, particularly the CD4(+)/CD8(-) one. Finally, a significative dose-dependent inhibition of the T-cell mitogen-induced cell proliferation was observed in thymocytes exposed to 1 and 8 microM TPTA. These results are indicative of the TPTA immunotoxic properties, according to previous published reports concerning the in vitro and in vivo toxicity of some di- and triorganotin compounds.

Induction of apoptosis in the blue mussel Mytilus galloprovincialis by tri-n-butyltin chloride

Induction of apoptosis by tri-n-butyltin (TBT) in gill tissue of the mussel Mytilus galloprovincialis was investigated. The terminal dUTP nick-end labeling technique (TUNEL) was used to detect cells displaying DNA fragmentation within gill structures. Genomic DNA fragmentation was detected as characteristically ladder-like pattern of DNA fragments induced by single injection of different doses of TBT (1-5 microg/g) below the mantle, directly into the pallial fluid, after 24 h of incubation. DNA degradation of higher order DNA structure, as well as reduced G(0)/G(1) cell cycle region (the sub-G(1) region) was detectable after 1.5 h of TBT incubation. Presence of apoptotic cells in mussels' gills was indicated by the selective loss of G(2)/M cells concomitant with the appearance of cells with decreased DNA content in S and G(0)/G(1) cell cycle regions. The effect of the TBT on cell cycle in a mussel gill was a dose related and exposure time depending. The possible mechanism of induction of apoptosis in vivo in gill tissue of mussel treated with TBT is discussed.

Organotins induce apoptosis by disturbance of [Ca(2+)](i) and mitochondrial activity, causing oxidative stress and activation of caspases in rat thymocytes

Di-n-butyltin dichloride (DBTC) and tri-n-butyltin chloride (TBTC) cause thymus atrophy in rodents. At low doses, antiproliferative modes of action have been shown to be involved, whereas at higher doses apoptosis seems to be the mechanism of thymotoxicity by these chemicals. In vitro, a similar concentration-dependency has been observed. The purpose of the present research was to investigate the mechanisms underlying DNA fragmentation induced by these organotin compounds in freshly isolated rat thymocytes. As previously shown for TBTC, DBTC is also able to significantly increase intracellular Ca(2+) level ([Ca(2+)](i)). The rise in [Ca(2+)](i), already evident 5 min after treatment, was followed by a dose- and time-dependent generation of reactive oxygen species (ROS) at the mitochondrial level. Simultaneously, organotins induced the release of cytochrome c from the mitochondrial membrane into the cytosol. ROS production and the release of cytochrome c were reduced by BAPTA, an intracellular Ca(2+) chelator, or rotenone, an inhibitor of the electron entry from complex I to ubiquinone, indicating the important role of Ca(2+) and mitochondria during these early intracellular events. Furthermore, we demonstrated that rotenone prevents apoptosis induced by 3 microM DBTC or TBTC and, in addition, that both BAPTA and Z-DEVD FMK (mainly a caspase-3 inhibitor) decreased apoptosis by DBTC (already shown for TBTC). Taken together these data show the apoptotic pathway followed by organotin compounds starts with an increase of [Ca(2+)](i), then continues with release of ROS and cytochrome c from mitochondria, activation of caspases, and finally results in DNA fragmentation.

The mechanisms of hsp27 antibody-mediated apoptosis in retinal neuronal cells

Although elevated titers of serum antibodies to hsp27 accompany human diseases such as cancer and glaucoma, evidence of their pathogenic effects is lacking. Here we present novel evidence that exogenously applied hsp27 antibody enters neuronal cells in human retina by an endocytic mechanism. Subsequent to internalization, hsp27 antibody facilitates apoptotic cell death as characterized by morphological assessment, DNA fragmentation, and the activation of cysteine aspartic acid proteases. In addition, we demonstrate that after internalization, hsp27 antibody is detected in discrete cytoplasmic and nuclear structures and colocalizes to actin cytoskeleton. Hsp27 antibody binding to actin results in depolymerization and proteolytic cleavage of actin in a dose-dependent manner. These results suggest that exogenous hsp27 antibody may induce neuronal apoptosis by inactivating or attenuating the ability of native hsp27 to stabilize actin cytoskeleton, thereby providing a novel mechanism by which autoantibodies to hsp27 may impair cell survival in selective human diseases.

Effect of adrenalectomy on ethanol-associated changes in lymphocyte cell numbers and subpopulations in thymus, spleen, and gut-associated lymphoid tissues

Consumption of ethanol (ETOH) by experimental animals and human beings is associated with elevated serum levels of corticosteroids. One of the most robust findings associated with ETOH consumption is a loss of lymphocytes from thymus and spleen, as well as from peripheral lymphoid organs to include mesenteric lymph nodes and Peyer's patches, which are lymphoid organs associated with the gastrointestinal tract. To study the role of corticosteroids in loss of cells from thymus, spleen, and gut-associated lymphoid organs, adrenalectomized (ADX) or intact C57Bl/6 mice were fed a liquid diet containing ETOH (to supply 36% of calories as ETOH) or an isocaloric control diet with a pair-feeding protocol. Loss of lymphocytes from all lymphoid organs was associated closely with serum corticosterone levels in both ETOH-fed and pair-fed groups. ETOH-fed ADX animals showed much less cell loss than did ETOH-fed intact animals. However, there was still an association between ETOH consumption and cell loss when cell loss in ETOH-fed ADX animals was compared with that in ADX pair-fed and ADX chow-fed groups. In both intact and ADX animals ETOH consumption was associated with a loss of immature (CD4(+) and CD8(+)) cells from the thymus. These data lead to the suggestion that corticosteroids are responsible for most of the cell loss from thymus, spleen, mesenteric lymph nodes, and Peyer's patches in association with ETOH consumption. Some cell loss, however, is independent of corticosteroids. The data presented here also support the suggestion that cell loss from lymphoid organs could be the result of nutritional factors.

Flow cytometric analysis on tri-n-butyltin-induced increase in annexin V binding to membranes of rat thymocytes

Effects of tri-n-butyltin chloride (TBT) on rat thymocytes were examined by using a flow cytometer and three fluorescent dyes (annexin V-FITC, ethidium bromide and fluo-3-AM) to further characterize its cytotoxic action. TBT at concentrations of 100 nM or greater, time- and dose-dependently increased the population of annexin V-positive live cells in the cell suspension. Most of cells became to be annexin V-positive within 60 min after the start of application of 300 nM TBT. Some of annexin V-positive live cells were further stained with ethidium, indicating that some of the cells were killed, in continued presence of TBT at 300 nM or greater. When the cells were exposed to 300 nM TBT only for 15 min, the population of annexin V-positive live cells increased after removal of TBT from incubation medium. TBT-induced increase in the population of annexin V-positive live cells was partly attenuated under Ca(2+)-free condition, although that was not the case for the dead cells. TBT at 30 nM or greater increased [Ca(2+)]i in a dose-dependent manner. Triethyltin and trimethyltin even at 1 μM did not increase the [Ca(2+)]i and the population of annexin V-positive live cells. The population of annexin V-positive live cells increased as the [Ca(2+)]i was increased by ionomycin, a calcium ionophore. Results suggest an involvement of Ca(2+) in some of TBT-induced cytotoxicity.

In vitro exposure effects of cyclosporin A and bis(tri-n-butyltin)oxide on lymphocyte proliferation, cytokine (receptor) mRNA expression, and cell surface marker expression in rat thymocytes and splenocytes

Rat thymocytes and splenocytes were exposed in vitro to the model compounds Cyclosporin A (CsA), an immunosuppressive drug, and bis(tri-n-butyltin)oxide (TBTO), an immunotoxic environmental contaminant. The lymphocyte transformation test (LTT), cytokine (receptor) mRNA expression (RT-PCR and dot blot hybridisation), and flow cytometry were evaluated as assays for in vitro immunotoxicity, at dose levels that did not show effects on viability, this being the aim of the study. LTT and RT-PCR proved useful assays. Lymphocyte transformation was suppressed by both compounds, while IL-2 mRNA expression was suppressed by CsA but not by TBTO, and both compounds suppressed IL-2R mRNA expression in splenocytes but not in thymocytes. Furthermore, the data obtained suggest that antiproliferative effects may be more relevant than apoptosis induction for TBTO induced thymus atrophy.

A positively charged alpha-lipoic acid analogue with increased cellular uptake and more potent immunomodulatory activity

alpha-Lipoic acid (LA) is taken up by cells and reduced to its potent dithiol form, dihydrolipoate(DHLA), much of which is rapidly effluxed out from cells. To improve retention in cells, the LA molecule was modified to confer a positive charge at physiological pH. N,N-dimethyl,N'-2-amidoethyl-lipoate was synthesized. The protonated form of the new molecule is referred to as LA-Plus. The uptake of LA-Plus by human Wurzburg T cells was higher compared to that of LA. Several-fold higher amounts of DHLA-Plus, the corresponding reduced form of LA-Plus, were detected in LA-Plus treated cells compared to the amount of DHLA found in cells treated with LA. At 100 microM, LA did not but LA-Plus inhibited H2O2 induced NF-kappaB activation and NF-kappaB directed IL-2 receptor expression. Both LA and LA-Plus synergised with selenium in inhibiting H2O2 induced NF-kappaB activation. At 150 microM LA-Plus, but not LA, inhibited TNFalpha induced NF-kappaB activation. At 5 microM LA-Plus, but not LA, protected against both spontaneous and etoposide induced apoptosis in rat thymocytes. LA-Plus is thus an improved form of LA with increased therapeutic potential.

Proteasome activities decrease during dexamethasone-induced apoptosis of thymocytes

The induction of apoptosis in thymocytes by the glucocorticoid dexamethasone was used as a model system to investigate whether there are changes in 20 S and 26 S proteasome activities during apoptosis. We observed that thymocytes contain high concentrations of proteasomes and that following treatment with dexamethasone, cell extracts showed a decrease in proteasome chymotrypsin-like activity which correlated with the degree of apoptosis observed. The decrease in chymotrypsin-like activity of 20 S and 26S proteasomes was still apparent after these complexes had been partially purified from apoptotic thymocyte extracts and was therefore not due to competition resulting from a general increase in protein turnover. The trypsin-like and peptidylglutamylpeptide hydrolase activities of proteasome complexes were also observed to decrease during apoptosis, but these decreases were reversed by the inhibition of apoptosis by the caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone. However, the chymotrypsin-like activity of proteasomes decreased further in the presence of the apoptosis inhibitor. Val-Ala-Asp-fluoromethylketone was found to inhibit the chymotrypsin- and trypsin-like activity of 26 S proteasomes in vitro. The decrease in proteasome activities in apoptosis did not appear to be due to a decrease in the concentration of total cellular proteasomes. Thus, the early decreases in 20 S and 26 S proteasome activities during apoptosis appear to be due to a down-regulation of their proteolytic activities and not to a decrease in their protein concentration. These data suggest that proteasomes may be responsible, in thymocytes, for the turnover of a protein that functions as a positive regulator of apoptosis.

Apoptosis and necrosis in toxicology: a continuum or distinct modes of cell death?

Mounting evidence indicates that apoptosis rather than necrosis predominates in many cytolethal toxic injuries. Associated cell death models of apoptosis and necrosis are either: (1) totally separate death modes, (2) a continuum whereby they are extremes of biochemically overlapping death pathways, or (3) essentially distinct processes with only limited molecular and cell biology overlap. We conclude that the current balance of evidence favours the third of these options. The established axiom that, even when considering the same toxicant, injury amplitude (dose) is a primary determinant of whether cells die via active cell death (apoptosis) or failure of homeostasis (necrosis) remains valid. Tissue selectivity of toxicants can stem from the apoptotic or necrotic thresholds at which different cells die, as well as targeting factors such as toxicokinetics, receptor recognition, bioactivation, and cell-specific lesions.

Differential effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, bis(tri-n-butyltin) oxide and cyclosporine on thymus histophysiology

Recent advances in the histophysiology of the normal thymus have revealed its complex architecture, showing distinct microenvironments at the light and electron microscopic level. The epithelium comprising the major component of the thymic stroma is not only involved in the positive selection of thymocytes, but also in their negative selection. Dendritic cells, however, are more efficient than epithelial cells in mediating negative selection. Thymocytes are dependent on the epithelium for normal development. Conversely, epithelial cells need the presence of thymocytes to maintain their integrity. The thymus rapidly responds to immunotoxic injury. Both the thymocytes and the nonlymphoid compartment of the organ can be targets of exposure. Disturbance of positive and negative thymocyte selection may have a major impact on the immunological function of the thymus. Suppression of peripheral T-cell-dependent immunity as a consequence of thymus toxicity is primarily seen after perinatal exposure when the thymus is most active. Autoimmunity may be another manifestation of chemically mediated thymus toxicity. Although the regenerative capacity of thymus structure is remarkable, it remains to be clarified whether this also applies to thymus function. In-depth mechanistic studies on chemical-induced dysfunction of the thymus have been conducted with the environmental contaminants 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and bis(tri-n-butyltin)oxide (TBTO) as well as the pharmaceutical immunosuppressant cyclosporine (CsA). Each of these compounds exerts a differential effect on the morphology of the thymus, depending on the cellular targets for toxicity. TCDD and TBTO exposure results in cortical lymphodepletion, albeit by different mechanisms. An important feature of TCDD-mediated thymus toxicity is the disruption of epithelial cells in the cortex. TBTO primarily induces cortical thymocyte cell death. In contrast CsA administration results in major alterations in the medulla, the cortex remaining largely intact. Medullary epithelial cells and dendritic cells are particularly sensitive to CsA. The differential effects of these three immunotoxicants suggest unique susceptibilities of the various cell types and regions that make up the thymus.

Comparative analysis of flow cytometric methods for apoptosis quantitation in murine thymocytes and human peripheral lymphocytes from controls and HIV-infected persons. Evidence for interference by granulocytes and erythrocytes

The present article reports a multiparametric cytofluorimetric analysis of apoptosis in murine thymocytes and human PBMC from healthy donors or HIV-infected patients. We have evaluated four previously described cytofluorimetric methods of apoptosis quantification, each of them detecting distinct cellular alterations of the apoptosis process. Reduced DNA stainability was detected with the PI assay on nuclei and the AO/EB dual staining method was evaluated on entire and non-fixed cells. DNA strand breaks were detected following in situ nick translation, and alterations in membrane integrity were evaluated following 7-AAD incorporation. When apoptosis was quantified in murine thymocytes under various conditions of induction, the combined analysis of FSC/SSC criteria and 7-AAD or AO/EB staining on the same samples permitted the identification of distinct steps in the apoptosis process. Moreover these four methods proved to be reliable and gave statistically similar results both on murine thymocytes and PBMC from healthy donors. However, in HIV-infected persons, some discordant apoptosis determinations were observed with PI and 7-AAD staining assays. We found that after Ficoll isolation, PBMC from AIDS patients were enriched in erythrocytes and granulocytes. On the one hand, granulocytes were found to be responsible for a poor apoptosis estimation with the PI assay whereas erythrocytes were responsible for an underestimation rate of apoptosis in the 7-AAD assay. To prevent such interference, we propose some modifications which render these methods more suitable for application to PBMC from HIV-infected patients. Taken together these observations indicate that it is essential to assess critically the apoptosis quantification methods with respect to their applicability to complex lymphoid populations such as those from AIDS patients.

Cytotoxic action of tri-n-butyltin on dissociated rat cerebellar neurones: a flow-cytometric study

The effects of tri-n-butyltin on cell viability and intracellular Ca2+ concentration ([CA2+]i) were examined by a flow cytometer in rat cerebellar neurones to reveal the contribution of tri-n-butyltin-induced increased in the [Ca2+]i to its cytotoxicity. Tri-n-Butyltin decreased the cell viability in association with increased [Ca2+]i at concentrations of 0.3 microM or more. Decrease or increase of the extracellular Ca2+ concentration ([Ca2+]o) respectively decreased or increased the cell viability. However, cell viability in the presence of ionomycin which increased [Ca2+]i more significantly than tri-n-butyltin was higher than that in the presence of tri-n-butyltin. Tri-n-butyltin also decreased cell viability under nominally [Ca2+]o-free conditions although the [Ca2+]i increase by tri-n-butyltin was still lower than the control [Ca2+]i under normal [Ca2+]o (2 mM). Therefore, it is unlikely that neuronal death induced by tri-n-butyltin is entirely dependent on the tri-n-butyltin-induced increase in [Ca2+]i.

Cleavage of actin by interleukin 1 beta-converting enzyme to reverse DNase I inhibition

Three of the predominant features of apoptosis are internucleosomal DNA fragmentation, plasma membrane bleb formation, and retraction of cell processes. We demonstrate that actin is a substrate for the proapoptotic cysteine protease interleukin 1beta-converting enzyme. Actin cleaved by interleukin 1beta-converting enzyme can neither inhibit DNase I nor polymerize to its filamentous form as effectively as intact actin. These findings suggest a mechanism for the coordination of the proteolytic, endonucleolytic, and morphogenetic aspects of apoptosis.

Triphenyltin acetate toxicity: a biochemical and ultrastructural study on mouse thymocytes

1. Triphenyltin acetate (TPTA) has been shown to exert in vivo a selective toxic effect on the immune system. To assess in vitro possible alterations induced by TPTA exposure, primary cultures of mouse thymocytes were incubated up to 24 h with graded amounts (1-12 microM) of the organotin. 2. The cytotoxic activity has been evaluated with the MTT colorimetric assay, the neutral red (NR) assay and the lactic dehydrogenase (LDH) cellular release. Cell pellets were fixed with 2.5% glutaraldehyde, resin-embedded and ultrathin sections were observed through transmission electron microscopy. 3. After 2 h of incubation, dose-dependent increases of cytotoxicity were observed in thymocytes submitted to MTT and NR tests (up to 41.43% and 18.9%, respectively), while 22 h later this overt effect on cell viability was noticed merely in cells exposed to 12 microM TPTA. Dose-dependent increases of LDH leakage in the culture medium were observed all throughout the study. 4. Morphological investigations revealed features (chromatin condensation, cell membranes fragmentation and formation of membrane bound apoptotic bodies) suggestive of apoptosis. 5. This study indicates that TPTA is cytotoxic to mouse thymocytes: morphologically, the rising of apoptosis is likely to be recognized, as previously reported in different in vitro studies with other immunosuppressive agents as dioxin and corticosteroids.

F-actin levels but not actin polymerization are affected by triphenyltin in HL-60 cells

The toxin triphenyl tin (TPT), Sn(C(6)H(5))(+)(3) caused a rapid decrease in the F-actin content of promyelocytic human leukemia cells (HL-60) chemically differentiated to neutrophils. Prior incubation (2 min) of the cells with 10 μM TPT did not modify the extent of actin polymerization inducible either by a receptor-mediated stimulus (chemotactic peptide fMLP) or by a direct activator of G proteins (AlF(-)(4)). The inorganic tin salts SnCl(2) and SnCl(4) did not affect F-actin content or production of HL-60 cells. Microfilament thiol groups were not reduced by exposure of cells to TPT, but even increased. When F-actin was exposed to 10 |GmM triphenyltin in a cell-free system, the depolymerizing effect was not detectable. Thus, TPT does not affect cytoskeletal protein directly but depends for its toxicity on some other induced change, probably ionic/osmotic in the intact cell.

Characteristics of cancer cell death after exposure to cytotoxic drugs in vitro

The characteristics of cell death were investigated after exposure of CCRF-CEM.f2 cells to five drugs over a broad concentration range; these were the glucocorticoid dexamethasone (DXM), the mitotic inhibitor vincristine (VIN) and three antimetabolites, methotrexate (MTX), 5'-fluoro-2'-deoxyuridine (FUdR) and 5'-fluorouracil (5-FU). Drug-treated cells were monitored for cell death mechanisms at different times by examining the pattern of DNA degradation, cell morphology and flow cytometric profile, together with effects on cell growth over 72 h. At growth-inhibitory drug concentrations, the first changes were cell cycle perturbations detectable after 4-6 h of drug exposure. The appearance of features characteristic of apoptotic cell death was noted after all drug treatments in the CCRF-CEM.f2 cell line, but the pattern and kinetics varied considerably. VIN induced apoptotic changes by 12 h, while DXM treatment caused apoptosis only after 48 h. Both MTX and FUdR induced morphological changes characteristic of apoptosis at least 24 h before internucleosomal DNA cleavage, which was detectable only after 48 h. In contrast, 5-FU did not cause internucleosomal DNA cleavage by 48 h at any concentration, despite the presence of morphologically apoptotic cells 24 h earlier. These data suggest that disruption of the cell cycle caused by drug treatment may be the common trigger initiating the drug-specific apoptotic sequence of dying cells.

An interleukin-1 beta-converting enzyme-like protease is a common mediator of apoptosis in thymocytes

Apoptosis was induced in thymocytes using diverse stimuli in order to identify events within a common apoptotic pathway. Benzyloxycarbonyl-valinyl-alaninyl-aspartyl fluoromethyl ketone (Z-VAD.FMK), an interleukin-1 beta-converting enzyme (ICE)-like protease inhibitor, inhibited apoptosis assessed by flow cytometry, proteolysis of poly (ADP)-ribose polymerase of DNA to both large kilobase pair fragments (30-50 and 200-300 kbp) and to nucleosomal fragments. Z-VAD.FMK also blocked all the classical ultrastructural features of apoptosis including chromatin condensation to one pole of the nucleus, nucleolar disintegration and cytoplasmic vacuolation. These results suggest the involvement of an ICE-like protease as a common mediator of apoptosis in thymocytes.

Discrimination of apoptotic thymocytes by forward light scatter

The discrimination of apoptotic and normal rat thymocytes by forward-angle light scatter in the UV depended on the angle over which the scattered light was collected. To achieve good separation, the light needed to be collected over a narrow angle. It was also observed that apoptotic thymocytes gave less forward light scatter than normal cells, whereas, under identical conditions, apoptotic cells from a murine cell line gave more light scatter than their normal counterparts. We suggest that the forward light scatter of apoptotic cells is affected by several factors, including changes in cell size and in the internal structure of the cell.

Alterations in mitochondrial structure and function are early events of dexamethasone-induced thymocyte apoptosis

In this paper we used a multiparametric approach to analyze extensively the events occurring during apoptotic cell death of thymocytes, and furthermore, we asked whether alterations in mitochondrial structure and function are occurring in early stages of apoptosis. A multiparametric quantitative analysis was performed on normal or apoptotic thymocytes emerging from a few-hour culture performed in culture medium or in the presence of dexamethasone. Simultaneous detection of light scattering properties, integrity of plasma membrane (trypan blue exclusion), chromatin condensation (AO/EB staining of entire cells or PI staining of nuclei), and DNA fragmentation (in situ nick-translation in apoptotic cells) allowed a precise analysis of the preapoptotic and apoptotic stages. Moreover a thorough study of mitochondrial transmembrane potential (delta psi m) assessed following in a time course study the uptake by apoptotic cells of the cationic lipophilic dye DiOC6(3) or the J-aggregate-forming cation JC-1, indicates that a drop in delta psi m occurs very early in thymocyte apoptosis, before DNA fragmentation. This is associated with alteration in mitochondrial structure assessed by cytofluorimetric study of NAO uptake in apoptotic cells. Finally these dramatic alterations in mitochondrial structure and function occurring in early stages of apoptosis were confirmed by confocal and electron microscopy analysis.

Analysis of internucleosomal DNA fragmentation in apoptotic thymocytes by dynamic sieving capillary electrophoresis

The analysis, by slab gel electrophoresis, of internucleosomal DNA cleavage or laddering, characteristic of apoptosis in many cell systems, is labour intensive, difficult to automate and at best only semi-quantitative. In this report we show that CE, using dilute solutions of hydroxyethylcellulose as a replaceable sieving matrix, can be applied to the relatively rapid analysis of DNA laddering in whole digests of apoptotic rat thymocytes. Also, using the sensitivity of laser-induced fluorescence detection and the highly sensitive nuclei acid stain YO-PRO-1, the CE method reported here can use 1000-2000 fold fewer cells than needed for traditional slab gel methods.

A pre-existing protease is a common effector of thymocyte apoptosis mediated by diverse stimuli

Data from a number of model systems support a role for proteolysis in apoptotic cell death. Using immature rat thymocytes, we demonstrate that the inhibitors N-tosyl-L-lysyl chloromethylketone (TLCK) and N-tosyl-L-phenylalanyl chloromethylketone (TPCK) have very different effects on apoptosis. TLCK inhibits apoptosis induced by diverse stimuli at an early stage prior to both DNA fragmentation and cytoplasmic changes. We show that the TLCK-sensitive target is pre-existing and not synthesized in response to apoptotic stimuli. The contrasting effects of TLCK and TPCK support the hypothesis that the TLCK target is a trypsin-like protease which is a common effector of thymocyte apoptosis.

A role for metals and free radicals in the induction of apoptosis in thymocytes

Recent reports have implicated a possible but undefined role for reactive oxygen species in the induction and mediation of apoptosis. In the present study, the role of free radicals and metal ions in apoptosis induced in rat thymocytes by dexamethasone and etoposide was examined. Copper chelators, but not iron specific chelators, inhibited apoptosis induced by both these stimuli. Several antioxidants also possessed potent inhibitory effects. We therefore propose that diverse agents may induce apoptosis in thymocytes by a common mechanism involving a copper mediated Fenton reaction, generating site specific hydroxyl radicals, possibly as a result of activation of the redox sensitive transcription factor NF-kappa B.

Dexamethasone and etoposide induce apoptosis in rat thymocytes from different phases of the cell cycle

Dexamethasone and etoposide both induce apoptosis in immature rat thymocytes. We investigated the dependence of apoptosis on the phase of the cell cycle after incubation with these drugs. Cell cycle progression was followed by a combination of pulse labelling with 5-bromo-2'-deoxyuridine (BrdU), labelling fixed cells with an anti-BrdU antibody and flow cytometry. Dexamethasone had little effect on the cell cycle progression of proliferating thymocytes, while etoposide caused cell cycle arrest. Normal and apoptotic thymocytes were separated by centrifugation on discontinuous Percoll gradients into four fractions (F1-F4). It was found that both dexamethasone and etoposide induced apoptosis in cells in G0/G1 and G2/M of the cell cycle, whereas only etoposide induced apoptosis of cells in S phase. These results demonstrated that dexamethasone induced apoptosis in quiescent cells while only etoposide could induce apoptosis in cells from the proliferative compartment. Following treatment of thymocytes with etoposide, some of the proliferating thymocytes (F1) were converted to cells with intermediate size and density (F3). We have recently identified these cells as a population of preapoptotic thymocytes, at an early stage of apoptosis. These cells then further progressed to fully apoptotic cells (F4). These data support the hypothesis that normal thymocytes (F1) became apoptotic (F4) via an intermediate population (F3).

Immunotoxic organotins as possible model compounds in studying apoptosis and thymocyte differentiation

In the mid-seventies it appeared that some organotin compounds selectively caused thymus atrophy. From that time onward efforts were made to reveal molecular and cellular mechanisms involved. In this review recent studies into organotin-sensitive stages and processes of thymocyte maturation are discussed. Together these studies resulted in the recognition of organotin compounds as possible model compounds in studying immature thymocyte differentiation and protein synthesis-independent apoptotic cell death of thymocytes.

Effect of tri-n-butyltin on intracellular Ca2+ concentration of mouse thymocytes under Ca(2+)-free condition

Effect of tri-n-butyltin at concentrations ranging from 100 nM to 1 microM on the intracellular Ca2+ concentration of mouse thymocytes was examined under Ca(2+)-free conditions in comparison with those of 50 nM A23187, 100 nM thapsigargin and 10 microM cyclopiazonic acid, using the fluorescent dye for intracellular Ca2+, fluo-3. Tri-n-butyltin persistently increased the intensity of fluo-3 fluorescence while A23187, thapsigargin and cyclopiazonic acid produced a transient augmentation of the fluorescence. Pretreatment with A23187 greatly decreased the fluorescence responses induced by 1 microM tri-n-butyltin. However, the effect of thapsigargin and cyclopiazonic acid on the tri-n-butyltin-induced response was much weaker than that of A23187. In the presence of tri-n-butyltin, the transient response produced by A23187 was greatly prolonged. Results may suggest that tri-n-butyltin increases the membrane Ca2+ permeability of the intracellular organelles (cellular calcium stores) and decreases the Ca2+ pump activity of thymocyte membrane, resulting in a sustained increase in the intracellular Ca2+ concentration under Ca(2+)-free concentration.