Tributyltin increases cytosolic free Ca2+ concentration in thymocytes by mobilizing intracellular Ca2+, activating a Ca2+ entry pathway, and inhibiting Ca2+ efflux

Tri-n-butyltin-induced change in cellular level of glutathione in rat thymocytes: a flow cytometric study

Since some of organotins, accumulated in edible mollusks of aquatic environments, exert a variety of toxic actions on experimental animals, it causes concern for the health of humans. We examined the effects of tri-n-butyltin chloride (TBT) and other organotins (triethyltin chloride, trimethyltin chloride, triphenyltin chloride and tetrabutyltin) on cellular content of glutathione (GSH) in rat thymocytes using a flow cytometer to further characterize the toxicity of TBT. When the cells were incubated with TBT at concentrations of 3 nM or more for 15 min, the cellular content of GSH dose-dependently decreased. However, it completely or partly recovered until 180 min even in the continued presence of TBT. This recovery was temperature-sensitive, suggesting an involvement of metabolic process. The efficacy of TBT to decrease the cellular content of GSH was greater than those of other organotins. Results suggest that TBT and some organotins at environmentally relevant (nanomolar) concentrations significantly reduce the cellular content of GSH, suggesting that they increase the vulnerability to some biological and chemical insults.

Activation of mitogen-activated protein kinases by tributyltin in CCRF-CEM cells: role of intracellular Ca(2+)

Effects of tributyltin chloride (TBT) and other organotin compounds on mitogen-activated protein kinases (MAPKs) were examined in CCRF-CEM human T lymphoblastoid cells. In response to the incubation with 0.25-2 microM TBT for 1 h, the levels of the phosphorylated form of extracellular signal-regulated protein kinase (ERK), c-Jun NH(2)-terminal kinase (JNK), and p38 MAPK increased in a dose-dependent manner. The phosphorylation was observed after 15 min and lasted for 4 h following exposure to 1 microM TBT, while the cell viability was not lowered significantly within 6 h. On the other hand, no clear changes were found in the total protein levels of ERK, JNK, and p38 MAPK. The in vitro activities of MAPKs also increased in response to TBT exposure. The potentials of MAPKs phosphorylation and of cellular damage were TBT > dibutyltin dichloride (DBT) > monobutyltin trichloride (MBT). When compared to other triorganotin compounds such as trimethyltin chloride (TMT), triphenyltin chloride (TPT), and triethyltin bromide (TET), TBT exposure induced the most marked phosphorylation of MAPKs. Chelation of intracellular Ca(2+) suppressed TBT-induced MAPKs phosphorylation almost completely, but removal of external Ca(2+) did not. The present results showed that tributyltin is a potent activator of ERK, JNK, and p38 MAPK pathways, and Ca(2+) mobilized from intracellular stores plays an important role for the phosphorylation of MAPKs in this human T cell line.

Cortical astrocytes exposed to tributyltin undergo morphological changes in vitro

We investigated the effect of tributyltin (TBT), an endocrine-disrupting chemical, on the morphology and viability of cultured rat cortical astrocytes. Cultured astrocytes exhibited smooth and planiform morphology under normal conditions. Following exposure to TBT, however, they showed rapid morphological changes that are characterized by asteriated cell bodies and process formation in a time- and concentration-dependent manner. Higher concentrations of TBT produced progressive cell death of the astrocytes. In serum-free medium, TBT at a concentration as low as 200 nM induced the stellation. Pharmacological studies revealed that the morphological changes were alleviated by application of diverse free radical scavengers or antioxidants such as catalase, superoxide dismutase, Trolox, ascorbic acid and N-acetyl-L-cysteine, suggesting that TBT-induced stellation is caused by oxidative stress involving free radicals, particularly reactive oxygen species. Furthermore, we found that the astrocyte stellation was abolished by treatment with inhibitors of phospholipase C, mitogen-activated protein kinase kinase or tyrosine phosphatase. The data suggest that TBT causes the stellation through intracellular signaling cascades rather than its non-specific toxicity. These findings provide an important insight for reconciling the problems in assumed aversive actions of this environmental pollutant for mammals.

Molecular mechanisms of apoptosis induced by cytotoxic chemicals

The purpose of this review article is to discuss established molecular mechanisms of apoptosis and their relevance to cell death induced by environmental toxicants. Apoptosis is a highly regulated form of cell death distinguished by the activation of a family of cysteine-aspartate proteases (caspases) that cleave various proteins resulting in morphological and biochemical changes characteristic of this form of cell death. Abundant evidence supports a role for mitochondria in regulating apoptosis. Specifically, it seems that a number of death stimuli target these organelles and stimulate, by an unknown mechanism, the release of several proteins, including cytochrome c. Once released into the cytosol, cytochrome c binds to its adaptor molecule, Apaf-1, which oligomerizes and then activates pro-caspase-9. Caspase-9 can signal downstream and activate pro-caspase-3 and -7. The release of cytochrome c can be influenced by different Bcl-2 family member proteins, including, but not limited to, Bax, Bid, Bcl-2, and Bcl-X(L). Bax and Bid potentiate cytochrome c release, whereas Bcl-2 and Bcl-X(L) antagonize this event. Although toxicologists have traditionally associated cell death with necrosis, emerging evidence suggests that different types of environmental contaminants exert their toxicity, at least in part, by triggering apoptosis. The mechanism responsible for eliciting the pro-apoptotic effect of a given chemical is often unknown, although in many instances mitochondria appear to be key participants. This review describes our current understanding of the role of apoptosis in environmental toxicant-induced cell death, using dioxin, metals (cadmium and methylmercury), organotin compounds, dithiocarbamates, and benzene as specific examples. Finally, we conclude with a critical discussion of the current knowledge in this area and provide recommendations for future directions.

Cytotoxicity of tributyltin in rat hippocampal slice cultures

The neurotoxic effects of tributyltin (TBT), an endocrine-disrupting chemical, were evaluated in organotypic slice cultures of immature rat hippocampus. Confocal microscopy study with propidium iodide showed that TBT induced severe neuronal death in a concentration- and time-dependent manner with CA3 > CA1 > dentate gyrus ranking of vulnerability of the hippocampal subfields. Dead or damaged neurons exhibited chromatin condensation, which is one of the morphological characteristics of apoptosis, as revealed by acridine orange staining. TBT neurotoxicity was alleviated by application of free radical scavengers or antioxidants, such as catalase, superoxide dismutase, Trolox and alpha-tocopherol but not by ascorbic acid or N-acetyl-L-cysteine, which suggests an involvement of free radicals, particularly reactive oxygen species. Neurons displayed a long-lasting increase in intracellular Ca2+ concentrations after TBT treatment. Although neither N-methyl-D-aspartate (NMDA) receptor inhibitors nor voltage-sensitive Ca2+ channel blockers protected hippocampal neurons against TBT neurotoxicity, non-NMDA receptor antagonist completely prevented TBT-induced neurodegeneration. These data suggest that TBT provokes apoptosis-like neuronal cell death, which might be mediated by intracellular Ca2+ elevation and free radical generation via non-NMDA receptor activation.

The role of calcium in pre- and postmitochondrial events in tributyltin-induced T-cell apoptosis

Using a novel dual-channel FACS methodology, the organotin compound TBT (2 microM) was shown to induce rapid (maximal by 3 min) and sustained elevations in intracellular calcium levels [Ca(2+)](i) in Jurkat T cells. This was preceded by mitochondrial hyperpolarization (maximal at 1 min), with subsequent loss of membrane potential, (Deltapsi(m)) over the next 15 min and was associated with the release of mitochondrial cytochrome c and the activation of type II caspases. The activation of the caspases was blocked by calcium chelation with EGTA and/or BAPTA. Interestingly, changes in Deltapsi(m) caused by TBT were not affected by chelation of intra- and extracellular calcium or by performing the experiments in a Ca(2+)-free medium. TBT also caused rapid elevation of [Ca(2+)](i) in cells lacking glycolytic ATP production. Despite this, the loss of Deltapsi(m) and the activation of type II caspases were delayed (maximal by 2 h) in these cells. Further, there was a failure to activate type II caspases in cells treated with TBT in a Ca(2+)-free medium, despite rapid release of mitochondrial cytochrome c. Consequently, these cells evaded the induction of apoptosis and were diverted to delayed necrotic deletion. Taken together, these data strongly suggest that the rapid rise in [Ca(2+)](i) caused by TBT in Jurkat T cells is not directly coupled to the induction of mitochondrial permeability transition, which rather results from a direct interaction of TBT with mitochondrial component(s) controlling pore transition. However, the rise in [Ca(2+)](i) is a prerequisite for postmitochondrial events involved in caspase activation prior to the induction of apoptosis.

Tributyltin-induced apoptosis requires glycolytic adenosine trisphosphate production

The toxicity of tributyltin chloride (TBT) involves Ca(2+) overload, cytoskeletal damage, and mitochondrial failure leading to cell death by apoptosis or necrosis. Here, we examined whether the intracellular ATP level modulates the mode of cell death after exposure to TBT. When Jurkat cells were energized by the mitochondrial substrate, pyruvate, low concentrations of TBT (1-2 microM) triggered an immediate depletion of intracellular ATP followed by necrotic death. When ATP levels were maintained by the addition of glucose, the mode of cell death was typically apoptotic. Glycolytic ATP production was required for apoptosis at two distinct steps. First, maintenance of adequate ATP levels accelerated the decrease of mitochondrial membrane potential, and the release of the intermembrane proteins adenylate kinase and cytochrome c from mitochondria. A possible role of the adenine nucleotide exchanger in this first ATP-dependent step is suggested by experiments performed with the specific inhibitor, bongkrekic acid. This substance delayed cytochrome c release in a manner similar to that caused by ATP depletion. Second, caspase activation following cytochrome c release was only observed in ATP-containing cells. Bcl-2 had only a minor effect on TBT-triggered caspase activation or cell death. We conclude that intracellular ATP concentrations control the mode of cell death in TBT-treated Jurkat cells at both the mitochondrial and caspase activation levels.

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.

Organotin compounds decrease in vitro survival, proliferation and differentiation of normal human B lymphocytes

Organotin compounds (OTC) are organometallic compounds with vast industrial and agriculture applications that give rise to ubiquitous environmental contamination. OTC are immunotoxic, but most studies have been performed in rodents and almost exclusively focused on T cell immunity. Humans can be exposed to OTC by inhalation, absorption, and consumption of contaminated food and water. To analyse the effects of OTC in human immune tissue, we isolated B cells from tonsils and exposed them to five OTC at various concentrations, during in vitro culture. Non-stimulated B cells were killed by 100 nM of all tested OTC after 8 h in vitro culture, under sub-optimal conditions, except TET. OTC also decreased the proliferation of tonsillar B lymphocytes stimulated with Staphylococcus aureus Cowan 1 (SAC) and IL-2, when present at 100 nM and higher concentrations. IgM secretion was reduced in stimulated cell cultures exposed to 100 nM dibutyltin chloride (DBT). Accordingly, increased phosphatidylserine exposure demonstrated that 100 nM TPT and DBT induced B cells to die by apoptosis. These data indicate that human B cells are diminished in their capacity to survive, proliferate and differentiate in the presence of OTC in vitro.

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.

Caspase involvement in the induction of apoptosis by the environmental toxicants tributyltin and triphenyltin

Organotin compounds such as tributyltin (TBT) and triphenyltin (TPT) can kill target cells by triggering apoptosis. The mechanism by which these environmental toxicants activate the apoptotic program is currently unclear. We have studied the effect of TBT and TPT in the human Hut-78 and Jurkat T-lymphocyte cell lines. Within 1 h there was a 30-fold increase in caspase activity, as measured by the cleavage of the fluorescent peptide DEVD-AMC. Morphological changes characteristic of apoptosis, such as membrane blebbing and nuclear fragmentation, were readily detectable. Blocking caspase activity with the peptide inhibitor z-VAD-fmk prevented all subsequent apoptotic changes. The optimal concentration range for induction of apoptosis was 0.5 to 5 microM TBT. TPT was also able to trigger caspase activity in the lymphocyte cell lines, but it took over 2 h to detect and occurred at a lower concentration range of 0.01 to 1 microM. Higher concentrations of TBT and TPT caused cell necrosis, and we showed that these concentrations were able to inhibit caspase activity in apoptotic cells. TBT and TPT were able interact with a vicinal thiol compound, similar to the known caspase inhibitor phenylarsine oxide, providing a potential mechanism for caspase inhibition. We propose that vicinal thiol proteins may be a general biological target of these organotin compounds, leading to the induction of caspase activity and apoptosis at low concentrations, and more extensive cell damage and necrotic cell death at higher concentrations.

Suppression of calcium oscillation by tri-n-butyltin chloride in cultured rat hepatocytes

The effects of tri-n-butyltin chloride (TBT), an environmental pollutant, on cytoplasmic free calcium ion concentration ([Ca2+]i) were investigated in primary cultured rat hepatocytes. A high concentration (4.0 microM) of TBT increased resting levels of [Ca2+]i and then induced cell blebs resulting in cell death within 2 h. The increase in [Ca2+]i, but not the cell death, depended on the presence of extracellular Ca2+, suggesting that the increase in [Ca2+]i is not critical for the cytotoxicity of TBT. A low concentration (0.1 microM) of TBT did not have any toxic effect (decrease in ATP content, decrease in viability, and shape change) on cultured hepatocytes and did not change [Ca2+]i. However, the calcium responses induced by phenylephrine, [Arg8]-vasopressin, and ATP were suppressed in the cells pretreated with 0.1 microM TBT for 30 min. The suppression was not observed in the cells pretreated with 0.1 microM TBT for only 1 min. Pretreatment with 0.1 microM TBT for 30 min had no effect on the inositol 1,4,5-triphosphate content or its increase in response to hormonal stimulation. These results suggest that TBT suppresses hormone-induced calcium responses at nontoxic low concentrations.

Tributyltin triggers apoptosis in trout hepatocytes: the role of Ca2+, protein kinase C and proteases

The purpose of the present study was to study the mechanisms involved in the induction of apoptosis and by tributyltin (TBT) in rainbow trout hepatocytes, and to examine the role of intracellular Ca2+, protein kinase C (PKC) and proteases in the apoptotic process. The intracellular Ca2+ chelator BAPTA-AM has a suppressive effect on TBT-mediated apoptosis. However, exposure to the ionophore A23187 is not sufficient to induce apoptosis in trout hepatocytes. The results obtained also show that TBT stimulates PKC gamma and delta translocation from cytosol to the plasma membrane in trout hepatocytes after 30 min of exposure. However, PKC gamma translocation is down-regulated after 90 min of treatment. The addition of protein kinase inhibitors (staurosporine and H-7) not only fails to inhibit apoptosis induced by TBT, but also leads to enhancement of DNA fragmentation. These inhibitors also afford a remarkable protection against the loss of plasma membrane integrity caused by TBT exposure. PMA, a direct activator of PKC, fails to stimulate DNA fragmentation. In addition, Z-VAD.FMK is an extremely potent inhibitor of TBT-induced apoptosis in trout hepatocytes, indicating that the activation of ICE-like proteases is a key event in this process. The cysteine protease inhibitor N-ethylmaleimide also prevented TBT-induced DNA fragmentation. Taken together, these data allow for the first time to suggest a mechanistic model of TBT-induced apoptosis. We propose that TBT could trigger apoptosis through a step involving Ca2+ efflux from the endoplasmic reticulum or other intracellular pools and by mechanisms involving cysteine proteases, such as calpains, as well as the phosphorylation status of apoptotic proteins such as Bcl-2 homologues.

Cytochrome c release and caspase activation in hydrogen peroxide- and tributyltin-induced apoptosis

The ability of H2O2 and tributyltin (TBT) to trigger pro-caspase activation via export of cytochrome c from mitochondria to the cytoplasm was investigated. Treatment of Jurkat T lymphocytes with H2O2 resulted in the appearance of cytochrome c in the cytosol within 2 h. This was at least 1 h before caspase activation was observed. TBT caused cytochrome c release already after 5 min, followed by caspase activation within 1 h. Measurement of mitochondrial membrane potential (delta psi(m)) showed that both H2O2 and TBT dissipated delta psi(m), but with different time courses. TBT caused a concomitant loss of delta psi(m) and release of cytochrome c, whereas cytochrome c release and caspase activation preceded any apparent delta psi(m) loss in H2O2-treated cells. Thus, our results suggest that different mechanisms are involved in triggering cytochrome c release with these apoptosis-inducing agents.

Comparative studies on the induction of muscle contracture in mouse diaphragm and Ca2+ release from sarcoplasmic reticulum vesicles by organotin compounds

Effects of organotins, including triethyltin and tributyltin, on skeletal muscle were studied with diaphragm and isolated sarcoplasmic reticulum membrane vesicles. Triethyltin induced muscle contracture in mouse diaphragm while tributyltin had comparatively less potency and efficacy in inducing the muscle contracture. The contracture induced by tributyltin was inhibited when the diaphragm was pretreated with low Ca2+ medium or caffeine while the contracture induced by triethyltin persisted in the Ca2+-free medium but was inhibited by pretreatment of caffeine. Pretreatment of dithiothreitol blocked the contracture induced by tributyltin but not that by triethyltin. Triethyltin dose-dependently induced Ca2+ release from sarcoplasmic reticulum vesicles and inhibited the Ca2+-ATPase activity. These results suggested that triethyltin induced contracture in mouse diaphragm was mainly by induction of Ca2+ release and inhibition of Ca2+ uptake of the internal Ca2+ storage site the sarcoplasmic reticulum, while the tributyltin induced contracture might be due to enhancement of extracellular Ca2+ influx which further induce the release of internal Ca2+ through the Ca2+-induced Ca2+ release mechanism.

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.

Effect of in vitro exposure to tributyltin on generation of oxygen metabolites by oyster hemocytes

Mollusks depend chiefly on hemocyte-mediated cytotoxic mechanisms such as reactive oxygen species (ROS) to defend against pathogenic microorganisms. The effect of in vitro tributyltin chloride (TBT) exposure on ROS generation by oyster (Crassostrea virginica) blood phagocytes is quantified in this study. Luminol-augmented chemiluminescence (LCL) was used to measure ROS activity of resting and zymosan-stimulated cells after 1 or 20 hr TBT exposure. LCL is thought to measure primarily the activity of the myeloperoxidase/hydrogen peroxide/ halide antimicrobial pathway. Hemocytes in TBT-free medium (controls) produced low level LCL, which was markedly stimulated by the addition of zymosan particles. Both resting and zymosan-stimulated LCL values were significantly inhibited by > or = 80 ppb TBT after either 1 or 20 hr of exposure. Exposure to < or = 2 ppb TBT concentrations for 20 hr produced slightly enhanced LCL activity, suggesting a hormesis-like effect. Partial reversibility of TBT suppression of LCL took place when previously exposed cells were put in TBT-free medium. The TBT concentrations used in these studies were not cytolethal in vitro and were considerably less than oyster tissue levels recorded after chronic, sublethal in vitro exposures. The data suggest that the common aquatic contaminant TBT can interact rapidly with C. virginica hemocytes to produce a partially reversible immunotoxicological lesion. Xenobiotic-induced suppression of ROS production by hemocytes may increase host susceptibility to infectious diseases.

Induction of c-fos expression by tributyltin in PC12 cells: involvement of intracellular Ca(2+)

The effects of tributyltin on the expression of immediate early genes, c-fos and c-jun, were examined in PC12 cells using reverse transcription polymerase chain reaction analysis. Tributyltin at concentrations of more than 0.4 μM induced the expression of c-fos after 15 min exposure. The induction of c-fos was accompanied with c-jun expression. Tributyltin-induced c-fos expression was abolished completely by actinomycin D, indicating it was due to transcriptional activation of the gene. Chelation of intracellular Ca(2+) suppressed the expression of c-fos markedly, while removal of external Ca(2+) did not. These results suggest that Ca(2+) mobilized from intracellular stores played a major role in the tributyltin-induced transcriptional activation of c-fos in PC12 cells.

Protease inhibitors block apoptosis at intermediate stages: a compared analysis of DNA fragmentation and apoptotic nuclear morphology

The possible correlation between DNA digestion and changes in nuclear morphology in apoptosis was studied by blocking the apoptotic process at intermediate stages. The apoptogenic action of three drugs: etoposide, puromycin, tributyltin, was contrasted with protease inhibitors with different specificity on U937 cells. The inhibitors interfered with the development of the apoptotic features without shifting cell death to necrosis: treated cells showed abnormal morphologies, which could be recognized as intermediate stages of apoptosis; accordingly, DNA analysis showed an inhibitor-dependent block of the apoptotic DNA digestion. The comparison between size of DNA fragments and nuclear morphology suggested the following correlations: loss of normal nuclear shape with the appearance of a > or = 2 Mb DNA band; ongoing chromatin condensation with the progressive DNA digestion up to 50 kb; nuclear fragmentation with DNA laddering. Protease inhibitors in etoposide-treated cells did not allow the formation of 700-300 kb fragments, suggesting that they possibly derive from a cell-mediated effect.

Tributyltin affects phagocytic activity of Ciona intestinalis hemocytes

Organotin compounds have been used in marine anti-fouling paints as biocides. Because tunicates are vulnerable to these compounds in their natural habitats, we used Ciona intestinalis to establish an assay for phagocytosis in vitro of yeast by hemocytes after exposure to different concentrations (0.0015, 0.015, 0.15 and 1.5 microM) of four organotin compounds: tributyltin (TBT), triphenyltin (TPT), dibutyltin (DBT) and diphenyltin (DPT). To evaluate the phagocytic activity, we used a method based on fluorescence excitation of yeast pre-treated with eosin-Y. The percentage of phagocytosis decreased from 45.1 +/- 3.49 to 22.4 +/- 5.14 at 1.5 microM of TBT (P < 0.001); it was significantly reduced in presence of the ionophore A23187. TPT, DPT and DBT did not show significant effects on phagocytosis. Because the effect of TBT was irreversible, differences between the inhibitory mechanisms of ionophore and TBT are suggested. These results indicate that for future analyses, tunicates should become excellent models for dissecting events such as phagocytosis that are associated with immunosuppression after exposure to xenobiotics.

Ca(2+)-dependent mechanisms of cytotoxicity and programmed cell death

There is increasing evidence that the calcium ion plays a critical role in both toxic cell killing and programmed cell death. Thus, in a variety of experimental systems a perturbation of intracellular Ca2+ homeostasis due to increased Ca2+ influx and/or inhibition of Ca2+ extrusion has been found to be an early event in the development of cell injury. It is clear that sustained increases in intracellular Ca2+ can activate cytotoxic mechanisms which result in perturbations of cellular structure and function. For example, the stimulation of Ca(2+)-dependent proteases can result in a disruption of cytoskeletal organization and the formation of surface protrusions (blebs) and Ca(2+)-mediated phospholipase activation can result in an impairment of mitochondrial function with collapse of membrane potential and cessation of ATP synthesis. The activation of a Ca2+, Mg(2+)-dependent nuclear endonuclease is associated with chromatin cleavage and appears to play a crucial role in programmed cell death (apoptosis) in the immune system and other tissues. There is also recent evidence that this process may be responsible for the immunotoxicity of dioxins and organotin compounds and involved in the killing of adenocarcinoma cells by tumor necrosis factor alpha. Although calcium ions appear to be required for endonuclease activity during apoptosis, this process is also influenced by other factors, e.g. protein kinase C activity, intracellular polyamine and Zn2+ levels, chromatin structure, etc. Thus, the regulation of endonuclease activity under both physiological and toxicological conditions appears to be complex and to involve multiple factors.