Role of mitochondria and calcium ions in tributyltin-induced gene regulatory pathways

Tributyltin induces premature hatching and reduces locomotor activity in zebrafish (Danio rerio) embryos/larvae at environmentally relevant levels

Tributyltin (TBT) is an organotin compound that is the active ingredient of many biocides and antifouling agents. In addition to its well established role as an endocrine disruptor, TBT is also associated with adverse effects on the nervous system and behavior. In this study, zebrafish (Danio rerio) embryos were exposed to environmentally relevant concentrations of TBT (0.01, 0.1, 1 nM) to determine how low levels affected development and behavior. Fish exposed to 1 nM TBT hatched earlier when compared to controls. Following a 96-h exposure, total swimming distance, velocity, and activity of zebrafish larvae were reduced compared to controls. To identify putative mechanisms for these altered endpoints, we assessed embryo bioenergetics and gene expression. We reasoned that the accelerated hatch time could be related to ATP production and energy, thus embryos were exposed to TBT for 24 and 48-h exposure prior to hatch. There were no differences among groups for endpoints related to bioenergetics (i.e. basal, ATP-dependent, and maximal respiration). To address mechanisms related to changes in behavioral activity, we measured transcripts associated with muscle function (myf6, myoD, and myoG) and dopamine signaling (th, dat, dopamine receptors) as dopamine regulates behavior. No transcript was altered in expression by TBT in larvae, suggesting that other mechanisms exist that may explain changes in higher level endpoints. These results suggest that endpoints related to the whole animal (i.e. timing of hatch and locomotor behavior) are more sensitive to environmentally-relevant concentrations of TBT compared to the molecular and metabolic endpoints examined here.

Evaluation of toxicological effects induced by tributyltin in clam Ruditapes decussatus using high-resolution magic angle spinning nuclear magnetic resonance spectroscopy: Study of metabolic responses in heart tissue and detection of a novel metabolite

Tributyltin (TBT) is a highly toxic pollutant present in many aquatic ecosystems. Its toxicity in mollusks strongly affects their performance and survival. The main purpose of this study was to elucidate the mechanisms of TBT toxicity in clam Ruditapes decussatus by evaluating the metabolic responses of heart tissues, using high-resolution magic angle-spinning nuclear magnetic resonance (HRMAS NMR), after exposure to TBT (10^-9, 10^-6 and 10^-4 M) during 24 h and 72 h. Results show that responses of clam heart tissue to TBT exposure are not dose dependent. Metabolic profile analyses indicated that TBT 10^-6 M, contrary to the two other doses tested, led to a significant depletion of taurine and betaine. Glycine levels decreased in all clam groups treated with the organotin. It is suggested that TBT abolished the cytoprotective effect of taurine, betaine and glycine thereby inducing cardiomyopathie. Moreover, results also showed that TBT induced increase in the level of alanine and succinate suggesting the occurrence of anaerobiosis particularly in clam group exposed to the highest dose of TBT. Taken together, these results demonstrate that TBT is a potential toxin with a variety of deleterious effects on clam and this organotin may affect different pathways depending to the used dose. The main finding of this study was the appearance of an original metabolite after TBT treatment likely N-glycine-N'-alanine. It is the first time that this molecule has been identified as a natural compound. Its exact role is unknown and remains to be elucidated. We suppose that its formation could play an important role in clam defense response by attenuating Ca²⁺ dependent cell death induced by TBT. Therefore this compound could be a promising biomarker for TBT exposure.

Toxicity of tributyltin (TBT) to terrestrial organisms and its species sensitivity distribution

The contamination of the terrestrial environment by disposal of tributyltin (TBT) by contaminated harbour sediments, sewage sludge and/or biocide products has been raising concerns and it may pose a risk to soil invertebrates and plants. This study aimed to improve the amount and quality of data for TBT toxicity in soils in order to assess the ecological risk of TBT to the terrestrial ecosystems. For this, bioassays were performed with the species Porcellionides pruinosus, Folsomia candida, Brassica rapa and Triticum aestivum to evaluate the toxic effects of TBT (as chloride) on these species. Additionally, this study contributed to increase the amount of data concerning TBT toxicity on soil dwelling organisms. The results showed a dose-response relationship between TBT concentration and the increase of toxicity in all species tested. These results were collated with results from literature to construct species sensitivity distributions (SSDs) and to calculate the hazardous concentration at 5% (HC₅) for all data, for each type of soil and TBT formulation used. The HC₅ value for TBT in soil was 2.06 mg TBT/kg soil dw. Little information is available concerning the concentrations of TBT in soils. In addition the predicted no-effect concentration (PNEC) value was determined to be 30 μg/kg soil. Only one study was found referring to TBT contaminated soils, and where TBT concentrations were lower than 0.024 μg TBT/kg for the wetland soil. Therefore it can be concluded that the real TBT concentrations determined represent low risk for environmental effects. In conclusion, the construction of SSDs and the calculation of HC5 using all the data available showed to be a more suitable method rather than the construction of several SSDs for each soil and TBT types. Further investigations concerning TBT concentrations and toxicity on soil organisms need to be performed to increase data and improve risk calculations.

Role of NF-κB activation in LPS-induced endothelial barrier breakdown

Endothelial barrier breakdown contributes to organ failure in sepsis. The key mechanism by which the potent sepsis inductor lipopolysaccharide (LPS) disrupts the endothelial barrier is controversial. Here, we tested the hypothesis that NF-κB activation is critically involved in endothelial barrier breakdown. Application of LPS to monolayers of porcine pulmonary artery endothelial cells (PAEC) and human dermal microvascular endothelial cells (HDMEC) induced a rapid and sustained activation of NF-κB as revealed by translocation of its subunit p65 into the nuclei in nuclear extraction assays and by immunostaining. Measurements of transendothelial electrical resistance (TER) and intercellular gap formation demonstrated significant breakdown of endothelial barrier properties following LPS treatment for 3 h. Interestingly, monolayers recovered spontaneously beginning after 10 h. Increased cAMP prevented LPS-induced loss of endothelial barrier properties, but did not block NF-κB activation. Application of the cell-permeable NEMO-binding domain (NBD) synthetic peptide was effective to prevent NF-κB activation, but did neither block LPS-induced loss of TER nor intercellular gap formation. NBD peptide alone did not alter endothelial barrier properties, but enhanced the barrier-compromising effects when applied in combination with LPS. Similarly, siRNA-mediated knock-down of p65 in HDMECs did not prevent LPS-induced barrier breakdown. Known targets of NF-κB-derived protein expression of caveolin or vasodilator-stimulated phosphoprotein (VASP) remained unaltered by LPS treatment of endothelial cells. In summary, our data indicate that NF-κB activation by LPS is not critically involved in disruption of endothelial barrier properties. Rather, our data suggest that NF-κB activation acts as a part of a rescue mechanism.

Dibutyltin exposure decreases granzyme B and perforin in human natural killer cells

Natural killer (NK) cells are a subset of lymphocytes that are capable of killing tumor and virally-infected cells. Dibutyltin (DBT) is a catalyst in the production of PVC plastics and a breakdown product of tributyltin (TBT). DBT is a significant environmental contaminant. This study investigates the mechanism by which DBT exposure decreases the immune function of human NK cells. NK cells destroy their target cells by releasing cytotoxic proteins, perforin, and granzyme B. We examined the effect of DBT exposures on the levels of cytotoxic proteins and their mRNAs. Exposure of NK cells to DBT for 1h caused significant decreases in the mRNAs for granzyme B and perforin but not in protein levels. A 24h exposure to DBT decreased mRNAs as well as protein levels for both granzyme B and perforin. Exposure to DBT for 1h followed by either a 24 or 48h period in DBT-free media, decreased levels of granzyme B and perforin. The results indicate that decreases in granzyme B and perforin levels in NK cells are consequences of DBT exposure. Additionally, DBT causes rapid decreases in mRNAs for perforin and granzyme B, suggesting decreases in transcription and/or increases in mRNA degradation.

Environmental chemical tributyltin augments adipocyte differentiation

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

Elevated Ca2+i transients induced by trimethyltin chloride in HeLa cells: types and levels of response

Humans are exposed to organotins, like trimethyltin (TMT) chloride via air, water and food, and intoxication might result in severe health complications. Toxic effects of organotin compounds are well documented, but possible mechanisms remain unclear and only little information is available how organometallic species interact with calcium controlling mechanisms. Therefore, the aim of this work was to investigate the effects of TMT on calcium homeostasis in HeLa S3 cells. Dynamic changes of cytosolic calcium (Ca2+(i)) were monitored using laser-scanning microscopy and fluo-4 loaded cells. Application of TMT resulted in sustained as well as in transient elevations of Ca2+(i). The number of reacting cells was directly correlated to the concentration of TMT used: with 500 microM TMT all cells reacted, with 50 microM TMT 80% and with 5 microM 74%. The fast Ca2+(i)-transients (spikes), measured in single cells, occurred even with 0.25 microM TMT and varied in size and duration. The sustained increase of Ca2+(i), measured as the average over all cells, was dose dependent with an approximately 8% increase for 5 microM TMT, approximately 12.3% for 50 microM and approximately 145% for 500 microM TMT. Moreover, this effect was partly reversible. A second application resulted in a similar sustained rise of Ca2+(i) compared to the first application of TMT, there was also no difference when no calcium was added to the external solution (151+/-10% compared to 145+/-15%; 500 microM TMT). This rise of Ca2+(i) was highly reduced (<10% increase) when the internal calcium stores were depleted before TMT (500 microM) was applied. Our data suggest that TMT influences Ca2+(i)-homeostasis of HeLa S3 cells, which might be related to its toxicity in this cell line.

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

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

Involvement of mitochondrial and death receptor pathways in tributyltin-induced apoptosis in rat hepatocytes

Tri-n-butyltin (TBT), a biocide, is known for its immunotoxicity and hepatotoxicity and is a well-characterised mitochondrial toxin. This report investigates the mechanisms involved in induction of apoptosis by TBT in primary cultures of rat hepatocytes. Release of cytochrome c from mitochondria into the cytosol was apparent after 15 min of exposure to 2.5 microM TBT. In addition, activity of initiator caspase-9 increased after 30 min, representing activation of the mitochondrial pathway in hepatocytes. The death receptor pathway was also activated by TBT, as indicated by recruitment of the adaptor protein FADD from the cytosol to the membrane as soon as 15 min after treatment. In addition, levels of the pro-apoptotic protein Bid decreased in the cytosol, while there was an increase in levels of the cleaved form tBid, in TBT-treated hepatocytes. Activity of initiator caspase-8 increased after 30 min. The principal effector caspase-3 was activated following 30 min of treatment with TBT. Activation was confirmed by immunodetection of a 17-kDa cleaved fragment. Apoptotic substrates such as Poly(ADP-ribose) polymerase and DNA fragmentation factor-45 are cleaved by caspase-3 to ensure the dismantlement of the cell. Cleavage of Poly(ADP-ribose) polymerase into a 85-kDa fragment appeared after 30 min of TBT treatment. DNA fragmentation factor-45 disappeared in TBT-exposed rat hepatocytes. This is the first detailed study reporting the involvement of initiator and effector caspases, cleavage of their intracellular substrates and activation of both death receptor and mitochondrial pathways in TBT-induced apoptosis in rat hepatocytes. The comprehension of molecular events of apoptosis is important for the evaluation of the risk to humans and animals.

Effects of tributyltin on barrier functions in human intestinal Caco-2 cells

The effect of tributyltin (TBT) on human intestinal epithelial cell functions was investigated by using human intestinal Caco-2 cell monolayers. We paid particular attention to the effect of TBT on two barrier functions: the tight junction as a physical barrier and MDR1/P-glycoprotein as a biological barrier. A loss of monolayer integrity was apparent from the TBT treatment and the paracellular permeability was increased by TBT. On the other hand, the activity of P-glycoprotein, which was examined by measuring the accumulation of Rhodamine-123 and daunomycin, was increased by prolonged TBT treatment in a concentration-dependent manner (1-100 nM). Furthermore, it was clarified by Western and Northern blots that this increase was accompanied by the increased expression of MDR1 mRNA and protein. The activation of a multidrug resistance transporter P-glycoprotein by TBT would cause a disorder of the human intestines by changing the drug pharmacokinetics.

Induction of adipose differentiation related protein and neutral lipid droplet accumulation in keratinocytes by skin irritants

Keratinocytes play an important role in skin irritation. In an attempt to investigate mechanistic bases of human skin irritation response, we recently identified the upregulation by skin irritants of adipose differentiation related protein (ADRP) in reconstituted human epidermis. ADRP is a lipid-storage-droplet-associated protein, governing deposition and release of lipids from droplets. The purpose of this study was to characterize, in a human keratinocyte cell line (NCTC 2544), sodium-dodecyl-sulfate-induced ADRP expression, to identify the biochemical events that lead to ADRP expression, and to understand its function in sodium dodecyl sulfate cytotoxicity. Sodium dodecyl sulfate induced a concentration- and time-related production of ADRP that was associated with lipid droplet accumulation. Lipid accumulation following sodium dodecyl sulfate treatment was due to intracellular redistribution rather than lipid neosynthesis, as indicated by equivalent 14C-oleate and 14C-acetate incorporations. Other skin irritants, namely benzalkonium chloride, tributyltin, and 12-O-tetradecanoylphorbol 13-acetate, also induce lipid droplet accumulation. Sodium-dodecyl-sulfate-induced ADRP expression and lipid droplet accumulation were modulated by the calcium chelator BAPTA, indicating a role of calcium in ADRP induction. Decrease of sodium-dodecyl-sulfate-induced ADRP expression by specific ADRP antisense oligonucleotide resulted in increased cytotoxicity, indicating a protective role of ADRP and lipid accumulation in the process of cell damage induced by skin irritants. ADRP expression was also induced in vivo following treatment with sodium dodecyl sulfate in an experimental model of skin irritation, indicating that the in vitro model represents irritation.

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

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

Nuclear factor kappaB (NF-kappaB) activation by hydrogen peroxide in human epidermal keratinocytes and the restorative effect of interleukin-10

The heterodimeric form of nuclear factor kappaB (NF-kappaB), NF-kappaB1/RelA, is one of the pluripotential transcription factors that activates various genes encoding cytokines and cell adhesion molecules. To clarify the involvement of radical oxygen species in the NF-kappaB activation pathway in keratinocytes, we examined the effect of hydrogen peroxide (H(2)O(2)) on the activation of NF-kappaB in cultured normal human epidermal keratinocytes. After the treatment of keratinocytes with 300 microM H(2)O(2), a translocation of NF-kappaB from the cytoplasm to nucleus was observed in an immunofluorescence study using anti-human NF-kappaB1 and anti-human RelA antibodies. Specific DNA binding was observed with the nuclear extract prepared from the H(2)O(2)-treated keratinocytes by the electrophoretic mobility shift assay. The presence of N-acetyl-L-cysteine or pyrrolidine dithiocarbamate during H(2)O(2) treatment prevented the nuclear localization of NF-kappaB. The involvement of radical oxygen species in the NF-kappaB activation pathway was suggested. Pretreatment of keratinocytes with 10 ng/ml of recombinant human interleukin-10 (IL-10) for 24 h suppressed the nuclear translocation of NF-kappaB induced by H(2)O(2). IL-10, which increases in ultraviolet-irradiated skin and suppresses delayed type hypersensitivity in vivo, may play an inhibitory role in cutaneous inflammation by inhibiting the NF-kappaB activation pathway in keratinocytes.

Effect of tributyltin chloride on the release of calcium ion from intracellular calcium stores in rat hepatocytes

The effects of tri-n-butyltin chloride (TBT), an environmental pollutant, on the release of Ca(2+) from intracellular stores were investigated in isolated rat hepatocytes. Isolated hepatocytes permeabilized with digitonin were suspended in solution, and the concentration of extracellular Ca(2+) was measured, using a fluorescent Ca(2+) dye, fura-2. In the solution containing permeabilized hepatocytes that had been preincubated with 4.0 microM TBT for 30 min, the extracellular Ca(2+) concentration was high, but the inositol 1,4,5-trisphosphate (InsP(3))-induced increase in Ca(2+) concentration was suppressed, suggesting that the extracellular release of Ca(2+) in response to TBT treatment was from intracellular stores. Images of the Ca(2+) concentration in the intracellular stores of primary cultured hepatocytes loaded with fura-2 were obtained after digitonin-permeabilization, using digitalized fluorescence microscopy. The permeabilized hepatocytes that had been preincubated with 4.0 microM TBT for 30 min had a very low fura-2 fluorescence ratio (340/380 nm), suggesting that stored Ca(2+) was released. When the hepatocytes were treated with 4.0 microM TBT after digitonin-permeabilization, the decrease in the fura-2 fluorescence ratio was very small. However, when the permeabilized hepatocytes were incubated with 4.0 microM TBT and 2.0 microM NADPH, the decrease was enhanced, raising the possibility that TBT might be metabolized to the active form(s), thus releasing Ca(2+) from intracellular stores. When the hepatocytes were preincubated with 0.1 microM TBT for 30 min and then were permeabilized, the fura-2 fluorescence ratio was almost the same as that in the control permeabilized hepatocytes. However, the InsP(3)-induced decrease in the fluorescence ratio was suppressed significantly in the permeabilized hepatocytes. These results suggest that TBT released Ca(2+) from the intracellular stores at high concentrations, and suppressed the InsP(3)-induced Ca(2+) release at non-toxic low concentrations. It is probable that the latter effect was responsible for the previously reported suppression of Ca(2+) response induced by hormonal stimulations (Kawanish et al., Toxicol Appl Pharmacol 1999;155:54-61).

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.

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.

Pyrrolidine dithiocarbamate prevents ethanol-induced elevation of [Ca2+]i in cultured canine cerebral vascular smooth muscle cells

Pyrrolidine dithiocarbamate (PDTC) has been shown to block nuclear transcription factor (NF-kappaB) activation induced by a wide range of stimuli in different cell lines. NF-kappaB is a common element of the promoter region of inflammatory cytokines which can be stimulated by ethanol. Recently, we have shown that PDTC can ameliorate cerebrovascular damage, brain cortical damage, leukocyte adhesion and rolling, and stroke induced by ethanol. We, therefore, tested the effects of preincubation with PDTC on alcohol-induced changes in intracellular free calcium ions ([Ca2+]i) in cultured canine cerebral smooth muscle cells. These vascular cells, chronically treated with ethanol (10-100 mM) for 24 and 72 h, exhibited concentration-dependent rises in [Ca2+]i. PDTC (0.1 microM) itself failed to influence resting levels of [Ca2+]i in these vascular muscle cells. PDTC (0.1 microM) pretreatment, however, inhibited completely the elevations in [Ca2+]i induced by chronic ethanol (10-100 mM). The present results suggest that ethanol-induced production of reactive oxygen species and elevation of [Ca2+]i in cultured canine cerebral vascular smooth muscle cells triggers induction of transcription factor NF-kappaB, which could play an important role in alcohol-induced brain damage and stroke.

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.