In vitro interactions of thallium with components of the glutathione-dependent antioxidant defence system

Thallium - poisoner's poison: An overview and review of current knowledge on the toxicological effects and mechanisms

Thallium (Tl) is one of the most toxic metals and its historic use in homicides has led it to be known as "the poisoner's poison." This review summarizes the methods for identifying Tl and determining its concentrations in biological samples in recently reported poisoning cases, as well as the toxicokinetics, toxicological effects, toxicity mechanisms, and detoxication methods of Tl. Recent findings regarding Tl neurotoxicological pathways and toxicological effects of Tl during pregnancy are also presented. Confirmation of elevated Tl concentrations in blood, urine, or hair is indispensable for diagnosing Tl poisoning. The kidneys show the highest Tl concentration within 24 h after ingestion, while the brain shows the highest concentration thereafter. Tl has a very slow excretion rate due to its large distribution volume. Following acute exposure, gastrointestinal symptoms are observed at an early stage, and neurological dysfunction is observed later: Tl causes the most severe damage in the central nervous system. Alopecia and Mees' lines in the nails are observed within 1 month after Tl poisoning. The toxicological mechanism of Tl is considered to be interference of vital potassium-dependent processes with Tl+ because its ionic radius is similar to that of K+, as well as inhibition of enzyme reactions by the binding of Tl to -SH groups, which disturbs vital metabolic processes. Tl toxicity is also related to reactive oxygen species generation and mitochondrial dysfunction. Prussian blue is the most effective antidote, and metallothionein alone or in combination with Prussian blue was recently reported to have cytoprotective effects after Tl exposure. Because Tl poisoning cases are still reported, early determination of Tl in biological samples and treatment with an antidote are essential.

Thallium Induces Antiproliferative and Cytotoxic Activity in Glioblastoma C6 and U373 Cell Cultures via Apoptosis and Changes in Cell Cycle

Thallium (Tl⁺) is a heavy metal that causes toxicity in several organs, including the brain. Its cytotoxic profile, combined with its affinity for tumor cells when used as a radioligand for labeling these cells, suggests its potential use as antitumor therapy. In this study, glioblastoma cell lines C6 (from rat) and U373 (from human) were exposed to increased concentrations of thallium(I) acetate (5, 10, 50, 100, or 200 µM) and several toxic endpoints were evaluated, including loss of confluence and morphological changes, loss of cell viability, changes in cell cycle, and apoptosis. Tl⁺ was detected in cells exposed to thallium(I) acetate, demonstrating efficient uptake mechanism. Confluence in both cell lines decreased in a concentration-dependent manner (50-200 µM), while morphological changes (cell shrinkage and decreased cell volume) were more evident at exposures to higher Tl⁺ concentrations. For both parameters, the effects of Tl⁺ were more prominent in C6 cells compared to U373 cells. The same trend was observed for cell viability, with Tl⁺ affecting this parameter in C6 cells at low concentrations, whereas U373 cells showed greater resistance, with significant changes observed only at the higher concentrations. C6 and U373 cells treated with Tl⁺ also showed morphological characteristics corresponding to apoptosis. The cytotoxic effects of Tl⁺ were also assessed in neural and astrocytic primary cultures from the whole rat brain. Primary neural and astrocytic cultures were less sensitive than C6 and U373 cells, showing changes in cell viability at 50 and 100 µM concentrations, respectively. Cell cycle in both brain tumor cell lines was altered by Tl⁺ in G1/G2 and S phases. In addition, when combined with temozolamide (500 µM), Tl⁺ elicited cell cycle alterations, increasing SubG1 population. Combined, our novel results characterize and validate the cytotoxic and antiproliferative effects of Tl⁺ in glioblastoma cells.

Protective effect of isoflavones and triterpenoid saponins from pueraria lobata on liver diseases: A review

In recent years, with the improvement of people's living standard and the change of diet structure, liver disease and its related complications have become a significant public health problem globally. Pueraria lobata (Pueraria montana var. lobata (Willd.) Sanjappa & Pradeep) belongs to the genus Pueraria, which is widely planted and used as medicine and food in Asia with a long history. A variety of natural active products, including puerarin, daidzein, formononetin, genistein, and soyasaponin, have been isolated and identified from pueraria lobata. A large number of studies have shown that various natural active products of pueraria lobata can play a protective role in different types of liver diseases by regulating oxidative stress, inflammatory response, lipid metabolism, etc. In this review, we focused on the protective effects of isoflavones and triterpenoid saponins from pueraria lobata on the liver through different targeted therapeutic mechanisms. What's more, we summarized their therapeutic potential for different types of liver diseases to provide evidence for their clinical application.

Thallium Toxicity in Caenorhabditis elegans: Involvement of the SKN-1 Pathway and Protection by S-Allylcysteine

Monovalent thallium (Tl⁺) is a cation that can exert complex neurotoxic patterns in the brain by mechanisms that have yet to be completely characterized. To learn more about Tl⁺ toxicity, it is necessary to investigate its major effects in vivo and its ability to trigger specific signaling pathways (such as the antioxidant SKN-1 pathway) in different biological models. Caenorhabditis elegans (C. elegans) is a nematode constituting a simple in vivo biological model with a well-characterized nervous system, and high genetic homology to mammalian systems. In this study, both wild-type (N2) and skn-1 knockout (KO) mutant C. elegans strains subjected to acute and chronic exposures to Tl⁺ [2.5-35 μM] were evaluated for physiological stress (survival, longevity, and worm size), motor alterations (body bends), and biochemical changes (glutathione S-transferase regulation in a gst-4 fluorescence strain). While survival was affected by Tl⁺ in N2 and skn-1 KO (worms lacking the orthologue of mammalian Nrf2) strains in a similar manner, the longevity was more prominently decreased in the skn-1 KO strain compared with the wild-type strain. Moreover, chronic exposure led to a greater compromise in the longevity in both strains compared with acute exposure. Tl⁺ also induced motor alterations in both skn-1 KO and wild-type strains, as well as changes in worm size in wild-type worms. In addition, preconditioning nematodes with the well-known antioxidant S-allylcysteine (SAC) reversed the Tl⁺-induced decrease in survival in the N2 strain. GST fluorescent expression was also decreased by the metal in the nematode, and recovered by SAC. Our results describe and validate, for the first time, features of the toxic pattern induced by Tl⁺ in an in vivo biological model established with C. elegans, supporting an altered redox component in Tl⁺ toxicity, as previously described in mammal models. We demonstrate that the presence of the orthologous SKN-1 pathway is required for worms in evoking an efficient antioxidant defense. Therefore, the nematode represents an optimal model to reproduce mammalian Tl⁺ toxicity, where toxic mechanisms and novel therapeutic approaches of clinical value may be successfully pursued.

Thallium-Induced Toxicity in Rat Brain Crude Synaptosomal/Mitochondrial Fractions is Sensitive to Anti-excitatory and Antioxidant Agents

The mechanisms by which the heavy metal thallium (Tl⁺) produces toxicity in the brain remain unclear. Herein, isolated synaptosomal/mitochondrial P2 crude fractions from adult rat brains were exposed to Tl⁺ (5-250 μM) for 30 min. Three toxic endpoints were evaluated: mitochondrial dysfunction, lipid peroxidation, and Na⁺/K⁺-ATPase activity inhibition. Concentration-response curves for two of these endpoints revealed the optimum concentration of Tl⁺ to induce damage in this preparation, 5 μM. Toxic markers were also estimated in preconditioned synaptosomes incubated in the presence of the N-methyl-D-aspartate receptor antagonist kynurenic acid (KYNA, 50 μM), the cannabinoid receptor agonist WIN 55,212-2 (1 μM), or the antioxidant S-allyl-L-cysteine (SAC, 100 μM). All these agents prevented Tl⁺ toxicity, though SAC did it with lower efficacy. Our results suggest that energy depletion, oxidative damage, and Na⁺/K⁺-ATPase activity inhibition account for the toxic pattern elicited by Tl⁺ in nerve terminals. In addition, the efficacy of the drugs employed against Tl⁺ toxicity supports an active role of excitatory/cannabinoid and oxidative components in the toxic pattern elicited by the metal.

Effects of endocrine disrupting chemicals on estrogen receptor alpha and heat shock protein 60 gene expression in primary cultures of loggerhead sea turtle (Caretta caretta) erythrocytes

The loggerhead turtle (Caretta caretta) can be considered a good indicator species for studying the ecological impact of endocrine disrupting chemicals (EDCs) on wildlife. However, the effect of these environmental pollutants on nuclear steroid hormone signaling has not yet been addressed in sea turtles mainly due to the legal constraints of their endangered status. Here we describe the use of primary erythrocyte cell cultures as in vitro models for evaluating the effects of different EDCs on the expression of estrogen receptor α (ERα). In addition, we evaluated erythrocyte toxicity caused by EDCs using Alamar Blue assay and heat shock proteins 60 (HSP60) expression. Primary cultures of erythrocytes were exposed to increasing concentrations of 4-nonylphenol (4NP), Diisodecyl phthalate (DiDP), Tri-m-cresyl phosphate (TMCP) and Tributyltin (TBT) for 48h. Alamar Blue demonstrated that exposure of erythrocytes to each contaminant for up to 48h led to a significant impairment of cellular metabolic activity at 100μM, with the exception of TBT. Moreover, our data indicate that loggerhead erythrocytes constitutively express ERα and HSP60 at the transcript level and respond to EDCs by up-regulating their expression. In this regard, ERα was up-regulated in a dose-dependent manner after 48h exposure to both 4NP and TMCP. Interestingly, the dosage-dependent effects of DiDP on ERα expression were opposite in comparison to that obtained following exposure to the other tested compounds. This work provides the first indication regarding the potential of primary erythrocytes as study models for evaluating the effects of EDCs on sea turtles.

Early response of glutathione- and thioredoxin-dependent antioxidant defense systems to Tl(I)- and Tl(III)-mediated oxidative stress in adherent pheochromocytoma (PC12adh) cells

Thallium (Tl) is a toxic heavy metal that causes oxidative stress both in vitro and in vivo. In this work, we evaluated the production of oxygen (ROS)- and nitrogen (RNS)-reactive species in adherent PC12 (PC12adh) cells exposed for 0.5-6 h to Tl(I) or Tl(III) (10-100 µM). In this system, Tl(I) induced mostly H₂O₂ generation while Tl(III) induced H₂O₂ and ONOO^·- generation. Both cations enhanced iNOS expression and activity, and decreased CuZnSOD expression but without affecting its activity. Tl(I) increased MnSOD expression and activity but Tl(III) decreased them. NADPH oxidase (NOX) activity remained unaffected throughout the period assessed. Oxidant levels returned to baseline values after 6 h of incubation, suggesting a response of the antioxidant defense system to the oxidative insult imposed by the cations. Tl also affected the glutathione-dependent system: while Tl(III) increased glutathione peroxidase (GPx) expression and activity, Tl(I) and Tl(III) decreased glutathione reductase (GR) expression. However, GR activity was mildly enhanced by Tl(III). Finally, thioredoxin-dependent system was evaluated. Only Tl(I) increased 2-Cys peroxiredoxins (2-Cys Prx) expression, although both cations increased their activity. Tl(I) increased cytosolic thioredoxin reductase (TrxR1) and decreased mitochondrial (TrxR2) expression. Tl(III) had a biphasic effect on TrxR1 expression and slightly increased TrxR2 expression. Despite of this, both cations increased total TrxR activity. Obtained results suggest that in Tl(I)-exposed PC12adh cells, there is an early response to oxidative stress mainly by GSH-dependent system while in Tl(III)-treated cells both GSH- and Trx-dependent systems are involved.

Overlapping toxic effect of long term thallium exposure on white mustard (Sinapis alba L.) photosynthetic activity

BACKGROUND

Heavy metal exposure affect plant productivity by interfering, directly and indirectly, with photosynthetic reactions. The toxic effect of heavy metals on photosynthetic reactions has been reported in wide-ranging studies, however there is paucity of data in the literature concerning thallium (Tl) toxicity. Thallium is ubiquitous natural trace element and is considered the most toxic of heavy metals; however, some plant species, such as white mustard (Sinapis alba L.) are able to accumulate thallium at very high concentrations. In this study we identified the main sites of the photosynthetic process inhibited either directly or indirectly by thallium, and elucidated possible detoxification mechanisms in S. alba.

RESULTS

We studied the toxicity of thallium in white mustard (S. alba) growing plants and demonstrated that tolerance of plants to thallium (the root test) decreased with the increasing Tl(I) ions concentration in culture media. The root growth of plants exposed to Tl at 100 μg L(-1) for 4 weeks was similar to that in control plants, while in plants grown with Tl at 1,000 μg L(-1) root growth was strongly inhibited. In leaves, toxic effect became gradually visible in response to increasing concentration of Tl (100 - 1,000 μg L(-1)) with discoloration spreading around main vascular bundles of the leaf blade; whereas leaf margins remained green. Subsequent structural analyses using chlorophyll fluorescence, microscopy, and pigment and protein analysis have revealed different effects of varying Tl concentrations on leaf tissue. At lower concentration partial rearrangement of the photosynthetic complexes was observed without significant changes in the chloroplast structure and the pigment and protein levels. At higher concentrations, the decrease of PSI and PSII quantum yields and massive oxidation of pigments was observed in discolored leaf areas, which contained high amount of Tl. Substantial decline of the photosystem core proteins and disorder of the photosynthetic complexes were responsible for disappearance of the chloroplast grana.

CONCLUSIONS

Based on the presented results we postulate two phases of thallium toxicity on photosynthesis: the non-destructive phase at early stages of toxicant accumulation and the destructive phase that is restricted to the discolored leaf areas containing high toxicant content. There was no distinct border between the two phases of thallium toxicity in leaves and the degree of toxicity was proportional to the migration rate of the toxicant outside the vascular bundles. The three-fold (nearly linear) increase of Tl(I) concentration was observed in damaged tissue and the damage appears to be associated with the presence of the oxidized form of thallium - Tl(III).

To involvement the conformation of the adenine nucleotide translocase in opening the Tl(+)-induced permeability transition pore in Ca(2+)-loaded rat liver mitochondria

The conformation of adenine nucleotide translocase (ANT) has a profound impact in opening the mitochondrial permeability transition pore (MPTP) in the inner membrane. Fixing the ANT in 'c' conformation by phenylarsine oxide (PAO), tert-butylhydroperoxide (tBHP), and carboxyatractyloside as well as the interaction of 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) with mitochondrial thiols markedly attenuated the ability of ADP to inhibit the MPTP opening. We earlier found (Korotkov and Saris, 2011) that calcium load of rat liver mitochondria in medium containing TlNO3 and KNO3 stimulated the Tl(+)-induced MPTP opening in the inner mitochondrial membrane. The MPTP opening as well as followed increase in swelling, a drop in membrane potential (ΔΨmito), and a decrease in state 3, state 4, and 2,4-dinitrophenol-uncoupled respiration were visibly enhanced in the presence of PAO, tBHP, DIDS, and carboxyatractyloside. However, these effects were markedly inhibited by ADP and membrane-penetrant hydrophobic thiol reagent, N-ethylmaleimide (NEM) which fix the ANT in 'm' conformation. Cyclosporine A additionally potentiated these effects of ADP and NEM. Our data suggest that conformational changes of the ANT may be directly involved in the opening of the Tl(+)-induced MPTP in the inner membrane of Ca(2+)-loaded rat liver mitochondria. Using the Tl(+)-induced MPTP model is discussed in terms finding new transition pore inhibitors and inducers among different chemical and natural compounds.

The N-Methyl-d-Aspartate Receptor Antagonist MK-801 Prevents Thallium-Induced Behavioral and Biochemical Alterations in the Rat Brain

Thallium (Tl+) is a toxic heavy metal capable of increasing oxidative damage and disrupting antioxidant defense systems. Thallium invades the brain cells through potassium channels, increasing neuronal excitability, although until now the possible role of glutamatergic transmission in this event has not been investigated. Here, we explored the possible involvement of a glutamatergic component in the Tl+-induced toxicity through the N-methyl-d-aspartate (NMDA) receptor antagonist dizocilpine (MK-801) in rats. The effects of MK-801 (1 mg/kg, intraperitoneally [ip]) on early (24 hours) motor alterations, lipid peroxidation, reduced glutathione (GSH) levels, and GSH peroxidase activity induced by Tl+ acetate (32 mg/kg, ip) were evaluated in adult rats. MK-801 attenuated the Tl+-induced hyperactivity and lipid peroxidation in the rat striatum, hippocampus and midbrain, and produced mild effects on other end points. Our findings suggest that glutamatergic transmission via NMDA receptors might be involved in the Tl+-induced altered regional brain redox activity and motor performance in rats.

Protective Effects of Diallyl Sulfide and Curcumin Separately against Thallium-Induced Toxicity in Rats

Thallium acetate (TI) is a cumulative poison intimately accompanied by an increase in reactive oxygen species (ROS) formation that represents an important risk factor for tissue injury and malfunction. This study aims to determine the possible hepatoprotective and antioxidant effects of diallyl sulfide (DAS) from garlic and curcumin from turmeric against TI-induced liver injury and oxidative stress (OS) in rats. This in vivo animal study divided rats into six groups of 8 rats per group. The first group received saline and served as the control group. The second and third groups received DAS or curcumin only at a dose of 200 mg/kg. The fourth group received TI at a dose of 6.4 mg/kg for 5 consecutive days. The fifth and sixth groups received DAS or curcumin orally 1 hour before TI intoxication at the same dose as the second and third groups. Liver integrity serum enzymes aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), lactate dehydrogenase (LDH), and γ-glutamyltransferase (γ-GT) were evaluated. Serum and liver tissue homogenate lipid peroxidation and OS biomarkers were measured. The data were analyzed by one-way ANOVA followed by Duncan's multiple range test for post hoc analysis using SPSS version 16. TI induced marked oxidative liver damage as shown by significantly (P≤0.05) elevated serum AST, ALT, ALP, LDH and γ-GT levels. There were significant (P≤0.05) increases in serum and hepatic malondialdehyde (MDA) and serum nitric oxide (NO) as well as decreased hepatic glutathione (GSH) and catalase (CAT) activities. There were significantly (P≤0.05) less serum and hepatic superoxide dismutase (SOD) and total antioxidant capacity (TAC). Pre-treatment with DAS or curcumin ameliorated the changes in most studied biochemical parameters. DAS and curcumin effectively reduced TI-induced liver toxicity.

Evaluation of cytogenetic and DNA damage caused by thallium(I) acetate in human blood cells

Although thallium is detrimental to all living organisms, information regarding the mutagenic and genotoxic effects of this element and its compounds remains scarce. Therefore, we tested the genotoxic and cytotoxic effects of thallium(I) acetate on human peripheral blood cells in vitro using structural chromosomal aberrations (SCAs), sister chromatid exchanges (SCEs), and single-cell gel electrophoresis (at pH >13 or 12.1) analysis. Whole blood samples were incubated with 0.5, 1, 5, 10, 50, or 100 µg/mL thallium salt. Exposure to this metal compound resulted in a clear dose-dependent reduction in the mitotic and replicative indices. An increase in SCAs was evident in the treated group compared with the control group, and significant differences were observed in the percentage of cells with SCAs when metaphase cells were treated with 0.5-10 µg/mL of thallium(I). The SCE test did not reveal any significant differences. We observed that a 1-h treatment with thallium(I) at pH > 13 significantly increased the comet length for all the concentrations tested; however, at pH 12.1, only the two highest concentrations affected the comet length. These results suggested that thallium(I) acetate induces cytotoxic, cytostatic, and clastogenic effects, as well as DNA damage.

Genetic toxicology of thallium: a review

This review summarizes the current knowledge about the general toxicity of thallium (Tl) and its environmental sources, with special emphasis placed on its potential mutagenic, genotoxic, and cytotoxic effects on both eukaryotic and prokaryotic cells. Tl is a nonessential heavy metal that poses environmental and occupational threats as well as therapeutic hazards because of its use in medicine. It is found in two oxidation states, thallous (Tl(+)) and thallic (Tl(3+)), both of which are considered highly toxic to human beings and domestic and wild organisms. Many Tl compounds are colorless, odorless and tasteless, and these characteristics, combined with the high toxicity of TI compounds, have led to their use as poisons. Because of its similarity to potassium ions (K(+)), plants and mammals readily absorb Tl(+) through the skin and digestive and respiratory systems. In mammals, it can cross the placental, hematoencephalic, and gonadal barriers. Inside cells, Tl can accumulate and interfere with the metabolism of potassium and other metal cations, mimicking or inhibiting their action. The effects of Tl on genetic material have not yet been thoroughly explored, and few existing studies have focused exclusively on Tl(+). Both in vivo and in vitro studies indicate that Tl compounds can have a weak mutagenic effect, but no definitive effect on the induction of primary DNA damage or chromosomal damage has been shown. These studies have demonstrated that Tl compounds are highly toxic and lead to changes in cell-cycle progression.

Whole blood thallium determination by GFAAS with high-frequency modulation polarization Zeeman effect background correction

A new technique of blood thallium direct determination based on graphite furnace atomic absorption spectrometry with Zeeman background absorption correction system was designed. The developed technique does not require sample digestion. Sample treatment includes only a fivefold per volume dilution of blood sample with 0.1% (m/v) Triton X-100. L'vov integrated platform was modified with 400 μg of Rh. Matrix modifier (200 μg NH(4)NO(3) and 160 μg Pd(NO(3))(2)) was suggested for coping chloride and blood organic matter interferences. Standard reference material (Clincheck® Plasma Control for trace elements) analysis was used for validation. Additional validation was performed by analyzing spiked blood samples in the whole dynamic range. The dynamic range was 2-50 μg/L. Precision (RSD) was found <12%. Blood thallium limit of detection was 0.2 μg/L.

Chlorpyrifos induces apoptosis and DNA damage in Drosophila through generation of reactive oxygen species

The present study investigated the apoptosis and DNA damage inducing potential of chlorpyrifos (CP) in Drosophila melanogaster. Third instar larvae of Drosophila were treated with different concentrations of CP (0.015-15.0 microg/L) for 2-48 h. Reactive oxygen species (ROS) generation, oxidative stress markers, DNA damage and apoptotic cell death end points were measured in them. A significant increase in DNA damage was concomitant with apoptotic mode of cell death in 15.0 microg/L CP-treated organisms for 24 and 48 h. Depolarization in mitochondrial membrane potential and increased casapase-3 and caspase-9 activities in these organisms indicated both as potential targets of CP. A significant positive correlation was observed among ROS generation, apoptosis and DNA damage. The study suggests that (i) ROS may be involved in inducing apoptosis and DNA damage in the CP-exposed larvae of Drosophila and (ii) D. melanogaster may be used as an alternative in vivo animal model for xenobiotics hazard assessment.

Effects of exogenous metallothionein against thallium-induced oxidative stress in rat liver

Metallothionein (MT) is a low-molecular weight sulfur-rich protein that plays role in metal homeostasis/detoxification and radical scavenging. The following study investigated the ability of exogenous MT to protect against oxidative damage induced by thallium (TI) in rat liver. Male Wistar rats were divided into four groups; a control and three experimental groups. The control group received physiological saline. Group 1 animals were injected with thallium acetate intraperitoneally (i.p.) at a single dose of LD(50) (32 mg/kg). In group 2 and group 3, metallothionein I was administrated once at two different doses (1 or 2.5mg/kg i.p., respectively) 1h before TI intoxication. Levels of endogenous antioxidants, oxidative stress markers were measured and histopathological examinations were performed 4 days after TI administration. TI accumulation in liver decreased related to the dose of MT. Mostly all of the alterations in the levels antioxidants restored to normal levels in MT administrated animals. H(2)O(2) levels and lipid peroxidation decreased, integrity of hepatocytes and membranous structures inside the cells were preserved. The toxic effects of TI were modulated in MT administrated animals particularly at the dose of 2.5mg/kg. These findings suggest an active role of exogenous MT against TI-induced oxidative stress in rat liver.

Tl(I) and Tl(III) activate both mitochondrial and extrinsic pathways of apoptosis in rat pheochromocytoma (PC12) cells

Thallium (Tl) is a highly toxic metal though yet its mechanisms are poorly understood. Previously, we demonstrated that rat pheochromocytoma (PC12) cells exposure to thallous (Tl(I)) or thallic (Tl(III)) cations leads to mitochondrial damage and reduced cell viability. In the present work we comparatively characterized the possible pathways involved in Tl(I)- and Tl(III)- (10-100 muM) mediated decrease in PC12 cells viability. We observed that these cations do not cause cell necrosis but significantly increased the number of cells with apoptotic features. Both cations lead to Bax oligomerization and caused apoptosis inducing factor (AIF), endonuclease G (Endo G), and cytochrome c release from mitochondria, but they did not activate caspase dependent DNAse (CAD). Tl(I)- and Tl(III)-dependent caspases 9 and 3 activation followed similar kinetics, with maximal effects at 18 h of incubation. In addition, Tl(I) promoted phosphatidylserine (PS) exposure. Tl(III) induced 2- and 18-fold increase in Fas content and caspase 8 activity, respectively. Together, experimental results show that Tl(I) and Tl(III) induce PC12 cells apoptosis, although differential pathways are involved. While Tl(I)-mediated cell apoptosis was mainly associated with mitochondrial damage, Tl(III) showed a mixed effect triggering both the intrinsic and extrinsic pathways of apoptosis. These findings contribute to a better understanding of the mechanisms underlying Tl-induced loss of cell viability in PC12 cells.

Relationship between thallium(I)-mediated plasma membrane fluidification and cell oxidants production in Jurkat T cells

The effects of thallous cation (Tl(+)) on: (a) the production of oxidant species and (b) membrane fluidity were evaluated in human leukemia T cells (Jurkat). After 72 h of incubation in the presence of Tl(+) (5-100 microM), no significant changes in cell viability were observed, although the average cell size was decreased as evaluated by steady-state light scattering. Tl(+) (5-100 microM) caused a significant increase in the concentration of cellular oxidants as measured with the probe 5(6)-carboxy-2',7'-dichlorodihydrofluorescein diacetate (DCDCDHF). Similarly, a higher lipid oxidation products release was observed as measured by TBARS production. Both Tl(+)-mediated DCDCDHF oxidation and TBARS production were prevented when cells were supplemented with 2mM Trolox. Tl(+) (5-100 microM) also induced a concentration-dependent increase in plasma membrane fluidity, evaluated with the probe 6-(9-anthroyloxy)stearic acid (6-AS). This effect of Tl(+) was neither associated to the externalization of phosphatidylserine, nor observed in Trolox-supplemented cells. Significant correlations were found between the increase in plasma membrane fluidity and TBARS production and DCDCDHF oxidation. Together, the present results suggest that the increase in cellular oxidants caused by Tl(+) could oxidize membrane fatty acids, resulting in an increase in membrane fluidity. These effects could underlie the pathology associated with Tl(+) toxicity.