Epidermal growth factor prevents thallium(I)- and thallium(III)-mediated rat pheochromocytoma (PC12) cell apoptosis

Tl(I) and Tl(III)-induce genotoxicity, reticulum stress and autophagy in PC12 Adh cells

Thallium (Tl) and its two cationic species, Tl(I) and Tl(III), are toxic for most living beings. In this work, we investigated the effects of Tl (10-100 µM) on the viability and proliferation capacity of the adherent variant of PC12 cells (PC12 Adh cells). While both Tl(I) and Tl(III) halted cell proliferation from 24 h of incubation, their viability was ~ 90% even after 72 h of treatment. At 24 h, increased levels of γH2AX indicated the presence of DNA double-strand breaks. Simultaneously, increased expression of p53 and its phosphorylation at Ser15 were observed, which were associated with decreased levels of p-AKTSer473 and p-mTORSer2448. At 72 h, the presence of large cytoplasmic vacuoles together with increased autophagy predictor values suggested that Tl may induce autophagy in these cells. This hypothesis was corroborated by images obtained by transmission electron microscopy (TEM) and from the decreased expression at 72 h of incubation of SQSTM-1 and increased LC3β-II to LC3β-I ratio. TEM images also showed enlarged ER that, together with the increased expression of IRE1-α from 48 h of incubation, indicated that Tl-induced ER stress preceded autophagy. The inhibition of autophagy flux with chloroquine increased cell mortality, suggesting that autophagy played a cytoprotective role in Tl toxicity in these cells. Together, results indicate that Tl(I) or Tl(III) are genotoxic to PC12 Adh cells which respond to the cations inducing ER stress and cytoprotective autophagy.

The Impact of Thallium Exposure in Public Health and Molecular Toxicology: A Comprehensive Review

This review offers a synthesis of the current understanding of the impact of low-dose thallium (Tl) on public health, specifically emphasizing its diverse effects on various populations and organs. The article integrates insights into the cytotoxic effects, genotoxic potential, and molecular mechanisms of thallium in mammalian cells. Thallium, a non-essential heavy metal present in up to 89 different minerals, has garnered attention due to its adverse effects on human health. As technology and metallurgical industries advance, various forms of thallium, including dust, vapor, and wastewater, can contaminate the environment, extending to the surrounding air, water sources, and soil. Moreover, the metal has been identified in beverages, tobacco, and vegetables, highlighting its pervasive presence in a wide array of food sources. Epidemiological findings underscore associations between thallium exposure and critical health aspects such as kidney function, pregnancy outcomes, smoking-related implications, and potential links to autism spectrum disorder. Thallium primarily exerts cellular toxicity on various tissues through mitochondria-mediated oxidative stress and endoplasmic reticulum stress. This synthesis aims to shed light on the intricate web of thallium exposure and its potential implications for public health, emphasizing the need for vigilant consideration of its risks.

Mitochondrial Oxidative Stress Is the General Reason for Apoptosis Induced by Different-Valence Heavy Metals in Cells and Mitochondria

This review analyzes the causes and consequences of apoptosis resulting from oxidative stress that occurs in mitochondria and cells exposed to the toxic effects of different-valence heavy metals (Ag+, Tl+, Hg2+, Cd2+, Pb2+, Al3+, Ga3+, In3+, As3+, Sb3+, Cr6+, and U6+). The problems of the relationship between the integration of these toxic metals into molecular mechanisms with the subsequent development of pathophysiological processes and the appearance of diseases caused by the accumulation of these metals in the body are also addressed in this review. Such apoptosis is characterized by a reduction in cell viability, the activation of caspase-3 and caspase-9, the expression of pro-apoptotic genes (Bax and Bcl-2), and the activation of protein kinases (ERK, JNK, p53, and p38) by mitogens. Moreover, the oxidative stress manifests as the mitochondrial permeability transition pore (MPTP) opening, mitochondrial swelling, an increase in the production of reactive oxygen species (ROS) and H2O2, lipid peroxidation, cytochrome c release, a decline in the inner mitochondrial membrane potential (ΔΨmito), a decrease in ATP synthesis, and reduced glutathione and oxygen consumption as well as cytoplasm and matrix calcium overload due to Ca2+ release from the endoplasmic reticulum (ER). The apoptosis and respiratory dysfunction induced by these metals are discussed regarding their interaction with cellular and mitochondrial thiol groups and Fe2+ metabolism disturbance. Similarities and differences in the toxic effects of Tl+ from those of other heavy metals under review are discussed. Similarities may be due to the increase in the cytoplasmic calcium concentration induced by Tl+ and these metals. One difference discussed is the failure to decrease Tl+ toxicity through metallothionein-dependent mechanisms. Another difference could be the decrease in reduced glutathione in the matrix due to the reversible oxidation of Tl+ to Tl3+ near the centers of ROS generation in the respiratory chain. The latter may explain why thallium toxicity to humans turned out to be higher than the toxicity of mercury, lead, cadmium, copper, and zinc.

Thallium(III) exposure alters diversity and co-occurrence networks of bacterial and fungal communities and intestinal immune response along the digestive tract in mice

The gut microbiota, which includes fungi and bacteria, plays an important role in maintaining gut health. Our previous studies have shown that monovalent thallium [Tl(I)] exposure is associated with disturbances in intestinal flora. However, research on acute Tl(III) poisoning through drinking water and the related changes in the gut microbiota is insufficient. In this study, we showed that Tl(III) exposure (10 ppm for 2 weeks) reduced the alpha diversity of bacteria in the ileum, colon, and feces of mice, as well as the alpha diversity of fecal fungi. In addition, principal coordinate analysis showed that Tl(III) exposure had little effect on the bacterial and fungal beta diversity. LEfSe analyses revealed that Tl(III) exposure altered the abundance of intestinal bacteria in the digestive tract and feces. Moreover, Tl(III) exposure had little effect on fungal abundance in the ileum, cecum, and colon, but had a considerable effect on fungal abundance in feces. After Tl(III) exposure, the fungal composition was more disrupted in feces than in the intestinal tract, suggesting that feces can serve as a representative of the gut mycobiota in Tl(III) exposure studies. Intra-kingdom network analyses showed that Tl(III) exposure affected the complexity of bacterial-bacterial and fungal-fungal co-occurrence networks along the digestive tract. The bacterial-fungal interkingdom co-occurrence networks exhibited increased complexity after Tl(III) exposure, except for those in the colon. Additionally, Tl(III) exposure altered the intestinal immune response. These results reveal the perturbation in gut bacterial and fungal diversity, abundance, and co-occurrence network complexity, as well as the gut immune response, caused by Tl(III) exposure.

Thallium(I and III) exposure leads to liver damage and disorders of fatty acid metabolism in mice

Thallium (Tl), a highly toxic and priority pollutant heavy metal, exposure can damage mitochondria and disrupt their function. The liver is the central organ that controls lipid homeostasis and contains a large number of mitochondria. So far, there is no study investigating the effects of Tl exposure on hepatic fatty acid metabolism. Here, we showed that 10 ppm of Tl(I) and Tl(III) exposures for two weeks did not significantly affect the body weight and water/food intake in mice. However, it decreased the ratio of liver/weight and induced hepatic sinus congestion and hepatocyte necrosis. Inductively coupled plasma-mass spectrometry (ICP-MS) analysis revealed Tl accumulation in the liver. Gas chromatography-mass spectrometry (GC-MS) results showed that Tl(I) exposure significantly increased hepatic C18:0 concentration, while significantly decreased the concentrations of C16:1n-7, C20:1n-9, C18:3n-6, and C20:2n-9. Tl(III) exposure significantly reduced hepatic concentrations of C20:0, C22:0, C20:1n-9, C18:3n-6, and C20:3n-6. In addition, Tl(I) exposure upregulated the genes related to antioxidation (HO-1, GPX1, and GPX4), fatty acid synthesis (FADS2 and Elovl2), and fatty acid oxidation pathway (PPARα, ACADM, ACADVL, ACAA2, and CPT1A) in the liver. Tl(III) exposure did not significantly affect the transcript levels of liver antioxidative/metabolic enzymes and fatty acid synthesis-related genes, but upregulated fatty acid oxidation pathway-related genes (CYP4A10 and CPT1A). These results suggest that Tl(I) and Tl(III) exposures can cause liver damage and disrupt hepatic fatty acid metabolism, which provide new insights into Tl exposure-induced energy depletion from the perspective of fatty acid metabolism.

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.

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.