Endoplasmic reticulum stress is involved in arsenite-induced oxidative injury in rat brain

Endoplasmic reticulum stress responses and epigenetic alterations in arsenic carcinogenesis

Arsenic is a well-known human carcinogen whose environmental exposure via drinking water, food, and air impacts millions of people across the globe. Various mechanisms of arsenic carcinogenesis have been identified, ranging from damage caused by excessive production of free radicals and epigenetic alterations to the generation of cancer stem cells. A growing body of evidence supports the critical involvement of the endoplasmic stress-activated unfolded protein response (UPR) in promoting as well as suppressing cancer development/progression. Various in vitro and in vivo models have also demonstrated that arsenic induces the UPR via activation of the PERK, IRE1α, and ATF6 proteins. In this review, we discuss the mechanisms of arsenic-induced endoplasmic reticulum stress and the role of each UPR pathway in the various cancer types with a focus on the epigenetic regulation and function of the ATF6 protein. The importance of UPR in arsenic carcinogenesis and cancer stem cells is a relatively new area of research that requires additional investigations via various omics-based and computational tools. These approaches will provide interesting insights into the mechanisms of arsenic-induced cancers for prospective target identification and development of novel anti-cancer therapies.

Mechanisms of genotoxicity and proteotoxicity induced by the metalloids arsenic and antimony

Arsenic and antimony are metalloids with profound effects on biological systems and human health. Both elements are toxic to cells and organisms, and exposure is associated with several pathological conditions including cancer and neurodegenerative disorders. At the same time, arsenic- and antimony-containing compounds are used in the treatment of multiple diseases. Although these metalloids can both cause and cure disease, their modes of molecular action are incompletely understood. The past decades have seen major advances in our understanding of arsenic and antimony toxicity, emphasizing genotoxicity and proteotoxicity as key contributors to pathogenesis. In this review, we highlight mechanisms by which arsenic and antimony cause toxicity, focusing on their genotoxic and proteotoxic effects. The mechanisms used by cells to maintain proteostasis during metalloid exposure are also described. Furthermore, we address how metalloid-induced proteotoxicity may promote neurodegenerative disease and how genotoxicity and proteotoxicity may be interrelated and together contribute to proteinopathies. A deeper understanding of cellular toxicity and response mechanisms and their links to pathogenesis may promote the development of strategies for both disease prevention and treatment.

Mechanisms Associated with Cognitive and Behavioral Impairment Induced by Arsenic Exposure

Arsenic (As) is a metalloid naturally present in the environment, in food, water, soil, and air; however, its chronic exposure, even with low doses, represents a public health concern. For a long time, As was used as a pigment, pesticide, wood preservative, and for medical applications; its industrial use has recently decreased or has been discontinued due to its toxicity. Due to its versatile applications and distribution, there is a wide spectrum of human As exposure sources, mainly contaminated drinking water. The fact that As is present in drinking water implies chronic human exposure to this metalloid; it has become a worldwide health problem, since over 200 million people live where As levels exceed safe ranges. Many health problems have been associated with As chronic exposure including cancer, cardiovascular diseases, gastrointestinal disturbances, and brain dysfunctions. Because As can cross the blood-brain barrier (BBB), the brain represents a target organ where this metalloid can exert its long-term toxic effects. Many mechanisms of As neurotoxicity have been described: oxidative stress, inflammation, DNA damage, and mitochondrial dysfunction; all of them can converge, thus leading to impaired cellular functions, cell death, and in consequence, long-term detrimental effects. Here, we provide a current overview of As toxicity and integrated the global mechanisms involved in cognitive and behavioral impairment induced by As exposure show experimental strategies against its neurotoxicity.

Mitochondrial ROS, ER Stress, and Nrf2 Crosstalk in the Regulation of Mitochondrial Apoptosis Induced by Arsenite

Long-term ingestion of arsenicals, a heterogeneous group of toxic compounds, has been associated with a wide spectrum of human pathologies, which include various malignancies. Although their mechanism of toxicity remains largely unknown, it is generally believed that arsenicals mainly produce their effects via direct binding to protein thiols and ROS formation in different subcellular compartments. The generality of these mechanisms most probably accounts for the different effects mediated by different forms of the metalloid in a variety of cells and tissues. In order to learn more about the molecular mechanisms of cyto- and genotoxicity, there is a need to focus on specific arsenic compounds under tightly controlled conditions. This review focuses on the mechanisms regulating the mitochondrial formation of ROS after exposure to low concentrations of a specific arsenic compound, NaAsO₂, and their crosstalk with the nuclear factor (erythroid-2 related) factor 2 antioxidant signaling and the endoplasmic reticulum stress response.

Mechanism for antiParkinsonian effect of resveratrol: Involvement of transporters, synaptic proteins, dendrite arborization, biochemical alterations, ER stress and apoptosis

The present study was undertaken to evaluate the mechanism for antiParkinsonian effect of resveratrol employing 6-hydroxydopamine (6-OHDA) induced experimental model of Parkinson's disease (PD). Resveratrol treatment significantly protects the PD related pathological markers like level of tyrosine hydroxylase, dopamine and apoptotic proteins (Bax and cleaved caspase-3). Disease pathology involves significantly decreased level of dopamine transporter, synaptophysin and postsynaptic density protein 95 (PSD-95) along with augmented level of vesicular monoamine transporter and considerably affected the dendrite arborization. Such affected neuronal communication was significantly restored with resveratrol treatment. Biochemical alterations include the depleted level of glutathione (GSH), mitochondrial complex-I activity with concomitant increased level of lipid peroxidation, nitrite level and calcium levels, which were also significantly inhibited with resveratrol treatment. Altered calcium level induces the endoplasmic reticulum (ER) stress related signalling and phosphorylated Nuclear factor erythroid 2-related factor 2 (Nrf2), and with resveratrol treatment the level of phosphorylated Nrf2 was further increased. The concurrent depleted level of proteasome activity was observed which was attenuated with resveratrol treatment. Proinflammatory cytokines and activated astrocytes were observed which was inhibited with resveratrol treatment. In conclusion, findings suggested that resveratrol exhibits the interference in neuronal communication, oxidative stress, mitochondrial pathophysiology, ER stress, protein degradation mechanism and inflammatory responses and could be utilize in clinics to treat the PD patients.

MicroRNA-124 Reduces Arsenic-induced Endoplasmic Reticulum Stress and Neurotoxicity and is Linked with Neurodevelopment in Children

Arsenic (As) exposure adversely affects neurodevelopment in children. Accumulation of misfolded proteins in cells exposed to As leads to endoplasmic reticulum (ER) stress response, which, if not relieved, results in cell death. Despite the potential role of ER stress for As-induced neurotoxicity, the underlying mechanisms remain poorly understood. Here we aimed to investigate the roles of microRNA(miR)-124, a novel ER stress suppressor, in As-induced ER stress response and cytotoxicity in neural cells. We further aimed to link these in vitro findings to neurodevelopmental outcomes in children who were exposed to As. Using Quantitative RT-PCR and Cyquant assay, we showed that miR-124 protects against As-induced cytotoxicity in neural cells with concomitant suppression of As-induced ER stress. In addition, As-induced cytotoxicity was exacerbated in miR-124 knockout cells generated by CRISPR-based gene editing compared scramble control. Furthermore, we identified two miR-124 SNPs rs67543816 (p = 0.0003) and rs35418153 (p = 0.0004) that are significantly associated with a mental composite score calculated from the Bayley Scales of Infant Development III in Bangladesh children. Our study reveals As-induced ER stress as a crucial mechanism underlying the toxic effects of As on neural cell function and neurodevelopment and identifies miR-124 as a potential preventative and therapeutic target against detrimental effects of As exposure in children.

Fatal gadolinium-induced encephalopathy following accidental intrathecal administration: a case report and a comprehensive evidence-based review

Gadolinium-based contrast agents (GBCAs) have been suggested as off-label alternatives to iodine-based contrast agents for fluoroscopic imaging during interventional pain procedures. We report a case of accidental intrathecal administration of a GBCA during a neuraxial interventional pain procedure leading to acute gadolinium neurotoxicity, which resulted in encephalopathy and ultimately death. To our knowledge, it is the first published case of fatal intrathecal gadolinium-induced encephalopathy and the first published case of intrathecal gadoteridol causing serious neurologic complications. In addition, the case presented here is placed in context with an associated comprehensive, evidence-based review of the use of gadolinium in interventional pain procedures, addressing gadolinium chemistry and pharmacologic properties, neurotoxicity and radiology. Physicians must be aware that gadolinium poses a significant risk of acute neurotoxicity even in small doses. Until further safety research is performed, GBCAs should not be considered a safe alternative for use in neuraxial interventional spine procedures when there is a risk of inadvertent intrathecal administration.

Neurotoxicity Linked to Dysfunctional Metal Ion Homeostasis and Xenobiotic Metal Exposure: Redox Signaling and Oxidative Stress

SIGNIFICANCE

Essential metals such as copper, iron, manganese, and zinc play a role as cofactors in the activity of a wide range of processes involved in cellular homeostasis and survival, as well as during organ and tissue development. Throughout our life span, humans are also exposed to xenobiotic metals from natural and anthropogenic sources, including aluminum, arsenic, cadmium, lead, and mercury. It is well recognized that alterations in the homeostasis of essential metals and an increased environmental/occupational exposure to xenobiotic metals are linked to several neurological disorders, including neurodegeneration and neurodevelopmental alterations. Recent Advances: The redox activity of essential metals is key for neuronal homeostasis and brain function. Alterations in redox homeostasis and signaling are central to the pathological consequences of dysfunctional metal ion homeostasis and increased exposure to xenobiotic metals. Both redox-active and redox-inactive metals trigger oxidative stress and damage in the central nervous system, and the exact mechanisms involved are starting to become delineated.

CRITICAL ISSUES

In this review, we aim to appraise the role of essential metals in determining the redox balance in the brain and the mechanisms by which alterations in the homeostasis of essential metals and exposure to xenobiotic metals disturb the cellular redox balance and signaling. We focus on recent literature regarding their transport, metabolism, and mechanisms of toxicity in neural systems.

FUTURE DIRECTIONS

Delineating the specific mechanisms by which metals alter redox homeostasis is key to understand the pathological processes that convey chronic neuronal dysfunction in neurodegenerative and neurodevelopmental disorders. Antioxid. Redox Signal. 28, 1669-1703.

Acrolein acts as a neurotoxin in the nigrostriatal dopaminergic system of rat: involvement of α-synuclein aggregation and programmed cell death

Clinical studies report significant increases in acrolein (an α,β-unsaturated aldehyde) in the substantia nigra (SN) of patients with Parkinson’s disease (PD). In the present study, acrolein-induced neurotoxicity in the nigrostriatal dopaminergic system was investigated by local infusion of acrolein (15, 50, 150 nmoles/0.5 μl) in the SN of Sprague-Dawley rats. Acrolein-induced neurodegeneration of nigrostriatal dopaminergic system was delineated by reductions in tyrosine hydroxylase (TH) levels, dopamine transporter levels and TH-positive neurons in the infused SN as well as in striatal dopamine content. At the same time, apomorphine-induced turning behavior was evident in rats subjected to a unilateral infusion of acrolein in SN. Acrolein was pro-oxidative by increasing 4-hydroxy-2-nonenal and heme oxygenase-1 levels. Furthermore, acrolein conjugated with proteins at lysine residue and induced α-synuclein aggregation in the infused SN. Acrolein was pro-inflammatory by activating astrocytes and microglia. In addition, acrolein activated caspase 1 in the infused SN, suggesting acrolein-induced inflammasome formation. The neurotoxic mechanisms underlying acrolein-induced neurotoxicity involved programmed cell death, including apoptosis and necroptosis. Compared with well-known Parkinsonian neurotoxins, including 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and rotenone which do not exist in the SN of PD patients, our in vivo study shows that acrolein acts as a Parkinsonian neurotoxin in the nigrostriatal dopaminergic system of rat brain.

New Insights Into Cellular Stress Responses to Environmental Metal Toxicants

Exposures to metal toxicants in the environment disrupt normal physiological functions and have been linked to the development of a myriad of human diseases. While the molecular and cellular mechanisms underlying metal toxicities remain to be fully understood, it is well appreciated that metal toxicants induce cellular stresses and that how cells respond to the stresses plays an important role in metal toxicity. In this review, we focus on how metal exposures induce stresses in the endoplasmic reticulum (ER) to elicit the unfolded protein response (UPR). We document the emerging evidence that induction of ER stress and UPR in the development of human diseases is associated with metal exposures. We also discuss the role of the interplay between ER stress and oxidative stress in metal toxicity. Finally, we review recent advances in functional genomics approaches and discuss how applications of these new tools could help elucidate the molecular mechanisms underlying cellular stresses induced by environmental metal toxicants.

Endoplasmic Reticulum Stress Instigates the Rotenone Induced Oxidative Apoptotic Neuronal Death: a Study in Rat Brain

The present study was conducted to evaluate the involvement of endoplasmic reticulum stress in rotenone-induced oxidative neuronal death in rat brain. Rotenone (6 μg/3 μl) was administered intranigrally, unilaterally (right side) in SD rat brain. Neuronal morphology, expression level of tyrosine hydroxylase (TH) and endoplasmic reticulum (ER) stress markers like glucose-regulated protein 78 (GRP78), growth arrest and DNA damage-inducible gene 153 (GADD153), eukaryotic translation initiation factor 2α (p-eIF2α/eIF2α) and cleaved caspase-12 were estimated in the rat brain. Levels of reactive oxygen species (ROS), reduced glutathione (GSH) and enzymatic activities of glutathione peroxidase (GPx) and glutathione reductase (GRd) were estimated to assess the rotenone induced oxidative stress. Apoptotic death of neurons was assessed by estimating the mRNA level of caspase-3. Rotenone administration caused altered neuronal morphology, decreased expression of TH, augmented ROS level, decreased level of GSH and decreased activities of GPx and GRd enzymes which were significantly attenuated with the pretreatment of ER stress inhibitor, salubrinal (1 mg/kg, intraperitoneal). Significantly increased levels of GRP78, GADD, dephosphorylated eIF2α and cleaved caspase-12 was also observed after rotenone administration, which was inhibited with the pretreatment of salubrinal. Rotenone-induced increased mRNA level of caspase-3 was also attenuated by pretreatment of salubrinal. Findings suggested that salubrinal treatment significantly inhibited the rotenone-induced neurotoxicity implicating that ER stress initiates the rotenone-induced oxidative stress and neuronal death.

Perfluorooctanoic acid exposure induces endoplasmic reticulum stress in the liver and its effects are ameliorated by 4-phenylbutyrate

Perfluoroalkyl acids (PFAAs) are a group of widely used anthropogenic compounds. As one of the most dominant PFAAs, perfluorooctanoic acid (PFOA) has been suggested to induce hepatotoxicity and several other toxicological effects. However, details on the mechanisms for PFOA-induced hepatotoxicity still need to be elucidated. In this study, we observed the occurrence of endoplasmic reticulum (ER) stress in mouse livers and HepG2 cells after PFOA exposure using several familiar markers for the unfolded protein response (UPR). ER stress in HepG2 cells after PFOA exposure was not significantly influenced by autophagy inhibition or stimulation. The antioxidant defense system was significantly disturbed in mouse livers after PFOA exposure, and reactive oxygen species (ROS) were increased in cells exposed to PFOA for 24 h. However, N-acetyl-L-cysteine (NAC) pretreatment did not satisfactorily alleviate the UPR in cells exposed to PFOA even though the increase of ROS was less evident. Furthermore, exposure of HepG2 cells to PFOA in the presence of sodium 4-phenylbutyrate (4-PBA), a chemical chaperone and ER stress inhibitor, suggested that 4-PBA alleviated the UPR and autophagosome accumulation induced by PFOA in cells. In addition, several toxicological effects attributed to PFOA exposure, including cell cycle arrest, proteolytic activity impairment, and neutral lipid accumulation, were also improved by 4-PBA cotreatment in cells. In vivo study demonstrated that PFOA-induced lipid metabolism perturbation and liver injury were partially ameliorated by 4-PBA in mice after 28 days of exposure. These findings demonstrated that PFOA-induced ER stress leading to UPR might play an important role in PFOA-induced hepatotoxic effects, and chemical chaperone 4-PBA could ameliorate the effects.

Roles of autophagy in MPP+-induced neurotoxicity in vivo: the involvement of mitochondria and α-synuclein aggregation

Macroautophagy (also known as autophagy) is an intracellular self-eating mechanism and has been proposed as both neuroprotective and neurodestructive in the central nervous system (CNS) neurodegenerative diseases. In the present study, the role of autophagy involving mitochondria and α-synuclein was investigated in MPP⁺ (1-methyl-4-phenylpyridinium)-induced oxidative injury in chloral hydrate-anesthetized rats in vivo. The oxidative mechanism underlying MPP⁺-induced neurotoxicity was identified by elevated lipid peroxidation and heme oxygenase-1 levels, a redox-regulated protein in MPP⁺-infused substantia nigra (SN). At the same time, MPP⁺ significantly increased LC3-II levels, a hallmark protein of autophagy. To block MPP⁺-induced autophagy in rat brain, Atg7siRNA was intranigrally infused 4 d prior to MPP⁺ infusion. Western blot assay showed that in vivo Atg7siRNA transfection not only reduced Atg7 levels in the MPP⁺-infused SN but attenuated MPP⁺-induced elevation in LC3-II levels, activation of caspase 9 and reduction in tyrosine hydroxylase levels, indicating that autophagy is pro-death. The immunostaining study demonstrated co-localization of LC3 and succinate dehydrogenase (a mitochondrial complex II) as well as LC3 and α-synuclein, suggesting that autophagy may engulf mitochondria and α-synuclein. Indeed, in vivo Atg7siRNA transfection mitigated MPP⁺-induced reduction in cytochrome c oxidase. In addition, MPP⁺-induced autophagy differentially altered the α-synuclein aggregates in the infused SN. In conclusion, autophagy plays a prodeath role in the MPP⁺-induced oxidative injury by sequestering mitochondria in the rat brain. Moreover, our data suggest that the benefits of autophagy depend on the levels of α-synuclein aggregates in the nigrostriatal dopaminergic system of the rat brain.

Arsenic trioxide induces unfolded protein response in vascular endothelial cells

Chronic arsenic exposure has been linked to endothelial dysfunction and apoptosis. We investigate the involvement of unfolded protein response (UPR) signaling in the arsenic-mediated cytotoxicity of the SVEC4-10 mouse endothelial cells. The SVEC4-10 cells underwent apoptosis in response to As2O3 dose- and time-dependently, accompanied by increased accumulation of calcium, and activation of caspase-3. These phenomena were completely inhibited by α-lipoic acid (LA), which did not scavenge ROS over-production, but were only partially or not ameliorated by tiron, a potent superoxide scavenger. Moreover, arsenic activated UPR, leading to phosphorylation of eukaryotic translation initiation factor 2 subunit α (eIF2α), induction of ATF4, and processing of ATF6. Treatment with arsenic also triggered the expression of endoplasmic reticulum (ER) stress markers, GRP78 (glucose-regulated protein), and CHOP (C/EBP homologous protein). The activation of eIF2α, ATF4 and ATF6 and expression of GRP78 and CHOP are repressed by both LA and tiron, indicating arsenic-induced UPR is mediated through ROS-dependent and ROS-independent pathways. Arsenic also induced ER stress-inducible genes, BAX, PUMA (p53 upregulated modulator of apoptosis), TRB3 (tribbles-related protein 3), and SNAT2 (sodium-dependent neutral amino acid transporter 2). Consistent with intracellular calcium and cell viability data, ROS may not be important in arsenic-induced death, because tiron did not affect the expression of these pro-apoptotic genes. In addition, pretreatment with salubrinal, a selective inhibitor of eIF2α dephosphorylation, enhanced arsenic-induced GRP78 and CHOP expression and partially prevented arsenic cytotoxicity in SVEC4-10 cells. Taken together, these results suggest that arsenic-induced endothelial cytotoxicity is associated with ER stress, which is mediated by ROS-dependent and ROS-independent signaling.

Neuroprotective effects of longan ( Dimocarpus longan Lour.) flower water extract on MPP+-induced neurotoxicity in rat brain

In this study, the neuroprotective effect of Dimocarpus longan Lour. flower water extract (LFWE) was investigated. First, an in vitro study showed that LFWE concentration-dependently inhibited lipid peroxidation of brain homogenates incubated at 37 °C. The antioxidative activity of LFWE was more potent than that of glutathione or Trolox. Furthermore, an ex vivo study found that the basal lipid peroxidation (0 °C) and lipid peroxidation incubated at 37 °C were lower in the brain homogenates of LFWE-treated (500 mg/day) rats, indicating that the brain of LFWE-treated rats was more resistant to oxidative stress. Moreover, a Parkinsonian animal model was employed to demonstrate that oral administration of LFWE (125-500 mg/kg/day) dose-dependently attenuated 1-methyl-4-phenylpyridinium (MPP(+))-induced neurotoxicity in the nigrostriatal dopaminergic system of rat brain. In conclusion, this study suggests that LFWE is antioxidative, anti-inflammatory, and anti-apoptotic. Furthermore, oral administration of LFWE appears to be useful in preventing and/or treating central nervous system neurodegenerative diseases, including Parkinsonism.

bFGF inhibits ER stress induced by ischemic oxidative injury via activation of the PI3K/Akt and ERK1/2 pathways

Extensive research has focused on finding effective strategies to prevent or improve recovery from brain ischemia and reperfusion (I/R) injury. The basic fibroblast growth factor (bFGF) has been shown to have therapeutic potential in some central nervous system (CNS) disorders, including ischemic injury. In this study, we demonstrate that bFGF administration can improve locomotor activity and inhibit the ER stress induced in the CA1 region of the hippocampus in a mouse model of I/R injury. In vitro, bFGF exerts a protective effect by inhibiting the ER stress response proteins CHOP, XBP-1, ATF-6 and caspase-12 that are induced by H(2)O(2) treatment. Both of these in vivo and in vitro effects are related to the activation of two downstream signaling pathways, PI3K/Akt and ERK1/2. Inhibition of the PI3K/Akt and ERK1/2 pathways by specific inhibitors, LY294002 and U0126, respectively, partially reduce the protective effect of bFGF. Taken together, our results indicate that the neuroprotective role of bFGF involves the suppression of ER stress in the ischemic oxidative damage models and oxidative stress-induced PC12 cell injury, and these effects is underlying the activation of the PI3K/Akt and ERK1/2 signal pathway.

Functional RNA interference (RNAi) screen identifies system A neutral amino acid transporter 2 (SNAT2) as a mediator of arsenic-induced endoplasmic reticulum stress

Exposure to the toxic metalloid arsenic is associated with diabetes and cancer and causes proteotoxicity and endoplasmic reticulum (ER) stress at the cellular level. Adaptive responses to ER stress are implicated in cancer and diabetes; thus, understanding mechanisms of arsenic-induced ER stress may offer insights into pathogenesis. Here, we identify genes required for arsenite-induced ER stress response in a genome-wide RNAi screen. Using an shRNA library targeting ∼20,000 human genes, together with an ER stress cell model, we performed flow cytometry-based cell sorting to isolate cells with defective response to arsenite. Our screen discovered several genes modulating arsenite-induced ER stress, including sodium-dependent neutral amino acid transporter, SNAT2. SNAT2 expression and activity are up-regulated by arsenite, in a manner dependent on activating transcription factor 4 (ATF4), an important mediator of the integrated stress response. Inhibition of SNAT2 expression or activity or deprivation of its primary substrate, glutamine, specifically suppressed ER stress induced by arsenite but not tunicamycin. Induction of SNAT2 is coincident with the activation of the nutrient-sensing mammalian target of rapamycin (mTOR) pathway, which is at least partially required for arsenite-induced ER stress. Importantly, inhibition of the SNAT2 or the System L transporter, LAT1, suppressed mTOR activation by arsenite, supporting a role for these transporters in modulating amino acid signaling. These findings reveal SNAT2 as an important and specific mediator of arsenic-induced ER stress, and suggest a role for aberrant mTOR activation in arsenic-related human diseases. Furthermore, this study demonstrates the utility of RNAi screens in elucidating cellular mechanisms of environmental toxins.

Evaluation of the use of human Mesenchymal Stem Cells for acute toxicity tests

In vitro cytotoxicity tests are typically carried out with transformed, immortalized cell lines or primary cells. Immortalized cells are readily available and easily maintained, although they usually show anomalous behavior and phenotypes, which do not reflect the mechanisms observed in their normal homologous cells. Primary cells are indeed considered a better option as model systems for predicting toxicological behavior, although they are limited in quantity and suffer from batch-to-batch variation due to the need to isolate them freshly for each study. In particular, human Mesenchymal Stem Cells (hMSCs) have never been adopted in order to develop in vitro model systems for acute toxicity tests of chemicals. Therefore, the aim of this study was to verify the possibility of using hMSCs as an alternative method to estimate in vivo starting dose for acute toxicity. As suggested by ICCVAM, 12 reference chemicals were assessed in the present study and a Neutral Red Uptake assay was performed. It is shown for the first time that MSCs isolated from human bone marrow can be confidently used in this area of toxicology. MSCs represent a good promise for the development of in vitro human assays and could ultimately replace, improve or overtake current predictive models in toxicology.

Arsenic-induced oxidative stress and its reversibility

This review summarizes the literature describing the molecular mechanisms of arsenic-induced oxidative stress, its relevant biomarkers, and its relation to various diseases, including preventive and therapeutic strategies. Arsenic alters multiple cellular pathways including expression of growth factors, suppression of cell cycle checkpoint proteins, promotion of and resistance to apoptosis, inhibition of DNA repair, alterations in DNA methylation, decreased immunosurveillance, and increased oxidative stress, by disturbing the pro/antioxidant balance. These alterations play prominent roles in disease manifestation, such as carcinogenicity, genotoxicity, diabetes, cardiovascular and nervous systems disorders. The exact molecular and cellular mechanisms involved in arsenic toxicity are rather unrevealed. Arsenic alters cellular glutathione levels either by utilizing this electron donor for the conversion of pentavalent to trivalent arsenicals or directly binding with it or by oxidizing glutathione via arsenic-induced free radical generation. Arsenic forms oxygen-based radicals (OH(•), O(2)(•-)) under physiological conditions by directly binding with critical thiols. As a carcinogen, it acts through epigenetic mechanisms rather than as a classical mutagen. The carcinogenic potential of arsenic may be attributed to activation of redox-sensitive transcription factors and other signaling pathways involving nuclear factor κB, activator protein-1, and p53. Modulation of cellular thiols for protection against reactive oxygen species has been used as a therapeutic strategy against arsenic. N-acetylcysteine, α-lipoic acid, vitamin E, quercetin, and a few herbal extracts show prophylactic activity against the majority of arsenic-mediated injuries in both in vitro and in vivo models. This review also updates the reader on recent advances in chelation therapy and newer therapeutic strategies suggested to treat arsenic-induced oxidative damage.

High endoplasmic reticulum activity renders multiple myeloma cells hypersensitive to mitochondrial inhibitors

Multiple myeloma (MM) cells continuously secrete large amounts of immunoglobulins that are folded in the endoplasmic reticulum (ER) whose function depend on the Ca(2+) concentration inside its lumen. Recently, it was shown that the ER membrane leaks Ca(2+) that is captured and delivered back by mitochondria in order to prevent its loss. Thus, we hypothesized that the highly active and abundant ER in MM cells results in greater Ca(2+)-regulation by mitochondria which would render them sensitive to mitochondrial inhibitors. Here, we indeed find that Ca(2+) leak is greater in 3 MM, when compared to 2 B-cell leukemia cell lines. Moreover, this greater leak in MM cells is associated with hypersensitivity to various mitochondrial inhibitors, including CCCP. Consistent with our hypothesis, CCCP is more potent in inducing the unfolded protein response marker, CHOP/GADD153 in MM versus B-cell leukemia lines. Additionally, MM cells are found to be significantly more sensitive to clinically used fenofibrate and troglitazone, both of which were recently shown to have inhibitory effects on mitochondrial function. Overall, our results demonstrate that the unusually high ER activity in MM cells may be exploited for therapeutic benefit through the use of mitochondrial inhibitors including troglitazone and fenofibrate.

Melatonin inhibits arsenite-induced peripheral neurotoxicity

In this study, the effect of melatonin on sodium arsenite (arsenite)-induced peripheral neurotoxicity was investigated using dorsal root ganglion (DRG) explants. After 24-hr incubation, arsenite (30 microm) consistently elevated the expression of heat shock protein 70 and haeme oxygenase-1, two well-known stress proteins, in the treated DRG explants. Co-incubation with melatonin (4 and 20 mm) concentration-dependently attenuated arsenite-induced elevation in stress proteins. Furthermore, melatonin inhibited arsenite-induced phosphorylation of p38 and DNA fragmentation. Inhibition by melatonin of arsenite-induced apoptosis was mediated via inactivating both endoplasmic reticulum (ER) and mitochondrial pathways. In the ER pathway, melatonin suppressed arsenite-induced elevation in activating transcription factor-6 and CCAAT/enhancer-binding protein homologous protein in the nuclear fraction of the treated DRG explants. Moreover, melatonin attenuated arsenite-induced activation of caspase 12, an ER-specific enzyme. In the mitochondrial pathway, arsenite-induced increases in Bcl-2 levels and cytosolic cytochrome c were reduced by melatonin. At the same time, melatonin inhibited arsenite-induced activation of caspase 3 in the treated DRG explants. Compared with glutathione and N-acetyl cysteine, melatonin was more potent than either in inhibiting arsenite-induced elevation in stress proteins. Taken together, our study demonstrates that melatonin is protective against arsenite-induced neurotoxicity in DRG explants. In addition, melatonin prevented arsenite-induced apoptosis via suppression of ER and mitochondrial activation. Our data suggest that melatonin is potentially a therapy for arsenite-induced peripheral neuropathy.

Chronic low-level arsenic exposure causes gender-specific alterations in locomotor activity, dopaminergic systems, and thioredoxin expression in mice

Arsenic (As) is a toxic metalloid widely present in the environment. Human exposure to As has been associated with the development of skin and internal organ cancers and cardiovascular disorders, among other diseases. A few studies report decreases in intelligence quotient (IQ), and sensory and motor alterations after chronic As exposure in humans. On the other hand, studies of rodents exposed to high doses of As have found alterations in locomotor activity, brain neurochemistry, behavioral tasks, and oxidative stress. In the present study both male and female C57Bl/6J mice were exposed to environmentally relevant doses of As such as 0.05, 0.5, 5.0, or 50 mg As/L of drinking water for 4 months, and locomotor activity was assessed every month. Male mice presented hyperactivity in the group exposed to 0.5 mg As/L and hypoactivity in the group exposed to 50 mg As/L after 4 months of As exposure, whereas female mice exposed to 0.05, 0.5, and 5.0 mg As/L exhibited hyperactivity in every monthly test during As exposure. Furthermore, striatal and hypothalamic dopamine content was decreased only in female mice. Also decreases in tyrosine hydroxylase (TH) and cytosolic thioredoxin (Trx-1) mRNA expression in striatum and nucleus accumbens were observed in male and female mice, respectively. These results indicate that chronic As exposure leads to gender-dependent alterations in dopaminergic markers and spontaneous locomotor activity, and down-regulation of the antioxidant capacity of the brain.

Arsenite-induced cytotoxicity in dorsal root ganglion explants

Peripheral neuropathy is common in people chronically overexposed to arsenic. We studied sodium arsenite (arsenite)-induced cytotoxicity in dorsal root ganglion (DRG) explants. Incubation with arsenite concentration- and time-dependently increased the expression of stress proteins, heat shock protein 70, and heme oxygenase-1 in DRG explants. Furthermore, apoptosis was involved in the arsenite-induced cytotoxicity in the treated DRG. Elevation in cytosolic cytochrome c levels and reduction in procaspase 3 levels suggested an involvement of the mitochondrial pathway in arsenite-induced apoptosis in this preparation. At the same time, increases in the activating transcription factor-4 and C/EBP homologous protein and reduction in procaspase 12 levels indicated activation of the endoplasmic reticulum (ER) pathway in the arsenite-induced cytotoxicity in DRG explants. Salubrinal (30 microM), an ER inhibitor, was found to attenuate arsenite-induced DNA fragmentation and reduction in procaspase 12 in DRG explants. Cytotoxic effects by arsenite, sodium arsenate (arsenate), monomethylarsonic acid (MMA), and dimethylarsinic acid (DMA) were compared, and the potency was as follows: arsenite >>> arsenate>MMA and DMA. Recombinant adenovirus vectors encoding glial-cell-derived neurotrophic factor (AdGDNF) genes allowed a stable delivery of GDNF genes to the infected cells in DRG explants. Applied in this manner, AdGDNF was found to inhibit arsenite-induced DNA fragmentation in DRG explants. Moreover, AdGDNF attenuated the arsenite-induced reduction in procaspases 3 and 12 levels. Taken together, our study demonstrates that arsenite is capable of inducing cytotoxicity in DRG explants. Both ER and mitochondria pathways are involved in the arsenite-induced apoptosis in DRG explants. Glial-cell-derived neurotrophic factor appears to be protective against arsenite-induced peripheral neuropathy.