Use of Human Central Nervous System Cell Cultures in Neurotoxicity Testing

Participation of protein kinases in cytotoxic and proapoptotic effects of ethylene glycol ethers and their metabolites in SH-SY5Y cells

Ethylene glycol ethers (EGEs) are compounds widely used in many branches of industry. Their toxicological profile in the peripheral tissues is relatively well described, but little is known about their action on the central nervous system (CNS). In this study, we evaluated the effect of 2-ethoxyethanol (EE), 2-butoxyethanol (BE), 2-phenoxyethanol (PHE) and their metabolites on necrotic (estimated by cell viability and lactate dehydrogenase release) and apoptotic (caspase-3 activity and mitochondrial membrane potential) processes and reactive oxygen species' (ROS) production in human neuroblastoma (SH-SY5Y) cells. We have shown that, similar to the peripheral tissues, EGE metabolites in most of the performed assays revealed greater potential to damage than the parent compounds in the CNS cells. Subsequently, we investigated the participation of some selected protein kinases in the degenerative activity of PHE and its main metabolite, phenoxyacetic acid (PHA). It has been found that a GSK3β inhibitor weakened the damaging effects of PHE and PHA in each of the performed assays. Furthermore, the kinases, p38-MAPK, JNK-MAPK and PKC, had a significant role in the cytotoxic and proapoptotic effects of PHA. These results indicate that the neurotoxic effect of EGEs may stem from their impact on many intracellular signal transduction pathways.

Activation of neuronal nitric oxide synthase (nNOS) signaling pathway in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced neurotoxicity

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has been reported to cause alterations in cognitive and motor behavior during both development and adulthood. In this study, the neuronal nitric oxide synthase (nNOS) signaling pathway was investigated in differentiated pheochromocytoma (PC12) cells to better understand the mechanisms of TCDD-induced neurotoxicity. TCDD exposure induced a time- and dose-dependent increase in nNOS expression. High levels of nitric oxide (NO) production by nNOS activation induced mitochondrial cytochrome c (Cyt-c) release and down-regulation of Bcl-2. Additionally, TCDD increased the expression of active caspase-3 and significantly led to apoptosis in PC12 cells. However, these effects above could be effectively inhibited by the addition of 7-nitroindazole (7-NI), a highly selective nNOS inhibitor. Moreover, in the brain cortex of Sprague-Dawley (SD) rats, nNOS was also found to have certain relationship with TCDD-induced neuronal apoptosis. Together, our findings establish a role for nNOS as an enhancer of TCDD-induced apoptosis in PC12 cells.

The PERK-eIF2α signaling pathway is involved in TCDD-induced ER stress in PC12 cells

Studies have shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces apoptotic cell death in neuronal cells. However, whether this is the result of endoplasmic reticulum (ER) stress-mediated apoptosis remains unknown. In this study, we determined whether ER stress plays a role in the TCDD-induced apoptosis of pheochromocytoma (PC12) cells and primary neurons. PC12 cells were exposed to different TCDD concentrations (1, 10, 100, 200, or 500nM) for varying lengths of time (1, 3, 6, 12, or 24h). TCDD concentrations much higher than 10nM (100, 200, or 500nM) markedly increased glucose-regulated protein (GRP78) and C/EBP homologous protein (CHOP) levels, which are hallmarks of ER stress. We also evaluated the effects of TCDD on ER morphology in PC12 cells and primary neurons that were treated with different TCDD concentrations (1, 10, 50, or 200nM) for 24h. Ultrastructural ER alterations were observed with transmission electron microscopy in PC12 cells and primary neurons treated with high concentrations of TCDD. Furthermore, TCDD-induced ER stress significantly promoted the activation of the PKR-like ER kinase (PERK), a sensor for the unfolded protein response (UPR), and its downstream target eukaryotic translation initiation factor 2 α (eIF2α); in contrast, TCDD did not appear to affect inositol-requiring enzyme 1 (IRE1) and activating transcription factor 6 (ATF6), two other UPR sensors. Importantly, TCDD significantly inhibited eIF2α phosphorylation and triggered apoptosis in PC12 cells after 6-24h of treatment. Salubrinal, which activates the PERK-eIF2α pathway, significantly enhanced eIF2α phosphorylation in PC12 cells and attenuated the TCDD-induced cell death. In contrast, knocking down eIF2α using small interfering RNA markedly enhanced TCDD-induced cell death. Together, these results indicate that the PERK-eIF2α pathway plays an important role in TCDD-induced ER stress and apoptosis in PC12 cells.

2,3,7,8-Tetrachlorodibenzo-p-dioxin induces apoptosis by disruption of intracellular calcium homeostasis in human neuronal cell line SHSY5Y

The persistent xenobiotic agent 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces neurotoxic effects that alters neurodevelopment and behavior both during development and adulthood. There are many ongoing efforts to determine the molecular mechanisms of TCDD-mediated neurotoxicity, the signaling pathways involved and its molecular targets in neurons. In this work, we have used SHSY5Y human neuroblastoma cells to characterize the TCDD-induced toxicity. TCDD produces a loss of viability linked to an increased caspase-3 activity, PARP-1 fragmentation, DNA laddering, nuclear fragmentation and hypodiploid (apoptotic) DNA content, in a similar way than staurosporine, a prototypical molecule of apoptosis induction. In addition, TCDD produces a decrease of mitochondrial membrane potential and an increase of intracellular calcium concentration (P < 0.05). Finally, based on the high lipophilic properties of the dioxin, we test the TCDD effect on the membrane integrity using sarcoplasmic reticulum vesicles as a model. TCDD produces calcium efflux through the membrane and an anisotropy decrease (P < 0.05) that reflects an increase in membrane fluidity. Altogether these results support the hypothesis that TCDD toxicity in SHSY5Y neuroblastoma cells provokes the disruption of calcium homeostasis, probably affecting membrane structural integrity, leading to an apoptotic process.

Major components of energy drinks (caffeine, taurine, and guarana) exert cytotoxic effects on human neuronal SH-SY5Y cells by decreasing reactive oxygen species production

Scope. To elucidate the morphological and biochemical in vitro effects exerted by caffeine, taurine, and guarana, alone or in combination, since they are major components in energy drinks (EDs). Methods and Results. On human neuronal SH-SY5Y cells, caffeine (0.125–2 mg/mL), taurine (1–16 mg/mL), and guarana (3.125–50 mg/mL) showed concentration-dependent nonenzymatic antioxidant potential, decreased the basal levels of free radical generation, and reduced both superoxide dismutase (SOD) and catalase (CAT) activities, especially when combined together. However, guarana-treated cells developed signs of neurite degeneration in the form of swellings at various segments in a beaded or pearl chain-like appearance and fragmentation of such neurites at concentrations ranging from 12.5 to 50 mg/mL. Swellings, but not neuritic fragmentation, were detected when cells were treated with 0.5 mg/mL (or higher doses) of caffeine, concentrations that are present in EDs. Cells treated with guarana also showed qualitative signs of apoptosis, including membrane blebbing, cell shrinkage, and cleaved caspase-3 positivity. Flow cytometric analysis confirmed that cells treated with 12.5–50 mg/mL of guarana and its combinations with caffeine and/or taurine underwent apoptosis. Conclusion. Excessive removal of intracellular reactive oxygen species, to nonphysiological levels (or “antioxidative stress”), could be a cause of in vitro toxicity induced by these drugs.

Twenty-First Century Challenges for In Vitro Neurotoxicity

During the last 40 years, studies incorporating in vitro methodologies have greatly advanced our understanding of human nerve cell biology. Attempts have been made to apply these to investigations of neurotoxicity. Due to the complexity of the nervous system, underpinned by an array of integrated interactions between a host of cell types, it is concluded that, at present, alternative neural models are most successful in determining the underlying mechanisms which can cause perturbation of normal functioning of the nervous system, both in adults and during the embryonic period. The use of tiered batteries of test models has been proposed in screening programmes for neurotoxicity, with the generation of much encouraging data in laboratories across the globe. This review aims to discuss the development of neural alternatives, considers the various model systems available, and highlights specific neuronal endpoints which can be tested, in addition to the cytotoxic evaluation of neuronal viability. Developments in molecular and stem cell biology, which are appropriate to neural tissue, and which offer the prospect of exciting advances for the next decade, are cited.

The flavonoid rutin induces astrocyte and microglia activation and regulates TNF-alpha and NO release in primary glial cell cultures

Astrocyte and microglia cells play an important role in the central nervous system (CNS). They react to various external aggressions by becoming reactive and releasing neurotrophic and/or neurotoxic factors. Rutin is a flavonoid found in many plants and has been shown to have some biological activities, but its direct effects on cells of the CNS have not been well studied. To investigate its potential effects on CNS glial cells, we used both astrocyte primary cultures and astrocyte/microglia mixed primary cell cultures derived from newborn rat cortical brain. The cultures were treated for 24 h with rutin (50 or 100 micromol/L) or vehicle (0.5% dimethyl sulfoxide). Mitochondrial function on glial cells was not evidenced by the MTT test. However, an increased lactate dehydrogenase activity was detected in the culture medium of both culture systems when treated with 100 micromol/L rutin, suggesting loss of cell membrane integrity. Astrocytes exposed to 50 micromol/L rutin became reactive as revealed by glial fibrillary acidic protein (GFAP) overexpression and showed a star-like phenotype revealed by Rosenfeld's staining. The number of activated microglia expressing OX-42 increased in the presence of rutin. A significant increase of nitric oxide (NO) was observed only in mixed cultures exposed to 100 micromol/L rutin. Enhanced TNFalpha release was observed in astrocyte primary cultures treated with 100 micromol/L rutin and in mixed primary cultures treated with 50 and 100 micromol/L, suggesting different sensitivity of both activated cell types. These results demonstrated that rutin affects astrocytes and microglial cells in culture and has the capacity to induce NO and TNFalpha production in these cells. Hence, the impact of these effects on neurons in vitro and in vivo needs to be studied.

Neurotoxic effects of trimethyltin and triethyltin on human fetal neuron and astrocyte cultures: a comparative study with rat neuronal cultures and human cell lines

Trimethyltin (TMT) and triethyltin (TET) caused cell death in cultures of primary human neurons and astrocytes, rat neurons and human neuroblastoma cell lines. Human neurons and astrocytes showed a delayed response to TMT cytotoxicity. After 24h of TMT exposure, LC50 values were 148.1, 335.5 and 609.7 microM for SK-N-MC neuroblastoma cell line, neurons and astrocytes, respectively. Over 5 days of exposure, the cytotoxic potency of TMT increased about 70-fold in human cortical neurons. Rat hippocampal neurons were the most vulnerable cells to TMT cytotoxicity, exhibiting an LC50 value 30-fold lower (1.4 microM) than that of rat cerebellar granule cells (44.28 microM). With the exception of rat hippocampal neurons, TET was more potent than TMT in inducing cell death (LC50 values of 3.5-16.9 microM). Moreover, TET was more effective than TMT in increasing intracellular free Ca2+ concentration in human and rat neurons. This work shows that human fetal neuron and astrocyte cultures are a useful model for studying the neurotoxic effects of these environmental contaminants and, thus, predicting their impact on human health.

Down's syndrome astrocytes have greater antioxidant capacity than euploid astrocytes

Down's syndrome (trisomy 21) brain tissue is considered to be susceptible to oxidative injury, mainly because its increased Cu/Zn-superoxide dismutase (SOD1) activity is not followed by an adaptive rise in hydrogen peroxide metabolizing enzymes. In vitro, trisomic neurons suffer oxidative stress and degenerate. We studied the response of trisomy 21 neuron and astrocyte cultures to hydrogen peroxide injury and found that they were, respectively, more and less vulnerable than their euploid counterparts. Differences were detected 24 h after exposures in the region of 50 microm and 500 microm hydrogen peroxide for neuron and astrocyte cultures, respectively. Cytotoxicity results were paralleled by a decrease in cellular glutathione. In addition, trisomic astrocytes showed a lower basal content of superoxide ion and a higher clearance of hydrogen peroxide from the culture medium. In the presence of hydrogen peroxide, trisomic astrocytes maintained their concentration of intracellular superoxide and hydroperoxides at a lower level than euploid astrocytes. Consistent with these results, trisomic astrocytes in neuron coculture were more neuroprotective than euploid astrocytes against hydrogen peroxide injury. We suggest that SOD1 overexpression has beneficial effects on astrocytes, as it does in other systems with similarly high disposal of hydroperoxides. In addition to a higher enzymatic activity of SOD1, cultures of trisomic astrocytes showed slightly higher glutathione reductase activity than euploid cultures. Thus, trisomy 21 astrocytes showed a greater antioxidant capacity against hydrogen peroxide than euploid astrocytes, and they partially counteracted the oxidative vulnerability of trisomic neurons in culture.

Evaluation of fluorescent dyes for measuring intracellular glutathione content in primary cultures of human neurons and neuroblastoma SH-SY5Y

BACKGROUND

Reduced glutathione (GSH) protects cells against oxidative injury and maintains a range of vital functions. To study GSH content in human neuronal cell cultures, thiol-sensitive fluorescent techniques requiring a small number of cells may be of great value, but their GSH specificity has not been established in these cells.

METHODS

We tested the efficiency of four currently available GSH fluorescent stains in human neurons and SH-SY5Y neuroblastoma cells, both cultured in microwells, by using a fluorescence plate reader. Cultures were treated with the inhibitor of the GSH synthesis, buthionine sulfoximine (BSO), and progressive GSH depletion was assayed with monochlorobimane (mBCl), monobromobimane (mBBr), 5-chloromethylfluorescein diacetate (CMFDA), and 7-amino-4-chloromethylcoumarin (CMAC). GSH was also determined by a biochemical method in cell homogenates to obtain quantitative reference values.

RESULTS

Neurons and SH-SY5Y neuroblastoma had basal GSH contents of 27.1 +/- 3.2 and 14.5 +/- 1.7 nmol/mg protein (n = 5), respectively. An approximate 90% depletion of GSH was obtained after 3 days of exposure to 1,000 microM of BSO in neurons and after 1 day in SH-SY5Y cells. Cell death through an apoptotic pathway appeared 1-2 days after total GSH depletion. The assayed stains had different degrees of background fluorescence and sensitivity to GSH content, with similar results in both neuronal cell types. The probes mBCl and CMAC showed the lowest background, and the GSH-depletion curves were most similar to that of the reference method.

CONCLUSIONS

Both mBCl and CMAC are useful fluorescent stains to determine semiquantitative GSH concentration in human neuronal cell cultures.

Methylmercury neurotoxicity in cultures of human neurons, astrocytes, neuroblastoma cells

Neurotoxic effects of methylmercury, were investigated in vitro in primary cultures of human neurons and astrocytes isolatedfrom human fetal brain and in the human neuroblastoma cell line SH-SY5Y. The protection provided by agents with antioxidant properties was tested in these cultures to examine the oxidative stress mechanism of methylmercury poisoning. After 24 h of exposure to methylmercury, LC50 values were 6.5, 8.1 and 6.9 microM for human neurons, astrocytes and neuroblastoma cells, respectively, and the degree of cell damage increased at longer exposure times. Depletion of the cellular pool of reduced glutathione (GSH) by treatment with buthionine sulfoximine potentiated methylmercury cytotoxicity in all three cell types; neuroblastoma cells were the most sensitive. Addition of GSH extracellularly blocked methylmercury neurotoxicity in all cell types. The major beneficial effect of GSH could be attributed to its capacity to form conjugates with methylmercury, which reduces the availability of these organometallic molecules to the cells and facilitates their efflux. Cysteine protected astrocytes and neuroblastoma cells from methylmercury neurotoxicity, while selenite, Vitamin E and catalase produced some minor protective effects in three cell types, particularly in neurons. The present study showed that the human neural cells tested had differential responses to methylmercury: astrocytes were resistant to methylmercury neurotoxicity and neurons were more most responsive to protection afforded by antioxidants among the three cell types.