The SH-SY5Y human neuroblastoma cell line, a relevant in vitro cell model for investigating neurotoxicology in human: focus on organic pollutants

Investigation of the toxicity triggered by chemicals on the human brain has traditionally relied on approaches using rodent in vivo models and in vitro cell models including primary neuronal cultures and cell lines from rodents. The issues of species differences between humans and rodents, the animal ethical concerns and the time and cost required for neurotoxicity studies on in vivo animal models, do limit the use of animal-based models in neurotoxicology. In this context, human cell models appear relevant in elucidating cellular and molecular impacts of neurotoxicants and facilitating prioritization of in vivo testing. The SH-SY5Y human neuroblastoma cell line (ATCC® CRL-2266^TM) is one of the most used cell lines in neurosciences, either undifferentiated or differentiated into neuron-like cells. This review presents the characteristics of the SH-SY5Y cell line and proposes the results of a systematic review of literature on the use of this in vitro cell model for neurotoxicity research by focusing on organic environmental pollutants including pesticides, 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD), flame retardants, PFASs, parabens, bisphenols, phthalates, and PAHs. Organic environmental pollutants are widely present in the environment and increasingly known to cause clinical neurotoxic effects during fetal & child development and adulthood. Their effects on cultured SH-SY5Y cells include autophagy, cell death (apoptosis, pyroptosis, necroptosis, or necrosis), increased oxidative stress, mitochondrial dysfunction, disruption of neurotransmitter homeostasis, and alteration of neuritic length. Finally, the inherent advantages and limitations of the SH-SY5Y cell model are discussed in the context of chemical testing.

In vitro methods for detecting cytotoxicity

Toxicant-induced damage to cells (cytotoxicity) can depress cell growth, compromise intracellular metabolic processes, and/or cause loss of cell viability. Methods that indicate these cytotoxic changes following toxicant exposures are provided, including [³H]thymidine uptake to measure cell growth, MTT dye conversion to detect changes in metabolic activity, and trypan blue uptake to indicate loss of cell viability.

Integration of in vitro neurotoxicity data with biokinetic modelling for the estimation of in vivo neurotoxicity

Risk assessment of neurotoxicity is mainly based on in vivo exposure, followed by tests on behaviour, physiology and pathology. In this study, an attempt to estimate lowest observed neurotoxic doses after single or repeated dose exposure was performed. Differentiated human neuroblastoma SH-SY5Y cells were exposed to acrylamide, lindane, parathion, paraoxon, phenytoin, diazepam or caffeine for 72 hours. The effects on protein synthesis and intracellular free Ca2+ concentration were studied as physiological endpoints. Voltage operated Ca2+ channel function, acetylcholine receptor function and neurite degenerative effects were investigated as neurospecific endpoints for excitability, cholinergic signal transduction and axonopathy, respectively. The general cytotoxicity, determined as the total cellular protein levels after the 72 hours exposure period, was used for comparison to the specific endpoints and for estimation of acute lethality. The lowest concentration that induced 20% effect (EC20) obtained for each compound, was used as a surrogate for the lowest neurotoxic level (LOEL) at the target site in vivo. The LOELs were integrated with data on adsorption, distribution, metabolism and excretion of the compounds in physiologically-based biokinetic (PBBK) models of the rat and the lowest observed effective doses (LOEDs) were estimated for the test compounds. A good correlation was observed between the estimated LOEDs and experimental LOEDs found in literature for rat for all test compounds, except for diazepam. However, when using in vitro data from the literature on diazepam's effect on gamma-amino butyric acid (GABA)A receptor function for the estimation of LOED, the correlation between the estimated and experimental LOEDs was improved from a 10,000-fold to a 10-fold difference. Our results indicate that it is possible to estimate LOEDs by integrating in vitro toxicity data as surrogates for lowest observed target tissue levels with PBBK models, provided that some knowledge about toxic mechanisms is known.

Gliotoxin induces caspase-dependent neurite degeneration and calpain-mediated general cytotoxicity in differentiated human neuroblastoma SH-SY5Y cells

In this study, a significant increase by 50% in intracellular free calcium concentration ([Ca(2+)](i)) was observed in differentiated human neuroblastoma (SH-SY5Y) cells after exposure to 0.25microM of the fungal metabolite gliotoxin for 72h. Further, the involvement of caspases and calpains was demonstrated to underlie the gliotoxin-induced cytotoxic and neurite degenerative effects. The caspase inhibitor Z-VAD-fmk almost completely reduced the neurite degeneration from 40% degeneration of neurites to 5% as compared to control. Inhibition of calpains with calpeptin significantly attenuated gliotoxin-induced cytotoxicity, determined as reduction in total cellular protein content, from 43% to 14% as compared to control cells. Western blot analyses of alphaII-spectrin breakdown fragments confirmed activity of the proteases, and that alphaII-spectrin was cleaved by caspases in gliotoxin-exposed cells. These results show that calpains and caspases have a role in the toxicity of gliotoxin in differentiated SH-SY5Y cells and that the process may be Ca(2+)-mediated.

The applicability of in vitro-derived data in hazard identification and characterisation of chemicals

Toxicological hazard and risk assessments for chemicals presently are mainly based on highly standardised protocols for animal experimentation and exposure assessment. In this paper the possibilities are being discussed of developing systems in which the systemic (acute and chronic) toxicity of chemicals can be quantified, without the heavy reliance on animal experiments. On the basis of a chemical's structure, in vitro data on its toxicity, and biokinetic modelling a decision/flow scheme is presented. Key elements are the evaluation of chemical functionalities representing structural alerts for toxic actions, the construction of biokinetic models on the basis of non-animal data (e.g. tissue-blood partition coefficients (PCs), in vitro biotransformation parameters), tests or batteries of tests for determining basal cytotoxicity and more specific tests for evaluating tissue- or organ toxicity. It is concluded that such a flow chart is a useful tool for different steps in the toxicological hazard and risk assessment, especially for those forms of toxicity for which validated in vitro and other non-animal tests have already been developed.

Toxicodynamic modelling and the interpretation of in vitro toxicity data

The results of in vitro toxicity experiments are not easily extrapolated to 'toxicological risk' for an intact organism. One of the most obvious differences between the situation in vitro and in vivo is the absence of the processes of absorption, distribution, metabolism and excretion that govern the exposure of the target tissues of the organism in vivo. The development of biokinetic models is aimed at estimating the relevant target tissue concentration of a compound. In our study, biokinetic models were constructed, where possible, solely on the basis of in vitro derived parameters for biotransformation as well as on partition coefficients determined or calculated from physicochemical structures. Another requirement is the existence of appropriate in vitro biological systems for the measurement of relevant effects. This requires a thorough knowledge of the possible mechanisms of toxic action, and of the physiology of the target organs. When these prerequisites are met (i.e. when the appropriate parameters can be quantified in a non-animal system), then an estimate of the dynamics in vitro can be made (e.g. as a critical active concentration). This will then result in a model describing a compound's dynamics. Eventually, the result of biokinetic and toxicodynamic models will need to be integrated in a compound's hazard and/or risk evaluation. A study carried out in the ECITTS programme showed promising results for the estimation of the acute and chronic systemic toxicity of a number of neurotoxic compounds.