IDENTIFICATION OF DIFFERENT CELL DEATH PATHWAYS ACTIVATED BY NEUROTOXIC FOOD CONTAMINANTS

Food can be the most significant source of exposure to chemical hazards that pose considerable risks to public health. Contamination may occur through environmental pollution, such as the case with PCBs, dioxins and toxic metals, or through the intentional use of chemicals, such as pesticides. Several of these chemicals have been suggested as possible risk factors for neurodevelopmental and neurodegenerative disorders. The central nervous system is particularly sensitive to neurotoxic stimuli, especially during early development, when there are windows of susceptibility to adverse interference that are not present in the mature brain. Neurotoxic injuries to the nervous system may result in apoptosis, an active cell death process requiring energy, and/or necrosis, a passive form of death. In the last years, we have investigated the mechanisms leading to neural cell death induced by neurotoxic food contaminants, including methylmercury (MeHg), PCB 153 and PCB 126. Different cell lines, including the mouse hippocampal neuronal cell line HT22, the pituitary cell line AtT20, the neural stem cell line C17.2, as well as primary cultures of rat embryonic and mouse adult neural stem cells (NSCs) were used as experimental models. The studies provided evidence for the activation of multiple cell death pathways in cells exposed to MeHg and PCBs. The Ca 2+ -dependent proteases calpains were activated in HT22 cells, as shown by the increase in the 150 kDa α-fodrin, a calpain breakdown product. Pre-treatment with the calpain inhibitor E64d exerted a partial protection in both HT22 and AtT20 cells against all three toxicants. There was no activation of caspases, as further supported by the lack of protective effects of the pan-caspase inhibitor z-VAD-fmk. In contrast, NSCs exposed to MeHg underwent apoptotic cell death via Bax oligomerization, cytochrome c release from the mitochondria with subsequent activation of caspase-3. In addition to caspases, calpain was also activated in parallel during MeHg-induced NSC cytotoxicity. Lysosomal disruption, visualized by a decreased uptake of the lysomotrophic vital dye acridine orange, was observed in the HT22 cells exposed to MeHg or PCBs, and in NSC but only after exposure to PCBs. In agreement, pre-incubation with the cathepsin inhibitor pepstatin protected the exposed cells suggesting a critical role of lysosomal proteases in the death process. In conclusion, different cell types are differently susceptible to food contaminants such as MeHg and PCBs, with embryonic NSC being the most sensitive cell type investigated. Interestingly, the same toxic stimulus can activate diverse intracellular responses. The use of multiple in vitro experimental models is a first choice approach to perform studies aimed at dissecting the molecular mechanisms of neurotoxic damage. Understanding the mechanisms of apoptotic cell death may offer new approaches to neuroprotective strategies against the damage induced by neurotoxicants.