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Efeito da pré-exposição a selénio na toxicidade do metilmercúrio em astrócitos e células da microglia
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Resumo(s)
O metilmercúrio (MeHg) é a principal forma de exposição humana a mercúrio (Hg), acontecendo principalmente pelo consumo de peixe do topo da cadeia alimentar (peixe-espada, atum, tubarão, etc.). O consumo excessivo de peixe causa uma exposição crónica ao MeHg causando efeitos neurotóxicos nefastos sobretudo na população infantil e durante desenvolvimento fetal. Tendo sido relatado que em Portugal foram encontrados níveis de MeHg acima do estipulado como seguro pela European Food Safety Authority (EFSA) em crianças e grávidas. É por isso importante a prevenção da exposição ao MeHg e estabelecer novas estratégias de prevenção do risco de toxicidade por MeHg.
O principal mecanismo de toxicidade do MeHg a nível molecular relaciona-se com elevada afinidade para o grupo selenol (SeH) dos resíduos Sec das selenoproteínas do sistema redox reduzindo a atividade antioxidante destas enzimas. No organismo, o selénio (Se) ingerido é reduzido à forma de seleneto (Se2-) que por sua vez é importante na atividade antioxidante das selenoproteínas ao ser incorporado no resíduo de selenocisteína (Sec) do sítio ativo destas enzimas. Compreender o efeito da interação entre o MeHg e o Se ao nível dos sistemas redox poderá ser uma forma de mitigar os efeitos negativos do MeHg.
Este trabalho teve por objetivo avaliar o efeito da suplementação com selénio na viabilidade celular de astrócitos e células da microglia expostas a metilmercúrio e estabelecer a sua ligação à síntese de selenoproteínas.
A viabilidade de ambas as linhas celulares diminuiu em função da concentração de MeHg e foi dependente do tempo de exposição. Ademais a microglia foi a mais vulnerável à citotoxicidade induzida pelo MeHg devido a uma menor quantidade de selenoproteínas disponível nestas células. Verificou-se que a exposição a Se teve um efeito protetor da viabilidade de ambas as linhas celulares expostas a MeHg, em particular a níveis mais reduzidos de Se (0,1 vs 0,5 μM). A pré-exposição a Se aumentou consideravelmente a expressão das selenoproteínas do sistema da glutationa, GPx1 e GPx4. Com efeito na microglia verificou-se um aumento de 3 e 40 vezes na GPx4 e GPx1 enquanto nos astrócitos esse aumento foi de 10 e 5 vezes respetivamente. Curiosamente a expressão da TrxR1, um dos principais alvos moleculares do MeHg teve apenas um pequeno aumento resultante da suplementação com Se, o que se pode explicar pelo facto dos seus níveis basais serem já mais elevados.
Por fim, observou-se que enquanto o MeHg inibe translocação nuclear do Nrf-2, a pré-exposição a Se promove-a, o que explica por sua vez o aumento da síntese das selenoproteínas. No seu conjunto estes resultados mostram que suplementação com Se e a otimização da síntese das selenopoteínas são um fator essencial na proteção das células contra a toxicidade do MeHg.
Methylmercury (MeHg) is the main form of human exposure to mercury (Hg), occurring mainly through the consumption of fish at the top of the food chain (swordfish, tuna, shark, etc.). Excessive fish consumption causes chronic exposure to MeHg, causing harmful neurotoxic effects, especially in child population and fetal development. It has been reported that in Portugal, MeHg levels were found above stipulated as safe by the European Food Safety Authority (EFSA) in children and pregnant women. It is therefore important to prevent MeHg exposure and establish new ways to prevent the risk of MeHg toxicity. The main mechanism of MeHg toxicity at molecular level is related to its high affinity for the selenol group (SeH) of the Sec residues in selenoproteins within the redox system, thereby reducing the antioxidant activity of these enzymes. In the body, ingested selenium (Se) is reduced to selenide form (Se2-), which in turn is crucial for the antioxidant activity of selenoproteins by being incorporated into selenocysteine residue (Sec) at the active site of these enzymes. Understanding the effect of the interaction between MeHg and Se at the level of redox systems could be a way to mitigate MeHg negative effects. This work aimed to evaluate the effect of selenium supplementation on the cell viability of astrocytes and microglial cells exposed to methylmercury and establish its link to selenoproteins’ synthesis. The viability of both cells decreased as a function of MeHg concentration and was dependent on the exposure time. Furthermore, microglia were more vulnerable to cytotoxicity induced by MeHg due to a smaller amount of selenoproteins available in these cells. It was found that Se exposure had a protective effect on both cells’ lines viability exposed to MeHg, particularly at lower Se levels (0,1 vs 0,5 μM). Pre-exposure to Se considerably increased selenoproteins’ expression of the glutathione system, Gpx1 and Gpx4. In effect, in microglia there was an increase of 3 and 40 times in Gpx4 and Gpx1, while in astrocytes this increase was 10 and 5 times respectively. Interestingly, TrxR1 expression, one of the main targets of MeHg, had only a small increase resulting from Se supplementation, which can be explained by the fact that its basal levels are already higher. Finally, it was observed that while MeHg inhibits Nrf-2 nuclear translocation, pre-exposure to Se promotes it, which in turn explains the increase in selenoproteins’ synthesis. Taken together, these results show that Se supplementation and optimization of selenoproteins’ synthesis are an essential factor in protecting cells against MeHg toxicity.
Methylmercury (MeHg) is the main form of human exposure to mercury (Hg), occurring mainly through the consumption of fish at the top of the food chain (swordfish, tuna, shark, etc.). Excessive fish consumption causes chronic exposure to MeHg, causing harmful neurotoxic effects, especially in child population and fetal development. It has been reported that in Portugal, MeHg levels were found above stipulated as safe by the European Food Safety Authority (EFSA) in children and pregnant women. It is therefore important to prevent MeHg exposure and establish new ways to prevent the risk of MeHg toxicity. The main mechanism of MeHg toxicity at molecular level is related to its high affinity for the selenol group (SeH) of the Sec residues in selenoproteins within the redox system, thereby reducing the antioxidant activity of these enzymes. In the body, ingested selenium (Se) is reduced to selenide form (Se2-), which in turn is crucial for the antioxidant activity of selenoproteins by being incorporated into selenocysteine residue (Sec) at the active site of these enzymes. Understanding the effect of the interaction between MeHg and Se at the level of redox systems could be a way to mitigate MeHg negative effects. This work aimed to evaluate the effect of selenium supplementation on the cell viability of astrocytes and microglial cells exposed to methylmercury and establish its link to selenoproteins’ synthesis. The viability of both cells decreased as a function of MeHg concentration and was dependent on the exposure time. Furthermore, microglia were more vulnerable to cytotoxicity induced by MeHg due to a smaller amount of selenoproteins available in these cells. It was found that Se exposure had a protective effect on both cells’ lines viability exposed to MeHg, particularly at lower Se levels (0,1 vs 0,5 μM). Pre-exposure to Se considerably increased selenoproteins’ expression of the glutathione system, Gpx1 and Gpx4. In effect, in microglia there was an increase of 3 and 40 times in Gpx4 and Gpx1, while in astrocytes this increase was 10 and 5 times respectively. Interestingly, TrxR1 expression, one of the main targets of MeHg, had only a small increase resulting from Se supplementation, which can be explained by the fact that its basal levels are already higher. Finally, it was observed that while MeHg inhibits Nrf-2 nuclear translocation, pre-exposure to Se promotes it, which in turn explains the increase in selenoproteins’ synthesis. Taken together, these results show that Se supplementation and optimization of selenoproteins’ synthesis are an essential factor in protecting cells against MeHg toxicity.
Descrição
Trabalho Final de Mestrado Integrado, Ciências Farmacêuticas, 2024, Universidade de Lisboa, Faculdade de Farmácia.
Palavras-chave
Metilmercúrio Selénio Selenoproteínas Nrf-2 Células da glia Mestrado Integrado - 2024