Efeitos de compostos orgânicos de selênio na toxicidade induzida por metilmercúrio: estudos in vitro

Abstract

Methylmercury (MeHg) is a known environmental pollutant that affects a variety of cellular functions, and the oxidative stress is one of the proposed mechanisms to explain its toxicity. Inhibition of selenoenzymes (Glutathione peroxidase (GPx) and Thioredoxin Reductase (TrxR)) has been demonstrated as an important factor related to oxidative stress induced by MeHg. On the other hand, synthetic selenium compounds have important antioxidant activity, mainly attributed to the GPx-like activity and the fact that they are substrates for TrxR, may even exert protection against MeHg toxicity. Whereas the exact mechanism by which MeHg exerts toxicity remains to be further investigated and that there are no effective therapies for this organometal toxicity, this study evaluated the different mechanisms involved in MeHg-induced toxicity as well a possible protective role of organic selenium compounds (ebselen, diphenyl diselenide (PhSe)2 and analogues (3’3 ditrifluoromethyldiphenyl diselenide (DFD), p-chloro-diphenyl diselenide (CLD) and p-methoxy-diphenyl diselenide(MD)) against toxic effects of this organometal at different in vitro models of biological systems targets of MeHg action. In Article 1 it was observed that MeHg caused an increase in oxidative stress markers in a model of mouse brain mitochondrial-enriched fraction. This result was evidenced by an increase in lipid peroxidation, hydroperoxide formation and a significant decrease in mitochondrial activity. Furthermore, it was also demonstrated a significant depletion of thiols. (PhSe)2 and DFD were able to protect against the decrease in mitochondrial activity, and lipid peroxidation caused by MeHg. The compound CLD was also effective in reversing the lipid peroxidation caused by exposure of mouse brain mitochondrial fraction to MeHg. The (PhSe)2 was also able to degrade hydrogen peroxide through a comparative study with catalase. These compounds showed protective activity according their GPx-like activity intensity ((PhSe) 2> DFD> CLD> MD). In a second analysis (manuscript 1), MeHg caused a significant inhibition of antioxidant enzymes, TrxR and GPx, in neuroblastoma cells (SH-SY5Y). In this study (PhSe)2 showed a better protective activity than ebselen. (PhSe)2 caused a substantial increase in the activity and expression of TrxR, and it was able to protect against the inhibition of this enzyme caused by MeHg. Under the same experimental conditions, ebselen exhibited a similar effect, though more modestly compared to (PhSe)2. Diphenyl diselenide also caused an increased in GPx expression, which was higher when compared to the effect of ebselen. In in vitro model of human leukocytes (manuscript 2), MeHg caused cell death and DNA damage. In parallel, there was a significant increase in Nrf2expression, the main cellular regulator of antioxidant response. Co-treatment with (PhSe)2 was able to reverse these damages, and normalizing the expression of Nrf2. Furthermore, (PhSe)2 caused a substantial increase in expression of TrxR enzyme, response that has been already demonstrated in the previous study in a model of human neuroblastoma. Together, the results presented in this thesis reinforce the central role of oxidative stress and inhibition of selenoezimas in MeHg-induced toxicity. Furthermore, the results indicate that the organic selenium compounds, especially (PhSe)2, play an important protective role against MeHg toxicity. The reversal of TrxR inhibition caused by (PhSe)2 appears to be an important mechanism involved in the protection against the deleterious effects caused by exposure to MeHg, a toxic environmental agent.