Envolvimento do sistema da tiorredoxina nas alterações induzidas pelo chumbo in vitro e in vivo: implicações na toxicidade do chumbo

• Autor(a) principal: Conterato, Greicy Michelle Marafiga
• Data de Publicação: 2011
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: Manancial - Repositório Digital da UFSM
• dARK ID: ark:/26339/00130000121kv
• Texto Completo: http://repositorio.ufsm.br/handle/1/4445
• topic: Chumbo; Cádmio; Exposição ocupacional; Estresse oxidativo; Antioxidantes; Tiorredoxina; Tiorredoxina redutase; Glutationa; Glutationa redutase; Glutationa S-transferase; Lead; Cadmium; Occupational exposure; Oxidative stress; Antioxidants; Thioredoxin; Thioredoxin reductase; Glutathione; Glutathione reductase; CNPQ::CIENCIAS BIOLOGICAS::BIOQUIMICA

Oxidative stress is an important molecular mechanism of lead (Pb) toxicity. The thioredoxin system (selenoenzyme thioredoxin reductase-TrxR, thioredoxin protein- Trx and NADPH) is essential for the antioxidant defense and cellular redox control. In our previous study, it was showed that cytosolic renal TrxR1 activity of rats increased after acute and long-term exposure to Pb and this was the only parameter that changed after both exposures to low Pb doses. Then, it was suggested that TrxR1 could operate in the early defense against Pb toxicity and it could also be used as a bioindicator of early effects of Pb. Thus, the main objective of this thesis was to investigate the role of thioredoxin system in Pb-induced changes, evaluating: I) in vitro the activity of purified TrxR1, as well as the activity and the protein expression of TrxR1 and Trx1 in renal HEK 293 culture cells exposed to Pb; II) in vivo, the effects of Pb exposure in rats and in occupationally-exposed humans on renal (only in rats) and blood TrxR1 activity (both rats and humans), comparing these effects to oxidative stress parameters, as well as to classical bioindicators of Pb effect and exposure. The results of the in vitro study showed that lead is a less potent inhibitor of the purified TrxR1 activity (IC50 = 0.27 TM) than its structural homologous glutathione reductase (IC50 = 0.048 TM). TrxR1 inhibition was independent on the selenocysteine residue of the active site and was reversible by bovine serum albumin and by the EDTA chelating. TrxR1 inhibition also occurred in HEK 293 cells exposed to the highest Pb acetate concentration (60 TM), without alterations in protein expression. However, under glutathione (GSH) depletion after pre-incubation of cells with L-buthionine-[S,R]-sulfoximine (BSO) and further exposure to Pb, the activity and expression of both TrxR1 and Trx1 increased in the absence of cytotoxicity and of changes in GR and glutathione S-transferase activities, which indicates Trx system as an important protective mechanism against Pb toxic effects in GSH-depleted cells. On the other hand, blood TrxR1 activity did not change either after acute exposure of rats or long-term exposure of humans to Pb. However, the increase of renal TrxR1 activity in rats exposed to the highest dose of Pb acetate (25 mg/kg) occurred concomitantly with the increase of blood and renal Pb levels over time (6, 24 e 48 h), whereas the erythrocyte δ-ALA-D inhibition, which is a classical indicator of Pb effects, occurred after 6 h of exposure and the activity was further recovered (at 24 and 48 h). Moreover, the increase of renal TrxR1 activity occurred without renal histopathologycal damage, which corroborates the increase of this enzyme as an early event of Pb toxicity. Overall, the results of the current study point out the thioredoxin system as a target for Pb, but mainly as a protective mechanism against Pb toxicity. However, the absence of changes in blood TrxR1 activity in Pb-exposed animals and humans indicates that this enzyme is not an appropriate bioindicator of the toxic effects of Pb in exposed populations.