Selenite causes cytotoxicity in rainbow trout (Oncorhynchus mykiss) hepatocytes by inducing oxidative stress

Selenium is an essential micronutrient to freshwater fish, but can be very toxic at slightly above the threshold level. The liver is known to be the major site of selenium accumulation and metabolism in fish. Recent evidence from mammalian systems suggests that oxidative damage is an important mechanism of selenium toxicity; however this phenomenon has not been investigated in-depth in fish, either in vivo or in vitro. Therefore, the present study was designed to investigate whether selenium (as selenite) exposure causes cytotoxicity in fish by inducing oxidative stress. We used isolated hepatocytes in primary culture from freshwater rainbow trout (Oncorhynchus mykiss) as the model in vitro experimental system. The 24 h LD50 of selenite to trout hepatocytes was found to be 587 μM. In order to evaluate the dose-dependent response patterns of various oxidative stress parameters, the trout hepatocytes were exposed to three different doses of selenite [50, 100 and 200 μM (corresponding to approximately 10%, 20% and 35% of 24 h LD50)] in addition to control (0 μM of selenite) for 24 h. We observed an induction of catalase (CAT) and superoxide dismutase (SOD) activities at 50 and 100 μM of selenite exposure, but not at 200 μM, relative to the control. In contrast, the induction of glutathione peroxidase (GPx) activity was recorded at 100 and 200 μM exposure doses, but not at 50 μM. We also demonstrated that selenite exposure (100–200 μM) increased intracellular ROS formation at an early stage (2 h). The reduced to oxidized glutathione ratio (GSH:GSSG) decreased sharply with increasing selenite dose, indicating the loss of cellular reducing capacity. The cellular lipid peroxidation tended to increase with increasing selenite exposure dose, indicating the occurrence of membrane damage. A 20–40% decrease in cell viability was observed at 100 and 200 μM of selenite exposure. The increase in cell death was associated with a significant increase of caspase-3/7 activity, suggesting the induction of apoptosis. Overall, the present study suggests that selenite exposure at high level causes oxidative damage to trout hepatocytes, probably by inducing the imbalance of intracellular glutathione (GSH) redox.