Contrasting effects of a classic Nrf2 activator, tert-butylhydroquinone, on the glutathione-related antioxidant defenses in Pacific oysters, Crassostrea gigas

Nrf2 is a well-known transcription factor controlling a number of antioxidant defense-related genes, which is understudied in bivalves. In this study, oysters Crassostrea gigas were exposed for 24, 48 and 96 h to 10 or 30 μM tert-butylhydroquinone (tBHQ), a classic Nrf2 activator. At 96 h, a clear induction of GSH-related antioxidant defenses was observed in gills of tBHQ-exposed animals, including GSH, glutathione S-transferase (GST), glutathione peroxidase (GPx) and glutathione reductase (GR). Unexpectedly, the activities of GST, GPx and GR were significantly decreased 24 h after tBHQ treatment, suggesting a possible inhibition, which was supported by in vitro experiments. GR mRNA (24 h) and protein levels (24 and 96 h) were increased by tBHQ treatment, confirming its induction, possibly by the Nrf2 pathway. The conserved domains at C. gigas Keap1 and Nrf2 proteins and the clear induction of GSH-related antioxidant defenses by tBHQ, a classical Nrf2 inducer, support the idea of a functional Nrf2/Keap1 pathway in bivalves. tBHQ also proved to be a tool to explore redox regulatory mechanisms in bivalves.