Upregulating Nrf2-dependent antioxidant defenses in Pacific oysters Crassostrea gigas: Investigating the Nrf2/Keapl pathway in bivalves

Analysis of the Pacific oyster Crassostrea gigas annotated genome revealed genes with conserved sequences belonging to typical cap 'n' collar Nrf2 domain, a major player in antioxidant protection, and domains belonging to Nrf2 cytoplasmic repressor (Keapl), but little is known about Nrf2/Keapl induction in bivalves. C. gigas were exposed to waterborne 10 and 30 mu M curcumin, a known inducer of the mammalian Nrf2. Curcumin disappeared from the seawater after 10 h, and accumulated in the gills (10 h) and digestive gland (10-96 h). A clear induction of glutathione (GSH)-related antioxidant defenses was observed at 96 h in the gills of curcumin exposed animals (10 and 30 mu M), including GSH levels, and the activity of glutathione reductase (GR), glutathione peroxidase (GPx), and glutathione S-transferase (GST). This response was completely absent in the digestive gland, in line with the idea that bivalve gills act as a major site for antioxidant protection under acute exposure. The relative mRNA levels coding glutamate-cysteine ligase, GR, GPx2 and GSTpi were clearly induced by curcumin treatment (30 mu M, 24 h). Curcumin pre-treatment for 96 h increased oyster resistance to cumene hy-droperoxide, but neither Nrf2 nor Keapl genes were modulated by curcumin. However, the conserved sequences belonging to typical Nrf2 and Keapl domains, and the notorious induction of antioxidant defense-related genes known to be controlled by Nrf2 in mammals, indicates a functional Nrf2/Keapl pathway in bivalves, and curcumin seems to be a new tool to investigate the antioxidant response in bivalves. (C) 2017 Elsevier Inc. All rights reserved.