Tetracycline antibiotics as PI3K inhibitors in the Nrf2-mediated regulation of antioxidative stress in zebrafish larvae

Scientific concern about veterinary antibiotics (VAs) residues in the aquatic environment has increased in recent years. However, little is known about the underlying molecular mechanism of antioxidative stress caused by VAs in fish. In this study, zebrafish larvae were exposed to two representatives of VAs, chlortetracycline (CTC) and oxytetracycline (OTC), for 48 h. The oxidative stress responses and possible molecular mechanism of action were investigated. The results showed that the activities of CAT, SOD and GPx were significantly inhibited and the contents of GSH and MDA increased after CTC exposure. Moreover, SOD and CAT activity were parallel to their mRNA and protein levels. Under OTC exposure, CAT and GST activity were inhibited, while GPx activity was induced, and MDA content decreased significantly. After treatment with CTC and OTC, glucose levels and Nrf2 mRNA and protein levels in zebrafish larvae were significantly downregulated. Further molecular docking and molecular dynamics simulations revealed that CTC and OTC are capable of docking into the binding pocket of zebrafish PI3K, an important molecule in the activation of Nrf2, and can form stable interactions through hydrogen bonds. The overall results indicated that CTC and OTC significantly induced oxidative stress responses in zebrafish larvae, and both CTC and OTC act as inhibitors of PI3K to inhibit the activation of the Nrf2/ARE signaling pathway, thus reducing the antioxidant capacity of fish.