In Vitro Cytotoxic Mechanism and Exposure Risk Assessments of Banned Organochlorine Pesticides in Indo-Pacific Humpback Dolphins

The persistent detrimental effects of banned organochlorine pesticide (dichlorodiphenyltrichloroethanes (DDTs)) exposure are a concern for cetaceans, yet their toxicological effects and risks have been difficult to handle due to technical and ethical issues. Here, we measured the concentrations of DDTs in the blubber of Indo-Pacific humpback dolphin (Sousa chinensis) (n = 128) collected from the Pearl River Estuary (PRE) during 2003–2020 and studied by an in vitro assay that used the S. chinensis skin fibroblasts to answer questions about the cytotoxicity mechanism and risks of DDTs. DDT levels in humpback dolphins exceed those reported in other marine mammal studies. As the fraction of DDEs within ∑DDTs increased, ∑DDT levels rose from 2004 to 2010, followed by a gradual decline from 2010 to 2020. Fibroblasts were treated with p,p′-DDT and their metabolites (p,p′-DDE and p,p′-DDD) and evaluated for cytotoxicity, metabolic efficiency, and oxidative stress response. In vitro cytotoxicity testing revealed that accumulation of DDTs is likely to pose adverse effects at the cellular level in 90% of the studied dolphins as their tissue concentrations exceeded EC50 values. Analyses of DDTs and their metabolites DDDs and DDEs in the cells and media indicated that a substantial amount of p,p′-DDT is metabolized into other products besides p,p′-DDE and p,p′-DDD. Furthermore, analyses of antioxidant capacity, oxidative damage, and expression of related genes indicated that p,p′-DDT exposure induced oxidative stress likely through the Keap1-Nrf2 pathway. This study deepens our understanding of the adverse effects of DDTs on dolphin health at cellular levels while also highlighting the ongoing threat that persistent environmental contaminants pose to the conservation of cetaceans.