Combined effects of polystyrene microplastics and cadmium on oxidative stress, apoptosis, and GH/IGF axis in zebrafish early life stages

The toxicological interactions of microplastics (MPs) and heavy metals have been paid much attention in aquatic organism. The mechanisms are not fully clear, particularly in fish early life stages. To the end, zebrafish embryos were exposed to 500 μg/L MPs, 5 μg/L cadmium (Cd), and their combination for 30 days. Body weight, adsorption characteristics of Cd onto MPs, Cd accumulation, oxidative stress, apoptosis, and growth hormone/insulin-like growth factor-I (GH/IGF) axis were examined. Exposure to MPs and Cd alone reduced body weight, which was aggravated by co-exposure. An increase in reactive oxygen species (ROS) levels was observed in larvae exposed to Cd or MPs + Cd, suggesting an induction of oxidative stress. Lipid peroxidation levels were not affected by exposure to MPs and Cd alone but dramatically enhanced by co-exposure, which may be explained by the reduction of total antioxidant capacity (TAOC) and activity levels of Mn-superoxide dismutase (Mn-SOD) and catalase (CAT) after co-exposure. Increased apoptotic cells were observed in the vertebral body of larvae exposed to Cd, the esophagus of larvae exposed to MPs, and both organs of larvae exposed to MPs + Cd, which was further confirmed by changes in the activities of Caspase-3, Caspase-8 and Caspase-9. PCR array on the transcription of genes related to growth, oxidative stress and apoptosis was examined, showing that the combined exposure resulted in greater magnitude of changes than MPs and Cd alone. The results indicate that MPs can enhance the negative effects of Cd on growth, oxidative damage and apoptosis in early life stages of zebrafish. However, the adsorption of Cd onto MPs was not observed and the combined exposure did not increase the Cd content in larvae compared to the single Cd exposure, implying that vector role of MPs in Cd uptake is negligible. [Display omitted] •MPs enhanced the negative effects of Cd on growth, oxidative stress and apoptosis in zebrafish early stages.•The vector role of MPs in Cd uptake was negligible.•The vertebral body and esophagus of fish were the most vulnerable to Cd and MPs alone respectively.•The combined exposure resulted in increased apoptotic cells in both vertebral body and esophagus.•The mechanisms were involved in Nrf2/Keap1, p53 and GH/IGF signaling.