Size-dependent internalization of micro- and nanoplastics induces pro-inflammatory and oxidative stress responses in marine and freshwater fish cell lines.

Microplastics (MPs) are ubiquitously detected in aquatic ecosystems and represent a growing environmental concern due to their persistence, accumulative toxicity, and ability to cross biological barriers, posing substantial risks to fish species. Although numerous studies have investigated the toxicity of MPs in fish, there is limited data on the potential toxic effects of MP exposure at the cellular level. In this study, we aimed to compare the cellular toxicity of polystyrene MPs of different shapes and sizes in fin and muscle cells derived from marine (red sea bream: PMF) and freshwater (fathead minnow: FHM) fish. Our results showed that exposure to MPs of various shapes and sizes did not cause significant changes in cell viability in either cell line. No significant differences were observed, although exposure to spherical MPs (1 μm) induced a decrease in cell viability at relatively high concentrations. The cellular uptake of MPs was observed following exposure to spherical MPs (0.2 and 1 μm), with internalization occurring at 6 h for 1 μm and 24 h for 0.2 μm of incubation. Internalization efficiency was higher for 1 μm MPs than for 0.2 μm MPs. Exposure to spherical MPs (0.2 and 1 μm) induced alterations in intracellular ROS levels and triggered the up-regulation of the NRF2 gene, a transcription factor involved in the stress response. The expression of pro-inflammatory cytokines, including TNF-α and IL1-β, was increased by exposure to spherical MPs (0.2 and 1 μm). In addition, this exposure changed the transcriptional responses of immune defense-related genes. Overall, our results suggest that acute exposure to spherical MPs (0.2 and 1 μm) may be related to the disruption of the immune defense system through continuous inflammation at the cellular level in fish, although it does not directly affect cell viability.