Interactions between polystyrene nanoparticles and human intestinal epithelial Caco-2 cells

Nanoplastics enter the human body mainly by ingestion through the gastrointestinal tract and thus the uptake and release of nanoplastics in intestinal cells have been studied. However, the fate of nanoplastics in intestinal cells remains poorly understood, particularly how they are exocytosed. Herein, we investigated the uptake, distribution, and exocytosis of nanoplastics in Caco-2 cells using 70 nm red fluorescent polystyrene (R70PS) as a nanoplastic model. The results show that R70PS readily enters Caco-2 cells and the content per cell peaks at around 24 h, but the total intracellular content in all cells increases continuously over 72 h. In addition, the uptake mechanisms change over incubation time, i.e. R70PS entered Caco-2 cells via both the energy-independent pathway and the energy-dependent caveolae-mediated endocytosis and macropinocytosis at 4 h incubation, but almost all R70PS entered cells in an energy-dependent manner via caveolae-mediated endocytosis, macropinocytosis, and clathrin-mediated endocytosis at 12 h incubation. Most of the intracellular R70PS accumulated in lysosomes, but R70PS also entered the mitochondria and its level increased over time. Approximately 45 % of the intracellular R70PS could be cleared from the cells within 12 h, mainly via the lysosomal pathway. Exocytosis was also associated with autophagy and was facilitated by the increase in the number of mitochondria and lysosomes, but inhibited by serum in the medium. Our findings deepen the understanding of the interaction between nanoplastics and intestinal cells, which is helpful for the risk assessment of nanoplastics.