Involvement of Na⁺/H⁺ exchangers (NHE) in the polystyrene particle uptake mechanism in Caco-2 cells

<p>Regarding the toxicity of orally ingested polystyrene, it is now known that relatively large polystyrene particles of about 0.1 μm in diameter, which are included in the category of microplastics, are taken up by Caco-2 cells, used as a model of the intestinal epithelium. Some of these particles accumulate in the lysosomal lumen, suggesting that endocytosis is involved. In this study, we evaluated the effect of Na⁺/H⁺ exchangers (NHE), which are involved in the regulation of endocytosis, on the uptake of polystyrene particles. 200 μg/ml of 0.1 µm diameter fluorescent polystyrene beads with amino group modification were treated in Caco-2 cells for 24 hours. Polystyrene particles were uptaken into the lysosome-like organelle lumen and an increase in the number and size of lysosomes, suggesting swelling. The NHE inhibitor amiloride significantly reduced the polystyrene particle uptake into the lysosome. In NHE-1 knockdown cells, the number of polystyrene particles incorporated into the lysosome was also significantly reduced. A slight decrease in the lysosomal luminal localization of the enzyme cathepsin D and partial accumulation of Galectin-3 in lysosomes were observed after treatment with polystyrene particles, suggesting that lysosomal dysfunction with increased lysosomal membrane permeabilization occurred. The increased lysosomal membrane permeabilization induced by polystyrene particle treatment was suppressed by amiloride treatment or NHE-1 knockdown. Thus, NHE-1 is involved in the uptake of polystyrene particles and lysosomal dysfunction.</p>