Background: Micro- and nanoplastic pollution has become a global environmental problem. Nanoplastics in the environment are still hard to detect because of analysis technology limitations. It is believed that when microplastics are found in the environment, more undetected nanoplastics are around. The current "microplastic exposure" is in fact the mixture of micro- and nanoplastic exposures. Therefore, the biological interaction between organisms among different sizes of micro- and nanoplastics should not be neglected.
Results: We measured the biodistribution of three polystyrene (PS) particles (50 nm PS, PS50; 500 nm PS, PS500; 5000 nm PS, PS5000) under single and co-exposure conditions in mice. We explored the underlying mechanisms by investigating the effects on three major components of the intestinal barrier (the mucus layer, tight junctions and the epithelial cells) in four intestine segments (duodenum, jejunum, ileum and colon) of mice. We found that the amounts of both PS500 and PS5000 increased when they were co-exposed with PS50 for 24 h in the mice. These increased amounts were due primarily to the increased permeability in the mouse intestines. We also confirmed there was a combined toxicity of PS50 and PS500 in the mouse intestines. This manifested as the mixture of PS50 and PS500 causing more severe dysfunction of the intestinal barrier than that caused by PS50 or PS500 alone. We found that the combined toxicity of PS micro- and nanoplastics on intestinal barrier dysfunction was caused primarily by reactive oxygen species (ROS)-mediated epithelial cell apoptosis in the mice. These findings were further confirmed by an oxidants or antioxidants pretreatment study. In addition, the combined toxicity of PS micro- and nanoplastics was also found in the mice after a 28-day repeated dose exposure.
Conclusions: There is a combined toxicity of PS50 and PS500 in the mouse intestines, which was caused primarily by ROS-mediated epithelial cell apoptosis in the mice. Considering that most recent studies on PS micro- and nanoplastics have been conducted using a single particle size, the health risks of exposure to PS micro- and nanoplastics on organisms may be underestimated.
Keywords: Combined effect; Health risk; Intestinal barrier; Microplastic; Mixture; Nanoplastic.