The enhancement in toxic potency of oxidized functionalized polyethylene-microplastics in mice gut and Caco-2 cells

Microplastics (MPs) are inevitably oxidized in the environment, however, to date, no studies have discussed the biological toxicity of oxidized polyethylene (Ox-PE) MPs. In this study, oxidized low-density polyethylene (Ox-LDPE), a representative Ox-PE, was prepared using a selective oxidation method. The difference in toxicity between LDPE-MPs and Ox-LDPE-MPs were evaluated in C57BL/6 mice and Caco-2 cells. The proton nuclear magnetic resonance (¹H NMR) and Fourier transform infrared (FTIR) spectroscopy analyses revealed that some hydrocarbon-containing groups were transformed into carboxyl and ketone groups during selective oxidation. In vivo experiment results showed that LDPE-MPs and Ox-LDPE-MPs exists in the intestinal (duodenum and colon) of mice, and Ox-LDPE-MPs caused more severe intestinal histological changes, oxidative stress, and inflammatory response. The gut microbiota data showed that the relative abundance of Lactobacillus decreased significantly in the LDPE-MP- and Ox-LDPE-MP-exposed groups (P < 0.05). The predicted Kyoto Encyclopedia of Genes and Genomes (KEGG) metabolic pathway suggested that exposure to LDPE-MPs or Ox-LDPE-MPs inhibited glycan biosynthesis and metabolism in the flora (P < 0.05). In vitro experiment results showed that selective oxidation to LDPE promoted its uptake by cells and aggravated adverse effects on cells, including reduced cell viability, damaged cell membrane, oxidative stress, and mitochondrial depolarization. The major mechanism of the increased toxicity of Ox-LDPE-MPs may be its easier accumulation and the ionic effect of oxygen-containing functional groups. Overall, these findings provide insights on the differences in toxicity between LDPE-MPs and Ox-LDPE-MPs. They also provide new perspectives for understanding the biohazards of MPs, which are necessary to accurately assess the potential environmental and health risks of these plastic pollutants.