Objective  To investigate potential inflammatory effect of polystyrene microplastics (PS-MPs) in small intestine of mice and its molecular mechanisms.

  Methods  Thirty-five male BABL/c mice were randomly assigned into five groups and gavaged daily with H₂O and PS-MPs of 50 nm, 500 nm, 5 μm, 50 μm in diameter at the dosage of 1mg/day continuously for 42 days. Body weight of the mice were measured during the administration and pathological changes in ileum of the mice were observed using hematoxylin and eosin (HE) staining at the end of the treatments. Murine intestinal epithelial MODE-K cells were cultured with fluorescent PS-MPs of 50 nm, 5 μm, 50 μm in diameter at concentration of 10 μg/ml. The internalization of the PS-MPs in the cultured MODE-K cells was examined with flow cetometry after the treatments of 10 mins, 30 mins, 1 h, 2 hs, and 4 hs and the viability of the MODE-K cells treated with PS-MPs at various concentrations was assessed with 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. The expressions of cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway-related mRNAs and proteins in the in small intestine tissues and the MODE-K cells after PS-MPs treatment were determined with real-time PCR and Western blot test.

  Results  Pathological changes of different severity were observed in small intestinal tissues of the mice exposed to PS-MPs. PS-MPs of 50 nm was internalized more rapidly and efficiently into the cytoplasm of MODE-K cells than 5 μm and 50 μm PS-MPs, and the PS-MPs significantly affected the viability of the MODE-K cells. Compared with those of the control mice, significant upregulations of the mRNAs of pro-inflammatory cytokines IL-6 (increased by 589%), IL-10 (573%), IL-22 (475%) and TNF-β (177%) were detected in small intestinal tissues of the mice treated with 50 nm PS-MP (P < 0.01 for all); meanwhile, protein expressions of IL-1β, IL-6 and IL-10 also increased significantly in the tissues of the mice (P < 0.05 for all). In MODE-K cells treated with PS-MPs of 50 nm at high dosage (20μg/mL), the expressions of IL-6, IL-10, IL-22, TNF-α, and TNF-β were significantly higher than those in the control cells, with the increases of 74%, 85%, 83%, 75% and 69%, respectively (P < 0.05 for all). Then, compared with those of the control mice, the mRNA expressions of cGAS and STING increased by 485% and 136% in small intestinal tissues of mice treated with PS-MPs of 50 nm ( both P < 0.05), and the protein expressions of STING and p-NF-κB also increased by 61% and 54%, respectively (both P < 0.05). In addition, the protein expressions of STING and p-NF-κB increased by 96% and 178% in the MODE-K cells treated with high-dose (20 μg/mL) 50 nm PS-MPs, respectively.

  Conclusion  PS-MPs exposure could trigger small intestinal inflammation in mouse and exposure to PS-MPs of 50 nm in diameter could activate cGAS-STING signaling pathway through cellular internalization and then act on related downstream inflammatory factors, finally contributing to chronic inflammation in small intestine.