Uptake and intracellular effects of different size-shape polystyrene microplastics in a Caco-2 monolayer

Our current understanding of the human toxicity of plastic particles constitutes a major knowledge gap. MPs can vary largely in their characteristics, and parameters such as size, shape, and chemical composition need to be included in risk assessment strategies. In the present study, we aimed to evaluate the impact of shape on uptake and intracellular effects of MPs comparing both spherical and irregularly shaped polystyrene particles in a human epithelial colorectal adenocarcinoma (Caco-2) cell line. Two sizes of carboxylated polystyrene spheres (200 nm and 2 μm) and self-synthesized irregularly shaped polystyrene particles (average length and diameter: 8.9 μm and 1.14 μm respectively) were used to study on one hand their ability to penetrate cells, and on the other hand their potential to induce changes in metabolic activity, membrane integrity, general reactive oxygen species levels, mitochondrial DNA content and expression of oxidative stress-related genes. Altogether, our study demonstrates that both spheres and irregularly-shaped MPs are internalized by Caco-2 cells in a size-dependent manner, i.e., smaller particles are more easily taken up after 24 h exposure. Intracellularly, a lowering in H2O2 levels was observed for all MPs. This was complemented with mitochondrial
changes after exposure to the 200 nm spheres and irregularly-shaped particles, while differential expression of pro- and anti- oxidative genes, namely HMOX1, CAT and GPX1,was observed during exposure to the 2 μm spheres. These adverse (anti)-oxidative responses indicate that effects are related to the physicochemical and toxicokinetic characteristics of the particles, and highlight the importance of particle characterization to acquire fundamental insights vital for proper hazard profiling.