Polystyrene Microplastics of Varying Sizes and Shapes Induce Distinct Redox and Mitochondrial Stress Responses in a Caco-2 Monolayer

Currently, we lack crucial knowledge on how the physicochemical properties of particles affect cellular health, resulting in an important gap in our understanding of the human toxicity of microplastics (MPs). Our aim was to evaluate the impact of the size and the shape of MPs on uptake and the intracellular effects in a human epithelial colorectal adenocarcinoma (Caco-2) cell line. Spherical (200 nm and 2 mu m) and fibre-/fragment-shaped (8.9 +/- 10.1 mu m by 1.14 +/- 0.97 mu m) polystyrene microplastics (PS-MPs) were used to study their uptake and the potential to induce redox and mitochondrial stress responses after 24 h of exposure. We demonstrated the cellular uptake of both spherical and fibre-/fragment-shaped MPs in a size-dependent manner. In response to 2 mu m spheres, we observed differential expressions of redox-related genes, including HMOX1, CAT, and GPX1. All PS-MPs decreased the intracellular H2O2 levels, which can be attributed to mitochondrial stress responses, such as increased mitochondrial DNA content, footprint, and morphology. Altogether, we demonstrated uptakes and changes in redox and mitochondrial parameters for all PS-MPs, with the 200 nm spheres showing the most profound effects. This suggests that the induction of defensive responses in Caco-2 cells mainly correlates with the number of particles taken up. This research was supported by a grant from the ZonMw Microplastics and Health Program—project number 458001012 (40-45800-98-118); BOF-UHasselt funding—project numbers R9782, and 22KP16; FWO—project number N15227910. The research leading to the results presented in this publication was carried out with infrastructure funded by EMBRC Belgium—FWO project GOH3817N We thank Naveen Reddy and Jorgo Merchiers (IMO-IMOMEC, UHasselt) for kindly providing access to their centrifugal spinning tool and their assistance with the polystyrene fibre synthesis. We also thank Wouter Marchal and Martine Vanhamel (Institute for Materials Research, UHasselt) for their assistance with the SLS measurements.