Uptake of Micro- and Nano-Sized Plastic Particles in Human Cells: Size-Dependent Endocytosis and Cellular Responses

Micro- and nanoplastics (MNPs) are increasingly detected in human tissues, yet the mechanisms governing their cellular uptake remain poorly understood. This study evaluated size-dependent cellular association of polystyrene MNPs (0.25-1.12 μm) across seven human cell lines representing skin, intestine, lung, kidney, liver, brain, and vascular endothelium. No cytotoxicity was observed in six cell lines after 24 or 48 h at concentrations up to 100 μg/mL, and epithelial and endothelial barrier integrity remained unaffected. Several cell types showed increased metabolic activity after 48 h. Cell-associated MNP fluorescence exhibited cell-type- and particle-size-dependent patterns. HEK001 keratinocytes and HMC-3 microglia showed rapid increases in cell-associated particle signals, with an apparent plateau for 0.25 μm particles; whereas HEK293-hTLR2 kidney cells and MRC-5 lung fibroblasts exhibited increasing cell-associated fluorescence over time across several particle sizes. Caco-2 intestinal cells showed limited interaction, whereas HepaRG liver and HMEC-1 endothelial cells exhibited size-dependent responses. Inhibition studies indicated that actin-dependent pathways contributed predominantly to the association of 0.54-1.12 μm particles with cells, whereas clathrin-mediated endocytosis contributed to smaller-particle interactions in a highly cell-specific and size-specific manner. Polydisperse mixtures exhibited reduced cell-associated particle signals but were unaffected by endocytosis inhibitors, further highlighting the complexity of nanoplastic internalization and the importance of both tissue identity and particle size in determining cellular interactions.