Size-dependent toxicity of polystyrene microplastics in lung cells: An in vivo and in vitro study

Microplastics (MPs) are emerging pollutants with pervasive respiratory exposure routes, yet their lung-specific toxicity mechanisms remain poorly defined. This study aims to investigate the size-dependent detrimental effects on pulmonary systems using in vivo (mice) and in vitro (coculture) models, simulating acute (single dose) and subchronic (28-day) exposures. Crucially, we identify epithelial-mesenchymal transition (EMT) as a novel cytotoxic mechanism and delineate the ECM-MMP signaling cascade as the primary driver of PS-MP-induced lung injury. In vivo, acute intratracheal exposure to 12 mg/kg PS-MPs (with 1 µm particles) resulted in weight loss (9.09 % vs. controls). PS-MPs accumulated dominantly in lungs, with 1 µm particles depositing 1.38-fold higher than 10 µm particles (quantitative result). Subchronic exposure (8 mg/kg) triggered particle-size-dependent pathology. 1 µm PS-MPs increased lung injury scores by 2.5-fold vs. 10 µm. Myeloperoxidase (MPO) and malondialdehyde (MDA) rose by 2.13-fold (1 µm) vs. 1.81-fold (10 µm). In vitro, 1 µm PS-MP-exposed lung cells induced mitochondrial depolarization (ΔΨm loss: 50 %) and apoptosis (17 % increase). Critically, 1 µm PS-MPs potently activated MMPs (MMP-2↑180 %, MMP9↑250 %) via ECM-MMP dysregulation. Our findings reveal that PS-MPs drive lung injury through oxidative stress, cell apoptosis, and mitochondrial dysfunction in a strict size-dependent manner (1 µm > 5 µm > 10 µm), with the ECM-MMP axis as a central pathway. The signaling pathway activated by PS-MPs in lung injury suggested that PS-MPs induced proliferation inhibition, oxidative stress, and EMT via activating the ECM-MMP signaling cascade. In addition, EMT activation suggested a novel mechanism for the cytotoxicity of PS-MPs, hinting at the potential carcinogenic effect of these pollutants.