Health Risks of Pristine and Leached Polystyrene Micro- and Nanoplastics: An In Vitro Study on Human Dental Pulp Stem Cells

The toxicity of micro- and nanoplastics in aquatic life is well documented, yet limited information is available on their effects in humans; moreover, most in vitro nanotoxicology studies rely on cancer cells. This study examined the effects of pristine and aged polystyrene micro- and nanoparticles on human dental pulp stem cells. While both particle sizes were internalized by the cells, primarily through endocytosis, they did not affect cell viability. In contrast, leachates from particles, aged for one month in culture medium, significantly reduced viability, indicating that toxicity arises from degradation byproducts rather than the particles themselves. Atomic force microscopy confirmed surface changes in aged plastics. Both particle sizes disorganized the cytoskeleton, leading to reduced actomyosin cortex integrity. Gene expression analysis revealed that leachates and aged particles activated inflammatory pathways, markedly increasing IL-8 and TGF-β1 expression, while also decreasing SOD levels associated with oxidative stress. No notable effects were observed on genes related to stemness or senescence. These results suggest that, while pristine micro- and nanoplastics may be relatively inert, their degradation products pose greater toxicological risks to human health. The findings highlight the importance of considering leachate toxicity in plastic pollution studies and demonstrate the value of stem cell-based models for evaluating the cellular and molecular impacts of environmental contaminants on human health.