Nanoplastic toxicity and uptake in kidney cells: differential effects of concentration, particle size, and polymer type.

Nanoplastics (NPs, < 1 µm) are emerging environmental contaminants capable of crossing biological barriers and interacting at the cellular and subcellular level. Despite evidence of microplastics in human kidney tissue and urine, the renal effects of NPs remain poorly understood. This study investigated the short-term effects of NPs polymer type, size, and concentration on human kidney proximal tubule cells (HK-2). Cells were exposed for 24-h to carboxylated polystyrene (PS), poly(methyl methacrylate) (PMMA), and polyethylene (PE) NPs (15-100 nm) at concentrations from 0.1 to 200 µg/mL. NPs morphology, size, and charge were characterised by scanning electron microscopy, dynamic light scattering, and zeta potential. Cell morphology, viability, cell cycle distribution, and NPs internalisation were assessed by microscopy and flow cytometry. Low-concentration exposures had minimal effects, whereas 100 and 200 µg/mL induced marked morphological changes, including cytoplasmic granularity. Viability decreased significantly at 200 µg/mL for several NPs types, with PE NPs causing the largest reduction (79.4%). Polymer type influenced outcomes, with PE and PMMA NPs causing greater morphological disruption than PS. Size effects were most evident in cell cycle analysis: 15 nm and 20 nm PS NPs and 100 nm PMMA NPs induced phase arrest without major viability loss. NPs internalisation increased with concentration but varied with polymer type, with PE NPs showing preferential perinuclear localisation. These findings demonstrate that NPs effects on kidney cells depend on polymer chemistry, particle size, concentration, and highlight the need for long-term studies using environmentally relevant NPs to better assess kidney toxicity risk.