Surface-driven endocrine activity of nanoplastics: polymer- and size-dependent estrogen and androgen receptors modulation without steroidogenesis perturbation.

Growing production and use of plastics have led to significant environmental pollution including the formation and accumulation of plastic nanoparticles (PNPs). Due to their small size, PNPs easily enter the human food chain; however, humans are also exposed to plastics through other consumer pathways, such as the use of cosmetic products. Despite considerable efforts to investigate the potential adverse effects of plastics, their impact on human health is not yet fully understood. In particular, endocrine disruption has emerged as a potential mechanism underlying reported reproductive and hormonal effects of micro- and nanoplastics. We applied an OECD-aligned in vitro test guidelines (TGs) to a factorial panel of eight PNPs spanning four common polymers (polystyrene (PS), polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET)) with size-resolved materials and polymer-matched mixtures. Thus, estrogen receptor a (ERα) transactivation (TG 455), androgen receptor (AR) transactivation (TG 458, antagonist mode), and H295R steroidogenesis (TG 456) assays were performed using HeLa-9903, AR-EcoScreen GR KO M1, and NCI-H295R cell models, respectively. Across 0.1-10 mg L⁻¹, no cytotoxicity was observed. PENPs (350 nm) and PPNPs (180 nm) acted as ER agonists, whereas PPNPs (50 and 180 nm) and PENPs (350 nm) antagonized AR; PSNPs and PETNPs showed no activity when tested individually. Notably, several mixtures elicited ER and AR responses even when constituent singles were inactive, indicating mixture-dependent potentiation. In contrast, the H295R assay did not meet the OECD decision rule for altered steroidogenesis: sporadic shifts in pathway intermediates did not propagate to estradiol or testosterone. Altogether, the data support a surface- and polymer-dependent, receptor-proximal mode of action for PNPs and highlight mixture effects as a critical, underappreciated driver. These results move endocrine hazard evaluation beyond polystyrene surrogates and provide decision-useful guidance on which polymers/sizes and mixture contexts merit priority in exposure monitoring and risk assessment.