The aim of the present study was to investigate the concentration- and size-dependent effects of non-functionalized polystyrene nanoparticles (PS-NPs) of varying diameters (29 nm, 44 nm, and 72 nm) on specific epigenetic modifications and gene expression profiles related to carcinogenesis in human peripheral blood mononuclear cells (PBMCs) in vitro. This in vitro human-cell-based model is used to investigate the epigenetic effect of various environmental xenobiotics. PBMCs were exposed to PS-NPs at concentrations ranging from 0.001 to 100 µg/mL for 24 h period. The analysis encompassed epigenetic DNA modifications, including levels of 5-methyl-2′-deoxycytidine (5-mdC) and 5-(hydroxymethyl)-2′-deoxycytidine (5-hmdC), as well as the levels of 2′-deoxyuridine (dU) and 5-(hydroxymethyl)-2′-deoxyuridine (5-hmdU) by mass spectrometry methods, methylation in the promoter regions of selected tumor suppressor genes TP53 (P53), CDKN2A (P16), and CDKN1A (P21) and proto-oncogenes (CCND1, BCL2, BCL6), along with the expression profile of the indicated genes by real-time PCR assays. The results obtained revealed no significant changes in global DNA methylation/demethylation levels in PBMCs after short-term exposure to non-functionalized PS-NPs. Furthermore, there were no changes observed in the level of dU, a product of cytosine deamination. However, the level of 5-hmdU, a product of both 5-hmdC deamination and thymine oxidation, was increased at the highest concentrations of larger PS-NPs (72 nm). None of the PS-NPs caused a change in the methylation pattern of the promoter regions of the TP53, CDKN2A, CDKN1A, CCND1, BCL2 and BCL6 genes. However, gene profiling indicated that PS-NPs with a diameter of 29 nm and 44 nm altered the expression of the TP53 gene. The smallest PS-NPs with a diameter of 29 nm increased the expression of the TP53 gene at a concentration of 10 µg/mL, while PS-NPs with a diameter of 44 nm did so at a concentration of 100 µg/mL. An increase in the expression of the CDKN2A gene was also observed when PBMCs were exposed to PS-NPs with 29 nm in diameter at the highest concentration. The observed effect depended on both the concentration and the size of the PS-NPs.

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