Adaptation of HepG2 cells to silver nanoparticles-induced stress is based on the pro-proliferative and anti-apoptotic changes in gene expression.

Silver nanoparticles (AgNPs) are one of the most widely used nanomaterials due to their antibacterial properties. Owing to the recent boost in the usage of AgNPs-containing products, human exposure to AgNPs is increasing, highlighting the need for careful evaluation of AgNPs toxicity in humans. We used two cellular models, hepatic HepG2 and epithelial A549 cell lines, to study the mechanism of AgNPs-induced toxicity at the cellular level. These two cell lines differ significantly in their response to AgNPs treatment. In the case of A549 cells, a minor decrease in viability and increase in the extent of DNA breakage were observed. A markedly different response to AgNPs was observed in HepG2 cells. In short term, a massive induction of DNA breakage was observed, suggesting that the basal activity of antioxidant defence in these cells was not sufficient to effectively protect them from the nanoparticle-induced oxidative stress. After prolonged exposure, the extent of DNA breakage decreased to the level observed in the control cells proving that a successful adaptation to the new conditions had taken place. The cells that were unable to adapt must have died, as revealed by the Neutral Red assay that indicated less than half viable cells after 24-h treatment with 100 µg/ml of 20nm AgNPs. The gene expression analysis revealed that the observed adaptation was underlain by a pro-proliferative, anti-apoptotic signal leading to up-regulation of the genes promoting proliferation and inflammatory response (EGR1, FOS, JUN, HK2, IL4, MMP10, VEGFA, WISP1, CEBPB, IL8, SELPLG), genes coding the anti-apoptotic proteins (BCL2A1, CCL2) and factors involved in the response to stress (HSPB1, GADD45A). Such a selection of highly resistant population of cells should be taken into account in the case of medical applications of nanoparticles since the sustained proliferative signalling and resistance to cell death are hallmarks of cancer, acquired by the cells in the process of carcinogenesis.