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Organically modified silica (ORMOSIL) nanoparticles (NPs) are widely used in biomedicine. However, their cell uptake process has not yet been characterised in detail. Here, we investigated the mechanism underlying endocytosis and subcellular localisation of ORMOSIL NPs. Exposure to ORMOSIL NPs induced a decrease in cell viability and increase in lactate dehydrogenase release in a dose-dependent manner in A549 cells. Once internalised, ORMOSIL NPs were translocated from early endosomes to the lysosomes, where they accumulated. Furthermore, deficiency of autophagosomal/lysosomal fusion failed to block lysosomal localisation of ORMOSIL NPs, suggesting that autophagy was not involved in the final lysosomal accumulation of ORMOSIL NPs. Meanwhile, an inhibitor of caveolae-mediated endocytosis, rather than inhibitors of phagocytosis or clathrin-mediated endocytosis, succeeded in blocking ORMOSIL NP cell uptake, indicating the involvement of caveolae-mediated endocytosis. Together, these results provide a new understanding of the toxicity, and suggest better biomedical applications, of ORMOSIL NPs.

Background: Most nanoparticles (NPs) reportedly block autophagic flux, thereby upregulating p62/SQSTM1 through degradation inhibition. p62 also acts as a multifunctional scaffold protein with multiple domains, and is involved in various cellular processes. However, the autophagy substrate-independent role of p62 and its regulation at the transcriptional level upon NPs exposure remain unclear. Results: In this work, we exposed BEAS-2b cells and mice to silica nanoparticles (SiNPs), and found that SiNPs increased p62 protein levels in vivo and vitro. Then, we further explored the role and mechanism of SiNPs-stimulated p62 in vitro, and found that p62 degradation was inhibited due to autophagic flux blockade. Mechanistically, SiNPs blocked autophagic flux through impairment of lysosomal capacity rather than defective autophagosome fusion with lysosomes. Moreover, SiNPs stimulated translocation of NF-E2-related factor 2 (Nrf2) to the nucleus from the cytoplasm, which upregulated p62 transcriptional activation through direct binding of Nrf2 to the p62 promoter. Nrf2 siRNA dramatically reduced both the mRNA and protein levels of p62. These two mechanisms led to p62 protein accumulation, thus increasing interleukin (IL)-1 and IL-6 expression. SiNPs activated nuclear factor kappa B (NF-κB), and this effect could be alleviated by p62 knockdown. Conclusion: SiNPs caused accumulation of p62 through both pre-and post-translational mechanisms, resulting in airway inflammation. These findings improve our understanding of SiNP-induced pulmonary damage and the molecular targets available to mitigate it.

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