p62/SQSTM1 Accumulation Resulting from Degradation Inhibition and Transcriptional Activation is Essential in Silica Nanoparticle-Induced Pulmonary Inflammation Through NF-κB Activation

Most nanoparticles (NPs) are reported to block autophagic flux, accompanied by accumulated p62/SQSTM1 resulting from degradation inhibition. p62 also acts as a multifunctional scaffold protein that contains multiple domains, involved in various cellular processes. However, the autophagy substrate-independent role and regulation at a transcriptional level of p62 upon NPs exposure are ignored. Here, we exposed BEAS-2b cells to silica nanoparticles (SiNPs), and found that p62 degradation was inhibited due to autophagic flux blockade. Mechanically, SiNPs blocked autophagy flux through lysosomal capacity impairment rather than defective autophagosome fusion with lysosomes. Moreover, SiNPs stimulated translocation of NF-E2-related factor 2 (Nrf2) to the nucleus from the cytoplasm, and upregulated p62 transcriptional activation through direct binding of Nrf2 to p62 promoter. Nrf2 siRNA dramatically decreased both mRNA and protein levels of p62. Above two mechanisms led to p62 protein accumulation, therefore increasing IL-1 and IL-6 expression. SiNPs activated nuclear Factor kappa B (NF-κB), which can be alleviated by p62 knockdown. In summary, SiNPs accumulated p62 by both pre- and post-translational mechanisms, resulting in pulmonary inflammation. These findings improve our understanding of SiNP-induced pulmonary damage and molecular targets to antagonise it.