Silica Nanoparticle-Induced Blockage of Autophagy Leads to Autophagic Cell Death in HepG2 Cells

Many efforts have been made to address the toxicity issue of silica nanoparticles (SiNPs). Previously, we found that the SiNPs could induce autophagy. It was still not clear about the relative mechanisms of SiNPs-induced autophagy and whether SiNPs would lead to autophagy blockage and subsequent autophagy dysfunction. Autophagy is recognized as a protective condition to maintain cellular homeostasis, while autophagy blockage could result in autophagy dysfunction which was proposed as a toxic mechanism of nanomaterials. In this study, autophagy induction was characterized by autophagic vacuoles detection, LC3-II expression, immunofluorescence and TEM ultrastructural observation. The cellular co-localization of fluorescein-labeled SiNPs and LC3 protein showed the interconnection of endosomes with the autophagic vacuoles. In addition, the detectable lysosome membrane destabilization and accumulated autophagic substrate p62 protein demonstrated the decrease of autophagic degradation, suggesting blockade of autophagosomes and lysosomes fusion in autophagic flux. Cell death detection and cellular co-staining further determined autophagic cell death. The inhibited expression of autophagy-involved PI3K/Akt/mTOR pathway clarified the underlying molecular signaling of autophagy in HepG2 cells after SiNPs exposure. In summary, this study represents a potential toxic mechanism involving autophagy dysfunction and ultimate autophagic cell death. It provides novel insights into the mechanisms of SiNPs by which they achieve their biological effects.