Interactions of silica nanoparticles with lung epithelial cells and the association to flotillins

Amorphous silica nanoparticles (aSNPs) gain increasing popularity for industrial and therapeutic claims. The lung with its surface area of 100-140 m² displays an ideal target for therapeutic approaches, but it represents also a serious area of attack for harmful nanomaterials. The exact nature of the cytotoxic effects of NPs is still unknown. Furthermore, cellular pathways and the destiny of internalized NPs are still poorly understood. Therefore, we examined the cytotoxicity (MTS, LDH) and inflammatory responses (IL-8) for different-sized aSNPs (30, 70, 300 nm) on our lung epithelial cells line NCI H441 and endothelial cell line ISO-HAS-1. Additionally, colocalization studies have been conducted via immunofluorescence staining for flotillin-1- and flotillin-2-bearing endocytic vesicles. Subsequently, the relevance of flotillins concerning the viability of aSNP-exposed epithelial cells has been evaluated using flotillin-1/2 depleted cells (siRNA). This study reveals the relevance of the nanoparticle size regarding cytotoxicity (MTS, LDH) and inflammatory responses (IL-8), whereat the smaller the size of the nanoparticle is, the more harmful are the effects. All different aSNP sizes have been incorporated in flotillin-1- and flotillin-2-labelled vesicles in lung epithelial and endothelial cells, which display a marker for late endosomal or lysosomal structures and appear to exhibit a clathrin- or caveolae-independent mode of endocytosis. Flotillin-depleted H441 showed a clearly decreased uptake of aSNPs. Additionally, the viability of aSNP-exposed cells was reduced in these cells. These findings indicate a contribution of flotillins in as yet unknown (clathrin or caveolae-independent) endocytosis mechanisms and (or) endosomal storage.