Figure 2. F-actin together with the factors driving firm adhesion of amorphous silica nanoparticles (ASN) to AEC, is responsible for ASN cellular uptake. ( A ) Electron tomogram captures the internalization of ASN by an A549 cell with the uptake compartment components marked as follows: F-actin (green); the plasma membrane (dark blue); ASN agglomerate (red); caveolae (cyan); the trans-Golgi network (TGN, beige); and mitochondria (light blue). ( B ) Ultracellular topography of the ASN uptake compartment components (structures annotated in panel A). ( C ) Identification of the protein-lined cup structures of 50 nm to 100 nm diameter, consistent with caveolae formation (black arrow heads) at the interface of ASN uptake. ( D ) mCherry-caveolin-1-transfected A549 cells form caveolae-lined invagination (pseudo-colored green) in a juxtaposition to the far-red-fluorescent ASN (red) during the uptake captured via confocal live cell imaging (LCI). ( E ) The involvement of actin during ASN internalization is revealed by LCI of eGFP-LifeAct-transfected A549. Note that actin bundles following or preceding the ASN agglomerates as they become internalized (cell boundary represented by white line). ( F ) Quantitation of ASN uptake in the absence and in the presence of endocytic inhibitors by A549 and primary hAECs. Note the ASN uptake dependence on all firm-cell-adhesion factors (per Fig. 1) as well as a significant uptake suppression by the actin inhibitor (Cytochalasin B). Mean ± SD, n = 4 **p < 0.01. Scale bar: ( A – C ) 500 nm, ( D , E ): 2