Endocytosis inhibitors prevent poly(amidoamine) dendrimer internalization and permeability across Caco-2 cells

Previous studies from our group demonstrated visual evidence that endocytosis mechanism(s) contribute to the internalization and intracellular trafficking of cationic and anionic poly(amidoamine) (PAMAM) dendrimers across Caco-2 cells. These dendrimers colocalized with established endocytosis markers, which suggested PAMAM dendrimers may be internalized by a clathrin-dependent endocytosis mechanism and are rapidly trafficked to endosomal and lysosomal compartments. In the present study, generation 4 PAMAM-NH2 (G4NH2) dendrimer was labeled with tritium to measure the rate of uptake and permeability in Caco-2 cells. The effect of endocytosis inhibitors brefeldin A, colchicine, filipin, and sucrose on G4NH2 absorption and transport was examined to give further insight into the endocytosis mechanisms that transport PAMAM dendrimers across Caco-2 cell monolayers. G4NH2 showed linear uptake at lower concentrations, and rapidly increased as a function of concentration. The rate of G4NH2 uptake significantly declined at high concentrations in the presence of the endocytosis inhibitors, and the apparent permeability similarly reduced in the presence of these inhibitors. A significant reduction in G4NH2 permeability was observed in the presence of brefeldin A and colchicine, which generally disrupt vesicular trafficking and formation during the endocytosis process. Coincubation with filipin and sucrose reduced G4NH2 permeability to a lesser extent, which suggests G4NH2 could be nonspecifically internalized in coated vesicles at the plasma membrane. The observations from this study further confirm that G4NH2 internalization and transport involves an endocytosis pathway.