High density of octaarginine stimulates macropinocytosis leading to efficient intracellular trafficking for gene expression.

The mechanism of the arginine-rich peptide-mediated cellular uptake is currently a controversial issue. Several factors, including the type of peptide, the nature of the cargo, and the linker between them, appear to affect uptake. One of the less studied factors, which may affect the uptake mechanism, is the effect of peptide density on the surface of the cargo. Here, we examined the mechanism of cellular uptake and intracellular trafficking of liposomes modified with different densities of the octaarginine (R8) peptide. Liposomes modified with a low R8 density were taken up mainly through clathrin-mediated endocytosis, leading to extensive lysosomal degradation, whereas those modified with a high R8 density were taken up mainly through macropinocytosis and were less subject to lysosomal degradation. Furthermore, the high density R8-liposomes were able to stimulate the macropinocytosis-mediated uptake of other particles. When plasmid DNA was condensed and encapsulated in R8-liposomes, the levels of gene expression were three orders of magnitude higher for the high density liposomes. The enhanced gene expression by the high density R8-liposomes was highly impaired by blocking uptake through macropinocytosis. The different extents of gene expression from different densities of the R8 peptide on the liposomes could be explained principally by the existence of an intracellular trafficking route, but not by the uptake amount, of internalized liposomes. These results show that the density of the R8 peptide on liposomes determines the uptake mechanism and that this is directly linked to intracellular trafficking, resulting in different levels of gene expression.