Cellular Uptake Mechanism of Cationic Branched Polypeptides with Poly[l-Lys] Backbone

Cationic macromolecular carriers can be effective carriers for small molecular compounds, drugs, epitopes, or nucleic acids. Polylysine-based polymeric branched polypeptides have been systematically studied on the level of cells and organisms as well. In the present study, we report our findings on the cellular uptake characteristics of nine structurally related polylysine-based polypeptides with cationic side chains composed of (i) single amino acid (poly[Lys(X_i)], X_iK) or (ii) oligo[dl-alanine] (poly[Lys(dl-Ala_m)], AK) or (iii) oligo[dl-alanine] with an additional amino acid (X) at the terminal position (poly[Lys(X_i-dl-Ala_m)] (XAK)) or (iv) at the position next to the polylysine backbone (poly[Lys(dl-Ala_m-X_i)] (AXK)). In vitro cytotoxicity and cellular uptake were characterized on HT-29 human colon carcinoma and HepG2 human hepatocarcinoma cell lines. Data indicate that the polycationic polypeptides studied are essentially nontoxic in the concentration range studied, and their uptake is very much dependent on the side chain structure (length, identity of amino acid X, and distance between the terminal positive charges) and also on the cell lines. Our findings in uptake inhibition studies suggest that predominantly macropinocytosis and caveole/lipid raft mediated endocytosis are involved. The efficacy of their internalization is markedly influenced by the hydrophobicity and charge properties of the amino acid X. Interestingly, the uptake properties of the these polypeptides show certain similarities to the entry pathways of several cell penetrating peptides.