Apolipoprotein E peptide-modified colloidal carriers: The design determines the mechanism of uptake in vascular endothelial cells

Supramolecular structures, particularly micelles and liposomes equipped with uptake-mediating address compounds, have attracted much attention as pharmaceutical formulations. Their development requires an understanding of the mechanism by which the carrier systems interact with and translocate into the target cells. We developed an apolipoprotein E-derived peptide, called A2, that efficiently translocates across cell membranes. Upon coupling of two palmitoyl chains (P2), the highly cationic sequence acquires detergent-like properties such as a strong tendency to self-associate and the ability to integrate into lipid bilayers. Confocal laser scanning microscopy and fluorescence activated cell sorting were used to compare the internalization of the fluorescence-labeled monomeric A2 with the uptake of the colloidal P2A2 micelles and P2A2-tagged liposomes into endothelial cells of blood vessels. Specific inhibitors of endocytosis were used to identify the underlying mechanisms. b.End3 and BAEC cells as example of endothelial cells of small capillaries and large vascular vessels, respectively, were examined. The uptake of monomeric A2 was characterized by poor cellular selectivity. A2 was efficiently internalized into both cell lines via at least two different mechanisms. Besides an endocytotic uptake route, a second passive pathway exists, that leads to a rapid distribution of A2 within the cytoplasm. Also liposomes tagged with P2A2 were non-selectively internalized into both b.End3 and BAEC cells. Their nonselective uptake was mediated by clathrin- and caveolin-independent endocytosis. In contrast, micellar P2A2 entered b.End3 cells via clathrin-mediated endocytosis, while no uptake of P2A2 into BAEC cells was observed. In conclusion, the specific clathrin-mediated uptake mode of P2A2 micelles might provide the basis for a blood brain barrier-specific targeting.