Possible mechanism of polycation liposome (PCL)-mediated gene transfer.

A novel gene transfer system utilizing polycation liposomes (PCLs), obtained by modifying liposomes with cetyl polyethylenimine (PEI), was previously developed (Gene Ther. 7 (2002) 1148). PCLs show notable transfection efficiency with low cytotoxicity. However, the mechanism of PCL-mediated gene transfer is still unclear. In this study, we examined the intracellular trafficking of PCL-DNA complexes by using HT1080 cells, fluorescent probe-labeled materials, and confocal laser scan microscopy. We found that the PCL-DNA complexes were taken up into cells by the endosomal pathway, since both cellular uptake of the complex and gene expression were blocked by wortmannin, an inhibitor of this pathway. We also observed that the plasmid DNA and cetyl PEI complex became detached from the PCL lipids and was preferentially transferred into the nucleus in the form of the complex, whereas the PCL lipids remained in the cytoplasmic area, possibly in the endosomes. In fact, nigericin, which dissipates the pH gradient across the endosomal membrane, inhibited the detachment of lipids from the PCL-DNA complex and subsequent gene expression. Taken together, our data indicate the following mechanism for gene transfer by PCLs: PCLs effectively transfer DNA to endosomes and release cetyl PEI-DNA complexes into the cytosol. Furthermore, cetyl PEI also contributes to gene entry into the nucleus.