Cationic
lipids are widely used for gene delivery, and inclusion of
dioleoylphosphatidylethanolamine (DOPE) as a helper
lipid in cationic
lipid-
DNA formulations often promotes transfection efficacy. To investigate the significance of DOPE's preference to adopt a hexagonal phase in the mechanism of transfection, the properties and transfection efficiencies of
SAINT-2/DOPE lipoplexes were compared to those of lipoplexes containing lamellar-phase-forming dipalmitoylphosphatidylethanolamine (
DPPE). After interaction with anionic vesicles, to simulate lipoplex-endosomal membrane interaction,
SAINT-2/DOPE lipoplexes show a perfect hexagonal phase, whereas
SAINT-2/
DPPE lipoplexes form a mixed lamellar-hexagonal phase. The transition to the hexagonal phase is crucial for dissociation of
DNA or
oligonucleotides (ODN) from the lipoplexes. However, while the efficiencies of
nucleic acid release from either complex were similar,
SAINT-2/DOPE lipoplexes displayed a two- to threefold higher transfection efficiency or nuclear ODN delivery. Interestingly,
rupture of endosomes following a cellular incubation with ODN-containing
SAINT-2/
DPPE complexes dramatically improved nuclear ODN delivery to a level that was similar to that observed for
SAINT-2/DOPE complexes. Our data demonstrate that although hexagonal phase formation in lipoplexes is a prerequisite for
nucleic acid release from the complex, it appears highly critical for accomplishing efficient translocation of
nucleic acids across the endosomal membrane into the cytosol for transport to the nucleus.