S. Simoes et al., On the mechanisms of internalization and intracellular delivery mediated by pH-sensitive liposomes, BBA-BIOMEMB, 1515(1), 2001, pp. 23-37
We investigated the molecular mechanisms by which pH-sensitive liposomes su
rpass the cytoplasmic and endosomal membranes to deliver their aqueous cont
ents into the cytoplasm. Various liposome formulations were evaluated for t
heir efficacy to mediate intracellular delivery of encapsulated material, i
ncluding a novel sterically stabilized pH-sensitive formulation ((DOPE: CHE
M S: DSPE-PEG(2000) (6:4:0.3)) that was previously developed in our laborat
ories. In an attempt to fully characterize the nature of liposome-cell inte
ractions different approaches based on a dual-labeling fluorescence assay w
ere used. Our results indicate that the efficacy of interaction of pH-sensi
tive liposomes, both plain and sterically stabilized, with cells is strongl
y determined by the inclusion of DOPE in their composition, independently o
f the type of the amphiphilic stabilizer used. In fact, DOPE-containing lip
osomes shown to be non-pH sensitive by biophysical assays, mediated cytopla
smic delivery of their contents as efficiently as well known pH-sensitive f
ormulations (e.g. DOPE:CHEMS). However, among the different formulations st
udied, DOPE:CHEMS liposomes were those exhibiting the highest extent of cel
l association. Moreover, our results with cells pretreated with metabolic i
nhibitors or lysosomotropic agents clearly indicate that DOPE-containing li
posomes are internalized essentially by endocytosis and that acidification
of the endosomes is not the only mechanism involved in the destabilization
of the liposomes inside the cell. (C) 2001 Elsevier Science B.V. All rights
reserved.