Lipid flow phenomena between liposomes, lipoproteins and cell membranes; application in drug delivery

The bilayer structure making up the physical boundary of the liposome bears a strong resemblance to the biological membranes forming the boundaries of cells and subcellular particles. Similarly, the extracellular lipid layers in the stratum corneum of the skin basically possess this bimolecular leaflet structure. Essentially half such a bilayer is found at the interface at the air-water interface in the lung and at the lipid-water interface of the surface of lipoprotein particles. In contrast to the bilayer, which separates two aqueous phases, these monolayers constitute the boundary between an aqueous and an air phase or an aqueous and a lipid phase, respectively. A fundamental difference between artificial (phospho)lipid membranes such as in liposomes and biological membranes is the presence in the latter of a host of membrane-spanning integral proteins as well as more loosely associated peripheral proteins. Proteins are likely to play an important role in interactions between biological membranes. Such interactions may, for example, lead to the induction of membrane fusion and lipid or protein exchange phenomena. During the interaction of liposomes with the membranous structures of the skin similar phenomena may be involved. In this paper observations will be discussed which have been made in our laboratories over the past few years and which may bear relevance to the understanding of such phenomena in which transfer of lipids between membranes plays a role.