Cascade liposomal triggering: light-induced Ca2+ release from diplasmenylcholine liposomes triggers PLA2-catalyzed hydrolysis and contents leakage from DPPC liposomes.

We have previously reported a direct triggering approach [Thompson, D. H., et al. (1996) Biochim. Biophys. Acta 1279, 25-34; Gerasimov, O. V., et al. (1997) Biochim. Biophys. Acta 1324, 200-214] based on the facile degradation of plasmenylcholine and diplasmenylcholine vinyl ether linkages by either photooxidation or low-pH environments. This report describes a novel, cascade-type triggering technique that utilizes liposome photooxidation and contents release to activate an enzyme capable of destabilizing conventional phosphatidylcholine liposomes. Our application of this concept employs a mixture of two different liposome populations, one composed of synthetic diplasmenylcholine (1, 2-dihexadec-1'-enyl-sn-glycero-3-phosphocholine, DPPlsCho) containing Ca2+ as a signaling agent for phospholipase A2 (PLA2) and the second composed of 1, 2-dihexadecanoyl-sn-glycero-3-phosphocholine (DPPC) with encapsulated calcein as the reporter molecule. Bacteriochlorophyll (BChl)-sensitized photorelease of Ca2+ from PLA2-resistant DPPlsCho liposomes activates extravesicular PLA2, thereby promoting catalyzed DPPC hydrolysis in a secondary triggering reaction, leading to calcein release. BChl/DPPlsCho/DHC/DPPE-PEG5000/Ca2+IN (0.5:85:10:5) liposomes can be phototriggered using 800 nm excitation, resulting in Ca2+ release (t50% release = 15 min) that cocatalyzes the release of calcein (t50% release = 40 min) from DPPC liposomes (1.5 mM total lipid in DPPlsCho liposomes, 0.18 mM DPPC, 210 micro M final Ca2+ concentration, 90 units of PLA2/ml, 50 mM calcein, and 36 micro M EDTA). No appreciable calcein release occurs in the absence of either PLA2 or BChl/DPPlsCho/DHC/DPPE-PEG5000/CaIN liposomes. The implications of this cascade triggering technique on drug delivery approaches are briefly discussed.