Phospholipase activation triggers apoptosis in photosensitized mouse lymphoma cells

Photodynamic therapy (PDT), an experimental cancer treatment employing a photosensitizer and visible light, is a highly efficient inducer of apoptosis (or programmed cell death) in mouse L5178Y lymphoma cells, resulting in extensive DNA fragmentation within 1-2 h. The major targets for PDT are in cellular membranes, and we now find that PDT sensitized by aluminum phthalocyanine causes the rapid (< 1 min) activation of phospholipase C and the breakdown of membrane phosphoinositides, as well as a similarly rapid release of Ca2+ from intracellular pools. A phospholipase C inhibitor, U73122, blocks the rapid transient increases in both inositol-1,4,5-trisphosphate and intracellular Ca2+ levels as well as the subsequent fragmentation of nuclear DNA, whereas the analogue U73343 is much less effective against all of the aforementioned responses. In addition, p-bromphenacyl bromide, an inhibitor of phospholipase A2, blocks DNA fragmentation, and PDT stimulates the release of arachidonic acid, probably by phospholipase A2-dependent breakdown of membrane phospholipids. Thus, photodynamic damage to cell membranes can mimic natural stimuli of phospholipases and initiate apoptosis in L5178Y cells.