Phospholipase Activation Triggers Apoptosis in Photosensitized Mouse Lymphoma Cells

Abstract

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.