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 targe
ts for PDT are in cellular membranes, and we now find that PDT sensiti
zed by aluminum phthalocyanine causes the rapid (<1 min) activation of
phospholipase C and the breakdown of membrane phosphoinositides, as w
ell as a similarly rapid release of Ca2+ from intracellular pools. A p
hospholipase 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 anal
ogue U73343 is much less effective against all of the aforementioned r
esponses. In addition, p-bromphenacyl bromide, an inhibitor of phospho
lipase 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 c
an mimic natural stimuli of phospholipases and initiate apoptosis in L
5178Y cells.