During apoptosis, a number of physical changes occur in the cell membrane including a gradual increase in permeability to vital stains such as
propidium iodide. This study explored the possibility that one consequence of membrane changes concurrent with early modest permeability is vulnerability to degradation by secretory
phospholipase A(2). The activity of this hydrolytic
enzyme toward mammalian cells depends on the health of the cell; healthy cells are resistant, but they become susceptible early during programmed death. Populations of S49
lymphoma cells during programmed death were classified by flow cytometry based on permeability to
propidium iodide and susceptibility to secretory
phospholipase A(2). The apoptotic inducers
thapsigargin and
dexamethasone caused modest permeability to
propidium iodide and increased staining by
merocyanine 540, a
dye sensitive to membrane perturbations. Various secretory
phospholipase A(2)
isozymes (human groups IIa, V, X, and
snake venom) preferentially hydrolyzed the membranes of cells that displayed enhanced permeability. In contrast, cells exposed briefly to a
calcium ionophore showed the increase in cell staining intensity by
merocyanine 540 without accompanying uptake of
propidium iodide. Under that condition, only the
snake venom and human group X
enzymes hydrolyzed cells that were dying. These results suggested that cells showing modest permeability to
propidium iodide during the early phase of apoptosis are substrates for secretory
phospholipase A(2) and that specificity among
isoforms of the
enzyme depends on the degree to which the membrane has been perturbed during the death process. This susceptibility to hydrolysis may be important as part of the signal to attract macrophages toward apoptotic cells.