The mode of cell death induced by photodynamic treatment depends on cell density

Madison Darby canine kidney II (MDCK II) cells were seeded out at two diffe rent densities and incubated with 125 IJ mu g/mL of the photosensitizer mes o-tetra(4-sulfonatophenyl)porphine (TPPS4) for 18 h, washed and irradiated with blue light. Four hours later the cells were studied by fluorescence mi croscopy. Apoptotic cells were detected by virtue of the distinct condensat ion and fragmentation of chromatin, and necrotic cells were detected by upt ake of propidium iodide. In addition apoptosis was measured by the TdT assa y. The fraction of apoptotic cells and the fraction of necrotic cells were determined for both cell densities at various levels of survival. With <55% total cell death the apoptotic fraction was significantly higher for cells in confluent monolayers than for cells growing in microcolonies at equitox ic doses. Confluent cells were 2.9 times more sensitive than cells in micro colonies partly due to a 1.5 times higher uptake of TPPS4 in monolayer cell s. The difference in mode of cell death for the different cell densities wa s not related to any observable difference in subcellular localization patt ern of TPPS4 at equitoxic doses of photodynamic treatment.