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 different densities and incubated with 125 micrograms/mL of the photosensitizer meso-tetra(4-sulfonatophenyl)porphine (TPPS4) for 18 h, washed and irradiated with blue light. Four hours later the cells were studied by fluorescence microscopy. Apoptotic cells were detected by virtue of the distinct condensation and fragmentation of chromatin, and necrotic cells were detected by uptake of propidium iodide. In addition apoptosis was measured by the TdT assay. 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 equitoxic doses. Confluent cells were 2.9 times more sensitive than cells in microcolonies partly due to a 1.5 times higher uptake of TPPS4 in monolayer cells. The difference in mode of cell death for the different cell densities was not related to any observable difference in subcellular localization pattern of TPPS4 at equitoxic doses of photodynamic treatment.