Hypericin incubation: Aliquots of hypericin (PhytoLab GmbH & Co.KG) stock solutions in dimethyl sulfoxide (DMSO; Molecular Probes™ D12345) with a concentration of 0.1 mM were freshly thawed for each cell incubation. Dilutions of this stock in complete cultivation medium and additional 10 % FCS were prepared to achieve a final hypericin concentration of 1 µM. This procedure was adapted from . Dilutions of DMSO in complete cultivation medium and additional 10 % FCS were used for controls, resulting in 1 % DMSO within the final cultivation mixture. The hypericin and DMSO cultivation mixtures were added to the cells and incubated for 30 min at 37 °C and 5 % CO2 conditions, respectively. Both incubations were disrupted by removing the cultivation mixtures and washing three times with phosphate buffered saline (PBS). Afterwards, fresh cultivation medium was added to the hypericin--incubated and control cells. All preparation steps and incubation experiments were performed in the dark. Cells were kept at 37 °C and 5 % CO2 atmosphere until resorufin and PI treatments or FLIM measurements.