The binding characteristics and intracellular localization of temoporfin (mTHPC) in myeloid leukemia cells: Phototoxicity and mitochondrial damage

The state of aggregation of the photosensitizer meso-tetrahydroxyphenylchlo rin (mTHPC) in both cell free and intracellular environment was elucidated by comparing its absorption and excitation spectra, In methanol, mTHPC exis ted as monomers and strongly fluoresced, In aqueous solutions such as phosp hate-buffered saline (PBS), mTHPC formed nonfluorescent aggregates. Some po rtion of mTHPC monomerized in the presence of 10% fetal calf serum PBS, In murine myeloid leukemia M1 and WEHI-3B (JCS) cells, cytoplasmic mTHPC were monomeric, By using organelle-specific fluorescent probes, it was found tha t mTHPC localized preferentially at the mitochondria and the perinuclear re gion. Photodynamic treatment of mTHPC-sensitized leukemia cells caused rapi d appearance of the apoptogenic protein cytochrome c in the cytosol, Result s from flow cytometric analysis showed that the release of cytochrome c was especially pronounced in JCS cells, and well correlated with the extent of apoptotic cell death as reported earlier. Electron microscopy revealed the loss of integrity of the mitochondrial membrane and the appearance of chro matin condensation as early as 1 h after light irradiation. We conclude tha t rapid release of cytochrome c from photodamaged mitochondria is responsib le for the mTHPC-induced apoptosis in the myeloid leukemia JCS and M1 cells .