Mitochondrial phototoxicity sensitized by doxycycline in cultured human carcinoma cells

Cultured MGH-U1 (human urinary bladder carcinoma) cells were treated with doxycycline (DOTC) and long-wave UV radiation (UVA). At UVA doses of 1 J/cm2 and above, the cells showed mitochondrial damage, reflected by altered localization of the fluorescent probe rhodamine-123, and striking vacuolization of the cytoplasm. Cell membrane integrity, as monitored by exclusion of ethidium bromide, was maintained for several hours after mitochondrial damage was evident. These changes were potentiated by irradiation in the presence of deuterium oxide, and diminished by irradiation in the presence of sodium azide. Addition of catalase, superoxide dismutase, or mannitol did not alter the damage threshold. These data indicate that the mitochondrion is an earlier target of DOTC photosensitization than the cell membrane. The critical photochemistry appears to involve singlet oxygen.