Photosensitization of phycocyanin extracted from Microcystis in human hepatocellular carcinoma cells: Implication of mitochondria-dependent apoptosis

The aim of this study was to explore the possibility that Microcystis phycocyanin (MC-PC) functions as a photosensitizer and to investigate the mechanism for the apoptosis induced by Microcystis phycocyanin-mediated photodynamic therapy (MC-PC-PDT) in human hepatocellular carcinoma cells (HepG2). After incubation with MC-PC, HepG2 cells were exposed to a He–Ne Laser beam and the cell survival rate was detected by MTT and Colony forming assay. The mechanism of apoptosis was determined by ultrastructural observation, reactive oxygen species (ROS) and mitochondrial membrane potential (Δψm) assay, activity detection of caspase-3 and cytosol cytochrome c and flow cytometry (FCM) for cell cycle analysis. Our results demonstrated that MC-PC-PDT effectively inhibits HepG2 proliferation with a half-lethal dose of 100 μg/mL and induces apoptosis at 24 h with a dose of 200 μg/mL MC-PC. MC-PC was found to localized in mitochondria, it could induce a high level of ROS accumulation at 16 h after PDT treatment, cause mitochondrial damage and the release of mitochondrial cytochrome c into the cytosol. These cellular changes are accompanied by a reduction of the Δψm, activation of caspase-3 and G2/M phase arrest, finally leading to apoptosis through a mitochondria-dependent pathway after 24 h. Meanwhile, necrosis was also contributed to cell death in MC-PC PDT process. The present study also identified a new source of phycocyanin from Microcystis as a safe and effective photosensitizer.