Photodynamic therapy (PDT) is a promising treatment modality for a variety of cancers. It utilizes light-absorbing compounds combined with direct illumination to generate reactive oxygen species in photosensitizer-targeted tumor cells resulting in the final photodamage of tumors. Recently, we demonstrated that a combination modality of 9-hydroxypheophorbide α (9-HPbD)-based PDT and carboplatin exerts enhanced cytotoxic and apoptotic effects on AMC-HN-3 laryngeal cancer cells. The present study aimed to investigate the potential apoptotic pathways initiated by 9-HPbD-PDT-mediated reactive oxygen species (ROS) in AMC-HN-3 cells. Cytotoxicity and apoptosis induced by 9-HPbD-PDT were exhibited in a ROS-dependent manner. Mitochondria and the endoplasmic reticulum (ER) were observed as preferential sites of 9-HPbD accumulation in AMC-HN-3 cells. ROS induced by 9-HPbD-PDT directly led to downregulated expression of Bcl-2, loss of mitochondrial membrane potential, release of cytochrome c from mitochondria, elevation of intracellular calcium due to ER stress, as well as induction of CHOP and activation of caspase-3, -8, -9 and -12. Our results demonstrated that ROS induced by 9-HPbD-PDT play a causative role in triggering mitochondrial events, ER stress and probable involvement of the extrinsic apoptotic pathway in AMC-HN-3 cells.

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