New insights of mitochondria reactive oxygen species generation and cell apoptosis induced by low dose photodynamic therapy

Photodynamic therapy (PDT) is an approved therapeutic procedure that exerts cytotoxic activity towards tumour cells by irradiating photosensitisers with light exposure to produce reactive oxygen species (ROS). In the current study, we have observed that there is an additional production of intracellular ROS during low dose PDT. A mitochondrial respiration-deficient cell line (ρ ⁰ cells) was investigated to determine the involvement of electron transfer chain (ETC). The production of ROS was significantly different between ASTC-a-1 and ρ ⁰ASTC-a-1 cells after an identical PDT treatment. Yet, with an increasing Photofrin dose, the difference gradually diminished. Pretreatment of the ASTC-a-1 cells with the ETC inhibitor rotenone lead the corresponding ROS production to a similar level from ρ ⁰ASTC-a-1 cells subjected to identical PDT protocols. Moreover, we found that the difference in intracellular ROS productions between ASTC-a-1 and ρ ⁰ASTC-a-1 cells started during a PDT treatment, while the irradiation was still being delivered. A cytotoxicity assay showed that, the ASTC-a-1 cells were more sensitive to PDT than ρ ⁰ASTC-a-1 cells. ROS scavenger n-acetyl-l-cysteine (NAC) attenuated the toxicity of PDT in both cell lines. Altogether, these results indicate that low dose PDT can induce an endogenous ROS production via the ETC. This additional endogenous ROS, on top of that from PDT photochemical reactions, contributes to an increased cell apoptosis. Thus, mitochondria are not only targets but also can be a source of ROS during low dose PDT. These results may provide a novel approach to improve PDT applications by maximising the efficiency of currently available photosensitisers.