Background: Protoporphyrin IX (PpIX) and its derivatives are widely used in photodynamic therapy (PDT) to kill cancer cells. Studies showed that the application of these drugs could cause systemic toxic effects in human. However, the molecular pathways involved in PpIX-induced cytotoxicity are not well-defined. Macrophages represent the primary system for protecting tissues from toxicants and initiating the resolution of inflammation. Thus, this study aims to investigate the toxicity of PpIX on macrophages and provide strategies to prevent the toxic effects.
Methods: THP-1 macrophages were incubated with PpIX and cell death was measured by MTT assay and Annexin V-PI staining. Intracellular reactive oxygen species (ROS) were evaluated by 2', 7'-Dichlorodihydrofluorescin diacetate (DCFH-DA) and MitoSOX® Red staining and mitochondrial membrane potential (ΔΨm) was detected by tetramethylrhodamine methyl ester (TMRM) staining. Mitogen-activated protein (MAP) kinase activation was assayed by western blotting. Mitochondrial permeability transition pore (mPTP) opening was measured by calcein loading/Co(2+) quenching technique and evaluating the release of mitochondrial content.
Results: PpIX reduced cell viability in a dose- and time-dependent manner. The cell death was characterized by increasing PI-positive cells, ATP depletion, LDH releasing and rapid ΔΨm loss favoring necrotic features. In addition, PpIX successively induced ROS production, c-Jun N-terminal protein kinase (JNK) activation and mPTP opening. ROS scavengers, N-acetylcysteine (NAC) and deferoxamine (DFX), JNK inhibitor, SP600125, and mPTP inhibitor, cyclosporin A (CsA), all significantly rescued this cell death. Furthermore, mPTP opening was directly regulated by ROS/JNK pathway.
Conclusion: PpIX induces a necrotic cell death in THP-1 macrophages through ROS production, JNK activation, and mPTP opening. It is tempting to speculate that blocking the pathways involved in the cytotoxic effects of PpIX will alleviate its side effects.