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.