Photodynamic therapy (
PDT) with endogenous
protoporphyrin IX derived from 5-aminolevulinic
acid or its derivatives has been established for treatments of several premalignancies and
malignancies; however, the mechanism of the modality is not fully elucidated. The
mitochondrial permeability transition pore consists mainly of the mitochondrial outer membrane
voltage-dependent anion channel and the peripheral
benzodiazepine receptor (PBR) and the mitochondrial inner membrane
adenine nucleotide translocator (ANT). These
mitochondrial proteins are responsible for the permeability transition that leads to apoptosis. In the present study, the human
leukemia cell line, Reh, was treated with
PDT using
hexaminolevulinate (HAL). More than 80% of apoptotic Reh cells were found after HAL-mediated
PDT (HAL-
PDT) with high-molecular-weight (50 kbp) DNA fragmentation. Addition of
PK11195 or
Ro5-4864, two
ligands of PBR, during HAL-
PDT significantly inhibited the apoptotic effect.
Bongkrekic acid, a
ligand for ANT, also reduced the
PDT effect. Although the mitochondrial transmembrane potential collapsed, neither cytosolic translocation of mitochondrial
cytochrome c nor activation of
caspase-9,
caspase-8,
caspase-3, and
poly(ADP-ribose) polymerase were found. However, nuclear translocation of
mitochondrial apoptosis-inducing factor (AIF) was shown by both immunoblotting and immunocytochemistry. Because AIF is the sole one among all proapoptotic factors involved in
caspase-dependent and
caspase-independent pathways that induces the high-molecular-weight DNA fragmentation, we conclude that HAL-
PDT specifically targets PBR, leading to apoptosis of the Reh cells through nuclear translocation of mitochondrial AIF. This study suggests PBR as a possible novel therapeutic target for HAL-based
PDT of
cancer.