Simultaneously targeting mitochondria and endoplasmic reticulum by photodynamic therapy induces apoptosis in human lymphoma cells

Photodynamic therapy (PDT) and photodetection with protoporphyrin IX (PpIX) precursors have widely been used in the diseases with abnormally proliferative cells, but the mechanism of the modality is not fully understood yet. In this study 70-95% of apoptotic cells after PDT with PpIX precursor, hexaminolevulinate (HAL) in two human lymphoma cell lines, Namalwa and Bjab, were confirmed by fluorescence microscopy, electron microscopy and flow cytometry. HAL-derived PpIX was mainly distributed in the mitochondria and endoplasmic reticulum (ER), both of which were initial targets after light exposure causing two major pathways simultaneously involved in the apoptotic induction. One was the mitochondrial pathway including the release of cytochrome c, cleavage of caspases-9/-3, poly(ADP-ribose) polymerase and DNA fragmentation factor. The other was the ER stress-mediated pathway triggering a transient increase in the cytosolic Ca(2+) level after photodamage to the ER calcium pump protein SERCA2. The released Ca(2+) further initiated the caspase-8 cleavage. The use of both extracellular Ca(2+) chelator EGTA and intracellular Ca(2+) chelator BAPTA-AM confirmed that such cytosolic Ca(2+) originated from the ER rather than extracellular Ca(2+)-containing medium. About 30% of the apoptosis was blocked with BAPTA-AM alone; while a complete inhibition of such apoptosis was achieved with a combination of the caspase-9 inhibitor Z-LEHD-FMK and caspase-8 inhibitor Z-IETD-FMK, thus quantifying each role of the mitochondrial and ER pathways.