Hypericin induced death receptor-mediated apoptosis in photoactivated tumor cells

Nasopharyngeal carcinoma (NPC) is a malignant disease of the head/neck region with a 5-year survival level of approximately 65%. To explore the novel therapeutic strategies in the management of this disease, the potential effects of photodynamic therapy (PDT) in NPC cells were investigated. PDT, a new mode of treatment, is based on the combined use of light-absorbing compounds and light irradiation. Two human NPC cells such as, poorly differentiated (NPC/CNE2) and moderately differentiated (NPC/TW0-1) and other types of tumor cells like colon (CCL-220.1) and bladder (SD) undergo rapid apoptosis when treated with PDT sensitized with hypericin (HY). It has been shown that this compound has a strong photodynamic effect on tumors and viruses. However, the initiating events of PDT sensitized HY-induced apoptosis are not identified completely. In this study, we sought to determine whether Fas/FasL upregulation and involvement of mitochondrial events are an early event in HY-treated PDT induced apoptosis. Loss of mitochondrial transmembrane potential, release of cytochrome c, involvement of caspases 8 and 3 and the status caspase-3 specific substrate PARP, were evaluated in PDT treated tumor cells. Photosensitization of HY enhanced both CD95/CD95L expression and induced CD95-signaling dependent cell death in all tumor cell lines studied. CD95/CD95L expression appeared within 2 h following light irradiation and appeared to be a principal event in PDT induced apoptosis. Furthermore, these results indicate that release of mitochondrial cytochrome c into the cytoplasm within 2-3 h post PDT is a secondary event following the activation of initiator caspase-8 preceding Apaf-1, caspase-9 and caspase-3 activation, cleavage of PARP and DNA fragmentation.