Abstract
Potent photosensitizers hypocrellin A (HA), hypocrellin B (HB) and hypericin (HY) are lipid-soluble perylquinone derivatives of the genus
Hypericum and have a strong photodynamic effect on tumors and viruses. However, the mechanisms of tumor cell death induced by HA, HB and HY are still unclear. Moreover, no reports have mentioned cell apoptosis induced by HA, HB and HY in human nasopharyngeal carcinoma (NPC) and other mucosal cells. In this study, we attempt to clarify the photodynamic effects of HA, HB and HY compounds in poorly differentiated (CNE2) and moderately differentiated (TW0-1) human NPC cells as well as human mucosal colon and bladder cells. Using these cell lines we investigated few hallmarks of apoptotic commitments in a drug dose dependent manner. Tumor cells photo-activated with HA, HB and HY showed cell size shrinkage and an increase in the sub-diploid DNA content. A loss of membrane phospholipid asymmetry associated with apoptosis was induced by all tumor cell lines as evidenced by the externalization of phosphatidylserine. Under apoptotic conditions, Western blot analysis of poly(ADP-ribose) polymerase, a caspases substrate, showed the classical cleavage pattern (116 to 85 kDa) associated with apoptosis in HA, HB and HY-treated cell lysates. In addition, 85 kDa cleaved product was blocked by the tetrapepdide caspase inhibitors such as DEVD-CHO or z-VAD-fmk. Both inhibitors protect tumor cells from apoptosis. These results demonstrate that tumor cell death induced by HA, HB and HY is mediated by caspase proteases. This study also identifies HB as a more potent and promising photosensitizer for the treatment of mucosal cancer cells.