Induction of apoptosis by Hypericin through activation of caspase-3 in human carcinoma cells

Potent photosensitizer Hypericin (HY), is a lipid soluble perylquinone derivative of the genus Hypericum and has a strong photodynamic effect on tumors and viruses. However, the mechanisms of tumor cell death induced by this compound is still unclear. Furthermore, there are no reports on mechanisms in cell apoptosis induced by perylquinones in human nasopharyngeal carcinoma (NPC) and other mucosal cells. We studied the photodynamic effects of HY compound in poorly differentiated (CNE2) and moderately differentiated (TW0-1) human NPC cells as well as human mucosal colon (CCL-220.1) and bladder (SD) cells. Using these cell lines we investigated few hall marks of apoptotic commitments in a drug and light dose dependent manner. Tumor cells photoactivated with 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 in all tumor cell lines as evidenced by the externalization of phosphatidylserine. Under apoptotic conditions, Western blot analysis of poly (ADP-ribose) polymerase, a caspase substrate, showed the classical cleavage pattern (116-85 kDa) associated with apoptosis in PDT-treated cell lysates. In addition, 85 kDa cleaved product was blocked by using tetrapeptide caspase inhibitors such as DEVD-CHO or z-VAD-fmk. These results demonstrate that tumor cell death induced by photoactivated HY is mediated by caspase proteases. This study also identifies that CNE2, CCL-220.1 (colon) and SD (bladder) cell lines are more sensitive than TW0-1 cell line to PDT using perylquinone HY.