It has been reported that novel photosensitizers Hypocrellin A and B, lipid soluble perylquinone derivatives of the genus Hypericum have a strong photodynamic effect on tumors and viruses. The molecular mechanisms of tumor cell death induction by Hypocrellin A and B are poorly understood. In this study, we have examined the photodynamic effects of Hypocrellin A and B compounds in poorly differentiated (CNE2) and moderately differentiated (TW0-1) human nasopharyngeal carcinoma (NPC) cells. Using these cell lines we investigated the role of the apoptotic pathway in photosensitized Hypocrellin A and B-mediated cell death. Tumor cells photoactivated with Hypocrellin A and B 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 both tumor cell lines as evidenced by the externalization of phosphatidylserine (PS). A dose-dependent increase in caspases-3 protease activity inhibitable by the tetrapeptide inhibitor DEVD-CHO was also observed in both cell lines. Western blot analysis of poly (ADP-ribose) polymerase, a caspase substrate, showed the classical cleavage pattern (116 to 85 kDa) associated with apoptosis in Hypocrellin A and B-treated cell lysates. In addition, caspase inhibition blocked the externalization of membrane PS, indicating that the loss of membrane phospholipid asymmetry is a downstream event of caspases activation. These results demonstrate that tumor cell death induced by Hypocrellin A and B is mediated by caspase proteases. In conclusion, this study identifies both Hypocrellins (A and B) as potent and promising photosensitizers for the treatment of NPC.

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