Mechanisms involved in Korean mistletoe lectin-induced apoptosis of cancer cells

AIM： To investigate the anti-cancer mechanisms of Korean mistletoe lectin （Viscum album coloratura agglutinin, VCA） using a human colon cancer cell line （COLO）. METHODS： Cytotoxic effects of VCA on COLO cells were determined by 3-（4,5-dimethylthiazol-2-yl）-2,5- diphenyltetrazolium bromide （MTI-） assay in vitro and tumor-killing effects in vivo. To study the mechanisms involved, the expression of various pro-caspases, anti- apoptotic proteins, and death receptors was determined by western blot. To determine which death receptor is involved in VCA-induced apoptosis of COLO cells, cytotoxicity was examined by MTT assay after treatment with agonists or antagonists of death receptors. RESULTS： VCA killed COLO cells in a time- and dosedependent manner and induced complete regression of tumors in nude mice transplanted with COLO cells. Treatment of COLO cells with VCA activated caspase-2, -3, -8, and -9 and decreased expression of anti-apoptotic molecules including receptor interacting protein, nuclear factor-κB, X-linked inhibitor of apoptosis protein, and Akt/protein kinase B. We then examined the involvement of death receptors in VCA-induced apoptosis. Only tumor necrosis factor receptor 1, among the death receptors examined, was involved in apoptosis of COLO cells, evidenced by inhibition of VCA-induced apoptosis and decreased activation of caspases, particularly caspase-8, by tumor necrosis factor receptor 1 antagonizing antibody.CONCLUSION： VCA-induced apoptotic COLO cell death is due to the activation of caspases and inhibition of antiapoptotic proteins, in part through the tumor necrosis factor receptor 1 signaling pathway.