Abrin P2 suppresses proliferation and induces apoptosis of colon cancer cells via mitochondrial membrane depolarization and caspase activation

To explore the cytotoxic mechanism of abrin P2 on human colon cancer HCT-8 cells, abrin P2 was isolated from the seed of Abrus precatorius L. It was found that abrin P2 exhibited cytotoxicity toward 12 different human cancer cell lines. Our results demonstrated that abrin P2 suppressed the proliferation of human colon cancer cells (HCT-8 cells) and induced cell cycle arrest at the S and G2/M phases. The mechanism by which abrin P2 inhibited cell proliferation was via the down-regulation of cyclin B1, proliferating cell nuclear antigen and Ki67, as well as the up-regulation of P21. In addition, abrin P2 induced a dose-and time-dependent increase in the rate of HCT-8 cell apoptosis. Treatment with both Z-VAD-FMK, a broad-spectrum caspase inhibitor, and abrin P2 demonstrated that abrin P2 induced HCT-8 cell apoptosis via the activation of caspases. Together, our results revealed that abrin P2-induced apoptosis in HCT-8 cells was associated with the activation of caspases-3/-8/-9, the reduction in the Bcl-2/Bax ratio, the loss of mitochondrial membrane potential, and the increase in cytochrome c release. We further showed that abrin P2 administration effectively suppressed the growth of colon cancer xenografts in nude mice. This is the first report that abrin P2 effectively inhibits colon cancer cell growth in vivo and in vitro by suppressing proliferation and inducing apoptosis.