Apoptosis-inducing activity of plant lectins and specific application to tumor cells

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Lectins are proteins of non-immune origin with specific binding affinity for carbohydrate moiety of glycoconjugates. We have developed a new method to isolate lectins using affinity gels conjugated with porcine plasma proteins. In this study, we examined biological activities of lectins isolated by our method. Aralia cordate lectin (ACL) from udo showed activity to enhance gene and protein expression of proinflammatory cytokine IL-8 in Caco-2 cells. Our results also showed apoptosis inducing activity of Japanese mistletoe lectin, iris lectin, and muscari lectin in human leukemia U937 cells. In addition, we found that Boletopsis leucomelas lectin (BLL) from the edible mushroom kurokawa induced apoptosis in U937 cells by the mechanism incuding the G2/M cell cycle arrest in assocation with up-regulation of p21/Waf1 gene expression. The observed decrease in expression of bcl-2 was ascribable to the up-regulated expression of p21/Waf1 and correlated to the enhanced release of cytochrome c from mitochondria to cytoplasm, leading to the activation of caspase-9 in BLL-treated cells. These data suggest usefulness of our new method for lectin isolation and importance of examining biological activities of plant lectins in various animal systems.