Apoptosis induction associated with cell cycle dysregulation by rice bran agglutinin

Effects of rice bran agglutinin (RBA) on human monoblastic leukemia U937 cells were examined in comparison with those of wheat germ agglutinin (WGA) and Viscum album agglutinin (VAA). These lectins inhibit cell growth, and several lines of evidence indicate that the growth inhibition is caused by the induction of apoptosis. We observed that RBA induces chromatin condensation, externalization of membrane phosphatidylserine, and DNA ladder formation, features of apoptosis. DNA ladder formation was inhibited by a general inhibitor against caspases, which are known to play essential roles in apoptosis. Flow cytometric analysis revealed that RBA and WGA cause G2/M phase cell cycle arrest with increased expression of Waf1/p21, while cell cycle arrest was not observed for VAA. These data indicate that RBA induces apoptosis associated with cell cycle arrest in U937 cells, and suggest that the induction mechanism for RBA is similar to that for WGA, but different from that for VAA.