Apoptosis is a systematic suicide of cells within an organism during normal morphogenesis, tissue remodeling, and in response to viral infections or other irreparable cell damage. 1, 2 In addition, failure in an apoptosis program often leads to an imbalance in cell number, and some of those abnormalities might lead to tumorigenesis. Based on this concept, control of apoptosis has emerged as an important strategy in cancer chemotherapy.

We recently isolated a novel anticancer drug, cytotrienin A (FIG. 1), that induces apoptosis in human promyelocytic leukemia HL-60 cells. 3, 4 Cytotrienin A induced marked morphologic changes in the cell, including membrane blebbing, cell shrinkage, chromatin condensation, internucleosomal degradation of chromosomal DNA, and fragmentation of the cell into apoptotic bodies. To gain insight into the mode of action of cytotrienin A-induced apoptosis, we performed an in-gel kinase assay using myelin basic protein (MBP) as a substrate and found activation of a kinase with an apparent molecular mass of 36 kD (termed p36 MBP kinase). The dose of cytotrienin A required to activate the p36 MBP kinase was consistent with that required to induce apoptotic DNA fragmentation in HL-60 cells. This p36 MBP kinase was activated with kinetics distinct from the activation of JNK (c-Jun N-terminal kinase)/SAPK (stress-activated protein kinase) and p38 MAPK. By contrast, p36 MBP kinase activation and apoptotic DNA fragmentation were inhibited by antioxidants such as N-acetylcysteine and reduced-form glutathione. The p36 MBP kinase activation was also observed during hydrogen peroxide (H2O2)-induced apoptosis. These results suggest that reactive oxygen species (ROS) including H2O2 play a key role in the activation of p36 MBP kinase and the subsequent apoptotic pathway induced by cytotrienin A.