Background MST1 is an upstream kinase of the JNK and p38 MAPK pathways whose expression induces apoptotic morphological changes such as nuclear condensation. During apoptosis, caspase cleavage of MST1 removes a C‐terminal regulatory domain, increasing the kinase activity of the MST1 N‐terminal domain. Downstream pathways of MST1 in the induction of apoptosis remain to be clarified.

Results In this study, we found that the expression of MST1 resulted in caspase‐3 activation. Therefore, MST1 is not only a target of caspases but also an activator of caspases. This caspase activation and apoptotic changes occur through JNK, since the co‐expression of a dominant‐negative mutant of JNK inhibited MST1‐induced morphological changes as well as caspase activation. In contrast, neither a dominant‐negative p38 nor the p38 inhibitor SB203580 inhibited them. MST1 induced nucleosomal DNA fragmentation, which was suppressed by caspase inhibitors or ICAD (Inhibitor of Caspase‐Activated DNase). Surprisingly, however, other changes such as membrane blebbing and chromatin condensation were not inhibited by caspase inhibitors.

Conclusion These results suggest that MST1 most likely promotes two events through JNK activation; first, MST1 induces the activation of caspases, resulting in CAD‐mediated DNA fragmentation, and second, MST1 induces chromatin condensation and membrane blebbing without utilizing downstream caspases.