Requirement of protein kinase (Krs/MST) activation for MT-21-induced apoptosis

Fas is a well characterized apoptosis-inducing factor. One of our synthetic compounds, MT-21, induced apoptosis in human leukemia HL-60 cells similar to Fas. MT-21 activated caspase-3, an important cysteine aspartic protease for apoptosis induction. MT-21 also activated c-Jun-NH2-terminal kinase (JNK), a member of mitogen activated protein kinase (MAPK) superfamily that is involved in the regulation of cell growth, differentiation and cell death. Moreover, MT-21 treatment resulted in the activation of a 36 kDa kinase which uses myelin basic protein (MBP) as a substrate. However, MAPK and p38 were not activated by treatment with MT-21. The 36 kDa MBP kinase was shown to be a proteolytic product derived from the Krs protein with a molecular weight of 60 kDa. The Krs protein is a Ser/Thr protein kinase whose activity is enhanced by digestion of its C-terminal regulatory domain by caspase-3. When a kinase-inactive mutant form of Krs protein was overexpressed in HL-60 cells, JNK activation and apoptosis induction by MT-21 were suppressed. Furthermore, overexpression of dominant negative c-Jun also suppressed apoptosis induction by MT-21. These findings indicate that MT-21 induces apoptosis by the activation of JNK via the Krs protein, which is activated by caspase cleavage.