Activation of c-jun N-terminal kinase (JNK) through c-kit-mediated phosphatidylinositol 3 (PI3) and Src kinase pathways plays an important role in cell proliferation and survival in mast cells. Gain-of-function mutations in c-kit are found in several human neoplasms. Constitutive activation of c-kit has been observed in human mastocytosis, acute myeloid leukemia, lymphoma, germ tumor and gastrointestinal stromal tumor. In the present study, we demonstrate that an anthrapyrazole SP600125, a reversible ATP-competitive inhibitor of JNK inhibits proliferation of human HMC-1 mast cells expressing constitutively activated c-kit mutant. We found that JNK/c-Jun was constitutively activated in HMC-1 cells without stimulation. When spontaneous activation of JNK/c-Jun was inhibited by treatment with SP600125, cell proliferation was suppressed. The concentration which effectively inhibited JNK/c-Jun activity in our experiment had no effect on SCF-induced phosphorylation of Akt or Erk, suggesting that SP600125 specifically inhibited JNK/c-Jun activity in HMC-1 cells. Moreover, we demonstrated that SP600125 induced HMC-1 cell apoptosis in dose- and time-dependent manner. Caspase-3 and PARP were cleaved as early as 12 h after treatment with SP600125, but caspase-9 was not. Also, cell cycle arrest in G1 phase was observed in SP600125 treated cells. Thus, the inhibitory effect of SP600125 on cell proliferation was associated with cell cycle arrest at the G1 phase and apoptosis accompanied by cleavage of caspase-3 and PARP. Caspase-3 inhibitor Z-DEVD-FMK almost completely inhibited SP600125-induced apoptosis of HMC-1 cells. In contrast, caspase-9 inhibitor Z-LEHD-FMK failed to block SP600125-induced apoptosis, suggesting that apoptosis induced by SP600125 was caspase-3 dependent. Following SP600125 treatment, down-regulation of cyclin D3 protein expression, but not p53 was also observed. Take together, JNK/c-Jun is essential for proliferation and survival of HMC-1 cells. The results obtained from the present study suggest the possibility that JNK/c-Jun may be a therapeutic target in diseases associated with c-kit mutant.

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