Growth suppression of human mast cells expressing constitutively active c‐kit receptors by JNK inhibitor SP600125

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 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 showed constitutive activation of JNK/c-Jun, and the inhibitory effect of SP600125 on cell proliferation was associated with cell cycle arrest at the G1 phase and apoptosis accompanied by the 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. Following Sp600125 treatment, down-regulation of cyclin D3 protein expression, but not p53 was also observed. Thus, 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 mutations in the catalytic domain of c-kit.