Therapeutic effects of a novel STAT3 inhibitor, JSI-124 (Cucurbitacin I) in gastrointestinal stromal tumor cells Free

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Gastrointestinal Stromal tumors (GIST) are the most common mesenchymal tumors of the intestinal tract. They are characterized by abundant uniform expression of KIT in more than 90% of tumors. The majority of tumors contain a mutation in KIT, causing constitutive activation of the protein and leading to continued cell growth and division. Recently imatinib mesylate (STI571, Gleevec), a selective inhibitor of the ABL, PDGFR, and KIT tyrosine kinases, has been shown to be effective in GIST clinical trials. However, this inhibition is temporary and does not cause tumor apoptosis and overtime, many patients become refractory to imatinib. Therefore, newer therapeutic approaches alone or in combination are needed. We have observed that a member of Signal Transducers and Activators of Transcription (STATs) family, STAT3, is constitutively activated in GIST cells. STATs are transcription factors activated in response to cytokines and growth factors and function in transducing signals from cell surface receptors to the cytoplasm and then translocating to the nucleus, where they act to regulate gene expression. To search for new drugs to aid in the possible cure of GIST, we have evaluated JSI-124 (Cucurbitacin I, NSC#521777), a newly identified inhibitor of STAT3. To establish the potential therapeutic benefits of JSI-124 in GIST, we evaluated the activity of the drug on GIST882 cells, which constitutively express activated STAT3. Treatment of these cells with JSI-124 resulted in a significant growth inhibition as compared to the untreated controls. Dramatic changes in cell morphological as well as induction of apoptosis in GIST882 cells was detected as early as 2 hr after treatment with clinically achievable concentrations of drug. We also demonstrated that JSI-124 rapidly inhibited the phosphorylation level of STAT3 but not that of ERK1/2 or AKT. In comparison, STAT phosphorylation was unaffected by treatment with imatinib mesylate, which also induces cell growth arrest, and leads to down-regulation of both ERK1/2 and AKT. Interestingly, after JSI-124 treatment we observed an increased gene expression of MAFbx (muscle atrophy F-box) and RTP801 (hypoxia-inducible factor 1-responsive gene). The former one is responsible for cell proteolysis and the latter is induced in ischemic injury and oxidative stress. These two genes are considered as possible candidate genes involved in the balance between cell survival and death. Our studies demonstrate that JSI-124, rapidly suppresses the levels of phosphorylated STAT3, without affecting the total level of this protein and that inhibition of cell growth, induction of apoptosis, and changes in gene expression in GIST cells was not dependent on either ERK1/2 or AKT signaling pathways. These studies suggest that JSI-124 may be an alternative or complementary drug for the treatment of primary and refractory GIST.