Gastrointestinal stromal tumors (GIST) can be successfully treated with imatinib mesylate (Gleevec); however, complete remissions are rare and patients frequently achieve disease stabilization in the presence of residual tumor masses. The clinical observation that discontinuation of treatment can lead to tumor progression suggests that residual tumor cells are, in fact, quiescent and, therefore, able to re-enter the cell-division cycle. In line with this notion, we have previously shown that imatinib induces GIST cell quiescence in vitro through the APC CDH1 -SKP2-p27Kip1 signaling axis. Here, we provide evidence that imatinib induces GIST cell quiescence in vivo and that this process also involves the DREAM complex, a multisubunit complex that has recently been identified as an additional key regulator of quiescence. Importantly, inhibition of DREAM complex formation by depletion of the DREAM regulatory kinase DYRK1A or its target LIN52 was found to enhance imatinib-induced cell death. Our results show that imatinib induces apoptosis in a fraction of GIST cells while, at the same time, a subset of cells undergoes quiescence involving the DREAM complex. Inhibition of this process enhances imatinib-induced apoptosis, which opens the opportunity for future therapeutic interventions to target the DREAM complex for more efficient imatinib responses. Cancer Res; 73(16); 5120-9. Ó2013 AACR.
Most gastrointestinal stromal tumors (GIST) are caused by oncogenic KIT or platelet-derived growth factor receptor activation, and the small molecule kinase inhibitor imatinib mesylate is an effective first-line therapy for metastatic or unresectable GIST. However, complete remissions are rare and most patients ultimately develop resistance, mostly because of secondary mutations in the driver oncogenic kinase. Hence, there is a need for novel treatment options to delay failure of primary treatment and restore tumor control in patients who progress under therapy with targeted agents. Historic data suggest that GISTs do not respond to classical chemotherapy, but systematic unbiased screening has not been performed. In screening a compound library enriched for U.S. Food and Drug Administration (FDA)-approved chemotherapeutic agents (NCI Approved Oncology Drugs Set II), we discovered that GIST cells display high sensitivity to transcriptional inhibitors and topoisomerase II inhibitors. Mechanistically, these compounds exploited the cells' dependency on continuous KIT expression and/or intrinsic DNA damage response defects, explaining their activity in GIST. Mithramycin A, an indirect inhibitor of the SP1 transcription factor, and mitoxantrone, a topoisomerase II inhibitor, exerted significant antitumor effects in mouse xenograft models of human GIST. Moreover, these compounds were active in patient-derived imatinib-resistant primary GIST cells, achieving efficacy at clinically relevant concentrations. Taken together, our findings reveal that GIST cells have an unexpectedly high and specific sensitivity to certain types of FDAapproved chemotherapeutic agents, with immediate implications for encouraging their clinical exploration. Cancer Res; 74(4); 1200-13. Ó2014 AACR.
end. Proteins destined to be degraded are selectively targeted to the proteasome by the addition of a series of covalently attached ubiquitin molecules. Deubiquitinating enzymes (DUBs) associated with the 19S regulatory subunit remove these ubiquitin chains before proteins can enter the proteolytic subunit. The 20S core contains three major proteolytic activities (β5 chymotrypsin-like, β1 caspase-like, β2 trypsin-like). Inhibitors of the 20S proteasome core particle, such as the prototype proteasome inhibitor bortezomib (Velcade ®), have gained clinical importance for the treatment of multiple myeloma and certain lymphomas 5. Previous studies from our laboratory have shown that targeting the ubiquitin-proteasome machinery with bortezomib is highly effective in GIST cells 6. We could demonstrate that bortezomib-induced apoptosis is mediated by a dual mechanism of action: increased levels of soluble, non-chromatin-bound pro-apoptotic histone H2AX and a dramatic downregulation of KIT expression mediated by inhibition of active gene transcription 6-8. It is known that loss of KIT expression is a strong inducer of apoptosis in GIST cells 7,9. Although bortezomib has not shown significant clinical activity in many solid tumors, including an array of sarcomas 10 , there are recent reports of its clinical activity in GIST. For example, a study evaluating a novel subcutaneous administration regimen of bortezomib in various solid tumors, noted the most significant response in a patient with GIST 11. In another study testing bortezomib in combination with vorinostat, one of the two GIST patients achieved stable disease 12. Nevertheless, bortezomib is associated with marked adverse effects, most importantly irreversible neuropathy, as well as a standard intravenous route of administration warranting the evaluation of second-generation proteasome inhibitors in GIST 11,13. Carfilzomib (Kyprolis ® , PR-171), ixazomib (Ninlaro ® , MLN-9708), and delanzomib (CEP-18770) are inhib
The majority of gastrointestinal stromal tumors (GISTs) are driven by oncogenic KIT signaling and can therefore be effectively treated with the tyrosine kinase inhibitor (TKI) imatinib mesylate. However, most GISTs develop imatinib resistance through secondary KIT mutations. The type of resistance mutation determines sensitivity to approved second-/third-line TKIs but can show high inter-and intratumoral heterogeneity.Therefore, therapeutic strategies that target KIT independently of the mutational status are intriguing. Inhibiting the ubiquitin-proteasome machinery with bortezomib is effective in GIST cells through a dual mechanism of KIT transcriptional downregulation and upregulation of the pro-apoptotic histone H2AX but clinically problematic due to the drug's adverse effects. We therefore tested second-generation inhibitors of the 20S proteasome (delanzomib, carfilzomib and ixazomib) with better pharmacologic profiles as well as compounds targeting regulators of ubiquitination (b-AP15, MLN4924) for their effectiveness and mechanism of action in GIST. All three 20S proteasome inhibitors were highly effective in vitro and in vivo, including in imatinib-resistant models. In contrast, b-AP15 and MLN4924 were only effective at high concentrations or had mostly cytostatic effects, respectively. Our results confirm 20S proteasome inhibitors as promising strategy to overcome TKI resistance in GIST, while highlighting the complexity of the ubiquitinproteasome machinery as therapeutic target.
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