Abstract 3855: Inhibitors of apoptosis (IAPs) in gastrointestinal stromal tumors

Abstract

Abstract Introduction Gastrointestinal stromal tumors (GIST) are characterized by gain-of-function-mutations of c-KIT or PDGFRA. Despite long-lasting remissions with the KIT inhibitor imatinib (IM) most patients eventually progress. While inhibitors of apoptosis proteins (IAPs) may confer resistance in other cancers little is known about the role of IAPs in GIST. Material and Methods IAP expression (cIAPs, XIAP, survivin) in patient samples (TMA, n=163), IM-sensitive (GIST-T1, GIST882, GIST430) and IM-resistant (GIST48B, GIST48) GIST cell lines (RT-PCR, western blot) was analyzed. The impact of KIT and growth inhibitory drugs on IAP expression (24h of treatment) was determined by western blot. Biological consequences of IAP modulation (YM155: transcriptional repressor of survivin; Embelin: biochemical XIAP inhibitor; lentiviral shRNA knockdown [KD] of survivin, XIAP) were investigated alone and in combination with KIT-related pathway inhibitors (IM, LY294002, 17-AAG). Results cIAPs, XIAP and survivin were expressed at various levels in all KIT-positive and negative GIST cell lines. Resistant cell lines did not exhibit higher levels of IAPs. Survivin expression was found to vary over time in GIST-T1 and 48B (2- and 4-fold within 5 days. Survivin expression (immunoreactivity score α3) was found in 40% of GIST patients. PI3K inhibition led to survivin downregulation of 20 to 65% (24h) in all tested cell lines whereas KIT inhibition (IM, 24h) reduced survivin and XIAP expression only in GIST-T1 (52%). The protein levels of both XIAP and survivin were decreased following HDAC (SAHA: 44-71%, except GIST430) and MDM2 inhibition (survivin: 40 - 90%). YM155 downregulated survivin and exhibited proapoptotic and antiproliferative effects in all cell lines at low nM levels (IC50: GIST-T1: 4nM, GIST48P: 40nM, GIST48B, GIST 882: 80nM) except GIST430 (IC50>1µM). No antagonistic effects were seen with combinational treatment. Embelin treatment resulted in IC50 levels of 40µM - 300µM while lacking a specific XIAP inhibition. A stable XIAP KD was established in GIST-T1 resulting in slower growth (25% in 3d). Conclusion IAPs are commonly expressed in GIST cell lines regardless of resistance to IM. Expression levels are mostly independent from KIT inhibition but proapoptotic effects from HDAC and MDM2 inhibitors may in part be conferred by IAPs. The survivin repressor YM155 showed antiproliferative and proapoptotic effects at levels that are therapeutically achievable suggesting a potential therapeutic role in GIST. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3855. doi:1538-7445.AM2012-3855