Abstract 3968: Polycomb-mediated suppression of the voltage-gated potassium channel KCNA5 promotes Ewing sarcoma cell survival

Abstract

Abstract In Ewing sarcoma (ES) relapse is prevalent indicating that some cancer cells are resistant to treatment; however, the mechanism(s) governing treatment resistance and relapse are unknown. Voltage-gated potassium (Kv) channels play an essential role in maintaining a balance between cell survival and apoptosis. In particular, the Kv1.5 channel is a key sensor of oxygen tension and regulator of apoptosis, controlling potassium efflux and caspase activation. It was recently discovered that brain tissue can be tolerized to conditions of hypoxia and nutrient deprivation and that this ischemic tolerance is mediated by upregulation of the polycomb (PcG) protein BMI-1 and epigenetic repression of the Kv1.5-encoding locus KCNA5. Given that Kv1.5 expression is often downregulated in human cancers and that ES cells upregulate the PcG proteins BMI-1 and EZH2, we hypothesized that polycomb-dependent suppression of KCNA5 contributes to ES cell survival under conditions of stress. To test this hypothesis, we exposed ES and non-transformed cells to conditions of hypoxia (1% O2) and growth factor deprivation and measured cell viability in the presence and absence of potassium channel blockers 4′-aminopyridine (4′AP) and diphenyl phosphine oxide-1 (DPO-1). These studies were repeated in ES cells following BMI-1 knockdown, treatment with the EZH2 inhibitor GSK-126 and in ES cells that were genetically altered to ectopically express active wild-type (WT) or inactive (Pore-Dead (PD)) Kv1.5 channel. Chromatin immunoprecipitation (ChIP) was used to determine PcG protein binding at the KCNA5 locus. Exposure of non-transformed cells to hypoxia and growth factor deprivation resulted in significant cell death. In contrast, ES cell viability was unaffected. Significantly, however, knockdown of BMI-1 and inhibition of EZH2 resulted in diminished ES cell viability in conditions of stress and this cell death was blocked by 4′AP and DPO-1. Likewise, pharmacologic inhibition of Kv1.5 also inhibited stress-induced death of non-transformed cells. ES cells transduced to ectopically express WT- but not PD-Kv1.5 were sensitized to stress-induced cell death and this reduction in cell viability was again reversed by 4′AP and DPO-1. Caspase activation assays confirmed that cell death was mediated by apoptosis. Finally, ChIP studies revealed the presence of BMI-1 and the EZH2-dependent histone modification, H3K27me3, at the KCNA5 promoter in ES cells. Together these data show that ES cells are resistant to hypoxia and growth factor induced cell death and that this resistance is mechanistically linked to downregulation of Kv1.5 channel activity. Further, we have determined that suppression of the Kv1.5 channel is mediated epigenetically, by polycomb-dependent repression of the KCNA5 locus. Together these studies reveal a novel mechanism of ES cell survival under conditions of physiologic stress. Citation Format: Katherine Ryland, Laurie Svoboda, Jeffrey R. Martens, Elizabeth R. Lawlor. Polycomb-mediated suppression of the voltage-gated potassium channel KCNA5 promotes Ewing sarcoma cell survival. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3968. doi:10.1158/1538-7445.AM2014-3968