Ewing sarcomas (ES) are highly malignant mesenchymal tumors, which most often occur in children and adolescents. The current treatment of choice comprises wide resection in combination with multimodal chemotherapy including etoposide (Eto). Due to the serious side effects associated with common chemotherapeutics and prevalent multidrug resistance in recurrent and metastatic ES, there is a growing demand for alternative strategies and add‑on drugs. Previous research has demonstrated efficient cell death induction by Eto in combination with arsenic trioxide (ATO) in ES cell lines. The aim of the present study was to investigate the effect of different temporal sequences of ATO and Eto administration on apoptosis induction and to explore the effect of both drugs on inhibitory glycogen synthase kinase‑3β (GSK3‑β) phosphorylation as well as multidrug transporter gene expression. The intensity of caspase activation was mainly determined by the Eto doses in A673 and TC‑71 cells, whereas in RD‑ES cells ATO application actively suppressed Eto‑induced apoptosis. This coincided with an increase in inhibitory GSK‑3β phosphorylation in ATO‑treated RD‑ES cells. Inherent mRNA expression of multidrug resistance‑associated protein 1 (MRP1) was low in the ES cell lines compared to that observed in the mesenchymal stem cells (MSC), whereas multidrug resistance protein 1 (MDR1) gene expression was considerably increased in the ES cell lines. ATO treatment reduced MRP1 mRNA expression in the A673 and TC‑71 cells, while expression was induced in the MSC and RD‑ES cells. In contrast, MDR1 mRNA expression was specifically induced by ATO in the A673 and TC‑71 cells, reinforcing the expression differences between MSC and the ES cell lines. Although a reliable cell death induction by the combination of ATO and Eto has been previously shown in ES cell lines, the present study showed marked heterogeneity of the ES cell response to ATO and Eto treatment, illustrating the difficulty of prediction of individual treatment outcome in ES.
Keywords: Ewing sarcoma; etoposide; arsenic trioxide; apoptosis; multidrug resistance; glycogen synthase kinase-3; glioma associated oncogene family 1.