## 4 Discussion  
Survivin belongs to the family of inhibitors of apoptosis and contributes to cell survival by blocking the activation of executive caspases [8]. However, survivin signalization is rather complex, as it simultaneously ensures proper cell division [7], and the variability of these roles is reflected by versatile survivin activity. We showed, in this study, that survivin is upregulated in the nucleoplasm of AML cells treated with Ara-C (Figure 2), although Ara-C considerably lowers cell proliferation and slightly decreases cell viability (Figure 1). The mechanism of Ara-C-induced survivin upregulation remains unknown. However, it was reported in hematopoietic cells that low dose of Ara-C induced S-phase arrest followed by an activation of cell cycling after 72 h [50]. Simultaneously, survivin downregulation sensitized AML cells to cytarabine treatment [33]. We, thus, speculate that increased survivin expression contributes to overcome a transient lack of cytidine trisphosphate during a single dose of Ara-C treatment. A combination of Ara-C with an anthracycline is used to treat most of AML cases. We, thus, measured an effect of IDA on survivin level in a panel of AML cell lines and we detected different survivin regulation in individual cell lines (Figures 3 and 4, discussed below in detail). We also investigated an impact of specific survivin inhibitor YM-155 on the same set of AML lines, and we found attenuated survivin levels in the lines which did not downregulate survivin after IDA treatment (Figures 5-7). Simultaneously, we observed altered expression of several other apoptosis-related proteins, particularly XIAP, Mcl1, p53, and p21. No correlation was found between the protein content of cultivation medium (% of FBS), recommended for individual cell lines by the providers, and an effect of cytostatics. Survivin expression was found to be activated by the AML1/ETO fusion gene [35], which is present in the Kasumi-1 cell line. Simultaneously, enhanced survivin and XIAP levels were suggested to account for the IDA resistance of the Kasumi-1 cell line [39]. On the other hand, survivin downregulation by YM-155 was observed in this cell line [22; 53]. Our results confirmed the IDA-induced survivin overexpression and drug resistance, as well as survivin level attenuation, after YM-155 treatment in the Kasumi-1 cell line. A moderate decrease in XIAP expression was observed in both cases. On the other hand, we detected no change in Mcl1 expression and a stable high expression of p53, which is mutated (R248Q) in Kasumi-1 cells. Pterocarbapquinone LQB-118, an inhibitor of IAP proteins, induced deregulation of survivin and XIAP in Kasumi-1, as well as in primary cells [39]. However, whereas XIAP attenuation was observed together with apoptosis, inhibition of survivin expression alone did not correlate with apoptosis induction [39]. In summary, complex IAP inhibition, but not that of the survivin alone, seems to be a promising alternative treatment for these IDA-resistant cells. Survivin expression is regulated by the activity of STAT3/5 and \(\beta\)-catenin, which are both targets of Flt3. Internal tandem duplication in Flt3 (Flt3-ITD), one of the AML-characteristic mutations, constitutively activates Flt3 kinase, resulting in aberrantly high phosphorylation of STAT3/5 and survivin overexpression [24]. Targeting Flt3 activity was shown to decrease survivin level [24; 54]. This suggests that survivin inhibition may bring some benefits to AML patients with Flt3-ITD. MV4-11, a cell line derived from an AML patient with Flt3-ITD, exhibited relatively high survivin expression. Despite a substantial effect on cell viability, YM-155 did not affect survivin expression in this line (Figure 5), indicating superiority of the Flt3-ITD-induced survivin overexpression over the YM-155-mediated inhibition. However, YM-155 considerably lowered XIAP and Mcl1 expressions, likely owing to a known off-target effect [39; 46]. Interestingly, survivin, together with XIAP and Mcl1, was attenuated in IDA-treated MV4-11.