Ewing sarcoma family of tumors (ESFT) is a group of aggressive pediatric 
malignancies driven by the EWS-FLI1 fusion protein, an aberrant transcription 
factor up-regulating specific target genes, such as neuropeptide Y (NPY) and its 
Y1 and Y5 receptors (Y5Rs). Previously, we have shown that both exogenous NPY 
and endogenous NPY stimulate ESFT cell death via its Y1 and Y5Rs. Here, we 
demonstrate that this effect is prevented by dipeptidyl peptidases (DPPs), which 
cleave NPY to its shorter form, NPY(3-36), not active at Y1Rs. We have shown 
that NPY-induced cell death can be abolished by overexpression of DPPs and 
enhanced by their down-regulation. Both NPY treatment and DPP blockade activated 
the same cell death pathway mediated by poly(ADP-ribose) polymerase (PARP-1) and 
apoptosis-inducing factor (AIF). Moreover, the decrease in cell survival induced 
by DPP inhibition was blocked by Y1 and Y5R antagonists, confirming its 
dependence on endogenous NPY. Interestingly, similar levels of NPY-driven cell 
death were achieved by blocking membrane DPPIV and cytosolic DPP8 and DPP9. 
Thus, this is the first evidence of these intracellular DPPs cleaving releasable 
peptides, such as NPY, in live cells. In contrast, another membrane DPP, 
fibroblast activation protein (FAP), did not affect NPY actions. In conclusion, 
DPPs act as survival factors for ESFT cells and protect them from cell death 
induced by endogenous NPY. This is the first demonstration that intracellular 
DPPs are involved in regulation of ESFT growth and may become potential 
therapeutic targets for these tumors.