The cellular origin of Ewing tumor (ET), a tumor of bone or soft tissues characterized by specific fusions between EWS and ETS genes, is highly debated. Through gene expression analysis comparing ETs with a variety of normal tissues, we show that the profiles of different EWS-FLI1-silenced Ewing cell lines converge toward that of mesenchymal stem cells (MSC). Moreover, upon EWS-FLI1 silencing, two different Ewing cell lines can differentiate along the adipogenic lineage when incubated in appropriate differentiation cocktails. In addition, Ewing cells can also differentiate along the osteogenic lineage upon long-term inhibition of EWS-FLI1. These in silico and experimental data strongly suggest that the inhibition of EWS-FLI1 may allow Ewing cells to recover the phenotype of their MSC progenitor.

Direct reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) provides a unique opportunity to derive patient-specific stem cells with potential applications in tissue replacement therapies and without the ethical concerns of human embryonic stem cells (hESCs). However, cellular senescence, which contributes to aging and restricted longevity, has been described as a barrier to the derivation of iPSCs. Here we demonstrate, using an optimized protocol, that cellular senescence is not a limit to reprogramming and that age-related cellular physiology is reversible. Thus, we show that our iPSCs generated from senescent and centenarian cells have reset telomere size, gene expression profiles, oxidative stress, and mitochondrial metabolism, and are indistinguishable from hESCs. Finally, we show that senescent and centenarianderived pluripotent stem cells are able to redifferentiate into fully rejuvenated cells. These results provide new insights into iPSC technology and pave the way for regenerative medicine for aged patients.

Ewing tumors are characterized by abnormal transcription factors resulting from the oncogenic fusion of EWS with members of the ETS family, most commonly FLI-1. RNA interference targeted to the junction between EWS and FLI-1 sequences was used to inactivate the EWS/FLI-1 fusion gene in Ewing cells and to explore the resulting phenotype and alteration of the gene expression profile. Loss of expression of EWS/FLI-1 resulted in the complete arrest of growth and was associated with a dramatic increase in the number of apoptotic cells. Gene profiling of Ewing cells in which the EWS/FLI-1 fusion gene had been inactivated identified downstream targets which could be grouped in two major functional clusters related to extracellular matrix structure or remodeling and regulation of signal transduction pathways. Among these targets, the insulin-like growth factor binding protein 3 gene (IGFBP-3), a major regulator of insulin-like growth factor 1 (IGF-1) proliferation and survival signaling, was strongly induced upon treating Ewing cells with EWS/FLI-1-specific small interfering RNAs. We show that EWS/FLI-1 can bind the IGFBP-3 promoter in vitro and in vivo and can repress its activity. Moreover, IGFBP-3 silencing can partially rescue the apoptotic phenotype caused by EWS/FLI-1 inactivation. Finally, IGFBP-3-induced Ewing cell apoptosis relies on both IGF-1-dependent and -independent pathways. These findings therefore identify the repression of IGFBP-3 as a key event in the development of Ewing's sarcoma.Ewing tumors, the second most frequent bone tumors in adolescents and young adults, are characterized by the presence of specific gene fusions which most frequently involve the EWS gene on chromosome 22 and the FLI-1 gene on chromosome 11 (8). Less frequently, EWS is fused with other members of the ETS family, including ERG, ETV1, E1A-F, or FEV (1). As a result of these gene fusions, tumor cells express a chimeric protein that contains the amino-terminal part of EWS and the DNA binding domain of the ETS transcription factor. EWS belongs to the TET family of proteins, which also includes TAF II 68 and TLS/FUS (4). All three proteins from this family have been shown to be involved in cancer-specific translocations, following a general scheme, which fuses the N-terminal domain of the TET partner to a variety of DNA binding domains, each fusion being highly specific for a tumor type. Numerous reports have addressed the mechanisms of tumoral transformation induced by TET fusion proteins and particularly EWS/FLI-1, the most frequent representative.Considerable attention has focused on the search for specific downstream target genes that may mediate EWS/FLI-1 transforming properties. Although the specificity of the EWS/FLI-1 fusion gene for Ewing tumors suggests that the cell context is critical for EWS/FLI-1-induced oncogenesis, the parental Ewing cell of origin is currently unknown. This issue precludes the development of homologous cell systems that may be particularly helpful to design strategies aimed at the identification of...