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EWS/FLI1 suppresses retinoblastoma protein function and senescence in Ewing's sarcoma cells
Hsien-Ming Hu1, Anna Zielinska-Kwiatkowska, Karen Munro
1Department of Orthopedics and Sports Medicine, School of Medicine, University of Washington, 1660 S. Columbian Way, ORT112, Seattle, Washington 98108, USA.
Abstract:
Ewing's Family Tumors (EFTs) most commonly harbor a specific t(11;22) translocation that generates the EWS/FLI1 fusion protein responsible for malignant transformation. Many potential downstream targets of EWS/FLI1 have been identified but a detailed mechanism by which the fusion protein brings about transformation remains unknown. In this report, we show that depletion of EWS/FLI1 in Ewing's cell lines results in a senescence phenotype, a marked increase in expression of the G1/S regulatory proteins p27(kip1) and p57(kip2), and a significant decrease in cyclin D1 and CDK2. We also demonstrate for the first time, to our knowledge, that knockdown of EWS/FLI1 leads to hypophosphorylation and functional activation of the retinoblastoma (pRb) family of proteins. Consistent with activation of the pRb proteins, E2F-responsive genes such as cyclin A are repressed in EWS/FLI1-depleted cells. Together, these results support the role of EWS/LI1 as an inhibitor of cellular senescence and implicate the retinoblastoma family of proteins as key mediators of this inhibition.
Insights
Ewing's sarcoma tumors rely on the EWS/FLI1 fusion protein. Depleting this protein triggers cellular senescence by activating the retinoblastoma (pRb) pathway, revealing a key mechanism in tumor development.
Area of Science:
- Oncology
- Molecular Biology
- Cellular Biology
Background:
- Ewing's Family Tumors (EFTs) are driven by the EWS/FLI1 fusion protein.
- The precise mechanism of malignant transformation by EWS/FLI1 is not fully understood.
Purpose of the Study:
- To investigate the downstream effects of EWS/FLI1.
- To elucidate the role of EWS/FLI1 in cellular senescence and transformation.
Main Methods:
- Depletion of EWS/FLI1 in Ewing's cell lines.
- Analysis of cell cycle regulatory proteins (p27kip1, p57kip2, cyclin D1, CDK2).
- Assessment of retinoblastoma (pRb) family protein phosphorylation and E2F-responsive gene expression.
Main Results:
- EWS/FLI1 depletion induced a senescence phenotype.
- Increased expression of p27kip1 and p57kip2, decreased cyclin D1 and CDK2.
- EWS/FLI1 knockdown led to pRb family hypophosphorylation and activation, repressing E2F-responsive genes like cyclin A.
Conclusions:
- EWS/FLI1 acts as an inhibitor of cellular senescence.
- The retinoblastoma (pRb) protein family mediates EWS/FLI1's inhibition of senescence.
- Understanding this pathway offers potential therapeutic targets for EFTs.
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