Editors' ChoiceCancer

ZEB1 Teams Up with the DNA Damage Response

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Science Signaling  09 Sep 2014:
Vol. 7, Issue 342, pp. ec245
DOI: 10.1126/scisignal.2005879

Radiation therapy (radiotherapy) kills cancer cells by inducing DNA damage. As epithelial cancers progress, cells undergo epithelial-to-mesenchymal transition (EMT), which endows cells with prometastatic traits, such as decreased cell adhesion and increased motility, and stem cell–like traits, such as decreased properties of fully differentiated cells. These stemlike cells (often referred to as cancer stem cells) exhibit increased DNA repair capacity and resistance to radiotherapy (radioresistance). Zhang et al. investigated whether EMT itself or an EMT mediator had a role in radioresistance. Overexpression and knockdown assays identified the EMT-associated transcription factor ZEB1, but not others (SNAIL or TWIST), as a mediator of resistance to ionizing radiation (IR) in various cultured cells, regardless of whether EMT occurred. Knocking down ZEB1 in radioresistant breast cancer cells (called SUM159-P2) delayed the repair of IR-induced DNA damage in culture and sensitized mouse xenografts to IR. The kinase ATM is a critical upstream component of the DNA damage response (DDR) after IR. ATM, but not ATR (another DDR kinase), coimmunoprecipitated and phosphorylated ZEB1, enhancing the stability of ZEB1. Expression of a phosphomimetic ZEB1 mutant decreased the sensitivity of parental SUM159 cells to IR. The cell cycle checkpoint kinases CHK1 and CHK2 are critical effectors of the DDR. Knocking down ZEB1 decreased the abundance of CHK1, but not CHK2, at the protein (but not mRNA) level. Knocking down CHK1 sensitized SUM159-P2 cells to IR and prevented ZEB1 overexpression–induced radioresistance in the parental cell line. CHK1 undergoes ubiquitin-dependent degradation. ZEB1 knockdown increased the polyubiquitylation of CHK1. Affinity purification mass spectrometry and coimmunoprecipitation techniques identified the deubiquitylating enzyme USP7 as a direct ZEB1-interacting protein and showed an interaction between ZEB1, USP7, and CHK1. Knocking down USP7 in SUM159-P2 cells decreased CHK1 abundance and sensitized cells to IR. The stability of other USP7 substrates was not affected by knocking down ZEB1, and, of those tested, only its interaction with CHK1 was enhanced or decreased upon ZEB1 overexpression or knockdown, respectively. The abundances of ZEB1 and CHK1 correlated with each other in a tissue microarray of breast carcinomas and with metastatic relapse after radiation therapy in a cohort of patients. Thus, the findings show that—independent of its role in promoting EMT—ZEB1 enhances cell survival in response to IR by mediating a branch of the DDR through CHK1, revealing new targets for overcoming resistance to radiation therapy.

P. Zhang, Y. Wei, L. Wang, B. G. Debeb, Y. Yuan, J. Zhang, J. Yuan, M. Wang, D. Chen, Y. Sun, W. A. Woodward, Y. Liu, D. C. Dean, H. Liang, Y. Hu, K. K. Ang, M.-C. Hung, J. Chen, L. Ma, ATM-mediated stabilization of ZEB1 promotes DNA damage response and radioresistance through CHK1. Nat. Cell Biol. 16, 864–875 (2014). [PubMed]