Ku70 and poly(ADP-ribose) polymerase-1 (PARP-1) competetively regulate β-catenin/TCF-4-mediated gene transactivation Free

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Regulation of transcriptional activity of the T-cell factor-4 (TCF-4) and β-catenin nuclear complex is considered crucial for embryonic development and colorectal carcinogenesis. We previously reported that poly(ADP-ribose) polymerase-1 (PARP-1) was a component and enhancer of the TCF-4 and β-catenin complex. (Idogawa, et al. Gastroenterology 2005 128:1919-36) However, its biological significance remains explained. Using immunoprecipitation and mass spectrometry we found that two Ku proteins, Ku70 and Ku80, were also associated with the TCF-4 and β-catenin complex. Ku70, but not Ku80, was an inhibitor of the transcriptional activity of TCF/LEF. Knock-down of Ku70 by RNA interference (RNAi) increased the amount of β-catenin associated with TCF-4 and enhanced the transcriptional activity of TCF/LEF. Ku competed with PARP-1 for binding to TCF-4. Treatment with a DNA-damaging alkylating agent bleomycin induced the polyADP-ribosylation of PARP-1 protein and inhibited its interaction with TCF-4. The bleomycin treatment increased the amount of Ku70 co-immunoprecipitated with TCF-4 and conversely decreased β-catenin co-immunoprecipitated with TCF-4. We propose a working model in which the transcriptional activity of TCF-4 is regulated by the relative amount of Ku70, PARP-1, and β-catenin proteins binding to TCF-4. PARP-1 is activated by DNA strand breaks and facilitates DNA repair. The Ku heterodimer recognizes DNA double strand breaks (DSB) and mediates nonhomologous end joining (NHEJ). Identification of the functional interaction of Ku70 as well as PARP-1 with the TCF-4 and β-catenin transcriptional complex may provide insights into a novel linkage between DNA damage recognition/repair and Wnt signaling. (Cancer Research 2007, in press)