Abstract A02: Beta-catenin-independent activation of TCF1/LEF1 in human hematopoietic tumor cells through interaction with ATF2 transcription factors
Authors: Grumolato Luca , Liu Guizhong , Haremaki Tomomi , Mungamuri Sathish Kumar , Mong Phyllus , Akiri Gal , Lopez-Bergami Pablo , Arita Adriana , Youssef Anouar , Mlodzik Marek , Ronai Ze'ev A. , Brody Joshua , Weinstein Daniel C. , Aaronson Stuart A. 
Published: August 2014

Abstract
The role of Wnt signaling in embryonic development and stem cell maintenance is well established and aberrations leading to the constitutive up-regulation of this pathway are frequent in several types of human cancers. Upon ligand-mediated activation, Wnt receptors promote the stabilization of beta-catenin, which translocates to the nucleus and binds to the T-cell factor/lymphoid enhancer factor (TCF/LEF) family of transcription factors to regulate the expression of Wnt target genes. When not bound to beta-catenin, the TCF/LEF proteins are believed to act as transcriptional repressors. Using a specific lentiviral reporter, we identified hematopoietic tumor cells displaying constitutive TCF/LEF transcriptional activation in the absence of beta-catenin stabilization. Suppression of TCF/LEF activity in these cells mediated by an inducible dominant-negative TCF4 (DN-TCF4) inhibited both cell growth and the expression of Wnt target genes. Further, expression of TCF1 and LEF1, but not TCF4, stimulated TCF/LEF reporter activity in certain human cell lines independently of beta-catenin. By a complementary approach in vivo, TCF1 mutants, which lacked the ability to bind to beta-catenin, induced Xenopus embryo axis duplication, a hallmark of Wnt activation, and the expression of the Wnt target gene Xnr3. Through generation of different TCF1-TCF4 fusion proteins, we identified three distinct TCF1 domains that participate in the beta-catenin-independent activity of this transcription factor. TCF1 and LEF1 physically interacted and functionally synergized with members of the activating transcription factor 2 (ATF2) family of transcription factors. Moreover, knockdown of ATF2 expression in lymphoma cells phenocopied the inhibitory effects of DN-TCF4 on the expression of target genes associated with the Wnt pathway and on cell growth. Together, our findings indicate that, through interaction with ATF2 factors, TCF1/LEF1 promote the growth of hematopoietic malignancies in the absence of beta-catenin stabilization, thus establishing a new mechanism for TCF1/LEF1 transcriptional activity distinct from that associated with canonical Wnt signaling.
Citation Format: Luca Grumolato, Guizhong Liu, Tomomi Haremaki, Sathish Kumar Mungamuri, Phyllus Mong, Gal Akiri, Pablo Lopez-Bergami, Adriana Arita, Anouar Youssef, Marek Mlodzik, Ze'ev A. Ronai, Joshua Brody, Daniel C. Weinstein, Stuart A. Aaronson. Beta-catenin-independent activation of TCF1/LEF1 in human hematopoietic tumor cells through interaction with ATF2 transcription factors. [abstract]. In: Proceedings of the Third AACR International Conference on Frontiers in Basic Cancer Research; Sep 18-22, 2013; National Harbor, MD. Philadelphia (PA): AACR; Cancer Res 2013;73(19 Suppl):Abstract nr A02.