Molecular Pathways Molecular Pathways: Epigenetic Modulation of Wnt–Glycogen Synthase Kinase-3 Signaling to Target Human Cancer Stem Cells Yannick D. Benoit 1 , Borhane Guezguez 1 , Allison L. Boyd 1,2 , and Mickie Bhatia 1,2 Abstract Aberrant regulation of the canonical Wnt signaling pathway (Wnt–b-catenin–GSK3 axis) has been a prevalent theme in cancer biology since earlier observations until recent genetic discoveries gleaned from tumor genome sequencing. During the last few decades, a large body of work demonstrated the involvement of the Wnt–b-catenin–GSK3 signaling axis in the formation and maintenance of cancer stem cells (CSC) responsible for tumor growth in several types of human malignancies. Recent studies have elucidated epigenetic mechanisms that control pluripotency and stemness, and allow a first assessment on how embryonic and normal tissue stem cells are dysregulated in cancer to give rise to CSCs, and how canonical Wnt signaling might be involved. Here, we review emerging concepts highlighting the critical role of epigenetics in CSC development through abnormal canonical Wnt signaling. Finally, we refer to the characterization of novel and powerful inhibitors of chromatin organization machinery that, in turn, restore the Wnt–b-catenin–GSK3 signaling axis in malignant cells, and describe attempts/relevance to bring these compounds into preclinical and clinical studies. Clin Cancer Res; 20(21); 5372–8. Ó2014 AACR. Background The Wnt family of secreted glycoproteins act as ligands to activate multiple signal transduction pathways (1). Upon activation, Wnt signaling promotes mainly b-cate- nin nuclear translocation to regulate expression of target genes via T-cell factor/lymphoid enhancer factor (TCF/ LEF) transcription factors (2). The Wnt–b-catenin path- way acts in a context-dependent manner to regulate cell proliferation and differentiation in both embryonic development and adults (2). Perturbations in the levels of Wnt–b-catenin signaling are linked to many disease processes, including solid tumors and leukemia (3, 4). The activity of the Wnt–b-catenin signaling pathway depends primarily on the activity of glycogen synthase kinase-3 (GSK3), which plays a key role in controlling b-catenin stability/degradation. GSK3-dependent phos- phorylation of b-catenin restricts its nuclear translocation by inducing proteasome-dependent proteolysis (5). Active b-catenin complexes recruit transcriptional coacti- vator cAMP responsive element binding protein (CREB)– binding protein (CBP) or its closely related homolog p300 (6) to potentiate the expression of downstream Wnt target genes (Fig. 1A). Consequently, GSK3 acts as a tumor suppressor by curbing canonical Wnt–b-catenin signaling. Recent advances in cancer genomics identified the Wnt– GSK3–b-catenin pathway as one of the most prevalent signaling mechanisms studied in cancer biology since mul- tiple genetic alterations of its components were recurrently associated with human tumorigenesis, including medullo- blastoma, hepatocellular cancer, colorectal cancers, and leukemia (7–10). To date, several reports also highlighted the importance of Wnt–GSK3–b-catenin signaling on self- renewal in both normal and cancer stem cells (CSC). Specifically, CSCs were identified as rare populations of cancer cells within a hierarchical model of tumorigenesis, displaying the ability to sustain long-term neoplastic dis- semination in both leukemia and solid cancers (11, 12). Considering their malignant and metastatic properties that might cause relapse, the CSC represents, to date, the major clinical obstacle for effective cancer eradication by conventional therapeutic measures (13). Examples of the participation of Wnt–GSK3 signaling in CSC development include cases of BCR-ABL chronic myeloid leukemia (CML; refs. 14, 15) presenting an aberrant nonfunctional form of GSK3 showing neoplastic progression toward an aggressive stage of the disease, marked by the progressive accumula- tion of nuclear active b-catenin within BCR-ABL CSCs (10). 1 Stem Cell and Cancer Research Institute, McMaster University, Hamilton, Ontario, Canada. 2 Department of Biochemistry, McMaster University, Hamilton, Ontario, Canada. Y.D. Benoit and B. Guezguez contributed equally to this article. Corresponding Author: Mickie Bhatia, Stem Cell and Cancer Research Institute, McMaster University, Faculty of Health Sciences, 1280 Main Street West, MDCL 5029, Hamilton, ON L8S 4K1, Canada. Phone: 905- 525-9140, ext. 28687; Fax: 905-522-7772; E-mail: mbhatia@mcmaster.ca doi: 10.1158/1078-0432.CCR-13-2491 Ó2014 American Association for Cancer Research. Clinical Cancer Research Clin Cancer Res; 20(21) November 1, 2014 5372 on May 22, 2016. © 2014 American Association for Cancer Research. clincancerres.aacrjournals.org Downloaded from Published OnlineFirst July 8, 2014; DOI: 10.1158/1078-0432.CCR-13-2491