Modulation of LEF/TCFs and Wnt Signaling in Colon Cancer
- Author(s): Tanio, Stephanie Sprowl
- Advisor(s): Waterman, Marian L
- et al.
Oncogenic Wnt signaling is implicated as the major driving force in colon cancer. Actions of misregulated Wnt signaling rely on the upregulation of Lymphoid Enhancer Factor/ T-cell Factor (LEF/TCF) transcription factor-dependent Wnt target genes. In colon cancer, Wnt-activating, full-length LEF/TCFs are expressed, while their dominant negative, Wnt-suppressing counterparts (dnLEF/TCFs) are not. Therefore, full-length isoforms act unopposed, leading to overactive, inappropriate expression of target genes. We use re-expression of dnLEF/TCFs as a tool to modulate Wnt signaling in colon cancer and discover what cancer phenotypes and gene programs are contributing to the oncogenic drive to cell transformation. Using expression of dnLEF-1 and dnTCF-1, this thesis illustrates how Wnt/β-catenin signaling directs Warburg metabolism. We identify Pyruvate dehydrogenase kinase 1 (PDK1) - a kinase that directs the conversion of pyruvate to lactate, and SLC16A1/MCT-1 - a transporter of small metabolites such as lactate, as important direct targets within a larger gene program for metabolism. Given recent preclinical development of small molecules that target Wnt signaling and metabolism, and given interest in developing new combination therapies for cancer treatment, we tested how Wnt inhibition affects the ability of 3-bromopyruvate to kill cancer cells in vitro. We report that this toxic molecule kills colon cancer cells, but that Wnt signaling inhibition lowers its efficacy. We conclude that both PDK1 and MCT-1 are part of a core Wnt gene program for glycolysis in colon cancer and that modulation of this program could play an important role in shaping sensitivity to drugs that target cancer metabolism.
Wnt ligands are secreted morphogens that control multiple developmental processes during embryogenesis and adult homeostasis. A diverse set of receptors and signals have been linked to individual Wnts, but the lack of tools for comparative analysis has limited the ability to determine which of these signals are general for the entire Wnt family, and which define subsets of differently acting ligands. We have created a versatile Gateway expression library of clones for all 19 human Wnts. An analysis comparing epitope-tagged and untagged versions of each ligand shows that despite their similar expression at the mRNA level, Wnts exhibit considerable variation in stability, processing and secretion. This comprehensive toolkit provides critical tools and new insights into human Wnt gene expression and function.