Role of Wnt Signaling in Colon Cancer Cell Metabolism

Over eighty percent of colon cancers derive from mutations that overactivate the Wnt pathway and cause cell transformation. These mutations stabilize β‐catenin protein, leading to its accumulation and nuclear translocation. Nuclear localization enables interaction with LEF/TCF transcription factors and the transcriptional overactivation of Wnt target genes, or gene programs. Identifying these gene programs and defining how they drive both the cancer cell phenotype and the tumor response to drug treatment is critical for developing therapies that block tumor progression. We and others have discovered that metabolism is one of the gene programs regulated by Wnt. Constitutively active Wnt signaling drives cells toward a glycolytic phenotype even in the presence of oxygen, a phenomenon known as Warburg metabolism. Using Gene Expression and ChIP‐seq analysis of colon cancer cells expressing dominant negative LEF‐1 and TCF‐1 isoforms (dnLEF/TCFs) we identified direct Wnt target genes that regulate this metabolic phenotype. Gene ontology analysis identified a subset of metabolism‐linked genes, including PDK1 (Pyruvate Dehydrogenase Kinase 1) and SLC16A1/ MCT1 (monocarboxylic acid transporter 1). PDK1 is a mitochondrial kinase that suppresses respiration and increases rates of glycolysis. MCT1 is a membrane‐bound transporter critical for maintaining the homeostasis of glycolytic cancer cells through import and/or efflux of lactate. PDK1 and MCT1 RNA and protein levels are significantly and specifically down regulated by dnLEF/TCFs in multiple colon cancer cell lines (SW480, SW620, HCT116 and DLD1). ChIP‐seq analysis with dnTCF1 identified regulatory regions with Wnt Response Elements (WREs) in distal and promoter proximal locations for each locus. Each peak of occupancy could confer Wnt regulation in luciferase reporter assays. c‐Myc, a Wnt target gene and known regulator of SLC16A1 transcription, regulates the locus additively but not synergistically with LEF/TCFs. Work by others has shown MCT1 to be necessary for the import of 3‐bromopyruvate, a potential cancer therapeutic that inhibits glycolysis through a mechanism not well understood. Our data suggests that colon cancer cells are sensitive to 3‐bromopyruvate in a Wnt dependent manner, and that co‐treatment of cells with Wnt inhibitors (e.g. XAV939, a tankyrase inhibitor) promotes resistance and recovery from 3‐bromopyruvate treatment. Overall this data suggests that Wnt signaling is important for maintaining a Warbug phenotype through PDK1 and MCT1, and that MCT1 may serve as an important indicator for drug sensitivity in colon cancer therapy.Support or Funding InformationNIH R01 CA108697 (PI: Waterman), President's Dissertation Year Fellowship, UC Irvine 2015–2016