title: Smad4 cooperates with lymphoid enhancer-binding factor 1/T cell-specific factor to increase c-myc expression in the absence of TGF-beta signaling.
abstract: The c-myc protooncogene is a key regulator of cell proliferation whose expression is reduced in normal epithelial cells in response to the growth inhibitory cytokine TGF-beta. Smad4 mediates this inhibitory effect of TGF-beta by forming a complex with Smad3, E2F4/5, and p107 at the TGF-beta inhibitory element (TIE) element on the c-myc promoter. In contrast, cell proliferation and c-myc expression are increased in response to Wnt ligands; this effect is mediated through the lymphoid enhancer-binding factor 1/T cell-specific factor (LEF/TCF) family of transcription factors on the c-myc promoter LEF/TCF-binding elements (TBE1 and TBE2). We report that a peptide aptamer designed to inhibit the binding between Smad4 and LEF/TCF reduced c-myc expression and the growth rate of HepG2 cells. Further analysis demonstrated that, in the absence of TGF-beta, Smad4 was bound to the positive regulatory element TBE1 from the c-myc promoter and activated c-myc promoter activity. Smad4 binding to the positive TBE1 c-myc element was reduced by TGF-beta, consistent with Smad4's inhibitory role on c-myc expression in response to TGF-beta. Reduction of Smad4 levels by RNAi knockdown also reduced c-myc expression levels and sensitized hepatocytes to cell death by serum deprivation. Two tumor-derived mutant Smad4 proteins that fail to mediate TGF-beta responses were still competent to cooperate with LEF1 to activate the c-myc promoter. These results support a previously unreported TGF-beta-independent function for Smad4 in cooperating with LEF/TCF to activate c-myc expression.
authors.affiliation: McArdle Laboratory for Cancer Research and Laboratory of Genetics, University of Wisconsin, Madison, WI 53706, USA.
authors.forename: S Kyun
authors.initials: SK
authors.lastname: Lim
authors.forename: F Michael
authors.initials: FM
authors.lastname: Hoffmann
mesh_terms: D045744:Cell Line, Tumor; D049109:Cell Proliferation; D005786:Gene Expression Regulation / Q000502:physiology*; D006801:Humans; D051821:Lymphoid Enhancer-Binding Factor 1 / Q000502:physiology*; D016271:Proto-Oncogene Proteins c-myc / Q000096:biosynthesis* / Q000235:genetics*; D015398:Signal Transduction / Q000502:physiology*; D051901:Smad4 Protein / Q000502:physiology*; D016212:Transforming Growth Factor beta / Q000502:physiology*; D015854:Up-Regulation / Q000502:physiology*
chemical_list: D051821:Lymphoid Enhancer-Binding Factor 1; C489427:MYC protein, human; D016271:Proto-Oncogene Proteins c-myc; C497796:SMAD4 protein, human; D051901:Smad4 Protein; D016212:Transforming Growth Factor beta