Oncogenic ras represses transforming growth factor-beta /Smad signaling by degrading tumor suppressor Smad4.

Academic Article


  • The loss of growth-inhibitory responses to transforming growth factor-beta (TGF-beta) is a frequent consequence of malignant transformation. Smad2, Smad3, and Smad4 proteins are important mediators of the antiproliferative responses to TGF-beta and may become inactivated in some human cancers. Epithelial cells harboring oncogenic Ras mutations often exhibit a loss of TGF-beta antiproliferative responses. To further investigate the effect of oncogenic Ras in TGF-beta signaling, we used an isopropyl-1-thio-beta-d-galactopyranoside-inducible expression system to express Ha-Ras(Val-12) in intestinal epithelial cells. Induction of Ha-Ras(Val-12) caused a decrease in the level of Smad4 expression, inhibited TGF-beta-induced complex formation between Smad2/Smad3 and Smad4, blocked Smad4 nuclear translocation, inhibited the TGF-beta-mediated decrease in [(3)H]thymidine incorporation, and repressed TGF-beta-activated transcriptional responses. The withdrawal of isopropyl-1-thio-beta-d-galactopyranoside or the addition of an inhibitor of the ubiquitin-proteasome pathway restored the Smad4 level and TGF-beta-induced Smad complex formation. Forced expression of Smad4 resulted in partial recovery of the TGF-beta-mediated growth inhibition and transcriptional responses in the presence of oncogenic Ras. Further, PD98059, a specific inhibitor of the MEK/ERK/mitogen-activated protein kinase pathway prevented the Ras-induced decrease in Smad4 expression and complex formation. Our results suggest a novel mechanism by which oncogenic Ras represses TGF-beta signaling by mitogen-activated protein kinase-dependent down-regulation of Smad4, thereby subverting the tumor suppressor function of TGF-beta.
  • Published In


  • Animals, Cell Division, Cell Line, Transformed, Cell Nucleus, Cell Transformation, Neoplastic, DNA-Binding Proteins, Gene Expression Regulation, Genes, Tumor Suppressor, Genes, ras, Isopropyl Thiogalactoside, Kinetics, Mutation, Protein Transport, Signal Transduction, Thymidine, Trans-Activators, Transcription, Genetic, Transforming Growth Factor beta
  • Digital Object Identifier (doi)

    Author List

  • Saha D; Datta PK; Beauchamp RD
  • Start Page

  • 29531
  • End Page

  • 29537
  • Volume

  • 276
  • Issue

  • 31