title: Oncogenic ras represses transforming growth factor-beta /Smad signaling by degrading tumor suppressor Smad4.
abstract: 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.
authors.affiliation: Department of Surgery, Vanderbilt University Medical Center, Vanderbilt-Ingram Cancer Center, Nashville, Tennessee 37232, USA.
authors.forename: D
authors.initials: D
authors.lastname: Saha
authors.forename: P K
authors.initials: PK
authors.lastname: Datta
authors.forename: R D
authors.initials: RD
authors.lastname: Beauchamp
mesh_terms: D000818:Animals; D002455:Cell Division / Q000187:drug effects; D002461:Cell Line, Transformed; D002467:Cell Nucleus / Q000187:drug effects / Q000378:metabolism; D002471:Cell Transformation, Neoplastic; D004268:DNA-Binding Proteins / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000187:drug effects; D016147:Genes, Tumor Suppressor; D011905:Genes, ras / Q000187:drug effects; D007544:Isopropyl Thiogalactoside / Q000494:pharmacology; D007700:Kinetics; D009154:Mutation; D021381:Protein Transport; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D013936:Thymidine / Q000378:metabolism; D015534:Trans-Activators / Q000378:metabolism*; D014158:Transcription, Genetic / Q000187:drug effects*; D016212:Transforming Growth Factor beta / Q000494:pharmacology*
chemical_list: D004268:DNA-Binding Proteins; D015534:Trans-Activators; D016212:Transforming Growth Factor beta; D007544:Isopropyl Thiogalactoside; D013936:Thymidine