Smad7 abrogates transforming growth factor-beta1-mediated growth inhibition in COLO-357 cells through functional inactivation of the retinoblastoma protein.
Smad7 is overexpressed in 50% of human pancreatic cancers. COLO-357 pancreatic cancer cells engineered to overexpress Smad7 are resistant to the actions of transforming growth factor-beta1 (TGF-beta1) with respect to growth inhibition and cisplatin-induced apoptosis but not with respect to modulation of gene expression. To delineate the mechanisms underlying these ... divergent consequences of Smad7 overexpression, we studied the effects of Smad7 on TGF-beta1-dependent signaling pathways and cell cycle regulating proteins. TGF-beta1 induced the phosphorylation of MAPK, p38 MAPK, and AKT2 irrespective of the levels of Smad7, and inhibitors of these pathways did not alter TGF-beta1 actions on cell growth. By contrast, Smad7 overexpression interfered with TGF-beta1-mediated attenuation of cyclin A and B levels, inhibition of cdc2 dephosphorylation and CDK2 inactivation, up-regulation of p27, and the maintenance of the retinoblastoma protein (RB) in a hypophosphorylated state. Smad7 also suppressed TGF-beta1-mediated inhibition of E2F activity but did not alter TGF-beta1-mediated phosphorylation of Smad2, the nuclear translocation of Smad2/3/4, or DNA binding of the Smad2/3/4 complex. Although Smad7 did not associate with the type I TGF-beta receptor (TbetaRI), SB-431542, an inhibitor of the kinase activity of this receptor, blocked TGF-beta1-mediated effects on Smad-2 phosphorylation. These findings point toward a novel paradigm whereby Smad7 acts to functionally inactivate RB and de-repress E2F without blocking the activation of TbetaRI and the nuclear translocation of Smad2/3, thereby allowing for TGF-beta1 to exert effects in a cancer cell that is resistant to TGF-beta1-mediated growth inhibition.
Mesh Terms:
Active Transport, Cell Nucleus, Animals, COS Cells, Cell Cycle, Cell Cycle Proteins, Cell Line, Tumor, Cell Nucleus, Cell Proliferation, Cell Separation, DNA-Binding Proteins, Dose-Response Relationship, Drug, E2F Transcription Factors, Flow Cytometry, Gene Expression Regulation, Humans, Immunoblotting, Immunoprecipitation, Luciferases, Microscopy, Fluorescence, Models, Biological, Phosphorylation, Protein Transport, Retinoblastoma Protein, Smad7 Protein, Subcellular Fractions, Tetrazolium Salts, Thiazoles, Time Factors, Trans-Activators, Transcription Factors, Transforming Growth Factor beta, Transforming Growth Factor beta1, p38 Mitogen-Activated Protein Kinases
Active Transport, Cell Nucleus, Animals, COS Cells, Cell Cycle, Cell Cycle Proteins, Cell Line, Tumor, Cell Nucleus, Cell Proliferation, Cell Separation, DNA-Binding Proteins, Dose-Response Relationship, Drug, E2F Transcription Factors, Flow Cytometry, Gene Expression Regulation, Humans, Immunoblotting, Immunoprecipitation, Luciferases, Microscopy, Fluorescence, Models, Biological, Phosphorylation, Protein Transport, Retinoblastoma Protein, Smad7 Protein, Subcellular Fractions, Tetrazolium Salts, Thiazoles, Time Factors, Trans-Activators, Transcription Factors, Transforming Growth Factor beta, Transforming Growth Factor beta1, p38 Mitogen-Activated Protein Kinases
J. Biol. Chem.
Date: Jun. 10, 2005
PubMed ID: 15811853
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