Abstract
Resistance to growth inhibitory effects of transforming growth factor (TGF)-beta is a frequent consequence of malignant transformation. On the other hand, serum concentrations of TGF-beta, plasminogen activator inhibitor type 1 (PAI-1), and vascular endothelial growth factor (VEGF) are elevated as tumor progresses. The molecular mechanism of autocrine TGF-beta signaling and its effects on PAI-1 and VEGF production in human hepatocellular carcinoma (HCC) is unknown. TGF-beta signaling involves TGF-beta type I receptor-mediated phosphorylation of serine residues within the conserved SSXS motif at the C-terminus of Smad2 and Smad3. To investigate the involvement of autocrine TGF-beta signal in cell growth, PAI-1 and VEGF production of HCC, we made stable transfectants of human HCC line (HuH-7 cells) to express a mutant Smad2(3S-A), in which serine residues of SSXS motif were changed to alanine. The transfectants demonstrated an impaired Smad2 signaling. Along with the resistance to growth inhibition by TGF-beta, forced expression of Smad2(3S-A) induced endogenous TGF-beta secretion. Moreover, this increased TGF-beta enhanced ligand-dependent signaling through the activated Smad3 and Smad4 complex, and transcriptional activities of PAI-1 and VEGF genes. In conclusion, distortion of autocrine TGF-beta signals in human HCC accelerates their malignant potential by enhancing cell growth as well as PAI-1 and VEGF production.
MeSH terms
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Amino Acid Motifs
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Amino Acid Substitution
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Autocrine Communication / physiology*
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Carcinoma, Hepatocellular / genetics
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Carcinoma, Hepatocellular / pathology*
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DNA Replication
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DNA-Binding Proteins / chemistry
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DNA-Binding Proteins / genetics
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DNA-Binding Proteins / metabolism
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Disease Progression
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Endothelial Growth Factors / biosynthesis*
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Endothelial Growth Factors / genetics
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Gene Expression Regulation, Neoplastic / drug effects*
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Humans
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Intercellular Signaling Peptides and Proteins / biosynthesis*
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Intercellular Signaling Peptides and Proteins / genetics
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Liver Neoplasms / genetics
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Liver Neoplasms / pathology*
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Lymphokines / biosynthesis*
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Lymphokines / genetics
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Macromolecular Substances
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Mutagenesis, Site-Directed
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Neoplasm Proteins / biosynthesis
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Neoplasm Proteins / genetics
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Neoplasm Proteins / physiology*
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Plasminogen Activator Inhibitor 1 / biosynthesis*
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Plasminogen Activator Inhibitor 1 / genetics
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Recombinant Fusion Proteins / physiology
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Signal Transduction
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Smad2 Protein
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Smad3 Protein
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Smad4 Protein
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Trans-Activators / chemistry
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Trans-Activators / genetics
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Trans-Activators / metabolism
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Transfection
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Transforming Growth Factor beta / physiology*
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Tumor Cells, Cultured / drug effects
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Tumor Cells, Cultured / metabolism
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Vascular Endothelial Growth Factor A
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Vascular Endothelial Growth Factors
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alpha-Fetoproteins / biosynthesis
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alpha-Fetoproteins / genetics
Substances
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DNA-Binding Proteins
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Endothelial Growth Factors
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Intercellular Signaling Peptides and Proteins
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Lymphokines
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Macromolecular Substances
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Neoplasm Proteins
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Plasminogen Activator Inhibitor 1
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Recombinant Fusion Proteins
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SMAD2 protein, human
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SMAD3 protein, human
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SMAD4 protein, human
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Smad2 Protein
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Smad3 Protein
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Smad4 Protein
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Trans-Activators
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Transforming Growth Factor beta
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Vascular Endothelial Growth Factor A
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Vascular Endothelial Growth Factors
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alpha-Fetoproteins