Abstract
Activin A and osteogenic protein-1 (OP-1) exerted antagonistic effects on each other's responses on the human Tera-2 embryonal carcinoma cell line. OP-1 dose dependently inhibited activin A-induced activation of p3TP-Lux transcriptional reporter, containing part of the human plasminogen activator inhibitor-1 (PAI-1) promoter, while activin A inhibited OP-1-mediated alkaline phosphatase induction. Approximately equimolar concentrations of both growth factors resulted in 50% inhibition of the respective biological responses. Affinity cross-linking studies using 125I-activin A or 125I-OP-1 followed by receptor-immunoprecipitations revealed that both ligands bound to the activin type II receptor (ActR-II), but recruited different type I receptors. In addition, OP-1 competed with binding of 125I-activin A, and activin A competed with binding of 125I-OP-1 to ActR-II. Transient transfection studies showed that competition between activin A and OP-1 also occurred at the type I receptor (ActR-1) level; constitutively active (CA)-ActR-I inhibited CA-ActR-IB-mediated p3TP-Lux reporter induction. There was no competition between activin A and OP-1 for availability of Smad4, indicating that the concentration of this common signal transducer is not limiting for generating the observed biological responses. Overexpression of ActR-II abolished the inhibitory effect of OP-1 on activin A-induced p3TP-Lux activation and, surprisingly, led to OP-1-induced transcriptional reporter activity. Whereas the exact mechanism of competition is unclear, the role of ActR-II in the competition between activin A and OP-1 is discussed in light of the observed interference in downstream signaling by CA-ActR-I and CA-ActR-IB.
Publication types
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Research Support, Non-U.S. Gov't
MeSH terms
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Activin Receptors, Type I
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Activin Receptors, Type II
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Activins
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Alkaline Phosphatase / metabolism
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Blotting, Northern
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Bone Morphogenetic Protein 7
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Bone Morphogenetic Proteins / analysis
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Bone Morphogenetic Proteins / genetics
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Bone Morphogenetic Proteins / metabolism*
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DNA-Binding Proteins / genetics
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Embryonal Carcinoma Stem Cells
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Enzyme Activation / physiology
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Gene Expression Regulation, Enzymologic / physiology
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Gene Expression Regulation, Neoplastic / physiology
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Genes, Reporter
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Growth Substances / analysis
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Growth Substances / genetics
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Growth Substances / metabolism*
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Humans
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Inhibins / analysis
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Inhibins / genetics
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Inhibins / metabolism*
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Neoplastic Stem Cells / chemistry
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Neoplastic Stem Cells / enzymology*
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Plasminogen Activator Inhibitor 1 / genetics
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Promoter Regions, Genetic / physiology
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Protein Binding / physiology
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Proto-Oncogene Proteins c-jun / genetics
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RNA, Messenger / analysis
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Receptors, Growth Factor / metabolism
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Smad6 Protein
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Smad7 Protein
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Trans-Activators / genetics
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Transcription, Genetic / physiology
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Transforming Growth Factor beta / analysis
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Transforming Growth Factor beta / genetics
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Transforming Growth Factor beta / metabolism
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Tumor Cells, Cultured / chemistry
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Tumor Cells, Cultured / enzymology
Substances
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BMP7 protein, human
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Bone Morphogenetic Protein 7
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Bone Morphogenetic Proteins
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DNA-Binding Proteins
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Growth Substances
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Plasminogen Activator Inhibitor 1
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Proto-Oncogene Proteins c-jun
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RNA, Messenger
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Receptors, Growth Factor
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SMAD6 protein, human
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SMAD7 protein, human
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Smad6 Protein
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Smad7 Protein
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Trans-Activators
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Transforming Growth Factor beta
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Activins
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Inhibins
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Activin Receptors, Type I
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Activin Receptors, Type II
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Alkaline Phosphatase