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Functional antagonism between activin and osteogenic protein-1 in human embryonal carcinoma cells
E Piek1, M Afrakhte, K Sampath
1Department of Cell Biology, University of Nijmegen, The Netherlands.
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.
Insights
Activin A and osteogenic protein-1 (OP-1) show opposing effects on human cells, competing for receptor binding. This competition influences cellular responses, impacting gene activation and differentiation pathways.
Area of Science:
- Cell biology
- Molecular signaling
- Developmental biology
Background:
- Activin A and Osteogenic Protein-1 (OP-1) are growth factors with significant roles in cellular processes.
- Understanding their interactions is crucial for deciphering complex signaling networks.
- The human Tera-2 embryonal carcinoma cell line serves as a model for studying these interactions.
Purpose of the Study:
- To investigate the antagonistic effects of Activin A and OP-1 on human Tera-2 cells.
- To elucidate the molecular mechanisms underlying their competitive interactions at the receptor level.
- To explore the downstream signaling consequences of this cross-talk.
Main Methods:
- Utilized p3TP-Lux transcriptional reporter assays to measure gene activation.
- Employed affinity cross-linking and receptor-immunoprecipitation to study ligand-receptor binding.
- Conducted transient transfection studies with constitutively active (CA) receptor constructs.
- Assessed Smad4 availability and the role of activin type II receptor (ActR-II) and type I receptors (ActR-I, ActR-IB).
Main Results:
- Activin A and OP-1 exhibited reciprocal inhibition of each other's biological responses.
- Both ligands bound to ActR-II but recruited distinct type I receptors.
- Competition was observed for ActR-II binding and also at the type I receptor level.
- Smad4 availability was not a limiting factor in the observed signaling.
- Overexpression of ActR-II modulated the inhibitory effects and induced OP-1 activity.
Conclusions:
- Activin A and OP-1 engage in significant cross-talk, primarily mediated through their shared use of ActR-II and distinct type I receptors.
- The competition occurs at multiple levels of the receptor complex, influencing downstream signaling pathways.
- Further investigation is needed to fully elucidate the precise mechanisms of competition and their implications for cellular function.