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Rapid flux in transforming growth factor-beta receptors on bone cells
M Centrella1, C Ji, S Casinghino
1Section of Plastic Surgery, Department of Surgery, Yale University School of Medicine, New Haven, Connecticut 06520-8041, USA.
Abstract:
The proportion of transforming growth factor-beta (TGF-beta) binding among conventional membrane receptors on bone cells can vary with hormone or growth factor treatment or with the state of osteoblast-like activity and appears to determine the nature of its biological effects. Therefore, functional TGF-beta receptor stability could be an important aspect of regulation. Suppression of protein synthesis reduced TGF-beta binding to types I and II receptors with t1/2 of 2 h and to betaglycan with t1/2 of 6 h. In contrast, suppression of mRNA transcription reduced TGF-beta binding at least 3-fold more slowly at each receptor site. Preexposure to TGF-beta decreased its binding at all three sites within 4 h in osteoblast-enriched cultures. This effect was transient with lower TGF-beta concentrations, where the receptor profile was nearly fully restored within 24-48 h. In contrast, less differentiated bone cells were less sensitive to ligand-dependent receptor down-regulation. Agents that alter protein kinase and phosphatase activity also modified the TGF-beta binding profile in specific ways. Together, these results indicate that cell surface TGF-beta receptors turn over rapidly by ligand-independent and ligand-dependent mechanisms, demonstrate that the binding capacity of TGF-beta receptors is less stable than their mRNAs, and that functional receptor levels may be determined in part by post-transcriptional events.
Insights
Transforming growth factor-beta (TGF-beta) receptors on bone cells are rapidly regulated by protein turnover, not mRNA transcription. This rapid receptor turnover influences TGF-beta
Area of Science:
- Cell biology
- Molecular signaling
- Bone biology
Background:
- Transforming growth factor-beta (TGF-beta) binding to bone cell receptors varies with treatment and cell activity, influencing its biological effects.
- Functional TGF-beta receptor stability is crucial for regulating cellular responses.
Purpose of the Study:
- To investigate the regulation and stability of TGF-beta receptors on bone cells.
- To determine the mechanisms controlling TGF-beta receptor levels and their impact on signaling.
Main Methods:
- Suppression of protein synthesis and mRNA transcription to assess receptor turnover rates.
- Exposure of osteoblast-enriched cultures to TGF-beta to study ligand-dependent down-regulation.
- Analysis of TGF-beta binding to types I and II receptors and betaglycan.
Main Results:
- Protein synthesis suppression rapidly reduced TGF-beta binding (t1/2 of 2-6 h), while mRNA suppression acted more slowly.
- Pre-exposure to TGF-beta induced transient down-regulation of receptor binding in osteoblast-enriched cultures.
- Less differentiated bone cells showed reduced sensitivity to ligand-dependent receptor down-regulation.
Conclusions:
- Cell surface TGF-beta receptors exhibit rapid turnover via ligand-dependent and independent pathways.
- TGF-beta receptor binding capacity is less stable than its mRNA, suggesting post-transcriptional regulation.
- Functional receptor levels are significantly influenced by post-transcriptional events and protein turnover.