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Differentiation and cell surface expression of transforming growth factor-beta receptors are regulated by interaction
Y Takeuchi1, K Nakayama, T Matsumoto
1Fourth Department of Internal Medicine, University of Tokyo School of Medicine, 3-28-6 Mejirodai, Bunkyo-ku, Tokyo 112, Japan.
Abstract:
Although transforming growth factor (TGF)-beta enhances bone formation, it inhibits the differentiation of osteoblasts. To clarify the regulatory mechanism of osteoblastic differentiation and TGF-beta actions, the relationship among differentiation, TGF-beta actions, and matrix protein synthesis was examined using murine osteoblast-like MC3T3-E1 cells. Alkaline phosphatase (ALP) activity continued to increase during long-term cultures, and the increase was closely associated with a reduction in cell surface TGF-beta receptors competent to bind TGF-beta. Both the stimulation of proteoglycan synthesis and the inhibition of ALP activity by TGF-beta were also suppressed. Collagen synthesis inhibitors and an anti-alpha2beta1 integrin blocking antibody blocked the changes in ALP activity and TGF-beta receptors, and a DGEA peptide that interferes binding of collagen to alpha2beta1 integrin also blocked the increase in ALP activity. Furthermore, when MC3T3-E1 cells were cultured on extracellular matrix layers obtained from these cells, all the differentiation-associated changes could be observed without collagen production, and the extracellular matrix-induced differentiation was also blocked by an anti-alpha2beta1 integrin antibody. These results demonstrate that the interaction of cell surface alpha2beta1 integrin with matrix collagen synthesized by osteoblasts themselves is involved in the osteoblastic differentiation and the reduction in cell surface receptors and actions of TGF-beta. It is suggested that matrix collagen synthesized under the stimulation by TGF-beta plays an important role in the regulation of osteoblastic differentiation and TGF-beta actions by differentiation-associated down-regulation of TGF-beta receptors.
Insights
Transforming growth factor-beta (TGF-β) impacts bone formation and osteoblast differentiation. This study reveals that osteoblast interaction with their own collagen matrix via alpha2beta1 integrin regulates differentiation and TGF-β signaling.
Area of Science:
- Cell Biology
- Biochemistry
- Orthopedics
Background:
- Transforming growth factor-beta (TGF-β) has a dual role in bone metabolism, promoting formation but inhibiting osteoblast differentiation.
- The precise mechanisms regulating osteoblast differentiation and TGF-β's complex actions remain incompletely understood.
Purpose of the Study:
- To elucidate the interplay between osteoblastic differentiation, TGF-β signaling, and matrix protein synthesis.
- To investigate the role of cell-matrix interactions in mediating these processes.
Main Methods:
- Utilized murine osteoblast-like MC3T3-E1 cells in long-term cultures.
- Assessed alkaline phosphatase (ALP) activity, TGF-β receptor expression, proteoglycan, and collagen synthesis.
- Employed collagen synthesis inhibitors, anti-alpha2beta1 integrin antibodies, and DGEA peptides.
- Cultured cells on pre-formed extracellular matrix (ECM) layers.
Main Results:
- Osteoblastic differentiation, indicated by increasing ALP activity, correlated with reduced cell-surface TGF-β receptors.
- TGF-β's effects on proteoglycan synthesis and ALP activity were diminished during differentiation.
- Alpha2beta1 integrin interaction with synthesized collagen was crucial for differentiation-associated changes in ALP activity and TGF-β receptor levels.
- Culturing on ECM induced differentiation, which was blocked by anti-alpha2beta1 integrin antibodies.
Conclusions:
- Osteoblast differentiation involves the interaction of cell-surface alpha2beta1 integrin with self-synthesized matrix collagen.
- This interaction mediates the reduction in cell-surface TGF-β receptors and modulates TGF-β actions.
- Matrix collagen, potentially stimulated by TGF-β, plays a key regulatory role in osteoblast differentiation and TGF-β signaling through differentiation-dependent downregulation of TGF-β receptors.