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Signal transductions induced by bone morphogenetic protein-2 and transforming growth factor-beta in normal human
1Division of Bone and Mineral Diseases, Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
Abstract:
Transforming growth factor beta (TGF-beta) activates Ras/MAPK signaling in many cell types. Because TGF-beta and BMP-2 exert similar effects, we examined if this signaling is stimulated by both factors and analyzed the relationship between this signaling and the Smads in osteoblasts. BMP-2 and TGF-beta stimulated Ras, MAPK, and AP-1 activities. The DNA binding activities of c-Fos, FosB/Delta FosB, Fra-1, Fra-2, and JunB were up-regulated whereas JunD activity was decreased. c-Fos, FosB/Delta FosB, and JunB were associated with Smad4. The stimulation of AP-1 by BMP-2 and TGF-beta was dependent on Smad signaling, and anti-Smad4 antibody interfered with AP-1 activity. Thus, BMP-2 and TGF-beta activate both Ras/MAPK/AP-1 and Smad signaling in osteoblasts with Smads modulating AP-1 activity. To determine the roles of MAPK in BMP-2 and TGF-beta function, we analyzed the effect of ERK and p38 inhibitors on the regulation of bone matrix protein expression and JunB and JunD levels by these two factors. ERK and p38 mediated TGF-beta suppression of osteocalcin and JunD as well as stimulation of JunB. p38 was essential in BMP-2 up-regulation of type I collagen, fibronectin, osteopontin, osteocalcin, and alkaline phosphatase activity whereas ERK mediated BMP-2 stimulation of fibronectin and osteopontin. Thus, ERK and p38 differentially mediate TGF-beta and BMP-2 function in osteoblasts.
Insights
Bone morphogenetic protein-2 (BMP-2) and transforming growth factor beta (TGF-beta) activate Ras/MAPK and Smad signaling pathways in osteoblasts. These pathways differentially regulate bone matrix protein expression via ERK and p38 kinases.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Transforming growth factor beta (TGF-beta) is known to activate Ras/MAPK signaling in various cell types.
- Bone morphogenetic protein-2 (BMP-2) and TGF-beta exhibit similar biological effects, suggesting overlapping signaling pathways.
- Osteoblasts are crucial cells for bone formation and remodeling, responding to various growth factors.
Purpose of the Study:
- To investigate whether BMP-2 and TGF-beta stimulate Ras/MAPK signaling in osteoblasts.
- To analyze the relationship between Ras/MAPK signaling, Smad signaling, and AP-1 activity in osteoblasts.
- To determine the specific roles of ERK and p38 kinases in mediating the effects of BMP-2 and TGF-beta on bone matrix protein expression.
Main Methods:
- Stimulation of osteoblasts with BMP-2 and TGF-beta.
- Analysis of Ras, MAPK, and AP-1 activities using DNA binding assays.
- Investigation of Smad4 association with transcription factors.
- Application of ERK and p38 inhibitors to assess their role in regulating gene expression.
Main Results:
- Both BMP-2 and TGF-beta stimulated Ras, MAPK, and AP-1 activities, with differential regulation of AP-1 components (c-Fos, FosB/Delta FosB, Fra-1, Fra-2, JunB, JunD).
- Smad4 was found to be associated with c-Fos, FosB/Delta FosB, and JunB, and Smad signaling was essential for AP-1 stimulation.
- ERK and p38 kinases differentially mediated the effects of BMP-2 and TGF-beta on the expression of bone matrix proteins (osteocalcin, type I collagen, fibronectin, osteopontin, alkaline phosphatase) and transcription factors (JunB, JunD).
Conclusions:
- BMP-2 and TGF-beta activate both Ras/MAPK/AP-1 and Smad signaling pathways in osteoblasts.
- Smad signaling modulates AP-1 activity in response to BMP-2 and TGF-beta.
- ERK and p38 kinases play distinct roles in mediating the cellular responses to BMP-2 and TGF-beta in osteoblasts, impacting bone matrix protein production.