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Receptor tyrosine kinases inhibit bone morphogenetic protein-Smad responsive promoter activity and differentiation of

Konosuke Nakayama1, Yasuhiro Tamura, Miyuki Suzawa

  • 1Division of Endocrinology, Department of Medicine, University of Tokyo School of Medicine, Tokyo, Japan. pumpkin-tky@umin.ac.jp

Journal of Bone and Mineral Research : the Official Journal of the American Society for Bone and Mineral Research
|May 8, 2003
PubMed

Insights

Growth factors like FGF-2 and EGF suppress osteoblast differentiation by inhibiting Smad1 transactivity via the RTK-ERK pathway. This mechanism involves interference with BMP signaling, not direct Smad1 phosphorylation.

Area of Science:

  • Cell Biology
  • Molecular Biology
  • Biochemistry

Background:

  • Fibroblast growth factor-2 (FGF-2) and epidermal growth factor (EGF) activate receptor tyrosine kinases (RTKs), leading to extracellular signal-regulated kinase (ERK) activation.
  • RTK-ERK signaling promotes osteoblast proliferation but inhibits their differentiation.

Purpose of the Study:

  • To elucidate the mechanism by which FGF-2 and EGF suppress osteoblastic differentiation.
  • To investigate the role of Smad1 transactivity in the inhibitory action of growth factors on osteoblast differentiation.

Main Methods:

  • Utilized MC3T3-E1 osteoblast-like cells and reporter assays for Smad6 promoter activity.
  • Assessed alkaline phosphatase activity and Smad6 promoter activity stimulated by bone morphogenetic protein-2 (BMP-2).
  • Employed adenoviral overexpression of constitutively active MEK, dominant-negative Ras, and a MEK1 inhibitor (PD098059).
  • Investigated the role of BMP-responsive elements (BMPREs) and utilized mutant Smad1 constructs.

Main Results:

  • FGF-2 and EGF inhibited BMP-2-stimulated alkaline phosphatase activity and Smad6 promoter activity.
  • The RTK-Ras-ERK pathway mimicked and reversed these inhibitory effects.
  • Inhibition occurred via BMPREs on the Smad6 promoter.
  • The RTK-ERK pathway suppressed BMP signaling by interfering with Smad1 transactivity, independent of direct Smad1 phosphorylation.

Conclusions:

  • The RTK-Ras-ERK pathway suppresses osteoblastic differentiation by inhibiting Smad1 transactivity.
  • This suppression mechanism involves interference with BMP signaling pathways.
  • Other ERK-phosphorylated transcriptional factors may regulate osteoblastic differentiation.

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