Ubiquitin Ligase Cbl-b Downregulates Bone Formation Through Suppression of IGF-I Signaling in Osteoblasts During Denervation

J Bone Miner Res. 2006 May;21(5):722-34. doi: 10.1359/jbmr.060207.

Abstract

Unloading can prevent bone formation by osteoblasts. To study this mechanism, we focused on a ubiquitin ligase, Cbl-b, which was highly expressed in osteoblastic cells during denervation. Our results suggest that Cbl-b may mediate denervation-induced osteopenia by inhibiting IGF-I signaling in osteoblasts.

Introduction: Unloading, such as denervation (sciatic neurectomy) and spaceflight, suppresses bone formation by osteoblasts, leading to osteopenia. The resistance of osteoblasts to growth factors contributes to such unloading-mediated osteopenia. However, a detailed mechanism of this resistance is unknown. We first found that a RING-type ubiquitin ligase, Cbl-b, was highly expressed in osteoblastic cells after sciatic neurectomy in mice. In this study, we reasoned that Cbl-b played an important role in the resistance of osteoblasts to IGF-I.

Materials and methods: Cbl-b-deficient (Cbl-b(-/-)) or wildtype (Cbl-b(+/+)) mice were subjected to sciatic neurectomy. Bone formation in these mice was assessed by calcein labeling and histomorphometric analyses. We examined IGF-I signaling molecules in femora of these mice by Western blot and immunohistochemical analyses. We also examined the mitogenic response of Cbl-b-overexpressing or -deficient osteoblastic cells to various growth factors.

Results: In Cbl-b(+/+) mice, denervation decreased femur mass and bone formation, whereas it increased the expression of Cbl-b protein in osteoprogenitor cells and in osteocalcin-positive cells (osteoblastic cells) in hindlimb bone. In contrast, in Cbl-b(-/-) mice, bone mass and bone formation were sustained during denervation. Denervation inhibited the mitogenic response of osteoprogenitor cells most significantly to IGF-I. Therefore, we focused on Cbl-b-mediated modification of IGF-I signaling. Denervation decreased the amounts of insulin receptor substrate-1 (IRS-1), phosphatidly inositol 3-phosphate kinase (PI3K), and Akt-1 proteins in femora of Cbl-b(+/+) mice, whereas the amounts of these IGF-I signaling molecules in femora of Cbl-b(-/-) mice were constant after denervation. On a cellular level, primary osteoblastic cells from Cbl-b(-/-) mice were more stimulated to proliferate by IGF-I treatment compared with those from Cbl-b(+/+) mice. Furthermore, overexpression of Cbl-b increased ubiquitination and degradation of IRS-1 in primary Cbl-b(-/-) osteoblastic cells, leading to their impaired mitogenic response to IGF-I.

Conclusions: These results suggest that Cbl-b induces resistance of osteoblasts to IGF-I during denervation by increasing IRS-1 degradation and that Cbl-b-mediated modification of IGF-I signaling may contribute to decreased bone formation during denervation.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Base Sequence
  • Blotting, Western
  • Bone Development / physiology*
  • Cells, Cultured
  • DNA Primers
  • Denervation*
  • Down-Regulation / physiology*
  • Hydrolysis
  • Immunohistochemistry
  • Insulin-Like Growth Factor I / metabolism*
  • Mice
  • Mice, Inbred C57BL
  • Osteoblasts / metabolism*
  • Protein Binding
  • Reverse Transcriptase Polymerase Chain Reaction
  • Signal Transduction / physiology*
  • Ubiquitin / metabolism
  • Ubiquitin-Protein Ligases / physiology*

Substances

  • DNA Primers
  • Ubiquitin
  • Insulin-Like Growth Factor I
  • Ubiquitin-Protein Ligases