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  • Oral presentation
  • Open Access

Src activation by cGMP/PKG II in osteoblasts: characterization of a mechano-sensitive signalling complex

  • 1,
  • 1,
  • 1,
  • 1,
  • 2,
  • 2,
  • 1,
  • 1 and
  • 1Email author
BMC Pharmacology201111 (Suppl 1) :O24

https://doi.org/10.1186/1471-2210-11-S1-O24

  • Published:

Keywords

  • Nitric Oxide
  • Fluid Shear Stress
  • Osteoblast Proliferation
  • Endochondral Bone Formation
  • Interstitial Fluid Flow

Background

Mechanical stimulation of bone cells is critical for maintaining bone mass and strength, and a better understanding of how mechanical stimuli are converted into intracellular signals to activate an anabolic program in osteoblasts/cytes is fundamental to improving treatments for osteo-degenerative diseases [1, 2]. Weight bearing and locomotion stimulate interstitial fluid flow through the bone canalicular system, and the resultant shear stress is thought to be a major mechanism whereby mechanical forces stimulate osteoblast/osteocyte growth and differentiation [1, 2]. In primary osteoblasts and osteoblast/cyte-like cell lines, fluid shear stress induces rapid expression of c-fos, fra-1, fra-2, and fosB/ΔfosB mRNAs [3]; these genes encode transcriptional regulators important for osteoblast proliferation and differentiation, as demonstrated by the phenotypes of mice that over-express or lack these proteins, respectively [4]. We have previously shown that fluid shear stress increases osteoblast/cyte nitric oxide (NO) production, leading to increased cGMP synthesis and activation of cGMP-dependent protein kinases (PKGs). The NO/cGMP/PKG signaling pathway is required for shear-induced expression of all four fos family genes, and induction of these genes is mediated through activation of the mitogen-activated protein kinases Erk1/2 [3]. However, molecular mechanisms leading to Erk activation in shear-stressed osteoblasts are largely unknown.

Results

We have now defined the events leading from shear stress activation of NO/cGMP/PKG II to the activation of Src [5]; we show that this pathway is required for Erk activation, and controls osteoblast proliferation and survival. We found a novel link between NO/cGMP/PKG and β3 integrins, the key mechano-sensors in bone, and show that PKG II activates β3-associated Src by activating the tyrosine phosphatase Shp-1. PKG II directly phosphorylates and stimulates Shp-1 activity, which de-phosphorylates a C-terminal, inhibitory phosphorylation site on Src, leading to Src activation. Fluid shear stress triggers PKG II, Src, and Shp-1/2 recruitment to a mechano-sensitive complex containing β3 integrins, defining a novel “mechanosome”. We found that PKG II-null mice have defective osteoblast Src/Erk signaling, decreased Erk-dependent gene expression in bone, and impaired osteoblast-dependent, membranous bone formation. These results complement previous studies in PKG II-deficient mice, which showed defective chondroblast differentiation and endochondral bone formation, and studies in NO synthase-deficient mice, which demonstrated an important role of NO in osteoblast biology [6, 7].

Conclusion

Our findings reveal crosstalk between NO/cGMP/PKG and integrins, establish a new mechanism of Src activation, and fill a gap in our understanding of how mechanical forces acting on cell-matrix adhesions are translated into cellular responses. Since Src controls Erk, a key regulator of osteoblast growth and survival, our results support using PKG-activating drugs as mechano-mimetics for treating osteoporosis.

Authors’ Affiliations

(1)
Department of Medicine, University of California at San Diego, La Jolla, CA 92093, USA
(2)
Institute for Pharmacology and Biomedical Center, University of Bonn, 53105 Bonn, Germany

References

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Copyright

© Rangaswami et al; licensee BioMed Central Ltd. 2011

This article is published under license to BioMed Central Ltd. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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