Publications

UNLABELLED

Integrin alphavbeta5 is expressed on osteoclast precursors and is capable of recognizing the same amino acid motif as alphavbeta3. Three-month-old beta5(-/-) female OVX mice had increased osteoclastogenesis ex vivo, and microCT assessment of trabecular bone volume was 53% lower than WT-OVX animals. These preliminary data suggest alphavbeta5 integrin's presence on osteoclast precursors may inhibit of osteoclast formation.

INTRODUCTION

Osteoclasts are unique resorptive skeletal cells, capable of degrading bone on contact to the juxtaposed matrix. Integrin alphavbeta5 is expressed on osteoclast precursors, structurally similar to alphavbeta3, and capable of recognizing the same amino acid motif. Given the structural relationship and reciprocal regulation of alphavbeta3 and alphavbeta5, the purpose of this study was to evaluate how alphavbeta5 might contribute to osteoclast maturation and activity.

MATERIALS AND METHODS

Three-month-old wildtype (WT) and beta5(-/-) female mice had ovariectomy (OVX) or sham operations. The osteoclastogenic capacity of marrow-derived precursors, the kinetic, the circulating, and structural parameters of bone remodeling, was determined after 6 weeks of paired feeding.

RESULTS AND CONCLUSIONS

OVX increased osteoclastogenesis ex vivo and in vivo. Osteoclast formation and prolonged pre-osteoclast survival were substantially enhanced in cultures containing beta5(-/-) cells whether obtained from sham-operated or OVX mice. Expression of cathepsin K, beta3 integrin subunit, and calcitonin receptor were accelerated in cultured beta5(-/-)osteoclasts. beta5(-/-) osteoclasts from OVX animals showed a 3-fold enhancement of net resorptive activity, with quantitative muCT showing trabecular bone volume loss after OVX 53% greater in beta5(-/-) OVX compared with similarly treated WT OVX mice (p < 0.05). alpha5beta3 seems to be an inhibitor of osteoclast formation, in contrast to alphavbeta3. In addition, loss of alphavbeta5 seems to accelerate osteoclast formation in the OVX model. Further examination of alphavbeta5 signaling pathways may enhance our understanding of the activation of bone resorption.

BACKGROUND

Phosphorylation of sphingosine by sphingosine kinase (SK) is the rate-limiting step in the cellular synthesis of sphingosine 1-phosphate (S1P). The monoganglioside GM1, which stimulates SK, is cardioprotective in part through increased generation of S1P that protects myocytes by diverse mechanisms. Because protein kinase C (PKC)epsilon activation is necessary for myocardial ischemic preconditioning (IPC) and PKC activators increase SK activity, we tested the hypothesis that SK may be a central mediator of IPC.

METHODS AND RESULTS

In adult murine hearts, IPC sufficient to reduce infarct size significantly increased cardiac SK activity, induced translocation of SK protein from the cytosol to membranes, and enhanced cardiac myocyte survival. IPC did not increase SK activity in PKCepsilon-null mice. The SK antagonist N,N-dimethylsphingosine inhibited PKCepsilon activation and directly abolished the protective effects of IPC and the enhanced SK activity induced by IPC.

CONCLUSIONS

These findings demonstrate that PKCepsilon is thus recruited by IPC and induces activation of SK that then mediates IPC-induced cardioprotection in murine heart.