Journal of Biological Chemistry
Volume 286, Issue 46, 18 November 2011, Pages 39871-39881
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Cell Biology
Osteopontin Signals through Calcium and Nuclear Factor of Activated T Cells (NFAT) in Osteoclasts: A NOVEL RGD-DEPENDENT PATHWAY PROMOTING CELL SURVIVAL*

https://doi.org/10.1074/jbc.M111.295048Get rights and content
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Osteopontin (OPN), an integrin-binding extracellular matrix glycoprotein, enhances osteoclast activity; however, its mechanisms of action are elusive. The Ca2+-dependent transcription factor NFATc1 is essential for osteoclast differentiation. We assessed the effects of OPN on NFATc1, which translocates to nuclei upon activation. Osteoclasts from neonatal rabbits and rats were plated on coverslips, uncoated or coated with OPN or bovine albumin. OPN enhanced the proportion of osteoclasts exhibiting nuclear NFATc1. An RGD-containing, integrin-blocking peptide prevented the translocation of NFATc1 induced by OPN. Moreover, mutant OPN lacking RGD failed to induce translocation of NFATc1. Thus, activation of NFATc1 is dependent on integrin binding through RGD. Using fluorescence imaging, OPN was found to increase the proportion of osteoclasts exhibiting transient elevations in cytosolic Ca2+ (oscillations). OPN also enhanced osteoclast survival. The intracellular Ca2+ chelator 1,2-bis(O-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA) suppressed Ca2+ oscillations and inhibited increases in NFATc1 translocation and survival induced by OPN. Furthermore, a specific, cell-permeable peptide inhibitor of NFAT activation blocked the effects of OPN on NFATc1 translocation and osteoclast survival. This is the first demonstration that OPN activates NFATc1 and enhances osteoclast survival through a Ca2+-NFAT-dependent pathway. Increased NFATc1 activity and enhanced osteoclast survival may account for the stimulatory effects of OPN on osteoclast function in vivo.

Bone
Calcium Imaging
Cell Death
NFAT Transcription Factor
Osteoclast
Osteopontin

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*

This work was supported by the Canadian Institutes of Health Research (CIHR).

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1–S3 and Videos S1–S3.

1

Supported in part by the Sato Fund, Nihon University School of Dentistry, Tokyo, Japan.

2

Recipient of a Frederick Banting and Charles Best Canada Graduate Scholarship from the CIHR.