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J. Cell Biol.,
Volume 145, Number 3, May 3, 1999 527-538








* Department of Mammalian Cell Molecular Biology, Osteoprotegerin (OPG) and OPG-ligand
(OPGL) potently inhibit and stimulate, respectively, osteoclast differentiation (Simonet, W.S., D.L. Lacey, C.R.
Dunstan, M. Kelley, M.-S. Chang, R. Luethy, H.Q.
Nguyen, S. Wooden, L. Bennett, T. Boone, et al. 1997. Cell. 89:309-319; Lacey, D.L., E. Timms, H.-L. Tan,
M.J. Kelley, C.R. Dunstan, T. Burgess, R. Elliott,
A. Colombero, G. Elliott, S. Scully, et al. 1998. Cell. 93:
165-176), but their effects on mature osteoclasts are not
well understood. Using primary cultures of rat osteoclasts on bone slices, we find that OPGL causes approximately sevenfold increase in total bone surface erosion. By scanning electron microscopy, OPGL-treated
osteoclasts generate more clusters of lacunae on bone
suggesting that multiple, spatially associated cycles of
resorption have occurred. However, the size of individual resorption events are unchanged by OPGL treatment. Mechanistically, OPGL binds specifically to mature OCs and rapidly (within 30 min) induces actin ring
formation; a marked cytoskeletal rearrangement that
necessarily precedes bone resorption. Furthermore, we
show that antibodies raised against the OPGL receptor,
RANK, also induce actin ring formation. OPGL-treated mice exhibit increases in blood ionized Ca++
within 1 h after injections, consistent with immediate
OC activation in vivo. Finally, we find that OPG blocks
OPGL's effects on both actin ring formation and bone
resorption. Together, these findings indicate that, in addition to their effects on OC precursors, OPGL and
OPG have profound and direct effects on mature OCs
and indicate that the OC receptor, RANK, mediates
OPGL's effects.
Department of Pathology, § Department of Protein Chemistry, and
Department of Cell Biology, Amgen Inc., Thousand Oaks, California 91320-1789
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