Cellular activity and signaling induced by osteoprotegerin in
osteoclasts: involvement of receptor activator of nuclear
factor nB ligand and MAPK
S. Theoleyre
a,1
, Y. Wittrant
a,1
, S. Couillaud
a
, P. Vusio
b
, M. Berreur
a
,
C. Dunstan
c
, F. Blanchard
a
,F.Re
´
dini
a
, D. Heymann
a,
*
a
EE 99-01, Pathophysiology of Bone Resorption Laboratory and Therapy of Primitive Bone Tumors,
Medicine Faculty, 1 rue G. Veil, 44035 Nantes cedex 01, France
b
IFR 26, Institut de Biologie, Nantes Hospital, France
c
Amgen Inc., One Amgen Center Drive, Thousand Oaks, CA 91320, USA
Received 22 July 2003; received in revised form 19 September 2003; accepted 15 October 2003
Abstract
Osteoprotegerin (OPG) is a decoy receptor for receptor activator of nuclear factor nB ligand (RANKL), an inducer of osteoclastogenesis
via its receptor RANK. We recently demonstrated that OPG also exerts a direct effect in osteoclasts by regulating protease expression. Herein,
we showed that OPG-induced pro-matrix metalloproteinase-9 activity was abolished by ras/MAPK inhibitors in purified osteoclasts. OPG
induced the phosphorylation of p38 and ERK1/2 in RAW264.7 cells. Only p38 activation was totally abolished by a blocking anti-RANKL
antibody or an excess of RANKL. Surface plasmon resonance experiments revealed that RANK, RANKL and OPG are able to form a tertiary
complex. These results suggested a potential formation of a tertiary complex RANK –RANKL–OPG on osteoclasts. Thus, OPG is not only a
soluble decoy receptor for RANKL but must be also considered as a direct effector of osteoclast functions.
D 2003 Elsevier B.V. All rights reseved.
Keywords: Osteoprotegerin; RANKL; Osteoclast; Metalloproteinase; Signaling
1. Introduction
Among the cellular and molecular parameters involved in
the bone resorption process [1–3], receptor activator of
nuclear factor nB ligand (RANKL), a member of the tumor
necrosis factor family is the main regulator of osteoclast
differentiation [4,5]. Within the bone system, soluble and
membrane forms of RANKL expressed by osteoblasts exert
their activities through binding to their receptor RANK on
osteoclasts [6,7]. The third protagonist, osteoprotegerin
(OPG) produced by osteoblasts acts as a decoy receptor
for RANKL, preventing it from binding to and activating
RANK [8]. It also inhibits the development of osteoclasts
[9] and down-regulates the RANKL signaling through
RANK [8]. The biological effects of OPG on bone cells
include the inhibition of terminal stages of osteoclast
differentiation, suppression of mature osteoclast activation,
and induction of apoptosis [7–9].
We recently demonstrated that OPG can also exert a
direct biological effect in osteoclasts by inducing proteases
and protease inhibitors expression suggesting a more com-
plex regulation of bone resorption by OPG than originally
described [10]. These results are consistent with those
obtained by Hakeda et al. [11] who reported a direct effect
of OPG on isolated osteoclasts via a 140 kDa OPG-binding
protein on cell membrane. Whereas OPG exhibits no
transmembrane domain, Yun et al. [12] reported the pres-
ence of a membrane-bound form of OPG in dendritic cells
that may correspond to either a matrix-bound or/and a
transmembrane form of the protein. More recently, Standal
et al. presented evidence that myeloma cells internalize and
0167-4889/$ - see front matter D 2003 Elsevier B.V. All rights reseved.
doi:10.1016/j.bbamcr.2003.10.005
* Corresponding author. Tel.: +33-2-4041-2845; fax: +33-2-4041-
2860.
E-mail address: dominique.heymann@sante.univ-nantes.fr
(D. Heymann).
1
Equally contributed to the work.
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Biochimica et Biophysica Acta 1644 (2004) 1 – 7