Biology Reference
In-Depth Information
osteoclast formation is the RANKL cytokine produced by osteoblasts in response to
hormonal stimulation. RANKL causes the CFU-M to commit to becoming an
osteoclast and soon becomes a mononucleated osteoclast. Osteoclasts must fuse
together and become multinucleated (three or more nuclei) and fully mature
before they are able to perform their bone-resorbing functions (Bar-Shavit
2007
;
Nakashima and Takayanagi
2009
).
Elevated cAMP, increased through treatment with PGE
2
, has been reported to
both increase and decrease osteoclast formation. These discrepancies seem to be
attributable to a number of factors: the dose of PGE
2
being administered, the timing
of PGE
2
administration, the type of cells being studied, and the methods used to
prepare osteoclasts and/or osteoblasts. It has been demonstrated that addition of a
cAMP-elevating agonist such as PGE
2
or forskolin or an inhibition of PDE activity
can increase osteoclast formation from either human or mouse bone marrow
cultures. Several groups have reported an increase in osteoclast formation from
bone marrow with PGE
2
(Fujita et al.
2003
; Noh et al.
2009
; Take et al.
2005
; Wani
et al.
1999
), 8-isoprostaglandin E2 (an oxidized form of PGE
2
) (Tintut et al.
2002
),
forskolin (Ransj
o et al.
1999
), a cAMP analog (Wani et al.
1999
), or PDE inhibition
(Noh et al.
2009
; Park and Yim
2007
; Takami et al.
2005
; Yamagami et al.
2003
).
This increase in osteoclast number and function can derive from two separate
mechanisms: the stimulation of osteoblasts to produce RANKL, and direct effects
on osteoclast precursors.
The concentration of PGE
2
is vitally important in the regulation of osteoclast
differentiation from bone marrow. Moderate doses of PGE
2
, ranging from 1 to
100 nM, seem to stimulate osteoclastogenesis in adherent cell-depleted hemato-
poietic monocultures in the presence of M-CSF and RANKL for both mouse and
human bone marrow cells (Fujita et al.
2003
; Ono et al.
2005
). One group reported a
synergistic increase in osteoclast formation when stimulated with RANKL and
1
m
M PGE
2
in bone marrow cultures (Wani et al.
1999
). However, several groups
report that when the concentration of PGE
2
is 1
m
M or higher, osteoclastogenesis is
inhibited and subsequent bone resorption is decreased (Fujita et al.
2003
; Ono et al.
2005
; Park and Yim
2007
). This does not hold true for all cell types as in
RAW 264.7 cells, a mouse monocyte-like cell line, PGE
2
concentrations at 1
m
M
stimulate osteoclast formation through PKA-dependent phosphorylation of TAK1
(Kobayashi et al.
2005a
).
A second model of osteoclast generation uses CD14+ monocytes from human
blood differentiated in the presence of M-CSF and RANKL to form osteoclasts.
This monocyte to osteoclast transformation is inhibited by PGE
2
at all concentra-
tions of PGE
2
tested, 0.1-100 nM (Itonaga et al.
1999
; Take et al.
2005
). However,
it was found that the timing of administration of PGE
2
also played a critical role in
the ability of osteoclast precursors to differentiate into osteoclasts. If PGE
2
was
added at days 0-2 of culture, differentiation was prevented, if added later in the
maturation process at days 6-8 there was no change in the number of mature
osteoclasts after 8 days (Take et al.
2005
). Similarly, the differentiation of bone
marrow cells was stimulated when PGE
2
was present during days 0-2 of culture,
but no change was observed when added at days 6-8.
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