15-01-2013, 12:20 PM
Bone effects of vitamin D – Discrepancies between in vivo and in vitro studies
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ABSTRACT
Vitamin D was discovered as an anti-rachitic agent, but even at present, there is no direct evidence to support
the concept that vitamin D directly stimulates osteoblastic bone formation and mineralization. It
appears to be paradoxical, but vitamin D functions in the process of osteoclastic bone resorption. In
1952, Carlsson reported that administration of vitamin D3 to rats fed a vitamin D-deficient, low calcium
diet raised serum calcium levels. Since the diet did not contain appreciable amounts of calcium, the rise
in serum calcium was considered to be derived from bone. Since then, this assay has been used as a standard
bioassay for vitamin D compounds. Osteoclasts, the cells responsible for bone resorption, develop
from hematopoietic cells of the monocyte-macrophage lineage. Several lines of evidence have shown that
the active form of vitamin D3, 1a,25-dihydroxyvitamin D3 [1a,25(OH)2D3] is one of the most potent inducers
of receptor activator of NF-jB ligand (RANKL), a key molecule for osteoclastogenesis, in vitro. In fact,
1a,25(OH)2D3 strongly induced osteoclast formation and bone resorption in vitro. Nevertheless,
1a,25(OH)2D3 and its prodrug, Alfacalcidol (1a-hydroxyvitamin D3) have been used as therapeutic agents
for osteoporosis since 1983, because they increase bone mineral density and reduce the incidence of bone
fracture in vivo. Furthermore, a new vitamin D analog, Eldecalcitol [2b-(3-hydroxypropoxy)-1a,25
(OH)2D3], has been approved as a new drug for osteoporosis in Japan in January 2011.
Introduction
Bone remodeling is a dynamic process orchestrated by boneforming
osteoblasts and bone-resorbing osteoclasts. In normal
bone remodeling, osteoclastic bone resorption is followed by
osteoblastic bone formation through a coupling mechanism [1].
Osteoporosis is a common skeletal disease involving a decrease
in bone mineral density (BMD),1 bone quality, and bone strength
[2,3]. Osteoporosis is caused by an imbalance of bone resorption
and bone formation, with the former exceeding the latter.
Discovery of the RANKL–RANK–OPG system
In 1981, Rodan and Martin [20] proposed that osteoblasts or
bone marrow stromal cells may intervene in the process of osteoclastic
bone resorption. Their argument such a mechanism was
based on the observations that first, bone-resorbing hormones
and cytokines have their receptors in osteoblastic cells but not in
osteoclasts, and second, the relative binding potencies of these
bone-resorbing factors to their respective receptors in osteoblasts
resemble those in inducing bone resorption. Based on the concept
proposed by Rodan and Martin, Takahashi et al. [21] established an
efficient mouse co-culture system of primary osteoblasts isolated
from calvaria and spleen cells to recruit osteoclasts. A number of
multinucleated osteoclasts were formed in response to
1a,25(OH)2D3 in this co-culture system. Cell-to-cell contact between
spleen cells and osteoblastic cells appeared important for
both osteoclast formation and activation [21].
In 1992, we proposed a working hypothesis for the mechanism of
osteoclastogenesis based on the extensive studies using the co-culture
system [22] (Fig. 2). Various bone-resorbing factors including
1a,25(OH)2D3, PTH and interleukin 6 (IL-6) together with soluble
IL-6 receptor (IL-6 + sIL-6R) appeared to act commonly on osteoblastic
cells, but not hematopoietic osteoclast precursors in the cocultures.
These bone-resorbing factors were classified into three categories
in terms of their signal transduction pathways: VDR-mediated
signals [1a,25(OH)2D3, protein kinase A-mediated signals
[PTH, prostaglandin E2 (PGE2)], and gp130-mediated signals [IL-6,
IL-11 oncostatin M (OSM), and leukemia inhibitory factor (LIF)].
We proposed that a membrane-bound factor named osteoclast differentiation
factor (ODF) which is commonly induced on the plasma
membrane of osteoblastic cells in response to these bone-resorbing
factors, mediates an essential signal to osteoclast progenitors for
their differentiation into mature osteoclasts [22]. Chambers
Discrepancy of bone effect of vitamin D compounds between
in vivo and in vitro
As described above, 1a,25(OH)2D3 is one of the most potent
inducers of RANKL in vitro. Nevertheless, 1a,25(OH)2D3 and its
pro-drug, 1a(OH)D3, have been used as therapeutic agents for osteoporosis,
since they increase BMD and reduce the incidence of
bone fracture in vivo. Daily administration of Eldecalcitol increased
BMD and reduced the incidence of bone fracture with greater efficacy
than Alfacalcidol [1a(OH)D3] [68]. Thus, there is a big discrepancy
between in vitro and in vivo effects of vitamin D compounds
on bone resorption (Fig. 8). How do we explain the discrepancies
of bone effects of vitamin D compounds between in vivo and
in vitro?
There are three possible explanations for the discrepancies between
in vivo and in vitro studies: the first, vitamin D compounds
may decrease the number of osteoclast precursors in vivo, the second,
they decrease RANKL expression in osteoblasts in vivo, and the
third, the differences of the concentration of vitamin D compounds
used in in vivo and in vitro studies.
Conclusion
Daily in vivo administration of active analogs of vitamin D3
inhibits osteoclast differentiation by suppressing RANKL expression
in osteoblasts. Eldecalcitol is more effective than Alfacalcidol
[1a(OH)D3] in inhibiting bone resorption in vivo. This may be
due to the fact that the higher affinity of Eldecalcitol for DBP extends
its half life in plasma. We raise the possibility that the
administration of Eldecalcitol in vivo reduces the number of
RANKL-producing osteoblasts in trabecular bone. Control of the
number of RANKL-producing osteoblastic cells by vitamin D
compounds appears physiologically important to maintain serum
calcium homeostasis.