Effects of menaquinone-4 on bone metabolism

Akedo et al. (1992) reported that MK-4 suppresses
the proliferation of osteoblastic cells in vitro. Warfarin
reversed this effect, implicating the γ-carboxylation
system in the modulation of proliferation. Koshihara
et al. (1996) reported that MK-4 enhanced
1,25-dihydroxyvitamin D3-induced mineralization
by human osteoblasts in vitro. This was due to enhanced
γ-carboxylation of the osteocalcin induced
by 1,25-dihydroxyvitamin D3, and accumulation of
carboxylated osteocalcin in the extracellular matrix,
causing mineralization (Koshihara & Hoshi, 1997).
Hara et al. (1993) reported that MK-4 inhibited the
bone resorption induced by interleukin-1α, prostaglandin
E2, parathyroid hormone and 1,25-dihydroxyvitamin
D3 in a dose-dependent manner in vitro.
MK-4 also inhibited the prostaglandin E2 production
stimulated by interleukin-1α. Koshihara et al. (1993)
showed that MK-4-induced inhibition of prostaglandin
synthesis in cultured human osteoblast-like
periosteal cells was reduced by cycloheximide, indicating
that newly synthesized protein participates in
the inhibitory effect.
Akiyama et al. (1994) examined the effects of MK-
4 on osteoclast-like multinucleated cell formation in
bone marrow cell cultures. MK-4 showed the most potent
inhibitory effect on cell formation when present
in cultures during the last 3 days, suggesting that the
vitamin blocks cell differentiation and/or cell fusion.
MK-4 did not infl uence 1,25-dihydroxyvitamin D3-
induced osteoclast-like cell formation when present
in the culture during the fi rst 4 days, indicating that
it does not affect proliferation of osteoclast precursor
cells. MK-4 did not affect the proliferation of many
other cell types in the bone marrow culture, suggesting
that the observed inhibitory effect of MK-4 on
osteoclast-like cells was not a result of cytotoxicity.
Hara et al. (1995) compared the effects of phylloquinone
and MK-4 on bone resorption in vitro.
Calcium concentration in the medium was used as
a parameter of bone resorption. MK-4 inhibited the
calcium release from mouse calvaria organ cultures
induced by 1,25-dihydroxyvitamin D3 or prostaglandin
E2, and it also inhibited osteoclast-like cell
formation induced by 1,25-dihydroxyvitamin D3
in co-culture of spleen cells and stromal cells at the
same concentrations. In contrast, the same doses of
phylloquinone had no effects on bone resorption and
osteoclast-like cell formation in these in vitro systems.
The inhibitory effect of MK-4 on the calcium release
from calvaria was not affected by the addition of warfarin,
suggesting that the effect of MK-4 is not due to
γ-carboxylation coupling with the vitamin K epoxide
cycle. The structures of MK-4 and phylloquinone differ
only in their side chains (see Fig. 10.1), therefore
whether the difference in their effects is related to
the differences in side chain structure was evaluated
in the co-culture system. Geranylgeraniol inhibited
osteoclast-like cell formation to almost the same degree
as MK-4, whereas the effect of phytol was weak.
Moreover, multi-isoprenyl alcohols of two to seven
units, except the four-unit geranylgeraniol, did not
affect osteoclast-like cell formation. Thus the specifi c
inhibitory effect of MK-4 is attributable to the geranylgeranyl
side chain.
Kameda et al. (1996) demonstrated that MK-4, but
not phylloquinone, inhibits bone resorption by targeting
osteoclasts to undergo programmed cell death
(apoptosis). MK-4 did not induce apoptosis in other
cell types in unfractionated bone cells. Calcitonin,
which strongly inhibits osteoclastic bone resorption
via calcitonin receptors, did not cause osteoclast apoptosis.
MK-4 might be an appropriate therapeutic
drug against bone diseases with excess bone resorp-
Vitamin K 267
tion, because of its selective and direct induction of
osteoclast apoptosis.
Clinical use of menaquinone-4 in osteoporosis
A number of Japanese studies have claimed benefi cial
results using synthetic MK-4 (menatetranone) in the
treatment of osteoporosis. The rationale includes the
possibility that MK-4 may have different effects on
bone metabolism than phylloquinone. The dosage
currently used (45 mg per day) is far in excess of daily
vitamin K requirements and any effect must be regarded
as pharmacological rather than a dietary correction
of a nutritional defi ciency. MK-4 was shown
to be effective in increasing bone mineral density of
cortical bone in osteoporotic patients (Orimo et al.,
1998) as well as preventing the occurrence of new
fractures and sustaining lumbar bone mineral density
(Shiraki et al., 2000). In the latter study, MK-4 treatment
enhanced γ-carboxylation of the osteocalcin
molecule. There were no signifi cant changes in bone
resorption markers, therefore the prevention of bone
fractures by MK-4 may not be caused entirely by inhibition
of bone resorption.