CARDIOVASCULAR JOURNAL OF AFRICA • Vol 23, No 6, July 2012
e8
AFRICA
that p53 has an inhibitory effect on osteogenic differentiation of
VSMCs in CKD mice, as evidenced by an increase in markers
of osteogenic differentiation and a decrease in expression of the
smooth muscle-specific marker. However, the mechanisms by
which p53 inhibited osteogenic differentiation of VSMCs were
not clear.
We then investigated the possible mechanisms whereby
p53 inhibited osteogenic differentiation of VSMCs in CKD
mice. Previous studies suggest that vascular calcification and
bone formation may share common regulatory mechanisms.
25-28
In bone, BMP-2 promotes osteoblast differentiation and
mineralisation. BMP-2 is a potent osteogenic protein required
for osteoblast differentiation and bone formation, which has
been implicated in vascular calcification.
25
BMP-2 promoted
transition of the osteochondrogenic phenotype of VSMCs, also
evidenced by an increase in marker expression of RUNX2 and a
decrease in marker expression of VSMC.
25
Runx2 is a transcription factor critical for osteogenesis
and bone formation and is expressed during ectopic vascular
calcification. A study by Tanaka
et al.
29
demonstrated that
RUNX2 could repress myocardin-induced differentiation and
concomitantly promote the osteogenic conversion of VSMCs.
The expression level or activity of the RUNX2 protein is critical
for the osteoblastic differentiation of VSMCs.
Osx is genetically downstream of RUNX2.
30
The studies by
Wang
et al
.
31
and Lengner
et al.
12
provide compelling evidence
that p53 suppresses osteoblast differentiation by repressing the
expression of either RUNX2 or Osx. The subtle discrepancy
that exists between the two studies (whether RUNX2 or Osx
is the target of p53 action) may be related to how p53 activity
is targeted and whether this mechanism alters the stage of cell
differentiation. In either case, the concept is that the absence of
a tumour-suppressor gene can enhance cell proliferation while
favouring differentiation.
30
Fujita
et al
.
32
found that BMP-2 could induce new bone
formation
in vivo
by the BMP–p53–Cbfa1–Osterix axis in
the osteoblast lineage. p53 is a negative regulator of Osx, and
osteoblasts deficient in p53 exhibited an enhanced ability to
promote osteoblast-dependent osteoclastogenesis.
32
In our study, no expression of BMP-2 was found in the
VSMCs from sham-operated mice, but in the VSMCs from
p53–/– mice, positive staining of BMP-2 proteins was increased
along with the increase in expression of RUNX2, Osx and ALP.
Conversely, positive staining of BMP-2 proteins was decreased
along with the decrease in expression of RUNX2, Osx and ALP
in p53
+
/
+
mice. The level of expression of BMP-2 correlated
negatively with that of p53, while
α
-SMA correlated negatively
with the level of expression of calcification-related proteins and
positively with that of p53 (Tables 2, 3, 4).
From the data above, our results imply that p53 may repress
osteogenic differentiation of VSMCs in CKD by inhibiting the
expression of BMP-2 and/or RUNX2 and Osx protein directly.
This indicates that the BMP–p53–Cbfa1(RUNX2)–Osterix axis
in osteoblast-dependent osteoclastogenesis is involved in the
mechanism of osteogenic differentiation of VSMCs in CKD
mice.
Clinically, a percentage of CKD patients in dialysis do not
have vascular calcification and also do not show development
or progression of calcification, implying that there are most
likely genetic factors that predispose to protection from vascular
calcification. p53 may play a role in these patients. The precise
molecular targets of p53 in these mechanism remains to be
elucidated in further
in vitro
study.
Conclusion
We found that a p53 deficiency resulted in phenotype changes
and elevated phosphate-induced mineralisation in VSMCs
from 5/6 Nx mice. The effect of p53 repressing osteogenic
differentiation of VSMCs is most likely to be mediated in part
by down-regulation of BMP-2 expression and/or expression of
RUNX2 and Osx directly. This study has provided a possible
novel mechanism to show that p53 could be a negative regulator
of osteogenic differentiation of VSMCs in CKD mice. It raises
an interesting question regarding future therapeutic strategies
for vascular calcification in CKD patients by up-regulating the
expressson of p53.
This study was supported by the Natural Science Foundation Project of CQ
CSTC (no. CSTC, 2008BB5275).
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