CARDIOVASCULAR JOURNAL OF AFRICA • Vol 21, No 5, September/October 2010
AFRICA
261
sion in the Zn group was significantly increased (2.11
±
0.25
vs 1.59
±
0.16,
p
<
0.01).
Discussion
Blood vessel endothelium is regulated by both NO-dependent
and -independent vascular relaxing factors. Endogenous CO is
an NO-independent vascular relaxing factor. CO and NO play
complementary or equivalent physiological roles in the main-
tenance of normal physiological function of blood vessels.
8,9
NO has 50-fold more affinity to cGMP than CO, therefore, NO
plays the major role under physiological conditions.
10
During the
process of atherosclerosis development, the endothelial function
is impaired. Therefore, the endothelium-derived cNOS expres-
sion or activity as well as the production of endothelial NO is
decreased, whereas the activity of vascular smooth muscle cell
(VSMC)-expressed iNOS is increased,
11,12
resulting in increased
NO free-radical production, which stimulates cell apoptosis and
collagen degradation and promotes the development of athero-
sclerosis.
11,13
NO free radicals could also synthesise peroxynitrite
through a reaction with superoxide anion, thus causing further
tissue damage.
12
During the development of atherosclerosis, HO-1 shows
inducible expression, and the production of CO is increased. On
one hand, the activity and production of iNOS is inhibited and
tissue damage is reduced.
14
Possible mechanisms are based on
the fact that iNOS contains a haeme domain, the latter a substrate
of HO-1. (1) Increased activity of HO-1 therefore accelerates the
degradation of iNOS. (2) An iNOS active site needs two haem
molecules. The increased activity of HO-1 accelerates the degra-
dation of haem and decreases iNOS synthesis. (3) CO combines
with iNOS to make it inactive.
15
(4) Free iron released during
haeme decomposition further inhibits iNOS production by
inhibiting nuclear transcription. Increased CO enhances cGMP
levels, relaxes blood vessels and compensates for the insufficient
functioning of endothelial cNOS producing NO.
The relationship between all factors involved in atheroscle-
rotic plaque formation is shown in Fig. 5. Of significance is the
decrease in cNOS expression and activity in the pathological
condition. Our study showed that cNOS activity and NO produc-
tion were significantly decreased in atherosclerotic aortic tissue.
Simultaneously, HO-1 expression and activity and CO produc-
tion were significantly increased. The area of aortic plaque in
the Ch group was less than that in the Zn group. However, in the
Zn group, the HO activity and expression and CO production
were significantly decreased, the iNOS activity was signifi-
cantly increased, and the area of aortic plaque was the largest.
This suggests that the NOS/NO system was inhibited during
the atherosclerotic process, and that the HO/CO system had a
compensatory protective effect.
The HO inhibitor Znpp-IX could reduce the compensation of
the HO/CO system, thereby aggravating atherosclerosis. It can
be seen that the NOS/NO and HO/CO systems had regulatory
and compensatory effects on each other during the atherosclerot-
ic process. The HO/CO system inhibited atherosclerotic progres-
sion through the regulation and compensation for NOS and NO.
Therefore, the effect of the HO/CO system is more important in
pathological conditions such as atherosclerosis.
VSMCs proliferation is an important pathological feature of
atherosclerosis. ET-1 stimulates pro-oncogene expressions of
c-myc and c-fos and DNA synthesis in vascular smooth muscle
cells in a dose-dependent manner through strong and long-last-
ing vasoconstrictor effects, thereby promoting the proliferation
of VSMCs and participating in the formation of atherosclerosis.
16
With the co-culture of endothelial cells and VSMCs, Morita
et
al
.
6
found that HO-1 mRNA expression was increased in hypoxic
VSMCs, and that hypoxic VSMCs could inhibit the high expres-
sion of ET-1 and PDGF-B mRNA and the secretion of ET-1 in
hypoxia-induced human umbilical vein endothelial cells. This
effect may be removed by the strong HO inhibitor, ZnPP-IX, and
also by the CO scavenger, haemoglobin. Therefore, it is thought
that VSMC-derived CO could inhibit the expressions of ET-1
and PDGF-B in endothelial cells via paracrine factors, thereby
further inhibiting the self-proliferation of VSMCs.
Conclusion
In our study, after interference with atherosclerotic plaque forma-
tion by the HO inducer, haemin, HO-1 activity was significantly
increased, HO-1 mRNA and protein expression were signifi-
cantly increased, and CO generation also increased significantly,
whereas the ET-1 level and expressions of ET-1 mRNA and
protein were significantly reduced compared to all other groups.
The area of aortic plaque was significantly decreased, indicating
that the progression of atherosclerosis was effectively inhib-
ited. The inhibitory effect of haemin on ET-1 expression may
have been related to the inhibition of HO-1 on the cytochrome
P450 mono-oxygenase, on which ET-1 synthesis depends. ET-1
expression is related to VSMCs proliferation. Therefore the anti-
atherogenic target of the HO-1/CO system is further identified.
Our study indicated that the HO-1/CO system had anti-
atherosclerotic effects, which were not achieved through its
regulation of serum lipids or ox-LDL but was probably related to
regulation and compensation of the NOS/NO system and down-
regulation of ET-1 expression.
This work was supported by the National Science Foundation of China (No.
39800065) the National Science Foundation of Guizhou Province (No. 2006-
2074), and the Shengzhang Fund of Excellent Technological and Educational
Talents of Guizhou Province (No.2007-72). We thank Mr Qianglin Duan, a
skilled English proof-reader from Tongji Hospital, for language editing and
article revision.
Fig. 5. Diagram showing the relationship between factors involved in atherosclerotic plaque formation.
TC, TG, LDL-C, ox-LDL
ET-1, iNOS
eNOS/NO system
HO-1/CO system
Atherosclerosis
Induces
Down-regulates
Down-regulates
Activates
Compensates
Induces
