Integrative Physiology |
From the Department of Biochemistry and Molecular Biology, Medical University of South Carolina, Charleston, SC.
Correspondence to Julie Chao, Department of Biochemistry and Molecular Biology, Medical University of South Carolina, Charleston, SC 29425-2211. E-mail chaoj{at}musc.edu
| Abstract |
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Key Words: kallistatin vascular smooth muscle cell neointima proliferation migration
| Introduction |
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Kallistatin is a serine proteinase inhibitor (serpin), which was first discovered as a tissue kallikrein binding protein (KBP) and was capable of inhibiting the enzymatic activity of kallikrein.5 The structure and organization of kallistatin gene are similar to those of other serpins, with typically 5 exons and 4 introns.6 Two putative activator protein-1 (AP-1) binding sites and hormone response elements were identified in its 5'-flanking region.7 The expression of kallistatin in rats was upregulated in the liver by estrogen, progesterone, growth hormone, and thyroxin7 8 9 and was induced in crushed muscle tissues after injury.10 In addition to its function as a proteinase inhibitor, kallistatin also has a potent vasodilatory effect on rat vasculature.11 Kallistatin reduced mean arterial blood pressure in anesthetized rats and renal perfusion pressure in isolated rat kidneys and induced vasorelaxation in rat aortic rings.11 Specific kallistatin-binding sites in the aortic membrane proteins were identified by a kallistatin-ligand binding assay.11 Furthermore, kallistatin was localized in the endothelial and smooth muscle cells of human blood vessels of various sizes.12 These findings suggest that kallistatin may play a role in the regulation of vascular function in autocrine and/or paracrine mechanisms. To explore the potential roles of kallistatin in vascular biology, we evaluated the effects of exogenous kallistatin on VSMC proliferation and migration in vitro as well as the expression and localization of endogenous kallistatin in balloon-injured rat arteries in vivo. The results of the present study provide new insights into the biological function of kallistatin in vascular cell growth and migration.
| Materials and Methods |
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Preparation of Adenovirus Carrying Rat Kallistatin Antisense
cDNA
Rat kallistatin cDNA7 was cloned in antisense
orientation into the adenoviral shuttle vector
pAdTrack-CMV.14 The resultant adenoviral plasmid carrying
rat kallistatin antisense cDNA or GFP under control of CMV
promoter/enhancer along with all Ad5 sequences except for the E1 and E3
genes was transfected into human embryonic kidney 293 packaging cells
(Quantum, Quebec, Canada). Large quantities of high-titer adenoviruses
were produced in human embryonic kidney 293 cells and purified by CsCl
banding as previously described.15
Primary Aortic Smooth Muscle Cell Culture
Rat primary VSMCs were isolated from the normal aorta of male
Sprague-Dawley rats (200 to 250 g) by the combined
collagenase and elastase digestion
method.16 Cells were serially passaged and used between
passages 3 and 10. Intimal smooth muscle cells were isolated from
neointima in injured aorta at 2 weeks after balloon
angioplasty as described.17
[3H]Thymidine Incorporation
Quiescent VSMCs in 24-well plates were treated with different
concentrations of kallistatin in serum-free medium for 18 hours and
then pulse-labeled with 1 µCi/mL of
[3H]thymidine (DuPont NEN) for another 6 hours.
Cells were then washed 3 times with PBS, precipitated with 10%
trichloroacetic acid at 4°C for 30 minutes, washed twice with 95%
ethanol, solubilized with 0.25 mol/L NaOH plus 0.1% SDS, and
neutralized with 1 mol/L acetic acid. Radioactivity was determined
using a liquid scintillation counter (Packard).
Cell Migration Assays
VSMC migration was assessed using modified Boyden chambers
(Corning Inc),18 19 which were coated with a solution of 5
µg/mL fibronectin and 100 µg/mL type I collagen (Sigma). VSMCs
(2x105 cells) suspended in DMEM containing 0.1%
BSA were added to the upper chamber, and tested samples were placed in
the bottom chamber. After 4 hours of incubation at 37°C, cells were
fixed and stained with hematoxylin and eosin. The average number of
cells from 4 randomly chosen high-power (x400) fields on the lower
surface of the filter was counted.
Statistical Analysis
Statistical significance was determined by 1-way ANOVA with the
Fisher multiple comparison test. All data are expressed as mean±SEM,
and differences are considered significant at a value of
P<0.05.
An expanded Materials and Methods section is available online at http://www.circresaha.org.
| Results |
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Induction of Endogenous Kallistatin Expression in
Balloon-Injured Artery
The expression of endogenous kallistatin in rat aortas
after balloon angioplasty was analyzed by competitive
polymerase chain reaction (PCR) and ELISA specific for rat kallistatin.
The representative competitive PCR image was shown in
Figure 2A
and kallistatin mRNA levels
calculated from the linear regression plot of the ratio plotted
logarithmically against the initial input of competitor DNA were shown
in Figure 2B
. At 1 week after balloon angioplasty,
kallistatin mRNA levels in rat abdominal aortas increased up to 10-fold
as compared with control sham-operated rats (11.43±1.05 versus
0.91±0.05 pg/µg total RNA, Figure 2
). Immunoreactive
kallistatin levels in the aortas were increased by 2.3-fold and
1.9-fold, respectively, at 1 and 2 weeks after balloon angioplasty as
compared with control sham-operated rats (n=5 and 3,
P<0.01, Figure 3
).
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Expression and Cellular Localization of Kallistatin in Rat Artery
After Balloon Angioplasty
To further explore the role of rat kallistatin in
neointima formation after balloon angioplasty,
time-dependent expression and cellular localization of rat kallistatin
mRNA in balloon-injured carotid arteries were identified by in situ
hybridization using the kallistatin antisense riboprobe (Figure 4
). Normal artery showed a very weak
hybridization signal in the medial layer (data not shown). In the
injured artery at 2 days after balloon angioplasty, a strong
hybridization signal was noted in the media, particularly the inner
layer. At 7 and 14 days after balloon angioplasty, intense staining of
kallistatin mRNA was identified in the neointima with
relatively low expression in the underlying media (Figure 4
).
Kallistatin mRNA was identified in the cytoplasm or around nuclei of
the proliferating VSMCs. Only background levels of nonspecific
hybridization signals were detected in the serial sections stained with
the kallistatin sense riboprobe (Figure 4
) or in the RNase
Apretreated sections stained with the kallistatin antisense riboprobe
(data not shown). These negative results confirmed the specificity of
in situ hybridization signals of these experiments. Proliferating cell
nuclear antigen (PCNA) and smooth muscle
-actin in the proliferating
VSMCs of balloon-injured rat arteries were identified
immunohistochemically using their respective antibodies (Figure 4
). The results show that the site of kallistatin expression was
spatially and temporally colocalized with PCNA and
-actin in serial
sections.
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Adenovirus-Mediated Kallistatin Antisense cDNA Delivery Inhibited
Kallistatin mRNA Expression and Neointima Formation in the
Balloon-Injured Artery
To further investigate the role of kallistatin in
neointima formation in vivo, adenovirus Ad.CMV-AS.KBP or
control virus Ad.CMV-GFP was delivered locally into the balloon-injured
rat carotid arteries. Competitive PCR showed that adenovirus-mediated
delivery of kallistatin antisense cDNA significantly reduced
kallistatin mRNA levels to those of sham rats (n=4, P<0.01,
Figure 5
). Reduction of kallistatin
expression by antisense inhibition was accompanied by significant
suppression of neointima formation in balloon-injured
arteries when compared with that in those arteries infected with
control virus (cross-sectional area: 83±5 [n=7] versus 115±11
µm2 [n=4], mean±SEM, P<0.01,
Figure 6A
). There was a 25% reduction in
intima/media ratio in rats receiving kallistatin antisense cDNA
delivery as compared with rats receiving or not receiving control virus
at 2 weeks after balloon angioplasty (0.90±0.04 [n=7] versus
1.17±0.05 [n=4] or 1.20±0.08 [n=6], mean±SEM,
P<0.01, Figure 6B
). No statistical difference was
found between injured rat carotid arteries after angioplasty with or
without control virus infection.
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PDGF-BB Increased Endogenous Kallistatin Synthesis and
Kallistatin Antisense cDNA Inhibited PDGF-BBInduced Cell
Proliferation and p42/44 Mitogen-Activated Protein Kinase
(MAPK) Activity in Cultured VSMCs
PDGF-BB markedly induced endogenous kallistatin
synthesis in cultured VSMCs as compared with the control (304±23
versus 57±13 pg/mg total protein [n=3], P<0.01). To
confirm the possibility that kallistatin may be involved in the
PDGF-induced MAPK pathway on the proliferation of VSMCs, cell
proliferation and MAPK activity in VSMCs with or without infection of
Ad.CMV-AS.KBP or Ad.CMV-GFP were examined after treatment of PDGF-BB or
kallistatin. Figure 7
shows that PDGF-BB
stimulated proliferation of VSMCs with or without control virus
infection, whereas adenovirus-mediated transfer of kallistatin
antisense cDNA attenuated PDGF-BBinduced VSMC proliferation as
compared with the control (n=4, P<0.01). Furthermore,
Figure 8
shows that kallistatin
and PDGF-BB increased MAPK activity by 2- and 3-fold, respectively,
compared with the control basal level, whereas kallistatin antisense
cDNA inhibited 35% of PDGF-BBinduced MAPK activity (n=3,
P<0.01). No significant downregulation of MAPK activity was
observed in VSMCs treated with PDGF-BB and infected with control virus.
Specific MAPK induced by kallistatin in VSMCs was further identified by
Western blot analysis using phosphospecific MAPK antibodies.
Activation of p42/44 MAPK, but neither p38 kinase nor stress-activated
protein kinase/c-Jun N-terminal kinase (SAPK/JNK), was induced
in VSMCs treated with kallistatin. Kallistatin increased
phosphorylation of p42/44 MAPK compared with the
control, and kallistatin antisense cDNA inhibited PDGF-BBinduced
phosphorylation of p42/44 MAPK (Figure 9A
). The levels of total p42/44 MAPK were
identical in all of the samples (Figure 9B
).
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| Discussion |
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-actin in proliferating VSMCs
during cell migration from media to neointima. Inhibition
of kallistatin expression by its antisense cDNA significantly
suppressed neointima formation in the balloon-injured
artery in vivo. The potential role of kallistatin as a growth factor or
a mediator of growth factors in neointima formation was
further confirmed by in vitro studies. Exogenous kallistatin
significantly stimulated the proliferation and migration of cultured
primary VSMCs, and antisense inhibition of kallistatin expression
attenuated PDGF-induced p42/44 MAPK activity and proliferation in
VSMCs. These results indicate that kallistatin in proliferative VSMCs
may function as a mediator of growth factors in the pathogenesis of
vascular injury. In this study, we show that both mRNA and protein levels of endogenous kallistatin were markedly increased at the injured sites after balloon angioplasty. This indicates that kallistatin is upregulated at the transcriptional level in the injured vessels. The elevated expression of kallistatin mRNA in the proliferating VSMCs during the process of neointima formation was also identified by in situ hybridization histochemistry. In response to mitogens and growth factors stimulated by vascular injuries, medial VSMCs may enter into the growth cycle between 2 and 3 days after balloon angioplasty. The majority of medial cells would complete their proliferation and migration within 7 days after angioplasty, whereas the neointima area would show a dramatic increase from 7 to 14 days. No significant growth of neointima occurs beyond 14 days after balloon angioplasty in rats.2 20 The intimal cell proliferation contributes considerably to the subsequent accumulation of neointima mass.20 Our results show that the sites of kallistatin expression were localized in the media at 2 days after angioplasty and in neointima at 7 and 14 days after angioplasty during the migration of proliferative VSMCs from media to neointima. The time and spatial coordination between kallistatin expression and cellular proliferation suggests that kallistatin may play the important role of autocrine growth factors in mediating VSMC proliferation and migration and in neointima formation after balloon angioplasty.
Unlike other mitogens, such as PDGF, bFGF, and epidermal growth
factor,17 kallistatin not only stimulated medial VSMC
proliferation but also had mitogenic effects on intimal
smooth muscle cell proliferation. A previous study showed that
50%
of medial VSMCs activated by balloon injury migrated and
underwent division. These cells constituted eight ninths of the final
neointimal cell population, whereas the other 50% migrated
without further proliferation and made up one ninth of the
neointimal cell population.20 These results
suggest that balloon injury stimulates a proportion of the medial VSMCs
to enter the growth cycle, and proliferation of intimal smooth muscle
cells accounts for most of intimal accumulation of VSMCs. We isolated
intimal smooth muscle cells from neointima at 2 weeks after
balloon angioplasty and showed that kallistatin stimulated the
proliferation of these cells. The stimulatory activity of kallistatin
on cell proliferation is specific because it was neutralized by its
specific antibody. Taken together, these results suggest that
kallistatin could also play an important role in intimal smooth muscle
cell proliferation and accumulation.
Our studies show that PDGF stimulated the expression of endogenous kallistatin in cultured VSMCs. PDGF is one of the crucial growth factors induced by vascular injury, and it has both mitogenic and chemotactic activities on medial VSMCs during intimal hyperplasia.2 4 However, PDGF alone can not optimally stimulate cell proliferation. It requires a second group of growth factors, termed "progression factors," to initiate DNA synthesis and cell division.4 21 22 A number of progression factors, such as bFGF,23 epidermal growth factor,24 and osteopontin,25 have been identified for intimal hyperplasia. Our results indicate that the expression of kallistatin is spatially and temporally colocalized with proliferating VSMCs of balloon-injured arteries in vivo, and kallistatin can stimulate the proliferation and migration of VSMCs independent of PDGF in vitro. These results suggest that kallistatin may function as an autocrine progression growth factor in response to vascular injury.
Using kallistatin antisense strategy, we demonstrate a potential role of kallistatin in mediating the PDGF-induced MAPK pathway resulting in proliferation of VSMCs. Our results show that PDGF induced kallistatin expression in cultured VSMCs. Moreover, kallistatin significantly increased p42/44 MAPK activity and stimulated VSMC proliferation in the absence of PDGF. MAPK-mediated cell proliferation is one of the major pathways for the regulation of PDGF-induced VSMC proliferation and growth.4 Activation of MAPK induces expression of proto-oncogenes c-fos and c-jun, which form transcription factors such as AP-1. Functional AP-1 further initiates the transcription of cyclins cell cycleregulatory genes, and cyclins stimulate cell proliferation.26 27 We observed that adenovirus-mediated kallistatin antisense cDNA delivery inhibited PDGF-induced p42/44 MAPK activity and cell proliferation in cultured VSMCs in vitro. Moreover, kallistatin antisense cDNA also suppressed neointima formation in balloon-injured arteries in vivo. Collectively, these results indicate that kallistatin may participate in mediating PDGF-induced p42/44 MAPK pathway. The detailed mechanisms by which kallistatin mediates PDGF-induced p42/44 MAPK pathway remain to be elucidated.
In summary, our results indicate that inhibition of endogenous kallistatin expression in injured blood vessels may have protective effects on neointima formation by inhibiting VSMC proliferation and migration. This study suggests that kallistatin may serve as a new potential therapeutic target for neointimal hyperplasia and restenosis after angioplasty.
| Acknowledgments |
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Received August 11, 1999; accepted December 7, 1999.
| References |
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