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From the Departments of Pathology (D.H.) and Surgery (R.F., A.W.C.), University of Washington, Seattle.
| Abstract |
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Key Words: smooth muscle cell plasminogen activator inhibitor type 1 tissue inhibitor of metalloproteinases-2 protease arterial injury
| Introduction |
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Vascular SMCs are known to produce both serine proteases and MMPs. These proteases are capable of degrading all the components of the ECM. Plasmin, a serine protease, is formed from plasminogen by PAs (tissue PA and urokinase PA). Studies from this as well as other laboratories have shown that PA activity increases after balloon injury and that by blocking plasmin generation, neointima formation is reduced.2 3 Plasmin is a serine protease of broad specificity that is also able to activate other MMPs as well as degrade matrix. Plasmin generation is primarily controlled by the balance between the PAs and their physiological inhibitors, the PAIs. In addition to PAs, MMPs are produced by vascular SMCs in response to balloon injury.4 5 Inhibition of MMP activity reduces neointima formation5 and limits invasiveness in tumor invasion models.6 7 8 These previous results show that inhibiting protease activity can limit neointima formation and suggest that protease inhibitors may serve to regulate SMC migration.
We now have investigated whether protease inhibitors are modulated in the injured rat carotid artery and whether their expression may serve to alter the proteolytic balance at key time points during the formation of the neointima. In the present study, we examined two classes of inhibitors of PAs and MMPs and demonstrate that in these two classes of inhibitors only two specific protease inhibitors, PAI-1 and TIMP-2, are increased in injured arteries and that their location and time of expression in the vessel wall may help define their role in the response to injury.
| Materials and Methods |
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Immunohistochemistry
Histological sections (4 µm) were cut from
paraffin-embedded rat carotid arteries, prepared, and placed on
poly-L-lysinecoated microscope slides. The sections were
deparaffinized by three changes in xylene for 5 minutes each. Slides
were rehydrated in a graded alcohol series as follows: three changes of
100% ethanol for 5 minutes total, two changes of 95% ethanol for 4
minutes total, and one change of 75% ethanol for 1 minute. Slides were
prepared for immunohistochemistry using the Vectastain Elite ABC kit
(Vector Laboratories). Endogenous peroxidase activity was blocked by a
5-minute incubation in 3% hydrogen peroxide. Slides were washed in two
changes of PBS, pH 7.2. Nonspecific binding of the secondary antibody
was blocked by incubation in 10% normal goat serum for 10 minutes,
followed by two changes of PBS. The rabbit anti-rat PAI-1 antibody
(American Diagnostica) or control rabbit IgG (Vector Laboratories) was
added at a concentration of 25 µg/mL in 1% BSA. PAI-1 antibody and
control rabbit IgG were incubated overnight at 4°C. The primary
antibody was washed in three changes of PBS for 1 minute each.
Biotinylated anti-rabbit secondary antibody was added for 30 minutes.
Slides were washed in three changes of PBS for 1 minute each, followed
by avidin-biotin-alkaline phosphatase for 30 minutes. The slides were
then washed in one change of PBS, followed by one wash in 0.05 mol/L
Tris, pH 7.6, at 37°C for 5 minutes each. Slides were incubated at
37°C for 30 minutes in 175 mL 0.05 Tris with 4 mL DAB (132 mL of 0.05
mol/L Tris, pH 7.6, and 5 g DAB; aliquot stored at -70°C) and
alkaline phosphatase substrate (Vector Laboratories), followed by one
wash in distilled water for 1 minute. Slides were counterstained in
hematoxylin, cleared in Histoclear (National Diagnostics), and mounted
in Histomount (National Diagnostics). Immunohistochemical staining was
visualized under bright-field illumination on a Leitz Dialux 20 EB and
photographed using Ectachrome Elite 100 ASA.
Isolation of RNA and Northern Blot Analysis
Total cellular RNA was isolated from 12 rat common carotid
arteries in pooled groups of four per time point by guanidinium
isothiocyanatephenolchloroform extraction as previously
described,10 except that tissue was first finely crushed
under liquid nitrogen in a mortar and pestle and then homogenized for
30 seconds in a Brinkman PT10-35 homogenizer. Isolated RNA was
resuspended in 0.5% SDS and quantified spectrophotometrically. RNA
samples were separated in a 1% agarose/formaldehyde
gel.11 RNA was transferred to Zeta-probe nylon membrane
(Bio-Rad) as described by the manufacturer and cross-linked to the
membrane using UV light (Stratagene). Filter hybridizations were
carried out as described by Church and Gilbert.12 In
general, hybridizations were performed in a solution of 1% BSA, 7%
SDS, 0.5 mol/L sodium phosphate, pH 7.0, 1 mmol/L EDTA, and 20%
formamide at 65°C. Filters were washed to a final stringency of 0.1x
SSPE at 65°C (0.33 mol/L NaCl and 0.2 mol/L
NaH2PO4, pH 7.4) and exposed to Kodak XAR x-ray
film at -70°C using an intensifying screen and quantified on the
PhosphorImager at the Markey Molecular Medicine Facility, Seattle,
Wash.
Isolation of Arterial Protein and Western Analysis
Seven days after injury, carotid arteries were removed, and the
intima, media, and adventitia were separated. The intima was separated
by cutting open the artery longitudinally and then peeling out the
intima with forceps. The media was separated from the adventitia by
holding onto the media and pulling the adventitia; this process leaves
some tightly connected adventitia remaining with the media. Tissue was
then snap-frozen and processed as described for reverse zymography.
Protein (10 µg per lane) was loaded and run on a 10%
SDS-polyacrylamide gel and transferred to nitrocellulose. Rabbit
antiTIMP-2 antibody was added at 1:1000 for 30 minutes and visualized
with 1:7500 dilution of anti-rabbit IgG (Fc)-alkaline phosphatase
conjugate and subsequent color development (Promega).
Probes
The rat PAI-1 cDNA probe used in the present study was
generously provided by Dr T. Gelehrter.13 The TIMP-2 cDNA
and TIMP-2 antibody were provided by Dr Keith Langley (Amgen, Thousand
Oaks, Calif). For Northern hybridizations, plasmids were radiolabeled
by the nick-translation method14 using the large fragment
of Klenow DNA polymerase and [32P]dCTP (3000
Ci/mmol).
Reverse Zymography
Carotid arteries were removed, stored, and homogenized in 0.05
mol/L Tris, 0.01 mol/L CaCl2, 2.0 mol/L guanidine HCl, and
0.2% Triton X-100, pH 7.5, as described by Dean et al.15
Homogenate was centrifuged at 10 000g for 1 minute, and the
supernatant was dialyzed against 3 vol of 0.05 mol/L Tris-HCl, pH 7.5,
and 0.2% Triton X-100 over 2 days (SpectraPore, Spectrum). Aliquots
were frozen at -70°C. Protein concentration was determined by the
BCA assay (Pierce) as described by the manufacturer. For PAI analysis,
20-µg samples were run on a 10% SDS-PAGE as described by
Laemmli16 and then washed for 10 minutes in 2.5% Triton
X-100 (Sigma Chemical Co) and 10 minutes in PBS. The gel was overlaid
onto a 1.25% agar (Difco) containing 40 µg/mL plasminogen (American
Diagnostica), 250 mU/mL human recombinant HMW urokinase (Calbiochem),
and 2% nonfat dry milk powder (wt/vol) (Carnation) in 0.1 mol/L Tris,
pH 8.1. The reverse zymogram was developed at 4°C overnight and then
at 37°C for 3 to 6 hours. Visualization was by dark-field
illumination. For TIMP reverse zymography, 100 µg of protein was run
on a 10% SDS-polyacrylamide gel containing 1 mg/mL gelatin and 1 mL of
MMP solution (University Technologies Intl Inc). The gel was washed in
2.5% Triton X-100 and developed for 22 hours at 37°C before staining
with Coomassie blue.
Statistics
PhosphorImager data from Northern blots were normalized to the
signal from 28S rRNA labeling, as a measure of loading, and expressed
as fold increase compared with uninjured carotid arteries. The mean
increase is reported with two-tailed 95% confidence intervals by
one-way ANOVA compared with the 0-hour injured control vessel. Error
bars are standard deviations of the mean.
| Results |
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Northern Analysis of PAI-1 mRNA
To determine the kinetics of PAI-1 induction in the rat carotid,
total mRNA from the carotids of balloon-injured rats was analyzed on
Northern blots using 32P-labeled cDNA probes specific for
rat PAI-1. Rat PAI-1 mRNA has only one transcript, whereas PAI-1 in
many other species has 3.2- and 2.3-kb transcripts.13 17
PAI-1 mRNA was induced by injury in the rat common carotid as shown in
Fig 2
. This induction was transient and returned to
baseline levels by 3 days after injury. The induction of message was
maximal at 6 hours (18-fold increase; n=3; SD, 10.6; two-tailed 95%
confidence interval, -7 to 45). Although the mRNA levels were
increased on three independent blots, the increase was not
statistically different from that for the control 0-hour injured
carotid artery. This induction was dramatically decreased by 24 hours
and returned to baseline levels by day 3 after injury.
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Immunohistochemical Analysis of PAI-1 Antigen in Injured Rat
Carotid Arteries
To determine the cell type responsible for the increase in PAI-1
expression, cross sections of injured carotid arteries were stained for
PAI-1. PAI-1 expression was increased in a subpopulation of cells in
the media at 6 hours after injury (Fig 3B
). PAI-1
staining was not seen by 3 days (Fig 3C
). PAI-1 was absent in the
15-minute injured vessel and in the normal uninjured vessel.
Adventitial staining was present in the IgG controls (Fig 3D
, 3E
, and 3F
). Adventitial staining was also present in sections stained with
antiPAI-1 preincubated with human PAI-1 antigen (data not shown).
Taken together, these data from Northern blotting, reverse zymography,
and immunohistochemistry clearly demonstrate that medial SMCs increase
transiently the expression of PAI-1 in response to injury.
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Effect of Arterial Injury on Inhibitors of MMPs
The carotid artery extracts were also analyzed for inhibitors of
MMPs. TIMP-2 (18 kD) was the only MMP inhibitor detected by reverse
zymography and appeared to be modulated by injury (Fig 4
). TIMP-1 (Mr, 28 kD) activity
was not detectable in the carotid extracts (Fig 4
). Immediately after
injury through 24 hours, TIMP-2 was present at low levels. Three days
after injury, TIMP-2 inhibitory activity increased and reached maximal
activity around day 5. By 14 days after injury, TIMP-2 was
decreased.
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Northern Analysis of TIMP-2 mRNA After Arterial Injury
TIMP-2 levels began to increase 24 hours after balloon injury and
reached a maximal 3-fold induction at day 7 (n=3; SD, 0.65; two-tailed
95% confidence interval, 1.4 to 4.7) (Fig 5
). TIMP-2
mRNA was not significantly elevated at 2 weeks. Rat TIMP-2 has two
transcripts, 1.0 and 3.5 kb. In response to injury, only the 1.0-kb
transcript was significantly induced, whereas the 3.5-kb transcript was
not modulated and remained constitutively expressed.
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Western Analysis of TIMP-2 Antigen in the Injured Carotid
Artery
The location of TIMP-2 after injury was determined by physically
separating the intima, media, and adventitia. Protein from each region
was then extracted and analyzed by Western blotting for TIMP-2 antigen
(Fig 6
). Seven days after injury, TIMP-2 antigen was
confined to the intima and was not detected in the media or the
adventitia. The data from Northern blotting, reverse zymography, and
Western blotting clearly demonstrate that TIMP-2 is induced by balloon
injury in the rat carotid artery and that TIMP-2 expression is limited
to the intima at day 7 after injury.
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| Discussion |
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PAI-1 mRNA, antigen, and activity were induced rapidly after injury, reached a maximum by 6 hours, and then quickly returned to baseline after 24 hours. TIMP-2 was induced at 1 day, reached maximal levels in the intima around day 7, and remained elevated through 2 weeks. TIMP-2 might be complexed at late time points with proteases (eg, MMP-2). This possibility might explain the apparent decline in TIMP-2 activity despite the presence of TIMP-2 mRNA at 14 days. The increase in protease inhibitors, in conjunction with previous observations of regulated protease activity, suggests that changes in proteolytic and fibrinolytic activity may be central events in the response to injury.
There is strong experimental evidence both in vivo and in vitro that regulation of matrix proteolysis is important for tumor invasion, tissue remodeling, angiogenesis, and development. Changes in the proteolytic balance between protease and anti-protease expression have been associated with diseases such as atherosclerosis,18 19 metastasis,20 and thromboembolism.21 Proteinases and their inhibitors also play an important role in tissue remodeling,22 23 wound repair,24 and many other normal processes related to reproduction, such as lactation,25 uterine implantation,26 and gestation.27 All of these events require controlled expression of PAs or MMPs or both in conjunction with their respective inhibitors. Vascular repair in response to injury might also require a highly controlled pattern of protease and anti-protease expression by vascular SMCs. Vascular SMCs degrade ECM by plasminogen-dependent and plasminogen-independent pathways.28 Plasmin and the MMPs form an interactive proteolytic cascade, since they affect the activation of one another. This proteolytic cascade is able to degrade most of the ECM components, including collagen types I, III, and IV, laminin, fibronectin, elastin, and proteoglycans. A shift in the balance between the proteases and their respective inhibitors is probably necessary for vascular remodeling to occur.
PAI-1 Expression After Arterial Injury
PAI-1 is a single-chain polypeptide with an
Mr of 50 kD that inhibits urokinase PA as well
as single- and two-chain forms of tissue PA by rapidly forming a 1:1
stoichiometric complex. PAI-1 is an early growth response gene in
fibroblasts and hepatocytes and is induced during the transition from
the quiescent state (G0) to the G1 phase of the
cell cycle.29 In the mouse, the highest concentrations of
PAI-1 are found in the aorta, consistent with observations that PAI-1
is a major product of smooth muscle cells.30 The 18-fold
increase in expression of PAI-1 at 6 hours after injury is consistent
with the time course of PAI-1 induction seen by other investigators
using cytokines31 or with other forms of
injury.29 PAI-1 is found in both active and latent forms.
Northern analysis, reverse zymography, and immunohistochemistry do not
distinguish between the two forms. This limits our ability to determine
whether PAI-1 is having a physiological effect after injury. However,
PAI-1 as measured in the present study might be biologically active,
because it can be stabilized in the vessel wall through
association with serum-derived vitronectin.32
Plasma PAI-1 increases after trauma and inflammation as one of several APR proteins.33 PAI-1 is synthesized by a wide variety of tissues in vivo, unlike most APR proteins, which are synthesized in the liver.30 In the rat, PAI-1 is a tissue-associated inhibitor of fibrinolysis at sites of vascular disruption and inflammation. Balloon catheter injury inflicts extensive damage on the vessel wall. The immediate priority of the vessel after this type of extensive injury is to prevent hemorrhage. Control of vascular disruption and hemorrhage is achieved through the formation of thrombi. At early times after arterial injury, there is platelet adherence and limited fibrin deposition on the luminal surface, but the luminal surface clears after 1 day. An early increase in tissue-associated PAI-1 may serve to limit fibrinolysis during this early repair period. Additionally, PAs are expressed in the injured rat carotid artery and are necessary but not sufficient for SMC migration. Tranexamic acid, a synthetic PAI, blocks SMC migration.3 PAI-1 may have a similar effect by limiting proteolytic activity at early time points after injury and as a result block migration.
TIMP-2 and Arterial Injury
MMPs, as well as PAs, are induced in rat injured carotid
arteries.4 5 A 92-kD collagenolytic activity thought to be
MMP-9 is increased 24 hours after injury and then decreases to baseline
levels. Gelatinase (72 kD) is expressed constitutively, and its
activated form is increased 5 days after injury and decreases after 14
days.4 5 A low-molecular-weight MMP (24.5 kD) with broad
proteolytic activity and high elastinolytic activity is found in the
adventitia and is increased 5 days after injury. This 24.5-kD MMP may
be involved in elastin turnover in the adventitia. This increased
activity of MMPs coincides with increased TIMP-2 expression. Increased
protease inhibitors could block cell migration from the media. This
theory is supported by experiments in which a synthetic MMP inhibitor,
GM 6001, is able to block 97% of the SMCs migrating into the intima by
blocking MMP activity.5
TIMP-2 is a nonglycosylated 21-kD protein that selectively forms a 1:1 complex with the latent and activated form of 72-kD type IV collagenase. TIMP-2 preferentially binds to the 72-kD proenzyme, whereas TIMP-1 preferentially binds to the 92-kD proenzyme form of type IV collagenase. Therefore, increased TIMP-2 expression may specifically limit the activity of 72-kD type IV collagenase. Although TIMP-1 and TIMP-2 have preferential binding patterns, they are able to inhibit all members of the MMP subclasses that have been tested.34 The preference of inhibitors for specific types and forms of MMPs confers an added level of control for proteolytic degradation.
There are two TIMP-2 transcripts expressed by SMCs. The 3.5-kb transcript of TIMP-2 is constitutively expressed after injury, whereas the 1.0-kb transcript is increased 3-fold. The two transcripts are thought to arise from alternative splicing of 5'-untranslated regions.35 Both transcripts have been shown to encode identical proteins.36 37 The opposite pattern of differential regulation of the TIMP-2 message has been described in vitro by Testa,38 who reported that the 3.5-kb transcript was modulated and the 1.0-kb transcript was minimally induced. Khokha et al39 have shown that even a 29% increase in TIMP-1 mRNA expression has a significant effect on the invasive phenotype. These results support the conclusion that the increase in TIMP-2 mRNA and activity seen in the present study is likely to produce a strong biological effect in the vessel wall and may serve to limit migration.
We have also examined TIMP-1 and TIMP-3 mRNA expression in the rat carotid artery after balloon injury and found no significant modulation of TIMP-1 and no detectable TIMP-3 expression (data not shown). These findings indicate that TIMP-1 and TIMP-2 are not regulated in parallel, a conclusion reached by other investigators.35 TIMP-1 is present at lower levels in rat arteries than in baboon arteries.40 This observation may suggest that different species preferentially use different proteases and their respective inhibitors to perform similar functions.
In summary, we have demonstrated that two endogenous inhibitors of protease activity, PAI-1 and TIMP-2, are increased in response to arterial injury in the rat carotid artery. Taken together with our previous observations on the induction of proteases during SMC migration, these results support our conclusion that changes in proteolytic activity are necessary for SMC migration and may be a critical part of the response to injury in the carotid artery.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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| Footnotes |
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Received December 13, 1995; accepted December 19, 1996.
| References |
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