Clinical Research |
From the Atherosclerosis Research Unit, King Gustaf V Research Institute, Department of Medicine (S.J., J.M., A.H., P.E.), Karolinska Institute, Karolinska Hospital, Stockholm, Sweden; Department of Cardiovascular Medicine (S.Y., A.H.), University of Oxford, United Kingdom; and Department of Internal Medicine (D.H.W., S.D., A.M.Z.), University of Frankfurt, Germany.
Correspondence to Sofia Jormsjö, King Gustaf V Research Institute, Karolinska Hospital, S-171 76 Stockholm, Sweden. E-mail sofiaj{at}instmed.ks.se
| Abstract |
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Key Words: metalloproteinase-12 matrix polymorphism insulin activator protein-1
| Introduction |
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induce MMPs in vitro, whereas other agents, such as transforming growth
factor-ß, heparin, and corticosteroids have an
inhibitory effect.1 8 9 Elastin is the major component of elastic fibers that provide resilience to tissues exposed to dynamic stress. MMP-12 (macrophage elastase) was first identified as an elastolytic metalloproteinase secreted by activated macrophages.10 11 Besides elastase activity, MMP-12 displays a broad substrate specificity, including extracellular matrix (ECM) proteins such as fibronectin, laminin, vitronectin, type IV collagen, and heparan sulfate.12 13 Thus, MMP-12 not only digests elastin but also degrades the basement membrane, which enables macrophages to penetrate injured tissues during inflammation. It is notable that lipid-laden macrophages located in the boundary between the acellular lipid core and fibrous areas in atherosclerotic plaques express MMP-12.7
In the present study, we have identified a common polymorphism within the MMP-12 gene promoter (an adenosine [A]toguanosine [G] substitution at position -82) that influences the binding of the transcription factor activator protein-1 (AP-1). There is also an allele-specific response of MMP-12 promoter activity to both insulin and PMA. Furthermore, the MMP-12 -82 A/G polymorphism is associated with coronary artery luminal dimensions in diabetic patients requiring percutaneous transluminal coronary angioplasty (PTCA) with stent implantation.
| Materials and Methods |
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Genotyping
A polymerase chain reaction (PCR)based restriction fragment
length polymorphism (RFLP) introducing a PvuII
restriction site using mismatch PCR primer was performed, and
products were separated on a 3% Metaphor agarose gel (FMC
BioProducts).
Cell Culture
U937 and MonoMac cells were maintained in RPMI 1640 with 7.5%
or 10% FCS, respectively. Murine lung macrophage (MALU) cells
were obtained from the Sir William Dunn School of Pathology Sciences
Cell Bank and cultured in RPMI 1640 with 2 mmol/L
L-glutamine and 10% FCS.
Electromobility Shift Assay (EMSA)
Double-stranded 26-mer oligonucleotides
corresponding to the sequence from 91 to 66 in the MMP-12 promoter
were used. Nuclear extract preparations and EMSA were as
described.15
Transfection and Transient Gene Expression Assay
Two DNA fragments covering the region from 155 to +28
harboring either the A or the G site were ligated into pGL2-Basic
Vector (Promega). Cells (20x106) were
transfected with 50 µg of construct and 15 µg of ß-galactosidase
plasmid, and 100 nmol/L insulin or 1 µmol/L PMA was added. Cells
were harvested after 24 hours. Luciferase levels were expressed in
arbitrary units after standardization against ß-galactosidase levels
(U937 cells) or Renilla luciferase levels (MALU cells). Data are
presented as percentage of unstimulated controls because of the
difference in control vectors and because of variations in transfection
efficiency between the different experiments.
Western Blot
Cytosolic extracts from U937 cells were prepared from
1x106 cells. Samples (20 µg) were subsequently
subjected to 10% SDS-PAGE. Mouse anti-hMMP12 antibody (0.5 µg/mL)
(R&D) and anti-mouse IgG conjugated with alkaline phosphatase were
added and visualized with the enhanced chemiluminescence (Amersham
Pharmacia Biotech) detection system.
Reverse Transcriptase (RT)PCR
To determine MMP-12 mRNA levels, total RNA was isolated from
7x106 cells 16 hours after treatment with PMA or
insulin. One microgram of total RNA was reverse transcribed. Levels of
GAPDH mRNA were estimated as an internal control.
Human Subjects
Allele frequencies were determined in 183 healthy men ages
23 to 45 years (40.3±3.4, mean±SD). Genotype relationships to
the vascular response after PTCA with stent implantation were
investigated in 367 consecutive patients undergoing this procedure in
the Department of Internal Medicine of the University Hospital, Johann
Wolfgang Goethe University (Frankfurt, Germany).16 A total
of 59 patients (47 nondiabetic and 12 diabetic) did not undergo
reangiography according to the study protocol. Of the remaining 308
patients, 71 had manifest diabetes (6 were treated with diet only, 36
with diet and oral antidiabetic drugs, and 17 with insulin alone or in
combination with oral antidiabetics).
Statistical Methods
Differences in continuous variables between groups were
tested either by ANOVA with the Scheffé F test used as a post hoc
test or by the Student paired t test. Values are given as
mean±SE. A
2 test was used to compare the
observed numbers of each MMP-12 genotype with those expected
for a population in Hardy-Weinberg equilibrium.
An expanded Materials and Methods section is available online at http://www.circresaha.org.
| Results |
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The -82 A/G Polymorphic Site Influences the Binding of
Transcription Factor AP-1
Database analysis of the sequence around the
polymorphic site revealed a potential binding site for the
transcription factor AP-1 (Figure 1C
). An EMSA was therefore
carried out using a probe corresponding to the sequence from -91 to
-66 of the A allele, which was incubated with nuclear extracts
prepared from U937 cells. As can be seen in Figure 2A
, 2
major protein-DNA complexes were
formed. Antibodies against c-Jun, a component of the AP-1 complex,
resulted in a supershift (Figure 2A
). Furthermore, when an
excessive amount of an unlabeled oligonucleotide
corresponding to a consensus AP-1 site as a competitor was included in
the assay, the upper complex was abolished, whereas it was not when an
oligonucleotide corresponding to a mutated AP-1 site
was used (Figure 2A
). Taken together, these results demonstrate
that AP-1 (the upper complex indicated by an arrow in Figure 2A
)
binds to the polymorphic site.
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To study the possibility of allele-specific differences in
the binding of AP-1, increasing amounts of either unlabeled A or G
sites were used as competitors in an EMSA. As demonstrated in Figure 2B
, the A allele resulted in a stronger competition
of the AP-1 complex than the G allele. Allele-specific
differences in AP-1 binding were also studied in the presence of either
PMA (Figure 3A
) or insulin (Figure 3B
), 2 known activators of AP-1
binding.9 17 18 Both PMA and insulin increased the binding
of AP-1 to the MMP-12 promoter, with higher affinity for the A
allele (mean±SE, 57±37% [n=4] and 47±13% [n=3] higher
binding affinity for the A probe than the G probe when stimulated with
PMA or insulin, respectively) (Figures 3A
and 3B
). In accordance
with the results obtained under basal conditions (Figure 2B
),
competition experiments using increasing amounts of either unlabeled A
or G probe in assays with extracts from insulin-stimulated U937 cells
demonstrated a stronger affinity of the AP-1 complex to the A
allele (data not shown). EMSAs using nuclear extracts derived from
MonoMac 6 cells showed a similar allele-specific difference in
binding of AP-1 (data not shown).
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Allele-Specific Regulation of the MMP-12 Gene by PMA and
Insulin
The results of the EMSAs suggest that the 82 A/G
polymorphism may have an impact on the transcriptional activity of
the MMP-12 promoter. To study allele-specific differences in
promoter activity, DNA constructs covering the region from 155 to +28
of either the A or G genotype were placed upstream of a
luciferase reporter gene and transfected into MALU or U937 cells. In
transfection experiments using MALU cells, there was no difference in
basal promoter activity between the A and G alleles
(P=0.3). In contrast, induction of the cells with 1
µmol/L PMA resulted in a significant difference in promoter activity,
the A allele showing higher activity than the G allele
(274±18% versus 241±27% [n=9] of the luciferase activity of
unstimulated control, P<0.01) (Figure 4
). Transfection experiments were also
performed using U937 cells with essentially the same results (275±64%
versus 234±43% [n=8] of the luciferase activity of unstimulated
control for the A and the G alleles, respectively,
P=0.07) (Figure 4
). Furthermore, both the A and G
alleles responded to insulin in transfection experiments using U937
cells. The promoter activity of the A allele tended to be slightly
higher than that of the G allele when transfected cells were
stimulated with 100 nmol/L insulin (143±16% versus 123±8% [n=7]
of unstimulated controls, P=0.17) (Figure 4
). RT-PCR
and Western blot analysis were performed to study whether
insulin also increased the endogenous production of
MMP-12 mRNA and protein in U937 cells. As demonstrated in Figure 5
, 100 nmol/L insulin resulted in an
increase in both the MMP-12 mRNA (Figure 5B
) and protein
production (Figure 5A
).
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Association of the MMP-12 -82 A/G Polymorphism With
Coronary Artery Luminal Dimensions in Diabetic Patients
Undergoing PTCA With Stent Implantation
To analyze whether the 82 A/G polymorphism is
associated with coronary artery luminal dimensions, 367
consecutive patients who underwent PTCA with stent implantation were
genotyped, and genotype groups were compared with
respect to quantitative coronary angiography measurements of
the coronary segment subjected to intervention. The
analyses were performed on coronary angiograms taken
immediately before and after the PTCA and at follow-up 4 to 6 months
later. The allele frequencies in the patient population were
0.84/0.16. No allele-specific differences were found when the
entire patient group was examined (data not shown). Because the 82
A/G polymorphism was associated with allele-specific responses
of MMP-12 promoter activity to insulin in vitro (Figures 3
and 4
), we focused on the subset of diabetic patients (71
individuals). An allele-specific difference in reference diameter
was found in the diabetic patients, the G allele being associated
with a greater luminal diameter compared with the A allele
(Table
). In contrast, the change in
minimal luminal diameter (MLD) between the postangioplasty and
follow-up angiographies did not differ according to MMP-12 -82 A/G
genotype.
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| Discussion |
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The transcription factor AP-1 appears to play an important and general role in regulating MMP expression. AP-1 consensus sequences have been demonstrated in the promoter regions of several MMP genes.19 Transfection studies have demonstrated that the AP-1 sites in the MMP-1, MMP-3, MMP-7, MMP-9, MMP-10, and MMP-13 promoters are functional.19 The present work provides evidence for a functional AP-1 site also in the MMP-12 promoter. In a previous study, it was demonstrated that PMA, a well-known inducer of AP-1 activity, enhances MMP-12 mRNA levels in the murine tumor cell line P388D1.20 It was suggested that PMA modifies the MMP-12 mRNA level by acting on the transcriptional activity of the gene rather than on transcript stability. A putative AP-1 binding site in the MMP-12 promoter was described when the genomic clone for MMP-12 was obtained.11 In the present study, using EMSA with antibodies against AP-1 and with competitor oligonucleotides harboring either a wild-type or a mutant AP-1 site, we show that this site is, indeed, a binding site for AP-1. Furthermore, the proximal promoter of the MMP-12 gene responds to PMA in transient transfection experiments using both U937 and MALU cell lines. This response is influenced by the 82 A/G polymorphism flanking the AP-1 binding site and influencing the binding affinity of AP-1. The differences in response to PMA or insulin between the 2 alleles were rather small. However, this is not surprising, because the polymorphism influences the binding affinity of AP-1 instead of creating a new response element in the promoter. In addition, a small change in promoter activity may have a significant effect on a long-term lesion. Taken together, these results argue in favor of the existence of an AP-1 response element within the MMP-12 promoter.
The present work also demonstrates that insulin influences the binding of the AP-1 complex and that insulin has a stimulatory effect on MMP-12 promoter activity in vitro. The insulin receptor signal transduction cascade involves the activity of numerous cytoplasmic and nuclear effectors. When insulin binds to its receptor, cytoplasmic protein kinases are activated, some of which are contained in the mitogen-activated protein kinase pathway. A number of potential nuclear substrates for mitogen-activated protein kinase have been identified, such as c-Jun and c-Fos, components of the transcription factor AP-1. Accordingly, it has been demonstrated that insulin enhances the binding of AP-1 to its consensus binding site and that insulin activates transcription through an AP-1 response element.17 18
Imbalance between synthesis and degradation of ECM proteins has been suggested to contribute to the atherosclerotic process and to restenosis after angioplasty.21 For example, matrix degradation is likely to occur when monocytes pass extracellular barriers, such as the basement membranes underlying the endothelium. Degradation of the dense network of proteoglycans will also influence the migration of smooth muscle cells. Furthermore, degradation of the ECM may also result in clinically silent plaque disruption, causing intraplaque thrombosis with a subsequent smooth muscle cell proliferative response that is analogous to that occurring after angioplasty.22 23 24 Another potential mechanism by which matrix turnover could influence vascular disease involves effects on the vascular tone, and it has been suggested that matrix degradation could influence vessel dilation through the NO synthase pathways.25 In the present study, we found that the A allele of the MMP-12 promoter, possessing increased transcriptional activity in vitro in monocytes/macrophages, is associated with a smaller coronary artery luminal diameter in vivo in patients with diabetes and manifest coronary artery disease requiring PTCA with stent implantation, as assessed by computer-based measurement of the reference diameter of the procedure-related coronary segment. The finding of an association between increased MMP-12 promoter activity and smaller, not larger, coronary luminal diameter is surprising. In studies of aneurysms, an increased elastolytic activity has been linked to widened luminal diameter. However, mechanisms leading to a smaller luminal diameter could also be envisioned. It has, for instance, been shown that some degradation products of elastin are chemotactic for leukocytes and thereby potentially contribute to inflammation and the development of atherosclerosis.26 27 Alternatively, the elastase may facilitate leukocyte entry into the vessel wall by breaking down mechanical barriers. However, it cannot be excluded that the allele-specific effect on the luminal diameter is a result of decreased elasticity of the vessel wall due either to altered MMP-12 elastolytic activity or to an effect on vascular tone. Further studies with analysis of multiple sites of the coronary arteries and analysis of patients with aneurysms are needed to resolve this issue. An association between the 82 A/G polymorphism and MLD was present after PTCA. This finding is difficult to interpret, because the stent implantation should prevent effects on the elasticity of the procedure-related coronary segment. A possible explanation is that allele-specific differences in vessel elasticity at the time of PTCA and stent implantation could be the cause of these results. The finding that there was no allele-specific difference in the change of MLD between the postangioplasty and follow-up angiographies suggests that luminal narrowing/restenosis after PTCA is not influenced by MMP-12 expression. Needless to say, the present study cannot distinguish whether the relationship of the MMP-12 polymorphism to the reference diameter reflects differences in atherosclerosis in this area or is a result of differences in vascular elasticity. It should also be stressed in this context that we have no direct evidence that the increased transcriptional activity associated with the A allele is accompanied by increased protease activity in vivo. Nevertheless, the association between the 82 A/G polymorphism and the luminal dimensions in diabetic patients requiring angiography argue that this may, indeed, be the case.
The fact that AP-1 binding sites are present in several promoters of the MMP gene family implies that insulin may have a stimulatory effect also on other MMP genes. Recently, the AP-1 motif of the MMP-1 promoter was shown to be the target for insulin signaling in vitro.9 Because hyperinsulinemia is a common finding among patients with type II diabetes as well as in nondiabetic patients with manifest coronary artery disease, this could have profound consequences for the atherosclerotic process and the risk of restenosis and/or the regulation of vascular tone in diabetic patients as well as in patients with other causes of elevated AP-1 activation. However, increased MMP expression in vivo remains to be demonstrated in diabetic patients. Large-scale clinical studies are also needed to determine whether the 82 A/G polymorphic site influences the atherosclerotic process or the risk of restenosis, particularly in patients with type II diabetes and in nondiabetic individuals with hyperinsulinemia secondary to insulin resistance.
In summary, the major findings of the present work are that (1) there is a functional AP-1 response element in the MMP-12 promoter; (2) transcription factor AP-1 is involved in insulin-enhanced MMP-12 expression; (3) a common A/G polymorphism in the MMP-12 promoter influences the binding affinity of AP-1; and (4) in a preliminary study, the A/G polymorphism influences the luminal diameter in patients with diabetes, thus suggesting a role for MMP-12 in vascular remodeling.
| Acknowledgments |
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Received February 8, 2000; accepted March 14, 2000.
| References |
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