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Am J Physiol Heart Circ Physiol 290: H232-H239, 2006. First published August 26, 2005; doi:10.1152/ajpheart.00457.2005
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Matrix metalloproteinase-9 gene deletion facilitates angiogenesis after myocardial infarction

Merry L. Lindsey,1 G. Patricia Escobar,1 Lawrence W. Dobrucki,2 Danielle K. Goshorn,1 Shenikqua Bouges,1 Joseph T. Mingoia,1 David M. McClister, Jr.,1 Haili Su,2 Joseph Gannon,3 Catherine MacGillivray,3 Richard T. Lee,3 Albert J. Sinusas,2 and Francis G. Spinale1,4

1Division of Cardiothoracic Surgery Research, Medical University of South Carolina, Charleston, South Carolina; 2Division of Cardiovascular Medicine, Department of Internal Medicine and Department of Diagnostic Radiology, Yale University School of Medicine, New Haven, Connecticut; 3Cardiovascular Division, Department of Medicine Brigham and Women's Hospital and Harvard Medical School, Cambridge, Massachusetts; and 4Ralph A. Johnson Veterans Administration Medical Center, Charleston, South Carolina

Submitted 6 May 2005 ; accepted in final form 19 August 2005


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Matrix metalloproteinases (MMPs) are postulated to be necessary for neovascularization during wound healing. MMP-9 deletion alters remodeling postmyocardial infarction (post-MI), but whether and to what degree MMP-9 affects neovascularization post-MI is unknown. Neovascularization was evaluated in wild-type (WT; n = 63) and MMP-9 null (n = 55) mice at 7-days post-MI. Despite similar infarct sizes, MMP-9 deletion improved left ventricular function as evaluated by hemodynamic analysis. Blood vessel quantity and quality were evaluated by three independent studies. First, vessel density was increased in the infarct of MMP-9 null mice compared with WT, as quantified by Griffonia (Bandeiraea) simplicifolia lectin I (GSL-I) immunohistochemistry. Second, preexisting vessels, stained in vivo with FITC-labeled GSL-I pre-MI, were present in the viable but not MI region. Third, a technetium-99m-labeled peptide (NC100692), which selectively binds to activated {alpha}v{beta}3-integrin in angiogenic vessels, was injected into post-MI mice. Relative NC100692 activity in myocardial segments with diminished perfusion (0–40% nonischemic) was higher in MMP-9 null than in WT mice (383 ± 162% vs. 250 ± 118%, respectively; P = 0.002). The unique finding of this study was that MMP-9 deletion stimulated, rather than impaired, neovascularization in remodeling myocardium. Thus targeted strategies to inhibit MMP-9 early post-MI will likely not impair the angiogenic response.

leukocytes; remodeling; imaging

REMODELING OF THE LEFT VENTRICLE (LV) postmyocardial infarction (post-MI) evokes changes to both cellular and extracellular matrix components to progressively alter LV structure and function (35). Matrix metalloproteinases (MMPs) comprise a family of zinc-dependent endopeptidases that can cleave all components of the extracellular matrix (ECM) and thereby exert influence on LV remodeling. MMPs are elevated after MI, and a cause and effect relationship between MMPs and LV remodeling has been demonstrated through the use of MMP inhibitors and MMP-null mice (23, 38, 55). In particular, MMP-9 is a 92-kDa gelatinase upregulated acutely post-MI, and MMP-9 gene deletion results in attenuated LV remodeling after MI (6, 22). Thus MMP-9 likely contributes to adverse LV remodeling post-MI.

For the purposes of this study, we use neovascularization and angiogenesis interchangeably according to the following previously assigned definition: the sprouting of new vessels at the capillary level (48). MMPs have also demonstrated roles in neovascularization, and MMP inhibition has been postulated to inhibit the angiogenic process (43). Clinical trials with MMP inhibitors, however, have suggested that MMP inhibition may promote, rather than inhibit, neovascularization (2). MMP-9 is a specific MMP that has been implicated in angiogenesis, and the macrophage is one of several cell types that express MMP-9 post-MI (46). The exact role of MMPs, particularly MMP-9, in post-MI neovascularization is not clear. The MMP-9 substrate portfolio is broad and includes both angiogenic and angiostatic cytokines, growth factors, and ECM components. For example, MMP-9 is able to break down collagen IV, which would stimulate endothelial cell sprouting (37). At the same time, MMP-9 can process plasminogen to form angiostatin, a potent inhibitor of angiogenesis (34). MMP-9 potentially regulates the balance between angiogenic and angiostatic factors. Therefore, we explored the alternative hypothesis that MMP-9 deletion would stimulate, rather than impair, neovascularization in response to MI. The three objectives of this study were to determine the effects of MMP-9 gene deletion on 1) macrophage infiltration, 2) neovascularization, and 3) angiogenic factor expression post-MI.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Mice. All animal procedures were conducted in accordance with the Guide for the Care and Use of Laboratory Animals (National Institutes of Health) and were approved by the Animal Research Committee of Harvard Medical School and the Institutional Animal Care and Use Committee at the Medical University of South Carolina.

MMP-9 null mice were generated by Vu and colleagues (50) by replacing part of exon 2 and all of intron 2 with a phosphoglycerate kinase-neomycin cassette. The mice have been backcrossed >6 generations into the FVB background. Homozygous wild-type FVB mice (WT; n = 63) and MMP-9 null FVB mice (MMP-9 null; n = 55) of both genders (8–16 wk old) were used for the MI protocols.

Experimental design. Subsets of surviving mice were randomized to the following terminal studies and downstream analyses: WT control (n = 10), WT post-MI (n = 6), and MMP-9 null post-MI (n = 9) mice for hemodynamic assessment; WT (n = 21) and MMP-9 null (n = 15) mice for in vivo Griffonia (Bandeiraea) simplicifolia lectin I (GSL-I) injection; WT (n = 10) and MMP-9 null (n = 7) mice for in vivo {alpha}v{beta}3 imaging; WT (n = 15) and MMP-9 null (n = 8) mice for infarct size and infarct-to-septal wall thickness ratio measurements; WT (n = 13) and MMP-9 null (n = 13) mice for GSL-I immunohistochemistry; WT (n = 8, 7, 7, and 7) and MMP-9 null (n = 9, 10, 8, and 7) mice at 1, 3, 5, and 7-days post-MI, respectively, for the macrophage infiltration time course; and WT (n = 8) and MMP-9 null (n = 9) mice for immunoblotting. No mice were excluded from any subset analysis. The sample sizes are shown in the tables and figures. For all analyses, the evaluator was blinded to genotype.

Coronary artery ligation. Coronary artery ligation was performed as described previously (3, 25, 30). Briefly, mice were anesthetized with 3–4% isoflurane and ventilated. Coronary ligation was made by placing an 8-0 Ethilon suture (VP-72-28086, Ethicon) underneath the left coronary artery. Infarction was confirmed by visible LV blanching and ST-segment elevation on the electrocardiogram. A subcutaneous injection of buprenorphine (0.01–0.05 mg/kg) was given before surgery and every 12 h for the next 48 h and longer as needed. The mice were placed in a 37°C incubator with oxygen during recovery. The mice were monitored closely until they were alert and ambulatory, at which time they were returned to their cages and monitored daily.

In vivo GSL-I staining. Endothelial cells were prelabeled by injecting 100 µg of fluorescein-labeled GSL-I isolectin B4 (Vector) into the tail vein 30 min before the MI surgery. The GSL-I was diluted in saline. On day 7 post-MI, the LV was collected as described in Noninvasive dual-isotope in vivo imaging and gamma well counting, and 6-µm frozen sections were cut and counterstained with the nuclear stain propidium iodide. Five random fields from the infarct and remote regions were scanned at low and high magnifications (x10 and x40) and evaluated for the presence of prelabeled vessels. All three regions of the myocardium (subendocardium, midmyocardium, and epicardium) were included in the evaluation. Because the qualitative assessment revealed no prestained vessels in the infarct regions, quantitation was not performed.

Noninvasive dual-isotope in vivo imaging and gamma well counting. To evaluate angiogenesis in WT (n = 9) and MMP-9 null (n = 6) mice, dual-isotope small animal single-photon emission-computed tomographic imaging (microSPECT) with a technetium-99m (99mTc)-labeled Arg-Gly-Asp (RGD) peptide (NC100692, Amersham Health) targeted at {alpha}v{beta}3-integrin, and a perfusion tracer thallium-201 (201Tl) was performed on day 7 post-MI.

After NC100692 injection (1.98 ± 0.58 mCi iv), a small animal X-ray computed tomography (microCT) scan was performed to localize the heart for subsequent fusion with the microSPECT image. At 60 min after radiotracer injection, microSPECT imaging with a 1-mm pinhole collimator was performed. Next, 201Tl (0.250 ± 0.025 mCi) was injected, and another microSPECT image was performed to delineate the infarct. After the imaging was complete, all animals were euthanized by potassium chloride injection for further tissue processing.

The mouse hearts were immediately dissected, washed with ice-cold PBS, and filled with dental-molding material (alginate, Quala Dental Products) to facilitate the cutting of the hearts into uniform 1-mm-thick slices. To validate the microSPECT imaging results, each heart slice was cut into four circumferential sections, each section was weighed, and gamma well counting (CobraPackard) of 201Tl and 99mTc activity was performed by using appropriate energy windows (201Tl, 50–90 keV; and 99mTc, 120–160 keV). Raw counts were corrected for radiotracer crosstalk, background, decay, and weight. Radiotracer spill-up and spill-down correction was performed by the counting of pure reference specimens of 201Tl and 99mTc activity using both energy windows. These reference samples were used to create an inversion matrix that was applied to the tissue specimens for the correction of radiotracer crosstalk. The corrected activity from selected segments within the central infarct area was normalized to activity of nonischemic regions of the heart, as previously reported (41).

Hemodynamics. For terminal hemodynamic studies, WT and MMP-9 null mice were anesthetized with 2% isoflurane, intubated, and ventilated as described in Coronary artery ligation. The right carotid artery was exposed, and a microtipped transducer 1.4-Fr catheter (SPR-671, Millar, Houston, TX) was advanced into the LV. Steady-state measurements were acquired to determine end-systolic pressure and maximum developed pressure over time (dP/dtmax) (8, 9).

Tissue collection. Mice were anesthetized with 5% inhalational isoflurane and euthanized by an injection of cardioplegic solution (13) into the LV by apical puncture. After the flushing of the coronary circulation was completed, the heart was removed and the LV was separated and weighed. The LV was sectioned into three segments, incubated in 1% triphenyltetrazolium chloride (Sigma Chemical), and photographed for infarct-size calculation (36). Infarct size and infarct-to-septal wall thickness ratio were calculated as described previously (24, 25). The LV for histology was fixed in zinc-formalin (Z-Fix; Anatech), paraffin-embedded, and sectioned at 5 µm. The LV for immunoblotting was separated into infarcted and noninfarcted remote sections and snap frozen.

Immunohistochemistry and immunoblotting. To determine the rate of macrophage infiltration, mice were euthanized at days 1, 3, 5, and 7 post-MI using the collection protocol described in Tissue collection. A rat anti-mouse Mac-3 antibody (Cedarlane) was used at 1:100 to stain for macrophages by the avidin-biotin complex technique using the 3,3'Diaminobenzidine (DAB) chromogen with eosin counterstain. For quantitative analysis of immunohistochemistry, measurements were performed by using Image-Pro software (Media Cybernetics) to calculate the number of positive cells per field for five random fields within the infarct region. The evaluator was blinded to time post-MI as well as to genotype.

Endothelial cells were detected by staining serial sections with biotinylated GSL-I (100 µg/ml, Vector). After cells were incubated with GSL-I, the avidin-biotinylated enzyme complex was added, followed by DAB chromogen with eosin counterstain. GSL-I staining was quantified by using Image-Pro software (Media Cybernetics) to calculate the total GSL area stained positive (brown DAB staining) per total section area (brown DAB and pink eosin staining). The data are presented as the area of GSL staining per total area (in mm2). To determine differences in vessel density between the infarct regions, sections were divided into the subendocardial, midmyocardial, and epicardial regions, and the individual regions were scanned at x40 magnification. Five random fields from each of the three regions were scanned and quantified for each section.

Frozen tissue from remote and infarct regions was homogenized, and immunoblotting was performed as described previously (53). Biological signaling molecules that mediate angiogenesis include the angiopoietins Ang-1 and Ang-2, the angiopoietin receptor Tie-2, and the VEGF (48). The antibodies for Ang-1 (Chemicon), Ang-2 (Chemicon), Tie-2 (Chemicon), and VEGF (Santa Cruz) were rabbit polyclonal antibodies that were used at a concentration of 0.2 µg/ml.

Data analysis. Data are means ± SE. Statistical analyses were performed by using Intercooled Stata 8.0 for Windows (Stata, College Station, TX). Survival analysis was performed by using Fisher exact test. Morphometric and hemodynamic data for WT and MMP-9 null post-MI mice were analyzed by Student's t-test, and hemodynamic data for WT controls and post-MI mice were compared by ANOVA with Bonferroni correction. The macrophage time-course numbers were analyzed by one-way ANOVA with Bonferroni correction. Vessel density and well-counting data were analyzed by Student's t-test. Immunoblotting data were compared by using a one-sample t-test. A two-tailed value of P < 0.05 was considered statistically significant.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 GRANTS
 REFERENCES
 
LV function and morphometric analyses. Morphometric and hemodynamic analyses are shown in Table 1. The 7-day mortality rate was 25% for WT and 21% for MMP-9 null mice (P = not significant). Infarct size was similar in both groups. The septal wall thickness was lower, and the infarct-to-septal wall thickness ratio was significantly higher in the MMP-9 null mice compared with WT mice. End-systolic pressure and dP/dtmax were higher in MMP-9 null mice post-MI compared with WT post-MI. Together, these data indicate attenuated wall thinning and improved LV function in the MMP-9 null mice at 7-days post-MI.


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  		Table 1. Morphometric and hemodynamic analyses

 
Early macrophage infiltration is reduced in MMP-9 null mice post-MI. Macrophages are key regulators of the early LV remodeling process, and MMP-9 is a predominant MMP expressed by macrophages. To determine the rate of macrophage infiltration post-MI, mice were euthanized at 1, 3, 5, and 7 days of post-MI, and the number of macrophages infiltrated into the infarct zone was quantified (Fig. 1). In the WT mice, macrophage infiltration was first observed at 3-days post-MI, peaked at 5 days, and began to decrease at day 7. In MMP-9 null mice, similar numbers infiltrated at day 3, but a further increase at day 5 did not occur. Levels were similar again at day 7. Peak macrophage infiltration, therefore, was attenuated in MMP-9 null mice.



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  		Fig. 1. Macrophage infiltration was impaired in matrix metalloproteinase (MMP)-9 null mice postmyocardial infarction (post-MI). After 1, 3, 5, and 7 days of MI, macrophages were stained for Mac-3 antibody. Wild-type (WT) mice (A) had significantly greater macrophage infiltration at 5-days post-MI compared with MMP-9 null mice (B). C: time course of macrophage infiltration. At days 1, 3, 5, and 7, WT mice were n = 8, 7, 7, and 7, respectively, and MMP-9 null mice were n = 9, 10, 8, and 7, respectively. *P < 0.05 vs. WT at 5-days post-MI.

 
Increased neovascularization in MMP-9 null mice post-MI. We evaluated whether vessel density at 7-days post-MI was different between the two groups (Fig. 2). In WT mice, GSL-I staining demonstrated that the predominant number of vessels within the infarct was localized to the midmyocardial region, with a paucity of vessels observed in the subendocardial and epicardial regions. Increased numbers of GSL-I-positive vessels were observed in both subendocardial and epicardial regions of MMP-9 null mice. In addition to an increase in total vessel density, vessel distribution between regions was also normalized in the absence of MMP-9.



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  		Fig. 2. MMP-9 null mice have increased vessel density post-MI compared with WT mice, due to increase in formation of new vessels. A: representative photomicrograph demonstrating Griffonia (Bandeiraea) simplicifolia lectin I (GSL-I)-positive staining in control left ventricle (LV). Both cross-sectionally and longitudinally oriented vessels are observed. B: GSL-I immunohistochemistry at 7 days post-MI. GSL-I-positive staining is lower in subendocardial (Endo) and epicardial (Epi) regions of WT mice (n = 13) compared with midmyocardial (Mid) region. GSL-I-positive staining is higher in Endo and Epi regions of MMP-9 null mice (n = 13), which serves to increase total vessel number as well as equilibrate levels between regions. *P < 0.05 vs. WT.

 
Vessels in MI region are not preexisting. The increase in vessel density observed in the infarct region of MMP-9 null mice may be due to an increase in the preservation of preexisting vessels. Alternatively, increased vessel density could be due to an increase in angiogenesis. To determine whether vessels were preexisting, FITC-labeled GSL-I was injected into the tail vein of mice pre-MI to label endothelial cells in vivo. The observation of GSL-I-positive staining only in regions of still viable myocardium confirmed that the label persisted for 7 days in vivo. However, at 7-days post-MI, no GSL-I labeling in the infarcted myocardium was seen in either WT or MMP-9 null mice. When histological sections were then stained for GSL-I, vessels within the infarct stained positive, suggesting that differences in vessel densities were due to an increased angiogenic potential in MMP-9 null mice (Fig. 3).



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  		Fig. 3. A: serial sections from remote region of MMP-9 null mouse at 7-days post-MI, showing longitudinally oriented vessels stained positive for GSL-I in vivo (left; green) and histologically (right; brown). Section on left was counterstained with propidium iodide (orange) to identify nuclei. B: representative images from WT (top; n = 21) and MMP-9 null mice (bottom; n = 15), demonstrating that preexisting vessels were only observed in regions of still viable myocardium (top area of the section) and not in regions of infarct (bottom area of the section). C: in infarct region of representative WT LV, vessels were not prestained (top) but were GSL-I positive by immunohistochemistry (bottom). *Orientation of location between serial sections.

 
Vessels in MI region are newly formed. The {alpha}v{beta}3-integrin is overexpressed and activated in angiogenic vessels and represents a marker of neovascularization (29). The uptake of a 99mTc-labeled chelate-peptide conjugate containing RGD (NC100692, GE Healthcare), which specifically binds to the activated conformation of {alpha}v{beta}3-integrin, was visualized in the infarct region of both WT and MMP-9 null mice at day 7 post-MI using a hybrid microSPECT-CT gamma camera (Fig. 3A). The uptake of NC100692 was observed in hypoperfused regions of the infarct area as defined by perfusion imaging with 201Tl. The gamma well counting of myocardial tissue for both NC100692 and 201Tl confirmed this observation (Fig. 3B). NC100692 activity in the myocardial segments with the greatest reduction in perfusion (0–40% nonischemic) was significantly higher in MMP-9 null mice compared with WT (383 ± 162% vs. 250 ± 118%, respectively, P = 0.002), indicating a higher density of newly formed {alpha}v{beta}3-positive vessels in the infarct region of MMP-9 null mice (Fig. 4).



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  		Fig. 4. A: representative in vivo dual isotope thallium-201 (201Tl) and technetium-99m (99mTc)-NC100692 small animal single-photon emission-computed tomographic imaging/small animal X-ray computed tomography (microSPECT/CT) images for WT and MMP-9 null mice. MicroSPECT short-axis images are shown in standard format, coregistered with microCT images. 99mTc-NC100692 microSPECT images (red) are fused with 201Tl (green) and CT (gray) images to define uptake of {alpha}v{beta}3-targeted radiotracer relative to 201Tl perfusion defect and anatomic structures within chest. Boxed areas (left) are enlarged (right). A, right: regions of increased 99mTc-NC100692 uptake in anterior-lateral infarct territory (arrows) and 99mTc-NC100692 activity associated with angiogenesis at thoracotomy site (*). 99mTc-NC100692 activity is also seen in liver. B: by well counting, relative uptake of the 99mTc-labeled {alpha}v{beta}3 peptide NC100692 was higher in MMP-9 null mice (n = 7) compared with WT mice (n = 10) in infarct region (0–40% nonischemic region). *P < 0.05 for WT vs. MMP-9 null mice.

 
Angiogenic factor differences. To determine whether levels of angiogenic factors were also different in MMP-9 null mice, levels of Ang-1, Ang-2, the Tie-2 receptor, and VEGF were measured in the remote and infarct regions (Fig. 5). In the WT mice, Ang-1 and Ang-2 levels were significantly higher in the infarct region compared with the remote region. In the MMP-9 null mice, in contrast, Ang-1 and Ang-2 levels were not significantly different between the infarct and remote regions. Full-length Tie-2 levels were not different between regions or mice. Soluble Tie-2 levels, however, were significantly elevated in the infarct region of MMP-9 null mice compared with that of WT mice. VEGF levels were lower in the infarct regions of both WT and MMP-9 null mice and were lower in the remote region of MMP-9 null mice compared with that of WT mice.



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  		Fig. 5. Immunoblotting for angiogenic factors angiopoietin (Ang)-1 (A), Ang-2 (B), Tie-2 (C), and VEGF (D) in infarct and remote regions of WT and MMP-9 null mice at 7-days post-MI. Ang-1 and Ang-2 levels were increased in infarct region of WT mice compared with remote region. Soluble Tie-2 (sTie-2) levels were higher in infarct region of MMP-9 null mice compared with that of WT mice. VEGF levels were lower in infarct regions of both WT and MMP-9 null mice and were lower in remote region of MMP-9 null mice compared with that of WT mice. IOD, integrated optical density. *P < 0.05 vs. WT remote region; **P < 0.05 vs. WT infarct region.

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
MMPs in general and MMP-9 in particular have been identified as playing a role in myocardial remodeling in the context of MI. Induction of MMP-9 has specific functional consequences, including the attenuation of end-systolic and end-diastolic dimension increases and decreased collagen accumulation (23). Additionally, we have previously demonstrated that remodeling of the LV post-MI is attenuated in MMP-9 null mice (6). The effects of MMP-9 on angiogenesis, however, have not been fully addressed. MMP-9 is prominently expressed in macrophages, which have been shown to participate in the angiogenic process (27, 46). Here we examined the consequences of MMP-9 gene deletion on the angiogenic process and the kinetics of macrophage infiltration post-MI. The unique findings of this study are twofold. First, contrary to the generally accepted hypothesis, MMP-9 deletion yielded increased angiogenesis in the post-MI LV. The increase in angiogenesis was measured by three independent techniques. Second, macrophage infiltration is impaired in the absence of MMP-9. Based on these results, we propose that MMP-9 impedes neovascularization post-MI, likely as a secondary effect through the ability of MMP-9 to degrade substrates that modulate new vessel formation. Furthermore, this is the first study to demonstrate that MMP-9 is not necessary for neovascularization post-MI.

The angiogenic response is essential for adequate wound healing (15). In addition to restoring blood flow in the long term, angiogenesis also provides additional protection against further cell injury in the short term (28). Although the complete effects of angiogenic signals on the post-MI myocardium are not fully understood, a primary benefit of early angiogenesis may be the stimulation of signaling cascades that positively effect LV remodeling. For example, increased VEGF levels inhibit cardiomyocyte apoptosis (11). In the long term, post-MI angiogenesis provides a conduit for bioactive molecules to enter the ischemic tissue and serves to prevent additional ischemia (5). As such, the angiogenic response is critical for the healing response post-MI. The open artery hypothesis proposed by White and Braunwald (52) attests to the importance of restoring blood flow to the necrotic tissue, and previous studies (1, 18) demonstrate that late reperfusion attenuates LV dilation and loss of function even when myocardium can no longer be salvaged.

We measured the angiogenic response using three independent techniques: histological GSL-I quantification, in vivo GSL-I injection pre-MI, and in vivo injection of an {alpha}v{beta}3-targeted radiotracer post-MI. At 7-days post-MI, vessel density in the subendocardial and epicardial MI regions was increased in MMP-9 null mice compared with WT mice. The increase in these regions was not due to the preservation of preexisting vessels, because pre-MI injection of GSL-I demonstrated the absence of preexisting vessels within the infarct zone. Preexisting vessels were present in the remote noninfarcted region, which served as a positive-staining control. The {alpha}v{beta}3-integrin provides another independent marker of angiogenesis (7, 42, 49). Therefore, we also injected a 99mTc-labeled RGD peptide (NC100692), which targets {alpha}v{beta}3-integrin activation in angiogenic vessels, into post-MI mice. NC100692 provides high affinity [inhibitory constant (Ki), ~1 nM] and specific binding to the {alpha}v{beta}3-integrin (31). Recent studies (16) have employed a fluorescent analog of NC100692 to confirm binding specificity and localization of this {alpha}v{beta}3-targeted tracer in cultured endothelial cells. These investigators (16) also confirmed endothelial cell specificity on tissue sections using dual immunofluorescent staining of endothelium and the fluorescein-labeled analog of NC100692. One limitation of this study is that blood flow measurements were not performed to determine patency of the vessels within the infarct region. Nonetheless, all three techniques employed in the current study demonstrated increased vessel density in the infarct region of the MMP-9 null mice compared with WT post-MI controls.

Macrophage infiltration is a necessary component of the inflammatory response after an MI that serves to herald in a myriad of cytokines, growth factors, and MMPs that modulate several aspects of the LV remodeling process, including angiogenesis (10). Macrophages are essential for the development of collateral circulation (14). Although it is clear that macrophages play an obligatory role in vessel development, the exact mechanisms whereby they do so remain unknown. Macrophages have been shown to be a rich source of angiogenic factors (44) and produce a number of factors that regulate multiple steps in the angiogenic cascade (40, 45), including growth factors, cytokines, and proteases, such as VEGF, TNF{alpha}, interleukins, MMPs, and serine proteases (4, 33, 47). In addition, anti-angiogenic factors are also produced by macrophages, including angiostatin, thrombospondin-1, and tissue inhibitors of metalloproteinases (4). In this study, however, we demonstrate reduced macrophage infiltration in the MMP-9 null mice at 5-days post-MI, concomitant with increased levels of neovascularization. These results suggest that, contrary to previously held concepts, the overall influence of the macrophage post-MI may actually be to curtail rather than promote angiogenesis. Based on the results presented here, a study of longer duration would be warranted to determine whether a prolonged stimulation of neovascularization occurs in the absence of MMP-9.

Changes in the local balance among angiopoietins, Tie, and VEGF determine the overall angiogenic potential of a tissue by influencing vessel stability and maturation (20). Whereas Ang-1 is constitutively expressed by endothelial cells and pericytes, Ang-2 expression is transcriptionally regulated. Ang-2 is expressed by endothelial cells, neutrophils, and macrophages (19, 48). Whether or not Ang-2 is a natural antagonist of Ang-1 is controversial and may depend on concentration and localization (39, 51). Therefore, the present study measured Ang-1, Ang-2, Tie-2, and VEGF levels. Ang-1 and Ang-2 levels were both elevated in the infarct region of WT but not MMP-9 null mice compared with the respective remote region. The failure of Ang-2 levels to increase in the absence of MMP-9 may be due to the reduced macrophage infiltration and/or to the reduced generation of bioactive factors that would stimulate Ang-2 expression. In contrast, soluble Tie-2 levels were higher in the infarct region of MMP-9 null mice compared with WT. Several laboratories (20, 54) have reported the solubilization of Tie receptors, which may further stimulate signaling pathways involved in angiogenesis. VEGF levels in the remote region of MMP-9 null mice were decreased compared with WT mice. Lee and colleagues (20) observed changes in plasma Ang-1, Ang-2, Tie-2, and VEGF levels in patients with acute coronary syndromes (MI and unstable angina). The decreased Ang-2 level and increased soluble Tie-2 level in the MMP-9 null mice potentially yield a proangiogenic state and explain the increase in neovascularization.

It has been previously demonstrated that LV dilation was attenuated in MMP-9 null mice post-MI (6). In the present study, the dP/dtmax was improved in MMP-9 null mice compared with WT mice, despite similar MI sizes. Coupled with the observed increase in infarct-to-septal wall thickness ratio, wall stress is likely decreased, which would improve LV function. The preservation of wall thickness and subsequent improvement in LV function in the MMP-9 null mice post-MI, compared with WT mice, may be due in part to improved neovascularization. Alternatively, the converse may be true. Improved neovascularization may be due to preservation of wall thickenss secondary to reduced ECM degradation.

Several laboratories have previously investigated the angiogenic response to injury using MMP inhibition and/or the MMP-9 null mice. Apparently conflicting publications report increased or decreased angiogenesis in the absence of MMP-9. In models of hindlimb ischemia, ocular infection, early tumor formation, and spinal cord injury, MMP-9 deletion inhibits angiogenesis (17, 21, 32). Johnson et al. (17) demonstrated that femoral artery ligation stimulated angiogenesis in WT mice, whereas MMP-9 null mice displayed decreased capillary-branching density that is indicative of reduced neovascularization. Interestingly, the authors report that macrophage infiltration was totally obliterated in the absence of MMP-9, whereas in our study macrophage infiltration was attenuated but not absent. In cancer studies, the MMP inhibitor BB-94 inhibited angiogenesis when treatment was initiated at the premalignant stage (12). The MMP-9 null mice themselves were originally phenotyped by a delay in long-bone growth formation due to impaired angiogenesis (50). It is interesting to note that the angiogenic defect was localized to the long bone. In contrast, models of late-stage tumors and myocardial injury demonstrate increased angiogenesis in the absence of MMP-9. The MMP inhibitor BB-94 that inhibited angiogenesis at the premalignant stage increased angiogenesis in clinical studies of patients with non-small cell lung carcinoma (12). In a rabbit MI model, Lee's laboratory (24) previously demonstrated increased vessel density in the subendocardial layer of animals treated with a selective MMP inhibitor that did not inhibit MMP-1. Together, these data suggest that the net response of MMP-9 deletion is the inhibition of angiogenesis in tissues that are not highly vascularized. In contrast, the net response of MMP-9 deletion in tissues that are highly vascularized is the stimulation of angiogenesis. Similar balances are also seen for cell growth and apoptotic processes (26).

In summary, MMP-9 deletion stimulated neovascularization and improved LV function in a murine model of post-MI ventricular remodeling. Although MMP-9-mediated stimulation and inhibition of neovascularization likely occur in tandem, the balance of the two processes is potentially altered in a context-specific manner to yield vessel growth or inhibition. The unique results of this study suggest that targeted strategies to inhibit MMP-9 early post-MI are not likely to impair the angiogenic response.


   GRANTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
REFERENCES
 
This study was supported by National Heart, Lung, and Blood Institute Grants HL-10337 and HL-75360 (to M. L. Lindsey), HL-65273 (to R. T. Lee), HL-65662 (to A. J. Sinusas), HL-45024, HL-97012, P01–48788, and a Veterans Administration Career Development Award (to F. G. Spinale). NC-100692 was provided through a grant from GE Healthcare (to A. J. Sinusas).


   ACKNOWLEDGMENTS
 
The authors thank Jennifer K. Hendrick, Julie McLean, and Sarah E. Camens for excellent technical assistance and Dr. Zena Werb for supplying MMP-9 heterozygous mice.


   FOOTNOTES
 

Address for reprint requests and other correspondence: M. L. Lindsey, Cardiothoracic Surgery, Rm. 629, Strom Thurmond Research Bldg., 770 MUSC Complex, Medical Univ. of South Carolina, 114 Doughty St., PO Box 250778, Charleston, SC 29425 (e-mail: lindseym{at}uthscsa.edu)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 

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