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J Am Coll Cardiol, 2004; 44:661-666, doi:10.1016/j.jacc.2004.04.046 © 2004 by the American College of Cardiology Foundation |

* Third Department of Internal Medicine and Cardiovascular Research Institute, Kurume University, Kurume, Japan
Cardiovascular Medicine, Kyushu University Graduate School of Medicine, Fukuoka, Japan
Manuscript received January 7, 2004; revised manuscript received March 18, 2004, accepted April 20, 2004.
* Reprint requests and correspondence: Dr. Hisashi Kai, Third Department of Internal Medicine and Cardiovascular Research Institute, Kurume University, 67 Asahi-machi, Kurume 830-0011, Japan (Email: naikai{at}med.kurume-u.ac.jp).
| Abstract |
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BACKGROUND: Roles of inflammatory changes including macrophage infiltration are suggested in ischemic neovascularization.
METHODS: Unilateral hindlimb ischemia was induced by excising surgically the entire femoral artery and vein in mice. Immediately after operation, plasmid deoxyribonucleic acid encoding a dominant negative mutant of MCP-1 (7ND) or the empty plasmid (mock) was injected into the ipsilateral thigh adductor muscle.
RESULTS: In mock-treated mice, MCP-1 was upregulated transiently in ischemic hindlimb peaking at day 3. Serial laser Doppler blood flow (LDBF) analysis showed an abrupt decrease in blood flow, followed by a recovery to the near-normal levels in mock-treated mice; 7ND treatment had no effects on the initial decrease in LDBF but deteriorated the recovery. At day 3, macrophage infiltration and inductions of tumor necrosis factor (TNF)-alpha and vascular endothelial growth factor (VEGF) were prominent in the ischemic adductor muscle in mock-treated mice; 7ND treatment significantly reduced macrophage infiltration and suppressed TNF-alpha and VEGF inductions in response to ischemia. At day 21, postmortem angiography and anti-CD31 immunohistostaining revealed well-developed collateral vessels and capillary formation, respectively, in the ischemic muscle of mock-treated mice; 7ND overexpression remarkably suppressed the collateral vessel formation and capillary formation.
CONCLUSIONS: Endogenous MCP-1 may play a role in ischemia-induced neovascularization by recruiting macrophages that activate TNF-alpha and VEGF inductions.
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A mutant of MCP-1, which lacks the N-terminal amino acids 2 to 8 (7ND), has a potent dominant negative activity (11). A series of recent studies have shown that overexpression of 7ND is a useful strategy for blocking MCP-1 activity in vivo in various cardiovascular disease models (12,13). In the present study, we sought to determine the role of the endogenous MCP-1 in ischemia-induced neovascularization in mouse hindlimb. For this purpose, effects of blocking MCP-1 activity in the ischemic hindlimb were investigated by overexpressing 7ND in the ischemic tissue.
| Methods |
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Animal model and gene transfer. The study protocol was approved by the Institutional Animal Care and Treatment Committee. After being anesthetized with intraperitoneal pentobarbital (50 mg/kg), the entire left femoral artery and vein were excised surgically in eight-week-old male C57BL/6 mice (Clea Japan Inc., Tokyo, Japan), as described previously (14,15). Immediately after operation, 30 µl of saline containing denoted dose of plasmid encoding 7ND complementary DNA or the empty plasmid (mock) was injected into the exposed ipsilateral thigh adductor muscle. Sham mice underwent the same operational procedure except that the artery and vein were left intact.
Expression of transfected 7ND was determined in the thigh adductor muscle on the basis of Western blot analysis using an anti-FLAG antibody. After mice were killed by an overdose of intraperitoneal pentobarbital, the thigh adductor muscle was removed, snap-frozen in liquid nitrogen, and stored at 80°C until use. The tissue was homogenized by using FastPrep homogenizer (ThermoSavant, Holbrook, New York), and the tissue protein was extracted, separated on 10% SDS-PAGE, and subjected to Western blot analysis (16). As shown in Figure 1, 7ND expression was transiently observed in the adductor muscle, peaking at days 3 to 7 and returning to insignificant levels by day 14 after operation and gene transfer.
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Histologic analysis. After mice were killed with an overdose of pentobarbital, the thigh adductor muscle was excised, fixed in methanol, embedded in paraffin, and then sectioned into 5-µm slices.
Capillary density
Capillary endothelial cells (ECs) were identified by immunohistostaining with an anti-CD31 monoclonal antibody (Pharmingen, Franklin Lakes, New Jersey). The number of capillaries was counted and averaged in 15 random microscopic fields from three independent cross-sections of the adductor muscle in each animal (n = 12) at day 21, and then capillary density was expressed as the number of capillaries per high-power field (x400).
Macrophage density
Infiltrated macrophages were detected immunohistochemically by using an anti-F4/80 monoclonal antibody (Serotec, Raleigh, North Carolina) in the cross-section of the adductor muscle at day 3. Macrophages were counted in 15 random microscopic fields from three independent sections of each animal (n = 8), and macrophage density was described as the number of F4/80-labeled cells per high-power field (x400) (8).
Tissue contents of MCP-1, VEGF, and TNF-alpha. At denoted day, the thigh adductor muscle (n = 8 per group) was excised, snap-frozen in liquid nitrogen, and stored at 80°C until use. After homogenization by using FastPrep (ThermoSavant), the tissue proteins were extracted, and the contents of MCP-1, VEGF, and TNF-alpha were determined by using a commercially available ELISA kit for mouse MCP-1, mouse VEGF, and mouse TNF-alpha (R&D System, Minneapolis, Minnesota), respectively, according to the manufacturer's instruction.
Postmortem angiography. At day 21, mice (n = 12 per group) were killed by injecting intraperitoneally an overdose of pentobarbital. Postmortem angiography was performed by injecting 200 µl of contrast media through a 26-gauge soft-tip catheter, which was inserted into the abdominal aorta, at a perfusion pressure of 80 to 90 mm Hg. X-ray angiograms were taken using a mammography system (Senographe 500T, GE Medical Systems, Tokyo, Japan), and the extent of collateral vessel formation was quantified by angiographic score, as described previously (8,17).
Statistical analysis. Statistical analysis was adequately performed by unpaired Student t test or analysis of variance followed by Scheffé's F test. A value of p < 0.05 was considered statistically significant. Angiographic score, capillary density, and macrophage count were evaluated by two observers in a blind manner, and the values obtained by the two observers were averaged. The interobserver or intraobserver variation was <5% in each experiment.
| Results |
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Transient MCP-1 expression in ischemic hindlimb. To determine the change in endogenous MCP-1 expression, tissue content of MCP-1 was measured in the thigh adductor muscle. Tissue MCP-1 levels were quite low in sham mice during the observation period (data not shown). Endogenous MCP-1 was transiently and remarkably upregulated in the ischemic muscle with a peak at day 3, remaining relatively high at day 7, and returning to the baseline by day 14 (Fig. 2).
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Impaired neovascularization in 7ND-treated mice. The development of angiographically visible collateral vessels and capillary formation were studied to determine whether impaired neovascularization was associated with reduced LDBF in ischemic hindlimb treated with 7ND. First, at day 21, postmortem angiography was performed, and the angiographic score was investigated (Fig. 6A). Mock-treated mice showed well-developed collateral vessels in the ischemic hindlimb. The number of collateral vessels was significantly reduced by overexpressing 7ND, and 7ND decreased the angiographic score to 60% of mock-treated mice.
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| Discussion |
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As shown in Figure 2, unilateral hindlimb ischemia induced a transient and remarkable MCP-1 induction in the thigh adductor muscle, peaking at day 3. It was noteworthy that the peak of MCP-1 induction coincided with that of macrophage infiltration seen in our previous study (8). Thus, we sought to clarify the role of endogenous MCP-1 by overexpressing 7ND, a dominant negative mutant of MCP-1, in the ischemic adductor muscle, because this mutant can bind to the MCP-1 receptors on the target cells but does not induce the intracellular signaling activation (11). When complementary DNA encoding 7ND was injected into the adductor muscle at the time of the surgical removal of femoral artery and vein, a transient, yet significant, expression of 7ND protein was observed on the basis of Western blotting against the FLAG epitope tag (Fig. 1). Because transfected 7ND was expressed as long as ischemia-induced MCP-1 induction (Fig. 2) was present, it was expected that activity of endogenous MCP-1 would be blocked in ischemic hindlimb. Indeed, as shown in Figure 4, macrophage infiltration was remarkably inhibited in the ischemic muscle by 7ND overexpression. This finding indicates that MCP-1 is a major determinant of the macrophage recruitment into the ischemic hindlimb. Along with ischemic hindlimb, MCP-1 induction was documented in ischemic cerebral cortex (18), ischemia-reperfused heart (19), ischemia-reperfused kidney (20), and ischemic retinopathy (21). The precise mechanism whereby ischemia induces MCP-1 induction remainsunknown at present. Activation of local angiotensin II system may be involved in one of the mechanisms, because MCP-1 induction in the ischemic hindlimb is remarkably attenuated in the angiotensin II type 1 receptor knockout mice (8).
The present study demonstrated that when MCP-1 activity was blocked, inductions of VEGF and TNF-alpha were significantly reduced in the ischemic hindlimb (Fig. 5). The attenuation of VEGF and TNF-alpha inductions could be responsible for the decreased formation of capillaries and angiographically visible collaterals, resulting in deteriorated recovery of blood flow of the ischemic hindlimb in 7ND-treated mice. It is indicated that infiltrated macrophages may facilitate ischemia-induced arteriogenesis and angiogenesis by producing VEGF (8). Moreover, activated macrophages have been shown to predominantly produce TNF-alpha upon the hindlimb ischemia (4). Tumor necrosis factor-alpha is a major mediator of inflammatory reactions, and also participates in arteriogenesis characterized by the formation of angiographically visible collaterals in the ischemic hindlimb (4). Because TNF-alpha is responsible for adhesion and activation of additional macrophages via upregulation of cell adhesion molecules on both ECs and macrophages (22,23), the angiogenic properties of TNF-alpha may be attributable, in part, to the formation of positive feedback loop of the macrophage-mediated mechanisms. Taken together, inhibition of TNF-alpha induction may lead to the elimination of ischemia-induced neovascularization both through the prevention of TNF-alphamediated exaggeration of inflammatory process and through the attenuation of the angiogenic properties of TNF-alpha. In addition, it has been shown that ECs express the MCP-1 receptor CCR2 and that MCP-1 induces proliferation and migration of ECs (24). Thus, it is also possible that 7ND overexpression may inhibit the direct angiogenic effect of MCP-1. As shown in Figures 4B and 5, the inhibitory effect of 7ND on VEGF induction was smaller than those on macrophage infiltration. Our previous study demonstrated that, in addition to inflammatory cells, skeletal myocytes express VEGF in the adductor muscle of the ischemic hindlimb (8). Thus, the skeletal myocytes, which were not inhibited by 7ND, may be responsible for the 7ND-insensitive VEGF production.
It was apparent that the observed effects of 7ND are partial on the inhibition of ischemia-induced neovascularization, although macrophage infiltration was almost abolished. The reduction in LDBF ratio by 7ND was most evident at day 7, and thereafter the extent of the LDBF reduction was smaller in the later phase. It was possible that inflammatory process was activated after 7ND expression had decayed. However, this does not seem the case because there were only sparse interstitial cells in the ischemic thigh muscle after day 14, irrespectively of 7ND treatment (Fig. 4). Another possible explanation is that macrophage-mediated inflammation participates in the recovery of LDBF of the ischemic hindlimb, mainly in the early phase, as an initiator of ischemic neovascularization. Thus, the 7ND-insensitive neovascularization, especially in the later phase, may be dependent upon the mechanisms related to cells other than macrophages.
At day 21, 7ND overexpression remarkably reduced collateral vessel formation and the intramuscular capillary formation in the ischemic hindlimb, whereas the inhibitory effects on the recovery of LDBF were modest. In the present study, we used an LDBF imaging system that can detect blood flow of the tissue surface to a depth of 600 µm (14), indicating that LDBF images reflect mainly the superficial and subdermal blood flow and, to lesser extent, the intramural blood flow. This may explain the difference in the response to 7ND between LDBF image and other parameters of neovascularization (angiography and histology). This issue should be addressed in future studies.
Study limitations. It has been shown that 7ND is not only expressed in the transfected site (e.g., thigh muscle) but also is secreted into the systemic circulation and, in turn, blocks MCP-1 activity in the remote organ (12,13). Thus, we do not exclude the possibility that mobilization of endothelial progenitor cells from bone marrow is reduced by 7ND overexpression in ischemic hindlimb.
In conclusion, neovascularization and blood flow recovery in response to hindlimb ischemia were significantly impaired in mice with functional blockade of MCP-1 activity. Thus, it is indicated that the induction of endogenous MCP-1 participates in angiogenesis and arteriogenesis by recruiting macrophages into the ischemic tissue. The present study would provide an insight into better understanding of the underlying mechanism of ischemia-induced neovascularization.
| Acknowledgments |
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| Footnotes |
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| References |
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