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Vascular Biology |
From the Department of Surgery, Division of Vascular Surgery, Vanderbilt University Medical Center (X.Q., M.A.C., R.J.G.), Nashville, Tenn; Guangxi Medical University First Affiliated Hospital, China (X.Q.); and the Departments of Cell and Developmental Biology (R.J.G.) and Cancer Biology (L.M.M.), Vanderbilt University Medical Center, Nashville, Tenn.
Correspondence to Raul J. Guzman, MD, Division of Vascular Surgery, Vanderbilt University Hospital, D-5237, MCN Building, 1161 21st Ave S, Nashville, TN 37232. E-mail raul.guzman{at}vanderbilt.edu
| Abstract |
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Methods and Results Aortic calcification was first induced in male Sprague-Dawley rats by administration of vitamin D3. Treatment with doxycycline decreased aortic calcium and phosphorus accumulation, and it reduced aortic gelatinase levels; however, it also prevented the bone resorption associated with high doses of vitamin D3. Using an in vivo model of localized aortic calcification, systemic doxycycline treatment reduced aortic calcium accumulation without affecting serum calcium levels, suggesting a more specific effect of doxycycline in the arterial wall. In organ culture, doxycycline limited aortic calcification caused by exposure to alkaline phosphatase and inorganic phosphate. When GM6001, a synthetic and specific inhibitor of MMPs, was used instead of doxycycline, it had a similar effect. In vivo, periadventitial delivery of GM6001 to calcifying arteries significantly reduced calcification compared with controls.
Conclusions These results suggest that MMPs are involved in aortic calcification, and inhibiting MMP activity could reduce calcium accumulation in the arterial wall.
Arterial calcification is associated with MMP-mediated matrix degradation. Inhibiting MMP activity with doxycycline or the synthetic MMP-inhibitor GM6001 prevents calcium accumulation in the arterial wall.
Key Words: artery calcification MMPs doxycycline GM6001 rat
| Introduction |
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MMPs are involved in multiple processes in the vascular wall.16 They are upregulated in human atherosclerotic and restenotic lesions, and they are involved in aneurysm formation.1721 Inhibiting MMP activity prevents injury-induced arterial remodeling in rodent models.22,23 Recent studies demonstrated a correlation between MMP-mediated elastin degradation and aortic calcification. Inhibiting elastin degradation with aluminum ions prevented calcification in the aortas of rats after calcium chloridemediated injury; and mice deficient in MMP-2 and MMP-9 did not develop calcification in a similar model.24 Additionally, Lee et al demonstrated increased MMP activity associated with increased levels of soluble elastin peptides in a rat subdermal model of elastin calcification.25 In the present study, we sought to determine whether inhibiting MMP activity could prevent calcium accumulation in experimental models of aortic calcification. We demonstrate that doxycycline, an antibiotic that inhibits MMP activity and reduces MMP levels, and GM6001, a synthetic selective MMP inhibitor, can inhibit arterial calcification both in organ culture and in vivo.
| Materials and Methods |
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Experimental Procedures
Arterial calcification was induced in male Sprague-Dawley rats by subcutaneous injection of 7.5 mg/mL vitamin D3 for 3 consecutive days. Rats received daily doxycycline by subcutaneous injection beginning at 2 days before vitamin D3 injections and ending at the time of harvest. In another group of rats, aortic calcification was induced by periadventitial application of 0.15 mol/L CaCl2 as described recently.24
Calcium, Phosphorus, Bone Densitometry Measurements
Detailed methods for calcium, phosphorus, and bone densitometry measurements are available in the online supplement.
Organ Culture
Segments of rat aorta measuring 1 cm underwent calcification as described previously.26 Doxycycline (100 µg/mL) was pre-equilibrated in medium and then added to the culture medium. In a separate series of experiments, the synthetic MMP inhibitor GM6001 (12.5 or 25 µmol/L) or GM6001-negative control was used instead of doxycycline.
In Vivo Delivery of GM6001
For in vivo evaluation of GM6001, rats underwent periadventitial application of CaCl2 solution or normal saline. The synthetic MMP inhibitor GM6001 or GM6001-negative control was then administered via the catheter on a daily basis until the time of aortic harvest.
| Results |
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Effect of Doxycycline on Vitamin D3Induced Aortic MMP Expression, Serum Calcium, and Bone Mineral Density
In rats given vitamin D3, we observed a decrease in aortic gelatinase levels from rats treated with 60 mg/kg doxycycline compared with those treated with vehicle (Figure 2A). The band corresponded to the band known to be MMP-9 found in conditioned medium from the human HT 1080 cell line (supplemental Figure II). No consistent changes were observed in bands located where MMP-2 is usually seen.
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We then evaluated the changes in serum calcium concentration caused by vitamin D3 injection and 60 mg/kg doxycycline treatment. Serum calcium was significantly increased in vitamin D3injected rats compared with controls, and this was not affected by treatment with doxycycline. The calcium concentration was 10.58±0.54 mg/dL in control rats versus 15.23±0.44 in vitamin D3injected rats (P<0.05 versus controls) and 15.95±0.84 in vitamin D3injected plus doxycycline (P=NS compared with vitamin D3 alone; Figure 2B).
Bone density was significantly lower in vitamin D3injected rats than in controls, but doxycycline treatment partially reversed this decrease. Bone density was 0.120±0.002 g/cm2 in controls compared with 0.087±0.002 in vitamin D3injected rats (P<0.05 versus control) and 0.101±0.005 in vitamin D3injected plus doxycycline rats (P<0.05 versus controls; P<0.05 versus vitamin D3injected; Figure 2C). This suggests that some effects of doxycycline on aortic calcification could occur through altering the increased bone resorption caused by high doses of vitamin D3.
Doxycycline Inhibits Aortic Calcification Caused by Periadventitial CaCl2 Administration
We next evaluated whether the effects of doxycycline could be reproduced in a model of arterial calcification that is not characterized by alterations in bone resorption. We studied the effects of doxycycline treatment in rats that underwent periadvential administration of 0.15 mol/L CaCl2. Animals not treated with doxycycline demonstrated medial and adventitial accumulation of calcium by von Kossa staining. However, animals treated with doxycycline had nearly complete inhibition of calcification as assessed by the von Kossa stain and by measurement of aortic calcium in harvested specimens. (Figure 3A and 3B). The aortic calcium content was 0.06±0.015 µmol/L/mg for NaCl control rats (n=3), 0.33±0.06 for CaCl2 rats not given doxycycline (n=4), and 0.13±0.11 µmoles/mg for rats treated with 60 mg/kg doxycycline (n=5; P<0.05; Figure 3C). Additionally, immunohistochemical staining of these specimens using the macrophage-specific antibody ED-1 demonstrated macrophage infiltration in the adventitia of untreated rats, but this was not see in doxycycline-treated rats (supplemental Figure III). Serum calcium levels in doxycycline-treated rats were not different from control rats (12.0±0.13 for controls versus 11.27±0.35 for doxycycline-treated rats; P=NS).
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In Vitro Effect of Doxycycline and GM6001 on Arterial Calcification
We next investigated whether the inhibitory effects of doxycycline on in vivo calcification could be reproduced in an organ culture system. This has the benefit of excluding the effects of doxycycline on systemic, circulating factors. Aortas were calcified in an organ culture system by addition of calf intestinal alkaline phosphatase and sodium phosphate to DMEM without serum (calcification medium).26 von Kossastained sections demonstrated calcium accumulation in the adventitia and outer medial layers of vessels cultured in calcification medium but little calcification in those also treated with doxycycline (supplemental Figure IV). The calcium content of vessels cultured in DMEM alone was 0.284±0.090 µmol/L, and there was a 3-fold increase in calcium content to 0.953±0.280 µmol/L for those cultured in calcification medium. Arteries cultured in calcification medium with 100 µg/mL doxycycline demonstrated a significant decrease in calcium accumulation (0.365±0.028; P<0.05 compared with calcification medium alone; Figure 4A). Gelatinase levels in the medium of doxycycline-treated samples were also reduced (Figure 4B).
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Although doxycycline has been useful for evaluating a drug with MMP-inhibiting properties, its experimental use is limited because it has several biological effects that are not related to MMPs. To determine whether the effects of doxycycline could be reproduced using a compound with more specific anti-MMP activity, we used the synthetic MMP inhibitor GM6001. Histological evaluation of these segments by von Kossas stain failed to demonstrate calcium accumulation in GM6001-treated arteries (Figure 5A and 5B). Aortas exposed to calcification medium showed an increase in calcium content compared with control vessels (0.211±0.016 µmol/L for control compared with 2.525±0.670 for calcification medium). When GM6001 was added to the calcification medium, there was a dose-dependent reduction in aortic calcium accumulation. Aortic calcium was 2.331±0.456 µmol/L for 12.5 µmol/L (P=NS) and 0.434±0.088 for 25 µmol/L GM6001 (P<0.05 versus no GM6001; Figure 5C). Viability of cultured segments at 7 days was not affected by calcification medium or GM6001, as evidenced by measurement of lactic dehydrogenase secretion into the medium, which was not different from control segments (data not shown).
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GM6001 Inhibits Aortic Calcification In Vivo
Based on our findings in organ culture, we next evaluated whether GM6001 could be used to inhibit aortic calcification in vivo. Daily injections of GM6001 or its negative control (250 µg per day) were administered by catheter into the periaortic space after periadventitial application of calcium chloride. Aortas from control rats that underwent painting with NaCl had minimal calcification after 7 days. However, CaCl2-painted arteries treated with GM6001-negative control had a >10-fold increase in calcium accumulation, and this was significantly decreased by local delivery of GM6001 (0.166±0.026 µmol/L for normal saline controls; 1.983±0.477 for CaCl2 with GM6001-negative control; and 0.656±0.331 for CaCl2 painted vessels treated with GM6001; P<0.05). Aortic histology from rats that received the negative control compound demonstrated extensive calcification in the adventitia and in the outer medial layers, whereas there was only occasional calcium staining in aortas from rats treated with GM6001 (Figure 6).
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| Discussion |
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The changes in arterial histology associated with hypervitaminosis D have been well characterized.2729 Rats exposed to sublethal doses of vitamin D initially develop calcium phosphate crystals along the elastic laminae followed by spreading of calcification into the surrounding medial smooth muscle cells.27 The model is characterized by increased bone resorption and increased intestinal uptake of ingested calcium, leading to elevated serum calcium levels.28 Although the mechanisms related to arterial calcification in this model have not been fully elucidated, it is thought to occur through an interaction between degraded medial elastin and circulating factors from resorbed bone. Price et al demonstrated recently that a fetuin-matrix Gla proteinmineral complex is increased in the circulation of rats exposed to toxic doses of vitamin D, whereas serum levels of fetuin, a proposed inhibitor of calcification, are decreased.30 They propose that high levels of fetuin in complex may prevent it from inhibiting the growth of mineral components in the elastic layer of the vessel wall. Alternatively, the fetuinmineral complex might serve as a marker for a presently unknown causative factor.
In our experiments, administration of vitamin D3 increased MMP-9 levels in the aortic wall and increased serum calcium levels. Doxycycline treatment reduced aortic MMP-9 levels back to normal, but it had no effect on serum calcium levels. This suggests that doxycycline may act in a manner that is similar to ibandronate or osteoprotegrin, which inhibit bone resorption but do not affect serum calcium levels in vitamin D3treated rats.3133 However, we have not excluded the possibility that the effects of doxycycline are related to other mechanisms such as increases in inhibitory factors, decreases in positive regulatory factors, direct binding of doxycycline to hydroxyapatite, antibiotic properties, or other yet unknown factors. Additionally, there are known interactions between doxycycline and calcium, and these may have occurred in our model despite the lack of effect on serum calcium levels. Also notable is the significant weight loss seen in vitamin D3-treated rats. This weight loss was not reversed by treatment with doxycycline, which suggests that vitamin D3 administration might have indirect consequences. For example, leptin levels may have been affected in this model, and this may have resulted in indirect effects on vascular calcification.34,35
The importance of MMPs in arterial calcification has been suggested previously by Basalyga et al, who demonstrated that mice deficient in MMP-2 or MMP-9 failed to develop aortic calcification after perivascular application of CaCl2.24 Additionally, Lee et al recently demonstrated by gelatin zymography and RT-PCR that MMP-2 and MMP-9 levels are increased in subdermally placed elastin that is undergoing active calcification.25 In the present series of experiments, we demonstrated that MMP-9 levels are increased in calcifying aortas, and that decreased gelatinase levels are associated with a reduced amount of aortic calcium accumulation. However, other MMPs are also capable of degrading medial elastin, and it is possible that MMPs other than the gelatinases could be responsible for the elastin disruption seen in this model.
A mechanism for elastin calcification that involves a positive feedback loop between degraded elastin and MMP gene expression has been proposed recently.25 According to this model, physical or biochemical injury may disrupt the glycoproteins normally surrounding elastin. This may serve to expose elastin to cells that, in turn, produce MMPs or other serine proteases that can degrade elastin. MMPs can then further degrade both the protective glycoproteins in the arterial wall and the exposed elastin. Elastin degradation may also release matrix-bound cytokines that can recruit inflammatory cells and cause smooth muscle cells to undergo a phenotypic modulation into more osteoblastic-type cells. Our data on aortic gelatinase levels during calcification are consistent with this model and furthermore suggest that the enzymes required for elastin degradation can be generated within the arterial wall.
A recently described model of aortic calcification involves the periadventitial application of CaCl2.24 Because calcification in this model is related to local factors, it is not associated with altered circulating levels of calcium, phosphate, or other products of bone resorption. Using this model, we were also able to demonstrate a significant reduction in aortic calcium accumulation by systemic treatment with doxycycline. The inhibitory effects of doxycycline on aortic calcification in this model argue against a mechanism related specifically to circulating mineral complexes; however, there are ongoing low levels of bone resorption that may be inhibited by doxycycline, and even these minimal changes may be sufficient to affect arterial calcification. Additionally, we noted strong ED-1specific staining in the adventitia of calcifying aortas, but this was not seen in doxycycline-treated rats. Although the role of macrophages in this model has not been defined, it is possible that they are another source of matrix-degrading enzymes necessary for the calcification to proceed. The potential role of inflammatory cells and their secreted mediators on arterial calcification has been reviewed recently,36 and further studies in this regard are needed.
Our experiments in organ culture allowed us to focus on the direct effects of MMP inhibition on the aortic wall. In this system, calcification is thought to occur because of inhibition of pyrophosphate production and increased calciumxphosphorus product in the medium.26 Doxycycline reduced aortic calcium levels, and this effect was associated with decreased MMP levels in the culture medium. This likely occurred through the previously described inhibitory effects of doxycycline on MMP synthesis.37 However, the use of doxycycline in mechanistic studies is limited because of its diverse actions, many of which are not related to altering MMP levels. For this reason, we extended our experiments to include the synthetic compound GM6001 that specifically inhibits the MMPs. We found that addition of GM6001 to the calcification medium resulted in a dose-dependent decrease in arterial calcium accumulation. On histology, GM6001-treated vessels had less calcium deposition in the adventitia and outer medial layers. GM6001 is a potent, hydroxamic acidbased MMP inhibitor with activity against several MMPs, including collagenase, the gelatinases, and stromelysin. Both the gelatinases and stromelysin have elastolytic activity, and a possible role of MMPs in medial calcification is in initiating the process of elastin degradation that may serve to provide a nidus for hydroxyapatite crystals to develop. These in vitro results suggested that synthetic MMP inhibitors might prevent arterial calcification in vivo.
Our final series of experiments involved the use of GM6001 in vivo. For these studies, GM6001 was delivered locally, near the aorta at a concentration that was similar to that used in organ culture, and we found that daily administration reduced aortic calcium content compared with negative control treated vessels. Although the source of MMP activity in this model is presently unknown, the potential sources include medial smooth muscle cells, adventitial fibroblasts, or infiltrating inflammatory cells, all of which would have been affected by local delivery of the inhibitor. GM6001 is a synthetic inhibitor with activity against several MMPs, and thus, the identity of MMPs involved in arterial calcification remains to be determined. Based on the current model, it is likely to be an MMP with elastolytic properties that can be produced either within the aortic wall or by cells that can reside in the adventitia. One possibility is that GM6001 has direct effects on the CaCl2 solution used to inhibit calcification. However, the accumulation of aortic calcium we observed with the negative control compound argues against this point. Another limitation of this technique is that compound delivery is limited to 5 to 7 days because of the formation of a fibrous capsule around the catheter. However, the fact that catheter delivery was successful suggests that aortic calcification occurs early in this model and that it can be inhibited by drug administration during the first several days. Alternatively, sufficient compound may have diffused through the capsule to have an effect at later time points. Efforts to understand the temporal aspects of this model and the relative contributions of MMPs during the various phases of arterial calcification are under way.
Together, our data suggest that MMPs are involved in arterial calcification and that inhibiting MMPs may be a clinically useful method for reducing calcification in the vessel wall. Arterial calcification is a complex phenomenon that is likely regulated at several levels, and further efforts to understand the role of MMPs in this pathologic process are needed.
| Acknowledgments |
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Sources of Funding
This research was supported by mentored clinical scientist development award HL069926 from the National Heart, Lung, and Blood Institute (R.G.) and grants from the Lifeline Foundation and the William J. von Liebig Foundation.
Disclosures
None.
| Footnotes |
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| References |
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