Published online before print March 2, 2006, doi: 10.1161/01.ATV.0000216119.79008.ac
© 2006 American Heart Association, Inc.
Atherosclerosis and Lipoproteins |
Selective Cyclooxygenase-2 Inhibition With Celecoxib Decreases Angiotensin II-Induced Abdominal Aortic Aneurysm Formation in Mice
From the Cardiovascular Research Center (V.L.K.), Department of Internal Medicine (V.L.K.), Department of Pharmaceutical Sciences, College of Pharmacy (D.B.T., J.M.G., C.D.L.), University of Kentucky, Lexington.
Correspondence to Victoria L. King, Cardiovascular Research Center, Wethington Building, Room 562, University of Kentucky, Lexington, KY 40536-0020. E-mail vicky.king{at}uky.edu; or Charles D. Loftin, Department of Pharmaceutical Sciences, 725 Rose St, Room 414, University of Kentucky, Lexington, KY 40536-0082. E-mail cdloft2@uky.edu
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Abstract |
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Objective— Inflammation plays an integral role in the development of abdominal aortic aneurysms (AAAs), and the expression of cyclooxygenase (COX)-2 is increased in aneurysmal tissue compared with normal aorta. Nonsteroidal anti-inflammatory drugs, which inhibit the activity of COX-1 and COX-2, decrease AAA expansion in humans and animal models of the disease. In the current study, we investigated the effectiveness of selective inhibition of COX-1 or COX-2 in attenuating AAA formation.
Methods and Results— Eight-week-old male apolipoprotein E-deficient mice were treated with selective inhibitors of COX-1 or COX-2, SC-560 (
25 mg · kg–1 · day–1), or celecoxib (125 mg · kg–1 · day–1), respectively. COX inhibitors were administered 1 week before angiotensin II (Ang II; 1000 ng · kg–1 · min–1) or saline infusion and throughout the time course of the experiment. COX-1 inhibition had no effect on incidence (control: 90% [9:10] versus SC-560: 89% [8:9]) or severity of Ang II-induced AAA formation. In contrast, celecoxib decreased the incidence (control: 74% [22:30] versus celecoxib: 11% [2:19]; P<0.001) and severity (P=0.001) of AAA formation. Celecoxib also decreased the incidence and severity of AAAs in nonhyperlipidemic mice.
Conclusions— COX-2–derived prostanoids play a fundamental role in the development of Ang II-induced AAAs in both hyperlipidemic and nonhyperlipidemic mice.
COX-2 expression is increased in aneurysmal tissue, and nonselective inhibition of COXs decreases AAA expansion. The present study demonstrates that selective COX-1 inhibition does not alter Ang II-induced AAA formation. In contrast, selective COX-2 inhibition with celecoxib attenuates Ang II-induced AAA formation in both nonhyperlipidemic and hyperlipidemic mice.
Key Words: cyclooxygenase-1 • cyclooxygenase-2 • abdominal aortic aneurysms • celecoxib • prostaglandin E2
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Introduction |
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Abdominal aortic aneurysms (AAAs) are an increasing health concern, particularly in the male population >65 years of age. AAAs are a permanent dilation of the artery leading to extensive remodeling of the vessel wall with a potential for rupture and resulting mortality. Hallmarks of AAAs are proteolytic degradation and the presence of an inflammatory infiltrate within the vascular wall.1 Inflammatory cells within the vascular wall are a significant source of proteolytic enzymes that contribute to the disease.1,2 Thus, a putative cause of human AAAs may be cellular remodeling within the vascular wall resulting from chronic inflammation and matrix degradation.
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Prostanoids are a class of inflammatory mediators that are dramatically increased in aneurysmal tissue.3–5 Prostanoids, which include prostaglandins and thromboxane A2, are synthesized by the 2 known isoforms of prostaglandin G/H synthase, also known as cyclooxygenase (COX)-1 and COX-2.6–8 COX-1 is constitutively expressed in most tissues, whereas COX-2 expression is inducible and is primarily responsible for the synthesis of prostanoids that contribute to inflammation.6–8 COX-2 expression is induced during the development of aneurysms, whereas, COX-1 expression is not altered, suggesting a primary role for COX-2 in the development of this disease.3–5 Although a variety of inflammatory cells are present in aneurysmal tissue, macrophages are believed to have a pronounced role in the pathogenesis of AAAs.2,9,10 Activated macrophages within the inflammatory infiltrate of human AAAs are a significant source of COX-2 expression, which may be responsible for the synthesis of prostanoids contributing to the vascular inflammation.4,5
The prostanoid most often observed in human aneurysmal tissue is prostaglandin E2 (PGE2).4,5 PGE2 synthesized by macrophages and smooth muscle cells (SMCs) increases the production of matrix metalloproteinases (MMPs), which are suggested to play a prominent role in the degradation of the vascular wall.2,11–13 Furthermore, SMCs from human aneurysmal aortic explants are more sensitive to PGE2-induced cell death compared with SMCs derived from normal aortas.5 The increased synthesis of PGE2 by aneurysmal tissue also stimulates production of inflammatory cytokines, which may exacerbate the aneurysmal process by further recruitment of inflammatory cells.5
Although there is no established pharmacological therapy for treating AAAs, use of nonsteroidal anti-inflammatory drugs (NSAIDs) has been associated with a reduction in AAA expansion.3,5,14 The best characterized mechanism of action for this class of drugs is the nonselective inhibition of COX-1 and COX-2 enzymatic activity.15 However, because neither COX-1 mRNA nor protein is increased in human aneurysms or experimental aneurysm models, the therapeutic effects of NSAIDs have been attributed to the inhibition of COX-2.3,5 Because of the predominant expression of COX-2 in these tissues, drugs that selectively inhibit COX-2 activity have been proposed as a treatment for AAAs,3,5 but their effectiveness for treating this disease in humans has not been examined. A recent study suggests that selective COX-2 inhibition with rofecoxib does not alter AAA expansion in a model using elastase perfusion to induce aneurysms in rats.16 However, the authors were not able to reproduce the previously reported inhibitory effect of indomethacin on AAA expansion in the rat elastase model.3,14
Infusion of angiotensin II (Ang II) into hyperlipidemic mice is a robust and well-established model of AAA formation.17–22 This model displays many characteristics of human AAAs, including inflammation as a prominent pathological feature of the disease.23 Studies investigating inflammatory gene expression in response to Ang II infusion suggest that COX-2 gene expression may be upregulated in the vascular wall during AAA development.19 Using isoform-selective inhibitors of COX, we used this model to determine which COX isoform(s) generates the prostanoids that mediate AAA formation. Herein, we demonstrate that selective COX-2 inhibition with celecoxib significantly reduces the incidence and severity of Ang II-induced AAAs in hyperlipidemic and nonhyperlipidemic mice.
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Methods |
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Animals
Eight- to 10-week-old male C57BL/6J and apolipoprotein E-deficient (apoE–/–) mice that have been backcrossed 12 times in a C57BL/6J background were purchased from Jackson Laboratories (Bar Harbor, Me). Eight- to 10-week-old male mice on a hybrid C57BL/6J x129/Ola (C57/129) genetic background, as originally described by Morham et al,24 were obtained from Taconic Farms Inc. All mice were housed under barrier conditions with food and water provided ad libitum. Studies were performed with the approval of the University of Kentucky Institutional Animal Care and Use Committee.
Drug Administration
Mice were fed a normal laboratory diet (Harlan Teklad) until 8 weeks of age. SC-560 (Cayman Chemical; 200 ppm; 25 mg · kg–1 · day–1) or celecoxib (Pfizer or LKT Laboratories, Inc; 1,000 ppm; 125 mg · kg–1 · day–1) were pelleted into a normal laboratory diet (Research Diets). One week before pump implantation, mice were placed on a normal laboratory diet or a diet containing SC-560 or celecoxib.
Ang II Infusion
Ang II (1000 ng · kg–1 · min–1; Sigma) or saline was administered subcutaneously via Alzet osmotic minipumps (model 2004) as described previously.17 Body weight was measured weekly throughout the time course of the experiment.
Serum Lipids and Lipoprotein Determination
Serum total cholesterol concentrations were determined using enzymatic assay kits (Wako Chemical Co.).
Blood Pressure Measurement
Mean systolic blood pressures were measured in conscious mice using a computerized tail-cuff method (BP-2000 Visitech Systems).18 All mice were acclimated to the system for 1 week before the start of the study. Blood pressure was measured 5 days per week throughout the time course of the study.
Vascular Pathology
Aneurysms in the abdominal aorta were quantified by the percent incidence, in which a >50% increase in external diameter was used to define the occurrence of an AAA. AAA severity was visually classified as described previously18 and also assessed by the wet weight of the abdominal aorta.
Quantification of Dorsal Skin PGE2 Concentrations
Dorsal skin containing both dermis and epidermis was homogenized in Tris buffer containing 100 µmol/L indomethacin, to inhibit ex vivo PGE2 synthesis, and centrifuged at 12 000g (4°C). PGE2 concentrations in the supernatant were determined by radioimmunoassay (RIA; Amersham).
Quantification of Aortic Explant PGE2 and Prostaglandin D2 Concentrations
Freshly harvested aorta from 8-week-old C57/129 or C57BL/6 apoE–/– mice were divided into segments (arch, thorax, and abdominal). Segments were placed into separate wells of 96-well culture plates and incubated in the absence or presence of Ang II (1 µmol/L) in serum-free DMEM for 24 hours at 37°C and 5% CO2. Media was removed after 4 and 24 hours. PGE2 and prostaglandin D2 (PGD2) concentrations in the media were determined by RIA (Amersham) or competitive enzyme immunoassay (Cayman Chemical), respectively.
Statistics
Mean and SEM were calculated for each parameter. Data were analyzed by Student t test or two-way ANOVA and significant interactions analyzed using a Tukey post hoc test for all parametric and Holm Sidak for all nonparametric pair-wise comparisons (SigmaStat). Blood pressure data were analyzed using repeated measures (SAS statistical software). Fisher exact test or 
2 analysis was used to determine differences among groups in the incidence and classification of aneurysms (SigmaStat). Values of P<0.05 were considered statistically significant.
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Results |
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Selective COX-1 Inhibition Does Not Alter the Incidence or Severity of Ang II-Induced AAAs in ApoE–/– Mice
To determine the role of COX-1 in Ang II-induced AAAs, the selective COX-1 inhibitor SC-560 (200 ppm;
25 mg · kg–1 · day–1) was administered to 8-week-old male apoE–/– mice 1 week prior to Ang II (1000 ng · kg–1 · min–1) infusion and throughout the 28 days of infusion. To demonstrate the efficacy of the dose of SC-560 to inhibit COX-1 activity, we examined endogenous PGE2 concentrations in dorsal skin, which is a rich source of COX-1. SC-560 administration reduced skin PGE2 concentrations by 67% (Figure 1), indicating the effectiveness of the drug at this dose for inhibiting COX-1.
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Selective COX-1 inhibition had no effect on body weight (Table I, available online at http://atvb.ahajournals.org), basal systolic pressure, or Ang II-induced increases in systolic blood pressure (Table II, available online at http://atvb.ahajournals.org). SC-560 modestly increased plasma cholesterol concentrations (Table I). However, selective COX-1 inhibition had no effect on the incidence of Ang II-induced AAA formation (Figure 2A; control: 90% [9:10] versus SC-560: 89% [8:9]). Additionally, SC-560 did not alter the severity of Ang II-induced AAAs (Figure 2B) as defined by visual classification (Figure I, available online at http://atvb.ahajournals.org) or by the wet weight of the abdominal aorta (Figure 2C; control: 19.5±3.0 versus SC-560: 18.8±4.2 mg tissue wet weight) in mice infused with Ang II. These findings suggest that COX-1–generated prostanoids do not contribute to Ang II-induced AAAs.
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Selective COX-2 Inhibition Decreases the Incidence and Severity of Ang II-induced AAAs in ApoE–/– Mice
To determine the role of COX-2 in Ang II-induced AAAs, the selective COX-2 inhibitor celecoxib (1000 ppm; 125 mg · kg–1 · day–1) was administered to 8-week-old male apoE–/– mice 1 week prior to saline or Ang II (1000 ng · kg–1 · min–1) infusion and throughout the 28 days of infusion. This dose of celecoxib resulted in plasma concentrations that were within the expected therapeutic concentration range for selective inhibition of COX-2 (control: not detected; celecoxib: saline: 1.6±0.5; Ang II: 1.6±0.2 µg/mL). Additionally, celecoxib treatment did not alter PGE2 levels in skin homogenates (Figure 1; 12% reduction compared with control diet) from the Ang II-infused mice, demonstrating that the dose of celecoxib used in these studies resulted in celecoxib blood levels sufficient for COX-2 inhibition without concomitant inhibition of COX-1.
Celecoxib did not alter body weight (Table III, available online at http://atvb.ahajournals.org), plasma cholesterol concentrations (Table III), basal systolic pressure or Ang II-induced increases in systolic blood pressure (Table II). In contrast to COX-1 inhibition, selective COX-2 inhibition with celecoxib attenuated the incidence of Ang II-induced AAA formation in apoE–/– mice (Figure 3A; control: 73% [22:30] versus celecoxib: 11% [2:19]; P<0.001). Moreover, celecoxib decreased the severity of Ang II-induced AAAs by both visual classification (Figure 3B; P=0.001) and weights of the abdominal aorta (Figure 3C; Ang II: control 16.8±1.1 versus celecoxib 8.3±1.2 mg; P<0.001). Notably, although Ang II infusion increased the weight of the abdominal aorta in mice on the control diet (control: saline: 4.1±2.2 versus Ang II: 16.8±1.1 mg), celecoxib ablated Ang II-dependent increases in abdominal aortic weight (celecoxib: saline: 5.1±2.2 mg versus Ang II: 8.3±1.2 mg). Although COX-2 was not detected by immunohistochemistry in the abdominal aortas of saline-infused mice, significant COX-2 expression was observed in the abdominal aortas of Ang II-infused mice (Figure II, available online at http://atvb.ahajournals.org). The expression of COX-2 was primarily localized in SMCs in the vascular wall of the abdominal aorta after 7 days of Ang II infusion (Figure III, available online at http://atvb.ahajournals.org). These findings suggest that selective COX-2 inhibition with celecoxib markedly reduces both the incidence and severity of Ang II-induced AAA formation in apoE–/– mice.
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Ang II Infusion Induces AAA Formation in a Nonhyperlipidemic Mouse Strain
To investigate whether hypercholesterolemia was required for the observed effects of celecoxib on Ang II-induced AAAs, we performed initial experiments to determine the susceptibility of 2 nonhyperlipidemic mouse strains, C57BL/6J and C57/129, to Ang II-induced AAA formation. The incidence of Ang II-induced AAA formation in the C57BL/6J mice was 5% (Figure 4A). In contrast, Ang II induced a 75% incidence (Figure 4A) of AAAs in the C57/129 strain, which is equivalent to the incidence that we (Figure 3A) and others17,18 observe in hyperlipidemic mice on the C57BL/6J background. Moreover, the severity (Figure 4B) of AAAs in C57/129 mice was also equivalent to that observed in hyperlipidemic mice (Figures 2B and 3
B).
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Selective COX-2 Inhibition Reduces the Incidence and Severity of Ang II-Induced AAAs in Nonhyperlipidemic Mice
To define the role of COX-2 in Ang II-induced AAA formation in nonhyperlipidemic mice, we administered celecoxib to C57/129 mice using the experimental paradigm used in studies with the apoE–/– mice. Celecoxib administration did not alter plasma cholesterol concentrations (Table III) in either saline or Ang II-infused mice. In agreement with the findings in apoE–/– mice, celecoxib attenuated the incidence (Figure 5A; control: 70% [7:10] versus celecoxib: 7% [1:14]; P=0.002) and severity (Figure 5B; P<0.001) of Ang II-induced AAA formation in C57/129 mice. These findings suggest that COX-2–generated prostanoids also play an important role in Ang II-induced AAAs in a nonhyperlipidemic state.
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Ang II Increases PGE2 and PGD2 Production in the Abdominal Aorta Ex Vivo
We examined the effect of Ang II on prostanoid synthesis in arch, thoracic, and abdominal segments of the aorta. Individual segments were incubated in the absence or presence of Ang II (1 µmol/L) for 4 and 24 hours, and media were analyzed for prostanoids. After 24 hours of Ang II treatment, PGE2 (Figure 6A; P=0.001) and PGD2 (Figure 6B; P=0.001) levels were augmented in abdominal aortic segments; however, there was no observed effect on PGF2
and thromboxane B2 (data not shown). Therefore, Ang II augments PGE2 and PGD2 production in the abdominal segment, which is the region of the aorta that is vulnerable to Ang II-induced AAA formation.
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Discussion |
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Increased synthesis of prostanoids have been associated with human and experimental models of AAAs.3–5 In the UK Small Aneurysm Trial and in an experimental model of AAA formation, nonselective inhibition of the COXs with NSAIDs reduced the expansion of AAAs.3,5,14 Because of the presence of COX-2 in aneurysmal tissue and the inhibition of PGE2 production by NSAIDs in aneurysmal tissue, it was inferred that the reduction in AAA expansion in the experimental model of AAAs, was through the inhibition of COX-2 activity.3 However, it is known that both COX isoforms contribute to the levels of systemic PGE2, as well as other prostanoids. In the present study, we used the Ang II-induced AAA model to define the role of the COX isoforms in AAA development. We demonstrate that selective COX-2 inhibition with celecoxib markedly attenuates AAA incidence and severity, suggesting that COX-2–generated prostanoids play a fundamental role in AAA formation.
Interestingly, some studies have implicated a predominant role of COX-1–derived prostanoids in animal models of cardiovascular disease, including atherosclerosis and restenosis.25,26 However, the role of COX-2 has been unclear, with studies suggesting that COX-2 increases,27 decreases,28 or has no effect on atherosclerosis.25,29,30 The observed reduction in abdominal aortic diameter in studies using traditional NSAIDs was attributed to COX-2 inhibition because of the increased presence of the enzyme in aneurysmal tissues.3,5 Additionally, reductions in PGE2 secretion from abdominal aortic explants after indomethacin treatment were attributed to reductions in COX-2 activity based on the increased expression of the enzyme in aneurysmal tissues compared with normal tissues.5 However, both COX-1 and COX-2 contribute to PGE2 synthesis, therefore, inhibition of COX-1–derived prostanoids by indomethacin may have also contributed to the observed decreases in PGE2 synthesis and abdominal aortic expansion. Our finding that the selective inhibition of COX-1 by SC-560 did not alter the incidence or severity of Ang II-induced AAAs suggests that prostanoids derived from COX-1 activity do not play a significant role in this disease.
COX-2 localizes in macrophages in the inflammatory infiltrate of human aortic aneurysmal tissues, which also produces increased concentrations of PGE2 compared with normal aorta.4 Nonselective COX inhibition decreases PGE2 suggested to be produced by COX-2 expressed in macrophages from human aortic aneurysmal explants.31 The broad spectrum MMP inhibitor doxycycline attenuates Ang II-induced AAA formation, demonstrating that MMPs play a critical role in AAA development.32 PGE2 synthesized by macrophages increases production of MMPs, which play a role in the degradation of the vascular wall during aneurysm formation, and COX-2 inhibitors decrease the expression and activation of MMPs.33,34 Attenuation of aneurysm formation in indomethacin-treated rats was associated with reductions in MMP-9.14 Thus, in our studies, celecoxib may attenuate AAA formation by inhibiting COX-2–generated prostanoid production in macrophages, which may regulate MMP expression or activation.
Ang II has been shown to directly stimulate COX-2 expression in vascular SMCs in vitro35–37 and to increase COX-2 mRNA expression in the vascular wall.19 Furthermore, increased COX-2 expression and PGE2 production occur in aneurysmal tissues.4,5 Recent studies demonstrate the expression of PGE2 and PGD2 receptors in aneurysmal tissues.38,39 In our studies, Ang II increased PGD2 synthesis only in the abdominal aortic segment, which was also the region of the aorta with the greatest absolute level of PGD2. Although the arch and thoracic aortic explant segments showed the greatest absolute levels of PGE2, only the abdominal segment, which is the region sensitive to AAA formation, showed significantly increased PGE2 production in response to Ang II. We propose that COX-2–derived PGE2 or PGD2 acting on their respective receptors may mediate Ang II-induced AAA formation.
Hypercholesterolemia has been thought to be a significant risk factor for the development of AAAs. However, a recent study examining risk factors associated with the expansion of small AAAs demonstrates that plasma lipid levels are not associated with AAA expansion.40 Previous studies, as well as our present study, demonstrate that C57BL/6 mice develop a low incidence of AAAs, which is markedly enhanced in the presence of hyperlipidemia.17,18,21 Our findings of equivalent AAA incidence and severity in nonhyperlipidemic C57/129 mice and C57BL/6 apoE–/– mice suggest that hypercholesterolemia is not required for AAA formation in this mouse strain. Moreover, our finding that selective COX-2 inhibition decreased Ang II-induced AAA formation in both hyperlipidemic and nonhyperlipidemic mice suggests that COX-2–generated prostanoids play an important role in AAA development, irrespective of cholesterol concentrations.
Previous studies in conscious mice demonstrate that Ang II infusion rapidly increases systolic blood pressure, a response that is maintained throughout the infusion.21,41–44 During acute Ang II infusion, COX-1–derived prostanoids contribute to the hypertensive response in mice, whereas COX-2–derived prostanoids attenuate the acute pressor response.45 Furthermore, in mice, the increase in blood pressure occurring during chronic infusion of Ang II requires prostanoids derived from the activity of COX-1,42 whereas COX-2–selective inhibition with rofecoxib has been shown to increase systolic blood pressure in other rodent models.46 Therefore, in our current studies, potential alterations in blood pressure after treatment with celecoxib could influence aneurysm development. However, in contrast to previous reports, we observed no alterations in systolic blood pressure in response to celecoxib treatment, either before or after chronic Ang II infusion. The different experimental regimens between our studies and previous reports may account for the different effects of COX-2 inhibition on blood pressure. Thus, in our studies, attenuation of AAA pathology after COX-2 inhibition does not result from indirect effects on blood pressure.
In conclusion, these are the first studies to demonstrate that selective COX-2 inhibition with celecoxib attenuates the development of Ang II-induced AAA formation in both hyperlipidemic and nonhyperlipidemic mice. Future studies to identify the COX-2–generated prostanoid(s) and their respective receptor(s) that play a role in AAA development will be critical in defining therapeutic targets for treatment of this disease.
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Acknowledgments |
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V.L.K. was supported by a grant from the National Institutes of Health (HL 70239). We would like to thank Lisa Cassis and Alan Daugherty for their helpful suggestions and discussion of this project and Dr Robert Langenbach (NIH/NIEHS) for the C57/129 hybrid mice.
Received December 7, 2005; accepted February 17, 2006.
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