Cyclooxygenase-2 and atherosclerosis: friend or foe?

Ziad A. Massy1 and Suzanne K. Swan1,2,

1 INSERM U507, Necker Hospital and CH Beauvais, Paris, France and 2 Division of Nephrology, Hennepin County Medical Center, University of Minnesota, Minneapolis, Minnesota, USA

Keywords: atherosclerosis; cyclooxygenase-2

Introduction

Atherosclerosis, manifested by heart disease, stroke, and peripheral vascular disease, remains the leading cause of morbidity and mortality in industrialized countries despite aggressive vascular intervention and myriad blood pressure- and lipid-lowering agents. Over the past decade, however, our understanding of atherogenesis has evolved from one of occlusive lipid accumulation to one of chronic inflammation involving cellular proliferation [1].

Prostaglandins mediate inflammation locally and modulate physiological responses systemically. Nearly all tissues produce prostaglandins and increase production at sites of inflammation. Specifically, arachidonic acid is metabolized to prostaglandin G2 (PGG2) and then to prostaglandin H2 (PGH2) by cyclooxygenase. These moieties are then converted to PGD2, PGE2, PGF2, PGI2, or thromboxane (TBX). The specific prostaglandin produced is determined, in part, by the particular cell type under consideration. For example, endothelium primarily produces PGI2 or prostacyclin, while platelets produce TBX. Despite recognition of cyclooxygenase's mechanism of action 30 years ago [2], the enzyme was first cloned in 1988 (cyclooxygenase-1 or COX-1) [3]. A second isoform was reported 3 years later (cyclooxygenase-2 or COX-2) [4].

COX-1 is constitutively expressed, serving a so-called ‘housekeeping’ role, in many tissues under basal conditions. For example, COX-1 helps to maintain normal physiological functions such as mucus production in the gastric mucosa. Conversely, COX-2 is an inducible enzyme, generally not present (or minimally so) in most tissues. Rather, its expression is more often associated with inflammation and other pathophysiological states. This realization has driven the development of COX-2 inhibitors such as celecoxib (Celebrex) and rofecoxib (Vioxx) for antiinflammatory/analgesic therapy of osteo- and rheumatoid-arthritis as well as acute pain syndromes. Such agents would offer efficacy while minimizing unwanted side effects attributable to COX-1 inhibition (e.g., gastric ulceration).

More recent investigations, however, reveal that COX-2 plays a key role in a wide range of physiological processes including organogenesis, brain and nerve function, reproduction, bone metabolism, salt and water handling, renin release, angiogenesis, cell proliferation and apoptosis. Further, COX-2 has been implicated in several disease states such as: colonic polyposis [5]; various forms of cancer [6,7]; Alzheimer's disease [8]; and vascular restenosis following angioplasty [9].

This review will focus on the putative role of COX-2 in the development and maintenance of atherosclerosis, emphasizing the ‘inflammatory’ nature of the disease. The role of COX-2 in atherosclerosis, however, may not represent a simple cause-and-effect scenario. Conflicting data exist with regard to the relationship that COX-2 may have with atherogenesis. The following discussion will present arguments for and against COX-2 as an aetiological factor in atherosclerotic cardiovascular disease, as well as the therapeutic potential for its pharmacological manipulation.

COX-2 and atheromatous tissue

A detailed description of atheromatous plaque generation is beyond the scope of this review and can be found elsewhere [1]. As such, a simplified construct of this lesion focuses on three cell types—endothelial, monocytic/macrophage, and vascular smooth muscle. Disruption of the endothelial cell barrier between circulating blood and tissue, monocyte/macrophage sequestration at these sites with elaboration of inflammatory cytokines, and the transmigration and uncontrolled proliferation of vascular smooth muscle cells typify the initial phase of the atherosclerotic process. COX-2 expression has been found in each of these cell types in animal models as well as in human atherosclerotic tissue [1012]. Similarly, COX-2 can be induced in these cells by many, if not all, of the same proinflammatory mediators implicated in the development of atherosclerosis. Such mediators include tumour necrosis factor (TNF), interleukin-1 (IL-1), interferon-{alpha}, free radicals, endotoxin, platelet-derived growth factor, hypoxia, and sheer stress as recently reviewed [12]. The presence of COX-2 in cells which comprise the atheromatous lesion as well as its inducibility by mediators of atherogenesis are in keeping with experimental and clinical data showing that prostaglandin production is increased in atherosclerosis; just as it is in other inflammatory conditions [12,13].

Particularly compelling histologically, are the findings of Schonbeck et al. [10] and Baker et al. [11] in which COX-2 expression is found in human atheromatous plaque lesions in diseased coronary arteries resected at the time of surgical revascularization, but not in normal coronary arteries. Furthermore, COX-2 expression colocalizes with that of inducible nitric oxide synthase (iNOS), suggesting an interaction between the two inflammatory mediators. Lastly, augmented COX-2 expression is present in diseased, transplanted coronary arteries to the same extent as that found in diseased native coronary arteries [11]. Again, this emphasizes the inflammatory nature of atherosclerosis as opposed to the ‘degeneration-with-ageing’ model held earlier.

Oxidative stress and COX-2

Accumulation of low-density lipoprotein (LDL) in the subendothelial region of the vascular wall is a primary event in the initiation of atherosclerotic injury. Monocytes, attracted to these areas of accumulated LDL by increased expression of adhesion molecules on endothelial cell membranes, aggregate on the surface of the lumen and subsequently transmigrate into the intimal layer. Here they differentiate into macrophages which scavenge lipoproteins, ultimately forming foam cells. LDL, however, is first oxidatively modified which facilitates its uptake by the macrophage [14]. Oxidized LDL (oxLDL) accelerates the formation of foam cells that contribute their lipid contents to fatty streak formation, the histological hallmark of the initial stage of atherosclerosis. Experimentally, the ability of oxLDL to activate a macrophage is not only dependent on oxidation of either the protein moiety or lipid moiety of the LDL molecule, but also requires fully differentiated macrophages derived from monocytes [15].

The relationship between COX-2 and oxLDL is not straightforward. Activated macrophages produce a variety of inflammatory mediators, including prostaglandins. Thus, increased COX-2 activity would be expected in oxidatively stressed macrophages given its inducible nature. In animal models, however, macrophages exposed to oxidized lipoproteins decrease their prostaglandin production (i.e. PGE2 and PGI2) when stimulated by inflammatory cytokines such as lipopolysaccharide (LPS) [16]. As such, cholesterol-rich macrophages and foam cells appear to have an impaired or reduced inflammatory response, rather than an augmented one, following oxLDL exposure [17]. Recently, Eligini et al. [18] reported that oxLDL inhibited LPS-induced COX-2 expression in human macrophages.

The pathophysiological significance of oxLDL's ability to downregulate COX-2, specifically, and to diminish macrophage responsiveness to inflammatory stimuli, globally, remains unclear. One argument highlights the negative feedback exerted on macrophage colony stimulating factor (M-CSF) by PGE2 and PGI2 [19]. Enhanced M-CSF secretion by monocytes could foster proliferation and accumulation of macrophages in areas of atheroma formation, potentiating the atherosclerotic process. Another argument contends that normal resolution of inflammatory processes requires intact and robust cellular responses to such stimuli, ‘effective clean-up’ in essence. Blunted inflammatory responses by macrophages exposed to oxLDL, early in the development of atheromatous lesions, could limit tissue repair and result in a low-grade state of chronic inflammatory injury which is unable to repair itself. Pharmacological intervention with selective COX-2 inhibitors may help to delineate the relationship between oxLDL, COX-2, and the activated macrophage.

Another lipoprotein, high-density lipoprotein or HDL, exerts an ‘antiatherogenic’ or protective effect on the cardiovascular system. A strong inverse correlation exists between levels of HDL and the frequency as well as mortality of atherosclerotic disease [20]. Functionally, HDL is known to remove excess cholesterol from the circulation and inhibit oxidation of LDL. The latter function is accomplished by paraoxonase, an esterase that degrades oxidized phospholipids and which uses HDL as its carrier protein [21]. What is not clear is whether HDL's angioprotective effect is attributable to these or to other unrelated functions.

Cockerill et al. [22] have reported on the inhibitory effect of HDL on cytokine-induced adhesion molecule expression by human endothelial cells. Based upon previous observations that HDL stimulates endothelial cell PGI2 synthesis [23], these same investigators reported that HDL can synergistically increase COX-2 expression in human endothelial cells following cytokine stimulation with either IL-1ß or TNF{alpha}. The same synergistic increase in COX-2 protein is mirrored by increased PGI2 production in these cytokine-stimulated cells [24]. Although HDL clearly has opposing effects on two cytokine-stimulated responses by human endothelium, the results may be additive. On one hand, an increase in PGI2, mediated by a HDL-induced increase in COX-2 expression, serves to inhibit platelet aggregation, cholesterol accumulation, and vascular smooth muscle cell proliferation/contraction and thus may be vasoprotective. Concurrently, decreased expression of adhesion molecules on cytokine-induced endothelial cell membranes may also be consistent with an antiatherogenic effect of HDL.

Prostacyclin (PGI2) and thromboxane (TBX)

PGI2 is a metabolite of arachidonic acid and the major prostaglandin produced by endothelial cells. As mentioned earlier, PGI2 is a potent inhibitor of leukocyte activation and adhesion, platelet aggregation, and vascular smooth muscle cell proliferation, migration, and contraction. Its role in vivo, however, has remained unclear. Recently, a knockout mouse deficient in the PGI2 receptor, IP, has demonstrated an antithrombotic as well as antiinflammatory role for PGI2 [25]. It is also well established that PGI2 excretion is increased in patients with ‘activated platelet’ conditions such as unstable atherosclerotic disease as well as following vascular interventional procedures [13]. On one hand, it would not be unreasonable to predict a facilitative role for COX-2 in these conditions of increased PGI2 production, given its inducible nature and the fact that COX-2 is responsible for the majority of PGI2 produced in the body [26]. On the other hand, nonselective NSAID therapy (sulindac and indomethacin) inhibits intimal proliferation and prevents or slows vascular changes in atherosclerosis-prone mice while aspirin and nimesulide (a selective COX-2 inhibitor) do not [9,27].

Clinically, a nagging question arises regarding the lack of inhibition by selective COX-2 inhibitors of thromboxane (TBX), a COX-1 mediated prostaglandin, produced primarily by platelets. Given TBX's procoagulant and vasoconstricting effects, long-term therapy with COX-2 inhibitors may create a state of chronic, ‘unopposed’ TBX activity with potentially deleterious cardiovascular outcomes. In the VIGOR trial, for example, over 8000 patients with rheumatoid arthritis were randomized to receive either naproxen (500 mg po bid) or rofecoxib (50 mg po q day) for 12 months [28]. Aspirin therapy was excluded in all subjects since the mucosa of the GI tract was being evaluated. Although rofecoxib demonstrated a marked gastroprotective effect, a significantly increased incidence of myocardial infarction occurred in the rofecoxib-treated group. A large percentage of these adverse events occurred in a small subset of patients who were at risk for myocardial infarction and should have been receiving aspirin therapy but were unaware of their risk factors at the time of randomization. These results have been attributed to the lack of antiplatelet activity demonstrated by selective COX-2 inhibitors. As noted recently, selective COX-2 inhibition may offer gastroprotection but not cardioprotection [29].

Conclusion

Clearly, many questions remain to be answered before the role of COX-2 in the atherosclerotic process is understood. COX-2 would be expected to play a role in an inflammatory condition such as atherosclerosis given its inducible nature yet some findings outlined above would contradict such a role (Table 1Go). Large, long-term trials involving patients at risk for atherosclerotic disease as well as those who require chronic analgesic therapy with COX-2 inhibitors for arthritic and other inflammatory conditions are warranted.


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Table 1. Evidence for and against COX-2 inhibition in atherogenesis

 

Notes

Correspondence and offprint requests to: Suzanne K. Swan MD, Associate Professor of Medicine, Division of Nephrology, Hennepin County Medical Center, 701 Park Avenue, Minneapolis, MN 55415, USA. Email: swanx005{at}tc.umc.edu Back

References

  1. Ross R. Atherosclerosis—an inflammatory disease. N Engl J Med1999; 340: 115–126[Free Full Text]
  2. Vane JR. Inhibition of prostaglandin synthesis as a mechanism of action for aspirin-like drugs. Nature1971; 231: 232–235
  3. DeWitt DL, Smith WL. Primary structure of prostaglandin G/H synthase from sheep vesicular gland determined from the complementary DNA sequence. Proc Natl Acad Sci USA1988; 85: 1412–1416[Abstract]
  4. Kujubu DA, Fletcher BS, Varnum BC, Lim RW, Herschman HR. TIS 10, a phorbol ester tumor promoter—inducible mRNA from Swiss 3T3 cells, encodes a novel prostaglandin synthase/cyclooxygenase homologue. J Biol Chem1991; 266: 12866–12872[Abstract/Free Full Text]
  5. Eberhart CE, Coffey RJ, Radhika A, Giardiello FM, Ferrenbach S, DuBois RN. Up-regulation of cyclooxygenase-2 gene expression in human colorectal adenomas and adenocarcinomas. Gastroenterology1994; 107: 1183–1188[ISI][Medline]
  6. Tsujii M, Kawano S, DuBois RN. Cyclooxygenase-2 expression in human colon cancer cells increases metastatic potential. Proc Natl Acad Sci USA1997; 94: 3336–3340[Abstract/Free Full Text]
  7. Fosslien E. Molecular pathology of cyclooxygenase-2 in neoplasia. Ann Clin Lab Sci2000; 30: 3–21[Abstract]
  8. Lukiw WJ, Bazan NG. Neuroinflammatory signaling upregulation in Alzheimer's disease. Neurochem Res2000; 25: 1173–84[ISI][Medline]
  9. Reis ED, Roque M, Dansky H et al. Sulindac inhibits neointimal formation after arterial injury in wild-type and apolipoprotein E-deficient mice. Proc Natl Acad Sci USA2000; 97: 12764–12769[Abstract/Free Full Text]
  10. Schonbeck U, Sukhova GK, Graber P, Coulter S, Libby P. Augmented expression of cyclooxygenase-2 in human atherosclerotic lesions. Am J Pathol1999; 155: 1281–1291[Abstract/Free Full Text]
  11. Baker CSR, Hall RJC, Evans TJ et al. Cyclooxygenase-2 is widely expressed in atherosclerotic lesions affecting native and transplanted human coronary arteries and colocalizes with inducible nitric oxide synthase and nitrotyrosine particularly in macrophages. Arterioscler Thromb Vasc Biol1999; 19: 646–655[Abstract/Free Full Text]
  12. Belton O, Byrne D, Kearney D, Leahy A, Fitzgerald D. Cyclooxygenase-1 and -2-dependent prostacyclin formation in patients with atherosclerosis. Circulation2000; 102: 840–845[Abstract/Free Full Text]
  13. Fitzgerald GA, Pedersen AK, Patrono C. Analysis of prostacyclin and thromboxane biosynthesis in cardiovascular disease. Circulation1983; 67: 1174–1177[ISI][Medline]
  14. Rosenfeld ME, Khoo JC, Miller E, Parthasarathy S, Palinski W, Witzum JL. Macrophage-derived foam cells freshly isolated from rabbit atherosclerotic lesions degrade modified lipoproteins, promote oxidation of low-density lipoproteins and contain oxidation-specific lipid protein adducts. J Clin Invest1991; 87: 90–99[ISI][Medline]
  15. Nguyen-Khoa T, Massy ZA, Witko-Sarsat V et al. Oxidized low-density lipoprotein induces macrophage respiratory burst via its protein moiety: a novel pathway in atherogenesis? Biochem Biophys Res Commun1999; 263: 804–809[ISI][Medline]
  16. Arai H, Nagano Y, Narumiya S, Kita T. Decreased arachidonate metabolism in mouse peritoneal macrophages after foam cell transformation with oxidized low-density lipoproteins. J Biochem1992; 112: 482–487[Abstract]
  17. Hamilton TA, Major JA, Chisholm GM. The effects of oxidized low density lipoproteins on inducible mouse macrophage gene expression are gene and stimulus dependent. J Clin Invest1995; 95: 2020–2027[ISI][Medline]
  18. Eligini S, Colli S, Basso F, Sironi L, Tremoli E. Oxidized low-density lipoprotein suppresses expression of inducible cyclooxygenase in human macrophages. Arterioscler Thromb Vasc Biol1999; 19: 1719–1725[Abstract/Free Full Text]
  19. Moore RN, Pitruzello FJ, Deana DG, Rouse BT. Endogenous regulation of macrophage proliferation and differentiation by E prostaglandins and interferon alpha/beta. Lymphokine Res1985; 4: 43–50[ISI][Medline]
  20. Yaari S, Goldbourt U, Even-Zohar S, Neufeld HN. HDL and total cholesterol: association with total, cardiovascular and cancer mortality in a seven year prospective study of 10,000 men. Lancet1981; 1: 1011–1015[Medline]
  21. Hegele RA. Paraoxonase—gene and disease. Ann Med1999; 31: 217–224[ISI][Medline]
  22. Cockerill GW, Rye KA, Gamble JR, Vadas MA, Barter PJ. High density lipoproteins inhibit cytokine induced expression of endothelial cell adhesion molecules. Arterioscler Thromb Vasc Biol1995; 15: 1987–1994[Abstract/Free Full Text]
  23. Fleisher LN, Tall AR, Witt LD, Miller RW, Cannon PJ. Stimulation of arterial endothelial cell prostacyclin synthesis by high density lipoprotein. J Biol Chem1982; 257: 6653–6655[Abstract/Free Full Text]
  24. Cockerill GW, Saklatvala J, Ridley SH et al. High density lipoproteins differentially modulate cytokine induced expression of E-selectin and COX-2. Arterioscler Thromb Vasc Biol1999; 19: 910–917[Abstract/Free Full Text]
  25. Murata T, Ushikubi F, Matsuoka T et al. Altered pain perception and inflammatory response in mice lacking prostacyclin receptor. Nature1997; 388: 678–682[ISI][Medline]
  26. McAdam BF, Catella-Lawson F, Mardini IA, Kapoor S, Lawson JA, FitzGerald GA. Systemic biosynthesis of prostacyclin by cyclooxygenase (COX)-2: the human pharmacology of a selective inhibitor of COX-2. Proc Natl Acad Sci USA1999; 96: 272–277[Abstract/Free Full Text]
  27. Pratico D, Tillmann C, Zhi-Bing Z, Hongwei L, FitzGerald GA. Acceleration of atherogenesis by COX-1-dependent prostanoid formation in low density lipoprotein receptor knockout mice. Proc Natl Acad Sci USA2001; 98: 3358–3363[Abstract/Free Full Text]
  28. Bombardier C, Laine L, Reicin A et al. for the VIGOR Study Group. Comparison of upper gastrointestinal toxicity of rofecoxib and naproxen in patients with rheumatoid arthritis. N Engl J Med2000; 343: 1520–1528[Abstract/Free Full Text]
  29. Boers M. NSAIDs and selective COX-2 inhibitors: competition between gastroprotection and cardioprotection. Lancet2001; 357: 1222–1223[ISI][Medline]