| COMMENTARIES |
Pathobiology and Biomarkers of Atherosclerosis
René R. S. Packard, M.D.* and Peter Libby, M.D.*, *Leducq Center for Cardiovascular Research, Division of Cardiovascular Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts, U.S.A.
Please address correspondence to:
Dr. Peter Libby
Leducq Center for Cardiovascular Research
Cardiovascular Medicine
Brigham and Women's Hospital
Harvard Medical School,
77 Avenue Louis Pasteur, NRB 7
Boston, Massachusetts 02115
Email: plibby@rics.bwh.harvard.edu
Disclosure:
Dr. Libby is listed as co-inventor on patents held by the Brigham and Women’s Hospital that relate to the use of inflammatory biomarkers in cardiovascular disease.
Funding:
This work was supported by a grant from the Fondation Leducq,
For a more complete discussion of the present topic, please refer to:
Packard RRS, Libby P. Inflammation in atherosclerosis: from vascular biology to biomarker discovery and risk prediction. Clin Chem 2008 Jan; 54(1):24-38.
Introduction
Many individuals who experience myocardial infarction have average or below-average cholesterol levels [1], which underscores the need for novel strategies of risk stratification [2]. The involvement of inflammation in all stages of atherosclerosis [3] has stimulated the evaluation of inflammatory biomarkers for cardiovascular risk prediction.
Initiation, Progression, and Rupture of Atherosclerotic Lesions
Fatty streaks have focal increases in the content of lipoproteins within the intima, where they undergo oxidative modification [4]. This modification triggers endothelial expression of adhesion molecules such as vascular cell adhesion molecule-1 (VCAM-1) and P-selectin, and of chemoattractant factors including monocyte chemoattractant protein-1, which mediate the recruitment of monocytes and lymphocytes [5].
Monocytes interacting with the endothelium increase matrix metalloproteinase (MMP)-9 production, allowing their infiltration [6]. The inflamed intima overexpresses macrophage colony-stimulating factor [7], which increases the expression of scavenger receptors, allowing subsequent endocytosis of modified lipoproteins and formation of foam cells. In parallel, macrophages proliferate and amplify the inflammatory response through the secretion of numerous growth factors and cytokines, most notably tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β).
Atherosclerotic lesions contain cytokines that promote a Th1 response [8]. These lymphocytes amplify the local inflammatory activity by producing pro-inflammatory cytokines such as interferon (IFN)-γ and CD40 ligand (CD40L). Adiponectin, a product of adipose tissue whose level decreases in obese individuals, reduces endothelial VCAM-1 expression, macrophage scavenger receptor expression, TNF-α production, and T-cell recruitment [9].
In response to platelet-derived growth factor (PDGF) among other stimuli, smooth muscle cells (SMCs) migrate from the tunica media into the intima, where they proliferate and secrete extracellular matrix proteins, including interstitial collagen [10]. This process causes the lesion to evolve from a lipid-rich plaque to a fibrotic one. Human atheromata express interleukin-18, which evokes essential effectors such as adhesion molecules, chemokines, cytokines, and MMPs [11].
All the main cell types involved in atherosclerosis express CD40L as well as its receptor, CD40. CD40 ligation triggers the expression of adhesion molecules and the secretion of numerous cytokines and MMPs involved in extracellular matrix degradation [12]. Importantly, CD40L can promote thrombosis, for example by inducing the expression of tissue factor, which initiates the coagulation cascade by accelerating the activity of factor VIIa.
Advanced complex atheromata exhibit a paucity of SMCs and abundant macrophages at sites of rupture. Inflammation can interfere with the integrity of the cap’s interstitial collagen by stimulating the destruction of existing collagen fibers through MMP secretion regulated by inflammatory mediators such as CD40L and by blocking the creation of new collagen through IFN-? [13].
Acute coronary syndromes often result from a physical disruption of the fibrous cap, allowing the blood to make contact with thrombogenic material [14]. Importantly, the fluid phase of blood, in particular levels of circulating plasminogen activator inhibitor-1 (PAI-1) and fibrinogen, may influence the consequences of a given plaque disruption [15,16]. Indeed, impaired fibrinolysis can result from an imbalance between clot-dissolving enzymes and their endogenous inhibitors, primarily PAI-1.
The ubiquitous involvement of inflammation in atherosclerosis has stimulated the evaluation of certain key inflammatory markers in cardiovascular risk prediction.
C-Reactive Protein
Data from multiple large-scale prospective studies demonstrate that elevated CRP levels independently predict adverse cardiovascular events [17-21]. These results have led to the development of the Reynolds Risk Score for women [22], which adds CRP and dichotomously determined family to the
Retrospective evidence supports the hypothesis that plasma CRP measurement identifies individuals who, while apparently at low risk with relatively low lipid levels, may still benefit from lipid-lowering statin therapy in both primary [23] and secondary prevention [24]. A large-scale, randomized clinical trial – Justification for the Use of Statins in Primary Prevention: an Intervention Trial Evaluating Rosuvastatin (JUPITER) – tested the effects of statin therapy in subjects who have both plasma LDL levels below those currently used to target therapy and plasma CRP levels that indicate heightened risk of a cardiovascular event [25,26]. Importantly, JUPITER was terminated prematurely due to an overwhelming clinical benefit of the statin-treated arm, with final definitive results pending.
Plasminogen Activator Inhibitor-1
Elevated levels of the endogenous inhibitor of fibrinolysis PAI-1 predict the occurrence of a first acute myocardial infarction in middle-aged men and women with a high prevalence of coronary heart disease [27]. Inflammatory mediators raise levels of PAI-1, an acute phase reactant. Diabetes and the components of the metabolic syndrome associate with high levels of PAI-1. Interestingly, numerous studies indicate that angiotensin-converting enzyme inhibitors decrease PAI-1 concentrations across different ethnic groups in both primary [28] and secondary prevention [29].
Soluble CD40 Ligand
In stable coronary artery disease, an association exists between atheroma burden, stenosis, and abnormally elevated levels of sCD40L [30]. In addition, patients with evidence of a lipid pool on high-resolution magnetic resonance imaging of carotid stenoses have elevated sCD40L levels [31]. High plasma concentrations of sCD40L associate with increased vascular risk in apparently healthy women [32]. In asymptomatic patients with low-grade carotid stenosis [33], and those with end-stage renal disease [34], elevated sCD40L levels predict the risk of an adverse cardiovascular event.
Furthermore, high levels of sCD40L identify patients at increased risk for an adverse cardiovascular event who would likely benefit from antiplatelet treatment through glycoprotein IIb/IIIa inhibition with abciximab [35].
Adiponectin
Secretion of adiponectin diminishes as adipose tissue mass increases [9]. Healthy middle-aged subjects’ adiponectin levels independently and negatively associate with carotid artery intima-media thickness [36]. Patients with stable coronary artery disease have lower adiponectin serum concentrations compared with age- and gender-matched controls [37] across ethnic groups. In a nested case-control study of males free of diagnosed cardiovascular disease at the time of blood draw, participants in the highest quintile of adiponectin had a significantly decreased risk of myocardial infarction [38]. Measuring adiponectin levels in apparently healthy middle-aged men with low HDL values identifies individuals at very high risk for adverse cardiovascular events [39]. A low level of adiponectin also constitutes a significant risk factor for the development of adverse cardiovascular events in patients with type 2 diabetes [40] as well as those with end-stage renal disease [41]. Moreover, the adiponectin gene promoter region has peroxisome proliferator response elements, and ligands for peroxisome proliferator-activated receptor (PPAR)-α [42] and PPAR-γ [43] elevate adiponectin.
Interleukin-18
IL-18 serum concentration independently predicts cardiovascular death in patients with documented coronary artery disease [44]. In addition, baseline plasma IL-18 concentration associates with future coronary events in healthy middle-aged men [45].
Matrix Metalloproteinase-9
Patients with stable coronary artery disease have elevated circulating levels of MMP-9 [30,46]. In prospective studies, elevated baseline levels of MMP-9 in subjects with carotid stenosis [47] or coronary artery disease [48] associate with an increased risk of stroke or cardiovascular death.
Conclusion
The clinical application of the concept that inflammation participates in atherosclerosis has stimulated the adoption of biomarkers of inflammation in risk prediction and other applications, as noted above. As a downstream biomarker, CRP appears to integrate upstream inflammatory triggers. In addition, CRP, sCD40L, and adiponectin may serve as targets and guides for pharmacologic therapy.
References