Doi:10.1016/j.cardiores.2006.03.004

Cardiovascular Research 71 (2006) 30 – 39 C-reactive protein in atherosclerosis: A causal factor? aHemostasis and Thrombosis Research Centre, Dept. of Hematology, Leiden University Medical Centre, Leiden, The Netherlands bDepartment of Hematology, Room Ee 13.93, Erasmus University Medical Center, Dr. Molewaterplein 50, 3015 GE Rotterdam, The Netherlands Received 3 August 2005; received in revised form 3 March 2006; accepted 6 March 2006 Atherosclerosis is considered a to be multifactorial disease driven by inflammatory reactions. The process of inflammation also contributes to the pathogenesis of acute atherothrombotic events. C-reactive protein (CRP) is an acute phase protein and its concentration inserum reflects the inflammatory condition of the patient. Levels of CRP are consistently associated with cardiovascular disease (CVD) andpredict myocardial infarctions and stroke. Since CRP is present in the atherosclerotic lesion, it may actively contribute to the progression and/or instability of the atherosclerotic plaque. The role of CRP in inflammation and its causality in atherosclerosis are the subject of manyinvestigations but are not yet fully elucidated. This review focuses on recently identified mechanisms by which CRP may modulate andevolve the process of atherosclerosis. We discuss the function of CRP and review the most recent evidence for an independent role of CRP inthe development of atherosclerosis. Many studies suggest such a role, but a number of the described effects may be the result ofcontamination of the CRP preparations.
D 2006 European Society of Cardiology. Published by Elsevier B.V. All rights reserved.
Keywords: C-reactive protein; Azide; Atherosclerosis; Inflammation; Thrombosis predictive value in the pathogenesis of CVD. Many clinicaland population studies, with cross-sectional and nested Inflammation plays a key role in the pathogenesis of case – control designs, proved these inflammatory mediators cardiovascular disease (CVD), acute atherothrombotic events and atherosclerosis Inflammation also regu- Several reports describe human CRP to be involved in lates the production of the acute phase proteins such as C- the induction of ischemic tissue damage in the brain, reactive protein (CRP), fibrinogen and serum amyloid A myocardial infarction and the increase of stroke volume in The serum concentration of CRP can increase > 1000- Most clinical studies report that CRP is an independent fold upon inflammation and, with a half life of 19 h, CRP is predictor of risk of atherosclerosis cardiovascular a very stable downstream marker of the inflammatory process Because CRP is such a sensitive indicator of the myocardial infarction even after considering other inflammatory process, it has been extensively studied cardiovascular risk factors such as age, smoking, obesity, whether plasma concentrations of CRP and other circulating diabetes, hypercholesterolemia and hypertension. However, inflammatory proteins (e.g. fibrinogen, interleukin-6) have a absence of a relationship between CRP and risk ofmyocardial infarction has been reported as well, especiallyafter comprehensive adjustment for established risk factors An early indication that CRP may be more than just a * Corresponding author. Tel.: +31 10 4089448; fax: +31 10 4089470.
risk marker was the observation that, of several inflamma- 0008-6363/$ - see front matter D 2006 European Society of Cardiology. Published by Elsevier B.V. All rights reserved.
E. Paffen, M.P.M. deMaat / Cardiovascular Research 71 (2006) 30 – 39 tory markers studied (such as P-selectin, interleukin-6, the development, instability and eventual rupture of the interleukin-1, tumor necrosis factor-a (TNFa), soluble intercellular adhesion molecule-1 (sICAM-1), fibrinogen),CRP emerged as the most powerful inflammatory predictorof future cardiovascular risk Assuming that, in the physiological condition of human pathology and the atherosclerotic lesion, CRP exerts direct CRP is one of the substances present in the atheroscle- functions, the question is whether it is possible to rotic lesion, more specifically in the vascular intima, where discriminate between the direct effects of CRP and the it co-localizes with monocytes, monocyte-derived macro- parallel presence of other factors that determine the risk of phages and lipoproteins This localization makes a CVD. It was recently observed that several of the direct contribution to the atherosclerotic process possible.
biological effects contributed to CRP in vitro are in fact CRP is a phylogenetically highly conserved plasma caused by contamination of the CRP preparation by azide protein, with homologues in vertebrates and many inverte- or bacterial lipopolysaccharides However, in other brates, that is part of the systemic response to inflammation studies a contribution of contamination can be excluded, It is an acute phase protein and a member of the family indicating that a causal role of CRP is clearly present.
of pentraxins. CRP was originally observed in 1930 in the Examples are the recent ex vivo and in vivo studies plasma of patients with acute infections, where it reacted where CRP was administered to healthy volunteers with the C polysaccharide of pneumococcus and the in vitro experiments on the effect of CRP on The major part of the CRP present in the plasma comes tissue plasminogen activator activity, interleukin-1h and from the liver, where the synthesis of CRP is mainly tumor necrosis factor-a in human aortic endothelial cells regulated by interleukin-6, which in turn is upregulated by The functional effects of CRP that may be relevant other inflammatory cytokines such as interleukin-1 and for the development of CVD and will be discussed in this tumor necrosis factor-a Small amounts of CRP can also be produced locally For example, CRP has beendetected on the surface of about 4% of normal bloodlymphocytes and it has been demonstrated that this CRP is produced by the lymphocytes themselves CRP alsocan be produced locally in atherosclerotic lesions by SMCs The first step in the development of an atherosclerotic plaque and the resulting local inflammatory process is The structure of CRP is important for its stability and for endothelial dysfunction. Upon injury, endothelial cells the execution of its function CRP is composed of (ECs) express the vascular cell adhesion molecule-1 five identical, 21,500 Da subunits. Upon dissociation of its (VCAM-1), intracellular adhesion molecule-1 (ICAM-1) pentameric structure, CRP subunits undergo a spontaneous and the endothelial leukocyte adhesion molecule-1 and irreversible conformational change. The loss of the (ELAM) on the cell surface Leukocytes, especially pentameric structure of CRP results in modified or T-lymphocytes (CD8+ cytotoxic T-cells, CD4+ helper Th1/ monomeric CRP (mCRP), which is a naturally occurring Th2-cells) and monocytes, are then recruited from the form of CRP and it is a tissue-based rather than a serum- blood and cross the endothelial cell barrier via a process based molecule mCRP is less soluble than CRP and called diapedesis. Lipoprotein particles (LDLs and var- tends to aggregate, and it has been described to induce iants such as oxidized LDL) accumulate in the lesion, are mRNA of chemokines and the expression of adhesion taken up by monocyte-derived macrophages, which as a molecules in human cultured coronary artery endothelial result develop into foam cells, and form a fatty streak.
cells (HCAECs) Thus, next to circulating native Smooth muscle cells (SMCs) proliferate and extracellular pentameric CRP, mCRP can also promote a pro-inflamma- matrix components extend to form a fibrous cap, tory phenotype and exert atherogenic effects in human enclosing and defining the morphology of the atheroscle- endothelial cells, although it may be in less potent manner than native CRP In ApoE(À/À) mice, mCRP has been The cells involved in formation of the atherosclerotic described to have opposite effects on atherosclerosis plaque (e.g. ECs, monocytes, T-cells, SMCs) are stimulated compared to normal CRP These data may explain in to produce many different substances, such as inflamma- part the conflicting activities previously reported for CRP in tory mediators (interleukin-6, tumor necrosis factor-a, interleukin-1) complement factors (C1q, C3, C5 –C9) chemokines (monocyte chemoattractant pro-tein-1, interleukin-8), adhesive molecules (selectins P/E, 4. Functionality of CRP and purity of CRP preparations integrins CD18/CD11) metalloproteinases (MMP-1/9)collagenases, reactive oxygen species (such as nitric The assumption that CRP is a causal factor in the oxide (NO)) and CRP These mediators contribute to development of the atherosclerotic lesion is based on its E. Paffen, M.P.M. deMaat / Cardiovascular Research 71 (2006) 30 – 39 rapid accessibility to the plaque, localization in the plaque rosis. Therefore, in this review, we will discuss the possible and the results of in vitro studies, in which CRP has been direct roles of CRP in atherosclerosis.
demonstrated to actively contribute to inflammatory pro-cesses.
The involvement of CRP in several mechanisms execut- ing and maintaining the inflammatory process has beenwidely studied with the use of different commercially Inflammatory mechanisms play a central role in all human CRP preparations. These CRP preparations differ phases of atherosclerosis, from the initial recruitment of in source (serum, plasma, recombinant) and quality.
circulating leukocytes to the arterial wall to the rupture of Use of commercially CRP preparations introduces two unstable plaques, which results in the clinical manifestations means of interference: 1) by contamination with bacterial of the disease. CRP may be involved in each of these stages lipopolysaccharides (LPS) or 2) by contamination with the by direct influencing processes like complement activation, apoptosis, vascular cell activation, monocyte recruitment, Presence of endotoxin in commercially available CRP lipid accumulation and thrombosis. Each of these processes preparations has been eloquently described and we offers several mechanisms by which CRP may influence its ourselves measured concentrations between 3 and 600 pg progress, as will be described in more detail below.
endotoxin/100 Ag CRP in different purified and recombinantCRP preparations (with the Limulus Amebocyte test which is endotoxin-positive > 10 pg = 0.12 EU endotoxin present;unpublished data 2003). The presence of even very low Activation of the classical pathway of the complement concentrations of endotoxin can interact with the contents of system is a well known and direct biological function of the vessel wall activate gene expression and a cascade CRP Via this action, CRP directly amplifies and of reactions in monocytes activate procoagulant activity facilitates innate immunity a process that has in monocytes and macrophages through induction of already been associated with initiation and progression of tissue factor Endotoxins trigger an atherogenic re- CVD for a long time. In situ hybridization showed intense sponse in SMCs and block the induction of secretion of mRNA signals for CRP and complement component C4 in interleukin-1 and MCP-1 by human endothelial cells SMCs and macrophages present in the thickened intima of The amount of sodium azide present in different the lesion. CRP also co-localizes with C5 – C9, the commercially CRP preparations, varies from 0.05% to membrane attack complex, of complement Activation 1.00% of the CRP present. Recent studies describe an of this membrane attack complex (MAC) is initiated by the effect of this preservative, when tested in vitro, on several direct binding of CRP to C1q, also present in the processes involved in atherosclerotic development, which atherosclerotic lesion and characterized by elevated were formerly subscribed to CRP. Addition of sodium azide, levels of component C5a C5a itself exerts potent in amounts comparable to its concentration as a preserva- chemotactic and pro-inflammatory effects and its plasma tive, to cultures of endothelial cells resulted in a decrease of levels have been associated with increased cardiovascular migration, proliferation and angiogenic properties of these risk in patients with advanced atherosclerosis cells In SMCs, addition of sodium azide (without CRP is also involved in the inhibition of complement CRP) did induce vasorelaxation and evoked inducible activation through interaction with factor H (fH), which is nitric oxide synthase (iNOS) induction and nitric monoxide also present in injured areas. The CRP – fH complex (NO) release. These effects were initially ascribed to the interferes with the activity of C3b (see and CRP. It is not clear yet how many of the reported effects of thus will prevent formation of the MAC.
CRP can be ascribed to contamination and whether the Through the interaction with complement factors, CRP effects can be explained by contamination completely or exerts a direct effect on arterial endothelial cells, by whether there will simultaneous effects of contaminants and increasing the expression of complement inhibitory factors CRP. However, several studies showed direct effects of CRP on the endothelial cells. This suggests that CRP-mediated preparations that were free of contaminants, showing that complement activation is a system set to regulate the CRP may indeed have a direct role. Examples are studies inflammatory reaction, because it will result in promoting comparing the effect of plasma from CRP transgenic mice the removal of debris from tissues and the deleterious effects and wild-type mice or studies in which the effect of of complement activation in patients with CVD CRP containing a sodium azide contamination exceeds the However, since complement activation also leads to the effect of sodium azide alone which deliver strong production of a variety of pro-inflammatory molecules, this evidence of CRP having a direct and causal effect on several mechanism of CRP-mediated complement regulation might cell types and the inflammatory processes. Furthermore, the also aggravate the inflammatory status in the entire body as specific interaction of CRP with complement factors, cell well as in the atherosclerotic plaque. Therefore, the direct receptors, lipids and other inflammatory mediators assures interaction between CRP and complement can both activate the possibility of CRP being directly involved atheroscle- and inhibit inflammation in atherosclerotic lesions.
E. Paffen, M.P.M. deMaat / Cardiovascular Research 71 (2006) 30 – 39 Fig. 1. Schematic representation of CRP-mediated complement regulation. Binding of CRP to microbial polysaccharides or ligands exposed on damagedactivates the classical pathway of complement. Activation is however limited to C1, C4, C2 and C3 with little consumption of C5 – C9. Surface-bound CRPrecruits fH, which regulates complement activation at the level of the alternate pathway C3 convertase and C5 convertase of both the classical and alternatepathways. fH is therefore thought to be responsible for low consumption of terminal pathway components during the CRP-initiated complement activation(adapted from Giannakis et al. fH: factor H, MBL: mannose-binding lectin pathway of complement, MAC: membrane attack complex.
5.2. Interaction with cell surface receptors CRP through its interference with the binding of LDL toCD36 The close proximity of CRP to monocytic cells in the arterial intima, attenuates its possibilities for a direct contribution to the progression of atherosclerosis.
The observation that CRP is localized between monocytes Thrombosis contributes to the progression of the underlines the possibility of a direct interaction of CRP atherosclerotic lesion and to the precipitation of the with these cells and with monocyte-derived macrophages cardiovascular event. Direct actions of CRP which contrib- via binding to a specific receptor. CRP binds to several ute to the induction of a prothrombotic state may be the receptors on human monocytes; to FcRgIIa (CD32) with enhancement of the procoagulant activity or the high affinity and to FcRgI (CD64) with lower affinity reduction of fibrinolysis CRP has been suggested to increasing phagocytosis and the release of inflam- induce a prothrombotic state via induction of tissue factor matory cytokines The Fc receptors have been expression in human monocytes but only in the described to mediate the effect of CRP on human aortic presence of and through direct interaction with other blood endothelial cells FcRgIIa is known as the putative cells as T-lymphocytes, B-lymphocytes and natural killer CRP receptor for leukocytes and also has been found on bovine aortic endothelial cells CRP also binds to In transgenic mice expressing human CRP (hCRP), the the inhibitory receptor, FcgammaRIIb, blocking activating injury-induced occlusion of the femoral artery (75% after 28 signals The binding of CRP to a receptor suggests days) was enhanced compared to the amount of occlusion its capacity to induce a specific biological effect observed in wild-type mice (17% after 28 days) such as direct involvement in cell-mediation and opsoni- indicating a prothrombotic effect of hCRP. A direct effect zation. Addition of CRP with and without anti-CD32- of CRP on hemostasis was shown in a recent study, where antibody to endothelial cells demonstrated the partial recombinant human CRP was infused into human volun- mediation of CRP in regulation of cell surface protein teers, which resulted in the stimulation of both hemostasis expression, such as the endothelial protein C receptor, by CD32 However, the downstream effects of CRP CRP may also inhibit fibrinolysis by increasing the binding have not yet been elucidated. The interaction of expression and activity of the main inhibitor of fibrinolysis, CRP with CD36, a scavenger receptor which is expressed plasminogen activator inhibitor-1 (PAI-1) in human aortic by macrophages and is involved in uptake of low-density endothelial cells (HAEC) Since PAI-1 promotes lipoprotein particles (LDL), demonstrates a direct role of atherothrombosis and progression of acute coronary syn- E. Paffen, M.P.M. deMaat / Cardiovascular Research 71 (2006) 30 – 39 dromes, this effect of CRP may also affect CVD Also, promotes MCP-1 mediated chemotaxis through upregula- besides this effect on PAI-1, recently CRP has been tion of CC chemokine receptor 2 expression in human demonstrated to directly decrease antigen levels and the activity of tissue plasminogen activator (tPa) in HAEC. tPa The effect of CRP on T-lymphocytes is indirect. T- is the substance normally inhibited by PAI-1. In this study, lymphocytes are recruited to the atherosclerotic lesion as a the direct and specific role of CRP is demonstrated by using result of the ongoing inflammatory process. Through the CRP that was free from sodium azide and LPS contamina- stimulation of cytokine production and secretion by macro- phages, CRP exerts an indirect effect on T-lymphocytespresent in the atherosclerotic lesion. CRP induces macro- 5.4. Cellular modulation, recruitment and activation phages to express interleukin-12, which contributes to thedevelopment of CD4+ T-helper cells In turn, these cells CRP contributes to an arterial pro-inflammatory and pro- express interferon-g, which is synergistic with CRP in the atherosclerotic phenotype by directly upregulating adhesion execution of many functions contributing to the pro- molecules and chemoattractant chemokines in endothelial atherosclerotic phenotype. In contrast, during CRP induced cells, vascular SMCs and monocytic cells. On the activation of complement and opsonization of apoptotic endothelial cell surface, expression of adhesion molecules cells, the actively phagocyting macrophages reduce expres- such as ICAM-1, VCAM-1, and E-selectin is upregulated sion of IL-12 and thereby suppress T-lymphocytes by CRP Via these processes, CRP induces plateletadhesion to endothelial cells CRP stimulates endo- 5.5. Expression of inflammatory mediators: cytokines, thelial cell dysfunction and the recruitment of monocytes and T-lymphocytes towards the endothelial wall. Thesefindings were reported by several groups, who also showed CRP induces inflammatory cytokines in a dose-depen- that CRP induced monocyte chemoattractant chemokine-1 dent way which provides further support for the (MCP-1) production. This upregulation of adhesion mole- hypothesis that interaction with mononuclear phagocytes cules is partly mediated via the production of endothelin-1, constitutes an important biological role for this acute phase a potent endothelium-derived vasoactive factor, and by the protein. Quantitative analysis of the CRP-induced release of production of the inflammatory cytokines interleukin-6 and interleukin-6, interleukin-1 and tumor necrosis factor-a by interleukin-8. As to the effects of CRP on MCP-1 freshly isolated normal human monocytes, revealed slight expression, aortic endothelial cells seem to be unresponsive differences in time courses. All three cytokines were whereas venous endothelial cells or monocytes detected 4 h after CRP addition in vitro, with maximal show increased expression of this chemoattractant. Since levels of TNFa at 8 h and of interleukin-1 and interleukin-6 atherosclerosis mainly develops in the arteries, the clinical significance of the effect of CRP on venous cells is not Interleukin-8 (IL-8), a member of the CXC chemokines promotes monocyte – endothelial cell adhesion and arrest CRP is also known to activate the NF-nB signaling and is abundant in atherosclerotic plaques. In human aortic pathway in saphenous vein endothelial cells which, in endothelial cells in vitro, CRP increases IL-8 protein and recent light of possible azide contamination might be mRNA expression in a time- and dose-dependent manner an artefact. Also in vascular SMCs, CRP has been indicated via specific upregulation of NF-nB activity to activate NF-nB Therefore, CRP has been suggested CRP induces production and secretion of MCP-1 in to mediate proliferation and activation of vascular SMCs, human umbilical vein endothelial cells but not in aortic causing the accumulation of these cells in the vascular endothelial cells MCP-1 present in the atherosclerotic intima, which is a key event in the development of arterial lesion can also originate from monocytes. CRP induces lesions. Another manner in CRP directly affects the a 7-fold increase in the production of monocyte MCP-1 in activation and proliferation of vascular SMCs, is via purified peripheral monocytes In patients with acute upregulation of mRNA and protein and increased cell coronary syndromes, baseline level of this chemoattractant surface expression of the angiotensin type 1 receptor (AT1- were elevated. This elevated expression is associated with R). This was demonstrated in vitro in human vascular both traditional risk factors for atherosclerosis as well as SMCs and in vivo in a rat carotid artery angioplasty model increased risk of myocardial infarction, independent of CRP also appears to be involved in the infiltration of In atherosclerotic lesions, CRP directly upregulates monocytes into the vessel wall and their subsequent mRNA expression of the macrophage markers CD11b and development into foam cells. The deposition of CRP in HLA-DR, as well as their protein products the arterial wall precedes monocyte infiltration and direct Monocyte expression of CD11b increased significantly involvement of CRP in recruitment of blood monocytes up to twofold when exposed to CRP, while no significant has been demonstrated in vitro, suggesting CRP to be difference in CD32 expression was observed, whereas CRP chemotactic for human blood monocytes CRP also exposure decreases CD31 expression. CRP can affect E. Paffen, M.P.M. deMaat / Cardiovascular Research 71 (2006) 30 – 39 monocyte activation ex vivo and induce phenotypic changes that result in an altered recruitment to endothelial cells Another mechanism by which CRP influences the CRP is directly involved in the process of apoptosis development and maintenance of the athereosclerotic lesion It binds to apoptotic cells in a Ca2+-dependent is its involvement in the CD40 – CD40Ligand (CD40L or manner and augments the classical pathway of complement CD154) interaction. CD40L, a 33-kDa activation-induced T- activation but protects the cells from assembling the lymphocyte surface glycoprotein, binds to CD40, a phos- terminal complement components (C5 – C9). Furthermore, phorylated glycoprotein expressed on B-lymphocytes, vas- CRP enhances opsonization and phagocytosis of apoptotic cular endothelial cells, monocytes, macrophages and cells by macrophages associated with the expression of the fibroblasts. Like CRP, the amount of soluble CD40 anti-inflammatory cytokine transforming growth factor-h.
increases during inflammation and in the atherosclerotic CRP and the classical complement components act in lesion. Therefore CD40L has been suggested to be a marker concert to promote non-inflammatory clearance of apoptotic for inflammation and involved in risk of cardiovascular cells The inhibitory effect of CRP on the NO events as well CRP upregulates the cell surface expression of endothelial progenitor cells directly inhibits expression of CD40 and CD40L on human umbilical their mobilization and differentiation, survival and function, endothelial cells in time CD40L is shed into the whereby it facilitates EC apoptosis and blocks the process of vasculature. Elevated levels of this soluble CD40L angiogenesis Apoptosis of vascular SMCs also plays (sCD40L) identify patients with acute coronary syndromes an important role in progression of atherosclerotic lesions at increased risk of recurrent MI and death, independent of and contributes to increased plaque vulnerability. Silencing other variables as cardiac troponine T or CRP the CRP-regulated GADD153 gene in vascular SMCsindicated that CRP plays an essential role in induced CRP also binds to phosphatidylcholines, by which it CRP has been described to decrease the expression and participates directly in activation of macrophages and bioactivity of endothelial nitric oxide synthase (eNOS or neutrophils in the clearance of apoptotic and necrotic cells NOS3) which results in reduced bioavailability of However, neutrophils are not present in the nitric monoxide (NO) and a subsequent effect of vasodila- tation. It was demonstrated recently that this effect can becaused by sodium azide as well however, it is not clear whether there remains a role for CRP. It is thereforeuncertain whether there is a causal role for CRP in the The interaction between lipids and CRP is diverse. It has regulation of expression of NO and involvement in vascular been suggested that CRP could be the factor that links reactivity Nevertheless, CRP has been suggested to lipoprotein-deposition and complement activation in ath- exert a specific effect on endothelial eNOS expression erosclerotic plaques. Binding of tissue-deposited CRP to through binding to the CRP receptor FcRgIIa In enzymatically degraded LDL enhances complement activa- HAECs and human coronary artery endothelial cells tion, which may be relevant to the development and (HCAECs), CRP contributes to a proatherogenic and progression of the atherosclerotic lesion, particularly at prothrombotic state by decreasing the release of NO and early stages of atherosclerosis when low concentrations of of the vasodilator and inhibitor of platelet aggregation enzymatically degraded LDL are present And, prostacyclin (PGI2), through directly increasing both although direct involvement of CRP has not been demon- strated, through this binding of CRP to enzymatically In vascular smooth muscle cells, CRP reduces expression degraded LDL, CRP may be involved in the massive of the inducible variant of nitric oxide synthase (iNOS) release of MCP-1 from macrophages described to be caused and subsequent NO-synthesis as well. But again, this might be an artefact caused by sodium azide In these vascular Although the reports on interaction between CRP and ox- SMCs, CRP also has been described to induce activation of LDL are conflicting, complement activation as a result of the iNOS promoter which, despite the fact that CRP this interaction is generally considered unlikely. Neverthe- seems to be no more than a weak inducer of NO-production, less, CRP has been described to directly induce lectin-like contradicts the study of Ikeda and coworkers. Nevertheless, ox-LDL receptor-1 expression (LOX-1) in human aortic both studies demonstrate that CRP is likely to be involved in ECs, because this could be reduced with antibodies against the regulation of cellular NO-levels. Furthermore, interac- CD32/CD64, ET-1 or IL-6 Via LOX-1, CRP is tion between CRP and interferon-gamma appears to suggested to regulate monocytes adhesion to ECs and enhance the effect of CRP on NO-regulation, which indicates that there may be a direct effect of CRP because The majority of sub-endothelial foam cells show contaminants will not be able to exert these specific positive staining for CRP. Zwaka et al. demonstrated that native LDL that was co-incubated with CRP was taken up E. Paffen, M.P.M. deMaat / Cardiovascular Research 71 (2006) 30 – 39 by macrophages via macropinocytosis. It was concluded process was not possible but we discussed above that that foam cell formation in human atherogenesis might be one thing does not exclude the other.
caused in part by uptake of CRP-opsonized native LDL Recently, several studies have reported conflicting results on the direct contribution of CRP to atherosclerosis in mice High levels of high-density lipoprotein (HDL) are that transgenically expressed CRP. Since CRP is expressed atheroprotective since HDL is involved in transporting only at a very low concentration in mice and does not show cholesterol from the periphery to the liver. HDL might also an acute phase behavior, it is possible to study the role of protect the endothelium since the CRP-induced upregulation transgenic CRP in mice. The first report was from Paul and of inflammatory adhesion molecules in HUVECs was coworkers who observed larger aortic atherosclerotic lesions completely blocked by HDL. So, HDL neutralizes CRP in human CRP transgenic apolipoprotein (apo) E-knockout induced proinflammatory activity HDL also inhibits mice than in control mice but in this study the CRP atherosclerosis through prevention of oxidation of LDL. It is levels were very high. Recently, we did not see a difference not known whether CRP has an effect on the oxidative in lesion size or severity of atherosclerosis at the aortic root between apoE*3-Leiden mice that did express human CRP(< 10 mg/L) and controls Reifenberg and coworkersreported similar observations in apoE-knockout mice that expressed rabbit CRP and subsequently discussedwhether transgenic apoE-knockout mice would provide an CRP may contribute to development of the atheroscle- answer about the role of CRP in atherogenesis at all. In our rotic lesion and the subsequent acute cardiovascular events opinion, we believe that more studies are needed to via its role in a large number of biological pathways. And elucidate the differences, and we cannot ignore previous although a number of the effects of CRP may be clouded by studies in which mouse models were used to clearly contamination, we demonstrated that a direct role of CRP in demonstrate a direct role of CRP in atherogenesis many inflammatory processes is probable. We reviewed the In conclusion, it is now an established fact that elevated roles of CRP in complement activation, cell adhesion and CRP levels are associated with a worse prognosis for CVD, recruitment, thrombosis, the expression of regulatory such as myocardial infarction, stroke and unstable angina.
cytokines, apoptosis and lipids. All these mechanisms are But it is important to distinguish between a role as a marker part of or are compromised by the process of inflammation.
and a factor that directly causes a biological effect because CRP may thus contribute to the development of the this will determine the optimal therapeutic intervention.
atherosclerotic lesion via a direct pro-inflammatory effect.
CRP may be a causal factor as well as a marker for CRP increases the release of endothelin-1 and upregulates inflammation, depending on the concentration. This concen- adhesion molecules and chemoattractant chemokines in tration of CRP depends on the rates of production and endothelial cells and vascular SMC. Most studies focus on clearance. The fact that CRP is a very stable protein, which is the effects of CRP on aortic endothelial cells but a few not consumed to a significant extent in any process, and the studies have also determined the effect on venous endothe- clearance of which is not influenced by any known condition lial cells as HUVECS and it was observed that CRP induced is in agreement with its functioning as a causal factor, the expression of MCP-1 CD40 and increased attempting to prolong the stability of the atherosclerotic activity of NF-nB This suggests that CRP exerts different and specific effect on different cell types.
In the atherosclerotic lesion, many processes involving However, it is important to realize that CRP also exerts CRP and many different cell types have been described but activity in anti-inflammatory or so-called protective mech- there is also concern that part of the observed effects are not anisms, such as suppressing the formation of the C5 – C9 mediated by CRP itself, but are the result of contamination of complex. Thereby CRP is capable of maintaining a certain the preparations with endotoxin or azide. Therefore, currently balance in the inflammatory process and stability of the the exact role of CRP in the initiation and progression of atherosclerotic lesion. In the lesion, CRP, which stimulates atherosclerosis is still unclear and needs to be further studied.
immunity as well as inflammation, contributes to theprolongation of the stability of the plaque.
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