Portal de Periódicos da CAPES

Soluble LOX‐1: A Novel Biomarker in Patients With Coronary Artery Disease, Stroke, and Acute Aortic Dissection?

2020; Wiley; Volume: 9; Issue: 1 Linguagem: Inglês

10.1161/jaha.119.013803

2047-9980

Anja Hofmann, Coy Brunßen, Steffen Wolk, Christian Reeps, Henning Morawietz,

Coronary Interventions and Diagnostics

HomeJournal of the American Heart AssociationVol. 9, No. 1Soluble LOX‐1: A Novel Biomarker in Patients With Coronary Artery Disease, Stroke, and Acute Aortic Dissection? Open AccessResearch ArticlePDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citations ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toOpen AccessResearch ArticlePDF/EPUBSoluble LOX‐1: A Novel Biomarker in Patients With Coronary Artery Disease, Stroke, and Acute Aortic Dissection? Anja Hofmann, PhD, Coy Brunssen, PhD, Steffen Wolk, MD, Christian Reeps, MD and Henning Morawietz, PhD, FAHA Anja HofmannAnja Hofmann Division of Vascular Endothelium and Microcirculation, , Department of Medicine III, , Medical Faculty Carl Gustav Carus and University Hospital Carl Gustav Carus Dresden, , Technische Universität Dresden, , Dresden, , Germany Division of Vascular and Endovascular Surgery, , Department for Visceral‐, Thoracic and Vascular Surgery, , Medical Faculty Carl Gustav Carus and University Hospital Carl Gustav Carus Dresden, , Technische Universität Dresden, , Dresden, , Germany Search for more papers by this author , Coy BrunssenCoy Brunssen Division of Vascular Endothelium and Microcirculation, , Department of Medicine III, , Medical Faculty Carl Gustav Carus and University Hospital Carl Gustav Carus Dresden, , Technische Universität Dresden, , Dresden, , Germany Search for more papers by this author , Steffen WolkSteffen Wolk Division of Vascular and Endovascular Surgery, , Department for Visceral‐, Thoracic and Vascular Surgery, , Medical Faculty Carl Gustav Carus and University Hospital Carl Gustav Carus Dresden, , Technische Universität Dresden, , Dresden, , Germany Search for more papers by this author , Christian ReepsChristian Reeps Division of Vascular and Endovascular Surgery, , Department for Visceral‐, Thoracic and Vascular Surgery, , Medical Faculty Carl Gustav Carus and University Hospital Carl Gustav Carus Dresden, , Technische Universität Dresden, , Dresden, , Germany Search for more papers by this author and Henning MorawietzHenning Morawietz *Correspondence to: Henning Morawietz, PhD, FAHA, Division of Vascular Endothelium and Microcirculation, Department of Medicine III, Medical Faculty Carl Gustav Carus and University Hospital Carl Gustav Carus Dresden, Technische Universität Dresden, Fetscherstrasse 74, 01307 Dresden, Germany. E‐mail: E-mail Address: [email protected] Division of Vascular Endothelium and Microcirculation, , Department of Medicine III, , Medical Faculty Carl Gustav Carus and University Hospital Carl Gustav Carus Dresden, , Technische Universität Dresden, , Dresden, , Germany Search for more papers by this author Originally published4 Jan 2020https://doi.org/10.1161/JAHA.119.013803Journal of the American Heart Association. 2020;9:e013803Cardiovascular diseases are the leading cause of death worldwide. Approximately 85% of all deaths from cardiovascular disease are due to acute myocardial infarction (AMI) and stroke. Strokes are associated with high mortality, and 15% to 30% of patients have permanent disability.1 Although biomarkers like cardiac TnT (troponin T; cTnT) and creatine kinase (CK) are successfully used for diagnosis of AMI, there is an ongoing need for markers that, for example, help identify patients with vulnerable plaques. In addition, no biomarker exists for the diagnosis of stroke or the identification of patients at higher risk for stroke.1Atherosclerosis is the underlying cause of most cardiovascular disease and involves plaque formation, intima thickening caused by lipid deposition, inflammation, and thickening of the arterial wall.2, 3 The plaque development starts with early atherosclerotic lesions and proceeds to advanced atherosclerotic lesions, which can evolve into complicated and unstable vulnerable plaques. Vulnerable plaques prone to rupture have intraplaque hemorrhage, apoptosis, and calcification. Rupture of unstable plaques causes atherothrombogenic events and leads to severe clinical symptoms.2Oxidative modifications of LDL (low‐density lipoprotein) to oxidized LDL (oxLDL) play an important role in the initiation and progression of atherosclerosis. OxLDL promotes foam cell and fatty streak formation, induces proinflammatory pathways, triggers vascular smooth muscle cell migration and proliferation, and induces cell death and apoptosis. Thus, oxLDL contributes to plaque growth and instability.2 Several oxLDL receptors have been described. LOX‐1 (lectin‐like oxLDL receptor 1) was identified as the major oxLDL receptor in endothelial cells.4 In early atherosclerotic lesions, LOX‐1 is mainly expressed in endothelial cells but extends its expression to smooth muscle cells and macrophages in advanced lesions.5, 6 Under physiological conditions, LOX‐1 expression is very low, but its expression increases in response to oxLDL, proinflammatory cytokines, pro‐oxidative and biomechanical stimuli.6, 7, 8, 9, 10 LOX‐1 activation promotes secretion of proinflammatory cytokines and reactive oxygen species formation.6, 10 Furthermore, LOX‐1 deletion reduces MMP‐2 (matrix metalloproteinase 2) and MMP‐9 in atherosclerotic lesions, whereas activation of oxLDL–LOX‐1 pathways increases MMP activity.11, 12, 13 This connects LOX‐1 to advanced lesion formation and plaque vulnerability.6, 14, 15 LOX‐1 expression is increased in experimental and clinical studies of hypertension, myocardial infarction, carotid artery atherosclerosis, type 2 diabetes mellitus, and obesity.5, 16, 17, 18, 19A soluble form of LOX‐1 (sLOX‐1) has been identified and is generated by ectodomain shedding. This process is triggered by oxLDL, CRP (C‐reactive protein), TNF‐α (tumor necrosis factor α), IL‐8 (interleukin‐8), and IL‐18 and mediated by the action of MMPs and ADAMs (a disintegrin and metalloproteinase; Figure 1).20, 21, 22 A large number of studies have demonstrated the diagnostic potential of sLOX‐1 regarding severity of stable coronary artery disease (CAD) and acute coronary syndrome (ACS), even allowing discrimination between different types of AMI.17, 23, 24, 25, 26 Furthermore, sLOX‐1 concentrations are increased in ischemic and hemorrhagic stroke, and patients with high sLOX‐1 levels have an increased risk of future clinical events.22, 27, 28 Quantification of sLOX‐1 might be an interesting novel biomarker to improve the diagnosis of patients at risk.Download figureDownload PowerPointFigure 1. Generation of sLOX‐1 (soluble lectin‐like oxidized low‐density lipoprotein receptor 1) from the membrane‐bound form. LOX‐1 expression is found on endothelial cells, smooth muscle cells, macrophages, platelets, and nonvascular cells. Proinflammatory mediators oxidized LDL (oxLDL), CRP (C‐reactive protein), IL‐8 (interleukin 8), IL‐18, or TNF‐α (tumor necrosis factor α) induce the shedding of cell‐membrane‐bound LOX‐1. This process involves reactive oxygen species (ROS) and the activation of matrix‐degrading enzymes ADAMs (a disintegrin and metalloprotease) and MMPs (matrix metalloproteases). ROS are generated by different enzymatic sources in vascular cells. Their formation is increased on activation of LOX‐1 and direct effects of oxLDL or TNF‐α on ROS producing enzymes. The proteolytic cleavage in the extracellular neck domain of membrane‐bound LOX‐1 leads to the release of the shorter sLOX‐1. Elevated sLOX‐1 concentrations are shown in stable coronary artery disease (CAD), acute myocardial infarction (AMI), stroke, and acute aortic dissections (AAD). Proinflammatory mediators that stimulate sLOX‐1 release are increased in atherosclerosis, hypertension, hyperlipidemia or type 2 diabetes mellitus. In addition, sLOX‐1 may even present a pathophysiologic link among these diseases. Interpretation of results has to involve adjustment for other comorbidities and pharmacologic therapy with statins, angiotensin‐converting enzyme (ACE) inhibitors, angiotensin receptor II type 1 (ATR1) blockers, or calcium channel blockers that might affect LOX‐1 expression and sLOX‐1 release. Figure adapted from SMART—Servier Medical Art by Servier (https://smart.servier.com). UA indicates unstable angina.Acute aortic dissection (AAD) is a life‐threatening disease caused by a tear in the vessel wall. Risk factors are poorly controlled hypertension, age, and male sex.29, 30 LOX‐1 expression is increased in hypertension.6 Impaired aortic vascular function has been described in mouse models.31 Furthermore, effects on smooth muscle cell apoptosis were shown.32, 33 No experimental data on LOX‐1 and aortic disease are available, but LOX‐1 might be involved in the degeneration of the vessel wall. Therefore, sLOX‐1 could be a marker in AAD that helps to set the diagnosis and, furthermore, allows discrimination among patients with AMI.This review provides a detailed overview on studies of sLOX‐1 in CAD, AMI, stroke, and AAD (Table). The findings are described and critically discussed. The potential of sLOX‐1 as a biomarker is evaluated by comparison to current clinical biomarkers and imaging techniques. The aim of this review is to evaluate the potential of sLOX‐1 for the diagnosis of CAD and AMI and its implications to predict the prognosis of these patients. In addition, the use of sLOX‐1 as a novel biomarker in patients with stroke and AAD will be discussed.Table 1. sLOX‐1 Concentrations in CAD, ACS, Stroke, and AADDiseaseControlDiseaseSample (n)Time Point of Blood CollectionCAD34Control,268 (111–767) pg/mL1–2 diseased vessels,611 (346–1313) pg/mL;3–4 diseased vessels,2143 (824–3201) pg/mLControl: 29;CAD: 60Before CAGCAD26Simple lesion,0.426 (0.195–1.075) ng/mLComplex lesion,0.914 (0.489–1.296) ng/mLSimple: 72; complex: 50Before CAGCAD, ACS26Stable CAD,0.579 (0.256–1.172) ng/mLACS, 1.610 (0.941–2.264) ng/mLCAD: 122;ACS: 58Before CAGACS26No complex lesion,1.003 (0.783–1.668) ng/mL1 complex lesion,1.456 (0.923–2.124) ng/mL; multiple complex lesions,2.171 (1.067–3.247) ng/mLNo complex lesion: 11; 1 complex lesion: 23;Multiple complex lesions: 24Before CAGCAD35Distal segment LAD lesion,0.70±0.17 ng/mLProximal/middle segment LAD lesion, 1.07±0.33 ng/mLDistal: 51; proximal/middle: 64After CAGACS23Intact coronary,<0.5 (<0.5–1.3) ng/mLControlled CHD,<0.5 (<0.5–3.4) ng/mL; ischemic CHD,0.73 (<0.5–14.0) ng/mL;acute noncardiac illness,<0.5 (<0.5–6.4) ng/mL;chronic illness,<0.5 (<0.5–3.3) ng/mLACS,2.91 (<0.5–170) ng/mLACS: 80; intact coronary: 52; controlled CHD: 122; ischemic CHD: 173; acute cardiac illness: 34; chronic illness: 60At CAG or time of visit (acute and chronic illness)ACS36NSTEMI,133.3 (106.6–238.5) ng/mLSTEMI, 204.2 (135.7–456.0) ng/mLNSTEMI: 19;STEMI: 56Before CAGACS37Non‐ACS,104.1 (67.9–128.6) ng/mLNSTEMI, 143.9 (96.6–255.1) ng/mL;STEMI, 259.0 (134.5–488.9) ng/mLNon‐ACS: 40;NSTEMI: 44;STEMI: 116In ERACS25Non‐ACS,0.096 (0.0645–0.162) ng/mLACS, 1.13 (0.168–3.46) ng/mLNon‐ACS: 89;ACS: 18After CAGPCI38PCI without RPMI,99±68 pg/mLPCI+RPMI,167±89 pg/mLPCI without RPMI: 181;PCI+RPMI: 33Every 6 h after PCI; in total, up to 24 hACS39Non‐AMI,64.3 (54.4–84.3) ng/mLSTEMI, 241.0 (132.4–472.2) ng/mL;NSTEMI, 147.3 (92.9–262.4) ng/mLNon‐AMI: 125;NSTEMI: 44;STEMI: 125Before CAGACS40Event‐free survival,2.54 ng/mLRecurrence‐ACS/and death,6.60 ng/mLEvent‐free: 81; recurrence: 13During acute stageAAD41NSTEMIAADNSTEMI: 39; AAD: 19In ER and before CAGStroke, ischemic stroke42Mean: Q1, 558 ng/mL;Q2, 925 ng/mL;Q3, 1289 ng/mL;Q4, 2367 ng/mLQ1, n=21;Q2, n=20;Q3, n=25;Q4, n=25Median: 11 yStroke27Control (ischemic stroke),486 (321–703) ng/L;control (ICH),513 (307–770) ng/L;control (ABI),496 (337–781) ng/L;control (cardioembolic stroke),462 (333–652) ng/L;control (lacunar infarction),558 (302–850) ng/LIschemic stroke,526 (330–883) ng/L;ICH, 720 (459–1125) ng/L;ABI, 641 (429–1302) ng/L;cardioembolic stroke,442 (225–840) ng/L;lacunar infarction,529 (341–743) ng/LControl: 250; ischemic stroke: 250—control: 127; ICH: 127—control: 43;ABI: 43—control: 59;cardioembolic stroke: 59—control: 56;lacunar infarction: 563 d after onset of strokeStroke28Healthy control0.67 ng/mLCarotid atherosclerosis,0.99 ng/mL;TIA,0.95 (0.23–7.31) ng/mL;ischemic stroke,1.0 (0.11–2.63) ng/mLHealthy control: 81; carotid atherosclerosis: 232; TIA: 61; ischemic stroke: 10448 h before operationStroke28sLOX‐1: tertile 1,3.48±0.279 (au); tertile 2,4.04±0.134 (au); tertile 3,4.79±0.480 (au)Tertile 1, 1567; tertile 2, 1568; tertile 3, 1568Baseline until first hospitalization for acute stroke (median: 16.5±3.6 y)Summary of the most important studies analyzing sLOX‐1 in patients with CAD, AAD, and stroke. AAD indicates acute aortic dissection; ABI, atherothrombogenic brain infarction; ACS, acute coronary syndrome; AMI, acute myocardial infarction; CAD, coronary artery disease; CAG, coronary angiography; CHD, coronary heart disease; ER, emergency room; ICH, intracerebral haemorrhage; LAD, left anterior descending; NSTEMI, non‐ST‐segment–elevation myocardial infarction; PCI, percutaneous coronary intervention; Q, quartile; RPMI, related periprocedural myocardial infarction; sLOX‐1, soluble lectin‐like oxidized low‐density lipoprotein receptor‐1; STEMI, ST‐segment–elevation myocardial infarction; TIA, transient ischemic attack.Release of sLOX‐1 Involves Inflammatory Cytokines and Matrix‐Degenerating EnzymesThe polypeptide sLOX‐1 consists of 187 amino acids and derives from the proteolytic cleavage of LOX‐1 expressed on the cell surface.15 The molecular weight of sLOX‐1 is 35 kDa and reflects the shortening of the mature LOX‐1 protein with a size of 40 kDa.20 LOX‐1 structurally belongs to the C‐type lectin receptor family and consists of 4 functional domains: a short N‐terminal cytoplasmic domain, a connecting neck domain, a transmembrane domain, and the lectin‐like ligand binding domain, located extracellularly at the C‐terminus.16, 43 Detailed analysis showed that PMSF (phenylmethylsulfonyl fluoride)–sensitive serine proteases cleave at 2 possible sites in the LOX‐1 extracellularly located neck domain.20, 34, 44 In this study, PMSF was applied to cultured cells. Therefore, it is difficult to conclude that only PMSF‐sensitive proteases cleave LOX‐1 because most studies emphasize the importance of MMPs in this context. A PMSF‐sensitive process might be involved in sLOX‐1 generation but rather in an indirect way. A cleavage site between Arg88 and Gln89 has been identified in the human LOX‐1 protein. This cleavage site is located between 2 stable and highly conserved regions at the beginning of a flexible region. Increases in flexibility are mainly due to the replacement of hydrophobic amino acid residues in polar or charged amino acids, thereby mediating the proteolytic attack.44It has been postulated that elevations in sLOX‐1 may reflect increased expression of the membrane‐bound form.20, 24 However, no clear correlation between plasma sLOX‐1 and plaque LOX‐1 mRNA expression was demonstrated in carotid atherosclerosis.28 One explanation could be that sLOX‐1 is generated not only from atherosclerotic carotid arteries but also from other atherosclerotic vessels45 and nonvascular cells.46, 47 Instead of correlations with tissue LOX‐1 expression, positive correlations between markers of macrophages and endothelial cells were found.28 Therefore, sLOX‐1 might be an inflammatory marker and a surrogate marker of tissue expression. In vulnerable atherosclerotic plaques, LOX‐1 is dominantly expressed in smooth muscle cells and macrophages and contributes to apoptosis and production of MMPs.12, 13, 23 MMPs are involved in the proteolytic cleavage of cell surface–expressed LOX‐1 and thus may represent a possible link between tissue LOX‐1 and sLOX‐1.21, 23Ectodomain shedding is a proteolytic process at transmembrane proteins that is driven by catalytically active ADAMs. The function of ectodomain shedding is the modulation of signaling pathways between host and neighboring cells by downregulating the expression of cell surface receptors or by increasing their soluble ligands.48 For sLOX‐1, the proinflammatory cytokines TNF‐α, IL‐8,28 IL‐18,49 and CRP50 are known to stimulate its release. In macrophages, CRP stimulated sLOX‐1 release by a mechanism involving p47phox phosphorylation, production of reactive oxygen species, and activation of TACE (TNF‐α converting enzyme; also known as ADAM‐17).50 ADAM‐10 is also involved in the shedding of membrane‐bound LOX‐1.49 In endothelial cells, soluble sMMP‐1 and/or sMMP‐2 is secreted and may induce LOX‐1 ectodomain shedding.21 In addition, depletion of membrane cholesterol (eg, by statins) enhances the release of full‐length LOX‐1 and sLOX‐1 in exosomes that originated from endothelial cells.21 Although many of these studies revealed contributions of MMPs and ADAMs in the shedding of sLOX‐1, no direct evidence shows that these enzyme mediate the cleavage of LOX‐1. It is possible that they act as activators of the real and, to date, unknown proteases. A recent study provided evidence that oxLDL induces the release of sLOX‐1 from endothelial cells, supporting the fact that sLOX‐1 originates from cells that were pre‐exposed to oxLDL.22 Interestingly, the authors demonstrated that native LDL, VLDL (very low‐density lipoprotein), HDL (high‐density lipoprotein), TGF‐β (transforming growth factor β) and IL‐1β had no effect on sLOX‐1 in endothelial cells.22Methods for Quantification of sLOX‐1 and Interpretation of ResultsMost of the previous studies used a “self‐made” sandwich ELISA with 2 different anti–human LOX‐1 polyclonal antibodies. Antibodies were raised against the extracellular domain of LOX‐1. The capture antibody was coated on the plates, and the other was fragmented into Fab, with labeling of horseradish peroxidase for enzymatic quantification. In this assay, the lower limit of the detection was 0.5 ng/mL (500 pg/mL). A major disadvantage was the missing ability to detect sLOX‐1 in healthy patients. Therefore, Nakamura and colleagues developed a chemiluminescent enzyme immunoassay. This assay uses a combination of 2 different monoclonal antibodies and could detect sLOX‐1 at concentrations as low as 8 pg/mL.51 This assay was used by the majority of subsequent studies in patients with CAD, ACS, or AAD. Several commercially available ELISA kits exist. Their sensitivity is in the range of 1 to 5 pg/mL. Besides high sensitivity, a major advantage is the low interassay variation during analysis of a high numbers of patients.Markstadt et al discussed LOX‐1 cleavage at the 187 residue in the neck domain. Elevated sLOX‐1 levels could be caused by cleavage of the cell‐bound LOX‐1 at the same site.22 Furthermore, mechanical cleavage during tissue disruption may contribute to sLOX‐1 in tissue homogenates.22 Potential cross‐reactivity of sLOX‐1 antibodies with oxLDL was analyzed. Zhao et al added high concentrations of oxLDL and found no interference.50 To exclude this possibility, oxLDL should be added to the analyzed matrix while using the ELISAs for the first time.Finally, careful interpretation of findings is necessary with respect to comorbidities and medical treatments. Because sLOX‐1 is increased in obesity, type 2 diabetes mellitus, metabolic syndrome, and peripheral artery disease,17, 45, 52, 53, 54 adjustment of findings to these comorbidities is important. In addition, in vitro and in vivo studies revealed that LOX‐1 expression is affected by angiotensin‐converting enzyme inhibitors,55, 56 angiotensin II receptor type 1 blockers,57 and statins.57, 58, 59 These pharmacologic therapies may also have an impact on sLOX‐1 concentrations.Is Soluble LOX‐1 a Diagnostic, Prognostic or Inflammatory Marker in Patients With CAD and MI?The evaluation of patients with suspected CAD and its acute complications involves assessment of risk factors (age, sex, weight, blood pressure, plasma lipids, or smoking) and imaging tools in combination with biochemical laboratory markers.60 The most common strategy is coronary angiography (CAG), providing a high diagnostic specify. Nonetheless, this technique is invasive, expensive, might involve clinical complications and in rare cases, the death of patients. Measurement of cTnT or cardiac TnI (troponin I), FABP (fatty acid‐binding protein) or CK and CK‐muscle/brain (CK‐MB) are routinely used in combination with CAG.61, 62 These markers are released after cardiomyocyte injury and used for the diagnosis of AMI in patients with chest pain.60 The marker cTnT is widely used in diagnosing AMI and predicting death.61, 62 It is highly specific and sensitive for cardiac injury. Elevations in cTnT are associated with a nearly 90% frequency of CAD detected by CAG.61 Because cTnT or CK are released after ischemic injury, neither is useful in detecting plaque vulnerability or plaque rupture at a stage before myocardial damage. Developing markers that reflect plaque instability and rupture would allow diagnosis at early stages before ACS becomes clinically apparent.23In recent years, novel risk markers have been developed in the cardiovascular field.60 IL‐6, CRP, monocyte subsets, and microRNAs, for example, are under intensive investigation. However, there are still controversies about confounding factors, methodological limitations, and statistics used for data analysis.60 In addition, several specific requirements are necessary to make tests reliable and useful in clinics, involving test sensitivity, specificity, potential to make a correct diagnosis, and “pretest probability” that match noninvasive findings with imaging or descriptions of clinical symptoms.63Stable CAD encompasses patients who have stable angina pectoris, patients with CAD‐related symptoms who become asymptomatic because of medical treatments, and those who reported symptoms for the first time but are in a chronic stable condition.64 A typical symptom of stable CAD is chest discomfort during exercise, emotions, or other stressors. The symptoms are caused by a mismatch between myocardial oxygen demand and supply.64 Basic assessments involve fasting lipid profile, screening for type 2 diabetes mellitus, estimation of renal function, resting ECG, and transthoracic echocardiography.64 Low concentrations of TnT have good prognostic value in patients with stable CAD but require ultrasensitive laboratory assays.64 No additional prognostic markers are recommended to manage patients with stable CAD.64Lubrano and colleagues analyzed 60 patients with CAD who underwent coronary catheterization. Their study showed elevated sLOX‐1, which tended to be increased with the severity of CAD, reflected by the number of affected vessels (control: 268 pg/mL, n=29; 1–2 diseased vessels: 611 pg/mL, n=30; 3–4 diseased vessels: 2143 pg/mL, n=30). Furthermore, sLOX‐1 positively correlated with TNF‐α, IL‐6, CRP, and age.34 A cross‐sectional study by Zhao et al measured ≈2.8‐fold higher sLOX‐1 levels in patients with ACS (1.610 ng/mL, n=58) compared with stable CAD (0.579 ng/mL, n=122).26 In ACS patients, sLOX‐1 positively correlated with the number of complex coronary lesions and was independently associated with the presence of multiple complex lesions (odds ratio: 1.967). It is known that the type of lesion predicts plaque vulnerability.24, 26 Interestingly, sLOX‐1 was 2‐fold higher in patients with complex lesions (n=50) compared with simple lesions (0.914 versus 0.426 pg/mL, n=72). In addition, sLOX‐1 was an independent predictor of complex lesions (odds ratio: 1.964), as demonstrated by multivariate analysis.26 Limitations were the exclusion of patients with type 2 diabetes mellitus and the rather small sample size.26Patients with atherosclerosis in the proximal or middle segment of the left anterior descending artery (LAD) represent a high‐risk subgroup in CAD. Balin et al showed 1.5‐fold elevated sLOX‐1 (1.07±0.33 ng/mL) in patients with proximal‐ or middle‐segment atherosclerosis compared with distal lesions (0.70±0.17 ng/mL).35 Interestingly, sLOX‐1 allowed identification of plaques in the proximal segment and thus may represent a predictor of plaque vulnerability in patients with stable CAD.35 Lesions in the proximal segment become clinically more apparent than those in distal segments. Although the numbers of patients were limited (n=51–64), this finding underscores the importance of sLOX‐1 as a potential predictor of the complexity of coronary lesions. Another study enrolled 94 patients with suspected CAD who underwent CAG and demonstrated positive correlations between sLOX‐1 and markers of oxidative stress (extracellular superoxide dismutase and 8‐isoprostane).65 This study suggests, that sLOX‐1 levels may reflect increased vascular oxidative stress. However, no correlations were observed among sLOX‐1, high‐sensitivity CRP (hs‐CRP), or brain natriuretic peptide.65ACS occurs by myocardial ischemia in patients with CAD. Unstable angina pectoris, non–ST‐segment–elevation MI (NSTEMI) and ST‐segment‐elevation MI (STEMI) are part of ACS.66 Although the clinical symptoms have the same underlying mechanism, a specific diagnosis is required to separate patients with the overall symptom of chest pain. MI is defined as myocardial cell death due to prolonged ischemia and diagnosed by ECG changes and monitored using the rise and/or fall of cardiac troponin levels.66 Cardiac troponins are the most commonly used biomarker and their high‐sensitivity assays are recommend for clinical use.66 Concentrations of TnT peak 4 hours after the onset of symptoms. Before this period, negative results may be obtained.39 In addition, TnT remains high for several days, making it difficult to detect recurrent MI. Because of the shorter half‐life, CK‐MB is often used for detection of reinfarction and periprocedural MI.67 An adequate diagnosis of MI requires a specific description of the patient's symptoms, detection of ischemic ECG changes, echocardiography for wall motion abnormalities, and identification of the thrombus by CAG.66 The most common treatments are revascularization by percutaneous coronary intervention (PCI) or coronary artery bypass grafting.68A large‐scale study analyzing the relevance of sLOX‐1 in CAD patients was conducted by Hayashida and colleagues in 2005.23 This study demonstrated elevated sLOX‐1 in patients with ACS (n=80) compared with those with intact coronary arteries (n=52), controlled coronary heart disease (n=122), and stable angina pectoris (n=173). Serum sLOX‐1 distinguished patients with ACS from those with non‐ACS (odds ratio: 1.51) and identified patients before the typical TnT increase. In a second study, sLOX‐1 and TnT were serially analyzed in 40 patients with ACS. Blood was taken on admission to the emergency room (ER), after PCI, and at days 1, 3, 5, and 7. Levels of sLOX‐1 peaked on admission to the ER and after PCI, whereas TnT peaked on day 1. Interestingly, sLOX‐1 did not correlate with TnT or CK‐MB. The authors concluded that sLOX‐1 is not a marker for cardiomyocyte injury and may again reflect plaque stability before myocardial damage.23 This study clearly emphasized the potential of sLOX‐1 as an early marker for detection of plaque vulnerability before elevations of TnT.69Significant differences in sLOX‐1 have been shown in ACS with (n=116) and without (n=44) ST‐segment elevation. The highest concentrations were found in those with STEMI (median: 259.0 pg/mL) compared with those with NSTEMI (median: 143.9 pg/mL) and without ACS (median: 104.1 pg/mL). Interestingly, hs‐TnT did not differ between STEMI and NSTEMI at this early stage of ACS,37 which supports the concept of sLOX‐1 as an early marker of ACS. Over time, sLOX‐1 declined after onset of pain and arrival at the ER, whereas hs‐TnT levels increased.37 The authors concluded that sLOX‐1 and hs‐TnT alone did not have the diagnostic accuracy to determine early stages of ACS, but a combination of both parameters may be useful.37 A study by Kume and colleagues showed a similar increase in sLOX‐1 in patients with ACS (n=18) versus without ACS (n=89).25 Compared with TnT or heart‐type FABP, sLOX‐1 had the highest sensitivity and specificity to diagnose ACS. It was detectable even without elevations of TnT. No correlations of sLOX‐1 with age, total cholesterol, triglycerides, and LDL cholesterol were found. In addition, sLOX‐1 did not correlate with TnT or heart‐type FABP.24, 25 A major limitation was the small number of patients, but this study highlights the role of LOX‐1 in diagnosis of early ACS without an TnT increase. In addition, these data suggest that sLOX‐1 is important in the late stages of CAD, where atherosclerotic plaques are prone to rupture, and not during the early stages of initiation and progression of atherosclerosis, for which risk factors play a central role.Further studies were conducted to evaluate the role of sLOX‐1 in early stages of AMI. Elevated sLOX‐1 levels have been demonstrated in patients with STEMI (241.0 pg/mL, n=125) compared with those with NSTEMI (147.3 pg/mL, n=44) and without AMI (64.3 pg/mL, n=125). In contrast to other studies of CAD,26 sLOX‐1 was not affected by the number of diseased vessels. The second part of this study analyzed time‐dependent changes in sLOX‐1. The highest concentrations were found between the onset of symptoms and arrival at the ER. TnT and CK‐MB increased 6 hours after arrival at the ER.39 At this early stage of STEMI, sLOX‐1 had high sensitivity for diagnosis of ACS.24, 39 The authors concluded that sLOX‐1 is not a marker of cardiomyocyte injury but rather reflects plaque instability. In summary, sLOX‐1 may help improve diagnosis in the early stage of ACS.39Additional studies were performed to analyze a putative link between sLOX‐1 and plaque vulnerability. A study in patients with ACS (n=128) showed elevated sLOX‐1 in case of ruptured compared with nonruptured plaques and stable angina pectoris (n=20). Plaque morphology was examined in coronary vessels by optical coherence tomography, and patients were categorized according to