Current Molecular Medicine - Volume 6, Issue 5, 2006

Atherothrombosis continues to be the most important cause of death and disability in western countries, and for 2020 will also be the main cause in developing countries. Recent paramount research performed within the last few years highlight the importance of new pathophysiological, clinical, and therapeutic approaches to atherothrombosis, creating new pathways for diagnosis, treatment and prevention. This supplement organized in three sections is devoted to novel concepts covering the disease, from early atherogenesis to plaque rupture and thrombosis. The first section entitled "Basic and Experimental" is composed by six articles addressing the latest understanding of the cellular and molecular mechanisms of the disease. In the first review, entitled "Cell biology and lipoproteins in atherosclerosis", Dr. Badimon et al., review the complex interaction among inflammatory cells, vascular elements, and lipoproteins leading to early atherogenesis. Specific attention is paid to the role of the endothelial cell as the central player for cell adhesion, subendothelial retention and modification of LDL, intracellular lipid accumulation, and extracellular matrix formation in the vascular wall. In the second review entitled "Neovascularization in human atherosclerosis", Dr. Moreno et al., review the role of vasa vasorum in health and disease, covering the cellular and molecular aspects of atherothrombosis neovascularization. The role of neovessels from early atherogenesis to plaque rupture and thrombosis is covered. Furthermore, the role of neovessels in restenosis and plaque regression is also addressed, providing the reader extensive knowledge in this evolving and promising field. In the third article entitled "Intraplaque Hemorrhage" Dr. Levy et al., review the pathophysiological consequences of red blood cell extravasation within atherosclerotic lesions, focusing on the potent oxidative and pro-inflammatory effects of Hemoglobin, leading to macrophage activation and plaque destabilization. The main defense mechanism against hemoglobin, the protein Haptoglobin is also reviewed, with special attention to the diabetic population. In the fourth article entitled "Link between inflammation and thrombogenicity in atherosclerosis" Dr. Viles et al., review the concept of vulnerable plaque, vulnerable blood, and vulnerable patient. Of note, the role of tissue factor, platelets, apoptosis, and other pro-thrombotic mediators is elegantly addressed, summarizing the interactions between inflammation and coagulation within the setting of atherothrombotic disease. In the fourth article of this section entitled "Metabolic syndrome, diabetes, and peroxisomal proliferator activated receptors" Dr. Meerarani., et al review the complex interaction between insulin resistance, endothelial dysfunction, and pro-inflammatory gene regulation leading to an increased atherothrombotic state. The pivotal role of HDL and the emerging role of peroxisomal proliferator activator receptors in disease stabilization are also addressed, providing the reader with a comprehensive approach to atherothrombosis in patients with metabolic syndrome. Finally, in the last article of this section entitled "Molecular determinants of vascular calcification: a bench to bedside review" Dr. Dellegrottaglie et al., review the mechanisms triggering osteogenesis of the vessel wall associated with the loss of inhibitory factors responsible for enhanced bone turnover. Of note, the interactions between the receptor activator nuclear factor kB, the multiple pro-inflammatory cytokines and the non-collagenous proteins including osteopontin and osteoprotegrin, are reviewed. The second section entitled "Clinical imaging and biomarkers of atherosclerosis" is composed by four articles addressing the state-of-the-art imaging approach for non-invasive diagnosis and risk-stratification of patients with increased likelihood of developing the disease. In the first article entitled "Calcium scoring and contrastenhanced CT angiography" Dr. Sanz et al., review the clinical application of detecting coronary calcium, and the recent advances in non-invasive, CT coronary angiography. In the second article entitled "Plaque imaging and characterization using magnetic resonance imaging: toward molecular assessment" Sirol et al., review the use of high-resolution magnetic resonance for plaque detection and characterization and the potentials of "molecular Imaging". A variety of molecules present in atherosclerotic plaques serving as targets for specific contrast agents allow for the identification of high-risk atherosclerotic lesions. In the third article entitled "atherosclerosis neovascularization and imaging" Dr. Purushothaman et al., specifically review the role of multiple techniques to visualize neovessels in the vessel wall, with special emphasis in molecular imaging.........

Atherosclerosis is an inflammatory process, triggered by the presence of lipids in the vascular wall, and encompasses a complex interaction among inflammatory cells, vascular elements, and lipoproteins through expression of several adhesion molecules and cytokines. Subendothelial retention of lipoproteins is the key initiating event in atherosclerosis, provoking a cascade of events to pathogenic response. High levels of plasma lipids, particularly low-density (LDL) and very-lowdensity lipoproteins (VLDL) are among the pathophysiologic stimuli that induce endothelial dysfunction. Endothelial cells regulate coagulation, thrombosis and the fibrinolytic system; the endothelium modulates the activity of smooth muscle cells (vascular tone/proliferation) and controls the traffic of macromolecules and inflammatory cells to the vessel wall. Furthermore, LDLs have been implicated in the induction of changes in permeability, cell adhesion and secretion of vasoactive molecules (nitric oxide [NO]), while VLDLs seem to modulate the fibrinolytic system [tissue plasminogen activator (TPA) and plasminogen activator inhibitor-1 (PAI-1)]. In this review, we will focus on the pathophysiologic functions of lipoproteins in the vascular wall.

In the absence of disease, microvessels provide vessel wall nutrients to the tunica media, while the intima is fed by oxygen diffusion from the lumen. As disease evolves and the tunica intima thickens, oxygen diffusion is impaired, and microvessels become the major source for nutrients to the vessel wall. Microvessels serve as a port of entry for inflammatory cells, from the systemic circulation to the nascent atherosclerotic lesion. As disease progress, microvessels also play a role in intraplaque hemorrhage, lipid core expansion, and plaque rupture. In addition, microvessels are also involved in stent restenosis, and plaque regression. Therefore, microvessels are a pivotal component of atherosclerosis, and proper patient risk-stratification in the near future may include the detection of increased neovascularization in atherosclerotic lesions. This review divided in two parts summarizes the current understanding of atherosclerosis neovascularization, starting with the normal anatomy and physiology and progressing to more advanced stages of the disease. We will review the structure and function of vasa vasorum in health and disease, the mechanisms responsible for the angiogenic process, the role of the immune system, including inflammation and Toll-like receptors, and the pathology of microvessels in early atherosclerotic plaques. Furthermore, the review addresses the advanced stages of atherosclerosis, summarizing the progressive role for microvessels during disease progression, red blood cell extravasation, lipid core expansion, plaque rupture, healing, repair, restenosis, and disease regression, offering the clinician a state-of-the-art, "bench to bedside" approach to neovascularization in human atherosclerosis.

Intraplaque hemorrhage is a common feature of atherosclerotic plaques and is considered one of the identifying features of complex lesions preceding acute ischemic events. The cause of intraplaque hemorrhage is most often secondary to rupture of neovessels, which have invaded the plaque. However, inflammation and metabolic factors such as diabetes may also precipitate hemorrhage from mature microvessels by damaging the endothelium. The mechanism by which hemorrhage destabilizes the plaque is in large part secondary to the action of hemoglobin released from red blood cells at the site of the hemorrhage. Hemoglobin is a potent pro-inflammatory agent by virtue of its ability to promote formation of ROS. The major defense mechanism against the toxic effects of extracorpuscular hemoglobin is the protein haptoglobin, which tightly binds to hemoglobin and prevents it from catalyzing oxidative reactions. There exists a common allelic polymorphism in the haptoglobin gene, which has recently been strongly associated with the risk of cardiovascular disease in multiple independent cohorts. The protein products of the two different haptoglobin alleles differ in their ability to serve as an antioxidant against hemoglobin and also to activate the CD163 receptor. This review presents a unifying hypothesis whereby the haptoglobin genotype is proposed to modulate the response to intraplaque hemorrhage and thereby play a fundamental role in determining the morphological and metabolic features of complex plaques preceding acute ischemic events.

Plaque disruption and subsequent thrombus formation play a critical role in the clinical manifestations of atherothrombosis. Vulnerable lesions are characterized by the existence of core rich in lipid, macrophages and tissue factor (TF). Plaque disruption facilitates the interaction between flowing blood with the inner components (TF) of disrupted atherosclerotic lesions triggering the coagulation cascade. TF, thrombin, platelets, fibrin and inflammatory cells are involved in this process of acute thrombus formation. This pathologic process is significantly accelerated by several "cardiovascular risk factors" such as diabetes, smoking, dyslipemia, etc. We will review on the role of TF, plaque cell apoptosis and blood thrombogenicity acting as a thread of inflammatory and prothrombotic mediators. We will also review the role of activated platelets as source for pro-inflammatory cytokines and enunciation of thrombotic process. Overall, we will try to emphasize the most recent understanding of the concepts involved in the interaction between inflammation and coagulation within the setting of atherothrombotic disease.

Metabolic syndrome is characterized by the clustering of a number of metabolic abnormalities in the presence of underlying insulin resistance with a strong association with diabetes and cardiovascular disease morbidity and mortality. The disorder is defined in different ways, but the pathophysiology is attributable to insulin resistance. An increased release of free fatty acids (FFAs) from adipocytes block insulin signal transduction pathway, induce endothelial dysfunction due to increased reactive oxygen species (ROS) generation and oxidative stress. Dyslipidemia, associated with high levels of triglycerides and low concentrations of high density lipoproteins (HDLs), contributes to a proinflammatory state. Inflammation, the key pathogenic component of atherosclerosis, promotes thrombosis, a process that underlies acute coronary event and stroke. Tissue factor, a potent trigger of the coagulation cascade, is increased in diabetes with poor glycemic control. Therapeutic lifestyle changes (weight loss and physical activity) along with pharmacological interventions are recommended to prevent the complications of metabolic syndrome. In addition to statins, metformin, blood pressure lowering medications, interventions to increase HDLs are other important approaches to decrease the risk of cardiovascular disease. Furthermore, the peroxisome proliferator activated receptor (PPAR) - alpha and gamma agonists are potent anti-inflammatory and anti-atherogenic agents that could both improve insulin sensitivity and the long-term cardiovascular risk. In this review we focus on the molecular and pathophysiological basis of metabolic syndrome, which augments diabetes (insulin resistance) and the contribution of neovascularization in the plaque progression in diabetes, leading to rupture and coronary thrombosis.

Vascular calcification (VC) is an orchestrated event, evoking the programmed process of the osteogenesis and triggered by inflammatory cytokines active at vascular level. VC is a dynamic process in which the vessel wall intima, media and also cardiac valves may be involved. Intimal calcification is an endochondral ossification process in which type II collagen is mineralized by calcium deposition. In contrast, an intra-membranous ossification process leads to medial calcification, while a dystrophic calcification process is responsible for valvular calcification. Mechanisms involved in VC may be summarized as: 1. Activation of osteogenesis in the vessel wall, 2. Loss of inhibitory factors, 3. Enhanced bone turnover, and 4. Abnormalities in mineral metabolism. The signaling axis constituted by osteoprotegerin (OPG), receptor activator nuclear factor kB (RANK) and its ligand (RANKL), along with the monocyte colony stimulating factor (M-CSF) and the transcription factor core Binding protein (Cbfa-1), play a pivotal role in the control of VC. In contrast, fetuin-A, matrix G1a protein (MGP) and osteopontin (OPN) control the inhibition of VC. In addition, abnormal mineral metabolism with enhanced phosphates availability favors calcium deposition. The inflammatory cytokines interleukin (IL-1) and tumor necrosis factor (TNF)-α enhance OPG and RANKL function in the vessel wall leading to VC. VC is a controlled process, depending on the balance between osteoblastic and osteoclastic influences and further modulated by the influence of risk factors like diabetes, smoking, age, hypertension and dyslipidemia. Recent advances in diagnostic tools such as with multi-detector computed tomography (MDCT) and electron beam computed tomography (EBCT), may help diagnosis and delineation of VC in the clinical setting and aid in understanding its prognostic value.

Computed tomography (CT) technology has emerged as the most promising imaging modality for the noninvasive evaluation of the coronary circulation. Of the CT-based approaches, multidetectorrow computed tomography (MDCT) and to a lesser extent electron beam computed tomography offer the potential of providing not only data on the spatial extent and burden of coronary calcium content, but also angiographic data, and plaque composition characteristics with the potential for prediction of susceptibility to future cardiovascular events. A number of studies have now confirmed that CT-based assessment of the presence and amount of coronary artery calcium provides incremental prognostic information over traditional risk factors in patients with coronary artery disease and can be employed to refine risk stratification in both asymptomatic and symptomatic subjects. With the advent of several recent advances in CT imaging, it is now possible to provide high resolution (sub-millimeter, isotropic voxels) images of the coronary arteries obtained rapidly with iodinated contrast injected peripherally. MDCT is currently the preferred modality for noninvasive contrast angiography of the coronary arteries by most groups, with a new generation of 64-slice scanners promising to further improve the results of this technique. MDCT-derived angiographic information in conjunction with coronary calcium scoring and plaque characterization has the potential of replacing invasive angiography, as it potentially could provide better global assessment of risk.

Identification of high-risk atherosclerotic lesions prone to rupture and thrombosis may greatly decrease the morbidity and mortality associated with atherosclerosis. High-resolution magnetic resonance imaging (MRI) has recently emerged as one of the most promising techniques for the noninvasive study of atherothrombotic disease, as it can characterize plaque composition and monitor its progression. The development of MRI contrast agents that specifically target components of the atherosclerotic plaque may enable non-invasive detection of high-risk lesions. This review discusses the use of high-resolution MRI for plaque detection and characterization and the potentials of "Molecular Imaging" using a variety of molecules present in atherosclerotic plaques that may serve as targets for specific contrast agents to allow the identification of high-risk atherosclerotic lesions invivo. Ultimately, such agents may enable treatment of "high-risk" patients prior to lesion progression and occurrence of complications.

Neovascularization in atherosclerotic plaques is particularly prominent in complicated lesions, and has been recently identified as a marker of plaque vulnerability. This observation has led to a growing interest in the development of imaging techniques with the ability to visualize and quantify the extent of plaque neovascularization. Such feature may play an important role in identifying those lesions more prone to destabilization and rupture, and in the guidance and monitoring of therapeutic interventions. Several modalities have emerged as potential candidates for imaging neovessels in atherosclerotic lesions. They include magnetic resonance imaging, x-ray computed tomography, positron emission tomography, single photon emission computed tomography, ultrasound, or nearinfrared optical imaging. These techniques differ in their achievable spatial and temporal resolution, availability, cost, reproducibility, degree of intrusiveness, capability to image atherosclerotic plaques in various vascular territories and ability to discern different plaque components, specifically the presence of neovessels. Molecular imaging, a rapidly evolving multidisciplinary field devoted to the visualization of specific physiopathologic processes at the cellular or molecular level, appears particularly well suited for this purpose because of its ability to target and visualize individual molecules specific to neoangiogenesis. In this manuscript we will review current evidence on the potential application of the various modalities, with a particular emphasis in molecular imaging.

Inflammation has been implicated in all stages of cardiovascular disease. This has driven a very fruitful search for new biomarkers, which potentially can be used as tools in the diagnosis and prognosis of atherothrombotic disease. While these new markers might prove useful in predicting the onset of atherosclerosis in healthy individuals, the utility of biomarkers in risk assessment for events in those patients with established disease and/or those with acute coronary syndrome requires further work. Effective biomarkers must be standardized, logistically simple to analyze, and clinically useful. Understanding what impact sex, age, ethnicity, and comorbid conditions may have on biomarkers is also of importance. Unfortunately, many of the candidate markers have yet to satisfy these requirements.

Despite significant progress in the management of atherosclerosis and its resultant complications, cardiovascular disease remains the principal cause of death in the world. The National Cholesterol Education Project Adult Treatment Panel III (NCEP ATP III) recognizes low levels of highdensity lipoprotein cholesterol (HDL) as a risk factor for coronary heart disease (CHD) and high levels of HDL as a risk-reducing factor; however, the elevation of HDL as a specific therapeutic target for the prevention and treatment of CHD has yet to be accepted on the same level as low-density lipoprotein (LDL)-reducing therapies. Current HDL elevators including nicotinic acid, fibric acid derivatives, peroxisome proliferator activated receptor (PPAR) agonists and statins also affect other lipid constituents which make interpretation of the clinical trials of these drugs difficult in teasing out the independent effect of HDL elevation. Ample laboratory investigation suggests that HDL elevation would reduce atherosclerotic burden through multiple independent mechanisms. In this review, we explore HDL biology, its potential mechanisms in the treatment of atherosclerotic disease, and promising new drugs with HDL-raising activity.

Atherosclerosis is a slow, complex process involving many different cell types and their interactions producing excessive oxidant stress, increased inflammation, abnormal endotheliumdependent vasodilation, and localized increases in thrombogenic and decreases in fibrinolytic factors. Numerous factors incite these processes, but oxidized LDL particles seem the most essential component of this pathologic cascade. These oxidized particles set up a chain of biochemical events eventually leading to clinical atherosclerotic events. Statins have been shown to reduce such events by 40 to 50% when LDL cholesterol is lowered to less than 80 mg/dL. But in-vitro studies have shown that the provision of mevalonate, not cholesterol, can result in reversal of some of the beneficial effects of statins, suggesting that other mechanisms such as inhibiting the formation of the isoprenoids, farnesyl pyrophosphate and geranylgeranyl pyrophosphate, and thus prenylation of many cell-signaling proteins may be important in the preventive effects of statins. Clinically, these more rapid, non-lipid-altering effects may be more apparent in acute coronary syndromes. Now that we have more LDL-cholesterol lowering agents which lower LDL-cholesterol without blocking HMG CoA reductase, we may be better able to dissect and understand the importance of these non-lipid-altering effects of statins in human disease.

Coronary heart disease is the leading cause of death in the US and the industrialized world. Ever since DeWood in 1980 demonstrated that a thrombus was the primary event leading to acute myocardial infarction and that they may subsequently lyse, the mainstay of therapy for the past 25 years has been antithrombotic therapy aimed at coronary thrombosis. There have been numerous advances in the treatment of acute coronary syndrome with antiplatelet, antithrombotic, and fibrinolytic agents that have significantly reduced morbidity and mortality. The role of various pharmaceutical agents in the different phases of acute coronary syndrome is a complex and ever changing field.