Current Drug Targets - Cardiovascular & Hematological Disorders - Volume 5, Issue 6, 2005

The last decade has seen a number of important advances in the use of animal models of atherosclerosis progression. Small animal models, particularly mouse knockouts and rabbit models, are finding increasing use. This review discusses those models of particular research utility, highlights their advantages and limitations, and specifically addresses methodologies and current developments, in what is a rapidly changing field.

Besides classical risk factors such as hypercholesterolemia and hypertension, chronic subacute inflammation has recently been recognized as an important force driving the development of atherosclerosis, the most common underlying cause of myocardial infarction and stroke. There is compelling evidence that a disturbance of cholesterol homeostasis contributes to the development of a chronic inflammatory state and that inhibitors of HMG-CoA reductase (statins) may dampen inappropriate inflammatory responses. We review the evidence and suggest mechanisms by which dietary cholesterol can induce an atherogenic inflammatory response in liver and vessel wall, with particular emphasis on the time course of this inflammatory response during atherogenesis and the interplay between these tissues. We discuss how statins interfere in this process, and whether they may reduce chronic subacute inflammation via a) their cholesterollowering effect, and/or b) their cholesterol-independent (pleiotropic) vasculoprotective activities. Recent studies performed in (humanized) animal models allow us to distinguish the lipid-lowering??dependent from the lipidlowering ??independent functions of statins. Using these data, we discuss the degree to which the lipid-lowering??dependent and lipid-lowering??independent effects of statins contribute to a reduction of inflammation, allowing estimation of the relevance of pleiotropic statin effects for the human situation.

The reduction of circulating atherogenic lipoproteins through lifestyle modification and pharmacologic intervention is an important therapeutic goal in patients at risk for acute cardiovascular events. A large number of clinical trials have demonstrated that the reduction of low-density lipoprotein cholesterol (LDL-C) is associated with significant decreases in the incidence of all cause mortality, stroke, fatal and nonfatal myocardial infarction, and the need for revascularization with coronary artery bypass grafting and percutaneous transluminal coronary angioplasty. Therapy with 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase inhibitors (i.e., statins) are the agents of choice for treating a variety of dyslipidemias, particularly when LDL-C levels are elevated. The statins are highly efficacious; however, not all patients are able to tolerate the higher doses of these medications due to adverse side-effects such as hepatoxicity and myotoxicity. Moreover, many patients cannot achieve their various lipoprotein targets at even the highest doses of these medications. Ezetimibe is a novel cholesterol absorption inhibitor that blocks the translocation of dietary and biliary cholesterol from the gastrointestinal lumen into the intracellular space of jejunal enterocytes. Ezetimibe undergoes enterohepatic recirculation with minimal systemic exposure and not does not adversely impact the pharmacokinetic profile of statins. Ezetimibe significantly reduces serum LDL-C. It is safe when used as monotherapy or when used in combination with statins. Ezetimibe is indicated in the management of hyperlipidemia, familial hypercholesterolemia, and sitosterolemia and significantly increases the percentage of patients able to reach their lipidlowering goals.

Acyl-coenzyme A: cholesterol acyltransferase (ACAT) is an intracellular enzyme that catalyzes the formation of cholesterol esters from cholesterol and fatty acyl-coenzyme A. Animal experiments showed that ACAT inhibitors reduce plasma cholesterol levels by suppressing absorption of dietary cholesterol and by suppressing the assembly and secretion of apolipoprotein B-containing lipoproteins such as very low density lipoprotein in liver and chylomicron in intestine. Moreover, ACAT inhibitors were shown to prevent formation of macrophage-derived foam cells in the arterial walls. However, a recent double-blind, placebo-controlled, randomized trial of a potent ACAT inhibitor, avasimibe, failed to show significant beneficial effects on coronary atherosclerosis assessed by intravascular ultrasound. For clinical application of ACAT inhibitors, development of more potent compounds and improvements of the methods to evaluate their clinical efficacy are strongly needed.

Epidemiologic and experimental observations suggest high density lipoprotein (HDL) has a protective effect against atherothrombotic vascular disease. These findings have stimulated considerable interest to promote HDL as a potential therapeutic strategy. Several exciting therapeutic strategies have recently emerged and currently are the focus of intense research interest. One approach is the direct administration of HDL or its components such as apolipoprotein A-I (apoA-I). Recently, much attention has focused on a naturally occurring variant of apoA-I, apoA-IMilano (apoA-IM) characterized by a cysteine for arginine substitution that is associated with low rates of vascular disease and significant longevity in its carriers, despite markedly reduced HDL and elevated triglyceride levels. The mutation alters the characteristics of the protein resulting in apoA-IM being functionally more effective than normal apoA-I. A number of animal and laboratory studies have demonstrated significant antiatherogenic, antiproliferative, antirestenotic, antiplatelet, antithrombotic, antiinflammatory, and antioxidant properties of apoA-IM. Furthermore, apoA-IM has been shown to promote reverse cholesterol transport, improve endothelial dysfunction and induce rapid mobilization of tissue cholesterol resulting in regression and alteration of plaque composition in animal models of atherosclerosis. Recently, a pilot clinical trial of recombinant apoA-IM demonstrated significant and rapid regression of atherosclerosis as measured by intravascular ultrasound in patients with acute coronary syndromes. These promising data provide the rationale for the development of reconstituted HDL utilizing recombinant apoA-IM as a potential therapeutic approach for atherothrombotic vascular disease in humans.

Over the past decades, lowering of LDL-cholesterol (LDL-c) levels has been established as the foundation for preventing atherosclerotic disease. It is, however, widely accepted that additional risk reduction has to come from modifying other risk factors than LDL-c. In this context, increasing HDL-cholesterol (HDL-c) levels by pharmacological inhibition of the cholesteryl ester transfer protein (CETP) is currently under intense investigation. Two small-molecule compounds, JTT-705 and Torcetrapib, have been shown to effectively increase HDL-c levels in humans, without inducing clinically significant side effects when used as monotherapy or combined with statins. Whether this approach will translate into a reduction in risk of atherosclerotic disease has not yet been established. Data from studies focusing on genetic CETP deficiency as well as those studying the relationship between CETP plasma levels and risk of atherosclerosis do not provide clear answers. Several long-term clinical studies addressing this crucial issue have recently been initiated, results of which will follow within the next few years. This review focuses on CETP, its role in human lipid metabolism and its relation to atherosclerotic disease. Furthermore, it summarizes the currently available data regarding pharmacological CETP inhibition. Finally, it will highlight a number of issues basic to the considerations of whether CETP inhibition will fulfill its promises.

Calcium channel blockers and HMG-CoA reductase inhibitors are widely used for the management of hypertension and dyslipidemia, respectively. The use of these agents in the prevention and treatment of cardiovascular disease remains largely based on their actions in lowering blood pressure and lipids. Recent clinical trials, however, indicate that certain members of these two drug classes may slow progression of disease to an extent that cannot be solely attributed to risk factor reduction. The proposed mechanisms for such pleiotropic actions include enhancement of endothelial-dependent nitric oxide bioavailability, anti-inflammatory activity, and inhibition of oxidative stress. To understand the basis for such effects, along with potential synergies, we will review the basic and clinical evidence that indicate a broader opportunity for treatment and protection of cardiovascular events by atheroprotection with these agents beyond risk factor management.

The incidence of diabetes is increasing at an alarming rate to the point where it is becoming an epidemic. An ageing population, sedentary lifestyle and an unhealthy diet are considered to have contributed toward this. What we must now consider is not only the burden of the disease but the complications that arise from diabetes, in particular kidney and heart disease. Foremost, more than half of the diabetic population will die from cardiovascular-related causes. Whilst diabetes is most often associated with hypertension, dyslipidaemia and obesity, these factors do not fully account for the increased burden of cardiovascular disease in people with diabetes. This strengthens the need for comprehensive studies investigating the underlying mechanisms mediating diabetic cardiovascular disease, and more specifically, diabetesassociated atherosclerosis. In addition to the recognised metabolic abnormalities associated with diabetes, upregulation of putative pathological pathways such as advanced glycation endproducts, renin-angiotensin system, oxidative stress and increased expression of growth factors and cytokines have been observed in the setting of diabetes. All of these have been shown to play a causal role in atherosclerotic plaque formation and thus may explain the increased risk of macrovascular complications in those patients with diabetes. In this review the effect of inhibiting the renin-angiotensin system with angiotensin converting enzyme inhibition and a comparison to angiotensin II receptor antagonism is discussed, with the results of clinical trails reflecting the more recently discovered, non-haemodynamic, proatherogenic actions of angiotensin II. The need for experimental models of diabetes-associated atherosclerosis will be covered, with particular emphasis given to the streptozotocin-diabetic apolipoprotein E knockout mouse. Finally, growth factors, including vascular endothelial growth factor and platelet-derived growth factor are discussed in detail.

The peroxisome proliferator-activated receptors (PPARs) α, β/δ and γ are ligand-activated transcription factors belonging to the nuclear receptor superfamily. PPARs heterodimerize with the retinoid X receptor (RXR) and modulate the function of many target genes. They were originally described as regulators of various metabolic, pathways, but have been recently found to also exert modulating actions in the vascular wall. PPARα is activated by endogenous ligands, such as polyunsaturated fatty acids and by synthetic agonists such as the fibrates. PPARα is expressed mainly in the liver, kidney and skeletal muscle and is involved in fatty acid oxidation. However, it is also expressed in vascular cells such as the endothelial cells, vascular smooth muscle cells and macrophages, where it exerts anti-inflammatory and antioxidant effects. Since atherosclerosis is both a chronic inflammatory and a lipid disorder and since PPARα is expressed in vascular cells and regulates the expression of genes involved in lipid metabolism and inflammation, PPARα activators may constitute useful agents for the prevention of atherosclerosis, beyond their effects on lipid metabolism. This review will focus on the functions of PPARα on lipid metabolism, on vascular inflammation and its relationship to atherosclerosis. Furthermore, the currently available preclinical and clinical data on PPARα activators as well as their future perspectives will be discussed

Lipoprotein-associated phospholipase A2 (Lp-PLA2) is an enzyme that is produced by inflammatory cells, is bound to circulating LDL, and is involved in hydrolyzing polar phospholipids, including those found in oxidized lowdensity lipoproteins. To date, the biological role of Lp-PLA2 in atherogenesis has been controversial, with initial reports purporting an atheroprotective effect attributable to the degradation of platelet activating factor and similar molecules. However, more recent studies suggest a proatherogenic role for this enzyme, which is attributed to Lp-PLA2-mediated hydrolysis of oxidatively modified low-density lipoproteins that results in the accumulation of proinflammatory products. The liberation of lysophosphatidylcholine and oxidized nonesterified fatty acids from oxidized phospholipids by the action of Lp-PLA2 results in diverse inflammatory effects on various cell types involved in atherogenesis. This concept is further supported by a number of recently published epidemiology studies suggesting that plasma levels of the enzyme predict future cardiovascular events independent of conventional risk factors. The development of selective inhibitors of Lp-PLA2 that inhibit enzyme activity in the circulation as well as within human atherosclerotic lesions opens the possibility of therapeutic manipulation of vascular inflammatory processes to reduce residual cardiovascular events in high risk individuals who continue to suffer fatal and nonfatal events despite the current standard of care.

The Liver X Receptors, LXRα and LXRβ are members of the nuclear hormone receptor superfamily which have recently been implicated as novel pharmacological targets for the treatment of cardiovascular diseases. The identification of natural and synthetic ligands for LXRs and the generation of LXR-deficient mice have been crucial for our understanding of the function of these receptors and for the identification of LXR-regulated target genes, particularly with respect to the role of LXRs in regulating cholesterol homeostasis. Synthetic LXRα/β agonists induce cholesterol efflux and reverse cholesterol transport, improve glucose metabolism, inhibit macrophage-derived inflammation, and suppress the proliferation of vascular smooth muscle cells. By regulating the expression of multiple genes involved in these pathways, LXR agonists prevent the development and progression of atherosclerosis and inhibit neointima formation following angioplasty of the arterial wall. In this review, we will summarize the important roles of LXR in metabolism and vascular biology and discuss its implications as potential molecular drug target for the treatment of cardiovascular diseases.

Mature human atherosclerotic plaques are frequently characterized by a lipid-rich core covered by a fibrous cap composed of fibrillar collagens, elastin, proteoglycans and smooth muscle cells (SMC). Most sudden deaths due to acute myocardial infarction are caused by rupture of coronary atheroma, leading to a prothrombotic response followed by rapid occlusion of the artery. The accumulation of macrophage-derived foam cells in vulnerable shoulder regions of atherosclerotic plaques correlates with increased local release of matrix-degrading metalloproteinases (MMPs) and weak fibrous cap tissue. These findings suggest a potential role of macrophage-derived MMPs in the weakening and ultimate rupture of plaque structures. Consequently, several studies have focussed on the hypothesis that inhibiting MMP activity would reduce plaque volume and prevent plaque rupture and therefore would be useful in the treatment of atherosclerosis. However, current synthetic MMP inhibitors are not very specific and clinical results have so far been inconclusive. The development of selective inhibitors and focal gene transfer approaches may be better suited for the treatment of atherosclerosis.

Overweight and obesity are recognised as significant risk factors for coronary heart disease (CHD). Weight reduction leads to reduction in associated CHD risk factors. The discovery that endocannabinoid system is involved in regulation of food intake and other reward behaviours has led to development of cannabinoid receptor antagonists. Recent studies with rimonabant, a cannabinoid type 1 receptor (CB1) antagonist, demonstrate clinically significant weight loss as well as reduction in metabolic syndromme burden in obese patients.

Despite significant protection from cardiovascular events by HMG-CoA reductase inhibitors (statins), the extent of event reduction is incomplete and persistent lipid abnormalities remain prevalent. As such, the use of combination lipid modification regimens is a clinical reality. While we await large, randomized clinical outcomes trials assessing the benefits of combination or novel lipid management therapies, non-invasive imaging tests such as B-mode ultrasound to measure carotid intima-media thickness (CIMT), coronary computed tomography (CT) and cardiovascular magnetic resonance imaging (CMR) are increasingly being utilized to accurately assess the impact of lipid modifying therapies on atherosclerosis. Based on knowledge that atherosclerosis progression is a validated surrogate for increased cardiovascular risk, use of atherosclerosis imaging surrogates in studies allows for smaller sample sizes, shortens study duration, demonstrates clinically meaningful changes prior to clinical event and provides potentially useful pathophysiologic information. CIMT measurement is currently the most studied and validated non-invasive imaging study used to assess atherosclerosis longitudinally in response to lipid modification therapies. Quantification of coronary calcium using coronary CT has been utilized to study atherosclerosis longitudinally, however, calcium scoring to assess response to lipid modifying therapies is not recommended due to several limitations of this assessment of the atherosclerosis process. These include the fact that non-calcified atherosclerosis is not quantified, that the calcification process in atherosclerosis is diverse and that the effects of statins on tissue calcification are complex. Use of contrast coronary CT to image non-calcified coronary atherosclerosis is promising. Modern CMR imaging, aided by high reproducibility and image resolution, is rapidly advancing and early studies imaging non-coronary and coronary atherosclerosis are impressive.