Peroxisome proliferator-activated receptor alpha,gamma coagonist LY465608 inhibits macrophage activation and atherosclerosis in apolipoprotein E knockout mice

Role of Peroxisome Proliferator-Activated Receptor γ in Ocular Diseases

Peroxisome proliferator-activated receptor γ (PPAR γ), a member of the nuclear receptor superfamily, is a ligand-activated transcription factor that plays an important role in the control of a variety of physiological processes. The last decade has witnessed an increasing interest for the role played by the agonists of PPAR γ in antiangiogenesis, antifibrosis, anti-inflammation effects and in controlling oxidative stress response in various organs. As the pathologic mechanisms of major blinding diseases, such as age-related macular degeneration (AMD), diabetic retinopathy (DR), keratitis, and optic neuropathy, often involve neoangiogenesis and inflammation- and oxidative stress-mediated cell death, evidences are accumulating on the potential benefits of PPAR γ to improve or prevent these vision threatening eye diseases. In this paper we describe what is known about the role of PPAR γ in the ocular pathophysiological processes and PPAR γ agonists as novel adjuvants in the treatment of eye diseases.

The peroxisome proliferator-activated receptors in cardiovascular diseases: experimental benefits and clinical challenges

The peroxisome proliferator-activated receptors, PPARα, PPARβ/δ and PPARγ, are ligand-activated transcriptional factors belonging to the nuclear receptors superfamily and they are known to play important roles in glucose and lipid metabolism. Experimental studies in animal models of metabolic diseases have also revealed that activation of PPARs protects against the vascular complications of diabetes, hypertension, atherosclerosis, myocardial infarction and stroke, through exerting their anti-inflammatory, anti-atherogenic and antioxidant effects. In clinical trials and post-market surveillance, agonists of PPARs have been shown to effectively prevent cardiovascular events. However, adverse effects, particularly for PPARγ agonists, are also observed with the use of investigational PPAR agonists and even some approved drugs. Further exploration of underlying mechanisms is needed to develop novel ways of PPAR activation without causing serious side effects. This article reviews the cardiovascular effects of PPARs, with emphasis on the therapeutic potential of PPAR agonists in combating metabolic vascular diseases.

Nuclear receptor mediated mechanisms of macrophage cholesterol metabolism

Macrophages comprise a family of multi-faceted phagocytic effector cells that differentiate "in situ" from circulating monocytes to exert various functions including clearance of foreign pathogens as well as debris derived from host cells. Macrophages also possess the ability to engulf and metabolize lipids and this way connect lipid metabolism and inflammation. The molecular link between these processes is provided by certain members of the nuclear receptor family. For instance, peroxisome proliferator activated receptors (PPAR) and liver X receptors (LXR) are able to sense the dynamically changing lipid environment and translate it to gene expression changes in order to modulate the cellular phenotype. Atherosclerosis embodies both sides of this coin: it is a disease in which macrophages with altered cholesterol metabolism keep the arteries in a chronically inflamed state. A large body of publications has accumulated during the past few decades describing the role of nuclear receptors in the regulation of macrophage cholesterol homeostasis, their contribution to the formation of atherosclerotic plaques and their crosstalk with inflammatory pathways. This review will summarize the most recent findings from this field narrowly focusing on the contribution of various nuclear receptors to macrophage cholesterol metabolism.

Conjugated linoleic acid and inflammatory cell signalling

Conjugated linoleic acids (CLA) are a family of polyunsaturated fatty acids (PUFA), some isomers occurring naturally in beef and dairy products and others being formed as a result of bihydrogenation of vegetable oils to form margarine. Synthetic and natural sources of CLA may have beneficial effects in a range of inflammatory conditions including colitis, atherosclerosis, metabolic syndrome and rheumatoid arthritis. Most of the biological effects have been attributed to the cis9, trans11- (c9, t11-) and the trans10, cis12- (t10, c12-) isomers. Evidence suggests that c9, t11-CLA is responsible for the anti-inflammatory effect attributed to CLA while t10, t12-CLA appears to be responsible for anti-adipogenic effects. This review will focus on the effects of CLA on the inflammatory components associated with insulin resistance, atherosclerosis and Th1 mediated inflammatory disease, at a cellular, systemic and clinical level. Whist CLA may ameliorate certain aspects of the inflammatory response, particularly within cellular and animal models, the relevance of this has yet to be clarified within the context of human health.

Chemical analysis of atherosclerotic plaque cholesterol combined with histology of the same tissue

Sensitive method for chemical analysis of free cholesterol (FC) and cholesterol esters (CE) was developed. Mouse arteries were dissected and placed in chloroform-methanol without tissue grinding. Extracts underwent hydrolysis of cholesteryl esters and derivatization of cholesterol followed by liquid chromatography/mass spectrometry (LC/MS/MS) analysis. We demonstrated that FC and CE could be quantitatively extracted without tissue grinding and that lipid extraction simultaneously worked for tissue fixation. Delipidated tissues can be embedded in paraffin, sectioned, and stained. Microscopic images obtained from delipidated arteries have not revealed any structural alterations. Delipidation was associated with excellent antigen preservation compatible with traditional immunohistochemical procedures. In ApoE(-/-) mice, LC/MS/MS revealed early antiatherosclerotic effects of dual PPARalpha,gamma agonist LY465606 in brachiocephalic arteries of mice treated for 4 weeks and in ligated carotid arteries of animals treated for 2 weeks. Reduction in CE and FC accumulation in atherosclerotic lesions was associated with the reduction of lesion size. Thus, a combination of LC/MS/MS measurements of CE and FC followed by histology and immunohistochemistry of the same tissue provides novel methodology for sensitive and comprehensive analysis of experimental atherosclerotic lesions.

PPAR Agonists and Cardiovascular Disease in Diabetes

Peroxisome proliferators activated receptors (PPARs) are ligand-activated nuclear transcription factors that play important roles in lipid and glucose homeostasis. To the extent that PPAR agonists improve diabetic dyslipidaemia and insulin resistance, these agents have been considered to reduce cardiovascular risk. However, data from murine models suggests that PPAR agonists also have independent anti-atherosclerotic actions, including the suppression of vascular inflammation, oxidative stress, and activation of the renin angiotensin system. Many of these potentially anti-atherosclerotic effects are thought to be mediated by transrepression of nuclear factor-kB, STAT, and activator protein-1 dependent pathways. In recent clinical trials, PPARalpha agonists have been shown to be effective in the primary prevention of cardiovascular events, while their cardiovascular benefit in patients with established cardiovascular disease remains equivocal. However, the use of PPARgamma agonists, and more recently dual PPARalpha/gamma coagonists, has been associated with an excess in cardiovascular events, possibly reflecting unrecognised fluid retention with potent agonists of the PPARgamma receptor. Newer pan agonists, which retain their anti-atherosclerotic activity without weight gain, may provide one solution to this problem. However, the complex biologic effects of the PPARs may mean that only vascular targeted agents or pure transrepressors will realise the goal of preventing atherosclerotic vascular disease.

Mouse models for atherosclerosis and pharmaceutical modifiers

Atherosclerosis is a multifactorial highly-complex disease with numerous etiologies that work synergistically to promote lesion development. The ability to develop preventive and ameliorative treatments will depend on animal models that mimic the human subject metabolically and pathophysiologically and will develop lesions comparable to those in humans. The mouse is the most useful, economic, and valid model for studying atherosclerosis and exploring effective therapeutic approaches. Among the most widely used mouse models for atherosclerosis are apolipoprotein E-deficient (ApoE-/-) and LDL receptor-deficient (LDLr-/-) mice. An up-and-coming model is the ApoE*3Leiden (E3L) transgenic mouse. Here, we review studies that have explored how and to what extent these mice respond to compounds directed at treatment of the risk factors hypercholesterolemia, hypertriglyceridemia, hypertension, and inflammation. An important outcome of this survey is that the different models used may differ markedly from one another in their response to a specific experimental manipulation. The choice of a model is therefore of critical importance and should take into account the risk factor to be studied and the working spectrum of the compounds tested.

Underlying mechanisms of pharmacology and toxicity of a novel PPAR agonist revealed using rodent and canine hepatocytes

Marked species-specific responses to agonists of the peroxisome proliferator-activated alpha receptor (PPAR alpha) have been observed in rats and dogs, two species typically used to assess the potential human risk of pharmaceuticals in development. In this study, we used primary cultured rat and dog hepatocytes to investigate the underlying mechanisms of a novel PPAR alpha and -gamma coagonist, LY465608, relative to fenofibrate, a prototypical PPAR alpha agonist. As expected, rat hepatocytes incubated with these two agonists demonstrated an increase in peroxisome number as evaluated by electron microscopy, whereas the peroxisome number remained unchanged in dog hepatocytes. Biochemical analysis showed that rat hepatocytes responded to PPAR agonists with an induction of both peroxisomal and mitochondrial beta-oxidation (PBox and MBox) activities. Dog hepatocytes treated with both PPAR agonists, however, did not show increased PBox activity but did demonstrate increased MBox activity. Analysis of peroxisomal beta-oxidation gene expression markers by quantitative real-time PCR confirmed that PPAR agonists induced the peroxisomal enzymes, acyl-coenzyme A (CoA) oxidase (Acox), enoyl-CoA hydratase/L-3-hydroxyacyl-CoA dehydrogenase (Ehhadh), and 3-ketoacyl-CoA thiolase (Acaa1) at the transcriptional level in rat hepatocytes, but not dog hepatocytes. Expression of mRNA for the mitochondrial beta-oxidation gene hydroxyacyl-CoA dehydrogenase/3-ketoacyl-CoA thiolase (Hadhb), however, increased in both rat and dog hepatocytes, consistent with biochemical measurements of peroxisomal and mitochondrial beta-oxidation. Repeat-dose nonclinical safety studies of LY465608 revealed abnormities in mitochondrial morphology and evidence of single-cell necrosis following 30 days of dosing exclusively in dogs, but not in rats. Microarray analysis indicated that dog hepatocytes, but not rat hepatocytes, treated with LY465608 had an expression profile consistent with abnormalities in the regulation of cell renewal and death, oxidative stress, and mitochondrial bioenergetics, which may explain the canine-specific toxicity observed in vivo with this compound. This increased sensitivity to mitochondrial toxicity of canine hepatocytes relative to rat hepatocytes identified using gene expression was confirmed using the fluorescent indicator tetramethylrhodamine ethyl ester (TMRE) and flow cytometry. At doses of 0.1 microM LY465608, canine hepatocytes showed a greater shift in fluorescence indicative of mitochondrial damage than observed with rat hepatocytes treated at 10 microM. In summary, using rat and dog primary hepatocytes, we replicated the pharmacologic and toxicologic effects of LY465608 observed in vivo during preclinical development and propose an underlying mechanism for these species-specific effects.

Dual PPARα/γ Agonist Tesaglitazar Reduces Atherosclerosis in Insulin-Resistant and Hypercholesterolemic ApoE*3Leiden Mice

OBJECTIVE

We investigated whether the dual PPARalpha/gamma agonist tesaglitazar has anti-atherogenic effects in ApoE*3Leiden mice with reduced insulin sensitivity.

METHODS AND RESULTS

ApoE*3Leiden transgenic mice were fed a high-fat (HF) insulin-resistance-inducing diet. One group received a high-cholesterol (HC) supplement (1% wt/wt; HC group). A second group received the same HC supplement along with tesaglitazar (T) 0.5 micromol/kg diet (T group). A third (control) group received a low-cholesterol (LC) supplement (0.1% wt/wt; LC group). Tesaglitazar decreased plasma cholesterol by 20% compared with the HC group; cholesterol levels were similar in the T and LC groups. Compared with the HC group, tesaglitazar caused a 92% reduction in atherosclerosis, whereas a 56% reduction was seen in the cholesterol-matched LC group. Furthermore, tesaglitazar treatment significantly reduced lesion number beyond that expected from cholesterol lowering and induced a shift to less severe lesions. Concomitantly, tesaglitazar reduced macrophage-rich and collagen areas. In addition, tesaglitazar reduced inflammatory markers, including plasma SAA levels, the number of adhering monocytes, and nuclear factor kappaB-activity in the vessel wall.

CONCLUSIONS

Tesaglitazar has anti-atherosclerotic effects in the mouse model that go beyond plasma cholesterol lowering, possibly caused by a combination of altered lipoprotein profiles and anti-inflammatory vascular effects.

Selective PPAR modulators, dual and pan PPAR agonists: multimodal drugs for the treatment of Type 2 diabetes and atherosclerosis

More than 70% of patients with Type 2 diabetes mellitus (T2DM) die because of cardiovascular diseases. Current therapeutic strategies are based on separate treatment of insulin resistance and dyslipidaemia. Development of drugs with multimodal activities should improve management of the global cardiovascular risk of T2DM patients and result in better patient compliance. New therapeutic strategies are aimed at targeting the entire spectrum of dysfunctioning organs, cells and regulatory pathways implicated in the pathogenesis of T2DM, dyslipidaemia and atherosclerosis. PPAR family members play major roles in the regulation of lipid metabolism, glucose homeostasis and inflammatory processes, making these transcription factors ideal targets for therapeutic strategies against these diseases. This review discusses why PPARs and development of novel selective PPAR modulators, dual and pan PPAR agonists constitute promising approaches for the treatment of diabetes, dyslipidaemia and atherosclerosis.

Profound resolution of early atherosclerosis with conjugated linoleic acid

Conjugated linoleic acid (CLA) refers to a group of positional and geometric isomers of linoleic acid and has been shown to suppress the development of atherosclerosis in experimental models. However, the mechanism involved is unclear although it is believed it may act as a cyclooxygenase inhibitor or as an agonist of the nuclear receptors, peroxisome proliferator activated receptors (PPARs). In this study, we examined the effect of cis-9,trans-11:trans-10,cis-12-CLA (80:20 blend) on the regression of pre-established atherosclerosis. ApoE(-/-) mice fed a 1% cholesterol diet were randomized at 8 weeks to continue receiving the diet supplemented with 1% control saturated fat or 1% CLA blend for a further 8 weeks. CLA supplementation did not simply prevent progression but induced almost complete resolution of atherosclerosis. Although CLA inhibited platelet deposition, as detected by staining of platelet glycoprotein alpha11b beta111a, it did not inhibit COX-mediated generation of prostaglandins in this model. However, PPARalpha and PPARgamma expression was increased in the aorta of the CLA-treated animals. This was coincident with decreased macrophage accumulation and decreased expression of the macrophage scavenger receptor CD36 and increased apoptosis in the aorta in vivo. CLA induces the resolution of atherosclerosis by negatively regulating the expression of pro-inflammatory genes and inducing apoptosis in the atherosclerotic lesion.

PEROXISOME PROLIFERATOR-ACTIVATED RECEPTORS: How Their Effects on Macrophages Can Lead to the Development of a New Drug Therapy Against Atherosclerosis

Peroxisome proliferator-activated receptors (PPARs) alpha (alpha), beta/delta (beta/delta), and gamma (gamma) are members of the nuclear receptor superfamily, which also includes the estrogen, androgen, and glucocorticoid receptors. Recent evidence suggests that PPARs regulate genes involved in lipid metabolism, glucose homeostasis, and inflammation in various tissues; however, the mechanisms involved are not completely understood. Anti-diabetic drugs, called glitazones, can selectively activate PPARgamma, and hypolipidemic drugs, called fibrates, can weakly activate PPARalpha. Both classes of drugs can decrease insulin resistance and dyslipidemias, which also makes them attractive for treating the metabolic syndrome. The metabolic syndrome exhibits a constellation of risk factors for atherosclerosis that include obesity, insulin resistance, dyslipidemias, and hypertension. Interestingly, all three PPARs are present in macrophages and can therefore have a profound effect on several disease processes, including atherosclerosis. Macrophages are key players in atherosclerotic lesion development. Currently, the first line of defense in reducing the risk of atherosclerosis is aimed at lowering low-density lipoproteins (LDL) and raising high-density lipoproteins (HDL), but a large percentage of patients on statins still succumb to coronary artery disease. However, with the development of drugs selectively activating PPARs, a new arsenal of drugs specifically targeting to the macrophage/foam cell may potentially have a profound impact on how we treat cardiovascular disease.

Therapy insight: heart disease and the insulin-resistant patient

Insulin-resistance syndromes are of pandemic proportions; 150 million people worldwide and an estimated 43 million people in the US are currently affected by type 2 diabetes mellitus or metabolic syndrome respectively. Treatment of heart disease in the context of type 2 diabetes requires multifactorial risk-factor management, including lifestyle modification and drug treatment for comorbidities. Management of coronary risk extends beyond simple cholesterol lowering. Early use of cardiac imaging and, where appropriate, revascularization should be considered in high-risk or symptomatic patients. Traditionally, patients with type 2 diabetes and coronary arterial disease have been treated surgically, but percutaneous revascularization of these patients is increasingly common. Indeed, revascularization by use of drug-eluting coronary stents combined with administration of novel antiplatelet agents has revolutionized percutaneous coronary intervention in patients with type 2 diabetes. Despite these advances, there is no consensus of opinion regarding revascularization strategies or risk-factor management in insulin-resistant patients with symptomatic or prognostically important coronary arterial disease. Furthermore, specific therapies and preventative strategies for diabetic cardiomyopathy and heart failure in patients with type 2 diabetes remain elusive. The identification of optimized approaches for the prevention and treatment of the metabolic syndrome and heart disease in insulin-resistant, nondiabetic patients remains a major global challenge.

Peroxisome proliferator-activated receptor gamma as a drug target in the pathogenesis of insulin resistance

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that belong to the nuclear hormone receptor superfamily. The activation of PPAR-gamma, an isotype of PPARs, can either increase or decrease the transcription of target genes. The genes controlled by this form of PPAR have been shown to encode proteins or peptides that participate in the pathogenesis of insulin resistance. Insulin resistance is defined as a state of reduced responsiveness to normal circulating concentrations of insulin and it often co-exists with central obesity, hypertension, dyslipidemia, and atherosclerosis. There is substantial evidence that links obesity with insulin resistance and type-2 diabetes. The early phase of obesity-related insulin resistance has 2 components: (a) interruption of lipid homeostasis leading to the increased plasma concentration of fatty acids that is normally suppressed by the activation of PPAR-gamma, and (b) activation of factors such as cytokines depressed by PPAR-gamma that cause insulin resistance. Therefore, it is logical to suggest that activation of PPAR-gamma may partially reverse the state of insulin resistance. Evidently, activation of the nuclear receptor, PPAR-gamma, by thiazolidinediones has been reported to ameliorate insulin resistance. Although hepatotoxity and possibility to induce congestive heart failure (CHF) limit the widely use of thiazolodinediones, they are still powerful weapon to fight against insulin resistance and type-2 diabetes if use properly. This article reviews the physiology of PPAR-gamma and insulin-signaling transduction, the pathogenesis of insulin resistance in obesity-related type-2 diabetes, the pharmacological role of PPAR-gamma in insulin resistance, and additional effects of thiazolidinediones.

The PPARdelta agonist GW0742X reduces atherosclerosis in LDLR(-/-) mice

Several lines of evidence suggest a biological role for peroxisome proliferator-activated receptor (PPARdelta) in the pathogenesis of atherosclerosis. Administration of synthetic PPARdelta agonists to obese rhesus monkeys elevates serum high-density lipoprotein (HDL) cholesterol as a result of increased reverse cholesterol transport whilst in vitro studies have suggested a role for PPARdelta in lipid uptake into macrophages. Recent studies have found that PPARdelta depletion from macrophages in LDL receptor (LDLR(-/-)) mice decreases lesion area via modulation of the inflammatory status of the macrophage, an effect also seen on pharmacological activation of PPARdelta in vitro. We demonstrate here that the PPARdelta agonist, GW0742X has potent anti-atherogenic activity in the LDLR(-/-) mouse, decreasing lesion area by up to 50%. Administration of GW0742X had no effect on total cholesterol, HDL or LDL cholesterol and modest effects on very low-density lipoprotein (VLDL). Treatment with GW0742X resulted in decreased expression of monocyte chemoattractant protein 1 (MCP-1) and intracellular adhesion moleculae 1 (ICAM-1) in the aortae of treated mice. In addition, GW0742X decreased tumour necrosis factor-alpha (TNFalpha) expression in peritoneal macrophages, aortae and adipose tissue in comparison with control animals. Changes in gene expression were reflected in decreased plasma levels of MCP-1. These observations support an atheroprotective effect of PPARdelta agonists in vivo.

Understanding hyperlipidemia and atherosclerosis: lessons from genetically modified apoe and ldlr mice

Hyperlipidemia is the most important risk factor for atherosclerosis, which is the major cause of cardiovascular disease. The etiology of hyperlipidemia and atherosclerosis is complex and governed by multiple interacting genes. However, mutations in two genes have been shown to be directly involved, i.e., the low-density lipoprotein receptor (LDLR) and apolipoprotein E (ApoE). Genetically modified mouse models have been instrumental in elucidating the underlying molecular mechanisms in lipid metabolism. In this review, we focus on the use of two of the most widely used mouse models, ApoE- and LDLR-deficient mice. After almost a decade of applications, it is clear that each model has unique strengths and drawbacks when carrying out studies of the role of additional genes and environmental factors such as nutrition and lipid-lowering drugs. Importantly, we elaborate on mice expressing mutant forms of APOE, including the

Fibrates

Atherosclerosis of the large arteries is the main origin of cerebro- and cardiovascular diseases, the leading causes of mortality and morbidity in industrialized countries. The pathophysiology of coronary and cerebrovascular atherosclerosis is multifactorial and complex. Fibrates are hypolipidemic drugs that lower progression of atherosclerotic lesions mainly through activation of the nuclear receptor peroxisome-proliferator activated receptor-alpha. In addition, fibrates exert pleiotropic and anti-inflammatory actions. In this chapter, we will focus on the different effects of fibrates impacting on the development of atherosclerosis.

PPAR- and LXR-dependent pathways controlling lipid metabolism and the development of atherosclerosis

The nuclear receptor superfamily is composed of transcription factors that positively and negatively regulate gene expression in response to the binding of a diverse array of lipid-derived hormones and metabolites. Intense efforts are currently being directed at defining the biological roles and mechanisms of action of liver X receptors (LXRs) and peroxisome proliferator-activated receptors (PPARs). LXRs have been found to play essential roles in the regulation of whole body cholesterol absorption and excretion, in the efflux of cholesterol from peripheral cells, and in the biosynthesis and metabolism of very low density lipoproteins. PPARs have been found to regulate diverse aspects of lipid metabolism, including fatty acid oxidation, fat cell development, lipoprotein metabolism, and glucose homeostasis. Intervention studies indicate that activation of PPARalpha, PPARgamma, and LXRs by specific synthetic ligands can inhibit the development of atherosclerosis in animal models. Here, we review recent studies that provide new insights into the mechanisms by which these subclasses of nuclear receptors act to systemically influence lipid and glucose metabolism and regulate gene expression within the artery wall.

Atherosclerosis in the apolipoprotein-E-deficient mouse: a decade of progress

Arguably the most critical advancement in the elucidation of factors affecting atherogenesis has been the development of mouse models of atherosclerosis. Among available models, the apolipoprotein E-deficient (apoE-/-) mouse is particularly popular because of its propensity to spontaneously develop atherosclerotic lesions on a standard chow diet. A Medline search reveals over 645 articles dedicated to studies using this reliable and convenient "super" animal model since its inception (Piedrahita JA et al, Proc Natl Acad Sci U S A 1992;89:4471-4475; Plump AS et al, Cell 1992;71:343-353) with a more or less steady increase from year to year. This review will examine our present understanding of the pathology and progression of plaques in this animal and highlight some of the nutritional, pharmacological, and genetic studies that have enhanced this understanding.

Modulation of PPARalpha expression and inflammatory interleukin-6 production by chronic glucose increases monocyte/endothelial adhesion

OBJECTIVE

We have previously reported increased monocyte adhesion to human aortic endothelial cells (HAECs) cultured in 25 mmol/L glucose (HG) compared with normal glucose (NG) (5.5 mmol/L). In this study, we explored mechanisms that contribute to increased monocyte adhesion by elevated glucose.

METHODS AND RESULTS

We found that HAECs cultured in HG have increased production of the chemokine interleukin-6 (IL-6). We examined whether IL-6 directly modulated monocyte adhesion to EC. Inhibition of IL-6 using a neutralizing antibody significantly reduced glucose-mediated monocyte adhesion by 50%, and addition of IL-6 directly to human EC stimulated monocyte adhesion. PPARalpha has been reported to negatively regulate expression of IL-6 in vascular cells, so we examined PPARalpha-associated signaling in EC. A known PPARalpha agonist, Wy14,643, prevented glucose-mediated IL-6 production by EC and reduced glucose-mediated monocyte adhesion by 40%. HG-cultured HAEC had a 50% reduction in expression of PPARalpha compared with control EC. Primary aortic EC isolated from PPARalpha knockout (KO) mice showed increased monocyte adhesion compared with EC isolated from control mice. PPARalpha KO EC also had increased production of IL-6. Finally, we measured IL-6 levels in diabetic db/db mice and found significant 6-fold elevations in IL-6 levels in db/db EC.

CONCLUSIONS

These data indicate that IL-6 production is increased in diabetes and contributes to early vascular inflammatory changes. PPARalpha protects EC from glucose-mediated monocyte adhesion, in part through regulation of IL-6 production.

Pharmacotherapy for dyslipidaemia--current therapies and future agents

Current lipid-altering agents that lower low density lipoprotein cholesterol (LDL-C) primarily through increased hepatic LDL receptor activity include statins, bile acid sequestrants/resins and cholesterol absorption inhibitors such as ezetimibe, plant stanols/sterols, polyphenols, as well as nutraceuticals such as oat bran, psyllium and soy proteins; those currently in development include newer statins, phytostanol analogues, squalene synthase inhibitors, bile acid transport inhibitors and SREBP cleavage-activating protein (SCAP) activating ligands. Other current agents that affect lipid metabolism include nicotinic acid (niacin), acipimox, high-dose fish oils, antioxidants and policosanol, whilst those in development include microsomal triglyceride transfer protein (MTP) inhibitors, acylcoenzyme A: cholesterol acyltransferase (ACAT) inhibitors, gemcabene, lifibrol, pantothenic acid analogues, nicotinic acid-receptor agonists, anti-inflammatory agents (such as Lp-PLA(2) antagonists and AGI1067) and functional oils. Current agents that affect nuclear receptors include PPAR-alpha and -gamma agonists, while in development are newer PPAR-alpha, -gamma and -delta agonists, as well as dual PPAR-alpha/gamma and 'pan' PPAR-alpha/gamma/delta agonists. Liver X receptor (LXR), farnesoid X receptor (FXR) and sterol-regulatory element binding protein (SREBP) are also nuclear receptor targets of investigational agents. Agents in development also may affect high density lipoprotein cholesterol (HDL-C) blood levels or flux and include cholesteryl ester transfer protein (CETP) inhibitors (such as torcetrapib), CETP vaccines, various HDL 'therapies' and upregulators of ATP-binding cassette transporter (ABC) A1, lecithin cholesterol acyltransferase (LCAT) and scavenger receptor class B Type 1 (SRB1), as well as synthetic apolipoprotein (Apo)E-related peptides. Fixed-dose combination lipid-altering drugs are currently available such as extended-release niacin/lovastatin, whilst atorvastatin/amlodipine, ezetimibe/simvastatin, atorvastatin/CETP inhibitor, statin/PPAR agonist, extended-release niacin/simvastatin and pravastatin/aspirin are under development. Finally, current and future lipid-altering drugs may include anti-obesity agents which could favourably affect lipid levels.

Peroxisome proliferator-activated receptors: are they involved in atherosclerosis progression?

Peroxisome proliferator-activated receptors (PPAR) are nuclear receptors present in several organs and cell types. They are subdivided into PPAR alpha, PPAR gamma and PPAR delta (or beta). PPAR alpha and gamma are the two main categories of these receptors, which are both characterized by their ability to influence lipid metabolism, glucose homeostasis, cell proliferation, differentiation and apoptosis, as well as the inflammatory response, by transcriptional activation of target genes. PPAR alpha are activated by fatty acids, eicosanoids and fibrates, while PPAR gamma activators include arachidonic acid metabolites, oxidized low density lipoprotein and thiazolidinediones. Atherosclerosis is now considered a chronic inflammatory condition. Thus, PPAR activation appears a promising approach to favorably affect atherosclerosis development through both metabolic and anti-inflammatory effects. However, the clinical data in favor of an anti-atherosclerotic action of PPAR agonists are still scanty, and some experimental data would even indicate possible pro-atherogenic effects, or a lack of effect in the female sex. New controlled clinical studies will provide the information necessary to understand the true significance and usefulness of PPAR alpha, gamma and delta activators in the control of atherosclerotic disease.

New lipid-modifying therapies

Lipid abnormalities are central among the risk factors for the development of cardiovascular disease and their correction remains a major target for the medical community. Inhibitors of 3-hydroxy-3-methyl glutaryl coenzyme A reductase (statins) are the most widely prescribed and best tolerated of the currently available lipid-modifying therapies. Newer agents in this class (e.g., rosuvastatin) have proven to be more effective at lowering levels of low-density lipoprotein cholesterol. New formulations of drugs such as nicotinic acid, which improve treatment regimens and reduce unpleasant side effects, may result in improved patient compliance with this therapy. The development of novel drugs such as cholesterol absorption inhibitors (e.g., ezetimibe) and acyl-coenzyme A cholesterol acyltransferase inhibitors (e.g., avasimibe) will provide clinicians with therapeutic options that exploit different pathways to those currently being utilised. By combining these agents with statins, greater improvements in the lipid profile than those seen to date could be produced. In addition, advances in our understanding of the pathophysiology of dyslipidaemia have enabled other novel therapeutic targets to be identified and studies with experimental drugs underscore the potential of these approaches.

Arterial retention of apolipoprotein B(48)- and B(100)-containing lipoproteins in atherogenesis

PURPOSE OF REVIEW

The "response to retention" hypothesis of atherosclerosis suggests that the arterial deposition of cholesterol is directly proportional to the concentration of circulating plasma lipoproteins. However, there is increasing evidence to support the concept that specific lipoproteins may be preferentially retained within the arterial wall, possibly as a result of greater affinity for cell surface and extracellular matrices.

RECENT FINDINGS

Recently, key studies have provided insight into mechanisms involved in the interaction of apolipoprotein B (apoB)-containing lipoproteins with extracellular matrices. In addition, novel methods and innovative experimental design has enabled us to differentiate between the delivery, retention and efflux of apoB(48)- and apoB(100)-containing lipoproteins. Other studies have demonstrated a relationship between extracellular matrix proteoglycan expression and the development of atherosclerosis. Discussion in the present review also extends to the mechanisms that are involved in the relative intimal retention of apoB(48)- and apoB(100)-containing lipoproteins in order to explain the atherogenicity of these macromolecules.

SUMMARY

The perspective of this review is to highlight recent advances in the area of arterial lipoprotein retention and the physiological significance these processes may have in the aetiology of cardiovascular disease. Importantly, an understanding of the mechanisms responsible for the retention of apoB(48)/B(100)-containing lipoproteins will enable new strategies to be developed for the future management of cardiovascular disease.