Human apolipoprotein A-I and A-I mimetic peptides: potential for atherosclerosis reversal

Peptides derived from serum amyloid A prevent, and reverse, aortic lipid lesions in apoE−/− mice

Macrophages (Mphi) at sites of acute tissue injury accumulate and export cholesterol quickly. This metabolic activity is likely dependent on the physiological function of a major acute-phase protein, serum amyloid A 2.1 (SAA2.1), that is synthesized by hepatocytes as part of a systemic response to acute injury. Our previous studies using cholesterol-laden J774 mouse Mphi showed that an N-terminal domain of SAA2.1 inhibits acyl-CoA:cholesterol acyltransferase activity, and a C-terminal domain enhances cholesteryl ester hydrolase activity. The net effect of this enzymatic regulation is to drive intracellular cholesterol to its unesterified state, the form readily exportable to an extracellular acceptor such as HDL. Here, we demonstrate that these domains from mouse SAA2.1, when delivered in liposomal formulation, are effective at preventing and reversing aortic lipid lesions in apolipoprotein E-deficient mice maintained on high-fat diets. Furthermore, mouse SAA peptides, in liposomal formulation, are effective at regulating cholesterol efflux in THP-1 human Mphi, and homologous domains from human SAA are effective in mouse J774 cells. These peptides operate at the level of the foam cell in the reverse cholesterol pathway and therefore may be used in conjunction with other agents that act more distally in this process. Such human peptides, or small molecule mimics of their structure, may prove to be potent antiatherogenic agents in humans.

Inhibition of Lipopolysaccharide-Induced Inflammatory Responses by an Apolipoprotein AI Mimetic Peptide

Previous studies suggest that high-density lipoprotein and apoAI inhibit lipopolysaccharide (LPS)-induced inflammatory responses. The goal of the current study was to test the hypothesis that the apoAI mimetic peptide L-4F exerts antiinflammatory effects similar to apoAI. Pretreatment of human umbilical vein endothelial cells (HUVECs) with LPS induced the adhesion of THP-1 monocytes. Incubation of cells with LPS and L-4F (1 to 50 μg/mL) reduced THP-1 adhesion in a concentration-dependent manner. This response was associated with a significant reduction in the synthesis of cytokines, chemokines, and adhesion molecules. L-4F reduced vascular cell adhesion molecule-1 expression induced by LPS or lipid A, whereas a control peptide (Sc-4F) showed no effect. In contrast to LPS treatment, L-4F did not inhibit IL-1β- or tumor necrosis factor-α–induced vascular cell adhesion molecule-1 expression. The inhibitory effect of L-4F on LPS induction of inflammatory markers was associated with reduced binding of LPS to its plasma carrier molecule, lipopolysaccharide binding protein, and decreased binding of LPS to HUVEC monolayers. LPS and L-4F in HUVEC culture medium were fractionated by fast protein liquid chromatography and were localized to the same fractions, suggesting a physical interaction between these molecules. Proinflammatory responses to LPS are associated with the binding of lipid A to cell surface receptors. The current studies demonstrate that L-4F reduces the expression of inflammatory markers induced by LPS and lipid A and suggest that apoAI peptide mimetics may be useful in the treatment of inflammation associated with endotoxemia.

Emerging role of high-density lipoprotein in the prevention of cardiovascular disease

In major statin trials, the relative risk reduction is typically in the range 25-35%, thus indicating that the majority of cardiac events continues to occur despite statin therapy. Hence, there is a considerable interest in identifying novel therapies capable of further reducing cardiovascular disease risk. One such potential therapeutic target is a low level of high-density lipoprotein (HDL) cholesterol. Emerging targets involved in HDL metabolism are: (i) liver X receptor and peroxisome proliferator-activated receptor agonists; (ii) cholesteryl ester transfer protein inhibitors; (iii) HDL mimetics (ETC-216); (iv) apolipoprotein A-I synthetic peptides; and (v) HDL delipidation and reinfusion. Although they are at various stages of development, each of these therapies has promise for the treatment of cardiovascular disease in humans.

Apolipoprotein A-I Mimetic Peptides

Despite identical amino acid composition, differences in class A amphipathic helical peptides caused by differences in the order of amino acids on the hydrophobic face results in substantial differences in antiinflammatory properties. One of these peptides is an apolipoprotein A-I (apoA-I) mimetic, D-4F. When given orally to mice and monkeys, D-4F caused the formation of pre-β high-density lipoprotein (HDL), improved HDL-mediated cholesterol efflux, reduced lipoprotein lipid hydroperoxides, increased paraoxonase activity, and converted HDL from pro-inflammatory to antiinflammatory. In apolipoprotein E (apoE)-null mice, D-4F increased reverse cholesterol transport from macrophages. Oral D-4F reduced atherosclerosis in apoE-null and low-density lipoprotein (LDL) receptor-null mice. In vitro when added to human plasma at nanomolar concentrations, D-4F caused the formation of pre-β HDL, reduced lipoprotein lipid hydroperoxides, increased paraoxonase activity, and converted HDL from pro-inflammatory to antiinflammatory. Physical-chemical properties and the ability of various class A amphipathic helical peptides to activate lecithin cholesterol acyltransferase (LCAT) in vitro did not predict biologic activity in vivo. In contrast, the use of cultured human artery wall cells in evaluating these peptides was more predictive of their efficacy in vivo. We conclude that the antiinflammatory properties of different class A amphipathic helical peptides depends on subtle differences in the configuration of the hydrophobic face of the peptides, which determines the ability of the peptides to sequester inflammatory lipids. These differences appear to be too subtle to predict efficacy based on physical-chemical properties alone. However, understanding these physical-chemical properties provides an explanation for the mechanism of action of the active peptides.

Drugs in development: targeting high-density lipoprotein metabolism and reverse cholesterol transport

PURPOSE OF REVIEW

This review summarizes currently available therapies for raising high-density lipoprotein cholesterol (HDL-C) and expands on therapies currently in development that target high-density lipoprotein cholesterol.

RECENT FINDINGS

In the realm of new high-density lipoprotein-raising therapies, there is a strong focus on high-density lipoprotein metabolism and the reverse cholesterol transport pathway. Several infusions of recombinant apoA-I Milano/phospholipid complexes appeared to reduce atheroma volume as measured by intravascular ultrasound. Both intravenous and oral apoA-I mimetic peptides are in early clinical trials. Next generation PPAR-alpha agonists are more potent at high-density lipoprotein-raising than currently available fibrates, and dual PPAR-alpha/PPAR-gamma agonists are under investigation to help correct atherogenic dyslipidemia seen in many diabetics. Two small molecule inhibitors of the cholesteryl ester transfer protein have shown promise in clinical trials at substantially raising high-density lipoprotein cholesterol.

SUMMARY

Larger scale clinical trials, including those with additional surrogate outcome measures as well as cardiovascular event outcomes are needed to further assess the benefit of newer high-density lipoprotein-raising therapies. Additional therapeutics are currently in development that target other parts of the reverse cholesterol transport pathway and, in addition to providing new potential pharmaceuticals, will help to further elucidate the atheroprotective mechanisms of high-density lipoprotein cholesterol.

An oral apoJ peptide renders HDL antiinflammatory in mice and monkeys and dramatically reduces atherosclerosis in apolipoprotein E-null mice

OBJECTIVE

To determine the properties of a peptide synthesized from D-amino acids corresponding to residues 113 to 122 in apolipoprotein (apo) J.

METHODS AND RESULTS

In contrast to D-4F, D- [113-122]apoJ showed minimal self-association and helicity in the absence of lipids. D-4F increased the concentration of apoA-I with pre-beta mobility in apoE-null mice whereas D- [113-122]apoJ did not. After an oral dose D- [113-122]apoJ more slowly associated with lipoproteins and was cleared from plasma much more slowly than D-4F. D- [113-122]apoJ significantly improved the ability of plasma to promote cholesterol efflux and improved high-density lipoprotein (HDL) inflammatory properties for up to 48 hours after a single oral dose in apoE-null mice, whereas scrambled D- [113-122]apoJ did not. Oral administration of 125 microg/mouse/d of D- [113-122]apoJ reduced atherosclerosis in apoE-null mice (70.2% reduction in aortic root sinus lesion area, P=4.3 x 10(-13); 70.5% reduction by en face analysis, P=1.5 x 10(-6)). In monkeys, oral D- [113-122]apoJ rapidly reduced lipoprotein lipid hydroperoxides (LOOH) and improved HDL inflammatory properties. Adding 250 ng/mL of D-[113-122]apoJ (but not scrambled D- [113-122]apoJ) to plasma in vitro reduced LOOH and increased paraoxonase activity.

CONCLUSIONS

Oral D- [113-122]apoJ significantly improves HDL inflammatory properties in mice and monkeys and inhibits lesion formation in apoE-null mice.

Statins and LDL-cholesterol lowering: an overview

Statins have become a cornerstone of treatment for dyslipidaemia primarily due to their marked lowering of low-density lipoprotein cholesterol (LDL-C). Studies show that statin treatment typically reduces relative risk of cardiovascular disease by 24-37%, regardless of age, sex, prior history of coronary heart disease (CHD), or other co-morbid conditions. There is also a growing body of evidence that statins can be effective in people whose LDL-C is not considered elevated under current guidelines. In both the Anglo-Scandinavian Cardiac Outcomes Trial (ASCOT) and the Collaborative Atorvastatin Diabetes Study (CARDS), participants randomised to atorvastatin (10 mg/day) experienced at least a one-third reduction in major cardiovascular events, even though at baseline, their LDL-C was within the normal range. Other studies have also provided evidence that more intensive lipid-lowering regimens could provide additional clinical benefits. In the Reversal of Atherosclerosis with Aggressive Lipid Lowering (REVERSAL) trial, the first active-control clinical trial of CHD progression, an intensive lipid-lowering regimen using atorvastatin (80 mg/day) decreased atherogenic lipoproteins and atheroma volume in patients with established CHD, compared with a moderate regimen using pravastatin (40 mg/day). Furthermore, relative to baseline, there was no measurable atheroma progression in the atorvastatin group. While statin therapy does offer significant clinical benefit, 60-70% of major cardiovascular events are still not prevented, which underscores the need for alternative interventions. Targeting inflammatory mediators of atherosclerosis such as C-reactive protein (CRP), as well as combination therapy to simultaneously raise high-density lipoprotein cholesterol (HDL-C) and lower LDL-C, are among the promising new strategies for primary and secondary prevention of atherosclerotic disease. This article will summarise data concerning use of statins in patients without markedly elevated LDL-C. The issue of the ideal LDL-C target will also be considered before addressing future treatment options for dyslipidaemia.