Beyond high-density lipoprotein cholesterol levels evaluating high-density lipoprotein function as influenced by novel therapeutic approaches. J Am Coll Cardiol. 2008;51:2199-2211. [PMID: 18534265 DOI: 10.1016/j.jacc.2008.03.016]

Thyroid hormones and thyroid hormone receptors: Effects of thyromimetics on reverse cholesterol transport

Reverse cholesterol transport (RCT) is a complex process which transfers cholesterol from peripheral cells to the liver for subsequent elimination from the body via feces. Thyroid hormones (THs) affect growth, development, and metabolism in almost all tissues. THs exert their actions by binding to thyroid hormone receptors (TRs). There are two major subtypes of TRs, TRα and TRβ, and several isoforms (e.g. TRα1, TRα2, TRβ1, and TRβ2). Activation of TRα1 affects heart rate, whereas activation of TRβ1 has positive effects on lipid and lipoprotein metabolism. Consequently, particular interest has been focused on the development of thyromimetic compounds targeting TRβ1, not only because of their ability to lower plasma cholesterol but also due their ability to stimulate RCT, at least in pre-clinical models. In this review we focus on THs, TRs, and on the effects of TRβ1-modulating thyromimetics on RCT in various animal models and in humans.

Nanotechnology for synthetic high-density lipoproteins

Atherosclerosis is the disease mechanism responsible for coronary heart disease (CHD), the leading cause of death worldwide. One strategy to combat atherosclerosis is to increase the amount of circulating high-density lipoproteins (HDL), which transport cholesterol from peripheral tissues to the liver for excretion. The process, known as reverse cholesterol transport, is thought to be one of the main reasons for the significant inverse correlation observed between HDL blood levels and the development of CHD. This article highlights the most common strategies for treating atherosclerosis using HDL. We further detail potential treatment opportunities that utilize nanotechnology to increase the amount of HDL in circulation. The synthesis of biomimetic HDL nanostructures that replicate the chemical and physical properties of natural HDL provides novel materials for investigating the structure-function relationships of HDL and for potential new therapeutics to combat CHD.

Apolipoprotein A-I mimetic peptides

PURPOSE OF REVIEW

To review published data related to the potential applicability of apolipoprotein A-I mimetic peptides.

RECENT FINDINGS

Despite a wealth of information on HDL-C levels and risk for cardiovascular disease (CVD), little evidence is present to suggest that raising HDL-C levels per se will result in CVD risk reduction. Rather, increasing HDL functionality might be a more successful strategy to reverse the process of atherosclerosis. In as such, apoA-I mimetic peptides, either in single or tandem formulation, hold great promise. Evidence gathered over the last years has provided insight in the extent to which mimetics influence several cardio metabolic pathways. ApoA-I mimetics have shown to have anti-inflammatory, antioxidant, and antiatherogenic effects. Direct comparisons between different mimetics have provided insight in factors influencing the differential beneficial consequences of these peptides. Data derived from recent studies suggest that mimetics might gain their position as a therapeutic intervention in the treatment of septicaemia, transplantation rejection, diabetes and auto-immune diseases.

SUMMARY

This review provides a summary of the current literature on the potential application of apoA-I mimetics as therapeutic agents. There is increasing evidence that these mimetics should be considered as a promising supplement to current strategies. Results from human studies addressing the in-vivo effects of the different apoA-I mimetics are eagerly awaited.

The versatility of HDL: a crucial anti-inflammatory regulator

BACKGROUND

Low levels of plasma high-density lipoprotein (HDL) represent a major cardiovascular risk factor and therefore raising HDL has been proposed to positively affect patients with atherosclerotic heart disease. However, the current evidence that raising HDL per se will reduce atherosclerosis and thereby cardiovascular events still remains controversial.

AIMS

In this review, we discuss the diverse anti-atherogenic and anti-inflammatory properties of HDL in the light of recent findings indicating that the quality rather than the mere quantity of HDL determines its beneficial effects against atherosclerosis. More specifically, we will focus on the conspicuous anti-inflammatory properties of HDL as this might contribute to the overall beneficial effects of HDL in diseased patients such as modulation of costimulatory/adhesion molecule expression, cytokine production and inhibition of the prototypical proinflammatory transcription factor NF-κB.

RESULTS

A range of clinical disorders share permanent inflammation as a characteristic hallmark including coronary artery disease, chronic kidney disease, diabetes mellitus or rheumatoid arthritis and also display distinct qualitative changes in the HDL compartment. Loss of anti-inflammatory functions of HDL is emerging as an important risk factor for disease progression and survival in these clinical entities.

CONCLUSIONS

It will be important to define the anti-inflammatory effects of HDL at the molecular level and to dissect the manifold functional implications to develop both novel functional assays that enable meaningful outcome studies and foster new therapeutic concepts in patients with altered HDL function.

High-density lipoprotein suppresses the type I interferon response, a family of potent antiviral immunoregulators, in macrophages challenged with lipopolysaccharide

BACKGROUND

High-density lipoprotein (HDL) protects the artery wall by removing cholesterol from lipid-laden macrophages. However, recent evidence suggests that HDL might also inhibit atherogenesis by combating inflammation.

METHODS AND RESULTS

To identify potential antiinflammatory mechanisms, we challenged macrophages with lipopolysaccharide, an inflammatory microbial ligand for Toll-like receptor 4. HDL inhibited the expression of 30 (277 of 911) of the genes normally induced by lipopolysaccharide, microarray analysis revealed. One of its major targets was the type I interferon response pathway, a family of potent viral immunoregulators controlled by Toll-like receptor 4 and the TRAM/TRIF signaling pathway. Unexpectedly, the ability of HDL to inhibit gene expression was independent of macrophage cholesterol stores. Immunofluorescent studies suggested that HDL promoted TRAM translocation to intracellular compartments, which impaired subsequent signaling by Toll-like receptor 4 and TRIF. To examine the potential in vivo relevance of the pathway, we used mice deficient in apolipoprotein A-I, the major protein of HDL. After infection with Salmonella typhimurium, a Gram-negative bacterium that expresses lipopolysaccharide, apolipoprotein A-I-deficient mice had 6-fold higher plasma levels of interferon-β, a key regulator of the type I interferon response, than did wild-type mice.

CONCLUSIONS

HDL inhibits a subset of lipopolysaccharide-stimulated macrophage genes that regulate the type I interferon response, and its action is independent of sterol metabolism. These findings raise the possibility that regulation of macrophage genes by HDL might link innate immunity and cardioprotection.

Future therapeutic directions in reverse cholesterol transport

Despite a robust inverse association between high-density lipoprotein (HDL) cholesterol levels and atherosclerotic cardiovascular disease, the development of new therapies based on pharmacologic enhancement of HDL metabolism has proven challenging. Emerging evidence suggests that static measurement of HDL levels has inherent limitations as a surrogate for overall HDL functionality, particularly with regard to the rate of flux through the macrophage reverse cholesterol transport (RCT) pathway. Recent research has provided important insight into the molecular underpinnings of RCT, the process by which excess cellular cholesterol is effluxed from peripheral tissues and returned to the liver for ultimate intestinal excretion. This review discusses the critical importance and current strategies for quantifying RCT flux. It also highlights therapeutic strategies for augmenting macrophage RCT via three conceptual approaches: 1) improved efflux of cellular cholesterol via targeting the macrophage; 2) enhanced cholesterol efflux acceptor functionality of circulating HDL; and 3) increased hepatic uptake and biliary/intestinal excretion.

Biological properties of apolipoprotein a-I mimetic peptides

Apolipoprotein A-I (apoA-I) mimetic peptides resemble the physiochemical properties of the helices of apoA-I and show promise for the treatment of atherosclerotic vascular diseases and other chronic inflammatory disorders. These peptides have numerous properties, such as the ability to remodel high-density lipoprotein, sequester oxidized lipids, promote cholesterol efflux, and activate an anti-inflammatory process in macrophages, any or all of which may contribute to their antiatherogenic properties. In murine models, the 4F peptide attenuates early atherosclerosis but seems to require the addition of statins to influence more mature lesions. A recently developed method for the oral delivery of the peptides that protects them from proteolysis will facilitate further research on the mechanism of action of these peptides. This review focuses on the properties of the 4F peptide, although numerous apoA-I mimetics are under investigation and a single "best" peptide that mimics all of the properties of the antiatherogenic protein apoA-I has not been identified.

High-density lipoprotein therapeutics and cardiovascular prevention

The field of cardiovascular prevention has long anticipated the evolution of high-density lipoprotein (HDL) therapy from unproven metabolic tweaking to pillar of risk reduction on par with low-density lipoprotein control. However, the convincing epidemiologic data linking HDL cholesterol (HDL-C) and cardiovascular disease risk in an inverse correlation has not yet translated into clinical trial evidence supporting linearity between HDL-C increases and risk reduction, or identifying obvious goals of therapy. Although HDL-C-increasing lifestyle maneuvers and established HDL drugs such as niacin and fibrates are likely to protect the vasculature, the negative results obtained in trials of a cholesteryl ester transfer protein inhibitor remind us that HDL-C increases are not always beneficial. It is becoming clear that a functional HDL is a more desirable target than simply increasing HDL-C levels. The larger objective of improving HDL functionality (with or without HDL-C level changes) is bound to become the guiding principle for pharmaceutical research in this area. Several new compounds currently being tested bridge the classical aim of increasing HDL-C levels with the novel target of improving HDL function.

HDL cholesterol and residual risk of first cardiovascular events after treatment with potent statin therapy: an analysis from the JUPITER trial

BACKGROUND

HDL-cholesterol concentrations are inversely associated with occurrence of cardiovascular events. We addressed, using the JUPITER trial cohort, whether this association remains when LDL-cholesterol concentrations are reduced to the very low ranges with high-dose statin treatment.

METHODS

Participants in the randomised placebo-controlled JUPITER trial were adults without diabetes or previous cardiovascular disease, and had baseline concentrations of LDL cholesterol of less than 3.37 mmol/L and high-sensitivity C-reactive protein of 2 mg/L or more. Participants were randomly allocated by a computer-generated sequence to receive rosuvastatin 20 mg per day or placebo, with participants and adjudicators masked to treatment assignment. In the present analysis, we divided the participants into quartiles of HDL-cholesterol or apolipoprotein A1 and sought evidence of association between these quartiles and the JUPITER primary endpoint of first non-fatal myocardial infarction or stroke, hospitalisation for unstable angina, arterial revascularisation, or cardiovascular death. This trial is registered with ClinicalTrials.gov, number NCT00239681.

FINDINGS

For 17,802 patients in the JUPITER trial, rosuvastatin 20 mg per day reduced the incidence of the primary endpoint by 44% (p<0.0001). In 8901 (50%) patients given placebo (who had a median on-treatment LDL-cholesterol concentration of 2.80 mmol/L [IQR 2.43-3.24]), HDL-cholesterol concentrations were inversely related to vascular risk both at baseline (top quartile vs bottom quartile hazard ratio [HR] 0.54, 95% CI 0.35-0.83, p=0.0039) and on-treatment (0.55, 0.35-0.87, p=0.0047). By contrast, among the 8900 (50%) patients given rosuvastatin 20 mg (who had a median on-treatment LDL-cholesterol concentration of 1.42 mmol/L [IQR 1.14-1.86]), no significant relationships were noted between quartiles of HDL-cholesterol concentration and vascular risk either at baseline (1.12, 0.62-2.03, p=0.82) or on-treatment (1.03, 0.57-1.87, p=0.97). Our analyses for apolipoprotein A1 showed an equivalent strong relation to frequency of primary outcomes in the placebo group but little association in the rosuvastatin group.

INTERPRETATION

Although measurement of HDL-cholesterol concentration is useful as part of initial cardiovascular risk assessment, HDL-cholesterol concentrations are not predictive of residual vascular risk among patients treated with potent statin therapy who attain very low concentrations of LDL cholesterol.

FUNDING

AstraZeneca.

Cilostazol enhances macrophage reverse cholesterol transport in vitro and in vivo

OBJECTIVE

Recent failure of an HDL-cholesterol raising strategy using a cholesteryl ester transfer protein inhibitor highlights the importance of the anti-atherogenic function rather than plasma concentration of HDL. Cilostazol, a selective inhibitor of phosphodiesterase 3, has been widely used in patients with atherosclerotic diseases and is known to increase HDL-cholesterol. However, it remains unclear whether cilostazol enhances anti-atherogenic properties by promoting reverse cholesterol transport (RCT), a major anti-atherogenic function of HDL.

METHODS AND RESULTS

We observed that treatment of THP-1 macrophages, human monocyte-derived macrophages, and RAW264.7 cells with cilostazol increased ABCA1 and ABCG1 expression in a concentration-dependent manner, translating into enhanced apoA-I- and HDL-mediated cholesterol efflux from the macrophages. However, other cyclic AMP (cAMP)-elevating agents did not increase ABCA1 gene expression in THP-1 macrophages. Cilostazol did not change intracellular cAMP levels in THP-1 macrophages and RAW264.7 cells, and a protein kinase A (PKA) inhibitor did not affect cilostazol-induced ABCA1 and ABCG1 expression. To further investigate RCT in vivo, (3)H-cholesterol-labeled and acetyl LDL-loaded RAW264.7 cells were intraperitoneally injected into mice and the appearance of the (3)H-tracer was monitored in plasma, liver, and feces. Supporting the in vitro data, cilostazol was found to significantly increase (3)H-tracer levels in both plasma and feces.

CONCLUSIONS

These findings indicate that cilostazol might provide anti-atherosclerotic effects by promoting RCT through increased ABCA1/G1 expression in macrophages.

Biliary sterol secretion is not required for macrophage reverse cholesterol transport

Recent evidence suggests that the intestine may play a direct facilitative role in reverse cholesterol transport (RCT), independent of hepatobiliary secretion. In order to understand the nonbiliary pathway for RCT, we created both genetic and surgical models of biliary cholesterol insufficiency. To genetically inhibit biliary cholesterol secretion, we generated mice in which Niemann-Pick C1-Like 1 (NPC1L1) was overexpressed in the liver. Compared to controls, NPC1L1(Liver-Tg) mice exhibit a >90% decrease in biliary cholesterol secretion, yet mass fecal sterol loss and macrophage RCT are normal. To surgically inhibit biliary emptying into the intestine, we have established an acute biliary diversion model. Strikingly, macrophage RCT persists in mice surgically lacking the ability to secrete bile into the intestine. Collectively, these studies demonstrate that mass fecal sterol loss and macrophage RCT can proceed in the absence of biliary sterol secretion, challenging the obligate role of bile in RCT.

Cholesteryl ester transfer protein in patients with coronary heart disease

BACKGROUND

The impact of cholesteryl ester transfer protein (CETP) in the development of atherosclerosis is a matter for ongoing debate. In this study, we analyse associations of CETP with cardiovascular endpoints in a cohort of patients with stable coronary artery disease (CAD).

DESIGN

KAROLA is a prospective observational study of patients with CAD and a median follow-up of 8 years (n = 1132). CETP levels were measured using an enzyme-linked immunosorbent assay.

RESULTS

Cholesteryl ester transfer protein levels were lower in men (P = 0.0016), positively correlated to low-density lipoprotein cholesterol, and inversely correlated to triglyceride levels (P < 0.0001 and P = 0.011 respectively). There was no significant difference in mortality between patients in different CETP quartiles; the hazard ratio of lowest vs. highest quartile was 1.33 (95% confidence interval (CI): 0.77-2.30) for mortality and 1.24 (95% CI: 0.75-2.03) for secondary events. In post hoc analyses, comparing nondiabetic subjects with CETP below vs. above median, the adjusted hazard ratio for death in patients with low CETP levels was 1.84 (95% CI: 1.10-3.09).

CONCLUSION

Although statistically significant associations were found only in post hoc analyses, the effect sizes in this study were in line with previous findings in the Framingham and LURIC population. In combination, the emerging evidence challenges the concept of pharmacological CETP inhibition.

Discovery and validation of new molecular targets in treating dyslipidemia: the role of human genetics

Several high-profile failures of lipid-related therapeutics in clinical trials have led to intense interest in improved discovery and preclinical prioritization of potential targets. The careful study of patients with rare monogenic disorders has played a key role in establishing the causal role of cholesterol in atherosclerosis and highlighting viable drug targets. Systematic efforts to extend the association of common variants linked with lipid levels to coronary disease enable assessment of the vascular consequences of lifelong differences in lipids due to variation in specific molecules. This application of genetic epidemiology, termed Mendelian randomization, may prove useful in informing ongoing drug development efforts.

5A apolipoprotein mimetic peptide promotes cholesterol efflux and reduces atherosclerosis in mice

Intravenous administration of apolipoprotein (apo) A-I complexed with phospholipid has been shown to rapidly reduce plaque size in both animal models and humans. Short synthetic amphipathic peptides can mimic the antiatherogenic properties of apoA-I and have been proposed as alternative therapeutic agents. In this study, we investigated the atheroprotective effect of the 5A peptide, a bihelical amphipathic peptide that specifically effluxes cholesterol from cells by ATP-binding cassette transporter 1 (ABCA1). 5A stimulated a 3.5-fold increase in ABCA1-mediated efflux from cells and an additional 2.5-fold increase after complexing it with phospholipid (1:7 mol/mol). 5A-palmitoyl oleoyl phosphatidyl choline (POPC), but not free 5A, was also found to promote cholesterol efflux by ABCG1. When incubated with human serum, 5A-POPC bound primarily to high-density lipoprotein (HDL) but also to low-density lipoprotein (LDL) and promoted the transfer of cholesterol from LDL to HDL. Twenty-four hours after intravenous injection of 5A-POPC (30 mg/kg) into apoE-knockout (KO) mice, both the cholesterol (181%) and phospholipid (219%) content of HDL significantly increased. By an in vivo cholesterol isotope dilution study and monitoring of the flux of cholesterol from radiolabeled macrophages to stool, 5A-POPC treatment was observed to increase reverse cholesterol transport. In three separate studies, 5A when complexed with various phospholipids reduced aortic plaque surface area by 29 to 53% (n = 8 per group; p < 0.02) in apoE-KO mice. No signs of toxicity from the treatment were observed during these studies. In summary, 5A promotes cholesterol efflux both in vitro and in vivo and reduces atherosclerosis in apoE-KO mice, indicating that it may be a useful alternative to apoA-I for HDL therapy.

The HDL hypothesis: does high-density lipoprotein protect from atherosclerosis?

There is unequivocal evidence of an inverse association between plasma high-density lipoprotein (HDL) cholesterol concentrations and the risk of cardiovascular disease, a finding that has led to the hypothesis that HDL protects from atherosclerosis. This review details the experimental evidence for this "HDL hypothesis". In vitro studies suggest that HDL has a wide range of anti-atherogenic properties but validation of these functions in humans is absent to date. A significant number of animal studies and clinical trials support an atheroprotective role for HDL; however, most of these findings were obtained in the context of marked changes in other plasma lipids. Finally, genetic studies in humans have not provided convincing evidence that HDL genes modulate cardiovascular risk. Thus, despite a wealth of information on this intriguing lipoprotein, future research remains essential to prove the HDL hypothesis correct.

Molecular pathways and agents for lowering LDL-cholesterol in addition to statins

Recent guidelines in North America and Europe recommend lowering low density lipoprotein associated cholesterol (LDLC) to achieve optimal coronary heart disease risk reduction. Statins have been the therapy of choice and proven successful and relatively safe. However, we are now facing new challenges and it appears that additional or alternative drugs are urgently needed. This boosts research in the field, reopening old cases like other inhibitors of cholesterol synthesis or making attractive tools from the latest technologies like gene silencing by anti-sense oligonucleotides. LDLs are cholesterol-enriched lipoproteins stabilized by the hepatic apolipoprotein B100, and derived from TG rich very low density lipoprotein. This review focuses on the molecular pathways involved in plasma LDLC production and elimination, in particular cholesterol absorption and the hepatobiliary route, apoB100 and VLDL production, and LDL clearance via the LDL receptor. We will identify important or rate-limiting proteins (including Niemann-Pick C1-like 1 (NPC1L1), microsomal TG transfer protein (MTP), acyl-coenzyme A/cholesterol acyltransferase (ACAT), Acyl-CoA:diacylglycerol acyltransferases 2 (DGAT2), proprotein convertase subtilisin kexin type 9 (PCSK9)), and nuclear receptors (farnesoid X receptor (FXR), thyroid hormone receptor (TR)) that constitute interesting therapeutic targets. Numerous compounds already in use modulate these pathways, such as phytosterols, ezetimibe, bile acids sequestrants, niacin, and fibrates. Many pathways can be considered to lower LDLC, but the road has been paved with disappointments and difficulties. With new targets identified and diversification of the drugs, a new era for better LDLC management is plausible.

Impact of circulating esterified eicosanoids and other oxylipins on endothelial function

Eicosanoids, including epoxyeicosatrienoic acids, hydroxyeicosatetraenoic acids, and other oxylipins derived from polyunsaturated fatty acids, have emerging roles in endothelial inflammation and subsequent atherosclerosis. Unlike eicosanoids in the prostanoid series, they are known to be esterified in cell lipids such as phospholipids and triglycerides; however, our understanding of these reservoirs is in its infancy. This review focuses on recent work identifying circulating oxylipins, primarily esterified with lipoprotein lipids, and their effects on markers of endothelial dysfunction. These oxylipins are known to be released by at least one lipase (lipoprotein lipase) and to mediate increased expression of inflammatory markers in endothelial cells, which coincides with the known roles of lipoproteins in endothelial dysfunction. The implications of the lipolytic release of lipoprotein-bound oxylipins for the inflammatory response, challenges to analysis of this oxylipin compartment, and the potential importance of non-arachidonate-derived oxylipins are discussed.

Cholesteryl ester transfer protein and mortality in patients undergoing coronary angiography: the Ludwigshafen Risk and Cardiovascular Health study

BACKGROUND

The role of cholesteryl ester transfer protein (CETP) in the development of atherosclerosis is still open to debate. In the Investigation of Lipid Level Management to Understand its Impact in Atherosclerotic Events (ILLUMINATE) trial, inhibition of CETP in patients with high cardiovascular risk was associated with increased high-density lipoprotein levels but increased risk of cardiovascular morbidity and mortality. In this report, we present a prospective observational study of patients referred to coronary angiography in which CETP was examined in relation to morbidity and mortality.

METHODS AND RESULTS

CETP concentration was determined in 3256 participants of the Ludwigshafen Risk and Cardiovascular Health (LURIC) study who were referred to coronary angiography at baseline between 1997 and 2000. Median follow-up time was 7.75 years. Primary and secondary end points were cardiovascular and all-cause mortality, respectively. CETP levels were higher in women and lower in smokers, in diabetic patients, and in patients with unstable coronary artery disease, respectively. In addition, CETP levels were correlated negatively with high-sensitivity C-reactive protein and interleukin-6. After adjustment for age, sex, medication, coronary artery disease status, cardiovascular risk factors, and diabetes mellitus, the hazard ratio for death in the lowest CETP quartile was 1.33 (1.07 to 1.65; P=0.011) compared with patients in the highest CETP quartile. Corresponding hazard ratios for death in the second and third CETP quartiles were 1.17 (0.92 to 1.48; P=0.19) and 1.10 (0.86 to 1.39; P=0.46), respectively.

CONCLUSIONS

We interpret our data to suggest that low endogenous CETP plasma levels per se are associated with increased cardiovascular and all-cause mortality, challenging the rationale of pharmacological CETP inhibition.

Standardization of a method to evaluate the antioxidant capacity of high-density lipoproteins

BACKGROUND

A method to evaluate the antioxidant capacity of high-density lipoprotein (HDL) was developed and standardized.

METHODS

This method measure conjugated diene (CD) formation and electrophoretic mobility of low-density lipoprotein (LDL) in agarose gels in the presence and absence of HDL. HDL was isolated from 1 mL of plasma within 24 hours and oxidation assays were performed within 6 hours. Oxidation was induced by adding CuSO4. The lag phase increase in CD kinetics and the inhibition of electrophoretic mobility were defined as the HDL antioxidant capacity.

RESULTS

The optimal conditions for the CD assay were 2.5 μM CuSO4, LDL at 0.1 g apoB/L, HDL at 0.1 g apoA-I/L, at 37°C and for 3h 50 min. Agarose electrophoresis at 100 V, at 4°C for 50 min was then performed immediately. CD formation variability was 21.1% for inter-assay CV and 12.7% for intra-assay CV. Electrophoretic mobility was 26.5% for inter-assay CV and 2.4% for intra-assay CV. Correlation analysis showed a significant association between the antioxidant capacity of HDL and its neutral/polar lipid ratio.

CONCLUSIONS

The method herein described measures of the HDL antioxidant capacity in a reproducible and rapid manner that can be applied to a relatively high number of samples.

An apoA-I mimetic peptide facilitates off-loading cholesterol from HDL to liver cells through scavenger receptor BI

Apolipoprotein A-I (apoA-I) mimetic peptides have been pursued as new therapeutic agents for the treatment of atherosclerosis, yet their precise mechanism responsible for atheroprotection remains unclear. Like apoA-I itself, most of these peptides are capable of stimulating cholesterol efflux from macrophages or foam cells, and some of them stimulate lecithin cholesterol acyltransferase (LCAT) activity in the reverse cholesterol transport (RCT) pathway. However, the ability of mimetic peptides to deliver cholesterol into hepatocytes (off-loading), the last step of the RCT pathway, has not been demonstrated. In this study, we compared a mimetic peptide D-4F to purified apoA-I, to address the role that mimetics play during the off-loading process. Both D-4F and apoA-I formed spherical nano-particles when reconstituted with cholesteryl ester and phospholipids. Compared to apoA-I, D-4F particles were 20 times more efficient in off-loading cholesterol to HepG2 hepatocytes with an apparent K_t (transport) of 0.74 μg/mL. Furthermore, D-4F also facilitated cholesteryl ester offloading from HDL particles into HepG2 cells when it was pre-incubated with these HDL particles. Using an inducible HEK293 cell line, we demonstrated that these nano-particles were able to be taken up through SR-BI, a HDL selective receptor. Cholesterol uptake by HepG2 cells was completely blocked by a neutralizing monoclonal antibody against SR-BI, demonstrating that D-4F particles, similar to HDL, specifically off-loaded cholesterol through SR-BI. Overall our data provides evidence that D-4F is capable of mimicking apoA-I to form HDL-like particles, and off-loads cholesterol for catabolism and excretion, thus completing RCT.

Impaired serum capacity to induce cholesterol efflux is associated with endothelial dysfunction in type 2 diabetes mellitus

OBJECTIVE

Reverse cholesterol transport (RCT) plays a protective role against atherosclerosis and cholesterol efflux from cells is an early step in the RCT pathway. We investigated whether the capacity of serum to induce cholesterol efflux was associated with endothelial dysfunction in type 2 diabetes.

METHODS

Endothelium-dependent and -independent vasodilation of the brachial artery was measured by high-resolution vascular ultrasound and serum cholesterol efflux capacity was determined by measuring the transfer of [3H]cholesterol from Fu5AH cells to serum in 137 patients with type 2 diabetes and 75 controls.

RESULTS

Serum cholesterol efflux capacity was lower in diabetic patients than in the controls (13.6+/-2.5% vs. 14.6+/-3.4%, respectively, p=0.02), and both endothelium-dependent vasodilation (4.9+/-2.2% vs. 8.8+/-4.1%, respectively, p<0.01) and endothelium-independent vasodilation were impaired (13.4+/-4.3% vs. 16.3+/-5.5%. respectively, p<0.01). Endothelium-dependent vasodilation correlated with serum cholesterol efflux capacity (r=0.26, p=0.003) in diabetic patients and controls (r=0.24, p=0.037). On general linear model analysis, the presence of diabetes, brachial artery diameter and serum cholesterol efflux capacity were significant independent determinants of endothelium-dependent vasodilation.

CONCLUSION

Impaired serum cholesterol efflux capacity was associated with endothelial dysfunction independent of other cardiovascular risk factors.

Proteomics of apolipoproteins and associated proteins from plasma high-density lipoproteins

Proteomics studies have extended the list of identified apolipoproteins and associated proteins present in HDL and its subclasses. These proteins appear to cluster around specific functions related to lipid metabolism, inflammation, the immune system, hormone-binding, hemostasis, and antioxidant properties. Small studies suggest that there are substantial differences between the HDL proteome from cardiovascular disease patients and that from controls. Furthermore, dyslipidemia therapy shifts the HDL proteome from patients toward the profile observed in healthy controls. In addition, the proteome of HDL and LDL from patients with insulin resistance and peripheral atherosclerosis show significant differences with that of matched healthy controls. The proteome of HDL and LDL density subclasses have apolipoproteins and associated proteins profiles that suggest subclass-specific functions. However, proteomics studies of lipoproteins are few and small and should be interpreted with caution. Nonetheless rapid technical progress in proteomic platforms suggest that soon analysis time will be reduced and precise measurement of identified proteins will be possible. This, combined with controlled purification steps of HDL and its subclasses should provide further information about proteins involved in the particles postulated spectrum of functions, including those believed to be atheroprotective.

HDL-Cholesterol, Blood Pressure, and Asymmetric Dimethylarginine Are Significantly Associated With Arterial Wall Thickness in Children

Objective— Atherosclerosis is found at autopsy in the arteries of adolescents and young adults. Arterial wall thickening may be assessed in vivo by ultrasound measurement of the carotid intima media thickness (CIMT), a marker of subclinical atherosclerosis. As the determinants of arterial wall thickness in childhood are unknown, we assessed the influence of cardiovascular risk factors on CIMT in 8-year-old children.

Methods and Results— A community-based sample of 405 children (age 8.0±0.1 years, 49% girls) had anthropometry, family history, blood pressure (BP), and CIMT measured. A blood sample was collected for HDL and non-HDL cholesterol, apolipoproteins A1 and B, high-sensitivity C-reactive protein, bilirubin, and asymmetric dimethylarginine (ADMA, an endogenous nitric oxide inhibitor). CIMT was significantly associated with systolic BP ( r =0.17, P <0.001), diastolic BP ( r =0.10, P =0.04), HDL ( r =−0.13, P =0.02), and ADMA ( r =0.18, P =0.001). CIMT was significantly higher in children with premature parental CHD (0.63±0.07 versus 0.59±0.06 mm, P =0.03). On multivariate analysis, HDL (β coefficient=−0.02, P =0.04), ADMA (β coefficient=0.05, P <0.001), and systolic BP (β coefficient=0.001, P =0.003) were significantly and independently associated with CIMT.

Conclusions— Lower HDL-cholesterol, higher levels of ADMA, and systolic BP are significantly associated with greater arterial wall thickness in early childhood.

Vasculoprotective Effects of Apolipoprotein Mimetic Peptides: An Evolving Paradigm In Hdl Therapy (Vascular Disease Prevention, In Press.)

Anti-atherogenic effects of high density lipoprotein (HDL) and its major protein component apolipoprotein A-I (apoA-I) are principally thought to be due to their ability to mediate reverse cholesterol transport. These agents also possess anti-oxidant properties that prevent the oxidative modification of low density lipoprotein (LDL) and anti-inflammatory properties that include inhibition of endothelial cell adhesion molecule expression. Results of the Framingham study revealed that a reduction in HDL levels is an independent risk factor for coronary artery disease (CAD). Accordingly, there has been considerable interest in developing new therapies that specifically elevate HDL cholesterol. However, recent evidence suggests that increasing circulating HDL cholesterol levels alone is not sufficient as a mode of HDL therapy. Rather, therapeutic approaches that increase the functional properties of HDL may be superior to simply raising the levels of HDL per se. Our laboratory has pioneered the development of synthetic, apolipoprotein mimetic peptides which are structurally and functionally similar to apoA-I but possess unique structural homology to the lipid-associating domains of apoA-I. The apoA-I mimetic peptide 4F inhibits atherogenic lesion formation in murine models of atherosclerosis. This effect is related to the ability of 4F to induce the formation of pre-β HDL particles that are enriched in apoA-I and paraoxonase. 4F also possesses anti-inflammatory and anti-oxidant properties that are independent of its effect on HDL quality per se. Recent studies suggest that 4F stimulates the expression of the antioxidant enzymes heme oxygenase and superoxide dismutase and inhibits superoxide anion formation in blood vessels of diabetic, hypercholesterolemic and sickle cell disease mice. The goal of this review is to discuss HDL-dependent and -independent mechanisms by which apoA-I mimetic peptides reduce vascular injury in experimental animal models.

Apolipoprotein A-I mimetic peptides

Recent publications reveal the mechanism of action of apolipoprotein A-I (apoA-I) mimetic peptides to be the remarkable binding affinity that oxidized lipids have for these peptides compared with apoA-I. There was no difference in the binding affinity of oxidized lipids or in peptide efficacy in reducing inflammation and atherosclerosis in rabbits injected with peptides synthesized from all D- or all L-amino acids. The apoA-I mimetic peptide 4F increased the formation of pre-beta high-density lipoprotein, increased cholesterol efflux, and reduced lipoprotein oxidation in vitro; it increased antioxidants and vascular repair in type 1 diabetic rats; it improved vasodilation, oxidative stress, myocardial inflammation, and angiogenic potential in a mouse model of scleroderma; it reduced renal inflammation in low-density lipoprotein receptor-null mice fed a Western diet; it reduced arthritis in a rat model; it reduced adiposity, increased adiponectin levels, and improved insulin sensitivity in obese mice; and it improved high-density lipoprotein inflammatory properties in humans with coronary heart disease.

Templated spherical high density lipoprotein nanoparticles

We report the synthesis of high density lipoprotein (HDL) biomimetic nanoparticles capable of binding cholesterol. These structures use a gold nanoparticle core to template the assembly of a mixed phospholipid layer and the adsorption of apolipoprotein A-I. These synthesized structures have the general size and surface composition of natural HDL and, importantly, bind free cholesterol (K(d) = 4 nM). The determination of the K(d) for these particles, with respect to cholesterol complexation, provides a key starting and comparison point for measuring and evaluating the properties of subsequently developed synthetic versions of HDL.

In vivo macrophage-specific RCT and antioxidant and antiinflammatory HDL activity measurements: New tools for predicting HDL atheroprotection

The beneficial therapeutic effects of raising HDL cholesterol are proving difficult to confirm in humans. The evaluation of antiatherogenic functions of HDL is an important area of research which includes the role of HDL in reverse cholesterol transport (RCT), especially macrophage-specific RCT, and its antioxidant and antiinflammatory roles. The antioxidant and antiinflammatory functions of HDL can be assessed using cell-free and cell-based assays. Also, a new approach was developed to measure RCT from labeled-cholesterol macrophages to liver and feces of mice. Studies in genetically engineered animals indicate that these major HDL antiatherogenic functions are better predictors of atherosclerosis susceptibility than HDL cholesterol or total RCT. Thus, functional testing of the antiatherogenic functions of HDL in experimental animal models may facilitate the development of new strategies for the prevention and treatment of atherosclerosis.

Carotid artery stiffness, high-density lipoprotein cholesterol and inflammation in men with pre-hypertension

Low circulating levels of high-density lipoprotein cholesterol (HDL-C) are associated with increased risk for cardiovascular events. HDL-C has a variety of poorly understood atheroprotective effects, including altering lipid metabolism and reducing inflammation. Increased arterial stiffness is an important predictor of subsequent cardiovascular risk. Therefore, in this study, we sought to determine whether HDL-C levels are associated with carotid arterial stiffness. In addition, we examined potential correlates of this association, such as inflammatory factors, cardiorespiratory fitness and body fat percentage. Carotid artery beta-stiffness was measured by ultrasound in 47 (23 years old) healthy pre-hypertensive men. Low HDL-C was defined as <1.0 mmol l(-1). Body fat was measured by air displacement plethysmography. Cardiorespiratory fitness was measured using a maximal exercise test, with metabolic gas analysis and inflammatory markers consisting of C-reactive protein (CRP), white blood cell (WBC) count and absolute neutrophil count. Men with a low HDL-C had significantly higher carotid artery stiffness, CRP, WBC count, neutrophil count, body fat, fasting glucose and lower cardiorespiratory fitness (P<0.05). Co-varying for cardiorespiratory fitness, % body fat and glucose had no effect on group differences in carotid artery stiffness. Co-varying for inflammatory markers resulted in groups having similar carotid artery stiffness. Pre-hypertensive men with low HDL-C have a higher carotid artery stiffness when compared with those with higher HDL-C. The detrimental effects of low HDL-C on large artery stiffness in pre-hypertensive men may be mediated by inflammation and not by cardiorespiratory fitness or body fat levels.

Association between change in high density lipoprotein cholesterol and cardiovascular disease morbidity and mortality: systematic review and meta-regression analysis

OBJECTIVE

To investigate the association between treatment induced change in high density lipoprotein cholesterol and total death, coronary heart disease death, and coronary heart disease events (coronary heart disease death and non-fatal myocardial infarction) adjusted for changes in low density lipoprotein cholesterol and drug class in randomised trials of lipid modifying interventions.

DESIGN

Systematic review and meta-regression analysis of randomised controlled trials.

DATA SOURCES

Medline, Embase, Central, CINAHL, and AMED to October 2006 supplemented by contact with experts in the field.

STUDY SELECTION

In teams of two, reviewers independently determined eligibility of randomised trials that tested lipid modifying interventions to reduce cardiovascular risk, reported high density lipoprotein cholesterol and mortality or myocardial infarctions separately for treatment groups, and treated and followed participants for at least six months.

DATA EXTRACTION AND SYNTHESIS

Using standardised, pre-piloted forms, reviewers independently extracted relevant information from each article. The change in lipid concentrations for each trial and the weighted risk ratios for clinical outcomes were calculated.

RESULTS

The meta-regression analysis included 108 randomised trials involving 299 310 participants at risk of cardiovascular events. All analyses that adjusted for changes in low density lipoprotein cholesterol showed no association between treatment induced change in high density lipoprotein cholesterol and risk ratios for coronary heart disease deaths, coronary heart disease events, or total deaths. With all trials included, change in high density lipoprotein cholesterol explained almost no variability (<1%) in any of the outcomes. The change in the quotient of low density lipoprotein cholesterol and high density lipoprotein cholesterol did not explain more of the variability in any of the outcomes than did the change in low density lipoprotein cholesterol alone. For a 10 mg/dl (0.26 mmol/l) reduction in low density lipoprotein cholesterol, the relative risk reduction was 7.2% (95% confidence interval 3.1% to 11%; P=0.001) for coronary heart disease deaths, 7.1% (4.5% to 9.8%; P<0.001) for coronary heart disease events, and 4.4% (1.6% to 7.2%; P=0.002) for total deaths, when adjusted for change in high density lipoprotein cholesterol and drug class.

CONCLUSIONS

Available data suggest that simply increasing the amount of circulating high density lipoprotein cholesterol does not reduce the risk of coronary heart disease events, coronary heart disease deaths, or total deaths. The results support reduction in low density lipoprotein cholesterol as the primary goal for lipid modifying interventions.

High-density lipoprotein: why all the fuss?

The current search for new treatments to combat coronary heart disease (CHD) is centred on increasing HDL-cholesterol. The failure of the CETP inhibitor torcetrapib may force a rethink. This perspective briefly reviews the antiatherosclerotic properties of HDL and ways HDL-cholesterol concentration can be raised, but argues – in light of the fact that HDL-cholesterol concentration does not reflect the protective properties of HDL particles – that this approach is flawed and a different approach, targeting know antiatherosclerotic components of HDL, is required.

Is high HDL cholesterol always good?

Because of the obvious negative relation between high-density lipoprotein (HDL) cholesterol and cardiovascular disease and the substantial residual risk of this disease even during treatment with high-dose statin there has been an urgent need to investigate the possible therapeutic benefit of increasing HDL. Even if treatment with nicotinic acid with its marked HDL-increasing effect has been encouraging, there is no evidence so far that specific increase of HDL cholesterol results in less cardiovascular disease. Treatment with the cholesterol ester transfer protein (CETP) inhibitor and HDL-increasing drug torcetrapib resulted in increased risk of cardiovascular disease. These negative results were followed by a lively discussion regarding the possible benefit of HDL-increasing treatment in general and CETP inhibition in particular. Suggested possible causes for the negative outcome by torcetrapib treatment are off-target non-CETP-related effect of this particular inhibitor, inability of very high blood HDL cholesterol levels to protect, induction of dysfunctional HDL, and direct atherogenic effect of CETP inhibition. It is concluded that still today little is known about the effect of specific therapeutic elevation of HDL cholesterol, particularly so through CETP inhibition on cardiovascular risk. New interventional studies on this therapeutic principle are welcomed and under way.

Animal and cellular models for hypolipidemic drugs

BACKGROUND

The development of effective and safe lipid-lowering agents should set out from and rely on robust preclinical investigation.

OBJECTIVE

To accomplish this aim, the selection of proper cellular and animal models is crucial.

RESULTS

Because lipid-lowering agents are ultimately supposed to reduce the atherosclerotic burden in the arterial wall, they need to tackle directly or indirectly the multifactorial nature of atherosclerotic disease. Hence, these drugs may essentially prevent triglyceride-rich lipoprotein assembly or enhance low-density lipoprotein (LDL) clearance through the LDL or related receptors in the liver. Established animal models such as the apolipoprotein E- and the LDL-receptor knockout mice are widely used to test drug actions on these pathways. A different approach is testing the ability of candidate drugs to increase plasma high-density lipoprotein (HDL) levels. More recently, the focus has shifted to drugs enhancing HDL function rather than just plasma HDL levels. This in turn requires in vitro and particularly in vivo models of reverse cholesterol transport, which have become available by now.

CONCLUSION

A positive outcome of preclinical studies is necessary but not sufficient for an investigational new drug to be eventually approved for clinical use.

The role of reverse cholesterol transport in animals and humans and relationship to atherosclerosis

Reverse cholesterol transport (RCT) is a term used to describe the efflux of excess cellular cholesterol from peripheral tissues and its return to the liver for excretion in the bile and ultimately the feces. It is believed to be a critical mechanism by which HDL exert a protective effect on the development of atherosclerosis. In this paradigm, cholesterol is effluxed from arterial macrophages to extracellular HDL-based acceptors through the action of transporters such as ABCA1 and ABCG1. After efflux to HDL, cholesterol may be esterified in the plasma by the enzyme lecithin:cholesterol acyltransferase and is ultimately transported from HDL to the liver, either directly via the scavenger receptor BI or after transfer to apolipoprotein B-containing lipoproteins by the cholesteryl ester transfer protein. Methods for assessing the integrated rate of macrophage RCT in animals have provided insights into the molecular regulation of the process and suggest that the dynamic rate of macrophage RCT is more strongly associated with atherosclerosis than the steady-state plasma concentration of HDL cholesterol. Promotion of macrophage RCT is a potential therapeutic approach to preventing or regressing atherosclerotic vascular disease, but robust measures of RCT in humans will be needed in order to confidently advance RCT-promoting therapies in clinical development.

Comparison of treatment of severe high-density lipoprotein cholesterol deficiency in men with daily atorvastatin (20 mg) versus fenofibrate (200 mg) versus extended-release niacin (2 g)

To determine whether available lipid-modifying medication can increase high-density lipoprotein (HDL) cholesterol in well-defined genetic or familial HDL-deficiency states, we studied 19 men with HDL deficiency (HDL cholesterol <5th percentile for age and gender) 55 +/- 10 years of age. Concomitant risk factors included diabetes (n = 3) and hypertension (n = 7) and 8 patients had coronary artery disease. Molecular analysis revealed that 4 patients had a mutation in the ABCA1 gene. Patients were assigned to sequentially receive atorvastatin 20 mg/day, fenofibrate 200 mg/day, and extended-release niacin 2 g/day for 8 weeks, with a 4-week washout period between each treatment. Patients in whom a statin was required, according to current treatment guidelines, were kept on atorvastatin throughout the study. Baseline HDL cholesterol level was 0.63 +/- 0.12 mmol/L (24 +/- 5 mg/dl), triglycerides 2.01 +/- 0.98 mmol/L (180 +/- 86 mg/dl), and low-density lipoprotein (LDL) cholesterol 2.29 +/- 0.95 mmol/L (94 +/- 39 mg/dl). Mean percent changes in HDL cholesterol on atorvastatin, fenofibrate, and niacin were -6% (p = NS), +6% (p = NS), and +22% (p <0.05), respectively. Furthermore, niacin significantly increased the large alpha-1 apolipoprotein A-I-containing HDL subspecies (12 to 17 nm). In conclusion, niacin was the only effective drug to increase HDL cholesterol. The absolute increase in HDL cholesterol, approximately 0.10 mmol/L (3.9 mg/dl), is of uncertain clinical significance. Biomarkers of HDL-mediated cellular cholesterol efflux were not changed by niacin therapy. Atorvastatin or fenofibrate had little effect on HDL cholesterol; atorvastatin decreased the total cholesterol/HDL cholesterol ratio by 26%. Fenofibrate did not change HDL cholesterol levels and caused an increase in LDL cholesterol. Aggressive LDL cholesterol lowering may be the strategy of choice in such patients.

Identification of three loci affecting HDL-cholesterol levels in a screen for chemically induced recessive mutations in mice

We conducted a genome-wide screen using the mutagen N-ethyl-N-nitrosourea to identify recessive mutations in genes that lead to altered lipid traits in mice. We screened 7,546 G3 mice that were of mixed C57BL/6J (B6) x C3.SW-H2(b)/SnJ (C3) genomes and identified three pedigrees with differences in plasma HDL-cholesterol. Genome scan analyses mapped three distinct loci to chromosomes 3, 4, and 7. An S1748L missense mutation was identified in ABCA1 in one pedigree with undetectable levels of HDL-cholesterol and resulted in reduced protein levels. This phenotype was completely penetrant, semi-dominant, and cosegregated with high plasma triglycerides. Mice in a second pedigree had very high levels of plasma total cholesterol and HDL-cholesterol (up to 800 mg/dl total cholesterol). Despite a high degree of phenotype lability and reduced penetrance, an I68N missense mutation was identified in the transcription factor CCAAT/enhancer binding protein alpha (C/EBPalpha). Finally, a second high HDL-cholesterol pedigree of mice, again with a highly labile phenotype and reduced penetrance, was mapped to a 7 Mb locus on chromosome 3. These results illustrate the use of a hybrid background for simultaneous screening and mapping of mutagenized pedigrees of mice and identification of three novel alleles of HDL-cholesterol phenotypes.