CETP inhibitor torcetrapib promotes reverse cholesterol transport in obese insulin-resistant CETP-ApoB100 transgenic mice

Altered HDL metabolism in metabolic disorders: insights into the therapeutic potential of HDL

Metabolic disorders are associated with an increased risk of cardiovascular disease (CVD), and are commonly characterized by a low plasma level of high-density lipoprotein cholesterol (HDL-C). Although cholesterol lowering medications reduce CVD risk in these patients, they often remain at increased risk of CVD. Therapeutic strategies that raise HDL-C levels and improve HDL function are a potential treatment option for reducing residual CVD risk in these individuals. Over the past decade, understanding of the metabolism and cardioprotective functions of HDLs has improved, with preclinical and clinical studies both indicating that the ability of HDLs to mediate reverse cholesterol transport, inhibit inflammation and reduce oxidation is impaired in metabolic disorders. These cardioprotective effects of HDLs are supported by the outcomes of epidemiological, cell and animal studies, but have not been confirmed in several recent clinical outcome trials of HDL-raising agents. Recent studies suggest that HDL function may be clinically more important than plasma levels of HDL-C. However, at least some of the cardioprotective functions of HDLs are lost in acute coronary syndrome and stable coronary artery disease patients. HDL dysfunction is also associated with metabolic abnormalities. This review is concerned with the impact of metabolic abnormalities, including dyslipidemia, obesity and Type 2 diabetes, on the metabolism and cardioprotective functions of HDLs.

Bile Acid and Cholesterol Metabolism in Atherosclerotic Cardiovascular Disease and Therapy

Dysregulation of lipid metabolism is a major factor contributing to atherosclerotic cardiovascular disease (ACVD), which is the number one cause of death in western countries. The liver plays a central role in maintaining whole body cholesterol homeostasis via catabolism of cholesterol to bile acids, as well as biliary cholesterol excretion. The liver synthesizes lipoproteins that transport dietary cholesterol and fats to muscle and adipose tissue, directs reverse cholesterol transport of excess cholesterol from extrahepatic tissues and macrophages to the liver to convert to bile acids, and thus, protects against metabolism-related nonalcoholic fatty liver disease (NAFLD) and ACVD. Liver fibrosis/nonalcoholic steatohepatitis increases the risk and prevalence of cardiovascular disease morbidity and mortality. Bile acids are signaling molecules and metabolic regulators that activate farnesoid X receptor and G protein-coupled bile acid receptor-1 to regulate lipid, glucose, and energy metabolism. The bidirectional regulation of bile acids and the gut microbiota determine the rate of bile acid synthesis, the bile acid pool size, and the composition of the circulating bile acid pool. The liver-intestine-heart axis regulates lipid metabolism, inflammation, and the pathogenesis of metabolic diseases such as ACVD, NAFLD, diabetes, and obesity. This review focuses on the roles of liver-to-intestine, liver-to-heart and intestine-to-heart axes in cholesterol, lipoprotein, and bile acid metabolism; signaling in heart health and ACVD; and drug therapies for atherosclerosis.

Therapy with cholesteryl ester transfer protein (CETP) inhibitors and diabetes risk

BACKGROUND

Cholesteryl ester transfer protein (CETP) inhibitors are a class of drugs that targets the CETP enzyme to significantly increase serum high-density lipoprotein cholesterol (HDL-C) and decrease low-density lipoprotein cholesterol (LDL-C) levels. As HDL-C has potential antidiabetic properties, and the beneficial effects of CETP drugs on glucose homoeostasis have not been sufficiently studied, the aims of this study were: (1) to evaluate the effect of CETP inhibitors on the incidence of diabetes; and (2) to assess the association between CETP inhibitor-induced changes in HDL-C levels and incidence of diabetes.

METHODS

A meta-analysis was performed of randomized controlled clinical trials of CETP inhibitor therapy, either alone or combined with other lipid-lowering drugs, reporting data from new cases of diabetes with a minimum of 6 months of follow-up, after searching the PubMed/MEDLINE, Embase and Cochrane Controlled Trials databases. A fixed-effects meta-regression model was then applied.

RESULTS

Four eligible trials of CETP inhibitors, involving a total of 73,479 patients, were considered for the analyses, including 960 newly diagnosed cases of diabetes in the CTEP inhibitor group vs 1086 in the placebo group. CETP inhibitor therapy was associated with a significant 12% reduction in incidence of diabetes (OR: 0.88, 95% CI: 0.81-0.96; P=0.005). Assessment of the relationship between on-treatment HDL-C and the effect of CETP inhibitors showed a statistically non-significant trend (Z=-1.13, P=0.26).

CONCLUSION

CETP inhibitors reduced the incidence of diabetes. The improvement in glucose metabolism may have been related, at least in part, to the increase in HDL-C concentration.

The controversy over the use of cholesteryl ester transfer protein inhibitors: is there some light at the end of the tunnel?

BACKGROUND

According to epidemiological studies, there is no clear relationship between the plasma cholesteryl ester transfer protein (CETP) concentration and the development of atherosclerosis in human populations. Although some studies suggest that increased CETP activity relates to undesirable profiles of plasma lipoproteins, promoting an anti-atherogenic plasma lipoprotein profile by drugs that inhibit CETP has not succeeded in preventing atherosclerosis in humans.

MATERIALS AND METHODS

This review describes 28 investigations in human populations dealing with plasma CETP, 11 in mice that express human CETP and seven in animals (six in rabbits and one in mice) in which plasma CETP activity was inhibited by drugs.

RESULTS

Present review shows that models in mice expressing human CETP are not illuminating because they report increase as well reduction of atherosclerosis. However, investigations in rabbits and mice that develop severe hypercholesterolaemia clearly indicate that impairment of the plasma CETP activity elicits protection against the development of atherosclerosis; in all of these experiments are attained substantial reductions of the atherogenic lipoproteins, namely, plasma apoB containing lipoproteins.

CONCLUSION

These models are strong indicators that the benefit in preventing atherosclerosis should be earned in cases of hyperlipidemia by CETP inhibitors.

HDL functionality in reverse cholesterol transport--Challenges in translating data emerging from mouse models to human disease

Whereas LDL-derived cholesterol accumulates in atherosclerotic lesions, HDL particles are thought to facilitate removal of cholesterol from the lesions back to the liver thereby promoting its fecal excretion from the body. Because generation of cholesterol-loaded macrophages is inherent to atherogenesis, studies on the mechanisms stimulating the release of cholesterol from these cells and its ultimate excretion into feces are crucial to learn how to prevent lesion development or even induce lesion regression. Modulation of this key anti-atherogenic pathway, known as the macrophage-specific reverse cholesterol transport, has been extensively studied in several mouse models with the ultimate aim of applying the emerging knowledge to humans. The present review provides a detailed comparison and critical analysis of the various steps of reverse cholesterol transport in mouse and man. We attempt to translate this in vivo complex scenario into practical concepts, which could serve as valuable tools when developing novel HDL-targeted therapies.

A selective peroxisome proliferator-activated receptor δ agonist PYPEP suppresses atherosclerosis in association with improvement of the serum lipoprotein profiles in human apolipoprotein B100 and cholesteryl ester transfer protein double transgenic mice

OBJECTIVE

Although peroxisome proliferator-activated receptor (PPAR) δ agonists have been shown to improve the serum lipoprotein profiles in humans, the impact of the changes in these lipoprotein profiles on atherosclerosis remains to be elucidated. The aim of this study was to investigate the relationship between the selective PPARδ agonist-induced alterations of serum lipoprotein profiles and the development of atherosclerosis in human apolipoprotein B100 and cholesterol ester transfer protein double transgenic (hApoB100/hCETP-dTg) mice with human-like hypercholesterolemic dyslipidemia.

METHODS

hApoB100/hCETP-dTg mice fed an atherogenic diet received a novel PPARδ agonist (PYPEP) or vehicle for 18 weeks, followed by evaluation of atherosclerosis. Serum samples were collected during the treatment period at least at 3-week intervals to determine the lipoprotein levels and the levels of an inflammatory marker, macrophage chemotactic protein-1 (MCP-1), and to analyze the lipoprotein profile by fast protein liquid chromatography. The cholesterol efflux capacity of high-density lipoprotein (HDL) was examined using [(3)H]-cholesterol labeled macrophages.

RESULTS

Compared with vehicle treatment, PYPEP treatment caused increases in the serum levels of HDL cholesterol and apolipoprotein A-I (ApoA-I), as well as reductions in the serum non-HDL cholesterol and MCP-1 levels. The HDL fraction from the PYPEP-treated group maintained its cholesterol efflux capacity and showed an increased population of smaller HDL particles. PYPEP substantially suppressed atherosclerotic lesion progression, and the lesion areas had significant correlations with non-HDL cholesterol, HDL cholesterol, ApoA-I and MCP-1 by Pearson's correlation analysis. A multiple regression analysis revealed that non-HDL cholesterol and ApoA-I were significantly associated with the atherosclerotic lesion area.

CONCLUSION

A novel PPARδ agonist, PYPEP, suppressed atherosclerotic lesion progression by improving the serum lipoprotein profiles, including increased levels of ApoA-I and functional HDL particles, as well as a reduced non-HDL cholesterol level, in hApoB100/hCETP-dTg mice with human-like hypercholesterolemic dyslipidemia.

Cholesterol efflux and reverse cholesterol transport

Both alterations of lipid/lipoprotein metabolism and inflammatory events contribute to the formation of the atherosclerotic plaque, characterized by the accumulation of abnormal amounts of cholesterol and macrophages in the artery wall. Reverse cholesterol transport (RCT) may counteract the pathogenic events leading to the formation and development of atheroma, by promoting the high-density lipoprotein (HDL)-mediated removal of cholesterol from the artery wall. Recent in vivo studies established the inverse relationship between RCT efficiency and atherosclerotic cardiovascular diseases (CVD), thus suggesting that the promotion of this process may represent a novel strategy to reduce atherosclerotic plaque burden and subsequent cardiovascular events. HDL plays a primary role in all stages of RCT: (1) cholesterol efflux, where these lipoproteins remove excess cholesterol from cells; (2) lipoprotein remodeling, where HDL undergo structural modifications with possible impact on their function; and (3) hepatic lipid uptake, where HDL releases cholesterol to the liver, for the final excretion into bile and feces. Although the inverse association between HDL plasma levels and CVD risk has been postulated for years, recently this concept has been challenged by studies reporting that HDL antiatherogenic functions may be independent of their plasma levels. Therefore, assessment of HDL function, evaluated as the capacity to promote cell cholesterol efflux may offer a better prediction of CVD than HDL levels alone. Consistent with this idea, it has been recently demonstrated that the evaluation of serum cholesterol efflux capacity (CEC) is a predictor of atherosclerosis extent in humans.

Efficacy, safety, tolerability, and pharmacokinetic profile of evacetrapib administered as monotherapy or in combination with atorvastatin in Japanese patients with dyslipidemia

The cholesteryl ester transfer protein (CETP) inhibitor evacetrapib has been previously shown to increase high-density lipoprotein cholesterol (HDL-C) and decrease low-density lipoprotein cholesterol (LDL-C) levels, as monotherapy or in combination with statins. In this study, 165 Japanese patients with elevated LDL-C or low HDL-C levels were randomly assigned to receive placebo, evacetrapib monotherapy 30 mg, 100 mg, or 500 mg, atorvastatin 10 mg, or evacetrapib 100 mg in combination with atorvastatin 10 mg. After 12 weeks, evacetrapib monotherapy increased HDL-C levels by 74%, 115%, and 136% and decreased LDL-C levels by 15%, 23%, and 22% and CETP activity by 50%, 83%, and 95% (for the 30-mg, 100-mg, and 500-mg dose groups, respectively) versus placebo. In combination with atorvastatin 10 mg, evacetrapib 100 mg increased HDL-C levels by 103% and decreased LDL-C levels by 15% and CETP activity by 68% versus atorvastatin alone. After a 4- to 6-week washout, HDL-C, LDL-C, and CETP mass and activity returned to baseline levels in the evacetrapib-treated groups, and most patients had evacetrapib concentrations below the quantitation limit. Evacetrapib monotherapy or in combination with atorvastatin was not likely to be associated with any significant change in blood pressure and did not have any adverse effects on mineralocorticoid or glucocorticoid measures. Notably, plasma evacetrapib concentrations were mostly undetectable, and all pharmacodynamic biomarkers (HDL-C and LDL-C levels and CETP mass and activity) returned to baseline after a 4- to 6-week washout. In conclusion, evacetrapib as monotherapy or in combination with atorvastatin effectively decreased CETP activity and LDL-C levels and increased HDL-C levels after 12 weeks in Japanese patients with dyslipidemia.

[A history and review of cholesterol ester transfer protein inhibitors and their contribution to the understanding of the physiology and pathophysiology of high density lipoprotein]

There is irrefutable evidence that statins reduce the risk of cardiovascular events in a magnitude proportional to the intensity of the decrease in cholesterol transport by the low density lipoproteins. Despite this great advance there is still a residual risk of cardiovascular events. For this reason, an increase in the levels of high density lipoprotein is considered in order to boost the main action of this lipoprotein, which is reverse cholesterol transport. Distinct classes of evidence (epidemiological, genetic, and pathophysiological) show that the inhibition and/or modulation of cholesterol ester transfer protein increases plasma high density lipoprotein-cholesterol levels. The main reason for presenting this review is to look at the physiology of cholesterol ester transfer protein, its interrelationship with high density lipoproteins, and to give an update on the development of different cholesterol ester transfer protein inhibitor/modulator molecules.

Antisense oligonucleotide inhibition of cholesteryl ester transfer protein enhances RCT in hyperlipidemic, CETP transgenic, LDLr-/- mice

Due to their ability to promote positive effects across all of the lipoprotein classes, cholesteryl ester transfer protein (CETP) inhibitors are currently being developed as therapeutic agents for cardiovascular disease. In these studies, we compared an antisense oligonucleotide (ASO) inhibitor of CETP to the CETP small molecule inhibitor anacetrapib. In hyperlipidemic CETP transgenic (tg) mice, both drugs provided comparable reductions in total plasma cholesterol, decreases in CETP activity, and increases in HDL cholesterol. However, only mice treated with the antisense inhibitor showed an enhanced effect on macrophage reverse cholesterol transport, presumably due to differences in HDL apolipoprotein composition and decreases in plasma triglyceride. Additionally, the ASO-mediated reductions in CETP mRNA were associated with less accumulation of aortic cholesterol. These preliminary findings suggest that CETP ASOs may represent an alternative means to inhibit that target and to support their continued development as a treatment for cardiovascular disease in man.

Assessing the functional properties of high-density lipoproteins: an emerging concept in cardiovascular research

Although plasma concentrations of high-density lipoprotein (HDL) cholesterol correlate inversely with the incidence of atherosclerotic cardiovascular disease, results from recent epidemiological, genetic and pharmacological intervention studies resulted in a shift of concept. Rather than HDL cholesterol mass levels, the functionality of HDL particles is increasingly regarded as potentially clinically important. This review provides an overview of four key functional properties of HDL, namely cholesterol efflux and reverse cholesterol transport; antioxidative activities; anti-inflammatory activities; and the ability of HDL to increase vascular nitric oxide production resulting in vasorelaxation. Currently available assays are put into context with different HDL isolation procedures yielding compositional heterogeneity of the particle. Gathered knowledge on the impact of different disease states on HDL function is discussed together with potential underlying causative factors modulating HDL functionalities. In addition, a perspective is provided regarding how a better understanding of the determinants of (dys)functional HDL might impact clinical practice and the future design of rational and specific therapeutic approaches targeting atherosclerotic cardiovascular disease.

Upregulating Reverse Cholesterol Transport With Cholesteryl Ester Transfer Protein Inhibition Requires Combination With the LDL-Lowering Drug Berberine in Dyslipidemic Hamsters

Objective—

This study aimed to investigate whether cholesteryl ester transfer protein inhibition promotes in vivo reverse cholesterol transport in dyslipidemic hamsters.

Methods and Results—

In vivo reverse cholesterol transport was measured after an intravenous injection of 3 H-cholesteryl-oleate–labeled/oxidized low density lipoprotein particles ( 3 H-oxLDL), which are rapidly cleared from plasma by liver-resident macrophages for further 3 H-tracer egress in plasma, high density lipoprotein (HDL), liver, and feces. A first set of hamsters made dyslipidemic with a high-fat and high-fructose diet was treated with vehicle or torcetrapib 30 mg/kg (TOR) over 2 weeks. Compared with vehicle, TOR increased apolipoprotein E–rich HDL levels and significantly increased 3 H-tracer appearance in HDL by 30% over 72 hours after 3 H-oxLDL injection. However, TOR did not change 3 H-tracer recovery in liver and feces, suggesting that uptake and excretion of cholesterol deriving from apolipoprotein E-rich HDL is not stimulated. As apoE is a potent ligand for the LDL receptor, we next evaluated the effects of TOR in combination with the LDL-lowering drug berberine, which upregulates LDL receptor expression in dyslipidemic hamsters. Compared with TOR alone, treatment with TOR+berberine 150 mg/kg resulted in lower apolipoprotein E–rich HDL levels. After 3 H-oxLDL injection, TOR+berberine significantly increased 3 H-tracer appearance in fecal cholesterol by 109%.

Conclusion—

Our data suggest that cholesteryl ester transfer protein inhibition alone does not stimulate reverse cholesterol transport in dyslipidemic hamsters and that additional effects mediated by the LDL-lowering drug berberine are required to upregulate this process.

Sitagliptin promotes macrophage-to-faeces reverse cholesterol transport through reduced intestinal cholesterol absorption in obese insulin resistant CETP-apoB100 transgenic mice

Dipeptidyl peptidase-4 inhibitors (DPP-4i) improve glycaemic control in type 2 diabetes, but their benefits on reverse cholesterol transport (RCT) remain unknown. We evaluated the effects of DPP-4i sitagliptin 500 mg/kg/day on RCT in obese insulin-resistant CETP-apoB100 transgenic mice. Metformin 300 mg/kg/day orally was used as a reference compound. Both metformin and sitagliptin showed the expected effects on glucose parameters. Although no significant effect was observed on total cholesterol and high-density lipoprotein (HDL) cholesterol levels, sitagliptin, but not metformin, increased faecal cholesterol mass excretion by 132% (p < 0.001 vs. vehicle), suggesting a potent effect on cholesterol metabolism. Mice were then injected i.p. with (3) H-cholesterol labelled macrophages to measure RCT over 48 h. Compared with vehicle, sitagliptin significantly increased macrophage-derived (3) H-cholesterol faecal excretion by 39%. Administration of (14) C-cholesterol labelled olive oil orally showed a significant reduction of (14) C-tracer plasma appearance over time with sitagliptin, indicating that this drug promotes RCT through reduced intestinal cholesterol absorption.

Regulation of reverse cholesterol transport - a comprehensive appraisal of available animal studies

Plasma levels of high density lipoprotein (HDL) cholesterol are strongly inversely correlated to the risk of atherosclerotic cardiovascular disease. A major recognized functional property of HDL particles is to elicit cholesterol efflux and consequently mediate reverse cholesterol transport (RCT). The recent introduction of a surrogate method aiming at determining specifically RCT from the macrophage compartment has facilitated research on the different components and pathways relevant for RCT. The current review provides a comprehensive overview of studies carried out on macrophage-specific RCT including a quick reference guide of available data. Knowledge and insights gained on the regulation of the RCT pathway are summarized. A discussion of methodological issues as well as of the respective relevance of specific pathways for RCT is also included.