HDL-targeted therapies: progress, failures and future

Effects of low-carbohydrate diets v. low-fat diets on body weight and cardiovascular risk factors: a meta-analysis of randomised controlled trials

The effects of low-carbohydrate (LC) diets on body weight and cardiovascular risk are unclear, and previous studies have found varying results. Our aim was to conduct a meta-analysis of randomised controlled trials (RCT), assessing the effects of LC diets v. low-fat (LF) diets on weight loss and risk factors of CVD. Studies were identified by searching MEDLINE, Embase and Cochrane Trials. Studies had to fulfil the following criteria: a RCT; the LC diet was defined in accordance with the Atkins diet, or carbohydrate intake of <20% of total energy intake; twenty subjects or more per group; the subjects were previously healthy; and the dietary intervention had a duration of 6 months or longer. Results from individual studies were pooled as weighted mean difference (WMD) using a random effect model. In all, eleven RCT with 1369 participants met all the set eligibility criteria. Compared with participants on LF diets, participants on LC diets experienced a greater reduction in body weight (WMD -2·17 kg; 95% CI -3·36, -0·99) and TAG (WMD -0·26 mmol/l; 95% CI -0·37, -0·15), but a greater increase in HDL-cholesterol (WMD 0·14 mmol/l; 95% CI 0·09, 0·19) and LDL-cholesterol (WMD 0·16 mmol/l; 95% CI 0·003, 0·33). This meta-analysis demonstrates opposite change in two important cardiovascular risk factors on LC diets--greater weight loss and increased LDL-cholesterol. Our findings suggest that the beneficial changes of LC diets must be weighed against the possible detrimental effects of increased LDL-cholesterol.

Systematic review and meta-analysis deciphering the impact of fibrates on paraoxonase-1 status

OBJECTIVE

A significant residual cardiovascular risk is consistently observed in patients treated with statins. A combined treatment with fibrates reduces cardiovascular events in very high-risk patients. Because this is apparently unconnected to an improvement in lipid-related outcomes we hypothesized that the cardioprotective effects of fibrates might be associated with an improvement in paraoxonase-1 (PON1) status.

METHOD

The search for existing evidence, using the Medline, Scopus and Cochrane databases, was systematic and followed the PRISMA statement without restrictions on publication date. We excluded non-clinical and observational studies and we extracted data on baseline and post-treatment values of serum PON1 activity and other measurements of PON1 status.

RESULTS

Nine studies (including 12 treatment arms) in patients with hyperlipidemia, diabetes or metabolic syndrome treated with fibrates, alone or in combination with statins, were included to synthesize results. A meta-analysis of the data using a random-effects model revealed a significant increase in serum PON1 activity following fibrate therapy (WMD: 15.64U/L, 95% CI: 6.94, 24.34, p<0.001), an effect that was robust and not sensitive to any particular study. Subgroup analysis indicated differences in the effect size among types of fibrates and that PON1 alterations were associated with high-density lipoprotein cholesterol changes following fibrate therapy.

CONCLUSIONS

Results indicate a significant PON1-enhancing effect of fibrates. Whether this effect is associated with a clinical benefit, although likely, remains to be further investigated.

ApoA-I-Directed Therapies for the Management of Atherosclerosis

Several recent reports have raised doubts about the atheroprotective role of high-density lipoprotein cholesterol (HDL-C). Nevertheless, a substantial body of work supports the validity of pharmacological interventions able to enhance HDL function, as opposed to raising HDL-C levels per se. In this article, we briefly review the development of pharmacological interventions that target apoA-I and HDL function as a means of reducing atherosclerotic risk: small molecule pharmaceuticals, small HDL mimetic peptides, and infusion of apoA-I-containing particles.

Oxidized LDL induced extracellular trap formation in human neutrophils via TLR-PKC-IRAK-MAPK and NADPH-oxidase activation

Neutrophil extracellular traps (NETs) formation was initially linked with host defence and extracellular killing of pathogens. However, recent studies have highlighted their inflammatory potential. Oxidized low density lipoprotein (oxLDL) has been implicated as an independent risk factor in various acute or chronic inflammatory diseases including systemic inflammatory response syndrome (SIRS). In the present study we investigated effect of oxLDL on NETs formation and elucidated the underlying signalling mechanism. Treatment of oxLDL to adhered PMNs led to a time and concentration dependent ROS generation and NETs formation. OxLDL induced free radical formation and NETs release were significantly prevented in presence of NADPH oxidase (NOX) inhibitors suggesting role of NOX activation in oxLDL induced NETs release. Blocking of both toll like receptor (TLR)-2 and 6 significantly reduced oxLDL induced NETs formation indicating requirement of both the receptors. We further identified Protein kinase C (PKC), Interleukin-1 receptor associated kinase (IRAKs), mitogen-activated protein kinase (MAPK) pathway as downstream intracellular signalling mediators involved in oxLDL induced NETs formation. OxLDL components such as oxidized phospholipids (lysophosphatidylcholine (LPC) and oxidized 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphorylcholine (oxPAPC)) were most potent NETs inducers and might be crucial for oxLDL mediating NETs release. Other components like, oxysterols, malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) were however less potent as compared to oxidized phospholipids. This study thus demonstrates for the first time that treatment of human PMNs with oxLDL or its various oxidized phopholipid component mediated NETs release, implying their role in the pathogenesis of inflammatory diseases such as SIRS.

Novel therapies for severe dyslipidemia originating from human genetics

PURPOSE OF REVIEW

Novel therapies for severe dyslipidemia target a wide range of unmet medical needs: severe familial hypercholesterolemia, severe hypertriglyceridemia and chylomicronemia, elevated lipoprotein (a), lipodystrophies, high-density lipoprotein particle diseases, lysosomal acid lipase deficiency and storage diseases, nonalcoholic fatty liver disease and others. The purpose of this review is to describe the contribution of human genetics to the development of therapeutic approaches targeting severe dyslipidemia.

RECENT FINDINGS

Recent advances in human genetics and the identification of rare genetic variants having strong effects on disease risk not only accelerated the development of therapies for severe dyslipidemia, they also revealed new pathways, genes and mechanisms of health, disease or drug response, and facilitated molecular diagnosis, which may prove essential as the authorized use of some of these novel drugs is limited to specific conditions. In addition, the dissection of the gene and cell machinery gave rise to new technologies, gene-based therapies and biodrugs covering a broad range of novel agents currently available or in clinical development to treat severe lipid disorders.

SUMMARY

Several novel therapies are recently available or under development to treat severe dyslipidemia and associated risk stem directly from genetic research. Altogether, these therapies target a broad variety of severe dyslipidemia pathways or mechanisms and illustrate that clinical lipidology has now entered the era of precision medicine.

High-Density Lipoproteins: Nature's Multifunctional Nanoparticles

High-density lipoproteins (HDL) are endogenous nanoparticles involved in the transport and metabolism of cholesterol, phospholipids, and triglycerides. HDL is well-known as the "good" cholesterol because it not only removes excess cholesterol from atherosclerotic plaques but also has anti-inflammatory and antioxidative properties, which protect the cardiovascular system. Circulating HDL also transports endogenous proteins, vitamins, hormones, and microRNA to various organs. Compared with other synthetic nanocarriers, such as liposomes, micelles, and inorganic and polymeric nanoparticles, HDL has unique features that allow them to deliver cargo to specific targets more efficiently. These attributes include their ultrasmall size (8-12 nm in diameter), high tolerability in humans (up to 8 g of protein per infusion), long circulating half-life (12-24 h), and intrinsic targeting properties to different recipient cells. Various recombinant ApoA proteins and ApoA mimetic peptides have been recently developed for the preparation of reconstituted HDL that exhibits properties similar to those of endogenous HDL and has a potential for industrial scale-up. In this review, we will summarize (a) clinical pharmacokinetics and safety of reconstituted HDL products, (b) comparison of HDL with inorganic and other organic nanoparticles,

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.

New Therapeutic Approaches to the Treatment of Dyslipidemia

Dyslipidemia is a risk factor for atherosclerotic cardiovascular disease (ASCVD). Abundant data indicate that low-density lipoproteins (LDL) are causal for ASCVD; a new class of LDL-lowering medicines, the PCSK9 inhibitors, will address much unmet medical need. Human genetics suggest that triglyceride-rich lipoproteins (TRL) are pro-atherogenic and have pointed to a number of protein regulators of lipoprotein lipase activity that are candidates for therapeutic targeting. Finally, high-density lipoprotein (HDL) cholesterol does not appear to be causally associated with protection from ASCVD, reinforced by the failure of three CETP inhibitors in CV outcome trials, but HDL function remains of interest.

Does high-density lipoprotein protect vascular function in healthy pregnancy?

The maternal adaptation to pregnancy includes hyperlipidaemia, oxidative stress and chronic inflammation. In non-pregnant individuals, these processes are usually associated with poor vascular function. However, maternal vascular function is enhanced in pregnancy. It is not understood how this is achieved in the face of the adverse metabolic and inflammatory environment. Research into cardiovascular disease demonstrates that plasma HDL (high-density lipoprotein), by merit of its functionality rather than its plasma concentration, exerts protective effects on the vascular endothelium. HDL has vasodilatory, antioxidant, anti-thrombotic and anti-inflammatory effects, and can protect against endothelial cell damage. In pregnancy, the plasma HDL concentration starts to rise at 10 weeks of gestation, peaking at 20 weeks. The initial rise in plasma HDL occurs around the time of the establishment of the feto-placental circulation, a time when the trophoblast plugs in the maternal spiral arteries are released, generating oxidative stress. Thus there is the intriguing possibility that new HDL of improved function is synthesized around the time of the establishment of the feto-placental circulation. In obese pregnancy and, to a greater extent, in pre-eclampsia, plasma HDL levels are significantly decreased and maternal vascular function is reduced. Wire myography studies have shown an association between the plasma content of apolipoprotein AI, the major protein constituent of HDL, and blood vessel relaxation. These observations lead us to hypothesize that HDL concentration, and function, increases in pregnancy in order to protect the maternal vascular endothelium and that in pre-eclampsia this fails to occur.

Lipoprotein hydrophobic core lipids are partially extruded to surface in smaller HDL: "Herniated" HDL, a common feature in diabetes

Recent studies have shown that pharmacological increases in HDL cholesterol concentrations do not necessarily translate into clinical benefits for patients, raising concerns about its predictive value for cardiovascular events. Here we hypothesize that the size-modulated lipid distribution within HDL particles is compromised in metabolic disorders that have abnormal HDL particle sizes, such as type 2 diabetes mellitus (DM2). By using NMR spectroscopy combined with a biochemical volumetric model we determined the size and spatial lipid distribution of HDL subclasses in a cohort of 26 controls and 29 DM2 patients before and after two drug treatments, one with niacin plus laropiprant and another with fenofibrate as an add-on to simvastatin. We further characterized the HDL surface properties using atomic force microscopy and fluorescent probes to show an abnormal lipid distribution within smaller HDL particles, a subclass particularly enriched in the DM2 patients. The reduction in the size, force cholesterol esters and triglycerides to emerge from the HDL core to the surface, making the outer surface of HDL more hydrophobic. Interestingly, pharmacological interventions had no effect on this undesired configuration, which may explain the lack of clinical benefits in DM2 subjects.

Antilipidemic Drug Therapy Today and in the Future

The armamentarium for the treatment of dyslipidemia today comprises six different modes of action with overall around 24 different drugs. The treatment of lipid disorders was revolutionized with the introduction of statins which have become the most important therapeutic option available today to reduce and prevent atherosclerosis and its detrimental consequences like cardiovascular diseases and stroke. With and optimized reduction of elevated LDL levels with statins, the risk for cardiovascular diseases (CVD) can be reduced by 30%, indicating a residual remaining risk of 70% for the development and progression of CVD notifying still a high medical need for more effective antilipidemic drugs. Consequently, the search for novel lipid-modifying drugs is still one of the most active areas in research and development in the pharmaceutical industry. Major focus lies on approaches to LDL-lowering drugs superior to statins with regard to efficacy, safety, and patient compliance and on approaches modifying plasma levels and functionality of HDL particles based on the clinically validated inverse relationship between high-plasma HDL levels and the risk for CVD. The available drugs today for the treatment of dyslipidemia are small organic molecules or nonabsorbable polymers for binding of bile acids to be applied orally. Besides small molecules for novel targets, biological drugs such as monoclonal antibodies, antisense or gene-silencing oligonucleotides, peptidomimetics, reconstituted synthetic HDL particles and therapeutic proteins are novel approaches in clinical development are which have to be applied by injection or infusion. The promising clinical results of several novel drug candidates, particularly for LDL cholesterol lowering with monoclonal antibodies raised against PCSK9, may indicate more than a decade after the statins, the entrance of new breakthrough therapies to treat lipid disorders.

Large HDL Subfraction But Not HDL-C Is Closely Linked With Risk Factors, Coronary Severity and Outcomes in a Cohort of Nontreated Patients With Stable Coronary Artery Disease: A Prospective Observational Study

High-density lipoprotein (HDL) is highly heterogeneous in its size and composition. Till now, the link of HDL subfractions to coronary risk is less clear. We aimed to investigate the associations of HDL subfractions with traditional risk factors (RFs), coronary severity, and outcomes in a cohort of nontreated patients with stable coronary artery disease (CAD). We prospectively enrolled 591 eligible patients. Baseline HDL subfractions were separated by Lipoprint system. HDL subfractions (large, medium, and small) and HDL-cholesterol (HDL-C) levels were dichotomized into low and high group according to the 50 percentile. Coronary severity was evaluated by SYNTAX, Gensini, and Jeopardy scoring systems. Patients were followed up annually for major adverse cardiovascular events (MACEs). Cox proportional hazards' models were used to evaluate the risk of HDL subfractions on MACEs. Patients with high large HDL-C levels had a decreased number of RFs. Significantly, large HDL-C levels were negatively associated with coronary severity assessed by SYNTAX and Gensini score (both P < 0.05). New MACEs occurred in 67 (11.6%) patients during a median 17.0 months follow-up. Moreover, the log-rank test revealed that there was a significant difference between high and low large HDL-C groups in event-free survival analysis (P = 0.013), but no differences were observed in total HDL-C groups and medium or small HDL-C groups (both P > 0.05). In particular, the multivariate Cox-proportional hazards model revealed that high large HDL-C was associated with lower MACEs risk (hazard ratio [95% confidence interval] 0.531 [0.295-0.959]) independent of potential confounders. Higher large HDL-C but not medium, small, or total HDL-C is associated with lower cardiovascular risk, highlighting the potential beneficial of HDL subfractionation.

Proteomic analysis of HDL from inbred mouse strains implicates APOE associated with HDL in reduced cholesterol efflux capacity via the ABCA1 pathway

Cholesterol efflux capacity associates strongly and negatively with the incidence and prevalence of human CVD. We investigated the relationships of HDL’s size and protein cargo with its cholesterol efflux capacity using APOB-depleted serum and HDLs isolated from five inbred mouse strains with different susceptibilities to atherosclerosis. Like humans, mouse HDL carried >70 proteins linked to lipid metabolism, the acute-phase response, proteinase inhibition, and the immune system. HDL’s content of specific proteins strongly correlated with its size and cholesterol efflux capacity, suggesting that its protein cargo regulates its function. Cholesterol efflux capacity with macrophages strongly and positively correlated with retinol binding protein 4 (RBP4) and PLTP, but not APOA1. In contrast, ABCA1-specific cholesterol efflux correlated strongly with HDL’s content of APOA1, APOC3, and APOD, but not RBP4 and PLTP. Unexpectedly, APOE had a strong negative correlation with ABCA1-specific cholesterol efflux capacity. Moreover, the ABCA1-specific cholesterol efflux capacity of HDL isolated from APOE-deficient mice was significantly greater than that of HDL from wild-type mice. Our observations demonstrate that the HDL-associated APOE regulates HDL’s ABCA1-specific cholesterol efflux capacity. These findings may be clinically relevant because HDL’s APOE content associates with CVD risk and ABCA1 deficiency promotes unregulated cholesterol accumulation in human macrophages.

Associations between intensive diabetes therapy and NMR-determined lipoprotein subclass profiles in type 1 diabetes

Our objective is to define differences in circulating lipoprotein subclasses between intensive versus conventional management of type 1 diabetes during the randomization phase of the Diabetes Control and Complications Trial (DCCT). NMR-determined lipoprotein subclass profiles (NMR-LSPs), which estimate molar subclass concentrations and mean particle diameters, were determined in 1,294 DCCT subjects after a median of 5 years (interquartile range: 4-6 years) of randomization to intensive or conventional diabetes management. In cross-sectional analyses, we compared standard lipids and NMR-LSPs between treatment groups. Standard total, LDL, and HDL cholesterol levels were similar between randomization groups, while triglyceride levels were lower in the intensively treated group. NMR-LSPs showed that intensive therapy was associated with larger LDL diameter (20.7 vs. 20.6 nm, P = 0.01) and lower levels of small LDL (median: 465 vs. 552 nmol/l, P = 0.007), total IDL/LDL (mean: 1,000 vs. 1,053 nmol/l, P = 0.01), and small HDL (mean: 17.3 vs. 18.6 μmol/l, P < 0.0001), the latter accounting for reduced total HDL (mean: 33.8 vs. 34.8 μmol/l, P = 0.01). In conclusion, intensive diabetes therapy was associated with potentially favorable changes in LDL and HDL subclasses in sera. Further research will determine whether these changes contribute to the beneficial effects of intensive diabetes management on vascular complications.

Agonistic Human Antibodies Binding to Lecithin-Cholesterol Acyltransferase Modulate High Density Lipoprotein Metabolism

Drug discovery opportunities where loss-of-function alleles of a target gene link to a disease-relevant phenotype often require an agonism approach to up-regulate or re-establish the activity of the target gene. Antibody therapy is increasingly recognized as a favored drug modality due to multiple desirable pharmacological properties. However, agonistic antibodies that enhance the activities of the target enzymes are rarely developed because the discovery of agonistic antibodies remains elusive. Here we report an innovative scheme of discovery and characterization of human antibodies capable of binding to and agonizing a circulating enzyme lecithin cholesterol acyltransferase (LCAT). Utilizing a modified human LCAT protein with enhanced enzymatic activity as an immunogen, we generated fully human monoclonal antibodies using the XenoMouse^TM platform. One of the resultant agonistic antibodies, 27C3, binds to and substantially enhances the activity of LCAT from humans and cynomolgus macaques. X-ray crystallographic analysis of the 2.45 Å LCAT-27C3 complex shows that 27C3 binding does not induce notable structural changes in LCAT. A single administration of 27C3 to cynomolgus monkeys led to a rapid increase of plasma LCAT enzymatic activity and a 35% increase of the high density lipoprotein cholesterol that was observed up to 32 days after 27C3 administration. Thus, this novel scheme of immunization in conjunction with high throughput screening may represent an effective strategy for discovering agonistic antibodies against other enzyme targets. 27C3 and other agonistic human anti-human LCAT monoclonal antibodies described herein hold potential for therapeutic development for the treatment of dyslipidemia and cardiovascular disease.

Vaccenic acid and trans fatty acid isomers from partially hydrogenated oil both adversely affect LDL cholesterol: a double-blind, randomized controlled trial

BACKGROUND

Adverse effects of industrially produced trans fatty acids (iTFAs) on the risk of coronary artery disease are well documented in the scientific literature; however, effects of naturally occurring trans fatty acids (TFAs) from ruminant animals (rTFA), such as vaccenic acid (VA) and cis-9,trans-11 conjugated linoleic acid (c9,t11-CLA), are less clear. Although animal and cell studies suggest that VA and c9,t11-CLA may be hypocholesterolemic and antiatherogenic, epidemiologic data comparing rTFAs and iTFAs are inconsistent, and human intervention studies have been limited, underpowered, and not well controlled.

OBJECTIVE

We determined the effects of VA, c9,t11-CLA, and iTFA, in the context of highly controlled diets (24 d each), on lipoprotein risk factors compared with a control diet.

RESULTS

We conducted a double-blind, randomized, crossover feeding trial in 106 healthy adults [mean ± SD age: 47 ± 10.8 y; body mass index (in kg/m(2)): 28.5 ± 4.0; low-density lipoprotein (LDL) cholesterol: 3.24 ± 0.63 mmol/L]. Diets were designed to have stearic acid replaced with the following TFA isomers (percentage of energy): 0.1% mixed isomers of TFA (control), ∼3% VA, ∼3% iTFA, or 1% c9,t11-CLA. Total dietary fat (34% of energy) and other macronutrients were matched. Total cholesterol (TC), LDL cholesterol, triacylglycerol, lipoprotein(a), and apolipoprotein B were higher after VA than after iTFA; high-density lipoprotein (HDL) cholesterol and apolipoprotein AI also were higher after VA. Compared with control, VA and iTFA both increased TC, LDL cholesterol, ratio of TC to HDL cholesterol, and apolipoprotein B (2-6% change; P < 0.05); VA also increased HDL cholesterol, apolipoprotein AI, apolipoprotein B, and lipoprotein(a) (2-6% change; P < 0.05), whereas iTFA did not. c9,t11-CLA lowered triacylglycerol (P ≤ 0.01) and had no effect on other lipoprotein risk factors.

CONCLUSIONS

With respect to risk of cardiovascular disease, these results are consistent with current nutrition labeling guidelines, with the requirement of VA, but not c9,t11-CLA, to be listed under TFA on the Nutrition Facts Panel. This trial was registered at clinicaltrials.gov as NCT00942656.

Gene-based therapies in lipidology: current status and future challenges

PURPOSE OF REVIEW

Gene-based therapies are designed to modulate gene expression in specific tissues by introducing into cells transgenes, antisense oligonucleotides, RNA interference, microRNAs, or a variety of other oligonucleotide-based compounds and their delivery systems. Several types of gene-based therapies are already available or in clinical development to treat severe lipid-related disorders and associated risk. The review briefly presents the current status and future challenges of these therapies in clinical lipidology, focusing on most advanced and promising agents or mechanisms.

RECENT FINDINGS

Gene-based agents address several unmet medical needs in lipidology such as homozygous familial hypercholesterolemia, familial or multifactorial chylomicronemia, severe hypertriglyceridemia, elevated lipoprotein (a), familial partial lipodystrophy, nonalcoholic fatty liver disease, and hypoalphalipoproteinemia. Most advanced antisense oligonucleotide drugs target apolipoprotein C-III, apolipoprotein (a), angiopoietin-like 3, and diacylglycerol o-acyltransferase-2. Long-term efficacy and safety data are now available for two gene-based agents, mipomersen, approved in the USA for homozygous familial hypercholesterolemia and Glybera, AAV1-LPLS447X gene therapy, conditionally approved in Europe for lipoprotein lipase deficiency.

SUMMARY

Although positive to date, the overall benefit-risk ratio of gene-based therapies is yet to be documented long term across additional patients and conditions. The next generation of such therapies might improve their therapeutic index.

Structural and Functional Analysis of the ApolipoproteinA-I A164S Variant

Apolipoprotein A-I (apoA-I) is the main protein involved in the formation of high-density lipoprotein (HDL), it is the principal mediator of the reverse cholesterol transfer (RCT) pathway and provides cardio-protection. In addition to functional wild-type apoA-I, several variants have been shown to associate with hereditary amyloidosis. In this study we have performed biophysical and biochemical analyses of the structure and functional properties of the A164S variant of apoA-I (1:500 in the Danish general population), which is the first known mutation of apoA-I that leads to an increased risk of ischaemic heart disease (IHD), myocardial infarction and mortality without associated low HDL cholesterol levels. Despite the fact that epidemiologically IHD is associated with low plasma levels of HDL, the A164S mutation is linked to normal plasma levels of lipids, HDL and apoA-I, suggesting impaired functionality of this variant. Using biophysical techniques (e.g., circular dichroism spectroscopy and electron microscopy) to determine secondary structure, stability and pro-amyloidogenic property of the lipid free A164S apoA-I variant, our observations suggest similarity in structural properties between apoA-I WT and apoA-I A164S. However, the A164S apoA-I variant exhibits lower binding affinity to lipids but forms similar sized HDL particles to those produced by WT.

Therapeutic applications of reconstituted HDL: When structure meets function

Reconstituted forms of HDL (rHDL) are under development for infusion as a therapeutic approach to attenuate atherosclerotic vascular disease and to reduce cardiovascular risk following acute coronary syndrome and ischemic stroke. Currently available rHDL formulations developed for clinical use contain apolipoprotein A-I (apoA-I) and one of the major lipid components of HDL, either phosphatidylcholine or sphingomyelin. Recent data have established that quantitatively minor molecular constituents of HDL particles can strongly influence their anti-atherogenic functionality. Novel rHDL formulations displaying enhanced biological activities, including cellular cholesterol efflux, may therefore offer promising prospects for the development of HDL-based, anti-atherosclerotic therapies. Indeed, recent structural and functional data identify phosphatidylserine as a bioactive component of HDL; the content of phosphatidylserine in HDL particles displays positive correlations with all metrics of their functionality. This review summarizes current knowledge of structure-function relationships in rHDL formulations, with a focus on phosphatidylserine and other negatively-charged phospholipids. Mechanisms potentially underlying the atheroprotective role of these lipids are discussed and their potential for the development of HDL-based therapies highlighted.

Evaluation of HDL-modulating interventions for cardiovascular risk reduction using a systems pharmacology approach

The recent failures of cholesteryl ester transport protein inhibitor drugs to decrease CVD risk, despite raising HDL cholesterol (HDL-C) levels, suggest that pharmacologic increases in HDL-C may not always reflect elevations in reverse cholesterol transport (RCT), the process by which HDL is believed to exert its beneficial effects. HDL-modulating therapies can affect HDL properties beyond total HDL-C, including particle numbers, size, and composition, and may contribute differently to RCT and CVD risk. The lack of validated easily measurable pharmacodynamic markers to link drug effects to RCT, and ultimately to CVD risk, complicates target and compound selection and evaluation. In this work, we use a systems pharmacology model to contextualize the roles of different HDL targets in cholesterol metabolism and provide quantitative links between HDL-related measurements and the associated changes in RCT rate to support target and compound evaluation in drug development. By quantifying the amount of cholesterol removed from the periphery over the short-term, our simulations show the potential for infused HDL to treat acute CVD. For the primary prevention of CVD, our analysis suggests that the induction of ApoA-I synthesis may be a more viable approach, due to the long-term increase in RCT rate.

HDL and glucose metabolism: current evidence and therapeutic potential

High-density lipoprotein (HDL) and its principal apolipoprotein A-I (ApoA-I) have now been convincingly shown to influence glucose metabolism through multiple mechanisms. The key clinically relevant observations are that both acute HDL elevation via short-term reconstituted HDL (rHDL) infusion and chronically raising HDL via a cholesteryl ester transfer protein (CETP) inhibitor reduce blood glucose in individuals with type 2 diabetes mellitus (T2DM). HDL may mediate effects on glucose metabolism through actions in multiple organs (e.g., pancreas, skeletal muscle, heart, adipose, liver, brain) by three distinct mechanisms: (i) Insulin secretion from pancreatic beta cells, (ii) Insulin-independent glucose uptake, (iii) Insulin sensitivity. The molecular mechanisms appear to involve both direct HDL signaling actions as well as effects secondary to lipid removal from cells. The implications of glucoregulatory mechanisms linked to HDL extend from glycemic control to potential anti-ischemic actions via increased tissue glucose uptake and utilization. Such effects not only have implications for the prevention and management of diabetes, but also for ischemic vascular diseases including angina pectoris, intermittent claudication, cerebral ischemia and even some forms of dementia. This review will discuss the growing evidence for a role of HDL in glucose metabolism and outline related potential for HDL therapies.

HDL-S1P: cardiovascular functions, disease-associated alterations, and therapeutic applications

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid contained in High-density lipoproteins (HDL) and has drawn considerable attention in the lipoprotein field as numerous studies have demonstrated its contribution to several functions inherent to HDL. Some of them are partly and some entirely due to the S1P contained in HDL (HDL-S1P). Despite the presence of over 1000 different lipids in HDL, S1P stands out as it possesses its own cell surface receptors through which it exercises key physiological functions. Most of the S1P in human plasma is associated with HDL, and the amount of HDL-S1P influences the quality and quantity of HDL-dependent functions. The main binding partner of S1P in HDL is apolipoprotein M but others may also exist particularly under conditions of acute S1P elevations. HDL not only exercise functions through their S1P content but have also an impact on genuine S1P signaling by influencing S1P bioactivity and receptor presentation. HDL-S1P content is altered in human diseases such as atherosclerosis, coronary artery disease, myocardial infarction, renal insufficiency and diabetes mellitus. Low HDL-S1P has also been linked to impaired HDL functions associated with these disorders. Although the pathophysiological and molecular reasons for such disease-associated shifts in HDL-S1P are little understood, there have been successful approaches to circumvent their adverse implications by pharmacologically increasing HDL-S1P as means to improve HDL function. This mini-review will cover the current understanding of the contribution of HDL-S1P to physiological HDL function, its alteration in disease and ways for its restoration to correct HDL dysfunction.

Small, dense high-density lipoprotein 3 particles exhibit defective antioxidative and anti-inflammatory function in familial hypercholesterolemia: Partial correction by low-density lipoprotein apheresis

BACKGROUND

Familial hypercholesterolemia (FH) features elevated oxidative stress and accelerated atherosclerosis driven by elevated levels of atherogenic lipoproteins relative to subnormal levels of atheroprotective high-density lipoprotein (HDL). Small, dense HDL3 potently protects low-density lipoprotein (LDL) against proinflammatory oxidative damage.

OBJECTIVE

To determine whether antioxidative and/or anti-inflammatory activities of HDL are defective in FH and whether such defects are corrected by LDL apheresis.

METHODS

Antioxidative and antiinflammatory activities of HDL were evaluated as protection of reference LDL from oxidative stress and capacity to prevent accumulation of proinflammatory oxidised lipids, respectively. Lipid surface rigidity of HDL was assessed using a fluorescent probe. HDL components were measured by analytical approaches. Systemic oxidative stress was characterized as plasma 8-isoprostanes.

RESULTS

Pre-LDL-apheresis, FH patients (n = 10) exhibited elevated systemic oxidative stress (3.3-fold, P < 0.001) vs. sex- and age-matched normolipidemic controls (n = 10). Both antioxidative and antiinflammatory activity of HDL3 were impaired (up to -91%, P < 0.01) in FH. Sphingomyelin and saturated fatty acid contents were elevated in FH HDL3, resulting in enhanced lipid surface rigidity. The surface lipid content (phospholipids, free cholesterol) was reduced in FH (up to -15%, P < 0.001), whereas content of core lipids (cholesteryl esters, triglycerides) was elevated (up to +17%, P < 0.001). Molar apolipoprotein A-I content of HDL3 was subnormal in FH. A single LDL-apheresis session partially corrected (by up to 76%) deficient HDL antiatherogenic activities, attenuated systemic oxidative stress and partially normalised both the lipid composition and surface rigidity of HDL particles.

CONCLUSIONS

FH features elevated oxidative stress and deficient antioxidative and anti-inflammatory activities of small, dense HDL3; such functional deficiency is intimately linked to anomalies in lipid and protein composition, which may impair the capacity of HDL to acquire and inactivate oxidized lipids.

FR171456 is a specific inhibitor of mammalian NSDHL and yeast Erg26p

FR171456 is a natural product with cholesterol-lowering properties in animal models, but its molecular target is unknown, which hinders further drug development. Here we show that FR171456 specifically targets the sterol-4-alpha-carboxylate-3-dehydrogenase (Saccharomyces cerevisiae—Erg26p, Homo sapiens—NSDHL (NAD(P) dependent steroid dehydrogenase-like)), an essential enzyme in the ergosterol/cholesterol biosynthesis pathway. FR171456 significantly alters the levels of cholesterol pathway intermediates in human and yeast cells. Genome-wide yeast haploinsufficiency profiling experiments highlight the erg26/ERG26 strain, and multiple mutations in ERG26 confer resistance to FR171456 in growth and enzyme assays. Some of these ERG26 mutations likely alter Erg26 binding to FR171456, based on a model of Erg26. Finally, we show that FR171456 inhibits an artificial Hepatitis C viral replicon, and has broad antifungal activity, suggesting potential additional utility as an anti-infective. The discovery of the target and binding site of FR171456 within the target will aid further development of this compound.

FR171456 is a bioactive chemical produced by some microorganisms. Here, the authors identify the enzyme NSDHL of the sterol synthesis pathway as the molecular target of FR171456, rendering it the first compound to specifically target this class of enzyme in yeast and mammalian cells.

Reconstituted high-density lipoproteins acutely reduce soluble brain Aβ levels in symptomatic APP/PS1 mice

Many lines of evidence suggest a protective role for high-density lipoprotein (HDL) and its major apolipoprotein (apo)A-I in Alzheimer's Disease (AD). HDL/apoA-I particles are produced by the liver and intestine and, in addition to removing excess cholesterol from the body, are increasingly recognized to have vasoprotective functions. Here we tested the ability of reconstituted HDL (rHDL) consisting of human apoA-I reconstituted with soy phosphatidylcholine for its ability to lower amyloid beta (Aβ) levels in symptomatic APP/PS1 mice, a well-characterized preclinical model of amyloidosis. Animals were treated intravenously either with four weekly doses (chronic study) or a single dose of 60mg/kg of rHDL (acute study). The major finding of our acute study is that soluble brain Aβ40 and Aβ42 levels were significantly reduced within 24h of a single dose of rHDL. By contrast, no changes were observed in our chronic study with respect to soluble or deposited Aβ levels in animals assessed 7days after the final weekly dose of rHDL, suggesting that beneficial effects diminish as rHDL is cleared from the body. Further, rHDL-treated animals showed no change in amyloid burden, cerebrospinal fluid (CSF) Aβ levels, neuroinflammation, or endothelial activation in the chronic study, suggesting that the pathology-modifying effects of rHDL may indeed be acute and may be specific to the soluble Aβ pool. That systemic administration of rHDL can acutely modify brain Aβ levels provides support for further investigation of the therapeutic potential of apoA-I-based agents for AD. This article is part of a Special Issue entitled: Vascular Contributions to Cognitive Impairment and Dementia edited by M. Paul Murphy, Roderick A. Corriveau and Donna M. Wilcock.

Immune-mediated and lipid-mediated platelet function in atherosclerosis

PURPOSE OF REVIEW

Cardiovascular disease (CVD) is the leading cause of death and morbidity worldwide. Detailed knowledge of the mechanisms of atherosclerosis, the main underlying disease of CVD, will enable improved preventive and therapeutic options, thus potentially limiting the burden of vascular disease in aging societies. A large body of evidence illustrates the contribution of platelets to processes beyond their traditionally recognized role as mediators in thrombosis and hemostasis. Recent advances in molecular biology help to understand the complexity of atherosclerosis.

RECENT FINDINGS

This article outlines the role of platelets as modulators of immune responses in the context of atherosclerosis. It provides a short overview of interactions between platelets and endothelial cells or immune cells via direct cell contact or soluble factors during atherogenesis. By means of some well examined, exemplary pathways (e.g. CD40/CD40L dyad), this article will discuss recent discoveries in immune-related function of platelets. We also focus on the relationship between platelets and the lipid metabolism highlighting potential consequences to atherosclerosis and dyslipidemia.

SUMMARY

A better understanding of the molecular mechanisms of platelet-related immune activity allows their utilization as powerful diagnostic tools or targets of therapeutic intervention. Those findings might help to develop new classes of drugs which may supplement or replace classical anticoagulants and help clinicians to tackle CVD more efficiently.

Cholesterol efflux capacity, macrophage reverse cholesterol transport and cardioprotective HDL

PURPOSE OF REVIEW

Randomized clinical trials provide strong evidence that pharmacological elevation of HDL-cholesterol (HDL-C) fails to reduce cardiovascular disease (CVD) risk in statin-treated humans. It is thus critical to identify new metrics that capture HDL's cardioprotective effects.

RECENT FINDINGS

We review recent evidence that HDL's cholesterol efflux capacity is a strong inverse predictor of incident and prevalent CVD in humans. In light of those findings, we assess the proposal that impaired macrophage cholesterol efflux to HDL contributes to disease risk. We also discuss recent studies implicating small HDL particles in cholesterol efflux from macrophages.

SUMMARY

These observations lay the foundation for a new approach to understanding mechanistically how HDL's functional properties help reduce CVD risk.

Dietary Almonds Increase Serum HDL Cholesterol in Coronary Artery Disease Patients in a Randomized Controlled Trial

BACKGROUND

More than one-half of coronary artery disease (CAD) patients have low HDL cholesterol despite having well-managed LDL cholesterol. Almond supplementation has not been shown to elevate circulating HDL cholesterol concentrations in clinical trials, perhaps because the baseline HDL cholesterol of trial subjects was not low.

OBJECTIVE

This clinical trial was designed to test the effect of almond supplementation on low HDL cholesterol in CAD patients.

METHODS

A total of 150 CAD patients (50 per group), with serum LDL cholesterol ≤100 mg/dL and HDL cholesterol ≤40 mg/dL in men and ≤50 mg/dL in women, were recruited from the Aga Khan University Hospital. After recording vital signs and completing a dietary and physical activity questionnaire, patients were randomly assigned to 1 of the following 3 groups: the no-intervention group (NI), the Pakistani almonds group (PA), and the American almonds group (AA). The respective almond varieties (10 g/d) were given to patients with instructions to soak them overnight, remove the skin, and eat them before breakfast. Blood samples for lipid profiling, body weight, and blood pressure were collected, and assessment of dietary patterns was done at baseline, week 6, and week 12.

RESULTS

Almonds significantly increased HDL cholesterol. At weeks 6 and 12, HDL cholesterol was 12-14% and 14-16% higher, respectively, in the PA and AA than their respective baselines. In line with previous reports, serum concentrations of total cholesterol, triglycerides, LDL cholesterol, and VLDL cholesterol; total-to-HDL and LDL-to-HDL cholesterol ratios, and the atherogenic index were reduced in both the PA and AA at weeks 6 and 12 compared with baseline (P < 0.05). Effects on serum lipids did not differ between the 2 almond groups. Dietary patterns, body weight, and blood pressure did not change in any of the 3 groups during the trial.

CONCLUSION

A low dose of almonds (10 g/d) consumed before breakfast can increase HDL cholesterol, in addition to improving other markers of abnormal lipid metabolism in CAD patients with low initial HDL cholesterol. This trial was registered at the Australian New Zealand Clinical Trial Registry as ACTRN12614000036617.

Mass spectrometry meets the challenge of understanding the complexity of the lipoproteome: recent findings regarding proteins involved in dyslipidemia and cardiovascular disease

Despite the fact that link between dyslipidemia and atherosclerosis was made over 100 years ago, atherosclerosis remains a major cause of morbidity and mortality worldwide. Major efforts focus towards understanding lipid metabolism, particularly by studying its particle compartments in circulation: the lipoproteins. In recent years, mass spectrometry has played an integral role in the deep sequencing of the lipoproteome and in metabolism studies conducted in vivo. This review highlights the path of lipoprotein research towards state-of-the-art mass spectrometry with special emphasis on the method of selected reaction monitoring and its impact on apolipoprotein metabolism studies. Also presented is what is expected for the lipoprotein field with the recent advent of high resolution/accurate mass parallel reaction monitoring mass spectrometry. The benefits of high resolution/accurate mass measurements are demonstrated by example instrument workflows and by detailing a novel method to quantify very low levels of circulating proprotein convertase subtilisin-kexin type 9 in rabbit. It is anticipated that future clinical studies or clinical trials aimed to treat dyslipidemia by manipulating key regulatory proteins will benefit from the new and exciting opportunities afforded by the latest technologies in mass spectrometry.

HDL/ApoA-1 infusion and ApoA-1 gene therapy in atherosclerosis

The HDL hypothesis stating that simply raising HDL cholesterol (HDL-C) may produce cardiovascular benefits has been questioned recently based on several randomized clinical trials using CETP inhibitors or niacin to raise HDL-C levels. However, extensive pre-clinical data support the vascular protective effects of administration of exogenous ApoA-1 containing preβ-HDL like particles. Several small proof-of-concept clinical trials using such HDL/ApoA-1 infusion therapy have shown encouraging results but definitive proof of efficacy must await large scale clinical trials. In addition to HDL infusion therapy an alternative way to exploit beneficial cardiovascular effects of HDL/ApoA-1 is to use gene transfer. Preclinical studies have shown evidence of benefit using this approach; however clinical validation is yet lacking. This review summarizes our current knowledge of the aforementioned strategies.

Infusion of Reconstituted High-Density Lipoprotein, CSL112, in Patients With Atherosclerosis: Safety and Pharmacokinetic Results From a Phase 2a Randomized Clinical Trial

Background

CSL112 is a new formulation of human apolipoprotein A-I (apoA-I) being developed to reduce cardiovascular events following acute coronary syndrome. This phase 2a, randomized, double-blind, multicenter, dose-ranging trial represents the first clinical investigation to assess the safety and pharmacokinetics/pharmacodynamics of a CSL112 infusion among patients with stable atherosclerotic disease.

Methods and Results

Patients were randomized to single ascending doses of CSL112 (1.7, 3.4, or 6.8 g) or placebo, administered over a 2-hour period. Primary safety assessments consisted of alanine aminotransferase or aspartate aminotransferase elevations >3× upper limits of normal and study drug–related adverse events. Pharmacokinetic/pharmacodynamic assessments included apoA-I plasma concentration and measures of the ability of serum to promote cholesterol efflux from cells ex vivo. Of 45 patients randomized, 7, 12, and 14 received 1.7-, 3.4-, and 6.8-g CSL112, respectively, and 11 received placebo. There were no clinically significant elevations (>3× upper limit of normal) in alanine aminotransferase or aspartate aminotransferase. Adverse events were nonserious and mild and occurred in 5 (71%), 5 (41%), and 6 (43%) patients in the CSL112 1.7-, 3.4-, and 6.8-g groups, respectively, compared with 3 (27%) placebo patients. The imbalance in adverse events was attributable to vessel puncture/infusion-site bruising. CSL112 resulted in rapid (T_max≈2 hours) and dose-dependent increases in apoA-I (145% increase in the 6.8-g group) and total cholesterol efflux (up to 3.1-fold higher than placebo) (P<0.001).

Conclusions

CSL112 infusion was well tolerated in patients with stable atherosclerotic disease. CSL112 immediately raised apoA-I levels and caused a rapid and marked increase in the capacity of serum to efflux cholesterol. This potential novel approach for the treatment of atherosclerosis warrants further investigation.

Clinical Trial Registration

URL: http://www.ClinicalTrials.gov. Unique identifier: NCT01499420.

Reconstituted High-Density Lipoprotein Attenuates Cholesterol Crystal-Induced Inflammatory Responses by Reducing Complement Activation

Chronic inflammation of the arterial wall is a key element in the development of atherosclerosis, and cholesterol crystals (CC) that accumulate in plaques are associated with initiation and progression of the disease. We recently revealed a link between the complement system and CC-induced inflammasome caspase-1 activation, showing that the complement system is a key trigger in CC-induced inflammation. HDL exhibits cardioprotective and anti-inflammatory properties thought to explain its inverse correlation to cardiovascular risk. In this study, we sought to determine the effect of reconstituted HDL (rHDL) on CC-induced inflammation in a human whole blood model. rHDL bound to CC and inhibited the CC-induced complement activation as measured by soluble terminal C5b-9 formation and C3c deposition on the CC surface. rHDL attenuated the amount of CC-induced complement receptor 3 (CD11b/CD18) expression on monocytes and granulocytes, as well as reactive oxygen species generation. Moreover, addition of CC to whole blood resulted in release of proinflammatory cytokines that were inhibited by rHDL. Our results support and extend the notion that CC are potent triggers of inflammation, and that rHDL may have a beneficial role in controlling the CC-induced inflammatory responses by inhibiting complement deposition on the crystals.

High-density lipoprotein-based drug discovery for treatment of atherosclerosis

INTRODUCTION

Although there has been great progress achieved by the use of intensive statin therapy, the burden of atherosclerotic cardiovascular disease (CVD) remains high. This has initiated the search for novel high-density lipoprotein (HDL)-based therapeutics. Recent years have witnessed a shift from traditional raising HDL-C levels to enhancing HDL functionality, in which the process of reverse cholesterol transport (RCT) has acquired much attention.

AREAS COVERED

In this review, the authors describe the key factors involved in RCT process for potential drug targets to reduce the CVD risk. Furthermore, the review provides a summary of the effective screening methods that have been developed to target RCT and their applications. This review also introduces some new strategies currently being clinically developed, which have the potential to improve HDL function in the RCT process.

EXPERT OPINION

It is rational that the functionality of HDL is more important than the plasma HDL-C level in the evaluation of pharmacological treatment in atherosclerosis. HDL-based strategies designed to promote macrophage RCT are a major area of current drug discovery and development for atherosclerotic diseases. A better understanding of the functionality of HDL and its relationship with atherosclerosis will expand our knowledge of the role of HDL in lipid metabolism, holding promise for a future successful HDL-based therapy.

Innovative pharmaceutical interventions in cardiovascular disease: Focusing on the contribution of non-HDL-C/LDL-C-lowering versus HDL-C-raising: A systematic review and meta-analysis of relevant preclinical studies and clinical trials

Non-HDL-cholesterol is well recognised as a primary causal risk factor in cardiovascular disease. However, despite consistent epidemiological evidence for an inverse association between HDL-C and coronary heart disease, clinical trials aimed at raising HDL-C (AIM-HIGH, HPS2-THRIVE, dal-OUTCOMES) failed to meet their primary goals. This systematic review and meta-analysis investigated the effects of established and novel treatment strategies, specifically targeting HDL, on inhibition of atherosclerosis in cholesteryl ester transfer protein-expressing animals, and the prevention of clinical events in randomised controlled trials. Linear regression analyses using data from preclinical studies revealed associations for TC and non-HDL-C and lesion area (R(2)=0.258, P=0.045; R(2)=0.760, P<0.001), but not for HDL-C (R(2)=0.030, P=0.556). In clinical trials, non-fatal myocardial infarction risk was significantly less in the treatment group with pooled odd ratios of 0.87 [0.81; 0.94] for all trials and 0.85 [0.78; 0.93] after excluding some trials due to off-target adverse events, whereas all-cause mortality was not affected (OR 1.05 [0.99-1.10]). Meta-regression analyses revealed a trend towards an association between between-group differences in absolute change from baseline in LDL-C and non-fatal myocardial infarction (P=0.066), whereas no correlation was found for HDL-C (P=0.955). We conclude that the protective role of lowering LDL-C and non-HDL-C is well-established. The contribution of raising HDL-C on inhibition of atherosclerosis and the prevention of cardiovascular disease remains undefined and may be dependent on the mode of action of HDL-C-modification. Nonetheless, treatment strategies aimed at improving HDL function and raising apolipoprotein A-I may be worth exploring.

Effects of Hedan Tablet () on lipid profile, proprotein convertase subtilisin/kexin type 9 and high-density lipoprotein subfractions in patients with hyperlipidemia: A primary study

OBJECTIVE

To investigate the effects of Hedan Tablet () on serum lipid profile, proprotein convertase subtilisin/kexin type 9 (PSCK9) and high-density lipoprotein (HDL) subfractions in patients with hyperlipidemia.

METHODS

Thirty-seven patients with hyperlipidemia were randomized to treatment with Hedan Tablet 4.38 g/day as Hedan group (18 cases) or placebo (19 cases) as control group for 8 weeks. The lipid profile, PCSK9 and HDL subfractions were determined at day 0 and week 8 in both groups respectively.

RESULTS

Hedan treatment for 8 weeks mildly decreased serum low-density lipoprotein cholesterol (LDL-C) levels, while no changes were found in total cholesterol (TC), triglycerides (TG) and PCSK9 concentrations. Furthermore, Hedan treatment increased the concentration of large high-density lipoprotein cholesterol (HDL-C) and the percentage of large HDL subfraction, while decreased the concentration of small HDL-C and the percentage of small HDL subfraction without changing serum HDL-C levels in patients with hyperlipidemia.

CONCLUSION

Hedan treatment of 4.38 g per day for 8 weeks could confer a favorable effects on serum LDL-C concentration as well as HDL subfractions.

Niacin as antidyslipidemic drug

Niacin is an important vitamin (B3) that can be used in gram doses to positively modify pathogenetically relevant lipid disorders: elevated LDL cholesterol, elevated non-HDL cholesterol, elevated triglycerides, elevated lipoprotein(a), and reduced HDL cholesterol. This review reports the latest published findings with respect to niacin's mechanisms of action on these lipids and its anti-inflammatory and anti-atherosclerotic effects. In the pre-statin era, niacin was shown to have beneficial effects on cardiovascular end-points; but in recent years, two major studies performed in patients whose LDL cholesterol levels had been optimized by a statin therapy did not demonstrate an additional significant effect on these end-points in the groups where niacin was administered. Both studies have several drawbacks that suggest that they are not representative for other patients. Thus, niacin still plays a role either as an additive to a statin or as a substitute for a statin in statin-intolerant patients. Moreover, patients with elevated triglyceride and low HDL cholesterol levels and patients with elevated lipoprotein(a) concentrations will possibly benefit from niacin, although currently the study evidence for these indications is rather poor. Niacin may be useful for compliant patients, however possible side effects (flushing, liver damage) and contraindications should be taken into consideration.

Analysis of "On/Off" Kinetics of a CETP Inhibitor Using a Mechanistic Model of Lipoprotein Metabolism and Kinetics

RG7232 is a potent inhibitor of cholesteryl-ester transfer protein (CETP). Daily oral administration of RG7232 produces a dose- and time-dependent increase in high-density lipoprotein-cholesterol (HDL-C) and apolipoproteinA-I (ApoA-I) levels and a corresponding decrease in low-density lipoprotein-cholesterol (LDL-C) and apolipoproteinB (ApoB) levels. Due to its short plasma half-life (∼3 hours), RG7232 transiently inhibits CETP activity during each dosing interval ("on/off" kinetics), as reflected by the temporal effects on HDL-C and LDL-C. The influence of RG7232 on lipid-poor ApoA-I (i.e., pre-β 1) levels and reverse cholesterol transport rates is unclear. To investigate this, a published model of lipoprotein metabolism and kinetics was combined with a pharmacokinetic model of RG7232. After calibration and validation of the combined model, the effect of RG7232 on pre-β 1 levels was simulated. A dose-dependent oscillation of pre-β 1, driven by the "on/off" kinetics of RG7232 was observed. The possible implications of these findings are discussed.

The influence of dysfunctional signaling and lipid homeostasis in mediating the inflammatory responses during atherosclerosis

Atherosclerosis, the underlying cause of myocardial infarction and thrombotic cerebrovascular events, is responsible for the majority of deaths in westernized societies. Mortality from this disease is also increasing at a marked rate in developing countries due to the acquisition of a westernized lifestyle accompanied with elevated rates of obesity and diabetes. Atherosclerosis is recognized as a chronic inflammatory disorder associated with lipid accumulation and the development of fibrotic plaques within the walls of medium and large arteries. A range of immune cells, such as macrophages and T-lymphocytes, through the action of various cytokines, such as interleukins-1 and -33, transforming growth factor-β and interferon-γ, orchestrates the inflammatory response in this disease. The disease is also characterized by marked dysfunction in lipid homeostasis and signaling pathways that control the inflammatory response. This review will discuss the molecular basis of atherosclerosis with particular emphasis on the roles of the immune cells and cytokines along with the dysfunctional lipid homeostasis and cell signaling associated with this disease.

[New agents for hypercholesterolemia]

An elevated proportion of high cardiovascular risk patients do not achieve the therapeutic c-LDL goals. This owes to physicians' inappropriate or insufficient use of cholesterol lowering medications or to patients' bad tolerance or therapeutic compliance. Another cause is an insufficient efficacy of current cholesterol lowering drugs including statins and ezetimibe. In addition, proprotein convertase subtilisin kexin type 9 inhibitors are a new cholesterol lowering medications showing safety and high efficacy to reduce c-LDL in numerous already performed or underway clinical trials, potentially allowing an optimal control of hypercholesterolemia in most patients. Agents inhibiting apolipoprotein B synthesis and microsomal transfer protein are also providing a new potential to decrease cholesterol in patients with severe hypercholesterolemia and in particular in homozygote familial hypercholesterolemia. Last, cholesteryl ester transfer protein inhibitors have shown powerful effects on c-HDL and c-LDL, although their efficacy in cardiovascular prevention and safety has not been demonstrated yet. We provide in this article an overview of the main characteristics of therapeutic agents for hypercholesterolemia, which have been recently approved or in an advanced research stage.

Increased atherosclerosis in P2Y13/apolipoprotein E double-knockout mice: contribution of P2Y13 to reverse cholesterol transport

AIMS

High-density lipoproteins (HDLs) protect against atherosclerosis mainly due to their function in hepatobiliary reverse cholesterol transport (RCT). This is a process whereby excess cholesterol from peripheral tissues is transported by HDL particles to the liver for further metabolism and biliary excretion. Hepatic uptake of HDL holoparticles involves the P2Y13 receptor, independently of the selective cholesteryl ester uptake mediated by scavenger receptor class B, type I (SR-BI). Accordingly, P2Y13-deficient mice (P2Y13 (-/-)) have impaired RCT. This study assessed whether P2Y13 deficiency would affect atherosclerotic development.

METHODS AND RESULTS

P2Y13 (-/-) mice were crossbred with atherosclerosis-prone apoE(-/-) mice. When 15 weeks old, P2Y13 (-/-)/apoE(-/-) mice had more aortic sinus lesions than apoE(-/-) mice. Bone marrow transplantation showed that the absence of the P2Y13 receptor in blood cells did not lead to significantly greater atherosclerotic plaque size formation compared with control apoE(-/-) reconstituted animals. Conversely, the absence of the P2Y13 receptor, except in blood cells, resulted in lesion sizes similar to that in P2Y13 (-/-)/apoE(-/-) reconstituted mice, pointing to a role for non-haematopoietic-derived P2Y13. Unexpectedly, P2Y13 (-/-)/apoE(-/-) mice displayed a lower HDL-cholesterol level than apoE(-/-) mice, which might be due to greater SR-BI expression in the liver. However, P2Y13 deficiency in apoE(-/-) mice was translated into reduced biliary and faecal sterol excretion and impaired RCT from macrophage to faeces, suggesting that an alteration in hepatobiliary RCT could be solely responsible for the greater atherosclerosis observed.

CONCLUSION

The P2Y13 receptor protects against atherosclerosis, primarily through its role in hepatobiliary RCT.

Structural stability and functional remodeling of high-density lipoproteins

Lipoproteins are protein-lipid nanoparticles that transport lipids in circulation and are central in atherosclerosis and other disorders of lipid metabolism. Apolipoproteins form flexible structural scaffolds and important functional ligands on the particle surface and direct lipoprotein metabolism. Lipoproteins undergo multiple rounds of metabolic remodeling that is crucial to lipid transport. Important aspects of this remodeling, including apolipoprotein dissociation and particle fusion, are mimicked in thermal or chemical denaturation and are modulated by free energy barriers. Here we review the biophysical studies that revealed the kinetic mechanism of lipoprotein stabilization and unraveled its structural basis. The main focus is on high-density lipoprotein (HDL). An inverse correlation between stability and functions of various HDLs in cholesterol transport suggests the functional role of structural disorder. A mechanism for the conformational adaptation of the major HDL proteins, apoA-I and apoA-II, to the increasing lipid load is proposed. Together, these studies help understand why HDL forms discrete subclasses separated by kinetic barriers, which have distinct composition, conformation and functional properties. Understanding these properties may help improve HDL quality and develop novel therapies for cardiovascular disease.