Extended-Release Niacin/Laropiprant Improves Overall Efficacy of Postprandial Reverse Cholesterol Transport

Reduced Capacity of High-Density Lipoprotein to Acquire Free Cholesterol From Triglyceride-Rich Lipoproteins Is Associated With Elevated Postprandial Hypertriglyceridemia in Healthy Men

BACKGROUND

The capacity of high-density lipoprotein cholesterol (HDL) to acquire free cholesterol (FC) from triglyceride-rich lipoproteins during lipoprotein lipase-dependent lipolysis in a process of reverse remnant cholesterol transport, has been proposed as a key biological function of HDL particles that underlies the U-shaped relationship between HDLcholesterol and cardiovascular diseases. Although reverse remnant cholesterol transport has been evaluated in a fasting state, it has never been explored under nonfasting conditions.

METHODS AND RESULTS

FC transfer was evaluated in healthy men (n=78) before and throughout the postprandial phase up to 8 hours after consumption of a test meal. Postprandially, the capacity of HDL to acquire FC increased progressively, reaching a maximal mean value of 98.5%±22.5% 6 hours after meal intake (P<0.05). Analysis of the study population according to tertiles of postprandial variation of FC transfer identified subjects exhibiting reduced capacity of HDL to acquire FC (tertile 1), those for whom the capacity of HDL to acquire FC remained unchanged (tertile 2), and subjects characterized by an enhanced FC transfer during the postprandial phase (tertile 3). Across the tertiles, we found an inverse relationship between the maximal postprandial change in FC transfer to HDL and the degree of postprandial triglyceride response.

CONCLUSIONS

Healthy individuals exhibiting exacerbated postprandial triglyceride response and reduced HDL cholesterol levels feature reduced FC transfer to HDL during the postprandial state. These data suggest that to normalize postprandial triglyceride response, 2 conditions need to be fulfilled: notably elevated FC transfer to HDL in the postprandial phase and increased levels of acceptor HDL particles.

Postprandial Lipid Metabolism in Normolipidemic Subjects and Patients with Mild to Moderate Hypertriglyceridemia: Effects of Test Meals Containing Saturated Fatty Acids, Mono-Unsaturated Fatty Acids, or Medium-Chain Fatty Acids

Fasting and postprandial hypertriglyceridemia are causal risk factors for atherosclerosis. The prevalence of hypertriglyceridemia is approximately 25–30% and most hypertriglyceridemic patients suffer from mild to moderate hypertriglyceridemia. Data regarding dietary interventions on postprandial triglyceride metabolism of mildly to moderately hypertriglyceridemic patients is, however, sparse. In a randomized controlled trial, eight mildly hypertriglyceridemic patients and five healthy, normolipidemic controls received three separate standardized fat-meals containing either saturated fatty acids (SFA), mono-unsaturated fatty acids (MUFA), or medium-chain fatty acids (MCFA) in a randomized order. Fasting and postprandial lipid parameters were determined over a 10 h period and the (incremental) area under the curve (AUC/iAUC) for plasma triglycerides and other parameters were determined. MCFA do not lead to a significant elevation of postprandial total plasma triglycerides and other triglyceride parameters, while both SFA (patients: p = 0.003, controls: p = 0.03 compared to MCFA) and MUFA (patients: p = 0.001; controls: p = 0.14 compared to MCFA) do lead to such an increase. Patients experienced a significantly more pronounced increase of plasma triglycerides than controls (SFA: patients iAUC = 1006 mg*h/dL, controls iAUC = 247 mg*h/dL, p = 0.02; MUFA: patients iAUC = 962 mg*h/dL, controls iAUC = 248 mg*h/dL, p = 0.05). Replacing SFA with MCFA may be a treatment option for mildly to moderately hypertriglyceridemic patients as it prevents postprandial hypertriglyceridemia.

High-density lipoprotein: our elusive friend

PURPOSE OF REVIEW

Despite advances in the research on HDL composition (lipidomics and proteomics) and functions (cholesterol efflux and antioxidative capacities), the relationship between HDL compositional and functional properties is not fully understood. We have reviewed the recent literature on this topic and pointed out the difficulties which limit our understanding of HDL's role in cardiovascular disease (CVD).

RECENT FINDINGS

Though current findings strongly support that HDL has a significant role in CVD, the underlying mechanisms by which HDL mitigates CVD risk are not clear. This review focuses on studies that investigate the cell-cholesterol efflux capacity and the proteomic and lipidomic characterization of HDL and its subfractions especially those that analyzed the relationship between HDL composition and functions.

SUMMARY

Recent studies on HDL composition and HDL functions have greatly contributed to our understanding of HDL's role in CVD. A major problem in HDL research is the lack of standardization of both the HDL isolation and HDL functionality methods. Data generated by different methods often produce discordant results on the particle number, size, lipid and protein composition, and the various functions of HDL.

High-Density Lipoprotein Particles, Cell-Cholesterol Efflux, and Coronary Heart Disease Risk

Objective- The cell-cholesterol efflux capacity of HDL (high-density lipoprotein) is inversely associated with coronary heart disease risk. ABCA1 (ATP-binding cassette transporter A1) plays a crucial role in cholesterol efflux from macrophages to preβ-1-HDL. We tested the hypothesis that coronary heart disease patients have functionally abnormal preβ-1-HDL. Approach and Results- HDL cell-cholesterol efflux capacity via the ABCA1 and the SR-BI (scavenger receptor class B type I) pathways, HDL antioxidative capacity, apo (apolipoprotein) A-I-containing HDL particles, and inflammatory- and oxidative-stress markers were measured in a case-control study of 100 coronary heart disease cases and 100 sex-matched controls. There were significant positive correlations between ABCA1-dependent cholesterol efflux and the levels of small lipid-poor preβ-1 particles ( R²=0.535) and between SR-BI-dependent cholesterol efflux and the levels of large lipid-rich (α-1+α-2) HDL particles ( R²=0.712). Cases had significantly higher (87%) preβ-1 concentrations than controls, but the functionality of their preβ-1 particles (preβ-1 concentration normalized ABCA1-dependent efflux capacity) was significantly lower (-31%). Cases had significantly lower (-12%) mean concentration of large HDL particles, but the functionality of their particles (α-1+α-2 concentration normalized SR-BI-dependent efflux capacity) was significantly higher (22%) compared with that of controls. HDL antioxidative capacity was significantly lower (-16%) in cases than in controls. There were no significant correlations between either preβ-1 functionality or large HDL particle functionality with HDL antioxidative capacity or the concentrations of inflammatory- and oxidative-stress markers. Conclusions- HDL cell-cholesterol efflux capacity is significantly influenced by both the concentration and the functionality of specific HDL particles participating in cell-cholesterol efflux. Coronary heart disease patients have higher than normal preβ-1 concentrations with decreased functionality and lower than normal large HDL particle concentrations with enhanced functionality.

A critical appraisal of the measurement of serum 'cholesterol efflux capacity' and its use as surrogate marker of risk of cardiovascular disease

The 'cholesterol efflux capacity (CEC)' assay is a simple in vitro measure of the capacities of individual sera to promote the first step of the reverse cholesterol transport pathway, the delivery of cellular cholesterol to plasma HDL. This review describes the cell biology of this model and critically assesses its application as a marker of cardiovascular risk. We describe the pathways for cell cholesterol export, current cell models used in the CEC assay with their limitations and consider the contribution that measurement of serum CEC provides to our understanding of HDL function in vivo.

Where next with HDL assays?

PURPOSE OF REVIEW

The inverse association between HDL cholesterol (HDL-C) and cardiovascular disease (CVD) has been unequivocally proven in the past several decades. However, some interventions aiming to increase HDL-C failed to reduce CVD risk. HDL is structurally and functionally complex and HDL-associated metrics other than HDL-C, such as the concentration, composition, and functionality of HDL particles, have been considered as better determinants of CVD risk. A large body of recent research has addressed changes in HDL functions and HDL subpopulations in CVD with the goal of discovering novel and reliable biomarkers and targets for the treatment or prevention of CVD.

RECENT FINDINGS

We have reviewed recent findings on HDL composition, HDL particle concentrations, and cell-cholesterol efflux capacity that have lately contributed to our understanding of HDL's role in CVD.

SUMMARY

We point out that a major problem in HDL research is the lack of standardization of HDL assays that has led to discrepancies among studies. Therefore, there is a need for new standardized assays that capture the complexities of key HDL parameters.

HDL Cholesterol Metabolism and the Risk of CHD: New Insights from Human Genetics

PURPOSE OF REVIEW

Elevated high-density lipoprotein cholesterol levels in the blood (HDL-C) represent one of the strongest epidemiological surrogates for protection against coronary heart disease (CHD), but recent human genetic and pharmacological intervention studies have raised controversy about the causality of this relationship. Here, we review recent discoveries from human genome studies using new analytic tools as well as relevant animal studies that have both addressed, and in some cases, fueled this controversy.

RECENT FINDINGS

Methodologic developments in genotyping and sequencing, such as genome-wide association studies (GWAS), exome sequencing, and exome array genotyping, have been applied to the study of HDL-C and risk of CHD in large, multi-ethnic populations. Some of these efforts focused on population-wide variation in common variants have uncovered new polymorphisms at novel loci associated with HDL-C and, in some cases, CHD risk. Other efforts have discovered loss-of-function variants for the first time in genes previously implicated in HDL metabolism through common variant studies or animal models. These studies have allowed the genetic relationship between these pathways, HDL-C and CHD to be explored in humans for the first time through analysis tools such as Mendelian randomization. We explore these discoveries for selected key HDL-C genes CETP, LCAT, LIPG, SCARB1, and novel loci implicated from GWAS including GALNT2, KLF14, and TTC39B. Recent human genetics findings have identified new nodes regulating HDL metabolism while reshaping our current understanding of known candidate genes to HDL and CHD risk through the study of critical variants across model systems. Despite their effect on HDL-C, variants in many of the reviewed genes were found to lack any association with CHD. These data collectively indicate that HDL-C concentration, which represents a static picture of a very dynamic and heterogeneous metabolic milieu, is unlikely to be itself causally protective against CHD. In this context, human genetics represent an extremely valuable tool to further explore the biological mechanisms regulating HDL metabolism and investigate what role, if any, HDL plays in the pathogenesis of CHD.

Time to ditch HDL-C as a measure of HDL function?

PURPOSE OF REVIEW

Epidemiological and clinical studies link low levels of HDL cholesterol (HDL-C) with increased risk of atherosclerotic cardiovascular disease (CVD). However, genetic polymorphisms linked to HDL-C do not associate consistently with CVD risk, and randomized clinical studies of drugs that elevate HDL-C via different mechanisms failed to reduce CVD risk in statin-treated patients with established CVD. New metrics that capture HDL's proposed cardioprotective effects are therefore urgently needed.

RECENT FINDINGS

Recent studies demonstrate cholesterol efflux capacity (CEC) of serum HDL (serum depleted of cholesterol-rich atherogenic lipoproteins) is an independent and better predictor of incident and prevalent CVD risk than HDL-C. However, it remains unclear whether therapies that increase CEC are cardioprotective. Other key issues are the impact of HDL-targeted therapies on HDL particle size and concentration and the relationship of those changes to CEC and cardioprotection.

SUMMARY

It is time to end the clinical focus on HDL-C and to understand how HDL's function, protein composition and size contribute to CVD risk. It will also be important to link variations in function and size to HDL-targeted therapies. Developing new metrics for quantifying HDL function, based on better understanding HDL metabolism and macrophage CEC, is critical for achieving these goals.

Role of Niacin in Current Clinical Practice: A Systematic Review

BACKGROUND

Niacin, a potent high-density lipoprotein cholesterol-raising drug, seems an attractive approach to reduce cardiac events in patients with or at risk of atherosclerotic cardiovascular disease. However, previous evidence for niacin has been challenged recently by negative outcomes in 2 large, randomized, controlled trials comparing niacin to placebo with background statin therapy. We studied the currently available evidence for the role of niacin treatment for reducing the risk of cardiovascular events in current practice.

METHODS

A systematic review of randomized controlled trials in the MEDLINE, EMBASE, CINAHL, and Cochrane databases comparing niacin alone or combined with statin therapy was performed. We extracted trial level data, including basic characteristics and number of patients enrolled, duration of follow up, occurrence of adverse events, and cardiovascular-related outcomes. Random effects meta-analysis was conducted to estimate the risk ratio (RR) for individual trial endpoints.

RESULTS

Thirteen trials (N = 35,206) were selected for final analysis. The mean follow-up duration was 32.8 months. Overall, niacin led to significant increases in serum high-density lipoprotein cholesterol levels from baseline trial enrolment by 21.4%, 9.31 (95% confidence interval [CI] 5.11-13.51) mg/dL. However, we did not observe any differences in all-cause mortality rates (RR 0.99; 95% CI 0.88-1.12) between niacin and control arms. Further, niacin treatment was associated with a trend toward lower risk of cardiovascular mortality (RR 0.91; 95% CI 0.81-1.02), coronary death (RR 0.93; 95% CI 0.78-1.10), nonfatal myocardial infarction (RR 0.85; 95% CI 0.73-1.0), revascularization (coronary and noncoronary) (RR 0.83; 95% CI 0.65-1.06), and stroke (RR 0.89; 95% CI 0.72-1.10), compared with control.

CONCLUSION

Niacin therapy does not lead to significant reductions in total or cause-specific mortality or recurrent cardiovascular events among persons with or at risk of atherosclerotic cardiovascular disease.

Nicotinic acid timed to feeding reverses tissue lipid accumulation and improves glucose control in obese Zucker rats[S]

Nicotinic acid (NiAc) is a potent inhibitor of lipolysis, acutely reducing plasma free fatty acid (FFA) concentrations. However, a major FFA rebound is seen during rapid NiAc washout, and sustained exposure is associated with tolerance development, with FFAs returning to pretreatment levels. Our aim was to find a rational NiAc dosing regimen that preserves FFA lowering, sufficient to reverse nonadipose tissue lipid accumulation and improve metabolic control, in obese Zucker rats. We compared feeding-period versus fasting-period NiAc dosing for 5 days: 12 h subcutaneous infusion (programmable, implantable mini-pumps) terminated by gradual withdrawal. It was found that NiAc timed to feeding decreased triglycerides in liver (-47%; P < 0.01) and heart (-38%; P < 0.05) and reduced plasma fructosamine versus vehicle. During oral glucose tolerance test, plasma FFA levels were reduced with amelioration of hyperglycemia and hypertriglyceridemia. Furthermore, timing NiAc to feeding resulted in a general downregulation of de novo lipogenesis (DNL) genes in liver. By contrast, NiAc timed to fasting did not reduce tissue lipids, ameliorate glucose intolerance or dyslipidemia, or alter hepatic DNL genes. In conclusion, NiAc dosing regimen has a major impact on metabolic control in obese Zucker rats. Specifically, a well-defined NiAc exposure, timed to feeding periods, profoundly improves the metabolic phenotype of this animal model.

Oxidative stress, HDL functionality and effects of intravenous iron administration in women with iron deficiency anemia

BACKGROUND AND AIMS

Iron deficiency anemia (IDA) affects around 20-30% of adults worldwide. An association between IDA and cardiovascular disease (CVD) has been reported. Oxidative stress, inflammation and low concentration of high-density lipoproteins (HDL) were implicated on endothelial dysfunction and CVD in IDA. We studied the effects of iron deficiency and of an intravenous iron administration on oxidative stress and HDL characteristics in IDA women.

METHODS

Two studies in IDA women are presented: a case-control study, including 18 patients and 18 age-matched healthy women, and a follow-up study 72hr after the administration of intravenous iron (n = 16). Lipids, malondialdehyde, cholesteryl ester transfer protein (CETP), paraoxonase-1 (PON-1) and HDL chemical composition and functionality (cholesterol efflux and antioxidative activity) were measured. Cell cholesterol efflux from iron-deficient macrophages to a reference HDL was also evaluated.

RESULTS

IDA patients showed higher triglycerides and CETP activity and lower HDL-C than controls (all p < 0.001). HDL particles from IDA patients showed higher triglyceride content (+30%,p < 0.05) and lower antioxidative capacity (-23%,p < 0.05). Although HDL-mediated cholesterol efflux was similar between the patients and controls, iron deficiency provoked a significant reduction in macrophage cholesterol efflux (-25%,p < 0.05). Arylesterase activity of PON-1 was significantly lower in IDA patients than controls (-16%,p < 0.05). The intravenous administration of iron was associated with a decrease in malondialdehyde levels and an increase in arylesterase activity of PON-1 (-22% and +18%, respectively, p < 0.05).

CONCLUSION

IDA is associated with oxidative stress and functionally deficient HDL particles. It remains to be determined if such alterations suffice to impair endothelial function in IDA.