Niacin increases HDL biogenesis by enhancing DR4-dependent transcription of ABCA1 and lipidation of apolipoprotein A-I in HepG2 cells

Effects of niacin on apo A1 and B levels: a systematic review and meta-analysis of randomised controlled trials

Niacin has been investigated for its potential impact on lipid metabolism and cardiovascular health. This meta-analysis aims to systematically evaluate the effects of niacin interventions on apo A1 and apo B levels, key regulators of lipoprotein metabolism and markers of cardiovascular risk. A comprehensive search of the literature was performed on five databases of PubMed, Scopus, Web of Science, Embase and Cochrane library, from inception up to 15 July 2023. This search identified 1452 publications, from which twelve randomised controlled trials met the inclusion criteria. The intervention dosages ranged from 500 to 3000 mg/d, and the study durations spanned from 6 to 102·8 weeks. The niacin intervention demonstrated a significant reduction in apo B levels (weighted mean differences (WMD): -24·37 mg/dl, P = 0·01). Subgroup analyses indicated that intervention duration played a role, with trials of ≤ 16 weeks showing a greater reduction in apo B. Regarding apo A1, niacin significantly increased its levels (WMD: 8·23 mg/dl, P < 0·001). Subgroup analyses revealed that the beneficial effects of niacin on apo A1 were observed at a dosage of > 1500 mg/d (P < 0·001), and extended-release niacin was more effective compared with other forms (P < 0·001). According to the Begg's regression test, no publication bias was observed in this systematic review and meta-analysis. This meta-analysis highlights niacin's potential role in improving lipid profiles and cardiovascular health. Further well-designed clinical trials are needed to elucidate and confirm optimal dosages and durations of niacin interventions for influencing apo A1 and B.

Potential Therapeutic Agents That Target ATP Binding Cassette A1 (ABCA1) Gene Expression

The cholesterol efflux protein ATP binding cassette protein A1 (ABCA) and apolipoprotein A1 (apo A1) are key constituents in the process of reverse-cholesterol transport (RCT), whereby excess cholesterol in the periphery is transported to the liver where it can be converted primarily to bile acids for either use in digestion or excreted. Due to their essential roles in RCT, numerous studies have been conducted in cells, mice, and humans to more thoroughly understand the pathways that regulate their expression and activity with the goal of developing therapeutics that enhance RCT to reduce the risk of cardiovascular disease. Many of the drugs and natural compounds examined target several transcription factors critical for ABCA1 expression in both macrophages and the liver. Likewise, several miRNAs target not only ABCA1 but also the same transcription factors that are critical for its high expression. However, after years of research and many preclinical and clinical trials, only a few leads have proven beneficial in this regard. In this review we discuss the various transcription factors that serve as drug targets for ABCA1 and provide an update on some important leads.

Role of ABCA1 in Cardiovascular Disease

Cholesterol homeostasis plays a significant role in cardiovascular disease. Previous studies have indicated that ATP-binding cassette transporter A1 (ABCA1) is one of the most important proteins that maintains cholesterol homeostasis. ABCA1 mediates nascent high-density lipoprotein biogenesis. Upon binding with apolipoprotein A-I, ABCA1 facilitates the efflux of excess intracellular cholesterol and phospholipids and controls the rate-limiting step of reverse cholesterol transport. In addition, ABCA1 interacts with the apolipoprotein receptor and suppresses inflammation through a series of signaling pathways. Thus, ABCA1 may prevent cardiovascular disease by inhibiting inflammation and maintaining lipid homeostasis. Several studies have indicated that post-transcriptional modifications play a critical role in the regulation of ABCA1 transportation and plasma membrane localization, which affects its biological function. Meanwhile, carriers of the loss-of-function ABCA1 gene are often accompanied by decreased expression of ABCA1 and an increased risk of cardiovascular diseases. We summarized the ABCA1 transcription regulation mechanism, mutations, post-translational modifications, and their roles in the development of dyslipidemia, atherosclerosis, ischemia/reperfusion, myocardial infarction, and coronary heart disease.

Remembering your A, B, C's: Alzheimer's disease and ABCA1

The function of ATP binding cassette protein A1 (ABCA1) is central to cholesterol mobilization. Reduced ABCA1 expression or activity is implicated in Alzheimer's disease (AD) and other disorders. Therapeutic approaches to boost ABCA1 activity have yet to be translated successfully to the clinic. The risk factors for AD development and progression, including comorbid disorders such as type 2 diabetes and cardiovascular disease, highlight the intersection of cholesterol transport and inflammation. Upregulation of ABCA1 can positively impact APOE lipidation, insulin sensitivity, peripheral vascular and blood–brain barrier integrity, and anti-inflammatory signaling. Various strategies towards ABCA1-boosting compounds have been described, with a bias toward nuclear hormone receptor (NHR) agonists. These agonists display beneficial preclinical effects; however, important side effects have limited development. In particular, ligands that bind liver X receptor (LXR), the primary NHR that controls ABCA1 expression, have shown positive effects in AD mouse models; however, lipogenesis and unwanted increases in triglyceride production are often observed. The longstanding approach, focusing on LXRβ vs. LXRα selectivity, is over-simplistic and has failed. Novel approaches such as phenotypic screening may lead to small molecule NHR modulators that elevate ABCA1 function without inducing lipogenesis and are clinically translatable.

Regulation of NAD+ metabolism in aging and disease

More than a century after discovering NAD⁺, information is still evolving on the role of this molecule in health and diseases. The biological functions of NAD⁺ and NAD⁺ precursors encompass pathways in cellular energetics, inflammation, metabolism, and cell survival. Several metabolic and neurological diseases exhibit reduced tissue NAD⁺ levels. Significantly reduced levels of NAD⁺ are also associated with aging, and enhancing NAD⁺ levels improved healthspan and lifespan in animal models. Recent studies suggest a causal link between senescence, age-associated reduction in tissue NAD⁺ and enzymatic degradation of NAD⁺. Furthermore, the discovery of transporters and receptors involved in NAD⁺ precursor (nicotinic acid, or niacin, nicotinamide, and nicotinamide riboside) metabolism allowed for a better understanding of their role in cellular homeostasis including signaling functions that are independent of their functions in redox reactions. We also review studies that demonstrate that the functional effect of niacin is partially due to the activation of its cell surface receptor, GPR109a. Based on the recent progress in understanding the mechanism and function of NAD⁺ and NAD⁺ precursors in cell metabolism, new strategies are evolving to exploit these molecules' pharmacological potential in the maintenance of metabolic balance.

Hepatic cholesterol transport and its role in non-alcoholic fatty liver disease and atherosclerosis

Non-alcoholic fatty liver disease (NAFLD) is a quickly emerging global health problem representing the most common chronic liver disease in the world. Atherosclerotic cardiovascular disease represents the leading cause of mortality in NAFLD patients. Cholesterol metabolism has a crucial role in the pathogenesis of both NAFLD and atherosclerosis. The liver is the major organ for cholesterol metabolism. Abnormal hepatic cholesterol metabolism not only leads to NAFLD but also drives the development of atherosclerotic dyslipidemia. The cholesterol level in hepatocytes reflects the dynamic balance between endogenous synthesis, uptake, esterification, and export, a process in which cholesterol is converted to neutral cholesteryl esters either for storage in cytosolic lipid droplets or for secretion as a major constituent of plasma lipoproteins, including very-low-density lipoproteins, chylomicrons, high-density lipoproteins, and low-density lipoproteins. In this review, we describe decades of research aimed at identifying key molecules and cellular players involved in each main aspect of hepatic cholesterol metabolism. Furthermore, we summarize the recent advances regarding the biological processes of hepatic cholesterol transport and its role in NAFLD and atherosclerosis.

Dysfunctional High-Density Lipoproteins in Type 2 Diabetes Mellitus: Molecular Mechanisms and Therapeutic Implications

High density lipoproteins (HDLs) are commonly known for their anti-atherogenic properties that include functions such as the promotion of cholesterol efflux and reverse cholesterol transport, as well as antioxidant and anti-inflammatory activities. However, because of some chronic inflammatory diseases, such as type 2 diabetes mellitus (T2DM), significant changes occur in HDLs in terms of both structure and composition. These alterations lead to the loss of HDLs’ physiological functions, to transformation into dysfunctional lipoproteins, and to increased risk of cardiovascular disease (CVD). In this review, we describe the main HDL structural/functional alterations observed in T2DM and the molecular mechanisms involved in these T2DM-derived modifications. Finally, the main available therapeutic interventions targeting HDL in diabetes are discussed.

The Role of GPR109a Signaling in Niacin Induced Effects on Fed and Fasted Hepatic Metabolism

Signaling through GPR109a, the putative receptor for the endogenous ligand β-OH butyrate, inhibits adipose tissue lipolysis. Niacin, an anti-atherosclerotic drug that can induce insulin resistance, activates GPR109a at nM concentrations. GPR109a is not essential for niacin to improve serum lipid profiles. To better understand the involvement of GPR109a signaling in regulating glucose and lipid metabolism, we treated GPR109a wild-type (+/+) and knockout (-/-) mice with repeated overnight injections of saline or niacin in physiological states characterized by low (ad libitum fed) or high (16 h fasted) concentrations of the endogenous ligand, β-OH butyrate. In the fed state, niacin increased expression of apolipoprotein-A1 mRNA and decreased sterol regulatory element-binding protein 1 mRNA independent of genotype, suggesting a possible GPR109a independent mechanism by which niacin increases high-density lipoprotein (HDL) production and limits transcriptional upregulation of lipogenic genes. Niacin decreased fasting serum non-esterified fatty acid concentrations in both GPR109a +/+ and -/- mice. Independent of GPR109a expression, niacin blunted fast-induced hepatic triglyceride accumulation and peroxisome proliferator-activated receptor α mRNA expression. Although unaffected by niacin treatment, fasting serum HDL concentrations were lower in GPR109a knockout mice. Surprisingly, GPR109a knockout did not affect glucose or lipid homeostasis or hepatic gene expression in either fed or fasted mice. In turn, GPR109a does not appear to be essential for the metabolic response to the fasting ketogenic state or the acute effects of niacin.

The efficacy of niacin supplementation in type 2 diabetes patients: Study protocol of a randomized controlled trial

BACKGROUND

Dyslipidemia is a main risk factor of cardiovascular disease in the diabetic patients. Niacin was found acutely to decrease the plasma concentration of free fatty acids by inhibiting their mobilization from adipose tissue. This present study is a double blinded, randomized, and prospective trial to determine the effect of niacin during dyslipidemia in type 2 diabetic patients.

METHODS

This randomized controlled, double-blinded, single center trial is carried out according to the principles of Declaration of Helsinki. This present study was approved in institutional review committee of the Second Affiliated Hospital of Dalian Medical University. All the patients received the informed consent. Diabetic patients were randomized (1:1) to receive 3-month treatment with extended-release niacin or matching placebo. The major outcome of our present study was the change in the level of HbA1c from the baseline to week 12. Secondary outcome measures contained the levels of fasting blood glucose, the concentrations of serum transaminase, the other laboratory variables, and self-reported adverse events. The P < .05 was regarded as statistically significant.

RESULTS

We assumed that adding the niacin to the medication in patients with type 2 diabetes would reduce dyslipidemia and achieve target lipid levels.

TRIAL REGISTRATION

This study protocol was registered in Research Registry (researchregistry5925).

Brothers in Arms: ABCA1- and ABCG1-Mediated Cholesterol Efflux as Promising Targets in Cardiovascular Disease Treatment

Atherosclerosis is a leading cause of cardiovascular disease worldwide, and hypercholesterolemia is a major risk factor. Preventive treatments mainly focus on the effective reduction of low-density lipoprotein cholesterol, but their therapeutic value is limited by the inability to completely normalize atherosclerotic risk, probably due to the disease complexity and multifactorial pathogenesis. Consequently, high-density lipoprotein cholesterol gained much interest, as it appeared to be cardioprotective due to its major role in reverse cholesterol transport (RCT). RCT facilitates removal of cholesterol from peripheral tissues, including atherosclerotic plaques, and its subsequent hepatic clearance into bile. Therefore, RCT is expected to limit plaque formation and progression. Cellular cholesterol efflux is initiated and propagated by the ATP-binding cassette (ABC) transporters ABCA1 and ABCG1. Their expression and function are expected to be rate-limiting for cholesterol efflux, which makes them interesting targets to stimulate RCT and lower atherosclerotic risk. This systematic review discusses the molecular mechanisms relevant for RCT and ABCA1 and ABCG1 function, followed by a critical overview of potential pharmacological strategies with small molecules to enhance cellular cholesterol efflux and RCT. These strategies include regulation of ABCA1 and ABCG1 expression, degradation, and mRNA stability. Various small molecules have been demonstrated to increase RCT, but the underlying mechanisms are often not completely understood and are rather unspecific, potentially causing adverse effects. Better understanding of these mechanisms could enable the development of safer drugs to increase RCT and provide more insight into its relation with atherosclerotic risk. SIGNIFICANCE STATEMENT: Hypercholesterolemia is an important risk factor of atherosclerosis, which is a leading pathological mechanism underlying cardiovascular disease. Cholesterol is removed from atherosclerotic plaques and subsequently cleared by the liver into bile. This transport is mediated by high-density lipoprotein particles, to which cholesterol is transferred via ATP-binding cassette transporters ABCA1 and ABCG1. Small-molecule pharmacological strategies stimulating these transporters may provide promising options for cardiovascular disease treatment.

Niacin: an old lipid drug in a new NAD+ dress

Niacin, the first antidyslipidemic drug, has been at the center stage of lipid research for many decades before the discovery of statins. However, to date, despite its remarkable effects on lipid profiles, the clinical outcomes of niacin treatment on cardiac events is still debated. In addition to its historically well-defined interactions with central players of lipid metabolism, niacin can be processed by eukaryotic cells to synthesize a crucial cofactor, NAD+ NAD+ acts as a cofactor in key cellular processes, including oxidative phosphorylation, glycolysis, and DNA repair. More recently, evidence has emerged that NAD+ also is an essential cosubstrate for the sirtuin family of protein deacylases and thereby has an impact on a wide range of cellular processes, most notably mitochondrial homeostasis, energy homeostasis, and lipid metabolism. NAD+ achieves these remarkable effects through sirtuin-mediated deacetylation of key transcriptional regulators, such as peroxisome proliferator-activated receptor gamma coactivator 1-α, LXR, and SREBPs, that control these cellular processes. Here, we present an alternative point of view to explain niacin's mechanism of action, with a strong focus on the importance of how this old drug acts as a control switch of NAD+/sirtuin-mediated control of metabolism.

[Pharmacological targets for dislipidemies correction. Opportunities and prospects of therapeutic usage]

Literature data on influence of existing and new groups of drug preparations for dyslipidemias correction are systemized, and molecular mechanisms of their effects are reviewed. The results of experimental and clinical investigations aimed at revealing of new pharmacological targets of dyslipidemias correction were analyzed. The approaches for activation of high density lipoproteins functionality are described. The implementation of alternative preparations with new alternative mechanisms of action may be suggested to improve the effectiveness of traditional treatment in the future.

Apolipoprotein A1 Inhibits the TGF-β1-Induced Endothelial-to-Mesenchymal Transition of Human Coronary Artery Endothelial Cells

OBJECTIVE

Transforming growth factor β1 (TGF-β1) is the major cytokine for stimulating endothelial cells (ECs) to transdifferentiate to mesenchymal cells (MCs) in the process known as endothelial-to-mesenchymal transition (EndMT). Recently, TGF-β1-induced EndMT has been implicated in the pathogenesis of atherosclerosis (AS). It has been identified that apolipoprotein A1 (ApoA-I) obstructs TGF-β1-induced endothelial dysfunction, providing a protective effect for ECs and also anti-AS activity. However, the exact role of ApoA-I in TGF-β1-induced EndMT is not clear. In this study, we aimed to investigate whether ApoA-I can modulate TGF-β1-induced EndMT in human coronary artery ECs (HCAECs).

METHODS AND RESULTS

The HCAECs were treated with TGF-β1 with or without ApoA-I. Morphological changes in HCAECs and the expression of EndMT-related markers were evaluated. HCAECs treated with TGF-β1 were found to transform to MC morphology, with inconspicuous expression of EC markers such as vascular endothelial cadherin and CD31, and conspicuous expression of fibroblast-specific protein 1 (FSP-1) and α-smooth muscle actin. The treatment of HCAECs with ApoA-I inhibited the TGF-β1-induced EndMT, and elevated expression of EC markers was observed but reduced expression of MC markers. Moreover, ApoA-I impeded the expression level of Slug and Snail, crucial transcriptional factors of EndMT, and it inhibited the TGF-β1-induced phosphorylation of Smad2 and Smad3 which affected the EC morphology. In addition, the knockdown of ABCA1 by RNA interference eliminated the inhibition effect of ApoA-I on TGF-β1-induced EndMT.

CONCLUSIONS

Our findings revealed a novel mechanism for the ApoA-I protective effect on endothelium function via the inhibition of TGF-β1-induced EndMT. This might provide new insights for developing strategies for modulating AS and vascular remodeling.

Niacin Promotes Cardiac Healing after Myocardial Infarction through Activation of the Myeloid Prostaglandin D2 Receptor Subtype 1

Niacin is a well established drug used to lower cholesterol and prevent cardiovascular disease events. However, niacin also causes cutaneous flushing side effects due to release of the proresolution mediator prostaglandin D₂ (PGD₂). Recent randomized clinical trials have demonstrated that addition of niacin with laropiprant [a PGD₂ receptor subtype 1 (DP1) blocker] to statin-based therapies does not significantly decrease the risk of cardiovascular disease events, but increases the risk of serious adverse events. Here, we tested whether, and how, niacin beneficial effects on myocardial ischemia require the activation of the PGD₂/DP1 axis. Myocardial infarction (MI) was reproduced by ligation of the left anterior descending branch of the coronary artery in mice. We found that niacin increased PGD₂ release in macrophages and shifted macrophages to M2 polarization both in vitro and in vivo by activation of DP1 and accelerated inflammation resolution in zymosan-induced peritonitis in mice. Moreover, niacin treatment facilitated wound healing and improved cardiac function after MI through DP1-mediated M2 bias and timely resolution of inflammation in infarcted hearts. In addition, we found that niacin intake also stimulated M2 polarization of peripheral monocytes in humans. Collectively, niacin promoted cardiac functional recovery after ischemic myocardial infarction through DP1-mediated M2 polarization and timely resolution of inflammation in hearts. These results indicated that DP1 inhibition may attenuate the cardiovascular benefits of niacin.

Pleiotropic effects of niacin: Current possibilities for its clinical use

Niacin was the first hypolipidemic drug to significantly reduce both major cardiovascular events and mortality in patients with cardiovascular disease. Niacin favorably influences all lipoprotein classes, including lipoprotein[a],and belongs to the most potent hypolipidemic drugs for increasing HDL-C. Moreover, niacin causes favorable changes to the qualitative composition of lipoprotein HDL. In addition to its pronounced hypolipidemic action, niacin exerts many other, non-hypolipidemic effects (e.g., antioxidative, anti-inflammatory, antithrombotic), which favorably influence the development and progression of atherosclerosis. These effects are dependent on activation of the specific receptor HCA2. Recent results published by the two large clinical studies, AIM-HIGH and HPS2-THRIVE, have led to the impugnation of niacin's role in future clinical practice. However, due to several methodological flaws in the AIM-HIGH and HPS2-THRIVE studies, the pleiotropic effects of niacin now deserve thorough evaluation. This review summarizes the present and possible future use of niacin in clinical practice in light of its newly recognized pleiotropic effects.

Niacin Alternatives for Dyslipidemia: Fool's Gold or Gold Mine? Part II: Novel Niacin Mimetics

Two cardiovascular outcome trials established niacin 3 g daily prevents hard cardiac events. However, as detailed in part I of this series, an extended-release (ER) alternative at only 2 g nightly demonstrated no comparable benefits in two outcome trials, implying the alternative is not equivalent to the established cardioprotective regimen. Since statins leave a significant treatment gap, this presents a major opportunity for developers. Importantly, the established regimen is cardioprotective, so the pathway is likely beneficial. Moreover, though effective, the established cardioprotective regimen is cumbersome, limiting clinical use. At the same time, the ER alternative has been thoroughly discredited as a viable substitute for the established cardioprotective regimen. Therefore, by exploiting the pathway and skillfully avoiding the problems with the established cardioprotective regimen and the ER alternative, developers could validate cardioprotective variations facing little meaningful competition from their predecessors. Thus, shrewd developers could effectively tap into a gold mine at the grave of the ER alternative. The GPR109A receptor was discovered a decade ago, leading to a large body of evidence commending the niacin pathway to a lower cardiovascular risk beyond statins. While mediating niacin's most prominent adverse effects, GPR109A also seems to mediate anti-lipolytic, anti-inflammatory, and anti-atherogenic effects of niacin. Several developers are investing heavily in novel strategies to exploit niacin's therapeutic pathways. These include selective GPR109A receptor agonists, niacin prodrugs, and a niacin metabolite, with encouraging early phase human data. In part II of this review, we summarize the accumulated results of these early phase studies of emerging niacin mimetics.

[Consecutive formation of the functions of high-, low-density and very-low-density lipoproteins during phylogenesis. Unique algorithm of the effects of lipid-lowering drugs]

During phylogenesis, all fatty acids (FA) were initially transported to cells by apoA-I high-density lipoproteins (HDL) in polar lipids. Later, active cellular uptake of saturated, monoenoic and unsaturated FA occurred via triglycerides (TG) in low-density lipoproteins (LDL). Active uptake of polyenoic FA (PUFA) required the following: a) PUFA re-esterified from polar phospholipids into nonpolar cholesteryl polyesters (poly-CLE), b) a novel protein, cholesteryl ester transfer protein (CETP), initiated poly-CLE transformation from HDL to LDL. CETP formed blood HDL-CETP-LDL complexes in which poly-CLE spontaneously came from polar lipids of TG in HDL to nonpolar TG in LDL. Then ligand LDLs formed and the cells actively absorbed PUFA via apoB-100 endocytosis. Some animal species (rats, mice, dogs) developed a spontaneous CETP-minus mutation followed by population death from atherosclerosis. However, there was another active CETP-independent uptake formed during phylogenesis; the cells internalized poly-CLE in HDL. Since apoA-I had no domain-ligand, another apoE/A-I ligand formed; the cells began synthesizing apoE/A-1 receptors. In cells of rabbits and primates absorbed cells PUFA consecutively: HDL-->LDL-->apoB-100 endocytosis; those of rats and dogs did HDL directly: HDL-->anoE/A-I endocytosis. In the rabbits, CETP was high, apoE in HDL was low, and the animals were sensitive to exogenous hypercholesterolemia. In the rats, CETP was low and ApoE in HDL-was high, and the animals were resistant to hypercholesterolemia. Reduced bioavailability of PUFA during their consecutive cellular uptake and develdpment of intercellular PUFA deficiency are fundamental to the pathogenesis of atherosclerosis.

Vitamin D Protects Against Atherosclerosis via Regulation of Cholesterol Efflux and Macrophage Polarization in Hypercholesterolemic Swine

OBJECTIVE

Prevalence of vitamin D (VD) deficiency and its association with the risk of cardiovascular disease prompted us to evaluate the effect of VD status on lipid metabolism and atherosclerosis in hypercholesterolemic microswine.

APPROACH AND RESULTS

Yucatan microswine were fed with VD-deficient (0 IU/d), VD-sufficient (1000 IU/d), or VD-supplemented (3000 IU/d) high-cholesterol diet for 48 weeks. Serum lipids and 25(OH)-cholecalciferol levels were measured biweekly. Histology and biochemical parameters of liver and arteries were analyzed. Effect of 1,25(OH)2D3 on cholesterol metabolism was examined in human hepatocyte carcinoma cell line (HepG2) and human monocytic cell line (THP-1) macrophage-derived foam cells. VD deficiency decreased plasma high-density lipoprotein levels, expression of liver X receptors, ATP-binding membrane cassette transporter A1, and ATP-binding membrane cassette transporter G1 and promoted cholesterol accumulation and atherosclerosis in hypercholesterolemic microswine. VD promoted nascent high-density lipoprotein formation in HepG2 cells via ATP-binding membrane cassette transporter A1-mediated cholesterol efflux. Cytochrome P450 (CYP)27B1 and VD receptor were predominantly present in the CD206(+) M2 macrophage foam cell-accumulated cores in coronary artery plaques. 1,25(OH)2D3 increased the expression of liver X receptors, ATP-binding membrane cassette transporter A1, and ATP-binding membrane cassette transporter G1 and promoted cholesterol efflux in THP-1 macrophage-derived foam cells. 1,25(OH)2D3 decreased intracellular free cholesterol and polarized macrophages to M2 phenotype with decreased expression of tumor necrosis factor-α, interleukin-1β, interleukin-6 under lipopolysaccharide stimulation. 1,25(OH)2D3 markedly induced CYP27A1 expression via a VD receptor-dependent c-Jun N-terminal kinase (JNK) 1/2 signaling pathway and increased 27-hydroxycholesterol levels, which induced liver X receptors, ATP-binding membrane cassette transporter A1, and ATP-binding membrane cassette transporter G1 expression and stimulated cholesterol efflux that was inhibited by VD receptor antagonist and JNK1/2 signaling inhibitor in THP-1 macrophage-derived foam cell.

CONCLUSIONS

VD protects against atherosclerosis in hypercholesterolemic swine via controlling cholesterol efflux and macrophage polarization via increased CYP27A1 activation.

Nicotinic Acid Accelerates HDL Cholesteryl Ester Turnover in Obese Insulin-Resistant Dogs

AIM

Nicotinic acid (NA) treatment decreases plasma triglycerides and increases HDL cholesterol, but the mechanisms involved in these change are not fully understood. A reduction in cholesteryl ester transfer protein (CETP) activity has been advanced to explain most lipid-modulating effects of NA. However, due to the central role of CETP in reverse cholesterol transport in humans, other effects of NA may have been hidden. As dogs have no CETP activity, we conducted this study to examine the specific effects of extended-release niacin (NA) on lipids and high-density lipoprotein (HDL) cholesteryl ester (CE) turnover in obese Insulin-Resistant dogs with increase plasma triglycerides.

METHODS

HDL kinetics were assessed in fasting dogs before and four weeks after NA treatment through endogenous labeling of cholesterol and apolipoprotein AI by simultaneous infusion of [1,2 13C2] acetate and [5,5,5 2H3] leucine for 8 h. Kinetic data were analyzed by compartmental modeling. In vitro cell cholesterol efflux of serum from NA-treated dogs was also measured.

RESULTS

NA reduced plasma total cholesterol, low-density lipoprotein cholesterol, HDL cholesterol, triglycerides (TG), and very-low-density lipoprotein TG concentrations (p < 0.05). The kinetic study also showed a higher cholesterol esterification rate (p < 0.05). HDL-CE turnover was accelerated (p < 0.05) via HDL removal through endocytosis and selective CE uptake (p < 0.05). We measured an elevated in vitro cell cholesterol efflux (p < 0.05) with NA treatment in accordance with a higher cholesterol esterification.

CONCLUSION

NA decreased HDL cholesterol but promoted cholesterol efflux and esterification, leading to improved reverse cholesterol transport. These results highlight the CETP-independent effects of NA in changes of plasma lipid profile.

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.

Will Lipidation of ApoA1 through Interaction with ABCA1 at the Intestinal Level Affect the Protective Functions of HDL?

The relationship between levels of high-density lipoprotein cholesterol (HDL-C) and cardiovascular (CV) risk is well recognized; however, in recent years, large-scale phase III studies with HDL-C-raising or -mimicking agents have failed to demonstrate a clinical benefit on CV outcomes associated with raising HDL-C, casting doubt on the "HDL hypothesis." This article reviews potential reasons for the observed negative findings with these pharmaceutical compounds, focusing on the paucity of translational models and relevant biomarkers related to HDL metabolism that may have confounded understanding of in vivo mechanisms. A unique function of HDL is its ability to interact with the ATP-binding cassette transporter (ABC) A1 via apolipoprotein (Apo) A1. Only recently, studies have shown that this process may be involved in the intestinal uptake of dietary sterols and antioxidants (vitamin E, lutein and zeaxanthin) at the basolateral surface of enterocytes. This parameter should be assessed for HDL-raising drugs in addition to the more documented reverse cholesterol transport (RCT) from peripheral tissues to the liver. Indeed, a single mechanism involving the same interaction between ApoA1 and ABCA1 may encompass two HDL functions previously considered as separate: antioxidant through the intestinal uptake of antioxidants and RCT through cholesterol efflux from loaded cells such as macrophages.

High Density Lipoprotein Cholesterol Increasing Therapy: The Unmet Cardiovascular Need

Despite aggressive strategies are now available to reduce LDL-cholesterol, the risk of cardiovascular events in patients with coronary artery disease remains substantial. Several preclinical and clinical studies have shown that drug therapy ultimately leads to a regression of the angiographic lesions but also results in a reduction in cardiovascular events. The dramatic failure of clinical trials evaluating the cholesterol ester transfer protein (CEPT) inhibitors, torcetrapib and dalcetrapib, has led to considerable doubt about the value of the current strategy to raise high-density lipoprotein cholesterol (HDL-C) as a treatment for cardiovascular disease. These clinical results, as well as animal studies, have revealed the complexity of HDL metabolism, assessing a more important role of functional quality compared to circulating quantity of HDL. As a result, HDL-based therapeutic interventions that maintain or enhance HDL functionality, such as improving its main property, the reverse cholesterol transport, require closer investigation. In this review, we will discuss HDL metabolism and function, clinical-trial data available for HDL-raising agents, and potential strategies for future HDL-based therapies.

High-density lipoprotein metabolism, composition, function, and deficiency

PURPOSE OF REVIEW

To examine the recent advances in our knowledge of HDL metabolism, composition, function, and coronary heart disease (CHD), as well as marked HDL deficiency states because of mutations in the apolipoprotein (apo) A-I, ATP-binding cassette transfer protein A1 and lecithin cholesterol acyltransferase (LCAT) gene loci.

RECENT FINDINGS

It has been documented that apoA-I, myeloperoxidase and paraoxonase 1 (PON1) form a complex in HDL that is critical for HDL binding and function. Myeloperoxidase has a negative impact on HDL function, whereas PON1 has a beneficial effect. Patients who lack apoA-I develop markedly premature CHD. Patients who lack ATP-binding cassette transfer protein A1 transporter function have only very small discoidal preβ-1 HDL, and develop hepatosplenomegaly, intermittent neuropathy and premature CHD, although significant heterogeneity for these disorders has been reported. Patients with LCAT deficiency have abnormal small discoidal LDLs and HDL particles, and develop kidney failure. Enzyme replacement therapy is being developed for the latter disorder.

SUMMARY

Recent data indicates that proteins other than apoA-I and apoA-II such as MPO and PON1 have important effects on HDL function. There has been considerable recent progress made in our understanding of HDL protein content and function.

Effect of Niacin on High-Density Lipoprotein Apolipoprotein A-I Kinetics in Statin-Treated Patients With Type 2 Diabetes Mellitus

Objective—

To investigate the effect of extended-release (ER) niacin on the metabolism of high-density lipoprotein (HDL) apolipoprotein A-I (apoA-I) in men with type 2 diabetes mellitus on a background of optimal statin therapy.

Approach and Results—

Twelve men with type 2 diabetes mellitus were recruited for a randomized, crossover design trial. Patients were randomized to rosuvastatin or rosuvastatin plus ER niacin for 12 weeks and then crossed over to the alternate therapy after a 3-week washout period. Metabolic studies were performed at the end of each treatment period. HDL apoA-I kinetics were measured after a standardized liquid mixed meal and a bolus injection of d3-leucine for 96 hours. Compartmental analysis was used to model the data. ER niacin significantly decreased plasma triglyceride, plasma cholesterol, non-HDL cholesterol, low-density lipoprotein cholesterol, and apoB (all P <0.05) and significantly increased HDL cholesterol and apoA-I concentrations ( P <0.005 and P <0.05, respectively). ER niacin also significantly increased HDL apoA-I pool size (6088±292 versus 5675±305 mg; P <0.001), and this was attributed to a lower HDL apoA-I fractional catabolic rate (0.33±0.01 versus 0.37±0.02 pools/d; P <0.005), with no significant changes in HDL apoA-I production (20.93±0.63 versus 21.72±0.85 mg/kg per day; P =0.28).

Conclusions—

ER niacin increases HDL apoA-I concentration in statin-treated subjects with type 2 diabetes mellitus by lowering apoA-I fractional catabolic rate. The effect on HDL metabolism was independent of the reduction in plasma triglyceride with ER niacin treatment. Whether this finding applies to other dyslipidemic populations remains to be investigated.

The high-fat high-fructose hamster as an animal model for niacin's biological activities in humans

OBJECTIVE

Niacin has been used for more than 50 years to treat dyslipidemia, yet the mechanisms underlying its lipid-modifying effects remain unknown, a situation stemming at least in part from a lack of validated animal models. The objective of this study was to determine if the dyslipidemic hamster could serve as such a model.

MATERIALS/METHODS

Dyslipidemia was induced in Golden Syrian hamsters by feeding them a high-fat, high-cholesterol, and high-fructose (HF/HF) diet. The effect of high-dose niacin treatment for 18 days and 28 days on plasma lipid levels and gene expression was measured.

RESULTS

Niacin treatment produced significant decreases in plasma total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), triglycerides (TG), and free fatty acids (FFA), but had no measureable effect on high-density lipoprotein cholesterol (HDL-C) in the dyslipidemic hamster. Niacin treatment also produced significant increases in hepatic adenosine ATP-Binding Cassette A1 (ABCA1) mRNA, ABCA1 protein, apolipoprotein A-I (Apo A-I) mRNA, and adipose adiponectin mRNA in these animals.

CONCLUSIONS

With the exception of HDL-C, the lipid effects of niacin treatment in the dyslipidemic hamster closely parallel those observed in humans. Moreover, the effects of niacin treatment on gene expression of hepatic proteins related to HDL metabolism are similar to those observed in human cells in culture. The HF/HF-fed hamster could therefore serve as an animal model for niacin's lowering of proatherogenic lipids and mechanisms of action relative to lipid metabolism.

Pharmacogenomics of high-density lipoprotein-cholesterol-raising therapies

High levels of HDL cholesterol (HDL-C) have traditionally been linked to lower incidence of cardiovascular disease, prompting the search for effective and safe HDL-C raising pharmaceutical agents. Although drugs such as niacin and fibrates represent established therapeutic approaches, HDL-C response to such therapies is variable and heritable, suggesting a role for pharmacogenomic determinants. Multiple genetic polymorphisms, located primarily in genes encoding lipoproteins, cholesteryl ester transfer protein, transporters and CYP450 proteins have been shown to associate with HDL-C drug response in vitro and in epidemiologic studies. However, few of the pharmacogenomic findings have been independently validated, precluding the development of clinical tools that can be used to predict HDL-C response and leaving the goal of personalized medicine to future efforts.

Relationship of lipoproteins to cardiovascular events: the AIM-HIGH Trial (Atherothrombosis Intervention in Metabolic Syndrome With Low HDL/High Triglycerides and Impact on Global Health Outcomes)

OBJECTIVES

This study sought to examine the relationship between niacin treatment, lipoproteins, and cardiovascular (CV) outcomes in this secondary analysis of the AIM-HIGH (Atherothrombosis Intervention in Metabolic Syndrome With Low HDL/High Triglycerides and Impact on Global Health Outcomes) trial.

BACKGROUND

During a 3-year follow-up in 3,414 patients with established CV disease and low high-density lipoprotein cholesterol (HDL-C) levels, combined niacin + low-density lipoprotein cholesterol (LDL-C)-lowering therapy did not reduce CV events compared with LDL-C-lowering therapy alone.

METHODS

Subjects taking simvastatin and/or ezetimibe were randomized to receive extended-release (ER) niacin 1,500 to 2,000 mg or minimal immediate-release niacin (≤ 150 mg) as placebo at bedtime. LDL-C levels in both groups were maintained from 40 to 80 mg/dl. Hazard ratios were estimated by using Cox proportional hazards models for relationships between lipoproteins and the composite endpoint of CV death, myocardial infarction, acute coronary syndrome, ischemic stroke, or symptom-driven revascularization.

RESULTS

CV outcomes were not associated with ER niacin in any baseline lipoprotein tertile. In a subset of patients in both the highest triglyceride (≥ 198 mg/dl) and lowest HDL-C (<33 mg/dl) tertiles, ER niacin showed a trend toward benefit (hazard ratio: 0.74, p = 0.073). In-trial LDL-C levels, non-HDL-C levels, and the total cholesterol/HDL-C ratio were positively associated with CV events in the control group, but these relationships were absent in the ER niacin group.

CONCLUSIONS

Baseline lipoprotein tertiles did not predict differential benefit or harm with ER niacin added to LDL-C-lowering therapy, but a small dyslipidemic subgroup may benefit. ER niacin attenuated expected relationships of lipoprotein risk factors with CV events, raising the possibility that nonlipoprotein actions of niacin could affect risk. (Niacin Plus Statin to Prevent Vascular Events [AIM-HIGH]; NCT00120289).

Recent advances in niacin and lipid metabolism

PURPOSE OF REVIEW

This review focuses on the current understanding of the physiological mechanisms of action of niacin on lipid metabolism and atherosclerosis.

RECENT FINDINGS

Emerging findings indicate that niacin decreases hepatic triglyceride synthesis and subsequent VLDL/LDL secretion by directly and noncompetitively inhibiting hepatocyte diacylglycerol acyltransferase 2. Recent studies in mice lacking niacin receptor GPR109A and human clinical trials with GPR109A agonists disproved the long believed hypothesis of adipocyte triglyceride lipolysis as the mechanism for niacin's effect on serum lipids. Niacin, through inhibiting hepatocyte surface expression of β-chain ATP synthase, inhibits the removal of HDL-apolipoprotein (apo) AI resulting in increased apoAI-containing HDL particles. Additional recent findings suggest that niacin by increasing hepatic ATP-binding cassette transporter A1-mediated apoAI lipidation increases HDL biogenesis, thus stabilizing circulation of newly secreted apoAI. New concepts have also emerged on lipid-independent actions of niacin on vascular endothelial oxidative and inflammatory events, myeloperoxidase release from neutrophils and its impact on HDL function, and GPR109A-mediated macrophage inflammatory events involved in atherosclerosis.

SUMMARY

Recent advances have provided physiological mechanisms of action of niacin on lipid metabolism and atherosclerosis. Better understanding of niacin's actions on multiple tissues and targets may be helpful in designing combination therapy and new treatment strategies for atherosclerosis.

Single nucleotide polymorphisms in CETP, SLC46A1, SLC19A1, CD36, BCMO1, APOA5, and ABCA1 are significant predictors of plasma HDL in healthy adults

Background

In a marker-trait association study we estimated the statistical significance of 65 single nucleotide polymorphisms (SNP) in 23 candidate genes on HDL levels of two independent Caucasian populations. Each population consisted of men and women and their HDL levels were adjusted for gender and body weight. We used a linear regression model. Selected genes corresponded to folate metabolism, vitamins B-12, A, and E, and cholesterol pathways or lipid metabolism.

Methods

Extracted DNA from both the Sacramento and Beltsville populations was analyzed using an allele discrimination assay with a MALDI-TOF mass spectrometry platform. The adjusted phenotype, y, was HDL levels adjusted for gender and body weight only statistical analyses were performed using the genotype association and regression modules from the SNP Variation Suite v7.

Results

Statistically significant SNP (where P values were adjusted for false discovery rate) included: CETP (rs7499892 and rs5882); SLC46A1 (rs37514694; rs739439); SLC19A1 (rs3788199); CD36 (rs3211956); BCMO1 (rs6564851), APOA5 (rs662799), and ABCA1 (rs4149267). Many prior association trends of the SNP with HDL were replicated in our cross-validation study. Significantly, the association of SNP in folate transporters (SLC46A1 rs37514694 and rs739439; SLC19A1 rs3788199) with HDL was identified in our study.

Conclusions

Given recent literature on the role of niacin in the biogenesis of HDL, focus on status and metabolism of B-vitamins and metabolites of eccentric cleavage of β-carotene with lipid metabolism is exciting for future study.

Apelin-13 increases expression of ATP-binding cassette transporter A1 via activating protein kinase C α signaling in THP-1 macrophage-derived foam cells

Apelin has an antiatherogenic function through activating protein kinase C (PKC) to initiate a series of cellular signaling pathways. PKC phosphorylates and stabilizes ATP-binding cassette transporter A1 (ABCA1) through inhibiting its degradation mediated by calpain. Thus, in the present study, we investigated whether apelin-13 affects expression of ABCA1 through PKC signaling. The results showed that apelin-13 dramatically increased cholesterol efflux from THP-1 macrophage-derived foam cells and reduced cellular cholesterol levels. ABCA1 protein but not mRNA levels were dramatically increased by apelin-13, and calpain-induced degradation of ABCA1 and calpain activity were suppressed with treatment of apelin-13. However, the effects of apelin-13 on ABCA1 protein expression, cellular cholesterol efflux and calpain activity were abolished by depletion of PKCα, suggesting the potential important role of PKCα. In addition, apelin-13 was shown to phosphorylate serine residues in ABCA1 through the PKCα pathway. Thus, apelin-13 appears to activate PKCα, phosphorylate ABCA1 and inhibit calpain-mediated proteolysis, thereby promoting cholesterol efflux and reducing foam cell formation. Our study herein described a possible mechanism for understanding the antiatherogenic effects of apelin on attenuating the progression of atherosclerosis.

Regression of atherosclerosis

Atherosclerotic cardiovascular disease (CVD) is a complex disorder that leads to premature death and hospitalization. Several drugs have been, or are currently being tested for their ability to reduce cardiovascular mortality and/or promote regression of atherosclerotic lesions. In addition to "hard end point" clinical trials in which total and cardiovascular mortality as well as risk of incident myocardial infarction are considered as outcomes, trials with surrogate end points using imaging biomarkers can rapidly assess the efficacy of new cardiovascular drugs. Low-density lipoprotein-based therapies with statins have been shown to promote atherosclerosis regression, and several other drugs targeting high-density lipoproteins or inflammation/oxidation are currently being tested in both outcomes and imaging trials in which atherosclerosis regression is anticipated. In this review, we focus on the latest progress in CVD and highlight novel drugs that tackle atherosclerosis as well as the currently used and upcoming imaging techniques to optimally measure atherosclerosis progression.