The ATP-binding cassette transporter A1 (ABCA1) is a key mediator of cellular cholesterol efflux and HDL maturation. ABCA1 mRNA has an unusually long 3’ untranslated region, which makes it highly susceptible to microRNA (miRNA) targeting and repression. As such, multiple miRNAs have been reported to directly target ABCA1, including miR-33a/b, miR-26, miR-106b, and miR-758. Many of these miRNAs participate in feed-forward or feedback networks in controlling cholesterol and lipid homeostasis. Antisense oligonucleotide-based inhibition of miR-33 was found to increase HDL-C levels and regress atherosclerosis in mice and non-human primates. In this edition of Circulation Research, two separate studies identified novel miRNA networks driven by nuclear receptor induced miR-144 targeting of ABCA1 and cholesterol efflux. The first study reports that miR-144 serves to buffer uncontrolled ABCA1 activation in response to high cholesterol states and liver X receptor (LXR) activation in macrophages and liver. The second study highlights the role of miR-144 and ABCA1 in promotion of bile acid secretion in response to farensoid X receptor (FXR) activation in the liver. These studies suggest that anti-miR-144, like anti-miR-33, could be a novel approach to targeting HDL and reverse cholesterol transport.
Editorials; high-density lipoprotein cholesterol; lipoproteins; obesity; adipose tissue; Reverse Cholesterol Transport
Recent studies involving HDL-raising therapeutics have greatly changed our understanding of this field. Despite effectively raising HDL-C levels, niacin remains of uncertain clinical benefit. Synthetic niacin receptor agonists are unlikely to raise HDL-C or have other beneficial effects on plasma lipids. Despite the failure in phase 3 of two CETP inhibitors, two potent CETP inhibitors that raise HDL-C levels by > 100% (and reduce LDL-C substantially) are in late stage clinical development. Infusions of recombinant HDL containing ‘wild-type’ apoA-I or apoA-I Milano, as well as autologous delipidated HDL, all demonstrated promising early results and remain in clinical development. A small molecule that causes upregulation of endogenous apoA-I production is also in clinical development. Finally, upregulation of macrophage cholesterol efflux pathways through agonism of liver X receptors or antagonism of miR-33 remains of substantial interest. The field of HDL therapeutics is poised to transition from the ‘HDL-cholesterol hypothesis’ to the ‘HDL flux hypothesis’ in which the impact on flux from macrophage to feces is deemed to be of greater therapeutic benefit than the increase in steady-state concentrations of HDL cholesterol.
Lipids; HDL; reverse cholesterol transport; niacin; GPR109A; cholesteryl ester transfer protein; CETP; anacetrapib; torcetrapib; dalcetrapib; evacetrapib; apoA-I; recombinant HDL; RVX-208; Liver X receptor; miR-33; cardiovascular disease
Obesity and obstructive sleep apnea tend to coexist and are associated with inflammation, insulin resistance, dyslipidemia, and high blood pressure, but their causal relation to these abnormalities is unclear.
We randomly assigned 181 patients with obesity, moderate-to-severe obstructive sleep apnea, and serum levels of C-reactive protein (CRP) greater than 1.0 mg per liter to receive treatment with continuous positive airway pressure (CPAP), a weight-loss intervention, or CPAP plus a weight-loss intervention for 24 weeks. We assessed the incremental effect of the combined interventions over each one alone on the CRP level (the primary end point), insulin sensitivity, lipid levels, and blood pressure.
Among the 146 participants for whom there were follow-up data, those assigned to weight loss only and those assigned to the combined interventions had reductions in CRP levels, insulin resistance, and serum triglyceride levels. None of these changes were observed in the group receiving CPAP alone. Blood pressure was reduced in all three groups. No significant incremental effect on CRP levels was found for the combined interventions as compared with either weight loss or CPAP alone. Reductions in insulin resistance and serum triglyceride levels were greater in the combined-intervention group than in the group receiving CPAP only, but there were no significant differences in these values between the combined-intervention group and the weight-loss group. In per-protocol analyses, which included 90 participants who met prespecified criteria for adherence, the combined interventions resulted in a larger reduction in systolic blood pressure and mean arterial pressure than did either CPAP or weight loss alone.
In adults with obesity and obstructive sleep apnea, CPAP combined with a weight-loss intervention did not reduce CRP levels more than either intervention alone. In secondary analyses, weight loss provided an incremental reduction in insulin resistance and serum triglyceride levels when combined with CPAP. In addition, adherence to a regimen of weight loss and CPAP may result in incremental reductions in blood pressure as compared with either intervention alone.
Recombinant adeno-associated viral vectors based on serotype 8 (AAV8) transduce liver with superior tropism following intravenous (IV) administration. Previous studies conducted by our lab demonstrated that AAV8-mediated transfer of the human low-density lipoprotein receptor (LDLR) gene driven by a strong liver-specific promoter (thyroxin-binding globulin [TBG]) leads to high level and persistent gene expression in the liver. The approach proved efficacious in reducing plasma cholesterol levels and resulted in the regression of atherosclerotic lesions in a murine model of homozygous familial hypercholesterolemia (hoFH). Prior to advancing this vector, called AAV8.TBG.hLDLR, to the clinic, we set out to investigate vector biodistribution in an hoFH mouse model following IV vector administration to assess the safety profile of this investigational agent. Although AAV genomes were present in all organs at all time points tested (up to 180 days), vector genomes were sequestered mainly in the liver, which contained levels of vector 3 logs higher than that found in other organs. In both sexes, the level of AAV genomes gradually declined and appeared to stabilize 90 days post vector administration in most organs although vector genomes remained high in liver. Vector loads in the circulating blood were high and close to those in liver at the early time point (day 3) but rapidly decreased to a level close to the limit of quantification of the assay. The results of this vector biodistribution study further support a proposed clinical trial to evaluate AAV8 gene therapy for hoFH patients.
Although high-density lipoprotein (HDL) is known to inhibit endothelial adhesion molecule expression, the mechanism for this anti-inflammatory effect remains obscure. Surprisingly, we observed that HDL no longer decreased adhesion of U937 monocytoid cells to tumor necrosis factor (TNF)α-stimulated human endothelial cells (EC) in the presence of the general lipase inhibitor tetrahydrolipstatin. In considering endothelial mechanisms responsible for this effect, we found that endothelial lipase (EL) overexpression in both EC and non-EL–expressing NIH/3T3 mouse embryonic fibroblasts cells significantly decreased TNFα-induced VCAM1 expression and promoter activity in a manner dependent on HDL concentration and intact EL activity. Given recent evidence for lipolytic activation of peroxisome proliferator-activated receptors (PPARs)—nuclear receptors implicated in metabolism, atherosclerosis, and inflammation—we hypothesized HDL hydrolysis by EL is an endogenous endothelial mechanism for PPAR activation. In both EL-transfected NIH cells and bovine EC, HDL significantly increased PPAR ligand binding domain activation in the order PPAR-α≫-γ>-δ. Moreover, HDL stimulation induced expression of the canonical PPARα-target gene acyl-CoA-oxidase (ACO) in a PPARα-dependent manner in ECs. Conditioned media from EL-adenovirus transfected cells but not control media exposed to HDL also activated PPARα. PPARα activation by EL was most potent with HDL as a substrate, with lesser effects on LDL and VLDL. Finally, HDL inhibited leukocyte adhesion to TNFα-stimulated ECs isolated from wild-type but not PPARα-deficient mice. This data establishes HDL hydrolysis by EL as a novel, distinct natural pathway for PPARα activation and identifies a potential mechanism for HDL-mediated repression of VCAM1 expression, with significant implications for both EL and PPARs in inflammation and vascular biology.
adhesion molecules; endothelial cells; HDL cholesterol; high-density lipoproteins; lipase; PPARs; transcriptional regulation
Bone marrow derived endothelial progenitor cells (EPCs) are early precursors of mature endothelial cells which replenish aging and damaged endothelial cells. The authors studied a diabetic swine model to determine if induction of DM adversely affects either bone marrow or circulating EPCs and whether a HMG-CoA reductase inhibitor (statin) improves development and recruitment of EPCs in the absence of cholesterol lowering. Streptozotocin was administered to Yorkshire pigs to induce DM. One month after induction, diabetic pigs were treated with atorvastatin (statin, n = 10), ezetimibe (n = 10) or untreated (n = 10) and evaluated for number of bone marrow and circulating EPCs and femoral artery endothelial function. There was no effect of either medication on cholesterol level. One month after induction of DM prior to administration of drugs, the number of bone marrow and circulating EPCs significantly decreased (P < 0.0001) compared to baseline. Three months after DM induction, the mean proportion of circulating EPCs significantly increased in the atorvastatin group, but not in the control or ezetimibe groups. The control group showed progressive reduction in percentage of flow mediated vasodilatation (no dilatation at 3 months) whereas the atorvastatin group and ezetimibe exhibited vasodilatation, 6% and 4% respectively. DM results in significant impairment of bone marrow and circulating EPCs as well as endothelial function. The effect is ameliorated, in part, by atorvastatin independent of its cholesterol lowering effect. These data suggest a model wherein accelerated atherosclerosis seen with DM may, in part, result from reduction in EPCs which may be ameliorated by treatment with a statin.
diabetes mellitus; endothelium; endothelial progenitor cells; statin; inflammation; cytometry
Identification of the CETP, LIPG (encoding endothelial lipase) and APOC3 genes, and ana lysis of rare genetic variants in them, have allowed researchers to increase understanding of HDL metabolism significantly. However, development of cardiovascular risk-reducing therapeutics targeting the proteins encoded by these genes has been less straightforward. The failure of two CETP inhibitors is complex but illustrates a possible over-reliance on HDL cholesterol as a marker of therapeutic efficacy. The case of endothelial lipase exemplifies the importance of utilizing population-wide genetic studies of rare variants in potential therapeutic targets to gain information on cardiovascular disease end points. Similar population-wide studies of cardiovascular end points make apoC-III a potentially attractive target for lipid-related drug discovery. These three cases illustrate the positives and negatives of single-gene studies relating to HDL-related cardiovascular drug discovery; such studies should focus not only on HDL cholesterol and other components of the lipid profile, but also on the effect genetic variants have on cardiovascular end points.
anacetrapib; apoC-III; CETP; cholesterol; dalcetrapib; endothelial lipase; evacetrapib; genetics; HDL; ISI-APOCIIIRX; torcetrapib
Various pathological changes lead to the development of heart failure (HF). HDL is dysfunctional in both acute coronary syndrome, as measured by the HDL inflammatory index (HII) assay, and stable coronary disease, as measured by cholesterol efflux capacity. We therefore hypothesized that these functions of HDL are also impaired in subjects with ischaemic cardiomyopathy.
Methods and results
A case–control study was performed on subjects in the University of Pennsylvania Catheterization Study (PennCath) cohort of patients with angina. Cases had EF <50% and angiographic CAD (≥70% stenosis of any vessel; n = 23); controls included those with EF ≥55% and no CAD (n = 46). Serum from subjects was apolipoprotein-B depleted to isolate an HDL fraction. To measure HDL anti-oxidative capacity, the HDL fraction was incubated with LDL and a reporter lipid that fluoresces when oxidized. To measure cholesterol efflux capacity, the HDL fraction was also incubated with macrophages and tritium-labelled cholesterol. Mean HII was higher and efflux capacity lower in subjects with ischaemic cardiomyopathy (HII 0.26 vs. –0.028; efflux 0.80 vs. 0.92; P < 0.05). In a multivariable logistic regression model, both high HII and low efflux capacity were significant risk factors for HF [HII odds ratio (OR) 2.8, 95% confidence interval (CI) 2.0–3.9, P = 0.002; efflux OR 2.1, 95% CI 1.5–3.0, P = 0.03]. These effects persisted after adjustment for covariates and traditional risk factors for HF.
Subjects with reduced EF from ischaemia have lower HDL concentration and also impaired HDL function. HDL is a versatile lipoprotein particle with various anti-inflammatory and vasoprotective functions, whose impairment may contribute to ischaemic heart failure.
Heart failure; Coronary artery disease; HDL cholesterol
Postprandial triglyceridemia predicts cardiovascular events. Niacin might lower postprandial triglycerides (TG) by restricting free fatty acid (FFA). Immediate-release niacin reduced postprandial TGs, but extended-release niacin failed to do so when dosed the night before a fat challenge.
1) Determine whether extended-release niacin dosed before a fat challenge suppresses postprandial TG. 2) Determine whether postprandial TG is related to FFA restriction.
Double-blinded, placebo-controlled, random-order crossover experiment, where healthy volunteers took 2 g extended-release niacin or placebo 1 hour before heavy cream. We sampled blood over 12 hours, and report TG and FFA as means±SD for incremental area under the curve (iAUC) and nadir.
Combining 43 fat challenges from 22 subjects, postprandial TG iAUC was +312±200 on placebo vs +199±200 mg/dL*h on extended-release niacin (33% drop, p= 0.02). The incremental nadir for FFA was −0.07±0.15 on placebo vs −0.27±0.13 mmol/L on extended-release niacin (p<0.0001), and FFA iAUC fell from +2.9±1.5 to +1.5±1.5 mmol/L*h on extended-release niacin (20% drop, p=0.0015). The TG iAUC was strongly related to the post-dose drop in FFA (r=+0.58, p=0.0007).
Given right before a fat meal, even a single dose of extended-release niacin suppresses postprandial triglyceridemia. This establishes that postprandial TG suppression is an acute pharmacodynamic effect of extended-release niacin, probably the result of marked FFA restriction. Further study is warranted to determine whether mealtime dosing would augment the clinical efficacy of extended-release niacin therapy.
adult; African Americans; clinical trial; dietary fats; drug; evaluation; free fatty acids; humans; hydroxybutyrate; ketones; lipoprotein; lipids; niacin; niacin/pharmacology; niacin/therapeutic use; postprandial; randomized controlled trial; triglycerides
High density lipoprotein; cholesterol efflux; reverse cholesterol transport; atherosclerosis; cardiovascular disease; vascular imaging
We performed a genome-wide association study on 1,292 individuals with abdominal aortic aneurysms (AAAs) and 30,503 controls from Iceland and The Netherlands, with a follow-up of top markers in up to 3,267 individuals with AAAs and 7,451 controls. The A allele of rs7025486 on 9q33 was found to associate with AAA, with an odds ratio (OR) of 1.21 and P = 4.6 × 10−10. In tests for association with other vascular diseases, we found that rs7025486[A] is associated with early onset myocardial infarction (OR = 1.18, P = 3.1 × 10−5), peripheral arterial disease (OR = 1.14, P = 3.9 × 10−5) and pulmonary embolism (OR = 1.20, P = 0.00030), but not with intracranial aneurysm or ischemic stroke. No association was observed between rs7025486[A] and common risk factors for arterial and venous diseases—that is, smoking, lipid levels, obesity, type 2 diabetes and hypertension. Rs7025486 is located within DAB2IP, which encodes an inhibitor of cell growth and survival.
Despite the critical importance of plasma lipoproteins in the development of atherosclerosis, varying degrees of evidence surround the causal associations of lipoproteins with coronary artery disease (CAD). These causal contributions can be assessed by employing genetic variants as unbiased proxies for lipid levels. A relatively large number of low-density lipoprotein cholesterol (LDL-C) variants strongly associate with CAD, confirming the causal impact of this lipoprotein on atherosclerosis. Although not as firmly established, genetic evidence supporting a causal role of triglycerides (TG) in CAD is growing. Conversely, high-density lipoprotein cholesterol (HDL-C) variants not associated with LDL-C or TG have not yet been shown to be convincingly associated with CAD, raising questions about the causality of HDL-C in atherosclerosis. Finally, genetic variants at the LPA locus associated with lipoprotein(a) [Lp(a)] are decisively linked to CAD, indicating a causal role for Lp(a). Translational investigation of CAD-associated lipid variants may identify novel regulatory pathways with therapeutic potential to alter CAD risk.
Genetics; GWAS; Mendelian randomization; Lipids; LDL-C; Lipoprotein(a); TG; HDL-C; PCSK9; SORT1; TRIB1; Apolipoprotein A5; Apolipoprotein C3; Lipoprotein lipase; Cholesterol ester transfer protein; Coronary artery disease; Lipid Traits
To identify loci for coronary artery calcification (CAC) in patients with chronic kidney disease (CKD).
CKD is associated with increased CAC and subsequent coronary heart disease (CHD) but the mechanisms remain poorly defined. Genetic studies of CAC in CKD may provide a useful strategy for identifying novel pathways in CHD.
We performed a candidate gene study (~2,100 genes; ~50,000 SNPs) of CAC within the Chronic Renal Insufficiency Cohort (CRIC) Study (n=1,509; 57% European, 43% African ancestry). SNPs with preliminary evidence of association with CAC in CRIC were examined for association with CAC in PennCAC (n=2,560) and Amish Family Calcification Study (AFCS; n=784) samples. SNPs with suggestive replication were further analyzed for association with myocardial infarction (MI) in the Pakistan Risk of Myocardial Infarction study (PROMIS) (n=14,885).
Of 268 SNPs reaching P <5×10−4 for CAC in CRIC, 28 SNPs in 23 loci had nominal support (P <0.05 and in same direction) for CAC in PennCAC or AFCS. Besides chr9p21 and COL4A1, known loci for CHD, these included SNPs having reported GWAS association with hypertension (e.g., ATP2B1). In PROMIS, four of the 23 suggestive CAC loci (chr9p21, COL4A1, ATP2B1 and ABCA4) had significant associations with MI consistent with their direction of effect on CAC.
We identified several loci associated with CAC in CKD that also relate to MI in a general population sample. CKD imparts a high risk of CHD and may provide a useful setting for discovery of novel CHD genes and pathways.
Coronary artery calcification (CAC); chronic kidney disease (CKD); Chronic Renal Insufficiency Cohort Study (CRIC); myocardial infarction (MI); risk factors; candidate genes; single nucleotide polymorphisms (SNPs)
Atherosclerosis is a complex and heritable disease involving multiple cell types and the interactions of many different molecular pathways. The genetic and molecular mechanisms of atherosclerosis have in part been elucidated by mouse models; at least 100 different genes have been shown to influence atherosclerosis in mice. Importantly, unbiased genome-wide association studies have recently identified a number of novel loci robustly associated with atherosclerotic coronary artery disease (CAD). Here we review the genetic data elucidated from mouse models of atherosclerosis, as well as significant associations for human CAD. Furthermore, we discuss in greater detail some of these novel human CAD loci. The combination of mouse and human genetics has the potential to identify and validate novel genes that influence atherosclerosis, some of which may be candidates for new therapeutic approaches.
CAD; Lipids; Mice; GWAS; Genome-wide
Inflammation is a critical component of atherosclerosis. IL-1 is a classic proinflammatory cytokine that has been linked to atherosclerosis. A clinical trial has been launched in which an antibody specific for IL-1β is being studied for its effects on cardiovascular events in patients with atherosclerosis. In this issue of the JCI, Alexander et al. report that mice lacking the receptor for IL-1 unexpectedly have features of advanced atherosclerosis that suggest the atherosclerotic plaques may be less stable. These findings illustrate the complexity of inflammatory pathways in atherosclerosis and suggest the need for careful calibration of antiinflammatory approaches to atherosclerosis.
In addition to extensively characterized role of high density lipoprotein (HDL) in reverse cholesterol transport, bioactive lipids bound to HDL can also exert diverse vascular effects. Despite this, integration of HDL action in the vasculature with pathways that metabolize HDL and release bioactive lipids has been much less explored. The effects of HDL on endothelial cells (ECs) are mediated in part by HDL-associated sphingosine 1-phosphate (S1P), which binds to S1P1 receptors and promotes activation of eNOS and the kinase Akt. In these studies, we characterized the role of endothelial lipase (EL) in the control of endothelial signaling and biology, including those mediated by HDL-associated S1P.
Approach and Results
HDL-induced angiogenesis in aortic rings from EL-deficient (EL−/−) mice was markedly decreased versus wild-type controls. In cultured ECs, siRNA-mediated knockdown of EL abrogated HDL-promoted EC migration and tube formation. siRNA-mediated EL knockdown also attenuated HDL-induced phosphorylation of eNOS1179 and Akt473. S1P stimulation restored HDL-induced endothelial migration and Akt/eNOS phosphorylation that had been blocked by siRNA-mediated EL knockdown. HDL-induced EC migration and Akt/eNOS phosphorylation were completely inhibited by the S1P1 antagonist W146 but not by the S1P3 antagonist CAY10444.
Endothelial lipase is a critical determinant of the effects of HDL on S1P-mediated vascular responses and acts on HDL to promote activation of S1P1, leading to Akt/eNOS phosphorylation and subsequent endothelial migration and angiogenesis. The role of EL in HDL-associated S1P effects provides new insights into EL action, the responses seen through EL and HDL interaction, and S1P signaling.
HDL; endothelial lipase; endothelial cells; sphingosine 1-phosphate; angiogenesis
Endothelial–vascular smooth muscle cell communication has a critical role in cardiovascular homeostasis and the pathogenesis of atherosclerosis. A study now demonstrates extracellular-vesicle-mediated transfer of the atheroprotective microRNAs miR-143/145 between endothelial and vascular smooth muscle cells, providing compelling evidence that intercellular transport of miRNAs can influence a pathological process, namely atherosclerosis.
Genome-wide association studies (GWAS) have identified a genetic variant at a locus on chromosome 1p13 that is associated with reduced risk of myocardial infarction, reduced plasma levels of LDL cholesterol (LDL-C), and markedly increased expression of the gene sortilin-1 (SORT1) in liver. Sortilin is a lysosomal sorting protein that binds ligands both in the Golgi apparatus and at the plasma membrane and traffics them to the lysosome. We previously reported that increased hepatic sortilin expression in mice reduced plasma LDL-C levels. Here we show that increased hepatic sortilin not only reduced hepatic apolipoprotein B (APOB) secretion, but also increased LDL catabolism, and that both effects were dependent on intact lysosomal targeting. Loss-of-function studies demonstrated that sortilin serves as a bona fide receptor for LDL in vivo in mice. Our data are consistent with a model in which increased hepatic sortilin binds intracellular APOB-containing particles in the Golgi apparatus as well as extracellular LDL at the plasma membrane and traffics them to the lysosome for degradation. We thus provide functional evidence that genetically increased hepatic sortilin expression both reduces hepatic APOB secretion and increases LDL catabolism, providing dual mechanisms for the very strong association between increased hepatic sortilin expression and reduced plasma LDL-C levels in humans.
Autosomal recessive hypercholesterolemia (ARH) is a rare inherited disorder characterized by extremely high total and low-density lipoprotein cholesterol levels that has been previously linked to mutations in LDLRAP1. We identified a family with ARH not explained by mutations in LDLRAP1 or other genes known to cause monogenic hypercholesterolemia. The aim of this study was to identify the molecular etiology of ARH in this family.
Approach and Results
We used exome sequencing to assess all protein coding regions of the genome in three family members and identified a homozygous exon 8 splice junction mutation (c.894G>A, also known as E8SJM) in LIPA that segregated with the diagnosis of hypercholesterolemia. Since homozygosity for mutations in LIPA is known to cause cholesterol ester storage disease (CESD), we performed directed follow-up phenotyping by non-invasively measuring hepatic cholesterol content. We observed abnormal hepatic accumulation of cholesterol in the homozygote individuals, supporting the diagnosis of CESD. Given previous suggestions of cardiovascular disease risk in heterozygous LIPA mutation carriers, we genotyped E8SJM in >27,000 individuals and found no association with plasma lipid levels or risk of myocardial infarction, confirming a true recessive mode of inheritance.
By integrating observations from Mendelian and population genetics along with directed clinical phenotyping, we diagnosed clinically unapparent CESD in the affected individuals from this kindred and addressed an outstanding question regarding risk of cardiovascular disease in LIPA E8SJM heterozygous carriers.
hypercholesterolemia; genetics; myocardial infarction
Peroxisome proliferator-activate receptorα (PPARα) activation has been shown in vitro to increase macrophage cholesterol efflux, the initial step in reverse cholesterol transport (RCT). However, it remains unclear whether PPARα activation promotes macrophage RCT in vivo.
Methods and Results
We demonstrated that a specific potent PPARα agonist GW7647 inhibited atherosclerosis and promoted macrophage RCT in hypercholesterolemic mice expressing the human apoA-I gene. We compared the effect of GW7647 on RCT in human apoA-I transgenic (hA-ITg) mice with wild-type (WT) mice and showed that the PPARα agonist promoted RCT in hA-ITg mice to a much greater extent than in WT mice, indicating that human apoA-I expression is important for PPARα-induced RCT. We further investigated the dependence of the macrophage PPARα-LXR pathway on the promotion of RCT by GW7647. Primary murine macrophages lacking PPARα or LXR abolished the ability of GW7647 to promote RCT in hA-ITg mice. In concert, the PPARα agonist promoted cholesterol efflux and ABCA1/ABCG1 expression in primary macrophages and this was also by the PPARα-LXR pathway.
Our observations demonstrate that a potent PPARα agonist promotes macrophage RCT in vivo in a manner that is enhanced by human apoA-I expression and dependent on both macrophage PPARα and LXR expression.
PPARα; LXR; cholesterol efflux; reverse cholesterol transport; apolipoprotein A-I
The niacin receptor GPR109A is a Gi-protein coupled receptor which mediates the effects of niacin on inhibiting intracellular triglyceride lipolysis in adipocytes. However, the role of GPR109A in mediating the effects of niacin on high density lipoprotein (HDL) metabolism is unclear. We found niacin has no effect on HDL-C in GPR109A knockout mice. Furthermore, niacin lowered intracellular cAMP in primary hepatocytes mediated by GPR109A. We used an adeno-associated viral (AAV) serotype 8 vector encoding GPR109A under the control of the hepatic-specific thyroxine-binding globulin promoter to specifically overexpress GPR109A in mouse liver. Plasma HDL-C, hepatic ABCA1 and the HDL cholesterol production rate were significantly reduced in mice overexpressing GPR109A. Overexpression of GPR109A reduced primary hepatocyte free cholesterol efflux to apoA-I; conversely, GPR109A deficient hepatocytes had increased ABCA1-mediated cholesterol efflux. These data support the concept that the HDL-C lowering effect of niacin in wild-type mice is mediated through stimulation of GPR109A in hepatocytes; such an effect then leads to reduced hepatocyte ABCA1 expression and activity, decreased cholesterol efflux to nascent apoA-I, and reduced HDL-C levels. These results indicate that niacin-mediated activation of GP109A in liver lowers ABCA1 expression leading to reduced hepatic cholesterol efflux to HDL.
niacin; GPR109A; HDL; ABCA1; AAV
Several high profile failures of lipid-related therapeutics in clinical trials have led to intense interest in improved discovery and preclinical prioritization of potential targets. The careful study of patients with rare monogenic disorders has played a key role in establishing the causal role of cholesterol in atherosclerosis and highlighting viable drug targets. Systematic efforts to extend the association of common variants linked with lipid levels to coronary disease allow assessment of the vascular consequences of lifelong differences in lipids due to variation in specific molecules. This application of genetic epidemiology, termed Mendelian randomization, may prove useful in informing ongoing drug development efforts.