HDL-mediated Reverse Cholesterol Transport Research Articles

Understanding HDL Metabolism and Biology Through In Vivo Tracer Kinetics.

HDL (high-density lipoprotein), owing to its high protein content and small size, is the densest circulating lipoprotein. In contrast to lipid-laden VLDL (very-low-density lipoprotein) and LDL (low-density lipoprotein) that promote atherosclerosis, HDL is hypothesized to mitigate atherosclerosis via reverse cholesterol transport, a process that entails the uptake and clearance of excess cholesterol from peripheral tissues. This process is mediated by APOA1 (apolipoprotein A-I), the primary structural protein of HDL, as well as by the activities of additional HDL proteins. Tracer-dependent kinetic studies are an invaluable tool to study HDL-mediated reverse cholesterol transport and overall HDL metabolism in humans when a cardiovascular disease therapy is investigated. Unfortunately, HDL cholesterol-raising therapies have not been successful at reducing cardiovascular events suggesting an incomplete picture of HDL biology. However, as HDL tracer studies have evolved from radioactive isotope- to stable isotope-based strategies that in turn are reliant on mass spectrometry technologies, the complexity of the HDL proteome and its metabolism can be more readily addressed. In this review, we outline the motivations, timelines, advantages, and disadvantages of the various tracer kinetics strategies. We also feature the metabolic properties of select HDL proteins known to regulate reverse cholesterol transport, which in turn underscore that HDL lipoproteins comprise a heterogeneous particle population whose distinct protein constituents and kinetics likely determine its function and potential contribution to cholesterol clearance.

ApoE and apoC-III-defined HDL subtypes: a descriptive study of their lecithin cholesterol acyl transferase and cholesteryl ester transfer protein content and activity

BackgroundThe functionality of high-density lipoproteins (HDL) is a better cardiovascular risk predictor than HDL concentrations. One of the key elements of HDL functionality is its apolipoprotein composition. Lecithin-cholesterol acyl transferase (LCAT) and cholesterol-ester transfer protein (CETP) are enzymes involved in HDL-mediated reverse cholesterol transport. This study assessed the concentration and activity of LCAT and CETP in HDL subspecies defined by their content of apolipoproteins E (apoE) and C-III (apoC-III) in humans.MethodsEighteen adults (ten women and eight men, mean age 55.6, BMI 26.9 Kg/m2, HbA1c 5.4%) were studied. HDL from each participant were isolated and divided into four subspecies containing respectively: No apoE and no apoC-III (E-C-), apoE but not apoC-III (E + C-), apoC-III but no apoE (E-C+) and both apoE and apoC-III (E + C+). The concentration and enzymatic activity of LCAT and CETP were measured within each HDL subspecies using immunoenzymatic and fluorometric methods. Additionally, the size distribution of HDL in each apolipoprotein-defined fraction was determined using non-denaturing electrophoresis and anti-apoA-I western blotting.ResultsHDL without apoE or apoC-III was the predominant HDL subtype. The size distribution of HDL was very similar in all the four apolipoprotein-defined subtypes. LCAT was most abundant in E-C- HDL (3.58 mg/mL, 59.6% of plasma LCAT mass), while HDL with apoE or apoC-III had much less LCAT (19.8, 12.2 and 8.37% of plasma LCAT respectively for E + C-, E-C+ and E + C+). LCAT mass was lower in E + C- HDL relative to E-C- HDL, but LCAT activity was similar in both fractions, signaling a greater activity-to-mass ratio associated with the presence of apoE. Both CETP mass and CETP activity showed only slight variations across HDL subspecies. There was an inverse correlation between plasma LCAT activity and concentrations of both E-C+ pre-beta HDL (r = − 0.55, P = 0.017) and E-C- alpha 1 HDL (r = − 0.49, P = 0.041). Conversely, there was a direct correlation between plasma CETP activity and concentrations of E-C+ alpha 1 HDL (r = 0.52, P = 0.025).ConclusionsThe presence of apoE in small HDL is correlated with increased LCAT activity and esterification of plasma cholesterol. These results favor an interpretation that LCAT and apoE interact to enhance anti-atherogenic pathways of HDL.

MTORC1-dependent protein synthesis and autophagy uncouple in the regulation of Apolipoprotein A-I expression

BackgroundApolipoprotein A-I (ApoA-I) is involved in reverse cholesterol transport as a major component of HDL, but also conveys anti-thrombotic, anti-oxidative, anti-inflammatory and immune-regulatory properties that are pertinent to its protective roles in cardiovascular, inflammatory and malignant pathologies. Despite the pleiotropy in ApoA-I functions, the regulation of intracellular ApoA-I levels remains poorly explored. MethodsHepG2 hepatoma cells and primary mouse hepatocytes were used as in vitro models to study the impact of genetic and chemical inhibitors of autophagy and the proteasome on ApoA-I by immunoblot, immunofluorescence and electron microscopy. Different growth conditions were implemented in conjunction with mTORC inhibitors to model the influence of nutrient scarcity versus sufficiency on ApoA-I regulation. Hepatic ApoA-I expression was also evaluated in high fat diet-fed mice displaying blockade in autophagy. ResultsUnder nutrient-rich conditions, basal ApoA-I levels in liver cells are sustained by the balancing act of autophagy and of mTORC1-dependent de novo protein synthesis. ApoA-I proteolysis occurs through a canonical autophagic pathway involving Beclin1 and ULK1 and the receptor protein p62/SQSTM1 that targets ApoA-I to autophagosomes. However, upon aminoacid insufficiency, suppression of ApoA-I synthesis prevails, rendering mTORC1 inactivation dispensable for autophagy-mediated ApoA-I proteolysis. ConclusionThese data underscore the major contribution of post-transcriptional mechanisms to ApoA-I levels which differentially involve mTORC1-dependent signaling to protein synthesis and autophagy, depending on nutrient availability. Given the established role of ApoA-I in HDL-mediated reverse cholesterol transport, this mode of ApoA-I regulation may reflect a hepatocellular response to the organismal requirement for maintenance of cholesterol and lipid reserves under conditions of nutrient scarcity.

Abstract 554: FoxO Transcription Factors are Required for Hepatic HDL-Cholesterol Clearance

Insulin resistance and type 2 diabetes are associated with low levels of high-density lipoprotein-cholesterol (HDL-C). The insulin-repressible FoxO transcription factors are potential mediators of insulin’s effect on HDL-C. FoxOs mediate a substantial portion of insulin-regulated transcription, and poor FoxO repression is thought to contribute to the excessive glucose production in diabetes. In this work, we show that mice with liver-specific triple FoxO knockout (L-FoxO1,3,4), which are known to have reduced hepatic glucose production, also have increased HDL-C. This was associated with decreased expression of HDL-C clearance factors, scavenger receptor class B type I (SR-BI) and hepatic lipase, and defective selective uptake of HDL-cholesteryl ester by the liver. The phenotype could be rescued by re-expression of SR-BI. These findings demonstrate that hepatic FoxOs are required for cholesterol homeostasis and HDL-mediated reverse cholesterol transport to the liver.

FoxO transcription factors are required for hepatic HDL cholesterol clearance.

Insulin resistance and type 2 diabetes are associated with low levels of high-density lipoprotein cholesterol (HDL-C). The insulin-repressible FoxO transcription factors are potential mediators of the effect of insulin on HDL-C. FoxOs mediate a substantial portion of insulin-regulated transcription, and poor FoxO repression is thought to contribute to the excessive glucose production in diabetes. In this work, we show that mice with liver-specific triple FoxO knockout (L-FoxO1,3,4), which are known to have reduced hepatic glucose production, also have increased HDL-C. This was associated with decreased expression of the HDL-C clearance factors scavenger receptor class B type I (SR-BI) and hepatic lipase and defective selective uptake of HDL cholesteryl ester by the liver. The phenotype could be rescued by re-expression of SR-BI. These findings demonstrate that hepatic FoxOs are required for cholesterol homeostasis and HDL-mediated reverse cholesterol transport to the liver.

Abstract 190: The Role of Hepatic FoxO Transcription Factors in HDL-Mediated Reverse Cholesterol Transport

Insulin resistance and type 2 diabetes are associated with low levels of high-density lipoprotein-cholesterol (HDL-C). The insulin-repressible FoxO transcription factors are potential mediators of insulin’s effect on HDL-C. FoxOs mediate a substantial portion of insulin-regulated transcription, and poor FoxO repression is thought to contribute to the excessive glucose production in diabetes. In this work, we show that mice with liver-specific triple FoxO knockout (L-FoxO1,3,4), which are known to have reduced hepatic glucose production, also have increased HDL-C. This was associated with decreased expression of HDL-C clearance factors, scavenger receptor class B type I (SR-BI) and hepatic lipase, and defective selective uptake of HDL-cholesteryl ester by the liver. The phenotype could be rescued by re-expression of SR-BI. These findings demonstrate that hepatic FoxOs are required for cholesterol homeostasis and HDL-mediated reverse cholesterol transport to the liver.

Effects of hyperlipidaemia on plasma apolipoprotein M levels in patients with type 2 diabetes mellitus: an independent case-control study.

BackgroundApolipoprotein M (apoM) is mainly enriched in high-density lipoprotein (HDL) cholesterol and is slightly present in low-density lipoprotein (LDL) cholesterol and very low-density lipoprotein cholesterol. apoM is involved in HDL formation and HDL-mediated reverse cholesterol transport. apoM is also associated with hyperlipidaemia and type 2 diabetes mellitus (T2DM). Significantly high plasma apoM levels are detected in hyperlipidaemia mice with a defective LDL receptor. By contrast, low plasma apoM levels are observed in patients with T2DM, which is often accompanied with hyperlipidaemia. However, the underlying mechanism of this condition is poorly understood. This research aims to examine the changes in apoM levels in patients with hyperlipidaemia and to determine the effects of hyperlipidaemia on plasma apoM levels in patients with T2DM.MethodsThis study included patients with hyperlipidaemia (n = 79), patients with T2DM but without hyperlipidaemia (n = 125), patients with T2DM and hyperlipidaemia (n = 98), and healthy controls (n = 105). Their plasma apoM concentrations were measured with enzyme-linked immunosorbent assay.ResultsThe average plasma apoM concentrations were 18 % higher in the hyperlipidaemia group (26.63 ± 10.35 ng/μL) than in the healthy controls (22.61 ± 10.81 ng/μL, P <0.01). The plasma apoM concentrations were lower in the T2DM without hyperlipidaemia group (18.54 ± 10.33 ng/μL, P <0.01) and the T2DM with hyperlipidaemia group (19.83 ± 7.41 ng/μL, P <0.05) than in the healthy controls. Similar to apoA-I (1.29 ± 0.33 g/L vs. 1.28 ± 0.31 g/L, P >0.05), the plasma apoM concentrations in the T2DM with hyperlipidaemia group did not significantly differ from those in the T2DM without hyperlipidaemia group (P >0.05). Multivariate linear regression analysis showed that hyperlipidaemia (β = 5.18, P = 0.007) is an independent promoting factor of plasma apoM levels and diabetes (β = −3.09, P = 0.005) is an inhibiting factor of plasma apoM levels.ConclusionPlasma apoM concentrations are higher in patients with hyperlipidaemia than in healthy controls. Low plasma apoM levels in patients with T2DM are likely caused by diabetes but are not induced by hyperlipidaemia.

Abstract 442: Generation of an Abcg1 Knock Out Mouse on the Reversa Background

Background: Clinically HDL mediated reverse cholesterol transport (RCT) from macrophages has been shown to be inversely associated with carotid intima media thickness. Cholesterol efflux to mature HDL is mediated by ATP binding cassette transporter G1 (Abcg1). Abcg1 pays a key role in cholesterol transport with loss of function in macrophages and endothelial cells associated with significant cholesterol accumulation. However, mechanistic studies into the role of Abgc1 in plaque regression have been restricted due to the limitations of current regression models. Aims: To use TALENS mediated genomic editing to generate an Abcg1 knockout mouse on the REVERSA background to enable the investigation of its role in plaque regression. Methods and results: TALENs constructs were targeted to exon 3 upstream of the phosphate binding Walker A domain. TALEN mRNA was injected into REVERSA oocytes which were then implanted into foster mice. Founders were screened by Cel1 nuclease assay and sequencing. Three independent alleles were identified two of which create frameshift mutations (predicted to be null alleles) and one which resulted in a 3 amino acid deletion and a one amino acid substitution near the Walker A domain (potential hypomorphic allele). The two founder lines with frame shift mutations (KO 145 and 171) were taken forward for additional analysis. RNA extracted from primary macrophages from WT (REVERSA) and homozygous Abcg1 knockout mice was used to confirm the mutation was transcribed to RNA. Intron-spanning primers were designed and a product of the expected size was obtained and sequence analysis confirmed the insertion (KO-145) and deletion (KO-171) within the WALKER A domain of Abcg1. To ensure the mutations resulted in loss of function, a radioactive RCT assay was carried out in bone marrow derived primary macrophages. A significant decrease in RCT to HDL was observed in macrophages from both the KO-145 and KO-171 lines as expected there was no difference in RCT to ApoA1-I. Conclusions: Using genomic editing we have generated a gene specific knockout on the REVERSA background which will enable, for the first time, the investigation of the role of Abcg1 in plaque regression.

Abstract 11563: Proteomic Analysis of Adult Congenital Heart Disease Associated Pulmonary Hypertension

Introduction: Pulmonary arterial hypertension (PAH) is a relative common complication in patients with congenital heart disease (CHD), affecting both disease progression and prognosis. The pathogenic mechanisms that lead to “irreversible” PAH are therefore poorly understood in subjects with CHD. Hypothesis: The aim of this study was to apply proteomics to discover protein regulations in the lungs of irreversible PAH patients, which could lead to generation of new hypotheses concerning signaling pathways and elucidating the pathophysiology of irreversible PAH. Methods: Label-free liquid chromatography tandem mass spectrometry was used to compare protein profiles in surgical samples of lungs from 4 patients with irreversible CHD-PAH, 4 reversible CHD-PAH and 4 control subjects. Result: A total of 133 proteins were differently expressed. The majority of differential expressed proteins were associated with cytoskeleton remodeling, integrin-mediated cell adhesion and migration, endothelial cell contacts, blood coagulation and HDL-mediated reverse cholesterol transport. Novel findings included increased expression of filamin A and caveolin-1 and decreased expression of glutathione S-transferase (GST) family. Filamin A, a major isoform of the filamin family which is a family of cytoskeletal proteins that have been implicated as critical mediators of organ development and cell signaling and migration. In the present study, the density and cellular location of filamin A immunoreactivity was accordant with caveolin-1. The results were confirmed by Western blot and immunohistochemistry. In reversible CHD-PAH and control lung tissue, filamin A and caveolin-1 were prominent in endothelium of pulmonary arteriole. In irreversible CHD-PAH lung tissue, they were prominent in the vascular smooth muscle cell. Conclusions: Label-free proteomics identified differences in the expression of several proteins in the CHD-PAH lung, many of which are relevant to the cell proliferation, differentiation and migration. Increased expression of filamin A and caveolin-1 may be pertinent to the proliferation and migration of vascular smooth muscle cell. Decreased expression of GST indicated impaired anti-oxidative function in irreversible CHD-PAH.

MicroRNAs regulates the high density lipoprotein biogenesis and its transport

High density lipoprotein (HDL) has been considered as an important mediator in favoring cardioprotective effects. However, recent studies revealed that HDL-C-raising therapeutics alone failed to reduce the risk of cardiovascular diseases, suggesting that the HDLfunctionalitymay be more critical for its cardioprotective properties than the simple HDL-C levels. microRNAs (miRNAs)have been identified as the novel regulators of lipid metabolism and played essential roles in the key steps of reverse cholesterol transport, involving in HDL biogenesis, cellular cholesterol mobilization, hepatic HDL uptake and excretion. The intensive research on lipid metabolism-related miRNAs may provide further clarification on the molecular regulatory mechanisms for HDL-mediated reverse cholesterol transport and plasma HDL-C levels, advancing our knowledge on the effects of HDL in pathogenesis and progression of cardiovascular diseases.(Chin J Lab Med, 2015, 38: 436-438) Key words: Lipoproteins, HDL; Biological transport; MicroRNA

AIBP: A Novel Molecule at the Interface of Cholesterol Transport, Angiogenesis, and Atherosclerosis.

Cardiovascular disease, which is often driven by hypercholesterolemia and subsequent coronary atherosclerosis, is the number-one cause of morbidity and mortality in the United States. Based on long-term epidemiological studies, high-density lipoprotein cholesterol (HDL-C) levels are inversely correlated with risk for coronary artery disease (CAD). HDL-mediated reverse cholesterol transport (RCT) is responsible for cholesterol removal from the peripheral tissues and return to the liver for final elimination.1 In atherosclerosis, intraplaque angiogenesis promotes plaque growth and increases plaque vulnerability. Conceivably, the acceleration of RCT and disruption of intraplaque angiogenesis would inhibit atherosclerosis and reduce CAD. We have identified a protein called apoA-I binding protein (AIBP) that augments HDL functionality by accelerating cholesterol efflux. Furthermore, AIBP inhibits vascular endothelial growth factor receptor 2 activation in endothelial cells and limits angiogenesis.2 The following discusses the prospect of using AIBP as a novel therapeutic approach for the treatment of CAD.

Intralipid decreases apolipoprotein M levels and insulin sensitivity in rats.

BackgroundApolipoprotein M (ApoM) is a constituent of high-density lipoproteins (HDL). It plays a crucial role in HDL-mediated reverse cholesterol transport. Insulin resistance is associated with decreased ApoM levels.AimsTo assess the effects of increased free fatty acids (FFAs) levels after short-term Intralipid infusion on insulin sensitivity and hepatic ApoM gene expression.MethodsAdult male Sprague-Dawley (SD) rats infused with 20% Intralipid solution for 6 h. Glucose infusion rates (GIR) were determined by hyperinsulinemic-euglycemic clamp during Intralipid infusion and plasma FFA levels were measured by colorimetry. Rats were sacrificed after Intralipid treatment and livers were sampled. Human embryonic kidney 293T cells were transfected with a lentivirus mediated human apoM overexpression system. Goto-Kakizaki (GK) rats were injected with the lentiviral vector and insulin tolerance was assessed. Gene expression was assessed by real-time RT-PCR and PCR array.ResultsIntralipid increased FFAs by 17.6 folds and GIR was decreased by 27.1% compared to the control group. ApoM gene expression was decreased by 40.4% after Intralipid infusion. PPARβ/δ expression was not changed by Intralipid. Whereas the mRNA levels of Acaca, Acox1, Akt1, V-raf murine sarcoma 3611 viral oncogene homolog, G6pc, Irs2, Ldlr, Map2k1, pyruvate kinase and RBC were significantly increased in rat liver after Intralipid infusion. The Mitogen-activated protein kinase 8 (MAPK8) was significantly down-regulated in 293T cells overexpressing ApoM. Overexpression of human ApoM in GK rats could enhance the glucose-lowering effect of exogenous insulin.ConclusionThese results suggest that Intralipid could decrease hepatic ApoM levels. ApoM overexpression may have a potential role in improving insulin resistance in vivo and modulating apoM expression might be a future therapeutic strategy against insulin resistance in type 2 diabetes.

Future of High-Density Lipoprotein Infusion Therapies

The field of high-density lipoprotein (HDL) therapeutics is broadly aimed at combating the significant residual cardiovascular risk remaining after effective low-density lipoprotein (LDL) cholesterol reduction.1 After the recent failure of 3 orally active HDL-raising agents in prospective intervention trials, including 2 in the cholesteryl ester transfer protein (CETP) inhibitor class2,3 and niacin,4,5 the goals of HDL therapy are currently undergoing reassessment. Key opinion leaders are advocating a shift in the target of HDL therapy from elevation in circulating HDL cholesterol levels to enhancement of the functional properties of HDL, especially HDL-mediated reverse cholesterol transport and cellular cholesterol efflux.6,7 These events have focused the spotlight on HDL infusion therapies that transiently increase HDL particle number and enhance efflux capacity for cellular cholesterol; other defective features of HDL functionality may potentially be normalized.8 However, evaluation of HDL infusion therapies has been limited to date to small surrogate end-point trials; indeed, large phase 3 outcome trials to test the efficacy of these agents in reducing definitive morbidity and mortality end points are eagerly awaited. If proven effective, HDL infusion agents would have potential application across a wide range of vascular diseases but would be particularly amenable to delivery in the context of acute coronary syndromes. Coronary artery disease morbidity and mortality relate largely to either physical disruption or endothelial erosion of the fibrous cap of atherosclerotic plaques in the coronary arteries, triggering luminal or intraplaque thrombus formation.9 An acute or unstable coronary syndrome (sudden death, nonfatal myocardial infarction, or unstable angina) results, the nature of which depends in large part on the degree of vessel occlusion. Plaques at greatest risk for rupture are characterized by a marked abundance of both extracellular and intracellular lipid (cholesterol); immunoinflammatory cells, including monocyte-macrophages and T …

Differential gene expression in adipose tissue from obese human subjects during weight loss and weight maintenance

Differential gene expression in adipose tissue during diet-induced weight loss followed by a weight stability period is poorly characterized. Markers of these processes may provide a deeper understanding of underlying mechanisms. We aimed to identify differentially expressed genes in human adipose tissue during weight loss and weight maintenance after weight loss. RNA from subcutaneous abdominal adipose tissue from 9 obese subjects was analyzed by using a complementary DNA microarray at baseline after weight loss on a low-calorie diet and after weight maintenance. Subjects lost 18.8 ± 1.8% of weight and maintained this loss during weight maintenance (1.1 ± 2.1%; range: -9.3 to 10.6%). Most differentially expressed genes exhibited a reciprocal regulation and returned to baseline after weight loss (2163 genes) and weight maintenance (3175 genes). CETP and ABCG1, both of which participate in the HDL-mediated reverse cholesterol transport (RCT), were among the most upregulated of the 750 genes that were differentially expressed after both processes. Several genes involved in inflammation were downregulated. The use of real-time polymerase chain reaction confirmed or partially confirmed the previously implicated genes TNMD and MMP9 (both downregulated), PNPLA3 (upregulated), and CIDEA and SCD (both reciprocally regulated). The beneficial effects of weight loss should be investigated after long-term weight maintenance. The processes of weight loss and weight maintenance should be viewed as biologically distinct. CETP and ABCG1 may be important mediators of these effects through HDL-mediated RCT.

Type I diabetes mellitus decreases in vivo macrophage-to-feces reverse cholesterol transport despite increased biliary sterol secretion in mice

Type I diabetes mellitus (T1DM) increases atherosclerotic cardiovascular disease; however, the underlying pathophysiology is still incompletely understood. We investigated whether experimental T1DM impacts HDL-mediated reverse cholesterol transport (RCT). C57BL/6J mice with alloxan-induced T1DM had higher plasma cholesterol levels (P < 0.05), particularly within HDL, and increased hepatic cholesterol content (P < 0.001). T1DM resulted in increased bile flow (2.1-fold; P < 0.05) and biliary secretion of bile acids (BA, 10.5-fold; P < 0.001), phospholipids (4.5-fold; P < 0.001), and cholesterol (5.5-fold; P < 0.05). Hepatic cholesterol synthesis was unaltered, whereas BA synthesis was increased in T1DM (P < 0.001). Mass fecal BA output was significantly higher in T1DM mice (1.5-fold; P < 0.05), fecal neutral sterol excretion did not change due to increased intestinal cholesterol absorption (2.1-fold; P < 0.05). Overall in vivo macrophage-to-feces RCT, using [(3)H]cholesterol-loaded primary mouse macrophage foam cells, was 20% lower in T1DM (P < 0.05), mainly due to reduced tracer excretion within BA (P < 0.05). In vitro experiments revealed unchanged cholesterol efflux toward T1DM HDL, whereas scavenger receptor class BI-mediated selective uptake from T1DM HDL was lower in vitro and in vivo (HDL kinetic experiments) (P < 0.05), conceivably due to increased glycation of HDL-associated proteins (+65%, P < 0.01). In summary, despite higher mass biliary sterol secretion T1DM impairs macrophage-to-feces RCT, mainly by decreasing hepatic selective uptake, a mechanism conceivably contributing to increased cardiovascular disease in T1DM.

Lipotoxicity and Impaired High Density Lipoprotein-Mediated Reverse Cholesterol Transport in Chronic Kidney Disease

Chronic kidney disease (CKD) is associated with a high risk of death from cardiovascular disease. Inflammation, oxidative stress, and dyslipidemia, which are common consequences of CKD, contribute to the pathogenesis of atherosclerosis and cardiovascular disease in this population. Dyslipidemia of CKD is characterized by diminished plasma high density lipoprotein (HDL) concentration, impaired HDL anti-oxidant and anti-inflammatory activities, and elevated plasma triglyceride, very low density lipoprotein (VLDL), intermediate density lipoprotein, chylomicron remnants, and oxidized lipids and lipoproteins. The constellation of inflammation, HDL deficiency, and oxidative modification of lipoproteins can cause atherosclerosis and progression of renal disease. We have recently found lipid accumulation in the remnant kidney and the wall of aorta in rats with CKD induced by 5/6 nephrectomy. This was mediated by up-regulation of scavenger receptors involved in the influx of oxidized lipids or lipoproteins, tubular reabsorption of lipid binding proteins through megalin-cubilin complexes, upregulation of fatty acid synthesis, and downregulation of fatty acid oxidation pathways. The combination of increased lipid influx, elevated production and reduced catabolism of lipids, and impaired HDL-mediated reverse cholesterol transport can promote atherosclerosis, glomerulosclerosis, and tubulointerstitial damage. Although statins can be effective in slowing CKD progression in patients with mild-to-moderate CKD, they have consistently failed to mitigate oxidative stress, inflammation, HDL deficiency, or cardiovascular mortality in the end-stage renal disease populations. Similarly, high doses of antioxidant vitamins have failed to either ameliorate oxidative stress, inflammation, or improve overall mortality in end-stage renal disease. This article is intended to provide a brief review of the effects of CKD on HDL structure and function and pathways of lipid influx, efflux, synthesis, and catabolism in the artery wall and the diseased kidney.

Intestinal Specific LXR Activation Stimulates Reverse Cholesterol Transport and Protects from Atherosclerosis

Several steps of the HDL-mediated reverse cholesterol transport (RCT) are transcriptionally regulated by the nuclear receptors LXRs in the macrophages, liver, and intestine. Systemic LXR activation via synthetic ligands induces RCT but also causes increased hepatic fatty acid synthesis and steatosis, limiting the potential therapeutic use of LXR agonists. During the last few years, the participation of the intestine in the control of RCT has appeared more evident. Here we show that while hepatic-specific LXR activation does not contribute to RCT, intestinal-specific LXR activation leads to decreased intestinal cholesterol absorption, improved lipoprotein profile, and increased RCT in vivo in the absence of hepatic steatosis. These events protect against atherosclerosis in the background of the LDLR-deficient mice. Our study fully characterizes the molecular and metabolic scenario that elects the intestine as a key player in the LXR-driven protective environment against cardiovascular disease.

High-Density Lipoprotein at the Interface of Type 2 Diabetes Mellitus And Cardiovascular Disorders

Type 2 diabetes mellitus is associated with low high-density lipoprotein (HDL) cholesterol levels, which is an independent cardiovascular risk factor. Low HDL cholesterol concentrations reflect a dysregulation in HDL metabolism, which is determined by the concerted action of different proteins, including cholesterol ester transfer protein, lecithin: cholesterol acyltransferase, endothelial and lipoprotein lipase, phospholipid transfer protein, and hepatic lipase, as well as different receptors, including the scavenger receptor class B type I and ATP-binding cassette transporter A1 and G1. Type 2 diabetes mellitus is besides a dysregulation in HDL metabolism, also associated with dysfunctional HDL: HDL-mediated reverse cholesterol transport as well as the anti-oxidative and endothelial-protective features of HDL are impaired in type 2 diabetes mellitus. The first part of the present review gives an overview of how type 2 diabetes mellitus affects the expression and/or activity of receptors and proteins involved in HDL metabolism and how different diabetes-associated factors influence the functionality of HDL. The second part of the review focuses on describing the newest insights in the impact of HDL on glucose metabolism and on diabetes-associated cardiovascular complications.

Innovation in the Treatment of Uremia: Proceedings from the Cleveland Clinic Workshop: Causes of Dysregulation of Lipid Metabolism in Chronic Renal Failure

End-stage renal disease (ESRD) is associated with accelerated atherosclerosis and premature death from cardiovascular disease. These events are driven by oxidative stress inflammation and lipid disorders. ESRD-induced lipid abnormalities primarily stem from dysregulation of high-density lipoprotein (HDL), triglyceride-rich lipoprotein metabolism, and oxidative modification of lipoproteins. In this context, production and plasma concentration of Apo-I and Apo-II are reduced, HDL maturation is impaired, HDL composition is altered, HDL antioxidant and anti-inflammatory functions are depressed, clearance of triglyceride-rich lipoproteins and their atherogenic remnants is impaired, their composition is altered, and their plasma concentration is elevated in ESRD. The associated defect in HDL maturation is largely caused by acquired lecithin-cholesterol acyltransferase deficiency while its triglyceride enrichment is due to hepatic lipase deficiency. Hypertriglyceridemia, abnormal composition, and impaired clearance of triglyceride-rich lipoproteins and their remnants are mediated by down-regulation of lipoprotein lipase, hepatic lipase, very low-density lipoprotein (VLDL) receptor, and LDL receptor-related protein, relative reduction in ApoC-II/ApoC-III ratio, up-regulation of acyl-CoA cholesterol acyltransferase, and elevated plasma level of cholesterol ester-poor prebeta HDL. Impaired clearance and accumulation of oxidation-prone VLDL and chylomicron remnants and abnormal LDL composition in the face of oxidative stress and inflammation favors their uptake by macrophages and resident cells in the artery wall. The effect of heightened influx of lipids is compounded by impaired HDL-mediated reverse cholesterol transport leading to foam cell formation which is the central event in atherosclerosis plaque formation and subsequent plaque rupture, thrombosis, and tissue damage.

Abstract 1952: ABCA1 Deficency Diminishes Dysfunctional HDL And Inhibits Early Atherosclerotic Lesion Development In SR-BI Knockout Mice

Objective: ABCA1 and SR-BI play an essential role in reverse cholesterol transport (RCT) from macrophages in the arterial wall to the liver. The aim of this study was to investigate the effect of combined total-body ABCA1 and SR-BI deficiency on lipoprotein metabolism and atherosclerosis. Methods &amp; Results: ABCA1/SR-BI double knockout (dKO), ABCA1 KO (AKO), SR-BI KO (SKO), and wildtype (WT) littermates were fed an atherogenic diet for 10 weeks to induce atherosclerosis. Under these conditions, SR-BI deficiency leads to largely increased serum cholesterol (C) levels (SKO: 470±68 mg/dL vs WT: 181±9 mg/dL, p&lt;0.001), due to an impaired removal of HDL-C and VLDL-C, while hampered HDL production in AKO mice resulted in cholesterol levels of only 23±4 mg/dL (p&lt;0.0001). In dKO mice, HDL-C was similarly low as in single AKO mice. Total serum cholesterol levels, however, were 3.8-fold (p&lt;0.001) higher, due to increased VLDL-C levels. Interestingly, more extreme foam cell accumulation was observed in the peritoneal cavity of dKO mice ((30±6%, p&lt;0.01), as compared to WT (1.7±0.2%), SKO (2.5±0.3%), and AKO (5±2%) mice. Combined deletion of ABCA1 and SR-BI thus induces macrophage foam cell formation in vivo . The accumulation of HDL-C and VLDL-C in SKO mice dramatically induced atherosclerotic lesion formation (206±16x10 3 μm 2 vs 12±2x10 3 μm 2 for WT mice, p&lt;0.001)), while lesions were only 33±12x10 3 μm 2 (p&lt;0.05) in AKO and 51±19x10 3 μm 2 (p&lt;0.01) in dKO mice with low HDL-C levels. Thus, despite enhanced foam cell formation in dKO mice, ABCA1 deletion reduces the atherosclerosis susceptibility of SKO mice. Next, the serum atherogenicity was tested by incubation of RAM cells with 3% serum of different types of mice. Serum from SKO mice induced more extreme foam cell formation (89±2%, p&lt;0.001), compared to serum from WT (31±3%), AKO (2.3±0.1%), and dKO (3.1±0.5%) mice. Interestingly, this effect highly correlated with both HDL-C (r=0.98, p&lt;0.0001) and VLDL/LDL-C levels (r=0.96, p&lt;0.0001). Conclusion: Total-body ABCA1 deficiency inhibits early atherosclerotic lesion development in SR-BI KO mice, which might be attributable to reduced circulating levels of pro-atherogenic HDL. Furthermore, both SR-BI and ABCA1 are essential for maintaining HDL-mediated RCT.