Differential effects of scavenger receptor BI deficiency on lipid metabolism in cells of the arterial wall and in the liver

MicroRNA-24 aggravates atherosclerosis by inhibiting selective lipid uptake from HDL cholesterol via the post-transcriptional repression of scavenger receptor class B type I

BACKGROUND AND AIMS

Liver scavenger receptor class B type I (SR-BI) exerts atheroprotective effects through selective lipid uptake (SLU) from high-density lipoprotein cholesterol (HDL-C). Low hepatic SR-BI expression leads to high HDL-C levels in the circulation and an increased risk of atherosclerosis. Furthermore, macrophage SR-BI mediates bidirectional cholesterol flux and may protect against atherogenesis. Previous studies have revealed that miR-24 is closely related to cardiovascular disease (CVD) progression. We aimed to investigate the molecular mechanisms by which miR-24 participates in SR-BI-mediated selective HDL cholesteryl ester (HDL-CE) uptake and further atherogenesis in apoE^-/- mice.

METHODS

Bioinformatic predictions and luciferase reporter assays were utilized to detect the association between miR-24 and the SR-BI 3' untranslated region (3' UTR), and RT-PCR and western blotting were used to evaluate SR-BI mRNA and protein expression, respectively. The effects of miR-24 on Dil-HDL uptake were determined by flow cytometry assay. Double-radiolabeled HDL (¹²⁵I-TC-/[³H] CEt-HDL) was utilized to measure the effects of miR-24 on HDL and CE binding and SLU in HepG2 and PMA-treated THP-1 cells. In addition, total cholesterol (TC) levels in HepG2 cells were analyzed using enzymatic methods, and macrophage lipid content was evaluated by high-performance liquid chromatography (HPLC) assay. Small interfering RNA (siRNA) and pcDNA3.1(-)-hSR-BI plasmid transfection procedures were utilized to confirm the role of SR-BI in the effects of miR-24 on Dil-HDL uptake, SLU and cholesterol levels in both cell types. Hepatic SR-BI level in apoE^-/- mice was measured by western blotting. Liver TC, FC and CE levels and plasma triglycerides (TG), TC and HDL-C levels were evaluated enzymatically using commercial test kits. Atherosclerotic lesion sizes were measured using Oil Red O and hematoxylin-eosin staining.

RESULTS

miR-24 directly repressed SR-BI expression by targeting its 3'UTR. In addition, miR-24 decreased Dil-HDL uptake and SLU in HepG2 and THP-1 macrophages. In the presence of HDL, miR-24 decreased TC levels in HepG2 cells and TC, free cholesterol (FC) and CE levels in macrophages. Overexpression and down-regulation assays showed that SR-BI mediated the effects of miR-24 on Dil-HDL uptake, SLU and cholesterol levels. Lastly, miR-24 administration decreased hepatic SR-BI expression and promoted atheromatous plaque formation in apoE^-/- mice, findings in line with those of our in vitro studies.

CONCLUSIONS

These findings indicate that miR-24 accelerates atherogenesis by repressing SR-BI-mediated SLU from HDL-C.

SR-B1: A Unique Multifunctional Receptor for Cholesterol Influx and Efflux

The scavenger receptor, class B type 1 (SR-B1), is a multiligand membrane receptor protein that functions as a physiologically relevant high-density lipoprotein (HDL) receptor whose primary role is to mediate selective uptake or influx of HDL-derived cholesteryl esters into cells and tissues. SR-B1 also facilitates the efflux of cholesterol from peripheral tissues, including macrophages, back to liver. As a regulator of plasma membrane cholesterol content, SR-B1 promotes the uptake of lipid soluble vitamins as well as viral entry into host cells. These collective functions of SR-B1 ultimately affect programmed cell death, female fertility, platelet function, vasculature inflammation, and diet-induced atherosclerosis and myocardial infarction. SR-B1 has also been identified as a potential marker for cancer diagnosis and prognosis. Finally, the SR-B1-linked selective HDL-cholesteryl ester uptake pathway is now being evaluated as a gateway for the delivery of therapeutic and diagnostic agents. In this review, we focus on the regulation and functional significance of SR-B1 in mediating cholesterol movement into and out of cells.

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

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

HDL as a Causal Factor in Atherosclerosis: Insights from Human Genetics

High-density lipoprotein cholesterol (HDL-C) levels are inversely related to risk of atherosclerotic cardiovascular disease (ASCVD). However, the simplistic assumption that HDL-C levels directly and causally impact atherogenesis has been challenged in recent years. The purpose of this article is to review the current state of knowledge regarding genetically determined HDL-C levels and ASCVD risk and determine what insight these studies provide into the causal relationship between HDL and atherosclerosis.

Dysfunctional HDL in diabetes mellitus and its role in the pathogenesis of cardiovascular disease

Coronary artery disease, the leading cause of death in the developed and developing countries, is prevalent in diabetes mellitus with 68% cardiovascular disease (CVD)-related mortality. Epidemiological studies suggested inverse correlation between HDL and CVD occurrence. Therefore, low HDL concentration observed in diabetic patients compared to non-diabetic individuals was thought to be one of the primary causes of increased risks of CVD. Efforts to raise HDL level via CETP inhibitors, Torcetrapib and Dalcetrapib, turned out to be disappointing in outcome studies despite substantial increases in HDL-C, suggesting that factors beyond HDL concentration may be responsible for the increased risks of CVD. Therefore, recent studies have focused more on HDL function than on HDL levels. The metabolic environment in diabetes mellitus condition such as hyperglycemia-induced advanced glycation end products, oxidative stress, and inflammation promote HDL dysfunction leading to greater risks of CVD. This review discusses dysfunctional HDL as one of the mechanisms of increased CVD risks in diabetes mellitus through adversely affecting components that support HDL function in cholesterol efflux and LDL oxidation. The dampening of reverse cholesterol transport, a key process that removes cholesterol from lipid-laden macrophages in the arterial wall, leads to increased risks of CVD in diabetic patients. Therapeutic approaches to keep diabetes under control may benefit patients from developing CVD.

SR-BI: A Multifunctional Receptor in Cholesterol Homeostasis and Atherosclerosis

The HDL receptor scavenger receptor class B type I (SR-BI) plays crucial roles in cholesterol homeostasis, lipoprotein metabolism, and atherosclerosis. Hepatic SR-BI mediates reverse cholesterol transport (RCT) by the uptake of HDL cholesterol for routing to the bile. Through the selective uptake of HDL lipids, hepatic SR-BI modulates HDL composition and preserves HDL's atheroprotective functions of mediating cholesterol efflux and minimizing inflammation and oxidation. Macrophage and endothelial cell SR-BI inhibits the development of atherosclerosis by mediating cholesterol trafficking to minimize atherosclerotic lesion foam cell formation. SR-BI signaling also helps limit inflammation and cell death and mediates efferocytosis of apoptotic cells in atherosclerotic lesions thereby preventing vulnerable plaque formation. SR-BI is emerging as a multifunctional therapeutic target to reduce atherosclerosis development.

Rediscovering scavenger receptor type BI: surprising new roles for the HDL receptor

PURPOSE OF REVIEW

Scavenger receptor BI (SR-BI) is classically known for its role in antiatherogenic reverse cholesterol transport as it selectively takes up cholesterol esters from HDL. Here, we have highlighted recent literature that describes novel functions for SR-BI in physiology and disease.

RECENT FINDINGS

A large population-based study has revealed that patients heterozygous for the P376L mutant form of SR-BI showed significantly increased levels of plasma HDL-cholesterol and had increased risk of cardiovascular disease, demonstrating that SR-BI in humans is a significant determinant of cardiovascular disease. Furthermore, SR-BI has been shown to modulate the susceptibility to LPS-induced tissue injury and the ability of sphingosine 1 phosphate to interact with its receptor, linking SR-BI to the regulation of inflammation. In addition, important domains within the molecule (Trp-415) as well as novel regulators (procollagen C-endopeptidase enhancer protein 2) of SR-BI's selective uptake function have recently been identified. Moreover, relatively high expression levels of the SR-BI protein have been observed in a variety of cancer tissues, which is associated with a reduced overall survival rate.

SUMMARY

The HDL receptor SR-BI is a potential therapeutic target not only in the cardiovascular disease setting, but also in inflammatory conditions as well as in cancer.

Scavenger receptor B1 (SR-B1) profoundly excludes high density lipoprotein (HDL) apolipoprotein AII as it nibbles HDL-cholesteryl ester

Reverse cholesterol transport (transfer of macrophage-cholesterol in the subendothelial space of the arterial wall to the liver) is terminated by selective high density lipoprotein (HDL)-cholesteryl ester (CE) uptake, mediated by scavenger receptor class B, type 1 (SR-B1). We tested the validity of two models for this process: "gobbling," i.e. one-step transfer of all HDL-CE to the cell and "nibbling," multiple successive cycles of SR-B1-HDL association during which a few CEs transfer to the cell. Concurrently, we compared cellular uptake of apoAI with that of apoAII, which is more lipophilic than apoAI, using HDL-[³H]CE labeled with [¹²⁵I]apoAI or [¹²⁵I]apoAII. The studies were conducted in CHO-K1 and CHO-ldlA7 cells (LDLR^-/-) with (CHO-SR-B1) and without SR-B1 overexpression and in human Huh7 hepatocytes. Relative to CE, both apoAI and apoAII were excluded from uptake by all cells. However, apoAII was more highly excluded from uptake (2-4×) than apoAI. To distinguish gobbling versus nibbling mechanisms, media from incubations of HDL with CHO-SR-B1 cells were analyzed by non-denaturing PAGE, size-exclusion chromatography, and the distribution of apoAI, apoAII, cholesterol, and phospholipid among HDL species as a function of incubation time. HDL size gradually decreased, i.e. nibbling, with the concurrent release of lipid-free apoAI; apoAII was retained in an HDL remnant. Our data support an SR-B1 nibbling mechanism that is similar to that of streptococcal serum opacity factor, which also selectively removes CE and releases apoAI, leaving an apoAII-rich remnant.

SR-B1 and PDZK1: partners in HDL regulation

PURPOSE OF REVIEW

To outline the roles of SR-B1 and PDZK1 in hepatic selective HDL cholesterol uptake and reverse cholesterol transport and the consequences for atherosclerosis development.

RECENT FINDINGS

Much of our understanding of the physiological roles of SR-B1 and PDZK1 in HDL metabolism and atherosclerosis comes from studies of genetically manipulated mice. These show SR-B1 and PDZK1 play key roles in HDL metabolism and protection against atherosclerosis. The recent identification of rare loss of function mutations in the human SCARB1 gene verifies that it plays similar roles in HDL metabolism in humans. Other rare mutations in both the human SCARB1 and PDZK1 genes remain to be characterized but may have potentially devastating consequences to SR-B1 function.

SUMMARY

Identification of carriers of rare mutations in human SCARB1 and PDZK1 that impair the function of their gene products and characterization of the effects of these mutations on HDL cholesterol levels and atherosclerosis will add to our understanding of the importance of HDL function and cholesterol flux, as opposed to HDL-cholesterol levels, per se, for protection against cardiovascular disease.

Effects of Dietary Flavonoids on Reverse Cholesterol Transport, HDL Metabolism, and HDL Function

Strong experimental evidence confirms that HDL directly alleviates atherosclerosis. HDL particles display diverse atheroprotective functions in reverse cholesterol transport (RCT), antioxidant, anti-inflammatory, and antiapoptotic processes. In certain inflammatory disease states, however, HDL particles may become dysfunctional and proatherogenic. Flavonoids show the potential to improve HDL function through their well-documented effects on cellular antioxidant status and inflammation. The aim of this review is to summarize the basic science and clinical research examining the effects of dietary flavonoids on RCT and HDL function. Based on preclinical studies that used cell culture and rodent models, it appears that many flavonoids (e.g., anthocyanidins, flavonols, and flavone subclasses) influence RCT and HDL function beyond simple HDL cholesterol concentration by regulating cellular cholesterol efflux from macrophages and hepatic paraoxonase 1 expression and activity. In clinical studies, dietary anthocyanin intake is associated with beneficial changes in serum biomarkers related to HDL function in a variety of human populations (e.g., in those who are hyperlipidemic, hypertensive, or diabetic), including increased HDL cholesterol concentration, as well as HDL antioxidant and cholesterol efflux capacities. However, clinical research on HDL functionality is lacking for some flavonoid subclasses (e.g., flavanols, flavones, flavanones, and isoflavones). Although there has been a tremendous effort to develop HDL-targeted drug therapies, more research is warranted on how the intake of foods or specific nutrients affects HDL function.

Deacetylated Konjac Glucomannan Is Less Effective in Reducing Dietary-Induced Hyperlipidemia and Hepatic Steatosis in C57BL/6 Mice

Konjac gel foods that mainly consist of deacetylated konjac glucomannan (Da-KGM) are considered to have the same health benefits as native konjac glucomannan (KGM); however, no definitive data support this notion. The objective of this study was to compare the effects of Da-KGM and KGM on the hyperlipidemia and liver steatosis induced by high-fat diet feeding and to investigate the underlying molecular mechanisms. C57BL/6 mice were fed (1) normal chow diet, (2) high-fat diet, (3) HFD with KGM, or (4) HFD with Da-KGM for 10 weeks. KGM, but not Da-KGM, showed decreased fat accumulation, improved blood and liver lipid profiles, and prevention of liver lipid droplet deposition compared with HFD. Compared with Da-KGM, KGM increased the outputs of fecal bile acid (KGM 22.5 ± 2.34 mg/g vs Da-KGM 19.3 ± 1.87 mg/g), fat (KGM 5.56 ± 0.68 mg/g vs Da-KGM 4.42 ± 0.57 mg/g) and cholesterol (KGM2.67 ± 0.43 mg/g vs Da-KGM 1.78 ± 0.28 mg/g), fecal concentrations of total short-chain fatty acids (KGM 103 ± 14.8 μmol/g vs Da-KGM 74.5 ± 8.49 μmol/g), and improved hepatic antioxidant status and upregulated CYP7A1 and LDLR gene expression. These findings suggest that deacetylation of KGM negatively affects its fermentation characteristics and its inhibition of lipid absorption, which thereby reduces Da-KGM's health benefits.

Regulation of lipid metabolism by obeticholic acid in hyperlipidemic hamsters

The farnesoid X receptor (FXR) plays critical roles in plasma cholesterol metabolism, in particular HDL-cholesterol (HDL-C) homeostasis. Obeticholic acid (OCA) is a FXR agonist being developed for treating various chronic liver diseases. Previous studies reported inconsistent effects of OCA on regulating plasma cholesterol levels in different animal models and in different patient populations. The mechanisms underlying its divergent effects have not yet been thoroughly investigated. The scavenger receptor class B type I (SR-BI) is a FXR-modulated gene and the major receptor for HDL-C. We investigated the effects of OCA on hepatic SR-BI expression and correlated such effects with plasma HDL-C levels and hepatic cholesterol efflux in hyperlipidemic hamsters. We demonstrated that OCA induced a time-dependent reduction in serum HDL-C levels after 14 days of treatment, which was accompanied by a significant reduction of liver cholesterol content and increases in fecal cholesterol in OCA-treated hamsters. Importantly, hepatic SR-BI mRNA and protein levels in hamsters were increased to 1.9- and 1.8-fold of control by OCA treatment. Further investigations in normolipidemic hamsters did not reveal OCA-induced changes in serum HDL-C levels or hepatic SR-BI expression. We conclude that OCA reduces plasma HDL-C levels and promotes transhepatic cholesterol efflux in hyperlipidemic hamsters via a mechanism involving upregulation of hepatic SR-BI.

Carboxy-terminal deletion of the HDL receptor reduces receptor levels in liver and steroidogenic tissues, induces hypercholesterolemia, and causes fatal heart disease

The HDL receptor SR-BI mediates the transfer of cholesteryl esters from HDL to cells and controls HDL abundance and structure. Depending on the genetic background, loss of SR-BI causes hypercholesterolemia, anemia, reticulocytosis, splenomegaly, thrombocytopenia, female infertility, and fatal coronary heart disease (CHD). The carboxy terminus of SR-BI (⁵⁰⁵QEAKL⁵⁰⁹) must bind to the cytoplasmic adaptor PDZK1 for normal hepatic-but not steroidogenic cell-expression of SR-BI protein. To determine whether SR-BI's carboxy terminus is also required for normal protein levels in steroidogenic cells, we introduced into SR-BI's gene a ⁵⁰⁷Ala/STOP mutation that produces a truncated receptor (SR-BIΔCT). As expected, the dramatic reduction of hepatic receptor protein in SR-BIΔCT mice was similar to that in PDZK1 knockout (KO) mice. Unlike SR-BI KO females, SR-BIΔCT females were fertile. The severity of SR-BIΔCT mice's hypercholesterolemia was intermediate between those of SR-BI KO and PDZK1 KO mice. Substantially reduced levels of the receptor in adrenal cortical cells, ovarian cells, and testicular Leydig cells in SR-BIΔCT mice suggested that steroidogenic cells have an adaptor(s) functionally analogous to hepatic PDZK1. When SR-BIΔCT mice were crossed with apolipoprotein E KO mice (SR-BIΔCT/apoE KO), pathologies including hypercholesterolemia, macrocytic anemia, hepatic and splenic extramedullary hematopoiesis, massive splenomegaly, reticulocytosis, thrombocytopenia, and rapid-onset and fatal occlusive coronary arterial atherosclerosis and CHD (median age of death: 9 wk) were observed. These results provide new insights into the control of SR-BI in steroidogenic cells and establish SR-BIΔCT/apoE KO mice as a new animal model for the study of CHD.

Rare variant in scavenger receptor BI raises HDL cholesterol and increases risk of coronary heart disease

Scavenger receptor BI (SR-BI) is the major receptor for high-density lipoprotein (HDL) cholesterol (HDL-C). In humans, high amounts of HDL-C in plasma are associated with a lower risk of coronary heart disease (CHD). Mice that have depleted Scarb1 (SR-BI knockout mice) have markedly elevated HDL-C levels but, paradoxically, increased atherosclerosis. The impact of SR-BI on HDL metabolism and CHD risk in humans remains unclear. Through targeted sequencing of coding regions of lipid-modifying genes in 328 individuals with extremely high plasma HDL-C levels, we identified a homozygote for a loss-of-function variant, in which leucine replaces proline 376 (P376L), in SCARB1, the gene encoding SR-BI. The P376L variant impairs posttranslational processing of SR-BI and abrogates selective HDL cholesterol uptake in transfected cells, in hepatocyte-like cells derived from induced pluripotent stem cells from the homozygous subject, and in mice. Large population-based studies revealed that subjects who are heterozygous carriers of the P376L variant have significantly increased levels of plasma HDL-C. P376L carriers have a profound HDL-related phenotype and an increased risk of CHD (odds ratio = 1.79, which is statistically significant).

HDL is redundant for adrenal steroidogenesis in LDLR knockout mice with a human-like lipoprotein profile

The contribution of HDL to adrenal steroidogenesis appears to be different between mice and humans. In the current study, we tested the hypothesis that a difference in lipoprotein profile may be the underlying cause. Hereto, we determined the impact of HDL deficiency on the adrenal glucocorticoid output in genetically modified mice with a human-like lipoprotein profile. Genetic deletion of APOA1 in LDL receptor (LDLR) knockout mice was associated with HDL deficiency and a parallel increase in the level of cholesterol associated with nonHDL fractions. Despite a compensatory increase in the adrenal relative mRNA expression levels of the cholesterol synthesis gene, HMG-CoA reductase, adrenals from APOA1/LDLR double knockout mice were severely depleted of neutral lipids, as compared with those of control LDLR knockout mice. However, basal corticosterone levels and the adrenal glucocorticoid response to stress were not different between the two types of mice. In conclusion, we have shown that HDL is not critical for proper adrenal glucocorticoid function when mice are provided with a human-like lipoprotein profile. Our findings provide the first experimental evidence that APOB-containing lipoproteins may facilitate adrenal steroidogenesis, in an LDLR-independent manner, in vivo in mice.

Lipoprotein profiles in human heterozygote carriers of a functional mutation P297S in scavenger receptor class B1

The scavenger receptor class B type 1 (SR-B1) is an important HDL receptor involved in cholesterol uptake and efflux, but its physiological role in human lipoprotein metabolism is not fully understood. Heterozygous carriers of the SR-B1(P297S) mutation are characterized by increased HDL cholesterol levels, impaired cholesterol efflux from macrophages and attenuated adrenal function. Here, the composition and function of lipoproteins were studied in SR-B1(P297S) heterozygotes.Lipoproteins from six SR-B1(P297S) carriers and six family controls were investigated. HDL and LDL/VLDL were isolated by ultracentrifugation and proteins were separated by two-dimensional gel electrophoresis and identified by mass spectrometry. HDL antioxidant properties, paraoxonase 1 activities, apoA-I methionine oxidations and HDL cholesterol efflux capacity were assessed.Multivariate modeling separated carriers from controls based on lipoprotein composition. Protein analyses showed a significant enrichment of apoE in LDL/VLDL and of apoL-1 in HDL from heterozygotes compared to controls. The relative distribution of plasma apoE was increased in LDL and in lipid-free form. There were no significant differences in paraoxonase 1 activities, HDL antioxidant properties or HDL cholesterol efflux capacity but heterozygotes showed a significant increase of oxidized methionines in apoA-I.The SR-B1(P297S) mutation affects both HDL and LDL/VLDL protein compositions. The increase of apoE in carriers suggests a compensatory mechanism for attenuated SR-B1 mediated cholesterol uptake by HDL. Increased methionine oxidation may affect HDL function by reducing apoA-I binding to its targets. The results illustrate the complexity of lipoprotein metabolism that has to be taken into account in future therapeutic strategies aiming at targeting SR-B1.

Genome-Edited Human Pluripotent Stem Cell-Derived Macrophages as a Model of Reverse Cholesterol Transport--Brief Report

OBJECTIVE

To create isogenic human pluripotent stem cell-derived macrophages with and without ABCA1 expression as a model for reverse cholesterol transport.

APPROACH AND RESULTS

The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated 9 (Cas9) genome-editing system was used to introduce frameshift mutations into the coding sequence of ATP-binding cassette, subfamily A, member 1. Individual human pluripotent stem cell clones with deleterious mutations were identified, expanded, and differentiated into mature macrophages with a cytokine-based, feeder-free differentiation protocol. Wild-type cells demonstrated effective cholesterol efflux to apoAI acceptor, whereas ABCA1(-/-) cells displayed significantly reduced efflux ability and increased expression of proinflammatory cytokines.

CONCLUSIONS

Human pluripotent stem cell-derived macrophages capable of reverse cholesterol transport can be rapidly generated and genetically edited with CRISPR/Cas9. Introduction of homozygous frameshift mutations results in loss of ABCA1 expression in differentiated macrophages and subsequent reduction of cholesterol efflux capability. This facile genome-editing approach and differentiation protocol pave the way for future studies of the molecular determinants of reverse cholesterol transport and other macrophage properties.

Xanthine-based KMUP-1 improves HDL via PPARγ/SR-B1, LDL via LDLRs, and HSL via PKA/PKG for hepatic fat loss

The phosphodiesterase inhibitor (PDEI)/eNOS enhancer KMUP-1, targeting G-protein coupled receptors (GPCRs), improves dyslipidemia. We compared its lipid-lowering effects with simvastatin and explored hormone-sensitive lipase (HSL) translocation in hepatic fat loss. KMUP-1 HCl (1, 2.5, and 5 mg/kg/day) and simvastatin (5 mg/kg/day) were administered in C57BL/6J male mice fed a high-fat diet (HFD) by gavage for 8 weeks. KMUP-1 inhibited HFD-induced plasma/liver TG, total cholesterol, and LDL; increased HDL/3-hydroxy-3-methylglutaryl-CoA reductase (HMGR)/Rho kinase II (ROCK II)/PPARγ/ABCA1; and decreased liver and body weight. KMUP-1 HCl in drinking water (2.5 mg/200 ml tap water) for 1–14 or 8–14 weeks decreased HFD-induced liver and body weight and scavenger receptor class B type I expression and increased protein kinase A (PKA)/PKG/LDLRs/HSL expression and immunoreactivity. In HepG2 cells incubated with serum or exogenous mevalonate, KMUP-1 (10⁻⁷∼10⁻⁵ M) reversed HMGR expression by feedback regulation, colocalized expression of ABCA1/apolipoprotein A-I/LXRα/PPARγ, and reduced exogenous geranylgeranyl pyrophosphate/farnesyl pyrophosphate (FPP)-induced RhoA/ROCK II expression. A guanosine 3′,5′-cyclic monophosphate (cGMP) antagonist reversed KMUP-1-induced ROCK II reduction, indicating cGMP/eNOS involvement. KMUP-1 inceased PKG and LDLRs surrounded by LDL and restored oxidized LDL-induced PKA expresion. Unlike simvastatin, KMUP-1 could not inhibit ¹⁴C mevalonate formation. KMUP-1 could, but simvastatin could not, decrease ROCK II expression by exogenous FPP/CGPP. KMUP-1 improves HDL via PPARγ/LXRα/ABCA1/Apo-I expression and increases LDLRs/PKA/PKG/HSL expression and immunoreactivity, leading to TG hydrolysis to lower hepatic fat and body weight.

Adrenocortical LDL receptor function negatively influences glucocorticoid output

Over 50% of the cholesterol needed by adrenocortical cells for the production of glucocorticoids is derived from lipoproteins. However, the overall contribution of the different lipoproteins and associated uptake pathways to steroidogenesis remains to be determined. Here we aimed to show the importance of LDL receptor (LDLR)-mediated cholesterol acquisition for adrenal steroidogenesis in vivo. Female total body LDLR knockout mice with a human-like lipoprotein profile were bilaterally adrenalectomized and subsequently provided with one adrenal either expressing or genetically lacking the LDLR under their renal capsule to solely modulate adrenocortical LDLR function. Plasma total cholesterol levels and basal plasma corticosterone levels were identical in the two types of adrenal transplanted mice. Strikingly, restoration of adrenal LDLR function significantly reduced the ACTH-mediated stimulation of adrenal steroidogenesis (P<0.001), with plasma corticosterone levels that were respectively 44-59% lower (P<0.01) as compared to adrenal LDLR negative controls. In addition, LDLR positive adrenal transplanted mice exhibited a significant decrease (-39%; P<0.001) in their plasma corticosterone level under fasting stress conditions. Biochemical analysis did not show changes in the expression of genes involved in cholesterol mobilization. However, LDLR expressing adrenal transplants displayed a marked 62% reduction (P<0.05) in the transcript level of the key steroidogenic enzyme HSD3B2. In conclusion, our studies in a mouse model with a human-like lipoprotein profile provide the first in vivo evidence for a novel inhibitory role of the LDLR in the control of adrenal glucocorticoid production.

High-density lipoprotein-mediated transcellular cholesterol transport in mouse aortic endothelial cells

Accumulation of unesterified cholesterol-rich lipid vesicles in the subendothelial space contributes to atherogenesis. Transport of cholesterol from the subendothelial intima back to the circulating blood inhibits atherosclerosis development; however, the mechanism for this process has not been fully defined. Using cultured mouse aortic endothelial cells (MAECs), we observed that unesterified cholesterol can be transported across the endothelial cell monolayer from the basolateral to the apical compartment. Administration of high-density lipoprotein (HDL) or apolipoprotein AI (apoAI) to the apical compartment enhanced transendothelial cholesterol transport in a concentration-dependent manner. Knockdown of ATP-binding cassette transporter G1 (ABCG1) or scavenger receptor class B type I (SR-B1), or inhibition of SR-B1 diminished HDL-induced transendothelial cholesterol transport; while knockdown of ABCA1 reduced apoAI-mediated cholesterol transport. HDL enhanced phosphorylation of phosphatidylinositol 3-kinase (PI3K) and Akt in MAECs. However, inhibition of PI3K or Akt did not reduce HDL-induced transendothelial cholesterol transport. These results suggest that HDL enhances transendothelial cholesterol transport by activation of a mechanism involving ABCA1, ABCG1 and SR-B1 but not involving PI3K and Akt.

Increased plasma cholesterol esterification by LCAT reduces diet-induced atherosclerosis in SR-BI knockout mice

LCAT, a plasma enzyme that esterifies cholesterol, has been proposed to play an antiatherogenic role, but animal and epidemiologic studies have yielded conflicting results. To gain insight into LCAT and the role of free cholesterol (FC) in atherosclerosis, we examined the effect of LCAT over- and underexpression in diet-induced atherosclerosis in scavenger receptor class B member I-deficient [Scarab(-/-)] mice, which have a secondary defect in cholesterol esterification. Scarab(-/-)×LCAT-null [Lcat(-/-)] mice had a decrease in HDL-cholesterol and a high plasma ratio of FC/total cholesterol (TC) (0.88 ± 0.033) and a marked increase in VLDL-cholesterol (VLDL-C) on a high-fat diet. Scarab(-/-)×LCAT-transgenic (Tg) mice had lower levels of VLDL-C and a normal plasma FC/TC ratio (0.28 ± 0.005). Plasma from Scarab(-/-)×LCAT-Tg mice also showed an increase in cholesterol esterification during in vitro cholesterol efflux, but increased esterification did not appear to affect the overall rate of cholesterol efflux or hepatic uptake of cholesterol. Scarab(-/-)×LCAT-Tg mice also displayed a 51% decrease in aortic sinus atherosclerosis compared with Scarab(-/-) mice (P < 0.05). In summary, we demonstrate that increased cholesterol esterification by LCAT is atheroprotective, most likely through its ability to increase HDL levels and decrease pro-atherogenic apoB-containing lipoprotein particles.

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

AIMS

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

METHODS AND RESULTS

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

CONCLUSION

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

Mouse models of disturbed HDL metabolism

High-density lipoprotein (HDL) is considered to be an anti-atherogenic lipoprotein moiety. Generation of genetically modified (total body and tissue-specific knockout) mouse models has significantly contributed to our understanding of HDL function. Here we will review data from knockout mouse studies on the importance of HDL's major alipoprotein apoA-I, the ABC transporters A1 and G1, lecithin:cholesterol acyltransferase, phospholipid transfer protein, and scavenger receptor BI for HDL's metabolism and its protection against atherosclerosis in mice. The initial generation and maturation of HDL particles as well as the selective delivery of its cholesterol to the liver are essential parameters in the life cycle of HDL. Detrimental atherosclerosis effects observed in response to HDL deficiency in mice cannot be solely attributed to the low HDL levels per se, as the low HDL levels are in most models paralleled by changes in non-HDL-cholesterol levels. However, the cholesterol efflux function of HDL is of critical importance to overcome foam cell formation and the development of atherosclerotic lesions in mice. Although HDL is predominantly studied for its atheroprotective action, the mouse data also suggest an essential role for HDL as cholesterol donor for steroidogenic tissues, including the adrenals and ovaries. Furthermore, it appears that a relevant interaction exists between HDL-mediated cellular cholesterol efflux and the susceptibility to inflammation, which (1) provides strong support for the novel concept that inflammation and metabolism are intertwining biological processes and (2) identifies the efflux function of HDL as putative therapeutic target also in other inflammatory diseases than atherosclerosis.

Ablating L-FABP in SCP-2/SCP-x null mice impairs bile acid metabolism and biliary HDL-cholesterol secretion

On the basis of their abilities to bind bile acids and/or cholesterol, the physiological role(s) of liver fatty acid-binding protein (L-FABP) and sterol carrier protein (SCP) 2/SCP-x (SCP-2/SCP-x) gene products in biliary bile acid and cholesterol formation was examined in gene-ablated male mice. L-FABP (LKO) or L-FABP/SCP-2/SCP-x [triple-knockout (TKO)] ablation markedly decreased hepatic bile acid concentration, while SCP-2/SCP-x [double-knockout (DKO)] ablation alone had no effect. In contrast, LKO increased biliary bile acid, while DKO and TKO had no effect on biliary bile acid levels. LKO and DKO also altered biliary bile acid composition to increase bile acid hydrophobicity. Furthermore, LKO and TKO decreased hepatic uptake and biliary secretion of high-density lipoprotein (HDL)-derived 22-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3β-ol (NBD-cholesterol), while DKO alone had no effect. Finally, LKO and, to a lesser extent, DKO decreased most indexes contributing to cholesterol solubility in biliary bile. These results suggest different, but complementary, roles for L-FABP and SCP-2/SCP-x in biliary bile acid and cholesterol formation. L-FABP appears to function more in hepatic retention of bile acids as well as hepatic uptake and biliary secretion of HDL-cholesterol. Conversely, SCP-2/SCP-x may function more in formation and biliary secretion of bile acid, with less impact on hepatic uptake or biliary secretion of HDL-cholesterol.

Susceptibility gene for stroke or cerebral infarction in the Han population in Hunan Province of China

The scavenger receptor class B type I gene can protect against atherosclerosis; a mononucleotide polymorphism is associated with differences in blood lipid metabolism, postprandial serum lipid levels, insulin resistance, coronary artery disease and familial hyperlipidemia. In this study, the scavenger receptor class B type I gene exon 1 G4A gene polymorphism in atherosclerotic cerebral infarction patients, cerebral hemorrhage patients and normal controls was detected using the polymerase chain reaction-restriction fragment length polymorphism method. The results showed that the GA + AA genotype frequency of scavenger receptor class B type I gene G4A in atherosclerotic cerebral infarction patients was similar to that in cerebral hemorrhage patients and normal controls; however, the A allele frequency was significantly lower than that in normal controls. The serum level of high-density lipoprotein cholesterol in patients with the scavenger receptor class B type I gene G4A GA + AA genotype was significantly higher, while the serum level of low-density lipoprotein cholesterol was significantly lower than that in patients with the GG genotype, in both the atherosclerotic cerebral infarction and cerebral hemorrhage groups. The serum level of high-density lipoprotein cholesterol in patients with the scavenger receptor class B type I gene G4A GA + AA genotype was significantly higher, while the serum levels of low-density lipoprotein cholesterol and total cholesterol were significantly lower than those in normal controls with the GG genotype. Our experimental results suggest that the G4A polymorphism of the scavenger receptor class B type I gene is a possible predisposing risk factor for atherosclerotic cerebral infarction, and that it has no association with cerebral hemorrhage in the Han population in Hunan province of China. The A allele is possibly associated with the metabolism of high-density and low-density lipoprotein cholesterol.

Scavenger receptor class B type I (SR-BI): a versatile receptor with multiple functions and actions

Scavenger receptor class B type I (SR-BI), is a physiologically relevant HDL receptor that mediates selective uptake of lipoprotein (HDL)-derived cholesteryl ester (CE) in vitro and in vivo. Mammalian SR-BI is a 509-amino acid, ~82 kDa glycoprotein that contains N- and C-terminal cytoplasmic domains, two-transmembrane domains, as well as a large extracellular domain containing 5-6 cysteine residues and multiple sites for N-linked glycosylation. The size and structural characteristics of SR-BI, however, vary considerably among lower vertebrates and insects. Recently, significant progress has been made in understanding the molecular mechanisms involved in the posttranscriptional/posttranslational regulation of SR-BI in a tissue specific manner. The purpose of this review is to summarize the current body of knowledge about the events and molecules connected with the posttranscriptional/posttranslational regulation of SR-BI and to update the molecular and functional characteristics of the insect SR-BI orthologs.

A computational model for the analysis of lipoprotein distributions in the mouse: translating FPLC profiles to lipoprotein metabolism

Disturbances of lipoprotein metabolism are recognized as indicators of cardiometabolic disease risk. Lipoprotein size and composition, measured in a lipoprotein profile, are considered to be disease risk markers. However, the measured profile is a collective result of complex metabolic interactions, which complicates the identification of changes in metabolism. In this study we aim to develop a method which quantitatively relates murine lipoprotein size, composition and concentration to the molecular mechanisms underlying lipoprotein metabolism. We introduce a computational framework which incorporates a novel kinetic model of murine lipoprotein metabolism. The model is applied to compute a distribution of plasma lipoproteins, which is then related to experimental lipoprotein profiles through the generation of an in silico lipoprotein profile. The model was first applied to profiles obtained from wild-type C57Bl/6J mice. The results provided insight into the interplay of lipoprotein production, remodelling and catabolism. Moreover, the concentration and metabolism of unmeasured lipoprotein components could be determined. The model was validated through the prediction of lipoprotein profiles of several transgenic mouse models commonly used in cardiovascular research. Finally, the framework was employed for longitudinal analysis of the profiles of C57Bl/6J mice following a pharmaceutical intervention with a liver X receptor (LXR) agonist. The multifaceted regulatory response to the administration of the compound is incompletely understood. The results explain the characteristic changes of the observed lipoprotein profile in terms of the underlying metabolic perturbation and resultant modifications of lipid fluxes in the body. The Murine Lipoprotein Profiler (MuLiP) presented here is thus a valuable tool to assess the metabolic origin of altered murine lipoprotein profiles and can be applied in preclinical research performed in mice for analysis of lipid fluxes and lipoprotein composition.

Hepatic scavenger receptor BI protects against polymicrobial-induced sepsis through promoting LPS clearance in mice

Recent studies revealed that scavenger receptor BI (SR-BI or Scarb1) plays a critical protective role in sepsis. However, the mechanisms underlying this protection remain largely unknown. In this study, using Scarb1(I179N) mice, a mouse model specifically deficient in hepatic SR-BI, we report that hepatic SR-BI protects against cecal ligation and puncture (CLP)-induced sepsis as shown by 75% fatality in Scarb1(I179N) mice, but only 21% fatality in C57BL/6J control mice. The increase in fatality in Scarb1(I179N) mice was associated with an exacerbated inflammatory cytokine production. Further study demonstrated that hepatic SR-BI exerts its protection against sepsis through its role in promoting LPS clearance without affecting the inflammatory response in macrophages, the glucocorticoid production in adrenal glands, the leukocyte recruitment to peritoneum or the bacterial clearance in liver. Our findings reveal hepatic SR-BI as a critical protective factor in sepsis and point out that promoting hepatic SR-BI-mediated LPS clearance may provide a therapeutic approach for sepsis.

Scavenger receptor class B type I and immune dysfunctions

PURPOSE OF REVIEW

To summarize the recent findings about the roles of scavenger receptor class B type I (SR-BI) in immunity and discuss the underlying mechanisms by which SR-BI prevents immune dysfunctions.

RECENT FINDINGS

SR-BI is well known as a high-density lipoprotein (HDL) receptor playing key roles in HDL metabolism and in protection against atherosclerosis. Recent studies have indicated that SR-BI is also an essential modulator in immunity. SR-BI deficiency in mice causes immune dysfunctions, including increased atherosclerosis, elevated susceptibility to sepsis, impaired lymphocyte homeostasis, and autoimmune disorders. SR-BI exerts its protective roles through a variety of HDL-dependent and HDL-independent mechanisms. SR-BI is also involved in hepatitis C virus cell entry. A deficiency of SR-BI in humanized mice has been shown to decrease hepatitis C virus infectivity.

SUMMARY

SR-BI regulates immunity via multiple mechanisms and its deficiency causes numerous diseases. A comprehensive understanding of the roles of SR-BI in protection against immune dysfunctions may provide a therapeutic target for intervention against its associated diseases.

Scavenger receptor BI and high-density lipoprotein regulate thymocyte apoptosis in sepsis

OBJECTIVE

Thymocyte apoptosis is a major event in sepsis; however, how this process is regulated remains poorly understood.

APPROACH AND RESULTS

Septic stress induces glucocorticoids production which triggers thymocyte apoptosis. Here, we used scavenger receptor BI (SR-BI)-null mice, which are completely deficient in inducible glucocorticoids in sepsis, to investigate the regulation of thymocyte apoptosis in sepsis. Cecal ligation and puncture induced profound thymocyte apoptosis in SR-BI(+/+) mice, but no thymocyte apoptosis in SR-BI(-/-) mice because of lack of inducible glucocorticoids. Unexpectedly, supplementation of glucocorticoids only partly restored thymocyte apoptosis in SR-BI(-/-) mice. We demonstrated that high-density lipoprotein (HDL) is a critical modulator for thymocyte apoptosis. SR-BI(+/+) HDL significantly enhanced glucocorticoid-induced thymocyte apoptosis, but SR-BI(-/-) HDL had no such activity. Further study revealed that SR-BI(+/+) HDL modulates glucocorticoid-induced thymocyte apoptosis via promoting glucocorticoid receptor translocation, but SR-BI(-/-) HDL loses such regulatory activity. To understand why SR-BI(-/-) HDL loses its regulatory activity, we analyzed HDL cholesterol contents. There was 3-fold enrichment of unesterified cholesterol in SR-BI(-/-) HDL compared with SR-BI(+/+) HDL. Normalization of unesterified cholesterol in SR-BI(-/-) HDL by probucol administration or lecithin cholesteryl acyltransferase expression restored glucocorticoid-induced thymocyte apoptosis, and incorporating unesterified cholesterol into SR-BI(+/+) HDL rendered SR-BI(+/+) HDL dysfunctional. Using lckCre-GR(fl/fl) mice in which thymocytes lack cecal ligation and puncture-induced thymocyte apoptosis, we showed that lckCre-GR(fl/fl) mice were significantly more susceptible to cecal ligation and puncture-induced septic death than GR(fl/fl) control mice, suggesting that glucocorticoid-induced thymocyte apoptosis is required for protection against sepsis.

CONCLUSIONS

The findings in this study reveal a novel regulatory mechanism of thymocyte apoptosis in sepsis by SR-BI and HDL.

ATP Synthase β-Chain Overexpression in SR-BI Knockout Mice Increases HDL Uptake and Reduces Plasma HDL Level

HDL cholesterol is known to be inversely correlated with cardiovascular disease due to its diverse antiatherogenic functions. SR-BI mediates the selective uptake of HDL-C. SR-BI knockout diminishes but does not completely block the transport of HDL; other receptors may be involved. Ectopic ATP synthase β-chain in hepatocytes has been previously characterized as an apoA-I receptor, triggering HDL internalization. This study was undertaken to identify the overexpression of ectopic ATP synthase β-chain on DIL-HDL uptake in primary hepatocytes in vitro and on plasma HDL levels in SR-BI knockout mice. Human ATP synthase β-chain cDNA was delivered to the mouse liver by adenovirus and GFP adenovirus as control. The adenovirus-mediated overexpression of β-chain was identified at both mRNA and protein levels on mice liver and validated by its increasing of DiL-HDL uptake in primary hepatocytes. In response to hepatic overexpression of β-chain, plasma HDL-C levels and cholesterol were reduced in SR-BI knockout mice, compared with the control. The present data suggest that ATP synthase β-chain can serve as the endocytic receptor of HDL, and its overexpression can reduce plasma HDL-C.

Genetics of HDL-C: a causal link to atherosclerosis?

Prospective epidemiological studies have consistently reported an inverse association between HDL cholesterol (HDL-C) levels and the risk of cardiovascular disease (CVD). However, large intervention trials on HDL-C-increasing drugs and recent Mendelian randomization studies have questioned a causal relationship between HDL-C and atherosclerosis. HDL-C levels have been shown to be highly heritable, and the combination of HDL-C-associated SNPs in recent large-scale genome-wide association studies (GWAS) only explains a small proportion of this heritability. As a large part of our current understanding of HDL metabolism comes from genetic studies, further insights in this research field may aid us in elucidating HDL functionality in relation to CVD risk. In this review we focus on the question of whether genetically defined HDL-C levels are associated with risk of atherosclerosis. We also discuss the latest insights for HDL-C-associated genes and recent GWAS data.

High-density lipoprotein endocytosis in endothelial cells

AIM: To describe the way stations of high-density lipoprotein (HDL) uptake and its lipid exchange in endothelial cells in vitro and in vivo.

METHODS: A combination of fluorescence microscopy using novel fluorescent cholesterol surrogates and electron microscopy was used to analyze HDL endocytosis in great detail in primary human endothelial cells. Further, HDL uptake was quantified using radio-labeled HDL particles. To validate the in vitro findings mice were injected with fluorescently labeled HDL and particle uptake in the liver was analyzed using fluorescence microscopy.

RESULTS: HDL uptake occurred via clathrin-coated pits, tubular endosomes and multivesicular bodies in human umbilical vein endothelial cells. During uptake and resecretion, HDL-derived cholesterol was exchanged at a faster rate than cholesteryl oleate, resembling the HDL particle pathway seen in hepatic cells. In addition, lysosomes were not involved in this process and thus HDL degradation was not detectable. In vivo, we found HDL mainly localized in mouse hepatic endothelial cells. HDL was not detected in parenchymal liver cells, indicating that lipid transfer from HDL to hepatocytes occurs primarily via scavenger receptor, class B, type I mediated selective uptake without concomitant HDL endocytosis.

CONCLUSION: HDL endocytosis occurs via clathrin-coated pits, tubular endosomes and multivesicular bodies in human endothelial cells. Mouse endothelial cells showed a similar HDL uptake pattern in vivo indicating that the endothelium is one major site of HDL endocytosis and transcytosis.

SR-BI in bone marrow derived cells protects mice from diet induced coronary artery atherosclerosis and myocardial infarction

SR-BI deficient mice that are also hypomorphic for apolipoprotein E expression develop diet induced occlusive coronary artery atherosclerosis, myocardial infarction and early death. To test the role of SR-BI in bone marrow derived cells, we used bone marrow transplantation to generate SR-BI-null; apoE-hypomorphic mice in which SR-BI expression was restored solely in bone marrow derived cells. SR-BI-null; apoE-hypomorphic mice were transplanted with SR-BI^+/+apoE-hypomorphic, or control, autologous SR-BI-null; apoE-hypomorphic bone marrow. Four weeks later, mice were fed a high-fat, high-cholesterol, cholate-containing diet to induce coronary artery atherosclerosis. Mice transplanted with autologous bone marrow developed extensive aortic atherosclerosis and severe occlusive coronary artery atherosclerosis after 4 weeks of feeding. This was accompanied by myocardial fibrosis and increased heart weights. In contrast, restoration of SR-BI expression in bone marrow derived-cells reduced diet induced aortic and coronary artery atherosclerosis, myocardial fibrosis and the increase in heart weights in SR-BI-null; apoE-hypomorphic mice. Restoration of SR-BI in bone marrow derived cells did not, however, affect steady state lipoprotein cholesterol levels, but did reduce plasma levels of IL-6. Monocytes from SR-BI-null mice exhibited a greater capacity to bind to VCAM-1 and ICAM-1 than those from SR-BI^+/+ mice. Furthermore, restoration of SR-BI expression in bone marrow derived cells attenuated monocyte recruitment into atherosclerotic plaques in mice fed high fat, high cholesterol cholate containing diet. These data demonstrate directly that SR-BI in bone marrow-derived cells protects against both aortic and CA atherosclerosis.

Functional genomics of the human high-density lipoprotein receptor scavenger receptor BI: an old dog with new tricks

PURPOSE OF REVIEW

The athero-protective role of scavenger receptor BI (SR-BI) is primarily attributed to its ability to selectively transfer cholesteryl esters from high-density lipoproteins (HDLs) to the liver during reverse cholesterol transport (RCT). In this review, we highlight recent findings that reveal the impact of SR-BI on lipid levels and cardiovascular disease in humans. Moreover, additional responsibilities of SR-BI in modulating adrenal and platelet function, as well as female fertility in humans, are discussed.

RECENT FINDINGS

Heterozygote carriers of P297S, S112F and T175A-mutant SR-BI receptors were identified in patients with high HDL-cholesterol levels. HDL from P297S-SR-BI carriers was unable to mediate macrophage cholesterol efflux, whereas hepatocytes expressing P297S-SR-BI were unable to mediate the selective uptake of HDL-cholesteryl esters. S112F and T175A-mutant receptors exhibited similar impaired cholesterol transport functions in vitro. Reduced SR-BI function in P297S carriers was also associated with decreased steroidogenesis and altered platelet function. Further, human population studies identified SCARB1 variants associated with female infertility.

SUMMARY

Identification of SR-BI variants confirms the key role of this receptor in influencing lipid levels and RCT in humans. A deeper understanding of the contributions of SR-BI to steroidogenesis, platelet function and fertility is required in light of exploration of HDL-raising therapies aimed at reducing cardiovascular risk.

The myeloperoxidase product hypochlorous acid generates irreversible high-density lipoprotein receptor inhibitors

OBJECTIVE

Elevated levels of advanced oxidation protein products have been described in several chronic inflammatory diseases, like chronic renal insufficiency, rheumatoid arthritis, and atherosclerosis. Recent findings revealed that advanced oxidation protein products are inhibitors of the major high-density lipoprotein receptor, scavenger receptor class B, type 1 (SR-BI). Here, we investigated which oxidation-induced structural alterations convert plasma albumin into a high-density lipoprotein-receptor inhibitor.

APPROACH AND RESULTS

Exposure of albumin to the physiological oxidant, hypochlorous acid, generated high-affinity SR-BI ligands. Protection of albumin-lysine residues before exposure to hypochlorous acid as well as regeneration of N-chloramines after oxidation of albumin completely prevented binding of oxidized albumin to SR-BI, indicating that modification of albumin-lysine residues is required to generate SR-BI ligands. Of particular interest, N-chloramines within oxidized albumin promoted irreversible binding to SR-BI, resulting in permanent receptor blockade. We observed that the SR-BI inhibitory activity of albumin isolated from chronic kidney disease patients correlated with the content of the myeloperoxidase-specific oxidation product 3-chlorotyrosine and was associated with alterations in the composition of high-density lipoprotein.

CONCLUSIONS

Given that several potential atheroprotective activities of high-density lipoprotein are mediated by SR-BI, the present results raise the possibility that oxidized plasma albumin, through permanent SR-BI blockade, contributes to the pathophysiology of cardiovascular disease.

Regulation of expression and function of scavenger receptor class B, type I (SR-BI) by Na+/H+ exchanger regulatory factors (NHERFs)

Scavenger receptor class B, type I (SR-BI) binds HDL and mediates selective delivery of cholesteryl esters (CEs) to the liver, adrenals, and gonads for product formation (bile acids and steroids). Because relatively little is known about SR-BI posttranslational regulation in steroidogenic cells, we examined the roles of Na(+)/H(+) exchanger regulatory factors (NHERFs) in regulating SR-BI expression, SR-BI-mediated selective CE uptake, and steroidogenesis. NHERF1 and NHERF2 mRNA and protein are expressed at varying levels in model steroidogenic cell lines and the adrenal, with only low expression of PDZK1 (NHERF3) and NHERF4. Dibutyryl cyclic AMP decreased NHERF1 and NHERF2 and increased SR-BI mRNA expression in primary rat granulosa cells and MLTC-1 cells, whereas ACTH had no effect on NHERF1 and NHERF2 mRNA levels but decreased their protein levels in rat adrenals. Co-immunoprecipitation, colocalization, bimolecular fluorescence complementation, and mutational analysis indicated that SR-BI associates with NHERF1 and NHERF2. NHERF1 and NHERF2 down-regulated SR-BI protein expression through inhibition of its de novo synthesis. NHERF1 and NHERF2 also inhibited SR-BI-mediated selective CE transport and steroidogenesis, which were markedly attenuated by partial deletions of the PDZ1 or PDZ2 domain of NHERF1, the PDZ2 domain of NHERF2, or the MERM domains of NHERF1/2 or by gene silencing of NHERF1/2. Moreover, an intact COOH-terminal PDZ recognition motif (EAKL) in SR-BI is needed. Transient transfection of hepatic cell lines with NHERF1 or NHERF2 caused a significant reduction in endogenous protein levels of SR-BI. Collectively, these data establish NHERF1 and NHERF2 as SR-BI protein binding partners that play a negative role in the regulation of SR-BI expression, selective CE transport, and steroidogenesis.

Adrenal-Specific Scavenger Receptor BI Deficiency Induces Glucocorticoid Insufficiency and Lowers Plasma Very-Low-Density and Low-Density Lipoprotein Levels in Mice

Objective—

We determined the physiological consequences of adrenocortical-specific deletion of scavenger receptor BI (SR-BI) function in C57BL/6 wild-type mice.

Methods and Results—

One adrenal from 10-day-old SR-BI knockout (KO) mice or wild-type controls was transplanted under the renal capsule of adrenalectomized C57BL/6 recipient mice. The fasting plasma corticosterone level increased over time in transplanted mice. Corticosterone values in SR-BI KO transplanted mice remained ≈50% lower ( P <0.001) as compared with wild-type transplanted mice, which coincided with adrenocortical lipid depletion. A 6.5-fold higher ( P <0.01) plasma adrenocorticotropic hormone level was present in SR-BI KO transplanted mice reminiscent of primary glucocorticoid insufficiency. On feeding with cholic acid-containing high cholesterol/high fat diet, SR-BI KO transplanted mice exhibited a 26% ( P <0.05) reduction in their liver triglyceride level. Hepatic myosin regulatory light chain interacting protein/inducible degrader of the low-density lipoprotein receptor mRNA expression was 48% ( P <0.01) decreased in adrenal-specific SR-BI KO mice, which was paralleled by a marked decrease (–46%; P <0.01) in proatherogenic very-low-density and low-density lipoprotein levels.

Conclusion—

Adrenal-specific disruption of SR-BI function induces glucocorticoid insufficiency and lowers plasma very-low-density and low-density lipoprotein levels in atherogenic diet-fed C57BL/6 mice. These findings further highlight the interaction between adrenal high-density lipoprotein-cholesterol uptake by SR-BI, adrenal steroidogenesis, and the regulation of hepatic lipid metabolism.

Chronic pharmacological activation of P2Y13 receptor in mice decreases HDL-cholesterol level by increasing hepatic HDL uptake and bile acid secretion

High level of high-density lipoprotein cholesterol (HDL-cholesterol) is inversely correlated to the risk of atherosclerotic cardiovascular disease. The protective effect of HDL is mostly attributed to their metabolic functions in reverse cholesterol transport (RCT), a process whereby excess cell cholesterol is taken up from peripheral cells and processed in HDL particles, and is later delivered to the liver for further metabolism and bile excretion. We have previously demonstrated that P2Y13 receptor is critical for RCT and that intravenous bolus injection of cangrelor (AR-C69931MX), a partial agonist of P2Y13 receptor, can stimulate hepatic HDL uptake and subsequent lipid biliary secretion without any change in plasma lipid levels. In the present study, we investigated the effect of longer-term treatment with cangrelor on lipoprotein metabolism in mice. We observed that continuous delivery of cangrelor at a rate of 35μg/day/kg body weight for 3days markedly decreased plasma HDL-cholesterol level, by increasing the clearance of HDL particles by the liver. These effects were correlated to an increase in the rate of biliary bile acid secretion. An increased expression of SREBP-regulated genes of cholesterol metabolism was also observed without any change of hepatic lipid levels as compared to non-treated mice. Thus, 3-day cangrelor treatment markedly increases the flux of HDL-cholesterol from the plasma to the liver for bile acid secretion. Taken together our results suggest that P2Y13 appears a promising target for therapeutic intervention aimed at preventing or reducing cardiovascular risk.

Lecithin:cholesterol acyltransferase: old friend or foe in atherosclerosis?

Lecithin:cholesterol acyltransferase (LCAT) is a key enzyme that catalyzes the esterification of free cholesterol in plasma lipoproteins and plays a critical role in high-density lipoprotein (HDL) metabolism. Deficiency leads to accumulation of nascent preβ-HDL due to impaired maturation of HDL particles, whereas enhanced expression is associated with the formation of large, apoE-rich HDL(1) particles. In addition to its function in HDL metabolism, LCAT was believed to be an important driving force behind macrophage reverse cholesterol transport (RCT) and, therefore, has been a subject of great interest in cardiovascular research since its discovery in 1962. Although half a century has passed, the importance of LCAT for atheroprotection is still under intense debate. This review provides a comprehensive overview of the insights that have been gained in the past 50 years on the biochemistry of LCAT, the role of LCAT in lipoprotein metabolism and the pathogenesis of atherosclerosis in animal models, and its impact on cardiovascular disease in humans.

Farnesoid X receptor induces murine scavenger receptor Class B type I via intron binding

Farnesoid X receptor (FXR) is a nuclear receptor and a key regulator of liver cholesterol and triglyceride homeostasis. Scavenger receptor class B type I (SR-BI) is critical for reverse cholesterol transport (RCT) by transporting high-density lipoprotein (HDL) into liver. FXR induces SR-BI, however, the underlying molecular mechanism of this induction is not known. The current study confirmed induction of SR-BI mRNA by activated FXR in mouse livers, a human hepatoma cell line, and primary human hepatocytes. Genome-wide FXR binding analysis in mouse livers identified 4 putative FXR response elements in the form of inverse repeat separated by one nucleotide (IR1) at the first intron and 1 IR1 at the downstream of the mouse Sr-bi gene. ChIP-qPCR analysis revealed FXR binding to only the intronic IR1s, but not the downstream one. Luciferase assays and site-directed mutagenesis further showed that 3 out of 4 IR1s were able to activate gene transcription. A 16-week high-fat diet (HFD) feeding in mice increased hepatic Sr-bi gene expression in a FXR-dependent manner. In addition, FXR bound to the 3 bona fide IR1s in vivo, which was increased following HFD feeding. Serum total and HDL cholesterol levels were increased in FXR knockout mice fed the HFD, compared to wild-type mice. In conclusion, the Sr-bi/SR-BI gene is confirmed as a FXR target gene in both mice and humans, and at least in mice, induction of Sr-bi by FXR is via binding to intronic IR1s. This study suggests that FXR may serve as a promising molecular target for increasing reverse cholesterol transport.