Role Of Scavenger Receptor BI Research Articles

Novel role for scavenger receptor B-I in neutrophilic asthma (IRC10P.419)

Abstract Asthma is a heterogeneous airway disease that has risen significantly in recent decades. While there has been a major research focus on eosinophilic asthma, the neutrophilic, Th17-predominant disease that characterizes severe asthmatics is less understood. Scavenger Receptor B-I (SR-BI) is a receptor mostly studied in the context of cellular uptake of cholesterol. Recently, we reported that SR-BI regulates the pulmonary innate immune response, but the role of SR-BI in asthma is unknown. Therefore, SR-BI+/+ and SR-BI-/- mice were sensitized on days 0 and 7 by oropharyngeal aspiration of ovalbumin (OVA) together with LPS and then challenged on day 14 with an aerosol OVA. Airway inflammation and cytokines were quantified 48hrs after challenge. Bone marrow-derived dendritic cells (BMDCs) from SR-BI+/+ and SR-BI-/- mice were co-cultured in the presence of OVA/LPS with CD4+ T cells from OT-II mice that carry a transgenic TCR specific for the MHC class II-restricted OVA peptide (OT-II Tg). Compared to SR-BI+/+ counterparts, SR-BI-/- mice had increased pulmonary neutrophils, Th17 cells, G-CSF and CXCL5 after OVA challenge, but had equivalent amounts of IL-4, IL-5, and eosinophils. BMDCs from SR-BI-/- mice have elevated levels of CD80 and CD86, and CD4+ OT-II Tg T cells produced more IL-17 after culture with OVA/LPS. SR-BI modulates the lung’s response to environmental adjuvants such as LPS by regulating Th17 polarization, suggesting a novel role for SR-BI in neutrophilic asthma.

Key role for scavenger receptor B-I in the integrative physiology of host defense during bacterial pneumonia.

Scavenger receptor B-I (SR-BI) is a multirecognition receptor that regulates cholesterol trafficking and cardiovascular inflammation. Although it is expressed by neutrophils (PMNs) and lung-resident cells, no role for SR-BI has been defined in pulmonary immunity. Herein, we report that, compared to SR-BI+/+ counterparts, SR-BI−/− mice suffer markedly increased mortality during bacterial pneumonia associated with higher bacterial burden in lung and blood, deficient induction of the stress glucocorticoid corticosterone, higher serum cytokines, and increased organ injury. SR-BI−/− mice had significantly increased PMN recruitment and cytokine production in the infected airspace. This was associated with defective hematopoietic cell-dependent clearance of lipopolysaccharide from the airspace and increased cytokine production by SR-BI−/− macrophages. Corticosterone replacement normalized alveolar neutrophilia but not alveolar cytokines, bacterial burden, or mortality, suggesting that adrenal insufficiency derepresses PMN trafficking to the SR-BI−/− airway in a cytokine-independent manner. Despite enhanced alveolar neutrophilia, SR-BI−/− mice displayed impaired phagocytic killing. Bone marrow chimeras revealed this defect to be independent of the dyslipidemia and adrenal insufficiency of SR-BI−/− mice. During infection, SR-BI−/− PMNs displayed deficient oxidant production and CD11b externalization, and increased surface L-selectin, suggesting defective activation. Taken together, SR-BI coordinates several steps in the integrated neutrophilic host defense response to pneumonia.

Formation of a protein corona on silver nanoparticles mediates cellular toxicity via scavenger receptors.

Addition of a protein corona (PC) or protein adsorption layer on the surface of nanomaterials following their introduction into physiological environments may modify their activity, bio-distribution, cellular uptake, clearance, and toxicity. We hypothesize that silver nanoparticles (AgNPs) will associate with proteins common to human serum and cell culture media forming a PC that will impact cell activation and cytotoxicity. Furthermore, the role of scavenger receptor BI (SR-BI) in mediating this toxicity was evaluated. Citrate-suspended 20 nm AgNPs were incubated with human serum albumin (HSA), bovine serum albumin (BSA), high-density lipoprotein (HDL), or water (control) to form a PC. AgNPs associated with each protein (HSA, BSA, and HDL) forming PCs as assessed by electron microscopy, hyperspectral analysis, ζ-potential, and hydrodynamic size. Addition of the PC decreased uptake of AgNPs by rat lung epithelial and rat aortic endothelial cells. Hyperspectral analysis demonstrated a loss of the AgNP PC following internalization. Cells demonstrated concentration-dependent cytotoxicity following exposure to AgNPs with or without PCs (0, 6.25, 12.5, 25 or 50 μg/ml). All PC-coated AgNPs were found to activate cells by inducing IL-6 mRNA expression. A small molecule SR-BI inhibitor was utilized to determine the role of SR-BI in the observed effects. Pretreatment with the SR-BI inhibitor decreased internalization of AgNPs with or without PCs, and reduced both cytotoxicity and IL-6 mRNA expression. This study characterizes the formation of a PC on AgNPs and demonstrates its influence on cytotoxicity and cell activation through a cell surface receptor.

Cooperation between hepatic cholesteryl ester hydrolase and scavenger receptor BI for hydrolysis of HDL-CE

Liver is the sole organ responsible for the final elimination of cholesterol from the body either as biliary cholesterol or bile acids. High density lipoprotein (HDL)-derived cholesterol is the major source of biliary sterols and represents a mechanism for the removal of cholesterol from peripheral tissues including artery wall-associated macrophage foam cells. Via selective uptake through scavenger receptor BI (SR-BI), HDL-cholesterol is thought to be directly secreted into bile, and HDL cholesteryl esters (HDL-CEs) enter the hepatic metabolic pool and need to be hydrolyzed prior to conversion to bile acids. However, the identity of hepatic CE hydrolase (CEH) as well as the role of SR-BI in bile acid synthesis remains elusive. In this study we examined the role of human hepatic CEH (CES1) in facilitating hydrolysis of SR-BI-delivered HDL-CEs. Over-expression of CEH led to increased hydrolysis of HDL-[³H]CE in primary hepatocytes and SR-BI expression was required for this process. Intracellular CEH associated with BODIPY-CE delivered by selective uptake via SR-BI. CEH and SR-BI expression enhanced the movement of [³H]label from HDL-[³H]CE to bile acids in vitro and in vivo. Taken together, these studies demonstrate that SR-BI-delivered HDL-CEs are hydrolyzed by hepatic CEH and utilized for bile acid synthesis.

Abstract 126: Identification of an Individual With Extremely High HDL-C Who is Homozygous for a Loss-of-function Mutation in Scavenger Receptor BI (SR-BI) and Functional Characterization of the Mutation

Background The role of SR-BI in human HDL metabolism and physiology has yet to be fully clarified and no humans with homozygous SR-BI deficiency have been reported to date. We report the identification of an individual with HDL-C > 160 mg/dL who is homozygous for a Pro376Leu mutation in the SCARB1 gene. Methods In silico tests were performed to evaluate the effect of the variant on protein function (Polyphen 2 and Condel) and on protein secondary structure (Raptor X and NetSurfP). HDL binding, cholesteryl-ether (CE) uptake and the effect on cholesterol efflux were assessed in vitro in COS7 cells. Serum efflux was measured using J774 macrophages. Subcellular distribution of the mutant protein was assessed through biotinylation of cell surface proteins and trypsin sensitivity. Results HDL-C was 166mg/dl and apoA-I was and 275 mg/dl in the homozygous subject. HDL particles were much larger and enriched in cholesterol and triglycerides as compared with a control subject matched for HDL-C levels. In silico analysis predicted that the variant would cause secondary structure modifications and be deleterious for function. In vitro studies in COS7 cells transfected with the P376L mutation showed a strongly diminished HDL binding (-69%) and CE uptake (-79%) as compared to cells transfected with the WT form of SR-BI. Efflux was also impaired (-59%). The biotinylation and trypsin sensitivity assays showed a defect in the delivery of the mutant protein to the cell surface. Conclusions We report the first identification of a homozygote for a loss-of-function mutation in SR-BI; the phenotype includes extremely elevated HDL-C levels. The P376L mutation in SCARB1 gene results in marked loss of SR-BI function at least in part by impairing the transport of the protein to the cell surface.

Functional genomics of the human high-density lipoprotein receptor scavenger receptor BI

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. 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. 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.

Abstract 327: SR-BI Deficiency and Hypercholesterolemia Synergistically Lead to Impaired Erythropoiesis

Recent studies showed that mice deficient in scavenger receptor BI (SR-BI) exhibit impaired erythropoiesis and it has been proposed that the high cholesterol environment plays a major role in the abnormal erythropoiesis. In this study, we utilized three animal models to assess the role of SR-BI and hypercholesterolemia in erythropoiesis. First, we used a high fat diet-induced hypercholesterolemia model. High fat diet caused a 2-fold increase in plasma cholesterol levels in SR-BI +/+ (172 mg/dl to 350 mg/dl) and SR-BI -/- mice (313 mg/dl to 716 mg/dl). The high fat diet treatment markedly exacerbated the impaired erythropoiesis in SR-BI -/- mice as shown by 4-fold increase in reticulocyte percentage and 2.5-fold increase in early-to-late erythroblast ratio. Unexpectedly, high fat feeding did not induce abnormal erythropoiesis in SR-BI +/+ mice despite of hypercholesterolemia in these mice. We then used SR-BI/LDLR double knockout mice to further elucidate the contribution of hypercholesterolemia to erythropoiesis. SR-BI/LDLR double knockout mice had 2-fold increase in plasma cholesterol levels and exhibited severer impaired erythropoiesis, similar to high fat-fed SR-BI -/- mice. Interestingly, despite of hyperlipidemia, LDLR single knockout mice did not display impaired erythropoiesis. Finally, we investigated the contribution of hepatic SR-BI using ScarbI I179N mutant mice, whose hepatic SR-BI expression has been knocked down by 90% and therefore has a 1.7-fold increase in plasma cholesterol levels compared to wild type controls. ScarbI I179N mice displayed normal erythropoiesis, similar to wild type controls. These findings indicate that hypercholesterolemia does not cause abnormal erythropoiesis in the presence of SR-BI, but markedly impairs erythropoiesis in the absence of SR-BI. We conclude that SR-BI is essential for normal erythropoiesis, and that hypercholesterolemia and SR-BI deficiency synergistically exacerbated impaired erythropoiesis.

Abstract 3736: The HDL Receptor SR-BI Modulates Platelet Function and Susceptibility to Thrombosis in vivo

Objective: Scavenger receptor BI (SR-BI) is a cell surface receptor that promotes the selective uptake of cholesteryl esters from HDL by the liver. In mice, SR-BI deficiency leads to increased plasma HDL cholesterol levels and enhanced susceptibility to atherosclerosis. Recently, a family was identified in which heterozygous carriers of a mutation (P297S) in SR-BI showed elevated HDL cholesterol levels, suggesting that SR-BI is equally important in controlling HDL cholesterol levels in humans. Interestingly, SR-BI is also expressed on platelets, which play a crucial role in atherothrombosis. The aim of this study was to investigate the role of SR-BI in platelet function in both SR-BI knockout (KO) mice and human P297S carriers. Methods & Results: Hematological analysis showed that SR-BI deficiency in mice caused thrombocytopenia (381±69x10 9 SR-BI KO platelets/L vs 878±162x10 9 wildtype (WT) platelets/L, p=0.001). Correspondingly, SR-BI deficiency increased tail bleeding times from 85±8 to 134±16 sec (p=0.02). SR-BI KO platelets were abnormally large (7.1±0.3 fl vs 5.8±0.0 fl for WT platelets, p=0.0004), probably caused by an increased cholesterol content. Platelet count and size of the human P297S carriers were, however, not significantly different from matched controls. The FeCl 3 acute injury model was used to study the effect of SR-BI deficiency in mice on the susceptibility to arterial thrombosis. WT mice developed total arterial occlusion after 24±2 min. In SR-BI KO mice, however, the time to total arterial occlusion was reduced 2-fold to 13±1 min (p=0.02). In line with this finding, SR-BI KO and P297S platelets circulated in an activated state and showed increased adherence to immobilized fibrinogen. The activated state of the platelets in vivo led to desensitization of the receptors for ADP and thromboxane that are essential for positive feedback mechanisms to enhance the platelet response. As a result, ex vivo aggregation of platelets from SR-BI KO mice and P297S carriers was inhibited in response to agonist stimulation as compared to control platelets. Conclusion: SR-BI is not only essential for HDL cholesterol homeostasis and atherosclerosis susceptibility, but also for maintaining normal platelet aggregation and prevention of thrombosis.

Abstract 3753: Critical Role of Scavenger Receptor-BI Expressing Bone Marrow-Derived Endothelial Progenitor Cells in the Attenuation of Allograft Vasculopathy by Human Apo A-I Transfer

Background: Accelerated endothelial regeneration mediated by enhanced endothelial progenitor cell (EPC) incorporation may attenuate the development of allograft vasculopathy. Hypothesis: We investigated the hypothesis that modulation of EPC biology and attenuation of allograft vasculopathy by increased HDL cholesterol following human apo A-I (AdA-I) transfer requires scavenger receptor (SR)-BI expression in bone marrow-derived EPCs. Methods: Bone marrow transplantations with SR-BI+/+ or SR-BI−/− bone marrow were performed 4 weeks before gene transfer or saline injection. E1E3E4-deleted vectors containing a hepatocyte-specific human apo A-I expression cassette or containing no expression cassette were injected via the tail vein. Two weeks later, a common carotid artery of a female Balb/c donor mouse was transplanted paratopically into male recipient C57BL/6 mice. To analyse EPC incorporation, sex mismatch bone marrow transplantations were performed in female C57BL/6 mice and incorporated EPCs were quantified by in situ hybridization for the murine Y-chromosome. Results: Following AdA-I transfer, the number of circulating EPCs increased 2.0-fold (p<0.0001) at different time-points in C57BL/6 mice transplanted with SR-BI+/+ bone marrow but was unaltered in mice with SR-BI−/− bone marrow. The effect of HDL on EPC migration in vitro requires signaling via SR-BI and extracellular signal-regulated kinases (ERK) and is dependent on increased NO production in EPCs. Human apo A-I transfer 2 weeks before paratopic artery transplantation reduced intimal area at day 21 3.7-fold (p<0.001) in mice with SR-BI+/+ bone marrow but had no effect in mice with SR-BI−/− bone marrow. The number of CD31 positive endothelial cells lining the lumen and the number of incorporated EPCs was increased 3.0-fold (p<0.001) and 9.7-fold (p<0.001), respectively, in AdA-I treated chimeric SR-BI+/+ mice compared to control mice with SR-BI+/+ bone marrow. Endothelial regeneration and EPC incorporation was not increased after AdA-I transfer in chimeric SR-BI−/−mice. Conclusion: Human apo A-I transfer-mediated endothelial regeneration to prevent allograft vasculopathy is strictly dependent on SR-BI expressing bone marrow-derived EPCs.

Endothelial lipase releases saturated and unsaturated fatty acids of high density lipoprotein phosphatidylcholine

We assessed the ability of endothelial lipase (EL) to hydrolyze the sn-1 and sn-2 fatty acids (FAs) from HDL phosphatidylcholine. For this purpose, reconstituted discoidal HDLs (rHDLs) that contained free cholesterol, apolipoprotein A-I, and either 1-palmitoyl-2-oleoylphosphatidylcholine, 1-palmitoyl-2-linoleoylphosphatidylcholine, or 1-palmitoyl-2-arachidonylphosphatidylcholine were incubated with EL- and control (LacZ)-conditioned media. Gas chromatography analysis of the reaction mixtures revealed that both the sn-1 (16:0) and sn-2 (18:1, 18:2, and 20:4) FAs were liberated by EL. The higher rate of sn-1 FA cleavage compared with sn-2 FA release generated corresponding sn-2 acyl lyso-species as determined by MS analysis. EL failed to release sn-2 FA from rHDLs containing 1-O-1'-hexadecenyl-2-arachidonoylphosphatidylcholine, whose sn-1 position contained a nonhydrolyzable alkyl ether linkage. The lack of phospholipase A(2) activity of EL and its ability to liberate [(14)C]FA from [(14)C]lysophosphatidylcholine (lyso-PC) led us to conclude that EL-mediated deacylation of phosphatidylcholine (PC) is initiated at the sn-1 position, followed by the release of the remaining FA from the lyso-PC intermediate. Thin-layer chromatography analysis of cellular lipids obtained from EL-overexpressing cells revealed a pronounced accumulation of [(14)C]phospholipid and [(14)C]triglyceride upon incubation with 1-palmitoyl-2-[1-(14)C]linoleoyl-PC-labeled HDL(3), indicating the ability of EL to supply cells with unsaturated FAs.

A role of scavenger receptor BI (SR-BI), endothelial lipase (EL) and fatty acid translocase (fat)/CD36 in PUFA supply at the blood-brain barrier

A role of scavenger receptor BI (SR-BI), endothelial lipase (EL) and fatty acid translocase (fat)/CD36 in PUFA supply at the blood-brain barrier

Immunolocalization and funtional role of scavenger receptor BI (SR-BI) and ATP-binding cassette A1 (ABC1)

Immunolocalization and funtional role of scavenger receptor BI (SR-BI) and ATP-binding cassette A1 (ABC1)

Immunolocalization and funtional role of scavenger receptor BI (SR-BI) and ATP-binding cassette A1 (ABC1)

Immunolocalization and funtional role of scavenger receptor BI (SR-BI) and ATP-binding cassette A1 (ABC1)

Scavenger Receptor BI Transfers Major Lipoprotein-associated Phospholipids into the Cells

The phospholipids of lipoproteins can be transferred to cells by an endocytosis-independent uptake pathway. We analyzed the role of scavenger receptor BI (SR-BI) for the selective cellular phospholipid import. Human monocytes rapidly acquired the pyrene (py)-labeled phospholipids sphingomyelin (SM), phosphatidylcholine, and phosphatidylethanolamine from different donors (low and high density lipoproteins (LDL, HDL), lipid vesicles). The anti-SR-BI antibody directed against the extracellular loop of the membrane protein lowered the cellular import of the phospholipids by 40-80%. The phospholipid transfer from the lipid vesicles into the monocytes was suppressed by LDL, HDL, and apoprotein AI. Transfection of BHK cells with the cDNA for human SR-BI enhanced the cellular import of the vesicle-derived py-phospholipids by 5-6-fold. In the case of the LDL donors, transfer of py-SM to the transfected cells was stimulated to a greater extent than the uptake of the other py-phospholipids. Similar differences were not observed when the vesicles and HDL were used as phospholipid donors. The concentration of LDL required for the half-maximal phospholipid import was close to the previously reported apparent dissociation constant for LDL binding to SR-BI. The low activation energy of the SR-BI-mediated py-phospholipid import indicated that the transfer occurs entirely in a hydrophobic environment. Disruption of cell membrane caveolae by cyclodextrin treatment reduced the SR-BI-catalyzed incorporation of py-SM, suggesting that intact caveolae are necessary for the phospholipid uptake. In conclusion, SR-BI mediates the selective import of the major lipoprotein-associated phospholipids into the cells, the transfer efficiency being dependent on the structure of the donor lipoprotein.

Comparison of Class B Scavenger Receptors, CD36 and Scavenger Receptor BI (SR-BI), Shows That Both Receptors Mediate High Density Lipoprotein-Cholesteryl Ester Selective Uptake but SR-BI Exhibits a Unique Enhancement of Cholesteryl Ester Uptake

Scavenger receptor BI (SR-BI) mediates the selective uptake of high density lipoprotein (HDL) cholesteryl ester (CE), a process by which HDL CE is taken into the cell without internalization and degradation of the HDL particle. The biochemical mechanism by which SR-BI mediates the selective uptake of HDL CE is poorly understood. Given that CE transfer will occur to some extent from HDL to protein-free synthetic membranes, one hypothesis is that the role of SR-BI is primarily to tether HDL close to the cell surface to facilitate CE transfer from the particle to the plasma membrane. In the present study, this hypothesis was tested by comparing the selective uptake of HDL CE mediated by mouse SR-BI (mSR-BI) with that mediated by rat CD36 (rCD36), a closely related class B scavenger receptor. Both mSR-BI and rCD36 bind HDL with high affinity, and both receptors mediate HDL CE selective uptake. However, SR-BI mediates selective uptake of HDL CE with a 7-fold greater efficiency than rCD36. HDL CE selective uptake mediated by rCD36 is dependent on HDL binding to the receptor, since a mutation that blocks HDL binding also blocks HDL CE selective uptake. These data lead us to hypothesize that one component of HDL CE selective uptake is the tethering of HDL particles to the cell surface. To explore the molecular domains responsible for the greater efficiency of selective uptake by mSR-BI, we compared binding and selective uptake among mSR-BI, scavenger receptor BII, and various chimeric receptors formed from mSR-BI and rCD36. The results show that the extracellular domain of mSR-BI is essential for efficient HDL CE uptake, but the C-terminal cytoplasmic tail also has a major influence on the selective uptake process.