Scavenger Receptor Class B Type I Function Research Articles

HDL mediates reverse cholesterol transport from ram spermatozoa and induces hyperactivated motility.

Reverse cholesterol transport or cholesterol efflux is part of an extensive plasma membrane remodeling process in spermatozoa that is imperative for fertilization. For ram spermatozoa, sheep serum is well known to support in vitro fertilization (IVF), but knowledge of its explicit role is limited. Though, it is postulated to elicit cholesterol efflux owing to the presence of high-density lipoproteins (HDLs) that interact with transmembrane cholesterol transporters, such as adenosinetriphosphate (ATP)-binding cassette transporter A1 (ABCA1) and scavenger receptor class B, type I (SR-BI). In this study, we report that both sheep serum and HDLs were able to elicit cholesterol efflux alone by up to 20–40% (as measured by the boron dipyrromethene (BODIPY)-cholesterol assay). Furthermore, when the antagonists glibenclamide and valspodar were used to inhibit the function of ABCA1 and SR-BI or ABCA1 alone, respectively, cholesterol efflux was only marginally reduced (8–15%). Nevertheless, it is likely that in ram spermatozoa, a specific facilitated pathway of cholesterol efflux is involved in the interaction between cholesterol acceptors and transporters. Interestingly, exposure to HDLs also induced hyperactivated motility, another critical event required for successful fertilization. Taken together, this study details the first report of the dual action of HDLs on ram spermatozoa, providing both an insight into the intricacy of events leading up to fertilization in vivo as well as demonstrating the possible application of HDL supplementation in media for IVF.

Hyperalphalipoproteinemic scavenger receptor BI knockout mice exhibit a disrupted epidermal lipid barrier

Scavenger receptor class B type I (SR-BI) mediates the selective uptake of cholesteryl esters (CE) from high-density lipoproteins (HDL). An impaired SR-BI function leads to hyperalphalipoproteinemia with elevated levels of cholesterol transported in the HDL fraction. Accumulation of cholesterol in apolipoprotein B (apoB)-containing lipoproteins has been shown to alter skin lipid composition and barrier function in mice. To investigate whether these hypercholesterolemic effects on the skin also occur in hyperalphalipoproteinemia, we compared skins of wild-type and SR-BI knockout (SR-BI−/−) mice. SR-BI deficiency did not affect the epidermal cholesterol content and induced only minor changes in the ceramide subclasses. The epidermal free fatty acid (FFA) pool was, however, enriched in short and unsaturated chains. Plasma CE levels strongly correlated with epidermal FFA C18:1 content. The increase in epidermal FFA coincided with downregulation of cholesterol and FFA synthesis genes, suggesting a compensatory response to increased flux of plasma cholesterol and FFAs into the skin. Importantly, the SR-BI−/− epidermal lipid barrier showed increased permeability to ethyl-paraminobenzoic acid, indicating an impairment of the barrier function. In conclusion, increased HDL-cholesterol levels in SR-BI−/− mice can alter the epidermal lipid composition and lipid barrier function similarly as observed in hypercholesterolemia due to elevated levels of apoB-containing lipoproteins.

Abstract 045: SR-BI and PCPE2 Modulate Lipid Trafficking in Adipocytes

Adipose tissue undergoes distinct structural remodeling in response to increases in fat mass, as in obesity. During this process, the extracellular matrix (ECM) plays a pivotal role with respect to both adipocyte receptor signaling and structural remodeling. One ECM protein, procollagen endopeptidase enhancer 2 (PCPE2), has been shown by our lab to work in partnership with the scavenger receptor class B type I (SR-BI) to regulate adipocyte cholesterol homeostasis. To investigate the functional impact of the partnership between SR-BI and PCPE2, we studied mature adipocytes differentiated from murine embryonic fibroblasts (MEF) and compared to our adipose specific PCPE2 knockout mice. Our studies showed the presence of an SR-BI-PCPE2 protein complex based on the co-immunoprecipitation of PCPE2 with SR-BI in both differentiated MEFs and mouse adipose tissue. Furthermore, stimulated emission depletion (STED) microscopy showed SR-BI and PCPE2 localized on the lipid droplet surface separated by distances ranging from 24 – 90 nm, suggesting that the SR-BI-PCPE2 complex may contain other proteins. Most significantly, a loss of SR-BI function was shown in PCPE2-deficient adipocytes by the SR-BI mediated HDL binding and cholesteryl ester (CE) uptake assays. Compared to differentiated MEFs from Ldlr -/- mice, binding of HDL to differentiated MEFs from Ldlr -/- Pcpe2 -/- mice was reduced by 75%, and HDL cholesteryl oleyl ether uptake was reduced by 50%. Disruption of adipocyte SR-BI function in the absence of PCPE2 also reduced free cholesterol efflux indicating substantial disruption to intracellular cholesterol homeostasis. Moreover, analysis of RNAseq from several human clinical studies show that PCPE2 mRNA expression is positively correlated with fat mass in specific adipose depots, while negatively correlated with plasma insulin, glucose and triglyceride levels, with the latter two more robust in type 2 diabetic populations (FDR < 0.004). Overall, these results suggest that the partnership between SR-BI and PCPE2 in adipocytes plays an important role in cholesterol trafficking and homeostasis which influences fat mass expansion, glucose tolerance and insulin resistance.

Role of SR-BI in atherosclerosis, malignancies, and infectious diseases

Abstract Scavenger receptor class B type I (SR-BI) is a high-affinity receptor for high-density lipoprotein (HDL). The primary role of this receptor is the selective uptake of HDLs in the liver through reverse cholesterol transport. SR-BI interacts with HDL to regulate lipid metabolism and affects various vascular cell functions involved in atherosclerosis (As). In addition, SR-BI is involved in the development of malignant tumors and infectious diseases. This article reviews the function and potential therapeutic targets of SR-BI in As, malignancies, and infectious diseases.

The unfolded protein response is a negative regulator of scavenger receptor class B, type I (SR-BI) expression

Scavenger receptor class B, type I (SR-BI) is the main receptor for high-density lipoprotein (HDL) and an emerging atheroprotective candidate. A central function of SR-BI is the delivery of HDL-derived cholesterol to the liver for subsequent excretion into the bile. Here, we investigated the regulation of SR-BI by the unfolded protein response (UPR), an adaptive mechanism induced by endoplasmic reticulum (ER) stress, which is frequently activated in metabolic disorders.We provide evidence that induction of acute ER stress by well-characterized chemical inducers leads to decreased SR-BI expression in hepatocyte-derived cell lines. This results in a functional reduction of selective lipid uptake from HDL. However, the regulation of SR-BI by ER stress is not a direct consequence of altered cellular cholesterol metabolism. Finally, we show that SR-BI down-regulation by the UPR might be a compensatory mechanism to provide partial adaption to ER stress.The observed down-regulation of SR-BI by ER stress in hepatic cells might contribute to the unfavorable effects of metabolic disorders on cholesterol homeostasis and cardiovascular diseases.

Abstract 320: Procollagen C-Endopeptidase Enhancer Protein 2 Enhances SR-BI Mediated HDL Cholesterol Uptake and Reduces Atherosclerosis in Mice

Epidemiological studies have shown an inverse correlation between plasma high density lipoprotein (HDL) concentrations and cardiovascular disease risk. At variance with these observations, clinical trials that significantly raised plasma HDL-C levels did not have improved clinical outcomes, emphasizing the importance of understanding HDL function. Recently, a significant correlation has been reported between human procollagen c-endopeptidase enhancer protein 2 (PCPE2) single nucleotide polymorphisms and HDL. PCPE2, a 52 kDa glycoprotein found in the extracellular matrix enhances cleavage of C-terminal procollagen by bone morphogenetic protein 1. Mice lacking PCPE2 have elevated concentrations of enlarged plasma HDL, a phenomenon associated with defective cholesterol efflux. HDL synthesis depends on ABCA1-mediated lipid efflux to lipid-poor apoA-I balanced by cholesterol uptake through hepatic scavenger receptor class B type I (SR-BI). Our studies focused on investigating if the elevated concentration of enlarged plasma HDL in PCPE2 deficient mice was atheroprotective. PCPE2 deficient mice were crossed with LDLr -/- mice (SKO) giving LDLr -/- , PCPE2 -/- (DKO) mice that had elevated HDL levels compared to SKO mice. Despite elevated HDL levels, we found that DKO mice had significantly more lipid and CD68+ infiltration into the aortic root, similar to that reported for LDLr -/- , apoA-I -/- mice that lack plasma apoA-I/HDL. Furthermore, DKO mice showed reduced HDL apoA-I fractional clearance and reverse cholesterol transport rates compared to SKO mice suggesting PCPE2 plays a significant role in HDL remodeling and/or cholesterol uptake by the liver. To test the effect of PCPE2 on SR-BI function we incubated 3 H-cholesteryl ether ( 3 H-CE) enriched HDL from SKO and DKO mice with CHO cells overexpressing PCPE2. Compared to CHO control cells, overexpression of PCPE2 increased 3 H-CE HDL uptake that was independent of HDL particle origin. Western Blot analysis showed no difference in SR-BI expression between control and PCPE2 transfected cells, suggesting that PCPE2 enhanced SR-BI function and promoted HDL cholesterol ester uptake. We conclude that PCPE2 is atheroprotective and an essential component of the reverse cholesterol transport system.

PDZK1 prevents neointima formation via suppression of breakpoint cluster region kinase in vascular smooth muscle.

Scavenger receptor class B, type I (SR-BI) and its adaptor protein PDZK1 mediate responses to HDL cholesterol in endothelium. Whether the receptor-adaptor protein tandem serves functions in other vascular cell types is unknown. The current work determined the roles of SR-BI and PDZK1 in vascular smooth muscle (VSM). To evaluate possible VSM functions of SR-BI and PDZK1 in vivo, neointima formation was assessed 21 days post-ligation in the carotid arteries of wild-type, SR-BI-/- or PDZK1-/- mice. Whereas neointima development was negligible in wild-type and SR-BI-/-, there was marked neointima formation in PDZK1-/- mice. PDZK1 expression was demonstrated in primary mouse VSM cells, and compared to wild-type cells, PDZK1-/- VSM displayed exaggerated proliferation and migration in response to platelet derived growth factor (PDGF). Tandem affinity purification-mass spectrometry revealed that PDZK1 interacts with breakpoint cluster region kinase (Bcr), which contains a C-terminal PDZ binding sequence and is known to enhance responses to PDGF in VSM. PDZK1 interaction with Bcr in VSM was demonstrated by pull-down and by coimmunoprecipitation, and the augmented proliferative response to PDGF in PDZK1-/- VSM was abrogated by Bcr depletion. Furthermore, compared with wild-type Bcr overexpression, the introduction of a Bcr mutant incapable of PDZK1 binding into VSM cells yielded an exaggerated proliferative response to PDGF. Thus, PDZK1 has novel SR-BI-independent function in VSM that affords protection from neointima formation, and this involves PDZK1 suppression of VSM cell proliferation via an inhibitory interaction with Bcr.

Editorial introductions

Current Opinion in Endocrinology and Diabetes was launched in 1994, with Obesity added to the title in 2007. It is one of a successful series of review journals whose unique format is designed to provide a systematic and critical assessment of the literature as presented in the many primary journals. The fields of endocrinology and diabetes are divided into 12 sections that are reviewed once a year. Each section is assigned a Section Editor, a leading authority in the area, who identifies the most important topics at that time. Here we are pleased to introduce one of the Journal's Section Editors for this issue. SECTION EDITORS David M. HarlanDavid M. HarlanDr David M. Harlan is a graduate of the University of Michigan (Bachelor of Science) and the Duke University School of Medicine, USA. He completed his internal medicine internship and residency at the Duke University Medical Center. This was before serving for 4 years as a staff internist at the Navy Hospital, San Diego, CA, USA. He then returned to Duke for his fellowship training in diabetes, endocrinology, and metabolism. In 1991 he established his laboratory at the Navy Medical Research Institute, USA, where he rose to become the Director of Combat Casualty Care. He was then appointed to the faculty of the Uniformed Services University of the Health Sciences. In 1999, he moved his laboratory to the National Institutes of Health, where he became the Diabetes Branch Chief within the National Institute of Diabetes, Digestive, and Kidney Diseases. In 2010, he moved to the University of Massachusetts School of Medicine, USA, where he is the William and Doris Krupp Professor of Medicine. Dr Harlan co-directs the university's Diabetes Center of Excellence. His research interests all relate to diabetes and include the immunopathogenesis underlying type 1 diabetes and improved care delivery models, including transplant-based approaches, for the disease. Annabelle RodriguezAnnabelle RodriguezDr Annabelle Rodriguez is Professor of Cell Biology and the Linda and David Roth Chair of Cardiovascular Research at the University of Connecticut Health Center in the Center of Vascular Biology, USA. Dr Rodriguez is also an Adjunct Associate Professor of Medicine at the Johns Hopkins University School of Medicine, USA. Dr Rodriguez received her MD from New Jersey Medical School in Newark, USA, and then continued with postdoctoral fellowships at the Medical College of Pennsylvania in Philadelphia (in the Department of Physiology and Biochemistry), USA and then John Hopkins University School of Medicine in the Division of Endocrinology and Metabolism. Dr Rodriguez is a member of professional societies including the American Heart Association, the American Diabetes Association, and the Endocrine Society, and she is an immediate past member of the NIH Atherosclerosis and Inflammation Cardiovascular study section. Dr Rodriguez conducts basic science research in the area of lipid metabolism, with a particular emphasis on aspects of lipoprotein scavenger receptor class B type I (SR-BI) in atherosclerosis and infertility. In mice, deficiency of SR-BI is associated with higher HDL cholesterol levels, but paradoxically with increased risk for atherosclerosis and female infertility. Utilizing in vitro and in vivo approaches, Dr Rodriguez's laboratory was the first to demonstrate that two human SR-BI gene (SCARB1) variants were significantly associated with reduced SR-BI function and expression in macrophages. Moreover, Dr Rodriguez's laboratory showed that a synonymous variant in the exon 8 of the human SCARB1 gene significantly affected the RNA secondary structure, and this in turn, significantly reduced SR-BI protein expression. Dr Rodriguez recently completed studies funded by a Burroughs Wellcome Clinical Translational Award to study the effects of SR-BI deficiency on human female fertility, and continues to be funded by NIH RO1 grant mechanism that examines the functional effect of SR-BI gene variation on SR-BI protein expression, and its association with cardiovascular outcomes. In October 2013 Dr Rodriguez was awarded Scientist-of-the-Year by Great Minds in STEM in recognition of her work in the area of genomic medicine and SCARB1. Dr Rodriguez will also be editing the first edition of the textbook, Translational Cardiometabolic Genomic Medicine, which will highlight the translational aspects of genomic medicine.

SR-BI mediates high density lipoprotein (HDL)-induced anti-inflammatory effect in macrophages

High density lipoprotein (HDL) receptor, scavenger receptor class B, type I (SR-BI), mediates selective cholesteryl ester uptake from lipoproteins into the liver as well as cholesterol efflux from macrophages to HDL. Recently, strong evidence has demonstrated the anti-inflammatory effect of HDL, although the mechanism of action is not fully understood. In this study, we showed that the anti-inflammatory effects of HDL are dependent on SR-BI expression in THP-1 macrophages. Consistent with earlier findings, pretreatment of macrophages with HDL abolished LPS-induced TNFα production. HDL also inhibited LPS-induced NF-κB activation. In addition, knockdown of SR-BI or inhibition of SR-BI ligand binding abolished the anti-inflammatory effect of HDL. SR-BI is a multi-ligand receptor that binds to modified lipoproteins as well as native HDL. Since modified lipoproteins have pro-inflammatory properties, it is unclear whether SR-BI activated by modified HDL has an anti- or pro-inflammatory effect. Glycated HDL induced NF-κB activation and cytokine production in macrophages in vitro, suggesting a pro-inflammatory effect for modified HDL. Moreover, inhibition of SR-BI function or expression potentiated glycated HDL-induced TNF-α production, suggesting an anti-inflammatory effect for SR-BI. In conclusion, SR-BI plays an important function in regulating HDL-mediated anti-inflammatory response in macrophages.

SR-BI/CD36 chimeric receptors define extracellular subdomains of SR-BI critical for cholesterol transport.

High-density lipoproteins (HDLs) are athero-protective, primarily because of their ability to promote cholesterol flux from peripheral tissues to the liver by reverse cholesterol transport (RCT). The delivery of HDL-cholesteryl esters (CE) into cells is mediated by the HDL receptor, scavenger receptor class B type I (SR-BI), a promising target for enhancing whole body cholesterol disposal and preventing cardiovascular disease. A detailed understanding of the structural determinants underlying proper SR-BI/HDL alignment that supports the selective uptake of HDL-CE into cells remains lacking. To this end, we exploited CD36, a class B scavenger receptor with a predicted topology similar to that of SR-BI that binds HDL but is unable to mediate efficient selective uptake of HDL-CE. We generated a series of SR-BI/CD36 chimeric receptors that span the extracellular (EC) domain of SR-BI to delineate regions that are essential for SR-BI’s cholesterol transport functions. All 16 SR-BI/CD36 chimeras were transiently expressed in COS-7 cells, and their plasma membrane localization was confirmed. The majority of SR-BI/CD36 chimeric receptors displayed significant reductions in their ability to (i) bind HDL, (ii) deliver HDL-CE to cells, (iii) mediate efflux of free cholesterol (FC) to HDL, and (iv) redistribute plasma membrane domains of FC. We also demonstrated that changes in SR-BI function were independent of receptor oligomerization. Altogether, we have identified discrete subdomains, particularly in the N-terminal and C-terminal regions of the EC domain of SR-BI, that are critical for productive receptor–ligand interactions and the various cholesterol transport functions of SR-BI.

Role of hypervariable region 1 for the interplay of hepatitis C virus with entry factors and lipoproteins.

Hepatitis C virus (HCV) particles associate with lipoproteins and infect cells by using at least four cell entry factors. These factors include scavenger receptor class B type I (SR-BI), CD81, claudin 1 (CLDN1), and occludin (OCLN). Little is known about specific functions of individual host factors during HCV cell entry and viral domains that mediate interactions with these factors. Hypervariable region 1 (HVR1) within viral envelope protein 2 (E2) is involved in the usage of SR-BI and conceals the viral CD81 binding site. Moreover, deletion of this domain alters the density of virions. We compared lipoprotein interaction, surface attachment, receptor usage, and cell entry between wild-type HCV and a viral mutant lacking this domain. Deletion of HVR1 did not affect CD81, CLDN1, and OCLN usage. However, unlike wild-type HCV, HVR1-deleted viruses were not neutralized by antibodies and small molecules targeting SR-BI. Nevertheless, modulation of SR-BI cell surface expression altered the infection efficiencies of both viruses to similar levels. Analysis of affinity-purified virions revealed comparable levels of apolipoprotein E (ApoE) incorporation into viruses with or without HVR1. However, ApoE incorporated into these viruses was differentially recognized by ApoE-specific antibodies. Thus, SR-BI has at least two functions during cell entry. One of them can be neutralized by SR-BI-targeting molecules, and it is critical only for wild-type HCV. The other one is important for both viruses but apparently is not inactivated by the SR-BI binding antibodies and small molecules evaluated here. In addition, HVR1 modulates the conformation and/or epitope exposure of virus particle-associated ApoE. HCV cell entry is SR-BI dependent irrespective of the presence or absence of HVR1. Moreover, this domain modulates the properties of ApoE on the surface of virus particles. These findings have implications for the development of SR-BI-targeting antivirals. Furthermore, these findings highlight separable functions of SR-BI during HCV cell entry and reveal a novel role of HVR1 for the properties of virus-associated lipoproteins.

Gender- and region-specific alterations in bone metabolism in Scarb1-null female mice.

A positive correlation between plasma levels of HDL and bone mass has been reported by epidemiological studies. As scavenger receptor class B, type I (SR-BI), the gene product of Scarb1, is known to regulate HDL metabolism, we recently characterized bone metabolism in Scarb1-null mice. These mice display high femoral bone mass associated with enhanced bone formation. As gender differences have been reported in HDL metabolism and SR-BI function, we investigated gender-specific bone alterations in Scarb1-null mice by microtomography and histology. We found 16% greater relative bone volume and 39% higher bone formation rate in the vertebrae from 2-month-old Scarb1-null females. No such alteration was seen in males, indicating gender- and region-specific differences in skeletal phenotype. Total and HDL-associated cholesterol levels, as well as ACTH plasma levels, were increased in both Scarb1-null genders, the latter being concurrent to impaired corticosterone response to fasting. Plasma levels of estradiol did not differ between null and WT females, suggesting that the estrogen metabolism alteration is not relevant to the higher vertebral bone mass in female Scarb1-null mice. Constitutively, high plasma levels of leptin along with 2.5-fold increase in its expression in white adipose tissue were measured in female Scarb1-null mice only. In vitro exposure of bone marrow stromal cells to ACTH and leptin promoted osteoblast differentiation as evidenced by increased gene expression of osterix and collagen type I alpha. Our results suggest that hyperleptinemia may account for the gender-specific high bone mass seen in the vertebrae of female Scarb1-null mice.

Editorial introductions

Current Opinion in Endocrinology and Diabetes was launched in 1994, with Obesity added to the title in 2007. It is one of a successful series of review journals whose unique format is designed to provide a systematic and critical assessment of the literature as presented in the many primary journals. The fields of endocrinology and diabetes are divided into 12 sections that are reviewed once a year. Each section is assigned a Section Editor, a leading authority in the area, who identifies the most important topics at that time. Here we are pleased to introduce one of the Journal's Section Editors for this issue. SECTION EDITORS David M. HarlanDavid M. HarlanDr David M. Harlan is a graduate of the University of Michigan (Bachelor of Science) and the Duke University School of Medicine, USA. He completed his internal medicine internship and residency at the Duke University Medical Center. This was before serving for 4 years as a staff internist at the Navy Hospital, San Diego, CA, USA. He then returned to Duke for his fellowship training in diabetes, endocrinology, and metabolism. In 1991 he established his laboratory at the Navy Medical Research Institute, USA, where he rose to become the Director of Combat Casualty Care. He was then appointed to the faculty of the Uniformed Services University of the Health Sciences. In 1999, he moved his laboratory to the National Institutes of Health, where he became the Diabetes Branch Chief within the National Institute of Diabetes, Digestive, and Kidney Diseases. In 2010, he moved to the University of Massachusetts School of Medicine, USA, where he is the William and Doris Krupp Professor of Medicine. Dr Harlan co-directs the university's Diabetes Center of Excellence. His research interests all relate to diabetes and include the immunopathogenesis underlying type 1 diabetes and improved care delivery models, including transplant-based approaches, for the disease. Annabelle RodriguezAnnabelle RodriguezDr Annabelle Rodriguez is Professor of Cell Biology and the Linda and David Roth Chair of Cardiovascular Research at the University of Connecticut Health Center in the Center of Vascular Biology, USA. Dr Rodriguez is also an Adjunct Associate Professor of Medicine at the Johns Hopkins University School of Medicine, USA. Dr Rodriguez received her MD from New Jersey Medical School in Newark, USA, and then continued with postdoctoral fellowships at the Medical College of Pennsylvania in Philadelphia (in the Department of Physiology and Biochemistry), USA, and then John Hopkins University School of Medicine in the Division of Endocrinology and Metabolism. Dr Rodriguez is a member of professional societies including the American Heart Association, the American Diabetes Association, and the Endocrine Society, and she is an immediate past member of the NIH Atherosclerosis and Inflammation Cardiovascular study section. Dr Rodriguez conducts basic science research in the area of lipid metabolism, with a particular emphasis on aspects of lipoprotein scavenger receptor class B type I (SR-BI) in atherosclerosis and infertility. In mice, deficiency of SR-BI is associated with higher HDL cholesterol levels, but paradoxically with increased risk for atherosclerosis and female infertility. Utilizing in vitro and in vivo approaches, Dr Rodriguez's laboratory was the first to demonstrate that two human SR-BI gene (SCARB1) variants were significantly associated with reduced SR-BI function and expression in macrophages. Moreover, Dr Rodriguez's laboratory showed that a synonymous variant in the exon 8 of the human SCARB1 gene significantly affected the RNA secondary structure, and this in turn, significantly reduced SR-BI protein expression. Dr Rodriguez recently completed studies funded by a Burroughs Wellcome Clinical Translational Award to study the effects of SR-BI deficiency on human female fertility, and continues to be funded by NIH RO1 grant mechanism that examines the functional effect of SR-BI gene variation on SR-BI protein expression, and its association with cardiovascular outcomes. In October 2013 Dr Rodriguez was awarded Scientist-of-the-Year by Great Minds in STEM in recognition of her work in the area of genomic medicine and SCARB1. Dr Rodriguez will also be editing the first edition of the textbook, Translational Cardiometabolic Genomic Medicine, which will highlight the translational aspects of genomic medicine.

Editorial introductions

Current Opinion in Endocrinology and Diabetes was launched in 1994, with Obesity added to the title in 2007. It is one of a successful series of review journals whose unique format is designed to provide a systematic and critical assessment of the literature as presented in the many primary journals. The fields of endocrinology and diabetes are divided into 12 sections that are reviewed once a year. Each section is assigned a Section Editor, a leading authority in the area, who identifies the most important topics at that time. Here we are pleased to introduce one of the Journal's Section Editors for this issue. SECTION EDITORS David M. HarlanDavid M. HarlanDr David M. Harlan is a graduate of the University of Michigan (Bachelor of Science) and the Duke University School of Medicine, USA. He completed his internal medicine internship and residency at the Duke University Medical Center. This was before serving for 4 years as a staff internist at the Navy Hospital, San Diego, CA, USA. He then returned to Duke for his fellowship training in diabetes, endocrinology, and metabolism. In 1991 he established his laboratory at the Navy Medical Research Institute, USA, where he rose to become the Director of Combat Casualty Care. He was then appointed to the faculty of the Uniformed Services University of the Health Sciences. In 1999, he moved his laboratory to the National Institutes of Health, where he became the Diabetes Branch Chief within the National Institute of Diabetes, Digestive, and Kidney Diseases. In 2010, he moved to the University of Massachusetts School of Medicine, USA, where he is the William and Doris Krupp Professor of Medicine. Dr Harlan co-directs the university's Diabetes Center of Excellence. His research interests all relate to diabetes and include the immunopathogenesis underlying type 1 diabetes and improved care delivery models, including transplant-based approaches, for the disease. Annabelle RodriguezAnnabelle RodriguezDr Rodriguez is the Linda and David Roth Chair of Cardiovascular Research at the University of Connecticut Health Center in the Center of Vascular Biology, USA. Dr Rodriguez is also an Adjunct Associate Professor of Medicine at the Johns Hopkins University School of Medicine, USA. Dr Rodriguez received her MD from New Jersey Medical School in Newark, USA, and then continued with postdoctoral fellowships at the Medical College of Pennsylvania in Philadelphia (in the Department of Physiology and Biochemistry), USA and then John Hopkins University School of Medicine in the Division of Endocrinology and Metabolism. Dr Rodriguez is a member of professional societies including the American Heart Association, the American Diabetes Association, and the Endocrine Society, and she is an immediate past member of the NIH Atherosclerosis and Inflammation Cardiovascular study section. Dr Rodriguez conducts basic science research in the area of lipid metabolism, with a particular emphasis on aspects of lipoprotein scavenger receptor class B type I (SR-BI) in atherosclerosis and infertility. In mice, deficiency of SR-BI is associated with higher HDL cholesterol levels, but paradoxically with increased risk for atherosclerosis and female infertility. Utilizing in vitro and in vivo approaches, Dr Rodriguez's laboratory was the first to demonstrate that two human SR-BI gene (SCARB1) variants were significantly associated with reduced SR-BI function and expression in macrophages. Moreover, Dr Rodriguez's laboratory recently showed that a synonymous variant in the exon 8 of the human SCARB1 gene significantly affected the RNA secondary structure, and this in turn, significantly reduced SR-BI protein expression. Dr Rodriguez recently completed studies funded by a Burroughs Wellcome Clinical Translational Award to study the effects of SR-BI deficiency on human female fertility, and continues to be funded by NIH RO1 grant mechanism that examines the functional effect of SR-BI gene variation on SR-BI protein expression, and its association with cardiovascular outcomes.

The postbinding activity of scavenger receptor class B type I mediates initiation of hepatitis C virus infection and viral dissemination

Scavenger receptor class B type I (SR-BI) is a high-density lipoprotein (HDL) receptor highly expressed in the liver and modulating HDL metabolism. Hepatitis C virus (HCV) is able to directly interact with SR-BI and requires this receptor to efficiently enter into hepatocytes to establish productive infection. A complex interplay between lipoproteins, SR-BI and HCV envelope glycoproteins has been reported to take place during this process. SR-BI has been demonstrated to act during binding and postbinding steps of HCV entry. Although the SR-BI determinants involved in HCV binding have been partially characterized, the postbinding function of SR-BI remains largely unknown. To uncover the mechanistic role of SR-BI in viral initiation and dissemination, we generated a novel class of anti-SR-BI monoclonal antibodies that interfere with postbinding steps during the HCV entry process without interfering with HCV particle binding to the target cell surface. Using the novel class of antibodies and cell lines expressing murine and human SR-BI, we demonstrate that the postbinding function of SR-BI is of key impact for both initiation of HCV infection and viral dissemination. Interestingly, this postbinding function of SR-BI appears to be unrelated to HDL interaction but to be directly linked to its lipid transfer function. Taken together, our results uncover a crucial role of the SR-BI postbinding function for initiation and maintenance of viral HCV infection that does not require receptor-E2/HDL interactions. The dissection of the molecular mechanisms of SR-BI-mediated HCV entry opens a novel perspective for the design of entry inhibitors interfering specifically with the proviral function of SR-BI.

Functions of scavenger receptor class B, type I in atherosclerosis

This review highlights the diverse roles of the high-affinity HDL receptor scavenger receptor class B, type I (SR-BI) in the modulation of global cholesterol homeostasis and vascular cell function, and the potential implications of these processes in atherosclerosis. SR-BI in the liver plays a critical role in reverse cholesterol transport and it dramatically impacts the characteristics of the HDL particle, and through reverse cholesterol transport it promotes an antiatherogenic lipid environment in the vascular wall. SR-BI in macrophages may influence their inflammatory phenotype. In endothelial cells, SR-BI mediates HDL-induced endothelial nitric oxide synthase activation and proliferation and migration, and in platelets SR-BI may be prothrombotic in the setting of dyslipidemia. Several polymorphisms of SR-BI have been reported in humans that influence receptor expression or function. In addition to regulating global lipid metabolism, SR-BI influences the functions of a variety of vascular cells relevant to atherosclerosis. Studies of SR-BI genetics in humans partially support the conclusions drawn from experimental models. However, because of the multiple functions of SR-BI, the diversity of cell types in which it is expressed, and the influence of the receptor on the characteristics of its own ligand, our understanding of the biology of the receptor is just emerging.

A Genetic Variant of the Scavenger Receptor BI in Humans

Background In mice, the scavenger receptor class B type I (SR-BI) is essential for the delivery of high-density lipoprotein (HDL) cholesterol to the liver and steroidogenic organs. Paradoxically, elevated HDL cholesterol levels are associated with increased atherosclerosis in SR-BI-knockout mice. It is unclear what role SR-BI plays in human metabolism. Methods We sequenced the gene encoding SR-BI in persons with elevated HDL cholesterol levels and identified a family with a new missense mutation (P297S). The functional effects of the P297S mutation on HDL binding, cellular cholesterol uptake and efflux, atherosclerosis, platelet function, and adrenal function were studied. Results Cholesterol uptake from HDL by primary murine hepatocytes that expressed mutant SR-BI was reduced to half of that of hepatocytes expressing wild-type SR-BI. Carriers of the P297S mutation had increased HDL cholesterol levels (70.4 mg per deciliter [1.8 mmol per liter], vs. 53.4 mg per deciliter [1.4 mmol per liter] in noncarriers; P Conclusions We identified a family with a functional mutation in SR-BI. The mutation carriers had increased HDL cholesterol levels and a reduction in cholesterol efflux from macrophages but no significant increase in atherosclerosis. Reduced SR-BI function was associated with altered platelet function and decreased adrenal steroidogenesis. (Funded by the European Community and others.).

In Vitro and in Vivo Analysis of the Binding of the C Terminus of the HDL Receptor Scavenger Receptor Class B, Type I (SR-BI), to the PDZ1 Domain of Its Adaptor Protein PDZK1

The PDZ1 domain of the four PDZ domain-containing protein PDZK1 has been reported to bind the C terminus of the HDL receptor scavenger receptor class B, type I (SR-BI), and to control hepatic SR-BI expression and function. We generated wild-type (WT) and mutant murine PDZ1 domains, the mutants bearing single amino acid substitutions in their carboxylate binding loop (Lys(14)-Xaa(4)-Asn(19)-Tyr-Gly-Phe-Phe-Leu(24)), and measured their binding affinity for a 7-residue peptide corresponding to the C terminus of SR-BI ((503)VLQEAKL(509)). The Y20A and G21Y substitutions abrogated all binding activity. Surprisingly, binding affinities (K(d)) of the K14A and F22A mutants were 3.2 and 4.0 μM, respectively, similar to 2.6 μM measured for the WT PDZ1. To understand these findings, we determined the high resolution structure of WT PDZ1 bound to a 5-residue sequence from the C-terminal SR-BI ((505)QEAKL(509)) using x-ray crystallography. In addition, we incorporated the K14A and Y20A substitutions into full-length PDZK1 liver-specific transgenes and expressed them in WT and PDZK1 knock-out mice. In WT mice, the transgenes did not alter endogenous hepatic SR-BI protein expression (intracellular distribution or amount) or lipoprotein metabolism (total plasma cholesterol, lipoprotein size distribution). In PDZK1 knock-out mice, as expected, the K14A mutant behaved like wild-type PDZK1 and completely corrected their hepatic SR-BI and plasma lipoprotein abnormalities. Unexpectedly, the 10-20-fold overexpressed Y20A mutant also substantially, but not completely, corrected these abnormalities. The results suggest that there may be an additional site(s) within PDZK1 that bind(s) SR-BI and mediate(s) productive SR-BI-PDZK1 interaction previously attributed exclusively to the canonical binding of the C-terminal SR-BI to PDZ1.

Role of the adaptor protein PDZK1 in controlling the HDL receptor SR-BI

Regulation of lipoprotein receptor activity influences lipoprotein metabolism, related physiology and pathophysiology. Adaptor proteins that bind to the LDL or HDL receptors apparently link these receptors to cellular components essential for their normal functioning. Here, we focus on the influence of PDZK1 on the HDL receptor scavenger receptor class B type I (SR-BI), with emphasis on the roles played by its individual PDZ domains, the impact in regulating HDL metabolism and the relevance for cardiovascular disease. PDZK1 plays an essential role in maintaining hepatic SR-BI levels and controlling HDL metabolism, protects against the development of atherosclerosis in a murine model and also mediates SR-BI-dependent regulation of endothelial cell biology by HDL, suggesting that PDZK1 plays multiple roles in normal physiology and may influence associated disorder. All four PDZ domains of PDZK1 appear necessary to promote normal hepatic expression, function and intracellular localization of SR-BI. SR-BI mediates several features of HDL metabolism and function, some of which depend on SR-BI's interaction with PDZK1. Exploration of the structure and function of PDZK1 and the mechanisms by which it controls SR-BI will provide additional insights into HDL metabolism and may provide the basis for new therapeutic modalities for cardiovascular disease.

Normal Hepatic Cell Surface Localization of the High Density Lipoprotein Receptor, Scavenger Receptor Class B, Type I, Depends on All Four PDZ Domains of PDZK1

PDZK1 is a four PDZ domain-containing scaffold protein that binds to scavenger receptor class B, type I (SR-BI), the high density lipoprotein receptor, by its first PDZ domain (PDZ1). PDZK1 knock-out mice exhibit a >95% decrease in hepatic SR-BI protein and consequently an approximately 70% increase in plasma cholesterol in abnormally large high density lipoprotein particles. These defects are corrected by hepatic overexpression of full-length PDZK1 but not the PDZ1 domain alone, which partially restores SR-BI protein abundance but not cell surface expression or function. We have generated PDZK1 knock-out mice with hepatic expression of four PDZK1 transgenes encoding proteins with nested C-terminal truncations: pTEM, which lacks the three C-terminal residues (putative PDZ-binding motif), and PDZ1.2, PDZ1.2.3, or PDZ1.2.3.4, which contain only the first two, three, or four N-terminal PDZ domains, respectively, but not the remaining C-terminal sequences. Hepatic overexpression of pTEM restored normal hepatic SR-BI abundance, localization, and function. Hepatic overexpression of PDZ1.2 or PDZ1.2.3 partially restored SR-BI abundance ( approximately 12 or approximately 30% of wild type, respectively) but did not (PDZ1.2) or only slightly (PDZ1.2.3) restored hepatic SR-BI cell surface localization and function. Hepatic overexpression of PDZ1.2.3.4 completely restored SR-BI protein abundance, cell surface expression, and function (normalization of plasma cholesterol levels). Thus, all four PDZ domains in PDZK1, but not PDZ1-3 alone, are sufficient for its normal control of the abundance, localization, and therefore function of hepatic SR-BI, whereas the residues C-terminal to the PDZ4 domain, including the C-terminal putative PDZ-binding domain, are not required.