17β-Estradiol promotes the up-regulation of SR-BII in HepG2 cells and in rat livers

Scavenger Receptor Class A to E Involved in Various Cancers

Scavenger receptors typically bind to multiple ligands on a cell surface, including endogenous and modified host-derived molecules and microbial pathogens. They promote the elimination of degraded or harmful substances such as non-self or altered-self targets through endocytosis, phagocytosis, and adhesion. Currently, scavenger receptors are subdivided into eight classes based on several variations in their sequences due to alternative splicing. Since recent studies indicate targeting scavenger receptors has been involved in cancer prognosis and carcinogenesis, we will focus on the current knowledge about the emerging role of scavenger receptor classes A to E in cancer progression.

Human SR-BI and SR-BII Potentiate Lipopolysaccharide-Induced Inflammation and Acute Liver and Kidney Injury in Mice

The class B scavenger receptors BI (SR-BI) and BII (SR-BII) are high-density lipoprotein receptors that recognize various pathogens, including bacteria and their products. It has been reported that SR-BI/II null mice are more sensitive than normal mice to endotoxin-induced inflammation and sepsis. Because the SR-BI/II knockout model demonstrates multiple immune and metabolic disorders, we investigated the role of each receptor in the LPS-induced inflammatory response and tissue damage using transgenic mice with pLiv-11-directed expression of human SR-BI (hSR-BI) or human SR-BII (hSR-BII). At 6 h after i.p. LPS injection, transgenic hSR-BI and hSR-BII mice demonstrated markedly higher serum levels of proinflammatory cytokines and 2- to 3-fold increased expression levels of inflammatory mediators in the liver and kidney, compared with wild-type (WT) mice. LPS-stimulated inducible NO synthase expression was 3- to 6-fold higher in the liver and kidney of both transgenic strains, although serum NO levels were similar in all mice. Despite the lower high-density lipoprotein plasma levels, both transgenic strains responded to LPS by a 5-fold increase of plasma corticosterone levels, which were only moderately lower than in WT animals. LPS treatment resulted in MAPK activation in tissues of all mice; however, the strongest response was detected for hepatic extracellular signal-regulated protein kinase 1 and 2 and kidney JNK of both transgenic mice. Histological examination of hepatic and renal tissue from LPS-challenged mice revealed more injury in hSR-BII, but not hSR-BI, transgenic mice versus WT controls. Our findings demonstrate that hSR-BII, and to a lesser extent hSR-BI, significantly increase LPS-induced inflammation and contribute to LPS-induced tissue injury in the liver and kidney, two major organs susceptible to LPS toxicity.

Scavenger Receptors: Emerging Roles in Cancer Biology and Immunology

Scavenger receptors constitute a large family of evolutionally conserved protein molecules that are structurally and functionally diverse. Although scavenger receptors were originally identified based on their capacity to scavenge modified lipoproteins, these molecules have been shown to recognize and bind to a broad spectrum of ligands, including modified and unmodified host-derived molecules or microbial components. As a major subset of innate pattern recognition receptors, scavenger receptors are mainly expressed on myeloid cells and function in a wide range of biological processes, such as endocytosis, adhesion, lipid transport, antigen presentation, and pathogen clearance. In addition to playing a crucial role in maintenance of host homeostasis, scavenger receptors have been implicated in the pathogenesis of a number of diseases, e.g., atherosclerosis, neurodegeneration, or metabolic disorders. Emerging evidence has begun to reveal these receptor molecules as important regulators of tumor behavior and host immune responses to cancer. This review summarizes our current understanding on the newly identified, distinct functions of scavenger receptors in cancer biology and immunology. The potential of scavenger receptors as diagnostic biomarkers and novel targets for therapeutic interventions to treat malignancies is also highlighted.

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.

Cellular cholesterol delivery, intracellular processing and utilization for biosynthesis of steroid hormones

Steroid hormones regulate diverse physiological functions such as reproduction, blood salt balance, maintenance of secondary sexual characteristics, response to stress, neuronal function and various metabolic processes. They are synthesized from cholesterol mainly in the adrenal gland and gonads in response to tissue-specific tropic hormones. These steroidogenic tissues are unique in that they require cholesterol not only for membrane biogenesis, maintenance of membrane fluidity and cell signaling, but also as the starting material for the biosynthesis of steroid hormones. It is not surprising, then, that cells of steroidogenic tissues have evolved with multiple pathways to assure the constant supply of cholesterol needed to maintain optimum steroid synthesis. The cholesterol utilized for steroidogenesis is derived from a combination of sources: 1) de novo synthesis in the endoplasmic reticulum (ER); 2) the mobilization of cholesteryl esters (CEs) stored in lipid droplets through cholesteryl ester hydrolase; 3) plasma lipoprotein-derived CEs obtained by either LDL receptor-mediated endocytic and/or SR-BI-mediated selective uptake; and 4) in some cultured cell systems from plasma membrane-associated free cholesterol. Here, we focus on recent insights into the molecules and cellular processes that mediate the uptake of plasma lipoprotein-derived cholesterol, events connected with the intracellular cholesterol processing and the role of crucial proteins that mediate cholesterol transport to mitochondria for its utilization for steroid hormone production. In particular, we discuss the structure and function of SR-BI, the importance of the selective cholesterol transport pathway in providing cholesterol substrate for steroid biosynthesis and the role of two key proteins, StAR and PBR/TSO in facilitating cholesterol delivery to inner mitochondrial membrane sites, where P450scc (CYP11A) is localized and where the conversion of cholesterol to pregnenolone (the common steroid precursor) takes place.

Impact of gene variants on sex-specific regulation of human Scavenger receptor class B type 1 (SR-BI) expression in liver and association with lipid levels in a population-based study

BACKGROUND

Several studies have noted that genetic variants of SCARB1, a lipoprotein receptor involved in reverse cholesterol transport, are associated with serum lipid levels in a sex-dependent fashion. However, the mechanism underlying this gene by sex interaction has not been explored.

METHODS

We utilized both epidemiological and molecular methods to study how estrogen and gene variants interact to influence SCARB1 expression and lipid levels. Interaction between 35 SCARB1 haplotype-tagged polymorphisms and endogenous estradiol levels was assessed in 498 postmenopausal Caucasian women from the population-based Rancho Bernardo Study. We further examined associated variants with overall and SCARB1 splice variant (SR-BI and SR-BII) expression in 91 human liver tissues using quantitative real-time PCR.

RESULTS

Several variants on a haplotype block spanning intron 11 to intron 12 of SCARB1 showed significant gene by estradiol interaction affecting serum lipid levels, the strongest for rs838895 with HDL-cholesterol (p=9.2x10(-4)) and triglycerides (p=1.3x10(-3)) and the triglyceride:HDL cholesterol ratio (p=2.7x10(-4)). These same variants were associated with expression of the SR-BI isoform in a sex-specific fashion, with the strongest association found among liver tissue from 52 young women<45 years old (p=0.002).

CONCLUSIONS

Estrogen and SCARB1 genotype may act synergistically to regulate expression of SCARB1 isoforms and impact serum levels of HDL cholesterol and triglycerides. This work highlights the importance of considering sex-dependent effects of gene variants on serum lipid levels.

Scavenger receptor class B type I protein as an independent predictor of high-density lipoprotein cholesterol levels in subjects with hyperalphalipoproteinemia

CONTEXT

In mice, scavenger receptor class B, type I (SR-BI) receptor protein deficiency is associated with elevated high-density lipoprotein (HDL)-cholesterol (HDL-C) levels.

OBJECTIVE

Our objective was to determine the relationship between SR-BI protein and HDL-C levels in humans.

DESIGN

This was a prospective study of adults with hyperalphalipoproteinemia. Fasting blood was obtained for lipid and lipoprotein measurement, genomic DNA, and monocyte-derived macrophages. SR-BI protein levels were measured by Western blots, and SR-BI activity was measured by cholesteryl ester (CE) uptake of each donor's radiolabeled HDL with their monocyte-derived macrophages, or by degradation and specific cell association of dual-labeled HDL in vitro.

SETTING

The study was performed in a tertiary university teaching hospital.

RESULTS

The mean age was 57.2 +/- 10.9 yr (n = 65). SR-BI protein levels were inversely associated with HDL-C levels (P < 0.002), HDL particle size (P < 0.05), and positively associated with CE uptake (P < 0.004); there was no association with plasma apolipoprotein levels. SR-BI protein levels (P = 0.01) were independent predictors of HDL-C levels. Subjects who were carriers of the A allele for the rs4238001 (glycine to serine at position 2) polymorphism [single nucleotide polymorphism (SNP)] had lower SR-BI protein levels (P = 0.01), whereas carriers of the C allele for the rs2278986 SNP also had lower SR-BI protein levels (P = 0.02). Body mass index (P = 0.05), rs4238001 (P = 0.01), and rs2278986 (P = 0.01) SNPs were independent predictors of SR-BI protein levels. In vitro studies of murine macrophages stably expressing the glycine to serine at position 2 SNP showed less degradation (P < 0.0004) and specific cell association (P < 0.0004) of [(125)I, (3)H]-CE-labeled HDL.

CONCLUSIONS

SR-BI protein has an independent effect on HDL-C levels in women with hyperalphalipoproteinemia. Two SNPs were significantly associated with lower SR-BI protein levels.

Regulation of alternative splicing of liver scavenger receptor class B gene by estrogen and the involved regulatory splicing factors

The scavenger receptor class B isoforms (SR-B) type I and type II mediate the selective uptake of high-density lipoprotein cholesterol and promote reverse cholesterol transport, an important atherosclerosis protection mechanism, in the liver. Previously it was shown that the hepatic expression of SR-BI and SR-BII is regulated by estrogen. In the present study, we demonstrate that estrogen differentially regulates expression of the glycosylated and nonglycosylated forms of SR-BI and SR-BII in rat liver and hepatic cells. We report that estrogen mainly induces the down-regulation of glycosylated SR-BI and the up-regulation of nonglycosylated SR-BII. To study how estrogen regulates expression of the SR-B isoforms, we constructed a SR-B minigene containing minimal genomic sequences and were able to demonstrate that estrogen directly regulates the pre-mRNA alternative splicing of the exogenously expressed SR-B minigene in hepatic cells. Furthermore, we showed that the overexpression of splicing factors alternative splicing factor/splicing factor 2, Transformer (Tra)-2alpha, and Tra2beta changes the splicing pattern of SR-B dramatically, whereas other splicing factors, such as heterogeneous nuclear ribonucleoprotein-G, SC-35, and arginine/serine-rich p40, had no effect. We also demonstrate that estrogen regulates Tra2beta expression levels in liver cells. These studies suggest that estrogen may regulate SR-B isoform expression at both the RNA splicing and posttranslational modification levels and that, for alternative splicing regulation, estrogen may function by regulating the expression of the splicing factors alternative splicing factor/splicing factor 2, Tra2alpha, and especially Tra2beta.

CLA-1 and its splicing variant CLA-2 mediate bacterial adhesion and cytosolic bacterial invasion in mammalian cells

CD36 and LIMPII analog 1, CLA-1, and its splicing variant, CLA-2 (SR-BI and SR-BII in rodents), are human high density lipoprotein receptors with an identical extracellular domain which binds a spectrum of ligands including bacterial cell wall components. In this study, CLA-1- and CLA-2-stably transfected HeLa and HEK293 cells demonstrated several-fold increases in the uptake of various bacteria over mock-transfected cells. All bacteria tested, including both Gram-negatives (Escherichia coli K12, K1 and Salmonella typhimurium) and Gram-positives (Staphylococcus aureus and Listeria monocytogenes), demonstrated various degrees of lower uptake in control cells. This result is consistent with the presence of high-density lipoprotein-receptor-independent bacterial uptake that is enhanced by CLA-1/CLA-2 overexpression. Bacterial lipopolysaccharides, lipoteichoic acid, and synthetic amphipathic helical peptides (L-37pA and D-37pA) competed with E. coli K12 for CLA-1 and CLA-2 binding. Transmission electron microscopy and confocal microscopy revealed cytosolic accumulation of bacteria in CLA-1/CLA-2-overexpressing HeLa cells. The antibiotic protection assay confirmed that E. coli K12 was able to survive and replicate intracellularly in CLA-1- and CLA-2-overexpressing HeLa, but both L-37pA and D-37pA prevented E. coli K12 invasion. Peritoneal macrophages isolated from SR-BI/BII-knockout mice demonstrated a 30% decrease in bacterial uptake when compared with macrophages from normal mice. Knockout macrophages were also characterized by decreased bacterial cytosolic invasion, ubiquitination, and proteasome mobilization while retaining bacterial lysosomal accumulation. These results indicate that, by facilitating bacterial adhesion and cytosolic invasion, CLA-1 and CLA-2 may play an important role in infection and sepsis.

Estrogen regulation of the scavenger receptor class B gene: Anti-atherogenic or steroidogenic, is there a priority?

High density lipoprotein (HDL) participates in reverse cholesterol transport and in the delivery of cholesterol to the liver and steroidogenic tissues by a mechanism called "selective lipid uptake" which is mediated by the HDL receptor, scavenger receptor B type I (SR-BI). Overexpression of SR-BI suppresses atherosclerosis by increasing reverse cholesterol transport. In contrast, genetic ablation of SR-BI has a negative effect on cardiovascular physiology in both males and females and a gender specific negative impact on female fertility. Cholesterol is essential for mammalian embryonic development as a necessary component of cell membranes and as a substrate for steroidogenesis. The SR-BI receptor is highly expressed in the human placenta allowing the growing fetus to obtain a considerable portion of cholesterol from maternal lipoproteins. Estrogen, which plays an important role in maintaining pregnancy, has been shown to enhance plasma HDL levels and promote reverse cholesterol transport. Since SR-BI is the major determinant of serum HDL levels, direct regulation of the SR-BI gene by estrogen is theorized. The objective of this manuscript is to summarize the current information related to estrogen regulation of the gene that codes for the SR-BI receptor.

Opposite effect of caveolin-1 in the metabolism of high-density and low-density lipoproteins

Receptors of the scavenger class B family were reported to be localized in caveolae, the cell surface microdomains rich in free cholesterol and glycosphyngolipids, which are characterized by the presence of caveolin-1. Parenchymal hepatic and hepatoma HepG2 cells express very low levels of caveolin-1. In the present study, stable transformants of HepG2 cells expressing caveolin-1 were generated to address the effect of caveolin-1 on receptor activity. Compared to normal cells, these cells show higher (125)I-bovine serum albumin (BSA) uptake and cholesterol efflux, two indicators of functional caveolae. By immunoprecipitation, cell fractionation and confocal analyses, we found that caveolin-1 is well colocalized with the cluster of differentiation-36 (CD36) and the low-density lipoprotein (LDL) receptor (LDLr) but to a lesser extent with the scavenger receptor class B type I (SR-BI) in HepG2 cells expressing caveolin-1. However, caveolin-1 expression favors the dimerization of SR-BI. Two clones of cells expressing caveolin-1 were investigated for their lipoprotein metabolism activity. Compared to normal cells, these cells show a 71-144% increase in (125)I-LDL degradation. The analysis of the cholesteryl esters (CE)-selective uptake (CE association minus protein association) revealed that the expression of caveolin-1 in HepG2 cells decreases by 59%-73% LDL-CE selective uptake and increases high-density lipoprotein (HDL)-CE selective uptake by 44%-66%. We conclude that the expression of caveolin-1 in HepG2 cells moves the balance of LDL degradation/CE selective uptake towards degradation and favors HDL-CE selective uptake. Thus, in the normal hepatic parenchymal situation where caveolin-1 is poorly expressed, LDL-CE selective uptake is the preferred pathway.

Bile acids reduce SR-BI expression in hepatocytes by a pathway involving FXR/RXR, SHP, and LRH-1

Hepatic SR-BI mediates uptake of circulating cholesterol into liver hepatocytes where a part of the cholesterol is metabolised to bile acids. In the hepatocytes, bile acids reduce their own synthesis by a negative feedback loop to prevent toxic high levels of bile acids. Bile acid-activated FXR/RXR represses expression of CYP7A1, the rate-limiting enzyme during bile acid synthesis, by inducing the expression of SHP, which inhibits LXR/RXR and LRH-1-transactivation of CYP7A1. The present paper presents data indicating that CDCA suppresses SR-BI expression by the same pathway. As previously reported, LRH-1 induces SR-BI promoter activity. Here we show that CDCA or over-expression of SHP inhibit this transactivation. No FXR-response element was identified in the bile acid-responsive region of the SR-BI promoter (-1200bp/-937bp). However, a binding site for LRH-1 was characterised and shown to specifically bind LRH-1. The present study shows that also the SR-BI-mediated supply of cholesterol, the substrate for bile acid synthesis, is feedback regulated by bile acids.

Cholesteryl esters in malignancy

Cholesteryl esters, formed by the esterification of cholesterol with long-chain fatty acids, on one hand, are the means by which cholesterol is transported through the blood by lipoproteins, on the other, the way cholesterol itself can be accumulated in the cells. Therefore, these important molecules play an active part in metabolic pathways that form the basis of cholesterol trafficking and homeostasis. The role of different regulatory mechanisms in cholesterol homeostasis in physiologic and neoplastic conditions with emphasis on intracellular content of cholesteryl esters is here reviewed. Numerous studies carried out on tumor cell lines, experimental tumors, and human tumors have shown an abnormal cholesterol metabolism that is reflected by an increase in intracellular cholesteryl esters due to an alteration in all the mechanisms that form the basis of regulation, in particular: cholesterol de novo biosynthesis; uptake of exogenous cholesterol LDL receptor mediated; cholesterol esterification mediated by the ACAT activity; cholesterol efflux HDL receptor mediated. The most recent analytic-spectroscopic applications that permit cholesteryl ester determination on tumor lipidic extracts and directly in vivo are also reported. This review gives an overview of cholesterol homeostasis in physiological and pathological conditions where cholesteryl esters are over-expressed.

The selective estrogen receptor modulator acolbifene reduces cholesterolemia independently of its anorectic action in control and cholesterol-fed rats

The cancer-preventing selective estrogen receptor modulator (SERM) acolbifene (ACOL) exerts a potent and pure antiestrogenic action in the mammary gland and uterus, yet it displays beneficial, estrogen-like actions on energy and lipid metabolism in rodents. The compound reduces food intake and strongly decreases cholesterolemia in rats fed a cholesterol-free diet. This study was designed to establish whether the anorectic effect of ACOL is involved in its cholesterol-lowering action, and whether the compound retains its ability to lower cholesterol concentrations in rats with diet-induced hypercholesterolemia. Female rats were fed a purified diet devoid of cholesterol (reference diet) or containing 2% cholesterol (C-diet); they were either not treated or treated daily with ACOL or not treated and pair-fed to the ACOL-treated rats. The C-diet did not affect food intake or weight and fat gains. ACOL reduced food intake (16%) and weight gain (45%, mainly fat) similarly in both dietary cohorts. ACOL, but not pair feeding, reduced cholesterolemia by 33% in rats fed the reference diet. As expected, the C-diet raised serum total cholesterol almost 3-fold and this increase was largely prevented by ACOL but not by pair feeding. Cholesterol was reduced by ACOL, mainly in the HDL fraction, in rats fed the reference diet, but only in the non-HDL fraction in those fed the C-diet. In livers of rats fed the reference diet, ACOL, but not pair feeding, increased protein abundance of the scavenger receptor, class B, type 1, and the LDL receptor, thought to be involved in ACOL-mediated cholesterol lowering. These findings demonstrate that the potent hypocholesterolemic action of ACOL is independent of the concomitant reduction in food intake and fat accretion, and that such action occurs in rats with overt diet-induced hypercholesterolemia.

Physiological importance of SR-BI in the in vivo metabolism of human HDL and LDL in male and female mice

The physiological role of murine scavenger receptor class B type I (SR-BI) was evaluated by in vivo clearances of human HDL3 and LDL in normal and SR-BI knockout (KO) mice. In normal mice, cholesteryl esters (CEs) were removed faster than proteins, indicating a selective uptake process from both HDL3 and LDL. SR-BI KO mice showed 80% losses of HDL-CE selective uptake and the complete loss of LDL-CE selective uptake in the first phase of clearance. However, the second phase was characterized by an acceleration of CE disappearance in SR-BI KO mice. Thus, SR-BI is the only murine receptor mediating HDL-CE selective uptake, whereas a SR-BI-independent pathway specific to LDL can rescue SR-BI deficiency. The analysis of LDL recovered 3 h after injection in mice from different genotypes revealed that LDLs are significantly depleted in CE (reduction from 19% to 50% of the CE/protein ratios). A smaller LDL size in comparison with that of noninjected LDL was also detectable but was more evident for LDL recovered from normal mice. All LDL preparations migrate faster than noninjected LDL on agarose-barbital gels. Thus, both SR-BI-dependent and -independent pathways lead to substantial changes in LDL.

Modulation of high-density lipoprotein cholesterol metabolism and reverse cholesterol transport

Low high-density lipoprotein (HDL)-cholesterol (C) is an important risk factor for coronary heart disease. In vitro, HDL exerts several potentially anti-atherogenic effects including reverse cholesterol transport (RCT) from peripheral cells to the liver. Hence, raising HDL-C has become an interesting target for anti-atherosclerotic drug therapy. Levels of HDL-C and the composition of HDL subclasses in plasma are regulated by apolipoproteins, lipolytic enzymes, lipid transfer proteins, receptors, and cellular transporters. The interplay of these factors leads to RCT and determines the composition and thereby the anti-atherogenic properties of HDL. Recent findings suggest that the mechanism of HDL modification rather than a sole increase in HDL-C determines the efficacy of anti-atherosclerotic drug therapy. In several controlled and prospective intervention studies, patients with low HDL-C and additional risk factors benefited from treatment with fibrates or statins. However, in only some of the fibrate trials was prevention of coronary events in patients with low HDL-C and hypertriglyceridaemia related to an increase in HDL-C. This may be because currently available drugs increase HDL-C levels only moderately and because HDL levels per se do not necessarily correlate with the functionality of HDL. However, several novel targets to modify RCT have emerged from the recent understanding of HDL synthesis, maturation and catabolism. The four major targets for an anti-atherogenic strategy in HDL metabolism include stimulation of apoA-I synthesis and secretion, the stimulation of ABCA1 expression, the inhibition of cholesterol ester transfer protein, and the up-regulation of scavenger receptor BI. These and other modulations of HDL metabolism are thought to result in improved RCT making them attractive targets for the development of new regimens of anti-atherogenic drug therapy.

The role of scavenger receptor class B type I (SR-BI) in lipid trafficking. defining the rules for lipid traders

The scavenger receptor class B type I (SR-BI) is a 509-amino acid, 82 kDa glycoprotein, with two cytoplasmic C- and N-terminal domains separated by a large extracellular domain. The aim of this review is to define the role of SR-BI as a lipoprotein receptor responsible for selective uptake of cholesteryl esters (CE) from high density lipoprotein (HDL) and low density lipoprotein (LDL) and free cholesterol (FC) efflux to lipoprotein acceptors. These activities depend on lipoprotein binding to its extracellular domain and subsequent lipid exchange at the plasma membrane. CE selective uptake supplies cholesterol to liver and steroidogenic tissues, for biliary cholesterol secretion and steroid hormone synthesis. Genetically modified mice have confirmed SR-BI's major role in tissue cholesterol uptake and in reverse cholesterol transport, i.e. cholesterol turnover. Accordingly, cellular cholesterol level, estrogens and trophic hormones regulate SR-BI expression by both transcriptional and post-transcriptional mechanisms. Importantly, mouse SR-BI overexpression has both corrective and preventive effects on atherosclerosis. Human SR-BI has very similar tissue distribution, binding properties and lipid transfer activities compared to rodent SR-BI. However, human plasma has most of its cholesterol in LDL. Thus, there is considerable interest to develop anti-atherogenic strategies involving human SR-BI-mediated increases in reverse cholesterol transport through HDL and/or LDL.

High density lipoprotein uptake by scavenger receptor SR-BII

Scavenger receptor class B, type I (SR-BI) mediates selective uptake of high density lipoprotein (HDL) lipids. It is unclear whether this process occurs at the cell membrane or via endocytosis. Our group previously identified an alternative mRNA splicing variant of SR-BI, named SR-BII, with an entirely different, yet highly conserved cytoplasmic C terminus. In this study we aimed to compare HDL uptake by both isoforms. Whereas SR-BI was mainly ( approximately 70%) localized on the surface of transfected Chinese hamster ovary cells, as determined by biotinylation, HDL binding at 4 degrees C, and studies of enhanced green fluorescent protein-tagged SR-BI/II fusion proteins, the majority of SR-BII ( approximately 80-90%) was expressed intracellularly. The cellular distribution of SR-BI was not affected by deletion of the C terminus, which suggests that the distinct C terminus of SR-BII is responsible for its intracellular expression. Pulse-chase experiments showed that SR-BII rapidly internalized HDL protein, whereas in the case of SR-BI most HDL protein remained surface bound. Like its ligand, SR-BII was more rapidly endocytosed compared with SR-BI. Despite more rapid HDL uptake by SR-BII than SR-BI, selective cholesteryl ether uptake was significantly lower. Relative to their levels of expression at the cell surface, however, both isoforms mediated selective uptake with similar efficiency. HDL protein that was internalized by SR-BII largely co-localized with transferrin in the endosomal recycling compartment. Within the endosomal recycling compartment of SR-BII cells, there was extensive co-localization of internalized HDL lipid and protein. These results do not support a model that selective lipid uptake by SR-BI requires receptor/ligand recycling within the cell. We conclude that SR-BII may influence cellular cholesterol trafficking and homeostasis in a manner that is distinct from SR-BI.

Interaction of scavenger receptor class B type I with peroxisomal targeting receptor Pex5p

Scavenger receptor class B type I (SR-BI) is an HDL receptor that mediates selective HDL lipid uptake. Peroxisomes play an important role in lipid metabolism and peroxisomal targeting signal type 1 (PTS1)-containing proteins are translocated to peroxisomes by the peroxisomal targeting import receptor, Pex5p. We have previously identified a PTS1 motif in the intracellular domain of rat SR-BI. Here, we examine the possible interaction between Pex5p and SR-BI. Expression of a Flag-tagged intracellular domain of SR-BI resulted in translocation to the peroxisome as demonstrated by double labeling with anti-Flag IgG and anti-catalase IgG analyzed by confocal microscopy. Immunoprecipitation experiments with anti-SR-BI antibody showed that Pex5p co-precipitated with SR-BI. However, when an antibody against Pex5p was used for immunoprecipitation, only the 57kDa, non-glycosylated form, of SR-BI co-precipitated. We conclude that the PTS1 domain of SR-BI is functional and can mediate peroxisomal interaction via Pex5p, in vitro.

Low high-density lipoprotein cholesterol: physiological background, clinical importance and drug treatment

Low high-density lipoprotein (HDL) cholesterol is an important risk factor for coronary heart disease (CHD). In vitro, HDL exerts several potentially anti-atherogenic activities. HDLs mediate the reverse cholesterol transport (RCT) from peripheral cells to the liver, inhibit oxidation of low-density lipoprotein (LDL), adhesion of monocytes to the endothelium, apoptosis of vascular endothelial and smooth muscle cells and platelet activation, and stimulate the endothelial secretion of vasoactive substances as well as smooth muscle cell proliferation. Hence, raising HDL-cholesterol levels has become an interesting target for anti-atherosclerotic drug therapy. Levels of HDL cholesterol and the composition of HDL subclasses in plasma are regulated by apolipoproteins, lipolytic enzymes, lipid transfer proteins, receptors and cellular transporters. The interplay of these factors leads to RCT and determines the composition and, thereby, the anti-atherogenic properties of HDL. Several inborn errors of metabolism, as well as genetic animal models, are characterised by both elevated HDL cholesterol and increased rather than decreased cardiovascular risk. These findings suggest that the mechanism of HDL modification rather than simply increasing HDL cholesterol determine the efficacy of anti-atherosclerotic drug therapy. In several controlled and prospective intervention studies, patients with low HDL cholesterol and additional risk factors benefited from treatment with fibric acid derivatives (fibrates) or HMG-CoA reductase inhibitors (statins). However, only in some trials was prevention of coronary events in patients with low HDL cholesterol and hypertriglyceridaemia related to an increase in HDL cholesterol. We discuss the clinical and metabolic effects of fibrates, statins, nicotinic acid and sex steroids, and present novel therapeutic strategies that show promise in modifying HDL metabolism. In conclusion, HDL-cholesterol levels increase only moderately after treatment with currently available drugs and do not necessarily correlate with the functionality of HDL. Therefore, the anti-atherosclerotic therapy of high-risk cardiovascular patients should currently be focused on the correction of other risk factors present besides low HDL cholesterol. However, modification of HDL metabolism and improvement of RCT remain an attractive target for the development of new regimens of anti-atherogenic drug therapy.

The role of the high-density lipoprotein receptor SR-BI in the lipid metabolism of endocrine and other tissues

Because cholesterol is a precursor for the synthesis of steroid hormones, steroidogenic tissues have evolved multiple pathways to ensure adequate supplies of cholesterol. These include synthesis, storage as cholesteryl esters, and import from lipoproteins. In addition to endocytosis via members of the low-density lipoprotein receptor superfamily, steroidogenic cells acquire cholesterol from lipoproteins by selective lipid uptake. This pathway, which does not involve lysosomal degradation of the lipoprotein, is mediated by the scavenger receptor class B type I (SR-BI). SR-BI is highly expressed in steroidogenic cells, where its expression is regulated by various trophic hormones, as well as in the liver. Studies of genetically manipulated strains of mice have established that SR-BI plays a key role in regulating lipoprotein metabolism and cholesterol transport to steroidogenic tissues and to the liver for biliary secretion. In addition, analysis of SR-BI-deficient mice has shown that SR-BI expression is important for alpha-tocopherol and nitric oxide metabolism, as well as normal red blood cell maturation and female fertility. These mouse models have also revealed that SR-BI can protect against atherosclerosis. If SR-BI plays similar physiological and pathophysiological roles in humans, it may be an attractive target for therapeutic intervention in cardiovascular and reproductive diseases.

Biphasic estradiol-induced AKT phosphorylation is modulated by PTEN via MAP kinase in HepG2 cells

We reported previously in HepG2 cells that estradiol induces cell cycle progression throughout the G1-S transition by the parallel stimulation of both PKC-alpha and ERK signaling molecules. The analysis of the cyclin D1 gene expression showed that only the MAP kinase pathway was involved. Here, the presence of rapid/nongenomic, estradiol-regulated, PI3K/AKT signal transduction pathway, its modulation by the levels of the tumor suppressor PTEN, its cross-talk with the ERK pathway, and its involvement in DNA synthesis and cyclin D1 gene promoter activity have all been studied in HepG2 cells. 17beta-Estradiol induced the rapid and biphasic phosphorylation of AKT. These phosphorylations were independent of each other, being the first wave of activation independent of the estrogen receptor (ER), whereas the second was dependent on ER. Both activations were dependent on PI3K activity; furthermore, the ERK pathway modulated AKT phosphorylation by acting on the PTEN levels. The results showed that the PI3K pathway, as well as ER, were strongly involved in both G1-S progression and cyclin D1 promoter activity by acting on its proximal region (-254 base pairs). These data indicate that in HepG2 cells, different rapid/nongenomic estradiol-induced signal transduction pathways modulate the multiple steps of G1-S phase transition.

Scavenger receptor class B, type I on non-malignant and malignant human epithelial cells mediates cholesteryl ester-uptake from high density lipoproteins

Hepatoma cell lines serve as a suitable model to study hepatic clearance of lipoprotein-associated cholesteryl esters (CEs). The present study aimed at investigating holoparticle-association of and selective CE-uptake from human high density lipoprotein subclass 3 (HDL3) by non-malignant adult (Chang-liver) and non-malignant fetal (WRL-68) epithelial cell lines as well as a hepatocellular carcinoma (HUH-7) cell line. Binding properties of 125I-HDL3 at 4 and 37 degrees C were similar for all three cell lines while degradation rates were highest for Chang-liver cells. Calculating the selective uptake of HDL3-associated CEs as the difference between [3H]CE- and 125I-HDL3 cell-association revealed that the selective lipid uptake and holoparticle-association was similar in Chang-liver while in WRL-68 and HUH-7 cells pronounced capacity for lipid tracer uptake in excess of holoparticle uptake was measured. Using RT-PCR, Northern and Western blot analysis, as well as immunocytochemical technique pronounced expression of scavenger receptor class B, type I (SR-BI) but not SR-BII (a splice variant of SR-BI less efficient for selective CE-uptake than SR-BI) could be identified in HUH-7 and WRL-68 cells. A polyclonal antiserum raised against SR-BI significantly decreased cell-association of [3H]CE-HDL3 in HUH-7 and WRL-68. The present findings suggest that the capacity for selective cholesteryl ester-uptake from high density lipoprotein by malignant and normal epithelial cells from the liver depends on expression of the scavenger receptor class B, type I.

Cholesteryl ester transfer protein expression attenuates atherosclerosis in ovariectomized mice

Reduced estrogen levels result in loss of protection from coronary heart disease in postmenopausal women. Enhanced and diminished atherosclerosis have been associated with plasma levels of cholesteryl ester transfer protein (CETP); however, little is known about the role of CETP-ovarian hormone interactions in atherogenesis. We assessed the severity of diet-induced atherosclerosis in ovariectomized (OV) CETP transgenic mice crossbred with LDL receptor knockout mice. Compared with OV CETP expressing ((+)), OV CETP non-expressing ((-)) mice had higher plasma levels of total, VLDL-, LDL-, and HDL-cholesterol, as well as higher antibodies titers against oxidized LDL. The mean aortic lesion area was 2-fold larger in OV CETP(-) than in OV CETP(+) mice (147 +/- 90 vs. 73 +/- 42 x 10(3) micro m(2), respectively). Estrogen therapy in OV mice blunted the CETP dependent differences in plasma lipoproteins, oxLDL antibodies, and atherosclerosis severity. Macrophages from OV CETP(+) mice took up less labeled cholesteryl ether (CEt) from acetyl-LDL than macrophages from OV CETP(-) mice. Estrogen replacement induced a further reduction in CEt uptake and an elevation in HDL mediated cholesterol efflux from pre-loaded OV CETP(+) as compared with OV CETP(-) macrophages. These findings support the proposed anti-atherogenic role of CETP in specific metabolic settings.

Distinct nongenomic signal transduction pathways controlled by 17beta-estradiol regulate DNA synthesis and cyclin D(1) gene transcription in HepG2 cells

Estrogens induce cell proliferation in target tissues by stimulating progression through the G1 phase of the cell cycle. Activation of cyclin D(1) gene expression is a critical feature of this hormonal action. The existence of rapid/nongenomic estradiol-regulated protein kinase C (PKC-alpha) and extracellular signal-regulated kinase (ERK) signal transduction pathways, their cross talk, and role played in DNA synthesis and cyclin D(1) gene transcription have been studied herein in human hepatoma HepG2 cells. 17Beta-estradiol was found to rapidly activate PKC-alpha translocation and ERK-2/mitogen-activated protein kinase phosphorylation in this cell line. These actions were independent of each other, preceding the increase of thymidine incorporation into DNA and cyclin D(1) expression, and did not involve DNA binding by estrogen receptor. The results obtained with specific inhibitors indicated that PKC-alpha pathway is necessary to mediate the estradiol-induced G1-S progression of HepG2 cells, but it does not exert any effect(s) on cyclin D(1) gene expression. On the contrary, ERK-2 cascade was strongly involved in both G1-S progression and cyclin D(1) gene transcription. Deletion of its activating protein-1 responsive element motif resulted in attenuation of cyclin D(1) promoter responsiveness to estrogen. These results indicate that estrogen-induced cyclin D(1) transcription can occur in HepG2 cells independently of the transcriptional activity of estrogen receptor, sustaining the pivotal role played by nongenomic pathways of estrogen action in hormone-induced proliferation.

Scavenger receptor class BI and selective cholesteryl ester uptake: partners in the regulation of steroidogenesis

The steroidogenic tissues have a special requirement for cholesterol, which is used as a substrate for steroid hormone biosynthesis. In many species this cholesterol is obtained from plasma lipoproteins by a unique pathway in which circulating lipoproteins bind to the surface of the steroidogenic cells and contribute their cholesteryl esters to the cells by a 'selective' process in which the whole lipoprotein particle does not enter the cell. This review describes the lipoprotein selective cholesteryl ester uptake process and its specific partnership with the HDL receptor, scavenger receptor class BI (SR-BI). It describes the characteristics of the selective pathway, and the molecular properties, localization, regulation, anchoring sites and potential mechanisms of action of SR-BI in facilitating cholesteryl ester uptake by steroidogenic cells.

Estrogen activates the high-density lipoprotein receptor gene via binding to estrogen response elements and interaction with sterol regulatory element binding protein-1A

The effects of E2 on the high-density lipoprotein receptor (HDL-R) scavenger receptor class B type I (SR-BI) gene were examined. Four putative estrogen response element half-site motifs (ERE(1/2)) (-2176, -1726, -1622, and -1211, designated ERE(1/2)-1, 2, 3, and 4, respectively) were identified in the HDL-R SR-BI promoter. Transfection studies and mutation analysis demonstrated that E2 significantly increased HDL-R SR-BI promoter activity and that mutating ERE(1/2)-1, 2, and 4 resulted in a loss of E2 responsiveness. Both ER alpha and ER beta formed specific complexes with ERE(1/2)-1, 2, and 4 but did not bind ERE(1/2)-3 in vitro. Interestingly, ERE(1/2)-3 was the motif shown not to be important for E2-activation of the HDL-R SR-BI promoter in the mutational analysis studies. The influence of SREBP-1a (sterol regulatory element binding protein-1a) on E2 regulation of the HDL-R SR-BI gene was also examined. SREBP-1a was able to bind directly to the ERE(1/2) motifs and enhanced ER binding when both ER subtypes were present. ER alpha and beta also bound to a sterol response element motif, but they did not enhance SREBP-1a binding. Cotransfection studies demonstrated that the presence of the three factors, ER alpha, ER beta, and SREBP-1a, enhanced the overall luciferase activity produced from the HDL-R SR-BI promoter construct in the presence of only one of the factors. Interaction of SREBP-1a with both ERs was demonstrated using a mammalian two-hybrid assay. The data confirmed that E2 through the ERs can positively regulate the HDL-R SR-BI through binding and activation of three ERE(1/2) motifs and identified SREBP-1a as a potential coactivator of the E2-ER-dependent effects on the HDL-R SR-BI gene.