Characterization of SR-B2a and SR-B2b genes and their ability to promote GCRV infection in grass carp (Ctenopharyngodon idellus)

Scavenger receptor class B type 2 (SR-B2) is a pattern recognition receptor involved in innate immunity in mammals; however, the immunological function of SR-Bs in fish remains unclear. In this study, the full-length cDNA sequences of SR-B2a and SR-B2b from grass carp (Ctenopharyngodon idellus) were cloned and designated as CiSR-B2a and CiSR-B2b. Multiple alignments and phylogenetic analyses deduced that CiSR-B2a and CiSR-B2b had the highest evolutionary conservation and were closely related to the zebrafish (Danio rerio) homologs, DrSR-B2a and DrSR-B2b, respectively. Both CiSR-B2a and CiSR-B2b were expressed in all the tested tissues, with the highest expression levels found in the hepatopancreas. In Ctenopharyngodon idellus kidney cells (CIK), CiSR-B2a and CiSR-B2b were mainly located in the cytoplasm, and a small amount located on the plasma membrane. After challenge with Grass Carp Reovirus (GCRV), the expression of CiSR-B2a and CiSR-B2b were significantly upregulated in the spleen (about 10.27 and 27.19 times higher than that at 0 day, p < 0.01). With CiSR-B2a or CiSR-B2b overexpressed in CIK, the relative copy number of GCRV in the cells was both significantly increased compared to that in the control group, indicating that CiSR-B2a and CiSR-B2b may be important proteins during the infection processes of GCRV.

The high-density lipoprotein receptor Scarb1 is required for normal bone differentiation in vivo and in vitro

We examined bone formation and turnover in high-density lipoprotein (HDL) receptor, scavenger receptor type I (Scarb1), knockout animals relative to wild-type (WT) controls. Scarb1-/- animals have elevated serum adrenocorticotropic hormone (ACTH) due to the role of Scarb1 in glucocorticoid production, which might cause increased bone mass. However, this was not observed: Scarb1-/- mice, with ACTH, over 1000 pg/ml relative to wild-type ACTH ~ 25 pg/ml, bone of the knockout animals was osteopenic relative to the wild type at 16 weeks, including bone volume/total volume and trabecular thickness. Other serum parameters of WT and Scarb1-/- animals in cortisol or calcium were unaffected, although Scarb1-/- animals had significantly elevated PTH and decreased phosphate. Osteoblast and osteoclast-related mRNAs extracted from bone were greatly decreased at 8 or 16 weeks. Importantly, in normal ACTH, osteogenic differentiation in vitro from mesenchymal stem cells showed reduced alkaline phosphatase and mineralization. In Scarb1-/- cells relative to WT, mRNAs for RunX2, alkaline phosphatase, type I collagen, and osteocalcin were reduced 40-90%, all p < 0.01, indicating a role of Scarb1 in osteoblast differentiation independent of ACTH. Additionally, in vitro osteoblast differentiation at variable ACTH in WT cells confirmed ACTH increasing bone differentiation, mineralization, alkaline phosphatase, and osteocalcin mRNA at 0-10 nM ACTH, but reduced bone differentiation at 100-1000 nM ACTH. Overall Scarb1-/- animals show inhibited bone formation with age. This may be a mixed effect on direct bone formation and of very high ACTH. Further, this work shows that both ACTH concentration and the HDL receptor Scarb1 play important independent roles in osteoblast differentiation.

The atherogenic Scarb1 null mouse model shows a high bone mass phenotype

Scavenger receptor class B, type I (SR-BI), the Scarb1 gene product, is a receptor associated with cholesteryl ester uptake from high-density lipoproteins (HDL), which drives cholesterol movement from peripheral tissues toward the liver for excretion, and, consequently, Scarb1 null mice are prone to atherosclerosis. Because studies have linked atherosclerosis incidence with osteoporosis, we characterized the bone metabolism in these mice. Bone morphometry was assessed through microcomputed tomography and histology. Marrow stromal cells (MSCs) were used to characterize influence of endogenous SR-BI in cell functions. Total and HDL-associated cholesterol in null mice were increased by 32-60%, correlating with its role in lipoprotein metabolism. Distal metaphyses from 2- and 4-mo-old null mice showed correspondingly 46 and 37% higher bone volume fraction associated with a higher number of trabeculae. Histomorphometric analyses in 2-mo-old null male mice revealed 1.42-fold greater osteoblast surface, 1.37-fold higher percent mineralizing surface, and 1.69-fold enhanced bone formation rate. In vitro assays for MSCs from null mice revealed 37% higher proliferation rate, 48% more alkaline phosphatase activity, 70% greater mineralization potential and a 2-fold osterix (Sp7) expression, yet a 0.5-fold decrease in caveolin-1 (Cav1) expression. Selective uptake levels of HDL-associated cholesteryl oleate and estradiol were similar between MSC from wild-type and Scarb1 null mice, suggesting that its contribution to this process is not its main role in these cells. However, Scarb1 knockout stunted the HDL-dependent regulation of Cav1 genic expression. Scarb1 null mice are not prone to osteoporosis but show higher bone mass associated with enhanced bone formation.

Hepatitis C virus infection of a thyroid cell line: implications for pathogenesis of hepatitis C virus and thyroiditis

BACKGROUND

Autoimmune and non-autoimmune thyroiditis frequently occur in persons with hepatitis C virus (HCV) infection. Treatment with interferon alpha (IFNα) is also associated with significant risk for the development of thyroiditis. To explore HCV-thyroid interactions at a cellular level, we evaluated whether a human thyroid cell line (ML1) could be infected productively with HCV in vitro.

METHODS AND RESULTS

ML1 cells showed robust surface expression of the major HCV receptor CD81. Using a highly sensitive, strand-specific reverse transcription polymerase chain reaction assay, positive-sense and negative-sense HCV RNA were detected in ML1 cell lysates at days 3, 7, and 14 postinfection with HCV. HCV core protein was expressed at high levels in ML1 supernatants at days 1, 3, 5, 7, and 14 postinfection. The nonstructural protein NS5A was also detected in ML1 cell lysates by Western blotting. HCV entry into ML1 cells was shown to be dependent on the HCV entry factors CD81 and SR-B1/CLA1, while IFNα inhibited HCV replication in ML1 cells in a dose-dependent manner. Supernatants from HCV-infected ML1 cells were able to infect fresh ML1 cells productively, suggesting that infectious virions could be transferred from infected to naïve thyroid cells in vivo. Additionally, HCV infection of ML1 cells led to increased expression of the pro-inflammatory cytokine IL-8.

CONCLUSIONS

For the first time, we have demonstrated that HCV can infect human thyroid cells in vitro. These findings strongly suggest that HCV infection of thyrocytes may play a role in the association between chronic HCV infection and thyroid autoimmunity. Furthermore, the thyroid may serve as an extrahepatic reservoir for HCV viral replication, thus contributing to the persistence of viral infection and to the development of thyroid autoimmunity.

[HDL level or HDL function as the primary target in preventive cardiology]

The risk for myocardial infarction can be reduced by almost 50% solely by lowering LDL cholesterol. Despite success reducing LDL and cholesterol, atherosclerosis and myocardial infarction remain significant challenges. However, mechanisms of the reverse cholesterol transport system might be used more effectively in the foreseeable future. Although the benefit of high HDL cholesterol appears to be obvious, most clinical trials aimed at increasing HDL cholesterol failed to generate convincing results. Therefore, the question arises as to whether indeed only HDL level or perhaps rather more HDL function is of considerable therapeutic relevance. If function is the crucial issue drugs such as CETP (cholesteryl ester transfer protein) activators or SR-B1 (scavenger receptor type B-1) upregulators could be beneficial. These types of drugs could improve HDL metabolism and might have beneficial effects despite the fact that they lower HDL levels. Ongoing studies on next generation CETP inhibitors and nicotinic acid will clarify this question and might help in our struggle against atherosclerosis.

Genetic studies in mice and humans reveal new physiological roles for the high-density lipoprotein receptor scavenger receptor class B type I

PURPOSE OF REVIEW

Scavenger receptor class B type I (BI) is primarily known for its role in the selective uptake of cholesteryl esters from HDL, the final step in reverse cholesterol transport. Here, we will discuss findings that highlight the recently established novel link between scavenger receptor BI and adrenal and platelet function.

RECENT FINDINGS

Human heterozygote carriers of a functional P297S mutation in the scavenger receptor BI gene show an attenuated adrenal glucocorticoid output and an altered platelet function. Scavenger receptor BI knockout mice lack adrenal cholesteryl ester stores and suffer from primary adrenal glucocorticoid insufficiency, indicating that adrenal HDL cholesteryl ester uptake by scavenger receptor BI is needed for generating the cholesterol pool used for steroidogenesis. Scavenger receptor BI knockout mice exhibit thrombocytopenia, an impaired platelet aggregation response, and higher susceptibility for arterial thrombosis. Bone marrow-specific deletion of scavenger receptor BI in mice indicates that scavenger receptor BI indirectly modulates platelet function through regulation of plasma cholesterol levels.

SUMMARY

Scavenger receptor BI is not merely a crucial mediator of reverse cholesterol transport, but rather acts as a multipurpose player in cholesterol and steroid metabolism. Further understanding of the contribution of scavenger receptor BI's roles in adrenal steroidogenesis and platelet function to the pathogenesis of atherosclerosis and atherothrombosis will hopefully show its potential as a therapeutic target for cardiovascular benefit.

Effect of bariatric surgery-induced weight loss on SR-BI-, ABCG1-, and ABCA1-mediated cellular cholesterol efflux in obese women

AIM

We tested the hypothesis that quantitative changes in high-density lipoprotein (HDL) particles weight loss induced by Roux-en-Y bypass (RYGBP) in morbidly obese subjects might be associated with improved functionality of these particles in the reverse cholesterol transport pathway.

METHODS AND RESULTS

Thirty-four morbidly obese women were recruited and followed up before and 6 months after RYGBP. After surgery, along with a major weight loss (-20%; P < 0.0001), we observed a significant increase in HDL mass concentration (+14%; P < 0.04), reflecting a specific increase in large HDL2 subfraction levels (+42%; P < 0.01), whereas those of HDL3 remained unchanged. Cholesterol ester transfer protein activity decreased significantly (-15%; P < 0.0001). Efflux capacity of total plasma increased significantly via both scavenger receptor class B type I (SR-BI) (+58%; P < 0.0001) and ATP binding cassette G1 (ABCG1) (+26%; P < 0.0001) pathways. Such enhanced capacity resulted from increased capacity of HDL2 particles to mediate cholesterol efflux through the SR-BI pathway (+56%, P < 0.001) and from the increase plasma level of cholesteryl ester-rich HDL2 particles for the ABCG1 pathway.

CONCLUSION

RYGBP-induced weight loss results in improvement in atherogenic lipid profile including a shift toward a more cardioprotective HDL subfraction profile. In addition, our in vitro studies demonstrated an increased in plasma efflux capacity via both SR-BI and ABCG1 after surgery.

PPARγ activation redirects macrophage cholesterol from fecal excretion to adipose tissue uptake in mice via SR-BI

PPARγ agonists, used in the treatment of Type 2 diabetes, can raise HDL-cholesterol, therefore could potentially stimulate macrophage-to-feces reverse cholesterol transport (RCT). We aimed to test whether PPARγ activation promotes macrophage RCT in vivo. Macrophage RCT was assessed in mice using cholesterol loaded/(3)H-cholesterol labeled macrophages. PPARγ agonist GW7845 (20 mg/kg/day) did not change (3)H-tracer plasma appearance, but surprisingly decreased fecal (3)H-free sterol excretion by 43% (P<0.01) over 48h. Total free cholesterol efflux from macrophages to serum (collected from control and GW7845 groups) was not different, although ABCA1-mediated efflux was significantly higher with GW7845. To determine the effect of PPARγ activation on HDL cholesterol uptake by different tissues, the metabolic fate of HDL labeled with (3)H-cholesteryl ether (CE) was also measured. We observed two-fold increase in HDL derived (3)H-CE uptake by adipose tissue (P<0.005) with concomitant 22% decrease in HDL derived (3)H-CE uptake by the liver (P<0.05) in GW7845 treated wild type mice. This was associated with a significant increase in SR-BI protein expression in adipose tissue, but not liver. The same experiment in SR-BI knockout mice, showed no difference in HDL derived (3)H-CE uptake by adipose tissue or liver. In conclusion, PPARγ activation decreases the fecal excretion of macrophage derived cholesterol in mice. This is not due to inhibition of cholesterol efflux from macrophages, but rather involves redirection of effluxed cholesterol from liver towards adipose tissue uptake via SR-BI. This represents a novel mechanism for regulation of RCT and may extend the therapeutic implications of these ligands.

Adaptation of hepatitis C virus to mouse CD81 permits infection of mouse cells in the absence of human entry factors

Hepatitis C virus (HCV) naturally infects only humans and chimpanzees. The determinants responsible for this narrow species tropism are not well defined. Virus cell entry involves human scavenger receptor class B type I (SR-BI), CD81, claudin-1 and occludin. Among these, at least CD81 and occludin are utilized in a highly species-specific fashion, thus contributing to the narrow host range of HCV. We adapted HCV to mouse CD81 and identified three envelope glycoprotein mutations which together enhance infection of cells with mouse or other rodent receptors approximately 100-fold. These mutations enhanced interaction with human CD81 and increased exposure of the binding site for CD81 on the surface of virus particles. These changes were accompanied by augmented susceptibility of adapted HCV to neutralization by E2-specific antibodies indicative of major conformational changes of virus-resident E1/E2-complexes. Neutralization with CD81, SR-BI- and claudin-1-specific antibodies and knock down of occludin expression by siRNAs indicate that the adapted virus remains dependent on these host factors but apparently utilizes CD81, SR-BI and occludin with increased efficiency. Importantly, adapted E1/E2 complexes mediate HCV cell entry into mouse cells in the absence of human entry factors. These results further our knowledge of HCV receptor interactions and indicate that three glycoprotein mutations are sufficient to overcome the species-specific restriction of HCV cell entry into mouse cells. Moreover, these findings should contribute to the development of an immunocompetent small animal model fully permissive to HCV.

The hepatitis C virus (HCV) infects only humans and chimpanzees, which has hampered development of suitable animal models. The inability of HCV to penetrate non-human cells is primarily due to inefficient usage of non-human CD81 and occludin. In this study we adapted HCV to mouse CD81. Efficient utilization of mouse CD81 is conferred by a combination of three mutations in the viral glycoproteins. These changes also permit entry via rat or hamster CD81, and lower viral dependence on additional HCV entry factors. Strikingly, mouse CD81 adapted HCV glycoproteins mediate entry into mouse cells in the absence of human entry factors. The adaptive mutations are not resident in viral domains implicated in direct CD81 binding. Nevertheless, they enhance binding to human CD81, increase susceptibility to different neutralizing antibodies and facilitate induction of viral cell fusion by low pH. This suggests that structural changes accompanied by exposure of the CD81 binding site and neutralizing epitopes have “unlocked” the viral envelope protein complex facilitating infection through non-human entry factors. These results highlight mechanisms of HCV receptor usage and tropism. They also demonstrate that HCV can be adapted to using non-human host factors, which may ultimately facilitate the development of small animal models.

Protein mediators of sterol transport across intestinal brush border membrane

Dysregulation of cholesterol balance contributes significantly to atherosclerotic cardiovascular disease (ASCVD), the leading cause of death in the United States. The intestine has the unique capability to act as a gatekeeper for entry of cholesterol into the body, and inhibition of intestinal cholesterol absorption is now widely regarded as an attractive non-statin therapeutic strategy for ASCVD prevention. In this chapter we discuss the current state of knowledge regarding sterol transport across the intestinal brush border membrane. The purpose of this work is to summarize substantial progress made in the last decade in regards to protein-mediated sterol trafficking, and to discuss this in the context of human disease.

rs5888 variant of SCARB1 gene is a possible susceptibility factor for age-related macular degeneration

Major genetic factors for age-related macular degeneration (AMD) have recently been identified as susceptibility risk factors, including variants in the CFH gene and the ARMS2 LOC387715/HTRA1locus. Our purpose was to perform a case-control study in two populations among individuals who did not carry risk variants for CFHY402H and LOC387715 A69S (ARMS2), called “study” individuals, in order to identify new genetic risk factors. Based on a candidate gene approach, we analyzed SNP rs5888 of the SCARB1 gene, coding for SRBI, which is involved in the lipid and lutein pathways. This study was conducted in a French series of 1241 AMD patients and 297 controls, and in a North American series of 1257 patients with advanced AMD and 1732 controls. Among these individuals, we identified 61 French patients, 77 French controls, 85 North American patients and 338 North American controls who did not carry the CFH nor ARMS2 polymorphisms. An association between AMD and the SCARB1 gene was seen among the study subjects. The genotypic distribution of the rs5888 polymorphism was significantly different between cases and controls in the French population (p<0.006). Heterozygosity at the rs5888 SNP increased risk of AMD compared to the CC genotypes in the French study population (odds ratio (OR) = 3.5, CI95%: 1.4–8.9, p<0.01) and after pooling the 2 populations (OR = 2.9, 95% CI: 1.6–5.3, p<0.002). Subgroup analysis in exudative forms of AMD revealed a pooled OR of 3.6 for individuals heterozygous for rs5888 (95% CI: 1.7–7.6, p<0.0015). These results suggest the possible contribution of SCARB1, a new genetic factor in AMD, and implicate a role for cholesterol and antioxidant micronutrient (lutein and vitamin E) metabolism in AMD.

Macrophage reverse cholesterol transport in mice expressing ApoA-I Milano

OBJECTIVE

To compare the abilities of human wild-type apoA-I (WT apoA-I) and human apoA-I(Milano) (apoA-I(M)) to promote macrophage reverse cholesterol transport (RCT) in apoA-I-null mice infected with adeno-associated virus (AAV) expressing either WT apoA-I or apoA-I(M).

METHODS AND RESULTS

WT apoA-I- or apoA-I(M)-expressing mice were intraperitoneally injected with [H(3)]cholesterol-labeled J774 mouse macrophages. After 48 hours, no significant difference was detected in the amount of cholesterol removed from the macrophages and deposited in the feces via the RCT pathway between the WT apoA-I and apoA-I(M) groups. Analysis of the individual components of the RCT pathway demonstrated that the apoA-I(M)-expressing mice promoted ATP-binding cassette transporter A1 (ABCA1)-mediated cholesterol efflux as efficiently as WT apoA-I but that apoA-I(M) had a reduced ability to promote cholesterol esterification via lecithin cholesterol-acyltransferase (LCAT). This resulted in reduced cholesteryl ester (CE) and increased free cholesterol (FC) levels in the plasma of mice expressing apoA-I(M) compared to WT apoA-I. These differences did not affect the rate of delivery of labeled cholesterol to the liver via SR-BI-mediated selective uptake or its subsequent excretion in the feces.

CONCLUSIONS

Within the limits of the in vivo assay, WT apoA-I and apoA-I(M) are equally efficient at promoting macrophage RCT, suggesting that if apoA-I(M) is more atheroprotective than WT apoA-I it is not attributable to an enhancement of macrophage RCT.

Novel ENU-induced point mutation in scavenger receptor class B, member 1, results in liver specific loss of SCARB1 protein

Cardiovascular disease (CVD) is the largest cause of premature death in human populations throughout the world. Circulating plasma lipid levels, specifically high levels of LDL or low levels of HDL, are predictive of susceptibility to CVD. The scavenger receptor class B member 1 (SCARB1) is the primary receptor for the selective uptake of HDL cholesterol by liver and steroidogenic tissues. Hepatic SCARB1 influences plasma HDL-cholesterol levels and is vital for reverse cholesterol transport. Here we describe the mapping of a novel N-ethyl-N-nitrosourea (ENU) induced point mutation in the Scarb1 gene identified in a C57BL/6J background. The mutation is located in a highly conserved amino acid in the extracellular loop and leads to the conversion of an isoleucine to an asparagine (I179N). Homozygous mutant mice express normal Scarb1 mRNA levels and are fertile. SCARB1 protein levels are markedly reduced in liver (∼90%), but not in steroidogenic tissues. This leads to ∼70% increased plasma HDL levels due to reduced HDL cholesteryl ester selective uptake. Pdzk1 knockout mice have liver-specific reduction of SCARB1 protein as does this mutant; however, in vitro analysis of the mutation indicates that the regulation of SCARB1 protein in this mutant is independent of PDZK1. This new Scarb1 model may help further our understanding of post-translational and tissue-specific regulation of SCARB1 that may aid the important clinical goal of raising functional HDL.

Anti-atherogenic actions of high-density lipoprotein through sphingosine 1-phosphate receptors and scavenger receptor class B type I

Plasma high-density lipoprotein (HDL) is a potent anti-atherogenic factor, a critical role of which is thought to be reverse cholesterol transport through the lipoprotein-associated apolipoprotein A-I (apoA-I). HDL also carries a potent bioactive lipid mediator, sphingosine 1-phophate (S1P), which exerts diverse physiological and pathophysiological actions in a variety of biological systems, including the cardiovascular system. In addition, HDL-associated apoA-I is known to stimulate intracellular signaling pathways unrelated to transporter activity. Mounting evidence indicates that multiple antiatherogenic or anti-inflammatory actions of HDL independent of cholesterol metabolism are mediated by the lipoprotein-associated S1P through S1P receptors and by apoA-I through scavenger receptor class B type I.

Effects of oleic acid and macrophage recruitment on cholesterol efflux in cell culture and in vivo

BACKGROUND AND AIM

Monounsaturated fatty acids in diets are beneficial for the plasma lipoprotein profile, but studies in cell culture point out that they may also be detrimental by inhibiting cholesterol efflux to apo AI.

METHODS AND RESULTS

In the present study we used mouse peritoneal macrophages, loaded with cholesterol and upregulated by cyclic AMP or by LXR/RXR ligands and compared the effect of oleic acid on cholesterol efflux to 3 different acceptors. Inhibition of cholesterol efflux by oleic acid ranged from 10 to 25% with HDL or 2.5% mouse serum, while efflux to phosphatidyl choline vesicles was not affected. Previously we reported that the LXR ligand, TO901317, retarded cholesterol removal in vivo from a modified LDL depot in muscle. This could have resulted from inhibition by unsaturated fatty acids or from reduction in macrophage recruitment due to the anti-inflammatory action of LXR.

CONCLUSIONS

Our current findings, of retardation of cholesterol clearance from the depot in the presence of low macrophage recruitment, support the latter possibility.

Hepatitis C virus entry into host cells

The recent development of functional models to analyze the early steps of the hepatitis C virus (HCV) life cycle has highlighted that HCV entry is a slow and complex multistep process involving the presence of several entry factors. Initial host cell attachment may involve glycosaminoglycans and the low-density lipoprotein receptor, after which the particle appears to interact sequentially with three entry factors: the scavenger receptor class B type I, the tetraspanin CD81 and the tight-junction protein claudin-1. Several serum components may also modulate HCV entry, while the recently discovered CD81 partner EWI-2wint can block the interaction of the viral particle with CD81, potentially preventing infection in the cell types in which it is expressed. After binding to the host cell, the HCV particle is internalized by clathrin-mediated endocytosis, with fusion likely occurring in early endosomes. This review summarizes our current knowledge on HCV entry.

[Advancement of the study of the mechanisms of cells invades by HCV]

Hepatitis C virus may be an important human pathogen, it may infect human associated with hepatocytes invaded by HCV. The construction features of HCV and its receptor candidates and the mechanisms of cells invaded by HCV were reviewed.

Functional involvement of scavenger receptor class B, type I, in the uptake of alpha-tocopherol using cultured rat retinal capillary endothelial cells

PURPOSE

Alpha-Tocopherol is an essential micronutrient acting as an antioxidant in the retina. However, the molecular mechanism of its retinal uptake from the circulating blood remains to be determined. The purpose of this study was to elucidate the contribution of scavenger receptor class B, type I (SR-BI), to the uptake of high-density lipoprotein (HDL)-associated alpha-tocopherol (alpha-tocopherol-HDL) using a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2 cells), as an in vitro inner blood-retinal barrier model.

METHODS

An uptake study of alpha-tocopherol-HDL was performed using TR-iBRB2 cells. The expression of SR-BI protein was determined by immunoblot and immunohistochemical analyses. RNA interference was done to clarify the relationship between SR-BI protein expression and the uptake of alpha-tocopherol-HDL by TR-iBRB2 cells.

RESULTS

[(14)C]alpha-tocopherol-HDL uptake by TR-iBRB2 cells exhibited a time-dependent increase and a temperature-dependence with an 88% reduction for 90 min at 4 degrees C compared with that at 37 degrees C. The uptake of [(14)C]alpha-tocopherol-HDL was inhibited by BLT-1, a specific inhibitor of the SR-BI-mediated lipid transfer between HDL and cells, in a concentration-dependent manner with an IC(50) of 23.2 nM. SR-BI protein expression was detected in TR-iBRB2 cells and SR-BI immunostaining was observed along the rat retinal capillaries. Inhibition of SR-BI protein expression by SR-BI siRNA resulted in a 24.4% reduction in [(14)C]alpha-tocopherol-HDL uptake.

CONCLUSIONS

Our findings strongly suggest that SR-BI at the inner blood-retinal barrier is responsible for alpha-tocopherol uptake from the circulating blood and plays a key role in maintaining alpha-tocopherol in the neural retina.

The roles of different pathways in the release of cholesterol from macrophages

Cholesterol efflux occurs by different pathways, including transport mediated by specific proteins. We determined the effect of enriching cells with free cholesterol (FC) on the release of FC to human serum. Loading Fu5AH cells with FC had no effect on fractional efflux, whereas enriching mouse peritoneal macrophages (MPMs) resulted in a doubling of fractional efflux. Efflux from cholesterol-normal MPM and Fu5AH cells to 15 human sera correlated well with HDL parameters. However, these relationships were reduced or lost with cholesterol-loaded MPMs. Using macrophages from scavenger receptor class B type I (SR-BI)-, ABCA1-, and ABCG1-knockout mice, together with inhibitors of SR-BI- and ABCA1-mediated efflux, we were able to quantitate efflux upon loading macrophages with excess cholesterol and to establish the contributions of the various efflux pathways in cholesterol-normal and -enriched cells. The removal of ABCA1 had essentially no effect on the total efflux when cell cholesterol levels were normal. However, in cholesterol-enriched cells, the removal of ABCA1 reduced efflux by 50%. Approximately 20% of the efflux stimulated by FC-loading MPM is attributable to ABCG1. The SR-BI contribution to efflux was small. Another pathway that is present in all cells is aqueous diffusion. Our studies demonstrate that this mechanism is one of the major contributors to efflux, particularly in cholesterol-normal cells.

Age-related influence of the HDL receptor SR-BI on synaptic plasticity and cognition

Dysregulated cholesterol metabolism is a major risk factor for atherosclerosis and other late-onset disorders, such as Alzheimer's disease. The scavenger receptor, class B, type I (SR-BI) is critical in maintaining the homeostasis of cholesterol and alpha-tocopherol. SR-BI binds high-density lipoproteins (HDL) and mediates the selective transfer of cholesteryl esters and alpha-tocopherol from circulating HDL to cells. SR-BI is also involved in reverse cholesterol transport from peripheral tissues into the liver. Previous studies using SR-BI genetic knockout mice indicated that the deletion of SR-BI resulted in an accelerated onset of atherosclerosis. We hypothesized that SR-BI-dependent lipid dysregulation might disrupt brain function leading to cognitive impairment. Here, we report that very old SR-BI knockout mice show deficient synaptic plasticity (long-term potentiation) in the CA1 region of the hippocampus. Very old SR-BI KO mice also display selective impairments in recognition memory and spatial memory. Thus, SR-BI influences neural and cognitive processes, a finding that highlights the contribution of cholesterol and alpha-tocopherol homeostasis in proper cognitive function.

Role of High-Density Lipoprotein and Scavenger Receptor B Type I in the Promotion of Endothelial Repair

There is considerable experimental evidence that high-density lipoprotein (HDL) cholesterol and the principal high-affinity HDL receptor scavenger receptor B type I (SR-BI) afford cardiovascular protection. However, the fundamental mechanisms underlying the protection remain complex and not well understood. Recent work in cell culture indicates that the HDL-SR-BI tandem stimulates endothelial cell migration. Further studies have revealed that this entails Src-mediated, phosphatidylinositol 3-kinase-mediated, and mitogen-activated protein kinase-mediated signaling that leads to the activation of Rac guanosine triphosphate hydrolase and the resultant rearrangement of the actin cytoskeleton. Furthermore, assessment of reendothelialization after perivascular electric injury in mice indicates that HDL-SR-BI-mediated stimulation of endothelial migration is operative in vivo. Recent additional work in mice also indicates that HDL activates the recruitment of endothelial progenitor cells into the intimal layer in the setting of endothelial injury. As such, signaling initiated by HDL-SR-BI promotes endothelial repair, and this novel mechanism of action may be critically involved in the impact of the lipoprotein on vascular health and disease.

Intestinal absorption of dietary carotenoids

The assessment of carotenoid bioavailability has long been hampered by the limited knowledge of their absorption mechanisms. However, recent reports have elucidated important aspects of carotenoid digestion and absorption. Disruption of food matrix and increasing amounts of fat seem to enhance the absorption of carotenes to a larger extent than that of xanthophylls. Comparing different carotenoid species, xanthophylls seem to be more easily released from the food matrix and more efficiently micellized than the carotenes. On the other hand, carotenes are more efficiently taken up by the enterocytes. However, carotenoid emulsification and micellization steps are largely affected by the food matrix and dietary components, being the main determinant of carotenoid bioavailability from foodstuffs. Although the intestinal uptake of carotenoids has been thought to occur by simple diffusion, recent studies reported the existence of receptor-mediated transport of carotenoids in enterocytes. Comparisons between the intestinal absorption of a wide array of carotenoids would be useful to elucidate the absorption mechanism of each carotenoid species, in view of the recent indications that intestinal carotenoid uptake may involve the scavenger receptor class B type I and possibly other epithelial transporters. The unraveling of the whole mechanism underlying the absorption of carotenoids will be the challenge for future studies.

Effects of amino acid substitutions at glycine 420 on SR-BI cholesterol transport function

Scavenger receptor class B type I (SR-BI) facilitates the uptake of HDL cholesteryl esters (CEs) in a two-step process involving binding of HDL to its extracellular domain and transfer of HDL core CEs to a metabolically active membrane pool, where they are subsequently hydrolyzed by a neutral CE hydrolase. Recently, we characterized a mutant, G420H, which replaced glycine 420 in the extracellular domain of SR-BI with a histidine residue and had a profound effect on SR-BI function. The G420H mutant receptor exhibited a reduced ability to mediate selective HDL CE uptake and was unable to deliver HDL CE for hydrolysis, despite the fact that it retained the ability to bind HDL. This did not hold true if glycine 420 was replaced with an alanine residue; G420A maintained wild-type HDL binding and cholesterol transport activity. To further understand the role that glycine 420 plays in SR-BI function and why there was a disparity between replacing glycine 420 with a histidine versus an alanine, we generated a battery of point mutants by substituting glycine 420 with amino acids possessing side chains that were charged, hydrophobic, polar, or bulky and tested the resulting mutants for their ability to support HDL binding, HDL cholesterol transport, and delivery for hydrolysis. The results indicated that substitution with a negatively charged residue or a proline impaired cell surface expression of SR-BI or its interaction with HDL, respectively. Furthermore, substitution of glycine 420 with a positively charged residue reduced HDL CE uptake as well as its subsequent hydrolysis.

The low-density lipoprotein receptor plays a role in the infection of primary human hepatocytes by hepatitis C virus

BACKGROUND/AIMS

The direct implication of low-density lipoprotein receptor (LDLR) in hepatitis C virus (HCV) infection of human hepatocyte has not been demonstrated. Normal primary human hepatocytes infected by serum HCV were used to document this point.

METHODS

Expression and activity of LDLR were assessed by RT-PCR and LDL entry, in the absence or presence of squalestatin or 25-hydroxycholesterol that up- or down-regulates LDLR expression, respectively. Infection was performed in the absence or presence of LDL, HDL, recombinant soluble LDLR peptides encompassing full-length (r-shLDLR4-292) or truncated (r-shLDLR4-166) LDL-binding domain, monoclonal antibodies against r-shLDLR4-292, squalestatin or 25-hydroxycholesterol. Intracellular amounts of replicative and genomic HCV RNA strands used as end point of infection were assessed by RT-PCR.

RESULTS

r-shLDLR4-292, antibodies against r-shLDLR4-292 and LDL inhibited viral RNA accumulation, irrespective of genotype, viral load or liver donor. Inhibition was greatest when r-shLDLR4-292 was present at the time of inoculation and gradually decreased as the delay between inoculation and r-shLDLR4-292 treatment increased. In hepatocytes pre-treated with squalestatin or 25-hydroxycholesterol before infection, viral RNA accumulation increased or decreased in parallel with LDLR mRNA expression and LDL entry.

CONCLUSIONS

LDLR is involved at an early stage in infection of normal human hepatocytes by serum-derived HCV virions.

Serum amyloid A has antiviral activity against hepatitis C virus by inhibiting virus entry in a cell culture system

Serum amyloid A (SAA) is an acute phase protein produced by the liver. SAA concentration increases markedly in the serum following inflammation and infection. Large increases in SAA concentration during the acute phase response suggest that SAA has a beneficial role in host defense. This study sought to determine the effect of SAA on hepatitis C virus (HCV) infectivity using retroviral particles pseudotyped with HCV envelope glycoproteins (HCVpp) and the recently developed cell culture system for HCV (HCVcc). SAA inhibited HCVpp and HCVcc infection in a dose-dependent manner by affecting an early step of the virus life cycle. Further characterization with HCVpp indicated that SAA blocks virus entry by interacting with the viral particle. In addition, the antiviral activity of SAA was strongly reduced when high-density lipoproteins (HDL) were coincubated with SAA. However, HDL had only a slight effect on the antiviral activity of SAA when HCVpp was first preincubated with SAA. Furthermore, analyses of SAA in sera of chronic HCV patients revealed the presence of variable levels of SAA with abnormally elevated concentrations in some cases. However, no obvious clinical correlation was found between SAA levels and HCV viral loads. In conclusion, our data demonstrate an antiviral activity for SAA and suggest a tight relationship between SAA and HDL in modulating HCV infectivity.

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.

Characterization of scavenger receptor class B, type I in Atlantic salmon (Salmo salar L.)

The scavenger receptor class B, type I (SR-BI) is an important player in regulation of mammalian lipid homeostasis. We therefore wanted to study this receptor in Atlantic salmon (Salmo salar L.), which requires a diet with particular high lipid content. We have for the first time cloned and characterized SR-BI from a salmonid fish. The predicted 494 amino acid protein contained two transmembrane domains, several putative N-glycosylation sites, and showed 72% sequence identity with the predicted homolog from zebrafish. SR-BI expression was analyzed by reverse transcription Real-Time PCR in several tissues, and a high relative expression in salmon midgut was detected, which may suggest that SR-BI has a role in uptake of lipids from the diet. We also expressed a construct of salmon myc-tagged SR-BI in salmon TO cells and HeLa cells, which gave a protein of approximately 80 kDa on reducing SDS-PAGE using an antibody against the myc-epitope. Immunofluorescence microscopy analyses of the salmon SR-BI protein in transiently transfected HeLa cells revealed staining in the cell periphery and in some intracellular membranes, but not in the nucleus, which indicated that the salmon protein may be a functional membrane protein. We also observed a high degree of co-localization using an anti-peptide SR-BI antiserum. We found that 20 microg mL(-1) insulin up-regulated the SR-BI mRNA levels in primary cultures of salmon hepatocytes relative to untreated cells. Oleic acid, EPA, DHA, or dexamethasone did not affect the relative expression of SR-BI in this liver model system. In conclusion, the salmon SR-BI cDNA encoded a protein with several features common to those of mammalian species. SR-BI gene expression was high in the intestine, which leads us to propose that SR-BI may contribute to the uptake of lipids from the diet.

High-density lipoproteins reduce the neutralizing effect of hepatitis C virus (HCV)-infected patient antibodies by promoting HCV entry

The neutralizing activity of anti-hepatitis C virus (HCV) antibodies is attenuated by a factor present in human sera, which has been proposed to be high-density lipoproteins (HDLs). HDLs have also been shown to facilitate the entry of HCV pseudoparticles (HCVpp) into target cells. Here, the aim of the study was to determine whether HDL-mediated facilitation of HCVpp and infectious HCV (HCVcc) entry and attenuation of neutralization are two related phenomena. The data indicated that HDLs attenuate neutralization at a constant rate. In addition, as for HDL-mediated facilitation of HCVpp entry, attenuation of neutralization depended on the expression of the scavenger receptor BI (SR-BI) and its selective lipid-uptake function. Finally, kinetic experiments showed that HDL-mediated facilitation of HCVpp entry is more rapid than virus neutralization. Altogether, these observations indicate that HCV is exploiting the physiological activity of SR-BI for promoting its entry into target cells, which consequently also protects the virus against neutralizing antibodies.

Continued inhibition of atherosclerotic lesion development in long term Western diet fed CD36o /apoEo mice

We previously determined that absence of CD36 inhibited atherosclerosis lesion development in 12-week Western diet fed apoE degrees mice, and this was due largely to absence of macrophage CD36. It is possible that at later stages of disease this effect would be lost due to the progressive nature of lesion development and involvement of other factors. However, lesion development continues to be characterized by recruitment of macrophages and foam cell formation, thus it is also possible that delay in lipid accumulation as a result of absence of CD36 would continue to retard lesion development. The objective of this study was to determine if absence of CD36 continued to inhibit lesion formation. Background matched apoE degrees and CD36 degrees /apoE degrees mice were fed a Western diet for up to 35 weeks. At 20 and 35 weeks, lesion area was 25 and 35% less, respectively, in CD36 degrees /apoE degrees mice. Most impressive was the difference in gross appearance of the aortas at 35 weeks: apoE degrees aortas were sclerotic and nearly occluded by lesion, whereas aortas from CD36 degrees /apoE degrees mice had smaller lesions that were more punctate. We conclude that absence of CD36 continues to reduce lesion burden even at late stages of disease in the apoE degrees model.

Quantitative analysis of SR-BI-dependent HDL retroendocytosis in hepatocytes and fibroblasts

Previous studies have suggested that HDL retroendocytosis may play a role in scavenger receptor class B type I (SR-BI)-dependent selective lipid uptake in a cell-specific manner. To investigate this possibility, we developed methods to quantitatively measure HDL uptake and resecretion in fibroblast (COS-7) and hepatocyte (HepG2) cells expressing exogenous SR-BI. Approximately 17% and 24% of HDL associated in an SR-BI-dependent manner with COS-7 and HepG2 cells, respectively, accumulates intracellularly after a 10 min incubation. To determine whether this intracellular HDL undergoes retroendocytosis, we developed a pulse-chase assay whereby internalized biotinylated (125)I-HDL(3) secreted from cells is quantitatively precipitated from cell supernatants using immobilized streptavidin. Our results show a rapid secretion of a portion of intracellular HDL from both cell types (representing 4-7% of the total cell-associated HDL) that is almost complete within 30 min (half-life approximately 10 min). In COS-7 cells, the calculated rate of HDL secretion ( approximately 0.5 ng HDL/mg/min) was >30-fold slower than the rate of SR-BI-dependent selective cholesteryl ester (CE) uptake ( approximately 17 ng HDL/mg/min), whereas the rate of release of HDL from the cell surface ( approximately 19 ng HDL/mg/min) was similar to the rate of selective CE uptake. Notably, the rate of SR-BI-dependent HDL resecretion in COS-7 and HepG2 cells was similar. BLT1, a compound that inhibits selective CE uptake, does not alter the amount of SR-BI-mediated HDL retroendocytosis in COS-7 cells. From these data, we conclude that HDL retroendocytosis in COS-7 and HepG2 cells is similar and that the vast majority of SR-BI-dependent selective uptake occurs at the cell surface in both cell types.

Effects of hepatic expression of the high-density lipoprotein receptor SR-BI on lipoprotein metabolism and female fertility

The etiology of human female infertility is often uncertain. The sterility of high-density lipoprotein (HDL) receptor-negative (SR-BI(-/-)) female mice suggests a link between female infertility and abnormal lipoprotein metabolism. SR-BI(-/-) mice exhibit elevated plasma total cholesterol [with normal-sized and abnormally large HDL and high unesterified to total plasma cholesterol (UC:TC) ratio]. We explored the influence of hepatic SR-BI on female fertility by inducing hepatic SR-BI expression in SR-BI(-/-) animals by adenovirus transduction or stable transgenesis. For transgenes, we used both wild-type SR-BI and a double-point mutant, Q402R/Q418R (SR-BI-RR), which is unable to bind to and mediate lipid transfer from wild-type HDL normally, but retains virtually normal lipid transport activities with low-density lipoprotein. Essentially wild-type levels of hepatic SR-BI expression in SR-BI(-/-) mice restored to nearly normal the HDL size distribution and plasma UC:TC ratio, whereas approximately 7- to 40-fold overexpression dramatically lowered plasma TC and increased biliary cholesterol secretion. In contrast, SR-BI-RR overexpression had little effect on SR-BI(+/+) mice, but in SR-BI(-/-) mice, it substantially reduced levels of abnormally large HDL and normalized the UC:TC ratio. In all cases, hepatic transgenic expression restored female fertility. Overexpression in SR-BI(-/-) mice of lecithin:cholesterol acyl transferase, which esterifies plasma HDL cholesterol, did not normalize the UC:TC ratio, probably because the abnormal HDL was a poor substrate, and did not restore fertility. Thus, hepatic SR-BI-mediated lipoprotein metabolism influences murine female fertility, raising the possibility that dyslipidemia might contribute to human female infertility and that targeting lipoprotein metabolism might complement current assisted reproductive technologies.

High-density lipoprotein is a potential growth factor for adrenocortical cells

The entry of cholesterol contained within high-density lipoprotein (HDL) into adrenocortical cells is mediated by a human homologue of SR-BI, CD36, and LIMPII Analogous-1 (CLA-1) and thus augmenting their growth. To address the role of CLA-1, we created a mutant mCLA that lacked the C-terminal tail. HDL CE selective uptake by cells carrying the mCLA-1 receptor was fully active and equivalent to those transfected with full-length CLA-1 (fCLA-1). Expression of mCLA inhibited the proliferation of an adrenocortical cell line and the incorporation of [(3)H]thymidine into the cells. This effect was sensitive to wortmannin, an inhibitor of phosphoinositide 3-kinase (PI3K). Our transcriptional studies revealed that the inhibitory action of mCLA required the transcriptional factor AP-1 and the effect of HDL on AP-1 activation was also abrogated by wortmannin. These findings raise the possibility that the inhibitors of the effects of HDL may be of therapeutic value for adrenocortical tumor.

Cholesterol substrate pools and steroid hormone levels are normal in the face of mutational inactivation of NPC1 protein

Mutational inactivation of NPC1 largely blocks the movement of LDL-derived cholesterol from the lysosome to the metabolically active, cytosolic pool of sterol that is the substrate for steroid hormone production. Such a block might, in theory, lead to deficiencies in circulating levels of testosterone, progesterone, and corticosterone. However, there are at least two other sources for cellular cholesterol, de novo synthesis and scavenger receptor class B type I-mediated uptake of HDL cholesteryl ester (CE). In this study, we measured the rates of net cholesterol acquisition by these three pathways in the adrenal, ovary, and testis. In all three organs, the majority (81-98%) of cholesterol acquisition came from the selective uptake of CE from HDL and de novo synthesis. Furthermore, in the npc1(-/-)mouse, the cytosolic storage pool of CE in a tissue such as the adrenal remained constant (approximately 25 mg/g). As a result of these alternative pathways, the plasma concentrations of testosterone (3.5 vs. 2.5 ng/ml), progesterone (8.5 vs. 6.7 ng/ml), and corticosterone (391 vs. 134 ng/ml) were either the same or elevated in the npc1(-/-)mouse, compared with the control animal. Thus, impairment of cholesterol acquisition through the NPC1-dependent, clathrin-coated pit pathway did not limit the availability of cholesterol substrate for steroid hormone synthesis in the steroidogenic cells.

SR-BI mediates cholesterol efflux via its interactions with lipid-bound ApoE. Structural mutations in SR-BI diminish cholesterol efflux

Apolipoprotein E (apoE) and the lipoprotein receptor SR-BI play critical roles in lipid and lipoprotein metabolism. We have examined the cholesterol efflux from wild-type (WT) and mutant forms of SR-BI expressed in ldlA-7 cells using reconstituted discoidal particles consisting of apoE, 1-palmitoyl-2-oleoyl-l-phospatidylcholine (POPC), and cholesterol (C) as acceptors. POPC/C-apoE particles generated using apoE2, apoE3, apoE4, or carboxy-terminally truncated forms apoE4-165, apoE4-202, apoE4-229, and apoE4-259 caused similar (20-25%) cholesterol efflux from WT SR-BI. Cholesterol efflux mediated by POPC/C-apoE was not enhanced in the presence of lipid-free apoE. The rate of cholesterol efflux mediated by particles containing the WT or carboxy-terminally truncated forms of apoE was decreased to approximately 30% of the WT control with the Q402R/Q418R mutant SR-BI form that is unable to bind native HDL normally but binds LDL. The rate of cholesterol efflux was further decreased to approximately 7% of the WT control with another SR-BI mutant (M158R) that binds neither HDL nor LDL. The level of binding of POPC/C-apoE particles (150 microg/mL) to SR-BI mutant forms Q402R/Q418R and M158R was 70 and 8% of the WT control, respectively. SR-BI-dependent binding of lipid-free apoE to cells was undetectable, and cholesterol efflux was less than 0.5%. The findings establish that only lipid-bound apoE promotes SR-BI-mediated cholesterol efflux and that the amino-terminal region of residues 1-165 of apoE is sufficient for both receptor binding and cholesterol efflux. The SR-BI-apoE interactions may contribute to overall cholesterol homeostasis in cells and tissues that express SR-BI and apoE.

The role of cyclic AMP response element binding protein in transactivation of scavenger receptor class B type I promoter in transfected cells and in primary cultures of rat theca-interstitial cells

In the ovary, lutropin (LH) stimulates the selective uptake and transport of cholesterol for steroid biosynthesis from HDL particles via the scavenger receptor class B type I (SR-BI). Furthermore the expression of SR-BI mRNA in the ovary is stimulated by LH and cyclic AMP (cAMP). Since the promoter of the rat SR-BI gene is devoid of consensus cyclic AMP response element (CRE) sequences, this study examined if cAMP response element binding protein (CREB) plays a role in the transactivation of SR-BI promoter (SR-BIpr). The transactivation of SR-BIpr was examined in transfected 293T cells and human granulosa SVOG-4o cells, and in primary cultures of rat theca-interstitial cells infected with adenoviral constructs containing the SR-BIpr and a luciferase reporter gene. Dose-related increases in SR-BRpr activity ranging from 2- to 4-fold was induces by 293T cells co-transfected with the catalytic subunit of protein kinase A (cPKA). Co-transfections with CREB and cPKA produced a concentration-dependent increase ranging from 6- to 32-fold. The cAMP-mediated transactivation was significantly attenuated by co-transfection with CREB M1, a non-phosphorylatable, dominant-negative form of CREB. An increase in transactivation of SR-BIpr activity was also seen in SVOG-4o cells co-transfected with CREB. In primary cultures of rat theca-interstitial (T-I) cells infected with an adenoviral construct of SR-BIpr, forskolin produced a marked increase in promoter activity. These data indicate that stimulation of the cAMP-PKA-CREB pathway enhances rat SR-BIpr activity and substantiate the role of CREB as an intermediary in this process. The absence of canonical CRE sequences in the rat SR-BIpr suggests that the activation of SR-BI by CREB may occur either through non-canonical CRE sequences or through additional transcription factors that cooperate with CREB in the activation of SR-BI promoter activity.

High-density lipoprotein promotes endothelial cell migration and reendothelialization via scavenger receptor-B type I

Vascular disease risk is inversely related to circulating levels of high-density lipoprotein (HDL) cholesterol. However, the mechanisms by which HDL provides vascular protection are unclear. The disruption of endothelial monolayer integrity is an important contributing factor in multiple vascular disorders, and vascular lesion severity is tempered by enhanced endothelial repair. Here, we show that HDL stimulates endothelial cell migration in vitro in a nitric oxide-independent manner via scavenger receptor B type I (SR-BI)-mediated activation of Rac GTPase. This process does not require HDL cargo molecules, and it is dependent on the activation of Src kinases, phosphatidylinositol 3-kinase, and p44/42 mitogen-activated protein kinases. Rapid initial stimulation of lamellipodia formation by HDL via SR-BI, Src kinases, and Rac is also demonstrable. Paralleling the in vitro findings, carotid artery reendothelialization after perivascular electric injury is blunted in apolipoprotein A-I(-/-) mice, and reconstitution of apolipoprotein A-I expression rescues normal reendothelialization. Furthermore, reendothelialization is impaired in SR-BI(-/-) mice. Thus, HDL stimulates endothelial cell migration via SR-BI-initiated signaling, and these mechanisms promote endothelial monolayer integrity in vivo.

Human scavenger receptor class B type I is expressed with cell-specific fashion in both initial and terminal site of reverse cholesterol transport

The reverse cholesterol transport (RCT) is one of the major protective systems against atherosclerosis, in which high-density lipoprotein (HDL) removes cholesterol from lipid-laden cells and delivers it to the liver. Scavenger receptor class B type I (SR-BI) is a HDL receptor in the liver and adrenal glands and is involved in the selective uptake of cholesteryl ester from HDL, which has been extensively, analyzed using rodent models. However, the expression and regulation of the human homologue of this receptor are not known yet. We previously reported that this receptor is expressed in in vitro differentiated macrophages and its expression is up-regulated by the addition of modified lipoproteins into the medium [Hirano K, Yamashita S, Nakagawa Y, et al. Expression of human scavenger receptor class B type I in cultured human monocyte-derived macrophages and atherosclerotic lesions. Circ Res 1999;85:108-16]. In order to further investigate the physiological significance of this receptor in humans, we have performed extensive immunohistochemical analyses with specimens of the liver and adrenal glands as well as arteries with different stages of atherosclerotic lesions. In human liver and adrenal glands, a positive SR-BI immunoreactivity was detected in both hepatic and adrenal parenchymal cells as well as Kupffer cells. These parenchymal cells had a strong signal on the cell surface, whereas Kupffer cells showed a heterogeneous and punctate pattern. In human aorta and coronary arteries, SR-BI was highly expressed in atherosclerotic plaques, but not in non-atherosclerotic lesions. Double immunostaining revealed that SR-BI was expressed in a subpopulation of macrophages, of which staining pattern was similar to that observed in Kupffer cells. These data clearly demonstrated that SR-BI was expressed with cell-specific fashions in both the initial and terminal step of RCT in humans. Thus, SR-BI might be physiologically relevant and have distinct tissue-specific functions.

Serum Amyloid A Promotes Cholesterol Efflux Mediated by Scavenger Receptor B-I

Serum amyloid A (SAA) is an acute phase protein whose expression is markedly up-regulated during inflammation and infection. The physiological function of SAA is unclear. In this study, we reported that SAA promotes cellular cholesterol efflux mediated by scavenger receptor B-I (SR-BI). In Chinese hamster ovary cells, SAA promoted cellular cholesterol efflux in an SR-BI-dependent manner, whereas apoA-I did not. Similarly, SAA, but not apoA-I, promoted cholesterol efflux from HepG2 cells in an SR-BI-dependent manner as shown by using the SR-BI inhibitor BLT-1. When SAA was overexpressed in HepG2 cells using adenovirus-mediated gene transfer, the endogenously expressed SAA promoted SR-BI-dependent efflux. To assess the effect of SAA on SR-BI-mediated efflux to high density lipoprotein (HDL), we compared normal HDL, acute phase HDL (AP-HDL, prepared from mice injected with lipopolysaccharide), and AdSAA-HDL (HDL prepared from mice overexpressing SAA). Both AP-HDL and AdSAA-HDL promoted 2-fold greater cholesterol efflux than normal HDL. Lipid-free SAA was shown to also stimulate ABCA1-dependent cholesterol efflux in fibroblasts, in line with an earlier report (Stonik, J. A., Remaley, A. T., Demosky, S. J., Neufeld, E. B., Bocharov, A., and Brewer, H. B. (2004) Biochem. Biophys. Res. Commun. 321, 936-941). When added to cells together, SAA and HDL exerted a synergistic effect in promoting ABCA1-dependent efflux, suggesting that SAA may remodel HDL in a manner that releases apoA-I or other efficient ABCA1 ligands from HDL. SAA also facilitated efflux by a process that was independent of SR-BI and ABCA1. We conclude that the acute phase protein SAA plays an important role in HDL cholesterol metabolism by promoting cellular cholesterol efflux through a number of different efflux pathways.

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.