The cellular biology of scavenger receptor class B type I

The expression of hSR-B1 on platelets of patients with coronary artery disease (CAD)

INTRODUCTION

The human scavenger receptor class B type 1 (hSR-B1), which serves as a high affinity receptor for HDL, is expressed on platelet surface and mediates various anti-atherogenic functions. Based on the anti-thrombotic effect of HDL and the importance of HDL-SR-B1 in the formation of atherosclerotic plaque, the present study was aimed to investigate and compare the expression level of hSR-B1on platelets of CAD patients with that of normal controls.

METHODS

The expression of the hSR-B1 on platelets of 31 CAD patients with atherosclerotic plaque and 20 healthy controls were detected using flowcytometry and western blotting. Moreover, platelet function in response to the agonists was examined by aggregometry, and the lipid panel tests were assayed using chemistry autoanalyzer.

RESULTS

Our findings showed that the expression of hSR-B1 was significantly reduced on the surface of platelets from CAD patients with atherosclerotic disease, as compared with healthy controls (6/8% vs. 13/6%) (P < 0,001). Of particular of interest, we also found that the formation of aggregates after stimulation of the platelets with ADP was higher in patients with atherosclerotic disease than the controls; indicating an inverse relationship between hSR-B1 expression and the function of human platelets.

CONCLUSION

Taken together, the results of the present study raise the possibility that the measurement of hSR-B1 expression on human platelets may provide a valuable insight that reflects the status of RCT in patients with atherosclerosis.

Scavenger receptor B type 1: expression, molecular regulation, and cholesterol transport function

Cholesterol is required for maintenance of plasma membrane fluidity and integrity and for many cellular functions. Cellular cholesterol can be obtained from lipoproteins in a selective pathway of HDL-cholesteryl ester (CE) uptake without parallel apolipoprotein uptake. Scavenger receptor B type 1 (SR-B1) is a cell surface HDL receptor that mediates HDL-CE uptake. It is most abundantly expressed in liver, where it provides cholesterol for bile acid synthesis, and in steroidogenic tissues, where it delivers cholesterol needed for storage or steroidogenesis in rodents. SR-B1 transcription is regulated by trophic hormones in the adrenal gland, ovary, and testis; in the liver and elsewhere, SR-B1 is subject to posttranscriptional and posttranslational regulation. SR-B1 operates in several metabolic processes and contributes to pathogenesis of atherosclerosis, inflammation, hepatitis C virus infection, and other conditions. Here, we summarize characteristics of the selective uptake pathway and involvement of microvillar channels as facilitators of selective HDL-CE uptake. We also present the potential mechanisms of SR-B1-mediated selective cholesterol transport; the transcriptional, posttranscriptional, and posttranslational regulation of SR-B1; and the impact of gene variants on expression and function of human SR-B1. A better understanding of this unique pathway and SR-B1's role may yield improved therapies for a wide variety of conditions.

Reconstituted high-density lipoproteins: novel biomimetic nanocarriers for drug delivery

High-density lipoproteins (HDL) are naturally-occurring nanoparticles that are biocompatible, non-immunogenic and completely biodegradable. These endogenous particles can circulate for an extended period of time and transport lipids, proteins and microRNA from donor cells to recipient cells. Based on their intrinsic targeting properties, HDL are regarded as promising drug delivery systems. In order to produce on a large scale and to avoid blood borne pollution, reconstituted high-density lipoproteins (rHDL) possessing the biological properties of HDL have been developed. This review summarizes the biological properties and biomedical applications of rHDL as drug delivery platforms. It focuses on the emerging approaches that have been developed for the generation of biomimetic nanoparticles rHDL to overcome the biological barriers to drug delivery, aiming to provide an alternative, promising avenue for efficient targeting transport of nanomedicine.

Debris buster is a Drosophila scavenger receptor essential for airway physiology

Scavenger receptors class B (SR-B) are multifunctional transmembrane proteins, which in vertebrates participate in lipid transport, pathogen clearance, lysosomal delivery and intracellular sorting. Drosophila has 14 SR-B members whose functions are still largely unknown. Here, we reveal a novel role for the SR-B family member Debris buster (Dsb) in Drosophila airway physiology. Larvae lacking dsb show yeast avoidance behavior, hypoxia, and severe growth defects associated with impaired elongation and integrity along the airways. Furthermore, in dsb mutant embryos, the barrier function of the posterior spiracles, which are critical for gas exchange, is not properly established and liquid clearance is locally impaired at the spiracular lumen. We found that Dsb is specifically expressed in a group of distal epithelial cells of the posterior spiracle organ and not throughout the entire airways. Furthermore, tissue-specific knockdown and rescue experiments demonstrate that Dsb function in the airways is only required in the posterior spiracles. Dsb localizes in intracellular vesicles, and a subset of these associate with lysosomes. However, we found that depletion of proteins involved in vesicular transport to the apical membrane, but not in lysosomal function, causes dsb-like airway elongation defects. We propose a model in which Dsb sorts components of the apical extracellular matrix which are essential for airway physiology. Since SR-B LIMP2-deficient mice show reduced expression of several apical plasma membrane proteins, sorting of proteins to the apical membrane is likely an evolutionary conserved function of Dsb and LIMP2. Our data provide insights into a spatially confined function of the SR-B Dsb in intracellular trafficking critical for the physiology of the whole tubular airway network.

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

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

A Novel Anti-Inflammatory Effect for High Density Lipoprotein

High density lipoprotein has anti-inflammatory effects in addition to mediating reverse cholesterol transport. While many of the chronic anti-inflammatory effects of high density lipoprotein (HDL) are attributed to changes in cell adhesion molecules, little is known about acute signal transduction events elicited by HDL in endothelial cells. We now show that high density lipoprotein decreases endothelial cell exocytosis, the first step in leukocyte trafficking. ApoA-I, a major apolipoprotein of HDL, mediates inhibition of endothelial cell exocytosis by interacting with endothelial scavenger receptor-BI which triggers an intracellular protective signaling cascade involving protein kinase C (PKC). Other apolipoproteins within the HDL particle have only modest effects upon endothelial exocytosis. Using a human primary culture of endothelial cells and murine apo-AI knockout mice, we show that apo-AI prevents endothelial cell exocytosis which limits leukocyte recruitment. These data suggest that high density lipoprotein may inhibit diseases associated with vascular inflammation in part by blocking endothelial exocytosis.

High scavenger receptor class B type I expression is related to tumor aggressiveness and poor prognosis in breast cancer

Scavenger receptor class B type I (SR-BI) has been linked to the development and progression of breast cancer. However, its clinical significance in breast cancer remains unclear. Here, we evaluated SR-BI expression in a well-characterized breast cancer tissue microarray by immunohistochemistry. High SR-BI expression was observed in 54 % of all breast cancer cases and was significantly associated with advanced pTNM stage (P = 0.002), larger tumor size (P = 0.023), lymph node metastasis (P = 0.012), and the absence of ER (P = 0.014). The Kaplan-Meier survival analysis revealed that patients with high SR-BI expression had significantly shorter overall survival (OS) (P = 0.004). Moreover, multivariate analysis with adjustment for other prognostic factors confirmed that SR-BI was an independent prognostic factor for patient outcome (P = 0.017). Overall, our study demonstrated that high SR-BI expression was related to conventional parameters indicative of more aggressive tumor type and may serve as a new prognostic marker for poor clinical outcome in human breast cancer.

Kimchi methanol extract and the kimchi active compound, 3'-(4'-hydroxyl-3',5'-dimethoxyphenyl)propionic acid, downregulate CD36 in THP-1 macrophages stimulated by oxLDL

Macrophage foam cell formation by oxidized low-density lipoprotein (oxLDL) is a key step in the progression of atherosclerosis, which is involved in cholesterol influx and efflux in macrophages mediated by related proteins such as peroxisome proliferator-activated receptor γ (PPARγ), CD36, PPARα, liver-X receptor α (LXRα), and ATP-binding cassette transporter A1 (ABCA1). The aim of this study was to investigate the beneficial effects of kimchi methanol extract (KME) and a kimchi active compound, 3-(4'-hydroxyl-3',5'-dimethoxyphenyl)propionic acid (HDMPPA) on cholesterol flux in THP-1-derived macrophages treated with oxLDL. The effects of KME and HDMPPA on cell viability and lipid peroxidation were determined. Furthermore, the protein expression of PPARγ, CD36, PPARα, LXRα, and ABCA1 was examined. OxLDL strongly induced cell death and lipid peroxidation in THP-1-derived macrophages. However, KME and HDMPPA significantly improved cell viability and inhibited lipid peroxidation induced by oxLDL in THP-1-derived macrophages (P<.05). Moreover, KME and HDMPPA suppressed CD36 and PPARγ expressions, both of which participate in cholesterol influx. In contrast, KME and HDMPPA augmented LXRα, PPARα, and ABCA1 expression, which are associated with cholesterol efflux. Consequently, KME and HDMPPA suppressed lipid accumulation. These results indicate that KME and HDMPPA may inhibit lipid accumulation, in part, by regulating cholesterol influx- and efflux-related proteins. These findings will thus be useful for future prevention strategies against atherosclerosis.

Neutralizing antibody-resistant hepatitis C virus cell-to-cell transmission

Hepatitis C virus (HCV) can initiate infection by cell-free particle and cell-cell contact-dependent transmission. In this study we use a novel infectious coculture system to examine these alternative modes of infection. Cell-to-cell transmission is relatively resistant to anti-HCV glycoprotein monoclonal antibodies and polyclonal immunoglobulin isolated from infected individuals, providing an effective strategy for escaping host humoral immune responses. Chimeric viruses expressing the structural proteins representing the seven major HCV genotypes demonstrate neutralizing antibody-resistant cell-to-cell transmission. HCV entry is a multistep process involving numerous receptors. In this study we demonstrate that, in contrast to earlier reports, CD81 and the tight-junction components claudin-1 and occludin are all essential for both cell-free and cell-to-cell viral transmission. However, scavenger receptor BI (SR-BI) has a more prominent role in cell-to-cell transmission of the virus, with SR-BI-specific antibodies and small-molecule inhibitors showing preferential inhibition of this infection route. These observations highlight the importance of targeting host cell receptors, in particular SR-BI, to control viral infection and spread in the liver.

Biliary cholesterol secretion: More than a simple ABC

Biliary cholesterol secretion is a process important for 2 major disease complexes, atherosclerotic cardiovascular disease and cholesterol gallstone disease. With respect to cardiovascular disease, biliary cholesterol secretion is regarded as the final step for the elimination of cholesterol originating from cholesterol-laden macrophage foam cells in the vessel wall in a pathway named reverse cholesterol transport. On the other hand, cholesterol hypersecretion into the bile is considered the main pathophysiological determinant of cholesterol gallstone formation. This review summarizes current knowledge on the origins of cholesterol secreted into the bile as well as the relevant processes and transporters involved. Next to the established ATP-binding cassette (ABC) transporters mediating the biliary secretion of bile acids (ABCB11), phospholipids (ABCB4) and cholesterol (ABCG5/G8), special attention is given to emerging proteins that modulate or mediate biliary cholesterol secretion. In this regard, the potential impact of the phosphatidylserine flippase ATPase class I type 8B member 1, the Niemann Pick C1-like protein 1 that mediates cholesterol absorption and the high density lipoprotein cholesterol uptake receptor, scavenger receptor class B type I, is discussed.

The hepatitis C virus and its hepatic environment: a toxic but finely tuned partnership

Twenty years after its discovery, HCV (hepatitis C virus) still infects 170 million people worldwide and cannot be properly treated due to the lack of efficient medication. Its life cycle must be better understood to develop targeted pharmacological arsenals. HCV is an enveloped virus bearing two surface glycoproteins, E1 and E2. It only infects humans through blood transmission, and hepatocytes are its only target cells. Hepatic trabeculae are formed by hepatocyte rows surrounded by sinusoid capillaries, irrigating hepatic cells. Hepatocytes are polarized and have basolateral and apical poles, separated by tight junctions in contact with blood and bile respectively. In blood, HCV remains in contact with lipoproteins. It then navigates through hepatic microenvironment and extracellular matrix, composed of glycosaminoglycans and proteins. HCV then encounters the hepatocyte basolateral membrane, where it interacts with its entry factors: the low-density lipoprotein receptor, CD81 tetraspanin, and the high-density lipoprotein (scavenger) receptor SR-BI (scavenger receptor BI). How these molecules interact with HCV remains unclear; however, a tentative sequence of events has been proposed. Two essential factors of HCV entry are the tight junction proteins claudin-1 and occludin. Cell polarity therefore seems to be a key for HCV entry. This raises several exciting questions on the HCV internalization pathway. Clathrin-dependent endocytosis is probably the route of HCV transport to intracellular compartments, and the ultimate step of its entry is fusion, which probably takes place within endosomes. The mechanisms of HCV membrane fusion are still unclear, notably the nature of the fusion proteins is unknown and the contribution of HCV-associated lipoproteins to this event is currently under investigation.

Hepatoma cell density promotes claudin-1 and scavenger receptor BI expression and hepatitis C virus internalization

Hepatitis C virus (HCV) entry occurs via a pH- and clathrin-dependent endocytic pathway and requires a number of cellular factors, including CD81, the tight-junction proteins claudin 1 (CLDN1) and occludin, and scavenger receptor class B member I (SR-BI). HCV tropism is restricted to the liver, where hepatocytes are tightly packed. Here, we demonstrate that SR-BI and CLDN1 expression is modulated in confluent human hepatoma cells, with both receptors being enriched at cell-cell junctions. Cellular contact increased HCV pseudoparticle (HCVpp) and HCV particle (HCVcc) infection and accelerated the internalization of cell-bound HCVcc, suggesting that the cell contact modulation of receptor levels may facilitate the assembly of receptor complexes required for virus internalization. CLDN1 overexpression in subconfluent cells was unable to recapitulate this effect, whereas increased SR-BI expression enhanced HCVpp entry and HCVcc internalization, demonstrating a rate-limiting role for SR-BI in HCV internalization.

A role for hepatic scavenger receptor class B, type I in decreasing high density lipoprotein levels in mice that lack phosphatidylethanolamine N-methyltransferase

Phosphatidylethanolamine N-methyltransferase (PEMT) is a liver-specific enzyme that converts phosphatidylethanolamine to phosphatidylcholine (PC). Mice that lack PEMT have reduced plasma levels of PC and cholesterol in high density lipoproteins (HDL). We have investigated the mechanism responsible for this reduction with experiments designed to distinguish between a decreased formation of HDL particles by hepatocytes or an increased hepatic uptake of HDL lipids. Therefore, we analyzed lipid efflux to apoA-I and HDL lipid uptake using primary cultured hepatocytes isolated from Pemt(+/+) and Pemt(-/-) mice. Hepatic levels of the ATP-binding cassette transporter A1 are not significantly different between Pemt genotypes. Moreover, hepatocytes isolated from Pemt(-/-) mice released cholesterol and PC into the medium as efficiently as did hepatocytes from Pemt(+/+) mice. Immunoblotting of liver homogenates showed a 1.5-fold increase in the amount of the scavenger receptor, class B, type 1 (SR-BI) in Pemt(-/-) compared with Pemt(+/+) livers. In addition, there was a 1.5-fold increase in the SR-BI-interacting protein PDZK1. Lipid uptake experiments using radiolabeled HDL particles revealed a greater uptake of [(3)H]cholesteryl ethers and [(3)H]PC by hepatocytes derived from Pemt(-/-) compared with Pemt(+/+) mice. Furthermore, we observed an increased association of [(3)H]cholesteryl ethers in livers of Pemt(-/-) compared with Pemt(+/+) mice after tail vein injection of [(3)H]HDL. These results strongly suggest that PEMT is involved in the regulation of plasma HDL levels in mice, mainly via HDL lipid uptake by SR-BI.

Inconsistencies in the genetic prediction of HDL cholesterol versus atherosclerosis

PURPOSE OF REVIEW

High-density lipoprotein (HDL) is generally perceived as having a protective role with respect to cardiovascular disease. The metabolism of HDL is mediated through a complex network of apoproteins, enzymes and transfer proteins. Genetic variants within this network can increase plasma HDL, but not with uniformly beneficial clinical outcomes. The purpose of this review is to explore and propose mechanisms for these discrepant observations.

RECENT FINDINGS

Recent developments in this area include new observations of genetic variants that paradoxically increase both HDL and cardiovascular risk. Also discussed are newly observed, function-altering modifications of the HDL particle. Proposed explanations include the segregation of the genetic variants associated with the respective endpoints of plasma HDL and cardiovascular risk. Functionally impaired but quantitatively robust plasma HDL and the emerging understanding of proinflammatory HDL also may contribute to our understanding of discordant observations.

SUMMARY

Enhanced understanding of these relationships may allow a more accurate assessment of clinical risk based on plasma HDL and help explain why HDL may, in some circumstances, be an inappropriate therapeutic target.

NPC1L1 (Niemann-Pick C1-like 1) mediates sterol-specific unidirectional transport of non-esterified cholesterol in McArdle-RH7777 hepatoma cells

Recent evidence suggests that NPC1L1 (Niemann-Pick C1-like 1) is critical for intestinal sterol absorption in mice, yet mechanisms by which NPC1L1 regulates cellular sterol transport are lacking. In the study we used a McArdle-RH7777 rat hepatoma cell line stably expressing NPC1L1 to examine the sterol-specificity and directionality of NPC1L1-mediated sterol transport. As previously described, cholesterol-depletion-driven recycling of NPC1L1 to the cell surface facilitates cellular uptake of non-esterified (free) cholesterol. However, it has no impact on the uptake of esterified cholesterol, indicating free sterol specificity. Interestingly, the endocytic recycling of NPC1L1 was also without effect on beta-sitosterol uptake, indicating that NPC1L1 can differentiate between free sterols of animal and plant origin in hepatoma cells. Furthermore, NPC1L1-driven free cholesterol transport was unidirectional, since cellular cholesterol efflux to apolipoprotein A-I, high-density lipoprotein or serum was unaffected by NPC1L1 expression or localization. Additionally, NPC1L1 facilitates mass non-esterified-cholesterol uptake only when it is located on the cell surface and not when it resides intracellularly. Finally, NPC1L1-dependent cholesterol uptake required adequate intracellular K(+), yet did not rely on intracellular Ca(2+), the cytoskeleton or signalling downstream of protein kinase A, protein kinase C or pertussis-toxin-sensitive G-protein-coupled receptors. Collectively, these findings support the notion that NPC1L1 can selectively recognize non-esterified cholesterol and promote its unidirectional transport into hepatoma cells.

Apolipoprotein A-I stimulates AMP-activated protein kinase and improves glucose metabolism

AIMS/HYPOTHESIS

In humans, one of the hallmarks of type 2 diabetes is a reduced plasma concentration of HDL and its major protein component, apolipoprotein A-I (APOA-I). However, it is unknown whether APOA-I directly protects against diabetes. The aim of this study was to characterise the functional role of APOA-I in glucose homeostasis.

METHODS

The effects of APOA-I on phosphorylation of AMP-activated protein kinase (AMPK) and acetyl-coenzyme A carboxylase (ACC), glucose uptake and endocytosis were analysed in C2C12 myocytes. Glucose metabolism was investigated in Apoa-I knockout (Apoa-I (-/-)) mice.

RESULTS

APOA-I was able to stimulate the phosphorylation of AMPK and ACC, and elevated glucose uptake in C2C12 myocytes. APOA-I could be endocytosed into C2C12 myotubes through a clathrin-dependent endocytotic process. Inhibition of endocytosis abrogated APOA-I-stimulated AMPK phosphorylation. In Apoa-I (-/-) mice, AMPK phosphorylation was reduced in skeletal muscle and liver, and expression of gluconeogenic enzymes was increased in liver. In addition, the Apoa-I (-/-) mice had increased fat content and compromised glucose tolerance.

CONCLUSIONS/INTERPRETATION

Our data indicate that APOA-I has a protective effect against diabetes via activation of AMPK. ApoA-I deletion in the mouse led to increased fat mass and impaired glucose tolerance.

Stimulation of lipolysis enhances the rate of cholesterol efflux to HDL in adipocytes

Adipose tissue constitutes a major location for cholesterol storage and, as such, it may play a role in the regulation of circulating cholesterol levels. A possible metabolic link between the lipolytic activity of adipocytes and their ability to release cholesterol to reconstituted human high density lipoprotein, HDL, was investigated in 3T3-L1 adipocytes. In the presence of HDL, composed of human apoA-I and phosphatidylcholine, adipocytes release cholesterol in a lipoprotein-dose and time dependent fashion. beta-adrenergic activation of the lipolysis promotes a 22% increase in the extent of cholesterol efflux to reconstituted discoidal HDL particles. Activation of lipolysis promotes a rapid decrease in the cholesterol content of the plasma membrane and a concomitant increase in lipid droplet cholesterol. This change is independent of the presence of HDL. Activation of the lipolysis does not affect the levels of ABCA1 and SR-BI. Therefore, the enhancement of cholesterol efflux is not due to the level of plasma membrane cholesterol, or to the levels of the cholesterol transporters ABCA1 and scavenger receptor SR-BI. Brefeldin A did not affect the rate of cholesterol efflux under basal lipolytic conditions, but it abolished the lipolysis-dependent enhancement of cholesterol efflux to HDL. This study suggests that activation of lipolysis is accompanied by an increase in BFA-sensitive vesicular transport that in turn enhances cholesterol efflux to HDL. The study supports a metabolic link between the lipolytic activity of adipocytes and the rate of cellular cholesterol efflux to HDL.

Enhanced scavenging of lipid substances is a possible effect of corticosteroids in the treatment of cholesterol crystal embolism

Cholesterol crystal embolism (CCE) is a systemic refractory disease especially prevalent amongst elderly patients suffering from atherosclerosis. Treatment of this condition remains controversial due to difficulties in diagnosis. Corticosteroid therapy may be an important treatment option despite its elusive mechanisms. To clarify the role of corticosteroid in CCE therapy, we collected the samples from six autopsied subjects with CCE, three of whom were clinically given various doses of corticosteroid to investigate stable atherosclerosis-related substances, advanced glycation end-products (AGE), and several AGE receptors such as scavenger receptor class B type 1 (SR-B1), receptor for AGE (RAGE), and galectin-3 in the liver tissues and atherosclerotic areas by immunostaining using a tissue macro-array technique. An intense expression of AGE and its receptors was identified in the enlarged Kupffer cells of CCE cases, which were given relatively high doses of corticosteroid. In addition, numerous mononuclear cells in the intimal atheromatous plaque presented strong expressions of AGE and SR-B1. In conclusion, we speculated that corticosteroid treatment for CCE may upregulate the activations, including phagocytic capacity of Kupffer cells mediated by overexpression of RAGE and scavenger receptors, resulting in efficient clearance of the lipid substances from the blood circulation released from atherosclerotic areas.

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.

Defective insulin receptor activation and altered lipid rafts in Niemann-Pick type C disease hepatocytes

Niemann-Pick type C (NPC) disease is a neuro-visceral cholesterol storage disorder caused by mutations in the NPC-1 or NPC-2 gene. In the present paper, we studied IR (insulin receptor) activation and the plasma-membrane lipid assembly in primary hepatocytes from control and NPC1-/- mice. We have previously reported that, in hepatocytes, IR activation is dependent on cholesterol-sphingolipid rafts [Vainio, Heino, Mansson, Fredman, Kuismanen, Vaarala and Ikonen (2002) EMBO Rep. 3, 95-100]. We found that, in NPC hepatocytes, IR levels were up-regulated and the receptor activation was compromised. Defective IR activation was reproduced in isolated NPC plasma-membrane preparations, which displayed an increased cholesterol content and saturation of major phospholipids. The NPC plasma membranes were less fluid than control membranes as indicated by increased DPH (1,6-diphenyl-1,3,5-hexatriene) fluorescence anisotropy values. Both in NPC hepatocytes and plasma-membrane fractions, the association of IR with low-density DRMs (detergent-resistant membranes) was increased. Moreover, the detergent resistance of both cholesterol and phosphatidylcholine were increased in NPC membranes. Finally, cholesterol removal inhibited IR activation in control membranes but restored IR activation in NPC membranes. Taken together, the results reveal a lipid imbalance in the NPC hepatocyte, which increases lipid ordering in the plasma membrane, alters the properties of lipid rafts and interferes with the function of a raft-associated plasma-membrane receptor. Such a mechanism may participate in the pathogenesis of NPC disease and contribute to insulin resistance in other disorders of lipid metabolism.

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.

Regulation of the selective uptake of cholesteryl esters from high density lipoproteins by sphingomyelin

Although sphingomyelin (SM) is a major phospholipid in lipoproteins as well as in the membrane rafts where the scavenger receptor class B type I (SR-BI) is localized, its possible role in the selective uptake of cholesteryl ester (CE) by the SR-BI-mediated pathway is unknown. We investigated the effect of SM in lipoproteins and cell membranes on the selective uptake in three different cell lines: SR-BI-transfected CHO cells, hepatocytes (HepG2), and adrenocortical cells (Y1BS1). Incorporation of SM into recombinant high density lipoprotein (rHDL) containing labeled CE resulted in up to 50% inhibition of the selective uptake of CE in all three cell lines. This inhibition was completely reversed by treatment of rHDL with sphingomyelinase (SMase). Selective uptake from plasma HDL was activated by 22-72% after treatment of HDL with SMase. In addition, pretreatment of the cells with SMase resulted in stimulation of CE uptake from rHDL by CHO and Y1BS1, although not by HepG2. Incorporation of ceramide into rHDL resulted in up to 2-fold stimulation of CE uptake, although pretreatment of cells with egg ceramide had no significant effect. These results show that SM and ceramide in the lipoproteins and the cell membranes regulate the SR-BI-mediated selective uptake of CE, possibly by interacting with the sterol ring or with SR-BI itself.

ATP-binding cassette transporter A1: a cell cholesterol exporter that protects against cardiovascular disease

Blood high-density lipoprotein (HDL) levels are inversely related to risk for cardiovascular disease, implying that factors associated with HDL metabolism are atheroprotective. One of these factors is ATP-binding cassette transporter A1 (ABCA1), a cell membrane protein that mediates the transport of cholesterol, phospholipids, and other metabolites from cells to lipid-depleted HDL apolipoproteins. ABCA1 transcription is highly induced by sterols, a major substrate for cellular export, and its expression and activity are regulated posttranscriptionally by diverse processes. Liver ABCA1 initiates formation of HDL particles, and macrophage ABCA1 protects arteries from developing atherosclerotic lesions. ABCA1 mutations can cause a severe HDL deficiency syndrome characterized by cholesterol deposition in tissue macrophages and prevalent atherosclerosis. Genetic manipulations of ABCA1 expression in mice also affect plasma HDL levels and atherogenesis. Metabolites elevated in individuals with the metabolic syndrome and diabetes destabilize ABCA1 protein and decrease cholesterol export from macrophages. Moreover, oxidative modifications of HDL found in patients with cardiovascular disease reduce the ability of apolipoproteins to remove cellular cholesterol by the ABCA1 pathway. These observations raise the possibility that an impaired ABCA1 pathway contributes to the enhanced atherogenesis associated with common inflammatory and metabolic disorders. The ABCA1 pathway has therefore become an important new therapeutic target for treating cardiovascular disease.

Analysis of the binding of hepatitis C virus genotype 1a and 1b E2 glycoproteins to peripheral blood mononuclear cell subsets

Hepatitis C virus (HCV) binding to hepatocytes is thought to be mediated via interaction of the E2 glycoprotein with (co-)receptors including CD81 and scavenger receptor class B type I (SR-BI). Here, the expression of CD81 and SR-BI was analysed on peripheral blood mononuclear cell (PBMC) subsets, and the binding of genotype 1 soluble truncated E2 (sE2) proteins to these cells was investigated. All PBMC subsets expressed CD81, although at varying levels. In contrast, SR-BI was only detected on monocytes and dendritic cells (DCs). The genotype 1a H77c sE2 protein showed higher PBMC binding than other genotype 1a/b sE2s. H77c sE2 binding to different PBMC subsets largely paralleled their level of CD81 expression, and could be inhibited by blocking E2–CD81 interaction. However, those PBMC subsets reported to be infected by HCV in vivo (monocytes, DCs and B cells) also exhibited residual, CD81-independent binding, indicating roles for SR-BI/other receptor(s) in mediating haematopoietic cell infection.

CD36 overexpression in ritonavir-treated THP-1 cells is reversed by alpha-tocopherol

Therapies with antiretroviral protease inhibitors (ARPI) are correlated with a higher risk for dyslipidemia, hypercholesterolemia, and atherosclerosis. The original aim of this study was to establish whether alpha-tocopherol can reduce CD36 scavenger receptor overexpression occurring after treatment of monocytes with the ARPI ritonavir. We show here that treatment of THP-1 monocytes with ritonavir increases total protein and surface expression of CD36; however, only weak changes are observed at the mRNA level, suggesting that CD36 overexpression occurs mainly at the posttranscriptional level. Concentrations of ritonavir that upregulate CD36 expression inhibit proteasome activity in THP-1 cells, indicating a possible regulatory role of the proteasome in CD36 overexpression. Similar to ritonavir, the proteasome inhibitor ALLN increases the CD36 surface expression on THP-1 cells. alpha-Tocopherol efficiently normalizes CD36 protein overexpression after ritonavir treatment and reduces oxLDL uptake. Furthermore, in THP-1 monocytes, alpha-tocopherol reverses the proteasome activity inhibited by ritonavir. This study indicates that an increased CD36 protein expression in THP-1 monocytes induced by ritonavir can be normalized by alpha-tocopherol. CD36 overexpression is caused by inhibition of proteasome activity by ritonavir, which is efficiently restored by alpha-tocopherol.

Lipoproteins, macrophage function, and atherosclerosis: beyond the foam cell?

Atherogenesis requires and is highly influenced by the interaction between lipoproteins and macrophages. Most of the focus to date has been on the ability of atherogenic lipoproteins (such as low-density lipoproteins, LDL) to promote and of anti-atherogenic lipoproteins (such as high-density lipoproteins, HDL) to prevent the development of the cholesteryl ester-enriched macrophage-derived foam cell. However, lipoprotein-macrophage interactions have the potential to modulate macrophage function in a variety of additional ways that may impact on atherosclerosis. These include modulating cellular cholesterol and oxysterol content, providing fatty acids as ligands for PPARs, and acting as ligands for macrophage scavenger and Toll-like receptors. We suggest that atherogenic lipoproteins promote and anti-atherogenic lipoproteins inhibit atherogenesis by modulating macrophage function in a variety of ways beyond cholesteryl ester accumulation and foam cell formation.

Hypolipidemic action of the SERM acolbifene is associated with decreased liver MTP and increased SR-BI and LDL receptors

This study aimed to identify the mechanisms of the hypolipidemic action of the selective estrogen receptor modulator (SERM) acolbifene (ACOL). Four weeks of treatment with ACOL reduced fasting and postprandial plasma triglycerides (TGs), an effect associated with lower VLDL-TG secretion rate (-25%), and decreased mRNA of microsomal triglyceride transfer protein (MTP; -29%). ACOL increased liver TG concentration (+100%) and amplified the feeding-induced increase in the master lipogenic regulators sterol-regulatory element binding protein-1a (SREBP-1a) and SREBP-1c. ACOL decreased total, HDL, and non-HDL cholesterol (CHOL) by 50%. SREBP-2 mRNA and HMG-CoA reductase activity were minimally affected by ACOL. However, in the fasted state, liver concentration of scavenger receptor class B type I (SR-BI) protein, but not mRNA, was 3-fold higher in ACOL-treated than in control animals and correlated with plasma HDL-CHOL levels (r = 0.80, P < 0.002). Liver LDL receptor (LDLR) protein, but not mRNA, was increased 2-fold by ACOL, independently of the nutritional status. This study demonstrates that ACOL possesses the unique ability among SERMs to reduce VLDL-TG secretion, likely by reducing MTP expression, and strongly suggests that the robust hypocholesterolemic action of ACOL is related to increased removal of CHOL from the circulation as a consequence of enhanced liver SR-BI and LDLR abundance.

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.

Selective uptake of high density lipoproteins cholesteryl ester in the dog, a species lacking in cholesteryl ester transfer protein activity

Amongst the processes involved in the reverse cholesterol transport (RCT) from organs to liver, including high density lipoproteins-apolipoprotein AI (HDL-apoAI) dependent tissue uptake and cholesteryl ester transfer protein (CETP)-mediated transfers, the selective uptake of cholesteryl ester (CE) is of increasing interest through its antiatherogenic role. The purpose of this report is to develop a simple protocol allowing study of this process in an animal model with easier quantification of CE selective uptake. The dog was chosen essentially because this animal has a low CETP activity and an appropriate size to conduce a kinetic study. Tracer kinetics were performed to estimate in vivo the contributions of the pathways involved in HDL-CE turnover in dogs. Stable isotopes, 13C-acetate and D3-leucine as labeled precursors of CE and apoAI, were infused to fasting dogs. Isotopic enrichments were monitored in plasma unesterified cholesterol and in HDL-CE and apoAI by mass spectrometry. Kinetics were analyzed using compartmental modeling. Results concerned the measurement of the activity of cholesterol esterification (0.13+/-0.032 h(-1)), rate of HDL-apoAI catabolism (0.024+/-0.012 h(-1)), HDL-CE turnover (0.062+/-0.010 h(-1)) and CE selective uptake (0.038+/-0.014 h(-1)). Our results show that CE in dogs is mainly eliminated by selective uptake of HDL-CE (60% of HDL-CE turnover), unlike in other species studied by similar methods in our laboratory. This study shows that among species used to analyze cholesterol metabolism, the dog appears to be the animal in whom HDL-CE selective uptake represents the largest part of HDL-CE turnover.

Changes in plasma membrane properties and phosphatidylcholine subspecies of insect Sf9 cells due to expression of scavenger receptor class B, type I, and CD36

In mammalian cells scavenger receptor class B, type I (SR-BI), mediates the selective uptake of high density lipoprotein (HDL) cholesteryl ester into hepatic and steroidogenic cells. In addition, SR-BI has a variety of effects on plasma membrane properties including stimulation of the bidirectional flux of free cholesterol (FC) between cells and HDL and changes in the organization of plasma membrane FC as indicated by increased susceptibility to exogenous cholesterol oxidase. Recent studies in SR-BI-deficient mice and in SR-BI-expressing Sf9 insect cells showed that SR-BI has significant effects on plasma membrane ultrastructure. The present study was designed to test the range of SR-BI effects in Sf9 insect cells that typically have very low cholesterol content and a different phospholipid profile compared with mammalian cells. The results showed that, as in mammalian cells, SR-BI expression increased HDL cholesteryl ester selective uptake, cellular cholesterol mass, FC efflux to HDL, and the sensitivity of membrane FC to cholesterol oxidase. These activities were diminished or absent upon expression of the related scavenger receptor CD36. Thus, SR-BI has fundamental effects on cholesterol flux and membrane properties that occur in cells of evolutionarily divergent origins. Profiling of phospholipid species by electrospray ionization mass spectrometry showed that scavenger receptor expression led to the accumulation of phosphatidylcholine species with longer mono- or polyunsaturated acyl chains. These changes would be expected to decrease phosphatidylcholine/cholesterol interactions and thereby enhance cholesterol desorption from the membrane. Scavenger receptor-mediated changes in membrane phosphatidylcholine may contribute to the increased flux of cholesterol and other lipids elicited by these receptors.

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.

Scavenger receptor BI: a scavenger receptor with a mission to transport high density lipoprotein lipids

PURPOSE OF REVIEW

This review will survey recent findings on the cholesterol transport and scavenger functions of scavenger receptor BI. Although scavenger receptor BI and CD36 bind many of the same ligands, these two receptors have very specific lipid transport functions: CD36 facilitates the uptake of long chain fatty acids and SR-BI mediates the transport of cholesterol and cholesteryl ester from HDL particles. Scavenger receptor BI is a physiologically relevant HDL receptor that, along with HDL, is protective against cardiovascular disease. Its atheroprotective role has been hypothesized to be due to its function in the reverse cholesterol transport pathway.

RECENT FINDINGS

Recent studies suggest that scavenger receptor BI function is not only crucial for cholesterol delivery to the liver but is also important for cholesterol efflux at the vessel wall. Therefore, the receptor acts at both ends of the reverse cholesterol transport pathway. In addition, it stimulates nitric oxide production in endothelial cells, which may also contribute to its positive influence on the vasculature. Lastly, the glycoprotein was cloned as a scavenger receptor and in some cases is still thought to operate in this fashion.

SUMMARY

It will be interesting to follow future research on scavenger receptor BI that will delineate its functions in cholesterol transport as well as its scavenger functions. Additionally, we are only beginning to learn of the glycoprotein's effects on disease states besides atherosclerosis and cardiovascular disease.

Cholesterol depletion induces PKA-mediated basolateral-to-apical transcytosis of the scavenger receptor class B type I in MDCK cells

Cholesterol-based membrane microdomains, or lipid rafts, are believed to play important, yet poorly defined, roles in protein trafficking and signal transduction. In polarized epithelial cells, the current view is that rafts are involved in apical but not in basolateral protein transport from the trans-Golgi network (TGN). We report here that cholesterol is required in a post-TGN mechanism of basolateral regionalization. Permanently transfected Madin-Darby canine kidney cells segregated the caveolae/raft-associated high-density lipoprotein scavenger receptor class B type I (SR-BI) predominantly to the basolateral domain where it was constitutively internalized and recycled basolaterally. Acute cholesterol depletion did not significantly alter SR-BI internalization, implying a cholesterol depletion-insensitive endocytic process but instead induced its transcytosis through a protein kinase A (PKA)- and microtubule-dependent mechanism. Forskolin also elicited SR-BI transcytosis. The basolateral distribution of endogenous epidermal growth factor receptor remained unaffected. Strikingly, cholesterol depletion induced PKA activity without increasing the cAMP levels. Thus, our results are consistent with a scenario in which cholesterol-based rafts promote internalization and basolateral recycling of internalized SR-BI whereas a PKA pool sensitive to cholesterol depletion mediates SR-BI transcytosis. Regulated transcytosis of SR-BI may provide an additional mechanism to control cholesterol homeostasis. These results disclose relationships between cholesterol-based rafts and PKA activity operating in a post-TGN mechanism of regulated apical-to-basolateral cell surface protein distribution.

Dimerization of the scavenger receptor class B type I: formation, function, and localization in diverse cells and tissues

This study has examined the dimeric/oligomeric forms of scavenger receptor class B type I (SR-BI) and its alternatively spliced form, SR-BII, in a diverse group of cells and tissues: i.e., normal and hormonally altered tissues of mice and rats as well as tissues of transgenic animals and genetically altered steroidogenic and nonsteroidogenic cells overexpressing the SR-B proteins. Using both biochemical and morphological techniques, we have seen that these dimeric and higher order oligomeric forms of SR-BI expression are strongly associated with both functional and morphological expression of the selective HDL cholesteryl ester uptake pathway. Rats and mice show some species differences in expression of SR-BII dimeric forms; this difference does not extend to the use of SR-B cDNA types for transfection purposes. In a separate study, cotransfection of HEK293 cells with cMyc and V5 epitope-tagged SR-BI permitted coprecipitation and quantitative coimmunocytochemical measurements at the electron microscope level, suggesting that much of the newly expressed SR-BI protein in stimulated cells dimerizes and that the SR-BI dimers are localized to the cell surface and specifically to microvillar or double membraned intracellular channels. These combined data suggest that SR-BI self-association represents an integral step in the selective cholesteryl ester uptake process.

ABCA1-dependent lipid efflux to apolipoprotein A-I mediates HDL particle formation and decreases VLDL secretion from murine hepatocytes

High levels of expression of the ATP binding cassette transporter A1 (ABCA1) in the liver and the need to over- or underexpress hepatic ABCA1 to impact plasma HDL levels in mice suggest a major role of the liver in HDL formation and in determining circulating HDL levels. Cultured murine hepatocytes were used to examine the role of hepatic ABCA1 in mediating the lipidation of apolipoprotein A-I (apoA-I) for HDL particle formation. Exogenous apoA-I stimulated cholesterol efflux to the medium from wild-type hepatocytes, but not from ABCA1-deficient (abca1(-/-)) hepatocytes. ApoA-I induced the formation of new HDL particles and enhanced the lipidation of endogenously secreted murine apoA-I in ABCA1-expressing but not abca1(-/-) hepatocytes. ABCA1-dependent cholesterol mobilization to apoA-I increased new cholesterol synthesis, indicating depletion of the regulatory pool of hepatocyte cholesterol during HDL formation. Secretion of triacylglycerol and apoB was decreased following apoA-I incubation with ABCA1-expressing but not abca1(-/-) hepatocytes. These results support a major role for hepatocyte ABCA1 in generating a critical pool of HDL precursor particles that enhance further HDL generation and passive cholesterol mobilization in the periphery. The results also suggest that diversion of hepatocyte cholesterol into the "reverse" cholesterol transport pathway diminishes cholesterol availability for apoB-containing lipoprotein secretion by the liver.

Normal and pathological human testes express hormone-sensitive lipase and the lipid receptors CLA-1/SR-BI and CD36

Numerous studies have demonstrated the important role of cholesterol and cholesteryl esters in tumor cell proliferation and progression of cancer. However, few studies have focused on the role of lipid transporters and lipases in cancer development and progression. The present study examined the expression of hormone-sensitive lipase (HSL) and the scavenger receptors CLA-1/SR-BI and CD36 in normal human testis and in nontumor and tumor testicular disorders by immunohistochemistry and Western blotting analysis. In normal young testes, immunoreaction to CLA-1/SR-BI was found in the spermatid acrosomic vesicle and on the surface of Sertoli and Leydig cells. HSL was detected in spermatogonia, the Golgi region of spermatocytes, the nucleus of spermatids, and the cytoplasm of both Sertoli and Leydig cells. Elderly testes and testes with hypospermatogenesis showed a similar staining pattern to that of normal young testes except for CD36, which was expressed in Sertoli cells. Cryptorchid testes demonstrated intense labeling to HSL and weak labeling to SR-BI in Sertoli cells (nucleus and cytoplasm) and Leydig cells (cytoplasm). Seminiferous tubules with intratubular germ cell neoplasia exhibited intense immunolabeling to the 3 lipid receptors in the surface of neoplastic cells and to HSL in the nucleus. In seminoma and spermatocytic seminoma, neoplastic cells labeled to HSL but failed to stain with antilipid receptors; in the seminiferous tubules at the periphery of the tumour, Charcot-Böttcher crystalloids of Sertoli cells were strongly positive to CLA-1. Testes with mature teratoma showed a weak reaction to CD36 and SR-BI in some cells of enteric-type glands, and immature teratoma were exclusively immunolabeled with HSL. Western blotting analysis revealed that multiple bands were immunolabeled, with differences seen between normal and pathological testes. The results of this study indicate that the presence of lipid receptors (CLA-1/SR-BI) and hormone-sensitive lipase in Leydig cells suggests a role of these proteins in steroidogenesis. Also, these proteins seem to be involved in spermiogenesis, as their labeling in spermatids suggests. In nonmalignant and malignant pathologies, cholesterol metabolism is probably altered, and HSL labeling in neoplastic germ cell nuclei suggests a still-unknown function of this enzyme, probably related to cell cycle regulation.

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.

Scavenger receptor BI (SR-BI) clustered on microvillar extensions suggests that this plasma membrane domain is a way station for cholesterol trafficking between cells and high-density lipoprotein

Receptor-mediated trafficking of cholesterol between lipoproteins and cells is a fundamental biological process at the organismal and cellular levels. In contrast to the well-studied pathway of LDL receptor-mediated endocytosis, little is known about the trafficking of high-density lipoprotein (HDL) cholesterol by the HDL receptor, scavenger receptor BI (SR-BI). SR-BI mediates HDL cholesteryl ester uptake in a process in which HDL lipids are selectively transferred to the cell membrane without the uptake and degradation of the HDL particle. We report here the cell surface locale where the trafficking of HDL cholesterol occurs. Fluorescence confocal microscopy showed SR-BI in patches and small extensions of the cell surface that were distinct from sites of caveolin-1 expression. Electron microscopy showed SR-BI in patches or clusters primarily on microvillar extensions of the plasma membrane. The organization of SR-BI in this manner suggests that this microvillar domain is a way station for cholesterol trafficking between HDL and cells. The types of phospholipids in this domain are unknown, but SR-BI is not strongly associated with classical membrane rafts rich in detergent-resistant saturated phospholipids. We speculate that SR-BI is in a more fluid membrane domain that will favor rapid cholesterol flux between the membrane and HDL.

The role of drug transporters at the blood-brain barrier

The blood-brain barrier (BBB) is a dynamic interface between the blood and the brain. It eliminates (toxic) substances from the endothelial compartment and supplies the brain with nutrients and other (endogenous) compounds. It can be considered as an organ protecting the brain and regulating its homeostasis. Until now, many transport systems have been discovered that play an important role in maintaining BBB integrity and brain homeostasis. In this review, we focus on the role of carrier- and receptor-mediated transport systems (CMT, RMT) at the BBB. These include CMT systems, such as P-glycoprotein, multidrug-resistance proteins 1-7, nucleoside transporters, organic anion transporters, and large amino-acid transporters; RMT systems, such as the transferrin-1 and -2 receptors; and the scavenger receptors SB-AI and SB-BI.

Identification of developmentally regulated genes in the somatic cells of the mouse testis using serial analysis of gene expression

To identify genes developmentally regulated in the somatic cells of the testis, serial analysis of gene expression (SAGE) has been used to generate gene expression profiles from these cells in the fetal and adult mouse. To avoid germ cell transcripts, a fetal SAGE library was generated from germ cell-free fetal Wv/Wv mice, and an adult SAGE library was generated from adult testes depleted of germ cells with busulfan. The combined SAGE libraries contained 147570 tags identifying 12976 unique transcripts. Of these transcripts, 3607 were present in only the fetal library and 3941 were present in only the adult library. Most of the abundant differentially expressed tags in the adult testis library were from characterized genes, whereas 3' rapid amplification of complementary ends was required to identify most differentially expressed tags in the fetal library. These fetal tags were mostly associated with uncharacterized UniGene clusters. These data provide a comprehensive and quantitative analysis of gene expression in the somatic cells of the fetal and adult testis (including unknown transcripts) and identify genes differentially expressed in these cells during testis development. These differentially regulated genes are likely to provide insight into mechanisms regulating testis function both during development and in the adult animal.

Identification of uteroglobin-related protein 1 and macrophage scavenger receptor with collagenous structure as a lung-specific ligand-receptor pair

High in normal (HIN)-1 is a secreted protein highly expressed in normal breast epithelium and down-regulated in breast carcinomas. By searching GenBank expressed sequence tag databases, we identified HIN-2, a protein homologous to HIN-1. HIN-2 is identical with a recently identified protein called uteroglobin-related protein 1 (UGRP1). Northern blot analysis demonstrated that UGRP1 is specifically expressed by lung, but not by the other tissues examined. By in situ hybridization experiments, UGRP1 was shown to be expressed by lung Clara-like cells in the bronchial epithelium and to be up-regulated in cystic fibrosis. In a mammalian expression system, secreted recombinant UGRP1 was copurified with apolipoprotein A-I. Using a retroviral vector-mediated expression cloning approach, we identified macrophage scavenger receptor with collagenous structure (MARCO) as a receptor for UGRP1. Northern blot and in situ hybridization experiments indicated that MARCO is expressed by alveolar macrophages in the lung. UGRP1 also bound to bacteria and yeast. LPS, a previously identified MARCO ligand, competed with UGRP1 for binding to MARCO and bacteria. Our findings suggest that UGRP1-MARCO is a ligand-receptor pair that is probably involved in inflammation and pathogen clearance in the lung.

Separation of lipid transport functions by mutations in the extracellular domain of scavenger receptor class B, type I

Scavenger receptor class B, type I (SR-BI) shows a variety of effects on cellular cholesterol metabolism, including increased selective uptake of high density lipoprotein (HDL) cholesteryl ester, stimulation of free cholesterol (FC) efflux from cells to HDL and phospholipid vesicles, and changes in the distribution of plasma membrane FC as evidenced by increased susceptibility to exogenous cholesterol oxidase. Previous studies showed that these multiple effects require the extracellular domain of SR-BI, but not the transmembrane and cytoplasmic domains. To test whether 1) the extracellular domain of SR-BI mediates multiple activities by virtue of discrete functional subdomains, or 2) the multiple activities are, in fact, secondary to and driven by changes in cholesterol flux, the extracellular domain of SR-BI was subjected to insertional mutagenesis by strategically placing an epitope tag into nine sites. These experiments identified four classes of mutants with disruptions at different levels of function. Class 4 mutants showed a clear separation of function between HDL binding, HDL cholesteryl ester uptake, and HDL-dependent FC efflux on one hand and FC efflux to small unilamellar vesicles and an increased cholesterol oxidase-sensitive pool of membrane FC on the other. Selective disruption of the latter two functions provides evidence for multiple functional subdomains in the extracellular receptor domain. Furthermore, these findings uncover a difference in the SR-BI-mediated efflux pathways for FC transfer to HDL acceptors versus phospholipid vesicles. The loss of the cholesterol oxidase-sensitive FC pool and FC efflux to small unilamellar vesicle acceptors in Class 4 mutants suggests that these activities may be mechanistically related.

Lipoprotein enhancement of ovarian theca-interstitial cell steroidogenesis: relative contribution of scavenger receptor class B (type I) and adenosine 5'-triphosphate- binding cassette (type A1) transporter in high-density lipoprotein-cholesterol transport and androgen synthesis

The theca-interstitial cells take up plasma high-density lipoprotein (HDL)- and low-density-lipoprotein-derived cholesterol to convert into steroid hormones. The uptake of HDL-derived cholesterol is mediated by the scavenger receptor, class B, type I (SR-BI). In nonsteroidogenic cells, HDL-stimulated efflux of cholesterol has been shown to be mediated by the ATP-binding cassette A1 (ABCA1) transporter. Its expression has not been documented in steroidogenic cells. The goal of the present study was to determine: 1) the role of SR-BI in theca-interstitial cell androgen production; 2) whether theca-interstitial cells express ABCA1 transporter mRNA; and 3) the relative roles of SR-BI and ABCA1 transporter in androgen production. The ABCA1 transporter mRNA expression in rat theca-interstitial cells was shown using RT-PCR and Northern blot analyses. The role of SR-BI and ABCA1 in androstenedione production was also examined by treating cells with anti-SR-BI and 2-hydroxypropyl-beta-cyclodextrin in the presence and absence of human chorionic gonadotropin and/or human HDL(3). The treatment of theca-interstitial cells with anti-SR-BI antibody blocked more than 90% of HDL plus human chorionic gonadotropin-stimulated androstenedione production, and selective HDL-CE uptake. On the other hand, the use of inhibitors of ABCA1 transporter function had no discernible effect on HDL-supported androgen production. These data demonstrate that, although theca-interstitial cells express both SR-BI and ABCA1 transporter mRNA, the SR-BI pathway supplies the majority of the cholesterol required for androgen production. Furthermore, the present study presents evidence for a crucial role for SR-BI in HDL-mediated androgen production.

Expression of human scavenger receptor B1 on and in human platelets

OBJECTIVE

The abundance of HDL particles correlates inversely with the incidence of coronary heart disease. The human scavenger receptor B1 (hSR-B1/CLA-1) is a receptor for HDL. Expression of hSR-B1/CLA-1 mRNA and protein in human platelets was determined using reverse transcriptase-polymerase chain reaction and Western blot, respectively. Presence of the protein on the surface of platelets was shown using flow cytometry.

METHODS AND RESULTS

Immunochemical staining for hSR-B1/CLA-1 showed that it was expressed in megakaryocytes, the platelet precursors of human bone marrow. These findings prompted us to ask whether hSR-B1/CLA-1 was differentially expressed on platelets obtained from patients with atherosclerotic disease compared with those in control subjects. Our findings showed that abundance of hSR-B1/CLA-1 was significantly reduced on the surface of platelets from patients with atherosclerotic disease. The reduced levels of hSR-B1/CLA-1 were associated with increased cholesterol ester content in platelets from patients with atherosclerotic disease compared with control subjects. A negative correlation existed between hSR-B1/CLA-1 expression and platelet aggregation. In summary, our studies show that the HDL receptor hSR-B1/CLA-1 is expressed in platelets and their precursor, the megakaryocyte. The levels of hSR-B1/CLA-1 expression correlate inversely with cholesterol ester content and platelet aggregation.

CONCLUSIONS

These findings suggest that determining the level of hSR-B1/CLA-1 expression on the platelets may be a useful clinical marker for atherosclerotic diseases.

Recycling of apoprotein E is associated with cholesterol efflux and high density lipoprotein internalization

After receptor-mediated endocytosis of triglyceride-rich lipoproteins (TRL) into the liver, TRL particles are immediately disintegrated in peripheral endosomal compartments. Whereas core lipids and apoprotein B are delivered for degradation into lysosomes, TRL-derived apoE is efficiently recycled back to the plasma membrane. This is followed by apoE re-secretion and association of apoE with high density lipoproteins (HDL). Because HDL and apoE can independently promote cholesterol efflux, we investigated whether recycling of TRL-derived apoE in human hepatoma cells and fibroblasts could be linked to intracellular cholesterol transport. In this study we demonstrate that HDL(3) does not only act as an extracellular acceptor for recycled apoE but also stimulates the recycling of internalized TRL-derived apoE. Furthermore, radioactive pulse-chase experiments indicate that apoE recycling is accompanied by cholesterol efflux. Confocal imaging reveals co-localization of apoE and cholesterol in early endosome antigen 1-positive endosomes. During apoE re-secretion, HDL(3)-derived apoA-I is found in these early endosome antigen 1, cholesterol-containing endosomes. As shown by time-lapse fluorescence microscopy, apoE recycling involves the intracellular trafficking of apoA-I to pre-existing and TRL-derived apoE/cholesterol-containing endosomes in the periphery. Thus, these studies provide evidence for a new intracellular link between TRL-derived apoE, cellular cholesterol transport, and HDL metabolism.

SR-BI-directed HDL-cholesteryl ester hydrolysis

We have examined the metabolic fate of HDL cholesteryl ester (CE) delivered to cells expressing scavenger receptor class B type I (SR-BI). Comparison of SR-BI with a related class B scavenger receptor, CD36, showed a greater uptake and a more rapid and extensive hydrolysis of HDL-CE when delivered by SR-BI. In addition, hydrolysis of HDL-CE delivered by both receptors was via a neutral CE hydrolase. These data indicate that SR-BI, but not CD36, can efficiently direct HDL-CE to a neutral CE hydrolytic pathway. In contrast, LDL-CE was delivered and hydrolyzed equally well by SR-BI and CD36. Hydrolysis of LDL-CE delivered by SR-BI was via a neutral CE hydrolase but that delivered by CD36 occurred via an acidic CE hydrolase, indicating that SR-BI and CD36 deliver LDL-CE to different metabolic pathways. Comparison of inhibitor sensitivities in Y1-BS1 adrenal, Fu5AH hepatoma, and transfected cells suggests that hydrolysis of HDL-CE delivered by SR-BI occurs via cell type-specific neutral CE hydrolases. Furthermore, HDL-CE hydrolytic activity was recovered in a membrane fraction of Y1-BS1 cells. These findings suggest that SR-BI efficiently delivers HDL-CE to a metabolically active membrane compartment where CE is hydrolyzed by a neutral CE hydrolase.