Scavenger receptor BI and cholesterol trafficking

Regulation of the steroidogenic acute regulatory protein gene expression: present and future perspectives

Steroid hormones are synthesized in the adrenal gland, gonads, placenta and brain and are critical for normal reproductive function and bodily homeostasis. The steroidogenic acute regulatory (StAR) protein regulates the rate-limiting step in steroid biosynthesis, i.e. the delivery of cholesterol from the outer to the inner mitochondrial membrane. The expression of the StAR protein is predominantly regulated by cAMP-dependent mechanisms in the adrenal and gonads. Whereas StAR plays an indispensable role in the regulation of steroid biosynthesis, a complete understanding of the regulation of its expression and function in steroidogenesis is not available. It has become clear that the regulation of StAR gene expression is a complex process that involves the interaction of a diversity of hormones and multiple signaling pathways that coordinate the cooperation and interaction of transcriptional machinery, as well as a number of post-transcriptional mechanisms that govern mRNA and protein expression. However, information is lacking on how the StAR gene is regulated in vivo such that it is expressed at appropriate times during development and is confined to the steroidogenic cells. Thus, it is not surprising that the precise mechanism involved in the regulation of StAR gene has not yet been established, which is the key to understanding the regulation of steroidogenesis in the context of both male and female development and function.

HDL-replacement therapy: mechanism of action, types of agents and potential clinical indications

HDL-replacement therapy is a promising new treatment strategy involving the acute administration of HDL to rapidly stabilize patients at imminent risk for developing a myocardial infarction, such as those with acute coronary syndrome. This review will first focus on the anti-atherogenic mechanisms for HDL, such as the stimulation of the reverse cholesterol transport pathway, and then discuss the other potential beneficial biological effects of HDL on atherosclerosis. The various types of HDL-replacement therapies that are being investigated and developed will be reviewed and ongoing clinical trials and other possible clinical indications for HDL-replacement therapy besides the prevention of myocardial infarction will also be described. Finally, HDL-replacement therapy will be put into perspective by summarizing the current gaps in our knowledge of HDL metabolism and identifying challenges for future research in this area.

JTT-705: is there still future for a CETP inhibitor after torcetrapib?

BACKGROUND

Despite reduction in low-density lipoprotein cholesterol, there is still a considerable amount of residual atherosclerosis-related disease. Epidemiological and pathophysiological data strongly favour increasing plasma high-density lipoprotein (HDL) cholesterol levels as antiatherogenic therapy, for example with cholesteryl ester transfer inhibition (CETP). However, negative Phase III studies on clinical end points with the CETP inhibitor torcetrapib challenge the future perspectives of other CETP inhibitors such as JTT-705.

OBJECTIVE

Is there potential for CETP inhibition with JTT-705 after torcetrapib's collapse?

METHODS

Search of articles in Pubmed citing JTT-705, torcetrapib and anacetrapib, or citing effects of pharmacological HDL-cholesterol raising or CETP inhibition.

RESULTS/CONCLUSION

There is possibly a future for HDL-cholesterol raising therapies. Phase III clinical studies with either JTT-705 or anacetrapib will determine whether CETP inhibition is beneficial.

Steroidogenic capacity of residual ovarian tissue in 4-vinylcyclohexene diepoxide-treated mice

Menopause is an important public health issue because of its association with a number of disorders. Androgens produced by residual ovarian tissue after menopause could impact the development of these disorders. It has been unclear, however, whether the postmenopausal ovary retains steroidogenic capacity. Thus, an ovary-intact mouse model for menopause that uses the occupational chemical 4-vinylcyclohexene diepoxide (VCD) was used to characterize the expression of steroidogenic genes in residual ovarian tissue of follicle-depleted mice. Female B6C3F1 mice (age, 28 days) were dosed daily for 20 days with either vehicle or VCD (160 mg kg(-1) day(-1)) to induce ovarian failure. Ovaries were collected on Day 181 and analyzed for mRNA and protein. Acyclic aged mice were used as controls for natural ovarian senescence. Relative to cycling controls, expression of mRNA encoding steroidogenic acute regulatory protein (Star); cholesterol side-chain cleavage (Cyp11a1); 3beta-hydroxysteroid dehydrogenase (Hsd3b); 17alpha-hydroxylase (Cyp17a1); scavenger receptor class B, type 1 (Scarb1); low-density lipoprotein receptor (Ldlr); and luteinizing hormone receptor (Lhcgr) was enriched in VCD-treated ovaries. In acyclic aged ovaries, mRNA expression for only Cyp17a1 and Lhcgr was greater than that in controls. Compared to cycling controls, ovaries from VCD-treated and aged mice had similar levels of HSD3B, CYP17A1, and LHCGR protein. The pattern of protein immunofluorescence staining for HSD3B in follicle-depleted (VCD-treated) ovaries was homogeneous, whereas that for CYP17A1 was only seen in residual interstitial cells. Circulating levels of FSH and LH were increased, and androstenedione levels were detectable following follicle depletion in VCD-treated mice. These findings support the idea that residual ovarian tissue in VCD-treated mice retains androgenic capacity.

Effects of acceptor composition and mechanism of ABCG1-mediated cellular free cholesterol efflux

Among the known mechanisms of reverse cholesterol transport (RCT), ATP binding cassette transporter G1 (ABCG1)-mediated free cholesterol (FC) transport is the most recent and least studied. Here, we have characterized the efficiencies of different acceptors using baby hamster kidney (BHK) cells transfected with human ABCG1 cDNA, which is inducible upon treatment with mifepristone. When normalized on particle number and particle surface area, the acceptor efficiency for FC efflux was as follows: small unilamellar vesicles (SUV)>LDL>reconstituted HDL>HDL(2) = HDL(3). Based on phospholipid content, the order was reversed. ABCG1 also mediated phospholipid efflux to human serum and HDL(3). ABCG1-mediated FC efflux correlated significantly with a number of HDL subfractions and components in serum collected from 25 normolipidemic individuals: apolipoprotein A-II (apoA-II) (r(2) = 0.7), apolipoprotein A-I (apoA-I) (r(2) = 0.5), HDL-C (r(2) = 0.4), HDL-PL (r(2) = 0.4), alpha-2 HDL (r(2) = 0.4), and prebeta HDL (r(2) = 0.2). ABCG1 did not enhance influx of FC or cholesteryl oleyl ether (COE) when cells were incubated with radiolabeled HDL(3). ABCG1 expression did not increase the association of HDL(3) with cells. Compared with control cells, ABCG1 expression significantly increased the FC pool available for efflux and the rate constant for efflux. In conclusion, composition and particle size determine the acceptor efficiency for ABCG1-mediated efflux. ABCG1 increases cell membrane FC pools and changes its rate of desorption into the aqueous phase without enhancing the association with the acceptor.

Coronary artery diseases in South Asian immigrants: an update on high density lipoprotein role in disease prevention

Over the past several years, the overall prevalence and incidence of cardiovascular diseases (CVD) including coronary artery diseases (CAD) have declined in the United States (US) and in many developed countries. However, among South Asian in general and South Asian immigrants (SAIs) in particular, a disturbing trend toward high rates of CAD has been noted. This trend is associated with a high prevalence of conventional risk factors and metabolic syndrome in this population, yet these conventional risk factors may not account for the greater CAD risk among SAIs. A search for additional markers is warranted, to enable early detection and prevention of CAD in this high risk group. High density lipoprotein (HDL) is one of the predictor of CAD and is considered to be cardio-protective. However, some of the recent studies have shown that HDL is not only ineffective as an antioxidant but, paradoxically, appears to be pro-oxidant, and has been found to be associated with CAD. Such HDL is called dysfunctional HDL. We present here an overview CAD and CAD risk factors in general and dyslipidemias in particular in SAIs. In addition, the evolving theories on dysfunctional HDL and its impact on CAD are also briefly presented.

Failure of adrenal corticosterone production in POMC-deficient mice results from lack of integrated effects of POMC peptides on multiple factors

Production of corticosteroids from the adrenal gland is a multistep process in which corticosterone is enzymatically processed from its precursor cholesterol. The main hormone regulating the production of corticosterone is the proopiomelanocortin (POMC)-derived adrenocorticotropic hormone (ACTH). Adrenals of POMC-deficient (POMC(-/-)) mice do not produce corticosterone either at basal levels or in response to acute stimulation with ACTH. However, pharmacological amounts of ACTH delivered continuously elicit corticosterone production over time. To define the relative effects of ACTH on individual factors involved in corticosterone production, parameters of adrenal cholesterol metabolism and steroidogenesis were examined in POMC(-/-) mice compared with wild-type and ACTH-treated mutant mice. POMC(-/-) adrenals lack cholesterol esters (CE); adrenal CE is restored with ACTH treatment. However, discontinuation of ACTH treatment stops corticosterone production despite the presence of adrenal CE. Failure of corticosterone production by POMC(-/-) adrenals occurs despite the constitutive presence of transcripts of genes required for cholesterol metabolism and steroidogenesis. Levels of key proteins involved in selective cholesterol uptake and steroidogenesis were attenuated; ACTH treatment increased these protein levels, most significantly those of the receptor responsible for selective uptake of CE, scavenger receptor class B, type I (SR-BI). Our studies reveal that failure of corticosterone production of POMC(-/-) adrenal glands and its pharmacological reconstitution by ACTH are not mediated by any one individual protein, but rather as an integrated effect on multiple factors from import of the substrate cholesterol to its conversion to corticosterone.

Serum lecithin: cholesterol acyltransferase activity, HDL2 and HDL3 composition in hypertensive mothers and their small for gestational age newborns

The aim of this study was to determine serum lecithin:cholesterol acyltransferase (LCAT) activity in parallel with HDL2 and HDL3 amounts and composition in pregnancy induced hypertension (PIH) and chronic hypertensive (CH) mothers and in their small for gestational age (SGA) newborns. LCAT activity was assayed by conversion of [3H] cholesterol to labelled cholesteryl ester. HDL2 and HDL3 were separated by ultracentrifugation. At term, cholesterol values were similar in PIH, CH and controls. However, higher levels of triglycerides were observed in PIH and CH (+20% and +21%, respectively) as compared with normotensive control mothers (NC). HDL2 and HDL3-phospholipids, HDL2-cholesterol concentrations and LCAT activity were lower in PIH and CH mothers than in NC mothers. Similar changes were also observed in SGA newborns of PHI mothers and in SGA newborns of CH mothers when compared to appropriate for gestational age newborns of control mothers (AGA-NC). In addition, SGA newborns showed low HDL2 and HDL3 apoA-I contents. Maternal hypertension and foetal intrauterine growth retardation are associated with profound abnormalities in HDL metabolism, consistent with an atherogenic risk. SGA lipoprotein profiles appear to implicate later metabolic diseases.

Excess coronary artery disease risk in South Asian immigrants: can dysfunctional high-density lipoprotein explain increased risk?

BACKGROUND

Coronary artery disease (CAD) is the leading cause of mortality and morbidity in the United States (US), and South Asian immigrants (SAIs) have a higher risk of CAD compared to Caucasians. Traditional risk factors may not completely explain high risk, and some of the unknown risk factors need to be explored. This short review is mainly focused on the possible role of dysfunctional high-density lipoprotein (HDL) in causing CAD and presents an overview of available literature on dysfunctional HDL.

DISCUSSION

The conventional risk factors, insulin resistance parameters, and metabolic syndrome, although important in predicting CAD risk, may not sufficiently predict risk in SAIs. HDL has antioxidant, antiinflammatory, and antithrombotic properties that contribute to its function as an antiatherogenic agent. Recent Caucasian studies have shown HDL is not only ineffective as an antioxidant but, paradoxically, appears to be prooxidant, and has been found to be associated with CAD. Several causes have been hypothesized for HDL to become dysfunctional, including Apo lipoprotein A-I (Apo A-I) polymorphisms. New risk factors and markers like dysfunctional HDL and genetic polymorphisms may be associated with CAD.

CONCLUSIONS

More research is required in SAIs to explore associations with CAD and to enhance early detection and prevention of CAD in this high risk group.

B-Type natriuretic peptide inhibited angiotensin II-stimulated cholesterol biosynthesis, cholesterol transfer, and steroidogenesis in primary human adrenocortical cells

In this study, we demonstrate that B-type natriuretic peptide (BNP) opposed angiotensin II (Ang II)-stimulated de novo cholesterol biosynthesis, cellular cholesterol uptake, cholesterol transfer to the inner mitochondrial membrane, and steroidogenesis, which are required for biosynthesis of steroid hormones such as aldosterone and cortisol in primary human adrenocortical cells. BNP dose-dependently stimulated intracellular cGMP production with an EC(50) of 11 nm, implying that human adrenocortical cells express the guanylyl cyclase A receptor. cDNA microarray and real-time RT-PCR analyses revealed that BNP inhibited Ang II-stimulated genes related to cholesterol biosynthesis (acetoacetyl coenzyme A thiolase, HMG coenzyme A synthase 1, HMG coenzyme A reductase, isopentenyl-diphosphate Delta-isomerase, lanosterol synthase, sterol-4C-methyl oxidase, and emopamil binding protein/sterol isomerase), cholesterol uptake from circulating lipoproteins (scavenger receptor class B type I and low-density lipoprotein receptor), cholesterol transfer to the inner mitochondrial membrane (steroidogenic acute regulatory protein), and steroidogenesis (ferredoxin 1,3beta-hydroxysteroid dehydrogenase, glutathione transferase A3, CYP19A1, CYP11B1, and CYP11B2). Consistent with the microarray and real-time PCR results, BNP also blocked Ang II-induced binding of (125)I-labeled low-density lipoprotein and (125)I-labeled high-density lipoprotein to human adrenocortical cells. Furthermore, BNP markedly inhibited Ang II-stimulated release of estradiol, aldosterone, and cortisol from cultured primary human adrenocortical cells. These findings demonstrate that BNP opposes Ang II-induced steroidogenesis via multiple steps from cholesterol supply and transfer to the final formation of steroid hormones. This study provides new insights into the cellular mechanisms by which BNP modulates Ang II-induced steroidogenesis in the adrenal gland.

Wild-type ApoA-I and the Milano variant have similar abilities to stimulate cellular lipid mobilization and efflux

OBJECTIVE

The present study is a comparative investigation of cellular lipid mobilization and efflux to lipid-free human apoA-I and apoA-I(Milano), reconstituted high-density lipoprotein (rHDL) particles containing these proteins and serum isolated from mice expressing human apoA-I or apoA-I(Milano).

METHODS AND RESULTS

Cholesterol and phospholipid efflux to these acceptors was measured in cell systems designed to assess the contributions of ATP-binding cassette A1 (ABCA1), scavenger receptor type BI (SRBI), and cellular lipid content to cholesterol and phospholipid efflux. Acceptors containing the Milano variant of apoA-I showed no functional increase in lipid efflux in all assays when compared with wild-type apoA-I. In fact, in some systems, acceptors containing the Milano variant of apoA-I promoted significantly less efflux than the acceptors containing wild-type apoA-I (apoA-I(wt)). Additionally, intracellular cholesteryl ester hydrolysis in macrophage foam cells was not different in the presence of either apoA-I(Milano) or apoA-I(wt).

CONCLUSION

Collectively these studies suggest that if the Milano variant of apoA-I offers greater atheroprotection than wild-type apoA-I, it is not attributable to greater cellular lipid mobilization.

HDL Cholesterol: Physiology, Pathophysiology, and Management

Numerous epidemiological studies have identified high-density lipoprotein cholesterol (HDL) to be an independent risk factor for coronary heart disease (CHD). HDL is an emerging therapeutic target that could rival the impact of 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) reductase inhibitors (statins) on LDL and CHD risk reduction. HDL metabolism, HDL kinetics, the concentration of various HDL subclasses, and other genetic factors affecting HDL functionality may all contribute to the anti-atherogenic properties of HDL; thus, standard plasma measurement may not capture the full range of HDL effects. Algorithms have been suggested to treat low HDL levels in subgroups of patients; however, no formal HDL target goals or treatment guidelines have been implemented as there is a lack of strong clinical evidence to support effective pharmacologic therapy for primary risk reduction. Available therapies have a modest impact on serum HDL levels; however, emerging therapies could have a more significant influence.

[New insights and perspectives in the management of dyslipidemia]

PURPOSE

Recent end point trials of lipid-lowering drugs have shown that patients at very high-risk for coronary disease benefit from treatments that lowers low density lipoprotein cholesterol (LDL cholesterol) plasma levels< or =70 mg/dl and that patients with at least 2 risk factors benefit from LDL cholesterol levels< or =100 mg/dl. Epidemiologic studies have shown that the concentration of high density lipoprotein cholesterol (HDL cholesterol) is a strong, independent, inverse predictor of coronary disease risk. Innovative pharmacological approaches to raise low HDL cholesterol levels are currently of considerable interest, especially for patients with type 2 diabetes or metabolic syndrome.

RESULTS

Rosuvastatin has shown superior efficacy in lowering LDL cholesterol, although evidence of clinical benefit is actually lacking. Ezetimibe is a lipid-lowering drug that inhibits absorption of dietary and biliary cholesterol. Its co-administration with statin has given very interesting results. Niacin is the most effective of currently available options for raising HDL cholesterol, although tolerability can be an issue, with serious side effects such as loss of glucose control and liver toxicity. Flushing may occur in 80% of treated patients. Two CETP inhibitors have shown therapeutical efficacy to raise HDL cholesterol, but clinical benefit remains uncertain.

The LDL receptor is not necessary for acute adrenal steroidogenesis in mouse adrenocortical cells

Steroid hormones are synthesized using cholesterol as precursor. To determine the functional importance of the low density lipoprotein (LDL) receptor and hormone-sensitive lipase (HSL) in adrenal steroidogenesis, adrenal cells were isolated from control, HSL(-/-), LDLR(-/-), and double LDLR/HSL(-/-) mice. The endocytic and selective uptake of apolipoprotein E-free human high density lipoprotein (HDL)-derived cholesteryl esters did not differ among the mice, with selective uptake accounting for >97% of uptake. In contrast, endocytic uptake of either human LDL- or rat HDL-derived cholesteryl esters was reduced 80-85% in LDLR(-/-) and double-LDLR/HSL(-/-) mice. There were no differences in the selective uptake of either human LDL- or rat HDL-derived cholesteryl esters among the mice. Maximum corticosterone production induced by ACTH or dibutyryl cyclic AMP and lipoproteins was not altered in LDLR(-/-) mice but was reduced 80-90% in HSL(-/-) mice. Maximum corticosterone production was identical in HSL(-/-) and double-LDLR/HSL(-/-) mice. These findings suggest that, although the LDL receptor is responsible for endocytic delivery of cholesteryl esters from LDL and rat HDL to mouse adrenal cells, it appears to play a negligible role in the delivery of cholesterol for acute adrenal steroidogenesis in the mouse. In contrast, HSL occupies a vital role in adrenal steroidogenesis because of its link to utilization of selectively delivered cholesteryl esters from lipoproteins.

Adrenal cholesterol utilization

There are multiple systems for cellular cholesterol delivery for steroidogenesis, including uptake of lipoprotein-derived cholesterol via LDL receptor mediated endocytic pathways and SR-BI mediated "selective" pathways, as well as from endogenous cholesterol synthesis and the mobilization of stored cholesteryl esters. The vast majority of lipoprotein-derived cholesterol utilized for murine adrenal steroidogenesis is obtained via SR-BI mediated "selective" uptake of cholesteryl esters. Hormone-sensitive lipase (HSL) is responsible for neutral cholesteryl ester hydrolase activity in the adrenal and is critical for hydrolyzing stored cholesteryl esters, as well as cholesteryl esters that are selectively delivered from lipoproteins via SR-BI. Marked defects in steroid production are observed in adrenal cells from HSL knockout mice, due to an inability to process and utilize cholesteryl esters selectively derived from lipoproteins. Although the LDL receptor is responsible for receptor-mediated endocytic delivery of cholesteryl esters, adrenal steroid hormone production is normal in mice lacking LDL receptors.

Familial dyslipidaemias: an overview of genetics, pathophysiology and management

Plasma lipid disorders can occur either as a primary event or secondary to an underlying disease or use of medications. Familial dyslipidaemias are traditionally classified according to the electrophoretic profile of lipoproteins. In more recent texts, this phenotypic classification has been replaced with an aetiological classification. Familial dyslipidaemias are generally grouped into disorders leading to hypercholesterolaemia, hypertriglyceridaemia, a combination of hyper-cholesterolaemia and hypertriglyceridaemia, or abnormal high-density lipoprotein-cholesterol (HDL-C) levels. The management of these disorders requires an understanding of plasma lipid and lipoprotein metabolism. Lipid transport and metabolism involves three general pathways: (i) the exogenous pathway, whereby chylomicrons are synthesised by the small intestine, and dietary triglycerides (TGs) and cholesterol are transported to various cells of the body; (ii) the endogenous pathway, whereby very low-density lipoprotein-cholesterol (VLDL-C) and TGs are synthesised by the liver for transport to various tissues; and (iii) the reverse cholesterol transport, whereby HDL cholesteryl ester is exchanged for TGs in low-density lipoptrotein (LDL) and VLDL particles through cholesteryl ester transfer protein in a series of steps to remove cholesterol from the peripheral tissues for delivery to the liver and steroidogenic organs. The plasma lipid profile can provide a framework to guide the selection of appropriate diet and drug treatment. Many patients with hyperlipoproteinaemia can be treated effectively with diet. However, dietary regimens are often insufficient to bring lipoprotein levels to within acceptable limits. In this article, we review lipid transport and metabolism, discuss the more common lipid disorders and suggest some management guidelines. The choice of a particular agent depends on the baseline lipid profile achieved after 6-12 weeks of intense lifestyle changes and possible use of dietry supplements such as stanols and plant sterols. If the predominant lipid abnormality is hypertriglyceridaemia, omega-3 fatty acids, a fibric acid derivative (fibrate) or nicotinic acid would be considered as the first choice of therapy. In subsequent follow-up, when LDL-C is >130 mg/dL (3.36 mmol/L) then an HMG-CoA reductase inhibitor (statin) should be added as a combination therapy. If the serum TG levels are <500 mg/dL (2.26 mmol/L) and the LDL-C values are over 130 mg/dL (3.36 mmol/L) then a statin would be the first drug of choice. The statin dose can be titrated up to achieve the therapeutic goal or, alternatively, ezetimibe can be added. A bile acid binding agent is an option if the serum TG levels do not exceed 200 mg/dL (5.65 mmol/L), otherwise a fibrate or nicotinic acid should be considered. The decision to treat a particular person has to be individualised.

Comparison of different cellular models measuring in vitro the whole human serum cholesterol efflux capacity

BACKGROUND

Fu5AH rat hepatoma cells and cAMP (cyclic AMP)-pretreated J774 mouse macrophages are commonly used as models for SR-BI (scavenger receptor class B type I) and ABCA1 (ATP binding cassette transporter 1)-mediated free cholesterol efflux to whole serum, respectively. However, the responsiveness of Fu5AH, control or cAMP pretreated J774 cells to the various lipids and HDL (high-density lipoprotein)-parameters from both normo- and dyslipidaemic subjects has never been compared within the same study.

MATERIALS AND METHODS

Fifty-eight men were classified into four groups: type IIa hypercholesterolaemic (n = 12), type IIb dyslipidaemic (n = 13), type IV hypertriglyceridaemic (n = 18) and normolipidaemic (n = 15) were recruited. A complete lipid profile including prebeta-HDL was performed. Cholesterol efflux from Fu5AH cells as well as from control or cAMP pretreated J774 cells were measured; the difference between these two latter values being taken as the ABCA1-mediated efflux.

RESULTS

The Fu5AH and the control J774 cells delivered cholesterol to mature HDLs, especially to phospholipid (PL)-rich HDL. Using cAMP pretreated cells, the ABCA1-dependent efflux was highly sensitive to prebeta-HDL, which appeared to be a factor in determining the efflux. Consistent with the dependence of the SR-BI-mediated efflux on HDL-PL levels, which are not different between groups, all sera displayed similar efflux capacities from the Fu5AH cells. Conversely, in accordance with their high prebeta-HDL levels, the ABCA1-dependent efflux highlighted the efficiency of type IV sera.

CONCLUSION

Two complementary cellular models providing SR-BI and ABCA1-dependent efflux should be used to measure the capacity of a biological fluid which contains a wide variety of components to promote cholesterol efflux.

The predominance of one of the SR-BI isoforms is associated with increased esterified cholesterol levels not apoptosis in mink testis

Scavenger receptor class B type I (SR-BI) contributes to HDL-mediated cellular cholesterol efflux and is a phagocytosis-inducing phospholipid phosphatidylserine receptor in rat Sertoli cells, whereas the spliced variant of the SR-B gene, SR-BII, is implicated in the efflux of free cholesterol in macrophages. This study aimed to assess whether spontaneous autoimmune orchitis (AIO), which causes impaired clearance of apoptotic germ cells and spermatogenic arrest, involves SR-BI, SR-BII, and/or cholesterol. The levels measured during development and the annual reproductive cycle in normal mink were compared with those in mink with spontaneous AIO. Time periods with lowest tubular esterified cholesterol (EC) levels showed maximal SR-BI and SR-BII levels, and the periods when one or the other SR-BI isoform predominated showed increased EC levels and spermatogenic arrest in normal mink seminiferous tubules. In tubules with AIO, the predominance of only one or the other SR-BI isoform was the reverse of that measured in normal tubules, and it was associated with an increase in EC levels but not with apoptosis levels. SR-BI and SR-BII levels were not correlated with serum testosterone levels. SR-BI mainly localized to the Leydig cell, germ cell, and Sertoli cell surface, where its distribution was stage-specific. SR-BII was principally intracellular. Tubules from testes with AIO showed a deregulation of cholesterol homeostasis and SR-BI expression but relatively unchanged apoptosis levels. These results suggest that the expression of both SR-BI isoforms is required for the maintenance of low EC levels and that the predominance of only one isoform is associated with the accumulation of EC but not with apoptosis in the tubules.

Therapeutic elevation of HDL-cholesterol to prevent atherosclerosis and coronary heart disease

Innovative pharmacological approaches to raise anti-atherogenic high-density lipoprotein-cholesterol (HDL-C) are currently of considerable interest, particularly in atherogenic dyslipidemias characterized by low levels of HDL-C, such as type 2 diabetes, the metabolic syndrome, and mixed dyslipidemia, but equally among individuals with or at elevated risk for premature cardiovascular disease (CVD). Epidemiological and observational studies first demonstrated that HDL-C was a strong, independent predictor of coronary heart disease (CHD) risk, and suggested that raising HDL-C levels might afford clinical benefit. Accumulating data from clinical trials of pharmacological agents that raise HDL-C levels have supported this concept. In addition to the pivotal role that HDL-C plays in reverse cholesterol transport and cellular cholesterol efflux, HDL particles possess a spectrum of anti-inflammatory, anti-oxidative, anti-apoptotic, anti-thrombotic, vasodilatory and anti-infectious properties, all of which potentially contribute to their atheroprotective nature. Significantly, anti-atherogenic properties of HDL particles are attenuated in common metabolic diseases that are characterized by subnormal HDL-C levels, such as type 2 diabetes and metabolic syndrome. Inhibition of cholesteryl ester transfer protein (CETP), a key player in cholesterol metabolism and transport, constitutes an innovative target for HDL-C raising. In lipid efficacy trials, 2 CETP inhibitors-JTT-705 and torcetrapib-induced marked elevation in HDL-C levels, with torcetrapib displaying greater efficacy. Moreover, both agents attenuate aortic atherosclerosis in cholesterol-fed rabbits. Clinical trial data demonstrating the clinical benefits of these drugs on atherosclerosis and CHD are eagerly awaited.

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.

Relative Contributions of ABCA1 and SR-BI to Cholesterol Efflux to Serum From Fibroblasts and Macrophages

Objectives— Cholesterol efflux is achieved by several mechanisms. This study examines contributions of these pathways to efflux to human serum.

Methods and Results— Human fibroblasts were stably transfected with SR-BI while ABCA1 was upregulated. Quantitation of cholesterol efflux to human serum demonstrated that there was efflux from cells without either protein. Expression of ABCA1 produced a small increase in efflux, whereas SR-BI expression had a dramatic impact. To quantitate ABCA1 and SR-BI contribution, fibroblasts were pretreated with Probucol and BLT-1 to, respectively, inhibit these efflux proteins. Exposing SR-BI–expressing fibroblasts to BLT-1 inhibited efflux by 67%. Probucol pretreatment of ABCA1-expressing fibroblasts reduced efflux to serum by 26%. A large fraction of total efflux was uninhibited. For both J774 and mouse peritoneal macrophages, contributions of either ABCA1 or SR-BI to efflux to serum were low, with background/uninhibited efflux contributing from 70% to 90% of total efflux.

Conclusions— We have shown that ABCA1-mediated efflux to serum responds to the pool of lipid-free/poor apolipoproteins, whereas phospholipid-containing particles mediate SR-BI efflux. Although SR-BI and ABCA1 contribute to efflux from fibroblasts and cholesterol-enriched macrophages, a large proportion of the total efflux to human serum is mediated by a mechanism that is neither SR-BI nor ABCA1.

Targeting cholesteryl ester transfer protein for the prevention and management of cardiovascular disease

Epidemiologic studies have shown that the concentration of high-density lipoprotein cholesterol (HDL-C) is a strong, independent, inverse predictor of coronary heart disease risk. This identifies HDL-C as a potential therapeutic target. Compared with low-density lipoprotein cholesterol (LDL-C)-lowering agents, however, currently available HDL-raising drugs are relatively ineffective. Consequently, recent years have seen considerable efforts expended on identifying new drugs that can raise HDL-C. Cholesteryl ester transfer protein (CETP) plays an important role in cholesterol metabolism, being responsible for the transfer of cholesteryl esters from HDL to very low-density lipoproteins and LDLs. The observation that Japanese populations with CETP deficiency exhibited high levels of HDL-C has led to the concept that drugs targeting CETP activity may elevate HDL-C levels and potentially decrease cardiovascular risk. Support of this proposition has been obtained in rabbits where inhibition of CETP activity is markedly antiatherogenic. Two CETP inhibitors-torcetrapib and JTT-705-are currently in the preliminary stages of clinical development. Initial studies with these drugs in humans show that they substantially increase HDL-C levels and modestly decrease LDL-C levels. Larger, long-term, randomized, clinical end point trials are required to determine whether the beneficial effects of CETP inhibitors on lipoprotein metabolism can translate into reductions in cardiovascular events.

Cholesteryl Esters of Aggregated LDL Are Internalized by Selective Uptake in Human Vascular Smooth Muscle Cells

Objective— Low-density lipoprotein (LDL) receptor-related protein (LRP1) mediates the internalization of aggregated LDL (agLDL)–LDL trapped in the arterial intima bound to proteoglycans–into human vascular smooth muscle cells (VSMC). LRP1-mediated agLDL uptake induces high-intracellular cholesteryl ester (CE) accumulation. The aim of this study was to characterize the mechanism of agLDL internalization in human VSMC.

Methods and Results— The lipidic component of LDL was labeled with [ 3 H] and the apolipoprotein component with [ 125 I]. We found that >90% of intracellular CE derived from agLDL uptake was not associated with apoB100 degradation but was selectively taken up from agLDL. The inhibition of LRP1 expression by small interfering RNA treatment led to a decrease of 80±0.05% in agLDL-CE selective uptake. AgLDL induced intracellular CE accumulation without a concomitant CE synthesis. Cytosolic and cytoskeletal proteins were not required for CE transport. Electron and confocal microscopy experiments indicate that CE derived from agLDL accumulated in adipophilin-stained lipid droplets that were not removable by high-density lipoprotein.

Conclusions— Taken together, these results demonstrate that LRP1 mediates the selective uptake of CE from agLDL and that CE derived from agLDL is not intracellularly processed but stored in lipid droplets in human VSMC.

Distinct Cellular Loci for the ABCA1-Dependent and ABCA1-Independent Lipid Efflux Mediated by Endogenous Apolipoprotein E Expression

OBJECTIVE

Macrophage expression of both apolipoprotein E (apoE) and ABCA1 have been shown to modulate lipid efflux from these cells and to play an important atheroprotective role in vivo. We evaluated the relationship between apoE and ABCA1 for regulating cellular sterol efflux.

METHODS AND RESULTS

ApoE-mediated, but ABCA1-independent, lipid efflux was demonstrated in 3 model systems. First, adenoviral-mediated expression of apoE in dermal fibroblasts isolated from ABCA1(-/-) mice significantly increased both sterol and phospholipid efflux. Second, expression of human apoE in a macrophage cell line increased sterol efflux, and this increment in efflux was not reduced by suppressing ABCA1 expression. Third, reduction of apoE expression using an apoE small interfering RNA significantly reduced sterol efflux from ABCA1(-/-) mouse peritoneal macrophages. ApoE-mediated, but ABCA1-independent, lipid efflux could be differentiated from lipid efflux that was dependent on the extracellular accumulation of secreted apoE, because exogenous cell-derived apoE stimulated efflux only from cells expressing ABCA1. Sterol efflux was usually highest in cells expressing both ABCA1 and apoE, likely representing a summation of the ABCA1-dependent and -independent pathways for apoE-mediated sterol efflux.

CONCLUSIONS

ABCA1 expression is required for apoE-mediated efflux when endogenously synthesized apoE accumulates extracellularly. Our results, however, establish the existence of an ABCA1-independent pathway for lipid efflux that requires the intracellular synthesis and/or transport of apoE.

Hepatic expression of scavenger receptor class B type I (SR-BI) is a positive regulator of macrophage reverse cholesterol transport in vivo

Hepatic expression of the scavenger receptor class B type I (SR-BI) promotes selective uptake of HDL cholesterol by the liver and is believed to play a role in the process of reverse cholesterol transport (RCT). We hypothesized that hepatic SR-BI expression is a regulator of the rate of integrated macrophage-to-feces RCT and used an in vivo model to test this hypothesis. Cholesterol-loaded and [3H]cholesterol-labeled J774 macrophages were injected intraperitoneally into mice, after which the appearance of the [3H]cholesterol in the plasma, liver, and feces over 48 hours was quantitated. Mice overexpressing SR-BI in the liver had significantly reduced [3H]cholesterol in the plasma but markedly increased [3H] tracer excretion in the feces over 48 hours. Conversely, mice deficient in SR-BI had significantly increased [3H]cholesterol in the plasma but markedly reduced [3H] tracer excretion in the feces over 48 hours. These studies demonstrate that hepatic SR-BI expression, despite its inverse effects on steady-state plasma HDL cholesterol concentrations, is an important positive regulator of the rate of macrophage RCT.

Reverse cholesterol transport and cholesterol efflux in atherosclerosis

Reverse cholesterol transport (RCT) is a pathway by which accumulated cholesterol is transported from the vessel wall to the liver for excretion, thus preventing atherosclerosis. Major constituents of RCT include acceptors such as high-density lipoprotein (HDL) and apolipoprotein A-I (apoA-I), and enzymes such as lecithin:cholesterol acyltransferase (LCAT), phospholipid transfer protein (PLTP), hepatic lipase (HL) and cholesterol ester transfer protein (CETP). A critical part of RCT is cholesterol efflux, in which accumulated cholesterol is removed from macrophages in the subintima of the vessel wall by ATP-binding membrane cassette transporter A1 (ABCA1) or by other mechanisms, including passive diffusion, scavenger receptor B1 (SR-B1), caveolins and sterol 27-hydroxylase, and collected by HDL and apoA-I. Esterified cholesterol in the HDL is then delivered to the liver for excretion. In patients with mutated ABCA1 genes, RCT and cholesterol efflux are impaired and atherosclerosis is increased. In studies with transgenic mice, disruption of ABCA1 genes can induce atherosclerosis. Levels of HDL are inversely correlated with incidences of cardiovascular disease. Supplementation with HDL or apoA-I can reverse atherosclerosis by accelerating RCT and cholesterol efflux. On the other hand, pro-inflammatory factors such as interferon-gamma (IFN-gamma), endotoxin, tumour necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1beta), can be atherogenic by impairing RCT and cholesterol efflux, according to in vitro studies. RCT and cholesterol efflux play a major role in anti-atherogenesis, and modification of these processes may provide new therapeutic approaches to cardiovascular disease. Further research on new modifying factors for RCT and cholesterol efflux is warranted.

Cholesteryl esters in malignancy

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

Differential effects of HDL subpopulations on cellular ABCA1- and SR-BI-mediated cholesterol efflux

Our objective was to evaluate the associations of individual apolipoprotein A-I (apoA-I)-containing HDL subpopulation levels with ABCA1- and scavenger receptor class B type I (SR-BI)-mediated cellular cholesterol efflux. HDL subpopulations were measured by nondenaturing two-dimensional gel electrophoresis from 105 male subjects selected with various levels of apoA-I in pre-beta-1, alpha-1, and alpha-3 HDL particles. ApoB-containing lipoprotein-depleted serum was incubated with [(3)H]cholesterol-labeled cells to measure efflux. The difference in efflux between control and ABCA1-upregulated J774 macrophages was taken as a measure of ABCA1-mediated efflux. SR-BI-mediated efflux was determined using cholesterol-labeled Fu5AH hepatoma cells. Fractional efflux values obtained from these two cell systems were correlated with the levels of individual HDL subpopulations. A multivariate analysis showed that two HDL subspecies correlated significantly with ABCA1-mediated efflux: small, lipid-poor pre-beta-1 particles (P=0.0022) and intermediate-sized alpha-2 particles (P=0.0477). With regard to SR-BI-mediated efflux, multivariate analysis revealed significant correlations with alpha-2 (P=0.0004), alpha-1 (P=0.0030), pre-beta-1 (P=0.0056), and alpha-3 (P=0.0127) HDL particles. These data demonstrate that the small, lipid-poor pre-beta-1 HDL has the strongest association with ABCA1-mediated cholesterol even in the presence of all other HDL subpopulations. Cholesterol efflux via the SR-BI pathway is associated with several HDL subpopulations with different apolipoprotein composition, lipid content, and size.

HDL: the 'new' target of cardiovascular medicine

Clinical, experimental and epidemiological research has shown the undeniable causal relationship between low HDL plasma concentrations and cardiovascular disease. Low HDL levels are present in about 10% of the general population and represent the most frequent form of dyslipidemia in patients with coronary disease. Reduced HDL concentrations seem to be unable to eliminate efficiently the cholesterol excess at vascular wall level, contributing to the onset of the inflammatory response that typically occurs in the pathogenesis of atherosclerosis right from its earliest stages. The results of numerous studies quite convincingly suggest that HDL is capable of exerting anti-inflammatory activity either directly or by modulating the expression of a number of acute phase proteins. Although the therapeutic options currently available for raising HDL levels still show modest efficacy, both in experimental and pre-clinical fields, genetic investigation and specifically aimed pharmacological treatment have produced more encouraging results, shedding some light on the concrete possibility of being able to treat this disease in the very near future.

Dietary fatty acids and cholesterol differentially modulate HDL cholesterol metabolism in Golden-Syrian hamsters

Dietary fatty acids alter HDL cholesterol concentrations, presumably through mechanisms related to reverse cholesterol transport. The effect of dietary fats (coconut oil, butter, traditional stick margarine, soybean oil, canola oil) differing in fatty acid profile on this antiatherogenic process was assessed with respect to plasma lipids; exogenous and endogenous lecithin-cholesterol acyltransferase (LCAT), cholesterol ester transfer protein (CETP), phospholipid transfer protein (PLTP) activities; and LCAT, apolipoprotein (apo) A-I and scavenger receptor B class-1 (SR-B1) mRNA abundance. Golden-Syrian hamsters were fed a nonpurified (6.25 g/100 g fat) diet containing an additional 10 g/100 g experimental fat and 0.1 g/100 g cholesterol for 6 wk. Canola and soybean oils significantly lowered serum HDL cholesterol concentrations relative to butter. Canola oil, relative to butter, resulted in higher exogenous LCAT activity, and both soybean and canola oils significantly increased hepatic apo A-I and SR-B1 mRNA abundance. Butter, relative to margarine, coconut and soybean oils, significantly increased serum non-HDL cholesterol concentrations. Endogenous and exogenous LCAT, CETP, and PLTP activities did not differ in hamsters fed margarine or saturated fat diets, despite lower hepatic LCAT, apo A-I, and SR-B1 mRNA abundance, suggesting that changes in available substrate and/or modification to the LCAT protein may have been involved in lipoprotein changes. These results suggest that lower HDL cholesterol concentrations, as a result of canola and soybean oil feeding, may not be detrimental due to increases in components involved in the reverse cholesterol transport process in these hamsters and may retard the progression of atherosclerosis.

Progestogens of varying androgenicity and cardiovascular risk factors in postmenopausal women receiving oestrogen replacement therapy

OBJECTIVE

Medroxyprogesterone (MP) was used as the progestogen in randomized clinical trials of postmenopausal hormone replacement on cardiovascular risk. To attempt to understand the lack of benefit in these trials, we have examined the effects of MP and two other progestogens, the less androgenic desogestrel (DG) and the more androgenic norethisterone (NE), on cardiovascular risk factors against a background of oestrogen therapy.

DESIGN AND MEASUREMENTS

Thirty-four women were treated with conjugated equine oestrogens (CEE) 0.625 mg daily alone for 12 weeks, followed in random order by each of the three progestogens (DG 75 microg, MP 10 mg and NE 1 mg daily) given sequentially for three 12-week cycles while maintaining the same CEE treatment. We measured serum lipoproteins, paraoxonase activity, C-reactive protein (CRP), fibrinogen, fasting glucose and insulin levels at baseline, at the end of the oestrogen-only phase and at the end of each of the combined oestrogen and progestogen phases.

RESULTS

The addition of progestogens to CEE maintained the oestrogen-induced reduction in apolipoprotein B (apo B) and lipoprotein (a) [Lp(a)], and further lowered total cholesterol (P < 0.01) and fibrinogen (P < 0.001). CEE raised serum triglyceride (P < 0.001) and CRP (P < 0.01) concentrations, which reverted towards pre-oestrogen levels with progestogens. Progestogens significantly reduced high density lipoprotein (HDL) cholesterol (P < 0.05). NE was associated with the greatest reduction in HDL cholesterol and apo A1, but was most effective in preserving paraoxonase activity and reducing the potentially unfavourable oestrogen-induced increases in triglycerides and CRP.

CONCLUSION

Preconceptions that more androgenic progestogens necessarily have more unfavourable effects on cardiovascular risk factors may require revision.

Probucol inhibits ABCA1-mediated cellular lipid efflux

OBJECTIVE

ATP-binding cassette transporter A1 (ABCA1) mediates the efflux of lipids from cells to lipid-poor apolipoproteins. In this article, we characterize the effect of probucol on cellular ABCA1-mediated lipid efflux.

METHODS AND RESULTS

Probucol inhibited cholesterol efflux up to 80% in J774 macrophages expressing ABCA1. In Fu5AH hepatoma cells that contain scavenger receptor class B, type I, but not functional ABCA1, we observed no effect of probucol on cholesterol efflux. Probucol inhibited cholesterol efflux from normal human skin fibroblasts but not from fibroblasts from a Tangier patient. Fluorescent confocal microscopy and biotinylation assay demonstrated that in J774 cells probucol impaired the translocation of ABCA1 from intracellular compartments to the plasma membrane. Probucol also inhibited the formation of an ABCA1-linked cholesterol oxidase sensitive plasma membrane domain. Consistent with the inhibitory effect on ABCA1 translocation to the plasma membrane, probucol reduced cell surface-specific [125I]-labeled apolipoprotein-AI binding.

CONCLUSIONS

We conclude that probucol is an effective inhibitor of ABCA1-mediated cholesterol efflux without influencing scavenger receptor class B type I-mediated efflux. The inhibition of ABCA1 translocation to the plasma membrane may in part explain the reported in vivo high-density lipoprotein-lowering action of probucol.

Regulation of plasma high-density lipoprotein levels by the ABCA1 transporter and the emerging role of high-density lipoprotein in the treatment of cardiovascular disease

High-density lipoproteins (HDL) protect against cardiovascular disease. HDL removes and transports excess cholesterol from peripheral cells to the liver for removal from the body. HDL also protects low-density lipoproteins (LDL) from oxidation and inhibits expression of adhesion molecules in endothelial cells, preventing monocyte movement into the vessel wall. The ABCA1 transporter regulates intracellular cholesterol levels in the liver and in peripheral cells by effluxing excess cholesterol to lipid-poor apoA-I to form nascent HDL, which is converted to mature alpha-HDL by esterification of cholesterol to cholesteryl esters (CE) by lecithin cholesterol acyltransferase. The hepatic ABCA1 transporter and apoA-I are major determinants of levels of plasma alpha-HDL cholesterol as well as poorly lipidated apoA-I, which interact with ABCA1 transporters on peripheral cells in the process of reverse cholesterol transport. Cholesterol in HDL is transported directly back to the liver by HDL or after transfer of CE by the cholesteryl ester transfer protein (CETP) by the apoB lipoproteins. Current approaches to increasing HDL to determine the efficacy of HDL in reducing atherosclerosis involve acute HDL therapy with infusions of apoA-I or apoA-I mimetic peptides and chronic long-term therapy with selective agents to increase HDL, including CETP inhibitors.

Scavenger receptor class B type I is solely responsible for the selective uptake of cholesteryl esters from HDL by the liver and the adrenals in mice

Scavenger receptor class B type I (SR-BI) has been identified as a functional HDL binding protein that can mediate the selective uptake of cholesteryl ester (CE) from HDL. To quantify the in vivo role of SR-BI in the process of selective uptake, HDL was labeled with cholesteryl ether ([(3)H] CEt-HDL) and (125)I-tyramine cellobiose ([(125)I]TC-HDL) and injected into SR-BI knockout (KO) and wild-type (WT) mice. In SR-BI KO mice, the clearance of HDL-CE from the blood circulation was greatly diminished (0.043 +/- 0.004 pools/h for SR-BI KO mice vs. 0.106 +/- 0.004 pools/h for WT mice), while liver and adrenal uptake were greatly reduced. Utilization of double-labeled HDL ([(3)H]CEt and [(125)I]TC) indicated the total absence in vivo of the selective decay and liver uptake of CE from HDL in SR-BI KO mice. Parenchymal cells isolated from SR-BI KO mice showed similar association values for [(3)H]CEt and [(125)I]TC in contrast to WT cells, indicating that in parenchymal liver cells SR-BI is the only molecule exerting selective CE uptake from HDL. Thus, in vivo and in vitro, SR-BI is the sole molecule mediating the selective uptake of CE from HDL by the liver and the adrenals, making it the unique target to modulate reverse cholesterol transport.

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.

Focus on high-density lipoproteins in reducing cardiovascular risk

Low high-density lipoprotein (HDL) cholesterol is associated with increased risk of coronary heart disease (CHD). Ongoing investigation into the mechanisms whereby HDL cholesterol might provide protection from atherosclerosis and clinical disease has resulted in improved understanding of the role of HDL in removal of cholesterol from the arterial wall and has suggested a number of strategies for augmenting the beneficial activities of the lipoprotein. Current drug options for increasing HDL cholesterol levels include the statins, fibrates, and niacin. Strategies in development for increasing the function of HDL or apolipoprotein A-I and thereby reducing atherosclerotic progression include use of agents to upregulate the adenosine triphosphate-binding cassette transporter in vessel wall macrophages to increase cholesterol efflux from these cells; use of agents to stimulate endogenous apoA-I synthesis; administration of apoA-I, apoA-I Milano, apoA-I-mimetic peptides, or delipidated HDL; and use of cholesteryl ester transfer protein inhibitors.

Contributions of domain structure and lipid interaction to the functionality of exchangeable human apolipoproteins

Exchangeable apolipoproteins function in lipid transport as structural components of lipoprotein particles, cofactors for enzymes and ligands for cell-surface receptors. Recent findings with apoA-I and apoE suggest that the tertiary structures of these two members of the human exchangeable apolipoprotein gene family are related. Characteristically, these proteins contain a series of proline-punctuated, 11- or 22-amino acid, amphipathic alpha-helical repeats that can adopt a helix bundle conformation in the lipid-free state. The amino- and carboxyl-terminal regions form separate domains with the latter being primarily responsible for lipid binding. Interaction with lipid induces changes in the conformation of the amino-terminal domain leading to alterations in function; for example, opening of the amino-terminal four-helix bundle in apolipoprotein E upon lipid binding is associated with enhanced receptor-binding activity. The concept of a two-domain structure for the larger exchangeable apolipoproteins is providing new molecular insights into how these apolipoproteins interact with lipids and other proteins, such as receptors. The ways in which structural changes induced by lipid interaction modulate the functionality of these apolipoproteins are reviewed.

SR-BI- and ABCA1-mediated cholesterol efflux to serum from patients with Alagille syndrome

Alagille syndrome is associated with bile duct paucity resulting in liver disease. Patients can be divided into mildly and severely icteric groups, with both groups having altered lipoproteins. The incidence of ischemic heart disease is rare in severely cholestatic children despite increased total cholesterol and decreased high density lipoprotein cholesterol (HDL-C). The present studies examine the impact of altered lipid and lipoproteins on scavenger receptor class B type I (SR-BI)- and ABCA1-mediated efflux to serum from both groups. Efflux was compared with serum from 29 patients (15 with normal plasma cholesteryl ester, 14 with low cholesteryl ester). Efflux via SR-BI and ABCA1 was studied using cell systems having either low or high expression levels of these receptors. SR-BI efflux was lower (P = 0.04) with serum from severely icteric patients (3.9 +/- 1.4%) compared with serum from mildly icteric patients (5.1 +/- 1.4%) and was positively correlated with HDL-C and its apolipoproteins. SR-BI-mediated efflux was not correlated with any particular mature HDL but was negatively correlated with small lipid-poor prebeta-1 HDL. Consistent with severely icteric patients having high prebeta-1 HDL levels, the ABCA1 efflux was significantly higher with their serum (4.8 +/- 2.2%) compared with serum from mildly icteric patients (2.0 +/- 0.6%) and was positively correlated with prebeta-1 HDL. These studies demonstrated that prebeta-1 HDL is the preferred acceptor for ABCA1 efflux, whereas many particles mediate SR-BI efflux.

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.

The control by angiotensin II of cholesterol supply for aldosterone biosynthesis

In the adrenal glomerulosa cell, aldosterone is synthesized from cholesterol, which is supplied to the cell and stored under the form of cholesterol esters, then hydrolyzed to be transferred to the mitochondrial outer membrane and finally transported to the inner membrane where the P450 side-chain cleavage enzyme will convert it to pregnenolone. Angiotensin II (AngII), one of the major physiological regulators of mineralocorticoid synthesis, appears to affect most of the steps along this cascade and thus to exert a powerful control over the use of cholesterol for aldosterone production.

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.

Scavenger receptor class B type I-mediated cholesteryl ester-selective uptake and efflux of unesterified cholesterol. Influence of high density lipoprotein size and structure

Scavenger receptor (SR)-BI catalyzes the selective uptake of cholesteryl ester (CE) from high density lipoprotein (HDL) by a two-step process that involves the following: 1) binding of HDL to the receptor and 2) diffusion of the CE molecules into the cell plasma membrane. We examined the effects of the size of discoidal HDL particles containing wild-type (WT) apoA-I on selective uptake of CE and efflux of cellular free (unesterified) cholesterol (FC) from COS-7 cells expressing SR-BI to determine the following: 1) the influence of apoA-I conformation on the lipid transfer process, and 2) the contribution of receptor binding-dependent processes to the overall efflux of cellular FC. Large (10 nm diameter) reconstituted HDL bound to SR-BI better (B(max) approximately 420 versus 220 ng of apoA-I/mg cell protein), delivered more CE, and promoted more FC efflux than small ( approximately 8 nm) particles. When normalized to the number of reconstituted HDL particles bound to the receptor, the efficiencies of either CE uptake or FC efflux with these particles were the same indicating that altering the conformation of WT apoA-I modulates binding to the receptor (step 1) but does not change the efficiency of the subsequent lipid transfer (step 2); this implies that binding induces an optimal alignment of the WT apoA-I.SR-BI complex so that the efficiency of lipid transfer is always the same. FC efflux to HDL is affected both by binding of HDL to SR-BI and by the ability of the receptor to perturb the packing of FC molecules in the cell plasma membrane.

Hormone-sensitive lipase is required for high-density lipoprotein cholesteryl ester-supported adrenal steroidogenesis

Steroid hormones are synthesized using cholesterol as precursor, with a substantial portion supplied by the selective uptake of lipoprotein-derived cholesteryl esters. Adrenals express a high level of neutral cholesteryl ester hydrolase activity, and recently hormone-sensitive lipase (HSL) was shown to be responsible for most adrenal neutral cholesteryl ester hydrolase activity. To determine the functional importance of HSL in adrenal steroidogenesis, adrenal cells were isolated from control and HSL-/- mice, and the in vitro production of corticosterone was quantified. Results show that, even though adrenal cholesteryl ester content was substantially elevated in both male and female HSL-/- mice, basal corticosterone production was reduced approximately 50%. The maximum corticosterone production induced by dibutyryl cAMP, and lipoproteins was approximately 75-85% lower in adrenal cells from HSL-/- mice compared with control. There is no intrinsic defect in the conversion of cholesterol into steroids in HSL-/- mice. Dibutyryl cAMP-stimulated conversion of high-density lipoprotein cholesteryl esters into corticosterone was reduced 97% in HSL-/- mice. An increase in low-density lipoprotein receptor expression appears to be one of the compensatory mechanisms for cholesterol delivery in HSL-/- mice. These findings suggest that HSL is functionally linked to the selective pathway and is critically involved in the intracellular processing and availability of cholesterol for adrenal steroidogenesis.

SR-BI and cholesterol uptake into steroidogenic cells

Scavenger receptor class B, type I (SR-BI) is a receptor for high-density lipoprotein (HDL) that mediates cellular uptake of HDL cholesteryl ester (HDL CE) and is the major route for cholesterol delivery to the steroidogenic pathway. SR-BI is localized in specialized microvillar channels in the plasma membrane that retain HDL and are sites of selective uptake of HDL CE. The formation of microvillar channels in the adrenal gland requires SR-BI and is regulated by adrenocorticotropin hormone. SR-BI-mediated uptake of HDL CE is a two-step process that requires high-affinity binding of HDL followed by transfer of CE to the membrane. CE uptake is followed by hydrolysis to free cholesterol by a neutral CE hydrolase. In this review, we describe new information on the mechanism of transfer of cholesterol from plasma HDL to the steroidogenic pathway in endocrine cells.

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.

Identification and characterization of rodent ABCA1 in isolated type II pneumocytes

ATP-binding cassette transporter A1 (ABCA1) promotes transfer of cholesterol and phospholipid from cells to lipid-free serum apolipoproteins. ABCA1 mRNA and protein expression in primary cultures of rodent type II cells was sensitive to upregulation with 5 microM 9-cis-retinoic acid (9cRA) and 6.2 microM 22-hydroxycholesterol (22-OH). The increase in ABCA1 protein levels was time dependent and was maximal after 16 h of exposure to 9cRA + 22-OH. Inducible ABCA1 was also found in transformed cell lines of lung origin: WI38/VA13, A549, and NIH-H441 cells. Stimulation of ABCA1 in rat type II cells by 9cRA + 22-OH resulted in a four- or fivefold enhancement of efflux of radioactive phospholipid or cholesterol, respectively, from the pneumocytes to apolipoprotein AI (apo AI), whereas cAMP (0.3 mM) had no effect. ABCA1-mediated lipid efflux to apo AI was independent of the surfactant secretion pathway, inasmuch as upregulation of ABCA1 resulted in a reduction of secretagogue-stimulated surfactant phospholipid release. These studies demonstrate the presence of functional ABCA1 in type II cells from the lung.