Liver-specific overexpression of scavenger receptor BI decreases levels of very low density lipoprotein ApoB, low density lipoprotein ApoB, and high density lipoprotein in transgenic mice

Apolipoprotein E regulates dietary cholesterol absorption and biliary cholesterol excretion: Studies in C57BL/6 apolipoprotein E knockout mice

The present study examined the role of apolipoprotein E (apoE) in the regulation of dietary cholesterol absorption and biliary cholesterol excretion. Increasing dietary cholesterol from 0.02% to 0.5% in C57BL/6 wild-type mice decreased the percentage of dietary cholesterol that is absorbed by 25%, and this decrease was associated with a 2-fold increase in gallbladder biliary cholesterol concentration. In contrast, increasing dietary cholesterol from 0. 02% to 0.5% in C57BL/6 apoE knockout mice produced no significant suppression of the percentage dietary cholesterol absorption and increased gallbladder biliary cholesterol concentration only 16%. Whereas in wild-type mice, the increase in dietary cholesterol increased the hepatic excretion of biliary cholesterol 4-fold, there was only a 2-fold increase in apoE knockout mice. On both the low- and the high-cholesterol diets, whole liver and isolated hepatocyte cholesterol content was higher in the apoE knockout mice. These results suggest that, in response to dietary cholesterol, apoE may play a critical role in decreasing the percentage absorption of dietary cholesterol and increasing biliary cholesterol excretion. These observations suggest a mechanism whereby the absence of apoE contributes to the propensity for tissue cholesterol deposition and accelerated atherogenesis in apoE knockout mice.

Gene transfer and hepatic overexpression of the HDL receptor SR-BI reduces atherosclerosis in the cholesterol-fed LDL receptor-deficient mouse

HDL cholesterol levels in humans are inversely correlated with the risk of atherosclerosis. The class B scavenger receptor type I (SR-BI) is the first molecularly well-defined HDL receptor, and hepatic overexpression of SR-BI in normal mice has been shown to result in decreased plasma HDL cholesterol levels. To determine whether SR-BI overexpression is proatherogenic or is protective against atherosclerosis, LDL receptor-deficient mice were placed on a high-fat/high-cholesterol diet for 2 or 12 weeks to induce atherosclerotic lesions of different stages and then were injected with a recombinant adenovirus encoding murine SR-BI. Transient hepatic overexpression of SR-BI in mice with both early and advanced lesions significantly decreased atherosclerosis. SR-BI expression was associated with markedly decreased HDL cholesterol and either unchanged or only modestly reduced non-HDL cholesterol levels; in all experiments, the mean HDL cholesterol levels were significantly correlated with atherosclerotic lesion size. These data suggest that interventions that promote HDL cholesterol transport and lower plasma HDL cholesterol levels can suppress atherosclerosis, even when initiated after significant lesion development. Thus, stimulation of hepatic SR-BI activity may provide a novel target for therapeutic intervention in atherosclerotic cardiovascular disease.

Scavenger receptor regulation and atherosclerosis

Atherosclerosis and its complications, such as coronary heart disease, heart infarction and stroke, are the leading causes of death in the developed world. High blood pressure, diabetes, smoking and a diet high in cholesterol and lipids clearly increase the likelihood of premature atherosclerosis, albeit other factors, such as the individual genetic makeup, may play an additional role. During atherosclerosis, uncontrolled cholesterol and lipid accumulation in macrophages and smooth muscle cells leads to foam cell formation and to the progression of the atherosclerotic plaque. This review will focus on foam cell formation within the atherosclerotic lesion, the involvement of the scavenger receptor genes in this process, and the possibility to interfere with scavenger receptor function to reduce the progression of atherosclerosis. To date, the regulatory mechanisms for the expression of scavenger receptor genes and their role in atherosclerosis are not well characterized. Knowledge on this subject could lead to a better understanding of the process, prevention and therapy of this disease.

Mechanism of action of niacin on lipoprotein metabolism

It is generally accepted that the increased concentrations of apolipoprotein (apo) B containing very low-density lipoproteins (VLDL) and low-density lipoproteins (LDL), and decreased levels of apo AI containing high-density lipoproteins (HDL) are correlated to atherosclerotic cardiovascular disease. Current evidence indicates that the post-translational apo-B degradative processes regulate the hepatic assembly and secretion of VLDL and the subsequent generation of LDL particles. The availability of triglycerides (TG) for the addition to apo B during intracellular processing appears to play a central role in targeting apo B for either intracellular degradation or assembly and secretion as VLDL particles. Based on the availability of TG, the liver secretes either dense TG-poor VLDL2 or large TG-rich VLDL1 particles, and these particles serve as precursors for the formation of more buoyant or small, dense LDL particles by lipid transfer protein- and hepatic lipase-mediated processes. HDLs are a heterogenous class of lipoproteins, and apo AI (the major protein of HDL) participates in reverse cholesterol transport, a process by which excess cholesterol is eliminated. Recent studies indicate that HDL particles containing only apo A-I (LPA-I) are more effective in reverse cholesterol transport and more anti-atherogenic than HDL particles containing both apo A-I and apo A-II (LPA-I + A-II).

Influence of the HDL receptor SR-BI on atherosclerosis

The scavenger receptor class B, type I (SR-BI) is an HDL receptor that mediates selective cholesterol uptake from HDL to cells. In rodents, SR-BI has a critical influence on plasma HDL-cholesterol concentration and structure, the delivery of cholesterol to steroidogenic tissues, female fertility, and biliary cholesterol concentration. SR-BI can also serve as a receptor for non-HDL lipoproteins and appears to play an important role in reverse cholesterol transport. Recent studies involving the manipulation of SR-BI expression in mice, either using adenovirus-mediated or transgenic hepatic overexpression or using homologous recombination for complete functional ablation, indicate that the expression of SR-BI protects against atherosclerosis. If SR-BI has a similar activity in humans, it may become an attractive target for therapeutic intervention.

The HDL receptor SR-BI: a new therapeutic target for atherosclerosis?

Although high-density lipoprotein (HDL) metabolism is a crucial process for cholesterol homeostasis and coronary heart disease, therapeutic approaches for selective modification of plasma HDL levels are not currently available. The discovery of well-defined cell-surface HDL receptors should provide new avenues for treatment of atherosclerotic cardiovascular disease. In fact, SR-BI, a recently identified receptor for selective HDL cholesterol uptake, is relevant for physiological processes (for example, HDL metabolism, steroidogenesis and biliary cholesterol secretion) and pathophysiological conditions (for example, atherosclerosis) in animal models. If SR-BI has similar activities in humans, it might represent a new therapeutic target for atherosclerosis.

Hepatic scavenger receptor BI promotes rapid clearance of high density lipoprotein free cholesterol and its transport into bile

The clearance of free cholesterol from plasma lipoproteins by tissues is of major quantitative importance, but it is not known whether this is passive or receptor-mediated. Based on our finding that scavenger receptor BI (SR-BI) promotes free cholesterol (FC) exchange between high density lipoprotein (HDL) and cells, we tested whether SR-BI would effect FC movement in vivo using [(14)C]FC- and [(3)H]cholesteryl ester (CE)-labeled HDL in mice with increased (SR-BI transgenic (Tg)) or decreased (SR-BI attenuated (att)) hepatic SR-BI expression. The initial clearance of HDL FC was increased in SR-BI Tg mice by 72% and decreased in SR-BI att mice by 53%, but was unchanged in apoA-I knockout mice compared with wild-type mice. Transfer of FC to non-HDL and esterification of FC were minor and could not explain differences. The hepatic uptake of FC was increased in SR-BI Tg mice by 34% and decreased in SR-BI att mice by 22%. CE clearance and uptake gave similar results, but with much slower rates. The uptake of HDL FC and CE by SR-BI Tg primary hepatocytes was increased by 2.2- and 2.6-fold (1-h incubation), respectively, compared with control hepatocytes. In SR-BI Tg mice, the initial biliary secretion of [(14)C]FC was markedly increased, whereas increased [(3)H]FC appeared after a slight delay. Thus, in the mouse, a major portion of the clearance of HDL FC from plasma is mediated by SR-BI.

Scavenger receptor class B, type I, mediates selective uptake of low density lipoprotein cholesteryl ester

Scavenger receptor, class B, type I (SR-BI) is a cell-surface glycoprotein that mediates selective uptake of high density lipoprotein cholesteryl ester (CE) without the concomitant uptake and degradation of the particle. We have investigated the endocytic and selective uptake of low density lipoprotein (LDL)-CE by SR-BI using COS-7 cells transiently transfected with mouse SR-BI. Analysis of lipoprotein uptake data showed a concentration-dependent LDL-CE-selective uptake when doubly labeled LDL particles were incubated with SR-BI-expressing COS-7 cells. In contrast to vector-transfected cells, SR-BI-expressing COS-7 cells showed marked increases in LDL cell association and CE uptake by the selective uptake pathway, but only a modest increase in CE uptake by the endocytic pathway. SR-BI-mediated LDL-CE-selective uptake exceeded LDL endocytic uptake by 50-100-fold. SR-BI-mediated LDL-CE-selective uptake was not inhibited by the proteoglycan synthesis inhibitor, p-nitrophenyl-beta-D-xylopyranoside or by the sulfation inhibitor sodium chlorate, indicating that SR-BI-mediated LDL-CE uptake occurs independently of LDL interaction with cell-surface proteoglycan. Analyses with subclones of Y1 adrenocortical cells showed that LDL-CE-selective uptake was proportional to the level of SR-BI expression. Furthermore, antibody directed to the extracellular domain of SR-BI blocked LDL-CE-selective uptake in adrenocortical cells. Thus, in cells that normally express SR-BI and in transfected COS-7 cells SR-BI mediates the efficient uptake of LDL-CE via the selective uptake mechanism. These results suggest that SR-BI may influence the metabolism of apoB-containing lipoproteins in vivo by mediating LDL-CE uptake into SR-BI-expressing cells.

Low density lipoprotein uptake: holoparticle and cholesteryl ester selective uptake

Low density lipoproteins (LDL) contain apolipoprotein B-100 and are cholesteryl ester-rich, triglyceride-poor macromolecules, arising from the lipolysis of very low density lipoproteins. This review will describe the receptors responsible for uptake of whole LDL particles (holoparticle uptake), and the selective uptake of LDL cholesteryl ester. The LDL-receptor mediates the internalization of whole LDL through an endosomal-lysosomal pathway, leading to complete degradation of LDL. Increasing LDL-receptor expression by pharmacological intervention efficiently reduces blood LDL concentrations. The lipolysis stimulated receptor and LDL-receptor related protein may also lead to complete degradation of LDL in presence of free fatty acids and apolipoprotein E- or lipase-LDL complexes, respectively. Selective uptake of LDL cholesteryl ester has been demonstrated in the liver, especially in rodents and humans. This activity brings five times more LDL cholesteryl ester than the LDL-receptor to human hepatoma cells, suggesting that it is a physiologically significant pathway. The lipoprotein binding site of HepG2 cells mediates this process and recognizes all lipoprotein classes. Scavenger receptor class B type I and CD36, which mediate the selective uptake of high density lipoprotein cholesteryl ester, are potentially involved in LDL cholesteryl ester selective uptake, since they both bind LDL with high affinity. It is not known whether they are identical to the uncloned lipoprotein binding site and if the selective uptake of LDL cholesteryl ester produces a less atherogenic particle. If this is verified, pharmacological up-regulation of LDL cholesteryl ester selective uptake may become another therapeutic approach for reducing blood LDL-cholesterol levels and the risk of atherosclerosis.

Influence of the high density lipoprotein receptor SR-BI on reproductive and cardiovascular pathophysiology

The high density lipoprotein (HDL) receptor SR-BI (scavenger receptor class B type I) mediates the selective uptake of plasma HDL cholesterol by the liver and steroidogenic tissues. As a consequence, SR-BI can influence plasma HDL cholesterol levels, HDL structure, biliary cholesterol concentrations, and the uptake, storage, and utilization of cholesterol by steroid hormone-producing cells. Here we used homozygous null SR-BI knockout mice to show that SR-BI is required for maintaining normal biliary cholesterol levels, oocyte development, and female fertility. We also used SR-BI/apolipoprotein E double homozygous knockout mice to show that SR-BI can protect against early-onset atherosclerosis. Although the mechanisms underlying the effects of SR-BI loss on reproduction and atherosclerosis have not been established, potential causes include changes in (i) plasma lipoprotein levels and/or structure, (ii) cholesterol flux into or out of peripheral tissues (ovary, aortic wall), and (iii) reverse cholesterol transport, as indicated by the significant reduction of gallbladder bile cholesterol levels in SR-BI and SR-BI/apolipoprotein E double knockout mice relative to controls. If SR-BI has similar activities in humans, it may become an attractive target for therapeutic intervention in a variety of diseases.

Hepatocellular transport and secretion of biliary lipids

Bile is the route for elimination of cholesterol from the body. Recent studies have begun to elucidate hepatocellular, molecular and physical-chemical mechanisms whereby bile salts stimulate biliary secretion of cholesterol together with phospholipids, which are enriched (up to 95%) in phosphatidylcholines. Active translocation of bile salts and phosphatidylcholines across the hepatocyte's canalicular plasma membrane provides the driving force for biliary lipid secretion. This facilitates physical-chemical interactions between detergent-like bile salt molecules and the ectoplasmic leaflet of the canalicular membrane, which result in biliary secretion of cholesterol and phosphatidylcholines as vesicles. Within the hepatocyte, separate molecular pathways function to resupply bile salts, phosphatidylcholines and cholesterol to the canalicular membrane for ongoing biliary lipid secretion.

Scavenger receptor BI and cholesterol trafficking

Scavenger receptor BI (SR-BI) mediates the selective uptake of HDL cholesteryl ester into steroidogenic cells and the liver and is a major determinant of the plasma HDL concentration in the mouse. Recent studies indicate that SR-BI also alters the metabolism of apolipoprotein B-containing particles and influences the development of atherosclerosis in several animal models. These results and the similar pattern of SR-BI expression in humans emphasize that it is important to learn how this receptor influences lipoprotein metabolism and atherosclerosis in people.

Atheroprotective mechanisms of HDL

The aim of this review was to bring together results obtained from studies on different aspects of HDL as related to CHD and atherosclerosis. As atherosclerosis is a multistep process, the various components of HDL can intervene at different stages, such as induction of monocyte adhesion molecules, prevention of LDL modification and removal of excess cholesterol by reverse cholesterol transport. Transgenic technology has provided a model for atherosclerosis, and permitted evaluation of the contributions of different HDL components towards the global effect. The availability of apo AIV transgenic mice amplified the results obtained from apo AI overexpressors with respect to prevention of atherosclerosis. Prevention of atherosclerosis in apo E deficient mice by relatively small amounts of macrophage derived apo E may open new possibilities for therapeutic intervention. Contrary to early notions, increased plasma levels of CETP, even in the presence of low but functionally normal HDL, were atheroprotective. The extent to which paraoxonase and apo J participate in prevention of human atherosclerosis needs further evaluation. The findings that LCAT overexpression in rabbits was atheroprotective in contrast to increase in atherosclerosis in h LCAT tg mice, which was only partially corrected by CETP expression, call for some caution in the extrapolation of results from transgenic animals to humans. The important discovery of SR-BI as the receptor for selective uptake of CE from HDL revived interest in the clearance of CE from plasma. This pathway supplies also the vital precursor for steroidogenesis in adrenals and gonads and was shown to be dependent on apo AI.

Probucol enhances selective uptake of HDL-associated cholesteryl esters in vitro by a scavenger receptor B-I-dependent mechanism

Recently, the class B, type I scavenger receptor (SR-BI) has been shown to mediate the selective uptake of high density lipoprotein (HDL) cholesteryl esters (CEs), ie, lipid uptake independent of HDL holoparticle uptake. In vivo, this selective uptake delivers CEs to the liver for excretion and to steroidogenic tissues for hormone synthesis. Probucol, a hydrophobic antioxidant drug, lowers plasma cholesterol in humans and rodents and may inhibit progression of atherosclerosis and postangioplasty restenosis. In this study, the effect of probucol on HDL selective CE uptake was investigated in mice and in cells expressing SR-BI. Probucol feeding lowered plasma HDL cholesterol and markedly increased selective CE uptake from HDL in the liver and adrenal glands. However, probucol did not alter SR-BI protein levels in membranes from these organs. When incubated with control Chinese hamster ovary (CHO) cells, HDL isolated from probucol-treated mice (P-HDL) and HDL from control mice (C-HDL) showed similar low selective uptake of CEs. However, when incubated with SR-BI-transfected CHO cells, P-HDL showed a 2-fold increase in selective uptake compared with C-HDL. In an adrenal cell line (Y1-BS1), which expresses SR-BI in an adrenocorticotropic hormone-inducible manner, P-HDL showed significantly greater selective CE uptake than did C-HDL, and the differential response was amplified by adrenocorticotropic hormone treatment. In contrast to P-HDL, incorporation of this compound into HDL in vitro did not result in stimulation of selective CE uptake by SR-BI-transfected CHO cells, even though a significant mass of probucol could be detected in the HDL preparation. The specific interaction of P-HDL with SR-BI in cell culture could be observed after only 24 hours of probucol feeding, when there were minimal changes in HDL size and composition. Thus, probucol or one of its metabolites increases selective CE uptake in vivo by modifying HDL in a way that causes enhanced interaction with SR-BI. The increased interaction of P-HDL with SR-BI in the liver and arterial wall may be partly responsible for the effects of probucol on atherosclerosis and restenosis.

Decreased atherosclerosis in heterozygous low density lipoprotein receptor-deficient mice expressing the scavenger receptor BI transgene

Scavenger receptor type B class I (SR-BI), initially identified as a receptor that recognizes low density lipoprotein (LDL), was recently shown to mediate the selective uptake of high density lipoprotein (HDL) cholesteryl esters in liver and steroidogenic tissues. To evaluate effects on atherosclerosis, transgenic mice with liver-specific overexpression of SR-BI (SR-BI Tg mice) have been crossed onto LDL receptor-deficient backgrounds. To induce atherosclerosis in a setting of moderate hypercholesterolemia, heterozygous LDL receptor-deficient mice (LDLR1) were fed a high fat/cholesterol/bile salt diet, and homozygous LDL receptor knock-outs (LDLR0) were fed a high fat/cholesterol diet. LDLR1/SR-BI Tg mice showed decreases in VLDL, LDL, and HDL cholesterol and a significant 80% decrease in mean lesion area in the aortic root compared with LDLR1 mice (female LDLR1 74, 120 micrometers(2) versus LDLR1/SR-BI Tg 12, 667 micrometers(2); male 25, 747 micrometers(2)++ versus 5, 448 micrometers(2), respectively). LDLR0/SR-BI Tg mice showed decreased LDL and HDL cholesterol but increased VLDL cholesterol and no significant difference in extent of atherosclerosis compared with LDLR0 mice. Combined data analysis showed a strong correlation between atherosclerotic lesion area and the VLDL+LDL cholesterol level but no correlation with HDL level. These studies demonstrate a strong anti-atherogenic potential of hepatic SR-BI overexpression. In mice with marked overexpression of SR-BI, the protective effect appears to be primarily related to the lowering of VLDL and LDL cholesterol levels.