Biomedical subjects

A R Tall

Publications and source records attributed to A R Tall.

At least 55 records · Page 3Linked to original sources

PubMed1999

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.

Administration, Oral↗

PubMed1999

Targeted mutation of plasma phospholipid transfer protein gene markedly reduces high-density lipoprotein levels.

It has been proposed that the plasma phospholipid transfer protein (PLTP) facilitates the transfer of phospholipids and cholesterol from triglyceride-rich lipoproteins (TRL) into high-density lipoproteins (HDL). To evaluate the in vivo role of PLTP in lipoprotein metabolism, we used homologous recombination in embryonic stem cells and produced mice with no PLTP gene expression. Analysis of plasma of F2 homozygous PLTP-/- mice showed complete loss of phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, sphingomyelin, and partial loss of free cholesterol transfer activities. Moreover, the in vivo transfer of [3H]phosphatidylcholine ether from very-low-density proteins (VLDL) to HDL was abolished in PLTP-/- mice. On a chow diet, PLTP-/- mice showed marked decreases in HDL phospholipid (60%), cholesterol (65%), and apo AI (85%), but no significant change in non-HDL lipid or apo B levels, compared with wild-type littermates. On a high-fat diet, HDL levels were similarly decreased, but there was also an increase in VLDL and LDL phospholipids (210%), free cholesterol (60%), and cholesteryl ester (40%) without change in apo B levels, suggesting accumulation of surface components of TRL. Vesicular lipoproteins were shown by negative-stain electron microscopy of the free cholesterol- and phospholipid-enriched IDL/LDL fraction. Thus, PLTP is the major factor facilitating transfer of VLDL phospholipid into HDL. Reduced plasma PLTP activity causes markedly decreased HDL lipid and apoprotein, demonstrating the importance of transfer of surface components of TRL in the maintenance of HDL levels. Vesicular lipoproteins accumulating in PLTP-/- mice on a high-fat diet could influence the development of atherosclerosis.

Animals↗

PubMed1999

Remodeling of HDL by CETP in vivo and by CETP and hepatic lipase in vitro results in enhanced uptake of HDL CE by cells expressing scavenger receptor B-I.

The transport of HDL cholesteryl esters (CE) from plasma to the liver involves a direct uptake pathway, mediated by hepatic scavenger receptor B-I (SR-BI), and an indirect pathway, involving the exchange of HDL CE for triglycerides (TG) of TG-rich lipoproteins by cholesteryl ester transfer protein (CETP). We carried out HDL CE turnover studies in mice expressing human CETP and/or human lecithin:cholesterol acyltransferase (LCAT) transgenes on a background of human apoA-I expression. The fractional clearance of HDL CE by the liver was delayed by LCAT transgene, while the CETP transgene increased it. However, there was no incremental transfer of HDL CE radioactivity to the TG-rich lipoprotein fraction in mice expressing CETP, suggesting increased direct removal of HDL CE in the liver. To evaluate the possibility that this might be mediated by SR-BI, HDL isolated from plasma of the different groups of transgenic mice was incubated with SR-BI transfected or control CHO cells. HDL isolated from mice expressing CETP showed a 2- to 4-fold increase in SR-BI-mediated HDL CE uptake, compared to HDL from mice lacking CETP. The addition of pure CETP to HDL in cell culture did not lead to increased selective uptake of HDL CE by cells. However, when human HDL was enriched with TG by incubation with TG-rich lipoproteins in the presence of CETP, then treated with hepatic lipase, there was a significant enhancement of HDL CE uptake. Thus, the remodeling of human HDL by CETP, involving CE;-TG interchange, followed by the action of hepatic lipase (HL), leads to the enhanced uptake of HDL CE by cellular SR-BI. These observations suggest that in animals such as humans in which both the selective uptake and CETP pathways are active, the two pathways could operate in a synergistic fashion to enhance reverse cholesterol transport.

Animals↗

PubMed1999

Regulation of adrenal scavenger receptor-BI expression by ACTH and cellular cholesterol pools.

Scavenger receptor BI (SR-BI) mediates selective uptake of high density lipoprotein (HDL) cholesteryl ester in the liver and adrenal gland. Adrenal SR-BI is increased both in adrenocorticotropic hormone (ACTH)-treated mice and also in apolipoprotein A-I knock-out (apoA-I0) mice which have depleted adrenal cholesterol stores. The goal of the present study was to determine whether adrenal cholesterol stores and ACTH have independent effects on SR-BI expression in adrenal gland. Adrenal SR-BI levels were 5-fold higher in apoA-I0 than wild-type mice when killed under low stress condition, and plasma ACTH levels were similar in both strains. After male apoA-I0 or wild-type mice were treated with dexamethasone to suppress ACTH release, adrenal SR-BI protein levels were decreased in both groups but remained 13-fold higher in apoA-I0 than in wild-type mice. By contrast, uncontrolled stress or supplemental ACTH treatment increased SR-BI levels but narrowed the difference in SR-BI expression between apoA-I0 and wild-type. Cholesterol depletion by beta-cyclodextrin in cultured Y1-BS1 adrenal cells also led to a rapid 2- to 3-fold increase in SR-BI mRNA and protein levels, in association with a significant depletion of cellular free cholesterol. These results indicate that depletion of adrenal cholesterol stores can act independently from ACTH to increase SR-BI expression, but in vivo this effect is diminished under high ACTH conditions. Both stimuli may increase selective uptake via increased SR-BI as a means of replenishing cholesterol stores for steroid hormone synthesis.

Adrenal Glands↗

PubMed1998

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.

Scavenger receptor BI (SR-BI) is known to mediate the selective uptake of high density lipoprotein (HDL) cholesteryl ester (CE) in liver and steroidogenic tissues. To evaluate the role of SR-BI in plasma lipoprotein metabolism, we have generated transgenic mice with liver-specific overexpression of murine SR-BI. On a chow diet SR-BI transgenic (SR-BI Tg) mice have decreased HDL-CE, apoA-I, and apoA-II levels; plasma triglycerides, low density lipoprotein (LDL) cholesterol, and very low density lipoprotein (VLDL) and LDL apoB were also decreased, compared with control mice. Turnover studies using non-degradable CE and protein labels showed markedly increased total and selective uptake of HDL-CE in the liver and increased HDL protein catabolism in both liver and kidney. To evaluate the changes in apoB further, mice were challenged with high fat, high cholesterol diets. In SR-BI Tg mice plasma apoB levels were only 3-15% of control levels, and the dietary increase in VLDL and LDL apoB was virtually abolished. These studies show that steady state overexpression of hepatic SR-BI reduces HDL levels and increases reverse cholesterol transport. They also indicate that SR-BI can play a role in the metabolism of apoB-containing lipoproteins. The dual effects of increased reverse cholesterol transport and lowering of apoB-containing lipoproteins that result from hepatic SR-BI overexpression could have anti-atherogenic consequences.

Animals↗

PubMed1998

Sterol regulatory element binding protein-1 activates the cholesteryl ester transfer protein gene in vivo but is not required for sterol up-regulation of gene expression.

The plasma cholesteryl ester transfer protein (CETP) plays a central role in high density lipoprotein metabolism and reverse cholesterol transport. Plasma CETP levels are increased in response to dietary or endogenous hypercholesterolemia as a result of increased gene transcription in liver and periphery. Deletional analysis in human CETP transgenic mice localized this response to a region of the proximal promoter which contains a tandem repeat of the sterol regulatory element (SRE) of the 3-hydroxy-3-methylglutaryl-CoA reductase gene. The purpose of the present study was to evaluate the role of the SRE-like element in CETP promoter activity. Gel shift assays using CETP promoter fragments containing these elements showed binding of the transcription factors, sterol regulatory element-binding protein-1 (SREBP-1) and Yin Yang-1 (YY-1). Point mutations in the SRE-like element, designated MUT1 and MUT2, resulted in decreased binding of SREBP-1 (MUT1) or SREBP-1 and YY-1 (MUT2). To determine the in vivo significance of this binding activity, CETP transgenic mice were prepared containing these promoter point mutations. MUT1 and MUT2 transgenic mice expressed CETP activity and mass in plasma. In response to high fat, high cholesterol diets, both MUT1-CETP and MUT2-CETP transgenic mice displayed induction of plasma CETP activity similar to that observed in natural flanking region (NFR) CETP transgenic mice. Moreover, in stably transfected adipocyte cell lines, MUT1 and MUT2 CETP promoter-reporter genes showed significant induction of reporter activity in response to sterols. To evaluate transactivation by SREBP-1, NFR- and MUT1-CETP transgenic mice were crossed with SREBP-1 transgenic mice. Induction of the SREBP transgene in the liver with a low carbohydrate diet resulted in a 3-fold increase in plasma CETP activity in NFR-CETP/SREBP transgenic mice, but there was no significant change in activity in MUT1-CETP/SREBP transgenic mice. Thus, SREBP-1 transactivates the NFR-CETP transgene in vivo, as a result of interaction with the CETP promoter SREs. However, this interaction is not required for positive sterol induction of CETP gene transcription. The results suggest independent regulation of the CETP gene by SREBP-1 and a distinct positive sterol response factor.

Animals↗

PubMed1998

Biliary cholesterol excretion: a novel mechanism that regulates dietary cholesterol absorption.

The regulation of dietary cholesterol absorption was examined in C57BL/6 and transgenic mice with liver overexpression of the scavenger receptor BI (SR-BI Tg). In C57BL/6 animals, feeding 0.02 to 1% (wt/wt) dietary cholesterol resulted in a dose-dependent decrease in the percentage of dietary cholesterol absorbed. A plot of total daily mass of dietary cholesterol absorbed versus the percentage by weight of cholesterol in the diet yielded a curve suggesting a saturable process with a Km of 0.4% (wt/wt) and a Vmax of 0.65 mg cholesterol/g body weight per day. Dietary cholesterol suppressed hepatic 3-hydroxy-3-methylglutaryl CoA reductase activity, stimulated cholesterol 7alpha-hydroxylase activity, and enhanced fecal excretion of bile acids, but none of these changes correlated with the percentage of dietary cholesterol absorption. Dietary cholesterol also caused an increase in biliary cholesterol concentration, and in this case the concentration of biliary cholesterol was strongly and inversely correlated with the percentage dietary cholesterol absorption (r = -0.63, P < 0.0001). Biliary cholesterol concentration was also directly correlated with daily cholesterol intake, dietary cholesterol mass absorption, and liver cholesterol ester content. Transgene-induced overexpression of SR-BI resulted in a stimulation of excretion of cholesterol into the bile and suppressed percentage dietary cholesterol absorption. Furthermore, biliary cholesterol levels in SR-BI Tg mice were strongly and inversely correlated with the percentage of dietary cholesterol absorbed (r = -0.99, P < 0.0008). In summary, these results suggest that the excretion of cholesterol into the bile plays an important role in regulating the percentage absorption of dietary cholesterol.

Animals↗

PubMed1998

Targeted mutation reveals a central role for SR-BI in hepatic selective uptake of high density lipoprotein cholesterol.

Scavenger receptor BI (SR-BI) is a cell surface receptor that binds high density lipoproteins (HDL) and mediates selective uptake of HDL cholesteryl esters (CE) in transfected cells. To address the physiological role of SR-BI in HDL cholesterol homeostasis, mice were generated bearing an SR-BI promoter mutation that resulted in decreased expression of the receptor in homozygous mutant (designated SR-BI att) mice. Hepatic expression of the receptor was reduced by 53% with a corresponding increase in total plasma cholesterol levels of 50-70% in SR-BI att mice, attributable almost exclusively to elevated plasma HDL. In addition to increased HDL-CE, HDL phospholipids and apo A-1 levels were elevated, and there was an increase in HDL particle size in mutant mice. Metabolic studies using HDL bearing nondegradable radiolabels in both the protein and lipid components demonstrated that reducing hepatic SR-BI expression by half was associated with a decrease of 47% in selective uptake of CE by the liver, and a corresponding reduction of 53% in selective removal of HDL-CE from plasma. Taken together, these findings strongly support a pivotal role for hepatic SR-BI expression in regulating plasma HDL levels and indicate that SR-BI is the major molecule mediating selective CE uptake by the liver. The inverse correlation between plasma HDL levels and atherosclerosis further suggests that SR-BI may influence the development of coronary artery disease.

Animals↗

PubMed1998

Scavenger receptor class B type I as a mediator of cellular cholesterol efflux to lipoproteins and phospholipid acceptors.

We recently reported that the rate of efflux of cholesterol from cells to high density lipoprotein (HDL) was related to the expression level of scavenger receptor class B type I (SR-BI). Moreover, the expression of this receptor in atheromatous arteries raises the possibility that SR-BI mediates cholesterol efflux in the arterial wall (Ji, Y., Jian, B., Wang, N., Sun, Y., de la Llera Moya, M., Phillips, M. C., Rothblat, G. H., Swaney, J. B., and Tall, A. R. (1997) J. Biol. Chem. 272, 20982-20985). In this paper we describe studies that suggest that the presence of phospholipid on acceptor particles plays an important role in modulating interaction with the SR-BI. Specifically, enrichment of serum with phospholipid resulted in marked stimulation of cholesterol efflux from cells that had higher levels of SR-BI expression, like Fu5AH or Y1-BS1 cells, and little or no stimulation in cells with low SR-BI levels, such as Y-1 cells. Stimulation of efflux by phospholipid enrichment was also a function of SR-BI levels in Chinese hamster ovary cells transfected with the SR-BI gene. Efflux to protein-free vesicles prepared with 1-palmitoyl-2-oleoylphosphatidyl-choline also correlated with SR-BI levels, suggesting that phospholipid, as well as protein, influences the interaction that results in cholesterol efflux. By contrast, cholesterol efflux from a non-cell donor showed no stimulation consequent to phospholipid enrichment of the serum acceptor. These results may help to explain observations in the literature that document an increased risk of atherosclerosis in patients with depressed levels of HDL phospholipid even in the face of normal HDL cholesterol levels.

Animals↗

PubMed1998

Increased sphingomyelin content of plasma lipoproteins in apolipoprotein E knockout mice reflects combined production and catabolic defects and enhances reactivity with mammalian sphingomyelinase.

Apolipoprotein E knockout (apoE0) mice accumulate atherogenic remnant lipoproteins in plasma. We now provide evidence that these particles are enriched in sphingomyelin (SM), and explore the mechanisms and possible pathophysiological consequences of this finding. The phosphatidylcholine/sphingomyelin (PC/SM) ratio was reduced in all lipoproteins in apoE0 mice compared with wild-type (Wt) mice (2.0+/-0.2 vs. 4.7+/-0.5; 2.8+/-0.5 vs. 5.5+/-0.9; 1.9+/-0. 5 vs. 4.6+/-0.5 for VLDL, LDL, and HDL), reflecting 400 and 179% increases in plasma pools of SM and PC, respectively. Turnover studies using [14C]PC/[3H]SM VLDL or HDL showed that the fractional catabolic rate (FCR) of VLDL-SM and HDL-SM were markedly reduced in the apoE0 mice compared with Wt mice, while the FCRs of VLDL-PC and HDL-PC were similar. By contrast, the FCRs of [3H]PC ether and [14C]SM were identical in apoE0 and Wt mice. The production rates of VLDL-SM and HDL-SM in apoE0 mice were much higher than in Wt mice, while the production rates of lipoprotein PC were similar. To assess the underlying mechanisms, we also measured the PC/SM ratio in VLDL and LDL of LDL receptor knockout (LDLr0) and hepatic LDL receptor-related protein knockout/LDLr0 mice, but found no difference with Wt mice. Using S-sphingomyelinase, an enzyme secreted by macrophages and endothelial cells, we found that VLDL and LDL from apoE0, but not from Wt or LDLr0 mice, were significantly aggregated, and that aggregation was not prevented by adding back apoE. We then enriched the apoE0-VLDL and Wt-VLDL with different amounts of SM, and found that VLDL aggregation was enhanced. Thus, the increased SM content of lipoproteins in apoE0 mice is due to combined synthesis and clearance defects. Impaired SM clearance reflects resistance to intravascular enzymes and delayed removal by a non-LDLr, non-LDLr related protein pathway. The increased SM content in slowly cleared remnant lipoproteins may enhance their susceptibility to arterial wall SMase and increase their atherogenic potential.

Acyltransferases↗

PubMed1998

The implications of the structure of the bactericidal/permeability-increasing protein on the lipid-transfer function of the cholesteryl ester transfer protein.

The cholesteryl ester transfer protein (CETP) is evolutionarily related to the bactericidal/permeability-increasing protein (BPI). The recently solved structure of BPI shows an elongated, boomerang-shaped molecule, with two hydrophobic pockets opening to its concave side. These pockets each contain a phospholipid molecule. A model of CETP, based on the recently solved crystal structure of BPI, provides the basis for interpreting functional studies on CETP. In this model, C-terminal residues 461-476, which were shown to be required for neutral lipid transfer between plasma lipoproteins, from an amphipathic helix covering the opening of the N-terminal pocket. A possible lipid-transfer mechanism for CETP, with the initial step involving the disordering of lipids in the lipoprotein surface, followed by the flipping and entry of a lipid molecule into the hydrophobic lipid-binding pocket, is hypothesized in light of structural evidence and recent studies.

Antimicrobial Cationic Peptides↗

PubMed1998

Plasma lipid transfer proteins, high-density lipoproteins, and reverse cholesterol transport.

Cholesteryl ester transfer protein (CETP) and phospholipid transfer protein (PLTP) are members of the lipid transfer/lipopolysaccharide binding protein gene family. Recently, the crystal structure of one of the members of the gene family, bactericidal permeability increasing protein, was solved, providing potential insights into the mechanisms of action of CETP and PLTP. These molecules contain intrinsic lipid binding sites and appear to act as carrier proteins that shuttle between lipoproteins to redistribute lipids. The phenotype of human CETP genetic deficiency states and CETP transgenic mice indicates that CETP plays a major role in the catabolism of high-density lipoprotein (HDL) cholesteryl esters and thereby influences the concentration, apolipoprotein content, and size of HDL particles in plasma. PLTP also appears to have an important role in determining HDL levels and speciation. Recent data indicate that genetic CETP deficiency is associates with an excess of coronary heart disease in humans, despite increased HDL levels. Also, CETP expression is anti-atherogenic in many mouse models, even while lowering HDL. These data tend to support the reverse cholesterol transport hypothesis, i.e., that anti-atherogenic properties of HDL are related to its role in reverse cholesterol transport. Recently, another key molecule involved in this pathway was identified, scavenger receptor BI; this mediates the selective uptake of HDL cholesteryl esters in the liver and thus constitutes a pathway of reverse cholesterol transport parallel to that mediated by CETP. Reflecting its role in reverse cholesterol transport, the CETP gene is up-regulated in peripheral tissues and liver in responses to dietary or endogenous hypercholesterolemia. An analysis of the CETP proximal promoter indicates that it contains sterol regulatory elements highly homologous to those present in 3-hydroxy-3-methylglutaryl-coenzyme A reductase; the CETP gene is transactivated by the binding of SREBP-1 to these elements. A challenge for the future will be the manipulation of components of the reverse cholesterol transport pathway, such as CETP, PLTP, or scavenger receptor BI for therapeutic benefit.

Animals↗

PubMed1998

An overview of reverse cholesterol transport.

Reverse cholesterol transport is a multi-step process resulting in the net movement of cholesterol from peripheral tissues back to the liver via the plasma compartment. Cellular cholesterol efflux is mediated by HDL, acting in conjunction with the cholesterol esterifying enzyme, lecithin: cholesterol acyltransferase. Cholesteryl ester accumulating in HDL can then follow a number of different fates: uptake in the liver in HDL containing apolipoprotein (particle uptake) by LDL receptors, selective uptake of HDL cholesteryl ester in liver or other tissues involving scavenger receptor B1, or transfer to triglyceride-rich lipoproteins as a result of the activity of cholesteryl ester transfer protein, with subsequent uptake of triglyceride-rich lipoprotein remnants in the liver. Recently, we and others have taken a molecular approach to analysing the different components of reverse cholesterol transport, by over- or under-expression of individual molecules in induced mutant mouse models, or by the study of human mutations involving molecules of reverse cholesterol transport. Such studies reveal that over-expression of the major HDL apoprotein, apolipoprotein A-I, is clearly anti-atherogenic. However, over- or under-expression of molecules such as cholesteryl ester transfer protein, which have opposite effects on HDL levels and reverse cholesterol transport, suggest that both HDL levels as well as the dynamics of cholesterol movement through HDL are involved in the anti-atherogenic actions of HDL.

Animals↗

PubMed1998

Relationship of HDL and coronary heart disease to a common amino acid polymorphism in the cholesteryl ester transfer protein in men with and without hypertriglyceridemia.

Plasma triglyceride (TG) levels are inversely related to HDL-cholesterol levels and subjects with high TG and low HDL cholesterol have increased coronary heart disease (CHD) risk. Plasma cholesteryl ester transfer protein (CETP) transfers cholesteryl esters from HDL to TG-rich lipoproteins. In this study we determined the relationship between a common CETP amino acid polymorphism (I405V) and CETP and HDL levels and CHD prevalence in 576 men of Japanese ancestry in the Honolulu Heart Program cohort. This conservative substitution was associated with altered plasma CETP concentration (1.95 +/- 0.54, 1.91 +/- 0.57, and 1.77 +/- 0.57 microg/ml for the II, IV and VV genotypes, respectively). The distribution of plasma CETP concentrations among the VV, but not II, men appeared bimodal (P < 0.01), suggesting the presence of a functionally significant CETP gene mutation(s) in a subset of V alleles. HDL-C levels were higher in VV than IV for II men (55.4 +/- 17.4, 51.3 +/- 16.6, 51.1 +/- 17.0 mg/dl, P < 0.04). However, the increase in HDL was only significant in VV men with plasma TG > 165 mg/dl. Although CHD prevalence was not significantly different among the three genotypes in this population, in the subpopulation with high plasma TG, CHD prevalence appeared higher among VV than IV or II subjects (38% vs. 27% vs. 18%, P < 0.05 for an interaction of genotype and plasma TG levels). In fresh plasma from a separate group of normolipidemic subjects, the V/I polymorphism was not associated with any change in plasma CETP specific activity. The data suggest that a widespread and common CETP gene mutation(s) in linkage disequilibrium with 405V causes low CETP. Among hypertriglyceridemic men this is associated with higher HDL and possibly with increased CHD.

Amino Acids↗

PubMed1997

Scavenger receptor BI promotes high density lipoprotein-mediated cellular cholesterol efflux.

Scavenger receptor BI (SR-BI) binds high density lipoproteins (HDL) with high affinity and mediates the selective uptake of HDL cholesteryl ester. We examined the potential role of SR-BI in mediating cellular cholesterol efflux. In Chinese hamster ovary cells stably transfected with murine SR-BI, overexpression of SR-BI resulted in a 3-4-fold stimulation of initial cholesterol efflux rates. Efflux rates correlated with SR-BI expression in cells and HDL concentration in the medium. When incubated with synthetic cholesterol-free HDL, SR-BI-transfected cells showed approximately 3-fold increases in initial rates of efflux compared with control cells, indicating that SR-BI expression enhances net cholesterol efflux mediated by discoidal HDL. In six different cell types, including cultured macrophages, the rate of efflux of cholesterol mediated by HDL or serum was well correlated with cellular SR-BI expression level. In addition, in situ hybridization experiments revealed that SR-BI mRNA was expressed in the thickened intima of atheromatous aorta of apolipoprotein E knockout mice. Thus, SR-BI is an authentic HDL receptor mediating cellular cholesterol efflux. SR-BI may facilitate the initial steps of HDL-mediated cholesterol efflux in the arterial wall as well as later steps of reverse cholesterol transport involving uptake of HDL cholesterol in the liver.

Animals↗

PubMed1997

IFN-gamma potentiates atherosclerosis in ApoE knock-out mice.

The early colocalization of T cells and the potent immunostimulatory cytokine IFN-gamma to atherosclerotic lesions suggest that the immune system contributes to atherogenesis. Since mice with a targeted disruption of the apoE gene (apoE 0 mice) develop profound atherosclerosis, we examined the role of IFN-gamma in this process. First, the presence of CD4(+) and CD8(+) cells, which secrete lesional IFN-gamma, was documented in apoE 0 atheromata. Then, the apoE 0 mice were crossed with IFN-gamma receptor (IFNgammaR) 0 mice to generate apoE 0/IFNgammaR 0 mice. Compared to the apoE 0 mice, the compound knock-out mice exhibited a substantial reduction in atherosclerotic lesion size, a 60% reduction in lesion lipid accumulation, a decrease in lesion cellularity, but a marked increase in lesion collagen content. Evaluation of the plasma lipoproteins showed that the compound knockout mice had a marked increase in potentially atheroprotective phospholipid/apoA-IV rich particles as well. This correlated with an induction of hepatic apoA-IV transcripts. These observations suggest that IFN-gamma promotes and modifies atherosclerosis through both local effects in the arterial wall as well as a systemic effect on plasma lipoproteins. Therefore, therapeutic inhibition of IFN-gamma signaling may lead to the formation of more lipid-poor and stable atheromata.

Animals↗

PubMed1997

A mouse model with features of familial combined hyperlipidemia.

Familial combined hyperlipidemia (FCHL) is a common inherited lipid disorder, affecting 1 to 2 percent of the population in Westernized societies. Individuals with FCHL have large quantities of very low density lipoprotein (VLDL) and low density lipoprotein (LDL) and develop premature coronary heart disease. A mouse model displaying some of the features of FCHL was created by crossing mice carrying the human apolipoprotein C-III (APOC3) transgene with mice deficient in the LDL receptor. A synergistic interaction between the apolipoprotein C-III and the LDL receptor defects produced large quantities of VLDL and LDL and enhanced the development of atherosclerosis. This mouse model may provide clues to the origin of human FCHL.

Animals↗

PubMed1997

Plasma kinetics of cholesteryl ester transfer protein in the rabbit. Effects of dietary cholesterol.

The plasma kinetics of recombinant human cholesteryl ester transfer protein (rCETP) were studied in six rabbits before and after cholesterol feeding (0.5% wt/wt). The rCETP, labeled with the use of the Bolton Hunter reagent, was shown to retain neutral lipid transfer activity. After intravenous infusion, labeled rCETP associated with rabbit lipoproteins to an extent similar to endogenous rabbit CETP (62% to 64% HDL associated). The plasma kinetics of CETP, modeled with the use of SAAM-II, conformed to a two-pool model, likely representing free and loosely HDL-associated CETP (fast pool) and a tightly apo (apolipoprotein) AI-associated (slow pool) CETP. The plasma residency time (chow diet) of the fast pool averaged 7.1 hours and of the slow pool, 76.3 hours. The production rate (PR) into and the fractional catabolic rate (FCR) of the fast pool were 20 and 10 times the PR and FCR, respectively, of the slow pool. In response to cholesterol feeding, CETP PR, FCR, and plasma mass increased by 416%, 60%, and 230%, respectively. There was a strong correlation (r = .95, P = .003) between the increase in rabbit plasma CETP and the modeled increase in CETP PR in response to cholesterol feeding, suggesting that labeled human rCETP is a satisfactory tracer for rabbit plasma CETP. CETP is catabolized by distinct pools, likely corresponding to an apo AI-associated (slow) pool and a free and/or loosely HDL-associated (fast) pool. Factors that alter the affinity of CETP for HDL would be predicted to result in altered CETP catabolism. The effect of dietary cholesterol on plasma CETP mass can be explained largely by the effects on CETP synthesis, consistent with the observed effects of cholesterol on tissue mRNA levels.

Animals↗