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

Clinical presentation, laboratory values, and coronary heart disease risk in marked high-density lipoprotein-deficiency states

Our purpose is to provide a framework for diagnosing the inherited causes of marked high-density lipoprotein (HDL) deficiency (HDL cholesterol levels <10 mg/dL in the absence of severe hypertriglyceridemia or liver disease) and to provide information about coronary heart disease (CHD) risk for such cases. Published articles in the literature on severe HDL deficiencies were used as sources. If apolipoprotein (Apo) A-I is not present in plasma, then three forms of ApoA-I deficiency, all with premature CHD,and normal low-density lipoprotein (LDL) cholesterol levels have been described: ApoA-I/C-III/A-IV deficiency with fat malabsorption, ApoA-I/C-III deficiency with planar xanthomas, and ApoA-I deficiency with planar and tubero-eruptive xanthomas (pictured in this review for the first time). If ApoA-I is present in plasma at a concentration <10 mg/dL, with LDL cholesterol that is about 50% of normal and mild hypertriglyceridemia, a possible diagnosis is Tangier disease due to mutations at the adenosine triphosphate binding cassette protein A1 (ABCA1) gene locus. These patients may develop premature CHD and peripheral neuropathy, and have evidence of cholesteryl ester-laden macrophages in their liver, spleen, tonsils, and Schwann cells, as well as other tissues. The third form of severe HDL deficiency is characterized by plasma ApoA-I levels <40 mg/dL, moderate hypertriglyceridemia, and decreased LDL cholesterol, and the finding that most of the cholesterol in plasma is in the free rather than the esterified form, due to a deficiency in lecithin:cholesterol acyltransferase activity. These patients have marked corneal opacification and splenomegaly, and are at increased risk of developing renal failure, but have no clear evidence of premature CHD. Marked HDL deficiency has different etiologies and is generally associated with early CHD risk.

Cardiovascular disease risk in type 2 diabetes mellitus: insights from mechanistic studies

Individuals with type 2 diabetes mellitus have increased cardiovascular disease risk compared with those without diabetes. Treatment of the residual risk, other than blood pressure and LDL-cholesterol control, remains important as the rate of diabetes increases worldwide. The accelerated atherosclerosis and cardiovascular disease in diabetes is likely to be multifactorial and therefore several therapeutic approaches can be considered. Results of mechanistic studies done in vitro and in vivo--animals and people--can provide important insights with the potential to improve clinical management decisions and outcomes. In this Review, we focus on three areas in which pathophysiological considerations could be particularly informative--ie, the roles of hyperglycaemia, diabetic dyslipidaemia (other than the control of LDL-cholesterol concentrations), and inflammation (including that in adipose tissue) in the acceleration of vascular injury.

Computational lipidology: predicting lipoprotein density profiles in human blood plasma

Monitoring cholesterol levels is strongly recommended to identify patients at risk for myocardial infarction. However, clinical markers beyond "bad" and "good" cholesterol are needed to precisely predict individual lipid disorders. Our work contributes to this aim by bringing together experiment and theory. We developed a novel computer-based model of the human plasma lipoprotein metabolism in order to simulate the blood lipid levels in high resolution. Instead of focusing on a few conventionally used predefined lipoprotein density classes (LDL, HDL), we consider the entire protein and lipid composition spectrum of individual lipoprotein complexes. Subsequently, their distribution over density (which equals the lipoprotein profile) is calculated. As our main results, we (i) successfully reproduced clinically measured lipoprotein profiles of healthy subjects; (ii) assigned lipoproteins to narrow density classes, named high-resolution density sub-fractions (hrDS), revealing heterogeneous lipoprotein distributions within the major lipoprotein classes; and (iii) present model-based predictions of changes in the lipoprotein distribution elicited by disorders in underlying molecular processes. In its present state, the model offers a platform for many future applications aimed at understanding the reasons for inter-individual variability, identifying new sub-fractions of potential clinical relevance and a patient-oriented diagnosis of the potential molecular causes for individual dyslipidemia.

Is raising HDL a futile strategy for atheroprotection?

The dramatic failure of clinical trials evaluating the cholesterol ester transfer protein inhibitor torcetrapib has led to considerable doubt about the value of raising high-density lipoprotein cholesterol (HDL-C) as a treatment for cardiovascular disease. These results have underscored the intricacy of HDL metabolism, with functional quality perhaps being a more important consideration than the circulating quantity of HDL. As a result, HDL-based therapeutics that maintain or enhance HDL functionality warrant closer investigation. In this article, we review the complexity of HDL metabolism, discuss clinical-trial data for HDL-raising agents, including possible reasons for the failure of torcetrapib, and consider the potential for future HDL-based therapies.

Relation of gemfibrozil treatment and high-density lipoprotein subpopulation profile with cardiovascular events in the Veterans Affairs High-Density Lipoprotein Intervention Trial

The significant cardiovascular disease (CVD) event reduction in the Veterans Affairs High-Density Lipoprotein Intervention Trial (VA-HIT) could not be fully explained by the 6% increase in high-density lipoprotein (HDL) cholesterol with the fibrate gemfibrozil. We examined whether measurement of HDL subpopulations provided additional information relative to CVD risk reduction. The HDL subpopulations were characterized by 2-dimensional gel electrophoresis in subjects who were treated with gemfibrozil (n = 754) or placebo (n = 741). In this study, samples obtained at the 3-month visit were used; and data were analyzed prospectively using CVD events (coronary heart disease death, myocardial infarction, or stroke) during the 5.1 years of follow-up. Analyses in the gemfibrozil arm showed that subjects with recurrent CVD events had significantly higher prebeta-1 and had significantly lower alpha-1 and alpha-2 HDL levels than those without such events. Prebeta-1 level was a significant positive predictor; alpha-1 and alpha-2 levels were significant negative risk factors for future CVD events. alpha-2 level was superior to HDL cholesterol level in CVD-risk assessment after adjustment for established risk factors. Gemfibrozil treatment was associated with 3% to 6% decreases in the small, lipid-poor prebeta-1 HDL and in the large, lipid-rich alpha-1 and alpha-2 HDL and with increases in the small alpha-3 (3%) and prealpha-3 (16%) HDLs. Although the use of gemfibrozil has been associated with reduction in CVD events in VA-HIT, HDL subpopulation analysis indicates that gemfibrozil-mediated improvement in CVD risk might not be the result of its effects on HDL. It is quite possible that much of the cardiovascular benefits of gemfibrozil are due to a much wider spectrum of effects on metabolic processes that is not reflected by changes in blood lipids and HDL subpopulations.

Increasing high-density lipoprotein cholesterol, inhibition of cholesteryl ester transfer protein, and heart disease risk reduction

Our purpose is to review recent research in the area of high-density lipoprotein (HDL) cholesterol raising and coronary artery disease (CAD) risk reduction. It is known that a decreased HDL cholesterol level is an important CAD risk factor and that raising HDL cholesterol has been associated with CAD risk reduction. A relative new strategy for raising HDL cholesterol, inhibition of cholesteryl ester transfer protein (CETP), is markedly effective. CETP inhibitors prevent the transfer of cholesteryl ester from HDL to triglyceride-rich lipoproteins in exchange for triglyceride. One inhibitor, torcetrapib, binds to CETP on HDL, markedly increases HDL cholesteryl ester, has no effect on fecal cholesterol excretion, but can raise blood pressure. A large clinical trial in patients with CAD who were taking atorvastatin was recently stopped prematurely because of excess mortality in those receiving torcetrapib versus placebo, and 2 other trials reported no benefit of torcetrapib on coronary atherosclerosis or carotid intima-media thickness as compared with subjects on atorvastatin alone. The adverse effects of torcetrapib may be compound specific, and because the crystal structure of CETP is now known, it should be possible to develop more optimal CETP inhibitors that do not form a nonproductive complex with CETP on the HDL particle, as has been reported for torcetrapib. Another alternative for increasing HDL levels is to develop more effective and better tolerated niacin preparations.

Clair RW, Rudel LL, Ghosh S. Macrophage-specific transgenic expression of cholesteryl ester hydrolase significantly reduces atherosclerosis and lesion necrosis in Ldlr mice

Accumulation of cholesteryl esters (CEs) in macrophage foam cells, central to atherosclerotic plaque formation, occurs as a result of imbalance between the cholesterol influx and efflux pathways. While the uptake, or influx, of modified lipoproteins is largely unregulated, extracellular acceptor-mediated free cholesterol (FC) efflux is rate limited by the intracellular hydrolysis of CE. We previously identified and cloned a neutral CE hydrolase (CEH) from human macrophages and demonstrated its role in cellular CE mobilization. In the present study, we examined the hypothesis that macrophage-specific overexpression of CEH in atherosclerosis-susceptible Ldlr(-/-) mice will result in reduction of diet-induced atherosclerosis. Transgenic mice overexpressing this CEH specifically in the macrophages (driven by scavenger receptor promoter/enhancer) were developed and crossed into the Ldlr(-/-) background (Ldlr(-/-)CEHTg mice). Macrophage-specific overexpression of CEH led to a significant reduction in the lesion area and cholesterol content of high-fat, high-cholesterol diet-induced atherosclerotic lesions. The lesions from Ldlr(-/-)CEHTg mice did not have increased FC, were less necrotic, and contained significantly higher numbers of viable macrophage foam cells. Higher CEH-mediated FC efflux resulted in enhanced flux of FC from macrophages to gall bladder bile and feces in vivo. These studies demonstrate that by enhancing cholesterol efflux and reverse cholesterol transport, macrophage-specific overexpression of CEH is antiatherogenic.

Effects of niacin and Niaspan on HDL lipoprotein cellular SR-BI-mediated cholesterol efflux

BACKGROUND

Niacin, the lipid-regulating agent with the longest therapeutic experience, has been demonstrated to both raise high-density lipoprotein cholesterol (HDL-C) levels and to diminish the risk of atherosclerosis and its vascular complications.

OBJECTIVE

The present study was carried out to explore niacin's effect on scavenger receptor class B type I (SR-BI)-mediated cholesterol efflux, a component of reverse cholesterol transport, using an in vitro model system.

METHODS

Thirty frozen samples from a large randomized, multicenter trial comparing crystalline niacin, extended-release niacin (Niaspan), and placebo were analyzed for SR-BI efflux.

RESULTS

Both the extended-release and crystalline niacin demonstrated significant increases in HDL-C (approximately 50%) over baseline values compared to the placebo group (14%). This was associated with a significant increase in SR-BI efflux of 2.7% and 3.4% for extended-release niacin and niacin, respectively, compared to placebo (0.4%). Although, there was no relationship between HDL-C and SR-BI efflux at baseline or at the end of treatment, there was a linear relation between the changes in HDL-C and SR-BI efflux (r = 0.58, P < 0.002).

CONCLUSION

This study suggests that niacin has a beneficial effect on SR-BI efflux that is related to the change in level of HDL-C.

Apolipoprotein A-I but not high-density lipoproteins are internalised by RAW macrophages: roles of ATP-binding cassette transporter A1 and scavenger receptor BI

Accumulation of lipid-loaded macrophages (foam cells) within the vessel wall is an early hallmark of atherosclerosis. High-density lipoproteins (HDL) and apolipoprotein A-I (apoA-I) can efficiently promote cholesterol efflux from macrophages. Therefore, the interaction of HDL and apoA-I with macrophages appears to be important in the initial steps of reverse cholesterol transport, i.e. the transport of excess cholesterol from foam cells to the liver. However, although several cellular apoA-I and HDL receptors and transporters have been identified, it is as yet controversial how these interactions lead to cholesterol efflux from foam cells. In this study, we show that RAW264.7 macrophages bind HDL and apoA-I in a compatible manner. Furthermore, cell surface biotinylation experiments revealed that apoA-I but not HDL is specifically internalised. Binding of HDL to macrophages is decreased by reducing the expression of scavenger receptor BI (SR-BI) with cyclic adenosine monophosphate (cAMP), acetylated low-density lipoprotein (acLDL) or RNA interference. In contrast, apoA-I cell association and internalisation is modulated in parallel with ATP-binding cassette transporter A1 (ABCA1) expression which is altered by stimulating cells with cAMP and acLDL or expressing short hairpin RNA (shRNA) against ABCA1. Consistent with this, cell surface trapping of ABCA1 with cyclosporin A (CsA) results in increased apoA-I binding but reduced internalisation. Furthermore, blocking apoA-I uptake inhibits cholesterol efflux to apoA-I but not to HDL. Taken together, these data suggest that apoA-I- but not HDL-mediated cholesterol efflux may involve retroendocytosis.

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

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

Indices of reverse cholesterol transport in subjects with metabolic syndrome after treatment with rosuvastatin

OBJECTIVE

The effects of the statin, rosuvastatin on indices of reverse cholesterol transport were studied in a randomized, placebo-controlled, cross-over trial in 25 overweight subjects with defined metabolic syndrome.

RESULT

Four weeks' treatment with 40 mg/day rosuvastatin significantly reduced levels of plasma cholesterol (44%), LDL cholesterol (60%) and triglyceride (38%). HDL cholesterol (mean [S.D.]) rose (0.97[0.17] to 1.05[0.17]mmol/L; P<0.05) and the LpA-I component of HDL from 39[7] to 45[9]mg/dL (P<0.05). LCAT activity fell (0.55[0.13] to 0.35[0.07]nmol/mL/h; P<0.05); CETP activity and mass fell from 89[13] to 80[11]nmol//L/h and from 1.66[0.57] to 1.28[0.41]mug/mL respectively, (P<0.05). Cholesterol efflux in vitro (to plasmas from THP-1 activated cells) fell from 7.1[1.8]% (placebo) to 6.2[1.7]% (rosuvastatin); P<0.05, but when plasmas depleted of apoB lipoproteins were studied, the difference in efflux was no longer statistically significant. During placebo efflux was paradoxically inversely correlated with HDL-C (P=0.016) and LpA-I (P=0.035) concentrations but these correlations were absent after rosuvastatin.

CONCLUSIONS

The data suggest possible HDL dysfunctionality in subjects with metabolic syndrome. The reduced capacity of plasmas following statin treatment to stimulate cholesterol efflux in vitro occurred in association with reduction in apoB lipoproteins and reduced activities of CETP and LCAT, and despite increased levels of HDL cholesterol.

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.

Where are we with high-density lipoprotein raising and inhibition of cholesteryl ester transfer for heart disease risk reduction?

PURPOSE OF REVIEW

To review recent research in the area of high-density lipoprotein raising and coronary heart disease risk reduction.

RECENT FINDINGS

A decreased high-density lipoprotein-cholesterol is an important coronary heart disease risk factor and raising high-density lipoprotein-cholesterol has been associated with coronary heart disease risk reduction. A relative new strategy for raising high-density lipoprotein-cholesterol, i.e. inhibition of cholesteryl ester transfer protein, is markedly effective. Cholesteryl ester transfer protein inhibitors prevent the transfer of cholesteryl ester from high-density lipoprotein to triglyceride-rich lipoproteins in exchange for triglyceride. One inhibitor, torcetrapib, binds to cholesteryl ester transfer protein on high-density lipoprotein, markedly raises high-density lipoprotein-cholesteryl ester and has no effect on fecal cholesterol excretion, but can raise blood pressure. A large clinical trial in coronary heart disease patients on atorvastatin was recently stopped prematurely because of excess mortality in those receiving torcetrapib vs. placebo and two other trials reported no benefit of torcetrapib on coronary atherosclerosis or carotid artery intimal medial thickness as compared with subjects on atorvastatin alone.

SUMMARY

The adverse effects of torcetrapib may be compound-specific and, since the crystal structure of cholesteryl ester transfer protein is now known, it should be possible to develop more optimal cholesteryl ester transfer protein inhibitors that do not form a nonproductive complex with cholesteryl ester transfer protein on the high-density lipoprotein particle, as has been reported for torcetrapib. The alternative for high-density lipoprotein raising is to develop more effective and better tolerated niacin preparations.

Comparison of the effects of high doses of rosuvastatin versus atorvastatin on the subpopulations of high-density lipoproteins

Atorvastatin and rosuvastatin are both highly effective in decreasing low-density lipoprotein cholesterol and triglyceride levels. However, rosuvastatin was shown to be more effective in increasing high-density lipoprotein (HDL) cholesterol levels. The purpose of the study is to compare the effects of daily doses of rosuvastatin 40 mg with atorvastatin 80 mg during a 6-week period on HDL subpopulations in 306 hyperlipidemic men and women. We previously showed that increased levels of large alpha-1 and alpha-2 HDLs decrease the risk of coronary heart disease and protect against progression of coronary atherosclerosis (superior to HDL cholesterol). In this study, both statins caused significant increases in large alpha-1 (p <0.001) and alpha-2 (p <0.001 for rosuvastatin, p <0.05 for atorvastatin) and significant (p <0.001) decreases in small pre-beta-1 HDL levels; however, increases in the 2 large HDL particles were significantly higher for rosuvastatin than atorvastatin (alpha-1, 24% vs 12%; alpha-2, 13% vs 4%; p <0.001). Statin-induced increases in alpha-1 and alpha-2 correlated with increases in HDL cholesterol, whereas decreases in pre-beta-1 were associated with decreases in triglycerides. In subjects with low HDL cholesterol (<40 mg/dl for men, <50 mg/dl for women, n = 99), increases in alpha-1 were 32% versus 11%, and in alpha-2, 21% versus 5% for rosuvastatin and atorvastatin, respectively. In conclusion, our data show that both statins, given at their maximal doses, favorably alter the HDL subpopulation profile, but also that rosuvastatin is significantly more effective in this regard than atorvastatin.

High-density lipoprotein and transport of cholesterol and triglyceride in blood

High-density lipoproteins (HDL) contain approximately 25% of the cholesterol and <5% of the triglyceride in the plasma of human blood. However, the dynamic exchange of lipids and lipid-binding proteins is not revealed by simply considering the mass of material at any point in time. HDL are the most complex of lipoprotein species with multiple protein constituents, which facilitate cholesterol secretion from cells, cholesterol esterification in plasma, and transfer of cholesterol to other lipoproteins and to the liver for excretion. They also play a major role in triglyceride transport by providing for activation of lipoprotein lipase, exchange of triglyceride among the lipoproteins, and removal of triglyceride rich remnants of chylomicrons and very-low-density lipoproteins after lipase action. In addition, antioxidative enzymes and phospholipid transfer proteins are important components of HDL. Many of the proteins of HDL are exchangeable with other lipoproteins, including chylomicrons and very-low-density lipoproteins. The constantly changing content of lipids and apolipoproteins in HDL particles generate a series of structures that can be analyzed by using separation techniques that depend on size or charge of the particles. Interaction of these various structures can be very different with cell surfaces depending on the size or apolipoprotein content. A series of different transport proteins preferentially exchange lipids with specific structures among the HDL but interact poorly or not at all with others. The role of these differing forms of HDL and their interactions with cells and other lipoprotein species in plasma is the subject of intense study stimulated by the potential for reducing atherogenesis. The strength of this is only partially indicated by the correlation of higher total levels of the HDL particles with reduced incidence of vascular disease in various clinical trials and epidemiological studies.

Increased cholesterol efflux from cultured fibroblasts to plasma from hypertriglyceridemic type 2 diabetic patients: roles of pre beta-HDL, phospholipid transfer protein and cholesterol esterification

We tested whether hypertriglyceridemia associated with type 2 diabetes mellitus is accompanied by alterations in pre beta-HDL, which are considered to be initial acceptors of cell-derived cholesterol, and by changes in the ability of plasma to promote cellular cholesterol efflux. In 28 hypertriglyceridemic and 56 normotriglyceridemic type 2 diabetic patients, and in 56 control subjects, we determined plasma lipids, HDL cholesterol and phospholipids, plasma pre beta-HDL and pre beta-HDL formation, phospholipid transfer protein (PLTP) activity, plasma cholesterol esterification (EST) and cholesteryl ester transfer (CET) and the ability of plasma to stimulate cholesterol efflux out of cultured human fibroblasts. HDL cholesterol and HDL phospholipids were lower, whereas plasma PLTP activity, EST and CET were higher in hypertriglyceridemic diabetic patients than in the other groups. Pre beta-HDL levels and pre beta-HDL formation were unaltered, although the relative amount of pre beta-HDL (expressed as % of total plasma apo A-I) was increased in hypertriglyeridemic diabetic patients. Cellular cholesterol efflux to plasma from hypertriglyceridemic diabetic patients was increased compared to efflux to normotriglyceridemic diabetic and control plasma, but efflux to normotriglyceridemic diabetic and control plasma did not differ. Multiple linear regression analysis demonstrated that cellular cholesterol efflux to plasma was positively and independently related to pre beta-HDL formation, PLTP activity and EST (multiple r=0.48), but not to the diabetic state. In conclusion, cholesterol efflux from fibroblasts to normotriglyceridemic diabetic plasma is unchanged. Efflux to hypertriglyceridemic diabetic plasma is enhanced, in association with increased plasma PLTP activity and cholesterol esterification. Unaltered pre beta-HDL formation in diabetic hypertriglyceridemia, despite low apo A-I, could contribute to maintenance of cholesterol efflux.

A Unique Protease-sensitive High Density Lipoprotein Particle Containing the Apolipoprotein A-IMilano Dimer Effectively Promotes ATP-binding Cassette A1-mediated Cell Cholesterol Efflux

Carriers of the apolipoprotein A-I(Milano) (A-I(M)) variant present with severe reductions of plasma HDL levels, not associated with premature coronary heart disease (CHD). Sera from 14 A-I(M) carriers and matched controls were compared for their ability to promote ABCA1-driven cholesterol efflux from J774 macrophages and human fibroblasts. When both cell types are stimulated to express ABCA1, the efflux of cholesterol through this pathway is greater with A-I(M) than control sera (3.4 +/- 1.0% versus 2.3 +/- 1.0% in macrophages; 5.2 +/- 2.4% versus 1.9 +/- 0.1% in fibroblasts). A-I(M) and control sera are instead equally effective in removing cholesterol from unstimulated cells and from fibroblasts not expressing ABCA1. The A-I(M) sera contain normal amounts of apoA-I-containing prebeta-HDL and varying concentrations of a unique small HDL particle containing a single molecule of the A-I(M) dimer; chymase treatment of serum degrades both particles and abolishes ABCA1-mediated cholesterol efflux. The serum content of chymase-sensitive HDL correlates strongly and significantly with ABCA1-mediated cholesterol efflux (r = 0.542, p = 0.004). The enhanced capacity of A-I(M) serum for ABCA1 cholesterol efflux is thus explained by the combined occurrence in serum of normal amounts of apoA-I-containing prebeta-HDL, together with a unique protease-sensitive, small HDL particle containing the A-I(M) dimer, both effective in removing cell cholesterol via ABCA1.

Risk factors for coronary artery disease in patients with elevated high-density lipoprotein cholesterol

High high-density lipoprotein (HDL) levels protect against coronary artery disease (CAD) development. We hypothesized that patients with CAD and high HDL levels would have higher prevalence of other CAD risk factors compared with patients with CAD and normal HDL. We identified 41,982 patients from a single center with normal levels (40 to 60 mg/dl in men, 50 to 70 mg/dl in women) or high HDL levels (> or =70 mg/dl in men, > or =80 mg/dl in women) when last measured between January 2000 and April 2004. From this overall population, we characterized a cohort of 1,610 patients with CAD, including 98 patients with high HDL levels. We measured prevalence of traditional CAD risk factors by comparing these 98 patients with patients with CAD and normal HDL levels (n = 1,512). We performed manual chart review in patients (n = 196) matched 1:1 by age, gender, and HDL level to obtain further detail with regard to differences in family history and lifestyle factors. In patients with CAD, those with high HDL levels (98 of 1,610, 6.1%) were of similar age (71.1 vs 69.6 years, p = 0.23), had similar prevalence of hypertension (78.6% vs 88.7%, p = 0.30), lower levels of low-density lipoprotein (85.3 vs 90.9 mg/dl, p = 0.04) and triglycerides (87.1 vs 141.2 mg/dl, p <0.01), and a lower prevalence of diabetes (28.6% vs 38.4%, p = 0.05) compared with patients with normal HDL levels. In logistic regression models, patients with high HDL levels and CAD were less likely to have diabetes (adjusted odds ratio 0.60, 95% confidence interval 0.38 to 0.95, p = 0.03) or obesity (adjusted odds ratio 0.50, 95% confidence interval 0.25 to 0.99, p = 0.046) than patients with normal HDL levels and CAD. In conclusion, patients with high HDL and CAD had a similar or lower prevalence of traditional CAD risk factors compared with patients with normal HDL levels and CAD.

LDL particle subspecies are distinct in their capacity to mediate free cholesterol efflux via the SR-BI/Cla-1 receptor

The human scavenger receptor SR-BI/Cla-1 promotes efflux of free cholesterol from cells to both high-density and low-density lipoproteins (HDL, LDL). SR-BI/Cla-1-mediated cholesterol efflux to HDL is dependent on particle size, lipid content and apolipoprotein conformation; in contrast, the capacity of LDL subspecies to accept cellular cholesterol via this receptor is indeterminate. Cholesterol efflux assays were performed with CHO cells stably transfected with Cla-1 cDNA. Expression of Cla-1 in CHO cells induced elevation in total cholesterol efflux to plasma, LDL and HDL. Such Cla-1-specific efflux was abrogated by addition of anti-Cla-1 antibody. LDL were fractionated into five subspecies either on the basis of hydrated density or size. Among LDL subfractions, small dense LDL (sdLDL) were 1.5-to 3-fold less active acceptors for Cla-1-mediated cellular cholesterol efflux. Equally, sdLDL markedly reduced Cla-1-specific cholesterol efflux to large buoyant LDL in a dose-dependent manner. Conversely, sdLDL did not influence efflux to HDL(2). These findings provide evidence that LDL particles are heterogeneous in their capacity to promote Cla-1-mediated cholesterol efflux. Relative to HDL(2), large buoyant LDL may constitute physiologically-relevant acceptors for cholesterol efflux via Cla-1.

Functionally defective high-density lipoprotein: a new therapeutic target at the crossroads of dyslipidemia, inflammation, and atherosclerosis

High-density lipoproteins (HDL) possess key atheroprotective biological properties, including cellular cholesterol efflux capacity, and anti-oxidative and anti-inflammatory activities. Plasma HDL particles are highly heterogeneous in physicochemical properties, metabolism, and biological activity. Within the circulating HDL particle population, small, dense HDL particles display elevated cellular cholesterol efflux capacity, afford potent protection of atherogenic low-density lipoprotein against oxidative stress and attenuate inflammation. The antiatherogenic properties of HDL can, however be compromised in metabolic diseases associated with accelerated atherosclerosis. Indeed, metabolic syndrome and type 2 diabetes are characterized not only by elevated cardiovascular risk and by low HDL-cholesterol (HDL-C) levels but also by defective HDL function. Functional HDL deficiency is intimately associated with alterations in intravascular HDL metabolism and structure. Indeed, formation of HDL particles with attenuated antiatherogenic activity is mechanistically related to core lipid enrichment in triglycerides and cholesteryl ester depletion, altered apolipoprotein A-I (apoA-I) conformation, replacement of apoA-I by serum amyloid A, and covalent modification of HDL protein components by oxidation and glycation. Deficient HDL function and subnormal HDL-C levels may act synergistically to accelerate atherosclerosis in metabolic disease. Therapeutic normalization of attenuated antiatherogenic HDL function in terms of both particle number and quality of HDL particles is the target of innovative pharmacological approaches to HDL raising, including inhibition of cholesteryl ester transfer protein, enhanced lipidation of apoA-I with nicotinic acid and infusion of reconstituted HDL or apoA-I mimetics. A preferential increase in circulating concentrations of HDL particles possessing normalized antiatherogenic activity is therefore a promising therapeutic strategy for the treatment of common metabolic diseases featuring dyslipidemia, inflammation, and premature atherosclerosis.

Cholesteryl Ester Transfer Protein Decreases High-Density Lipoprotein and Severely Aggravates Atherosclerosis in APOE*3-Leiden Mice

Objective— The role of cholesteryl ester transfer protein (CETP) in the development of atherosclerosis is still undergoing debate. Therefore, we evaluated the effect of human CETP expression on atherosclerosis in APOE*3-Leiden ( E3L ) mice with a humanized lipoprotein profile.

Methods and Results— E3L mice were crossbred with human CETP transgenic mice. On a chow diet, CETP expression increased plasma total cholesterol (TC) (+43%; P <0.05). To evaluate the effects of CETP on the development of atherosclerosis, mice were fed a Western-type diet containing 0.25% cholesterol, leading to 4.3-fold elevated TC levels in both E3L and CETP.E3L mice ( P <0.01). On both diets, CETP expression shifted the distribution of cholesterol from high-density lipoprotein (HDL) toward very-low-density lipoprotein (VLDL)/low-density lipoprotein (LDL). Moreover, plasma of CETP.E3L mice had reduced capacity (−39%; P <0.05) to induce SR-BI–mediated cholesterol efflux from Fu5AH cells than plasma of E3L mice. After 19 weeks on the Western-type diet, CETP.E3L mice showed a 7.0-fold increased atherosclerotic lesion area in the aortic root compared with E3L mice ( P <0.0001).

Conclusions— CETP expression in E3L mice shifts the distribution of cholesterol from HDL to VLDL/LDL, reduces plasma-mediated SR-BI–dependent cholesterol efflux, and represents a clear pro-atherogenic factor in E3L mice. We anticipate that the CETP.E3L mouse will be a valuable model for the preclinical evaluation of HDL-raising interventions on atherosclerosis development.

Both hepatic and extrahepatic ABCA1 have discrete and essential functions in the maintenance of plasma high-density lipoprotein cholesterol levels in vivo

BACKGROUND

Extrahepatic tissues have long been considered critical contributors of cholesterol to nascent HDL particles in the reverse cholesterol transport pathway, in which ABCA1 plays the crucial role. Recent studies, however, including both overexpression and deletion of ABCA1 selectively in the liver, have highlighted the primary role of the liver in the maintenance of HDL levels in vivo.

METHODS AND RESULTS

The availability of mice with complete deletion of ABCA1 (total knockout [TKO]) and with liver-specific deletion of ABCA1 (LSKO) has enabled us to dissect the discrete roles of hepatic relative to extrahepatic ABCA1 in HDL biogenesis. Delivery of adenoviral ABCA1 resulted in selective expression of physiological levels of ABCA1 in the livers of both LSKO and TKO mice, resulting in increased HDL cholesterol (HDL-C). Expression of ABCA1 in the liver of LSKO mice resulted in plasma HDL-C levels that were similar to those in wild-type mice and significantly above those seen in similarly treated TKO mice. HDL particles from ABCA1-expressing LSKO mice were larger and contained significantly increased cholesterol compared with TKO mice. Infusion of human apolipoprotein A-I/phospholipid reconstituted HDL particles normalized plasma HDL-C levels in LSKO mice but had no effect on HDL-C levels in TKO mice.

CONCLUSIONS

Although hepatic ABCA1 appears crucial for phospholipid transport, extrahepatic tissues play an important role in cholesterol transfer to nascent HDL particles. These data highlight the discrete and specific roles of both liver and extrahepatic ABCA1 in HDL biogenesis in vivo and indicate that ABCA1 shows lipid cargo selectivity depending on its site of expression.

Relationship between the concentration and antiatherogenic activity of high-density lipoproteins

PURPOSE OF REVIEW

The relationship between the concentration of high-density lipoprotein cholesterol and their protective function is addressed.

RECENT FINDINGS

Strong epidemiologic evidence indicates that the concentration of high-density lipoproteins is a powerful inverse predictor of cardiovascular risk. This is consistent with the fact that high levels of high-density lipoprotein are generally associated with an increased concentration of large high-density lipoprotein particles that are now known to be the preferred acceptors of cholesterol released from macrophages via the adenosine triphosphate-binding cassette A1 pathway. Some of the protective activity, however, of high-density lipoproteins may reflect functions of specific subpopulations or variations in the 'quality' of high-density lipoprotein particles and may be unrelated to the concentration of the total high-density lipoprotein fraction. This review summarizes the cardiovascular protective role of high-density lipoproteins and addresses how the concentration and antiatherogenic activity of high-density lipoproteins are related.

SUMMARY

Some of the protective functions of high-density lipoprotein are closely related to the simple concentration of high-density lipoproteins; others may not be related. Given the current high level of interest in therapies that raise the concentration of high-density lipoprotein, it is essential to conduct further research to identify precisely how high-density lipoproteins protect.

Cholesteryl ester transfer protein inhibition, high-density lipoprotein metabolism and heart disease risk reduction

PURPOSE OF REVIEW

Cholesteryl ester transfer protein (CETP) inhibitors (JTT-705 and torcetrapib) are currently in clinical testing, and significantly raise high-density lipoprotein (HDL) cholesterol levels. Low HDL cholesterol is a significant independent predictor of coronary heart disease (CHD) and HDL raising has been associated with coronary heart disease risk reduction, but there is debate about whether CETP inhibition will reduce coronary heart disease risk.

RECENT FINDINGS

It has been documented in transgenic mouse models that apolipoprotein (apo) C-I inhibits CETP, and that high mono-unsaturated fat diets prevent the normal stimulation of CETP activity by dietary cholesterol. In rabbits, torcetrapib markedly decreases clearance of HDL cholesteryl ester via an indirect pathway, but has no effect on total plasma cholesteryl ester clearance. In humans, torcetrapib raises HDL apoA-I by modestly decreasing its fractional catabolic rate, while having a very profound effect on raising HDL cholesterol and large alpha-1 migrating HDL particles by more than 50%, with no effect on fecal cholesterol excretion. When JTT-705 at 600 mg/day was given to hypercholesterolemic patients already on pravastatin 40 mg/day, the combination was well tolerated and increases in HDL cholesterol of 28% were noted.

SUMMARY

In our view, CETP inhibitors in combination with statins will be profoundly beneficial in reducing human atherosclerosis, primarily because they normalize HDL particles and prevent the transfer of cholesteryl ester from HDL to atherogenic lipoproteins.

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.

Roles of ATP binding cassette transporters A1 and G1, scavenger receptor BI and membrane lipid domains in cholesterol export from macrophages

PURPOSE OF REVIEW

The initial steps of reverse cholesterol transport involve export of cholesterol from peripheral cells to plasma lipoproteins for subsequent delivery to the liver. The review discusses recent developments in our understanding of how these steps occur, with particular emphasis on the macrophage, the major site of cellular cholesterol accumulation in atherosclerosis.

RECENT FINDINGS

ATP binding cassette transporter (ABC) A1 exports cholesterol and phospholipid to lipid-free apolipoproteins, while ATP binding cassette transporter G1 and scavenger receptor BI export cholesterol to phospholipid-containing acceptors. ABCA1-dependent cholesterol export involves an initial interaction of apolipoprotein AI with lipid raft membrane domains, although ABCA1 and most exported cholesterol are not raft associated. ABCG1 exports cholesterol to HDL and other phospholipid-containing acceptors. These include particles generated during lipidation of apoAI by ABCA1, suggesting that the two transporters cooperate in cholesterol export. Scavenger receptor BI is atheroprotective, mediating clearance of HDL cholesterol by the liver. The relative contributions of scavenger receptor BI and ABCG to cholesterol export to HDL from macrophages is unclear and may depend on cellular cholesterol status and the cholesterol gradient between cell and acceptor.

SUMMARY

The presence of distinct pathways for cholesterol efflux to lipid-free apolipoprotein AI and phospholipid-containing HDL species clarifies our understanding of reverse cholesterol transport, and provides new opportunities for its therapeutic manipulation.

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.

Measurement of cholesterol bidirectional flux between cells and lipoproteins

We developed an assay that quantitates bidirectional cholesterol flux between cells and lipoproteins. Incubating Fu5AH cells with increasing concentrations of human serum resulted in increased influx and efflux; however, influx was 2- to 3-fold greater at all serum concentrations. With apolipoprotein B (apoB)-depleted serum, the ratio of influx to efflux (I/E) was close to 1, indicating cholesterol exchange. The apoB fraction of serum induced influx and little efflux, with I/E > 1. Using block lipid transport-1 to block scavenger receptor class B type I (SR-BI)-mediated flux with different acceptors, we determined that 50% to 70% of efflux was via SR-BI. With HDL, 90% of influx was via SR-BI, whereas with LDL or serum, 20% of influx was SR-BI-mediated. Cholesterol-enriched hepatoma cells produced increased efflux without a change in influx, resulting in reduced I/E. The assay was applied to cholesterol-normal and -enriched mouse peritoneal macrophages exposed to serum or LDL. The enrichment enhanced efflux without shifts in influx. With cholesterol-enriched macrophages, HDL efflux was enhanced and influx was greatly reduced. With all lipoproteins, cholesterol enrichment of murine peritoneal macrophages led to a reduced I/E. We conclude that this assay can simultaneously and accurately quantitate cholesterol bidirectional flux and can be applied to a variety of cells exposed to isolated lipoproteins or serum.

Cholesteryl ester transfer protein (CETP) inhibition beyond raising high-density lipoprotein cholesterol levels: pathways by which modulation of CETP activity may alter atherogenesis

Raising high-density lipoprotein cholesterol (HDL-C) is a promising strategy in the struggle to prevent cardiovascular disease, and cholesteryl ester transfer protein (CETP) inhibitors have been developed to accomplish this. The first results are encouraging, and, in fact, in rabbits, inhibition of CETP reduces atherosclerosis. Because human data regarding the reduction of atheroma burden require more time, the biochemical mechanisms underlying the putative atheroprotection of CETP inhibitors are currently dissected, and several pathways have emerged. First, CETP inhibition increases HDL-C and reduces low-density lipoprotein cholesterol (LDL-C) levels consistent with CETP lipid transfer activity and its role in reverse cholesterol transport (RCT). This coincides with putative beneficial increases in both HDL and LDL size. However, many aspects regarding the impact of CETP inhibition on the RCT pathway remain elusive, in particular whether the first step concerning cholesterol efflux from peripheral tissues to HDL is influenced. Moreover, the relevance of scavenger receptor BI and consequently the central role of HDL in human RCT is still unclear. Second, CETP inhibition was shown recently to increase antioxidant enzymes associated with HDL, in turn associated with decreased oxidation of LDL. Atheroprotection in man is currently anticipated based on the improvement of these biochemical parameters known to influence atherosclerosis, but final confirmation regarding the impact of CETP inhibition on cardiovascular outcome will have to come from trials evaluating clinical end points.

Kinetics of prebeta1 HDL and alphaHDL in type II diabetic patients

BACKGROUND

The aim of this study was to analyze the recycling of high density lipoprotein (HDL) in six type II diabetic patients compared with six control subjects by endogenous labelling of apolipoprotein A-I (Apo A-I) with stable isotope Apo A.

MATERIALS AND METHODS

The -I-HDL kinetics were performed by infusion of (5.5.5-(2)H3)-leucine for 14 h. The prebeta1 and alphaHDL were separated by gel filtration fast protein liquid chromatrography system (FPLC). Kinetics of isotopic enrichment of Apo A-I were analyzed with a multi-compartmental model software (SAAM II, SAAM Institute, Seattle, WA).

RESULTS

Plasma Apo A-I concentration was decreased in patients with type II diabetes as a result of a decrease in Apo A-I-alphaHDL (P < 0.05). Diabetic patients were also characterized by an increased relative contribution of Apo A-I in prebeta1 HDL (18.3 +/- 2.8% vs 11.9 +/- 3.7%, P < 0.01). The synthetic rate of prebeta1 HDL was slightly increased in diabetic patients compared with control (NS) and an increase of recycling rate of alpha to prebeta1 HDL was observed (11.67 +/- 3.14 d(-1) vs 7.09 +/- 4.51 d(-1), P < 0.05). The clearance rate of Apo A-I was higher in diabetic patients (P < 0.05 for Apo A-I-prebeta1 HDL and P < 0.005 for Apo A-I-alphaHDL).

CONCLUSION

This study suggests that the usual increase in prebeta1 HDL in type II diabetic patients is mainly related to an increased conversion rate of alpha to prebeta1 HDL.

The effects of statins on high-density lipoproteins

Statins inhibit cholesterol synthesis and are effective in lowering total cholesterol levels in plasma or serum due to reductions in low-density lipoprotein and very low-density lipoproteins, as well as reducing progression of coronary atherosclerosis, coronary heart disease, and stroke morbidity and mortality. These agents also modestly raise levels of high-density lipoprotein (HDL) cholesterol and its major protein, apolipoprotein (apo) A-I. The more effective statins can also raise the levels of large alpha-1 HDL particles as assessed by two-dimensional gel electrophoresis. High levels of these particles promote reverse cholesterol transport and protect against coronary heart disease and progression of coronary atherosclerosis. The mechanism whereby statins alter HDL and its subspecies appears to be due to reduction of triglyceride-rich lipoproteins, with a secondary decrease in cholesteryl ester transfer protein activity, and less transfer of HDL cholesterol to triglyceride-rich lipoprotein acceptor particles. Increasingly, statins will be combined with other agents such as ezetimibe, fibrates, niacin, and cholesteryl ester transfer protein inhibitors to optimize the entire lipoprotein profile to alter not only low-density lipoprotein, but also HDL, triglycerides, lipoprotein(a), and C-reactive protein, and also to reduce cardiovascular morbidity and mortality.