Acceleration of reverse cholesterol transport

Molecules that mimic apolipoprotein A-I: potential agents for treating atherosclerosis

Certain amphipathic α-helical peptides can functionally mimic many of the properties of full-length apolipoproteins, thereby offering an approach to modulate high-density lipoprotein (HDL) for combating atherosclerosis. In this Perspective, we summarize the key findings and advances over the past 25 years in the development of peptides that mimic apolipoproteins, especially apolipoprotein A-I (apoA-I). This assemblage of information provides a reasonably clear picture of the state of the art in the apolipoprotein mimetic field, an appreciation of the potential for such agents in pharmacotherapy, and a sense of the opportunities for optimizing the functional properties of HDL.

Positive association of the hepatic lipase gene polymorphism c.514C > T with estrogen replacement therapy response

Background

Hepatic lipase (HL), an enzyme present in the hepatic sinusoids, is responsible for the lipolysis of lipoproteins. Human HL contains four polymorphic sites: G-250A, T-710C, A-763G, and C-514T single-nucleotide polymorphism (SNPs). The last polymorphism is the focus of the current study. The genotypes associated with the C-514T polymorphism are CC (normal homozygous - W), CT (heterozygous - H), and TT (minor-allele homozygous - M). HL activity is significantly impaired in individuals of the TT and CT genotypes. A total of 58 post-menopausal women were studied. The subjects were hysterectomized women receiving hormone replacement therapy consisting of 0.625 mg of conjugated equine estrogen once a day. The inclusion criteria were menopause of up to three years and normal blood tests, radiographs, cervical-vaginal cytology, and densitometry. DNA was extracted from the buccal and blood cells of all 58 patients using a commercially available kit (GFX^® - Amersham-Pharmacia, USA).

Results

Statistically significant reductions in triglycerides (t = 2.16; n = 58; p = 0.03) but not in total cholesterol (t = 0.14; n = 58; p = 0.89) were found after treatment. This group of good responders were carriers of the T allele; the CT and TT genotypes were present significantly more frequently than in the group of non-responders (p = 0.02 or p = 0.07, respectively). However, no significant difference in HDL-C (t = 0.94; n = 58; p = 0.35) or LDL-C (t = -0.83; n = 58; p = 0.41) was found in these patients.

Conclusions

The variation in lipid profile associated with the C-514T polymorphism is significant, and the T allele is associated with the best response to ERT.

Antiatherogenic effects of newly developed apolipoprotein A-I mimetic peptide/phospholipid complexes against aortic plaque burden in Watanabe-heritable hyperlipidemic rabbits

This study analyzed the antiatherogenic effects of newly developed apolipoprotein A-I (ApoA-I) mimetic peptide/phospholipid complexes (ETC-642) against the aortic plaque burden in vivo. We used human macrophage cells to analyze cholesterol efflux by ETC-642. Watanabe-heritable hyperlipidemic (WHHL) rabbits were divided into 3 groups: low- (15mg/kg) and high-dose ETC-642 (50mg/kg), and placebo. The test material was injected twice/week for 12 weeks. The aortic plaque burden was assessed by intravascular ultrasound (IVUS) at 0 and 12 weeks. Plasma lipid profiles were analyzed by capillary isotachophoresis every 4 weeks. ETC-642 had an effect on cholesterol efflux comparable to that of conventional rHDL. In WHHL rabbits, high-dose ETC-642 inhibited the progression of aortic atherosclerosis compared to placebo. There was no change in the percentage of plaque volume (%PV) in the high-dose group between before (30.9%) and after infusion (28.6%), whereas there was a significant increase in the control group from 27.8% to 37.9%. ETC-642 significantly reduced charge-modified low-density lipoprotein (LDL) by converting more negative-charged modified LDL to less negative-charged LDL, and reduced small dense (sd) LDL by converting it into large, buoyant (lb) LDL. Changes in the %PV were positively correlated with changes in negative-charged modified LDL (r=0.61, p<0.01) and sdLDL (r=0.59, p<0.01), and negatively correlated with changes in less negative-charged LDL (r=-0.43, p<0.01) and lbLDL (r=-0.57, p<0.01). In conclusion, the ETC-642-induced remodeling of sdLDL to large and lbLDL and the enhancement of cholesterol efflux may prevent progression of the aortic plaque burden. HDL-based therapy may be useful for preventing the progression of plaque volume.

Relationship between high density lipoprotein antioxidant activity and carotid arterial intima-media thickness in patients with essential hypertension

The objective of this study was to evaluate high-density lipoprotein (HDL) antioxidative activity and its possible influencing factors in patients with essential hypertension and to investigate the correlations between HDL antioxidative activity and the carotid arterial intima-media thickness (CIMT). Thirty-three patients with essential hypertension and 32 healthy people as control were included. High- and low-density lipoprotein in plasma were isolated by one-step density gradient ultracentrifugation, and induced oxidation with external Cu(2+). Antioxidant activity of HDL, lag time, and lipid peroxidation degree were determined by spectrophotometric and thiobarbituric acid reactive substances (TBARS) methods. Paraoxonase 1 (PON1) activity in serum was measured with continuous monitoring using phenylacetate as a substrate. The CIMT was measured with a high-resolution ultrasound Doppler system. In patients with essential hypertension, the inhibitory effect of HDL on low-density lipoprotein (LDL) oxidation and the PON1 activity were reduced (72.29 +/- 2.03)% vs. (80.91 +/- 2.06)%, and (112.21 +/- 8.64)u/ml vs. (146.43 +/- 8.79)u/ml (all P < 0.05). The lag time of oxidation and the lipid peroxidation between the hypertensive group and the control group did not show a statistically significant difference. Multiple stepwise regression analysis revealed that HDL antioxidative activity might be affected by PON1 activity (P = 0.004), diastolic pressure (P = 0.004), sex (P = 0.006), and that CIMT might be affected by HDL antioxidative activity (P = 0.030), systolic pressure (P = 0.026), and total cholesterol level (P = 0.033). The HDL antioxidative activity is reduced in patients with essential hypertension and significantly affected by sex. The CIMT was negatively correlated with HDL antioxidative activity, which suggests that decreased HDL antioxidative activity may be one of the important determinants for the development of atherosclerosis in patients with essential hypertension.

Signaling by the high-affinity HDL receptor scavenger receptor B type I

Scavenger receptor B type I (SR-BI) plays an important role in mediating cholesterol exchange between cells, high-density lipoprotein (HDL) cholesterol, and other lipoproteins. SR-BI in hepatocytes is essential for reverse cholesterol transport and biliary secretion of HDL cholesterol; thus, it is atheroprotective. More recently, it has been discovered that the HDL-SR-BI tandem serves other functions that also likely contribute to HDL-related cardiovascular protection. A number of the latter mechanisms, particularly in endothelial cells, involve unique direct signal initiation by SR-BI that leads to the activation of diverse kinase cascades. SR-BI signaling occurs in response to plasma membrane cholesterol flux. It requires the C-terminal PDZ-interacting domain of the receptor, which mediates direct interaction with the adaptor molecule PDZK1; and the C-terminal transmembrane domain, which directly binds membrane cholesterol. In endothelium, direct SR-BI signaling in response to HDL results in enhanced production of the antiatherogenic molecule nitric oxide; in a nitric oxide-independent manner, it serves to maintain endothelial monolayer integrity. The role of SR-BI signaling in the numerous other cellular targets of HDL, including hepatocytes, macrophages, and platelets, and the basis by which SR-BI senses plasma membrane cholesterol movement to modify cell behavior are unknown. Further understanding of signaling by SR-BI will optimize the capacity to harness the mechanisms of action of HDL-SR-BI for cardiovascular benefit.

Reconstituted high-density lipoprotein attenuates postinfarction left ventricular remodeling in rats

Since little is known about the effects of reconstituted high-density lipoprotein (rHDL) in left ventricular (LV) remodeling, these effects were examined in rats after acute myocardial infraction (MI). Sixteen male Wistar rats were randomly divided into three groups: Sham-operated (n=6), and MI rats that received a permanent ligation around the proximal left coronary artery and infusions of placebo (MI group, n=5) or rHDL (containing as apolipoproteinA-I 6mg/kg) administered intravenously (MI+rHDL group, n=5). rHDL was infused once a week for 4 weeks. In addition, in vitro assays were performed to examine the effect of rHDL. The MI+rHDL group showed a significant increase in LV ejection fraction (EF) between weeks 1 and 4, a decrease in LV end-systolic diameter, compared with the progressive deterioration of LV size and function in the MI group. In addition, the MI+rHDL group showed a significant decrease in fibrotic area of MI in LV compared to that in the MI group, while there were no significant increases in capillary density or cell size in LV in the MI+rHDL group. Interestingly, the MI+rHDL group showed a significant activation of retinoblastoma and ERK (extracellular-signal-regulated kinase) but not cleaved caspase-3, p38 MAPK or Jun N-terminal kinase. rHDL suppressed H(2)O(2)-induced arrest of cell growth in myocytes. This effect was blocked by PD98059, an ERK inhibitor. In conclusions, rHDL-promoted cell survival has beneficial morphological effects that help to prevent LV remodeling and improve function after MI, and may prevent arrest of cell growth through ERK pathway in myocytes.

Effects of the progestagen-only contraceptive implant Implanon on cardiovascular risk factors

OBJECTIVE

Epidemiological studies on the cardiovascular risk of progestagen-only contraceptives are rare. With the present study we aimed to investigate the effect of the low-dose etonogestrel-releasing contraceptive implant Implanon on cardiovascular risk factors, including markers of inflammation.

DESIGN

Longitudinal study.

SETTING

Family planning centre of a University Hospital.

SUBJECTS

Thirty-six healthy, nonsmoking women with regular cycles (n = 18 controls without hormonal contraception; n = 18 cases requesting the insertion of Implanon.

MEASUREMENTS

Blood samples for the determination of C-reactive protein (CRP), nitric oxide (NO), sex hormones and plasma lipids were taken in the early follicular phase of the cycle in both groups. A second sample was taken 12 weeks after Implanon insertion or in the controls during the early follicular phase of cycle 4.

RESULTS

Implanon treatment caused a 36% decrease in CRP (P < 0.06) and a significant decrease in high density lipoprotein (HDL) (P < 0.007), low density lipoprotein (LDL) (P < 0.001), cholesterol (P < 0.001), testosterone (P < 0.05) and SHBG (P < 0.002). Levels of NO, oestradiol and progesterone were not affected in either group. The cholesterol/HDL ratio did not change in Implanon carriers. There was a significant correlation between the cardiovascular risk factors CRP, cholesterol/HDL ratio and NO.

CONCLUSION

The progestagen-only implant Implanon does not exert a negative effect on the cardiovascular risk factors CRP, cholesterol/HDL ratio and NO. These results suggest that the use of a progestagen-only contraception does not increase cardiovascular risk factors in healthy young women.

Inconsistencies in the genetic prediction of HDL cholesterol versus atherosclerosis

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Newly developed reconstituted high-density lipoprotein containing sphingosine-1-phosphate induces endothelial tube formation

Reconstituted high-density lipoprotein (rHDL) has been shown to produce a rapid regression of atherosclerosis in animal models and humans. Sphingosine-1-phosphate (S1P), which is a bioactive lipid in HDL, plays a role in mitogenesis, endothelial cell motility, and cell survival, as well as organization and differentiation into a vessel. In this study, we examined the direct role of a newly developed rHDL, [POPC(1-palmitoyl-2-oleoyl phosphatidylcholine)/S1P/apolipoproteinA-I(A-I)]rHDL containing S1P in tube formation in endothelial cells (ECs) as well as cholesterol efflux in macrophage. The effect of (POPC/S1P/A-I)rHDL on cholesterol efflux in macrophage was similar to that of conventional rHDL, (POPC/A-I)rHDL. In addition, (POPC/S1P/A-I)rHDL induced EC proliferation through the activation of phospho-Akt and phospho-extracellular-signal-regulated kinases (p-ERK) 1/2 and EC tube formation, and this effect was blocked by inhibitors of Akt, ERK and endothelial nitric-oxide synthase (eNOS). In addition, (POPC/S1P/A-I)rHDL-induced p-ERK1/2 activation and EC tube formation can be mainly attributed to S1P-stimulated signaling through S1P2 and S1P3 as determined by an anti-sense strategy. In conclusion, (POPC/S1P/A-I)rHDL induces cholesterol efflux independently of S1P but has additional S1P-mediated effects on EC tube formation mediated by Akt/ERK/NO through S1P2 and S1P3. In the future, these new discs may be useful for the treatment of atherosclerotic and ischemic cardiovascular disease, such as acute coronary syndrome and atherosclerosis obliterans.

Reverse cholesterol transport and cholesterol efflux in atherosclerosis

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

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

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

The expression of intact and mutant human apoAI/CIII/AIV/AV gene cluster in transgenic mice

The apoAI/CIII/AIV gene cluster is involved in lipid metabolism and has a complex pattern of gene expression modulated by a common regulatory element, the apoCIII enhancer. A new member of this cluster, apolipoprotein (apo) AV, has recently been discovered as a novel modifier in triglyceride metabolism. To determine the expression of all four apo genes in combination and, most importantly, whether the transcription of apoAV is coregulated by the apoCIII enhancer in the cluster, we generated an intact transgenic line carrying the 116-kb human apoAI/CIII/AIV/AV gene cluster and a mutant transgenic line in which the apoCIII enhancer was deleted from the 116-kb structure. We demonstrated that the apoCIII enhancer regulated hepatic and intestinal apoAI, apoCIII, and apoAIV expression; however, it did not direct the newly identified apoAV in the cluster. Furthermore, human apo genes displayed integrated position-independent expression and a closer approximation of copy number-dependent expression in the intact transgenic mice. Because apoCIII and apoAV play opposite roles in triglyceride homeostasis, we analyzed the lipid profiles in our transgenic mice to assess the effects of human apoAI gene cluster expression on lipid metabolism. The triglyceride level was elevated in intact transgenic mice but decreased in mutant ones compared with nontransgenic mice. In addition, the expression of human apoAI and apoAIV elevated high density lipoprotein cholesterol in transgenic mice fed an atherogenic diet. In conclusion, our studies with human apoAI/CIII/AIV/AV gene cluster transgenic models showed that the apoCIII enhancer regulated expression of apoAI, apo-CIII, and apoAIV but not apoAV in vivo and showed the influences of expression of the entire cluster on lipid metabolism.

HDL cholesterol and protective factors in atherosclerosis

A low level of high-density lipoprotein cholesterol (HDL-C) is an important risk factor for cardiovascular disease. Epidemiological and clinical studies provide evidence that HDL-C levels are linked to rates of coronary events. The cardioprotective effects of HDL-C have been attributed to its role in reverse cholesterol transport, its effects on endothelial cells, and its antioxidant activity. Although some clinical trials suggest a benefit of raising HDL-C to reduce risk, further studies are needed, and HDL-C is still not considered a primary target of therapy in the National Cholesterol Education Program guidelines. However, HDL-C should be considered as part of the patient's overall profile of established risk factors in determining treatment strategies.

A comparison of HDL and LDL cholesterol for prevalent coronary calcification

BACKGROUND

Coronary calcification is a marker for coronary atherosclerosis. It has been postulated that high levels of high density lipoprotein cholesterol (HDL-C) are associated with a reduced amount of atherosclerotic disease while previous reports have found a lack of association between low density lipoprotein cholesterol (LDL-C) and coronary calcification (CAC). The purpose of this study was to compare the correlation and predictive power of HDL-C with LDL-C for prevalent coronary calcification.

METHODS

A total of 6093 subjects were studied with respect to coronary calcification, serum cholesterol indices, personal health history and body morphology. Analyses consisted of correlation coefficients, logistic regression and sensitivity analysis to determine the strength of association between HDL-C and coronary calcification after controlling for covariates.

RESULTS

The correlation between HDL-C and coronary calcium score (CCS) was three times that of LDL-C. Individuals with an HDL-C level <40 mg/dl had significantly higher calcium scores while increases in HDL-C were associated with a significant reduction in risk for the presence of any calcified plaque. Results of multivariate logistic regression revealed that HDL-C is predictive of calcified plaque development independent of LDL-C. Sensitivities and positive predictive values for both HDL-C and LDL-C were low.

CONCLUSIONS

Increasing levels of HDL-C were associated with less coronary calcification and a smaller probability of having any calcified disease supporting the antiatherogenic hypothesis for HDL-C. HDL-C predicts the presence of any calcified atherosclerotic plaque independently of LDL-C. However, neither parameter seems suitable as a screening tool for predicting prevalent calcified atheromatous disease.

Involvement of Cdc42 signaling in apoA-I-induced cholesterol efflux

Cholesterol efflux, an important mechanism by which high density lipoproteins (HDL) protect against atherosclerosis, is initiated by docking of apolipoprotein A-I (apoA-I), a major HDL protein, to specific binding sites followed by activation of ATP-binding cassette transporter A1 (ABCA1) and translocation of cholesterol from intracellular compartments to the exofacial monolayer of the plasma membrane where it is accessible to HDL. In this report, we investigated potential signal transduction pathways that may link apoA-I binding to cholesterol translocation to the plasma membrane and cholesterol efflux. By using pull-down assays we found that apoA-I substantially increased the amount of activated Cdc42, Rac1, and Rho in human fibroblasts. Moreover, apoA-I induced actin polymerization, which is known to be controlled by Rho family G proteins. Inhibition of Cdc42 and Rac1 with Clostridium difficile toxin B inhibited apoA-I-induced cholesterol efflux, whereas inhibition of Rho with Clostridium botulinum C3-exoenzyme exerted opposite effects. Adenoviral expression of a Cdc42(T17N) dominant negative mutant substantially reduced apoA-I-induced cholesterol efflux, whereas dominant negative Rac1(T17N) had no effect. We further found that two downstream effectors of Cdc42/Rac1 signaling, c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (p38 MAPK), are activated by apoA-I. Pharmacological inhibition of JNK but not p38 MAPK decreased apoA-I-induced cholesterol efflux, whereas anisomycin and hydrogen peroxide, two direct JNK activators, could partially substitute for apoA-I in its ability to induce cholesterol efflux. These results for the first time demonstrate activation of Rho family G proteins and stress kinases by apoA-I and implicate the involvement of Cdc42 and JNK in the apoA-I-induced cholesterol efflux.

Atheroprotective effects of high-density lipoproteins

Observational studies provide overwhelming evidence that a low high-density lipoprotein (HDL)-cholesterol level increases the risk of coronary events, both in healthy subjects and in patients with coronary heart disease. Based on in vitro experiments, several mechanistic explanations for the atheroprotective function of HDL have been suggested. However, few of these were verified in vivo in humans or in experiments with transgenic animals. The HDL functions currently most widely held to account for the antiatherogenic effect include participation in reverse cholesterol transport, protection against endothelial dysfunction, and inhibition of oxidative stress. This review summarizes current views on the molecular mechanism underlying these atheroprotective effects of HDL.

High density lipoprotein-induced angiogenesis requires the activation of Ras/MAP kinase in human coronary artery endothelial cells

OBJECTIVE

Plasma high density lipoprotein (HDL) levels have been shown to be inversely correlated with coronary artery disease, but the mechanisms of the direct protective effect of HDL on endothelial cells (ECs) are not fully understood. In this study, we investigated the role of the HDL-mediated promotion of angiogenesis in human coronary artery ECs (HCECs).

METHODS AND RESULTS

We developed an in vitro model of HCEC tube formation on a matrix gel. We optimized the maximum dose of HDL required to induce tube formation in initial experiments, in which the dose response showed that the maximum effective dose of HDL was 100 microg/mL. PD98059, an inhibitor of p42/44 mitogen-activated protein kinase (MAPK) activity, but not SB203580, an inhibitor of p38 MAPK activity, suppressed HDL-induced tube formation. Dominant-negative Ras N17 inhibited HDL-induced tube formation. HDL activated Ras according to a ras pull-down assay, and this effect was inhibited by pertussis toxin. Moreover, HDL activated phospho(p)-p42/44 MAPK, whereas Ras N17 blocked HDL-induced pp42/44 MAPK.

CONCLUSIONS

These results indicate that HDL induced a potent signal through a Ras/MAPK pathway mediated by a pertussis toxin-sensitive G-protein coupled receptor to the angiogenic phenotype in HCECs.

HDL and triglyceride as therapeutic targets

PURPOSE OF REVIEW

Epidemiological studies have shown that plasma HDL-cholesterol is inversely related to coronary artery disease and that there is an inverse relationship between HDL-cholesterol and triglyceride levels, but it is now demonstrated that hypertriglyceridemia is an independent risk factor for coronary heart disease (CHD). The goal of this review is to discuss if triglycerides and HDL-cholesterol could be therapeutic targets to reduce cardiovascular risk.

RECENT FINDINGS

Triglyceride measurement is not informative on the specificity of the triglyceride-rich lipoproteins present in the plasma because some of these are not atherogenic (chylomicrons, large VLDLs) while others are highly atherogenic (small VLDLs, remnants, IDL...). Statins, in addition to reducing LDL-cholesterol, significantly reduced atherogenic remnant lipoprotein cholesterol levels. 4S, CARE+LIPID, and AFCAPS/TexCAPS studies, suggested enhanced therapeutic potential of statins for improving triglyceride and HDL-cholesterol levels in patients with CHD. A fibrate (gemfibrozil) was shown to reduce death from CHD and non-fatal myocardial infarction in secondary prevention of CHD in men with low levels of HDL-cholesterol (VA-HIT); during the treatment these levels predicted the magnitude of reduction in risk for CHD events.

SUMMARY

ATP III recommendations state, on triglycerides and HDL-cholesterol as targets to reduce cardiovascular risk: (1) that lowering LDL-cholesterol levels is the primary target of therapy, (2) a secondary target is to achieve a triglyceride level < 150 mg/dL and (3) clinical trial data are considered to be insufficient to support recommended a specific HDL-cholesterol goal even if HDL-cholesterol < 40 mg/dL is considered to be a major risk factor of CHD.

Lipid free apolipoprotein E binds to the class B Type I scavenger receptor I (SR-BI) and enhances cholesteryl ester uptake from lipoproteins

The Class B type I scavenger receptor I (SR-BI) is a physiologically relevant high density lipoprotein (HDL) receptor that can mediate selective cholesteryl ester (CE) uptake by cells. Direct interaction of apolipoprotein E (apoE) with this receptor has never been demonstrated, and its implication in CE uptake is still controversial. By using a human adrenal cell line (NCI-H295R), we have addressed the role of apoE in binding to SR-BI and in selective CE uptake from lipoproteins to cells. This cell line does not secrete apoE and SR-BI is its major HDL-binding protein. We can now provide evidence that 1) free apoE is a ligand for SR-BI, 2) apoE associated to lipids or in lipoproteins does not modulate binding or CE-selective uptake by the SR-BI pathway, and 3) the direct interaction of free apoE to SR-BI leads to an increase in CE uptake from lipoproteins of both low and high densities. We propose that this direct interaction could modify SR-BI structure in cell membranes and potentiate CE uptake.

Hepatic lipase: a pro- or anti-atherogenic protein?

Hepatic lipase (HL) plays a role in the metabolism of pro- and anti-atherogenic lipoproteins affecting their plasma level and composition. However, there is controversy regarding whether HL accelerates or retards atherosclerosis. Its effects on different lipoprotein classes show that, potentially, HL may promote as well as decrease atherogenesis. Studies in animals with genetically modulated HL expression show that it depends on the model used whether HL acts pro- or anti-atherogenic. In humans, HL activity seems to correlate inversely with atherosclerosis in (familial) hypercholesterolemia, and positively in hypertriglyceridemia. In normolipidemia, HL activity is weakly associated with coronary artery disease (CAD). Genetically low or absent HL activity is usually associated with increased CAD risk, especially if plasma lipid transport is impaired due to other factors. Since HL promotes the uptake of lipoproteins and lipoprotein-associated lipids, HL may affect intracellular lipid content. We hypothesize that the prime role of HL is to maintain, in concert with other factors (e.g., lipoprotein receptors), intracellular lipid homeostasis. This, and the uncertainties about its impact on human atherosclerosis, makes it difficult to predict whether HL is a suitable target for intervention to lower CAD risk. First, the physiological meaning of changes in HL activity under different conditions should be clarified.

HDL and arteriosclerosis: beyond reverse cholesterol transport

The inverse correlation between serum levels of high density lipoprotein (HDL) cholesterol and the risk of coronary heart disease, the protection of susceptible animals from atherosclerosis by transgenic manipulation of HDL metabolism, and several potentially anti-atherogenic in vitro-properties have made HDL metabolism an interesting target for pharmacological intervention in atheroslcerosis. We have previously reviewed the concept of reverse cholesterol transport, which describes both the metabolism and the classic anti-atherogenic function of HDL (Arterioscler. Thromb. Vasc. Biol. 20 2001 13). We here summarize the current understanding of additional biological, potentially anti-atherogenic properties of HDL. HDL inhibits the chemotaxis of monocytes, the adhesion of leukocytes to the endothelium, endothelial dysfunction and apoptosis, LDL oxidation, complement activation, platelet activation and factor X activation but also stimulates the proliferation of endothelial cells and smooth muscle cells, the synthesis of prostacyclin and natriuretic peptide C in endothelial cells, and the activation of proteins C and S. These anti-inflammatory, anti-oxidative, anti-aggregatory, anti-coagulant, and pro-fibrinolytic activities are exerted by different components of HDL, namley apolipoproteins, enzymes, and even specific phospholipids. This complexity further emphasizes that changes in the functionality of HDL rather than changes of plasma HDL-cholesterol levels determine the anti-atherogenicity of therapeutic alterations of HDL metabolism.